ABSTRACT

Gonadotropin hormone-releasing hormone (GnRH) is the key regulator of the reproductive axis.  Its pulsatile secretion determines the pattern of secretion of the gonadotropins, follicle stimulating hormone and luteinizing hormone, which then regulate both the endocrine function and gamete maturation in the gonads. Recent years have seen rapid developments in how GnRH secretion is regulated, with the discovery of the kisspeptin-neurokinin-dynorphin neuronal network in the hypothalamus. This mediates both positive and negative sex steroid feedback control of GnRH secretion, in conjunction with other neuropeptides and neurotransmitters. This chapter describes the main features of this regulatory system, including how its anatomical arrangements interact with functional control, and describes key differences between rodent and larger mammalian systems.

INTRODUCTION

Since the discovery of Gonadotropin Releasing Hormone (GnRH), an extensive body of literature has established it as the pivotal central regulator of human reproduction. However, the GnRH neuronal network, per se lacks the cellular machinery to fully integrate developmental, environmental, endocrine, and metabolic factors that influence its secretion. For example, GnRH neurons do not express the principal estrogen receptor alpha (ER-alpha), which is required for sex-steroid mediated control of gonadotropin secretion (1). Intermediate signaling pathways must therefore exist to mediate gonadal steroid feedback. Current evidence, accumulated since the discovery of Kisspeptin-Neurokinin B-Dynorphin (KNDy) neuronal network in the last decade, suggests a pivotal role for this network in the regulation of pulsatile GnRH secretion by integrating nutrient, endocrine, and environmental signals, and thus the control of downstream hypothalamic-pituitary-gonadal (HPG) axis.

The HPG axis anatomically comprises:

1. The hypothalamus (especially the infundibular nucleus, the human homologue of the arcuate nucleus) where the KNDy and GnRH-producing neurons are located.
2. The anterior pituitary, where Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) are secreted by gonadotropes.
3. The gonads, responsible for the production of both sex steroids and gametes, under the influences of LH and FSH.

As with other endocrine systems, positive and negative feedback regulate HPG axis (2,3). In this chapter, we have focused on human data. Where human data is limited, data from other species are leveraged.

GONADOTROPIN RELEASING HORMONE (GnRH) – THE PRINCIPAL REGULATOR OF REPRODUCTION

The Discovery of GnRH

GnRH was isolated from porcine hypothalami and structurally identified as a decapeptide (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly·NH2) five decades ago (4-6). This decapeptide was shown to potently stimulate LH and FSH release from the pituitary in a number of mammalian species (6,7). Early literature referred to this peptide as the ‘Luteinizing Hormone-Releasing Hormone (LH-RH)’, but more recently, it is widely referenced as Gonadotropin Releasing Hormone (GnRH) -reflecting the stimulatory role on the secretion of both gonadotropins – i.e., LH and FSH (8).

Diverse forms of GnRH and its receptor exist among vertebrates, with over twenty primary structures across species, suggesting that GnRH system developed early in the evolutionary sequence (9,10). The GnRH structure was first identified in mammals and is therefore referred to as GnRH I (9,11).  Subsequently, another structurally different vertebrate GnRH sequence was first identified from chicken brain -this is now referred to as GnRH II (pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2) (10,12). A third form has also been described in fish - GnRH III (9,12). In mammals, hypophysiotropic functions are limited to GnRH I, therefore in the human context GnRH I is referred to as GnRH (13) and we will use this terminology for this review.

Neuroanatomy of GnRH Neurons

GnRH neurons originate in the medial olfactory placode during embryological development and migrate along the olfactory bulb to their final positions within the hypothalamus (14,15). A number of factors contributing to this GnRH neuron migratory process have been identified: anosmin-1 (the product of KAL gene) (16), neuropilins (17), leukemia inhibitory factor (18), fibroblast growth factor receptor 1 (19), fibroblast growth factor receptor 8 (20), polysialic form of neural adhesion molecule (PSA-NCAM) (21), among others (22). Defective GnRH migration leads to Kallmann syndrome, characterized by hypogonadotropic hypogonadism due to GnRH deficiency and anosmia (15). Mutations in prokineticin genes (PROK1 and PROK2) lead to hypogonadotropic hypogonadism without anosmia, suggesting that factors other than suboptimal migration can also lead to functional deficiencies in GnRH (15,23,24).

GnRH cell bodies are located in the medial preoptic area (POA) and in the arcuate/infundibular nucleus of the hypothalamus, forming a neuronal network with projections to the median eminence (25). GnRH is secreted from the median eminence into the fenestrated capillaries of portal circulation, carried to the anterior pituitary (25). In humans, the number of GnRH neurons has been estimated to range between 1000 to 1500 (9,14). The co-location of GnRH neurons with other central regulators allows the GnRH network to be influenced by a range of neuroendocrine and metabolic inputs (26).

GnRH Secretion and Pulsatility

Two distinct modes of GnRH secretion have been described: pulsatile and surge modes (26). Pulsatile mode refers to episodic release of GnRH, with distinct pulses of GnRH secretion into the portal circulation with undetectable GnRH concentrations between pulses. The surge mode of GnRH secretion occurs in females, during the pre-ovulatory phase, in which the presence of GnRH in the portal circulation appears to be persistent (26,27).

Direct pulsatile GnRH release was initially demonstrated in ovariectomized rhesus monkeys using serial samples of portal blood (28). Pulsatile pattern of GnRH secretion was demonstrated subsequently in humans through serial blood sampling during pituitary surgery (29). Abolishment of LH pulses by GnRH antisera (30,31) and its reestablishment with GnRH analogues (30) suggest that LH pulses are determined by the underlying GnRH pulsatility.  The LH pulsatility was first detected during an attempt to validate a radioimmunoassay to measure serum LH in rhesus monkeys, where marked variations in LH levels was noticed (32). Further studies confirmed the pulsatile nature of LH secretion (33-35). In women, the frequency and amplitude of LH pulses were noted to be dependent on the menstrual cycle phase, with pulses every 1 to 2 hours during the early follicular phase eventually merging into a continuous mid-cycle surge, and decreased pulse frequency to every 4 hours during the luteal phase (36). In humans, LH pulse frequency is used as a surrogate of GnRH pulsatility, as ethical considerations and technical challenges preclude sampling of hypophyseal blood or cerebrospinal fluid to measure GnRH concentrations directly (37,38).

The importance of GnRH pulsatility on LH and FSH secretion was first demonstrated in rhesus monkeys, where endogenous GnRH secretion was abolished by hypothalamic radiofrequency. Pulsatile GnRH reinstated gonadotropin secretion in these animals, whereas continuous GnRH only elicited a transient response. Moreover, the switch from continuous to pulsatile GnRH administration allowed recovery of gonadotropin secretion (39).

GnRH neurons coordinate their activity, but the precise mechanism of this remains unclear (27,40), and is the subject of continuing investigations. Episodic multi-unit electrical activity at medial basal hypothalami (MBH) is correlated with LH release, suggesting that ‘GnRH pulse generator’ is anatomically located at MBH – or closely linked to it neurohormonally(41,42). GnRH neurons show intrinsic electrical pulsatility. GnRH cell lines derived from mouse hypothalamic and fetal olfactory placode GnRH neurons both demonstrate intrinsic pulsatility in vitro (26,43,44). Functionally, the ‘GnRH pulse generator’ relies on complex relations between glutamatergic cells, GnRH and other neurons, and likely other elements are involved, of which the kisspeptin-neurokinin B-opioid pathway may have a pivotal intermediary role in the regulation of GnRH pulsatility (45).

Differential Regulation of LH and FSH

The stimulatory effects of GnRH on LH and FSH secretion are not identical (46).  FSH secretion is more irregular than LH in both humans and sheep, which is essentially related to the pulsatility and different stimulatory effects of GnRH, but other factors also might be relevant, such as differences in LH and FSH storage (more scarce for the FSH), existence of different gonadotropes subpopulations, or diverse response times to GnRH (47). In ovariectomized sheep administered GnRH antisera, pulsatile secretion of LH was completely inhibited (undetectable LH levels within 24 hours), while the FSH concentration fell more slowly and remained detectable (30). It has been estimated that 93% of the GnRH pulses were associated with FSH pulses and, unlike LH, a constitutive secretion of FSH appears to exist (48). The frequency of GnRH input has been demonstrated to selectively regulate gonadotropin subunit gene transcription: rapid GnRH pulse rates increase α and LH-β and slow GnRH pulse frequency increases FSH-β gene transcription (49-51). Moreover, with progressive increases in GnRH frequency (from one pulse every 120 to 60 min, from 60 to 30 min, and from 30 to 15 min) in GnRH deficient men, mean LH rose concurrently with a decrease in LH pulse amplitude, while FSH remained unchanged (52).

Biological and Clinical Relevance of GnRH Pulsatility

Appropriate modulation of LH pulse frequency is essential for pubertal maturation and reproductive function. In infancy, LH pulsatile secretion is increased (often termed mini-puberty), likely reflecting pulsatile GnRH secretion, but soon becomes quiescent (53). This pre-pubertal suppression of HPG axis has been shown to occur in agonadal humans (54)and primates (55), suggesting that hypothalamo-hypophyseal factors play a role in post-natal quiescence of the reproductive axis, until puberty sets in.

The onset of pubertal maturation is heralded by the development of a pattern of steady acceleration in LH pulsatility (56). In children, higher basal and GnRH stimulated LH concentrations are observed in early childhood (<5 years). This is subdued mid-childhood (5-11 years) and increase thereafter with pubertal development (54,57).  Conceptually, an abnormal reactivation of GnRH pulse frequency is the central mechanism associated with precocious or delayed puberty (14).

In women, the pattern of GnRH secretion is essential for the regulation of the menstrual cycle (Figure 1) (58,59). LH pulse frequency is slow in the luteal phase, and increasingly speeds up during the follicular and the pre-ovulatory phases, presumably reflecting changes in GnRH pulse frequency (60). Abnormalities in GnRH - and hence LH pulse frequency - are associated with a number of reproductive endocrine disorders. In hypothalamic amenorrhea, a condition associated with anovulatory amenorrhea and hypoestrogenism, LH pulse frequency (and by inference GnRH) is lower than expected for the prevailing steroid profile and is comparable to luteal phase pulsatility (37). LH pulse frequency in hyperprolactinemic women is also lower than in healthy women, requiring dopaminergic agonist preparations, such as bromocriptine to regulate prolactin secretion and restore LH pulse frequency (38). In polycystic ovary syndrome LH pulse frequency and amplitude are higher throughout the menstrual cycle in comparison to that observed in healthy women, contributing to chronic anovulation (61-64).

Figure 1. Hormonal oscillations through the menstrual cycle. In the early follicular phase of the menstrual cycle, the initial increase in FSH stimulates follicular recruitment and maturation. The consequent secretion of estradiol (E2) selectively inhibits FSH release (needed for selection of the dominant follicle) and maintains rapid GnRH pulsatility during the late follicular phase. The persistent rapid GnRH pulses increase LH, which further stimulates E2 secretion, culminating in positive E2 feedback to produce the mid-cycle LH surge. During the LH surge, GnRH levels appear to be consistently elevated and remain elevated as LH declines, suggesting that the frequency of GnRH pulse has become very rapid or continuous, which results in desensitization of LH secretion (possibly the mechanism to terminate the LH surge). After ovulation, luteinization of the ruptured follicle results in progesterone secretion which reduces the frequency of GnRH pulses. With the demise of corpus luteum, E2, progesterone and inhibin levels fall, and the GnRH pulse frequency increases, leading to follicular maturation in the next cycle. (Adapted from: Marshall JC, Dalkin AC, Haisenleder DJ, Paul SJ, Ortolano GA, Kelch RP. Gonadotropin-releasing hormone pulses: regulators of gonadotropin synthesis and ovulatory cycles. Recent Prog Horm Res. 1991;47:155-187).

NEURONAL REGULATION OF GnRH SECRETION: THE KISSPEPTIN-NEUROKININ B-DYNORPHIN (KNDy) NEURONAL NETWORK

Whilst the central role attributed to GnRH remains undisputed, its effective function requires input from other neuronal networks. For instance, the absence of estrogen receptor alpha (ER-alpha) expression on GnRH neurons suggests the need for an intermediate signaling pathway to mediate gonadal steroid feedback (1). The discovery of kisspeptin signaling in neuroendocrine regulation of human reproduction revolutionized the current understanding of the HPG axis. Kisspeptin signaling pathway is increasingly recognized as essential for normal puberty, gonadotropin secretion, and regulation of reproduction (65-67). Other relevant kisspeptin roles have been identified such as regulation of sexual and social behavior, emotional brain processing, mood, audition, olfaction, metabolism, body composition, cardiac function, among others (68-74).

Discovery of KNDy Neuronal Network

KiSS1, the gene encoding kisspeptins, was first described in 1996 as a suppressor of metastasis in human malignant melanoma (75,76). This gene was discovered in Hershey and named in accordance with the famous chocolates ‘Hershey’s Kisses’; the inclusion of ‘SS’ is indicative of ‘suppressor sequence’. The KiSS1 gene maps to chromosome 1q32 and includes four exons of which the first two are not translated (77). The gene encodes the precursor 145 amino acid peptide, which is cleaved down to a 54 amino acid peptide. This peptide can be truncated to 14, 13 and 10 amino-acid peptides, all sharing the C-terminal sequence (78,79). These peptides are collectively referred to as kisspeptins - and Kp-10, Kp-13, Kp-14 and Kp-54 are suggested abbreviations for human kisspeptins (80). In 2001, kisspeptins were identified as ligands for the orphan G–protein receptor 54 (GPR54) (81-83), currently named KISS1R (80). KISS1R is localized to human chromosome 19p13.3 and it has five exons, encoding a 398-amino acid protein with seven trans-membrane domains (79,82). Upon binding by kisspeptin, KISS1R activates phospholipase C and recruits intracellular messengers, inositol triphosphate and diacylglycerol, which in turn lead to the release of calcium and activation of protein kinase C (82-84).

A reproductive role for kisspeptin in humans became apparent from patients with pubertal disorders which were associated with KISS1R mutations (85-87). A number of inactivating mutations of Kiss1 and Kiss1r have since been reported in animal models with phenotypes characterized by pubertal delay (88). An activating mutation in KISS1R has been described in a girl with precocious puberty: when compared to cells with wild-type transfected GPR54, cells with this mutation showed prolonged inositol phosphate accumulation and phosphorylation of extracellular signal–regulated kinase, suggesting extended activation of intracellular signaling by the mutant GPR54 (89). Missense mutations have also been reported in KISS1 gene in three unrelated children with central precocious puberty (90). Functional studies of these mutant peptides demonstrated higher resistance to in vitro degradation but normal affinity to KISS1R, thus suggestive of increased bioavailability as the mechanism by which these abnormal kisspeptins induce precocious puberty (90).

Recently in an Asian cohort, potentially regulatory polymorphisms, as rs5780218 and rs12998, in KiSS1 gene were significantly associated to genetic susceptibility to central precocious puberty in Chinese girls by single-locus analysis (91). Nevertheless, these findings are inconsistently reported in literature and require additional validation in functional studies.

A role for neurokinin B in the hypothalamic regulation was also demonstrated when genetic studies in patients from consanguineous families with hypogonadotropic hypogonadism were found to have missense mutations in TAC3 (encodes neurokinin B) and TACR3 (encodes neurokinin B receptor) (92). Other cases have been reported since (93-96).

There is also long-standing evidence for the role of opioid systems in reproduction. In 1980, Wilkes reported the localization of β endorphin in the human hypothalamus (97). Studies involving the administration of naloxone and naltrexone (opioid antagonists) to humans showed stimulatory effects on LH secretion (98,99), and other studies supported the notion that endogenous opioids play a role in the control of HPG axis (100-104). In 2007, it was demonstrated that dynorphin and kisspeptin are co-localized along with neurokinin B in the same hypothalamic neuronal population in sheep, therefore termed KNDy (Kisspeptin-Neurokinin B-Dynorphin) neurons, highlighting the possible interconnection between these neuropeptides in the control of GnRH and gonadotropin secretion (105-107). The co-localization of kisspeptin, neurokinin B and dynorphin has also been demonstrated in humans (108).

Kisspeptin neurons have also other important neuroanatomical relationships, such as with neuronal nitric oxide synthase neurons as demonstrated in prepubertal female sheep (109), or with somatostatin neurons in the rat hypothalamus (110).

Neuroanatomy of KNDy Neuronal Network

The location of kisspeptin neurons is different between rodents and human species. In humans, kisspeptin neurons are distributed in the rostral Pre-optic Area (POA) and in the infundibular nucleus in the hypothalamus (Figure 2) (108,111). In both male and female autopsy samples, the majority of kisspeptin cell bodies are identified in the infundibular nucleus, and a second dense population of kisspeptin neurons in the rostral POA (108). The infundibular nucleus (arcuate nucleus in non-human species) is similar across species, but the rostral region is more species specific (108,112,113). In rodents, the rostral population is located in the anteroventral periventricular nucleus (AVPV) and the periventricular nucleus (PeN), the continuum of this region named as the rostral periventricular region of the third ventricle (RP3V) (112,114). Humans and ruminants lack this well-defined RP3V population of kisspeptin neurons, which are more scattered within the preoptic region (113,115).

Kisspeptin axons form dense plexuses in the human infundibular stalk, where the secretion of GnRH occurs (108). Axo-somatic, axo-dendritic, and axo-axonal contacts between kisspeptin and GnRH axons were demonstrated at this level, showing that kisspeptin and GnRH networks are in close proximity (108,116). Moreover, GnRH neurons express Kiss1rmRNA, reinforcing the notion of kisspeptin involvement in GnRH secretion (117-119).

Figure 2. Neuroanatomy of kisspeptin-GnRH pathway and the control of HPG axis in humans and rodents. Kisspeptin signals directly to GnRH neurons, which express KISS1R. The location of kisspeptin neurons within the hypothalamus is species specific, residing within the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus in rodents, and within the preoptic area (POA) and the infundibular nucleus in humans. Kisspeptin neurons in the infundibular nucleus (humans)/arcuate nucleus (rodents) co-express neurokinin B and dynorphin (KNDy neurons), which autosynaptically regulate kisspeptin secretion (via neurokinin B receptor and kappa opioid peptide receptor). In humans, infundibular KNDy neurons relay negative (red) and positive (green) feedback, whereas in rodents the negative and positive steroid feedback are mediated via arcuate nucleus and AVPV respectively. The role of human POA kisspeptin neurons in sex steroid feedback is not yet clear. (Adapted from: Skorupskaite K, George JT, Anderson RA. The kisspeptin-GnRH pathway in human reproductive health and disease. Human Reproduction Update. 2014;20:485-500).

Three-quarters of kisspeptin-immunoreactive cells in the human infundibular nucleus of the hypothalamus co-express neurokinin B and dynorphin (KNDy neurons) (108,120). KNDy neurons in rodents and ruminants are localized in the arcuate nucleus of the hypothalamus. However, neurokinin B and dynorphin are absent from kisspeptin neurons in the hypothalamic POA (Figure 2) (67,115). This differential expression of neuropeptides may reflect distinct functions of these two kisspeptin populations with kisspeptin neurons in the AVPV acting as LH surge generators, while those in the ARC (including KNDy neurons) acting as LH pulse generators.

Significant kisspeptin expression was also demonstrated in central extra-hypothalamic sites, including in limbic and paralimbic brain regions, such as medial amygdala, cingulate, globus pallidus, hippocampus, putamen and thalamus, key areas of neurobiological control of sexual and emotional behaviors (reviewed in detail in (121)), as well as peripherally in organs like ovary, testis, uterus and placenta where the kisspeptin system may also play a part in reproduction function (122,123).

Apart from reproductive and central kisspeptin expression, the kisspeptin signaling system has been demonstrated in several peripheral tissues, namely, in pancreas (involved in glucose-stimulated insulin secretion); in endothelial cells of different vascular beds as coronary artery, aorta and umbilical vein (triggering vasoconstriction); in the kidney, namely, in tubular cells, collecting duct cells and vascular smooth muscle cells (involved in function and renal morphogenesis); as well as in bone, fat and liver tissue (78,124,125).

Interactions Between Kisspeptin, Neurokinin B and Dynorphin

KDNy neurons act synergistically to induce coordinated and pulsatile GnRH secretion by regulating the neuroactivity of other KDNy cells. This is supported by the existence of neurokinin B and kappa opioid peptide receptors (receptor for dynorphin) within the KNDy cells, but not kisspeptin receptors, which are predominantly expressed on GnRH neurons (107,120,126). Neuron-neuron and neuron-glia communications via gap junctions contribute for the synchronized activities among KNDy neurons (127).  

Neurokinins (A and B) are members of the tachykinin family of peptides, which stimulate three related GPCRs (encoded by TACR1, TACR2 and TACR3) (128) This family also contains substance P, neuropeptide K, neuropeptide γ, hemokinin-1, and more recently endokinins. Neurokinin B acts predominantly on TACR3. Neurokinin B increases the membrane potential of KNDy neurons, leading to an increase in KNDy neuron pulsatile activity which, in turn, will promote the secretion of kisspeptin leading, ultimately, to GnRH secretion (67,129,130). Neurokinin B signaling regulates GnRH/LH secretion in healthy women, and it is crucial for the mediation of the estrogenic positive and negative feedback on LH secretion (131-133). There is rapidly increasing interest in the therapeutic value of neurokinin antagonists in several indications in reproductive health, recently reviewed in (134).

In women with polycystic ovary syndrome, the relationship between kisspeptin and gonadotropin levels has been widely explored in those with anovulatory cycles (135-138). Most studies have shown higher serum levels of kisspeptin and LH when the oligomenorrhea phenotype is present, despite the high heterogeneity observed. In this context, potential treatments targeting neuroendocrine dysfunction emerged. The administration of neurokinin 3 receptor antagonists markedly reduced serum LH concentration and pulse frequency, as well as serum testosterone (139-141). A recent study confirmed a complex crosstalk between neurokinin B and kisspeptin pathways in the regulation of GnRH secretion in polycystic ovary syndrome. In this study, kisspeptin-10 infusion given to women with polycystic ovary syndrome increased LH secretion with a direct relationship to estradiol exposure. Neurokinin 3 receptor antagonism reduced LH secretion and pulsatility, and whilst LH response to kisspeptin-10 was preserved, its relationship with circulating estradiol was not. More interestingly, although kisspeptin-10 increased LH pulse frequency, changes in other parameters of LH secretory pattern were prevented when co-administered with neurokinin 3 receptor antagonists (141).

In postmenopausal women,  seven day treatment with neurokinin 3 receptor antagonist decreased LH secretion, but not FSH secretion, as well as lead to a remarkable reduction in hot flushes (142). Neurokinin 3 receptor antagonism efficiency in treating menopausal hot flushes has been also demonstrated in other clinical trials (143,144). Fezolinetant administrated in a single daily dose regimen (30 or 45mg/daily) for treatment of moderate to severe vasomotor symptoms reduced these symptoms by over 50% from baseline within the first week and persistently during the 52-week treatment period, and is now approved for the treatment of menopausal vasomotor symptoms (145-148).

A second comparable drug, elinzanetant, which differs in its pharmacology in that it is an antagonist at the NK1 as well as NK3 receptor, has also been recently demonstrated to reduce the severity and frequency of moderate-to-severe vasomotor symptoms and also to improve sleep quality and menopause-related quality of life (149). The importance of NK1 receptor antagonism in these effects is unclear.

In healthy men, neurokinin B signaling display a central role for the reproductive function, and this is functionally upstream of kisspeptin-mediated GnRH secretion: LH, FSH and testosterone secretion decreased during the administration of a neurokinin 3 receptor antagonist, while kisspeptin-10 administration restored LH secretion to the same degree before and during neurokinin 3 receptor antagonist treatment (150).

An increase in the expression of Kiss1 in the hypothalamic neurons was observed following senktide (agonist of neurokinin B) administration (151), and its stimulatory effects were abolished in Gpr54 knock-out male (152). In ovariectomized goats, neurokinin B stimulated LH secretion through electrical multi-unit activity corresponded to LH secretion, suggesting a hypothalamic site for this GnRH pulse generation (153). GnRH antagonists abolished the stimulatory effect of neurokinin B, demonstrating its site of action to be functionally higher than the GnRH receptor (154,155).

Dynorphins act as the decelerator that inhibits KNDy neurons pulsatility. Studies involving the administration of opioid antagonists to humans have shown stimulatory effects on LH secretion in late follicular and mid-luteal phase (98,99), and together with other studies (100-104), highlight the inhibitory input by dynorphins on kisspeptin signalling, and consequently on GnRH/gonadotropin secretion. Through the stimulatory effects of neurokinin B and kisspeptin, and the inhibitory action of dynorphin, these neuropeptides coordinate pulsatile GnRH and LH secretion (Figure 2) (156,157).

Kisspeptin-mediated GnRH secretion is sex steroid dependent. Estrogen and progesterone directly modulate kisspeptin activity though the sex-steroid receptors expressed on kisspeptin neurons at both AVPV and the arcuate nucleus (158-160). Furthermore, two distinct populations of kisspeptin neurons, the infundibular/arcuate region of which interacts with neurokinin B and dynorphin, appear to mediate distinct sex-steroid pathways (discussed in more detail in sections 4.1-4.4).

Briefly, in humans, KNDy neurons in the infundibular nucleus alone are involved in negative and positive sex-steroid feedback, whereas in rodents positive sex-steroid feedback seems to be mediated via kisspeptin neurons in the AVPV region and negative sex-steroid feedback via the arcuate KNDy neurons (Figure 2) (67,111,160,161).

Stimulatory Effect of Kisspeptin on GnRH and Gonadotropin Secretion

Kisspeptin is a potent stimulator of the HPG axis – and in fact, it is the most potent GnRH secretagogue currently known. Kisspeptin signals directly to the hypothalamic GnRH neurons via kisspeptin receptor to release GnRH into the portal circulation, which in turn stimulates the anterior pituitary gonadotropes to produce LH and FSH (129,162).

The stimulatory effects of kisspeptin on LH secretion have been documented in animal models (163-166). This is consistent with human studies, where kisspeptin increases both LH and FSH secretion with the preferential stimulatory effect on the former (67,167-177). Kissppetin-54 was first administered in healthy men as an intravenous infusion with dose-dependent rise in LH secretion (169). Since then kisspeptin was administered in different isoforms (kisspeptin-54 and kisspeptin-10), different routes (subcutaneous and intravenous), different types of exposure (continuous and bolus), to healthy men and women and in endocrine disease models with low gonadotropin output, all showing stimulatory effect of kisspeptin on LH secretion (fully reviewed in (67)).

Pulsatile GnRH secretion correlates with LH pulsatility, prompting investigation of the effect of kisspeptin on regulating LH pulse frequency. LH pulse frequency and amplitude were increased following intravenous infusion of kisspeptin-10 in healthy men (172), and subcutaneous bolus of kisspeptin-54 in healthy women (174). The hypothalamic response to kisspeptin-54 and the pituitary response to GnRH are preserved in healthy older men (178). Kisspeptin also stimulates LH pulse frequency in reproductive endocrine disorders of low LH pulsatility, including hypothalamic amenorrhea, defects in the neurokinin B pathway and hypogonadal men with diabetes (96,179,180). Indeed, kisspeptin-54 and kisspeptin-10, as well as kisspeptin agonists like MVT-602 (previously known as TAK-448) are able to stimulate physiological reproductive hormone secretion in individuals with functional hypogonadism related to deficient GnRH secretion, such as in hypothalamic amenorrhea or polycystic ovary syndrome (181,182).

In addition, recent findings have explored further the effects of MVT-602. LH concentration increased in a dose-dependent manner, resembling the amplitude and duration found in the physiological mid-cycle LH surge, proving to be safe and well tolerated throughout the dose range (0.3 – 3.0 mg) (183). This approach to mimicking the physiological response during oocyte maturation and ovulation may have clinical utility for women during medically assisted reproduction.

Kisspeptin regulates GnRH and subsequently gonadotropin secretion through Kiss1r, as suggested by Messager who demonstrated no detectable LH levels in response to kisspeptin in Kiss1r knockout mice (119). The prevention of the stimulatory effect of kisspeptin on LH secretion by GnRH antagonists indicate that kisspeptin action is GnRH-mediated (118,164,184-186). This is further supported by the observation that kisspeptin cause depolarization of GnRH neurons (117) and stimulate GnRH release from hypothalamic explants (187,188). The expression of GnRH mRNA is upregulated in GnRH neurons following kisspeptin administration (189). Moreover, in patients with impaired functional capacity of GnRH neurons (idiopathic hypogonadotropic hypogonadism), the same dose of kisspeptin failed to induce LH response seen in healthy men and women (190). In female rats, ablation of KNDy neurons resulted in hypogonadotropic hypogonadism, confirming its role in the maintenance of normal LH levels and to estrous cyclicity (191).

Some investigators have demonstrated a direct stimulatory effect of kisspeptin on gonadotropes, but this direct stimulatory action of kisspeptin on gonadotropes remains debatable (192-196). Kiss1 and Kiss1r gene expression has been shown in gonadotropes, and gonadotropin secretion from the pituitary explants was observed following exposure to kisspeptin (78,192-195). Moreover, LHβ and FSHβ gene expression was upregulated in the primary pituitary cells treated with kisspeptin. Whilst kisspeptin can directly regulate gonadotropins at the transcriptional level, it appears to be less relevant than the GnRH-mediated action (67,195,196).

Desensitization Effect of Chronic or Continuous Exposure to Kisspeptin

Continuous administration of GnRH desensitizes the HPG axis by downregulation of GnRH receptors and desensitization of gonadotropes, following an initial stimulatory effect (39). It is therefore important to ascertain the effects of continuous exposure to kisspeptin on the HPG axis. Efforts have been made to assess the impact of continuous infusions of kisspeptin in a number of animal experiments (119,197-200).

In adult rats, continuous administration of kisspeptin-54 increased serum LH and free testosterone on day one, but this stimulatory effect was lost after 2 days, indicative of kisspeptin receptor desensitization (200). In rhesus monkeys, the continuous administration of kisspeptin-10 resulted in suppression of LH secretion, indicating desensitization of kisspeptin receptor (198). The kisspeptin receptor has been shown to desensitize in vitro (197). In sheep, infusion of kisspeptin-10 resulted in acute increase in serum LH levels, which declined by the end of 4-hour infusion, while GnRH remained elevated following the discontinuation of kisspeptin-10 administration. This suggests that desensitization to GnRH could be occurring at the level of pituitary gonadotropes (119).  

Consistent with animal studies, Jayasena et al. demonstrated that in women with hypothalamic amenorrhea an initial increase in LH and FSH secretion was not sustained following twice daily subcutaneous kisspeptin-54 administration for two weeks (176). Other studies in humans employing continuous or repeated kisspeptin administration provide conflicting evidence for kisspeptin-mediated desensitization and appear to be dose-related (172,180). High doses of kisspeptin may induce desensitization, but this is not apparent at lower doses (67). Sustained LH secretion and increased LH pulsatility was demonstrated with lower dose of kisspeptin-54 (0.01-1nmol/kg/h) infusion for 8 hours in women with hypothalamic amenorrhea (180) and kisspeptin-10 (3.1 nmol/kg/h) infusion for 22.5 hours in healthy men (172). In contrast, LH secretion was not maintained in three healthy men during the 24 hour infusion of kisspeptin-10 at 9.2 nmol/kg/h (the highest dose used in humans so far), although serum LH did not fall to the castrate levels and remained well above baseline at end of infusion (201).

Kisspeptin receptor agonist analogues, TAK-488 and TAK-683, induce desensitization when administered to healthy men (202,203). However, the ability of natural kisspeptin fragments to downregulate the HPG axis in humans remains to be established, and is to date complicated by differences in study protocols, in terms of isoform of kisspeptin used, duration (8 hours-2 weeks), mode and route of kisspeptin administration, lower doses of kisspeptin in human studies compared to animal, and the endocrine profile of the study participants (men versus women versus hypothalamic amenorrhea). 

Sexual Dimorphism in Kisspeptin Signaling

The response to kisspeptin is different in men and women. In men, kisspeptin potently stimulates the release of LH, but in women the effect of kisspeptin is variable and dependent on the phase of menstrual cycle (67). Whilst men respond to the modest doses of kisspeptin, LH response to kisspeptin in healthy women is minimal and inconsistent in the early follicular phase but greatest in the pre-ovulatory phase of the menstrual cycle (169-171,177). This indicates that in addition to the fluctuations in sex-steroid milieu, other mechanisms, such as changes in pituitary sensitivity to GnRH or GnRH network responsiveness to kisspeptin regulate the sensitivity to kisspeptin throughout the menstrual cycle (67,126,204).

Not only there is sexual dimorphism in gonadotropin response to kisspeptin, but there are also anatomical differences. Female hypothalami have significantly more kisspeptin fibers and kisspeptin cell bodies than men (173). Only a few kisspeptin cell bodies are present in the male infundibular nucleus and none in the rostral periventricular nucleus, which is on contrary to the female hypothalami with abundant kisspeptin network in both of these hypothalamic nuclei (108). These sex differences in kisspeptin neurons appear to be established early during perinatal development through the action of sex steroids (126,205).

These marked functional and anatomical differences may reflect sexually dimorphic roles of kisspeptin between both sexes, influencing their reproductive functions, namely the sex steroid feedback in GnRH and gonadotropin secretion (67).

Kisspeptin, GnRH and Puberty

Kisspeptin is crucial for normal pubertal development, the discovery of which formed the basis for the obligate role of kisspeptin signaling in the control of reproductive function (206). More than a decade ago two independent groups identified ‘inactivating’ mutations in KISS1R in patients with hypogonadotropic hypogonadism presenting with pubertal delay (85,86). Recently, a male patient with a biallelic loss-of-function KISS1R mutation was described who had undergone a normal and timely puberty, although as a child he had presented with microphallus and bilateral cryptorchidism. This suggests different levels of dependence of the hypothalamic-pituitary-gonadal axis on kisspeptin signaling during the reproductive life span, with the mini-puberty of infancy appearing more dependent on the kisspeptin system than is adolescent puberty (207). On the other hand, activating mutations in KISS1R and KISS1 were then described in children with central precocious puberty (89,90).

Hypothalamic expression of Kiss1 and Kiss1R mRNA is upregulated at puberty (117,165,208), and the percentage of GnRH neurons depolarizing in response to kisspeptin increases from juvenile (25%) to pubertal (50%) and to adult mice (>90%) (117), suggesting that GnRH neurons may acquire sensitivity to kisspeptin across puberty. In monkeys, kisspeptin-54 secretion and pulsatility increased at the onset of puberty (209). Moreover, the exogenous administration of kisspeptin resulted in earlier puberty in rats and monkeys (208,210), whereas kisspeptin antagonists delayed puberty in rats (186) and inhibited GnRH release in pubertal monkeys (211). In other study, daily injections of a synthetic kisspeptin analogue have been shown to significantly advance puberty in prepubertal female mice (212). GnRH neuron-specific Kiss1r knockout mouse showed a delay in pubertal onset, abnormal estrous cyclicity in female and abnormal external genitalia in male (microphallus, decreased anogenital distance associated with failure of preputial gland separation) (213).

Exogenous kisspeptin stimulated GnRH-induced LH secretion in patients with hypogonadotropism resulted in a spontaneous and permanent activation of their hypothalamic-pituitary-gonadal axis, whereas patients with idiopathic hypogonadotropic hypogonadism and no spontaneous LH pulsatility did not respond to kisspeptin, suggesting that the reversal of hypogonadism, sexual maturation and puberty may well be associated with the acquisition of kisspeptin responsiveness which in turn signals the emergence of reproductive endocrine activity (214). A recent study, 15 children with delayed puberty were administered intravenous kisspeptin and displayed divergent responses, with seven subjects having no response to kisspeptin, whereas others having either robust response (comparable to those of adults) or intermediate responses as perceived in one case (215).

GnRH release during puberty appears to require a cooperative mechanism between the kisspeptin/NKB networks in close interaction with different neuropeptides, as substance P, NKA, RFRP-3 and alpha-MSH, working as partners to regulate puberty timing influenced, naturally, by a combination of genetic, environmental, and gene-environment interactions (216).

Agonists and antagonists of kisspeptin and NKB were administered into the stalk-median eminence (region with high concentration of GnRH, kisspeptin and NKB neuroterminal fibers), and it was found that both kisspeptin-10 and the NK3R agonist senktide stimulated GnRH release in a dose-responsive manner in prepubertal and pubertal monkeys. However, senktide-induced GnRH release was blocked in the presence of a KISS1R antagonist and the kisspeptin-induced GnRH release was blocked in the presence of NK3R antagonist in pubertal monkeys, leading to the notion that a reciprocal signaling mechanism between kisspeptin and NKB exists and is possibly necessary for a normal puberty (217). These data together emphasizes that disrupted kisspeptin-GPR54-NKB signaling leads to hypogonadotropic hypogonadism, reinforcing the critical role of kisspeptin in puberty.

REGULATION OF GnRH AND GONADOTROPIN SECRETION

Development and maintenance of normal reproductive function requires a coordinated interplay between neuroendocrine, metabolic, and environmental factors. The GnRH-gonadotropin system plays a central role in the regulation of reproduction by integrating different signals and factors (Figure 3) (126,204). 

Figure 3. Neuroendocrine regulation of GnRH/gonadotropin secretion.
The GnRH-gonadotropin system plays a central role in the regulation of reproduction by integrating different neuroendocrine, metabolic and environmental signals/factors. The KNDy signaling has a key role in this process by integrating some of these signals and by regulating GnRH neurons.

Overview of Sex Steroid Feedback

A crucial role for sex steroids in the regulation of GnRH neurons and/or gonadotropes in humans was initially proposed as serial blood sampling and gonadotropin assays in women through phases of menstrual cycle showed an uneven distribution, with a clear mid-cycle surge in LH and FSH. Two mechanisms were proposed to mediate this effect: first, GnRH secretion is altered in response to the steroid milieu; second, sensitivity of the gonadotropes to a GnRH input is sex-steroid dependent, although the exact mechanism remains controversial due to inter-species variation (218).

Hypothalamic secretion of GnRH increases during proesterus in rats (219), sheep (220), and non-human primates (221). Pulsatile once hourly administration of exogenous GnRH restored ovulation in Rhesus monkeys with hypothalamic lesions which abolished GnRH secretion, suggesting that it was the ‘ebb and flow’ of ovarian estrogen feedback acting directly on the pituitary which triggered an LH surge (222). In humans, endogenous GnRH secretion is potentially diminished during the pre-ovulatory LH surge and the suppression of gonadotropin secretion is greater with lower doses of a GnRH receptor antagonist during the mid-cycle surge in comparison to the other phases of the menstrual cycle (223). This suggests that pituitary gonadotrope sensitivity to GnRH is enhanced during the mid-cycle surge. Administration of exogenous estradiol or testosterone in men with hypogonadotropic hypogonadism receiving pulsatile GnRH therapy, decreased gonadotropin concentrations, demonstrating inhibitory effects of sex-steroids at the level of pituitary (224). A direct effect of estrogen on gonadotropes is further demonstrated by the inhibition of LH secretion from rat pituitary gonadotropes in vitro (225). Literature to date suggests that there is dual-site sex-steroid feedback in the regulation of gonadotropin secretion, occurring at the level of both pituitary and hypothalamus (226-231).

Estrogen Feedback

Patterns of GnRH and LH secretion across the menstrual cycle are modulated by estradiol feedback. A biphasic effect of estradiol on gonadotropin secretion has long been established and it is essential for normal menstrual cycle, with an initial negative feedback (greater suppression of FSH) and a subsequent positive feedback (more prominent for LH) (32). However, the basis for estrogen feedback has been unclear for a long time. GnRH neurons do not express estrogen receptor alpha (ER-alpha) (232,233), and therefore a mediator between gonads and hypothalamus was missed. Recent evidence suggests that kisspeptin and neurokinin B (132) appears to be providing this “missing link” as a key regulator of both negative and positive estrogen feedback (67,126).

KNDy neurons in the infundibular nucleus in humans and the arcuate nucleus in other mammals mediate negative estrogen feedback. Estrogen suppresses kisspeptin and neurokinin B release from KNDy neurons, which reduce their stimulatory input to GnRH neurons. Simultaneously, there is a relative deficiency in dynorphin signaling as part of this negative feedback, releasing the inhibitory action on kisspeptin signaling (Figure 2) (67). Immunohistochemical staining of the postmenopausal women hypothalami showed up-regulated expression of KISS1 mRNA and hypertrophy of kisspeptin neurons in the infundibular nucleus when compared to the premenopausal women (111). These hypertrophied kisspeptin neurons co-localized with ER-alpha, had increased expression of neurokinin B and decreased levels of prodynorphin mRNA (234-236). The above evidence for the involvement of the infundibular KNDy system in mediating negative estrogen feedback in humans is consistent with animal studies. Kisspeptin neurons in the arcuate nucleus show frequent co-localization with ER-alpha (160,237). In ovariectomized animals, the expression of Kiss1 and neurokinin B mRNA was up-regulated but prodynorphin mRNA reduced in the arcuate nucleus (equivalent to the infundibular nucleus in humans), and this was reversed by estrogen replacement (102,115,120,238-242). Postmenopausal women are resistant to the stimulatory effect of kisspeptin on LH secretion (142,243), but postmenopausal women receiving estradiol replacement therapy are only resistant to kisspeptin initially and then they do demonstrate a remarkable increase in LH pulse amplitude with direct correlation to the circulating levels of estradiol and duration of kisspeptin administration (243). However, neurokinin B regulates gonadotropin secretion in postmenopausal women, and antagonizing the neurokinin 3 receptor modestly decreases LH secretion in this context (142).

Interestingly, the use of neurokinin 3 receptor antagonists has been shown to effectively reduce the frequency and severity of menopause-related vasomotor symptoms owing to their inhibitory effect in the hypothalamic thermoregulatory center, and thus presenting a potential non-hormonal treatment option for menopausal women (144,148,149).

Negative estrogen feedback switches to positive feedback in the late follicular phase of menstrual cycle, in order to induce the pre-ovulatory LH surge. Ovarian estradiol seems to be the predominant signal to trigger this switch, via ER-alpha, stimulating RP3V kisspeptin neurons while it inhibits arcuate kisspeptin neurons. Recent evidence supports the role of kisspeptin in generating the LH surge: during an assisted conception cycle, kisspeptin-54, used instead of a routinely administered human chorionic gonadotropin, induced an LH surge, and oocyte maturation, with a subsequent live term birth (241). Repeated twice-daily administration of kisspeptin-54 shortened the menstrual cycle, suggesting that the onset of LH surge was advanced (173). This is further supported by antagonistic studies in animal models, where the administration of kisspeptin antiserum or antagonists blunt/prevent LH peak, whilst kisspeptin advances LH surge (211,244,245).

However, kisspeptin-mediated positive estrogen feedback has marked anatomical variations between humans and other species. In rodents, positive estrogen feedback is mediated via the AVPV nucleus, which is absent in humans, other primates and sheep (Figure 2). AVPV neurons are sexually dimorphic, with higher density of ER-alpha described in females and AVPV kisspeptin neurons, as a subset of AVPV neurons, share this pattern (246). There seems to be functional specialization, since only a subset of AVPV kisspeptin neurons (~1/3) are synaptically connected to GnRH cell bodies, but of these, nearly all express estrogen sensitivity and most co-express tyrosine hydroxylase to facilitate positive feedback (247). The expression of Kiss1 mRNA in the AVPV nucleus is low following an ovariectomy but is dramatically increased with estrogen treatment and at the time of LH surge (160,161). In sheep, positive estrogen feedback is mediated though the arcuate nucleus, where the expression of Kiss1 mRNA is the greatest at the pre-ovulatory LH surge (195).

There are no studies looking at the anatomical region of estrogen mediating positive feedback in humans. Although there does not appear to be two distinct anatomical populations of kisspeptin neurons to relay negative and positive sex-steroid feedback in humans, it is possible that separate signaling pathways exists to mediate gonadal steroid feedback.

Whilst it is clear that kisspeptin is involved in estrogen-induced mid-cycle gonadotropin surge, the role of KNDy neurons in positive estrogen feedback is less obvious. In sheep, the expression of neurokinin B mRNA was increased during the LH surge, and neurokinin B receptor agonist senktide induced LH secretion mimicking its mid-cycle surge (248,249). However, this has not been reproduced in other species, including humans (180). In summary, KNDy neurons mediate negative estrogen feedback in the infundibular nucleus in humans and the arcuate nucleus in other species. Positive estrogen feedback is mediated via kisspeptin neurons, which show marked inter-species anatomical variation.

In addition to the gonads, the brain is one of the major organs producing estradiol, and recently a number of studies demonstrated that estradiol is synthesized and released in the hypothalamus (i.e. neuroestradiol) contributing to the regulation of GnRH release, particularly regarding its positive feedback effect during the preovulatory GnRH/LH surge (250).

Progesterone Feedback

Progesterone reduces LH pulse frequency in healthy women. LH secretory pattern in women exposed to exogenous progesterone was comparable to LH profile observed in the mid-luteal phase, demonstrating that progesterone plays a central role in the luteal phase of menstrual cycle (251). These inhibitory effects of progesterone on gonadotropin secretion are mediated by the progesterone receptor (PR) (252). The suppressive effect of progesterone on LH secretion was diminished in the context of estrogen deficiency, while co-administration of estradiol restored it (252), suggesting an interplay between these sex steroids. However, the presence of PR on only a small subset of GnRH neurons (253-255)led to the notion that intermediaries are involved in mediating inhibitory progesterone signal to GnRH neurons.

There is evidence that KNDy neurons play a role in mediating progesterone feedback on GnRH through dynorphin signaling (Figure 2) (102,120). PR have been demonstrated to be co-localized with dynorphin in the KNDy neurons (159)and progesterone increased dynorphin concentrations (256). Moreover, the number of preprodynorphin mRNA expressing cells decreased in postmenopausal women (236) and in ovariectomized ewes, but normalized with exogenous progesterone to luteal levels (256).

Testosterone Feedback

Testosterone exerts negative feedback on gonadotropin secretion. Early studies verified that LH and FSH pulse frequency are enhanced in hypogonadal men and exogenous testosterone decreases gonadotropin secretion, suggesting that testosterone have an inhibitory effect on GnRH secretion (230,257).

Few GnRH neurons express androgen receptors (AR) (258). GnRH neurons were thus considered to be reliant on an intermediary neuronal population to mediate testosterone feedback. A key role for KNDy neurons in this mediation has been suggested, as these neurons express AR which directly mediate the androgen feedback. The androgen feedback may also rely on the aromatization of testosterone, as testosterone-induced suppression of Kiss1 mRNA in the rodent arcuate nucleus is identical to that observed with estradiol, but more than that observed with dihydrotestosterone administration (259). The cross-talk between AR and ER was suggested from animal studies: AR expression was downregulated in the prostate following neonatal estrogen exposure (260), and AR transcription was modulated following a co-transfection of AR and ER (261).

Navarro has described a role for KNDy neurons in mediating the negative testosterone feedback on GnRH secretion, and provided evidence that neurokinin B released from KNDy neurons is part of an auto-feedback loop that generates the pulsatile secretion of Kiss1 and GnRH in male mice: Kiss1 and dynorphin mRNA are regulated by testosterone through estrogen and androgen receptor-dependent pathways; KNDy neurons express neurokinin B receptor whereas GnRH neurons do not, and senktide (an agonist for the neurokinin B receptor) activates KNDy neurons leading to gonadotropin secretion but has no discernible effect on GnRH neurons (262). Other studies demonstrated that the suppression of gonadotropin secretion using testosterone is associated with a reduction of Kiss1 mRNA in the hypothalamus (118,208,263). Moreover, post-orchidectomy rise in LH in rodents can be blocked by kisspeptin antagonists, further suggesting that kisspeptin system mediates the hypothalamic androgen feedback (186).

Stress and Glucocorticoids

Physical and psychological stress is associated with hypothalamic amenorrhea, possibly though the activation of hypothalamic-pituitary-adrenal (HPA) axis (264,265). Experimental evidence points towards a cortisol-mediated suppression of gonadotropin secretion as the main key pathway to explain stress-induced gonadotropin suppression(55,266-273). The negative effect of cortisol on HPG axis is recognized to occur at both pituitary and hypothalamic levels. There are also data suggesting that upstream factors in the HPA axis, such as Corticotropin Releasing Hormone (CRH) and vasopressin may play a mediatory role (274,275).

Cortisol secretion in women with hypothalamic amenorrhea is elevated (267), and evening adrenocorticotropic hormone (ACTH) and cortisol concentrations are higher in excessive exercise (266,270). Administration of exogenous glucocorticoids to eugonadal women was associated with a decrease in LH pulse frequency, suggesting that glucocorticoids have a negative action on GnRH secretion (273). In ovine portal blood, cortisol administration led to a decrease in GnRH pulse frequency (272). Inferences of cortisol effects on gonadotropin secretion were also derived from observations in women and men with Cushing’s syndrome (condition associated with excessive cortisol secretion), where exogenous GnRH preferentially stimulates FSH whilst LH remains unchanged (268,271). The resolution of male hypogonadotropic hypogonadism was also observed in men with remission of Cushing’s disease (271). This negative input of cortisol on the HPG axis may be modulated by sex-steroid hormones, and kisspeptin signaling has also been implicated in the process.

Cortisol alone had no impact on GnRH pulsatility in ovariectomized ewes, but the co-administration of estradiol and progesterone led to a 70% decrease in GnRH secretion (272). Decreased hypothalamic Kiss1 mRNA expression has been observed during exposure to stress or exogenous glucocorticoids. The role of kisspeptin in mediating stress inputs is further supported by the expression of glucocorticoid receptor on murine kisspeptin neurons (276). Colocalization by immunohistochemistry of CRH receptor (CRH-R) in most hypothalamic kisspeptin neurons in the AVPV/PeN and ARC nuclei as well as glucocorticoid receptor (GR) in AVPV/PeN kisspeptin neurons support a relevant direct role of kisspeptin neurons in the inhibitory effects of CRH/ glucocorticoids (277).

Hypothalamic CRH neurons, important regulators of the stress response, also directly modulate GnRH excitability in a dose-dependent and receptor-specific manner, and the GnRH response to CRH is influenced by estrogens (278). Intracerebroventricular administration of CRH in female rats suppressed LH pulsatility and the LH surge, and this suppression was enhanced by estrogens (279).

Animal models have also linked increased exposure to RFamide-related peptide-3 (RFRP-3) during acute and chronic stress and hypothalamic expression of GnIH mRNA. Along these lines, the surface of GnIH neurons has glucocorticoid receptors and hydrocortisone administration was associated to an increased GnIH mRNA expression, ultimately leading to lower GnRH activity and dysregulation of the HPG axis (280). Together, these findings emphasize that kisspeptin as GnIH provide relevant inputs that contribute to an inhibitory effect of corticosteroids on gonadal axis during stress.

Prolactin

Prolactin is a well-known inhibitor of GnRH release and a suppressor of the HPG axis. The association between hyperprolactinemia and reproductive dysfunction has long been established, accounting for 14% of secondary amenorrhea and hypogonadism cases (281) and for a third of women presenting with infertility (282,283). Hyperprolactinemia is evident in 16% of men with erectile dysfunction and in 11% of men with oligospermia (284). The decreased pulsatility of LH in hyperprolactinemia responds to bromocriptine (285). GnRH therapy has restored ovulation and normal luteal function in bromocriptine resistant hyperprolactinemia women (286,287), suggesting that prolactin exerts inhibition through direct reduction of GnRH secretion.

The neuroendocrine pathway by which prolactin inhibits GnRH pulse frequency remains to be fully elucidated. A direct action of prolactin on the GnRH neuronal network is possible (288,289). Prolactin has also been demonstrated to influence other systems, including GABA (290), β endorphins (291), neuropeptide Y (292) and dopaminergic systems (via tuberoinfundibular dopamine (TIDA) neurons) (293).

Nevertheless, data suggest that prolactin receptors are expressed in most kisspeptin neurons but only in a small proportion of GnRH neurons, indicating that kisspeptin signaling may have a role in this context (288,294). In rodent models, kisspeptin neurons in the arcuate nucleus modulate dopamine release from dopaminergic neurons, thereby regulating prolactin secretion (295). Kiss1 expression is decreased in lactation, a physiological state associated with hyperprolactinemia (296). Prolactin-sensitive GABA and kisspeptin neurons were identified in regions of the rat hypothalamus (294). Moreover, in a mouse model of anovulatory hyperprolactinemia (induced by a continuous infusion of prolactin), Kiss1 mRNA levels were diminished and peripheral administration of kisspeptin restored gonadotropin secretion and ovarian cyclicity (297). There are also other animal studies reporting an inhibitory effect of prolactin on Kiss1 expression (298,299). This data suggests that kisspeptin is a possible link between hyperprolactinemia and GnRH deficiency. The administration of kisspeptin-10 reactivated the gonadotropin secretion in women with hyperprolactinemia-induced hypogonadotropic amenorrhea, suggesting that GnRH deficiency in the context of hyperprolactinemia is, at least in part, mediated by an impaired hypothalamic kisspeptin secretion (300).

On the other hand, kisspeptins appears to have a stimulatory effect on prolactin release, as demonstrated in a recent study in ovariectomized rats which had intracerebroventricular injections of kisspeptin-10 with subsequent increase in prolactin release, and this required the estrogen receptor-alpha and was potentiated by progesterone via progesterone receptor activation (301).

Nutrition and Metabolism

A link between energy balance and reproductive function enables organisms to survive to reproductive maturity and to withstand the energy needs of parturition, lactation, and other parental behaviors. This link optimizes reproductive success under fluctuating metabolic conditions (302). Kisspeptin signaling may link nutrition/metabolic status and reproduction by sensing energy stores and translate this information into GnRH secretion (303). These relations elucidate further associations between reproductive dysfunction and metabolic disturbances, such as diabetes, obesity or anorexia nervosa (67,304,305).

Food deprivation impairs GnRH and gonadotropin secretion, and leptin (a satiety hormone secreted by adipose tissue, the levels of which drop in response to fasting) plays a role in this inter-regulation by stimulating LH release (67,306-308). Periods of fasting and calorie restriction decrease LH pulse frequency and increase pulse amplitude (302,309-311). Administration of recombinant leptin increased LH pulse frequency in women with hypothalamic amenorrhea (312) and prevented fasting-induced drop in testosterone and LH pulsatility in healthy men (313). Moreover, humans with mutations in leptin or in leptin receptor show hypogonadism (314). Thus, the crosstalk between kisspeptin and leptin is relevant for reproduction and fertility (71), including in the setting of assisted reproduction techniques (315).

Kisspeptin neurons may have a role in mediating the metabolic signals of leptin on the control of HPG axis, as 40% of the arcuate kisspeptin neurons express leptin receptors in contrast to the GnRH neurons, where leptin receptors are absent (316-319). Food deprivation is associated with a decrease in kisspeptin, and subsequent reduction in gonadotropin secretion (320-323). Levels of low Kiss1 mRNA expression in the leptin-deficient ob/ob mice are partially upregulated by exogenous leptin (161). Moreover, exogenous kisspeptin restored vaginal opening (marker of sexual maturation) in malnourished rodents (320). Animal models of type 1 diabetes, characterized by insulin deficiency and impaired cellular nutrition, had hypogonadotropic hypogonadism and decreased Kiss1 mRNA expression. Repeated administration of kisspeptin to these rodents increased prostate and testis weight (324). It is plausible that a relative deficiency of kisspeptin secretion is a mechanism for hypogonadotropic hypogonadism in patients with obesity and diabetes (179). In hypogonadal men with type 2 diabetes, kisspeptin-10 increased LH secretion and pulse frequency (179). Although early studies appeared to suggest a direct link between kisspeptin and leptin, it seems that the neuronal pathway whereby leptin modulates GnRH is far more complex (325,326). Only partial restoration in Kiss1 mRNA in leptin-deficiency and normal pubertal development and fertility observed in selective leptin receptor deletion from kisspeptin neurons suggest that kisspeptin may link reproduction and metabolism through other ways than leptin (161,327). Proopiomelanocortin (POMC), agouti-related peptide, neuropeptide Y, ghrelin, and cocaine- and amphetamine-regulated transcript (CART) expressing neurons have been linked to this process (303,319). Kisspeptin neurons communicate with POMC and neuropeptide Y neurons and are able to modulate the expression of relevant genes in these cells (316). This link between kisspeptin and other peptides classically associated to food intake (as POMC and neuropeptide Y) was explored due to the anorexigenic effect of intracerebroventricular administration of KP-10 in male rats mediated via anorectic neuropeptides, nesfatin-1 and oxytocin, expressed in various hypothalamic nuclei. Diminished food intake and anorexia was significantly abolished by pretreatment with oxytocin receptor antagonist (328,329).

Several studies have also suggested that ghrelin can interact directly with hypothalamic neurons leading to suppression of gonadotropins release, and thus impairing fertility, an effect that is dependent of the estradiol milieu (303,330-332).

GABA (Gamma-Amino Butyric Acid)

GABA has also been implicated as a regulator of GnRH secretion. Although GABA is classically an inhibitory neurotransmitter in the central nervous system, most mature GnRH neurons are stimulated by GABA, which has attributed to GABA an excitatory action in HPG axis. The precise physiology of this mechanism is still unclear (333-337), but it may be related to the bidirectional interactions between GABA and kisspeptin pathways, as well as between these and GnRH neurons, in a variety of ways throughout development (338). In early development, GABA seems to increase KISS1 expression in embryonic phase and early postnatally, while in the absence of GABAergic input the expression of KISS1 declines (338,339). In the prepubertal period, the central restraint on GnRH secretion seems to be mediated by GABA possibly acting directly via kisspeptin neurons (338). In the peri-pubertal phase, the antagonism of GABA and the intrinsic disinhibition of kisspeptin neurons seem to be critical in puberty initiation and development (340,341). In adulthood, the interactions between GnRH-GABA-kisspeptin become more complex with HPG axis function critically dependent on such interactions. For instance, the preovulatory surge does not occur in the absence of GABA signaling, thus neurons co-expressing GABA and kisspeptin seem crucial in providing double excitatory input to GnRH neurons at the time of ovulation (338,342).

Additionally, in healthy men, total endogenous GABA levels in the anterior cingulate cortex, a key limbic structure, significantly decreased after intravenous infusion of kisspeptin (1 nmol/kg/h) demonstrating a potent inhibitory effect of kisspeptin on GABA levels which could be a fundamental concept in understanding the central limbic effect of kisspeptin in the human brain (343).

Other Neuropeptides

In addition to KNDy system and GABA, other peptides and neurotransmitters have been shown to influence GnRH-gonadotrope system: vasoactive intestinal polypeptide (VIP), vasopressin, catecholamines, nitric oxide, neurotensin, gonadotropin-inhibitory hormone (GnIH) /RFamide related peptide-3 (RFRP-3) (337), nucleobindin-2/nesfatin-1 (344). Excitatory inputs to the HPG axis may be mediated by VIP, catecholamines, glutamate and possibly vasopressin, whereas GnIH in birds, or its mammalian homolog RFRP-3, provide inhibitory inputs (345-349). RFRP neuronal populations have been detected mainly in the hypothalamic dorsomedial nucleus or adjacent regions, and they have projections to several hypothalamic areas including the arcuate nucleus, paraventricular nucleus, ventromedial nucleus and the lateral hypothalamus, all areas with major roles in the regulation of reproduction and energy balance (350,351). RFRP-3, encoded by the gene Rfrp, inhibits the electric firing of GnRH and kisspeptin neurons (346,352), which results in a suppression of GnRH-induced gonadotropin release with consequent inhibition of the reproductive axis (353). This RFRP-3 inhibitory input on the gonadotropin release is influenced by estrogens and may well be involved in their negative feedback. Estrogens reduce RFRP-3 expression and RFRP-3 neuronal activation (354,355).

Particular attention has been paid to the role of glutamate as a stimulatory modulator of the activity of ARC kisspeptin neurons, reaffirming the role of kisspeptin as a major neural integrator of inputs to GnRH neurons. Data from Kiss1 KO rats showed failure to increase GnRH/LH secretion following monosodium glutamate/NMDA administration (356).

SUMMARY

Complex neuroendocrine networks coordinate the regulation of reproduction, integrating a wide range of internal and external environmental inputs and signals. GnRH, the principal regulator of reproduction integrates cues from sex steroids, stress, glucocorticoids, nutritional and metabolic status, prolactin and other peptides, to controls gonadotropin secretion and subsequently gonadal function. Recently, the KNDy neuronal network has emerged as essential gatekeeper of GnRH release and thus reproduction, fertility and puberty. Translational clinical studies, exploring kisspeptin and neurokinin B activity in various physiological and pathological states are pivotal to explore potential clinical applications for these novel neuropeptides and their agonists as well as antagonists, may underpin future management of some disorders with dysfunctional GnRH pulsatility, such polycystic ovary syndrome, hypothalamic amenorrhea, infertility, obesity, pubertal disorders and menopause-related symptoms.

REFERENCES

1. Herbison AE, Theodosis DT. Localization of oestrogen receptors in preoptic neurons containing neurotensin but not tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone in the male and female rat. Neuroscience. 1992;50(2):283-298.
2. Fink G. Neuroendocrine regulation of pituitary function: general principles. Neuroendocrinology in Physiology and Medicine. 2000:p.107-134.
3. Tena-Sempere M. Hypothalamic KiSS-1: the missing link in gonadotropin feedback control? Endocrinology. 2005;146(9):3683-3685.
4. Baba Y, Matsuo H, Schally AV. Structure of the porcine LH- and FSH-releasing hormone. II. Confirmation of the proposed structure by conventional sequential analyses. Biochem Biophys Res Commun. 1971;44(2):459-463.
5. Matsuo H, Arimura A, Nair RM, Schally AV. Synthesis of the porcine LH- and FSH-releasing hormone by the solid-phase method. Biochem Biophys Res Commun. 1971;45(3):822-827.
6. Schally AV, Arimura A, Kastin AJ, Matsuo H, Baba Y, Redding TW, Nair RM, Debeljuk L, White WF. Gonadotropin-releasing hormone: one polypeptide regulates secretion of luteinizing and follicle-stimulating hormones. Science. 1971;173(4001):1036-1038.
7. Schally AV, Arimura A, Baba Y, Nair RM, Matsuo H, Redding TW, Debeljuk L. Isolation and properties of the FSH and LH-releasing hormone. Biochem Biophys Res Commun. 1971;43(2):393-399.
8. Schally AV. Use of GnRH in preference to LH-RH terminology in scientific papers. Hum Reprod. 2000;15(9):2059-2061.
9. Millar RP. GnRHs and GnRH receptors. Anim Reprod Sci. 2005;88(1-2):5-28.
10. Okubo K, Nagahama Y. Structural and functional evolution of gonadotropin-releasing hormone in vertebrates. Acta Physiol (Oxf). 2008;193(1):3-15.
11. Miyamoto K, Hasegawa Y, Nomura M, Igarashi M, Kangawa K, Matsuo H. Identification of the second gonadotropin-releasing hormone in chicken hypothalamus: evidence that gonadotropin secretion is probably controlled by two distinct gonadotropin-releasing hormones in avian species. Proc Natl Acad Sci U S A. 1984;81(12):3874-3878.
12. White SA, Bond CT, Francis RC, Kasten TL, Fernald RD, Adelman JP. A second gene for gonadotropin-releasing hormone: cDNA and expression pattern in the brain. Proc Natl Acad Sci U S A. 1994;91(4):1423-1427.
13. Gault PM, Maudsley S, Lincoln GA. Evidence that gonadotropin-releasing hormone II is not a physiological regulator of gonadotropin secretion in mammals. J Neuroendocrinol. 2003;15(9):831-839.
14. Balasubramanian R, Dwyer A, Seminara SB, Pitteloud N, Kaiser UB, Crowley WF, Jr. Human GnRH deficiency: a unique disease model to unravel the ontogeny of GnRH neurons. Neuroendocrinology. 2010;92(2):81-99.
15. Mitchell AL, Dwyer A, Pitteloud N, Quinton R. Genetic basis and variable phenotypic expression of Kallmann syndrome: towards a unifying theory. Trends Endocrinol Metab. 2011;22(7):249-258.
16. Cariboni A, Pimpinelli F, Colamarino S, Zaninetti R, Piccolella M, Rumio C, Piva F, Rugarli EI, Maggi R. The product of X-linked Kallmann's syndrome gene (KAL1) affects the migratory activity of gonadotropin-releasing hormone (GnRH)-producing neurons. Hum Mol Genet. 2004;13(22):2781-2791.
17. Cariboni A, Hickok J, Rakic S, Andrews W, Maggi R, Tischkau S, Parnavelas JG. Neuropilins and their ligands are important in the migration of gonadotropin-releasing hormone neurons. J Neurosci. 2007;27(9):2387-2395.
18. Magni P, Dozio E, Ruscica M, Watanobe H, Cariboni A, Zaninetti R, Motta M, Maggi R. Leukemia inhibitory factor induces the chemomigration of immortalized gonadotropin-releasing hormone neurons through the independent activation of the Janus kinase/signal transducer and activator of transcription 3, mitogen-activated protein kinase/extracellularly regulated kinase 1/2, and phosphatidylinositol 3-kinase/Akt signaling pathways. Mol Endocrinol. 2007;21(5):1163-1174.
19. Dode C, Levilliers J, Dupont JM, De Paepe A, Le Du N, Soussi-Yanicostas N, Coimbra RS, Delmaghani S, Compain-Nouaille S, Baverel F, Pecheux C, Le Tessier D, Cruaud C, Delpech M, Speleman F, Vermeulen S, Amalfitano A, Bachelot Y, Bouchard P, Cabrol S, Carel JC, Delemarre-van de Waal H, Goulet-Salmon B, Kottler ML, Richard O, Sanchez-Franco F, Saura R, Young J, Petit C, Hardelin JP. Loss-of-function mutations in FGFR1 cause autosomal dominant Kallmann syndrome. Nat Genet. 2003;33(4):463-465.
20. Kim SH, Hu Y, Cadman S, Bouloux P. Diversity in fibroblast growth factor receptor 1 regulation: learning from the investigation of Kallmann syndrome. J Neuroendocrinol. 2008;20(2):141-163.
21. Yoshida K, Rutishauser U, Crandall JE, Schwarting GA. Polysialic acid facilitates migration of luteinizing hormone-releasing hormone neurons on vomeronasal axons. J Neurosci. 1999;19(2):794-801.
22. Kaprara A, Huhtaniemi IT. The hypothalamus-pituitary-gonad axis: Tales of mice and men. Metabolism. 2017.
23. Martin C, Balasubramanian R, Dwyer AA, Au MG, Sidis Y, Kaiser UB, Seminara SB, Pitteloud N, Zhou QY, Crowley WF, Jr. The role of the prokineticin 2 pathway in human reproduction: evidence from the study of human and murine gene mutations. Endocr Rev. 2011;32(2):225-246.
24. Pitteloud N, Zhang C, Pignatelli D, Li JD, Raivio T, Cole LW, Plummer L, Jacobson-Dickman EE, Mellon PL, Zhou QY, Crowley WF, Jr. Loss-of-function mutation in the prokineticin 2 gene causes Kallmann syndrome and normosmic idiopathic hypogonadotropic hypogonadism. Proc Natl Acad Sci U S A. 2007;104(44):17447-17452.
25. Clifton DKS, R.A. . Neuroendocrinology of reproduction. Yen & Jaffe's Reproductive Endocrinology JF Strauss and RL Barberi, Elsevier. 2009.
26. Maeda K, Ohkura S, Uenoyama Y, Wakabayashi Y, Oka Y, Tsukamura H, Okamura H. Neurobiological mechanisms underlying GnRH pulse generation by the hypothalamus. Brain Res. 2010;1364:103-115.
27. Moenter SM, DeFazio AR, Pitts GR, Nunemaker CS. Mechanisms underlying episodic gonadotropin-releasing hormone secretion. Front Neuroendocrinol. 2003;24(2):79-93.
28. Carmel PW, Araki S, Ferin M. Pituitary stalk portal blood collection in rhesus monkeys: evidence for pulsatile release of gonadotropin-releasing hormone (GnRH). Endocrinology. 1976;99(1):243-248.
29. Antunes JL, Carmel PW, Housepian EM, Ferin M. Luteinizing hormone-releasing hormone in human pituitary blood. J Neurosurg. 1978;49(3):382-386.
30. Caraty A, Martin GB, Montgomery G. A new method for studying pituitary responsiveness in vivo using pulses of LH-RH analogue in ewes passively immunized against native LH-RH. Reprod Nutr Dev. 1984;24(4):439-448.
31. Lincoln GA, Fraser HM. Blockade of episodic secretion of luteinizing hormone in the ram by the administration of antibodies to luteinizing hormone releasing hormone. Biol Reprod. 1979;21(5):1239-1245.
32. Knobil E, Plant TM, Wildt L, Belchetz PE, Marshall G. Control of the rhesus monkey menstrual cycle: permissive role of hypothalamic gonadotropin-releasing hormone. Science. 1980;207(4437):1371-1373.
33. Bolt DJ. Changes in the concentration of luteinizing hormone in plasma of rams following administration of oestradiol, progesterone or testosterone. J Reprod Fertil. 1971;24(3):435-438.
34. Dierschke DJ, Bhattacharya AN, Atkinson LE, Knobil E. Circhoral oscillations of plasma LH levels in the ovariectomized rhesus monkey. Endocrinology. 1970;87(5):850-853.
35. Naftolin F, Yen SS, Tsai CC. Rapid cycling of plasma gonadotrophins in normal men as demonstrated by frequent sampling. Nat New Biol. 1972;236(64):92-93.
36. Yen SS, Tsai CC. The biphasic pattern in the feedback action of ethinyl estradiol on the release of pituitary FSH and LH. J Clin Endocrinol Metab. 1971;33(6):882-887.
37. Reame NE, Sauder SE, Case GD, Kelch RP, Marshall JC. Pulsatile gonadotropin secretion in women with hypothalamic amenorrhea: evidence that reduced frequency of gonadotropin-releasing hormone secretion is the mechanism of persistent anovulation. J Clin Endocrinol Metab. 1985;61(5):851-858.
38. Sauder SE, Frager M, Case GD, Kelch RP, Marshall JC. Abnormal patterns of pulsatile luteinizing hormone secretion in women with hyperprolactinemia and amenorrhea: responses to bromocriptine. J Clin Endocrinol Metab. 1984;59(5):941-948.
39. Belchetz PE, Plant TM, Nakai Y, Keogh EJ, Knobil E. Hypophysial responses to continuous and intermittent delivery of hypopthalamic gonadotropin-releasing hormone. Science. 1978;202(4368):631-633.
40. Rasmussen DD. Episodic gonadotropin-releasing hormone release from the rat isolated median eminence in vitro. Neuroendocrinology. 1993;58(5):511-518.
41. Thiery JC, Pelletier J. Multiunit activity in the anterior median eminence and adjacent areas of the hypothalamus of the ewe in relation to LH secretion. Neuroendocrinology. 1981;32(4):217-224.
42. Wilson RC, Kesner JS, Kaufman JM, Uemura T, Akema T, Knobil E. Central electrophysiologic correlates of pulsatile luteinizing hormone secretion in the rhesus monkey. Neuroendocrinology. 1984;39(3):256-260.
43. Martinez de la Escalera G, Choi AL, Weiner RI. Generation and synchronization of gonadotropin-releasing hormone (GnRH) pulses: intrinsic properties of the GT1-1 GnRH neuronal cell line. Proc Natl Acad Sci U S A. 1992;89(5):1852-1855.
44. Terasawa E, Keen KL, Mogi K, Claude P. Pulsatile release of luteinizing hormone-releasing hormone (LHRH) in cultured LHRH neurons derived from the embryonic olfactory placode of the rhesus monkey. Endocrinology. 1999;140(3):1432-1441.
45. Ezzat A, Pereira A, Clarke IJ. Kisspeptin is a component of the pulse generator for GnRH secretion in female sheep but not the pulse generator. Endocrinology. 2015;156(5):1828-1837.
46. Millar RP, Lu ZL, Pawson AJ, Flanagan CA, Morgan K, Maudsley SR. Gonadotropin-releasing hormone receptors. Endocr Rev. 2004;25(2):235-275.
47. Pincus SM, Padmanabhan V, Lemon W, Randolph J, Rees Midgley A. Follicle-stimulating hormone is secreted more irregularly than luteinizing hormone in both humans and sheep. J Clin Invest. 1998;101(6):1318-1324.
48. Padmanabhan V, McFadden K, Mauger DT, Karsch FJ, Midgley AR, Jr. Neuroendocrine control of follicle-stimulating hormone (FSH) secretion. I. Direct evidence for separate episodic and basal components of FSH secretion. Endocrinology. 1997;138(1):424-432.
49. Dalkin AC, Haisenleder DJ, Ortolano GA, Ellis TR, Marshall JC. The frequency of gonadotropin-releasing-hormone stimulation differentially regulates gonadotropin subunit messenger ribonucleic acid expression. Endocrinology. 1989;125(2):917-924.
50. Haisenleder DJ, Dalkin AC, Ortolano GA, Marshall JC, Shupnik MA. A pulsatile gonadotropin-releasing hormone stimulus is required to increase transcription of the gonadotropin subunit genes: evidence for differential regulation of transcription by pulse frequency in vivo. Endocrinology. 1991;128(1):509-517.
51. Kaiser UB, Jakubowiak A, Steinberger A, Chin WW. Differential effects of gonadotropin-releasing hormone (GnRH) pulse frequency on gonadotropin subunit and GnRH receptor messenger ribonucleic acid levels in vitro. Endocrinology. 1997;138(3):1224-1231.
52. Spratt DI, Finkelstein JS, Butler JP, Badger TM, Crowley WF, Jr. Effects of increasing the frequency of low doses of gonadotropin-releasing hormone (GnRH) on gonadotropin secretion in GnRH-deficient men. J Clin Endocrinol Metab. 1987;64(6):1179-1186.
53. Waldhauser F, Weissenbacher G, Frisch H, Pollak A. Pulsatile secretion of gonadotropins in early infancy. Eur J Pediatr. 1981;137(1):71-74.
54. Conte FA, Grumbach MM, Kaplan SL, Reiter EO. Correlation of luteinizing hormone-releasing factor-induced luteinizing hormone and follicle-stimulating hormone release from infancy to 19 years with the changing pattern of gonadotropin secretion in agonadal patients: relation to the restraint of puberty. J Clin Endocrinol Metab. 1980;50(1):163-168.
55. Pohl CR, deRidder CM, Plant TM. Gonadal and nongonadal mechanisms contribute to the prepubertal hiatus in gonadotropin secretion in the female rhesus monkey (Macaca mulatta). J Clin Endocrinol Metab. 1995;80(7):2094-2101.
56. Boyar R, Finkelstein J, Roffwarg H, Kapen S, Weitzman E, Hellman L. Synchronization of augmented luteinizing hormone secretion with sleep during puberty. N Engl J Med. 1972;287(12):582-586.
57. Roth JC, Kelch RP, Kaplan SL, Grumbach MM. FSH and LH response to luteinizing hormone-releasing factor in prepubertal and pubertal children, adult males and patients with hypogonadotropic and hypertropic hypogonadism. J Clin Endocrinol Metab. 1972;35(6):926-930.
58. Marshall JC, Dalkin AC, Haisenleder DJ, Griffin ML, Kelch RP. GnRH pulses--the regulators of human reproduction. Trans Am Clin Climatol Assoc. 1993;104:31-46.
59. Marshall JC, Dalkin AC, Haisenleder DJ, Paul SJ, Ortolano GA, Kelch RP. Gonadotropin-releasing hormone pulses: regulators of gonadotropin synthesis and ovulatory cycles. Recent Prog Horm Res. 1991;47:155-187; discussion 188-159.
60. Yen SS, Tsai CC, Naftolin F, Vandenberg G, Ajabor L. Pulsatile patterns of gonadotropin release in subjects with and without ovarian function. J Clin Endocrinol Metab. 1972;34(4):671-675.
61. Arroyo A, Laughlin GA, Morales AJ, Yen SS. Inappropriate gonadotropin secretion in polycystic ovary syndrome: influence of adiposity. J Clin Endocrinol Metab. 1997;82(11):3728-3733.
62. Morales AJ, Laughlin GA, Butzow T, Maheshwari H, Baumann G, Yen SS. Insulin, somatotropic, and luteinizing hormone axes in lean and obese women with polycystic ovary syndrome: common and distinct features. J Clin Endocrinol Metab. 1996;81(8):2854-2864.
63. Waldstreicher J, Santoro NF, Hall JE, Filicori M, Crowley WF, Jr. Hyperfunction of the hypothalamic-pituitary axis in women with polycystic ovarian disease: indirect evidence for partial gonadotroph desensitization. J Clin Endocrinol Metab. 1988;66(1):165-172.
64. Yen SS, Vela P, Rankin J. Inappropriate secretion of follicle-stimulating hormone and luteinizing hormone in polycystic ovarian disease. J Clin Endocrinol Metab. 1970;30(4):435-442.
65. Beltramo M, Dardente H, Cayla X, Caraty A. Cellular mechanisms and integrative timing of neuroendocrine control of GnRH secretion by kisspeptin. Mol Cell Endocrinol. 2014;382(1):387-399.
66. Jayasena CN, Abbara A, Izzi-Engbeaya C, Comninos AN, Harvey RA, Gonzalez Maffe J, Sarang Z, Ganiyu-Dada Z, Padilha AI, Dhanjal M, Williamson C, Regan L, Ghatei MA, Bloom SR, Dhillo WS. Reduced levels of plasma kisspeptin during the antenatal booking visit are associated with increased risk of miscarriage. J Clin Endocrinol Metab. 2014;99(12):E2652-2660.
67. Skorupskaite K, George JT, Anderson RA. The kisspeptin-GnRH pathway in human reproductive health and disease. Hum Reprod Update. 2014;20(4):485-500.
68. Clarke SA, Dhillo WS. Kisspeptin across the human lifespan:evidence from animal studies and beyond. J Endocrinol. 2016;229(3):R83-98.
69. Comninos AN, Dhillo WS. Emerging Roles of Kisspeptin in Sexual and Emotional Brain Processing. Neuroendocrinology. 2018;106(2):195-202.
70. Dudek M, Ziarniak K, Sliwowska JH. Kisspeptin and Metabolism: The Brain and Beyond. Front Endocrinol (Lausanne). 2018;9:145.
71. Wahab F, Atika B, Ullah F, Shahab M, Behr R. Metabolic Impact on the Hypothalamic Kisspeptin-Kiss1r Signaling Pathway. Front Endocrinol (Lausanne). 2018;9:123.
72. Wolfe A, Hussain MA. The Emerging Role(s) for Kisspeptin in Metabolism in Mammals. Front Endocrinol (Lausanne). 2018;9:184.
73. Yang L, Comninos AN, Dhillo WS. Intrinsic links among sex, emotion, and reproduction. Cell Mol Life Sci. 2018;75(12):2197-2210.
74. Mills EG, Yang L, Abbara A, Dhillo WS, Comninos AN. Current Perspectives on Kisspeptins Role in Behaviour. Front Endocrinol (Lausanne). 2022;13:928143.
75. Lee JH, Miele ME, Hicks DJ, Phillips KK, Trent JM, Weissman BE, Welch DR. KiSS-1, a novel human malignant melanoma metastasis-suppressor gene. J Natl Cancer Inst. 1996;88(23):1731-1737.
76. Ciaramella V, Della Corte CM, Ciardiello F, Morgillo F. Kisspeptin and Cancer: Molecular Interaction, Biological Functions, and Future Perspectives. Front Endocrinol (Lausanne). 2018;9:115.
77. West A, Vojta PJ, Welch DR, Weissman BE. Chromosome localization and genomic structure of the KiSS-1 metastasis suppressor gene (KISS1). Genomics. 1998;54(1):145-148.
78. Kotani M, Detheux M, Vandenbogaerde A, Communi D, Vanderwinden JM, Le Poul E, Brezillon S, Tyldesley R, Suarez-Huerta N, Vandeput F, Blanpain C, Schiffmann SN, Vassart G, Parmentier M. The metastasis suppressor gene KiSS-1 encodes kisspeptins, the natural ligands of the orphan G protein-coupled receptor GPR54. J Biol Chem. 2001;276(37):34631-34636.
79. Pasquier J, Kamech N, Lafont AG, Vaudry H, Rousseau K, Dufour S. Molecular evolution of GPCRs: Kisspeptin/kisspeptin receptors. J Mol Endocrinol. 2014;52(3):T101-117.
80. Gottsch ML, Navarro VM, Zhao Z, Glidewell-Kenney C, Weiss J, Jameson JL, Clifton DK, Levine JE, Steiner RA. Regulation of Kiss1 and dynorphin gene expression in the murine brain by classical and nonclassical estrogen receptor pathways. J Neurosci. 2009;29(29):9390-9395.
81. Lee DK, Nguyen T, O'Neill GP, Cheng R, Liu Y, Howard AD, Coulombe N, Tan CP, Tang-Nguyen AT, George SR, O'Dowd BF. Discovery of a receptor related to the galanin receptors. FEBS Lett. 1999;446(1):103-107.
82. Muir AI, Chamberlain L, Elshourbagy NA, Michalovich D, Moore DJ, Calamari A, Szekeres PG, Sarau HM, Chambers JK, Murdock P, Steplewski K, Shabon U, Miller JE, Middleton SE, Darker JG, Larminie CG, Wilson S, Bergsma DJ, Emson P, Faull R, Philpott KL, Harrison DC. AXOR12, a novel human G protein-coupled receptor, activated by the peptide KiSS-1. J Biol Chem. 2001;276(31):28969-28975.
83. Ohtaki T, Shintani Y, Honda S, Matsumoto H, Hori A, Kanehashi K, Terao Y, Kumano S, Takatsu Y, Masuda Y, Ishibashi Y, Watanabe T, Asada M, Yamada T, Suenaga M, Kitada C, Usuki S, Kurokawa T, Onda H, Nishimura O, Fujino M. Metastasis suppressor gene KiSS-1 encodes peptide ligand of a G-protein-coupled receptor. Nature. 2001;411(6837):613-617.
84. Liu X, Lee K, Herbison AE. Kisspeptin excites gonadotropin-releasing hormone neurons through a phospholipase C/calcium-dependent pathway regulating multiple ion channels. Endocrinology. 2008;149(9):4605-4614.
85. de Roux N, Genin E, Carel JC, Matsuda F, Chaussain JL, Milgrom E. Hypogonadotropic hypogonadism due to loss of function of the KiSS1-derived peptide receptor GPR54. Proc Natl Acad Sci U S A. 2003;100(19):10972-10976.
86. Seminara SB, Messager S, Chatzidaki EE, Thresher RR, Acierno JS, Jr., Shagoury JK, Bo-Abbas Y, Kuohung W, Schwinof KM, Hendrick AG, Zahn D, Dixon J, Kaiser UB, Slaugenhaupt SA, Gusella JF, O'Rahilly S, Carlton MB, Crowley WF, Jr., Aparicio SA, Colledge WH. The GPR54 gene as a regulator of puberty. N Engl J Med. 2003;349(17):1614-1627.
87. Tenenbaum-Rakover Y, Commenges-Ducos M, Iovane A, Aumas C, Admoni O, de Roux N. Neuroendocrine phenotype analysis in five patients with isolated hypogonadotropic hypogonadism due to a L102P inactivating mutation of GPR54. J Clin Endocrinol Metab. 2007;92(3):1137-1144.
88. Chan YM, Broder-Fingert S, Wong KM, Seminara SB. Kisspeptin/Gpr54-independent gonadotrophin-releasing hormone activity in Kiss1 and Gpr54 mutant mice. J Neuroendocrinol. 2009;21(12):1015-1023.
89. Teles MG, Bianco SD, Brito VN, Trarbach EB, Kuohung W, Xu S, Seminara SB, Mendonca BB, Kaiser UB, Latronico AC. A GPR54-activating mutation in a patient with central precocious puberty. N Engl J Med. 2008;358(7):709-715.
90. Silveira LG, Noel SD, Silveira-Neto AP, Abreu AP, Brito VN, Santos MG, Bianco SD, Kuohung W, Xu S, Gryngarten M, Escobar ME, Arnhold IJ, Mendonca BB, Kaiser UB, Latronico AC. Mutations of the KISS1 gene in disorders of puberty. J Clin Endocrinol Metab. 2010;95(5):2276-2280.
91. Dong L, Zhou W, Lin Z, Tang L, Deng X, Chen B, Huang W, Xiong Q. Polymorphism rs5780218, rs12998 and rs10158616 in KISS1 gene among the Hubei province Chinese girls with the central precocious puberty. Endocrine. 2024;84(3):1229-1237.
92. Topaloglu AK, Reimann F, Guclu M, Yalin AS, Kotan LD, Porter KM, Serin A, Mungan NO, Cook JR, Imamoglu S, Akalin NS, Yuksel B, O'Rahilly S, Semple RK. TAC3 and TACR3 mutations in familial hypogonadotropic hypogonadism reveal a key role for Neurokinin B in the central control of reproduction. Nat Genet. 2009;41(3):354-358.
93. Gianetti E, Tusset C, Noel SD, Au MG, Dwyer AA, Hughes VA, Abreu AP, Carroll J, Trarbach E, Silveira LF, Costa EM, de Mendonca BB, de Castro M, Lofrano A, Hall JE, Bolu E, Ozata M, Quinton R, Amory JK, Stewart SE, Arlt W, Cole TR, Crowley WF, Kaiser UB, Latronico AC, Seminara SB. TAC3/TACR3 mutations reveal preferential activation of gonadotropin-releasing hormone release by neurokinin B in neonatal life followed by reversal in adulthood. J Clin Endocrinol Metab. 2010;95(6):2857-2867.
94. Guran T, Tolhurst G, Bereket A, Rocha N, Porter K, Turan S, Gribble FM, Kotan LD, Akcay T, Atay Z, Canan H, Serin A, O'Rahilly S, Reimann F, Semple RK, Topaloglu AK. Hypogonadotropic hypogonadism due to a novel missense mutation in the first extracellular loop of the neurokinin B receptor. J Clin Endocrinol Metab. 2009;94(10):3633-3639.
95. Young J, Bouligand J, Francou B, Raffin-Sanson ML, Gaillez S, Jeanpierre M, Grynberg M, Kamenicky P, Chanson P, Brailly-Tabard S, Guiochon-Mantel A. TAC3 and TACR3 defects cause hypothalamic congenital hypogonadotropic hypogonadism in humans. J Clin Endocrinol Metab. 2010;95(5):2287-2295.
96. Young J, George JT, Tello JA, Francou B, Bouligand J, Guiochon-Mantel A, Brailly-Tabard S, Anderson RA, Millar RP. Kisspeptin restores pulsatile LH secretion in patients with neurokinin B signaling deficiencies: physiological, pathophysiological and therapeutic implications. Neuroendocrinology. 2013;97(2):193-202.
97. Wilkes MM, Watkins WB, Stewart RD, Yen SS. Localization and quantitation of beta-endorphin in human brain and pituitary. Neuroendocrinology. 1980;30(2):113-121.
98. Quigley ME, Yen SS. The role of endogenous opiates in LH secretion during the menstrual cycle. J Clin Endocrinol Metab. 1980;51(1):179-181.
99. Veldhuis JD, Rogol AD, Samojlik E, Ertel NH. Role of endogenous opiates in the expression of negative feedback actions of androgen and estrogen on pulsatile properties of luteinizing hormone secretion in man. J Clin Invest. 1984;74(1):47-55.
100. Boukhliq R, Goodman RL, Berriman SJ, Adrian B, Lehman MN. A subset of gonadotropin-releasing hormone neurons in the ovine medial basal hypothalamus is activated during increased pulsatile luteinizing hormone secretion. Endocrinology. 1999;140(12):5929-5936.
101. Genazzani AD, Gastaldi M, Petraglia F, Battaglia C, Surico N, Volpe A, Genazzani AR. Naltrexone administration modulates the neuroendocrine control of luteinizing hormone secretion in hypothalamic amenorrhoea. Hum Reprod. 1995;10(11):2868-2871.
102. Goodman RL, Coolen LM, Anderson GM, Hardy SL, Valent M, Connors JM, Fitzgerald ME, Lehman MN. Evidence that dynorphin plays a major role in mediating progesterone negative feedback on gonadotropin-releasing hormone neurons in sheep. Endocrinology. 2004;145(6):2959-2967.
103. Shoupe D, Montz FJ, Lobo RA. The effects of estrogen and progestin on endogenous opioid activity in oophorectomized women. J Clin Endocrinol Metab. 1985;60(1):178-183.
104. Walsh JP, Clarke IJ. Effects of central administration of highly selective opioid mu-, delta- and kappa-receptor agonists on plasma luteinizing hormone (LH), prolactin, and the estrogen-induced LH surge in ovariectomized ewes. Endocrinology. 1996;137(9):3640-3648.
105. Cheng G, Coolen LM, Padmanabhan V, Goodman RL, Lehman MN. The kisspeptin/neurokinin B/dynorphin (KNDy) cell population of the arcuate nucleus: sex differences and effects of prenatal testosterone in sheep. Endocrinology. 2010;151(1):301-311.
106. Goodman RL, Lehman MN, Smith JT, Coolen LM, de Oliveira CV, Jafarzadehshirazi MR, Pereira A, Iqbal J, Caraty A, Ciofi P, Clarke IJ. Kisspeptin neurons in the arcuate nucleus of the ewe express both dynorphin A and neurokinin B. Endocrinology. 2007;148(12):5752-5760.
107. Burke MC, Letts PA, Krajewski SJ, Rance NE. Coexpression of dynorphin and neurokinin B immunoreactivity in the rat hypothalamus: Morphologic evidence of interrelated function within the arcuate nucleus. J Comp Neurol. 2006;498(5):712-726.
108. Hrabovszky E, Ciofi P, Vida B, Horvath MC, Keller E, Caraty A, Bloom SR, Ghatei MA, Dhillo WS, Liposits Z, Kallo I. The kisspeptin system of the human hypothalamus: sexual dimorphism and relationship with gonadotropin-releasing hormone and neurokinin B neurons. Eur J Neurosci. 2010;31(11):1984-1998.
109. Bedenbaugh MN, O'Connell RC, Lopez JA, McCosh RB, Goodman RL, Hileman SM. Kisspeptin, gonadotrophin-releasing hormone and oestrogen receptor alpha colocalise with neuronal nitric oxide synthase neurones in prepubertal female sheep. J Neuroendocrinol. 2018;30(1).
110. Dufourny L, Delmas O, Teixeira-Gomes AP, Decourt C, Sliwowska JH. Neuroanatomical connections between kisspeptin neurones and somatostatin neurones in female and male rat hypothalamus: a possible involvement of SSTR1 in kisspeptin release. J Neuroendocrinol. 2018:e12593.
111. Rometo AM, Krajewski SJ, Voytko ML, Rance NE. Hypertrophy and increased kisspeptin gene expression in the hypothalamic infundibular nucleus of postmenopausal women and ovariectomized monkeys. J Clin Endocrinol Metab. 2007;92(7):2744-2750.
112. Clarkson J, Herbison AE. Postnatal development of kisspeptin neurons in mouse hypothalamus; sexual dimorphism and projections to gonadotropin-releasing hormone neurons. Endocrinology. 2006;147(12):5817-5825.
113. Pompolo S, Pereira A, Estrada KM, Clarke IJ. Colocalization of kisspeptin and gonadotropin-releasing hormone in the ovine brain. Endocrinology. 2006;147(2):804-810.
114. Clarkson J, d'Anglemont de Tassigny X, Colledge WH, Caraty A, Herbison AE. Distribution of kisspeptin neurones in the adult female mouse brain. J Neuroendocrinol. 2009;21(8):673-682.
115. Oakley AE, Clifton DK, Steiner RA. Kisspeptin signaling in the brain. Endocr Rev. 2009;30(6):713-743.
116. Uenoyama Y, Inoue N, Pheng V, Homma T, Takase K, Yamada S, Ajiki K, Ichikawa M, Okamura H, Maeda KI, Tsukamura H. Ultrastructural evidence of kisspeptin-gonadotrophin-releasing hormone (GnRH) interaction in the median eminence of female rats: implication of axo-axonal regulation of GnRH release. J Neuroendocrinol. 2011;23(10):863-870.
117. Han SK, Gottsch ML, Lee KJ, Popa SM, Smith JT, Jakawich SK, Clifton DK, Steiner RA, Herbison AE. Activation of gonadotropin-releasing hormone neurons by kisspeptin as a neuroendocrine switch for the onset of puberty. J Neurosci. 2005;25(49):11349-11356.
118. Irwig MS, Fraley GS, Smith JT, Acohido BV, Popa SM, Cunningham MJ, Gottsch ML, Clifton DK, Steiner RA. Kisspeptin activation of gonadotropin releasing hormone neurons and regulation of KiSS-1 mRNA in the male rat. Neuroendocrinology. 2004;80(4):264-272.
119. Messager S, Chatzidaki EE, Ma D, Hendrick AG, Zahn D, Dixon J, Thresher RR, Malinge I, Lomet D, Carlton MB, Colledge WH, Caraty A, Aparicio SA. Kisspeptin directly stimulates gonadotropin-releasing hormone release via G protein-coupled receptor 54. Proc Natl Acad Sci U S A. 2005;102(5):1761-1766.
120. Lehman MN, Coolen LM, Goodman RL. Minireview: kisspeptin/neurokinin B/dynorphin (KNDy) cells of the arcuate nucleus: a central node in the control of gonadotropin-releasing hormone secretion. Endocrinology. 2010;151(8):3479-3489.
121. Mills EGA, Dhillo WS, Comninos AN. Kisspeptin and the control of emotions, mood and reproductive behaviour. J Endocrinol. 2018;239(1):R1-R12.
122. Cao Y, Li Z, Jiang W, Ling Y, Kuang H. Reproductive functions of Kisspeptin/KISS1R Systems in the Periphery. Reprod Biol Endocrinol. 2019;17(1):65.
123. Hu KL, Chang HM, Zhao HC, Yu Y, Li R, Qiao J. Potential roles for the kisspeptin/kisspeptin receptor system in implantation and placentation. Hum Reprod Update. 2019;25(3):326-343.
124. Bhattacharya M, Babwah AV. Kisspeptin: beyond the brain. Endocrinology. 2015;156(4):1218-1227.
125. Guzman S, Dragan M, Kwon H, de Oliveira V, Rao S, Bhatt V, Kalemba KM, Shah A, Rustgi VK, Wang H, Bech PR, Abbara A, Izzi-Engbeaya C, Manousou P, Guo JY, Guo GL, Radovick S, Dhillo WS, Wondisford FE, Babwah AV, Bhattacharya M. Targeting hepatic kisspeptin receptor ameliorates nonalcoholic fatty liver disease in a mouse model. J Clin Invest. 2022;132(10).
126. Pinilla L, Aguilar E, Dieguez C, Millar RP, Tena-Sempere M. Kisspeptins and reproduction: physiological roles and regulatory mechanisms. Physiol Rev. 2012;92(3):1235-1316.
127. Ikegami K, Minabe S, Ieda N, Goto T, Sugimoto A, Nakamura S, Inoue N, Oishi S, Maturana AD, Sanbo M, Hirabayashi M, Maeda KI, Tsukamura H, Uenoyama Y. Evidence of involvement of neurone-glia/neurone-neurone communications via gap junctions in synchronised activity of KNDy neurones. J Neuroendocrinol. 2017;29(6).
128. Topaloglu AK, Semple RK. Neurokinin B signalling in the human reproductive axis. Mol Cell Endocrinol. 2011;346(1-2):57-64.
129. Navarro VM, Bosch MA, Leon S, Simavli S, True C, Pinilla L, Carroll RS, Seminara SB, Tena-Sempere M, Ronnekleiv OK, Kaiser UB. The integrated hypothalamic tachykinin-kisspeptin system as a central coordinator for reproduction. Endocrinology. 2015;156(2):627-637.
130. Ikegami K, Watanabe Y, Nakamura S, Goto T, Inoue N, Uenoyama Y, Tsukamura H. Cellular and molecular mechanisms regulating the KNDy neuronal activities to generate and modulate GnRH pulse in mammals. Front Neuroendocrinol. 2022;64:100968.
131. Skorupskaite K, George JT, Veldhuis JD, Anderson RA. Neurokinin B Regulates Gonadotropin Secretion, Ovarian Follicle Growth, and the Timing of Ovulation in Healthy Women. J Clin Endocrinol Metab. 2018;103(1):95-104.
132. Skorupskaite K, George JT, Veldhuis JD, Millar RP, Anderson RA. Interactions Between Neurokinin B and Kisspeptin in Mediating Estrogen Feedback in Healthy Women. J Clin Endocrinol Metab. 2016;101(12):4628-4636.
133. Skorupskaite K, George J, Anderson RA. Role of a neurokinin B receptor antagonist in the regulation of ovarian function in healthy women. Lancet. 2015;385 Suppl 1:S92.
134. Skorupskaite K, Anderson RA. Hypothalamic neurokinin signalling and its application in reproductive medicine. Pharmacol Ther. 2021:107960.
135. Akad M, Socolov R, Furnica C, Covali R, Stan CD, Crauciuc E, Pavaleanu I. Kisspeptin Variations in Patients with Polycystic Ovary Syndrome-A Prospective Case Control Study. Medicina (Kaunas). 2022;58(6).
136. de Assis Rodrigues NP, Lagana AS, Zaia V, Vitagliano A, Barbosa CP, de Oliveira R, Trevisan CM, Montagna E. The role of Kisspeptin levels in polycystic ovary syndrome: a systematic review and meta-analysis. Arch Gynecol Obstet. 2019;300(5):1423-1434.
137. Zhao T, Zhang Q, Xiao X, Tao X, Gao M, He W, Wu X, Yuan T. Associations of the KiSS-1 and GPR54 genetic polymorphism with polycystic ovary syndrome in Yunnan, China. Gynecol Endocrinol. 2022;38(9):790-794.
138. Zhao T, Xiao X, Li L, Tao X, He W, Zhang Q, Wu X, Yuan T. Role of kisspeptin in polycystic ovarian syndrome: A metabolomics study. Clin Endocrinol (Oxf). 2023;99(3):315-325.
139. George JT, Kakkar R, Marshall J, Scott ML, Finkelman RD, Ho TW, Veldhuis J, Skorupskaite K, Anderson RA, McIntosh S, Webber L. Neurokinin B Receptor Antagonism in Women With Polycystic Ovary Syndrome: A Randomized, Placebo-Controlled Trial. J Clin Endocrinol Metab. 2016;101(11):4313-4321.
140. Fraser GL, Obermayer-Pietsch B, Laven J, Griesinger G, Pintiaux A, Timmerman D, Fauser B, Lademacher C, Combalbert J, Hoveyda HR, Ramael S. Randomized Controlled Trial of Neurokinin 3 Receptor Antagonist Fezolinetant for Treatment of Polycystic Ovary Syndrome. J Clin Endocrinol Metab. 2021.
141. Skorupskaite K, George JT, Veldhuis JD, Millar RP, Anderson RA. Kisspeptin and neurokinin B interactions in modulating gonadotropin secretion in women with polycystic ovary syndrome. Hum Reprod. 2020;35(6):1421-1431.
142. Skorupskaite K, George JT, Veldhuis JD, Millar RP, Anderson RA. Neurokinin 3 Receptor Antagonism Reveals Roles for Neurokinin B in the Regulation of Gonadotropin Secretion and Hot Flashes in Postmenopausal Women. Neuroendocrinology. 2018;106(2):148-157.
143. Prague JK, Roberts RE, Comninos AN, Clarke S, Jayasena CN, Nash Z, Doyle C, Papadopoulou DA, Bloom SR, Mohideen P, Panay N, Hunter MS, Veldhuis JD, Webber LC, Huson L, Dhillo WS. Neurokinin 3 receptor antagonism as a novel treatment for menopausal hot flushes: a phase 2, randomised, double-blind, placebo-controlled trial. Lancet. 2017;389(10081):1809-1820.
144. Depypere H, Timmerman D, Donders G, Sieprath P, Ramael S, Combalbert J, Hoveyda HR, Fraser GL. Treatment of Menopausal Vasomotor Symptoms With Fezolinetant, a Neurokinin 3 Receptor Antagonist: A Phase 2a Trial. J Clin Endocrinol Metab. 2019;104(12):5893-5905.
145. Prague JK, Dhillo WS. Treating hot flushes with a neurokinin 3 receptor antagonist. Oncotarget. 2017;8(63):106153-106154.
146. Prague JK, Dhillo WS. Neurokinin 3 receptor antagonism - the magic bullet for hot flushes? Climacteric. 2017;20(6):505-509.
147. Anderson RA, Skorupskaite K, Sassarini J. The neurokinin B pathway in the treatment of menopausal hot flushes. Climacteric. 2019;22(1):51-54.
148. Johnson KA, Martin N, Nappi RE, Neal-Perry G, Shapiro M, Stute P, Thurston RC, Wolfman W, English M, Franklin C, Lee M, Santoro N. Efficacy and Safety of Fezolinetant in Moderate to Severe Vasomotor Symptoms Associated With Menopause: A Phase 3 RCT. J Clin Endocrinol Metab. 2023;108(8):1981-1997.
149. Pinkerton JV, Simon JA, Joffe H, Maki PM, Nappi RE, Panay N, Soares CN, Thurston RC, Caetano C, Haberland C, Haseli Mashhadi N, Krahn U, Mellinger U, Parke S, Seitz C, Zuurman L. Elinzanetant for the Treatment of Vasomotor Symptoms Associated With Menopause: OASIS 1 and 2 Randomized Clinical Trials. JAMA. 2024.
150. Skorupskaite K, George JT, Veldhuis JD, Millar RP, Anderson RA. Neurokinin 3 receptor antagonism decreases gonadotropin and testosterone secretion in healthy men. Clin Endocrinol (Oxf). 2017;87(6):748-756.
151. Navarro VM, Castellano JM, McConkey SM, Pineda R, Ruiz-Pino F, Pinilla L, Clifton DK, Tena-Sempere M, Steiner RA. Interactions between kisspeptin and neurokinin B in the control of GnRH secretion in the female rat. Am J Physiol Endocrinol Metab. 2011;300(1):E202-210.
152. Garcia-Galiano D, van Ingen Schenau D, Leon S, Krajnc-Franken MA, Manfredi-Lozano M, Romero-Ruiz A, Navarro VM, Gaytan F, van Noort PI, Pinilla L, Blomenrohr M, Tena-Sempere M. Kisspeptin signaling is indispensable for neurokinin B, but not glutamate, stimulation of gonadotropin secretion in mice. Endocrinology. 2012;153(1):316-328.
153. Wakabayashi Y, Nakada T, Murata K, Ohkura S, Mogi K, Navarro VM, Clifton DK, Mori Y, Tsukamura H, Maeda K, Steiner RA, Okamura H. Neurokinin B and dynorphin A in kisspeptin neurons of the arcuate nucleus participate in generation of periodic oscillation of neural activity driving pulsatile gonadotropin-releasing hormone secretion in the goat. J Neurosci. 2010;30(8):3124-3132.
154. Ramaswamy S, Seminara SB, Ali B, Ciofi P, Amin NA, Plant TM. Neurokinin B stimulates GnRH release in the male monkey (Macaca mulatta) and is colocalized with kisspeptin in the arcuate nucleus. Endocrinology. 2010;151(9):4494-4503.
155. Ramaswamy S, Seminara SB, Plant TM. Evidence from the agonadal juvenile male rhesus monkey (Macaca mulatta) for the view that the action of neurokinin B to trigger gonadotropin-releasing hormone release is upstream from the kisspeptin receptor. Neuroendocrinology. 2011;94(3):237-245.
156. Navarro VM. Interactions between kisspeptins and neurokinin B. Adv Exp Med Biol. 2013;784:325-347.
157. Navarro VM, Gottsch ML, Chavkin C, Okamura H, Clifton DK, Steiner RA. Regulation of gonadotropin-releasing hormone secretion by kisspeptin/dynorphin/neurokinin B neurons in the arcuate nucleus of the mouse. J Neurosci. 2009;29(38):11859-11866.
158. Ciofi P, Krause JE, Prins GS, Mazzuca M. Presence of nuclear androgen receptor-like immunoreactivity in neurokinin B-containing neurons of the hypothalamic arcuate nucleus of the adult male rat. Neurosci Lett. 1994;182(2):193-196.
159. Foradori CD, Coolen LM, Fitzgerald ME, Skinner DC, Goodman RL, Lehman MN. Colocalization of progesterone receptors in parvicellular dynorphin neurons of the ovine preoptic area and hypothalamus. Endocrinology. 2002;143(11):4366-4374.
160. Smith JT, Cunningham MJ, Rissman EF, Clifton DK, Steiner RA. Regulation of Kiss1 gene expression in the brain of the female mouse. Endocrinology. 2005;146(9):3686-3692.
161. Smith JT, Acohido BV, Clifton DK, Steiner RA. KiSS-1 neurones are direct targets for leptin in the ob/ob mouse. J Neuroendocrinol. 2006;18(4):298-303.
162. George JT, Seminara SB. Kisspeptin and the hypothalamic control of reproduction: lessons from the human. Endocrinology. 2012;153(11):5130-5136.
163. Arreguin-Arevalo JA, Lents CA, Farmerie TA, Nett TM, Clay CM. KiSS-1 peptide induces release of LH by a direct effect on the hypothalamus of ovariectomized ewes. Anim Reprod Sci. 2007;101(3-4):265-275.
164. Gottsch ML, Cunningham MJ, Smith JT, Popa SM, Acohido BV, Crowley WF, Seminara S, Clifton DK, Steiner RA. A role for kisspeptins in the regulation of gonadotropin secretion in the mouse. Endocrinology. 2004;145(9):4073-4077.
165. Shahab M, Mastronardi C, Seminara SB, Crowley WF, Ojeda SR, Plant TM. Increased hypothalamic GPR54 signaling: a potential mechanism for initiation of puberty in primates. Proc Natl Acad Sci U S A. 2005;102(6):2129-2134.
166. Smith JT, Li Q, Pereira A, Clarke IJ. Kisspeptin neurons in the ovine arcuate nucleus and preoptic area are involved in the preovulatory luteinizing hormone surge. Endocrinology. 2009;150(12):5530-5538.
167. Chan YM, Butler JP, Pinnell NE, Pralong FP, Crowley WF, Jr., Ren C, Chan KK, Seminara SB. Kisspeptin resets the hypothalamic GnRH clock in men. J Clin Endocrinol Metab. 2011;96(6):E908-915.
168. Chan YM, Butler JP, Sidhoum VF, Pinnell NE, Seminara SB. Kisspeptin administration to women: a window into endogenous kisspeptin secretion and GnRH responsiveness across the menstrual cycle. J Clin Endocrinol Metab. 2012;97(8):E1458-1467.
169. Dhillo WS, Chaudhri OB, Patterson M, Thompson EL, Murphy KG, Badman MK, McGowan BM, Amber V, Patel S, Ghatei MA, Bloom SR. Kisspeptin-54 stimulates the hypothalamic-pituitary gonadal axis in human males. J Clin Endocrinol Metab. 2005;90(12):6609-6615.
170. Dhillo WS, Chaudhri OB, Thompson EL, Murphy KG, Patterson M, Ramachandran R, Nijher GK, Amber V, Kokkinos A, Donaldson M, Ghatei MA, Bloom SR. Kisspeptin-54 stimulates gonadotropin release most potently during the preovulatory phase of the menstrual cycle in women. J Clin Endocrinol Metab. 2007;92(10):3958-3966.
171. George JT, Anderson RA, Millar RP. Kisspeptin-10 stimulation of gonadotrophin secretion in women is modulated by sex steroid feedback. Hum Reprod. 2012;27(12):3552-3559.
172. George JT, Veldhuis JD, Roseweir AK, Newton CL, Faccenda E, Millar RP, Anderson RA. Kisspeptin-10 is a potent stimulator of LH and increases pulse frequency in men. J Clin Endocrinol Metab. 2011;96(8):E1228-1236.
173. Jayasena CN, Comninos AN, Nijher GM, Abbara A, De Silva A, Veldhuis JD, Ratnasabapathy R, Izzi-Engbeaya C, Lim A, Patel DA, Ghatei MA, Bloom SR, Dhillo WS. Twice-daily subcutaneous injection of kisspeptin-54 does not abolish menstrual cyclicity in healthy female volunteers. J Clin Endocrinol Metab. 2013;98(11):4464-4474.
174. Jayasena CN, Comninos AN, Veldhuis JD, Misra S, Abbara A, Izzi-Engbeaya C, Donaldson M, Ghatei MA, Bloom SR, Dhillo WS. A single injection of kisspeptin-54 temporarily increases luteinizing hormone pulsatility in healthy women. Clin Endocrinol (Oxf). 2013;79(4):558-563.
175. Jayasena CN, Nijher GM, Abbara A, Murphy KG, Lim A, Patel D, Mehta A, Todd C, Donaldson M, Trew GH, Ghatei MA, Bloom SR, Dhillo WS. Twice-weekly administration of kisspeptin-54 for 8 weeks stimulates release of reproductive hormones in women with hypothalamic amenorrhea. Clin Pharmacol Ther. 2010;88(6):840-847.
176. Jayasena CN, Nijher GM, Chaudhri OB, Murphy KG, Ranger A, Lim A, Patel D, Mehta A, Todd C, Ramachandran R, Salem V, Stamp GW, Donaldson M, Ghatei MA, Bloom SR, Dhillo WS. Subcutaneous injection of kisspeptin-54 acutely stimulates gonadotropin secretion in women with hypothalamic amenorrhea, but chronic administration causes tachyphylaxis. J Clin Endocrinol Metab. 2009;94(11):4315-4323.
177. Jayasena CN, Nijher GM, Comninos AN, Abbara A, Januszewki A, Vaal ML, Sriskandarajah L, Murphy KG, Farzad Z, Ghatei MA, Bloom SR, Dhillo WS. The effects of kisspeptin-10 on reproductive hormone release show sexual dimorphism in humans. J Clin Endocrinol Metab. 2011;96(12):E1963-1972.
178. Abbara A, Narayanaswamy S, Izzi-Engbeaya C, Comninos AN, Clarke SA, Malik Z, Papadopoulou D, Clobentz A, Sarang Z, Bassett P, Jayasena CN, Dhillo WS. Hypothalamic Response to Kisspeptin-54 and Pituitary Response to Gonadotropin-Releasing Hormone Are Preserved in Healthy Older Men. Neuroendocrinology. 2018;106(4):401-410.
179. George JT, Veldhuis JD, Tena-Sempere M, Millar RP, Anderson RA. Exploring the pathophysiology of hypogonadism in men with type 2 diabetes: kisspeptin-10 stimulates serum testosterone and LH secretion in men with type 2 diabetes and mild biochemical hypogonadism. Clin Endocrinol (Oxf). 2013;79(1):100-104.
180. Jayasena CN, Abbara A, Veldhuis JD, Comninos AN, Ratnasabapathy R, De Silva A, Nijher GM, Ganiyu-Dada Z, Mehta A, Todd C, Ghatei MA, Bloom SR, Dhillo WS. Increasing LH pulsatility in women with hypothalamic amenorrhoea using intravenous infusion of Kisspeptin-54. J Clin Endocrinol Metab. 2014;99(6):E953-961.
181. Abbara A, Eng PC, Phylactou M, Clarke SA, Richardson R, Sykes CM, Phumsatitpong C, Mills E, Modi M, Izzi-Engbeaya C, Papadopoulou D, Purugganan K, Jayasena CN, Webber L, Salim R, Owen B, Bech P, Comninos AN, McArdle CA, Voliotis M, Tsaneva-Atanasova K, Moenter S, Hanyaloglu A, Dhillo WS. Kisspeptin receptor agonist has therapeutic potential for female reproductive disorders. J Clin Invest. 2020;130(12):6739-6753.
182. Szeliga A, Podfigurna A, Bala G, Meczekalski B. Kisspeptin and neurokinin B analogs use in gynecological endocrinology: where do we stand? J Endocrinol Invest. 2020;43(5):555-561.
183. Abbara A, Ufer M, Voors-Pette C, Berman L, Ezzati M, Wu R, Lee TY, Ferreira JCA, Migoya E, Dhillo WS. Endocrine profile of the kisspeptin receptor agonist MVT-602 in healthy premenopausal women with and without ovarian stimulation: results from 2 randomized, placebo-controlled clinical tricals. Fertil Steril. 2024;121(1):95-106.
184. Li XF, Kinsey-Jones JS, Cheng Y, Knox AM, Lin Y, Petrou NA, Roseweir A, Lightman SL, Milligan SR, Millar RP, O'Byrne KT. Kisspeptin signalling in the hypothalamic arcuate nucleus regulates GnRH pulse generator frequency in the rat. PLoS One. 2009;4(12):e8334.
185. Millar RP, Roseweir AK, Tello JA, Anderson RA, George JT, Morgan K, Pawson AJ. Kisspeptin antagonists: unraveling the role of kisspeptin in reproductive physiology. Brain Res. 2010;1364:81-89.
186. Roseweir AK, Kauffman AS, Smith JT, Guerriero KA, Morgan K, Pielecka-Fortuna J, Pineda R, Gottsch ML, Tena-Sempere M, Moenter SM, Terasawa E, Clarke IJ, Steiner RA, Millar RP. Discovery of potent kisspeptin antagonists delineate physiological mechanisms of gonadotropin regulation. J Neurosci. 2009;29(12):3920-3929.
187. Thompson EL, Patterson M, Murphy KG, Smith KL, Dhillo WS, Todd JF, Ghatei MA, Bloom SR. Central and peripheral administration of kisspeptin-10 stimulates the hypothalamic-pituitary-gonadal axis. J Neuroendocrinol. 2004;16(10):850-858.
188. Tovar S, Vazquez MJ, Navarro VM, Fernandez-Fernandez R, Castellano JM, Vigo E, Roa J, Casanueva FF, Aguilar E, Pinilla L, Dieguez C, Tena-Sempere M. Effects of single or repeated intravenous administration of kisspeptin upon dynamic LH secretion in conscious male rats. Endocrinology. 2006;147(6):2696-2704.
189. Novaira HJ, Ng Y, Wolfe A, Radovick S. Kisspeptin increases GnRH mRNA expression and secretion in GnRH secreting neuronal cell lines. Mol Cell Endocrinol. 2009;311(1-2):126-134.
190. Chan YM, Lippincott MF, Butler JP, Sidhoum VF, Li CX, Plummer L, Seminara SB. Exogenous kisspeptin administration as a probe of GnRH neuronal function in patients with idiopathic hypogonadotropic hypogonadism. J Clin Endocrinol Metab. 2014;99(12):E2762-2771.
191. Mittelman-Smith MA, Krajewski-Hall SJ, McMullen NT, Rance NE. Ablation of KNDy Neurons Results in Hypogonadotropic Hypogonadism and Amplifies the Steroid-Induced LH Surge in Female Rats. Endocrinology. 2016;157(5):2015-2027.
192. Gutierrez-Pascual E, Martinez-Fuentes AJ, Pinilla L, Tena-Sempere M, Malagon MM, Castano JP. Direct pituitary effects of kisspeptin: activation of gonadotrophs and somatotrophs and stimulation of luteinising hormone and growth hormone secretion. J Neuroendocrinol. 2007;19(7):521-530.
193. Richard N, Galmiche G, Corvaisier S, Caraty A, Kottler ML. KiSS-1 and GPR54 genes are co-expressed in rat gonadotrophs and differentially regulated in vivo by oestradiol and gonadotrophin-releasing hormone. J Neuroendocrinol. 2008;20(3):381-393.
194. Smith JT, Coolen LM, Kriegsfeld LJ, Sari IP, Jaafarzadehshirazi MR, Maltby M, Bateman K, Goodman RL, Tilbrook AJ, Ubuka T, Bentley GE, Clarke IJ, Lehman MN. Variation in kisspeptin and RFamide-related peptide (RFRP) expression and terminal connections to gonadotropin-releasing hormone neurons in the brain: a novel medium for seasonal breeding in the sheep. Endocrinology. 2008;149(11):5770-5782.
195. Smith JT, Rao A, Pereira A, Caraty A, Millar RP, Clarke IJ. Kisspeptin is present in ovine hypophysial portal blood but does not increase during the preovulatory luteinizing hormone surge: evidence that gonadotropes are not direct targets of kisspeptin in vivo. Endocrinology. 2008;149(4):1951-1959.
196. Witham EA, Meadows JD, Hoffmann HM, Shojaei S, Coss D, Kauffman AS, Mellon PL. Kisspeptin regulates gonadotropin genes via immediate early gene induction in pituitary gonadotropes. Mol Endocrinol. 2013;27(8):1283-1294.
197. Pampillo M, Camuso N, Taylor JE, Szereszewski JM, Ahow MR, Zajac M, Millar RP, Bhattacharya M, Babwah AV. Regulation of GPR54 signaling by GRK2 and {beta}-arrestin. Mol Endocrinol. 2009;23(12):2060-2074.
198. Ramaswamy S, Seminara SB, Pohl CR, DiPietro MJ, Crowley WF, Jr., Plant TM. Effect of continuous intravenous administration of human metastin 45-54 on the neuroendocrine activity of the hypothalamic-pituitary-testicular axis in the adult male rhesus monkey (Macaca mulatta). Endocrinology. 2007;148(7):3364-3370.
199. Seminara SB, Dipietro MJ, Ramaswamy S, Crowley WF, Jr., Plant TM. Continuous human metastin 45-54 infusion desensitizes G protein-coupled receptor 54-induced gonadotropin-releasing hormone release monitored indirectly in the juvenile male Rhesus monkey (Macaca mulatta): a finding with therapeutic implications. Endocrinology. 2006;147(5):2122-2126.
200. Thompson EL, Murphy KG, Patterson M, Bewick GA, Stamp GW, Curtis AE, Cooke JH, Jethwa PH, Todd JF, Ghatei MA, Bloom SR. Chronic subcutaneous administration of kisspeptin-54 causes testicular degeneration in adult male rats. Am J Physiol Endocrinol Metab. 2006;291(5):E1074-1082.
201. Lippincott MFS, V.F.; Seminara, S. Continuously administered kisspeptin as a pseudo-antagonist of the kisspeptin receptor. The Endocrine Society´s 95th Annual Meeting, ENDO 2013, San Francisco, Abstract SAT 2134-2163. 2013.
202. MacLean DB, Matsui H, Suri A, Neuwirth R, Colombel M. Sustained exposure to the investigational Kisspeptin analog, TAK-448, down-regulates testosterone into the castration range in healthy males and in patients with prostate cancer: results from two phase 1 studies. J Clin Endocrinol Metab. 2014;99(8):E1445-1453.
203. Scott G, Ahmad I, Howard K, MacLean D, Oliva C, Warrington S, Wilbraham D, Worthington P. Double-blind, randomized, placebo-controlled study of safety, tolerability, pharmacokinetics and pharmacodynamics of TAK-683, an investigational metastin analogue in healthy men. Br J Clin Pharmacol. 2013;75(2):381-391.
204. Christian CA, Moenter SM. The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges. Endocr Rev. 2010;31(4):544-577.
205. Kauffman AS. Coming of age in the kisspeptin era: sex differences, development, and puberty. Mol Cell Endocrinol. 2010;324(1-2):51-63.
206. Terasawa E, Garcia JP, Seminara SB, Keen KL. Role of Kisspeptin and Neurokinin B in Puberty in Female Non-Human Primates. Front Endocrinol (Lausanne). 2018;9:148.
207. Shahab M, Lippincott M, Chan YM, Davies A, Merino PM, Plummer L, Mericq V, Seminara S. Discordance in the Dependence on Kisspeptin Signaling in Mini Puberty vs Adolescent Puberty: Human Genetic Evidence. J Clin Endocrinol Metab. 2018;103(4):1273-1276.
208. Navarro VM, Castellano JM, Fernandez-Fernandez R, Barreiro ML, Roa J, Sanchez-Criado JE, Aguilar E, Dieguez C, Pinilla L, Tena-Sempere M. Developmental and hormonally regulated messenger ribonucleic acid expression of KiSS-1 and its putative receptor, GPR54, in rat hypothalamus and potent luteinizing hormone-releasing activity of KiSS-1 peptide. Endocrinology. 2004;145(10):4565-4574.
209. Keen KL, Wegner FH, Bloom SR, Ghatei MA, Terasawa E. An increase in kisspeptin-54 release occurs with the pubertal increase in luteinizing hormone-releasing hormone-1 release in the stalk-median eminence of female rhesus monkeys in vivo. Endocrinology. 2008;149(8):4151-4157.
210. Plant TM, Ramaswamy S, Dipietro MJ. Repetitive activation of hypothalamic G protein-coupled receptor 54 with intravenous pulses of kisspeptin in the juvenile monkey (Macaca mulatta) elicits a sustained train of gonadotropin-releasing hormone discharges. Endocrinology. 2006;147(2):1007-1013.
211. Pineda R, Garcia-Galiano D, Roseweir A, Romero M, Sanchez-Garrido MA, Ruiz-Pino F, Morgan K, Pinilla L, Millar RP, Tena-Sempere M. Critical roles of kisspeptins in female puberty and preovulatory gonadotropin surges as revealed by a novel antagonist. Endocrinology. 2010;151(2):722-730.
212. Decourt C, Robert V, Anger K, Galibert M, Madinier JB, Liu X, Dardente H, Lomet D, Delmas AF, Caraty A, Herbison AE, Anderson GM, Aucagne V, Beltramo M. A synthetic kisspeptin analog that triggers ovulation and advances puberty. Sci Rep. 2016;6:26908.
213. Novaira HJ, Sonko ML, Hoffman G, Koo Y, Ko C, Wolfe A, Radovick S. Disrupted kisspeptin signaling in GnRH neurons leads to hypogonadotrophic hypogonadism. Mol Endocrinol. 2014;28(2):225-238.
214. Lippincott MF, Chan YM, Delaney A, Rivera-Morales D, Butler JP, Seminara SB. Kisspeptin Responsiveness Signals Emergence of Reproductive Endocrine Activity: Implications for Human Puberty. J Clin Endocrinol Metab. 2016;101(8):3061-3069.
215. Chan YM, Lippincott MF, Kusa TO, Seminara SB. Divergent responses to kisspeptin in children with delayed puberty. JCI Insight. 2018;3(8).
216. Sobrino V, Avendano MS, Perdices-Lopez C, Jimenez-Puyer M, Tena-Sempere M. Kisspeptins and the neuroendocrine control of reproduction: Recent progress and new frontiers in kisspeptin research. Front Neuroendocrinol. 2022;65:100977.
217. Garcia JP, Guerriero KA, Keen KL, Kenealy BP, Seminara SB, Terasawa E. Kisspeptin and Neurokinin B Signaling Network Underlies the Pubertal Increase in GnRH Release in Female Rhesus Monkeys. Endocrinology. 2017;158(10):3269-3280.
218. Midgley AR, Jr., Jaffe RB. Regulation of human gonadotropins. X. Episodic fluctuation of LH during the menstrual cycle. J Clin Endocrinol Metab. 1971;33(6):962-969.
219. Levine JE, Bauer-Dantoin AC, Besecke LM, Conaghan LA, Legan SJ, Meredith JM, Strobl FJ, Urban JH, Vogelsong KM, Wolfe AM. Neuroendocrine regulation of the luteinizing hormone-releasing hormone pulse generator in the rat. Recent Prog Horm Res. 1991;47:97-151; discussion 151-153.
220. Clarke IJ. Gonadotrophin-releasing hormone secretion (GnRH) in anoestrous ewes and the induction of GnRH surges by oestrogen. J Endocrinol. 1988;117(3):355-360.
221. Pau KY, Berria M, Hess DL, Spies HG. Preovulatory gonadotropin-releasing hormone surge in ovarian-intact rhesus macaques. Endocrinology. 1993;133(4):1650-1656.
222. Knobil E, Plant TM. The hypothalamic regulation of LH and FSH secretion in the rhesus monkey. Res Publ Assoc Res Nerv Ment Dis. 1978;56:359-372.
223. Hall JE, Taylor AE, Martin KA, Rivier J, Schoenfeld DA, Crowley WF, Jr. Decreased release of gonadotropin-releasing hormone during the preovulatory midcycle luteinizing hormone surge in normal women. Proc Natl Acad Sci U S A. 1994;91(15):6894-6898.
224. Bagatell CJ, Dahl KD, Bremner WJ. The direct pituitary effect of testosterone to inhibit gonadotropin secretion in men is partially mediated by aromatization to estradiol. J Androl. 1994;15(1):15-21.
225. Emons G, Ortmann O, Thiessen S, Knuppen R. Effects of estradiol and some antiestrogens (clomiphene, tamoxifen, and hydroxytamoxifen) on luteinizing hormone secretion by rat pituitary cells in culture. Arch Gynecol. 1986;237(4):199-211.
226. Finkelstein JS, O'Dea LS, Whitcomb RW, Crowley WF, Jr. Sex steroid control of gonadotropin secretion in the human male. II. Effects of estradiol administration in normal and gonadotropin-releasing hormone-deficient men. J Clin Endocrinol Metab. 1991;73(3):621-628.
227. Hayes FJ, Seminara SB, Decruz S, Boepple PA, Crowley WF, Jr. Aromatase inhibition in the human male reveals a hypothalamic site of estrogen feedback. J Clin Endocrinol Metab. 2000;85(9):3027-3035.
228. Kerin JF, Liu JH, Phillipou G, Yen SS. Evidence for a hypothalamic site of action of clomiphene citrate in women. J Clin Endocrinol Metab. 1985;61(2):265-268.
229. Kesner JS, Wilson RC, Kaufman JM, Hotchkiss J, Chen Y, Yamamoto H, Pardo RR, Knobil E. Unexpected responses of the hypothalamic gonadotropin-releasing hormone "pulse generator" to physiological estradiol inputs in the absence of the ovary. Proc Natl Acad Sci U S A. 1987;84(23):8745-8749.
230. Pitteloud N, Dwyer AA, DeCruz S, Lee H, Boepple PA, Crowley WF, Jr., Hayes FJ. Inhibition of luteinizing hormone secretion by testosterone in men requires aromatization for its pituitary but not its hypothalamic effects: evidence from the tandem study of normal and gonadotropin-releasing hormone-deficient men. J Clin Endocrinol Metab. 2008;93(3):784-791.
231. Veldhuis JD, Evans WS, Rogol AD, Thorner MO, Stumpf P. Actions of estradiol on discrete attributes of the luteinizing hormone pulse signal in man. Studies in postmenopausal women treated with pure estradiol. J Clin Invest. 1987;79(3):769-776.
232. Herbison AE. Multimodal influence of estrogen upon gonadotropin-releasing hormone neurons. Endocr Rev. 1998;19(3):302-330.
233. McDevitt MA, Glidewell-Kenney C, Jimenez MA, Ahearn PC, Weiss J, Jameson JL, Levine JE. New insights into the classical and non-classical actions of estrogen: evidence from estrogen receptor knock-out and knock-in mice. Mol Cell Endocrinol. 2008;290(1-2):24-30.
234. Rance NE, McMullen NT, Smialek JE, Price DL, Young WS, 3rd. Postmenopausal hypertrophy of neurons expressing the estrogen receptor gene in the human hypothalamus. J Clin Endocrinol Metab. 1990;71(1):79-85.
235. Rance NE, Young WS, 3rd. Hypertrophy and increased gene expression of neurons containing neurokinin-B and substance-P messenger ribonucleic acids in the hypothalami of postmenopausal women. Endocrinology. 1991;128(5):2239-2247.
236. Rometo AM, Rance NE. Changes in prodynorphin gene expression and neuronal morphology in the hypothalamus of postmenopausal women. J Neuroendocrinol. 2008;20(12):1376-1381.
237. Franceschini I, Lomet D, Cateau M, Delsol G, Tillet Y, Caraty A. Kisspeptin immunoreactive cells of the ovine preoptic area and arcuate nucleus co-express estrogen receptor alpha. Neurosci Lett. 2006;401(3):225-230.
238. Abel TW, Voytko ML, Rance NE. The effects of hormone replacement therapy on hypothalamic neuropeptide gene expression in a primate model of menopause. J Clin Endocrinol Metab. 1999;84(6):2111-2118.
239. Eghlidi DH, Haley GE, Noriega NC, Kohama SG, Urbanski HF. Influence of age and 17beta-estradiol on kisspeptin, neurokinin B, and prodynorphin gene expression in the arcuate-median eminence of female rhesus macaques. Endocrinology. 2010;151(8):3783-3794.
240. Goodman RL, Holaskova I, Nestor CC, Connors JM, Billings HJ, Valent M, Lehman MN, Hileman SM. Evidence that the arcuate nucleus is an important site of progesterone negative feedback in the ewe. Endocrinology. 2011;152(9):3451-3460.
241. Jayasena CN, Abbara A, Comninos AN, Nijher GM, Christopoulos G, Narayanaswamy S, Izzi-Engbeaya C, Sridharan M, Mason AJ, Warwick J, Ashby D, Ghatei MA, Bloom SR, Carby A, Trew GH, Dhillo WS. Kisspeptin-54 triggers egg maturation in women undergoing in vitro fertilization. J Clin Invest. 2014;124(8):3667-3677.
242. Sandoval-Guzman T, Rance NE. Central injection of senktide, an NK3 receptor agonist, or neuropeptide Y inhibits LH secretion and induces different patterns of Fos expression in the rat hypothalamus. Brain Res. 2004;1026(2):307-312.
243. Lippincott MF, Chan YM, Rivera Morales D, Seminara SB. Continuous Kisspeptin Administration in Postmenopausal Women: Impact of Estradiol on Luteinizing Hormone Secretion. J Clin Endocrinol Metab. 2017;102(6):2091-2099.
244. Clarkson J, d'Anglemont de Tassigny X, Moreno AS, Colledge WH, Herbison AE. Kisspeptin-GPR54 signaling is essential for preovulatory gonadotropin-releasing hormone neuron activation and the luteinizing hormone surge. J Neurosci. 2008;28(35):8691-8697.
245. Kinoshita M, Tsukamura H, Adachi S, Matsui H, Uenoyama Y, Iwata K, Yamada S, Inoue K, Ohtaki T, Matsumoto H, Maeda K. Involvement of central metastin in the regulation of preovulatory luteinizing hormone surge and estrous cyclicity in female rats. Endocrinology. 2005;146(10):4431-4436.
246. Watson RE, Jr., Langub MC, Jr., Engle MG, Maley BE. Estrogen-receptive neurons in the anteroventral periventricular nucleus are synaptic targets of the suprachiasmatic nucleus and peri-suprachiasmatic region. Brain Res. 1995;689(2):254-264.
247. Kumar D, Candlish M, Periasamy V, Avcu N, Mayer C, Boehm U. Specialized subpopulations of kisspeptin neurons communicate with GnRH neurons in female mice. Endocrinology. 2015;156(1):32-38.
248. Billings HJ, Connors JM, Altman SN, Hileman SM, Holaskova I, Lehman MN, McManus CJ, Nestor CC, Jacobs BH, Goodman RL. Neurokinin B acts via the neurokinin-3 receptor in the retrochiasmatic area to stimulate luteinizing hormone secretion in sheep. Endocrinology. 2010;151(8):3836-3846.
249. Li Q, Millar RP, Clarke IJ, Smith JT. Evidence that Neurokinin B Controls Basal Gonadotropin-Releasing Hormone Secretion but Is Not Critical for Estrogen-Positive Feedback in Sheep. Neuroendocrinology. 2015;101(2):161-174.
250. Terasawa E. Neuroestradiol in regulation of GnRH release. Horm Behav. 2018.
251. Soules MR, Steiner RA, Clifton DK, Cohen NL, Aksel S, Bremner WJ. Progesterone modulation of pulsatile luteinizing hormone secretion in normal women. J Clin Endocrinol Metab. 1984;58(2):378-383.
252. Skinner DC, Evans NP, Delaleu B, Goodman RL, Bouchard P, Caraty A. The negative feedback actions of progesterone on gonadotropin-releasing hormone secretion are transduced by the classical progesterone receptor. Proc Natl Acad Sci U S A. 1998;95(18):10978-10983.
253. Fox SR, Harlan RE, Shivers BD, Pfaff DW. Chemical characterization of neuroendocrine targets for progesterone in the female rat brain and pituitary. Neuroendocrinology. 1990;51(3):276-283.
254. King JC, Tai DW, Hanna IK, Pfeiffer A, Haas P, Ronsheim PM, Mitchell SC, Turcotte JC, Blaustein JD. A subgroup of LHRH neurons in guinea pigs with progestin receptors is centrally positioned within the total population of LHRH neurons. Neuroendocrinology. 1995;61(3):265-275.
255. Skinner DC, Caraty A, Allingham R. Unmasking the progesterone receptor in the preoptic area and hypothalamus of the ewe: no colocalization with gonadotropin-releasing neurons. Endocrinology. 2001;142(2):573-579.
256. Foradori CD, Goodman RL, Adams VL, Valent M, Lehman MN. Progesterone increases dynorphin a concentrations in cerebrospinal fluid and preprodynorphin messenger ribonucleic Acid levels in a subset of dynorphin neurons in the sheep. Endocrinology. 2005;146(4):1835-1842.
257. Matsumoto AM, Bremner WJ. Modulation of pulsatile gonadotropin secretion by testosterone in man. J Clin Endocrinol Metab. 1984;58(4):609-614.
258. Herbison AE, Skinner DC, Robinson JE, King IS. Androgen receptor-immunoreactive cells in ram hypothalamus: distribution and co-localization patterns with gonadotropin-releasing hormone, somatostatin and tyrosine hydroxylase. Neuroendocrinology. 1996;63(2):120-131.
259. Smith JT, Dungan HM, Stoll EA, Gottsch ML, Braun RE, Eacker SM, Clifton DK, Steiner RA. Differential regulation of KiSS-1 mRNA expression by sex steroids in the brain of the male mouse. Endocrinology. 2005;146(7):2976-2984.
260. Woodham C, Birch L, Prins GS. Neonatal estrogen down-regulates prostatic androgen receptor through a proteosome-mediated protein degradation pathway. Endocrinology. 2003;144(11):4841-4850.
261. Kumar MV, Leo ME, Tindall DJ. Modulation of androgen receptor transcriptional activity by the estrogen receptor. J Androl. 1994;15(6):534-542.
262. Navarro VM, Gottsch ML, Wu M, Garcia-Galiano D, Hobbs SJ, Bosch MA, Pinilla L, Clifton DK, Dearth A, Ronnekleiv OK, Braun RE, Palmiter RD, Tena-Sempere M, Alreja M, Steiner RA. Regulation of NKB pathways and their roles in the control of Kiss1 neurons in the arcuate nucleus of the male mouse. Endocrinology. 2011;152(11):4265-4275.
263. Shibata M, Friedman RL, Ramaswamy S, Plant TM. Evidence that down regulation of hypothalamic KiSS-1 expression is involved in the negative feedback action of testosterone to regulate luteinising hormone secretion in the adult male rhesus monkey (Macaca mulatta). J Neuroendocrinol. 2007;19(6):432-438.
264. Chrousos GP, Torpy DJ, Gold PW. Interactions between the hypothalamic-pituitary-adrenal axis and the female reproductive system: clinical implications. Ann Intern Med. 1998;129(3):229-240.
265. Kalantaridou SN, Makrigiannakis A, Zoumakis E, Chrousos GP. Stress and the female reproductive system. J Reprod Immunol. 2004;62(1-2):61-68.
266. Barron JL, Noakes TD, Levy W, Smith C, Millar RP. Hypothalamic dysfunction in overtrained athletes. J Clin Endocrinol Metab. 1985;60(4):803-806.
267. Berga SL, Mortola JF, Girton L, Suh B, Laughlin G, Pham P, Yen SS. Neuroendocrine aberrations in women with functional hypothalamic amenorrhea. J Clin Endocrinol Metab. 1989;68(2):301-308.
268. Boccuzzi G, Angeli A, Bisbocci D, Fonzo D, Giadano GP, Ceresa F. Effect of synthetic luteinizing hormone releasing hormone (LH-RH) on the release of gonadotropins in Cushing's disease. J Clin Endocrinol Metab. 1975;40(5):892-895.
269. Breen KM, Oakley AE, Pytiak AV, Tilbrook AJ, Wagenmaker ER, Karsch FJ. Does cortisol acting via the type II glucocorticoid receptor mediate suppression of pulsatile luteinizing hormone secretion in response to psychosocial stress? Endocrinology. 2007;148(4):1882-1890.
270. Luger A, Deuster PA, Kyle SB, Gallucci WT, Montgomery LC, Gold PW, Loriaux DL, Chrousos GP. Acute hypothalamic-pituitary-adrenal responses to the stress of treadmill exercise. Physiologic adaptations to physical training. N Engl J Med. 1987;316(21):1309-1315.
271. Luton JP, Thieblot P, Valcke JC, Mahoudeau JA, Bricaire H. Reversible gonadotropin deficiency in male Cushing's disease. J Clin Endocrinol Metab. 1977;45(3):488-495.
272. Oakley AE, Breen KM, Clarke IJ, Karsch FJ, Wagenmaker ER, Tilbrook AJ. Cortisol reduces gonadotropin-releasing hormone pulse frequency in follicular phase ewes: influence of ovarian steroids. Endocrinology. 2009;150(1):341-349.
273. Saketos M, Sharma N, Santoro NF. Suppression of the hypothalamic-pituitary-ovarian axis in normal women by glucocorticoids. Biol Reprod. 1993;49(6):1270-1276.
274. Breen KM, Karsch FJ. New insights regarding glucocorticoids, stress and gonadotropin suppression. Front Neuroendocrinol. 2006;27(2):233-245.
275. Chen MD, Ordog T, O'Byrne KT, Goldsmith JR, Connaughton MA, Knobil E. The insulin hypoglycemia-induced inhibition of gonadotropin-releasing hormone pulse generator activity in the rhesus monkey: roles of vasopressin and corticotropin-releasing factor. Endocrinology. 1996;137(5):2012-2021.
276. Kinsey-Jones JS, Li XF, Knox AM, Wilkinson ES, Zhu XL, Chaudhary AA, Milligan SR, Lightman SL, O'Byrne KT. Down-regulation of hypothalamic kisspeptin and its receptor, Kiss1r, mRNA expression is associated with stress-induced suppression of luteinising hormone secretion in the female rat. J Neuroendocrinol. 2009;21(1):20-29.
277. Takumi K, Iijima N, Higo S, Ozawa H. Immunohistochemical analysis of the colocalization of corticotropin-releasing hormone receptor and glucocorticoid receptor in kisspeptin neurons in the hypothalamus of female rats. Neurosci Lett. 2012;531(1):40-45.
278. Phumsatitpong C, Moenter SM. Estradiol-Dependent Stimulation and Suppression of Gonadotropin-Releasing Hormone Neuron Firing Activity by Corticotropin-Releasing Hormone in Female Mice. Endocrinology. 2018;159(1):414-425.
279. Mitchell JC, Li XF, Breen L, Thalabard JC, O'Byrne KT. The role of the locus coeruleus in corticotropin-releasing hormone and stress-induced suppression of pulsatile luteinizing hormone secretion in the female rat. Endocrinology. 2005;146(1):323-331.
280. Choi YJ, Habibi HR, Kil GS, Jung MM, Choi CY. Effect of cortisol on gonadotropin inhibitory hormone (GnIH) in the cinnamon clownfish, Amphiprion melanopus. Biochem Biophys Res Commun. 2017;485(2):342-348.
281. Bohnet HG, Dahlen HG, Wuttke W, Schneider HP. Hyperprolactinemic anovulatory syndrome. J Clin Endocrinol Metab. 1976;42(1):132-143.
282. Diaz S, Seron-Ferre M, Cardenas H, Schiappacasse V, Brandeis A, Croxatto HB. Circadian variation of basal plasma prolactin, prolactin response to suckling, and length of amenorrhea in nursing women. J Clin Endocrinol Metab. 1989;68(5):946-955.
283. Molitch ME. Pregnancy and the hyperprolactinemic woman. N Engl J Med. 1985;312(21):1364-1370.
284. De Rosa M, Zarrilli S, Di Sarno A, Milano N, Gaccione M, Boggia B, Lombardi G, Colao A. Hyperprolactinemia in men: clinical and biochemical features and response to treatment. Endocrine. 2003;20(1-2):75-82.
285. Moult PJ, Rees LH, Besser GM. Pulsatile gonadotrophin secretion in hyperprolactinaemic amenorrhoea an the response to bromocriptine therapy. Clin Endocrinol (Oxf). 1982;16(2):153-162.
286. Lecomte P, Lecomte C, Lansac J, Gallier J, Sonier CB, Simonetta C. Pregnancy after intravenous pulsatile gonadotropin-releasing hormone in a hyperprolactinaemic woman resistant to treatment with dopamine agonists. Eur J Obstet Gynecol Reprod Biol. 1997;74(2):219-221.
287. Polson DW, Sagle M, Mason HD, Adams J, Jacobs HS, Franks S. Ovulation and normal luteal function during LHRH treatment of women with hyperprolactinaemic amenorrhoea. Clin Endocrinol (Oxf). 1986;24(5):531-537.
288. Grattan DR, Jasoni CL, Liu X, Anderson GM, Herbison AE. Prolactin regulation of gonadotropin-releasing hormone neurons to suppress luteinizing hormone secretion in mice. Endocrinology. 2007;148(9):4344-4351.
289. Milenkovic L, D'Angelo G, Kelly PA, Weiner RI. Inhibition of gonadotropin hormone-releasing hormone release by prolactin from GT1 neuronal cell lines through prolactin receptors. Proc Natl Acad Sci U S A. 1994;91(4):1244-1247.
290. Kolbinger W, Beyer C, Fohr K, Reisert I, Pilgrim C. Diencephalic GABAergic neurons in vitro respond to prolactin with a rapid increase in intracellular free calcium. Neuroendocrinology. 1992;56(2):148-152.
291. Sarkar DK, Yen SS. Hyperprolactinemia decreases the luteinizing hormone-releasing hormone concentration in pituitary portal plasma: a possible role for beta-endorphin as a mediator. Endocrinology. 1985;116(5):2080-2084.
292. Li C, Chen P, Smith MS. Neuropeptide Y and tuberoinfundibular dopamine activities are altered during lactation: role of prolactin. Endocrinology. 1999;140(1):118-123.
293. Moore KE. Interactions between prolactin and dopaminergic neurons. Biol Reprod. 1987;36(1):47-58.
294. Kokay IC, Petersen SL, Grattan DR. Identification of prolactin-sensitive GABA and kisspeptin neurons in regions of the rat hypothalamus involved in the control of fertility. Endocrinology. 2011;152(2):526-535.
295. Szawka RE, Ribeiro AB, Leite CM, Helena CV, Franci CR, Anderson GM, Hoffman GE, Anselmo-Franci JA. Kisspeptin regulates prolactin release through hypothalamic dopaminergic neurons. Endocrinology. 2010;151(7):3247-3257.
296. Yamada S, Uenoyama Y, Kinoshita M, Iwata K, Takase K, Matsui H, Adachi S, Inoue K, Maeda KI, Tsukamura H. Inhibition of metastin (kisspeptin-54)-GPR54 signaling in the arcuate nucleus-median eminence region during lactation in rats. Endocrinology. 2007;148(5):2226-2232.
297. Sonigo C, Bouilly J, Carre N, Tolle V, Caraty A, Tello J, Simony-Conesa FJ, Millar R, Young J, Binart N. Hyperprolactinemia-induced ovarian acyclicity is reversed by kisspeptin administration. J Clin Invest. 2012;122(10):3791-3795.
298. Araujo-Lopes R, Crampton JR, Aquino NS, Miranda RM, Kokay IC, Reis AM, Franci CR, Grattan DR, Szawka RE. Prolactin regulates kisspeptin neurons in the arcuate nucleus to suppress LH secretion in female rats. Endocrinology. 2014;155(3):1010-1020.
299. Brown RS, Herbison AE, Grattan DR. Prolactin regulation of kisspeptin neurones in the mouse brain and its role in the lactation-induced suppression of kisspeptin expression. J Neuroendocrinol. 2014;26(12):898-908.
300. Millar RP, Sonigo C, Anderson RA, George J, Maione L, Brailly-Tabard S, Chanson P, Binart N, Young J. Hypothalamic-Pituitary-Ovarian Axis Reactivation by Kisspeptin-10 in Hyperprolactinemic Women With Chronic Amenorrhea. J Endocr Soc. 2017;1(11):1362-1371.
301. Aquino NSS, Araujo-Lopes R, Henriques PC, Lopes FEF, Gusmao DO, Coimbra CC, Franci CR, Reis AM, Szawka RE. alpha-Estrogen and Progesterone Receptors Modulate Kisspeptin Effects on Prolactin: Role in Estradiol-Induced Prolactin Surge in Female Rats. Endocrinology. 2017;158(6):1812-1826.
302. Schneider JE. Energy balance and reproduction. Physiol Behav. 2004;81(2):289-317.
303. Yeo SH, Colledge WH. The Role of Kiss1 Neurons As Integrators of Endocrine, Metabolic, and Environmental Factors in the Hypothalamic-Pituitary-Gonadal Axis. Front Endocrinol (Lausanne). 2018;9:188.
304. Dandona P, Dhindsa S. Update: Hypogonadotropic hypogonadism in type 2 diabetes and obesity. J Clin Endocrinol Metab. 2011;96(9):2643-2651.
305. George JT, Millar RP, Anderson RA. Hypothesis: kisspeptin mediates male hypogonadism in obesity and type 2 diabetes. Neuroendocrinology. 2010;91(4):302-307.
306. Bergendahl M, Aloi JA, Iranmanesh A, Mulligan TM, Veldhuis JD. Fasting suppresses pulsatile luteinizing hormone (LH) secretion and enhances orderliness of LH release in young but not older men. J Clin Endocrinol Metab. 1998;83(6):1967-1975.
307. Bergendahl M, Veldhuis JD. Altered pulsatile gonadotropin signaling in nutritional deficiency in the male. Trends Endocrinol Metab. 1995;6(5):145-159.
308. Weigle DS, Duell PB, Connor WE, Steiner RA, Soules MR, Kuijper JL. Effect of fasting, refeeding, and dietary fat restriction on plasma leptin levels. J Clin Endocrinol Metab. 1997;82(2):561-565.
309. Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab. 2003;88(1):297-311.
310. Loucks AB, Verdun M, Heath EM. Low energy availability, not stress of exercise, alters LH pulsatility in exercising women. J Appl Physiol (1985). 1998;84(1):37-46.
311. Schreihofer DA, Amico JA, Cameron JL. Reversal of fasting-induced suppression of luteinizing hormone (LH) secretion in male rhesus monkeys by intragastric nutrient infusion: evidence for rapid stimulation of LH by nutritional signals. Endocrinology. 1993;132(5):1890-1897.
312. Welt CK, Chan JL, Bullen J, Murphy R, Smith P, DePaoli AM, Karalis A, Mantzoros CS. Recombinant human leptin in women with hypothalamic amenorrhea. N Engl J Med. 2004;351(10):987-997.
313. Chan JL, Heist K, DePaoli AM, Veldhuis JD, Mantzoros CS. The role of falling leptin levels in the neuroendocrine and metabolic adaptation to short-term starvation in healthy men. J Clin Invest. 2003;111(9):1409-1421.
314. Farooqi IS, O'Rahilly S. Leptin: a pivotal regulator of human energy homeostasis. Am J Clin Nutr. 2009;89(3):980S-984S.
315. Rehman R, Jamil Z, Khalid A, Fatima SS. Cross talk between serum Kisspeptin-Leptin during assisted reproduction techniques. Pak J Med Sci. 2018;34(2):342-346.
316. Backholer K, Smith JT, Rao A, Pereira A, Iqbal J, Ogawa S, Li Q, Clarke IJ. Kisspeptin cells in the ewe brain respond to leptin and communicate with neuropeptide Y and proopiomelanocortin cells. Endocrinology. 2010;151(5):2233-2243.
317. Cunningham MJ, Clifton DK, Steiner RA. Leptin's actions on the reproductive axis: perspectives and mechanisms. Biol Reprod. 1999;60(2):216-222.
318. Fernandez-Fernandez R, Martini AC, Navarro VM, Castellano JM, Dieguez C, Aguilar E, Pinilla L, Tena-Sempere M. Novel signals for the integration of energy balance and reproduction. Mol Cell Endocrinol. 2006;254-255:127-132.
319. Quennell JH, Mulligan AC, Tups A, Liu X, Phipps SJ, Kemp CJ, Herbison AE, Grattan DR, Anderson GM. Leptin indirectly regulates gonadotropin-releasing hormone neuronal function. Endocrinology. 2009;150(6):2805-2812.
320. Castellano JM, Navarro VM, Fernandez-Fernandez R, Nogueiras R, Tovar S, Roa J, Vazquez MJ, Vigo E, Casanueva FF, Aguilar E, Pinilla L, Dieguez C, Tena-Sempere M. Changes in hypothalamic KiSS-1 system and restoration of pubertal activation of the reproductive axis by kisspeptin in undernutrition. Endocrinology. 2005;146(9):3917-3925.
321. Luque RM, Kineman RD, Tena-Sempere M. Regulation of hypothalamic expression of KiSS-1 and GPR54 genes by metabolic factors: analyses using mouse models and a cell line. Endocrinology. 2007;148(10):4601-4611.
322. Roa J, Garcia-Galiano D, Varela L, Sanchez-Garrido MA, Pineda R, Castellano JM, Ruiz-Pino F, Romero M, Aguilar E, Lopez M, Gaytan F, Dieguez C, Pinilla L, Tena-Sempere M. The mammalian target of rapamycin as novel central regulator of puberty onset via modulation of hypothalamic Kiss1 system. Endocrinology. 2009;150(11):5016-5026.
323. Wahab F, Ullah F, Chan YM, Seminara SB, Shahab M. Decrease in hypothalamic Kiss1 and Kiss1r expression: a potential mechanism for fasting-induced suppression of the HPG axis in the adult male rhesus monkey (Macaca mulatta). Horm Metab Res. 2011;43(2):81-85.
324. Castellano JM, Navarro VM, Fernandez-Fernandez R, Roa J, Vigo E, Pineda R, Dieguez C, Aguilar E, Pinilla L, Tena-Sempere M. Expression of hypothalamic KiSS-1 system and rescue of defective gonadotropic responses by kisspeptin in streptozotocin-induced diabetic male rats. Diabetes. 2006;55(9):2602-2610.
325. Elias CF. Leptin action in pubertal development: recent advances and unanswered questions. Trends Endocrinol Metab. 2012;23(1):9-15.
326. Hill JW, Elmquist JK, Elias CF. Hypothalamic pathways linking energy balance and reproduction. Am J Physiol Endocrinol Metab. 2008;294(5):E827-832.
327. Donato J, Jr., Cravo RM, Frazao R, Gautron L, Scott MM, Lachey J, Castro IA, Margatho LO, Lee S, Lee C, Richardson JA, Friedman J, Chua S, Jr., Coppari R, Zigman JM, Elmquist JK, Elias CF. Leptin's effect on puberty in mice is relayed by the ventral premammillary nucleus and does not require signaling in Kiss1 neurons. J Clin Invest. 2011;121(1):355-368.
328. Schalla MA, Stengel A. Central mechanisms of kisspeptin-induced inhibition of food intake. Peptides. 2021;135:170475.
329. Saito R, Tanaka K, Nishimura H, Nishimura K, Sonoda S, Ueno H, Motojima Y, Yoshimura M, Maruyama T, Yamamoto Y, Kusuhara K, Ueta Y. Centrally administered kisspeptin suppresses feeding via nesfatin-1 and oxytocin in male rats. Peptides. 2019;112:114-124.
330. Frazao R, Dungan Lemko HM, da Silva RP, Ratra DV, Lee CE, Williams KW, Zigman JM, Elias CF. Estradiol modulates Kiss1 neuronal response to ghrelin. Am J Physiol Endocrinol Metab. 2014;306(6):E606-614.
331. Kluge M, Schussler P, Schmidt D, Uhr M, Steiger A. Ghrelin suppresses secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in women. J Clin Endocrinol Metab. 2012;97(3):E448-451.
332. Martini AC, Fernandez-Fernandez R, Tovar S, Navarro VM, Vigo E, Vazquez MJ, Davies JS, Thompson NM, Aguilar E, Pinilla L, Wells T, Dieguez C, Tena-Sempere M. Comparative analysis of the effects of ghrelin and unacylated ghrelin on luteinizing hormone secretion in male rats. Endocrinology. 2006;147(5):2374-2382.
333. Donoso AO, Lopez FJ, Negro-Vilar A. Cross-talk between excitatory and inhibitory amino acids in the regulation of luteinizing hormone-releasing hormone secretion. Endocrinology. 1992;131(3):1559-1561.
334. Leranth C, MacLusky NJ, Sakamoto H, Shanabrough M, Naftolin F. Glutamic acid decarboxylase-containing axons synapse on LHRH neurons in the rat medial preoptic area. Neuroendocrinology. 1985;40(6):536-539.
335. Mijiddorj T, Kanasaki H, Sukhbaatar U, Oride A, Kyo S. DS1, a delta subunit-containing GABA(A) receptor agonist, increases gonadotropin subunit gene expression in mouse pituitary gonadotrophs. Biol Reprod. 2015;92(2):45.
336. Moore AM, Prescott M, Marshall CJ, Yip SH, Campbell RE. Enhancement of a robust arcuate GABAergic input to gonadotropin-releasing hormone neurons in a model of polycystic ovarian syndrome. Proc Natl Acad Sci U S A. 2015;112(2):596-601.
337. Watanabe M, Fukuda A, Nabekura J. The role of GABA in the regulation of GnRH neurons. Front Neurosci. 2014;8:387.
338. Di Giorgio NP, Bizzozzero-Hiriart M, Libertun C, Lux-Lantos V. Unraveling the connection between GABA and kisspeptin in the control of reproduction. Reproduction. 2019;157(6):R225-R233.
339. Kanasaki H, Tumurbaatar T, Oride A, Hara T, Okada H, Kyo S. Gamma-aminobutyric acidA receptor agonist, muscimol, increases KiSS-1 gene expression in hypothalamic cell models. Reprod Med Biol. 2017;16(4):386-391.
340. Kurian JR, Keen KL, Guerriero KA, Terasawa E. Tonic control of kisspeptin release in prepubertal monkeys: implications to the mechanism of puberty onset. Endocrinology. 2012;153(7):3331-3336.
341. Terasawa E, Kurian JR, Guerriero KA, Kenealy BP, Hutz ED, Keen KL. Recent discoveries on the control of gonadotrophin-releasing hormone neurones in nonhuman primates. J Neuroendocrinol. 2010;22(7):630-638.
342. Piet R, Kalil B, McLennan T, Porteous R, Czieselsky K, Herbison AE. Dominant Neuropeptide Cotransmission in Kisspeptin-GABA Regulation of GnRH Neuron Firing Driving Ovulation. J Neurosci. 2018;38(28):6310-6322.
343. Comninos AN, Yang L, O'Callaghan J, Mills EG, Wall MB, Demetriou L, Wing VC, Thurston L, Owen BM, Abbara A, Rabiner EA, Dhillo WS. Kisspeptin modulates gamma-aminobutyric acid levels in the human brain. Psychoneuroendocrinology. 2021;129:105244.
344. Hatef A, Unniappan S. Gonadotropin-releasing hormone, kisspeptin, and gonadal steroids directly modulate nucleobindin-2/nesfatin-1 in murine hypothalamic gonadotropin-releasing hormone neurons and gonadotropes. Biol Reprod. 2017;96(3):635-651.
345. Christian CA, Moenter SM. Vasoactive intestinal polypeptide can excite gonadotropin-releasing hormone neurons in a manner dependent on estradiol and gated by time of day. Endocrinology. 2008;149(6):3130-3136.
346. Ducret E, Anderson GM, Herbison AE. RFamide-related peptide-3, a mammalian gonadotropin-inhibitory hormone ortholog, regulates gonadotropin-releasing hormone neuron firing in the mouse. Endocrinology. 2009;150(6):2799-2804.
347. Lee EJ, Moore CT, Hosny S, Centers A, Jennes L. Expression of estrogen receptor-alpha and c-Fos in adrenergic neurons of the female rat during the steroid-induced LH surge. Brain Res. 2000;875(1-2):56-65.
348. Miller BH, Olson SL, Levine JE, Turek FW, Horton TH, Takahashi JS. Vasopressin regulation of the proestrous luteinizing hormone surge in wild-type and Clock mutant mice. Biol Reprod. 2006;75(5):778-784.
349. Palm IF, Van Der Beek EM, Wiegant VM, Buijs RM, Kalsbeek A. Vasopressin induces a luteinizing hormone surge in ovariectomized, estradiol-treated rats with lesions of the suprachiasmatic nucleus. Neuroscience. 1999;93(2):659-666.
350. Leon S, Tena-Sempere M. Dissecting the Roles of Gonadotropin-Inhibitory Hormone in Mammals: Studies Using Pharmacological Tools and Genetically Modified Mouse Models. Front Endocrinol (Lausanne). 2015;6:189.
351. Qi Y, Oldfield BJ, Clarke IJ. Projections of RFamide-related peptide-3 neurones in the ovine hypothalamus, with special reference to regions regulating energy balance and reproduction. J Neuroendocrinol. 2009;21(8):690-697.
352. Liu X, Herbison AE. Kisspeptin Regulation of Neuronal Activity throughout the Central Nervous System. Endocrinol Metab (Seoul). 2016;31(2):193-205.
353. Acevedo-Rodriguez A, Kauffman AS, Cherrington BD, Borges CS, Roepke TA, Laconi M. Emerging insights into Hypothalamic-pituitary-gonadal (HPG) axis regulation and interaction with stress signaling. J Neuroendocrinol. 2018.
354. Kriegsfeld LJ, Mei DF, Bentley GE, Ubuka T, Mason AO, Inoue K, Ukena K, Tsutsui K, Silver R. Identification and characterization of a gonadotropin-inhibitory system in the brains of mammals. Proc Natl Acad Sci U S A. 2006;103(7):2410-2415.
355. Molnar CS, Kallo I, Liposits Z, Hrabovszky E. Estradiol down-regulates RF-amide-related peptide (RFRP) expression in the mouse hypothalamus. Endocrinology. 2011;152(4):1684-1690.
356. Uenoyama Y, Nakamura S, Hayakawa Y, Ikegami K, Watanabe Y, Deura C, Minabe S, Tomikawa J, Goto T, Ieda N, Inoue N, Sanbo M, Tamura C, Hirabayashi M, Maeda KI, Tsukamura H. Lack of pulse and surge modes and glutamatergic stimulation of luteinising hormone release in Kiss1 knockout rats. J Neuroendocrinol. 2015;27(3):187-197.

ABSTRACT

Empty sella is a radiological finding of a flattened pituitary in a sellar space filled with cerebrospinal fluid. It may be primary or secondary consequent to various processes causing injury and shrinkage of the pituitary gland (postpartum hemorrhage, pituitary surgery, irradiation, apoplexy, infection, head trauma, hypophysitis, etc.). The mechanisms involved in the pathogenesis of the so called “primary empty sella” may range from continuously or intermittently increased intracranial pressure due to idiopathic benign intracranial hypertension, obesity, arterial hypertension, or multiple pregnancies in female patients with accompanying insufficiency of the sellar diaphragm and changes in pituitary gland volume (hyperplasia during pregnancy, lactation, menopause etc.). Primary empty sella can be an incidental radiological finding in an asymptomatic patient with preserved pituitary function. In symptomatic patients with the so called “empty sella syndrome” (headache, visual disturbances, and hormonal dysfunction), the radiological finding of an empty sella is important in the differential diagnosis of other sellar lesions. Hypopituitarism, partial or complete, and hyperprolactinemia are not uncommon in these patients. The treatment of hypopituitarism and hyperprolactinemia is advocated in all patients with confirmatory results. In patients with the secondary empty sella, hypopituitarism is more common and more readily recognized due to damage caused by surgery, radiation therapy, or various pathological causes. Rarely, an empty sella can be associated with hormonal hypersecretion from an “invisible” micro adenoma producing prolactin, growth hormone (acromegaly), or ACTH (Cushing’s disease). A wide range of radiological findings in patients with secondary and primary empty sella coupled with clinical data (important hints from the history and data on endocrine function) are presented for further illustration of this topic.

HISTORY

The term ‘empty sella’ was first used by Bush in 1951 (1) to describe a peculiar anatomical condition, observed in 40 of 788 human cadavers, particularly females, characterized by a sella turcica with an incomplete diaphragm sellae that forms only a small peripheral rim, with a pituitary gland not absent, but flattened in such a manner as to form a thin layer of tissue at the bottom of the sella turcica. Kaufman (2), in 1968, speculated that ‘…empty sella is a distinct anatomical and radiographic entity, function of an incompleteness of the diaphragma sellae and of the cerebrospinal fluid (CSF) pressure, normal or elevated’. The role of the normal fluctuations of CSF pressures and the effect of a superimposed prolonged increase in CSF pressure were related to the anatomic changes involving the bony wall of the empty sella.

DEFINITION, ETIOLOGY, AND PREVALENCE OF EMPTY SELLA

Empty sella is defined as herniation of subarachnoid space into the sella turcica (arachnoidocele). It is a term for the radiological finding of “empty sellar space” on magnetic resonance imaging (MRI) and computerized tomography (CT) with a flattened pituitary and elongated stalk. It can be partial if less than 50% of sellar space is filled with cerebro-spinal fluid (CSF), or complete if CSF fills more than 50% of space in the sella and gland thickness is less than 2mm (3,4).

Regarding its etiology, it can be primary if there is no pathological process in the sellar region preceding the pituitary damage or secondary if it is consequent to a specific pathological process. Primary empty sella (PES) can be an incidental finding or may arise during imaging for headache, endocrine disorders, neurological symptoms, visual disturbances, abnormal sella turcica radiograph, and other reasons.

Primary empty sella (PES) can be caused by intracranial hypertension and/or insufficiency of the sellar diaphragm in subjects with no previous history of pituitary disease. Insufficiency of the sellar diaphragm, a deflection of dura matter separating the suprasellar cistern from the pituitary fossa, allows unobstructed pulsatile movements of CSF from chiasmatic cistern causing flattening of the pituitary to the sellar floor. In extreme cases bone erosion of the sellar floor and CSF leak (rhinorrhea) may occur, increasing the risk of meningitis. Partial or complete absence of the sellar diaphragm has been demonstrated in patients with PES.

Intracranial pressure can be intermittently increased due to obesity, sleep apnea, arterial hypertension, pregnancy, and labor. Intracranial hypertension may be idiopathic or associated with other intracranial processes such as tumors, venous thrombosis, infections, or malformations. Idiopathic intracranial hypertension (IIH) or “pseudo-tumor cerebri” is a rare condition affecting 1 in 100,000 persons. It can be due to impaired CSF absorption, increased CSF secretion, and/or increased capillary permeability (5). Impaired CSF dynamics and absorption have been found in up to 77% and 84% of patients with PES, respectively (6), The prevalence of PES is very high in patients with IIH ranging from 70-94% (4).

Changes in the pituitary gland volume may also be involved in the pathogenesis of empty sella syndrome including hyperplasia during pregnancy and lactation and pituitary involution after menopause accounting for the significantly higher prevalence of this condition in female patients (female to male ratio 5:1).

Factors involved in the pathogenesis of primary empty sella are shown in Figure 1.

Secondary empty sella is more common and is related to various pathological processes of the sellar region. Among many causes, pituitary tumor shrinkage occurring after medical treatment, surgery, radiotherapy, and apoplexy of a pituitary adenoma are frequent causes of secondary empty sella. Likewise, postpartum pituitary necrosis, pituitary infection, hypophysitis, and traumatic brain injury may lead to pituitary atrophy. The diagnosis of secondary empty sella is more difficult if there is no known underlying pathology involving the pituitary gland. In these cases, the sella is normal in size, and the function of the flattened pituitary gland may or may not be compromised. Such is the case in congenital causes of hypopituitarism both acquired and genetic, presenting with a hypoplastic pituitary gland and ectopic posterior lobe (7). Large intracranial tumors such as slow-growing meningiomas can also cause increased intracranial pressure and secondary empty sella in a significant number of patients. Pituitary MRI images of patients with secondary and primary empty sella are presented in the section dedicated to radiological appearance of an empty sella (Fig. 2-6). Associated risk factors for primary and secondary empty sella are shown in Table 1.

The reported prevalence of empty sella depends on techniques used for detection. In autopsy studies empty sella has been found in up to 5.5-12% cases (1,2), On imaging the overall incidence has been estimated at 12% (4). The prevalence of PES is very high in patients with IIH (8)

Figure 1. Factors involved in pathogenesis of primary empty sella include the upper-sellar factors, incompetence or incomplete formation of the sellar diaphragm, and pituitary factors associated with the variation in the pituitary volume. Modified from Bioscientifica Ltd., Chiloiro S et al, European Journal of Endocrinology (2017) 177, R275–R285

RADIOLOGICAL APPEARANCE OF EMPTY SELLA

Very commonly an empty sella is incidentally discovered during MRI or CT imaging or during evaluation for headache, endocrine, neurological or visual disturbances. Less commonly it is observed after additional imaging for abnormal sella turcica radiographs. Chronic intracranial hypertension can lead to sellar remodeling and enlargement with thinning of the sellar floor and rarely rhinorrhea.

On typical presentation CSF filling is in continuity with overlying subarachnoid spaces and the residual pituitary gland is flattened against the sellar floor of an enlarged bony sella with pituitary volume usually less than 611.21 mm³ (9). The differential diagnosis with cystic lesions and congenital pituitary abnormalities may pose a challenge. The stalk is usually thinned and located in the midline. Asymmetry is a frequent sign of the secondary empty sella. In rare cases the chiasm can herniate into the sella in cases of both primary and secondary etiology. Indirect signs of intracranial hypertension may also be present such as flattening of the posterior sclera, prominent subarachnoid spaces along the optic nerves, vertical tortuosity of the optic nerve sheath complex, and increased width of the optic nerve sheaths (10).

Sagittal and coronal T1-weighted (T1W) contrast enhanced images and coronal T2-weighted (T2W) images are strongly indicated for MR studies because they show CSF within the sella. On FLAIR sequences the intrasellar fluid completely suppresses and it presents without restriction in DWI sequences. After contrast T1W images show normal enhancement of residual pituitary gland and the stalk in PES in contrast to scaring and distortion in SES.

In patients with congenital hypopituitarism an ectopic posterior pituitary, stalk duplication or absence may be present in combination with other midline defects (agenesis of corpus callosum, supraoptic dysplasia, etc.) (7).

Figures 2, 3, 4, 5 represent various MRI findings of patients with secondary empty sella due to postpartum hemorrhage (Sheehan’s syndrome Fig. 2), lymphocytic hypophysitis (Fig. 3), shrinkage of a macroprolactinoma after successful treatment with cabergoline (Fig.4), and congenital hypopituitarism with empty sella (Fig.5)

Figure 2. Sagittal T1W images of two patients with empty sella and Sheehan’s syndrome
Left Panel (sagittal T1W image without contrast) and Right (sagittal T1W image after contrast enhancement) represent the MRI appearance of empty sella in two patients with Sheehan’s syndrome. Both patients presented with hyponatremic coma due to unrecognized panhypopituitarism and infection 3 and 20 years after delivery complicated by postpartum hemorrhage.

Figure 3. Contrast enhanced coronal and sagittal T1W images of lymphocytic hypophysitis spontaneous evolution from the presentation (panel A, B), after 4 (panel C) and 10 years (panel D) of follow-up resulting in secondary empty sella.

Figure 4. Coronal T1W images demonstrating a pituitary tumor (macroprolactinoma) shrinkage in a patient treated with the dopamine agonist cabergoline for 10 years. The patient presented with hyperprolactinemia causing galactorrhea-amenorrhea, secondary adrenal insufficiency and central hypothyroidism. Hyperprolactinemia and adrenal insufficiency completely recovered during follow up.

Figure 5. Sagittal T1W showing small pituitary gland found at the bottom of sella, thin stalk and ectopic posterior lobe in a young adult with isolated childhood-onset growth hormone deficiency (congenital).

Figure 6. Sagittal T1W images of pituitary stalk pressed against the dorsum sellae causing mild hyperprolactinemia in a patient with primary empty sella.

PRIMARY EMPTY SELLA (PES) AND EMPTY SELLA SYNDROME

Epidemiologically, this is associated with female sex (female to male ratio 5:1), multiple pregnancies, obesity, arterial hypertension, and middle age. It may present with headaches, endocrine dysfunction, and visual disturbances due to pressure on the neighboring structures.

A typical clinical picture consists of headache and obesity. Women in the reproductive age may be affected by menstrual irregularities, galactorrhea, and sterility. Man can develop gynecomastia and sexual disturbances. Primary empty sella due to the syndrome of increased intracranial pressure can also be associated with symptoms of intracranial hypertension.

Pathogenesis of Primary Empty Sella (PES)

Pregnancy may trigger the onset of PES. It is associated with pituitary hyperplasia and CSF hypertension especially in multiple pregnancies. PES has also been associated with CSF hypertension related to obesity and arterial hypertension. In the largest study with 175 patients, multiple pregnancies were reported in 58.3% women with PES, while obesity and arterial hypertension were recorded in 49.5% and 27.3% of patients (10). In patients with benign intracranial hypertension, empty sella is a common finding (8, 11).

Endocrine Dysfunction in PES

Hyperprolactinemia, usually mild (less than 50 ng/ml), is present in approximately 10% of patients (10). It is often due to increased pressure of the CSF on the pituitary stalk and diminished dopamine inhibitory effect. Prolactin dynamics in PES may be influenced by gonadal status, intracranial pressure, neurotransmitters, and stalk integrity. Rarely, pituitary microadenomas causing acromegaly and Cushing’s disease may be associated with empty sella. In the recent study by Himes et al. empty sella was associated with an increased rate of MRI negative Cushing’s disease (12). Pituitary compression causing a relative reduction in the volume of the pituitary for imaging is a plausible cause for not detecting the tumor mass with MRI (12). The presence of ES was associated with lower preoperative prolactin and nadir GH responses to OGTT in patients with acromegaly (13) Functional imaging, if available may identify occult pituitary tumors as therapeutic targets in such settings.

The prevalence of hypopituitarism in patients with PES is variable. In a study by Guitelman et al. it was found in 28% of patients (11). Panhypopituitarism was present in 40% of these patients, while partial or isolated hormone deficiencies were diagnosed in 60% of hypopituitary patients (11). The most prevalent pituitary deficiency was growth hormone deficiency.

In a pooled meta-analysis, which included four studies, the frequency of hypopituitarism was 52% (14). Multiple pituitary hormone deficiencies were present in 30%, isolated in 21% of patients with PES. Growth hormone and gonadotropins were the most common isolated insufficiencies (14). Significant correlation was reported between IGF-1 values and pituitary volume measurements (15).

In a retrospective single-center study of 765 patients by Ekhzaimy et al. 79% of PES was diagnosed incidentally on MRI. The majority of patients were evaluated by general practitioners with suboptimal hormonal evaluation while only 20 % were referred to endocrinologists for hormonal evaluation (16).

A multicenter retrospective study from Italy, detected hormonal alterations in 29% of incidental PES suggesting the need for careful initial evaluation since on follow-up hormonal deterioration was uncommon (3%) (17). Hypopituitarism was associated with male sex (17, 18) and radiological finding of complete PES (18).

Hormonal assessment is advocated in all patients with ES. In case of borderline results or suspected isolated or partial insufficiency stimulatory tests are recommended if clinically relevant for hormone replacement.

Other symptoms in patients with ES include headache and visual and neurological disturbances. In patients with intracranial hypertension these symptoms are more common.

Neurological and Ophthalmic Dysfunction in PES

Headache is present in approximately 80% (3,6). In 20% of patients it may be accompanied by visual disturbances, even papilledema in intracranial hypertension (3,6).

Visual disturbances including worsening of visual acuity, blurred vision, diplopia, defects of oculomotor nerve, and optical neuritis were also reported in patients with PES. In case of benign intracranial hypertension ophthalmic echography and computerized visual field with evoked potentials should be performed in consultation with ophthalmologists. Study by Yilmaz et al. evaluated retinal optic nerve fiber thickness by optic coherence tomography in asymptomatic patients with empty sella and healthy controls (19). Reduced values in asymptomatic patients provide valuable data for monitoring of these patients (19). In case of chiasmal herniation into the empty sella with acute visual deterioration, new neurosurgical techniques of chiasmal transsphenoidal elevation are available (20).

Neurological disturbances including dizziness, syncope, cranial nerve disorders, convulsions, and depression were reported in approximately 40% of patients with PES (3, 6). Rhinorrhea increases the risk of meningitis and surgery involving osseous remodeling techniques (21).

An association of PES with periventricular white matter hyperintensities, and enlarged perivascular spaces, common features of cerebral small vessel disease, were reported confirming the previous clinical observation and possible common underlying mechanism (22).

TREATMENT

In patients with increased idiopathic intracranial pressure osmotic diuretics or acetazolamide (Diamox) are advocated. Weight loss may be helpful in obese and overweight patients especially if accompanied by sleep apnea. Neurosurgical techniques may be indicated for rhinorrhea and some symptomatic secondary causes of empty sella syndrome with increased intracranial pressure and acute visual disturbances. If hypopituitarism is present it should be treated following the current recommendations (23) and hyperprolactinemia treated with dopamine agonists.

CONCLUSION

Primary empty sella can be heterogeneous in origin and presentations range from an asymptomatic incidental radiological finding to endocrine and neuro-ophthalmological manifestations. Female sex, multiple pregnancies, obesity, and arterial hypertension are associated risk factors as well as the syndrome of benign intracranial hypertension. Secondary empty sella is caused by various pathological processes resulting in shrinkage of the pituitary gland. Routine hormonal status assessment and regular follow-up are indicated in all patients since the prevalence of pituitary dysfunction is significant.

REFERENCES

1. Busch W. Die Morphologie der Sella turcica und ihre Beziehungen zur Hypophyse. Virchows Archiv für Pathologische Anatomie und Physiologie und für klinische Medizin 1951; 320: 437–458.
2. Kaufman B. The ‘empty’ sella turcica-a manifestation of the intrasellar subarachnoid space. Radiology 1968; 90: 931–941.
3. De Marinis L, Bonadonna S, Bianchi A, Giulio M, Gustina A. Extensive clinical experience: primary empty sella. Journal of Clinical Endocrinology and Metabolism 2005:5471-5477.
4. Chiloiro S, Giampietro A. Bianchi A, Tartaglione T, Capobianco A, Anile C, De Marinis L. Primary empty sella: a comprehensive review. European Journal of Endocrinology 2017; 177:6; R275-R285.
5. Friedman DI, Jacobson DM. Diagnostic criteria for idiopathic intracranial hypertension. Neurology2002; 59: 1492–1495.
6. Maira G, Anile C, Mangiola A. Primary empty sella syndrome in a series of 142 patients. Journal of Neurosurery 2005; 103: 831–836.
7. Besci Ö ,Yaşar E, Mert Erbaş I, Yüksek Acınıklı K, Korcan Demir K, Böber E, Abacı A. Clinical course of primary empty sella in children: a single center experience. The Turkish Journal of Pediatrics 2022; 64: 900-908
8. Bhawna S, Naveen S, Vikas S, Ashok P, Goel Clinical and Radiological Profile of 122 Cases of Idiopathic Intracranial Hypertension in a Tertiary Care Centre of India. Neurology India 2022; 70(2):704-709.
9. Hoffmann J, Schmidt C, Kunte H, Klingebiel R, Harms L, Huppertz HJ, Lüdemann L & Wiener E. Volumetric assessment of optic nerve sheath and hypophysis in idiopathic intracranial hypertension. American Journal of Neuroradiology 2014;35: 513–518.
10. Degnan AJ, Levy LM. Pseudotumor cerebri: brief review of clinical syndrome and imaging findings. American Journal of Neuroradiology 2012; doi 10.3174/ajnrA2.404
11. Guitelman M, Basalvibaso NG, Vitale M, Chervin A, Katz D, Miragaya K, Herrera J, Cornalo D, Servido M, Boero L, Manavela M, Danilowicz K, Alfieri A, Stalldecker G, Glerean M, Fainstein Day P, Ballarino C, Mallea Gil MS, Rogozinski A. Primary empty sella (PES): a review of 175 cases. Pituitary 2013, 16:270-274.
12. Himes BT, Bhargav AG, Brown DA, Kaufmann TJ, Bancos I, Van Gompel JJ. Does pituitary compression/empty sella syndrome contribute to MRI-negative Cushing's disease? A single-institution experience. Neurosurg Focus. 2020 Jun;48(6):E3. doi: 10.3171/2020.3.FOCUS2084. PMID: 32480375
13. Urhan E, Hacioglu A, Okcesiz I, Karaca Z,  Kara CS, Unluhizarci The coexistence of newly diagnosed acromegaly with primary empty sella: More frequent than expected? Growth Horm IGF Res 2023 Feb:68:101521.doi: 10.1016/j.ghir.2022.101521. Epub 2022 Nov 11.
14. Auer MK, MR Stieg, Crispin A, Sievers C, Stalla GK, Kopczak A. Primary empty sella syndrome and the prevalence of hormonal dysregulation. Deutsches Artzteblatt International 2018; 115: 99-105.
15. Akkus G, Sözütok S , Odabaş F, Onan B, Evran M, Karagun B, Sert M, Tetiker T. Volume in Patients with Primary Empty Sella and Clinical Relevance to Pituitary Hormone Secretion Current Medical Imaging, 2021, 17, 1018-1024.
16. Ekhzaimy AA, Mujammami M, Tharkar S, Alansary MA, Al Otaibi D. Clinical presentation, evaluation and case management of primary empty sella syndrome: a retrospective analysis of 10-year single-center patient data. BMC Endocr Disord. 2020 Sep 17;20(1):142. doi: 10.1186/s12902-020-00621-5. PMID: 32943019; PMCID: PMC7495892.
17. Carosi G, Brunetti A, Mangone A, Baldelli R , Tresoldi A, Del Sindaco G, Lavezzi E , Sala E, Mungari R , Fatti LM, Galazzi E, Ferrante E, Indirli R, Biamonte E, Arosio M, Cozzi R, Lania A, Mazziotti G, Mantovani G. A Multicenter Cohort Study in Patients With Primary Empty Sella: Hormonal and Neuroradiological Features Over a Long Follow-Up. Frontiers in Endocrinology doi: 10.3389/fendo.2022.925378
18. Lu M, Ye J, Gao F^. Analysis of clinical features of primary empty sella. Ann Endocrinol (Paris) 2023 Apr;84(2):249-253.
19. Yilmaz A, Gok M, Altas H, Yildirim T, Kaygisiz S, Isik HS. Retinal nerve fibre and ganglion cell inner plexiform layer analysis by optical coherence tomography in asymptomatic empty sella patients. Int J Neurosci. 2020 Jan;130(1):45-51. doi: 10.1080/00207454.2019.1660328. Epub 2019 Sep 13. PMID:31462116.
20. Barzaghi LR, Donofrio CA, Panni P, Losa M, Mortini P. Treatment of empty sella associated with visual impairment: a systematic review of chiasmapexy techniques. Pituitary. 2018 Feb;21(1):98-106. doi: 10.1007/s11102-017-0842-6. Review
21. Guinto G, Nettel B, Hernández E, Gallardo D, Aréchiga N, Mercado M. Osseous Remodeling Technique of the Sella Turcica: A New Surgical Option for Primary Empty Sella Syndrome. World Neurosurg. 2019 Jun;126:e953-e958. doi:10.1016/j.wneu.2019.02.195. Epub 2019 Mar 12. PMID: 30877013.
22. Chen T, Li G, Wu D, Xie B , Feng Y, Xiao S, Li J, Liu Y, Yang J, Li X. Primary empty sella: The risk factors and associations with the cerebral small vessel diseases: An observational study. Clinical Neurology and Neurosurgery. Volume 203, April 2021, 106586
23. Flesseriu M, Hashim IA, Karavitaki N, Melmed S, Murad MH, Salvatori R, Samuels MH: Hormonal replacement in hypoituitarism in adults: an endocrine society clinical practice guideline. Journal of Clinical Endocrinology and metabolism 2016; 101:3888-3921.

ABSTRACT

Infections of the hypothalamic-pituitary region are rare lesions, accounting for less than 1% of all pituitary lesions. The clinical diagnosis of these infections can be difficult due to the nonspecific nature of the disease (in many patients without symptoms of infection) and may be misdiagnosed as other pituitary lesions. The risk factors for infections of the hypothalamic-pituitary region are meningitis, paranasal sinusitis, head surgery, and immunocompromised host (diabetes mellitus, Cushing’s syndrome, HIV infections, solid organ transplantation, malignancy). Infections can develop in a normal pituitary gland or in pre-existing pituitary lesions (adenoma, Rathke´s cleft cyst, craniopharyngioma). There are several modes of dissemination of the infection to the hypothalamic-pituitary region: hematogenous, iatrogenic (after neurosurgical procedures), and spread from paranasal or nasal cavity (through venous channels of the sphenoid bone). Hypothalamic-pituitary infections most commonly present with visual disturbances and headache, in some cases with fever and leukocytosis. A significant proportion of patients develop hypothalamic-pituitary dysfunction during the acute phase of the disease or months and years after successful antimicrobial therapy. Diagnosis can be challenging and the hypothalamic-pituitary infection with formation of abscess or granuloma may be misdiagnosed as a pituitary tumor. Transsphenoidal drainage followed by antibiotics, antimycotics, or anti-tuberculous drugs are usually efficient in successful treatment of these patients.

INTRODUCTION

Infections of the hypothalamic-pituitary region are rare and commonly described in case reports or small case series. These infections include bacterial infections (pituitary abscess), tuberculosis, fungal, viral, and parasitic infections (Table 1). An infection in the hypothalamic-pituitary region may present as a sella/suprasellar mass and may be misinterpreted as a pituitary tumor. Also, these infections may cause hypopituitarism and be misdiagnosed as post-encephalitic syndrome (1-4).

Infections of the hypothalamic-pituitary region may be primary (without an identifiable source) or secondary in origin (3, 5). The more common is a primary pituitary infection, which occurs in previously healthy normal pituitary glands. Secondary pituitary infections occur in patients with a pre-existing lesion in the pituitary region (pituitary adenoma, Rathke´s cleft cyst, craniopharyngioma, or prior pituitary surgery).

There are several sources of infections in the hypothalamic-pituitary region (Table 2). Dissemination from the sphenoid sinus to the pituitary is possible by direct contact and through shared venous drainage.

Infections in the hypothalamic-pituitary region are rare and several predisposing factors have been identified (2) (Table 3).

Infections of the hypothalamic-pituitary region may present with neurological signs and symptoms and signs of neuroendocrine dysfunction (Table 4).

HYPOTHALAMIC-PITUITARY BACTERIAL INFECTIONS

Pituitary Abscess

Pituitary abscesses are rare pituitary lesions accounting for less than 1% of all pituitary lesions (6, 7). The first case of a pituitary abscess was described in 1848 and since then it has been mostly described in case reports or small series. In two-thirds of patients, pituitary abscesses occur in previously healthy normal glands (primary pituitary abscess) (8). In other patients, there is a preexisting lesion in the pituitary region, such as a pituitary adenoma, Rathke´s cleft cyst, granulomatous hypophysitis, or craniopharyngioma or prior pituitary surgery (secondary pituitary abscess) (5, 9-12). The infection can be caused by hematogenous dissemination or by direct extension from surrounding structures (meningitis, sphenoid sinusitis, cavernous sinus thrombophlebitis) (Table 2). Pituitary surgery and immunocompromised condition are also risk factors for pituitary abscesses (Table 3).

According to the clinical presentation and duration of the disease, pituitary abscesses can be acute, subacute (the disease course less than 1 month), or chronic (disease course longer than 1 month) (13). Infective manifestations (fever, leukocytosis, meningism) have been reported in patients with acute and subacute pituitary abscesses, while chronic pituitary abscesses have a more indolent course.

Many patients with pituitary abscess were misdiagnosed as having a pituitary adenoma, pituitary adenoma with apoplexy, or Rathke´s cleft cyst prior to surgery (14). The diagnosis of this potentially life-threatening disease is based on intraoperative detection of pus and postoperative histopathological analysis. Clinically, pituitary abscesses usually present with neurological signs and symptoms (headache, visual disturbances), signs of neuroendocrine dysfunction (anterior hypopituitarism, AVP deficiency) and signs and symptoms related to infections (fever, leukocytosis) (5, 8, 14). The largest series of primary pituitary abscesses (84 patients) during a 20-year period reported asthenia as the most common clinical presentation (75%), followed with visual impairment (71%), and headache (50%) (6).

Compared to patients with a pituitary adenoma who rarely have neuroendocrine dysfunction, most patients with pituitary abscesses have hypopituitarism (8, 9, 13). The large case series of 66 pituitary abscesses reported anterior pituitary hypopituitarism in 81.8% of patients, while AVP deficiency was diagnosed in 47.9% of patients (8). Nine percent of patients (9.3%) had isolated hypogonadism, 3.7% had isolated ACTH deficiency, 1.8% had isolated hypothyroidism, and 1.8% had combined hypogonadism and ACTH deficiency (8). The possible source of the pituitary infection was found in 14 out of 66 patients (sepsis, sinusitis, pulmonary tuberculosis) (8). Recently published the largest series of 84 patients with primary pituitary abscesses confirmed the high incidence (73%) of preoperative neuroendocrine dysfunction in patients with pituitary abscess: panhypopituitarism in 46%, isolated corticotropic insufficiency in 13%, isolated thyrotropic insufficiency in 10%, isolated gonadotropic insufficiency in 8%, and combined two pituitary axes insufficiencies in 22% (6).

On MRI, pituitary abscesses present as a sella masses, hypointense or isointense on T1-weighted imaging, hyperintense or isointense on T2-weighed imaging, with typical rim enhancement after gadolinium injection, mimicking apoplexy of the pituitary adenoma or other cystic sella lesions (7-9, 13, 15-17; Fig. 1). Diffusion-weighted imaging (DWI) sequences of pituitary abscesses often demonstrate high signal intensity with a reduction in the apparent diffusion coefficient, different from necrotic brain tumors (18).

Figure. 1. Pituitary abscess: gadolinium-enhanced T1-weighted MRI scan (sagittal view) shows sellar and suprasellar mass with peripheral contrast enhancement.

Neuroimaging with nuclear medicine investigations (18-FDG PET scan, labelled leukocyte scintigraphy) could increase the preoperative diagnostic rate in challenging patients with pituitary abscess (6).

The majority of patients are treated with transsphenoidal surgery, rarely with a transcranial approach (8). Intraoperatively, pus is found in the sella (19, 20). In patients with secondary pituitary abscesses, the sphenoid sinus is the most common site of extrasellar invasion (5).

On histopathological analysis, there is evidence of acute or chronic inflammation, while Gram staining and bacterial cultures in some cases may identify the infecting pathogen. In most cases, the etiological agents cannot be isolated (6, 12). In the two largest studies on pituitary abscesses, positive results on gram staining or bacterial cultures were found in only 19.7% and 25% of patients, respectively (6, 8). The most prevalent organisms are Gram-positive cocci (Staphylococcus Aureus and Streptococcus species), but also Gram-negative bacteria (Neisseria, Esherichia coli, Pseudomonas, Brucella) (5, 6, 9, 21).

Patients with bacterial pituitary abscesses are treated with intravenous and oral antibiotics for three to six weeks to prevent the recurrence of the pituitary abscesses, but in some cases, reoperation was required. In rare cases, AVP deficiency and hypopituitarism are reversible, occasionally followed by secondary empty sella (22). In most cases, neuroendocrine dysfunction and AVP deficiency are irreversible findings (6, 13). The preoperative diagnosis of pituitary abscess represented a protective factor for pituitary function recovery (in 23% of patients) (6).

Although pituitary abscesses are more indolent than other intracranial abscesses, secondary pituitary abscesses in patients with pituitary adenomas (not in Rathke´s cleft cyst) is associated with high mortality rate (26%) due to the dissemination of the infection or meningitis (5). In patients with infected Rathke´s cleft cyst, clinical manifestations are commonly subacute, without septic symptoms (9, 11).

Recently published the largest systematic review of 488 cases of pituitary abscess examined presentation, radiological findings, endocrinological abnormalities, treatment, and mortality of these patients (23). The most common symptoms were headache (76%) and visual field loss. The median time from onset of symptoms to presentation was 120 days. Symptoms and biochemical markers of infection were absent in 57% of cases. The appearance of hypointensity on T1 weighted images and hyperintensity on T2 weighted images, as well as peripheral contrast enhancement of the pituitary on MRI were the most common radiological findings. Fifty-five percent of patients had negative culture results. Endocrinological abnormalities were present in 84.5% of cases (panhypopituitarism in 41%, AVP deficiency in 25%) and persisted in over half of cases. The mortality rate was 5.1%, with delayed presentation increasing risk of mortality (23).

Hypopituitarism Caused by Treponema Pallidum Infection (Syphilis)

Syphilis is a sexually transmitted chronic bacterial infection caused by Treponema Pallidum which progresses over years through a series of clinical stages. Syphilis is a well-recognized cause of hypopituitarism, with granulomatous hypophysitis (noncaseating giant cell granuloma), syphilitic gumma in sella region, or congenital syphilis causing hypothalamic-pituitary dysfunction. The first cases of hypopituitarism caused by syphilis were described almost 70 years ago, mostly in postmortem cases (24). The use of penicillin caused a decline in syphilis presentations and late complications, including congenital syphilis. Nowadays, the incidence of syphilis has been rising again and this sexually transmitted disease should be considered again in the differential diagnosis of neurological, psychiatric, and endocrine cases in high-income countries in risk groups (HIV positive patients, men-men relationships, crack cocaine users, and among intravenous drug users) (25, 26). Spirochete Treponema Pallidum is also described as a cause of hypophysitis and pituitary gland enlargement with hypopituitarism, mostly in immunocompromised patients (HIV-infected patients) with syphilitic meningitis (27). Syphilis may cause a sella mass with suprasellar extension mimicking a pituitary tumor and causing severe headache and hypopituitarism (28). The diagnosis is confirmed by positive treponemal antibody or by detection of Treponema Pallidum by immunohistochemistry or PCR on the resected pituitary. This disorder is treated with antibiotics.

Hypopituitarism Caused by Leptospira Interrogans Infection (Sy Weil)

Leptospirosis is a common tropical febrile, zoonotic infectious disease caused by spirochete Leptospira Interrogans. The bacteria are spread through the urine of infected animals (cattle, pigs, horses, dogs, rodents, wild animals). Humans can become infected through contact with urine or other body fluids from infected animals or through contact with water, soil, or food contaminated with the urine of infected animals. This disease presents with hepatorenal syndrome and systemic hemorrhagic manifestations. The first case of pituitary apoplexy and panhypopituitarism caused by leptospirosis in a 56-year-old male with type 2 diabetes mellitus was recently published (29). The patient developed fever, nausea, vomiting and acute kidney injury. Leptospirosis was diagnosed by positive leptospira antibody test, and he started treatment with antibiotics. After five days of admission, he developed signs and symptoms of pituitary apoplexy. A brain MRI scan was consistent with apoplexy in a pituitary adenoma (the mass showed T2W hyper intensity and TIW isointensity with hypo intense areas which suggested hemorrhage). The patient developed hypopituitarism and was replaced with glucocorticoids and thyroid hormones. The follow-up MRI scan showed resolution of the hemorrhagic focus and regression of the pituitary adenoma. The proposed mechanism of pituitary apoplexy is platelet dysfunction (caused by uremia and directly by leptospira) and non-inflammatory vasculopathy (increased vascular permeability due to disruption of endothelial cell-cell junctions, cell retraction and opening of intercellular gaps) (29). Leptospirosis clinically presents very similar to another zoonosis, hantavirus infection, is more common worldwide and serology and PCR are necessary to distinguish between these two diseases (30).

Hypothalamic-Pituitary Dysfunction Following Bacterial CNS Infections

Hypothalamic-pituitary dysfunction is a well-recognized complication of acute infectious diseases of the central nervous system (meningitis and encephalitis) and may occur in the acute phase or in the late stage of these diseases (1, 4, 31, 32). The clinical spectrum of neuroendocrine dysfunction may range from an isolated pituitary hormone deficiency to panhypopituitarism. Endocrine dysfunction in the acute phase of meningitis may return to normal after the acute period or be irreversible (32). The most common deficit is isolated GH deficiency diagnosed 6-48 months after the infection, reported at a rate of 28.6% (31).

Hypopituitarism following acute viral or bacterial meningitis in children is not as common as in adulthood (33, 34). The GH neurosecretory dysfunction (low IGF1 with normal GH response in clonidine test) was found in 3 out of 37 children tested at least 6 months following the diagnosis of bacterial meningitis (34). There are rare case reports on hypopituitarism during acute meningitis caused by Streptococcus Group B or sepsis caused by Salmonella enteritidis in a neonatal period (35, 36).

The pathophysiological mechanism responsible for hypothalamic-pituitary dysfunction following acute meningitis is not fully understood. In some patients, anti-pituitary and anti-hypothalamus antibodies are detected (37). It is proposed that acute infection provokes an autoimmune process and may cause axonal injury with consequent neuroendocrine dysfunction (38).

Idiopathic Granulomatous Inflammation of the Cavernous Sinus - the Tolosa-Hunt Syndrome      

Tolosa-Hunt syndrome is defined as an idiopathic granulomatous inflammation of the cavernous sinus, therefore not infectious in origin, with variable extension into the superior orbital fissure/orbital apex, usually unilateral. The diagnosis is made by exclusion of other more common causes of cavernous sinus lesions (thrombosis, tumors, fungal infections, systemic granulomatous diseases-sarcoidosis, tuberculosis, Wegener´s granulomatosis) (39). In less than 5% of cases it can be bilateral, mimicking a pituitary adenoma in imaging studies (40). The etiology of Tolosa-Hunt syndrome is not fully understood. The disease is characterized by nonspecific granulomatous inflammation with infiltration of lymphocytes and plasmocytes. The patient presents with severe unilateral orbital pain and ipsilateral ocular motor neuropathy. The paralysis of one or more cranial nerves passing through the cavernous sinus (III, IV, VI) develops with orbital pain after less than 2 weeks. The signs of infection (fever, leukocytosis) are usually present. Granulomatous inflammation develops within the cavernous sinus causing acute throbbing orbital pain and disordered eye movement. Brain MRI scan demonstrate the inflammation in the cavernous sinus, orbital apex, and rarely in the sella. MR venography of the brain is important to exclude cavernous sinus thrombosis. Treatment consists of high dose corticosteroids and antibiotics. Refractory and steroid-intolerant cases may be treated with immunosuppressants (Methotrexate or Azathioprine) and gamma knife radiotherapy (41). Periorbital pain intensity is rapidly decreasing and resolving within 72h, while the resolution of ophthalmoplegia improves gradually and takes a longer time to resolve (several weeks) (42). If the patient is not responding to standard therapy, biopsy of the lesion is necessary to exclude other diseases, such as lymphoma.

Hypothalamic-Pituitary Tuberculosis

The incidence of tuberculosis is rising not only in developing countries, but also in developed countries, especially given the increasing population migration. Mortality has begun to increase after years of decline and since 2018 more than 7 million people have died of tuberculosis (43). Extrapulmonary tuberculosis may affect the brain causing tuberculous meningitis and tuberculoma of the central nervous cases. Tuberculous meningitis has a tendency to affect basal parts of the brain from where it can spread to the sella region. In rare cases, CNS tuberculosis may present as tuberculous hypophysitis or sella/suprasellar tuberculoma mimicking a pituitary adenoma or pituitary apoplexy (44-48). It may occur in the absence of systemic tuberculosis, but the majority of patients have a past history of pulmonary tuberculosis or tuberculosis of other organs (spine). Tuberculosis may affect the hypothalamus, pituitary, paranasal sinuses (sphenoid sinus), or tuberculoma may be located only in the pituitary stalk. Hypothalamic-pituitary dysfunction and AVP deficiency during the acute phase or years after recovery from acute tuberculous meningitis suggests a more destructive and more extensive hypothalamic and pituitary damage compared to other causes of acute viral and bacterial meningitis.

A significant proportion of patients with sella tuberculoma or tuberculous meningitis develop hypothalamic-pituitary dysfunction. In 18 cases of histologically proved sella tuberculoma (5 of them with past history of tuberculosis), 5 patients had hypopituitarism and 3 had hyperprolactinemia due to pituitary stalk compression (44).

In patients with tuberculous meningitis half of them developed neuroendocrine dysfunction: hyperprolactinemia (23-50% of patients), hypocortisolism (13-43%), hypothyroidism (17-31%), hypogonadism (34%), SIADH (10%) (4, 49, 50) during the acute phase. Multiple hormonal axes were affected in 23.5% of patients (49, 50). In young adult patients who survived tuberculous meningitis in childhood and were tested several years after recovery, hypopituitarism was diagnosed in 20% of patients (51). This is a consequence of fibrosis, gliosis, and calcification in the hypothalamus and pituitary after recovery of active tuberculous brain infection. In rare cases, pituitary function recovered after successful treatment with anti-tuberculous drugs (52).

The diagnosis of sella tuberculoma is a challenge, especially in cases without systemic tuberculosis. The signs and symptoms are nonspecific: fever, neurological abnormalities (headache, visual disturbances), and neuroendocrine dysfunction (44, 53). Sellar tuberculoma on MRI scans presents as thickening of the pituitary stalk or abnormal enhancement pattern of the sella lesion (54) (Fig. 2).

Figure 2. Tubercular hypophysitis: sellar MRI scan (coronal view) shows stalk thickening.

If the correct diagnosis is established, anti-tuberculosis drugs are effective and surgery is not indicated for tuberculous hypophysitis. With surgery the histological examination shows granulomas with central caseous necrosis surrounded by giant Langhans cells. In cases in whom Ziehl-Nielsen staining for acid fast bacilli and the culture on Lowenstein-Jensen media are negative, PCR for detection of mycobacterial DNA in tissue or CSF may help.

HYPOTHALAMIC-PITUITARY FUNGAL INFECTIONS

Hypothalamic-pituitary fungal infections are extremely rare and occur usually in immunocompromised patients (diabetes mellitus, granulocytopenia, solid organ transplantation). There are only few case reports of Candida and Aspergillus sella abscesses and reviews of fungal sellar abscesses (55-61). Aspergillus is a ubiquitous saprophitic fungus found in the nasal mucosa of healthy people and patients with chronic sinusitis. This fungus may cause CNS infection, such as meningitis, encephalitis, brain abscess, subdural abscess, pituitary abscess, and mycotic arteritis with thrombosis and aneurysm (61-63). Fungal sella abscesses have a nonspecific presentation, with neurological signs and symptoms (headache, visual disturbances) and hypothalamic-pituitary dysfunction. Sella MRI images are nonspecific, with a T1W hypointense or isointense mass with rim enhancement and may be misdiagnosed as a pituitary adenoma (Fig. 3). It is proposed that a low signal on T2W images due to iron deposition may be a more specific sign of fungal abscess (58, 64; Fig. 4). Diagnosis of fungal pituitary abscess is made by histopathological finding (Grocott–Gömöri's methenamine silver stain demonstrates septate fungal hyphae), cultivation, or PCR identification of fungus DNA. A combination of transsphenoidal drainage and antifungal therapy (liposomal Amphotericin B, itraconazole, voriconazole, caspofungin, micafungin) can result in a good prognosis (55, 59, 60, 65). Endocrinopathies caused by fungal abscess have a low rate of recovery (59).

Figure 3. Fungal pituitary abscess spreading from fungal sinusitis: sellar MRI scan (coronal and sagittal views) shows a large sellar mass pushing the pituitary upwards.

Figure 4. Fungal infections in the sella: a) CT scan of the sinuses (axial view) shows a large sellar mass, erosion in sellar floor, propagation of the pathological process and opacification of the sinuses, and B) sellar MRI scan (coronal view) shows a giant hypointense lesion in the sellar region.

An unusual form of allergic fungal sinusitis which expands from the sphenoid sinus through a bone erosion to the sella in immunocompetent patient has been described (64) (Fig. 4). This patient had hyperprolactinemia (in the setting with no pituitary stalk compression), which resolved after successful transsphenoidal operation followed by anti-mycotics and corticosteroids (64). It is speculated that fungal glucans may directly stimulate glucan-specific receptors on somatomammotroph cells to stimulate prolactin secretion (66).

HYPOTHALAMIC-PITUITARY VIRAL INFECTIONS

Hypothalamic-pituitary dysfunction (hypopituitarism and cranial diabetes insipidus) may develop in the acute phase of viral infections of the CNS (meningitis and encephalitis) or in the late stage of these diseases (1, 31, 32). Infectious agents which cause CNS viral infections are listed in Table 5.

The investigation of hypothalamic-pituitary function at least 6 months after recovery from mild-to-moderate meningitis/encephalitis showed that 21% patients developed isolated corticotroph deficiency, while other neuroendocrine abnormalities or AVP deficiency were not found (1).

Hantavirus

Hemorrhagic fever with renal syndrome (HFRS), caused by Hantaviruses in the Bunyaviridae family, is an endemic zoonotic disease transmitted by rodents. There are several serotypes of these RNA viruses causing systemic infection, milder form called nephropathia endemica (Puumala) or severe form (Dobrava, Belgrade). The disease is endemic in Europe (Balkans, Finland, Germany) and Asia (Korea), where several outbreaks have been recorded. Farmers and solders are exposed to the virus by inhalation of infected rodent urine, feces, or saliva. Hantavirus infiltrates the vascular system causing increased capillary permeability, renal failure, thrombocytopenia, hemorrhage, fever, hypotension, and shock. The mortality rate is 6.6%. Autopsy findings reported a slightly enlarged pituitary with ischemia/infarction, hemorrhage, and necrosis (67-69). Direct viral invasion was confirmed in the pituitary causing viral hypophysitis (69). Hypothalamic-pituitary dysfunction has been reported during the acute phase of the disease or after long-term follow-up (70-79). A milder form of HFRS infection caused by Puumala virus (nephropathia epidemica) is associated with a lower incidence of hypopituitarism (80). It is speculated that in some patients with no signs of hemorrhage in the sella, hantavirus may cause autoimmune hypophysitis and hypopituitarism (81). Sellar MRI imaging in hypopituitary patients reveals an edematous pituitary gland or increased signal intensity in the pituitary due to hemorrhage during the acute phase, while pituitary atrophy and secondary empty sella develops months and years after acute infection (76, 79-81) (Fig 5). A retrospective study of 60 adults who had recovered from HFRS reported that 18% of patients developed hypopituitarism (82). Ten percent of patients had a single pituitary deficit (three GH, two gonadal, and one adrenal), and 8.3% had multiple pituitary hormone deficiencies (82). In rare cases, HFRS may cause injury of the pituitary stalk or acute/subacute hemorrhage in the pituitary gland and AVP deficiency with panhypopituitarism (30, 77).

Figure. 5. Hemorrhagic fever with renal syndrome: sellar MRI scan (sagittal view) shows pituitary atrophy and secondary empty sella.

SARS-CoV-2

The novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified as a cause of coronavirus disease 2019 (COVID-19). This virus is responsible for a variety of clinical manifestations, ranging from an asymptomatic stage to severe pulmonary disease (respiratory distress syndrome) and various extrapulmonary manifestations, including endocrine axes (83-85).

SARS-CoV-2 is a neuroinvasive virus which enters the brain through the nasopharyngeal epithelium via the olfactory nerve, passes through the blood-brain barrier, or enters the brain through the median eminence where this barrier is absent (86). Neurons, glial cells, and cerebrovascular endothelia cells, as well as hypothalamus and pituitary cells express an angiotensin-converting enzyme 2 (ACE2) receptor responsible for the entry of the virus into these cells which induce neuroinflammation (86-88). SARS-CoV-2 virus is also detected in cerebrospinal fluid of patients with encephalitis caused by COVID-19 and in pituitary tissues from patients who died from SARS (89, 90). SARS-CoV-2 is responsible for a variety of neurological complications, including headache, anosmia, confusion, ataxia, neuropathic pain, seizures, and delirium (89).

There are also indirect systemic effects of COVID-19 virus: an altered immune response (cytokine storm), infection-induced thrombocytopenia, platelet dysfunction, coagulopathy (hypercoagulable state), endothelitis, and endothelial dysfunction (91). All these direct and indirect systemic effects of COVID-19 virus may be responsible for ischemic and hemorrhagic vascular syndromes, affecting also the hypothalamo-pituitary region. Another possible indirect effect of COVID-19 may be mediated by cytokines that can trigger hypothalamo-pituitary inflammation with consequent neuroendocrine dysfunction (92). Also, it has been shown that SARS-CoV-2 express an amino-acid sequence that mimics human ACTH and induces the production of autoantibodies against ACTH causing cortisol insufficiency and inadequate adrenal response to the stress (83).

SARS-CoV-2 infection may involve the hypothalamo-pituitary axis causing pituitary apoplexy, hypophysitis, and hyponatremia (83, 85, 91, 93-97).

There are nine case reports of pituitary adenoma (6 males/3 females, between 27 and 56 years of age, eight with pituitary macroadenoma) and two series of three patients with pituitary adenomas, complicated by apoplexy during COVID-19 infection (93-102). In one patient this occurred during the third trimester of pregnancy. Four patients had transsphenoidal surgery and recovered, one patient had transcranial resection, three patients were conservatively managed, and one patient died 12 hours after admission. There is also a report of a 65-year-old woman with no underlying pituitary disease who developed acute pituitary apoplexy one month after the initial diagnosis of COVID-19 (103). She developed anterior hypopituitarism with no evidence of AVP deficiency. An MRI pituitary scan showed the resolution of intrapituitary hemorrhage, with normal size of the pituitary gland after six months of follow-up. A small case series reported three patients with previously unknown pituitary adenoma complicated by apoplexy during COVID-19 infection: a 54-year-old female with null cell adenoma, a 52-year-old and a 56-year-old men, both obese with hypertension, both with a lactotroph pituitary adenoma (94). In another series of three patients with pituitary macroadenoma complicated with apoplexy following a symptomatic COVID-19 infection, two of these patients’ developed symptoms of pituitary apoplexy days following the viral infection, whereas the third patient developed pituitary apoplexy after a 2-month period (96).

In some patients COVID-19 caused an acute lymphocytic hypophysitis with temporal evolution of symptoms, suggesting an immune-mediated parainfectious pattern of disease (104, 105). An 18-year-old previously healthy girl with a history of symptomatic COVID-19 three weeks prior to the acute onset of headache and dizziness, presented with diffuse thickening and enlargement of the infundibulum with homogenous contrast enhancement of the pituitary (104). She was treated with glucocorticoids with a significant clinical improvement on day 3 and complete resolution of MRI finding on day 5. A similar case is a  16-year-old girl who presented with headaches, polyuria/polydipsic syndrome, and impaired visual acuity three weeks after COVID-19 infection (105). She had pituitary enlargement on MRI, was treated with methylprednisolone and improved on day 5. Such a rapid headache resolution after steroid treatment suggests a transitory acute hypophysitis, an immune-mediated process, triggered by viral infection. It has been hypothesized that SARS-CoV-2 may induce hypothalamo-pituitary autoimmunity, with positive anti-hypothalamus (AHA) and anti-pituitary (APA) antibodies (106). The current results in the literature about antigens which are targets of AHA and APA are still contradictory (83). It is possible that patients at high-risk for such a complication are carriers of specific HLA alleles. The human leukocyte antigen (HLA) complex has a central role in the recognition and presentation of viral antigens to immune system. HLA class I molecules mediates innate defense strategies against viral infection. HLA-c 04:01, DRB1 08, DQB1 06 carriers have an increased risk of a severe clinical course of COVID-19 and generate a robust response and susceptibility to autoimmune diseases (107-110).

Even these two acute pituitary disorders (pituitary apoplexy with transition to acute hypophysitis) can occur in patients with COVID-19 and preexisting unknown pituitary adenoma (97).

Several cases of AVP deficiency as a late complication of COVID-19 infection were published (111-114). All patients (2 males/2 females, 28-60 years of age) suddenly developed polyuria, nocturia, and polydipsia four to eight weeks after the diagnosis of COVID-19 infection. In one of the published cases, a sellar MRI scan showed an enlarged pituitary with an absent posterior pituitary bright spot on T1W images associated with thickening of the pituitary stalk of 3.5 mm suggesting infundibulo-neurohypophysitis (113). Other pituitary hormone evaluations were normal. They were treated with oral desmopressin.

A case of a young woman with hypothalamic amenorrhea following COVID-19, with normal appearance of the sella turcica and regular dimensions of the pituitary was reported (95). This case suggests clinicians need to follow patients for possible delayed neuroendocrine dysfunctions after COVID-19 infection.

Hyponatremia was reported in 30-60% of patients with SARS-CoV-1 infection and was associated with worse outcomes and increased mortality (115). The syndrome of inappropriate anti-diuretic hormone secretion (SIADH) was the most common reason for hyponatremia, followed by adrenal insufficiency (116).

There are also data on the association between COVID-19 vaccination and the subsequent development of pituitary diseases (85, 117-120). A recently published systematic review analyzed 23 case reports of post COVID-19 vaccination pituitary diseases: hypophysitis in 9 patients, pituitary apoplexy in 6 patients, SIADH in 5 patients, and isolated ACTH deficiency in two patients (119). Symptoms of pituitary disease typically occurred shortly (several days) after vaccine administration and the pathogenetic mechanisms potentially include molecular mimicry, vaccine adjuvants, and vaccine-induced thrombotic thrombocytopenia (83, 119). The presence of ACE2 receptors in the hypothalamo-pituitary system contributes to these post vaccinal pituitary diseases. Isolated infundibulo-neurohypophysitis and AVP deficiency or isolated ACTH deficiency may also develop several days after immunization with BNT 162b2 mRNA COVID-19 vaccine (117, 121). In some cases of panhypopituitarism and AVP deficiency due to hypophysitis after COVID-19 vaccination, hormonal secretion partially improved during follow-up (120).

Other Viruses

Cytomegalovirus, herpes simplex, varicella zoster, and enterovirus have also been described in very rare cases of central diabetes insipidus, mainly in immunocompromised patients with encephalitis (such as HIV infection, Cushing´s syndrome, lymphoma or immunosuppressive therapy) (122-126). Direct cytomegalovirus invasion and reduction in the number of AVP and oxytocin cells was confirmed in the hypothalamus (123).

HYPOTHALAMIC-PITUITARY PARASITIC INFECTIONS

Parasitic infections of the pituitary are rare and infections in the sellar region caused by Toxoplasma gondii, Echinococcus species, and Taenia solium have been reported anecdotally (4).

Toxoplasmosis is a worldwide zoonosis, caused by the protozoan parasite Toxoplasma gondii. This is one of the most common parasitic infections of warm-blooded animals and humans. Approximately one-third of humans have been exposed to T. gondii, mostly with no serious diseases, except in immunocompromised patients (HIV) and congenital toxoplasmosis. Two cases of prolactinomas with T. gondii cysts among tumor cells were reported (127). Toxoplasmosis is the most common CNS infection in immunocompromised patients (patients with HIV infection) and may cause hypopituitarism, accompanied by focal neurological deficits, headache, and fever (128, 129). The brain MRI shows lesions with significant enhancement of T2W images and peri-lesional edema, which may be misdiagnosed as intracranial metastasis. The diagnosis is based on brain biopsy with confirmed presence of T. gondii by PCR. Infection with T. gondii is treated with antimicrobial therapy and with hormone replacement therapy as needed.

The most common parasitic infestation of the brain is neurocysticercosis, caused by Taenia solium. A systematic review of 23 patients with intrasellar cysticercosis reported endocrine abnormalities (panhypopituitarism, hyperprolactinemia, AVP deficiency, and isolated hypothyroidism) in 56% of the affected population (130). These infections present with a cystic mass in the sella with hypopituitarism caused by compression, subarachnoid cysts, obstructive hydrocephalus, or neuroinflammation (ventriculitis, focal arachnoiditis) (4, 131). Neurocysticercosis may involve the pituitary stalk too (132). Transsphenoidal or transcranial operation is required for the definitive histopathological diagnosis and cure, because medical therapy with praziquantel is usually ineffective (130).

ACKNOWLEDGMENT

This study was supported by The Science Fund of the Republic of Serbia, Grant No: 7754282 — Prediction, prevention and patient’s participation in diagnosis of selected fungal infections (FI): an implementation of a novel method for obtaining tissue specimens, “FungalCaseFinder”.

REFERENCES

1. Schaefer S, Boegershausen N, Meyer S, Ivan D, Schepelmann K, Kann PH. Hypothalamic-pituitary insufficiency following infectious diseases of the central nervous system. Eur J Endocrinol 2008; 158: 3-9.
2. Pekic S, Popovic V. Alternative causes of hypopituitarism: traumatic brain injury, cranialirradiation, and infections.Handb Clin Neurol 2014; 124: 271-290
3. Pekic S, Popovic V. Diagnosis of endocrine disease: Expanding the cause of hypopituitarism. Eur J Endocrinol2017; 176: R269-R282
4. Das L, Dutta P. Approach to the patient: a case with an unusual cause of hypopituitarism. J Clin Endocrinol Metab 2023; 108: 1488-1504. 
5. Awad AJ, Rowland NC, Mian M, Hiniker A, Tate M, Aghi MK. Etiology, prognosis, and management of secondary pituitary abscesses forming in underlying pituitary adenomas. J Neurooncol 2014; 117: 469–476
6. Mallereau CH, Todeschi J, Ganau M, Cebula H, Bozzi MT, Romano A, Le Van T, Ollivier I, Zaed I, Spatola G, Nannavecchia B, Mahoudeau P, Djennaoui I, Debry C, Signorelli F, Ligarotti GKI, Pop R, Baloglu S, Fasciglione E, Goichot B, Bund C, Gaudias J, Proust F, Chibbaro S. Pituitary abscess: a challenging preoperative diagnosis-a multicenter study. Medicina (Kaunas) 2023; 59: 565.
7. Naser PV, Papadopoulou P, Teuber J, Kopf S, Jesser J, Unterberg AW, Beynon C. Characteristics of inflammatory and infectious diseases of the pituitary gland in patients undergoing transsphenoidal surgery. Pituitary 2023; 26: 451-460. 
8. GaoL, Guo X, Tian R, Wang Q, Feng M, Bao X, Deng K, Yao Y, Lian W, Wang R, Xing B. Pituitary abscess: clinical manifestations, diagnosis and treatment of 66 cases from a large pituitary center over 23 years. Pituitary 2017; 20: 189-194
9. Aranda F, Garcia R, Guarda FJ, Nilo F, Cruz JP, Callejas C, Balcells ME, Gonzales G, Rojas R, Villanueva P. Rathke´s cleft cyst infections and pituitary absesses: case series and review of the literature. Pituitary 2021; 24: 374-383
10. Warmbier J, Lüdecke DK, Flitsch J, Buchfelder M, Fahlbusch R, Knappe UJ, Kreutzer J, Buslei R, Bergmann M, Heppner F, Glatzel M, Saeger W. Typing of inflammatory lesions of the pituitary. Pituitary 2022; 25: 131-142.
11. Rabiei MM, Ebrahimzadeh K, Davoudi Z, Bidari Zerehpoosh F, Javandoust Gharehbagh F, Sedaghati R, Lotfollahi L, Kalhor F, Alavi Darazam I. A case of pituitary gland abscess associated with granulomatous hypophysitis. BMC Neurol 2023; 23:11.
12. Inoue E, Kesumayadi I, Fujio S, Makino R, Hanada T, Masuda K, Higa N, Kawade S, Niihara Y, Takagi H, Kitazono I, Takahashi Y, Hanaya R. Secondary hypophysitis associated with Rathke's cleft cyst resembling a pituitary abscess. Surg Neurol Int 2024; 15: 69.
13. LiuF, Li G, Yao Y, Yang Y, Ma W, Li Y, Chen G, Wang R. Diagnosis and management of pituitary abscess: experiences from 33 cases. Clin Endocrinol 2011; 74: 79-88
14. Xue Q, Shi X, Fu X, Yin Y, Zhou H, Liu S, Sun Q, Meng J, Bian L, Tan H, He H. Pituitary abscess: a descriptive analysis of a series of 19 patients-a multi-center experience. Eur J Med Res 2024; 29: 262.
15. Vates GE, Berger MS, Wilson CB. Diagnosis and management of pituitary abscess: a review of twenty-four cases. J Neurosurg 2001; 95: 233-241
16. Ciappetta P, Calace A, D'Urso PI, De Candia N. Endoscopic treatment of pituitary abscess: two case reports and literature review. Neurosurg Rev 2008; 31: 237-246
17. Dalan R, Leow MK. Pituitary abscess: our experience with a case and a review of the literature. Pituitary 2008; 11: 299-306
18. Takao H, Doi I, Watanabe T. Diffusion-weighted magnetic resonance imaging in pituitary abscess. J Comp Assist Tomogr 2006; 30: 514–516
19. Zhang X, Sun J, Shen M, Shou X, Qiu H, Qiao N, Zhang N, Li S, Wang Y, Zhao Y. Diagnosis and minimally invasive surgery for the pituitary abscess: A review of twenty nine cases. Clin Neurol Neurosurg 2012; 114: 957-961
20. Al Salman JM, Al Agha RAMB, Helmy M. Pituitary abscess. BMJ Case Rep 2017; 2017
21. De la Peña-Sosa G, Cabello-Hernández AI, Gómez-Ruíz RP, Gómez-Sámano MA, Gómez-Pérez FJ. Pituitary abscess causing panhypopituitarism in a patient with neurobrucellosis: case report. AACE Clin Case Rep 2023; 10: 10-13.  
22. Pepene CE, Ilie I, Mihu D, Stan H, Albu S, Duncea I. Primary pituitary abscess followed by empty sella syndrome in an adolescent girl. Pituitary 2010; 13: 385-389
23. Stringer F, Foong YC, Tan A, Hayman S, Zajac JD, Grossmann M, Zane JNY, Zhu J, Ayyappan S. Pituitary abscess: a case report and systematic review of 488 cases. Orphanet J Rare Dis 2023; 18: 165.
24. Oelbaum M. Hypopituitarism in male subjects due to syphilis with a discussion of androgen treatment. Q J Med 1952; 83: 249–264
25. Noblett J, Roberts E. The importance of not jumping to conclusions: syphilisas an organic cause of neurological, psychiatric and endocrine presentations. BMJ Case Rep 2015; 2015.
26. Hook EW 3rd. Syphilis. Lancet 2017; 389: 1550-1557.
27. Spinner CD, Noe S, Schwerdtfeger C, Todorova A, Gaa J, Schmid RM, Busch DH, Neuenhahn M. Acute hypophysitis and hypopituitarism in early syphilitic meningitis in a HIV-infected patient: a case report. BMC Infect Dis 2013; 13: 481-484
28. Bricaire L, Van Haecke C, Laurent-Roussel S, Jrad G, Bertherat J, Bernier M, Gaillard S, Groussin L, Dupin N. The great imitator in endocrinology: a painful hypophysitis mimicking a pituitarytumor. J Clin Endocrinol Metab 2015; 100: 2837-2840
29. Gohil J, Gowda A, George T, Easwer HV, George A, Nair P. Pituitary apoplexy and panhypopituitarism following acute leptospirosis. Pituitary 2021 May 21; 1-5.
30. Schwab S, Lissmann S, Schafer N, Isaak A, Klingmuller D, Attenberger U, Eis-Hubinger AM, Hofmann J, Strassburg C, Lutz P. When polyuria does not stop: a case report on an unusual complication of hantairus infection. BMC Infectious Diseases 2020; 20: 713
31. Tanriverdi F, Alp E, Demiraslan H, Dokmetas HS, Unluhizarci K, Doganay M, Casanueva FF, Kelestimur F. Investigation of pituitary functions in patients with acute meningitis: a pilot study. J Endocrinol Invest 2008; 31: 489-491
32. Tsiakalos A, Xynos ID, Sipsas NV, Kaltsas G. Pituitary insufficiency after infectious meningitis: a prospective study.J Clin Endocrinol Metab 2010; 957: 3277-3281
33. Giavoli C, Tagliabue C, Profka E, Senatore L, Bergamaschi S, Rodari G, Spada A, Beck-Peccoz P, Esposito S. Evaluation of pituitary function after infectious meningitis in childhood. BMC Endocr Disord2014; 14: 80-84
34. Karadag-Oncel E, Cakir M, Kara A, Gonc N, Cengiz AB, Ozon A, Ciftci E, Alikasifoglu A, Ceyhan M, Kandemir N. Evaluation of hypothalamic-pituitary function in children following acute bacterial meningitis. Pituitary 2015; 18: 1-7
35. Saranac L, Bjelakovic B, Djordjevic D, Novak M, Stankovic T. Hypopituitarism occurring in neonatal sepsis. J Pediatr Endocrinol Metab 2012; 25: 847-848
36. Ferreira AS, Fernandes AL, Guaragna-Filho G. Hypopituitarism as consequence of late neonatal infection by Group B streptococcus: a case report. Pan Afr Med J2015; 20: 308
37. Tanriverdi F, De Bellis A, Teksahin H, Alp E, Bizzarro A, Sinisi AA, Bellastella G, Paglionico VA, Bellastella A, Unluhizarci K, Doganay M, Kelestimur F. Prospective investigation of pituitary functions in patients with acute infectious meningitis: is acute meningitis induced pituitary dysfunction associated with autoimmunity? Pituitary 2012; 5: 579–588
38. Gerber J, Seitz RC, Bunkowski S, Bruck W, Nau R. Evidence for frequent focal and diffuse acute axonal injury in human bacterial meningitis. Clin Neuropathol 2009; 28:33–39
39. BhatkarS, Goyal MK, Takkar A, Mukherjee KK, Singh P, Singh R, Lal V. Cavernous sinus syndrome: A prospective study of 73 cases at a tertiary care centre in Northern India. Clin Neurol Neurosurg 2017; 155: 63-69
40. Swiątkowska-Stodulska R, Stodulski D, Babinska A, Piskunowicz M, Sworczak K. Bilateral Tolosa-Hunt syndrome mimicking pituitary adenoma. Endocrine 2017; 58: 582-586
41. LeeJM, Park JS, Koh EJ. Gamma Knife radiosurgery in steroid-intolerant Tolosa-Hunt syndrome: case report. Acta Neurochir (Wien) 2016; 158: 143-145
42. Arshad A, Nabi S, Panhwar MS, Rahil A.Tolosa-Huntsyndrome: an arcane pathology of cavernous venous sinus. BMJ Case Rep 2015; 2015.
43. Reid M, Agbassi YJP, Arinaminpathy N, Bercasio A, Bhargava A, Bhargava M, Bloom A,Cattamanchi A, Chaisson R, Chin D, Churchyard G, Cox H, Denkinger CM, Ditiu L, Dowdy D, Dybul M, Fauci A, Fedaku E, Gidado M, Harrington M, Hauser J, Heitkamp P, Herbert N, Herna Sari A, Hopewell P, Kendall E, Khan A, Kim A, Koek I, Kondratyuk S, Krishnan N, Ku CC, Lessem E, McConnell EV, Nahid P, Oliver M, Pai M, Raviglione M, Ryckman T, Schäferhoff M, Silva S, Small P, Stallworthy G, Temesgen Z, van Weezenbeek K, Vassall A, Velásquez GE, Venkatesan N, Yamey G, Zimmerman A, Jamison D, Swaminathan S, Goosby E. Scientific advances and the end of tuberculosis: a report from the Lancet Commission on Tuberculosis. Lancet 2023; 402: 1473-1498.
44. Sharma MC, Arora R, Mahapatra AK, Sarat-Chandra P, Gaikwad SB, Sarkar C. Intrasellar tuberculoma--an enigmatic pituitary infection: a series of 18 cases. Clin Neurol Neurosurg 2000; 102: 72-77
45. Arunkumar MJ, Rajshekhar V. Intrasellar tuberculoma presenting as pituitary apoplexy. Neurol India 2001, 49: 407-410
46. Husain N, Husain M, Rao P. Pituitary tuberculosis mimicking idiopathic granulomatous hypophysitis. Pituitary 2008; 11: 313-315
47. Aziz Khan A, Ahuja S, Malik S, Naaz S, Zaheer S. Pituitary tuberculoma with panhypopituitarism masquerading as a pituitary adenoma. Neuropathology 2023; 43: 496-499.
48. Elfarissi MA, Dahamou M, Dehneh Y, Lhamlili M, Khoulali M, Oulali N, Moufid F. Pediatric sellar-suprasellar tuberculosis: A case report and review of the literature. Surg Neurol Int 2023; 14: 379. 
49. Dhanwal DK, Vyas A, Sharma A, Saxena A. Hypothalamic pituitary abnormalities in tubercular meningitis at the time of diagnosis. Pituitary 2010; 13: 304-310
50. More A, Verma R, Garg RK, Malhotra HS, Sharma PK, Uniyal R, Pandey S, Mittal M. A study of neuroendocrine dysfunction in patients of tuberculous meningitis. J Neurol Sci2017; 379: 198-206
51. Lam KS, Sham MM, Tam SC, Ng MM, Ma HT. Hypopituitarism after tuberculous meningitis in childhood. Ann Intern Med 1993; 118: 701-706
52. Tanimoto K, Imbe A, Shishikura K, Imbe H, Hiraiwa T, Miyata T, Ikeda N, Kuroiwa T, Terasaki J, Hanafusa T. Reversible hypopituitarism with pituitarytuberculoma. Intern Med 2015; 54: 1247-1251
53. Jain A, Dhanwal DK, Kumar J. A rare case of pituitarytuberculoma-diagnosed and managed conservatively.Pituitary 2015; 18: 579-580
54. Bonifacio-Delgadillo D, Aburto-Murrieta Y, Salinas-Lara C, Sotelo J, Montes-Mojarro I, Garcia-Marquez A. Clinical presentation and magnetic resonance findings in sellar tuberculomas. Case Rep Med2014; 2014: 961913. 
55. Liu W, Chen H, Cai B, Li G, You C, Li H. Successful treatment of sellar aspergillus abscess. J Clin Neurosci 2010; 17: 1587-1589
56. Vijayvargiya P, Javed I, Moreno J, Mynt MA, Kotapka M, Zaki R, Ortiz J. Pituitaryaspergillosis in a kidney transplant recipient and review of the literature. Transpl Infect Dis 2013; 15(5): E196-200
57. Hong W, Liu Y, Chen M, Lin K, Liao Z, Huang S. Secondary headache due to aspergillus sellar abscesssimulating a pituitaryneoplasm: case report and review of literature. Springerplus 2015; 4: 550
58. Lim JC, Rogers TW, King J, Gaillard F. Successful treatment of pituitarysella Aspergillus abscess in a renal transplant recipient. J Clin Neurosci 2017; 45: 138-140
59. Strickland BA, Pham M, Bakhsheshian J, Carmichael J, Weiss M, Zada G. Endoscopic endonasal transsphenoidal drainage of a spontaneous candidaglabrata pituitary abscess. World Neurosurg 2018; 109: 467-470
60. Saffarian A, Derakhshan N, Taghipour M, Eghbal K, Roshanfarzad M, Dehghanian A. Sphenoid Aspergilloma with headache and acute vision loss. World Neurosurg 2018; 115: 159-161
61. Funakoshi S, Nishiyama M, Komori M, Hyodo M, Kawanishi Y, Ueba T, Fujimoto S, Terada Y. Hypopituitarism due to CNS Aspergillus infection. Intern Med 2024 Mar 11. doi: 10.2169/internalmedicine.3390-23.
62. Iplikcioglu AC, Bek S, Bikmaz K,Ceylan D, Gökduman CA. Aspergillus pituitary abscess. Acta Neurochir 2004; 146: 521-524
63. Hao L, Jing C, Bowen C, Min H, Chao Y. Aspergillus sellar abscess: case report and review of the literature. Neurol India 2008; 56: 186-188
64. Pekic S, Arsenijevic VA, Gazibara MS, Milojevic T, Pendjer I, Stojanovic M, Popovic V. What lurks in the sellar?Lancet 2010; 375: 432
65. Ouyang T, Zhang N, Wang L, Jiao J, Zhao Y, Li Z, Chen J. Primary Aspergillus sellar abscesssimulating pituitary tumor in immunocompetent patient. J Craniofac Surg 2015; 26: e86-88
66. Breuel KF, Kougias P, Rice PJ, Wei D, De Ponti K, Wang J, Laffan JJ, Li C, Kalbfleisch J, Williams DL. Anterior pituitary cells express pattern recognition receptors for fungal glucans: implications for neuroendocrine immune involvement in response to fungal infections. Neuroimmunomodulation 2004; 11: 1–9
67. Klebanov IuA. Analysis of fatal outcome in hemorrhagic fever with renal syndrome. Klin Med 1990; 68: 64-67
68. Valtonen M, Kauppila M, Kotilainen P, Lähdevirta J, Svartbäck CM, Kosunen O, Nurminen J, Sarkkinen H, Brummer-Korvenkontio M. Four fatal cases of nephropathia epidemica. Scand J Infect Dis 1995; 27: 515-517
69. Hautala T, Sironen T, Vapalahti O, Pääkkö E, Särkioja T, Salmela PI, Vaheri A, Plyusnin A, Kauma H. Hypophyseal hemorrhage and panhypopituitarism during Puumala Virus Infection: Magnetic Resonance Imaging and detection of viral antigen in the hypophysis. Clin Infect Dis 2002; 35: 96-101
70. Lim TH, Chang KH, Han MC, Chang YB, Lim SM, Yu YS, Chun YH, Lee JS. Pituitary atrophy in Korean (epidemic) hemorrhagic fever: CT correlation with pituitary function and visual field. Am J Neuroradiol 1986; 7: 633-637
71. Forslund T, Saltevo J, Anttinen J, Auvinen S, Brummer-Korvenkontio M, Korhonen A, Poutiainen M. Complications of nephropathia epidemica: three cases. J Intern Med 1992; 232: 87-90
72. Settergren B, Boman J, Linderholm M, Wiström J, Hägg E, Arvidsson PA. A case of nephropathia epidemica associated with panhypopituitarism and nephrotic syndrome. Nephron 1992; 61: 234-235
73. Suh DC, Park JS, Park SK, Lee HK, Chang KH. Pituitary hemorrhage as a complication of hantaviral disease. Am J Neuroradiol 1995; 16: 175-178
74. Park JE, Pyo HJ. Delayed onset of diuresis in a patient with acute renal failure due to hemorrhagic fever with renal syndrome who also developed anterior hypopituitarism. Clin Nephrol 1996; 46: 141-145
75. Kim NH, Cho JG, Ahn YK, Lee SU, Kim KH, Cho JH, Kim HG, Kim W, Jeong MH, Park JC, Kang JC. A case of torsade de pointes associated with hypopituitarism due to hemorrhagic fever with renal syndrome. J Korean Med Sci 2001; 16: 355-359
76. Pekic S, Cvijovic G, Stojanovic M, Kendereski A, Micic D, Popovic V. Hypopituitarism as a late complication of hemorrhagic fever. Endocrine 2005; 26: 79-82
77. Ahn HJ, Chung JH, Kim DM, Yoon NR, Kim CM. Hemorrhagic fever with renal syndrome accompanied by panhypopituitarism and central diabetes insipidus: a case report. J Neurovirol  2018; 24: 382-387
78. Bhoelan S, Langerak T, Noack D, van Schinkel L, van Nood E, van Gorp ECM, Rockx B, Goeijenbier M. Hypopituitarism after Orthohantavirus infection: what is currently known? Viruses 2019; 11: 340
79. Shi S, Zhang A, Zhang J, Xu S. Partial hypopituitarism with ACTH deficiency as the main manifestation as a complication of hemorrhagic fever with renal syndrome. BMC Endocr Disord 2024; 24: 61.
80. Partanen T, Koivikko M, Leisti P, Salmela P, Pääkkö E, Karttunen A, Sintonen H, Risteli L, Hautala N, Vapalahti O, Vaheri A, Kauma H, Hautala T. Long-term hormonal follow-up after human Puumala hantavirus infection. Clin Endocrinol2016; 84: 85-91
81. Tarvainen M, Mäkelä S, Mustonen J, Jaatinen P. Autoimmune polyendocrinopathy and hypophysitis after Puumalahantavirus infection. Endocrinol Diabetes Metab Case Rep 2016; 2016
82. Stojanovic M, Pekic S, Cvijovic G, Miljic D, Doknic M, Nikolic-Djurovic M, Micic D, Hrvacevic R, Nesic V, Popovic V.High risk of hypopituitarism in patients who recovered from hemorrhagic fever with renal syndrome. J Clin Endocrinol Metab 2008; 93: 2722-2728
83. Bellastella G, Cirillo P, Carbone C, Scappaticcio L, Maio A, Botta G, Tomasuolo M, Longo M, Pontillo A, Bellastella A, Esposito K, De Bellis A. Neuroimmunoendocrinology of SARS-CoV 2 Infection. Biomedicines 2022; 10: 2855. 
84. Clarke SA, Abbara A, Dhillo WS. Impact of COVID-19 on the endocrine system: a mini-review. Endocrinology 2022; 163: bqab203. 
85. Capatina C, Poiana C, Fleseriu M. Pituitary and SARS CoV-2: An unremitting conundrum. Best Pract Res Clin Endocrinol Metab 2023; 37: 101752
86. Chigr F, Merzouki M, Najimi M Autonomic brain centers and pathophysiology of COVID-19. ACS Chem Neurosci 2020; 11: 1520–1522
87. Conde Cardona G, Quintana Pajaro LD, Quintero Marzola ID, Ramos Villegas Y, Moscote Salazar LR. Neurotropism of SARS-CoV 2: mechanisms and manifestations. J Neurol Sci 2020; 412: 116824
88. Han T, Kang J, Li G, Ge J, Gu J. Analysis of 2019-nCoV receptor ACE2 expression in different tissues and its significance study. Ann Transl Med 2020; 8: 1077
89. Abdel-Mannan O, Eyre M, Lobel U, Bamford A, Eltze C, Hameed B, Hemingway C, Hacohen Y. Neurologic and radiographic findings associated with COVID-19 infection in children. JAMA Neurol 2020; 77: 1440-1445
90. Ding Y, He L, Zhang Q, Huang Z, Che X, Hou J, Wang H, Shen H, Qiu L, Li Z, Geng J, Cai J, Han H, Li X, Kang W, Weng D, Liang P, Jiang S. Organ distribution of severe acute respiratory syndrome (SARS) associated coronavirus (SARS-CoV) in SARS patients: implications for pathogenesis and virus transmission pathways. J Pathol 2004; 203: 622–630
91. Frara S, Allora A, Castellino L, di Filippo L, Loli P, Giustina A. COVID-19 and the pituitary. Pituitary, 2021; 24: 465-481
92. Puig-Domingo M, Marazuela M, Yildiz BO, Giustina A. COVID-19 and endocrine and metabolic diseases. An updated statement from the European Society of Endocrinology. Endocrine 2021; 72: 301-316.
93. Ghosh R, Roy D, Roy D, Mandal A, Dutta A, Naga D, Benito-Leon J. A rare case of SARS-CoV-2 infection associated with pituitary apoplexy without comorbidities. J Endocr Soc 2021; 5: bvaa203.
94. Martinez-Perez R, Kortz MW, Carroll BW, Duran D, Neill JS, Luzardo GD, Zachariah MA. Coronavirus disease 2019 and pituitary apoplexy: a single-center case series and review of the literature. World Neurosurg 2021; 152: e678-e687.
95. Facondo P, Maltese V, Delbarba A, Pirola I, Rotondi M, Ferlin A, Cappelli C. Case report: hypothalamic amenorrhea following COVID-19 infection and review of literatures. Front Endocrinol (Lausanne) 2022; 13: 840749.
96. Hazzi C, Villemure-Poliquin N, Nadeau S, Champagne PO. SARS-CoV-2 infection, a risk factor for pituitary apoplexy? A case series and literature review. Ear Nose Throat J 2024; 103: 153S-161S.  
97. Racca G, D'Agnano S, Fasano N, Gianotti L. Pituitary apoplexy with transition to acute hypophysitis in a patient with Sars-CoV-2 pneumonia. JCEM Case Rep 2022; 1: luac010.
98. Bray DP, Solares CA, Oyesiku NM. Rare case of a disappearing pituitary adenoma during the COVID-19 pandemic. World Neurosurg 2020; 146: 148-149
99. Chan JL, Gregory KD, Smithson SS, Naqvi M, Mamelak AN. Pituitary apoplexy associated with acute COVID-19 infection and pregnancy. Pituitary 2020; 23: 716-720
100. Santos CDSE, Filho LMDCL, Santos CAT, Neill JS, Vale HF, Kurnutala LN. Pituitary tumor resection in a patient with SARSCoV-2 (COVID-19) infection. A case report and suggested airway management guidelines. Braz J Anesthesiol 2020; 70: 165-170
101. Solorio-Pineda S, Almendarez-Sanchez CA, Tafur-Grandett AA, Ramos-Martinez GA, Huato-Reyes R, Ruiz-Flores MI, Sosa-Najera A. Pituitary macroadenoma apoplexy in a severe acute respiratory syndrome-coronavirus-2-positive testing: causal or casual? Surg Neurol Int 2020; 11: 304
102. LaRoy M & McGuire M. Pituitary apoplexy in the setting of COVID-19 infection: a case report. Am J Emerg Med 2021; S0735-6757(21)00153-4
103. Bordes SJ, Phang-Lyn S, Najera E, Borghei-Razavi H, Adada B. Pituitary apoplexy attributed to COVID-19 infection in the absence of an underlying macroadenoma or other identifiable cause. Cureus 2021; 13: e13315
104. Joshi M, Gunawardena S, Goenka A, Ey E, Kumar G. Post COVID-19 Lymphocytic hypophysitis: a rare presentation. Child Neurol Open 2022; 9: 2329048X221103051
105. Zerrouki D, Assarrar I, Rami I, Rouf S, Latrech H. Coronavirus as a trigger of lymphocytic hypophysitis in an adolescent girl: An exceptional case report. Int J Surg Case Rep 2024; 115: 109218.
106. Gonen MS, De Bellis A, Durcan E, Bellastella G, Cirillo P, Scappaticcio L, Longo M, Bircan BE, Sahin S, Sulu C, Ozkaya HM, Konukoglu D, Kartufan FF, Kelestimur F. Assessment of neuroendocrine changes and hypothalamo-pituitary autoimmunity in patients with COVID-19. Horm Metab Res 2022; 54: 153-161. 
107. Littera R, Campagna M, Deidda S, Angioni G, Cipri S, Melis M, Firinu D, Santus S, Lai A, Porcella R, Lai S, Rassu S, Scioscia R, Meloni F, Schirru D, Cordeddu W, Kowalik MA, Serra M, Ragatzu P, Carta MG, Del Giacco S, Restivo A, Deidda S, Orrù S, Palimodde A, Perra R, Orrù G, Conti M, Balestrieri C, Serra G, Onali S, Marongiu F, Perra A, Chessa L. Human leukocyte antigen complex and other immunogenetic and clinical factors influence susceptibility or protection to SARS-CoV-2 infection and severity of the disease course. The Sardinian experience. Front Immunol 2020; 11: 605688. 
108. Weiner J, Suwalski P, Holtgrewe M, Rakitko A, Thibeault C, Müller M, Patriki D, Quedenau C, Krüger U, Ilinsky V, Popov I, Balnis J, Jaitovich A, Helbig ET, Lippert LJ, Stubbemann P, Real LM, Macías J, Pineda JA, Fernandez-Fuertes M, Wang X, Karadeniz Z, Saccomanno J, Doehn JM, Hübner RH, Hinzmann B, Salvo M, Blueher A, Siemann S, Jurisic S, Beer JH, Rutishauser J, Wiggli B, Schmid H, Danninger K, Binder R, Corman VM, Mühlemann B, Arjun Arkal R, Fragiadakis GK, Mick E, Comet C, Calfee CS, Erle DJ, Hendrickson CM, Kangelaris KN, Krummel MF, Woodruff PG, Langelier CR, Venkataramani U, García F, Zyla J, Drosten C, Alice B, Jones TC, Suttorp N, Witzenrath M, Hippenstiel S, Zemojtel T, Skurk C, Poller W, Borodina T, Pa-Covid SG, Ripke S, Sander LE, Beule D, Landmesser U, Guettouche T, Kurth F, Heidecker B. Increased risk of severe clinical course of COVID-19 in carriers of HLA-C*04:01. EClinicalMedicine 2021; 40: 101099.
109. Hovhannisyan A, Madelian V, Avagyan S, Nazaretyan M, Hyussyan A, Sirunyan A, Arakelyan R, Manukyan Z, Yepiskoposyan L, Mayilyan KR, Jordan F. HLA-C*04:01 affects HLA class I heterozygosity and predicted affinity to SARS-CoV-2 peptides, and in combination with age and sex of armenian patients contributes to COVID-19 severity. Front Immunol 2022; 13: 769900.
110. Yasuda S, Suzuki S, Yanagisawa S, Morita H, Haisa A, Satomura A, Nakajima R, Oikawa Y, Inoue I, Shimada A. HLA typing of patients who developed subacute thyroiditis and Graves' disease after SARS-CoV-2 vaccination: a case report. BMC Endocr Disord 2023; 23: 54.
111. Rajevac H, Bachan M, Khan Z. Diabetes insipidus as a symptom of covid-19 infection: case report. Chest 2020, 158: A2576
112. Sheikh AB, Javed N, Sheikh AA, Upadhyay S, Shekhar R. Diabetes insipidus and concomitant myocarditis: a late sequelae of COVID-19 infection. J Investig Med High Impact Case Rep 2021, 9: 1–4
113. Misgar RA, Rasool A, Wani AI, Bashir MI. Central diabetes insipidus (infudibuloneuro hypophysitis): a late complication of COVID-19 infection. J Endocrinol Invest 2021 Jul 3; 1-2
114. Yavari A, Sharifan Z, Larijani B, Mosadegh Khah A. Central diabetes insipidus secondary to COVID-19 infection: a case report. BMC Endocr Disord 2022; 22: 134.
115. Berni A, Malandrino D, Corona G, Maggi M, Parenti G, Fibbi B, Poggesi L, Bartoloni A, Lavorini F, Fanelli A, Scocchera G, Nozzoli C, Peris A, Pieralli F, Pini R, Ungar A, Peri A. Serum sodium alterations in SARS CoV-2 (COVID-19) infection: impact on patient outcome. Eur J Endocrinol 2021; 185: 137-144. 
116. Khidir RJY, Ibrahim BAY, Adam MHM, Hassan RME, Fedail ASS, Abdulhamid RO, Mohamed SOO. Prevalence and outcomes of hyponatremia among COVID-19 patients: A systematic review and meta-analysis. Int J Health Sci (Qassim) 2022; 16: 69-84.
117. Morita S, Tsuji T, Kishimoto S, Uraki S, Takeshima K, Iwakura H, Furuta H, Nishi M, Inaba H, Matsuoka TA. Isolated ACTH deficiency following immunization with the BNT162b2 SARS-CoV-2 vaccine: a case report. BMC Endocr Disord 2022; 22:185.
118. Panos LD, Bargiotas P, Hadjigeorgiou G, Panos GD. Neurovascular adverse effects of Sars-Cov-2 vaccination. Drug Des Devel Ther 2024; 18: 1891-1905.
119. Verrienti M, Marino Picciola V, Ambrosio MR, Zatelli MC. Pituitary and COVID-19 vaccination: a systematic review. Pituitary 2024 May 18. doi: 10.1007/s11102-024-01402-2.
120. Watanabe S, Tamura Y, Oba K, Kitayama S, Sato M, Kodera R, Toyoshima K, Chiba Y, Araki A. Hypopituitarism with secondary adrenocortical insufficiency and arginine vasopressin deficiency due to hypophysitis after COVID-19 vaccination: a case report. BMC Endocr Disord 2024; 24(1): 71.
121. Bouça B, Roldão M, Bogalho P, Cerqueira L, Silva-Nunes J. Central diabetes insipidus following immunization with BNT162b2 mRNA COVID-19 vaccine: a case report. Front Endocrinol (Lausanne) 2022; 13: 889074.
122. Torres AM, Kazee AM, Simon H, Knudson PE, Weinstock RS. Central diabetes insipidus due to herpes simplex in a patient immunosuppressed by Cushing's syndrome. Endocr Pract 2000; 6: 26-28
123. Moses AM, Thomas DG, Canfield MC, Collins GH. Central diabetes insipidus due to cytomegalovirus infection of the hypothalamus in a patient with acquired immunodeficiency syndrome: a clinical, pathological, and immunohistochemical case study. J Clin Endocrinol Metab 2003; 88: 51-54.
124. Scheinpflug K, Schalk E, Reschke K, Franke A, Mogren M. Diabetes insipidus due to herpes encephalitis in a patient with diffuse large cell lymphoma. A case report. Exp Clin Endocrinol Diabetes 2006; 114: 31-34
125. Jones G, Muriello M, Patel A, Logan L. Enteroviral meningoencephalitis complicated by central diabetes insipidusin a neonate: a case report and review of the literature. J Pediatric Infect Dis Soc 2015; 4: 155-158.
126. Tsuji H, Yoshifuji H, Fujii T, Matsuo T, Nakashima R, Imura Y, Yukawa N, Ohmura K, Sumiyoshi S, Mimori T. Visceral disseminated varicella zoster virus infection after rituximab treatment for granulomatosis with polyangiitis. Mod Rheumatol  2017; 27: 155-161
127. Zhang X, Li Q, Hu P, Cheng H, Huang G. Two case reports of pituitary adenoma associated with Toxoplasma gondii infection. J Clin Pathol 2002; 55: 965-966
128. Milligan SA, Katz MS, Craven PC, Strandberg DA, Russell IJ, Becker RA. Toxoplasmosis presenting as panhypopituitarism in a patient with the acquired immune deficiency syndrome. Am J Med 1984; 77: 760-764
129. Hamdeh S, Abbas A, Fraker J, Lambrecht JE. Intracranial toxoplasmosis presenting as panhypopituitarism in an immunocompromised patient. Am J Emerg Med2015; 33: 1848.e1-2
130. Del Brutto OH, Del Brutto VJ. Intrasellar cysticercosis: a systematic review. Acta Neurol Belg 2013; 113: 225-227.
131. Dutta D, Kumar M, Ghosh S, Mukhopadhyay S, Chowdhury S. Pituitary hormone deficiency due to racemose neurocysticercosis. Lancet Diabetes Endocrinol 2013; 1: e13.
132. Cheong JH, Kim JM, Kim CH. Neurocysticercosis involving the pituitary stalk: case report and literature review. J Korean Neurosurg Soc 2010; 48: 91-93