Figure 1.Schematic of feedback control of hypothalamic-pituitary-ovarian axis. NE=norepinephrine, E=epinephrine, DA=dopamine and EOP=endogenous opioids. From Marshall JC: Regulation of gonadotropin secretion. In DeGroot LJ (Eds): Endocrinology (ed3). Philadelphia, Saunders Press, 1995. pp 1993-2007