THE PREOVULATORY GONADOTROPIN-RELEASING-HORMONE SURGE - A NEUROENDOCRINE SIGNAL FOR OVULATION

Recent studies have demonstrated that an important component of the po sitive feedback response to oestradiol in mammals is an action within the central nervous system to induce a large surge in the secretion of gonadotrophin-releasing hormone (GnRH). This oestradiol-induced neuro endocrine signal for ovulation has been best characterized in ewes. Th e GnRH surge is high in amplitude; the amount secreted increases on av erage more than 40 times above the pre-surge baseline value. The initi al increment in GnRH secretion precedes or coincides with the onset of the LH surge. The GnRH surge is of extended duration, lasting far lon ger than the preovulatory LH surge. A molecular variant of GnRH, which is less active biologically than native GnRH, is co-secreted at the t ime of the surge, but termination of the LH surge cannot be accounted for by a change in biological activity of the secreted GnRH. Generatio n of the GnRH surge appears to follow a characteristic progressive cha nge in the pattern of GnRH in portal blood. High concentrations of oes tradiol initially stimulate the secretion of GnRH between pulses; this is followed by augmentation of both pulsatile and interpulse GnRH rel ease producing the rising limb of the surge. Finally, recent experimen ts have indicated that the local application of oestradiol to the vent romedial nucleus of the hypothalamus is sufficient to stimulate the Gn RH surge, suggesting a key role for this hypothalamic area in the gene ration of this neuroendocrine signal for ovulation.