The preovulatory gonadotrophin-releasing hormone surge: a neuroendocrine signal for ovulation.

Recent studies have demonstrated that an important component of the positive 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 neuroendocrine signal for ovulation has been best characterized in ewes. The GnRH surge is high in amplitude; the amount secreted increases on average more than 40 times above the pre-surge baseline value. The initial increment in GnRH secretion precedes or coincides with the onset of the LH surge. The GnRH surge is of extended duration, lasting far longer than the preovulatory LH surge. A molecular variant of GnRH, which is less active biologically than native GnRH, is co-secreted at the time of the surge, but termination of the LH surge cannot be accounted for by a change in biological activity of the secreted GnRH. Generation of the GnRH surge appears to follow a characteristic progressive change in the pattern of GnRH in portal blood. High concentrations of oestradiol initially stimulate the secretion of GnRH between pulses; this is followed by augmentation of both pulsatile and interpulse GnRH release producing the rising limb of the surge. Finally, recent experiments have indicated that the local application of oestradiol to the ventromedial nucleus of the hypothalamus is sufficient to stimulate the GnRH surge, suggesting a key rol for this hypothalamic area in the generation of this neuroendocrine signal for ovulation.