THE NEUROENDOCRINE SIGNAL FOR OVULATION

The central nervous system plays a crucial role in determining if and when ovulation occurs. This neural regulation of fertility is achieved. in large measure, by means of the secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus. This report addresses changes in the secretion of GnRH into hypophyseal portal blood, and the regulation of these changes. leading up to ovulation in the domestic sheep. During the estrous cycle, the process of ovulation is heralded by a large preovulatory surge in the secretion of luteinizing hormone (LH). Recent studies have provided strong evidence that this LH discharge is induced by an unambiguous surge in the secretion of GnRH into hypophyseal portal blood. This surge of GnRH is massive and sustained, and is triggered by the increase in circulating estradiol secreted from the developing preovulatory ovarian follicles. Although the mechanism by which the increment in estradiol activates the GnRH neuronal network remains to be determined, this process involves a switch in the pattern of GnRH secretion. At the time of this switch, a pattern of GnRH release that is strictly episodic at other times of the estrous cycle gives way to a surge that sustains a continuous elevation of GnRH in portal blood for many hours. This switch in the operation of the GnRH neurosecretory system, and the resultant massive release of GnRH, constitutes the neuroendocrine signal for ovulation.