FEEDBACK ACTIONS OF ESTRADIOL ON GNRH SECRETION DURING THE FOLLICULARPHASE OF THE ESTROUS-CYCLE

The pattern of GnRH secretion during the follicular phase of the estro us cycle of sheep is characterized by an initial marked change in epis odic secretion (increased frequency and decreased amplitude) followed by a massive and sustained discharge - the preovulatory GnRH surge. St udies employing a physiological model for the follicular phase have re vealed that estradiol has profound and complex feedback effects on GnR H release during the preovulatory period. These include both quantitat ive effects on pulses (stimulation of frequency, inhibition of amplitu de) and qualitative effects (altering pulse shape, stimulating interpu lse secretion), in addition to inductng a preovulatory GnRH surge. In stimulating the surge, estradiol causes a highly characteristic change in the minute-to-minute pattern of GnRH in hypophyseal portal blood. Initially, a strictly episodic pattern gives way to one in which GnRH is consistently elevated between pulses. Then, following enhancement o f both pulsatile and interpulse components, GnRH becomes extremely hig h and variable for the majority of the surge. From this point, a regul ar and well organized pulse pattern is not apparent. The characteristi c time course of GnRH at surge onset provides insight into possible me chanistic changes in the GnRH neurosecretory system. Such changes incl ude quantitative and qualitative alterations in the pulse generating m echanism, recruitment of a surge specific population of GnRH neurones, morphologic alterations in GnRH neurones and neighboring cells, and c hanges in efficiency or route of delivery of GnRH from its site of rel ease to the portal vasculature. These possibilities, while untested an d speculative, provide a conceptual framework for future research.