Feedback action of estradiol on GnRH secretion during the follicular phase of the estrus cycle
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The pattern of GnRH secretion during the follicular phase of the estrus cycle of sheep is characterized by an initial marked in episodic secretion followed by a massive and sustained discharge - the preovulatory GnRH surge.Studies emploing a physiological model for the follicular phase have revealed that estradiol has profound and complex feedback effects on GnRH release during the preovulatory period.These include both quantitative efects on pulses and qualitative effects, in addition to inducting 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 of both pulsatile and interpulse components, GnRH becomes extramely high and variable for the majority of the surge.From this point, a regular and well organized pulse pattern is not apparent.The characteristic time course of GnRH at surge onset provides insight into possible mechanistic changes in the GnRH neurosecretory system.Such changes include quantitative and qualitative alternations in the pulse generating mechanism, recruiment of a surge specific population of GnRH neurones, morphologic alternations in GnRH neurones , morphologic alternations in GnRH neurones and neighboring cells, and changes in efficiency or route of delivery of GnRH from its siet of release to the portal vasculature.These possibilities, while untested and speculative, provide a conceptual framework for future research.
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