Dynamics of gonadotropin-releasing hormone (GnRH) secretion during the GnRH surge: insights into the mechanism of GnRH surge induction.

Recent studies demonstrate unequivocally that a preovulatory surge of GnRH is secreted into pituitary portal blood during the estrous cycle of the ewe and that this surge is induced by the follicular phase rise in estradiol. These data, obtained at 10-min intervals, suggested the surge results from a continuous elevation of GnRH rather than from a sequence of discrete pulses. This study examines the dynamics of GnRH secretion in more detail to determine if the surge results from strictly episodic release of the decapeptide. Our approach was to monitor GnRH secretion into pituitary portal blood at very frequent intervals during several "windows" of the GnRH surge induced using a physiological model for the estrous cycle. Samples of portal blood were obtained at either 2-min intervals (6 ewes), or 30-sec intervals (12 ewes) at several times during the surge; at other times portal blood was sampled less often to monitor progression of the GnRH surge. All ewes had an unambiguous GnRH surge; amplitude ranged from 100- to 500-fold over pressure levels. Regardless of sampling interval, our results provide no convincing evidence to indicate the enhanced secretion of GnRH is strictly episodic; values remained continuously elevated in portal blood. Our findings are consistent with the hypothesis that the GnRH surge is not composed entirely of discrete synchronous secretory events, and they raise the possibility that one action of estradiol in inducing the GnRH surge may be to switch the pattern of GnRH secretion into portal blood from episodic to continuous.