SEASONAL-CHANGES IN GONADOTROPIN-RELEASING-HORMONE SECRETION IN THE EWE - ALTERATION IN RESPONSE TO THE NEGATIVE FEEDBACK ACTION OF ESTRADIOL

Two experiments were performed to test the hypothesis that there is a seasonal change in the negative feedback effect of estradiol on episod ic secretion of GnRH in the ewe. The first experiment identified a spe cific estradiol treatment (delivered by s.c. Silastic implant) that pr oduced a 50% decrease in the frequency of pulsatile secretion of LH in ovariectomized ewes during the anestrous season. In the second experi ment, theis estradiol treatment was administered to ovariectomized ewe s during the mid-breeding and anestrous seasons. Separate groups of ov ariectomized ewes not treated with estradiol were included during each season to test for a seasonal difference in the effect of estradiol o n episodic GnRH and LH secretion. Samples of hypophyseal portal blood (for GnRH) and jugular blood (for LH) were obtained at 5-min intervals approximately one month after placement of the estradiol implants. Du ring the breeding season, no effect of estradiol was observed on eithe r the frequency or size of GnRH and LH pulses. During anestrus, howeve r, estradiol produced a profound suppression of the frequency of GnRH and LH pulses, and an increase in GnRH pulse size. No significant seas onal change was observed in the characteristics of GnRH and LH pulses in ovariectomized ewes in the absence of estradiol treatment. These fi ndings lead to the conclusion that there is a marked seasonal change i n the negative feedback effect of estradiol on episodic GnRH secretion in the ewe, with the steroid being maximally effective during anestru s.