Progesterone priming is essential for the full expression of the positive feedback effect of estradiol in inducing the preovulatory gonadotropin-releasing hormone surge in the ewe

The luteal phase elevation in circulating progesterone (P) powerfully inhibits GnRH and, consequently, LH release, thereby preventing premature preovulatory LH surges in the ewe. Whether luteal phase P modulates the response of the GnRH system to the positive feedback effect of estradiol is unknown. To investigate this possibility, two experiments were conducted during the anestrous season using an artificial model of the follicular phase in ovariectomized ewes bearing 10-mm sc 17 beta-estradiol SILASTIC brand implants (Dow Coming Corp.). In Exp 1, ewes (n = 10) were run through four successive artificial cycles during which a luteal phase level of P was either replaced (cycles 1 and 3) or not replaced (cycles 2 and 4). GnRH and LH secretions were monitored by sampling cerebrospinal fluid (CSF) and jugular blood from 10-35 h after four 30-mm 17 beta-estradiol SILASTIC implants were inserted sc. CSF could be collected from only four ewes over the four cycles. There was no P-dependent difference in the onset of the GnRH and LH surges, which may have been due to a progressive delay in the surge onsets over the four cycles (by ANOVA, P < 0.05). Due to this delay, it was not possible to obtain an accurate estimate of the duration of the GnRH and LH surges in all ewes, but the size of the GnRH surge was always greater when animals had been treated with P, resulting in a significant increase in the maximum (P < 0.01) and mean (P < 0.05) levels during the surge. In contrast, there was no effect on any parameter of LH secretion. In Exp 2, ewes (n = 10) were run through two artificial estrous cycles during which luteal phase P was either replaced or not replaced, using a cross-over experimental design. CSF was collected from seven ewes over the two cycles. GnRH and LH secretions were monitored from 10-53 h after estradiol administration. As in Exp 1, a clear significant increase in the maximal and mean GnRH levels (P < 0.05 for both) was observed during the surge when ewes had been pretreated with P. Again, no changes were observed in LH release during the surge. P priming did, however, delay the onsets of the GnRH (P < 0.01) and LH surges (P < 0.01). Our data show that the increase in P during the luteal phase of the estrous cycle is essential for the full expression of the positive feedback effect of estradiol in inducing the preovulatory GnRH surge in the ewe.