Duration and amplitude of the luteal phase progesterone increment times the estradiol-induced luteinizing: Hormone surge in ewes

Progesterone (P) powerfully inhibits the neuroendocrine reproductive axis, but the mechanisms and site or sites of action of this steroid remain poorl y understood. Progesterone exposure during the luteal phase also alters the responsiveness of the hypothalamus to increased concentrations of estrogen (E) during the follicular phase. Using an ovariectomized ovine follicular phase model, we investigated whether the amplitude and duration of the lute al phase increase in circulating P affects the E-induced surge in LH. Treat ment of ewes for 10 days with two, one, or half an intravaginal P-releasing implant or with an empty implant demonstrated that P concentrations signif icantly (P < 0.0001) delayed the time to surge onset upon exposure to an eq ual concentration of E. This delay was not due to a time-related difference in responsiveness to E after P clearance because the time of surge onset w as not different when E treatment began 6, 12, or 24 h after the withdrawal of two P implants that had been present for 10 days. The final study demon strated that the duration of P before treatment (5, 10, or 30 days) signifi cantly (P < 0.0001) delayed the responsiveness of the estradiol-dependent s urge-generating system. There was no effect on surge amplitude or duration in any experiment. Thus, the amplitude and duration of exposure to luteal p hase P significantly affect the neural elements targeted by E to induce the preovulatory LH surge.