Progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by inhibiting the surge of gonadotropin-releasing hormone.

Previous studies indicate an elevation of circulating progesterone blocks the positive feedback effect of a rise in circulating estradiol. This explains the absence of gonadotropin surges in the luteal phase of the menstrual or estrous cycle despite occasional rises in circulating estradiol to a concentration sufficient for surge induction. Recent studies demonstrate estradiol initiates the LH surge in sheep by inducing a large surge of GnRH secretion, measurable in the hypophyseal portal vasculature. We tested the hypothesis that progesterone blocks the estradiol-induced surge of LH and FSH in sheep by preventing this GnRH surge. Adult Suffolk ewes were ovariectomized, treated with Silastic implants to produce and maintain midluteal phase concentrations of circulating estradiol and progesterone, and an apparatus was surgically installed for sampling of pituitary portal blood. One week later the ewes were allocated to two groups: a surge-induction group (n = 5) in which the progesterone implants were removed to simulate luteolysis, and a surge-block group (n = 5) subjected to a sham implant removal such that the elevation in progesterone was maintained. Sixteen hours after progesterone-implant removal (or sham removal), all animals were treated with additional estradiol implants to produce a rise in circulating estradiol as seen in the follicular phase of the estrous cycle. Hourly samples of pituitary portal and jugular blood were obtained for 24 h, spanning the time of the expected hormone surges, after which an iv bolus of GnRH was injected to test for pituitary responsiveness to the releasing hormone. All animals in the surge-induction group exhibited vigorous surges of GnRH, LH, and FSH, but failed to show a rise in gonadotropin secretion in response to the GnRH challenge given within hours of termination of the gonadotropin surges. The surges of GnRH, LH, and FSH were blocked in all animals in which elevated levels of progesterone were maintained. These animals in the surge-block group, however, did secrete LH in response to the GnRH challenge. We conclude progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by acting centrally to inhibit the surge of GnRH secreted into the hypophyseal portal vasculature.