Preovulatory Gonadotropin Surge System of Prepubertal Female Sheep Is Exquisitely Sensitive to the Stimulatory Feedback Action of Estradiol*

Abstract

The sensitivity of the luteinizing hormone [LH] surge mechanism to estradiol-positive feedback was examined in immature lambs (19 wk of age) several weeks before 1st ovulation (31 wk of age). To minimize differences in basal estradiol concentrations, lambs were pretreated with a low level of estradiol(2 pg/ml; Silastic capsule) for 4 days and were ovariectomized before they were challenged with 1 of 5 levels of estradiol (n = 5 lambs/level). The estradiol increments produced after insertion of the 2nd set of estradiol implants ranged from 2-11 pg/ml. All estradiol increments, even those as low as 2 pg/ml, produced LH surges; in the absence of an estradiol increment (2nd implant not inserted; n = 5 lambs), a LH surge did not occur. The similarity of the dose-response curve for estradiol-induced LH release in the lamb to that which was previously reported for the mature female indicates that the lamb is equally sensitive to estradiol stimulatory feedback action long before the age of 1st ovulation. Low estradiol secretion, rather than reduced sensitivity of the surge mechanism to estradiol-positive feedback, apparently is responsible for the anovulatory condition of the immature female sheep.