Preovulatory gonadotropin surge system of prepubertal female sheep is exquisitely sensitive to the stimulatory feedback action of estradiol.

The sensitivity of the LH surge mechanism to estradiol-positive feedback was examined in immature lambs (19 weeks of age) several weeks before first ovulation (31 weeks 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 one of five levels of estradiol (n = 5 lambs/level). The estradiol increments produced after insertion of the second 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 (second 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 we previously reported for the mature female indicates that the lamb is equally sensitive to estradiol stimulatory feedback action long before the age of first ovulation. This suggests that low estradiol secretion, rather than reduced sensitivity of the surge mechanism to estradiol-positive feedback, is responsible for the anovulatory condition of the immature female sheep.