Endocrine Mechanisms Governing Transition into Adulthood: A Marked Decrease in Inhibitory Feedback Action of Estradiol on Tonic Secretion of Luteinizing Hormone in the Lamb during Puberty*

The hypothesis was tested that a decrease in response to the inhibitory feedback action of estradiol on tonic LH secretion occurs in the female lamb during puberty. Ovaries were removed from 10 female lambs at 19 weeks of age, which was approximately 11 weeks before the expected first ovulation. At ovariectomy, Silastic implants containing crystalline estradiol were inserted sc in 6 lambs to maintain chronic low levels of circulating estradiol. In estradiol-treated ovariectomized lambs, serum LH was suppressed to undetectable concentrations until the age when first ovulations were initiated in untreated intact lambs. At this time, circulating LH increased dramatically to castrate levels in 5 of 6 estradiol-treated ovariectomized females despite maintenance of constant levels of serum estradiol. The ”escape“ from estradiol negative feedback on tonic LH secretion in ovariectomized lambs and the first ovulation in intact lambs occurred over a wide range of body weights. In the slowest growing estradiol-treate lamb, a decrease in response to estradiol feedback did not occur; similarly, the slowest growing intact lamb did not exhibit ovulation during the study. In 4 ovariectomized lambs not treated with estradiol, serum LH remained at low castrate levels (7 ng/ml) until the time of first ovulation in intact lambs, when a secondary rise in circulating LH to adult castrate levels (15 ng/ml) occurred. With respect to season, the decrease in response to estradiol in ovariectomized lambs and the first ovulation in intact lambs began in late September, which was 7 weeks after the onset of the annual breeding season (early August) when similar phenomena were manifest in fully mature ewes. These results suggest that 1) a decrease in response to estradiol inhibition of tonic LH secretion occurs during puberty in the lamb; 2) a stimulus to tonic LH secretion occurs during puberty which is independent of ovarian steroid control (secondary rise in circulating LH in untreated ovariectomized lambs); and 3) the final processes governing the onset of puberty in the lamb and the onset of the annual breeding season in the adult appear to share a common mechanism, a decrease in the inhibitory feedback response to estradiol on tonic LH secretion.