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*

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Abstract
Ovaries were removed from 10 female lambs at 19 wk of age, which was approximately 11 wk before the expected 1st ovulation. At ovariectomy, Silastic implants containing crystalline estradiol [E2] were inserted s.c. in 6 lambs to maintain chronic low levels of circulating E2. In E2-treated ovariectomized lambs, serum LH [luteinizing hormone] was suppressed to undetectable concentrations until the age when 1st ovulations were initiated in untreated intact lambs. At this time, circulating LH increased dramatically to castrate levels in 5 of 6 E2-treated ovariectomized females despite maintenance of constant levels of serum E2. The escape from E2 negative feedback on tonic LH secretion in ovariectomized lambs and the 1st ovulation in intact lambs occurred over a wide range of body weights. In the slowest growing E2-treated lamb, a decrease in response to E2 feedback did not occur; similarly, the slowest growing intact lamb did not exhibit ovulation during the study. In 4 ovariectomized lambs not treated with E2, serum LH remained at low castrate levels (7 ng/ml) until the time of 1st 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 E2 in ovariectomized lambs and the 1st ovulation in intact lambs began in late Sept., which was 7 wk after the onset of the annual breeding season (early Aug.), when similar phenomena were manifest in fully mature ewes. A decrease in response to E2 inhibition of tonic LH secretion apparently occurs during puberty in the lamb; 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); 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 E2 of tonic LH secretion.

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