Premature seasonal inhibition of tonic LH secretion by oestradiol in the female lamb and its consequences

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Abstract
Patterns of circulating LH [luteinizing hormone] were examined in ovariectomized lambs and adults bearing Silastic capsules containing estradiol-17.beta. and in untreated ovariectomized lambs. A decrease in serum LH in estradiol-treated lambs occurred coincidentally with the cessation of ovulations in intact lambs and in the absence of any decrease in circulating LH in untreated ovariectomized lambs. On average, these phenomena occurred 3 wk before the seasonal reduction in serum LH levels and onset of anestrus in estradiol-treated ovariectomized and intact adults, respectively. An early seasonal increase in responsiveness to estradiol negative feedback on tonic LH secretion apparently is responsible for the premature cessation of ovulations in the lamb, accounting, in part, for the shor breeding season during the 1st year of life and, in some lambs, the failure to initiate cyclicity until the 2nd year after birth; and onset of anestrus in lambs, in contrast to puberty, is not associated with steroid-independent changes in LH secretion.