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

Abstract

Patterns of circulating LH were examined in ovariectomized lambs and adults bearing Silastic capsules containing oestradiol-17 beta and in untreated ovariectomized lambs. A decrease in serum LH in oestradiol-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 weeks before the seasonal reduction in serum LH levels and onset of anoestrus in oestradiol-treated ovariectomized and intact adults, respectively. The results suggest that (1) an early seasonal increase in responsiveness to oestradiol negative feedback on tonic LH secretion is responsible for the premature cessation of ovulations in the lamb, accounting, in part, for the short breeding season during the first year of life and, in some lambs, the failure to initiate cyclicity until the second year after birth; and (2) onset of anoestrus in lambs, in contrast to puberty, is not associated with steroid-independent changes in LH secretion.