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The endocrin control of seasonal reproductive function in the ewe: a marked change in response to the negative feedback action of estradiol on luteinizing hormone secretion.

A group of 6 ewes was ovariectomized and treated sc for 20 months with Silastic capsules containing estradiol-17β. Although this treatment maintained serum estradiol concentrations relatively constant at mid-luteal levels (3–5 pg/ml), there was a dramatic seasonal variation in serum LH concentrations. Throughout anestrus, LH levels remained undetectable. At the onset of the breeding season, there was a striking increase in LH concentrations to levels which approached those in untreated ovariectomized ewes. Mean LH concentrations remained high until the beginning of the next anestrous season, when they decreased precipitously to undetectable levels where they remained until the subsequent breeding season. These biannual changes in the negative feedback action of estradiol occurred within 2–3 weeks in each ewe and coincided with the transitions between the breeding season and anestrus in intact ewes. In contrast to the biphasic annual pattern in LH secretion observed in estradiol-treated ovariectomized ewes...