Photoperiodic Control of Seasonal Breeding in Ewes: Modulation of the Negative Feedback Action of Estradiol12

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Abstract
In ewes, a pivotal neuroendocrine event regulating seasonal breeding is a change in response of the hypothalamo-pituitary axis to the negative feedback action of estradiol. Further, the major environmental parameter regulating seasonal breeding is photoperiod. This study was performed to examine whether changes in photoperiod regulate breeding seasons by modulating response to steroid feedback. Six intact ewes, five estradiol-treated ovariectomized ewes, and one vasectomized ram were housed under artificial short-day (8L:16D) and long-day (16L:8D) photoperiods which were alternated every 90 days. Under these conditions, the intact ewes underwent two breeding and anestrous seasons in 1 year. Further, the transitions between breeding seasons coincided with striking fluctuations in serum LH (between <0.3 and 10 ng/ml) and FSH (50-170 ng/ml) in the estradiol-treated ovariectomized ewes, long days causing anestrus and an increased response to estradiol (low LH and FSH), short days resulting in breeding season and a decreased response to estradiol (high LH and FSH). Similar results were obtained in identical groups of ewes, which were subjected to 120-day alternations between long and short days, except that there were only three seasonal transitions and FSH was not measured. These results provide strong support for the hypothesis that in ewes the mechanism whereby photoperiod regulates seasonal breeding includes modulation of response to the negative feedback action of estradiol on gonadotropin secretion.

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