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Seasonal variations in the behavioural and feedback actions of oestradiol in the ewe were examined by determining the ability of various physiological oestradiol concentrations to elicit oestrous behaviour, induce an LH surge, and suppress tonic LH secretion at four times of the year. These tests were performed in acutely ovariectomized animals pretreated with oestradiol and progesterone to minimize seasonal differences in their endocrine status. Although smaller amplitude LH surges were observed in anoestrus, the dose–response curves for the induction of LH surges were virtually identical at all times of the year. Oestradiol was slightly less effective in eliciting oestrous behaviour in anoestrus and during the transition to the breeding season than at other times. This seasonal variation was, however, observed only with relatively low oestradiol concentrations; serum oestradiol levels of 3 pg/ml or greater induced oestrus in almost all ewes regardless of season. In contrast, there was a marked seasonal change in the negative feedback action of oestradiol. In anoestrus, basal oestradiol levels of 1–3 pg/ml suppressed LH to low levels (0·3 ng/ml), whereas in the breeding season, even peak oestradiol concentrations of 10 pg/ml were not able to produce this degree of inhibition. These results thus support the hypothesis that the annual breeding cycle of the ewe is governed primarily by shifts in the extent to which oestradiol can suppress tonic LH secretion.

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