Sexual differentiation of the mechanism controlling the preovulatory discharge of luteinizing hormone in sheep.

Silastic capsules containing estradiol-17beta were implanted subcutaneously into male and female sheep which had been gonadectomized as adults, In both sexes, this treatment resulted in a prompt decrease in serum LH concentrations reflecting a negative feedback suppression of tonic LH secretion. In females but not males, this decrease was followed, within 24 h, by an LH surge after which circulating LH declined to low levels resembling those in intact ewes during the luteal phase of the estrous cycle. In males, only a sustained reduction in circulating LH was observed. Three weeks after placement of the initial estradiol implants, while serum LH levels were low and stable, additional estradiol implants were inserted. This additional treatment effected step-like increments in circulating estradiol to approximately 20 or 55 pg/ml (depending on the number of implants) and elicited single or multiple LH discharges in females but failed to induce LH surges in males. These findings lead to the conclusion that the mechanism which governs the cyclic mode of LH secretion in sheep, as in rats, undergoes sexual differentiation. Further, the presence of multiple LH surges in some females suggests that an increment and sustained elevation in circulating estradiol can induce the occurrence, or permit the expression, of consecutive signals for the LH surge in the ovariectomized ewe.