Effect of the introduction of rams during the anoestrous season on the pulsatile secretion of LH in ovariectomized ewes

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Abstract
Introduction of rams to ovariectomized ewes treated with estradiol implants (n = 10) increased the frequency of LH [luteinizing hormone] pulses from 4.8 to 10.6 pulses per 12 h. This effect was reflected by increases in mean levels of LH and the basal levels on which the pulses were superimposed. In ewes that had not been treated with estradiol (n = 5), there was no significant increase in pulse frequncy but mean and basal levels of LH increased slightly after the introduction of rams. In a 2nd experiment, similar effects of the introduction of rams were seen in ovariectomized ewes treated with estradiol or estradiol + androstenedione (n = 16), but no significant effects of the rams were observed in untreated ewes (n = 8) or ewes treated only with androstenedione (n = 7). No preovulatory surges of LH were observed in the 30 h period after the introduction of rams. The ram stimulus probably evokes the increase in pulse frequency by inhibiting the negative feedback action of estradiol, and the surge normally observed in entire ewes is dependent on the ovarian response to these pulses. The observation of responses in some ewes not treated with estradiol also raises the possibility that the ram stimulus can act directly on the hypothalamic neurons that control the secretion of LH, and that this effect is enhanced in the presence of estrogen.

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