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Sex Difference in the Suppressive Effect of Cortisol on Pulsatile Secretion of Luteinizing Hormone in Sheep

Department of Physiology (C.A.S., A.I.T., A.J.T.), Monash University, Victoria 3800, Australia; Prince Henry’s Institute of Medical Research (I.J.C.), Clayton 3187, Australia; and Reproductive Sciences Program (K.M.B., F.J.K.), University of Michigan, Ann Arbor, Michigan 48109

Address all correspondence and requests for reprints to: Dr. Alan J. Tilbrook, Department of Physiology, P.O. Box 13F, Monash University, Victoria 3800, Australia. E-mail: alan.tilbrook{at}med.monash.edu.au.

We tested the hypothesis that there are sex differences in the inhibitory effect of cortisol on pulsatile LH secretion and pituitary responsiveness to GnRH in gonadectomized sheep. In experiment 1, pulsatile LH secretion was examined in gonadectomized ewes and rams infused with either saline, a low (250 µg/kg·h) or a high (500 µg/kg·h) dose of cortisol for 30 h. In experiment 2, direct pituitary actions of cortisol were assessed by monitoring LH pulse amplitude in response to exogenous GnRH in hypothalamo-pituitary disconnected ewes and rams infused with the low dose of cortisol. In experiment 1, the mean (±SEM) plasma LH concentration was (P < 0.05) reduced significantly during cortisol infusion in both sexes, but the effect was greater in rams. In ewes, LH pulse amplitude and frequency were reduced (P < 0.05) at the high, but not the low, cortisol dose, whereas total LH output (LH pulse amplitude multiplied by frequency) was reduced (P < 0.05) at both doses. In rams, LH pulse frequency and amplitude and total LH output were (P < 0.05) reduced significantly at both cortisol doses. In experiment 2, plasma LH concentration and pulse amplitude in response to exogenous GnRH were not affected by infusion of cortisol in either sex. We conclude that gonadectomized rams are more sensitive than gonadectomized ewes to the effects of cortisol to inhibit LH secretion and that sex differences exist in the specific actions of cortisol on LH pulses. The results of experiment 2 suggest that intact hypothalamic input to the pituitary is necessary for cortisol to inhibit pituitary responsiveness to GnRH.

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