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Seasonal Differences in the Effect of Isolation and Restraint Stress on the Luteinizing Hormone Response to Gonadotropin-Releasing Hormone in Hypothalamopituitary Disconnected, Gonadectomized Rams and Ewes¹

Department of Physiology,³ Monash University, Clayton, Victoria 3800, Australia Prince Henry's Institute of Medical Research,⁴ Clayton, Victoria 3168, Australia Baker Medical Research Institute,⁵ Prahran, Victoria 3181, Australia

Stress responses are thought to act within the hypothalamopituitary unit to impair the reproductive system, and the sites of action may differ between sexes. The effect of isolation and restraint stress on pituitary responsiveness to GnRH in sheep was investigated, with emphasis on possible sex differences. Experiments were conducted during the breeding season and the nonbreeding season. In both experiments, 125 ng of GnRH was injected i.v. every 2 h into hypothalamopituitary disconnected, gonadectomized rams and ewes on 3 experimental days, with each day divided into two periods. During the second period on Day 2, isolation and restraint stress was imposed for 5.5 h. Plasma concentrations of LH and cortisol were measured in samples of blood collected from the jugular vein. In the second experiment (nonbreeding season), plasma concentrations of epinephrine, norepinephrine, 3,4-dihydroxyphenylalanine, and 3,4-dihydroxyphenylglycol were also measured. In both experiments, there was no effect of isolation and restraint stress on plasma concentrations of cortisol in either sex. During the breeding season, there was no effect of isolation and restraint stress on plasma concentrations of LH in either sex. During the nonbreeding season, the amplitude of the first LH pulse after the commencement of stress was significantly reduced (P < 0.05) in rams and ewes. In the second experiment, during stress there was a significant increase (P < 0.05) in plasma concentrations of epinephrine in rams and ewes and significantly higher (P < 0.05) basal concentrations of norepinephrine in ewes than in rams. These results suggest that in sheep stress reduces responsiveness of the pituitary gland to exogenous GnRH during the nonbreeding season but not during the breeding season, possibly because of mediators of the stress response other than those of the hypothalamus-pituitary-adrenal gland axis.

¹ This work was funded by grants from the National Health and Medical Research Council of Australia and Monash University.

² Correspondence: Alan Tilbrook, Department of Physiology, Building 13F, Monash University, Clayton, Victoria 3800, Australia. FAX: 61 3 9905 2547; alan.tilbrook{at}med.monash.edu.au

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