Stress-Induced Suppression of Luteinizing Hormone Concentrations in Wild Baboons: Role of Opiates*

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Abstract
Numerous stressors disrupt male reproductive physiology; previous studies of a population of wild baboons, living freely in a national park in East Africa, indicated that the stress of anesthetization by phencyclidine darting decreased both LH secretion and testicular sensitivity to LH. This study was undertaken to determine the mechanism(s) of the decreased LHsecretion in these animals. Neither stress-induced glucocorticoid nor catecholamine release was responsible, since neither blockade of glucocorticoid secretion with the adrenal steroidogenesis inhibitor metyrapone nor blockade of catecholamine secretion with the sympathetic ganglionic blocking drug chlorisondamine prevented the stress-induced decline in serum LHconcentrations. Administration of the opiate receptor antagonist naloxone (0.5 mg/kg BW), however, not only prevented the decline, but also transiently elevated serum LH concentrations, suggesting that opiates play a role in tonic as well as stressinduced decreases in LH secretion. Administration of a small dose of naloxone (0.03 mg/kg BW) commensurate with occupancy of only fi-opiate receptors slowed the stress-induced decline in LH concentrations, as did administration of the Kreceptor antagonist MR 1452. These data suggest that opiates inhibit LH release via the combined occupancy of both fi- and K-receptors.