OPIOID MODULATION OF THE EFFECTS OF REPEATED STRESS ON ACTH, CORTISOL, PROLACTIN, AND GROWTH-HORMONE IN PIGS

Prepubertal gilts (n = 16) were restrained with a nose snare for 15 min each day over 9 days. At the beginning of the first and last nose snare, the animals were also injected IV with 1 mg/kg of naloxone. Blood samples were taken before and after restraint at 15-min intervals and plasma assayed for ACTH, cortisol, prolactin, and GH. The initial restraint led to significant increases in ACTH, cortisol, prolactin, and GH concentrations. There was no evidence of a reduction in the magnitude of endocrine responses with repeated restraint. Indeed, the response of GH was more apparent after the final restraint. In the absence of the restraint, naloxone elevated cortisol and ACTH concentrations. Naloxone, given with the first restraint, enhanced the increase in ACTH, cortisol, and prolactin. Naloxone, given during the last restraint, inhibited the increase in GH, but had less effect upon cortisol and prolactin concentrations than during the initial restraint. Gilts genetically selected for a high cortisol response to ACTH injections had a higher basal cortisol concentration and a higher cortisol response to restraint than gilts selected for a low cortisol response to ACTH. However, there were no differences between these groups in ACTH, prolactin, or GH concentrations, or in any endocrine response to naloxone. Endogenous opioids can inhibit pituitary-adrenocortical responses and enhance GH responses of pigs to stress. Measures of cortisol concentrations are poor predictors of prolactin and GH responses to stress.