Naltrexone does not reverse the inhibitory effect of chronic restraint on gonadotropin secretion in the intact male rat

Abstract

There is considerable evidence suggesting that endogenous opioids may play an important role in acute stress-induced decreases in luteinizing hormone (LH) release. Studies were undertaken to analyze the role of endogenous opioids in chronic stress-induced decrease in circulating LH and follicle-stimulating hormone (FSH). Chronic restraint (6 h daily over 4 days) evoked a decrease in circulating LH and FSH. Naltrexone treatment, (2 mg/kg three times daily) during the 4 days of restraint, caused an increase in plasma concentrations of LH and FSH, and antagonized the LH suppressory effect of morphine (10 mg/kg) administration. Despite this, naltrexone treatment was ineffective in preventing the inhibitory effect of chronic restraint stress on circulating LH and FSH. Chronic restraint also induced a decrease in hypothalamic LH-releasing hormone (LHRH) content in saline-treated rats. On the contrary, in naltrexone-treated rats, chronic restraint evoked an increase in hypothalamic LHRH content. Thus endogenous opioids and chronic stress seem to act by different mechanisms on the hypothalamic LHRH neuron. In unstressed orchidectomized rats, naltrexone administration did not modify circulating LH, but increased plasma concentrations of LH in acutely restrained rats. These data suggest that endogenous opioids may mediate gonadotropin secretion during acute stress, but not during chronic stress.