OPIOID-INDUCED SUPPRESSION OF RAT TESTICULAR FUNCTION

The effects of opioids on testicular function were assessed in the rat through measurements of serum testosterone levels, testicular interst itial fluid (TIF) formation and TIF testosterone levels after morphine and opioid antagonist (naloxone, naltrexone) treatment. Serum and TIF levels of testosterone were significantly decreased 1 to 6 h after mo rphine (10 mg/kg) injection, and TIF volumes were decreased 2-3 h afte r injection morphine. Each of these decreases was dose-related. In con trast to the effects of morphine, the opioid antagonist naloxone incre ased TIF testosterone but did not alter TIF volumes. Moreover, the opi oid antagonist naltrexone totally blocked morphine's effects on both t estosterone secretion and TIF volume, suggesting that morphine's testi cular effects were mediated by naltrexone-sensitive opioid receptors i n the testes. The possible role of morphine-induced reductions in gona dotropin secretion in morphine's testicular effects was also examined. Morphine suppressed testosterone secretion and TIF volumes after pret reatment with human chorionic gonadotropin, which reverses morphine's suppression of luteinizing hormone (LH). Our results, therefore, indic ate that morphine exerts effects on testicular function that are indep endent of its effects on LH. They furthermore support the hypothesis t hat both endogenous and exogenous opioids disrupt two major aspects of testicular function: Testosterone secretion and TIF formation. Becaus e of the role of TIF in maintaining testicular function, our results s uggest that opioid-induced changes in testosterone secretion into TIF and TIF formation may, at least in part, explain the well-established effects of opioids on reproductive endocrinology and function in the m ale.