In Vivo Evidence for a Direct Effect of Naloxone on Testicular Steroidogenesis in the Male Rat

Abstract It has been suggested that endogenous opioid peptides (EOP) exert paracrine or autocrine effects in the testes. To assess this hypothesis, we examined whether naloxone, by blocking the effects of EOP, influenced serum testosterone levels, apart from its effects on LHRH/LH, in the intact male rat. We found that naloxone increased serum LH and testosterone levels over essentially the same time course and produced dosedependent increases in serum testosterone levels even though LH levels were maximally elevated at all doses. These data are not consistent with the view that naloxone exerts its effects on testosterone exclusively by altering LHRH/LH release. Additional, perhaps more definitive, evidence of a direct effect of naloxone on testosterone's biosynthesis was provided by our observations that 1) naloxone generated increases in serum testosterone levels in male rats in which the naloxone-induced surge in LH was blocked by nembutal; and 2) intratesticular injections of naloxone increased serum ...