These studies examined whether nitric oxide (NO) mediates opioid suppression of testicular steroidogenesis, Adult male rats were treated with various combinations of a NO synthase (NOS) inhibitor (N-G-nitro-L-arginine methyl ester; NAME), a NO donor (isosorbide dinitrate; ISDN), an opioid agonist (morphine), and an opioid antagonist (naltrexone), Serum LH and testosterone and testicular interstitial fluid (TIF) testosterone concentrations were then measured, Inhibition of NO production by NAME reversed morphine-suppressed testosterone secretion; treatment with the NO donor, ISDN, reversed naltrexone-stimulated testosterone secretion, NAME did not alter morphine's effects on LH secretion and attenuated morphine's suppression of hCG-stimulated testosterone secretion, indicating that these effects occur directly in the testes and are not dependent on LH secretion, Even though these effects suggested possible interactions between NO and opioid systems, no additive or synergistic effects were found with suppressive combinations of morphine and ISDN, or with stimulatory combinations of naltrexone and NAME at doses that had little effect on testosterone secretion when given alone. These results indicate that opioid and NO exert independent effects on testicular steroidogenesis through separate pathways or mechanisms and that NO does not mediate opioid-induced testicular suppression.
