Abstract: AIM: To investigate the effect of opioid μ receptor on testosterone and its peripheral mechanism.  METHODS: Levels of testosterone and testosterone synthetase mRNA in testis plasma and testis homogenate, IGF1 protein and mRNA expression in testis homogenate were detected by intratesticular morphine injection in vivo animal model and treatment of specific μ receptor antagonists and agonists in vitro cultured testis. RESULTS:Serum testosterone level was decreased after testis was injected with morphine ［(722.11±121.02) vs. (346.91±75.31) pg/mL; t=7.898, P=0.001］. Testis homogenate testosterone level ［(133.61±16.82) vs. (66.56±14.96) pg/mg protein; t=8.941, P=0.001］ and expression of testosterone synthetase were decreased. Testis IGF1 level ［(7.93±2.13) vs. (2.54±0.74) ng/mg protein, t=7.155, P=0.001］ and IGF1 mRNA ［(3.22±1.12) vs. (1.66±0.55), t=3.751, P=0.001］ expression were decreased in intratesticular morphine injection model. Testosterone level in culture media, IGF1 and expression of testosterone synthetase were decreased after testis was injected with DAMGO in vitro model. And testosterone level, IGF1 and expression of testosterone synthetase were increased after testis was injected with CTOP.CONCLUSION: Morphine acts on opioid μ receptor on the surface of testicular Sertoli cells and inhibits the synthesis of IGF1. It decreases expression of testosterone synthetase Scarb1, Star, 3βHsd, Cyp17α1 and 17βHsd from testicular Leydig cells. which leads to a decrease in testosterone synthesis.

Key words: opioid receptors, testosterone, peripheral mechanism