Does Melatonin Really Suppress the Testosterone Release in Rat Leydig Cell In Vitro?

Abstract

The effect of melatonin on testosterone release was investigated in Sprague Dawley rats in vivo and in vitro. When plasma testosterone levels were determined in rats 2 weeks after pinealectomy (Px) or in rats given daily melatonin injections (MI), no significant differences were observed compared with those in intact or sham-operated rats (Sham). Although luteinizing hormone (LH) stimulated testosterone release by Leydig cells from intact, Px and MI rats in a dose-dependent manner, there was no significant difference in the magnitude of the response to LH. In addition, neither basal nor LH (5 mIU/ml) -stimulated testosterone release were affected by co-culture of various doses of melatonin (1 nM-10 μM) for 4 h and 12 h. When various doses of LH (10 μIU-100 mIU/ml) were co-administered with 0.5 or 5 μM melatonin, no significant change was observed in testosterone release compared with that in the presence of LH alone. Although forskolin, an adenylate cyclase activator, stimulated testosterone release by Leydig cells from intact rats in a dose-dependent manner, melatonin did not reduce forskolin-stimulated testosterone release in 4 h and 12 h cultures. These results do not support the hypothesis that melatonin directly suppresses LH-stimulated testosterone release by rat Leydig cells.