Effects of melatonin on reproductive and accessory reproductive organs in male rats

Effects of daily injections of melatonin on male rat reproductive and accessory reproductive organs were studied. The weight of the prostate was decreased by treatment with a high dose of melatonin (8.0 mg/kg, s.c., injection time; 17:00) once daily for 30 d. The weights of the other accessory reproductive organs and testes were not influenced by melatonin. Lower doses (0.8, 2.4, 4.8 mg/kg), had no effect. After the successive treatment with melatonin (8.0 mg/kg), the testosterone levels in testes and in serum and the conversion rate of [3H]testosterone to [3H]dihydrotestosterone in the prostate were not influenced. The activity of acid phosphatase and the uptake of [3H]testosterone by the prostate, in contrast, were significantly decreased after the successive treatment with melatonin. These data suggest that melatonin may have a direct inhibitory action on male rat prostate, though only at a high dose.