Melatonin administered in the afternoon decreases next-day luteinizing hormone levels in men - Lack of antagonism by flumazenil

The role of melatonin in the regulation of human reproduction remains uncle ar. In the present study, we examined the influence of exogenous melatonin on pulsatile luteinizing hormone (LW), diurnal rhythm of testosterone, and endogenous melatonin profile in six healthy young adult males. To test the hypothesis that the effect of melatonin on LH or testosterone secretory pat terns may be mediated through the benzodiazepine-(BNZ) gamma-amino-butyric acid (GABA) receptor complex, a benzodiazepine receptor antagonist (Flumaze nil) was administered. The study design comprised four 10-h (4:00 PM-2:00 A M) testing periods. During each experimental period, subjects were given an oral dose of placebo, or 3 mg melatonin or 10 mg flumazenil, at 5:00 PM, i n a randomized, double-blind, partially repeated Latin square design in the following combinations: placebo-placebo, placebo-melatonin, flumazenil-pla cebo, and flumazenil-melatonin. The following day, serum samples were obtai ned every 20 min between 4:00 PM and 2:00 AM in a controlled light-dark env ironment for the determination of LH and melatonin levels. Serum testostero ne concentrations were determined every 20 min between 7:00 and 8:00 AM and 7:00 and 8:00 PM. A significant decrease in mean serum LH levels (p < 0.02 ) was observed in the melatonin-treated groups as compared with placebo-flu mazenil groups. There was no change in LH pulse frequency, testosterone lev els, or in melatonin onset time and amplitude. No additional effect of flum azenil on LH or testosterone levels was observed. These data indicate that an evening melatonin administration decreases the next-day LH secretion in normal adult males without altering testosterone l evels or the endogenous nocturnal melatonin secretory pattern. This effect of melatonin is not mediated through the benzodiazepine-GABA receptor compl ex.