Apparent dualistic effects of twice daily melatonin administration on reproductive organs in male golden hamsters.

Daily subcutaneous injections of various dosages of melatonin were given early in the light phase to groups of adult male golden hamsters maintained in a long photoperiodic cycle (14 h light). This injection was followed by a second daily injection of melatonin late in the light phase at a concentration sufficient to induce reproductive organ atrophy. An afternoon injection of melatonin resulted in reproductive organ atrophy in animals that received 0, 1 or 5 microgram of melatonin in the morning. However, morning injections of 25 or 100 microgram of melatonin partially prevented the inhibitory effects of the afternoon injection of melatonin, and a morning injection of 500 microgram of melatonin prevented the reduction of testicular weights in animals which concurrently received the afternoon injection of melatonin. Serum testosterone levels were depressed in animals that received afternoon injections of melatonin regardless of the dosage of melatonin given in the morning. The greatest concentration of melatonin given in the morning (500 microgram) did not completely prevent the depressive effect of the afternoon injection of melatonin on serum testosterone. Since melatonin-induced gonadal atrophy is not prevented by a morning injection of melatonin of less than 25 microgram, the data suggest that there is a lower limit of the concentration of melatonin when given in the morning that is incapable of "down-regulating" the melatonin receptors to the atrophy-inducing injection of the indole given later in the photoperiod.