Melatonin: Failure of pharmacological doses to induce testicular atrophy in the male golden hamster

Effects of graded doses of melatonin on reproductive function of the male golden hamster were studied. Daily injections of melatonin at 17:00 h brought about complete testicular regression when the hamster received 10, 25, and 100 ug melatonin; larger doses of 1,000 ug or 2,500 ug had a partial or no effect on testicular or accessory sex organ weights. The dose response relationship of testicular and accessory sex organ weights to melatonin treatment somewhat resembled an inverted bell-shaped curve. Plasma, LH and prolactin concentrations followed the pattern of the testicular and accessory sex organ weights; small doses of melatonin suppressed plasma, LH and prolactin while larger doses had no suppressive effects. These results indicate that melatonin can exert a reproductive inhibitory effect if injected with small, but not with larger doses.