Reproductive consequences of melatonin in mammals

Melatonin (25 μg) injected every afternoon into intact male hamsters induces a collapse of the reproductive system and a drop in both pituitary and plasma levels of luteinizing hormone and prolactin. Injections of melatonin every weekday in the afternoon is nearly as effective as daily injections in inhibiting reproduction and associated hormones. However, when melatonin is administered even less frequently the effectiveness of the antigonadotrophin decreases. An injection of a large dose of melatonin in the morning (when the animals are insensitive to melatonin) prevents the afternoon injections of melatonin from inhibiting reproductive physiology; presumably, melatonin administered in the morning down regulates the melatonin receptors. Thrice daily injections (25 μg each) of melatonin (at 1000, 1300 and 1600 h) into pinealectomized male hamsters induces atrophy of the gonads and depresses plasma testosterone levels. The action of thrice daily injections is prevented by either anterior or total hypothalamic cuts. These findings suggest that melatonin normally acts in the brain rather than on peripheral organs to suppress reproduction.