Central melatonin receptors: implications for a mode of action

The influence of melatonin on circadian and photoperiodic functions in numerous species is well documented. It is known that the effect of melatonin on circadian rhythmicity is mediated via the suprachiasmatic nucleus (SCN), the biological clock of the brain. It is not known however where the photoperiodic effects of melatonin are mediated. Evidence from brain lesioning and melatonin implant studies point to a site in or near the medial hypothalamus. In contrast to these studies, melatonin receptors have been reported in widespread areas of the brain, the pituitary and in peripheral tissues. The characteristics of the reported melatonin receptors vary widely between studies and consequently no definitive description of a physiologically relevant melatonin receptor has received universal recognition. This review marshals recent evidence for the localization and characterization of the melatonin receptor and discusses these findings in the context of the known effects of the hormone in different species.