Melatonin circadian rhythm in the retina of mammals

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated tha t melatonin synthesis occurs also in the retina (and in other organs as wel l) of several vertebrate classes, including mammals. As in the pineal, mela tonin synthesis in the retina is elevated at night and reduced during the d ay. Since melatonin receptors are present in the retina and retinal melaton in does not contribute to the circulating levels, retinal melatonin probabl y acts locally as a neuromodulator. Melatonin synthesis in the retinas of m ammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have b een described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compens ated. The recent cloning of the gene responsible for the synthesis of the e nzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the mel atonin synthetic pathway) will facilitate localizing the cellular site of m elatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal m elatonin is involved in several functions, but its precise role is yet to b e understood.