Melatonin receptor distribution in the brain and retina of a lizard, Anolis carolinensis

Melatonin binding sites were identified in the brain and retina of the lizard Anolis carolinensis using in vitro autoradiography. Radioactive labeling was observed in areas which receive primary, secondary, and tertiary visual input: the superficial layers of the optic tectum, lateral geniculate nucleus, nucleus rotundus, dorsal ventricular ridge, and striatum. Other areas that demonstrated binding included the left medial habenular nucleus, the interpeduncular nucleus, medial cortex, dorsal cortex, mammillary nucleus, and septum. In the retina, melatonin binding was localized in the inner plexiform layer. Radioactive melatonin binding to the optic tectum was reduced in the presence of a nonhydrolyzable cyclic GMP analog, indicating that the melatonin receptor in the brain of this lizard is associated with a G-protein. These results suggest that melatonin receptor binding sites are widely distributed in the forebrain and midbrain of the iguanid lizard, and are prominent in areas of the nervous system that are associated with visual processing. The highest degree of melatonin binding appeared in the left medial habenular nucleus, interpeduncular nucleus, and dorsal ventricular ridge. This suggests that these brain regions may be important targets for the actions of melatonin, such as its effects on circadian rhythmicity, thermoregulation and photoperiodic reproduction.