Asymmetric distribution of melatonin receptors in the brain of the lizard Anolis carolinensis.

The pineal hormone melatonin may regulate seasonal reproduction and entrainment of circadian rhythms by binding to specific brain receptors. An analysis of melatonin receptor distribution in the lizard brain revealed an asymmetry of melatonin binding in the diencephalon. A high degree of melatonin binding was present in the left habenular nucleus, but no binding was observed in the habenulum of the right brain hemisphere. It is intriguing that the left habenular nucleus, in contrast to the right habenulum, both possesses a high density of melatonin receptors and receives primary photic input from the parietal eye. Similarly, the optic tectum, which receives primary visual input from the retina, is also rich in melatonin receptors. These observations suggest that the left habenulum is under dual control (neuronal and hormonal) of the parietal eye/pineal complex, and that melatonin may play a significant role in neural processing of visual information.