The effect of light on melatonin secretion in the cultured pineal glands of Anolis lizards.

Melatonin, a hormone produced by the pineal gland, is important for regulating circadian rhythms in many animals. Light at night causes an acute suppression of melatonin in nearly all vertebrate species. A previous study found that light failed to suppress melatonin in the lizard Anolis carolinensis. This is a surprising result given that the Anolis pineal gland is intrinsically photosensitive, is a key pacemaker controlling locomotor activity, and can be directly entrained to a light-dark cycle. To find out if the lack of photic suppression is widespread in the Anolis genus, we investigated the acute effects of light on melatonin secretion in five different species of Anolis using flow-through tissue culture. We administered a two-hour pulse of bright light to isolated pineal glands during the night. The results show photic suppression of melatonin in all five Anolis species, but the suppression is weak relative to that seen in other vertebrates. Moreover, Anolis species differ in the magnitude of the effect. These findings are discussed in the context of vertebrate pineal evolution and the ecology of Anolis lizards. Given their extensive phylogenetic and ecological divergence, Anolis lizards provide a promising system for investigating the ecology and evolution of circadian organization.