2-[125I]-melatonin binding sites in the central nervous system and neural retina of the frog Rana perezi: regulation by light and temperature.

The objective of the present study is to test daily and seasonal changes in 2-[125I]-Melatonin ([125I]-Mel) binding in different brain areas and the retina of the frog Rana perezi as well as the possible effect of light and temperature on melatonin receptors. During the day-night cycle, binding of [125I]-Mel showed a clear rhythm in the optic tectum, diencephalon, telencephalon, and neural retina, the binding being higher in the light phase than in the dark phase. By contrast, melatonin receptors did not show any significant summer-winter differences in any of the four tissues studied. In the neural retina, but not in the brain, exposure of frogs to 24 h darkness for one week leads to significantly less [125I]-Mel binding than 24 h light exposure. This darkness-induced reduction of [125I]-Mel binding is not due to a desensitisation of binding sites by high melatonin levels. Thermal acclimation to either 5 or 22 degrees C for one month did not change the affinity (Kd) and density (Bmax) of [125I]-Mel binding sites either in the brain or the retina. All these results indicate that there is a daily rhythm in melatonin receptors in the frog brain and retina, and that the light/dark cycle can drive this rhythm in [125I]-Mel binding in the retina. Temperature apparently did not modify [125I]-Mel binding in frogs.