In vivo microdialysis studies of pineal and ocular melatonin rhythms in birds

Pineal and retinal melatonin has an important role in the control of avian circadian rhythms. In order to study the mechanisms of circadian rhythms of melatonin synthesis in the pineal and in the eye, in vivo microdialysis was applied to these organs. In both pigeons and Japanese quails, pineal and ocular melatonin levels were high during the dark and low during the day under light-dark (LD) cycles. These rhythms persisted under constant dim light (LLdim) conditions indicating the circadian nature of pineal and ocular melatonin release. Light has two effects on melatonin synthesis. One is acute inhibition of melatonin synthesis and the other is entrainment of circadian melatonin rhythms. We have examined photoreceptors mediating these effects in the pigeon. The results have indicated that the eyes are not involved in light-induced suppression and photic entrainment of pineal melatonin release, and pineal photoreceptors themselves are likely to mediate these effects. Concerning ocular melatonin, retinal photoreceptors seem to mediate light-induced suppression and photic entrainment and no evidence supporting mediation of extraretinal photoreceptors was obtained. Because dopamine is implicated in retinal melatonin synthesis, we measured dopamine and melatonin release simultaneously from the eye of pigeon. In contrast to melatonin rhythms, dopamine increased during the day and decreased during the dark. This antiphase relationship between melatonin and dopamine persisted in LLdim, suggesting an interaction between these two rhythms. The results of an intraocular injection of dopamine or melatonin in the phase of melatonin and dopamine rhythms indicated that the interaction is required for maintaining the antiphase relationship between the two rhythms.