The relationship between ocular melatonin and dopamine rhythms in the pigeon: effects of melatonin inhibition on dopamine release

Our previous study has shown that the phases of circadian rhythms of ocular melatonin and dopamine are always opposite and intraocular melatonin injec tion suppresses dopamine release. Therefore, it is possible that dopamine r hythms result from inhibitory action of melatonin. We have examined this po ssibility in the following experiments. In the first experiment effects of continuous light on melatonin and dopamine release were examined. The data indicated that continuous light exposure resulted in loss of circadian rhyt hmicity of melatonin and dopamine by suppressing melatonin and enhancing do pamine levels throughout the day. To further examine the effects of Light i n the second experiment, 2 h light pulse was applied during the night, then temporal changes of melatonin and dopamine release were studied. The light pulse rapidly suppressed melatonin release, whereas it rapidly increased d opamine release. These changes occurred within 30 min in both melatonin and dopamine. However, the recovery after the cessation of the Light stimulus was slower in melatonin than dopamine. In the third experiment it was teste d if dopamine release was increased by lowering melatonin release with an i ntraocular injection of the D2 agonist, quinpirol. Although quinpirol stron gly inhibited melatonin release independently of the time of injection, dop amine did not always increase by the inhibition of melatonin. These results indicate that ocular dopamine rhythms are not simply produced by melatonin inhibitory action. (C) 1999 Elsevier Science B.V. All rights reserved.