Phase-relationship and mutual effects between circadian rhythms of ocular melatonin and dopamine in the pigeon

In order to study the mechanisms of ocular circadian rhythms in the pigeon, we measured melatonin and dopamine simultaneously from the eye using in vivo microdialysis. In experiment 1, the phase relationship between circadian rhythms of ocular melatonin and dopamine under light-dark cycles (LD) and continuous dim light (LLdim) was examined. Under LD, melatonin was high during the dark and low during the light. On the other hand dopamine was high during the light and low during the dark. These rhythms with the anti-phase relationship were maintained after the birds were transferred from LD to LLdim. In experiment 2, effects of a single light pulse on melatonin and dopamine rhythms were examined. A light pulse at CT18 rapidly suppressed melatonin release to the daytime level, whereas it rapidly increased dopamine release to the daytime level. The light pulse also affected the phases of melatonin and dopamine rhythms, inducing phase advances of both rhythm without changing the anti-phase relationship before the light pulse. In experiment 3, effects of an intraocular injection of dopamine or melatonin on their circadian rhythms were examined. A dopamine injection during the subjective night suppressed melatonin release and induced a light-pulse type phase shift in both melatonin and dopamine rhythms. On the other hand, a melatonin injection during the subjective day suppressed dopamine release and induced a dark-pulse type phase shift. These results are compatible with either one or two oscillator models, but the interaction between melatonin and dopamine is, in either case considered as an important mechanism regulating ocular circadian rhythms of the pigeon.