Circadian oscillators in vertebrate retinal photoreceptor cells.

Recent progress in research on retinal circadian rhythmicity is reviewed. Important advances include the discovery that circadian oscillators are present in the retinas of diverse vertebrate species, and evidence that circadian rhythmicity is generated by the photoreceptor cells. Research on the cellular and molecular mechanisms of photoreceptor circadian rhythms has revealed that expression of a subset of genes associated with photoreception, melatonin synthesis and transcriptional control are regulated by a circadian oscillator. Finally, it has been found that cAMP mimics darkness in resetting the phase of the retinal photoreceptor circadian oscillator, suggesting that it may be a component of a transduction pathway for entrainment of the oscillator.