CIRCADIAN CLOCK FUNCTIONS LOCALIZED IN XENOPUS RETINAL PHOTORECEPTORS

A circadian oscillator that regulates visual function is located somew here within the vertebrate eye. To determine whether circadian rhythmi city is generated by retinal photoreceptors, we isolated and cultured photoreceptor layers from Xenopus retina. On average, 94% of the viabl e cells in these preparations were rod or cone photoreceptors. Photore ceptor layers produced melatonin rhythmically, with an average period of 24.3 hr, in constant darkness. The phase of the melatonin rhythm wa s reset by in vitro exposure of the photoreceptor layers to cycles of either light or quinpirole, a D2 dopamine receptor agonist. These data indicate that other parts of the eye are not necessary for generation or entrainment of retinal circadian melatonin rhythms and suggest tha t rod and/or cone photoreceptors are circadian clock cells.