Light-sensitive melatonin synthesis by Xenopus photoreceptors after destruction of the inner retina

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Abstract
Several lines of evidence indicate that retinal photoreceptors produce melatonin. However, there are other potential melatonin sources in the retina, and melatonin synthesis can be regulated by feedback from the inner retina. To analyze cellular mechanisms of melatonin regulation in retinal photoreceptors, we have developed an in vitro method for destruction of the inner retina that preserves functional photoreceptors in contact with the pigment epithelium. Eyecups, which include the neural retina, retinal pigment epithelium, choroid, and sclera were prepared. The vitreal surface of the retina in each eyecup was washed sequentially with 1% Triton X-100, water, and culture medium. This lysed the ganglion cells and neurons and glia of the inner nuclear layer, causing the retina to split apart within the inner nuclear layer. The damaged inner retina was peeled away, leaving photoreceptors attached to the pigment epithelium. The cell density of the inner nuclear layer was reduced 94% by this method, but there was little apparent damage to the photoreceptors. Lesioned eyecups produced normal melatonin levels in darkness at night, and melatonin production was inhibited by light. These results indicate that the inner retina is not necessary for melatonin production nor for regulation of photoreceptor melatonin synthesis by light. The lesion method used in this study may be useful for other physiological and biochemical studies of photoreceptors.