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AF Wiechmann, K Aoki, MJ Vrieze, CR Wirsig-Wiechmann; Immunolocalization of Melatonin Receptor Subtypes in Photoreceptors and Inner Retinal Neurons of Xenopus laevis Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1362.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The hormone melatonin is an output signal of the endogenous circadian clock in retinal photoreceptors, and may act as a paracrine and/or intracrine neurohormone by binding to specific receptors in the eye. The goal of this study was to identify the retinal cells that express the Mel1a and Mel1c receptor subtype proteins, and to confirm the presence of receptor expression in photoreceptor cells by measuring photoreceptor membrane currents in response to applied melatonin. Methods: Sections of adult Xenopus laevis neural retina and retinal pigment epithelium (RPE) were analyzed by immunocytochemistry and confocal microscopy, using antibodies prepared against specific sequences of the Mel1a and Mel1c receptor proteins. Other retinal cell markers that label dopaminergic amacrine cells (TOH) and GABA-ergic (GAD) amacrine cells were used in double-label experiments. To test the hypothesis that functional melatonin receptors are present on photoreceptor cells, rod photoreceptors in retinal slice preparations were analyzed by whole cell patch clamp recording. The cells were voltage clamped at -60 mV and a series of voltage steps were applied in the presence or absence of 100 nM melatonin, or vehicle control solution. Results: The distribution of Mel1a and Mel1c receptor subtype immunoreactivity was similar in some regards, since the two subtypes were co-localized in the inner plexiform layer (IPL). However, The Mel1c showed a much higher intensity of immunoreactivity in the photoreceptor cells, and the Mel1a subtype did not reveal any photoreceptor labeling. The Mel1c antibody, but not the Mel1a, labeled ganglion cell soma. While both receptor subtypes were localized to the outer plexiform layer (OPL), they did not appear to co-localize to the identical cell types. Melatonin reduced the magnitude of the inward current in most rod cells studied, and also suppressed the overall magnitude of the rod membrane current in retinal slices. Conclusion: The Mel1a and Mel1c receptor proteins are present in the Xenopus laevis retina, and their distribution in the photoreceptors and inner retina is very similar to that reported in the human retina, and supports the previous report that melatonin receptor RNA is localized to photoreceptors. The melatonin-induced change in membrane currents in rod cells supports the concept that photoreceptor cells are direct targets for melatonin.
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