Direct Modulation of Rod Photoreceptor Responsiveness through a Mel1c Melatonin Receptor in Transgenic Xenopus laevis Retina

PURPOSE. Retinal circadian signals may have a role in maintaining the normal function and health of photoreceptors. Melatonin is an output of the retinal circadian oscillator and provides nocturnal signaling that is mediated through specific G-protein- coupled receptors. Melatonin receptors are expressed in retinal photoreceptor cells, and this study was undertaken to test the hypothesis that melatonin directly increases photoreceptor responses through melatonin receptors. METHODS. Transgenic Xenopus laevis frogs were generated using a DNA construct containing a Xenopus opsin promoter driving expression of a melatonin Me1 1c receptor-green fluorescent protein (GFP) fusion protein (XoP-MEL 1c -GFP). Electroretinogram (ERG) analysis on transgenic and normal tadpole eyes was performed in response to melatonin treatment, and the eyes were subsequently examined by confocal microscopy and GFP immunocytochemistry. RESULTS. XOP-MEL 1c -GFP transgenic frogs demonstrated GFP immunoreactivity in rod photoreceptor inner segments throughout the retina, indicating the rod-specific expression of the Mel 1c -GFP fusion protein. ERG analysis of transgenic tadpole eyes showed that 1 to 100 nM melatonin increased the a-and b-wave amplitudes. Control transgenic (XOP-GFP) and normal frogs exhibited only modest ERG responses to 100-nM melatonin treatment. The effect of melatonin on a- and b-wave amplitudes in XOP-MEL 1c -GFP transgenic frogs was dose dependent, with ERG responses occurring at physiological concentrations. CONCLUSIONS. The results suggest that melatonin, acting through Mel 1c receptors on rod photoreceptor membranes, directly stimulates the responsiveness of rod photoreceptors to light. This supports the hypothesis that melatonin acts both as an intracrine and paracrine circadian signal of darkness, and binds to specific receptors in photoreceptors and other retinal cells to increase visual sensitivity.