Purchase this article with an account.
A.F. Wiechmann, M.J. Vrieze, R.K. Dighe, Y. Hu; Melatonin Directly Influences Rod Photoreceptor Sensitivity to Light via a Specific Melatonin Receptor: A Transgenic Xenopus laevis Model . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3267.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: Retinal circadian signals may have a prominent 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-coupled receptors. Based on our previous observations that melatonin receptors are expressed in retinal photoreceptor cells, we tested the hypothesis that melatonin directly increases photoreceptor responses via melatonin receptors. Methods: Transgenic frogs were generated using a DNA construct containing a Xenopus opsin promoter (XOP) driving expression of a melatonin Mel1c receptor-green fluorescent protein (GFP) fusion protein (XOP-Mel1c-GFP). Normal tadpoles and control transgenic tadpoles that were created using the Xenopus opsin promoter-GFP construct (XOP-GFP) were analyzed in an identical manner as the receptor fusion protein transgenic frogs. Electroretinogram (ERG) analysis in response to melatonin was performed on transgenic and normal tadpoles, and the eyes were examined by fluorescent microscopy. Results: XOP-Mel1c-GFP transgenic frogs demonstrated GFP fluorescence in rod photoreceptor inner segments throughout the retina, indicating the rod-specific expression of the Mel1c-GFP fusion protein. ERG analysis of normal and transgenic tadpole eyes showed that 1-100 nM melatonin increased the a-wave (photoreceptor response) and b-wave (cone and inner retina response) amplitudes in transgenic frogs that over-expressed Mel1c receptor-GFP fusion proteins in rod photoreceptors. Control transgenic (XOP-GFP) and normal frogs exhibited very modest ERG responses to 100 nM melatonin treatment. The effect of melatonin on a-wave and b-wave amplitudes in transgenic frogs was dose-dependent, with ERG responses occurring at physiological concentrations. Conclusions: This study provides direct evidence that melatonin, acting through the Mel1c receptors on rod photoreceptor membranes, increases the responsiveness of rod photoreceptors to light. This supports our hypothesis that melatonin, synthesized by photoreceptors at night, 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, thus facilitating dark adaptation and other diurnal events that occur in the retina.
This PDF is available to Subscribers Only