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Nange Jin, Christophe Ribelayga; Melatonin Modulates Rod Photoreceptor Electrical Coupling in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5006.
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© ARVO (1962-2015); The Authors (2016-present)
In the vertebrate retina, melatonin, whose levels are under the control of a retinal clock and peak at night, plays a key role in the regulation of circadian physiology. In the melatonin-proficient CBA/Ca mouse, a circadian clock controls the state of electrical coupling between rod photoreceptors, so that rod coupling is weak during the day and strong at night (Jin and Ribelayga, 2013 ARVO abstract). Here we tested 1) whether the state of rod coupling is sensitive to melatonin, and 2) whether a circadian rhythm in rod coupling exists in the retina of the melatonin-deficient C57/Bl6 mouse strain.
Perforated patch clamp recordings from single rod inner segments were performed in intact mouse retinas maintained by superfusion at 32°C. The kinetics and reliability of the rod light responses were recorded using brief dim full-field stimuli, and the receptive field was mapped using a computer-generated stimulus projected onto the retina by a Lucivid camera. Recorded cells were labeled by iontophoresis of the biotinylated tracer Neurobiotin. Melatonin was dissolved and applied in the perfusion system.
In CBA/Ca retinas, application of physiological doses (3-300 pM) of melatonin during the subjective day, when endogenous melatonin levels are low and rod coupling is weak, mimicked the nighttime state. That is, the rod light responses became more reliable compared to the daytime control, indicating an increase in rod electrical coupling; the space constant, a measure of the receptive field size of the recorded rod, increased by 2- to 3-fold; and tracer coupling was observed in many neighboring rods while it was consistently restricted to the recorded cell under control conditions. In the C57/Bl6 mouse, the rod light response reliability and kinetics, receptive field size and tracer coupling size were similar to the daytime state observed in the CBA/Ca mouse, during both day and night. Yet, physiological doses of melatonin increased rod electrical and tracer coupling in C57/Bl6 retinas, regardless of time of day.
Our data provide electrophysiological and tracer coupling evidence that melatonin increases rod electrical coupling in the mouse retina. Together with the constitutively weak rod coupling observed in the C57Bl/6J retina, our results suggest that melatonin is a nighttime effector of the circadian clock that controls rod electrical coupling.
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