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Y. Wen, S.C. Mangel; A Circadian Clock in the Fish Retina Regulates Rod–Cone Coupling . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4541.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: A circadian (24–hr) clock in the fish retina, by increasing dopamine release and activation of dopamine D2–like receptors in the day, regulates rod and cone input to cone horizontal cells (HCs) so that cone input dominates during the day and rod input dominates at night (Wang & Mangel, 1996; Ribelayga et al., 2002, 2004). However, it is unclear how rod input reaches cone HCs at night, because cone HCs receive synaptic contact from cones, but not from rods (Stell & Lightfoot, 1975). Because rods and cones are connected by gap junctions (Raviola & Gilula, 1973), it has been suggested that rod input reaches cone HCs at night via rod–cone gap junctions (Wang & Mangel, 1996; Ribelayga et al., 2002, 2004). We therefore studied whether this is so. Methods: Whole–cell patch clamp recordings from the inner segments of cones in intact neural goldfish retinas were obtained under continuous dark–adapted conditions in the subjective day (CT8–9) and night (CT16–20). Responses to dim lights were measured to assess light and spectral sensitivity and the cones injected with biocytin or neurobiotin (both 0.3%) to assess the extent of photoreceptor cell coupling. Results: The light responses of cones in the subjective night (n=11) had a threshold that was 1.5 log units lower and a longer time–to–peak, than those of cones in the subjective day (n=14). Spectral sensitivity measurements distinguished three cone types in the subjective day, but all cones were most sensitive to green wavelength light in the subjective night. Following injection into single cones, tracer was observed on average in 52.2 +/– 16.0 (SEM) rods and 5.8 +/– 2.9 cones during the subjective night (n=5), but remained in the cone injected during the subjective day (n=5). Moreover, application of spiperone (10 µM), a D2–like receptor antagonist, during the subjective day (n=7) shifted the light responses and tracer coupling to closely match those observed during the subjective night. Conclusions: These findings indicate that the circadian clock in the fish retina increases the coupling of rod–cone gap junctions at night so that rod input reaches cone HCs. The findings also indicate that endogenous activation of dopamine D2–like receptors during the subjective day, due to the circadian–mediated increase in dopamine release during the subjective day, closes rod–cone gap junctions so that only cone input reaches cone HCs.
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