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Andrey V. Dmitriev, Stuart C. Mangel; Rod-Cone Electrical Coupling Affects Cone Light Responses Significantly More Than Rod Light Responses in Mammalian Retinas. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1184.
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Rods and cones are anatomically connected by gap junctions that are opened at night and closed in the day by the retinal circadian clock (Ribelayga et al., 2008; Ribelayga & Mangel, 2010). In this work we computationally investigated the consequences of this daily rhythm in rod-cone coupling.
In the model, rods and cones were represented by a simple electrical circuit that included a light-sensitive part (outer segment) and a light-insensitive part, which were connected by a pair of resistors, one for a hemichannel on the rod side of the gap junction and the other for a hemichannel on the cone side. We assumed that one cone could be coupled to 2 to 30 rods, but that each rod makes contact with only one cone (Tsukamoto et al., 2001). The electrical responses of rods and cones to brief light flashes over a range (from dim near-threshold scotopic to bright photopic) of intensities were simulated and the effects of changes in the conductance of the rod-cone gap junctions on these light responses were analyzed. A wide range of coupling conductances, including biophysically realistic values for closed, partly open and completely open gap junctions, were used in the model. The model was calculated with a custom made computer program.
Increases in rod-cone electrical coupling affected the light responses of both photoreceptor types, but the magnitude of the effect depended on the number of rods connected to each cone. When the rod/cone ratio was approximately 10, an increase in rod-cone coupling significantly changed cone light response amplitudes, so that they were similar to rod response amplitudes, while the rod responses changed very little. The largest coupling-mediated increase in cone light response amplitude (>20 mV) occurred to light intensities in the mesopic range. The coupling-mediated decrease in rod response amplitude did not exceed 3 mV. The much stronger effect of increased coupling on cones vs. rods was even more striking when the relative changes in the light response amplitudes of rods and cones were compared.
Computational analysis shows that the circadian clock-induced increase in rod-cone coupling at night has a dramatic effect on cone light responses, but not on rod light responses, so that cones and rods responded similarly to light stimuli in the scotopic and mesopic ranges at night. The main reason for the difference in how increased coupling affects rods and cones is the fact that rods significantly outnumber cones in most mammalian retinas.
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