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Sabrina Asteriti, Claudia Gargini, Luigi Cervetto, Lorenzo Cangiano; Rod-Cone Coupling Studied in Single Mouse Cones: Impact and Regulation. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2492.
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© ARVO (1962-2015); The Authors (2016-present)
Rod-cone gap junctions (GJs) in mammals are thought of being an escape route for rod signals when incoming light is greater than a few sparse photons and the rod-rod bipolar synapse is saturated. While indirect evidence suggest that this pathway is dynamically regulated by light and circadian rhythm, the only direct functional tests so far, done in Macaque, have found rod-cone coupling to be static. We optimized patch recordings from mouse cones to investigate the impact and regulatory latitude of rod-cone coupling.
Cones (n=55) were recorded with perforated patch-clamp in retinal slices from wt mice, and from animals lacking connexin isoform 36 (Cx36-KO). Photovoltage responses were evoked with green/UV stimulation protocols, consisting of dim (16.6 ph/µm2) and bright (1570 or 3140 ph/µm2) flashes, designed to dissect and quantify rod input through differences in light sensitivity, kinetics of recovery from saturation and spectral preference. Control recordings in rods were also performed.
Most wt cones expressed rod-like features, including: (1) responses to dim flashes (> 1 mV in 23 of 42); (2) responses to bright flashes followed by slow plateaus and a transient suppression of dim flash responses; (3) slow recovery of dim flash responses and plateaus upon return to darkness after prolonged rod-saturating light background. While some cones expressed rod features at the beginning of the recording, most cones acquired them progressively starting from seal formation, with a time course of minutes. Irrespective of their intrinsic spectral type (G- or UV-preferring, as determined with rod-saturating pre-flashes) for dim flashes cones preferred G, in line with rods (n=15). Medium-large dim flash responses were observed in both M and S/M cones, while, of two S cones recorded, only one had a small response. Dim and bright flash responses had different reversal potentials, consistent with an origin in separate electrotonic compartments. The involvement of GJs was confirmed with the specific antagonist meclofenamic acid (100 µM), which abolished dim flash responses. In contrast to wt animals, in Cx36-KO mice cones did not appear to be able to couple to rods.
Mouse cones can couple remarkably well to rods and acquire a rod-like functional phenotype. Coupling strength varies among cones and is able to undergo rapid up-regulation triggered by perturbation of the intracellular milieu.
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