Light-evoked current responses in rod bipolar cells, cone depolarizing bipolar cells and AII amacrine cells in dark-adapted mouse retina

Abstract

Light-evoked excitatory cation current (ΔI_C) and inhibitory chloride current (ΔI_Cl) of rod and cone depolarizing bipolar cells (DBC_Rs and DBC_Cs) and AII amacrine cells (AIIACs) in dark-adapted mouse retinal slices were studied by whole-cell voltage-clamp recording techniques, and the cell morphology was revealed by Lucifer yellow fluorescence with a confocal microscope. ΔI_C of all DBC_Rs exhibited similar high sensitivity to 500 nm light, but two patterns of ΔI_Cl were observed in DBC_Rs with slightly different axon morphology. At least two types of DBC_Cs were identified: one with axon terminals ramified in 70–85% of the depth of the inner plexiform layer (IPL) and DBC_R-like ΔI_C sensitivity, whereas the other with axon terminals ramified in 55–75% of IPL depth and much lower ΔI_C sensitivity. The relative rod/cone inputs to DBCs and AIIACs were analysed by comparing the ΔI_C and ΔI_Cl thresholds and dynamic ranges with the corresponding values of rods and cones. On average, the sensitivity of a DBC_R to the 500 nm light is about 20 times higher than that of a rod. The sensitivity of an AIIAC is more than 1000 times higher than that of a rod, suggesting that AIIAC responses are pooled through a coupled network of about 40 AIIACs. Interactions of rod and cone signals in dark-adapted mouse retina appear asymmetrical: rod signals spread into the cone system more efficiently than cone signals into the rod system. The mouse synaptic circuitry allows small rod signals to be highly amplified, and effectively transmitted to the cone system via rod–cone and AIIAC–DBC_C coupling.