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

Light-evoked excitatory cation current (DeltaI(C)) and inhibitory chloride current (DeltaI(Cl)) of rod and cone depolarizing bipolar cells (DBC(R)s and DBC(C)s) and All 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. DeltaI(C) of all DBCRs exhibited similar high sensitivity to 500 nm light, but two patterns of DeltaI(Cl) were observed in DBCRS with slightly different axon morphology. At least two types of DBC(C)s were identified: one with axon terminals ramified in 70-85% of the depth of the inner plexiform layer (IPL) and DBCR-like DeltaI(C) sensitivity., whereas the other with axon terminals ramified in 55-75% of IPL depth and much lower DeltaI(C) sensitivity. The relative rod/cone inputs to DBCs and AIIACs were analysed by comparing the DeltaI(C) and DeltaI(Cl) thresholds and dynamic ranges with the corresponding values of rods and cones. On average, the sensitivity of a DBCR 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-DBCC coupling.