Synaptic circuitry mediating light-evoked signals in dark-adapted mouse retina

Light-evoked excitatory cation current (DeltaI(C)) and inhibitory chloride current (DeltaI(Cl)) of rod and cone bipolar cells and AII amacrine cells (AIIACs) were recorded from slices of dark-adapted mouse retinas, and alpha ganglion cells were recorded from flatmounts of dark-adapted mouse retinas. The cell morphology was revealed by Lucifer yellow fluorescence with a confocal microscope. DeltaI(C) of all rod depolarizing bipolar cells (DBC(R)s) exhibited similar high sensitivity to 500nm light, but two patterns of Deltal(Cl) were observed-with slightly different axon morphologies. At least two types of cone depolarizing bipolar cells (DBC(C)s) were identified: one with axon terminals ramified in 70-85% of IPL depth and DBCR-like DeltaI(C) sensitivity, and 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 analyzed 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 retinas 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. Three types of alpha ganglion cells (alphaGCs) were identified. (1) ONGCs exhibits no spike activity in darkness, increased spikes in light, sustained inward DeltaI(C), sustained outward DeltaI(Cl), of varying amplitude, and large soma (20-25 mum in diameter) with an alpha-cell-like dendritic field about 180-350 mum stratifying near 70%, of the IPL depth. (2) Transient OFFalphaGCs (tOFFalphaGCs) exhibit no spike activity in darkness, transient increased spikes at light offset, small sustained outward DeltaI(C) in light, a large transient inward DeltaI(C) at light offset, a sustained outward DeltaI(Cl), and a morphology similar to the ONalphaGCs except for that their dendrites stratified near 30% of the IPL depth. (3) Sustained OFFalphaGCs (sOFFalphaGCs) exhibit maintained spike activity of 5-10 Hz in darkness, sustained decrease of spikes in light, sustained outward DeltaI(C), sustained outward DeltaI(Cl), and a morphology similar to the tOFFalphaGCs. By comparing the response thresholds and dynamic ranges of alphaGCs with those of the pre-ganglion cells, our data suggest that the light responses of each type of alphaGCs are mediated by different sets of bipolar cells and amacrine cells. (C) 2004 Elsevier Ltd. All rights reserved.