Light-evoked excitatory and inhibitory synaptic inputs to ON and OFF α ganglion cells in the mouse retina

Bipolar cell and amacrine cell synaptic inputs to alpha ganglion cells (alphaGCs) in dark-adapted mouse retinas were studied by recording the light-evoked excitatory cation current (DeltaI(C)) and inhibitory chloride current (DeltaI(Cl)) under voltage-clamp conditions, and the cell morphology was revealed by Lucifer yellow fluorescence with a confocal microscope. Three types of alphaGCs were identified. (1) ONalphaGCs 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 alpha-cell-like dendritic field similar to 180 - 350 mum stratifying near 70% of the inner plexiform layer (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 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 preganglion cells, our data suggest that the light responses of each type of alphaGCs are mediated by different sets of bipolar cells and amacrine cells. This detailed physiological analysis complements the existing anatomical results and provides new insights on the functional roles of individual synapses in the inner mammalian retina.