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 α ganglion cells (αGCs) in dark-adapted mouse retinas were studied by recording the light-evoked excitatory cation current (ΔIC) and inhibitory chloride current (ΔICl) under voltage-clamp conditions, and the cell morphology was revealed by Lucifer yellow fluorescence with a confocal microscope. Three types of αGCs were identified. (1) ONαGCs exhibits no spike activity in darkness, increased spikes in light, sustained inward ΔIC, sustained outward ΔICl of varying amplitude, and large soma (20–25 μm in diameter) withα-cell-like dendritic field ∼180–350 μm stratifying near 70% of the inner plexiform layer (IPL) depth. (2) Transient OFFαGCs (tOFFαGCs) exhibit no spike activity in darkness, transient increased spikes at light offset, small sustained outward ΔIC in light, a large transient inward ΔIC at light offset, a sustained outward ΔICl, and a morphology similar to the ONαGCs except for that their dendrites stratified near 30% of the IPL depth. (3) Sustained OFFαGCs exhibit maintained spike activity of 5–10 Hz in darkness, sustained decrease of spikes in light, sustained outward ΔIC, sustained outward ΔICl, and a morphology similar to the tOFFαGCs. By comparing the response thresholds and dynamic ranges of αGCs with those of the preganglion cells, our data suggest that the light responses of each type of αGCs 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.