One unique subtype of retinal ganglion cell is the direction selective (DS) cell, which responds vigorously to stimulus movement in a preferred direction, but weakly to movement in the opposite or null direction. Here we show that the application of the GABA receptor blocker picrotoxin unmasks a robust excitatory OFF response in ON DS ganglion cells. Similar to the characteristic ON response of ON DS cells, the masked OFF response is also direction selective, but its preferred direction is opposite to that of the ON component. Given that the OFF response is unmasked with picrotoxin, its direction selectivity cannot be generated by a GABAergic mechanism. Alternatively, we find that the direction selectivity of the OFF response is blocked by cholinergic drugs, suggesting that acetylcholine release from presynaptic starburst amacrine cells is crucial for its generation. Finally, we find that the OFF response is abolished by application of a gap junction blocker, suggesting that it arises from electrical synapses between ON DS and polyaxonal amacrine cells. Our results suggest a novel role for gap junctions in mixing excitatory ON and OFF signals at the ganglion cell level. We propose that OFF inputs to ON DS cells are normally masked by a GABAergic inhibition, but are unmasked under certain stimulus conditions to mediate optokinetic signals in the brain.
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