Cells sensitive to the orientation of edges are ubiquitous in visual systems, and have been described in the vertebrate retina, yet the synaptic mechanisms that generate orientation selectivity in the retina are largely unknown. Here, we analyze the synaptic mechanisms that generate selective responses to vertically and horizontally oriented stimuli in rabbit retinal ganglion cells. The data indicate that the excitatory and inhibitory inputs to orientation-selective ganglion cells are rendered orientation selective within the presynaptic circuitry. In accordance with previous extracellular recordings, presynaptic GABAergic inhibition is critical to generate orientation selectivity, and we show that it includes lateral inhibition of glutamatergic bipolar cells and serial inhibitory connections between GABAergic and glycinergic amacrine cells. Despite very similar spiking properties, vertically and horizontally selective ganglion cells (VS-GCs and HS-GCs, respectively) show marked differences in their underlying synaptic mechanisms. Both cell types receive glutamatergic inputs via non-NMDA (AMPA/kainate) and NMDA receptors, while VS-GCs receive additional excitation mediated by glycinergic disinhibition. A striking difference between these cells is that during nonpreferred simulation, excitation is suppressed and direct glycinergic inhibition is increased in HS-GCs, whereas for VS-GCs, both excitatory and inhibitory inputs are suppressed. Thus, orientation selectivity is generated presynaptically both by modulation of bipolar cell output and by serial inhibitory connections between amacrine cells. Minimal circuit models are proposed that account for these observations.
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