Pattern of synaptic excitation and inhibition upon direction-selective retinal ganglion cells

The distributions of excitatory and inhibitory synapses upon the dendritic arbor of a direction-selective retinal ganglion cell were compared by triple-labeling techniques. The dendrites were visualized by confocal microscopy after injection of Lucifer yellow. Excitatory inputs from bipolar cells were located by using antibodies against kinesin II, a component of synaptic ribbons. Inhibitory inputs were identified by antibodies against gamma-aminobutyric acid-A receptors. The combined images were examined by visual inspection and by formal, automated analyses, in a search for anisotropies that might contribute to a directional preference of the ganglion cell. Within the limits of our analysis, none was found. If an anatomic specialization underlies direction selectivity, it appears to lie in the geometry and spatial positioning of the neurons afferent to the ganglion cell and/or the microcircuitry among its afferent synapses.