Two types of ON direction-selective ganglion cells in rabbit retina

Direction-selective ganglion cells (DSGCs) respond with robust spiking to image motion in a particular direction. Previously, two main types of DSGCs have been described in rabbit retina: the ON-OFF DSGCs respond to both increases and decreases in illumination, whereas the ON DSGCs respond only to increases in illumination. In this study, we show that there are two distinct types of ON DSGCs, which can be separated by differences in their receptive-field properties, dendritic morphology and tracer-coupling pattern. While both types show robust direction-selectivity, one type responds to increases in illumination with sustained firing, whereas the other responds with relatively transient firing. The two types of ON DSGCs also have distinct dendritic morphologies: the sustained cells give rise to shorter and more numerous terminal dendrites, which are distributed throughout the dendritic field forming a space-filling lattice. In addition, the transient ON DSGCs, but not the sustained ON DSGCs, show tracer-coupling to a mosaic of amacrine cells when filled with Neurobiotin. Both types of ON DSGCs have been encountered in previous studies but were not recognized as distinct types. We propose that the two types also differ in their central projections, with only the sustained cells projecting to the medial terminal nucleus (MTN) of the accessory optic system (AOS).