Rod signals are routed through specific Off cone bipolar cells in primate retina

Abstract

Adapting between scotopic and photopic illumination involves switching the routing of retinal signals between rod and cone-dominated circuits. In the daytime, cone signals pass through parallel On and Off cone bipolar cells, that are sensitive to increments and decrements in luminance, respectively. At night, rod signals are routed into these cone-pathways via a key glycinergic interneuron, the AII amacrine cell (AII-AC). In primates, it is not known whether AII-ACs contact all Off-bipolar cell types indiscriminately, or whether their outputs are biased towards specific Off-bipolar cell types. Here, we show that the rod-driven glycinergic output of AII-ACs is strongly biased towards a subset of macaque Off-cone bipolar cells. The Off-bipolar types that receive this glycinergic input have sustained physiological properties and include the Off-midget bipolar cells, which provide excitatory input to the Off-midget ganglion cells (parvocellular pathway). The kinetics of the glycinergic events are consistent with the involvement of the 1 glycine receptor subunit. Taken together with results in mouse retina, our findings point towards a conserved motif whereby rod signals are preferentially routed into sustained Off signaling pathways. Significance StatementVisual signals pass through different retinal neurons depending on the prevailing level of illumination. Under night-time light levels, signals from rods pass through the AII amacrine cell, an inhibitory interneuron that routes rod signals into On and Off bipolar cells to detect increments and decrements in light intensity, respectively. Here, we show in primate retina that the output of AII amacrine cells is strongly biased towards specific Off bipolar cell types, which suggests that rod signals reach the brain via specific neural channels. Our results further our understanding of how visual signals are routed through visual circuits during night-time vision.