eNeuro. 2020 Nov 13:ENEURO.0432-20.2020. doi: 10.1523/ENEURO.0432-20.2020. Online ahead of print.
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). AII-ACs also provide On-pathway driven crossover inhibition to Off cone bipolar cells under photopic conditions. In primates, it is not known whether all Off-bipolar cell types receive functional inputs from AII-ACs. Here, we show that select Off-cone bipolar cell types receive significantly higher levels of On-pathway driven glycinergic input than others. The rise and decay kinetics of the glycinergic events are consistent with involvement of the α1 glycine receptor subunit, a result supported by a higher level of GLRA1 transcript in these cells. The Off-bipolar types that receive glycinergic input have sustained physiological properties and include the flat midget bipolar cells, which provide excitatory input to the Off-midget ganglion cells (parvocellular pathway). Our results suggest that only a subset of Off-bipolar cells have the requisite receptors to respond to AII-AC input. Taken together with results in mouse retina, our findings suggest a conserved motif whereby signal output from AII-ACs is preferentially routed into sustained Off-bipolar signaling pathways.Significance Statement Visual signals pass through different retinal neurons depending on the prevailing level of illumination. The AII-ACs are a key inhibitory neuron involved in signaling during daytime and night-time vision. Here, we show that only select Off bipolar cell types are equipped with receptors to receive signals from AII-ACs. These results suggest that rod signals may reach the brain via specific output channels. Our results further our understanding of how visual signals are routed through retinal circuits during night-time and day-time vision.