A2 amacrine cell output synapses to alpha ganglion cells in the mouse retina.

A2 amacrine cells (AIIACs) are the most important amacrine cells (ACs) in the mammalian retina, as they are the "hubs" for merging rod and cone signals in the inner retina and segregate them into ON and OFF ganglion cell (GC) pathways. It is unclear, however, whether AIIACs send their signals to all types of GCs in the mammalian retina. A major reason for this knowledge gap is that it has not been possible to selectively stimulate AIIACs without stimulating other retinal neurons. In this study, we will use a newly-developed mouse retinal preparation in which channelrhodopsin (a ChR2 [H134R]-Venus fusion protein) is selectively expressed in AIIACs via intravitreal injection of AAV serotype 2 (AAV2) carrying the ACAGW-ChR2-Venus construct (Addgene 20,071). We have identified a titer range within which ChR2 expresses ONLY in AIIACs and not in any other cells, yet large and clear ChR2-elicited postsynaptic signals can be observed. We examined effects of ChR2-elicited AIIAC depolarization on four types of alpha GCs, the ON-sustained αRGC (sONαGC), OFF-sustained αRGC (sOFFαGCs), ON-transient αRGC (tONαGCs) and OFF-transient αRGC (tOFFαGCs), and found that AIIAC depolarization mediates the sONαGC and sOFFαGC responses via the AIIAC-cone DBCR/MC (rod/M-cone ON Bipolar Cell) gap junction pathway and the AIIAC-OFFαGC glycinergic pathway, respectively. AIIAC depolarization did not affect the tONαGCs and tOFFαGCs, whose responses might be mediated by other ACs. These experiments demonstrate that AIIAC depolarization mediates the responses of a subpopulation of ON and OFF GC types, suggesting that signals from any single retinal neuron type participate in restricted, cell-type-specific circuitries linked to retinal output pathways.