An alternative pathway for signal flow from rod photoreceptors to ganglion cells in mammalian retina

Rod signals in the mammalian retina are thought to reach ganglion cells over the circuit rod arrow right rod depolarizing bipolar cell arrow right AII amacrine cell arrow right cone bipolar cells arrow right ganglion cells. A possible alternative pathway involves gap junctions linking the rods and cones, the circuit being rod arrow right cone arrow right cone bipolar cells arrow right ganglion cells. It is not clear whether this second pathway indeed relays rod signals to ganglion cells. We studied signal flow in the isolated rabbit retina with a multielectrode array, which allows the activity of many identified ganglion cells to be observed simultaneously while the preparation is stimulated with light and/or exposed to drugs. When transmission between rods and rod depolarizing bipolar cells was blocked by the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), rod input to all On-center and briskly responding Off-center ganglion cells was dramatically reduced as expected. Off responses persisted, however, in Off-center sluggish and On-Off direction-selective ganglion cells. Presumably these responses were generated by the alternative pathway involving rod-cone junctions. This APB-resistant pathway may carry the major rod input to Off-center sluggish and On-Off direction-selective ganglion cells.