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 → rod depolarizing bipolar cell → AII amacrine cell → cone bipolar cells → ganglion cells. A possible alternative pathway involves gap junctions linking the rods and cones, the circuit being rod → cone → cone bipolar cells → 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 directionselective 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.