Microcircuits for night vision in mouse retina.

Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod --> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod --> rod bipolar --> AII amacrine --> cone bipolar --> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).