An alternative pathway for rod signals in the rodent retina: Rod photoreceptors, cone bipolar cells, and the localization of glutamate receptors

In the mammalian retina, extensive processing of spatiotemporal and chromat ic information occurs. One key principle in signal transfer through the ret ina is parallel processing. Two of these parallel pathways are the ON- and OFF-channels transmitting light and dark signals. This dual system is creat ed in the outer plexiform layer, the first relay station in retinal signal transfer. Photoreceptors release glutamate onto ON- and OFF-type bipolar ce lls, which are functionally distinguished by their postsynaptic expression of different types of glutamate receptors, namely ionotropic and metabotrop ic glutamate receptors. in the current concept, rad photoreceptors connect only to rod bipolar cells (ON-type) and cone photoreceptors connect only to cone bipolar cells (ON- and OFF-type). We have studied the distribution of (RS)-alpha-aminoJhydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutama te receptor subunits at the synapses in the auter plexiform layer of the ro dent retina by immunoelectron microscopy and serial section reconstruction. We report a non-classical synaptic contact and an alternative pathway for rod signals in the retina. Rod photoreceptors made synaptic contact with pu tative OFF-cone bipolar cells that expressed the AMPA glutamate receptor su bunits GluR1 and GluR2 on their dendrites. Thus, in the retina of mouse and rat, an alternative pathway for rod signals exists, where rod photorecepto rs bypass the rod bipolar cell and directly excite OFF-cone bipolar cells t hrough an ionotropic sign conserving AMPA glutamate receptor.