Glutamate receptors at bipolar synapses in the inner plexiform layer of primate retina: light microscopic analysis

At least 10 different types of bipolar cells have been distinguished in the primate retina. The axon terminals of these cells stratify in distinct strata in the inner plexiform layer and are involved in parallel pathways to distinct types of ganglion cells. Ionotropic glutamate receptor (GluR) subunits also show a stratified distribution in the inner plexiform layer. Here, we investigated whether different types of bipolar cells are associated with different types of ionotropic glutamate receptors in the inner retina of a New World primate, the common marmoset Callithrix jacchus. Vertical cryostat sections through central retina were double labeled with immunohistochemical markers for bipolar cell types and with antibodies to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits GluR1 to 4, kainate receptor subunits GluR6/7, and the NR1C2' subunit of the N-methyl-D-aspartate (NMDA) receptor. The axon terminals of bipolar cell types were reconstructed from confocal sections, and the colocalized immunoreactive puncta were quantified. For all bipolar cell types, immunoreactive puncta for the AMPA receptor subunits GluR2, 2/3, and 4 were colocalized at highest densities, whereas GluR1-immunoreactive puncta were expressed at very low densities. The kainate receptor subunits GluR6/7 were predominantly associated with diffuse bipolar (DB6) and rod bipolar cells. The NMDA receptor subunit NR1C2' was specifically colocalized with flat midget and DB3 axons. These findings suggest that rod and cone bipolar cell types contribute to multiple but distinct glutamate receptor pathways in primate retina.