AMPA-selective glutamate receptors play a major role in glutamatergic neurotransmission in the retina and are expressed in a variety of neuronal subpopulations. In the present study, immunocytochemical techniques were used to visualize the distribution of GluR2 and GluR4 subunits in the cat retina. Results were compared with previous localizations of GluR1 and GluR2/3. Staining for GluR2 was limited to a small number of amacrine and ganglion cells whereas GluR4 staining was present in A-type horizontal cells, many amacrine cells including type AII amacrine cells, and the majority of the cells in the ganglion cell layer. Analysis of synaptic relationships in the outer plexiform layer showed the GluR4 subunit to be concentrated at the contacts of cone photoreceptors with A-horizontal cells. In the inner plexiform layer, both GluR2 and GluR4 were postsynaptic to cone bipolar cells at dyad contacts although GluR2 staining was limited to one of the postsynaptic elements whereas GluR4 immunoreactivity was often seen in both postsynaptic elements. Unlike GluR2, GluR4 was also postsynaptic to rod bipolar cells where it could be visualized in processes of AII amacrine cells. The data indicate that GluR3 and GluR4 subunits are colocalized in a number of cell types including A-type horizontal cells, AII amacrine cells, and alpha ganglion cells, but whether they are combined in the same multimeric receptors remains to be determined.

The distribution of AMPA-selective glutamate receptor subunits was studied in the cat retina using antisera against GluR1 and GluR2/3. Both antisera were localized in postsynaptic sites in the outer plexiform layer (OPL) as well as the inner plexiform layer (IPL). Immunoreactivity for GluR1 was seen in a subpopulation of OFF cone bipolar cells and a number of amacrine and ganglion cells. Within the IPL, processes staining for GluR1 received input from OFF and ON cone bipolar cells but not from rod bipolars. Labeling for GluR2/3 was seen in horizontal cells, an occasional cone bipolar cell, and numerous amacrine and ganglion cells. In the IPL, GluR2/3 staining was postsynaptic to cone bipolar cells in both sublaminae. AII amacrine cells which receive rod bipolar input were also labeled for GluR2/3. With both antisera, staining was limited to a single member of the bipolar dyad complex, providing morphological evidence for functional diversity in glutamatergic pathways.