Localization of metabotropic glutamate receptors mGluR1alpha and mGluR2/3 in the cat retina

The distribution of metabotropic glutamate receptors 1alpha (mGluR1alpha) and mGluR2/3 in the cat retina was studied through the use of preembedding immunocytochemistry for light and electron microscopy. Staining for mGluR1alpha in the outer plexiform layer was seen in numerous punctate structures that were identified as rod spherules. Cone pedicles remained unlabeled. A number of amacrine and ganglion cell somata also were stained with processes ramifying throughout the inner plexiform layer. These processes were postsynaptic to cone bipolar cells in both sublaminae, where they comprised one but not both of the postsynaptic elements at dyad contacts. Immunostaining for mGluR2/3 was observed in horizontal cells as well as in numerous amacrine and displaced amacrine cells. Labeled amacrine processes were postsynaptic to cone bipolar cells in both sublaminae but, similar to mGluR1alpha, comprised only one of the postsynaptic elements. Staining for mGluR2/3 also was seen in amacrine processes postsynaptic to rod bipolar terminals; these processes were identified as belonging to type A17 amacrine cells. The distribution patterns indicate that both mGluR1alpha and mGluR2/3 are positioned for postsynaptic function, whereas mGluR1alpha also may contribute to the presynaptic regulation of glutamate release from rod photoreceptors.