DISTRIBUTION OF GLUTAMATE-RECEPTOR SUBTYPES IN THE VERTEBRATE RETINA

The distribution of glutamate receptor subunit/subtypes in the vertebr ate retina was investigated by immunocytochemistry using anti-peptide antibodies against AMPA (GluR1-4), kainate (GluR6/7) and metabotropic (mGluR1 alpha) receptors. All receptor subtypes examined are present i n the mammalian retina, but they are distributed differentially. GluR1 is present in the inner plexiform layer as well as amacrine and gangl ion cell bodies. GluR2 is present mainly in the outer plexiform layer and bipolar cells. An anti-GluR2/3 antibody labels both plexiform laye rs and various cell bodies in the inner nuclear layer and the ganglion cell layer. GluR4 is present on Muller glial cells. In the goldfish r etina, GluR2 immunoreactivity is prominent in the Mb type of ON-bipola r dells, including the dendrites and the large synaptic terminal. The putative dendritic localization is surprising, because no depolarizing conductance increase induced by glutamate is thought to be present in these cells. An AMPA receptor at a presynaptic terminal;is also unusu al, and probably provides feedback control of glutamate release. GluR6 /7 is most widespread in the retina, being present in horizontal, bipo lar, amacrine and ganglion cells. Ion channels composed of GluR6 are n ow known to be phosphorylated by protein kinase A, resulting in curren t potentiation. This property and our present observation together sug gest that the glutamate receptors previously studied electrophysiologi cally by others in horizontal cells may contain GluR6. mGluR1 alpha is found mostly in the inner plexiform layer; its localization partially overlaps with that of the inositol trisphosphate receptor in the reti na. Our results suggest that, in the retina, glutamate receptor subtyp es may be expressed in selective cell types according to their specifi c functions.