EXPRESSION OF GLUTAMATE RECEPTOR GENES IN THE MAMMALIAN RETINA - THE LOCALIZATION OF GLUR1 THROUGH GLUR7 MESSENGER-RNAS

Seven distinct cDNAs encoding functional subunits of the AMPA/kainate- type glutamate receptors have been recently cloned. This in situ hybri dization study was done to determine which subunits are expressed in t he retina and, where possible, which neurons express them. Hybridizati on of S-35-UTP-labeled cRNA probes with transverse sections revealed t hat mRNAs for all seven receptor subunits (GluR1-GluR7) are expressed in both cat and rat retinas. GluR1 and GluR2 produced labeling over th e entire inner nuclear layer (INL) and ganglion cell layer (GCL). GluR 3-GluR7 have more limited distributions, indicative of expression by o nly a subset of neurons. All of the subunits are expressed by the cell s at the inner edge of the INL, where amacrine cells reside, yet the l ayers with the horizontal, bipolar, and ganglion cells contain differe nt subsets of subunits. These findings suggest that these glutamate re ceptor subunits are employed at many of the retinal synapses, includin g the photoreceptor input to the outer plexiform layer and the bipolar cell's contacts with the processes at the INL. It is also possible th at some glial cells in the INL express some of the subunits. Since dif ferent combinations of GluR1-GluR3 have been shown to play an importan t role in the calcium permeability in response to glutamate, we invest igated whether single cells coexpressed those subunits. By hybridizing adjacent semithin (1 mum) sections of the cat retina with probes for GluR1-GluR3, it was possible to observe coexpression of all three subu nits, or of pairs of these subunits, in cells within the INL and GCL.