Expression of glutamate receptor genes in the mammalian retina: the localization of GluR1 through GluR7 mRNAs.

Seven distinct cDNAs encoding functional subunits of the AMPA/kainate-type glutamate receptors have been recently cloned. This in situ hybridization study was done to determine which subunits are expressed in the retina and, where possible, which neurons express them. Hybridization of 35S-UTP-labeled cRNA probes with transverse sections revealed that mRNAs for all seven receptor subunits (GluR1-GluR7) are expressed in both cat and rat retinas. GluR1 and GluR2 produced labeling over the entire inner nuclear layer (INL) and ganglion cell layer (GCL). GluR3-GluR7 have more limited distributions, indicative of expression by only a subset of neurons. All of the subunits are expressed by the cells at the inner edge of the INL, where amacrine cells reside, yet the layers with the horizontal, bipolar, and ganglion cells contain different subsets of subunits. These findings suggest that these glutamate receptor subunits are employed at many of the retinal synapses, including the photoreceptor input to the outer plexiform layer and the bipolar cell's contacts with the processes at the INL. It is also possible that some glial cells in the INL express some of the subunits. Since different combinations of GluR1-GluR3 have been shown to play an important role in the calcium permeability in response to glutamate, we investigated whether single cells coexpressed those subunits. By hybridizing adjacent semithin (1 micron) sections of the cat retina with probes for GluR1-GluR3, it was possible to observe coexpression of all three subunits, or of pairs of these subunits, in cells within the INL and GCL.