Expression of NMDA and High-affinity Kainate Receptor Subunit mRNAs in the Adult Rat Retina

The expression patterns of nine genes encoding the N-methyl-d-aspartate (NMDA) receptor subunits NR1 and NR2A, NR2B, NR2C and NR2D, and the high-affinity kainate receptor subunits KA1, KA2, GluR6 and GluR7, were studied in the adult rat retina by in situ hybridization. Hybridization with [³⁵S]dATP-labelled oligonucleotide probes revealed the expression of four of the NMDA receptor subunits (NR1, NR2A, NR2B and NR2C) and three of the high-affinity kainate receptor subunits (KA2, GluR6 and GluR7) in the retina. The NMDA receptor subunit NR2D and the high-affinity kainate receptor subunit KA1 could not be detected. In the ganglion cell layer, virtually every ganglion cell or displaced amacrine cell expressed the receptor subunits NR1, NR2A, NR2B, NR2C, KA2 and GluR7. The GluR6 subunit was expressed in a more restricted manner in the ganglion cell layer. In the inner nuclear layer, the receptor subunits NR1 and KA2 were homogeneously distributed, and therefore are most likely expressed by all cell types in this layer. The GluR6, NR2A, NR2B and NR2C subunits were expressed by subsets of amacrine cells. Labelling for NR2C was also found above the middle of the inner nuclear layer, corresponding to the location of bipolar cell somata. The GluR7 subunit was expressed by most amacrine and bipolar cells. These findings suggest that NMDA and high-affinity kainate receptor subunits could be present at a majority of glutamatergic retinal synapses.