Expression of NMDA receptor subunit 1 in the rat retina.

N-methyl-D-aspartate receptors (NMDARs) belong to the ionotropic glutamate receptors, which play key roles in neuronal communication in the retina. NMDA receptors are tetrameric protein complexes usually comprising two obligatory NMDA receptor 1 (NR1) subunits and modulatory NMDA receptor 2/3 (NR2/3) subunits. Although the expression patterns of different NMDA receptor subunits have been extensively studied, in this study we focused on NR1 protein expression in the rat retina by immunofluorescence double labeling. We show that NR1 labeling is diffusely distributed in the outer plexiform layer (OPL) and throughout the whole inner plexiform layer (IPL). The NR1-immunoreactivity (IR) was displayed in a variety of cells in the inner nuclear layer (INL) and the ganglion cell layer (GCL). Interestingly, NR1 was expressed in both rod and cone bipolar cells identified by specific bipolar cell markers Chx10, protein kinase C (PKC) and recoverin. All the amacrine cells that we studied, including cholinergic, dopaminergic, GABAergic and glycinergic amacrine cells, were NR1-IR positive. In the ganglion cell layer, NR1-IR was expressed in all cells that were positive for the ganglion cell marker Brn3a. Our study suggests that the NR1 subunit is expressed more widely than was previously appreciated.