The distribution and developmental regulation of NMDA receptor subunit proteins in the outer and inner retina of the rat

In order to investigate whether N-methyl-D-aspartate (NMDA) receptors with distinct pharmacological properties are differentially distributed within the retinal layers, the spatial distribution and temporal regulation of all NMDA receptor subunits was analyzed in parallel on the protein level in the rat retina during development. Immunohistochemistry was performed on retinal sections at different developmental ages between embryonic (E) days 20/21 and the adult stage using specific antibodies against NMDA subunits (NR1, NR2A-D). All NMDA subunits were expressed in the rat retina postnatally but showed different spatial patterns. In particular, and in contrast to previous in situ hybridization studies, labeling of NR2 subunits was observed in horizontal cell bodies and in the outer plexiform layer, indicating that functional NMDA receptors are expressed in this retinal cell type in the rat. Expression of NR2D was restricted to the inner retina and seemed to be involved in neurotransmission within the rod pathway. In the inner plexiform layer (IPL), distinct patterns of labeling were observed for different NMDA subunits. NR1 was found in two bands which can be related to the off- and on-signal pathways, whereas NR2A and NR2B were located in two bands within the off-sublaminae of the IPL. The antibody against NR2C was distributed throughout the whole IPL, and NR2D was expressed exclusively in the innermost part of the IPL where rod bipolar cell terminals terminate. Distinct bands of immunoreactivity in the IPL were observed only from P14 on. In conclusion, there are clear differences in the spatial distribution and temporal expression of NMDA receptor subtypes in the rodent retina. This indicates that specific retinal cells selectively express glutamate receptors composed of different subunit combinations and thus display different pharmacological and kinetic properties.