Synaptic localization of NMDA receptor subunits in the rat retina

The distribution and synaptic clustering of N-methyl-D-aspartate (NMDA) rec eptors were studied in the rat retina by using subunit specific antisera. A punctate immunofluorescence was observed in the inner plexiform layer (IPL ) for all subunits tested, and electron microscopy confirmed that the immun oreactive puncta represent labeling of receptors clustered at postsynaptic sites. Double labeling of sections revealed that NMDA receptor clusters wit hin the IPL are composed of different subunit combinations: NR1/NR2A, NR1/N R2B, and in a small number of synapses NR1/NR2A/NR2B. The majority of NMDA receptor clusters were colocalized with the postsynaptic density proteins P SD-95, PSD-93, and SAP 102. Double labeling of the NMDA receptor subunit sp ecific antisera with protein kinase C (PKC), a marker of rod bipolar cells, revealed very little colocalization at the rod bipolar cell axon terminal. This suggests that NMDA receptors are important in mediating neurotransmis sion within the cone bipolar cell pathways of the IPL. The postsynaptic neu rons are a subset of amacrine cells and most ganglion cells. Usually only o ne of the two postsynaptic processes at the bipolar cell ribbon synapses ex pressed NMDA receptors. In the outer plexiform layer (OPL), punctate immuno fluoresence was observed for the NR1C2' subunit, which was shown by electro n microscopy to be localized presynaptically within both rod and cone photo receptor terminals. J. Comp. Neurol. 420:98-112, 2000. (C) 2000 Wiley-Liss, Inc.