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.