DISTRIBUTION OF THE EXCITATORY AMINO-ACID RECEPTOR SUBUNITS GLUR2(4) IN MONKEY HIPPOCAMPUS AND COLOCALIZATION WITH SUBUNITS GLUR5-7 AND NMDAR1

Ionotropic excitatory amino acid (EAA) receptors are divided pharmacol ogically into three categories termed NMDA, AMPA/kainate, and high aff inity kainate receptors. Each of these receptor subtypes is composed o f a specific subset of subunits termed GluR1-4 (AMPA/kainate), GluR5-7 , KA1-2 (high affinity kainate), and NMDAR1, 2 A-D (NMDA). Although co localization of NMDA and non-NMDA receptors has been previously demons trated electrophysiologically in rat, comprehensive analyses of subuni t specific colocalization patterns have not been possible until the ad vent of appropriate antibodies. The present study investigates such im munocytochemical colocalization of several EAA receptor subunits withi n individual cells as well as dendritic spines in the monkey hippocamp us. Double-label immunohistochemical experiments using antibodies whic h are specific for GluR2(4), GluR5-7, and NMDAR1 demonstrated that vir tually all projection neurons in each subfield of the hippocampus cont ain subunits from the AMPA/kainate, kainate, and NMDA receptor familie s. In addition, confocal microscopy has demonstrated that individual s pines may contain subunits representative of multiple EAA receptor fam ilies. Furthermore, detailed regional, cellular, and ultrastructural d istribution patterns of the EAA receptor subunits GluR2 and GluR4 in m onkey hippocampus are presented based on the use of a monoclonal antib ody (mAb), 3A11, which was generated against the putative extracellula r N-terminal domain of GluR2. Since this antibody recognizes only GluR 2 in Western blots, and GluR2 as well as GluR4 in fixed transiently tr ansfected cells, it has been designated anti-GluR2(4). Immunocytochemi cal labeling with mAb 3A11 revealed pyramidal cell somata and dendrite s in each field of the hippocampus, as well as granule cells and polym orphic hilar cells in the dentate gyrus. Small cells with the morpholo gic characteristics of astroglia were also immunolabeled for GluR2(4) within the alveus and fimbria. Immunoreactivity at the ultrastructural level was localized to postsynaptic densities on dendritic spines and shafts and within the somatodendritic cytoplasm in all major hippocam pal regions, as well as in a subset of dentate granule cell axons with in the messy fiber projection.