DISTRIBUTION AND SYNAPTIC LOCALIZATION OF IMMUNOCYTOCHEMICALLY IDENTIFIED NMDA RECEPTOR SUBUNIT PROTEINS IN SENSORY-MOTOR AND VISUAL CORTICES OF MONKEY AND HUMAN

NMDA receptors are composed of multiple receptor subunit proteins, of which NMDAR1 appears to be a critical component for normal receptor fu nction (Nakanishi, 1992). In this study, quantitative immunocytochemic al methods were used at the light and electron microscopic levels to l ocalize NMDAR1 subunits in the primary motor (M1) and somatic sensory (S1) cortex of monkeys, and in the primary visual cortices (V1) of mon key and human. Three principal features of NMDAR1 subunit organization were examined in detail in the monkey cortex: (1) the laminar and cel lular distribution patterns, relying in part on double-labeling paradi gms with the calcium-binding proteins parvalbumin (PV) and calretinin (CR) as markers for discrete subpopulations of GABAergic interneurons; (2) the codistribution of NMDAR1 subunits with non-NMDA ionotropic re ceptor subunits; (3) a quantitative assessment of the percentages of a symmetrical synapses in layers II/III, IV, and V/VI that were NMDAR1 i mmunoreactive. In monkey M1, S1, and V1, NMDAR1 immunoreactivity was p resent in all layers, localized primarily to large numbers of pyramida l cell somata and proximal apical dendrites, to presumptive spiny stel late cells in layer IV of V1, and to the vast majority (similar to 80- 90%) of PV-immunoreactive cells. By contrast, NMDAR1 immunoreactivity was present in only a very small percentage of the CR-immunoreactive c ells (similar to 6-9%). Colocalization with non-NMDA receptor subunits showed that all cells (100%) that contained G1uR2/3 subunits were als o NMDAR1 immunoreactive. In addition, the complete codistribution of G luR5/6/7 subunits with GluR2/3 subunits suggests, indirectly, that all GluR5/6/7-immunoreactive cells are also NMDAR1 immunoreactive. The la minar and cellular distribution patterns of immunostaining in human V1 were very similar to those in monkey V1. Electron microscopy of monke y sections confirmed an extensive dendritic and synaptic localization of NMDAR1 subunits. Labeling of synapses was present on asymmetrical p ostsynaptic densities associated with both dendritic shafts and spines . In supragranular layers of V1, a greater percentage of asymmetrical synapses were NMDAR1 immunopositive (44%) in comparison to layer IVC b eta (34% ) or deep layers (19%). In contrast, in area 3b of S1, the pe rcentage of labeled synapses was greatest in layer IV (45%) in compari son to superficial (26%) and deep (37%) layers, while in M1, the perce ntages of labeled synapses were similar between superficial (46%) and deep (40%) layers. Taken together, these data indicate that NMDAR1-imm unoreactive cells in neocortex represent a morphologically, functional ly, and neurochemically heterogeneous population. in addition, the NMD AR1-immunopositive synapses represent a major proportion of the asymme trical synapses in primary sensory and motor cortex, and have a distri bution suggesting that in primates, they play a major role in mediatin g a diverse set of excitatory afferents.