Expression of NR2 receptor subunit in rat somatic sensory cortex: Synapticdistribution and colocalization with NR1 and PSD-95

Functional N-methyl-D-aspartate (NMDA) receptors comprise heteromeric combi nations of NR1 and NR2 subunits. In the present study we employed light and electron microscopic immunocytochemistry to study the expression of NR2A a nd NR2B (NR2A/B) protein in somatic sensory cortex of adult rats. To relate this distribution to that of NR1 and to the NMDA receptor anchoring protei n PSD-95, we documented extensive cellular colocalization of NR2A/B with NR 1 at the light microscopic level. In contrast, PSD-95 exhibited little soma tic staining, being restricted mainly to dendrites and neuropil. We employe d postembedding immunocytochemistry to study the ultrastructural expression of NR2A/B. Labeling in neuronal perikarya was associated with rough endopl asmic reticulum and Golgi apparatus; in dendrites, gold particles labeled m icrotubules. The preponderance of labeling was associated with asymmetric s ynapses. Double immunolabeling revealed that NR2 colocalized in many synaps es with NR1 and with PSD-95. Quantitative measurements revealed that densit y of gold particles coding for both NR2 and PSD-95 was highest just inside the postsynaptic membrane. Tangentially along the membrane, gold particles were concentrated at the synaptic specialization. These data provide struct ural evidence in neocortex for heteromeric NMDA receptors anchored at the p ostsynaptic membrane. (C) 1999 Wiley-Liss, Inc.