MOLECULAR DISSECTION OF NATIVE MAMMALIAN FOREBRAIN NMDA RECEPTORS CONTAINING THE NR1 C2 EXON - DIRECT DEMONSTRATION OF NMDA RECEPTORS COMPRISING NR1, NR2A, AND NR2B SUBUNITS WITHIN THE SAME COMPLEX

The subunit compositions of the NR1 C2 exon-containing N-methyl-D-aspa rtate (NMDA) receptors of adult mammalian forebrain were determined by using a combination of immunoaffinity chromatography and immunoprecip itation studies with NMDA receptor subunit-specific antibodies. NMDA r eceptors were solubilised by sodium deoxycholate, pH 9, and purified b y anti-NR1 C2 antibody affinity chromatography. The purified receptor subpopulation showed immunoreactivity with anti-NR1 C2, anti-NR1 N1, a nti-NR1 C2', anti-NR2A, and anti-NR2B NMDA receptor antibodies. The NR 1 C2-receptor subpopulation was subjected to immunoprecipitation using anti-NR2B antibodies and the resultant immune pellets analysed by imm unoblotting where anti-NR1 C2, anti-NR1 C2', anti-NR2A, and anti-NR2B immunoreactivities were all found. Quantification of the immunoblots s howed that 46% of the NR1 C2 immunoreactivity was associated with the NR2B subunit. Of this, 87% (i.e., 40% of total) were NR1 C2/NR2B recep tors and 13% (6% of total) were NR1 C2/NR2A/NR2B, thus identifying the triple combination as a minor receptor subset. These results demonstr ate directly, for the first time, the coexistence of the NR2A and NR2B subunits in native NMDA receptors. They show the coexistence of two s plice forms of the NR1 subunit, i.e., NR1 C2 and NR1 C2', in native re ceptors and, in addition, they imply an NMDA receptor subpopulation co ntaining four types of NMDA receptor subunit, NR1 02, NR1 C2', NR2A, a nd NR2B, which, in accord with molecular size determinations, predicts that the NMDA receptor is at least tetrameric. These results are the first quantitative study of NMDA receptor subtypes and demonstrate mol ecular heterogeneity for both the NR1 and the NR2 subunits in native f orebrain NMDA receptors.