NMDA receptor subunit gene expression in the rat brain: a quantitative analysis of endogenous mRNA levels of NR1Com, NR2A, NR2B, NR2C, NR2D and NR3A.

Electrophysiological recordings have shown NMDA receptors to be heterogenous structures capable of responding to selected antagonists and agonists in multiple ways. This diversity in functional response has led investigators to conclude that these channels are comprised of unique combinations of receptor subunits which determine a cell's functional NMDA-signature [H. Meguro, H. Mori, K. Araki, E. Kushiya, T. Kutsuwada, M. Yamazaki, T. Kumanishi, M. Arakawa, K. Sakimura, M. Mishina, Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs, Nature (London) 357 (1992) 70-74; T. Ishii, K. Moriyoshi, H. Sugihara, K. Sakurada, H. Kadotani, M. Yokoi, C. Akazawa, R. Shigemoto, N. Mizuno, S. Nakanishi, Molecular characterization of the family of the N-methyl-d-aspartate receptor subunits, J. Biol. Chem. 268 (1993) 2836-2843; K.A. Wafford, C.J. Bain, B. Le Bourdelles, P.J. Whiting, J.A. Kemp, Preferential co-assembly of recombinant NMDA receptors composed of three different subunits, NeuroReport 4 (1993) 1347-1349; T. Priestley, P. Laughton, J. Myers, B. Le Bourdelles, J. Kerby, P.J. Whiting, Pharmacological properties of recombinant human N-methyl-d-aspartate receptors comprising NR1a/NR2A and NR1a/NR2B subunit assemblies expressed in permanently transfected mouse fiberblast cells, Mol. Pharmacol. 48 (1995) 841-848; P.H. Seeburg, N. Burnashev, G. Kohr, T. Kuner, R. Sprengel, H. Monyer, The NMDA receptor channel: molecular design of a coincidence detector, Recent Prog. Horm. Res. 50 (1995) 19-34; A.L. Buller, D.T. Monagahan, Pharmacological heterogeneity of NMDA receptors: characterization of NR1a/NR2D heteromers expressed in Xenopus oocytes, Eur. J. Pharmacol. 320 (1997) 87-94]. In situ hybridization and immunocytochemical studies have shown that there is a spatio-temporal level of expression throughout the brain for each of the receptor subunits with some regions showing a strong preference for a particular subunit. Although these studies collectively show that there are regional differences with respect to NMDA receptor subunit expression in the brain, it has not been determined at what level(s) these genes are expressed or whether each region displays a unique NMDA-subunit signature. The present study was undertaken to examine the level of gene expression for the NR1, NR2A, NR2B, NR2C, NR2D and NR3A receptor subunits in isolated regions of rat brain using the nuclease protection assay. Results show that each of the brain regions examined expresses all six NMDA receptor subunits. The level of message expression for NR1 greatly exceeded that of the other subunits combined, with values ranging from 67-88% of the total subunit gene expression. The relative proportions of the other subunits (NR2A-D and NR3A) varied widely, suggesting that NMDA receptor composition is unique to each region of the brain.