CLONING, FUNCTIONAL COEXPRESSION, AND PHARMACOLOGICAL CHARACTERIZATION OF HUMAN CDNAS ENCODING NMDA RECEPTOR NR1 AND NR2A SUBUNITS

Using expression cloning, and more recently using polymerase chain rea ction cloning approaches, a family of rat N-methyl-D-aspartate (NMDA) receptor subunit cDNAs has been described (NR1, NR2A, NR2B, NR2C, and NR2D). Here we report cloning and sequencing of cDNAs encoding isoform s of the human NR1 subunit (NR1a, NR1d, and NR1e) that differ at their C-terminal end as a result of alternative splicing and also of a cDNA encoding the human NR2A subunit. The deduced amino acid sequences of the human NR1 subunit isoforms differed from the published rat NR1 sub unit sequences at only eight positions, all of which were N-terminal t o the alternatively spliced domains, The human NR2A subunit deduced am ino acid sequence differed from the published rat NR2A subunit sequenc e at 81 of the 1,464 amino acids, with most of the substitutions being located in the C-terminal half of the subunit. The gene for NR2A has been localised to human chromosome 16. We also report the expression a nd pharmacological characterisation of recombinant human NR1a/NR2A het eromeric receptors in Xenopus oocytes. These receptors had EC(50) valu es of 2.14 and 2.05 mu M for glutamate and glycine, respectively, and an IC50 of 46.8 mu M for Mg2+. Responses were antagonised by D-2-amino -5-phosphonovalerate, L-689,560, pH 6.3, zinc, and MK-801. No modulato ry effect was observed on application of ifenprodil, confirming previo us observations with rat NR1 + NR2A recombinant receptors.