Pharmacological properties of recombinant human N-methyl-D-aspartate receptors comprising NR1a/NR2A and NR1a/NR2B subunit assemblies expressed in permanently transfected mouse fibroblast cells.

The pharmacological properties of two recombinant human N-methyl-D-aspartate (NMDA) receptor subtypes, comprising either NR1a/NR2A or NR1a/NR2B subunits permanently transfected into mouse L(tk-) cells, have been compared using whole-cell voltage-clamp electrophysiology. Glutamate was a full agonist at both receptors, having a modestly but statistically significant (p < 0.002) higher affinity for the NR2B- than the NR2A-containing receptor (microscopic Kd [mKd] = 0.76 and 0.43 microM, respectively). In comparison to glutamate, NMDA, quinolinic acid, and cis-2,3-piperidinedicarboxylic acid were partial agonists at both receptor subtypes. Maximal amplitude currents resulted when glutamate-site agonists were combined with either glycine or D-serine; both of these amino acids were, therefore, defined as full agonists at the glycine site. Glycine had an approximately 10-fold higher affinity (p < 0.0001) for NR2B- than for NR2A-containing receptors (mKd = 0.057 and 0.53 microM, respectively). D-Cycloserine, (+)-(3R)-3-amino-1-hydroxypyrrolidin-2-one, (+)-cis-(4R)-methyl-(3R)-amino-1-hydroxypyrrolidin-2-one, and 1-aminocyclobutanecarboxylic acid also had higher affinities for the NR2B-containing receptor but were partial agonists, at both receptor subtypes, unlike glycine. Agonist-evoked whole-cell currents were antagonized by D-(-)-2-amino-5-phosphonopentanoic acid, cis-4-(phosphonomethyl)piperidine-2-carboxylic acid, and 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, all of which had slightly, but statistically significant, higher affinities (2.2-, 2.8-, and 5.5-fold, respectively) for the NR2A-containing receptor. Responses were also antagonized by the glycine-site antagonists 7-chlorokynurenic acid, 7-chloro-4-hydroxy-3-(3-phenoxy)phenylquinolin-2-(1H)-one, and (+/=)-4-(trans)-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino- 1,2,3,4- tetrahydroquinoline. The atypical NMDA antagonist ifenprodil showed the largest separation in functional affinity (IC50 values, 0.6 and 175 microM at NR2B- and NR2A-containing receptors, respectively). These experiments demonstrate the usefulness of permanently transfected L(tk-) cells for electrophysiological studies of recombinant NMDA receptor function and provide the first detailed functional pharmacological analysis of human NMDA receptor subtypes.