LIGAND AFFINITIES AT RECOMBINANT N-METHYL-D-ASPARTATE RECEPTORS DEPEND ON SUBUNIT COMPOSITION

The ligand preferences of recombinant NR1 homomeric and NR1-NR2 heteromeric NMDA receptors were examined by homogenate binding assay. The binding affinities for most ligands were similar to those reported for native NMDA receptors. The order of affinity for [H-3]glutamate was NR1-NR2B > NR1-NR2A approximate to NR1-NR2D > NR1-NR2C > NR1. NMDA had approximately equal affinity for all heteromeric types (K-i approximate to 5 mu M), but the competitive antagonists CGS 19755 (cis-4-(phosphonomethyl)piperidine-2-carboxylic acid) and CGP 39653 (D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) displayed the affinity order NR1-NR2A > NR1-NR2B > NR1-NR2D > NR1-NR2C. Binding of [H-3]CGP 39653 could only be detected at the NR1-NR2A receptor type (K-d approximate to 6 nM). The glycine site antagonist [H-3]5,7-dichlorokynurenate bound with good affinity to all recombinant receptors (K-d approximate to 50-100 nM), while glycine exhibited an affinity order of NR1-NR2C >> NR1 = NR1-NR2B = NR1-NR2D > NR1-NR2A. The channel-site ligand [H-3]MK 801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] 5,10-imine hydrogen maleate) showed the affinity ranking NR1-NR2A = NR1-NR2B >> NR1 > NR1-NR2C = NR1-NR2D. Thus the ligand binding affinities of recombinant NMDA receptors is dependent on their subunit composition. The NR1-NR2A, NR1-NR2B, NR1-NR2C and NR1-NR2D receptors may account for the antagonist-preferring, agonist-preferring, cerebellar, and medial thalamic subtypes of native NMDA receptors, respectively.