[3H]DL 105,519 binds with equal high affinity to both assembled and unassembled NR1 subunits of the NMDA receptor

[H-3]MDL 105,519 (((E)-3-(2-phenyl-2-carboxyethenyl)-4,6-dichloro-1[H-3]-indole-2-carboxylic acid) is a novel radioligand which binds with high affinity, K-d = 2.5 nM, to the glycine site of adult rodent forebrain, N-methyl-D-aspartate subtype of glutamate receptors. As with other glycine site antagonists, the major determinants for high-affinity binding of [H-3]MDL 105,519 resides upon the NRI subunit, and not the NR2 subunits. [H-3]MDL 105,519 binds with equal affinity, K-d = 3 nM, to both NR1-1a or NR1-4b splice variants, as well as the NR1-1a/NR2A receptor expressed in human embryonic kidney (HEK) 293 cells. One percent Triton X-100/1 M NaCl solubilises with a recovery of 15 +/- 3%, a mixed pool of assembled and unassembled forebrain NR1 subunit polypeptides. in this preparation, the recovery of [H-3]MK801 ((+)-5-[H-3]methyl-10,11-dihydrodibenzo[a, d]cyclohepten-5,10-imine binding activity (7 +/- 1%) reflects the amount of assembled NR1 subunits whereas [H-3]MDL 105,519 binds quantitatively, with a recovery of 19 +/- 4% and K-d = 3 nM, to both assembled and unassembled NR1 subunits. Therefore, [H-3]MDL 105,519 should prove a useful ligand, in conjunction with immunopurification approaches, to address the question of NMDA receptor subunit stoichiometry. (C) 1998 Elsevier Science B.V. All rights reserved.