TY - JOUR T1 - Equilibrium Constants for (<em>R</em>)-[(<em>S</em>)-1-(4-Bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic Acid (NVP-AAM077) Acting at Recombinant NR1/NR2A and NR1/NR2B <em>N</em>-Methyl-<span class="sc">d</span>-aspartate Receptors: Implications for Studies of Synaptic Transmission JF - Molecular Pharmacology JO - Mol Pharmacol SP - 1022 LP - 1032 DO - 10.1124/mol.106.024042 VL - 70 IS - 3 AU - Pamela A. Frizelle AU - Philip E. Chen AU - David J. A. Wyllie Y1 - 2006/09/01 UR - http://molpharm.aspetjournals.org/content/70/3/1022.abstract N2 - We have quantified the effects of the N-methyl-d-aspartate (NMDA) receptor antagonist (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) at rat recombinant N-methyl-d-aspartate receptor (NR)1/NR2A and NR1/NR2B NMDA receptors expressed in Xenopus laevis oocytes. We observed no difference in the steady-state levels of inhibition produced by NVP-AAM077 when it was either preapplied or coapplied with glutamate. The IC50 values for NVP-AAM077 acting at NR1/NR2A NMDA receptors were, as expected, dependent on the glutamate concentration used to evoke responses, being 31 ± 2 nM (with glutamate at its EC50 concentration) and 214 ± 10 nM (at 10 times the EC50 concentration). Schild analysis confirmed that the antagonism produced by NVP-AAM077 at NR1/NR2A NMDA receptors was competitive and gave an estimate of its equilibrium constant (KB) of 15 ± 2 nM. Furthermore, Schild analysis of an NMDA receptor carrying a threonine-to-alanine point mutation in the NR2A ligand binding site indicated that NVP-AAM077 still acted in a competitive manner but with its KB increased by around 15-fold. At NR1/NR2B NMDA receptors, NVP-AAM077 displayed reduced potency. An IC50 value of 215 ± 13 nM was obtained in the presence of the EC50 concentration of glutamate (1.5 μM), whereas a value of 2.2 ± 0.14 μM was obtained with higher (15 μM) glutamate concentrations. Schild analysis gave a KB for NVP-AAM077 at NR2B-containing receptors of 78 ± 3 nM. Finally, using a kinetic scheme to model “synaptic-like” activation of NMDA receptors, we show that the difference in the equilibrium constants for NVP-AAM077 is not sufficient to discriminate between NR2A-containing or NR2B-containing NMDA receptors. The American Society for Pharmacology and Experimental Therapeutics ER -