Characterization of glutamate binding sites in receptors assembled from transfected NMDA receptor subunits

Previous studies in brain and recombinant NMDA receptors have observed heterogeneity in NMDA-sensitive glutamate binding site. We further characterized the glutamate site assembled from NR1a, NR2A, and NR2B NMDA receptor subunits using L-[3H]glutamate and [3H]CGP 39653 binding assays. In contrast to earlier reports, we demonstrate a unique pharmacology for the NR2A subunit alone, which has high affinity for agonists but low affinity for competitive antagonists compared with heteromeric combinations of NR1a + NR2A and NR1a + NR2B. Similar to previous reports, we find unequal antagonist affinity between heteromeric combinations of NR1a + NR2A and NR1a + NR2B. However, unlike earlier reports, we describe two binding components within each heteromeric transfection that more closely resemble data obtained for binding to brain membranes. In addition, we show Mg2+ can alter [3H]CGP 39653 binding in both the NR1a + NR2A and the NR1a + NR2B combination, thus allowing comparison of the [3H]CGP 39653-labeled site between the two heteromeric combinations. Agonist inhibition of [3H]CGP 39653 binding revealed differences between the heteromeric combinations as well as within each heteromeric combination, the latter of which more closely resembled results from brain. These results further determine components of the agonist and antagonist binding sites of the NMDA receptor as well as suggest additional possible mechanisms of heterogeneity of the glutamate site in the brain.