The novel anticonvulsant MK‐801 binds to the activated state of the N‐methyl‐ d ‐aspartate receptor in rat brain

1 The influence of endogenous and exogenous acidic amino acids on the binding of [ 3 H]‐MK‐801, a selective, non‐competitive antagonist of N‐methyl‐ d ‐aspartate (NMDA) receptors, has been investigated in rat cerebral cortex crude synaptic membranes (CSM). 2 Removal of endogenous glutamate and aspartate from CSM by repeated washing reduced the affinity of [ 3 H]‐MK‐801 for its binding site (with no change in the total number of binding sites) and increased NMDA‐sensitive l ‐[ 3 H]‐glutamate binding. 3 In washed CSM, competitive NMDA antagonists of the dl ‐α‐amino‐ω‐phosphonocarboxylate series reduced [ 3 H]‐MK‐801 binding and NMDA‐sensitive L‐[ 3 H]‐glutamate binding, the most active compounds being 2‐amino‐5‐phosphonovalerate (AP5) and 2‐amino‐7‐phosphono‐heptanoate (AP7). 4 Exogenous excitatory amino acid agonists enhanced the binding of [ 3 H]‐MK‐801 to washed CSM by up to 700%. A selective involvement of NMDA receptors in these effects was indicated by the excellent correlation between EC 50 s for stimulation of [ 3 H]‐MK‐801 binding and IC 50 s for inhibition of NMDA‐sensitive l ‐[ 3 H]‐glutamate binding in the same membranes. 5 The selective, competitive NMDA receptor antagonist D‐AP5 blocked the l ‐glutamate‐induced increase in [ 3 H]‐MK‐801 binding in a competitive manner with a pA 2 value of 6.0. 6 These results seem to reflect a molecular interaction between two distinct components of the NMDA receptor complex: the transmitter recognition site and the site through which MK‐801 exerts its antagonist effects, possibly the ion channel.