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

Abstract

The influence of endogenous and exogenous acidic amino acids on the binding of [3H]‐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). Removal of endogenous glutamate and aspartate from CSM by repeated washing reduced the affinity of [3H]‐MK‐801 for its binding site (with no change in the total number of binding sites) and increased NMDA‐sensitive l‐[3H]‐glutamate binding. In washed CSM, competitive NMDA antagonists of the dl‐α‐amino‐ω‐phosphonocarboxylate series reduced [3H]‐MK‐801 binding and NMDA‐sensitive L‐[3H]‐glutamate binding, the most active compounds being 2‐amino‐5‐phosphonovalerate (AP5) and 2‐amino‐7‐phosphono‐heptanoate (AP7). Exogenous excitatory amino acid agonists enhanced the binding of [3H]‐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 EC50s for stimulation of [3H]‐MK‐801 binding and IC50s for inhibition of NMDA‐sensitive l‐[3H]‐glutamate binding in the same membranes. The selective, competitive NMDA receptor antagonist D‐AP5 blocked the l‐glutamate‐induced increase in [3H]‐MK‐801 binding in a competitive manner with a pA2 value of 6.0. 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. 1987 British Pharmacological Society