[3H]MK‐801 Labels a Site on the N‐Methyl‐D‐Aspartate Receptor Channel Complex in Rat Brain Membranes

: The potent noncompetitive N‐methyl‐D‐aspartate (NMDA) receptor antagonist [3H]MK‐801 bound with nanomolar affinity to rat brain membranes in a reversible, saturable, and stereospecific manner. The affinity of [3H]‐MK‐801 was considerably higher in 5 mM Tris‐HCl (pH 7.4) than in previous studies using Krebs‐Henseleit buffer. [3H]MK‐801 labels a homogenous population of sites in rat cerebral cortical membranes with KD of 6.3 nM and Bmax of 2.37 pmol/mg of protein. This binding was unevenly distributed among brain regions, with hippocampus > cortex > olfactory bulb = striatum > medulla‐pons, and the cerebellum failing to show significant binding. Detailed pharmacological characterization indicated [3H]HIMK‐801 binding to a site which was competitively and potently inhibited by known noncompetitive NMDA receptor antagonists, such as phencyclidine, thienylcyclohexylpiperidine (TCP), ketamine, N‐allylnormetazocine (SKF 10,047), cyclazocine, and etoxadrol, a specificity similar to sites labelled by [3H]TCP. These sites were distinct from the high‐affinity sites labelled by the σ receptor ligand (+)‐[3H]SKF 10,047. [3H]MK‐801 binding was allosterically modulated by the endogenous NMDA receptor antagonist Mg2+ and by other active divalent cations. These data suggest that [3H]MK‐801 labels a high‐affinity site on the NMDA receptor channel complex, distinct from the NMDA recognition site, which is responsible for the blocking action of MK‐801 and other noncompetitive NMDA receptor antagonists.