[3H]MK801 binding to the N-methyl-D-aspartate receptor reveals drug interactions with the zinc and magnesium binding sites.

We examined the ability of a range of tricyclic antidepressants (TCADs) and phenothiazine derivatives and their metabolites to inhibit the N-methyl-D-aspartate (NMDA) receptor complex using a [3H]MK801 binding assay. Desmethylation of the side chain of both TCADs and phenothiazines increased their potency against [3H]MK801 binding, as did removal of Cl substituted on the conjugated ring. Other side chain modifications further increased the potency of phenothiazines such as in the case of ethopropazine. Generally, the increase in potency of drugs at the NMDA receptor complex was associated with a decrease in the potency at other sites of action of these compounds. This finding suggests that it may be possible to separate the established actions of these compounds from their NMDA inhibitory effects. We also examined the mechanism of action of a number of compounds by monitoring drug effects on the dissociation rate of [3H]MK801 in the presence of Mg++. Phenothiazines and TCADs generally slow the dissociation of [3H]MK801, although to differing extents. Drugs such as 9-aminoacridine, cyproheptadine and ethopropazine also slowed the dissociation rate. These findings suggest a Zn++-like action of these compounds. In contrast, mecamylamine, methapyrilene and procyclidine had very little effect on the dissociation rate, suggesting a competitive action at the [3H]MK801/phencyclidine binding site. Chlorpromazine at low concentrations slowed the dissociation rate, while increasing it at higher concentrations. Thus, chlorpromazine demonstrated both Zn++ and Mg++-like effects. These studies demonstrate novel inhibitory actions of TCAD and phenothiazine derivatives at the NMDA receptor complex that are apparently mediated by the Zn++ binding site.