Mechanism of phencyclidine/N-methyl-D-aspartate receptor interaction: Implications for neuropsychiatric illness

Abstract

Phencyclidine (PCP,"angel dust") induces a psychotomimetic state closely resembling schizophrenia. The neurobehavioral effects of PCP are mediated at a specific PCP receptor which is postulated to represent a site within the ionophore associated with the N-methyl- scD-aspartate (NMDA) receptor complex. In order to test this hypothesis, the mechanism of interaction between PCP receptors and NMDA was studied using isolated rat brain membranes. In initial studies, binding of the radiolabelled PCP derivative ({dollar}\sp3{dollar}H) TCP was shown to be inhibited by competitive NMDA receptor antagonists such as scD({dollar}-{dollar})-2-amino-5-phosphonovaleric acid (scD({dollar}-{dollar})AP5). In saturation studies, scD({dollar}-{dollar})AP5 decreased the apparent maximal receptor density (Bmax) without affecting the apparent dissociation constant ({dollar}K\sb{lcub}\rm d{rcub}{dollar}) for ({dollar}\sp3{dollar}H) TCP binding, suggesting that NMDA receptor activation regulates accessibility of PCP reactor ligands to their binding site. Subsequent studies using the higher-affinity PCP receptor ligand ({dollar}\sp3{dollar}H) MK-801 demonstrated two discrete components of binding. A high-affinity (1-2 nM) component was found to display rapid kinetics of association (t{dollar}\sb{lcub}1\over2{rcub}{dollar} = 5{dollar}-{dollar}15 min). Binding to this high affinity component was inhibited by scD({dollar}-{dollar})AP5 and stimulated by the NMDA receptor agonist scL-glutamate and additively by the allosteric NMDA potentiators scD-serine or glycine. By contrast, a component with apparent low affinity (40{dollar}-{dollar}80 nM) and slow kinetics of association (t{dollar}\sb{lcub}1\over2{rcub}{dollar} = 90 min), was affected in a biphasic fashion by NMDA receptor agonists. These findings suggested that the high affinity component represented binding to PCP receptors within activated NMDA complexes whereas the low affinity component of ({dollar}\sp3{dollar}H) MK-801 binding represented its binding within closed, agonist-associated NMDA receptor complexes, and that multiple molecules of agonist (scL-glutamate) might be required for NMDA receptor activation. A final study was performed in which it was demonstrated that the Hill coefficient for activation of NMDA receptor complexes by scL-glutamate was 2.0 suggesting that 2 or more molecules of agonist are required for NMDA receptor activation. These findings suggest that NMDA receptor activation may conform to a model previously proposed to account for functioning of receptors such as the nicotinic cholinergic and GABA{dollar}\sb{lcub}\rm A{rcub}{dollar} receptors which belong to the Class I superfamily of ligand-gated channels.