The N-Methyl-D-Aspartate Receptor Complex

Abstract

The effects of agonists of the N-methyl-D-aspartate (NMDA) receptor can be blocked by dissociative anesthetics such as phencyclidine (PCP) in a non-competitive manner. This finding together with the fact that ligand binding to the PCP receptor is dependent on the presence of L-glutamate has led to the suggestion that there may exist an NMDA/PCP receptor complex in mammalian brain tissue. This concept has been extended to the inclusion of a cation channel based on the inhibitory actions of the divalent cation, magnesium. Evaluation of the binding of tritiated TCP (thienylcyclohexylpiperidine) a high affinity ligand for the PCP receptor, under four conditions: in basal, well washed rat cortical membranes; in the presence of L-glutamate; in the presence of magnesium; and in the presence of both magnesium and L-glutamate, with NMDA antagonists and dissociative anesthetics showed that these agents had distinct profiles of activity at the PCP receptor. Furthermore, while both classes of compound could modulate TCP binding, only NMDA receptor antagonists inhibited the binding of tritiated CPP (3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid) which labels central NMDA recognition sites. The present data support the existence of an NMDA/PCP receptor complex in mammalian brain tissue. The data currently available would suggest however, that the interface is sequentially NMDA to PCP with the latter site affecting NMDA-mediated responses at a step intermediate between receptor activation and physiological response.