Activation-related and activation-independent effects of polyamines on phencyclidine receptor binding within the N-methyl-D-aspartate receptor complex.

The phencyclidine (PCP) receptor is located within the N-methyl-D-aspartate (NMDA) receptor-gated ion channel. The functional state of the NMDA receptor complex thus influences parameters of radioligand binding to the PCP receptor, and PCP receptor ligands can serve as in vitro probes for elucidation of NMDA receptor activation mechanisms. PCP receptor binding is stimulated by NMDA receptor agonists such as L-glutamate and also by distinct classes of modulatory agents such as glycine-like amino acids and polyamines such as spermidine (SPD). The present study utilizes a kinetic approach permitting differentiation of PCP receptor binding within closed and activated conformations of the NMDA receptor complex. The results demonstrate that SPD increases radioligand binding to the PCP receptor through two distinct mechanisms. First, SPD, like glycine, increases the percentage of time that NMDA channels remain in the open state in the presence of L-glutamate, consistent with a role as a positive allosteric modulator of NMDA receptor activation. Second, unlike glycine, SPD increases the affinity of the PCP receptor for its ligands. The latter effect does not appear to reflect increased NMDA receptor activation. SPD does not induce glycine-like alteration of the EC50 value for stimulation of PCP receptor binding by L-glutamate, suggesting that the effects of SPD cannot be attributed solely to augmentation of glycine binding. These findings demonstrate first that total specific PCP receptor binding cannot, of itself, be used as an index of NMDA receptor activation and second, glycine and polyamines differ in the mechanisms by which they potentiate PCP receptor binding.