Equilibrium analysis of [³H]TCP binding: effects of glycine, magnesium and N-methyl-D-aspartate agonists

It has been reported that glutamate can increase the binding of [³H]TCP to phencyclidine (PCP) receptors by an action on receptors which are selective for N-methyl-D-aspartate (NMDA). Recently this laboratory has reported that glycine and magnesium can amplify this effect of NMDA agonists in well-washed, lysed cortical membranes. Here we report that maximally effective concentrations of glutamate (10 μM), NMDA (300 μM), MgCl₂ (300 μM) and glycine (10 μM) increase the affinity of the PCP receptor for [³H]TCP by approximately 4-fold in the absence of any change in the density of PCP receptors. However, in combination with glutamate, magnesium had the further effect of increasing the B_max by about 75%. Finally, a synaptosomal P₂ preparation, which had not been washed to minimize the concentration of endogenous effectors had a B_max value similar to the well-washed preparation, but had a K_D value 8-fold lower. These data indicate that the primary effect of NMDA agonists, glycine, and low concentrations of magnesium ions is to convert the PCP receptor from a low-affinity to a high-affinity state. These data are discussed in relation to the functional regulation of the NMDA ionophore.