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 microM), NMDA (300 microM), MgCl2 (300 microM) and glycine (10 microM) 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 Bmax by about 75%. Finally, a synaptosomal P2 preparation, which had not been washed to minimize the concentration of endogenous effectors had a Bmax value similar to the well-washed preparation, but had a KD 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.