Modulation of Mg(2+)-dependent [3H]TCP binding by L-glutamate, glycine, and guanine nucleotides in rat cerebral cortex.

Biochemical and electrophysiological studies have demonstrated that phencyclidine (PCP) recognition site exists in the ion channel of the N-methyl-D-aspartate (NMDA) receptor ion channel complex. Using an extensively washed rat cortical membrane preparation, the effects of Mg2+ and guanylylimidodiphosphate (GppNHp) were examined on the binding of [3H]-N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine ([3H]TCP). Low concentrations of Mg2+ (EC50 = 11 microM) stimulated [3H]TCP binding under the basal condition and high concentrations of Mg2+ (IC50 = 1 mM) inhibited it. In the presence of 10 microM L-glutamate and 10 microM glycine, their EC50 values for Mg2+ enhancement of [3H]TCP binding were markedly reduced (to 1.9 microM or 8.4 microM), respectively. By contrast, the IC50 values for Mg2+ inhibition of [3H]TCP binding were reduced in the presence of L-glutamate, but not glycine. Furthermore, a stimulatory effect of Mg2+ on [3H]TCP binding was additional to the [3H]TCP binding stimulated by a maximally effective concentration of L-glutamate (10 microM) or glycine (10 microM). In the kinetic study, 300 microM Mg2+ produced an increase in the rates of both association and dissociation of [3H]TCP. Similar results were obtained with L-glutamate (10 microM) and glycine (10 microM); 10 mM Mg2+ also caused an acceleration of the association rate but strongly decreased [3H]TCP binding at equilibrium. Compared with [3H]TCP binding under the basal condition, K+ (10 mM) alone decreased the maximal binding without producing any change in the association rate; 10 mM K+ also significantly decreased Mg(2+)-stimulated [3H]TCP binding but caused no change in the acceleration of the association rate caused by Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)