Lead Inhibits the Rat N-Methyl-[formula omitted]-Aspartate Receptor Channel by Binding to a Site Distinct from the Zinc Allosteric Site

Abstract

Due to the importance of the NMDA receptor in cognitive function and in models of synaptic plasticity, the effect of Pb+2 on this receptor has been one focus of attempts to define the bases of Pb-induced cognitive impairments seen in young children. The following study was performed to identify the effects on access to the NMDA receptor channel of acute exposure to free Pb+2in vitro. Cerebrocortical membranes were prepared from adult male Sprague–Dawley rats, and binding was measured in 50 mM Tris–acetate with 3H-MK-801 in the presence of saturating concentrations of glutamate and glycine. The potency of Pb+2 to inhibit access to the receptor channel (IC50 = 0.55 μM) was greater than that of Zn+2 (IC50 = 1.30 μM). Dissociation of MK-801 from its binding site exhibited two-component kinetics, and both rate constants were significantly slowed in the presence of Pb+2 or Zn+2. To directly address the question of whether Pb+2 inhibited the receptor channel by binding to the Zn+2 modulatory site, changes in inhibitory potency for the receptor channel were measured when both metals were present. The results demonstrate that multiple levels of Pb+2 produce a concentration-dependent downward shift of the Zn+2 inhibition curve, indicating a noncompetitive inhibition of MK-801 binding by Pb+2 with respect to that of Zn+2. Moreover, Zn+2 IC50 values significantly decreased as a function of increasing Pb+2 concentrations. Analogous results were obtained when Pb+2 inhibition curves were determined in the presence of multiple levels of Zn+2. These findings indicate that the inhibitory properties of free Pb+2 and Zn+2 on the NMDA receptor channel are similar in nature but are exerted via independent allosteric binding sites.