NMDA receptors as targets of heavy metal interaction and toxicity

The N-methyl-D-aspartate (NMDA) receptor (NR) is a ligand-gated channel that carries the slow component of the glutamate-activated postsynaptic current. Divalent metal ions can affect the NR channel activity in a voltage-dependent (Mg2+-like) or voltage-independent (Zn2+-like) manner. We have studied the effect of two toxic metals, lead (Pb2+) and nickel (Ni2+) on recombinant NR1a-NR2A and NR1a-NR2B channels expressed in RNA-injected Xenopus laevis oocytes or in transiently transfected mammalian HEK293 cells. Pb2+ caused a dose-dependent, but voltage-independent reversible inhibition of NMDA-activated channel activity similar for NR2A- and NR2B-containing receptors; it did not modify the single channel conductance, indicating that its binding site is located out of the ionic pathway of permeation. On the contrary, Ni2+ had multiple and complex effects on NR channels. It determined a voltage-dependent, Mg2+-like block by which the single channel amplitude and the mean open time were reduced in both NR2A- and NR2B-containing channels. While high (> 100 mu M) concentrations caused a dose-dependent reduction of the activity in both channel types, 30 mu M determined a voltage-independent decrease in the frequency of NR1a-NR2A channel openings, but an increase in the frequency of NR1a-NR2B channel openings, confirming previous observations of a subunit-dependent effect of this metal. These results were interpreted under the hypothesis that Pb(2+)mediates a Zn2+-like voltage-independent allosteric modulation that, different from Zn2+, is subunit-independent. In contrast, Ni2+ has different modes of action, which are dependent on the NR2 subunit type present in the receptor and are likely to be related to different interaction sites. The NR2B-dependent facilitation bears close similarities with the polyamine-mediated potentiation.