Differential Sensitivity of RecombinantN-Methyl-d-Aspartate Receptor Subtypes to Zinc Inhibition

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Abstract
Zinc has been shown to be present in synaptic vesicles of a subset of glutamatergic boutons and is believed to be coreleased with glutamate at these synapses. A variety of studies have suggested that zinc might play a role in modulation of excitatory transmission, as well as excitotoxicity, by inhibitingN-methyl-d-aspartate (NMDA)-type glutamate receptors. To further investigate the modulatory effects of zinc on NMDA receptors of different subunit compositions, we coexpressed the recombinant subunit NR1 with NR2A and/or NR2B in HEK 293 cells. In whole-cell patch-clamp recordings from these transfected cells, zinc inhibited peak glutamate-evoked current responses in a noncompetitive manner, but there were significant differences between the receptor subtypes in sensitivity to zinc inhibition. For NR1/NR2A, ∼40% of the peak current was inhibited by zinc in a voltage-independent manner with an IC50 value of 5.0 ± 1.6 nm and at a VH value of −60 mV; the remainder was blocked at a second, voltage-dependent site with an IC50 value of 79 ± 18 μm. In contrast, NR1/NR2B currents showed nearly complete inhibition at a voltage-independent site with an IC50 value of 9.5 ± 3.3 μm. Cells cotransfected with NR1, NR2A, and NR2B showed zinc sensitivity intermediate between that characteristic of NR1/NR2A and that of NR1/NR2B. Furthermore, zinc accelerated the macroscopic desensitization of both NR1/NR2A and NR1/NR2B in a dose-dependent manner, apparently independently of glycine-sensitive desensitization and Ca2+-dependent inactivation; maximal effects were to decrease desensitization time constants for NR1/NR2A by ∼75% and for NR1/NR2B by ∼90%. Differential modulation of NR1/NR2A and NR1/NR2B currents by zinc may play a role in regulating NMDA receptor-induced synaptic plasticity and neurotoxicity.