Multiple structural determinants of voltage-dependent magnesium block in recombinant NMDA receptors

Neuropharmacology. 1993 Nov;32(11):1203-11. doi: 10.1016/0028-3908(93)90014-t.


The voltage-dependent block of NMDA channels by Mg2+ is an important functional element of NMDA receptors, since relief of block by depolarization plays a key role in some forms of ischemic neurodegeneration and synaptic plasticity. To identify the relevant structural domains responsible for block by Mg2+ and TCP, we used site-directed mutagenesis to change individual amino acids of the rat NR1A subunit in a transmembrane region (599-DALTLSSAMWFSWGVLLNSGIGE-621, mutated residues underlined) previously shown to donate residues that influence ionic selectivity. Ten mutant NR1A subunits were co-expressed in Xenopus oocytes with either the epsilon 1 or NR2A subunits, and receptor properties were analyzed under two-electrode voltage clamp. The mutation N616R virtually abolished voltage-dependent Mg2+ block, reduced Zn2+ block 5-fold and greatly reduced Ba2+ permeability in confirmation of previous reports. This mutation also reduced the potency of TCP as a use-dependent blocker by 200-fold. The remaining low-affinity TCP block did not appear to be use-dependent, suggesting two blocking sites for TCP. None of the other mutations differed significantly from NR1A itself except S617N, which displayed a 6-fold reduction in Mg2+ block. A well-barrier model of permeation through the NMDA receptor channel is presented that quantitatively reproduces voltage-dependent Mg2+ block. This model demonstrates that only minimum changes energy profiles experienced by permeating ions, equivalent to the energy of a single hydrogen or ionic bond, are required to abolish Mg2+ block. These findings indicate that only small structural changes are needed to convert a Mg(2+)-insensitive ion channel to a channel with pronounced voltage-dependent Mg2+ block.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Barium / metabolism
  • DNA, Complementary / genetics
  • Electrophysiology
  • Energy Metabolism
  • In Vitro Techniques
  • Magnesium / pharmacology*
  • Molecular Sequence Data
  • Mutagenesis
  • Oocytes / metabolism
  • Phencyclidine / analogs & derivatives
  • Phencyclidine / pharmacology
  • Receptors, N-Methyl-D-Aspartate / antagonists & inhibitors*
  • Receptors, N-Methyl-D-Aspartate / genetics
  • Receptors, N-Methyl-D-Aspartate / physiology
  • Recombination, Genetic
  • Xenopus
  • Zinc / metabolism


  • DNA, Complementary
  • Receptors, N-Methyl-D-Aspartate
  • Barium
  • tenocyclidine
  • Magnesium
  • Phencyclidine
  • Zinc