Regular ArticleThe mechanism of magnesium block of NMDA receptors

The block of NMDA receptor channels by external magnesium (Mg2+) is believed to be of great physiological importance. The original model of a Mg2+ binding site deep inside the pore, near the cytoplasmic side of the channel has, however, become more and more questionable. In the experiments reported here, Mg2+ block was studied in inside-out patches and inside-out giant patches from Xenopus oocytes expressing recombinant NR1/NR2A receptors. The conductance of single NR1/NR2A receptor channels was rather weakly dependent on the concentration of permeant ions, and if only internal permeant ion concentration was changed the voltage for half-maximal block by Mg2+ shifted with the reversal potential. This suggests that NMDA receptor channels are strongly negatively charged. The charged spider venom Argiotoxin 636 (ATX) exhibited both external and internal block of NR1/NR2A receptors, with apparent electrical distances of almost 100% from both sides of the channel. This gives strong evidence against a considerable asymmetry of the channel. The electrical distance and voltage for half maximal block by Mg2+ was dependent on the monovalent ion species, especially on the cytoplasmic side of the membrane. This effect was independent of the reversal potential, suggesting a direct interaction between Mg2+ and other ions in the channel. A model is presented which explains the excessive voltage-dependence for block by Mg2+ and ATX by the interaction of monovalent ions with the blocking ion.