Modulation by permeant ions of Mg 2+ inhibition of NMDA‐activated whole‐cell currents in rat cortical neurons

Whole‐cell N ‐methyl‐ d ‐aspartate (NMDA)‐activated currents were recorded from cultured rat cortical neurons. We report here a powerful effect of changing permeant ion concentrations on the voltage‐dependent inhibition by external Mg 2+ (Mg 2+ (Mg o 2+ ) of these currents. Internal Cs + (Cs i + ) affected Mg o 2+ inhibition of the NMDA‐activated currents in a voltage‐dependent manner. A decrease in Cs i + concentration ([Cs + ] i ) from 125 to 8 m m reduced Mg o 2+ IC 50 by 1.4‐fold at −105 mV and by 11.5‐fold at – 15 mV. A decrease in external Na + (Na o + concentration ([Na + ] o ) also reduced Mg o 2+ IC 50 . This effect was voltage independent. A decrease in [Na + ] o from 140 to 70 m m reduced Mg o 2+ IC 50 by 1.4‐fold at–105 mV and by 1.6‐fold at–15 mV. Varying external Ca 2+ (Ca o 2+ ) concentrations ([Ca 2+ ] o ) from 0.1 to1 m m did not affect Mg o 2+ inhibition, even though changing [Ca 2+ ] o in the same range strongly influenced the magnitude of NMDA‐activated currents in the absence of Mg o 2+ . However, increasing [Ca 2+ ] o to higher concentrations (2–20 m m ) greatly increased Mg o 2+ IC 50 at hyperpolarized voltages. These data are consistent with a model in which Na i + and Cs i + modulate Mg o 2+ inhibition of NMDA‐activated currents by occupying external permeant ion binding sites. The Mg o 2+ IC 50 values reported here are similar to Mg o 2+ K D values calculated from previous single‐channel measurements of Mg o 2+ blocking kinetics. This similarity implies that Mg o 2+ does not affect gating while blocking the channel.