Abstract
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+)(o)) of these currents. Internal Cs(+) (Cs(+)(i)) affected Mg(2+)(o) inhibition of the NMDA-activated currents in a voltage-dependent manner. A decrease in Cs(+)(i) concentration ([Cs(+)](i)) from 125 to 8 mM reduced Mg(2+)(o) 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(2+)(o) IC(50). This effect was voltage independent. A decrease in [Na(+)](o) from 140 to 70 mM reduced Mg(2+)(o) IC(50) by 1.4-fold at -105 mV and by 1.6-fold at -15 mV. Varying external Ca(2+) (Ca(2+)(o)) concentrations ([Ca(2+)](o)) from 0.1 to 1 mM did not affect Mg(2+)(o) inhibition, even though changing [Ca(2+)](o) in the same range strongly influenced the magnitude of NMDA-activated currents in the absence of Mg(2+)(o). However, increasing [Ca(2+)](o) to higher concentrations (2-20 mM) greatly increased Mg(2+)(o) IC(50) at hyperpolarized voltages. These data are consistent with a model in which Na(+)(i) and Cs(+)(i) modulate Mg(2+)(o) inhibition of NMDA-activated currents by occupying external permeant ion binding sites. The Mg(2+)(o) IC(50) values reported here are similar to Mg(2+)(o) K(D) values calculated from previous single-channel measurements of Mg(2+)(o) blocking kinetics. This similarity implies that Mg(2+)(o) does not affect gating while blocking the channel.