Permeant ion effects on external Mg2+ block of NR1/2D NMDA receptors

Voltage-dependent channel block by external Mg2+ (Mg-o(2+)) of NMDA receptors is an essential determinant of synaptic function. The resulting Mg-o(2+) inhibition of NMDA responses depends strongly on receptor subunit composition: NR1/2A and NR1/2B receptors are more strongly inhibited by Mg-o(2+) than are NR1/2C or NR1/2D receptors. Previous work showed that permeant ions have profound effects on Mg-o(2+) block of NMDA receptors composed of NR1, NR2A, and NR2B subunits. Whether permeant ions affect Mg-o(2+) inhibition of NR1/2C or NR1/2D receptors is unknown. We investigated the effects of permeant ions on Mg-o(2+) block of NR1/2D receptors by integrating results from whole-cell recordings, single-channel recordings, and kinetic modeling. Lowering internal [ Cs+] caused a voltage-dependent decrease in the Mg-o(2+) IC50 and in the apparent Mg-o(2+) unblocking rate, and increase in the apparent Mg-o(2+) blocking rate (k(+), app) of NR1/2D receptors. Lowering external [ Na+] caused modest voltage-dependent changes in the Mg-o(2+) IC50 and k(+), app. These data can be explained by a kinetic model in which occupation of either of two external permeant ion binding sites prevents Mg-o(2+) entry into the channel. Occupation of an internal permeant ion binding site prevents Mg-o(2+) permeation and accelerates Mg-o(2+) unblock to the external solution. We conclude that variations in permeant ion site properties shape the NR2 subunit dependence of Mg-o(2+) block. Furthermore, the external channel entrance varies little among NMDA receptor subtypes. Differences in the Mg-o(2+) blocking site, and particularly in the selectivity filter and internal channel entrance, are principally responsible for the subunit dependence of Mg-o(2+) block.