Figure 1. Correction for potentiation by ${\text{Mg}}_{\text{o}}^{\text{2+}}$.

A, example of a cell in which the NMDA-activated current was potentiated by ${\text{Mg}}_{\text{o}}^{\text{2+}}$. Bars above the traces indicate times of agonist and ${\text{Mg}}_{\text{o}}^{\text{2+}}$ application. At −25 mV, the inward current was inhibited by 1 mm ${\text{Mg}}_{\text{o}}^{\text{2+}}$; at +55 mV, 1 mm ${\text{Mg}}_{\text{o}}^{\text{2+}}$ potentiated the outward current. B, calculated ${\text{Mg}}_{\text{o}}^{\text{2+}}$ IC₅₀ values before (○) and after (□) correction for ${\text{Mg}}_{\text{o}}^{\text{2+}}$ potentiation are plotted here for comparison. At depolarized voltages, where ${\text{Mg}}_{\text{o}}^{\text{2+}}$ IC₅₀ reaches the millimolar range, ${\text{Mg}}_{\text{o}}^{\text{2+}}$ potentiation leads to overestimation of IC₅₀ values and exaggeration of the voltage dependence of ${\text{Mg}}_{\text{o}}^{\text{2+}}$ inhibition.