Slow voltage-dependent changes in channel open-state probability underlie hysteresis of NMDA responses in Mg²⁺-free solutions

Many single-channel studies rely on the assumption that the channels are functioning under steady-state conditions. In examining the basis for nonlinear whole-cell current-voltage curves in Mg²⁺-free solutions we discovered that N-methyl-d-aspartate (NMDA) channels in excised patches reversibly shifted their open-state probability (P_o) in a voltage-dependent way, exhibiting approximately 3- to 4-fold greater Pa at positive potentials than at rest. Changes in P_o were mainly attributable to shifts in frequency of channel opening. P_o changed remarkably slowly (2-15 min), explaining the hysteresis of wholecell current-voltage curves obtained in nonequilibrium conditions. The slow increase in P_o provides a mechanism by which NMDA channels can substantially increase Ca²⁺ influx in cells depolarized for prolonged periods of time and may play a role in excitotoxicity.