Differential effects of Zn2+ on activation, deactivation,and inactivation kinetics in neuronal voltage-gatedNa+ channels

Whole-cell, patch-clamp recordings were carried out in acutely dissociated neurons from entorhinal cortex (EC) layer II to study the effects of Zn2+ on Na+ current kinetics and voltage dependence. In the presence of 200 μM extracellular Cd2+ to abolish voltage-dependent Ca2+ currents, and 100 mM extracellular Na+, 1 mM Zn2+ inhibited the transient Na+ current, INaT, only to a modest degree (~17% on average). A more pronounced inhibition (~36%) was induced by Zn2+ when extracellular Na+ was lowered to 40 mM. Zn2+ also proved to modify INaT voltage-dependent and kinetic properties in multiple ways. Zn2+ (1 mM) shifted the voltage dependence of INaT activation and that of INaT onset speed in the positive direction by ~5 mV. The voltage dependence of INaT steadystate inactivation and that of INaT inactivation kinetics were markedly less affected by Zn2+. By contrast, INaT deactivation speed was prominently accelerated, and its voltage dependence was shifted by a significantly greater amount (~8 mVon average) than that of INaT activation. In addition, the kinetics of INaT recovery from inactivation were significantly slowed by Zn2+. Zn2+ inhibition of INaT showed no signs of voltage dependence over the explored membrane-voltage window, indicating that the above effects cannot be explained by voltage dependence of Zn2+- induced channel-pore block. These findings suggest that the multiple, voltage-dependent state transitions that the Na+ channel undergoes through its activation path are differentially sensitive to the gating-modifying effects of Zn2+, thus resulting in differential modifications of the macroscopic current’s activation, inactivation, and deactivation. Computer modeling provided support to this hypothesis.