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

Abstract

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, I _NaT, 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 I _NaT voltage-dependent and kinetic properties in multiple ways. Zn²⁺ (1 mM) shifted the voltage dependence of I _NaT activation and that of I _NaT onset speed in the positive direction by ~5 mV. The voltage dependence of I _NaT steady-state inactivation and that of I _NaT inactivation kinetics were markedly less affected by Zn²⁺. By contrast, I _NaT deactivation speed was prominently accelerated, and its voltage dependence was shifted by a significantly greater amount (~8 mV on average) than that of I _NaT activation. In addition, the kinetics of I _NaT recovery from inactivation were significantly slowed by Zn²⁺. Zn²⁺ inhibition of I _NaT 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.