ION CHANNELS, RECEPTORS AND TRANSPORTERS Differential effects of Zn on activation, deactivation, and inactivation kinetics in neuronal voltage-gated Na channels

Whole-cell, patch-clamp recordings were carried out in acutely dissociated neurons from entorhinal cortex (EC) layer II to study the effects of Zn on Na current kinetics and voltage dependence. In the presence of 200 μM extracellular Cd to abolish voltage-dependent Ca currents, and 100 mM extracellular Na, 1 mM Zn inhibited the transient Na current, INaT, only to a modest degree (~17% on average). A more pronounced inhibition (~36%) was induced by Zn when extracellular Na was lowered to 40 mM. Zn also proved to modify INaT voltage-dependent and kinetic properties in multiple ways. Zn (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 Zn. 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 Zn. Zn 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 Zninduced 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 Zn, thus resulting in differential modifications of the macroscopic current’s activation, inactivation, and deactivation. Computer modeling provided support to this