Time and Voltage-Dependent Slowing in the Off Gating Currents from Kv3.1

Abstract

Kv3.1 channels are characterized by fast activation and deactivation kinetics with a quite positive threshold of activation. In the central nervous system they are involved in the high-frequency firing pattern of neurons. Given some controversy on Kv3.1 channel gating, we performed an in-depth analysis of WT Kv3.1 gating currents elicited in mammalian Ltk- cells. The Q/V curves determined from integrating either QON or QOFF were comparable and were best fitted with a double Boltzmann distribution. The Q/V curves were shifted by approximately 15 mV towards more hyperpolarized potentials compared to the ionic G/V curve, indicating that a substantial charge was moved during closed-state transitions. The double component in the Q/V curve and a bi-exponential decay of IgON indicated that the charge was carried by at least two distinct transition steps, contrary to earlier reports for Kv3.2b, but more consistent with Shaker, Kv1.5 and Kv2.1 channels. Since the integrals of QON and QOFF were identical, there was no charge immobilization within a 125 ms depolarization. The time course of IgOFF was fast after short depolarizations, but displayed Shaker-like slowing with increasing step duration for depolarizations positive to the threshold for ionic current activation. Since 4-AP blocks the concerted opening step in Shaker, we used it to separate the two components of IgOFF. In the presence of 3 mM 4-AP only the fast IgOFF remained, indicating that the slowing is linked to a final concerted step in channel opening which appears to be a general gating feature of Kv channels.