Neuroprotective agent riluzole dramatically slows inactivation of Kv1.4 potassium channels by a voltage-dependent oxidative mechanism

The modulation of Kv1.4 K+ channels by the neuroprotective agent riluzole was studied in bovine adrenal zona fasciculata cells by using whole-cell patch clamp. At concentrations ranging from 1 to 100 muM, riluzole reversibly inhibited Kv1.4 channels (IC50 = 70 muM) and irreversibly slowed Kv1.4 inactivation. Riluzole (100 muM) increased the inactivation time constant (T-i) from a control value of 28.9 +/-3.9 to 623 +/- 47.6 ms (n=13). The slowing of bKv1.4 inactivation was not affected by substituting poorly hydrolyzable nucleotides for ATP in the pipette solution, or by the addition of cAMP. Riluzole-induced slowing of bKv1.4 inactivation was nearly eliminated by the presence of the antioxidant reduced glutathione (3 mM) or dithiothreitol (3-5 mM) in the recording pipette, or when cells were superfused with riluzole at a holding potential of -40 mV rather than -80 mV. These results are consistent with a model in which riluzole inhibits bKv1.4 currents and slows inactivation by separate mechanisms. Slowing of inactivation is independent of protein kinases, but probably involves oxidation of a cysteine in the N-terminal inactivation domain. Failure of riluzole to slow inactivation when applied to a depolarized cell suggests that this cysteine is protected in an inactivated Kv1.4 channel. The neuroprotective action of riluzole involves inhibition of glutamate release from presynaptic terminals within the central nervous system. Kv1.4 K+ channels are distributed throughout the brain in axons and nerve terminals, including those from which glutamate is released. The pronounced slowing of Kv1.4 inactivation by riluzole in these neurons could be an important mechanism underlying the inhibition of glutamate release and the therapeutic actions of this drug.