The α-like scorpion toxin BmK I enhances membrane excitability via persistent sodium current by preventing slow inactivation and deactivation of rNav1.2a expressed in Xenopus Oocytes

BmK I is classified as α-like scorpion toxin that specifically binds the voltage-gated sodium channels via receptor site-3. Previous results showed BmK I induced epileptiform responses in rats via intra-hippocampal injection, but the mechanism has yet to be clarified. In this study, using two-electrode voltage/current clamp technique, we determined the effects of BmK I on rNav1.2a expressed in Xenopus oocytes. The results showed that BmK I prevented the development of slow inactivation of rNav1.2a from the open-state and enhanced the persistent sodium current (INaP) at suprathreshold potentials in concentration-dependence, whereas it hardly affected the fast inactivation. BmK I was also able to augment the subthreshold INaP at high concentrations (>100 nM) with disruption of the open-state deactivation. The increased INaP accelerated the firing frequency in the oocytes that fired repetitively after electrode punctures, as well as raised the baseline potential and induced bursting of spikes in the quiescent oocytes. These results demonstrated that BmK I could target rNav1.2a and induce the INaP by preventing the development of slow inactivation and deactivation from the open-state, leading to the enhancement of membrane excitability, which may be involved in the BmK I-induced epilepsy.