Activation of sodium channels by α-scorpion toxin, BmK NT1, produced neurotoxicity in cerebellar granule cells: an association with intracellular Ca2+ overloading

Voltage-gated sodium channels (VGSCs) are responsible for the action potential generation in excitable cells including neurons and involved in many physiological and pathological processes. Scorpion toxins are invaluable tools to explore the structure and function of ion channels. BmK NT1, a scorpion toxin from Buthus martensii Karsch, stimulates sodium influx in cerebellar granule cells (CGCs). In this study, we characterized the mode of action of BmK NT1 on the VGSCs and explored the cellular response in CGC cultures. BmK NT1 delayed the fast inactivation of VGSCs, increased the Na+ currents, and shifted the steady-state activation and inactivation to more hyperpolarized membrane potential, which was similar to the mode of action of alpha-scorpion toxins. BmK NT1 stimulated neuron death (EC50 = 0.68 A mu M) and produced massive intracellular Ca2+ overloading (EC50 = 0.98 A mu M). TTX abrogated these responses, suggesting that both responses were subsequent to the activation of VGSCs. The Ca2+ response of BmK NT1 was primary through extracellular Ca2+ influx since reducing the extracellular Ca2+ concentration suppressed the Ca2+ response. Further pharmacological evaluation demonstrated that BmK NT1-induced Ca2+ influx and neurotoxicity were partially blocked either by MK-801, an NMDA receptor blocker, or by KB-R7943, an inhibitor of Na+/Ca2+ exchangers. Nifedipine, an L-type Ca2+ channel inhibitor, slightly suppressed both Ca2+ response and neurotoxicity. A combination of these three inhibitors abrogated both responses. Considered together, these data ambiguously demonstrated that activation of VGSCs by an alpha-scorpion toxin was sufficient to produce neurotoxicity which was associated with intracellular Ca2+ overloading through both NMDA receptor- and Na+/Ca2+ exchanger-mediated Ca2+ influx.