Ca 2+ entry through Na V channels generates submillisecond axonal Ca 2+ signaling

Calcium ions (Ca ) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca ] ) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca ] changes were sensitive to the specific voltage-gated sodium (Na ) channel blocker tetrodotoxin. Consistent with the conjecture that Ca enters through the Na channel pore, the optically resolved in the axon initial segment overlapped with the activation kinetics of Na channels and heterologous expression of Na 1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca ] rise. Finally, computational simulations predicted that axonal [Ca ] transients reflect a 0.4% Ca conductivity of Na channels. The findings indicate that Ca permeation through Na channels provides a submillisecond rapid entry route in Na -enriched domains of mammalian axons.