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

Calcium ions (Ca(2+)) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca(2+) imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca(2+)](i)) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca(2+)](i) changes were sensitive to the specific voltage-gated sodium (Na(V)) channel blocker tetrodotoxin. Consistent with the conjecture that Ca(2+) enters through the Na(V) channel pore, the optically resolved I(Ca) in the axon initial segment overlapped with the activation kinetics of Na(V) channels and heterologous expression of Na(V)1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca(2+)](i) rise. Finally, computational simulations predicted that axonal [Ca(2+)](i) transients reflect a 0.4% Ca(2+) conductivity of Na(V) channels. The findings indicate that Ca(2+) permeation through Na(V) channels provides a submillisecond rapid entry route in Na(V)-enriched domains of mammalian axons.