Ca2+ Entry Through Na-V Channels Generates Submillisecond Axonal Ca2+ Signaling

Calcium ions (Ca2+) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca2+ imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca2+](i)) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca2+](i) changes were sensitive to the specific voltage-gated sodium (Na-V) channel blocker tetrodotoxin. Consistent with the conjecture that Ca2+ 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 [Ca2+](i) rise. Finally, computational simulations predicted that axonal [Ca2+](i) transients reflect a 0.4% Ca2+ conductivity of Na-V channels. The findings indicate that Ca2+ permeation through Na-V channels provides a submillisecond rapid entry route in Na-V-enriched domains of mammalian axons.