Ca2+ entry through NaV channels generates submillisecond axonal Ca2+ signaling

Abstract

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+]ichanges were sensitive to the specific voltage-gated sodium (NaV) channel blocker tetrodotoxin. Consistent with the conjecture that Ca2+enters through the NaVchannel pore, the optically resolvedICain the axon initial segment overlapped with the activation kinetics of NaVchannels and heterologous expression of NaV1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca2+]irise. Finally, computational simulations predicted that axonal [Ca2+]itransients reflect a 0.4% Ca2+conductivity of NaVchannels. The findings indicate that Ca2+permeation through NaVchannels provides a submillisecond rapid entry route in NaV-enriched domains of mammalian axons.