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Action potential generation requires a high sodium channel density in the axon initial segment

Abstract

The axon initial segment (AIS) is a specialized region in neurons where action potentials are initiated. It is commonly assumed that this process requires a high density of voltage-gated sodium (Na+) channels. Paradoxically, the results of patch-clamp studies suggest that the Na+ channel density at the AIS is similar to that at the soma and proximal dendrites. Here we provide data obtained by antibody staining, whole-cell voltage-clamp and Na+ imaging, together with modeling, which indicate that the Na+ channel density at the AIS of cortical pyramidal neurons is 50 times that in the proximal dendrites. Anchoring of Na+ channels to the cytoskeleton can explain this discrepancy, as disruption of the actin cytoskeleton increased the Na+ current measured in patches from the AIS. Computational models required a high Na+ channel density (2,500 pS μm−2) at the AIS to account for observations on action potential generation and backpropagation. In conclusion, action potential generation requires a high Na+ channel density at the AIS, which is maintained by tight anchoring to the actin cytoskeleton.

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Figure 1: Divergence of AIS Na+ channel density estimates using cell-attached recording and Na+ channel antibody staining.
Figure 2: Comparison of AIS and proximal apical dendritic whole-cell Na+ current indicates a high Na+ channel density in the AIS.
Figure 3: Changes in intracellular Na+ during action potentials are largest in the AIS.
Figure 4: The actin cytoskeleton influences AIS Na+ channel density and action potential properties.
Figure 5: Simulation of axonal action potential rate-of-rise requires a high Na+ channel density in the AIS.
Figure 6: Simulation of action potential initiation and backpropagation requires a high Na+ channel density in the AIS.

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Acknowledgements

We thank W. Catterall for the gift of the pan-alpha sodium channel antibody. This work was supported by the Alexander von Humboldt Foundation (G.J.S.) and a NRSA Senior Fellowship (P.C.R.).

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M.H.P.K. performed the patch and whole-cell current-clamp experiments, as well as simulations, and wrote the paper; S.U.I. performed the antibody experiments; B.M.K. helped with the sodium imaging experiments and performed the associated simulations; S.R.W. and P.C.R. helped with the patch experiments; and G.J.S. performed the whole-cell voltage-clamp and sodium imaging experiments and wrote the paper.

Corresponding author

Correspondence to Greg J Stuart.

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Kole, M., Ilschner, S., Kampa, B. et al. Action potential generation requires a high sodium channel density in the axon initial segment. Nat Neurosci 11, 178–186 (2008). https://doi.org/10.1038/nn2040

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