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 approximately 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 (approximately 2,500 pS microm(-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.

Axonal bleb recording

“…Indeed, recordings from patches excised from axonal blebs reveal a much higher current density than those obtained from the soma [1] , consistent with immunostaining results and the finding of significantly larger peak Na + currents in cell-attached patches from intact AIS than those at the soma after depolymerizing actin filaments [19] . Recordings from blebs formed at the AIS also revealed that axonal Neurosci Bull August 1, 2012, 28(4): 342-350 346 action potentials have higher peak amplitudes and faster rising phases than somatic ones [33] , indicating a high density of axonal Na + channels.…”

“…For example, the AIS of layer-5 pyramidal cells has a larger diameter (1-2.5 μm) than the rest of the axonal compartment (0.1-1 μm) and thus is accessible to patch pipettes [18][19][20][21] .…”

“…First, the size of blebs is folds-larger than the axon, overcoming the greatest limitation -small diameter -for applying However, one should note that the axonal bleb is an injury site, and reorganization of the intracellular cytoskeleton participates in the formation of this structure. Since membrane proteins, including ion channels, are anchored to the intracellular cytoskeleton, the disorganization of microtubules and microfilaments may set ion channels free and ready to be drawn into the tip of a patch pipette [19,30] .…”

“…The small local capacitance of the AIS allows rapid changes in local membrane potential and action potential initiation in response to a small amount of Na + influx [6,21] . Previous immunostaining and theoretical studies suggested that a high density of Na + channels at the AIS determines it as the initiation site [18,19,[45][46][47][48][49] . Since the total Na + channel densities are similar at the proximal and distal AISs [18,19,46] , why do action potentials prefer to initiate at the distal end?…”

“…Previous immunostaining and theoretical studies suggested that a high density of Na + channels at the AIS determines it as the initiation site [18,19,[45][46][47][48][49] . Since the total Na + channel densities are similar at the proximal and distal AISs [18,19,46] , why do action potentials prefer to initiate at the distal end? Consistent with previous reports, recordings from outside-out patches excised from axonal blebs of cortical pyramidal neurons indicate that the AIS has the highest Na + channel density, ~34-fold greater than the soma (Fig.…”