Nav1.2 and BK channels interaction shapes the action potential in the axon initial segment.

In neocortical layer-5 pyramidal neurons, the action potential (AP) is generated in the axon initial segment (AIS) when the membrane potential (V m ) reaches the threshold for activation of the voltage-gated Na + channels (VGNCs) Na v 1.2 and Na v 1.6. Yet, whereas it is known that these VGNCs differ in spatial distribution along the AIS and in biophysical properties, the understanding of the functional differences between the two channels remains elusive. Here, using ultrafast Na + , V m and Ca 2+ imaging in combination with the partial block of Na v 1.2 by the peptide G 1 G 4 -huwentoxin-IV, we demonstrate an exclusive role of Na v 1.2 in shaping the generating AP. Precisely, we show that a selective block of ∼30% of Na v 1.2 widens the AP in the distal part of the AIS and we demonstrate that this effect is due to a loss of activation of BK Ca 2+ -activated K + channels (CAKCs). Indeed, Ca 2+ influx via Na v 1.2 activates BK CAKCs determining the amplitude and the early phase of repolarisation of the AP in the AIS. By mean of control experiments using 4,9-Anhydrotetrodotoxin, which is a moderately selective inhibitor of Na v 1.6, we concluded that the Ca 2+ influx shaping the early phase of the AP is exclusive of Na v 1.2. Hence, we mimicked this result with a NEURON model where the role of the different ion channels tested reproduced the experimental evidence. The exclusive role of Na v 1.2 reported here is important for understanding the physiology and pathology of neuronal excitability. KEY POINTS: We optically analysed the action potential that generates in the axon initial segment of mouse layer-5 neocortical pyramidal neurons and its associated Na+ and Ca2+ currents using ultrafast imaging techniques. We found that the partial selective block of the voltage-gated Na+ channel Nav1.2, produced by a recently developed peptide, widens the shape of the action potential in the distal part of the axon initial segment. We demonstrate that this effect is due to the reduction of the Ca2+ influx through Nav1.2 that activates BK Ca2+-activated K+ channels. To validate our conclusions, we generated a NEURON model that reproduces the ensemble of our experimental results. The present results indicate a specific role of Nav1.2 in the axon initial segment for the shaping of the action potential during its generation. Abstract figure legend In the axon initial segment of neocortical pyramidal neurons the voltage-gated Na + channel Na v 1.2 contributes to the generation of the action potential by providing a Na + current and a Ca 2+ current that activates BK Ca 2+ -activated K + channels shaping the kinetics of the action potential. This article is protected by copyright. All rights reserved.