Nav1.2 and BK channel 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 these VGNCs are known to differ in spatial distribution along the AIS and in biophysical properties, our understanding of the functional differences between the two channels remains elusive. Here, using ultrafast Na⁺ , V_m and Ca²⁺ imaging in combination with partial block of Na_v 1.2 by the peptide G¹ G⁴ -huwentoxin-IV, we demonstrate an exclusive role of Na_v 1.2 in shaping the generating AP. Precisely, we show that 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 Ca²⁺ -activated K⁺ channels (CAKCs). Indeed, Ca²⁺ influx via Na_v 1.2 activates BK CAKCs, determining the amplitude and the early phase of repolarization of the AP in the AIS. By using control experiments using 4,9-anhydrotetrodotoxin, a moderately selective inhibitor of Na_v 1.6, we concluded that the Ca²⁺ influx shaping the early phase of the AP is exclusive of Na_v 1.2. Hence, we mimicked this result with a neuron model in which 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 generated 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 partial selective block of the voltage-gated Na⁺ channel Na_v 1.2, 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 a reduction of the Ca²⁺ influx through Na_v 1.2 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 Na_v 1.2 in the axon initial segment for shaping of the action potential during its generation.