Neurons with Multiple Axons Have Functional Axon Initial Segments.

Neurons grow multiple axons after treatment with glycogen synthase kinase-3 (GSK-3) inhibitors. However, whether they are electrically active is not known. Here, we examined the role of multiple axons as electrophysiological components during neuronal firing. Combining pharmacological, immunofluorescence, and electrophysiological methods, we found that more neurons had multiple axon initial segments (AISs) after inhibition of GSK-3 activity with SB415286. The multiple AISs induced by GSK-3 inhibition were enriched with voltage-gated sodium channels. The depolarization rate of the multiple-AIS neurons was increased, but their action potential threshold and half-width were normal. By calculating derivatives of the action-potential rising phase, an extra d2 V/dt 2 peak from the extra AIS was distinguished; this indicated that the extra AIS fired ahead of the soma and increased the rate of depolarization. Our study demonstrates that the multiple axons induced by GSK-3 inhibition have AIS structures that are electrically active, and provides insight for axon and AIS studies.