Spike initiation properties of pyramidal neuron axons revealed by channelrhodopsin-based photostimulation

Mohammad Amin Kamaleddin (1,2), Stéphanie Ratté (1,3), Steven Prescott (1,2,3)

1: Neurosciences and Mental Health, The Hospital for Sick Children, Toronto, Ontario, Canada

2: Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada

3: Department of Physiology, University of Toronto, Toronto, Ontario, Canada

Spikes can be initiated in different parts of a neuron. In pyramidal neurons, spikes normally originate near the soma, in the axon initial segment (AIS), because this is the most excitable region of the neuron. The AIS converts sustained depolarization into repetitive spiking. Recordings from the cut end of axons (i.e. blebs) have suggested that axons do not spike repetitively during depolarization but, instead, spike only at the onset of abrupt depolarization. This transient spike pattern is consistent with class 3 excitability and is well suited for supporting spike propagation, but it remains unclear whether transient spiking accurately reflects axon excitability or is an artifact of axon damage. Recording intracellularly from an intact axon is prohibitively difficult because of its small caliber. To overcome this technical challenge, we evoked spikes from different parts of CA1 pyramidal neurons using localized photoactivation of channelrhodopsin-2 (ChR2) while recording the resulting spike train at the soma. We found that photostimulation of the soma evoked repetitive spiking, like during current injection, whereas photostimulation of the axon several hundred microns from the soma often evoked variable patterns of spiking. Careful dissection of those spike pattern revealed that only the first spike originated in the axon whereas later spikes were due to stray light exciting the soma and/or dendrites. Overall, our results confirm that axons spike transiently in response to sustained depolarization, consistent with class 3 excitability.