Ectopic action potentials in regular spiking neurons in the anesthetized mouse.

Action potentials (APs) in the central nervous system typically arise at the axon initial segment (AIS) near the cell body. "AIS-APs" actively propagate down the axon to the synaptic terminals, where they elicit vesicular release. Under certain conditions, APs can also emerge in axons' distal branches and terminals. These "Ectopic" APs (EAPs) then travel antidromically toward the cell body and propagate in the axonal arbor, where they also can trigger vesicular release. Mammalian EAPs have previously been detected in pathological tissue and have been increasingly observed in nonpathological tissue. We recently found that most excitatory pyramidal neurons in layers 2/3 and 4 in the mouse orbitofrontal cortex can generate EAPs in acute slices. Here, we test whether EAPs occur in layer 2/3 cortical pyramidal neurons in the anesthetized mouse. We obtained whole-cell current-clamp recordings of layer 2/3 regular spiking (RS) cells in the somatosensory and visual cortices of ketamine-anesthetized mice. High-frequency trains of AIS-APs repeatedly elicited by direct current injection were successful at inducing EAPs in most tested cells. The proportion of RS cells exhibiting EAPs and their ectopic firing profile were comparable with what we had previously observed in vitro. This study demonstrates that pyramidal neurons in layer 2/3 can fire EAPs in an in vivo mouse model and suggests that studies utilizing in vitro preparations are valid for determining whether populations of neurons are capable of firing EAPs.