Change in the position of the action potential initiation site in Granule Cells of the Dentate Gyrus during repetitive firing

Abstract

from the soma, closer compared to other neurons. In layer V pyramidal neurons, where AIS is located 60-100 μm from the soma, two peaks appear in the second derivative of Vm, where the first peak correspond to action potential initiated at the AIS, and the second corresponds to the somatic action potential. Because of a closer initiation site in granule cells, the second derivative of single action potential does not show the usual two separate peaks corresponding to axonal and somatic initiations. However, in the latest action potentials within a successive train, two peaks appears in the second derivative, which is more evident as “hump” by inspecting phase plots. The apparition of the hump in the second derivative could be due to a movement of the initiation site away from the soma due to inactivation of voltage dependent sodium channels in the proximal AIS. In order to investigate this behavior, we used a computational model for granule cells that can reproduce features of single action potential (Schmidt-Hieber C. and Bischofberger J. J Neurosci. 30:10233-42), and modify it in order to reproduce features of spike trains. This model allowed us to explore several combination of values and position in the axon for several conductances, in order to explain the hump in the second derivative under repetitive firing.Our results suggest that action potential initiation in granule cells is a dynamic process that can account for plastic changes in firing properties by moving position and length of initiation site, as has been proposed for other brain structures.