Modulation Of Dendritic Action Currents Decreases The Reliability Of Population Spikes. L. López, J. M. Ibarz, and O. Herreras. Dpto. de Investigación, Hospital Ramón y Cajal, Ctra. Colmenar km 9, 28034 Madrid, Spain.
APStracts 6:0519N, 1999.
During synchronous action potential (AP) firing of CA1 pyramidal cells a population spike (PS) is recorded in the extracellular space whose amplitude is considered a reliable quantitative index of the population output. Since the AP can be actively conducted and differentially modulated along the soma and dendrites, the extracellular part of the dendritic inward currents variably contribute to the somatic PS by spreading in the volume conductor to adjacent strata. We have studied this contribution by current-source density analysis and intracellular recordings in vivo during repetitive backpropagation that induces their selective fading. Both the PS and the ensemble action currents declined during high frequency activation, although at different rates and timings. The decline was much stronger in dendrites than in the somatic region. At specific frequencies and for a short number of impulses the decrease of the somatic PS was neither due to fewer firing cells nor to decreased somatic action currents, but to the blockade of dendritic action currents. The dendritic contribution to the peak of the somatic antidromic PS was estimated around 30-40%, and up to 100% at later times in the positive-going limb. The blockade of AP dendritic invasion is in part due to a decreased transfer of current from the soma which undergoes a cumulative increase of conductance and slow depolarization during the train that eventually extended into the axon. The possibility of differential modulation of soma and dendritic action currents during APs should be checked when using the PS as a quantitative parameter.
Received 21 July 1999; accepted in final form 11 October 1999.
APS Manuscript Number J608-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999