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The Journal of Neurophysiology Vol. 86 No. 4 October 2001, pp. 1523-1545
Copyright ©2001 by the American Physiological Society
1Physics Department, University of Ottawa, Ottawa, Ontario K1N 6N5; 2Department of Cell Biology and Anatomy, Neuroscience Research Group, University of Calgary, Calgary, Alberta T2N 4N1; and 3Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
Doiron, Brent,
André Longtin,
Ray W. Turner, and
Leonard Maler.
Model of Gamma Frequency Burst Discharge Generated by Conditional
Backpropagation. J. Neurophysiol. 86: 1523-1545, 2001. Pyramidal cells of the electrosensory lateral
line lobe (ELL) of the weakly electric fish Apteronotus
leptorhynchus have been shown to produce oscillatory burst
discharge in the
-frequency range (20-80 Hz) in response to
constant depolarizing stimuli. Previous in vitro studies have shown
that these bursts arise through a recurring spike backpropagation from
soma to apical dendrites that is conditional on the frequency of action
potential discharge ("conditional backpropagation"). Spike bursts
are characterized by a progressive decrease in inter-spike intervals
(ISIs), and an increase of dendritic spike duration and the amplitude
of a somatic depolarizing afterpotential (DAP). The bursts are
terminated when a high-frequency somatic spike doublet exceeds the
dendritic spike refractory period, preventing spike backpropagation. We present a detailed multi-compartmental model of an ELL basilar pyramidal cell to simulate somatic and dendritic spike discharge and
test the conditions necessary to produce a burst output. The model
ionic channels are described by modified Hodgkin-Huxley equations and
distributed over both soma and dendrites under the constraint of
available immunocytochemical and electrophysiological data. The
currents modeled are somatic and dendritic sodium and potassium
involved in action potential generation, somatic and proximal apical
dendritic persistent sodium, and KV3.3 and fast transient A-like potassium channels distributed over the entire model
cell. The core model produces realistic somatic and dendritic spikes,
differential spike refractory periods, and a somatic DAP. However, the
core model does not produce oscillatory spike bursts with constant
depolarizing stimuli. We find that a cumulative inactivation of
potassium channels underlying dendritic spike repolarization is a
necessary condition for the model to produce a sustained
-frequency
burst pattern matching experimental results. This cumulative
inactivation accounts for a frequency-dependent broadening of dendritic
spikes and results in a conditional failure of backpropagation when the
intraburst ISI exceeds dendritic spike refractory period, terminating
the burst. These findings implicate ion channels involved in
repolarizing dendritic spikes as being central to the process of
conditional backpropagation and oscillatory burst discharge in this
principal sensory output neuron of the ELL.
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