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Journal of Neuroscience, Vol 15, 6720-6734, Copyright © 1995 by Society for Neuroscience
Consequences and mechanisms of spike broadening of R20 cells in Aplysia californica
M Ma and J Koester
Center for Neurobiology and Behavior, College of Physicians and Surgeons, Columbia University, New York, New York 10032, USA.
We studied frequency-dependent spike broadening in the two electrically
coupled R20 neurons in the abdominal ganglion of Aplysia. The peptidergic
R20 cells excite the R25/L25 interneurons (which trigger respiratory
pumping) and inhibit the RB cells. When fired at 1-10 Hz, the duration of
the falling phase of the action potential in R20 neurons increases 2-10
fold during a spike train. Spike broadening recorded from the somata of the
R20 cells affected synaptic transmission to nearby follower cells.
Chemically mediated synaptic output was reduced by approximately 50% when
recorded trains of nonbroadened action potentials were used as command
signals for a voltage-clamped R20 cell. Electrotonic EPSPs between the R20
cells, which normally facilitated by two- to fourfold during a high
frequency spike train, showed no facilitation when spike broadening was
prevented under voltage-clamp control. To examine the mechanism of
frequency- dependent spike broadening, we applied two-electrode
voltage-clamp and pharmacological techniques to the somata of R20 cells.
Several voltage- gated ionic currents were isolated, including INa, a
multicomponent ICa, and three K+ currents--a high threshold, fast transient
A-type K+ current (IAdepol), a delayed rectifier K+ current (IK-V), and a
Ca(2+)- sensitive K+ current (IK-Ca), made up of two components. The
influences of different currents on spike broadening were determined by
using the recorded train of gradually broadening action potentials as the
command for the voltage clamp. We found the following. (1) IAdepol is the
major outward current that contributes to repolarization of nonbroadened
spikes. It undergoes pronounced cumulative inactivation that is a critical
determinant of spike broadening. (2) Activity-dependent changes in IK-V,
IK-Ca, and ICa have complex effects on the kinetics and extent of
broadening. (3) The time integral of ICa during individual action
potentials increases approximately threefold during spike broadening.
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