ROLE OF VOLTAGE-GATED K+ CURRENTS IN MEDIATING THE REGULAR SPIKING
PHENOTYPE OF CALLOSAL-PROJECTING RAT VISUAL CORTICAL NEURONS
Rachel E. Locke and Jeanne M. Nerbonne
Department of Molecular Biology and Pharmacology Washington University
School of Medicine, St. Louis, MO 63110 USA
APStracts 4:119N, 1997.
ABSTRACT
Whole-cell current- and voltage-clamp recordings were combined to examine
action potential waveforms, repetitive firing patterns and the functional
roles of voltage-gated K+ currents (IA, ID and IK) in identified callosal-
projecting (CP) neurons from postnatal (day 7 to 13) rat primary visual
cortex. Brief (1 ms) depolarizing current injections evoke single action
potentials in CP neurons with mean ñ SD (n = 60) durations at 50 % and 90 %
repolarization of 1.9 ñ 0.5 and 5.5 ñ 2.0 ms, respectively; action potential
durations in individual cells are inversely correlated with peak outward
current density. During prolonged threshold depolarizing current injections,
CP neurons fire repetitively, and two distinct, noninterconverting "regular
spiking" firing patterns are evident: weakly adapting CP cells fire
continuously, whereas strongly adapting CP cells cease firing during
maintained depolarizing current injections. Action potential repolarization is
faster and afterhyperpolarizations are more pronounced in strongly than in
weakly adapting CP cells. In addition, input resistances are lower and plateau
K+ current densities are higher in strongly than in weakly adapting CP cells.
Functional studies reveal that blockade of ID reduces the latency to firing an
action potential, and increases action potential durations at 50 % and 90 %
repolarization. Blockade of ID also increases firing rates in weakly adapting
cells and results in continuous firing of strongly adapting cells. Following
applications of mM concentrations of 4-aminopyridine to suppress IA (as well
as block ID), action potential durations at 50 % and 90 % repolarization are
further increased, and firing rates are accelerated over those observed when
only ID is blocked. Using VClamp/CClamp (Huguenard & McCormick, 1992;
McCormick & Huguenard, 1992) and the voltage-clamp data in the preceding
manuscript (Locke & Nerbonne, 1997), mathematical descriptions of IA, ID and
IK are generated and a model of the electrophysiological properties of rat
visual cortical CP neurons is developed. The model is used to simulate the
firing properties of strongly adapting and weakly adapting CP cells and to
explore the functional roles of IA, ID and IK in shaping the waveforms of
individual action potentials and controlling the repetitive firing properties
of these cells.
Received 3 September 1996; accepted in final form 30 June 1997.
APS Manuscript Number J706-6
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997