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The Journal of Neurophysiology Vol. 85 No. 2 February 2001, pp. 1013-1016
Copyright ©2001 by the American Physiological Society
RAPID COMMUNICATION
Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204-5513
Pan, Enhui and
Costa M. Colbert.
Subthreshold Inactivation of Na+ and K+
Channels Supports Activity-Dependent Enhancement of Back-Propagating
Action Potentials in Hippocampal CA1. J. Neurophysiol. 85: 1013-1016, 2001. Back-propagating action potentials in CA1 pyramidal neurons
may provide the postsynaptic dendritic depolarization necessary for the
induction of long-term synaptic plasticity. The amplitudes of
back-propagating action potentials are not all or none but are limited
in amplitude by dendritic A-type K+ channels.
Previous studies of back-propagating action potentials have suggested
that prior depolarization of the dendritic membrane reduces A-type
channel availability through inactivation, resulting in an enhanced, or
boosted, dendritic action potential. However, inactivation kinetics in
the subthreshold potential range have not been directly measured.
Furthermore, the corresponding rates of Na+
channel inactivation with depolarization have not been considered. Here
we report in cell-attached patches (150-220 µm from the soma, 32°C) that at 20-mV positive to rest, A-type K+
channels inactivated with a single exponential time constant of 6 ms,
whereas Na+ channels inactivated with a time
constant of 37 ms. The ratio of available Na+ to
K+ current increased as the duration of the
depolarization increased. Thus the subthreshold properties of
Na+ and A-type K+ channels
provide a mechanism by which information about the level of synaptic
activity may be encoded in the amplitude of back-propagating action potentials.
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