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Journal of Neuroscience, Vol 13, 4936-4948, Copyright © 1993 by Society for Neuroscience
Low-threshold Ca2+ current and its role in spontaneous elevations of intracellular Ca2+ in developing Xenopus neurons [published erratum appears in J Neurosci 1994 Mar;14(3):following table of contents]
X Gu and NC Spitzer
Department of Biology, University of California at San Diego, La Jolla 92093.
Amphibian spinal neurons exhibit spontaneous elevations of intracellular
calcium at early stages of development. The underlying calcium influx
involves high-voltage-activated (HVA) currents. To begin to understand how
they are triggered, we have studied the biophysical properties and
developmental function of low-voltage-activated (LVA) T- type calcium
current of neurons cultured from the embryonic neural plate. T current was
recorded from young neurons (6-9 hr in vitro) and from mature neurons
(18-48 hr in vitro) using whole-cell voltage clamp. For both young and
mature neurons, T current has a low threshold and is activated at membrane
potentials positive to -60 mV in 2 mM extracellular calcium. The current is
maximal at -35 mV with a mean peak amplitude of approximately 50 pA. Nickel
blocks both LVA and HVA currents, but the former are 20-fold more
sensitive. Amiloride also blocks T current selectively. T current is
recorded in 87% of young neurons. This percentage drops to 67% in mature
neurons after 1 d in culture and to 35% in mature neurons after 2 d in
culture. There are no significant developmental changes in T current
threshold, peak density, time course of activation and inactivation, and
pharmacological sensitivity to blockers from 6 to 48 hr in culture.
Spontaneous transient calcium elevations in young neurons assayed by fluo-3
fluorescence are blocked by nickel or amiloride at concentrations that
specifically block T current. T current has the lowest threshold among
other inward currents in young neurons. Moreover, mathematical simulations
show that T current lowers the threshold of the action potential by 15 mV.
We conclude that T current can depolarize cells and trigger action
potentials, and constitutes part of the cascade of events leading to
spontaneous elevations of intracellular calcium in cultured neurons at
early stages of differentiation.
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