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J Neurophysiol (January 1, 2003). 10.1152/jn.00241.2002
Submitted on Submitted 3 April 2002; accepted in final form 26 August 2002
Department of Physiology and Biophysics, University of Washington, School of Medicine, Seattle, Washington 98195
Powers, Randall K. and
Marc D. Binder.
Persistent Sodium and Calcium Currents in Rat Hypoglossal
Motoneurons. J. Neurophysiol. 89: 615-624, 2003. Voltage-dependent persistent inward currents are thought to
make an important contribution to the input-output properties of
motoneurons, influencing both the transfer of synaptic current to
the soma and the effects of that current on repetitive discharge. Recent studies have paid particular attention to the contribution of
L-type calcium channels, which are thought to be widely distributed on
both the somatic and the dendritic membrane. However, the relative contribution of different channel subtypes as well as their
somatodendritic distribution may vary among motoneurons of different
species, developmental stages, and motoneuron pools. In this study, we have characterized persistent inward currents in juvenile (10- to
24-day-old) rat hypoglossal (HG) motoneurons. Whole-cell, voltage-clamp recordings were made from the somata of visualized rat HG motoneurons in 300-µm brain stem slices. Slow (10 s), triangular voltage-clamp commands from a holding potential of 70 to 0 mV and back elicited whole-cell currents that were dominated by outward, potassium currents,
but often showed a region of negative slope resistance on the rising
phase of the command. In the presence of potassium channel blockers
(internal cesium and external 4-aminopyridine and tetraethylammonium),
net inward currents were present on both the rising and falling phases
of the voltage-clamp command. A portion of the inward current present
on the ascending phase of the command was mediated by TTX-sensitive
sodium channels, whereas calcium channels mediated the remainder of the
current. We found roughly the same relative contributions of P-, N-,
and L-type channels to the calcium currents recorded at the soma that
had previously been found in neonatal rat HG motoneurons. In most cells, the somatic voltage thresholds for calcium current onset and
offset were similar and the peak current was largest on the ascending
phase of the clamp command. However, about one-third of the cells
exhibited a substantial clockwise current hysteresis, i.e., inward
currents were present at lower voltages on the descending phase of the
clamp command. In the same cells, 1-s depolarizing voltage-clamp
commands were followed by prolonged tail currents, consistent with a
prominent contribution from dendritic channels. In contrast to previous
reports on turtle and mouse motoneurons, blocking L-type calcium
channels did not eliminate these presumed dendritic currents.
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