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The Journal of Neurophysiology Vol. 86 No. 4 October 2001, pp. 1955-1971
Copyright ©2001 by the American Physiological Society
Division of Neuroscience, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
Bennett, David J.,
Yunru Li, and
Merek Siu.
Plateau Potentials in Sacrocaudal Motoneurons of Chronic Spinal
Rats, Recorded In Vitro. J. Neurophysiol. 86: 1955-1971, 2001. Intracellular
recordings were made from sacrocaudal tail motoneurons of acute and
chronic spinal rats to examine whether plateau potentials contribute to
spasticity associated with chronic injury. The spinal cord was
transected at the S2 level, causing, over time,
exaggerated long-lasting reflexes (hyperreflexia) associated with a
general spasticity syndrome in the tail muscles of chronic spinal rats
(1-5 mo postinjury). The whole sacrocaudal spinal cord of chronic or
acute spinal rats was removed and maintained in vitro in normal
artificial cerebral spinal fluid (ACSF). Hyperreflexia in chronic
spinal rats was verified by recording the long-lasting ventral root
responses to dorsal root stimulation in vitro. The intrinsic properties
of sacrocaudal motoneurons were studied using intracellular injections
of slow triangular current ramps or graded current pulses. In
chronic spinal rats, the current injection triggered
sustained firing and an associated sustained depolarization (plateau potential; 34/35 cells; mean, 5.5 mV; duration >5
s; normal ACSF). The threshold for plateau initiation was low and usually corresponded to an acceleration in the membrane potential just
before recruitment. After recruitment and plateau activation, the
firing rate changed linearly with current during the slow ramps [63%
of cells had a linear frequency-current (F-I) relation] despite the presence of the plateau. The persistent inward
current (IPIC) producing the
plateau and sustained firing was estimated to be on average 0.8 nA as
determined by the reduction in injected current needed to stop the
sustained firing [
I = 
0.8 ± 0.6 (SD) nA], compared with the current needed to start firing
(I = 1.7 ± 1.5 nA; 47% reduction). In
motoneurons of acute spinal rats, plateaus were rarely seen
(3/22), although they could be made to occur with bath application of
serotonin. In motoneurons of chronic spinal rats there were
no significant changes in the mean passive input resistance,
rheobase or amplitude of the spike afterhyperpolarization (AHP) as
compared with acute spinal rats. However, there were significant
increases in AHP duration and initial firing rate at recruitment and
decreases in minimum firing rate and F-I slope. We suggest
that the higher initial firing rate resulted from the plateau
activation at recruitment and the lower F-I slope resulted from an increase in active conductance during firing, due to
IPIC. Brief dorsal root stimulation
also triggered a plateau and sustained discharge (long-lasting
reflexes; 2-5 s) in motoneurons of chronic (but not acute) spinal
rats. When the plateau was eliminated by a hyperpolarizing current
bias, the reflex response was significantly shortened (to 1 s).
Thus plateaus contributed substantially to the long-lasting reflexes in
vitro and therefore should contribute significantly to the
corresponding exaggerated reflexes and spasticity in awake chronic
spinal rats.
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