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Journal of Neuroscience, Vol 9, 2455-2461, Copyright © 1989 by Society for Neuroscience
Disuse-induced enhancement of Ia synaptic transmission in spinal motoneurons of the rat
T Manabe, S Kaneko and M Kuno
Department of Physiology, Kyoto University Faculty of Medicine, Japan.
Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in rat
spinal motoneurons by stimulation of a muscle nerve are enhanced during the
first few days after section of the muscle nerve before subsiding to
subnormal levels. We have examined whether this biphasic alteration results
from changes of different factors involved in the regulation of central
synaptic function. We chronically blocked the conduction of the intact
peripheral nerve with tetrodotoxin (TTX). Under this disuse condition,
enhancement of monosynaptic EPSPs was fully expressed within 2 d, and
synaptic function remained elevated for at least 3 d, even after resumption
of the impulse activity following discontinuance of the TTX. The magnitude
of synaptic enhancement and the minimal period of nerve block required for
synaptic enhancement were both comparable to those observed following nerve
section. Thus, the early synaptic enhancement produced by section of the
peripheral nerve could be accounted for entirely by the deprivation of
sensory impulse activity. However, a prolonged nerve block up to 6 d still
maintained synaptic enhancement. This was in contrast with synaptic
depression observed after the first 3-4 d following section of the
peripheral nerve. The late synaptic depression can thus be attributed to
changes in some factors other than sensory impulse activity. Therefore,
sensory impulse activity and the peripheral axonal continuity of sensory
fibers appear to be 2 distinct factors for the regulation of central
synaptic function.
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