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The Journal of Neurophysiology Vol. 87 No. 5 May 2002, pp. 2225-2236
Copyright ©2002 by the American Physiological Society
Department of Pharmacology, College of Medicine, The University of Iowa, Iowa City, Iowa 52242
Zhuo, M.,
J. N. Sengupta, and
G. F. Gebhart.
Biphasic Modulation of Spinal Visceral Nociceptive Transmission
From the Rostroventral Medial Medulla in the Rat. J. Neurophysiol. 87: 2225-2236, 2002. Descending
inhibitory and facilitatory influences from the rostroventral medulla
(RVM) on responses of lumbosacral spinal neurons to noxious colorectal
distension (CRD, 80 mmHg, 20 s) were studied. At 25 sites in the
RVM, electrical stimulation produced biphasic effects, facilitating
responses of spinal neurons to CRD at lesser intensities of stimulation
(5-25 µA) and inhibiting responses of the same neurons at greater
intensities of stimulation (50-100 µA). At 38 other sites in the
RVM, electrical stimulation produced only intensity-dependent
inhibition of neuron responses to CRD. At another 13 sites in the RVM,
electrical stimulation (5-100 µA) produced only facilitatory effects
on responses to CRD. Descending modulatory effects were selective for
distension-evoked activity; spontaneous activities of the same spinal
neurons were not significantly affected by electrical stimulation
that either facilitated or inhibited neuron responses to CRD. Neuron
responses to graded CRD (20-100 mmHg) were positively accelerating
functions that were shifted leftward or rightward, respectively, by
lesser, facilitatory intensities or greater, inhibitory intensities of RVM stimulation. L-glutamate microinjection into the RVM
replicated the effects of electrical stimulation, producing similar
biphasic modulatory effects as produced by electrical stimulation.
Microinjection of glutamate into the RVM at a low dose (5 nmoles)
facilitated responses of spinal neurons to CRD and inhibited responses
of the same neurons at a greater dose (50 nmoles). In some experiments, microinjection of lidocaine (0.5 µl of 4% solution) or the
neurotoxin ibotenic acid (0.5 µl, 10 µg) into the RVM produced
reversible or long-lasting, respectively, decreases in spontaneous
activity and responses of spinal neurons to CRD. These results reveal
that spinal visceral nociceptive transmission is subject to a tonic descending excitatory influence from the RVM and that descending modulatory effects from the RVM on visceral nociceptive transmission are qualitatively similar to modulation of cutaneous nociceptive transmission.
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