The Journal of Neurophysiology Vol. 88 No. 3 September 2002, pp. 1185-1196
Copyright ©2002 by the American Physiological Society
Effect of Tactile Inputs on Thalamic Responses to Noxious
Colorectal Distension in Rat
Hong-Qi
Zhang,1
Elie D.
Al-Chaer,2 and
William D.
Willis3
1School of Chinese Medicine, Hong Kong
Baptist University, Kowloon Tong, Hong Kong; and Departments of
2Internal Medicine and
3Anatomy and Neurosciences, University of Texas
Medical Branch, Galveston, Texas 77555
Zhang, Hong-Qi,
Elie D. Al-Chaer, and
William D. Willis.
Effect of Tactile Inputs on Thalamic Responses to Noxious
Colorectal Distension in Rat. J. Neurophysiol. 88: 1185-1196, 2002. Recent discoveries of visceral nociceptive
inputs sharing the classical tactile pathway in the dorsal-column
medial lemniscus system have opened a new venue for the investigation
of somatovisceral interactions. The current study was designed to
determine whether somatic innocuous inputs modulate visceral
nociceptive transmission at the thalamic level. The investigation was
carried out by means of extracellular single-unit recordings in the
ventroposterior lateral nucleus of the thalamus in rats anesthetized
with pentobarbital. Noxious visceral stimulation was achieved by
reproducible colorectal distension (CRD, 20-80 mmHg) with a balloon
catheter. Tactile stimulation was delivered by means of a
feedback-controlled mechanical stimulator. The response of the neurons
to CRD was compared before and after the conditioning procedure by
giving tactile stimulation either immediately before CRD or overlapping
it. Twenty-five ventroposterior lateral (VPL) thalamic neurons were
found among numerous tactile-only neurons to have convergent inputs
from both tactile and visceral sources. Their responses to CRD were
excitatory (19), inhibitory (4), or bimodal. When cutaneous tactile
stimuli were delivered before CRD, the responses were reduced in 18 cases. The reduction, however, was usually short-lasting, immediately
following tactile stimulation and could not be enhanced by a prolonged
conditioning procedure. It was unlikely to be attributable to neuronal
habituation as the inverted procedure, CRD stimulation before tactile,
often produced the opposite effect, that is, an enhanced response to skin stimulation. Repeated CRD could bring about sensitization of the
responses of thalamic neurons as manifested by increased spontaneous
discharge, lowered response threshold, and increased response level.
Under such circumstances, the original effect of tactile stimulation on
CRD responses could be weakened. In conclusion, tactile stimulation may
in most circumstances inhibit thalamic neuronal responses to visceral
nociceptive input produced by CRD. However, the effect appears to be
mild and short-lasting at the individual neuronal level in the VPL thalamus.
