Involvement of the caudal medulla in the negative feed-back mechanisms
triggered by spatial summation of nociceptive inputs.
Olivier Gall, Didier Bouhassira, Djamel Chitour, and Daniel Le Bars.
INSERM U- 161, 75014 Paris, France..
APStracts 4:210N, 1997.
ABSTRACT
In the rat, applying noxious heat stimuli to the excitatory receptive fields
and simultaneously to adjacent, much larger, areas of the body results in a
surface-related reduction in the responses of lumbar dorsal horn convergent
neurones. These inhibitory effects induced by spatial summation of nociceptive
inputs have been shown to involve a supraspinally mediated negative feedback
loop. The aim of the present study was to determine the anatomical level of
integration of these controls and hence to ascertain what relationships they
might share with other descending controls modulating the transmission of
nociceptive signals. The responses of lumbar convergent neurones to noxious
stimulation (15 s immersion in a 48 C water bath) applied to increasing areas
of the ipsilateral hindlimb were examined in several anaesthetized
preparations: sham-operated rats, rats with acute transections performed at
various levels of the brainstem and spinal rats. The effects of heterotopic
noxious heat stimulation (tail immersion in a 52 C water bath) on the C-fibre
responses of these neurones were also analysed. The electrophysiological
properties of dorsal horn convergent neurones including their responses to
increasing stimulus surface areas were not different in sham-operated animals
and in animals whose brainstems had been completely transected rostral to a
plane -2.8 mm remote from interaural line (200 æm caudal to the caudal end of
the rostral ventromedial medulla). In these animals, increasing the stimulated
area size from 4.8 cm2 to 18 cm2 resulted in a 35-45 % reduction in the
responses. In contrast, relative to responses elicited by 4.8 cm2 stimuli,
responses to 18 cm2 were unchanged or even increased in animals with
transections at more caudal level and in spinal animals. Inhibitions of the C-
fibre responses elicited by heterotopic noxious heat stimulation were in the
70-80% range during conditioning in sham-operated animals and in animals with
rostral brainstem transections. Such effects were reduced significantly
(residual inhibitions in the 10-20 % range) in animals with transections more
than 500 æm caudal to the caudal end of the rostral ventromedial medulla and
in spinal animals.
It is concluded that the caudal medulla constitutes a key region for the
expression of negative feed-back mechanisms triggered by both spatial
summation of noxious inputs and heterotopic noxious inputs.
Received 7 May 1997; accepted in final form 21 July 1997.
APS Manuscript Number J368-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 September 1997