INFLUENCE OF SPINALIZATION ON SPINAL WITHDRAWAL REFLEX RESPONSES VARIES DEPENDING ON THE SUBMODALITY OF THE TEST STIMULUS AND THE EXPERIMENTAL PATHOPHYSIOLOGICAL CONDITION IN THE RAT

The influence of midthoracic spinalization on thermally and mechanical ly induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats w ith a spinal nerve ligation-induced unilateral neuropathy, and rats wi th a canageenan-induced inflammation of one hindpaw. Tail flick respon se was induced by radiant heat. Hindlimb withdrawal was induced blv ra diant heat, ice water, and innocuous or noxious mechanical stimulation of Me paw. Prior to spinalization, spinal nerve ligated and carrageen an-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heal-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature chan ge did not explain the facilitation of noxious heat-evoked reflexes. I n contrast, spinal withdrawal responses induced by noxious cold or mec hanical stimulation were significantly suppressed following spinalizat ion. The spinalization-induced facilitatory effects as well as inhibit ory ones on spinal reflexes were enhanced in inflamed/neuropathic anim als. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs, hindlimb), and o n the pathophysiological condition. (C) 1998 Elsevier Science B.V. All rights reserved.