INFLUENCE OF THE SYMPATHETIC NERVOUS-SYSTEM IN THE DEVELOPMENT OF ABNORMAL PAIN-RELATED BEHAVIORS IN A RAT MODEL OF NEUROPATHIC PAIN

This study evaluated the effect of surgical sympathectomy on pain-rela ted behaviours in a well established model of peripheral mononeuropath y produced by loose ligatures around the common sciatic nerve in the r at. Behavioural abnormalities include spontaneous abnormal position of the hindpaw after the nerve constriction, indicative of ''spontaneous pain'', and changes in responses to mechanical or thermal stimuli app lied to this paw. These changes are usually maximal at week 2 after th e surgery, stable until weeks 3-4, and disappear between weeks 8 and 1 2. To assess the role of the sympathetic nervous system in the develop ment and persistence of these abnormalities, four groups of rats were behaviourally tested: (i) rats receiving a complete sham surgery, (ii) rats with a sciatic nerve constriction produced by loose ligatures ar ound the common nerve trunk plus a sham sympathectomy, (iii) rats rece iving a lumbar sympathectomy with a sham nerve ligature, and (iv) rats receiving a simultaneous surgical lumbar sympathectomy and a sciatic nerve constriction. The efficacy of the sympathectomy was assessed by the measure of the noradrenaline level in the sciatic nerve. Sympathec tomy reduced selectively or even prevented the abnormal reaction to co ld temperature and to heat (45 degrees C) in rats with a peripheral mo noneuropathy. In contrast, the abnormal reaction to mechanical pressur e was not influenced, and the behavioural abnormalities indicating spo ntaneous pain were still present. Sympathectomy alone resulted in a re duction of the vocalization threshold to pressure on both hindpaws, bu t also a short-lasting increased tolerance to cold immersion. This stu dy confirms the selective role of the sympathetic nervous system in af fecting the development and maintenance of some abnormal pain-related behaviours to thermal stimuli in rats with a moderate, but persistent, constriction of one sciatic nerve.