SUBMODALITY-SELECTIVE HYPERALGESIA ADJACENT TO PARTIALLY INJURED SCIATIC-NERVE IN THE RAT IS DEPENDENT ON CAPSAICIN-SENSITIVE AFFERENT-FIBERS AND INDEPENDENT OF COLLATERAL SPROUTING OR A DORSAL-ROOT REFLEX

We studied submodality dependence of sensory changes produced by unila teral ligation of the sciatic or the saphenous nerve in the rat. We fo cused especially on sensory changes in the skin area adjacent to the i nnervation area of the injured nerve. Moreover, we examined the roles of capsaicin-sensitive nociceptive fibers, collateral sprouting and a dorsal root reflex in sensory changes observed behaviorally. Assessmen t of sensory changes was performed by a pattern of behavioral tests: h ot-plate test and hindlimb withdrawal responses induced by radiant hea t, hot-water bath, innocuous mechanical stimuli, and noxious mechanica l stimuli. In one group, the saphenous nerve ipsilateral to the sciati c ligation was topically treated with capsaicin (1%) at the time of th e surgery. A proximal stump of a saphenous nerve strand was orthodromi cally stimulated to induce a dorsal root reflex (an antidromic volley) in nociceptive fibers of the saphenous nerve trunk. For visualization of plasma extravasation induced by a dorsal root reflex, a dye-labeli ng (Evans blue) technique was used. A collateral sprouting of nocicept ive fibers of the uninjured saphenous nerve was evaluated by determini ng the plasma extravasation response induced by antidromic stimulation of the saphenous nerve. Three and 10 days following the sciatic const riction injury, the hindlimb withdrawal threshold evoked by noxious me chanical stimulation of the medial side of the paw (the innervation ar e of the intact saphenous nerve) was significantly decreased. There wa s no corresponding thermal hyperalgesia adjacent to the injured sciati c nerve. Chronic constriction of the saphenous nerve did not produce a ny significant hyper-or hypoalgesia to mechanical or thermal stimulati on of the uninjured sciatic nerve area. Topical treatment of the ipsil ateral (intact) saphenous nerve at the time of the sciatic nerve ligat ion completely prevented the development of mechanical hyperalgesia in the medial side of the paw (the innervation area of the saphenous ner ve). No dorsal root reflex in nociceptive fibers mediating the adjacen t hyperalgesia could be evoked. No collateral sprouting of the uninjur ed nociceptive fibers of the saphenous nerve was observed. The results indicate that the constriction injury of the sciatic nerve produced a selective hyperalgesia to mechanical stimulation in the innervation a rea of the neighboring saphenous nerve. At the peripheral level, the m echanical hyperalgesia adjacent to the innervation area of the injured nerve was mediated by capsaicin-sensitive nociceptive fibers. Collate ral sprouting of nociceptive fibers from the uninjured to the injured innervation area did not contribute to the present sensory findings. T he sciatic nerve injury did not induce a dorsal root reflex in nocicep tive fibers innervating the hyperalgesic saphenous nerve area. (C) 199 7 Elsevier Science Inc.