INFLUENCE OF VARIOUS EXPERIMENTAL PARAMETERS ON THE INCIDENCE OF THERMAL AND MECHANICAL HYPERALGESIA INDUCED BY A CONSTRICTION MONONEUROPATHY OF THE SCIATIC-NERVE IN LIGHTLY ANESTHETIZED RATS

In the current investigation we examined a peripheral mononeuropathy i nduced by four loose ligatures around the sciatic nerve. The nerve was treated with lidocaine or saline before implementing the ligatures (s ilk or chromic gut). The latency of the hindlimb withdrawal to noxious mechanical and radiant heat stimuli was tested under light pentobarbi tal anesthesia 3-10 days following the surgery, A latency/threshold di fference greater than or equal to 156 between the hindlimbs was consid ered to represent a change in the nocifensive response. The adapting: skin temperature of the hindlimbs was also measured. Pieces of hindpaw skin and the sciatic nerve were taken for histological evaluation. Th e results indicate that the incidence of hyperalgesia induced by a con striction mononeuropathy depended on the noxious submodality tested, a nd that the thermal and mechanical hyperalgesia were not coupled in al l cases. Use of only one test modality led to false negative results. Furthermore, mononeuropathy-induced changes in the adapting skin tempe rature produced a considerable number of false positive results (an ar tefactual hyperalgesia) when radiant heat alone was used to determine the nocifensive withdrawal latency without paying attention to the abn ormality of the skin temperature. A preemptive lidocaine treatment of the sciatic nerve before the nerve ligation significantly reduced the incidence of mononeuropathy-induced hyperalgesia, The nerve ligation m aterial (silk vs chromic gut) was not a significant factor for the dev elopment of hyperalgesia induced by a constriction injury of a periphe ral nerve. In histological evaluation the constriction injury-induced damage of the sciatic nerve was verified, and the inflammatory reactio n caused by chromic gut was not stronger than that caused by silk liga tures. In general, immunohistochemical staining for substance P decrea sed whereas that for VIP increased. The results support the hypothesis that ligation-induced mechanical trauma and the afferent barrage indu ced by it, especially during the perioperative period, plays an import ant role in the development of postoperative hyperalgesia. (C) 1994 Ac ademic Press, Inc.