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 induced by four loose ligatures around the sciatic nerve. The nerve was treated with lidocaine or saline before implementing the ligatures (silk or chromic gut). The latency of the hindlimb withdrawal to noxious mechanical and radiant heat stimuli was tested under light pentobarbital anesthesia 3-10 days following the surgery. A latency/threshold difference > or = 15% between the hindlimbs was considered 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. The results indicate that the incidence of hyperalgesia induced by a constriction mononeuropathy depended on the noxious submodality tested, and that the thermal and mechanical hyperalgesia were not coupled in all cases. Use of only one test modality led to false negative results. Furthermore, mononeuropathy-induced changes in the adapting skin temperature produced a considerable number of false positive results (an artefactual hyperalgesia) when radiant heat alone was used to determine the nocifensive withdrawal latency without paying attention to the abnormality 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 material (silk vs chromic gut) was not a significant factor for the development of hyperalgesia induced by a constriction injury of a peripheral nerve. In histological evaluation the constriction injury-induced damage of the sciatic nerve was verified, and the inflammatory reaction caused by chromic gut was not stronger than that caused by silk ligatures. In general, immunohistochemical staining for substance P decreased whereas that for VIP increased. The results support the hypothesis that ligation-induced mechanical trauma and the afferent barrage induced by it, especially during the perioperative period, plays an important role in the development of postoperative hyperalgesia.