Lumbar sympathectomy failed to reverse mechanical allodynia- and hyperalgesia-like behavior in rats with L5 spinal nerve injury

The L5 spinal nerve ligation model of neuropathic pain in rats has been pro posed as a model for sympathetically maintained pain (SMP) based on the eff ects of surgical or chemical sympathectomy on nerve injury induced behavior . In an attempt to confirm that the lesion produces an animal model of SMP, surgical sympathectomies were independently conducted in two different lab oratories (Johns Hopkins and University Kiel) using male Sprague-Dawley (n = 30) or Wister rats (n = 14). The L5 spinal nerve was ligated or cut and l igated. Using von Frey hairs, paw withdrawal threshold and incidence of paw withdrawal were tested concurrently before and after the sympathectomy. Th e sympathectomy was either verified by (a) glyoxylic acid staining of perip heral blood vessels of the hindpaw or (b) skin temperature measurements of the hindpaws. To blind the experimenter, surgeries and behavioral tests wer e performed by two different investigators and a sham sympathectomy was per formed at Johns Hopkins. Decreased paw withdrawal thresholds and increased frequencies of paw withdrawal on the lesioned side were observed after the L5 lesion. Thus, the L5 spinal nerve ligation resulted in behavioral signs of allodynia and hyperalgesia to mechanical stimuli. Lumbar surgical sympat hectomy 1-3 weeks after the lesion or prior to lesion with bilateral remova l of the sympathetic ganglia L2-L4, however, did not reverse or prevent the behavioral changes induced by the nerve injury. The lack of effect of the sympathectomies was independent of the testing paradigm used. Experiments i n Wistar and Sprague-Dawley rats yielded the same results. Potential reason s for the discrepancies between the present study and earlier reports are d iscussed. These results indicate that an L5 spinal nerve injury rat model i s not a reliable model for SMP. (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.