Spinal nerve ligation-induced neuropathy in the rat: sensory disorders andcorrelation between histology of the peripheral nerves

We studied the effect of unilateral ligation of two spinal nerves on behavi oral pain responses evoked by various types of cutaneous stimuli in the adu lt rat. Furthermore, we determined the effect of spinal nerve ligation on m orphology of the peripheral nerves. The most consistent behavioral finding (83%) was a marked decrease in monofilament-induced hindlimb withdrawal thr esholds (mechanical allodynia) ipsilateral to the spinal nerve ligation. Th is mechanical allodynia was observed as early as during the Ist post-operat ive day and it persisted up to 2 months (the maximum length of the observat ion period). In contrast, hyperalgesia to noxious mechanical stimulation (R andal-Sellitto test) was not observed in allodynic rats until the 3rd post- operative day. In a minority of rats (13%), spinal nerve ligation-induced m echanical hyperalgesia without a concomitant mechanical allodynia. There wa s no corresponding heat hyperalgesia in the injured hindlimb (hot water imm ersion; radiant heat- or hot-plate-induced hindlimb withdrawal tests). In c ontrast, hypoalgesia to heat was observed on the Ist postoperative day, but not later. Neuropathological analysis of the peripheral nerves revealed a dramatic decrease in the number of myelinated nerve fibers distal to the sp inal nerve ligation site. The results support the previous evidence indicat ing that ligation of spinal nerves induces a marked allodynia to mechanical stimulation. However, this mechanical allodynia may differentially dissoci ate from mechanical and thermal hyperalgesia at various post-operative time points. The marked mechanical allodynia together with a dramatic decrease in the number of myelinated nerve fibers is paradoxical, since the activati on of myelinated nerve fibers by monofilaments produced abnormally strong b ehavioral responses. This paradox may be explained by spinal nerve ligation -induced amplification or disinhibition of tactile signals at central level s. (C) 1999 International Association for the Study of Pain. Published by E lsevier Science B.V.