Behavioral signs of ongoing pain and cold allodynia in a rat model of neuropathic pain.

Previous studies by our laboratory established a rat model of neuropathic pain which displayed long-lasting heat hyperalgesia and mechanical allodynia that are sympathetically maintained. The present study was undertaken to extend our earlier findings by examining additional behavioral signs of ongoing pain and cold allodynia in our animal model and testing their sympathetic dependency. Neuropathic surgery was done by tightly ligating the L5 and L6 segmental spinal nerves of rats unilaterally. In addition to the behavioral signs of heat hyperalgesia and mechanical allodynia observed before, these rats displayed signs of ongoing pain (lasting at least 10 weeks) and cold allodynia (lasting at least 16 weeks). These behaviors were reduced markedly after surgical lumbar sympathectomy. The results of the present study, together with the previous study, suggest that our animal model exhibits neuropathic pain behaviors including ongoing pain, heat hyperalgesia, mechanical allodynia and cold allodynia. Since all of these behavioral signs are sympathetically maintained, our model represents a model for sympathetically maintained pain.