Injury Prior to Neurectomy Alters the Pattern of Autotomy in Rats Behavioral Evidence of Central Neural Plasticity

A common property of phantom limb pain is that a preamputation lesion continues to be felt in the same location of the phantom limb after amputation. A model of the phantom limb in the rat is provided by sectioning the sciatic and saphenous nerves. This procedure leads to self-mutilation of the denervated hindpaw, a behavior known as autotomy. There is strong evidence that autotomy is a response to painful or dysesthetic sensations referred to the anesthetic limb. The present study examined the hypothesis that the site of autotomy behavior can be altered by an injury given prior to denervation. Experiment 1 evaluated the effects of a thermal injury applied (under sodium pentobarbital anesthesia) to the medial or lateral hindpaw and digits before or after sciatic and saphenous nerve transections in order to determine whether autotomy is directed specifically to a previously injured site. The results revealed that autotomy onset occurred in the injured region of the paw in a significantly greater proportion of rats, compared to an uninjured control group, if the thermal injury had been induced befor denervation: 100% of the rats with medial paw injury induced prior to neurectomy initiated autotomy in the medial digits, and 55.6% of rats with lateral paw injury initiated autotomy in the lateral digits. Rats injured after autotomy showed no such preference (medial, 33.3% and lateral, 37.5%) relative to the uninjured controls (medial, 33.3% and lateral, 17%). These results suggest that central cells, sensitized by the thermal injury, contribute to enhanced autotomy in the absence of further inputs from the injured paw. However, rats injured after neurectomy displayed enhanced autotomy compared to uninjured controls, suggesting that peripheral, injury-related factors may also contribute to the autotomy among injured animals. In order to minimize the contribution of peripheral factors produced by cutaneous injury, experiment 2 examined the effects, on autotomy behavior, of noxious electrical or mechanical stimulation of the sciatic nerve (under sodium pentobarbital anesthesia) prior to neurectomy. Electrical stimulation of the sciatic nerve prior to denervation altered the pattern of self-mutilation compared to control rats. These experiments suggest that the alteration in the site of autotomy onset by a prior injury depends in part on a sensory memory mechanism in the central nervous system that is sustained in the absence of further inputs from the injured region and that long outlasts the duration of noxious stimulation. The results imply that a preoperative anesthetic block of the relevant peripheral nerves and/or spinal cord cells involved in nociceptive transmission would decrease the intensity of postoperative pain and reduce the incidence of chronic pain syndromes such as phantom limb pain.