Dorsal root section elicits signs of neuropathic pain rather than reversing them in rats with L5 spinal nerve injury

Mechanical allodynia- and hyperalgesia-like behavior which develops in rats after L5 spinal nerve lesion has been suggested to be due to ectopic activ ity in the lesioned afferent neurons originating at the lesion site and/or in the dorsal root ganglion because it is eliminated by section of the dors al root. Here we reevaluated the effect of a dorsal rhizotomy in rats after L5 spinal nerve lesion. Using calibrated von Prey hairs, paw withdrawal th reshold to single stimuli and paw withdrawal incidence to repetitive stimul ation were tested before and after nerve section. Neuropathic pain behavior of similar time course and magnitude also developed after cutting the L5 d orsal root, and L5 spinal nerve lesion-induced abnormal behavior could not be reversed by dorsal rhizotomy. The neuropathic pain behavior elicited by dorsal root section also developed when impulse conduction in the dorsal ro ot axons was blocked during rhizotomy by a local anesthetic, i.e. when the immediate injury discharge was prevented from reaching the spinal cord. The se results challenge the widely accepted idea that neuropathic pain behavio r developing after spinal nerve lesion is dependent on ectopic activity in the lesioned afferent neurons. However, the present results do not rule out the possibility that after the two nerve lesions the mechanisms generating neuropathic pain behavior are different. After dorsal rhizotomy neuropathi c pain behavior may be related to deafferentation whereas after spinal nerv e lesion it may be caused by ectopic activity. (C) 2000 International Assoc iation for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.