CONTRIBUTIONS OF INJURED AND INTACT AFFERENTS TO NEUROPATHIC PAIN IN AN EXPERIMENTAL RAT MODEL

This study was conducted to determine the contribution of peripheral i nputs from injured and intact afferent fibers to behavioral signs of n europathic pain, using a previously developed neuropathic rat model. N europathic injury was produced by tightly ligating the left L5 and L6 spinal nerves; this procedure induced rats to display neuropathic pain behaviors in the ipsilateral hindlimb. The behaviors included signs o f mechanical and cold allodynia, as well as ongoing pain. Five days af ter neuropathic injury, peripheral inputs from injured segments (L5 an d L6) or intact segments (L3 and L4) were blocked by either transectio n of the dorsal roots or application of a local anesthetic (bupivacain e) to the roots. Dorsal rhizotomy of the injured segments reduced all components of neuropathic pain behaviors. In contrast, dorsal rhizotom y of the uninjured segments abolished behavioral signs of mechanical a nd cold allodynia, but signs of ongoing pain were preserved, Blocking afferent inputs by application of bupivacaine mimicked the results of dorsal rhizotomy, in a reversible manner. These results suggest that a fferent signals from injured and intact fibers play distinctively diff erent roles in neuropathic pain: inputs from injured afferents maintai n all components of neuropathic pain, while those from intact afferent s mediate evoked pain such as mechanical and cold allodynia. An hypoth esis is proposed to explain the results of the present as well as othe r published studies.