Early onset of spontaneous activity in uninjured C-fiber nociceptors afterinjury to neighboring nerve fibers

Ligation and transection of the L5 spinal nerve in the rat lead to behavior al signs of pain and hyperalgesia. Discharge of injured nociceptors has bee n presumed to play a role in generating the pain. However, A fibers, but no t C fibers, in the injured L5 spinal nerve have been shown to develop spont aneous activity. Moreover, an L5 dorsal root rhizotomy does not reverse thi s pain behavior, suggesting that signals from other uninjured spinal nerves are involved. We asked if abnormal activity develops in an adjacent, uninj ured root. Single nerve fiber recordings were made from the L4 spinal nerve after ligation and transection of the L5 spinal nerve. Within 1 d of the l esion, spontaneous activity developed in approximately half of the C fiber afferents. This spontaneous activity was at a low level (median rate, seven action potentials/5 min), originated distal to the dorsal root ganglion, a nd was present in nociceptive fibers with cutaneous receptive fields. The i ncidence and level of spontaneous activity were similar 1 week after injury . The early onset of spontaneous activity in uninjured nociceptive afferent s could be the signal that produces the central sensitization responsible f or the development of mechanical hyperalgesia. Because L4 afferents comingl e with degenerating L5 axons in the peripheral nerve, we hypothesize that p roducts associated with Wallerian degeneration lead to an alteration in the properties of the adjacent, uninjured afferents.