EXAGGERATED C-FIBER ACTIVATION PREVENTS PERIPHERAL-NERVE INJURY-INDUCED HYPERINDUCIBILITY OF C-FOS IN PARTIALLY DEAFFERENTED SPINAL DORSAL HORN

Dorsal horn neurons chronically deafferented by peripheral nerve injur ies acquire hypersensitivity to noxious input from outside the origina l receptive field. This study examines the effect of electrical nerve stimulation at the time of injury on such injury-induced hypersensitiv ity. The medial 3/8 of the dorsal horn laminae I/II around the junctio n of 4th and 5th lumbar segments (the tibial territory) was deafferent ed by transection of the ipsilateral tibial nerve in rats. At 2 days o r 3 weeks postinjury, the hindpaw was injected with formalin to induce c-fos. At 2 days, neurons with induced c-Fos protein-like immunoreact ivity (fos-neurons) were largely confined in the lateral 5/8 of lamina e I/II (the peroneal and hip, thus P and H territory). At 3 weeks, fos -neurons significantly increased in the deafferented tibial territory. A similar increase was also noted in the P and H territory. Thus the dorsal horn neurons exhibited c-fos hyperinducibility, an indication o f hypersensitivity. Electrical stimulation with a train of 150 shocks (10 V, 2 ms) of the proximal nerve stump immediately after transection prevented the c-fos hyperinducibility. The effect was greater with th e stimulation frequency of 0.5 Hz than 0.1 Hz or 10 Hz. The stimulatio n had no effect on the c-fos inducibility at 2 days postinjury. (C) 19 97 Elsevier Science Ireland Ltd.