Excitability of spinal cord and gracile nucleus neurons in rats with chronically injured sciatic nerve examined by c-fos expression

Low-threshold sensory pathways have been suggested to have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. Fos-like immunoreactive (Fos-LI) neurons are expressed in spinal cord laminae III-TV and the gracile nucleus by electrically stimulating the injured nerves at AP strength after sciatic nerve transection in rats. This suggests that the excitability of these neurons is increased by nerve injury. In this study, we investigated which receptors are involved in the regulation of the increased excitability in spinal and gracile nucleus neurons. The sciatic nerve of Sprague-Dawley rats (150 g) was transected 7 days before the experiment day. The rats were administered morphine, muscimol, baclofen, MK-801, CNQX, N-G-nitro-L-arginine methyl ester hydrochloride (L-NAME) or clonidine i.p., and then electrically stimulated at 0.1 mA to the proximal region to the nerve injury site under urethane anesthesia. Two hours after the stimulation, Fos-LI expression was increased in the spinal cord dorsal horn and the gracile nucleus in control rats. Baclofen inhibited the Fos-LI expression both in the spinal cord and the gracile nucleus. Morphine inhibited only the Fos-LI expression in the posterior cutaneous (PC) nerve territory of laminae I-II, but not in the sciatic nerve (SC) territory, laminae III-IV nor the gracile nucleus. MK-801 had an inhibitory but complicated effect in laminae I-II and the gracile nucleus. The other drugs were not effective on Fos-LI expression. It is suggested that the GABAB receptor has a pivotal role in the regulation of Fos-LI expression after electrical stimulation to the injured low-threshold sensory fibers, and other receptors have little effect on the Fos-LI expression. (C) 1999 Elsevier Science B.V. All rights reserved.