Effects of midazolam in the spinal nerve ligation model of neuropathic pain in rats

Potential changes in the spinal GABAergic activity after nerve injury were studied by comparing the effects of systemic administration of the benzodiazepine midazolam on the noxious evoked responses of dorsal horn in rats with spinal nerve ligation of neuropathy and control animals. The tight ligation of the L(5) and L6 spinal nerves was performed in adult male Sprague-Dawley rats and resulting mechanical and cold allodynia were assessed with von Prey hairs and the acetone drop test. Single unit extracellular recordings of dorsal horn neurones were performed 15-18 days after the surgery under halothane anaesthesia using transcutaneous electrical stimulation of the receptive field at three times the C-fibre threshold. The rats in the spinal nerve ligation group, but not in the sham-operated control group developed mechanical and cold allodynia. Subcutaneous administration of midazolam 0.1-3.0 mg/kg reduced the A delta-fibre evoked activity in a dose-related manner in all study groups, but the C-fibre evoked activity was significantly reduced only in the spinal nerve ligation group. The inhibitory effects of s.c. midazolam were significantly reversed by i.t. administration of flumazenil, suggesting a spinal site of action. Midazolam reduced C-fibre evoked firing significantly more in the spinal nerve ligation model than in the non-operated or sham controls. These results indicate changes in the spinal GABAergic system in the neuropathic animals and could be of importance in the development of new treatments for neuropathic pain. (C) 2000 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.