Electrophysiologic evidence for increased endogenous GABAergic but not glycinergic inhibitory tone in the rat spinal nerve ligation model of neuropathy

Background: Changes in the inhibitory activity mediated by gamma -aminobutyric acid (GABA) and glycine, acting at spinal GABA(A) receptors and strychnine-sensitive glycine receptors, are of interest in the development of neuropathic pain. There is anatomic evidence for changes in these transmitter systems after nerve injuries, and blocking either GABA(A) or glycine receptors has been shown to produce allodynia-like behavior in awake normal animals. Methods: In this study, the possible changes in GABAergic and glycinergic inhibitory activity in the spinal nerve Ligation model of neuropathic pain mere studied by comparing the effects of the GABA(A)-receptor antagonist bicuculline and the glycine-receptor antagonist strychnine in neuropathic rats to their effects in sham-operated and nonoperated control rats. Results: Bicuculline produced a dose-related facilitation of the A delta -fiber-evoked activity in all study groups and increased C-fiber-mediated activity in the spinal nerve ligation group but not in either of the control groups. There were no differences in the effect of bicuculline on low threshold responses between the study groups. The glycine receptor antagonist strychnine did not have a statistically significant effect on any of the parameters studied in any of the control groups. Conclusions: These results support the idea of an increased GABAergic inhibitory tone in the spinal cord of neuropathic rats, possibly as compensation for increased excitability after nerve injury.