The antinociceptive effect of systemic gabapentin is related to the type of sensitization-induced hyperalgesia

Background: Gabapentin is a structural analogue of gamma- aminobutyric acid with strong anticonvulsant and analgesic activities. Important discrepancies are observed on the effectiveness and potency of gabapentin in acute nociception and sensitization due to inflammation and neuropathy. There is also some controversy in the literature on whether gabapentin is only active in central areas of the nervous system or is also effective in the periphery. This is probably due to the use of different experimental models, routes of administration and types of sensitization. The aim of the present study was to investigate the influence of the spinal cord sensitization on the antinociceptive activity of gabapentin in the absence and in the presence of monoarthritis and neuropathy, using the same experimental protocol of stimulation and the same technique of evaluation of antinociception. Methods: We studied the antinociceptive effects of iv. gabapentin in spinal cord neuronal responses from adult male Wistar rats using the recording of single motor units technique. Gabapentin was studied in the absence and in the presence of sensitization due to arthritis and neuropathy, combining noxious mechanical and repetitive electrical stimulation ( wind- up). Results: The experiments showed that gabapentin was effective in arthritic ( max. effect of 41 +/- 15% of control and ID50 of 1,145 +/- 14 micromol/ kg; 200 mg/ kg) and neuropathic rats ( max. effect of 20 +/- 8% of control and ID50 of 414 +/- 27 micromol/ kg; 73 mg/ kg) but not in normal rats. The phenomenon of wind- up was dose- dependently reduced by gabapentin in neuropathy but not in normal and arthritic rats. Conclusion: We conclude that systemic gabapentin is a potent and effective antinociceptive agent in sensitization caused by arthritis and neuropathy but not in the absence of sensitization. The potency of the antinociception was directly related to the intensity of sensitization in the present experimental conditions. The effect is mainly located in central areas in neuropathy since wind- up was significantly reduced, however, an action on inflammation- induced sensitized nociceptors is also likely.