Minocycline prevents the development of neuropathic pain, but not acute pain: Possible anti-inflammatory and antioxidant mechanisms

Glia, particularly astrocytes and microglia, are known to play an important role in central sensitization and are strongly implicated in the exaggerated pain states. In the present study, we determined the effect of minocycline, an inhibitor of microglial activation, in acute nociception, peritonitis, and the development and maintenance of hypersensitivity following chronic constriction injury of the sciatic nerve in rats. A single dose of minocycline (30 or 100 mg/kg, i.p.) 30 min before acetic acid or zymosan injection did not attenuate the nociceptive behavior in mice. It had no effect on the early events of peritoneal inflammation (vascular permeability, inflammatory cell infiltration, and release of pro-inflammatory cytokines) in acetic acid or zymosan-injected mice. In addition, minocycline (30 or 100 mg/kg, i.p.) did not alter basal nociceptive responses in the tail immersion test. Chronic administration of minocycline (10 or 30 mg/kg, i.p.) for 7 days started before nerve injury significantly prevented the development of neuropathic pain, interestingly, it further delayed the development of hypersensitivity. In contrast, single injection of minocycline failed to reverse hypersensitivity when administered during the development of neuropathic pain. No significant effects were observed on hypersensitivity when treatment was started once neuropathic state was established. Pre-treatment, but not post-treatment, with minocycline markedly attenuated increased pro-inflammatory cytokines release and oxidative and nitrosative stress in mononeuropathic rats. These results suggest that minocycline had no effect on acute peritoneal inflammation, nociception, and chronic administration of minocycline when started early before peripheral nerve injury could attenuate and further delays the development of neuropathic pain. Concluding, this study clearly shows minocycline, an inhibitor of microglial activation, by inhibiting the release of pro-inflammatory mediators and reducing oxidative stress prevented the development of neuropathic pain.