Locally Injected Dexmedetomidine Inhibits Carrageenin-Induced Inflammatory Responses in the Injected Region

Dexmedetomidine, a highly selective agonist of α2-adrenoceptors, is a commonly used sedative; however, a potent anti-inflammatory effect has also been found. In the present study we evaluated the inhibitory effect of locally injected dexmedetomidine on inflammatory responses in the injected region. Local inflammation was induced in the hindpaws of male mice (aged 6-8 weeks) by intraplantar injection of lambda-carrageenin. To offset the central effect of tested agents, different agents were blindly injected into the left and right paws in the pairs of comparison. The effect of dexmedetomidine on edema (increase in paw volume), the accumulation of leukocytes, and production of tumor necrosis factor-α (TNF-α) and cyclooxygenase-2 (COX-2) were evaluated after carrageenin injection, using water displacement plethysmometry, histological imaging, immunohistochemistry, and Western blotting analysis. Furthermore, we also evaluated the effect of yohimbine, a full antagonist of α2-adrenoceptors, and phenylephrine, an agonist of the α1-adrenoceptor, on dexmedetomidine's action on inflammatory responses. Paw volume and amount of leukocytes in the injected region significantly increased after the injection of carrageenin. Similarly, TNF-α and COX-2 production was found in the subcutaneous region injected with carrageenin, 4 hours after injection. Dexmedetomidine significantly inhibited all increases in paw volume, leukocytes, and production of TNF-α and COX-2. Furthermore, yohimbine significantly antagonized the anti-inflammatory effects of dexmedetomidine, whereas phenylephrine did not significantly alter them. The findings suggest that locally injected dexmedetomidine exhibits an anti-inflammatory effect against local acute inflammatory responses, mediated by α2-adrenoceptors.