• Sinyoung Kang, Chi Hyo Kim, Heeseung Lee, Dong Yeon Kim, Jong In Han, Rack Kyung Chung, and Guie Yong Lee.
• Department of Anesthesiology and Pain Medicine, Ewha Womans University Medical College, Seoul, Korea.
• Anesth. Analg. 2007 Mar 1; 104 (3): 719-25.

BackgroundThe analgesic interaction between cannabinoids and local anesthetics has not been investigated. We sought to determine the nature of the interaction between the intrathecal cannabinoid receptor agonist (WIN 55,212-2) and bupivacaine using the formalin test.MethodsLumbar intrathecal catheters were implanted in male Sprague-Dawley rats. After intrathecal administration of WIN 55,212-2, bupivacaine, or their combination, 50 microL of 5% formalin was injected subcutaneously into the hindpaw. Dose-response curves were established and the respective ED50 (50% effective dose) values were determined for each agent alone. Fixed-ratio combinations of WIN 55,212-2 and bupivacaine were tested for combined antinociceptive effects in the formalin test and an isobolographic analysis was performed to characterize the pharmacologic interaction of both drugs.ResultsIntrathecally administered WIN 55,212-2, bupivacaine, or their combination produced a dose-dependent decrease in the number of flinches during Phase 1 and 2 of the formalin test. Isobolographic analysis revealed a synergistic interaction between intrathecal WIN 55,212-2 and bupivacaine in both phases of the formalin test. In combination, the ED50 value was significantly smaller than the theoretical additive value (P < 0.05).ConclusionsThese results demonstrate that intrathecally coadministered WIN 55,212-2 and bupivacaine provide synergistic antinociceptive interaction in both phases of the formalin test.

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