Analgesic effects of intravenous curcumin in the rat formalin test

Curcumin (Curcuma longa, [1E,6E]-1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione) has been reported to have anti-inflammatory, antioxidant, antiviral, antifungal, antitumor, and antinociceptive action. In this study, the analgesic efficacy of intravenous curcumin for inflammatory pain was investigated using adult male Sprague-Dawley rats weighing between 200 and 300 g. Curcumin (1, 2.5, 5, and 10 mg/kg) and normal saline solution (0.2 mL) or DMSO (1 %, 0.2 mL) were intravascularly administered, respectively, to each group of five rats 30 s before the formalin test was performed, where phase I and II data were collected. Immunohistochemical analysis was performed to assess the localization of inflammatory cytokines (IL-1β, IL-6, and TNF-α) in the L4–L6 spinal cord of three rats from each of 1 mg/kg, 10 mg/kg and the control (DMSO, 1 %, 0.2 mL) group. It was found that the number of flinching responses were significantly decreased for 1 mg/kg and 10 mg/kg groups in phase I (p < 0.05) compared to the control group. However, there were no significant differences between the curcumin and control groups in phase II. In this phase, appearance of the peak point shifted to the right in the curcumin groups. The sum of the numbers of the responses from phase I and II did not appear to be significantly decreased by the amount of curcumin. As the peak point of the Time Effect Curve for phase I progressed over time over phase II, the mean plaque shifted to the right. The peak at 30 min was significantly delayed more than 10 min in 5 and 10 mg/kg groups compared to the controls. Immunohistochemistry results do not seem to be significantly different in each group except in IL-1β in curcumin 10 mg/kg group.