Noxious behaviours from injection of formalin and morphine tolerance of addicted male rats

Abstract Background: Formalin as a chemical noxious stimulus evokes biphasic pain that its latter phase as a chronic inflammation pain is similar to clinical pain. Objective: Assessment of noxious behaviours from subcutaneous injection of formalin and morphine tolerance effects on the responses of addicted male rats. Methods: Flinching and licking behaviours scored as quantitative pain. Experiments were carried out in normal and morphine addicted rats. Behaviour scores measured quantitative after injection of formalin in plantar region of hind paw. Assessment of morphine tolerance was carried out by intrapritoneal injection of morphine (10mg/Kg) 10 minute before formalin injection. Findings: In this study, we observed biphasic model of formalin test. Morphine injection in normal animals supressed noxious behaviours. Flinching and licking responses evoked by formalin not affected by morphine injection. Licking scores in addicted rats have significant difference from normal rats (p<0.05). Conclusion: Addicted rats showed tolerance to morphine analgesia after formalin injection. Hyperalgesic effects of formalin injection observed in addicted rats.