ABSTRACT
BACKGROUND: The stimulus provided by a subcutaneous injection of formalin is tonic, moderate, continuous pain. We evaluated the effect of formalin at three concentrations, 1.5%, 2.5%, and 5% - 50nl to determine the relationship between formalin concentration and pain behaviors. METHODS: Male Sprague-Dawley rats (250 - 300 g body weight) were used. Following formalin (n = 8 each group) or saline (control group, n = 6) injection, flinching, licking, lifting, and favoring responses were recorded during the early (0 - 5 minutes after injection; phase 1) and late phases (20 - 60 minutes after injection, phase 2). Sham-injected rats (n = 4) underwent subcutaneous insertion of the needle, but no substance was injected. RESULTS: During both phases, flinching was more frequent in the 5% formalin group than in the control group (P < 0.05). As for the licking behavior, phase 1 of 2.5% and 5% formalin groups and phase 2 of all three groups showed longer durations than those in the control group, respectively (P < 0.05). As for the lifting behavior, phase 2 of the 2.5% and 5% group showed a longer duration than the control group (P < 0.05). There was no significant biphasic response of favoring in each group. CONCLUSIONS: Our findings indicate that the formalin concentration (2.5% or higher) plays an important role in inducing the biphasic response. Flinching and licking were the more spontaneous and robust biphasic parameters. In both phases, flinching was robust for 5% formalin.