Change of nociceptive response and hind paw edema following ethyl pyruvate (EP) administration. (A) Effects of the systemic administration of ethyl pyruvate (EP) (10, 50, and 100 mg/kg) on paw licking and lifting responses following intraplantar injection of formalin (5%, 50 μl) into the hind paw. The rats receiving saline vehicle showed typical biphasic nociceptive behavior. While phase I nociceptive response was similar between the saline- and EP-injected rats, phase II nociceptive behavior was significantly reduced by EP administration 1 hour prior to formalin injection. All results are presented as mean ± SEM. Student's t test was performed at each time point after formalin injection. Values are expressed as mean ± SEM. *P < 0.01; **P < 0.05 vs. control rats (saline-pretreated and formalin-treated). Saline (n = 15), EP 10 mg/kg (n = 14), 50 mg/kg (n = 10), and 100 mg/kg (n = 13). (B) Total time of nociceptive behaviors during phase II. Total times of licking and lifting were attenuated by EP in a dose-related fashion following intraplantar injection of formalin. Values are expressed as mean ± SEM. *P < 0.01 vs. control rats (saline-pretreated and formalin-treated). (C) Effects of EP on the magnitude of hind paw edema following formalin injection. An index of paw edema was calculated as the mean difference of paw thickness (thickness of the ipsilateral paw after injection/thickness of the ipsilateral paw before injection × 100). EP significantly reduced formalin-induced edema compared to the control. Values are expressed as mean ± SEM. *P < 0.01 vs. control rats (saline-pretreated and formalin-treated).