Mechanical hyperalgesia/allodynia induced by injection of CFA into the right masseter muscle of the rat. The EF50s were derived from the respective stimulus-response frequency function curves and are plotted against time. Note log scales for the ordinate and abscissa. A. Ipsilateral site. Note significant decreases in EF50s at 30 min-7 d after CFA injection, indicating inflammatory hyperalgesia/allodynia. B. Contralateral site. There were significant reductions in EF50s at 180 min-3 d after CFA, indicating the development of secondary hyperalgesia after masseter inflammation. C. To verify muscle hyperalgesia after injection of CFA into the masseter, local anesthesia was induced in the masseter or the overlying skin ipsilateral to inflammation by bupivacaine (0.75%, 0.1 ml) at 20 min before CFA injection. Compared to saline-treated rats (Sal Mass, n = 5), hyperalgesia was eliminated when the masseter was anesthetized (Bupi Mass, n = 5) and reappeared at 24 h after CFA. Local anesthesia of the skin overlying masseter (Bupi Skin, n = 8) did not produce a significant effect on the development of hyperalgesia. Asterisks in panels A and B denote significant differences from the baseline values (*, p < 0.05; **, p < 0.01). Pound signs in C indicate significant differences from the saline control (Sal Mass) (p < 0.01). (ANOVA with repeated-measures and post hoc tests).