Blockade of spinal 5-HT3 receptors attenuates formalin-evoked hyperalgeisa. (A) Flinches/minute plotted versus time after injection of formalin in rats treated with IT injection of ondansetron (ODS, 1 µg), or vehicle (saline, 10 µl). Each point represents the number of flinches per min. (B) Histogram shows inhibitory effects of IT ondasertron at doses of 1 and 3 µg on phase I and phase II flinching behavior. The bars represent the total number of flinches in each phase. The data are presented as mean ± SEM of 4–11 rats/group. * p < 0.05, ODS vs. saline, one-way ANOVA.