Fig. 1. Pain-related behaviors (A ) and referred mechanical hyperalgesia (B ) induced by intracolonic administration of different concentrations of capsaicin (0.01–1%) or its solvent (0) in WT and σ1-KO mice. The behavioral pain responses (licking of abdomen, stretching, abdominal retractions) were recorded during the first 20 min after instillation of capsaicin, and referred mechanical hyperalgesia (evaluated by stimulation of the abdomen with von Frey filaments) was measured at 20 min after instillation of capsaicin. Each point and vertical line represents the mean ± SEM of values obtained in 12 WT or 10 σ1-KO mice per group. Statistically significant differences between the values obtained in capsaicin- and solvent-treated animals: ##P < 0.01. Statistically significant differences between the values obtained in WT and σ1-KO animals at the same concentration of capsaicin: **P < 0.01 (two-way ANOVA followed by Bonferroni test). WT, wild type; σ1-KO, σ1receptor knockout.