Figure 1.

ICR, CB₁ wild-type (CB₁WT) or knock-out (CB₁KO) mice received capsaicin or vehicle (100% ethanol) into the plantar surface of the left hind paw. Volume of intraplantar paw administration was 5 μl. A, Number of flinches induced by capsaicin (1, 5, or 10 μg) or vehicle in ICR mice every minute for 5 min. B, Total number of flinches evoked by capsaicin or vehicle in 5 min in ICR mice. Capsaicin induced paw flinches in a dose-dependent manner, resulting in A₅₀ of 2.5 μg. C, Number of flinches induced by capsaicin (1, 5, or 10 μg) or vehicle in CB₁WT mice every minute for 5 min. D, Total number of flinches evoked by capsaicin or vehicle in 5 min in CB₁WT mice. Capsaicin induced paw flinches in a dose-dependent manner, resulting in A₅₀ of 3.2 μg. E, Number of flinches induced by capsaicin (1, 5, 10, 30, 100 μg) or vehicle in CB₁KO mice every minute for 5 min. F, Total number of flinches evoked by the different doses of capsaicin in 5 min was significantly abolished in CB₁KO mice (n = 6–12 in all studies).