Fig. 1

Capsaicin-induced adrenaline secretion is completely impaired in TRPV1 knockout mice. We investigated the effect of capsaicin on adrenaline secretion in male adult C57BL/6 wild-type mice and TRPV1 knockout mice (obtained from Dr. D. Julius, University of California, San Francisco) according to the previous report with a slight modification [89]. A mouse anesthetized with α-chloralose and urethane (0.1 and 1 g/kg, respectively) was placed on heated pad, and the rectal temperature was maintained at 36.5–37.5 °C. Heparinized saline (500 IU/ml, 25 µl) was injected through the femoral vein, then sampling of adrenal blood from the adrenal vein was started with a 2-min interval. Immediately after collecting the first fraction, capsaicin (0.05 mg/kg) or vehicle (saline containing 2% ethanol and 10% Tween-80) was administered into the femoral vein. Plasma adrenaline was purified with active alumina and measured by HPLC-electrochemical detection. Intravenous administration of capsaicin significantly increased adrenaline secretion in wild-type mice (a) but not TRPV1 knockout mice (b). There was a significant difference between vehicle and capsaicin in a but not b (treatment effect, p < 0.01 by two-way ANOVA). **p < 0.01 by Bonferroni’s test. c Total adrenaline secretion for 20 min in a and b. *p < 0.05, **p < 0.01 by one-way ANOVA followed by Tukey’s test. Each value is the mean ± SEM (n = 5)