TRPA1 expression and distribution in the trachea and skin tissues of guinea pigs, as detected by immunohistochemistry. (A) TRPA1 expression and distribution (shown as brown staining) in the trachea of different groups of guinea pigs. (B) TRPA1 expression and distribution (shown as brown staining) in the skin of different groups of guinea pigs. (C) Digital quantitative presentation of TRPA1 expression in the trachea of different groups of guinea pigs, demonstrating that TRPA1 overexpression in the trachea was induced by the repetitive inhalation of citric acid (model) and further enhanced, following exposure of the whole body (model, whole-body exposure) and trunk-limbs (model, trunk-limb exposure), but not the head (model head exposure), to a cold environment. (D) Digital quantitative presentation of TRPA1 expression in the skin of different groups of guinea pigs, showing that TRPA1 overexpression in the skin was induced in healthy guinea pigs (whole-body control) and further elevated, following exposure of the whole body (model, whole-body exposure) and trunk-limbs (model, trunk-limb exposure), but not the head (model, head exposure), to a cold environment. The original magnification was 400× for panels (A,B), and the quantitative values (mean ± SD) of stained epithelia are presented for panels (C,D) (n = 5; *p < 0.05, **p < 0.01, ****p < 0.0001).