Compounds with species-specific effects on TRPA1. (A) The electrophilic thioaminals (with a reactive sulfur atom labeled by an asterisk) block human TRPA1, but activate rat TRPA1 (AMG9090, AMG5445, CMP1, CMP2, and CMP3) or have almost no effects (AMG7160 and AMG2504) (Klionsky et al., 2007; Chen et al., 2008). CMP4, with a sulfur-to-nitrogen substitution, is not reactive and has no effects on human or rat channel. (B) Menthol is an activator of mouse TRPA1 at low concentrations and a blocker at high concentrations; but it only activates human TRPA1 (Xiao et al., 2008). Caffeine activates mouse TRPA1 but blocks human TRPA1 (Nagatomo and Kubo, 2008).