Evolutionary tuning of TRPA1 and TRPV1 thermal and chemical sensitivity in vertebrates.

Thermal perception is an essential sensory system for survival since temperature fluctuations affect various biologic processes. Therefore, evolutionary changes in thermosensory systems may have played important roles in adaptation processes. Comparative analyses of sensory receptors among different species can provide us with important clues to understand the molecular basis for adaptation. Several ion channels belonging to the transient receptor potential (TRP) superfamily serve as thermal sensors in a wide variety of animal species. These TRP proteins are multimodal receptors that are activated by temperature as well as other sensory stimuli. Among them TRPV1 and TRPA1 are activated by noxious ranges of thermal stimuli and irritating chemicals, and are mainly expressed in nociceptive sensory neurons. Comparative analyses of TRPV1 and TRPA1 among various vertebrate species revealed evolutionary changes that likely contributed to diversification of sensory perception. Whereas heat-induced TRPV1 responses have been conserved across many vertebrates, TRPA1 varied among species. Mutagenesis experiments using these two channels from various species also helped characterize the molecular basis for their activation and inhibition. Meanwhile, recent detailed comparative analyses using closely related species showed shifts in TRPV1 and TRPA1 thermal sensitivity that allowed adaptation to different thermal environments. Changes in TRPV1 heat responses appear to arise from just a few amino acid differences among species. These observations suggest that evolutionary changes in peripheral sensors are likely driving force for shifting thermal perception in adaptation processes.