Temperature is one of the most critical environmental factors affecting survival, and thus species that inhabit different thermal niches have evolved thermal sensitivities suitable for their respective habitats. During the process of shifting thermal niches, various types of genes expressed in diverse tissues, including those of the peripheral to central nervous systems, are potentially involved in the evolutionary changes in thermosensation. To elucidate the molecular mechanisms behind the evolution of thermosensation, thermal responses were compared between two species of clawed frogs (Xenopus laevis and Xenopus tropicalis) adapted to different thermal environments. X. laevis was much more sensitive to heat stimulation than X. tropicalis at the behavioral and neural levels. The activity and sensitivity of the heat-sensing TRPA1 channel were higher in X. laevis compared with those of X. tropicalis. The thermal responses of another heat-sensing channel, TRPV1, also differed between the two Xenopus species. The species differences in Xenopus TRPV1 heat responses were largely determined by three amino acid substitutions located in the first three ankyrin repeat domains, known to be involved in the regulation of rat TRPV1 activity. In addition, Xenopus TRPV1 exhibited drastic species differences in sensitivity to capsaicin, contained in chili peppers, between the two Xenopus species. Another single amino acid substitution within Xenopus TRPV1 is responsible for this species difference, which likely alters the neural and behavioral responses to capsaicin. These combined subtle amino acid substitutions in peripheral thermal sensors potentially serve as a driving force for the evolution of thermal and chemical sensation.Animals have evolved sophisticated physiological systems for sensing ambient temperatures, as fluctuations in environmental temperature significantly affect various biological processes. Species adapted to different thermal niches are likely to have acquired specific thermal sensitivities suitable for their respective habitats. Recent progress and understanding of the molecular mechanisms behind thermosensation has enabled us to elucidate the molecular basis for the evolution of thermosensation in the process of shifting thermal niches (1-3).In vertebrates, peripheral sensory neurons such as dorsal root ganglion (DRG) 3 and trigeminal ganglion neurons relay temperature information to the central nervous system. During the initiation of signal transduction, thermal stimuli are transduced into electrical signals and a subset of ion channels that are thermally activated plays crucial roles in this process. These ion channels belong to the transient receptor potential (TRP) ion channel superfamily and are called "thermoTRP" (1-3). Animals possess several kinds of thermoTRP channels, which have distinctive temperature ranges for activation. For example, TRPV1 is activated by heat (4 -7), whereas TRPM8 is activated by cold (1-3, 8).Temperature sensitivity among orthologous thermoTRP channels has changed duri...

Background: TRPV1 is a thermosensitive channel in mammalian, but little information is available on amphibian TRPV1 function. Results: Capsaicin, heat, and acid stimulated cloned and endogenous frog TRPV1 channels, and the former two stimuli evoked nocifensive behaviors. Conclusion: Frog TRPV1 functions as a capsaicin-, heat-, and acid-sensitive channel. Significance: TRPV1 may have acquired a physiological role for noxious detection soon after its evolutionary origin.

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