TRPA1, a member of the transient receptor potential (TRP) family of ion channels, is expressed by dorsal root ganglion neurons and by cells of the inner ear, where it has proposed roles in sensing sound, painful cold, and irritating chemicals. To test the in vivo roles of TRPA1, we generated a mouse in which the essential exons required for proper function of the Trpa1 gene were deleted. Knockout mice display behavioral deficits in response to mustard oil, to cold ( approximately 0 degrees C), and to punctate mechanical stimuli. These mice have a normal startle reflex to loud noise, a normal sense of balance, a normal auditory brainstem response, and normal transduction currents in vestibular hair cells. TRPA1 is apparently not essential for hair-cell transduction but contributes to the transduction of mechanical, cold, and chemical stimuli in nociceptor sensory neurons.

TRPA1 functions as an excitatory ionotropic receptor in sensory neurons. It was originally described as a noxious cold-activated channel, but its cold sensitivity has been disputed in later studies, and the contribution of TRPA1 to thermosensing is currently a matter of strong debate. Here, we provide several lines of evidence to establish that TRPA1 acts as a cold sensor in vitro and in vivo. First, we demonstrate that heterologously expressed TRPA1 is activated by cold in a Ca 2؉ -independent and Ca 2؉ store-independent manner; temperature-dependent gating of TRPA1 is mechanistically analogous to that of other temperature-sensitive TRP channels, and it is preserved after treatment with the TRPA1 agonist mustard oil. Second, we identify and characterize a specific subset of cold-sensitive trigeminal ganglion neurons that is absent in TRPA1-deficient mice. Finally, cold plate and tail-flick experiments reveal TRPA1-dependent, cold-induced nociceptive behavior in mice. We conclude that TRPA1 acts as a major sensor for noxious cold.cold sensing ͉ pain ͉ sensory neurons ͉ TRP channels S ensing the environmental temperature is essential for animals to maintain thermal homeostasis and to avoid prolonged contact with harmfully hot or cold objects (1). Our understanding of the molecular basis of thermosensation has made great strides with the discovery that several members of the transient receptor potential (TRP) cation channel family exhibit highly temperature-sensitive gating and are expressed in cells of the sensory system (1). Mice lacking specific temperature-sensitive TRP channels illustrate how these channels serve as molecular thermometers in the peripheral sensory system (2). At least 3 heat-activated members of the TRPV subfamily (TRPV1, TRPV3, and TRPV4) are critically involved in sensing hot temperatures. TRPM8, a channel activated by cold temperatures and cooling compounds, such as menthol, plays a major role in cold sensing (1). Importantly, although TRPM8-deficient mice exhibit significant deficits in cold sensing in the temperature range between 28°C and 15°C, they retain a normal response to noxious cold temperatures, demonstrating the existence of TRPM8-independent mechanisms to detect noxious cold (3-5). TRPA1 has been put forward as a potential candidate to mediate detection of noxious cold, based on its expression in nociceptive neurons, and on the finding that heterologously expressed TRPA1 in CHO cells is activated by cold temperatures with a lower temperature threshold for activation than TRPM8 (6-8).At this point, however, the role of TRPA1 in (noxious) cold sensing is highly controversial. First, there is no consensus as to whether TRPA1 is directly gated by cold temperatures. Two groups have reported that they failed to detect cold-induced activation of heterologously expressed TRPA1 (9, 10), and a third report suggested that cold-induced activation of TRPA1 in overexpression systems is an indirect effect, caused by cold-induced Ca 2ϩ release from intracellular stores and subsequent Ca 2ϩ -d...

Mechanical deflection of the sensory hair bundles of receptor cells in the inner ear causes ion channels located at the tips of the bundle to open, thereby initiating the perception of sound. Although some protein constituents of the transduction apparatus are known, the mechanically gated transduction channels have not been identified in higher vertebrates. Here, we investigate TRP (transient receptor potential) ion channels as candidates and find one, TRPA1 (also known as ANKTM1), that meets criteria for the transduction channel. The appearance of TRPA1 messenger RNA expression in hair cell epithelia coincides developmentally with the onset of mechanosensitivity. Antibodies to TRPA1 label hair bundles, especially at their tips, and tip labelling disappears when the transduction apparatus is chemically disrupted. Inhibition of TRPA1 protein expression in zebrafish and mouse inner ears inhibits receptor cell function, as assessed with electrical recording and with accumulation of a channel-permeant fluorescent dye. TRPA1 is probably a component of the transduction channel itself.