TRPA1 is a calcium-permeable nonselective cation transient receptor potential (TRP) channel that functions as an excitatory ionotropic receptor in nociceptive neurons. TRPA1 is robustly activated by pungent substances in mustard oil, cinnamon, and garlic and mediates the inflammatory actions of environmental irritants and proalgesic agents. Here, we demonstrate a bimodal sensitivity of TRPA1 to menthol, a widely used cooling agent and known activator of the related cold receptor TRPM8. In whole-cell and single-channel recordings of heterologously expressed TRPA1, submicromolar to low-micromolar concentrations of menthol cause channel activation, whereas higher concentrations lead to a reversible channel block. In addition, we provide evidence for TRPA1-mediated menthol responses in mustard oil-sensitive trigeminal ganglion neurons. Our data indicate that TRPA1 is a highly sensitive menthol receptor that very likely contributes to the diverse psychophysical sensations after topical application of menthol to the skin or mucous membranes of the oral and nasal cavities.

Here we provide evidence for a critical role of the transient receptor potential cation channel, subfamily V, member 4 (TRPV4) in normal bladder function. Immunofluorescence demonstrated TRPV4 expression in mouse and rat urothelium and vascular endothelium, but not in other cell types of the bladder. Intracellular Ca 2+ measurements on urothelial cells isolated from mice revealed a TRPV4-dependent response to the selective TRPV4 agonist 4α-phorbol 12,13-didecanoate and to hypotonic cell swelling. Behavioral studies demonstrated that TRPV4 -/-mice manifest an incontinent phenotype but show normal exploratory activity and anxietyrelated behavior. Cystometric experiments revealed that TRPV4 -/-mice exhibit a lower frequency of voiding contractions as well as a higher frequency of nonvoiding contractions. Additionally, the amplitude of the spontaneous contractions in explanted bladder strips from TRPV4 -/-mice was significantly reduced. Finally, a decreased intravesical stretch-evoked ATP release was found in isolated whole bladders from TRPV4 -/-mice. These data demonstrate a previously unrecognized role for TRPV4 in voiding behavior, raising the possibility that TRPV4 plays a critical role in urothelium-mediated transduction of intravesical mechanical pressure.

Acute pain represents a crucial alarm signal to protect us from injury. Whereas the nociceptive neurons that convey pain signals were described more than a century ago, the molecular sensors that detect noxious thermal or mechanical insults have yet to be fully identified. Here we show that acute noxious heat sensing in mice depends on a triad of transient receptor potential (TRP) ion channels: TRPM3, TRPV1, and TRPA1. We found that robust somatosensory heat responsiveness at the cellular and behavioural levels is observed only if at least one of these TRP channels is functional. However, combined genetic or pharmacological elimination of all three channels largely and selectively prevents heat responses in both isolated sensory neurons and rapidly firing C and Aδ sensory nerve fibres that innervate the skin. Strikingly, Trpv1Trpm3Trpa1 triple knockout (TKO) mice lack the acute withdrawal response to noxious heat that is necessary to avoid burn injury, while showing normal nociceptive responses to cold or mechanical stimuli and a preserved preference for moderate temperatures. These findings indicate that the initiation of the acute heat-evoked pain response in sensory nerve endings relies on three functionally redundant TRP channels, representing a fault-tolerant mechanism to avoid burn injury.

Glucose homeostasis is critically dependent on insulin release from pancreatic β-cells, which is strictly regulated by glucose-induced oscillations in membrane potential (V m ) and the cytosolic calcium level ([Ca 2+ ] cyt ] cyt ) (1-4). This pattern, consisting of slow waves of depolarized plateaus on which bursts of action potentials are superimposed and separated by electrically silent intervals, plays a critical role in the regulation of insulin secretion. Indeed, in the absence of depolarization, no insulin is released, and the "extent" of electrical activity largely determines the amount of released insulin (1,5 ] cyt ), and the cellular metabolism of the β-cell (1, 6). The increase in [Ca 2+ ] cyt originates from glucoseinduced Ca 2+ influx through voltage-gated L-type Ca 2+ channels and, possibly, Ca 2+ mobilization from intracellular stores, the latter promoted by activation of the phospholipase C system and generation of inositol 1,4,5-trisphosphate (1,7,8). Despite intensive investigation, several aspects of the rhythmic electrical activity of β-cells, such as the origin of the variability in oscillation pattern, remain unclear. Indeed, glucose stimulation can result in highfrequency short bursts, low-frequency long bursts, or a combination of these two patterns, also known as compound bursts (9, 10).In this study we identified TRPM5 as a player in the electrical activity of glucose-stimulated pancreatic β-cells. TRPM5 is one of 28 members of the large transient receptor potential (TRP) superfamily (11-13). TRPM5, and its close homologue TRPM4, are Ca 2+ -activated cation channels that are permeable for monovalent cations, but not divalent cations, with a conductance of approximately 20 to 25 pS (14-16). Using Trpm5−/− mice we show here that this channel promotes high-frequency glucose-induced oscillations in V m and [Ca 2+ ] cyt in pancreatic β-cells. Loss of TRPM5 expression, and high-frequency bursting, is functionally relevant as this leads to reduced glucose-induced insulin release from isolated islets and impaired glucose tolerance. Results and DiscussionExpression of the Trpm5 Gene in Pancreatic β-Cells. Previously, Trpm5 expression was shown on the mRNA level in several β-cell lines and in human and mouse tissues, including taste buds, intestine, and pancreatic islets (13,(17)(18)(19)(20)(21)(22). Here we describe immunostaining of TRPM5 protein in pancreatic islets with a specific antibody (Fig. 1). TRPM5 is colocalized with insulin in WT islets, strongly suggesting expression of TRPM5 in insulin secreting β-cells. Specific staining with the TRPM5 antibody is absent in Trpm5 −/− islets. Quantitative PCR experiments in a purified β-cell sample confirmed expression of Trpm5 in the β-cells. Expression of Trpm5 could also be detected in purified α-cells, although to a lower level compared with β-cells (Fig. S1). Characterization of a Ca2+ Release-Activated Cation Current in Pancreatic Islet Cells. To determine whether TRPM5 is part of the Ca 2+-activated monovalent cation current descri...

Steviol glycosides (SGs), such as stevioside and rebaudioside A, are natural, non-caloric sweet-tasting organic molecules, present in extracts of the scrub plant Stevia rebaudiana, which are widely used as sweeteners in consumer foods and beverages. TRPM5 is a Ca2+-activated cation channel expressed in type II taste receptor cells and pancreatic β-cells. Here we show that stevioside, rebaudioside A and their aglycon steviol potentiate the activity of TRPM5. We find that SGs potentiate perception of bitter, sweet and umami taste, and enhance glucose-induced insulin secretion in a Trpm5-dependent manner. Daily consumption of stevioside prevents development of high-fat-diet-induced diabetic hyperglycaemia in wild-type mice, but not in Trpm5−/− mice. These results elucidate a molecular mechanism of action of SGs and identify TRPM5 as a potential target to prevent and treat type 2 diabetes.