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.

Transient receptor potential (TRP) cation channel subfamily M member 3 (TRPM3), a member of the TRP channel superfamily, was recently identified as a nociceptor channel in the somatosensory system, where it is involved in the detection of noxious heat; however, owing to the lack of potent and selective agonists, little is known about other potential physiological consequences of the opening of TRPM3. Here we identify and characterize a synthetic TRPM3 activator, CIM0216, whose potency and apparent affinity greatly exceeds that of the canonical TRPM3 agonist, pregnenolone sulfate (PS). In particular, a single application of CIM0216 causes opening of both the central calcium-conducting pore and the alternative cation permeation pathway in a membrane-delimited manner. CIM0216 evoked robust calcium influx in TRPM3-expressing somatosensory neurons, and intradermal injection of the compound induced a TRPM3-dependent nocifensive behavior. Moreover, CIM0216 elicited the release of the peptides calcitonin generelated peptide (CGRP) from sensory nerve terminals and insulin from isolated pancreatic islets in a TRPM3-dependent manner. These experiments identify CIM0216 as a powerful tool for use in investigating the physiological roles of TRPM3, and indicate that TRPM3 activation in sensory nerve endings can contribute to neurogenic inflammation.TRP channel | TRPM3 | peptide release | nociceptor T ransient receptor potential (TRP) channels represent a large and diverse family of nonselective cation channels that respond to a wide range of chemical and physical stimuli and biophysical properties (1). TRP cation channel subfamily M member 3 (TRPM3), a calcium-permeable nonselective cation channel (2), is a typical example of a polymodally gated TRP channel, in that it can be activated by ligands, such as pregnenolone sulfate (PS) and nifedipine, as well as by heat and membrane depolarization (3, 4). Interestingly, recent evidence indicates that combined stimulation with PS and clotrimazole (Clt) leads to the activation of two distinct permeation pathways in TRPM3: the central pore, which is Ca 2+ -permeable and carries an outwardly rectifying current, and an alternative ion permeation pathway that mediates an inwardly rectifying monovalent cation current (5).TRPM3 is highly expressed in somatosensory neurons, where it plays decisive roles in the nocifensive response to PS and heat, as well as in the development of heat hyperalgesia during inflammation (3, 6). In these neurons, TRPM3 is frequently coexpressed with TRPA1 and TRPV1, two TRP channels that have emerged as key regulators of neurogenic inflammation by triggering neuropeptide release from sensory nerve endings (7,8). Whether activation of TRPM3 can also initiate the release of neuropeptides, such as substance P or calcitonin gene-related peptide (CGRP), which elicit vasodilation, vascular leakage, and other responses in peripheral cell types, is unclear, however. In addition, TRPM3 is expressed in pancreatic beta cells, where it is involved in controlling insulin rel...

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TRPM3, also known as melastatin 2 (MLSN2), LTRPC3 (long TRPC3) and KIAA1616, is a member of the TRPM subfamily of transient receptor potential (TRP) ion channels. The channel was originally identified as a volume-regulated ion channel that can be activated upon reduction of the extracellular osmolality. Later, the channel was proposed to be involved in the modulation of insulin release in pancreatic islets. However, new evidence has uncovered a role of TRPM3 as a thermosensitive nociceptor channel implicated in the detection of noxious heat. The channel is functionally expressed in a subset of neurons of the somatosensory system and can be activated by heat. The purpose of the present review is to summarize existing knowledge of the expression, biophysics and pharmacology of TRPM3 and to serve as a guide for future studies aimed at improving the understanding of the mechanism of thermosensation and proposed physiological functions of TRPM3.