Regulation of Pain and Itch by TRP Channels

TRPA1 is a Universal Chemo-Irritant Receptor Involved in Neuropathic Cold-Pain

TRPA1 is a polymodal nocisensor gated by a wide range of chemical irritants. It is present in a subpopulation of Aδ- and C-fiber nociceptive neurons in dorsal root, trigeminal, and vagal (nodose or jugular) ganglia. TRPA1 is activated by pain-inducing natural products, including mustard oil (allyl isothiocyanate), the pungent ingredient in mustard, wasabi, and horseradish, cinnamaldehyde from cinnamon, and pungent compounds in garlic (allicin) and onions (diallyl disulfide) [17, 20–22]. These natural products evoke pain-related behaviors in animals, and pain and irritation in humans (Fig. 1). The chemical reactivity and structural diversity of these compounds suggest that TRPA1 does not fit into the mold of the traditional definition of a pharmacological receptor, but may in fact be a chemical reactivity detector, acting as a chemical warning sensor that translates chemical reactivity into a pain signal. Indeed, reactive cysteine residues in the intracellular N-terminal domain of TRPA1 are essential for the majority of electrophilic agonists to gate the ion channel [23, 24]. More recent studies have discovered additional sites within the ion channel protein, including lysine residues, that also contribute to reactivity detection [24–26]. TRPA1 is also sensitive to non-reactive irritating natural products, including carvacrol (from clove), thymol (from thyme), gingerol (from ginger), and menthol (mint) [27–29].

Chemosensory nerve endings in the cornea, nose, and larynx constantly monitor the environment for airborne chemical threats, which initiate defensive reflexes such as lachrymation, sneezing, and coughing. The upper and lower respiratory tract are innervated by primary afferent C-fibers and Aδ-fibers. The airway-innervating trigeminal and vagal ganglia show appreciable TRPA1 expression [20, 30–32]. A significant number of airborne chemicals including industrial pollutants, but also natural substances, induce airway irritation by activating sensory neurons innervating the lung/airway in a TRPA1-dependent manner [28, 33–36]. TRPA1 expressed in airway-innervating neurons mediates acute irritation responses to tobacco smoke and smoke from fires, activated by the smoke irritants acrolein and croton aldehyde [28, 32, 36–38].

The most potent TRPA1 agonists identified so far are tear gas agents such as 2-chloroacetophenone and 2-chlorobenzalmalononitrile that activate TRPA1 in the low nanomolar or picomolar range [39, 40]. TRPA1-deficient mice lack acute nocifensive responses to these extremely painful agents [40]. TRPA1 is also sensitive to oxidizing chemical exposure, including chlorine gas, hydrogen peroxide, and ozone [32, 34]. Respiratory reflex responses to such exposure are absent in TRPA1-deficient mice [32, 34]. TRPA1 activity also contributes to the toxicological effects of particulates and metals such as zinc, cadmium, and copper, underscoring the essential role of TRPA1 in the detection of toxic environmental agents [41–43]. Airborne environmental exposure to contaminants such as ozone, particulates, and acrolein affect cardiovascular function even at very low levels, initiating changes in blood pressure and heart rate. TRPA1 activation by inhaled diesel exhaust and acrolein has been demonstrated to initiate cardiac arrhythmia in mice and rats, likely through changes in vagal sensory-autonomic control of cardiac function [44, 45].

While the function of TRPA1 as a chemosensor is firmly established, its role in thermal sensing remains a matter of debate. In mammals, exposure to temperatures below ~ 15 °C can elicit pain. TRPA1 was initially identified as a sensor for noxious cold; however, this study under-reported the prevalence of TRPA1-expressing sensory neurons that turned out to be much larger than the population responding to noxious cold [20, 46]. Studies in TRPA1-deficient mice have reported contradictory data, some finding no difference in noxious cold-induced nocifensive behavior [36, 47–49], and also when TRPA1 expression is specifically ablated in sensory neurons or inhibited by a selective antagonist [50, 51]. The complete ablation of TRPA1-expressing sensory neurons does not change the sensitivity of trigeminal nerves to cold [52]. Other studies have detected diminished responses to cold stimuli in TRPA1-deficient mice; however, the mice had been exposed to freezing temperatures that may have damaged tissue and induced inflammation [13, 53]. The thermal sensitivity of TRPA1 is clearly species-specific, some reports finding that human and other primate orthologs lack cold sensitivity, while others finding bi-modal (cold and heat) sensitivity [12, 54–56].

TRPA1 channels are highly sensitive to Ca²⁺, which has bimodal effects on channel function. Ca²⁺ potentiates and activates TRPA1, but is also essential for TRPA1 desensitization and can be indirectly regulated by G protein-coupled receptors (GPCRs), such as bradykinin receptors and proteinase-activated receptors (PAR2) or CGRP-signaling [57].

Less controversial is the role of TRPA1 in the cold allodynia associated with inflammatory and neuropathic conditions, including chemotherapy-induced neuropathies after treatment with platinum or taxol therapeutics [25, 58–61] In this case, TRPA1 may serve as an amplifier, increasing the excitability of cold-sensitive neurons [58, 62]. A proposed mechanism involves reactive oxygen species (ROS) signaling induced by cooling [63]. Inhibition of hydroxylation of a proline residue in a human TRPA1 N-terminal ankyrin repeat either by mutation or using a prolyl hydroxylase inhibitor potentiates the cold sensitivity of TRPA1 in the presence of hydrogen peroxide. The same study showed that inhibiting prolyl-hydroxylase in mice triggers TRPA1 sensitization which is sufficient to sense cold-evoked ROS, hence causing cold hypersensitivity. Furthermore, this mechanism is thought to underlie the acute cold hypersensitivity induced by the chemotherapeutic agent oxaliplatin or its metabolite oxalate [63].

TRPA1 has also been implicated in diabetic neuropathic pain, caused by chronic reactive chemical stress due to metabolic imbalance. In rodent models, TRPA1 inhibitors have been shown to protect nerves from peripheral degeneration and reduce nocifensive responses [64, 65]. Methyl glyoxal, a reactive metabolic product that is increased in diabetics, is a TRPA1 agonist that may contribute to chronic heightened channel activity, leading to increased pain sensation, Ca²⁺ overload, and subsequent peripheral degeneration [66, 67].

TRPA1 in Inflammatory Pain

TRPA1 sensitization leads to hyperalgesia in response to various stimuli. Its pathophysiological role in inflammation is based on its ability to activate in response to various mediators and metabolites produced under inflammatory conditions. Pro-inflammatory mediators released from non-excitable cells include ATP, bradykinin (BK), prostaglandins, leukotrienes, histamine, tumor necrosis factor-α, interleukin-1β (IL-1β), proteases, and glutamate.

BK indirectly activates TRPA1 channels to mediate pain and the inflammatory response. This requires BK to interact with its receptor (B2R), which further activates the phospholipase C (PLC)-dependent signaling pathway and promotes phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolysis to produce inositol triphosphate (IP₃) and diacylglycerol (DAG). Both lipids IP₃ and DAG can activate TRPA1 [17]. Trpa1^−/− mice have reduced thermal and mechanical pain responses to intraplantar injection of BK or allyl isothiocyanate [36].

Agents such as Complete Freund’s Adjuvant (CFA) are widely used to evoke inflammatory pain equivalents in preclinical models. This leads to increased expression of TRPA1 in dorsal root ganglion (DRG) neurons [30]. Furthermore, the TRPA1 antagonist HC-030031 reduces nocifensive behaviors induced by paw injection of formalin and suppresses mechanical hyperalgesia in the CFA model of inflammatory pain [68].

Human TRPA1 is also activated by acidosis which occurs in tissue ischemia such as myocardial infarction or peripheral vascular occlusive disease [69]. However, initial attempts to combat acidosis-evoked pain by targeting TRPA1 have not been met with success in humans [70]. Ischemia leads to an increase in ROS generation which results in the formation of reactive carbonyl species like 4-hydroxynonenal and 4-oxononenal, which act directly on TRPA1 [71, 72].

Recent studies suggest that TRPA1 may also be expressed in non-neuronal tissues. The strongest evidence for non-neuronal expression was provided by studies on intestinal enterochromaffin cells that respond to a TRPA1 agonist with the release of serotonin that, in turn, activates underlying sensory nerve endings, a mechanism regulating gastrointestinal motility [73, 74]. TRPA1 expression has also been reported in some permanent cell lines, including mast cells, fibroblasts, and epithelial lines. Quantitative PCR studies have shown that neuronal transcript levels are at least several orders of magnitude higher than those in extra-neuronal tissues, making validation of extra-neuronal expression in other organs difficult [75]. Tissue detection of TRPA1 is also hampered by the poor specificity and validation of the available antibodies, and the non-specific effects of the chemically-reactive TRPA1 agonists. Recent studies using strategies to specifically ablate TRPA1 expression in sensory neurons have recapitulated most of the findings from global TRPA1-deficient mice, including the absence of acute and inflammatory pain modalities and hypersensitivity responses, suggesting that neuronal TRPA1 accounts for most of the TRPA1-dependent physiological mechanisms [51, 52, 76]. However, as in the case of enterochromaffin cells, the expression of TRPA1 in small cell populations may be difficult to detect. Additional sites with reported TRPA1 expression are melanocytes, odontoblasts, insulin-producing β-cells of the islets of Langerhans in the pancreas, and vascular endothelial cells. Via its endothelial expression, TRPA1 might regulate vascular tone, and might also play a role in the development of atherosclerotic disease [77].

Trigeminally-mediated pain involving TRPA1 channels could be relevant to the pathogenesis of headaches, especially migraine pain. TRPA1 channels have been shown to signal in response to the plant-derived headache- and migraine-inducing volatile compound umbellone, derived from the “headache tree”, Umbellaria californica [78, 79]. Since migraine is thought to occur when meningeal trigeminal nerves are activated, intraganglionic signaling between nasal airway-innervating and meningeal trigeminal branches would be a prerequisite [80]. Such a mechanism has been proposed for other environmental irritants that trigger headaches, including acrolein, the TRPA1 agonist in smoke and air pollution. Extended inhalational exposure of mice to acrolein has been shown to increase meningeal vascular blood flow, supporting such a mechanism [81, 82]. A common pro-migraine pathogenic mechanism is the release of calcitonin gene-related peptide (CGRP) from primary trigeminal sensory neurons, known to be triggered after TRPA1 activation [83]. CGRP is a strong meningeal vasodilator, increasing blood flow, and is a target of a novel antimigraine treatment successfully tested in clinical trials. Several clinically-effective antimigraine medicines, including herbal compounds such as petasin and parthenolides, have been shown to modulate TRPA1 channels, implicating TRPA1 as an important contributor to migraine and headaches, and as an additional target for innovative migraine drug development [83–86].

The pro-inflammatory actions of TRPA1 extend beyond pain. Recent studies suggest that TRPA1 is involved in inflammatory pathology of the respiratory system, including allergic and chemically-induced asthma and smoking-induced inflammation [37, 87, 88]. TRPA1 inhibitors, or deletion of the TRPA1 gene, strongly reduce asthmatic airway contractions and pulmonary inflammation in mice and rats allergic to the experimental allergen, ovalbumin [87].

Animal studies have revealed that the combined role of TRPA1 as a pain transducer and regulator of inflammation extends to many other inflammatory pain conditions, including arthritis and gout [89–91]. The gastrointestinal tract is also innervated by TRPA1-expressing nerve fibers. Here, TRPA1 plays a prominent role in acute mechanical distension pain of the colon and in gastrointestinal inflammatory pain and hypersensitivity during pancreatitis as well as in esophagitis due to chemical injury or allergic sensitization [92–103]. A recent study of colonic pain mechanisms reported that capsazepine desensitizes colonic afferents after the induction of colonic inflammation, yet this process is surprisingly independent of Trpv1, and apparently functions via the desensitization of TRPA1 expression in sensory neurons innervating the colon. In visceral pain of the colon and pancreas, there appears to be a particular synergy between TRPV4 and TRPA1 (see also the section on TRPV4 below) [104].

TRPA1 in an Inherited Episodic Pain Syndrome and Other Forms of Pain

The discovery of a human gain-of-function mutation in TRPA1 has greatly validated the role of TRPA1 as a “pain-TRP”. This missense mutation is associated with a familial episodic pain syndrome [105] which is characterized by the presence of severe pain in the upper body accompanied by breathing difficulty, tachycardia, sweating, generalized pallor, and stiffness of the abdominal wall [105]. These painful episodes are more likely if a carrier of the mutation has experienced a challenge through fasting, fatigue, and exposure to cold and/or physical stress. Up to this point, among the entire TRP superfamily of channels, this hereditary pain syndrome caused by a gain-of-function point mutation in TRPA1 is the only TRP-channelopathy in humans associated exclusively with pain.

This mutation is located in the linker region that connects transmembrane segments 4 and 5, thus intracellularly, where substitution of the asparagine 855 by a serine (N855S) changes the voltage-dependence to more negative potentials, thus increasing TRPA1 activity without modifying its affinity for some activators [105, 106].

Interestingly, the N855 position in TRPA1 has been shown to be involved in the inhibitory action of the TRPA1 antagonist HC-030031 in species-selectivity studies of human versus frog TRPA1 [107]. This raises the question of the correlation between the mutated amino-acid associated with human disease and the amino-acid changes found in frog TRPA1 and zebrafish TRPA1b, both of which are insensitive to HC-030031. Evolutionary comparison and these structure-activity-pharmacology studies suggest that N855 in human TRPA1 is a potential high-priority target for developing effective TRPA1-modulating compounds.

The study reporting the N855 mutation in inherited episodic pain syndrome also noted that patients experience breathing difficulties during painful episodes [105]. While no further details were provided, this finding suggests that TRPA1 may be involved in respiratory control and, potentially, in asthmatic responses. This function has been further supported by a large longitudinal study that found several polymorphisms in the TRPA1 gene to be associated with childhood asthma [108].

Adding further weight to the concept that TRPA1 is a pain-TRP in humans, the human TRPA1 gene was found to have methylation differences in identical twins who had different levels of pain sensitivity [109]. Increased expression of TRPA1 was found in the skin. This is consistent with an attenuating effect of DNA methylation on the TRPA1 promoter affecting TRPA1 gene expression.

We are optimistic that more widespread application of human DNA sequencing, especially in patients suffering from various forms of chronic pain affecting multiple generations will uncover additional polymorphisms and mutations in pain-TRPs, such as TRPA1, that co-contribute to pathological pain [109].

Two non-hereditary forms of specific pain stand out in which TRPA1 is involved, the challenging-to-manage pain [89, 90, 110–117] of sickle-cell disease [118, 119], and the largely unmet medical need of arthritis pain.

TRPA1 in Itch

Itch is defined as an unpleasant sensation that evokes the desire/reflex to scratch. TRPA1 has been suggested to mediate histamine-independent itch in response to exogenous pruritogens such as chloroquine and cowhage spicules, and to endogenous pruritic mediators produced in the skin and other organs, including bile acid, serotonin, ROS, leukotriene-B4, thymic stromal lymphopoietin, and IL-13, IL-22, IL-31, and IL-33 [3, 120–125].

Endothelin (ET-1), also known as a vasoconstricting peptide, can evoke itch by stimulating nociceptors and pruriceptors and sensitizing them to noxious and pruritic stimuli. ET-1 has been implicated in both pain and itch via TRPA1, TRPV1, and TRPV4 [94, 126–131]. Serotonin-induced itch has been shown to be mediated via activation of the serotonergic receptor HTR7 which signals to TRPA1 [132] (but see subsection “TRPV4 in itch”). Aberrant serotonin signaling has long been linked to a variety of human chronic itch conditions, including atopic dermatitis. In a mouse model of this highly prevalent condition (10% in the USA and Europe), mice lacking HTR7 or TRPA1 display reduced scratching and skin lesion severity [132]. These data highlight a role for HTR7 and TRPA1 in acute and chronic itch, and suggest that their antagonists may be useful for treating a variety of pathological itch conditions. Recent studies have also shown that TRPA1 contributes to pruritus due to allergic contact dermatitis, including that elicited by the poison ivy allergen, urushiol [75, 123].

Similarly, itch associated with severe liver disease, known as cholestatic itch, appears to be TRPA1-dependent. Bile acids induce itch in mice by activating the bile-acid receptor TGR5 and TRPA1. TRPA1 acts downstream of G-protein-coupled bile receptors such as TGR5. Antagonists of TGR5 and TRPA1, or inhibitors of the signaling mechanism by which TGR5 activates TRPA1, might be developed for the treatment of cholestatic itch [133, 134]. However, the serum levels of bile acids do not correlate with the presence and intensity of cholestatic itch in liver patients, whereas the bioreactive phospholipid lysophosphatidic acid (LPA) does. Of note, LPA also signals to pruriceptor neurons via TRPA1 [120].

Loss- and gain-of-function studies in mice have demonstrated that the G-protein-coupled receptor MrgprA3 is required for chloroquine-responsiveness in mice [135]. TRPA1 has been found to signal downstream of MrgprA3 in cultured sensory neurons and heterologous cells [122].

Again, TRPA1 not only serves as a sensor for pruritogens, but is also essential for maintaining skin inflammation, as shown in models of contact dermatitis and atopic dermatitis in which treatment with TRPA1 inhibitors reduces skin swelling, epidermal water loss, and leukocyte infiltration [75, 136, 137].

In terms of the development of TRPA1 inhibitory/modulatory compounds for human clinical use, as of today, TRPA1 inhibitors have a shared feature in that they can all be improved in terms of better efficacy and fewer side effects [138, 139]. At the basic science level, on the other hand, elegant work has been conducted. TRPA1 has been found to be evolutionarily conserved, with the involvement of TRPA1 orthologues in pain-related behavior and inflammation in invertebrate model organisms [56, 140–157].

TRPM2 Pro-pain Role

The thermo-TRP TRPM2 plays a role in pain signaling as recently discovered [54, 158–166].

TRPM3 Pro-pain Role

This is also true for the pregnenolone receptor, TRPM3 [167–170].

TRPM8 in Cold Hypersensitivity and Neuropathic Pain

Among the TRPM members in the pain pathway, TRPM8 ion channels have been researched most in-depth. TRPM8 expression is restricted to a subset of small-diameter sensory neurons of the trigeminal and dorsal root ganglia [171–173]. This channel is activated by cool and noxious cold temperatures between 10 °C and 23 °C, and by natural compounds that evoke a cold sensation such as menthol and eucalyptol (Fig. 1) [171, 172, 174]. Similar to TRPV1 and TRPA1, TRPM8 activation evokes an influx of Ca²⁺ into the primary sensory neuron, leading to its activation and the propagation of action potentials. The role of TRPM8 in cold thermosensation has been widely explored in vivo in Trpm8^−/− mice. These mice lack a response to cold stimuli and chemical compounds that cause a cold sensation such as icilin, as well as lacking a response to acetone-evoked evaporative cooling [174–176]. TRPM8 is also involved in orofacial and visceral (colonic) pain [47, 177–180].

Nerve injury can result in cold allodynia, and it has been suggested that TRPM8 channels are a key substrate thereof [177, 181]. Rats subjected to constriction nerve injury show enhanced withdrawal reflexes in response to evaporative cooling with acetone. They also have an increase in the number of neurons immunoreactive to the TRPM8 channel and more neurons that are stimulated by menthol and cold [177].

TRPM8 has been mechanistically linked to cold hypersensitivity [181]. Unlike heat hyperalgesia which is produced by several algogenic agents, TRPM8 cold sensitization appears to be evoked by neurotrophic factors such as artemin and nerve growth factor which produce their effects through specific receptors co-expressed with TRPM8 in a subset of sensory neurons [182, 183]. This concept was convincingly demonstrated using the evaporative cooling assay, where neurotrophic factors were injected into the foot-pad with subsequent application of acetone-evoked evaporative cooling to the paw. Wild-type mice showed an increase in cold-response with growth factors, while Trpm8^−/− mice did not [182, 183].

The sensitizing effects of TRPM8 ion channels on cold pain can be modulated by endogenous lipids. For example, TRPM8 activation by cold and menthol requires the presence of PIP₂ (a membrane phospholipid), which interacts directly with positively-charged amino-acids located in the TRP box of the channel [184]. Furthermore, PIP₂ by itself can activate the channel [184]. Therefore, TRPM8-mediated cold allodynia and hypersensitivity could be attenuated by regulating PIP₂ levels such as via the activation of Ca²⁺-dependent PLC which then hydrolyzes PIP₂ to produce DAG and IP₃ [184]. Moreover, DAG can activate the PKC pathway which then attenuates TRPM8 gating. This effect is mediated by specific TRPM8 serine-phosphorylation events [185].

In addition, TRPM8 activity is endogenously regulated by the iPLA₂ pathway, a calcium-insensitive phospholipase which produces lysophospholipids and free polyunsaturated fatty acids (PUFA) from the hydrolysis of sn-2-glycerophospholipids [186]. Interestingly, inhibition of iPLA₂ attenuates the TRPM8 currents evoked by cold, menthol, and icilin [186]. Moreover, lysophosphatidylcholine (LPC) released from the iPLA₂ pathway can activate TRPM8 even at 37 °C [186]. Furthermore, LPC evokes cold hypersensitivity in a TRPM8-dependent manner since this behavior is absent in Trpm8^−/− mice [187].

Pointing to endogenous regulatory mechanisms, PUFAs such as arachidonic, eicosapentaenoic, and docosahexaenoic acids, which also are released by iPLA₂, are endogenous inhibitors of TRPM8 [186].

Therefore, the iPLA₂ pathway can have contrasting effects on TRPM8 activation: an activating effect by lysophospholipids and a modulating inhibitory effect by PUFAs. Is one pathway “dominant” over the other? Equimolar application of LPC and arachidonic acid favors the activation of TRPM8 by LPC, possibly implicating this mechanism in cold pain and cold hypersensitivity phenomena [186].

Selective TRPM8 inhibitors are effective in reducing cold sensing and also cold pain, suggesting that TRPM8 contributes to both innocuous and noxious thermal sensing [47, 188]. Another study using a TRPM8-specific inhibitor reported no significant effects on neuropathic mechanical allodynia, but the effects on cold allodynia were not investigated [189].

While TRPM8 inhibition may be effective in reducing neuropathic cold pain, TRPM8 agonists have long been known to have analgesic properties. Menthol is widely used as a topical analgesic and inhaled antitussive with respiratory counterirritant properties. Menthol and TRPM8-selective menthol analogs have been shown to suppress nocifensive responses to several acute pain stimuli, including capsaicin, acrolein, and acid [178]. The analgesic effects of menthol are absent in TRPM8-deficient mice [178]. Menthol also suppresses inflammatory pain in a TRPM8-dependent manner [178]. Icilin has been demonstrated to diminish colitis-associated pain in a TRPM8-dependent manner [190]. Eucalyptol, the TRPM8 agonist in eucalyptus oil, also suppresses pain and respiratory inflammation and irritation [180, 191]. TRPM8-selective agonists may have improved analgesic properties compared to menthol that can cause irritation in some patients [178].

Possibly indicative of a role for TRPM8 in controlling itch, menthol has been reported to function as an anti-pruritic in therapy-resistant pruritus in lichen amyloidosis [192] and hydroxyethyl starch-induced itch [193]. However, these clinical findings leave open the possibility of menthol also acting on TRPA1 that is also sensitive to menthol. A more recent study has demonstrated that the anti-pruritic actions of menthol depend on inhibitory interneurons in the spinal cord that receive input from menthol-sensitive fibers and dampen input from pruriceptors [194].

TRPV1 is an Extensively Studied Pain-TRP

The TRPV1 channel is a representative member of the TRPV subfamily and is the most well-characterized TRP channel. TRPV1 can be activated by various stimuli, including temperature (~ 42 °C), pH, and a wide range of both endogenous and exogenous compounds (Fig. 1). Its main exogenous ligand is capsaicin, a compound from pungent peppers that activates the channel. The beneficial effects of capsaicin on nociception were identified before the TRPV1 channel was discovered [195]. TRPV1 was cloned in 1997 from a cDNA library isolated from capsaicin and temperature-stimulated nociceptor neurons [196]. Subsequent characterization of TRPV1 has revealed its key role in nociception. In addition, with the use of Trpv1^−/− mice, TRPV1 has been shown to be important in pain sensation [197]. Due to its importance in these physiological processes, several compounds have been developed to modulate the activity of TRPV1 to eliminate or reduce pain.

TRPV3 in Pain and Itch

A member of the TRP vanilloid subfamily important for temperature detection is the TRPV3 channel. This non-selective cation channel is abundantly expressed in skin (specifically in keratinocytes) [242] and has also been detected in neurons from the DRG and TG [243]. TRPV3 protein has 43% identity to TRPV1 and is a warm-sensitive but capsaicin- and low pH-insensitive ion channel [242, 243].

Among the ligands for TRPV3 activation are plant-derived compounds such as camphor, carvacrol, and thymol [244]; furthermore, TRPV3 activity is enhanced by endogenously produced compounds such as arachidonic acid which is an unsaturated fatty acid released during inflammation [245]. Unlike TRPV1 activation which requires the metabolism of this fatty acid to evoke currents, the enhancement of TRPV3 function is independent of arachidonic acid oxidation [245]. It is possible that in skin pathologies such as psoriasis where arachidonic acid levels are elevated [246], TRPV3 activation could contribute to chronic dermatitis.

Another TRPV3 agonist is farnesyl pyrophosphate (FPP), an endogenously produced intermediate metabolite of the mevalonate pathway [247]; it produces isoprenoids which can also activate TRPV3. FPP has been identified as a novel pain-producing compound through specific TRPV3 activation [248].

TRPV4 Functions as a Pain-TRP and an Itch-TRP

As a member of the TRPV subfamily, TRPV4 is a multi-modally activated, nonselective cation channel. Functional expression of TRPV4 has been detected in nerve cells and non-neuronal cells [254]. Importantly for this review, one of the initial discovery papers lays out a rationale of how TRPV4 can function in pain by demonstrating its expression in TG sensory neurons, in small-diameter neurons, and reasoning about a role for TRPV4 in pain signaling [254]. This initial conjecture has now been confirmed, e.g. a search for “TRPV4 pain” generated 185 references out of a total of 1085 references for “TRPV4”. Primary sensory neurons with TRPV4 expression include pain-sensing neurons in the DRG [8, 255, 256] and TG [257, 258] [94, 259], most recently also satellite cells in sensory ganglia [260], in the CNS for pain transmission in astrocytes [261], microglial cells [262], and neurons [263, 264, 265]. Outside the nervous system, TRPV4 has been found in innervated cells such as prominently in chondrocytes [266], vascular endothelia [267], and innervated epithelia such as skin keratinocytes [129, 268], airway epithelial cells [26, 269, 270], colonic epithelia [271], and odontoblasts [272]. Co-labeling experiments with neuronal markers and size determination have revealed that TRPV4-expressing sensory neurons are rather nociceptive [129, 273, 274], a finding in keeping with evidence that suggests that TRPV4 is involved in nociception both physiologically and in sensitized states such as inflammation and nerve injury [8, 273]. Of note, TRPV4 expression in TGs appears to be higher than that in DRGs; this is still not fully explained and how this difference correlates with sensory functions is not yet known [275, 276].