Identification of transmembrane domain 5 as a critical molecular determinant of menthol sensitivity in mammalian TRPA1 channels

TRPA1 is a polyunsaturated fatty acid sensor in mammals

Fatty acids can act as important signaling molecules regulating diverse physiological processes. Our understanding, however, of fatty acid signaling mechanisms and receptor targets remains incomplete. Here we show that Transient Receptor Potential Ankyrin 1 (TRPA1), a cation channel expressed in sensory neurons and gut tissues, functions as a sensor of polyunsaturated fatty acids (PUFAs) in vitro and in vivo. PUFAs, containing at least 18 carbon atoms and three unsaturated bonds, activate TRPA1 to excite primary sensory neurons and enteroendocrine cells. Moreover, behavioral aversion to PUFAs is absent in TRPA1-null mice. Further, sustained or repeated agonism with PUFAs leads to TRPA1 desensitization. PUFAs activate TRPA1 non-covalently and independently of known ligand binding domains located in the N-terminus and 5^th transmembrane region. PUFA sensitivity is restricted to mammalian (rodent and human) TRPA1 channels, as the drosophila and zebrafish TRPA1 orthologs do not respond to DHA. We propose that PUFA-sensing by mammalian TRPA1 may regulate pain and gastrointestinal functions.

TRPA1 agonist activity of probenecid desensitizes channel responses: consequences for screening

The transient receptor potential channel subtype A member 1 (TRPA1) is a nonselective cation channel widely viewed as having therapeutic potential, particularly for pain-related indications. Realization of this potential will require potent, selective modulators; however, currently the pharmacology of TRPA1 is poorly defined. As TRPA1 is calcium permeable, calcium indicators offer a simple assay format for high-throughput screening. In this report, we show that probenecid, a uricosuric agent used experimentally in screening to increase loading of calcium-sensitive dyes, activates TRPA1. Prolonged probenecid incubation during the dye-loading process reduces agonist potency upon subsequent challenge. When Chinese Hamster Ovary (CHO)-hTRPA1 or STC-1 cells, which endogenously express TRPA1, were dye loaded in the presence of 2 mM probenecid TRPA1, agonists appeared less potent; EC(50) for allyl isothiocyante agonists in CHO-hTRPA1 was increased from 1.5±0.19 to 7.32±1.20 μM (P<0.01). No significant effect on antagonist potency was observed when using the agonist EC(80) concentration determined under the appropriate dye-loading conditions. We suggest an alternative protocol for calcium imaging using another blocker of anion transport, sulfinpyrazone. This blocker significantly augments indicator dye loading and the screening window, but is not a TRPA1 agonist and has no effect on agonist potency.

Differential roles of galanin on mechanical and cooling responses at the primary afferent nociceptor

Background

Galanin is expressed in a small percentage of intact small diameter sensory neurons of the dorsal root ganglia and in the afferent terminals of the superficial lamina of the dorsal horn of the spinal cord. The neuropeptide modulates nociception demonstrating dose-dependent pro- and anti-nociceptive actions in the naïve animal. Galanin also plays an important role in chronic pain, with the anti-nociceptive actions enhanced in rodent neuropathic pain models. In this study we compared the role played by galanin and its receptors in mechanical and cold allodynia by identifying individual rat C-fibre nociceptors and characterising their responses to mechanical or acetone stimulation.

Results

Mechanically evoked responses in C-fibre nociceptors from naive rats were sensitised after close intra-arterial infusion of galanin or Gal2-11 (a galanin receptor-2/3 agonist) confirming previous data that galanin modulates nociception via activation of GalR2. In contrast, the same dose and route of administration of galanin, but not Gal2-11, inhibited acetone and menthol cooling evoked responses, demonstrating that this inhibitory mechanism is not mediated by activation of GalR2. We then used the partial saphenous nerve ligation injury model of neuropathic pain (PSNI) and the complete Freund’s adjuvant model of inflammation in the rat and demonstrated that close intra-arterial infusion of galanin, but not Gal2-11, reduced cooling evoked nociceptor activity and cooling allodynia in both paradigms, whilst galanin and Gal2-11 both decreased mechanical activation thresholds. A previously described transgenic mouse line which inducibly over-expresses galanin (Gal-OE) after nerve injury was then used to investigate whether manipulating the levels of endogenous galanin also modulates cooling evoked nociceptive behaviours after PSNI. Acetone withdrawal behaviours in naive mice showed no differences between Gal-OE and wildtype (WT) mice. 7-days after PSNI Gal-OE mice demonstrated a significant reduction in the duration of acetone-induced nociceptive behaviours compared to WT mice.

Conclusions

These data identify a novel galaninergic mechanism that inhibits cooling evoked neuronal activity and nociceptive behaviours via a putative GalR1 mode of action that would also be consistent with a TRP channel-dependent mechanism.

CGRPα-expressing sensory neurons respond to stimuli that evoke sensations of pain and itch

Calcitonin gene-related peptide (CGRPα, encoded by Calca) is a classic marker of nociceptive dorsal root ganglia (DRG) neurons. Despite years of research, it is unclear what stimuli these neurons detect in vitro or in vivo. To facilitate functional studies of these neurons, we genetically targeted an axonal tracer (farnesylated enhanced green fluorescent protein; GFP) and a LoxP-stopped cell ablation construct (human diphtheria toxin receptor; DTR) to the Calca locus. In culture, 10–50% (depending on ligand) of all CGRPα-GFP-positive (+) neurons responded to capsaicin, mustard oil, menthol, acidic pH, ATP, and pruritogens (histamine and chloroquine), suggesting a role for peptidergic neurons in detecting noxious stimuli and itch. In contrast, few (2.2±1.3%) CGRPα-GFP⁺ neurons responded to the TRPM8-selective cooling agent icilin. In adult mice, CGRPα-GFP⁺ cell bodies were located in the DRG, spinal cord (motor neurons and dorsal horn neurons), brain and thyroid—reproducibly marking all cell types known to express Calca. Half of all CGRPα-GFP⁺ DRG neurons expressed TRPV1, ∼25% expressed neurofilament-200, <10% contained nonpeptidergic markers (IB4 and Prostatic acid phosphatase) and almost none (<1%) expressed TRPM8. CGRPα-GFP⁺ neurons innervated the dorsal spinal cord and innervated cutaneous and visceral tissues. This included nerve endings in the epidermis and on guard hairs. Our study provides direct evidence that CGRPα⁺ DRG neurons respond to agonists that evoke pain and itch and constitute a sensory circuit that is largely distinct from nonpeptidergic circuits and TRPM8⁺/cool temperature circuits. In future studies, it should be possible to conditionally ablate CGRPα-expressing neurons to evaluate sensory and non-sensory functions for these neurons.

Modulation of thermoreceptor TRPM8 by cooling compounds

ThermoTRPs, a subset of the Transient Receptor Potential (TRP) family of cation channels, have been implicated in sensing temperature. TRPM8 and TRPA1 are both activated by cooling. TRPM8 is activated by innocuous cooling (<30 °C) and contributes to sensing unpleasant cold stimuli or mediating the effects of cold analgesia and is a receptor for menthol and icilin (mint-derived and synthetic cooling compounds, respectively). TRPA1 (Ankyrin family) is activated by noxious cold (<17 °C), icilin, and a variety of pungent compounds. Extensive amount of medicinal chemistry efforts have been published mainly in the form of patent literature on various classes of cooling compounds by various pharmaceutical companies; however, no prior comprehensive review has been published. When expressed in heterologous expression systems, such as Xenopus oocytes or mammalian cell lines, TRPM8 mediated currents are activated by a number of cooling compounds in addition to menthol and icilin. These include synthetic p-menthane carboxamides along with other class of compounds such as aliphatic/alicyclic alcohols/esters/amides, sulphones/sulphoxides/sulphonamides, heterocyclics, keto-enamines/lactams, and phosphine oxides. In the present review, the medicinal chemistry of various cooling compounds as activators of thermoTRPM8 channel will be discussed according to their chemical classes. The potential of these compounds to emerge as therapeutic agents is also discussed.

Sweet taste and menthol increase cough reflex thresholds

Cough is a vital protective reflex that is triggered by both mechanical and chemical stimuli. The current experiments explored how chemosensory stimuli modulate this important reflex. Cough thresholds were measured using a single-inhalation capsaicin challenge. Experiment 1 examined the impact of sweet taste: Cough thresholds were measured after rinsing the mouth with a sucrose solution (sweet) or with water (control). Experiment 2 examined the impact of menthol: Cough thresholds were measured after inhaling headspace above a menthol solution (menthol vapor) or headspace above the mineral oil solvent (control). Experiment 3 examined the impact of rinsing the mouth with a (bitter) sucrose octaacetate solution. Rinsing with sucrose and inhaling menthol vapor significantly increased measured cough thresholds. Rinsing with sucrose octaacete caused a non-significant decrease in cough thresholds, an important demonstration of specificity. Decreases in cough reflex sensitivity from sucrose or menthol could help explain why cough syrups without pharmacologically active ingredients are often almost as effective as formulations with an added drug. Further, the results support the idea that adding menthol to cigarettes might make tobacco smoke more tolerable for beginning smokers, at least in part, by reducing the sensitivity of an important airway defense mechanism.

Olfactory and trigeminal interaction of menthol and nicotine in humans

The purpose of the study was to investigate the interactions between two stimuli—menthol and nicotine—both of which activate the olfactory and the trigeminal system. More specifically, we wanted to know whether menthol at different concentrations modulates the perception of burning and stinging pain induced by nicotine stimuli in the human nose. The study followed an eightfold randomized, double-blind, cross-over design including 20 participants. Thirty phasic nicotine stimuli at one of the two concentrations (99 and 134 ng/mL) were applied during the entire experiment every 1.5 min for 1 s; tonic menthol stimulation at one of the three concentrations (0.8, 1.5 and 3.4 μg/mL) or no-menthol (placebo control conditions) was introduced after the 15th nicotine stimulus. The perceived intensities of nicotine’s burning and stinging pain sensations, as well as perceived intensities of menthol’s odor, cooling and pain sensations, were estimated using visual analog scales. Recorded estimates of stinging and burning sensations induced by nicotine initially decreased (first half of the experiment) probably due to adaptation/habituation. Tonic menthol stimulation did not change steady-state nicotine pain intensity estimates, neither for burning nor for stinging pain. Menthol-induced odor and cooling sensations were concentration dependent when combined with low-intensity nicotine stimuli. Surprisingly, this dose dependency was eliminated when combining menthol stimuli with high-intensity nicotine stimuli. There was no such nicotine effect on menthol’s pain sensation. In summary, we detected interactions caused by nicotine on menthol perception for odor and cooling but no effect was elicited by menthol on nicotine pain sensation.

The thermo-TRP ion channel family: properties and therapeutic implications

The thermo-transient receptor potentials (TRPs), a recently discovered family of ion channels activated by temperature, are expressed in primary sensory nerve terminals where they provide information about thermal changes in the environment. Six thermo-TRPs have been characterised to date: TRP vanilloid (TRPV) 1 and 2 are activated by painful levels of heat, TRPV3 and 4 respond to non-painful warmth, TRP melastatin 8 is activated by non-painful cool temperatures, while TRP ankyrin (TRPA) 1 is activated by painful cold. The thermal thresholds of many thermo-TRPs are known to be modulated by extracellular mediators, released by tissue damage or inflammation, such as bradykinin, PG and growth factors. There have been intensive efforts recently to develop antagonists of thermo-TRP channels, particularly of the noxious thermal sensors TRPV1 and TRPA1. Blockers of these channels are likely to have therapeutic uses as novel analgesics, but may also cause unacceptable side effects. Controlling the modulation of thermo-TRPs by inflammatory mediators may be a useful alternative strategy in developing novel analgesics.

Sustained TRPA1 activation in vivo

Transient receptor potential A1 (TRPA1) is a calcium permeable non-selective cation channel that is selectively localized to peptidergic C-fibres in the pain pathway. TRPA1 is highly conserved across the animal kingdom and it is able to detect a wide range of potentially toxic environmental chemicals. An unusual mechanism of TRPA1 activation was recently elucidated in which reactive agonists bind covalently to cysteines and lysine in the intracellular N-terminus. Despite a covalent activation mechanism, only transient TRPA1 activation is seen in the maintained presence of reactive agonists in whole-cell patch clamp experiments. I suggest that previous patch clamp studies are performed under conditions that do not fully mimic all aspects of TRPA1 activation. Here, I argue that compelling evidence exists for sustained TRPA1 activation in several chronic (neuropathic) pain-related pathophysiological conditions in vivo. I discuss briefly putative mechanisms that are likely to contribute to and maintain sustained TRPA1 agonist levels through increased production and/or decreased metabolism and inactivation. Chronic pain can be understood as a false alarm evoked by sustained and increased levels of endogenous TRPA1 agonists in various pathophysiological conditions.

Comparing ion conductance recordings of synthetic lipid bilayers with cell membranes containing TRP channels

In this article we compare electrical conductance events from single channel recordings of three TRP channel proteins (TRPA1, TRPM2 and TRPM8) expressed in human embryonic kidney cells with channel events recorded on synthetic lipid membranes close to melting transitions. Ion channels from the TRP family are involved in a variety of sensory processes including thermo- and mechano-reception. Synthetic lipid membranes close to phase transitions display channel-like events that respond to stimuli related to changes in intensive thermodynamic variables such as pressure and temperature. TRP channel activity is characterized by typical patterns of current events dependent on the type of protein expressed. Synthetic lipid bilayers show a wide spectrum of electrical phenomena that are considered typical for the activity of protein ion channels. We find unitary currents, burst behavior, flickering, multistep-conductances, and spikes behavior in both preparations. Moreover, we report conductances and lifetimes for lipid channels as described for protein channels. Non-linear and asymmetric current-voltage relationships are seen in both systems. Without further knowledge of the recording conditions, no easy decision can be made whether short current traces originate from a channel protein or from a pure lipid membrane.

TRPA1 antagonists as potential analgesic drugs

The necessity of safe and effective treatments for chronic pain has intensified the search for new analgesic drugs. In the last few years, members of a closely-related family of ion channels, called transient receptor potential (TRP) have been identified in different cell types and their functions in physiological and pathological conditions have been characterized. The transient receptor potential ankyrin 1 (TRPA1), originally called ANKTM1 (ankyrin-like with transmembrane domains protein 1), is a molecule that has been conserved in different species during evolution; TRPA1 is a cation channel that functions as a cellular sensor, detecting mechanical, chemical and thermal stimuli, being a component of neuronal, epithelial, blood and smooth muscle tissues. In mammals, TRPA1 is largely expressed in primary sensory neurons that mediate somatosensory processes and nociceptive transmission. Recent studies have described the role of TRPA1 in inflammatory and neuropathic pain. However, its participation in cold sensation has not been agreed in different studies. In this review, we focus on data that support the relevance of the activation and blockade of TRPA1 in pain transmission, as well as the mechanisms underlying its activation and modulation by exogenous and endogenous stimuli. We also discuss recent advances in the search for new analgesic medicines targeting the TRPA1 channel.

Pokes, sunburn, and hot sauce: Drosophila as an emerging model for the biology of nociception

The word "nociception" is derived from the Latin "nocere," which means "to harm." Nociception refers to the sensory perception of noxious stimuli that have the potential to cause tissue damage. Since the perception of such potentially harmful stimuli often results in behavioral escape responses, nociception provides a protective mechanism that allows an organism to avoid incipient (or further) damage to the tissue. It appears to be universal in metazoans as a variety of escape responses can be observed in both mammalian and non-mammalian vertebrates, as well as diverse invertebrates such as leeches, nematodes, and fruit flies (Sneddon [2004] Brain Research Review 46:123-130; Tobin and Bargmann [2004] Journal of Neurobiology 61:161-174; Smith and Lewin [2009] Journal of Comparative Physiology 195:1089-1106). Several types of stimuli can trigger nociceptive sensory transduction, including noxious heat, noxious chemicals, and harsh mechanical stimulation. Such high-threshold stimuli induce the firing of action potentials in peripheral nociceptors, the sensory neurons specialized for their detection (Basbaum et al. [2009] Cell 139:267-284). In vertebrates, these action potentials can either be relayed directly to a spinal motor neuron to provoke escape behavior (the so-called monosynaptic reflex) or can travel via spinal cord interneurons to higher-order processing centers in the brain. This review will cover the establishment of Drosophila as a system to study various aspects of nociceptive sensory perception. We will cover development of the neurons responsible for detecting noxious stimuli in larvae, the assays used to assess the function(s) of these neurons, and the genes that have been found to be required for both thermal and mechanical nociception. Along the way, we will highlight some of the genetic tools that make the fly such a powerful system for studies of nociception. Finally, we will cover recent studies that introduce new assays employing adult Drosophila to study both chemical and thermal nociception and provide an overview of important unanswered questions in the field.

Activation of TRPA1 by membrane permeable local anesthetics

Background

Low concentrations of local anesthetics (LAs) suppress cellular excitability by inhibiting voltage-gated Na⁺ channels. In contrast, LAs at high concentrations can be excitatory and neurotoxic. We recently demonstrated that LA-evoked activation of sensory neurons is mediated by the capsaicin receptor TRPV1, and, to a lesser extent by the irritant receptor TRPA1. LA-induced activation and sensitization of TRPV1 involves a domain that is similar, but not identical to the vanilloid-binding domain. Additionally, activation of TRPV1 by LAs involves PLC and PI(4,5)P₂-signalling. In the present study we aimed to characterize essential structural determinants for LA-evoked activation of TRPA1.

Results

Recombinant rodent and human TRPA1 were expressed in HEK293t cells and investigated by means of whole-cell patch clamp recordings. The LA lidocaine activates TRPA1 in a concentration-dependent manner. The membrane impermeable lidocaine-derivative QX-314 is inactive when applied extracellularly. Lidocaine-activated TRPA1-currents are blocked by the TRPA1-antagonist HC-030031. Lidocaine is also an inhibitor of TRPA1, an effect that is more obvious in rodent than in human TRPA1. This species-specific difference is linked to the pore region (transmembrane domain 5 and 6) as described for activation of TRPA1 by menthol. Unlike menthol-sensitivity however, lidocaine-sensitivity is not similarly determined by serine- and threonine-residues within TM5. Instead, intracellular cysteine residues known to be covalently bound by reactive TRPA1-agonists seem to mediate activation of TRPA1 by LAs.

Conclusions

The structural determinants involved in activation of TRPA1 by LAs are disparate from those involved in activation by menthol or those involved in activation of TRPV1 by LAs.

Recent advances in the biology and medicinal chemistry of TRPA1

The transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is a nonselective cation channel that is highly expressed in small-diameter sensory neurons, where it functions as a polymodal receptor, responsible for detecting potentially harmful chemicals, mechanical forces and temperatures. TRPA1 is also activated and/or sensitized by multiple endogenous inflammatory mediators. As such, TRPA1 likely mediates the pain and neurogenic inflammation caused by exposure to reactive chemicals. In addition, it is also possible that this channel may mediate some of the symptoms of chronic inflammatory conditions such as asthma. We review recent advances in the biology of TRPA1 and summarize the evidence for TRPA1 as a therapeutic drug target. In addition, we provide an update on TRPA1 medicinal chemistry and the progress in the search for novel TRPA1 antagonists.

The effect of menthol vapor on nasal sensitivity to chemical irritation

INTRODUCTION

Among other effects, menthol added to cigarettes may modulate sensory response to cigarette smoke either by masking "harshness" or contributing to a desirable "impact." However, harshness and impact have been imprecisely defined and assessed using subjective measures. Thus, the current experiments used an objective measure of sensitivity to chemical irritation in the nose to test the hypothesis that menthol vapor modulates sensitivity to chemical irritation in the airways.

METHODS

Nasal irritation thresholds were measured for 2 model compounds (acetic acid and allyl isothiocyanate) using nasal lateralization. In this technique, participants simultaneously sniff clean air in one nostril and chemical vapor in the other and attempt to identify the stimulated nostril. People cannot lateralize based on smell alone but can do so when chemicals are strong enough to feel. In one condition, participants were pretreated by sniffing menthol vapor. In a control condition, participants were pretreated by sniffing an odorless blank (within-subjects design).

RESULTS

Pretreatment with menthol vapor decreased sensitivity to nasal irritation from acetic acid (participants required higher concentrations to lateralize) but increased sensitivity to allyl isothiocyanate (lower concentrations were required).

CONCLUSIONS

The current experiments provide objective evidence that menthol vapor can modulate sensitivity to chemical irritation in the upper airways in humans. Cigarette smoke is a complex mixture of chemicals and particulates, and further work will be needed to determine exactly how menthol modulates smoking sensation. A better understanding could lead to treatments tailored to help menthol smokers quit by replacing the sensation of mentholated cigarettes.

Novel menthol-derived cooling compounds activate primary and second-order trigeminal sensory neurons and modulate lingual thermosensitivity

We presently investigated 2 novel menthol derivatives GIV1 and GIV2, which exhibit strong cooling effects. In previous human psychophysical studies, GIV1 delivered in a toothpaste medium elicited a cooling sensation that was longer lasting compared with GIV2 and menthol carboxamide (WS-3). In the current study, we investigated the molecular and cellular effects of these cooling agents. In calcium flux studies of TRPM8 expressed in HEK cells, both GIV1 and GIV2 were approximately 40- to 200-fold more potent than menthol and WS-3. GIV1 and GIV2 also activated TRPA1 but at levels that were 400 times greater than those required for TRPM8 activation. In calcium imaging studies, subpopulations of cultured rat trigeminal ganglion and dorsal root ganglion cells responded to GIV1 and/or GIV2; the majority of these were also activated by menthol and some were additionally activated by the TRPA1 agonist cinnamaldehyde and/or the TRPV1 agonist capsaicin. We also made in vivo single-unit recordings from cold-sensitive neurons in rat trigeminal subnucleus caudalis (Vc). GIV 1 and GIV2 directly excited some Vc neurons, GIV1 significantly enhanced their responses to cooling, and both GIV1 and GIV2 reduced responses to noxious heat. These novel cooling compounds provide additional molecular tools to investigate the neural processes of cold sensation.

Temperature-dependent STIM1 activation induces Ca²+ influx and modulates gene expression

Intracellular Ca²⁺ is essential for diverse cellular functions. Ca²⁺ entry into many cell types including immune cells is triggered by depleting endoplasmic reticulum (ER) Ca²⁺, a process termed store-operated Ca²⁺ entry (SOCE). STIM1 is an ER Ca²⁺ sensor. Upon Ca²⁺ store depletion, STIM1 clusters at ER-plasma membrane junctions where it interacts with and gates Ca²⁺-permeable Orai1 ion channels. Here we show that STIM1 is also activated by temperature. Heating cells caused clustering of STIM1 at temperatures above 35°C without depleting Ca²⁺ stores, and led to STIM1/Orai1-mediated Ca²⁺ influx as a heat off-response (response after cooling). Interestingly, the functional coupling of STIM1 and Orai1 is prevented at high temperatures, potentially explaining the heat off-response. Importantly, physiologically-relevant temperature shifts modulates STIM1-dependent gene expression in Jurkat T-cells. Therefore, temperature is an important regulator of STIM1 function.

Scraping through the ice: uncovering the role of TRPM8 in cold transduction

The proper detection of environmental temperatures is essential for the optimal growth and survival of organisms of all shapes and phyla, yet only recently have the molecular mechanisms for temperature sensing been elucidated. The discovery of temperature-sensitive ion channels of the transient receptor potential (TRP) superfamily has been pivotal in explaining how temperatures are sensed in vivo, and here we will focus on the lone member of this cohort, TRPM8, which has been unequivocally shown to be cold sensitive. TRPM8 is expressed in somatosensory neurons that innervate peripheral tissues such as the skin and oral cavity, and recent genetic evidence has shown it to be the principal transducer of cool and cold stimuli. It is remarkable that this one channel, unlike other thermosensitive TRP channels, is associated with both innocuous and noxious temperature transduction, as well as cold hypersensitivity during injury and, paradoxically, cold-mediated analgesia. With ongoing research, the field is getting closer to answering a number of fundamental questions regarding this channel, including the cellular mechanisms of TRPM8 modulation, the molecular context of TRPM8 expression, as well as the full extent of the role of TRPM8 in cold signaling in vivo. These findings will further our understanding of basic thermotransduction and sensory coding, and may have important implications for treatments for acute and chronic pain.

The Met268Pro mutation of mouse TRPA1 changes the effect of caffeine from activation to suppression

The transient receptor potential A1 channel (TRPA1) is activated by various compounds, including isothiocyanates, menthol, and cinnamaldehyde. The sensitivities of the rodent and human isoforms of TRPA1 to menthol and the cysteine-attacking compound CMP1 differ, and the molecular determinants for these differences have been identified in the 5th transmembrane region (TM5) for menthol and TM6 for CMP1. We recently reported that caffeine activates mouse TRPA1 (mTRPA1) but suppresses human TRPA1 (hTRPA1). Here we aimed to identify the molecular determinant that is responsible for species-specific differences in the response to caffeine by analyzing the functional properties of various chimeras expressed in Xenopus oocytes. We initially found that the region between amino acids 231 and 287, in the distal N-terminal cytoplasmic region of mTRPA1, is critical. In a mutagenesis study of this region, we subsequently observed that introduction of a Met268Pro point mutation into mTRPA1 changed the effect of caffeine from activation to suppression. Because the region including Met-268 is different from other reported ligand-binding sites and from the EF-hand motif, these results suggest that the caffeine response is mediated by a unique mechanism, and confirm the importance of the distal N-terminal region for regulation of TRPA1 channel activity.

Cooling-sensitive TRPM8 is thermostat of skin temperature against cooling

We have shown that cutaneous cooling-sensitive receptors can work as thermostats of skin temperature against cooling. However, molecule of the thermostat is not known. Here, we studied whether cooling-sensitive TRPM8 channels act as thermostats. TRPM8 in HEK293 cells generated output (y) when temperature (T) was below threshold of 28.4°C. Output (y) is given by two equations: At T >28.4°C, y = 0; At T <28.4°C, y  =  -k(T - 28.4°C). These equations show that TRPM8 is directional comparator to elicits output (y) depending on negative value of thermal difference (ΔT  =  T - 28.4°C). If negative ΔT-dependent output of TRPM8 in the skin induces responses to warm the skin for minimizing ΔT recursively, TRPM8 acts as thermostats against cooling. With TRPM8-deficient mice, we explored whether TRPM8 induces responses to warm the skin against cooling. In behavioral regulation, when room temperature was 10°C, TRPM8 induced behavior to move to heated floor (35°C) for warming the sole skin. In autonomic regulation, TRPM8 induced activities of thermogenic brown adipose tissue (BAT) against cooling. When menthol was applied to the whole trunk skin at neutral room temperature (27°C), TRPM8 induced a rise in core temperature, which warmed the trunk skin slightly. In contrast, when room was cooled from 27 to 10°C, TRPM8 induced a small rise in core temperature, but skin temperature was severely reduced in both TRPM8-deficient and wild-type mice by a large heat leak to the surroundings. This shows that TRPM8-driven endothermic system is less effective for maintenance of skin temperature against cooling. In conclusion, we found that TRPM8 is molecule of thermostat of skin temperature against cooling.

Self- and cross-desensitization of oral irritation by menthol and cinnamaldehyde (CA) via peripheral interactions at trigeminal sensory neurons

Menthol and cinnamaldehyde (CA) are plant-derived spices commonly used in oral hygiene products, chewing gum, and many other applications. However, little is known regarding their sensory interactions in the oral cavity. We used a human psychophysics approach to investigate the temporal dynamics of oral irritation elicited by sequential application of menthol and/or CA, and ratiometric calcium imaging methods to investigate activation of rat trigeminal ganglion (TG) cells by these agents. Irritancy decreased significantly with sequential oral application of menthol and CA (self-desensitization). Menthol cross-desensitized irritation elicited by CA, and vice versa, over a time course of at least 60 min. Seventeen and 19% of TG cells were activated by menthol and CA, respectively, with ∼50% responding to both. TG cells exhibited significant self-desensitization to menthol applied at a 5, but not 10, min interval. They also exhibited significant self-desensitization to CA at 400 but not 200 μM. Menthol cross-desensitized TG cell responses to CA. CA at a concentration of 400 but not 200 μM also cross-desensitized menthol-evoked responses. The results support the argument that the perceived reductions in oral irritancy and cross-interactions between menthol and CA and menthol observed (at least at short interstimulus intervals) can be largely accounted for by the properties of trigeminal sensory neurons innervating the tongue.

The general anesthetic propofol excites nociceptors by activating TRPV1 and TRPA1 rather than GABAA receptors

Anesthetic agents can induce a paradox activation and sensitization of nociceptive sensory neurons and, thus, potentially facilitate pain processing. Here we identify distinct molecular mechanisms that mediate an activation of sensory neurons by 2,6-diisopropylphenol (propofol), a commonly used intravenous anesthetic known to elicit intense pain upon injection. Clinically relevant concentrations of propofol activated the recombinant transient receptor potential (TRP) receptors TRPA1 and TRPV1 heterologously expressed in HEK293t cells. In dorsal root ganglion (DRG) neurons, propofol-induced activation correlated better to expression of TRPA1 than of TRPV1. However, pretreatment with the protein kinase C activator 4β-phorbol 12-myristate 13-acetate (PMA) resulted in a significantly sensitized propofol-induced activation of TRPV1 in DRG neurons as well as in HEK293t cells. Pharmacological and genetic silencing of both TRPA1 and TRPV1 only partially abrogated propofol-induced responses in DRG neurons. The remaining propofol-induced activation was abolished by the selective γ-aminobutyric acid, type A (GABA(A)) receptor antagonist picrotoxin. Propofol but not GABA evokes a release of calcitonin gene-related peptide, a key component of neurogenic inflammation, from isolated peripheral nerves of wild-type but not TRPV1 and TRPA1-deficient mice. Moreover, propofol but not GABA induced an intense pain upon intracutaneous injection. As both the release of calcitonin gene-related peptide and injection pain by propofol seem to be independent of GABA(A) receptors, our data identify TRPV1 and TRPA1 as key molecules for propofol-induced excitation of sensory neurons. This study warrants further investigations into the role of anesthetics to induce nociceptor sensitization and to foster postoperative pain.

Honey bee thermal/chemical sensor, AmHsTRPA, reveals neofunctionalization and loss of transient receptor potential channel genes

Insects are relatively small heterothermic animals, thus they are highly susceptible to changes in ambient temperature. However, a group of honey bees is able to maintain the brood nest temperature between 32°C and 36°C by either cooling or heating the nest. Nevertheless, how honey bees sense the ambient temperature is not known. We identified a honey bee Hymenoptera-specific transient receptor potential A (HsTRPA) channel (AmHsTRPA), which is activated by heat with an apparent threshold temperature of 34°C and insect antifeedants such as camphor in vitro. AmHsTRPA is expressed in the antennal flagellum, and ablation of the antennal flagella and injection of AmHsTRPA inhibitors impair warmth avoidance of honey bees. Gustatory responses of honey bees to sucrose are suppressed by noxious heat and insect antifeedants, but are relieved in the presence of AmHsTRPA inhibitors. These results suggest that AmHsTRPA may function as a thermal/chemical sensor in vivo. As shown previously, Hymenoptera has lost the ancient chemical sensor TRPA1; however, AmHsTRPA is able to complement the function of Drosophila melanogaster TRPA1. These results demonstrate that HsTRPA, originally arisen by the duplication of Water witch, has acquired thermal- and chemical-responsive properties, which has resulted in the loss of ancient TRPA1. Thus, this is an example of neofunctionalization of the duplicated ion channel gene followed by the loss of the functionally equivalent ancient gene.

Sensory detection and responses to toxic gases: mechanisms, health effects, and countermeasures

The inhalation of reactive gases and vapors can lead to severe damage of the airways and lung, compromising the function of the respiratory system. Exposures to oxidizing, electrophilic, acidic, or basic gases frequently occur in occupational and ambient environments. Corrosive gases and vapors such as chlorine, phosgene, and chloropicrin were used as warfare agents and in terrorist acts. Chemical airway exposures are detected by the olfactory, gustatory, and nociceptive sensory systems that initiate protective physiological and behavioral responses. This review focuses on the role of airway nociceptive sensory neurons in chemical sensing and discusses the recent discovery of neuronal receptors for reactive chemicals. Using physiological, imaging, and genetic approaches, Transient Receptor Potential (TRP) ion channels in sensory neurons were shown to respond to a wide range of noxious chemical stimuli, initiating pain, respiratory depression, cough, glandular secretions, and other protective responses. TRPA1, a TRP ion channel expressed in chemosensory C-fibers, is activated by almost all oxidizing and electrophilic chemicals, including chlorine, acrolein, tear gas agents, and methyl isocyanate, the highly noxious chemical released in the Bhopal disaster. Chemicals likely activate TRPA1 through covalent protein modification. Animal studies using TRPA1 antagonists or TRPA1-deficient mice confirmed the role of TRPA1 in chemically induced respiratory reflexes, pain, and inflammation in vivo. New research shows that sensory neurons are not merely passive sensors of chemical exposures. Sensory channels such as TRPA1 are essential for maintenance of airway inflammation in asthma and may contribute to the progression of airway injury following high-level chemical exposures.

Menthol response and adaptation in nociceptive-like and nonnociceptive-like neurons: role of protein kinases

Menthol-sensitive/capsaicin-insensitive neurons (MS/CI) and menthol-sensitive/capsaicin-sensitive neurons (MS/CS) are thought to represent two functionally distinct populations of cold-sensing neurons that use TRPM8 receptors to convey innocuous and noxious cold information respectively. However, TRPM8-mediated responses have not been well characterized in these two neuron populations. Using rat dorsal root ganglion neurons, here we show that MS/CI neurons had larger menthol responses with greater adaptation. In contrast, MS/CS neurons had smaller menthol responses with less adaptation. All menthol-sensitive neurons showed significant reduction of menthol responses following the treatment of cells with the protein kinase C (PKC) activator PDBu (Phorbol 12,13-dibutyrate). PDBu-induced reduction of menthol responses was completely abolished in the presence of PKC inhibitors BIM (bisindolylmaleimide) or staurosporine. When menthol responses were examined in the presence of protein kinase inhibitors, it was found that the adaptation was significantly attenuated by either BIM or staurosporine and also by the Ca²⁺/calmodulin-dependent protein kinase (CamKII) inhibitor KN62 (N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine) in MS/CI neurons. In contrast, in MS/CS neurons menthol response was not affected significantly by BIM, staurosporine or KN62. In both MS/CI and MS/CS neurons, the menthol responses were not affected by PKA activators forskolin and 8-Br-cAMP (8-Bromoadenosine-3', 5'-cyclic monophosphate) or by protein kinase A (PKA) inhibitor Rp-cAMPs (Rp-Adenosine-3',5'-cyclic monophosphorothioate). Taken together, these results suggest that TRPM8-mediated responses are significantly different between non-nociceptive-like and nociceptive-like neurons.

A gain-of-function mutation in TRPA1 causes familial episodic pain syndrome

Human monogenic pain syndromes have provided important insights into the molecular mechanisms that underlie normal and pathological pain states. We describe an autosomal-dominant familial episodic pain syndrome characterized by episodes of debilitating upper body pain, triggered by fasting and physical stress. Linkage and haplotype analysis mapped this phenotype to a 25 cM region on chromosome 8q12-8q13. Candidate gene sequencing identified a point mutation (N855S) in the S4 transmembrane segment of TRPA1, a key sensor for environmental irritants. The mutant channel showed a normal pharmacological profile but altered biophysical properties, with a 5-fold increase in inward current on activation at normal resting potentials. Quantitative sensory testing demonstrated normal baseline sensory thresholds but an enhanced secondary hyperalgesia to punctate stimuli on treatment with mustard oil. TRPA1 antagonists inhibit the mutant channel, promising a useful therapy for this disorder. Our findings provide evidence that variation in the TRPA1 gene can alter pain perception in humans.

Agonist-induced changes in Ca(2+) permeation through the nociceptor cation channel TRPA1

The Ca(2+)-permeable cation channel TRPA1 acts as an ionotropic receptor for various pungent compounds and as a noxious cold sensor in sensory neurons. It is unclear what proportion of the TRPA1-mediated current is carried by Ca(2+) ions and how the permeation pathway changes during stimulation. Here, based on the relative permeability of the nonstimulated channel to cations of different size, we estimated a pore diameter of approximately 11 A. Combined patch-clamp and Fura-2 fluorescence recordings revealed that with 2 mM extracellular Ca(2+), and at a membrane potential of -80 mV, approximately 17% of the inward TRPA1 current is carried by Ca(2+). Stimulation with mustard oil evoked an apparent dilatation of the pore of 3 A and an increase in divalent cation selectivity and fractional Ca(2+) current. Mutations in the putative pore that reduced the divalent permeability and fractional Ca(2+) current also prevented mustard-oil-induced increases in Ca(2+) permeation. It is interesting that fractional Ca(2+) currents for wild-type and mutant TRPA1 were consistently higher than values predicted based on biionic reversal potentials using the Goldman-Hodgkin-Katz equation, suggesting that binding of Ca(2+) in the pore hinders monovalent cation permeation. We conclude that the pore of TRPA1 is dynamic and supports a surprisingly large Ca(2+) influx.

Analysis of Drosophila TRPA1 reveals an ancient origin for human chemical nociception

Chemical nociception, the detection of tissue-damaging chemicals, is important for animal survival and causes human pain and inflammation, but its evolutionary origins are largely unknown. Reactive electrophiles are a class of noxious compounds humans find pungent and irritating, like allyl isothiocyanate (in wasabi) and acrolein (in cigarette smoke)1–3. Insects to humans find reactive electrophiles aversive1–3, but whether this reflects conservation of an ancient sensory modality has been unclear. Here we identify the molecular basis of reactive electrophile detection in flies. We demonstrate that dTRPA1, the Drosophila melanogaster ortholog of the human irritant sensor, acts in gustatory chemosensors to inhibit reactive electrophile ingestion. We show that fly and mosquito TRPA1 orthologs are molecular sensors of electrophiles, using a mechanism conserved with vertebrate TRPA1s. Phylogenetic analyses indicate invertebrate and vertebrate TRPA1s share a common ancestor that possessed critical characteristics required for electrophile detection. These findings support emergence of TRPA1-based electrophile detection in a common bilaterian ancestor, with widespread conservation throughout vertebrate and invertebrate evolution. Such conservation contrasts with the evolutionary divergence of canonical olfactory and gustatory receptors and may relate to electrophile toxicity. We propose human pain perception relies on an ancient chemical sensor conserved across ~500 million years of animal evolution.

Pharmacology of transient receptor potential melastatin channels in the vasculature

Mammalian transient receptor potential melastatin (TRPM) non-selective cation channels, the largest TRP subfamily, are widely expressed in excitable and non-excitable cells where they perform diverse functions ranging from detection of cold, taste, osmolarity, redox state and pH to control of Mg(2+) homeostasis and cell proliferation or death. Recently, TRPM gene expression has been identified in vascular smooth muscles with dominance of the TRPM8 channel. There has been in parallel considerable progress in decoding the functional roles of several TRPMs in the vasculature. This research on native cells is aided by the knowledge of the activation mechanisms and pharmacological properties of heterologously expressed TRPM subtypes. This paper summarizes the present state of knowledge of vascular TRPM channels and outlines several anticipated directions of future research in this area.

The transient receptor potential channel TRPM8 is inhibited via the alpha 2A adrenoreceptor signaling pathway

The transient receptor potential channel melastatin member 8 (TRPM8) is expressed in sensory neurons, where it constitutes the main receptor of environmental innocuous cold (10-25 degrees C). Among several types of G protein-coupled receptors expressed in sensory neurons, G(i)-coupled alpha 2A-adrenoreceptor (alpha 2A-AR), is known to be involved in thermoregulation; however, the underlying molecular mechanisms remain poorly understood. Here we demonstrated that stimulation of alpha 2A-AR inhibited TRPM8 in sensory neurons from rat dorsal root ganglia (DRG). In addition, using specific pharmacological and molecular tools combined with patch-clamp current recordings, we found that in heterologously expressed HEK-293 (human embryonic kidney) cells, TRPM8 channel is inhibited by the G(i) protein/adenylate cyclase (AC)/cAMP/protein kinase A (PKA) signaling cascade. We further identified the TRPM8 S9 and T17 as two key PKA phosphorylation sites regulating TRPM8 channel activity. We therefore propose that inhibition of TRPM8 through the alpha 2A-AR signaling cascade could constitute a new mechanism of modulation of thermosensation in both physiological and pathological conditions.

The roles of iPLA2, TRPM8 and TRPA1 in chemically induced cold hypersensitivity

Background

The cooling agents menthol and icilin act as agonists at TRPM8 and TRPA1. In vitro, activation of TRPM8 by icilin and cold, but not menthol, is dependent on the activity of a sub-type of phospholipase A2, iPLA2. Lysophospholipids (e.g. LPC) produced by PLA2 activity can also activate TRPM8. The role of TRPA1 as a primary cold sensor in vitro is controversial, although there is evidence that TRPA1 plays a role in behavioural responses to noxious cold stimuli. In this study, we have investigated the roles of TRPM8 and TRPA1 and the influence of iPLA2 on noxious cold sensitivities in naïve animals and after local administration of menthol, icilin and LPC. The roles of the channels in cold sensitivity were investigated in mice lacking either TRPM8 (Trpm8^-/-) or TRPA1 (Trpa1^-/-).

Results

Intraplantar administration of icilin evoked a dose-dependent increase in sensitivity to a 10°C stimulus that was inhibited by iPLA2 inhibition with BEL. In contrast the cold hypersensitivities elicited by intraplantar menthol and LPC were not inhibited by BEL treatment. BEL had no effect on basal cold sensitivity and mechanical hypersensitivities induced by the TRPV1 agonist, capsaicin, and the P2X3 agonist α,β-methylene ATP. Both Trpm8^-/- and Trpa1^-/- mice showed longer latencies for paw withdrawal from a 10°C stimulus than wild-type littermates. Cold hypersensitivities induced by either icilin or LPC were absent in Trpm8^-/- mice but were retained in Trpa1^-/- mice. In contrast, cold hypersensitivity evoked by menthol was present in Trpm8^-/- mice but was lost in Trpa1^-/- mice.

Conclusions

The findings that iPLA2 inhibition blocked the development of cold hypersensitivity after administration of icilin but failed to affect menthol-induced hypersensitivity agree well with our earlier in vitro data showing a differential effect of iPLA2 inhibition on the agonist activities of these agents. The ability of LPC to induce cold hypersensitivity supports a role for iPLA2 in modulating TRPM8 activity in vivo. Studies on genetically modified mice demonstrated that the effects of icilin and LPC were mediated by TRPM8 and not TRPA1. In contrast, menthol-induced cold hypersensitivity was dependent on expression of TRPA1 and not TRPM8.

Distinct expression of cold receptors (TRPM8 and TRPA1) in the rat nodose-petrosal ganglion complex

TRPM8 and TRPA1 are cold-activated transient receptor potential (TRP) cation channels. TRPM8 is activated by moderate cooling, while TRPA1 is activated by extreme, noxious cold temperatures. These cold receptors are expressed in different subpopulations of primary afferent neurons. TRPA1 is co-expressed in a subpopulation of somatosensory neurons expressing TRPV1, which is activated by heat. However, the distribution and co-expression of these channels in the nodose-petrosal ganglion complex, which contains the jugular (JG), petrosal (PG), and nodose ganglia (NG) (mainly involved in putative somatic, chemo- and somato-sensation, and somato and visceral sensation, respectively), remain unknown. Here, we conducted in situ hybridization analysis of the rat nodose-petrosal ganglion complex using specific riboprobes for TRPM8, TRPA1, and TRPV1 to compare the features of the cranial sensory ganglia. Hybridization signals for TRPA1 were diffusely observed throughout these ganglia, whereas TRPM8 transcripts were seen in the JG and PG but not in the NG. We retrogradely labeled cranial nerve X with Fast Blue (fluorescent dye) and found TRPM8 transcripts in the jugular-vagal ganglion but not the NG neurons. TRPA1 transcripts were not detected in TRPM8-expressing neurons but were present in the subpopulation of TRPV1-expressing visceral sensory neurons. Taken together, these findings support that in the vagal system the expression of cold-activated TRP channels differs between nodose- and jugular-ganglion neurons suggesting different mechanisms of cold-transduction and that the TRPA1 distribution is consistent with its proposed function as a cold-sensing receptor in the visceral system.

The contribution of TRPM8 and TRPA1 channels to cold allodynia and neuropathic pain

Cold allodynia is a common feature of neuropathic pain however the underlying mechanisms of this enhanced sensitivity to cold are not known. Recently the transient receptor potential (TRP) channels TRPM8 and TRPA1 have been identified and proposed to be molecular sensors for cold. Here we have investigated the expression of TRPM8 and TRPA1 mRNA in the dorsal root ganglia (DRG) and examined the cold sensitivity of peripheral sensory neurons in the chronic construction injury (CCI) model of neuropathic pain in mice.

In behavioral experiments, chronic constriction injury (CCI) of the sciatic nerve induced a hypersensitivity to both cold and the TRPM8 agonist menthol that developed 2 days post injury and remained stable for at least 2 weeks. Using quantitative RT-PCR and in situ hybridization we examined the expression of TRPM8 and TRPA1 in DRG. Both channels displayed significantly reduced expression levels after injury with no change in their distribution pattern in identified neuronal subpopulations. Furthermore, in calcium imaging experiments, we detected no alterations in the number of cold or menthol responsive neurons in the DRG, or in the functional properties of cold transduction following injury. Intriguingly however, responses to the TRPA1 agonist mustard oil were strongly reduced.

Our results indicate that injured sensory neurons do not develop abnormal cold sensitivity after chronic constriction injury and that alterations in the expression of TRPM8 and TRPA1 are unlikely to contribute directly to the pathogenesis of cold allodynia in this neuropathic pain model.

Nicotine activates the chemosensory cation channel TRPA1

Topical application of nicotine, as used in nicotine replacement therapies, causes irritation of the mucosa and skin. This reaction has been attributed to activation of nicotinic acetylcholine receptors (nAChRs) in chemosensory neurons. In contrast with this view, we found that the chemosensory cation channel transient receptor potential A1 (TRPA1) is crucially involved in nicotine-induced irritation. We found that micromolar concentrations of nicotine activated heterologously expressed mouse and human TRPA1. Nicotine acted in a membrane-delimited manner, stabilizing the open state(s) and destabilizing the closed state(s) of the channel. In the presence of the general nAChR blocker hexamethonium, nociceptive neurons showed nicotine-induced responses that were strongly reduced in TRPA1-deficient mice. Finally, TRPA1 mediated the mouse airway constriction reflex to nasal instillation of nicotine. The identification of TRPA1 as a nicotine target suggests that existing models of nicotine-induced irritation should be revised and may facilitate the development of smoking cessation therapies with less adverse effects.

Compounds from Sichuan and Melegueta peppers activate, covalently and non-covalently, TRPA1 and TRPV1 channels

BACKGROUND AND PURPOSE

Oily extracts of Sichuan and Melegueta peppers evoke pungent sensations mediated by different alkylamides [mainly hydroxy-alpha-sanshool (alpha-SOH)] and hydroxyarylalkanones (6-shogaol and 6-paradol). We assessed how transient receptor potential ankyrin 1 (TRPA1) and TRP vanilloid 1 (TRPV1), two chemosensory ion channels, participate in these pungent sensations.

EXPERIMENTAL APPROACH

The structure-activity relationships of these molecules on TRPA1 and TRPV1 was measured by testing natural and synthetic analogues using calcium and voltage imaging on dissociated dorsal root ganglia neurons and human embryonic kidney 293 cells expressing the wild-type channels or specific cysteine mutants using glutathione trapping as a model to probe TRPA1 activation. In addition, using Trpv1 knockout mice, the compounds' aversive responses were measured in a taste brief-access test.

KEY RESULTS

For TRPA1 activation, the cis C6 double bond in the polyenic chain of alpha-SOH was critical, whereas no structural specificity was required for activation of TRPV1. Both 6-shogaol and 6-paradol were found to activate TRPV1 and TRPA1 channels, whereas linalool, an abundant terpene in Sichuan pepper, activated TRPA1 but not TRPV1 channels. Alkylamides and 6-shogaol act on TRPA1 by covalent bonding whereas none of these compounds activated TRPV1 through such interactions. Finally, TRPV1 mutant mice retained sensitivity to 6-shogaol but were not responsive to alpha-SOH.

CONCLUSIONS AND IMPLICATIONS

The pungent nature of components of Sichuan and Melegueta peppers was mediated via interactions with TRPA1 and TRPV1 channels and may explain the aversive properties of these compounds.

TRPV1 is activated by both acidic and basic pH

Maintaining physiological pH is required for survival, and exposure to alkaline chemicals such as ammonia (smelling salts) elicits severe pain and inflammation through unknown mechanisms. TRPV1, the capsaicin receptor, is an integrator of noxious stimuli including heat and extracellular acidic pH. Here, we report that ammonia activates TRPV1, TRPA1 (another polymodal nocisensor), and other unknown receptor(s) expressed in sensory neurons. Ammonia and intracellular alkalization activate TRPV1 through a mechanism that involves a cytoplasmic histidine residue, not used by other TRPV1 agonists such as heat, capsaicin or low pH. Our studies show that TRPV1 detects both acidic and basic deviations from homeostatic pH.