Evolutionary tuning of TRPA1 and TRPV1 thermal and chemical sensitivity in vertebrates

Identification of the TRPA1 cannabinoid-binding site

Chronic pain accounts for nearly two-thirds of conditions eligible for medical cannabis licenses, yet the mechanisms underlying cannabis-induced analgesia remain poorly understood. The principal phytocannabinoids, the psychoactive Δ9-tetrahydrocannabinol (THC) and non-psychoactive cannabidiol (CBD), exhibit comparable efficacy in pain management. Notably, THC functions as an agonist of cannabinoid receptor 1 (CB1), whereas CBD shows minimal activity on CB1 and CB2 receptors. Elucidating the molecular targets through which phytocannabinoids modulate the pain system is required for advancing our understanding of the pain pathway and optimizing medical cannabis therapies. Transient receptor potential ankyrin 1 (TRPA1), a pivotal chemosensor in the pain pathway, has been identified as a phytocannabinoid target. Unlike most TRPA1 activators, phytocannabinoid activation is not mediated through the electrophilic binding site, suggesting an alternative mechanism. Here, we identified the human TRPA1 channel cannabinoid-binding site (CBS) and demonstrated that mutations at residue Y840 abolished responses to both THC and CBD at saturating concentrations, indicating a shared primary binding site. Molecular modeling revealed distinct interactions of THC and CBD with the Y840 residue within the CBS. Additionally, CBD binds to the adjacent general anesthetic binding site at oversaturating concentrations. Our findings define the CBS of TRPA1 as overlapping with and adjacent to binding sites for other allosteric activators, suggesting that TRPA1 possesses a highly adaptable domain for binding non-electrophilic activators. This underscores its unique role as a chemosensor in the pain pathway. Furthermore, our results provide new insights into the molecular mechanisms of cannabinoid-induced analgesia and identify novel targets for pain management therapies.

A journey from molecule to physiology and in silico tools for drug discovery targeting the transient receptor potential vanilloid type 1 (TRPV1) channel

The heat and capsaicin receptor TRPV1 channel is widely expressed in nerve terminals of dorsal root ganglia (DRGs) and trigeminal ganglia innervating the body and face, respectively, as well as in other tissues and organs including central nervous system. The TRPV1 channel is a versatile receptor that detects harmful heat, pain, and various internal and external ligands. Hence, it operates as a polymodal sensory channel. Many pathological conditions including neuroinflammation, cancer, psychiatric disorders, and pathological pain, are linked to the abnormal functioning of the TRPV1 in peripheral tissues. Intense biomedical research is underway to discover compounds that can modulate the channel and provide pain relief. The molecular mechanisms underlying temperature sensing remain largely unknown, although they are closely linked to pain transduction. Prolonged exposure to capsaicin generates analgesia, hence numerous capsaicin analogs have been developed to discover efficient analgesics for pain relief. The emergence of in silico tools offered significant techniques for molecular modeling and machine learning algorithms to indentify druggable sites in the channel and for repositioning of current drugs aimed at TRPV1. Here we recapitulate the physiological and pathophysiological functions of the TRPV1 channel, including structural models obtained through cryo-EM, pharmacological compounds tested on TRPV1, and the in silico tools for drug discovery and repositioning.

Role of TRPA1 in Painful Cold Hypersensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that plays a pivotal role in pain generation after exposure to irritant chemicals and is involved in the sensation of a wide variety of pathological pain. TRPA1 was first reported to be sensitive to noxious cold, but its intrinsic cold sensitivity still remains under debate. To address this issue, we focused on cold hypersensitivity induced by oxaliplatin, a platinum-based chemotherapeutic drug, as a peculiar adverse symptom of acute peripheral neuropathy. We and other groups have shown that oxaliplatin enhances TRPA1 sensitivity to its chemical agonists and reactive oxygen species (ROS). Our in vitro and animal model studies revealed that oxaliplatin, or its metabolite oxalate, inhibits hydroxylation of a proline residue within the N-terminus of human TRPA1 (hTRPA1) via inhibition of prolyl hydroxylase domain-containing protein (PHD), which induces TRPA1 sensitization to ROS. Although hTRPA1 is insensitive to cold, PHD inhibition endows hTRPA1 with cold sensitivity through sensing the small amount of ROS produced after exposure to cold. Hence, we propose that PHD inhibition can unveil the cold sensitivity of hTRPA1 by converting ROS signaling into cold sensitivity. Furthermore, in this review, we summarize the role of TRPA1 in painful cold hypersensitivity during peripheral vascular impairment.

Understanding the role of environmental temperature on sex determination through comparative studies in reptiles and amphibians

In vertebrates, species exhibit phenotypic plasticity of sex determination that the sex can plastically be determined by the external environmental temperature through a mechanism, temperature-dependent sex determination (TSD). Temperature exerts influence over the direction of sexual differentiation pathways, resulting in distinct primary sex ratios in a temperature-dependent manner. This review provides a summary of the thermal sensitivities associated with sex determination in reptiles and amphibians, with a focus on the pattern of TSD, gonadal differentiation, temperature sensing, and the molecular basis underlying thermal sensitivity in sex determination. Comparative studies across diverse lineages offer valuable insights into comprehending the evolution of sex determination as a phenotypic plasticity. While evidence of molecular mechanisms governing sexual differentiation pathways continues to accumulate, the intracellular signaling linking temperature sensing and sexual differentiation pathways remains elusive. We emphasize that uncovering these links is a key for understanding species-specific thermal sensitivities in TSD and will contribute to a more comprehensive understanding of ecosystem and biodiversity conservations.

Lipid-Mediated Mechanisms of Thermal Adaptation and Thermoregulatory Behavior in Animals

Temperature affects a variety of cellular processes because the molecular motion of cellular constituents and the rate of biochemical reactions are sensitive to temperature changes. Thus, the adaptation to temperature is necessary to maintain cellular functions during temperature fluctuation, particularly in poikilothermic organisms. For a wide range of organisms, cellular lipid molecules play a pivotal role during thermal adaptation. Temperature changes affect the physicochemical properties of lipid molecules, resulting in the alteration of cell membrane-related functions and energy metabolism. Since the chemical structures of lipid molecules determine their physicochemical properties and cellular functions, cellular lipids, particularly fatty acid-containing lipid molecules, are remodeled as a thermal adaptation response to compensate for the effects of temperature change. In this chapter, we first introduce the structure and biosynthetic pathway of fatty acid-containing lipid molecules, such as phospholipid and triacylglycerol, followed by a description of the cellular lipid-mediated mechanisms of thermal adaptation and thermoregulatory behavior in animals.

Canine, mouse, and human transient receptor potential ankyrin 1 (TRPA1) channels show different sensitivity to menthol or cold stimulation

Transient receptor potential ankyrin 1 (TRPA1) is a nonselective cation channel that is activated by a variety of stimuli and acts as a nociceptor. Mouse and human TRPA1 exhibit different reactivity to some stimuli, including chemicals such as menthol as well as cold stimuli. The cold sensitivity of TRPA1 in mammalian species is controversial. Here, we analyzed the reactivity of heterologously expressed canine TRPA1 as well as the mouse and human orthologs to menthol or cold stimulation in Ca2+-imaging experiments. Canine and human TRPA1 exhibited a similar response to menthol, that is, activation in a concentration-dependent manner, even at the high concentration range in contrast to the mouse ortholog, which did not respond to high concentration of menthol. In addition, the response during the removal of menthol was different; mouse TRPA1-expressing cells exhibited a typical response with a rapid and clear increase in [Ca2+]i ("off-response"), whereas [Ca2+]i in human TRPA1-expressing cells was dramatically decreased by the washout of menthol and [Ca2+]i in canine TRPA1-expressing cells was slightly decreased. Finally, canine TRPA1 as well as mouse and human TRPA1 were activated by cold stimulation (below 19-20°C). The sensitivity to cold stimulation differed between these species, that is, human TRPA1 activated at higher temperatures compared with the canine and mouse orthologs. All of the above responses were suppressed by the selective TRPA1 inhibitor HC-030031. Because the concentration-dependency and "off-response" of menthol as well as the cold sensitivity were not uniform among these species, studies of canine TRPA1 might be useful for understanding the species-specific functional properties of mammalian TRPA1.

Pharmacological Potential of cis-jasmone in Adult Zebrafish (Danio rerio)

This study evaluates the pharmacological potential of cis-jasmone (CJ) in adult zebrafish (Danio rerio; aZF). Initially, aZF (n = 6/group) were pretreated (20 µL; p. o.) with CJ (0.1 or 0.3 or 1.0 mg/mL) or vehicle (0.5% Tween 80). The animals were submitted to acute toxicity and locomotion tests, pentylenetetrazole-induced seizure, carrageenan-induced abdominal edema, and cinnamaldehyde-, capsaicin-, menthol-, glutamate-, and acid saline-induced orofacial nociception. The possible mechanisms of anticonvulsant, anxiolytic, and antinociceptive action were evaluated. The involvement of central afferent fibers sensitive to cinnamaldehyde and capsaicin and the effect of CJ on the relative gene expression of TRPA1 and TRPV1 in the brain of aZF were also analyzed, in addition to the study of molecular docking between CJ and TRPA1, TRPV1 channels, and GABA_A receptors. CJ did not alter the locomotor behavior and showed pharmacological potential in all tested models with no toxicity. The anticonvulsant effect of CJ was prevented by flumazenil (GABAergic antagonist). The anxiolytic-like effect of CJ was prevented by flumazenil and serotonergic antagonists. The antinociceptive effect was prevented by TRPA1 and TRPV1 antagonists. Chemical ablation with capsaicin and cinnamaldehyde prevented the orofacial antinociceptive effect of CJ. Molecular docking studies indicate that CJ interacted with TRPA1, TRPV1, and GABA_A receptors. CJ inhibited the relative gene expression of TRPA1 and TRPV1. CJ has pharmacological potential for the treatment of seizures, anxiety, inflammation, and acute orofacial nociception. These effects are obtained by modulating the GABAergic and serotonergic systems, as well as the TRPs and ASIC channels.

Air Pollutant impacts on the brain and neuroendocrine system with implications for peripheral organs: a perspective

Air pollutants are being increasingly linked to extrapulmonary multi-organ effects. Specifically, recent studies associate air pollutants with brain disorders including psychiatric conditions, neuroinflammation and chronic diseases. Current evidence of the linkages between neuropsychiatric conditions and chronic peripheral immune and metabolic diseases provides insights on the potential role of the neuroendocrine system in mediating neural and systemic effects of inhaled pollutants (reactive particulates and gases). Autonomically-driven stress responses, involving sympathetic-adrenal-medullary and hypothalamus-pituitary-adrenal axes regulate cellular physiological processes through adrenal-derived hormones and diverse receptor systems. Recent experimental evidence demonstrates the contribution of the very stress system responding to non-chemical stressors, in mediating systemic and neural effects of reactive air pollutants. The assessment of how respiratory encounter of air pollutants induce lung and peripheral responses through brain and neuroendocrine system, and how the impairment of these stress pathways could be linked to chronic diseases will improve understanding of the causes of individual variations in susceptibility and the contribution of habituation/learning and resiliency. This review highlights effects of air pollution in the respiratory tract that impact the brain and neuroendocrine system, including the role of autonomic sensory nervous system in triggering neural stress response, the likely contribution of translocated nano particles or metal components, and biological mediators released systemically in causing effects remote to the respiratory tract. The perspective on the use of systems approaches that incorporate multiple chemical and non-chemical stressors, including environmental, physiological and psychosocial, with the assessment of interactive neural mechanisms and peripheral networks are emphasized.

Intracellular Helix-Loop-Helix Domain Modulates Inactivation Kinetics of Mammalian TRPV5 and TRPV6 Channels

TRPV5 and TRPV6 are calcium-selective ion channels expressed at the apical membrane of epithelial cells. Important for systemic calcium (Ca2+) homeostasis, these channels are considered gatekeepers of this cation transcellular transport. Intracellular Ca2+ exerts a negative control over the activity of these channels by promoting inactivation. TRPV5 and TRPV6 inactivation has been divided into fast and slow phases based on their kinetics. While slow inactivation is common to both channels, fast inactivation is characteristic of TRPV6. It has been proposed that the fast phase depends on Ca2+ binding and that the slow phase depends on the binding of the Ca2+/Calmodulin complex to the internal gate of the channels. Here, by means of structural analyses, site-directed mutagenesis, electrophysiology, and molecular dynamic simulations, we identified a specific set of amino acids and interactions that determine the inactivation kinetics of mammalian TRPV5 and TRPV6 channels. We propose that the association between the intracellular helix-loop-helix (HLH) domain and the TRP domain helix (TDh) favors the faster inactivation kinetics observed in mammalian TRPV6 channels.

The role of TRPA1 channels in thermosensation

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal nonselective cation channel sensitive to different physical and chemical stimuli. TRPA1 is associated with many important physiological functions in different species and thus is involved in different degrees of evolution. TRPA1 acts as a polymodal receptor for the perceiving of irritating chemicals, cold, heat, and mechanical sensations in various animal species. Numerous studies have supported many functions of TRPA1, but its temperature-sensing function remains controversial. Although TRPA1 is widely distributed in both invertebrates and vertebrates, and plays a crucial role in tempreture sensing, the role of TRPA1 thermosensation and molecular temperature sensitivity are species-specific. In this review, we summarize the temperature-sensing role of TRPA1 orthologues in terms of molecular, cellular, and behavioural levels.

Functional analysis of thermo-sensitive TRPV1 in an aquatic vertebrate, masu salmon (Oncorhynchus masou ishikawae)

Transient receptor potential vanilloid 1 (TRPV1) is mainly expressed in nociceptive primary sensory neurons and acts as a sensor for heat and capsaicin. The functional properties of TRPV1 have been reported to vary among species and, in some cases, the species difference in its thermal sensitivity is likely to be associated with thermal habitat conditions. To clarify the functional properties and physiological roles of TRPV1 in aquatic vertebrates, we examined the temperature and chemical sensitivities of TRPV1 in masu salmon (Oncorhynchus masou ishikawae, Om) belonging to a family of salmonids that generally prefer cool environments. First, behavioral experiments were conducted using a video tracking system. Application of capsaicin, a TRPV1 agonist, induced locomotor activities in juvenile Om. Increasing the ambient temperature also elicited locomotor activity potentiated by capsaicin. RT-PCR revealed TRPV1 expression in gills as well as spinal cord. Next, electrophysiological analyses of OmTRPV1 were performed using a two-electrode voltage-clamp technique with a Xenopus oocyte expression system. Heat stimulation evoked an inward current in heterologously expressed OmTRPV1. In addition, capsaicin produced current responses in OmTRPV1-expressing oocytes, but higher concentrations were needed for its activation compared to the mammalian orthologues. These results indicate that Om senses environmental stimuli (heat and capsaicin) through the activation of TRPV1, and this channel may play important roles in avoiding environments disadvantageous for survival in aquatic vertebrates.

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Capsaicin evoked behavioral responses of Oncorhynchus masou ishikawae (Om).

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The behavioral response to heat was potentiated by capsaicin.

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Heterologously expressed OmTRPV1 was activated by heat and capsaicin.

Evolutionary tuning of TRPA1 underlies the variation in heat avoidance behaviors among frog species inhabiting diverse thermal niches

Environmental temperature is a critical factor for all forms of life, and thermal tolerance defines the habitats utilized by a species. Moreover, the evolutionary tuning of thermal perception can also play a key role in habitat selection. Yet, the relative importance of thermal tolerance and perception in environmental adaptation remains poorly understood. Thermal conditions experienced by anuran tadpoles differ among species due to the variation in breeding seasons and water environments selected by parental frogs. In the present study, heat tolerance and avoidance temperatures were compared in tadpoles from five anuran species that spatially and temporally inhabit different thermal niches. These two parameters were positively correlated with each other and were consistent with the thermal conditions of habitats. The species difference in avoidance temperature was 2.6 times larger than that in heat tolerance, suggesting the importance of heat avoidance responses in habitat selection. In addition, the avoidance temperature increased after warm acclimation, especially in the species frequently exposed to heat in their habitats. Characterization of the heat-sensing transient receptor potential ankyrin 1 (TRPA1) ion channel revealed an amphibian-specific alternatively spliced variant containing a single valine insertion relative to the canonical alternative spliced variant of TRPA1, and this novel variant altered the response to thermal stimuli. The two alternatively spliced variants of TRPA1 exhibited different thermal responses in a species-specific manner, which are likely to be associated with a difference in avoidance temperatures among species. Together, our findings suggest that the functional change in TRPA1 plays a crucial role in thermal adaptation processes.

Temperature acclimation in hot-spring snakes and the convergence of cold response

Animals have evolved sophisticated temperature-sensing systems and mechanisms to detect and respond to ambient temperature changes. As a relict species endemic to the Qinghai-Tibet Plateau, hot-spring snake (Thermophis baileyi) survived the dramatic changes in climate that occurred during plateau uplift and ice ages, providing an excellent opportunity to explore the evolution of temperature sensation in ectotherms. Based on distributional information and behavioral experiments, we found that T. baileyi prefer hot-spring habitats and respond more quickly to warmth than other two snakes, suggesting that T. baileyi may evolve an efficient thermal-sensing system. Using high-quality chromosome-level assembly and comparative genomic analysis, we identified cold acclimation genes experiencing convergent acceleration in high-altitude lineages. We also discovered significant evolutionary changes in thermosensation- and thermoregulation-related genes, including the transient receptor potential (TRP) channels. Among these genes, TRPA1 exhibited three species-specific amino acid replacements, which differed from those found in infrared imaging snakes, implying different temperature-sensing molecular strategies. Based on laser-heating experiments, the T. baileyi-specific mutations in TRPA1 resulted in an increase in heat-induced opening probability and thermal sensitivity of the ion channels under the same degree of temperature stimulation, which may help the organism respond to temperature changes more quickly. These results provide insight into the genetic mechanisms underpinning the evolution of temperature-sensing strategies in ectotherms as well as genetic evidence of temperature acclimation in this group.

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Hot-spring snakes prefer hot-spring habitats on the Qinghai-Tibet Plateau

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Genetic variation in the snakes contribute to the temperature acclimation

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Unique mutations in TRPA1 increase thermal sensitivity of the ion channel

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Different temperature-sensing strategies existed across snakes

Neuronal and non-neuronal TRPA1 as therapeutic targets for pain and headache relief

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, has a major role in different types of pain. TRPA1 is primarily localized to a subpopulation of primary sensory neurons of the trigeminal, vagal, and dorsal root ganglia. This subset of nociceptors produces and releases the neuropeptide substance P (SP) and calcitonin gene-related peptide (CGRP), which mediate neurogenic inflammation. TRPA1 is characterized by unique sensitivity for an unprecedented number of reactive byproducts of oxidative, nitrative, and carbonylic stress and to be activated by several chemically heterogenous, exogenous, and endogenous compounds. Recent preclinical evidence has revealed that expression of TRPA1 is not limited to neurons, but its functional role has been reported in central and peripheral glial cells. In particular, Schwann cell TRPA1 was recently implicated in sustaining mechanical and thermal (cold) hypersensitivity in mouse models of macrophage-dependent and macrophage-independent inflammatory, neuropathic, cancer, and migraine pain. Some analgesics and herbal medicines/natural products widely used for the acute treatment of pain and headache have shown some inhibitory action at TRPA1. A series of high affinity and selective TRPA1 antagonists have been developed and are currently being tested in phase I and phase II clinical trials for different diseases with a prominent pain component. Abbreviations: 4-HNE, 4-hydroxynonenal; ADH-2, alcohol dehydrogenase-2; AITC, allyl isothiocyanate; ANKTD, ankyrin-like protein with transmembrane domains protein 1; B2 receptor, bradykinin 2 receptor; CIPN, chemotherapeutic-induced peripheral neuropathy; CGRP, calcitonin gene related peptide; CRISPR, clustered regularly interspaced short palindromic repeats; CNS, central nervous system; COOH, carboxylic terminal; CpG, C-phosphate-G; DRG, dorsal root ganglia; EP, prostaglandins; GPCR, G-protein-coupled receptors; GTN, glyceryl trinitrate; MAPK, mitogen-activated protein kinase; M-CSF, macrophage-colony stimulating factor; NAPQI, N-Acetyl parabenzoquinone-imine; NGF, nerve growth factor; NH2, amino terminal; NKA, neurokinin A; NO, nitric oxide; NRS, numerical rating scale; PAR2, protease-activated receptor 2; PMA, periorbital mechanical allodynia; PLC, phospholipase C; PKC, protein kinase C; pSNL, partial sciatic nerve ligation; RCS, reactive carbonyl species; ROS, reactive oxygen species; RNS, nitrogen oxygen species; SP, substance P; TG, trigeminal ganglion; THC, Δ9-tetrahydrocannabinol; TrkA, neurotrophic receptor tyrosine kinase A; TRP, transient receptor potential; TRPC, TRP canonical; TRPM, TRP melastatin; TRPP, TRP polycystin; TRPM, TRP mucolipin; TRPA, TRP ankyrin; TRPV, TRP vanilloid; VG, vagal ganglion.

Anomalous Temperature Interdicts the Reproductive Activity in Fish: Neuroendocrine Mechanisms of Reproductive Function in Response to Water Temperature

Fish are poikilotherm and small changes in water temperature can greatly affect physiological processes including reproduction, which is regulated by complex neuroendocrine mechanisms that respond to climatic events. This review provides evidence that anomalous high and low temperature may directly affect reproduction in fish by suppressing the expression of genes in the reproductive neuroendocrine system. The grass puffer, Takifugu alboplumbeus, is an excellent animal model for studying the thermal regulation of reproduction, for they exhibit periodic spawning activities, which are synchronized with seasonal, lunar and daily cycles. In the grass puffer, the expression of the genes encoding gonadotropin-releasing hormone (GnRH) 1, kisspeptin, gonadotropin-inhibitory hormone (GnIH) and their receptors were markedly suppressed in the diencephalon of fish exposed to high temperature (28°C) when compared to normal temperature (21°C), followed by the decrease in the pituitary mRNA levels for follicle-stimulating hormone (FSH), luteinizing hormone (LH) and growth hormone (GH). On the other hand, the exposure to low temperature (14°C) also inhibited the expression of gnrh1, kiss2, gnih and their receptor genes in the brain and fshb, lhb, gh and prl in the pituitary. Taken together, it is plausible that anomalous high and low temperature may be a proximate driver of termination of reproduction by suppressing the activity of the reproductive GnRH/kisspeptin/GnIH system, possibly through direct action of temperature signals at transcription level.

OUP accepted manuscript

Animals rely on their sensory systems to inform them of ecologically relevant environmental variation. In the Southern Ocean, the thermal environment has remained between −1.9 and 5 °C for 15 Myr, yet we have no knowledge of how an Antarctic marine organism might sense their thermal habitat as we have yet to discover a thermosensitive ion channel that gates (opens/closes) below 10 °C. Here, we investigate the evolutionary dynamics of transient receptor potential (TRP) channels, which are the primary thermosensors in animals, within cryonotothenioid fishes—the dominant fish fauna of the Southern Ocean. We found cryonotothenioids have a similar complement of TRP channels as other teleosts (∼28 genes). Previous work has shown that thermosensitive gating in a given channel is species specific, and multiple channels act together to sense the thermal environment. Therefore, we combined evidence of changes in selective pressure, gene gain/loss dynamics, and the first sensory ganglion transcriptome in this clade to identify the best candidate TRP channels that might have a functional dynamic range relevant for frigid Antarctic temperatures. We concluded that TRPV1a, TRPA1b, and TRPM4 are the likeliest putative thermosensors, and found evidence of diversifying selection at sites across these proteins. We also put forward hypotheses for molecular mechanisms of other cryonotothenioid adaptations, such as reduced skeletal calcium deposition, sensing oxidative stress, and unusual magnesium homeostasis. By completing a comprehensive and unbiased survey of these genes, we lay the groundwork for functional characterization and answering long-standing thermodynamic questions of thermosensitive gating and protein adaptation to low temperatures.

Thermodynamic and structural basis of temperature-dependent gating in TRP channels

Living organisms require detecting the environmental thermal clues for survival, allowing them to avoid noxious stimuli or find prey moving in the dark. In mammals, the Transient Receptor Potential ion channels superfamily is constituted by 27 polymodal receptors whose activity is controlled by small ligands, peptide toxins, protons and voltage. The thermoTRP channels subgroup exhibits unparalleled temperature dependence -behaving as heat and cold sensors. Functional studies have dissected their biophysical features in detail, and the advances of single-particle Cryogenic Electron microscopy provided the structural framework required to propose detailed channel gating mechanisms. However, merging structural and functional evidence for temperature-driven gating of thermoTRP channels has been a hard nut to crack, remaining an open question nowadays. Here we revisit the highlights on the study of heat and cold sensing in thermoTRP channels in the light of the structural data that has emerged during recent years.

A molecular perspective on identifying TRPV1 thermosensitive regions and disentangling polymodal activation

TRPV1 is a polymodal receptor ion channel that is best known to function as a molecular thermometer. It is activated in diverse ways, including by heat, protons (low pH), and vanilloid compounds, such as capsaicin. In this review, we summarize molecular studies of TRPV1 thermosensing, focusing on the cross-talk between heat and other activation modes. Additional insights from TRPV1 isoforms and non-rodent/non-human TRPV1 ortholog studies are also discussed in this context. While the molecular mechanism of heat activation is still emerging, it is clear that TRPV1 thermosensing is modulated allosterically, i.e., at a distance, with contributions from many distinct regions of the channel. Similarly, current studies identify cross-talk between heat and other TRPV1 activation modes, such as protons and capsaicin, and that these modes can generally be selectively disentangled. In aggregate, this suggests that future TRPV1 molecular studies should define allosteric pathways and provide mechanistic insight, thereby enabling mode-selective manipulation of the polymodal receptor. These advances are anticipated to have significant implications in both basic and applied biomedical sciences.

Heat Stress Responses in Birds: A Review of the Neural Components

Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world's food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.

Thermal Sensitivity of Heat Sensor TRPA1 Correlates With Temperatures Inducing Heat Avoidance Behavior in Terrestrial Ectotherms

Temperature is an essential environmental factor that controls an organism’s performances. As ectothermic animals largely rely on external heat sources for adjusting their body temperature, thermal perception is a primary process of behavioral thermoregulation. Transient receptor potential ankyrin 1 (TRPA1) is a heat sensitive ion channel in most non-mammalian species, and its heat activation has been suggested to induce heat avoidance behaviors in ectothermic animals. However, associations between TRPA1 and ecologically relevant temperatures have not been investigated, and the analyses including diverse taxa will provide robust support for understanding the associations. Here, I conducted extensive literature review, and assembled published data on thermal threshold of TRPA1 and three physiological parameters: the experimental voluntary maximum (EVM), which is body temperatures when heat avoidance behaviors are induced; the critical thermal maximum (CTmax), which is a point in temperature beyond which an organism becomes incapacitated; and average body temperature (Tmean) recorded in the field. Then, I examined the relationships between thermal threshold of TRPA1 and each of the three physiological parameters. As phylogenetically closely related species tend to show similar trait values among species, I conducted the regression analyses by accounting for phylogenetic distances among species. This study supports previous research by affirming that thermal threshold of TRPA1 is substantially correlated with body temperature that the animals escaped from the heat source, represented here as EVM. Nevertheless, thermal threshold of TRPA1 showed a statistically insignificant correlation with CTmax and Tmean. The results suggest that although thermal threshold of TRPA1 is evolutionarily labile, its associations with EVM is highly conserved among diverse terrestrial ectotherms. Therefore, thermal threshold of TRPA1 could be a useful parameter to evaluate species vulnerability to thermal stress particularly in the recent climate warming scenario.

Generation of GCaMP6s-Expressing Zebrafish to Monitor Spatiotemporal Dynamics of Calcium Signaling Elicited by Heat Stress

The ability of organisms to quickly sense and transduce signals of environmental stresses is critical for their survival. Ca²⁺ is a versatile intracellular messenger involved in sensing a wide variety of stresses and regulating the subsequent cellular responses. So far, our understanding for calcium signaling was mostly obtained from ex vivo tissues and cultured cell lines, and the in vivo spatiotemporal dynamics of stress-triggered calcium signaling in a vertebrate remains to be characterized. Here, we describe the generation and characterization of a transgenic zebrafish line with ubiquitous expression of GCaMP6s, a genetically encoded calcium indicator (GECI). We developed a method to investigate the spatiotemporal patterns of Ca²⁺ events induced by heat stress. Exposure to heat stress elicited immediate and transient calcium signaling in developing zebrafish. Cells extensively distributed in the integument of the head and body trunk were the first batch of responders and different cell populations demonstrated distinct response patterns upon heat stress. Activity of the heat stress-induced calcium signaling peaked at 30 s and swiftly decreased to near the basal level at 120 s after the beginning of exposure. Inhibition of the heat-induced calcium signaling by LaCl₃ and capsazepine and treatment with the inhibitors for CaMKII (Ca²²/calmodulin-dependent protein kinase II) and HSF1 (Heat shock factor 1) all significantly depressed the enhanced heat shock response (HSR). Together, we delineated the spatiotemporal dynamics of heat-induced calcium signaling and confirmed functions of the Ca²⁺-CaMKII-HSF1 pathway in regulating the HSR in zebrafish.

Transient receptor potential ankyrin 1 channel: An evolutionarily tuned thermosensor

The discovery of the role of the transient receptor potential ankyrin 1 (TRPA1) channel as a polymodal detector of cold and pain-producing stimuli almost two decades ago catalyzed the consequent identification of various vertebrate and invertebrate orthologues. In different species, the role of TRPA1 has been implicated in numerous physiological functions, indicating that the molecular structure of the channel exhibits evolutionary flexibility. Until very recently, information about the critical elements of the temperature-sensing molecular machinery of thermosensitive ion channels such as TRPA1 had lagged far behind information obtained from mutational and functional analysis. Current developments in single-particle cryo-electron microscopy are revealing precisely how the thermosensitive channels operate, how they might be targeted with drugs, and at which sites they can be critically regulated by membrane lipids. This means that it is now possible to resolve a huge number of very important pharmacological, biophysical and physiological questions in a way we have never had before. In this review, we aim at providing some of the recent knowledge on the molecular mechanisms underlying the temperature sensitivity of TRPA1. We also demonstrate how the search for differences in temperature and chemical sensitivity between human and mouse TRPA1 orthologues can be a useful approach to identifying important domains with a key role in channel activation.

The diversity in sensitivity of TRPA1 and TRPV1 of various animals to polyphenols

The perception of tastes is sensed by the receptors that stimulate sensory cells. We previously reported that TRPA1 and TRPV1 channels expressed in the oral cavity of mammals, are activated by the auto-oxidized product of epigallocatechin gallate (oxiEGCG), a major astringent catechin in green tea. Here, we investigated and compared the sensitivity of TRPA1 and TRPV1 from various animals to astringent polyphenols. We selected three polyphenols, oxiEGCG, tannic acid and myricetin. HEK293T cells expressing TRPA1 or TRPV1 from mammal, bird, reptile, amphibian, and fish, were analyzed for their activation by the Ca²⁺-imaging. We found the apparent diversity in the polyphenol-sensitivity among various animals. Mammalian TRPs showed relatively higher sensitivity to polyphenols, and especially, human TRPA1 and TRPV1 could be activated by all of three polyphenols at 20 μM. Reptile TRP channels, however, were insensitive to any polyphenols examined. Moreover, the polyphenol-sensitivity of zebrafish TRPA1 and TRPV1 was quite different from that of medaka TRP channels. Since many polyphenols are present in plants and the sensing of polyphenols using TRP channels in the oral cavity might cause astringent taste, the observed diversity of the polyphenol-sensitivity of TRP channels might be involved in the divergence in the food habit of various animals.

Functional expression of Δ12 fatty acid desaturase modulates thermoregulatory behaviour in Drosophila

Polyunsaturated fatty acids (PUFAs) play crucial roles in adaptation to cold environments in a wide variety of animals and plants. However, the mechanisms by which PUFAs affect thermoregulatory behaviour remain elusive. Thus, we investigated the roles of PUFAs in thermoregulatory behaviour of Drosophila melanogaster. To this end, we generated transgenic flies expressing Caenorhabditis elegans Δ12 fatty acid desaturase (FAT-2), which converts mono-unsaturated fatty acids to PUFAs such as linoleic acid [C18:2 (n-6)] and linolenic acid [C18:3 (n-3)]. Neuron-specific expression of FAT-2 using the GAL4/UAS expression system led to increased contents of C18:2 (n-6)-containing phospholipids in central nerve system (CNS) and caused significant decreases in preferred temperature of third instar larvae. In genetic screening and calcium imaging analyses of thermoreceptor-expressing neurons, we demonstrated that ectopic expression of FAT-2 in TRPA1-expressing neurons led to decreases in preferred temperature by modulating neuronal activity. We conclude that functional expression of FAT-2 in a subset of neurons changes the thermoregulatory behaviour of D. melanogaster, likely by modulating quantities of PUFA-containing phospholipids in neuronal cell membranes.

A paradigm of thermal adaptation in penguins and elephants by tuning cold activation in TRPM8

Sensing temperature is critical for the survival of all living beings. Here, we show that during cold-induced activation of the archetypical temperature-sensitive TRPM8 ion channel, there are hydrophobic residues in the pore domain stabilized in the exposed state. Tuning hydrophobicity of these residues specifically alters cold response in TRPM8. Furthermore, TRPM8 orthologs in vertebrates evolved to employ such a mechanism, which physiologically tunes cold tolerance for better thermal adaptation. Our findings not only advance the understanding of the cold-induced activation mechanism of TRPM8 but also bring insights to the molecular evolution strategy for ambient-temperature adaptation in vertebrates.

To adapt to habitat temperature, vertebrates have developed sophisticated physiological and ecological mechanisms through evolution. Transient receptor potential melastatin 8 (TRPM8) serves as the primary sensor for cold. However, how cold activates TRPM8 and how this sensor is tuned for thermal adaptation remain largely unknown. Here we established a molecular framework of how cold is sensed in TRPM8 with a combination of patch-clamp recording, unnatural amino acid imaging, and structural modeling. We first observed that the maximum cold activation of TRPM8 in eight different vertebrates (i.e., African elephant and emperor penguin) with distinct side-chain hydrophobicity (SCH) in the pore domain (PD) is tuned to match their habitat temperature. We further showed that altering SCH for residues in the PD with solvent-accessibility changes leads to specific tuning of the cold response in TRPM8. We also observed that knockin mice expressing the penguin’s TRPM8 exhibited remarkable tolerance to cold. Together, our findings suggest a paradigm of thermal adaptation in vertebrates, where the evolutionary tuning of the cold activation in the TRPM8 ion channel through altering SCH and solvent accessibility in its PD largely contributes to the setting of the cold-sensitive/tolerant phenotype.

Hyperthermia induced by transient receptor potential vanilloid-1 (TRPV1) antagonists in human clinical trials: Insights from mathematical modeling and meta-analysis

Antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel alter body temperature (T_b) in laboratory animals and humans: most cause hyperthermia; some produce hypothermia; and yet others have no effect. TRPV1 can be activated by capsaicin (CAP), protons (low pH), and heat. First-generation (polymodal) TRPV1 antagonists potently block all three TRPV1 activation modes. Second-generation (mode-selective) TRPV1 antagonists potently block channel activation by CAP, but exert different effects (e.g., potentiation, no effect, or low-potency inhibition) in the proton mode, heat mode, or both. Based on our earlier studies in rats, only one mode of TRPV1 activation - by protons - is involved in thermoregulatory responses to TRPV1 antagonists. In rats, compounds that potently block, potentiate, or have no effect on proton activation cause hyperthermia, hypothermia, or no effect on T_b, respectively. A T_b response occurs when a TRPV1 antagonist blocks (in case of hyperthermia) or potentiates (hypothermia) the tonic TRPV1 activation by protons somewhere in the trunk, perhaps in muscles, and - via the acido-antithermogenic and acido-antivasoconstrictor reflexes - modulates thermogenesis and skin vasoconstriction. In this work, we used a mathematical model to analyze T_b data from human clinical trials of TRPV1 antagonists. The analysis suggests that, in humans, the hyperthermic effect depends on the antagonist's potency to block TRPV1 activation not only by protons, but also by heat, while the CAP activation mode is uninvolved. Whereas in rats TRPV1 drives thermoeffectors by mediating pH signals from the trunk, but not T_b signals, our analysis suggests that TRPV1 mediates both pH and thermal signals driving thermoregulation in humans. Hence, in humans (but not in rats), TRPV1 is likely to serve as a thermosensor of the thermoregulation system. We also conducted a meta-analysis of T_b data from human trials and found that polymodal TRPV1 antagonists (ABT-102, AZD1386, and V116517) increase T_b, whereas the mode-selective blocker NEO6860 does not. Several strategies of harnessing the thermoregulatory effects of TRPV1 antagonists in humans are discussed.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Drosophila menthol sensitivity and the Precambrian origins of transient receptor potential-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that transient receptor potential melastatin (TRPM) and transient receptor potential ankyrin (TRPA) thermal and electrophile sensitivities predate the protostome-deuterostome split (greater than 550 Ma). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g. menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural and phylogenetic approaches, we tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. We found that topical application of menthol to Drosophila melanogaster larvae elicits a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, we put forth the hypotheses that three previously undescribed TRPM channel clades (basal, αTRPM and βTRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Non-Analgesic Symptomatic or Disease-Modifying Potential of TRPA1

TRPA1, a versatile ion channel of the Transient Receptor Potential (TRP) channel family, detects a large variety of chemicals and can contribute to signal processing of other stimuli, e.g., due to its sensitivity to cytosolic calcium elevation or phosphoinositolphosphate modulation. At first, TRPA1 was found on sensory neurons, where it can act as a sensor for potential or actual tissue damage that ultimately may elicit pain or itch as warning symptoms. This review provides an update regarding the analgesic and antipruritic potential of TRPA1 modulation and the respective clinical trials. Furthermore, TRPA1 has been found in an increasing amount of other cell types. Therefore, the main focus of the review is to discuss the non-analgesic and particularly the disease-modifying potential of TRPA1. This includes diseases of the respiratory system, cancer, ischemia, allergy, diabetes, and the gastrointestinal system. The involvement of TRPA1 in the respective pathophysiological cascades is so far mainly based on pre-clinical data.

Elucidating the functional evolution of heat sensors among Xenopus species adapted to different thermal niches by ancestral sequence reconstruction

Ambient temperature fluctuations are detected via the thermosensory system which allows animals to seek preferable thermal conditions or escape from harmful temperatures. Evolutionary changes in thermal perception have thus potentially played crucial roles in niche selection. The genus Xenopus (clawed frog) is suitable for investigating the relationship between thermal perception and niche selection due to their diverse latitudinal and altitudinal distributions. Here we performed comparative analyses of the neuronal heat sensors TRPV1 and TRPA1 among closely related Xenopus species (X. borealis, X. muelleri, X. laevis, and X. tropicalis) to elucidate their functional evolution and to assess whether their functional differences correlate with thermal niche selection among the species. Comparison of TRPV1 among four extant Xenopus species and reconstruction of the ancestral TRPV1 revealed that TRPV1 responses to repeated heat stimulation were specifically altered in the lineage leading to X. tropicalis which inhabits warmer niches. Moreover, the thermal sensitivity of TRPA1 was lower in X. tropicalis than the other species, although the thermal sensitivity of TRPV1 and TRPA1 was not always lower in species that inhabit warmer niches than the species inhabit cooler niches. However, a clear correlation was found in species differences in TRPA1 activity. Heat-evoked activity of TRPA1 in X. borealis and X. laevis, which are adapted to cooler niches, was significantly higher than in X. tropicalis and X. muelleri which are adapted to warmer niches. These findings suggest that the functional properties of heat sensors changed during Xenopus evolution, potentially altering the preferred temperature ranges among species.

A crayfish Ras gene is involved in the defense against bacterial infection under high temperature

Temperature is an important environmental factor influencing crustacean resistance to pathogen infection. However, the mechanism underlying immune regulation by temperature remains unclear in crustacean. Here, we report a Ras gene of crayfish (designated as PcRAS1) which is involved in immune regulation of crayfish under high temperature. PcRAS1 is induced by both high temperature and bacterial infection and the induction by bacterial infection is associated with temperature. Significant changes of PcRAS1 expression was observed at 32 °C and 24 °C after infection with Aeromonas hydrophila, but relative moderate alternation was found at 16 °C after challenged with A. hydrophila. PcRAS1 silencing significantly reduced crayfish survival from high temperature (32 °C and 24 °C) or bacterial infection at 32 °C, but there was no significant effect on survival from bacterial infection at 24 °C or 16 °C. Further analysis reveals that PO activity is reduced by high temperature or enhanced by bacterial infection. Moreover, both the decreased PO activity and the enhanced PO activity are affected by PcRAS1 expression. PcRAS1 silencing further reduces PO activity under high temperature and compromises the enhanced PO activity by bacterial infection. Lipid peroxidation (LPO) and total antioxidant capacity (TAC) are also involved in the responses to high temperature. LPO is enhanced by lower temperature. TAC is reduced by high temperature and TAC change resulting from high temperature is amplified by PcRAS1 silencing. These results collectively indicate that PcRAS1 is involved in immune regulation against bacterial infection mediated by temperature.

Molecular Evolution Bioinformatics Toward Structural Biology of TRPV1-4 Channels

Bioinformatics is a very resourceful tool to understand evolution of membrane proteins, such as transient receptor potential channels. Expert bioinformatics users rely on specialized scripting and programming skills. Several web servers and standalone tools are available for nonadvanced users willing to develop projects to understand their system of choice. In this case, we present a desktop-based protocol to develop evostructural hypotheses based on basic bioinformatics skills and resources, specifically for a small subgroup of TRPV channels, which can be further implemented for larger datasets.

Effects of TRPA1 activation and inhibition on TRPA1 and CGRP expression in dorsal root ganglion neurons

Transient receptor potential ankyrin 1 (TRPA1) is a key player in pain and neurogenic inflammation, and is localized in nociceptive primary sensory dorsal root ganglion (DRG) neurons. TRPA1 plays a major role in the transmission of nociceptive sensory signals. The generation of neurogenic inflammation appears to involve TRPA1-evoked release of calcitonin gene-related peptide (CGRP). However, it remains unknown whether TRPA1 or CGRP expression is affected by TRPA1 activation. Thus, in this study, we examined TRPA1 and CGRP expression in DRG neurons in vitro after treatment with the TRPA1 activator formaldehyde or the TRPA1 blocker menthol. In addition, we examined the role of extracellular signal-regulated protein kinase 1/2 (ERK1/2) in this process. DRG neurons in culture were exposed to formaldehyde, menthol, the ERK1/2 inhibitor PD98059 + formaldehyde, or PD98059 + menthol. After treatment, real-time polymerase chain reaction, western blot assay and double immunofluorescence labeling were performed to evaluate TRPA1 and CGRP expression in DRG neurons. Formaldehyde elevated mRNA and protein levels of TRPA1 and CGRP, as well as the proportion of TRPA1- and CGRP-positive neurons. In contrast, menthol reduced TRPA1 and CGRP expression. Furthermore, the effects of formaldehyde, but not menthol, on CGRP expression were blocked by pretreatment with PD98059. PD98059 pretreatment did not affect TRPA1 expression in the presence of formaldehyde or menthol.

TRP Channels as Drug Targets to Relieve Itch

Although acute itch has a protective role by removing irritants to avoid further damage, chronic itch is debilitating, significantly impacting quality of life. Over the past two decades, a considerable amount of stimulating research has been carried out to delineate mechanisms of itch at the molecular, cellular, and circuit levels. There is growing evidence that transient receptor potential (TRP) channels play important roles in itch signaling. The purpose of this review is to summarize our current knowledge about the role of TRP channels in the generation of itch under both physiological and pathological conditions, thereby identifying them as potential drug targets for effective anti-itch therapies.

Effect of single-nucleotide polymorphisms in TRPV1 on burning pain and capsaicin sensitivity in Japanese adults

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that is expressed in the sensory neurons and responds to various noxious stimuli including heat and capsaicin. The molecular properties of TRPV1 have been clearly examined; however, there are obvious individual differences in human sensitivity to thermal stimuli and capsaicin. Here, we examined the possibility that different genome sequence of human TRPV1 caused the different sensitivity to heat or capsaicin. The sensitivities to burning pain and capsaicin of Japanese adult subjects were compared with their TRPV1 genome sequence, and we detected 6 single-nucleotide polymorphisms and 11 single-nucleotide polymorphisms related to burning pain and capsaicin sensitivity, respectively. In particular, homozygous I585V, a single-nucleotide polymorphism with amino acid substitution, significantly related to higher capsaicin sensitivity.

Molecular mechanism of the tree shrew's insensitivity to spiciness

Spicy foods elicit a pungent or hot and painful sensation that repels almost all mammals. Here, we observe that the tree shrew (Tupaia belangeri chinensis), which possesses a close relationship with primates and can directly and actively consume spicy plants. Our genomic and functional analyses reveal that a single point mutation in the tree shrew’s transient receptor potential vanilloid type-1 (TRPV1) ion channel (tsV1) lowers its sensitivity to capsaicinoids, which enables the unique feeding behavior of tree shrews with regards to pungent plants. We show that strong selection for this residue in tsV1 might be driven by Piper boehmeriaefolium, a spicy plant that geographically overlaps with the tree shrew and produces Cap2, a capsaicin analog, in abundance. We propose that the mutation in tsV1 is a part of evolutionary adaptation that enables the tree shrew to tolerate pungency, thus widening the range of its diet for better survival.

Most mammals cannot tolerate the pungent sensation, such as that evoked by eating chili peppers. Here, we show that unexpectedly, the tree shrew, a mammal closely related to primates, can consume pungent plants. We determined that this tolerance is caused by an amino acid change in the tree shrew’s transient receptor potential vanilloid type-1 (TRPV1) ion channel, which lowers the channel’s sensitivity to capsaicinoids—the substances that make plants spicy. We attribute the strong selection for this amino acid to an adaptation to consuming Piper boehmeriaefolium, a spicy plant that geographically overlaps with the tree shrew and produces Cap2, a substance similar to capsaicin, the pungent agent found in chili peppers. Our study suggests an evolutionary and molecular mechanism adopted by the tree shrew to expand its nutritional repertoire.