Aop: 196

Title

A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the AO. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Volatile Organic Chemicals Activate TRPA1 Receptor to Induce Sensory Pulmonary Irritation

Short name
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TRPA1 activation leads to pulmonary sensory irritation

Graphical Representation

A graphical representation of the AOP.This graphic should list all KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs. More help
Click to download graphical representation template Explore AOP in a Third Party Tool
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Authors

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Jeanelle Martinez and Thomas Eling

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section.   More help
Jeanelle Martinez   (email point of contact)

Contributors

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  • Jeanelle Martinez

Status

Provides users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. OECD Status - Tracks the level of review/endorsement the AOP has been subjected to. OECD Project Number - Project number is designated and updated by the OECD. SAAOP Status - Status managed and updated by SAAOP curators. More help
Author status OECD status OECD project SAAOP status
Under development: Not open for comment. Do not cite Under Development
This AOP was last modified on June 04, 2021 14:17

Revision dates for related pages

Page Revision Date/Time
TRPA1 activation, TRPA1 Receptor September 16, 2017 10:17
Opening of calcium channel, Calcium influx October 25, 2017 16:59
Trigeminal nerve activation September 24, 2018 12:08
SP (Substance P) release, Local increase of SP September 16, 2017 10:17
Trigeminal and/or vagal nerve excitation causes Airway Hyper-responsiveness,Cough, Dyspnea September 24, 2018 12:14
Increased Respiratory irritability and Chronic Cough, September 24, 2018 12:51
Increased CGRP, neuronal release of CGRP September 16, 2017 10:17
Irritation of nasal mucosa inducing sneeze reflex September 16, 2017 10:17
Increased neurokinin A (NKA) by neuronal cells September 16, 2017 10:17
TRPA1 activation, TRPA1 Receptor leads to Opening of calcium channel, Calcium influx November 29, 2016 21:13
Opening of calcium channel, Calcium influx leads to Excitation, Trigeminal nerve excitation December 03, 2016 16:38
Excitation, Trigeminal nerve excitation leads to SP (Substance P) release, Local increase of SP June 23, 2017 11:14
Excitation, Trigeminal nerve excitation leads to Increased CGRP June 23, 2017 13:12
Excitation, Trigeminal nerve excitation leads to Increased NKA June 23, 2017 16:37
SP (Substance P) release, Local increase of SP leads to Respiratory irritability September 24, 2018 12:47
Increased CGRP leads to Respiratory irritability June 27, 2017 18:23
Increased NKA leads to Respiratory irritability September 24, 2018 12:48
Respiratory irritability leads to Increased Airway Hyper-responsiveness,Cough, Dyspnea September 24, 2018 12:20
Excitation, Trigeminal nerve excitation leads to Irritation induced sneezing June 23, 2017 14:42
Dibenzo[B,F][1,4]Oxazepine January 17, 2017 14:50
2-Chlorobenzalmalononitrile January 20, 2017 11:39

Abstract

A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

Pulmonary irritation induced by chemical exposure can be divided into two types. One type of irritation, corrosive irritation causes inflammation from tissue destruction and appears to be limited to inhalation of industrial chemicals for example hydrochloric acid, chlorine gas, and ammonia. The second type, sensory irritation is caused by sensory stimulation mediated by activation of specific receptors located in the trigeminal nerve endings and occurs at concentrations that do not result in tissue destruction. Our focus is to develop an adverse outcome pathway (AOP) for sensory irritants.  Many inhalable chemicals are sensory irritants (Alarie et al 1966, 1973, Schaper et al., 1993) that can cause physiological responses including pain, mucus production, nasal obstruction, sneezing, coughing, and decreases in respiration rate (Brunning et al., 2014). The target organ of respiratory irritants is ultimately the airways causing inflammation of the trachea, bronchitis, and bronchiolitis. A receptor present in the upper airways is the TRPA1 (transient receptor potential cation channel, subfamily A, member 1) that induces a physiological response to make the subject aware of the presence of chemicals and initiate several defensive biological responses. A wide range of chemicals activates TRPA1 (Bessac and Jordt, 2010). The activation of TRPA1 acts as a gatekeeper of inflammation/irritation and is a critical endpoint in the regulation of exposure to volatile organic chemicals (Bautista et al., 2013; Lehman et al., 2015).

AOP Development Strategy

Context

Used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development.The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. More help

Exposure to airborne pollutants can cause a number of harmful effects including irritation. Airborne pollutants are typically associated with industrial activities, cigarette smoke and automobile exhaust. However, inhalation exposure to irritant gases and vapors frequently occurs in occupational and ambient environments. Each year new and untested inhalable chemicals are added to the environment and the lack of appropriate toxic information may result in an underestimation of occupation risks due to exposure of airborne irritants. Assays or tests are needed to first determine and then characterize the potential irritant respiratory response of uncharacterized current and newly added volatile chemicals. To develop suitable assays it is necessary to understand the fundamental biochemical and physiology processes initiating the development and progression of irritation. The presented AOP is focused on the critical biochemical event, the activation of the TRP receptors, then describing the subsequent biochemical processes and pathways and how they interact with each other resulting in pulmonary irritation.

Strategy

Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components.  Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help

Events:

Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a prototypical stressor and the biological system) of an AOP. More help
Key Events (KE)
A measurable event within a specific biological level of organisation. More help
Adverse Outcomes (AO)
An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help
Type Event ID Title Short name
MIE 1215 TRPA1 activation, TRPA1 Receptor TRPA1 activation, TRPA1 Receptor
KE 1218 Opening of calcium channel, Calcium influx Opening of calcium channel, Calcium influx
KE 1220 Trigeminal nerve activation Excitation, Trigeminal nerve excitation
KE 1222 SP (Substance P) release, Local increase of SP SP (Substance P) release, Local increase of SP
KE 1433 Increased CGRP, neuronal release of CGRP Increased CGRP
KE 1435 Increased neurokinin A (NKA) by neuronal cells Increased NKA
AO 1223 Trigeminal and/or vagal nerve excitation causes Airway Hyper-responsiveness,Cough, Dyspnea Increased Airway Hyper-responsiveness,Cough, Dyspnea
AO 1434 Irritation of nasal mucosa inducing sneeze reflex Irritation induced sneezing
AO 1226 Increased Respiratory irritability and Chronic Cough, Respiratory irritability

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP.Each table entry acts as a link to the individual KER description page. More help

Network View

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (WoE) summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Prototypical Stressors

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available. More help

Sex Applicability

The sex for which the AOP is known to be applicable. More help
Sex Evidence
Female Moderate

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

Life Stage Applicability: Exposure to toxic gases and vapor and in particular reactive chemicals activate TRPV1, ASICs and TRPA1 and induce neurogenic inflammation and pain and coughing responses. The activation of TRPA1 and TRPV1 also stimulates the release of inflammatory peptide SP, NKA and CGRP, which play a role in the asthmatic and irritation symptoms, observed after one-time and (Caceres et al., 2009) frequent exposure to inhaled irritants. These receptors may become hyper sensitized leading to chronic airway inflammation and cough. Exposure to high levels of TRPA1 agonists can induce reactive airway dysfunction syndrome or RADS, characterized by asthma like symptoms (Brooks et al., 1985). Bessac and Jordt (2008) proposed that exposure to an irritant and hence activation of TRPA1 may sensitize TRPA1 through inflammatory pathways and thereby establishing a hypersensitivity to other reactive irritants. TRPA1 but not TRPV1 appear to play an important role in allergic airway inflammation and hyper-reactivity associated with asthma. In a mouse model of asthma, the ovalbumin challenged mouse, ovalbumin induces a Th2 directed allergic response that included increase in leukocyte in the bronchoalveolar lavage fluid, increases in inflammatory cytokines and increase mucus production. In TRPA1 (-/-) mice, however these responses to ovalbumin were significantly reduced. Treatment of mice with a tear gas and TRPA1 agonist increased CGRP, SP and neurokinin A in wild type mice with greatly reduced levels in the TRPA1 (-/-) mice appearing in the alveolar fluid. Likewise treatment of wild type mice with ovalbumin also increase increased with level of neurokinin A with a diminished level in the TRPA1 (-/-) mice. These results indicate TRPA1 has a critical role in the development of asthma after allergen challenge (Caceres et al., 2009

Taxonomic Applicability,: The data used to support the KERs in this AOP derives from experimental studies conducted in rats, knockout mice or cell cultures from fibroblasts, ovary epithelium. TRP channels are present in primary sensory neruons, and also non-nueuronal tissues including epithelium, fibroblasts, and smooth muscle (Kissin, 2008). The majority of the KEs in this AOP seem to be highly conserved across species. It is unclear if these KERs of the present AOP are also applicable for other species rather than human, primates, rats and mice.

Sex Applicability: There is definitely potential for sex applicability to play a role in airway irritation and chemical sensitization. It has been demonstrated that TRP channels in sensory neurons are regulated by prolactin, in a sex-dependent manner. Prolactin plays a role in the sensitization of TRP channels and may promote antinocioceeptive effects (Patel, et al., 2013).In female mice dorsal root ganglia neurons, TRPV1 activity was enhanced with prolactin at 10-25 ng/ml whereas male mice required >0.8-1 mcg/ml PRL. ASICs activity has also been shown to be upregulated by prolactin in female rats (Li et al., 2016). </em>

Essentiality of the Key Events

The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently, evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence. The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs. More help

Molecular Initiating Event Summary, Key Event Summary

Macro-molecular:

The critical importance of the activation of TRPA1 receptor in the pulmonary response to irritants is strongly supported by the results of studies (Bautista et.al., 2006; Caceres et al., 2009) with mice deficient in TRPA1 (TRPA1-/- mice). The TRPA1 -/- mice have significant lower decrease in the respiration rate and symptoms of irritation after exposure to inhalable irritants than wild type mice. Many electrophiles and oxidants are very potent irritants and covalently link to and modify and thus activate the critical receptor(s) that initiate the irritation response (Bessac and Jordt, 2010, Bautista et al., 2013). Some members of this reactive group are isocyanates, α, β-unsaturated aldehydes, heavy metals and peroxides.

Cell/Tissue:

The trigeminal chemosensory nerve endings in the nasal mucosa and airway are a first line of defense against irritant chemicals. In these nerve endings are TRP calcium ion channels, TRPV1 and TRPA1, with TRPA1 acting as the major sensor to noxious chemicals. Irritants, electrophiles, oxidants, as well as endogenous chemicals, activate TRPA1 causing an influx of Ca+2 into the nerve. This results in excitation of the trigeminal nerves and the initiation of airway reflex responses coughing, sneezing and pain. A clear time dependent relationship occurs between the activation of the receptor and Ca ion flux and the biological effects (Bessac et aal., 2008).

Reactive irritants like the isothiocyanates form adducts with thiol residues present in the receptor active site causing prolonged nerve activation (Hinman et al., 2006; Peterlin et al., 2007; Macpherson et al., 2007). The activation of TRPA1 with electrophilic irritants results in the covalent binding of the chemicals to cysteine and lysine residues of TRPA1 forming a “foreign protein” or hapten leading sensitization and enhanced responses to subsequent exposure to the irritant (Vandebriel et al., 2011).

The cellular responses include excitation of the trigeminal nerves causes the release of neuropeptides substance P, NK and CGRP that promote neurogenic inflammation, vasodilation, and fluid leakage. Substance P and NK bind to NK receptors that are expressed in the plasma membrane of cell bodies and dendrites of neurons.

Organ/Organ System:

As the irritant reach the lower airways the sensory nerve activation causing the following organ response or pulmonary responses: bronchial constriction spams and increased mucus production and further neurogenic inflammation. Increased eosinophils and T helper cells and the associated inflammatory cytokines (IL-2, Il-4, IL-10, Il-13) are observed (Caceres et al., 2009; Belvisi et al., 2011). Trigeminal activation also results in a vagal response that slows the respiration rate, which is the basis for the use of RD50, or the irritant concentration that induces a 50% decrease in respiration rates in mice (Alarie, 1981).

Individual

Exposure to irritants the activation of TRPA1 couples with its interaction with TRPV1 can eventually lead to the following organism responses or clinical manifestations: chronic cough, pain, airway inflammation, COPD and asthmatic like conditions (Bessac and Jordt, 2008, Chen and Haclos, 2015, Baraldi et al., 2010.)

Evidence Assessment

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

Summary Table 1. Concordance of dose-response relationships Strong in vitro dose response relationships for the activation of TRPA1 by known irritants are published in the literature. In general, a good correlation exists between the potency as measure in vitro and the irritation observed mice in as measure by the Alarie test for sensory irritation.

2. Temporal concordance among the key events and adverse effect; There is good agreement between the sequences of biochemical and physiological events leading to pulmonary irritation ( Bassec and Jordt 2009, Bessac and Jordt, 2010, Baustista et al, 2006).

3. Strength, consistency, and specificity of association of adverse effect and initiating event There is excellent strength, as well as good consistency and high specificity, of the association between the activation of TRP receptors and irritation. The critical importance of the activation of TRPA1 receptor in the pulmonary responses to irritants is strongly supported by the results of studies with mice deficient in TRPA1 (TRPA1-/- mice) (Bautista et.al., 2006; Caceres et al., 2009).

4. Biological plausibility, coherence, and consistency of the experimental evidence The in vitro, and in vivo experimental evidence is logical and consistent with the hypothesis that the activation of the TRP receptors, in particular, the activation of TRPA1 is the key initiation event in pulmonary irritation (Bautista et al 2013, Bessac and Jirdt, 2009).

5. Uncertainties, inconsistencies, and data gaps. A database containing more than 350 RD50 values that is a measure of irritation in vivo obtained from experimental animals is available (Schaper 1993) but only a few of these irritants has been tested in in vitro assays to directly measure the activation of TRPA1 (Bessac and Jordt, 2010; Bos et al., 2002, Lehmann 2016). A real gap in the information of irritation and activation by TRPA1 is the lack of potency data on the activation of TRPA1 by known irritants.

Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors.  More help

Summary Table The most effective chemicals with the lowest EC50 to activate the TRPA1 calcium channel are unsurprisingly the electrophiles like tear gases, acrolein, or cinnamaldehyde (Bandell et al., 2004; Jordt et al., 2004; Macpherson et al., 2007; and Mukhopadhyaty et al., 2011). Other chemicals with higher EC50’s include hydrogen peroxide, formaldehye, and Crotonaldehyde (Mukhopadhyaty et al., 2011; Sawada et al., 2008).

Considerations for Potential Applications of the AOP (optional)

Addressess potential applications of an AOP to support regulatory decision-making.This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. More help

The AOP would be enhanced by the addition of results confirming the activation of TRPA1 by the chemicals established as irritants with reported RD50. This information would confirm what irritants and their toxicity is mediate by these biochemical processes initiated by TRPA1 activation. If a significant number of these irritants are not activators of TRP receptors, it would stimulate the search for additional biochemical mechanisms for irritants. Furthermore the additional data would permit a comparison of the RD50 and EC50 . This comparison may allow a determination if an in vitro assay for TRP activation using, for example, fibroblasts can be used determine if an uncharacterized chemical is potential chemical irritant and if the values are useful in setting exposure limits.

References

List of the literature that was cited for this AOP. More help

Alarie, Y. Dose-response analysis in animal studies: prediction of human responses. Environ Health Perspect 42, 9-13 (1981).

Baraldi, P. G., Preti, D., Materazzi, S. & Geppetti, P. Transient receptor potential ankyrin 1 (TRPA1) channel as emerging target for novel analgesics and anti-inflammatory agents. J Med Chem 53, 5085-5107, doi:10.1021/jm100062h (2010).

Bautista, D. M. et al. TRPA1 mediates the inflammatory actions of environmental irritants and proalgesic agents. Cell 124, 1269-1282, doi:10.1016/j.cell.2006.02.023 (2006).

Belvisi, M. G., Dubuis, E. & Birrell, M. A. Transient receptor potential A1 channels: insights into cough and airway inflammatory disease. Chest 140, 1040-1047, doi:10.1378/chest.10-3327 (2011).

Bandell, M. et al. Noxious Cold Ion Channel TRPA1 Is Activated by Pungent Compounds and Bradykinin. Neuron 41, 849-857, doi:10.1016/S0896-6273(04)00150-3 (2004).

Bessac, B. F. & Jordt, S. E. Breathtaking TRP channels: TRPA1 and TRPV1 in airway chemosensation and reflex control. Physiology (Bethesda) 23, 360-370, doi:10.1152/physiol.00026.2008 (2008).

Bessac, B. F. & Jordt, S. E. Sensory detection and responses to toxic gases: mechanisms, health effects, and countermeasures. Proc Am Thorac Soc 7, 269-277, doi:10.1513/pats.201001-004SM (2010).

Bos, P. M., Busschers, M. & Arts, J. H. Evaluation of the sensory irritation test (Alarie test) for the assessment of respiratory tract irritation. J Occup Environ Med 44, 968-976 (2002).

Caceres, A. I. et al. A sensory neuronal ion channel essential for airway inflammation and hyperreactivity in asthma. Proc Natl Acad Sci U S A 106, 9099-9104, doi:10.1073/pnas.0900591106 (2009).

Chen, J. & Hackos, D. H. TRPA1 as a drug target--promise and challenges. Naunyn Schmiedebergs Arch Pharmacol 388, 451-463, doi:10.1007/s00210-015-1088-3 (2015).

Hinman, A., Chuang, H. H., Bautista, D. M. & Julius, D. TRP channel activation by reversible covalent modification. Proc Natl Acad Sci U S A 103, 19564-19568, doi:10.1073/pnas.0609598103 (2006).

Jordt, S.-E. et al. Mustard oils and cannabinoids excite sensory nerve fibres through the TRP channel ANKTM1. Nature 427, 260-265, doi:http://www.nature.com/nature/journal/v427/n6971/suppinfo/nature02282_S1.html (2004).

Kissin, I. Vanilloid-Induced Conduction Anelgesia: Selective, Dose-Dependent, Long-Lasting, with a Low Level of Potential Neurotoxicity. Anesthesia and analgesia 107, 271-281, doi:10.1213/ane.0b013e318162cfa3 (2008).

Kuwabara, Y., Alexeeff, G. V., Broadwin, R., and Salmon, A. G. (2007). Evaluation and application of the RD50 for determining acceptable exposure levels of airborne sensory irritants for the public. Environ Health Perspect 115(11), 1609-16.

Lehmann, R., Hatt, H. & van Thriel, C. Alternative in vitro assays to assess the potency of sensory irritants—Is one TRP channel enough? NeuroToxicology, doi:http://dx.doi.org/10.1016/j.neuro.2016.08.010.

Liu, T. T. et al. Prolactin potentiates the activity of acid-sensing ion channels in female rat primary sensory neurons. Neuropharmacology 103, 174-182, doi:10.1016/j.neuropharm.2015.07.016 (2016).

Macpherson, L. J. et al. Noxious compounds activate TRPA1 ion channels through covalent modification of cysteines. Nature 445, 541-545, doi:10.1038/nature05544 (2007).\

Moldoveanu, B., Otmishi, P., Jani, P., Walker, J., Sarmiento, X., Guardiola, J., … Yu, J. (2009). Inflammatory mechanisms in the lung. Journal of Inflammation Research2, 1–11.

Mukhopadhyay, I. et al. Expression of functional TRPA1 receptor on human lung fibroblast and epithelial cells. Journal of Receptors and Signal Transduction 31, 350-358, doi:10.3109/10799893.2011.602413 (2011).

Patil, M. J., Ruparel, S. B., Henry, M. A. & Akopian, A. N. Prolactin regulates TRPV1, TRPA1, and TRPM8 in sensory neurons in a sex-dependent manner: Contribution of prolactin receptor to inflammatory pain. Am. J. Physiol.-Endocrinol. Metab. 305, E1154-E1164, doi:10.1152/ajpendo.00187.2013 (2013).

Peterlin, Z., Chesler, A. & Firestein, S. A painful trp can be a bonding experience. Neuron 53, 635-638, doi:10.1016/j.neuron.2007.02.011 (2007).

Sawada, Y., Hosokawa, H., Matsumura, K. & Kobayashi, S. Activation of transient receptor potential ankyrin 1 by hydrogen peroxide. Eur J Neurosci 27, 1131-1142, doi:10.1111/j.1460-9568.2008.06093.x (2008).

Schaper, M. Development of a Database for Sensory Irritants and its Use in Establishing Occupational Exposure Limits. American Industrial Hygiene Association journal 54, 488-544, doi:10.1080/15298669391355017 (1993).

Vandebriel, R. et al. Respiratory sensitization: advances in assessing the risk of respiratory inflammation and irritation. Toxicol In Vitro 25, 1251-1258, doi:10.1016/j.tiv.2011.04.027 (2011).