13.2.1. TRPA1

Transient receptor potential ankyrin 1 (TRPA1) is one of the most extensively studied channels in the airway, and emerging evidence points toward a major role for TRPA1 in the lung's defense system. Under normal conditions, the main functions of TRPA1 are protecting the airway by evoking cough associated with respiratory irritants and minimizing exposure by controlling airway tone causing bronchoconstriction.

Much of the evidence comes from electrophysiology and calcium imaging experiments and utilizing TRPA1 knockout mice and selective TRPA1 antagonists. TRPA1 is referred to as an irritant receptor as it can be activated by exogenous irritants including mustard oil, wasabi, cinnamaldehyde, cigarette smoke, chlorine, aldehydes, and scents (Caceres et al., 2009; Jordt et al., 2004). Activation by endogenous stimuli has also been identified, including reactive oxygen species, hypochlorites, lipid peroxidation products, isoprostanes, and prostaglandins (Bandell et al., 2004; Jha et al., 2015; Bautista, et al., 2005; Andersson et al., 2008).

Quantitative analysis from mouse tissue has shown that TRPA1 channels are mainly expressed in the spinal dorsal root ganglia (DRG) and trigeminal ganglia (TG) (Jang et al., 2012). Expression has also been demonstrated in mouse bronchopulmonary afferent neurons (jugular/nodose) with apparent coexpression of TRPA1 and TRPV1 (Story et al., 2003; Nassenstein et al., 2008). As well as neuronal tissue, evidence also points toward the presence of TRPA1 channels in nonneuronal cells. Using immunohistochemistry, TRPA1 staining has been observed in cultured human and mouse airway epithelial cells and smooth muscle cells (Nassini, 2012). Functional expression has also been demonstrated in human lung fibroblast cells (CCD19-Lu) and in the human pulmonary alveolar epithelial cell line (A549) at both mRNA and protein levels (Mukhopadhyay et al., 2011; Büch et al., 2013).

Activation of TRPA1 leads to neuronal depolarisation and action potential discharge in the afferent fiber. Exogenous compounds such as mustard oil and cinnamaldehyde have been shown to activate TRPA1 channels directly, by covalent modification of reactive cysteine residues in the amino terminus of the protein (Macpherson, 2007; Hinman, 2006). Inflammatory mediators such as bradykinin activate TRPA1 indirectly via G protein-coupled receptors. This leads to a second messenger cascade and increase in intracellular Ca²⁺ (Bandell, 2004). Several studies have demonstrated the importance of Ca²⁺ in the regulation of TRPA1 activity; intracellular Ca²⁺ release may directly activate cation influx through TRPA1 channels (Zurgorg, 2007).

Recent data have demonstrated the importance of the TRPA1 pathway in initiating the normal cough reflex (Birrell et al., 2009; Andrè et al., 2009; Brozmanova et al., 2012; Grace et al., 2012). Isolated vagal nerve preparations have shown that tussive agents activate vagal nerve endings in mouse, guinea pig, and human tissue. A role for TRAP1 was confirmed using TRPA1^−/– mice and TRPA1 inhibitors. Furthermore, cough evoked in an in vivo guinea pig model and human volunteers was shown to be reduced in the presence of TRPA1 antagonists (Birrell et al., 2009). Studies have identified several triggers of the TRPA1-mediated cough response including exogenous irritants such as those present in cigarette smoke, agents produced endogenously by oxidative stress (Birrell et al., 2009; Andrè et al., 2009; Brozmanova et al., 2012), and the endogenous inflammatory mediators prostaglandin E2 and bradykinin (Grace et al., 2012). As such endogenous mediators are known to be elevated in certain pathological conditions, these findings have implications for the role of TRPA1 channels in disease-associated cough. This is discussed along with the role of pollution in Section 13.3.

A further protective role for TRAP1 has been reported by the modulation of airway tone. Components of cigarette smoke have been shown to cause contraction of isolated guinea pig bronchial rings by activation of TRPA1 channels on sensory neurons. Channel activation triggers tachykinin release, which in turn acts on smooth muscle cells to initiate contraction, a protective mechanism, referred to as the nocifensor system (Andrè, 2008). In addition, the TRPA1 agonist 4-oxo-2-nonenal (4-ONE) and the general anesthetics isoflurane and desflurane have been shown to induce TRPA1-dependent bronchial contraction by a similar mechanism (Satoh et al., 2009). In a more recent study, acrolein a chemical component of smoke, has been shown to relax isolated tracheal smooth muscle (Cheah et al., 2014). Authors suggest relaxation may serve as a defensive mechanism by counteracting the effect of uncontrolled neurogenic excitation and airway narrowing by irritants evoking inflammation and bronchoconstriction. This effect is thought to occur via a pathway involving cross-talk between TRPA1 expressing sensory C-fibers, epithelial cells, and smooth muscle (Cheah et al., 2014). TRPA1 channels have also been implicated in the release of proinflammatory mediators from nonneuronal cells including epithelium, smooth muscle cells, and fibroblasts (Grace et al., 2014).

13.2.2. TRPV1

TRPV1-expressing neurons represent the largest subpopulation of primary sensory neurons. TRPV1-positive neurons belong to cells with C- and Aδ- fibers and are functionally identified as nociceptors. They innervate the entire respiratory tract from the nose to the alveoli including smooth muscle and blood vessels (Watanabe et al., 2006; Grace et al., 2014). TRPA1-positive neurons are most notably stretch insensitive and peptidergic, and are coexpressed with TRPV1 channels. However, TRPV1-postive neurons can be both stretch insensitive and stretch sensitive and either peptidergic or nonpeptidergic. They are also not exclusively expressed with TRPA1 (Bhattacharya et al., 2008).

Observations have reported a positive interaction between TRPA1 and TRPV1 regulating the sensitivity of sensory neurons during an inflammatory reaction (Lee and Hsu, 2015). Like TRPA1, TRPV1 stimulation may also be the consequence of G protein-coupled receptor (GPCR) activation by inflammatory mediators, such as bradykinin (Bautista et al., 2006; Grace et al., 2012). Low levels of TRPV1 in the airway may suggest that TRPV1 has limited physiological function, but its ability to be activated by proinflammatory mediators suggests that this channel may play an important role in inflammatory conditions and be of pathological importance. As is the case with TRPA1, TRPV1 has also been implicated in playing a role in airway defense, and several studies have been carried out investigating the role of TRPV1 sensory nerves in the initiation of the normal cough reflex. Cough induced in human and animal models by substances that activate TRPV1 such as capsaicin and citric acid is inhibited by TRPV1 antagonists (Bhattacharya et al., 2007; Grace et al., 2014). Investigators have also proposed that TRPV1 is involved in modulation of airway tone, and bronchoconstriction occurs by a mechanism involving activation of TRPV1 by inflammatory mediators (Delescluse et al., 2012). In this study, increased airway contraction in a disease model was inhibited by TRPV1 antagonists. Further evidence for a role in defense mechanisms has been shown by a study demonstrating that environmental prototype particulate matter (PM) is sensed by TRPV1 (Deering-Rice et al., 2012). This study showed that TRPV1 mediates the induction of several important proinflammatory cytokine/chemokine genes in lung epithelial cells in response to PM.

It is also important to note the presence and function of TRPV1 in nonneuronal cells in the airway. Functional TRPV1 protein has been demonstrated in cultured primary bronchial epithelial cells by patch-clamp experiments (McGarvey et al., 2014). Stimulation by capsaicin induced a dose-dependent release of interleukin 8 (IL-8), which, being blocked by capsazepine, suggests TRPV1 activation (McGarvey et al., 2014). TRPV1 has also been shown to be expressed in smooth muscle cells and to play a role in airway smooth muscle physiology, including proliferation and apoptosis (Zhao, 2013) and control of airway tone (Grace et al., 2014). Several recent studies have demonstrated expression of TRP channels in inflammatory and immune cells (Parenti et al., 2016). As discussed later in the chapter, emerging evidence suggests that TRP channels may be responsible at least in part for the transition of early defensive immune and inflammatory responses to chronic responses and disease pathology (Parenti et al., 2016). In relation to TRPV1, murine CD4⁺ T lymphocytes have been shown to express functional TRPV1, which was activated on stimulation of the T cell antigen receptor (TCR), contributed to Ca²⁺ influx and TCR signaling, resulting in T-cell activation. Inhibition of TRPV1 in mouse and human CD4⁺ T cells, with antagonists or by genetic manipulation, resulted in a Trpv1^−/− CD4⁺ T-cell-like phenotype (Bertin et al., 2014).

13.2.3. TRPV4

Compared to TRPA1 and TRPV1, little is known about TRPV4 expression in peripheral sensory neurons that innervate the lung. However, a recent study identifies the TRPV4-ATP-P2X3 interaction as a key osmosensing pathway involved in airway sensory nerve reflexes (Bonvini, 2016). TRPV4 ligands and hypoosmotic solutions caused depolarization of murine, guinea pig, and human vagus nerves, and firing of Aδ-fibers, which was inhibited by TRPV4 and P2X3 receptor antagonists; both antagonists blocked TRPV4-induced cough. In another study, TRPV4 has also been implicated in the development of neurogenic inflammation (Vergnolle et al., 2010). Hypotonic solutions and 4αPDD have been shown to induce neuropeptide release from afferent nerves in isolated murine airways (Vergnolle et al., 2010). These data suggest that TRPV4 also plays a role in airway defense.

Several studies have demonstrated that TRPV4 is widely expressed in nonneuronal cells in the airways, including mRNA expression in human airway smooth muscle cells (Jia et al., 2004; Dietrich et al., 2006), confirmation of expression by immunohistochemistry in the alveolar septal wall of human, rat and mouse lung (Alvarez et al., 2006), and human nasal, tracheal and bronchial epithelial cells (Alenmyr et al., 2014; Fernández-Fernández et al., 2008). TRPV4 is also present in lung fibroblasts (Rahaman et al., 2014) and inflammatory cells including alveolar macrophages (Hamanaka et al., 2010) and mononuclear cells (Delany et al., 2001).

Compared to other TRP channels, less is also known about external and endogenous ligands that may be responsible for TRPV4 activation and function, and indeed the mechanisms involved. However, known channel activators include osmotic changes, for example, increasing activity in hypotonic solutions, and mechanical stimuli such as shear stress (Garcia-Elias et al., 2014). Both stimuli depend on phospholipase A2 activation and subsequent production of arachidonic acid (AA) metabolites. TRPV4 channels can also be directly activated by AA (Zheng et al., 2013). Other TRPV4 channel activators include moderate heat (24°C–38°C) and the synthetic phorbol ester 4α-PDD (Watanabe et al., 2002a). Evidence suggests that TRPV4 channel activity is regulated by Ca²⁺, depending on its concentration. Ca²⁺ can either potentiate or inhibit channel activity (Garcia-Elias et al., 2014).

A role for TRPV4 in airway smooth muscle function has been demonstrated in human bronchial smooth muscle cells, where 4α-PDD, stretch, and hypotonic solutions caused increases in Ca²⁺ and subsequent contraction (Jia et al., 2004). An indirect role for TRPV4 in contraction has been shown in human bronchial smooth muscle segments using the TRPV4 agonist GSK1016790 (McAlexander et al., 2014). This agonist was shown to cause contraction via 5-lipoxygenase activation and production of cysteinyl leukotrienes (McAlexander et al., 2014).

Further evidence for the involvement of TRPV4 in lung defense has been shown by studies demonstrating activation of TRP4 modulating ciliary beat frequency (Lorenzo et al., 2008) and the control of epithelial and endothelial barrier function (Alvarez et al., 2006; Li et al., 2011a). A potential role in macrophage activation by mechanical stress (Hamanaka et al., 2010) has also been suggested.

13.2.4. TRPM8

Evidence to date suggests TRPM8 may play a minimal role in airway physiology with contradictory findings often reported (Grace et al., 2014). One study demonstrated that TRPM8 is not expressed in mouse vagal afferents innervating the airways, and menthol, a TRPM8 agonist, was ineffective at increasing intracellular Ca²⁺ in lung-specific vagal sensory neurons (Nassenstein et al., 2008). Other studies have found evidence of a subpopulation of rat airway vagal afferent nerves expressing TRPM8 receptors and that these receptors are activated by cold temperatures (Xing et al., 2008). It is implied from these data that activation is likely to trigger responses such as airway constriction in response to a reduction in temperature. Molecular analysis has also identified expression of TRPM8 in a subset of nasal trigeminal afferent neurons (Plevkova et al., 2013). However, authors have suggested that activation of TRPM8 has an inhibitory effect; they conclude that menthol suppresses cough evoked in the lower airways primarily through this subset of neurons initiated from the nose (Buday et al., 2012; Plevkova et al., 2013).

TRPM8 is also expressed in airway epithelial cells and has recently been shown to inhibit proliferation and migration in a rat asthma model (Zhang, 2016). TRPM8 has been identified primarily within endoplasmic reticulum membranes of epithelial cells (Sabnis et al., 2008) and may play a role in production of mucus and inflammatory mediators from airway epithelium (Sabnis et al., 2008; Grace et al., 2014).

13.3.1. Asthma

Asthma is a chronic inflammatory airway disease affecting an estimated 300 million people worldwide. The number of annual deaths attributed to the disease is approximately 250,000. Physiological hallmarks of asthma include AHR and variable airflow obstruction. The disease is defined by symptoms including wheeze, chest tightness, shortness of breath, cough, and dyspnea. The airway inflammatory response can vary between asthmatic patients, and as the disease becomes more persistent, further airway obstruction can occur from edema, mucus hypersecretion, and airway remodeling. Repeated exposure to environmental allergens is thought to be the underlying cause of asthma, the early asthmatic response occurring minutes after initial exposure to allergen. Recognition of the allergen by IgE on the surface of mast cells induces degranulation and release of inflammatory mediators such as histamine and cysteinyl leukotriene (CysLT), causing acute bronchoconstriction (Adelroth et al., 1986). A late asthmatic response (LAR) can follow 3–8 hours after allergen exposure and only occurs in approximately 50% of patients (O'Byrne et al., 2009). The late asthmatic response results in further bronchoconstriction and is also associated with a complex immune and inflammatory response. This response is linked to an increase in inflammatory cells including Th-2 cells, eosinophils, airway, basophils, and less consistently neutrophils and trafficking and activation of myeloid dendritic cells into the airways (Holgate et al., 2008; Gauvreau et al., 2015). Prominent inflammatory mediators include IL-4, IL-5, IL-13, eotaxin (CCL11), and eicosanoids. The LAR phenotype can also occur in response to other triggers such as exercise, cold air, irritants, and stress. The mechanisms regulating the airway response to these factors are less well defined. Isocyanate is another known trigger in patients suffering from occupational asthma (Kenyon et al., 2012). Disease exacerbations in asthmatic patients can also be caused by respiratory infections.

TRP channels are likely candidates in the pathophysiology of asthma, as several exogenous and endogenous TRP channel agonists such as cigarette smoke and reactive oxygen species are also known asthma triggers. Irritants can trigger asthma-type symptoms and subsequent hypersensitivity to chemical and physical stimuli (Preti et al., 2012). Endogenous mediators produced by infiltrating immune cells or inflamed airway tissue can reach elevated levels high enough to chronically activate TRPA1 in airway sensory neurons (Trevisani et al., 2007). A critical role for TRPA1 in the pathogenesis of asthma has been shown in several studies using the TRPA1 knockout mouse. These mice have been shown to be deficient in the neuronal detection of proinflammatory asthma agents (Bessac et al., 2008; Andrè et al., 2008). Lack of TRPA1 may prevent neuronal excitation and Ca²⁺ influx normally activated by these mediators, which is likely to occur during inflammatory progression in asthma (Caceres et al., 2009). Further evidence of a role for TRPA1 in asthmatic airway inflammation has been shown in the mouse ovalbumin model of asthma (Caceres et al., 2009). In this study, pharmacological inhibition and genetic deletion of TRPA1 diminish allergen-induced inflammatory leukocyte infiltration, mucus production, cytokine and chemokine levels, and AHR. TRPAP1 knockout mice also show impaired acute and inflammatory neuropeptide release in the airways required for leukocyte infiltration. A role for TRPV1 is less clear as data from asthma models have reported conflicting results since lack of TRPV1 did not demonstrate any change in the mouse ovalbumin model (Caceres et al., 2009). However, other asthma models have shown that modulation of TRPV1 attenuated the asthma model inflammatory phenotype (Mori et al., 2011; Rehman et al., 2013).

Evidence of a role for TRP channels in control of airway tone was discussed earlier in the chapter. It is therefore not surprising to find studies linking TRP channel activation and AHR in asthma models. The results of one such study using ovalbumin-sensitized animals, show abolished LAR response in the presence of TRP channel inhibitors (Raemdonck et al., 2012). The authors conclude that allergen challenge activates TRPA1 channels on sensory neurons, which in turn triggers a central reflex leading to parasympathetic cholinergic activation of airway smooth muscle. Although this study ruled out a role for TRPV1 in AHR, data from unanesthetized, ovalbumin-sensitized guinea pig showed inhibition of AHR by TRPV1 antagonists (Delescluse et al., 2012). A recent study has demonstrated a link between TRPA1 and TRPV1 in the development of AHR in response to the chemical sensitizer toluene-2,4-diisocyanate (TDI) (Devos et al., 2016). Authors used a mouse model of TDI sensitization to induce AHR, and blocked TRPA1- and TRPV1-channel activity using pharmacological methods and knockout mice. They put forward a neuroimmune pathway as a possible mechanism after demonstrating that TRPA1 and TRPV1 channels and mast cell knockout mice did not exhibit AHR despite being sensitized by TDI.

The discovery of genetic variants of certain TRP channels has led to studies demonstrating a link between the presence of certain polymorphisms and altered disease presentation. The outcome of one such study showed an association between a TRPV1 single nucleotide polymorphism (SNP), and a protective effect against the presence of wheezing in a group of asthma patients (Cantero-Recasens et al., 2010). Fluorescent microscopy revealed a lower increase in Ca²⁺ concentration upon stimulation indicating a decrease in channel activity. These results provide evidence for a role of TRPV1 channels in altered Ca²⁺ signaling underlying asthma pathophysiology. Another study has implicated TRPA1 in the modulation of asthma in children exposed to high levels of pollution (Deering-Rice et al., 2015). Expression of variant forms of TRPA1 may increase the sensitivity of TRPA1 to insoluble particulate matter including 3,5-ditert butylphenol, a soluble nonelectrophilic agonist and component of diesel exhaust particles. Such polymorphisms of the TRPA1 channel may lead to an increase in activation and a correlation with reduced asthma control.

As the role of TRP channels in asthma pathophysiology becomes more apparent, changes in expression levels or patterns of TRP channel expression become increasingly likely. A search of the literature revealed a study demonstrating altered TRPV1 expression in a rat model of chronic inflammation induced by an ovalbumin sensitization (Zhang et al., 2008). Authors report an increase in the proportion of TRPV1 expressing neurons in pulmonary myelinated afferents in the nodose ganglia. Data also showed an increase in sensitivity to capsaicin of vagal bronchopulmonary myelinated afferents. Further studies using a similar model in guinea pigs have shown that inflammation causes a “phenotypic switch in vagal tracheal cough-causing, low-threshold mechanosensitive Aδ neurons, such that they begin expressing TRPV1 channels” (Lieu et al., 2012). Authors also provide evidence to suggest a role for neurotrophic factors in the modulation of gene expression of TRPV1 and nerve phenotype, which could contribute to excessive coughing caused by allergens. Other results have shown an increase in sensitivity to continuous capsaicin inhalation in cough-variant asthma (Nakajima et al., 2006). Other evidence has highlighted the role of TRPV1 and TRPA1 in sensitization resulting in exaggerated sensory responses to ROS causing airway hypersensitivity—a characteristic feature of asthma (Ruan et al., 2014).

13.3.2. Chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a term used to describe a number of conditions including emphysema that affects the alveoli, and chronic bronchitis affecting the bronchi. According to the Global Initiative for COPD guidelines, it is characterized by persistent airflow limitation that is usually progressive and associated with an enhanced chronic inflammatory response to noxious particles or gases (Pauwels et al., 2001). It is well established that tobacco smoke increases susceptibility to COPD, but other factors including genetic factors and prolonged exposure to occupational dusts and chemicals can also increase the risk of developing the disease (Salvi and Barnes, 2009). Symptoms include chronic cough, dyspnea, and excess sputum production; exacerbations also often occur. Examination of smokers with chronic bronchitis has shown an increased number of CD8⁺ T lymphocytes, neutrophils, and macrophages in surgical pulmonary specimens (Saetta et al., 1997; Saetta et al., 1998). Increased levels of proinflammatory mediators have also been shown in patients with COPD, which are significantly associated with disease severity (Barnes, 2016; Selvarajah et al., 2016).

Recent evidence suggests that TRPA1 is the primary TRP channel implicated in the pathogenesis of COPD. As described earlier, it is activated by several components of cigarette smoke, including acrolein and crotonaldehyde and nicotine (Lin et al., 2010; Andrè et al., 2008; Talavera et al., 2009), so a role of TRPA1 is highly likely. Cigarette smoke aqueous extract (CSE) has been shown to mobilize Ca²⁺ in cultured guinea pig jugular ganglia neurons and promote contraction of isolated guinea pig bronchi (Andrè et al., 2008). CSE and unsaturated aldehydes contract the guinea pig bronchus by a neurogenic mechanism mediated by TRPA1 stimulation, Ca²⁺ mobilization, and release of neuropeptides. Another study by Shapiro and colleagues has investigated the effects of wood smoke particle material (WSPM), which has been shown to have a clear impact on human health causing a progressive decline in lung function and development of chronic COPD (Naeher et al., 2007; Laumbach et al., 2012; Shapiro et al., 2013). This study demonstrated that TRPA1 in trigeminal sensory neurons was activated by WSPM resulting in Ca²⁺ flux shown by imaging experiments. Continued exposure to WSPM therefore has implications for the development of neurogenic inflammation, decreased respiratory drive, chronic cough, and bronchoconstriction by activating this pathway (Shapiro et al., 2013).

There is emerging evidence to suggest the involvement of other TRP channels in respiratory symptoms associated with COPD. The suggestion of a possible role for TRPV1 comes from the effects of tiotropium, a widely prescribed drug for its bronchodilator effects in COPD and asthma. It has also been shown to attenuate cough in preclinical and tussive challenge studies (Bateman et al., 2009). Recent data have shown that tiotropium inhibited capsaicin-induced Ca²⁺ responses and currents in isolated guinea pig vagal tissue, implying a TRPV1 neuronal mediated effect (Birrell et al., 2014). Increased TRPV1 and TRPV4 mRNA expression has also been demonstrated in patients with COPD (Baxter et al., 2014). In the same study, TRPV1 and TRPV4 were shown to play a role in cigarette smoke (CS)–induced release of extracellular ATP, which is elevated in the COPD airway and is thought to be intrinsic to CS-induced inflammation. Moreover, SNPs in TRVP4 have been associated with COPD (Zhu et al., 2009).

13.3.3. Chronic cough

Chronic cough is typically defined as a cough that lasts longer than 8 weeks and occurs in approximately 40% of the population. As discussed above, chronic cough is present in diseases such as COPD and asthma, but can also occur as a persistent cough after a viral or bacterial infection or in isolation with an unknown cause. The underlying mechanisms of chronic cough are complex, and causes of exaggerated cough in disease pathology are not completely understood. As already described, TRP channels play an important role in the cough reflex as a protective function in airway defense so are likely to be involved in abnormal cough presentation. A role for TRPV1 in chronic cough is a likely scenario due to the protussive effects of the TRPV1 agonist capsaicin. Increased expression of TRPV1 in sensory nerves has been demonstrated in patients with chronic cough (Mitchell et al., 2005; Groneberg et al., 2004). Moreover, in a recent study an association has been made between the presence of TRPV1 SNPs and a higher risk of developing chronic cough in patients who smoke, or with occupational exposure (Smit et al., 2012). Studies have also shown an increase in sensitivity to capsaicin in patients with chronic cough (Doherty et al., 2000; Pecova et al., 2008). The TRPA1 channel is also involved in cough associated with asthma and COPD as described above. TRPA1 is activated by reactive electrophilic molecules including acrolein that is a by-product of oxidative stress that can lead to activation of the cough reflex (Bautista et al., 2006; Trevisani et al., 2007). As oxidative stress can be initiated by environmental irritants and as a by-product of inflammation, this is a possible pathway whereby exaggerated cough is initiated. This is made more likely during inflammation by the increase of other endogenous ligands that directly activate TPA1, such as prostaglandins (Grace and Belvisi, 2011).

Patients suffering from idiopathic cough have been reported to have suffered from a viral infection preceding the onset of their cough (Haque et al., 2005). Human rhinovirus is the major cause of the common cold and has also been shown to be responsible for asthma exacerbations (Arden et al., 2006; Nicholson et al., 1993). A recent study has demonstrated the ability of rhinovirus to infect neuronal cells, and that infection causes an increase in expression of TRPV1, TRPA1, and TRPM8 mRNA and protein (Abdullah et al., 2014). Virus-induced soluble factors, for example, IL-8, IL-16, and nerve growth factor, were shown to be sufficient for TRPV1 and TRPA1 upregulation in culture, Interestingly, mechanisms of upregulation differed for TRPM8 which required replicating virus. This suggests that direct interaction with virus particles and neuronal cells is required for TRPM8 upregulation (Abdullah et al., 2014). Taken together these data suggest a role for certain TRP channels on sensory afferents in postviral cough.

13.3.4. Other respiratory diseases

Nonallergic rhinitis (NAR) is inflammation of the nose that affects approximately 10% of people worldwide. Symptoms include sneezing, irritation, and nasal congestion that are induced by hypersensitivity to stimuli including smoke, temperature changes, and irritants. TRPV1 channels on C-fiber afferents innervating the nasal mucosa are thought to play a role in NAR, since the activation of this ion channel causes the release of neuropeptides substance P and calcitonin G-related peptides from nerve terminals resulting in a local inflammatory response (Van Gerven et al., 2012). Nasal application of capsaicin has been shown to alleviate the symptoms of TRPV1. This is thought to occur by desensitization after strong excitation of the TRPV1 neurons, or massive influx of Ca²⁺ resulting in nonfunctional afferents or degeneration of nerve terminals (Anand and Bley, 2011; Van Gerven et al., 2014). The role of neuronal TRP channels in allergic rhinitis is less clear. It has been hypothesized that allergic rhinitis induced by exposure to allergens may have increased sensitivity to stimulation of TRP channels (Alenmyr, 2009). This study went on to demonstrate an increase in itch response to stimulation of TRPV1 at seasonal allergen exposure. However, the TRPV1 blocker SB-705498 had no effect on nasal symptoms of patients with seasonal allergic rhinitis including itch (Alenmyr et al., 2012). More recent studies have concentrated on alternative target cells, demonstrating localization of TRPV1 expression to CD4⁺ T cells (Samivel et al., 2016). TRPV1 has been shown to be involved in T-cell receptor signaling that is altered in TRPV1 knockout allergic rhinitis mice (Samivel et al., 2016).

Apneic responses (lethal ventilator arrest) have recently been shown to be associated with upregulation of TRPV1 channels (Zhuang et al., 2015). Prenatal exposure to nicotine has been shown to trigger apneic responses during severe hypoxia (Zhuang et al., 2014). This is caused by lack of inspiratory drive from the CNS but the underlying mechanisms are poorly understood. The study by Zhuang et al. (2015) has demonstrated an association between apneic responses and sensitization of bronchopulmonary C-fibers shown by increased firing rate. Upregulation of TRPV1 was associated with increased gene expression of the tropomyosin receptor kinase A (TrkA) in the nodose/jugular ganglia.