1. Cross Talk between TRPA1 and TRPV1

Both TRPV1 and TRPA1 are co-expressed in a large subset of sensory nerves, where they integrate numerous noxious stimuli. Both channels have also been shown to co-express in keratinocytes, mast cells, dendritic cells, and endothelial cells. In cells that co-express TRPA1 and TRPV1, the two TRP channels seem to form a complex at the plasma membrane and influence each other’s function ^[1][2]. TRPV1-mediated currents in TRPA1-positive neurons characterize small current densities with slow decay, which is caused by TRPA1 channel activities and intracellular Ca²⁺ mobilization. On the other hand, desensitization of TRPV1-mediated current in TRPA1-positive neurons is apparently slow, due to appending TRPA1-mediated current ^[3]. This cross-communication between TRPA1 and TRPV1 may involve the concentration of cytosolic Ca²⁺, one of the main sensitizers/desensitizers of TRPA1 and TRPV1. These channels can also sensitize each other via a Ca²⁺-dependent signaling pathway ^[4]. In peripheral sensory fibers, opening TRPA1 and TRPV1 channels stimulates the release of substance P and calcitonin gene-related peptide via axon reflex ^[5]. Stimulating TRPV1 in uroepithelial keratinocytes causes them to secrete ATP and excite nociceptors ^[6]. It is suggested that opening TRPA1 channels in keratinocytes may also trigger ATP release and stimulate neighboring somatosensory nerve endings ^[7][8][9].

2. TRPA1 and TRPV1 as a Bridge between Taste and Chemesthesis

Taste is a composite of 5 primary taste qualities, salty, sour, sweet, bitter and umami. These taste qualities are detected by a distinct subset of cells within a taste bud. Taste buds are a conglomeration of 50–100 taste receptor cells. Three different subsets of taste receptor cells within the taste bud are characterized as Type I through Type III cells. Among the Type II cells, sweet and umami cells express heterodimeric receptors that are made up of taste receptor type 1 (T1R) G-protein coupled receptors, T1R2/T1R3 and T1R1/T1R3, respectively, while bitter cells express ~25–30 taste receptor type 2 (T2R) G-protein coupled receptors. Downstream of receptors, Type II sweet, bitter and umami taste cells share an intracellular signal transduction mechanism comprising PLCβ2, inositol triphosphate receptor type 3 (IP3), Ca²⁺-dependent monovalent cation channel TRPM5, and voltage-dependent ATP release heterooligomeric channel composed of calcium homeostasis modulator 1 (CALHM1) and 3 (CALHM3). Type III cells express polycystin 2 Like 1 (PKD2L1), TRP cation channel, sour sensing H⁺ channel Otopetrin-1, and carbonic anhydrase 4 ^[10][11][12]. In fungiform papillae and the soft palate, a subset of Type II cells shows similar but not identical molecular feature with sweet, umami, and bitter taste cells. This newly discovered Type II cell sub-population expresses PLCβ2, IP3, CALHM3, a transcription factor skin head (SKN)-1a, and α subunit of the epithelial Na⁺ channel (ENaC). However, this sub-population of Type II cells lack TRPM5 and guanine nucleotide-binding protein G(t) subunit alpha-3 (GNAT3). SKN-1a-deficient taste buds are predominantly composed of putative non-sensory Type I cells and Type III cells, whereas wild-type taste buds include Type II (i.e., sweet, umami, and bitter taste) cells and Na⁺-taste cells ^[13]. These subsets of cells with ENaC activity fire action potentials in response to ENaC-mediated Na⁺ influx without changing the intracellular Ca²⁺ concentration and form a channel synapse with afferent neurons involving CALHM1/3 ^[14].

3. Chemesthesis and Sour Taste as Co-Mediators of Acid-Evoked Aversion

Sour taste elicits an aversive response to acids. Animals lacking sour taste receptor cells that express PKD2L1 were found to display strong aversion to acids even though the neuronal responses from sour taste receptor cells were completely abolished ^[15]. Only the animals in which trigeminal TRPV1-expressing neurons were ablated, and that also lacked sour sensing H⁺ channel Otopetrin-1, exhibited a major loss of behavioral aversion to acid. These results suggest that the somatosensory and taste systems serve as co-mediators of acid-evoked aversion ^[11]. Since TRPA1 channels are also opened by acidic stimuli and are expressed in trigeminal nerve fibers ^[16], further studies are needed in TRPA1 knockout mice to determine if these channels also contribute to the aversion to sour taste stimuli.

4. Chemesthesis and Bitter Taste as Co-Mediators of Bitter-Evoked Aversion

Nicotine, present in tobacco plant, is bitter and in behavioral experiments is aversive to animals. Similar to the case with sour taste, the aversive behavior to nicotine involves both somatosensory input and sensing of its bitter taste via the T2Rs/PLCβ2/TRPM5 pathway. TRPM5 pathway is important in detecting most bitter compounds, including quinine and nicotine. In behavioral studies, TRPM5 knockout mice in which trigeminal TRPV1-expressing neurons were ablated still found nicotine aversive, indicating the presence of a TRPM5-independent bitter tasting mechanism. In TRPM5 knockout mice, both behavioral and neural responses were inhibited in the presence of nicotinic acetylcholine receptor (nAChR) blocker, mecamylamine ^[17]. Thus, with regard to nicotine, its aversive response is due to somatosensory input and its detection by both TRPM5-dependent and a TRPM5-independent pathway that involves nAChRs. A similar mechanism may also be involved in the aversive responses to ethanol at high concentrations ^[18]. In support of this mechanism, both in rodent and human taste receptor cells, nAChRs are expressed in TRPM5 positive cells ^[19][20]. These studies suggest that both TRPM5-dependent and TRPM5-independent but nAChR-dependent pathways are present in bitter taste cells. It is important to emphasize that the TRPM5-independent mechanism for detecting the bitter taste of nicotine was revealed after silencing the trigeminal component by injecting capsaicin in neonate rats. Smokers tend to demonstrate higher taste thresholds. This may stem from a reduction in number of fungiform papillae on the tongue ^[21], suggesting a link between taste and cigrette smoking. Menthol activates TRPM8, and menthol and nicotine are TRPA1 agonists. Menthol decreases oral nicotine aversion in C57BL/6 mice through a TRPM8-dependent mechanism ^[22]. Allelic variants in TRPA1 might contribute to individual differences in preference for mentholated cigarettes, tendency to smoke cigarettes with higher nicotine levels, extract more nicotine from each cigarette, and/or display greater difficulties in quitting smoking ^{[23][24][25][26]}.

5. Chemesthesis and Salt Taste

Calcitonin gene-related peptide (CGRP) receptor has been shown to localize in a subset of Type III taste receptor cells, while CGRP was localized in the nerve fibers around the taste buds ^[15]. A subset of Type III taste receptor cells responded to sub-micromolar concentration of CGRP with an increase in intracellular Ca²⁺ in a PLC-dependent manner. These results raise the possibility that CGRP released from trigeminal nerves can modulate responses in a subset of Type III taste receptor cells that most likely also express the amiloride-insensitive salt receptor ^[27]. However, at present it is not known if directly opening TRPA1 in nerve fibers around the taste buds will activate CGRP positive Type III cells.

Although, TRPV1 is not expressed in rodent taste receptor cells, TRPV1 agonists capsaicin and resiniferatoxin produced dose-dependent increase in the amiloride-insensitive NaCl chorda tympani taste nerve responses in rodents ^[28]; at low concentrations enhancing and at high concentrations inhibiting the NaCl chorda tympani responses. TRPV1 knockout mice did not elicit amiloride-insensitive NaCl chorda tympani taste nerve responses. It was hypothesized that in the absence of TRPV1 expression in rodent taste receptor cells, capsaicin and resiniferatoxin most likely produce these effects indirectly via the release of CGRP from trigeminal neurons around the taste bud cells. CGRP then acts on the calcitonin gene-related peptide receptor expressed in Type III cells that presumably also harbor the amiloride-insensitive salt taste receptor(s) ^[29][30][31]. The effects of capsaicin and resiniferatoxin on amiloride-insensitive NaCl chorda tympani responses are also modulated by N-geranyl cyclopropylcarboxamide, an amiloride-insensitive salt taste enhancer that was also found to open both TRPV1 and TRPA1 channels ^[32][33]. However, at present amiloride-insensitive NaCl chorda tympani responses have not been recorded from TRPA1 knockout mice in the absence and presence of capsaicin, resiniferatoxin, N-geranyl cyclopropylcarboxamide and capsiate.

In addition to nerve fibers, TRPV1 is co-expressed with ENaC in kidney cortical-collecting duct cells. Inhibiting TRPV1 activity by high salt in kidney cortical-collecting duct cells enhances α-ENaC expression and opening TRPV1 by capsaicin inhibits the high salt-induced increase in α-ENaC ^[34]. TRPV1 channel opening prevents high-salt diet-induced hypertension in mice ^[35]. TRPV1 can be activated when blood pressure increases throughout the body, thereby activating the serine/threonine protein kinase 1 and serum and glucocorticoid-regulated kinase 1 signaling pathways. This results in decreased expression of α-ENaC, which is crucial to the regulation of renal Na⁺ absorption and systemic blood pressure. This effect reduces the reabsorption of Na⁺ and water, ultimately reducing cardiac output and systemic vascular pressure ^[34]. Although TRPV1 does not seem to be expressed in rodent anterior fungiform taste receptor cells ^[36], TRPV1 mRNA has been shown to be expressed in stably proliferating human taste cells isolated from fungiform papillae ^[37]. More recent studies indicate that TRPV1 is also expressed in adult human cultured fungiform taste cells ^[38]. Cultured human fungiform taste cells in high-salt media increased δ- and γ-ENaC mRNA and protein expression. Capsaicin inhibited the high-salt-induced increase in δ- and γ-ENaC. These effects were TRPV1-dependent. The effects of capsaicin are also mimicked by its non-pungent analogues, capsiate and olvanil. It is interesting to note that non-pungent capsaicin like compounds such as capsiate also open the TRPA1 channel ^[39].

At present, some studies have implicated a role of TRPV1 in taste perception. However, such studies have not been performed with respect to TRPA1. Since two different TRPA1 knockout mouse models have been generated, these knockout mice models should be used to explore further the relation between chemesthesis and taste and the relationship between TRPV1 and TRPA1. AITC, the powerful plant-derived pungent principal which sensitizes both TRPA1 and TRPV1, significantly suppressed high-sodium responses in chorda tympani taste nerve in mice ^[40]. This study, again, suggests that chemical that are involved in chemesthesis can modulate primary taste qualities, for example modifying an aversive taste response to high salt.

6. Effect of Single-Nucleotide Polymorphisms in TRPA1 and TRPV1 on Chemesthesis and Taste

More recently, it has been suggested that TRP channels provide a link between pain and taste, as they play a role in sensory signaling for taste, thermosensation, mechanosensation, and nociception ^[41]. Single-nucleotide polymorphisms in TRPA1 and TRPV1 have been shown to affect and modulate both chemesthesis and taste perception. This is illustrated by the observations that genomic variability in the TRPV1 gene correlates with altered capsaicin sensitivity ^[42][43], alterations in various pain conditions, and with alterations in salty taste sensitivity and salt preference ^[41][44] in humans. Intronic TRPV1 variants are associated with insensitivity to capsaicin ^[45], while the coding TRPV1 variant rs8065080 are associated with altered responses to experimentally induced pain ^[46]. In addition, gain-of-function mutations in TRPV1 are associated with increased pain sensitivity ^[47]. The effect of TRPV1 modulators on salt taste may involve acute effects on the amiloride-insensitive salt taste responses or long-term effects on ENaC expression in human taste receptor cells. Sweet taste and smell are generally associated with lower pain intensity perception and unpleasantness related with phasic pain compared with bitter taste ^[48].

In addition to salt taste, genetic variation in TRPV1 and TRPA1 also affects other taste qualities. Genetic variation TRPV1 and T2Rs can alter sensations from sampled ethanol and can potentially influence how individuals initially respond to alcoholic beverages ^[49]. Several variants of the hTRPA1 gene alter the functional properties of the channel in a significant way that are associated with altered pain perception or chemosensory functions in human subjects ^[50][51][52]. A genetic variant of TRPA1, along with variants in T2R50, and GNAT3, have been linked to personal differences in the taste perception of cilantro ^[53]. Perception of smell has also been linked to TRPA1 genetic variants. TRPA1 has been shown to be expressed in mouse olfactory bulb ^[54]. People carrying a TRPA1 SNP (rs11988795) scored better on odor discrimination and had a higher reported H₂S stimulus intensity. In a Guelph family health study, the rs713598 in the T2R38 bitter taste receptor gene and rs236514 in the potassium inwardly rectifying channel subfamily J member 2 (KCNJ2) sour taste-associated gene showed significant association with phenylthiocarbamide supra threshold sensitivity, and sour taste preference in parents, respectively. In children, rs173135 in KCNJ2 and rs4790522 in the TRPV1 salt taste-associated gene showed significant association with sour and salt taste preferences, respectively ^[55].

The E179K variant of TRPA1 also appears essential for abnormal heat sensation in pain patients. Single amino acid exchange at position 179 in the ankyrin repeat 4 of hTRPA1 seems to be the culprit. TRPA1 wild-type Lys-179 protein when expressed in human embryonic kidney (HEK) cells exhibited normal biochemical properties. It is opened by cold, shows normal trafficking into the plasma membrane and is capable of forming large protein complexes. Its protein expression in HEK cells could be increased at low (4 °C) and high temperatures (49 °C). However, HEK cells expressing the variant Lys179 TRPA1 did not show opening at 4 °C. This can be attributed to its inability to interact with other proteins or other TRPA1 monomers during oligomerization ^[56]. At present, it not clear how protein genetics of TRPA1 alter the relationship between pain, chemesthesis and taste.