TRPM8 protein localization in trigeminal ganglion and taste papillae

Neuromodulators in Acute and Chronic Cough in Children: An Update from the Literature

Cough is one of the most common reasons leading to pediatric consultations, negatively impacting the quality of life of patients and caregivers. It is defined as a sudden and forceful expulsion of air from the lungs through the mouth, typically triggered by irritation or the stimulation of sensory nerves in the respiratory tract. This reflex is controlled by a neural pathway that includes sensory receptors, afferent nerves, the brainstem's cough center, efferent nerves, and the muscles involved in coughing. Based on its duration, cough in children may be classified as acute, lasting less than four weeks, and chronic, persisting for more than four weeks. Neuromodulators have shown promise in reducing the frequency and severity of cough by modulating the neural pathways involved in the cough reflex, although they require careful monitoring and patient selection to optimize the outcomes. This review aims to examine the rationale for using neuromodulators in the management of cough in children.

Chief Complaint: Nasal Congestion

Nasal obstruction is the subjective perception and objective state of insufficient airflow through the nose. Nasal congestion, conversely, describes a state of not just inadequate airflow or obstructive phenomena but also pressure- and mucus-related states to the patient. Nasal receptors belonging to the transient receptor potential (TRP) protein family mediate the sense of nasal patency via the trigeminal nerve. The transient receptor potential melastatin-8 (TRPM8) responds to temperatures around 8°C to 22°C, and is stimulated by menthol and other cooling agents. The radiant effects of airflow create heat loss to activate these receptors and humans perceive this as nasal patency rather than the direct detection of airflow. The thermovascular state of the mucosa, in conditions such as rhinitis, influence TRPM8 activation. Nasal endoscopy can show signs of rhinitis and should be considered an essential part of the workup of nasal congestion. Efforts to relieve nasal congestion need to manage the mucosal state and surgery needs to ensures that the nasal cavity mucosa is exposed to the cooling effects of airflow rather than simply creating a passage to the nasopharynx.

Separation of Oral Cooling and Warming Requires TRPM8

Cooling sensations arise inside the mouth during ingestive and homeostasis behaviors. Oral presence of cooling temperature engages the cold and menthol receptor TRPM8 (transient receptor potential melastatin 8) on trigeminal afferents. Yet, how TRPM8 influences brain and behavioral responses to oral temperature is undefined. Here we used in vivo neurophysiology to record action potentials stimulated by cooling and warming of oral tissues from trigeminal nucleus caudalis neurons in female and male wild-type and TRPM8 gene deficient mice. Using these lines, we also measured orobehavioral licking responses to cool and warm water in a novel, temperature-controlled fluid choice test. Capture of antidromic electrophysiological responses to thalamic stimulation identified that wild-type central trigeminal neurons showed diverse responses to oral cooling. Some neurons displayed relatively strong excitation to cold <10°C (COLD neurons) while others responded to only a segment of mild cool temperatures below 30°C (COOL neurons). Notably, TRPM8 deficient mice retained COLD-type but lacked COOL cells. This deficit impaired population responses to mild cooling temperatures below 30°C and allowed warmth-like (≥35°C) neural activity to pervade the normally innocuous cool temperature range, predicting TRPM8 deficient mice would show anomalously similar orobehavioral responses to warm and cool temperatures. Accordingly, TRPM8 deficient mice avoided both warm (35°C) and mild cool (≤30°C) water and sought colder temperatures in fluid licking tests, whereas control mice avoided warm but were indifferent to mild cool and colder water. Results imply TRPM8 input separates cool from warm temperature sensing and suggest other thermoreceptors also participate in oral cooling sensation.

Inducible desensitization to capsaicin with repeated low-dose exposure in human volunteers

Responses to capsaicin are reduced following repeated exposure, a phenomenon known as capsaicin desensitization. Heavy consumers of chilies consistently report reduced oral burn relative to infrequent consumers, presumably due to chronic desensitization. However, the mechanism(s) underlying capsaicin desensitization remain poorly understood. We hypothesized that reduced response to capsaicin due to repeated oral exposure may result from a change in the expression of the capsaicin receptor (TRPV1) gene. To test this, we conducted two longitudinal desensitization studies in healthy human volunteers. In Study 1, 51 adults completed a 17-day capsaicin desensitization protocol. The study consisted of three in-person visits where they were asked to sample stimuli, including 3, 6, and 9 ppm capsaicin, and rate intensity on a general labeled magnitude scale (gLMS). Between days 3 & 17, participants rinsed at home with 6 ppm capsaicin (n = 31) or a control (n = 20) solution (20 uM sucrose octaccetate; SOA) twice a day. Before and after the oral exposure protocol, a clinician collected fungiform papillae. Participants randomized to the capsaicin rinse showed a statistically significant reduction in oral burn ratings that was not observed in controls, indicating repeated low-dose exposure can systematically induce desensitization. TRPV1 expression was not associated with reported capsaicin burn, and there was no evidence of a decrease in TRPV1 expression following capsaicin exposure. In Study 2, participants (n = 45) rinsed with 6 ppm capsaicin in a similar protocol, rating capsaicin, vanillyl butyl ether (VBE), cinnamaldehyde, ethanol, menthol, and sucrose on days 1, 3, & 17. Burn from capsaicin, VBE, cinnamaldehyde, and ethanol all showed a statistically significant change - capsaicin, VBE and cinnamaldehyde burn all dropped ∼20 %, and a larger reduction was seen for ethanol - while menthol cooling and sucrose sweetness did not change. Collectively, this suggests reductions in oral burn following chronic capsaicin exposure generalizes to other stimuli (i.e., cross desensitization) and this cannot be explained by a change in TRPV1 mRNA expression. More work is needed to elucidate the underlying mechanism for capsaicin desensitization in the oral cavity.

Transient receptor potential channels as an emerging therapeutic target for oropharyngeal dysphagia

Oropharyngeal dysphagia is a serious health concern in older adults and patients with neurological disorders. Current oropharyngeal dysphagia management largely relies on compensatory strategies with limited efficacy. A long-term goal in swallowing/dysphagia-related research is the identification of pharmacological treatment strategies for oropharyngeal dysphagia. In recent decades, several pre-clinical and clinical studies have investigated the use of transient receptor potential (TRP) channels as a therapeutic target to facilitate swallowing. Various TRP channels are present in regions involved in the swallowing process. Animal studies have shown that local activation of these channels by their pharmacological agonists initiates swallowing reflexes; the number of reflexes increases when the dose of the agonist reaches a particular level. Clinical studies, including randomized clinical trials involving patients with oropharyngeal dysphagia, have demonstrated improved swallowing efficacy, safety, and physiology when TRP agonists are mixed with the food bolus. Additionally, there is evidence of plasticity development in swallowing-related neuronal networks in the brain upon TRP channel activation in peripheral swallowing-related regions. Thus, TRP channels have emerged as a promising target for the development of pharmacological treatments for oropharyngeal dysphagia.

Interaction of Calmodulin with TRPM: An Initiator of Channel Modulation

Transient receptor potential melastatin (TRPM) channels, a subfamily of the TRP superfamily, constitute a diverse group of ion channels involved in mediating crucial cellular processes like calcium homeostasis. These channels exhibit complex regulation, and one of the key regulatory mechanisms involves their interaction with calmodulin (CaM), a cytosol ubiquitous calcium-binding protein. The association between TRPM channels and CaM relies on the presence of specific CaM-binding domains in the channel structure. Upon CaM binding, the channel undergoes direct and/or allosteric structural changes and triggers down- or up-stream signaling pathways. According to current knowledge, ion channel members TRPM2, TRPM3, TRPM4, and TRPM6 are directly modulated by CaM, resulting in their activation or inhibition. This review specifically focuses on the interplay between TRPM channels and CaM and summarizes the current known effects of CaM interactions and modulations on TRPM channels in cellular physiology.

Inhibitory effects of linalool, an essential oil component of lavender, on nociceptive TRPA1 and voltage-gated Ca2+ channels in mouse sensory neurons

Linalool, an essential oil component of lavender is commonly used in fragrances. It is known that linalool has anxiolytic, sedative, and analgesic actions. However, the mechanism of its analgesic action has not yet been fully clarified. Pain signals elicited by the activation of nociceptors on peripheral neurons are transmitted to the central nervous system. In the present study, we investigated the effects of linalool on transient receptor potential (TRP) channels and voltage-gated channels, both of which are important for pain signaling via nociceptors in somatosensory neurons. For detection of channel activity, the intracellular Ca2+ concentration ([Ca2+]i) was measured using a Ca2+-imaging system, and membrane currents were recorded using the whole-cell patch-clamp technique. Analgesic actions were also examined in vivo. In mouse sensory neurons linalool at concentrations that did not induce [Ca2+]i increases did not affect [Ca2+]i responses to capsaicin and acids, TRPV1 agonists, but suppressed those induced by allyl isothiocyanate (AITC) and carvacrol, TRPA1 agonists. Similar inhibitory effects of linalool were observed in cells that heterologously expressed TRPA1. Linalool attenuated the [Ca2+]i increases induced by KCl and voltage-gated Ca2+ currents but only slightly suppressed voltage-gated Na＋currents in mouse sensory neurons. Linalool diminished TRPA1-mediated nociceptive behaviors. The present data suggest that linalool exerts an analgesic action via the suppression of nociceptive TRPA1 and voltage-gated Ca2+ channels.

Irritant-evoked reflex tachyarrhythmia in spontaneously hypertensive rats is reduced by inhalation of TRPM8 agonists l-menthol and WS-12

Inhalation of noxious irritants activates nociceptive sensory afferent nerves innervating the airways, inducing reflex regulation of autonomic networks and the modulation of respiratory drive and cardiovascular (CV) parameters such as heart rate and blood pressure. In healthy mammals, irritant-evoked pulmonary-cardiac reflexes cause parasympathetic-mediated bradycardia. However, in spontaneously hypertensive (SH) rats, irritant inhalation also increases sympathetic drive to the heart. This remodeled pulmonary-cardiac reflex may contribute to cardiovascular risk caused by inhalation of air pollutants/irritants in susceptible individuals with cardiovascular disease (CVD). Previous studies have shown that the cooling mimic l-menthol, an agonist for the cold-sensitive transient receptor potential melastatin 8 (TRPM8), can alleviate nasal inflammatory symptoms and respiratory reflexes evoked by irritants. Here, we investigated the impact of inhalation of TRPM8 agonists l-menthol and WS-12 on pulmonary-cardiac reflexes evoked by inhalation of the irritant allyl isothiocyanate (AITC) using radiotelemetry. l-Menthol, but not its inactive analog d-menthol, significantly reduced the AITC-evoked reflex tachycardia and premature ventricular contractions (PVCs) in SH rats but had no effect on the AITC-evoked bradycardia in either SH or normotensive Wistar-Kyoto (WKY) rats. WS-12 reduced AITC-evoked tachycardia and PVCs in SH rats, but this more potent TRPM8 agonist also reduced AITC-evoked bradycardia. l-Menthol had no effect on heart rate when given alone, whereas WS-12 evoked a minor bradycardia in WKY rats. We conclude that stimulation of TRPM8-expressing afferents within the airways reduces irritant-evoked pulmonary-cardiac reflexes, especially the aberrant reflex tachyarrhythmia in SH rats. Airway menthol treatment may be an effective therapy for reducing pollution-associated CV exacerbations.NEW & NOTEWORTHY Irritant-evoked pulmonary-cardiac reflexes are remodeled in spontaneously hypertensive (SH) rats-causing de novo sympathetic reflexes that drive tachyarrhythmia. This remodeling may contribute to air pollution-associated risk in susceptible individuals with cardiovascular disease. We found that inhalation of TRPM8 agonists, l-menthol and WS-12, but not the inactive analog d-menthol, selectively reduces the reflex tachyarrhythmia evoked by allyl isothiocyanate (AITC) inhalation in SH rats. Use of menthol may protect susceptible individuals from pollution-associated CV exacerbations.

Localization of TRP Channels in Healthy Oral Mucosa from Human Donors

The oral cavity is exposed to a remarkable range of noxious and innocuous conditions, including temperature fluctuations, mechanical forces, inflammation, and environmental and endogenous chemicals. How such changes in the oral environment are sensed is not completely understood. Transient receptor potential (TRP) ion channels are a diverse family of molecular receptors that are activated by chemicals, temperature changes, and tissue damage. In non-neuronal cells, TRP channels play roles in inflammation, tissue development, and maintenance. In somatosensory neurons, TRP channels mediate nociception, thermosensation, and chemosensation. To assess whether TRP channels might be involved in environmental sensing in the human oral cavity, we investigated their distribution in human tongue and hard palate biopsies. TRPV3 and TRPV4 were expressed in epithelial cells with inverse expression patterns where they likely contribute to epithelial development and integrity. TRPA1 immunoreactivity was present in fibroblasts, immune cells, and neuronal afferents, consistent with known roles of TRPA1 in sensory transduction and response to damage and inflammation. TRPM8 immunoreactivity was found in lamina propria and neuronal subpopulations including within the end bulbs of Krause, consistent with a role in thermal sensation. TRPV1 immunoreactivity was identified in intraepithelial nerve fibers and end bulbs of Krause, consistent with roles in nociception and thermosensation. TRPM8 and TRPV1 immunoreactivity in end bulbs of Krause suggest that these structures contain a variety of neuronal afferents, including those that mediate nociception, thermosensation, and mechanotransduction. Collectively, these studies support the role of TRP channels in oral environmental surveillance and response.

Transient receptor potential melastatin-7 in the rat dorsal root ganglion

AIMS

Transient receptor potential melastatin-7 (TRPM7) is a selective cation permeable channel which plays important roles in cellular and developmental biology such as cell proliferation, survival, differentiation and migration. This channel is also known to be necessary for transmitter release in the peripheral nervous system. In this study, immunohistochemistry for TRPM7 was conducted in the rat lumbar dorsal root ganglion (DRG).

METHODS

Triple immunofluorescence methods were used to demonstrate distribution of TRPM7 and its relationship to other TRP channels in the DRG. Retrograde tracing and double immunofluorescence methods were also performed to know peripheral targets of DRG neurons containing TRPM7 and TRP vanilloid 1 (TRPV1). In addition, transection of the sciatic nerve was conducted to demonstrate an effect of the nerve injury on TRPM7expression in the DRG.

RESULTS

TRPM7-immunoreactivity was expressed by 53.9% of sensory neurons in the 4th lumbar DRG. TRPM7-immunoreactive (-IR) DRG neurons mostly had small (<600 µm²) and medium-sized (600-1200 µm²) cell bodies. By triple and double immunofluorescence methods, approximately 70% of TRPM7-IR DRG neurons contained TRPV1-immunoreactivity. Although the number of DRG neurons co-expressing TRPM7 and TRPM8 was small in the DRG, almost all of TRPM8-IR DRG neurons co-expressed TRPM7-immunoreactivity. By combination of retrograde tracing method and immunohistochemistry, TRPM7 was expressed by half of DRG neurons innervating the plantar skin (61.9%) and gastrocnemius muscle (51.2%), and 79.6% of DRG neurons innervating the periosteum. Co-expression of TRPM7 and TRPV1 among periosteum DRG neurons (75.7%) was more abundant than among cutaneous (53.2%) and muscular (40.4%) DRG neurons. DRG neurons which co-expressed these ion channels in the periosteum had smaller cell bodies compared to the skin and muscle. In addition, the sciatic nerve transection decreased the number of TRPM7-IR neurons in the DRG (approximately 60% reduction). The RT-qPCR analysis also demonstrated reduction of TRPM7 mRNA in the injured DRG.

CONCLUSION

The present study suggests that TRPM7 is mainly located in small nociceptors in the DRG. The content of TRPM7 in DRG neurons is probably different among their peripheral targets. TRPM7 in DRG neurons may be able to respond to noxious stimulation from their peripheral tissues. The nerve injury can decrease the level of TRPM7 mRNA and protein in DRG neurons.

Trigeminal Stimulus Menthol Masks Bitter Off-Flavor of Artificial Sweetener Acesulfame-K

Consumer health concerns and regulatory policies lead to a growing demand for sugar-sweetened beverage alternatives. A reduced energy content can be achieved using artificial sweeteners, which often also convey a metallic or bitter off-flavor. Therefore, the alteration of sweetness perception and masking of potential off-flavors are paramount for improving sweet beverages. Trigeminal stimuli, such as capsaicin (spicy) or menthol (cooling), have been used to influence taste perception in food items, although their use in beverages has not yet been systematically investigated. Here, the influence of menthol on sweetness perception in an aqueous solution is examined both on the sensory and psychophysiological level. The addition of menthol had no sensory effect on sweetness perception; however, psychophysiological measurements suggest a boost in the physiological response to cold perception through the addition of sugar. Moreover, menthol addition shifted the recognition threshold of unpleasant bitterness of the sweetener acesulfame-K from 21.35 to 36.93 mg/L, masking the off-flavor. These findings illuminate the complexity of trigeminal perception influences on taste. Further investigation of these effects can render trigeminal stimuli an effective tool to enhance beverage aroma and flavor.

The effects of certain TRP channels and voltage-gated KCNQ/Kv7 channel opener retigabine on calcitonin gene-related peptide release in the trigeminovascular system

BACKGROUND

Calcitonin gene-related peptide release in trigeminovascular system is a pivotal component of neurogenic inflammation underlying migraine pathophysiology. Transient receptor potential channels and voltage-gated KCNQ/Kv7 potassium channels expressed throughout trigeminovascular system are important targets for modulation of calcitonin gene-related peptide release. We investigated the effects of certain transient receptor potential (TRP) channels the vanilloid 1 and 4 (TRPV1 and TRPV4), the ankyrin 1 (TRPA1), and metastatin type 8 (TRPM8), and voltage-gated potassium channel (Kv7) opener retigabine on calcitonin gene-related peptide release from peripheral (dura mater and trigeminal ganglion) and central (trigeminal nucleus caudalis) trigeminal components of rats.

METHODS

The experiments were carried out using well-established in-vitro preparations (hemiskull, trigeminal ganglion and trigeminal nucleus caudalis) from male Wistar rats. Agonists and antagonists of TRPV1, TRPV4, TRPA1 and TRPM8 channels, and also retigabine were tested on the in-vitro release of calcitonin gene-related peptide. Calcitonin gene-related peptide concentrations were measured using enzyme-linked immunosorbent assay.

RESULTS

Agonists of these transient receptor potential channels induced calcitonin gene-related peptide release from hemiskull, trigeminal ganglion and trigeminal nucleus caudalis, respectively. The transient receptor potential channels-induced calcitonin gene-related peptide releases were blocked by their specific antagonists and reduced by retigabine. Retigabine also decreased basal calcitonin gene-related peptide releases in all preparations.

CONCLUSION

Our findings suggest that favorable antagonists of these transient receptor potential channels, or Kv7 channel opener retigabine may be effective in migraine therapy by inhibiting neurogenic inflammation that requires calcitonin gene-related peptide release.

Genotypic Differences in the Effects of Menthol on Nicotine Intake and Preference in Mice

Menthol has been shown to exacerbate elements of nicotine addiction in humans and rodents; however, the mechanisms mediating its effects are not fully understood. This study examined the impact of genetic factors in menthol’s effects on oral nicotine consumption by comparing two inbred mouse strains with differing sensitivities to nicotine. C57BL/6J (B6J) mice are nicotine-preferring, while DBA/2J (D2J) mice are not. While the effects of menthol on oral nicotine consumption have been highlighted in B6J mice, it is unknown if they extend to the D2J strain as well. Consequently, adolescent (PND 21) and adult (PND 63), male and female D2J mice were subjected to the nicotine two-bottle choice (2BC) paradigm with orally and systemically administered menthol. Then, we evaluated its impact on nicotine pharmacological responses in conditioned reward and nociception after systemic administration and, lastly, investigated the potential involvement of the TAAR1 gene and α7 nAChRs in menthol’s effects. Menthol failed to enhance oral nicotine consumption in adult and adolescent female and male D2J mice. Moreover, this lack in effect was not due to nicotine concentration, oral aversion to menthol, or basal preference for nicotine. Menthol also failed to augment nicotine reward or enhance nicotine-induced antinociception in D2J mice, demonstrating that genetic background plays a significant role in sensitivity to menthol’s effects on nicotine. Furthermore, TAAR1 or α7 nAChRs did not seem to mediate menthol’s differential effects in D2J mice. These findings support the existence of genotype-specific mechanisms that may contribute to the variable effects of menthol in different populations.

PACAP-38 Induces Transcriptomic Changes in Rat Trigeminal Ganglion Cells Related to Neuroinflammation and Altered Mitochondrial Function Presumably via PAC1/VPAC2 Receptor-Independent Mechanism

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a broadly expressed neuropeptide which has diverse effects in both the peripheral and central nervous systems. While its neuroprotective effects have been shown in a variety of disease models, both animal and human data support the role of PACAP in migraine generation. Both PACAP and its truncated derivative PACAP(6-38) increased calcium influx in rat trigeminal ganglia (TG) primary sensory neurons in most experimental settings. PACAP(6-38), however, has been described as an antagonist for PACAP type I (known as PAC1), and Vasoactive Intestinal Polypeptide Receptor 2 (also known as VPAC2) receptors. Here, we aimed to compare the signaling pathways induced by the two peptides using transcriptomic analysis. Rat trigeminal ganglion cell cultures were incubated with 1 µM PACAP-38 or PACAP(6-38). Six hours later RNA was isolated, next-generation RNA sequencing was performed and transcriptomic changes were analyzed to identify differentially expressed genes. Functional analysis was performed for gene annotation using the Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Reactome databases. We found 200 common differentially expressed (DE) genes for these two neuropeptides. Both PACAP-38 and PACAP(6-38) treatments caused significant downregulation of NADH: ubiquinone oxidoreductase subunit B6 and upregulation of transient receptor potential cation channel, subfamily M, member 8. The common signaling pathways induced by both peptides indicate that they act on the same target, suggesting that PACAP activates trigeminal primary sensory neurons via a mechanism independent of the identified and cloned PAC1/VPAC2 receptor, either via another target structure or a different splice variant of PAC1/VPAC2 receptors. Identification of the target could help to understand key mechanisms of migraine.

Activation of Drosophila melanogaster TRPA1 Isoforms by Citronellal and Menthol

BACKGROUND

The transient receptor potential ankyrin 1 (TRPA1) cation channels function as broadly-tuned sensors of noxious chemicals in many species. Recent studies identified four functional TRPA1 isoforms in Drosophila melanogaster (dTRPA1(A) to (D)), but their responses to non-electrophilic chemicals are yet to be fully characterized.

METHODS

We determined the behavioral responses of adult flies to the mammalian TRPA1 non-electrophilic activators citronellal and menthol, and characterized the effects of these compounds on all four dTRPA1 channel isoforms using intracellular Ca²⁺ imaging and whole-cell patch-clamp recordings.

RESULTS

Wild type flies avoided citronellal and menthol in an olfactory test and this behavior was reduced in dTrpA1 mutant flies. Both compounds activate all dTRPA1 isoforms in the heterologous expression system HEK293T, with the following sensitivity series: dTRPA1(C) = dTRPA1(D) > dTRPA1(A) ≫ dTRPA1(B) for citronellal and dTRPA1(A) > dTRPA1(D) > dTRPA1(C) > dTRPA1(B) for menthol.

CONCLUSIONS

dTrpA1 was required for the normal avoidance of Drosophila melanogaster towards citronellal and menthol. All dTRPA1 isoforms are activated by both compounds, but the dTRPA1(B) is consistently the least sensitive. We discuss how these findings may guide further studies on the physiological roles and the structural bases of chemical sensitivity of TRPA1 channels.

TRPM8 Channels: Advances in Structural Studies and Pharmacological Modulation

The transient receptor potential melastatin subtype 8 (TRPM8) is a cold sensor in humans, activated by low temperatures (>10, <28 °C), but also a polymodal ion channel, stimulated by voltage, pressure, cooling compounds (menthol, icilin), and hyperosmolarity. An increased number of experimental results indicate the implication of TRPM8 channels in cold thermal transduction and pain detection, transmission, and maintenance in different tissues and organs. These channels also have a repercussion on different kinds of life-threatening tumors and other pathologies, which include urinary and respiratory tract dysfunctions, dry eye disease, and obesity. This compendium firstly covers newly described papers on the expression of TRPM8 channels and their correlation with pathological states. An overview on the structural knowledge, after cryo-electron microscopy success in solving different TRPM8 structures, as well as some insights obtained from mutagenesis studies, will follow. Most recently described families of TRPM8 modulators are also covered, along with a section of molecules that have reached clinical trials. To finalize, authors provide an outline of the potential prospects in the TRPM8 field.

Cell death modulation by transient receptor potential melastatin channels TRPM2 and TRPM7 and their underlying molecular mechanisms

Transient receptor potential melastatin (TRPM) channels are members of the transient receptor potential (TRP) channels, a family of evolutionarily conserved integral membrane proteins. TRPM channels are nonselective cation channels, mediating the influx of various ions including Ca²⁺, Na⁺ and Zn²⁺. The function of TRPM channels is vital for cell proliferation, cell development and cell death. Cell death is a key procedure during embryonic development, organism homeostasis, aging and disease. The category of cell death modalities, beyond the traditionally defined concepts of necrosis, autophagy, and apoptosis, were extended with the discovery of pyroptosis, necroptosis and ferroptosis. As upstream signaling regulators of cell death, TRPM channels have been involved inrelevant pathologies. In this review, we introduced several cell death modalities, then summarized the contribution of TRPM channels (especially TRPM2 and TRPM7) to different cell death modalities and discussed the underlying regulatory mechanisms. Our work highlighted the possibility of TRPM channels as potential therapeutic targets in cell death-related diseases.

Linalool odor-induced analgesia is triggered by TRPA1-independent pathway in mice

We had recently reported that linalool odor exposure induced significant analgesic effects in mice and that the effects were disappeared in olfactory-deprived mice in which the olfactory epithelium was damaged, thus indicating that the effects were triggered by chemical senses evoked by linalool odor exposure. However, the peripheral neuronal mechanisms, including linalool receptors that contribute toward triggering the linalool odor-induced analgesia, still remain unexplored. In vitro studies have shown that the transient receptor potential ankyrin 1 (TRPA1) responded to linalool, thus raising the possibility that TRPA1 expressed on the trigeminal nerve terminal detects linalool odor inhaled into the nostril and triggers the analgesic effects. To address this hypothesis, we measured the behavioral pain threshold for noxious mechanical stimulation in TRPA1-deficient mice. In contrast to our expectation, we found a significant increase in the threshold after linalool odor exposure in TRPA1-deficient mice, indicating the analgesic effects of linalool odor even in TRPA1-deficient mice. Furthermore, intranasal application of TRPA1 selective antagonist did not alter the analgesic effect of linalool odor. These results showed that the linalool odor-induced analgesia was triggered by a TRPA1-independent pathway in mice.

Implications of Transient Receptor Potential Cation Channels in Migraine Pathophysiology

Migraine is a common and debilitating headache disorder. Although its pathogenesis remains elusive, abnormal trigeminal and central nervous system activity is likely to play an important role. Transient receptor potential (TRP) channels, which transduce noxious stimuli into pain signals, are expressed in trigeminal ganglion neurons and brain regions closely associated with the pathophysiology of migraine. In the trigeminal ganglion, TRP channels co-localize with calcitonin gene-related peptide, a neuropeptide crucially implicated in migraine pathophysiology. Many preclinical and clinical data support the roles of TRP channels in migraine. In particular, activation of TRP cation channel V1 has been shown to regulate calcitonin gene-related peptide release from trigeminal nerves. Intriguingly, several effective anti-migraine therapies, including botulinum neurotoxin type A, affect the functions of TRP cation channels. Here, we discuss currently available data regarding the roles of major TRP cation channels in the pathophysiology of migraine and the therapeutic applicability thereof.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

Taste the Pain: The Role of TRP Channels in Pain and Taste Perception

Transient receptor potential (TRP) channels are a superfamily of cation transmembrane proteins that are expressed in many tissues and respond to many sensory stimuli. TRP channels play a role in sensory signaling for taste, thermosensation, mechanosensation, and nociception. Activation of TRP channels (e.g., TRPM5) in taste receptors by food/chemicals (e.g., capsaicin) is essential in the acquisition of nutrients, which fuel metabolism, growth, and development. Pain signals from these nociceptors are essential for harm avoidance. Dysfunctional TRP channels have been associated with neuropathic pain, inflammation, and reduced ability to detect taste stimuli. Humans have long recognized the relationship between taste and pain. However, the mechanisms and relationship among these taste-pain sensorial experiences are not fully understood. This article provides a narrative review of literature examining the role of TRP channels on taste and pain perception. Genomic variability in the TRPV1 gene has been associated with alterations in various pain conditions. Moreover, polymorphisms of the TRPV1 gene have been associated with alterations in salty taste sensitivity and salt preference. Studies of genetic variations in TRP genes or modulation of TRP pathways may increase our understanding of the shared biological mediators of pain and taste, leading to therapeutic interventions to treat many diseases.

Exploration of TRPM8 Binding Sites by β-Carboline-Based Antagonists and Their In Vitro Characterization and In Vivo Analgesic Activities

Transient receptor potential melastatin 8 (TRPM8) ion channel represents a valuable pharmacological option for several therapeutic areas. Here, a series of conformationally restricted derivatives of the previously described TRPM8 antagonist N,N′-dibenzyl tryptophan 4 were prepared and characterized in vitro by Ca²⁺-imaging and patch-clamp electrophysiology assays. Molecular modeling studies led to identification of a broad and well-defined interaction network of these derivatives inside the TRPM8 binding site, underlying their antagonist activity. The (5R,11aS)-5-(4-chlorophenyl)-2-(4-fluorobenzyl)-5,6,11,11a-tetrahydro-1H-imidazo[1′,5′:1,6]pyrido[3,4-b]indole-1,3(2H)-dione (31a) emerged as a potent (IC₅₀ = 4.10 ± 1.2 nM), selective, and metabolically stable TRPM8 antagonist. In vivo, 31a showed significant target coverage in an icilin-induced WDS (at 11.5 mg/kg ip), an oxaliplatin-induced cold allodynia (at 10–30 μg sc), and CCI-induced thermal hyperalgesia (at 11.5 mg/kg ip) mice models. These results confirm the tryptophan moiety as a solid pharmacophore template for the design of highly potent modulators of TRPM8-mediated activities.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.

Menthol, Nicotine, and Flavoring Content of Capsule Cigarettes in the US

Objectives

Characterize physical design features of cigarette brands sold in the US according to the delivery method of menthol that may affect sensory perception among users.

Methods

Twelve cigarette brands, mentholated and non-mentholated, were purchased for analyses of the physical design characteristics, quantification of nicotine and menthol, and identification of flavor additives.

Results

Physical design characteristics did not differ significantly between the various cigarette brands. However, significant differences were seen in levels of menthol. Menthol levels were greatest in products that had dual delivery methods of menthol (6.7mg/cigarette; SE=0.27) followed by products mentholated in a filter capsule only (5.7mg/cigarette; SE=0.25), and those mentholated in the tobacco only (3.8mg/cigarette; SE=0.12); products that were not mentholated had the least (0.38mg/cigarette; SE=0.31). Finally, flavor additives with a mint flavor profile other than menthol were identified, such as pulegone and limonene, and differed between cigarette brands, which are likely contributing to the menthol flavor experience associated with use of these products.

Conclusions

The regulation of menthol delivery method, flavorings added to the capsule, and/or menthol concentration may be beneficial for the public health as these factors are likely creating unique sensory experiences.

Seasonal Variation of Overactive Bladder Symptoms in Female Patients

Purpose

To evaluate seasonal variations of overactive bladder (OAB) symptoms in women who visited hospital clinics.

Methods

Medical records of female patients treated for OAB symptoms from January 2011 to December 2017 were retrospectively reviewed. Patients with pyuria at the first visit, those who did not complete the questionnaire, and those with <3 overactive bladder symptom scores (OABSS) were excluded. Uroflowmetric parameters, 3-day micturition diary, and OABSS were analyzed.

Results

A total of 582 patients with OAB symptoms who visited the hospital were enrolled in this study. Patients were grouped into 1 of the 3 season groups (cold, intermediate, and hot) depending on the average temperature of the month that the patient first visited the urologic department outpatient clinic. The total OABSS was significantly different between the 3 season groups (cold [7.25±3.20] vs. intermediate [6.24±3.40] vs. hot [5.51±3.20], P=0.001). The proportion of patients who had moderate OAB symptoms (6≤OABSS) was higher in the cold season group (56.2%) than in the other season groups (intermediate, 42.1%; hot, 31.8%; P=0.002). Differences in the number of micturitions (12.12±4.56 vs. 10.95±4.39, P=0.021) and number of urgent urinary incontinence episodes (2.06±0.94 vs. 2.48±0.87, P=0.001) between the cold and hot season groups were also significant. However, differences in the nocturia episode, total daytime voided volume, and mean voided volume between season groups were not significant.

Conclusions

Different urinary symptoms and uroflowmetric parameters were correlated with seasonal variation. OAB symptoms might be worse in cold season than in other seasons.

Tobacco use and chemosensory impairments among current adult tobacco users in the US: Data from NHANES 2013-2014

INTRODUCTION

Among US adults 40 years and older, about 23% report problems with their ability to smell, and 19% report problems with their ability to taste. Chemosenses are a first line of defence against environmental hazards (e.g. fires and leaking gas). A potential risk factor of chemosensory disorders includes nicotine product use, such as cigarette use. This study aims to assess the relationship of taste and smell alterations with type of recent nicotine product use (e.g. inhaled versus smokeless), recent cigarette use, and mentholation status based on data from NHANES 2013–2014.

METHODS

A total of 3186 men and women, 40 years and older, from NHANES 2013–2014 were assessed for smell and taste impairment, according to their recent nicotine product use. Taste impairment was identified as inability to identify quinine as bitter in the whole-mouth taste test. Impairment of smell was defined as failing to identify six or more of eight specific odors. Logistic regression models were adjusted for age, gender, and race/ethnicity.

RESULTS

Approximately 13% of participants failed the smell examination. No significant association between smell examination outcome and recent nicotine product use was found, though recent cigarette use showed a trend toward positive association (OR=1.66, 95% CI: 0.76–3.63) and mentholation status showed a trend toward negative association (OR=0.57, 95% CI: 0.22–1.49) on smell examination results. About 17% of participants failed the taste examination, and trends toward positive association were seen between taste examination outcomes and both recent nicotine product use (OR=1.28, 95% CI: 0.99–1.65) and recent cigarette use (OR=1.30, 95% CI: 0.50–3.40).

CONCLUSIONS

Findings indicate that recent use of nicotine products has an inconsistent relationship to dysfunctions in taste and smell. However, limiting the use of inhaled nicotine products, such as from cigarette use, could prove beneficial to a person’s taste and smell ability.

Influence of Temperature on Motor Behaviors in Newborn Opossums (Monodelphis domestica): An In Vitro Study

External thermosensation is crucial to regulate animal behavior and homeostasis, but the development of the mammalian thermosensory system is not well known. We investigated whether temperature could play a role in the control of movements in a mammalian model born very immature, the opossum (Monodelphis domestica). Like other marsupials, at birth the opossum performs alternate and rhythmic movements with its forelimbs (FLs) to reach a teat where it attaches in order to continue its development. It was shown that FL movements can be induced by mechanical stimulation of the snout in in vitro preparations of newborns consisting of the neuraxis with skin and FLs intact. In the present study, we used puff ejections of cold, neutral (bath temperature) and hot liquid directed toward the snout to induce FL responses in such preparations. Either the responses were visually observed under a microscope or triceps muscle activity was recorded. Cold liquid systematically induced FL movements and triceps contractions, but neutral and hot temperatures were less potent to do so. Sections of the trigeminal nerves and removal of the facial skin diminished responses to cold and nearly abolished those to hot and neutral stimulations. Transient receptor potential melastatin 8 (TRPM8) being the major cold receptor cation channel in adult mammals, we employed immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to test for its expression, but found that it is not expressed before 13 postnatal days. Overall our results indicate that cold thermosensation exerts a strong influence on motor behaviors in newborn opossums.

Altered Salivary Flow, Protein Composition, and Rheology Following Taste and TRP Stimulation in Older Adults

Taste and smell perceptions diminish in older age, impacting upon quality of life and nutrition, yet the causes of taste loss are largely unknown. Transient receptor potential channels (TRP) found on the oral mucosa are also involved in oral sensations including cooling and burning and may contribute to the eating experience of older people. Older adults often have reduced salivary flow and the physical properties of saliva may change, but the role of saliva in oral sensations of older adults is yet to be elucidated. Here, the effect of older age on subjective (perception) and objective (stimulated salivary response) measures of TRP stimulants, odors, and basic tastants was investigated. Whole mouth saliva was collected from younger (mean age 24 years) and older adults (mean age 72 years) following stimulation of taste [mono sodium glutamate (MSG) and caffeine], olfaction (menthol), and TRP receptors (capsaicin). Participants rated perceived intensity of each stimulus, and salivary properties were assessed. Older age was associated with 15% lower umami taste and 26% lower menthol odor perception, coupled with 17% lower salivary response to MSG. Interestingly, there were no differences for perception of TRP stimulants, so chemo-sensation was not affected by age. Younger adults had four times greater elasticity (Spinnbarkeit) with MUC7 levels almost double and 66% greater resting salivary flow rate. Stimulated salivary responses in the younger group were also higher compared to the older group, with changes in protein and viscoelasticity in response to taste and TRP stimulation. These results show the impact of older age upon taste and smell sensation which may lead to changes in the physical and compositional properties of saliva in response to taste/odor stimulation. Measurement of stimulated salivary flow and rheology provides an objective measure of taste in addition to subjective perceptions which can be influenced by participant bias. Chemo-sensation may be retained with age and trigeminal stimuli such as chili could be employed in future studies to enhance meals for an age group at risk of malnutrition. Alteration in salivary properties due to advanced age could impact on ability to taste due to poor diffusion of tastants and reduced oral surface protection.

Dry eye sensitizes cool cells to capsaicin-induced changes in activity via TRPV1

Corneal cool cells are sensitive to the ocular fluid status of the corneal surface and may be responsible for the regulation of basal tear production. Previously, we have shown that dry eye, induced by lacrimal gland excision (LGE) in rats, sensitized corneal cool cells to the transient receptor potential melastatin 8 (TRPM8) agonist menthol and to cool stimulation. In the present study, we examined the effect of dry eye on the sensitivity of cool cells to the transient receptor potential vanilloid 1 (TRPV1) agonist capsaicin. Single-unit recordings in the trigeminal ganglion were performed 7-10 days after LGE. At a concentration of 0.3 μM, capsaicin did not affect ongoing or cool-evoked activity in control animals yet facilitated ongoing activity and suppressed cool-evoked activity in LGE animals. At higher concentrations (3 μM), capsaicin continued to facilitate ongoing activity in LGE animals but suppressed ongoing activity in control animals. Higher concentrations of capsaicin also suppressed cool-evoked activity in both groups of animals, with an overall greater effect in LGE animals. In addition to altering cool-evoked activity, capsaicin enhanced the sensitivity of cool cells to heat in LGE animals. Capsaicin-induced changes were prevented by the application of the TRPV1 antagonist capsazepine. With the use of fluorescent in situ hybridization, TRPV1 and TRPM8 expression was examined in retrograde tracer-identified corneal neurons. The coexpression of TRPV1 and TRPM8 in corneal neurons was significantly greater in LGE-treated animals when compared with sham controls. These results indicate that LGE-induced dry eye increases TRPV1-mediated responses in corneal cool cells at least in part through the increased expression of TRPV1. NEW & NOTEWORTHY Corneal cool cells are known to detect drying of the ocular surface. Our study is the first to report that dry eye induced alterations in cool cell response properties, including the increased responsiveness to noxious heat and activation by capsaicin. Along with the changes in cell response properties, it is possible these neurons also function differently in dry eye, relaying information related to the perception of ocular irritation in addition to regulating tearing and blinking.

TRP Channels and Migraine: Recent Developments and New Therapeutic Opportunities

Migraine is the second-most disabling disease worldwide, and the second most common neurological disorder. Attacks can last many hours or days, and consist of multiple symptoms including headache, nausea, vomiting, hypersensitivity to stimuli such as light and sound, and in some cases, an aura is present. Mechanisms contributing to migraine are still poorly understood. However, transient receptor potential (TRP) channels have been repeatedly linked to the disorder, including TRPV1, TRPV4, TRPM8, and TRPA1, based on their activation by pathological stimuli related to attacks, or their modulation by drugs/natural products known to be efficacious for migraine. This review will provide a brief overview of migraine, including current therapeutics and the link to calcitonin gene-related peptide (CGRP), a neuropeptide strongly implicated in migraine pathophysiology. Discussion will then focus on recent developments in preclinical and clinical studies that implicate TRP channels in migraine pathophysiology or in the efficacy of therapeutics. Given the use of onabotulinum toxin A (BoNTA) to treat chronic migraine, and its poorly understood mechanism, this review will also cover possible contributions of TRP channels to BoNTA efficacy. Discussion will conclude with remaining questions that require future work to more fully evaluate TRP channels as novel therapeutic targets for migraine.

Transient Receptor Potential Melastatin 8 (TRPM8)-Based Mechanisms Underlie Both the Cold Temperature-Induced Inflammatory Reactions and the Synergistic Effect of Cigarette Smoke in Human Bronchial Epithelial (16HBE) Cells

Transient receptor potential melastatin 8 (TRPM8) is a major receptor of cold environment. Recently, we found that cigarette smoke extract (CSE) upregulated TRPM8 mRNA and protein expression in bronchial tissues that made them more sensitive to cold stimuli. In our present study, we found that cold temperature (18°C)-induced activation of TRPM8 in 16HBE (human bronchial epithelial) cells facilitated Ca²⁺ influx and subsequently led to the increased expression of interleukin (IL)-6, IL-8, and tumor necrosis factor (TNF)-α via the upregulation of p-extracellular signal-regulated kinase (ERK) and the activation of NF-κB. In addition, 16HBE cells that co-stimulated with 18°C and CSE were used to explore the synergistic effect of CSE on cold temperature-induced inflammatory cytokine production as well as the possible involved signaling pathway. RT-PCR and western blot analysis revealed that CSE upregulated TRPM8 mRNA and protein level in 16HBE cells. Ca²⁺ imaging, western blot, and luciferase assay showed more robust increase in intracellular Ca²⁺ and promoted phosphorylated ERK, P38, and NF-κB activity, respectively, in 16HBE cells co-stimulated with CSE and cold temperature, and such alteration was attenuated by TRPM8 short hairpin RNA (shRNA) transfection and BCTC pretreatment. Furthermore, enhanced levels of IL-6, IL-8, and TNF-α showed by enzyme-linked immunosorbent assay (ELISA) were reduced by specific inhibitors of ERK and NF-κB. Collectively, our results suggest that mitogen-activated protein kinase (MAPK)/NF-κB signaling is involved in TRPM8-mediated cold temperature-induced inflammatory cytokine expression. In addition, CSE synergistically amplifies cold temperature-induced inflammatory factors release via upregulating TRPM8 expression and enhancing MAPK/NF-κB signaling pathway.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.

Mouse Parabrachial Neurons Signal a Relationship between Bitter Taste and Nociceptive Stimuli

Taste and somatosensation both mediate protective behaviors. Bitter taste guides avoidance of ingestion of toxins while pain sensations, such as noxious heat, signal adverse conditions to ward off harm. Although brain pathways for taste and somatosensation are typically studied independently, prior data suggest that they intersect, potentially reflecting their common protective role. To investigate this, we applied electrophysiologic and optogenetic techniques in anesthetized mice of both sexes to evaluate relationships between oral somatosensory and taste activity in the parabrachial nucleus (PbN), implicated for roles in gustation and pain. Spikes were recorded from taste-active PbN neurons tested with oral delivery of thermal and chemesthetic stimuli, including agonists of nocisensitive transient receptor potential (TRP) ion channels on somatosensory fibers. Gustatory neurons were also tested to follow electrical pulse stimulation of an oral somatosensory region of the spinal trigeminal subnucleus caudalis (Vc), which projects to the PbN. Neurons composed classic taste groups, including sodium, electrolyte, appetitive, or bitter cells. Across groups, most neurons spiked to Vc pulse stimulation, implying that trigeminal projections reach PbN gustatory neurons. Among such cells, a subpopulation responsive to the bitter taste stimuli quinine and cycloheximide, and aversive concentrations of sodium, cofired to agonists of nocisensitive TRP channels, including capsaicin, mustard oil, and noxious heat. Such neurons populated the lateral PbN. Further, nociceptive activity in PbN bitter taste neurons was suppressed during optogenetic-assisted inhibition of the Vc, implying convergent trigeminal input contributed to such activity. Our results reveal a novel role for PbN gustatory cells in cross-system signaling related to protection.SIGNIFICANCE STATEMENT Prior data suggest that gustatory and trigeminal neural pathways intersect and overlap in the parabrachial area. However, no study has directly examined such overlap and why it may exist. Here we found that parabrachial gustatory neurons can receive afferent projections from trigeminal nuclei and fire to oral nociceptive stimuli that excite somatosensory receptors and fibers. Activation to aversive nociceptive stimuli in gustatory cells was associated with responding to behaviorally avoided bitter tastants. We were further able to show that silencing trigeminal projections inhibited nociceptive activity in parabrachial bitter taste neurons. Our results imply that in the parabrachial area, there is predictable overlap between taste and somatosensory processing related to protective coding and that classically defined taste neurons contribute to this process.

TRPM8, ASIC1, and ASIC3 localization and expression in the human oropharynx

BACKGROUND

Oropharyngeal dysphagia (OD) is a prevalent disease with poor prognosis among older people and has no pharmacological treatment. Polymodal sensory receptors like the TRP or ASIC family receptors are potential targets to treat OD. TRPM8 agonists and acidic solutions can improve the swallow response in patients with OD, but little is known about the expression of TRPM8, ASIC1, and ASIC3 in the human oropharynx. The aim of this study was to assess the expression and localization of TRPM8, ASIC1, and ASIC3 in human samples of the oropharynx to lay the basis for new pharmacological treatments for OD.

METHODS

Pathology-free samples from oropharyngeal regions innervated by cranial nerves V, IX, and X were obtained during major ENT surgery and processed to obtain mRNA (20 patients) or to be used in immunohistochemical assays (12 patients). TRPM8, ASIC1, and ASIC3 expression and localization were studied with RT-qPCR and fluorescent immunohistochemistry.

KEY RESULTS

ASIC3 was expressed in the 3 regions studied with similar levels and was localized on sensory fibers innervating the mucosa below the basal lamina of all studied regions. TRPM8 was also co-localized on the sensory fibers innervating the mucosa below the basal lamina of all studied regions. In contrast, ASIC1 was only found in the nerves innervating the tongue muscular fibers.

CONCLUSIONS & INFERENCES

TRPM8 and ASIC3 are found on submucosal sensory nerves in the human oropharynx. Our study lays the basis to use oropharyngeal TRPM8 and ASIC3 receptors as therapeutic targets to develop new active pharmacological treatments for OD patients.

Constitutive activity of transient receptor potential vanilloid type 1 triggers spontaneous firing in nerve growth factor-treated dorsal root ganglion neurons of rats

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We examined the role of TRPV1 in the generation of spontaneous APs in NGF-treated cultured DRG neurons of rats.

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Spontaneous firing in the on-cell configuration was abolished by TRPV1 antagonists capsazepine and BCTC.

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Chronic treatment with NGF induced capsazepine- and BCTC-sensitive cation conductance.

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NGF-induced cation conductance through TRPV1 causes spontaneous firing.

Dorsal root ganglion (DRG) neurons cultured in the presence of nerve growth factor (NGF, 100 ng/ml) often show a spontaneous action potential. Underlying mechanisms of this spontaneous firing were examined using the patch clamp technique. The spontaneous firing in the on-cell configuration was abolished by a decrease in the Na⁺ concentration and by the TRPV1 antagonists capsazepine (10 μM) and BCTC (1 μM). These responses were accompanied by hyperpolarization of the resting potential. The holding current observed in neurons voltage clamped at –60 mV in the whole-cell configuration was significantly larger in the neurons that fired spontaneously, indicating that these neurons had an additional cation conductance that caused depolarization and triggered action potentials. The holding current in the firing neurons was decreased by extracellular Na⁺ reduction, capsazepine and BCTC. The amplitudes of the capsazepine- or BCTC-sensitive component of the holding current in the spontaneously firing neurons were ten times as large as those recorded in the other neurons showing no spontaneous firing. However, the amplitudes of the current responses to capsaicin (1 μM) were not different regardless of the presence of spontaneous firing or treatment with NGF. These results indicate that chronic NGF treatment of cultured DRG neurons in rats induces a constitutively active cation conductance through TRPV1, which depolarizes the neurons and triggers spontaneous action potentials in the absence of any stimuli. Since NGF in the DRG is reported to increase after nerve injury, this NGF-mediated regulation of TRPV1 may be a cause of the pathogenesis of neuropathic pain.

[Menthol in the control of bladder activity: A review]

INTRODUCTION

Menthol is a natural compound, of which the known effects on human physiology are manifold (a feeling of freshness, decongestant, bowel antispasmodic). Its implication in vesico-sphincteral physiopathology has been studied since the nineties.

METHOD

Literature review of the previous studies having implied menthol in pelvi-perineal physiology through the articles indexed on the Pubmed database, with keywords menthol, menthol and bladder, menthol and toxicity, and TRPM8. Only articles in English were selected.

RESULTS

Of the 30 articles that were included, most demonstrated the existence of a micturition reflex to menthol and cold, mediated by the C-type nerve to the spine through activation of TRPM8 urothelial receptors. More recent experiments paradoxically showed an inhibitory effect of menthol on detrusor contractility, independently of TRPM8, when muscle tissue is directly exposed to the compound. However, similar effects of targeted cutaneous exposure or urothelial exposure on detrusorian function have also been demonstrated through TRPM8. This receptor also appears to be involved in interstitial cystitis and idiopathic detrusor overactivity. Lastly, the potential toxicity of menthol appears negligible. Most of the referenced studies are related to animal experiments. Of the three studies that implied humans, only one elucidates some therapeutic applications.

CONCLUSION

It seems that menthol and its receptors are involved in vesico-sphincteral physiopathology and could provide therapeutic potential in detrusorian overactivity and interstitial cystitis with reduced toxicity.

Current understanding of trigeminal ganglion structure and function in headache

INTRODUCTION

The trigeminal ganglion is unique among the somatosensory ganglia regarding its topography, structure, composition and possibly some functional properties of its cellular components. Being mainly responsible for the sensory innervation of the anterior regions of the head, it is a major target for headache research. One intriguing question is if the trigeminal ganglion is merely a transition site for sensory information from the periphery to the central nervous system, or if intracellular modulatory mechanisms and intercellular signaling are capable of controlling sensory information relevant for the pathophysiology of headaches.

METHODS

An online search based on PubMed was made using the keyword "trigeminal ganglion" in combination with "anatomy", "headache", "migraine", "neuropeptides", "receptors" and "signaling". From the relevant literature, further references were selected in view of their relevance for headache mechanisms. The essential information was organized based on location and cell types of the trigeminal ganglion, neuropeptides, receptors for signaling molecules, signaling mechanisms, and their possible relevance for headache generation.

RESULTS

The trigeminal ganglion consists of clusters of sensory neurons and their peripheral and central axon processes, which are arranged according to the three trigeminal partitions V1-V3. The neurons are surrounded by satellite glial cells, the axons by Schwann cells. In addition, macrophage-like cells can be found in the trigeminal ganglion. Neurons express various neuropeptides, among which calcitonin gene-related peptide is the most prominent in terms of its prevalence and its role in primary headaches. The classical calcitonin gene-related peptide receptors are expressed in non-calcitonin gene-related peptide neurons and satellite glial cells, although the possibility of a second calcitonin gene-related peptide receptor in calcitonin gene-related peptide neurons remains to be investigated. A variety of other signal molecules like adenosine triphosphate, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion cells and may act at receptors on adjacent neurons or satellite glial cells.

CONCLUSIONS

The trigeminal ganglion may act as an integrative organ. The morphological and functional arrangement of trigeminal ganglion cells suggests that intercellular and possibly also autocrine signaling mechanisms interact with intracellular mechanisms, including gene expression, to modulate sensory information. Receptors and neurotrophic factors delivered to the periphery or the trigeminal brainstem can contribute to peripheral and central sensitization, as in the case of primary headaches. The trigeminal ganglion as a target of drug action outside the blood-brain barrier should therefore be taken into account.

Morphological and functional changes in TRPM8-expressing corneal cold thermoreceptor neurons during aging and their impact on tearing in mice

Morphological and functional alterations of peripheral somatosensory neurons during the aging process lead to a decline of somatosensory perception. Here, we analyze the changes occurring with aging in trigeminal ganglion (TG), TRPM8-expressing cold thermoreceptor neurons innervating the mouse cornea, which participate in the regulation of basal tearing and blinking and have been implicated in the pathogenesis of dry eye disease (DED). TG cell bodies and axonal branches were examined in a mouse line (TRPM8^BAC -EYFP) expressing a fluorescent reporter. In 3 months old animals, about 50% of TG cold thermoreceptor neurons were intensely fluorescent, likely providing strongly fluorescent axons and complex corneal nerve terminals with ongoing activity at 34°C and low-threshold, robust responses to cooling. The remaining TRPM8⁺ corneal axons were weakly fluorescent with nonbeaded axons, sparsely ramified nerve terminals, and exhibited a low-firing rate at 34°C, responding moderately to cooling pulses as do weakly fluorescent TG neurons. In aged (24 months) mice, the number of weakly fluorescent TG neurons was strikingly high while the morphology of TRPM8⁺ corneal axons changed drastically; 89% were weakly fluorescent, unbranched, and often ending in the basal epithelium. Functionally, 72.5% of aged cold terminals responded as those of young animals, but 27.5% exhibited very low-background activity and abnormal responsiveness to cooling pulses. These morpho-functional changes develop in parallel with an enhancement of tear's basal flow and osmolarity, suggesting that the aberrant sensory inflow to the brain from impaired peripheral cold thermoreceptors contributes to age-induced abnormal tearing and to the high incidence of DED in elderly people.

Antinociceptive effect of two novel transient receptor potential melastatin 8 antagonists in acute and chronic pain models in rat

BACKGROUND AND PURPOSE

Transient receptor potential (TRP) channels are a superfamily of non-selective cation permeable channels involved in peripheral sensory signalling. Animal studies have shown that several TRPs are important players in pain modulation. Among them, the TRP melastatin 8 (TRPM8) has elicited more interest for its controversial role in nociception. This channel, expressed by a subpopulation of sensory neurons in dorsal root ganglia (DRG) and trigeminal ganglia (TG), is activated by cold temperatures and cooling agents. In experimental neuropathic pain models, an up-regulation of this receptor in DRG and TG has been observed, suggesting a key role for TRPM8 in the development and maintenance of pain. Consistent with this hypothesis, TRPM8 knockout mice are less responsive to pain stimuli.

EXPERIMENTAL APPROACH

In this study, the therapeutic potential and efficacy of two novel TRPM8 antagonists, DFL23693 and DFL23448, were tested.

KEY RESULTS

Two potent and selective TRPM8 antagonists with distinct pharmacokinetic profiles, DFL23693 and DFL23448, have been fully characterized in vitro. In vivo studies in well-established models, namely, the wet-dog shaking test and changes in body temperature, confirmed their ability to block the TRPM8 channel. Finally, TRPM8 blockage resulted in a significant antinociceptive effect in formalin-induced orofacial pain and in chronic constriction injury-induced neuropathic pain, confirming an important role for this channel in pain perception.

CONCLUSION AND IMPLICATIONS

Our findings, in agreement with previous literature, encourage further studies for a better comprehension of the therapeutic potential of TRPM8 blockers as novel agents for pain management.

The neurochemistry and morphology of functionally identified corneal polymodal nociceptors and cold thermoreceptors

It is generally believed that the unencapsulated sensory nerve terminals of modality specific C- and Aδ-neurons lack structural specialization. Here we determined the morphology of functionally defined polymodal receptors and cold thermoreceptors in the guinea pig corneal epithelium. Polymodal receptors and cold thermoreceptors were identified by extracellular recording at the surface of the corneal epithelium. After marking the recording sites, corneas were processed to reveal immunoreactivity for the transient receptor potential channels TRPV1 (transient receptor potential cation channel, subfamily V, member 1) or TPRM8 (transient receptor potential cation channel subfamily M member 8). Polymodal receptor nerve terminals (n = 6) were TRPV1-immunoreactive and derived from an axon that ascended from the sub-basal plexus to the squamous cell layer where it branched into fibers that ran parallel to the corneal surface and terminated with small bulbar endings (ramifying endings). Cold thermoreceptor nerve terminals were TRPM8-immunoreactive (n = 6) and originated from an axon that branched as it ascended through the wing cell and squamous cell layers and terminated with large bulbar endings (complex endings). These findings indicate that modality specific corneal sensory neurons with unencapsulated nerve endings have distinct nerve terminal morphologies that are likely to relate to their function.

St. Catherine of Siena (1347-1380 AD): one of the earliest historic cases of altered gustatory perception in anorexia mirabilis

St. Catherine of Siena suffered from an extreme form of holy fasting, a condition classified as anorexia mirabilis (also known as inedia prodigiosa). Historical and medical scholarships alike have drawn a comparison between this primaeval type of anorexia with a relatively common form of eating disorder among young women in the modern world, anorexia nervosa. St. Catherine's condition was characterised by a disgust for sweet taste, a condition also described in anorexia nervosa, and characterised by specific neurophysiological changes in the brain. St. Catherine's case may be considered one of the oldest veritable descriptions of altered gustation (dysgeusia). Moreover, a more compelling neurophysiological similarity between anorexia mirabilis and anorexia nervosa may be proposed.

The roles of TRPV1, TRPA1 and TRPM8 channels in chemical and thermal sensitivity of the mouse oral mucosa

Spices in food and beverages and compounds in tobacco smoke interact with sensory irritant receptors of the transient receptor potential (TRP) cation channel family. TRPV1 (vanilloid type 1), TRPA1 (ankyrin 1) and TRPM8 (melastatin 8) not only elicit action potential signaling through trigeminal nerves, eventually evoking pungent or cooling sensations, but by their calcium conductance they also stimulate the release of calcitonin gene-related peptide (CGRP). This is measured as an index of neuronal activation to elucidate the chemo- and thermosensory transduction in the isolated mouse buccal mucosa of wild types and pertinent knockouts. We found that the lipophilic capsaicin, mustard oil and menthol effectively get access to the nerve endings below the multilayered squamous epithelium, while cigarette smoke and its gaseous phase were weakly effective releasing CGRP. The hydrophilic nicotine was ineffective unless applied unprotonated in alkaline (pH9) solution, activating TRPA1 and TRPV1. Also, mustard oil activated both these irritant receptors in millimolar but only TRPA1 in micromolar concentrations; in combination (1 mm) with heat (45 °C), it showed supraadditive, that is heat sensitizing, effects in TRPV1 and TRPA1 knockouts, suggesting action on an unknown heat-activated channel and mustard oil receptor. Menthol caused little CGRP release by itself, but in subliminal concentration (2 mm), it enabled a robust cold response that was absent in TRPM8^-/- but retained in TRPA1^-/- and strongly reduced by TRPM8 inhibitors. In conclusion, all three relevant irritant receptors are functionally expressed in the oral mucosa and play their specific roles in inducing neurogenic inflammation and sensitization to heat and cold.

Analgesic-Like Activity of Essential Oil Constituents: An Update

The constituents of essential oils are widely found in foods and aromatic plants giving characteristic odor and flavor. However, pharmacological studies evidence its therapeutic potential for the treatment of several diseases and promising use as compounds with analgesic-like action. Considering that pain affects a significant part of the world population and the need for the development of new analgesics, this review reports on the current studies of essential oils’ chemical constituents with analgesic-like activity, including a description of their mechanisms of action and chemical aspects.

Quantitative Analysis of Mouse Dural Afferent Neurons Expressing TRPM8, VGLUT3, and NF200

OBJECTIVE

To quantify the abundance of dural afferent neurons expressing transient receptor potential channel melastatin 8 (TRPM8), vesicular glutamate transporter 3 (VGLUT3), and neurofilament 200 (NF200) in adult mice.

BACKGROUND

With the increasing use of mice as a model system to study headache mechanisms, it is important to understand the composition of dural afferent neurons in mice. In a previous study, we have measured the abundance of mouse dural afferent neurons that express neuropeptide calcitonin gene-related peptide as well as two TRP channels TRPV1 and TRPA1, respectively. Here, we conducted quantitative analysis of three other dural afferent subpopulations in adult mice.

METHODS

We used the fluorescent tracer Fluoro-Gold to retrogradely label dural afferent neurons in adult mice expressing enhanced green fluorescent protein in discrete subpopulations of trigeminal ganglion (TG) neurons. Mechanoreceptors with myelinated fibers were identified by NF200 immunoreactivity. We also conducted Ca²⁺ -imaging experiments to test the overlap between TRPM8 and VGLUT3 expression in mouse primary afferent neurons (PANs).

RESULTS

The abundance of TRPM8-expressing neurons in dural afferent neurons was significantly lower than that in total TG neurons. The percentages of dural afferent neurons expressing VGLUT3 and NF200 were comparable to those of total TG neurons, respectively. TRPM8 agonist menthol evoked Ca²⁺ influx in less than 7% VGLUT3-expressing PANs in adult mice.

CONCLUSIONS

TG neurons expressing TRPM8, VGLUT3, and NF200 all innervate adult mouse dura. TRPM8 and VGLUT3 are expressed in distinct subpopulations of PANs in adult mice. These results provide an anatomical basis to investigate headache mechanisms in mouse models.

Modulation of taste processing by temperature

Taste stimuli have a temperature that can stimulate thermosensitive neural machinery in the mouth during gustatory experience. Although taste and oral temperature are sometimes discussed as different oral sensory modalities, there is a body of literature that demonstrates temperature is an important component and modulator of the intensity of gustatory neural and perceptual responses. Available data indicate that the influence of temperature on taste, herein referred to as "thermogustation," can vary across taste qualities, can also vary among stimuli presumed to share a common taste quality, and is conditioned on taste stimulus concentration, with neuronal and psychophysical data revealing larger modulatory effects of temperature on gustatory responding to weakened taste solutions compared with concentrated. What is more, thermogustation is evidenced to involve interplay between mouth and stimulus temperature. Given these and other dependencies, identifying principles by which thermal input affects gustatory information flow in the nervous system may be important for ultimately unravelling the organization of neural circuits for taste and defining their involvement with multisensory processing related to flavor. Yet thermal effects are relatively understudied in gustatory neuroscience. Major gaps in our understanding of the mechanisms and consequences of thermogustation include delineating supporting receptors, the potential involvement of oral thermal and somatosensory trigeminal neurons in thermogustatory interactions, and the broader operational roles of temperature in gustatory processing. This review will discuss these and other issues in the context of the literature relevant to understanding thermogustation.

The identification of the TRPM8 channel on primary culture of human nasal epithelial cells and its response to cooling

BACKGROUND

It has been proposed that the transient receptor potential (TRP) channel Melastatin 8 (TRPM8) is a cold-sensing TRP channel. However, its presence and its role in the nasal cavity have not yet been fully studied.

METHODS

Immunohistology was used to study TRPM8 receptors in both the nasal mucosa tissue and the primary cultures of human nasal cells. Cells from primary cultures were immunostained with antibodies to TRPM8, mucin, cytokeratin (CK)-14, CK-18, and vimentin. Western blotting and real-time polymerase chain reaction (PCR) were used to determine the physiological role of TRPM8 in mucus production in the nasal cavity, with and without its agonist and antagonist.

RESULTS

The TRPM8 is clearly present in the epithelium, mucous glands, and vessels. No obvious TRPM8-immunoreactive cells were detected in the connective tissue. Immunostaining of cytospin preparations showed that epithelial cells test positive for CK-14, CK-18, TRPM8, and mucin 5AC (MUC5AC). Fibroblastic cells are stained negative for TRPM8. Secreted mucins in the cultured supernatant are detected after exposure to menthol and moderate cooling to 24°C. Both induce a statistically significant increase in the level of MUC5AC mRNA and mucin production. BCTC, a TRPM8 antagonist, has a statistically significant inhibitory effect on MUC5AC mRNA expression and MUC5AC protein production that is induced by menthol and moderate cooling to 24°C.

CONCLUSIONS

The study demonstrates that TRPM8 is present in the nasal epithelium. When it is activated by moderate cooling to 24°C or menthol, TRPM8 induces the secretion of mucin. This study shows that TRPM8 channels are important regulators of mucin production. Therefore, TRPM8 antagonists could be used to treat refractory rhinitis.

Corneal Sensitivity to Hyperosmolar Eye Drops: A Novel Behavioral Assay to Assess Diabetic Peripheral Neuropathy

Purpose

Diagnosis of peripheral neuropathy (PN), which affects approximately 50% of the diabetic population, is subjective, with many patients seeking a diagnosis only after presenting with symptoms. Recently, in vivo confocal microscopy of subepithelial corneal nerve density has been promoted as a surrogate marker for early detection of PN, but imaging of corneal nerves requires sophisticated instrumentation, expertise in confocal imaging, cooperative patients, and automated analysis tools to derive corneal nerve density. As an alternative, we developed a simple screening method that is based on the sensitivity of corneal nerves to cause reflex eyelid squinting in response to hyperosmolar eye drops.

Methods

Eyes of control and type 2 diabetic rats were given an eye drop of a 290- to 900-mOsm solution, and the ocular response was video recorded. Other neuropathic end points including nerve conduction velocity and subepithelial cornea nerve density were determined.

Results

Motor and sensory nerve conduction velocity and total nerve fiber length of corneal nerves in the subepithelial layer were significantly decreased in diabetic rats. Applying the hyperosmotic solutions to the ocular surface caused an osmolarity-dependent increase in squinting of the treated eye in control rats. Squinting was almost totally blocked by preapplication of proparacaine or N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide, a transient receptor potential melastatin-8 channel blocker. Squinting in response to the 900-mOsm solution was significantly reduced in diabetic rats.

Conclusions

Preclinical studies show that evaluation of corneal sensitivity may be an alternative method for the early detection of PN and has potential for translation to clinical studies.

Functional interactions between transient receptor potential M8 and transient receptor potential V1 in the trigeminal system: Relevance to migraine pathophysiology

Background

Recent genome-wide association studies have identified transient receptor potential M8 (TRPM8) as a migraine susceptibility gene. TRPM8 is a nonselective cation channel that mediates cool perception. However, its precise role in migraine pathophysiology is elusive. Transient receptor potential V1 (TRPV1) is a nonselective cation channel activated by noxious heat. Both TRPM8 and TRPV1 are expressed in trigeminal ganglion (TG) neurons.

Methods

We investigated the functional roles of TRPM8 and TRPV1 in a meningeal inflammation-based migraine model by measuring the effects of facial TRPM8 activation on thermal allodynia and assessing receptor coexpression changes in TG neurons. We performed retrograde tracer labeling to identify TG neurons innervating the face and dura.

Results

We found that pharmacological TRPM8 activation reversed the meningeal inflammation-induced lowering of the facial heat pain threshold, an effect abolished by genetic ablation of TRPM8. No significant changes in the heat pain threshold were seen in sham-operated animals. Meningeal inflammation caused dynamic alterations in TRPM8/TRPV1 coexpression patterns in TG neurons, and colocalization was most pronounced when the ameliorating effect of TRPM8 activation on thermal allodynia was maximal. Our tracer assay disclosed the presence of dura-innervating TG neurons sending collaterals to the face. Approximately half of them were TRPV1-positive. We also demonstrated functional inhibition of TRPV1 by TRPM8 in a cell-based assay using c-Jun N-terminal kinase phosphorylation as a surrogate marker.

Conclusions

Our findings provide a plausible mechanism to explain how facial TRPM8 activation can relieve migraine by suppressing TRPV1 activity. Facial TRPM8 appears to be a promising therapeutic target for migraine.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.