Expression of transient receptor potential vanilloid 1 and anoctamin 1 in rat trigeminal ganglion neurons innervating the tongue

Insights into the function and regulation of the calcium-activated chloride channel TMEM16A

The TMEM16A channel, a member of the TMEM16 protein family comprising chloride (Cl-) channels and lipid scramblases, is activated by the free intracellular Ca2+ increments produced by inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release after GqPCRs or Ca2+ entry through cationic channels. It is a ubiquitous transmembrane protein that participates in multiple physiological functions essential to mammals' lives. TMEM16A structure contains two identical 10-segment monomers joined at their transmembrane segment 10. Each monomer harbours one independent hourglass-shaped pore gated by Ca2+ ligation to an orthosteric site adjacent to the pore and controlled by two gates. The orthosteric site is created by assembling negatively charged glutamate side chains near the pore´s cytosolic end. When empty, this site generates an electrostatic barrier that controls channel rectification. In addition, an isoleucine-triad forms a hydrophobic gate at the boundary of the cytosolic vestibule and the inner side of the neck. When the cytosolic Ca2+ rises, one or two Ca2+ ions bind to the orthosteric site in a voltage (V)-dependent manner, thus neutralising the electrostatic barrier and triggering an allosteric gating mechanism propagating via transmembrane segment 6 to the hydrophobic gate. These coordinated events lead to pore opening, allowing the Cl- flux to ensure the physiological response. The Ca2+-dependent function of TMEM16A is highly regulated. Anions with higher permeability than Cl- facilitate V dependence by increasing the Ca2+ sensitivity, intracellular protons can replace Ca2+ and induce channel opening, and phosphatidylinositol 4,5-bisphosphate bound to four cytosolic sites likely maintains Ca2+ sensitivity. Additional regulation is afforded by cytosolic proteins, most likely by phosphorylation and protein-protein interaction mechanisms.

Function and Regulation of the Calcium-Activated Chloride Channel Anoctamin 1 (TMEM16A)

Various human tissues express the calcium-activated chloride channel Anoctamin 1 (ANO1), also known as TMEM16A. ANO1 allows the passive chloride flux that controls different physiological functions ranging from muscle contraction, fluid and hormone secretion, gastrointestinal motility, and electrical excitability. Overexpression of ANO1 is associated with pathological conditions such as hypertension and cancer. The molecular cloning of ANO1 has led to a surge in structural, functional, and physiological studies of the channel in several tissues. ANO1 is a homodimer channel harboring two pores - one in each monomer - that work independently. Each pore is activated by voltage-dependent binding of two intracellular calcium ions to a high-affinity-binding site. In addition, the binding of phosphatidylinositol 4,5-bisphosphate to sites scattered throughout the cytosolic side of the protein aids the calcium activation process. Furthermore, many pharmacological studies have established ANO1 as a target of promising compounds that could treat several illnesses. This chapter describes our current understanding of the physiological roles of ANO1 and its regulation under physiological conditions as well as new pharmacological compounds with potential therapeutic applications.

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.

R, Patel PH, Bazan DZ, Mora A, Samuel N, Mingle AC, Leon LR, Varon J, Surani S. Drug-Induced Hyperthermia Review

Humans maintain core body temperature via a complicated system of physiologic mechanisms that counteract heat/cold fluctuations from metabolism, exertion, and the environment. Overextension of these mechanisms or disruption of body temperature homeostasis leads to bodily dysfunction, culminating in a syndrome analogous to exertional heat stroke (EHS). The inability of this thermoregulatory process to maintain the body temperature is caused by either thermal stress or certain drugs. EHS is a syndrome characterized by hyperthermia and the activation of systemic inflammation. Several drug-induced hyperthermic syndromes may resemble EHS and share common mechanisms. The purpose of this article is to review the current literature and compare exertional heat stroke (EHS) to three of the most widely studied drug-induced hyperthermic syndromes: malignant hyperthermia (MH), neuroleptic malignant syndrome (NMS), and serotonin syndrome (SS). Drugs and drug classes that have been implicated in these conditions include amphetamines, diuretics, cocaine, antipsychotics, metoclopramide, selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants (TCAs), and many more. Observations suggest that severe or fulminant cases of drug-induced hyperthermia may evolve into an inflammatory syndrome best described as heat stroke. Their underlying mechanisms, symptoms, and treatment approaches will be reviewed to assist in accurate diagnosis, which will impact the management of potentially life-threatening complications.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Estrogen-dependent regulation of transient receptor potential vanilloid 1 (TRPV1) and P2X purinoceptor 3 (P2X3): Implication in burning mouth syndrome

Sex differences in the nervous system have gained recent academic interest. While the prominent differences are observed in mood and anxiety disorders, growing number of evidences also suggest sex difference in pain perception. This review focuses on estrogen as the key molecule underlying such difference, because estrogen plays many functions in the nervous system, including modulation of transient receptor potential vanilloid 1 (TRPV1) and P2X purinoceptor 3 (P2X3), two important nociceptive receptors. Estrogen was shown in various studies to up-regulate TRPV1 expression through two distinct pathways, resulting in pro-nociceptive effect. However, estrogen alleviated pain in other studies, by down-regulating nerve growth factor (NGF)-activated pathways and TRPV1. Estrogen may also attenuate nociception by inhibiting P2X3 receptors and ATP-signaling. Understanding the mechanism underlying the pro- and anti-nociceptive effect of estrogen might be crucial to understand pathophysiology of the burning mouth syndrome (BMS), a common chronic orofacial pain disorder in menopausal women. The involvement of TRPV1 is strongly suspected because of burning sensation. Reduced estrogen level of the BMS patient might have caused increased activity of P2X3 receptors. Interestingly, the increased expression of TRPV1 and P2X3 in oral mucosa of BMS patients was reported. The combinational impact of differential modulation of TRPV1/P2X3 during menopause might be an important contributing factor of etiology of BMS. Understanding the estrogen-dependent regulation of nociceptive receptors may provide a valuable insight toward the peripheral mechanism of sex-difference in pain perception.

Nociceptive receptors are expressed differently in trigeminal nociception after lingual nerve injury and unilateral external carotid artery occlusion in rats

OBJECTIVES

To investigate the different changes in nociceptive activity between two animal models of trigeminal neuropathic pain: unilateral external carotid artery ischemic reperfusion and lingual nerve crush in rats.

DESIGN

In this study, changes in nociceptive activity were investigated in unilateral external carotid artery ischemic reperfusion and lingual nerve crush models of trigeminal neuropathic pain in rats. Field excitatory postsynaptic potentials (fEPSPs) evoked by capsaicin application on the tongue of rats were recorded in the trigeminal nucleus caudalis. In addition, immunohistochemistry was performed in the trigeminal ganglia and trigeminal nucleus caudalis.

RESULTS

The fEPSP in unilateral external carotid artery ischemic reperfusion and lingual nerve crush rats was irregular relative to that in sham rats. In particular, the fEPSP spike in lingual nerve crush rats had a higher amplitude and shorter duration than that in sham rats. Unilateral external carotid artery ischemic reperfusion and lingual nerve crush also increased c-fos expression in the trigeminal nucleus caudalis. Upregulation of transient receptor potential vanilloid 1 in trigeminal ganglion was observed in unilateral external carotid artery ischemic reperfusion and lingual nerve crush rats, whereas upregulation of purinergic receptor subtype 3 in trigeminal ganglion was observed only in lingual nerve crush rats.

CONCLUSIONS

Although unilateral external carotid artery ischemic reperfusion and lingual nerve crush similarly increased nociceptive activity at the trigeminal nucleus caudalis, the fEPSPs and expression of nociceptive peripheral afferent neurons were different. Therefore, direct and indirect nerve injuries apparently induced the same nociceptive activity by different signaling responses dependent on nociceptive receptors.

Emerging Perspectives on Pain Management by Modulation of TRP Channels and ANO1

Receptor-type ion channels are critical for detection of noxious stimuli in primary sensory neurons. Transient receptor potential (TRP) channels mediate pain sensations and promote a variety of neuronal signals that elicit secondary neural functions (such as calcitonin gene-related peptide [CGRP] secretion), which are important for physiological functions throughout the body. In this review, we focus on the involvement of TRP channels in sensing acute pain, inflammatory pain, headache, migraine, pain due to fungal infections, and osteo-inflammation. Furthermore, action potentials mediated via interactions between TRP channels and the chloride channel, anoctamin 1 (ANO1), can also generate strong pain sensations in primary sensory neurons. Thus, we also discuss mechanisms that enhance neuronal excitation and are dependent on ANO1, and consider modulation of pain sensation from the perspective of both cation and anion dynamics.

TRPV1 activation alters the function of Aδ and C fiber sensory neurons that innervate bone

The Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a non-selective cation channel that is activated by capsaicin, low pH and noxious heat. It has been suggested to have a pro-algesic role in a range of conditions that present with bone pain, but the mechanisms by which this occurs are not yet clear. In this study we aimed to determine if TRPV1 is expressed in Aδ and/or C fiber bone afferent neurons, and to explore its role in the activation and/or sensitization of bone afferent neurons to mechanical stimulation. A combination of retrograde tracing and immunohistochemistry was used to determine expression of TRPV1 in the soma of bone afferent neurons that innervate the rat tibial marrow cavity. A novel, in vivo, electrophysiological bone-nerve preparation, recently developed in our laboratory, was used to make recordings of the activity and sensitivity of bone afferent neurons in response to application of the TRPV1 agonist capsaicin to the marrow cavity. We found that a substantial proportion of bone afferent neurons express TRPV1. These include both small-diameter myelinated (neurofilament rich) and unmyelinated (neurofilament poor) neurons that are likely to be Aδ and C fiber neurons, respectively. Electrophysiological recordings revealed that application of capsaicin to the marrow cavity increased ongoing activity of C fiber, and to a lesser extent Aδ fiber, bone afferent neurons. Capsaicin also sensitized both Aδ and C fiber bone afferent neurons to mechanical stimulation. This evidence supports a role for TRPV1 in the pathogenesis of pain associated with bone pathology or disease.

Characterization of sensory neuronal subtypes innervating mouse tongue

The tongue is uniquely exposed to water-soluble environmental chemicals that may lead to injury or tumorigenesis. However, comparatively little research has focused on the molecular and functional organization of trigeminal ganglia (TG) afferent neurons innervating the tongue. The current study identified and characterized lingual sensory neurons based on a neuronal subtype classification previously characterized in the dorsal root ganglion (DRG) neurons. We employed immunohistochemistry on transgenic reporter mouse lines as well as single-cell PCR of known markers of neuronal subtypes to characterize neuronal subtypes innervating the tongue. Markers expressed in retrogradely labeled TG neurons were evaluated for the proportion of neurons expressing each marker, intensity of expression, and overlapping genes. We found that tongue-innervating sensory neurons primarily expressed CGRP, TRPV1, TrkC, 5HT3A and Parvalbumin. These markers correspond to peptidergic and a subgroup of non-peptidergic C-nociceptors, peptidergic A nociceptors, proprioceptors and myelinated low-threshold mechanoreceptors (LTMRs). Interestingly, as reported previously, we also found several differences between TG and DRG neurons indicating the need for single-cell sequencing of neuronal types based on tissue type within all TG as well as DRG neurons.

Estrogens Exacerbate Nociceptive Pain via Up-Regulation of TRPV1 and ANO1 in Trigeminal Primary Neurons of Female Rats

Several trigeminal pain disorders show sex differences, and high levels of estrogens may underlie these differences. The interaction between transient receptor potential vanilloid 1 (TRPV1) and anoctamin 1 (ANO1) plays an important role in peripheral nociception. However, whether TRPV1 and ANO1 are involved in estrogen-modulated trigeminal pain sensitivity is unclear. In this study, we examined estradiol (E2) modulation of nociception through behavioral and immunohistological experiments after application of capsaicin (Cap), a selective TRPV1 agonist, onto the ocular surface in ovariectomized rats treated with high-dose E2 (HE) or low-dose E2 (LE) for 2 days. In addition, we used real-time PCR to study the effects of E2 on the expression levels of TRPV1 and ANO1 mRNA in trigeminal ganglia. In the behavioral experiment, the HE group showed significant potentiation of Cap-evoked nocifensive behavior compared with the LE group. Immunohistochemistry showed that Cap evoked a significantly greater number of cells that were immunoreactive for c-Fos, a marker of nociceptive activation, in the trigeminal subnucleus caudalis/upper cervical cord in the HE group than in the LE group. The number of c-Fos-immunoreactive cells in the ventral trigeminal interpolaris/caudalis were similar in the 2 groups. Real-time PCR showed that the levels of TRPV1 and ANO1 mRNA in the HE group were significantly higher than levels in the LE group. Thus, high levels of estrogens may be a risk factor for Cap-evoked nociceptive pain, and estrogen-dependent increases in TRPV1 and ANO1 are likely involved in modulating the nociceptive response in the trigeminal area.

Regulation of transient receptor potential vanilloid 1 expression in trigeminal ganglion neurons via methyl-CpG binding protein 2 signaling contributes tongue heat sensitivity and inflammatory hyperalgesia in mice

Background

Pain hypoalgesia has been reported in Rett syndrome patients, a severe neurodevelopmental disorder which can be attributed to mutations in the methyl-CpG binding protein 2 (MeCP2). Here, we examined the role of MeCP2 signaling in tongue heat sensitivity in the normal and inflamed state using Mecp2 heterozygous (Mecp2^+/−) mice.

Results

Heat hypoalgesia of the tongue occurred in Mecp2^+/− mice and submucosal injection of complete Freund’s adjuvant into the tongue produced a long-lasting heat hyperalgesia at the inflamed site in wild-type mice but not in Mecp2^+/− mice. Transient receptor potential vanilloid 1 was expressed in a large number of MeCP2-immunoreactive trigeminal ganglion neurons innervating the tongue in both wild-type and Mecp2^+/− mice (70.9% in wild type; 72.1% in Mecp2^+/−). The number of transient receptor potential vanilloid 1-immunoreactive trigeminal ganglion neurons innervating the tongue was smaller in Mecp2^+/− mice relative to wild-type mice (30.5% in wild type; 20.2% in Mecp2^+/−). Following complete Freund’s adjuvant injection, the number of transient receptor potential vanilloid 1- and MeCP2-immunoreactive trigeminal ganglion neurons innervating the tongue, as well as MeCP2 protein expression in trigeminal ganglion, was significantly increased in wild-type mice but not in Mecp2^+/− mice. Additionally, tongue heat hyperalgesia following complete Freund’s adjuvant injection was completely suppressed by the administration of SB366791, a transient receptor potential vanilloid 1 antagonist, in the tongue.

Conclusions

These findings indicate that tongue heat sensitivity and hypersensitivity are dependent on the expression of transient receptor potential vanilloid 1 which is regulated via MeCP2 signaling in trigeminal ganglion neurons innervating the tongue.

Eact, a small molecule activator of TMEM16A, activates TRPV1 and elicits pain- and itch-related behaviours

BACKGROUND AND PURPOSE

TMEM16A, also known as anoctamin 1 channel, is a member of the Ca(2+)-activated chloride channels family and serves as a heat sensor in the primary nociceptors. Eact is a recently discovered small molecule activator of the TMEM16A channel. Here, we asked if Eact produces pain- and itch-related responses in vivo and investigated the cellular and molecular basis of Eact-elicited responses in dorsal root ganglia (DRG) neurons.

EXPERIMENTAL APPROACH

We employed behavioural testing combined with pharmacological inhibition and genetic ablation approaches to identify transient receptor potential vanilloid 1 (TRPV1) as the prominent mediator for Eact-evoked itch- or pain-related responses. We investigated the effects of Eact on TRPV1 and TMEM16A channels expressed in HEK293T cells and in DRG neurons isolated from wild type and Trpv1(-/-) mice using Ca(2+) imaging and patch-clamp recordings. We also used site-directed mutagenesis to determine the molecular basis of Eact activation of TRPV1.

KEY RESULTS

Administration of Eact elicited both itch- and pain-related behaviours. Unexpectedly, the Eact-elicited behavioural responses were dependent on the function of TRPV1, as shown by pharmacological inhibition and genetic ablation studies. Eact activated membrane currents and increased intracellular free Ca(2+) in both TRPV1-expressing HEK293T cells and isolated DRG neurons in a TRPV1-dependent manner. Eact activation of the TRPV1 channel was severely attenuated by mutations disrupting the capsaicin-binding sites.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that Eact activates primary sensory nociceptors and produces both pain and itch responses mainly through direct activation of TRPV1 channels.

Modulating Ca²⁺ signals: a common theme for TMEM16, Ist2, and TMC

Since the discovery of TMEM16A (anoctamin 1, ANO1) as Ca(2+)-activated Cl(-) channel, the protein was found to serve different physiological functions, depending on the type of tissue. Subsequent reports on other members of the anoctamin family demonstrated a broad range of yet poorly understood properties. Compromised anoctamin function is causing a wide range of diseases, such as hearing loss (ANO2), bleeding disorder (ANO6), ataxia and dystonia (ANO3, 10), persistent borrelia and mycobacteria infection (ANO10), skeletal syndromes like gnathodiaphyseal dysplasia and limb girdle muscle dystrophy (ANO5), and cancer (ANO1, 6, 7). Animal models demonstrate CF-like airway disease, asthma, and intestinal hyposecretion (ANO1). Although present data indicate that ANO1 is a Ca(2+)-activated Cl(-) channel, it remains unclear whether all anoctamins form plasma membrane-localized or intracellular chloride channels. We find Ca(2+)-activated Cl(-) currents appearing by expression of most anoctamin paralogs, including the Nectria haematococca homologue nhTMEM16 and the yeast homologue Ist2. As recent studies show a role of anoctamins, Ist2, and the related transmembrane channel-like (TMC) proteins for intracellular Ca(2+) signaling, we will discuss the role of these proteins in generating compartmentalized Ca(2+) signals, which may give a hint as to the broad range of cellular functions of anoctamins.

Anoctamin Calcium-Activated Chloride Channels May Modulate Inhibitory Transmission in the Cerebellar Cortex

Calcium-activated chloride channels of the anoctamin (alias TMEM16) protein family fulfill critical functions in epithelial fluid transport, smooth muscle contraction and sensory signal processing. Little is known, however, about their contribution to information processing in the central nervous system. Here we examined the recent finding that a calcium-dependent chloride conductance impacts on GABAergic synaptic inhibition in Purkinje cells of the cerebellum. We asked whether anoctamin channels may underlie this chloride conductance. We identified two anoctamin channel proteins, ANO1 and ANO2, in the cerebellar cortex. ANO1 was expressed in inhibitory interneurons of the molecular layer and the granule cell layer. Both channels were expressed in Purkinje cells but, while ANO1 appeared to be retained in the cell body, ANO2 was targeted to the dendritic tree. Functional studies confirmed that ANO2 was involved in a calcium-dependent mode of ionic plasticity that reduces the efficacy of GABAergic synapses. ANO2 channels attenuated GABAergic transmission by increasing the postsynaptic chloride concentration, hence reducing the driving force for chloride influx. Our data suggest that ANO2 channels are involved in a Ca2+-dependent regulation of synaptic weight in GABAergic inhibition. Thus, in balance with the chloride extrusion mechanism via the co-transporter KCC2, ANO2 appears to regulate ionic plasticity in the cerebellum.

Anoctamin-1 Cl(-) channels in nociception: activation by an N-aroylaminothiazole and capsaicin and inhibition by T16A[inh]-A01

Background

Anoctamin 1 (ANO1 or TMEM16A) Ca²⁺-gated Cl⁻ channels of nociceptor neurons are emerging as important molecular components of peripheral pain transduction. At physiological intracellular Cl⁻ concentrations ([Cl⁻]_i) sensory neuronal Cl⁻ channels are excitatory. The ability of sensory neuronal ANO1 to trigger action potentials and subsequent nocifensive (pain) responses were examined by direct activation with an N-aroylaminothiazole. ANO1 channels are also activated by intracellular Ca²⁺ ([Ca²⁺]_i) from sensory neuronal TRPV1 (transient-receptor-potential vallinoid 1) ion channels and other noxicant receptors. Thus, sensory neuronal ANO1 can facilitate TRPV1 triggering of action potentials, resulting in enhanced nociception. This was investigated by reducing ANO1 facilitation of TRPV1 effects with: (1) T16A[inh]-A01 ANO1-inhibitor reagent at physiological [Cl⁻]_i and (2) by lowering sensory neuronal [Cl⁻]_i to switch ANO1 to be inhibitory.

Results

ANO1 effects on action potential firing of mouse dorsal root ganglia (DRG) neurons in vitro and mouse nocifensive behaviors in vivo were examined with an N-aroylaminothiazole ANO1-activator (E-act), a TRPV1-activator (capsaicin) and an ANO1-inhibitor (T16A[inh]-A01). At physiological [Cl⁻]_i (40 mM), E-act (10 µM) increased current sizes (in voltage-clamp) and action potential firing (in current-clamp) recorded in DRG neurons using whole-cell electrophysiology. To not disrupt TRPV1 carried-Ca²⁺ activation of ANO1 in DRG neurons, ANO1 modulation of capsaicin-induced action potentials was measured by perforated-patch (Amphotericin–B) current-clamp technique. Subsequently, at physiological [Cl⁻]_i, capsaicin (15 µM)-induced action potential firing was diminished by co-application with T16A[inh]-A01 (20 µM). Under conditions of low [Cl⁻]_i (10 mM), ANO1 actions were reversed. Specifically, E-act did not trigger action potentials; however, capsaicin-induced action potential firing was inhibited by co-application of E-act, but was unaffected by co-application of T16A[inh]-A01. Nocifensive responses of mice hind paws were dramatically induced by subcutaneous injections of E-act (5 mM) or capsaicin (50 µM). The nocifensive responses were attenuated by co-injection with T16A[inh]-A01 (1.3 mM).

Conclusions

An ANO1-activator (E-act) induced [Cl⁻]_i-dependent sensory neuronal action potentials and mouse nocifensive behaviors; thus, direct ANO1 activation can induce pain perception. ANO1-inhibition attenuated capsaicin-triggering of action potentials and capsaicin-induced nocifensive behaviors. These results indicate ANO1 channels are involved with TRPV1 actions in sensory neurons and inhibition of ANO1 could be a novel means of inducing analgesia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0061-y) contains supplementary material, which is available to authorized users.

Transient receptor potential cation channel subfamily V member 1 expressing corneal sensory neurons can be subdivided into at least three subpopulations

The cornea is innervated by three main functional classes of sensory neurons: polymodal nociceptors, pure mechano-nociceptors and cold-sensing neurons. Here we explored transient receptor potential cation channel subfamily V member 1 (TRPV1) expression in guinea pig corneal sensory neurons, a widely used molecular marker of polymodal nociceptors. We used retrograde tracing to identify corneal afferent neurons in the trigeminal ganglion (TG) and double label in situ hybridization and/or immunohistochemistry to determine their molecular profile. In addition, we used immunohistochemistry to reveal the neurochemistry and structure of TRPV1 expressing nerve endings in the corneal epithelium. Approximately 45% of corneal afferent neurons expressed TRPV1, 28% expressed Piezo2 (a marker of putative pure mechano-nociceptors) and 8% expressed the transient receptor potential cation channel subfamily M member 8 (TRPM8; a marker of cold-sensing neurons). There was no co-expression of TRPV1 and Piezo2 in corneal afferent neurons, but 6% of TRPV1 neurons co-expressed TRPM8. The TRPV1 expressing corneal afferent neurons could be divided into three subpopulations on the basis of calcitonin gene-related peptide (CGRP) and/or or glial cell line-derived neurotrophic factor family receptor alpha3 (GFRα3) co-expression. In the corneal epithelium, the TRPV1 axons that co-expressed CGRP and GFRα3 ended as simple unbranched endings in the wing cell layer. In contrast, those that only co-expressed GFRα3 had ramifying endings that branched and terminated in the squamous cell layer, whereas those that only co-expressed CGRP had simple endings in the basal epithelium. This study shows that the majority of TRPV1 expressing corneal afferent neurons (>90%) are likely to be polymodal nociceptors. Furthermore, TRPV1 expressing corneal afferent neurons can be subdivided into specific subpopulations based on their molecular phenotype, nerve terminal morphology and distribution in the corneal epithelium.

Released lipids regulate transient receptor potential channel (TRP)-dependent oral cancer pain

Background

Pain in the head neck area is an early symptom in oral cancer, supporting the hypothesis that cancer cells control the activities of surrounding nociceptors at the site of the tumor. Several reports implicate TRPV1 and TRPA1 in cancer pain, although there is a large gap in knowledge since the mechanisms for tumor-induced activation of these TRP receptors are unknown. Interestingly, TRP-active lipids such as linoleic acid, arachidonic acid, hydroxyoctadecadienoic acid and hydroxyeicosatetraenoic acid are significantly elevated in the saliva of oral cancer patients compared to normal patients, supporting a possible linkage between these lipids and oral cancer pain. We therefore hypothesize that oral squamous cell carcinomas release certain lipids that activate TRPV1 and/or TRPA1 on sensory neurons, contributing to the development of oral cancer pain.

Methods

Lipid extracts were made from conditioned media of three human oral squamous cell carcinoma (OSCC) cell lines as well as one normal human oral keratinocytes cell line. These were then injected intraplantarly into rat hindpaws to measure spontaneous nocifensive behavior, as well as thermal and mechanical allodynia. For interventional experiments, the animals were pretreated with AMG517 (TRPV1 antagonist) or HC030031 (TRPA1 antagonist) prior to extract injection.

Results

These studies demonstrate that lipids released from the three OSCC cell lines, but not the normal cell line, were capable of producing significant spontaneous nocifensive behaviors, as well as thermal and mechanical allodynia. Notably each of the cell lines produced a different magnitude of response for each of three behavioral assays. Importantly, pre-treatment with a TRPVI antagonist blocked lipid-mediated nocifensive and thermal hypersensitivity, but not mechanical hypersensitivity. In addition, pre-treatment with a TRPA1 antagonist only reversed thermal hypersensitivity without affecting lipid-induced nocifensive behavior or mechanical allodynia.

Conclusions

These data reveal a novel mechanism for cancer pain and provide strong direction for future studies evaluating the cellular mechanism regulating the TRP-active lipids by OSCC tumors.

Increased expression of TRPV1 in the trigeminal ganglion is involved in orofacial pain during experimental tooth movement in rats

To investigate whether transient receptor potential vanilloid type 1 (TRPV1) is involved in pain induced by experimental tooth movement, experiments were performed in male Sprague-Dawley rats weighing 200-250 g. Directed face-grooming behavior was used to evaluate nocifensive behavior in rats during experimental tooth movement. The distribution of TRPV1 in the trigeminal ganglion (TG) was evaluated by immunohistochemistry, and its expression was detected by western blotting at several time points following the application of various magnitudes of force during tooth movement. Immunohistochemical analysis revealed that TRPV1 was expressed in TG, and its expression was increased after experimental tooth movement. Western blot results also showed that experimental tooth movement led to a statistically significant increase in expression of TRPV1 protein in TG. Meanwhile, the time spent on directed face-grooming peaked on day 1 and thereafter showed a gradual decrease. In addition, both the change in TRPV1 expression in the TG and directed face-grooming behavior were modulated in a force-dependent manner and in concert with initial orthodontic pain responses. Our results reveal that TRPV1 expression is modulated by experimental tooth movement and is involved in tooth-movement pain.