Tooth injury increases expression of the cold sensitive TRP channel TRPA1 in trigeminal neurons

Spatiotemporal dynamics of brain function during the natural course in a dental pulp injury model

Purpose

Toothache, a common disorder afflicting most people, shows distinct features at different clinical stages. This study aimed to depict metabolic changes in brain and investigate the potential mechanism involved in the aberrant affective behaviors during the natural process of toothache.

Methods

We investigated the spatiotemporal patterns of brain function during the natural course of toothache in a rat model of dental pulp injury (DPI) by using positron emission tomography (PET).

Results

Glucose metabolism peaked on the 3rd day and gradually decreased in several brain regions after DPI, which was in line with the behavioral and histological results. PET imaging showed that visual pathway was involved in the regulation of toothache. Meanwhile, the process of emotional regulation underlying toothache was mediated by N-methyl-D-aspartic receptor subunit 2B (NR2B) in the caudal anterior cingulate cortex (cACC).

Conclusion

Our results revealed the spatiotemporal neurofunctional patterns during toothache process and preliminarily elucidated the role of NR2B in cACC in the regulation of toothache-related affective behaviors.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05764-2.

TRPA1 triggers hyperalgesia and inflammation after tooth bleaching

Hyperalgesia has become a major problem restricting the clinical application of tooth bleaching. We hypothesized that transient receptor potential ankyrin 1 (TRPA1), a pain conduction tunnel, plays a role in tooth hyperalgesia and inflammation after bleaching. Dental pulp stem cells were seeded on the dentin side of the disc, which was cut from the premolar buccal tissue, with 15% (90 min) or 40% (3 × 15 min) bleaching gel applied on the enamel side, and treated with or without a TRPA1 inhibitor. The bleaching gel stimulated intracellular reactive oxygen species, Ca²⁺, ATP, and extracellular ATP in a dose-dependent manner, and increased the mRNA and protein levels of hyperalgesia (TRPA1 and PANX1) and inflammation (TNFα and IL6) factors. This increment was adversely affected by TRPA1 inhibitor. In animal study, the protein levels of TRPA1 (P = 0.0006), PANX1 (P < 0.0001), and proliferation factors [PCNA (P < 0.0001) and Caspase 3 (P = 0.0066)] increased significantly after treated rat incisors with 15% and 40% bleaching gels as detected by immunohistochemistry. These results show that TRPA1 plays a critical role in sensitivity and inflammation after tooth bleaching, providing a solid foundation for further research on reducing the complications of tooth bleaching.

Satellite glial cells in sensory ganglia express functional transient receptor potential ankyrin 1 that is sensitized in neuropathic and inflammatory pain

Transient receptor potential ankyrin 1 (TRPA1) is well documented as an important molecule in pain hypersensitivity following inflammation and nerve injury and in many other cellular biological processes. Here, we show that TRPA1 is expressed not only by sensory neurons of the dorsal root ganglia (DRG) but also in their adjacent satellite glial cells (SGCs), as well as nonmyelinating Schwann cells. TRPA1 immunoreactivity is also detected in various cutaneous structures of sensory neuronal terminals, including small and large caliber cutaneous sensory fibers and endings. The SGC-expressed TRPA1 is functional. Like DRG neurons, dissociated SGCs exhibit a robust response to the TRPA1-selective agonist allyl isothiocyanate (AITC) by an increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i). These responses are abolished by the TRPA1 antagonist HC030031 and are absent in SGCs and neurons from global TRPA1 null mice. SGCs and neurons harvested from DRG proximal to painful tissue inflammation induced by plantar injection of complete Freund’s adjuvant show greater AITC-evoked elevation of [Ca²⁺]_i and slower recovery compared to sham controls. Similar TRPA1 sensitization occurs in both SGCs and neurons during neuropathic pain induced by spared nerve injury. Together, these results show that functional TRPA1 is expressed by sensory ganglia SGCs, and TRPA1 function in SGCs is enhanced after both peripheral inflammation and nerve injury, and suggest that TRPA1 in SGCs may contribute to inflammatory and neuropathic pain.

The combined use of systemic analgesic/anti-inflammatory drugs and a bioactive topical desensitizer for reduced in-office bleaching sensitivity without jeopardizing the hydrogen peroxide efficacy: a randomized, triple blinded, split-mouth clinical trial

OBJECTIVE

To evaluate the effect of combined systemic administration of paracetamol 500 mg/codeine phosphate 30 mg (PACO) and postoperative topical application of a bioactive desensitizer on in-office bleaching sensitivity and tooth color change.

MATERIALS AND METHODS

A randomized, triple-blind, split-mouth clinical trial was conducted. Forty volunteers ingested PACO (n = 20) or placebo (PLA) (n = 20). Their left/right hemiarches received topical application of a bioactive desensitizer [Nano-P™(NP)] and prophylactic paste (PAS), generating four treatment approaches: PACO/NP, PACO/PAS, PLA/NP, and PLA/PAS. Two bleaching sessions (35% hydrogen peroxide) were performed, and the PAS/NP were applied after the procedure. Sensitivity was obtained since the first bleaching session up to 7 days post-bleaching. The color change was evaluated using CIEDE2000 and whiteness index parameters up to 7 days post-bleaching. Data were analyzed using one- and two-way ANOVA/Tukey post hoc tests (p < 0.05).

RESULTS

The PLA/PAS showed a sensitivity average of at least two times higher than the PACO/NP. The treatment approaches promoted statistically similar bleaching patterns (p > 0.05).

CONCLUSION

The combined approach of systemic administration of PACO and postoperative topical application of NP reduced the level of in-office bleaching sensitivity without jeopardizing hydrogen peroxide efficacy.

CLINICAL RELEVANCE

Professionals can adopt the combined approach of systemic administration of analgesic/anti-inflammatory drugs and topical application of a bioactive desensitizer for decreased bleaching sensitivity caused by 35% hydrogen peroxide in-office.

Role of TRPA1 in Tissue Damage and Kidney Disease

TRPA1, a nonselective cation channel, is expressed in sensory afferent that innervates peripheral targets. Neuronal TRPA1 can promote tissue repair, remove harmful stimuli and induce protective responses via the release of neuropeptides after the activation of the channel by chemical, exogenous, or endogenous irritants in the injured tissue. However, chronic inflammation after repeated noxious stimuli may result in the development of several diseases. In addition to sensory neurons, TRPA1, activated by inflammatory agents from some non-neuronal cells in the injured area or disease, might promote or protect disease progression. Therefore, TRPA1 works as a molecular sentinel of tissue damage or as an inflammation gatekeeper. Most kidney damage cases are associated with inflammation. In this review, we summarised the role of TRPA1 in neurogenic or non-neurogenic inflammation and in kidney disease, especially the non-neuronal TRPA1. In in vivo animal studies, TRPA1 prevented sepsis-induced or Ang-II-induced and ischemia-reperfusion renal injury by maintaining mitochondrial haemostasis or via the downregulation of macrophage-mediated inflammation, respectively. Renal tubular epithelial TRPA1 acts as an oxidative stress sensor to mediate hypoxia-reoxygenation injury in vitro and ischaemia-reperfusion-induced kidney injury in vivo through MAPKs/NF-kB signalling. Acute kidney injury (AKI) patients with high renal tubular TRPA1 expression had low complete renal function recovery. In renal disease, TPRA1 plays different roles in different cell types accordingly. These findings depict the important role of TRPA1 and warrant further investigation.

Transient Receptor Potential (TRP) Ion Channels in Orofacial Pain

Orofacial pain, including temporomandibular joint disorders pain, trigeminal neuralgia, dental pain, and debilitating headaches, affects millions of Americans each year with significant population health impact. Despite the existence of a large body of information on the subject, the molecular underpinnings of orofacial pain remain elusive. Two decades of research has identified that transient receptor potential (TRP) ion channels play a crucial role in pathological pain. A number of TRP ion channels are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. Although there are many similarities, the orofacial sensory system shows some distinct peripheral and central pain processing and different sensitivities from the spinal sensory system. Relative to the extensive review on TRPs in spinally-mediated pain, the summary of TRPs in trigeminally-mediated pain has not been well-documented. This review focuses on the current experimental evidence involving TRP ion channels, particularly TRPV1, TRPA1, TRPV4, and TRPM8 in orofacial pain, and discusses their possible cellular and molecular mechanisms.

Sanguinarine is an agonist of TRPA1 channel

Sanguinarine, a benzyl isoquinoline alkaloid extracted from the root of Papaveraceae plants, shows extensive pharmacological activities including anti-microbial, anti-trypanosoma, anti-tumor, anti-platelet, anti-hypertensive effects, as well as inhibition of osteoclast formation. Here we demonstrate that TRPA1 channel (Transient receptor potential cation channel, member A1) is a potential target for sanguinarine. Electrophysiological recordings show that sanguinarine activates TRPA1 channel potently with an EC50 0.09 (0.04-0.13) μM, but has no effects on other examined TRP channels. Sanguinarine increases the intracellular calcium levels and upregulates the excitability of mouse dorsal root ganglion (DRG) neurons in vitro significantly. Plantar injection of sanguinarine evokes nociceptive behaviors similar to that elicited by allyl isothiocyanate (AITC), a classic agonist of TRPA1. Both the enhancement of excitability of DRG neurons and the nociceptive behaviors can be attenuated by treatment of TRPA1 channel antagonist HC030031 or knockout of trpa1 gene. Taken together, our data demonstrate that sanguinarine is a potent and relatively selective agonist of TRPA1 channel.

The functions of mechanosensitive ion channels in tooth and bone tissues

Tooth and bone are independent tissues with a close relationship. Both are composed of a highly calcified outer structure and soft inner tissue, and both are constantly under mechanical stress. In particular, the alveolar bone and tooth constitute an occlusion system and suffer from masticatory and occlusal force. Thus, mechanotransduction is a key process in many developmental, physiological and pathological processes in tooth and bone. Mechanosensitive ion channels such as Piezo1 and Piezo2 are important participants in mechanotransduction, but their functions in tooth and bone are poorly understood. This review summarizes our current understanding of mechanosensitive ion channels and their roles in tooth and bone tissues. Research in these areas may shed new light on the regulation of tooth and bone tissues and potential treatments for diseases affecting these tissues.

Evoked and spontaneous pain assessment during tooth pulp injury

Injury of the tooth pulp is excruciatingly painful and yet the receptors and neural circuit mechanisms that transmit this form of pain remain poorly defined in both the clinic and preclinical rodent models. Easily quantifiable behavioral assessment in the mouse orofacial area remains a major bottleneck in uncovering molecular mechanisms that govern inflammatory pain in the tooth. In this study we sought to address this problem using the Mouse Grimace Scale and a novel approach to the application of mechanical Von Frey hair stimuli. We use a dental pulp injury model that exposes the pulp to the outside environment, a procedure we have previously shown produces inflammation. Using RNAscope technology, we demonstrate an upregulation of genes that contribute to the pain state in the trigeminal ganglia of injured mice. We found that mice with dental pulp injury have greater Mouse Grimace Scores than sham within 24 hours of injury, suggestive of spontaneous pain. We developed a scoring system of mouse refusal to determine thresholds for mechanical stimulation of the face with Von Frey filaments. This method revealed that mice with a unilateral dental injury develop bilateral mechanical allodynia that is delayed relative to the onset of spontaneous pain. This work demonstrates that tooth pain can be quantified in freely behaving mice using approaches common for other types of pain assessment. Harnessing these assays in the orofacial area during gene manipulation should assist in uncovering mechanisms for tooth pulp inflammatory pain and other forms of trigeminal pain.

The effect of oral anti-inflammatory drugs on reducing tooth sensitivity due to in-office dental bleaching: A systematic review and meta-analysis

BACKGROUND

The authors' aim was to evaluate the effectiveness of anti-inflammatory drugs on tooth sensitivity (TS) during and after in-office dental bleaching procedures.

TYPES OF STUDIES REVIEWED

The authors selected randomized controlled trials in which the investigators compared anti-inflammatory drugs with a placebo to evaluate in-office dental bleaching TS. The authors performed an electronic search by using PubMed, ScienceDirect, and Embase. In addition, the authors searched other Web sites, such as ClinicalTrials.gov, to identify ongoing studies.

RESULTS

The authors included 7 randomized controlled trials (324 adults) in the review. According to the extracted data, the authors performed the meta-analysis by using risk ratios and their 95% confidence intervals or by using the mean difference with a 95% confidence interval. The authors used the Cochrane Collaboration's tool to assess study quality. After the evaluation, the authors considered 6 studies to be high quality and a single study to be low quality. The overall results of the evaluation process revealed the absence of a clinically significant effect of anti-inflammatory drugs.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

The results of this analytic process indicated that anti-inflammatory drugs have no clinically significant effect on the TS that occurs due to in-office bleaching. Readers must analyze these results carefully given the limitations of this review, such as the small samples size and the heterogeneity among the studies in some stages of the evaluation process. The results of this analytical study highlight the need for more clinical studies to reach a significant conclusion because TS is one of the most important reasons for the cessation of bleaching treatment.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Pathological Mechanisms and Therapeutic Targets for Trigeminal Neuropathic Pain

Trigeminal neuropathic pain is a chronic pain condition caused by damage or inflammation of the trigeminal nerve or its branches, with both peripheral and central nervous system dysfunction contributing to the disorder. Trigeminal pain conditions present with diagnostic and therapeutic challenges to healthcare providers and often require multiple therapeutic approaches for pain reduction. This review will provide the overview of pathophysiology in peripheral and central nociceptive circuits that are involved in neuropathic pain conditions involving the trigeminal nerve and the current therapeutics that are used to treat these disorders. Recent advances in treatment of trigeminal pain, including novel therapeutics that target ion channels and receptors, gene therapy and monoclonal antibodies that have shown great promise in preclinical studies and clinical trials will also be described.

Ion Channels Involved in Tooth Pain

The tooth has an unusual sensory system that converts external stimuli predominantly into pain, yet its sensory afferents in teeth demonstrate cytochemical properties of non-nociceptive neurons. This review summarizes the recent knowledge underlying this paradoxical nociception, with a focus on the ion channels involved in tooth pain. The expression of temperature-sensitive ion channels has been extensively investigated because thermal stimulation often evokes tooth pain. However, temperature-sensitive ion channels cannot explain the sudden intense tooth pain evoked by innocuous temperatures or light air puffs, leading to the hydrodynamic theory emphasizing the microfluidic movement within the dentinal tubules for detection by mechanosensitive ion channels. Several mechanosensitive ion channels expressed in dental sensory systems have been suggested as key players in the hydrodynamic theory, and TRPM7, which is abundant in the odontoblasts, and recently discovered PIEZO receptors are promising candidates. Several ligand-gated ion channels and voltage-gated ion channels expressed in dental primary afferent neurons have been discussed in relation to their potential contribution to tooth pain. In addition, in recent years, there has been growing interest in the potential sensory role of odontoblasts; thus, the expression of ion channels in odontoblasts and their potential relation to tooth pain is also reviewed.

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.

Botulinum toxin: A review of the mode of action in migraine

Botulinum toxin serotype A (BoNT/A) was originally used in neurology for the treatment of dystonia and blepharospasms, but is now clinically used worldwide for the treatment of chronic migraine. Still, the possible mode of action of BoNT/A in migraine is not fully known. However, the mode of action of BoNT/A has been investigated in experimental pain as well as migraine models, which may elucidate the underlying mechanisms in migraine. The aim of this study was to review studies on the possible mode of action of BoNT/A in relation to chronic migraine treatment. Observations suggest that the mode of action of BoNT/A may not be limited to the injection site, but also includes anatomically connected sites due to axonal transport. The mechanisms behind the effect of BoNT/A in chronic migraine may also include modulation of neurotransmitter release, changes in surface expression of receptors and cytokines as well as enhancement of opioidergic transmission. Clinical and experimental studies with botulinum toxin in the last decade have advanced our understanding of headache and other pain states. More research into botulinum toxin as treatment for headache is warranted as it can be an attractive alternative for patients who do not respond positively to other drugs.

Nocifensive Behaviors in Mice with Radiation-Induced Oral Mucositis

Oral mucositis can result in significant dysphagia, and is the most common dose-limiting acute toxicity in head and neck cancer patients receiving chemoradiotherapy. There is a critical need to determine the cellular and molecular mechanisms that underlie radiotherapy-associated discomfort in patients with mucositis. The objective was to induce oral mucositis in mice, using a clinical linear accelerator, and to quantify resultant discomfort, and characterize peripheral sensitization. A clinical linear accelerator was used to deliver ionizing radiation to the oral cavity of mice. Mucositis severity scoring, and various behavioral assays were performed to quantify bouts of orofacial wiping and scratching, bite force, gnawing behavior and burrowing activity. Calcium imaging was performed on neurons of the trigeminal ganglia. Glossitis was induced with a single fraction of at least 27 Gy. Body weight decreased and subsequently returned to baseline, in concert with development and resolution of mucositis, which was worst at day 10 and 11 postirradiation, however was resolved within another 10 days. Neither bite force, nor gnawing behavior were measurably affected. However, burrowing activity was decreased, and both facial wiping and scratching were increased while mice had visible mucositis lesions. Sensory nerves of irradiated mice were more responsive to histamine, tumor necrosis factor alpha and capsaicin. Radiation-induced glossitis is associated with hyper-reactivity of sensory neurons in the trigeminal ganglia of mice, and is accompanied by several behaviors indicative of both itch and pain. These data validate an appropriate model for cancer treatment related discomfort in humans.

TRPA1 receptor is upregulated in human oral lichen planus

OBJECTIVE

Oral lichen planus (OLP) is a chronic inflammatory disease of unknown etiology with antigen-specific and non-specific mechanisms. Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel activated by noxious stimuli such as oxidative stress products evoking pain and release of proinflammatory mediators from sensory nerve endings culminating in neurogenic inflammation. Extraneuronal TRPA1s, for example, on immune cells possess yet unknown functions.

SUBJECTS AND METHODS

We studied the buccal mRNA expression (qPCR) and protein localization (immunohistochemistry) of TRPA1 receptors and key OLP mediator transcripts in oral mucosa samples of healthy volunteers (n = 9), OLP patients (n = 43), and OLP-like hyperkeratotic patients (n = 12).

RESULTS

We measured 27.7- and 25.5-fold TRPA1 mRNA increase in OLP and OLP-like hyperkeratotic patients compared to healthy controls. TRPA1 transcripts elevated 2.4-fold in hypertensive OLP but not in hyperkeratotic patients compared to counterparts, reduced by 1.6-fold by angiotensin-convertase inhibitor intake. TRPA1 messenger RNA was more coexpressed with transcripts of tumor necrosis factor α than with interferon γ. Keratinocytes, macrophages but not T cells expressed TRPA1.

CONCLUSIONS

We provided evidence for the extraneuronal presence and upregulation of the proinflammatory TRPA1 receptor in buccal samples of patients with OLP. This may implicate the ion channel in the pathomechanism of OLP.

Nociceptive TRP Channels: Sensory Detectors and Transducers in Multiple Pain Pathologies

Specialized receptors belonging to the transient receptor potential (TRP) family of ligand-gated ion channels constitute the critical detectors and transducers of pain-causing stimuli. Nociceptive TRP channels are predominantly expressed by distinct subsets of sensory neurons of the peripheral nervous system. Several of these TRP channels are also expressed in neurons of the central nervous system, and in non-neuronal cells that communicate with sensory nerves. Nociceptive TRPs are activated by specific physico-chemical stimuli to provide the excitatory trigger in neurons. In addition, decades of research has identified a large number of immune and neuromodulators as mediators of nociceptive TRP channel activation during injury, inflammatory and other pathological conditions. These findings have led to aggressive targeting of TRP channels for the development of new-generation analgesics. This review summarizes the complex activation and/or modulation of nociceptive TRP channels under pathophysiological conditions, and how these changes underlie acute and chronic pain conditions. Furthermore, development of small-molecule antagonists for several TRP channels as analgesics, and the positive and negative outcomes of these drugs in clinical trials are discussed. Understanding the diverse functional and modulatory properties of nociceptive TRP channels is critical to function-based drug targeting for the development of evidence-based and efficacious new generation analgesics.

How Far Have We Come in the Field of Nerve Regeneration After Trigeminal Nerve Injury?

Patients suffering from nerve injury with sensory disturbances or orofacial pain have greatly reduced quality of life, and it is a big cost for the society. Abnormal sensations caused by trigeminal nerve injury often become chronic, severely debilitating, and extremely difficult to treat. In general, non-invasive treatment such as drug treatment has been insufficient, and there are currently few available effective treatments. Surgical interventions such as end-to-end connection or nerve grafting have disadvantages such as donor site morbidity or formation of neuroma. There is need for optimizing the technique for nerve repair, especially for the trigeminal nerve system, which has so far not yet been well explored. Recently, tissue engineering using biodegradable synthetic material and cell-based therapies represents a promising approach to nerve repair and it has been reported that mesenchymal stem cell (MSC) has an anti-inflammatory effect and seems to play an important role in nerve healing and regeneration.

Group III mGluR8 negatively modulates TRPA1

Several lines of evidence indicate group III metabotropic glutamate receptors (mGluRs) have systemic anti-hyperalgesic effects. We hypothesized this could occur through modulation of TRP channels on nociceptors. This study used a multifaceted approach to examine the interaction between group III mGluRs (mGluR8) and transient receptor potential ankyrin 1 (TRPA1) on cutaneous nociceptors in rats. Ca²⁺ imaging studies demonstrated co-localization and functional coupling of TRPA1 and mGluR8, since 1μM (S)-3,4-dicarboxyphenylglycine (DCPG) (mGluR8 agonist) significantly reduced Ca²⁺ mobilization produced by 30μM mustard oil (MO), a TRPA1 agonist. Behavioral studies demonstrated that 10mM MO produced mechanical hypersensitivity when topically applied to the hind paw, significantly decreasing paw withdrawal threshold (PWT) from 15g to 6g. However, administration of 30μM DCPG prior to 10mM MO reversed this hypersensitivity such that PWT was not significantly different from baseline. At the single-fiber level, compared to vehicle, 30μM MO significantly increased nociceptor activity and decreased mechanical threshold. However, 30μM DCPG reversed both of these MO-induced effects. Furthermore, DCPG significantly reduced the number of MO-induced mechanically sensitive fibers. Inhibition of protein kinase A (PKA) using Rp-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (RpCAMPS) (PKA inhibitor, 1 and 10μM) significantly reduced MO-induced Ca²⁺ mobilization. Taken together, these results show that group III mGluRs negatively modulate TRPA1 activity on cutaneous nociceptors. Furthermore, it is likely that this modulation occurs intracellularly at the level of the cAMP/PKA pathway. This study demonstrates that group III agonists may be effective in the treatment of mechanical hypersensitivity which can develop as a result of inflammation, nerve injury, chemotherapy and other disease states.

Transient receptor potential ankyrin 1 (TRPA1) antagonists

The transient receptor potential ankyrin 1 (TRPA1) channel is an irritant sensor highly expressed on nociceptive neurons. The clinical use of TRPA1 antagonists is based on the concept that TRPA1 is active during disease states like neuropathic pain. Indeed, in Phase 2a proof-of-concept studies the TRPA1 antagonist GRC17536 has shown efficacy in patients with painful diabetic neuropathy. Moreover, animal studies suggest that the therapeutic value of TRPA1 antagonists extends beyond pain to pruritus, asthma and cough with limited safety concerns. This review provides a comprehensive overview of the patent literature (since 2007) on small-molecule inhibitors of the TRPA1 channel. Despite the clear progress, many unanswered questions remain. Future advancement to Phase 3 studies will assess the real translational potential of this research field.

La(3+) Alters the Response Properties of Neurons in the Mouse Primary Somatosensory Cortex to Low-Temperature Noxious Stimulation of the Dental Pulp

Although dental pain is a serious health issue with high incidence among the human population, its cellular and molecular mechanisms are still unclear. Transient receptor potential (TRP) channels are assumed to be involved in the generation of dental pain. However, most of the studies were conducted with molecular biological or histological methods. In vivo functional studies on the role of TRP channels in the mechanisms of dental pain are lacking. This study uses in vivo cellular electrophysiological and neuropharmacological method to directly disclose the effect of LaCl₃, a broad spectrum TRP channel blocker, on the response properties of neurons in the mouse primary somatosensory cortex to low-temperature noxious stimulation of the dental pulp. It was found that LaCl₃ suppresses the high-firing-rate responses of all nociceptive neurons to noxious low-temperature stimulation and also inhibits the spontaneous activities in some nonnociceptive neurons. The effect of LaCl₃ is reversible. Furthermore, this effect is persistent and stable unless LaCl₃ is washed out. Washout of LaCl₃ quickly revitalized the responsiveness of neurons to low-temperature noxious stimulation. This study adds direct evidence for the hypothesis that TRP channels are involved in the generation of dental pain and sensation. Blockade of TRP channels may provide a novel therapeutic treatment for dental pain.

Temporal dynamics of anxiety phenotypes in a dental pulp injury model

Background

Accumulating clinical and preclinical evidence indicates that chronic pain is often comorbid with persistent low mood and anxiety. However, the mechanisms underlying pain-induced anxiety, such as its causality, temporal progression, and relevant neural networks are poorly understood, impeding the development of efficacious therapeutic approaches.

Results

Here, we have identified the sequential emergence of anxiety phenotypes in mice subjected to dental pulp injury (DPI), a prototypical model of orofacial pain that correlates with human toothache. Compared with sham controls, mice subjected to DPI by mechanically exposing the pulp to the oral environment exhibited significant signs of anxiogenic effects, specifically, altered behaviors on the elevated plus maze (EPM), novelty-suppressed feeding (NSF) tests at 1 but not 3 days after the surgery. Notably, at 7 and 14 days, the DPI mice again avoided the open arm, center area, and novelty environment in the EPM, open field, and NSF tests, respectively. In particular, DPI-induced social phobia and increased repetitive grooming did not occur until 14 days after surgery, suggesting that DPI-induced social anxiety requires a long time. Moreover, oral administration of an anti-inflammatory drug, ibuprofen, or an analgesic agent, ProTx-II, which is a selective inhibitor of Na_V1.7 sodium channels, both significantly alleviated DPI-induced avoidance in mice. Finally, to investigate the underlying central mechanisms, we pharmacologically blocked a popular form of synaptic plasticity with a GluA2-derived peptide, long-term depression, as that treatment significantly prevented the development of anxiety phenotype upon DPI.

Conclusions

Together, these results suggest a temporally progressive causal relationship between orofacial pain and anxiety, calling for more in-depth mechanistic studies on concomitant pain and anxiety disorders.

Sensory TRP channels: the key transducers of nociception and pain

Peripheral detection of nociceptive and painful stimuli by sensory neurons involves a complex repertoire of molecular detectors and/or transducers on distinct subsets of nerve fibers. The majority of such molecular detectors/transducers belong to the transient receptor potential (TRP) family of cation channels, which comprise both specific receptors for distinct nociceptive stimuli, as well as for multiple stimuli. This chapter discusses the classification, distribution, and functional properties of individual TRP channel types that have been implicated in various nociceptive and/or painful conditions.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

Activation of transient receptor potential ankyrin 1 by eugenol

Eugenol is a bioactive plant extract used as an analgesic agent in dentistry. The structural similarity of eugenol to cinnamaldehyde, an active ligand for transient receptor potential ankyrin 1 (TRPA1), suggests that eugenol might produce its effect via TRPA1, in addition to TRPV1 as we reported previously. In this study, we investigated the effect of eugenol on TRPA1, by fura-2-based calcium imaging and patch clamp recording in trigeminal ganglion neurons and in a heterologous expression system. As the result, eugenol induced robust calcium responses in rat trigeminal ganglion neurons that responded to a specific TRPA1 agonist, allyl isothiocyanate (AITC), and not to capsaicin. Capsazepine, a TRPV1 antagonist failed to inhibit eugenol-induced calcium responses in AITC-responding neurons. In addition, eugenol response was observed in trigeminal ganglion neurons from TRPV1 knockout mice and human embryonic kidney 293 cell lines that express human TRPA1, which was inhibited by TRPA1-specific antagonist HC-030031. Eugenol-evoked TRPA1 single channel activity and eugenol-induced TRPA1 currents were dose-dependent with EC50 of 261.5μM. In summary, these results demonstrate that the activation of TRPA1 might account for another molecular mechanism underlying the pharmacological action of eugenol.

Bimodal voltage dependence of TRPA1: mutations of a key pore helix residue reveal strong intrinsic voltage-dependent inactivation

Transient receptor potential A1 (TRPA1) is implicated in somatosensory processing and pathological pain sensation. Although not strictly voltage-gated, ionic currents of TRPA1 typically rectify outwardly, indicating channel activation at depolarized membrane potentials. However, some reports also showed TRPA1 inactivation at high positive potentials, implicating voltage-dependent inactivation. Here we report a conserved leucine residue, L906, in the putative pore helix, which strongly impacts the voltage dependency of TRPA1. Mutation of the leucine to cysteine (L906C) converted the channel from outward to inward rectification independent of divalent cations and irrespective to stimulation by allyl isothiocyanate. The mutant, but not the wild-type channel, displayed exclusively voltage-dependent inactivation at positive potentials. The L906C mutation also exhibited reduced sensitivity to inhibition by TRPA1 blockers, HC030031 and ruthenium red. Further mutagenesis of the leucine to all natural amino acids individually revealed that most substitutions at L906 (15/19) resulted in inward rectification, with exceptions of three amino acids that dramatically reduced channel activity and one, methionine, which mimicked the wild-type channel. Our data are plausibly explained by a bimodal gating model involving both voltage-dependent activation and inactivation of TRPA1. We propose that the key pore helix residue, L906, plays an essential role in responding to the voltage-dependent gating.

Cellular and molecular mechanisms of dental nociception

Due, in part, to the unique structure of the tooth, dental pain is initiated via distinct mechanisms. Here we review recent advances in our understanding of inflammatory tooth pain and discuss 3 hypotheses proposed to explain dentinal hypersensitivity: The first hypothesis, supported by functional expression of temperature-sensitive transient receptor potential channels, emphasizes the direct transduction of noxious temperatures by dental primary afferent neurons. The second hypothesis, known as hydrodynamic theory, attributes dental pain to fluid movement within dentinal tubules, and we discuss several candidate cellular mechanical transducers for the detection of fluid movement. The third hypothesis focuses on the potential sensory function of odontoblasts in the detection of thermal or mechanical stimuli, and we discuss the accumulating evidence that supports their excitability. We also briefly update on a novel strategy for local nociceptive anesthesia via nociceptive transducer molecules in dental primary afferents with the potential to specifically silence pain fibers during dental treatment. Further understanding of the molecular mechanisms of dental pain would greatly enhance the development of therapeutics that target dental pain.