NGF up-regulates TRPA1: implications for orofacial pain

Inflammation-induced mast cell-derived nerve growth factor: a key player in chronic vulvar pain?

Provoked vulvodynia (PV) is characterized by localized chronic vulvar pain. It is associated with a history of recurrent inflammation, mast cell (MC) accumulation and neuronal sprouting in the vulva. However, the mechanism of how vulvar-inflammation promotes neuronal sprouting and gene-expression adaptation in the spinal cord, leading to hypersensitivity and painful sensations, is unknown. Here, we found that vulvar tissue from women with PV (n = 8) is characterized by MC accumulation and neuronal sprouting compared to women without PV (n = 4). In addition, we observed these changes in an animal study of PV. Thus, we found that repeated vulvar zymosan-inflammation challenges lead to long-lasting mechanical and thermal vulvar hypersensitivity, which is mediated by MC accumulation, neuronal sprouting, overexpression of the pain channels (TRPV1 and TRPA1) in vulvar neurons, as well as a long-term increase of gene expression related to neuroplasticity, neuroinflammation and nerve growth factor (NGF) in the spinal cord/dorsal root ganglia (DRG) (L6-S3). However, regulation of the NGF pathway by stabilization of MC activity with ketotifen fumarate (KF) during vulvar inflammation attenuates the local increase of NGF and histamine, as well as the elevated transcription of pro-inflammatory cytokines and NGF pathway in the spinal cord. Additionally, KF treatment during inflammation modulates MC accumulation, neuronal hyperinnervation and overexpression of the TRPV1 and TRPA1 channels in the vulvar neurons, consequently preventing the development of vulvar pain. A thorough examination of the NGF pathway during inflammation revealed that blocking NGF activity by using an NGF-non-peptide-inhibitor (Ro08-2750) regulates the upregulation of genes related to neuroplasticity and the NGF pathway in the spinal cord, as well as modulating neuronal sprouting and overexpression of the pain channels, resulting in a reduced level of vulvar hypersensitivity. On the other hand, stimulation of the NGF pathway in the vulvar promotes neuronal sprouting, overexpression of pain channels and increase of gene expression related to neuroplasticity, neuroinflammation and NGF in the spinal cord, resulting in long-lasting vulvar hypersensitivity. In conclusion, our findings suggest that vulvar allodynia induced by inflammation is mediated by MC accumulation, neuronal sprouting and neuromodulation in the vulvar. Additionally, chronic vulvar pain may involve a long-term adaptation in gene expression in the spinal cord, which probably plays a critical role in central sensitization and pain maintenance. Strikingly, regulating the NGF pathway during the critical period of inflammation prevents vulvar pain development via modulating the neuronal changes in the vestibule and spinal cord, suggesting a fundamental role for the NGF pathway in PV development.

Recurrent infections drive persistent bladder dysfunction and pain via sensory nerve sprouting and mast cell activity

Urinary tract infections (UTIs) account for almost 25% of infections in women. Many are recurrent (rUTI), with patients frequently experiencing chronic pelvic pain and urinary frequency despite clearance of bacteriuria after antibiotics. To elucidate the basis for these bacteria-independent bladder symptoms, we examined the bladders of patients with rUTI. We noticed a notable increase in neuropeptide content in the lamina propria and indications of enhanced nociceptive activity. In mice subjected to rUTI, we observed sensory nerve sprouting that was associated with nerve growth factor (NGF) produced by recruited monocytes and tissue-resident mast cells. Treatment of rUTI mice with an NGF-neutralizing antibody prevented sprouting and alleviated pelvic sensitivity, whereas instillation of native NGF into naïve mice bladders mimicked nerve sprouting and pain behavior. Nerve activation, pain, and urinary frequency were each linked to the presence of proximal mast cells, because mast cell deficiency or treatment with antagonists against receptors of several direct or indirect mast cell products was each effective therapeutically. Thus, our findings suggest that NGF-driven sensory sprouting in the bladder coupled with chronic mast cell activation represents an underlying mechanism driving bacteria-independent pain and voiding defects experienced by patients with rUTI.

Can cannabidiol have an analgesic effect?

BACKGROUND

Cannabis, more commonly known as marijuana or hemp, has been used for centuries to treat various conditions. Cannabis contains two main components cannabidiol (CBD) and tetrahydrocannabinol (THC). CBD, unlike THC, is devoid of psychoactive effects and is well tolerated by the human body but has no direct effect on the receptors of the endocannabid system, despite the lack of action on the receptors of the endocannabid system.

OBJECTIVES AND METHODS

We have prepared a literature review based on the latest available literature regarding the analgesic effects of CBD. CBD has a wide range of effects on the human body. In this study, we will present the potential mechanisms responsible for the analgesic effect of CBD. To the best of our knowledge, this is the first review to explore the analgesic mechanisms of CBD.

RESULTS AND CONCLUSION

The analgesic effect of CBD is complex and still being researched. CBD models the perception of pain by acting on G protein-coupled receptors. Another group of receptors that CBD acts on are serotonergic receptors. The effect of CBD on an enzyme of potential importance in the production of inflammatory factors such as cyclooxygenases and lipoxygenases has also been confirmed. The presented potential mechanisms of CBD's analgesic effect are currently being extensively studied.

Chemotherapy for pain: reversing inflammatory and neuropathic pain with the anticancer agent mithramycin A

ABSTRACT

The persistence of inflammatory and neuropathic pain is poorly understood. We investigated a novel therapeutic paradigm by targeting gene networks that sustain or reverse persistent pain states. Our prior observations found that Sp1-like transcription factors drive the expression of TRPV1, a pain receptor, that is blocked in vitro by mithramycin A (MTM), an inhibitor of Sp1-like factors. Here, we investigate the ability of MTM to reverse in vivo models of inflammatory and chemotherapy-induced peripheral neuropathy (CIPN) pain and explore MTM's underlying mechanisms. Mithramycin reversed inflammatory heat hyperalgesia induced by complete Freund adjuvant and cisplatin-induced heat and mechanical hypersensitivity. In addition, MTM reversed both short-term and long-term (1 month) oxaliplatin-induced mechanical and cold hypersensitivity, without the rescue of intraepidermal nerve fiber loss. Mithramycin reversed oxaliplatin-induced cold hypersensitivity and oxaliplatin-induced TRPM8 overexpression in dorsal root ganglion (DRG). Evidence across multiple transcriptomic profiling approaches suggest that MTM reverses inflammatory and neuropathic pain through broad transcriptional and alternative splicing regulatory actions. Mithramycin-dependent changes in gene expression following oxaliplatin treatment were largely opposite to and rarely overlapped with changes in gene expression induced by oxaliplatin alone. Notably, RNAseq analysis revealed MTM rescue of oxaliplatin-induced dysregulation of mitochondrial electron transport chain genes that correlated with in vivo reversal of excess reactive oxygen species in DRG neurons. This finding suggests that the mechanism(s) driving persistent pain states such as CIPN are not fixed but are sustained by ongoing modifiable transcription-dependent processes.

Peripheral Neuroinflammation and Pain: How Acute Pain Becomes Chronic

The number of individuals suffering from severe chronic pain and its social and financial impact is staggering. Without significant advances in our understanding of how acute pain becomes chronic, effective treatments will remain out of reach. This mini review will briefly summarize how critical signaling pathways initiated during the early phases of peripheral nervous system inflammation/ neuroinflammation establish long-term modifications of sensory neuronal function. Together with the recruitment of non-neuronal cellular elements, nociceptive transduction is transformed into a pathophysiologic state sustaining chronic peripheral sensitization and pain. Inflammatory mediators, such as nerve growth factor (NGF), can lower activation thresholds of sensory neurons through posttranslational modification of the pain-transducing ion channels transient-receptor potential TRPV1 and TRPA1. Performing a dual role, NGF also drives increased expression of TRPV1 in sensory neurons through the recruitment of transcription factor Sp4. More broadly, Sp4 appears to modulate a nociceptive transcriptome including TRPA1 and other genes encoding components of pain transduction. Together, these findings suggest a model where acute pain evoked by peripheral injury-induced inflammation becomes persistent through repeated cycles of TRP channel modification, Sp4-dependent overexpression of TRP channels and ongoing production of inflammatory mediators.

A Novel Flp Reporter Mouse Shows That TRPA1 Expression Is Largely Limited to Sensory Neuron Subsets

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that is activated by electrophilic irritants, oxidative stress, cold temperature, and GPCR signaling. TRPA1 expression has been primarily identified in subsets of nociceptive sensory afferents and is considered a target for future analgesics. Nevertheless, TRPA1 has been implicated in other cell types including keratinocytes, epithelium, enterochromaffin cells, endothelium, astrocytes, and CNS neurons. Here, we developed a knock-in mouse that expresses the recombinase FlpO in TRPA1-expressing cells. We crossed the TRPA1Flp mouse with the R26ai65f mouse that expresses tdTomato in a Flp-sensitive manner. We found tdTomato expression correlated well with TRPA1 mRNA expression and sensitivity to TRPA1 agonists in subsets of TRPV1 (transient receptor potential vanilloid receptor type 1)-expressing neurons in the vagal ganglia and dorsal root ganglia (DRGs), although tdTomato expression efficiency was limited in DRG. We observed tdTomato-expressing afferent fibers centrally (in the medulla and spinal cord) and peripherally in the esophagus, gut, airways, bladder, and skin. Furthermore, chemogenetic activation of TRPA1-expressing nerves in the paw evoked flinching behavior. tdTomato expression was very limited in other cell types. We found tdTomato in subepithelial cells in the gut mucosa but not in enterochromaffin cells. tdTomato was also observed in supporting cells within the cochlea, but not in hair cells. Lastly, tdTomato was occasionally observed in neurons in the somatomotor cortex and the piriform area, but not in astrocytes or vascular endothelium. Thus, this novel mouse strain may be useful for mapping and manipulating TRPA1-expressing cells and deciphering the role of TRPA1 in physiological and pathophysiological processes.

Role of Neurotrophins in Orofacial Pain Modulation: A Review of the Latest Discoveries

Orofacial pain represents a multidisciplinary biomedical challenge involving basic and clinical research for which no satisfactory solution has been found. In this regard, trigeminal pain is described as one of the worst pains perceived, leaving the patient with no hope for the future. The aim of this review is to evaluate the latest discoveries on the involvement of neurotrophins in orofacial nociception, describing their role and expression in peripheral tissues, trigeminal ganglion, and trigeminal nucleus considering their double nature as "supporters" of the nervous system and as "promoters" of nociceptive transmission. In order to scan recent literature (last ten years), three independent researchers referred to databases PubMed, Embase, Google Scholar, Scopus, and Web of Science to find original research articles and clinical trials. The researchers selected 33 papers: 29 original research articles and 4 clinical trials. The results obtained by the screening of the selected articles show an interesting trend, in which the precise modulation of neurotrophin signaling could switch neurotrophins from being a "promoter" of pain to their beneficial neurotrophic role of supporting the nerves in their recovery, especially when a structural alteration is present, as in neuropathic pain. In conclusion, neurotrophins could be interesting targets for orofacial pain modulation but more studies are necessary to clarify their role for future application in clinical practice.

Cannabis Pharmacogenomics: A Path to Personalized Medicine

Cannabis and related compounds have created significant research interest as a promising therapy in many disorders. However, the individual therapeutic effects of cannabinoids and the incidence of side effects are still difficult to determine. Pharmacogenomics may provide the answers to many questions and concerns regarding the cannabis/cannabinoid treatment and help us to understand the variability in individual responses and associated risks. Pharmacogenomics research has made meaningful progress in identifying genetic variations that play a critical role in interpatient variability in response to cannabis. This review classifies the current knowledge of pharmacogenomics associated with medical marijuana and related compounds and can assist in improving the outcomes of cannabinoid therapy and to minimize the adverse effects of cannabis use. Specific examples of pharmacogenomics informing pharmacotherapy as a path to personalized medicine are discussed.

Human TrkAR649W mutation impairs nociception, sweating and cognitive abilities: a mouse model of HSAN IV

A functional nerve growth factor NGF-Tropomyosin Receptor kinase A (TrkA) system is an essential requisite for the generation and maintenance of long-lasting thermal and mechanical hyperalgesia in adult mammals. Indeed, mutations in the gene encoding for TrkA are responsible for a rare condition, named Hereditary Sensory and Autonomic Neuropathy type IV (HSAN IV), characterized by the loss of response to noxious stimuli, anhidrosis and cognitive impairment. However, to date, there is no available mouse model to properly understand how the NGF-TrkA system can lead to pathological phenotypes that are distinctive of HSAN IV. Here, we report the generation of a knock-in mouse line carrying the HSAN IV TrkAR649W mutation. First, by in vitro biochemical and biophysical analyses, we show that the pathological R649W mutation leads to kinase-inactive TrkA also affecting its membrane dynamics and trafficking. In agreement with the HSAN IV human phenotype, TrkAR649W/m mice display a lower response to thermal and chemical noxious stimuli, correlating with reduced skin innervation, in addition to decreased sweating in comparison to TrkAh/m controls. Moreover, the R649W mutation decreases anxiety-like behavior and compromises cognitive abilities, by impairing spatial-working and social memory. Our results further uncover unexplored roles of TrkA in thermoregulation and sociability. In addition to accurately recapitulating the clinical manifestations of HSAN IV patients, our findings contribute to clarifying the involvement of the NGF-TrkA system in pain sensation.

Pharmacological Potential of cis-jasmone in Adult Zebrafish (Danio rerio)

This study evaluates the pharmacological potential of cis-jasmone (CJ) in adult zebrafish (Danio rerio; aZF). Initially, aZF (n = 6/group) were pretreated (20 µL; p. o.) with CJ (0.1 or 0.3 or 1.0 mg/mL) or vehicle (0.5% Tween 80). The animals were submitted to acute toxicity and locomotion tests, pentylenetetrazole-induced seizure, carrageenan-induced abdominal edema, and cinnamaldehyde-, capsaicin-, menthol-, glutamate-, and acid saline-induced orofacial nociception. The possible mechanisms of anticonvulsant, anxiolytic, and antinociceptive action were evaluated. The involvement of central afferent fibers sensitive to cinnamaldehyde and capsaicin and the effect of CJ on the relative gene expression of TRPA1 and TRPV1 in the brain of aZF were also analyzed, in addition to the study of molecular docking between CJ and TRPA1, TRPV1 channels, and GABA_A receptors. CJ did not alter the locomotor behavior and showed pharmacological potential in all tested models with no toxicity. The anticonvulsant effect of CJ was prevented by flumazenil (GABAergic antagonist). The anxiolytic-like effect of CJ was prevented by flumazenil and serotonergic antagonists. The antinociceptive effect was prevented by TRPA1 and TRPV1 antagonists. Chemical ablation with capsaicin and cinnamaldehyde prevented the orofacial antinociceptive effect of CJ. Molecular docking studies indicate that CJ interacted with TRPA1, TRPV1, and GABA_A receptors. CJ inhibited the relative gene expression of TRPA1 and TRPV1. CJ has pharmacological potential for the treatment of seizures, anxiety, inflammation, and acute orofacial nociception. These effects are obtained by modulating the GABAergic and serotonergic systems, as well as the TRPs and ASIC channels.

Modulation of Glia Activation by TRPA1 Antagonism in Preclinical Models of Migraine

Preclinical data point to the contribution of transient receptor potential ankyrin 1 (TRPA1) channels to the complex mechanisms underlying migraine pain. TRPA1 channels are expressed in primary sensory neurons, as well as in glial cells, and they can be activated/sensitized by inflammatory mediators. The aim of this study was to investigate the relationship between TRPA1 channels and glial activation in the modulation of trigeminal hyperalgesia in preclinical models of migraine based on acute and chronic nitroglycerin challenges. Rats were treated with ADM_12 (TRPA1 antagonist) and then underwent an orofacial formalin test to assess trigeminal hyperalgesia. mRNA levels of pro- and anti-inflammatory cytokines, calcitonin gene-related peptide (CGRP) and glia cell activation were evaluated in the Medulla oblongata and in the trigeminal ganglia. In the nitroglycerin-treated rats, ADM_12 showed an antihyperalgesic effect in both acute and chronic models, and it counteracted the changes in CGRP and cytokine gene expression. In the acute nitroglycerin model, ADM_12 reduced nitroglycerin-induced increase in microglial and astroglial activation in trigeminal nucleus caudalis area. In the chronic model, we detected a nitroglycerin-induced activation of satellite glial cells in the trigeminal ganglia that was inhibited by ADM_12. These findings show that TRPA1 antagonism reverts experimentally induced hyperalgesia in acute and chronic models of migraine and prevents multiple changes in inflammatory pathways by modulating glial activation.

Lipoyl-Based Antagonists of Transient Receptor Potential Cation A (TRPA1) Downregulate Osteosarcoma Cell Migration and Expression of Pro-Inflammatory Cytokines

Osteosarcoma is a heterogeneous tumor intimately linked to its microenvironment, which promotes its growth and spread. It is generally accompanied by cancer-induced bone pain (CIBP), whose main component is neuropathic pain. The TRPA1 ion channel plays a key role in metastasis and is increasingly expressed in bone cancer. Here, a novel TRPA1 inhibitor is described and tested together with two other known TRPA1 antagonists. The novel lipoyl derivative has been successfully assessed for its ability to reduce human osteosarcoma MG-63 cell viability, motility, and gene expression of the CIBP pro-inflammatory cytokines interleukin 6 (IL-6) and tumor necrosis factor α (TNF-α). A putative three-dimensional (3D) model of the inhibitor covalently bound to TRPA1 is also proposed. The in vitro data suggest that the novel inhibitor described here may be highly interesting and stimulating for new strategies to treat osteosarcomas.

Antagonism of CGRP Receptor: Central and Peripheral Mechanisms and Mediators in an Animal Model of Chronic Migraine

Calcitonin-gene-related peptide (CGRP) plays a key role in migraine pathophysiology and more specifically in the mechanisms underlying peripheral and central sensitization. Here, we explored the interaction of CGRP with other pain mediators relevant for neuronal sensitization in an animal model of chronic migraine. Male Sprague-Dawley rats were exposed to nitroglycerin (NTG, 5 mg/kg, i.p.) or vehicle co-administered with the CGRP receptor antagonist olcegepant (2 mg/kg i.p.), or its vehicle, every other day over a 9-day period. Twenty-four hours after the last injection of NTG (or vehicle), behavioral test and ex vivo analysis were performed. Olcegepant attenuated NTG-induced trigeminal hyperalgesia in the second phase of the orofacial formalin test. Interestingly, it also reduced gene expression and protein levels of CGRP, pro-inflammatory cytokines, inflammatory-associated miRNAs (miR-155-5p, miR-382-5p, and miR-34a-5p), and transient receptor potential ankyrin channels in the medulla-pons area, cervical spinal cord, and trigeminal ganglia. Similarly, olcegepant reduced the NTG-induced increase in CGRP and inflammatory cytokines in serum. The findings show that the activation of the CGRP pathway in a migraine animal model was associated to the persistent activation of inflammatory pathways, which was paralleled by a condition of hyperalgesia. These molecular events are relevant for informing us about the mechanisms underlying chronic migraine.

Transient receptor potential Ankyrin 1: structure, function and ligands

Introduction: Transient receptor potential ankyrin 1 (TRPA1) is a protein expressed in many living organisms. During the study of TRPA1, its unique biological role as a universal and polymodal sensor of various altering agents was found. The aim of this study is to search and generalize information about structural features and molecular determinants, mechanisms of activation, action and modulation of TRPA1 as a universal pain and inflammation sensor, as well as the nature of activators and antagonists of this target and their therapeutic potential.

Materials and methods: This article presents an overview of the results of scientific research of TRPA1, its modulators, as well as an overview of their pharmacological potential over the period from the discovery of these channels to the present, with an emphasis on the last decade.

Results and discussion: The main collected data on expression, structural features and molecular determinants, mechanisms of activation and action of TRPA1 indicate its role as a universal and labile element of the primary response of the body to adverse exogenous and endogenous factors. Regardless of the nature of the stimulus, hyperstimulation of TRPA1 channels can lead to such phenomena as pain, inflammation, itching, edema and other manifestations of alteration, and therefore TRPA1 blockade can be used in the treatment of various diseases accompanied by these pathological conditions. Currently, TRPA1 antagonists are being actively searched for and studied, as evidenced by a high patent activity over the past 14 years; however, the molecular mechanisms of action and pharmacological properties of TRPA1 blockers remain understudied.

Conclusion: Acquire of new information about TRPA1 will help in the development of its modulators, which can become promising analgesics, anti-inflammatory drugs, bronchodilators, and agents for the treatment of cardiovascular diseases of new generations.

Characterization of Early Inflammatory Events Leading to Provoked Vulvodynia Development in Rats

Background

Provoked vulvodynia (PV) is the main cause of vulvar pain and dyspareunia. The etiology of PV has not yet been elucidated. However, PV is associated with a history of recurrent inflammation, and its often accompanied by increases in the numbers of mast cells (MCs) and sensory hyperinnervation in the vulva. Therefore, this study aimed to examine the role of MCs and the early inflammatory events in the development of chronic vulvar pain in a rat model of PV.

Methods

Mechanical and thermal vulvar sensitivity was measured for 5 months following zymosan vulvar challenges. Vulvar changes in glutamate and nerve growth factor (NGF) were analyzed using ELISA. Immunofluorescence (IF) staining of the vulvar section after 20, 81, and 160 days of the zymosan challenge were performed to test MCs accumulation, hyperinnervation, and expression of pain channels (transient receptor potential vanilloid/ankyrin-1-TRPV1 & TRPA1) in vulvar neurons. Changes in the development of vulvar pain were evaluated following the administration of the MCs stabilizer ketotifen fumarate (KF) during zymosan vulvar challenges.

Results

Zymosan-challenged rats developed significant mechanical and thermal vulvar sensitivity that persisted for over 160 days after the zymosan challenge. During inflammation, increased local concentrations of NGF and glutamate and a robust increase in MCs degranulation were observed in zymosan-challenged rats. In addition, zymosan-challenged rats displayed sensory hyperinnervation and an increase in the expression of TRPV1 and TRPA1. Treatment with KF attenuated the upregulated level of NGF during inflammation, modulated the neuronal modifications, reduced MCs accumulation, and enhanced mechanical hypersensitivity after repeated inflammation challenges.

Conclusion

The present findings suggest that vulvar hypersensitivity is mediated by MCs accumulation, nerve growth, and neuromodulation of TRPV1 and TRPA1. Hence, KF treatment during the critical period of inflammation contributes to preventing chronic vulvar pain development.

Evaluating the association of TRPA1 gene polymorphisms with pain sensitivity: a protocol for an adaptive recall by genotype study

BACKGROUND

Pain is a complex polygenic trait whose common genetic underpinnings are relatively ill-defined due in part to challenges in measuring pain as a phenotype. Pain sensitivity can be quantified, but this is difficult to perform at the scale required for genome wide association studies (GWAS). Existing GWAS of pain have identified surprisingly few loci involved in nociceptor function which contrasts strongly with rare monogenic pain states. This suggests a lack of resolution with current techniques. We propose an adaptive methodology within a recall-by-genotype (RbG) framework using detailed phenotyping to screen minor alleles in a candidate 'nociceptor' gene in an attempt to estimate their genetic contribution to pain.

METHODS/DESIGN

Participants of the Avon Longitudinal Study of Parents and Children will be recalled on the basis of genotype at five common non-synonomous SNPs in the 'nociceptor' gene transient receptor potential ankylin 1 (TRPA1). Those homozygous for the common alleles at each of the five SNPs will represent a control group. Individuals homozygous for the minor alleles will then be recruited in a series of three sequential test groups. The outcome of a pre-planned early assessment (interim) of the current test group will determine whether to continue recruitment or switch to the next test group. Pain sensitivity will be assessed using quantitative sensory testing (QST) before and after topical application of 10% cinnamaldehyde (a TRPA1 agonist).

DISCUSSION

The design of this adaptive RbG study offers efficiency in the assessment of associations between genetic variation at TRPA1 and detailed pain phenotypes. The possibility to change the test group in response to preliminary data increases the likelihood to observe smaller effect sizes relative to a conventional multi-armed design, as well as reducing futile testing of participants where an effect is unlikely to be observed. This specific adaptive RbG design aims to uncover the influence of common TRPA1 variants on pain sensation but can be applied to any hypothesis-led genotype study where costly and time intensive investigation is required and / or where there is large uncertainty around the expected effect size.

TRIAL REGISTRATION

ISRCTN, ISRCTN16294731. Retrospectively registered 25th November 2021.

TRP Channels as Molecular Targets to Relieve Cancer Pain

Transient receptor potential (TRP) channels are critical receptors in the transduction of nociceptive stimuli. The microenvironment of diverse types of cancer releases substances, including growth factors, neurotransmitters, and inflammatory mediators, which modulate the activity of TRPs through the regulation of intracellular signaling pathways. The modulation of TRP channels is associated with the peripheral sensitization observed in patients with cancer, which results in mild noxious sensory stimuli being perceived as hyperalgesia and allodynia. Secondary metabolites derived from plant extracts can induce the activation, blocking, and desensitization of TRP channels. Thus, these compounds could act as potential therapeutic agents, as their antinociceptive properties could be beneficial in relieving cancer-derived pain. In this review, we will summarize the role of TRPV1 and TRPA1 in pain associated with cancer and discuss molecules that have been reported to modulate these channels, focusing particularly on the mechanisms of channel activation associated with molecules released in the tumor microenvironment.

The Role of TRPA1 Channels in the Central Processing of Odours Contributing to the Behavioural Responses of Mice

Transient receptor potential ankyrin 1 (TRPA1), a nonselective cation channel, contributes to several (patho)physiological processes. Smell loss is an early sign in several neurodegenerative disorders, such as multiple sclerosis, Parkinson’s and Alzheimer’s diseases; therefore, we focused on its role in olfaction and social behaviour with the aim to reveal its potential therapeutic use. The presence of Trpa1 mRNA was studied along the olfactory tract of mice by combined RNAscope in situ hybridisation and immunohistochemistry. The aversive effects of fox and cat odour were examined in parallel with stress hormone levels. In vitro calcium imaging was applied to test if these substances can directly activate TRPA1 receptors. The role of TRPA1 in social behaviour was investigated by comparing Trpa1 wild-type and knockout mice (KO). Trpa1 mRNA was detected in the olfactory bulb and piriform cortex, while its expression was weak in the olfactory epithelium. Fox, but not cat odour directly activated TRPA1 channels in TRPA1-overexpressing Chinese Hamster Ovary cell lines. Accordingly, KO animals showed less aversion against fox, but not cat odour. The social interest of KO mice was reduced during social habituation–dishabituation and social interaction, but not during resident–intruder tests. TRPA1 may contribute to odour processing at several points of the olfactory tract and may play an important role in shaping the social behaviour of mice. Thus, TRPA1 may influence the development of certain social disorders, serving as a potential drug target in the future.

Therapeutic effects of nerve growth factor-targeting therapy on bladder overactivity in rats with prostatic inflammation

BACKGROUND

The present study examined the effect of liposomes conjugated with antisense oligonucleotide of nerve growth factor (NGF-OND) on local overexpression of NGF and bladder overactivity using rats with prostatic inflammation (PI).

METHODS

Male Sprague-Dawley rats were divided into three groups: (1) Control group; intact rats, (2) PI-NS group; rats with PI and intravesical instillation of normal saline (NS), (3) PI-OND group; rats with PI and intravesical instillation of NGF-OND. On Day 0, PI was induced by intraprostatic 5%-formalin injection. On Day 14, NGF-OND or NS was instilled directly into the bladder after laparotomy. On Day 28, therapeutic effects of NGF-OND were evaluated by awake cystometry and histological analysis as well as reverse-transcription polymerase chain reaction measurements of messenger RNA (mRNA) levels of NGF in the bladder and prostate, inflammatory markers in the prostate, C-fiber afferent markers, and an A-type K+ channel α-subunit (Kv 1.4) in L6-S1 dorsal root ganglia (DRG).

RESULTS

Intravesical NFG-OND treatment reduced PI-induced overexpression of NGF in both bladder and prostate, and reduced PI-induced bladder overactivity evident as longer intercontraction intervals in association with reductions of TRPV1 and TRPA1 mRNA expression levels in DRG. mRNA expression of Kv1.4 in DRG was reduced after PI, but improved in the PI-OND group.

CONCLUSIONS

These results indicate that NGF locally expressed in the bladder is an important mediator inducing bladder overactivity with upregulation of C-fiber afferent markers and downregulation of an A-type K+ channel subunit in DRG following PI, and that liposome-based, local NGF-targeting therapy could be effective for not only bladder overactivity and afferent sensitization, but also PI. Thus, local blockade of NGF in the bladder could be a therapeutic modality for male LUTS due to BPH with PI.

Pharmacological Management of Acute Endodontic Pain

Pain associated with infections of the tooth pulp and periapical tissues is intense and often the most common reason for patients seeking emergency dental care. Effective management of acute dental pain requires a deep understanding of pain mechanisms, which enables accurate diagnosis and definitive treatment. While drugs are only used as an adjunct to definitive dental treatment, a thorough understanding of their mechanism of action and effectiveness enables clinicians to effectively control intra-operative and post-operative pain and prevent persistent pain. This review describes how pain is detected, processed, and perceived. It also provides information on evidence-based strategies on the use of different classes of drugs to effectively manage endodontic pain.

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.

Selective targeting of peripheral cannabinoid receptors prevents behavioral symptoms and sensitization of trigeminal neurons in mouse models of migraine and medication overuse headache

ABSTRACT

Migraine affects ∼15% of the world's population greatly diminishing their quality of life. Current preventative treatments are effective in only a subset of migraine patients, and although cannabinoids seem beneficial in alleviating migraine symptoms, central nervous system side effects limit their widespread use. We developed peripherally restricted cannabinoids (PRCBs) that relieve chronic pain symptoms of cancer and neuropathies, without appreciable central nervous system side effects or tolerance development. Here, we determined PRCB effectiveness in alleviating hypersensitivity symptoms in mouse models of migraine and medication overuse headache. Long-term glyceryl trinitrate (GTN, 10 mg/kg) administration led to increased sensitivity to mechanical stimuli and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment, but not posttreatment, prevented behavioral and biochemical correlates of GTN-induced sensitization. Low pH-activated and allyl isothiocyanate-activated currents in acutely isolated trigeminal neurons were reversibly attenuated by PRCB application. Long-term GTN treatment significantly enhanced these currents. Long-term sumatriptan treatment also led to the development of allodynia to mechanical and cold stimuli that was slowly reversible after sumatriptan discontinuation. Subsequent challenge with a previously ineffective low-dose GTN (0.1-0.3 mg/kg) revealed latent behavioral sensitization and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment prevented all behavioral and biochemical correlates of allodynia and latent sensitization. Importantly, long-term PRCB treatment alone did not produce any behavioral or biochemical signs of sensitization. These data validate peripheral cannabinoid receptors as potential therapeutic targets in migraine and medication overuse headache.

Current Concepts of Dentinal Hypersensitivity

INTRODUCTION

Although many clinical studies have reported on the prevalence of dental pain, far fewer studies have focused on the mechanisms of dental pain. This is an important gap because increased understanding of dental pain mechanisms may lead to improved diagnostic tests or therapeutic interventions. The aim of this study was to comprehensively review the literature on the mechanisms of dentinal sensitivity.

METHODS

PubMed and Ovid were searched for articles that addressed dentinal pain and or pulpal sensitivity. Because of the breadth of research ranging from cellular/molecular studies to clinical trials, a narrative review on the mechanisms of dentinal sensitivity was constructed based on the literature.

RESULTS

Five various mechanisms for dentinal sensitivity have been proposed: (1) the classic hydrodynamic theory, (2) direct innervation of dentinal tubules, (3) neuroplasticity and sensitization of nociceptors, (4) odontoblasts serving as sensory receptors, and (5) algoneurons.

CONCLUSIONS

These theories are not mutually exclusive, and it is possible that several of them contribute to dentinal sensitivity. Moreover, pulpal responses to tissue injury may alter the relative contribution of these mechanisms. For example, pulpal inflammation may lead to neuronal sprouting and peripheral sensitization. Knowledge of these mechanisms may prompt the development of therapeutic drugs that aim to disrupt these mechanisms, leading to more effective treatments for pulpal pain.

Common and discrete mechanisms underlying chronic pain and itch: peripheral and central sensitization

Normally, an obvious antagonism exists between pain and itch. In normal conditions, painful stimuli suppress itch sensation, whereas pain killers often generate itch. Although pain and itch are mediated by separate pathways under normal conditions, most chemicals are not highly specific to one sensation in chronic pathologic conditions. Notably, in patients with neuropathic pain, histamine primarily induces pain rather than itch, while in patients with atopic dermatitis, bradykinin triggers itch rather than pain. Accordingly, repetitive scratching even enhances itch sensation in chronic itch conditions. Physicians often prescribe pain relievers to patients with chronic itch, suggesting common mechanisms underlying chronic pain and itch, especially peripheral and central sensitization. Rather than separating itch and pain, studies should investigate chronic itch and pain including neuropathic and inflammatory conditions. Here, we reviewed chronic sensitization leading to chronic pain and itch at both peripheral and central levels. Studies investigating the connection between pain and itch facilitate the development of new therapeutics against both chronic dysesthesias based on the underlying pathophysiology.

Pacific-Ciguatoxin-2 and Brevetoxin-1 Induce the Sensitization of Sensory Receptors Mediating Pain and Pruritus in Sensory Neurons

Ciguatera fish poisoning (CFP) and neurotoxic shellfish poisoning syndromes are induced by the consumption of seafood contaminated by ciguatoxins and brevetoxins. Both toxins cause sensory symptoms such as paresthesia, cold dysesthesia and painful disorders. An intense pruritus, which may become chronic, occurs also in CFP. No curative treatment is available and the pathophysiology is not fully elucidated. Here we conducted single-cell calcium video-imaging experiments in sensory neurons from newborn rats to study in vitro the ability of Pacific-ciguatoxin-2 (P-CTX-2) and brevetoxin-1 (PbTx-1) to sensitize receptors and ion channels, (i.e., to increase the percentage of responding cells and/or the response amplitude to their pharmacological agonists). In addition, we studied the neurotrophin release in sensory neurons co-cultured with keratinocytes after exposure to P-CTX-2. Our results show that P-CTX-2 induced the sensitization of TRPA1, TRPV4, PAR2, MrgprC, MrgprA and TTX-r NaV channels in sensory neurons. P-CTX-2 increased the release of nerve growth factor and brain-derived neurotrophic factor in the co-culture supernatant, suggesting that those neurotrophins could contribute to the sensitization of the aforementioned receptors and channels. Our results suggest the potential role of sensitization of sensory receptors/ion channels in the induction or persistence of sensory disturbances in CFP syndrome.

FAM19A5l Affects Mustard Oil-Induced Peripheral Nociception in Zebrafish

Family with sequence similarity 19 (chemokine (C-C motif)-like) member A5 (FAM19A5) is a chemokine-like secretory protein recently identified as involved in the regulation of osteoclast formation, post-injury neointima formation, and depression. Although roles for FAM19A5 have been described in nervous system development and psychiatric disorders, its role in the nervous system remains poorly understood. Here, we analyzed the evolutionary history of FAM19A genes in vertebrates and identified FAM19A5l, a paralogous zebrafish gene originating from a common ancestral FAM19A5 gene. Further, zebrafish FAM19A5l is expressed in trigeminal and dorsal root ganglion neurons as well as distinct neuronal subsets of the central nervous system. Interestingly, FAM19A5l⁺ trigeminal neurons are nociceptive neurons that localized with TRPA1b and TRPV1 and respond to mustard oil treatment. Behavioral analysis further revealed that the nociceptive response to mustard oil decreases in FAM19A5l-knockout zebrafish larvae. In addition, TRPA1b and NGFa mRNA levels are down- and upregulated in FAM19A5l-knockout and -overexpressing transgenic zebrafish, respectively. Together, our data suggest that FAM19A5l plays a role in nociceptive responses to mustard oil by regulating TRPA1b and NGFa expression in zebrafish.

Nerve Growth Factor Enhances Tooth Mechanical Hyperalgesia Through C-C Chemokine Ligand 19 in Rats

The nerve growth factor (NGF) plays an important role in the regulation of neuropathic pain. It has been demonstrated that calcitonin gene-related peptide (CGRP), a well-known contributor to neurogenic inflammation, increases neuroinflammatory pain induced by NGF. The inflammatory mediator that NGF most strongly induces is C-C chemokine ligand 19 (CCL19), which can recruit inflammatory cells by binding to the receptor CCR7 followed by promoting the response of neuroinflammation. However, the regulatory mechanism of NGF and CCL19 in tooth movement orofacial pain and the interaction between both are still unclear. In this study, male Sprague–Dawley rats were used to study the modulation of NGF on orofacial pain through CCL19 and the role of each in tooth movement pain in rats. The expression levels of CCL19 mRNA and protein were determined by real-time PCR and immunofluorescence, respectively. Pain levels were assessed by measuring the rats' bite force, which drops as pain rises. Meanwhile, by verifying the relationship between CGRP and CCL19, it was laterally confirmed that NGF could modulate tooth movement-induced mechanical hyperalgesia through CCL19. The results showed that the expression level of CCL19 rose with the increased NGF, and neurons expressing CGRP can express stronger CCL19. Compared with the baseline level, the bite force for all rats dropped sharply on day 1, reached its lowest level on day 3, and recovered gradually on day 5. All results indicated that NGF played an important role in tooth movement orofacial pain via positively regulating CCL19 expression in the trigeminal ganglia of rats. Additionally, CCL19 increased the sensitivity to experimental tooth movement orofacial pain. NGF can regulate CCL19 expression, although it may regulate other inflammatory pathways as well. This is the first report on the interactions and modulations of tooth movement orofacial pain by NGF through CCL19 in rats.

TRP channels in airway sensory nerves

Transient Receptor Potential (TRP) channels expressed in specific subsets of airway sensory nerves function as transducers and integrators of a diverse range of sensory inputs including chemical, mechanical and thermal signals. These TRP sensors can detect inhaled irritants as well as endogenously released chemical substances. They play an important role in generating the afferent activity carried by these sensory nerves and regulating the centrally mediated pulmonary defense reflexes. Increasing evidence reported in recent investigations has revealed important involvements of several TRP channels (TRPA1, TRPV1, TRPV4 and TRPM8) in the manifestation of various symptoms and pathogenesis of certain acute and chronic airway diseases. This mini-review focuses primarily on these recent findings of the responses of these TRP sensors to the biological stresses emerging under the pathophysiological conditions of the lung and airways.

Neurotrophic effects of dental pulp stem cells on trigeminal neuronal cells

Evidence indicates that dental pulp stem cells (DPSC) secrete neurotrophic factors which play an important role in neurogenesis, neural maintenance and repair. In this study we investigated the trophic potential of DPSC-derived conditioned medium (CM) to protect and regenerate isolated primary trigeminal ganglion neuronal cells (TGNC). DPSC and TGNC were harvested by enzymatic digestion from Wister-Hann rats. CM was collected from 72 h serum-free DPSC cultures and neurotrophic factors; nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and glial cell line-derived neurotrophic factor (GDNF) were analysed by specific enzyme-linked immunosorbent assays (ELISAs). Primary co-cultures of DPSC and TGNC were established to evaluate the paracrine effects of DPSC. In comparison, NGF was used to evaluate its neurotrophic and neuritogenic effect on TGNC. Immunocytochemistry was performed to detect the neuronal-markers; neuronal nuclei (NeuN), microtubule-associated protein-2 (MAP-2) and βIII-tubulin. Quantitative real time polymerase chain reaction (qRT-PCR) was used to analyse neuronal-associated gene expression of NeuN, MAP-2, βIII-tubulin in addition to growth-associated protein-43 (GAP-43), Synapsin-I and thermo-sensitive transient receptor potential vanilloid channel-1 (TRPV1). DPSC-CM contained significant levels of NGF, BDNF, NT-3 and GDNF. DPSC and DPSC-CM significantly enhanced TGNC survival with extensive neurite outgrowth and branching as evaluated by immunocytochemistry of neuronal markers. DPSC-CM was more effective in stimulating TGNC survival than co-cultures or NGF treated culture. In comparison to controls, DPSC-CM significantly upregulated gene expression of several neuronal markers as well as TRPV1. This study demonstrated that DPSC-derived factors promoted survival and regeneration of isolated TGNC and may be considered as cell-free therapy for TG nerve repair.

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

Roles of tumor necrosis factor-α and interleukin-6 in regulating bone cancer pain via TRPA1 signal pathway and beneficial effects of inhibition of neuro-inflammation and TRPA1

Background

Pain is one of the most common and distressing symptoms suffered by patients with progression of bone cancer; however, the mechanisms responsible for hyperalgesia are not well understood. The purpose of our current study was to determine contributions of the sensory signaling pathways of inflammatory tumor necrosis factor-α and interleukin-6 and downstream transient receptor potential ankyrin 1 (TRPA1) to neuropathic pain induced by bone cancer. We further determined whether influencing these pathways can improve bone cancer pain.

Methods

Breast sarcocarcinoma Walker 256 cells were implanted into the tibia bone cavity of rats to induce mechanical and thermal hyperalgesia. ELISA and western blot analysis were used to examine (1) the levels of tumor necrosis factor-α and interleukin-6 in dorsal root ganglion and (2) protein expression of tumor necrosis factor-α and interleukin-6 receptors (TNFR1 and IL-6R) and TRPA1 as well as intracellular signals (p38-MAPK and JNK).

Results

Tumor necrosis factor-α and interleukin-6 were elevated in the dorsal root ganglion of bone cancer rats, and expression of TNFR1, IL-6R, and TRPA1 was upregulated. In addition, inhibition of TNFR1 and IL-6R alleviated mechanical and thermal hyperalgesia in bone cancer rats, accompanied with downregulated TRPA1 and p38-MAPK and JNK.

Conclusions

We revealed specific signaling pathways leading to neuropathic pain during the development of bone cancer, including tumor necrosis factor-α-TRPA1 and interleukin-6-TRPA1 signal pathways. Overall, our data suggest that blocking these signals is beneficial to alleviate bone cancer pain.

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.

Is TRPA1 Burning Down TRPV1 as Druggable Target for the Treatment of Chronic Pain?

Over the last decades, a great array of molecular mediators have been identified as potential targets for the treatment of chronic pain. Among these mediators, transient receptor potential (TRP) channel superfamily members have been thoroughly studied. Namely, the nonselective cationic channel, transient receptor potential ankyrin subtype 1 (TRPA1), has been described as a chemical nocisensor involved in noxious cold and mechanical sensation and as rivalling TRPV1, which traditionally has been considered as the most important TRP channel involved in nociceptive transduction. However, few TRPA1-related drugs have succeeded in clinical trials. In the present review, we attempt to discuss the latest data on the topic and future directions for pharmacological intervention.

Neurotrophic Proteins in Dentin and Their Effect on Trigeminal Sensory Neurons

INTRODUCTION

A plethora of bioactive molecules present during tooth formation become sequestered in the mineralized dentin matrix and can be released into the pulp tissue after demineralization from carious lesions. However, neurotrophic factors are differentially expressed and secreted during various stages of odontogenesis. Thus, the aims of this study were (1) to investigate their presence and relative abundance in crown and root dentin and (2) to evaluate the bioactivity of dentin-derived proteins on neuronal cells.

METHODS

Dentin matrix proteins (DMPs) were isolated from matched roots and crowns of extracted healthy human third molars. The total protein amount as well as the concentration of growth factors and neurotrophic proteins were quantified. The impact on neuritogenesis was determined with mouse trigeminal neurons in vitro and by a hydrogel implant model in vivo. Transient receptor potential cation channel subfamily V member 1 (TRPV1) sensitization of DMP-conditioned neurons was evaluated by single-cell calcium imaging.

RESULTS

The relative concentration of neurotrophic molecules revealed that nerve growth factor is the most abundant neurotrophin with 3-fold increased expression in radicular dentin. Similarly, brain-derived neurotrophic factor and neurotrophin 3 are more abundant in radicular than coronal dentin. Conversely, glial cell line-derived neurotrophic factor is more abundant in coronal dentin, whereas neurotrophin 4 is equally distributed. Dentin matrix proteins promoted neurite outgrowth in vitro and axonal targeting in vivo, with a greater effect observed by radicular dentin extracts. Furthermore, DMPs sensitized TRPV1 responses in mouse trigeminal neurons with greater activity seen with extracts from root dentin.

CONCLUSIONS

Neurotrophic factors are differentially distributed between coronal and radicular dentin with different effects of dentin-derived proteins on axonal growth and targeting as well as the sensitization of TRPV1. Thus, extracellular proteins from the dentin matrix are likely involved in neurogenic responses to caries and could be exploited in clinical regenerative endodontics to promote reinnervation and enhance tissue regeneration.

Transcription factor Sp4 is required for hyperalgesic state persistence

Understanding how painful hypersensitive states develop and persist beyond the initial hours to days is critically important in the effort to devise strategies to prevent and/or reverse chronic painful states. Changes in nociceptor transcription can alter the abundance of nociceptive signaling elements, resulting in longer-term change in nociceptor phenotype. As a result, sensitized nociceptive signaling can be further amplified and nocifensive behaviors sustained for weeks to months. Building on our previous finding that transcription factor Sp4 positively regulates the expression of the pain transducing channel TRPV1 in Dorsal Root Ganglion (DRG) neurons, we sought to determine if Sp4 serves a broader role in the development and persistence of hypersensitive states in mice. We observed that more than 90% of Sp4 staining DRG neurons were small to medium sized, primarily unmyelinated (NF200 neg) and the majority co-expressed nociceptor markers TRPV1 and/or isolectin B4 (IB4). Genetically modified mice (Sp4+/-) with a 50% reduction of Sp4 showed a reduction in DRG TRPV1 mRNA and neuronal responses to the TRPV1 agonist-capsaicin. Importantly, Sp4+/- mice failed to develop persistent inflammatory thermal hyperalgesia, showing a reversal to control values after 6 hours. Despite a reversal of inflammatory thermal hyperalgesia, there was no difference in CFA-induced hindpaw swelling between CFA Sp4+/- and CFA wild type mice. Similarly, Sp4+/- mice failed to develop persistent mechanical hypersensitivity to hind-paw injection of NGF. Although Sp4+/- mice developed hypersensitivity to traumatic nerve injury, Sp4+/- mice failed to develop persistent cold or mechanical hypersensitivity to the platinum-based chemotherapeutic agent oxaliplatin, a non-traumatic model of neuropathic pain. Overall, Sp4+/- mice displayed a remarkable ability to reverse the development of multiple models of persistent inflammatory and neuropathic hypersensitivity. This suggests that Sp4 functions as a critical control point for a network of genes that conspire in the persistence of painful hypersensitive states.

Suppression of neovascularization in corneal stroma in a TRPA1-null mouse

Corneal neovascularization and inflammatory fibrosis induced by severe injury or infection leads to tissue opacification and even blindness. Transient receptor potential (TRP) channel subtypes contribute to mediating these maladaptive responses through their interactions with other receptors. TRPV1 is one of the contributing channel isoforms inducing neovascularization in an alkali burn mouse wound healing model. VEGF-A upregulation contributes to neovascularization through interaction with its cognate receptors (VEGFR). Since the TRP isoform in this tissue, TRPA1, is also involved, we determined here if one of the pathways mediating neovascularization and immune cell infiltration involve an interaction between VEGFR and TRPA1 in a cauterization corneal mouse wound healing model. Localization of TRPA1 and endothelial cell (EC) CD31 immunostaining pattern intensity determined if TRPA1 expression was EC delimited during cauterization induced angiogenesis. Quantitative RT-PCR evaluated the effects of the absence of TRPA1 function on VEGF-A and TGF-β1 mRNA expression during this process. Macrophage infiltration increased based on rises in F4/80 antigen immunoreactivity. TRPA1 immunostaining was absent on CD31-immunostained EC cells undergoing neovascularization, but it was present on other cell type(s) adhering to EC in vivo. Absence of TRPA1 expression suppressed both stromal neovascularization and inhibited macrophage infiltration. Similarly, the increases occurring in both VEGF-A and TGF-β1 mRNA expression levels in WT tissue were blunted in the TRPA1^-/- counterpart. On the other hand, in the macrophages their levels were invariant and their infiltration was inhibited. To determine if promotion by TRPA1 of angiogenesis was dependent on its expression on other unidentified cell types, the effects were compared of pharmacological manipulation of TRPA1 activity on EC proliferation tube formation and migration. In the presence and absence of a fibroblast containing feeder layer. Neither VEGF-induced increases in human vascular endothelial cell (HUVEC) proliferation nor migration were changed by a TRPA1 antagonist HC-030031 in the absence of a feeder layer. However, on a fibroblast feeder layer this antagonist suppressed HUVEC tube formation. In conclusion, during corneal wound healing transactivation by VEGFR of TRPA1 contributes to mediating neovascularization and macrophage infiltration. Such crosstalk is possible because of close proximity between VEGFR delimited expression on EC and TRPA1 expression restricted to cell types adhering to EC.

Role of intraganglionic transmission in the trigeminovascular pathway

Migraine is triggered by poor air quality and odors through unknown mechanisms. Activation of the trigeminovascular pathway by environmental irritants may occur via activation of transient receptor potential ankyrin 1 (TRPA1) receptors on nasal trigeminal neurons, but how that results in peripheral and central sensitization is unclear. The anatomy of the trigeminal ganglion suggests that noxious nasal stimuli are not being transduced to the meninges by axon reflex but likely through intraganglionic transmission. Consistent with this concept, we injected calcitonin gene-related peptide, adenosine triphosphate, or glutamate receptor antagonists or a gap junction channel blocker directly and exclusively into the trigeminal ganglion and blocked meningeal blood flow changes in response to acute nasal TRP agonists. Previously, we observed chronic sensitization of the trigeminovascular pathway after acrolein exposure, a known TRPA1 receptor agonist. To explore the mechanism of this sensitization, we utilized laser dissection microscopy to separately harvest nasal and meningeal trigeminal neuron populations in the absence or presence of acrolein exposure. mRNA levels of neurotransmitters important in migraine were then determined by reverse transcription polymerase chain reaction. TRPA1 message levels were significantly increased in meningeal cell populations following acrolein exposure compared to room air exposure. This was specific to TRPA1 message in meningeal cell populations as changes were not observed in either nasal trigeminal cell populations or dorsal root ganglion populations. Taken together, these data suggest an important role for intraganglionic transmission in acute activation of the trigeminovascular pathway. It also supports a role for upregulation of TRPA1 receptors in peripheral sensitization and a possible mechanism for chronification of migraine after environmental irritant exposure.

Effects of TRPA1 activation and inhibition on TRPA1 and CGRP expression in dorsal root ganglion neurons

Transient receptor potential ankyrin 1 (TRPA1) is a key player in pain and neurogenic inflammation, and is localized in nociceptive primary sensory dorsal root ganglion (DRG) neurons. TRPA1 plays a major role in the transmission of nociceptive sensory signals. The generation of neurogenic inflammation appears to involve TRPA1-evoked release of calcitonin gene-related peptide (CGRP). However, it remains unknown whether TRPA1 or CGRP expression is affected by TRPA1 activation. Thus, in this study, we examined TRPA1 and CGRP expression in DRG neurons in vitro after treatment with the TRPA1 activator formaldehyde or the TRPA1 blocker menthol. In addition, we examined the role of extracellular signal-regulated protein kinase 1/2 (ERK1/2) in this process. DRG neurons in culture were exposed to formaldehyde, menthol, the ERK1/2 inhibitor PD98059 + formaldehyde, or PD98059 + menthol. After treatment, real-time polymerase chain reaction, western blot assay and double immunofluorescence labeling were performed to evaluate TRPA1 and CGRP expression in DRG neurons. Formaldehyde elevated mRNA and protein levels of TRPA1 and CGRP, as well as the proportion of TRPA1- and CGRP-positive neurons. In contrast, menthol reduced TRPA1 and CGRP expression. Furthermore, the effects of formaldehyde, but not menthol, on CGRP expression were blocked by pretreatment with PD98059. PD98059 pretreatment did not affect TRPA1 expression in the presence of formaldehyde or menthol.

Antagonism of Transient Receptor Potential Ankyrin Type-1 Channels as a Potential Target for the Treatment of Trigeminal Neuropathic Pain: Study in an Animal Model

Transient receptor potential ankyrin type-1 (TRPA1) channels are known to actively participate in different pain conditions, including trigeminal neuropathic pain, whose clinical treatment is still unsatisfactory. The aim of this study was to evaluate the involvement of TRPA1 channels by means of the antagonist ADM_12 in trigeminal neuropathic pain, in order to identify possible therapeutic targets. A single treatment of ADM_12 in rats 4 weeks after the chronic constriction injury of the infraorbital nerve (IoN-CCI) significantly reduced the mechanical allodynia induced in the IoN-CCI rats. Additionally, ADM_12 was able to abolish the increased levels of TRPA1, calcitonin gene-related peptide (CGRP), substance P (SP), and cytokines gene expression in trigeminal ganglia, cervical spinal cord, and medulla induced in the IoN-CCI rats. By contrast, no significant differences between groups were seen as regards CGRP and SP protein expression in the pars caudalis of the spinal nucleus of the trigeminal nerve. ADM_12 also reduced TRP vanilloid type-1 (TRPV1) gene expression in the same areas after IoN-CCI. Our findings show the involvement of both TRPA1 and TRPV1 channels in trigeminal neuropathic pain, and in particular, in trigeminal mechanical allodynia. Furthermore, they provide grounds for the use of ADM_12 in the treatment of trigeminal neuropathic pain.

Molecular basis of TRPA1 regulation in nociceptive neurons. A review

Transient receptor potential A1 (TRPA1) is an excitatory ion channel that functions as a cellular sensor, detecting a wide range of proalgesic agents such as environmental irritants and endogenous products of inflammation and oxidative stress. Topical application of TRPA1 agonists produces an acute nociceptive response through peripheral release of neuropeptides, purines and other transmitters from activated sensory nerve endings. This, in turn, further regulates TRPA1 activity downstream of G-protein and phospholipase C-coupled signaling cascades. Despite the important physiological relevance of such regulation leading to nociceptor sensitization and consequent pain hypersensitivity, the specific domains through which TRPA1 undergoes post-translational modifications that affect its activation properties are yet to be determined at a molecular level. This review aims at providing an account of our current knowledge on molecular basis of regulation by neuronal inflammatory signaling pathways that converge on the TRPA1 channel protein and through modification of its specific residues influence the extent to which this channel may contribute to pain.

The role of the transient receptor potential ankyrin type-1 (TRPA1) channel in migraine pain: evaluation in an animal model

Background

Clinical and experimental studies have pointed to the possible involvement of the transient receptor potential ankyrin type-1 (TRPA1) channels in migraine pain. In this study, we aimed to further investigate the role of these channels in an animal model of migraine using a novel TRPA1 antagonist, ADM_12, as a probe.

Methods

The effects of ADM_12 on nitroglycerin-induced hyperalgesia at the trigeminal level were investigated in male rats using the quantification of nocifensive behavior in the orofacial formalin test. The expression levels of the genes coding for c-Fos, TRPA1, calcitonin gene-related peptide (CGRP) and substance P (SP) in peripheral and central areas relevant for migraine pain were analyzed. CGRP and SP protein immunoreactivity was also evaluated in trigeminal nucleus caudalis (TNC).

Results

In rats bearing nitroglycerin-induced hyperalgesia, ADM_12 showed an anti-hyperalgesic effect in the second phase of the orofacial formalin test. This effect was associated to a significant inhibition of nitroglycerin-induced increase in c-Fos, TRPA1 and neuropeptides mRNA levels in medulla-pons area, in the cervical spinal cord and in the trigeminal ganglion. No differences between groups were seen as regards CGRP and SP protein expression in the TNC.

Conclusions

These findings support a critical involvement of TRPA1 channels in the pathophysiology of migraine, and show their active role in counteracting hyperalgesia at the trigeminal level.

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

Effects of pulpotomy using mineral trioxide aggregate on prostaglandin transporter and receptors in rat molars

Mineral trioxide aggregate (MTA) is a commonly used dental pulp-capping material with known effects in promoting reparative dentinogenesis. However, the mechanism by which MTA induces dentine repair remains unclear. The aim of the present study was to investigate the role of prostaglandin E₂ (PGE₂) in dentine repair by examining the localisation and mRNA expression levels of its transporter (Pgt) and two of its receptors (Ep2 and Ep4) in a rat model of pulpotomy with MTA capping. Ep2 expression was detected in odontoblasts, endothelial cells, and nerve fibres in normal and pulpotomised tissues, whereas Pgt and Ep4 were immunolocalised only in the odontoblasts. Moreover, mRNA expression of Slco2a1 (encoding Pgt), Ptger2 (encoding Ep2), and Ptger4 (encoding Ep4) was significantly upregulated in pulpotomised dental pulp and trigeminal ganglia after MTA capping. Our results provide insights into the functions of PGE₂ via Pgt and Ep receptors in the healing dentine/pulp complex and may be helpful in developing new therapeutic targets for dental disease.

Evaluation of mesenchymal stem cell modulation of trigeminal neuronal responses to cold

Tissue engineering protocols, such as regenerative endodontic procedures (REPs), comprise biologically based procedures designed to restore normal physiologic function. For REPs, the goal is reconstitution of the pulp-dentin complex by delivering mesenchymal stem cells (MSCs), including the stem cells of the apical papilla (SCAP) into a root canal system. Many patients regain cold sensitivity after REPs, but the mechanism is not understood. We hypothesized that SCAP modulate nociceptive function through a paracrine mechanism that activates cold-sensitive ion channels in neurons. We established a co-culture system with human SCAP and rat trigeminal (TG) sensory neurons in order to determine the effect of SCAP co-culture on neuronal responses using whole-cell patch-clamp electrophysiology. TG neurons co-cultured with SCAP demonstrated increased TRPA1-mediated (p<0.01) and TRPM8-mediated inward current densities (p<0.01) at 24h in co-culture. Cold stimulation to SCAP significantly increased ATP release (p<0.01), and supernatant collected after cold stimulation to SCAP was able to activate cultured TG neurons. Co-culture with SCAP significantly increased sustained ATP-evoked inward current density (p<0.05). These data suggest that SCAP release trophic factors that act on afferent neurons to enhance cold-sensitive ion channel activity.

Effect of Intravesical Liposome-Based Nerve Growth Factor Antisense Therapy on Bladder Overactivity and Nociception in a Rat Model of Cystitis Induced by Hydrogen Peroxide

The aim of this study was to evaluate whether liposome-based local suppression of nerve growth factor (NGF) in the bladder has effects on bladder hypersensitivity in a rat cystitis model induced by intravesical instillation of hydrogen peroxide (HP). HP (1.5%) was intravesically administered to adult female Sprague-Dawley rats. Liposomes complexed with NGF antisense oligonucleotide (OND) labeled with TYE563 fluorescent tag were intravesically instilled on day 2. Red fluorescence from the TYE 563 tag was observed with fluorescent microscopy on day 3. Four separate groups of rats were used in the following experiments: (a) sham-liposome group, (b) sham-OND group, (c) cystitis-liposome group, and (d) cystitis-OND group. Saline or 1.5% HP was intravesically administered on day 0. Empty liposomes or liposomes-antisense OND were instilled into the bladder on day 2. The following experiments were conducted to evaluate the effect of NGF antisense treatment on day 7: (a) continuous cystometry was performed in an awake condition; (b) pain behavior induced by instillation of resiniferatoxin into the bladder, including licking behavior (lower abdominal licking) and freezing behavior (motionless head-turning toward lower abdomen), was observed; (c) immunohistochemical staining of the bladder and L6 DRG for NGF was performed; (d) the expression of several genes in the bladder was analyzed by reverse transcription polymerase chain reaction (RT-PCR); and (e) after Fast Blue was injected into the bladder wall, Fast Blue-positive or -negative cells in DRG neurons were separately collected by using a laser-capture microdissection method 7 days later. RT-PCR was performed to evaluate gene expressions in captured neuronal cells. The expression of TYE563 was identified only in the urothelial layer. In cystometric investigation, intercontraction intervals (ICI) were significantly (p = 0.001) shorter in the cystitis-liposome group in comparison to the sham-liposome group. ICI was significantly (p = 0.007) longer in the cystitis-OND group compared to the cystitis-liposome group. Comparisons of the sham-liposome and the sham-OND groups showed no significant difference in ICI (p = 0.56). Licking events did not significantly differ among the four groups. In contrast, the cystitis-liposome group showed significantly more freezing events than the sham-liposome group did (p = 0.002). A significant reduction in the number of freezing events was observed in the cystitis-OND group compared to the cystitis-liposome group (p = 0.04). Immunofluorescence staining demonstrated that NGF expression in the mucosa (p = 0.02) and L6 DRG (p = 0.01) was significantly higher in the cystitis-liposome group than it was in the sham-liposome group. The expression of NGF was significantly lower in the mucosa (p = 0.002) and L6 DRG (p = 0.01) in the cystitis-OND group compared to the cystitis-liposome group. RT-PCR showed that the expression of NGF and TRPV1 mRNA in the mucosa was significantly higher in the cystitis-liposome group than it was in the sham-liposome group (p = 0.001 and 0.03, respectively). On the other hand, these gene expressions were significantly lower in the cystitis-OND group than they were in the cystitis-liposome group (p = 0.007 and 0.02, respectively). The cystitis-liposome group showed significantly higher expression of TRPA1, P2X3, and BDNF mRNA in labeled bladder afferent neurons than the sham-liposome group did (p = 0.03, 0.01, and 0.001, respectively). These gene expressions were significantly lower in the cystitis-OND group compared to the cystitis-liposome group (p = 0.04, 0.006, and 0.03, respectively). The study indicated that intravesical application of liposome-NGF antisense OND significantly improved bladder hypersensitivity induced by chemical cystitis in rats. Intravesical treatment with liposome-OND conjugates could be a novel local therapy of hypersensitive bladder disorders such as bladder pain syndrome/interstitial cystitis.

TRPV1 and TRPA1 in cutaneous neurogenic and chronic inflammation: pro-inflammatory response induced by their activation and their sensitization

Cutaneous neurogenic inflammation (CNI) is inflammation that is induced (or enhanced) in the skin by the release of neuropeptides from sensory nerve endings. Clinical manifestations are mainly sensory and vascular disorders such as pruritus and erythema. Transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) are non-selective cation channels known to specifically participate in pain and CNI. Both TRPV1 and TRPA1 are co-expressed in a large subset of sensory nerves, where they integrate numerous noxious stimuli. It is now clear that the expression of both channels also extends far beyond the sensory nerves in the skin, occuring also in keratinocytes, mast cells, dendritic cells, and endothelial cells. In these non-neuronal cells, TRPV1 and TRPA1 also act as nociceptive sensors and potentiate the inflammatory process. This review discusses the role of TRPV1 and TRPA1 in the modulation of inflammatory genes that leads to or maintains CNI in sensory neurons and non-neuronal skin cells. In addition, this review provides a summary of current research on the intracellular sensitization pathways of both TRP channels by other endogenous inflammatory mediators that promote the self-maintenance of CNI.