Differential TRPV1 and TRPV2 channel expression in dental pulp

Transient receptor potential channels in dental inflammation and pain perception: A comprehensive review

Transient Receptor Potential (TRP) channels are a family of ion channels that play pivotal roles in various physiological processes, including sensory transduction, temperature regulation, and inflammation. In the context of dentistry, recent research has highlighted the involvement of TRP channels in mediating sensory responses and inflammation in dental tissues and temporo-mandibular joint (TMJ) structure. TRP channels have emerged as major contributors in the development of inflammatory conditions and pain affecting the oral cavity and related structures, such as periodontitis, dental erosion cause hypersensitivity, pulpitis, and TMJ disorders. These inflammatory conditions notably contribute to oral health challenges, often leading to sharp pain, dull aches, and compromised functionality. Pharmacological interventions and emerging strategies aimed at modulating TRP channel activity are critically evaluated. The therapeutic potential of targeting TRP channels in the management within dental practice is a focal point of view to alleviate pain and inflammation. In conclusion, this comprehensive review provides a valuable synthesis of current knowledge regarding the involvement of TRP channels in inflammatory conditions of dentistry underscoring the potential of TRP channels as promising targets for therapeutic intervention, and then paving the way for innovative strategies to address the complexities of inflammatory dental conditions.

Dental health concerns for patients suffering from facial, peri-oral burns, and inhalation injury: A persistent yet underappreciated challenge

Demographic data reveal a correlative relationship between facial burns and profoundly impaired dental health, while inhalation injury correlates with a high Periodontal Screening and Recording Index, both of which significantly reduce quality of life for the burn injury patient. Despite these facts, few if any burn centers surveyed in the U.S. or Germany enlist specialized services and well-defined follow-up protocols for patients who might well be at risk. Facial burns represent a severe injury, the consequence of which may tragically lead to significant and long lasting functional, esthetic, and psychological sequelae such as perioral scarring, deformity and microstomia. Inhalation injury from inhaled heated air, gas, and chemical exposure are well-known causes of early as well as late airway injury, morbidity, and death. Thermal injury patterns to perioral-related and dental structures is increasingly being recognized. This review delineates current burn-related dental health pathology and when available the underlying causation. These findings may help guide future research as well as therapeutic strategies to improve dental health and outcome for our patients.

Effects of Capsaicin on Migration and Alkaline Phosphatase Activity of Dental Pulp Cells

OBJECTIVES

 Dental pulp, a specialized mesenchymal tissue within teeth, is pivotal in dental health and tissue repair. Capsaicin, the primary pungent component of chili peppers, is known for its diverse pharmacological properties. While capsaicin's effects on various cell types have been studied, its impact on dental pulp cells remains relatively unexplored. This study investigated the influence of pure capsaicin extract on dental pulp cell behavior, focusing on cell viability, proliferation, migration, and alkaline phosphatase (ALP) activity.

MATERIALS AND METHODS

 Capsaicin solution was prepared and diluted to various concentrations (1 nM, 0.01 µM, 0.1 µM, 1 µM, 10 µM, and 100 µM), then was tested on rat dental pulp cells (RPC-C2A). Cell viability and proliferation were assessed using the MTT assay. Boyden chamber tests and wound healing were used for evaluating cell migration. The activity of ALP was determined to show cell function during dental pulp repair.

STATISTICAL ANALYSIS

 The data were analyzed using a one-way analysis of variance or an independent-sample Kruskal-Wallis, followed by multiple comparison tests.

RESULTS

 Capsaicin of 100 µM exhibited cytotoxicity, whereas those with lower concentrations stimulated cell proliferation. Wound healing assays revealed increased cell migration, particularly when cultured with 1 nM capsaicin (p = 0.002). Boyden chamber assays demonstrated enhanced cell invasion without statistical significance. ALP activity of dental pulp cells increased significantly at 1 nM (p < 0.001) and 1 µM (p = 0.021) capsaicin concentrations, indicating potential dentinogenesis and pulp repair.

CONCLUSION

 Capsaicin of lower concentrations, less than 10 µM, is likely to promote proliferation, migration, and ALP activity of dental pulp cells. Our findings offer potential applications for capsaicin as a medication for dental pulp repair.

TRPV2: a universal regulator in cellular physiology with a yet poorly defined thermosensitivity

Transient receptor potential (TRP) ion channels serve as sensors for variations in ambient temperature, modulating both thermoregulation and temperature responsive cellular processes. Among these, the vanilloid TRP subfamily (TRPV) comprises six members and at least four of these members (TRPV1-TRPV4) have been associated with thermal sensation. TRPV2 has been described as a sensor for noxious heat, but subsequent studies have unveiled a more complex role for TRPV2 beyond temperature perception. This comprehensive review aims to elucidate the intricate thermosensitivity of TRPV2 by synthesizing current knowledge on its biophysical properties, expression pattern and known physiological functions associated with thermosensation.

Efficacy of Preemptive Analgesia on Pain Perception After Simple Tooth Extraction: A Prospective Study

Background and objective This study aims to explore the concept of preemptive analgesia, which is the technique of administration of analgesic agents before the painful stimulus. This bridges the time gap between the onset of action of the analgesic agents and the wear-off of local anesthesia. Existing literature also brings up the concept of central sensitization, which is the hyper-activity of the nervous system in response to a noxious stimulus. Administration of preemptive analgesia prevents central sensitization and hence provides prolonged analgesia to the patient. For the benefit of this study, tab. Etoricoxib 90 mg was used as the analgesic agent. In addition, this study aims to investigate the effects of the administration of tab. Etoricoxib 90 mg 30 minutes before extraction of a single mandibular third molar on the effects of pain experienced by the patient after tooth extraction as compared to a placebo. Methodology This was a double-blinded, prospective, observational study. The pain experienced by 50 participants in each group was measured at 1 hour, 6 hours, 12 hours, and 24 hours postoperatively using a visual analog scale (VAS). The independent samples t-test was then conducted to evaluate the results and draw out conclusions. Results The average difference in pain experienced was maximum in the first hour after the procedure. The mean VAS score reported by patients was 3.14 in the study group but was 6.40 in the control group within the first hour. This difference was reduced in the first six hours after the procedure, with the average score being 3.82 in the study and 7.16 in the control group. The difference was the least after 12 hours, with the study group experiencing a VAS score of 4.64 and controls experiencing a VAS score of 6.14. After the first 24 hours, the mean VAS score was 3.80 in the study group and 5.60 in the control group. Conclusions Preemptive administration of tab. Etoricoxib 90 mg can reduce postextraction pain in healthy adult patients as compared to placebo tablets, with a maximum difference in pain reduction seen at the end of the first six hours (P = 0.012) and the minimum at the end of 12 hours (P = 0.0197).

The anatomy, neurophysiology, and cellular mechanisms of intradental sensation

Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces. The trigeminal sensory neurons underlie sensation originating from the tooth. Prior work has provided important physiological and molecular characterization of dental pulp sensory innervation. Clinical dental experiences have informed our conception of the consequence of activating these neurons. However, the biological role of sensory innervation within the tooth is yet to be defined. Recent transcriptomic data, combined with mouse genetic tools, have the capacity to provide important cell-type resolution for the physiological and behavioral function of pulp-innervating sensory neurons. Importantly, these tools can be applied to determine the neuronal origin of acute dental pain that coincides with tooth damage as well as pain stemming from tissue inflammation (i.e., pulpitis) toward developing treatment strategies aimed at relieving these distinct forms of pain.

Neuronal-immune axis alters pain and sensory afferent damage during dental pulp injury

ABSTRACT

Dental pulp tissue is densely innervated by afferent fibers of the trigeminal ganglion. When bacteria cause dental decay near the pulpal tissue, a strong neuronal and immune response occurs, creating pulpitis, which is associated with severe pain and pulp tissue damage. Neuroimmune interactions have the potential to modulate both the pain and pathological outcome of pulpitis. We first investigated the role of the neuropeptide calcitonin gene-related peptide (CGRP), released from peptidergic sensory afferents, in dental pain and immune responses by using Calca knockout (Calca -/- ) and wild-type (Calca +/+ ) mice, in a model of pulpitis by creating a mechanical exposure of the dental pulp horn. We found that the neuropeptide CGRP, facilitated the recruitment of myeloid cells into the pulp while also increasing spontaneous pain-like behavior 20% to 25% at an early time point. Moreover, when we depleted neutrophils and monocytes, we found that there was 20% to 30% more sensory afferent loss and increased presence of bacteria in deeper parts of the tissue, whereas there was a significant reduction in mechanical pain response scores compared with the control group at a later time point. Overall, we showed that there is a crosstalk between peptidergic neurons and neutrophils in the pulp, modulating the pain and inflammatory outcomes of the disease.

The value of the International Association for the Study of Pain to career development: perspectives of trainee and early career members

ABSTRACT

Supporting its young members has been a key priority of the International Association for the Study of Pain (IASP) for the past 5 decades. The IASP, along with its federations, chapters, and special interest groups, has provided benefits to its trainee and early career members for their career development. This article summarizes various key IASP initiatives and benefits offered to IASP members and how these benefits have positively impacted their careers, including examples from the authors of this article. Suggestions are made for future directions that the IASP could implement to enhance the value provided to its trainee and early career members, which will in turn contribute to IASP achieving its mission to stimulate and support the study of pain and to translate that knowledge into improved pain relief worldwide.

TRPC expression in human periodontal ligament cells and the periodontal tissue of periodontitis mice: a preliminary study

BACKGROUND

Transient receptor potential canonical (TRPC) channels are non-selective cationic channels with permeability to Ca2+ and Na+. Despite their importance, there are currently few studies on TRPC in the periodontal ligament (PDL) and bone cells in the dental field. To provide biological information regarding TRPC in PDL cells and periodontal tissue, we evaluated TRPC channels expression in the osteoblast differentiation of PDL cells and periodontitis-induced tissue. Human PDL cells were cultured in osteogenic differentiation media for 28 days, and the expression of Runx2, osteocalcin (OCN), and TRPC1, 3, 4, and 6 was evaluated by real-time PCR. In ligature-induced periodontitis mice, the alveolar bone and osteoid areas, the osteoclast number, and the expression of Runx2, OCN, TRPC3, and TRPC6 was evaluated by H&E staining, TRAP staining, and immunohistochemistry, respectively.

RESULTS

In the PDL cell differentiation group, TRPC6 expression peaked on day 7 and TRPC3 expression generally increased during differentiation. During the 28 days of periodontitis progression, alveolar bone loss and osteoclast numbers increased compared to the control group during the experimental period and the osteoid area increased from day 14. TRPC6 expression in the periodontitis group increased in the PDL area and in the osteoblasts compared to the control group, whereas TRPC3 expression increased only in the PDL area on days 7 and 28.

CONCLUSIONS

These results indicate changes of TRPC3 and TRPC6 expression in PDL cells that were differentiating into osteoblasts and in periodontitis-induced tissue, suggesting the need for research on the role of TRPC in osteoblast differentiation or periodontitis progression.

Alterations in the microenvironment and the effects produced of TRPV5 in osteoporosis

The pathogenesis of osteoporosis involves multiple factors, among which alterations in the bone microenvironment play a crucial role in disrupting normal bone metabolic balance. Transient receptor potential vanilloid 5 (TRPV5), a member of the TRPV family, is an essential determinant of the bone microenvironment, acting at multiple levels to influence its properties. TRPV5 exerts a pivotal influence on bone through the regulation of calcium reabsorption and transportation while also responding to steroid hormones and agonists. Although the metabolic consequences of osteoporosis, such as loss of bone calcium, reduced mineralization capacity, and active osteoclasts, have received significant attention, this review focuses on the changes in the osteoporotic microenvironment and the specific effects of TRPV5 at various levels.

Dihydroceramides Derived from Bacteroidetes Species Sensitize TRPV1 Channels

Bacterial colonization of open wounds is common, and patients with infected wounds often report significantly elevated pain sensitivity at the wound site. Transient Receptor Potential Vanilloid Type 1 (TRPV1) channels are known to play an important role in pain signaling and may be sensitized under pro-inflammatory conditions. Bacterial membrane components, such as phosphoethanolamine dihydroceramide (PEDHC), phosphoglycerol dihydroceramide (PGDHC), and lipopolysaccharide (LPS), are released in the environment from the Gram-negative bacteria of the Bacteroidetes species colonizing the infected wounds. Here, we used intracellular calcium imaging and patch-clamp electrophysiology approaches to determine whether bacterially derived PEDHC, PGDHC, or LPS can modulate the activity of the TRPV1 channels heterologously expressed in HEK cells. We found that PEDHC and PGDHC can sensitize TRPV1 in a concentration-dependent manner, whereas LPS treatment does not significantly affect TRPV1 activity in HEK cells. We propose that sensitization of TRPV1 channels by Bacteroidetes-derived dihydroceramides may at least in part underlie the increased pain sensitivity associated with wound infections.

The Oral Cavity Potentially Serving as a Reservoir for SARS-CoV-2 but Not Necessarily Facilitating the Spread of COVID-19 in Dental Practice

Intraoral tissues, secretions, and microenvironments may provide severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with the conditions necessary for viral cellular entry and inhabitation. The aim of the present study is to overview the oral cavity that potentially serves as a reservoir for SARS-CoV-2, and then discuss the possibility that such oral cavity facilitates the spread of coronavirus disease 2019 (COVID-19) in dental practice. Articles were retrieved from PubMed/Medline, LitCovid, ProQuest, Google Scholar, and preprint medRxiv databases. Results of the literature search indicated that SARS-CoV-2 host cell entry-relevant receptor and virus/cell membrane fusion mediators are expressed in major and minor salivary glands, tongue, taste bud, periodontal tissue, and dental pulp, which would be a target and reservoir for SARS-CoV-2. SARS-CoV-2 is present in saliva and gingival crevicular fluid of COVID-19 patients. These secretions would contaminate dental aerosol and droplet with SARS-CoV-2. SARS-CoV-2 inhabits periodontal pocket, gingival sulcus, and dental caries lesion, which could provide SARS-CoV-2 with a habitat. SARS-CoV-2 ribonucleic acid is preserved in dental calculus, which may inform of the previous infection with SARS-CoV-2. Despite involvement of the oral cavity in SARS-CoV-2 transmission and infection, to date, there have been no clusters of COVID-19 in dental practice. Dental settings are much less likely to facilitate the spread of COVID-19 compared with general medical settings, which may be explained by the situation of dentistry that the number of patients to visit dental offices/clinics was decreased during the COVID-19 pandemic, the characteristics of dentistry that dental professionals have maintained high awareness of viral infection prevention, adhered to a strict protocol for infection control, and been using personal protective equipment for a long time, the experimental results that dental devices generate only small amounts of aerosol responsible for the airborne viral transmission, irrigant from the dental unit contributes to the aerosol microbiota much rather than saliva, and the commonly used evacuation or suction system effectively reduces aerosol and droplet generation, and the possibility that human saliva exhibits the antiviral activity and the property to inhibit SARS-CoV-2 infection. It is considered that dental treatment and oral health care can be delivered safely in the COVID-19 era.

Cannabinoid Therapeutics in orofacial pain management: A Systematic Review

The objective of this paper is to investigate the published evidence regarding effects of cannabinoids (natural and synthetic) on post-operative and/or out-of-office pain management in patients suffering from orofacial pain that presents in the dental setting. Three online databases (Ovid (MEDLINE), PubMed (MEDLINE), Scopus) were searched (July 2021). Additional studies were sought through grey literature searching (Cochrane Library Trials and ClinicalTrials.gov) and hand-searching the reference lists of included articles. All studies that analysed cannabinoid products and pain management of conditions that present in the general or specialist dental setting in the English language were included. Of the five articles included, one reported a significant effect on temporomandibular disorder pain relief using a topical cannabidiol formulation compared to a placebo. Four articles reported no significant effects of cannabinoids for pain management across various orofacial pain conditions. Although one study reported a positive effect, insufficient evidence exists to support a tangible clinical benefit of cannabinoids in managing orofacial pain, further research is recommended to investigate the benefits of cannabinoids' use. © 2022 Australian Dental Association.

Trigeminal neurons control immune-bone cell interaction and metabolism in apical periodontitis

Abstract

Apical periodontitis (AP) is an inflammatory disease occurring following tooth infection with distinct osteolytic activity. Despite increasing evidence that sensory neurons participate in regulation of non-neuronal cells, their role in the development of AP is largely unknown. We hypothesized that trigeminal ganglia (TG) Nav1.8⁺ nociceptors regulate bone metabolism changes in response to AP. A selective ablation of nociceptive neurons in Nav1.8^Cre/Diphtheria toxin A (DTA)^Lox mouse line was used to evaluate the development and progression of AP using murine model of infection-induced AP. Ablation of Nav1.8⁺ nociceptors had earlier progression of AP with larger osteolytic lesions. Immunohistochemical and RNAscope analyses demonstrated greater number of macrophages, T-cells, osteoclast and osteoblast precursors and an increased RANKL:OPG ratio at earlier time points among Nav1.8^Cre/ DTA^Lox mice. There was an increased expression of IL-1α and IL-6 within lesions of nociceptor-ablated mice. Further, co-culture experiments demonstrated that TG neurons promoted osteoblast mineralization and inhibited osteoclastic function. The findings suggest that TG Nav1.8⁺ neurons contribute to modulation of the AP development by delaying the influx of immune cells, promoting osteoblastic differentiation, and decreasing osteoclastic activities. This newly uncovered mechanism could become a therapeutic strategy for the treatment of AP and minimize the persistence of osteolytic lesions in refractory cases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00018-022-04335-w.

Immunohistochemistry as a detection tool for ion channels involved in dental pain signaling

Background

Despite advances in pain detection, diagnosis, and management, the prevalence of dental pain is still on the rise. Although dental pain is not directly related to fatal outcomes, the two most common types of dental pain—dental caries and dentin hypersensitivity—have a significant impact on an individual’s quality of life. Understanding the mechanism of the pain pathway is one of the crucial steps in providing better treatment for these patients. Ion channels are critical biomolecules that have been the subject of dental study owing to their roles in the transmission and transduction of external stimuli, as well as in the control and perception of pain. Numerous immunohistochemical (IHC) staining approaches have also been used to identify the many ion channels implicated in peripheral pain signaling in dental pulp.

Highlight

This review highlights the critical steps in IHC and its role in the detection of ion channels involved in the dental pain signaling pathway.

Conclusion

The key ion channels identified using IHC and whose functions have been widely researched in dental tissues are addressed in this review article.

Neural signalling of gut mechanosensation in ingestive and digestive processes

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

Oral-Gut-Brain Axis in Experimental Models of Periodontitis: Associating Gut Dysbiosis With Neurodegenerative Diseases

Periodontitis is considered a non-communicable chronic disease caused by a dysbiotic microbiota, which generates a low-grade systemic inflammation that chronically damages the organism. Several studies have associated periodontitis with other chronic non-communicable diseases, such as cardiovascular or neurodegenerative diseases. Besides, the oral bacteria considered a keystone pathogen, Porphyromonas gingivalis, has been detected in the hippocampus and brain cortex. Likewise, gut microbiota dysbiosis triggers a low-grade systemic inflammation, which also favors the risk for both cardiovascular and neurodegenerative diseases. Recently, the existence of an axis of Oral-Gut communication has been proposed, whose possible involvement in the development of neurodegenerative diseases has not been uncovered yet. The present review aims to compile evidence that the dysbiosis of the oral microbiota triggers changes in the gut microbiota, which creates a higher predisposition for the development of neuroinflammatory or neurodegenerative diseases.The Oral-Gut-Brain axis could be defined based on anatomical communications, where the mouth and the intestine are in constant communication. The oral-brain axis is mainly established from the trigeminal nerve and the gut-brain axis from the vagus nerve. The oral-gut communication is defined from an anatomical relation and the constant swallowing of oral bacteria. The gut-brain communication is more complex and due to bacteria-cells, immune and nervous system interactions. Thus, the gut-brain and oral-brain axis are in a bi-directional relationship. Through the qualitative analysis of the selected papers, we conclude that experimental periodontitis could produce both neurodegenerative pathologies and intestinal dysbiosis, and that periodontitis is likely to induce both conditions simultaneously. The severity of the neurodegenerative disease could depend, at least in part, on the effects of periodontitis in the gut microbiota, which could strengthen the immune response and create an injurious inflammatory and dysbiotic cycle. Thus, dementias would have their onset in dysbiotic phenomena that affect the oral cavity or the intestine. The selected studies allow us to speculate that oral-gut-brain communication exists, and bacteria probably get to the brain via trigeminal and vagus nerves.

Upregulation of Toll-like Receptor 2 in Dental Primary Afferents Following Pulp Injury

Pulpitis (toothache) is a painful inflammation of the dental pulp and is a prevalent problem throughout the world. This pulpal inflammation occurs in the cells inside the dental pulp, which have host defense mechanisms to combat oral microorganisms invading the pulp space of exposed teeth. This innate immunity has been well studied, with a focus on Toll-like receptors (TLRs). The function of TLR4, activated by Gram-negative bacteria, has been demonstrated in trigeminal ganglion (TG) neurons for dental pain. Although Gram-positive bacteria predominate in the teeth of patients with caries and pulpitis, the role of TLR2, which is activated by Gram-positive bacteria, is poorly understood in dental primary afferent (DPA) neurons that densely innervate the dental pulp. Using Fura-2 based Ca²⁺ imaging, we observed reproducible intracellular Ca²⁺ responses induced by Pam₃CSK₄ and Pam₂CSK₄ (TLR2-specific agonists) in TG neurons of adult wild-type (WT) mice. The response was completely abolished in TLR2 knock-out (KO) mice. Single-cell RT-PCR detected Tlr2 mRNA in DPA neurons labeled with fluorescent retrograde tracers from the upper molars. Using the mouse pulpitis model, real-time RT-PCR revealed that Tlr2 and inflammatory-related molecules were upregulated in injured TG, compared to non-injured TG, from WT mice, but not from TLR2 KO mice. TLR2 protein expression was also upregulated in injured DPA neurons, and the change was corresponded with a significant increase in calcitonin gene-related peptide (CGRP) expression. Our results provide a better molecular understanding of pulpitis by revealing the potential contribution of TLR2 to pulpal inflammatory pain.

Pivotal role of Transient Receptor Potential Channels in oral physiology

BACKGROUND

Transient Receptor Potential (TRP) Channels constitute a large family of non-selective permeable ion channels involved in the perception of environmental stimuli with a central and continuously expanding role in oral tissue homeostasis. Recent studies indicate the regulatory role of TRPs in pulp physiology, oral mucosa sensation, dental pain nociception and salivary gland secretion. This review provides an update on the diverse functions of TRP channels in the physiology of oral cavity, with emphasis on their cellular location, the underlying molecular mechanisms and clinical significance.

METHODS

A structured search of bibliographic databases (PubMed and MEDLINE) was performed for peer reviewed studies on TRP channels function on oral cavity physiology the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.

RESULTS

TRPs expression has been detected in major cell types of the oral cavity, including odontoblasts, periodontal ligament, oral epithelial, salivary gland cells, and chondrocytes of temporomandibular joints, where they mediate signal perception and transduction of mechanical, thermal, and osmotic stimuli. They contribute to pulp physiology through dentin formation, mineralization, and periodontal ligament formation along with alveolar bone remodeling in dental pulp and periodontal ligament cells. TRPs are also involved in oral mucosa sensation, dental pain nociception, saliva secretion, swallowing reflex and temporomandibular joints' development.

CONCLUSION

Various TRP channels regulate oral cavity homeostasis, playing an important role in the transduction of external stimuli to intracellular signals in a cell type-specific manner and presenting promising drug targets for the development of pharmacological strategies to manage oral diseases.

Associations between Pain Severity, Clinical Findings, and Endodontic Disease: A Cross-Sectional Study

INTRODUCTION

Thorough pain assessment and thermal and mechanical testing are the primary diagnostic tools used to assess the status of pulp and periapical tissues in teeth with potential endodontic pathology. This study evaluated predictors of acute odontogenic pain to better understand the relationship between endodontic pain, clinical testing, endodontic disease, and diagnoses.

METHODS

Participants (N = 228) presenting with acute odontogenic pain underwent standardized clinical testing and reported their pain intensity. Univariate and multiple regression analyses were performed to evaluate the predictors of acute endodontic pain. Chi-square tests with Bonferroni adjustments were conducted to measure the frequency of endodontic diagnostic test findings and clinical observations in patients with different pulpal diagnoses.

RESULTS

A negative response to cold stimulation on the causative tooth and percussion hypersensitivity on the healthy adjacent tooth were the strongest predictors of higher levels of acute endodontic pain. Percussion hypersensitivity on the healthy adjacent tooth was present in a quarter of the cohort and was reported with equal frequency in teeth diagnosed with irreversible pulpitis, necrotic pulp, and previously initiated/treated teeth. Although painful percussion on the causative tooth was more frequently reported in teeth diagnosed with necrotic pulp, painful palpation was more frequently reported on teeth diagnosed with previously initiated/treated teeth.

CONCLUSIONS

Percussion hypersensitivity on the healthy adjacent tooth may reveal a lowered pain threshold and heightened pain sensitization. It is also possible that the 2 commonly performed mechanical sensory tests, percussion and palpation hypersensitivity, may detect different aspects of endodontic pathophysiology and pain processing.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Effects of CGRP-Primed Dental Pulp Stem Cells on Trigeminal Sensory Neurons

Dental pulp stem cells (DPSCs) are important in tooth physiology, contributing to development, repair, regeneration, and immunomodulatory processes. However, their role in inflammatory mechanisms underlying pulpitis is not well understood. We evaluated the influence of DPSCs stimulated with calcitonin gene-related peptide (CGRP), a proinflammatory neuropeptide, on the expression of mediators released from DPSCs and the effect of these mediators on sensory neuron activity. Human DPSCs were treated with either control media or media containing CGRP (10^-8 M) for 7 d, and the conditioned media (CM) containing DPSC-released mediators was collected. The expression of cytokines and chemokines from DPSCs was evaluated by reverse transcription quantitative polymerase chain reaction. The effects of the CM from CGRP-primed DPSCs (primed DPSC-CM) were evaluated on sensory afferents by using primary cultures of mouse trigeminal neurons and an organotypic model of cultured human pulp slices. Mouse trigeminal neurons and human pulp explants were pretreated for 24 h with control or primed DPSC-CM and then stimulated with capsaicin. Afferent activity was measured by quantifying the response to capsaicin via live cell calcium imaging in mouse neurons and CGRP released from pulp explants. Gene expression analysis showed that primed DPSCs overexpressed some proinflammatory cytokines and chemokines, including chemokines CXCL1 and CXCL8, which are both agonists of the receptor CXCR2 expressed in sensory neurons. Primed DPSC-CM increased human pulp sensory afferent activity as compared with control DPSC-CM. Similarly, primed DPSC-CM increased the intensity of calcium responses in cultured mouse trigeminal neurons. Furthermore, the CXCR2 antagonist SB225002 prevented trigeminal neuron sensitization to capsaicin induced by primed DPSC-CM. In conclusion, mediators released by DPSCs, primed with the proinflammatory mediator CGRP, induce neuronal sensitization through CXCR2 receptor. These data suggest that DPSCs might contribute to pain symptoms that develop in pulpitis.

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.

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.

Blockage of ventrolateral periaqueductal gray matter cannabinoid 1 receptor increases dental pulp pain and pain-related subsequent learning and memory deficits in rats

Cannabinoid 1 receptor (CB1R) signaling has a pivotal role in the modulation of both pain and cognitive responses. This study aims at investigating the role of CB1R in the ventrolateral periaqueductal gray matter (vlPAG) on both pulpal pain and pain-related subsequent changes in learning and memory performances in rats. The adult male Wistar rats were cannulated in the vlPAG. The rats were pretreated by intra-vlPAG administration of selective CB1R antagonist AM-251 (2, 4 and 8 µg/rat) and vehicle dimethylsulfoxide. The drugs were microinjected 20 min before the induction of capsaicin-induced pulpalgia. The nociceptive behaviors were recorded for 40 min. Then, passive avoidance and spatial learning and memory were assessed using the shuttle box and Morris water maze tests, respectively. Following the administration of intradental capsaicin, there was a significant nociceptive response that increased after an induced blockage of CB1R by AM-251 at 4 and 8 µg. In addition, capsaicin impaired passive avoidance and spatial memory performance of rats. Microinjection of AM-251, prior to capsaicin, could dose-dependently exaggerate capsaicin-related learning and memory deficits in both tests. The present data indicated that the vlPAG endocannabinoid system is involved in the modulation of pain signals from dental pulp. It was also accompanied by learning and memory impairments.

Transcriptomic Classification of Neurons Innervating Teeth

Toothache is a common painful consequence of damage to the teeth, particularly when coupled to infection. Clinical restoration of tooth integrity, sometimes involving physical and chemical sterilization of the tooth with nerve fiber ablation (i.e., endodontic therapy), generally alleviates pain and allows long-lasting dental function. These observations raise questions regarding the biological role of tooth-innervating fibers. Here, we determined the transcriptomic diversity of the sensory neurons that can be retrogradely labeled from mouse molar teeth. Our results demonstrate that individual molars are each targeted by a dedicated population of about 50 specialized trigeminal neurons. Transcriptomic profiling identifies the majority of these as expressing markers of fast-conducting neurons, with about two-thirds containing nociceptive markers. Our data provide a new view of dental innervation, extending previous reports that used candidate gene approaches. Importantly, almost all retrogradely labeled neurons, including nociceptors, express the recently characterized mechanosensor Piezo2, an ion channel that endows cells with sensitivity to gentle touch. Intriguingly, about a quarter of the labeled neurons do not appear to be nociceptors, perhaps insinuating a role for them in discriminative touch. We hypothesize that dental neurons are capable of providing mechanosensitive information to drive rapid behavioral responses and protect teeth from damage. Damage to the teeth and exposure of the large population of molar nociceptors may trigger prolonged or abnormal activation driving toothache. Future studies examining the responses of these transcriptomically defined classes of neurons will help define their significance in oral sensation.

Inhibition of carrageenan-induced dental inflammatory responses owing to decreased TRPV1 activity by Dexmedetomidine

Background

Dexmedetomidine (Dex) is a highly selective agonist of the α2 adrenergic receptor and a common sedative; however, its anti-inflammatory effect has been studied. In this study, the inhibitory effect of Dex on inflammation in dental pulp cells was assessed. For this, the effect of Dex on inflammation induced by carrageenan (Car) in human dental pulp cells (hDPCs) was evaluated. Car incubation induced a robust inflammatory response in hDPCs as well as activation of PKA–STAT3 and PKC–nuclear factor kappa B (NF-κB) signaling pathways.

Results

Dex reduced the expression of inflammatory cytokines in a dose-dependent manner. Meanwhile, the phosphorylation of PKA, PKC, STAT3, and NF-κB as well as the nuclear accumulation of STAT3 and NF-κB were significantly increased in Dex-treated Car-induced hDPCs. Western blotting results also showed that the phosphorylation level of transient receptor potential cation channel subfamily V member 1 (TRPV1) was downregulated as a result of Dex treatment. Furthermore, we found that administration of the TRPV1 agonist capsaicin (Cap) reversed the effects of Dex on proinflammatory cytokines; however, the expression and activation of PKA–STAT3 and PKC–NF-κB signals were not altered owing to Cap administration.

Conclusions

These results indicate that Dex plays a defensive role in dental pulp inflammation by regulating the TRPV1 channel and can be used as a potential target for human dental pulp inflammation intervention.

Orthodontic force-induced oxidative stress in the periodontal tissue and dental pulp elicits nociception via activation/sensitization of TRPA1 on nociceptive fibers

Orthodontic patients complain of pain for the first few days after insertion of appliances. Mechanical force has been reported to produce oxidants in periodontal ligament (PDL) cells. It has not been studied whether orthodontic force-induced oxidative stress elicits nociception. Herein, we focused on the role of the oxidant-sensitive channel TRPA1 on nociception in orthodontic pain. In a rat model of loaded orthodontic force between the maxillary first molar and incisor, the behavioral signs of orofacial nociception, facial rubbing and wiping, increased to a peak on day 1 and gradually diminished to the control level on day 5. Administration of free radical scavengers (Tempol and PBN) and TRPA1 antagonist (HC-030031) inhibited nociceptive behaviors on day 1. In the PDL, the oxidative stress marker 8-OHdG was highly detected on day 1 and recovered on day 5 to the sham-operated level. The dental pulp showed similar results as the PDL. TRPA1 mRNA was abundantly expressed in the trigeminal ganglion relative to PDL tissue, and there were TRPA1-immunopositive neuronal fibers in the PDL and pulp. In dissociated trigeminal ganglion neurons, H₂O₂ at 5 mM induced a Ca²⁺ response that was inhibited by HC-030031. Although H₂O₂ at 100 μM did not yield any response, it enhanced the mechanically activated TRPA1-dependent Ca²⁺ response. These results suggest that oxidative stress in the PDL and dental pulp following orthodontic force activates and/or mechanically sensitizes TRPA1 on nociceptive fibers, resulting in orthodontic nociception. Later, the disappearance of nociception seems to be related to a decrease in oxidative stress, probably due to tissue remodeling.

TRPM8 Mediates Hyperosmotic Stimuli-Induced Nociception in Dental Afferents

Hyperosmolar sweet foods onto exposed tooth dentin evoke sudden and intense dental pain, called dentin hypersensitivity. However, it remains unclear how hyperosmolar stimuli excite dental primary afferent (DPA) neurons and thereby lead to dentin hypersensitivity. This study elucidated whether TRPM8, which is well known as a cold temperature- or menthol-activated receptor, additionally mediates nociception in response to hyperosmolar stimuli in adult mouse DPA neurons, which are identified by a fluorescent retrograde tracer: DiI. Single-cell reverse transcription polymerase chain reaction revealed that TRPM8 was expressed in subsets of DPA neurons and that TRPM8 was highly colocalized with TRPV1 and Piezo2. Immunohistochemical analysis also confirmed TRPM8 expression in DPA neurons. By using Fura-2-based calcium imaging, application of hyperosmolar sucrose solutions elicited calcium transients in subsets of the trigeminal ganglion neurons, which was significantly abolished by a selective TRPM8 antagonist: N-(3-Aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)benzamide (AMTB) hydrochloride. When we further examined changes of c-fos expression (a neuronal activation marker) in the spinal trigeminal nucleus after hyperosmolar stimulation onto exposed tooth dentin, c-fos mRNA and protein expression were increased and were also significantly reduced by AMTB, especially in the spinal trigeminal interpolaris-caudalis transition zone (Vi/Vc). Taken together, our results provide strong evidence that TRPM8 expressed in DPA neurons might mediate dental pain as a hyperosmosensor in adult mice.

TRPV1 and TRPV1-Expressing Nociceptors Mediate Orofacial Pain Behaviors in a Mouse Model of Orthodontic Tooth Movement

Orthodontic force produces mechanical irritation and inflammation in the periodontium, which is inevitably accompanied by pain. Despite its prevalence, treatment of orthodontic pain is ineffective. Elucidating underlying neural mechanisms is critical to improving the management of orthodontic pain. We have assessed the contribution of transient receptor potential vanilloid subtype 1 (TRPV1) and the TRPV1-expressing subset of nociceptive afferents to pain behaviors induced by orthodontic force in mice. Microfocus X-ray computed tomography analysis showed that application of an orthodontic force of 10 g to the maxillary first molar produced reliable tooth movement in mice. Mouse grimace scale (MGS) was evaluated as an indication of non-evoked spontaneous pain and bite force (BF) was measured for assessing bite-evoked nocifensive behaviors. Orthodontic force increased MGS and decreased BF, both of which were interpreted as increased levels of pain. These behaviors peaked at 1d and returned near to the sham level at 7d. Retrograde labeling and immunohistochemical assays showed TRPV1-expressing peptidergic afferents are abundantly projected to the periodontium. Direct injection of resiniferatoxin into trigeminal ganglia (TG) decreased TRPV1-expressing afferents by half in the targeted region of TG. The chemical ablation of TRPV1-expressing afferents significantly attenuated orthodontic pain behaviors assessed by MGS and BF. Consistently, the knockout of TRPV1 also attenuated orthodontic force-induced changes in MGS and BF. These results suggest that TRPV1 and TRPV1-expressing trigeminal nociceptors constitute a primary pathway mediating orthodontic pain behaviors in mice. This model will be useful for mechanistic studies on orthodontic pain aimed at developing novel approaches for painless orthodontics.

TNF-α differently regulates TRPV2 and TRPV4 channels in human dental pulp cells

AIM

To investigate the influence of tumour necrosis factor (TNF)-α on transient receptor potential channel vanilloid subfamily type 2 (TRPV2) and TRPV4 channels in human dental pulp cells (HDPCs), and explore the potential downstream signalling pathway mediating this process.

METHODOLOGY

Immunofluorescence staining and ratiometric calcium imaging were used to confirm the expression and activation of TRPV2 and TRPV4 channels. Different regulations of 1 and 10 ng mL^-1 as well as short- and long-term TNF-α treatments to TRPV2 and TRPV4 response were examined by RT-qPCR, Western blot analysis, flow cytometry and ratiometric calcium imaging. Functions of TNF receptor (TNFR)1 and p38 MAPK signalling pathways in this process were also detected by respective inhibitors. Immunoelectron microscopy (IEM) was used to examine long-term effect of TNF-α on TRPV2 expression at the subcellular level. Data were analysed statistically with t-test, and one-way analysis of variance was used with the non-parametric Mann-Whitney and Kruskal-Wallis tests. The level of significance was set at P < 0.05.

RESULTS

TRPV2 and TRPV4 channels were activated by respective agonists in HDPCs. Neither TRPV2 nor TRPV4 channels were upregulated by 1 ng mL^-1 TNF-α (P > 0.05). TRPV2, but not TRPV4, was upregulated by 10 ng mL^-1 TNF-α (P < 0.05). Both short- and long-term treatments with 10 ng mL^-1 TNF-α significantly enhanced TRPV2 responses, whereas only short-term treatment of TNF-α increased TRPV4 response (P < 0.05). Moreover, the inhibitors of TNFR and p38 both significantly decreased the TNF-α-induced up-regulation of TRPV channels (P < 0.05). At the subcellular level, prolonged TNF-α treatment significantly increased the functional expression of the TRPV2 channel especially in the nucleus, endoplasmic reticulum and mitochondria.

CONCLUSIONS

Low and high concentrations, as well as short- and long-term TNF-α treatments regulated the activity of TRPV2 and TRPV4 channels in HDPCs differently, and this effect might be mediated by TNFR1 and p38 MAPK signalling pathways. IEM was used to confirm that prolonged TNF-α treatment significantly increased the functional expression of the TRPV2 channel at a subcellular level.

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

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

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 effect of central administration of alpha-pinene on capsaicin-induced dental pulp nociception

AIM

To assess the effects of central administration of α-pinene alone and in combination with either bicuculline or naloxone, as GABAA and μ-opioid receptor antagonists, respectively, on capsaicin-induced dental pulp stimulation in rats.

METHODOLOGY

Forty-eight adult male Wistar rats aged 2 months (230-270 g) were cannulated via their lateral ventricles for the central administration of the drugs. α-Pinene was injected at 0.1, 0.2 and 0.4 μmol L-1 . Then, dental pulp stimulation was induced by intradental application of capsaicin solution (100 μg), and nociceptive scores were recorded for up to 40 min. For investigation of the anti-inflammatory effects of α-pinene, expression of COX-2 in the subnucleolus caudalis (Vc) of rats was determined using immunofluorescence staining. Nonparametric repeated measure Friedman and Kruskal-Wallis tests as well as parametric one-way analysis of variance were used for the statistical analysis.

RESULTS

α-Pinene at 0.2 and 0.4 μmol L-1 was able to decrease capsaicin-induced nociception. Moreover, there was a significant increase in the expression of COX-2-positive cells in the Vc of capsaicin-treated rats (P < 0.01). This effect was prohibited by α-pinene (0.4 μmol L-1 ). Co-administration of bicuculline (1 μg per rat) or naloxone (6 μg per rat) with α-pinene (0.4 μmol L-1 ), however, prevented the inhibitory effects of α-pinene on both capsaicin-induced pulp nociception and COX-2 over-expression.

CONCLUSIONS

Pinene exhibited significant curable effects on capsaicin-induced pulpal nociception and inflammation mainly via pharmacological interfacing with GABAA and μ-opioid receptors.

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.

Conditioned medium from rat dental pulp reduces the number of osteoclasts via attenuation of adhesiveness in osteoclast precursors

Dental pulp is known to play crucial roles in homeostasis of teeth and periodontal tissue. Although resorption of bone around the roots of nonvital teeth is occasionally observed in clinical practice, little is known about the role of dental pulp in osteoclastogenesis. Here we evaluated the effects of conditioned medium (CM) from rat dental pulp on osteoclastogenesis. It was found that the CM reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts, but did not alter the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 and TRAP. To further understand the mechanism behind these results, we evaluated the effects of CM on osteoclast precursors and found that the CM removed cell processes, resulting in a significant reduction in the number of attached cells and an increase in the number of freely floating cells. Furthermore, the CM suppressed the mRNA levels of focal adhesion kinase and paxillin, which are involved in cell adhesiveness and spreading. Collectively, the present results show that CM from dental pulp serves as an inhibitor of osteoclastogenesis by reducing the number and adhesiveness of osteoclast precursors, suggesting novel therapeutic applicability for osteoporosis.

TRPM8 and TRPA1 do not contribute to dental pulp sensitivity to cold

Sensory neurons innervating the dental pulp have unique morphological and functional characteristics compared to neurons innervating other tissues. Stimulation of dental pulp afferents whatever the modality or intensity of the stimulus, even light mechanical stimulation that would not activate nociceptors in other tissues, produces an intense pain. These specific sensory characteristics could involve receptors of the Transient Receptor Potential channels (TRP) family. In this study, we compared the expression of the cold sensitive receptors TRPM8 and TRPA1 in trigeminal ganglion neurons innervating the dental pulp, the skin of the cheek or the buccal mucosa and we evaluated the involvement of these receptors in dental pulp sensitivity to cold. We showed a similar expression of TRPM8, TRPA1 and CGRP in sensory neurons innervating the dental pulp, the skin or the mucosa. Moreover, we demonstrated that noxious cold stimulation of the tooth induced an overexpression of cFos in the trigeminal nucleus that was not prevented by the genetic deletion of TRPM8 or the administration of the TRPA1 antagonist HC030031. These data suggest that the unique sensory characteristics of the dental pulp are independent to TRPM8 and TRPA1 receptors expression and functionality.

Molecular, cellular and behavioral changes associated with pathological pain signaling occur after dental pulp injury

Persistent pain can occur after routine dental treatments in which the dental pulp is injured. To better understand pain chronicity after pulp injury, we assessed whether dental pulp injury in mice causes changes to the sensory nervous system associated with pathological pain. In some experiments, we compared findings after dental pulp injury to a model of orofacial neuropathic pain, in which the mental nerve is injured. After unilateral dental pulp injury, we observed increased expression of activating transcription factor 3 (ATF3) and neuropeptide Y (NPY) mRNA and decreased tachykinin precursor 1 gene expression, in the ipsilateral trigeminal ganglion. We also observed an ipsilateral increase in the number of trigeminal neurons expressing immunoreactivity for ATF3, a decrease in substance P (SP) immunoreactive cells, and no change in the number of cells labeled with IB4. Mice with dental pulp injury transiently exhibit hindpaw mechanical allodynia, out to 12 days, while mice with mental nerve injury have persistent hindpaw allodynia. Mice with dental pulp injury increased spontaneous consumption of a sucrose solution for 17 days while mental nerve injury mice did not. Finally, after dental pulp injury, an increase in expression of the glial markers Iba1 and glial fibrillary acidic protein occurs in the transition zone between nucleus caudalis and interpolaris, ipsilateral to the injury. Collectively these studies suggest that dental pulp injury is associated with significant neuroplasticity that could contribute to persistent pain after of dental pulp injury.

The orofacial antinociceptive effect of Kaempferol-3-O-rutinoside, isolated from the plant Ouratea fieldingiana, on adult zebrafish (Danio rerio)

The aim of this study was to evaluate the antinociceptive effect of Kaempferol-3-O-rutinoside (KR), isolated from the plant Ouratea fieldingiana, on the orofacial nociception and possible mechanisms of action. Adult zebrafish (Danio rerio) were tested as a behavioral model to study formalin, glutamate, capsaicin, cinnamaldehyde and acidic saline-induced orofacial nociception, using as parameter the number of times the fish crossed the lines between the quadrants of a glass Petri dish during a specific time. Morphine was used as positive control. The effect of KR was tested for modulation by opioid (naloxone), nitrergic (L-NAME), TRPV1 (ruthenium red), TRPA1 (camphor) or ASIC (amiloride) antagonists. The effect of KR on zebrafish locomotor behavior was evaluated with the open field test. KR did not alter the fish's locomotor system and significantly reduced the orofacial nociceptive behavior induced by all noxious agents compared to the control group. The antinociceptive effect of KR was similar to morphine. All antagonists inhibited the antinociceptive effect of KR. KR has pharmacological potential for the treatment of acute orofacial pain and this effect is modulated by the opioid and nitrergic systems as well as TRPV1, TRPA1 and ASIC channels. These results can lead to the development of a new natural product for the treatment of orofacial pain and confirm the popular use of O. fieldingiana leaf for pain relief.

Transient receptor potential vanilloid 4 (TRPV4) expression on the nerve fibers of human dental pulp is upregulated under inflammatory condition

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) has been considered as a mechano-, thermo- and osmo-receptor. Under inflammatory conditions in dental pulp, teeth can become sensitive upon exposure to a variety of innocuous stimuli. The objective of the present study was to investigate the expression of the TRPV4 channel on nerve fibers in human dental pulp of non-symptomatic and symptomatic teeth associated with inflammatory conditions.

DESIGN

Dental pulp from extracted human permanent teeth was processed for fluorescence immunohistochemistry. Ten asymptomatic (normal) and 10 symptomatic (symptoms associated with pulpitis) teeth were used in this study. Nerve fibers were identified by immunostaining for a marker, protein gene product 9.5, and the cells were counterstained with 4',6-diamidino-2-phenylindole. An anti-TRPV4 antibody was used to trace TRPV4 expression.

RESULTS

TRPV4 expression was co-localized with the nerve fiber marker. Immunoreactivity for TRPV4 was more intense (p < 0.05) in the nerves of symptomatic teeth than those of normal teeth. The number of co-localization spots was increased significantly (p < 0.05) in the dental pulp of symptomatic teeth compared with that of asymptomatic (normal) teeth.

CONCLUSIONS

There is expression of TRPV4 channels on the nerve fibers of human dental pulp. Our findings suggest upregulation of TRPV4 expression under inflammatory conditions in the pulp. The upregulation of TRPV4 channels may be associated with the exaggerated response of dental pulp to innocuous mechanical, thermal and osmotic stimuli under inflammatory conditions.

The Role of Transient Receptor Potential Cation Channel, Subfamily C, Member 1 in the Odontoblast-like Differentiation of Human Dental Pulp Cells

INTRODUCTION

Calcium ions (Ca²⁺) actively participate in reparative dentin formation by promoting cellular proliferation and differentiation of human dental pulp cells (hDPCs). Transient receptor potential cation channel, subfamily C, member 1 (TRPC1) activates Ca²⁺ entry upon store depletion in a variety of cell types. However, the function of TRPC1 in hDPCs has not been reported. Therefore, we aimed to analyze the role of TRPC1 in hDPCs undergoing odontoblast-like differentiation.

METHODS

Immunohistochemical staining was used to determine the distribution of TRPC1 in pulp tissues. Western blot analysis was used to detect the protein level of TRPC1 in the odontoblast-like differentiation of hDPCs. Knockdown of TRPC1 was performed with an adenoviral vector to evaluate the role of TRPC1 in hDPCs during odontoblast-like differentiation.

RESULTS

The results showed that TRPC1 was highly expressed in the cytoplasm of dental pulp cells, especially in the odontoblast layer of the healthy pulp. Moreover, the protein level of TRPC1 increased in a time-dependent manner during the odontoblast-like differentiation of hDPCs. Importantly, knockdown of TRPC1 attenuated the process of odontoblast-like differentiation as indicated by the reduction in mineralized nodules and the down-regulation of dentin sialophosphoprotein and dentin matrix protein 1. Moreover, knockdown of TRPC1 decreased Ca²⁺ entry to the cytoplasm of hDPCs.

CONCLUSIONS

Our data indicated a pivotal role of TRPC1 in the odontoblastlike differentiation of hDPCs, which may be a therapeutic target to enhance reparative dentin formation.

Clinical Evaluation of Genotoxicity of In-office Bleaching

Objective: The aim of this study was to evaluate the genotoxicity of in-office bleaching with 35% hydrogen peroxide in epithelial cells from the gingival and lip tissues.

Methods and Materials: Thirty volunteers with central incisors shade A1 or darker were selected for this study. The gingival tissue of the teeth to be bleached was isolated with a light-polymerized resin dam, and the 35% hydrogen peroxide gel was administered during three 15-minute applications over the course of the 45-minute application period. Two bleaching sessions with a one-week interval in between were performed. Exfoliated oral mucosa gingival epithelial cells and upper lip lining were collected at baseline and one month after the in-office dental bleaching. The scraped cells were placed on clean glass slides and smears were prepared. After staining with Giemsa solution, two blinded examiners performed cell and micronuclei counts under a 100× optical microscope. Tooth sensitivity was evaluated using the Visual Analogue Scale (VAS). Shade evaluation was recorded before and one month after the bleaching treatment with the value-oriented shade guide Vita Bleachedguide 3D-MASTER and the spectrophotometer Vita Easyshade. Data from the shade guide units and the micronuclei (MN) frequency were subjected to a Mann-Whitney test (α=0.05). The overall difference between before and one month after the bleaching treatment (ΔE and ΔSGU), absolute risk, and intensity of tooth sensitivity (TS) were calculated, as was the 95% confidence interval (CI).

Results: The frequency of MN was not increased after bleaching with 35% hydrogen peroxide in both study groups (p&gt;0.05). The absolute risk of TS of the participants was 93% (95% CI, 79%-98%), with a mean VAS intensity of 5.7 ± 2.9 (95% CI, 4.6-6.8). Meaningful whitening was observed after bleaching. The change in shade guide units in the Bleachedguide 3D-MASTER was 2.3 ± 1.4. In terms of ΔE, the change in color was 7.7 ± 3.5.

Conclusions: The in-office bleaching did not induce DNA damage to the gingival and lip tissue during the bleaching period. Although effective whitening was observed, most of the participants experienced TS.

Neuronal TRPV1 activation regulates alveolar bone resorption by suppressing osteoclastogenesis via CGRP

The transient receptor potential vanilloid 1 (TRPV1) channel is abundantly expressed in peripheral sensory neurons where it acts as an important polymodal cellular sensor for heat, acidic pH, capsaicin, and other noxious stimuli. The oral cavity is densely innervated by afferent sensory neurons and is a highly specialized organ that protects against infections as well as physical, chemical, and thermal stresses in its capacity as the first part of the digestive system. While the function of TRPV1 in sensory neurons has been intensively studied in other organs, its physiological role in periodontal tissues is unclear. In this study we found that Trpv1^−/− mice developed severe bone loss in an experimental model of periodontitis. Chemical ablation of TRPV1-expressing sensory neurons recapitulated the phenotype of Trpv1^−/− mice, suggesting a functional link between neuronal TRPV1 signaling and periodontal bone loss. TRPV1 activation in gingival nerves induced production of the neuropeptide, calcitonin gene-related peptide (CGRP), and CGRP treatment inhibited osteoclastogenesis in vitro. Oral administration of the TRPV1 agonist, capsaicin, suppressed ligature-induced bone loss in mice with fewer tartrate-resistant acid phosphatase (TRAP)-positive cells in alveolar bone. These results suggest that neuronal TRPV1 signaling in periodontal tissue is crucial for the regulation of osteoclastogenesis via the neuropeptide CGRP.

Variability in Capsaicin-stimulated Calcitonin Gene-related Peptide Release from Human Dental Pulp

INTRODUCTION

The unique innervation and anatomic features of dental pulp contribute to the remarkable finding that any physical stimulation of pulpal tissue is painful. Furthermore, when pathological processes such as caries affect teeth and produce inflammation of the pulp, the pain experienced can be quite intense and debilitating. To better understand these underlying neurobiological mechanisms and identify novel analgesic targets for pulpally derived pain, we have developed a powerful ex vivo model using human tooth slices.

METHODS

Noncarious, freshly extracted teeth were collected and sectioned longitudinally into 1-mm-thick slices containing both dental pulp and the surrounding mineralized tissues. Tooth slices from 36 patients were exposed to 60 μmol/L capsaicin to stimulate the release of calcitonin gene-related peptide (CGRP) from nerve terminals in the pulp. Patient factors were analyzed for their effects on capsaicin-stimulated CGRP release using a mixed model analysis of variance.

RESULTS

Approximately one third of the variability observed in capsaicin-evoked CGRP release was attributable to differences between individuals. In terms of individual factors, there was no effect of anesthesia type, sex, or age on capsaicin-stimulated CGRP release. Using a within-subject study design, a significant effect of capsaicin on CGRP release was observed.

CONCLUSIONS

Capsaicin-stimulated CGRP release from dental pulp is highly variable between individuals. A within-subject study design improves the variability and maximizes the potential of this powerful translational model to test the efficacy of novel pharmacotherapeutic agents on human peripheral nociceptors.

Pre- and postoperative dexamethasone does not reduce bleaching-induced tooth sensitivity: A randomized, triple-masked clinical trial

BACKGROUND

Tooth sensitivity (TS) is the most common side effect of dental bleaching therapies. Dexamethasone has been used with tooth bleaching to reduce TS. The efficacy of dexamethasone for this purpose has not been well studied.

METHODS

The authors conducted a triple-masked, randomized, clinical trial with a parallel design involving 63 healthy participants who received either a placebo or dexamethasone. The placebo or dexamethasone (8 milligrams) was administered 1 hour before the in-office bleaching (35% hydrogen peroxide) and extra doses of 4 mg were administered every 6 hours for a total of 48 hours. TS was recorded on 2 scales: visual analog scale (0-10) and numeric rating scale (0-4) in different periods. The color evaluations were performed before and 1 month after dental bleaching with visual shade guides VITA Classical (VITA Zahnfabrik) and VITA Bleachedguide 3D-MASTER (VITA Zahnfabrik), and for a shade guide evaluation, the authors used a digital spectrophotometer, VITA Easyshade (VITA Zahnfabrik). The absolute risk of TS was evaluated by a Fisher exact test. Data of TS intensity using the NRS scale for the 2 groups were compared with Mann-Whitney and Friedman tests, whereas data from the visual analog scale were evaluated by 2-way repeated measures analysis of variance. The color changes between groups were compared using a t test (α = .05).

RESULTS

In both groups, the authors detected a high risk of TS, which was approximately 90%. No significant difference was observed in terms of TS intensity. A whitening of approximately 3 shade guide units of the VITA Classical was detected in both groups, which were statistically similar.

CONCLUSIONS

The use of dexamethasone before bleaching did not reduce the risk and intensity of bleaching-induced TS.

PRACTICAL IMPLICATIONS

The use of the steroidal anti-inflammatory agent dexamethasone was not capable of preventing TS arising from in-office dental bleaching.

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

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

Dentin and pulp sense cold stimulus

Dentin hypersensitivity is a common symptom, and recent convergent evidences have reported transient receptor potential (TRP) channels in odontoblasts act as mechanical and thermal molecular sensor, which detect stimulation applied on the exposed dentin surface, to drive multiple odontoblastic cellular functions, such as sensory transduction and/or dentin formation. In the present study, we confirmed expression of TRP melastatin subfamily member-8 (TRPM8) channels in primary cultured cells derived from human dental pulp cells (HPCs) and mouse odontoblast-lineage cells (OLCs) as well as in dentin matrix protein-1 (DMP-1) and dentin sialoprotein (DSP) positive acutely isolated rat odontoblasts from dental pulp tissue slice culture by immunohistochemical analyses. In addition, we detected TRPM8 channel expression on HPCs and OLCs by RT-PCR and Western blotting analyses. These results indicated that both odontoblasts and dental pulp cells express TRPM8 channels in rat, mouse and human, and therefore we hypothesize they may contribute as cold sensor in tooth.