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

Nociceptor Neurons Facilitate Orthodontic Tooth Movement via Piezo2 in Mice

Multiple sensory afferents, including mechanosensitive and nociceptive nerves, are projected to the periodontium. Peptidergic afferents expressing transient receptor potential vanilloid 1 (TRPV1), a receptor for capsaicin, mediate pain caused by orthodontic forces. However, their role in orthodontic force-induced alveolar bone remodeling is poorly understood as is the contribution of mechanosensitive ion channels such as Piezo2 in nociceptive nerves. To investigate this role, we studied orthodontic tooth movement and alveolar bone remodeling using neural manipulations and genetic mouse models. Chemical ablation of TRPV1-expressing afferents localized to the trigeminal ganglia decreased orthodontic force-induced tooth movement and the number of osteoclasts in alveolar bone on the compression side. The extent of the force-induced increase in the ratio of receptor activator of nuclear factor kappa-B ligand/osteoprotegerin in the periodontium was modestly decreased in the chemical ablation group. Furthermore, chemogenetic silencing of TRPV1-lineage afferents reduced orthodontic tooth movement and the number of osteoclasts. Piezo2 was expressed in most periodontal afferents, and chemogenetic inhibition of Piezo2-expressing neurons decreased orthodontic tooth movement and the number of osteoclasts. In addition, the conditional knockout of Piezo2 in TRPV1-lineage afferents decreased orthodontic tooth movement and the number of osteoclasts. Overall, these results suggest that nociceptor neurons play critical roles in orthodontic force-induced alveolar bone remodeling and that the mechanical activation of neuronal Piezo2 in nociceptive nerves facilitates orthodontic tooth movement and associated alveolar bone remodeling.

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.

The Role of TRPV1/CGRP Pathway Activated by Prevotella melaninogenica in Pathogenesis of Oral Lichen Planus

The distinctive clinicopathologic characteristics of OLP indicated that both microbial dysbiosis and neurogenic inflammation may be jointly involved in its progression, and transient receptor potential vanilloid receptor-1 (TRPV1) may be a crucial element. The purpose of this study was to explore how TRPV1 mediated P. melaninogenica-induced inflammation. Meanwhile, we aimed to unravel how IL-36γ dysregulated the barrier function in oral keratinocytes. Here, the expression of TRPV1, calcitonin gene-related peptide (CGRP), and its receptor receptor activity-modifying protein 1 (RAMP1) in OLP patients were detected. Prevotella melaninogenica (P. melaninogenica) was used to build a mouse model of oral chronic inflammation. Normal human oral keratinocytes (NHOKs) stimulated by P. melaninogenica were used to examine TRPV1 activation and CGRP release. To investigate the effect of exogenous CGRP on Interleukin-36 gamma (IL-36γ) expression in NHOKs and bacterial viability, P. melaninogenica and NHOKs were treated with it, respectively. Recombinant IL-36γ protein was used to probe its regulation of oral epithelial barrier function. TRPV1, CGRP, and RAMP1 were substantially expressed in OLP. P. melaninogenica increased TRPV1 expression in mice and caused the release of CGRP and an increase in pro-inflammatory cytokines via activating TRPV1 in NHOKs. Blockade of TRPV1 suppressed P. melaninogenica-induced inflammation. CGRP boosted the production of IL-36γ released by NHOKs, resulting in lower expression of zonula occludens-1 (ZO-1). Also, CGRP can decrease the viability of P. melaninogenica. Together, these findings provide fresh insight into the vital role performed by P. melaninogenica-induced functional changes in oral epithelial cells and neurons in an intricate OLP inflammatory process.

DNA methylation profiling of CpG islands in trigeminal ganglion of rats with orofacial pain induced by experimental tooth movement

BACKGROUND

Tooth movement induced orofacial pain is the most cited negative effect during orthodontic treatment, while treatment options without side effects are limited. The differential expression of pain-related genes due to DNA methylation and demethylation is instrumental in pain. The purpose of the study was to evaluate the DNA methylation profiling of CpG islands (CGI) and CGI shores in promoter regions in trigeminal ganglions (TG) of tooth movement induced orofacial pain rats, thus to further insight the DNA methylation regulation in orofacial pain.

MATERIALS AND METHODS

An orofacial pain rat model was constructed by ligating coil springs between the incisor and first maxillary molar with 40 g of force. The Rat Grimace Score (RGS) was used for pain evaluation. The genome methylation status was analyzed by the reduced representation bisulfite sequencing (RRBS) technique. Gene ontology (GO) and kyoto encyclopedia of genes and genomes (KEGG) analyses were conducted in the differentially methylated regions (DMRs). Moreover, a protein-protein interaction (PPI) network was established to detect annotated genes associated with pain.

RESULTS

RGS was significantly higher in orofacial pain rats than in sham rats. RRBS showed widespread methylation changes in CGI and CGI shores in TG promoter regions. Both 902 hypermethylated DMRs and 862 hypomethylated DMRs were found in the CGIs of promoter regions. KEGG analysis revealed that annotated genes are participated in endocrine, nervous, immune, and sensory systems. Moreover, the "Calcium signaling pathway", "Wnt signaling pathway" and "Neuroactive ligand-receptor interaction" were significantly enriched pathways. Furthermore, PPI network showed several genes (Ctnnb1, Dlg4, Creb1, Camk2g, Bmp2, etc.) with different methylation statuses were reported to be associated with pain.

CONCLUSIONS

This study demonstrated methylation changes were existed in CGI and CGI shores in TG promoter regions when pain occurs, thus providing a basis for further study on the mechanism of DNA methylation in orofacial pain.

Advancing Pain Understanding and Drug Discovery: Insights from Preclinical Models and Recent Research Findings

Despite major advancements in our understanding of its fundamental causes, pain-both acute and chronic-remains a serious health concern. Various preclinical investigations utilizing diverse animal, cellular, and alternative models are required and frequently demanded by regulatory approval bodies to bridge the gap between the lab and the clinic. Investigating naturally occurring painful disorders can speed up medication development at the preclinical and clinical levels by illuminating molecular pathways. A wide range of animal models related to pain have been developed to elucidate pathophysiological mechanisms and aid in identifying novel targets for treatment. Pain sometimes drugs fail clinically, causing high translational costs due to poor selection and the use of preclinical tools and reporting. To improve the study of pain in a clinical context, researchers have been creating innovative models over the past few decades that better represent pathological pain conditions. In this paper, we provide a summary of traditional animal models, including rodents, cellular models, human volunteers, and alternative models, as well as the specific characteristics of pain diseases they model. However, a more rigorous approach to preclinical research and cutting-edge analgesic technologies may be necessary to successfully create novel analgesics. The research highlights from this review emphasize new opportunities to develop research that includes animals and non-animals using proven methods pertinent to comprehending and treating human suffering. This review highlights the value of using a variety of modern pain models in animals before human trials. These models can help us understand the different mechanisms behind various pain types. This will ultimately lead to the development of more effective pain medications.

PDX1, a transcription factor essential for organ differentiation, regulates SERCA-dependent Ca2+ homeostasis in sensory neurons

Pancreatic and duodenal homeobox 1 (PDX1) is a transcription factor required for the development and differentiation of the pancreas. Previous studies indicated that PDX1 expression was restricted to the gastrointestinal tract. Using a cre-dependent reporter, we observed PDX1-dependent expression of tdtomato (PDX1-tom) in a subpopulation of sensory nerves. Many of these PDX1-tom afferents expressed the neurofilament 200 protein and projected to the skin. Tdtomato-labeled terminals were associated with hair follicles in the form of longitudinal and circumferential lanceolate endings suggesting a role in tactile and proprioceptive perception. To begin to examine the functional significance of PDX1 in afferents, we used Fura-2 imaging to examine calcium (Ca2+) handling under naïve and nerve injury conditions. Neuropathic injury is associated with increased intracellular Ca2+ signaling that in part results from dysregulation of the sarco/endoplasmic reticulum calcium transport ATPase (SERCA). Here we demonstrate that under naïve conditions, PDX1 regulates expression of the SERCA2B isoform in sensory neurons. In response to infraorbital nerve injury, a significant reduction of PDX1 and SERCA2B expression and dysregulation of Ca2+ handling occurs in PDX1-tom trigeminal ganglia neurons. The identification of PDX1 expression in the somatosensory system and its regulation of SERCA2B and Ca2+ handling provide a new mechanism to explain pathological changes in primary afferents that may contribute to pain associated with nerve injury.

Nociceptor mechanisms underlying pain and bone remodeling via orthodontic forces: toward no pain, big gain

Orthodontic forces are strongly associated with pain, the primary complaint among patients wearing orthodontic braces. Compared to other side effects of orthodontic treatment, orthodontic pain is often overlooked, with limited clinical management. Orthodontic forces lead to inflammatory responses in the periodontium, which triggers bone remodeling and eventually induces tooth movement. Mechanical forces and subsequent inflammation in the periodontium activate and sensitize periodontal nociceptors and produce orthodontic pain. Nociceptive afferents expressing transient receptor potential vanilloid subtype 1 (TRPV1) play central roles in transducing nociceptive signals, leading to transcriptional changes in the trigeminal ganglia. Nociceptive molecules, such as TRPV1, transient receptor potential ankyrin subtype 1, acid-sensing ion channel 3, and the P2X3 receptor, are believed to mediate orthodontic pain. Neuropeptides such as calcitonin gene-related peptides and substance P can also regulate orthodontic pain. While periodontal nociceptors transmit nociceptive signals to the brain, they are also known to modulate alveolar bone remodeling in periodontitis. Therefore, periodontal nociceptors and nociceptive molecules may contribute to the modulation of orthodontic tooth movement, which currently remains undetermined. Future studies are needed to better understand the fundamental mechanisms underlying neuroskeletal interactions in orthodontics to improve orthodontic treatment by developing novel methods to reduce pain and accelerate orthodontic tooth movement-thereby achieving "big gains with no pain" in clinical orthodontics.

The interleukin-6 signal regulates orthodontic tooth movement and pain

Orthodontic tooth movement (OTM) is accomplished by controlling the mechanical loading onto the bone around the roots of target teeth. The precise orthodontic force induces osteoclastic bone resorption on the compression side and osteoblastic bone formation on the tension side of the alveolar bone. Orthodontic intervention causes inflammation in the periodontal ligament (PDL), which manifests as acute pain. Because inflammation is deeply connected to bone remodeling, it has been indicated that the inflammation after orthodontic intervention affects both the movement of teeth and generation of pain. However, the precise mechanisms underlying the immune regulation of OTM and the related pain are not well elucidated. Here, we found from the search of a public database that the interleukin (IL)-6 family of cytokines are highly expressed in the PDL by mechanical loading. The IL-6 signal was activated in the PDL after orthodontic intervention. The signal promoted OTM by inducing osteoclastic bone resorption. IL-6 was found to increase the number of osteoclasts by suppressing apoptosis and increasing their responsiveness to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). Furthermore, IL-6 signal was shown to elicit orthodontic pain by inducing neuroinflammation in the trigeminal ganglion (TG). Taken together, it was demonstrated that the IL-6 signal regulates tooth movement and pain during orthodontic treatment. It was also indicated that local blockade of the IL-6 signal is a promising therapeutic option in orthodontic treatment, targeting both tooth movement and pain.

Brain-derived neurotrophic factor promotes orthodontic tooth movement by alleviating periodontal ligament stem cell senescence

Orthodontic treatment in older adults is more difficult than in younger adults, partially due to delayed osteogenesis caused by senescence of human periodontal ligament stem cells (hPDLSCs). The production of brain-derived neurotrophic factor (BDNF) which regulates the differentiation and survival of stem cells decreases with age. We aimed to investigate the relationship between BDNF and hPDLSC senescence and its effects on orthodontic tooth movement (OTM). We constructed mouse OTM models using orthodontic nickel‑titanium springs and compared the responses of wild-type (WT) and BDNF^+/- mice with or without addition of exogenous BDNF. In vitro, hPDLSCs subjected to the mechanical stretch were used to simulate the cell stretch environment during OTM. We extracted periodontal ligament cells from WT and BDNF^+/- mice to evaluate their senescence-related indicators. The application of orthodontic force increased BDNF expression in the periodontium of WT mice, while the mechanical stretch increased BDNF expression in hPDLSCs. Osteogenesis-related indicators, including RUNX2 and ALP decreased and cellular senescence-related indicators such as p16, p53 and β-galactosidase increased in BDNF^+/- mice periodontium. Furthermore, periodontal ligament cells extracted from BDNF^+/- mice exhibited more senescent compared with cells from WT mice. Application of exogenous BDNF decreased the expression of senescence-related indicators in hPDLSCs by inhibiting Notch3, thereby promoting osteogenic differentiation. Periodontal injection of BDNF decreased the expression of senescence-related indicators in periodontium of aged WT mice. In conclusion, our study showed that BDNF promotes osteogenesis during OTM by alleviating hPDLSCs senescence, paving a new path for future research and clinical applications.

Substance P aggravates ligature-induced periodontitis in mice

Periodontitis is one of the most common oral diseases in humans, affecting over 40% of adult Americans. Pain-sensing nerves, or nociceptors, sense local environmental changes and often contain neuropeptides. Recent studies have suggested that nociceptors magnify host response and regulate bone loss in the periodontium. A subset of nociceptors projected to periodontium contains neuropeptides, such as calcitonin gene-related peptide (CGRP) or substance P (SP). However, the specific roles of neuropeptides from nociceptive neural terminals in periodontitis remain to be determined. In this study, we investigated the roles of neuropeptides on host responses and bone loss in ligature-induced periodontitis. Deletion of tachykinin precursor 1 (Tac1), a gene that encodes SP, or treatment of gingiva with SP antagonist significantly reduced bone loss in ligature-induced periodontitis, whereas deletion of calcitonin related polypeptide alpha (Calca), a gene that encodes CGRP, showed a marginal role on bone loss. Ligature-induced recruitment of leukocytes, including neutrophils, and increase in cytokines leading to bone loss in periodontium was significantly less in Tac1 knockout mice. Furthermore, intra-gingival injection of SP, but not neurokinin A, induced a vigorous inflammatory response and osteoclast activation in alveolar bone and facilitated bone loss in ligature-induced periodontitis. Altogether, our data suggest that SP plays significant roles in regulating host responses and bone resorption in ligature-induced periodontitis.

Potential Mechanisms Underlying Marijuana-Associated Periodontal Tissue Destruction

While definitive evidence awaits, cannabis is emerging as a likely risk factor for periodontal tissue destruction. The mechanisms that underlie potential cannabis-induced or cannabis-enhanced periodontal diseases, however, remain to be elucidated. Herein, we 1) examine insights obtained from the endocannabinoid system, 2) summarize animal models of exposure to cannabinoid receptor agonists and antagonists, 3) review the evidence suggesting that cannabis and cannabis-derived molecules exert a profound influence on components of the oral microbiome, and 4) assess studies indicating that marijuana and phytocannabinoids compromise the immune response to plaque. Furthermore, we address how knowledge of cannabinoid influences in the oral cavity may be exploited to provide potential novel periodontal therapeutics, while recognizing that such medicinal approaches may be most appropriate for nonhabitual marijuana users. The suspected increase in susceptibility to periodontitis in marijuana users is multifaceted, and it is clear that we are only beginning to understand the complex toxicological, cellular, and microbial interactions involved. With marijuana consumption increasing across all societal demographics, periodontal complications of use may represent a significant, growing oral health concern. In preparation, an enhanced research response would seem appropriate.

Ablation of transient receptor potential vanilloid subtype 1-expressing neurons in rat trigeminal ganglia aggravated bone resorption in periodontitis with diabetes

OBJECTIVES

We aimed to investigate the contribution of neurons expressing transient receptor potential vanilloid subtype 1 (TRPV1) to alveolar bone homeostasis in periodontitis with diabetes.

DESIGN

Diabetes was induced by streptozotocin injection in Sprague-Dawley rats. Resiniferatoxin was injected into left trigeminal ganglia to ablate TRPV1-expressing neurons. 3-0 silks were tied around left maxillary second molars to induce experimental periodontitis. Alveolar bone was assessed by micro-computed tomography and tartrate-resistant acid phosphatase staining. Macrophages were detected by immunohistochemistry staining.

RESULTS

TRPV1 expression in trigeminal ganglia was increased in diabetic rats compared to non-diabetic counterparts. Local ablation of TRPV1 eliminated facial heat hyperalgesia but aggravated alveolar bone damage and osteoclastogenesis in experimental periodontitis in both diabetic and non-diabetic rats. Immunohistochemistry staining presented enhanced macrophage infiltration and M1 macrophage polarization in periodontal lesions in TRPV1-ablated groups.

CONCLUSIONS

These findings demonstrated that TRPV1 expression in trigeminal ganglia could be enhanced in diabetic condition, and the integrity of TRPV1-expressing neurons in trigeminal ganglia exerted a neuroprotective effect against alveolar bone resorption and inflammation in diabetic periodontitis.

Irradiation of the Normal Murine Tongue Causes Upregulation and Activation of Transient Receptor Potential (TRP) Ion Channels

Signal transduction at sensory neurons occurs via transmembrane flux of cations, which is largely governed by the transient receptor potential (TRP) family of ion channels. It is unknown whether TRP channel activation contributes to the pain that accompanies radiation-induced oral mucositis. This study sought to characterize changes in TRP channel expression and function that occur in the locally irradiated tissues and afferent neurons of mice. Female CD-1 mice received single high-dose (27 Gy) tongue irradiation, or sham irradiation. Animals were euthanized either before overt glossitis developed (days 1 and 5 postirradiation), when glossitis was severe (day 11), or after mice had recovered (days 21 and 45). Tongue irradiation caused upregulation of the Trpv1 gene in trigeminal ganglia (TG) neurons. Other TRP genes (Trpv2, Trpv4, Trpa1, Trpm8) and Gfrα3 (which acts upstream of several TRP channels) were also upregulated in TGs and/or tongue tissue, in response to radiation. Ex vivo calcium imaging experiments demonstrated that the proportions of TG neurons responding to histamine (an activator of TRPV1, TRPV4 and TRPA1), TNF-α (an activator of TRPV1, TRPV2 and TRPV4), and capsaicin (a TRPV1 agonist), were increased as early as one day after tongue irradiation; these changes persisted for at least 21 days. In a subsequent experiment, we found that genetic deletion of TRPV1 mitigated weight loss (a surrogate marker of pain severity) in mice with severe glossitis. The results intimate that various TRP channels, and TRPV1 in particular, should be explored as analgesic targets for patients experiencing pain after oral irradiation.

Orthodontic force induces nerve injury-like transcriptomic changes driven by TRPV1-expressing afferents in mouse trigeminal ganglia

Orthodontic force produces mechanical irritation and localized inflammation in the periodontium, which causes pain in most patients. Nocifensive behaviors resulting from orthodontic force in mice can be substantially attenuated by intraganglionic injection of resiniferatoxin (RTX), a neurotoxin that specifically ablates a subset of neurons expressing transient receptor potential vanilloid 1 (TRPV1). In the current study, we determined changes in the transcriptomic profiles in the trigeminal ganglia (TG) following the application of orthodontic force, and assessed the roles of TRPV1-expressing afferents in these transcriptomic changes. RTX or vehicle was injected into the TG of mice a week before the placement of an orthodontic spring exerting 10 g of force. After 2 days, the TG were collected for RNA sequencing. The application of orthodontic force resulted in 1279 differentially expressed genes (DEGs) in the TG. Gene ontology analysis showed downregulation of gliogenesis and ion channel activities, especially of voltage-gated potassium channels. DEGs produced by orthodontic force correlated more strongly with DEGs resulting from nerve injury than from inflammation. Orthodontic force resulted in the differential expression of multiple genes involved in pain regulation, including upregulation of Atf3, Adcyap1, Bdnf, and Csf1, and downregulation of Scn10a, Kcna2, Kcnj10, and P2ry1. Orthodontic force-induced DEGs correlated with DEGs specific to multiple neuronal and non-neuronal subtypes following nerve injury. These transcriptomic changes were abolished in the mice that received the RTX injection. These results suggest that orthodontic force produces transcriptomic changes resembling nerve injury in the TG and that nociceptive inputs through TRPV1-expressing afferents leads to subsequent changes in gene expression not only in TRPV1-positive neurons, but also in TRPV1-negative neurons and non-neuronal cells throughout the ganglia. Orthodontic force-induced transcriptomic changes might be an active regenerative program of trigeminal ganglia in response to axonal injury following orthodontic force.

Varied temporal expression patterns of trigeminal TRPA1 and TRPV1 and the neuropeptide CGRP during orthodontic force-induced pain

OBJECTIVE

The aim of this study was to quantify the temporal changes in inflammation and TRPA1, TRPV1 and CGRP expression in the trigeminal ganglion during force-induced orthodontic pain.

DESIGN

Orthodontic force was applied to both maxillary first molars in 8-week-old Wistar rats for 12 h, 24 h, 3 d or 7 d. The rat grimace scale (RGS) score and duration of face grooming were used to measure orthodontic pain. Western blotting was performed to assess TRPA1, TRPV1 and CGRP expression in trigeminal ganglia. NF-кB levels and colocalization of TRPA1, TRPV1 and CGRP were evaluated by immunofluorescent staining.

RESULTS

Application of continuous force significantly increased pain behaviours at 1 and 3 d. NF-кB significantly increased in periodontal ligament at 12 h until 3 d. TRPV1 was significantly elevated within 1 d; TRPA1 significantly increased from 1-3 d; CGRP expression significantly increased from 12 h to 3 d. The TRPV1/TRPA1 expression ratio was highest at 12 h; the TRPA1/TRPV1 ratio peaked at 3 d. The percentages of trigeminal neurons co-expressing TRPA1/TRPV1, TRPA1/CGRP, and TRPV1/CGRP significantly increased by 12 h and peaked at 24 h. CGRP expression had a stronger positive correlation with TRPV1 than TRPA1.

CONCLUSIONS

Inflammation induced by application of orthodontic force sensitizes trigeminal TRPV1 and TRPA1; TRPV1 is primarily activated as an early response, whereas TRPA1 is activated as a late response. Activation of both nociceptors results in CGRP release. Thus, blocking both TRPV1 and TRPA1 may represent a primary therapeutic target for relief of orthodontic pain.

Acute and Chronic Pain from Facial Skin and Oral Mucosa: Unique Neurobiology and Challenging Treatment

The oral cavity is a portal into the digestive system, which exhibits unique sensory properties. Like facial skin, the oral mucosa needs to be exquisitely sensitive and selective, in order to detect harmful toxins versus edible food. Chemosensation and somatosensation by multiple receptors, including transient receptor potential channels, are well-developed to meet these needs. In contrast to facial skin, however, the oral mucosa rarely exhibits itch responses. Like the gut, the oral cavity performs mechanical and chemical digestion. Therefore, the oral mucosa needs to be insensitive, to some degree, in order to endure noxious irritation. Persistent pain from the oral mucosa is often due to ulcers, involving both tissue injury and infection. Trigeminal nerve injury and trigeminal neuralgia produce intractable pain in the orofacial skin and the oral mucosa, through mechanisms distinct from those seen in the spinal area, which is particularly difficult to predict or treat. The diagnosis and treatment of idiopathic chronic pain, such as atypical odontalgia (idiopathic painful trigeminal neuropathy or post-traumatic trigeminal neuropathy) and burning mouth syndrome, remain especially challenging. The central integration of gustatory inputs might modulate chronic oral and facial pain. A lack of pain in chronic inflammation inside the oral cavity, such as chronic periodontitis, involves the specialized functioning of oral bacteria. A more detailed understanding of the unique neurobiology of pain from the orofacial skin and the oral mucosa should help us develop novel methods for better treating persistent orofacial pain.

Transient receptor potential vanilloid subtype 1 depletion mediates mechanical allodynia through cellular signal alterations in small-fiber neuropathy

Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodal nociceptor that monitors noxious thermal sensations. Few studies have addressed the role of TRPV1 in mechanical allodynia in small-fiber neuropathy (SFN) caused by sensory nerve damage. Accordingly, this article reviews the putative mechanisms of TRPV1 depletion that mediates mechanical allodynia in SFN. The intraepidermal nerve fibers (IENFs) degeneration and sensory neuronal injury are the primary characteristics of SFN. Intraepidermal nerve fibers are mainly C-polymodal nociceptors and Aδ-fibers, which mediated allodynic pain after neuronal sensitization. TRPV1 depletion by highly potent neurotoxins induces the upregulation of activating transcription factor 3 and IENFs degeneration which mimics SFN. TRPV1 is predominately expressed by the peptidergic than nonpeptidergic nociceptors, and these neurochemical discrepancies provided the basis of the distinct pathways of thermal analgesia and mechanical allodynia. The depletion of peptidergic nociceptors and their IENFs cause thermal analgesia and sensitized nonpeptidergic nociceptors respond to mechanical allodynia. These distinct pathways of noxious stimuli suggested determined by the neurochemical-dependent neurotrophin cognate receptors such as TrkA and Ret receptors. The neurogenic inflammation after TRPV1 depletion also sensitized Ret receptors which results in mechanical allodynia. The activation of spinal TRPV1(+) neurons may contribute to mechanical allodynia. Also, an imbalance in adenosinergic analgesic signaling in sensory neurons such as the downregulation of prostatic acid phosphatase and adenosine A1 receptors, which colocalized with TRPV1 as a membrane microdomain also correlated with the development of mechanical allodynia. Collectively, TRPV1 depletion-induced mechanical allodynia involves a complicated cascade of cellular signaling alterations.

Methods Used and Application of the Mouse Grimace Scale in Biomedical Research 10 Years on: A Scoping Review

The Mouse Grimace Scale (MGS) was developed 10 years ago as a method for assessing pain through the characterisation of changes in five facial features or action units. The strength of the technique is that it is proposed to be a measure of spontaneous or non-evoked pain. The time is opportune to map all of the research into the MGS, with a particular focus on the methods used and the technique's utility across a range of mouse models. A comprehensive scoping review of the academic literature was performed. A total of 48 articles met our inclusion criteria and were included in this review. The MGS has been employed mainly in the evaluation of acute pain, particularly in the pain and neuroscience research fields. There has, however, been use of the technique in a wide range of fields, and based on limited study it does appear to have utility for pain assessment across a spectrum of animal models. Use of the method allows the detection of pain of a longer duration, up to a month post initial insult. There has been less use of the technique using real-time methods and this is an area in need of further research.

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.

Local delivery of simvastatin maintains tooth anchorage during mechanical tooth moving via anti-inflammation property and AMPK/MAPK/NF-kB inhibition

Simvastatin (SMV) could increase tooth anchorage during orthodontic tooth movement (OTM). However, previous studies on its bone‐specific anabolic and anti‐inflammation properties were based on static in vitro and in vivo conditions. AMPK is a stress‐activated kinase that protects tissue against serious damage from overloading inflammation. Rat periodontal ligament cells (PDLCs) were subjected to a serial of SMV concentrations to investigate the optimization that promoted osteogenic differentiation. The PDLCs in static and/or tensile culturing conditions then received the proper concentration SMV. Related factors expression was measured by the protein array, real‐time PCR and Western blot. The 0.05UM SMV triggered osteogenic differentiation of PDLCs. The inhibition of AMPK activation through a pharmacological approach (Compound C) caused dramatic decrease in osteogenic/angiogenic gene expression and significant increase in inflammatory NF‐κB phosphorylation. In contrast, pharmacological activation of AMPK by AICAR significantly inhibited inflammatory factors expression and activated ERK1/2, P38 MAPK phosphorylation. Moreover, AMPK activation induced by SMV delivery significantly attenuated the osteoclastogenesis and decreased the expression of pro‐inflammatory TNF‐α and NF‐κB in a rodent model of OTM. The current studies suggested that SMV could intrigue intrinsic activation of AMPK in PDLCs that promote attenuate the inflammation which occurred under tensile irritation through AMPK/MAPK/NF‐kB Inhibition.

Fight fire with fire: Neurobiology of capsaicin-induced analgesia for chronic pain

Capsaicin, the pungent ingredient in chili peppers, produces intense burning pain in humans. Capsaicin selectively activates the transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptive primary afferents, and underpins the mechanism for capsaicin-induced burning pain. Paradoxically, capsaicin has long been used as an analgesic. The development of topical patches and injectable formulations containing capsaicin has led to application in clinical settings to treat chronic pain conditions, such as neuropathic pain and the potential to treat osteoarthritis. More detailed determination of the neurobiological mechanisms of capsaicin-induced analgesia should provide the logical rationale for capsaicin therapy and help to overcome the treatment's limitations, which include individual differences in treatment outcome and procedural discomfort. Low concentrations of capsaicin induce short-term defunctionalization of nociceptor terminals. This phenomenon is reversible within hours and, hence, likely does not account for the clinical benefit. By contrast, high concentrations of capsaicin lead to long-term defunctionalization mediated by the ablation of TRPV1-expressing afferent terminals, resulting in long-lasting analgesia persisting for several months. Recent studies have shown that capsaicin-induced Ca²⁺/calpain-mediated ablation of axonal terminals is necessary to produce long-lasting analgesia in a mouse model of neuropathic pain. In combination with calpain, axonal mitochondrial dysfunction and microtubule disorganization may also contribute to the longer-term effects of capsaicin. The analgesic effects subside over time in association with the regeneration of the ablated afferent terminals. Further determination of the neurobiological mechanisms of capsaicin-induced analgesia should lead to more efficacious non-opioidergic analgesic options with fewer adverse side effects.

The Utility of Grimace Scales for Practical Pain Assessment in Laboratory Animals

Simple Summary

Grimace scales for laboratory animals were first reported ten years ago. Yet, despite their promise as pain assessment tools it appears that they have not been implemented widely in animal research establishments for clinical pain assessment. We discuss potential reasons for this based on the knowledge gained to date on their use and suggest avenues for further research, which might improve uptake of their use in laboratory animal medicine.

Abstract

Animals’ facial expressions are widely used as a readout for emotion. Scientific interest in the facial expressions of laboratory animals has centered primarily on negative experiences, such as pain, experienced as a result of scientific research procedures. Recent attempts to standardize evaluation of facial expressions associated with pain in laboratory animals has culminated in the development of “grimace scales”. The prevention or relief of pain in laboratory animals is a fundamental requirement for in vivo research to satisfy community expectations. However, to date it appears that the grimace scales have not seen widespread implementation as clinical pain assessment techniques in biomedical research. In this review, we discuss some of the barriers to implementation of the scales in clinical laboratory animal medicine, progress made in automation of collection, and suggest avenues for future research.

Different Therapeutic Effects of CO2 and Diode Laser Irradiation on Tooth Movement-Related Pain

Although orthodontic treatment is common, orthodontic force often induced pain. Low-level laser therapy (LLLT) has been investigated to improve therapeutic comfort. In dentistry, LLLT is mainly applied using two types of lasers, CO₂ and diode lasers, whose biological actions are thought to be associated with wavelength (CO₂: 10,600 nm; diode: 808 nm). The analgesic effect of LLLT on orthodontic treatment-related pain is widely reported but inconsistent. This study aimed to (1) determine whether irradiation with a CO₂ or diode laser attenuates orthodontic treatment-related pain using the jaw-opening reflex model, (2) elucidate the optimal irradiation protocol for both lasers to obtain the maximal analgesic effect, (3) evaluate the effects of laser irradiation on other biological features [e.g., tooth movement, glial fibrillary acidic protein (GFAP) expression, and temperature alterations] and (4) investigate the mechanism underlying the analgesic effect of laser irradiation. In this animal model, orthodontic treatment-induced pain manifested as a significantly reduced the threshold for inducing the jaw-opening reflex on the orthodontically treated side compared with the contralateral side. GFAP expression in the bilateral trigeminal ganglia (TGs) was significantly increased by the application of orthodontic force. CO₂ laser irradiation of the orthodontically treated region significantly increased the threshold for inducing the jaw-opening reflex and the peripheral temperature. Similar reductions in jaw-opening reflex excitability were induced by surface anesthesia and thermal stimulation but not, the diode laser. Neither CO₂ nor diode laser irradiation altered GFAP expression in the TGs. Infiltration anesthesia also significantly increased the threshold for inducing the jaw-opening reflex on each anesthetized side. Irradiation (30 s) by either laser immediately after orthodontic force application (preirradiation) significantly decreased jaw-opening reflex excitability and GFAP expression in the bilateral TGs the next day. However, thermal stimulation immediately after orthodontic force application failed to alter jaw-opening reflex excitability the next day. Laser irradiation did not alter tooth movement; however, an optimized irradiation protocol for aiding tooth movement is suggested. In conclusion, both CO₂ and diode lasers are able to prevent orthodontic treatment-related pain. Furthermore, the involvement of temperature alterations and surface anesthesia in the analgesic effect induced by CO₂ laser irradiation is suggested.