Capsaicin-evoked iCGRP release from human dental pulp: a model system for the study of peripheral neuropeptide secretion in normal healthy tissue

CGRP Released by Corneal Sensory Nerve Maintains Tear Secretion of the Lacrimal Gland

Purpose

This study aims to elucidate the calcitonin gene-related peptide (CGRP) mediation and primary mechanism of corneal sensory nerves on tear production of the lacrimal gland.

Methods

Mouse corneal denervation models were constructed through surgical axotomy, pharmacologic treatment with capsaicin or resiniferatoxin, and Trpv1-Cre/DTR mice with diphtheria toxin injection. The capsaicin-treated mice received subconjunctival injection of CGRP or substance P, while the normal C57BL/6J mice were administered with CGRP receptor antagonist BIBN-4096. Furthermore, double immunostaining of c-FOS+ and choline acetyltransferase was used to evaluate the activation of the superior salivatory nucleus (SSN). Mouse lacrimal glands were collected for transcriptomic sequencing and subsequent RNA and protein expression analysis.

Results

The corneal denervated mice exhibited a significant reduction in corneal sensitivity and tear secretion. In capsaicin-treated mice, tear secretion decreased to 2.5 ± 0.5 mm compared to 6.3 ± 0.9 mm in control mice (P < 0.0001). However, exogenous administration of CGRP in capsaicin-treated mice increased tear secretion from 2.6 ± 0.5 mm to 4.5 ± 0.5 mm (P = 0.0009), while BIBN-4096 treatment reduced tear secretion to 3.4 ± 0.5 mm when compared to 7.3 ± 0.7 mm in control mice (P = 0.0022). Furthermore, c-FOS+ cell number in the SSN increased by twofold (P = 0.0168) after CGRP administration compared with capsaicin-treated mice. In addition, the expressions of CCNA2, Ki67, PCNA, and CDK1 in acinar cells of the lacrimal gland were impaired by corneal denervation and alleviated by CGRP administration.

Conclusions

CGRP released by corneal sensory nerves mediates tear secretion of the lacrimal gland, providing a new strategy for improving tear secretion in patients with neurotrophic keratitis.

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.

Exercise Improves Orofacial Pain and Modifies Neuropeptide Expression in a Rat Model of Parkinson's Disease

Pain is a common non-motor symptom of Parkinson's disease (PD), which often occurs in the early disease stages. Despite the high prevalence, it remains inadequately treated. In a hemi-parkinsonian rat model, we aimed to investigate the neurochemical factors involved in orofacial pain development, with a specific focus on pain-related peptides and cannabinoid receptors. We also evaluated whether treadmill exercise could improve orofacial pain and modulate these mechanisms. Rats were unilaterally injected in the striatum with either 6-hydroxydopamine (6-OHDA) or saline. Fifteen days after stereotactic surgery, the animals were submitted to treadmill exercise (EX), or remained sedentary (SED). Pain assessment was performed before the surgical procedure and prior to each training session. Pain-related peptides, substance P (SP), calcitonin gene-related peptide (CGRP), and transient receptor potential vanilloid type 1 (TRPV1) activation and cannabinoid receptor type 1 (CB1) and type 2 (CB2) were evaluated in the trigeminal nucleus. In order to confirm the possible involvement of cannabinoid receptors, we also injected antagonists of CB1 and CB2 receptors. We confirmed the presence of orofacial pain after unilateral 6-OHDA-injection, which improved after aerobic exercise training. We also observed increased pain-related expression of SP, CGRP and TRPV1 and decreased CB1 and CB2 in the trigeminal ganglion and caudal spinal trigeminal nucleus in animals with PD, which was reversed after aerobic exercise training. In addition, we confirm the involvement of cannabinoid receptors since both antagonists decreased the nociceptive threshold of PD animals. These data suggest that aerobic exercise effectively improved the orofacial pain associated with the PD model, and may be mediated by pain-related neuropeptides and cannabinoid receptors in the trigeminal system.

The Role of Asiatic Acid in Preventing Dental Pulp Inflammation: An in-vivo Study

Purpose

Acute dental pulp inflammation necessitates early treatment to alleviate inflammation and pain. In the inflammatory phase, a substance is required to lower the inflammatory mediators and reactive oxygen species that play a crucial role in that phase. Asiatic acid is a natural triterpene obtained from the Centella asiatica plant with a high antioxidant value. This study examined the effect of Asiatic acid's antioxidant, anti-inflammatory, and antinociceptive properties on dental pulp inflammation.

Methods

The research is an experimental laboratory, with a post-test only with a control group design. The study utilised 40 male Wistar rats weighing 200-250 grams and aged 8-10 weeks. Rats were divided into five groups (control, eugenol, Asiatic Acid 0.5%; 1%; 2% group). Dental pulp inflammation was created in the maxillary incisor after six hours of administration of lipopolysaccharides (LPS). The dental pulp treatment then continued with the administration of eugenol and three different Asiatic acid concentrations (0.5%, 1% and 2%). In the next 72 hours, the teeth were biopsied, and the dental pulp was analysed using the enzyme-linked immunosorbent assay (ELISA) to measure the level of MDA, SOD, TNF-α, beta-endorphins and CGRP. Histopathological examination and the Rat Grimace Scale were utilised to determine the level of inflammation and pain, respectively.

Results

The effect of Asiatic Acid on MDA, TNF-α, and CGRP levels decreased significantly compared to the control group (p=<0.001). On the SOD and beta-endorphin levels, Asiatic acid treatment resulted in a considerable rise (p =<0.001).

Conclusion

Due to its antioxidant, anti-inflammatory, and antinociceptive characteristics, Asiatic acid can reduce inflammation and pain in acute pulp inflammation due to its ability to decrease MDA, TNFα, and CGRP levels while raising SOD and beta-endorphin levels.

Gαs-Coupled CGRP Receptor Signaling Axis from the Trigeminal Ganglion Neuron to Odontoblast Negatively Regulates Dentin Mineralization

An inflammatory response following dental pulp injury and/or infection often leads to neurogenic inflammation via the axon reflex. However, the detailed mechanism underlying the occurrence of the axon reflex in the dental pulp remains unclear. We sought to examine the intracellular cyclic adenosine monophosphate (cAMP) signaling pathway in odontoblasts via the activation of G_s protein-coupled receptors and intercellular trigeminal ganglion (TG) neuron-odontoblast communication following direct mechanical stimulation of TG neurons. Odontoblasts express heterotrimeric G-protein α-subunit Gα_s and calcitonin receptor-like receptors. The application of an adenylyl cyclase (AC) activator and a calcitonin gene-related peptide (CGRP) receptor agonist increased the intracellular cAMP levels ([cAMP]_i) in odontoblasts, which were significantly inhibited by the selective CGRP receptor antagonist and AC inhibitor. Mechanical stimulation of the small-sized CGRP-positive but neurofilament heavy chain-negative TG neurons increased [cAMP]_i in odontoblasts localized near the stimulated neuron. This increase was inhibited by the CGRP receptor antagonist. In the mineralization assay, CGRP impaired the mineralization ability of the odontoblasts, which was reversed by treatment with a CGRP receptor antagonist and AC inhibitor. CGRP establishes an axon reflex in the dental pulp via intercellular communication between TG neurons and odontoblasts. Overall, CGRP and cAMP signaling negatively regulate dentinogenesis as defensive mechanisms.

Endogenous Mas-related G-protein-coupled receptor X1 activates and sensitizes TRPA1 in a human model of peripheral nerves

Mas-related G-protein-coupled receptor X1 (MrgprX1) is a human-specific Mrgpr and its expression is restricted to primary sensory neurons. However, its role in nociception and pain signaling pathways is largely unknown. This study aims to investigate a role for MrgprX1 in nociception via interaction with the pain receptor, Transient Receptor Potential Ankyrin 1 (TRPA1), using in-vitro and in-vivo human neuronal models. MrgprX1 protein expression in human trigeminal nociceptors was investigated by the immunolabeling of the dental pulp and cultured peripheral neuronal equivalent (PNE) cells. MrgprX1 receptor signaling was monitored by Fura-2-based Ca²⁺ imaging using PNEs and membrane potential responses were measured using FluoVolt^TM . Immunofluorescent staining revealed MrgprX1 expression in-vivo in dental afferents, which was more intense in inflamed compared to healthy dental pulps. Endogenous MrgprX1 protein expression was confirmed in the in-vitro human PNE model. MrgprX1 receptor signaling and the mechanisms through which it couples to TRPA1 were studied by Ca²⁺ imaging. Results showed that MrgprX1 activates TRPA1 and induces membrane depolarization in a TRPA1 dependent manner. In addition, MrgprX1 sensitizes TRPA1 to agonist stimulation via Protein Kinase C (PKC). The activation and sensitization of TRPA1 by MrgprX1 in a model of human nerves suggests an important role for this receptor in the modulation of nociception.

Trigeminal Sensory Neurons and Pulp Regeneration

The pulp-dentin complex is innervated by a high density of trigeminal neurons free nerve endings. These neuronal fibers are highly specialized to sense noxious stimuli such as thermal, mechanical, chemical, and biological cues. This robust alert system provides immediate feedback of potential or actual injury triggering reflex responses that protect the teeth from further injury. In the case of patients, pain is the most important experience that leads them to seek oral health care. The adequate removal of the etiology, such as caries, provides ample opportunity for the robust reparative and regenerative potential of the pulp-dentin complex to restore homeostasis. In addition to this elaborated surveillance system, evidence has accumulated that sensory neuronal fibers can potentially modulate various steps of the reparative and regenerative process through cellular communication processes. These include modulation of immunologic, angiogenic, and mineralization responses. Despite these orchestrated cellular events, the defense of the pulp-dentin complex may be overwhelmed, resulting in pulp necrosis and apical periodontitis. Regenerative endodontic procedures have evolved to restore the once lost function of the pulp-dentin complex. After these procedures, a large subset of successful cases demonstrates a positive response to sensitivity testing, suggesting reinnervation of the canal space. This process is likely mediated through cellular and noncellular release of neurotrophic factors such as brain-derived nerve growth factor. In addition, these newly recruited nerve fibers appear equipped to sense thermal stimuli through nonhydrodynamic mechanisms. Collectively, the significance of innervation in the normal physiology of the pulp-dentin complex and its role in regeneration need to be better appreciated to promote further research in this area that could potentially bring new therapeutic opportunities.

Expression of Neurotrophic Factors in Human Dentin and Their Regulation of Trigeminal Neurite Outgrowth

INTRODUCTION

Neurotrophic factors play a significant role in the innervation of the pulp-dentin complex during and after organogenesis. There have been numerous bioactive molecules identified in the dentin extracellular matrix; however, the expression of neurotrophic factors in the dentin matrix and their biological activity are largely unknown. The purpose of this study was to characterize the relative expression of neurotrophic factors in human dentin matrix proteins (DMPs) and their effect on neurite outgrowth of trigeminal (TG) neurons.

METHODS

Dentin was powdered in liquid nitrogen from noncarious human third molar teeth. DMPs were solubilized through an EDTA extraction method, dialyzed, and lyophilized until use. The relative expression of nerve growth factor, brain-derived neurotrophic factor, glial cell-line derived neurotrophic factor, neurotrophin 3, and neurotrophin 4/5 was determined by the enzyme-linked immunosorbent assay. Rat TG neurons were cultured and exposed to different concentrations of DMPs (1-10⁵ ng/mL) or vehicle, and a quantitative neurite outgrowth assay was performed.

RESULTS

Human DMPs contained all of the tested neurotrophic factors, with glial cell-line derived neurotrophic factor and neurotrophin 4/5 found at the highest levels. DMPs were able to promote the neurite outgrowth of rat TG neurons at an optimum concentration of 10-10² ng/mL, whereas the effect was partially inhibited at higher concentrations (>10³ ng/mL).

CONCLUSIONS

The human dentin extracellular matrix is a rich reservoir for neurotrophic factors that are key components for neuronal homeostasis, differentiation, and regeneration. These data suggest that neurotrophins in DMPs could play an important role as signaling molecules for the innervation of the pulp-dentin complex during the processes of tooth formation, repair, and regeneration.

Peripheral sympathetic mechanisms in orofacial pain

Sympathetic nervous system (SNS) is a part of the autonomic nervous system which involuntarily regulates internal body functions. It appears to modulate the processing of nociceptive information. Many orofacial pain conditions involve inflammation of orofacial tissues and/or injury of nerve, some of which might be attributed to SNS. Thus, the aim of this review was to bring together the data available regarding the peripheral sympathetic mechanisms involved in orofacial pain. A clearer understanding of SNS–sensory interactions in orofacial pain may provide a basis for novel therapeutic strategies for conditions that respond poorly to conventional treatments.

TRPV1-Like Immunoreactivity in the Human Locus K, a Distinct Subregion of the Cuneate Nucleus

The presence of transient receptor potential vanilloid type-1 receptor (TRPV1)-like immunoreactivity (LI), in the form of nerve fibres and terminals, is shown in a set of discrete gray matter subregions placed in the territory of the human cuneate nucleus. We showed previously that those subregions share neurochemical and structural features with the protopathic nuclei and, after the ancient name of our town, collectively call them Locus Karalis, and briefly Locus K. TRPV1-LI in the Locus K is codistributed, though not perfectly overlapped, with that of the neuropeptides calcitonin gene-related peptide and substance P, the topography of the elements immunoreactive to the three markers, in relation to each other, reflecting that previously described in the caudal spinal trigeminal nucleus. Myelin stainings show that myelinated fibres, abundant in the cuneate, gracile and trigeminal magnocellular nuclei, are scarce in the Locus K as in the trigeminal substantia gelatinosa. Morphometric analysis shows that cell size and density of Locus K neurons are consistent with those of the trigeminal substantia gelatinosa and significantly different from those of the magnocellular trigeminal, solitary and dorsal column nuclei. We propose that Locus K is a special component of the human dorsal column nuclei. Its functional role remains to be determined, but TRPV1 appears to play a part in it.

Calcitonin-gene related peptide is a potent inducer of oedema in rat orofacial tissue

BACKGROUND AND AIMS

This study aimed to assess the potential of calcitonin-gene related peptide (CGRP), a neuropeptide released from sensory nerves, to induce oedema in orofacial tissue.

EXPERIMENTAL APPROACH

Wistar rats (150-200 g) anesthetized with isoflurane were injected intraorally with CGRP (100 μl; 8-33 pmol) in the right side of the mouth. The contralateral side was injected with the same volume of physiological saline. Increased cheek thickness (in mm), as a measure of oedema formation, was assayed bilaterally with a digital caliper before (T = 0) and up to 24 h following injection of CGRP. Pretreatment with antagonists (CGRP_8-37, 10 nmol; pizotifen, 2 mg/kg) was given by intra-oral or subcutaneous injection, 10 or 30 min, respectively, before the inflammatory stimulus. CGRP and CGRP_8-37 were also injected into the rat hind paw to induce oedema. Data are presented as the mean (±SEM) difference in thickness between the right and the left sides at each time.

RESULTS

Following intra-oral injection, CGRP induced a rapidly developing (5-15 min) and long-lasting (6 h), dose-dependent oedema in the rat cheek, blocked by pre-treatment with CGRP_8-37 or pizotifen. CGRP induced a smaller oedematogenic effect in the rat hind paw also blocked by the CGRP antagonist. CGRP (16 pmol) potentiated the oedema induced by co-injected substance P (3.7 nmol) and contributed to the oedema following intraoral injection of carrageenan (100 μg). Injection of CGRP_8-37 alone induced an early but short-lasting oedema.

CONCLUSION

Local injection of CGRP potently induced oedema in the orofacial tissue of rats which was blocked by a CGRP receptor antagonist. The overall inhibition of carrageenan-induced oedema by CGRP_8-37 suggests that endogenous CGRP contributes to an oedematogenic response in orofacial tissues.

Orthodontic force application upregulated pain-associated prostaglandin-I2/PGI2-receptor/TRPV1 pathway-related gene expression in rat molars

This study aimed to analyze the mRNA expression and protein localization of prostaglandin I₂ (PGI₂) synthase (PGIS), the PGI₂ receptor (IP receptor) and transient receptor potential cation channel, subfamily V, member 1 (TRPV1) in force-stimulated rat molars, toward the elucidation of the PGI₂-IP receptor-TRPV1 pathway that is in operation in the pulp and possibly associated with orthodontic pain and inflammation. Experimental force was applied to the maxillary first and second molars by inserting an elastic band between them for 6-72 h. PGIS, PTGIR (the IP receptor gene), and TRPV1 mRNA levels in the coronal pulp were analyzed with real-time PCR. PGIS, IP receptor, and TRPV1 proteins were immunostained. The force stimulation induced significant upregulation of PGIS at 6-24 h, and PTGIR and TRPV1 at 6 and 12 h in the pulp. PGIS was immunolocalized in odontoblasts and some fibroblasts in the force-stimulated pulp. The IP receptor and TRPV1 immunoreactivities were detected on odontoblasts and some nerve fibers. It was concluded that PGIS, PTGIR, and TRPV1 in rat molar pulp were significantly upregulated shortly after the force application, and that the IP receptor was co-expressed on TRPV1-expressing nerves and odontoblasts. These findings suggest that the PGI₂-IP receptor-TRPV1 pathway is associated with the acute phase of force-induced pulp changes involving odontoblasts and nerves.

Agonist-dependence of functional properties for common nonsynonymous variants of human transient receptor potential vanilloid 1

Transient receptor potential vanilloid 1 (TRPV1) is a polymodal receptor activated by capsaicin, heat, and acid, which plays critical roles in thermosensation and pain. In addition, TRPV1 also contributes to multiple pathophysiological states in respiratory, cardiovascular, metabolic, and renal systems. These contributions are further supported by evidence that variations in the human TRPV1 (hTRPV1) gene are associated with various physiological and pathological phenotypes. However, it is not well understood how the variations in hTRPV1 affect channel functions. In this study, we examined functional consequences of amino acid variations of hTRPV1 induced by 5 nonsynonymous single-nucleotide polymorphisms (SNPs) that most commonly exist in the human population. Using electrophysiological assays in HEK293 cells, we examined 9 parameters: activation, Ca permeation, and desensitization after activation by capsaicin, acid, and heat. Our results demonstrated that the 5 SNPs differentially affected functional properties of hTRPV1 in an agonist-dependent manner. Based upon the directionality of change of each phenotype and cumulative changes in each SNP, we classified the 5 SNPs into 3 presumptive functional categories: gain of function (hTRPV1 Q85R, P91S, and T469I), loss of function (I585V), and mixed (M315I). These results reveal a spectrum of functional variation among common hTRPV1 polymorphisms in humans and may aid mechanistic interpretation of phenotypes associated with nonsynonymous hTRPV1 SNPs under pathophysiological conditions.

Capsaicin-sensitive Innervation Modulates the Development of Apical Periodontitis

INTRODUCTION

Nociceptive neurons play a critical role in the detection of stimuli evoking actual or potential tissue injury. In addition, they are involved in neurogenic inflammation by the peripheral release of neuropeptides such as calcitonin gene-related peptide (CGRP). The dental pulp and periradicular tissues are innervated by capsaicin-sensitive neurons known to release CGRP. However, the role of these capsaicin-sensitive neurons in the development of apical periodontitis is largely unknown. The aim of this study was to evaluate the contribution of peptidergic neurons to the development of apical periodontitis.

METHODS

Neonatal Sprague-Dawley rats were injected with vehicle (control group) or a single subcutaneous capsaicin dose to cause the selective ablation of peptidergic neurons (neonatal capsaicin group). Ablation of capsaicin-sensitive neurons was verified with confocal microscopy, capsaicin-induced eye-wipe nocifensive behavior test, and by measurement of immunoreactive CGRP levels in the dental pulp. Five weeks after ablation, standardized pulp exposures were made in the mandibular left first molars. Mandibles were harvested at 7, 14, 21, and 28 days after pulp exposure and imaged with micro-computed tomography (μCT) to quantify apical lesion volume. Data were analyzed by using 2-way ANOVA analysis with Bonferroni post hoc test.

RESULTS

Rats in the control group displayed a robust capsaicin-induced nocifensive behavior, which was nearly abolished in the neonatal capsaicin group. In addition, the neonatal capsaicin group showed a significant depletion of susceptible neurons and CGRP in the dental pulp compared with control. Importantly, micro-computed tomography analysis showed larger periradicular lesions at 7 and 14 days after pulp exposure in the neonatal capsaicin group when compared with control.

CONCLUSIONS

Results identify a protective role for capsaicin-sensitive neurons in the initial phase of apical periodontitis. Thus, interventions or disorders that alter activity of capsaicin-sensitive fibers are likely to alter the development of apical periodontitis.

TRPV1 receptor in the human trigeminal ganglion and spinal nucleus: immunohistochemical localization and comparison with the neuropeptides CGRP and SP

This work presents new data concerning the immunohistochemical occurrence of the transient receptor potential vanilloid type-1 (TRPV1) receptor in the human trigeminal ganglion (TG) and spinal nucleus of subjects at different ontogenetic stages, from prenatal life to postnatal old age. Comparisons are made with the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). TRPV1-like immunoreactive (LI) material was detected by western blot in homogenates of TG and medulla oblongata of subjects at prenatal and adult stages of life. Immunohistochemistry showed that expression of the TRPV1 receptor is mostly restricted to the small- and medium-sized TG neurons and to the caudal subdivision of the spinal trigeminal nucleus (Sp5C). The extent of the TRPV1-LI TG neuronal subpopulation was greater in subjects at early perinatal age than at late perinatal age and in postnatal life. Centrally, the TRPV1 receptor localized to fibre tracts and punctate elements, which were mainly distributed in the spinal tract, lamina I and inner lamina II of the Sp5C, whereas stained cells were rare. The TRPV1 receptor colocalized partially with CGRP and SP in the TG, and was incompletely codistributed with both neuropeptides in the spinal tract and in the superficial laminae of the Sp5C. Substantial differences were noted with respect to the distribution of the TRPV1-LI structures described in the rat Sp5C and with respect to the temporal expression of the receptor during the development of the rat spinal dorsal horn. The distinctive localization of TRPV1-LI material supports the concept of the involvement of TRPV1 receptor in the functional activity of the protopathic compartment of the human trigeminal sensory system, i.e. the processing and neurotransmission of thermal and pain stimuli.

Role of Oxidized Lipids and TRP Channels in Orofacial Pain and Inflammation

Acute or chronic inflammation comprises a highly prevalent type of orofacial pain and is mediated by the generation of endogenous agonists that activate numerous receptors expressed on terminals of trigeminal (TG) nociceptive afferent neurons. One such studied receptor is transient receptor potential vanilloid subtype 1 (TRPV1). TRPV1 is a ligand-gated cation channel that is expressed on a major subclass of nociceptors and is found in many orofacial tissues, including dental pulp. Antagonists to TRPV1 reveal an important role for this channel in mediating hypersensitivity in preclinical models of inflammatory or neuropathic pain. Recent studies have demonstrated that endogenous TRPV1 agonists are generated by oxidation of omega-6 polyunsaturated fatty acids, including both linoleic acid and arachidonic acid. A major mechanism triggering the release of oxidative linoleic acid metabolites (OLAMs) and oxidative arachidonic acid metabolites (OAAMs) is the action of oxidative enzymes. Oxidative enzymes such as cytochrome P450 isozymes are rapidly upregulated in TG neurons after orofacial inflammation and increase the capacity of TG neurons to generate OLAMs. Cytochrome P450 isozymes are also increased in immune cells in irreversibly inflamed human dental pulp, and extracts of this tissue have significantly increased capacity to generate OLAMs. Together, these studies point to a novel pain mechanism involving the enzymatic generation of endogenous OLAM and OAAM agonists of TRPV1. This finding provides a rationale for an entirely new class of analgesics by inhibition of oxidative enzyme activity.

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.

Accounting for the delay in the transition from acute to chronic pain: axonal and nuclear mechanisms

Acute insults produce hyperalgesic priming, a neuroplastic change in nociceptors that markedly prolongs inflammatory mediator-induced hyperalgesia. After an acute initiating insult, there is a 72 h delay to the onset of priming, for which the underlying mechanism is unknown. We hypothesized that the delay is due to the time required for a signal to travel from the peripheral terminal to the cell body followed by a return signal to the peripheral terminal. We report that when an inducer of hyperalgesic priming (monocyte chemotactic protein 1) is administered at the spinal cord of Sprague Dawley rats, priming is detected at the peripheral terminal with a delay significantly shorter than when applied peripherally. Spinally induced priming is detected not only when prostaglandin E2 (PGE2) is presented to the peripheral nociceptor terminals, but also when it is presented intrathecally to the central terminals in the spinal cord. Furthermore, when an inducer of priming is administered in the paw, priming can be detected in spinal cord (as prolonged hyperalgesia induced by intrathecal PGE2), but only when the mechanical stimulus is presented to the paw on the side where the priming inducer was administered. Both spinally and peripherally induced priming is prevented by intrathecal oligodeoxynucleotide antisense to the nuclear transcription factor CREB mRNA. Finally, the inhibitor of protein translation reversed hyperalgesic priming only when injected at the site where PGE2 was administered, suggesting that the signal transmitted from the cell body to the peripheral terminal is not a newly translated protein, but possibly a newly expressed mRNA.

A human capsaicin model to quantitatively assess salivary CGRP secretion

BACKGROUND

Capsaicin induces the release of calcitonin gene-related peptide (CGRP) via the transient receptor potential channel V1 (TRPV1). The CGRP response after capsaicin application on the tongue might reflect the "activation state" of the trigeminal nerve, since trigeminal CGRP-containing vesicles are depleted on capsaicin application. We tested (i) the quantitative CGRP response after oral capsaicin application; (ii) the optimal concentration of red chili homogenate; and (iii) the day-to-day variability in this response.

METHODS

Saliva was collected for two consecutive days after oral application of eight capsaicin dilutions (red chili homogenates) of increasing concentrations in 13 healthy individuals. Effects of homogenate concentration were assessed. Consecutively, saliva was sampled after application of vehicle and undiluted homogenates.

RESULTS

CGRP secretion (pg/ml) increased dose-dependently with homogenate concentration (p < 0.001). CGRP levels were highest after application of nondiluted homogenate (vs. baseline: 13.3 (5.0) vs. 9.7 (2.9); p = 0.003, as was total CGRP secretion in five minutes (pg) with undiluted (vs. baseline): 89.2 (44.1) vs. 14.1 (2.8); p < 0.001. The dose-dependent response in CGRP was not affected by day (p = 0.14) or day*concentration (p = 0.60). Increase in CGRP (undiluted - baseline; pg/ml) did not differ between measurements on dose-finding (p = 0.67) and follow-up days (p = 0.46).

CONCLUSION

Oral application of red chili homogenate is well tolerated and causes a dose-dependent CGRP release in saliva, without day-to-day effects in this response. This model could be used to noninvasively study the activation state of the trigeminal nerve innervating salivary glands.

Expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in the rat dental pulp and trigeminal ganglion following inflammation

Background

There is increasing evidence that peripheral glutamate signaling mechanism is involved in the nociceptive transmission during pathological conditions. However, little is known about the glutamate signaling mechanism and related specific type of vesicular glutamate transporter (VGLUT) in the dental pulp following inflammation. To address this issue, we investigated expression and protein levels of VGLUT1 and VGLUT2 in the dental pulp and trigeminal ganglion (TG) following complete Freund’s adjuvant (CFA) application to the rat dental pulp by light microscopic immunohistochemistry and Western blot analysis.

Results

The density of VGLUT2− immunopositive (+) axons in the dental pulp and the number of VGLUT2+ soma in the TG increased significantly in the CFA-treated group, compared to control group. The protein levels of VGLUT2 in the dental pulp and TG were also significantly higher in the CFA-treated group than control group by Western blot analysis. The density of VGLUT1+ axons in the dental pulp and soma in the TG remained unchanged in the CFA-treated group.

Conclusions

These findings suggest that glutamate signaling that is mediated by VGLUT2 in the pulpal axons may be enhanced in the inflamed dental pulp, which may contribute to pulpal axon sensitization leading to hyperalgesia following inflammation.

The neuroanatomy of sexual dimorphism in opioid analgesia

The influence of sex has been neglected in clinical studies on pain and analgesia, with the vast majority of research conducted exclusively in males. However, both preclinical and clinical studies indicate that males and females differ in both the anatomical and physiological composition of central nervous system circuits that are involved in pain processing and analgesia. These differences influence not only the response to noxious stimuli, but also the ability of pharmacological agents to modify this response. Morphine is the most widely prescribed opiate for the alleviation of persistent pain in the clinic; however, it is becoming increasingly clear that morphine is less potent in women compared to men. This review highlights recent research identifying neuroanatomical and physiological dimorphisms underlying sex differences in pain and opioid analgesia, focusing on the endogenous descending pain modulatory circuit.

Oxidized linoleic acid metabolite-cytochrome P450 system (OLAM-CYP) is active in biopsy samples from patients with inflammatory dental pain

Endogenous TRPV1 agonists such as oxidized linoleic acid metabolites (OLAMs) and the enzymes releasing them [eg, cytochrome P450 (CYP)] are up-regulated after inflammation in the rat. However, it is not known whether such agonists are elevated in human inflammatory pain conditions. Because TRPV1 is expressed in human dental pulp nociceptors, we hypothesized that OLAM-CYP machinery is active in this tissue type and is increased under painful inflammatory conditions such as irreversible pulpitis (IP). The aim of this study was to compare CYP expression and linoleic acid (LA) metabolism in normal vs inflamed human dental pulp. Our data showed that exogenous LA metabolism was significantly increased in IP tissues compared to normal tissues and that pretreatment with a CYP inhibitor, ketoconazole, significantly inhibited LA metabolism. Additionally, extracts obtained from LA-treated inflamed tissues evoked significant inward currents in trigeminal ganglia neurons and were blocked by pretreatment with the TRPV1 antagonist IRTX. Moreover, extracts obtained from ketoconazole-pretreated inflamed tissues significantly reduced inward currents in trigeminal ganglia neurons. These data suggest that LA metabolites produced in human inflamed tissues act as TRPV1 agonists and that the metabolite production can be targeted by CYP inhibition. In addition, immunohistochemical analysis of 2 CYP isoforms, CYP2J and CYP3A1, were shown to be predominately expressed in immune cells infiltrating the inflamed dental pulp, emphasizing the paracrine role of CYP enzymes in OLAM regulation. Collectively, our data indicate that the machinery responsible for OLAM production is up-regulated during inflammation and can be targeted to develop potential analgesics for inflammatory-induced dental pain.

In vitro pharmacological characterization of a novel TRPA1 antagonist and proof of mechanism in a human dental pulp model

AZ465 is a novel selective transient receptor potential cation channel, member A1 (TRPA1) antagonist identified during a focused drug discovery effort. In vitro, AZ465 fully inhibits activation by zinc, O-chlorobenzylidene malononitrile (CS), or cinnamaldehyde of the human TRPA1 channel heterologously expressed in human embryonic kidney cells. Our data using patch-clamp recordings and mouse/human TRPA1 chimeras suggest that AZ465 binds reversibly in the pore region of the human TRPA1 channel. Finally, in an ex vivo model measuring TRPA1 agonist-stimulated release of neuropeptides from human dental pulp biopsies, AZD465 was able to block 50%–60% of CS-induced calcitonin gene-related peptide release, confirming that AZ465 inhibits the native human TRPA1 channel in neuronal tissue.

Serotonergic neuromodulation of peripheral nociceptors

Nociception, the encoding and processing of noxious environmental stimuli by sensory neurons, functions to protect an organism from bodily damage. Activation of the terminal endings of certain sensory neurons, termed nociceptors, triggers a train of impulses to neurons in the spinal cord. Signals are integrated and processed in the dorsal spinal cord and then projected to the brain where they elicit the perception of pain. A number of neuromodulators that can affect nociceptors are released in the periphery during the inflammation that follows an initial injury. Serotonin (5-HT) is a one such proinflammatory mediator. This review discusses our current understanding of the neuromodulatory role of 5-HT, and specifically how this monoamine activates and sensitizes nociceptors. Potential therapeutic targets to treat pain are described.

Sex differences in serotonin enhancement of capsaicin-evoked calcitonin gene-related peptide release from human dental pulp

Serotonin (5HT) is a pronociceptive mediator in the periphery, and evidence implicates involvement in trigeminal pain processing. However, the mechanism(s) by which 5HT modulates trigeminal nociceptors remains unclear. Trigeminal pain can be evoked by the transient receptor potential V1 channel (TRPV1), which is expressed by nociceptive trigeminal neurons and induces release of proinflammatory calcitonin gene-related peptide (CGRP). In our preclinical models, 5HT evoked thermal hyperalgesia and enhanced calcium influx and CGRP release from the TRPV1 population of trigeminal nociceptors. Whether this occurs in humans is unknown. As dental pulp is densely innervated by trigeminal nociceptors, routine tooth extractions offer a unique opportunity to examine whether 5HT enhances CGRP release from human nociceptors. Pulpal tissue was collected from 140 extracted molar teeth from men and women, and basal release samples were collected before treatment with saline or 5HT 100μmol/L. CGRP release was then stimulated with the TRPV1 agonist capsaicin 1μmol/L and quantitated by enzyme immunoassay. Additional samples were collected for Western blots to examine 5HT receptor expression. We report that 5HT induced a significant increase in capsaicin-evoked CGRP release, and that this enhancement was observed only in female dental pulp, with no effect of 5HT on male dental pulp. The greatest amount of CGRP release occurred in dental pulp from women in the luteal phase of the menstrual cycle. These results indicate that 5HT enhances capsaicin-evoked CGRP release from human trigeminal nociceptors in a sexually dimorphic manner providing a mechanistic basis for prevalence of trigeminal pain disorders in women.

Anandamide induces matrix metalloproteinase-2 production through cannabinoid-1 receptor and transient receptor potential vanilloid-1 in human dental pulp cells in culture

INTRODUCTION

Anandamide (N-arachidonoylethanolamine [AEA]) is one of the main endocannabinoids. Endocannabinoids are implicated in various physiological and pathologic functions, inducing not only nociception but also regeneration and inflammation. The role of the endocannabinoid system in peripheral organs was recently described. The aim of this study was to investigate the effect of AEA on matrix metalloproteinase (MMP)-2 induction in human dental pulp cells (HPC).

METHODS

We examined AEA-induced MMP-2 production and the expression of AEA receptors (cannabinoid [CB] receptor-1, CB2, and transient receptor potential vanilloid-1 [TRPV1]) in HPC by Western blot. MMP-2 concentrations in supernatants were determined by enzyme-linked immunosorbent assay. We then investigated the role of the AEA receptors and mitogen-activated protein kinase in AEA-induced MMP-2 production in HPC.

RESULTS

AEA significantly induced MMP-2 production in HPC. HPC expressed all 3 types of AEA receptor (CB1, CB2, and TRPV1). AEA-induced MMP-2 production was blocked by CB1 or TRPV1 antagonists and by small interfering RNA for CB1 or TRPV1. Furthermore, c-Jun N-terminal kinase inhibitor also reduced MMP-2 production.

CONCLUSIONS

We demonstrated for the first time that AEA induced MMP-2 production via CB1 and TRPV1 in HPC.

MATHEMATICAL MODELING FOR THE PREDICTION AND IMPROVEMENT OF TOOTH THERMAL PAIN: A REVIEW

Tooth pain, especially tooth thermal pain, is one of the most important symptoms and signs in dental clinic and daily life. As a special sensation, pain has been studied extensively in both clinic and experimental research aimed at reducing or eliminating the possible negative effects of pain. Unfortunately, the full underlying mechanism of pain is still unclear, because the pain could be influenced by many factors, including physiological, psychological, physical, chemical, and biological factors and so on. Besides, most studies on pain mechanisms in the literature are based on skin pain sensation and only few are based on tooth pain. In this paper, we present a comprehensive review on both neurophysiology of tooth pain mechanism, and corresponding thermal, mechanical, and thermomechanical behaviors of teeth. We also describe a multiscale modeling approach for quantifying tooth thermal pain by integrating the mathematic methods of engineering into the neuroscience. The mathematical model of tooth thermal pain will enable better understanding of thermal pain mechanism and optimization of existing diagnosis and treatment in dental clinic.

Restriction of transient receptor potential vanilloid-1 to the peptidergic subset of primary afferent neurons follows its developmental downregulation in nonpeptidergic neurons

Primary afferent "pain" fibers (nociceptors) are divided into subclasses based on distinct molecular and anatomical features, and these classes mediate noxious modality-specific contributions to behaviors evoked by painful stimuli. Whether the heat and capsaicin receptor transient receptor potential vanilloid-1 (TRPV1) is expressed heterogeneously across several sensory populations, or is selectively expressed by a unique nociceptor subclass, however, is unclear. Here we used two lines of Trpv1 reporter mice to investigate the primary afferent expression of TRPV1, both during development and in the adult. We demonstrate, using Cre-induced lineage tracing, that during development TRPV1 is transiently expressed in a wide range of dorsal root ganglion neurons, and that its expression is gradually refined, such that TRPV1 transcripts become restricted to a specific subset of peptidergic sensory neurons. Finally, the remarkable sensitivity that is characteristic of these reporter mice revealed an innervation of central and peripheral targets by TRPV1+ primary afferents in the adult that is considerably more extensive than has previously been appreciated.

Differential TRPV1 and TRPV2 channel expression in dental pulp

Hypersensitivity to thermal and mechanical stimuli can occur in painful pulpitis. To explore the neuro-anatomical basis of heat and mechanical sensitivity, we evaluated expression of TRPV1 (heat) and TRPV2 (heat/mechanical) channels in the cell bodies and terminal arborizations of neurons that innervate the dental pulp (DP) and periodontal tissues (PDL). We report that ~50% of trigeminal ganglion (TG) neurons retrogradely labeled from the DP express TRPV2, and this was significantly greater than the general expression of this channel in the TG (15%) and slightly more than what is expressed in the PDL by retrograde labeling (40%). The TRPV1 receptor, however, was less prevalent in neurons innervating the DP than their general expression in the TG (17% vs. 26%) and was more extensively expressed in neurons innervating the PDL (26%). Co-labeling studies showed that 70% of neurons that innervate the DP are myelinated. Approximately 1/3 of the retrogradely labeled neurons from the DP were calcitonin-gene-related-peptide-positive (peptide-expressing), but very few expressed the IB4 marker of non-peptidergic unmyelinated afferents. These findings suggest that the DP has a unique neurochemical innervation with regard to TRP receptor expression, which has significant implications for the mechanisms contributing to odontogenic pain and management strategies.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).