Sodium channel Nav1.8 immunoreactivity in painful human dental pulp

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.

Efficacy of four local anaesthesia protocols for mandibular first molars with symptomatic irreversible pulpitis: A randomized clinical trial

AIM

To examine the efficacy rate of four anaesthetic protocols in mandibular first molars with symptomatic irreversible pulpitis (SIP).

METHODOLOGY

One hundred and sixty patients with a diagnosis of SIP were included in this randomized clinical trial. Patients were randomly allocated into four treatment groups (N = 40) according to the administered technique: Group 1 (IANB): standard inferior alveolar nerve block (IANB) injection; Group 2 (IANB + IO): standard IANB followed by a supplemental intraosseous infusion (IO) injection; Group 3 (IANB + PDL): standard IANB followed by a supplemental periodontal ligament (PDL) injection; Group 4 (IANB + BI): standard IANB followed by a supplemental buccal infiltration. Patients rated pain intensity using a verbal rating scale when the root canal treatment procedure was initiated, that is, during caries removal, access preparation and pulpectomy. Heart rate changes were recorded before, during and after each injection. The anaesthetic efficacy rates were analysed using chi-square tests, age differences using one-way anova, gender differences using Fischer Exact tests whilst heart rate changes were analysed using Kruskal-Wallis tests. Statistical significances were set at p < .05 level.

RESULTS

All the included patients were analysed. No differences in the efficacy rate were found in relation to the age or gender of the participants amongst the study groups (p > .05). IANB + IO injections had a significantly higher efficacy rate (92.5%) when compared to other techniques (p < .05), followed by IANB + PDL injections (72.5%), IANB + BI injections (65.0%), with no significant differences between the IANB + PDL or IANB + BI injections (p > .05). IANB injection alone had a significantly lower rate (40%) compared to the other techniques (p < .05). A transient but significant rise in the heart rate was recorded in 60% (24/40) of patients who received the IANB + IO injection compared to other groups (p < .05).

CONCLUSIONS

Inferior alveolar nerve block injection alone did not reliably permit pain-free treatment for mandibular molars with SIP. The use of an additional IO supplemental injection provided the most effective anaesthesia for patients requiring emergency root canal treatment for SIP in mandibular posterior teeth.

Dental nerves: a neglected mediator of pulpitis

As one of the most densely innervated tissues, the dental pulp contains abundant nerve fibres, including sensory, sympathetic and parasympathetic nerve fibres. Studies in animal models and human patients with pulpitis have revealed distinct alterations in protein expression and histological appearance in all types of dental nerve fibres. Various molecules secreted by neurons, such as classical neurotransmitters, neuropeptides and amino acids, not only contribute to the induction, sensitization and maintenance of tooth pain, but also regulate non-neuronal cells, including fibroblasts, odontoblasts, immune cells and vascular endothelial cells. Dental nerves are particularly important for the microcirculatory and immune responses in pulpitis via their release of a variety of functional substances. Further, nerve fibres are found to be involved in dental soft and hard tissue repair. Thus, understanding how dental nerves participate in pulpitis could have important clinical ramifications for endodontic treatment. In this review, the roles of dental nerves in regulating pulpal inflammatory processes are highlighted and their implications for future research on this topic are discussed.

Local anaesthesia in dentistry: a review

Over the past century, there is perhaps no greater contribution to the practice of clinical dentistry than the development and application of local anaesthesia. What were once considered painful procedures have now been made routine by the deposition and action of local anaesthetics. This article will serve as a review of basic pharmacological principles of local anaesthesia, subsequent sequelae that can arise from their use, considerations when using local anaesthetics, and recent advances in the delivery of local anaesthetics.

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.

Topical Ambroxol 20% for the Treatment of Classical Trigeminal Neuralgia - A New Option? Initial Clinical Case Observations

BACKGROUND

Trigeminal neuralgia is difficult to treat and shows upregulation of sodium channels. The expectorant ambroxol acts as a strong local anesthetic, about 40 times stronger than lidocaine. It preferentially inhibits the channel subtype Na_v 1.8, expressed especially in nociceptive C-fibers. It seemed reasonable to try ambroxol for the treatment with neuropathic facial pain unresponsive to other standard options.

MATERIAL AND METHODS

Medical records of patients suffering from classical trigeminal neuralgia (n = 5) and successful pain reduction following topical ambroxol 20% cream in addition to standard treatment are reported.

RESULTS

All patients reported pain attacks with pain intensity between 4 and 10 NRS (numeric pain scale). In all cases they could be triggered, 3 patients reported additional spontaneous pain. Attacks were reduced in all 5 patients. Pain reduction achieved following ambroxol 20% cream was 2-8 points (NRS) earliest within 15-30 minutes and lasted for 4-6 hours mostly. This was reproducible in all cases; in one case pain was eliminated after 1 week. No patient reported side effects or skin changes; oral medication was reduced in 2 patients.

CONCLUSION

For the first time, a clinically significant pain relief following topical ambroxol 20% cream in patients with trigeminal neuralgia is reported. In view of the positive side effect profile, topical ambroxol for patients with such a highly impaired quality of life should be investigated further as a matter of urgency.

Neuropathic pain in the orofacial region: The role of pain history. A retrospective study

INTRODUCTION

Orofacial neuropathic pain is often difficult to treat, mostly because of still unclear underlying mechanisms. The occurrence of such neuropathic pain varies depending on different factors, of which preexisting preoperative pain seems to be of high importance. The aim of this study was thus to test the hypothesis that prior history of pain could indeed be considered a risk factor for the development of orofacial neuropathic pain in the same region.

METHODS

The study was performed in the dental department of the Groupe Hospitalier Pitié-Salpêtrière (GHPS) in Paris, France. We investigated the presence of prior inflammatory pain before development of orofacial neuropathic pain in 56 patients. For each patient file, the following items were collected: age, gender; medical history; diagnosis; description of the pain (at time of consultation); presence or absence of prior dental treatment; date and type of dental treatment received.

RESULTS

41 patients (73%) of orofacial neuropathic pain patients had a history of pain compatible with an inflammatory condition; 4% (n=2) did not report any prior pain and 23% (n=13) could not remember. Among the patients with documented history of pain prior to neuropathy, 88% (n=36) received surgical treatment; 61%, (n=25) endodontic treatment and 22%, (n=9) restorative treatment. All eventually received endodontic treatment or tooth extraction. These dental treatments are compatible with the hypothesis of prior inflammatory pain in these patients.

CONCLUSION

These results support the hypothesis that prior inflammatory pain could favor the development of orofacial neuropathic pain. Prevention and treatment of inflammatory trigeminal pain may therefore play a key role in preventing future neuropathic pain development.

Temporal dynamics of anxiety phenotypes in a dental pulp injury model

Background

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

Results

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

Conclusions

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

The iontophoresis of lignocaine with epinephrine into carious dentine for pain control during cavity preparation in human molars

OBJECTIVE

To determine the effectiveness of the iontophoretic delivery of lignocaine with epinephrine through carious dentine for pain control during cavity preparation.

DESIGN

The experiments were carried out on 56 carious molars that required class I restorations in 42 subjects (aged 15-20 years). The overhanging enamel and soft caries were removed then the sensitivity of the exposed dentine was tested with drilling, probing and air blast stimuli. The subject indicated the intensity of any pain produced by marking a visual analogue scale (VAS). The cavity was then filled with 20% w/v lidocaine with 0.1% w/v epinephrine and a 200 μA iontophoretic current applied for 2 min after which the sensitivity of the dentine was re-tested. If the dentine was not anaesthetized, the treatment and testing were repeated up to 6 times.

RESULTS

The total duration (min) of iontophoresis required to anaesthetize the dentine was: 2 in 7 teeth, 4 in 17 teeth, 6 in 14 teeth, 8 in 4 teeth, and 10 in 7 teeth. The remaining 7 teeth were not anaesthetized even after 14 min of iontophoresis.

CONCLUSIONS

The iontophoretic delivery of lignocaine with epinephrine anaesthetized dentine for cavity preparation in 49 of 56 (87.5%) of carious molars.

A novel selective and orally bioavailable Nav 1.8 channel blocker, PF-01247324, attenuates nociception and sensory neuron excitability

BACKGROUND AND PURPOSE

NaV 1.8 ion channels have been highlighted as important molecular targets for the design of low MW blockers for the treatment of chronic pain. Here, we describe the effects of PF-01247324, a new generation, selective, orally bioavailable Nav 1.8 channel blocker of novel chemotype.

EXPERIMENTAL APPROACH

The inhibition of Nav 1.8 channels by PF-01247324 was studied using in vitro patch-clamp electrophysiology and the oral bioavailability and antinociceptive effects demonstrated using in vivo rodent models of inflammatory and neuropathic pain.

KEY RESULTS

PF-01247324 inhibited native tetrodotoxin-resistant (TTX-R) currents in human dorsal root ganglion (DRG) neurons (IC50 : 331 nM) and in recombinantly expressed h Nav 1.8 channels (IC50 : 196 nM), with 50-fold selectivity over recombinantly expressed TTX-R hNav 1.5 channels (IC50 : ∼10 μM) and 65-100-fold selectivity over TTX-sensitive (TTX-S) channels (IC50 : ∼10-18 μM). Native TTX-R currents in small-diameter rodent DRG neurons were inhibited with an IC50 448 nM, and the block of both human recombinant Nav 1.8 channels and TTX-R from rat DRG neurons was both frequency and state dependent. In vitro current clamp showed that PF-01247324 reduced excitability in both rat and human DRG neurons and also altered the waveform of the action potential. In vivo experiments n rodents demonstrated efficacy in both inflammatory and neuropathic pain models.

CONCLUSIONS AND IMPLICATIONS

Using PF-01247324, we have confirmed a role for Nav 1.8 channels in both inflammatory and neuropathic pain. We have also demonstrated a key role for Nav 1.8 channels in action potential upstroke and repetitive firing of rat and human DRG neurons.

How can sensitive dentine become hypersensitive and can it be reversed?

This paper reviews a number of studies in oral biology and endodontics that deal with the reactivity of the pulpo-dentine complex in response to mechanical and immunological stimuli. It can be hypothesized that these reactions could also apply to changes in dentine sensitivity following periodontal procedures. Some of these changes involve neurogenic inflammation of the pulp under exposed open tubules; this increases the rate of outward fluid flow through the tubules, making the overlying exposed dentine more sensitive. Other changes may be due to inflammation-related nerve sprouting of pulpal nerves, which can lead to innervation of more tubules than normal. Changes may also involve upregulation of new, more sensitive ion channels in the membranes of these nerves. The goal of the paper is to increase awareness of the complex issues involved in dentine sensitivity, so that future investigators may develop agents or techniques to stimulate mechanisms that mitigate dentine sensitivity, or to block mechanisms that aggravate the condition, for therapeutic effect.

The success rate of bupivacaine and lidocaine as anesthetic agents in inferior alveolar nerve block in teeth with irreversible pulpitis without spontaneous pain

Objectives

Achieving adequate anesthesia with inferior alveolar nerve blocks (IANB) is of great importance during dental procedures. The aim of the present study was to assess the success rate of two anesthetic agents (bupivacaine and lidocaine) for IANB when treating teeth with irreversible pulpitis.

Materials and Methods

Sixty volunteer male and female patients who required root canal treatment of a mandibular molar due to caries participated in the present study. The inclusion criteria included prolonged pain to thermal stimulus but no spontaneous pain. The patients were randomly allocated to receive either 2% lidocaine with 1:80,000 epinephrine or 0.5% bupivacaine with 1:200,000 epinephrine as an IANB injection. The sensitivity of the teeth to a cold test as well as the amount of pain during access cavity preparation and root canal instrumentation were recorded. Results were statistically analyzed with the Chi-Square and Fischer's exact tests.

Results

At the final step, fifty-nine patients were included in the study. The success rate for bupivacaine and lidocaine groups were 20.0% and 24.1%, respectively. There was no significant difference between the two groups at any stage of the treatment procedure.

Conclusions

There was no difference in success rates of anesthesia when bupivacaine and lidocaine were used for IANB injections to treat mandibular molar teeth with irreversible pulpitis. Neither agent was able to completely anesthetize the teeth effectively. Therefore, practitioners should be prepared to administer supplemental anesthesia to overcome pain during root canal treatment.

Correlation of Nav1.8 and Nav1.9 sodium channel expression with neuropathic pain in human subjects with lingual nerve neuromas

Background

Voltage-gated sodium channels Nav1.8 and Nav1.9 are expressed preferentially in small diameter sensory neurons, and are thought to play a role in the generation of ectopic activity in neuronal cell bodies and/or their axons following peripheral nerve injury. The expression of Nav1.8 and Nav1.9 has been quantified in human lingual nerves that have been previously injured inadvertently during lower third molar removal, and any correlation between the expression of these ion channels and the presence or absence of dysaesthesia investigated.

Results

Immunohistochemical processing and quantitative image analysis revealed that Nav1.8 and Nav1.9 were expressed in human lingual nerve neuromas from patients with or without symptoms of dysaesthesia. The level of Nav1.8 expression was significantly higher in patients reporting pain compared with no pain, and a significant positive correlation was observed between levels of Nav1.8 expression and VAS scores for the symptom of tingling. No significant differences were recorded in the level of expression of Nav1.9 between patients with or without pain.

Conclusions

These results demonstrate that Nav1.8 and Nav1.9 are present in human lingual nerve neuromas, with significant correlations between the level of expression of Nav1.8 and symptoms of pain. These data provide further evidence that changes in expression of Nav1.8 are important in the development and/or maintenance of nerve injury-induced pain, and suggest that Nav1.8 may be a potential therapeutic target.

The role of sodium channels in chronic pain

Here we review recent research into the mechanisms of chronic pain that has focused on neuronal sodium channels, a target of classic analgesic agents. We first discuss evidence that specific sodium channel isoforms are essential for the detection and conduction of normal acutely painful stimuli from nociceptors. We then review findings that show changes in sodium channel expression and localization in chronic inflammation and nerve injury in animal and human tissues. We conclude by discussing the role that myelination plays in organizing and maintaining sodium channel clusters at nodes of Ranvier in normal development and how inflammatory processes or nerve injury alter the characteristics of such clusters. Based on these findings, we suggest that chronic pain may in part result from partial demyelination of axons during chronic injury, which creates aberrant sodium channel clusters that serve as sites of ectopic sensitivity or spontaneous activity.

NaV 1.8, but not NaV 1.9, is upregulated in the inflamed dental pulp tissue of human primary teeth

AIM

To investigate alterations in Na(V) 1.8 and Na(V) 1.9 expression within inflamed dental pulp tissue of human primary teeth.

METHODOLOGY

Dental pulp tissue obtained from both normal and inflamed pulps in primary teeth as well as pulps from normal and inflamed permanent teeth was used. The quantity of Na(V) 1.8 and Na(V) 1.9 expression in the dental pulp tissue was investigated using Western blot analysis. General neuron marker (PGP9.5) was used to quantify for neural density, and an increase in metalloproteinase-9 was used to indicate pulpal inflammation in inflamed teeth. Statistically significant differences for each determined parameter between normal and inflamed teeth of both primary and permanent teeth were tested using the Mann-Whitney rank sum test.

RESULTS

There was no significant difference in neural density of normal and inflamed dental pulp tissue, although degrees of inflammation were increased in the inflamed dental pulp of both permanent and primary teeth (P < 0.05). Na(V) 1.8 and Na(V) 1.9 expression in inflamed pulps of permanent teeth increased significantly compared with normal permanent teeth (P < 0.05). However, only Na(V) 1.8 expression was increased significantly in the inflamed dental pulp of primary teeth (P < 0.05).

CONCLUSIONS

Na(V) 1.8 alone may be the therapeutic target for treatment of painful pulpitis in primary teeth.

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

OBJECTIVE

Transient receptor potential (TRP) channels, a family of structurally related proteins have been implicated in the sensation of pain and hyperalgesia caused by exogenous and endogenous agonists, as well as touch, pH, and temperature. The objective of this study was to determine the effects of tooth injury on the expression of the cold sensitive channel TRPA1, in the trigeminal ganglion, the primary source of sensory and nociceptive innervation of teeth.

DESIGN

We analyzed TRPA1 expression in a rodent model of tooth injury, by Western blot analyses of proteins extracted from trigeminal ganglia.

RESULTS

We found that TRPA1 was selectively increased in trigeminal ganglia innervating injured teeth when compared to TRPA1 expression in trigeminal ganglia innervating healthy teeth.

CONCLUSIONS

Our results provide the first evidence of increased expression of a cold-sensitive TRP channel in trigeminal ganglia after pulp exposure, and are consistent with the possibility that increased expression and function of TRPA1 in trigeminal neurons contributes to hyperalgesia and allodynia following tooth injury.

Cannabinoid receptor CB1-immunoreactive nerve fibres in painful and non-painful human tooth pulp

The cannabinoid receptor CB1 is involved in modulation of neuronal hypersensitivity and pain. The aim of this study was to evaluate CB1 receptor levels for the first time in dental pain. A total of 19 patients due for molar extraction were divided into two groups, those with existing dental pain (n=9), and those with no history of pain (n=10). Immunohistochemistry and computer image analysis was used to evaluate CB1-positive nerve fibres in tooth pulp, with neurofilament-immunostaining as a structural nerve marker. CB1-immunoreactive nerve fibres were scattered throughout the tooth pulp and often seen in nerve bundles, but the fibres did not penetrate the subodontoblastic layer. There was no statistically significant change in the CB1 nerve fibre percentage area in the painful group compared to the non-painful group (p=0.146); the neurofilament fibres were significantly reduced in the painful group compared to the controls (p=0.028), but there was no difference in the ratio of CB1 to neurofilaments between the two groups. Thus, CB1 expression is maintained by nerve fibres in painful human dental pulp, and peripherally-restricted CB1 agonists currently in development may advance the treatment of dental pain.

Sodium channel Na v 1.7 immunoreactivity in painful human dental pulp and burning mouth syndrome

BACKGROUND

Voltage gated sodium channels Na v 1.7 are involved in nociceptor nerve action potentials and are known to affect pain sensitivity in clinical genetic disorders.

AIMS AND OBJECTIVES

To study Na v 1.7 levels in dental pulpitis pain, an inflammatory condition, and burning mouth syndrome (BMS), considered a neuropathic orofacial pain disorder.

METHODS

Two groups of patients were recruited for this study. One group consisted of patients with dental pulpitis pain (n = 5) and controls (n = 12), and the other patients with BMS (n = 7) and controls (n = 10). BMS patients were diagnosed according to the International Association for the Study of Pain criteria; a pain history was collected, including the visual analogue scale (VAS). Immunohistochemistry with visual intensity and computer image analysis were used to evaluate levels of Na v 1.7 in dental pulp tissue samples from the dental pulpitis group, and tongue biopsies from the BMS group.

RESULTS

There was a significantly increased visual intensity score for Na v 1.7 in nerve fibres in the painful dental pulp specimens, compared to controls. Image analysis showed a trend for an increase of the Na v 1.7 immunoreactive % area in the painful pulp group, but this was not statistically significant. When expressed as a ratio of the neurofilament % area, there was a strong trend for an increase of Na v 1.7 in the painful pulp group. Na v 1.7 immunoreactive fibres were seen in abundance in the sub-mucosal layer of tongue biopsies, with no significant difference between BMS and controls.

CONCLUSION

Na v 1.7 sodium channel may play a significant role in inflammatory dental pain. Clinical trials with selective Na v 1.7 channel blockers should prioritize dental pulp pain rather than BMS.

Sodium channel expression and localization at demyelinated sites in painful human dental pulp

The expression of sodium channels (NaCh(s)) change after inflammatory and nerve lesions, and this change has been implicated in the generation of pain states. Here we examine NaCh expression within nerve fibers from normal and painful extracted human teeth with special emphasis on their localization within large accumulations, like those seen at nodes of Ranvier. Pulpal tissue sections from normal wisdom teeth and from teeth with large carious lesions associated with severe and spontaneous pain were double-stained with pan-specific NaCh antibody and caspr (paranodal protein used to visualize nodes of Ranvier) antibody, while additional sections were triple-stained with NaCh, caspr and myelin basic protein (MBP) antibodies. Z-series of images were obtained with the confocal microscope and evaluated with NIH ImageJ software to quantify the density and size of NaCh accumulations, and to characterize NaCh localization at caspr-identified typical and atypical nodal sites. Although the results showed variability in the overall density and size of NaCh accumulations in painful samples, a common finding included the remodeling of NaChs at atypical nodal sites. This remodeling of NaChs included prominent NaCh expression within nerve regions that showed a selective loss of MBP staining in a pattern consistent with a demyelinating process.

PERSPECTIVE

This study identifies the remodeling of NaChs at demyelinated sites within the painful human dental pulp and suggests that the contribution of NaChs to spontaneous pulpal pain generation may be dependant not only on total NaCh density but may also be related to NaCh expression at atypical nodal sites.

The peripheral pro-nociceptive state induced by repetitive inflammatory stimuli involves continuous activation of protein kinase A and protein kinase C epsilon and its Na(V)1.8 sodium channel functional regulation in the primary sensory neuron

In the present study, the participation of the Na(V)1.8 sodium channel was investigated in the development of the peripheral pro-nociceptive state induced by daily intraplantar injections of PGE(2) in rats and its regulation in vivo by protein kinase A (PKA) and protein kinase C epsilon (PKCvarepsilon) as well. In the prostaglandin E(2) (PGE(2))-induced persistent hypernociception, the Na(V)1.8 mRNA in the dorsal root ganglia (DRG) was up-regulated. The local treatment with dipyrone abolished this persistent hypernociception but did not alter the Na(V)1.8 mRNA level in the DRG. Daily intrathecal administrations of antisense Na(V)1.8 decreased the Na(V)1.8 mRNA in the DRG and reduced ongoing persistent hypernociception. Once the persistent hypernociception had been abolished by dipyrone, but not by Na(V)1.8 antisense treatment, a small dose of PGE(2) restored the hypernociceptive plateau. These data show that, after a period of recurring inflammatory stimuli, an intense and prolonged nociceptive response is elicited by a minimum inflammatory stimulus and that this pro-nociceptive state depends on Na(V)1.8 mRNA up-regulation in the DRG. In addition, during the persistent hypernociceptive state, the PKA and PKCvarepsilon expression and activity in the DRG are up-regulated and the administration of the PKA and PKCvarepsilon inhibitors reduce the hypernociception as well as the Na(V)1.8 mRNA level. In the present study, we demonstrated that the functional regulation of the Na(V)1.8 mRNA by PKA and PKCvarepsilon in the primary sensory neuron is important for the development of the peripheral pro-nociceptive state induced by repetitive inflammatory stimuli and for the maintenance of the behavioral persistent hypernociception.

Characterization of a model of chronic orofacial hyperalgesia in the rat: contribution of NA(V) 1.8

The purpose of this study was to develop and characterize a model of orofacial inflammatory hyperalgesia. Injection of complete Freund's adjuvant (CFA) into the upper lip/whisker pad of the rat produced significant and long-lasting thermal (> or =14 days) and mechanical (> or =28 days) hyperalgesia in the area of CFA injection. Both indomethacin and morphine, given systemically, significantly attenuated thermal hyperalgesia; the effect of morphine was shown to be opioid receptor-mediated. We also examined the contribution of the tetrodotoxin-resistant voltage-gated sodium channel Na(v)1.8 in CFA-produced orofacial mechanical hypersensitivity. Na(v)1.8 mRNA was increased > or =2.5-fold in trigeminal ganglion neurons 1 and 2 weeks after CFA treatment, and Na(v)1.8 protein was increased in the infraorbital nerve over a similar time course. The changes observed were time-dependent and had returned to baseline when examined 2 months after inflammation; there were no changes in Na(v)1.9 mRNA in trigeminal ganglion neurons after CFA treatment. In support of this, Na(v)1.8 antisense oligodeoxynucleotide treatment significantly attenuated CFA-produced mechanical hypersensitivity. These results document development of a model of inflammatory orofacial hyperalgesia, which, consistent with other reports, indicate a contribution of tetrodotoxin-resistant, voltage-gated sodium channel Na(v)1.8.

PERSPECTIVE

Orofacial hypersensitivity develops postoperatively as a routine course of orofacial surgery, and mechanical allodynia is characteristic of temporomandibular joint disorder. The results described in this report are novel with respect to the duration of orofacial hypersensitivity produced and suggest that pharmacological targeting of the voltage-gated sodium channel Na(v)1.8 may be useful in managing hypersensitivity.

Quantification of neural protein in extirpated tooth pulp

Because the pulp tissue extirpated during root canal procedures might serve as a valuable resource with which to assess underlying mechanisms of persistent pain, we sought to determine whether standard Western blotting techniques could be used to quantify neural proteins in pulp extirpated from teeth with irreversible pulpitis. Pulp harvested from healthy intact teeth extracted for orthodontic reasons was used for comparison. The neural marker PGP9.5 was detectable in all samples tested. A membrane enrichment protocol enabled detection of even low abundance, high molecular weight proteins such as the sodium channel alpha-subunit NaV1.8. Importantly, it was possible to quantify a approximately 6-fold increase in the relative density of NaV1.8 in inflamed pulp compared with control pulp. Our results suggest that it should be possible to use extirpated tooth pulp to validate mechanisms of persistent pain implicated in preclinical studies as well as evaluate the therapeutic efficacy of novel antinociceptive interventions.

Expression of Nav1.9 Channels in Human Dental Pulp and Trigeminal Ganglion

There is a higher incidence of local anesthetic failure in endodontic patients experiencing pulpal hyperalgesia. Up-regulation of Nav1.9, a voltage-gated sodium channel isoform, might play a key role in local anesthetic failure because Nav1.9 channels increase neuronal excitability and have low sensitivity to blockade by local anesthetics. Immunocytochemistry was used to examine Nav1.9 channel expression in axons of symptomatic (painful) versus asymptomatic human dental pulp and to determine Nav1.9 expression levels in neuronal somata of the human trigeminal ganglion. Nav1.9 channel immunoreactivity on pulpal axons was significantly increased in painful teeth. Nav1.9 channels were expressed in membranes and cytoplasm of human trigeminal ganglion neurons, with the highest expression in small neuronal somata. Nav1.9 expression in the trigeminal ganglion coupled with increased expression in symptomatic pulp might contribute to hypersensitivity of inflamed pulps and local anesthetic failure. Furthermore, the present study suggests that Nav1.9 channels are potential targets for novel anesthetics.

TRPM8 Axonal expression is decreased in painful human teeth with irreversible pulpitis and cold hyperalgesia

Pulpitis pain might be triggered by a cold stimulus, yet the cellular mechanisms responsible for this phenomenon are largely unknown. One possible mechanism involves the direct activation of cold-responsive thermoreceptors. The purpose of this study was to evaluate the possible role of the TRPM8 thermoreceptor in cold-mediated noxious pulpal pain mechanisms by comparing expression patterns in pulpal nerves from healthy control molars to cold-sensitive painful molars with irreversible pulpitis. Samples were identically processed with the indirect immunofluorescence method, and images were obtained with confocal microscopy. The immunofluorescence intensity and area occupied by TRPM8 within N52/PGP9.5-identified nerve fibers were quantified. Results showed that relative to normal samples, TRPM8 nerve area expression was significantly less in the cold-sensitive painful samples (34.9% vs 8%, P <0.03), but with no significant difference in immunofluorescence intensity between the 2 groups. These results suggest that TRPM8 is most likely not involved in cold-mediated noxious pulpal pain mechanisms.

Burning mouth syndrome as a trigeminal small fibre neuropathy: Increased heat and capsaicin receptor TRPV1 in nerve fibres correlates with pain score

Burning mouth syndrome (BMS) is often an idiopathic chronic and intractable pain condition, affecting 1.5-5.5% of middle-aged and elderly women. We have studied the heat and capsaicin receptor TRPV1, and its regulator nerve growth factor (NGF), in BMS. Patients with BMS (n=10) and controls (n=10) were assessed for baseline and post-topical capsaicin pain scores, and their tongue biopsies immunostained for TRPV1, NGF, and structural nerve markers neurofilament and peripherin. Nerve fibres penetrating the epithelium were less abundant in BMS (p<0.0001), indicating a small fibre neuropathy. TRPV1-positive fibres were overall significantly increased in BMS (p=0.0011), as were NGF fibres (p<0.0001) and basal epithelial cell NGF staining (p<0.0147). There was a significant correlation between the baseline pain score and TRPV1 (p=0.0143) and NGF fibres (p=0.0252). A significant correlation was observed between baseline and post-capsaicin pain (p=0.0006). Selective TRPV1 and NGF blockers may provide a new therapy for BMS.

Peripheral mechanisms of odontogenic pain

In this article, we review the key basic mechanisms associated with this phenomena and more recently identified mechanisms that are current areas of interest. Although many of these pain mechanisms apply throughout the body, we attempt to describe these mechanisms in the context of trigeminal pain.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.