Substance P sensitizes P2X3 in nociceptive trigeminal neurons

Role of neuropeptides in orofacial pain: A literature review

BACKGROUND

Neuropeptides play a critical role in regulating pain and inflammation. Despite accumulating evidence has further uncovered the novel functions and mechanisms of different neuropeptides in orofacial pain sensation and transmission, there is deficient systematic description of neuropeptides' pain modulation in the orofacial region, especially in the trigeminal system.

OBJECTIVES

The present review aims to summarise several key neuropeptides and gain a better understanding of their major regulatory roles in orofacial inflammation and pain.

METHODS

We review and summarise current studies related to calcitonin gene-related peptide (CGRP), substance P (SP), opioid peptide (OP), galanin (GAL) and other neuropeptides' functions and mechanisms as well as promising targets for orofacial pain control.

RESULTS

A number of neuropeptides are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. The functions, possible cellular and molecular mechanisms have been introduced and discussed. Neuropeptides and their agonists or antagonists which are widely studied to be potential treatment options of orofacial pain has been evaluated.

CONCLUSIONS

Various neuropeptides play important but distinct (pro-nociceptive or analgesic) roles in orofacial pain with different mechanisms. In summary, CGRP, SP, NPY, NKA, HK-1, VIP mainly play proinflammatory and pro-nociceptive effects while OP, GAL, OXT, OrxA mainly have inhibitory effects on orofacial pain.

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.

Transient Receptor Potential Channels, Natriuretic Peptides, and Angiotensin Receptor-Neprilysin Inhibitors in Patients With Heart Failure

Heart failure (HF) remains the leading cause of death, morbidity, and medical expenses worldwide. Treatments for HF with reduced ejection fraction have progressed in recent years; however, acute decompensated heart failure remains difficult to treat. The transient receptor potential (TRP) channel family plays roles in various cardiovascular diseases, responding to neurohormonal and mechanical load stimulation. Thus, TRP channels are promising targets for drug discovery, and many studies have evaluated the roles of TRP channels expressed on pain neurons. The natriuretic peptide (NP) family of proteins regulates blood volume, natriuresis, and vasodilation and can antagonize the renin-angiotensin-aldosterone system and participate in the pathogenesis of major cardiovascular diseases, such as HF, coronary atherosclerotic heart disease, and left ventricular hypertrophy. NPs are degraded by neprilysin, and the blood level of NPs has predictive value in the diagnosis and prognostic stratification of HF. In this review, we discuss the relationships between typical TRP family channels (e.g., transient receptor potential cation channel subfamily V member 1 andTRPV1, transient receptor potential cation channel subfamily C member 6) and the NP system (e.g., atrial NP, B-type NP, and C-type NP) and their respective roles in HF. We also discuss novel drugs introduced for the treatment of HF.

Molecular mechanism in trigeminal nerve and treatment methods related to orthodontic pain

BACKGROUND

Orthodontic treatment is the main treatment approach for malocclusion. Orthodontic pain is an inevitable undesirable adverse reaction during orthodontic treatment. It is reported orthodontic pain has become one of the most common reason that patients withdraw from orthodontic treatment. Therefore, understanding the underlying mechanism and finding treatment of orthodontic pain are in urgent need.

AIMS

This article aims to sort out the mechanisms and treatments of orthodontic pain, hoping to provide some ideas for future orthodontic pain relief.

MATERIALS

Tooth movement will cause local inflammation. Certain inflammatory factors and cytokines stimulating the trigeminal nerve and further generating pain perception, as well as drugs and molecular targeted therapy blocking nerve conduction pathways, will be reviewed in this article.

METHOD

We review and summaries current studies related to molecular mechanisms and treatment approaches in orthodontic pain control.

RESULTS

Orthodontics pain related influencing factors and molecular mechanisms has been introduced. Commonly used clinical methods in orthodontic pain control has been evaluated.

DISCUSSION

With the clarification of more molecular mechanisms, the direction of orthodontic pain treatment will shift to targeted drugs.

Perception of pain in Class II malocclusion children treated with cervical headgear: a randomized controlled trial

OBJECTIVES

The aims of this study were: 1. to evaluate the experience of pain perceived by children during separator placement and headgear wear; 2. to find possible associations between the perceived intensity of pain and the levels of Substance P (SP) and interleukin-1 beta (IL-1β) in the gingival crevicular fluid (GCF) during these procedures; 3. to identify other factors, such as previous pain experience, which could be associated to the patients' perceived discomfort or pain during treatment.

TRIAL DESIGN

Nine-month parallel-group randomized controlled trial.

METHODS

Forty Class II malocclusion children (8-12 years) were included, half of which received a cervical headgear while the other half did not receive any treatment during the study period. Baseline pain data were recorded including previous experience to general and dental pain, Corah's Dental Anxiety Scale, and baseline pain using a visual analogue scale (VAS). Elastic separators were placed in children for 1 week, followed by molar band and cervical headgear placement. Children were seen at various time points throughout the 9-month period where at each appointment, a VAS assessment of pain as well as GCF sampling was carried out to quantify the levels of SP and IL-1β. Multiple regression analysis was performed to ascertain the influence of factors including sex, age, time, headgear wear, and baseline pain data on pain severity.

RESULTS

Pain severity and SP and IL-1β levels in the GCF follow a similar pattern, with peaks being observed 1 day after orthodontic elastic separator placement. Pain was more severe after the placement of orthodontic separators than following cervical headgear wear. With regard to pain predictors, pain is more severe in older children, those with a worse previous general pain experience, and those with higher levels of IL-1β, particularly after elastic separator placement.

CONCLUSIONS

Orthodontic pain and discomfort following orthodontic separator placement and cervical headgear wear depends on factors including age, previous pain experience, and the level of IL-1β in the GCF.

Ocular surface inflammation induces de novo expression of substance P in the trigeminal primary afferents with large cell bodies

We evaluated the changes in substance P (SP)-expressing trigeminal neurons (TNs) innervating the cornea following ocular surface inflammation. Ocular surface inflammation was induced in Sprague-Dawley rats using 0.1% benzalkonium chloride (BAK). The corneal staining score, corneal epithelial apoptosis, conjunctival goblet cells, and density of corneal subbasal nerve plexus (SNP) were assessed, and the mRNA levels of SP, interleukin (IL)-1β, IL-6, and tumour necrosis factor-α were measured in corneas and ipsilateral trigeminal ganglia (TG). SP-immunoreactivity (IR) was measured in corneal intraepithelial nerves and TNs. The cell size of corneal TNs in the TG was calculated. All parameters were observed immediately (BAK group), at 1 week (1 w group), and 2 months (2 m group) after 2 weeks of BAK application. BAK caused an increase in the corneal staining score and the number of apoptotic cells, loss of conjunctival goblet cells, reduced density of corneal SNP, and upregulated expression of SP and inflammatory cytokines in both the cornea and TG in the BAK group but those changes were not observed in the 2 m group. On the other hand, SP-IR% and mean cell size of corneal TNs increased significantly in the BAK, 1 w, and 2 m groups, compared to the control. Our data suggest that following ocular surface inflammation, large-sized corneal TNs which normally do not express SP, expressed it and this phenotype switching lasted even after the inflammation disappeared. Long-lasting phenotypic switch, as well as changes in the expression level of certain molecules should be addressed in future studies on the mechanism of corneal neuropathic pain.

Ion Channels Involved in Substance P-Mediated Nociception and Antinociception

Substance P (SP), an 11-amino-acid neuropeptide, has long been considered an effector of pain. However, accumulating studies have proposed a paradoxical role of SP in anti-nociception. Here, we review studies of SP-mediated nociception and anti-nociception in terms of peptide features, SP-modulated ion channels, and differential effector systems underlying neurokinin 1 receptors (NK1Rs) in differential cell types to elucidate the effect of SP and further our understanding of SP in anti-nociception. Most importantly, understanding the anti-nociceptive SP-NK1R pathway would provide new insights for analgesic drug development.

P2X3 receptor-mediated visceral hyperalgesia and neuronal sensitization following exposure to PTSD-like stress in the dorsal root ganglia of rats

BACKGROUND

Patients with posttraumatic stress disorder (PTSD) often share co-morbidity with chronic pain conditions. Recent studies suggest a role of P2X3 receptors and ATP signaling in pain conditions. However, the underlying mechanisms of visceral hyperalgesia following exposure to PTSD-like stress conditions remain unclarified.

METHODS

The behavior and hormones relevant for PTSD were studied. Visceromotor responses (VMR) and the abdominal withdrawal reflexes (AWR) to colorectal distention (CRD) were recorded to determine P2X3-receptor-mediated alteration of hyperalgesia following single-prolonged stress (SPS) exposure. Immunofluorescence, Western blotting, and patch-clamp were used.

KEY RESULTS

The escape latency, adrenocorticotropic hormone and cortisol were increased on days 7-14. Visceromotor responses and AWR was reduced at day 1 in SPS rats but increased to higher levels than in controls after exposure to day 7. Intrathecal administration of the P2X3-receptor antagonist TNP-ATP abolished the CRD response. Based on immunofluorescence and Western blotting analysis, SPS-treated rats exhibited reduced P2X3 expression in dorsal root ganglia (DRG) after day 1 compared with controls. P2X3 expression in DRG was enhanced on day 7 after SPS and the increase of the P2X3 expression was maintained on day 14 and 21 compared with controls. The P2X3-receptor agonist α,β-me ATP (10 μM) induced a fast desensitizing inward current in DRG neurons of both control and SPS-treated rats. The average peak current densities in SPS-treated group were increased 3.6-fold. TNP-ATP (100 nM) markedly blocked all fast α,β-me ATP-induced inward currents in the DRG neurons both in control and SPS-treated rats.

CONCLUSIONS & INFERENCES

The data indicate an important role of P2X3 signaling in visceral hyperalgesia following PTSD-like stress.

The nervous system of airways and its remodeling in inflammatory lung diseases

Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.

[ROLE PHOSPHOINOSITID SIGNALING PATHWAY IN OPIOIDS CONTROL OF P2X3 RECEPTORS IN THE PRIMARY SENSORY NEURONS]

Homomeric P2X3 receptors expressed in primary nociceptive neurons are crucial elements in the pain signal generation. In turn, opioid system regulates the intensity of this signal in both CNS and PNS. Here we describe the effects of opioids on P2X3 receptors in DRG neurons studied by using patch clamp technique. Activation of G-protein coupled opioid receptors by endogenous opioid Leu-enkephalin (Leu), resulted in the two opposite effects on P2X3 receptor-mediated currents (P2X3 currents). In particular, application of 1 µM Leu lead to the complete inhibition of P2X3 currents. However, after pretreatment of the neurons with a Gi/o-protein inhibitor pertussis toxin (PT), the same concentration of Leu caused facilitation of P2X3 currents. PLC inhibitor U-73122 at concentration of 1 µM completely eliminated both facilitating and inhibitory effects of Leu on P2X3 currents. Thus, opioid receptor agonists cause two oppositely directed effects on P2X3 receptors in DRG neurons of rats and both of them are mediated through PLC signaling pathway. Our results point to a possible molecular basis of the mechanism for the well-known transition inhibitory action of opioids (analgesia) to facilitating (hyperalgesia).

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Craniofacial Pain as the Sole Sign of Prodromal Angina and Acute Coronary Syndrome: A Review and Report of a Rare Case

Orofacial pain can arise from different regions and etiologies. Some of the most debilitating pain conditions arise from the structures innervated by the trigeminal system (head, face, masticatory musculature, temporomandibular joint and associated structures). The problem with referred pain is the misdiagnosis and unnecessary therapy directed to the pain location instead of its origin. When craniofacial pain is the sole sign of myocardial ischemia, failure to recognize its cardiac source can endanger the patient. In particular, apart from unnecessary dental treatments, patients with acute myocardial infarction who do not experience chest pain run a very high risk of misdiagnosis and death. As endodontists, each of us may face many patients complaining of pain sensation in the teeth with the main source being other craniofacial/visceral organs. This review plots a diagnostically challenging case paving the way for further literature presentation in this regard. The aim of this compendious review was to gain knowledge about the prevalence, clinical characteristics and possible mechanisms of craniofacial pain of cardiac origin, in order to improve the clinician’s ability to make a correct diagnosis.

Molecular mechanism for opioid dichotomy: bidirectional effect of μ-opioid receptors on P2X₃ receptor currents in rat sensory neurones

Here, we describe a molecular switch associated with opioid receptors-linked signalling cascades that provides a dual opioid control over P2X3 purinoceptor in sensory neurones. Leu-enkephalin inhibited P2X3-mediated currents with IC50 ~10 nM in ~25% of small nociceptive rat dorsal root ganglion (DRG) neurones. In contrast, in neurones pretreated with pertussis toxin leu-enkephalin produced stable and significant increase of P2X3 currents. All effects of opioid were abolished by selective μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), nonselective inhibitor naloxone, and by PLC inhibitor U73122. Thus, we discovered a dual link between purinoceptors and μ-opioid receptors: the latter exert both inhibitory (pertussis toxin-sensitive) and stimulatory (pertussis toxin-insensitive) actions on P2X3 receptors through phospholipase C (PLC)-dependent pathways. This dual opioid control of P2X3 receptors may provide a molecular explanation for dichotomy of opioid therapy. Pharmacological control of this newly identified facilitation/inhibition switch may open new perspectives for the adequate medical use of opioids, the most powerful pain-killing agents known today.

Characteristics of dorsal root ganglia neurons sensitive to Substance P

BACKGROUND

Substance P modulates ion channels and the excitability of sensory neurons in pain pathways. Within the heterogeneous population of Dorsal Root Ganglia (DRG) primary sensory neurons, the properties of cells that are sensitive to Substance P are poorly characterized. To define this population better, dissociated rat DRG neurons were tested for their responsiveness to capsaicin, ATP and acid. Responses to ATP were classified according to the kinetics of current activation and desensitization. The same cells were then tested for modulation of action potential firing by Substance P.

RESULTS

Acid and capsaicin currents were more frequently encountered in the largest diameter neurons. P2X3-like ATP currents were concentrated in small diameter neurons. Substance P modulated the excitability in 20 of 72 cells tested (28%). Of the Substance P sensitive cells, 10 exhibited an increase in excitability and 10 exhibited a decrease in excitability. There was no significant correlation between sensitivity to capsaicin and to Substance P. Excitatory effects of Substance P were strongly associated with cells that had large diameters, fired APs with large overshoots and slowly decaying after hyperpolarizations, and expressed acid currents at pH 7. No neurons that were excited by Substance P presented P2X3-like currents. In contrast, neurons that exhibited inhibitory effects of Substance P fired action potentials with rapidly decaying after hyperpolarizations.

CONCLUSION

We conclude that excitatory effects of Substance P are restricted to a specific neuronal subpopulation with limited expression of putative nociceptive markers.

Are sensory TRP channels biological alarms for lipid peroxidation?

Oxidative stress induces numerous biological problems. Lipid oxidation and peroxidation appear to be important steps by which exposure to oxidative stress leads the body to a disease state. For its protection, the body has evolved to respond to and eliminate peroxidation products through the acquisition of binding proteins, reducing and conjugating enzymes, and excretion systems. During the past decade, researchers have identified a group of ion channel molecules that are activated by oxidized lipids: transient receptor potential (TRP) channels expressed in sensory neurons. These ion channels are fundamentally detectors and signal converters for body-damaging environments such as heat and cold temperatures, mechanical attacks, and potentially toxic substances. When messages initiated by TRP activation arrive at the brain, we perceive pain, which results in our preparing defensive responses. Excessive activation of the sensory neuronal TRP channels upon prolonged stimulations sometimes deteriorates the inflammatory state of damaged tissues by promoting neuropeptide release from expresser neurons. These same paradigms may also work for pathologic changes in the internal lipid environment upon exposure to oxidative stress. Here, we provide an overview of the role of TRP channels and oxidized lipid connections during abnormally increased oxidative signaling, and consider the sensory mechanism of TRP detection as an alert system.

Sensory TRP channel interactions with endogenous lipids and their biological outcomes

Lipids have long been studied as constituents of the cellular architecture and energy stores in the body. Evidence is now rapidly growing that particular lipid species are also important for molecular and cellular signaling. Here we review the current information on interactions between lipids and transient receptor potential (TRP) ion channels in nociceptive sensory afferents that mediate pain signaling. Sensory neuronal TRP channels play a crucial role in the detection of a variety of external and internal changes, particularly with damaging or pain-eliciting potentials that include noxiously high or low temperatures, stretching, and harmful substances. In addition, recent findings suggest that TRPs also contribute to altering synaptic plasticity that deteriorates chronic pain states. In both of these processes, specific lipids are often generated and have been found to strongly modulate TRP activities, resulting primarily in pain exacerbation. This review summarizes three standpoints viewing those lipid functions for TRP modulations as second messengers, intercellular transmitters, or bilayer building blocks. Based on these hypotheses, we discuss perspectives that account for how the TRP-lipid interaction contributes to the peripheral pain mechanism. Still a number of blurred aspects remain to be examined, which will be answered by future efforts and may help to better control pain states.

Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease

The tachykinins, exemplified by substance P, are one of the most intensively studied neuropeptide families. They comprise a series of structurally related peptides that derive from alternate processing of three Tac genes and are expressed throughout the nervous and immune systems. Tachykinins interact with three neurokinin G protein-coupled receptors. The signaling, trafficking, and regulation of neurokinin receptors have also been topics of intense study. Tachykinins participate in important physiological processes in the nervous, immune, gastrointestinal, respiratory, urogenital, and dermal systems, including inflammation, nociception, smooth muscle contractility, epithelial secretion, and proliferation. They contribute to multiple diseases processes, including acute and chronic inflammation and pain, fibrosis, affective and addictive disorders, functional disorders of the intestine and urinary bladder, infection, and cancer. Neurokinin receptor antagonists are selective, potent, and show efficacy in models of disease. In clinical trials there is a singular success: neurokinin 1 receptor antagonists to treat nausea and vomiting. New information about the involvement of tachykinins in infection, fibrosis, and pruritus justifies further trials. A deeper understanding of disease mechanisms is required for the development of more predictive experimental models, and for the design and interpretation of clinical trials. Knowledge of neurokinin receptor structure, and the development of targeting strategies to disrupt disease-relevant subcellular signaling of neurokinin receptors, may refine the next generation of neurokinin receptor antagonists.

Extracellular caspase-6 drives murine inflammatory pain via microglial TNF-α secretion

Increasing evidence indicates that the pathogenesis of neuropathic pain is mediated through spinal cord microglia activation. The intracellular protease caspase-6 (CASP6) is known to regulate neuronal apoptosis and axonal degeneration; however, the contribution of microglia and CASP6 in modulating synaptic transmission and pain is unclear. Here, we found that CASP6 is expressed specifically in C-fiber axonal terminals in the superficial spinal cord dorsal horn. Animals exposed to intraplantar formalin or bradykinin injection exhibited CASP6 activation in the dorsal horn. Casp6-null mice had normal baseline pain, but impaired inflammatory pain responses. Furthermore, formalin-induced second-phase pain was suppressed by spinal injection of CASP6 inhibitor or CASP6-neutralizing antibody, as well as perisciatic nerve injection of CASP6 siRNA. Recombinant CASP6 (rCASP6) induced marked TNF-α release in microglial cultures, and most microglia within the spinal cord expressed Tnfa. Spinal injection of rCASP6 elicited TNF-α production and microglia-dependent pain hypersensitivity. Evaluation of excitatory postsynaptic currents (EPSCs) revealed that rCASP6 rapidly increased synaptic transmission in spinal cord slices via TNF-α release. Interestingly, the microglial inhibitor minocycline suppressed rCASP6 but not TNF-α-induced synaptic potentiation. Finally, rCASP6-activated microglial culture medium increased EPSCs in spinal cord slices via TNF-α. Together, these data suggest that CASP6 released from axonal terminals regulates microglial TNF-α secretion, synaptic plasticity, and inflammatory pain.

B-type natriuretic peptide-induced delayed modulation of TRPV1 and P2X3 receptors of mouse trigeminal sensory neurons

Important pain transducers of noxious stimuli are small- and medium-diameter sensory neurons that express transient receptor vanilloid-1 (TRPV1) channels and/or adenosine triphosphate (ATP)-gated P2X3 receptors whose activity is upregulated by endogenous neuropeptides in acute and chronic pain models. Little is known about the role of endogenous modulators in restraining the expression and function of TRPV1 and P2X3 receptors. In dorsal root ganglia, evidence supports the involvement of the natriuretic peptide system in the modulation of nociceptive transmission especially via the B-type natriuretic peptide (BNP) that activates the natriuretic peptide receptor-A (NPR-A) to downregulate sensory neuron excitability. Since the role of BNP in trigeminal ganglia (TG) is unclear, we investigated the expression of BNP in mouse TG in situ or in primary cultures and its effect on P2X3 and TRPV1 receptors of patch-clamped cultured neurons. Against scant expression of BNP, almost all neurons expressed NPR-A at membrane level. While BNP rapidly increased cGMP production and Akt kinase phosphorylation, there was no early change in passive neuronal properties or responses to capsaicin, α,β-meATP or GABA. Nonetheless, 24 h application of BNP depressed TRPV1 mediated currents (an effect blocked by the NPR-A antagonist anantin) without changing responses to α,β-meATP or GABA. Anantin alone decreased basal cGMP production and enhanced control α,β-meATP-evoked responses, implying constitutive regulation of P2X3 receptors by ambient BNP. These data suggest a slow modulatory action by BNP on TRPV1 and P2X3 receptors outlining the role of this peptide as a negative regulator of trigeminal sensory neuron excitability to nociceptive stimuli.

Cellular and molecular mechanisms of dental nociception

Due, in part, to the unique structure of the tooth, dental pain is initiated via distinct mechanisms. Here we review recent advances in our understanding of inflammatory tooth pain and discuss 3 hypotheses proposed to explain dentinal hypersensitivity: The first hypothesis, supported by functional expression of temperature-sensitive transient receptor potential channels, emphasizes the direct transduction of noxious temperatures by dental primary afferent neurons. The second hypothesis, known as hydrodynamic theory, attributes dental pain to fluid movement within dentinal tubules, and we discuss several candidate cellular mechanical transducers for the detection of fluid movement. The third hypothesis focuses on the potential sensory function of odontoblasts in the detection of thermal or mechanical stimuli, and we discuss the accumulating evidence that supports their excitability. We also briefly update on a novel strategy for local nociceptive anesthesia via nociceptive transducer molecules in dental primary afferents with the potential to specifically silence pain fibers during dental treatment. Further understanding of the molecular mechanisms of dental pain would greatly enhance the development of therapeutics that target dental pain.

Effect of amitriptyline on tetrodotoxin-resistant Nav1.9 currents in nociceptive trigeminal neurons

Background

Amitriptyline (AMI) is tricyclic antidepressant that has been widely used to manage various chronic pains such as migraines. Its efficacy is attributed to its blockade of voltage-gated sodium channels (VGSCs). However, the effects of AMI on the tetrodotoxin-resistant (TTX-r) sodium channel Na_v1.9 currents have been unclear to present.

Results

Using a whole-cell patch clamp technique, this study showed that AMI efficiently inhibited Na_v1.9 currents in a concentration-dependent manner and had an IC₅₀ of 15.16 μM in acute isolated trigeminal ganglion (TG) neurons of the rats. 10 μM AMI significantly shifted the steady-state inactivation of Na_v1.9 channels in the hyperpolarizing direction without affecting voltage-dependent activation. Surprisingly, neither 10 nor 50 μM AMI caused a use-dependent blockade of Na_v1.9 currents elicited by 60 pulses at 1 Hz.

Conclusion

These data suggest that AMI is a state-selective blocker of Na_v1.9 channels in rat nociceptive trigeminal neurons, which likely contributes to the efficacy of AMI in treating various pains, including migraines.

Changes of substance P in the crevicular fluid in relation to orthodontic movement preliminary investigation

Substance P (SP) is a tachykinin released from both the central and the peripheral endings of primary afferent neurons and functions as a neurotransmitter. As a transmitter signaling pain, substance P is involved in nociception and is an extremely potent vasodilator. We found several studies about this neuropeptide especially in relation to parodontology and a few orthodontic reviews. This is because in the past the importance of this neuropeptide in dental element undergoing periodontal inflammation was observed. The aims of the present pilot study was to investigate whether the substance P was present in gingival crevicular fluid in dental elements undergoing orthodontic treatment with Invisalign technique compared to teeth belonging to the same series but not undergoing orthodontic movement. We analysed gengival crevicular fluid (GCF) collected from four young subjects, using a paper cone for a time of 60 seconds. The results showed that SP is present in the gengival sulcus in elements undergoing orthodontic forces during treatment with Invisalign technique and not in the control teeth. During the literature analysis, we have found a lot of papers describing involvement of SP in periodontitis and inflammatory diseases, but further studies are needed in order to demonstrate the role of this neuropeptide during teeth movement.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

Control of P2X3 channel function by metabotropic P2Y2 utp receptors in primary sensory neurons

Purinergic signaling contributes significantly to pain mechanisms, and the nociceptor-specific P2X3 ATP receptor channel is considered a target in pain therapeutics. Recent findings suggesting the coexpression of metabotropic P2Y receptors with P2X3 implies that ATP release triggers the activation of both ionotropic and metabotropic purinoceptors, with strong potential for functional interaction. Modulation of native P2X3 function by P2Y receptor activation was investigated in rat dorsal root ganglia (DRG) neurons using whole cell patch-clamp recordings. Application of the selective P2Y receptor agonist UTP decreased peak amplitudes of α,β-meATP-evoked homomeric P2X3-mediated currents, but had no effect on heteromeric P2X2/3-mediated currents. Treatment with phospholipase C inhibitor U73122 significantly reversed P2X3 current inhibition induced by UTP-sensitive P2Y receptor activation. We previously reported the modulation of P2X receptors by phospholipids in DRG neurons and injection of exogenous phosphatidylinositol-4,5-bisphosphate (PIP(2)) fully reverses UTP-mediated regulation of P2X3 channel activity. Pharmacological as well as functional screening of P2Y receptor subtypes indicates the predominant involvement of P2Y2 receptor in P2X3 inhibition, and immunolocalization confirms a significant cellular coexpression of P2X3 and P2Y2 in rat DRG neurons. In summary, the function of P2X3 ATP receptor can be inhibited by P2Y2-mediated depletion of PIP(2). We propose that expression of P2Y2 purinoceptor in nociceptive sensory neurons provides an homeostatic mechanism to prevent excessive ATP signaling through P2X3 receptor channels.

Peripheral mechanisms of dental pain: the role of substance P

Current evidence supports the central role of neuropeptides in the molecular mechanisms underlying dental pain. In particular, substance P, a neuropeptide produced in neuron cell bodies localised in dorsal root and trigeminal ganglia, contributes to the transmission and maintenance of noxious stimuli and inflammatory processes. The major role of substance P in the onset of dental pain and inflammation is increasingly being recognised. Well-grounded experimental and clinical observations have documented an increase in substance P concentration in patients affected by caries, pulpitis, or granulomas and in those undergoing standard orthodontic or orthodontic/dental care procedures. This paper focuses on the role of substance P in the induction and maintenance of inflammation and dental pain, in order to define future lines of research for the evaluation of therapeutic strategies aimed at modulating the complex effects of this mediator in oral tissues.

Mechanisms of nociceptive transduction and transmission: a machinery for pain sensation and tools for selective analgesia

Many surgical and dental procedures depend on use of local anesthetics to reversibly eliminate pain. By the blockade of voltage-gated sodium channels, local anesthetics prevent the transmission of nociceptive information. However, since all local anesthetics act non-selectively on all types of axons they also cause a loss of innocuous sensation, motor paralysis and autonomic block. Thus, approaches that produce only a selective blockade of pain fibers are of great potential clinical importance. In this chapter we will review the recent findings describing mechanisms of pain transduction and transmission and introduce novel therapeutic approaches to produce pain-selective analgesia.