Are Outcomes of Temporomandibular Joint Arthroscopy Influenced by Central Sensitization?

PURPOSE: A proportion of subjects with internal derangements of the temporomandibular joint (TMJ) may have a central sensitization disorder that may affect pain perception after surgery. This study aims to estimate the association between fibromyalgianess (FMness) score, a summed score of the Widespread Pain Index (WPI) and Symptom Severity Sore (SSS), and outcomes following TMJ arthroscopy. METHODS: A retrospective cohort study including individuals who received arthroscopy for TMJ internal derangement at Michigan Medicine between 2011 and 2020 was performed. A predictor variable, FMness score, was assigned via the sum of WPI and SSS. Univariate and bivariate analyses were performed. Linear-mixed effects models were used to analyze 6 different outcomes, each in their own model: pain, jaw functional limitation scale (JFLS), JFLS-mobility domain, pain-related disability, comfortable maximum interincisal opening, and active maximum interincisal opening. Covariance structure was selected based on null model fit separately for each outcome. RESULTS: Thirty-one subjects were included in the study sample. Twenty-eight subjects were female. Average age was 45.9 years. Bivariate analysis demonstrated that subject's FMness score was not correlated with pain (b = 0.03 [-0.10, 0.17] P = .59) or JFLS score (b = 1.00 [-.80, 2.81] P = .27). However, subject's FMness score was significant for predicting JFLS-mobility domain score (b = .61, [0.05, 1.18] P = .04). CONCLUSION: A greater extent of central sensitization was associated with lower comfortable mouth opening after surgery, greater limitations in opening wide enough to eat various foods (higher JFLS-mobility scores), and higher pain-related disability. Future studies with larger sample sizes and reconstructive TMJ operations such as total TMJ arthroplasty may help clarify the impact of SSS and WPI scores on outcomes of TMJ surgery.

Central Sensitization Is Modulated Following Trigger Point Anesthetization in Patients with Chronic Pain from Whiplash Trauma. A Double-Blind, Placebo-Controlled, Crossover Study.

Objective: Central sensitization (CS) with low peripheral pain thresholds (PPTs) is a common finding among patients with chronic pain after whiplash (CPWI). While it has been proposed that myofascial myofascial trigger points (MTrPs) may act as modulators of central sensitization, previously reported findings are conflicting and inconclusive. The present study was designed to investigate immediate responsiveness of CS to alterations in nociceptive input. Design: Controlled, double-blind, cross-over. Subjects: Thirty-one patients with chronic pain (trapezius myalgia) and CS after whiplash. Methods: Participants were referred by randomization to group A for injection of a single peripheral pain generator (MTrP or other discrete tender point) with local anesthetic or to group B for sham injection and cross-over. Documentation of PPT (Algometer), maximum jaw opening (caliper), and grip strength (Vigorimeter), as well as subjective overall pain (visual analog scale [VAS]), was made before and after each intervention. Results: Statistical analysis of data (Student's t test, analysis of variance) confirmed that peripheral pain thresholds were significantly higher and maximum jaw opening significantly greater after anesthetizing a focal pain generator in the trapezius, but not after a sham injection. In contrast with the objective variables, subjective generalized pain improved (VAS) after not only an injection of local anesthetic, but also, and to a similar extent, after a sham injection. Conclusions: CS, as expressed by lowered PPT, is a rapidly adjusting physiological response to nociceptive stimuli in some patients with chronic pain after whiplash. PPT are likely modulated by myofascial tender points in selected patients with CS. With reference to the present findings, surgical ablation of MTrPs is discussed as a potential treatment modality for CS.

Mechanisms of craniofacial pain.

Aim To provide an overview of mechanisms underlying craniofacial pain; to highlight peripheral and central adaptations that may promote chronification of pain in craniofacial pain states such as migraine and temporomandibular disorders (TMD). Background Pain is a common symptom associated with disorders involving craniofacial tissues including the teeth and their supporting structures, the temporomandibular joint and the muscles of the head. Most acute painful craniofacial conditions are easily recognized and well managed, but others, especially those that are chronic (e.g., migraine, TMD and trigeminal neuropathies), present clinical challenges. Preclinical studies have provided substantial information about the anatomical and physiological mechanisms related to the initiation and modulation of nociceptive signals in the trigeminal system. While knowledge of the mechanisms underlying chronic craniofacial pain remains limited, both clinical and preclinical investigations suggest that changes in afferent inputs to the brain as well as in brain structure and modulatory pathways occur in chronic pain. Collectively, these changes result in amplification of nociception that promotes and sustains craniofacial chronic pain states. Conclusions The increased understanding gained of the physiological and pathological processing of nociception in the trigeminal system has provided new perspectives for the mechanistic understanding of acute craniofacial pain conditions and the peripheral and central adaptations that are related to pain chronification. Such knowledge may contribute to improvements in currently available treatments as well as to the development of novel analgesic therapies.

Reduced hypothalamic-pituitary-adrenal axis activity in chronic multi-site musculoskeletal pain: partly masked by depressive and anxiety disorders.

BACKGROUND: Studies on hypothalamic-pituitary-adrenal axis (HPA-axis) function amongst patients with chronic pain show equivocal results and well-controlled cohort studies are rare in this field. The goal of our study was to examine whether HPA-axis dysfunction is associated with the presence and the severity of chronic multi-site musculoskeletal pain. METHODS: Data are from the Netherlands Study of Depression and Anxiety including 1125 subjects with and without lifetime depressive and anxiety disorders. The Chronic Pain Grade questionnaire was used to determine the presence and severity of chronic multi-site musculoskeletal pain. Subjects were categorized into a chronic multi-site musculoskeletal pain group (n = 471) and a control group (n = 654). Salivary cortisol samples were collected to assess HPA-axis function (awakening level, 1-h awakening response, evening level, diurnal slope and post-dexamethasone level). RESULTS: In comparison with the control group, subjects with chronic multi-site musculoskeletal pain showed significantly lower cortisol level at awakening, lower evening level and a blunted diurnal slope. Lower cortisol level at awakening and a blunted diurnal slope appeared to be restricted to those without depressive and/or anxiety disorders, who also showed a lower 1-h awakening response. CONCLUSIONS: Our results suggest hypocortisolemia in chronic multi-site musculoskeletal pain. However, if chronic pain is accompanied by a depressive or anxiety disorder, typically related to hypercortisolemia, the association between cortisol levels and chronic multi-site musculoskeletal pain appears to be partly masked. Future studies should take psychopathology into account when examining HPA-axis function in chronic pain.

Pain in fibromyalgia and related conditions.

Pain is the hallmark symptom of fibromyalgia (FM) and other related syndromes, but quite different from that of other rheumatic diseases, which depends on the degree of damage or inflammation in peripheral tissues. Sufferers are often defined as patients with chronic pain without an underlying mechanistic cause, and these syndromes and their symptoms are most appropriately described as "central pain", "neuropathic pain", "nonnociceptive pain" or "central sensitivity syndromes". The pain is particular, regional or widespread, and mainly relates to the musculoskeletal system; hyperalgesia or allodynia are typical. Its origin is currently considered to be distorted pain or sensory processing, rather than a local or regional abnormality. FM is probably the most important and extensively described central pain syndrome, but the characteristics and features of FM-related pain are similar in other disorders of particular interest for rheumatologists, such as myofascial pain syndromes and temporo-mandibular joint disorders, and there is also an intriguing overlap between FM and benign joint hypermobility syndrome. This suggests that the distinctive aspects of pain in these idiopathic or functional conditions is caused by central nervous system hypersensitivity and abnormalities. Pharmacological and non-pharmacological therapies have been suggested for the treatment of these conditions, but a multidisciplinary approach is required in order to reduce the abnormal cycle of pain amplification and the related maladaptive and self-limiting behaviours.

The prevalence of comorbid symptoms of central sensitization syndrome among three different groups of temporomandibular disorder patients.

AIMS: Symptoms of central sensitization syndrome (CSS) were evaluated among three different groups of temporomandibular disorder (TMD) patients. Additionally, TMD group differences in pain and pain-related disability were assessed, as well as emotional distress. METHODS: Participants were 250 patients with symptoms of acute TMD, recruited from dental clinics within a major metropolitan area. Sequential regressions and multivariate analyses of covariance were conducted in order to make group comparisons. RESULTS: Those with a TMD Muscle Disorder (ie, myofacial TMD [m-TMD]) and those with more than one TMD diagnosis had the most symptoms of CSS and higher reports of pain and pain-related disability. Moreover, emotional distress accounted for a substantial amount of the variance for physical symptoms and mediated all TMD comparisons. CONCLUSIONS: Myofacial TMD is characterized by a high degree of comorbidity of symptoms of CSS and associated emotional distress.

Cross-cultural adaptation and validation for central sensitization inventory: based on Chinese patients undergoing total knee arthroplasty for knee osteoarthritis.

BACKGROUND: This study was conducted to develop a simplified Chinese version of the central sensitization inventory (CSI-CV) and to evaluate its reliability and validity. METHODS: The CSI-CV was developed through a process involving the translation and back translation of the original CSI. Subsequently, experts reviewed and revised the content of the items to ensure their appropriateness. A total of 325 patients with knee osteoarthritis (KOA), who were scheduled to undergo total knee arthroplasty (TKA), completed the CSI-CV at a prominent orthopedic center in Xi'an, China. Afterward, a random selection of 100 participants was chosen for retesting after one week. The reliability and validity of the inventory were evaluated through exploratory factor analysis, correlation coefficient calculation and other methods. RESULTS: The CSI-CV consists of 25 items in five dimensions (emotional distress, headache and jaw symptoms, physical symptoms, urological symptoms, and fatigue and sleep problems). The cumulative variance contribution rate was 75.3%, the Cronbach's α coefficient was 0.83, the Guttman split-half reliability coefficient was 0.88 and the intraclass correlation coefficient was 0.965. The CSI-CV scores correlated moderately with the total scores of the brief pain inventory (r = 0.506), Western Ontario and McMaster Universities Osteoarthritis Index (r = 0.466) and EuroQoL Group's five-dimension questionnaire (r = 0.576). CONCLUSIONS: The findings demonstrate that the CSI was successfully trans-culturally adapted into a simplified Chinese version (CSI-CV) that was reliable and valid for Chinese-speaking patients who awaiting TKA for KOA.

Central sensitization in burning mouth syndrome: a practical approach using questionnaires.

OBJECTIVES: Some experts have suggested that burning mouth syndrome (BMS) should be included in the family of central sensitivity syndromes, a group of similar medical disorders linked by the central sensitization (CS) mechanism. Our objective is to assess the presence of CS in patients with BMS by performing a clinical examination and administering questionnaires to measure the generalized extent of pain, the presence of associated symptoms, and the number of other concurrent chronic pain conditions. STUDY DESIGN: We conducted a case-control study in 82 subjects (40 patients with BMS and 42 controls). Patients with BMS were diagnosed using The International Classification of Headache Disorders 3rd edition, beta version (ICHD-IIIß) criteria. The Widespread Pain Index (WPI) and Symptom Severity (SS) Score questionnaires were used to determine the degree of central sensitivity. The number of other concurrent chronic pain conditions was determined with the Neblett inventory. RESULTS: Data indicative of CS show a statistically significant association with BMS. Both SS Score and Widespread Pain Index scores higher in patients with BMS. Additionally, patients with BMS reported a significantly higher number of other central sensitivity syndromes. CONCLUSIONS: Patients with BMS could present a CS component as well as other chronic pain conditions. The use of questionnaires may be useful to determine the degree of central sensitivity in patients with BMS.

Modulation of astroglial glutamine synthetase activity affects nociceptive behaviour and central sensitization of medullary dorsal horn nociceptive neurons in a rat model of chronic pulpitis.

Previous studies indicate that the astroglial glutamate-glutamine shuttle may be involved in acute pulpal inflammatory pain by influencing central sensitization induced in nociceptive neurons in the trigeminal subnucleus caudalis [the medullary dorsal horn (MDH)] by application of an inflammatory irritant to the rat tooth pulp. The aim of this study was to test if intrathecal application to the rat medulla of the astroglial glutamine synthetase inhibitor methionine sulfoximine (MSO) can influence the central sensitization of MDH nociceptive neurons and the animal's associated behaviour that are manifested in a model of chronic pulpitis pain induced by exposure of a mandibular molar pulp. This model was found to be associated with nocifensive behaviour and enhanced reflex activity evoked by mechanical stimulation of the rat's facial skin and with immunocytochemical evidence of astroglial activation in the MDH. These features were apparent for up to 28 days post-operatively. During this post-operative period, the nocifensive behaviour and enhanced reflex activity were significantly attenuated by intrathecal application of MSO (5 µL, 10 mM) but not by vehicle application. In electrophysiological recordings of nociceptive neuronal activity in the MDH, central sensitization was also evident in pulp-exposed rats but not in intact rats and could be significantly attenuated by MSO application but not by vehicle application. These behavioural and neuronal findings suggest that the astroglial glutamate-glutamine shuttle is responsible for the maintenance of inflammation-induced nocifensive behavioural changes and the accompanying central sensitization in MDH nociceptive neurons in this chronic pulpitis pain model.

Quantitative Assessment of Mechanical Allodynia and Central Sensitization in Endodontic Patients.

INTRODUCTION: Patients seeking endodontic treatment commonly present with reduced mechanical pain thresholds (ie, mechanical allodynia [MA]) in the offending teeth. In patients with moderate to severe pain, MA may manifest in the teeth contralateral to the offending teeth because of the onset of central sensitization (CS). We hypothesize that there are quantitative differences in MA and CS in patients with different pulp and periradicular diagnoses. METHODS: Patients (n = 70) receiving endodontic treatment in the graduate endodontic clinic at the University of Texas Health Science Center at San Antonio and healthy volunteers (n = 10) were included in this cross-sectional study. The mechanical pain threshold from molar teeth was measured by a digital bite force transducer on the offending tooth (ipsilateral) and the contralateral tooth. Ipsi- and contralateral MA among different endodontic diagnoses were analyzed using the Kruskal-Wallis with Dunn post hoc test and the Student t test for differences between sexes. Multivariate regression models analyzed predictors for MA and CS. RESULTS: Periradicular diagnoses of asymptomatic apical periodontitis, symptomatic apical periodontitis, and chronic apical abscess cases were significantly associated with MA. CS, seen as contralateral MA, was only detected in pulpal diagnosis of symptomatic irreversible pulpitis, previously initiated treatment, symptomatic apical periodontitis, and chronic apical abscess. Females experienced significantly lower pain thresholds than males on both sides. MA and CS were significantly correlated in both sexes. The preoperative pain level and duration were significant predictors for MA and CS only in female patients. Lastly, age was a significant predictor for MA in females. CONCLUSIONS: The magnitude of MA and CS varied with different endodontic diagnoses, with CS being correlated with increases in MA. Only in female patients were age, preoperative pain duration, and intensity significant predictors for the development of MA and CS.

Central Sensitization-Based Classification for Temporomandibular Disorders: A Pathogenetic Hypothesis.

Dysregulation of Autonomic Nervous System (ANS) and central pain pathways in temporomandibular disorders (TMD) is a growing evidence. Authors include some forms of TMD among central sensitization syndromes (CSS), a group of pathologies characterized by central morphofunctional alterations. Central Sensitization Inventory (CSI) is useful for clinical diagnosis. Clinical examination and CSI cannot identify the central site(s) affected in these diseases. Ultralow frequency transcutaneous electrical nerve stimulation (ULFTENS) is extensively used in TMD and in dental clinical practice, because of its effects on descending pain modulation pathways. The Diagnostic Criteria for TMD (DC/TMD) are the most accurate tool for diagnosis and classification of TMD. However, it includes CSI to investigate central aspects of TMD. Preliminary data on sensory ULFTENS show it is a reliable tool for the study of central and autonomic pathways in TMD. An alternative classification based on the presence of Central Sensitization and on individual response to sensory ULFTENS is proposed. TMD may be classified into 4 groups: (a) TMD with Central Sensitization ULFTENS Responders; (b) TMD with Central Sensitization ULFTENS Nonresponders; (c) TMD without Central Sensitization ULFTENS Responders; (d) TMD without Central Sensitization ULFTENS Nonresponders. This pathogenic classification of TMD may help to differentiate therapy and aetiology.

Pain Mechanisms and Centralized Pain in Temporomandibular Disorders.

Until recently, most clinicians and scientists believed that the experience of pain is perceptually proportional to the amount of incoming peripheral nociceptive drive due to injury or inflammation in the area perceived to be painful. However, many cases of chronic pain have defied this logic, leaving clinicians perplexed as to how patients are experiencing pain with no obvious signs of injury in the periphery. Conversely, there are patients who have a peripheral injury and/or inflammation but little or no pain. What makes some individuals experience intense pain with minimal peripheral nociceptive stimulation and others experience minimal pain with serious injury? It is increasingly well accepted in the scientific community that pain can be generated and maintained or, through other mechanisms, suppressed by changes in the central nervous system, creating a complete mismatch between peripheral nociceptive drive and perceived pain. In fact, there is no known chronic pain condition where the observed extent of peripheral damage reproducibly engenders the same level of pain across individuals. Temporomandibular disorders (TMDs) are no exception. This review focuses on the idea that TMD patients range on a continuum-from those whose pain is generated peripherally to those whose pain is centralized (i.e., generated, exacerbated, and/or maintained by central nervous system mechanisms). This article uses other centralized chronic pain conditions as a guide, and it suggests that the mechanistic variability in TMD pain etiology has prevented us from adequately treating many individuals who are diagnosed with the condition. As the field moves forward, it will be imperative to understand each person's pain from its own mechanistic standpoint, which will enable clinicians to deliver personalized medicine to TMD patients and eventually provide relief in even the most recalcitrant cases.

Recognition and Treatment of Central Sensitization in Chronic Pain Patients: Not Limited to Specialized Care.

Modern pain neuroscience has substantially improved our understanding of the (development of) chronic musculoskeletal pain. The time has come for orthopaedic and sports physical therapists to implement modern pain neuroscience in specialized, but definitely also in primary, care settings, including the role of central sensitization (CS) in amplifying and explaining the presence of the pain experience. Central sensitization dominates the clinical picture in a subgroup of the musculoskeletal pain population, ranging from tennis elbow over shoulder pain to osteoarthritis and whiplash. Applying modern pain neuroscience to clinical practice implies (1) recognizing those patients having predominant CS pain, and (2) accounting for CS when designing the treatment plan in those with predominant CS pain. Future work in this area should (1) examine the validity of the proposed clinical classification algorithm for identifying CS pain in patients with orthopaedic and sports injuries, and (2) explore evidence-based treatment options for patients having predominant CS pain. J Orthop Sports Phys Ther 2016;46(12):1024-1028. doi:10.2519/jospt.2016.0612.

Experimental jaw muscle pain increases pain scores and jaw movement variability in higher pain catastrophizers.

AIMS: To investigate differences between higher and lower pain catastrophizers in the effects of hypertonic saline-evoked jaw muscle pain on pain perception and jaw movement. METHODS: Repetitive open/close jaw movements were recorded in 28 asymptomatic participants (20 men, 8 women; ages 25 to 62 years) during continuous infusion of 5% hypertonic saline or isotonic saline into the right masseter muscle. All participants completed the McGill Pain Questionnaire; the Depression, Anxiety and Stress Scales; and the Jaw Function Limitation Scale. They were divided into two groups depending on the median Pain Catastrophizing Scale score. Statistical analyses involved multivariate analysis of variance, independent samples or paired t tests, and Pearson correlations (statistical α: P < .05). RESULTS: Pain intensity, unpleasantness, perceived area, and pain rating indices were significantly (P < .05) elevated in higher pain catastrophizers during hypertonic saline-evoked pain in comparison with lower catastrophizers. The higher catastrophizers exhibited significantly (P < .05) slower jaw velocity than the lower catastrophizers during hypertonic saline infusion in comparison with IS infusion. In comparison with lower catastrophizers, there was a significantly greater change in the percentage of coefficient of variation between hypertonic saline and isotonic saline infusions in higher catastrophizers for closing velocity and opening and closing amplitude. CONCLUSION: The increased reported pain intensity, pain areas, and pain rating indices are consistent with enhanced central sensitization processes in high-catastrophizing individuals. The slower velocity and greater variability of repetitive jaw movements in higher pain catastrophizing individuals in acute experimental pain may reflect changes in motor coordination as an example of avoidance behavior for the jaw motor system.

Increased release of dopamine in the striata of young adults with hearing impairment and its relevance for the social defeat hypothesis of schizophrenia.

IMPORTANCE: An increased risk for psychosis is observed in people with hearing impairment. According to the social defeat hypothesis, the long-term experience of exclusion leads to enhanced baseline activity and/or sensitization of the dopamine system and puts the individual at increased risk for psychosis. OBJECTIVE: To investigate whether young adults with severe hearing impairment (SHI) experience more feelings of social defeat, show greater dopamine release in response to dexamphetamine, and report a stronger subjective reaction to this substance than normal-hearing young adults and to examine whether dopamine release is associated with both self-reported social exclusion and dexamphetamine-induced psychotic experiences. DESIGN, SETTING, AND PARTICIPANTS: A sample of 19 participants with SHI and 19 smoking-, age-, and sex-matched healthy controls underwent single-photon emission computed tomography with iodine 123-labeled iodobenzamide as a radiotracer before and after an amphetamine challenge at an academic hospital. EXPOSURES: Dexamphetamine sulfate (0.3 mg/kg) administered intravenously. MAIN OUTCOMES AND MEASURES: Baseline D2/3 receptor binding and endogenous dopamine release. RESULTS: The participants with SHI reported experiencing more feelings of social defeat (U=109, z=-2.09, P=.04) and loneliness (U=87.5, z=-2.72, P=<.001) than did healthy controls, but they did not differ from healthy controls with regard to baseline psychotic symptoms (U=156.5, z=-0.70, P=.48). There were no significant group differences in baseline D2/3 receptor binding. However, repeated-measures multivariate analysis of covariance with age (in months) and tobacco smoking (in pack-years) as covariates showed that there was a greater amphetamine-induced striatal dopamine release among the participants with SHI than among the healthy controls (F1,34=4.55, P=.04). After amphetamine administration, the participants with SHI reported more changes in affect than the healthy controls, but not a greater increase in psychotic symptoms. Likewise, reports of social exclusion and an increase in psychotic symptoms were not associated with dopamine release. CONCLUSIONS AND RELEVANCE: Our study presents preliminary evidence of dopamine sensitization in a socially excluded group of people with hearing impairment. If replicated by future studies in other excluded groups, this finding may have major implications for our understanding of the underlying mechanism and for prevention of psychotic disorders.

Central α-adrenoceptors contribute to mustard oil-induced central sensitization in the rat medullary dorsal horn.

Our previous studies have demonstrated that application of the inflammatory irritant mustard oil (MO) to the tooth pulp produces trigeminal central sensitization that includes increases in mechanoreceptive field size and responses to noxious stimuli and decrease in activation threshold in brainstem nociceptive neurons of trigeminal subnucleus caudalis (the medullary dorsal horn, MDH). The aim of the present study was to test if central noradrenergic processes are involved in the central sensitization of MDH neurons and if α1-adrenoceptors or α2-adrenoceptors or both are involved. In urethane/α-chloralose-anesthetized rats, the activity of extracellularly recorded and functionally identified single nociceptive neurons in the MDH was studied. Continuous intrathecal (i.t.) superfusion of the adrenergic modulator guanethidine and α-adrenoceptor blocker phentolamine or selective α1-adrenoceptor antagonist prazosin over the medulla strongly attenuated all three MO-induced parameters of central sensitization in the MDH nociceptive neurons, compared to phosphate-buffered saline (as vehicle control). In contrast, i.t. superfusion of the selective α2-adrenoceptor antagonist yohimbine had little effect on the mechanoreceptive field expansion and the decreased mechanical activation threshold, and indeed facilitated responses to noxious stimuli of sensitized nociceptive neurons. Superfusion of each of the four chemicals alone did not affect baseline nociceptive neuronal properties. These findings provide the first documentation of the involvement of central noradrenergic processes in MDH in the development of the central sensitization, and that α1- and α2-adrenoceptors may be differentially involved.

Central sensitization and MAPKs are involved in occlusal interference-induced facial pain in rats.

UNLABELLED: We previously developed a rat dental occlusal interference model of facial pain that was produced by bonding a crown onto the right maxillary first molar and was reflected in sustained facial hypersensitivity that was suggestive of the involvement of central sensitization mechanisms. The aim of the present study was to investigate potential central mechanisms involved in the occlusal interference-induced facial hypersensitivity. A combination of behavioral, immunohistochemical, Western blot, and electrophysiological recording procedures was used in 98 male adult Sprague Dawley rats that either received the occlusal interference or were sham-operated or naive rats. Immunohistochemically labeled astrocytes and microglia in trigeminal subnucleus caudalis (Vc) showed morphological changes indicative of astrocyte and microglial activation after the occlusal interference. Prolonged upregulation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) was also documented in Vc after placement of the occlusal interference and was expressed in both neurons and glial cells at time points when rats showed peak mechanical facial hypersensitivity. The intrathecal administration of the p38 MAPK inhibitor SB203580 to the medulla significantly inhibited the occlusal interference-induced hypersensitivity, and the ERK inhibitor PD98059 produced an even stronger effect. Central sensitization of functionally identified Vc nociceptive neurons following placement of the occlusal interference was also documented by extracellular electrophysiological recordings, and intrathecal administration of PD98059 could reverse the neuronal central sensitization. These novel findings suggest that central mechanisms including central sensitization of trigeminal nociceptive neurons and non-neuronal processes involving MAPKs play significant roles in the production of occlusal interference-induced facial pain. PERSPECTIVE: Central mechanisms including trigeminal nociceptive neuronal sensitization, non-neuronal processes involving glial activation, and MAPKs play significant roles in occlusal interference-induced facial pain. These mechanisms may be involved in clinical manifestations of facial pain that have been reported in patients with an occlusal interference.

Involvement of ATP in noxious stimulus-evoked release of glutamate in rat medullary dorsal horn: a microdialysis study.

Our electrophysiological studies have shown that both purinergic and glutamatergic receptors are involved in central sensitization of nociceptive neurons in the medullary dorsal horn (MDH). Here we assessed the effects of intrathecal administration of apyrase (a nucleotide degrading enzyme of endogenous adenosine 5-triphosphate [ATP]), a combination of apyrase and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, an adenosine A1 receptor antagonist), or 2,3-O-2,4,6-trinitrophenyl-adenosine triphosphate (TNP-ATP, a P2X1, P2X3, P2X2/3 receptor antagonist) on the release of glutamate in the rat MDH evoked by application of mustard oil (MO) to the molar tooth pulp. In vivo microdialysis was used to dialyse the MDH every 5 min, and included 3 basal samples, 6 samples after drug treatment and 12 samples following application of MO. Tooth pulp application of MO induced a significant increase in glutamate release in the MDH. Superfusion of apyrase or TNP-ATP alone significantly reduced the MO-induced glutamate release in the MDH, as compared to vehicle. Furthermore, the suppressive effects of apyrase on glutamate release were reduced by combining it with DPCPX. This study demonstrates that application of an inflammatory irritant to the tooth pulp induces glutamate release in the rat MDH in vivo that may be reduced by processes involving endogenous ATP and adenosine.

Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors.

Central sensitization is a crucial process underlying the increased neuronal excitability of nociceptive pathways following peripheral tissue injury and inflammation. Our previous findings have suggested that extracellular adenosine 5'-triphosphate (ATP) molecules acting at purinergic receptors located on presynaptic terminals (e.g., P2X2/3, P2X3 subunits) and glial cells are involved in the glutamatergic-dependent central sensitization induced in medullary dorsal horn (MDH) nociceptive neurons by application to the tooth pulp of the inflammatory irritant mustard oil (MO). Since growing evidence indicates that activation of P2X7 receptors located on glia is involved in chronic inflammatory and neuropathic pain, the aim of the present study was to test in vivo for P2X7 receptor involvement in this acute inflammatory pain model. Experiments were carried out in anesthetized Sprague-Dawley male rats. Single unit recordings were made in MDH functionally identified nociceptive neurons for which mechanoreceptive field, mechanical activation threshold and responses to noxious stimuli were tested. We found that continuous intrathecal (i.t.) superfusion over MDH of the potent P2X7 receptor antagonists brilliant blue G and periodated oxidized ATP could each significantly attenuate the MO-induced MDH central sensitization. MDH central sensitization could also be produced by i.t. superfusion of ATP and even more effectively by the P2X7 receptor agonist benzoylbenzoyl ATP. Superfusion of the microglial blocker minocycline abolished the MO-induced MDH central sensitization, consistent with reports that dorsal horn P2X7 receptors are mostly expressed on microglia. In control experiments, superfusion over MDH of vehicle did not produce any significant changes. These novel findings suggest that activation of P2X7 receptors in vivo may be involved in the development of central sensitization in an acute inflammatory pain model.