Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain: evidence for altered temporal summation of pain

Treatment of central sensitization in patients with 'unexplained' chronic pain: what options do we have?

INTRODUCTION

Central sensitization accounts for chronic 'unexplained' pain in a wide variety of disorders, including chronic whiplash-associated disorders, temporomandibular disorders, chronic low back pain, osteoarthritis, fibromyalgia, chronic fatigue syndrome and chronic tension-type headache among others. Given the increasing evidence supporting the clinical significance of central sensitization in those with unexplained chronic pain, the awareness is growing that central sensitization should be a treatment target in these patients.

AREAS COVERED

This article provides an overview of the treatment options available for desensitizing the CNS in patients with chronic pain due to central sensitization. It focuses on those strategies that specifically target pathophysiological mechanisms known to be involved in central sensitization. In addition, pharmacological options, rehabilitation and neurotechnology options are discussed.

EXPERT OPINION

Acetaminophen, serotonin-reuptake inhibitor drugs, selective and balanced serototin and norepinephrine-reuptake inhibitor drugs, the serotonin precursor tryptophan, opioids, N-methyl-d-aspartate (NMDA)-receptor antagonists, calcium-channel alpha(2)delta (a2δ) ligands, transcranial magnetic stimulation, transcutaneous electric nerve stimulation (TENS), manual therapy and stress management each target central pain processing mechanisms in animals that - theoretically - desensitize the CNS in humans. To provide a comprehensive treatment for 'unexplained' chronic pain disorders characterized by central sensitization, it is advocated to combine the best evidence available with treatment modalities known to target central sensitization.

Is it all central sensitization? Role of peripheral tissue nociception in chronic musculoskeletal pain

Fibromyalgia syndrome (FM) is a highly prevalent musculoskeletal disorder that is often accompanied by somatic hyperalgesia (enhanced pain from noxious stimuli). Neural mechanisms of somatic hyperalgesia have been analyzed via quantitative sensory testing of FM patients. Results of these studies suggest that FM pain is associated with widespread primary and secondary cutaneous hyperalgesia, which are dynamically maintained by tonic impulse input from deep tissues and likely by brain-to-spinal cord facilitation. Enhanced somatic pains are accompanied by mechanical hyperalgesia and allodynia in FM patients as compared with healthy controls. FM pain is likely to be at least partially maintained by peripheral impulse input from deep tissues. This conclusion is supported by results of several studies showing that injection of local anesthetics into painful muscles normalizes somatic hyperalgesia in FM patients.

The role of quantitative sensory testing in the evaluation of musculoskeletal pain conditions

Quantitative sensory testing (QST) is a noninvasive method of assessing sensory and pain perception that has been used in the past 30 years primarily for analysis of cutaneous and mucosal perception. In recent years, several published studies have demonstrated that QST may be useful in the analysis of painful musculoskeletal disorders as well. Based on the results of these studies, it can be postulated that QST may be useful in the analysis of the pathogenesis, classification, and differential diagnosis of musculoskeletal disorders. However, due to the diverse ethiopathogenetic basis of these disorders, a broad range of QST test batteries may be necessary to analyze the various musculoskeletal disease entities. This review analyzes published studies on this subject and summarizes current information on altered sensory and pain perception available for some of the most common musculoskeletal disorders. At present, QST remains primarily a research tool but may be useful in differential diagnosis in indicating the presence of central sensitization and for clinical monitoring of disease progression or treatment response.

Guidelines and recommendations for assessment of somatosensory function in oro-facial pain conditions--a taskforce report

The goals of an international taskforce on somatosensory testing established by the Special Interest Group of Oro-facial Pain (SIG-OFP) under the International Association for the Study of Pain (IASP) were to (i) review the literature concerning assessment of somatosensory function in the oro-facial region in terms of techniques and test performance, (ii) provide guidelines for comprehensive and screening examination procedures, and (iii) give recommendations for future development of somatosensory testing specifically in the oro-facial region. Numerous qualitative and quantitative psychophysical techniques have been proposed and used in the description of oro-facial somatosensory function. The selection of technique includes time considerations because the most reliable and accurate methods require multiple repetitions of stimuli. Multiple-stimulus modalities (mechanical, thermal, electrical, chemical) have been applied to study oro-facial somatosensory function. A battery of different test stimuli is needed to obtain comprehensive information about the functional integrity of the various types of afferent nerve fibres. Based on the available literature, the German Neuropathic Pain Network test battery appears suitable for the study of somatosensory function within the oro-facial area as it is based on a wide variety of both qualitative and quantitative assessments of all cutaneous somatosensory modalities. Furthermore, these protocols have been thoroughly described and tested on multiple sites including the facial skin and intra-oral mucosa. Standardisation of both comprehensive and screening examination techniques is likely to improve the diagnostic accuracy and facilitate the understanding of neural mechanisms and somatosensory changes in different oro-facial pain conditions and may help to guide management.

Neurobiology of estrogen status in deep craniofacial pain

Pain in the temporomandibular joint (TMJ) region often occurs with no overt signs of injury or inflammation. Although the etiology of TMJ-related pain may involve multiple factors, one likely risk factor is female gender or estrogen status. Evidence is reviewed from human and animal studies, supporting the proposition that estrogen status acts peripherally or centrally to influence TMJ nociceptive processing. A new model termed the "TMJ pain matrix" is proposed as critical for the initial integration of TMJ-related sensory signals in the lower brainstem that is both modified by estrogen status, and closely linked to endogenous pain and autonomic control pathways.

Effects of the N-methyl-D-aspartate receptor on temporal summation of second pain (wind-up) in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder in which the pathophysiological mechanisms of the pain and hypersensitivity are not well understood. IBS patients frequently complain of pain in body regions somatotopically distinct from the gut, suggesting that central hyperalgesic mechanisms may be involved. In the current study, during the wind-up testing session, a series of 6 heat pulses were presented with an interstimulus interval (ISI) of 3 seconds. Following the 1st, 3rd, and 6th thermal stimuli, subjects were asked to rate the late thermal sensation or second pain. IBS patients who demonstrated temporal summation of pain (TSSP) then received dextromethorphan and placebo in a randomized, double-blind, fashion to block wind-up. The results showed: 1) a subset of IBS patients, but not controls, showed TSSP in response to a series of noxious heat pulses; and 2) TSSP was blocked by administration of dextromethorphan, an NMDA receptor antagonist. In summary, these findings further elucidate mechanisms of somatic hypersensitivity in a subset of IBS patients. Our results also support an etiologic basis for abnormal NMDA receptor mechanisms in some IBS patients. Future studies are needed to determine if NMDA receptor antagonists may be used to treat IBS patients.

PERSPECTIVE

This study evaluates temporal summation of second pain in a subset of IBS patients that is blocked by Dextromethorphan, an NMDA receptor antagonist. Theses results could lead to the use of an NMDA receptor antagonist in the treatment of pain in a subset of IBS patients.

Pain variability in fibromyalgia is related to activity and rest: role of peripheral tissue impulse input

Because fibromyalgia (FM) patients frequently report activity-dependent deep tissue pains, impulse input from painful body regions may be relevant for their musculoskeletal complaints. In addition, peripheral impulse input may induce and maintain thermal and mechanical hyperalgesia of FM patients. If so, activity and rest may alternately enhance and diminish intensity of FM pain. However, the effects of exercise on pain are ambiguous in studies of FM. Whereas exercise-only studies demonstrated increased pain and hyperalgesia during and after physical activity, some exercise studies that included rest periods resulted in decreased FM pain and increased function. To further clarify these effects, we examined the effects of alternating exercise with rest on clinical pain and thermal/mechanical hyperalgesia of 34 FM patients and 36 age-matched healthy controls (NC). Using an ergometer, all subjects performed arm exercise to exhaustion twice alternating with 15-minute rest periods. Although strenuous muscle activity was reported as painful by most FM subjects, overall clinical pain consistently decreased during the rest periods. Additionally, FM subjects' pain sensitivity to mechanical pressure decreased after each exercise and rest session.

CONCLUSION

Alternating strenuous exercise with brief rest periods not only decreased overall clinical pain of FM subjects but also their mechanical hyperalgesia. No prolonged worsening of overall FM pain and hyperalgesia occurred despite vigorous muscle activity. Our findings contribute further evidence that FM pain and hyperalgesia are at least partially maintained by muscle impulse input, and that some types of exercises may be beneficial for FM.

PERSPECTIVE

FM is a pain-amplification syndrome that depends at least in part on peripheral tissue impulse input. Whereas muscle activity increased overall pain, short rest periods produced analgesic effects.

Ethnic differences in cardiovascular-somatosensory interactions and in the central processing of noxious stimuli

Ethnic differences in central sensitization of pain processing and stress-relevant endogenous pain regulatory mechanisms were examined. Forty-four African Americans (AAs; 50% women) and 44 non-Hispanic Whites (nHWs; 50% women) matched for socioeconomic status, were tested for pain responses to the temporal summation of heat pulses and ischemic and cold pain. Resting and stress blood pressure (BP) and norepinephrine (NE) were assessed. AAs had heightened pain responses to all 3 pain tasks relative to nHWs. In nHWs, higher BP and NE were related to reduced pain. In AAs, there was no relationship between BP and pain, but higher NE was related to increased pain. This study provides evidence for ethnic differences in centrally mediated pain and extends prior research demonstrating ethnic differences in endogenous pain regulatory mechanisms. These results have implications for understanding biobehavioral factors contributing to ethnic disparities in clinical pain.

Utility of the NEO-FFI in multi-dimensional assessment of orofacial pain conditions

The purpose of this study was to examine the utility of using the NEO-FFI personality assessment as part of multi-dimensional psychological assessment in orofacial pain patients during the initial diagnostic visit. All patients completed an orofacial pain questionnaire and a battery of psychological questionnaires that cover a wide range of symptoms and behaviors important to developing a comprehensive treatment plan. For purposes of the present study, the NEO-FFI was included. Personality characteristics were within normal ranges compared to NEO-FFI norms. Regression analyses demonstrated higher self-reported pain severity was associated with lower openness. Higher psychological distress was associated with higher neuroticism, lower extraversion, and lower openness. In patients reporting a traumatic life experience, neuroticism and extraversion mediated the relationship between trauma-symptoms and psychological distress. The addition of a global personality measure in standard assessment batteries may help the clinician develop coping strategies and techniques based on individual characteristics of the pain patient. Greater understanding of the individual's personality characteristics will likely influence the nature of the pain management program and predict treatment outcomes.

Suprathreshold heat pain response is associated with clinical pain intensity for patients with shoulder pain

Quantitative sensory testing (QST) has become commonly used for the assessment of pain in subjects with clinical conditions. However, there is no consensus about which type of QST is the best predictor of clinical pain responses. The purposes of this study were to determine: a) the QST measure with the strongest association with clinical pain intensity; and b) if the QST measure continued to predict clinical pain intensity in a model including relevant psychological factors. Fifty-nine patients seeking treatment for shoulder pain underwent experimental pain assessment involving heat and pressure stimuli. The patients also completed validated questionnaires for pain intensity, pain catastrophizing, anxiety, and depression. The 5th pain rating in a series of suprathreshold heat pain stimuli accounted for a significant amount of variance in clinical pain intensity, with no other QST measure contributing to the model. The 5th pain rating remained a significant contributor to clinical pain intensity when psychological factors were included in the model. Furthermore, subjects with elevated 5th pain rating, pain catastrophizing, and depression scores had higher clinical pain intensity ratings in pre- and postoperative assessments. These data suggest that assessment of pain should include suprathreshold heat stimuli and psychological factors separately, and a combination of these factors may be predictive of pain intensity outcomes.

PERSPECTIVE

The current study provides evidence for a suprathreshold heat pain response as a clinically relevant QST measure for patients with shoulder pain, even after psychological factors were considered. The present findings suggest that the 5th pain rating from a series of suprathreshold stimuli, pain catastrophizing, and depression might play a role in predicting pain intensity outcomes.

Sex differences in experimental and clinical pain sensitivity for patients with shoulder pain

Previous research demonstrates that men and women differ in the way that they perceive and process pain. Much of this work has been done in healthy adults with a lack of consensus in clinical pain populations. The purpose of this study was to investigate how men and women with shoulder pain differ in their experience of experimental and clinical pain and whether psychological processes differentially affect these responses. Fifty-nine consecutive subjects (24 women, 35 men) seeking operative treatment for shoulder pain were enrolled in this study. Subjects completed self report questionnaires to assess clinical pain, catastrophizing, anxiety and depression and underwent a series of experimental pain tests consisting of pressure pain, thermal pain (threshold and tolerance), and thermal temporal summation. Results indicated that women experienced greater clinical pain and enhanced sensitivity to pressure pain. Age did not affect the observed sex differences. There were no sex differences in psychological association with experimental and clinical pain in this cohort. The relationship between clinical and experimental pressure pain was stronger in women as compared to men. These findings offer insight into the interactions between biological and psychosocial influences of pain and how these interactions vary by sex.

Effect of catechol-O-methyltransferase polymorphism on response to propranolol therapy in chronic musculoskeletal pain: a randomized, double-blind, placebo-controlled, crossover pilot study

INTRODUCTION

Three common haplotypes in the gene encoding catechol-O-methyltransferase (COMT) have been associated with pain modulation and the risk of developing chronic musculoskeletal pain, namely temporomandibular disorder (TMD). Haplotypes coding for higher enzymatic activity were correlated with lower pain perception. Rodent studies showed that COMT inhibition increases pain sensitivity through beta2/3-adrenergic receptors. We hypothesized that the nonselective beta-adrenergic antagonist propranolol will reduce clinical and experimental pain in TMD patients in a manner dependent on the individuals' COMT diplotype.

METHODS

Forty Caucasian female participants meeting the Research Diagnostic Criteria for TMD were genotyped for COMT polymorphisms and completed a randomized, double-blind, placebo-controlled, two-period crossover pilot study. Each period consisted of a baseline assessment week followed by an intervention week (propranolol or placebo). Changes in clinical pain ratings, psychological status, and responses to heat and pressure stimuli between baseline and intervention weeks were compared across periods.

RESULTS

The number of patients reporting a reduction in pain intensity rating was greater during propranolol treatment (P=0.014) compared with placebo. Propranolol significantly reduced a composite pain index (P=0.02) but did not decrease other clinical and experimental pain ratings. When stratified by the COMT high activity haplotype, a beneficial effect of propranolol on pain perception was noted in patients not carrying this haplotype, a diminished benefit was observed in the heterozygotes, and no benefit was noted in the homozygotes.

CONCLUSION

COMT haplotypes may serve as genetic predictors of propranolol treatment outcome, identifying a subgroup of TMD patients who will benefit from propranolol therapy.

Referred pain from muscle trigger points in the masticatory and neck-shoulder musculature in women with temporomandibular disoders

Our aim was to describe the referred pain patterns and size of areas of trigger points (TrPs) in the masticatory and neck-shoulder muscles of women with myofascial temporomandibular disorders (TMD). Twenty-five women with myofascial TMD and 25 healthy matched women participated. Bilateral temporalis, deep masseter, superficial masseter, sternocleidomastoid, upper trapezius and suboccipital muscles were examined for TrPs by an assessor blinded to the subjects' condition. TrPs were identified with manual palpation and categorized into active and latent according to proposed criteria. The referred pain areas were drawn on anatomical maps, digitalized, and measured. The occurrence of active (P < .001) and latent TrPs (P = .04) were different between groups. In all muscles, there were significantly more active and latent TrP in patients than controls (P < .001). Significant differences in referred pain areas between groups (P < .001) and muscles (P < .001) were found: the referred pain areas were larger in patients (P < .001), and the referred pain area elicited by suboccipital TrPs was greater than the referred pain from other TrPs (P < .001). Referred pain areas from neck TrPs were greater than the pain areas from masticatory muscle TrPs (P < .01). Referred pain areas of masticatory TrPs were not different (P > .703). The local and referred pain elicited from active TrPs in the masticatory and neck-shoulder muscles shared similar pain pattern as spontaneous TMD, which supports the concept of peripheral and central sensitization mechanisms in myofascial TMD.

PERSPECTIVE

The current study showed the existence of multiple active muscle TrPs in the masticatory and neck-shoulder muscles in women with myofascial TMD pain. The local and referred pain elicited from active TrPs reproduced pain complaints in these patients. Further, referred pain areas were larger in TMD pain patients than in healthy controls. The results are also in accordance with the notion of peripheral and central sensitization mechanisms in patients with myofascial TMD.

The effect of fibromyalgia and widespread pain on the clinically significant temporomandibular muscle and joint pain disorders--a prospective 18-month cohort study

Although most cases of temporomandibular muscle and joint disorders (TMJD) are mild and self-limiting, about 10% of TMJD patients develop severe disorders associated with chronic pain and disability. It has been suggested that fibromyalgia and widespread pain play a significant role in TMJD chronicity. This paper assessed the effects of fibromyalgia and widespread pain on clinically significant TMJD pain (GCPS II-IV). Four hundred eighty-five participants recruited from the Minneapolis/St. Paul area through media advertisements and local dentists received examinations and completed the Graded Chronic Pain Scale (GCPS) at baseline and at 18 months. Baseline widespread pain (OR: 2.53, P = .04) and depression (OR: 5.30, P = .005) were associated with onset of clinically significant pain (GCPS II-IV) within 18 months after baseline. The risk associated with baseline fibromyalgia was moderate, but not significant (OR: 2.74, P = .09). Persistence of clinically significant pain was related to fibromyalgia (OR: 2.48, P = .02) and depression (OR: 2.48, P = .02). These results indicate that these centrally generated pain conditions play a role in the onset and persistence of clinically significant TMJD.

PERSPECTIVE

Fibromyalgia and widespread pain should receive important consideration when evaluating and developing a treatment plan for patients with TMJD.

Supra-threshold scaling, temporal summation, and after-sensation: relationships to each other and anxiety/fear

This study investigated the relationship of thermal pain testing from three types of quantitative sensory testing (ie, supra-threshold stimulus response scaling, temporal summation, and after-sensation) at three anatomical sites (ie, upper extremity, lower extremity, and trunk). Pain ratings from these procedures were also compared with common psychological measures previously shown to be related to experimental pain responses and consistent with fear-avoidance models of pain. Results indicated that supra-threshold stimulus response scaling, temporal summation, and after-sensation, were significantly related to each other. The site of stimulation was also an important factor, with the trunk site showing the highest sensitivity in all three quantitative sensory testing procedures. Supra-threshold response measures were highly related to measures of fear of pain and anxiety sensitivity for all stimulation sites. For temporal summation and after-sensation, only the trunk site was significantly related to anxiety sensitivity, and fear of pain, respectively. Results suggest the importance of considering site of stimulation when designing and comparing studies. Furthermore, psychological influence on quantitative sensory testing is also of importance when designing and comparing studies. Although there was some variation by site of stimulation, fear of pain and anxiety sensitivity had consistent influences on pain ratings.

Central processing of noxious somatic stimuli in patients with irritable bowel syndrome compared with healthy controls

OBJECTIVE

To compare a central analgesic mechanism known as diffuse noxious inhibitory controls (DNIC) using somatic test stimuli and somatic conditioning stimuli, (CS) in irritable bowel syndrome (IBS) patients and healthy controls.

METHODS

Participants were 48 premenopausal females (27 with IBS), mean age of 29 years. The phasic heat test stimulus (peak temperature, 50 degrees C) was applied to the left palm. The DNIC effect, which measured reductions in average pain ratings (APR) during counter irritation (submersion of the participant's right hand in painful 12 degrees C circulating water) compared with baseline, was compared between groups. In addition, a second, counterbalanced, CS protocol (right hand submerged in nonpainful 32 degrees C circulating water) was performed. Differences in APR between the 2 counterirritation protocols were compared between groups to control for nonspecific effects known to influence DNIC. Psychologic measures and cardiovascular reactivity were also assessed.

RESULTS

IBS patients demonstrated smaller DNIC than controls (P=0.011, repeated measures analysis of variance), and greater state-anxiety, depression, catastrophizing, and anger-out expression (P<0.05). Group differences in DNIC were enhanced after controlling for nonspecific effects occurring during the nonpainful CS, and for psychologic measures (P=0.001, repeated measures analysis of covariance). There were no group differences in age, cardiovascular reactivity, APR, or pain ratings for the 12 degrees C CS.

DISCUSSION

These data demonstrate deficient DNIC in IBS. This is the first study to adequately control for alternative explanations of pain reduction during counterirritation. Only by controlling for nonspecific effects can evidence of deficient DNIC be attributed to dysregulation in endogenous analgesic mechanisms.

Temporomandibular disorders and migraine chronification

Among the established and potential comorbidities of migraine, the temporomandibular disorders (TMD) are rarely discussed, although they are of importance for several reasons. TMD may cause headaches per se, worsen existent primary headaches, and add to the burden of headache disorders. This article explores the potential comorbidity between migraine and TMD and the role of TMD as a potential factor to induce chronic migraine. We discuss the similarities between both conditions, review evidence to support the idea that both disorders are comorbid, and highlight the limited evidence suggesting that TMD influence migraine progression. Finally, we discuss the importance of cutaneous allodynia mediating the TMD/frequent headache relationship.

Pathophysiology of TMD pain--basic mechanisms and their implications for pharmacotherapy

This article discusses the pathophysiology of temporomandibular disorders (TMD)-related pain and its treatment with analgesic drugs. Temporomandibular disorders are comprised of a group of conditions that result in temporomandibular joint pain (arthralgia, arthritis) and/or masticatory muscle pain (myofascial TMD). In at least some patients with TMD, a peripheral mechanism contributes to this pain. However, there is often a poor correlation between the severity of TMD-related pain complaints and evidence of definitive tissue pathology. This has led to the concept that pain in some patients with TMD may result from altered central nervous system pain processing and further that this altered pain processing may be attributable to specific genes that are heritable. Psychosocial stressors are also thought to contribute to the development of TMD-related pain, particularly masticatory muscle pain. Finally, substantially more women suffer from TMD than men. Although there are arguably multiple reasons for sex-related differences in the prevalence of TMD, one candidate for the increased occurrence of this disorder in women has been suggested to be the female sex hormone oestrogen. Analgesic drugs are an integral part of the primary treatment for TMD-related pain and dysfunction with more that 90% of treatment recommendations involving use of medications. The most commonly used agents include non-steroidal anti-inflammatory drugs, corticosteroids, muscle relaxants, anxiolytics, opiates and tricyclic antidepressants, however, evidence in support of the effectiveness of these drugs is lacking. Continued research into the pathophysiology of TMD-related pain and the effectiveness of analgesic treatments for this pain is required.

Mechanisms by which sleep disturbance contributes to osteoarthritis pain: a conceptual model

Sleep disturbance is prevalent in aging and painful rheumatologic populations, but it has largely been a neglected dimension of the routine clinical care of arthritis patients. Pain associated with osteoarthritis (OA) is a leading cause of disability worldwide, and factors that contribute to pain in OA are poorly understood. Sleep disturbance is not only a consequence of pain, it is also likely to play an integral role in pain expression. Emerging research suggests that many patients with OA demonstrate signs of generalized hyperalgesia and faulty central pain modulatory processing similar to other idiopathic pain disorders, such as fibromyalgia. Sleep disruption is increasingly recognized as a direct contributor to both hyperalgesia and impaired endogenous pain modulation. This article reviews the extant literature on sleep disturbance and hyperalgesia in patients with OA. We propose a conceptual working model describing pathways by which sleep disturbance interacts directly with central pain processing mechanisms and inflammatory processes, and indirectly with mood and physical functioning to augment clinical OA pain. The clinical and research implications of the model are discussed.

Temporomandibular disorders and cutaneous allodynia are associated in individuals with migraine

The objective of this study was to estimate and contrast the occurrence of ictal and interictal cutaneous allodynia (CA) in individuals with migraine with and without temporomandibular disorders (TMD). Both TMD and CA are common in migraine and may be associated with migraine transformation from episodic into a chronic form. Herein we hypothesize that TMD contributes to the development of CA and to more severe headaches. In a clinic-based sample of individuals with episodic migraine, the presence of TMD was assessed using the research diagnostic criteria for myofascial or mixed (myofascial and arthralgic) TMD. Ictal CA was quantified using the validated Allodynia Symptom Checklist (ASC-12). The ASC-12 measures CA over the preceding month by asking 12 questions about the frequency of allodynia symptoms during headaches. Interictal CA was assessed in the domains of heat, cold and mechanical static allodynia using quantitative sensory testing. Our sample consists of 55 individuals; 40 (73%) had TMD (23 with myofascial TMD and 17 with the mixed type). CA of any severity (as assessed by ASC-12) occurred in 40% of those without TMD (reference group), 86.9% of those with myofascial TMD (P = 0.041, RR = 3.2, 95% CI = 1.5-7.0) and in 82.3% of those with mixed TMD (P = 0.02, RR = 2.5, 95% CI = 1.2-5.3). Individuals with TMD were more likely to have moderate or severe CA associated with their headaches. Interictally (quantitative sensory testing), thresholds for heat and mechanical nociception were significantly lower in individuals with TMD. Cold nociceptive thresholds were not significantly different in migraine patients with and without TMD. TMDs were also associated with change in extra-cephalic pain thresholds. In logistical regression, TMD remained associated with CA after adjusting for aura, gender and age. TMD and CA are associated in individuals with migraine.

Bilateral thermal hyperalgesia in trigeminal and extra-trigeminal regions in patients with myofascial temporomandibular disorders

Our aim was to assess thermal sensitivity in both trigeminal and extra-trigeminal regions in patients with myofascial temporomandibular disorder (TMD) but without comorbid conditions as compared to age-matched controls. Twenty women (age 24 +/- 3 years) diagnosed with myofascial TMD according to the research diagnostic criteria for TMD and 20 healthy women (age 24 +/- 4 years) were included. Warm and cold detection thresholds (WDT and CDT, respectively) and heat and cold pain thresholds (HPT and CPT, respectively) were bilaterally assessed over the masseter and frontalis muscles (trigeminal regions) and the wrist (extra-trigeminal region). The mean of three determinations at each site was calculated and used for analysis. The order of the test sites was randomized. A two-way ANOVA was used to test for differences between groups and sides (most painful/contra-lateral side; dominant/non-dominant). No significant differences between groups for WDT or CDT in trigeminal and extra-trigeminal regions (ANOVA, P > 0.389) were found. There were significant differences between groups, but not between sides, for HPT and CPT in both trigeminal and extra-trigeminal areas (ANOVA, P < 0.001). CPT (P < 0.001) over the trigeminal area was positively correlated with both pain intensity and duration of pain symptoms: the longer the history of pain or the greater the pain intensity, the higher the CPT (i.e., the greater cold hyperalgesia) over the trigeminal region. Our findings revealed bilateral thermal hyperalgesia (lower HPT and higher CPT) but normal WDT and CDT in trigeminal and extra-trigeminal regions in women with myofascial TMD as compared to healthy controls. Bilateral heat/cold hyperalgesia in trigeminal and extra-trigeminal areas may reflect a dysfunction of thermal channels in myofascial TMD patients as result of some combination of peripheral sensitization, facilitation of central nociceptive processing and/or decreased descending inhibition.

Central sensitivity syndromes: mounting pathophysiologic evidence to link fibromyalgia with other common chronic pain disorders

The aim of this study was to review emerging data from the fields of nursing, rheumatology, dentistry, gastroenterology, gynecology, neurology, and orthopedics that support or dispute pathophysiologic similarities in pain syndromes studied by each specialty. A literature search was performed through PubMed and Ovid using the terms fibromyalgia, temporomandibular joint disorder, irritable bowel syndrome, irritable bladder/interstitial cystitis, headache, chronic low back pain, chronic neck pain, functional syndromes, and somatization. Each term was linked with pathophysiology and/or central sensitization. This paper presents a review of relevant articles with a specific goal of identifying pathophysiologic findings related to nociceptive processing. The extant literature presents considerable overlap in the pathophysiology of these diagnoses. Given the psychosomatic lens through which many of these disorders are viewed, demonstration of evidence-based links supporting shared pathophysiology between these disorders could provide direction to clinicians and researchers working to treat these diagnoses. "Central sensitivity syndromes" denotes an emerging nomenclature that could be embraced by researchers investigating each of these disorders. Moreover, a shared paradigm would be useful in promoting cross-fertilization between researchers. Scientists and clinicians could most effectively forward the understanding and treatment of fibromyalgia and other common chronic pain disorders through an appreciation of their shared pathophysiology.

Pain catastrophizing and salivary cortisol responses to laboratory pain testing in temporomandibular disorder and healthy participants

Pain catastrophizing is an important variable in the context of acute and chronic pain. The neurophysiological correlates of pain catastrophizing, however, have not been rigorously evaluated. We examined the relationship between trait-pain catastrophizing and morning salivary cortisol levels before and following a 45-minute laboratory pain-testing session in healthy, pain-free (n = 22), and temporomandibular disorder (TMD) participants (n = 39). We also examined whether TMD patients evidenced generalized hyperalgesia and hypercortisolism. Pain catastrophizing was associated with a flattened morning salivary cortisol profile in the context of pain testing, irrespective of pain status. Cortisol profiles did not differ between healthy and TMD participants. TMD was associated with mechanical hyperalgesia only at the masseter. These data are the first to show an association between pain catastrophizing and elevated salivary cortisol profiles in the context of standardized experimental pain testing. These findings in both healthy individuals and those with chronic orofacial pain suggest that aberrant adrenocortical responses to pain may serve as a neurophysiologic pathway by which pain catastrophizing enhances vulnerability for development of chronic pain and maintains and/or exaggerates existing pain and associated morbidity.

PERSPECTIVE

Neurophysiological mechanisms by which pain catastrophizing is related to acute and chronic pain recently have come under empirical study. Understanding of these mechanisms has the unique potential to shed light on key central-nervous-system factors that mediate catastrophizing-pain relations and therapeutic benefits associated with changes in catastrophizing and related cognitive processes.

NMDA receptor blockade reduces temporomandibular joint-evoked activity of trigeminal subnucleus caudalis neurons in an estrogen-dependent manner

Estrogen status is a risk factor in painful temporomandibular disorders (TMJD). Previously we reported that estradiol (E2) enhanced nociceptive processing of TMJ input by neurons in superficial laminae at the spinomedullary (Vc/C(1-2)) region; however, the mechanisms for this enhancement are not known. The present study determined if ionotropic glutamate receptors contribute to TMJ nociceptive processing in an E2-dependent manner. Ovariectomized (OvX) female rats were treated with high E2 (HE2) or low dose E2 (LE2) for 2 days and neural activity was recorded in laminae I-II at the Vc/C(1-2) region. TMJ-responsive units were activated by ATP injections into the joint space. ATP-evoked unit responses in HE2 rats were reduced significantly by topical application of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid (AP5) in a dose-related manner, while units from LE2 were not affected. Application of the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), inhibited the ATP-evoked responses in both groups. Spontaneous activity of TMJ units was not influenced by AP5, whereas it was reduced by DNQX similarly in both groups. The high threshold convergent cutaneous receptive field area of TMJ units was not changed by AP5, whereas DNQX caused a significant reduction in both groups. These results suggest that NMDA-dependent mechanisms contribute to the enhanced ATP-evoked responses of TMJ units in superficial laminae at the Vc/C(1-2) region under high E2 conditions, while non-NMDA-dependent mechanisms modify the encoding properties of TMJ units independent of E2 status.

Self-reported severity of taste disturbances correlates with dysfunctional grade of TMD pain

Altered central neural processing of sensory information may be associated with temporomandibular disorders (TMD) pain. The objectives of this study were to compare the prevalence of self-reported taste disturbances in TMD pain patients and in a control population, and to determine whether frequency of taste disturbances was correlated with dysfunctional grade of TMD pain. Subjects were 2026 people within a German population sample and 301 consecutive TMD patients diagnosed using the Research Diagnostic Criteria. Taste disturbances were measured using two questions from the Oral Health Impact Profile. Dysfunctional grade of TMD pain was measured with the Graded Chronic Pain Scale. A two-sample test of proportions revealed that TMD patients reported a greater frequency of taste disturbances, 6%, than did the general population subjects, 2% (P < 0.001). Moreover, the frequency of taste disturbances correlated with the dysfunctional grade of TMD pain. For each 1 unit increase in taste disturbance, the odds of observing a higher grade of TMD pain increased by 29% (95% CI: 3-63%, P = 0.03). Analysis by individual taste question and adjustment for age and gender did not substantially affect the results. These findings are consistent with a central neural dysfunction in TMD pain and suggest that a common neural substrate may underlie sensory disturbances of multiple modalities in chronic pain patients. Further research regarding taste disturbances and trigeminally mediated pains such as in TMD is warranted.

Pain sensitivity can be assessed by self-rating: Development and validation of the Pain Sensitivity Questionnaire

Experimental determination of pain sensitivity has received increasing attention because of emerging clinical applications (including prediction of postoperative pain and treatment response) and scientific implications (e.g. it has been proposed that above-average pain sensitivity is a risk factor for the development of chronic pain disorders). However, the use of experimental pain sensitivity assessment on a broad scale is hampered by its requirements on time, equipment and human resources and the fact that it is painful for the tested subject. Alternatives to experimental pain testing are currently lacking. Here we developed a self-rating instrument for the assessment of pain sensitivity, the Pain Sensitivity Questionnaire (PSQ) that is based on pain intensity ratings of daily life situations and takes 5-10min to complete. Adequate reliability of the PSQ was confirmed in 354 subjects. In a validation study comprising 47 healthy subjects, the results of comprehensive experimental pain testing, including different modalities (heat, cold, pressure, and pinprick) and different measures (pain thresholds, pain intensity ratings), were compared to the results of the PSQ. PSQ scores were significantly correlated to experimental pain intensity ratings (r = 0.56, p < 0.001) but not to pain thresholds (r = 0.03). Prediction of experimental pain intensity ratings by the PSQ was better than by pain-associated psychological factors (pain catastrophizing, depression, anxiety). This shows that the PSQ may be a simple alternative to experimental pain intensity rating procedures in healthy subjects and makes the PSQ a highly promising tool for clinical and experimental pain research.

Electric toothbrush application is a reliable and valid test for differentiating temporomandibular disorders pain patients from controls

BACKGROUND

Current methods for identifying patients with pain hypersensitivity are sufficiently complex to limit their widespread application in clinical settings. We assessed the reliability and validity of a simple multi-modal vibrotactile stimulus, applied using an electric toothbrush, to evaluate its potential as a screening tool for central sensitization.

METHODS

Fourteen female temporomandibular disorders (TMD) subjects with myofascial pain (RDC/TMD Ia or Ib) and arthralgia (RDC/TMD IIIa) were compared to 13 pain-free controls of matched age and gender. Vibrotactile stimulus was performed with an electric toothbrush, applied with 1 pound pressure for 30 seconds in four locations: over the lateral pole of the temporomandibular joint, masseter, temporalis, and mid-ventral surface of forearm. Pain intensity (0-10) was recorded following the stimulus at 0, 15, 30, and 60 seconds. Test-retest reliability was assessed with measurements from 8 participants, taken 2-12 hours apart. Case versus control differentiation involved comparison of area under the curve (AUC). A receiver operating characteristic (ROC) curve was used to determine cutoff AUC scores for maximum sensitivity and specificity for this multi-modal vibrotactile stimulus.

RESULTS

Test-retest reliability resulted in an ICC of 0.87 for all 4 pooled sites. ROC-determined AUC cutoff scores resulted in a sensitivity of 57% and specificity of 92% for all 4 pooled sites.

CONCLUSION

The electric toothbrush stimulus had excellent test-retest reliability. Validity of the scores was demonstrated with modest sensitivity and good specificity for differentiating TMD pain patients from controls, which are acceptable properties for a screening test.

Fatiguing exercise enhances hyperalgesia to muscle inflammation

Since many people with chronic fatigue present with pain and many people with chronic pain present with fatigue, we tested if fatigue would enhance the response to pain in male and female mice. We further tested for the activation of brainstem nuclei by the fatigue task using c-fos as a marker. Fatigue was induced by having mice spontaneously run in running wheel for 2h. Carrageenan (0.03%) was injected into the gastrocnemius muscle either 2h before or 2h after the fatigue task. The mechanical sensitivity of the paw (von Frey filaments), muscle (tweezers), grip force and running wheel activity was assessed before and 24h after injection of carrageenan. Both male and female mice that performed the fatigue task, either before or after intramuscular injection of carrageenan, showed an enhanced mechanical sensitivity of the paw, but not the muscle. Ovariectomized mice showed a similar response to male mice. There was a decrease in running wheel activity after carrageenan injection, but no change in grip force suggesting that mice had no deficit in motor performance induced by the carrageenan. C-fos expression was observed in the nucleus raphe pallidus, obscurus, and magnus after the fatigue task suggesting an increased activity in the raphe nuclei in response to the fatigue task. Therefore, widespread hyperalgesia is enhanced by the fatigue response but not hyperalgesia at the site of insult. We suggest that this effect is sex-dependent and involves mechanisms in the brainstem to result in an enhanced hyperalgesia.

Can we predict persistent postoperative pain by testing preoperative experimental pain?

PURPOSE OF REVIEW

Although it is recognized that medical and surgical procedures may lead to persistent postoperative pain, predicting which patients are at risk for developing chronic pain presents an ongoing challenge. Clinical observations indicate that similar invasive procedures associated with consequent peripheral tissue damage can cause a wide range of pain experience. This broad variability is likely a consequence of the diversity in the central pain processing of the peripherally generated noxious stimulation. Therefore, advanced psychophysical measures that dynamically represent central pain modulation mechanisms may be used to determine an individual's susceptibility to developing persistent postoperative pain. This review highlights how, and to what extent, preoperative experimental pain testing can be utilized in predicting persistent postoperative pain.

RECENT FINDINGS

Conflicting findings emerged regarding the role of traditional experimental pain tests, including pain threshold, supra-threshold magnitude estimation, and tolerance in the prediction of acute postoperative pain. Less efficient endogenous modulation and greater sensitization, as preoperatively assessed by advanced experimental pain tests, were associated with higher persistent postoperative pain scores.

SUMMARY

The preoperative identification of individuals who have enhanced pain sensitivity and are at risk for developing persistent postoperative pain is important to providing them with better treatment that is specifically tailored to their altered pain modulation, as represented psychophysically.

Sleep disorders and their association with laboratory pain sensitivity in temporomandibular joint disorder

STUDY OBJECTIVES

We characterized sleep disorder rates in temporomandibular joint disorder (TMD) and evaluated possible associations between sleep disorders and laboratory measures of pain sensitivity.

DESIGN

Research diagnostic examinations were conducted, followed by two consecutive overnight polysomnographic studies with morning and evening assessments of pain threshold.

SETTING

Orofacial pain clinic and inpatient sleep research facility.

PARTICIPANTS

Fifty-three patients meeting research diagnostic criteria for myofascial TMD.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

We determined sleep disorder diagnostic rates and conducted algometric measures of pressure pain threshold on the masseter and forearm. Heat pain threshold was measured on the forearm; 75% met self-report criteria for sleep bruxism, but only 17% met PSG criteria for active sleep bruxism. Two or more sleep disorders were diagnosed in 43% of patients. Insomnia disorder (36%) and sleep apnea (28.4%) demonstrated the highest frequencies. Primary insomnia (PI) (26%) comprised the largest subcategory of insomnia. Even after controlling for multiple potential confounds, PI was associated with reduced mechanical and thermal pain thresholds at all sites (P < 0.05). Conversely, the respiratory disturbance index was associated with increased mechanical pain thresholds on the forearm (P < 0.05).

CONCLUSIONS

High rates of PI and sleep apnea highlight the need to refer TMD patients complaining of sleep disturbance for polysomnographic evaluation. The association of PI and hyperalgesia at a nonorofacial site suggests that PI may be linked with central sensitivity and could play an etiologic role in idiopathic pain disorders. The association between sleep disordered breathing and hypoalgesia requires further study and may provide novel insight into the complex interactions between sleep and pain-regulatory processes.

Effects of upper extremity neural mobilization on thermal pain sensitivity: a sham-controlled study in asymptomatic participants

STUDY DESIGN

A single-blinded, quasi-experimental, within- and between-sessions assessment.

OBJECTIVES

To investigate potential mechanisms of neural mobilization (NM), using tensioning techniques in comparison to sham NM on a group of asymptomatic volunteers between the ages of 18 and 50.

BACKGROUND

NM utilizing tensioning techniques is used by physical therapists in the treatment of patients with cervical and/or upper extremity symptoms. The underlying mechanisms of potential benefits associated with NM tensioning techniques are unknown.

METHODS AND MEASURES

Participants (n = 62) received either a NM or sham NM intervention 2 to 3 times a week for a total of 9 sessions, followed by a 1-week period of no intervention to assess carryover effects. A-delta (first pain response) and C-fiber (temporal summation) mediated pain perceptions were tested via thermal quantitative sensory testing procedures. Elbow extension range of motion (ROM) and sensory descriptor ratings were obtained during a neurodynamic test for the median nerve. Data were analyzed with repeated-measures analysis of variance (ANOVA).

RESULTS

No group differences were seen for A-delta mediated pain perception at either immediate or carryover times. Group differences were identified for immediate C-fiber mediated pain perception (P = .032), in which hypoalgesia occurred for the NM group but not the sham NM group. This hypoalgesic effect was not maintained at carryover (P = .104). Group differences were also identified for the 3-week and carryover periods for elbow extension ROM (P = .004), and for the participant sensory descriptor ratings (P = .018), in which increased ROM and decreased sensory descriptor ratings were identified in participants in the NM group but not the sham NM group.

CONCLUSION

This study provides preliminary evidence that mechanistic effects of tensioning NM differ from sham NM for asymptomatic participants. Specifically, NM resulted in immediate, but not sustained, C-fiber mediated hypoalgesia. Also, NM was associated with increased elbow ROM and a reduction in sensory descriptor ratings at 3-week and carryover assessment times. These differences provide potentially important information on the mechanistic effects of NM, as well as the description of a sham NM for use in future clinical trials.

Adrenergic dysregulation and pain with and without acute beta-blockade in women with fibromyalgia and temporomandibular disorder

In patients with fibromyalgia syndrome (FMS) and temporomandibular disorder (TMD), stress and pain may chronically enhance sympathetic activity, altering cardiovascular responses and worsening pain. This study examined cardiovascular, epinephrine (EPI), norepinephrine (NE), cortisol and clinical pain responses in 54 female patients with these disorders and 34 controls. In a subsample of 10 FMS, 10 TMD patients and 16 controls, using a counterbalanced, double-blind, crossover design, the same responses were assessed after intravenous administration of low dose propranolol vs placebo. Testing included baseline, postural, speech and ischemic pain stressors. FMS patients showed lesser heart rate (HR) increases to posture challenge but greater blood pressure (BP) increases to postural and speech tasks than controls, as well as higher overall BP and greater total vascular resistance (TVR) than TMDs or controls. TMDs showed higher overall cardiac output and lower TVR than controls. Both FMS and TMD groups showed lower baseline NE than controls, and TMDs showed lower overall EPI and NE levels. Group differences in HR, EPI and NE were abolished after propranolol although BP, CO and TVR differences persisted. In both FMS and TMD, the number of painful body sites and ratings of total clinical pain obtained 4 times during each session were significantly lower after beta-blockade vs placebo.

PERSPECTIVE

These findings support the hypothesis that both FMS and TMD may frequently involve dysregulation of beta-adrenergic activity that contributes to altered cardiovascular and catecholamine responses and to severity of clinical pain. Acute treatment with low-dose propranolol led to short-term improvement in all these domains.

Emerging peripheral receptor targets for deep-tissue craniofacial pain therapies

While effective therapies are available for some types of craniofacial pain, treatments for deep-tissue craniofacial pain such as temporomandibular disorders are less efficacious. Several ion channels and receptors which are prominent in craniofacial nociceptive mechanisms have been identified on trigeminal primary afferent neurons. Many of these receptors and channels exhibit unusual distributions compared with extracranial regions. For example, expression of the ATP receptor P2X(3) is strongly implicated in nociception and is more abundant on trigeminal primary afferent neurons than analogous extracranial neurons, making them potentially productive targets specifically for craniofacial pain therapies. The initial part of this review therefore focuses on P2X(3) as a potential therapeutic target to treat deep-tissue craniofacial pain. In the trigeminal ganglion, P2X(3) receptors are often co-expressed with the nociceptive neuropeptides CGRP and SP. Therefore, we discuss the role of CGRP and SP in deep-tissue craniofacial pain and suggest that neuropeptide antagonists, which have shown promise for the treatment of migraine, may have wider therapeutic potential, including the treatment of deep-tissue craniofacial pain. P2X(3), TRPV1, and ASIC3 are often co-expressed in trigeminal neurons, implying the formation of functional complexes that allow craniofacial nociceptive neurons to respond synergistically to altered ATP and pH in pain. Future therapeutics for craniofacial pain thus might be more efficacious if targeted at combinations of P2X(3), CGRP, TRPV1, and ASIC3.

Deficiency in endogenous modulation of prolonged heat pain in patients with Irritable Bowel Syndrome and Temporomandibular Disorder

Females with Irritable Bowel Syndrome (IBS) and Temporomandibular Disorder (TMD) are characterized by enhanced sensitivity to experimental pain. One possible explanation for this observation is deficiencies in pain modulation systems such as Diffuse Noxious Inhibitory Control (DNIC). In a few studies that used brief stimuli, chronic pain patients demonstrate reduced DNIC. The purpose of this study was to compare sensitivity to prolonged heat pain and the efficacy of DNIC in controls to IBS and TMD patients. Heat pain (experimental stimulus; 44.0-49.0 degrees C), which was applied to left palm, was continuously rated during three 30-s trials across three separate testing sessions under the following conditions: without a conditioning stimulus; during concurrent immersion of the right foot in a 23.0 degrees C (control); and during noxious cold immersion in a (DNIC; 8.0-16.0 degrees C) water bath. Compared to controls, IBS and TMD patients reported an increased sensitivity to heat pain and failed to demonstrate pain inhibition due to DNIC. Controls showed a significant reduction in pain during the DNIC session. These findings support the idea that chronic pain patients are not only more pain sensitive but also demonstrate reduced pain inhibition by pain, possibly because of dysfunction of endogenous pain inhibition systems.

Morphine modulation of temporomandibular joint-responsive units in superficial laminae at the spinomedullary junction in female rats depends on estrogen status

The influence of analgesic agents on neurons activated by stimulation of the temporomandibular joint (TMJ) region is not well defined. The spinomedullary junction [trigeminal subnucleus caudalis (Vc)/C(1-2)] is a major site of termination for TMJ sensory afferents. To determine whether estrogen status influences opioid-induced modulation of TMJ units, the classical opioid analgesic, morphine, was given to ovariectomized (OvX) rats and OvX rats treated for 2 days with low-dose (LE2) or high-dose (HE2) 17beta-estradiol-3-benzoate. Under thiopental anesthesia, TMJ units in superficial and deep laminae at the Vc/C(1-2) junction were activated by injection of ATP (1 mm) directly into the joint space. In superficial laminae, morphine inhibited evoked activity in units from OvX and LE2 rats in a dose-related and naloxone-reversible manner, whereas units from HE2 rats were not inhibited. By contrast, in deep laminae, morphine reduced TMJ-evoked unit activity similarly in all groups. Morphine reduced the background activity of units in superficial and deep laminae and resting arterial pressure similarly in all groups. Morphine applied to the dorsal surface of the Vc/C(1-2) junction inhibited all units independently of E2 treatment. Quantitative polymerase chain reaction and immunoblots revealed a similar level of expression for mu-opioid receptors at the Vc/C(1-2) junction in LE2 and HE2 rats. These results indicated that estrogen status differentially affected morphine modulation of TMJ unit activity in superficial, but not deep, laminae at the Vc/C(1-2) junction in female rats. The site(s) for estrogen influence on morphine-induced modulation of TMJ unit activity was probably outside the medullary dorsal horn.

Perceived intensity and unpleasantness of cutaneous and auditory stimuli: an evaluation of the generalized hypervigilance hypothesis

According to the Generalized Hypervigilance Hypothesis (GHH) of McDermid et al., the unpleasantness of sensory stimuli, rather than their modality, determines whether they will be perceptually amplified in hypervigilant persons. In a test of this idea, ratings of the intensity of sensations evoked by cutaneous and auditory stimuli were obtained from individuals with chronic myofascial pain (fibromyalgia, temporomandibular disorders), and from (less hypervigilant) healthy control participants. In each modality, the stimuli spanned a wide intensity range, with the weakest stimuli being affectively neutral and the strongest being distinctly unpleasant, as determined by unpleasantness ratings. The pain patients showed robust perceptual amplification of the cutaneous pressure stimuli, and modest amplification of the auditory stimuli. In both cases, perceptual amplification extended to even the lowest stimulus intensities, a result that is not consistent with the predictions of the GHH. An alternative formulation, the attentional gain control model of hypervigilance, is proposed, according to which those types of stimuli that are associated with pain are amplified because of the attention that is habitually directed toward them.

Estradiol replacement modifies c-fos expression at the spinomedullary junction evoked by temporomandibular joint stimulation in ovariectomized female rats

The influence of estradiol (E2) treatment on temporomandibular joint (TMJ) nociceptive processing in the caudal trigeminal sensory brain stem complex was assessed in ovariectomized female rats by quantitative Fos-immunoreactivity (Fos-LI). After 2 days of daily injections of high (HE2) or low (LE2) dose E2 rats were anesthetized and the small fiber excitant, mustard oil (MO, 0-20%), was injected into the TMJ and after 2 h brains were processed for Fos-LI. TMJ-evoked Fos-LI in laminae I-II at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) junction and the dorsal paratrigeminal region (dPa5) was significantly greater in HE2 than LE2 rats, while Fos-LI produced at the ventral trigeminal interpolaris/caudalis transition region (Vi/Vc(vl)) was similar. E2 treatment also modified the influence of N-methyl-D-aspartate (NMDA) and AMPA receptor antagonists on TMJ-evoked Fos-LI. The NMDA antagonist, MK-801, dose-dependently reduced the Fos-LI response at the Vc/C1-2 junction in HE2 rats, while only high dose MK-801 was effective in LE2 rats. MK801 reduced equally the Fos-LI response at the Vi/Vc transition in both groups, while only minor effects were seen at the dPa5 region. The AMPA receptor antagonist, NBQX, reduced Fos-LI at the Vc/C(1-2) and Vi/Vc(vl) regions in HE2 rats, while only high dose NBQX was effective in LE2 rats. NBQX did not reduce Fos-LI at the dPa5 region in either group. These results suggest that estrogen status plays a significant role in TMJ nociceptive processing at the Vc/C1-2 junction mediated, in part, through ionotropic glutamate receptor-dependent mechanisms.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.