Spinal CCK contributes to somatic hyperalgesia induced by orofacial inflammation combined with stress in adult female rats.

In some chronic primary pain conditions such as temporomandibular disorder (TMD) and fibromyalgia syndrome (FMS), mild or chronic stress enhances pain. TMD and FMS often occur together, but the underlying mechanisms are unclear. The purpose of this study was to investigate the role of cholecystokinin (CCK) in the spinal cord in somatic hyperalgesia induced by orofacial inflammation combined with stress. Somatic hyperalgesia was detected by the thermal withdrawal latency and mechanical withdrawal threshold. The expression of CCK1 receptors, CCK2 receptors, ERK1/2 and p-ERK1/2 in the spinal cord was examined by Western blot. After the stimulation of orofacial inflammation combined with 3 day forced swim, the expression of CCK2 receptors and p-ERK1/2 protein in the L4-L5 spinal dorsal horn increased significantly, while the expression of CCK1 receptors and ERK1/2 protein remained unchanged. Intrathecal injection of the CCK2 receptor antagonist YM-022 or mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor PD98059 blocked somatic hyperalgesia induced by orofacial inflammation combined with stress. Intrathecal administration of the MEK inhibitor blocked somatic sensitization caused by the CCK receptor agonist CCK8. The CCK2 receptor antagonist YM-022 significantly reduced the expression of p-ERK1/2. These data indicate that upregulation of CCK2 receptors through the MAPK pathway contributes to somatic hyperalgesia in this comorbid pain model. Thus, CCK2 receptors and MAPK pathway may be potential targets for the treatment of TMD comorbid with FMS.

Aging-Related Phenotypic Conversion of Medullary Microglia Enhances Intraoral Incisional Pain Sensitivity.

Activated microglia involved in the development of orofacial pain hypersensitivity have two major polarization states. The aim of this study was to assess the involvement of the aging-related phenotypic conversion of medullary microglia in the enhancement of intraoral pain sensitivity using senescence-accelerated mice (SAM)-prone/8 (SAMP8) and SAM-resistant/1 (SAMR1) mice. Mechanical head-withdrawal threshold (MHWT) was measured for 21 days post palatal mucosal incision. The number of CD11c-immunoreactive (IR) cells [affective microglia (M1)] and CD163-IR cells [protective microglia (M2)], and tumor-necrosis-factor-α (TNF-α)-IR M1 and interleukin (IL)-10-IR M2 were analyzed via immunohistochemistry on days 3 and 11 following incision. The decrease in MHWT observed following incision was enhanced in SAMP8 mice. M1 levels and the number of TNF-α-IR M1 were increased on day 3 in SAMP8 mice compared with those in SAMR1 mice. On day 11, M1 and M2 activation was observed in both groups, whereas IL-10-IR M2 levels were attenuated in SAMP8 mice, and the number of TNF-α-IR M1 cells increased, compared to those in SAMR1 mice. These results suggest that the mechanical allodynia observed following intraoral injury is potentiated and sustained in SAMP8 mice due to enhancement of TNF-α signaling, M1 activation, and an attenuation of M2 activation accompanying IL-10 release.

Macrophages in trigeminal ganglion contribute to ectopic mechanical hypersensitivity following inferior alveolar nerve injury in rats.

BACKGROUND: Accidental mandibular nerve injury may occur during tooth extraction or implant procedures, causing ectopic orofacial pain. The exact mechanisms underlying ectopic orofacial pain following mandibular nerve injury is still unknown. Here, we investigated the role of macrophages and tumor necrosis factor alpha (TNFα) in the trigeminal ganglion (TG) in ectopic orofacial pain following inferior alveolar nerve transection (IANX). METHODS: IANX was performed and the mechanical head-withdrawal threshold (MHWT) in the whisker pad skin ipsilateral to IANX was measured for 15 days. Expression of Iba1 in the TG was examined on day 3 after IANX, and the MHWT in the whisker pad skin ipsilateral to IANX was measured following successive intra-ganglion administration of the macrophage depletion agent liposomal clodronate Clophosome-A (LCCA). TNFα expression in the TG and the MHWT in the whisker pad skin ipsilateral to IANX following successive intra-ganglion administration of the TNFα blocker etanercept were measured on day 3 after IANX, and tumor necrosis factor receptor-1 (TNFR1) immunoreactive (IR) cells in the TG were analyzed immunohistochemically on day 3. RESULTS: The MHWT in the whisker pad skin was significantly decreased for 15 days, and the number of Iba1-IR cells was significantly increased in the TG on day 3 after IANX. Successive intra-ganglion administration of the macrophage depletion agent LCCA significantly reduced the increased number of Iba1-IR cells in the TG and reversed the IANX-induced decrease in MHWT in the whisker pad skin. TNFα expression was increased in the TG on day 3 after IANX and was reduced following successive intra-ganglion administration of the TNFα inhibitor etanercept. The decreased MHWT was also recovered by etanercept administration, and TNFR1-IR cells in the TG were increased on day 3 following IANX. CONCLUSIONS: These findings suggest that signaling cascades resulting from the production of TNFα by infiltrated macrophages in the TG contributes to the development of ectopic mechanical allodynia in whisker pad skin following IANX.

Estrogen-Induced Monocytic Response Correlates with TMD Pain: A Case Control Study.

Temporomandibular disorders (TMD) are a set of conditions characterized by pain and dysfunction in the temporomandibular joint and muscles of mastication. These pain conditions are associated with considerable morbidity, societal costs, and reduced quality of life. The prevalence varies between 4% and 10%, with females at higher risk, and a higher prevalence occurs during reproductive years. The increased prevalence of TMD in females and low prevalence in childhood reinforce that sex hormones, like estrogen, play an important, complex role in the pathophysiology of these disorders. The goal of this study was to determine whether women with TMD exhibit a monocytic hyperinflammatory response compared with control women, and to examine associations of monocytic inflammatory responses with clinical pain. Eighteen women, aged 18 to 35 y, were seen during their follicular menstrual phase. A blood sample was collected, a clinical questionnaire about pain history was administered, and a Research Diagnostic Criteria (RDC) exam was performed. Extracted monocytes were stimulated with the toll-like receptor (TLR)-4 ligand, lipopolysaccharide (LPS), in the presence and absence of estrogen, and the levels of IL6 expression evaluated. Women with TMD showed a systemic hyperinflammatory phenotype, manifested by an increased monocytic release of cytokines after an inflammatory insult, and this was further increased by estrogen. In addition, monocytes from participants who self-reported more pain on the VAS scale produced higher levels of IL6 compared with those from participants who self-reported lower pain sensitivity. These data suggest that an estrogen-induced hyperinflammatory phenotype in women with TMD may at least in part contribute to heightened clinical pain, perhaps via central sensitization.

Role of prefrontal cortical calcium-independent phospholipase A2 in antinociceptive effect of the norepinephrine reuptake inhibitor antidepresssant maprotiline.

The prefrontal cortex is essential for executive functions such as decision-making and planning. There is also accumulating evidence that it is important for the modulation of pain. In this study, we investigated a possible role of prefrontal cortical calcium-independent phospholipase A2 (iPLA2) in antinociception induced by the norepinephrine reuptake inhibitor (NRI) and tetracyclic (tricyclic) antidepressant, maprotiline. Intraperitoneal injections of maprotiline increased iPLA2 mRNA and protein expression in the prefrontal cortex. This treatment also reduced grooming responses to von-Frey hair stimulation of the face after facial carrageenan injection, indicating decreased sensitivity to pain. The antinociceptive effect of maprotiline was abrogated by iPLA2 antisense oligonucleotide injection to the prefrontal cortex, indicating a role of this enzyme in antinociception. In contrast, injection of iPLA2 antisense oligonucleotide to the somatosensory cortex did not reduce the antinociceptive effect of maprotiline. Lipidomic analysis of the prefrontal cortex showed decrease in phosphatidylcholine species, but increase in lysophosphatidylcholine species, indicating increased PLA2 activity, and release of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) after maprotiline treatment. Differences in sphingomyelin/ceramide were also detected. These changes were not observed in maprotiline-treated mice that received iPLA2 antisense oligonucleotide to the prefrontal cortex. Metabolites of DHA and EPA may help to strengthen a known supraspinal antinociceptive pathway from the prefrontal cortex to the periaqueductal gray. Together, results indicate a role of prefrontal cortical iPLA2 and its enzymatic products in the antinociceptive effect of maprotiline.

Microglial Signaling in Chronic Pain with a Special Focus on Caspase 6, p38 MAP Kinase, and Sex Dependence.

Microglia are the resident immune cells in the spinal cord and brain. Mounting evidence suggests that activation of microglia plays an important role in the pathogenesis of chronic pain, including chronic orofacial pain. In particular, microglia contribute to the transition from acute pain to chronic pain, as inhibition of microglial signaling reduces pathologic pain after inflammation, nerve injury, and cancer but not baseline pain. As compared with inflammation, nerve injury induces much more robust morphologic activation of microglia, termed microgliosis, as shown by increased expression of microglial markers, such as CD11b and IBA1. However, microglial signaling inhibitors effectively reduce inflammatory pain and neuropathic pain, arguing against the importance of morphologic activation of microglia in chronic pain sensitization. Importantly, microglia enhance pain states via secretion of proinflammatory and pronociceptive mediators, such as tumor necrosis factor α, interleukins 1ß and 18, and brain-derived growth factor. Mechanistically, these mediators have been shown to enhance excitatory synaptic transmission and suppress inhibitory synaptic transmission in the pain circuits. While early studies suggested a predominant role of microglia in the induction of chronic pain, further studies have supported a role of microglia in the maintenance of chronic pain. Intriguingly, recent studies show male-dominant microglial signaling in some neuropathic pain and inflammatory pain states, although both sexes show identical morphologic activation of microglia after nerve injury. In this critical review, we provide evidence to show that caspase 6-a secreted protease that is expressed in primary afferent axonal terminals surrounding microglia-is a robust activator of microglia and induces profound release of tumor necrosis factor α from microglia via activation of p38 MAP kinase. The authors also show that microglial caspase 6/p38 signaling is male dominant in some inflammatory and neuropathic pain conditions. Finally, the authors discuss the relevance of microglial signaling in chronic trigeminal and orofacial pain.

Relationship of synovial tumor necrosis factor alpha and interleukin 6 to temporomandibular disorder.

PURPOSE: The purpose of this study was to elucidate the relationship of tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) to temporomandibular disorder with clinical symptoms such as pain, joint sounds, and mouth opening limitation by analysis of the level of these molecules in the synovial fluid. PATIENTS AND METHODS: Twenty-four patients with chief complaints of pain, mouth opening limitation, and clicking sounds were selected as the experimental group and compared with 5 healthy subjects. After joint lavage with arthrocentesis, diluted synovial fluid was collected and enzyme-linked immunosorbent assay was done for analysis of TNF-alpha and IL-6 in 24 experimental patients and 5 healthy subjects. RESULTS: The synovial levels of TNF-alpha and IL-6 were elevated in the experimental group compared with the healthy control group, but no significant correlation was established. The synovial levels of TNF-alpha and IL-6 were elevated in the acute pain group compared with the chronic pain group, but no significant correlation was established. CONCLUSION: In our analysis of 2 proinflammatory cytokines, TNF-alpha and IL-6, in the synovial fluid of temporomandibular disorder patients with symptoms of pain, mouth opening limitation, and clicking, both were elevated without statistical significance.

Temporomandibular joint pain and synovial fluid analysis: a review of the literature.

The pathophysiology of temporomandibular joint pain is not well understood. A significant amount of research has been conducted to evaluate synovial fluid in these patients and in healthy controls. Qualitative and quantitative analyses of the synovial fluid have shown a significant difference between these groups. A multitude of inflammatory mediators and degradation products have been identified. The concentration of these products has been shown to correlate with several clinical parameters including pain, chronicity, severity of degenerative change, and response to treatment. A common inflammatory pathway would appear to be involved in most patients. At the present time, synovial fluid analysis does not have the sensitivity or specificity to allow specific diagnoses and targeted treatment. Continued research with the specific aim of establishing more appropriate therapeutic modalities based on the biochemical pathways is warranted.

Neuroendocrine, immune, and local responses related to temporomandibular disorders.

Orofacial pain frequently originates from pathologic conditions in the masticatory muscles or temporomandibular joints (TMJs). The mediators and mechanisms that monitor pain and inflammation, centrally or peripherally, are of great interest in the search for new treatment modalities. The neuropeptides substance P (SP), calcitonin gene-related peptide (CGRP), and neuropeptide Y (NPY) have all been found at high levels in the synovial fluid of arthritic TMJs in association with spontaneous pain, while serotonin (5-HT) has been found in association with hyperalgesia/allodynia of the TMJ. Interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) have been found in arthritic TMJs, but not in healthy TMJs, in association with hyperalgesia/allodynia of the TMJ as well as spontaneous pain. Anterior open bite, which may be a clinical sign of TMJ destruction, has been found in association with high levels of CGRP, NPY, and IL-1 beta in the synovial fluid of the TMJ. Interleukin-1 beta has also been related to radiographic signs of joint destruction. Prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) are both present in the arthritic TMJ, and PGE2 has been shown to be associated with hyperalgesia/allodynia of the TMJ. Very little is known about pain and inflammatory mediators in muscles. However, we know that 5-HT and PGE2 are involved in the development of pain and hyperalgesia/allodynia of the masseter muscle in patients with fibromyalgia, whereas local myalgia (myofascial pain) seems to be modulated by other, as yet unknown mediators. Interaction between the peripheral nervous system (sensory and sympathetic nerves), the immune system, and local cells is probably of great importance for the modulation of pain and inflammation in the TMJ and orofacial musculature.

Tumor necrosis factor-alpha in synovial fluid and plasma from patients with chronic connective tissue disease and its relation to temporomandibular joint pain.

PURPOSE: The purpose of this study was to determine the level of tumor necrosis factor-alpha (TNF-alpha) in the temporomandibular joint (TMJ) synovial fluid (SF-TNF-alpha) and blood plasma (P-TNF-alpha) of patients with chronic inflammatory connective tissue disease and investigate its relation to TMJ pain, hyperalgesia, and allodynia. PATIENTS AND METHODS: Twenty-four patients with a diagnosis of chronic inflammatory connective tissue disease and TMJ pain were included in the study. Visual analog scale, tenderness of the TMJ, and pain at mandibular movements were registered, and the pressure pain threshold and pressure pain tolerance levels were measured. TMJ synovial fluid samples and blood plasma were analyzed for TNF-alpha and the levels related to TMJ pain, hyperalgesia, and allodynia. RESULTS: TNF-alpha was present in the TMJ synovial fluid of 8 of 24 patients at levels significantly exceeding those in plasma at the same visit. The presence of SF-TNF-alpha showed a significant positive correlation to TMJ pain at maximum voluntary mouth opening and tenderness to posterior palpation of the TMJ. CONCLUSION: Local production of TNF-alpha occurs in the TMJ synovium of patients with chronic inflammatory connective tissue disease. Pain on mandibular movement and tenderness on posterior palpation (allodynia) of the TMJ is related to the level of SF-TNF-alpha.

Effects of stretch-based progressive relaxation training on the secretion of salivary immunoglobulin A in orofacial pain patients.

There is a growing body of evidence that psychologic stressors can affect physical health and proneness to disease through depletion of the body's immune system. Relatively little research, however, has investigated the potential immunoenhancing effect of stress-relieving strategies such as progressive muscle relaxation. This study explored the relationship between immune functioning and relaxation training with persons experiencing persistent facial pain. In a single experimental session, 21 subjects either received relaxation training or rested for an equivalent time period. Salivary immunoglobulin A, mood, pain, and tension levels were measured before and after relaxation and rest periods. Results indicated that a greater proportion of those receiving relaxation training had increases in secretion of salivary immunoglobulin A. These findings suggest that immunoenhancement may be another potential benefit of progressive relaxation training for persons with chronic pain conditions.

Interleukin-6 in synovial fluid and HLA-DR expression in synovium from patients with temporomandibular disorders.

Interleukin-6-dependent mouse hybridoma cell line KD83 was used to test the biologic activity of interleukin-6 in synovial fluid from 37 patients with temporomandibular disorders. The results showed that the interleukin-6 level was greater than 100 U/mL in 13 of 18 patients with degenerative joint disease and in five of 12 patients with temporomandibular disc displacement. However, the interleukin-6 level was less than 100 U/mL (range, 20 to 75 U/mL) in all patients with masticatory muscle disorder. It has been found that degenerative joint disease tends to have acute and chronic stages, and interleukin-6 activity was probably related to the acute stage in the patients. Histologic studies of the synovium from seven patients with degenerative joint disease showed a variable degree of hyperplasia of the synovial lining cells and chronic inflammation in five of eight specimens. Immunostaining studies clearly showed the presence of significantly more HLA-DR-expressing cells (human leukocyte antigen-D-related) in synovium. Although it is unlikely that immune responses play an important primary role in initiating synovial inflammation and cartilage destruction, immune reactions may be one important factor in the maintenance and severity of some patients with temporomandibular disorders.

Tumor necrosis factor-alpha as a biochemical marker of pain and outcome in temporomandibular joints with internal derangements.

OBJECTIVE: Previous studies have demonstrated the presence of tumor necrosis factor-alpha (TNF) in human temporomandibular joint (TMJ) synovial fluid. The present study continues the investigation of the role of TNF in TMJs with internal derangements. MATERIALS AND METHODS: Synovial fluid was obtained from 18 TMJs in 12 patients undergoing either arthroscopy (14 joints) or arthrotomy (four joints) for internal derangements. Standardized clinical data were collected preoperatively, intraoperatively, and postoperatively. RESULTS: When pain on palpation was absent, the mean preoperative TNF level was 14 +/- 6 ng/mL. When pain on palpation was present, the mean TNF level was 42 +/- 39 ng/mL (significant difference at P = .05). When the surgical outcome was poor, the mean preoperative TNF level was 26 +/- 9 ng/mL. When the outcome was within the stated guidelines for a favorable result, the mean TNF level was 12 +/- 7 ng/mL (significant difference at P = .05). In addition, a significant reduction (P = .05) in TNF following joint lavage (preoperative, 48 ng/mL to postoperative, 7 ng/mL) was found. CONCLUSIONS: The finding of a positive correlation between preoperative pain and TNF values suggests a biochemical basis for the origin of the pain associated with internal derangements. The relationship between preoperative TNF levels and surgical outcome suggests that the prognosis for surgery may be predicted by measuring biochemical markers of joint disease.

Facial pain, distress, and immune function.

Chronic facial pain syndromes are associated with high levels of distress and depression. Immune system measures were investigated in otherwise healthy patients suffering from chronic temporomandibular pain and dysfunction syndrome (TMPDS) and in matched controls. No mean differences were found between TMPDS patients and the controls on any of the immune measures; however, both ConA and PWM responses in TMPDS patients were decreased in relation to the level of demoralization (P less than 0.05). Cognitive symptoms such as low self-esteem and perceptions of helplessness/hopelessness were implicated in these effects. In addition, among patients pain severity was independently associated with decreased ConA response (P less than 0.05). The data suggest possible correlates of stress-induced changes in the immune system.

Psychoneuroimmunology: potential relevance to chronic orofacial pain.

Studies undertaken over the past ten years have demonstrated that stress and depression can induce immune alterations, including decreased numbers of immunocompetent cells and impaired lymphocyte and natural killer cell activity. Factors such as age and severity of symptomatology influence these effects. The substantial stress and depression associated with chronic pain syndromes and the evidence for opioid involvement in immunomodulation suggest that immune system changes may occur in some patients with chronic facial pain.

[Basic cellular indices of immunity in patients with various pain syndromes of the face and head]. / Osnovnye pokazateli kletochnogo immuniteta u bol'nykh s razlichnymi bolevymi sindromami v oblasti litsa i golovy.

Studies of the major cell-mediated immunity parameters in patients with painful syndromes involving the face and neck have shown a reduction of the peripheral lymphocyte count.

[Immunological status of patients with painful syndromes in the face and head]. / Immunologicheskii status u bol'nykh s bolevymi sindromami v oblasti litsa i golovy.

As many as 145 patients with painful syndromes in the face and head underwent immunologic examination. They manifested certain changes in cellular and humoral immunity. An increase in the content of secretory IgA in the saliva and in the serum and a decrease of the concentration of IgM and IgG in the serum were detected in the majority of patients. In patients with NTN of mainly peripheral genesis and with Horton's syndrome, the concentration of IgE in the serum was significantly (P less than 0.01) elevated. The changes in the immunologic status, seen in patients after the treatment are discussed.