ß2-adrenergic receptor activation decreases the mechanical sensitivity of rat masticatory muscle afferent fibres.

BACKGROUND: Activation of ß2 adrenergic receptors reduces cutaneous mechanical pain thresholds in rats. While ß2 adrenergic receptor activation may contribute to mechanisms that underlie temporomandibular joint pain, its effect on masticatory muscle pain sensitivity is uncertain. OBJECTIVES: The current study sought to determine the extent to which ß adrenergic receptors are expressed by masticatory muscle afferent fibres, and to assess the effect of local activation of these receptors on the mechanical sensitivity of masticatory muscle afferent fibres in rats. METHODS: Trigeminal ganglion neurons that innervate the rat (n = 12) masseter muscle and lower lip were identified by tissue injection of fluorescent dyes and were then stained with antibodies against ß1 or ß2 adrenergic receptors. Extracellular recordings from 60 trigeminal ganglion neurons that innervate the masticatory muscle were undertaken in a second group of anaesthetised rats of both sexes (n = 37) to assess afferent mechanical activation thresholds. Thresholds were assessed before and after injection of the ß adrenergic receptor agonists into masticatory muscle. RESULTS: ß1 and ß2 adrenergic receptor expression was greater in labial skin than in masticatory muscle ganglion neurons (p < .05, one-way ANOVA, Holm-Sidak test). There was a higher expression of ß2 adrenergic receptors in masticatory muscle ganglion neurons in males than in females. The mixed ß agonist isoproterenol increased afferent mechanical activation threshold in male but not female rats (p < .05, Mann-Whitney test). In male rats, salbutamol, a ß2 selective agonist, also increased afferent mechanical activation threshold but hydralazine, a vasodilator, did not (p < .05, Mann-Whitney test). CONCLUSION: Activation of ß2 adrenergic receptors decreases the mechanical sensitivity of masticatory muscle afferent fibres in a sex-related manner.

The contribution of autonomic mechanisms to pain in temporomandibular disorders: A narrative review.

BACKGROUND: Temporomandibular disorders (TMD) are diagnosed based on symptom presentation and, like other functional pain disorders, often lack definitive pathology. There is a strong association between elevated stress levels and the severity of TMD-related pain, which suggests that alterations in autonomic tone may contribute to this pain condition. OBJECTIVES: This narrative review examines the association between altered autonomic function and pain in TMD. METHODS: Relevant articles were identified by searching PubMed and through the reference list of those studies. RESULTS: TMD sufferers report an increased incidence of orthostatic hypotension. As in other chronic musculoskeletal pain conditions, TMD is associated with increased sympathetic tone, diminished baroreceptor reflex sensitivity and decreased parasympathetic tone. It remains to be determined whether ongoing pain drives these autonomic changes and/or is exacerbated by them. To examine whether increased sympathetic tone contributes to TMD-related pain through ß2 adrenergic receptor activation, clinical trials with the beta blocker propranolol have been undertaken. Although evidence from small studies suggested propranolol reduced TMD-related pain, a larger clinical trial did not find a significant effect of propranolol treatment. This is consistent with human experimental pain studies that were unable to demonstrate an effect of ß2 adrenergic receptor activation or inhibition on masticatory muscle pain. In preclinical models of temporomandibular joint arthritis, ß2 adrenergic receptor activation appears to contribute to inflammation and nociception, whereas in masticatory muscle, α1 adrenergic receptor activation has been found to induce mechanical sensitisation. Some agents used to treat TMD, such as botulinum neurotoxin A, antidepressants and α2 adrenergic receptor agonists, may interact with the autonomic nervous system as part of their analgesic mechanism. CONCLUSION: Even if dysautonomia turns out to be a consequence rather than a causative factor of painful TMD, the study of its role has opened up a greater understanding of the pathogenesis of this condition.

Dysregulation of the peripheral glutamatergic system: A key player in migraine pathogenesis?

BACKGROUND: Although the role of glutamate in migraine pathogenesis remains uncertain, there has been significant interest in the development of drug candidates that target glutamate receptors. Activation of trigeminovascular afferent fibers is now recognized as a crucial step to the onset of a migraine episode. New evidence suggests a dysfunction in peripheral glutamate regulation may play a role in this process. OBJECTIVE: To provide a narrative review of the role of peripheral glutamate dysfunction in migraine. METHOD: A review of recent literature from neurobiological, pharmacological and genomic studies was conducted to support peripheral glutamate dysfunction as a potential element in migraine pathogenesis. RESULTS: Studies in rats suggest that elevated blood glutamate mechanically sensitizes trigeminal afferent fibers and stimulates the release of calcitonin-gene related peptide and other neuropeptides to promote and maintain neurogenic inflammation. These effects may be driven by upregulation of glutamate receptors, and modifications to reuptake and metabolic pathways of glutamate. Furthermore, genome wide association studies have found polymorphisms in glutamate receptor and transporter genes that are associated with migraine. CONCLUSION: The role of peripheral glutamate signalling in the onset and maintenance of migraine is not completely elucidated and future studies are still needed to confirm its role in migraine pathogenesis.

Density of nerve fibres and expression of substance P, NR2B-receptors and nerve growth factor in healthy human masseter muscle: An immunohistochemical study.

BACKGROUND: In skeletal muscle, free nerve endings are mostly located within the connective tissue. However, the distribution of sensory afferent fibres in healthy human masseter muscle tissues has not been studied. OBJECTIVES: Primarily to investigate human masseter muscle nerve fibre densities as well as expression of NR2B receptors, substance P (SP) and nerve growth factor (NGF), and secondarily to compare this between a) nerve fibres associated with myocytes and within connective tissue; b) sexes; and c) ages. METHODS: Microbiopsies of the masseter muscle were obtained from 60 sex- and age-matched healthy participants. Biopsy sections were analysed using immunohistochemistry and were visualised with a Leica TCS SPE confocal microscope. The Mann-Whitney U test was used for statistical analyses. RESULTS: The density of nerve fibres within connective tissue was significantly greater than in nerve fibres associated with myocytes (P < .001). Nerve fibres within connective tissue expressed SP alone or together with NR2B significantly more often than those associated with myocytes (P < .001). The frequency of nerve fibres, which expressed SP alone or in combination with NR2B or NGF, was significantly greater in women than in men (P < .050). Moreover, the co-expression of the three markers together was inversely correlated with age in women (P < .002). CONCLUSIONS: There is a higher density and greater expression of sensory nerve fibres within the connective tissue than associated with myocytes in healthy human masseter muscle. This suggests that nerve fibres within connective tissue are more involved in nociception than nerve fibres associated with myocytes.

Nerve growth factor alters the sensitivity of rat masseter muscle mechanoreceptors to NMDA receptor activation.

Intramuscular injection of nerve growth factor (NGF) into rat masseter muscle induces a local mechanical sensitization that is greater in female than in male rats. The duration of NGF-induced sensitization in male and female rats was associated with an increase in peripheral N-methyl-d-aspartate (NMDA) receptor expression by masseter muscle afferent fibers that began 3 days postinjection. Here, we investigated the functional consequences of increased NMDA expression on the response properties of masseter muscle mechanoreceptors. In vivo extracellular single-unit electrophysiological recordings of trigeminal ganglion neurons innervating the masseter muscle were performed in anesthetized rats 3 days after NGF injection (25 µg/ml, 10 µl) into the masseter muscle. Mechanical activation threshold was assessed before and after intramuscular injection of NMDA. NMDA injection induced mechanical sensitization in both sexes that was increased significantly following NGF injection in the male rats but not in the female rats. However, in female but not male rats, further examination found that preadministration of NGF induced a greater sensitization in slow Aδ-fibers (2-7 m/s) than fast Aδ-fibers (7-12 m/s). This suggests that preadministration of NGF had a different effect on slowly conducting mechanoreceptors in the female rats compared with the male rats. Although previous studies have found an association between estrogenic tone and NMDA activity, no correlation was observed between NMDA-evoked mechanical sensitization and plasma estrogen level. This study suggests NGF alters NMDA-induced mechanical sensitization in the peripheral endings of masseter mechanoreceptors in a sexually dimorphic manner.

Intraganglionic injection of a nitric oxide donator induces afferent mechanical sensitization that is attenuated by palmitoylethanolamide.

AIM: The aim of this article is to investigate whether the nitric oxide (NO) donator diethylenetriamine/nitric oxide (DETA/NO) affects trigeminal sensory processing through the trigeminal ganglion in part by activating trigeminal satellite glial cells (SGCs) and whether this effect is attenuated by the anti-inflammatory compound palmitoylethanolamide (PEA). METHODS: DETA/NO was administered to isolated rat trigeminal SGCs in vitro, and injected into the rat trigeminal ganglion in vivo, in the presence or absence of PEA. RESULTS: Administration of DETA/NO (1000 µM) increased the release of prostaglandin E2 by SGCs. PEA (1 and 10 µM) significantly attenuated prostaglandin E2 release. Two intraganglionic injections of DETA/NO (10 mM, 3 µl) or prostaglandin E2 at a 30-minute interval did not evoke discharge in trigeminal ganglion neurons that innervate the rat jaw-closer muscles, but did reduce the mechanical activation threshold of their peripheral endings by 30%-50%. Intravenous administration of PEA (1 mg/kg) or ketorolac (0.5 mg/kg) prevented DETA/NO-induced afferent mechanical sensitization. CONCLUSIONS: Elevation of NO in the trigeminal ganglion results in the sensitization of the peripheral endings of masticatory muscle nociceptors to mechanical stimulation through a mechanism that involves prostaglandin E2 release from SGCs. Attenuation of this sensitization by PEA suggests a possible option for acute management of craniofacial pain and headache.

Expression of 5-HT3 receptors and TTX resistant sodium channels (Na(V)1.8) on muscle nerve fibers in pain-free humans and patients with chronic myofascial temporomandibular disorders.

BACKGROUND: Previous studies have shown that 5-HT3-antagonists reduce muscle pain, but there are no studies that have investigated the expression of 5-HT3-receptors in human muscles. Also, tetrodotoxin resistant voltage gated sodium-channels (NaV) are involved in peripheral sensitization and found in trigeminal ganglion neurons innervating the rat masseter muscle. This study aimed to investigate the frequency of nerve fibers that express 5-HT3A-receptors alone and in combination with NaV1.8 sodium-channels in human muscles and to compare it between healthy pain-free men and women, the pain-free masseter and tibialis anterior muscles, and patients with myofascial temporomandibular disorders (TMD) and pain-free controls. METHODS: Three microbiopsies were obtained from the most bulky part of the tibialis and masseter muscles of seven and six healthy men and seven and six age-matched healthy women, respectively, while traditional open biopsies were obtained from the most painful spot of the masseter of five female patients and from a similar region of the masseter muscle of five healthy, age-matched women. The biopsies were processed by routine immunohistochemical methods. The biopsy sections were incubated with monoclonal antibodies against the specific axonal marker PGP 9.5, and polyclonal antibodies against the 5-HT3A-receptors and NaV1.8 sodium-channels. RESULTS: A similar percentage of nerve fibers in the healthy masseter (85.2%) and tibialis (88.7%) muscles expressed 5-HT3A-receptors. The expression of NaV1.8 by 5-HT3A positive nerve fibers associated with connective tissue was significantly higher than nerve fibers associated with myocytes (P < .001). In the patients, significantly more fibers per section were found with an average of 3.8 ± 3 fibers per section in the masseter muscle compared to 2.7 ± 0.2 in the healthy controls (P = .024). Further, the frequency of nerve fibers that co-expressed NaV1.8 and 5-HT3A receptors was significantly higher in patients (42.6%) compared to healthy controls (12.0%) (P < .001). CONCLUSIONS: This study showed that the 5-HT3A-receptor is highly expressed in human masseter and tibialis muscles and that there are more nerve fibers that express 5-HT3A-receptors in the masseter of women with myofascial TMD compared to healthy women. These findings indicate that 5-HT3-receptors might be up-regulated in myofascial TMD and could serve as potential biomarkers of chronic muscle pain.

MRGPRX2-mediated mast cell activation by substance P from overloaded human tenocytes induces inflammatory and degenerative responses in tendons.

Mast cells are immune cells minimally present in normal tendon tissue. The increased abundance of mast cells in tendinopathy biopsies and at the sites of tendon injury suggests an unexplored role of this cell population in overuse tendon injuries. Mast cells are particularly present in tendon biopsies from patients with more chronic symptom duration and a history of intensive mechanical loading. This study, therefore, examined the cross talk between mast cells and human tendon cells in either static or mechanically active conditions in order to explore the potential mechanistic roles of mast cells in overuse tendon injuries. A coculture of isolated human tenocytes and mast cells (HMC-1) combined with Flexcell Tension System for cyclic stretching of tenocytes was used. Additionally, human tenocytes were exposed to agonists and antagonists of substance P (SP) receptors. Mast cell degranulation was assessed by measuring ß-hexosaminidase activity. Transwell and cell adhesion assays were used to evaluate mast cell migration and binding to tendon extracellular matrix components (collagen and fibronectin), respectively. Gene expressions were analyzed using real time qRT-PCR. Our results indicate that mechanical stimulation of human tenocytes leads to release of SP which, in turn, activates mast cells through the Mas-related G-protein-coupled receptor X2 (MRGPRX2). The degranulation and migration of mast cells in response to MRGPRX2 activation subsequently cause human tenocytes to increase their expression of inflammatory factors, matrix proteins and matrix metalloproteinase enzymes. These observations may be important in understanding the mechanisms by which tendons become tendinopathic in response to repetitive mechanical stimulation.

Effect of propranolol on hypertonic saline-evoked masseter muscle pain and autonomic response in healthy women during rest and mental arithmetic task.

AIMS: To investigate in a randomized, double-blinded, placebo controlled, crossover study the effect of a single dose of the nonselective ß-adrenergic receptor antagonist propranolol (40 mg) on hypertonic saline (HS)-evoked masseter muscle pain and autonomic activity during rest and during a mental arithmetic task (Paced Auditory Serial Addition Task, PASAT). METHODS: Sixteen healthy women participated in two sessions in which propranolol or placebo was administered orally prior to two 5-minute infusions (30 minutes apart) of HS in the masseter muscle. The second HS infusion was combined with PASAT. HS-evoked pain intensity was scored on a numeric rating scale (NRS, 0 to 10). Heart rate variability and hemodynamic measures were recorded noninvasively (Task Force Monitor). Data were analyzed with repeated measurements analysis of variance (ANOVA). RESULTS: Propranolol did not reduce NRS pain scores compared with placebo but did induce significant autonomic changes with reduced heart rate and increased heart rate variability (standard deviations of all normal RR intervals; root mean square successive differences; low-frequency power; high-frequency power; and total power) independent of the mental task. CONCLUSION: A single dose of propranolol had no effect on acute HS-evoked pain levels during rest or during mental arousal. However, it influenced the tone of the autonomic nervous system, possibly reflecting an anxiolytic effect.

Headache and mechanical sensitization of human pericranial muscles after repeated intake of monosodium glutamate (MSG).

BACKGROUND: A single intake of monosodium glutamate (MSG) may cause headache and increased muscle sensitivity. We conducted a double-blinded, placebo-controlled, crossover study to examine the effect of repeated MSG intake on spontaneous pain, mechanical sensitivity of masticatory muscles, side effects, and blood pressure. METHODS: Fourteen healthy subjects participated in 5 daily sessions for one week of MSG intake (150 mg/kg) or placebo (24 mg/kg NaCl) (randomized, double-blinded). Spontaneous pain, pressure pain thresholds and tolerance levels for the masseter and temporalis muscles, side effects, and blood pressure were evaluated before and 15, 30, and 50 min after MSG intake. Whole saliva samples were taken before and 30 min after MSG intake to assess glutamate concentrations. RESULTS: Headache occurred in 8/14 subjects during MSG and 2/14 during placebo (P = 0.041). Salivary glutamate concentrations on Day 5 were elevated significantly (P < 0.05). Pressure pain thresholds in masseter muscle were reduced by MSG on Day 2 and 5 (P < 0.05). Blood pressure was significantly elevated after MSG (P < 0.040). CONCLUSION: In conclusion, MSG induced mechanical sensitization in masseter muscle and adverse effects such as headache and short-lasting blood pressure elevation for which tolerance did not develop over 5 days of MSG intake.

Race-based reporting and participation of Black individuals in registered pain clinical trials, United States, 2000 to 2019.

ABSTRACT: There are numerous, well-established racial disparities in the management of pain. The degree to which these are evident at the stage of conducting clinical trials is unknown. To address this knowledge gap, we examined race-based reporting, participation of Black individuals, and the factors associated with reporting and participation in pain clinical trials in the United States. Data were extracted from Clinicaltrials.gov and published articles. One thousand two hundred trials met our inclusion criteria; 482 (40.2%) reported participant race. More recent, publicly funded, and larger trials were more likely to report race. Of 82,468 participants included in pain clinical trials that reported race, 15,101 were Black individuals (18.3%). Participation of Black individuals was significantly associated with pain type (ß = +27% in cardiovascular disease pain compared with acute pain, P < 0.05), study population (ß = +33% and +7% in pain in minoritized populations and women, respectively, compared with general population, P < 0.05), pain intervention (ß = +7.5% for trials of opioid interventions compared with nonopioid interventions, P < 0.05), and a diverse team of investigators (ß = +8.0% for studies incorporating a visible non-White investigator compared with those that did not, P < 0.05). Our results indicate that representation of Black participants in pain clinical trials generally aligns with national demographics in the United States. Increased representation corresponds with health conditions more prevalent among Black individuals (eg, cardiovascular disease) and with a diverse study team composition. Despite these encouraging results, less than half of pain trials reported race, which introduces potential publication bias and limits external validity.

Intramuscular ketorolac inhibits activation of rat peripheral NMDA receptors.

The nonsteroidal anti-inflammatory drug (NSAID) diclofenac has local anesthetic-like and peripheral N-methyl-d-aspartate (NMDA) receptor antagonist characteristics when administered at higher concentrations to masticatory muscle. It is not known if the ability to inhibit NMDA receptors is unique to diclofenac or shared by other NSAIDs. This study was undertaken to determine whether intramuscular injection of ketorolac or naproxen at concentrations that do not induce local anesthetic-like effects could attenuate jaw-closer muscle nociceptor discharge in anesthetized Sprague-Dawley rats. It was found that ketorolac (5 mM) inhibited hypertonic saline-evoked nociceptor discharge, which suggests that at this concentration, ketorolac has local anesthetic-like properties. A lower concentration of ketorolac (0.5 mM), which did not affect hypertonic saline-evoked discharge, did inhibit nociceptor discharge evoked by NMDA. In contrast, naproxen (5 mM) did not alter hypertonic saline- or NMDA-evoked nociceptor discharge. Subsequent experiments revealed that ketorolac (0.5 mM) had no effect on nociceptor discharge evoked by αß-methylene ATP, 5-hydroxytryptamine, or AMPA. The inhibitory effect of ketorolac did not appear to be related to cyclooxygenase inhibition, because the concentration of prostaglandin E(2) in the masticatory muscles 10 min after injection of either NSAID was not significantly decreased. The present study indicates that in vivo, ketorolac, but not naproxen, can antagonize NMDA-evoked nociceptor discharge similarly to diclofenac. We speculate that structural similarities between ketorolac and diclofenac could account for the ability of these NSAIDs to inhibit NMDA-evoked nociceptor discharge. These properties may partly explain the analgesic effect of intramuscularly injected ketorolac in the clinic.

Experimental stressors alter hypertonic saline-evoked masseter muscle pain and autonomic response.

AIMS: To test in a randomized controlled trial, if hypertonic saline (HS)-evoked pain and autonomic function are modulated by either a cold pressor test (CPT) or mental arithmetic stress induced by a paced auditory serial addition task (PASAT). METHODS: Fourteen healthy women participated in three sessions. Pain was induced by two 5% HS infusions (5 minutes each, 30 minutes apart) infused into the masseter muscle. During the second HS infusion, pain was modulated by PASAT, CPT, or control (HS alone). HS-evoked pain intensity was scored on a 0 to 10 numeric rating scale (NRS). Heart rate variability (HRV) and hemodynamic measures were recorded noninvasively (Task Force Monitor). Data were analyzed using repeated measurements ANOVAs and Spearman correlation analysis. RESULTS: HS-evoked pain was significantly and similarly reduced by both PASAT (30.8 ± 27.6%; P < .001) and CPT (35.8 ± 26.6%; P < .001) compared with the control session (9.0 ± 30.5%; P > .05). PASAT and CPT increased the heart rate compared with control (P <.001). CPT reduced measures of vagal activity: Root mean square successive difference, high-frequency (HF) power, and coefficient of HF component variance compared with an internal control, ie, the first HS infusion (P < .05), while PASAT did not alter any of these HRV measures (P > .05). CONCLUSION: CPT and PASAT reduced HS-evoked masseter muscle pain and altered the autonomic response. The increase in heart rate following CPT and PASAT may be caused by different mechanisms. CPT reduced measures of efferent cardiac vagal (parasympathetic) activity, while the PASAT-induced increase in heart rate, but unchanged HRV, may suggest neurohumoral activation.

Efficacy of topical cannabinoids in the management of pain: a systematic review and meta-analysis of animal studies.

BACKGROUND/IMPORTANCE: Cannabinoids are emerging as an alternative pain management option, preliminarily supported by preclinical and clinical studies. Unwanted side effects from oral or inhaled cannabinoids remain, however, a major barrier to widespread use. Peripherally acting cannabinoids (eg, topically applied) may circumvent these side effects while providing localized pain management. OBJECTIVE: Our purpose was to systematically review the literature on the effectiveness of peripherally acting cannabinoids for pain management. EVIDENCE REVIEW: We searched MEDLINE, EMBASE, CENTRAL, CINAHL, and PubMed databases. Included studies examined the effect of topical/peripherally administered cannabinoids on pain ratings in humans, as well as pain-related outcomes in animals (eg, paw withdrawal). Due to a lack of trials, human studies were summarized in a narrative synthesis. Separate meta-analyses were performed for animal studies using radiant tail flick or paw withdrawal outcomes. FINDINGS: Our search yielded 1182 studies following removal of duplicates, with 46 studies (6 human, 40 animal) included. Human studies (one randomized controlled trial and five case studies/series) reported no adverse events to topical cannabinoids and preliminary evidence of decreased pain ratings. Animal studies reporting tail flick (5) (2.81, 95% CI 1.93 to 3.69, p<0.001) and mechanical withdrawal (11) (2.74, 95% CI 1.82 to 3.67, p<0.001) reported prolonged responses (analgesia) in peripheral cannabinoid groups compared with controls. CONCLUSIONS: Preclinical animal studies provided low-quality evidence for peripherally administered cannabinoids to provide regional, antinociceptive effects. The scarcity of high-quality human studies underscores the need to translate preclinical evidence into well-controlled human trials.

Peripheral N-methyl-D-aspartate receptor activation contributes to monosodium glutamate-induced headache but not nausea behaviours in rats.

Monosodium glutamate induces behaviors thought to reflect headache and nausea in rats. We explored the effects of the N-methyl-D-aspartate receptor antagonist (2R)-amino-5-phosphonovaleric acid, the inotropic glutamate receptor antagonist kynurenic acid, and the CGRP receptor antagonist olcegepant, on monosodium glutamate-induced increases in nocifensive, headache-like and nausea behaviours. Effects of these antagonists on motor function were examined with a rotarod. The effect of the dopamine receptor antagonist metoclopramide and the serotonin 3 receptor antagonist ondansetron on nausea behaviour was also assessed. (2R)-amino-5-phosphonovaleric acid, and to a lesser extent, kynurenic acid and olcegepant, reduced nocifensive and headache-like behaviours evoked by monosodium glutamate. No alteration in motor function by (2R)-amino-5-phosphonovaleric acid, kynurenic acid or olcegepant was observed. No sex-related differences in the effectiveness of these agents were identified. Nausea behaviour was significantly more pronounced in male than in female rats. Olcegepant, ondansetron and metoclopramide ameliorated this nausea behaviour in male rats. Ondansetron and metoclopramide also reduced headache-like behaviour in male rats. These findings suggest that peripheral N-methyl-D-aspartate receptor activation underlies monosodium glutamate-induced headache-like behaviour but does not mediate the nausea behaviour in rats.

Systemic administration of monosodium glutamate induces sexually dimorphic headache- and nausea-like behaviours in rats.

ABSTRACT: Ingestion of monosodium glutamate (MSG) causes headache, nausea, and craniofacial tenderness in healthy individuals. The present study explored whether MSG produces behavioural signs of headache, nausea, and changes in craniofacial sensitivity in rats. The behavior of male and female Sprague-Dawley rats was video recorded before and after intraperitoneal (i.p.) injections of MSG (1-1000 mg/kg), nitroglycerin (GTN, 10 mg/kg), or normal saline. Behaviors (grimace score, head-flicks, rearing, head scratches, facial grooming, lying-on-belly, and temporalis muscle region mechanical withdrawal threshold) were evaluated. Facial cutaneous temperature of the nose and forehead was measured before and after i.p. injections via infrared thermography. Plasma glutamate and calcitonin gene-related peptide concentrations after administration of 1000 mg/kg MSG were measured in anesthetized rats. Monosodium glutamate induced nocifensive, headache-like, and nausea-like behaviors in a dose-related manner but had no effect on mechanical threshold. Monosodium glutamate (1000 mg/kg) induced a significantly greater frequency of headache-like behavior in females but a longer duration of nausea-like behavior in males. Monosodium glutamate produced a prolonged increase in plasma glutamate and calcitonin gene-related peptide concentrations. Co-administration of the median effective dose of MSG (350 mg/kg) with GTN (10 mg/kg) amplified headache-like behaviors, induced significant craniofacial sensitivity, and produced increased nausea-like behaviour. Co-administration of sumatriptan or naproxen with MSG (1000 mg/kg) significantly attenuated MSG-induced nocifensive and headache-like behaviors. Our data suggest that systemic administration of MSG to rats induces behavioral correlates of headache and nausea. This model may offer another avenue for research on the mechanism and treatment of primary headache disorders such as migraine.

TNFα mechanically sensitizes masseter muscle nociceptors by increasing prostaglandin E2 levels.

TNFα induces mechanical sensitization of rat masseter muscle nociceptors, which takes 2-3 h to manifest and is mediated through activation of P55 and P75 receptors. This study was undertaken to determine whether TNFα induces nociceptor mechanical sensitization through the release of other algogenic substances such as glutamate, prostaglandin E(2) (PGE(2)), and/or nerve growth factor (NGF), which have been shown to induce mechanical sensitization of muscle nociceptors. Masseter muscle homogenate levels of PGE(2) and NGF were measured 3 h after injection of TNFα (1 µg) or vehicle control using commercially available kits. Interstitial glutamate concentration was measured after injection of TNFα or vehicle control using a glutamate-selective biosensor probe. Diclofenac, a cyclooxygenase inhibitor that blocks the synthesis of PGE(2), D-2-amino-5-phosphonovaleric acid (APV), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, and a tyrosine kinase A (TrkA) receptor antibody, which blocks NGF-induced masseter muscle nociceptor sensitization, were used to assess the contribution of PGE(2), glutamate, and NGF to TNFα-induced nociceptor sensitization. PGE(2) and glutamate concentrations were significantly elevated 3 h after TNFα injection into the masseter muscle. Injection of diclofenac partially reversed the TNFα-induced decreases in the mechanical threshold (MT) of masseter muscle nociceptors, whereas vehicle control, APV, and TrkA antibody did not significantly alter nociceptor MT. These results suggest that TNFα-induced mechanical sensitization of masseter muscle nociceptors is mediated in part by increased PGE(2) levels. The findings of this study support the hypothesis that TNFα induces a delayed mechanical sensitization of masseter muscle nociceptors indirectly by the release of PGE(2).

Attenuation of Sensory Transmission Through the Rat Trigeminal Ganglion by GABA Receptor Activation.

While the trigeminal ganglion is often considered a passive conduit of sensory transmission, neurons and satellite glial cells (SGCs) within it can release neurotransmitters and express neuroreceptors. Some trigeminal ganglion neurons contain the neurotransmitter γ-aminobutyric acid (GABA) and express GABA receptors. There is behavioral evidence that increased GABA levels in the trigeminal ganglion decreases nociception, while a loss of GABA receptors results in hyperalgesia, although the neural mechanisms for this remain to be investigated. In this study, the expression of GABA receptors by trigeminal ganglion neurons that innervate rat labial skin and masseter muscle was compared using immunohistochemistry. The effect of intraganglionic administration of GABA receptor agonists was investigated by single unit recording of trigeminal brainstem and ganglion neuron responses to stimulation of the labial skin and/or masseter muscle in anesthetized rats. The mean frequency of expression of GABAA and GABAB receptors by masseter and labial skin ganglion neurons was 62.5% and 92.7%, and 55.4% and 20.3%, respectively. The expression of both GABA receptors was significantly greater in skin ganglion neurons. Masticatory muscle evoked brainstem trigeminal neuron responses were significantly attenuated by intraganglionic injection of muscimol (GABAA) but not baclofen (GABAB). The mechanical sensitivity of slow and fast conducting masticatory muscle afferent fibers was decreased and increased, respectively, by intraganglionic injection of both muscimol and baclofen. Activation of GABAA receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

Sex-related differences in response to masseteric injections of glutamate and nerve growth factor in healthy human participants.

The neurophysiological mechanisms underlying NGF-induced masseter muscle sensitization and sex-related differences in its effect are not well understood in humans. Therefore, this longitudinal cohort study aimed to investigate the effect of NGF injection on the density and expression of substance P, NMDA-receptors and NGF by the nerve fibers in the human masseter muscle, to correlate expression with pain characteristics, and to determine any possible sex-related differences in these effects of NGF. The magnitude of NGF-induced mechanical sensitization and pain during oral function was significantly greater in women than in men (P < 0.050). Significant positive correlations were found between nerve fiber expression of NMDA-receptors and peak pain intensity (rs = 0.620, P = 0.048), and expression of NMDA-receptors by putative nociceptors and change in temporal summation pain after glutamate injection (rs = 0.561, P = 0.003). In women, there was a significant inverse relationship between the degree of NGF-induced mechanical sensitization and the change in nerve fiber expression of NMDA-receptors alone (rs = - 0.659, P = 0.013), and in combination with NGF (rs = - 0.764, P = 0.001). In conclusion, women displayed a greater magnitude of NGF-induced mechanical sensitization that also was associated with nerve fibers expression of NMDA-receptors, when compared to men. The present findings suggest that, in women, increased peripheral NMDA-receptor expression could be associated with masseter muscle pain sensitivity.

Nerve growth factor and glutamate increase the density and expression of substance P-containing nerve fibers in healthy human masseter muscles.

Nocifensive behavior induced by injection of glutamate or nerve growth factor (NGF) into rats masseter muscle is mediated, in part, through the activation of peripheral NMDA receptors. However, information is lacking about the mechanism that contributes to pain and sensitization induced by these substances in humans. Immunohistochemical analysis of microbiopsies obtained from human masseter muscle was used to investigate if injection of glutamate into the NGF-sensitized masseter muscle alters the density or expression of the NMDA receptor subtype 2B (NR2B) or NGF by putative sensory afferent (that express SP) fibers. The relationship between expression and pain characteristics was also examined. NGF and glutamate administration increased the density and expression of NR2B and NGF by muscle putative sensory afferent fibers (P < 0.050). This increase in expression was greater in women than in men (P < 0.050). Expression of NR2B receptors by putative sensory afferent fibers was positively correlated with pain characteristics. Results suggest that increased expression of peripheral NMDA receptors partly contributes to the increased pain and sensitivity induced by intramuscular injection of NGF and glutamate in healthy humans; a model of myofascial temporomandibular disorder (TMD) pain. Whether a similar increase in peripheral NMDA expression occurs in patients with painful TMDs warrants further investigation.