Sex-related differences in human pain and rat afferent discharge evoked by injection of glutamate into the masseter muscle

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Metabolic and behavioural effects of hermit crab shell removal techniques: Is heating less invasive than cracking?

Hermit crabs (Paguroidea; Latreille 1802) offer great opportunities to study animal behaviour and physiology. However, the animals' size and sex cannot be determined when they are inside their shell; information crucial to many experimental designs. Here, we tested the effects of the two most common procedures used to make crabs leave their shells: heating the shell apex and cracking the shell with a bench press. We compared the effects of each of the two procedures on the metabolic rate, hiding time, and duration of the recovery time relative to unmanipulated hermit crabs. The hermit crabs forced to abandon their shell through heating increased their respiratory rate shortly after the manipulation (1 h) and recovered their metabolic rate in less than 24 h, as occurs in individuals suddenly exposed to high temperatures in the upper-intertidal zone. Hermit crabs removed from their shells via cracking spent more time hiding in their new shells; this effect was evident immediately after the manipulation and lasted more than 24 h, similar to responses exhibited after a life-threatening predator attack. Both methods are expected to be stressful, harmful, or fear-inducing; however, the temperature required to force the crabs to abandon the shell is below the critical thermal maxima of most inhabitants of tropical tide pools. The wide thermal windows of intertidal crustaceans and the shorter duration of consequences of shell heating compared to cracking suggest heating to be a less harmful procedure for removing tropical hermit crabs from their shells.

Intramuscular injection of nerve growth factor as a model of temporomandibular disorder: nature, time-course, and sex differences characterising the pain experience

Background

Temporomandibular disorder (TMD) is a common condition that frequently transitions to chronic symptoms. Experimental pain models that mimic the symptoms of clinical TMD may be useful in understanding the mechanisms, and sex differences, present in this disorder. Here we aimed to comprehensively characterise the nature and time-course of pain, functional impairment and hyperalgesia induced by repeated intramuscular injection of nerve growth factor (NGF) into the masseter muscle, and to investigate sex differences in the NGF-induced pain experience.

Methods

94 healthy individuals participated in a longitudinal study with 30-day follow-up. NGF was injected into the right masseter muscle on Day 0 and Day 2. Participants attended laboratory sessions to assess pain (Numerical Rating Scale; NRS), functional limitation (mouth opening distance, Jaw Functional Limitation Scale; JFLS) and mechanical sensitization (pressure pain thresholds; PPTs) on Days 0, 2 and 5 and completed twice daily electronic pain dairies from Day 0 to day 30.

Results

Peak pain averaged 2.0/10 (95 % CI: 1.6-2.4) at rest and 4.3/10 (95 % CI: 3.9-4.8) on chewing. Pain-free mouth opening distance reduced from 5.0 cm (95 % CI: 4.8-5.1 cm) on Day 0 to 3.7 cm (95 % CI: 3.5-3.9 cm) on Day 5. The greatest reduction in PPTs was observed over the masseter muscle. Females experienced higher pain, greater functional impairment, and greater sensitivity to mechanical stimuli than males.

Conclusion

Intramuscular injection of NGF is a useful model with which to explore the mechanisms, and sex differences, present in clinical TMD.

[Hippocampus is involved in 17β-estradiol exacerbating experimental occlusal inter- ference-induced chronic masseter hyperalgesia in ovariectomized rats]

OBJECTIVE

To investigate the influence of chronic masseter hyperalgesia induced by 17β-estradiol (E2) and experimental occlusal interference (EOI) on underlying mechanism in hippocampus of ovariectomized (OVX) rats.

METHODS

In the study, 32 OVX rats were randomly divided into 4 groups (8 rats/group): The control group was OVX group, and 0 μg/d E2 (vehicle) injection was started 7 d after OVX without EOI; in the experimental group (1) OVX + E2 group, 80 μg/d E2 injection was started 7 d after OVX without EOI; in the experimental group (2) OVX + EOI group, vehicle injection was started 7 d after OVX and EOI was applied 17 d after OVX; in the experimental group (3) OVX + E2 + EOI group, 80 μg/d E2 injection was started 7 d after OVX and EOI was applied 17 d after OVX. Bilateral masseter muscle mechanical withdrawal thresholds were measured before OVX, 7 days after OVX (before E2 injection), 17 days after OVX (10 days after E2 injection and before EOI) and 24 days after OVX (7 days after EOI). Immunofluorescence staining was used to reveal phospho-extracellular signal regulated kinase 1/2 (p-ERK1/2)-positive neurons in CA3 of hippocampus. The protein expression of p-ERK1/2 in hippocampus was detected using Western Blot.

RESULTS

Compared with the control group [left side: (135.3±8.5) g, right side: (135.4±10.8) g], bilateral masseter muscle mechanical withdrawal thresholds of OVX+E2 group [left side: (113.3±5.6) g, right side: (112.5 ± 5.6) g] and OVX+EOI group [left side: (93.3±5.4) g, right side: 90.8±5.5) g] were decreased (P < 0.01). Bilateral masseter muscle mechanical withdrawal thresholds were significantly lower in OVX+E2+EOI group [left side: (81.2±6.2) g, right side: 79.8±7.7) g] than in the control, OVX+E2 and OVX+EOI groups (P < 0.05). The proportion of p-ERK1/2 positive neurons in the CA3 region of the hippocampus was increased in the control, OVX+E2, OVX+EOI and OVX+E2+EOI groups in turn, and the difference between the groups was statistically significant (P < 0.05). p-ERK1/2 protein expression was increased in the control, OVX+E2 and OVX+EOI groups in turn, but the difference was not statistically significant (P>0.05). p-ERK1/2 expression was significantly higher in OVX+E2+EOI group than in the other three groups (P < 0.05).

CONCLUSION

High concentration of E2 could exacerbated EOI-induced chronic masseter hyperalgesia in ovariectomized rats, and its central mechanism may be related to the upregulation of the phosphorylation of ERK1/2 in hippocampus.

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.

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.

Genistein reverses the effect of 17β-estradiol on exacerbating experimental occlusal interference-induced chronic masseter hyperalgesia in ovariectomised rats

BACKGROUND

Oro-facial pain is more prevalent in women than in men, and oestrogen may underlie this sex difference. Genistein reversed the potentiation of 17β-estradiol (E2) on glutamate-induced acute masseter nociceptive behaviour, but its role in dental experimental occlusal interference (EOI)-induced chronic masseter hyperalgesia remains unclear.

OBJECTIVE

This study aimed to investigate sex differences, and to explore the role and underlying mechanisms of genistein in E2-potentiated EOI-induced chronic masseter hyperalgesia in rats.

METHODS

Female and male rats were prepared to compare the sex differences of masseter hyperalgesia induced by EOI using a 0.4-mm-thick metal crown. Female rats were ovariectomised (OVX) and treated with E2 and genistein, followed by EOI. The head withdrawal threshold (HWT) was examined to assess masseter sensitivity. The protein expression of transient receptor potential vanilloid-1 (TRPV1) in the trigeminal ganglion (TG) was detected using western blotting. Immunofluorescence staining was used to reveal the colocalisation of oestrogen receptors (ERs) with TRPV1 and the percentage of TRPV1-positive neurons in the TG.

RESULTS

To some extent, female rats displayed enhanced sensitivity to EOI-induced chronic masseter hyperalgesia compared with males. Female rats showed the lowest HWT in the pro-oestrus phase. Pre-treatment with genistein antagonised E2 potentiation in EOI-induced masseter hyperalgesia and blocked the effect of E2 by downregulating TRPV1 protein expression and the percentage of TRPV1-positive neurons in the TG.

CONCLUSION

Female rats showed greater masseter hyperalgesia than males under EOI. Genistein antagonised the facilitation of EOI-induced chronic masseter hyperalgesia by E2 probably through inhibiting TRPV1 in the TG.

Drop homotopic effects of masseter-muscle pain on somatosensory sensitivity in healthy participants

Current pain classifications use 1.0-kg palpation of the masseter muscle to distinguish between “pain patients” and “healthy controls” but a thorough understanding of the normal physiological responses to various somatosensory stimuli is lacking. The aim of this study was to investigate somatosensory function of the skin over the masseter muscle in healthy participants that were divided into a masseter pain prone group (MPP) (n = 22) and non-MPP group (n = 22), according to the response to a 1.0-kg palpation. Quantitative sensory testing (QST) was performed at the skin above the right masseter muscle (homotopic). In an additional experiment, 13 individuals each from MPP and non-MPP received application of 60% topical lidocaine tape to the skin over the masseter muscle for 30 min. Immediately after, mechanical pain sensitivity (MPS), dynamic mechanical allodynia, and pressure pain threshold were tested. Homotopic MPS was significantly higher and PPTs significantly lower in MPP than in N-MPP (P < 0.05). Strikingly, no other differences in QST outcomes were observed between the groups (P > 0.05). After lidocaine application, no significant differences in homotopic MPS were observed between groups. The presence or absence of acute provoked pain in masseter muscle is exclusively associated with differences in homotopic MPS which is decreased following topical anesthesia.

The role of peripheral adenosine receptors in glutamate-induced pain nociceptive behavior

The role of peripheral adenosine receptors in pain is a controversial issue and seems to be quite different from the roles of spinal and central adenosine receptors. The present study is aimed at clarifying the role of these receptors in peripheral nociception. To clarify this, studies were done on Swiss mice with adenosine receptor agonists and antagonists. Nociceptive behavior was induced by subcutaneous injection of glutamate (10 μmol) into the ventral surface of the hind paw of mice. Statistical analyses were performed by one-way ANOVA followed by the Student-Newman-Keuls post hoc test. Results showed that intraplantar (i.pl.) administration of N6-cyclohexyl-adenosine (CHA), an adenosine A1 receptor agonist, at 1 or 10 μg/paw significantly reduced glutamate-induced nociception (p<0.01 and p<0.001 vs. vehicle, respectively, n=8-10). In contrast, i.pl. injection of hydrochloride hydrate (CGS21680, an adenosine A2A receptor agonist) (1 μg/paw) induced a significant increase in glutamate-induced nociception compared to the vehicle (p<0.05, n=8), while 4-(-2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a} {1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, an adenosine A2A receptor antagonist) (20 μg/paw) caused a significant reduction (p<0.05, n=7-8). There were no significant effects on i.pl. administration of four additional adenosine receptor drugs-8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A1 antagonist, 1-10 μg/paw), N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA, an A2B agonist, 1-100 μg/paw), alloxazine (an A2B antagonist, 0.1-3 μg/paw), and 2-hexyn-1-yl-N(6)-methyladenosine (HEMADO) (an A3 agonist, 1-100 μg/paw) (p>0.05 vs. vehicle for all tests). We also found that prior administration of DPCPX (3 μg/paw) significantly blocked the anti-nociceptive effect of CHA (1 μg/paw) (p<0.05, n=7-9). Similarly, ZM241385 (20 μg/paw) administered prior to CGS21680 (1 μg/paw) significantly blocked CGS21680-induced exacerbation of nociception (p<0.05, n=8). Finally, inosine (10 and 100 μg/paw), a novel endogenous adenosine A1 receptor agonist recently reported by our research group, was also able to reduce glutamate-induced nociception (p<0.001 vs. vehicle, n=7-8). Interestingly, as an A1 adenosine receptor agonist, the inosine effect was significantly blocked by the A1 antagonist DPCPX (3 μg/paw) (p<0.05, n=7-9) but not by the A2A antagonist ZM241385 (10 μg/paw, p>0.05). In summary, these results demonstrate for the first time that i.pl administration of inosine induces an anti-nociceptive effect, similar to that elicited by CHA and possibly mediated by peripheral adenosine A1 receptor activation. Moreover, our results suggest that peripheral adenosine A2A receptor activation presents a pro-nociceptive effect, exacerbating glutamate-induced nociception independent of inosine-induced anti-nociceptive effects.

Bone cancer-induced pain is associated with glutamate signalling in peripheral sensory neurons

We previously identified that several cancer cell lines known to induce nociception in mouse models release glutamate in vitro. Although the mechanisms of glutamatergic signalling have been characterized primarily in the central nervous system, its importance in the peripheral nervous system has been recognized in various pathologies, including cancer pain. We therefore investigated the effect of glutamate on intracellular electrophysiological characteristics of peripheral sensory neurons in an immunocompetent rat model of cancer-induced pain based on surgical implantation of mammary rat metastasis tumour-1 cells into the distal epiphysis of the right femur. Behavioural evidence of nociception was detected using von Frey tactile assessment. Activity of sensory neurons was measured by intracellular electrophysiological recordings in vivo. Glutamate receptor expression at the mRNA level in relevant dorsal root ganglia was determined by reverse transcription polymerase chain reaction using rat-specific primers. Nociceptive and non-nociceptive mechanoreceptor neurons exhibiting changes in neural firing patterns associated with increased nociception due to the presence of a bone tumour rapidly responded to sulphasalazine injection, an agent that pharmacologically blocks non-vesicular glutamate release by inhibiting the activity of the system x_C⁻ antiporter. In addition, both types of mechanoreceptor neurons demonstrated excitation in response to intramuscular glutamate injection near the femoral head, which corresponds to the location of cancer cell injection to induce the bone cancer-induced pain model. Therefore, glutamatergic signalling contributes to cancer pain and may be a factor in peripheral sensitization and induced tactile hypersensitivity associated with bone cancer-induced pain.

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Sterile water; a novel and promising human experimental craniofacial muscle pain model

BACKGROUND

The aim of this study was to investigate if intramuscular injection of sterile water can be used as a human experimental pain model that resembles clinical craniofacial muscle pain and to analyse if the effects differ between sexes.

METHODS

This randomised, double-blind, placebo-controlled cross-over study included 30 healthy age-matched women and men (23.6 ± 2.4 years). At three sessions, with at least one week of washout in between, 0.2 mL of either sterile water (test-substance), hypertonic saline (58.5 mg/mL; active control) or isotonic saline (0.9 mg/mL; passive control) was randomly injected into the right masseter muscle. Pain intensity (VAS) was continuously assessed during 5 min whereafter pain duration (s) and pain area (au) were calculated; pressure pain thresholds (PPT;kPa) were recorded every 5 minutes during 30 minutes.

RESULTS

Sterile water evoked pain of similar intensity (74.5 ± 49.9) as hypertonic saline (74.0 ± 50.5); whereas, isotonic saline evoked low-intensity pain (11.4 ± 23.4). The pain induced by sterile water and hypertonic saline had higher intensity (P < 0.001), longer duration (P < 0.001) and larger pain area (P < 0.001) than isotonic saline. There were no significant differences in any pain variable between sterile water and hypertonic saline. The PPT did not change significantly after any substance, except for in women 5 minutes after sterile water injection (P < 0.002). Pain duration was longer in the men for all substances (P < 0.006), while the pain area was larger in women after injection of hypertonic saline (P < 0.003).

CONCLUSION

These results indicate that pain evoked by sterile water resembles clinical muscle pain and may offer a novel and simpler alternative to hypertonic saline injections.

Kinematic and perceptual responses in heavy lifting and pulling: Are there differences between males and females?

This study investigated kinematic and perceptual differences between the sexes in a heavy lifting and pulling task. A 20 kg box was lifted from floor to chest height, and a 70 kg mannequin pulled across 20m. The effect of height, mass and average grip strength on kinematics and perceived workload was examined in 42 (19 females, 23 males) healthy individuals. A univariate linear regression analysis found females lifted with greater lumbar extension compared to males (p < 0.001), and adopted more hip (p = 0.006) and knee flexion (p = 0.036) in the pulling task. Females reported a greater perceived workload in both tasks (p < 0.001). After the multivariable analysis, only grip strength remained significant for perceived workload in the lift (p = 0.04), and height for knee flexion in the pull (p = 0.009). This highlights that height and strength are important factors driving kinematics and perceived workload. Clinicians may consider these factors in heavy manual tasks, more so than sex.

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.

Metabolomics in chronic pain research

BACKGROUND AND OBJECTIVE

Metabolomics deals with the identification and quantification of small molecules (metabolites) in biological samples. As metabolite levels can reflect normal or altered metabolic pathways, their measurement provides information to improve the understanding, diagnosis and management of diseases. Despite its immense potential, metabolomics applications to pain research have been sparse. This paper describes current metabolomics techniques, reviews published human metabolomics pain research and compares successful metabolomics research in other areas of medicine with the goal of highlighting opportunities offered by metabolomics to advance pain medicine.

DATABASES AND DATA TREATMENT

Non-systematic review.

RESULTS

Our search identified 19 studies that adopted a metabolomics approach in: fibromyalgia (7), chronic widespread pain (4), other musculoskeletal pain conditions (5), neuropathic pain (1), complex regional pain syndrome (1) and pelvic pain (1). The studies used either mass spectrometry or nuclear magnetic resonance. Most are characterized by small sample sizes. Some consistency has been found for alterations in glutamate and testosterone metabolism, and metabolic imbalances caused by the gut microbiome.

CONCLUSIONS

Metabolomics research in chronic pain is in its infancy. Most studies are at the pilot stage. Metabolomics research has been successful in other areas of medicine. These achievements should motivate investigators to expand metabolomics research to improve the understanding of the basic mechanisms of human pain, as well as provide tools to diagnose, predict and monitor chronic pain conditions. Metabolomics research can lead to the identification of biomarkers to support the development and testing of treatments, thereby facilitating personalized pain medicine.

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Factors associated with severity and resolution of intramuscular injection site pain following treatment for uncomplicated gonorrhoea: a prospective cohort study of GToG trial participants

Background:Intramuscular injection (IMI) pain reduces the acceptability of treatment, but data identifying those at highest risk of severe pain following an injection are limited. This study identified factors associated with the severity, duration and resolution of IMI pain following treatment for uncomplicated gonorrhoea. Methods: Data from a subset of participants (aged 16-70 years) recruited between October 2014 and November 2016 into the 'Gentamicin for the treatment of gonorrhoea' trial in England were analysed. Pain was measured using a visual analogue scale (VAS) immediately after injection. Self-reported duration of pain was collected at a scheduled 2-week follow-up visit. Results: Overall, 688 participants (82% male), reported a median pain VAS score of 23.5 (range 0-100), which was moderate-severe (VAS score >30) in 38% (262/688) of participants. Age [adjusted odds ratio (aOR) 0.86 per 5-yearly increase, (0.78-0.94)], gender [female cf. male, aOR 1.62 (1.18-2.24)], ethnicity [Asian cf. Caucasian, aOR 2.79 (1.72-4.51)], body mass index [aOR 0.80 per 5 kgm-2 increase, (0.72-0.89)] and antibiotic regimen [gentamicin cf. ceftriaxone, aOR 3.92, (3.12-4.93)] were associated with moderate-severe injection pain. The median duration of pain was 2 h (range 0-648). Gender [female cf. male, adjusted hazard ratio (aHR) 0.76 (0.57-1.00)], ethnicity [Mixed race cf. Caucasian, aHR 0.64 (0.45-0.92)], self-reported pain [aHR 0.97 per 10 mm VAS score increase, (0.95-0.99)] and symptom status [asymptomatic cf. symptomatic aHR 1.35 (1.12-1.64)] were associated with pain resolution. Conclusions: IMI pain is influenced by specific demographic, anthropometric and clinical factors. Knowledge of these factors will help identify those most at risk of pain.

The impact of statins on physical activity and exercise capacity: an overview of the evidence, mechanisms, and recommendations

PURPOSE

Statins are among the most widely prescribed medications worldwide. Considered the 'gold-standard' treatment for cardiovascular disease (CVD), statins inhibit HMG-CoA reductase to ultimately reduce serum LDL-cholesterol levels. Unfortunately, the main adverse event of statin use is the development of muscle-associated problems, referred to as SAMS (statin-associated muscle symptoms). While regular moderate physical activity also decreases CVD risk, there is apprehension that physical activity may induce and/or exacerbate SAMS. While much work has gone into identifying the epidemiology of SAMS, only recent research has focused on the extent to which these muscle symptoms are accompanied by functional declines. The purpose of this review is to provide an overview of possible mechanisms underlying SAMS and summarize current evidence regarding the relationship between statin treatment, physical activity, exercise capacity, and SAMS development.

METHODS

PubMed and Google Scholar databases were used to search the most relevant and up-to-date peer-reviewed research on the topic.

RESULTS

The mechanism(s) behind SAMS, including altered mitochondrial metabolism, reduced coenzyme Q10 levels, reduced vitamin D levels, impaired calcium homeostasis, elevated extracellular glutamate, and genetic polymorphisms, still lack consensus and remain up for debate. Our summation of the evidence leads us to suggest that the etiology of SAMS development is likely multifactorial. Our review also demonstrates that there is limited evidence for statins impairing exercise adaptations or reducing exercise capacity for the majority of the investigated populations.

CONCLUSION

The available evidence indicates that the benefits of engaging in physical activity while on statin medication largely outweigh the risks.

The pro-algesic effect of γ-aminobutyric acid (GABA) injection into the masseter muscle of healthy men and women

Background and aims

Preclinical studies have reported that activation of peripheral γ-aminobutyric acid A (GABAA) receptors may result in analgesia. The current study was conducted in young healthy men (n = 30) and women (n = 28) to determine whether injections of GABA into the masseter muscle reduce pain in a sex-related manner.

Methods

The effect of injection of GABA alone, or in combination with the non-inflammatory algogen glutamate, was assessed in two separate studies. Lorazepam, a positive allosteric modulator of the GABAA-receptor, was co-injected with GABA in both studies to explore the role of this receptor in muscle pain responses of healthy human volunteers. Masticatory muscle mechanical pain intensity was recorded on an electronic visual analogue scale (VAS) while muscle pain sensitivity was assessed by determining the pressure pain threshold (PPT), tolerance and maximal jaw opening (MJO) of the subjects prior to, and again after the various intramuscular injections.

Results

Intramuscular injection of GABA alone was reported to be significantly more painful, in a concentration related manner, than saline control injections, and this pain was further increased by co-injection of lorazepam with GABA. Co-injection of GABA with glutamate was found to significantly increase glutamate-evoked masseter muscle pain in men, but not in women. There was no effect of injections of either GABA alone, or GABA with glutamate, on PPT, tolerance or maximum jaw opening.

Conclusions

Injection of GABA into the human masseter muscle appears to excite nociceptors to produce muscle pain without a longer term effect on mechanical pain sensitivity in the muscle. The findings suggest that GABA-mediated pain in humans is produced through peripheral GABAA receptor activation. The mechanism underlying the sex-related difference in the effect of GABA on glutamate-evoked muscle pain was speculated to be due to a methodological artifact.

Implications

This study was designed to detect analgesic rather than algesic effects of peripherally administered GABA, and as a result, the concentration of glutamate chosen for injection was close to the maximal pain response for healthy women, based on previously determined pain-concentration response relationships for glutamate. This may explain the finding of greater pain in men than women, when GABA and glutamate were co-injected. Overall, the findings suggest that activation of peripheral GABAA receptors in human masticatory muscle produces pain, possibly due to depolarization of the masticatory muscle afferent fibers.

Comparison of masseter muscle referred sensations after mechanical and glutamate stimulation: a randomized, double-blind, controlled, cross-over study

Referred sensations (RS) are commonly found in various musculoskeletal pain conditions. Experimental studies have shown that RS can be elicited through glutamate injection and mechanical stimulation. Despite this, differences and similarities between these modalities in RS outcomes remain unclear. The aim of this study was to assess differences between mechanical-induced and glutamate injection-induced RS in the trigeminal region. The present randomized, double-blind, controlled, cross-over study recruited 60 healthy participants who were assessed in 2 different sessions. In both sessions, pressure was applied to the masseter muscle with 4 different forces (0.5, 1, 2, and 4 kg), and glutamate (1 mol/L or 0.25 mol/L) was injected into the same area. Participants rated their perceived masseter sensations and rated and drew any RS they experienced. No difference was found in number of participants reporting RS after glutamate injection compared with mechanical stimulation. More participants reported RS when the stimulus was painful compared with a nonpainful stimulus. Furthermore, it was shown that the more intense the stimulus, the higher the frequency of RS. Finally, RS centre-of-gravity location was similar between the 2 sessions. In summary, RS was elicited in healthy individuals through both modalities, and no differences in frequency of RS were observed in the orofacial region. Hence, RS does not seem to be modality-dependent, and only the painfulness of the stimulus caused an increase in frequency of RS. Finally, RS location for each participant was similar in both sessions possibly indicating a preferred location of referral. These findings may have implications for our understanding of RS in craniofacial pain conditions.

Amplification of glyceryl trinitrate-induced headache features by noxious craniofacial stimuli in pain-free healthy humans

AIM

Glyceryl trinitrate (GTN) provokes an immediate migraine-like headache, followed by a delayed migraine attack in migraineurs. In healthy volunteers, only an immediate, less severe and shorter headache occurs. The presence of an already sensitized nervous system in migraineurs may underlie the more intense and prolonged GTN-evoked headaches. We tested if in healthy humans, application of noxious cutaneous and/or mechanical stimulation within craniofacial region would enhance or prolong GTN-evoked headache.

MATERIALS & METHODS

Noxious stimuli with a capsaicin patch on forehead, a mechanical headband, or both were applied prior to sublingual GTN (0.5 mg) in 20 healthy volunteers. GTN-induced headache characteristics and sensory responsiveness were recorded.

RESULTS

A more intense GTN-evoked headache was produced following application of headband.

CONCLUSION

Noxious mechanical stimulation prior to GTN resulted in a more intense GTN-evoked headache.

Experimental noxious stimulation of the right masseter muscle does not affect bilateral masseter and temporalis muscle activity and force parameters during standardised isometric biting tasks

AIM

To determine if the electromyographic (EMG) activity of the left and right masseter and anterior temporalis muscles is altered by experimental right masseter muscle noxious stimulation during goal-directed isometric biting tasks in asymptomatic humans.

METHODS

Isometric biting tasks (slow and fast ramp biting tasks, 2-step biting task) were performed on an intraoral force transducer in 18 participants during the following blocks: baseline block, hypertonic saline infusion into the right masseter muscle (painful block) and isotonic saline infusion into the right masseter (control block). Bipolar surface electrodes recorded EMG activity from the bilateral masseter and anterior temporalis muscles. A 100-mm visual analogue scale (VAS) quantified pain intensity, and the McGill Pain Questionnaire (MPQ), the Depression, Anxiety and Stress Scales-21 (DASS-21) and the Pain Catastrophizing Scale (PCS) were completed. Repeated measures ANOVA assessed the effects of pain on the force rates (N/s), force amplitudes (N) and the root mean square (RMS) jaw muscle EMG activity across blocks. Statistical significance accepted at P < 0.05.

RESULTS

VAS scores were significantly (P < 0.001) higher during hypertonic than isotonic saline infusion blocks. There was no significant effect of pain on the force rates, or force levels or the RMS EMG activity of each masseter and anterior temporalis muscle.

CONCLUSION

The findings suggest that experimentally induced right masseter muscle pain does not modify force or surface jaw muscle EMG activity during isometric biting tasks.

Activation of rat masticatory muscle afferent fibres by acidic pH

Previous research findings have suggested an important role for acid sensing ion channels (ASICs) in muscle pain mechanisms. This study was conducted to determine if masticatory muscle afferent fibres express ASICs, if there are sex differences in this expression, and to compare the effects of low pH and hypertonic saline on afferent fibres that innervate the masticatory muscle in vivo. Immunohistochemistry methods were applied to examine the expression of ASICs in trigeminal ganglion neurons, while in vivo electrophysiology techniques were employed to examine changes in masticatory muscle afferent fibre excitability. Both ASIC₁ and ASIC₃ were expressed by predominantly larger masticatory muscle ganglion neurons, but the frequency of ASIC₃ expression (56%) was significantly greater than ASIC₁ (35%). No sex-related differences in expression were identified. Injection of pH 5.8, but not pH 6.8, phosphate buffered saline evoked afferent discharges that were significantly greater than those evoked by pH 7.4 buffer (control). Since ASIC₃ channels are not activated until the pH is around 6, these results indicate that activation of both channels contributes to excitation of masticatory muscle afferent fibres. The results further show that many masticatory muscle afferent fibres, which respond to low pH, are low threshold mechanoreceptors. These findings may explain why injection of low pH solutions into the masticatory muscles of healthy humans is not associated with significant muscle pain.

Dietary correlates of chronic widespread pain in Meru, Kenya

OBJECTIVE

To gather preliminary data examining whether dehydration and/or exposure to monosodium glutamate (MSG) may influence pain in Meru, Kenya.

METHODS

Two small pilot studies were conducted in Meru, Kenya after community report of high chronic pain in the region. First, a prevalence estimate was conducted among 89 participants from random households. Second, widespread chronic pain participants, recruited from the community, completed a demographic questionnaire, pain impact questionnaire, and 1-wk food/symptom diary. Those who returned the food/symptom diary 1 wk later were assigned to a treatment group based on their report of water/tea intake per day (<2 L versus ≥2 L) and reported use of mixed spices with MSG (yes/no). Groups received 1) water, 2) alternative spices without MSG, 3) water and alternative spices, or 4) acetaminophen (1000 mg twice daily) for those with neither exposure. Improvement was defined as ≥30% reduction in pain impact score.

RESULTS

Prevalence of chronic pain among 89 participants surveyed was 60%. Thirty participants were recruited for the intervention (90% female; average age 56 [ ± 18] y). The water and alternative spices and acetaminophen groups both experienced significant reductions in pain after 2 wk, with the greatest percentage of participants improving in the water and alternative spices group.

CONCLUSION

This pilot data suggests an abnormally high prevalence of pain in Meru, Kenya, and that MSG intake, combined with dehydration, may be contributing to chronic widespread pain in this region. Future research should include a formal pain prevalence estimate and a randomized controlled trial to further test this dietary intervention.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BACKGROUND

Chronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.

METHODS

Response properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.

RESULTS

Females have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.

CONCLUSIONS

Here, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

Does induced masseter muscle pain affect integrated jaw-neck movements similarly in men and women?

Normal jaw opening-closing involves simultaneous jaw and head-neck movements. We previously showed that, in men, integrated jaw-neck movements during jaw function are altered by induced masseter muscle pain. The aim of this study was to investigate possible sex-related differences in integrated jaw-neck movements following experimental masseter muscle pain. We evaluated head-neck and jaw movements in 22 healthy women and 16 healthy men in a jaw opening-closing task. The participants performed one control trial and one trial with masseter muscle pain induced by injection of hypertonic saline. Jaw and head movements were registered using a three-dimensional optoelectronic recording system. There were no significant sex-related differences in jaw and head movement amplitudes. Head movement amplitudes were significantly greater in the pain trials for both men and women. The proportional involvement of the neck motor system during jaw movements increased in pain trials for 13 of 16 men and for 18 of 22 women. Thus, acute pain may alter integrated jaw-neck movements, although, given the similarities between men and women, this interaction between acute pain and motor behaviour does not explain sex differences in musculoskeletal pain in the jaw and neck regions.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Trade-offs between predator avoidance and electric shock avoidance in hermit crabs demonstrate a non-reflexive response to noxious stimuli consistent with prediction of pain

Arthropods have long been thought to respond to noxious stimuli by reflex reaction. One way of testing if this is true is to provide the animal with a way to avoid the stimulus but to vary the potential cost of avoidance. If avoidance varies with potential cost then a decision making process is evident and the behaviour is not a mere reflex. Here we examine the responses of hermit crabs to electric shock within their shell when also exposed to predator or non-predator odours or to no odour. The electric shocks start with low voltage but increase in voltage with each repetition to determine how odour affects the voltage at which the shell is abandoned. There was no treatment effect on the voltage at which hermit crabs left their shells, however, those exposed to predator odours were less likely to evacuate their shells compared with no odour or low concentrations of non-predator odour. However, highly concentrated non-predator also inhibited evacuation. The data show that these crabs trade-off avoidance of electric shock with predator avoidance. They are thus not responding purely by reflex and the data are thus consistent with predictions of pain but do not prove pain.

Elevated Fractalkine (CX3CL1) Levels in the Trigeminal Ganglion Mechanically Sensitize Temporalis Muscle Nociceptors

It has been proposed that after nerve injury or tissue inflammation, fractalkine (CX3CL1) released from dorsal root ganglion neurons acts on satellite glial cells (SGCs) through CX3C receptor 1 (CX3CR1) to induce neuroplastic changes. The existence and importance of fractalkine/CX3CR1 signaling in the trigeminal ganglia has not yet been clarified. This study investigated (1) whether trigeminal ganglion neurons that innervate temporalis muscle and their associated SGCs contain fractalkine and/or express CX3CR1, (2) if intraganglionic injection of fractalkine increases the mechanical sensitivity of temporalis muscle afferent fibers, (3) whether complete Freund's adjuvant (CFA)-induced inflammation of the temporalis muscle alters the expression of fractalkine or its receptor in the trigeminal ganglion, and (4) if intraganglionic administration of CX3CR1 antibodies alters afferent mechanical sensitivity. Immunohistochemistry and in vivo electrophysiological recordings in male and female rats were used to address these questions. It was found that ∼50 % of temporalis ganglion neurons and ∼25 % of their associated SGCs express CX3CR1, while only neurons expressed fractalkine. Temporalis muscle inflammation increased the expression of fractalkine, but only in male rats. Intraganglionic injection of fractalkine (25 g/ml; 3 μl) induced prolonged afferent mechanical sensitization. Intraganglionic injection of CX3CR1 antibody increased afferent mechanical threshold, but this effect was greater in controls than in rats with CFA-induced muscle inflammation. These findings raise the possibility that basal fractalkine signalling within the trigeminal ganglion plays an important role in mechanical sensitivity of masticatory muscle sensory afferent fibers and that inhibition of CX3CR1 signaling within the trigeminal ganglia may induce analgesia through a peripheral mechanism.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1-2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1-2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2 μg/day, LE) or high-dose estradiol (20 μg/day, HE) for 2 days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1-2 surface at the site of recording 10 min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500 μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50 μM CPCCOEt, while 500 μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1-2 region and are largely independent of estrogen status.

Intradermal glutamate and capsaicin injections: intra- and interindividual variability of provoked hyperalgesia and allodynia

Intradermal injections of glutamate and capsaicin are attractive to use in human experimental pain models because hyperalgesia and allodynia mimic isolated aspects of clinical pain disorders. The aim of the present study was to investigate the reproducibility of these models. Twenty healthy male volunteers (mean age 24 years; range 18-38 years) received intradermal injections of glutamate and capsaicin in the volar forearm. Magnitudes of secondary pinprick hyperalgesia and brush-evoked allodynia were investigated using von Frey filaments (gauges 10, 15, 60 and 100 g) and brush strokes. Areas of secondary hyperalgesia and allodynia were quantified immediately after injection and after 15, 30 and 60 min. Two identical experiments separated by at least 7 days were performed. Reproducibility across and within volunteers (inter- and intra-individual variation, respectively) was assessed using intraclass correlation coefficient (ICC) and coefficient of variation (CV). Secondary pinprick hyperalgesia was observed as a marked increase in the visual analogue scale (VAS) response to von Frey gauges 60 and 100 g (P < 0.001) after glutamate injection. For capsaicin, secondary pinprick hyperalgesia was detected with all von Frey gauges (P < 0.001). Glutamate evoked reproducible VAS response to all von Frey gauges (ICC > 0.60) and brush strokes (ICC > 0.83). Capsaicin injection was reproducible for secondary hyperalgesia (ICC > 0.70) and allodynia (ICC > 0.71). Intra-individual variability was generally lower for the VAS response to von Frey and brush compared with areas of secondary hyperalgesia and allodynia. In conclusion, glutamate and capsaicin yield reproducible hyperalgesic and allodynic responses, and the present model is well suited for basic research, as well as for assessing the modulation of central phenomena.

Chronic musculoskeletal pain: review of mechanisms and biochemical biomarkers as assessed by the microdialysis technique

Chronic musculoskeletal pain conditions are multifaceted, and approximately 20% of the adult population lives with severe chronic pain, with a higher prevalence in women and in lower income groups. Chronic pain is influenced by and interacts with physical, emotional, psychological, and social factors, and a biopsychosocial framework is increasingly applied in clinical practice. However, there is still a lack of assessment procedures based on the activated neurobiological pain mechanisms (ie, the biological part of the biopsychosocial model of pain), which may be a necessary step for further optimizing outcomes after treatments for patients with chronic pain. It has been suggested that chronic pain conditions are mainly driven by alterations in the central nervous system with little or no peripheral stimuli or nociception. In contrast, other authors argue that such central alterations are driven by peripheral alterations and nociceptive input. Microdialysis is an in vivo method for studying local tissue alterations and allows for sampling of substances in the interstitium of the muscle, where nociceptor free nerve endings are found close to the muscle fibers. The extracellular matrix plays a key role in physiologic functions of cells, including the primary afferent nociceptor. The present review mainly concerns the results of microdialysis studies and how they can contribute to the understanding of activated peripheral nociceptive and pain mechanisms in humans with chronic pain. The primary aim was to review molecular studies using microdialysis for the investigation of human chronic muscle pain, ie, chronic masticatory muscle pain, chronic trapezius myalgia, chronic whiplash-associated disorders, and chronic widespread pain/fibromyalgia syndrome. Several studies clearly showed elevated levels of serotonin, glutamate, lactate, and pyruvate in localized chronic myalgias and may be potential biomarkers. These results indicate that peripheral muscle alterations are parts of the activated pain mechanisms in common chronic pain conditions. Muscle alterations have been reported in fibromyalgia syndrome and chronic widespread pain, but more studies are needed before definite conclusions can be drawn. For other substances, results are inconclusive across studies and patient groups.

Bone hyperalgesia after mechanical impact stimulation: a human experimental pain model

Hyperalgesia in different musculoskeletal structures including bones is a major clinical problem. An experimental bone hyperalgesia model was developed in the present study. Hyperalgesia was induced by three different weights impacted on the shinbone in 16 healthy male and female subjects. The mechanical impact pain threshold (IPT) was measured as the height from which three weights (165, 330, and 660 g) should be dropped to elicit pain at the shinbone. Temporal summation of pain to repeated impact stimuli was assessed. All these stimuli caused bone hyperalgesia. The pressure pain threshold (PPT) was assessed by a computerized pressure algometer using two different probes (1.0 and 0.5 cm(2)). All parameters were recorded before (0), 24, 72, and 96 h after the initial stimulations. The IPTs were lowest 24 h after hyperalgesia induction for all three weights and the effect lasted up to 72 h (p < 0.05). The PPT obtained with the 1.0 cm(2) probe was significantly lower than the PPT obtained with the 0.5 cm(2) probe, regardless of the time. Females developed more pronounced hyperalgesia reflected in reduced IPTs and PPTs (p < 0.05). Temporal summation was significantly (p < 0.05) facilitated after induction of hyperalgesia with the strongest facilitation in males. The developed bone pain and hyperalgesia model may provide the basis for studying this fundamental mechanism of bone-related hyperalgesia and be used for profiling compounds developed for this target.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

Time course analysis of the effects of botulinum neurotoxin type A on pain and vasomotor responses evoked by glutamate injection into human temporalis muscles

The effect of botulinum neurotoxin type A (BoNTA) on glutamate-evoked temporalis muscle pain and vasomotor responses was investigated in healthy men and women over a 60 day time course. Subjects participated in a pre-BoNTA session where their responses to injection of glutamate (1 M, 0.2 mL) and saline (0.2 mL) into the temporalis muscles were assessed. On Day 1, BoNTA (5 U) was injected into one temporalis muscle and saline into the contralateral temporalis muscle, in a randomized order. Subjects then received intramuscular injections of glutamate (1 M, 0.2 mL) into the left and right temporalis muscles at 3 h and subsequently 7, 30 and 60 days post-injection of BoNTA. Pain intensity, pain area, and neurogenic inflammation (skin temperature and skin blood perfusion) were recorded. Prior to BoNTA treatment, glutamate evoked significantly greater pain and vasomotor reactions (P < 0.001) than saline. BoNTA significantly reduced glutamate-evoked pain intensity (P < 0.05), pain area (P < 0.01), skin blood perfusion (P < 0.05), and skin temperature (P < 0.001). The inhibitory effect of BoNTA was present at 3 h after injection, peaked after 7 days and returned to baseline by 60 days. Findings from the present study demonstrated a rapid action of BoNTA on glutamate-evoked pain and neurogenic inflammation, which is in line with animal studies.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Pain and temporomandibular disorders: a pharmaco-gender dilemma

Gender is the biggest risk factor in the development of temporomandibular disorders (TMD) and orofacial pain. Gender differences in pain thresholds, temporal summation, pain expectations, and somatic awareness exist in patients with chronic TMD or orofacial pain. There are gender differences in pharmacokenetics and pharmacodynamics of medications used to treat pain. A better understanding of the mechanisms that contribute to the increased incidence and persistence of chronic pain in females is needed. Future research will elucidate the sex effects on factors that protect against developing pain or prevent debilitating pain. Gender-based treatments for TMD and orofacial pain treatment will evolve from the translational research stimulated by this knowledge.

Fatigue-enhanced hyperalgesia in response to muscle insult: induction and development occur in a sex-dependent manner

Chronic muscle pain affects 20-50% of the population, is more common in women than men, and is associated with increased pain during physical activity and exercise. Muscle fatigue is common in people with chronic muscle pain, occurs in response to exercise, and is associated with release of fatigue metabolites. Fatigue metabolites can sensitize muscle nociceptors, which could enhance pain with exercise. Using a mouse model we tested whether fatigue of a single muscle, induced by electrical stimulation, resulted in enhanced muscle hyperalgesia and if the enhanced hyperalgesia was more pronounced in female mice. Muscle fatigue was induced in combination with a sub-threshold muscle insult (2 injections of pH 5.0 saline) in male and female mice. We show that male and female mice, fatigued immediately prior to muscle insult in the same muscle, develop similar muscle hyperalgesia 24 hours later. However, female mice also develop hyperalgesia when muscle fatigue and muscle insult occur in different muscles, and when muscle insult is administered 24 hours after fatigue in the same muscle. Further, hyperalgesia lasts significantly longer in females. Finally, muscle insult with or without muscle fatigue results in minimal inflammatory changes in the muscle itself, and sex differences are not related to estradiol (ovariectomy) or changes in brainstem activity (pNR1). Thus, the current model mimics muscle fatigue-induced enhancement of pain observed in chronic muscle pain conditions in the human population. Interactions between fatigue and muscle insult may underlie the development of chronic widespread pain with an associated female predominance observed in human subjects.

Transdermal hormonal therapy in perimenstrual migraine: why, when and how?

Experimental and clinical evidence is strongly in favor of a role for estrogens in migraine. It is clear that estrogen fluctuations represent trigger factors for the attacks, while the resolution of these fluctuations (menopause) may be associated to the remission or, conversely, to the worsening of the disease. However, the exact mechanisms and mediators underlying the effects of estrogens in migraine are largely unknown. The exact mechanisms and mediators underlying the effects of estrogens in migraine are largely unknown. In this review, we summarize clinical and preclinical data that are relevant for the role of estrogens in migraine and we discuss how estrogen modulation can be exploited positively to improve hormonal-related migraine.