Brian E. Cairns

Injection of nerve growth factor into human masseter muscle evokes long-lasting mechanical allodynia and hyperalgesia

&NA; Nerve growth factor (NGF) is a neurotrophic protein with a pivotal role in development and maintenance of the nervous system on one side and inflammatory and neuropathic pain states on the other. NGF causes clear signs of behavioral hyperalgesia in animal models and following intradermal and systemic administration in humans. The present double‐blinded, placebo‐controlled study was designed to test quantitatively the effect and duration (1 h, 1, 7, 14, 21 and 28 days) of NGF (5 &mgr;g in 0.2 ml) injected into the masseter muscle. Pressure pain thresholds (PPT) and pressure tolerance thresholds (PTOL) were used as indices of mechanical allodynia and hyperalgesia in the jaw‐closing muscles. In addition, perceived pain intensity was assessed by the subjects on a 0–10 numerical rating scale (NRS) with the jaw at rest and in relation to various oral functions (chewing, yawning, talking, swallowing, drinking and smiling). Repeated measures analysis of variance (ANOVA) was used to test for significant effects. Injection of NGF into the masseter muscle was associated with significantly reduced PPT for 7 days (ANOVA: P<0.001) and PTOL for 1 day (P<0.001). Buffered isotonic saline injections into the masseter muscle also significantly lowered the PPT after 1 day but to a significantly smaller extent than the NGF injections (P<0.001) and isotonic saline had no significant effects on PTOL. In contrast, assessment of PPT and PTOL in the non‐injected temporalis muscles demonstrated a significant increase after 14–28 days (P<0.001), which may have reflected an adaptation to the test procedure. NRS scores of chewing and yawning were significantly increased for 7 days following NGF injection (P<0.001). Systemic adverse effects were noted in one subject who reported fever and slight discomfort about 8 h after the NGF injection. In conclusion, this is the first study to show that injection of NGF into the human masseter muscle causes local signs of mechanical allodynia and hyperalgesia that persist for at least 7 days as well as pain during strenuous jaw movement. The present pain model is safe and may be used to gain further insight into the neurobiological mechanisms of muscle pain and sensitization.

Glutamate-evoked pain and mechanical allodynia in the human masseter muscle

&NA; The present study examined the effect of peripheral administration of the excitatory amino acid (EAA) glutamate on the intensity of perceived pain and pressure pain thresholds (PPTs) in healthy young women (n=17) and men (n=18). Two injections separated by 25 min of 0.2 ml, 1.0 M glutamate into the masseter muscle produced significantly higher scores of pain on 0–10 cm visual analogue scales (VAS) in women than in men (analysis of variance, ANOVA: P<0.001). There was no significant difference between the VAS scores for the first and the second injections in either men or women. The PPTs determined in the masseter muscle were significantly reduced following the first injection and further significantly reduced after the second injection (ANOVA: P<0.001). Furthermore, the PPTs were reduced to a similar extent in both women and men (maximum 44–56%), suggesting that gender did not influence the process of sensitization. There were no significant difference in VAS scores or PPTs between women taking oral contraceptives (n=9) and those who did not (n=8) (ANOVAs: P=0.709, P=0.153). It is concluded that the VAS scores produced by intramuscular administration of 1.0 M glutamate may reflect a gender‐dependent activation of nociceptive pathways which, in part, may be mediated through peripheral EAA receptors. The reduction of PPTs in the masseter muscle following administration of glutamate in a concentration of 1.0 M may reflect allodynia to mechanical stimuli. This process of sensitization was not gender‐dependent. The present results suggest that injection of 1.0 M glutamate into the masseter muscle may provide a useful experimental method to test sensitization and efficacy of peripheral EAA receptor antagonists in human subjects.

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

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

Glutamate-induced sensitization of rat masseter muscle fibers.

In rats, intradermal or intraarticular injection of glutamate or selective excitatory amino acid receptor agonists acting at peripheral excitatory amino acid receptors can decrease the intensity of mechanical stimulation required to evoke nocifensive behaviors, an indication of hyperalgesia. Since excitatory amino acid receptors have been found on the terminal ends of cutaneous primary afferent fibers, it has been suggested that increased tissue glutamate levels may have a direct sensitizing effect on primary afferent fibers, in particular skin nociceptors. However, less is known about the effects of glutamate on deep tissue afferent fibers. In the present study, a series of experiments were undertaken to investigate the effect of intramuscular injection of glutamate on the excitability and mechanical threshold of masseter muscle afferent fibers in anesthetized rats of both sexes. Injection of 1.0 M, but not 0.1 M glutamate evoked masseter muscle afferent activity that was significantly greater than that evoked by isotonic saline. The mechanical threshold of masseter muscle afferent fibers, which was assessed with a Von Frey hair, was reduced by approximately 50% for a period of 30 min after injection of 1.0 M glutamate, but was unaffected by injections of 0.1 M glutamate or isotonic saline. Injection of 25% dextrose, which has the same osmotic strength as 1.0 M glutamate, did not evoke significant activity in or decrease the mechanical threshold of masseter muscle afferent fibers. Magnetic resonance imaging experiments confirmed that injection of 25% dextrose and 1.0 M glutamate produced similar edema volumes in the masseter muscle tissue. Co-injection of 0.1 M kynurenate, an excitatory amino acid receptor antagonist, and 1.0 M glutamate attenuated glutamate-evoked afferent activity and prevented glutamate-induced mechanical sensitization. When male and female rats were compared, no difference in the baseline mechanical threshold or in the magnitude of glutamate-induced mechanical sensitization of masseter muscle afferent fibers was observed; however, the afferent fiber activity evoked by injection of 1.0 M glutamate into the masseter muscle was greater in female rats. The results of the present experiments show that intramuscular injection of 1.0 M glutamate excites and sensitizes rat masseter muscle afferent fibers through activation of peripheral excitatory amino acid receptors and that glutamate-evoked afferent fiber activity, but not sensitization, is greater in female than male rats.

Neural mechanisms of temporomandibular joint and masticatory muscle pain: A possible role for peripheral glutamate receptor mechanisms

The purpose of the present review is to correlate recent knowledge of the role of peripheral ionotropic glutamate receptors in the temporomandibular joint and muscle pain from animal and human experimental pain models with findings in patients. Chronic pain is common, and many people suffer from chronic pain conditions involving deep craniofacial tissues such as temporomandibular disorders or fibromyalgia. Animal and human studies have indicated that the activation of peripheral ionotropic glutamate receptors in deep craniofacial tissues may contribute to muscle and temporomandibular joint pain and that sex differences in the activation of glutamate receptors may be involved in the female predominance in temporomandibular disorders and fibromyalgia. A peripheral mechanism involving autocrine and/or paracrine regulation of nociceptive neuronal excitability via injury or inflammation-induced release of glutamate into peripheral tissues that may contribute to the development of craniofacial pain is proposed.

Sex-related differences in NMDA-evoked rat masseter muscle afferent discharge result from estrogen-mediated modulation of peripheral NMDA receptor activity.

In the present study, the hypothesis that sex-related differences in glutamate-evoked rat masseter muscle afferent discharge may result from estrogen-related modulation of peripheral N-methyl-d-aspartate (NMDA) receptor activity and/or expression was tested by examining afferent fiber discharge in response to masseter injection of NMDA and the expression of NR2A/B subunits by masseter ganglion neurons in male and female rats. The results showed that injection of NMDA into the masseter muscle evoked discharges in putative mechanonociceptive afferent fibers and increased blood pressure that was concentration-dependent, however, a systemic action of NMDA appeared responsible for increased blood pressure. NMDA-evoked afferent discharge was significantly greater in female than in male rats, was positively correlated with plasma estrogen levels in females and was significantly greater in ovariectomized female rats treated with a high dose (5 mug/day) compared with a low dose (0.5 mug/day) of estrogen. Pre-treatment of high dose estrogen-treated-ovariectomized female rats with the Src tyrosine kinase inhibitor PP2 did not affect NMDA-evoked afferent discharge. NMDA-evoked afferent discharge was attenuated by the antagonists ketamine and ifenprodil, which is selective for NR2B containing NMDA receptors. Fewer masseter ganglion neurons expressed the NR2A (16%) subunit as compared with the NR2B subunit (38%), which was expressed at higher frequencies in intact female (46%) and high dose estrogen-treated ovariectomized female (60%) rats than in male (31%) rats. Taken together, these results suggest that sex-related differences in NMDA-evoked masseter afferent discharge are due, at least in part, to an estrogen-mediated increase in expression of peripheral NMDA receptors by masseter ganglion neurons in female rats.

Sex-related differences in pain

This article provides an overview of sex-related differences in musculoskeletal pain and the role sex hormones and response to analgesic drugs may play in these differences. Some common pain conditions that include temporomandibular disorders, rheumatoid arthritis, fibromyalgia syndrome and tension-type and migraine headaches, show fairly marked sex-related differences in their occurrence, however, with the exception of rheumatoid arthritis, these pain conditions are also characterized by a lack of understanding of their basic underlying pathophysiology. The association of pain symptoms of these musculoskeletal pain conditions with the reproductive cycle of women is strongly suggestive of a role of the estrogens and/or progesterones, the main female sex hormones, in sex-related differences in pain. Nevertheless, an alternative suggestion that testosterone, the major male sex hormone, protects men from these chronic musculoskeletal pain conditions, has also been made. Indeed, emerging evidence suggests that both male and female sex hormones may contribute to the marked sex-related differences in the occurrence of certain musculoskeletal pain conditions. Men and women also appear to differ in response to pain treatment with certain analgesic drugs. The mechanistic basis for these sex-related differences is not entirely understood but sex hormones are thought to be one of the influencing factors. An improved understanding of mechanisms which underlie sex-related differences in musculoskeletal pain and response to analgesic drugs should permit improved pain management strategies for male and female musculoskeletal pain patients in the clinical setting.

Effects of subcutaneous administration of glutamate on pain, sensitization and vasomotor responses in healthy men and women

Abstract The present study aimed to investigate if (1) subcutaneous injection of glutamate induces pain, sensitization and vasomotor responses in humans and (2) if sex differences exist in these responses. Thirty healthy volunteers (men‐15 and women‐15) were included. Each subject received four subcutaneous injections (0.1 ml; glutamate 100, 10, 1 mM and isotonic saline 0.9%) into the forehead skin in two sessions separated by one week. Assessments of pain intensity (VAS), quality, distribution; area of pinprick hyperalgesia; pressure pain threshold (PPT) at the injection site; surface skin temperature and local blood flow were performed at predetermined time points. The highest concentration of glutamate evoked the highest pain intensity, the longest duration of pain and the largest pain area under the VAS–time curve (P < 0.001) in both men and women, although responses in women were larger than in men (P < 0.05). The face‐chart pain area was the largest for the highest concentration of glutamate (P < 0.001) and women drew a larger pain area than men (P = 0.024). The area of pinprick hyperalgesia was the largest for glutamate 100 mM (P < 0.001) and women indicated a larger area than men (P < 0.001). Concentration‐dependent local vasomotor responses were found following the subcutaneous injection of glutamate but there was no sex difference in this effect. Glutamate 100 mM significantly reduced the PPT values (P < 0.001) without sex‐related differences. The present study demonstrates for the first time that subcutaneous injection of glutamate evokes pain, vasomotor responses and pinprick hyperalgesia in human volunteers and that there are sex‐related differences in some of these responses.

Botulinum neurotoxin type A (BoNTA) decreases the mechanical sensitivity of nociceptors and inhibits neurogenic vasodilation in a craniofacial muscle targeted for migraine prophylaxis

&NA; The mechanism by which intramuscular injection of BoNTA into the craniofacial muscles decreases migraine headaches is not known. In a blinded study, the effect of BoNTA on the mechanical and chemical responsiveness of individual temporalis muscle nociceptors and muscle neurogenic vasodilation was investigated in female rats. Mechanical threshold was measured for 3 h following intramuscular injection of BoNTA or vehicle, and for 10 min after a subsequent injection of the algogen glutamate. Injection of BoNTA significantly increased the mechanical threshold of muscle nociceptors without altering the muscle surface temperature and blocked glutamate‐induced mechanical sensitization and neurogenic vasodilation. None of these effects were reproduced by pancuronium‐induced muscle paralysis. Western blot analysis of temporalis muscles indicated that BoNTA significantly decreased SNAP‐25. Measurement of interstitial glutamate concentration with a glutamate biosensor indicated that BoNTA significantly reduced glutamate concentrations. The mechanical sensitivity of muscle nociceptors is modulated by glutamate concentration through activation of peripheral NMDA receptors. Immunohistochemical experiments were conducted and they indicated that half of the NMDA‐expressing temporalis nerve fibers co‐expressed substance P or CGRP. Additional electrophysiology experiments examined the effect of antagonists for NMDA, CGRP and NK1 receptors on glutamate‐induced effects. Glutamate‐induced mechanical sensitization was only blocked by the NMDA receptor antagonist, but muscle neurogenic vasodilation was attenuated by NMDA or CGRP receptor antagonists. These data suggest that injection of BoNTA into craniofacial muscles acts to decrease migraine headaches by rapidly decreasing the mechanical sensitivity of temporalis muscle nociceptors through inhibition of glutamate release and by attenuating the provoked release of CGRP from muscle nociceptors.

Osmotic effects on the T2 relaxation decay of in vivo muscle

Saline solutions are commonly employed as a vehicle for drugs administered intramuscularly. In this study, in vivo measurements of spin‐spin relaxation (T2) processes by magnetic resonance imaging (MRI) were performed to investigate the distribution of water in rat masseter muscle tissue after intramuscular injection of saline solutions of varying tonicity. Prior to saline injection, image‐based T2 relaxation decay of muscle was monoexponential. After injection of saline, the T2 relaxation decay became multiexponential. Non‐negative least squares (NNLS) analysis of the decay curves revealed two relaxation components: a fast component (T2 = 20–40 ms) and a slow component (T2 = 150–400 ms), which are assigned to intra‐ and extracellular water protons, respectively. Injection of hypertonic saline solutions significantly increased the extracellular water component in muscle tissue compared to isotonic saline solutions, an effect which lasted for more than 60 min. These findings suggest that MRI techniques may be useful to investigate the effect of hyper‐ or hypotonic solutions on muscle tissue in vivo. Magn Reson Med 46:592–599, 2001.