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

β2-adrenergic receptor activation decreases the mechanical sensitivity of rat masticatory muscle afferent fibres

BACKGROUND

Activation of β2 adrenergic receptors reduces cutaneous mechanical pain thresholds in rats. While β2 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 β 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 β1 or β2 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 β adrenergic receptor agonists into masticatory muscle.

RESULTS

β1 and β2 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 β2 adrenergic receptors in masticatory muscle ganglion neurons in males than in females. The mixed β agonist isoproterenol increased afferent mechanical activation threshold in male but not female rats (p < .05, Mann-Whitney test). In male rats, salbutamol, a β2 selective agonist, also increased afferent mechanical activation threshold but hydralazine, a vasodilator, did not (p < .05, Mann-Whitney test).

CONCLUSION

Activation of β2 adrenergic receptors decreases the mechanical sensitivity of masticatory muscle afferent fibres in a sex-related manner.

Pericranial Muscle Stiffness, Pain Thresholds, and Tenderness during a Treatment Cycle of OnabotulinumtoxinA for Chronic Migraine Prevention

BACKGROUND

Treatment with OnabotulinumtoxinA (BoNT-A) is effective as a preventive treatment for chronic migraine (CM). Preclinical studies suggest that the mechanism of action of BoNT-A in migraine is based on blocking unmyelinated C fibers. We aimed to investigate whether the muscle-relaxing effect of BoNT-A is associated with the preventive mechanism in patients with chronic migraine by measuring the stiffness, pain thresholds, and tenderness of the BoNT-A-applied muscles.

METHODS

A total of 22 patients with CM who were already in BoNT-A treatment participated in this longitudinal prospective study. Pericranial muscle stiffness was measured using ultrasound shear wave elastography, which measures the speed of shear waves propagating through the muscle. Pressure pain thresholds (PPT) were obtained via algometry, and muscle tenderness was measured via manual palpation. Measurements were made before BoNT-A injections and six weeks after the treatment. The measurements were performed while the muscles were maximally relaxed. The patients also completed daily diaries on headache and neck pain.

RESULTS

No change was observed in muscle stiffness (p = 0.737) or pericranial muscle tenderness (p = 0.400). The PPT over the trapezius muscles increased from 250 kPa before treatment to 304 kPa six weeks after treatment (p = 0.027). No change was observed on the temporalis muscles (p = 0.200) nor the non-dominant index finger (p = 0.067). BoNT-A decreased neck pain (p = 0.008) and headache (p = 0.007).

CONCLUSIONS

The findings suggest that BoNT-A leads to the desensitization of cutaneous and muscle nociceptors in the head and neck regions, whereas muscle relaxation might not be an important part of the anti-migraine effect.

Clinical efficacy of escitalopram combined with botulinum toxin A in patients with generalized anxiety disorder and comorbid headache

BACKGROUND

Generalized anxiety disorder (GAD) is a common mental disorder that happens comorbidly with other diseases. Headache is a common anxiety comorbidity. Previous reports have shown that the selection of therapeutic drugs for GAD patients and comorbid headache is challenging. Therefore, our study aimed to investigate the clinical efficacy of escitalopram combined with botulinum toxin A (BoNT/A) in patients with GAD and comorbid headache and seek an alternative treatment strategy for the comorbidity of GAD and headache.

METHODS

A prospective, randomized controlled, double-blind study was performed. The eligible GAD patients with comorbid headache were randomly assigned to the BoNT/A group and the placebo group. All the patients were given oral escitalopram therapy (10-20 mg/day) for the whole duration of the study. The BoNT/A group was given local injections of BoNT/A (50 U per person), whereas the placebo group was given local saline (0.9% NaCl) injections at the beginning and 3 months after the experiments. All participants were followed up for 6 months and relevant information was collected at months 0, 1, 2, 3, and 6. Primary outcomes included the following: (1) the Generalized Anxiety Disorder 7 (GAD-7); (2) the Self-rating Anxiety Scale (SAS); (3) the Hamilton Anxiety Rating Scales (HAMA); (4) days with headache per month; (5) visual analogue scale (VAS).

RESULTS

A total of 101 patients (the sex ratio of female to male: 3.39:1) were finally included. Compared with the placebo group, the BoNT/A group showed a significant decrease in GAD-7 scores, SAS scores, HAMA scores, days with headache per month, and VAS scores at months 1, 2, 3, and 6 of follow-up (all p < 0.05). The average time to complete remission of anxiety symptoms (HAMA< 7 points) in the BoNT/A group was less than the placebo group (2 months vs. 3 months). At the same time, the results of the survival analysis showed a clear beneficial effect of BoNT/A relative to placebo on the time to remission of anxiety (log-rank test, p < 0.001). Mean daily doses of escitalopram at the sixth month in the BoNT/A group was smaller than the placebo group (12.5 mg vs. 16.04 mg, p < 0.001). The number of patients who relapsed (HAMA total score ≥ 14 points) at 6 months of follow-up in the BoNT/A group was less than the placebo group (2.2% vs. 14.9%, p < 0.05). The rates of response (HAMA subtraction rate ≥ 50%) were 93.8% for the BoNT/A group and 75.5% for the placebo group (p < 0.05), and the rates of remission (HAMA < 7 points) were 87.5% for the BoNT/A group and 64.2% for the placebo group (p < 0.01) at the sixth month.

CONCLUSION

The combination of escitalopram with BoNT/A is a significantly effective intervention in improving clinical efficacy and reducing the recurrence in patients with GAD and comorbid headache, and we believe that this approach will be an additional treatment strategy for future treatment of comorbid headache in GAD. Therefore, we recommend that escitalopram combined with BoNT/A should be given as early as possible in GAD patients and comorbid headache.

New anatomical insights of the superficial branch of the zygomaticotemporal nerve for treating temporal migraines: An anatomical study

The zygomaticotemporal nerve is known to contribute to temporal migraines; however, its precise anatomy remains unknown. The potential accessory branches of the zygomaticotemporal nerve may be considered a cause of continued temporal migraines after surgical procedures. In this study, we defined the novel superficial branch of the zygomaticotemporal nerve (sZTN) and investigated its anatomical course, distribution, and clinical implications. Twenty-two hemifaces from 11 fixed Korean cadavers (six males, five females; mean age, 78.3 years) were used in this study. The piercing points of the sZTN through the deep and superficial layers of the deep temporal fascia, and the superficial temporal fascia were defined as P1, P2, and P3, respectively. The distance of each point from the zygomatic tubercle was measured using an image analysis software. The sZTN ascended between the bone and the temporalis after emerging from the zygomaticotemporal foramen. It then pierced the deep temporal fascia without penetrating the temporalis. After then, it pierced the superficial layer of the deep temporal fascia and turned superiorly toward the upper posterior temple. When the sZTN passed through the superficial temporal fascia, it intersected with the superficial temporal artery in every case. The novel findings of the sZTN may help in the treatment of intractable temporal migraines refractory to injection or surgical procedure. Based on our findings, targeting the sZTN may be applied as an alternative treatment strategy for patients who do not show significant improvement with treatment targeted to trigger sites.

OnabotulinumtoxinA effects on trigeminal nociceptors

BACKGROUND

OnabotulinumtoxinA (onabotA) is approved globally for prevention of chronic migraine; however, the classical mechanism of action of onabotA in motor and autonomic neurons cannot fully explain the effectiveness of onabotulinumtoxinA in this sensory neurological disease. We sought to explore the direct effects of onabotulinumtoxinA on mouse trigeminal ganglion sensory neurons using an inflammatory soup-based model of sensitization.

METHODS

Primary cultured trigeminal ganglion neurons were pre-treated with inflammatory soup, then treated with onabotulinumtoxinA (2.75 pM). Treated neurons were used to examine transient receptor potential vanilloid subtype 1 and transient receptor potential ankyrin 1 cell-surface expression, calcium influx, and neuropeptide release.

RESULTS

We found that onabotulinumtoxinA cleaved synaptosomal-associated protein-25 kDa in cultured trigeminal ganglion neurons; synaptosomal-associated protein-25 kDa cleavage was enhanced by inflammatory soup pre-treatment, suggesting greater uptake of toxin under sensitized conditions. OnabotulinumtoxinA also prevented inflammatory soup-mediated increases in TRPV1 and TRPA1 cell-surface expression, without significantly altering TRPV1 or TRPA1 protein expression in unsensitized conditions. We observed similar inhibitory effects of onabotulinumtoxinA on TRP-mediated calcium influx and TRPV1- and TRPA1-mediated release of calcitonin gene-related peptide and prostaglandin 2 under sensitized, but not unsensitized control, conditions.

CONCLUSIONS

Our data deepen the understanding of the sensory mechanism of action of onabotulinumtoxinA and support the notion that, once endocytosed, the cytosolic light chain of onabotulinumtoxinA cleaves synaptosomal-associated protein-25 kDa to prevent soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated processes more generally in motor, autonomic, and sensory neurons.

How Does Botulinum Toxin Inhibit Itch?

Two decades after reports of the anti-pruritic effects of botulinum neurotoxins (BoNTs), there is still no approved product for the anti-itch indication of BoNTs, and most clinical case reports still focus on the off-label use of BoNTs for various itchy conditions. Few randomized clinical trials have been conducted with controversial results, and the beneficial effects of BoNTs against itch are mainly based on case studies and case series. These studies are valuable in presenting the potential application of BoNTs in chronic pruritic conditions, but due to the nature of these studies, they are categorized as providing lower levels of evidence or lower grades of recommendation. To obtain approval for the anti-pruritic indication of BoNTs, higher levels of evidence are required, which can be achieved through conducting large-scale and well-designed studies with proper control groups and established careful and reliable primary and secondary outcomes. In addition to clinical evidence, presenting the mechanism-based antipruritic action of BoNTs can potentially strengthen, accelerate, and facilitate the current efforts towards further investments in accelerating the field towards the potential approval of BoNTs for itchy conditions. This review, therefore, aimed to provide the state-of-the-art mechanisms underlying the anti-itch effect of BoNTs from basic studies that resemble various clinical conditions with itch as a hallmark. Evidence of the neuronal, glial, and immune modulatory actions of BoNTs in reducing the transmission of itch are presented, and future potential directions are outlined.

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 β₂ 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 β₂ adrenergic receptor activation or inhibition on masticatory muscle pain. In preclinical models of temporomandibular joint arthritis, β₂ adrenergic receptor activation appears to contribute to inflammation and nociception, whereas in masticatory muscle, α₁ adrenergic receptor activation has been found to induce mechanical sensitization. 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.

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.

Trichodynia Revisited

Trichodynia refers to the painful sensation of the scalp related to the complaint of hair loss. Originally suggested to be distinguishing for telogen effluvium and related to hair loss activity and follicular inflammation, further studies have found trichodynia to be common in androgenetic alopecia as well and coexisting with psychopathologic findings. The respective studies failed to demonstrate correlations between trichodynia and quantifiable hair loss activity, nor histopathologic evidence for follicular inflammation. A symptomatic scalp is a frequent condition in specific dermatological conditions of the scalp. By definition of exclusion, we are not dealing with trichodynia in these cases. It is conceivable that neuropeptides are key players between the central nervous system and the skin immune and microvascular system. Such mechanisms would explain the noxious effects of both external stimuli and emotional distress in eliciting cutaneous nociception. Since we have begun to understand the diverse etiologies of trichodynia, and a single term does not measure up to this circumstance, it may be wiser to describe the condition depending on the type of scalp sensation and its specific disease association. Further studies are warranted into the neural/endothelial/follicular interactions both in hair growth and shedding and the psychosomatic diseases of the hair and scalp.

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.

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.

Double-Binding Botulinum Molecule with Reduced Muscle Paralysis: Evaluation in In Vitro and In Vivo Models of Migraine

With a prevalence of 15%, migraine is the most common neurological disorder and among the most disabling diseases, taking into account years lived with disability. Current oral medications for migraine show variable effects and are frequently associated with intolerable side effects, leading to the dissatisfaction of both patients and doctors. Injectable therapeutics, which include calcitonin gene-related peptide-targeting monoclonal antibodies and botulinum neurotoxin A (BoNT/A), provide a new paradigm for treatment of chronic migraine but are effective only in approximately 50% of subjects. Here, we investigated a novel engineered botulinum molecule with markedly reduced muscle paralyzing properties which could be beneficial for the treatment of migraine. This stapled botulinum molecule with duplicated binding domain-binary toxin-AA (BiTox/AA)-cleaves synaptosomal-associated protein 25 with a similar efficacy to BoNT/A in neurons; however, the paralyzing effect of BiTox/AA was 100 times less when compared to native BoNT/A following muscle injection. The performance of BiTox/AA was evaluated in cellular and animal models of migraine. BiTox/AA inhibited electrical nerve fiber activity in rat meningeal preparations while, in the trigeminovascular model, BiTox/AA raised electrical and mechanical stimulation thresholds in Aδ- and C-fiber nociceptors. In the rat glyceryl trinitrate (GTN) model, BiTox/AA proved effective in inhibiting GTN-induced hyperalgesia in the orofacial formalin test. We conclude that the engineered botulinum molecule provides a useful prototype for designing advanced future therapeutics for an improved efficacy in the treatment of migraine.

Periorbital botulinum toxin A improves photophobia and sensations of dryness in patients without migraine: Case series of four patients

Purpose

Individuals receiving botulinum toxin A (BoNT-A) injections in the head and neck for migraine treatment have reported decreases in photophobia and sensations of dryness, independent of ocular surface parameters. We hypothesized that patients without migraine but with similar ocular neuropathic-like symptoms would also experience symptomatic improvement with periocular BoNT-A injections, independent of ocular surface changes.

Observations

We identified four individuals without a history of migraine but with neuropathic ocular pain (symptoms of dryness, burning, and photophobia that were out of proportion to ocular surface findings and unresponsive to ongoing dry eye (DE) therapies). Individuals underwent 1 session of periocular BoNT-A injections. Validated questionnaires (Visual Light Sensitivity Questionnaire-8, Dry Eye Questionnaire-5) assessed photophobia and DE symptoms pre- and 1-month post-injections. All four reported improvements in frequency and severity of photophobia and eye discomfort following BoNT-A injections. Tear film parameters (phenol red thread test, tear break-up time, corneal staining, and Schirmer test) and eyelid (palpebral fissure height and levator palpebrae superioris function) and eyebrow (position) anatomy were also evaluated before and after injections. Despite a unanimous improvement in symptoms, there were no consistent changes in ocular surface parameters with BoNT-A injections across individuals.

Conclusions

Periocular BoNT-A shows promise in reducing photophobia and sensations of dryness in individuals with neuropathic-like DE symptoms without a history of migraine, independent of tear film, eyelid, or eyebrow parameters.

Modifications to the PREEMPT Protocol for OnabotulinumtoxinA Injections for Chronic Migraine in Clinical Practice

OBJECTIVE

To assess the PREEMPT protocol modifications that have developed in clinical practice over time.

BACKGROUND

The United States Food and Drug Administration approved the 155-unit fixed-dose, fixed-site PREEMPT protocol of onabotulinumtoxinA (BoNT-A) injections for migraine prevention 9 years ago.

METHODS

This is an anonymous survey with free text response options of Headache Medicine clinicians.

RESULTS

Out of the 878 contacted Headache Medicine clinicians, 182 (20.7%) completed the survey. Of the 182 respondents, 141 (77.5%) reported that they did not always follow the PREEMPT protocol. Of the 182 respondents, 128 (70%) changed the number of injections, 115 (63%) changed the total units of BoNT-A injected, 105 (57.7%) altered the location of injection sites (58%); 101 (55.5%) do not aspirate to ensure the absence of blood return; 22 (12.1%) changed the dilution; and 4 (2.2%) added lidocaine. The main reported reasons for changes in number, dose, and location of injections included adapting to the patients' pain, anatomy, and preferences.

CONCLUSIONS

The wide inter- and intra-personal variations in BoNT-A injections for chronic migraine prevention seen in this survey raise concerns about the standardization of the procedure and suggest that an advisory protocol containing more evidence and discussion of the reasoning behind the recommendations might be more helpful than the current prescriptive protocol.

Sex-Specific Pharmacotherapy for Migraine: A Narrative Review

Migraine is a common neurological disorder characterized by recurrent headache episodes that accompany sensory-motor disturbances, such as higher sensitivity to touch and light, extremity heaviness or weakness, and speech or language disabilities. Worldwide, migraine is one of the top 10 causes of disability and hence poses a huge economic burden to society. On average, migraine occurs in 12% of population but its occurrence is sexually dimorphic, as it is two to three times more prevalent in women than in men. This female to male ratio of migraine prevalence is age- and sex hormone-dependent. Advancements in understanding migraine pathogenesis have also revealed an association with both genetics and epigenetics. The severity of migraine, in terms of its attack duration, headache intensity, frequency, and occurrence of migraine-associated symptoms, has generally been reported to be greater in women. Sex differences in migraine disability and comorbidities, such as psychiatric disorders, have also been noted in some population-based studies. However, research on sex-related differences in response to migraine treatments is relatively scarce. Although a general observation is that women consume more medication than men for migraine treatment, strategies for the use of abortive and preventive medications for migraine are generally similar in both sexes. This narrative review summarizes available findings on sexually distinct responses to abortive and prophylactic pharmacotherapy of migraine. Basic experimental data and clinical findings will be presented, and potential mechanisms underlying sex-based responses will be discussed to highlight the importance and value of sex-based treatment in migraine research and practice.

Exploring the effects of extracranial injections of botulinum toxin type A on prolonged intracranial meningeal nociceptors responses to cortical spreading depression in female rats

Background

Botulinum neurotoxin type A, an FDA-approved prophylactic drug for chronic migraine, is thought to achieve its therapeutic effect through blocking activation of unmyelinated meningeal nociceptors and their downstream communications with myelinated nociceptors and potentially the vasculature and immune cells. Prior investigations to determine botulinum neurotoxin type A effects on meningeal nociceptors were carried out in male rats and tested with stimuli that act outside the blood brain barrier. Here, we sought to explore the effects of extracranial injections of botulinum neurotoxin type A on activation of meningeal nociceptors by cortical spreading depression, an event which occurs inside the blood brain barrier, in female rats.

Material and methods

Using single-unit recording, we studied myelinated C- and unmyelinated Aδ-meningeal nociceptors' responses to cortical spreading depression 7–14 days after injection of botulinum neurotoxin type A or saline along calvarial sutures.

Results

In female rats, responses to cortical spreading depression were typically more prolonged and, in some cases, began at relatively longer latencies post-cortical spreading depression, than had been observed in previous studies in male rats. Extracranial administration of botulinum neurotoxin type A reduced significantly the prolonged firing of the meningeal nociceptors, in the combined sample of Aδ- and C-fiber, but not their response probability.

Discussion

The findings suggest that the mechanism of action by which botulinum neurotoxin type A prevents migraine differ from the one by which calcitonin gene-related peptide monoclonal antibodies prevent migraine and that even when the origin of migraine is central (i.e. in the cortex), a peripherally acting drug can intercept/prevent the headache.

Botulinum neurotoxin type A combined with functional electrical stimulation for upper-limb poststroke spasticity with pain

Stroke is a complicated condition that leaves stroke survivors with prolonged disability. Common poststroke conditions are spasticity and pain, which directly affect daily activities and quality of life in these patients. Botulinum neurotoxin type A is an effective and well-tolerated treatment option for upper-limb spasticity. It reduces muscle tone and improves basic upper-limb activities. Functional electrical stimulation is also used widely as a therapeutic modality in the upper-limb muscle rehabilitation. Here, we present a case in which Botulinum neurotoxin type A injections combined with functional electrical stimulation were applied for upper-limb spasticity accompanied with pain in a poststroke patient.

Anatomical Look Into OnabotulinumtoxinA Injection for Chronic Migraine Headache

BACKGROUND AND OBJECTIVES

While existing studies about onabotulinumtoxinA for chronic migraines have focused on injection location and appropriate dosing, little consideration has been given to patient body habitus and its potential impact on efficacy. We hypothesized that with increasing patient body mass index (BMI) there would be more subcutaneous fat separating targeted muscle groups from the skin surface, such that standard 0.5-inch needles used in existing protocols may not allow intramuscular injection. This may have implications for treatment planning.

METHODS

Anatomically normal computed tomography scans of the head, neck, and face were randomly selected. Subjects were stratified into 4 groups based on BMI, with 30 patients in each group. Four standardized locations were chosen to obtain measurements from the skin surface to the underlying muscle fascia, including (1) frontalis, (2) temporalis, (3) semispinalis capitis, and (4) trapezius.

RESULTS

Median depth for the temporalis was 12.65 mm (Q1 = 9.32 mm, Q3 = 15.08 mm) for the BMI greater than 35 kg/m group. Median depth for the semispinalis capitis was 13.77 mm (Q1 = 10.3 mm, Q3 = 15.7 mm) for the BMI 30 to 35 kg/m group, and 14.75 mm (Q1 = 11.00, Q3 = 17.00 mm) for the BMI greater than 35 kg/m group. Median depth for the trapezius was 13.95 mm (Q1 = 10.18 mm, Q3 = 19.00 mm) for the BMI greater than 35 kg/m group. These medians exceeded the length of the standard 0.5-inch (12.-mm) needle used in existing protocols.

CONCLUSIONS

Our study demonstrates that with increasing BMI there is a greater distance between the skin surface and the muscle fascia of muscles that are targeted for injection in standard chronic migraine botulinum toxin injection protocols. Because of this, patient body habitus may be an important factor in injection technique.

Antipruritic Effects of Botulinum Neurotoxins

This review explores current evidence to demonstrate that botulinum neurotoxins (BoNTs) exert antipruritic effects. Both experimental and clinical conditions in which botulinum neurotoxins have been applied for pruritus relief will be presented and significant findings will be highlighted. Potential mechanisms underlying antipruritic effects will also be discussed and ongoing challenges and unmet needs will be addressed.

[Botulinum toxin type A in headache treatment : Established and experimental indications]

In recent years botulinum toxin type A has been used increasingly more in the treatment of specific headache disorders. Especially regarding chronic migraine with and without combined medication overuse, convincing randomized studies have proven the efficacy of this treatment option and have led to approval for this indication. Regarding other headache entities, such as episodic migraine, tension-type headache, trigeminal autonomic cephalalgia (TAC), neuralgic, neuropathic and myofascial pain, currently available scientific data on the efficacy of botulinum toxin type A are scarce and often ambiguous. The exact underlying mechanisms of the influence of botulinum toxin type A on the pathophysiology of headache are not completely clear but an influence on the release of calcitonin gene-related peptide (CGRP) seems to play a crucial role. This article summarizes the most important studies as well as experiences of treatment with botulinum toxin type A regarding different headache entities.

Chronic Migraine: An Update on Physiology, Imaging, and the Mechanism of Action of Two Available Pharmacologic Therapies

Several lines of research support the hypothesis that migraine is a spectrum of illness, with clinical symptoms that vary along a continuum from episodic migraine to chronic migraine. Physiologic changes may result in episodic migraine evolving into chronic migraine over months to years in susceptible individuals. With chronification, headache frequency increases, becoming more disabling and less responsive to therapy. Neurophysiologic and functional imaging research has reported that chronic migraine may be associated with severity‐specific metabolic, functional, and structural abnormalities in the brainstem. Without longitudinal studies, it is unclear whether these changes may represent a continuum of individual progression and/or are reversible. Furthermore, chronic migraine is associated with larger impairments in cortical processing of sensory stimuli when compared with episodic migraine, possibly caused by more pronounced cortical hyperexcitability.

Progressive changes in nociceptive thresholds and subsequent central sensitization due to recurrent migraine attacks in vulnerable individuals contribute to the chronic migraine state. This may result in changes to baseline neurologic function between headache attacks, evident in both electrophysiological and functional imaging research. Patients experiencing migraine chronification may report increased non‐headache pain, fatigue, psychiatric disorders (eg, depression, anxiety), gastrointestinal complaints, and other somatic conditions associated with their long‐term experience with migraine pain.

Recent research provides a foundation for differentiating episodic and chronic migraine based on neurophysiologic and neuroimaging tools. In this literature review, we consider these findings in the context of models designed to explain the physiology and progression of episodic migraine into chronic migraine, and consider treatment of chronic migraine in susceptible individuals. Advances in pharmacotherapy provide treatment options for chronic migraine. Of the currently available treatment options, only onabotulinumtoxinA and topiramate have received regulatory approval and have demonstrated efficacy in patients with chronic migraine, although the exact mechanisms of action are not fully elucidated.

Effects of OnabotulintoxinA on Habituation of Laser Evoked Responses in Chronic Migraine

Onabotulintoxin A (BontA) is an efficacious preventive treatment for chronic migraine, though the specific mechanism of action is still under discussion. The study aims: (1) To evaluate pain processing modifications in chronic migraine patients (CM) under single BontA administration in pericranial muscles, by means of CO² Laser Evoked Potentials (LEPs) obtained by the stimulation of the skin over the right frontal and trapezius injection sites and hand dorsum, in a double blind placebo controlled crossover design. (2) To correlate main LEPs findings with clinical outcome after one year of BontA treatment. Twenty refractory CM patients were included in the analysis. The LEPs were recorded in basal conditions and seven days after BontA (PREEMPT protocol) and saline solution injection. The N1, N2 and P2 amplitude and latencies and N2P2 habituation index were evaluated and correlated with the percent change of headache frequency after one year of toxin treatment. After seven days of BontA treatment, a normalization of the trigeminal habituation index was observed, which was correlated with the clinical outcome after one year of BontA therapy. Patients displaying trigeminal LEPs facilitation at T0 time showed a more efficient therapeutic outcome. Neurotoxin may exert a modulating effect on trigeminal nociception, normalizing central neurotransmission.

Increased pain and muscle glutamate concentration after single ingestion of monosodium glutamate by myofascial temporomandibular disorders patients

BACKGROUND

A randomized, double-blinded, placebo-controlled study was conducted to investigate if single monosodium glutamate (MSG) administration would elevate muscle/serum glutamate concentrations and affect muscle pain sensitivity in myofascial temporomandibular disorders (TMD) patients more than in healthy individuals.

METHODS

Twelve myofascial TMD patients and 12 sex- and age-matched healthy controls participated in two sessions. Participants drank MSG (150 mg/kg) or NaCl (24 mg/kg; control) diluted in 400 mL of soda. The concentration of glutamate in the masseter muscle, blood plasma and saliva was determined before and after the ingestion of MSG or control. At baseline and every 15 min after the ingestion, pain intensity was scored on a 0-10 numeric rating scale. Pressure pain threshold, pressure pain tolerance (PPTol) and autonomic parameters were measured. All participants were asked to report adverse effects after the ingestion.

RESULTS

In TMD, interstitial glutamate concentration was significantly greater after the MSG ingestion when compared with healthy controls. TMD reported a mean pain intensity of 2.8/10 at baseline, which significantly increased by 40% 30 min post MSG ingestion. At baseline, TMD showed lower PPTols in the masseter and trapezius, and higher diastolic blood pressure and heart rate than healthy controls. The MSG ingestion resulted in reports of headache by half of the TMD and healthy controls, respectively.

CONCLUSION

These findings suggest that myofascial TMD patients may be particularly sensitive to the effects of ingested MSG. WHAT DOES THIS STUDY ADD?': Elevation of interstitial glutamate concentration in the masseter muscle caused by monosodium glutamate (MSG) ingestion was significantly greater in myofascial myofascial temporomandibular disorders (TMD) patients than healthy individuals. This elevation of interstitial glutamate concentration in the masseter muscle significantly increased the intensity of spontaneous pain in myofascial TMD patients.

Influence of pro-algesic foods on chronic pain conditions

This paper examines current knowledge about putative "pro-algesic" dietary components, and discusses whether limiting the intake of these substances can help improve chronic pain. Although there is a common impression that numerous food components, natural and synthetic, can cause or worsen pain symptoms, very few of these substances have been investigated. This article focuses on four substances, monosodium glutamate, aspartame, arachidonic acid, and caffeine, where research shows that overconsumption may induce or worsen pain. For each substance, the mechanism whereby it may act to induce pain is examined, and any clinical trials examining the effectiveness of reducing the intake of the substance discussed. While all four substances are associated with pain, decreased consumption of them does not consistently reduce pain.

Structural and Functional Interactions between Transient Receptor Potential Vanilloid Subfamily 1 and Botulinum Neurotoxin Serotype A

BACKGROUND

Botulinum neurotoxins are produced by Clostridium botulinum bacteria. There are eight serologically distinct botulinum neurotoxin isoforms (serotypes A-H). Currently, botulinum neurotoxin serotype A (BoNT⁄A) is commonly used for the treatment of many disorders, such as hyperactive musculoskeletal disorders, dystonia, and pain. However, the effectiveness of BoNT⁄A for pain alleviation and the mechanisms that mediate the analgesic effects of BoNT⁄A remain unclear. To define the antinociceptive mechanisms by which BoNT/A functions, the interactions between BoNT⁄A and the transient receptor potential vanilloid subfamily 1 (TRPV1) were investigated using immunofluorescence, co-immunoprecipitation, and western blot analysis in primary mouse embryonic dorsal root ganglion neuronal cultures.

RESULTS

1) Three-week-old cultured dorsal root ganglion neurons highly expressed transient TRPV1, synaptic vesicle 2A (SV2A) and synaptosomal-associated protein 25 (SNAP-25). SV2A and SNAP-25 are the binding receptor and target protein, respectively, of BoNT⁄A. 2) TRPV1 colocalized with both BoNT⁄A and cleaved SNAP-25 when BoNT⁄A was added to dorsal root ganglia neuronal cultures. 3) After 24 hours of BoNT⁄A treatment (1 nmol⁄l), both TRPV1 and BoNT⁄A positive bands were detected in western blots of immunoprecipitated pellets. 4) Blocking TRPV1 with a specific antibody decreased the cleavage of SNAP-25 by BoNT⁄A.

CONCLUSION

BoNT/A interacts with TRPV1 both structurally and functionally in cultured mouse embryonic dorsal root ganglion neurons. These results suggest that an alternative mechanism is used by BoNT⁄A to mediate pain relief.

Current status and future directions of botulinum neurotoxins for targeting pain processing

Current evidence suggests that botulinum neurotoxins (BoNTs) A1 and B1, given locally into peripheral tissues such as skin, muscles, and joints, alter nociceptive processing otherwise initiated by inflammation or nerve injury in animal models and humans. Recent data indicate that such locally delivered BoNTs exert not only local action on sensory afferent terminals but undergo transport to central afferent cell bodies (dorsal root ganglia) and spinal dorsal horn terminals, where they cleave SNAREs and block transmitter release. Increasing evidence supports the possibility of a trans-synaptic movement to alter postsynaptic function in neuronal and possibly non-neuronal (glial) cells. The vast majority of these studies have been conducted on BoNT/A1 and BoNT/B1, the only two pharmaceutically developed variants. However, now over 40 different subtypes of botulinum neurotoxins (BoNTs) have been identified. By combining our existing and rapidly growing understanding of BoNT/A1 and /B1 in altering nociceptive processing with explorations of the specific characteristics of the various toxins from this family, we may be able to discover or design novel, effective, and long-lasting pain therapeutics. This review will focus on our current understanding of the molecular mechanisms whereby BoNTs alter pain processing, and future directions in the development of these agents as pain therapeutics.

Botulinum neurotoxin: Progress in negating its neurotoxicity; and in extending its therapeutic utility via molecular engineering. MiniReview

While the poisonous effects of botulinum neurotoxin (BoNT) have been recognized since antiquity, the overall actions and mechanisms of effects of BoNT have been elucidated primarily over the past several decades. The general utility of BoNT is described in the paper, but the focus is mainly on the approaches towards negating the toxic effects of BoNT, and on the projection of an engineered BoNT molecule serving as a Trojan Horse to deliver a therapeutic load for treatment of a host of medical disorders. The BoNT molecule is configured with a binding domain, a zinc-dependent protease with specificity primarily for vesicular proteins, and a translocation domain for delivery of the metalloprotease into the cytoplasm. The anti-toxin approaches for BoNT include the use of vaccines, antibodies, block of BoNT binding or translocation, inhibition of metalloprotease activity, impeded translocation of the protease/catalytic domain, and inhibition of the downstream Src signaling pathway. Projections of BoNT as a therapeutic include its targeting to non-cholinergic nerves, also targeting to non-neuronal cells for treatment of hypersecretory disorders (e.g., cystic fibrosis), and treatment of hormonal disorders (e.g., acromegaly). Still in the exploratory phase, there is the expectation of major advances in BoNT neuroprotective strategies and burgeoning utility of engineered BoNTs as therapeutics.

Botulinum Toxin Type a as a Therapeutic Agent against Headache and Related Disorders

Botulinum neurotoxin A (BoNT/A) is a toxin produced by the naturally-occurring Clostridium botulinum that causes botulism. The potential of BoNT/A as a useful medical intervention was discovered by scientists developing a vaccine to protect against botulism. They found that, when injected into a muscle, BoNT/A causes a flaccid paralysis. Following this discovery, BoNT/A has been used for many years in the treatment of conditions of pathological muscle hyperactivity, like dystonias and spasticities. In parallel, the toxin has become a “glamour” drug due to its power to ward off facial wrinkles, particularly frontal, due to the activity of the mimic muscles. After the discovery that the drug also appeared to have a preventive effect on headache, scientists spent many efforts to study the potentially-therapeutic action of BoNT/A against pain. BoNT/A is effective at reducing pain in a number of disease states, including cervical dystonia, neuropathic pain, lower back pain, spasticity, myofascial pain and bladder pain. In 2010, regulatory approval for the treatment of chronic migraine with BoNT/A was given, notwithstanding the fact that the mechanism of action is still not completely elucidated. In the present review, we summarize experimental evidence that may help to clarify the mechanisms of action of BoNT/A in relation to the alleviation of headache pain, with particular emphasis on preclinical studies, both in animals and humans. Moreover, we summarize the latest clinical trials that show evidence on headache conditions that may obtain benefits from therapy with BoNT/A.

Modulation of inflammatory mediators in the trigeminal ganglion by botulinum neurotoxin type A: an organ culture study

Background

Onabotulinumtoxin type A (BoNT-A) has been found to reduce pain in chronic migraine. The aim of the present study was to ask if BoNT-A can interact directly on sensory mechanisms in the trigeminal ganglion (TG) using an organ culture method.

Methods

To induce inflammation, rat TGs were incubated for 24 hrs with either the mitogen MEK1/2 inhibitor U0126, BoNT-A or NaCl. After this the TGs were prepared for immunohistochemistry. Sections of the TG were then incubated with primary antibodies against CGRP (neuronal transmitter), iNOS (inflammatory marker), IL-1β (Interleukin 1β), SNAP-25 (synaptic vesicle docking protein) or SV2-A (Botulinum toxin receptor element).

Results

We report that CGRP, iNOS, IL-1β, SNAP-25 and SV2-A were observed in fresh TG with a differential distribution. Interestingly, NaCl organ culture of the TG resulted in enhanced expression of CGRP and SNAP-25 in neurons and iNOS in SGCs. Co-incubation with U0126 or BoNT-A retained the increased expression of SNAP-25, while it decreased the IL-1β immunoreactivity in neurons. The iNOS expression in SGCs returned to levels observed in fresh specimens. Moreover, we observed no alteration SV2-A expression in SGCs. Thus, the overall picture is that both U0126 and BoNT-A have the ability to modify the expression of certain molecules in the TG.

Conclusion

We hypothesize that chronic migraine might be associated with some degree of inflammation in the TG that could involve both neurons and SGCs. It is clinically well recognized that treatment with corticosteroids will reduce the symptoms of chronic migraine; however this remedy is associated with long-term side effects. Understanding the mechanisms involved in the expressional alterations may suggest novel ways to modify the changes and indicate novel therapeutics. The results of the present work illustrate one way by which BoNT-A may modify these expressional alterations.

Blockade of glutamate release by botulinum neurotoxin type A in humans: a dermal microdialysis study

BACKGROUND

The analgesic action of botulinum neurotoxin type A (BoNTA) has been linked to the blockade of peripheral release of neuropeptides and neurotransmitters in animal models; however, there is no direct evidence of this in humans.

OBJECTIVES

To investigate the effect of BoNTA on glutamate release in humans, using an experimental model of pain and sensitization provoked by capsaicin plus mild heat.

METHODS

Twelve healthy volunteers (six men, six women) were pretreated with BoNTA (10 U) on the volar forearm and with a saline control on the contralateral side. Dermal microdialysis was applied one week later to collect interstitial samples before and after the application of a capsaicin patch (8%) plus mild heat (40°C⁄60 min) to provoke glutamate release, pain and vasodilation. Samples were collected every hour for 3 h using linear microdialysis probes (10 mm, 100 kD). Dialysate was analyzed for glutamate concentration. Pain intensity and skin vasomotor reactions (temperature and blood flow changes) were also recorded.

RESULTS

BoNTA significantly reduced glutamate release compared with saline (P<0.05). The provoked pain intensity was lower in the BoNTA-pretreated arm (P<0.01). The reduction in pain scores was not correlated with glutamate level. Cutaneous blood flow (P<0.05), but not cutaneous temperature (P≥0.05), was significantly reduced by BoNTA. There was a correlation between glutamate level and skin blood flow (r=0.58⁄P<0.05) but not skin temperature (P≥0.05). No differences according to sex were observed in any response.

CONCLUSIONS

The present study provided the first direct evidence supporting the inhibitory effect of BoNTA on glutamate release in human skin, which is potentially responsible for some of the analgesic action of BoNTA.

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.

An update on botulinum toxin A injections of trigger points for myofascial pain

Myofascial pain syndrome (MPS) is a common chronic pain condition that is characterized by distinct "trigger points." Despite current treatments with physical therapy, analgesics, anti-depressants and trigger-point injections, myofascial pain remains a challenging chronic pain condition in clinical practice. Botulinum toxin A (BTX-A) can cause prolonged muscle relaxation through inhibition of acetylcholine release. It may offer some advantages over the current treatments for MPS by providing a longer sustained period of pain relief. Despite numerous clinical trials, the efficacy of BTX-A in alleviating MPS is not well-established due to mixed results from recent clinical trials. Active trigger points are associated with referred pain and greatly impact many aspects of activities of daily living, mood, and health status. This review is designed to analyze the clinical trials regarding the efficacy of BTX-A injection of active trigger points as a treatment for MPS. The literature referenced was obtained via a computer search with Google Scholar, Pubmed, Medline and EMbase. Our search terms included "Botulinum toxin," "myofascial pain," "trigger points," "myofascial trigger points," "chronic pain." Additional references were retrieved from the reference list of the reports found via this search. Studies were considered eligible for inclusion if they were double-blinded, randomized, controlled trials evaluating the efficacy of BTX-A injections into trigger points for pain reduction, and if the trigger point selection in the trial included referred pain and/or local twitch response. Open-label studies, case reports, and other non-randomized studies were excluded. Eight trials were found according to the above criteria and are summarized in Table 1. There are well-designed clinical trials to support the efficacy of trigger-point injections with BTX-A for MPS. However, further clinical trials with considerations of minimizing placebo effect, repeated dosing, adequate coverage of trigger points, and using ultrasound confirmation and guidance are required to provide conclusive evidence for BTX-A in the treatment of myofascial pain.

Botulinum toxin A, brain and pain

Botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins known and a potential biological threat. At the same time, it is among the most widely used therapeutic proteins used yearly by millions of people, especially for cosmetic purposes. Currently, its clinical use in certain types of pain is increasing, and its long-term duration of effects represents a special clinical value. Efficacy of BoNT/A in different types of pain has been found in numerous clinical trials and case reports, as well as in animal pain models. However, sites and mechanisms of BoNT/A actions involved in nociception are a matter of controversy. In analogy with well known neuroparalytic effects in peripheral cholinergic synapses, presently dominant opinion is that BoNT/A exerts pain reduction by inhibiting peripheral neurotransmitter/inflammatory mediator release from sensory nerves. On the other hand, growing number of behavioral and immunohistochemical studies demonstrated the requirement of axonal transport for BoNT/A's antinociceptive action. In addition, toxin's enzymatic activity in central sensory regions was clearly identified after its peripheral application. Apart from general pharmacology, this review summarizes the clinical and experimental evidence for BoNT/A antinociceptive activity and compares the data in favor of peripheral vs. central site and mechanism of action. Based on literature review and published results from our laboratory we propose that the hypothesis of peripheral site of BoNT/A action is not sufficient to explain the experimental data collected up to now.

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.

Glutamate dysregulation in the trigeminal ganglion: a novel mechanism for peripheral sensitization of the craniofacial region

In the trigeminal ganglion (TG), satellite glial cells (SGCs) form a functional unit with neurons. It has been proposed that SGCs participate in regulating extracellular glutamate levels and that dysfunction of this SGC capacity can impact nociceptive transmission in craniofacial pain conditions. This study investigated whether SGCs release glutamate and whether elevation of TG glutamate concentration alters response properties of trigeminal afferent fibers. Immunohistochemistry was used to assess glutamate content and the expression of excitatory amino acid transporter (EAAT)1 and EAAT2 in TG sections. SGCs contained glutamate and expressed EAAT1 and EAAT2. Potassium chloride (10 mM) was used to evoke glutamate release from cultured rat SGCs treated with the EAAT1/2 inhibitor (3S)-3-[[3-[[4-(trifluoromethyl)ben zoyl]amino]phenyl]methoxy]-L-aspartic acid (TFB-TBOA) or control. Treatment with TFB-TBOA (1 and 10 μM) significantly reduced the glutamate concentration from 10.6 ± 1.1 to 5.8 ± 1.4 μM and 3.0 ± 0.8 μM, respectively (p<0.05). Electrophysiology experiments were conducted in anaesthetized rats to determine the effect of intraganglionic injections of glutamate on the response properties of ganglion neurons that innervated either the temporalis or masseter muscle. Intraganglionic injection of glutamate (500 mM, 3 μl) evoked afferent discharge and significantly reduced muscle afferent mechanical threshold. Glutamate-evoked discharge was attenuated bythe N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate (APV) and increased by TFB-TBOA, whereas mechanical sensitization was only sensitive to APV. Antidromic invasion of muscle afferent fibers by electrical stimulation of the caudal brainstem (10 Hz) or local anesthesia of the brainstem with lidocaine did not alter glutamate-induced mechanical sensitization. These findings provide a novel mechanism whereby dysfunctional trigeminal SGCs could contribute to cranial muscle tenderness in craniofacial pain conditions such as migraine headache.