Stimulation of craniofacial muscle afferents induces prolonged facilitatory effects in trigeminal nociceptive brain-stem neurones

Investigation of masticatory muscle thickness and mechanosensitivity of cervical and masticatory muscles in myofascial temporomandibular disorder patients with bruxism: A cross-sectional study

OBJECTIVE

Bruxism is a common problem associated with temporomandibular disorders (TMD). The aim of this study was to compare a patient group with Myofascial TMD and bruxism and a healthy control group in terms of masseter and temporal muscle thickness (clenching and resting), mechanosensitivity of neck and jaw muscles, craniofacial pain, and disability and emotional stress status.

METHODS

The study included 31 patients with myofascial TMD and bruxism (19 females, 12 males) with a mean age of 29.96 ± 8.12 years (range, 18-45 years), and a control group of 31 healthy subjects (19 females, 12 males) with a mean age of 27.58 ± 9.39 years years (range, 18-45 years). Masseter and temporal muscle thicknesses were evaluated with a mobile ultrasound device both at rest and when clenching the jaw. The mechanosensitivity values between the upper trapezius, obliquus capitis inferior, masseter and temporal muscles were measured with a digital algometer device. Craniofacial pain and disability level were evaluated with the Craniofacial Pain and Disability Index (CFPDI), and emotional stress levels with the Perceived Stress Scale-14 (PSS-14).

RESULTS

No difference was determined between the two groups in respect of the clenching and resting ratios of muscle thickness in any muscle (p > 0.05). The mechanosensitivity values in all muscles were lower in the myofascial TMD group than in the healthy group (p < 0.05). The CFPDI and PSS-14 scores were higher in the myofascial TMD group (p < 0.05). There was a moderate positive correlation between CFPDI, PSS-14 and Bruxism Frequency Score (p < 0.05).

CONCLUSION

The difference in mechanosensitivity and CFPDI values between the myofascial TMD patients with bruxism and the healthy control group indicates that the problem in this patient group has effects in the craniocervical and cervical regions. In addition, the correlation between CFPDI, PSS-14 and Bruxism Frequency Score in myofascial TMD patients suggests that this problem may be affected by the interaction of different parameters.

Effectiveness of Dry Needling versus Manual Therapy in Myofascial Temporomandibular Disorders: A Single-Blind Randomized Controlled Trial

Dry needling (DN) is an invasive physiotherapy technique employed for reducing myofascial pain. To compare the effectiveness of dry needling (DN) versus manual therapy (MT) in improving pain, active maximal mouth opening (AMMO) and cervical disability in patients with myofascial pain from temporomandibular disorders (TMDs) were investigated against these treatments. A single-blind, randomized controlled trial was carried out. Individuals (n = 50) with TMDs were randomly allocated in a 1:1 ratio to the DN (n = 25) or MT group (n = 25). Each group received three sessions, separated by 4 days, of either DN or MT. Outcomes were assessed according to pain intensity (Numeric Pain Rating Scale), AMMO (cm), disability (Neck Disability Index), and pressure-pain threshold (PPT) (digital algometry) from the active myofascial trigger points. In both groups, pain and neck disability were significantly lower at the end of treatment compared with those measured at baseline (pain: -2.52 with 95% CI: -3.43 to -1.60 for DN group; pain: -2.92 with 95% CI: -3.77 to -2.07 for MT group; disability: -3.2 with 95% CI: -4.31 to -2.09 for DN group; disability: -2.68 with 95% CI: -3.56 to -1.79 for MT group), but not were not lower after the first session, without differences between the groups. AMMO was significantly higher after the first session (0.16 with 95% CI: 0.03 to 0.29 for DN group; 0.30 with 95% CI: 0.20 to 0.41 for MT group) and at the end of treatment in both groups (0.27 with 95% CI: 0.14 to 0.41 for DN group; 0.37 with 95% CI: 0.22 to 0.52 for MT group) compared with the baseline measurements. Finally, PPT results for the masseter and pterygoid muscles were significantly higher at the end of treatment in both groups (without statistically significant differences between groups), but not after the first session. The assessed therapies, DN and MT, are equally effective in improving pain, AMMO, cervical disability, and PPT in the muscles directly involved in the temporomandibular joint biomechanics of patients with myofascial TMDs.

Ipsilateral Limb Extension of Referred Trigeminal Facial Pain due to Greater Occipital Nerve Entrapment: A Case Report

We report a very rare case of referred pain associated with entrapment of the greater occipital nerve (GON) occurring not only in the ipsilateral hemiface but also in the ipsilateral limb. There is an extensive convergence of cutaneous, tooth pulp, visceral, neck, and muscle afferents onto nociceptive and nonnociceptive neurons in the trigeminal nucleus caudalis (medullary dorsal horn). In addition, nociceptive input from trigeminal, meningeal afferents projects into trigeminal nucleus caudalis and dorsal horn of C1 and C2. Together, they form a functional unit, the trigeminocervical complex (TCC). The nociceptive inflow from suboccipital and high cervical structures is mediated with small-diameter afferent fibers in the upper cervical roots terminating in the dorsal horn of the cervical cord extending from the C2 segment up to the medullary dorsal horn. The major afferent contribution is mediated by the spinal root C2 that is peripherally represented by the greater occipital nerve (GON). Convergence of afferent signals from the trigeminal nerve and the GON onto the TCC is regarded as an anatomical basis of pain referral in craniofacial pain and primary headache syndrome. Ipsilateral limb pain occurs long before the onset of the referred facial pain. The subsequent severe hemifacial pain suggested GON entrapment. The occipital nerve block provided temporary relief from facial and extremity pain. Imaging studies found a benign osteoma in the ipsilateral suboccipital bone, but no direct contact with GON was identified. During GON decompression, severe entrapment of the GON was observed by the tendinous aponeurotic edge of the trapezius muscle, but the osteoma had no contact with the nerve. Following GON decompression, the referred trigeminal and extremity pain completely disappeared. The pain referral from GON entrapment seems to be attributed to the sensitization and hypersensitivity of the trigeminocervical complex (TCC). The clinical manifestations of TCC hypersensitivity induced by chronic entrapment of GONs are diverse when considering the occurrence of extremity pain as well as facial pain.

NOP01, a NOP receptor agonist, produced potent and peripherally restricted antinociception in a formalin-induced mouse orofacial pain model

Orofacial pain is one of the most common medical challenges. A preliminary report indicates that the NOP receptor may act as a therapeutic target in orofacial pain. Previous studies have shown that [(pF)Phe⁴, Aib⁷, Aib¹¹, Arg¹⁴, Lys¹⁵]N/OFQ-NH₂ (NOP01) functions as a potent NOP receptor peptide agonist. This work aims to investigate the antinociception of NOP01 and its possible action mechanisms in a formalin-induced mouse orofacial pain model at different levels. Our results demonstrated that local, intraperitoneal (i.p.) or intrathecal (i.t.) injection of NOP01 produced dose-related antinociception in both phases of the formalin pain, which could be inhibited by the NOP receptor antagonist but not the classical opioid receptor antagonist. Furthermore, the antinociception induced by systemic NOP01 was blocked by local but not spinal pretreatment with the NOP receptor antagonist, suggesting the involvement of the peripheral NOP receptor in NOP01-induced systemic antinociception. Moreover, local injection of NOP01 markedly suppressed the expression of c-Fos protein induced by formalin in ipsilateral trigeminal ganglion (TG) neurons. In conclusion, this work suggests that NOP01 exerts significant antinociception on orofacial pain at both peripheral and spinal levels via the NOP receptor. Notably, NOP01 cannot readily penetrate the blood-brain barrier. Thus, NOP01 may behave as a potential compound for developing peripherally restricted analgesics.

Responses of neurons in rostral ventromedial medulla to nociceptive stimulation of craniofacial region and tail in rats

The rostral ventromedial medulla (RVM) plays a key role in the endogenous modulation of nociceptive transmission in the central nervous system (CNS). The primary aim of this study was to examine whether the activities of RVM neurons were related to craniofacial nociceptive behaviour (jaw-motor response, JMR) as well as the tail-flick response (TF). The activities of RVM neurons and TF and JMR evoked by noxious heating of the tail or perioral skin were recorded simultaneously in lightly anaesthetized rats. Tail or perioral heating evoked the TF and JMR, and the latency of the JMR was significantly shorter (P < 0.001) than that of the TF. Of 89 neurons recorded in RVM, 40 were classified as ON-cells, 27 as OFF-cells, and 22 as NEUTRAL-cells based on their responsiveness to heating of the tail. Heating at either site caused an increase in ON-cell and decrease in OFF-cell activity before the occurrence of the TF and JMR, but did not alter the activity of NEUTRAL cells. Likewise, noxious stimulation of the temporomandibular joint had similar effects on RVM neurons. These findings reveal that the JMR is a measure of the excitability of trigeminal and spinal nociceptive circuits in the CNS, and that the JMR as well as TF can be used for studying processes related to descending modulation of pain. The findings also support the view that RVM ON- and OFF-cells play an important role in the elaboration of diverse nociceptive behaviours evoked by noxious stimulation of widely separated regions of the body.

Sex and Height Influence Neck Posture When Using Electronic Handheld Devices

With increased tablet ownership in the United States comes increased levels of neck flexion compared to desktop or laptop computer use, and these neck postures have been linked to increases in neck pain. Importantly, tablet viewing postures can be achieved in multiple ways and could be determined by the morphology of the individual and/or other extraneous factors. In this study, we aim to preliminarily evaluate how neck postures vary during tablet use among individuals and link this variation to other factors such as sex, height, weight, presence/absence of temporomandibular joint disorder (TMD), and morphology of the head and neck. We analyzed two-dimensional landmarks placed on lateral-view radiographs of 22 participants (10 female and 12 male) seated in neutral, upright, fully flexed, semi-reclined, and reclined postures. We utilize geometric morphometric techniques, which are advantageous for evaluating shape variation and have not been extensively applied to biomechanical analyses. We found skeletal morphology to be significantly related to sex and height in all but the neutral posture (P < 0.05), and weight was marginally significantly related to shape in the semi-reclined posture (P = 0.047). Morphologically, male participants exhibited more flexion at the articulatio atlantooccipitalis than females, and females showed greater mandibular protrusion than males, although this result is likely related to height. No relationship was found between posture and TMD. This research establishes a framework for future work that uses geometric morphometric analyses to evaluate how neck postures vary in relation to TMD. Clin. Anat. 32:1061-1071, 2019. © 2019 Wiley Periodicals, Inc.

Sustained and repeated mouth opening leads to development of painful temporomandibular disorders involving macrophage/microglia activation in mice

Temporomandibular disorder (TMD) is a set of heterogeneous musculoskeletal conditions involving the temporomandibular joint (TMJ) and/or the masticatory muscles. Up to 33% of the population has had at least 1 symptom of TMD with 5% to 10% of them requiring treatment. Common symptoms include limited jaw movement, joint sound, and pain in the orofacial area. Once TMD becomes chronic, it can be debilitating with comorbidities that greatly reduce one's overall quality of life. However, the underlying mechanism of TMD is unclear because of the multicausative nature of the disease. Here, we report a novel mouse model of TMD where a bite block was placed in between the upper and lower incisors such that the mouth was kept maximally open for 1.5 hours per day for 5 days. After sustained mouth opening, mice developed persistent orofacial mechanical allodynia and TMJ dysfunction. At the cellular level, we found masseter muscle dystrophy, and increased proteoglycan deposition and hypertrophic chondrocytes in the mandibular condyle. Increased F4/80 macrophages were also observed in the masseter muscles and the TMJ posterior synovium. We also found ATF3 neuronal injury and increased F4/80 macrophages in the trigeminal ganglia. Microglia activation was observed in the trigeminal subnucleus caudalis. Inhibiting macrophage and microglia activation with a colony stimulating factor-1 receptor inhibitor prevented the development of orofacial mechanical allodynia, but not TMJ dysfunction. This study suggests that mouth opening for an extended period during dental treatments or oral intubations may risk the development of chronic TMD and inflammation associated with macrophage and microglia in the tissue and trigeminal system contributes to the development of TMD pain.

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.

Analgesic activities of the mixed opioid and NPFF receptors agonist DN-9 in a mouse model of formalin-induced orofacial inflammatory pain

Orofacial pain is one of the most common pain conditions and compromises the quality of life of the sufferer. Several studies have shown that opioid agonists produced significant analgesia in the orofacial pain, and combination of opioids with drugs belonging to other classes induced synergism in the orofacial pain. However, combination therapy of different analgesic drugs improves the risk of drug-drug interactions. Against this background, we sought to investigate the analgesic effects of the multi-functional opioid peptide DN-9, a mixed opioid and NPFF receptors agonist that produced robust analgesia in acute and inflammatory pain models, on formalin-induced orofacial pain. Our results showed that formalin injection caused significant spontaneous pain behaviors and increased the expressions of the mu-opioid receptor, c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) in the ipsilateral trigeminal ganglion (TG). In mice pretreated with DN-9, there was a significant reduction in nociceptive behaviors, which was selectively mediated by the mu- and kappa-opioid receptors, independently of the NPFF system. Four hours after formalin injection, the level of c-Fos immunoreactivity in the ipsilateral TG neurons was much lower in mice pretreated with DN-9 or morphine. In addition, DN-9 exhibited a significant inhibition in the expression of p-ERK1/2, which was reversed by the selective antagonists of the mu- and kappa-opioid receptors. In conclusion, our present results demonstrate that central administration of DN-9 produces potential antinociceptive effects via the mu- and kappa-opioid receptors, independently of the NPFF system, and this antinociceptive action is tightly linked with the intracellular ERK activation in TG neurons.

Bilateral increases in ERK activation at the spinomedullary junction region by acute masseter muscle injury during temporomandibular joint inflammation in the rats

We determined the role of persistent monoarthritis of temporomandibular joint region (TMJ) on bilateral masseter muscle (MM) nociception in male rats using orofacial nocifensive behaviors, phosphorylated extracellular signal-regulated kinase and Fos induction at the trigeminal subnucleus caudalis/upper cervical spinal cord (Vc/C₂) region in response to formalin injection to the MM region. TMJ inflammation was induced by local injection of CFA into the left TMJ region. Orofacial nocifensive behaviors evoked by formalin injection ipsilateral or contralateral to the TMJ inflammation appeared to be increased at 1-14 days or at 1, 10 and 14 days after induction of TMJ inflammation, respectively, while increases in behavioral duration were seen mainly in the late phase rather than the early phase. The number of pERK positive cells was investigated in superficial laminae at the Vc/C₂ region at 3, 10, 20, 60 and 80 min after MM stimulation with formalin at 14 days after TMJ inflammation. TMJ-inflamed rats displayed greater responses of pERK expression by the ipsilateral MM stimulation at 3-60 min, while contralateral MM stimulation increased pERK expression at 3, 10 and 20 min compared to non-CFA rats. Fos expression by MM stimulation was increased at 14 days after induction of TMJ inflammation regardless of the affected side. These findings showed that persistent TMJ inflammation for 10 and 14 days is sufficient to enhance MM nociception indicated by behaviors and neural responses in superficial laminae at the Vc/C₂ region.

Mouse preferential incising force orientation changes during jaw closing muscle hyperalgesia and is sex dependent

INTRODUCTION

Mouse incising is controlled by a central pattern generator and this activity can change in the presence of pain. The incising frequency and maximum force generation decreases with pain. In this study, we used repetitive acidic injections in the left masseter muscle of male and female mice to determine differences between baseline and jaw muscle pain conditions and the effect of sex on preferential incising direction.

METHODS

A within subject design was used to evaluate data previously acquired using multi-axis force data (X, Y and Z) from the 4th baseline recording day and day 7 post-injection (day of maximal pain response) for each mouse of each sex. A total of 34 female and male (age 3-9months) CD-1 mice were evaluated. After mathematically rotating the X and Y axes to align the Y axis to be parallel to the wire struts of the cage top, data were analyzed to determine incising direction preference during baseline (non-pain) and pain (day 7) conditions and between sex. Radar plots of X-Y, X-Z and Y-Z axes depicted the average direction of incising preference between baseline and pain conditions for each sex. Statistical differences among groups were tested using a mixed model ANOVA.

RESULTS

Similar to previous findings, female mice had a more robust difference in incising direction preference when comparing male and female pain conditions and this was most evident in the X-Z axes. The incising frequencies most commonly affected were 5.3, 6.2 and 7.6Hz. Male mice varied little in their incising direction preference between the baseline and pain conditions. In addition, statistical comparison of ratios of the percent of time spent incising in the Z versus X axes for each incising frequency found that the incising preference was not different when comparing 5.3 and 7.6Hz frequencies. Finally, female mice used a novel approach to minimize pain while incising by rotating their head and body nearly 180 degrees while males did not use this strategy as frequently.

CONCLUSIONS

The preferred incising direction changes in a jaw muscle pain condition and this information can be used to further characterize functional pain in the masticatory muscle system. The changes were dependent on the incising frequency generated by the central pattern generator for incising.

Myofascial Trigger Points, Neck Mobility and Forward Head Posture in Unilateral Migraine

This paper describes the differences in the presence of myofascial trigger points (TrPs) in the upper trapezius, sternocleidomastoid, temporalis and suboccipital muscles between unilateral migraine subjects and healthy controls, and the differences in the presence of TrPs between the symptomatic side and the nonsymptomatic side in migraine subjects. In addition, we assess the differences in the presence of both forward head posture (FHP) and active neck mobility between migraine subjects and healthy controls and the relationship between FHP and neck mobility. Twenty subjects with unilateral migraine without side-shift and 20 matched controls participated. TrPs were identified when there was a hypersensible tender spot in a palpable taut band, local twitch response elicited by the snapping palpation of the taut band and reproduction of the referred pain typical of each TrP. Side-view pictures were taken in both sitting and standing positions to measure the cranio-vertebral angle. A cervical goniometer was employed to measure neck mobility. Migraine subjects showed a significantly greater number of active TrPs ( P < 0.001), but not latent TrPs, than healthy controls. Active TrPs were mostly located ipsilateral to migraine headaches ( P < 0.01). Migraine subjects showed a smaller cranio-vertebral angle than controls ( P < 0.001), thus presenting a greater FHP. Neck mobility in migraine subjects was less than in controls only for extension ( P = 0.02) and the total range of motion in flexion/extension ( P = 0.01). However, there was a positive correlation between the cranio-vertebral angle and neck mobility. Nociceptive inputs from TrPs in head and neck muscles may produce continuous afferent bombardment of the trigeminal nerve nucleus caudalis and, thence, activation of the trigeminovascular system. Active TrPs located ipsilateral to migraine headaches might be a contributing factor in the initiation or perpetuation of migraine.

Occipital Nerve Blockade in Chronic Cluster Headache Patients and Functional Connectivity Between Trigeminal and Occipital Nerves

Headache syndromes often involve occipital and neck symptoms, suggesting a functional connectivity between nociceptive trigeminal and cervical afferents. Although reports regarding effective occipital nerve blockades in cluster headache exist, the reason for the improvement of the clinical symptoms is not known. Using occipital nerve blockade and nociceptive blink reflexes, we were able to demonstrate functional connectivity between trigeminal and occipital nerves in healthy volunteers. The R2 components of the nociceptive blink reflex and the clinical outcome in 15 chronic cluster headache patients were examined before and after unilateral nerve blockade of the greater occipital nerve with 5 ml prilocain (1%) on the headache side. In contrast to recent placebo-controlled studies, only nine of the 15 cluster patients reported some minor improvement in their headache. Six patients did not report any clinical change. Exclusively on the injection side, the R2 response areas decreased and R2 latencies increased significantly after the nerve blockade. These neurophysiological and clinical data provide further evidence for functional connectivity between cervical and trigeminal nerves in humans. The trigeminocervical complex does not seem to be primarily facilitated in cluster headache, suggesting a more centrally located pathology of the disease. However, the significant changes of trigeminal function as a consequence of inhibition of the greater occipital nerve were not mirrored by a significant clinical effect, suggesting that the clinical improvement of occipital nerve blockades is not due to a direct inhibitory effect on trigeminal transmission.

The surgical management of cephalalgia

PURPOSE OF REVIEW

Management of headache disorders is not part of most craniomaxillofacial surgery practices; however there are certain indications for surgical management of headaches by the craniomaxillofacial surgeon.

RECENT FINDINGS

Migraine headaches are the most amenable to surgical management and while the exact mechanism of migraine is unknown, a central or peripheral trigger such as compressive neuropathy of trigeminal nerve branches leading to neurogenic inflammation has been suggested. The primary management for episodic migraine headache should be lifestyle modification and medication, whereas for chronic migraine (>15 headache days/month) use of medication and botulinum neurotoxin is effective, whereas some patients may choose to explore surgical options. Trigger site decompression for chronic migraine surgically relieves anatomic impingement at various sites and has been shown to reduce by at least 50% the frequency, intensity, and duration of headaches in over 85% and elimination of headaches in almost 60%. Trigger points may also lead to exacerbation of cluster headaches and treatment with botulinum neurotoxin may reduce attacks.

SUMMARY

Trigger site decompression is an effective treatment for chronic migraine, as are botulinum neurotoxin injections in reducing attacks in cluster headaches. The craniomaxillofacial surgeon is uniquely qualified to treat these primary headache disorders.

Scratching the surface: the processing of pain from deep tissues

Although most pain research focuses on skin, muscles, joints and viscerae are major sources of pain. We discuss the mechanisms of deep pains arising from somatic and visceral structures and how this can lead to widespread manifestations and chronification. We include how both altered peripheral and central sensory neurotransmission lead to deep pain states and comment on key areas such as top-down modulation where little is known. It is vital that the clinical characterization of deep pain in patients is improved to allow for back translation to preclinical models so that the missing links can be ascertained. The contribution of deeper somatic and visceral tissues to various chronic pain syndromes is common but there is much we need to know.

Sensitization and Interoception as Key Neurological Concepts in Osteopathy and Other Manual Medicines

Historically, approaches used in manual medicine to explain patient reported symptoms have been focused on the so-called exteroceptive paradigm. Arguably, this mindset lacks an appropriate "reading system" able to interpret musculoskeletal disorders from a different perspective, where the properties of the nervous system are embraced into a more holistic and functional-related context. Interestingly, if the underpinning mechanisms of a given treatment scenario/effect are taking into account, the majority of research outcomes focuses on a proprioceptive/exteroceptive explanation, leaving ting aside the additional or even central role of interoception. Currently, to date, the application of theoretical knowledge acquired on the relatively recent neuroscientific concepts and evidence concerning of interoception, sensitization, touch, autonomic functions, inflammation, and pain into a clinical/research manual medicine scenario is lacking, even if theoretically, the impact on the possible etiological mechanisms and treatment effects seems to be important. Here, we propose the conceptual foundations for a new way of interpreting and reading patients' clinical reported outcomes scenario based on interoception and sensitization. We argue that this will provide a foundation to create the ground for future research focusing on the hypotheses that manual therapies, specifically osteopathy, can intercede with sensitization states, at all levels, using interoceptive pathways.

Atypical temporomandibular joint pain: a case report

Atypical temporomandibular joint (TMJ) pain can consist of an unusual intensity, location or set of pain descriptors that do not match what is traditionally observed for TMJ capsular pain, disc displacements or arthritic conditions. Presented in this case report is an atypical pain report regarding a unilateral TMJ pain as the chief complaint. An overview of typical vs atypical TMJ pain is also reviewed to highlight unusual signs and symptoms so that the clinician can identify these atypical presentations and pursue further diagnostic approaches.

Involvement of astroglial glutamate-glutamine shuttle in modulation of the jaw-opening reflex following infraorbital nerve injury

To evaluate the mechanisms underlying orofacial motor dysfunction associated with trigeminal nerve injury, we studied the astroglial cell activation following chronic constriction injury (CCI) of the infraorbital nerve (ION) immunohistochemically, nocifensive behavior in ION-CCI rats, and the effect of the glutamine synthase (GS) blocker methionine sulfoximine (MSO) on the jaw-opening reflex (JOR), and also studied whether glutamate-glutamine shuttle mechanism is involved in orofacial motor dysfunction. GFAP-immunoreactive (IR) cells were observed in the trigeminal motor nucleus (motV) 3 and 14 days after ION-CCI, and the nocifensive behavior and JOR amplitude were also strongly enhanced at these times. The number of GS- and GFAP-IR cells was also significantly higher in ION-CCI rats on day 7. The amplitude and duration of the JOR were strongly suppressed after MSO microinjection (m.i.) into the motV compared with that before MSO administration in ION-CCI rats. After MSO administration, the JOR amplitude was strongly suppressed, and the duration of the JOR was shortened. Forty minutes after m.i. of glutamine, the JOR amplitude was gradually returned to the control level and the strongest attenuation of the suppressive effect of MSO was observed at 180 min after glutamine m.i. In addition, glutamine also attenuated the MSO effect on the JOR duration, and the JOR duration was extended and returned to the control level thereafter. The present findings suggest that astroglial glutamate-glutamine shuttle in the motV is involved in the modulation of excitability of the trigeminal motoneurons affecting the enhancement of various jaw reflexes associated with trigeminal nerve injury.

Projections from the dorsal peduncular cortex to the trigeminal subnucleus caudalis (medullary dorsal horn) and other lower brainstem areas in rats

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.

Lidocaine injection of pericranial myofascial trigger points in the treatment of frequent episodic tension-type headache

Background

The present study aimed to evaluate the efficacy of local lidocaine injections into the myofascial trigger points (TPs) located at the pericranial muscles in patients with episodic tension-type headache (ETTH).

Methods

The study included 108 patients with frequent ETTH that were randomized into 4 groups. One injection of saline (NaCl 0.9%) was administered to group 1 (n = 27), 1 injection of lidocaine (0.5%) was administered to group 2 (n = 27), group 3 (n = 27) received 5 injections of saline (NaCl 0.9%), and group 4 (n = 27) received 5 injections of lidocaine (0.5%); on alternate days 2 mL for each muscle was injected into the frontal, temporal, masseter, sternocleidomastoid, semispinalis capitis, trapezius and splenius capitis muscles bilaterally. The frequency of painful days per month (FPD) and the patients’ visual analogue scales (VAS) were evaluated before treatment, and 2, 4 and 6 months after treatment.

Results

Mean age of the patients was 36.28 ± 9.41 years (range: 18–54 years). FPD scores improved significantly in group 2, 3 and 4 at 2 months posttreatment compared to pre- treatment (all P < 0.05), and also VAS scores improved significantly in group 2 and 4 at 2 months posttreatment (P < 0.05) but this improvement insisted at the 6 month only in group 4. Group 2 had better VAS and FPD than group 1 only at 2. and 4. months after treatment (for VAS P < 0.0121, P = 0.0232; for FPD P = 0.0003, P = 0.0004, respectively). Group 4 had better scores than group 3 at the 2., 4. and 6. months after treatment in both parameters (all P < 0.05). Group 2 had better scores than group 1 in FPD at the 2. and 4. months posttreatment (P = 0.0003, P = 0.0004, respectively), but not at the 6. month.

Conclusion

Local lidocaine injections into the myofascial TPs located in the pericranial muscles could be considered as an effective alternative treatment for ETTH.

Central sensitization and MAPKs are involved in occlusal interference-induced facial pain in rats

We previously developed a rat dental occlusal interference model of facial pain that was produced by bonding a crown onto the right maxillary first molar and was reflected in sustained facial hypersensitivity that was suggestive of the involvement of central sensitization mechanisms. The aim of the present study was to investigate potential central mechanisms involved in the occlusal interference-induced facial hypersensitivity. A combination of behavioral, immunohistochemical, Western blot, and electrophysiological recording procedures was used in 98 male adult Sprague Dawley rats that either received the occlusal interference or were sham-operated or naive rats. Immunohistochemically labeled astrocytes and microglia in trigeminal subnucleus caudalis (Vc) showed morphological changes indicative of astrocyte and microglial activation after the occlusal interference. Prolonged upregulation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) was also documented in Vc after placement of the occlusal interference and was expressed in both neurons and glial cells at time points when rats showed peak mechanical facial hypersensitivity. The intrathecal administration of the p38 MAPK inhibitor SB203580 to the medulla significantly inhibited the occlusal interference-induced hypersensitivity, and the ERK inhibitor PD98059 produced an even stronger effect. Central sensitization of functionally identified Vc nociceptive neurons following placement of the occlusal interference was also documented by extracellular electrophysiological recordings, and intrathecal administration of PD98059 could reverse the neuronal central sensitization. These novel findings suggest that central mechanisms including central sensitization of trigeminal nociceptive neurons and non-neuronal processes involving MAPKs play significant roles in the production of occlusal interference-induced facial pain.

PERSPECTIVE

Central mechanisms including trigeminal nociceptive neuronal sensitization, non-neuronal processes involving glial activation, and MAPKs play significant roles in occlusal interference-induced facial pain. These mechanisms may be involved in clinical manifestations of facial pain that have been reported in patients with an occlusal interference.

The science of migraine

The cardinal symptom of migraine is headache pain. In this paper we review the neurobiology of this pain as it is currently understood. In recent years, we discovered that the network of neurons that sense pain signals from the dura changes rapidly during the course of a single migraine attack and that the treatment of an attack is a moving target. We found that if the pain is not stopped within 10-20 minutes after it starts, the first set of neurons in the network, those located in the trigeminal ganglion, undergo molecular changes that make them hypersensitive to the changing pressure inside the head, which explains why migraine headache throbs and is worsened by bending over and sneezing. We found that if the pain is not stopped within 60-120 minutes, the second group of neurons in the network, those located in the spinal trigeminal nucleus, undergoes molecular changes that convert them from being dependent on sensory signals they receive from the dura by the first set of neurons, into an independent state in which they themselves become the pain generator of the headache. When this happens, patients notice that brushing their hair, taking a shower, touching their periorbital skin, shaving, wearing earrings, etc become painful, a condition called cutaneous allodynia. Based on this scenario, we showed recently that the success rate of rendering migraine patients pain-free increased dramatically if medication was given before the establishment of cutaneous allodynia and central sensitization. The molecular shift from activity-dependent to activity-independent central sensitization together with our recent conclusion that triptans have the ability to disrupt communications between peripheral and central trigeminovascular neurons (rather than inhibiting directly peripheral or central neurons) explain their clinical effects. Both our clinical and pre-clinical findings of the last five years point to possible short- and long-term advantages in using an early-treatment approach in the treatment of acute migraine attacks.

Systemic pregabalin attenuates sensorimotor responses and medullary glutamate release in inflammatory tooth pain model

Our previous studies have demonstrated that application of inflammatory irritant mustard oil (MO) to the tooth pulp induces medullary glutamate release and central sensitization in the rat medullary dorsal horn (MDH), as well as nociceptive sensorimotor responses in craniofacial muscles in rats. There is recent evidence that anticonvulsant drugs such as pregabalin that influence glutamatergic neurotransmission are effective in several pain states. The aim of this study was to examine whether systemic administration of pregabalin attenuated glutamate release in the medulla as well as these nociceptive effects reflected in increased electromyographic (EMG) activity induced by MO application to the tooth pulp. Male adult rats were anesthetized with isofluorane (1.0-1.2%), and jaw and tongue muscle EMG activities were recorded by needle electrodes inserted bilaterally into masseter and anterior digastric muscles and into the genioglossus muscle, and also the medullary release of glutamate was assessed by in vivo microdialysis. Pregabalin or vehicle control (isotonic saline) was administered 30 min before the pulpal application of MO or vehicle control (mineral oil). Application of mineral oil to the maxillary first molar tooth pulp produced no change in baseline EMG activity and glutamate release. However, application of MO to the pulp significantly increased both the medullary release of glutamate and EMG activity in the jaw and tongue muscles for several minutes. In contrast, pre-medication with pregabalin, but not vehicle control, significantly and dose-dependently attenuated the medullary glutamate release and EMG activity in these muscles after MO application to the tooth pulp (analysis of variance (ANOVA), p<0.05). These results suggest that pregabalin may attenuate the medullary release of glutamate and associated nociceptive sensorimotor responses in this acute inflammatory pulpal pain model, and that it may prove useful for the treatment of orofacial inflammatory pain states.

Myofascial pain syndromes and their evaluation

This article reviews the available published knowledge about the diagnosis, pathophysiology and treatment of myofascial pain syndromes from trigger points. Furthermore, epidemiologic data and clinical characteristics of these syndromes are described, including a detailed account of sensory changes that occur at both painful and nonpainful sites and their utility for diagnosis and differential diagnosis; the identification/diagnostic criteria available so far are critically reviewed. The key role played by myofascial trigger points as activating factors of pain symptoms in other algogenic conditions--headache, fibromyalgia and visceral disease--is also addressed. Current hypotheses on the pathophysiology of myofascial pain syndromes are presented, including mechanisms of formation and persistence of primary and secondary trigger points as well as mechanisms beyond referred pain and hyperalgesia from trigger points. Conventional and most recent therapeutic options for these syndromes are described, and their validity is discussed on the basis of results from clinical controlled studies.

Quantification of local and referred pain in humans induced by intramuscular electrical stimulation

The basic knowledge related to referred muscle pain is limited. To study referred pain, an experimental model using intramuscular electrical stimulation has been developed. Four experiments were performed: (1) the thresholds for eliciting local (LPT) and referred pain (RPT) were determined; (2) stimulus-response functions relating stimulus intensity, pain intensity ratings and size of pain areas were determined; (3) inter- and intrasession variabilities were assessed; and (4) prolonged stimulations were given with a duration of 10 min to evaluate temporal aspects of the referred muscle pain. Intramuscular electrical stimulation of the tibialis anterior muscle elicited pain at the stimulation site in 94% of the subjects, and referred pain in 78% of the subjects. Referred pain was located in the anterior part of the ankle. The mean RPT was 72% higher than the mean LPT (p<0.01). Correlation was found between stimulus intensity, sensory/pain rating scores and size of pain areas (0.74< or =r< or =0.98,p<0.04). Size of pain areas and sensation/pain rating scores were correlated (0.86< or =r< or =0.97, p<0.01). Intersession variability showed that the LPTs were not significantly different (p>0.16), but the RPTs were disparate (p<0.02). Intrasession values revealed a significant difference between the five LPTs, RPTs, local and referred pain rating scores. The size of the local and referred pain areas remained constant. Prolonged stimulation at 150% of RPT showed that the onset (the first occurrence of pain) of referred pain occurred significantly later (43 s +/- 80 s) than at the local pain site (p<0.03). This study showed that local and referred muscle pain can be elicited by intramuscular electrical stimulation, and indicated that temporal and spatial summation may be involved in the elicitation of referred muscle pain.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis, upper cervical cord, NTS and Pa5 following capsaicin injection into masticatory and swallowing-related muscles in rats

Many phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) cells are expressed in the trigeminal spinal subnucleus caudalis (Vc), upper cervical spinal cord (C1-C2), nucleus tractus solitarii (NTS) and paratrigeminal nucleus (Pa5) after capsaicin injection into the whisker pad (WP), masseter muscle (MM), digastric muscle (DM) or sternohyoideus muscle (SM). The pERK-IR cells also showed NeuN immunoreactivity, indicating that ERK phosphorylation occurs in neurons. The pERK-IR cells were significantly reduced after intrathecal injection of MEK 1/2 inhibitor PD98059. The pERK-IR cells expressed bilaterally in the Vc and C1-C2 after capsaicin injection into the unilateral DM or SM, whereas unilaterally in the Vc and C1-C2 after unilateral WP or MM injection. After capsaicin injection into the WP or MM, the pERK-IR cell expression in the Vc was restricted rostrocaudally within a narrow area. However, the distribution of pERK-IR cells was more wide spread without a clear peak in the Vc and C1-C2 after capsaicin injection into the DM or SM. In the NTS, the unimodal pERK-IR cell expression peaked at 0-720μm rostral from the obex following capsaicin injection into WP, MM, DM or SM. In the ipsilateral Pa5, many pERK-IR cells were observed following capsaicin injection into the SM. The number of swallows elicited by distilled water administration was significantly smaller after capsaicin injection into the WP, MM or DM but not SM compared to that of vehicle-injected rats. Various noxious inputs due to the masticatory or swallowing-related muscle inflammation may be differentially involved in muscle pain and swallowing reflex activity.

Electromyographic activity of the cervical flexor muscles in patients with temporomandibular disorders while performing the craniocervical flexion test: a cross-sectional study

BACKGROUND

Most patients with temporomandibular disorders (TMD) have been shown to have cervical spine dysfunction. However, this cervical dysfunction has been evaluated only qualitatively through a general clinical examination of the cervical spine.

PURPOSE

The purpose of this study was to determine whether patients with TMD had increased activity of the superficial cervical muscles when performing the craniocervical flexion test (CCFT) compared with a control group of individuals who were healthy.

DESIGN

A cross-sectional study was conducted.

METHODS

One hundred fifty individuals participated in this study: 47 were healthy, 54 had myogenous TMD, and 49 had mixed TMD. All participants performed the CCFT. Data for electromyographic activity of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles were collected during the CCFT for all participants. A 3-way mixed-design analysis of variance for repeated measures was used to evaluate the differences in EMG activity for selected muscles while performing the CCFT under 5 incremental levels. Effect size values were calculated to evaluate the clinical relevance of the results.

RESULTS

Although there were no statistically significant differences in electromyographic activity in the SCM or AS muscles during the CCFT in patients with mixed and myogenous TMD compared with the control group, those with TMD tended to have increased activity of the superficial cervical muscles.

LIMITATIONS

The results obtained in this research are applicable for the group of individuals who participated in this study under the protocols used. They could potentially be applied to people with TMD having characteristics similar to those of the participants of this study.

CONCLUSION

This information may give clinicians insight into the importance of evaluation and possible treatment of the deep neck flexors in patients with TMD. However, future research should test the effectiveness of this type of program through a randomized controlled trial in people with TMD in order to determine the real value of treating this type of impairment in this population.

Modulation of astroglial glutamine synthetase activity affects nociceptive behaviour and central sensitization of medullary dorsal horn nociceptive neurons in a rat model of chronic pulpitis

Previous studies indicate that the astroglial glutamate-glutamine shuttle may be involved in acute pulpal inflammatory pain by influencing central sensitization induced in nociceptive neurons in the trigeminal subnucleus caudalis [the medullary dorsal horn (MDH)] by application of an inflammatory irritant to the rat tooth pulp. The aim of this study was to test if intrathecal application to the rat medulla of the astroglial glutamine synthetase inhibitor methionine sulfoximine (MSO) can influence the central sensitization of MDH nociceptive neurons and the animal's associated behaviour that are manifested in a model of chronic pulpitis pain induced by exposure of a mandibular molar pulp. This model was found to be associated with nocifensive behaviour and enhanced reflex activity evoked by mechanical stimulation of the rat's facial skin and with immunocytochemical evidence of astroglial activation in the MDH. These features were apparent for up to 28 days post-operatively. During this post-operative period, the nocifensive behaviour and enhanced reflex activity were significantly attenuated by intrathecal application of MSO (5 μL, 10 mM) but not by vehicle application. In electrophysiological recordings of nociceptive neuronal activity in the MDH, central sensitization was also evident in pulp-exposed rats but not in intact rats and could be significantly attenuated by MSO application but not by vehicle application. These behavioural and neuronal findings suggest that the astroglial glutamate-glutamine shuttle is responsible for the maintenance of inflammation-induced nocifensive behavioural changes and the accompanying central sensitization in MDH nociceptive neurons in this chronic pulpitis pain model.

Electrophysiological properties of spinal wide dynamic range neurons in neuropathic pain rats following spinal nerve ligation

OBJECTIVE

The present study aimed to investigate the electrophysiological properties of wide dynamic range (WDR) neurons in spinal dorsal horn of rats with neuropathic pain induced by lumber 5 (L5) spinal nerve ligation (SNL) in a large size of samples.

METHODS

Adult Sprague-Dawley rats were divided into normal and SNL groups. Electrophysiological technique was used to record the characteristics of WDR neurons in the spinal dorsal horn.

RESULTS

Compared with the WDR neurons in normal rats, the WDR neurons in SNL rats showed an increase in excitability, manifested by an enlargement of the receptive field size, an increase in the proportion of neurons that exhibited spontaneous activities, decreases in the C-response threshold and latency, and an increase in the C-response duration. In addition, the numbers of Aβ- and C-fiber-evoked discharges were smaller in SNL rats than in normal rats.

CONCLUSION

The excitability of spinal WDR neurons increased in rats with neuropathic pain induced by L5 SNL. The increase in excitability of WDR neurons may contribute to the development of neuropathic pain.

Facial injections of pruritogens and algogens excite partly overlapping populations of primary and second-order trigeminal neurons in mice

Intradermal cheek injection of pruitogens or algogens differentially elicits hindlimb scratching or forelimb wiping, suggesting that these behaviors distinguish between itch and pain. We studied whether pruritogens and algogens excite separate or overlapping populations of primary afferent and second-order trigeminal neurons in mice. Calcium imaging of primary sensory trigeminal ganglion (TG) cells showed that 15.4% responded to histamine, 5.8% to the protease-activated receptor (PAR)-2 agonist, 13.4% to allyl isothiocyanate (AITC), and 36.7% to capsaicin. AITC and/or capsaicin activated the vast majority of histamine- and PAR-2 agonist-sensitive TG cells. A chemical search strategy identified second-order neurons in trigeminal subnucleus caudalis (Vc) responsive to histamine, the PAR-2 agonist, or AITC. A minority of histamine or PAR-2 agonist-responsive Vc neurons responded to the other pruritogen, whereas a large majority of puritogen-responsive Vc neurons responded to capsaicin and/or AITC. A minority of AITC-responsive Vc neurons responded to pruritogens, whereas most responded to capsaicin. These data indicate that most primary and higher-order trigeminal sensory neurons are activated by both pruritic and algesic stimuli, although a minority exhibit selectivity. The results are discussed in terms of population codes for itch and pain that result in distinct behavioral responses of hindlimb scratching and forelimb wiping that are mediated at lumbar and cervical segmental levels, respectively.

Reduced endurance of the cervical flexor muscles in patients with concurrent temporomandibular disorders and neck disability

Subjects with temporomandibular disorders (TMDs) have been found to have clinical signs and symptoms of cervical dysfunction. Although many studies have investigated the relationship between the cervical spine and TMD, no study has evaluated the endurance capacity of the cervical muscles in patients with TMD. Thus the objective of this study was to determine whether patients with TMD had a reduced endurance of the cervical flexor muscles at any level of muscular contraction when compared with healthy subjects. One hundred and forty-nine participants provided data for this study (49 subjects were healthy, 54 had myogenous TMD, and 46 had mixed TMD). There was a significant difference in holding time at 25% MVC between subjects with mixed TMD when compared to subjects with myogenous TMD and healthy subjects. This implies that subjects with mixed TMD had less endurance capacity at a lower level of contraction (25% MVC) than healthy subjects and subjects with myogenous TMD. No significant associations between neck disability, jaw disability, clinical variables and neck flexor endurance test were found.

In vivo patch-clamp analysis of response properties of rat primary somatosensory cortical neurons responding to noxious stimulation of the facial skin

BACKGROUND

Although it has been widely accepted that the primary somatosensory (SI) cortex plays an important role in pain perception, it still remains unclear how the nociceptive mechanisms of synaptic transmission occur at the single neuron level. The aim of the present study was to examine whether noxious stimulation applied to the orofacial area evokes the synaptic response of SI neurons in urethane-anesthetized rats using an in vivo patch-clamp technique.

RESULTS

In vivo whole-cell current-clamp recordings were performed in rat SI neurons (layers III-IV). Twenty-seven out of 63 neurons were identified in the mechanical receptive field of the orofacial area (36 neurons showed no receptive field) and they were classified as non-nociceptive (low-threshold mechanoreceptive; 6/27, 22%) and nociceptive neurons. Nociceptive neurons were further divided into wide-dynamic range neurons (3/27, 11%) and nociceptive-specific neurons (18/27, 67%). In the majority of these neurons, a proportion of the excitatory postsynaptic potentials (EPSPs) reached the threshold, and then generated random discharges of action potentials. Noxious mechanical stimuli applied to the receptive field elicited a discharge of action potentials on the barrage of EPSPs. In the case of noxious chemical stimulation applied as mustard oil to the orofacial area, the membrane potential shifted depolarization and the rate of spontaneous discharges gradually increased as did the noxious pinch-evoked discharge rates, which were usually associated with potentiated EPSP amplitudes.

CONCLUSIONS

The present study provides evidence that SI neurons in deep layers III-V respond to the temporal summation of EPSPs due to noxious mechanical and chemical stimulation applied to the orofacial area and that these neurons may contribute to the processing of nociceptive information, including hyperalgesia.

Central sensitization induced in trigeminal and upper cervical dorsal horn neurons by noxious stimulation of deep cervical paraspinal tissues in rats with minimal surgical trauma

OBJECTIVE

This study investigated if central sensitization is induced in the trigeminal subnucleus caudalis (also termed the medullary dorsal horn) and C1 and C2 dorsal horns by noxious stimulation of deep upper cervical paraspinal tissues in a preparation relatively free of surgical trauma.

METHODS

Adult male Sprague-Dawley rats (275-450 g) were anesthetized intraperitoneally. Animals were then placed in a stereotaxic frame; a small cutaneous incision was made 3 to 4 mm near the bregma in the midline, and an opening into the skull was prepared by a 1/32-inch drill, 1 mm to the left from the midline. An epoxylite-coated tungsten microelectrode was introduced at an 18 degrees angle to enter this small opening on the skull and was then carefully advanced about 16 mm through cortex, cerebellum, and brainstem to reach subsequently histologically confirmed sites in the Vc and upper cervical (C1 and C2) dorsal horn region. Thirty-three, 27, and 15 neurons recorded in medullary, C1, and C2 dorsal horns, respectively, of chloralose/urethane-anesthetized rats were activated by noxious stimulation of mechanoreceptive fields involving V1, V2, and/or V3 trigeminal nerve territories. The inflammatory irritant mustard oil was injected into the deep paraspinal tissues at the level of the left C1-C2 joint. Pre and postinjection receptive field (RF) sizes were mapped by nonnoxious mechanical stimuli and noxious mechanical and heat stimuli.

RESULTS

A 30- to 50-minute increase (mean, 165% +/- 38.1%) in RF size postinjection for 62% of neurons tested was demonstrated, suggesting central sensitization; for most (>70%) neurons, the RF expanded caudally into cervically innervated tissues.

CONCLUSIONS

These findings provide the first documentation that deep cervical nociceptive inputs can induce central sensitization in medullary and C1/C2 dorsal horns and suggest that these effects may reflect mechanisms contributing to deep cervical pain and its referral.

Differential activation of the human trigeminal nuclear complex by noxious and non-noxious orofacial stimulation

There is good evidence from animal studies for segregation in the processing of non-nociceptive and nociceptive information within the trigeminal brainstem sensory nuclear complex. However, it remains unknown whether a similar segregation occurs in humans, and a recent tract tracing study suggests that this segregation may not exist. We used functional magnetic resonance imaging (fMRI) to define and compare activity patterns of the trigeminal brainstem nuclear complex during non-noxious and noxious cutaneous and non-noxious and noxious muscle orofacial stimulation in humans. We found that during cutaneous pain, signal intensity increased within the entire rostrocaudal extent of the spinal trigeminal nucleus (SpV), encompassing the ipsilateral oralis (SpVo), interpolaris (SpVi) and caudalis (SpVc) subdivisions. In contrast, muscle pain did not activate SpVi, but instead activated a discrete region of the ipsilateral SpVo and SpVc. Further, muscle noxious stimulation activated a region of the ipsilateral lateral pons in the region of the trigeminal principal sensory nucleus (Vp). Innocuous orofacial stimulation (lip brushing) also evoked a significant increase in signal intensity in the ipsilateral Vp; however, non-noxious muscle stimulation showed no increase in signal in this area. The data reveal that orofacial cutaneous and muscle nociceptive information and innocuous cutaneous stimulation are differentially represented within the trigeminal nuclear complex. It is well established that cutaneous and muscle noxious stimuli evoke different perceptual, behavioural and cardiovascular changes. We speculate that the differential activation evoked by cutaneous and muscle noxious stimuli within the trigeminal sensory complex may contribute to the neural basis for these differences.

Managing migraine associated with sensitization

The majority of migraineurs seeking secondary or tertiary medical care experience throbbing pain and cutaneous allodynia during the course of migraine. Underlying the origin of these symptoms are peripheral and central trigeminovascular neurons, whose cell bodies are located in the trigeminal ganglion and the spinal dorsal horn, respectively. The development of throbbing in the initial phase of migraine is mediated by sensitization of peripheral trigeminovascular neurons, whereas the development of cutaneous allodynia later in the attack is propelled by sensitization of central trigeminovascular neurons which, unfortunately, are not equipped to respond to triptans directly. Triptans appear to act presynaptically in the dorsal horn, such as to inhibit signal transmission from peripheral to central trigeminovascular neurons. Reining in the central neurons using triptan treatment is possible as long as their excitability remains driven by incoming signals from the meninges, but not after they develop autonomous activity. Accordingly, attacks with allodynia can be effectively terminated, provided that the patient vigilantly resorts to triptan therapy before or soon after the onset of allodynia, but not after allodynia has become firmly established. On the other hand, allodynic patients who missed the critical window for effective triptan therapy can still be rendered pain-free using an intravenous infusion of non-steroidal anti-inflammatory drugs.

Electrophysiological characterization of the rat trigeminal caudalis (Vc) neurons following intramuscular injection of capsaicin

Extracellular single unit recording experiments were performed to examine response characteristics of wide dynamic range neurons in the Vc that receive masseter afferent input in Sprague-Dawley rats. Capsaicin, or its vehicle, was directly administered into the masseter muscle and changes in resting discharge, responses to mechanical stimulation on the cutaneous receptive field and the electrical threshold for masseter nerve stimulation were assessed. Intramuscular capsaicin induced significant increase in the background discharge and mechanical hypersensitivity to the cutaneous stimulation and lowered the threshold for masseter nerve stimulation-evoked responses in the majority of neurons. The capsaicin-induced increase in evoked responses, but not the resting discharge, was partially attenuated when the muscle was pretreated with a mGluR antagonist. The present study suggests that injury or inflammation in the masseter muscle induce generalized hyperexcitability of central trigeminal neurons and that the blockade of peripherally localized mGluR5 can effectively attenuate muscular hypersensitivity.