Canadian Orofacial Pain Team workshop report on the global year against orofacial pain.

The year 2013-2014 has been designated the Global Year Against Orofacial Pain by the International Association for the Study of Pain. Accordingly, a multidisciplinary Canadian and international group of clinical, research and knowledge-transfer experts attended a workshop in Montreal, Quebec. The workshop had two aims: to identify new pathways for innovative diagnosis and management of chronic orofacial pain states; and to identify opportunities for further collaborative orofacial pain research and education in Canada. Three topics related to chronic orofacial pain were explored: biomarkers and pain signatures for chronic orofacial pain; misuse of analgesic and opioid pain medications for managing chronic orofacial pain; and complementary alternative medicine, topical agents and the role of stress in chronic orofacial pain. It was determined that further research is needed to: identify biomarkers of chronic orofacial post-traumatic neuropathic pain, with a focus on psychosocial, physiological and chemical-genetic factors; validate the short- and long-term safety (i.e., no harm to health, and avoidance of misuse and addiction) of opioid use for two distinct conditions (acute and chronic orofacial pain, respectively); and promote the use of topical medications as an alternative treatment in dentistry, and further document the benefits and safety of complementary and alternative medicine, including stress management, in dentistry. It was proposed that burning mouth syndrome, a painful condition that is not uncommon and affects mainly postmenopausal women, should receive particular attention.

Fractalkine signaling in microglia contributes to ectopic orofacial pain following trapezius muscle inflammation.

Fractalkine (FKN) signaling is involved in mechanical allodynia in the facial skin following trapezius muscle inflammation. Complete Freund's adjuvant (CFA) injection into the trapezius muscle produced mechanical allodynia in the ipsilateral facial skin that was not associated with facial skin inflammation and resulted in FKN but not FKN receptor (CX3CR1) expression, and microglial activation was enhanced in trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2). Intra-cisterna magna anti-CX3CR1 or anti-interleukin (IL)-1ß neutralizing antibody administration decreased the enhanced excitability of Vc and C1-C2 neurons in CFA-injected rats, whereas intra-cisterna magna FKN administration induced microglial activation and mechanical allodynia in the facial skin. IL-1ß expression and p38 mitogen-activated protein kinase phosphorylation were enhanced in activated microglia after CFA injection. The excitability of neurons whose receptive fields was located in the facial skin was significantly enhanced in CFA-injected rats, and the number of cells expressing phosphorylated extracellular signal-regulated kinase (pERK) following noxious mechanical stimulation of the facial skin was significantly increased in Vc and C1-C2. We also observed mechanical allodynia of the trapezius muscle as well as microglial activation and increased pERK expression in C2-C6 after noxious stimulation of the trapezius muscle in facial skin-inflamed rats. These findings suggest that FKN expression was enhanced in Vc and C1-C2 or C2-C6 following trapezius muscle or facial skin inflammation, microglia are activated via FKN signaling, IL-1ß is released from the activated microglia, and the excitability of neurons in Vc and C1-C2 or C2-C6 is enhanced, resulting in the ectopic mechanical allodynia.

Systemic pregabalin attenuates facial hypersensitivity and noxious stimulus-evoked release of glutamate in medullary dorsal horn in a rodent model of trigeminal neuropathic pain.

Pregabalin is effective in treating many neuropathic pain conditions. However, the mechanisms of its analgesic effects remain poorly understood. The aim of the present study was to determine whether pregabalin suppresses facial mechanical hypersensitivity and evoked glutamate release in the medullary dorsal horn (MDH) in a rodent model of trigeminal neuropathic pain. Nociceptive mechanical sensitivity was assessed pre-operatively, and then post-operatively 1h following pregabalin or vehicle (saline) treatment on post-operative days 2 and 5 following infraorbital nerve transection (IONX). In addition, an in vivo microdialysis probe was inserted into the exposed medulla post-operatively and dialysate samples were collected. Glutamate release was then evoked by mustard oil (MO) application to the tooth pulp, and the effects of pregabalin or vehicle were examined on the MDH glutamate release. Glutamate concentrations in the dialysated samples were determined by HPLC, and data analyzed by ANOVA. IONX animals (but not control animals) showed facial mechanical hypersensitivity for several days post-operatively. In addition, tooth pulp stimulation with MO evoked a transient release of glutamate in the MDH of IONX animals. Compared to vehicle, administration of pregabalin significantly attenuated the facial mechanical hypersensitivity as well as the MO-evoked glutamate release in MDH. This study provides evidence in support of recent findings pointing to the usefulness of pregabalin in the treatment of orofacial neuropathic pain.

Involvement of ATP in noxious stimulus-evoked release of glutamate in rat medullary dorsal horn: a microdialysis study.

Our electrophysiological studies have shown that both purinergic and glutamatergic receptors are involved in central sensitization of nociceptive neurons in the medullary dorsal horn (MDH). Here we assessed the effects of intrathecal administration of apyrase (a nucleotide degrading enzyme of endogenous adenosine 5-triphosphate [ATP]), a combination of apyrase and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, an adenosine A1 receptor antagonist), or 2,3-O-2,4,6-trinitrophenyl-adenosine triphosphate (TNP-ATP, a P2X1, P2X3, P2X2/3 receptor antagonist) on the release of glutamate in the rat MDH evoked by application of mustard oil (MO) to the molar tooth pulp. In vivo microdialysis was used to dialyse the MDH every 5 min, and included 3 basal samples, 6 samples after drug treatment and 12 samples following application of MO. Tooth pulp application of MO induced a significant increase in glutamate release in the MDH. Superfusion of apyrase or TNP-ATP alone significantly reduced the MO-induced glutamate release in the MDH, as compared to vehicle. Furthermore, the suppressive effects of apyrase on glutamate release were reduced by combining it with DPCPX. This study demonstrates that application of an inflammatory irritant to the tooth pulp induces glutamate release in the rat MDH in vivo that may be reduced by processes involving endogenous ATP and adenosine.

Corticomotor plasticity induced by tongue-task training in humans: a longitudinal fMRI study.

Corticomotor pathways may undergo neuroplastic changes in response to acquisition of new motor skills. Little is known about the motor control strategies for learning new tongue tasks. The aim of this study was to investigate the longitudinal effect of novel tongue-task training on corticomotor neuroplasticity. Thirteen healthy, right-handed men, aged 24-35 years (mean age ± SD: 27.3 ± 0.3 years), performed a training task consisting of standardized tongue protrusion onto a force transducer. The tongue task consisted of a relax-protrude-hold-relax cycle with 1.0 N as the target at the hold phase lasting for 1.5 s. Subjects repeated this task for 1 h. Functional magnetic resonance imaging was carried out before the tongue-task training (baseline), 1-h after the training, and one-day and one-week follow-up. During scanning, the subjects performed tongue protrusion in blocks interspersed with rest. A region-of-interest (ROI) approach and an explorative search were implemented for the analysis of corticomotor activity across conditions. All subjects completed the tongue-task training (mean success rate 43.0 ± 13.2%). In the baseline condition, tongue protrusion resulted in bilateral activity in regions most typically associated with a motor task including medial frontal gyrus (supplementary motor area [SMA]), precentral gyrus (tongue motor cortex), putamen, thalamus, and cerebellum. The ROI analysis revealed increased activity in the precentral gyrus already 1 h post-training. One day after the training, increased activity was observed in the precentral gyrus, SMA, putamen, and cerebellum. No increase was found 1 week after training. Correlation analyses between changes in success rates and changes in the numbers of voxels showed robust associations for left Area 4a in primary motor cortex 1 h, 1 day, and 1 week after the tongue-task training and for the left Area 4p in primary motor cortex and the left lateral premotor cortex 1 day after the training. In the unrestricted analysis, increased activity was found in the parahippocampal gyrus 1 h after the tongue-task training and remained for a week. Decreased activity was found in right post-central and middle frontal gyri 1 h and 1 week post-training. The results verified the involvement of specific corticomotor areas in response to tongue protrusion. Short-term tongue-task training was associated with longer-lasting (up to 1 week) changes in motor-related brain activity. The results suggested that primary motor areas are involved in the early and late stages, while other motor areas mainly are engaged in the later stage of corticomotor neuroplasticity of the tongue.

Central sensitization in medullary dorsal horn involves gap junctions and hemichannels.

Central sensitization is a fundamental mechanism contributing to acute and chronic pain conditions. Our previous studies have documented a glutamatergic, purinergic and glial-dependent central sensitization that can be induced in rat medullary dorsal horn nociceptive neurons by mustard oil application to the tooth pulp. This study showed that carbenoxolone, a potent gap junction and hemichannel blocker, completely blocked all parameters of mustard oil-induced central sensitization tested in functionally identified medullary dorsal horn nociceptive neurons. These results represent the first evidence suggesting that gap junctions and hemichannels may have a critical role in mediating central sensitization in dorsal horn nociceptive neurons and may account for the spread as well as development of central sensitization.

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.

Mechanisms involved in an increment of multimodal excitability of medullary and upper cervical dorsal horn neurons following cutaneous capsaicin treatment.

BACKGROUND: In order to evaluate mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc; the medullary dorsal horn) and upper cervical spinal cord (C1-C2) nociceptive neurons to heat, cold and mechanical stimuli following topical capsaicin treatment of the facial skin, nocifensive behaviors as well as phosphorylation of extracellular regulated-kinase (pERK) in Vc and C1-C2 neurons were studied in rats. RESULTS: Compared to vehicle application, capsaicin application to the lateral facial skin produced 1 hour later a flare in the skin, and also induced significantly greater nocifensive behaviors to heat, cold or mechanical stimulus of the lateral facial skin. The intrathecal (i.t.) injection of the MEK inhibitor PD98059 markedly attenuated the nocifensive behaviors to these stimuli in capsaicin-treated rats. Moreover, the number of pERK-like immunoreactive (pERK-LI) cells in Vc and C1-C2 was significantly larger following the heat, cold and mechanical stimuli in capsaicin-treated rats compared with vehicle-treated rats. The number of pERK-LI cells gradually increased following progressive increases in the heat or mechanical stimulus intensity and following progressive decrease in the cold stimulus. The ERK phosphorylation in Vc and C1-C2 neurons was strongly inhibited after subcutaneous injection of the capsaicin antagonist capsazepine in capsaicin-treated rats. CONCLUSION: The present findings revealed that capsaicin treatment of the lateral facial skin causes an enhancement of ERK phosphorylation in Vc and C1-C2 neurons as well as induces nocifensive behavior to heat, cold and mechanical simulation of the capsaicin-treated skin. The findings suggest that TRPV1 receptor mechanisms in rat facial skin influence nociceptive responses to noxious cutaneous thermal and mechanical stimuli by inducing neuroplastic changes in Vc and C1-C2 neurons that involve in the MAP kinase cascade.

Glutamine uptake contributes to central sensitization in the medullary dorsal horn.

Mustard oil application to tooth pulp produces central sensitization in rat medullary dorsal horn (MDH) nociceptive neurons, which has been implicated in persistent pain mechanisms. We found that superfusion onto MDH of methylaminoisobutyric acid, a competitive inhibitor of the neuronal system A transporter for presynaptic uptake of glutamine (a glutamate precursor released from astroglia), significantly depressed development of mustard oil-induced central sensitization in rat MDH nociceptive neurons. This finding indicates that the system A transporter is required for the expression of central sensitization and confirms the important roles of astroglia, glutamine and presynaptic modulation of glutamate release in the development of central sensitization.

Central sensitization induced in thalamic nociceptive neurons by tooth pulp stimulation is dependent on the functional integrity of trigeminal brainstem subnucleus caudalis but not subnucleus oralis.

We have previously demonstrated that application of the inflammatory irritant mustard oil (MO) to the rat molar tooth pulp induces central sensitization in nociceptive neurons within the contralateral ventroposterior medial (VPM) nucleus and posterior nuclear group (PO) of the thalamus as well as brainstem subnucleus caudalis (Vc) and subnucleus oralis (Vo). Since Vc and Vo are important relays of pulp afferent input to thalamus, the aim of this study was to test if local application of the synaptic blocker CoCl2 to Vc or Vo influences the pulp-induced thalamic central sensitization. The activity of 32 nociceptive-specific (NS) neurons within the rat VPM and immediately adjacent PO was recorded. Spontaneous activity, mechanoreceptive field (RF), mechanical activation threshold and evoked responses to graded mechanical stimuli were assessed before and after MO application to the pulp. MO application evoked immediate but short-lasting neuronal discharges in 21 of the 32 NS neurons tested, as well as central sensitization reflected in significant and long-lasting (> 60 min) RF expansion, decrease in activation threshold, and increase in graded pinch-evoked responses in all 32 NS neurons. CoCl2 applied to the ipsilateral Vc significantly attenuated these pulp-induced changes for 20 min or more. In contrast, CoCl2 applied to the ipsilateral Vo did not reverse this MO-induced central sensitization. Isotonic saline applied to Vc or Vo was also ineffective. These findings indicate that central sensitization induced in nociceptive neurons within VPM and PO by noxious stimulation of the tooth pulp is dependent upon the functional integrity of Vc but not Vo.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex.

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

Neck muscle length modulates nociceptive reflex evoked by noxious irritant application to rat neck tissues.

The application of mustard oil (MO), a small-fibre excitant and inflammatory irritant, into neck paraspinal muscles of the rat has been shown to produce a significant reflexive increase in electromyographic (EMG) activity in both neck and jaw muscles. It is possible that this nociceptive reflex activity is influenced by muscle length since recent evidence indicates that abnormal neck posture may be associated with cervical musculoskeletal disorders and pain. Therefore, the aim of this study was to test if muscle length modulates this nociceptive reflex response. Three different experimental procedures were employed in rats under halothane anesthesia: (1) MO injected into the left deep neck muscles with the rat placed in a straight body position (Straight group, n = 7); (2) MO injected into lengthened left deep neck muscles with the rat's neck rotated 45 degrees to the right with respect to the head (Stretched group, n = 11); and (3) MO injected into the right deep neck muscles with the rat's neck rotated 45 degrees to the right (Relaxed group, n = 9). The EMG activity of the deep neck, trapezius, and digastric muscles was bilaterally recorded, rectified and integrated into area under the curve (AUC). Control injections of the vehicle, mineral oil, did not evoke any muscle activity but MO evoked EMG activity in the ipsilateral deep neck and trapezius muscles of the Stretched group that was significantly greater than that evoked in the same muscles in the Straight and Relaxed groups. Also, the MO-evoked EMG activity in the contralateral deep neck muscles of the Stretched and Relaxed groups was greater than that of the corresponding muscles in the Straight group. The MO-evoked activity in the digastric, a jaw muscle whose length was not changed, did not show any significant difference between the three groups. These findings indicate that MO application to the rat deep neck muscles results in a larger nociceptive reflex in deep neck and trapezius muscles when they are stretched. This enhanced muscle activity could be associated with changes in the susceptibility of the neck muscles to pain or damage.

Plasticity in corticomotor control of the human tongue musculature induced by tongue-task training.

Transcranial magnetic stimulation (TMS) has been used to assess characteristics of the corticomotor control of the jaw muscles, but less is known about the cortical control of the human tongue and its modification by training. The aim of the present study was to determine the effect of training humans in a novel tongue-protrusion task for 1 week on corticomotor excitability as assessed by changes in electromyographic activity elicited in the tongue musculature by TMS, and in the tongue cortical motor map revealed by TMS. Eleven healthy subjects participated. Stimulus-response curves were generated from the motor evoked potentials (MEPs) recorded in the tongue musculature and, from the first dorsal interosseos (FDI) muscle as a control, at three time periods: at baseline, immediately after the 1-week training period, and at 2-weeks follow-up. In addition, the corticomotor representations of the tongue and FDI muscles were mapped on a 1 x 1 cm scalp grid. The tongue-training task required each subject to protrude the tongue onto a force transducer placed in front of the subject, and consisted of a relax-protrude-hold-relax cycle lasting 12.5 s with 1 N as the target at the hold phase. The subjects repeated this task for 60 min every day for 1 week. All subjects reported moderate levels of fatigue in the tongue during the first training day; however, these subjective reports decreased during the week (ANOVA P<0.001), and the subjects showed a progressive increase in their ability to perform the task successfully ( P<0.001). The threshold for evoking MEPs by TMS in the tongue musculature was significantly decreased after the last training day compared with baseline and the 2-weeks follow-up ( P<0.001). The amplitude of the MEPs in the tongue musculature was significantly increased at higher intensities of TMS after the last training day but returned to baseline values at the 2-weeks follow-up (P = 0.005). No significant effect of the training on MEPs in the FDI was observed (P = 0.493). Analysis of the corticomotor topographic maps revealed a significant ( P<0.05) increase in excitability and, hence, the cortical area from which TMS could evoke MEPs in the tongue, although the center of gravity representation for the tongue or FDI muscles remained stable. The present findings suggest that a specific and reversible plasticity of the corticomotor excitability related to tongue muscle control can be induced when humans learn to perform successfully a novel tongue task.

P2X receptors in trigeminal subnucleus caudalis modulate central sensitization in trigeminal subnucleus oralis.

This study investigated the role of trigeminal subnucleus caudalis (Vc) P2X receptors in the mediation of central sensitization induced in nociceptive neurons in subnucleus oralis (Vo) by mustard oil (MO) application to the tooth pulp in anesthetized rats. MO application produced a long-lasting central sensitization reflected in neuroplastic changes (i.e., increases in neuronal mechanoreceptive field size and responses to innocuous and noxious mechanical stimuli) in Vo nociceptive neurons. Twenty minutes after MO application, the intrathecal (i.t.) administration to the rostral Vc of the selective P2X(1), P2X(3), and P2X(2/3) receptor antagonist, 2'-(or 3'-)O-trinitrophenyl-ATP (TNP-ATP), significantly and reversibly attenuated the MO-induced central sensitization for more than 15 min; saline administration had no effect. Administration to the rostral Vc of the selective P2X(1), P2X(3), and P2X(2/3) receptor agonist, alpha,beta-methylene ATP (alpha,beta-meATP, i.t.) produced abrupt and significant neuroplastic changes in Vo nociceptive neurons, followed by neuronal desensitization as evidenced by the ineffectiveness of a second i.t. application of alpha,beta-meATP and subsequent MO application to the pulp. Administration to the rostral Vc of the selective P2X(1) receptor agonist beta,gamma-methylene ATP (beta,gamma-meATP, i.t.) produced no significant neuroplastic changes per se and did not affect the subsequent MO-induced neuroplastic changes in Vo nociceptive neurons. These results suggest that P2X(3) and possibly also the P2X(2/3) receptor subtypes in Vc may play a role in the initiation and maintenance of central sensitization in Vo nociceptive neurons induced by MO application to the pulp.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis.

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.