Temporomandibular joint inflammation activates glial and immune cells in both the trigeminal ganglia and in the spinal trigeminal nucleus

Molecular Aspects Involved in the Mechanisms of Bothrops jararaca Venom-Induced Hyperalgesia: Participation of NK1 Receptor and Glial Cells

Accidents caused by Bothrops jararaca (Bj) snakes result in several local and systemic manifestations, with pain being a fundamental characteristic. The inflammatory process responsible for hyperalgesia induced by Bj venom (Bjv) has been studied; however, the specific roles played by the peripheral and central nervous systems in this phenomenon remain unclear. To clarify this, we induced hyperalgesia in rats using Bjv and collected tissues from dorsal root ganglia (DRGs) and spinal cord (SC) at 2 and 4 h post-induction. Samples were labeled for Iba-1 (macrophage and microglia), GFAP (satellite cells and astrocytes), EGR1 (neurons), and NK1 receptors. Additionally, we investigated the impact of minocycline, an inhibitor of microglia, and GR82334 antagonist on Bjv-induced hyperalgesia. Our findings reveal an increase in Iba1 in DRG at 2 h and EGR1 at 4 h. In the SC, markers for microglia, astrocytes, neurons, and NK1 receptors exhibited increased expression after 2 h, with EGR1 continuing to rise at 4 h. Minocycline and GR82334 inhibited venom-induced hyperalgesia, highlighting the crucial roles of microglia and NK1 receptors in this phenomenon. Our results suggest that the hyperalgesic effects of Bjv involve the participation of microglial and astrocytic cells, in addition to the activation of NK1 receptors.

Cannabis Bioactive Compound-Based Formulations: New Perspectives for the Management of Orofacial Pain

The management of orofacial pain to alleviate the quality of life of affected patients is becoming increasingly challenging for scientific research and healthcare professionals. From this perspective, in addition to conventional therapies, new alternatives are being sought, increasingly looking at the use of both natural and synthetic products. Cannabis sativa L. represents an interesting source of bioactive compounds, including non-psychoactive cannabinoids, flavonoids, and terpenes, many of which are effective in improving pain intensity. Here, we aim to analyze the possible mechanisms of action of the bioactive natural and synthetic hemp-derived compounds responsible for the modulatory effects on pain-related pathways. The ability of these compounds to act on multiple mechanisms through a synergistic effect, reducing both the release of inflammatory mediators and regulating the response of the endocannabinoid system, makes them interesting agents for alternative formulations to be used in orofacial pain.

Temporomandibular disorder: A previously unreported complication of chronic suppurative otitis media

OBJECTIVE

The aim of the current clinical study was to test the hypothesis that chronic suppurative otitis media (CSOM) might be significantly associated with signs of temporomandibular joint (TMJ) internal derangement.

METHODS

The study involved 79 patients with CSOM and 79 control subjects. The TMJ was clinically examined in both groups.

RESULTS

Signs of internal derangement of the TMJ(s) were found in 67.1% of CSOM patients versus 26.6% of control subjects (p = .001).

CONCLUSION

CSOM may be associated with the extension of the inflammatory process into the TMJ, thereby predisposing to internal derangement of the joint.

Modulation of Glia Activation by TRPA1 Antagonism in Preclinical Models of Migraine

Preclinical data point to the contribution of transient receptor potential ankyrin 1 (TRPA1) channels to the complex mechanisms underlying migraine pain. TRPA1 channels are expressed in primary sensory neurons, as well as in glial cells, and they can be activated/sensitized by inflammatory mediators. The aim of this study was to investigate the relationship between TRPA1 channels and glial activation in the modulation of trigeminal hyperalgesia in preclinical models of migraine based on acute and chronic nitroglycerin challenges. Rats were treated with ADM_12 (TRPA1 antagonist) and then underwent an orofacial formalin test to assess trigeminal hyperalgesia. mRNA levels of pro- and anti-inflammatory cytokines, calcitonin gene-related peptide (CGRP) and glia cell activation were evaluated in the Medulla oblongata and in the trigeminal ganglia. In the nitroglycerin-treated rats, ADM_12 showed an antihyperalgesic effect in both acute and chronic models, and it counteracted the changes in CGRP and cytokine gene expression. In the acute nitroglycerin model, ADM_12 reduced nitroglycerin-induced increase in microglial and astroglial activation in trigeminal nucleus caudalis area. In the chronic model, we detected a nitroglycerin-induced activation of satellite glial cells in the trigeminal ganglia that was inhibited by ADM_12. These findings show that TRPA1 antagonism reverts experimentally induced hyperalgesia in acute and chronic models of migraine and prevents multiple changes in inflammatory pathways by modulating glial activation.

P2Y14 receptor in trigeminal ganglion contributes to neuropathic pain in mice

Trigeminal nerve injury is a common complication of various dental and oral procedures, which could induce trigeminal neuropathic pain but lack effective treatments. P2 purinergic receptors have emerged as novel therapeutic targets for such pain. Recent reports implied that the P2Y₁₄ receptor (P2Y₁₄R) was activated and promoted orofacial inflammatory pain and migraine. However, the role and mechanism of P2Y₁₄R in trigeminal neuropathic pain remain unknown. We induced an orofacial neuropathic pain model by chronic constriction injury of the infraorbital nerve (CCI-ION). Von-Frey tests showed that CCI-ION induced orofacial mechanical hypersensitivity. The increased activating transcription factor 3 (ATF3) expression in the trigeminal ganglion (TG) measured by immunofluorescence confirmed trigeminal nerve injury. Immunofluorescence showed that P2Y₁₄R was expressed in trigeminal ganglion neurons (TGNs) and satellite glial cells (SGCs). RT-qPCR and Western blot identified increased expression of P2Y₁₄R in TG after CCI-ION. CCI-ION also upregulated interleukin-1β (IL-1β), interleukin-6 (IL-6), C-C motif chemokine ligand 2 (CCL2), and tumor necrosis factor-α (TNF-α) in TG. Notably, CCI-ION-induced mechanical hypersensitivity and pro-inflammatory cytokines production were decreased by a P2Y₁₄R antagonist (PPTN). Trigeminal administration of P2Y₁₄R agonist (UDP-glucose) evoked orofacial mechanical hypersensitivity and increased pro-inflammatory cytokines above in TG. Furthermore, CCI-ION induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 in TG, which also were reduced by PPTN. The inhibitors of ERK1/2 (U0126) and p38 (SB203580) decreased these upregulated pro-inflammatory cytokines after CCI-ION. Collectively, this study revealed that P2Y₁₄R in TG contributed to trigeminal neuropathic pain via ERK- and p38-dependent neuroinflammation. Thus, P2Y₁₄R may be a potential drug target against trigeminal neuropathic pain.

Using dual polarities of transcranial direct current stimulation in global cerebral ischemia and its following reperfusion period attenuates neuronal injury

Multiple neuronal injury pathways are activated during cerebral ischemia and reperfusion (I/R). This study was designed to decrease potential neuronal injuries by using both transcranial direct current stimulation (tDCS) polarities in cerebral ischemia and its following reperfusion period. Ninety rats were randomly divided into six groups. In the sham group, rats were intact. In the I/R group, global cerebral I/R was only induced. In the I/R + c-tDCS and I/R + a-tDCS groups, cathodal and anodal currents were applied, respectively. In the I/R + c/a-tDCS, cathodal current was used in the cerebral ischemia and anodal in the reperfusion. In the I/R + a/c-tDCS group, cathodal and anodal currents were applied in the I/R, respectively. Hippocampal tissue was used to determine the levels of IL-1β, TNF-α, NOS, SOD, MDA, and NMDAR. Hot plate and open field tests evaluated sensory and locomotor performances. The cerebral edema was also measured. Histological assessment was assessed by H/E and Nissl staining of the hippocampal CA1 region. All tDCS modes significantly decreased IL-1β and TNF-α levels, especially in the c/a-tDCS. All tDCS caused a significant decrease in MDA and NOS levels while increasing SOD activity compared to the I/R group, especially in the c/a-tDCS mode. In the c-tDCS and a/c-tDCS groups, the NMDAR level was significantly decreased. The c/a-tDCS group improved sensory and locomotor performances more than other groups receiving tDCS. Furthermore, the least neuronal death was observed in the c/a-tDCS mode. Using two different polarities of tDCS could induce more neuroprotective versus pathophysiological pathways in cerebral I/R, especially in c/a-tDCS mode. HIGHLIGHTS: Multiple pathways of neuronal injury are activated in cerebral ischemia and reperfusion (I/R). Using tDCS could modulate neuroinflammation and oxidative stress pathways in global cerebral I/R. Using c/a-tDCS mode during cerebral I/R causes more neuroprotective effects against neuronal injuries of cerebral I/R.

The neuroprotective effects of transcranial direct current stimulation on global cerebral ischemia and reperfusion via modulating apoptotic pathways

BACKGROUND

Cerebral ischemia-reperfusion, subsequent hyperthermia, and hyperglycemia lead to neural damage. This study aimed to investigate the effects of using cathodal and/or anodal transcranial direct current stimulation (tDCS) in different stages of ischemia-reperfusion on apoptosis and controlling hyperthermia and hyperglycemia.

MATERIALS AND METHODS

A total of 78 male Wistar rats were randomly assigned into six groups (n=13), including sham, ischemia/reperfusion (I/R), anodal-tDCS (a-tDCS), cathodal-tDCS (c-tDCS), anodal/cathodal-tDCS (a/c-tDCS), and cathodal/anodal-tDCS (c/a-tDCS) groups. Global cerebral I/R was induced in all of the groups except for sham group. In a-tDCS and c-tDCS groups, the rats received anodal and cathodal currents in both I/R stages, respectively. In a/c-tDCS group, the rats received anodal current during the ischemia and cathodal current during the reperfusion. The c/a-tDCS group received the currents in the reverse order. The current intensity of 400µA was applied in ischemia phase (15min) and reperfusion phase (30min, twice a day). Body temperature and plasma blood sugar were measured daily. Rats were also tested for novel object recognition and passive avoidance memory. The apoptosis of hippocampal tissue was evaluated by measuring Bax, Bcl-2, Caspase-3, and TUNEL staining.

RESULTS

All tDCS significantly reduced hyperthermia and hyperglycemia, as well as Bax and Caspase-3 levels, it also increased Bcl-2 expression. The preliminary results from c/a-tDCS mode could improve the expression of apoptotic markers, memory function, hyperthermia, and hyperglycemia control and reduce DNA fragmentation compared to other stimulatory therapies.

CONCLUSION

All tDCS modes could save neurons by suppressing apoptotic and enhancing anti-apoptotic pathways, especially in the c/a tDCS mode.

A Single Immediate Use of the Cathodal Transcranial Direct Current Stimulation Induces Neuroprotection of Hippocampal Region Against Global Cerebral Ischemia

OBJECTIVES

Global cerebral ischemia (CI) causes severe neuronal injury, mainly in the hippocampal CA1 region. This study aimed to investigate an immediate using transcranial direct current stimulation (tDCS) in reducing neuronal injury induced by CI.

MATERIALS AND METHODS

The 32 Wistar male rats were randomly divided into four groups (n=8 per group). In the ischemia group (I), CI was induced via the 4-vessel occlusion model. In the sham group (Sh), rats did not receive any intervention. In the ischemia+cathodal group (I+c/tDCS), the cathodal current was applied during CI. In the ischemia+anodal group (I+a/tDCS), the anodal current was applied. The current intensity of 400 μA was applied for 15-min during the ischemia. Hippocampal tissue was used to assess levels of NMDAR, IL-1β, TNF-α, MDA, SOD, NOS, and apoptosis markers. Histological assessment and TUNEL staining were performed in CA1 hippocampal region.

RESULTS

The c/tDCS significantly decreased the levels of IL-1β and TNF-α than the I and a/tDCS groups. The c/tDCS significantly reduced MDA and NOS levels, while increasing the level of SOD than the I and a/tDCS. The c/tDCS caused a significant decrease in NMDAR level than the a/tDCS. Using c/tDCS significantly reduced the Bax and Caspase-3 expressions, while increasing the Bcl-2 expression than the I group. In the c/tDCS group, DNA fragmentation and neuronal death were significantly lower than the I and a/tDCS groups.

CONCLUSION

Using cathodal a direct current could attenuate primary pathophysiological pathways induced by CI, and it eventually reduced neurons death and apoptosis in the CA1 hippocampal region.

Discrepancy in the Usage of GFAP as a Marker of Satellite Glial Cell Reactivity

Satellite glial cells (SGCs) surrounding the neuronal somas in peripheral sensory ganglia are sensitive to neuronal stressors, which induce their reactive state. It is believed that such induced gliosis affects the signaling properties of the primary sensory neurons and is an important component of the neuropathic phenotype leading to pain and other sensory disturbances. Efforts to understand and manipulate such gliosis relies on reliable markers to confirm induced SGC reactivity and ultimately the efficacy of targeted intervention. Glial fibrillary acidic protein (GFAP) is currently the only widely used marker for such analyses. However, we have previously described the lack of SGC upregulation of GFAP in a mouse model of sciatic nerve injury, suggesting that GFAP may not be a universally suitable marker of SGC gliosis across species and experimental models. To further explore this, we here investigate the regulation of GFAP in two different experimental models in both rats and mice. We found that whereas GFAP was upregulated in both rodent species in the applied inflammation model, only the rat demonstrated increased GFAP in SGCs following sciatic nerve injury; we did not observe any such GFAP upregulation in the mouse model at either protein or mRNA levels. Our results demonstrate an important discrepancy between species and experimental models that prevents the usage of GFAP as a universal marker for SGC reactivity.

Involvement of P2Y12 receptors in an NTG-induced model of migraine in male mice

BACKGROUND AND PURPOSE

P2Y₁₂ receptors (P2Y₁₂ Rs) are known to regulate different forms of pain and inflammation. In this study we investigated the participation of P2Y₁₂ Rs in an animal model of migraine.

EXPERIMENTAL APPROACH

We tested the effect of the centrally administered selective P2Y₁₂ R antagonist PSB-0739, and P2Y₁₂ R gene deficiency in acute nitroglycerin (NTG)-treated mice. Additionally, platelet depletion was used to investigate the role of platelet P2Y₁₂ Rs during migraine-like pain.

KEY RESULTS

NTG induced sensory hypersensitivity of C57BL/6 wild-type (P2ry12^+/+ ) mice, accompanied by an increase in c-fos and CGRP expression in the upper cervical spinal cord (C1-C2) and trigeminal nucleus caudalis (TNC). Similar changes were also observed in P2Y₁₂ R gene-deficient (P2ry12^-/- ) mice. Prophylactic intrathecal application of PSB-0739 reversed thermal hyperalgesia and head grooming time in wild-type mice but had no effect in P2ry12^-/- mice; furthermore, it was also effective when applied as a post-treatment. PSB-0739 administration suppressed the expression of c-fos in C1-C2 and TNC, and decrease C1-C2 levels of dopamine and serotonin in wild-type mice. NTG treatment itself did not change adenosine diphosphate (ADP)-induced platelet activation measured by CD62P upregulation in wild-type mice. Platelet depletion by anti-mouse CD41 antibody and clopidogrel attenuated NTG-induced thermal hypersensitivity and head grooming time in mice.

CONCLUSION AND IMPLICATIONS

Taken together, our findings show that acute inhibition of P2Y₁₂ Rs alleviates migraine-like pain in mice, by modulating the expression of c-fos, and platelet P2Y₁₂ Rs might contribute to this effect. Hence, it is suggested that the blockade of P2Y₁₂ Rs may have therapeutic potential against migraine.

Role of Connexin 43 in an Inflammatory Model for TMJ Hyperalgesia

Temporomandibular joint disorders (TMD) consist of a heterogeneous group of conditions that present with pain in the temporomandibular joint (TMJ) region and muscles of mastication. This project assessed the role of connexin 43 (Cx43), a gap junction protein, in the trigeminal ganglion (TG) in an animal model for persistent inflammatory TMJ hyperalgesia. Experiments were performed in male and female rats to determine if sex differences influence the expression and/or function of Cx43 in persistent TMJ hyperalgesia. Intra-TMJ injection of Complete Freund's Adjuvant (CFA) caused a significant increase in Cx43 expression in the TG at 4 days and 10 days post-injection in ovariectomized (OvX) female rats and OvX females treated with estradiol (OvXE), while TG samples in males revealed only marginal increases. Intra-TG injection of interference RNA for Cx43 (siRNA Cx43) 3 days prior to recording, markedly reduced TMJ-evoked masseter muscle electromyographic (MMemg) activity in all CFA-inflamed rats, while activity in sham animals was not affected. Western blot analysis revealed that at 3 days after intra-TG injection of siRNA Cx43 protein levels for Cx43 were significantly reduced in TG samples of all CFA-inflamed rats. Intra-TG injection of the mimetic peptide GAP19, which inhibits Cx43 hemichannel formation, greatly reduced TMJ-evoked MMemg activity in all CFA-inflamed groups, while activity in sham groups was not affected. These results revealed that TMJ inflammation caused a persistent increase in Cx43 protein in the TG in a sex-dependent manner. However, intra-TG blockade of Cx43 by siRNA or by GAP19 significantly reduced TMJ-evoked MMemg activity in both males and females following TMJ inflammation. These results indicated that Cx43 was necessary for enhanced jaw muscle activity after TMJ inflammation in males and females, a result that could not be predicted on the basis of TG expression of Cx43 alone.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Increased substance P and synaptic remodeling occur in the trigeminal sensory system with sustained osteoarthritic temporomandibular joint sensitivity

Supplemental Digital Content is Available in the Text.

Increased substance P and a loss of inhibitory synapses occurs within the brain's trigeminal sensory system with persistent, but not transient, temporomandibular joint sensitivity.

Introduction:

Temporomandibular joint (TMJ) pain is among the most prevalent musculoskeletal conditions and can result from atypical joint loading. Although TMJ pain is typically self-resolving, 15% of patients develop chronic TMJ pain that is recalcitrant to therapy and may be attributed to changes in pain processing centers. Although TMJ overloading induces pain and osteoarthritis, whether neuronal modifications in the trigeminal sensory system contribute to persistent TMJ pain is unknown.

Objective:

This study investigates changes in excitatory neuropeptides and synaptic transmission proteins in cases of transient and persistent TMJ sensitivity in a rat model.

Methods:

Rats underwent repeated jaw loading that produces transient (2N-load) or persistent (3.5N-load) sensitivity. In both groups, immunolabeling was used to assess substance P in the spinal trigeminal nucleus caudalis (Sp5C) and glutamate transporter 1 in the ventroposteriomedial thalamus early after loading. Synaptosomal Western blots were used to measure synaptic proteins in the caudal medulla and thalamus at a later time after loading.

Results:

Substance P increases transiently in the Sp5C early after loading that induces persistent sensitivity. However, glutamate transporter 1 is unchanged in the ventroposteriomedial thalamus. At a later time, synaptosomal Western blots show loss of the presynaptic tethering protein, synapsin, and the inhibitory scaffolding protein, gephyrin, in the thalamus with persistent, but not transient, sensitivity. No changes are identified in synapsin, phosphorylated synapsin, homer, or gephyrin in the caudal medulla.

Conclusions:

Substance P in the Sp5C and later loss of inhibitory synapses in the thalamus likely contribute to, or indicate, persistent TMJ pain.

A Systematic Review of Rat Models With Temporomandibular Osteoarthritis Suitable for the Study of Emerging Prolonged Intra-Articular Drug Delivery Systems

PURPOSE

Development of minimally invasive therapies for temporomandibular joint osteoarthritis (TMJOA) has focused on drug intra-articular injections to avoid the systemic adverse effects experienced when these substances are administered orally. Therefore, we performed a systematic review to answer the question "Which method of induction of a TMJOA-related pain model in rats leads to prolonged painful symptoms, allowing the best assessment of a sustained drug delivery system?"

MATERIALS AND METHODS

Following the PRISMA guidelines, we searched MEDLINE for papers published from 1994 to July 2020 on a TMJ arthritis model using rats. We identified the means of pain induction and of nociception assessment. We assessed protocol bias using an adaptation of the QUADAS-2 tool. Animal selection, the reference standard method of pain assessment, applicability of a statistical assessment, and flow and timing were assessed.

RESULTS

Of the 59 full papers we reviewed, 41 performed no pain assessment after the first 7 days following induction of the TMJ-related pain model. We eventually identified 18 long-term TMJOA-related pain models. Pain was induced by injection of toxic substances, most commonly Freund's complete adjuvant (50 μg per 50 μl), formalin at various concentrations, or monosodium iodoacetate (0,5 mg per 50 μl), into the TMJ, or by physical methods. Few studies reported data on pain after 21 days of follow-up. Heterogeneity of induction methods, pain assessment methods, and flow and timing biases precluded a meta-analysis.

CONCLUSIONS

Given that pain is 1 of the main symptoms of TMJOA, experimental study protocols should include long-term pain assessment.

Therapeutic Agents for the Treatment of Temporomandibular Joint Disorders: Progress and Perspective

Temporomandibular joint disorders (TMD) are a common health condition caused by the structural or functional disorders of masticatory muscles and the temporomandibular joint (TMJ). Abnormal mandibular movement in TMD patients may cause pain, chronic inflammation, and other discomfort, which could be relieved by a variety of drugs through various delivery systems. In this study, we summarized commonly used therapeutic agents in the management of TMD as well as novel bioactive molecules in preclinical stage and clinical trials. The emerging therapy strategies such as novel intra-TMJ delivery systems and implants based on tissue engineering are also discussed. This comprehensive review will strengthen our understanding of pharmacological approaches for TMD therapy.

Preliminary evidence for conserved transcriptional response to adversity in adults with temporomandibular disorder

Temporomandibular disorder (TMD) is one of the most common orofacial pain conditions. Alteration in immune functioning is one promising biological mechanism underlying pain in TMD. However, there is a gap in the understanding of molecular bases contributing to altered immune functioning in these patients.

OBJECTIVES

In the current study, we investigated whether individuals with TMD would exhibit differential activity of 3 specific transcription factors involved in inflammatory (nuclear factor-kappa B, NF-kB), antiviral (interferon-regulatory factors, IRF), and sympathetic (cAMP response element-binding protein, CREB) processes using a promoter-based bioinformatics analysis, which is characterized as the "Conserved Transcriptional Response to Adversity."

METHODS

Adults with TMD (n = 19) and without (n = 17) underwent a standardized clinical examination for TMD. A blood sample was collected for genome-wide transcriptional RNA profiling. Bioinformatic analyses tested for differential prevalence of proinflammatory and antiviral transcription factor activity in core promoter sequences from all genes showing >1.2-fold differential expression in TMD vs controls.

RESULTS

Promoter-based bioinformatic analyses of genome-wide transcriptome profiles confirmed upregulation of genes bearing response elements for proinflammatory transcription factor (NF-kB, P = 0.002) and downregulation of genes with response elements for IRF (P = 0.037) in patients with TMD relative to controls. Results also indicated upregulated activity of CREB in patients with TMD (P = 0.08), consistent with increased activity of the sympathetic nervous system.

CONCLUSION

These results provide initial support that the regulation of immune pathways is altered in individuals with TMD. A shift of transcriptional resources to a proinflammatory state may be driven by psychosocial stress and contributes to symptoms associated with TMD.

From astrocytes to satellite glial cells and back: A 25 year-long journey through the purinergic modulation of glial functions in pain and more

Fundamental progresses have been made in pain research with a comprehensive understanding of the neuronal pathways which convey painful sensations from the periphery and viscera to the central nervous system and of the descending modulating pathways. Nevertheless, many patients still suffer from various painful conditions, which are often associated to other primary pathologies, and get no or poor relief from available painkillers. Thus, the interest of many researchers has concentrated on new and promising cellular targets and biochemical pathways. This is the case of glia cells, both in the peripheral and in the central nervous system, and of purinergic receptors. Starting from many intuitions and hypotheses raised by Prof. Geoffrey Burnstock, data have accumulated which clearly highlight the fundamental role exerted by several nucleotide and nucleoside receptors in the modulation of glial cell reaction to pain triggers and of their cross-talk with sensory neurons which significantly contributes to the transition from acute to chronic pain. The purinergic system has therefore become an appealing pharmacological target in pain research, also based on the quite unexpected discovery that purines are involved in ancient analgesic techniques such as acupuncture. A more in-depth understanding of the complex and intricated purine-orchestrated scenario in pain conditions will hopefully lead to the identification and clinical development of new and effective analgesics.

Glial cell activation and altered metabolic profile in the spinal-trigeminal axis in a rat model of multiple sclerosis associated with the development of trigeminal sensitization

Trigeminal neuralgia is often an early symptom of multiple sclerosis (MS), and it generally does not correlate with the severity of the disease. Thus, whether it is triggered simply by demyelination in specific central nervous system areas is currently questioned. Our aims were to monitor the development of spontaneous trigeminal pain in an animal model of MS, and to analyze: i) glial cells, namely astrocytes and microglia in the central nervous system and satellite glial cells in the trigeminal ganglion, and ii) metabolic changes in the trigeminal system. The subcutaneous injection of recombinant MOG_1-125 protein fragment to Dark Agouti male rats led to the development of relapsing-remitting EAE, with a first peak after 13 days, a remission stage from day 16 and a second peak from day 21. Interestingly, orofacial allodynia developed from day 1 post injection, i.e. well before the onset of EAE, and worsened over time, irrespective of the disease phase. Activation of glial cells both in the trigeminal ganglia and in the brainstem, with no signs of demyelination in the latter tissue, was observed along with metabolic alterations in the trigeminal ganglion. Our data show, for the first time, the spontaneous development of trigeminal sensitization before the onset of relapsing-remitting EAE in rats. Additionally, pain is maintained elevated during all stages of the disease, suggesting the existence of parallel mechanisms controlling motor symptoms and orofacial pain, likely involving glial cell activation and metabolic alterations which can contribute to trigger the sensitization of sensory neurons.

Retrograde Labeling of Different Distribution Features of DRG P2X2 and P2X3 Receptors in a Neuropathic Pain Rat Model

The distributions of P2X subtypes during peripheral neuropathic pain conditions and their differential roles are not fully understood. To explore these characteristics, the lumbosacral dorsal root ganglion (DRG) in the chronic constriction injury (CCI) sciatic nerve rat model was studied. Retrograde trace labeling combined with immunofluorescence technology was applied to analyze the distribution of neuropathic nociceptive P2X1-6 receptors. Our results suggest that Fluoro-Gold (FG) retrograde trace labeling is an efficient method for studying lumbosacral DRG neurons in the CCI rat model, especially when the DRG neurons are divided into small, medium, and large subgroups. We found that neuropathic nociceptive lumbosacral DRG neurons (i.e., FG-positive cells) were significantly increased in medium DRG neurons, while they declined in the large DRG neurons in the CCI group. P2X3 receptors were markedly upregulated in medium while P2X2 receptors were significantly decreased in small FG-positive DRG neurons. There were no significant changes in other P2X receptors (including P2X1, P2X4, P2X5, and P2X6). We anticipate that P2X receptors modulate nociceptive sensitivity primarily through P2X3 subtypes that are upregulated in medium neuropathic nociceptive DRG neurons and/or via the downregulation of P2X2 cells in neuropathic nociceptive small DRG neurons.

Orofacial skin inflammation increases the number of macrophages in the maxillary subregion of the rat trigeminal ganglion in a corticosteroid-reversible manner

Inflammation of the cutaneous orofacial tissue can lead to a prolonged alteration of neuronal and nonneuronal cellular functions in trigeminal nociceptive pathways. In this study, we investigated the effects of experimentally induced skin inflammation by dithranol (anthralin) on macrophage activation in the rat trigeminal ganglion. Tissue localization and protein expression levels of ionized calcium-binding adaptor molecule 1 (Iba1), a macrophage/microglia-specific marker, and proliferation/mitotic marker antigen identified by the monoclonal antibody Ki67 (Ki67), were quantitatively analyzed using immunohistochemistry and western blots in control, dithranol-treated, dithranol- and corticosteroid-treated, and corticosteroid-treated trigeminal ganglia. Chronic orofacial dithranol treatment elicited a strong pro-inflammatory effect in the ipsilateral trigeminal ganglion. Indeed, daily dithranol treatment of the orofacial skin for 3-5 days increased the number of macrophages and Iba1 protein expression in the maxillary subregion of the ipsilateral ganglion. In the affected ganglia, none of the Iba1-positive cells expressed Ki67. This absence of mitotically active cells suggested that the accumulation of macrophages in the ganglion was not the result of resident microglia proliferation but rather the extravasation of hematogenous monocytes from the periphery. Subsequently, when a 5-day-long anti-inflammatory corticosteroid therapy was employed on the previously dithranol-treated orofacial skin, Iba1 immunoreactivity was substantially reduced in the ipsilateral ganglion. Collectively, our findings indicate that both peripheral inflammation and subsequent anti-inflammatory therapy affect macrophage activity and thus interfere with the functioning of the affected sensory ganglion neurons.

Cathodal tDCS exerts neuroprotective effect in rat brain after acute ischemic stroke

BACKGROUND

Transcranial direct current stimulation (tDCS) is a non-invasive brain modulation technique that has been proved to exert beneficial effects in the acute phase of stroke. To explore the underlying mechanism, we investigated the neuroprotective effects of cathodal tDCS on brain injury caused by middle cerebral artery occlusion (MCAO).

RESULTS

We established the MCAO model and sham MCAO model with an epicranial electrode implanted adult male Sprague-Dawley rats, and then they were randomly divided into four groups (MCAO + tDCS, MCAO + sham tDCS (Sham), Control + tDCS and Control + Sham group). In this study, the severity degree of neurological deficit, the morphology of brain damage, the apoptosis, the level of neuron-specific enolase and inflammatory factors, the activation of glial cells was detected. The results showed that cathodal tDCS significantly improved the level of neurological deficit and the brain morphology, reduced the brain damage area and apoptotic index, and increased the number of Nissl body in MCAO rats, compared with MCAO + Sham group. Meanwhile, the high level of NSE, inflammatory factors, Caspase 3 and Bax/Bcl2 ratio in MCAO rats was reduced by cathodal tDCS. Additionally, cathodal tDCS inhibited the activation of astrocyte and microglia induced by MCAO. No difference was found in two Control groups.

CONCLUSION

Our results suggested that cathodal tDCS could accelerate the recovery of neurologic deficit and brain damage caused by MCAO. The inhibition of neuroinflammation and apoptosis resulted from cathodal tDCS may be involved in the neuroprotective process.

Progesterone and Allopregnanolone Rapidly Attenuate Estrogen-Associated Mechanical Allodynia in Rats with Persistent Temporomandibular Joint Inflammation

Temporomandibular joint disorder (TMD) is associated with pain in the joint (temporomandibular joint, TMJ) and muscles involved in mastication. TMD pain dissipates following menopause but returns in some women undergoing estrogen replacement therapy. Progesterone has both anti-inflammatory and antinociceptive properties, while estrogen's effects on nociception are variable and highly dependent on both natural hormone fluctuations and estrogen dosage during pharmacological treatments, with high doses increasing pain. Allopregnanolone, a progesterone metabolite and positive allosteric modulator of the GABAA receptor, also has antinociceptive properties. While progesterone and allopregnanolone are antinociceptive, their effect on estrogen-exacerbated TMD pain has not been determined. We hypothesized that removing the source of endogenous ovarian hormones would reduce inflammatory allodynia in the TMJ of rats and both progesterone and allopregnanolone would attenuate the estrogen-provoked return of allodynia. Baseline mechanical sensitivity was measured in female Sprague-Dawley rats (150-175 g) using the von Frey filament method followed by a unilateral injection of complete Freund's adjuvant (CFA) into the TMJ. Mechanical allodynia was confirmed 24 h later; then rats were ovariectomized or received sham surgery. Two weeks later, allodynia was reassessed and rats received one of the following subcutaneous hormone treatments over 5 days: a daily pharmacological dose of estradiol benzoate (E2; 50 μg/kg), daily E2 and pharmacological to sub-physiological doses of progesterone (P4; 16 mg/kg, 16 μg/kg, or 16 ng/kg), E2 daily and interrupted P4 given every other day, daily P4, or daily vehicle control. A separate group of animals received allopregnanolone (0.16 mg/kg) instead of P4. Allodynia was reassessed 1 h following injections. Here, we report that CFA-evoked mechanical allodynia was attenuated following ovariectomy and daily high E2 treatment triggered the return of allodynia, which was rapidly attenuated when P4 was also administered either daily or every other day. Allopregnanolone treatment, whether daily or every other day, also attenuated estrogen-exacerbated allodynia within 1 h of treatment, but only on the first treatment day. These data indicate that when gonadal hormone levels have diminished, treatment with a lower dose of progesterone may be effective at rapidly reducing the estrogen-evoked recurrence of inflammatory mechanical allodynia in the TMJ.

Cross-talk signaling in the trigeminal ganglion: role of neuropeptides and other mediators

The trigeminal ganglion with its three trigeminal nerve tracts consists mainly of clusters of sensory neurons with their peripheral and central processes. Most neurons are surrounded by satellite glial cells and the axons are wrapped by myelinating and non-myelinating Schwann cells. Trigeminal neurons express various neuropeptides, most notably, calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide (PACAP). Two types of CGRP receptors are expressed in neurons and satellite glia. A variety of other signal molecules like ATP, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion neurons and signal to neighboring neurons or satellite glial cells, which can signal back to neurons with same or other mediators. This potential cross-talk of signals involves intracellular mechanisms, including gene expression, that can modulate mediators of sensory information, such as neuropeptides, receptors, and neurotrophic factors. From the ganglia cell bodies, which are outside the blood–brain barrier, the mediators are further distributed to peripheral sites and/or to the spinal trigeminal nucleus in the brainstem, where they can affect neural transmission. A major question is how the sensory neurons in the trigeminal ganglion differ from those in the dorsal root ganglion. Despite their functional overlap, there are distinct differences in their ontogeny, gene expression, signaling pathways, and responses to anti-migraine drugs. Consequently, drugs that modulate cross-talk in the trigeminal ganglion can modulate both peripheral and central sensitization, which may potentially be distinct from sensitization mediated in the dorsal root ganglion.

P2Y12 receptor mediates microglial activation via RhoA/ROCK pathway in the trigeminal nucleus caudalis in a mouse model of chronic migraine

BACKGROUND

Microglial activation contributes to the development of chronic migraine (CM). The P2Y12 receptor (P2Y12R), a metabolic purinoceptor that is expressed on microglia in the central nervous system (CNS), has been indicated to play a critical role in the pathogenesis of chronic pain. However, whether it contributes to the mechanism of CM remains unknown. Thus, the present study investigated the precise details of microglial P2Y12R involvement in CM.

METHODS

Mice subjected to recurrent nitroglycerin (NTG) treatment were used as the CM model. Hyperalgesia were assessed by mechanical withdrawal threshold to electronic von Frey and thermal withdrawal latency to radiant heat. Western blot and immunohistochemical analyses were employed to detect the expression of P2Y12R, Iba-1, RhoA, and ROCK2 in the trigeminal nucleus caudalis (TNC). To confirm the role of P2Y12R and RhoA/ROCK in CM, we systemically administered P2Y12R antagonists (MRS2395 and clopidogrel) and a ROCK2 inhibitor (fasudil) and investigated their effects on microglial activation, c-fos, and calcitonin gene-related peptide (CGRP) expression in the TNC. To further confirm the effect of P2Y12R on microglial activation, we preincubated lipopolysaccharide (LPS)-treated BV-2 microglia with MRS2395 and clopidogrel. ELISA was used to evaluate the levels of inflammatory cytokines.

RESULTS

The protein levels of P2Y12R, GTP-RhoA, ROCK2, CGRP, c-fos, and inducible nitric oxide synthase (iNOS) in the TNC were increased after recurrent NTG injection. A double labeling study showed that P2Y12R was restricted to microglia in the TNC. MRS2395 and clopidogrel attenuated the development of tactile allodynia and suppressed the expression of CGRP, c-fos, and GTP-RhoA/ROCK2 in the TNC. Furthermore, fasudil also prevented hyperalgesia and suppressed the expression of CGRP in the TNC. In addition, inhibiting P2Y12R and ROCK2 activities suppressed NTG-induced microglial morphological changes (process retraction) and iNOS production in the TNC. In vitro, a double labeling study showed that P2Y12R was colocalized with BV-2 cells, and the levels of iNOS, IL-1β, and TNF-α in LPS-stimulated BV-2 microglia were reduced by P2Y12R inhibitors.

CONCLUSIONS

These data demonstrate that microglial P2Y12R in the TNC plays a critical role in the pathogenesis of CM by regulating microglial activation in the TNC via RhoA/ROCK pathway.

Basal astrocyte and microglia activation in the central nervous system of Familial Hemiplegic Migraine Type I mice

Background

Gain-of-function missense mutations in the α1A subunit of neuronal CaV2.1 channels, which define Familial Hemiplegic Migraine Type 1 (FHM1), result in enhanced cortical glutamatergic transmission and a higher susceptibility to cortical spreading depolarization. It is now well established that neurons signal to surrounding glial cells, namely astrocytes and microglia, in the central nervous system, which in turn become activated and in pathological conditions can sustain neuroinflammation. We and others previously demonstrated an increased activation of pro-algogenic pathways, paralleled by augmented macrophage infiltration, in both isolated trigeminal ganglia and mixed trigeminal ganglion neuron-satellite glial cell cultures of FHM1 mutant mice. Hence, we hypothesize that astrocyte and microglia activation may occur in parallel in the central nervous system.

Methods

We have evaluated signs of reactive glia in brains from naïve FHM1 mutant mice in comparison with wild type animals by immunohistochemistry and Western blotting.

Results

Here we show for the first time signs of reactive astrogliosis and microglia activation in the naïve FHM1 mutant mouse brain.

Conclusions

Our data reinforce the involvement of glial cells in migraine, and suggest that modulating such activation may represent an innovative approach to reduce pathology.

The effect of orofacial complete Freund's adjuvant treatment on the expression of migraine-related molecules

Background

Migraine is a neurovascular primary headache disorder, which causes significant socioeconomic problems worldwide. The pathomechanism of disease is enigmatic, but activation of the trigeminovascular system (TS) appears to be essential during the attack. Migraine research of recent years has focused on neuropeptides, such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide 1–38 (PACAP1–38) as potential pathogenic factors and possible therapeutic offensives. The goal of present study was to investigate the simultaneous expression of CGRP and precursor of PACAP1–38 (preproPACAP) in the central region of the TS in a time-dependent manner following TS activation in rats.

Methods

The right whisker pad of rats was injected with 50 μl Complete Freund’s Adjuvant (CFA) or saline. A mechanical allodynia test was performed with von Frey filaments before and after treatment. Transcardial perfusion of the animals was initiated 24, 48, 72 and 120 h after injection, followed by the dissection of the nucleus trigeminus caudalis (TNC). After preparation, the samples were stored at − 80 °C until further use. The relative optical density of CGRP and preproPACAP was analyzed by Western blot. One-way ANOVA and Kruskal-Wallis followed by Tukey post hoc test were used to evaluate the data. Regression analysis was applied to explore the correlation between neuropeptides expression and hyperalgesia.

Results

Orofacial CFA injection resulted in significant CGRP and preproPACAP release in the TNC 24, 48, 72 and 120 h after the treatment. The level of neuropeptides reached its maximum at 72 h after CFA injection, corresponding to the peak of facial allodynia. Negative, linear correlation was detected between the expression level of neuropeptides and value of mechanonociceptive threshold.

Conclusion

This is the first study which suggests that the expression of CGRP and preproPACAP simultaneously increases in the central region of activated TS and it influences the formation of mechanical hyperalgesia. Our results contribute to a better understanding of migraine pathogenesis and thereby to the development of more effective therapeutic approaches.

Experimental Methods to Inform Diagnostic Approaches for Painful TMJ Osteoarthritis

Temporomandibular joint (TMJ) osteoarthritis (OA) is a degenerative disease of the joint that can produce persistent orofacial pain as well as functional and structural changes to its bone, cartilage, and ligaments. Despite advances in the clinical utility and reliability of the Diagnostic Criteria for Temporomandibular Disorders, clinical tools inadequately predict which patients will develop chronic TMJ pain and degeneration, limiting clinical management. The challenges of managing and treating TMJ OA are due, in part, to a limited understanding of the mechanisms contributing to the development and maintenance of TMJ pain. OA is initiated by multiple factors, including injury, aging, abnormal joint mechanics, and atypical joint shape, which can produce microtrauma, remodeling of joint tissues, and synovial inflammation. TMJ microtrauma and remodeling can increase expression of cytokines, chemokines, and catabolic factors that damage synovial tissues and can activate free nerve endings in the joint. Although studies have separately investigated inflammation-driven orofacial pain, acute activity of the trigeminal nerve, or TMJ tissue degeneration and/or damage, the temporal mechanistic factors leading to chronic TMJ pain are undefined. Limited understanding of the interaction between degeneration, intra-articular chemical factors, and pain has further restricted the development of targeted, disease-modifying drugs to help patients avoid long-term pain and invasive procedures, like TMJ replacement. A range of animal models captures features of intra-articular inflammation, joint overloading, and tissue damage. Although those models traditionally measure peripheral sensitivity as a surrogate for pain, recent studies recognize the brain's role in integrating, modulating, and interpreting nociceptive inputs in the TMJ, particularly in light of psychosocial influences on TMJ pain. The articular and neural contributors to TMJ pain, imaging modalities with clinical potential to identify TMJ OA early, and future directions for clinical management of TMJ OA are reviewed in the context of evidence in the field.

The role of adenosine and P2Y receptors expressed by multiple cell types in pain transmission

The role of extracellular nucleotides and nucleosides as signaling molecules in cell-to-cell communication has now been clearly established. This is particularly true in the central and peripheral nervous system, where purines and pyrimidines are involved in both physiological and pathological interactions between neurons and surrounding glial cells. It can be thus foreseen that the purinergic system could represent a new potential target for the development of effective analgesics, also through the normalization of neuronal functions and the inhibition of glial cell activation. Research in the last 15 years has progressively confirmed this hypothesis, but no purinergic-based analgesics have reach the market so far; in the present review we have collected the more recent discoveries on the role of G protein-coupled P2Y nucleotide and of adenosine receptors expressed by both neurons and glial cells under painful conditions, and we have highlighted some of the challenges that must be faced to translate basic and preclinical studies to clinics.

The role of satellite glial cells in orofacial pain

Some chronic pain conditions in the orofacial region are common, the mechanisms underlying which are unresolved. Satellite glial cells (SGCs) are the glial cells of the peripheral nervous system. In the sensory ganglia, each neuronal body is surrounded by SGCs forming distinct functional units. The unique structural organization enables SGCs to communicate with each other and with their enwrapped neurons via a variety of ways. There is a growing body of evidence that SGCs can influence the level of neuronal excitability and are involved in the development and/or maintenance of pain. The aim of this review was to summarize the latest advances made about the implication of SGCs in orofacial pain. It may offer new targets for the development of orofacial pain treatment.

Peripheral sympathetic mechanisms in orofacial pain

Sympathetic nervous system (SNS) is a part of the autonomic nervous system which involuntarily regulates internal body functions. It appears to modulate the processing of nociceptive information. Many orofacial pain conditions involve inflammation of orofacial tissues and/or injury of nerve, some of which might be attributed to SNS. Thus, the aim of this review was to bring together the data available regarding the peripheral sympathetic mechanisms involved in orofacial pain. A clearer understanding of SNS–sensory interactions in orofacial pain may provide a basis for novel therapeutic strategies for conditions that respond poorly to conventional treatments.

Activation of trigeminal ganglion satellite glial cells in CFA-induced tooth pulp pain in rats

This study further investigated the mechanisms underlying the rat model of tooth pulp inflammatory pain elicited by complete Freund's adjuvant (CFA), in comparison to other pulpitis models. Pulps of the left maxillary first molars were accessed. In the CFA group, the pulps were exposed, and CFA application was followed by dental sealing. In the open group, the pulps were left exposed to the oral cavity. For the closed group, the pulps were exposed, and the teeth were immediately sealed. Naïve rats were used as negative controls. Several parameters were evaluated at 1, 2, 3 and 8 days. There was no statistical significant difference among the groups when body weight variation, food or water consumption were compared. Analysis of serum cytokines (IL-1β, TNF or IL-6) or differential blood cell counts did not reveal any evidence of systemic inflammation. The CFA group displayed a significant reduction in the locomotor activity (at 1 and 3 days), associated with an increased activation of satellite glial cells in the ipsilateral trigeminal ganglion (TG; for up to 8 days). Amygdala astrocyte activation was unaffected in any experimental groups. We provide novel evidence indicating that CFA-induced pulp inflammation impaired the locomotor activity, with persistent activation of ipsilateral TG satellite cells surrounding sensory neurons, without any evidence of systemic inflammation or amygdala astrogliosis.

Purple Corn Extract as Anti-allodynic Treatment for Trigeminal Pain: Role of Microglia

Natural products have attracted interest in the search for new and effective analgesics and coadjuvant approaches to several types of pain. It is in fact well known that many of their active ingredients, such as anthocyanins (ACNs) and polyphenols, can exert potent anti-inflammatory actions. Nevertheless, their potential beneficial effects in orofacial painful syndromes have not been assessed yet. Here, we have evaluated the preventive effect of an ACN-enriched purple corn extract against the development of orofacial allodynia, in comparison with isogenic yellow corn extract containing only polyphenols. Orofacial allodynia developed following induction of temporomandibular joint (TMJ) inflammation in male rats, due to the injection of Complete Freund’s Adjuvant (CFA), and was evaluated by von Frey filaments. Animals drank purple or yellow corn extracts or water starting from 11 days before induction of inflammation and up to the end of the experiment 3 days later. To highlight possible additive and/or synergic actions, some animals also received the anti-inflammatory drug acetyl salicylic acid (ASA). In parallel with the evaluation of allodynia, we have focused our attention on the activation of microglia cells in the central nervous system (CNS), as it is well-known that they significantly contribute to neuronal sensitization and pain. Our data demonstrate that purple corn extract is as effective as ASA in preventing the development of orofacial allodynia, and only partial additive effect is observed when the two agents are co-administered. Yellow corn exerted no effect. Multiple mechanisms are possibly involved in the action of purple corn, including reduction of trigeminal macrophage infiltration and the shift of microglia cell polarization to an anti-inflammatory phenotype. In fact, in rats receiving yellow corn or water microglia cells show thick, short cell processes typical of activated cells. Conversely, thinner and longer microglia cell processes are observed in the brainstem of animals drinking purple corn extract; shape changes are accompanied by a reduction in the expression of pro-inflammatory molecules and increased production of anti-inflammatory mediators. Administration of purple corn extracts therefore represents a possible low-cost and easy way to reduce trigeminal-associated pain in various pathological conditions also thanks to the modulation of microglia reactivity.

Grading facial expression is a sensitive means to detect grimace differences in orofacial pain in a rat model

Although pre-clinical models of pain are useful for defining relationships between biological mechanisms and pain, common methods testing peripheral sensitivity do not translate to the human pain experience. Facial grimace scales evaluate affective pain levels in rodent models by capturing and scoring spontaneous facial expression. But, the Rat Grimace Scale (RGS) has not assessed the common disorder of temporomandibular joint (TMJ) pain. A rat model of TMJ pain induced by jaw loading (1 hr/day for 7 days) was used to investigate the time course of RGS scores and compare them between different loading magnitudes with distinct peripheral sensitivity profiles (0N–no sensitivity, 2N–acute sensitivity, 3.5N–persistent sensitivity). In the 3.5N group, RGS is elevated over baseline during the loading period and one day after loading and is correlated with peripheral sensitivity (ρ = −0.48, p = 0.002). However, RGS is not elevated later when that group exhibits peripheral sensitivity and moderate TMJ condylar cartilage degeneration. Acutely, RGS is elevated in the 3.5N loading group over the other loading groups (p < 0.001). These findings suggest that RGS is an effective tool for detecting spontaneous TMJ pain and that spontaneous pain is detectable in rats that develop persistent TMJ sensitivity, but not in rats with acute resolving sensitivity.

Micronized palmitoylethanolamide reduces joint pain and glial cell activation

OBJECTIVE AND DESIGN

Temporomandibular disorder (TMD) is a common painful condition in the temporomandibular joint (TMJ). Joint inflammation is believed to be a chief cause of pain in patients with TMD, through the release of pro-inflammatory cytokines that induce peripheral sensitization of nerve terminals followed by microglial stimulation.

MATERIALS AND SUBJECT

TMJ was induced in rats with the injection of complete Freund's adjuvant (CFA) emulsion into the left TMJ capsule.

TREATMENT

The present study would assess the effects of micronized palmitoylethanolamide (m-PEA) on glial activation and trigeminal hypersensitivity.

METHODS

Ten mg/kg m-PEA or corresponding vehicle was administered 1 h after CFA and mechanical allodynia and edema were evaluated at 24 and 72 h after CFA injection.

RESULTS

CFA-injected animals showed TMJ edema and ipsilateral mechanical allodynia accompanied by a robust growth in GFAP protein-positive satellite glial cells and activation of resident macrophages in the TG. Moreover, m-PEA administration significantly reduced the degree of TMJ damage and pain, macrophage activation in TG and up-regulation of Iba1.

CONCLUSIONS

The results confirm that m-PEA could represent a novel approach for monitoring pain during trigeminal nerve sensitization.

NLRP3 inflammasome signaling as an early molecular response is negatively controlled by miR-186 in CFA-induced prosopalgia mice

The NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is the most frequently studied in the central nervous system and has been linked to neuropathic pain. In this study, a post-translational mechanism of microRNA (miR)-186 via regulating the expression of NLRP3 in the complete Freund's adjuvant (CFA)-treated mice was investigated. The injection of CFA was used to induce trigeminal neuropathic pain in mice. miRs microarray chip assay was performed in trigeminal ganglions (TGs). CFA treatment significantly increased the mRNA expression of NLRP3, interleukin (IL)-1β, and IL-18 in TGs compared to the control group. Moreover, 26 miRs were differentially expressed in TGs from trigeminal neuropathic pain mice, and the expression of miR-186 showed the lowest level of all the miRs. Further examination revealed that NLRP3 was a candidate target gene of miR-186. We delivered miR-186 mimics to CFA-treated mice. The head withdrawal thresholds of the CFA-treated mice were significantly increased by miR-186 mimics injection compared with CFA single treatment. The mRNA and protein expression of NLRP3, IL-1β, and IL-18 in TGs from trigeminal neuropathic pain mice were significantly inhibited by miR-186 mimics treatment compared to the CFA group. miR-186 was able to suppress the neuropathic pain via regulating the NLRP3 inflammasome signaling.

Current understanding of trigeminal ganglion structure and function in headache

INTRODUCTION

The trigeminal ganglion is unique among the somatosensory ganglia regarding its topography, structure, composition and possibly some functional properties of its cellular components. Being mainly responsible for the sensory innervation of the anterior regions of the head, it is a major target for headache research. One intriguing question is if the trigeminal ganglion is merely a transition site for sensory information from the periphery to the central nervous system, or if intracellular modulatory mechanisms and intercellular signaling are capable of controlling sensory information relevant for the pathophysiology of headaches.

METHODS

An online search based on PubMed was made using the keyword "trigeminal ganglion" in combination with "anatomy", "headache", "migraine", "neuropeptides", "receptors" and "signaling". From the relevant literature, further references were selected in view of their relevance for headache mechanisms. The essential information was organized based on location and cell types of the trigeminal ganglion, neuropeptides, receptors for signaling molecules, signaling mechanisms, and their possible relevance for headache generation.

RESULTS

The trigeminal ganglion consists of clusters of sensory neurons and their peripheral and central axon processes, which are arranged according to the three trigeminal partitions V1-V3. The neurons are surrounded by satellite glial cells, the axons by Schwann cells. In addition, macrophage-like cells can be found in the trigeminal ganglion. Neurons express various neuropeptides, among which calcitonin gene-related peptide is the most prominent in terms of its prevalence and its role in primary headaches. The classical calcitonin gene-related peptide receptors are expressed in non-calcitonin gene-related peptide neurons and satellite glial cells, although the possibility of a second calcitonin gene-related peptide receptor in calcitonin gene-related peptide neurons remains to be investigated. A variety of other signal molecules like adenosine triphosphate, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion cells and may act at receptors on adjacent neurons or satellite glial cells.

CONCLUSIONS

The trigeminal ganglion may act as an integrative organ. The morphological and functional arrangement of trigeminal ganglion cells suggests that intercellular and possibly also autocrine signaling mechanisms interact with intracellular mechanisms, including gene expression, to modulate sensory information. Receptors and neurotrophic factors delivered to the periphery or the trigeminal brainstem can contribute to peripheral and central sensitization, as in the case of primary headaches. The trigeminal ganglion as a target of drug action outside the blood-brain barrier should therefore be taken into account.

Glial interleukin-1β upregulates neuronal sodium channel 1.7 in trigeminal ganglion contributing to temporomandibular joint inflammatory hypernociception in rats

Background

The proinflammatory cytokine interleukin-1β (IL-1β) drives pain by inducing the expression of inflammatory mediators; however, its ability to regulate sodium channel 1.7 (Nav1.7), a key driver of temporomandibular joint (TMJ) hypernociception, remains unknown. IL-1β induces cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). We previously showed that PGE2 upregulated trigeminal ganglionic Nav1.7 expression. Satellite glial cells (SGCs) involve in inflammatory pain through glial cytokines. Therefore, we explored here in the trigeminal ganglion (TG) whether IL-1β upregulated Nav1.7 expression and whether the IL-1β located in the SGCs upregulated Nav1.7 expression in the neurons contributing to TMJ inflammatory hypernociception.

Methods

We treated rat TG explants with IL-1β with or without inhibitors, including NS398 for COX-2, PF-04418948 for EP2, and H89 and PKI-(6-22)-amide for protein kinase A (PKA), or with adenylate cyclase agonist forskolin, and used real-time PCR, Western blot, and immunohistofluorescence to determine the expressions or locations of Nav1.7, COX-2, cAMP response element-binding protein (CREB) phosphorylation, and IL-1β. We used chromatin immunoprecipitation to examine CREB binding to the Nav1.7 promoter. Finally, we microinjected IL-1β into the TGs or injected complete Freund’s adjuvant into TMJs with or without previous microinjection of fluorocitrate, an inhibitor of SGCs activation, into the TGs, and evaluated nociception and gene expressions. Differences between groups were examined by one-way analysis of variance (ANOVA) or independent samples t test.

Results

IL-1β upregulated Nav1.7 mRNA and protein expressions in the TG explants, whereas NS398, PF-04418948, H89, or PKI-(6-22)-amide could all block this upregulation, and forskolin could also upregulate Nav1.7 mRNA and protein expressions. IL-1β enhanced CREB binding to the Nav1.7 promoter. Microinjection of IL-1β into the TGs or TMJ inflammation both induced hypernociception of TMJ region and correspondingly upregulated COX-2, phospho-CREB, and Nav1.7 expressions in the TGs. Moreover, microinjection of fluorocitrate into the TGs completely blocked TMJ inflammation-induced activation of SGCs and the upregulation of IL-1β and COX-2 in the SGCs, and phospho-CREB and Nav1.7 in the neurons and alleviated inflammation-induced TMJ hypernociception.

Conclusions

Glial IL-1β upregulated neuronal Nav1.7 expression via the crosstalk between signaling pathways of the glial IL-1β/COX-2/PGE2 and the neuronal EP2/PKA/CREB/Nav1.7 in TG contributing to TMJ inflammatory hypernociception.

Activation of satellite glial cells in trigeminal ganglion following dental injury and inflammation

Satellite glial cells (SGCs), a peripheral neuroglial cell, surround neurons and form a complete envelope around individual sensory neurons in the trigeminal ganglia (TG), which may be involved in modulating neurons in inflammation. The purpose of this study was to determine the effect of dental injury and inflammation on SGCs in the TG. Pulp exposure (PX) was performed on the first maxillary molar of 28 rats. The neurons innervating injured tooth in TG were labeled by the retrograde transport of fluoro-gold (FG). Specimens were collected at 1, 3, 7, 14, 21 and 28 days after PX and stained immunohistochemically for glial fibrillary acid protein (GFAP), a marker of SGCs activation, in the TG. We observed that GFAP-immunoreactivity (IR) SGCs enclosed FG-labeled neurons increased in a time-dependent manner after PX. The neurons surrounded by GFAP-IR SGCs were mainly small and medium in size. The GFAP-IR SGCs encircled neurons increased significantly in the maxillary nerve region of the TG at 7-28 days following PX. The results show that dental injury and inflammation induced SGCs activation in the TG. It indicates that activation of SGCs might be implicated in the peripheral mechanisms of pain following dental injury and inflammation.

Central Role of Protein Kinase A in Promoting Trigeminal Nociception in an In Vivo Model of Temporomandibular Disorders

AIMS

To investigate cellular changes in the spinal trigeminal nucleus (STN) and trigeminal ganglion (TG) associated with trigeminal nociception mediated by inflammation in the temporomandibular joint (TMJ).

METHODS

Male Sprague-Dawley rats (n = 86) were utilized to investigate cellular and behavioral responses to prolonged TMJ inflammation caused by bilateral injection of Complete Freund's Adjuvant (CFA) in the TMJ capsules. To investigate the cellular effects of protein kinase A (PKA) in the STN, rats were injected intrathecally with the selective PKA inhibitor KT5720 prior to injection of CFA into both TMJ capsules. Levels of calcitonin gene-related peptide (CGRP), active PKA, and ionized calcium-binding adapter molecule 1 (Iba1) in the STN and expression of phosphorylated extracellular regulated kinases (p-ERK) in the TG were determined with immunohistochemistry (n ≥ 3 experiments per test condition). Nocifensive head withdrawal responses to mechanical stimulation of the cutaneous tissue over the TMJ were monitored following CFA injection in the absence or presence of KT5720 (n = 7). Statistical analysis was performed using parametric analysis of variance (ANOVA) tests.

RESULTS

Intrathecal injection of KT5720 significantly inhibited the stimulatory effect of CFA on levels of CGRP, PKA, and Iba1 in the STN. In addition, administration of KT5720 decreased the average number of CFA-induced nocifensive withdrawal responses to mechanical stimulation and the CFA-mediated increase in p-ERK expression in the ganglion.

CONCLUSION

These findings provide evidence that elevated PKA activity in the STN promotes cellular events temporally associated with trigeminal nociception caused by prolonged TMJ inflammation.

Orthodontic treatment-induced temporal alteration of jaw-opening reflex excitability

The impairment of orofacial motor function during orthodontic treatment needs to be addressed, because most orthodontic patients experience pain and motor excitability would be affected by pain. In the present study, the temporal alteration of the jaw-opening reflex excitability was investigated to determine if orthodontic treatment affects orofacial motor function. The excitability of jaw-opening reflex evoked by electrical stimulation on the gingiva and recorded bilaterally in the anterior digastric muscles was evaluated at 1 (D1), 3 (D3), and 7 days (D7) after orthodontic force application to the teeth of right side; morphological features (e.g., osteoclast genesis and tooth movement) were also evaluated. To clarify the underlying mechanism of orthodontic treatment-induced alteration of orofacial motor excitability, analgesics were administrated for 1 day. At D1 and D3, orthodontic treatment significantly decreased the threshold for inducing the jaw-opening reflex but significantly increased the threshold at D7. Other parameters of the jaw-opening reflex were also evaluated (e.g., latency, duration and area under the curve of anterior digastric muscles activity), and only the latency of the D1 group was significantly different from that of the other groups. Temporal alteration of the jaw-opening reflex excitability was significantly correlated with changes in morphological features. Aspirin (300 mg·kg^-1·day^-1) significantly increased the threshold for inducing the jaw-opening reflex, whereas a lower dose (75-150 mg·kg^-1·day^-1) of aspirin or acetaminophen (300 mg·kg^-1·day^-1) failed to alter the jaw-opening reflex excitability. These results suggest that an increase of the jaw-opening reflex excitability can be induced acutely by orthodontic treatment, possibly through the cyclooxygenase activation.NEW & NOTEWORTHY It is well known that motor function is affected by pain, but the effect of orthodontic treatment-related pain on the trigeminal motor excitability has not been fully understood. We found that, during orthodontic treatment, trigeminal motor excitability is acutely increased and then decreased in a week. Because alteration of trigeminal motor function can be evaluated quantitatively by jaw-opening reflex excitability, the present animal model may be useful to search for alternative approaches to attenuate orthodontic pain.

DNA damage mediates changes in neuronal sensitivity induced by the inflammatory mediators, MCP-1 and LPS, and can be reversed by enhancing the DNA repair function of APE1

Although inflammation-induced peripheral sensitization oftentimes resolves as an injury heals, this sensitization can be pathologically maintained and contribute to chronic inflammatory pain. Numerous inflammatory mediators increase the production of reactive oxygen (ROS) and nitrogen species (RNS) during inflammation and in animal models of chronic neuropathic pain. Our previous studies demonstrate that ROS/RNS and subsequent DNA damage mediate changes in neuronal sensitivity induced by anticancer drugs and by ionizing radiation in sensory neurons, thus we investigated whether inflammation and inflammatory mediators also could cause DNA damage in sensory neurons and whether that DNA damage alters neuronal sensitivity. DNA damage was assessed by pH2A.X expression and the release of the neuropeptide, calcitonin gene-related peptide (CGRP), was measured as an index of neuronal sensitivity. Peripheral inflammation or exposure of cultured sensory neurons to the inflammatory mediators, LPS and MCP-1, elicited DNA damage. Moreover, exposure of sensory neuronal cultures to LPS or MCP-1 resulted in changes in the stimulated release of CGRP, without altering resting release or CGRP content. Genetically enhancing the expression of the DNA repair enzyme, apurinic/apyrimidinic endonuclease (APE1) or treatment with a small-molecule modulator of APE1 DNA repair activity, both which enhance DNA repair, attenuated DNA damage and the changes in neuronal sensitivity elicited by LPS or MCP-1. In conclusion, our studies demonstrate that inflammation or exposure to inflammatory mediators elicits DNA damage in sensory neurons. By enhancing DNA repair, we demonstrate that this DNA damage mediates the alteration of neuronal function induced by inflammatory mediators in peptidergic sensory neurons.

Inflammatory pain assessment in the arthritis of the temporomandibular joint in rats: A comparison between two phlogistic agents

Temporomandibular joint (TMJ) disorders are a group of conditions that result in TMJ pain, which frequently limits basic daily activities. Experimental models that allow the study of the mechanisms underlying these inflammatory and pain conditions are of great clinical relevance. The aim of this study was to evaluate nociception, inflammation and participation of the macrophage/microglia cells in the arthritis of the TMJ induced by two phlogistic agents. 84 rats were divided into 2 groups: Zy, which received zymosan intra-articularly, or Cg, which received carrageenan intra-articularly. Mechanical nociception, total leukocyte influx to the synovial fluid and histopathological analyses were evaluated in the TMJ. The participation of macrophage/microglia located in trigeminal ganglia (TG) and in the subnucleus caudalis (V-SnC) was assessed immunohistochemically. Both agents induced mechanical hyperalgesia 6h after the induction, but a more persistent algesic state was perceived in the Cg group, which lasted for 120h. Even though both groups presented increased leukocyte influx, the Zy-group presented a more intense influx. Zymosan recruited resident macrophage in the trigeminal ganglia 24h after the injection. In the V-SnC, the group Cg presented a more prolonged immunolabeling pattern in comparison with the group Zy. It can be concluded that zymosan induced a more intense infiltrate and peripheral nervous changes, while Cg lead to a moderate TMJ inflammation with prominent changes in the V-SnC.

Mechanisms of craniofacial pain

Aim To provide an overview of mechanisms underlying craniofacial pain; to highlight peripheral and central adaptations that may promote chronification of pain in craniofacial pain states such as migraine and temporomandibular disorders (TMD). Background Pain is a common symptom associated with disorders involving craniofacial tissues including the teeth and their supporting structures, the temporomandibular joint and the muscles of the head. Most acute painful craniofacial conditions are easily recognized and well managed, but others, especially those that are chronic (e.g., migraine, TMD and trigeminal neuropathies), present clinical challenges. Preclinical studies have provided substantial information about the anatomical and physiological mechanisms related to the initiation and modulation of nociceptive signals in the trigeminal system. While knowledge of the mechanisms underlying chronic craniofacial pain remains limited, both clinical and preclinical investigations suggest that changes in afferent inputs to the brain as well as in brain structure and modulatory pathways occur in chronic pain. Collectively, these changes result in amplification of nociception that promotes and sustains craniofacial chronic pain states. Conclusions The increased understanding gained of the physiological and pathological processing of nociception in the trigeminal system has provided new perspectives for the mechanistic understanding of acute craniofacial pain conditions and the peripheral and central adaptations that are related to pain chronification. Such knowledge may contribute to improvements in currently available treatments as well as to the development of novel analgesic therapies.

Supradural inflammatory soup in awake and freely moving rats induces facial allodynia that is blocked by putative immune modulators

Facial allodynia is a migraine symptom that is generally considered to represent a pivotal point in migraine progression. Treatment before development of facial allodynia tends to be more successful than treatment afterwards. As such, understanding the underlying mechanisms of facial allodynia may lead to a better understanding of the mechanisms underlying migraine. Migraine facial allodynia is modeled by applying inflammatory soup (histamine, bradykinin, serotonin, prostaglandin E2) over the dura. Whether glial and/or immune activation contributes to such pain is unknown. Here we tested if trigeminal nucleus caudalis (Sp5C) glial and/or immune cells are activated following supradural inflammatory soup, and if putative glial/immune inhibitors suppress the consequent facial allodynia. Inflammatory soup was administered via bilateral indwelling supradural catheters in freely moving rats, inducing robust and reliable facial allodynia. Gene expression for microglial/macrophage activation markers, interleukin-1β, and tumor necrosis factor-α increased following inflammatory soup along with robust expression of facial allodynia. This provided the basis for pursuing studies of the behavioral effects of 3 diverse immunomodulatory drugs on facial allodynia. Pretreatment with either of two compounds broadly used as putative glial/immune inhibitors (minocycline, ibudilast) prevented the development of facial allodynia, as did treatment after supradural inflammatory soup but prior to the expression of facial allodynia. Lastly, the toll-like receptor 4 (TLR4) antagonist (+)-naltrexone likewise blocked development of facial allodynia after supradural inflammatory soup. Taken together, these exploratory data support that activated glia and/or immune cells may drive the development of facial allodynia in response to supradural inflammatory soup in unanesthetized male rats.

Crosstalk between Activated Microglia and Neurons in the Spinal Dorsal Horn Contributes to Stress-induced Hyperalgesia

Stress has been shown to enhance pain sensitivity resulting in stress-induced hyperalgesia. However, the underlying mechanisms have yet to be elucidated. Using single-prolonged stress combined with Complete Freund's Adjuvant injection model, we explored the reciprocal regulatory relationship between neurons and microglia, which is critical for the maintenance of posttraumatic stress disorder (PTSD)-induced hyperalgesia. In our assay, significant mechanical allodynia was observed. Additionally, activated neurons in spinal dorsal horn were observed by analysis of Fos expression. And, microglia were also significantly activated with the presence of increased Iba-1 expression. Intrathecal administration of c-fos antisense oligodeoxynucleotides (ASO) or minocycline (a specific microglia inhibitor) attenuated mechanical allodynia. Moreover, intrathecal administration of c-fos ASO significantly suppressed the activation of neurons and microglia. Interestingly, inhibition of microglia activation by minocycline significantly suppressed the activation of both neurons and microglia in spinal dorsal horn. P38 inhibitor SB203580 suppressed IL-6 production, and inhibition of IL-6 receptor (IL-6R) activation by tocilizumab suppressed Fos expression. Together, our data suggest that the presence of a "crosstalk" between activated microglia and neurons in the spinal dorsal horn, which might contribute to the stress-induced hyperactivated state, leading to an increased pain sensitivity.

Tumor necrosis factor-Alpha stimulates cytokine expression and transient sensitization of trigeminal nociceptive neurons

OBJECTIVE

Elevated levels of tumor necrosis factor- alpha (TNF-α) in the capsule of the temporomandibular joint (TMJ) are implicated in the underlying pathology of temporomandibular disorders (TMD). TMD are a group of conditions that result in pain in the TMJ and/or muscles of mastication, and are associated with significant social and economic burdens. The goal of this study was to investigate the effect of elevated TNF-α levels in the TMJ capsule on nocifensive behavioral response to mechanical stimulation of trigeminal neurons and regulation of cytokines within the trigeminal ganglion.

DESIGN

Male Sprague-Dawley rats were injected bilaterally in the TMJ capsule with TNF-α and changes in nocifensive head withdrawal responses to mechanical stimulation of cutaneous tissue directly over the capsule was determined using von Frey filaments. Cytokine levels in trigeminal ganglia were determined by protein array analysis at several time points post injection and correlated to nocifensive behavior.

RESULTS

TNF-α caused a significant increase in the average number of nocifensive responses when compared to naive and vehicle treated animals 2h post injection, but levels returned to control levels at 24h. Based on array analysis, the levels of eight cytokines were significantly elevated above vehicle control levels at 2h following TNF-α injection, but all eight had returned to the vehicle control levels after 24h.

CONCLUSIONS

Our findings provide evidence that elevated levels of TNF-α in the joint capsule, which is reported to occur in TMD, promotes nociception in trigeminal ganglia neurons via a mechanism that temporally correlates with differential regulation of several cytokines.

Elevated levels of calcitonin gene-related peptide in upper spinal cord promotes sensitization of primary trigeminal nociceptive neurons

Orofacial pain conditions including temporomandibular disorder (TMD) and migraine are characterized by peripheral and central sensitization of trigeminal nociceptive neurons. The goal of this study was to investigate the role of calcitonin gene-related peptide (CGRP) in promoting bidirectional signaling within the trigeminal system to mediate sensitization of primary nociceptive neurons. Adult male Sprague-Dawley rats were injected intercisternally with CGRP or co-injected with the receptor antagonist CGRP_8-37 or KT 5720, a protein kinase A (PKA) inhibitor. Nocifensive head withdrawal response to mechanical stimulation was investigated using von Frey filaments. Expression of PKA, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adapter molecule 1 (Iba1) in the spinal cord and phosphorylated extracellular signal-regulated kinase (P-ERK) in the ganglion was studied using immunohistochemistry. Some animals were co-injected with CGRP and Fast Blue dye and the ganglion was imaged using fluorescent microscopy. CGRP increased nocifensive responses to mechanical stimulation when compared to control. Co-injection of CGRP_8-37 or KT 5720 with CGRP inhibited the nocifensive response. CGRP stimulated PKA and GFAP expression in the spinal cord, and P-ERK in ganglion neurons. Seven days post injection, Fast Blue was observed in ganglion neurons and satellite glial cells. Our results demonstrate that elevated levels of CGRP in the upper spinal cord promote sensitization of primary nociceptive neurons via a mechanism that involves activation of PKA centrally and P-ERK in ganglion neurons. Our findings provide evidence of bidirectional signaling within the trigeminal system that facilitate increased neuron-glia communication within the ganglion associated with trigeminal sensitization.

Activity of botulinum toxin type A in cranial dura: implications for treatment of migraine and other headaches

BACKGROUND AND PURPOSE

Although botulinum toxin type A (BoNT/A) is approved for chronic migraine treatment, its mechanism of action is still unknown. Dural neurogenic inflammation (DNI) commonly used to investigate migraine pathophysiology can be evoked by trigeminal pain. Here, we investigated the reactivity of cranial dura to trigeminal pain and the mechanism of BoNT/A action on DNI.

EXPERIMENTAL APPROACH

Because temporomandibular disorders are highly comorbid with migraine, we employed a rat model of inflammation induced by complete Freund's adjuvant, followed by treatment with BoNT/A injections or sumatriptan p.o. DNI was assessed by Evans blue-plasma protein extravasation, cell histology and RIA for CGRP. BoNT/A enzymatic activity in dura was assessed by immunohistochemistry for cleaved synaptosomal-associated protein 25 (SNAP-25).

KEY RESULTS

BoNT/A and sumatriptan reduced the mechanical allodynia and DNI, evoked by complete Freund's adjuvant. BoNT/A prevented inflammatory cell infiltration and inhibited the increase of CGRP levels in dura. After peripheral application, BoNT/A-cleaved SNAP-25 colocalized with CGRP in intracranial dural nerve endings. Injection of the axonal transport blocker colchicine into the trigeminal ganglion prevented the formation of cleaved SNAP-25 in dura.

CONCLUSIONS AND IMPLICATIONS

Pericranially injected BoNT/A was taken up by local sensory nerve endings, axonally transported to the trigeminal ganglion and transcytosed to dural afferents. Colocalization of cleaved SNAP-25 and the migraine mediator CGRP in dura suggests that BoNT/A may prevent DNI by suppressing transmission by CGRP. This might explain the effects of BoNT/A in temporomandibular joint inflammation and in migraine and some other headaches.