Long lasting pain hypersensitivity following ligation of the tendon of the masseter muscle in rats: a model of myogenic orofacial pain

The role of voltage-gated calcium channel α2δ-1 in the occurrence and development in myofascial orofacial pain

Patients who suffer from myofascial orofacial pain could affect their quality of life deeply. The pathogenesis of pain is still unclear. Our objective was to assess Whether Voltage-gated calcium channel α2δ-1(Cavα2δ-1) is related to myofascial orofacial pain. Rats were divided into the masseter tendon ligation group and the sham group. Compared with the sham group, the mechanical pain threshold of the masseter tendon ligation group was reduced on the 4th, 7th, 10th and 14th day after operation(P < 0.05). On the 14th day after operation, Cavα2δ-1 mRNA expression levels in trigeminal ganglion (TG) and the trigeminal spinal subnucleus caudalis and C1-C2 spinal cervical dorsal horn (Vc/C2) of the masseter tendon ligation group were increased (PTG=0.021, PVc/C2=0.012). Rats were divided into three groups. On the 4th day after ligating the superficial tendon of the left masseter muscle of the rats, 10 ul Cavα2δ-1 antisense oligonucleotide, 10 ul Cavα2δ-1 mismatched oligonucleotides and 10 ul normal saline was separately injected into the left masseter muscle of rats in Cavα2δ-1 antisense oligonucleotide group, Cavα2δ-1 mismatched oligonucleotides group and normal saline control group twice a day for 4 days. The mechanical pain threshold of the Cavα2δ-1 antisense oligonucleotides group was higher than Cavα2δ-1 mismatched oligonucleotides group on the 7th and 10th day after operation (P < 0.01). After PC12 cells were treated with lipopolysaccharide, Cavα2δ-1 mRNA expression level increased (P < 0.001). Cavα2δ-1 may be involved in the occurrence and development in myofascial orofacial pain.

Neuroimaging and artificial intelligence for assessment of chronic painful temporomandibular disorders-a comprehensive review

Chronic Painful Temporomandibular Disorders (TMD) are challenging to diagnose and manage due to their complexity and lack of understanding of brain mechanism. In the past few decades' neural mechanisms of pain regulation and perception have been clarified by neuroimaging research. Advances in the neuroimaging have bridged the gap between brain activity and the subjective experience of pain. Neuroimaging has also made strides toward separating the neural mechanisms underlying the chronic painful TMD. Recently, Artificial Intelligence (AI) is transforming various sectors by automating tasks that previously required humans' intelligence to complete. AI has started to contribute to the recognition, assessment, and understanding of painful TMD. The application of AI and neuroimaging in understanding the pathophysiology and diagnosis of chronic painful TMD are still in its early stages. The objective of the present review is to identify the contemporary neuroimaging approaches such as structural, functional, and molecular techniques that have been used to investigate the brain of chronic painful TMD individuals. Furthermore, this review guides practitioners on relevant aspects of AI and how AI and neuroimaging methods can revolutionize our understanding on the mechanisms of painful TMD and aid in both diagnosis and management to enhance patient outcomes.

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Sensory Neuron-TRPV4 Modulates Temporomandibular Disorder Pain Via CGRP in Mice

Temporomandibular disorder (TMD) pain that involves inflammation and injury in the temporomandibular joint (TMJ) and/or masticatory muscle is the most common form of orofacial pain. We recently found that transient receptor potential vanilloid-4 (TRPV4) in trigeminal ganglion (TG) neurons is upregulated after TMJ inflammation, and TRPV4 coexpresses with calcitonin gene-related peptide (CGRP) in TMJ-innervating TG neurons. Here, we extended these findings to determine the specific contribution of TRPV4 in TG neurons to TMD pain, and examine whether sensory neuron-TRPV4 modulates TMD pain via CGRP. In mouse models of TMJ inflammation or masseter muscle injury, sensory neuron-Trpv4 conditional knockout (cKO) mice displayed reduced pain. Coexpression of TRPV4 and CGRP in TMJ- or masseter muscle-innervating TG neurons was increased after TMJ inflammation and masseter muscle injury, respectively. Activation of TRPV4-expressing TG neurons triggered secretion of CGRP, which was associated with increased levels of CGRP in peri-TMJ tissues, masseter muscle, spinal trigeminal nucleus, and plasma in both models. Local injection of CGRP into the TMJ or masseter muscle evoked acute pain in naïve mice, while blockade of CGRP receptor attenuated pain in mouse models of TMD. These results suggest that TRPV4 in TG neurons contributes to TMD pain by potentiating CGRP secretion. PERSPECTIVE: This study demonstrates that activation of TRPV4 in TG sensory neurons drives pain by potentiating the release of pain mediator CGRP in mouse models of TMJ inflammation and masseter muscle injury. Targeting TRPV4 and CGRP may be of clinical potential in alleviating TMD pain.

TMEM100, a regulator of TRPV1-TRPA1 interaction, contributes to temporomandibular disorder pain

There is an unmet need to identify new therapeutic targets for temporomandibular disorder (TMD) pain because current treatments are limited and unsatisfactory. TMEM100, a two-transmembrane protein, was recently identified as a regulator to weaken the TRPA1-TRPV1 physical association, resulting in disinhibition of TRPA1 activity in sensory neurons. Recent studies have also shown that Tmem100, Trpa1, and Trpv1 mRNAs were upregulated in trigeminal ganglion (TG) after inflammation of the temporomandibular joint (TMJ) associated tissues. These findings raise a critical question regarding whether TMEM100 in TG neurons is involved in TMD pain via regulating the TRPA1-TRPV1 functional interaction. Here, using two mouse models of TMD pain induced by TMJ inflammation or masseter muscle injury, we found that global knockout or systemic inhibition of TRPA1 and TRPV1 attenuated pain. In line with their increased genes, mice exhibited significant upregulation of TMEM100, TRPA1, and TRPV1 at the protein levels in TG neurons after TMD pain. Importantly, TMEM100 co-expressed with TRPA1 and TRPV1 in TG neurons-innervating the TMJ and masseter muscle and their co-expression was increased after TMD pain. Moreover, the enhanced activity of TRPA1 in TG neurons evoked by TMJ inflammation or masseter muscle injury was suppressed by inhibition of TMEM100. Selective deletion of Tmem100 in TG neurons or local administration of TMEM100 inhibitor into the TMJ or masseter muscle attenuated TMD pain. Together, these results suggest that TMEM100 in TG neurons contributes to TMD pain by regulating TRPA1 activity within the TRPA1-TRPV1 complex. TMEM100 therefore represents a potential novel target-of-interest for TMD pain.

Spinal CCK1 Receptors Contribute to Somatic Pain Hypersensitivity Induced by Malocclusion via a Reciprocal Neuron-Glial Signaling Cascade

Recent studies have shown that the incidence of chronic primary pain including temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) often exhibit comorbidities. We recently reported that central sensitization and descending facilitation system contributed to the development of somatic pain hypersensitivity induced by orofacial inflammation combined with stress. The purpose of this study was to explore whether TMD caused by unilateral anterior crossbite (UAC) can induce somatic pain hypersensitivity, and whether the cholecystokinin (CCK) receptor-mediated descending facilitation system promotes hypersensitivity through neuron-glia cell signaling cascade. UAC evoked thermal and mechanical pain hypersensitivity of the hind paws from day 5 to 70 that peaked at week 4 post UAC. The expression levels of CCK1 receptors, interleukin-18 (IL-18) and IL-18 receptors (IL-18R) were significantly up-regulated in the L4 to L5 spinal dorsal horn at 4 weeks post UAC. Intrathecal injection of CCK1 and IL-18 receptor antagonists blocked somatic pain hypersensitivity. IL-18 mainly co-localized with microglia, while IL-18R mainly co-localized with astrocytes and to a lesser extent with neurons. These findings indicate that the signaling transduction between neurons and glia at the spinal cord level contributes to the descending pain facilitation through CCK1 receptors during the development of the comorbidity of TMD and FMS. PERSPECTIVE: CCK1 receptor-dependent descending facilitation may mediate central mechanisms underlying the development of widespread somatic pain via a reciprocal neuron-glial signaling cascade, providing novel therapeutic targets for the clinical treatment of TMD and FMS comorbidities.

A novel rat model of temporomandibular disorder with improved face and construct validities

AIMS

Temporomandibular disorders are a cluster of orofacial conditions that are characterized by pain in the temporomandibular joint (TMJ) and surrounding muscles/tissues. Animal models of painful temporomandibular dysfunction (TMD) are valuable tools to investigate the mechanisms responsible for symptomatic temporomandibular joint and associated structures disorders. We tested the hypothesis that a predisposing and a precipitating factor are required to produce painful TMD in rats, using the ratgnawmeter, a device that determines temporomandibular pain based on the time taken for the rat to chew through two obstacles.

MATERIALS AND METHODS

Increased time in the ratgnawmeter correlated with nociceptive behaviors produced by TMJ injection of formalin (2.5%), confirming chewing time as an index of painful TMD. Rats exposed only to predisposing factors, carrageenan-induced TMJ inflammation or sustained inhibition of the catechol-O-methyltransferase (COMT) enzyme by OR-486, showed no changes in chewing time. However, when combined with a precipitating event, i.e., exaggerated mouth opening produced by daily 1-h jaw extension for 7 consecutive days, robust function impairment was produced.

KEY FINDINGS

These results validate the ratgnawmeter as an efficient method to evaluate functional TMD pain by evaluating chewing time, and this protocol as a model with face and construct validities to investigate symptomatic TMD mechanisms.

SIGNIFICANCE

This study suggests that a predisposition factor must be present in order for an insult to the temporomandibular system to produce painful dysfunction. The need for a combined contribution of these factors might explain why not all patients experiencing traumatic events, such as exaggerated mouth opening, develop TMDs.

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.

Occlusal disharmony and chronic oro-facial pain: from clinical observation to animal study

BACKGROUND

Occlusion can be viewed as the most sensitive susceptor of the central nervous system in the oro-facial region. Its inalienable relationships to the temporomandibular joint, the muscles, the stomatognathic system and even the central nervous system are self-evident. Almost all the dental treatments inevitably change the occlusion, potentially or actually, locally or extensively, and immediately or gradually.

OBJECTIVE

The objective of this study was to present a narrative literature on occlusal disharmony and chronic oro-facial pain.

METHODS

Literature reviews focusing on clinical studies about the relationship between occlusal disharmony and myofascial oro-facial pain, and related preclinical studies about the animal models of, as well as the peripheral and central mechanisms underlying this condition related to, occlusal disharmony were used as starting point and guidelines to describe the topics mentioned. A search of the PubMed database was performed mainly with the following search terms: "occlusion," "occlusal interference," "occlusal disharmony," "occlusal change," "oro-facial pain" and "myofascial pain."

RESULTS

Relevant literature from the past 70 years until the present day was meticulously studied. The literature review together with three related characteristic clinical cases revealed an intimate association between occlusal disharmony and chronic oro-facial pain, involving pathological changes, extending from the peripheral tissues to the central nervous system. The patients suffered from psychological distress, sleep disturbance and poor life quality.

CONCLUSION

Occlusal disharmony-related oro-facial pain is a clinical problem that deserves attention, although there are no universally accepted clinical protocols. The existing literature provides some constructive suggestions, but further research is needed.

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.

Temporomandibular Disorders and Fibromyalgia: A Narrative Review

Temporomandibular disorder (TMD) and fibromyalgia (FM) have some clinical characteristics in common, for instance the chronic evolution, the pathophysiology incompletely understood and a multifactorial genesis. The incidence and the relationship between TMD and FM patients are the aims of this review. A MEDLINE and Pubmed search was performed for the key words “temporomandibular disorder” AND “fibromyalgia” from 2000 to present. A total of 19 papers were included in our review, accounting for 5449 patients. Ten studies, reporting a total of 4945 patients with TMD, showed that only 16.5% of these patients had diagnosis of FM, whereas 12 studies, reporting a total of 504 patients with FM, demonstrated that 77.0% of these patients had diagnosis of TMD. A comorbid relationship exists between TMD and FM. The complexity of both diseases shows the importance of a multimodal and interdisciplinary.

Nociceptive behavioural assessments in mouse models of temporomandibular joint disorders

Orofacial pain or tenderness is a primary symptom associated with temporomandibular joint (TMJ) disorders (TMDs). To understand the pathological mechanisms underlying TMDs, several mouse models have been developed, including mechanical stimulus-induced TMD and genetic mouse models. However, a lack of feasible approaches for assessing TMD-related nociceptive behaviours in the orofacial region of mice has hindered the in-depth study of TMD-associated mechanisms. This study aimed to explore modifications of three existing methods to analyse nociceptive behaviours using two TMD mouse models: (1) mechanical allodynia was tested using von Frey filaments in the mouse TMJ region by placing mice in specially designed chambers; (2) bite force was measured using the Economical Load and Force (ELF) system; and (3) spontaneous feeding behaviour tests, including eating duration and frequency, were analysed using the Laboratory Animal Behaviour Observation Registration and Analysis System (LABORAS). We successfully assessed changes in nociceptive behaviours in two TMD mouse models, a unilateral anterior crossbite (UAC)-induced TMD mouse model and a β-catenin conditional activation mouse model. We found that the UAC model and β-catenin conditional activation mouse model were significantly associated with signs of increased mechanical allodynia, lower bite force, and decreased spontaneous feeding behaviour, indicating manifestations of TMD. These behavioural changes were consistent with the cartilage degradation phenotype observed in these mouse models. Our studies have shown reliable methods to analyse nociceptive behaviours in mice and may indicate that these methods are valid to assess signs of TMD in mice.

The neuro-pathophysiology of temporomandibular disorders-related pain: a systematic review of structural and functional MRI studies

Chronic pain surrounding the temporomandibular joints and masticatory muscles is often the primary chief complaint of patients with temporomandibular disorders (TMD) seeking treatment. Yet, the neuro-pathophysiological basis underlying it remains to be clarified. Neuroimaging techniques have provided a deeper understanding of what happens to brain structure and function in TMD patients with chronic pain. Therefore, we performed a systematic review of magnetic resonance imaging (MRI) studies investigating structural and functional brain alterations in TMD patients to further unravel the neurobiological underpinnings of TMD-related pain. Online databases (PubMed, EMBASE, and Web of Science) were searched up to August 3, 2019, as complemented by a hand search in reference lists. A total of 622 papers were initially identified after duplicates removed and 25 studies met inclusion criteria for this review. Notably, the variations of MRI techniques used and study design among included studies preclude a meta-analysis and we discussed the findings qualitatively according to the specific neural system or network the brain regions were involved in. Brain changes were found in pathways responsible for abnormal pain perception, including the classic trigemino-thalamo-cortical system and the lateral and medial pain systems. Dysfunction and maladaptive changes were also identified in the default mode network, the top-down antinociceptive periaqueductal gray-raphe magnus pathway, as well as the motor system. TMD patients displayed altered brain activations in response to both innocuous and painful stimuli compared with healthy controls. Additionally, evidence indicates that splint therapy can alleviate TMD-related symptoms by inducing functional brain changes. In summary, MRI research provides important novel insights into the altered neural manifestations underlying chronic pain in TMD.

A Pre-Existing Myogenic Temporomandibular Disorder Increases Trigeminal Calcitonin Gene-Related Peptide and Enhances Nitroglycerin-Induced Hypersensitivity in Mice

Migraine is commonly reported among patients with temporomandibular disorders (TMDs), especially myogenic TMD. The pathophysiologic mechanisms related to the comorbidity of the two conditions remain elusive. In the present study, we combined masseter muscle tendon ligation (MMTL)-produced myogenic TMD with systemic injection of nitroglycerin (NTG)-induced migraine-like hypersensitivity in mice. Facial mechanical allodynia, functional allodynia, and light-aversive behavior were evaluated. Sumatriptan, an FDA-approved medication for migraine, was used to validate migraine-like hypersensitivity. Additionally, we examined the protein level of calcitonin gene-related peptide (CGRP) in the spinal trigeminal nucleus caudalis using immunohistochemistry. We observed that mice with MMTL pretreatment have a prolonged NTG-induced migraine-like hypersensitivity, and MMTL also enabled a non-sensitizing dose of NTG to trigger migraine-like hypersensitivity. Systemic injection of sumatriptan inhibited the MMTL-enhanced migraine-like hypersensitivity. MMTL pretreatment significantly upregulated the protein level of CGRP in the spinal trigeminal nucleus caudalis after NTG injection. Our results indicate that a pre-existing myogenic TMD can upregulate NTG-induced trigeminal CGRP and enhance migraine-like hypersensitivity.

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.

NMDAR and JNK Activation in the Spinal Trigeminal Nucleus Caudalis Contributes to Masseter Hyperalgesia Induced by Stress

It is commonly accepted that psychological stress is closely associated with the occurrence and development of chronic orofacial pain. However, the pathogenesis underlying this process has not been fully elucidated. In the present study, we explored the role of N-methyl-D-aspartate receptors (NMDARs) and Jun N-terminal kinase (JNK) mediated intercellular communication between neurons and astrocytes in the spinal trigeminal nucleus caudalis (Vc) in the induction of masseter hyperalgesia by psychological stress in rats. We found that subjecting rats to 14 days of restraint stress (8 h/d) caused a significant decrease in body weight gain, behavioral changes and marked masseter hyperalgesia in the rats. We also found that exposure to restraint stress for 14 days caused the expression of pJNK in astrocytes in the Vc to significantly increase, and intrathecally infusing a JNK inhibitor significantly prevented restraint stress-induced masseter hyperalgesia in the rats. In addition, after exposure to restraint stress for 14 days, the stressed group exhibited a noticeably increased expression level of pNR2B in neurons in the Vc. Then, we intrathecally injected MK-801 (an NMDAR inhibitor) and ifenprodil (a selective NR2B subunit antagonist) and observed that the two types of inhibitors not only alleviated masseter hyperalgesia but also significantly inhibited the phosphorylation of JNK in the Vc after restraint stress; this indicates that the effect of NMDAR antagonists may influence the activation of astrocytic JNK. Furthermore, inhibitors of neuronal nitric oxide synthase (nNOS) activation and guanylate cyclase (GC) inhibitor could not only inhibit the expression of pJNK in the Vc, but also effectively alleviate masseter hyperalgesia induced by restraint stress. Taken together, our results suggest that NMDAR activation could increase JNK phosphorylation in astrocytes after restraint stress, which may depend on the nNOS-GC pathway. The intercellular communication between neurons and astrocytes in the Vc may play a key role in the induction of masseter muscle hyperalgesia by psychological stress in rats.

Voluntary biting behavior as a functional measure of orofacial pain in mice

INTRODUCTION

Pain-related behavior secondary to masticatory function can be assessed with the rodent bite force model. A reduction of the bite force has been shown to be related to pain associated with the masseter muscle and jaw activity, while an increase in bite force suggests improvement of muscle function and less pain. To evaluate the usefulness of the bite force measure in studying long-lasting orofacial pain we analyzed biting parameters during prolonged myofascial pain induced by ligation injury of the masseter muscle tendon (TL) in mice.

METHODS

C57Bl/6 mice were habituated to bite at a pair of aluminum plates attached to a force displacement transducer. The transduced voltage signals were amplified and converted to force through calibration with a standard weight set. Voluntary biting behavior was recorded for 100 s/session and those with bite forces ≥980 mN were analyzed. Nociception was also verified with von Frey, conditioned place avoidance (CPA) tests and mouse grimace scale. Persistent orofacial pain was induced with unilateral ligation of one tendon of the masseter muscle (TL).

RESULTS

To reduce interference of random bites of smaller forces, the top 5 or 15 bite forces (BF5/15) were chosen as a measure of masticatory function and related to pain behavior. Both male and female mice exhibited similar BF5/15. For the first nascent test of all mice, mean bite force was significantly and positively correlated with the body weight. However, this correlation was less clear in the latter tests (2-8 w). TL induced a reduction of BF5/15 that peaked at 1 w and returned to the baseline within 3 w. The von Frey and CPA tests indicated that mechanical allodynia/hyperalgesia persisted at the time when the BF had returned to the pre-injury level. Infusion of pain-relieving bone marrow stromal cells improved biting behavior in both male and female mice as shown by significantly increased BF5/15, compared to vehicle-treated mice.

CONCLUSIONS

Mouse voluntary biting behavior can be reliably measured and quantified with a simplified setup. The bite force showed an inverse relationship with the level of pain after TL and was improved by pain-relieving manipulations. However, the injury-induced reduction of bite force peaked early and did not parallel with other measures of nociception in the later phase of hyperalgesia. The results suggest that multiple factors such as the level of habituation, cognitive motive, physical status, and feeding drive may affect random voluntary biting and confound the biting parameters related to maintained hyperalgesia.

NF-KappaB Pathway Is Involved in Bone Marrow Stromal Cell-Produced Pain Relief

Bone marrow stromal cells (BMSCs) produce long-lasting attenuation of pain hypersensitivity. This effect involves BMSC's ability to interact with the immune system and activation of the endogenous opioid receptors in the pain modulatory circuitry. The nuclear factor kappa B (NF-κB) protein complex is a key transcription factor that regulates gene expression involved in immunity. We tested the hypothesis that the NF-κB signaling plays a role in BMSC-induced pain relief. We focused on the rostral ventromedial medulla (RVM), a key structure in the descending pain modulatory pathway, that has been shown to play an important role in BMSC-produced antihyperalgesia. In Sprague-Dawley rats with a ligation injury of the masseter muscle tendon (TL), BMSCs (1.5 M/rat) from donor rats were infused i.v. at 1 week post-TL. P65 exhibited predominant neuronal localization in the RVM with scattered distribution in glial cells. At 1 week, but not 8 weeks after BMSC infusion, western blot and immunostaining showed that p65 of NF-κB was significantly increased in the RVM. Given that chemokine signaling is critical to BMSCs' pain-relieving effect, we further evaluated a role of chemokine signaling in p65 upregulation. Prior to infusion of BMSCs, we transduced BMSCs with Ccl4 shRNA, incubated BMSCs with RS 102895, a CCR2b antagonist, or maraviroc, a CCR5 antagonist. The antagonism of chemokines significantly reduced BMSC-induced upregulation of p65, suggesting that upregulation of p65 was related to BMSCs' pain-relieving effect. We then tested the effect of a selective NF-κB activation inhibitor, BAY 11-7082. The mechanical hyperalgesia of the rat was assessed with the von Frey method. In the pre-treatment experiment, BAY 11-7082 (2.5 and 25 pmol) was injected into the RVM at 2 h prior to BMSC infusion. Pretreatment with BAY 11-7082 attenuated BMSCs' antihyperalgesia, but post-treatment at 5 weeks post-BMSC was not effective. On the contrary, in TL rats receiving BAY 11-7082 without BMSCs, TL-induced hyperalgesia was attenuated, consistent with dual roles of NF-κB in pain hypersensitivity and BMSC-produced pain relief. These results indicate that the NF-κB signaling pathway in the descending circuitry is involved in initiation of BMSC-produced behavioral antihyperalgesia.

Chronic orofacial pain animal models - progress and challenges

INTRODUCTION

Chronic orofacial pain is one of the most common pain conditions experienced by adults. Animal models are often selected as the most useful scientific methodology to explore the pathophysiology of the disorders that cause this disabling pain to facilitate the development of new treatments. The creation of new models or the improvement of existing ones is essential for finding new ways to approach the complex neurobiology of this type of pain. Areas covered: The authors describe and discuss a variety of animal models used in chronic orofacial pain (COFP). Furthermore, they examine in detail the mechanisms of action involved in orofacial neuropathic pain and orofacial inflammatory pain. Expert opinion: The use of animal models has several advantages in chronic orofacial pain drug discovery. Choosing an animal model that most closely represents the human disease helps to increase the chances of finding effective new therapies and is key to the successful translation of preclinical research to clinical practice. Models using genetically modified animals seem promising but have not yet been fully developed for use in chronic orofacial pain research. Although animal models have provided significant advances in the pharmacological treatment of orofacial pain, several barriers still need to be overcome for better treatment options.

Further observations on the behavioral and neural effects of bone marrow stromal cells in rodent pain models

BACKGROUND

Bone marrow stromal cells (BMSCs) have shown potential to treat chronic pain, although much still needs to be learned about their efficacy and mechanisms of action under different pain conditions. Here, we provide further convergent evidence on the effects of BMSCs in rodent pain models.

RESULTS

In an orofacial pain model involving injury of a tendon of the masseter muscle, BMSCs attenuated behavioral pain conditions assessed by von Frey filaments and a conditioned place avoidance test in female Sprague-Dawley rats. The antihyperalgesia of BMSCs in females lasted for <8 weeks, which is shorter than that seen in males. To relate preclinical findings to human clinical conditions, we used human BMSCs. Human BMSCs (1.5 M cells, i.v.) attenuated mechanical and thermal hyperalgesia induced by spinal nerve ligation and suppressed spinal nerve ligation-induced aversive behavior, and the effect persisted through the 8-week observation period. In a trigeminal slice preparation, BMSC-treated and nerve-injured C57B/L mice showed reduced amplitude and frequency of spontaneous excitatory postsynaptic currents, as well as excitatory synaptic currents evoked by electrical stimulation of the trigeminal nerve root, suggesting inhibition of trigeminal neuronal hyperexcitability and primary afferent input by BMSCs. Finally, we observed that GluN2A (N-methyl-D-aspartate receptor subunit 2A) tyrosine phosphorylation and protein kinase Cgamma (PKCg) immunoreactivity in rostral ventromedial medulla was suppressed at 8 weeks after BMSC in tendon-injured rats.

CONCLUSIONS

Collectively, the present work adds convergent evidence supporting the use of BMSCs in pain control. As PKCg activity related to N-methyl-D-aspartate receptor activation is critical in opioid tolerance, these results help to understand the mechanisms of BMSC-produced long-term antihyperalgesia, which requires opioid receptors in rostral ventromedial medulla and apparently lacks the development of tolerance.

Sex differences in peripheral mu-opioid receptor mediated analgesia in rat orofacial persistent pain model

Unilateral ligation of the tendon of anterior superficial part of rat masseter muscle (TASM) leads to long-lasting allodynia. Sex differences in peripheral mu-opioid receptor (MOR)-mediated analgesia under persistent myogenic pain are not well understood. In this study, we examined (1) whether locally applied MOR agonists attenuate persistent pain following TASM ligation in a sex dependent manner, (2) whether there are sex differences of MOR expression changes in rat trigeminal ganglia (TG). The effects of MOR agonist, D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin acetate salt (DAMGO), were assessed 14 days after TASM ligation in male, female and orchidectomized (GDX) male rats. MOR mRNA and protein levels in TG 14 days following tendon ligation were also determined. The mechanical thresholds of the injured side were significantly decreased in both male and female rats, from 3 days to 28 days after TASM ligation. A10 μg DAMGO significantly attenuated allodynia in male rats. A 10-fold higher dose of DAMGO was required in female and GDX male rats to produce the level of anti- allodynia achieved in male rats. The level of MOR mRNA in TG from male rats was significantly greater 14 days after TASM ligation compared with the sham-operated male rats, but not from female and GDX male rats. After TASM ligation, males had significantly more MOR immunoreactivity in TG compared to sham-operated males. The MOR levels increased to 181.8% of the sham level in male rats receiving tendon injury. But there was no significant change in female rats receiving tendon injury compared to the sham female rats. Taken together, our data suggest that there were sex differences in the effects of peripheral MOR agonists between male and female rats under TASM ligation developing long-lasting pain condition, which is partly mediated by sex differences in the changes of MOR expressions and testosterone is an important factor in the regulation of MOR.

Attenuation of myogenic orofacial nociception and mechanical hypersensitivity by viral mediated enkephalin overproduction in male and female rats

BACKGROUND

Clinical studies have tested the use of an engineered herpes virus to treat pain. We hypothesized that subcutaneous injections of an engineered herpes virus that expresses enkephalin would attenuate orofacial nociception and hypersensitivity in male and female rats by a central mechanism.

METHODS

Herpes virus was injected subcutaneously around the mouth of male and female rats seventy-two hours before ligatures were placed on the masseter tendon, control treatment groups received either no virus or no ligature. Enkephalin expression was measured and von Frey filament testing and meal duration were utilized to measure mechanical hypersensitivity and the nociceptive response, respectively. Naloxone or naloxone methiodide was administered to rats injected with the enkephalin expressing virus to test if enkephalin was acting peripherally or centrally.

RESULTS

Ligature significantly lengthened meal duration and reduced the threshold to von Frey filaments for 18 days. Infection with the enkephalin transgene significantly decreased this response for at least 11 days but only in male rats. Virus injection significantly increased expression of enkephalin in the mental nerve that innervates the mouth region, the trigeminal ganglia and the trigeminal nucleus caudalis but no increase was observed in the masseter nerve after virus injection. Naloxone but not naloxone methiodide reversed the response to the enkephaline expressing virus.

CONCLUSIONS

The data suggests that sex should be a considered when using this virus and that viral transfection of the mental nerve with an enkephalin transgene can reduce nociception and hypersensitivity through a central mechanism.

Involvement of trigeminal astrocyte activation in masseter hyperalgesia under stress

It is commonly accepted that psychological stress contributes to the development of temporomandibular joint disorders, in which chronic orofacial pain is the main symptom. However, the central mechanism underlying the development of these disorders has remained unclear. The current study was performed to determine the involvement of the glia in the trigeminal spinal subnucleus caudalis in stress-induced increases in masseter muscle hyperalgesia in rats. After being subjected to chronic restraint stress, the animals showed decreased body weight gain, behavioral changes and marked masseter allodynia. We also found that astrocytes, but not microglia, in the trigeminal subnucleus caudalis (Vc) were dramatically activated. A further analysis was undertaken to investigate the contribution of the glia; we intrathecally injected l-α-aminoadipate (astrocyte-specific inhibitor) and/or minocycline (microglia-specific inhibitor) into the stressed rats. Our results showed that l-α-aminoadipate (LAA), but not minocycline, could significantly attenuate the mechanical masseter allodynia and behavioral changes induced by restraint stress. In addition, the expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-d-aspartic acid receptor 1 (p-NR1) in the Vc was significantly increased after chronic restraint stress, whereas LAA dramatically inhibited the overexpression of IL-1β and p-NR1. Taken together, these results suggest that activated astrocytes in the Vc may be one of the most important factors in the pathophysiology of masseter hyperalgesia induced by restraint stress and the following overexpression of IL-1β and excessive NMDAR phosphorylation may ultimately contribute to masseter hyperalgesia. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for the treatment of orofacial pain induced by stress.

Infusion of Gabrα6 siRNA into the trigeminal ganglia increased the myogenic orofacial nociceptive response of ovariectomized rats treated with 17β-estradiol

High levels of 17β-estradiol (E2) have been found to reduce inflammatory temporomandibular joint (TMJ) pain. A search for genes effected by a high concentration of estradiol showed an increase in GABAA receptor subunit alpha 6 (Gabrα6) in the trigeminal ganglia (TG). Blockade of Gabrα6 expression in the TG increases masseter muscle nociception in male rats, but the relationship between estradiol's effect on nociception and Gabrα6 expression remains unclear in females. To address this knowledge gap we hypothesized that reducing Gabrα6 expression in the TG will increase the orofacial nociceptive response of ovariectomized female rats treated with estradiol. To administer hormone osmotic pumps were placed in rats that dispensed a low diestrus plasma concentration of 17β-estradiol, in addition, 17β-estradiol was injected to produce a high proestrus plasma concentration of estradiol. A ligature was then placed around the masseter tendon to induce a nociceptive response; a model for TMJ muscle pain. Gabrα6 small interfering RNA (siRNA) was later infused into the TG and the nociceptive response was measured using von Frey filaments and a meal duration assay. GABAA receptor expression was measured in the TG and trigeminal nucleus caudalis and upper cervical region (Vc-C1). Ligature significantly increased the nociceptive response but a high proestrus concentration of 17β-estradiol attenuated this response. Gabrα6 siRNA infusion decreased Gabrα6 expression in the TG and Vc-C1 but increased the nociceptive response after 17β-estradiol treatment. The results suggest estradiol decreased the orofacial nociceptive response, in part, by causing an increase in Gabrα6 expression.

Dentin hypersensitivity induces anxiety and increases corticosterone serum levels in rats

AIMS

Investigate the relationships between experimentally induced dentin hypersensitivity (DH) with behavioral, endocrine and dentin erosion data.

METHODS

Male Wistar rats divided into four groups, two controls and two experimental, received tap water or isotonic solution (Gatorade®, lemon, pH2.7) for 30 or 45 days. The DH test was performed by a cold water stimulus on molars. A score (0-3) was given to the rats' pain response. Anxiety was evaluated by the elevated plus maze model and by serum corticosterone levels. The dentin erosion was observed by scanning electron microscopy (SEM). Anatomopathological studies were performed on the stomach, adrenal, kidney, and liver.

RESULTS

Relative to control groups, experimental rats showed: 1) increased hypersensitivity scores (control group, 0; experimental groups, 2 (limits 0.5-3) on the 30th day and 2 (limits 1-3) on the 45th day); 2) reduced percentage of time and entries in the open arms and in serum corticosterone levels; 3) totally exposed dentinal tubules on the 30th day in SEM analysis of the teeth; and 4) no alterations in the anatomopathological and histological evaluations.

CONCLUSIONS

The treatment with isotonic solution for 30 days was able to induce DH after erosive challenge and severe DH was observed after isotonic solution treatment for 45 days. The pain induced by cold stimuli was consistent with the grade of DH. The close relationships between dental erosion, response to pain, serum levels of corticosterone and the EPM behavior responses reveal the effects of DH at several levels.

Meal duration as a measure of orofacial nociceptive responses in rodents

A lengthening in meal duration can be used to measure an increase in orofacial mechanical hyperalgesia having similarities to the guarding behavior of humans with orofacial pain. To measure meal duration unrestrained rats are continuously kept in sound attenuated, computerized feeding modules for days to weeks to record feeding behavior. These sound-attenuated chambers are equipped with chow pellet dispensers. The dispenser has a pellet trough with a photobeam placed at the bottom of the trough and when a rodent removes a pellet from the feeder trough this beam is no longer blocked, signaling the computer to drop another pellet. The computer records the date and time when the pellets were taken from the trough and from this data the experimenter can calculate the meal parameters. When calculating meal parameters a meal was defined based on previous work and was set at 10 min (in other words when the animal does not eat for 10 min that would be the end of the animal's meal) also the minimum meal size was set at 3 pellets. The meal duration, meal number, food intake, meal size and inter-meal interval can then be calculated by the software for any time period that the operator desires. Of the feeding parameters that can be calculated meal duration has been shown to be a continuous noninvasive biological marker of orofacial nociception in male rats and mice and female rats. Meal duration measurements are quantitative, require no training or animal manipulation, require cortical participation, and do not compete with other experimentally induced behaviors. These factors distinguish this assay from other operant or reflex methods for recording orofacial nociception.

Temporomandibular joint pain: a critical role for Trpv4 in the trigeminal ganglion

Temporomandibular joint disorder (TMJD) is known for its mastication-associated pain. TMJD is medically relevant because of its prevalence, severity, chronicity, the therapy-refractoriness of its pain, and its largely elusive pathogenesis. Against this background, we sought to investigate the pathogenetic contributions of the calcium-permeable TRPV4 ion channel, robustly expressed in the trigeminal ganglion sensory neurons, to TMJ inflammation and pain behavior. We demonstrate here that TRPV4 is critical for TMJ-inflammation-evoked pain behavior in mice and that trigeminal ganglion pronociceptive changes are TRPV4-dependent. As a quantitative metric, bite force was recorded as evidence of masticatory sensitization, in keeping with human translational studies. In Trpv4(-/-) mice with TMJ inflammation, attenuation of bite force was significantly less than in wildtype (WT) mice. Similar effects were seen with systemic application of a specific TRPV4 inhibitor. TMJ inflammation and mandibular bony changes were apparent after injections of complete Freund adjuvant but were remarkably independent of the Trpv4 genotype. It was intriguing that, as a result of TMJ inflammation, WT mice exhibited significant upregulation of TRPV4 and phosphorylated extracellular-signal-regulated kinase (ERK) in TMJ-innervating trigeminal sensory neurons, which were absent in Trpv4(-/-) mice. Mice with genetically-impaired MEK/ERK phosphorylation in neurons showed resistance to reduction of bite force similar to that of Trpv4(-/-) mice. Thus, TRPV4 is necessary for masticatory sensitization in TMJ inflammation and probably functions upstream of MEK/ERK phosphorylation in trigeminal ganglion sensory neurons in vivo. TRPV4 therefore represents a novel pronociceptive target in TMJ inflammation and should be considered a target of interest in human TMJD.

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

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

PERSPECTIVE

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

Reduced GABAA receptor α6 expression in the trigeminal ganglion enhanced myofascial nociceptive response

Activation of the GABAA receptor results in inhibition of neuronal activity. One subunit of this multi-subunit receptor termed alpha 6 (Gabrα6) contributed to inflammatory temporomandibular joint (TMJ) nociception but TMJ disorders often include myofascial pain. To address Gabrα6 role in myofascial pain we hypothesized that Gabrα6 has an inhibitory role in myofascial nociceptive responses similar to inflammatory TMJ arthritis. To test this hypothesis a, myofascial nociceptive response was induced by placing a ligature bilaterally on the tendon attachment of the anterior superficial part of a male rat's masseter muscle. Four days after ligature placement Gabrα6 expression was reduced by infusing the trigeminal ganglia (TG) with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). After siRNA infusion nociceptive behavioral responses were measured, i.e., feeding behavior and head withdrawal after pressing upon the region above the ligature with von Frey filaments. Neuronal activity in the TG and trigeminal nucleus caudalis and upper cervical region (Vc-C1) was measured by quantitating the amount of phosphorylated extracellular signal-regulated kinase (p-ERK). Total Gabrα6 and GABAA receptor contents in the TG and Vc-C1 were determined. Gabrα6 siRNA infusion reduced Gabrα6 and GABAA receptor expression and significantly increased the nociceptive response in both nociceptive assays. Gabrα6 siRNA infusion also significantly increased TG p-ERK expression of the ligated rats. From these results we conclude GABAA receptors consisting of the Gabrα6 subunit inhibit TG nociceptive sensory afferents in the trigeminal pathway and have an important role in the regulation of myofascial nociception.

Myoelectrical activity and muscle morphology in a rat model of myofascial trigger points induced by blunt trauma to the vastus medialis

OBJECTIVES

To explore myoelectrical activity and muscle morphology of myofascial trigger points (MTrPs) in an injury model of rats.

METHODS

A total of 24 male SD rats were randomly divided into a control group (group A) and model group (group B). A blunt striking injury and eccentric exercise were applied to the vastus medialis (VM) of rats in group B for 8 weeks. Later, the palpable taut band (TB), local twitch response, myoelectrical activities and morphology in the two groups were examined.

RESULTS

An average of 2.5 (30/12) palpable TBs were detected in the VM in group B compared with none in group A. The MTrPs had two types of abnormal potential. Their amplitudes were significantly higher than those in the control group (p<0.01) but their durations showed no significant differences. A series of reflex contractions appeared in groups A and B in response to external stimulation to the ear. Their amplitude and duration in group B were significantly lower than those in group A. A series of lower fibrillation potentials repeatedly occurred in model MTrPs in group B. The morphology of MTrPs showed abnormal muscle fibres with large round or ellipse shapes in cross-section and enlarged tapering shapes in longitudinal section.

CONCLUSIONS

Active MTrPs can be provoked by repeated blunt injury. Active MTrPs are a group of muscle fibres with abnormal shapes and abnormal myoelectrical potentials. External stimulation provokes low-voltage responses in MTrPs, which is different from the response of normal muscle fibres.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

A functional relationship between trigeminal astroglial activation and NR1 expression in a rat model of temporomandibular joint inflammation

OBJECTIVE

To examine the hypothesis that glial activation would regulate the expression of the N-methyl-D-aspartate receptor subunit 1 (NR1) in the trigeminal subnucleus caudalis (Sp5C) after temporomandibular joint (TMJ) inflammation.

METHODS

Inflammation of TMJ was produced in rats by injecting 50 μL complete Freund's adjuvant (CFA) into unilateral TMJ space. Sham control rats received incomplete Freund's adjuvant injection. Mechanical nociception in the affected and non-affected TMJ site was tested by using a digital algometer. Fractalkine, fluorocitrate, and/or MK801 were intracisternally administrated to examine the relationship between astroglial activation and NR1 upregulation.

RESULTS

CFA TMJ injection resulted in persistent ipsilateral mechanical hyperalgesia 1, 3, and 5 days after CFA injection. The inflammation also induced significant upregulation of CX3C chemokine receptor 1 and glial fibrillary acidic protein (GFAP) beginning on day 1 and of NR1 beginning on day 3 within the ipsilateral Sp5C. Intracisternal administration of fluorocitrate for 5 days blocked the development of mechanical hyperalgesia as well as the upregulation of GFAP and NR1 in the Sp5C. Conversely, intracisternal injection of fractalkine for 5 days exacerbated the expression of NR1 in Sp5C and mechanical hyperalgesia induced by TMJ inflammation. Moreover, once daily intracisternal fractalkine administration for 5 days in naïve rats induced the upregulation of NR1 and mechanical hyperalgesia.

CONCLUSIONS

These results suggest that astroglial activation contributes to the mechanism of TMJ pain through the regulation of NR1 expression in Sp5C.

Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Orofacial pain conditions are often very debilitating to the patient and difficult to treat. While clinical interest is high, the proportion of studies performed in the orofacial region in laboratory animals is relatively low, compared with other body regions. This is partly due to difficulties in testing freely moving animals and therefore lack of reliable testing methods. Here we present a comprehensive review of the currently used rodent models of inflammatory and neuropathic pain adapted to the orofacial areas, taking into account the difficulties and drawbacks of the existing approaches. We examine the available testing methods and procedures used for assessing the behavioral responses in the face in both mice and rats and provide a summary of some pharmacological agents used in these paradigms to date. The use of these agents in animal models is also compared with outcomes observed in the clinic.

Bone marrow stromal cells produce long-term pain relief in rat models of persistent pain

Chronic pain conditions are difficult to treat and are major health problems. Bone marrow stromal cells (BMSCs) have generated considerable interest as a candidate for cell-based therapy. BMSCs are readily accessible and are easy to isolate and expand ex vivo. Clinical studies show that direct injection of BMSCs does not produce unwanted side effects and is well tolerated and safe. Here, we show that a single systemic (intravenous) or local injection (into the lesion site) of rat primary BMSCs reversed pain hypersensitivity in rats after injury and that the effect lasted until the conclusion of the study at 22 weeks. The pain hypersensitivity was rekindled by naloxone hydrochloride, an opioid receptor antagonist that acts peripherally and centrally, when tested at 1-5 weeks after BMSC infusion. In contrast, naloxone methiodide, a peripherally acting opioid receptor antagonist, only rekindled hyperalgesia in the first 3 weeks of BMSC treatment. Focal downregulation of brainstem mu opioid receptors by RNA interference (RNAi) reversed the effect of BMSCs, when RNAi was introduced at 5- but not 1-week after BMSC transplantation. Thus, BMSCs produced long-term relief of pain and this effect involved activation of peripheral and central opioid receptors in distinct time domains. The findings prompt studies to elucidate the cellular mechanisms of the BMSC-induced pain relieving effect and translate these observations into clinical settings.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Contribution of Primary Afferent Input to Trigeminal Astroglial Hyperactivity, Cytokine Induction and NMDA Receptor Phosphorylation

We tested the hypothesis that primary afferent inputs play a role in astroglial hyperactivity after tissue injury. We first injected complete Freund's adjuvant (CFA, 0.05 ml, 1:1 oil/saline) into the masseter muscle, which upregulated glial fibrillary acidic protein (GFAP), a marker of astrocytes, interleukin (IL)-1β an inflammatory cytokine, and phosphorylation of serine896 of the NR1 subunit (P-NR1) of the NMDA receptor in the subnuclei interpolaris/caudalis (Vi/Vc) transition zone, an important structure for processing trigeminal nociceptive input. Local anesthetic block with lidocaine (2%) of the masseter muscle at 10 min prior to injection of CFA into the same site significantly reduced the CFA-induced increase in GFAP, IL-1β and P-NR1 (p<0.05, n=4/group). We then tested the effect of peripheral electrical stimulation (ES). The ES protocol was burst stimulation consisting of trains of 4 square pulses (10-100 Hz, 0.1-3 mA, 0.5 ms pulse width). Under pentobarbital anesthesia, an ES was delivered every 0.2 s for a total of 30 min. The Vi/Vc tissues were processed for immunohistochemistry or western blot analysis at 10-120 min after ES. Compared to naive and SHAM-treated rats, there was increased immunoreactivity against GFAP, IL-1β and P-NR1 in the Vi/Vc in rats receiving ES. Double staining showed that IL-1β was selectively localized in GFAP-positive astroglia, and P-NR1-immunoreactivity was localized to neurons. These findings indicate that primary afferent inputs are necessary and sufficient to induce astroglial hyperactivity and upregulation of IL-1β, as well as neuronal NMDA receptor phosphorylation.