Peripheral metabotropic glutamate receptor 5 mediates mechanical hypersensitivity in craniofacial muscle via protein kinase C dependent mechanisms

Peripheral role of glutamate in orofacial pain

Glutamate is the principal excitatory neurotransmitter in the central nervous system. In the periphery, glutamate acts as a transmitter and involves in the signaling and processing of sensory input. Glutamate acts at several types of receptors and also interacts with other transmitters/mediators under various physiological and pathophysiological conditions including chronic pain. The increasing amount of evidence suggests that glutamate may play a role through multiple mechanisms in orofacial pain processing. In this study, we reviewed the current understanding of how peripheral glutamate mediates orofacial pain, how glutamate is regulated in the periphery, and how these findings are translated into therapies for pain conditions.

Recent Advances in the Modulation of Pain by the Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluR or mGlu) are G-protein coupled receptors activated by the binding of glutamate, the main classical neurotransmitter of the nervous system. Eight different mGluR subtypes (mGluR1-8) have been cloned and are classified in three groups based on their molecular, pharmacological and signaling properties. mGluRs mediate several physiological functions such as neuronal excitability and synaptic plasticity, but they have also been implicated in numerous pathological conditions including pain. The availability of new and more selective allosteric modulators together with the canonical orthosteric ligands and transgenic technologies has led to significant advances in our knowledge about the role of the specific mGluR subtypes in the pathophysiological mechanisms of various diseases. Although development of successful compounds acting on mGluRs for clinical use has been scarce, the subtype-specific-pharmacological manipulation might be a compelling approach for the treatment of several disorders in humans, including pain; this review aims to summarize and update on preclinical evidence for the roles of different mGluRs in the pain system and discusses knowledge gaps regarding mGluR-related sex differences and neuroimmune signaling in pain.

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Serum response factor mediates nociceptor inflammatory pain plasticity

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Serum response factor upregulates A-Kinase Anchoring Protein 79/150 expression in afferent sensory neurons through metabotropic glutamate receptor signaling.

Introduction:

Chronic metabotropic glutamate receptor activation in nociceptive afferents may upregulate A-Kinase Anchoring Protein 150 (AKAP150) expression and/or function.

Objectives:

To quantify transcriptional changes in AKAP150 expression and/or function after long-term mGluR5 agonist exposure, and identify transcriptional elements responsible.

Methods:

Dorsal root ganglia (DRG) were dissected from Sprague-Dawley rats and cultured for biochemical analysis of AKAP150 expression after prolonged mGluR5 agonist exposure. Serum response factor (SRF) expression was knocked down through siRNA in cultures to demonstrate significance to AKAP150 upregulation. Serum response factor was also knocked down in vivo through intrathecal injections of specifically targeted oligonucleotides to demonstrate significance to hyperalgesic priming behavior in persistent mechanical hypersensitivity.

Results:

Serum response factor and AKAP150 are coexpressed in TRPV1(+) DRG neurons in intact DRG. Prolonged mGluR5 agonist exposure increases SRF-dependent transcription and AKAP150 expression in a manner sensitive to protein kinase C inhibition and SRF knock down. Serum response factor in vivo knock down reduces mechanical hyperalgesic priming.

Conclusion:

Serum response factor transcription plays an important role in transcriptional upregulation of AKAP and hyperalgesic priming behavior, and may contribute to the increased role of AKAP150 in the transition from acute to chronic pain.

Sensitization of TRPV1 and TRPA1 via peripheral mGluR5 signaling contributes to thermal and mechanical hypersensitivity

Peripheral tissue inflammation or injury causes glutamate release from nociceptive axons, keratinocytes, and Schwann cells, resulting in thermal hypersensitivity. However, the detailed molecular mechanisms underlying glutamate-induced thermal hypersensitivity are unknown. The aim of this study was to clarify the involvement of peripheral transient receptor potential (TRP) TRP vanilloid 1 (TRPV1), TRP ankyrin 1 (TRPA1), and protein kinase C epsilon (PKCε) in glutamate-induced pain hypersensitivity. The amount of glutamate in the facial tissue was significantly increased 3 days after facial Complete Freund's adjuvant injection. The head-withdrawal reflex threshold to heat, cold, or mechanical stimulation was significantly decreased on day 7 after continuous glutamate or metabotropic glutamate receptor 5 (mGluR5) agonist (CHPG) injection into the facial skin compared with vehicle-injected rats, and glutamate-induced hypersensitivity was significantly recovered by mGluR5 antagonist MTEP, TRPA1 antagonist HC-030031, TRPV1 antagonist SB366791, or PKCε translocation inhibitor administration into the facial skin. TRPV1 and TRPA1 were expressed in mGluR5-immunoreactive (IR) trigeminal ganglion (TG) neurons innervating the facial skin, and mGluR5-IR TG neurons expressed PKCε. There was no significant difference in the number of GluR5-IR TG neurons among glutamate-injected, saline-injected, and naive rats, whereas that of TRPV1- or TRPA1-IR TG neurons was significantly increased 7 days after continuous glutamate injection into the facial skin compared with vehicle injection. PKCε phosphorylation in TG was significantly enhanced following glutamate injection into the facial skin. Moreover, neuronal activity of TG neurons was significantly increased following facial glutamate treatment. The present findings suggest that sensitization of TRPA1 and/or TRPV1 through mGluR5 signaling via PKCε is involved in facial thermal and mechanical hypersensitivity.

Methylene blue relieves the development of osteoarthritis by upregulating lncRNA MEG3

Methylene blue (MB) is a long-term inhibitor of peripheral nerve axons, thereby alleviating or permanently eliminating pain. However, it remains unknown whether MB is safe and effective method of treating osteoarthritis (OA). MB was injected into the knee joints of rabbits and they were monitored for any histological structural changes. The results revealed no evident changes in the histological structure of the normal knee joint following injection of 1 mg/kg MB at 1, 4, 8 and 24 weeks post-injection. Compared with the vehicle control, MB treatment significantly enhanced the weight distribution and significantly decreased the swelling ratio of the rabbits. Additionally, levels of long non-coding RNA (lncRNA) maternally expressed 3 (MEG3) mRNA were significantly increased following treatment with MB, but the protein expression of P2X purinoceptor 3 (P2X3) was significantly suppressed compared with the vehicle control. The levels of interleukin (IL) 6, tumor necrosis factor (TNF)α, IL-1β and IL-8 were significantly suppressed following MB treatment, indicating that MB protects against OA progression. It was also revealed that MEG3 overexpression significantly suppresses levels of P2X3 protein. ELISA indicated that the MEG3-induced reduction of IL-6, TNFα, IL-1β and IL-8 expression was significantly reversed following P2X3 overexpression. Therefore, the results of the present study demonstrated that MB is an effective method of treating OA-associated pain by upregulating lncRNA MEG3 levels. Additionally, lncRNA MEG3 relieves the OA-associated pain and inflammation in a rabbit model of OA by inhibiting P2X3 expression.

Potentiation of acid-sensing ion channel activity by peripheral group I metabotropic glutamate receptor signaling

Glutamate activates peripheral group I metabotropic glutamate receptors (mGluRs) and contributes to inflammatory pain. However, it is still not clear the mechanisms are involved in group I mGluR-mediated peripheral sensitization. Herein, we report that group I mGluRs signaling sensitizes acid-sensing ion channels (ASICs) in dorsal root ganglion (DRG) neurons and contributes to acidosis-evoked pain. DHPG, a selective group I mGluR agonist, can potentiate the functional activity of ASICs, which mediated the proton-induced events. DHPG concentration-dependently increased proton-gated currents in DRG neurons. It shifted the proton concentration-response curve upwards, with a 47.3±7.0% increase of the maximal current response to proton. Group I mGluRs, especially mGluR5, mediated the potentiation of DHPG via an intracellular cascade. DHPG potentiation of proton-gated currents disappeared after inhibition of intracellular Gq/11 proteins, PLCβ, PKC or PICK1 signaling. Moreover, DHPG enhanced proton-evoked membrane excitability of rat DRG neurons and increased the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, peripherally administration of DHPG dose-dependently exacerbated nociceptive responses to intraplantar injection of acetic acid in rats. Potentiation of ASIC activity by group I mGluR signaling in rat DRG neurons revealed a novel peripheral mechanism underlying group I mGluRs involvement in hyperalgesia.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1-2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1-2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2 μg/day, LE) or high-dose estradiol (20 μg/day, HE) for 2 days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1-2 surface at the site of recording 10 min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500 μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50 μM CPCCOEt, while 500 μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1-2 region and are largely independent of estrogen status.

Peripheral group I metabotropic glutamate receptor activation leads to muscle mechanical hyperalgesia through TRPV1 phosphorylation in the rat

Elevated glutamate levels within injured muscle play important roles in muscle pain and hyperalgesia. In this study, we hypothesized that protein kinase C (PKC)-dependent TRPV1 phosphorylation contributes to the muscle mechanical hyperalgesia following activation of Group I metabotropic glutamate receptors (mGlu1/5). Mechanical hyperalgesia induced by (R,S)-3,5-dihydroxyphenylglycine (DHPG), an mGlu1/5 agonist, in the masseter muscle was attenuated by AMG9810, a specific TRPV1 antagonist. AMG9810 also suppressed mechanical hyperalgesia evoked by pharmacologic activation of PKC. DHPG-induced mechanical hyperalgesia was suppressed by pretreatment with a decoy peptide that disrupted interactions between TRPV1 and A-kinase-anchoring protein (AKAP), which facilitates phosphorylation of TRPV1. In dissociated trigeminal ganglia, DHPG upregulated serine phosphorylation of TRPV1 (S800), during which DHPG-induced mechanical hyperalgesia was prominent. The TRPV1 phosphorylation at S800 was suppressed by a PKC inhibitor. Electrophysiologic measurements in trigeminal ganglion neurons demonstrated that TRPV1 sensitivity was enhanced by pretreatment with DHPG, and this was prevented by a PKC inhibitor, but not by a protein kinase A inhibitor. These results suggest that mGlu1/5 activation in masseter afferents invokes phosphorylation of TRPV1 serine residues including S800, and that phosphorylation-induced sensitization of TRPV1 is involved in masseter mechanical hyperalgesia. These data support a role of TRPV1 as an integrator of glutamate receptor signaling in muscle nociceptors.

PERSPECTIVE

This article demonstrates that activation of mGlu1/5 leads to phosphorylation of a specific TRPV1 residue via PKC and AKAP150 in trigeminal sensory neurons and that functional interactions between glutamate receptors and TRPV1 mediate mechanical hyperalgesia in the muscle tissue.

Pharmacological Profiles of Oligomerized μ-Opioid Receptors

Opioids are widely prescribed pain relievers with multiple side effects and potential complications. They produce analgesia via G-protein-protein coupled receptors: μ-, δ-, κ-opioid and opioid receptor-like 1 receptors. Bivalent ligands targeted to the oligomerized opioid receptors might be the key to developing analgesics without undesired side effects and obtaining effective treatment for opioid addicts. In this review we will update the biological effects of μ-opioids on homo- or hetero-oligomerized μ-opioid receptor and discuss potential mechanisms through which bivalent ligands exert beneficial effects, including adenylate cyclase regulation and receptor-mediated signaling pathways.

Peripheral metabotropic glutamate receptor subtype 5 contributes to inflammation-induced hypersensitivity of the rat temporomandibular joint

Temporomandibular disorders (TMD) comprise an assortment of clinical conditions characterized by pain in the temporomandibular joint (TMJ). TMD patients have a variety of symptoms, including jaw movement disorder and TMJ pain. Metabotropic glutamate receptor subtype 5 (mGluR5) was reported to be involved in pain processing in several animal models of neuropathic and inflammatory pain. In this study, the head withdrawal threshold and mGluR5 expression were investigated in rats with complete Freund's adjuvant (CFA)-induced TMJ inflammatory pain. CFA injection into the TMJ significantly decreased the mechanical head withdrawal thresholds relative to vehicle injection, and the effects were blocked by pre-injection of 2-methyl-6-(phenylethynyl)-pyridine (MPEP). mGluR5 expression in the trigeminal ganglion was predominantly increased in the CFA-injected group compared with the normal control group. Pretreatment with MPEP, a selective mGluR5 antagonist, reduced mGluR5 expression in the trigeminal ganglion compared with the CFA group, as measured by immunohistochemistry, western blotting, and RT-PCR. Significant differences in the proportion or intensity of mGluR5 expression were found in animals with inflammation versus control animals at the examined time point. These findings indicate a role for peripheral mGluR5 in CFA-induced nociceptive behavior and TMJ inflammation. Peripheral application of mGluR5 antagonists could provide therapeutic benefits for inflammatory TMJ pain.

Ligands that interact with putative MOR-mGluR5 heteromer in mice with inflammatory pain produce potent antinociception

The low effectiveness of morphine and related mu opioid analgesics for the treatment of chronic inflammatory pain is a result of opioid-induced release of proinflammatory cytokines and glutamate that lower the pain threshold. In this regard, the use of opioids with metabotropic glutamate-5 receptor (mGluR5) antagonist has been reported to increase the efficacy of morphine and prevent the establishment of adverse effects during chronic use. Given the presence of opioid receptors (MORs) and mGluR5 in glia and neurons, together with reports that suggest coexpressed MOR/mGluR5 receptors in cultured cells associate as a heteromer, the possibility that such a heteromer could be a target in vivo was addressed by the design and synthesis of a series of bivalent ligands that contain mu opioid agonist and mGluR5 antagonist pharmacophores linked through spacers of varying length (10-24 atoms). The series was evaluated for antinociception using the tail-flick and von Frey assays in mice pretreated with lipopolysaccharide (LPS) or in mice with bone cancer. In LPS-pretreated mice, MMG22 (4c, 22-atom spacer) was the most potent member of the series (intrathecal ED50 ∼9 fmol per mouse), whereas in untreated mice its ED50 was more than three orders of magnitude higher. As members of the series with shorter or longer spacers have ≥500-fold higher ED50s in LPS-treated mice, the exceptional potency of MMG22 may be a result of the optimal bridging of protomers in a putative MOR-mGluR5 heteromer. The finding that MMG22 possesses a >10(6) therapeutic ratio suggests that it may be an excellent candidate for treatment of chronic, intractable pain via spinal administration.

Metabotropic glutamate receptor 5 contributes to inflammatory tongue pain via extracellular signal-regulated kinase signaling in the trigeminal spinal subnucleus caudalis and upper cervical spinal cord

Background

In the orofacial region, limited information is available concerning pathological tongue pain, such as inflammatory pain or neuropathic pain occurring in the tongue. Here, we tried for the first time to establish a novel animal model of inflammatory tongue pain in rats and to investigate the roles of metabotropic glutamate receptor 5 (mGluR5)-extracellular signal-regulated kinase (ERK) signaling in this process.

Methods

Complete Freund’s adjuvant (CFA) was submucosally injected into the tongue to induce the inflammatory pain phenotype that was confirmed by behavioral testing. Expression of phosphorylated ERK (pERK) and mGluR5 in the trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) were detected with immunohistochemical staining and Western blotting. pERK inhibitor, a selective mGluR5 antagonist or agonist was continuously administered for 7 days via an intrathecal (i.t.) route. Local inflammatory responses were verified by tongue histology.

Results

Submucosal injection of CFA into the tongue produced a long-lasting mechanical allodynia and heat hyperalgesia at the inflamed site, concomitant with an increase in the pERK immunoreactivity in the Vc and C1-C2. The distribution of pERK-IR cells was laminar specific, ipsilaterally dominant, somatotopically relevant, and rostrocaudally restricted. Western blot analysis also showed an enhanced activation of ERK in the Vc and C1-C2 following CFA injection. Continuous i.t. administration of the pERK inhibitor and a selective mGluR5 antagonist significantly depressed the mechanical allodynia and heat hyperalgesia in the CFA-injected tongue. In addition, the number of pERK-IR cells in ipsilateral Vc and C1-C2 was also decreased by both drugs. Moreover, continuous i.t. administration of a selective mGluR5 agonist induced mechanical allodynia in naive rats.

Conclusions

The present study constructed a new animal model of inflammatory tongue pain in rodents, and demonstrated pivotal roles of the mGluR5-pERK signaling in the development of mechanical and heat hypersensitivity that evolved in the inflamed tongue. This tongue-inflamed model might be useful for future studies to further elucidate molecular and cellular mechanisms of pathological tongue pain such as burning mouth syndrome.

Functional interactions between NMDA receptors and TRPV1 in trigeminal sensory neurons mediate mechanical hyperalgesia in the rat masseter muscle

The NMDA and TRPV1 receptors that are expressed in sensory neurons have been independently demonstrated to play important roles in peripheral pain mechanisms. In the present study, we investigated whether the 2 receptor-channel systems form a functional complex that provides the basis for the development of mechanical hyperalgesia. In the masseter muscle, direct application of NMDA induced a time-dependent increase in mechanical sensitivity, which was significantly blocked when the muscle was pretreated with a specific TRPV1 antagonist, AMG9810. The NR1 subunit of the NMDA receptor and TRPV1 were coexpressed in 32% of masseter afferents in trigeminal ganglia (TG). Furthermore, NR1 and NR2B formed protein-protein complexes with TRPV1 in TG as demonstrated by coimmunoprecipitation experiments. Calcium imaging analyses further corroborated that NMDA and TRPV1 receptors functionally interact. In TG culture, application of NMDA resulted in phosphorylation of serine, but not threonine or tyrosine, residues of TRPV1 in a time course similar to that of the development of NMDA-induced mechanical hyperalgesia. The NMDA-induced phosphorylation was significantly attenuated by CaMKII and PKC inhibitors, but not by a PKA inhibitor. Consistent with the biochemical data, the NMDA-induced mechanical hyperalgesia was also effectively blocked when the muscle was pretreated with a CaMKII or PKC inhibitor. Thus, NMDA receptors and TRPV1 functionally interact via CaMKII and PKC signaling cascades and contribute to mechanical hyperalgesia. These data offer novel mechanisms by which 2 ligand-gated channels in sensory neurons interact and reinforce the notion that TRPV1 functions as a signal integrator under pathological conditions.

Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

The cervical facet joint and its capsule are a common source of neck pain from whiplash. Mechanical hyperalgesia elicited by painful facet joint distraction is associated with spinal neuronal hyperexcitability that can be induced by transmitter/receptor systems that potentiate the synaptic activation of neurons. This study investigated the temporal response of a glutamate receptor and transporters in the dorsal root ganglia (DRG) and spinal cord. Bilateral C6/C7 facet joint distractions were imposed in the rat either to produce behavioral sensitivity or without inducing any sensitivity. Neuronal metabotropic glutamate receptor-5 (mGluR5) and protein kinase C-epsilon (PKCε) expression in the DRG and spinal cord were evaluated on days 1 and 7. Spinal expression of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1), was also quantified at both time points. Painful distraction produced immediate behavioral hypersensitivity that was sustained for 7 days. Increased expression of mGluR5 and PKCε in the DRG was not evident until day 7 and only following painful distraction; this increase was observed in small-diameter neurons. Only painful facet joint distraction produced a significant increase (p<0.001) in neuronal mGluR5 over time, and this increase also was significantly elevated (p≤0.05) over responses in the other groups at day 7. However, there were no differences in spinal PKCε expression on either day or between groups. Spinal EAAC1 expression was significantly increased (p<0.03) only in the nonpainful groups on day 7. Results from this study suggest that spinal glutamatergic plasticity is selectively modulated in association with facet-mediated pain.

Persistent orofacial muscle pain

The pathophysiology of persistent orofacial myalgia has been the centre of much controversy. In this article we suggest a novel descriptive term; 'persistent orofacial muscle pain' (POMP) and review current evidence that supports the hypothesis that the induction of POMP involves the interplay between a peripheral nociceptive source in muscle, a faulty central nervous system component and decreased coping ability. In this context it is widely accepted that a complex interaction of variable intrinsic and extrinsic factors act to induce POMP and dysfunction.

Activation of peripheral delta-opioid receptors leads to anti-hyperalgesic responses in the masseter muscle of male and female rats

In this project, we examined peripheral δ-opioid receptor (DOR)-mediated anti-hyperalgesic responses in the context of an acute orofacial muscle pain condition in both male and female rats. We also investigated whether the ATP-sensitive K+ channel (KATP), a downstream target of OR signaling, contributes to DOR-mediated anti-hyperalgesic responses. Local pretreatment of the masseter with a DOR agonist, [D-Pen², D-Pen⁶]-enkephalin (DPDPE), dose-dependently attenuated capsaicin-induced mechanical hypersensitivity in both male and female rats. However, there were sex differences in the potency of local DPDPE in that a 10-fold higher dose of DPDPE was required in female rats to produce the level of anti-hyperalgesia achieved in male rats. The sex differences in the DPDPE effect may not be fully explained by DOR expression level since there was no significant sex difference in DOR mRNA levels in trigeminal ganglia (TG). Finally, pretreatment of the masseter with the KATP antagonist, glibenclamide, significantly blocked the effects of DPDPE in male rats suggesting that the peripheral DOR effect is mediated by the KATP. These studies revealed novel information about sex differences with regards to peripherally localized DOR-mediated anti-hyperalgesia under an orofacial muscle pain condition.

Early continuous inhibition of group 1 mGlu signaling partially rescues dendritic spine abnormalities in the Fmr1 knockout mouse model for fragile X syndrome

RATIONALE

Abnormal dendritic spine morphology is a significant neuroanatomical defect in fragile X mental retardation. It has been suggested that overactive group 1 metabotropic glutamate receptor (mGlu) signaling is associated with the spine dysmorphology occurring in fragile X syndrome (FXS). Thus, group 1 mGlu became a new therapeutic target for the treatment of FXS.

OBJECTIVE

The purpose of this study was to identify the effect of inhibition of mGlu signaling in FXS.

METHODS

We observed the changes in dendritic spines after pharmacological modulation of mGlu signaling in an Fmr1 knockout (KO) mouse model.

RESULTS

The activation of group 1 mGlu resulted in elongation of dendritic spines in the cultured neurons derived from Fmr1 KO mice and wild-type (WT) mice. Antagonism of group 1 mGlu reduced the average spine length of Fmr1 KO neurons. Furthermore, systemic administration of the selective group 1 mGlu5 antagonist 2-methyl-6-phenylethynyl pyridine (MPEP) reduced the average spine length and density in the cortical neurons of Fmr1 KO mice at developmental age. For the adult mice, MPEP administration was less effective for the restoration of spine length. The percentage of immature spines showed a similar reduction in parallel to the changes of spine length. Temporary MPEP intervention with single-dose treatment did not show any effect.

CONCLUSION

These results show that MPEP administration could partially rescue the morphological deficits of dendritic spines in Fmr1 KO mice at developmental age.

Metabotropic glutamate receptor-5 and protein kinase C-epsilon increase in dorsal root ganglion neurons and spinal glial activation in an adolescent rat model of painful neck injury

There is growing evidence that neck pain is common in adolescence and is a risk factor for the development of chronic neck pain in adulthood. The cervical facet joint and its capsular ligament is a common source of pain in the neck in adults, but its role in adolescent pain remains unknown. The aim of this study was to define the biomechanics, behavioral sensitivity, and indicators of neuronal and glial activation in an adolescent model of mechanical facet joint injury. A bilateral C6-C7 facet joint distraction was imposed in an adolescent rat and biomechanical metrics were measured during injury. Following injury, forepaw mechanical hyperalgesia was measured, and protein kinase C-epsilon (PKCɛ) and metabotropic glutamate receptor-5 (mGluR5) expression in the dorsal root ganglion and markers of spinal glial activation were assessed. Joint distraction induced significant mechanical hyperalgesia during the 7 days post-injury (p < 0.001). Painful injury significantly increased PKCɛ expression in small- and medium-diameter neurons compared to sham (p < 0.05) and naïve tissue (p < 0.001). Similarly, mGluR5 expression was significantly elevated in small-diameter neurons after injury (p < 0.05). Spinal astrocytic activation after injury was also elevated over sham (p < 0.035) and naïve (p < 0.0001) levels; microglial activation was only greater than naïve levels (p < 0.006). Mean strains in the facet capsule during injury were 32.8 ± 12.9%, which were consistent with the strains associated with comparable degrees of hypersensitivity in the adult rat. These results suggest that adolescents may have a lower tissue tolerance to induce pain and associated nociceptive response than do adults.

Sex differences in the contribution of ATP-sensitive K+ channels in trigeminal ganglia under an acute muscle pain condition

In this study, we examined whether functional subunits of the ATP-dependent K+ channel (KATP) are expressed in trigeminal ganglia (TG), which contains sensory neurons that innervate oral and facial structures. We also investigated whether direct activation of the KATP effectively attenuates mechanical hypersensitivity in the context of an acute orofacial muscle pain condition. The KATP expression in TG and behavioral studies were conducted in age matched male and female Sprague-Dawley rats. RT-PCR experiments showed that the mRNAs for the inwardly rectifying pore-forming subunits, Kir6.1 and Kir6.2, as well as the regulatory sulfonylurea subunits, SUR1 and SUR2, were reliably detected in TG. Subsequent western blot analysis confirmed that proteins for all four subunits are expressed in TG, and showed that Kir6.2 is expressed at a significantly higher level in male TG compared to that of female rats. This observation was confirmed by the immunohistochemical demonstration of higher percentages of Kir6 positive masseter afferents in female rats. Masseteric injection of capsaicin evokes a time dependent increase in masseter sensitivity to noxious mechanical stimulation. A specific KATP agonist, pinacidil, dose-dependently attenuated the capsaicin-induced mechanical hypersensitivity in male rats. The dose of pinacidil (20 μg) that completely blocked the capsaicin responses in male rats was ineffective in female rats regardless of their estrus phases. Only at the highest dose (300 μg) we used, pinacidil was partially effective in female rats. Similarly, another KATP agonist, diazoxide which targets different KATP subunits also showed sex specific responses in attenuating capsaicin-induced masseter hypersensitivity. These data suggested that sex differences in functional KATP expression in TG may underlie sex specific responses to KATP agonists. The present study provided novel information on sex differences in KATP expression in TG and its contribution under an orofacial muscle pain condition.

PKC-1 acts with the ERK MAPK signaling pathway to regulate Caenorhabditis elegans mechanosensory response

In most animals, multiple genes encode protein kinase C (PKC) proteins. Pharmacological studies have revealed numerous roles for this protein family, yet the in vivo roles of specific PKC proteins and the functional targets of PKC activation are poorly understood. We find that in Caenorhabditis elegans, two PKC genes, pkc-1 and tpa-1, are required for mechanosensory response; the role of the nPKCε/η ortholog, pkc-1, was examined in detail. pkc-1 function is required for response to nose touch in adult C. elegans and pkc-1 likely acts in the interneurons that regulate locomotion which are direct synaptic targets of mechanosensory neurons. Previous studies have suggested numerous possible targets of pkc-1; our analysis indicates that pkc-1 may act via the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway. We find that ERK/MAPK pathway function is required for mechanosensory response in C. elegans and that at least one component of this pathway, lin-45 Raf, acts in interneurons of the mechanosensory circuit. Genetic analysis indicates that lin-45 and pkc-1 act together to regulate nose touch response. Thus, these results functionally link two conserved signaling pathways in adult C. elegans neurons and define distinct roles for PKC genes in vivo.

TNFα Mechanically Sensitizes Masseter Muscle Nociceptors by Increasing Prostaglandin E2 Levels

TNFα induces mechanical sensitization of rat masseter muscle nociceptors, which takes 2–3 h to manifest and is mediated through activation of P55 and P75 receptors. This study was undertaken to determine whether TNFα induces nociceptor mechanical sensitization through the release of other algogenic substances such as glutamate, prostaglandin E2 (PGE2), and/or nerve growth factor (NGF), which have been shown to induce mechanical sensitization of muscle nociceptors. Masseter muscle homogenate levels of PGE2 and NGF were measured 3 h after injection of TNFα (1 μg) or vehicle control using commercially available kits. Interstitial glutamate concentration was measured after injection of TNFα or vehicle control using a glutamate-selective biosensor probe. Diclofenac, a cycloxygenase inhibitor that blocks the synthesis of PGE2, d-2-amino-5-phophonovaleric acid (APV), a competitive N-methyl-d-aspartate (NMDA) receptor antagonist, and a tyrosine kinase A (TrkA) receptor antibody, which blocks NGF-induced masseter muscle nociceptor sensitization, were used to assess the contribution of PGE2, glutamate, and NGF to TNFα-induced nociceptor sensitization. PGE2 and glutamate concentrations were significantly elevated 3 h after TNFα injection into the masseter muscle. Injection of diclofenac partially reversed the TNFα-induced decreases in the mechanical threshold (MT) of masseter muscle nociceptors, whereas vehicle control, APV, and TrkA antibody did not significantly alter nociceptor MT. These results suggest that TNFα-induced mechanical sensitization of masseter muscle nociceptors is mediated in part by increased PGE2 levels. The findings of this study support the hypothesis that TNFα induces a delayed mechanical sensitization of masseter muscle nociceptors indirectly by the release of PGE2.

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

The mechanism by which intramuscular injection of BoNTA into the craniofacial muscles decreases migraine headaches is not known. In a blinded study, the effect of BoNTA on the mechanical and chemical responsiveness of individual temporalis muscle nociceptors and muscle neurogenic vasodilation was investigated in female rats. Mechanical threshold was measured for 3h following intramuscular injection of BoNTA or vehicle, and for 10 min after a subsequent injection of the algogen glutamate. Injection of BoNTA significantly increased the mechanical threshold of muscle nociceptors without altering the muscle surface temperature and blocked glutamate-induced mechanical sensitization and neurogenic vasodilation. None of these effects were reproduced by pancuronium-induced muscle paralysis. Western blot analysis of temporalis muscles indicated that BoNTA significantly decreased SNAP-25. Measurement of interstitial glutamate concentration with a glutamate biosensor indicated that BoNTA significantly reduced glutamate concentrations. The mechanical sensitivity of muscle nociceptors is modulated by glutamate concentration through activation of peripheral NMDA receptors. Immunohistochemical experiments were conducted and they indicated that half of the NMDA-expressing temporalis nerve fibers co-expressed substance P or CGRP. Additional electrophysiology experiments examined the effect of antagonists for NMDA, CGRP and NK1 receptors on glutamate-induced effects. Glutamate-induced mechanical sensitization was only blocked by the NMDA receptor antagonist, but muscle neurogenic vasodilation was attenuated by NMDA or CGRP receptor antagonists. These data suggest that injection of BoNTA into craniofacial muscles acts to decrease migraine headaches by rapidly decreasing the mechanical sensitivity of temporalis muscle nociceptors through inhibition of glutamate release and by attenuating the provoked release of CGRP from muscle nociceptors.

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

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

Allosteric modulation of metabotropic glutamate receptor 5 affects phosphorylation, internalization, and desensitization of the micro-opioid receptor

Recent evidence suggests that opioid analgesia and tolerance can be modulated by metabotropic glutamate receptors. Therefore, we studied the functional coupling and desensitization of the micro-opioid receptor (MOR) in human embryonic kidney (HEK) 293 cells which co-express metabotropic glutamate receptor 5 (mGluR5). As demonstrated by the D-Ala2,N-MePhe4,Gl-ol5-enkephalin (DAMGO)-induced inhibition of intracellular cAMP level and by binding studies, the co-expression of mGluR5 had no substantial effect on the agonist binding sites and functional coupling of the MOR. However, in MOR/ mGluR5 co-expressing cells, the non-competitive mGluR5 antagonist MPEP (2-methyl-6-(phenyl-ethynyl)-pyridine) decreases the DAMGO-induced MOR phosphorylation, internalization, and desensitization, whereas non-selective competitive mGluR antagonists or agonists had no effects. These findings indicate that an allosteric modulation of mGluR5 can affect the agonist-induced MOR signalling and regulation. As a mechanistic basis for the observed effects we suggested an interaction/heterodimerization of MOR and mGluR5, which is supported by the DAMGO-induced co-internalization of MOR and mGluR5 and by the increase of MPEP binding sites (Bmax) and a change of the binding affinity (K(D)) of mGluR5 receptors after the co-expression of MOR. In addition, co-immunoprecipitation experiments revealed evidence for an interaction between MOR and mGluR5 which is facilitated by MPEP treatment.

Signal transduction mechanisms underlying group I mGluR-mediated increase in frequency and amplitude of spontaneous EPSCs in the spinal trigeminal subnucleus oralis of the rat

Group I mGluRs (mGluR1 and 5) pre- and/or postsynaptically regulate synaptic transmission at glutamatergic synapses. By recording spontaneous EPSCs (sEPSCs) in the spinal trigeminal subnucleus oralis (Vo), we here investigated the regulation of glutamatergic transmission through the activation of group I mGluRs. Bath-applied DHPG (10 μM/5 min), activating the group I mGluRs, increased sEPSCs both in frequency and amplitude; particularly, the increased amplitude was long-lasting. The DHPG-induced increases of sEPSC frequency and amplitude were not NMDA receptor-dependent. The DHPG-induced increase in the frequency of sEPSCs, the presynaptic effect being further confirmed by the DHPG effect on paired-pulse ratio of trigeminal tract-evoked EPSCs, an index of presynaptic modulation, was significantly but partially reduced by blockades of voltage-dependent sodium channel, mGluR1 or mGluR5. Interestingly, PKC inhibition markedly enhanced the DHPG-induced increase of sEPSC frequency, which was mainly accomplished through mGluR1, indicating an inhibitory role of PKC. In contrast, the DHPG-induced increase of sEPSC amplitude was not affected by mGluR1 or mGluR5 antagonists although the long-lasting property of the increase was disappeared; however, the increase was completely inhibited by blocking both mGluR1 and mGluR5. Further study of signal transduction mechanisms revealed that PLC and CaMKII mediated the increases of sEPSC in both frequency and amplitude by DHPG, while IP₃ receptor, NO and ERK only that of amplitude during DHPG application. Altogether, these results indicate that the activation of group I mGluRs and their signal transduction pathways differentially regulate glutamate release and synaptic responses in Vo, thereby contributing to the processing of somatosensory signals from orofacial region.

Activation of TRPV1 and TRPA1 leads to muscle nociception and mechanical hyperalgesia

The involvement of TRPV1 and TRPA1 in mediating craniofacial muscle nociception and mechanical hyperalgesia was investigated in male Sprague-Dawley rats. First, we confirmed the expression of TRPV1 in masseter afferents in rat trigeminal ganglia (TG), and provided new data that TRPA1 is also expressed in primary afferents innervating masticatory muscles in double-labeling immunohistochemistry experiments. We then examined whether the activation of each TRP channel in the masseter muscle evokes acute nocifensive responses and leads to the development of masseter hypersensitivity to mechanical stimulation using the behavioral models that have been specifically designed and validated for the craniofacial system. Intramuscular injections with specific agonists for TRPV1 and TRPA1, capsaicin and mustard oil (MO), respectively, produced immediate nocifensive hindpaw responses followed by prolonged mechanical hyperalgesia in a concentration-dependent manner. Pretreatment of the muscle with a TRPV1 antagonist, capsazepine, effectively attenuated the capsaicin-induced muscle nociception and mechanical hyperalgesia. Similarly, pretreatment of the muscle with a selective TRPA1 antagonist, AP18, significantly blocked the MO-induced muscle nociception and mechanical hyperalgesia. We confirmed these data with another set of selective antagonist for TRPV1 and TRPA1, AMG9810 and HC030031, respectively. Collectively, these results provide compelling evidence that TRPV1 and TRPA1 can functionally contribute to muscle nociception and hyperalgesia, and suggest that TRP channels expressed in muscle afferents can engage in the development of pathologic muscle pain conditions.

Metabotropic glutamate receptor 5 activation inhibits microglial associated inflammation and neurotoxicity

The Group I metabotropic glutamate receptor 5 (mGluR5) can modulate addiction, pain, and neuronal cell death. Expression of some mGluRs, such as Group II and III mGluRs, has been reported in microglia and may affect their activation. However, the expression and role of mGluR5 in microglia is unclear. Using immunocytochemistry and Western blot, we demonstrate that mGluR5 protein is expressed in primary microglial cultures. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly reduces microglial activation in response to lipopolysaccharide, as indicated by a reduction in nitric oxide, reactive oxygen species, and TNFalpha production. Microglial induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are not observed in microglial cultures from mGluR5 knockout mice and are blocked by selective mGluR5 antagonists, suggesting that these actions are mediated by the mGluR5 receptor. Anti-inflammatory actions of mGluR5 activation are attenuated by phospholipase C and protein kinase C inhibitors, as well as by calcium chelators, suggesting that the mGluR5 activation in microglia involves the G(alphaq)-protein signal transduction pathway. These data indicate that microglial mGluR5 may represent a novel target for modulating neuroinflammation, an important component of both acute and chronic neurodegenerative disorders.

Algesic agents exciting muscle nociceptors

Morphologically, muscle nociceptors are free nerve endings connected to the CNS by thin myelinated (group III) or unmyelinated (group IV) afferent fibers. Not all of these endings are nociceptive; approximately 40% have a low mechanical threshold and likely fulfill non-nociceptive functions. Two chemical stimuli are particularly relevant as causes of muscle pain. The first is a drop in tissue pH, i.e. an increase in proton (H+) concentration. A large number of painful patho(physio)logical alterations of muscle tissue are associated with an acidic interstitial pH (e.g. tonic contractions, spasm, inflammation). The second important cause of muscle pain is a release of adenosine triphosphate (ATP). ATP is present in all body cells, but in muscle its concentration is particularly high. Any damage of muscle cells (trauma, necrotic myositis) is accompanied by a release of ATP from the cells. Therefore, ATP is considered a general pain stimulus by some. ATP and protons are relatively specific stimuli for muscle pain; in cutaneous pain they play a less important role. The numerous agents that are released in pathologically altered muscle include substances that desensitize mechanosensitive group IV receptors. Capsaicin has a long-lasting desensitizing action, brain-derived neurotrophic factor, and tumor necrosis factor-alpha, a short-lasting one. Most of the agents exciting group IV units (e.g. low pH, ATP, capsaicin) activate not only nociceptive endings but also non-nociceptive ones. The only substance encountered that excites exclusively nociceptive group IV receptors is nerve growth factor (NGF). In rat muscle chronically inflamed with complete Freund's adjuvant, most group IV endings are sensitized to mechanical (and to some) chemical stimuli. However, stimulants such as ATP, NGF, and solutions of low pH were found to be less effective in inflamed muscle. A possible explanation for this surprising finding is that in inflamed muscle the concentrations of ATP and NGF and H+ are increased. Therefore, experimental administration of these agents is a less effective stimulus.

Peripheral AMPA receptors contribute to muscle nociception and c-fos activation

In this study, involvement of peripheral AMPA receptors in mediating craniofacial muscle pain was investigated. AMPA receptor subunits, GluR1 and GluR2, were predominantly expressed in small to medium size neurons but more GluR2 positive labeling were encountered in trigeminal ganglia (TG) of male Sprague Dawley rats. A greater prevalence of GluR2 is reflected by the significantly higher percentage of GluR2 than GluR1 positive masseter afferents. Nocifensive behavior and c-fos immunoreactivity were assessed from the same animals that received intramuscular mustard oil (MO) with or without NBQX, a potent AMPA/KA receptor antagonist. Masseteric MO produced nocifensive hindpaw shaking responses that peaked in the first 30s and gradually diminished over a few minutes. There was a significant difference in both peak and overall MO-induced nocifensive responses between NBQX and vehicle pre-treated rats. Subsequent Fos studies also showed that peripheral NBQX pre-treatment effectively reduced the MO-induced neuronal activation in the subnucleus caudalis of the trigeminal nerve (Vc). These combined results provide compelling evidence that acute muscle nociception is mediated, in part, by peripherally located AMPA/KA receptors, and that blockade of multiple peripheral glutamate receptor subtypes may provide a more effective means of reducing muscular pain and central neuronal activation.

Interactions between glutamate and capsaicin in inducing muscle pain and sensitization in humans

The aim of the study was to investigate the interaction between glutamate and capsaicin in inducing muscle pain and sensitization in humans. Fifteen male volunteers participated. Glutamate or capsaicin or isotonic saline, in a paired-sequence order, was injected randomly into the right or left masseter muscle. Two injections were given in a double-blinded design 25 min apart in 1 session/week over 4 weeks: saline (A1) followed by glutamate (A2), capsaicin (B1) followed by glutamate (B2), saline (C1) followed by capsaicin (C2), and glutamate (D1) followed by capsaicin (D2). The subjects drew the area of perceived pain and scored pain intensity on a 0-10 visual analogue scale (VAS). Pressure pain threshold (PPT) at the injection site, at a site 2-cm away, and on the contralateral side, as well as pressure pain tolerance (PPTol) at the injection site and contralateral site, were also measured before and after injection and subsequently at 5-min intervals. Paired t-test analyses showed that the pain drawing area was significantly smaller in the B2 compared to the A2 condition (P=0.028), and significantly larger in the D2 compared to the C2 condition (P=0.027). It also revealed significantly lower VAS peak pain intensity (P=0.008) and smaller VAS area under the curve (P=0.003) for the B2 compared to the A2 condition, and significantly higher VAS peak pain (P=0.015) and larger VAS area under the curve (P=0.037) for the D2 compared to the C2 condition. There was a significant PPT and PPTol decrease at the injection site after glutamate or capsaicin injection (ANOVA: P<0.028). The percentage decrease in PPT or PPTol (at the injection site) was not significantly different for the B2 compared to the A2 condition (Paired t-test: P>0.682) or for the D2 compared to the C2 condition (P>0.133). Significant PPT changes were also observed at the site 2 cm away, but not on the contralateral side. In conclusion, these findings indicate that intramuscular administrations of glutamate and capsaicin interact and influence pain and sensitization of muscle nociceptors: glutamate causes a sensitization to subsequent administration of capsaicin, whereas capsaicin is associated with a desensitization to subsequent injection of glutamate. These findings support previous animal data.

Role of soluble guanylate cyclase in the trigeminal subnucleus caudalis in capsaicin-induced muscle hypersensitivity

Nitric oxide (NO) produces its effects by activating soluble guanylate cyclase (sGC). In the present study, we investigated the potential role of sGC in the subnucleus caudalis (Vc) in mediating masseter hypersensitivity under acute inflammatory condition in male Sprague-Dawley rats. First, our Western blot analysis revealed that sGC protein is reliably detected in the Vc. Subsequent immunohistochemical studies demonstrated that neuronal cell bodies in the superficial laminae of the Vc positively stained for sGC. Astrocytes in deeper lamina of the Vc also showed sGC immunoreactivity. We then tested whether intrathecal administration of sGC inhibitors, methylene blue (MB), and ODQ, in the Vc, attenuates masseter hypersensitivity induced by intramuscular injection of capsaicin. Intrathecal MB or ODQ significantly blocked the capsaicin-induced reduction of mechanical threshold to noxious stimulation of the masseter. These data indicate that the NO-sGC pathway in the Vc is involved in mediating orofacial muscle hypersensitivity under acute inflammatory conditions.

Differential regulation of glutamate receptors in trigeminal ganglia following masseter inflammation

The present study examined whether N-methyl-D-aspartate receptor (NMDAR), 5-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits and group I metabotropic glutamate receptors (mGluRs) are constitutively expressed in trigeminal ganglia (TG) using Western blot analysis in male Sprague-Dawley rats. We then investigated whether experimental induction of masseter inflammation influences glutamate receptor expressions by comparing the protein levels from naïve rats to those from complete Freund's adjuvant (CFA) inflamed rats. Our results showed that NMDAR1 (NR1), NMDAR2A (NR2A), NMDAR2B (NR2B), AMPAR1 (GluR1) and AMPAR2 (GluR2) subunits, and group I metabotropic glutamate receptor, mGluR5, are constitutively expressed in TG. Masseter inflammation significantly down-regulated NR1 subunit expression that persisted to 7 days post-CFA inflammation. NR2A and NR2B expressions were not significantly changed. GluR1 receptor subunit expression was slightly increased in TG 3 days after CFA-induced inflammation, but the change was not statistically significant. GluR2 protein level was not affected by CFA inflammation. The level of mGluR5 protein was significantly up-regulated in TG 3 days after CFA-induced masseter inflammation. There were no inflammation-induced changes in any of the proteins we analyzed in the contralateral, non-inflamed TG. These results suggested that muscle inflammation differentially modulates glutamate receptor subunits at the primary afferent level in male rats and that these inflammation-induced transcriptional changes may contribute to functionally different aspects of craniofacial muscle pain.