The role of peripheral N-methyl-D-aspartate receptors in muscle hyperalgesia

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.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

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.

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.

Early and late contributions of glutamate and CGRP to mechanical sensitization by endothelin-1

Intraplantar injection of endothelin-1 (ET-1) (1.5-10 muM) in the rat produces mechanical allodynia. Here we identify the receptor subtypes for ET-1, glutamate and CGRP critical to such allodynia. Antagonism of ET(A) or ET(B) receptors alone, by BQ123 or BQ788, respectively, only partially suppressed allodynia; the combined antagonists prevented allodynia, showing the involvement of both receptor subtypes. Co-injection of NMDA receptor antagonists, (+)MK-801 or D-AP5, with ET-1 also prevented allodynia. In contrast, co-injection of the CGRP1 antagonist CGRP(8-37) attenuated only the later phase of allodynia (>30 min). A mechanistic basis for these effects is shown by ET-1's ability to enhance basal release from cultured sensory neurons of glutamate and CGRP (2.4-fold and 5.7-fold, respectively, for 10 nM ET-1). ET(A) blockade reduced ET-1's enhancement of basal CGRP release by approximately 80%, but basal glutamate release by only approximately 30%. ET-1 also enhanced the capsaicin-stimulated release of CGRP (up to 2-fold for 0.3 nM ET-1), but did not change capsaicin-stimulated glutamate release. Release stimulated by elevated K+ was not altered by ET(A) blockade, nor did blockade of ET(B) reduce any type of release. Thus, ET-1 may induce release of glutamate and CGRP from nerve terminals innervating skin, thereby sensitizing primary afferents, accounting for ET-1-dependent tactile allodynia.

PERSPECTIVE

The endogenous endothelin peptides participate in a remarkable variety of pain-related processes. The present results provide evidence for the participation of ionotropic glutamatergic receptors and CGRP receptors in the hyperalgesic responses to exogenous ET-1 and suggest clinically relevant targets for further study of elevated pain caused by release of endogenous ET-1.

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.

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.

Hypertonic saline-induced muscle nociception and c-fos activation are partially mediated by peripheral NMDA receptors

In this study, the animal model of hypertonic saline (HS) infusion protocol was developed and utilized to test the hypothesis that HS causes peripheral release of glutamate, and that blockade of peripheral NMDA receptors significantly reduces HS-induced nocifensive behavior and central neuronal activation. Nocifensive behavior and c-fos immunoreactivity, as a marker of central neuronal activation, were assessed from the animals that received intramuscular HS infusion with and without the NMDA receptor antagonist, MK-801. HS infusion (20 microl/min for 10 min) in the rat masseter produced prolonged nocifensive hindpaw shaking responses that peaked in the first minute and gradually diminished over the infusion period. The HS induced nocifensive behavior was dose-dependently attenuated by MK-801 pretreatments (0.3 mg/kg and 0.1 mg/kg), but not by vehicle pretreatment (isotonic saline; ISO), in the masseter muscle. HS infusion produced a significant number of Fos positive neurons in the ispsilateral subnucleus caudalis (Vc). Subsequent immunohistochemical studies showed that peripheral MK-801 pretreatment effectively reduced the HS induced neuronal activation in the Vc. These results provide compelling evidence that HS-induced muscle nociception is mediated, in part, by peripheral release of glutamate, and that blockade of peripheral glutamate receptors may provide effective means of preventing central neuronal activation.