Role of metabotropic glutamate receptor subtype mGluR1 in brief nociception and central sensitization of primate STT cells

The influence of glutamate receptors on insulin release and diabetic neuropathy

Diabetes causes macrovascular and microvascular complications such as peripheral neuropathy. Glutamate regulates insulin secretion in pancreatic β-cells, and its increased activity in the central nervous system is associated with peripheral neuropathy in animal models of diabetes. One strategy to modulate glutamatergic activity consists in the pharmacological manipulation of metabotropic glutamate receptors (mGluRs), which, compared to the ionotropic receptors, allow for a fine-tuning of neurotransmission that is compatible with therapeutic interventions. mGluRs are a family of eight G-protein coupled receptors classified into three groups (I-III) based on sequence homology, transduction mechanisms, and pharmacology. Activation of group II and III or inhibition of group I represents a strategy to counteract the glutamatergic hyperactivity associated with diabetic neuropathy. In this review article, we will discuss the role of glutamate receptors in the release of insulin and the development/treatment of diabetic neuropathy, with particular emphasis on their manipulation to prevent the glutamatergic hyperactivity associated with diabetic neuropathy.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

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.

Neuro-immune signatures in chronic low back pain subtypes

We recently showed that patients with different chronic pain conditions (such as chronic low back pain, fibromyalgia, migraine, and Gulf War Illness) demonstrated elevated brain and/or spinal cord levels of the glial marker 18 kDa translocator protein, which suggests that neuroinflammation might be a pervasive phenomenon observable across multiple etiologically heterogeneous pain disorders. Interestingly, the spatial distribution of this neuroinflammatory signal appears to exhibit a degree of disease specificity (e.g. with respect to the involvement of the primary somatosensory cortex), suggesting that different pain conditions may exhibit distinct "neuroinflammatory signatures". To further explore this hypothesis, we tested whether neuroinflammatory signal can characterize putative etiological subtypes of chronic low back pain patients based on clinical presentation. Specifically, we explored neuroinflammation in patients whose chronic low back pain either did or did not radiate to the leg (i.e. "radicular" vs. "axial" back pain). Fifty-four chronic low back pain patients, twenty-six with axial back pain (43.7 ± 16.6 y.o. [mean±SD]) and twenty-eight with radicular back pain (48.3 ± 13.2 y.o.), underwent PET/MRI with [11C]PBR28, a second-generation radioligand for the 18 kDa translocator protein. [11C]PBR28 signal was quantified using standardized uptake values ratio (validated against volume of distribution ratio; n = 23). Functional MRI data were collected simultaneously to the [11C]PBR28 data 1) to functionally localize the primary somatosensory cortex back and leg subregions and 2) to perform functional connectivity analyses (in order to investigate possible neurophysiological correlations of the neuroinflammatory signal). PET and functional MRI measures were compared across groups, cross-correlated with one another and with the severity of "fibromyalgianess" (i.e. the degree of pain centralization, or "nociplastic pain"). Furthermore, statistical mediation models were employed to explore possible causal relationships between these three variables. For the primary somatosensory cortex representation of back/leg, [11C]PBR28 PET signal and functional connectivity to the thalamus were: 1) higher in radicular compared to axial back pain patients, 2) positively correlated with each other and 3) positively correlated with fibromyalgianess scores, across groups. Finally, 4) fibromyalgianess mediated the association between [11C]PBR28 PET signal and primary somatosensory cortex-thalamus connectivity across groups. Our findings support the existence of "neuroinflammatory signatures" that are accompanied by neurophysiological changes, and correlate with clinical presentation (in particular, with the degree of nociplastic pain) in chronic pain patients. These signatures may contribute to the subtyping of distinct pain syndromes and also provide information about inter-individual variability in neuro-immune brain signals, within diagnostic groups, that could eventually serve as targets for mechanism-based precision medicine approaches.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Differential expression of mGluRs in rat spinal dorsal horns and their modulatory effects on nocifensive behaviors

Glutamate is a neurotransmitter present in most excitatory synapses in the nervous system. It also plays a key role in the spinal cord’s physiological excitatory circuit and is involved in pathological neurotransmissions such as those observed in inflammatory and neuropathic pain conditions. The actions of glutamate are mediated by different types of ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). Although expressions of iGluRs are well studied, those of mGluRs are not fully elucidated in the spinal cord. In this study, we examined the expressions of mGluRs (mGluR1-8) and investigated which mGluR subtypes can modulate pain transmission in the dorsal horn of the spinal cord using an inflammatory pain model. Reverse transcription-polymerase chain reaction revealed that mGluR mRNAs, except for mGluR2 and 6, were detected in the spinal cord. Double labeling analysis, in situ hybridization histochemistry with immunohistochemistry, was used to examine the distribution of each mGluR in neurons or glial cells in the lamina I–II of the spinal dorsal horn. mGluR1, 5, and 7 were generally, and 4 and 8 were frequently, expressed in neurons. mGluR3 was expressed not only in neurons but also in oligodendrocytes. We next examined the distribution of mGluR4 and 8 were expressed in excitatory or inhibitory neurons. Both mGluR4 and 8 were preferentially expressed in inhibitory neurons rather than in excitatory neurons. Furthermore, intrathecal delivery of CPPG((RS)-α-cyclopropyl-4-phosphonophenylglycine), an antagonist for mGluR 4 and 8, attenuated nocifensive behaviors and the increase in fos-positive-excitatory neurons of the dorsal horn induced by intraplantar injection of formalin. These findings suggest that mGluR4 and 8, which are preferentially expressed in inhibitory neurons, may play roles in the modulation of pain transmission in the spinal dorsal horn.

Group I metabotropic glutamate receptor plasticity after peripheral inflammation alters nociceptive transmission in the dorsal of the spinal cord in adult rats

Abstract

The dorsal horn of the spinal cord is a crucial site for pain transmission and modulation. Dorsal horn neurons of the spinal cord express group I metabotropic glutamate receptors (group I mGluRs) that exert a complex role in nociceptive transmission. In particular, group I mGluRs promote the activation of L-type calcium channels, voltage-gated channels involved in short- and long-term sensitization to pain. In this study, we analyzed the role of group I mGluRs in spinal nociceptive transmission and the possible cooperation between these receptors and L-type calcium channels in the pathophysiology of pain transmission in the dorsal horn of the spinal cord. We demonstrate that the activation of group I mGluRs induces allodynia and L-type calcium channel-dependent increase in nociceptive field potentials following sciatic nerve stimulation. Surprisingly, in a model of persistent inflammation induced by complete Freund’s adjuvant, the activation of group I mGluRs induced an analgesia and a decrease in nociceptive field potentials. Among the group I mGluRs, mGluR1 promotes the activation of L-type calcium channels and increased nociceptive transmission while mGluR5 induces the opposite through the inhibitory network. These results suggest a functional switch exists in pathological conditions that can change the action of group I mGluR agonists into possible analgesic molecules, thereby suggesting new therapeutic perspectives to treat persistent pain in inflammatory settings.

The combination of postnatal maternal separation and social stress in young adulthood does not lead to enhanced inflammatory pain sensitivity and depression-related behavior in rats

The cumulative and match/mismatch hypotheses of stress are still under discussion regarding the effects of early life stress (ELS) on the vulnerability or resilience to psychopathology. In this context, an additional stress in later life (second hit) often leads to stress-related disorders that frequently include comorbid pain states. We previously observed that maternal separation (MS), a model of ELS, reduces vulnerability to neuropathic and inflammatory pain in rats. In the present study, we investigated the effects of an additional later stressor on the vulnerability to inflammatory pain. Sprague Dawley pups were divided into 4 groups: controls (CON, no stress), MS, social stress (SS) and MS+SS. At young adult age (from 7 to 15 weeks), stress- as well as pain-related parameters were evaluated prior and during 21 days following the induction of paw inflammation with complete Freund's adjuvant (CFA). Finally spinal glutamatergic transmission, immunocompetent cells, pro-inflammatory cytokines and growth factors were examined using qPCR. None of the stress conditions had a significant impact on corticosterone levels and anhedonia. In the forced swim test, MS and SS displayed increased passive coping whereas the combination of both stressors revoked this effect. The different stress conditions had no influence on basal mechanical thresholds and heat sensitivity. At 4 days post-inflammation all stress groups displayed lower mechanical thresholds than CON but the respective values were comparable at 7, 10, and 14 days. Only on day 21, MS rats were more sensitive to mechanical stimulation compared to the other groups. Regarding noxious heat sensitivity, MS+SS animals were significantly less sensitive than CON at 10 and 21 days after CFA-injection. qPCR results were mitigated. Despite the finding that stress conditions differentially affected different players of glutamatergic transmission, astrocyte activity and NGF expression, our biochemical results could not readily be related to the behavioral observations, precluding a congruent conclusion. The present results do neither confirm the cumulative nor corroborate or disprove the match/mismatch hypothesis.

Metaplasticity within the spinal cord: Evidence brain-derived neurotrophic factor (BDNF), tumor necrosis factor (TNF), and alterations in GABA function (ionic plasticity) modulate pain and the capacity to learn

Evidence is reviewed that behavioral training and neural injury can engage metaplastic processes that regulate adaptive potential. This issue is explored within a model system that examines how training affects the capacity to learn within the lower (lumbosacral) spinal cord. Response-contingent (controllable) stimulation applied caudal to a spinal transection induces a behavioral modification indicative of learning. This behavioral change is not observed in animals that receive stimulation in an uncontrollable manner. Exposure to uncontrollable stimulation also engages a process that disables spinal learning for 24-48 h. Controllable stimulation has the opposite effect; it engages a process that enables learning and prevents/reverses the learning deficit induced by uncontrollable stimulation. These observations suggest that a learning episode can impact the capacity to learn in future situations, providing an example of behavioral metaplasticity. The protective/restorative effect of controllable stimulation has been linked to an up-regulation of brain-derived neurotrophic factor (BDNF). The disruption of learning has been linked to the sensitization of pain (nociceptive) circuits, which is enabled by a reduction in GABA-dependent inhibition. After spinal cord injury (SCI), the co-transporter (KCC2) that regulates the outward flow of Cl^- is down-regulated. This causes the intracellular concentration of Cl^- to increase, reducing (and potentially reversing) the inward flow of Cl^- through the GABA-A receptor. The shift in GABA function (ionic plasticity) increases neural excitability caudal to injury and sets the stage for nociceptive sensitization. The injury-induced shift in KCC2 is related to the loss of descending serotonergic (5HT) fibers that regulate plasticity within the spinal cord dorsal horn through the 5HT-1A receptor. Evidence is presented that these alterations in spinal plasticity impact pain in a brain-dependent task (place conditioning). The findings suggest that ionic plasticity can affect learning potential, shifting a neural circuit from dampened/hard-wired to excitable/plastic.

A Wnt5a signaling pathway in the pathogenesis of HIV-1 gp120-induced pain

Pathological pain is one of the most common neurological complications in patients with HIV-1/AIDS. However, the pathogenic process is unclear. Our recent studies show that Wnt5a is upregulated in the spinal cord dorsal horn (SDH) of the patients with HIV who develop pain and that HIV-1 gp120, a potential causal factor of the HIV-associated pain, rapidly upregulates Wnt5a in the mouse SDH. Using a mouse model, we show here that a specific Wnt5a antagonist, Box-5, attenuated gp120-induced mechanical allodynia. Conversely, a Wnt5a agonist, Foxy5, facilitated the allodynia. To elucidate the molecular mechanism by which Wnt5a regulates gp120-induced allodynia, we tested the role of the JNK/TNF-α pathway. We observed that the JNK-specific inhibitor SP600125 blocked either gp120- or Foxy5-induced allodynia. Similarly, the TNF-α-specific antagonist Enbrel also reversed either gp120- or Foxy5-induced allodynia. These data suggest that JNK and TNF-α mediate the biological effects of Wnt5a in regulating gp120-induced allodynia. To investigate the cellular mechanism, we performed extracellular single-unit recording from SDH neurons in anesthetized mice. Both Box-5 and SP600125 negated gp120-induced potentiation of SDH neuron spiking evoked by mechanical stimulation of the hind paw. Furthermore, while Foxy5 potentiated spike frequency of SDH neurons, either SP600125 or Enbrel blocked the potentiation. The data indicate that Wnt5a potentiates the activity of SDH neurons through the JNK-TNF-α pathway. Collectively, our findings suggest that Wnt5a regulates the pathogenesis of gp120-induced pain, likely by sensitizing pain-processing SDH neurons through JNK/TNF-α signaling.

'Medusa-head ataxia': the expanding spectrum of Purkinje cell antibodies in autoimmune cerebellar ataxia. Part 1: Anti-mGluR1, anti-Homer-3, anti-Sj/ITPR1 and anti-CARP VIII

Serological testing for anti-neural autoantibodies is important in patients presenting with idiopathic cerebellar ataxia, since these autoantibodies may indicate cancer, determine treatment and predict prognosis. While some of them target nuclear antigens present in all or most CNS neurons (e.g. anti-Hu, anti-Ri), others more specifically target antigens present in the cytoplasm or plasma membrane of Purkinje cells (PC). In this series of articles, we provide a detailed review of the clinical and paraclinical features, oncological, therapeutic and prognostic implications, pathogenetic relevance, and differential laboratory diagnosis of the 12 most common PC autoantibodies (often referred to as 'Medusa-head antibodies' due to their characteristic somatodendritic binding pattern when tested by immunohistochemistry). To assist immunologists and neurologists in diagnosing these disorders, typical high-resolution immunohistochemical images of all 12 reactivities are presented, diagnostic pitfalls discussed and all currently available assays reviewed. Of note, most of these antibodies target antigens involved in the mGluR1/calcium pathway essential for PC function and survival. Many of the antigens also play a role in spinocerebellar ataxia. Part 1 focuses on anti-metabotropic glutamate receptor 1-, anti-Homer protein homolog 3-, anti-Sj/inositol 1,4,5-trisphosphate receptor- and anti-carbonic anhydrase-related protein VIII-associated autoimmune cerebellar ataxia (ACA); part 2 covers anti-protein kinase C gamma-, anti-glutamate receptor delta-2-, anti-Ca/RhoGTPase-activating protein 26- and anti-voltage-gated calcium channel-associated ACA; and part 3 reviews the current knowledge on anti-Tr/delta notch-like epidermal growth factor-related receptor-, anti-Nb/AP3B2-, anti-Yo/cerebellar degeneration-related protein 2- and Purkinje cell antibody 2-associated ACA, discusses differential diagnostic aspects and provides a summary and outlook.

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.

Metaplasticity and behavior: how training and inflammation affect plastic potential within the spinal cord and recovery after injury

Research has shown that spinal circuits have the capacity to adapt in response to training, nociceptive stimulation and peripheral inflammation. These changes in neural function are mediated by physiological and neurochemical systems analogous to those that support plasticity within the hippocampus (e.g., long-term potentiation and the NMDA receptor). As observed in the hippocampus, engaging spinal circuits can have a lasting impact on plastic potential, enabling or inhibiting the capacity to learn. These effects are related to the concept of metaplasticity. Behavioral paradigms are described that induce metaplastic effects within the spinal cord. Uncontrollable/unpredictable stimulation, and peripheral inflammation, induce a form of maladaptive plasticity that inhibits spinal learning. Conversely, exposure to controllable or predictable stimulation engages a form of adaptive plasticity that counters these maladaptive effects and enables learning. Adaptive plasticity is tied to an up-regulation of brain derived neurotrophic factor (BDNF). Maladaptive plasticity is linked to processes that involve kappa opioids, the metabotropic glutamate (mGlu) receptor, glia, and the cytokine tumor necrosis factor (TNF). Uncontrollable nociceptive stimulation also impairs recovery after a spinal contusion injury and fosters the development of pain (allodynia). These adverse effects are related to an up-regulation of TNF and a down-regulation of BDNF and its receptor (TrkB). In the absence of injury, brain systems quell the sensitization of spinal circuits through descending serotonergic fibers and the serotonin 1A (5HT 1A) receptor. This protective effect is blocked by surgical anesthesia. Disconnected from the brain, intracellular Cl^- concentrations increase (due to a down-regulation of the cotransporter KCC2), which causes GABA to have an excitatory effect. It is suggested that BDNF has a restorative effect because it up-regulates KCC2 and re-establishes GABA-mediated inhibition.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Metabotropic glutamate receptors as targets for analgesia: antagonism, activation, and allosteric modulation

The metabotropic glutamate receptors (mGluRs) are expressed pre- and post-synaptically throughout the nervous system where they serve as modulators of synaptic transmission and neuronal excitability. Activation of mGluRs can be pro- or anti-nociceptive, depending on their anatomic location and the signaling cascades to which they couple. Antagonists of Group I mGluRs and agonists of Group II and III mGluRs have shown therapeutic promise in animal pain models. This article reviews the potential therapeutic utility of several agents that act predominantly via mGluRs, specifically focusing on their analgesic efficacy and discussing possible off-target effects. Glutamate, the primary excitatory neurotransmitter in the vertebrate nervous system, mediates its effects via activation of two main classes of receptors: ligand-gated ion channels known as ionotropic receptors and G-protein coupled metabotropic receptors. Antagonists of ionotropic glutamate receptors, such as ketamine, have robust analgesic properties; however, their analgesic utility is limited to monitored clinical settings due to the potential for psychomimetic effects.

Blockade of metabotropic glutamate receptor 5 activation inhibits mechanical hypersensitivity following abdominal surgery

This study used the metabotropic glutamate 5 (mGlu5) receptor subtype-selective antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) to characterise the contribution of mGlu5 receptor activity to pain and hypersensitivity in an animal model of post-surgical pain. Adult female Wistar rats (200-250g) were anaesthetised with isoflurane (2%) and underwent a midline laparotomy with gentle manipulation of the viscera, and the effects of pre- (30min) or post- (5h) operative treatment with MPEP (1, 3 or 10mgkg(-1); i.p.) or drug-vehicle on hindpaw withdrawal latency (in seconds) to thermal stimulation (Hargreave's Test) and response threshold (in grams) to mechanical stimulation (using a dynamic plantar aesthesiometer) were measured. Animals that underwent surgery displayed significant hypersensitivity to mechanical stimulation of the hindpaws. Hypersensitivity was maximum at 6h post-surgery (44.5+/-2.4% decrease; p<0.01 vs. anaesthesia only controls) and persisted for 48h. Surgery had no effect on thermal withdrawal latency. Both pre-operative and post-operative administration of 10mgkg(-1)MPEP blocked mechanical hypersensitivity induced by surgery (p<0.01 vs. vehicle treatment). MPEP had no effect on acute nociceptive thresholds in naïve animals. These data suggest that activity at mGlu5 receptors contributes to development of pain and hypersensitivity following surgery.

Involvement of metabotropic glutamate receptor 5 in pudendal inhibition of nociceptive bladder activity in cats

This study used MTEP, a metabotropic glutamate receptor 5 (mGluR5) antagonist, to examine the role of mGluR5 in the neural control of the urinary bladder and in the inhibition of the micturition reflex by pudendal nerve stimulation (PNS). Experiments were conducted in 11 female cats under α-chloralose anaesthesia when the bladder was infused with either saline or 0.25% acetic acid (AA). AA irritated the bladder, induced bladder overactivity and significantly (P < 0.001) reduced bladder capacity to 14.9 ± 10.3% of the saline control capacity. MTEP (0.1-50 mg kg(-1), i.v.) significantly (P < 0.05) increased bladder capacity during saline distension but not during AA irritation. However, MTEP induced a transient inhibition of isovolumetric bladder contractions under both conditions. PNS (5 Hz), which was tested at the threshold (T) intensity for inducing a complete inhibition of isovolumetric bladder contractions and at an intensity of 3-4T, suppressed AA-induced bladder overactivity and significantly increased bladder capacity to 68.0 ± 31.3% at 1T (P < 0.05) and 98.5 ± 55.3% at 3-4T (P < 0.01) of the saline control capacity. MTEP dose dependently (0.1-50 mg kg(-1), i.v.) suppressed PNS inhibition of bladder overactivity at low intensity (1T) but not at high intensity (3-4T). During saline infusion PNS significantly (P < 0.05) increased bladder capacity to 167.7 ± 27.1% at 1T and 196.0 ± 37.4% at 3-4T. These inhibitory effects were not observed after MTEP (0.1-50 mg kg(-1), i.v.) which also increased bladder capacity. These results indicate that glutamic acid has a transmitter function in bladder and somato-bladder reflex mechanisms and raise the possibility that mGluR5 may be a target for pharmacological treatment of lower urinary tract disorders.

Allosteric modulation of metabotropic glutamate receptors

The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson's disease, and Fragile X syndrome.

Metabotropic glutamate and cannabinoid receptor crosstalk in periaqueductal grey pain processing

Metabotropic glutamate (mGlu) and cannabinoid receptors are G-protein coupled receptors which have shown synaptic co-operation through small lipid messengers in the central nervous system (CNS). A functional interaction between these two receptor families could have a relevant potential in the treatment of CNS disorders, including chronic pain. Indeed, both mGlu and cannabinoid receptors play a crucial role in the neurobiology of pain and their simultaneous manipulation could lead to novel strategies in pain management. In particular, as both mGlu and cannabinoid receptors have been found in the periaqueductal gray (PAG), a crucial station in the pain modulatory system, these receptors could be a substrate for producing analgesia at this level. In this review we aim to briefly illustrate the role of mGlu and can-nabinoid receptors in controlling nociceptive processes, some points of convergence, and their functional interaction in pain processing. Further insights into this functional linkage between the mGlu and cannabinoid receptors could pave the way to a new strategy for pain relief, such as a drug cocktail acting on cannabinoid/metabotropic glutamate receptors.

Cognitive impairment in pain through amygdala-driven prefrontal cortical deactivation

Cognitive deficits such as impaired decision-making can be a consequence of persistent pain. Normal functions of the intact amygdala and prefrontal cortex are required for emotion-based decision-making that relies on the ability to assess risk, attribute value, and identify advantageous strategies. We tested the hypothesis that pain-related cognitive deficits result from amygdala-driven impairment of medial prefrontal cortical (mPFC) function. To do this, we used electrophysiological single-unit recordings in vivo, patch clamp in brain slices, and various behavioral assays to show that increased neuronal activity in the amygdala in an animal model of arthritis pain was accompanied by decreased mPFC activation and impaired decision-making. Furthermore, pharmacologic inhibition (with a corticotropin-releasing factor 1 receptor antagonist) of pain-related hyperactivity in the basolateral amygdala (BLA), but not central amygdala (CeA), reversed deactivation of mPFC pyramidal cells and improved decision-making deficits. Pain-related cortical deactivation resulted from a shift of balance between inhibitory and excitatory synaptic transmission. Direct excitatory transmission to mPFC pyramidal cells did not change in the pain model, whereas polysynaptic inhibitory transmission increased. GABAergic transmission was reduced by non-NMDA receptor antagonists, suggesting that synaptic inhibition was glutamate driven. The results are consistent with a model of BLA-driven feedforward inhibition of mPFC neurons. In contrast to the differential effects of BLA versus CeA hyperactivity on cortical-cognitive functions, both amygdala nuclei modulate emotional-affective pain behavior. Thus, this study shows that the amygdala contributes not only to emotional-affective but also cognitive effects of pain. The novel amygdalo-cortical pain mechanism has important implications for our understanding of amygdala functions and amygdalo-cortical interactions.

Regulation of increased glutamatergic input to spinal dorsal horn neurons by mGluR5 in diabetic neuropathic pain

Diabetic neuropathic pain is associated with increased glutamatergic input in the spinal dorsal horn. Group I metabotropic glutamate receptors (mGluRs) are involved in the control of neuronal excitability, but their role in the regulation of synaptic transmission in diabetic neuropathy remains poorly understood. Here we studied the role of spinal mGluR5 and mGluR1 in controlling glutamatergic input in a rat model of painful diabetic neuropathy induced by streptozotocin. Whole-cell patch-clamp recordings of lamina II neurons were performed in spinal cord slices. The amplitude of excitatory post-synaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than in control rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) inhibited evoked EPSCs and sEPSCs more in diabetic than in control rats. Also, the percentage of neurons in which sEPSCs and evoked EPSCs were affected by MPEP or the group I mGluR agonist was significantly higher in diabetic than in control rats. However, blocking mGluR1 had no significant effect on evoked EPSCs and sEPSCs in either groups. The mGluR5 protein level in the dorsal root ganglion, but not in the dorsal spinal cord, was significantly increased in diabetic rats compared with that in control rats. Furthermore, intrathecal administration of MPEP significantly increased the nociceptive pressure threshold only in diabetic rats. These findings suggest that increased mGluR5 expression on primary afferent neurons contributes to increased glutamatergic input to spinal dorsal horn neurons and nociceptive transmission in diabetic neuropathic pain.

Spinal endocannabinoids and CB1 receptors mediate C-fiber-induced heterosynaptic pain sensitization

Diminished synaptic inhibition in the spinal dorsal horn is a major contributor to chronic pain. Pathways that reduce synaptic inhibition in inflammatory and neuropathic pain states have been identified, but central hyperalgesia and diminished dorsal horn synaptic inhibition also occur in the absence of inflammation or neuropathy, solely triggered by intense nociceptive (C-fiber) input to the spinal dorsal horn. We found that endocannabinoids, produced upon strong nociceptive stimulation, activated type 1 cannabinoid (CB1) receptors on inhibitory dorsal horn neurons to reduce the synaptic release of gamma-aminobutyric acid and glycine and thus rendered nociceptive neurons excitable by nonpainful stimuli. Our results suggest that spinal endocannabinoids and CB1 receptors on inhibitory dorsal horn interneurons act as mediators of heterosynaptic pain sensitization and play an unexpected role in dorsal horn pain-controlling circuits.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Imbalance between excitatory and inhibitory amino acids at spinal level is associated with maintenance of persistent pain-related behaviors

Although the postsynaptic events responsible for development of pathological pain have been intensively studied, the relative contribution of presynaptic neurotransmitters to the whole process remains less elucidated. In the present investigation, we sought to measure temporal changes in spinal release of both excitatory amino acids (EAAs, glutamate and aspartate) and inhibitory amino acids (IAAs, glycine, ?-aminobutyric acid and taurine) in response to peripheral inflammatory pain state. The results showed that following peripheral chemical insult induced by subcutaneous bee venom (BV) injection, there was an initial, parallel increase in spinal release of both EAAs and IAAs, however, the balance between them was gradually disrupted when pain persisted longer, with EAAs remaining at higher level but IAAs at a level below the baseline. Moreover, the EAAs-IAAs imbalance at the spinal level was dependent upon the ongoing activity from the peripheral injury site. Intrathecal blockade of ionotropic (NMDA and non-NMDA) and metabotropic (mGluRI, II, III) glutamate receptors, respectively, resulted in a differential inhibition of BV-induced different types of pain (persistent nociception vs. hyperalgesia, or thermal vs. mechanical hyperalgesia), implicating that spinal antagonism of any specific glutamate receptor subtype fails to block all types of pain-related behaviors. This result provides a new line of evidence emphasizing an importance of restoration of EAAs-IAAs balance at the spinal level to prevent persistence or chronicity of pain.

Group I metabotropic glutamate receptors control metaplasticity of spinal cord learning through a protein kinase C-dependent mechanism

Neurons within the spinal cord can support several forms of plasticity, including response-outcome (instrumental) learning. After a complete spinal transection, experimental subjects are capable of learning to hold the hindlimb in a flexed position (response) if shock (outcome) is delivered to the tibialis anterior muscle when the limb is extended. This response-contingent shock produces a robust learning that is mediated by ionotropic glutamate receptors (iGluRs). Exposure to nociceptive stimuli that are independent of limb position (e.g., uncontrollable shock; peripheral inflammation) produces a long-term (>24 h) inhibition of spinal learning. This inhibition of plasticity in spinal learning is itself a form of plasticity that requires iGluR activation and protein synthesis. Plasticity of plasticity (metaplasticity) in the CNS has been linked to group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) and activation of protein kinase C (PKC). The present study explores the role of mGluRs and PKC in the metaplastic inhibition of spinal cord learning using a combination of behavioral, pharmacological, and biochemical techniques. Activation of group I mGluRs was found to be both necessary and sufficient for metaplastic inhibition of spinal learning. PKC was activated by stimuli that inhibit spinal learning, and inhibiting PKC activity restored the capacity for spinal learning. Finally, a PKC inhibitor blocked the metaplastic inhibition of spinal learning produced by a group I mGluR agonist. The data strongly suggest that group I mGluRs control metaplasticity of spinal learning through a PKC-dependent mechanism, providing a potential therapeutic target for promoting use-dependent plasticity after spinal cord injury.

Metabotropic receptors for glutamate and GABA in pain

Glutamate and gamma-amino butyric acid (GABA) are respectively two major excitatory and inhibitory neurotransmitters of the adult mammalian central nervous system. These neurotransmitters exert their action through two types of receptors: ionotropic and metabotropic receptors. While ionotropic receptors are ligand gated ion channels involved in fast synaptic transmission, metabotropic receptors belong to the superfamily of G-protein coupled receptors (GPCRs) and are responsible for the neuromodulatory effect of glutamate and GABA. Metabotropic glutamate receptors (mGluRs) and metabotropic GABA receptors (GABA-B) are present at different levels of the pain neuraxis where they regulate nociceptive transmission and pain. The present review will focus on the role of these receptors in the modulation of pain perception.

Spinal mechanisms of antinociceptive effect caused by oral administration of bis-selenide in mice

The present study was designed to investigate further the mechanisms involved in the antinociception caused by bis-selenide in behavioral model of pain in mice. Bis-selenide (5-50 mg/kg), given orally, produced significant inhibition of the antinociceptive behavior induced by intrathecal (i.t.) injection of glutamate (175 nmol/site), kainate (110 pmol/site) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 50 nmol/site) and the maximal inhibitions observed were 57+/-5, 46+/-7 and 73+/-3%, respectively. Bis-selenide failed to affect the nociception induced by alpha-amino-3-hydroxy-5-mehtyl-4-isoxazolepropionic acid (AMPA; 135 pmol/site) and N-methyl-d-aspartate (NMDA; 450 pmol/site). This compound also reduced the nociceptive response induced by tumor necrosis factor-alpha (TNF-alpha; 0.1 pg/site), interleukin-1beta (IL-1beta; 1 pg/site), substance P (SP) (135 ng/site, i.t.) and capsaicin (30 ng/site) and the inhibitions observed were 81+/-3%, 88+/-1%, 77+/-3 and 67+/-3, respectively. The oral administration of bis-selenide (25-50 mg/kg) in mice caused a significant increase in the reaction time to thermal stimuli in the hot plate test and the mean ID(50) value (and the 95% confidence limits) was 20.37 (15.00-25.74) mg/kg. The antinociceptive effect caused by bis-selenide (50 mg/kg, p.o.) on the hot plate test in mice was reversed by intrathecal (i.t.) injection of some K(+) channel blockers such as tetraethylammonium (TEA, non-selective voltage-dependent K(+) channel inhibitor) and glibenclamide (ATP-sensitive K(+) channel inhibitor), but not apamin and charybdotoxin (large- and small-conductance Ca(2+)-activated K(+) channel inhibitors, respectively). Together, these results indicate that bis-selenide produces antinociception at spinal sites through the activation of ATP-sensitive and voltage-gated K(+) channels and interaction with kainate and trans-ACDP receptors as well as vanilloid and neuropeptide receptors and pro-inflammatory cytokines.

Metabotropic glutamate receptor 5 modulates nociceptive plasticity via extracellular signal-regulated kinase-Kv4.2 signaling in spinal cord dorsal horn neurons

Metabotropic glutamate receptors (mGluRs) play important roles in the modulation of nociception. The group I mGluRs (mGlu1 and mGlu5) modulate nociceptive plasticity via activation of extracellular signal-regulated kinase (ERK) signaling. We reported recently that the K+ channel Kv4.2 subunit underlies A-type K+ currents in the spinal cord dorsal horn and is modulated by the ERK signaling pathway. Kv4.2-mediated A-type currents are important determinants of dorsal horn neuronal excitability and central sensitization that underlies hypersensitivity after tissue injury. In the present study, we demonstrate that ERK-mediated phosphorylation of Kv4.2 is downstream of mGlu5 activation in spinal cord dorsal horn neurons. Activation of group I mGluRs inhibited Kv4.2-mediated A-type K+ currents and increased neuronal excitability in dorsal horn neurons. These effects were mediated by activation of mGlu5, but not mGlu1, and were dependent on ERK activation. Analysis of Kv4.2 phosphorylation site mutants clearly identified S616 as the residue responsible for mGlu5-ERK-dependent modulation of A-type currents and excitability. Furthermore, nociceptive behavior induced by activation of spinal group I mGluRs was impaired in Kv4.2 knock-out mice, demonstrating that, in vivo, modulation of Kv4.2 is downstream of mGlu5 activation. Altogether, our results indicate that activation of mGlu5 leads to ERK-mediated phosphorylation and modulation of Kv4.2-containing potassium channels in dorsal horn neurons. This modulation may contribute to nociceptive plasticity and central sensitization associated with chronic inflammatory pain conditions.

Effects of inflammation on the ultrastructural localization of spinal cord dorsal horn group I metabotropic glutamate receptors

Inflammatory pain is thought to induce functional plasticity of spinal dorsal horn neurons and may produce changes in glutamate receptor expression. Plasticity of group I metabotropic glutamate receptors (mGluR1 and mGluR5) is important in various neuronal systems, and these receptors are also known to modulate nociceptive neurotransmission in the spinal dorsal horn. The present study aimed at determining whether persistent inflammatory pain produces alterations in intracellular and plasma membrane-associated mGluR1alpha and mGluR5 in spinal cord dorsal horn. Persistent inflammation was induced in male Long Evans rats by a unilateral intraplantar injection of 100 muL of complete Freund's adjuvant (CFA). Three days after the CFA injection thermal withdrawal latencies were obtained prior to processing of transverse spinal cord sections for preembedding immunogold labeling after incubation in primary antibody for mGluR1alpha or mGluR5. Using electron microscopy, we quantified immunogold-labeled mGluR1alpha and mGluR5 profiles, located in lamina V and I-II, respectively, of both CFA-treated rats and untreated control rats. Compared to untreated rats, CFA-treated rats had a significant increase in the number of plasma membrane-associated mGluR5 immunogold-labeled particles in lamina I-II neurons of the spinal cord. Although no changes to mGluR1alpha expression were found in CFA-treated rats, plasma membrane-associated mGluR1alpha was significantly closer to the synapse. Therefore, in CFA-treated rats there was a specific increase in the ratio of plasma membrane-associated versus intracellular immunogold-labeled particles for mGluR5, and lateral movement of mGluR1alpha toward the synapse, indicating that peripheral inflammation-induced trafficking of group I mGluRs in spinal dorsal horn neurons may be an important factor in the development of plastic changes associated with inflammation-induced chronic pain.

Analgesic activity of metabotropic glutamate receptor 1 antagonists on spontaneous post-operative pain in rats

Activation of metabotropic glutamate (mGlu) receptors has previously been shown to play a role in inflammatory or neuropathic pain states. However, the role of mGlu type 1 receptors in post-operative pain remains to be investigated. In the present study, effects of potent and selective mGlu1 receptor antagonists A-841720, A-794282, A-794278, and A-850002 were evaluated in a skin incision-induced post-operative pain model in rats. Post-operative pain was examined 2 h following surgery using weight-bearing difference between injured and uninjured paws as a measure of spontaneous pain. In this model, A-841720, A-794282, A-794278, and A-850002 induced significant attenuation of spontaneous post-operative pain behavior, with ED50s of 10, 50, 50, and 65 micromol/kg i.p., respectively. Depending on the compound, significant motor side effects were also observed at 3 to 10 fold higher doses. These results support the notion that mGlu1 receptor activation plays a significant role in nociceptive transmission in post-operative pain, though motor impairment may be a limiting factor in developing mGlu1 receptor antagonists as novel analgesics.

Spinal CGRP1 receptors contribute to supraspinally organized pain behavior and pain-related sensitization of amygdala neurons

CGRP receptor activation has been implicated in peripheral and central sensitization. The role of spinal CGRP receptors in supraspinal pain processing and higher integrated pain behavior is not known. Here we studied the effect of spinal inhibition of CGRP1 receptors on supraspinally organized vocalizations and activity of amygdala neurons. Our previous studies showed that pain-related audible and ultrasonic vocalizations are modulated by the central nucleus of the amygdala (CeA). Vocalizations in the audible and ultrasonic range and hindlimb withdrawal thresholds were measured in awake adult rats before and 5-6h after induction of arthritis by intra-articular injections of kaolin and carrageenan into one knee. Extracellular single-unit recordings were made from neurons in the latero-capsular division of the CeA (CeLC) in anesthetized rats before and after arthritis induction. CGRP1 receptor antagonists were applied to the lumbar spinal cord intrathecally (5 microl/min) 6h postinduction of arthritis. Spinal administration of peptide (CGRP8-37, 1 microM) and non-peptide (BIBN4096BS, 1 microM) CGRP1 receptor antagonists significantly inhibited the increased responses of CeLC neurons to mechanical stimulation of the arthritic knee but had no effect under normal conditions. In arthritic rats, the antagonists also inhibited the audible and ultrasonic components of vocalizations evoked by noxious stimuli and increased the threshold of hindlimb withdrawal reflexes. The antagonists had no effect on vocalizations and spinal reflexes in normal rats. These data suggest that spinal CGRP1 receptors are not only important for spinal pain mechanisms but also contribute significantly to the transmission of nociceptive information to the amygdala and to higher integrated behavior.

Decrease in N-Methyl-d-aspartic Acid Receptor-NR2B Subunit Levels by Intrathecal Short-Hairpin RNA Blocks Group I Metabotropic Glutamate Receptor-Mediated Hyperalgesia

The present study characterizes the involvement of the N-methyl-D-aspartic acid receptors (NMDARs) in mediating thermal hyperalgesia induced by activation of group I metabotropic glutamate receptors (mGluRs). Intrathecal administration of the mGluR1/5 agonist (S)-3,5-DHPG [(S)-3,5-dihydroxyphenylglycine] to mice resulted in significant hyperalgesia as assessed by the tail immersion test. The pretreatment of mice i.t. with CGS 19755 (selective antagonist of the NMDAR), CGP 78608 [[(1S)-1-[[(7-bromo-1,2,3,4-tetrahydro-2,3-dioxo-5-quinoxalinyl)methyl]amino]ethyl]phosphonic acid] (selective antagonist at the glycine-binding site of the NMDAR), ifenprodil and Ro 25-6981 (selective antagonists of the NR2B subunit of the NMDAR), bisindolylmaleimide I and Go-7874 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] (inhibitors of protein kinase C), or PKI-(14-22)-amide [Myr-N-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile-NH(2)] (inhibitor of protein kinase A) dose-dependently inhibited the hyperalgesia induced by i.t. administration of the mGluR1/5 receptor agonist (S)-3,5-DHPG. In contrast, i.t. pretreatment of mice with NVP-AAM077 [[(R)-[(S)-1-(4-bromophenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid] (selective antagonist of the NR2A subunit of the NMDAR) or DT-3 [H-Arg-Gln-Ile-Lys-Ile-Trp-Phe-Gln-Asn-Arg-Arg-Met-Lys-Trp-Lys-Lys-Leu-Arg-Lys-Lys-Lys-Lys-Lys-His-OH] (inhibitor of protein kinase G) had no effect on (S)-3,5-DHPG-mediated hyperalgesia. We also show for the first time that i.t. injection of pSM2 (pShag Magic version 2)-grin2b (coding for an short-hairpin RNA to the NR2B subunit of the NMDAR) resulted in a dose-dependent decrease in the NR2B protein and blockade of hyperalgesia induced by activation of the mGluR1/5 in (S)-3,5-DHPG-treated mice. Taken together, our results suggest the hypothesis that mGluRs are coupled to the NMDAR channels through the NR2B subunit in the spinal cord and that this coupling involves the activation of protein kinase C and protein kinase A.

mGlu5 receptor and protein kinase C implicated in the development and induction of neuropathic pain following chronic ethanol consumption

The central mechanisms of neuropathic pain following chronic ethanol consumption are poorly understood. We previously reported that the levels of metabotropic glutamate 5 (mGlu5) receptor and phosphorylated-protein kinase C (PKC) were significantly increased in the spinal cord following chronic ethanol consumption. The aim of this study was to investigate whether mGlu5 receptor and PKC inhibitors directly attenuate the neuropathic pain-like state induced by chronic ethanol treatment in rats. A significant decrease in the mechanical nociceptive threshold was observed 5 weeks of chronic ethanol consumption. This hyperalgesia was significantly attenuated by repeated i.p. injection of (S)-2,6-diamino-N-[[1-(oxotridecyl)-2-piperidinyl]methyl] hexanamide dihydrochloride (NPC15437), a selective PKC inhibitor, once a day for a week after 4 weeks of ethanol treatment. Furthermore, this hyperalgesia was also significantly attenuated by repeated i.p. injection of 6-methyl-2-[phenylethynyl]-pyridine (MPEP), a selective mGlu5 receptor inhibitor, once a day for a week after 4 weeks of ethanol treatment. Furthermore, the hyperalgesia that developed after 5 weeks of ethanol treatment was significantly suppressed by a single i.p. post-injection with either NPC15437 or MPEP. These findings constitute direct evidence that spinal mGlu5 receptor and PKC play substantial roles in the development and maintenance of an ethanol-dependent neuropathic pain-like state in rats.

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

We previously demonstrated that peripherally located N-methyl-D-aspartic acid (NMDA) receptors contribute to acute muscle nociception and the development of chronic muscular hyperalgesia. In the present study, we investigated the potential role of peripheral group I metabotropic glutamate receptors (mGluRs 1/5) in the development of muscular hypersensitivity to mechanical stimulation, and attempted to elucidate intracellular signaling mechanisms associated with the mGluR activation in male Sprague-Dawley rats. First, our Western blot analyses revealed that mGluR 5 protein, but not mGluR 1 protein, is reliably detected in trigeminal ganglia and the masseter nerve. Subsequent behavioral studies demonstrated that the group I mGluR agonist, R,S-3,5-dihydroxyphenylglycol (DHPG), significantly decreased the mechanical threshold to noxious stimulation of the masseter, and that the DHPG-induced mechanical hypersensitivity can be effectively prevented by pretreatment of the masseter with 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGluR 5 antagonist, but not by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), a selective mGluR 1 antagonist. Moreover, the DHPG-induced mechanical hypersensitivity was significantly blocked by inhibiting either the alpha or epsilon isoform of protein kinase C (PKC). Collectively, these data provide evidence that peripherally located mGluR 5 may play an important role in the development of masseter hypersensitivity, and that PKC activation is required for the modulatory effect of peripheral mGluR 5 in the craniofacial muscle tissue. Thus, selective targeting of peripheral mGluR 5 and PKCalpha, as well as PKCepsilon, might serve as an effective therapeutic strategy in the management of chronic muscle pain conditions, such as temporomandibular disorders.

The peripheral role of group I metabotropic glutamate receptors on nociceptive behaviors in rats with knee joint inflammation

We examined whether the mGluR1 and mGluR5 were involved in development and maintenance of behavioral signs of non-evoked pain and secondary mechanical hyperalgesia induced by knee joint inflammation. Selective mGluR1 antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA: 50, 100, 200 microM/25 microl, n=10 per group) and selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP: 50, 100, 200 nM/25 microl, n=10 per group) was intra-articularly (i.a.) injected 30 min before and 4h after carrageenan injection and behavioral tests were conducted. In the pre-treatment, only a higher dose (200 nM) of MPEP significantly prevented the magnitude of weight load reduction, whereas AIDA (200 microM) and MEPE (50, 100 and 200 nM) significantly reduced the development of mechanical hyperalgesia compared to saline treated group. In the post-treatment, AIDA (200 microM) and MPEP at 100 and 200 nM partially reversed the reduction of weight load induced by carrageenan. MPEP significantly increased the withdrawal threshold to mechanical stimulation in a dose-dependent manner, whereas AIDA had significantly reversed the decreased the paw withdrawal threshold only at 200 microM. The present study demonstrated that i.a. MPEP, selective mGluR5 antagonist is more effective than selective mGluR1 antagonist, AIDA on non-evoked pain as well as mechanical hyperalgesia in both induction and maintenance phase in knee joint inflammation. It is suggested that peripheral mGlu5 receptors play a more prominent role in inflammatory pain including evoked and spontaneous pain. Thus, selective mGluR5 antagonist could be effective therapeutic tools in clinical setting.

Glutamate receptor ligands

Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neurotransmission also represents a key mechanism of pain-related plastic changes in the central and peripheral nervous system. This chapter will review the classification of glutamate receptors and their role in peripheral and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic value of certain glutamate receptor ligands will be discussed.

Glutamate and metabotropic glutamate receptors associated with innervation of the uterine cervix during pregnancy: Receptor antagonism inhibits c-fos expression in rat lumbosacral spinal cord at parturition

Dorsal root ganglia (DRG) neurons connect the spinal cord and uterine cervix, and are activated at parturition with subsequent stimulation of secondary neurons in the spinal dorsal horn and autonomic areas. Neuropeptide neurotransmitters and receptors have been studied in these areas, but amino acid transmitters, e.g., glutamate, an excitatory neurotransmitter involved in sensory and nociceptive processing, have not been characterized. To determine if glutamate is involved in innervation of the cervix, rats were examined for markers of glutamatergic neurons in the L6-S1 spinal cord, DRG and cervix. Metabotropic glutamate receptors mGluR5 in the spinal dorsal horn and their expression over pregnancy were examined in pregnant rats and pregnant rats treated continuously with an antagonist of mGluR5, 2-methyl-6-(phenylethynyl) pyridine (MPEP). Rats were allowed to deliver pups to determine if the antagonist altered the expression of an early response gene protein, Fos, in the L6-S1 cord. Immunohistochemistry showed glutamate- and vesicular glutamate transporter1 (VGluT1)-positive fibers in the cervix, glutamate- and VGluT1-expressing neurons in the DRG, some of which also exhibited retrograde tracer from cervical injections, and VGluT1 and mGluR5 immunoreactivities in the L6-S1 spinal dorsal horns. Expression of mGluR5 receptors increased over pregnancy. Fos-positive neurons were present among mGluR5-immunoreactivity in the spinal dorsal horn. Parturition-induced Fos-positive neurons in the spinal cords were abundant in control rats, but were reduced by 70% in MPEP-treated animals. These results suggest that glutamate is likely involved in the transmission of sensory signals, possibly pain, from the cervix to the spinal cord at parturition.

Structure–activity relationships of novel non-competitive mGluR1 antagonists: A potential treatment for chronic pain

A series of novel mGluR1 antagonists have been prepared. Incorporation of fragments derived from weak lead matter into a library led to enhanced potency in a new chemical series. A chemistry driven second library iteration, covering a greatly enhanced area of chemical space, maintained good potency and introduced metabolic stability.

Preemptive analgesia with lidocaine prevents Failed Back Surgery Syndrome

Failed Back Surgery Syndrome (FBSS) is commonly encountered in pain-treatment settings in the United States. We tested whether potential key factors in this syndrome, such as extracellular concentrations of excitatory amino acids (EAAs), are increased in the dorsal horn by synaptic release due to unintentional stretch and/or deformation/compression/transection of dorsal spinal structures during surgery. We hypothesized that pharmacological nerve block as a form of preemptive analgesia prior to any insult to dorsal root neurons will prevent an abnormally high increase in extracellular concentrations of EAAs in the dorsal horn and ultimately the establishment of central sensitization during back surgery. The L4 and L5 dorsal roots were cut bilaterally near the spinal cord to provide an adequate model to test for preemptive analgesia. Amino acid concentrations were measured by dorsal horn microdialysis sampling; EAAs aspartate and glutamate were significantly increased by 80% and 65% respectively, as were other amino acids compared to sham control values. Topical application of 1% Lidocaine, a voltage-gated Na(+) channel blocker, for 10 min prior to L4 and L5 bilateral dorsal rhizotomy (BDR) significantly attenuated the increase in EAA concentrations such that their values were not different from sham controls. Behavioral tests demonstrated significant hindlimb mechanical allodynia after BDRs that was significantly attenuated by Lidocaine pretreatment. Thus, Lidocaine pretreatment could offer a safe measure for prevention of chronic pain for back surgical procedures if given by intramuscular injection, topical administration onto spinal nerves and/or the dorsal spinal surface during surgical procedures that include nerve entrapment release, intervertebral disc modification and laminectomies.

Prolonged maintenance of capsaicin-induced hyperalgesia by brief daily vibration stimuli

This study tests the hypothesis that central sensitization initiated by nociceptive input can be maintained by repeated brief innocuous peripheral inputs. Capsaicin was injected intradermally into the hind paw of adult rats. Three different types of daily cutaneous mechanical stimulations (vibration, soft brush, or pressure) were applied to the capsaicin-injected paw for a period of 2 weeks. Daily stimulation consisted of a 10-s stimulation repeated every 30s for 30 min. Foot withdrawal thresholds to von Frey stimuli applied to the paw were measured once a day for 4 weeks. The capsaicin-only group (control rats without daily stimulation) showed hyperalgesia lasting for 3 days. In contrast, hyperalgesia persisted for 2 weeks in the group that received vibration stimulation. Neither the soft brush nor the pressure group showed a significant difference in mechanical threshold from the control group (capsaicin only). The vibration-induced prolonged hyperalgesia was significantly reduced by systemic injection of ifenprodil, an NMDA-receptor antagonist, but it was not influenced by either an AMPA-receptor blocker or a reactive oxygen species (ROS) scavenger. Furthermore, a dorsal column lesion did not interfere with the prolongation of hyperalgesia. Data suggest that vibration-induced prolongation of hyperalgesia is mediated by spinal NMDA-receptors, and a similar mechanism may underlie some forms of chronic pain with no obvious causes, such as complex regional pain syndrome type 1 (CRPS-1).

Involvement of mGluR5 in the ethanol-induced neuropathic pain-like state in the rat

Alcohol neuropathy has been thought to involve decreased nerve function following chronic ethanol consumption. However, there is no reliably successful therapy, largely due to a lack of understanding of the central underlying mechanisms. The aim of this study was to investigate the mechanisms that contribute to the neuropathic pain-like state induced by chronic ethanol treatment in rats. Rats were chronically treated with ethanol diet (1.25-5% of ethanol) for over 70 days. Mechanical hyperalgesia was observed during ethanol consumption and even after ethanol withdrawal. Under these conditions, an immunohistochemical study showed an increase in metabotropic glutamate receptor 5 (mGluR5) immunoreactivity in the superficial spinal dorsal horn of chronic ethanol-fed rats. Furthermore, immunoblot analysis revealed that the protein level of mGluR5 was clearly increased following chronic ethanol consumption. These findings support the idea that the increased levels of mGluR5 in the spinal cord may be, at least in part, involved in the induction of ethanol-dependent neuropathic pain-like state.

Glutamate receptors and pain

Pain is an important survival and protection mechanism for animals. However, chronic/persistent pain may be differentiated from normal physiological pain in that it confers no obvious advantage. An accumulating body of pharmacological, electrophysiological, and behavioral evidence is emerging in support of the notion that glutamate receptors play a crucial role in pain pathways and that modulation of glutamate receptors may have potential for therapeutic utility in several categories of persistent pain, including neuropathic pain resulting from injury and/or disease of central (e.g., spinal cord injury) or peripheral nerves (e.g., diabetic neuropathy, radiculopathy) and inflammatory or joint-related pain (e.g., rheumatoid arthritis, osteoarthritis). This review focuses on the role of glutamate receptors, including both ionotropic (AMPA, NMDA and kainate) and metabotropic (mGlu1-8) receptors in persistent pain states with particular emphasis on their expression patterns in nociceptive pathways and their potential as targets for pharmacological intervention strategies.

Metabotropic glutamate receptor subtype 5 antagonists MPEP and MTEP

Glutamate regulates the function of central nervous system (CNS), in part, through the cAMP and/or IP3/DAG second messenger-associated metabotropic glutamate receptors (mGluRs). The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has been extensively used to elucidate potential physiological and pathophysiological functions of mGluR5. Unfortunately, recent evidence indicates significant non-specific actions of MPEP, including inhibition of NMDA receptors. In contrast, in vivo and in vitro characterization of the newer mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) indicates that it is more highly selective for mGluR5 over mGluR1, has no effect on other mGluR subtypes, and has fewer off-target effects than MPEP. This article reviews literature on both of these mGluR5 antagonists, which suggests their possible utility in neurodegeneration, addiction, anxiety and pain management.

Central Hypersensitivity in Chronic Pain: Mechanisms and Clinical Implications

The available literature consistently shows increased pain sensitivity after sensory stimulation of healthy tissues in patients who have various chronic pain conditions. This indicates a state of hypersensitivity of the CNS that amplifies the nociceptive input arising from damaged tissues. Experimental data indicate that central hypersensitivity is probably induced primarily by nociceptive input arising from a diseased tissue. In patients, imbalance of descending modulatory systems connected with psychologic distress may play a role. There is experimental support in animal studies for the persistence of central hypersensitivity after complete resolution of tissue damage. This is particularly true for neuropathic pain conditions, whereby potentially irreversible plasticity changes of the CNS have been documented in animal studies. Whether such changes are present in musculoskeletal pain states is at present uncertain. Despite the likely importance of central hypersensitivity in the pathophysiology of chronic pain, this mechanism should not be used to justify the lack of understanding on the anatomic origin of the pain complaints in several pain syndromes, which is mostly due to limitations of the available diagnostic tools. Treatment strategies for central hypersensitivity in patients have been investigated mostly in neuropathic pain states. Possible therapy modalities for central hypersensitivity in chronic pain of musculoskeletal origin are largely unexplored. The limited evidence available and everyday practice show, at best, modest efficacy of the available treatment modalities for central hypersensitivity. The gap between basic knowledge and clinical benefits remains large and should stimulate further intensive research.