Metabotropic glutamate receptor involvement in models of acute and persistent pain: prospects for the development of novel analgesics

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.

Responses of algesic and metabolic substances to 8 h of repetitive manual work in myalgic human trapezius muscle

The trapezius muscle often develops pain as the result of repetitive and stressful work tasks although it is unclear to what extent this pain is due to alterations in muscle concentrations of algesic/nociceptive substances. Twenty women with chronic neck- and shoulder pain (TM) whose work required highly repetitive work tasks and 20 pain-free female colleagues (CON) were studied during and after a full 8-hour workday. We collected microdialysates from their dominant/most painful trapezius muscle; concentrations of serotonin, glutamate, lactate, pyruvate, potassium, bradykinin, and cytokines and blood flow were determined. In addition, we measured surface electromyogram, task exposure level, pain intensity, perceived mental stress, and urine-cortisol. In connection to the clinical neck and shoulder examination, we determined pressure pain thresholds (PPTs) over the trapezius and tibialis muscles. TM had higher concentrations of glutamate (71+/-42 vs. 36+/-15 micromol l(-1)) and pyruvate (187+/-89 vs. 125+/-63 micromol l(-1)) than CON. Interstitial serotonin was higher in TM (before work: 10.6+/-10.8 vs. 2.2+/-1.2 nM; after work: 9.2+/-8.3 vs. 1.5+/-2.9 nM). The trapezius blood flow during the working day was higher in TM than in CON. TM had lower PPT and higher pain intensity throughout the working day. No differences in EMG, task exposure level, mental stress, or urine-cortisol in the groups were found. These findings support the idea that peripheral nociceptive processes are activated in occupationally active subjects, who are diagnosed with trapezius myalgia. In contrast, no sign of low blood flow or increased stress or muscle activity markers were found in TM.

Group III mGluR7 and mGluR8 in the amygdala differentially modulate nocifensive and affective pain behaviors

The amygdala plays an important role in the emotional-affective component of pain and in pain modulation. Group III metabotropic glutamate receptors (mGluRs) regulate pain-related activity in the amygdala, but the behavioral consequence and contribution of individual subtypes are not known yet. This study determined the effects of mGluR7 and mGluR8 activation in the central nucleus of the amygdala (CeA) on nocifensive and affective pain responses and on pain-related anxiety-like behavior of adult rats. The pain state was induced by intraarticular injections of kaolin/carrageenan into one knee joint to produce a localized monoarthritis. Subtype-selective agonists were administered into the CeA by microdialysis in normal rats and in rats with arthritis. An mGluR7-selective agonist (N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride, AMN082, 25microM) decreased spinal withdrawal reflex thresholds and increased audible and ultrasonic vocalizations evoked by brief (15s) compression of the knee. AMN082 also decreased the open-arm preference in the elevated plus maze (EPM) test, suggesting anxiety-like behavior. In arthritic animals, however, AMN082 failed to modulate the increased spinal reflexes and vocalizations and anxiety-like behavior. An mGluR8-selective agonist (S-3,4-dicarboxyphenylglycine, S-3,4-DCPG, 10microM) had no effect in normal animals but inhibited the increased spinal reflex responses and audible and ultrasonic vocalizations of arthritic rats. S-3,4-DCPG also increased the open-arm choices of arthritic rats, suggesting anxiolytic effects. The results suggest that under normal conditions mGluR7, but not mGluR8, facilitates pain responses and has anxiogenic properties whereas mGluR8, but not mGluR7, can inhibit nocifensive and affective behaviors and anxiety in a model of arthritic pain.

Intracerebroventricular administration of N-acetylaspartylglutamate (NAAG) peptidase inhibitors is analgesic in inflammatory pain

Background

The peptide neurotransmitter N-Acetylaspartylglutamate (NAAG) is the third most prevalent transmitter in the mammalian central nervous system. Local, intrathecal and systemic administration of inhibitors of enzymes that inactivate NAAG decrease responses to inflammatory pain in rat models. Consistent with NAAG's activation of group II metabotropic glutamate receptors, this analgesia is blocked by a group II antagonist.

Results

This research aimed at determining if analgesia obtained following systemic administration of NAAG peptidase inhibitors is due to NAAG activation of group II mGluRs in brain circuits that mediate perception of inflammatory pain. NAAG and NAAG peptidase inhibitors, ZJ43 and 2-PMPA, were microinjected into a lateral ventricle prior to injection of formalin in the rat footpad. Each treatment reduced the early and late phases of the formalin-induced inflammatory pain response in a dose-dependent manner. The group II mGluR antagonist reversed these analgesic effects consistent with the conclusion that analgesia was mediated by increasing NAAG levels and the peptide's activation of group II receptors.

Conclusion

These data contribute to proof of the concept that NAAG peptidase inhibition is a novel therapeutic approach to inflammatory pain and that these inhibitors achieve analgesia by elevating synaptic levels of NAAG within pain processing circuits in brain.

Allosteric potentiators of metabotropic glutamate receptor subtype 1a differentially modulate independent signaling pathways in baby hamster kidney cells

Recent studies suggest that subtype specific activators of metabotropic glutamate receptors (mGluRs) have exciting potential for the development of novel treatment strategies for numerous psychiatric and neurological disorders. A number of positive allosteric modulators (PAMs) have been identified that are highly selective for mGluR1, including the compounds Ro 01-6128, Ro 67-4853, and Ro 67-7476. These PAMs have been previously found to interact with a site distinct from that of negative allosteric modulators (NAMs), typified by R214127. These mGluR1 PAMs do not have an effect on baseline calcium levels but induce leftward shifts in the concentration-response of mGluR1 to agonists. However, their effects on a variety of signaling pathways and their mechanism of action have not been fully explored and are of critical importance for further development of mGluR1 allosteric modulators as novel drugs. In baby hamster kidney (BHK) cells, mGluR1 activates calcium mobilization, cAMP production, and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation; signaling cascades which are distinct and differentially regulated. In contrast to their effects on calcium mobilization, these compounds were found to activate ERK1/2 phosphorylation in the absence of exogenously added agonist, an effect that was fully blocked by both orthosteric (LY341495) and allosteric (R214127) mGluR1 antagonists. The mGluR1 PAMs were also found to activate cAMP production in the absence of agonist. Thus, these mGluR1 PAMs have qualitatively different effects on a variety of mGluR1-mediated signal transduction cascades. Together, these data provide further evidence that allosteric compounds can differentially modulate the coupling of a single receptor to independent signaling pathways or act in a system-dependent manner.

Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain

Recent studies have indicated that metabotropic glutamate receptors mGluR5, mGluR2/3 and mGluR7 are present in the regions of central nervous system important for nociceptive transmission, but their involvement in neuropathic pain has not been well established. We demonstrated that acute and chronic administration of MPEP (mGluR5 antagonist), LY379268 (mGluR2/3 agonist), and AMN082 (mGluR7 agonist) attenuated allodynia (von Frey test) and hyperalgesia (cold plate test) as measured in Swiss albino mice on day seven after chronic constriction injury (CCI) to the sciatic nerve. Moreover, single administration of MPEP (30 mg/kg; i.p.) or LY379268 (10mg/kg; i.p.) injected 30 min before morphine potentiated morphine's effects (20mg/kg; i.p.) in the mouse CCI model, as measured by both the tests mentioned above. However, a single administration of AMN082 (3mg/kg; i.p.) potentiated the effects of a single morphine injection (20mg/kg; i.p.) in the von Frey test only. Chronic administration (7 days) of low doses of MPEP, LY379268 or AMN082 (all drugs at 3mg/kg; i.p.) potentiated the effects of single doses of morphine (3, 10, and 20mg/kg; i.p.) administered on day seven; however, AMN082 only potentiated the effect in the cold plate test. Additionally, the same doses of MPEP and LY379268 (but not AMN082) chronically co-administered with morphine (40 mg/kg; i.p.) attenuated the development of morphine tolerance in CCI-exposed mice. Our data suggest that mGluR5, mGluR2/3, and mGluR7 are involved in injury-induced plastic changes in nociceptive pathways and that the mGluR5 and mGluR2/3 ligands enhanced morphine's effectiveness in neuropathy, which could have therapeutic implications.

Transposition of three amino acids transforms the human metabotropic glutamate receptor (mGluR)-3-positive allosteric modulation site to mGluR2, and additional characterization of the mGluR2-positive allosteric modulation site

Glutamate is a major neurotransmitter in the central nervous system, and abnormal glutamate neurotransmission has been implicated in many neurological disorders, including schizophrenia, Alzheimer's disease, Parkinson's disease, addiction, anxiety, depression, epilepsy, and pain. Metabotropic glutamate receptors (mGluRs) activate intracellular signaling cascades in a G protein-dependent manner, which offer the opportunity for developing drugs that regulate glutamate neurotransmission in a functionally selective manner. In the present study, we further characterize the human mGluR2 (hmGluR2) potentiator binding site by showing that the substitution of the three amino acids found to be required for hmGluR2 potentiation, specifically Ser(688), Gly(689), and Asn(735), with the homologous hmGluR3 amino acids, inactivates the positive allosteric modulator activity of several structurally unique mGluR2 potentiators. Based on the characterization of the hmGluR2 potentiator binding site, we developed a novel scintillation proximity assay that was able to discriminate between compounds that were hmGluR2-specific potentiators, and those that were active on both hmGluR2 and hmGluR3. In addition, we substituted Ser(688), Gly(689), and Asn(735) into hmGluR3 and created an active hmGluR2 allosteric modulation site on the hmGluR3 receptor.

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.

Pharmacology of Glutamate Transport in the CNS: Substrates and Inhibitors of Excitatory Amino Acid Transporters (EAATs) and the Glutamate/Cystine Exchanger System x c −

As the primary excitatory neurotransmitter in the mammalian CNS, l-glutamateparticipates not only in standard fast synaptic communication, but also contributes to higher order signalprocessing, as well as neuropathology. Given this variety of functional roles, interest has been growingas to how the extracellular concentrations of l-glutamate surroundingneurons are regulated by cellular transporter proteins. This review focuses on two prominent systems, eachof which appears capable of influencing both the signaling and pathological actions of l-glutamatewithin the CNS: the sodium-dependent excitatory amino acid transporters (EAATs) and the glutamate/cystineexchanger, system x_c⁻(Sx_c⁻). Whilethe family of EAAT subtypes limit access to glutamate receptors by rapidly and efficiently sequesteringl-glutamate in neurons and glia, Sx_c⁻provides a route for the export of glutamate from cells into the extracellular environment. The primaryintent of this work is to provide an overview of the inhibitors and substrates that have been developedto delineate the pharmacological specificity of these transport systems, as well as be exploited as probeswith which to selectively investigate function. Particular attention is paid to the development of smallmolecule templates that mimic the structural properties of the endogenous substrates, l-glutamate,l-aspartate and l-cystine andhow strategic control of functional group position and/or the introduction of lipophilic R-groups can impactmultiple aspects of the transport process, including: subtype selectivity, inhibitory potency, and substrateactivity.

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.

Sensitization, glutamate, and the link between migraine and fibromyalgia

Recent advances have shed insight on the pathophysiologic mechanisms of fibromyalgia and migraine, especially in the chronic form. A growing body of evidence supports the involvement of peripheral and central sensitization disturbances of pain-related processes underlying both disorders. They involve increased glutamate transmission through interaction with its ionotropic and metabotropic receptors. Few studies supporting the implication of this excitatory amino acid in chronic migraine and primary fibromyalgia demonstrated increased levels of glutamate in the cerebrospinal fluid of affected patients. These findings have implications for future therapies directed against glutamate receptors (in particular, N-methyl-D-aspartate receptors). Limited clinical experience in this regard, although promising, does not exclude additional mechanisms contributing to the maintenance of pain, which can be the target of therapeutic approaches in both disorders.

Metabotropic glutamate receptors: beyond the regulation of synaptic transmission

Metabotropic glutamate (mGlu) receptors are G-protein coupled receptors activated by glutamate, the major excitatory neurotransmitter of the CNS. A growing body of evidence suggests that the function of mGlu receptors is not restricted to the regulation of synaptic transmission. mGlu receptors are expressed in a variety of peripheral cells, including inter alia hepatocytes, pancreatic cells, osteoblasts and immune cells. Within the immunological synapses, mGlu receptors expressed by T cells might contribute to the vast array of signals generated by the antigen-presenting cells. mGlu receptors are also found in embryonic and neural stem cells. This suggests their involvement in the pathophysiology of brain tumors, which likely originates from cancer stem cells similar to neural stem cells. Ligands of mGlu3 and mGlu4 receptors are potential candidates for the experimental treatment of malignant gliomas and medulloblastomas, respectively.

Suppression of spike-wave discharge activity and c-fos expression by 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine (Q5) in vivo

Antiepileptic and network inhibitory actions of Q5 (2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine) have recently been described in hippocampal slices. Here we present evidence on the in vivo antiabsence effect of Q5. All doses of Q5 tested (0.3 mg/kg, 0.9 mg/kg, 2.8 mg/kg) decreased the number, but not the duration and the frequency of absence spike-wave discharges (SWDs) in freely moving WAG/Rij rats. In vivo network inhibitory action of Q5 was monitored by following c-fos expression in different brain areas of Wistar rats. Significant depletion of c-fos expression was observed after single or repeated injections of Q5 (2.8 mg/kg and 2x2.8 mg/kg) in various brain areas, including hypothalamic paraventricular nucleus, medial amygdaloid nucleus, piriform cortex, somatosensory cortex, periventricular thalamic nucleus and periaqueductal central gray. Thus, our in vivo results demonstrate that in addition to the prevention of absence seizures, Q5 effectively suppresses neuronal activation in various stress- and pain-sensitive brain areas.

Metabotropic glutamate receptors in stem/progenitor cells

Functional mGlu receptor subtypes are found in stem/progenitor cells, and regulate proliferation, differentiation, and survival of these cells. Activation of mGlu5 receptors supports self-renewal of embryonic stem cells, which are pluripotent cells isolated from the blastocyst capable of generating all the body's cell lineages, including germ cells. Differentiation of embryonic stem cells into embryoid bodies is associated with the induction of mGlu4 receptors, the activation of which drives cell differentiation towards the mesoderm and endoderm lineages. Different mGlu receptor subtypes, mGlu3 and mGlu5 receptors in particular, are found in neural stem cells (stem cells resident in the CNS that give rise to neurons, astrocytes or oligodendrocytes) isolated from the developing brain or from regions of persistent neurogenesis of the adult brain (e.g. the subventricular zone lining the wall of the lateral ventricles). The evidence that activation of mGlu3 and mGlu5 receptors stimulates proliferation of these cells is particularly interesting because of the similarities between neural stem cells and putative cancer stem cells that support the growth of malignant gliomas. A link among mGlu receptors, stem cells and cancer is supported by the finding that mGlu4 receptors are expressed by cerebellar granule cell neuroprogenitors, which are the putative cells of origin of medulloblastomas. The study of mGlu receptors in stem/progenitor cells has potential applications in the optimisation of protocols of cell expansion and differentiation aimed at cell replacement strategies, and may gain new insights into the pathophysiology of neurodevelopmental disorders and brain tumours.

Metabotropic glutamate receptors: new targets for the control of tumor growth?

Cancer stem cells are currently a target for the treatment of malignant tumors. Transformed neural stem-progenitor cells of the brain subventricular zone and the external granular layer of the cerebellum are the putative cells of origin of malignant gliomas and medulloblastomas, which are the most frequent malignant brain tumors in adults and children, respectively. The proliferation of neural stem-progenitor cells is regulated by metabotropic glutamate (mGlu) receptors, which are G-protein-coupled receptors that are activated by glutamate, the major excitatory neurotransmitter of the CNS. At least two receptor subtypes - mGlu(3) and mGlu(4) receptors - control the proliferation of brain tumor cells, whereas mGlu(1) receptors have been implicated in the development of melanomas. We believe that individual mGlu receptor subtypes represent new potential targets for the treatment of several malignant tumors, including brain tumors.

Rational design, synthesis, and structure–activity relationship of benzoxazolones: New potent mglu5 receptor antagonists based on the fenobam structure

A novel class of potent and stable mGlu5 receptor antagonists was developed by combining information from a high-throughput screening campaign with the structure of the known anxiolytic fenobam. Representative compounds from this class show favorable pharmacokinetic properties and are active in an in vivo model of anxiety.

Interaction of novel positive allosteric modulators of metabotropic glutamate receptor 5 with the negative allosteric antagonist site is required for potentiation of receptor responses

Exciting advances have been made in the discovery of selective positive allosteric modulators of the metabotropic glutamate receptor (mGluR) mGluR5. These compounds may provide a novel approach that could be useful in the treatment of certain central nervous system disorders. However, because of their low potencies, previously described mGluR5 potentiators are not useful for functional studies in native preparations. In addition, binding sites at which these compounds act have not been identified. It has been suggested that two allosteric potentiators, 3,3'-difluorobenzaldazine and 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB), act by binding to the same allosteric site as the negative allosteric modulators of mGluR5 such as 2-methyl-6-(phenylethynyl)pyridine (MPEP). However, another mGluR5 potentiator, N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)m-ethyl]phenyl}-2-hydroxybenzamide, does not bind to this site, bringing this hypothesis into question. We have synthesized a series of CDPPB analogs and report that these compounds bind to the MPEP site with affinities that are closely related to their potencies as mGluR5 potentiators. Furthermore, allosteric potentiation is antagonized by a neutral ligand at the MPEP site and reduced by a mutation of mGluR5 that eliminates MPEP binding. Together, these data suggest that interaction with the MPEP site is important for allosteric potentiation of mGluR5 by CDPPB and related compounds. In addition, whole-cell patch-clamp studies in midbrain slices reveal that a highly potent analog of CDPPB, 4-nitro-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (VU-29), selectively potentiates mGluR5 but not mGluR1-mediated responses in midbrain neurons, whereas a previously identified allosteric potentiator of mGluR1 has the opposite effect.

Local administration of N-acetylaspartylglutamate (NAAG) peptidase inhibitors is analgesic in peripheral pain in rats

The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) selectively activates group II metabotropic glutamate receptors (mGluRs). Systemic administration of inhibitors of the enzymes that inactivate NAAG results in decreased pain responses in rat models of inflammatory and neuropathic pain. These effects are blocked by a group II mGluR antagonist. This research tested the hypothesis that some analgesic effects of NAAG peptidase inhibition are mediated by NAAG acting on sensory neurite mGluRs at the site of inflammation. Group II mGluR agonists, SLx-3095-1, NAAG and APDC, or NAAG peptidase inhibitors, ZJ-43 and 2-PMPA, injected into the rat footpad reduced pain responses in carrageenan or formalin models. The analgesic effects of SLx-3095-1, APDC, ZJ-43, 2-PMPA and NAAG were blocked by co-injection of LY341495, a selective group II mGluR antagonist. Injection of group II mGluR agonists, NAAG or the peptidase inhibitors into the contralateral rat footpad had no effect on pain perception in the injected paw. At 10-100 microm ZJ-43 and 2-PMPA demonstrated no consistent agonist activity at mGluR2 or mGluR3. Consistent with the conclusion that peripherally administered NAAG peptidase inhibitors increase the activation of mGluR3 by NAAG that is released from peripheral sensory neurites, we found that the tissue average concentration of NAAG in the unstimulated rat hind paw was about 6 microm. These data extend our understanding of the role of this peptide in sensory neurons and reveal the potential for treatment of inflammatory pain via local application of NAAG peptidase inhibitors at doses that may have little or no central nervous system effects.

Group I metabotropic glutamate receptor-induced Ca(2+)-gradients in rat superficial spinal dorsal horn neurons

Here, we investigated changes in the free cytosolic Ca(2+) concentration ([Ca(2+)](i)), induced by the pharmacological activation of metabotropic glutamate receptors (mGluRs), in nociceptive neurons of the superficial spinal dorsal horn. Microfluorometric Ca(2+) measurements with fura-2 in a lumbar spinal cord slice preparation from young rats were used. Bath application of the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) resulted in a distinct increase of [Ca(2+)](i) in most of the neurons in superficial dorsal horn. In contrast, activation of groups II or III mGluRs by DCG-IV or l-AP4, respectively, failed to evoke any significant change in [Ca(2+)](i). The effect of (S)-3,5-DHPG was mediated by both group I subtypes mGluR1 and mGluR5, since combined pre-treatment with the subtype antagonists (S)-4-CPG and MPEP was necessary to abolish the [Ca(2+)](i) increase. Depleting intracellular Ca(2+) stores with CPA or inhibiting IP(3)-receptors with 2-APB, respectively, reduced the (S)-3,5-DHPG-evoked [Ca(2+)](i) increase significantly. Inhibition of voltage-dependent L-type Ca(2+) channels (VDCCs) by verapamil or nicardipine reduced the (S)-3,5-DHPG-induced [Ca(2+)](i) rise likewise. Thus, in rat spinal cord, (S)-3,5-DHPG enhances Ca(2+) signalling in superficial dorsal horn neurons, mediated by the release of Ca(2+) from IP(3)-sensitive intracellular stores and by an influx through L-type VDCCs. This may be relevant to the processing of nociceptive information in the spinal cord.

Blockade of mGluR1 receptor results in analgesia and disruption of motor and cognitive performances: effects of A-841720, a novel non-competitive mGluR1 receptor antagonist

BACKGROUND AND PURPOSE

To further assess the clinical potential of the blockade of metabotropic glutamate receptors (mGluR1) for the treatment of pain.

EXPERIMENTAL APPROACH

We characterized the effects of A-841720, a novel, potent and non-competitive mGluR1 antagonist in models of pain and of motor and cognitive function.

KEY RESULTS

At recombinant human and native rat mGluR1 receptors, A-841720 inhibited agonist-induced calcium mobilization, with IC50 values of 10.7+/-3.9 and 1.0 +/- 0.2 nM, respectively, while showing selectivity over other mGluR receptors, in addition to other neurotransmitter receptors, ion channels, and transporters. Intraperitoneal injection of A-841720 potently reduced complete Freund's adjuvant-induced inflammatory pain (ED50 = 23 micromol kg(-1)) and monoiodoacetate-induced joint pain (ED50 = 43 micromol kg(-1)). A-841720 also decreased mechanical allodynia observed in both the sciatic nerve chronic constriction injury and L5-L6 spinal nerve ligation (SNL) models of neuropathic pain (ED50 = 28 and 27 micromol kg(-1), respectively). Electrophysiological studies demonstrated that systemic administration of A-841720 in SNL animals significantly reduced evoked firing in spinal wide dynamic range neurons. Significant motor side effects were observed at analgesic doses and A-841720 also impaired cognitive function in the Y-maze and the Water Maze tests.

CONCLUSIONS AND IMPLICATIONS

The analgesic effects of a selective mGluR1 receptor antagonist are associated with motor and cognitive side effects. The lack of separation between efficacy and side effects in pre-clinical models indicates that mGluR1 antagonism may not provide an adequate therapeutic window for the development of such antagonists as novel analgesic agents in humans.

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.

A new series of pyridinyl-alkynes as antagonists of the metabotropic glutamate receptor 5 (mGluR5)

Synthesis and some structure-activity relationships for a new series of propargyl ethers as mGluR5 antagonists are reported.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Enhanced group II mGluR-mediated inhibition of pain-related synaptic plasticity in the amygdala

Background

The latero-capsular part of the central nucleus of the amygdala (CeLC) is the target of the spino-parabrachio-amygdaloid pain pathway. Our previous studies showed that CeLC neurons develop synaptic plasticity and increased neuronal excitability in the kaolin/carrageenan model of arthritic pain. These pain-related changes involve presynaptic group I metabotropic glutamate receptors (mGluRs) and postsynaptic NMDA and calcitonin gene-related peptide (CGRP1) receptors. Here we address the role of group II mGluRs.

Results

Whole-cell current- and voltage-clamp recordings were made from CeLC neurons in brain slices from control rats and arthritic rats (>6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents from the pontine parabrachial (PB) area. A selective group II mGluR agonist (LY354740) decreased the amplitude of EPSCs more potently in CeLC neurons from arthritic rats (IC₅₀ = 0.59 nM) than in control animals (IC₅₀ = 15.0 nM). The inhibitory effect of LY354740 was reversed by a group II mGluR antagonist (EGLU) but not a GABA_A receptor antagonist (bicuculline). LY354740 decreased frequency, but not amplitude, of miniature EPSCs in the presence of TTX. No significant changes of neuronal excitability measures (membrane slope conductance and action potential firing rate) were detected.

Conclusion

Our data suggest that group II mGluRs act presynaptically to modulate synaptic plasticity in the amygdala in a model of arthritic pain.

Interactions between ephrin-B and metabotropic glutamate 1 receptors in brain tissue and cultured neurons

We examined the interaction between ephrins and metabotropic glutamate (mGlu) receptors in the developing brain and cultured neurons. EphrinB2 coimmunoprecipitated with mGlu1a receptors, in all of the brain regions examined, and with mGlu5 receptors in the corpus striatum. In striatal slices, activation of ephrinB2 by a clustered form of its target receptor, EphB1, amplified the mGlu receptor-mediated stimulation of polyphosphoinositide (PI) hydrolysis. This effect was abolished in slices treated with mGlu1 or NMDA receptor antagonists but was not affected by pharmacological blockade of mGlu5 receptors. An interaction among ephrinB2, mGlu1 receptor, and NMDA was supported by the following observations: (1) the NR1 subunit of NMDA receptors coimmunoprecipitated with mGlu1a receptors and ephrinB2 in striatal lysates; (2) clustered EphB1 amplified excitatory amino acid-stimulated PI hydrolysis in cultured granule cells grown under conditions that favored the expression of mGlu1a receptors; and (3) clustered EphB1 amplified the enhancing effect of mGlu receptor agonists on NMDA toxicity in cortical cultures, and its action was sensitive to mGlu1 receptor antagonists. Finally, fluorescence resonance energy transfer and coclustering analysis in human embryonic kidney 293 cells excluded a physical interaction between ephrinB2 and mGlu1a (or mGlu5 receptors). A functional interaction between ephrinB and mGlu1 receptors, which likely involves adaptor or scaffolding proteins, might have an important role in the regulation of developmental plasticity.

Phenyl ureas of creatinine as mGluR5 antagonists. A structure–activity relationship study of fenobam analogues

Fenobam (1) was developed by McNeil Laboratories as an anxiolytic agent with an unknown molecular target in the late 1970s. In a recent publication, it was revealed that fenobam is a non-competitive mGluR5 antagonist. Herein, we present the structure-activity relationship of fenobam and its analogues and similarities between the SAR of mGluR5 antagonism and the SAR of CNS properties originally reported by McNeil are discussed.

Structure-activity relationship of thiopyrimidines as mGluR5 antagonists

Structure-activity relationship investigations of the thiopyrimidine (1), an HTS hit with micromolar activity as a metabotropic glutamate receptor 5 (mGluR5) antagonist, led to compounds with sub-micromolar activity.

Alternative splicing of human metabotropic glutamate receptor 3

The metabotropic glutamate receptor 3 (GRM3, mGluR3) is important in regulating synaptic glutamate. Here, we report the existence of three splice variants of GRM3 in human brain arising from exon skipping events. The transcripts are expressed in prefrontal cortex, hippocampus and cerebellum, and in B lymphoblasts. We found no evidence for alternative splicing of GRM2. The most abundant GRM3 variant lacks exon 4 (GRM3Delta4). In silico translation analysis of GRM3Delta4 predicts a truncated protein with a conserved extracellular ligand binding domain, absence of a seven-transmembrane domain, and a unique 96-amino acid C-terminus. When expressed in rat hippocampal neurons, GRM3Delta4 is translated into a 60 kDa protein. Immunostaining and cell fractionation data indicate that the truncated protein is primarily membrane-associated. An antibody developed against the GRM3Delta4 C-terminus detects a protein of approximately 60 kDa in human brain lysates and in B lymphoblasts, suggesting translation of GRM3Delta4 in vivo. The existence of the GRM3Delta4 isoform is relevant in the light of the reported association of non-coding single nucleotide polymorphisms (SNPs) in GRM3 with schizophrenia, and with the potential of GRM3 as a therapeutic target for several neuropsychiatric disorders.

Analgesic effects of the selective group II (mGlu2/3) metabotropic glutamate receptor agonists LY379268 and LY389795 in persistent and inflammatory pain models after acute and repeated dosing

Group II (mGluR2/3) metabotropic glutamate receptors have been implicated in the mechanisms of persistent pain states. In the present study, the effects of the selective group II metabotropic glutamate receptor agonists LY379268 and LY389795 were evaluated in the formalin test, carrageenan-induced thermal hyperalgesia and mechanical allodynia, and capsaicin-induced mechanical allodynia in rats. The agonists LY379268 and LY389795 produced dose-dependent decreases in formalin-induced behaviors that were antagonized by the mGlu2/3 receptor antagonist LY341495. The group II antagonist LY341495 produced parallel shifts in the LY379268 dose-response curve, consistent with a competitive antagonism. LY379268 decreased formalin-induced behaviors after intracisternal but not intrathecal administration, suggesting primarily a supraspinal site of action. Both LY379268 and LY389795 produced a dose-related reversal of carrageenan-induced thermal hyperalgesia and capsaicin-induced mechanical allodynia, but had no effect on carrageenan-induced mechanical allodynia. Both agonists also increased response latencies in the hot plate test, but were without effect in the tail-flick test. However, both agonists produced motor impairment on the inverted screen at doses that were analgesic. Moreover, tolerance to the analgesic effects of LY379268 developed after 4 days of once-daily repeated administration in the formalin, carrageenan, capsaicin and hot plate tests. The present findings indicate that group II (mGluR2/3) metabotropic glutamate receptors may be involved in the mechanisms of hyperalgesia and allodynia, however tolerance rapidly develops to these effects.

NAAG peptidase inhibitors and their potential for diagnosis and therapy

Modulation of N-acetyl-L-aspartyl-L-glutamate peptidase activity with small-molecule inhibitors holds promise for a wide variety of diseases that involve glutamatergic transmission, and has implications for the diagnosis and therapy of cancer. This new class of compounds, of which at least one has entered clinical trials and proven to be well tolerated, has demonstrated efficacy in experimental models of pain, schizophrenia, amyotrophic lateral sclerosis, traumatic brain injury and, when appropriately functionalized, can image prostate cancer. Further investigation of these promising drug candidates will be needed to bring them to the marketplace. The recent publication of the X-ray crystal structure for the enzymatic target of these compounds should facilitate the development of other new agents with enhanced activity that could improve both the diagnosis and treatment of neurological disorders.

Antinociceptive effects and synergistic interaction with morphine of intrathecal metabotropic glutamate receptor 2/3 antagonist in the formalin test of rats

Spinal metabotropic glutamate receptors (mGluRs) have been known to be involved in the modulation of nociception. While the antinociceptive effects of the mGluR1/5 have been demonstrated, the role of mGluR2/3 for nociception is less clear. This study investigated the effects of an intrathecal mGluR2/3 agonist, APDC, and a mGluR2/3 antagonist, LY341495, for inflammatory and acute pain in the formalin test and thermal stimulation test. We also examined their interaction with intrathecal morphine for the antinociceptive effect. APDC had little effect on the formalin-induced nociception. In contrast, LY341495 caused a dose-dependent suppression of the phase 2 flinching response to the formalin stimulus without affecting phase 1 flinching response. Furthermore, the suppression of pain behavior by LY341495 during phase 2 was reduced significantly by pretreatment with APDC. LY341495 and morphine also showed synergistic drug interaction for antinociception during phase 2 in the formalin test.

3-[Substituted]-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitriles: Identification of highly potent and selective metabotropic glutamate subtype 5 receptor antagonists

Structure-activity relationship studies on the phenyl ring of 3-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile 2 led to the discovery that small, non-hydrogen bond donor substituents at the 3-position led to a substantial increase in in vitro potency. In particular, 3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile (7) is a highly potent and selective mGlu5 receptor antagonist with good rat pharmacokinetics, brain penetration, and in vivo receptor occupancy.

Cyclohexenyl- and dehydropiperidinyl-alkynyl pyridines as potent metabotropic glutamate subtype 5 (mGlu5) receptor antagonists

Structure-activity relationship studies leading to the discovery of novel mGlu5 receptor antagonists are described. These compounds show high in vitro potency, have good in vivo receptor occupancy, and a reasonable intravenous pharmacokinetic profile.

Dipyridyl amines: Potent metabotropic glutamate subtype 5 receptor antagonists

Modulation of the metabotropic glutamate subtype 5 (mGlu5) receptor may be useful in the treatment of a variety of central nervous system disorders. Herein, we report on the discovery, synthesis, and biological evaluation of dipyridyl amines as small molecule mGlu5 antagonists.

A close structural analog of 2-methyl-6-(phenylethynyl)-pyridine acts as a neutral allosteric site ligand on metabotropic glutamate receptor subtype 5 and blocks the effects of multiple allosteric modulators

The metabotropic glutamate receptor subtype 5 (mGlu5) activates calcium mobilization via binding of glutamate, the major excitatory neurotransmitter in the central nervous system. Allosteric modulation of the receptor has recently emerged as a promising alternative method of regulation to traditional regulation through orthosteric ligands. We now report three novel compounds that bind to the allosteric 2-methyl-6-(phenylethynyl)-pyridine (MPEP) site on mGlu5 but have only partial inhibition or no functional effects on the mGlu5 response. Two of these compounds, 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) and 2-(2-(5-bromopyridin-3-yl)ethynyl)-5-methylpyridine (Br-5MPEPy), act as partial antagonists of mGlu5 in that they only partially inhibit the response of this receptor to glutamate. The third compound, 5-methyl-6-(phenylethynyl)-pyridine (5MPEP), acts as a neutral allosteric site ligand that binds to the MPEP site and has no effects alone. However, 5MPEP blocks the effects of both the allosteric antagonist MPEP and potentiators 3,3'-difluorobenzaldazine and 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB). This compound also blocks depolarization effects of both MPEP and CDPPB in neurons in the subthalamic nucleus. These novel compounds provide valuable new insight into the pharmacology of allosteric sites on G protein-coupled receptors and provide valuable new tools for determining the effects of allosteric site ligands in native systems.

Activation of the ERK1/2 signaling cascade by excitotoxic spinal cord injury

The role of the ERK1/2 signal transduction pathway and related transcription factors in the regulation of gene expression and pain behavior following excitotoxic spinal cord injury (SCI) was examined. Specifically, phosphorylation of ERK1/2, activation of transcription factors NF-kB, ELK-1, and CREB, and gene expression of the neurokinin-1 receptor and NMDA receptor subunits NR1 and NR-2A were investigated. Excitotoxic injury was produced by intraspinal injection of quisqualic acid (QUIS) in male Sprague-Dawley rats. Western blots were used to evaluate phosphorylation and activation of ERK1/2 and transcription factors using phospho-specific or total antibodies. Real-time PCR was used to evaluate gene expression of NK-1R, NR-1, and NR-2A. Assessment of excessive grooming behavior was used to evaluate the presence of spontaneous pain behavior. Excitotoxic spinal injury resulted in: (1) increased phosphorylation of ERK1/2; (2) increased activation of NF-kB and phosphorylation of ELK-1; and (3) increased gene expression for the NK-1 receptor and NR1 and NR-2A subunits of the NMDA receptor. Blockade of the ERK cascade with the MEK inhibitor PD98059 inhibited phosphorylation of ELK-1, activation of NF-kB and gene expression of NR1, NR-2A and NK-1R, and prevented the development of excessive grooming behavior. The results have shown that excitotoxic spinal injury leads to the injury-induced activation of the ERK-->ELK-1 and NF-kB signaling cascades and transcriptional regulation of receptors important in the development of chronic pain. Blockade of this intracellular kinase cascade prevented the onset of injury-induced pain behavior.

Synthesis, structure-activity relationship, and receptor pharmacology of a new series of quinoline derivatives acting as selective, noncompetitive mGlu1 antagonists

We describe the discovery and the structure-activity relationship of a new series of quinoline derivatives acting as selective and highly potent noncompetitive mGlu1 antagonists. We first identified cis-10 as a fairly potent mGlu1 antagonist (IC(50) = 20 nM) in a cell-based signal transduction assay on the rat mGlu1 receptor expressed in CHO-K1 cells, and then we were able to design and synthesize highly potent compounds on both rat and human mGlu1 receptors as exemplified by compound cis-64a, which has an antagonist potency of 0.5 nM for the human mGlu1 receptor. We briefly present and discuss the in vitro metabolic stability of the compounds in human liver microsomes. We finally report the pharmacokinetic properties of our lead compound cis-64a.

cAMP-mediated mechanisms for pain sensitization during opioid withdrawal

Chronic opioid-induced drug dependence and withdrawal syndrome after opioid cessation remain a severe obstacle in clinical treatment of chronic pain and opioid drug addiction. One of the key symptoms during opioid withdrawal is a state of sensitized pain. The most significant molecular adaptation induced by chronic opioids in the brain is upregulation of the cAMP-signaling pathway. Although the cAMP system is known to have multiple effects on central neuron functions, how its upregulation mediates behavioral opioid dependence and withdrawal-induced pain in vivo remains unclear. In this study, we demonstrate that withdrawal from chronic morphine significantly upregulates the mRNA level of adenylyl cyclase (AC) VI and VIII isoforms and immunoreactivity of ACV/VI in the nucleus raphe magnus (NRM), a brainstem site critically involved in opioid modulation of pain. In cellular studies of NRM neurons containing mu-opioid receptors, we show that morphine withdrawal significantly increases glutamate synaptic transmission via a presynaptic mechanism mediated by an upregulated cAMP pathway. Morphine withdrawal also enhances the hyperpolarization-activated current in these neurons by increased intracellular cAMP. Both of the withdrawal-induced cAMP actions increase the excitability of these mu-receptor-containing neurons, which are thought to facilitate spinal pain transmission. Furthermore, in morphine-dependent rats in vivo, blocking the cAMP pathway significantly reduces withdrawal-induced pain sensitization. These results illustrate neurobiological mechanisms for the cAMP-mediated withdrawal pain and provide potential therapeutic targets for the treatment of opioid dependence and withdrawal-related problems.

5-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]-2,3'-bipyridine: a highly potent, orally active metabotropic glutamate subtype 5 (mGlu5) receptor antagonist with anxiolytic activity

Structure-activity relationship studies leading to the discovery of a new, orally active mGlu5 receptor antagonist are described. The title compound, 5-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-2,3'-bipyridine, is highly potent in vitro, has good in vivo receptor occupancy, and is efficacious in the rat fear-potentiated startle model of anxiety following oral dosing.

Dipyridyl amides: potent metabotropic glutamate subtype 5 (mGlu5) receptor antagonists

The mGlu5 receptor has been implicated in a number of CNS disorders. Herein, we report on the discovery, synthesis, and biological evaluation of dipyridyl amides as small molecules mGluR5 antagonists.

Synthesis, characterization, and first successful monkey imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET radiotracers

Three metabotropic glutamate receptor subtype 5 (mGluR5) PET tracers have been labeled with either carbon-11 or fluorine-18 and their in vitro and in vivo behavior in rhesus monkey has been characterized. Each of these tracers share the common features of high affinity for mGluR5 (0.08-0.23 nM vs. rat mGluR5) and moderate lipophilicity (log P 2.8-3.4). Compound 1b was synthesized using a Suzuki or Stille coupling reaction with [11C]MeI. Compounds 2b and 3b were synthesized by a SNAr reaction using a 3-chlorobenzonitrile precursor. Autoradiographic studies in rhesus monkey brain slices using 2b and 3b showed specific binding in cortex, caudate, putamen, amygdala, hippocampus, most thalamic nuclei, and lower binding in the cerebellum. PET imaging studies in monkey showed that all three tracers readily enter the brain and provide an mGluR5-specific signal in all gray matter regions, including the cerebellum. The specific signal observed in the cerebellum was confirmed by the autoradiographic studies and saturation binding experiments that showed tracer binding in the cerebellum of rhesus monkeys. In vitro metabolism studies using the unlabeled compounds showed that 1a, 2a, and 3a are metabolized slower by human liver microsomes than by monkey liver microsomes. In vivo metabolism studies showed 3b to be long-lived in rhesus plasma with only one other more polar metabolite observed.

Positive and negative allosteric modulation of metabotropic glutamate receptors: emerging therapeutic potential

Metabotropic glutamate receptors (mGluRs) modulate neuronal activity in the central and peripheral nervous systems, and since their discovery have attracted considerable attention as putative therapeutic targets for a range of neurological and psychiatric disorders. A number of competitive agonists and antagonists acting at the N-terminal glutamate binding site have been identified, the majority of which are conformationally constrained or substituted amino acid analogues. These ligands have greatly facilitated investigation of the physiological and pathological roles of the receptor family. However, their utility and therapeutic potential has been restricted by relatively poor bioavailability and central nervous system (CNS) penetration, as well as limited chemical tractability and, generally, a lack of selectivity for individual mGluRs. Recently, a number of non-competitive mGluR ligands have been identified which bind within the receptor transmembrane heptahelical domain. These include both positive and negative allosteric modulators. Positive allosteric modulators do not exhibit intrinsic agonism but facilitate agonist-mediated receptor activity. Negative allosteric modulators include both non-competitive antagonists and inverse agonists. Allosteric modulation offers the potential for improved selectivity, particularly for individual receptors within the mGluR family, and enhanced chemical tractability relative to competitive agonists/antagonists. In addition, positive allosteric modulation provides a distinct, and perhaps superior, profile to receptor agonism, offering the potential for facilitation of physiologically appropriate receptor activation with reduced liability for receptor desensitisation and/or tolerance. Thus, the emerging field of positive and negative allosteric modulation of the mGluR family offers considerable promise for the development of novel therapeutics.

Inflammation persistently enhances nocifensive behaviors mediated by spinal group I mGluRs through sustained ERK activation

Group I metabotropic glutamate receptors (mGluRs) and their downstream signaling pathways, which involve the extracellular signal-regulated kinases (ERKs), have been implicated as mediators of plasticity in several pain models. In this study, we report that inflammation leads to a long-lasting enhancement of behavioral responses induced by activation of spinal group I mGluRs. Thus, the nocifensive response to intrathecal injection of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG) is significantly potentiated seven days following Complete Freund's Adjuvant (CFA)-induced inflammation of the hind paw. This potentiation is not associated with increased mGlu1 or mGlu5 receptor expression but is associated with increased levels of phosphorylated ERK in dorsal horn neurons. We also tested whether the increased behavioral response to DHPG following inflammation may be explained by enhanced coupling of the group I mGluRs to ERK activation. DHPG-induced ERK phosphorylation in the dorsal horn is not potentiated following inflammation. However, inhibiting ERK activation using a MEK inhibitor, U0126, following inflammation attenuates the intrathecal DHPG-induced behavioral responses to a greater extent than in control animals. The results from this study indicate that persistent ERK activation is required for the enhanced behavioral responses to spinal group I mGluR activation following inflammation and suggest that tonic modulation of ERK activity may underlie a component of central sensitization in dorsal horn neurons.

Homer–PIKE complex: a novel link between mGluRI and PI 3-kinase

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, are thought to signal through the phospholipase and inositol (1,4,5)-trisphosphate receptor system, or through the adenylyl cyclase and protein kinase C system. Rong et al. have recently identified a new phosphoinositide (PI) 3-kinase enhancer (PIKE-L) that links group I mGlu receptors (mGluRI) to PI 3-kinase through Homer proteins, adaptors that bind mGluRI. mGluRI agonists enhanced mGluRI-Homer-PIKE-L complex formation, leading to activation of PI 3-kinase and inhibition of staurosporine-induced neuronal apoptosis. These results reveal a novel anti-apoptotic signaling mechanism that involves formation of an mGluRI signaling complex.