Group III mGluR regulation of synaptic transmission at the SC-CA1 synapse is developmentally regulated

Metabotropic glutamate receptor subtype 8 in the amygdala modulates thermal threshold, neurotransmitter release, and rostral ventromedial medulla cell activity in inflammatory pain

The amygdala is a crucial area in controlling the threshold of pain and its emotional component. The present study has evaluated the effect of a metabotropic glutamate 8 receptor (mGluR8) stimulation in the central nucleus of the amygdala (CeA) on the thermoceptive threshold and on CeA serotonin (5-HT), glutamate (Glu), and GABA release in normal and carrageenan-induced inflammatory pain conditions in rats. Furthermore, the activity of rostral ventromedial medulla (RVM) putative "pronociceptive" ON and "antinociceptive" OFF cells has been evaluated. (S)-3,4-Dicarboxyphenylglycine [(S)-3,4-DCPG], a selective mGluR8 agonist, administered into the CeA, did not change 5-HT, Glu, and GABA release, or the thermoceptive threshold, nor did it modify the activity of ON and OFF cells of the RVM in normal animals. In rats treated with carrageenan, intra-CeA (S)-3,4-DCPG perfusion produced antinociception, and increased 5-HT and Glu, whereas it decreased GABA release. Intra-CeA (S)-3,4-DCPG inhibited ON and increased OFF cell activities. Furthermore, an increase in mGluR8 gene, protein, and staining, the latter being associated with vesicular GABA transporter-positive profiles, has been found in the CeA after carrageenan-induced inflammatory pain. These results show that stimulation of mGluR8, which was overexpressed within the CeA in inflammatory pain conditions, inhibits nociceptive behavior. Such an effect is associated with an increase in 5-HT and Glu release, a decrease in GABA, and the inhibition of ON- and the stimulation of OFF-cell activities within RVM.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

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.

Role of GRM4 in idiopathic generalized epilepsies analysed by genetic association and sequence analysis

BACKGROUND

GRM4 encoding the group III metabotropic glutamate receptor 4 (mGluR4), is located on the chromosomal segment 6p21.3 where tentative susceptibility loci for Juvenile Myoclonic Epilepsy (JME) and Photoparoxysmal Response (PPR) have been mapped. The present candidate gene study examined if variation in GRM4 confers susceptibility to IGE.

PATIENTS AND METHODS

The case-control association sample included 564 unrelated IGE patients and 733 population controls of German descent. Association analysis was carried out for 17 single nucleotide polymorphisms (SNPs) covering the genomic GRM4 sequence for all IGE patients as well as for two common IGE subsyndromes [Juvenile Myoclonic Epilepsy (JME, n=215) and Childhood Absence Epilepsy (CAE, n=175)]. Sequence analysis was performed in 85 IGE and 42 PPR cases and 44 controls.

RESULTS

Nominally significant associations were detected between IGE and seven GRM4 SNPs (with P-values ranging from 0.037 to 0.0036), between JME and five SNPs (P=0.042-0.0106), and between CAE and two SNPs (P=0.0466-0.0021). Four novel SNPs were identified by sequence analysis.

CONCLUSIONS

Our association findings support the hypothesis that GRM4 sequence variants might confer low-risk effects to the etiology of IGE. A minor pathogenetic contribution of the examined variants is possible. These exploratory findings warrant further replication analyses.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

Context-dependent pharmacology exhibited by negative allosteric modulators of metabotropic glutamate receptor 7

Phenotypic studies of mice lacking metabotropic glutamate receptor subtype 7 (mGluR7) suggest that antagonists of this receptor may be promising for the treatment of central nervous system disorders such as anxiety and depression. Suzuki et al. (J Pharmacol Exp Ther 323:147-156, 2007) recently reported the in vitro characterization of a novel mGluR7 antagonist called 6-(4-methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[ 4,5-c]pyridin-4(5H)-one (MMPIP), which noncompetitively inhibited the activity of orthosteric and allosteric agonists at mGluR7. We describe that MMPIP acts as a noncompetitive antagonist in calcium mobilization assays in cells coexpressing mGluR7 and the promiscuous G protein G alpha(15). Assessment of the activity of a small library of MMPIP-derived compounds using this assay reveals that, despite similar potencies, compounds exhibit differences in negative cooperativity for agonist-mediated calcium mobilization. Examination of the inhibitory activity of MMPIP and analogs using endogenous G(i/o)-coupled assay readouts indicates that the pharmacology of these ligands seems to be context-dependent, and MMPIP exhibits differences in negative cooperativity in certain cellular backgrounds. Electrophysiological studies reveal that, in contrast to the orthosteric antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxyclycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), MMPIP is unable to block agonist-mediated responses at the Schaffer collateral-CA1 synapse, a location at which neurotransmission has been shown to be modulated by mGluR7 activity. Thus, MMPIP and related compounds differentially inhibit coupling of mGluR7 in different cellular backgrounds and may not antagonize the coupling of this receptor to native G(i/o) signaling pathways in all cellular contexts. The pharmacology of this compound represents a striking example of the potential for context-dependent blockade of receptor responses by negative allosteric modulators.

Metabotropic glutamate 7 receptor subtype modulates motor symptoms in rodent models of Parkinson's disease

Metabotropic glutamate (mGlu) receptors modulate synaptic transmission in the central nervous system and represent promising therapeutic targets for symptomatic treatment of Parkinson's disease (PD). Among the eight mGlu receptor subtypes, mGlu7 receptor is prominently expressed in the basal ganglia, but its role in restoring motor function in animal models of PD is not known. The effects of N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), the first selective allosteric activator of mGlu7 receptors, were thus tested in different rodent models of PD. Here, we show that oral (5 mg/kg) or intrastriatal administration (0.1 and 0.5 nmol) of AMN082 reverses haloperidol-induced catalepsy in rats. AMN082 (2.5 and 5 mg/kg) reduces apomorphine-induced rotations in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. In a more complex task commonly used to evaluate major akinetic symptoms of PD patients, 5 mg/kg AMN082 reverses the increased reaction time to respond to a cue of bilateral 6-OHDA-lesioned rats. In addition, AMN082 reduces the duration of haloperidol-induced catalepsy in a mGlu7 receptor-dependent manner in wild-type but not mGlu7 receptor knockout mice. Higher doses of AMN082 (10 and 20 mg/kg p.o.) have no effect on the same models of PD. Overall these findings suggest that mGlu7 receptor activation can reverse motor dysfunction associated with reduced dopamine activity. Selective ligands of mGlu7 receptor subtypes may thus be considered as promising compounds for the development of antiparkinsonian therapeutic strategies.

Electrophysiological and behavioral evidence that modulation of metabotropic glutamate receptor 4 with a new agonist reverses experimental parkinsonism

Developing nondopaminergic palliative treatments for Parkinson's disease represents a major challenge to avoid the debilitating side effects produced by L-DOPA therapy. Increasing interest is addressed to the selective targeting of group III metabotropic glutamate (mGlu) receptors that inhibit transmitter release at presumably overactive synapses in the basal ganglia. Here we characterize the functional action of a new orthosteric group III mGlu agonist, LSP1-2111, with a preferential affinity for mGlu4 receptor. In mouse brain slices, LSP1-2111 inhibits striatopallidal GABAergic transmission by selectively activating the mGlu4 receptor but has no effect at a synapse modulated solely by the mGlu7 and mGlu8 receptors. Intrapallidal LSP1-2111 infusion reverses the akinesia produced by nigrostriatal dopamine depletion in a reaction time task, whereas an mGlu8-receptor agonist has no effect. Finally, systemic administration of LSP1-2111 counteracts haloperidol-induced catalepsy, opening promising perspectives for the development of antiparkinsonian therapeutic strategies focused on orthosteric mGlu4-receptor agonists.

Allosteric modulators of GPCRs: a novel approach for the treatment of CNS disorders

Despite G-protein-coupled receptors (GPCRs) being among the most fruitful targets for marketed drugs, intense discovery efforts for several GPCR subtypes have failed to deliver selective drug candidates. Historically, drug discovery programmes for GPCR ligands have been dominated by efforts to develop agonists and antagonists that act at orthosteric sites for endogenous ligands. However, in recent years, there have been tremendous advances in the discovery of novel ligands for GPCRs that act at allosteric sites to regulate receptor function. These compounds provide high selectivity, novel modes of efficacy and may lead to novel therapeutic agents for the treatment of multiple psychiatric and neurological human disorders.

Activation of metabotropic glutamate receptors as a novel approach for the treatment of schizophrenia

In recent years, the metabotropic glutamate (mGlu) receptors have emerged as potential new drug targets for treatment of a range of CNS disorders. Some of the most compelling advances have been made in targeting specific mGlu receptor subtypes as a fundamentally new approach to the treatment of schizophrenia. Recent animal and clinical studies provide strong evidence that agonists of group II mGlu receptors (mGluR2 and mGluR3) are effective in the treatment of the positive symptoms of schizophrenia, and animal studies suggest that mGluR5 agonists could provide a novel approach for the treatment of all major symptom domains (positive, negative, and cognitive) of this disorder. Although the discovery of selective agonists of these receptors is a challenge, there have been recent advances in the discovery of highly selective positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These mGlu receptor-selective PAMs have properties needed for optimization as clinical candidates and have robust effects in animal models that predict efficacy in treatment of schizophrenia.

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.

Corequirement of PICK1 binding and PKC phosphorylation for stable surface expression of the metabotropic glutamate receptor mGluR7

The presynaptic metabotropic glutamate receptor (mGluR) mGluR7 modulates excitatory neurotransmission by regulating neurotransmitter release and plays a critical role in certain forms of synaptic plasticity. Although the dynamic regulation of mGluR7 surface expression governs a form of metaplasticity in the hippocampus, little is known about the molecular mechanisms regulating mGluR7 trafficking. We now show that mGluR7 surface expression is stabilized by both PKC phosphorylation and by receptor binding to the PDZ domain-containing protein PICK1. Phosphorylation of mGluR7 on serine 862 (S862) inhibits CaM binding, thereby increasing mGluR7 surface expression and receptor binding to PICK1. Furthermore, in mice lacking PICK1, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at mossy fiber-interneuron hippocampal synapses is impaired. These data support a model in which PICK1 binding and PKC phosphorylation act together to stabilize mGluR7 on the cell surface in vivo.

Metabotropic glutamate receptors mGluR4 and mGluR8 regulate transmission in the lateral olfactory tract-piriform cortex synapse

The piriform cortex (PC) is the primary terminal zone of projections from the olfactory bulb, termed the lateral olfactory tract (LOT). The PC plays a critical role in processing of olfactory stimuli and is also a highly seizure prone area thought to be involved in some forms of temporal lobe epilepsy. Pharmacological and immunohistochemical studies provide evidence for the localization of various metabotropic glutamate receptors (GluRs) in the PC. We employed whole-cell patch clamp recordings from PC pyramidal cells to determine the roles of group III mGluRs in modulating synaptic transmission at the LOT-PC synapse. The group III mGluR agonist, L-AP4, induced a concentration-dependent inhibition of synaptic transmission at the LOT-PC synapse at concentrations that activate mGluR4 and mGluR8, but not mGluR7 or other mGluR subtypes (EC50=473nM). In addition, the selective mGluR8 agonist, DCPG (300nM), also suppressed synaptic transmission at the LOT synapse. Furthermore, the inhibitory actions of L-AP4 and Z-cyclopentyl-AP4, a selective mGluR4 agonist, were potentiated by the mGluR4 positive allosteric modulator, PHCCC (30microM). The high potency of L-AP4, combined with the observed effects of DCPG and PHCCC, suggests that both mGluR4 and mGluR8 play a role in the l-AP4-induced inhibition of synaptic transmission at the LOT-PC synapse.