mGlu Agonist Research Articles

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.

Functional interaction between adenosine A2A and group III metabotropic glutamate receptors to reduce parkinsonian symptoms in rats

Non-dopaminergic drugs acting either on adenosine A2A or metabotropic glutamate (mGlu) receptors reduce motor impairment in animal models of Parkinson's disease (PD), suggesting a possible functional interaction between these receptors to regulate basal ganglia function. The present study therefore tested the behavioural effects of compounds acting selectively on A2A or on specific mGlu receptor subtypes, alone or in combination, in rodent models of PD. Acute administration of the adenosine A2A receptor antagonists CSC or MSX-3 at the highest doses tested (5 and 1.25 mg/kg, respectively) significantly reduces haloperidol-induced catalepsy. Furthermore, the anticataleptic effect of MSX-3 was enhanced by a 3-week treatment. Acute administration of the selective group III mGlu agonist ACPT-I produces potent anticataleptic effects and prolongs time on rotarod of 6-OHDA-lesioned rats. In contrast, acute or chronic administration of MPEP (mGlu5 receptor antagonist) has no anticataleptic action. Furthermore, the acute co-administration of ACPT-I 1 mg/kg, but not 5 mg/kg, with CSC markedly reduces catalepsy. Opposite effects are observed after a 3-week co-administration. The co-administration of ACPT-I with MSX-3 has anticataleptic effects both after acute or chronic treatment. In contrast, acute combination of subthreshold doses of CSC and MPEP has no effect. After a 3-week treatment, however, the combination of CSC and MPEP was found to reduce haloperidol-induced catalepsy. Altogether, these results show for the first time that systemic activation of group III mGlu receptors with ACPT-I provides benefits in parkinsonian rats and underlie a possible interaction with A2A receptors to regulate basal ganglia motor function.

Activation of NF-κB p65/c-Rel dimer is associated with neuroprotection elicited by mGlu5 receptor agonists against MPP+ toxicity in SK-N-SH cells

Nuclear factor-kappaB (NF-kappaB) is a transcriptional regulator of neuron survival eliciting diverse effects according to the specific composition of its active dimer. While p50/p65 mediates neurodegenerative events, c-Rel-containing dimers promote cell survival. Stimulation of metabotropic glutamate receptors type 5 (mGlu5) reduces neuron vulnerability to amyloid-beta through activation of anti-apoptotic, c-Rel-dependent transcription of Bcl-X(L) pathway. We here evaluated the protective activity of mGlu5 agonists in dopaminergic SK-N-SH cells exposed to 1-methyl-4-phenylpyridinium (MPP(+)), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causing parkinsonism in experimental animals. MPP(+) produced a concentration-dependent cell loss. Activation of mGlu5 receptors by CHPG (1 mM) and 3HPG (50 microM) abolished the toxic effect produced by 3 microM MPP(+). The neuroprotection was associated with activation of NF-kappaB p65/c-Rel dimer and reduction of p50/p65. These effects were prevented by the mGlu5 receptor antagonist MPEP (5 microM). It is suggested that mGlu5 receptor agonists through activation of a c-Rel-dependent anti-apoptotic pathway can rescue dopaminergic cell from mitochondrial toxicity.

Amino-Pyrrolidine Tricarboxylic Acids Give New Insight into Group III Metabotropic Glutamate Receptor Activation Mechanism

Like most class C G-protein-coupled receptors, metabotropic glutamate (mGlu) receptors possess a large extracellular domain where orthosteric ligands bind. Crystal structures revealed that this domain, called Venus FlyTrap (VFT), adopts a closed or open conformation upon agonist or antagonist binding, respectively. We have described amino-pyrrolidine tricarboxylic acids (APTCs), including (2S,4S)-4-amino-1-[(E)-3-carboxyacryloyl]pyrrolidine-2,4-dicarboxylic acid (FP0429), as new selective group III mGlu agonists. Whereas FP0429 is an almost full mGlu4 agonist, it is a weak and partial agonist of the closely related mGlu8 subtype. To get more insight into the activation mechanism of mGlu receptors, we aimed to elucidate why FP0429 behaves differently at these two highly homologous receptors by focusing on two residues within the binding site that differ between mGlu4 and mGlu8. Site-directed mutagenesis of Ser157 and Gly158 of mGlu4 into their mGlu8 homologs (Ala) turned FP0429 into a weak partial agonist. Conversely, introduction of Ser and Gly residues into mGlu8 increased FP0429 efficacy. Docking of FP0429 in mGlu4 VFT 3D model helped us characterize the role of each residue. Indeed, mGlu4 Ser157 seems to have an important role in FP0429 binding, whereas Gly158 may allow a deeper positioning of this agonist in the cavity of lobe I, thereby ensuring optimal interactions with lobe II residues in the fully closed state of the VFT. In contrast, the presence of a methyl group in mGlu8 (Ala instead of Gly) weakens the interactions with the lobe II residues. This probably results in a less stable or a partially closed form of the mGlu8 VFT, leading to partial receptor activation.

Can Selective Ligands for Glutamate Binding Proteins be Rationally Designed?

A major neurotransmitter, L-Glutamate must be stored, transported and received, and these processes are mediated by proteins that bind this simple yet essential amino acid. Detailed evidence continues to emerge on the structure of Glu binding proteins, which includes both receptors and transporters. It appears that receptors and transporters bind to Glu in different conformations, which may present a pharmacological opportunity. This review will compare and contrast information available on Glu Receptors (AMPA, NMDA, KA and mGlu), excitatory amino acid transporters (EAATs), the system Xc- transporter (XCT) and the vesicular Glutamate transporter (GVT). The cross-reactivity of ligands which have been previously used to characterize the glutamate binding proteins with system Xc- raises some fundamental interpretational issues regarding the mechanisms through which these analogues produce CNS damage. Although at one time it was thought that unraveling selectivity among glutamate binding proteins was an intractable problem, recently the NMDA antagonist (memantine) has been approved for general medical practice for treatment of Alzheimer's disease. Two other agents are in advanced clinical trials: an Ampakine for potential improvement of cognitive disorders, and a selective mGlu agonist for treatment of anxiety. The prospects for unraveling cross-reactivity will be weighed in light of a critical comparison of the glutamate binding protein targets.

Methyl Substitution of 2-Aminobicyclo[3.1.0]hexane 2,6-Dicarboxylate (LY354740) Determines Functional Activity at Metabotropic Glutamate Receptors: Identification of a Subtype Selective mGlu2 Receptor Agonist

LY354740 (1) is a highly potent and selective agonist of metabotropic glutamate (mGlu) receptors 2 and 3. In the present study, we have prepared C3- and C4-methyl-substituted variants of rac-1, compounds 5, 9, and 13. Each of these racemic methyl-substituted analogues displaced specific binding of the mGlu2/3 receptor antagonist (3)H-2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid ((3)H-LY341495) from membranes expressing mGlu2 or mGlu3 receptor subtypes. Evaluation of the functional effects of this series on second messenger responses in cells expressing human mGlu2 or mGlu3 receptors revealed C3beta-methyl analogue 5 to possess antagonist properties at both mGlu2 and mGlu3 receptors while C4beta-methyl analogue 9 acts as a full agonist at each of these targets. Unexpectedly, we found that incorporation of a methyl substituent at the C4alpha-position as in analogue 13 results in a mixed mGlu2 agonist/mGlu3 antagonist pharmacological profile. All of the mGlu2 agonist and mGlu3 antagonist activity of rac-13 was found to reside in its resolved (+)-isomer.

NF-κB factor c-Rel mediates neuroprotection elicited by mGlu5 receptor agonists against amyloid β-peptide toxicity

Opposite effects of nuclear factor-kappaB (NF-kappaB) on neuron survival rely on activation of diverse NF-kappaB factors. While p65 is necessary for glutamate-induced cell death, c-Rel mediates prosurvival effects of interleukin-1beta. However, it is unknown whether activation of c-Rel-dependent pathways reduces neuron vulnerability to amyloid-beta (Abeta), a peptide implicated in Alzheimer's disease pathogenesis. We show that neuroprotection elicited by activation of metabotropic glutamate receptors type 5 (mGlu5) against Abeta toxicity depends on c-Rel activation. Abeta peptide induced NF-kappaB factors p50 and p65. The mGlu5 agonists activated c-Rel, besides p50 and p65, and the expression of manganese superoxide dismutase (MnSOD) and Bcl-X(L). Targeting c-Rel expression by RNA interference suppressed the induction of both antiapoptotic genes. Targeting c-Rel or Bcl-X(L) prevented the prosurvival effect of mGlu5 agonists. Conversely, c-Rel overexpression or TAT-Bcl-X(L) addition rescued neurons from Abeta toxicity. These data demonstrate that mGlu5 receptor activation promotes a c-Rel-dependent antiapoptotic pathway responsible for neuroprotection against Abeta peptide.

Role of Group I Metabotropic Glutamate Receptors mGlu1 and mGlu5 in Nociceptive Signalling

Distribution, pharmacological and knockdown studies point to a role of group I receptors in nociceptive processing. mGlu1 and mGlu5 receptors are expressed on peripheral sensory afferents, in dorsal root ganglion neurons, dorsal and ventral horn of spinal cord, rostral ventromedial medulla, periaqeductal gray, amygdala, and ventrobasal thalamus. These receptors are functional: sensory neurons in these regions respond to pain stimuli and / or to mGlu1 and mGlu5 agonists. Intrathecal administration of group I agonists evokes spontaneous pain behaviour. Whereas mGlu1 / 5 antagonists do not alter normal sensation, their effect on acute nociception is controversial. Activation of spinal and peripheral mGlu1 or mGlu5 receptors is sufficient to evoke hyperalgesia and allodynia. Blockade of spinal or peripheral mGlu1 or peripheral mGlu5 receptors was antihyperalgesic in animal models of inflammatory pain. Although the role of supraspinal, spinal and peripheral mGlu1 and mGlu5 receptors in the development and maintenance of neuropathic pain states is not fully elucidated, spinal mGlu1 receptor blockade is effective in all pain states tested. In the periaqueductal gray, a group I agonist attenuates nociceptive responses, indicating that mGlu1 and / or mGlu5 receptors in this structure can activate the descending pathway to dampen pain. The data indicate that mGlu1 and mGlu5 antagonists have therapeutic potential for the treatment of chronic inflammatory and neuropathic pain states. More potent and bioavailable mGlu1 versus mGlu5 antagonists are needed to provide proof of concept. Keywords: mglu1, mglu5, metabotropic glutamate, nociception, pain, hyperalgesia

The Human Immunodeficiency Virus-1 Protein Tat and Its Discrete Fragments Evoke Selective Release of Acetylcholine from Human and Rat Cerebrocortical Terminals through Species-Specific Mechanisms

The effect of the human immunodeficiency virus-1 protein Tat was investigated on neurotransmitter release from human and rat cortical nerve endings. Tat failed to affect the release of several neurotransmitters, such as glutamate, GABA, norepinephrine, and others, but it evoked the release of [3H]ACh via increase of cytosolic [Ca2+]. In human nerve terminals, the Tat effect partly depends on Ca2+ entry through voltage-sensitive Ca2+ channels, because Cd2+ halved the Tat-evoked release. Activation of group I metabotropic glutamate receptors (mGluR) and mobilization of Ca2+ from IP3-sensitive intraterminal stores are also involved, because the Tat effect was prevented by mGluR antagonists 2-methyl-6-(phenylethynyl)pyridine hydrochloride and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester and by the IP3 receptor antagonists heparin and xestospongin C. Furthermore, the group I selective mGlu agonist (RS)-3,5-dihydroxyphenylglycine enhanced [3H]ACh release. In rat nerve terminals, the Tat-evoked release neither depends on external Ca2+ ions entry nor on IP3-mediated mechanisms. Tat seems to cause mobilization of Ca2+ from ryanodine-sensitive internal stores because its effect was prevented by both 8-bromo-cyclic adenosine diphosphate-ribose and dantrolene. The Tat-evoked release from human synaptosomes was mimicked by the peptide sequences Tat 32-62, Tat 49-86, and Tat 41-60. In contrast, the Tat 49-86 and Tat 61-80 fragments, but not the Tat 32-62 fragment, were active in rat synaptosomes. In conclusion, Tat elicits Ca2+-dependent [3H]ACh release by species-specific intraterminal mechanisms by binding via discrete amino acid sequences to different receptive sites on human and rat cholinergic terminals.

Role of G Protein-coupled Receptor Kinase 4 and ॆ-Arrestin 1 in Agonist-stimulated Metabotropic Glutamate Receptor 1 Internalization and Activation of Mitogen-activated Protein Kinases

The metabotropic glutamate 1 (mGlu(1)) receptor in cerebellar Purkinje cells plays a key role in motor learning and motor coordination. Here we show that the G protein-coupled receptor kinases (GRK) 2 and 4, which are expressed in these cells, regulate the mGlu(1) receptor by at least in part different mechanisms. Using kinase-dead mutants in HEK293 cells, we found that GRK4, but not GRK2, needs the intact kinase activity to desensitize the mGlu(1) receptor, whereas GRK2, but not GRK4, can interact with and regulate directly the activated Galpha(q). In cells transfected with GRK4 and exposed to agonist, beta-arrestin was first recruited to plasma membranes, where it was co-localized with the mGlu(1) receptor, and then internalized in vesicles. The receptor was also internalized but in different vesicles. The expression of beta-arrestin V53D dominant negative mutant, which did not affect the mGlu(1) receptor internalization, reduced by 70-80% the stimulation of mitogen-activated protein (MAP) kinase activation by the mGlu(1) receptor. The agonist-stimulated differential sorting of the mGlu(1) receptor and beta-arrestin as well as the activation of MAP kinases by mGlu(1) agonist was confirmed in cultured cerebellar Purkinje cells. A major involvement of GRK4 and of beta-arrestin in agonist-dependent receptor internalization and MAP kinase activation, respectively, was documented in cerebellar Purkinje cells using an antisense treatment to knock down GRK4 and expressing beta-arrestin V53D dominant negative mutant by an adenovirus vector. We conclude that GRK2 and GRK4 regulate the mGlu(1) receptor by different mechanisms and that beta-arrestin is directly involved in glutamate-stimulated MAP kinase activation by acting as a signaling molecule.

Group I mGlu receptors regulate the expression of the neuronal calcium sensor protein VILIP-1 in vitro and in vivo: implications for mGlu receptor-dependent hippocampal plasticity?

Metabotropic glutamate (mGlu) receptors are involved in several forms of synaptic plasticity in the rat hippocampus. Agonists which activate group I mGlu receptors induce slow-onset potentiation without prior tetanization in the hippocampal area CA1. Activation of group I mGlu receptors induces protein synthesis which may contribute to mGlu receptor-dependent forms of long-term plasticity. Calcium-binding proteins are widely considered to comprise key elements for synaptic plasticity. Therefore, we investigated whether the calcium sensor protein VILIP-1 is associated with group I mGlu receptor-mediated plasticity in the dentate gyrus (DG) in vivo. Application of either the group I and II mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) or the selective group I agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) resulted in slow-onset potentiation in the DG of adult rats. In hippocampal cell cultures both agonists elicited an enhanced expression of VILIP-1. In situ hybridization revealed strong hippocampal expression of VILIP-1 and intracerebral application of DHPG to adult rats significantly enhanced hippocampal VILIP-1 expression. The DHPG effects in both, hippocampal cultures and in vivo, were prevented by the group I mGlu receptor antagonist 4-Carboxyphenylglycine (4CPG). Calcium sensor proteins thus appear to be regulated by mGlu receptors in an activity-dependent manner. A specific role for group I mGlu receptors is evident. Furthermore, the sensor proteins may function as molecular switches for the long-term regulation of synaptic plasticity.

Identification and characterization of a novel splice variant of the metabotropic glutamate receptor 5 gene in human hippocampus and cerebellum

The G-protein coupled metabotropic glutamate receptor mGlu5 plays a pivotal role as a modulator of synaptic plasticity, ion channel activity and excitotoxicity. Two splice variants, hmGlu5a and -5b have been reported previously. During screening of a human brain cDNA library for hmGlu5a, we identified a novel variant (hmGlu5d) generated by alternative splicing at the C-terminal domain. The predicted hmGlu5d protein has a C-terminal 267 amino acid shorter than that of hmGlu5a. The pattern of mRNA expression of mGluR5 variants in human brain were analyzed by RT-PCR and in situ hybridization histochemistry. RT-PCR analysis demonstrated the presence of the hmGlu5d transcript, although at low level, in human whole brain, cerebellum, cerebral cortex and hippocampus. [ 3H]Quisqualate displayed similar affinity at the hmGlu5 splice variants ( K D values of 80±8 and 54±17 nM for hmGlu5a and -5d receptors, respectively). For the five mGlu agonists studied, a similar rank order of potency was observed on both hmGlu5a and -5d receptors: quisqualate>glutamate>DHPG>L-CCGI≈ACPD. MPEP inhibited the glutamate (2 μM)-induced [Ca 2+] i response in hmGlu5a and -5d-HEK293 cells also with similar potency (IC 50 values 25±1.5 and 20±1.4 nM, respectively). Therefore, the large truncation of the C-terminal tail of mGlu5 does not have any apparent major effect on the potency and efficacy of agonists as measured by the [Ca 2+] i responses or by activation of recombinant G-protein coupled inwardly rectifying K + (GIRK) channel currents. The only major functional difference is the increased sensitivity of hmGlu5d to protein kinase C (PKC)-mediated desensitization, relative to hmGlu5a.

Activation of mGlu1 but not mGlu5 metabotropic glutamate receptors contributes to postischemic neuronal injury in vitro and in vivo

In order to investigate the involvement of mGlu1 and mGlu5 metabotropic glutamate receptors in the development of postischemic neuronal death, we examined the effects of selective agonists and antagonists in models of cerebral ischemia in vitro and in vivo. In murine cortical cell cultures and rat organotypic hippocampal slices exposed to oxygen and glucose deprivation (OGD), the mGlu1 antagonists 1-aminoindan-1,5-dicarboxylic acid (AIDA; 300 μM), ( S)-(+)-2-(3′-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG; 300 μM), 7-hydroxyiminocyclopropan[ b]chromen-1 a-carboxylic acid ethyl ester (CPCCOEt; 10–30 μM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385; 30–100 μM) reduced neuronal loss when added to the medium during OGD and the subsequent 24-h recovery period. On the contrary, the potent and selective mGlu5 antagonist methyl-6-(phenylethynyl)-pyridine (MPEP; 0.1–1 μM) did not exhibit neuroprotection in any of these in vitro models. Incubation with the nonselective mGlu1 and mGlu5 agonist 3,5-dihydroxyphenylglycine (3,5-DHPG; 300 μM) but not with the mGlu5 agonist ( RS)-2-chloro-5-hydroxyphenylglycine (CHPG; 1 mM) enhanced the severity of OGD-induced neuronal damage. In gerbils subjected to global ischemia, intracerebroventricular administration of AIDA (100 nmol two times) or CBPG (300 nmol, two times) afforded consistent protection against CA1 pyramidal cell death, whereas MPEP (10 pmol i.c.v two times and 10 mg/kg i.p two times) failed to reduce postischemic hippocampal damage. Our results suggest that activation of mGlu1 but not mGlu5 receptor contributes to postischemic neuronal injury.

Modulation of DOI-induced increases in cortical BDNF expression by group II mGlu receptors

Previous studies have shown that 5-hydroxytryptamine 2A (5-HT 2A) receptor activation induces changes in the pattern of brain-derived neurotrophic factor (BDNF) mRNA expression in the neocortex and hippocampus, and that 5-HT 2A receptor blockade interferes with the induction of BDNF mRNA by stress. Recent studies have also shown that activation of metabotropic glutamate group II (mGlu2/3) receptors suppresses 5-HT 2A receptor-stimulated excitatory postsynaptic potentials/currents (EPSP/Cs) in pyramidal neurons in medial prefrontal cortex. Conversely, blockade of mGlu2/3 receptors enhances 5-HT-induced EPSCs. The current study examined the effects of the highly selective mGlu2/3 agonist (1 S,2 S,5 R,6 S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate (LY354740) and the mGlu2/3 antagonist 2 S-2-amino-2-(1 S,2 S-2-carboxycycloprop-1-yl)-3(xanthy-9-yl)propanoic acid (LY341495) on BDNF mRNA expression in medial prefrontal cortex induced by the hallucinogen and 5-HT 2A/2B/2C agonist 1-(2,5-dimethoxy-4-iodophenethyl)-2-aminopropane (DOI). LY354740 (0.1–10 mg/kg) dose-dependently suppressed DOI-induced BDNF mRNA levels in medial prefrontal cortex. In contrast, the mGlu2/3 antagonist LY341495 (1 mg/kg) enhanced DOI-induced BDNF mRNA levels. BDNF mRNA expression was not altered by administration of the mGlu agonist or the antagonist alone. These results are discussed with respect to a potential role for group II mGlu agonists in the treatment of depression and schizophrenia.

Group II mGluR receptor agonists are effective in persistent and neuropathic pain models in rats

The involvement of Group II metabotropic receptors in acute and persistent pain states was evaluated in several in vivo models of pain with selective and potent Group II metabotropic glutamate (mGlu) 2,3 agonists. LY354740, LY379268 and LY389795 attenuated late-phase paw-licking pain behavior in a dose-dependent manner in the formalin model of persistent pain. Effects occurred in the absence of overt neuromuscular deficits as measured by performance in the rotorod test for ataxia. The effects of LY354740 and LY379268 were also stereoselective. The order of potency of the agonists was LY389795>LY379268>LY354740. The attenuation of licking behavior by LY379268 (3 mg/kg) in the formalin model was reversed by a potent and selective mGlu2,3 receptor antagonist, LY341495 (1 mg/kg). In the L 5/L 6 spinal nerve ligation model of neuropathic pain in rats, LY379268 significantly reversed mechanical allodynia behavior in a dose-related manner. In contrast, LY379268 had no significant effects on the tail flick test or paw withdrawal test of acute thermal nociceptive function. These results support the involvement of Group II mGlu2,3 receptors in persistent pain mechanisms and suggest the potential utility of selective Group II mGlu agonists for the treatment of persistent pain.

Systemic pre-treatment with a group II mGlu agonist, LY379268, reduces hyperalgesia in vivo.

1. Previous studies investigating the role of metabotropic glutamate (mGlu) receptors in nociceptive processing have been hampered by the lack of systemically active, selective, ligands. This study investigates the possible analgesic and/or anti-hyperalgesic properties of the most potent compound to date that has systemic agonist activity at group II mGlu receptors, LY379268. 2. In testing the drug in rats as an analgesic to acute noxious stimuli, LY379268 (in doses up to 3 mg kg(-1) i.p.) did not affect withdrawal latencies to either mechanical or thermal stimulation. 3. However, when a 3 mg kg(-1) dose was given prior to an intraplantar injection of carrageenan, the inflammatory hyperalgesia that developed was significantly delayed compared to saline pre-treated controls, without affecting the inflammation of the paw. A similar dose of the mGlu-inactive enantiomer, LY379267, was not anti-hyperalgesic. 4. In a model of mouse tail withdrawal to warm water, LY379268 (12 mg kg(-1) i.p.), given before a subcutaneous tail injection of capsaicin, reduced the subsequent neurogenic hyperalgesia. 5. Rota-rod testing showed that the drug did not produce a motor impairment in rats at antihyperalgesic doses. 6. The results indicate that systemic activation of this group of mGlu receptors reduces both inflammatory and neurogenic thermal hyperalgesia.

Facilitation and inhibition of retrieval in two aversive tasks in rats by intrahippocampal infusion of agonists of specific glutamate metabotropic receptor subtypes.

The generic antagonist of glutamate metabotropic receptors (mGlus), MCPG, blocks retrieval of inhibitory avoidance when infused into the CA1 area of rat hippocampus. It was considered important to study the effect of agonists of different types of mGlus on retrieval both of this task and of a related one, contextual fear. To measure the effect of three mGlu agonists (3HPG, which is selective to mGlu1; LCCG, which binds to mGlu2 and mGlu3; and LAP-4, which binds to mGlu4 and mGlu6), infused bilaterally into CA1, on the retrieval of one-trial inhibitory avoidance and contextual fear in rats. Rats bilaterally implanted with cannulae in the CA1 region of the dorsal hippocampus were trained in one-trial step-down inhibitory avoidance or in a contextual fear task and tested for retention 24 h later. The drugs 3HPG, LCCG and LAP-4 were infused into CA1 at different concentration levels 10 min before retention testing. In addition, we studied the effect of these drugs on locomotor and exploratory activity measured in an open field, and on pro- and anti-conflict behaviour in an elevated plus-maze. 3HPG hindered, and LCCG and LAP-4 enhanced, retrieval of the two tasks. In all cases the effects were dose-dependent. The drugs had no effects on open field or plus maze behaviour. Retrieval of one-trial inhibitory avoidance and of contextual fear is regulated by mGlus in the CA1area of the rat hippocampus. The results suggest that mGlu2s, mGlu3s, mGlu4s and mGlu6s are necessary for retrieval and that mGlu1s play an inhibitory role. The effects are not explainable by nonspecific influences on locomotor or exploratory activity or anxiety levels.

Elevated levels of group‐III metabotropic glutamate receptors in the inferior colliculus of genetically epilepsy‐prone rats following intracollicular administration of l‐serine‐O‐phosphate

The selective group-III metabotropic glutamate receptor agonist, L-serine-O-phosphate (L-SOP), when injected bilaterally into the inferior colliculus of the sound sensitive genetically epilepsy-prone (GEP) rats produces a short proconvulsant excitation followed by a long phase of protection against sound-induced seizures lasting for 2-4 days. We have studied this prolonged suppression of audiogenic seizures using pharmacological and molecular biological approaches including semiquantitative RT-PCR and western blotting. The intracerebroventricular injection of the protein synthesis inhibitor cycloheximide (120 microg) 30 min beforehand significantly reduces the proconvulsant seizure activity and the prolonged anticonvulsant effect of intracollicular L-SOP (500 nmol/side). The sensitive semiquantitative RT-PCR revealed a significant up-regulation in mGlu(4) and mGlu(7) mRNA levels in the inferior colliculus at 2 days (maximum suppression of audiogenic seizures) after intracollicular L-SOP injection compared with the non-injected, 2-day post-vehicle treated and 7-day (return to expressing audiogenic seizures) post-drug or vehicle-treated groups. No significant changes were observed in mGlu(6) or mGlu(8) mRNA expression levels in drug-treated compared with control groups. Examination of mGlu(4a) and mGlu(7a) protein levels using western blotting showed a significant increase in mGlu(7a) but no significant change in mGlu(4a) protein levels 2 days after L-SOP treatment compared with the control groups (non-injected and 2-day vehicle-injected group). These results suggest that up-regulation of mGlu(7) receptors is involved in the prolonged anticonvulsant effect of L-SOP against sound-induced seizures in GEP rats. The potential use of mGlu(7) agonists as novel anti-epileptic agents merits investigation.

Synaptic activation of the group I metabotropic glutamate receptor mGlu1 on the thalamocortical neurons of the rat dorsal Lateral Geniculate Nucleus in vitro

Intracellular recordings were made from thalamocortical neurons in slices of rat dorsal lateral geniculate nucleus in vitro, where ionotropic glutamate receptors and ionotropic and metabotropic GABA receptors had been blocked. The activation of specific metabotropic glutamate receptors by exogenous agonists and by the electrical stimulation of the corticothalamic pathway was then assessed using selective antagonists. The specific group I agonist ( S)-3,5-dihydoxyphenylglycine and the non-selective agonist (1 S,3 R)-1-aminocyclo-pentane-1,3-dicarboxylic acid both caused a concentration-dependent depolarization of membrane potential. These effects were associated with an increase in the apparent input resistance, and a more robust expression of both the depolarizing sag of the voltage response and the low-threshold Ca 2+ potential and an increase in thalamocortical neuron excitability. However, group I agonists selective for the mGlu5 receptor and agonists selective for group II and III receptors did not have these effects. Consequently, these data suggested that these actions were mediated specifically by the group I mGlu1 receptor. The activation of cortical fibres, with trains of 50 stimuli at 50 Hz, resulted in a two-component depolarizing response. The first part of this synaptic response and the agonist-induced depolarization of membrane potential were depressed by the novel group I receptor antagonists LY367366 and LY367385, which are active at mGlu1 receptors. However, they were not blocked by 6-methyl-2-(phenylethyl)-pyridine, a highly selective mGlu5 receptor antagonist. Thus, the membrane potential depolarization of thalamocortical neurons caused either by exogenous agonists or by the stimulation of cortical fibres resulted from the specific activation of mGlu1 but not mGlu5 receptors. This result is consistent with the location of this receptor type on the distal dendrites of thalamocortical neurons in the dorsal lateral geniculate nucleus of the thalamus.

Anticonvulsant activity of two metabotropic glutamate Group I antagonists selective for the mGlu5 receptor: 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and (E)-6-methyl-2-styryl-pyridine (SIB 1893)

The selective mGlu5 antagonists, MPEP, 2-methyl-6-phenylethynyl-pyridine, and SIB1893, (E)-6-methyl-2-styryl-pyridine, have been evaluated as antiepileptic drugs in DBA/2 mice and lethargic mice. Clonic seizures induced by the selective mGlu5 agonist, ( R, S)-2-chloro-5-hydroxyphenylglycine (CHPG), 3 μmol intracerebroventricularly (i.c.v.), are potently suppressed by both compounds (MPEP, ED 50=0.42 [0.28–0.62] mg/kg intraperitoneally (i.p.); SIB 1893 ED 50=0.19 [0.11–0.33] mg/kg i.p.). Clonic seizures induced by the mGlu1,5 agonist, 3,5-dihydroxyphenylglycine (DHPG), 1.5 μmol i.c.v., are less potently suppressed by both compounds (MPEP, ED 50=22 [13–38] mg/kg i.p., 110 [67–180] nmol i.c.v.; SIB1893, ED 50=31 [18–54] mg/kg i.p., 95 [82–110] nmol i.c.v.). Sound-induced seizures in DBA/2 mice are suppressed at 15 min by MPEP and SIB 1893 (MPEP ED 50 clonic seizures=18 [10–32] mg/kg i.p., 93 [69–125] nmol i.c.v.; tonic seizures=6.1 [4.5–8.3] mg/kg i.p., 46 [26–80] nmol i.c.v.; SIB 1893 ED 50 clonic seizures=27 [17–44] mg/kg i.p., 825 [615–1108] nmol i.c.v., tonic seizures=5.4 [3.4–8.6] mg/kg i.p., 194 [113–332] nmol i.c.v.). The ED 50 for MPEP for impaired rotarod performance is 128 [83–193] mg/kg i.p., at 15 min, i.e. a therapeutic index for sound-induced seizures of 5–20. In lethargic mice ( lh/lh), a genetic absence model, MPEP, 50 mg/kg i.p., caused a marked reduction in the incidence of spontaneous spike-and-wave discharges. These selective antagonists of mGlu5 block seizures due to activation of mGlu5 at very low systemic doses. At rather higher doses they block convulsive and non-convulsive primary generalised seizures.