Defective group-II metaboropic glutamate receptors in the hippocampus of spontaneously depressed rats

Spontaneously depressed flinders sensitive line (FSL) rats showed a reduced expression of mGlu2/3 metabotropic glutamate receptors in the hippocampus, as compared to "non-depressed" flinders resistant line (FRL) rats. No changes in mGlu2/3 receptor protein levels were found in other brain regions, including the amygdala, hypothalamus, and cerebral cortex. Biochemical analysis of receptor signalling supported the reduction of mGlu2/3 receptors in the hippocampus of FSL rats. Accordingly, the selective mGlu2/3 receptor agonist, LY379268 (1muM) reduced forskolin-stimulated cAMP formation by 56% and 32% in hippocampal slices from FRL and FSL rats, respectively. In addition, LY379268 enhanced 3,5-dihydroxyphenylglycine-stimulated inositol phospholipid hydrolysis from 65% to 215% in hippocampal slices from FRL rats, whereas it was inactive in slices from FRL rats. We also examined the behavioural response of FSL rats to systemic injection of LY379268 (0.5mg/kg, i.p., once a day for 1-21 days) by measuring the immobility time in the forced swim test, which is known to be increased in these rats. LY379268 was administered alone or combined with the classical antidepressant, chlorimipramine (10mg/kg, i.p.). LY379268 alone had no effect at any of the selected time-points, whereas chlorimipramine alone reduced the immobility time only after 21 days of treatment. In contrast, when combined with LY379268, chlorimipramine reduced the immobility time during the first 14 days of treatment. These data support the view that mGlu2/3 receptors might be involved in the pathophysiology of depressive disorders, and that pharmacological activation of these receptors may shorten the latency of antidepressant medication.     

Protective role of group-II metabotropic glutamate receptors against nigro-striatal degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

To examine how mGlu2/3 metabotropic glutamate receptors affect nigro-striatal degeneration, we used the agonist, LY379268, and the antagonist, LY341495, in mice challenged with the nigro-striatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In control mice, high doses of MPTP (20 mg/kg, i.p., injected four times with 2 h of interval) induced a nearly total degeneration of the nigro-striatal pathway, as shown by measurements of striatal dopamine (DA) levels and by immunohistochemical analysis of tyrosine hydroxylase, high affinity dopamine transporter, and glial fibrillary acidic protein in the corpus striatum and substantia nigra. Lower cumulative doses of MPTP (30 mg/kg, i.p., injected only once) produced a partial lesion of the nigro-striatal pathway (about 50% reduction of striatal DA content). Systemic injection of LY379268 (1 mg/kg, i.p., 30 min prior to each injection of MPTP) partially reduced the extent of nigro-striatal degeneration induced by high doses of MPTP. Similar results were obtained by continuously delivering LY379268 (1 mg/kg/d for 7 d) by means of a subcutaneous osmotic minipump. The protective effect of LY379268 was antagonized by LY341495 (also delivered by the osmotic minipump). In mice challenged with the lower cumulative dose of MPTP, injection of LY379268 did not produce a significant neuroprotective effect. In contrast, the lesion was amplified by the antagonist, LY341495. Neither LY379268 nor LY341495 influenced the central bioavailability and the local half-life of MPTP, as shown by measurements of the toxin and its active metabolite, MPP(+), in the striatum. We conclude that mGlu2/3 receptors play a protective role against MPTP toxicity, and that the efficacy of the agonist, LY379268, critically depends on the extent of the nigro-striatal lesion.     

Targeting metabotropic glutamate receptors in the treatment of epilepsy: rationale and current status

Introduction: Several drugs targeting the GABAergic system are used in the treatment of epilepsy, but only one drug targeting glutamate receptors is on the market. This is surprising because an imbalance between excitatory and inhibitory neurotransmission lies at the core of the pathophysiology of epilepsy. One possible explanation is that drug development has been directed towards the synthesis of molecules that inhibit the activity of ionotropic glutamate receptors. These receptors mediate fast excitatory synaptic transmission in the central nervous system (CNS) and their blockade may cause severe adverse effects such as sedation, cognitive impairment, and psychotomimetic effects. Metabotropic glutamate (mGlu) receptors are more promising drug targets because these receptors modulate synaptic transmission rather than mediate it.

Areas covered: We review the current evidence that links mGlu receptor subtypes to the pathophysiology and experimental treatment of convulsive and absence seizures.

Expert opinion: While mGlu₅ receptor negative allosteric modulators have the potential to be protective against convulsive seizures and hyperactivity-induced neurodegeneration, drugs that enhance mGlu₅ and mGlu₇ receptor function may have beneficial effects in the treatment of absence epilepsy. Evidence related to the other mGlu receptor subtypes is more fragmentary; further investigations are required for an improved understanding of their role in the generation and propagation of seizures.     

Imipramine treatment up-regulates the expression and function of mGlu2/3 metabotropic glutamate receptors in the rat hippocampus

We examined the effect of a chronic imipramine treatment (10 mg/kg, i.p., once daily for 21 days) on the expression and function of metabotropic glutamate (mGlu) receptors in discrete regions of the rat brain. Chronic imipiramine treatment up-regulated the expression of mGlu2/3 receptor proteins in the hippocampus, nucleus accumbens, cerebral cortex and corpus striatum. Expression of mGlu1a receptor protein was increased exclusively in the hippocampus, whereas no changes in the expression of mGlu4 and mGlu5 receptors or Homer-1a protein were detected. Using hippocampal slices, we examined the stimulation of polyphosphoinositide (PI) hydrolysis induced by mGlu receptor agonists in control and imipramine-treated rats. Imipramine treatment amplified the PI response to the non subtype-selective mGlu receptor agonist, 1S,3R-aminocyclopentane-1,3-dicarboxylated (1S,3R-ACPD) in both hippocampal and cortical slices, but failed to affect the response to the selective mGlu1/5 receptor agonist, S-3,5-dihydroxyphenylglycine (DHPG). Amplification was restored when DHPG was combined with the selective mGlu2/3 receptor agonist, LY379268. In addition, 1S,3R-ACPD-stimulated PI hydrolysis was no longer enhanced in imipramine-treated rats when the mGlu2/3 component of the PI response was abrogated by the antagonist, LY341495. In contrast, the ability of LY379268 to inhibit forskolin-stimulated cAMP formation was reduced in hippocampal slices of rats chronically treated with imipramine. Taken together, these results suggest that neuroadaptive changes in the expression and function of mGlu2/3 receptors occur in response to chronic antidepressants.     

Constitutive endocytosis of the metabotropic glutamate receptor mGluR7 is clathrin-independent

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that are widely expressed throughout the brain and are involved in synaptic development, transmission, and plasticity. The endocytosis of several members of the GPCR superfamily of receptors, such as beta-adrenergic receptors, has been studied extensively. In contrast, the mechanisms regulating mGluR endocytosis and intracellular trafficking remain poorly defined. We describe here for the first time a distinct endocytic and intracellular sorting pathway utilized by mGluR7. We show that mGluR7 constitutively internalizes via a non-clathrin mediated pathway in heterologous cells and in neurons. Unlike clathrin-mediated NMDAR endocytosis, mGluR7 traffics via an Arf6-positive endosomal pathway, similar to other well-characterized proteins such as major histocompatibility complex class I (MHC I) and the GPI-anchored protein CD59. Thus constitutive endocytosis of mGluR7 in neurons is not regulated by clathrin-dependent mechanisms, and this clathrin-independent pathway ultimately determines the amount of receptor present on the plasma membrane available to bind and respond to glutamate.     

Blockade of glutamate transporters leads to potentiation of NMDA receptor current in layer V pyramidal neurons of the rat prefrontal cortex via group II metabotropic glutamate receptor activation

Membrane currents of layer V pyramidal cells in slices of the rat prefrontal cortex (PFC) were recorded with the patch-clamp technique. In an Mg(2+)-free superfusion medium l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a preferential blocker of astrocytic glutamate transporters, caused inward current due to the activation of NMDA receptors. The blockade of conducted action potentials by tetrodotoxin did not interfere with this effect. ATP was inactive when given alone and potentiated the NMDA-induced current in an Mg(2+)-containing but not Mg(2+)-free superfusion medium. Agonists of group I ((S)-3,5-dihydroxyphenylglycine; DHPG) and II ((1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid; LY 379268) metabotropic glutamate receptors (mGluRs) also potentiated responses to NMDA, whereas the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) did not affect them. In contrast to ATP, PDC evoked inward current in the absence but not in the presence of external Mg(2+), when given alone, and facilitated the NMDA effect Mg(2+)-independently. The PDC-induced facilitation of NMDA responses was blocked by group II ((2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid; LY 341495), but not group I ((RS)-1-aminoindan-1,5-dicarboxylic acid; AIDA) or III (alpha-methyl-3-methyl-4-phosphonophenylglycine; UBP 1112) mGluR antagonists. In conclusion, the blockade of astrocytic glutamate uptake by PDC may lead to a stimulation of group II mGluRs, while the triggering of exocytotic glutamate release from astrocytes by ATP may cause activation of group I mGluRs, both situated postsynaptically at layer V PFC pyramidal cells. Either group of mGluRs may interact with NMDA receptors in a positive manner.     

Blockade of metabotropic glutamate receptors inhibits cognition and neurodegeneration in an MPTP-induced Parkinson's disease rat model

Hyperactivity of the glutamatergic system is involved in excitotoxicity and neurodegeneration in Parkinson's disease (PD). Metabotropic glutamate receptor subtype 5 (mGluR5) modulates glutamatergic transmission and thus has been proposed as a potential target for neuroprotective drugs. The aim of this study was to determine the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), an mGluR5 antagonist, on working memory, object recognition, and neurodegeneration in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD rat model. Male Wistar rats were stereotaxically injected with MPTP into the substantia nigra pars compacta (SNc). Starting 1 day after lesioning (day 1), the rats were treated daily with MPEP (2 mg/kg/day, i.p.) for 14 days and rats underwent a T-maze test on days 8-10 and an object recognition test on days 12-14. MPTP-lesioned rats showed impairments of working memory in the T-maze test and of recognition function in the object recognition test and both effects were prevented by MPEP treatment. Furthermore, MPTP lesion-induced dopaminergic degeneration in the nigrostriatal system, microglial activation in the SNc, and cell loss in the hippocampal CA1 area were all inhibited by MPEP treatment. These data provide support for a role of mGluR5s in the pathophysiology of PD and suggest that MPEP is a promising pharmacological tool for the development of new treatments for dementia associated with PD.     

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.     

The possible involvement of metabotropic glutamate receptors in schizophrenia

Glutamate disruption is thought to have a major role in schizophrenia brain processes, possibly involving NMDA hypofunction. The metabotropic glutamate receptors are distributed in brain regions related to schizophrenia and seem to affect glutamate release in a moderate way. Compounds modulating these receptors are being investigated in animal models of schizophrenia, in an attempt to discover new antipsychotics. This article reviews the current research data regarding the role of these receptors in schizophrenia animal models. It was found that more research was done on Group I and II metabotropic receptors while investigation of group III receptors is still trailing behind. Accumulating evidence shows that mGluR5 antagonists by themselves do not necessarily disrupt pre-pulse inhibition (PPI), but can exacerbate disruption of PPI caused by MK-801 and PCP, while positive modulation of this receptor has beneficial effects on these models of psychosis. Group II agonists are also showing beneficial effects in animal models. It seems that metabotropic glutamate receptor modulators could be developed into a novel treatment of schizophrenia by altering glutamate release, thus overcoming the putative NMDA hypofunction. Although the implications from these pre-clinical studies to human schizophrenia patients are premature, the data obtained with some compounds point to promising results for drug development. More studies, with agents active at other mGluRs in animal models and schizophrenia patients as well as with human subjects are needed in order to clarify the role of the metabotropic glutamate receptors in the pathophysiology and pharmacotherapy of schizophrenia.     

Targeting striatal cholinergic interneurons in Parkinson's disease: focus on metabotropic glutamate receptors

In the early sixties, anticholinergic drugs were introduced in the pharmacological treatment of Parkinson's disease (PD). The rationale behind their utilisation in the treatment of the disease was based on the evidence of an imbalance between the dopaminergic inputs and the intrinsic cholinergic innervation within the striatum. Metabotropic glutamate (mGlu) receptors have been shown to play a key role in striatal function both in physiological conditions and in experimental models of diseases affecting this brain area. Indeed, compelling electrophysiological and morphological evidence shows that mGlu receptors are highly expressed at cellular level and exert a profound modulatory role on cholinergic interneurons excitability. This review will provide a brief survey of studies on the localization and function of mGlu receptors in cholinergic interneurons. The potential relevance of these findings in the control of motor function and in the treatment of PD will be discussed.     

Cocaine facilitates protein synthesis-dependent LTP: The role of metabotropic glutamate receptors

Cocaine addiction alters synaptic plasticity in many brain areas involved in learning and memory processes, including the hippocampus. Long-term potentiation (LTP) is one of the best studied examples of hippocampal synaptic plasticity and it is considered as one of the molecular basis of learning and memory. We previously demonstrated that in the presence of cocaine, a long lasting form of hippocampal LTP is induced by a single pulse of high frequency stimulation, which in normal conditions evokes only an early form of LTP. In this study, we further explore the molecular basis of this modulation of synaptic plasticity by cocaine. By performing pharmacological experiments on hippocampal slices, we were able to show that cocaine converts early LTP to a form of LTP dependent on protein synthesis, probably through the cAMP-dependent protein kinase and extracellular signal-regulated kinase signaling cascades. We also found that metabotropic glutamate receptors are involved in this phenomenon. These studies further clarify the molecular machinery used by cocaine to alter synaptic plasticity and modulate learning and memory processes.     

Surface expression of metabotropic glutamate receptor variants mGluR1a and mGluR1b in transfected HEK293 cells

Class C G-protein coupled receptors form obligatory dimers. Metabotropic glutamate receptors (mGluRs) are found commonly as homodimers. Alternative splicing of mGluR1 gene results in vivo in the expression of a long variant mGluR1a and at least two short variants mGluR1b and d. The amino acid sequences diverge within their carboxyl-termini six amino acid residues following RRKK motif. This four basic residue sequence was shown to have pronounced impact on function and trafficking of the short variants, while for mGluR1a the long C-terminus reduces the effects caused by presence of the RRKK motif. Here we investigated consequences of interactions between long mGluR1a and short mGluR1b variants. Our results show that mGluR1a interferes with mGluR1b trafficking to the cell surface in HEK293 transfected cells. Expression of a mGlu1a mutant incapable of activating G-proteins with mGluR1b mutated in the glutamate binding site led to the formation of a functional heterodimer. Moreover, we show that swapping long mGluR1a and/or short mGluR1b C-termini with corresponding regions in chimerical GB1 and GB2 gamma-amino butyric acid b (GABAb) receptor subunits do not exclude heterodimerization. These data reveal that the C-terminal ends of mGluR1 do not control subunit association, such that mGluR1 dimers with two distinct C-termini can form and function properly.     

An NMDAR-independent LTP mediated by group II metabotropic glutamate receptors and p42/44 MAP kinase in the dentate gyrus in vitro

The induction of long-term potentiation (LTP) under conditions of blockade of the N-methyl-D-aspartate receptor (NMDAR) was studied in the medial perforant path to granule cell synapse in the dentate gyrus. A small amplitude NMDAR-independent potentiation was induced by a single brief high frequency stimulation (HFS), and a summated larger LTP was induced by repeated spaced HFS. The NMDAR-independent LTP was mediated by activation of group II mGluR as it was inhibited by the group II antagonists EGLU and also low concentrations of LY341495, but not the group I mGluR antagonist MPEP. Perfusion of the group II mGluR agonist DCG-IV induced NMDAR-independent LTP in media containing an NMDAR antagonist. The NMDAR-independent LTP induced by HFS was mediated via activation of p42/44 MAP kinase as it was blocked by the selective inhibitor PD98059.     

Behavioral evidence supporting a differential role for spinal group I and II metabotropic glutamate receptors in inflammatory hyperalgesia in sheep

A differential role for metabotropic glutamate receptors (mGluRs) in spinal nociception in normal animals has previously been identified. The present study examined the contribution of group I and group II mGluRs to the development and maintenance of inflammatory hyperalgesia produced by unilateral intradermal injection of carrageenan into the lower forelimb in sheep. Carrageenan (7.5 mg in 500 micro l) produced a significant bilateral reduction in forelimb mechanical withdrawal thresholds. Intrathecal administration of saline-vehicle or the group II mGluR antagonist (2S)-alpha-ethylglutamate (EGLU; 570 nmol) had no effect on either the development or maintenance of hyperalgesia. However, intrathecal administration of the group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 450 nmol) before carrageenan blocked the development of ipsilateral hyperalgesia, and when given 2 h after carrageenan, reversed both ipsilateral and contralateral hyperalgesia. Intrathecal administration of the group II mGluR agonist (2S,1S,2S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 620 nmol) given either before or after carrageenan treatment produced analgesia and anti-hyperalgesia, an effect abolished by co-administration of EGLU (570 nmol). The magnitude of the analgesic response, assessed by the area under the response curve, was significantly greater than that produced by LCCG-I in normal animals. These data demonstrate that the development and maintenance of inflammatory hyperalgesia is dependent on activation of group I mGluRs in spinal cord. In addition, the analgesic and anti-hyperalgesic actions of group II mGluRs suggest that these receptors play a crucial role in modulating acute inflammatory hyperalgesia.     

Activation of amygdalar metabotropic glutamate receptors modulates anxiety, and risk assessment behaviors in ovariectomized estradiol-treated female rats

Anxiety disorders are more prevalent in females than males. The underlying reasons for this gender difference are unknown. Metabotropic glutamate receptors (mGluRs) have been linked to anxiety and it has been shown that interaction between estrogen receptors and mGluRs modulate sexual receptivity in rats. We investigated the role of mGluRs in anxiety-related behaviors in ovariectomized female rats with (OVX + EB) or without (OVX) estradiol implants. We centrally infused (s)-3,5-dihydroxyphenylglycine (DHPG), a group I mGluR agonist, into the basolateral amygdala (BLA) of OVX + EB and OVX rats at 0.1 and 1.0 μM. Male rats that normally have low estradiol levels were used to compare with OVX rats. Generalized anxiety, explorative activity and detection and analysis of threat were analyzed in the elevated plus maze (EPM) and risk assessment behaviors (RABs). DHPG (1.0 μM) increased the percentage of time spent in- and entries into- the open arms in OVX + EB, but not in OVX or male rats. Flat-back approaches and stretch-attend postures, two RABs, were significantly reduced by DHPG (0.1 and 1.0 μM) in OVX + EB rats only. DHPG did not modulate rearing and freezing, behaviors related to exploration and fear-like behavior, respectively. However, DHPG (1.0 μM) increased head dipping and decreased grooming behaviors in OVX rats, suggesting a weak explorative modulation. The effects of DHPG observed in OVX + EB, were blocked by 50 μM of (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), a mGluR1 antagonist. AIDA and/or estradiol did not modulate anxiety and or RABs. Our results show that intra-BLA infusion of DHPG exerts an anxiolytic-like effect in OVX + EB, but not in OVX or male rats. This effect seems to depend upon mGluR1 subtype activation. Our findings led us to suggest that the effects observed in OVX + EB rats might be due to an interaction at the membrane level of estrogen receptors with mGlu1 within the BLA.     

Involvement of group II metabotropic glutamate receptors in stress-induced behavioural sensitization

Rationale A short session of repeated foot shocks in rats causes long-lasting sensitization of behavioural, hormonal and autonomic responses to novel stressful challenges. The behavioural sensitization can be reduced by anxiolytics and mimics aspects of stress-induced changes in patients with post-traumatic stress disorder. Objectives The aim of this study was to test the efficacy of a group II metabotropic glutamate receptor (mGluR) agonist and assess altered brain mGluR receptor expression in shock-sensitized rats. Materials and methods Male Wistar rats were exposed to a 15-min session with ten 6-s foot shocks (preshocked). One and 2 weeks later, rats were intraperitoneally injected with the group II metabotropic glutamate receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or vehicle, and 30 min later exposed to 5 min of 85 dB noise. For in situ hybridization with probes for mGluR1, mGluR2, mGluR3 and mGluR5, preshocked and control rats were killed under basal conditions 2 weeks after foot shocks and their brains cryosectioned. Results APDC had no clear effect in controls, but dose-dependently reduced high immobility and increased low locomotion and rearing seen in preshocked rats to the levels of controls. mGluR3 expression was increased in the basolateral nucleus of the amygdala, and mGluR2 expression was increased in the agranular insular cortex of preshocked rats compared to controls. Conclusions Shock-induced behavioural sensitization in rats is reduced by acute treatment with a group II metabotropic glutamate receptor agonist. This effect may depend on the increased expression of amygdala mGluR3, which could be hypothesized as an endogenous mechanism to counteract stress-induced neuronal sensitization.     

Switch in the expression of mGlu1 and mGlu5 metabotropic glutamate receptors in the cerebellum of mice developing experimental autoimmune encephalomyelitis and in autoptic cerebellar samples from patients with multiple sclerosis

Recent evidence suggests that changes in the expression of membrane receptors/ion channels in cerebellar Purkinje cells contribute to the onset of cerebellar motor symptoms in patients with multiple sclerosis (MS). We examined the expression of group-I metabotropic glutamate receptors (mGlu1 and mGlu5 receptors) in the cerebellum of mice developing experimental autoimmune encephalomyelitis (EAE) and in autoptic cerebellar samples of MS patients. EAE was induced in mice by immunization with the 35-55 fragment of MOG (myelin oligodendrocyte glycoprotein). EAE mice showed a progressive loss of mGlu1a receptors in the cerebellum, associated with an increased expression of mGlu5 receptors. These changes were restricted to Purkinje cells and their dendritic arborization, as shown by immunohistochemistry. A reduced expression of mGlu1a receptors in cerebellar Purkinje cells was also found in 7 of 9 MS patients. In addition, a light/moderate to very strong mGlu5 receptor immunoreactivity was detected in Purkinje cells of 8 MS patients, but was always absent in non-MS control patients. In EAE mice, an acute treatment with the mGlu1 receptor enhancer, 9H-xanthene-9-carboxylic acid (4-trifluoromethyl-oxazol-2-yl)-amide (RO0711401), significantly improved motor coordination, whereas treatment with the mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 6-methyl-2-(phenylazo)-3-pyridinol (SIB-1757), had no effect. We conclude that mGlu1 receptor enhancers improve motor symptoms associated with EAE and might be helpful as symptomatic drugs in patients with MS.     

Identification of single nucleotide polymorphisms of the human metabotropic glutamate receptor 1 gene and pharmacological characterization of a P993S variant

mGluR1 receptors are believed to play major roles in the pathophysiology of diseases such as anxiety and chronic pain and are being actively investigated as targets for drug development. Sequence polymorphisms can potentially influence the efficacy of drugs in patient populations and are therefore an important consideration in the drug development process. To identify DNA sequence variants of the mGluR1 receptor, comparative DNA sequencing was performed on DNA samples (n = 186) from apparently healthy subjects representing two ethnic groups. In total, eight non-synonymous single nucleotide polymorphisms (SNPs) were identified and one SNP (c2977 > T) was found to be particularly common, this SNP results in a proline to serine substitution at residue 993 (P993S). The WT (P993) and S993 variants were expressed in an inducible system which allowed us to titrate gene expression to equivalent levels and were pharmacologically characterized. We determined the potency and affinity of standard antagonist compounds as well as the potency and efficacy of the endogenous ligand glutamate and other agonist compounds at both receptor variants. Agonist evoked increases in intracellular Ca²⁺ were measured by fluorometric imaging plate reader (FLIPR). The potency of mGluR1 antagonists was evaluated by their ability to inhibit quisqualate induced increases in intracellular Ca²⁺, while their affinities were determined by radio-ligand binding studies. This study demonstrates that the Pro993Ser amino acid exchange is highly frequent in the human mGluR1 gene. This polymorphism however, does not appear to affect the potency of agonist compounds or the potencies or affinities of small molecule antagonist compounds.     

L-CCG-I activates group III metabotropic glutamate receptors in the hippocampal CA3 region

Specific agonists of metabotropic glutamate receptors (mGluRs) provide powerful tools to discriminate afferent fibers originating from different presynaptic neurons. The group II mGluR agonists L-CCG-I ((2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine) and DCG-IV ((2S,2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl)glycine) are commonly used to distinguish between mossy fiber and associational-commissural (A/C) fiber input to the hippocampal CA3 region because only on the former group II mGluRs are expressed. Since previous reports indicated that L-CCG-I can activate group III mGluRs as well, we investigated whether L-CCG-I depresses A/C field potentials. L-CCG-I (10-300 microM) exhibited a significant dose-dependent and reversible reduction of A/C field potentials by 8 +/- 4% (10 microM), by 32 +/- 4% (100 microM, p < 0.001) and by 38 +/- 7% (300 microM, p < 0.05) that was accompanied by a concomitant increase in paired-pulse facilitation. Moreover, the selective group III antagonist (R,S)-alpha-methylserine-O-phosphate (MSOP; 100 microM) significantly reduced the field potential inhibition by L-CCG-I (100 microM) to 9 +/- 4% (p < 0.05). In contrast, DCG-IV did not affect A/C field potentials. In conclusion, the purported group II mGluR agonist L-CCG-I depresses A/C synaptic transmission by activation of group III mGluRs. For this reason, DCG-IV should be the drug of choice when aiming to discriminate between mossy fiber and A/C input to CA3 pyramidal cells.