The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates PCP-induced cognitive deficits in rats

Rationale Recent studies have shown that metabotropic glutamate receptor 5 (mGluR5) can modulate N-methyl-d-aspartate (NMDA) receptor function in vivo. For example, the mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) can potentiate PCP (phencyclidine)-evoked hyperactivity and PCP-induced disruptions in pre-pulse inhibition (PPI) in rats.Objective To extend these previous behavioral findings and determine whether the mGluR5 antagonist MPEP can modulate the disruptions in learning and memory induced by PCP in rats.Methods The effects of MPEP, alone and in combination with PCP, were evaluated in rats trained to perform a repeated acquisition procedure (learning) or a delayed non-matching to position (DNMTP) radial maze task (spatial memory).Results In the repeated acquisition task, MPEP (0–10 mg/kg, IP) dose-dependently decreased response rates but had no effect on response accuracy. In contrast, PCP (0.625–1.25 mg/kg, SC) reduced response rate and response accuracy in a dose-dependent manner. Although MPEP (10 mg/kg, IP) had no effect when administered alone, the mGluR5 antagonist potentiated the disruptions in learning induced by a low dose of PCP (0.625 mg/kg, SC). In the DNMTP maze task, MPEP (0–10 mg/kg, IP) had no effect on spatial memory, whereas PCP (1.25–2.5 mg/kg, SC) produced a dose-dependent disruption. MPEP (10 mg/kg, IP) potentiated the impairments in memory induced by PCP (1.25 mg/kg, SC).Conclusion The mGluR5 antagonist, MPEP, potentiated the disruptions in learning and memory induced by PCP. These behavioral data extend previous behavioral findings and further suggest that mGluR5 can modulate NMDA receptor function in vivo.     

The effects of the mGluR5 receptor antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) on behavioural responses to nicotine

Rationale  

Previous studies have shown that blockade of metabotropic glutamate 5 receptors (mGluR5) results in inhibition of nicotine self-administration in experimental animals. However, these studies have not established the behavioural mechanisms which mediate these effects or the extent to which the effects of mGluR5 antagonism on nicotine self-administration reflect a selective attenuation of nicotine reinforcement.     

Synthesis and pharmacological evaluation of phenylethynyl[1,2,4]methyltriazines as analogues of 3-methyl-6-(phenylethynyl)pyridine

Procedures were developed for the synthesis of 3-methyl-5-phenylethynyl[1,2,4]triazine (4), 6-methyl-3-phenylethynyl[1,2,4]triazine (5), and 5-methyl-3-phenylethynyl[1,2,4]triazine (6a) as analogues of 2-methyl-6-(phenylethynyl)pyridine (2). The compounds were evaluated for antagonism of glutamate-mediated mobilization of internal calcium in an mGluR5 in vitro efficacy assay. The most potent of the three analogues was 6a. Twenty additional analogues of 6a were synthesized and evaluated for mGluR5 antagonist efficacy. The most potent compounds were 3-(3-methylphenylethynyl)-5-methyl[1,2,4]triazine (6b), 5-(3-chlorophenylethynyl)-5-methyl[1,2,4]triazine (6c), and 3-(3-bromophenylethynyl)-5-methyl[1,2,4]triazine (6d).     

The metabotropic glutamate receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) blocks fear conditioning in rats.

Glutamate receptors play an essential role in fear-related learning and memory. The present study was designed to assess the role of the group I metabotropic glutamate receptor (mGluR) subtype 5 in the acquisition and retrieval of conditioned fear in rats. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was applied systemically (0.0, 0.3, 3.0, 30.0 mg/kg per os) 60 min before the acquisition training and before the expression of conditioned fear, respectively, in the fear-potentiated startle paradigm. MPEP dose-dependently blocked the acquisition of fear. This effect was not due to state-dependent learning. MPEP also prevented the expression of fear at a dose of 30.0 mg/kg. As a positive control for these effects, we showed that the benzodiazepine anxiolytic compound diazepam (1.25 mg/kg intraperitoneally) also blocked acquisition and expression of fear potentiated startle. MPEP did not affect the baseline startle magnitude, short-term habituation of startle, sensitisation of startle by footshocks or prepulse inhibition of startle. These data indicate a crucial role for mGluR5 in the regulation of fear conditioning. In the highest dose MPEP might exert anxiolytic properties.     

Effect of 2-methyl-6-(phenylethynyl) pyridine on intravenous self-administration of ketamine and heroin in the rat

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) may be beneficial for drug abuse treatment, as it has been found to reduce self-administration of ethanol, nicotine and cocaine in preclinical models. This study investigated whether this finding can be extended to dissociative anaesthetics and opioids. Long Evans rats were trained to intravenously self-administer ketamine (0.5 mg/kg/infusion, 2 h sessions, fixed ratio 3) or heroin (0.05 mg/kg/infusion, 1 h sessions, fixed ratio 10). After reaching stable responding, the effect of MPEP pretreatment (1.25-20 mg/kg, intraperitoneal; t=-30 min) on intravenous self-administration (IVSA) of each compound was investigated. Behavioural specificity of MPEP on IVSA was assessed using a food-reinforcement procedure. IVSA of ketamine was dose-dependently reduced by MPEP pretreatment, with a minimal effective dose of 5 mg/kg and a 75% reduction at the highest dose tested. IVSA of heroin was only modestly reduced by the highest dose of MPEP (20% reduction). Food-reinforced behaviour was not altered by MPEP, either given alone or in combination with ketamine or heroin, indicating that the effect in the IVSA paradigm was behaviourally specific. It is suggested that the differential effect of MPEP on IVSA of ketamine and heroin is related to the particular class of the self-administered drug or its relative reinforcing efficacy.     

Anxiolytic-like effects of group I metabotropic glutamate antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) in rats

We examined the anxiolytic-like activity of2-methyl-6-(phenylethynyl)-pyridine (MPEP) using the conflict drinking Vogel test in rats as a model. MPEP is a selective and brain-penetrable mGlu5 receptor antagonist, the most potent compound described so far. The results indicate that MPEP produced a dose-dependent anticonflict effect in rats. These data suggest that selective mGlu5 receptor antagonists may become a new class of anxiolytics.     

Metabotropic glutamate receptor 5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) microinfusions into the nucleus accumbens shell or ventral tegmental area attenuate the reinforcing effects of nicotine in rats

Systemic administration of the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was previously shown to selectively attenuate nicotine self-administration without affecting food-maintained responding in rats. Glutamatergic neurotransmission in the ventral tegmental area (VTA) and nucleus accumbens (NAcc) shell plays an important role in the reinforcing effects of nicotine. To determine the brain sites that may mediate the systemic effects of MPEP on nicotine self-administration, the present study investigated the effects of MPEP microinfusions into the VTA or the NAcc shell on nicotine and food self-administration in separate groups of rats. Administration of low MPEP doses (0, 0.5, 1, and 2 μg/0.5 μl/side) microinfused into the NAcc shell had no effect on nicotine self-administration, whereas higher MPEP doses (0, 10, 20, and 40 μg/0.5 μl/side) microinfused into the NAcc shell dose-dependently attenuated nicotine self-administration without affecting food-maintained responding. Microinfusions of MPEP into the VTA (0, 10, 20, and 40 μg/0.5 μl/side) significantly decreased both nicotine and food self-administration at 20 μg/0.5 μl/side but did not affect responding for either reinforcer at 40 μg/0.5 μl/side. This lack of effect of 40 μg/0.5 μl/side MPEP on either nicotine or food self-administration when administered into the VTA may be attributable either to actions of MPEP at presynaptic mGlu5 receptors or at targets other than mGlu5 receptors. Importantly, anatomical control injections 2 mm above the NAcc shell or the VTA using the most effective MPEP dose in the two regions did not result in attenuation of nicotine self-administration. In conclusion, MPEP microinfusions in the VTA or NAcc shell attenuates the reinforcing effects of nicotine possibly via blockade of mGlu5 receptors located in these regions.     

Antagonism of the mGlu5 agonist 2-chloro-5-hydroxyphenylglycine by the novel selective mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) in the thalamus.

Our previous work has shown that Group I mGlu receptors participate in thalamic sensory processing in vivo. However, unequivocal demonstration of mGlu5 participation has not been possible due to the lack of specific ligands. We have therefore made a preliminary study of the in vivo actions of the agonist (R,S)-2-Chloro-5-hydroxyphenylglycine [CHPG] and the novel mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine [MPEP] in order to characterize their suitability for functional studies. Iontophoretically administered MPEP selectively antagonized excitatory responses of single rat thalamic neurones to CHPG compared to the broad-spectrum mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate. In contrast, the established mGlu1 and mGlu5 antagonist (S)-4-carboxyphenylglycine reduced responses to both agonists. These findings are the first demonstration of an in vivo action of CHPG and its antagonism by a selective mGlu5 antagonist. Furthermore MPEP appears to be a good tool for functional studies of mGlu5.     

The mGluR5 antagonist MPEP decreases operant ethanol self-administration during maintenance and after repeated alcohol deprivations in alcohol-preferring (P) rats

Rationale Recent research indicates that blockade of mGluR5 modifies the reinforcing properties of ethanol.Objectives The present studies examined the effects of mGluR5 receptor blockade in a genetic model of high ethanol intake, the alcohol-preferring (P) rat, on the maintenance of operant ethanol self-administration. In addition, we determined the effect of 2-methyl-6-(phenylethyl)-pyridine (MPEP) on the repeated alcohol deprivation effect.Methods Twelve male (P) rats were trained in experimental sessions to self-administer 10% w/v ethanol via a sucrose-fading procedure. After the establishment of operant ethanol self-administration, subjects were treated with various metabotropic glutamate receptor (mGluR) subtype antagonists immediately prior to experimental sessions: the mGluR5 antagonist MPEP (1, 3, and 10 mg/kg); the mGluR2–3 antagonist LY-341495 (1, 3, and 10 mg/kg); and the mGluR1 antagonist CPCCOEt (1, 3, and 10 mg/kg). After determining the role of mGluR5 in the maintenance of operant ethanol self-administration, we examined the role of this receptor in relapse following repeated periods of alcohol deprivation by depriving subjects of ethanol exposure for three 2-week deprivation periods.Results The mGluR5 antagonist MPEP dose-dependently decreased operant ethanol self-administration. In addition, rats that received saline immediately prior to repeated alcohol deprivation sessions self-administered ethanol at increasing levels that were above those achieved in the last operant-conditioning session prior to the initial 2-week deprivation period. This repeated alcohol deprivation effect was prevented in subjects pretreated with MPEP (10 mg/kg).Conclusions These findings suggest that mGluR5 receptors may modulate both the maintenance of operant ethanol self-administration and abstinence-induced increases in ethanol intake.     

Effects of the prototypical mGlu(5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine on rotarod, locomotor activity and rotational responses in unilateral 6-OHDA-lesioned rats

In the present study, we evaluated the effect of the prototypical metabotropic glutamate receptor 5 (mGlu(5)) antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on motor behaviour in rats using the accelerating rotarod, spontaneous locomotor activity and the 6-hydroxy-dopamine (6-OHDA) lesion model to assess its treatment potential for Parkinson's disease. The data indicate that MPEP at doses between 7.5 and 300 mg/kg, p.o. did not disrupt endurance performance on the accelerating rotarod (4-40 rpm in 300 s) which indicates that MPEP has a relatively high safety margin. However, while ineffective at doses of 3.75, 7.5 and 15 mg/kg (p.o.) MPEP inhibited spontaneous locomotor activity at doses of 30 and 100 mg/kg (p.o.). In the 6-OHDA rat rotation model, at doses of 7.5, 15 and 30 mg/kg (p.o.), MPEP induced a dose-dependent ipsilateral rotational response that reached statistical significance at the highest dose tested. This effect was relatively small but consistent. In combination with direct or indirect dopamine agonists, i.e. apomorphine (0.25 mg/kg, s.c.) and D-amphetamine (2.5 mg/kg, i.p.), MPEP (7.5, 15 or 30 mg/kg, p.o.) was found to significantly inhibit these dopamine receptor mediated rotational responses. MPEP injected at a dose of 30 mg/kg also inhibited the rotational response induced by L-DOPA (25 mg/kg, i.p.). (+)MK-801 was used in these rotation experiments as the reference compound. In view of these findings, it could be concluded that MPEP and potentially other mGlu(5) receptor antagonists are probably not appropriate drug candidates for the symptomatic treatment of Parkinson's disease.     

2-Methyl-6-(phenylethynyl)-pyridine (MPEP), a potent, selective and systemically active mGlu5 receptor antagonist.

In the present paper we describe 2-methyl-6-(phenylethynyl)-pyridine (MPEP) as a potent, selective and systemically active antagonist for the metabotropic glutamate receptor subtype 5 (mGlu5). At the human mGlu5a receptor expressed in recombinant cells, MPEP completely inhibited quisqualate-stimulated phosphoinositide (PI) hydrolysis with an IC50 value of 36 nM while having no agonist or antagonist activities at cells expressing the human mGlu1b receptor at concentrations up to 30 microM. When tested at group II and III receptors, MPEP did not show agonist or antagonist activity at 100 microM on human mGlu2, -3, -4a, -7b, and -8a receptors nor at 10 microM on the human mGlu6 receptor. Electrophysiological recordings in Xenopus laevis oocytes demonstrated no significant effect at 100 microM on human NMDA (NMDA1A/2A), rat AMPA (Glu3-(flop)) and human kainate (Glu6-(IYQ)) receptor subtypes nor at 10 microM on the human NMDA1A/2B receptor. In rat neonatal brain slices, MPEP inhibited DHPG-stimulated PI hydrolysis with a potency and selectivity similar to that observed on human mGlu receptors. Furthermore, in extracellular recordings in the CA1 area of the hippocampus in anesthetized rats, the microiontophoretic application of DHPG induced neuronal firing that was blocked when MPEP was administered by iontophoretic or intravenous routes. Excitations induced by microiontophoretic application of AMPA were not affected.     

Glutamate release inhibiting properties of the novel mGlu(5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP): complementary in vitro and in vivo evidence.

We have previously demonstrated that neuronal release of the excitatory amino acid glutamate is facilitated by the selective activation of presynaptic Group I metabotropic autoreceptors. Here we report the release inhibiting actions of the novel mGlu(5) receptor-selective antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), both in vitro and in vivo. These data provide compelling evidence for the presence of functional positive modulatory mGlu(5) subtype autoreceptors in the mammalian central nervous system.     

2-(5-甲基-2-苯基-4-(口恶)唑)-乙醇的合成

以丙酰乙酸乙酯为起始原料,先溴化得到4-溴丙酰乙酸乙酯,再在甲苯中与苯甲酰胺回流环合得到2-(5-甲基-2-苯基-4-(口恶)唑)-乙酸乙酯,最后经氢化铝锂还原,得到目标杂环中间体2-(5-甲基-2-苯基-4-(口恶)唑)-乙醇.该合成方法反应步骤少,原料便宜,目标物的总收率为67.3%.     

Effect of the mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant properties of cocaine, <Emphasis Type="SmallCaps">d</Emphasis>-amphetamine,and the dopamine reuptake inhibitor GBR12909 in mice

Rationale Recent evidence suggests that, in addition to ascending monoaminergic systems, glutamate systems also play a role in psychostimulant-induced locomotor activity. The present study was conducted to examine the effects of the selective type-5 metabotropic glutamate receptor (mGluR5) antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant effects of cocaine, d-amphetamine, and the dopamine reuptake inhibitor GBR12909.Methods Male DBA/2J mice were treated with saline or MPEP (1, 5, 20 or 30 mg/kg i.p.) 10 min prior to the administration of cocaine (15 mg/kg or 30 mg/kg i.p.), d-amphetamine (3 mg/kg or 5 mg/kg i.p.) or GBR12909 (10 mg/kg or 20 mg/kg i.p.). Locomotor activity was then monitored in an open-field environment for 30 min. The effects of MPEP alone (1, 5, 20 and 30 mg/kg i.p.) on locomotor activity were also examined.Results MPEP dose dependently inhibited the acute locomotor stimulant effects of cocaine, d-amphetamine, and the 10-mg/kg dose of GBR12909. However, MPEP had no effect on the locomotor stimulant effects of the higher (20 mg/kg) dose of GBR12909. When tested alone, MPEP increased locomotor activity at doses of 5 mg/kg and 20 mg/kg.Conclusions Our data suggest that mGluR5 receptors not only mediate spontaneous locomotor activity in DBA/2J mice but also the acute locomotor stimulant effects of cocaine, d-amphetamine and lower doses of GBR12909. However, the fact that MPEP did not attenuate the locomotor stimulant effects of the high (20 mg/kg) dose of GBR12909 suggests complex interactions between metabotropic glutamate receptors, dopamine transporters and possibly other monoamines in the regulation of psychostimulant-induced locomotor activity.     

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 micromol 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 micromol 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.     

The mGluR5 antagonist MPEP selectively inhibits the onset and maintenance of ethanol self-administration in C57BL/6J mice

Rationale Many of the biochemical, physiological, and behavioral effects of ethanol are known to be mediated by ionotropic glutamate receptors. Emerging evidence implicates metabotropic glutamate receptors (mGluRs) in the biobehavioral effects of ethanol and other drugs of abuse, but there is little information regarding the role of mGluRs in the reinforcing effects of ethanol. Materials and methods Male C57BL/6J mice were trained to lever-press on a concurrent fixed ratio 1 schedule of ethanol (10% v/v) vs water reinforcement during 16-h sessions. Effects of mGluR1, mGluR2/3, and mGluR5 antagonists were then tested on parameters of ethanol self-administration behavior. Results The mGluR5 antagonist MPEP (1–10 mg/kg, i.p.) dose-dependently reduced ethanol-reinforced responding but had no effect on concurrent water-reinforced responding. Analysis of the temporal pattern of responding showed that MPEP reduced ethanol-reinforced responding during peak periods of behavior occurring during the early hours of the dark cycle. Further analysis showed that MPEP reduced the number of ethanol response bouts and bout-response rate. MPEP also produced a 13-fold delay in ethanol response onset (i.e., latency to the first response) with no corresponding effect on water response latency or locomotor activity. The mGluR1 antagonist CPCCOEt (1–10 mg/kg, i.p.) or the mGluR2/3 antagonist LY 341495 (1–30 mg/kg, i.p.) failed to alter ethanol- or water-reinforced responding. Conclusions These data indicate that mGlu5 receptors selectively regulate the onset and maintenance of ethanol self-administration in a manner that is consistent with reduction in ethanol’s reinforcement function.     

Synthesis of a mGluR5 antagonist using [11C]copper(I) cyanide

5‐(2‐Phenylethynyl)pyridine‐3‐[¹¹C]carbonitrile ([¹¹C]LY2232645), a metabotropic glutamate 5 receptor (mGluR5) antagonist, was synthesized by a no‐carrier‐added nucleophilic halogen displacement with [¹¹C]copper(I) cyanide. The average radiochemical yield was 2.5%, and the average specific activity was 1365 mCi/µmol at end‐of‐synthesis. The total time of synthesis, purification, and formulation was 26 min. Copyright © 2006 John Wiley & Sons, Ltd.     

2-Amino-1-methyl-6-(5-hydroxy-)phenylimidazo[4,5-b]pyridine (5-OH-PhIP), a biomarker for the genotoxic dose of the heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP).

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a mutagenic and carcinogenic heterocyclic amine formed during ordinary cooking. PhIP is metabolically activated by CYP P450 mediated N-hydroxylation followed by phase II esterification. The ultimate mutagenic metabolite reacts with DNA resulting in formation of adducts, and with proteins and other cellular constituents resulting in unstable products which are degraded to 5-hydroxy-PhIP. Rats were dosed orally with PhIP and urine and faeces were collected to 24, 48 and 72 h. Urine and faeces samples were hydrolysed with glucuronidase/sulfatase in order to release conjugated metabolites, followed by purification on C18 columns. The rats excreted 0.88% of the dose as 5-hydroxy-PhIP with the urine to 24 h and 0.04 and 0.01% to 48 and 72 h, respectively. The amounts of 5-hydroxy-PhIP found in faeces were 0.03, 0.1 and 0% to 24, 48 and 72 h. Urinary excretion of 5-hydroxy-PhIP showed a linear dose-response relationship in rats dosed orally with PhIP. This shows that 5-hydroxy-PhIP is also formed in vivo and that 91% is excreted with the urine in 24 h, indicating the possible use of 5-hydroxy-PhIP as a urinary biomarker for the bioactive dose of PhIP. In a preliminary study, using molecular imprinted polymer a specific sorbent for purification, after enzymatic hydrolysis and purification on a C18 column, we have identified 5-OH-PhIP in a 24-h urine sample from a male volunteer who had ingested a fried beef. This indicates that urinary 5-OH-PhIP could be used as an easily obtainable marker for the genotoxic dose of PhIP in human biomonitoring studies.     

Mutational analysis and molecular modeling of the binding pocket of the metabotropic glutamate 5 receptor negative modulator 2-methyl-6-(phenylethynyl)-pyridine

Metabotropic glutamate (mGlu) 5 is a G-protein-coupled metabotropic glutamate receptor that plays an important role as a modulator of synaptic plasticity, ion channel activity, and excitotoxicity. 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) is a highly potent, noncompetitive, selective, and systemically active antagonist of mGlu5 receptors. It binds to a novel allosteric site that resides within the seven-transmembrane domain of mGlu5 receptors. Using site-directed mutagenesis, [3H]MPEP binding, a functional Ca2+ mobilization assay, and rhodopsin-based homology modeling, we identified eight residues (Pro-6543.36, Tyr-6583.40, Leu-7435.47, Thr-7806.44, Trp-7846.48, Phe-7876.51, Tyr-7916.55, and Ala-8097.47) that are crucial for MPEP-binding to rat mGlu5 receptors. Four mutations, Y6583.40V, W7846.48A, F7876.51A, and A8097.47V, caused complete loss of [3H]MPEP binding and also blocked the MPEP-mediated inhibition of quisqualate-induced intracellular Ca2+ mobilization. To visualize these experimental findings, we have constructed a homology model based on the X-ray crystal of bovine rhodopsin and have suggested a possible binding mode of MPEP. We propose that MPEP via its interactions with a network of the aromatic residues including Phe-6583.40 in transmembrane (TM) 3 helix and Trp-7986.48, Phe-7876.51, and Tyr-7916.55 in TM6 helix prevents the movement of TM6 helix relative to TM3 helix, a step that is required for receptor activation, and consequently stabilizes the inactive conformation of mGlu5 receptor. In the TM6 region, we observed a striking similarity between the critical residues involved in MPEP-binding site with those of previously identified as 1-ethyl-2-methyl-6-oxo-4-(1,2,4,5-tetrahydro-benzo[d]azepin-3-yl)-1,6-dihydropyrimidine-5-carbonitrile-binding pocket of mGlu1, pointing to a common mechanism of inhibition shared by both antagonists.     

Stereoselective synthesis and preliminary evaluation of (+)- and (-)-3-methyl-5-carboxy-thien-2-yl-glycine (3-MATIDA): identification of (+)-3-MATIDA as a novel mGluR1 competitive antagonist

The synthesis of the (+)- and (-)-isomers of 3-methyl-5-carboxy-thyen-2-yl-glycine (3-MATIDA), heterocyle isosters of carboxyphenylglycines (CPGs), a known class of competitive metabotropic glutamate receptors, was accomplished by a stereoselective Ugi condensation. The two isomers were tested as potential rat mGluR1 ligand and the (+) isomer was found to be a moderately potent antagonist, while the (-) one was inactive.