MPEP阻断mGlu5抑制肝癌细胞HepG2增殖

代谢型谷氨酸受体5（metabotropic glutamate receptors 5, mGlu 5）在神经系统的多种病理生理过程中发挥重要作用. mGlu5与肝细胞癌的发生发展关系密切,其抑制剂2-甲基-6-(苯乙基）-吡啶（2-methyl-6-(phenylethyl)-pyridine, MPEP）能够促进肝癌细胞凋亡,抑制肝癌细胞的迁移.在此基础上,进一步探讨了MPEP与肝癌细胞增殖之间的关系.结果显示：在无血清和有血清的条件下,MPEP均能显著降低肝癌细胞HepG2的细胞活力.同时发现,有血清条件下MPEP能使HepG2细胞周期停滞在G1 期,显著降低HepG2细胞的DNA合成能力和克隆形成能力,并能下调细胞增殖信号ERK 通路的活性.该研究结果为进一步认识MPEP对肝癌细胞的抑制作用提供了新的实验证据.     

Neuroprotective effects of the mGlu5R antagonist MPEP towards quinolinic acid-induced striatal toxicity: involvement of pre- and post-synaptic mechanisms and lack of direct NMDA blocking activity

The aim of this work was to investigate the potential neuroprotective effects of the metabotropic glutamate receptor 5 (mGlu5R) antagonist 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) towards quinolinic acid (QA)-induced striatal excitoxicity. Intrastriatal MPEP (5 nmol/0.5 micro L) significantly attenuated the body weight loss, the electroencephalographic alterations, the impairment in spatial memory and the striatal damage induced by bilateral striatal injection of QA (210 nmol/0.7 micro L). In a second set of experiments, we aimed to elucidate the mechanisms underlying the neuroprotective effects of MPEP. In microdialysis studies in naive rats MPEP (80-250 micro m through the dialysis probe) significantly reduced the increase in glutamate levels induced by 5 mm QA. In primary cultures of striatal neurons MPEP (50 micro m) reduced the toxicity induced by direct application of glutamate [measured as release of lactate dehydrogenase [LDH]). Finally, we found that 50 micro m MPEP was unable to directly block NMDA-induced effects (namely field potential reduction in corticostriatal slices, as well as LDH release and intracellular calcium increase in striatal neurons). We conclude that: (i) MPEP has neuroprotective effects towards QA-induced striatal excitotoxicity; (ii) both pre- and post-synaptic mechanisms are involved; (iii) the neuroprotective effects of MPEP do not appear to involve a direct blockade of NMDA receptors.     

Metabotropic glutamate receptor 5 responses dictate differentiation of neural progenitors to NMDA‐responsive cells in fragile X syndrome

Disrupted metabotropic glutamate receptor 5 (mGluR5) signaling is implicated in many neuropsychiatric disorders, including autism spectrum disorder, found in fragile X syndrome (FXS). Here we report that intracellular calcium responses to the group I mGluR agonist (S)−3,5‐dihydroxyphenylglycine (DHPG) are augmented, and calcium‐dependent mGluR5‐mediated mechanisms alter the differentiation of neural progenitors in neurospheres derived from human induced pluripotent FXS stem cells and the brains of mouse model of FXS. Treatment with the mGluR5 antagonist 2‐methyl‐6‐(phenylethynyl)‐pyridine (MPEP) prevents an abnormal clustering of DHPG‐responsive cells that are responsive to activation of ionotropic receptors in mouse FXS neurospheres. MPEP also corrects morphological defects of differentiated cells and enhanced migration of neuron‐like cells in mouse FXS neurospheres. Unlike in mouse neurospheres, MPEP increases the differentiation of DHPG‐responsive radial glial cells as well as the subpopulation of cells responsive to both DHPG and activation of ionotropic receptors in human neurospheres. However, MPEP normalizes the FXS‐specific increase in the differentiation of cells responsive only to N ‐methyl‐d ‐aspartate (NMDA) present in human neurospheres. Exposure to MPEP prevents the accumulation of intermediate basal progenitors in embryonic FXS mouse brain suggesting that rescue effects of GluR5 antagonist are progenitor type‐dependent and species‐specific differences of basal progenitors may modify effects of MPEP on the cortical development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419–437, 2017     

The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester interact with overlapping binding pockets in the transmembrane region of group I metabotropic glutamate receptors

We have investigated the mechanism of inhibition and site of action of the novel human metabotropic glutamate receptor 5 (hmGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), which is structurally unrelated to classical metabotropic glutamate receptor (mGluR) ligands. Schild analysis indicated that MPEP acts in a non-competitive manner. MPEP also inhibited to a large extent constitutive receptor activity in cells transiently overexpressing rat mGluR5, suggesting that MPEP acts as an inverse agonist. To investigate the molecular determinants that govern selective ligand binding, a mutagenesis study was performed using chimeras and single amino acid substitutions of hmGluR1 and hmGluR5. The mutants were tested for binding of the novel mGluR5 radioligand [(3)H]2-methyl-6-(3-methoxyphenyl)ethynyl pyridine (M-MPEP), a close analog of MPEP. Replacement of Ala-810 in transmembrane (TM) VII or Pro-655 and Ser-658 in TMIII with the homologous residues of hmGluR1 abolished radioligand binding. In contrast, the reciprocal hmGluR1 mutant bearing these three residues of hmGluR5 showed high affinity for [(3)H]M-MPEP. Radioligand binding to these mutants was also inhibited by 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt), a structurally unrelated non-competitive mGluR1 antagonist previously shown to interact with residues Thr-815 and Ala-818 in TMVII of hmGluR1. These results indicate that MPEP and CPCCOEt bind to overlapping binding pockets in the TM region of group I mGluRs but interact with different non-conserved residues.     

Comparison of the binding pockets of two chemically unrelated allosteric antagonists of the mGlu5 receptor and identification of crucial residues involved in the inverse agonism of MPEP

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], a clinically validated non-benzodiazepine anxiolytic, has been shown to be a potent and non-competitive metabotropic glutamate (mGlu)-5 receptor antagonist. In the present study, we have used the site-directed mutagenesis coupled with three-dimensional receptor-based pharmacophore modelling to elucidate the interacting mode of fenobam within the seven-transmembrane domain (7TMD) of mGlu5 receptor and its comparison with that of 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototype antagonist. The common residues involved in the recognition of MPEP and fenobam include Pro654(3.36), Tyr658(3.40), Thr780(6.44), Trp784(6.48), Phe787(6.51), Tyr791(6.55) and Ala809(7.47). The differentiating residues between both modulators' interacting modes are Arg647(3.29), Ser657(3.39) and Leu743(5.47). Our data suggest that these chemically unrelated mGlu5 antagonists act similarly, probing a functionally unique region of the 7TMD. Using [3H]inositol phosphates accumulation assay, we have also identified the critical residues involved in the inverse agonist effect of MPEP. The mutation W784(6.48)A completely blocked the inverse agonist activity of MPEP; two mutations F787(6.51)A and Y791(6.55)A, caused a drastic decrease in the MPEP inverse agonism. Furthermore, these three mutations led to an increased efficacy of quisqualate without having any effect on its potency. The fact that the residues Trp784(6.48) and Phe787(6.51) are essential equally in antagonism and inverse agonism effects emphasizes again the key role of these residues and the involvement of a common transmembrane network in receptor inactivation by MPEP.     

Opposite influence of MPEP, an mGluR5 antagonist, on the locomotor hyperactivity induced by PCP and amphetamine.

Potential antipsychotic effects of a selective non-competitive antagonist of metabotropic glutamate receptor 5 (mGluR5), 2-methyl-6-phenylethynylpyridine (MPEP), was examined in two commonly used screening tests: (1) the hyperactivity induced by an NMDA receptor antagonist phencyclidine (PCP), and (2) the hyperactivity induced by an indirect dopamine agonist, D-amphetamine. PCP was administered at a dose of 2.5 mg/kg s.c. and D-amphetamine was given at a dose of 1 mg/kg s.c. MPEP (5 mg/kg i.p.) significantly enhanced the locomotor activity increased by PCP, but inhibited amphetamine-induced hyperactivity. The opposite effect of MPEP in the two above-mentioned models questions significance of the blockade of mGluR5 receptors to antipsychotic effects.     

Blockade of mGluR5 reverses abnormal firing of subthalamic nucleus neurons in 6-hydroxydopamine partially lesioned rats

Activation of metabotropic glutamate receptor 5 (mGluRs) in the subthalamic nucleus (STN) results in burst-firing activity of STN neurons, which is similar to that observed in Parkinson's disease (PD). We examined the effects of chronic and systemic treatment with 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective mGluR5 antagonist, in firing activity of STN neurons in partially lesioned rats by 6-hydroxydopamine (6-OHDA). In 6-OHDA-lesioned rats treated with vehicle, injection of 6-OHDA (4 microg) into the medial forebrain bundle produced a partial lesion causing 36% loss of tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the substantia nigra pars compacta (SNpc). The 6-OHDA lesion in vehicle-treated rats showed an increasing firing rate and a more irregular firing pattern of STN neurons. Whereas chronic, systemic treatment of MPEP (3 mg/kg/day, 14 days) produced neuroprotecive effects on the TH-ir neurons and normalized the hyperactive firing activity of STN neurons in 6-OHDA partially lesioned rats. These data demonstrate that partial lesion of the nigrostriatal pathway increases firing activity of STN neurons in the rat, and chronic, systemic MPEP treatment has the neuroprotective effect and reverses the abnormal firing activity of STN neurons, suggesting that MPEP has an important implication for the treatment of PD.     

Subtype selective antagonism of substantia nigra pars compacta Group I metabotropic glutamate receptors protects the nigrostriatal system against 6-hydroxydopamine toxicity in vivo

Evidence suggests that increased glutamatergic input to the substantia nigra pars compacta as a result of hyperactivity of subthalalmic nucleus output pathways may contribute to the progressive degeneration of nigral dopaminergic neurones in Parkinson's disease (PD), a debilitating neurodegenerative disorder which affects approximately 1% of people aged over 65. Substantial electrophysiological evidence suggests that the excitation of nigral dopaminergic neurones is regulated by the activation of Group I metabotropic glutamate receptors (mGluR), comprising mGluR1 and mGluR5 subtypes. As activation of these receptors by endogenous glutamate may promote multiple cascades leading to excitotoxic neuronal death, it may be hypothesised that functional antagonism of Group I mGluR should be neuroprotective and could form the basis of a novel neuroprotective treatment for PD. To investigate this hypothesis, the neuroprotective potential of the selective competitive mGlu1 antagonist (+)-2-methyl-4-carboxyphenylglycine ((S)-(+)-alpha-amino-4-carboxy-2-methlybenzeneacetic acid; LY367385) and the selective allosteric mGlu5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was tested in a rodent 6-hydroxydopamine (6-OHDA) model of PD in vivo. Both acute and subchronic intranigral administration of either LY367385 or MPEP resulted in significant neuroprotection of nigral tyrosine hydroxylase immunoreactive cell bodies, which correlated closely with prevention of striatal monoamine depletion following 6-OHDA lesioning. This neuroprotective action of LY367385 and MPEP displayed a clear concentration-dependent effect, suggesting a receptor-mediated mechanism of action. LY367385 produced robust neuroprotection at all concentrations tested (40, 200 and 1000 nmol in 4 microL), whilst MPEP displayed a bell-shaped neuroprotective profile with significant neuroprotection at low concentrations (2 and 10 nmol in 4 microL) but not at higher concentrations (50 nmol). Importantly, subchronic intranigral administration of MPEP and LY367385 appeared to slow the degeneration of remaining nigral dopaminergic neurones and prevented further striatal dopamine depletion in animals with established 6-OHDA induced nigrostriatal lesions, suggesting that these compounds may significantly influence disease progression in this model.     

Endogenously activated mGlu5 metabotropic glutamate receptors sustain the increase in c-Myc expression induced by leukaemia inhibitory factor in cultured mouse embryonic stem cells

We have shown that endogenous activation of type 5 metabotropic glutamate (mGlu5) receptors supports the maintenance of a pluripotent, undifferentiated state in D3 mouse embryonic stem cells cultured in the presence of leukaemia inhibitory factor (LIF). Here, we examined the interaction between LIF and mGlu5 receptors using as a read-out the immediate early gene, c-Myc. The selective mGlu5 receptor antagonist, 2-methyl-6-(phenylenthynyl)pyridine (MPEP; 1 mum), reduced the increase in c-Myc protein levels induced by LIF by enhancing c-Myc ubiquitination. A reduction in c-Myc levels was also observed following small interfering RNA-mediated mGlu5 receptor gene silencing. MPEP reduced glycogen synthase kinase-3beta phosphorylation on Ser9, but increased phosphorylation of the phosphatidylinositol-3-kinase (PI-3-K) substrate, AKT. In our hands, activated PI-3-K reduced the stability of c-Myc, because (i) the PI-3-K inhibitor, LY294002, prevented the reduction in c-Myc levels induced by MPEP; and (ii) over-expression of AKT promoted c-Myc ubiquitination. All effects of MPEP were mimicked by protein kinase C (PKC) inhibitors and reversed by the PKC activator, tetradecanoylphorbol-13-acetate. We conclude that endogenous activation of mGlu5 receptors sustains the increase in c-Myc induced by LIF in embryonic stem cells by inhibiting both glycogen synthase kinase-3beta and PI-3-K, both effects resulting from the activation of PKC.     

Antidepressant-like effect of MPEP,a potent,selective and systemically active mGlu5 receptor antagonist in the olfactory bulbectomized rats

Summary.  Using the olfactory bulbectomy model of depression, we examined the antidepressant-like activity of 2-methyl-6-(phenylethynyl)-pyridine (MPEP) in rats. Bulbectomized rats required a significantly greater number of trials to acquire the response similar to sham-operated controls in the passive avoidance model. Both the prolonged (but not acute) treatment with MPEP and with antidepressant drug-desipramine restored the learning deficit. The results indicate that the prolonged blockade of mGlu5 receptors exerts antidepressant-like effects in rats. Received July 2, 2001 Accepted August 6, 2001 Published online June 26, 2002     

Inhibition of human hepatic CYP isoforms by mGluR5 antagonists

Characterization of new chemical entities for their potential to produce drug-drug interactions is an important aspect of early drug discovery screening. In the present study, the potential for three metabotropic glutamate receptor antagonists to interact with recombinant human CYPs was investigated. 2-Methyl-6-(phenylethenyl) pyridine (SIB-1893), 2-methyl-6-(phenylethynyl) pyridine (MPEP) and 3-[2-methyl-1,3-thiazol-4-yl) ethynyl]-pyridine (MTEP) were moderate competitive inhibitors of recombinant human CYP1A2 (Ki, 0.5-1 microM). SIB-1893, but not MPEP or MTEP, was also a moderate competitive inhibitor of CYP1B1. MPEP and MTEP were weak inhibitors of CYP2C19. None of the three compounds tested were significant inhibitors (IC(50) values >50 microM) of CYP3A4, 2C9, 2D6, 2A6, 2B6 or 2E1. The results suggest that MTEP is a selective inhibitor of CYP1A2 and may prove to be a useful tool in studying drug-drug interactions involving this enzyme.     

The role of group I metabotropic glutamate receptors in memory consolidation and reconsolidation in the passive avoidance task in 1-day-old chicks

Although reconsolidation of memory after reminder does not seem to be the simple reiteration of the sequential stages occurring during memory consolidation, both phenomena probably employ similar mechanisms including activation of glutamate receptors and protein synthesis. It is known that group I metabotropic glutamate receptors (mGluRs) are involved in memory consolidation and modulation of protein synthesis. The aim of present study was to investigate the role of mGluR5 in memory consolidation and reconsolidation and to determine whether inhibition of these receptors may affect protein synthesis in these processes. The one-trial passive avoidance task on chicks was used as the experimental model of learning. Injection of the mGluR5 antagonist MPEP into a specific chick brain region IMM resulted in amnesia, provided the injection was made either shortly before or after training, or approximately 4 h after training. This amnesia was permanent, resembling the effects of protein synthesis inhibitors. MPEP injection immediately after reminder resulted in only a transient amnesia revealed 1h later. Increased expression of Zif/268 and c-Fos proteins 2 h after initial training was abolished bilaterally in chicks injected with MPEP. Injection of MPEP immediately after reminder did not inhibit c-Fos and Zif/268 expression, on the contrary, their expression was increased, specifically in left IMM and was similar to that observed after initial training. These results show that at least in the chick model mGluR5 play an important role in both consolidation and reconsolidation of memory but the mechanisms triggered by their activation in these processes differ. It is suggested that Ca(2+) signal derived from mGluR5 stimulation is necessary for complete memory consolidation, whereas during reconsolidation other mGluR5 triggered mechanisms of protein synthesis activation and regulation may be involved.     

Evidence for a single heptahelical domain being turned on upon activation of a dimeric GPCR

G-protein-coupled receptors (GPCRs) have been shown to form dimers, but the relevance of this phenomenon in G-protein activation is not known. Among the large GPCR family, metabotropic glutamate (mGlu) receptors are constitutive dimers. Here we examined whether both heptahelical domains (HDs) are turned on upon full receptor activation. To that aim, we measured G-protein coupling efficacy of dimeric mGlu receptors in which one subunit bears specific mutations. We show that a mutation in the third intracellular loop (i3 loop) known to prevent G-protein activation in a single subunit decreases coupling efficacy. However, when a single HD is blocked in its inactive state using an inverse agonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), no decrease in receptor activity is observed. Interestingly, in a receptor dimer in which the subunit that binds MPEP is mutated in its i3 loop, MPEP enhances agonist-induced activity, reflecting a 'better' activation of the adjacent HD. These data are consistent with a model in which a single HD is turned on upon activation of such homodimeric receptors and raise important issues in deciphering the functional role of GPCR dimer formation for G-protein activation.     

The mGluR5 Antagonist Fenobam Induces Analgesic Conditioned Place Preference in Mice with Spared Nerve Injury

Antagonists of metabotropic glutamate receptors (mGluRs) have the potential to act as analgesic drugs that may help alleviate chronic pain. This study was done to look at the possible rewarding properties of the mGluR5 antagonist, fenobam, in a cognitive assay. Analgesic conditioned place preference (aCPP) was used to examine the effects of fenobam (30 mg/kg) and the prototypical mGluR5 antagonist, MPEP, and these effects were compared to those of a drug with known analgesic properties, morphine (10 mg/kg). In each experiment, one group of mice received spared nerve injury (SNI) surgery to model chronic pain; the other group received a control sham surgery. Both fenobam and MPEP induced preference in the SNI mice, such that SNI mice spent significantly more time in the mGluR5 antagonist-paired chamber compared to a vehicle-paired chamber. No such preference developed for sham mice. Morphine induced preference in male and female mice in both the SNI and sham groups. The results showed that fenobam and MPEP likely reduced on-going distress in the SNI mice, causing them to prefer the chamber paired with the drug compared to the vehicle-paired chamber. Since sham animals did not prefer the drug-paired chamber, these data demonstrate that mGluR5 antagonism is non-rewarding in the absence of pain-like injury.     

Discovery of heterobicyclic templates for novel metabotropic glutamate receptor subtype 5 antagonists

Investigation of a series of heterobicyclic compounds with essential pharmacophoric features of the metabotropic glutamate receptor 5 (mGluR5) antagonists MPEP and MTEP provided novel structural templates with sub-micromolar affinities at the mGluR5.     

The Role of Metabotropic Glutamate Receptor 5 on the Stromal Cell-Derived Factor-1/CXCR4 System in Oral Cancer

We have demonstrated that blocking CXCR4 may be a potent anti-metastatic therapy for CXCR4-related oral cancer. However, as CXCR4 antagonists are currently in clinical use to induce the mobilization of hematopoietic stem cells, continuous administration as an inhibitor for the metastasis may lead to persistent leukocytosis. In this study, we investigated the novel therapeutic downstream target(s) of the SDF-1/CXCR4 system, using B88-SDF-1 cells, which have an autocrine SDF-1/CXCR4 system and exhibit distant metastatic potential in vivo. Microarray analysis revealed that 418 genes were upregulated in B88-SDF-1 cells. We identified a gene that is highly upregulated in B88-SDF-1 cells, metabotropic glutamate receptor 5 (mGluR5), which was downregulated following treatment with 1,1’ -[1,4-Phenylenebis(methylene)]bis-1,4,8,11-tetraazacyclotetradecane octahydrochloride (AMD3100), a CXCR4 antagonist. The upregulation of mGluR5 mRNA in the SDF-1/CXCR4 system was predominately regulated by the Ras-extracellular signal-regulated kinase (ERK)1/2 pathway. Additionally, the growth of B88-SDF-1 cells was not affected by the mGluR5 agonist (S)-3,5-DHPG (DHPG) or the mGluR5 antagonists 2-Methyl-6-(phenylethynyl)pyridine (MPEP) and 3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP). However, we observed that DHPG promoted B88-SDF-1 cell migration, whereas both MPEP and MTEP inhibited B88-SDF-1 cell migration. To assess drug toxicity, the antagonists were intraperitoneally injected into immunocompetent mice for 4 weeks. Mice injected with MPEP (5 mg/kg) and MTEP (5 mg/kg) did not exhibit any side effects, such as hematotoxicity, allergic reactions or weight loss. The administration of antagonists significantly inhibited the metastasis of B88-SDF-1 cells to the lungs of nude mice. These results suggest that blocking mGluR5 with antagonists such as MPEP and MTEP could prevent metastasis in CXCR4-related oral cancer without causing side effects.     

Peripheral versus central modulation of gastric vagal pathways by metabotropic glutamate receptor 5

Metabotropic glutamate receptors (mGluR) are classified into group I, II, and III mGluR. Group I (mGluR1, mGluR5) are excitatory, whereas group II and III are inhibitory. mGluR5 antagonism potently reduces triggering of transient lower esophageal sphincter relaxations and gastroesophageal reflux. Transient lower esophageal sphincter relaxations are mediated via a vagal pathway and initiated by distension of the proximal stomach. Here, we determined the site of action of mGluR5 in gastric vagal pathways by investigating peripheral responses of ferret gastroesophageal vagal afferents to graded mechanical stimuli in vitro and central responses of nucleus tractus solitarius (NTS) neurons with gastric input in vivo in the presence or absence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). mGluR5 were also identified immunohistochemically in the nodose ganglia and NTS after extrinsic vagal inputs had been traced from the proximal stomach. Gastroesophageal vagal afferents were classified as mucosal, tension, or tension-mucosal (TM) receptors. MPEP (1-10 microM) inhibited responses to circumferential tension of tension and TM receptors. Responses to mucosal stroking of mucosal and TM receptors were unaffected. MPEP (0.001-10 nmol icv) had no major effect on the majority of NTS neurons excited by gastric distension or on NTS neurons inhibited by distension. mGluR5 labeling was abundant in gastric vagal afferent neurons and sparse in fibers within NTS vagal subnuclei. We conclude that mGluR5 play a prominent role at gastroesophageal vagal afferent endings but a minor role in central gastric vagal pathways. Peripheral mGluR5 may prove a suitable target for reducing mechanosensory input from the periphery, for therapeutic benefit.     

Regulation of increased glutamatergic input to spinal dorsal horn neurons by mGluR5 in diabetic neuropathic pain

Diabetic neuropathic pain is associated with increased glutamatergic input in the spinal dorsal horn. Group I metabotropic glutamate receptors (mGluRs) are involved in the control of neuronal excitability, but their role in the regulation of synaptic transmission in diabetic neuropathy remains poorly understood. Here we studied the role of spinal mGluR5 and mGluR1 in controlling glutamatergic input in a rat model of painful diabetic neuropathy induced by streptozotocin. Whole-cell patch-clamp recordings of lamina II neurons were performed in spinal cord slices. The amplitude of excitatory post-synaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than in control rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) inhibited evoked EPSCs and sEPSCs more in diabetic than in control rats. Also, the percentage of neurons in which sEPSCs and evoked EPSCs were affected by MPEP or the group I mGluR agonist was significantly higher in diabetic than in control rats. However, blocking mGluR1 had no significant effect on evoked EPSCs and sEPSCs in either groups. The mGluR5 protein level in the dorsal root ganglion, but not in the dorsal spinal cord, was significantly increased in diabetic rats compared with that in control rats. Furthermore, intrathecal administration of MPEP significantly increased the nociceptive pressure threshold only in diabetic rats. These findings suggest that increased mGluR5 expression on primary afferent neurons contributes to increased glutamatergic input to spinal dorsal horn neurons and nociceptive transmission in diabetic neuropathic pain.     

Synthesis and biological evaluation of fenobam analogs as mGlu5 receptor antagonists

Optimization of affinity and microsomal stability led to identification of the potent, metabolically stable fenobam analog 4l. Robust in vivo efficacy of 4l was demonstrated in four different models of anxiety. Additionally, a ligand based pharmacophore alignment of fenobam and MPEP is proposed.     

The role of striatal metabotropic glutamate receptors in degeneration of dopamine neurons: review article

Summary.  Degeneration of dopaminergic nigrostriatal neurons is a primary cause of Parkinson's disease. Oxidative stress, excitotoxicity and mitochondrial failure are thought to be key mechanisms resposible for degeneration of dopaminergic cells. We found that the selective antagonist of the mGluR5 subtype MPEP in a dose of 5 mg/kg diminshed basal and veratridine (100 μM)-stimulated dopamine release in rat striatum in an in vivo model of microdialysis. In contrast, MPEP given intrastriatally in a high concentration (500 μM) enhanced the striatal extracellular concentration of dopamine. DCG-IV (100 μM), a non-selective agonist of group II mGluRs, inhibited the veratridine-stimulated striatal dopamine release. In an animal model of neuroxicity in vivo, methamphetamine (5 × 10 mg/kg, injected at 2 h intervals) produced deficits in the striatal content of dopamine and its metabolites DOPAC and HVA 72 h after the treatment. MPEP (5 × 5 mg/kg) given before each methamphetamine injection reversed the decrease in the striatal content of dopamine and diminished the methamphetamine-induced dopamine outflow from nigrostriatal terminals. It is concluded that the MPEP-produced blockade of mGluR5 situated on dopaminergic cells, or the suppression of glutamate release in the subthalamic nucleus or substantia nigra pars reticulata may directly and indirectly cause a decrease in striatal dopamine release. However, inhibitory effect of DCG-IV on dopamine release can be induced by attenuation of excitatory input from corticostriatal terminals by activation of mGluR2/3. Regulation of dopamine carriers by MPEP, an antagonist of group I mGluRs may be responsible for the reversal of toxicity induced by methamphetamine. Received July 7, 2001 Accepted August 6, 2001 Published online September 10, 2002