The antinociceptive and anxiolytic-like effects of the metabotropic glutamate receptor 5 (mGluR5) antagonists, MPEP and MTEP, and the mGluR1 antagonist, LY456236, in rodents: a comparison of efficacy and side-effect profiles

Rationale Modulation of metabotropic glutamate receptor (mGluR) subtypes represents a novel approach for the treatment of neurological and psychiatric disorders.Objectives This study was conducted to investigate the role of the mGluR5 and mGluR1 subtypes in the modulation of pain and anxiety.Methods The mGluR5 antagonists, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), and the mGluR1 antagonist, (4-methoxy-phenyl)-(6-methoxy-quinazolin-4-yl)-amine HCl (LY456236), were tested in models of pain [mouse formalin test, rat spinal nerve ligation (SNL)] and anxiety [Vogel conflict, conditioned lick suppression (CLS)], and their efficacious effects were compared to any associated side effects.Results The systemic administration of MPEP, MTEP, and LY456236 reduced hyperalgesia induced by formalin and mechanical allodynia following SNL. However, only LY456236 completely reversed the allodynia. In the anxiety models, MPEP (3–30 mg/kg), MTEP (3–10 mg/kg), and LY456236 (10–30 mg/kg) produced anxiolytic-like effects similar to the benzodiazepine, chlordiazepoxide (CDP, 6 mg/kg). However, only MPEP and MTEP were able to produce a level of anxiolysis comparable to CDP. In a series of tests examining potential side effects, MPEP and MTEP reduced body temperature and locomotor activity and impaired operant responding for food and rotarod performance at doses of 3–30 and 1–30 mg/kg, respectively. LY456236 reduced operant responding at 30 mg/kg.Conclusion Both mGluR5 and mGluR1 antagonists are effective in models of pain and anxiety. However, an mGluR1 antagonist was more efficacious than the two mGluR5 antagonists in the pain models, which, conversely, appeared more efficacious in the anxiety models. These findings support the potential utility of mGluR5 and mGluR1 antagonists for both the treatment of chronic pain and as novel anxiolytics.     

Divalent cations modulate the activity of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) and calcium receptors (CaR) are closely related G protein-coupled receptors (GPCRs). The similar structural and functional properties of mGluRs and CaRs include conserved amino acid residues involved in glutamate binding in mGluRs and Ca2+ binding in the CaR. Furthermore, recent findings have demonstrated that mGluRs can respond to high extracellular Ca2+ (Ca2+(o)) whereas CaR activity is potentiated by L-amino acids. We show that both mGluR1 and mGluR2 are activated by Ca2+(o) in the absence of glutamate in the extracellular media. This activation by Ca2+(o) is antagonized by Mg2+(o). Unlike the CaR, in which the intracellular carboxyl tail has been reported to be involved in Ca2+(o)-dependent activity, the carboxyl tail of mGluRs does not seem to play a role in mediating Ca2+(o) actions. On the other hand, we find that preservation of disulfide bonds in the N-terminal extracellular domain of mGluRs is essential for stimulation by Ca2+(o) as well as glutamate. Because the mGluR1 EC50 for Ca2+(o) is within the physiologic range of Ca2+ in the synaptic cleft, mGluR function is likely regulated by changes in divalent cations caused by synaptic activity under normal or pathologic conditions.     

Endocannabinoids contribute to short-term but not long-term mGluR-induced depression in the hippocampus

Activation of postsynaptic group 1 metabotropic glutamate receptors (mGluRs) by the agonist DHPG causes a long-term depression (DHPG-LTD) of excitatory transmission in the CA1 region of the hippocampus, as well as causing the release of endocannabinoids from pyramidal cells. As cannabinoid agonists cause a presynaptic inhibition at these synapses and DHPG-LTD is thought to be expressed, at least in part, by a presynaptic mechanism, we examined the possibility that endocannabinoids mediated DHPG-LTD. We find that antagonists of cannabinoid receptors reduce the acute depression induced by DHPG, but have no effect on the lasting depression. Furthermore, both the acute and the lasting effects of DHPG were unaffected in the CB1 knockout mouse. These findings suggest that endocannabinoids, acting on a non-CB1 cannabinoid receptor, contribute to the acute depression but not to DHPG-LTD. Presumably some other retrograde signalling mechanism is responsible for DHPG-LTD.     

Can we increase the speed and efficacy of antidepressant treatments? Part II. Glutamatergic and RNA interference strategies

In the second part we focus on two treatment strategies that may overcome the main limitations of current antidepressant drugs. First, we review the experimental and clinical evidence supporting the use of glutamatergic drugs as fast-acting antidepressants. Secondly, we review the involvement of microRNAs (miRNAs) in the pathophysiology of major depressive disorder (MDD) and the use of small RNAs (e.g.., small interfering RNAs or siRNAs) to knockdown genes in monoaminergic and non-monoaminergic neurons and induce antidepressant-like responses in experimental animals.The development of glutamatergic agents is a promising venue for antidepressant drug development, given the antidepressant properties of the non-competitive NMDA receptor antagonist ketamine. Its unique properties appear to result from the activation of AMPA receptors by a metabolite [(2 S,6 S;2 R,6 R)-hydroxynorketamine (HNK)] and mTOR signaling. These effects increase synaptogenesis in prefrontal cortical pyramidal neurons and enhance serotonergic neurotransmission via descending inputs to the raphe nuclei. This view is supported by the cancellation of ketamine's antidepressant-like effects by inhibition of serotonin synthesis.We also review existing evidence supporting the involvement of miRNAs in MDD and the preclinical use of RNA interference (RNAi) strategies to target genes involved in antidepressant response. Many miRNAs have been associated to MDD, some of which e.g., miR-135 targets genes involved in antidepressant actions. Likewise, SSRI-conjugated siRNA evokes faster and/or more effective antidepressant-like responses. Intranasal application of sertraline-conjugated siRNAs directed to 5-HT_1A receptors and SERT evoked much faster changes of pre- and postsynaptic antidepressant markers than those produced by fluoxetine.     

Positive AMPA receptor modulation in the treatment of neuropsychiatric disorders: A long and winding road

Glutamatergic transmission is widely implicated in neuropsychiatric disorders, and the discovery that ketamine elicits rapid-acting antidepressant effects by modulating α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) signaling has spurred a resurgence of interest in the field. This review explores agents in various stages of development for neuropsychiatric disorders that positively modulate AMPARs, both directly and indirectly. Despite promising preclinical research, few direct and indirect AMPAR positive modulators have progressed past early clinical development. Challenges such as low potency have created barriers to effective implementation. Nevertheless, the functional complexity of AMPARs sets them apart from other drug targets and allows for specificity in drug discovery. Additional effective treatments for neuropsychiatric disorders that work through positive AMPAR modulation may eventually be developed.     

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.     

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.     

Effects of MPEP on locomotion, sensitization and conditioned reward induced by cocaine or morphine

Exposure to environmental cues is considered a major cause of relapse in detoxified addicts. Recent findings showed an involvement of glutamate in cue-induced relapse and suggest that subtype 5 of metabotropic glutamate receptors (mGluR5) is involved in conditioned drug-reward. The present study applied the conditioned place preference (CPP) paradigm to examine the involvement of mGluR5 in cocaine- and morphine-induced behaviours. Results of previous mice-studies were extended into rats by using the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). As a result, the evaluated behavioural parameters were dose-relatedly affected by MPEP. Low-dosed MPEP (10 mg/kg, i.p.) did not affect spontaneous locomotion, reduced cocaine-induced hyperlocomotion and produced sensitized locomotion, while showing no effect on sensitized locomotion induced by repeated cocaine or morphine. Low-dosed MPEP did not genuinely block development of cocaine- and morphine-CPP, but rendered CPP expression state-dependent. The medium MPEP-dose (30 mg/kg) was most effective in reducing spontaneous locomotion. The high MPEP-dose (50 mg/kg) was most effective in reducing both body-weight and morphine-CPP expression. Cocaine-CPP expression was not affected by any MPEP-dose. In conclusion, mGluR5 are involved in modulation of spontaneous and cocaine-induced locomotion, in state-dependent learning and in expression of morphine-CPP. Thus, MPEP may be beneficial for relapse prevention in morphine-addicts.     

Ionotropic and metabotropic glutamate receptor structure and pharmacology

Rationale l-Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS) and mediates its actions via activation of both ionotropic and metabotropic receptor families. The development of selective ligands, including competitive agonists and antagonists and positive and negative allosteric modulators, has enabled investigation of the functional roles of glutamate receptor family members.Objective In this review we describe the subunit structure and composition of the ionotropic and metabotropic glutamate receptors and discuss their pharmacology, particularly with respect to selective tools useful for investigation of their function in the CNS.Results A large number of ligands are now available that are selective either for glutamate receptor subfamilies or for particular receptor subtypes. Such ligands have enabled considerable advances in the elucidation of the physiological and pathophysiological roles of receptor family members. Furthermore, efficacy in animal models of neurological and psychiatric disorders has supported the progression of several glutamatergic ligands into clinical studies. These include ionotropic glutamate receptor antagonists, which have entered clinical trials for disorders including epilepsy and ischaemic stroke, -amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor positive allosteric modulators which are under evaluation as cognitive enhancers, and metabotropic glutamate receptor 2 (mGluR2) agonists which are undergoing clinical evaluation as anxiolytics. Furthermore, preclinical studies have illustrated therapeutic potential for ligands selective for other receptor subtypes in various disorders. These include mGluR1 antagonists in pain, mGluR5 antagonists in anxiety, pain and drug abuse and mGluR5 positive allosteric modulators in schizophrenia.Conclusions Selective pharmacological tools have enabled the study of glutamate receptors. However, pharmacological coverage of the family is incomplete and considerable scope remains for the development of novel ligands, particularly those with in vivo utility, and for the their use together with existing tools for the further investigation of the roles of receptor family members in CNS function and as potentially novel therapeutics.     

铝电解工人神经行为功能及mGluR1表达的改变

[目的]探讨铝电解作业工人神经行为功能与外周血淋巴细胞中代谢性谷氨酸受体I亚型（mGluR1）表达的变化。[方法]①从太原铝厂选取电解车间退休工人35名作为接触组,选取无职业接触铝的该铝厂退休工人32名为对照组,询问调查对象的一般状况;②采用世界卫生组织推荐的神经行为测试组合（NCTB）对两组工人神经行为功能进行测试;③用蛋白印迹（Western blot）检测外周血淋巴细胞mGluR1蛋白的表达水平。[结果]①接触组和对照组的一般情况（年龄、性别、工龄、教育程度、吸烟、饮酒和退休年数）均衡,差别没有统计学意义（P〉0.05）;②接触组情感状态指标愤怒-敌意（POMSA）和疲惫-惰性（POMSF）得分较对照组明显增加（P〈0.05）;困惑-迷茫（POMSC）、忧郁-一沮丧（POMSD）、紧张-焦虑（POMST）和有力-好动（POMSV）的得分没有明显改变（P〉0.05）;数字广度（DSP）包括顺序数字广度（DSPF）和倒序数字广度（DSPB）、数字译码数（DSY）和目标追踪的正确打点数（PAC）均比对照组降低（P〈0.05）,简单反应时（SRT）、最快反应时（SRTF）、最慢反应时（SRTS）、非利手提转捷度（SANN）、利手提转捷度（SANP）、Benton视觉保留（BVR）和打点总数（PA）的得分两组差异无统计学意义（P〉0.05）,提示电解接触铝对退休的作业工人的情感状态和神经行为均产生了一定的负面影响;③Western blot结果显示接触组mGluR1蛋白表达量高于对照组,差异有统计学意义（P〈0.01）。[结论]铝电解工人神经行为功能受损,淋巴细胞mGluRl蛋白表达水平上调。