Protective effect of metabotropic glutamate mGluR5 receptor elimination in a 6-hydroxydopamine model of Parkinson's disease

Insights into the Metabolism, Disposition, and Quantitative Profile of mGlu5 NAM AE90015 with Metabolite Identification and a Novel Integration Method

AE90015 is a highly specific and effective negative allosteric modulator (NAM) for the human mGlu5 receptor, showing significant promise for treating Parkinson's disease. An in vivo rat oral dose study was conducted on AE90015, which involved the collection of urine and bile samples over a 24 h period. At the study's endpoint, plasma, liver, brain, and renal tissues were also collected. A total of 30 metabolites of AE90015 were identified and structurally characterized or detected using high-resolution LC-MS/MSn. These metabolites fall into four categories: mono-hydroxyl, di-hydroxyl, mono-hydroxyl glucuronide, and di-hydroxyl glucuronide. This study provided a comprehensive overview of the metabolism, excretion, and disposition of AE90015, a promising NAM. The primary clearance pathway for AE90015 is mono-oxidation, accounting for 96% of the total, while direct excretion via renal and bile routes accounted for only 0.5%. Bile emerged as the predominant excretion route, at 65%, for metabolites and a minor amount of parent compound, which contrasts with the common assumption that urine would be the primary excretion pathway, which accounted for 26%. Each adamantyl and pyrazine moiety of AE90015 undergoes a one-time oxidation, while the pyridyl portion remains unmetabolized. Secondary metabolites, such as di-hydroxylated forms and glucuronide conjugates, do not contribute to clearance. In this work, a new quantification method combining UV and mass spectra integration was developed, allowing for the quantification of overlapping metabolite peaks. This novel approach proved to be highly effective for metabolite identification in early preclinical studies.

Control of Dopamine Signal in High-Order Receptor Complex on Striatal Astrocytes

The receptor-receptor interaction (RRI) of G protein-coupled receptors (GPCRs) leads to new functional entities that are conceptually distinct from the simple addition of signals mediated by the activation of the receptors that form the heteromers. Focusing on astrocytes, there is evidence for the existence of inhibitory and facilitatory RRIs, including the heteromers formed by the adenosine A2A and the dopamine D2 receptors, by A2A and the oxytocin receptor (OTR), and the D2-OTR heteromers. The possible involvement of these receptors in mosaicism has never been investigated in striatal astrocytes. By biophysical and functional approaches, we focused our attention on the existence of an A2A-D2-OTR high-order receptor complex and its role in modulating cytosolic calcium levels and endogenous glutamate release, when striatal astrocyte processes were stimulated with 4-aminopyridine. Functional data indicate a permissive role of OTR on dopamine signaling in the regulation of the glutamatergic transmission, and an inhibitory control mediated by A2A on both the D2-mediated signaling and on the OTR-facilitating effect on D2. Imaging biochemical and bioinformatic evidence confirmed the existence of the A2A-D2-OTR complex and its ternary structure in the membrane. In conclusion, the D2 receptor appears to be a hotspot in the control of the glutamate release from the astrocytic processes and may contribute to the regulation and integration of different neurotransmitter-mediated signaling in the striatum by the A2A-D2-OTR heterotrimers. Considering the possible selectivity of allosteric interventions on GPCRs organized as receptor mosaics, A2A-D2-OTR heterotrimers may offer selective pharmacological targets in neuropsychiatric disorders and neurodegenerative diseases.

Effect of the Metabotropic Glutamate Receptor Type 5 Negative Allosteric Modulator Dipraglurant on Motor and Non-Motor Symptoms of Parkinson's Disease

Parkinson's disease (PD) patients suffer not only from the primary motor symptoms of the disease but also from a range of non-motor symptoms (NMS) that cause disability and low quality of life. Excessive glutamate activity in the basal ganglia resulting from degeneration of the nigrostriatal dopamine pathway has been implicated in the motor symptoms, NMS and dyskinesias in PD patients. In this study, we investigated the effects of a selective mGlu5 negative allosteric modulator (NAM), dipraglurant, in a rodent motor symptoms model of PD, but also in models of anxiety, depression and obsessive-compulsive disorder, all of which are among the most prevalent NMS symptoms. Dipraglurant is rapidly absorbed after oral administration, readily crosses the blood-brain barrier, and exhibits a high correlation between plasma concentration and efficacy in behavioral models. In vivo, dipraglurant dose-dependently reduced haloperidol-induced catalepsy, increased punished licks in the Vogel conflict-drinking model, decreased immobility time in the forced swim test, decreased the number of buried marbles in the marble-burying test, but had no effect on rotarod performance or locomotor activity. These findings suggest that dipraglurant may have benefits to address some of the highly problematic comorbid non-motor symptoms of PD, in addition to its antidyskinetic effect demonstrated in PD-LID patients.

The mGlu5 Receptor Protomer-Mediated Dopamine D2 Receptor Trans-Inhibition Is Dependent on the Adenosine A2A Receptor Protomer: Implications for Parkinson's Disease

The adenosine A_2A receptor (A_2AR), dopamine D₂ receptor (D₂R) and metabotropic glutamate receptor type 5 (mGluR₅) form A_2AR-D₂R-mGluR₅ heteroreceptor complexes in living cells and in rat striatal neurons. In the current study, we present experimental data supporting the view that the A_2AR protomer plays a major role in the inhibitory modulation of the density and the allosteric receptor-receptor interaction within the D₂R-mGluR₅ heteromeric component of the A_2AR-D₂R-mGluR₅ complex in vitro and in vivo. The A_2AR and mGluR₅ protomers interact and modulate D₂R protomer recognition and signalling upon forming a trimeric complex from these receptors. Expression of A_2AR in HEK293T cells co-expressing D₂R and mGluR₅ resulted in a significant and marked increase in the formation of the D₂R-mGluR₅ heteromeric component in both bioluminescence resonance energy transfer and proximity ligation assays. A highly significant increase of the the high-affinity component of D₂R (D2R_{Ki High)} values was found upon cotreatment with the mGluR₅ and A_2AR agonists in the cells expressing A_2AR, D₂R and mGluR₅ with a significant effect observed also with the mGluR₅ agonist alone compared to cells expressing only D₂R and mGluR₅. In cells co-expressing A_2AR, D₂R and mGluR₅, stimulation of the cells with an mGluR₅ agonist like or D₂R antagonist fully counteracted the D₂R agonist-induced inhibition of the cAMP levels which was not true in cells only expressing mGluR₅ and D₂R. In agreement, the mGluR₅-negative allosteric modulator raseglurant significantly reduced the haloperidol-induced catalepsy in mice, and in A_2AR knockout mice, the haloperidol action had almost disappeared, supporting a functional role for mGluR₅ and A_2AR in enhancing D₂R blockade resulting in catalepsy. The results represent a relevant example of integrative activity within higher-order heteroreceptor complexes.

Targeting the Type 5 Metabotropic Glutamate Receptor: A Potential Therapeutic Strategy for Neurodegenerative Diseases?

The type 5 metabotropic glutamate receptor, mGlu₅, has been proposed as a potential therapeutic target for the treatment of several neurodegenerative diseases. In preclinical neurodegenerative disease models, novel allosteric modulators have been shown to improve cognitive performance and reduce disease-related pathology. A common pathological hallmark of neurodegenerative diseases is a chronic neuroinflammatory response, involving glial cells such as astrocytes and microglia. Since mGlu₅ is expressed in astrocytes, targeting this receptor could provide a potential mechanism by which neuroinflammatory processes in neurodegenerative disease may be modulated. This review will discuss current evidence that highlights the potential of mGlu₅ allosteric modulators to treat neurodegenerative diseases, including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, this review will explore the role of mGlu₅ in neuroinflammatory responses, and the potential for this G protein-coupled receptor to modulate neuroinflammation.

Prevention of L-Dopa-Induced Dyskinesias by MPEP Blockade of Metabotropic Glutamate Receptor 5 Is Associated with Reduced Inflammation in the Brain of Parkinsonian Monkeys

Proinflammatory markers were found in brains of Parkinson’s disease (PD) patients. After years of L-Dopa symptomatic treatment, most PD patients develop dyskinesias. The relationship between inflammation and L-Dopa-induced dyskinesias (LID) is still unclear. We previously reported that MPEP (a metabotropic glutamate receptor 5 antagonist) reduced the development of LID in de novo MPTP-lesioned monkeys. We thus investigated if MPEP reduced the brain inflammatory response in these MPTP-lesioned monkeys and the relationship to LID. The panmacrophage/microglia marker Iba1, the phagocytosis-related receptor CD68, and the astroglial protein GFAP were measured by Western blots. The L-Dopa-treated dyskinetic MPTP monkeys had increased Iba1 content in the putamen, substantia nigra, and globus pallidus, which was prevented by MPEP cotreatment; similar findings were observed for CD68 contents in the putamen and globus pallidus. There was a strong positive correlation between dyskinesia scores and microglial markers in these regions. GFAP contents were elevated in MPTP + L-Dopa-treated monkeys among these brain regions and prevented by MPEP in the putamen and subthalamic nucleus. In conclusion, these results showed increased inflammatory markers in the basal ganglia associated with LID and revealed that MPEP inhibition of glutamate activity reduced LID and levels of inflammatory markers.

Multi-omics integration and interactomics reveals molecular networks and regulators of the beneficial effect of yoga and exercise

Background

Yoga is a multifaceted spiritual tool that helps in maintaining health, peace of mind, and positive thoughts. In the context of asana, yoga is similar to physical exercise. This study aims to construct a molecular network to find hub genes that play important roles in physical exercise and yoga.

Methodology

We combined differentially expressed genes (DEGs) in yoga and exercise using computational bioinformatics from publicly available gene expression omnibus (GEO) datasets and identified the codifferentially expressed mRNAs with GEO2R. The co-DEGs were divided into four different groups and each group was subjected to protein-protein interaction (PPI) network, pathways analysis, and gene ontology.

Results

Our study identified immunological modulation as a dominant target of differential expression in yoga and exercise. Yoga predominantly modulated genes affecting the Th1 and NK cells, whereas Cytokines, Macrophage activation, and oxidative stress were affected by exercise. We also observed that while yoga regulated genes for two main physiological functions of the body, namely Circadian Rhythm (BHLHE40) and immunity (LBP, T-box transcription factor 21, CEACAM1), exercise-regulated genes involved in apoptosis (BAG3, protein kinase C alpha), angiogenesis, and cellular adhesion (EPH receptor A1).

Conclusion

The dissimilarity in the genetic expression patterns in Yoga and exercise highlights the discrete effect of each in biological systems. The integration and convergences of multi-omics signals can provide deeper and comprehensive insights into the various biological mechanisms through which yoga and exercise exert their beneficial effects and opens up potential newer research areas.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Targeting Vesicular Glutamate Transporter Machinery: Implications on Metabotropic Glutamate Receptor 5 Signaling and Behavior

Cross talk between both pre- and postsynaptic components of glutamatergic neurotransmission plays a crucial role in orchestrating a multitude of brain functions, including synaptic plasticity and motor planning. Metabotropic glutamate receptor (mGluR) 5 exhibits promising therapeutic potential for many neurodevelopmental and neurodegenerative disorders as a consequence of its modulatory control over diverse neuronal networks required for memory, motor coordination, neuronal survival, and differentiation. Given these crucial roles, mGluR5 signaling is under the tight control of glutamate release machinery mediated through vesicular glutamate transporters (VGLUTs) that ultimately dictate glutamatergic output. A particular VGLUT isoform, VGLUT3, exhibits an overlapping, but unique, distribution with mGluR5, and the dynamic cross talk between mGluR5 and VGLUT3 is key for the function of specific neuronal networks involved in motor coordination, emotions, and cognition. Thus, aberrant signaling of the VGLUT3-mGluR5 axis is linked to various pathologies including, but not limited to, Parkinson disease, anxiety disorders, and drug addiction. We argue that a comprehensive profiling of how coordinated VGLUT3-mGluR5 signaling influences overall glutamatergic neurotransmission is warranted. SIGNIFICANCE STATEMENT: Vesicular glutamate receptor (VGLUT) 3 machinery orchestrates glutamate release, and its distribution overlaps with metabotropic glutamate receptor (mGluR) 5 in regional brain circuitries, including striatum, hippocampus, and raphe nucleus. Therefore, VGLUT3-mGluR5 cross talk can significantly influence both physiologic and pathophysiologic glutamatergic neurotransmission. Pathological signaling of the VGLUT3-mGluR5 axis is linked to Parkinson disease, anxiety disorders, and drug addiction. However, it is also predicted to contribute to other motor and cognitive disorders.

Novel genome-wide associations for anhedonia, genetic correlation with psychiatric disorders, and polygenic association with brain structure

Anhedonia is a core symptom of several psychiatric disorders but its biological underpinnings are poorly understood. We performed a genome-wide association study of state anhedonia in 375,275 UK Biobank participants and assessed for genetic correlation between anhedonia and neuropsychiatric conditions (major depressive disorder, schizophrenia, bipolar disorder, obsessive compulsive disorder and Parkinson's Disease). We then used a polygenic risk score approach to test for association between genetic loading for anhedonia and both brain structure and brain function. This included: magnetic resonance imaging (MRI) assessments of total grey matter volume, white matter volume, cerebrospinal fluid volume, and 15 cortical/subcortical regions of interest; diffusion tensor imaging (DTI) measures of white matter tract integrity; and functional MRI activity during an emotion processing task. We identified 11 novel loci associated at genome-wide significance with anhedonia, with a SNP heritability estimate (h2SNP) of 5.6%. Strong positive genetic correlations were found between anhedonia and major depressive disorder, schizophrenia and bipolar disorder; but not with obsessive compulsive disorder or Parkinson's Disease. Polygenic risk for anhedonia was associated with poorer brain white matter integrity, smaller total grey matter volume, and smaller volumes of brain regions linked to reward and pleasure processing, including orbito-frontal cortex. In summary, the identification of novel anhedonia-associated loci substantially expands our current understanding of the biological basis of state anhedonia and genetic correlations with several psychiatric disorders confirm the utility of this phenotype as a transdiagnostic marker of vulnerability to mental illness. We also provide the first evidence that genetic risk for state anhedonia influences brain structure, including in regions associated with reward and pleasure processing.

mGluR5 Allosteric Modulation Promotes Neurorecovery in a 6-OHDA-Toxicant Model of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease characterized by a loss of dopaminergic substantia nigra neurons and depletion of dopamine. To date, current therapeutic approaches focus on managing motor symptoms and trying to slow neurodegeneration, with minimal capacity to promote neurorecovery. mGluR5 plays a key role in neuroplasticity, and altered mGluR5 signaling contributes to synucleinopathy and dyskinesia in patients with Parkinson's disease. Here, we tested whether the mGluR5-negative allosteric modulator, (2-chloro-4-[2[2,5-dimethyl-1-[4-(trifluoromethoxy) phenyl] imidazol-4-yl] ethynyl] pyridine (CTEP), would be effective in improving motor deficits and promoting neural recovery in a 6-hydroxydopamine (6-OHDA) mouse model. Lesions were induced by 6-ODHA striatal infusion, and 30 days later treatment with CTEP (2 mg/kg) or vehicle commenced for either 1 or 12 weeks. Animals were subjected to behavioral, pathological, and molecular analyses. We also assessed how long the effects of CTEP persisted, and finally, using rapamycin, determined the role of the mTOR pathway. CTEP treatment induced a duration-dependent improvement in apomorphine-induced rotation and performance on rotarod in lesioned mice. Moreover, CTEP promoted a recovery of striatal tyrosine hydroxylase-positive fibers and normalized FosB levels in lesioned mice. The beneficial effects of CTEP were paralleled by an activation of mammalian target of rapamycin (mTOR) pathway and elevated brain-derived neurotrophic factor levels in the striatum of lesioned mice. The mTOR inhibitor, rapamycin (sirolimus), abolished CTEP-induced neurorecovery and rescue of motor deficits. Our findings indicate that mTOR pathway is a useful target to promote recovery and that mGluR5 allosteric regulators may potentially be repurposed to selectively target this pathway to enhance neuroplasticity in patients with Parkinson's disease.

Biased agonism and allosteric modulation of metabotropic glutamate receptor 5

Metabotropic glutamate receptors belong to class C G-protein-coupled receptors and consist of eight subtypes that are ubiquitously expressed throughout the central nervous system. In recent years, the metabotropic glutamate receptor subtype 5 (mGlu₅) has emerged as a promising target for a broad range of psychiatric and neurological disorders. Drug discovery programs targetting mGlu₅ are primarily focused on development of allosteric modulators that interact with sites distinct from the endogenous agonist glutamate. Significant efforts have seen mGlu₅ allosteric modulators progress into clinical trials; however, recent failures due to lack of efficacy or adverse effects indicate a need for a better understanding of the functional consequences of mGlu₅ allosteric modulation. Biased agonism is an interrelated phenomenon to allosterism, describing how different ligands acting through the same receptor can differentially influence signaling to distinct transducers and pathways. Emerging evidence demonstrates that allosteric modulators can induce biased pharmacology at the level of intrinsic agonism as well as through differential modulation of orthosteric agonist-signaling pathways. Here, we present key considerations in the discovery and development of mGlu₅ allosteric modulators and the opportunities and pitfalls offered by biased agonism and modulation.

Metabotropic glutamate receptor 5 mediates the suppressive effect of 6-OHDA-induced model of Parkinson's disease on liver cancer

Numerous epidemiological studies suggested that there is a variable cancer risk in patients with Parkinson's disease (PD). However, the underlying mechanisms remain unclear. In the present study, the role of metabotropic glutamate receptor 5 (mGluR5) has been investigated in 6-hydroxydopamine (6-OHDA)-induced PD combined with liver cancer both in vitro and in vivo. We found that PD cellular model from 6-OHDA-lesioned MN9D cells suppressed the growth, migration, and invasion of Hepa1-6 cells via down-regulation of mGluR5-mediated ERK and Akt pathway. The application of 2-methyl-6-(phenylethyl)-pyridine and knockdown of mGluR5 further decreased the effect on Hepa-1-6 cells when co-cultured with conditioned media. The effect was increased by (S)-3,5-dihydroxyphenylglycine and overexpression of mGluR5. Moreover, more release of glutamate from 6-OHDA-lesioned MN9D cells suppressed mGluR5-mediated effect of Hepa1-6 cells. Application of riluzole eliminated the increased glutamate release induced by 6-OHDA in MN9D cells and aggravated the suppressive effect on Hepa-1-6 cells. In addition, the growth of implanted liver cancer was inhibited in 6-OHDA induced PD-like rats, and was associated with increased glutamate release in the serum and down-regulation of mGluR5 in tumor tissue. Collectively, these results indicate that selective antagonism of glutamate and mGluR5 has a potentially beneficial effect in both liver cancer and PD, and thus may provide more understanding for the clinical investigation and further an additional therapeutic target for these two diseases.

Dynamic Changes in Striatal mGluR1 But Not mGluR5 during Pathological Progression of Parkinson's Disease in Human Alpha-Synuclein A53T Transgenic Rats: A Multi-PET Imaging Study

Parkinson's disease (PD) is a prevalent degenerative disorder affecting the CNS that is primarily characterized by resting tremor and movement deficits. Group I metabotropic glutamate receptor subtypes 1 and 5 (mGluR1 and mGluR5, respectively) are important targets for investigation in several CNS disorders. In the present study, we investigated the in vivo roles of mGluR1 and mGluR5 in chronic PD pathology by performing longitudinal positron emission tomography (PET) imaging in A53T transgenic (A53T-Tg) rats expressing an abnormal human α-synuclein (ASN) gene. A53T-Tg rats showed a dramatic decline in general motor activities with age, along with abnormal ASN aggregation and striatal neuron degeneration. In longitudinal PET imaging, striatal nondisplaceable binding potential (BPND) values for [(11)C]ITDM (N-[4-[6-(isopropylamino) pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]methylbenzamide), a selective PET ligand for mGluR1, temporarily increased before PD symptom onset and dramatically decreased afterward with age. However, striatal BPND values for (E)-[(11)C]ABP688 [3-(6-methylpyridin-2-ylethynyl)-cyclohex-2-enone-(E)-O-[(11)C]methyloxime], a specific PET ligand for mGluR5, remained constant during experimental terms. The dynamic changes in striatal mGluR1 BPND values also showed a high correlation in pathological decreases in general motor activities. Furthermore, declines in mGluR1 BPND values were correlated with decreases in BPND values for [(18)F]FE-PE2I [(E)-N-(3-iodoprop-2E-enyl)-2β-carbo-[(18)F]fluoroethoxy-3β-(4-methylphenyl) nortropane], a specific PET ligand for the dopamine transporter, a biomarker for dopaminergic neurons. In conclusion, our results have demonstrated for the first time that dynamic changes occur in mGluR1, but not mGluR5, that accompany pathological progression in a PD animal model.

SIGNIFICANCE STATEMENT

Synaptic signaling by glutamate, the principal excitatory neurotransmitter in the brain, is modulated by group I metabotropic glutamate receptors, including the mGluR1 and mGluR5 subtypes. In the brain, mGluR1 and mGluR5 have distinct functional roles and regional distributions. Their roles in brain pathology, however, are not well characterized. Using longitudinal PET imaging in a chronic rat model of PD, we demonstrated that expression of mGluR1, but not mGluR5, dynamically changed in the striatum accompanying pathological PD progression. These findings imply that monitoring mGluR1 in vivo may provide beneficial information to further understand central nervous system disorders.

The impact of metabotropic glutamate receptors into active neurodegenerative processes: A "dark side" in the development of new symptomatic treatments for neurologic and psychiatric disorders

Metabotropic glutamate (mGlu) receptor ligands are under clinical development for the treatment of CNS disorders with high social and economic burden, such as schizophrenia, major depressive disorder (MDD), and Parkinson's disease (PD), and are promising drug candidates for the treatment of Alzheimer's disease (AD). So far, clinical studies have shown symptomatic effects of mGlu receptor ligands, but it is unknown whether these drugs act as disease modifiers or, at the opposite end, they accelerate disease progression by enhancing neurodegeneration. This is a fundamental issue in the treatment of PD and AD, and is also an emerging theme in the treatment of schizophrenia and MDD, in which neurodegeneration is also present and contribute to disease progression. Moving from in vitro data and preclinical studies, we discuss the potential impact of drugs targeting mGlu2, mGlu3, mGlu4 and mGlu5 receptor ligands on active neurodegeneration associated with AD, PD, schizophrenia, and MDD. We wish to highlight that our final comments on the best drug candidates are not influenced by commercial interests or by previous or ongoing collaborations with drug companies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Blockade of metabotropic glutamate receptor 5 protects against DNA damage in a rotenone-induced Parkinson's disease model

Glutamate excitotoxicity contributes to the development of Parkinson's disease (PD) and pharmacological blockade of metabotropic glutamate receptor 5 (mGluR5) has beneficial anti-akinetic effects in animal models of PD; however, the mechanism by which these antagonists alleviate PD symptoms is largely unknown. In our study, the effects of mGluR5 inhibition on DNA damage were investigated in a rotenone-induced model of PD. We first found that the selective mGluR5 antagonist, 2-methyl-6- (phenylethynyl) pyridine, prevented rotenone-induced DNA damage in MN9D dopaminergic neurons through a mechanism involving the downregulation of intracellular calcium release which was associated with a reduction in endoplasmic reticulum stress and reactive oxygen species (ROS)-related mitochondrial dysfunction. Interestingly, the ROS-related mitochondrial dysfunction was accompanied by an increase in expression of the antioxidant protein, Trx2. Treatment of cells with the calcium chelating agent 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or the ROS scavenger N-acetyl-L-cysteine, also reduced rotenone-induced DNA damage, while transfection of a dominant-negative form of Trx2 increased it. In addition, mGluR5 inhibition altered the expression profiles of proteins involved in DNA repair activity. Specifically, the expression of phosphorylated ERK (p-ERK) and CREB, as well as APE1 and Rad51 were elevated after rotenone stimulation and were subsequently downregulated following blockade of mGluR5. These findings were confirmed in vivo in a rotenone-induced rat model of PD. Inhibition of mGluR5 protected against neurotoxicity by mitigating oxidative stress-related DNA damage associated with 8-hydroxy-2'-deoxyguanosine production and also reduced p-ERK activity and Trx2 expression. These findings provide a novel link between mGluR5 and DNA damage in a model of PD, and reveal a potential mechanism by which mGluR5 mediates DNA damage in neurodegenerative diseases.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Co-operative binding assay for the characterization of mGlu4 allosteric modulators

The interest in the role of metabotropic glutamate receptor 4 (mGlu4) in CNS related disorders has increased the need for methods to investigate the binding of allosteric drug candidates. Our aim is to present the first fully characterized in vitro binding assay of mGlu4 positive allosteric modulators (PAMs). Results suggest that mGlu4 PAMs have characteristic co-operative binding with orthosteric glutamate, which offers a notable insight to the further development of mGlu4 targeted therapies.

Mavoglurant as a treatment for Parkinson's disease

INTRODUCTION

A major unresolved issue in the Parkinson's disease (PD) treatment is the development of l-DOPA-induced dyskinesias (LIDs) as a side effect of chronic L-DOPA administration. Currently, LIDs are managed in part by reducing the L-DOPA dose or by the administration of amantadine. However, this treatment is only partially effective. A potential strategy, currently under investigation, is the coadministration of metabotropic glutamate receptor 5 (mGluR5) negative allosteric modulators (NAMs) and L-DOPA; a treatment that results in the improvement of dyskinesia symptoms and that permits reductions in l-DOPA dosage frequency.

AREAS COVERED

The authors examine the role of mGluR5 in the pathophysiology of PD and the potential use of mGluR5 NAM as an adjuvant therapy together with a primary treatment with L-DOPA. Specifically, the authors look at the mavoglurant therapy and the evidence presented through preclinical and clinical trials.

EXPERT OPINION

Interaction between mGluR5 NAM and L-DOPA is an area of interest in PD research as concomitant treatment results in the improvement of LID symptoms in humans, thus enhancing the patient's quality of life. However, few months ago, Novartis decided to discontinue clinical trials of mavoglurant for the treatment of LID, due to the lack of efficacy demonstrated in trials NCT01385592 and NCT01491529, although no safety concerns were involved in this decision. Nevertheless, the potential application of mGluR5 antagonists as neuroprotective agents must be considered and further studies are warranted to better investigate their potential.

Ceftriaxone increases glutamate uptake and reduces striatal tyrosine hydroxylase loss in 6-OHDA Parkinson's model

Excess glutamatergic neurotransmission may contribute to excitotoxic loss of nigrostriatal neurons in Parkinson's disease (PD). Here, we determined if increasing glutamate uptake could reduce the extent of tyrosine hydroxylase (TH) loss in PD progression. The beta-lactam antibiotic, ceftriaxone, increases the expression of glutamate transporter 1 (GLT-1), a glutamate transporter that plays a major role in glutamate clearance in central nervous system and may attenuate adverse behavioral or neurobiological function in other neurodegenerative disease models. In association with >80% TH loss, we observed a significant decrease in glutamate uptake in the established 6-hydroxydopamine (6-OHDA) PD model. Ceftriaxone (200 mg/kg, i.p.) increased striatal glutamate uptake with >5 consecutive days of injection in nonlesioned rats and lasted out to 14 days postinjection, a time beyond that required for 6-OHDA to produce >70% TH loss (∼9 days). When ceftriaxone was given at the time of 6-OHDA, TH loss was ∼57% compared to ∼85% in temporally matched vehicle-injected controls and amphetamine-induced rotation was reduced about 2-fold. This attenuation of TH loss was associated with increased glutamate uptake, increased GLT-1 expression, and reduced Serine 19 TH phosphorylation, a calcium-dependent target specific for nigrostriatal neurons. These results reveal that glutamate uptake can be targeted in a PD model, decrease the rate of TH loss in a calcium-dependent manner, and attenuate locomotor behavior associated with 6-OHDA lesion. Given that detection of reliable PD markers will eventually be employed in susceptible populations, our results give credence to the possibility that increasing glutamate uptake may prolong the time period before locomotor impairment occurs.

Hippocampal neuronal cells that accumulate α-synuclein fragments are more vulnerable to Aβ oligomer toxicity via mGluR5--implications for dementia with Lewy bodies

BACKGROUND

In dementia with Lewy bodies (DLB) abnormal interactions between α-synuclein (α-syn) and beta amyloid (Aβ) result in selective degeneration of neurons in the neocortex, limbic system and striatum. However, factors rendering these neurons selectively vulnerable have not been fully investigated. The metabotropic glutamate receptor 5 (mGluR5) has been shown to be up regulated in DLB and might play a role as a mediator of the neurotoxic effects of Aβ and α-syn in vulnerable neuronal populations. In this context, the main objective of the present study was to investigate the role of mGluR5 as a mediator of the neurotoxic effects of α-syn and Aβ in the hippocampus.

RESULTS

We generated double transgenic mice over-expressing amyloid precursor protein (APP) and α-syn under the mThy1 cassette and investigated the relationship between α-syn cleavage, Aβ, mGluR5 and neurodegeneration in the hippocampus. We found that compared to the single tg mice, the α-syn/APP tg mice displayed greater accumulation of α-syn and mGluR5 in the CA3 region of the hippocampus compared to the CA1 and other regions. This was accompanied by loss of CA3 (but not CA1) neurons in the single and α-syn/APP tg mice and greater loss of MAP 2 and synaptophysin in the CA3 in the α-syn/APP tg. mGluR5 gene transfer using a lentiviral vector into the hippocampus CA1 region resulted in greater α-syn accumulation and neurodegeneration in the single and α-syn/APP tg mice. In contrast, silencing mGluR5 with a lenti-shRNA protected neurons in the CA3 region of tg mice. In vitro, greater toxicity was observed in primary hippocampal neuronal cultures treated with Aβ oligomers and over-expressing α-syn; this effect was attenuated by down-regulating mGluR5 with an shRNA lentiviral vector. In α-syn-expressing neuronal cells lines, Aβ oligomers promoted increased intracellular calcium levels, calpain activation and α-syn cleavage resulting in caspase-3-dependent cell death. Treatment with pharmacological mGluR5 inhibitors such as 2-Methyl-6-(phenylethynyl)pyridine (MPEP) and 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine (MTEP) attenuated the toxic effects of Aβ in α-syn-expressing neuronal cells.

CONCLUSIONS

Together, these results support the possibility that vulnerability of hippocampal neurons to α-syn and Aβ might be mediated via mGluR5. Moreover, therapeutical interventions targeting mGluR5 might have a role in DLB.

Role of nucleus accumbens glutamatergic plasticity in drug addiction

Substance dependence is characterized by a group of symptoms, according to the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR). These symptoms include tolerance, withdrawal, drug consumption for alleviating withdrawal, exaggerated consumption beyond original intention, failure to reduce drug consumption, expending a considerable amount of time obtaining or recovering from the substance's effects, disregard of basic aspects of life (for example, family), and maintenance of drug consumption, despite facing adverse consequences. The nucleus accumbens (NAc) is a brain structure located in the basal forebrain of vertebrates, and it has been the target of addictive drugs. Different neurotransmitter systems at the level of the NAc circuitry have been linked to the different problems of drug addiction, like compulsive use and relapse. The glutamate system has been linked mainly to relapse after drug-seeking extinction. The dopamine system has been linked mainly to compulsive drug use. The glutamate homeostasis hypothesis centers around the dynamics of synaptic and extrasynaptic levels of glutamate, and their impact on circuitry from the prefrontal cortex (PFC) to the NAc. After repetitive drug use, deregulation of this homeostasis increases the release of glutamate from the PFC to the NAc during drug relapse. Glial cells also play a fundamental role in this hypothesis; glial cells shape the interactions between the PFC and the NAc by means of altering glutamate levels in synaptic and extrasynaptic spaces. On the other hand, cocaine self-administration and withdrawal increases the surface expression of subunit glutamate receptor 1 (GluA1) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors at the level of the NAc. Also, cocaine self-administration and withdrawal induce the formation of subunit glutamate receptor 2 (GluA2), lacking the Ca(2+)-permeable AMPA receptors (CP-AMPARs) at the level of the NAc. Antagonism of the CP-AMPARs reduces cravings. It is necessary to pursue further exploration of the AMPA receptor subunit composition and variations at the level of the NAc for a better understanding of glutamatergic plastic changes. It is known that cocaine and morphine are able to induce changes in dendritic spine morphology by modifying actin cycling. These changes include an initial increase in spine head diameter and increases in AMPA receptor expression, followed by a second stage of spine head diameter retraction and reduction of the AMPA receptors' expression in spines. Besides glutamate and dopamine, other factors, like brain-derived neurotrophic factor (BDNF), can influence NAc activity and induce changes in dendritic spine density. BDNF also induces drug-related behaviors like self-administration and relapse. Neither apoptosis nor neurogenesis plays a relevant role in the neurobiological processes subjacent to cocaine addiction in adults (rodent or human). Different therapeutic drugs like N-acetylcysteine (NAC), modafinil, acamprosate, and topiramate have been tested in preclinical and/or clinical models for alleviating drug relapse. Moreover, these therapeutic drugs target the glutamatergic circuitry between the PFC and the NAc. NAC and acamprosate have shown inconsistent results in clinical trials. Modafinil and topiramate have shown some success, but more clinical trials are necessary. Based on the current review findings, it could be recommendable to explore therapeutic approaches that include synergism between different drugs and neurotransmitter systems. The discrepancy in the results of some therapeutic drugs between preclinical versus clinical trials for alleviating relapse or drug dependence could be linked to the scarce exploration of preclinical models that mimic polydrug abuse patterns, for example, cocaine plus alcohol. At the clinical level, the pattern of polydrug consumption is a phenomenon of considerable frequency. Finally, as a complement at the end, an updated summary is included about the role of glutamate in other neuropsychiatric disorders (for example, mood disorders, schizophrenia, and others).

Targeting glutamate receptors to tackle the pathogenesis, clinical symptoms and levodopa-induced dyskinesia associated with Parkinson's disease

The appearance of levodopa-induced dyskinesia (LID) and ongoing degeneration of nigrostriatal dopaminergic neurons are two key features of Parkinson's disease (PD) that current treatments fail to address. Increased glutamate transmission contributes to the motor symptoms in PD, to the striatal plasticity that underpins LID and to the progression of neurodegeneration through excitotoxic mechanisms. Glutamate receptors have therefore long been considered as potential targets for pharmacological intervention in PD, with emphasis on either blocking activation of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA), N-methyl-D-aspartate (NMDA) or excitatory metabotropic glutamate (mGlu) 5 receptors or promoting the activation of group II/III mGlu receptors. Following a brief summary of the role of glutamate in PD and LID, this article explores the current status of pharmacological studies in pre-clinical rodent and primate models through to clinical trials, where applicable, that support the potential of glutamate-based therapeutic interventions. To date, AMPA antagonists have shown good efficacy against LID in rat and primate models, but the failure of perampanel to lessen LID in clinical trials casts doubt on the translational potential of this approach. In contrast, antagonists selective for NR2B-containing NMDA receptors were effective against LID in animal models and in small-scale clinical trials, though observed adverse cognitive effects need addressing. So far, mGlu5 antagonists or negative allosteric modulators (NAMs) look set to become the first introduced for tackling LID, with AFQ-056 reported to exhibit good efficacy in phase II clinical trials. NR2B antagonists and mGlu5 NAMs may subsequently prove to also be effective disease-modifying agents if their protective effects in rat and primate models of PD, respectively, are replicated in the next stages of investigation. Finally, group III mGlu4 agonists or positive allosteric modulators (PAMs), although in the early pre-clinical stages of investigation, are showing good efficacy against motor symptoms, neurodegeneration and LID. It is anticipated that the recent development of mGlu4 PAMs with improved systemic bioavailability will facilitate progression of these agents into the primate model of PD where their potential can be further explored.

Cognitive deficits in animal models of basal ganglia disorders

The two most common neurological disorders of the basal ganglia are Parkinson's disease (PD) and Huntington's disease (HD). The most overt symptoms of these diseases are motoric, reflecting the loss of the striatal medium spiny neurons in HD and ascending substantia nigra dopaminergic cells in PD. However, both disease processes induce insidious psychiatric and cognitive syndromes that can manifest well in advance of the onset of motor deficits. These early deficits provide an opportunity for prophylactic therapeutic intervention in order to retard disease progression from the earliest possible point. In order to exploit this opportunity, animal models of HD and PD are being probed for the specific cognitive deficits represented in the disease states. At the neuronal level, these deficits are typically, but not exclusively, mediated by disruption of parallel corticostriatal loops that integrate motor information with sensory and higher order, "executive" cognitive functions. Dysfunction in these systems can be probed with sensitive behavioural tests that selectively probe these cognitive functions in mouse models with focal lesions of striatal or cortical regions, or of specific neurotransmitter systems. Typically these tests were designed and validated in rats. With the advent of genetically modified mouse models of disease, validated tests provide an opportunity to screen mouse models of disease for early onset cognitive deficits. This review seeks to draw together the literature on cognitive deficits in HD and PD, to determine the extent to which these deficits are represented in the current animal models of disease, and to evaluate the viability of selecting cognitive deficits as potential therapeutic targets. This article is part of a Special Issue entitled 'Animal Models'.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.

Metabotropic glutamate receptors in neurodegeneration/neuroprotection: still a hot topic?

Moving from early studies, we here review the most recent evidence linking metabotropic glutamate (mGlu) receptors to processes of neurodegeneration/neuroprotection. The use of knockout mice and subtype-selective drugs has increased our knowledge of the precise role played by individual mGlu receptor subtypes in these processes. Activation of mGlu1 and mGlu5 receptors may either amplify or reduce neuronal damage depending on the context and the nature of the toxic insults. In contrast, mGlu1 and mGlu5 receptors antagonists are consistently protective in in vitro and in vivo models of neuronal death. A series of studies suggest that mGlu1 receptor antagonists or negative allosteric modulators (NAMs) are promising candidates for the treatment of ischemic brain damage, whereas mGlu5 receptor NAMs, which have been clinically developed for the treatment of Parkinson's disease (PD) and l-DOPA-induced dyskinesias, protect nigro-striatal dopaminergic neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice and monkeys. Activation of glial mGlu3 receptors promotes the formation of various neurotrophic factors, such as transforming growth factor-β (TGF-β), glial-derived neurotrophic factor (GDNF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF). Hence, selective mGlu3 receptor agonists or positive allosteric modulators (PAMs) (not yet available) are potentially helpful in the treatment of chronic neurodegenerative disorders such as PD, Alzheimer's disease (AD), and amyotrophic lateral sclerosis. Selective mGlu2 receptor PAMs should be used with caution in AD patients because these drugs are shown to amplify β-amyloid neurotoxicity. Finally, mGlu4 receptor agonists/PAMs share with mGlu5 receptor NAMs the ability to improve motor symptoms associated with PD and attenuate nigro-striatal degeneration at the same time. No data are yet available on the role of mGlu7 and mGlu8 receptors in neurodegeneration/neuroprotection.