Targeting metabotropic glutamate receptors (mGluRs) in Parkinson's disease

Glutamatergic dysfunction in neurodegenerative diseases focusing on Parkinson's disease: Role of glutamate modulators

Parkinson's disease (PD) is a prevalent neurodegenerative disorder resulting from the degeneration of dopamenergic neurons in the substantia nigra pars compacta (SNpc). Research has predominantly centered on understanding the dysfunction of dopaminergic neurotransmission in PD. Recently, more studies discussed the potential role of other neurotransmitters in PD neuropathology. One of the most important non-dopaminergic neurotransmitters involved in the pathogenesis of PD is glutamate, which is widely involved in glutamatergic neurotransmission in different brain regions, including SNpc. The development and progression of PD neuropathology and levodopa-induced dyskinesias (LID) are associated with glutamate neurotoxicity. Therefore, this review seeks to explore the possible involvement of glutamatergic signaling in PD development and assess the therapeutic potential of glutamate receptor antagonists in treating the disorder.

Cholesterol Dependence of the Conformational Changes in Metabotropic Glutamate Receptor 1

Metabotropic glutamate receptors (mGluRs) are class C G protein-coupled receptors that function as obligate dimers in regulating neurotransmission and synaptic plasticity in the central nervous system. The mGluR1 subtype has been shown to be modulated by the membrane lipid environment, particularly cholesterol, though the molecular mechanisms remain elusive. In this study, we employed all-atom molecular dynamics simulations to investigate the effects of cholesterol on the conformational dynamics of the mGluR1 seven-transmembrane (7TM) domain in an inactive state model. Simulations were performed with three different cholesterol concentrations (0%, 10%, and 25%) in a palmitoyl-oleoyl phosphatidylcholine (POPC) lipid bilayer system. Our results demonstrate that cholesterol induces conformational changes in the mGluR1 dimer more significantly than in the individual protomers. Notably, cholesterol modulates the dynamics and conformations of the TM1 and TM2 helices at the dimer interface. Interestingly, an intermediate cholesterol concentration of 10% elicits more pronounced conformational changes compared to both cholesterol-depleted (0%) and cholesterol-enriched (25%) systems. Specific electrostatic interaction unique to the 10% cholesterol system further corroborate these conformational differences. Given the high sequence conservation of the 7TM domains across mGluR subtypes, the cholesterol-dependent effects observed in mGluR1 are likely applicable to other members of this receptor family. Our findings provide atomistic insights into how cholesterol modulates the conformational landscape of mGluRs, which could impact their function and signaling mechanisms.

Identification of Novel Quinolone and Quinazoline Alkaloids as Phosphodiesterase 10A Inhibitors for Parkinson's Disease through a Computational Approach

Phosphodiesterases (PDEs) are vital in signal transduction, specifically by hydrolyzing cAMP and cGMP. Within the PDE family, PDE10A is notable for its prominence in the striatum and its regulatory function over neurotransmitters in medium-spiny neurons. Given the dopamine deficiency in Parkinson's disease (PD) that affects striatal pathways, PDE10A inhibitors could offer therapeutic benefits by modulating D1 and D2 receptor signaling. This study was motivated by the successful history of quinazoline/quinazoline scaffolds in the inhibition of PDE10A. This study involved detailed in silico evaluations through docking followed by pharmacological, pharmacophoric, and pharmacokinetic analyses, prioritizing central nervous system (CNS)-active drug criteria. Seven cyclic peptides, those featuring the quinazoline/quinazoline moiety at both termini, exhibited notably enhanced docking scores compared to those of the remaining alkaloids within the screened library. We identified 7 quinolines and 1 quinazoline including Lepadin G, Aspernigerin, CJ-13536, Aurachin A, 2-Undecyl-4(1H)-quinolone, Huajiaosimuline 3-Prenyl-4-prenyloxyquinolin-2-one, and Isaindigotone that followed the standard CNS active drug criteria. The dominant quinoline ring in our study and its related quinazoline were central to our evaluations; therefore, the pharmacophoric features of these scaffolds were highlighted. The top alkaloids met all CNS-active drug properties; while nonmutagenic and without PAINS alerts, many indicated potential hepatotoxicity. Among the compounds, Huajiaosimuline was particularly significant due to its alignment with lead-likeness and CNS-active criteria. Aspernigerin demonstrated its affinity for numerous dopamine receptors, which signifies its potential to alter dopaminergic neurotransmission that is directly related to PD. Interestingly, the majority of these alkaloids had biological targets primarily associated with G protein-coupled receptors, critical in PD pathophysiology. They exhibit superior excretion parameters and toxicity end-points compared to the standard. Notably, selected alkaloids demonstrated stability in the binding pocket of PDE10A according to the molecular dynamic simulation results. Our findings emphasize the potential of these alkaloids as PDE10A inhibitors. Further experimental studies may be necessary to confirm their actual potency in inhibiting PDE10A before exploring their therapeutic potential in PD.

Genome- and Exome-Wide Association Studies Revealed Candidate Genes Associated with DaTscan Imaging Features

Introduction

Despite remarkable progress in identifying Parkinson's disease (PD) genetic risk loci, the genetic basis of PD remains largely unknown. With the help of the endophenotype approach and using data from dopamine transporter single-photon emission computerized tomography (DaTscan), we identified potentially involved genes in PD.

Method

We conducted an imaging genetic study by performing exome-wide association study (EWAS) and genome-wide association study (GWAS) on the specific binding ratio (SBR) of six DaTscan anatomical areas between 489 and 559 subjects of Parkinson's progression markers initiative (PPMI) cohort and 83,623 and 36,845 single-nucleotide polymorphisms (SNPs)/insertion-deletion mutations (INDELs). We also investigated the association of cerebrospinal fluid (CSF) protein concentration of our significant genes with PD progression using PPMI CSF proteome data.

Results

Among 83,623 SNPs/INDELs in EWAS, one SNP (rs201465075) on 1 q32.1 locus was significantly (P value = 4.03 × 10-7) associated with left caudate DaTscan SBR, and 33 SNPs were suggestive. Among 36,845 SNPs in GWAS, one SNP (rs12450112) on 17 p.12 locus was significantly (P value = 1.34 × 10-6) associated with right anterior putamen DaTscan SBR, and 39 SNPs were suggestive among which 8 SNPs were intergenic. We found that rs201465075 and rs12450112 are most likely related to IGFN1 and MAP2K4 genes. The protein level of MAP2K4 in the CSF was significantly associated with PD progression in the PPMI cohort; however, proteomic data were not available for the IGFN1 gene.

Conclusion

We have shown that particular variants of IGFN1 and MAP2K4 genes may be associated with PD. Since DaTscan imaging could be positive in other Parkinsonian syndromes, caution should be taken when interpreting our results. Future experimental studies are also needed to verify these findings.

Metabotropic glutamate receptors and cognition: From underlying plasticity and neuroprotection to cognitive disorders and therapeutic targets

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors that play pivotal roles in mediating the activity of neurons and other cell types within the brain, communication between cell types, synaptic plasticity, and gene expression. As such, these receptors play an important role in a number of cognitive processes. In this chapter, we discuss the role of mGlu receptors in various forms of cognition and their underlying physiology, with an emphasis on cognitive dysfunction. Specifically, we highlight evidence that links mGlu physiology to cognitive dysfunction across brain disorders including Parkinson's disease, Alzheimer's disease, Fragile X syndrome, post-traumatic stress disorder, and schizophrenia. We also provide recent evidence demonstrating that mGlu receptors may elicit neuroprotective effects in particular disease states. Lastly, we discuss how mGlu receptors can be targeted utilizing positive and negative allosteric modulators as well as subtype specific agonists and antagonist to restore cognitive function across these disorders.

The role of excitatory amino acid transporter 2 (EAAT2) in epilepsy and other neurological disorders

Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS). Excitatory amino acid transporters (EAATs) have important roles in the uptake of glutamate and termination of glutamatergic transmission. Up to now, five EAAT isoforms (EAAT1-5) have been identified in mammals. The main focus of this review is EAAT2. This protein has an important role in the pathoetiology of epilepsy. De novo dominant mutations, as well as inherited recessive mutation in this gene, have been associated with epilepsy. Moreover, dysregulation of this protein is implicated in a range of neurological diseases, namely amyotrophic lateral sclerosis, alzheimer's disease, parkinson's disease, schizophrenia, epilepsy, and autism. In this review, we summarize the role of EAAT2 in epilepsy and other neurological disorders, then provide an overview of the therapeutic modulation of this protein.

Roles of metabotropic glutamate receptor 8 in neuropsychiatric and neurological disorders

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors. Among eight mGlu subtypes (mGlu1-8), mGlu8 has drawn increasing attention. This subtype is localized to the presynaptic active zone of neurotransmitter release and is among the mGlu subtypes with high affinity for glutamate. As a G_i/o-coupled autoreceptor, mGlu8 inhibits glutamate release to maintain homeostasis of glutamatergic transmission. mGlu8 receptors are expressed in limbic brain regions and play a pivotal role in modulating motivation, emotion, cognition, and motor functions. Emerging evidence emphasizes the increasing clinical relevance of abnormal mGlu8 activity. Studies using mGlu8 selective agents and knockout mice have revealed the linkage of mGlu8 receptors to multiple neuropsychiatric and neurological disorders, including anxiety, epilepsy, Parkinson's disease, drug addiction, and chronic pain. Expression and function of mGlu8 receptors in some limbic structures undergo long-lasting adaptive changes in animal models of these disorders, which may contribute to the remodeling of glutamatergic transmission critical for the pathogenesis and symptomatology of brain illnesses. This review summarizes the current understanding of mGlu8 biology and the possible involvement of the receptor in several common psychiatric and neurological disorders.

Structure-Based Discovery of Negative Allosteric Modulators of the Metabotropic Glutamate Receptor 5

Recently determined structures of class C G protein-coupled receptors (GPCRs) revealed the location of allosteric binding sites and opened new opportunities for the discovery of novel modulators. In this work, molecular docking screens for allosteric modulators targeting the metabotropic glutamate receptor 5 (mGlu₅) were performed. The mGlu₅ receptor is activated by the main excitatory neurotransmitter of the nervous central system, L-glutamate, and mGlu₅ receptor activity can be allosterically modulated by negative or positive allosteric modulators. The mGlu₅ receptor is a promising target for the treatment of psychiatric and neurodegenerative diseases, and several allosteric modulators of this GPCR have been evaluated in clinical trials. Chemical libraries containing fragment- (1.6 million molecules) and lead-like (4.6 million molecules) compounds were docked to an allosteric binding site of mGlu₅ identified in X-ray crystal structures. Among the top-ranked compounds, 59 fragments and 59 lead-like compounds were selected for experimental evaluation. Of these, four fragment- and seven lead-like compounds were confirmed to bind to the allosteric site with affinities ranging from 0.43 to 8.6 μM, corresponding to a hit rate of 9%. The four compounds with the highest affinities were demonstrated to be negative allosteric modulators of mGlu₅ signaling in functional assays. The results demonstrate that virtual screens of fragment- and lead-like chemical libraries have complementary advantages and illustrate how access to high-resolution structures of GPCRs in complex with allosteric modulators can accelerate lead discovery.

Correlation of Ferroptosis and Other Types of Cell Death in Neurodegenerative Diseases

Ferroptosis is defined as an iron-dependent, non-apoptotic cell death pathway, with specific morphological phenotypes and biochemical changes. There is a growing realization that ferroptosis has significant implications for several neurodegenerative diseases. Even though ferroptosis is different from other forms of programmed death such as apoptosis and autophagic death, they involve a number of common protein molecules. This review focuses on current research on ferroptosis and summarizes the cross-talk among ferroptosis, apoptosis, and autophagy that are implicated in neurodegenerative diseases. We hope that this information provides new ideas for understanding the mechanisms and searching for potential therapeutic approaches and prevention of neurodegenerative diseases.

Genes of the Glutamatergic System and Tardive Dyskinesia in Patients with Schizophrenia

Background: Tardive dyskinesia (TD) is an extrapyramidal side effect of the long-term use of antipsychotics. In the present study, the role of glutamatergic system genes in the pathogenesis of total TD, as well as two phenotypic forms, orofacial TD and limb-truncal TD, was studied. Methods: A set of 46 SNPs of the glutamatergic system genes (GRIN2A, GRIN2B, GRIK4, GRM3, GRM7, GRM8, SLC1A2, SLC1A3, SLC17A7) was studied in a population of 704 Caucasian patients with schizophrenia. Genotyping was performed using the MassARRAY Analyzer 4 (Agena Bioscience™). Logistic regression analysis was performed to test for the association of TD with the SNPs while adjusting for confounders. Results: No statistically significant associations between the SNPs and TD were found after adjusting for multiple testing. Since three SNPs of the SLC1A2 gene demonstrated nominally significant associations, we carried out a haplotype analysis for these SNPs. This analysis identified a risk haplotype for TD comprising CAT alleles of the SLC1A2 gene SNPs rs1042113, rs10768121, and rs12361171. Nominally significant associations were identified for SLC1A3 rs2229894 and orofacial TD, as well as for GRIN2A rs7192557 and limb-truncal TD. Conclusions: Genes encoding for mGlu3, EAAT2, and EAAT1 may be involved in the development of TD in schizophrenia patients.

Group I mGluRs in Therapy and Diagnosis of Parkinson’s Disease: Focus on mGluR5 Subtype

Metabotropic glutamate receptors (mGluRs; members of class C G-protein-coupled receptors) have been shown to modulate excitatory neurotransmission, regulate presynaptic extracellular glutamate levels, and modulate postsynaptic ion channels on dendritic spines. mGluRs were found to activate myriad signalling pathways to regulate synapse formation, long-term potentiation, autophagy, apoptosis, necroptosis, and pro-inflammatory cytokines release. A notorious expression pattern of mGluRs has been evident in several neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and schizophrenia. Among the several mGluRs, mGluR5 is one of the most investigated types of considered prospective therapeutic targets and potential diagnostic tools in neurodegenerative diseases and neuropsychiatric disorders. Recent research showed mGluR5 radioligands could be a potential tool to assess neurodegenerative disease progression and trace respective drugs’ kinetic properties. This article provides insight into the group I mGluRs, specifically mGluR5, in the progression and possible therapy for PD.

Metabotropic glutamate receptors and nitric oxide in dopaminergic neurotoxicity

Dopaminergic neurotoxicity is characterized by damage and death of dopaminergic neurons. Parkinson's disease (PD) is a neurodegenerative disorder that primarily involves the loss of dopaminergic neurons in the substantia nigra. Therefore, the study of the mechanisms, as well as the search for new targets for the prevention and treatment of neurodegenerative diseases, is an important focus of modern neuroscience. PD is primarily caused by dysfunction of dopaminergic neurons; however, other neurotransmitter systems are also involved. Research reports have indicated that the glutamatergic system is involved in different pathological conditions, including dopaminergic neurotoxicity. Over the last two decades, the important functional interplay between dopaminergic and glutamatergic systems has stimulated interest in the possible role of metabotropic glutamate receptors (mGluRs) in the development of extrapyramidal disorders. However, the specific mechanisms driving these processes are presently unclear. The participation of the universal neuronal messenger nitric oxide (NO) in the mechanisms of dopaminergic neurotoxicity has attracted increased attention. The current paper aims to review the involvement of mGluRs and the contribution of NO to dopaminergic neurotoxicity. More precisely, we focused on studies conducted on the rotenone-induced PD model. This review is also an outline of our own results obtained using the method of electron paramagnetic resonance, which allows quantitation of NO radicals in brain structures.

Impact of the apelin/APJ axis in the pathogenesis of Parkinson's disease with therapeutic potential

The pathogenesis of Parkinson's disease (PD) remains elusive. There is still no available disease-modifying strategy against PD, whose management is mainly symptomatic. A growing amount of preclinical evidence shows that a complex interplay between autophagy dysregulation, mitochondrial impairment, endoplasmic reticulum stress, oxidative stress, and excessive neuroinflammation underlies PD pathogenesis. Identifying key molecules linking these pathological cellular processes may substantially aid in our deeper understanding of PD pathophysiology and the development of novel effective therapeutic approaches. Emerging preclinical evidence indicates that apelin, an endogenous neuropeptide acting as a ligand of the orphan G protein-coupled receptor APJ, may play a key neuroprotective role in PD pathogenesis, via inhibition of apoptosis and dopaminergic neuronal loss, autophagy enhancement, antioxidant effects, endoplasmic reticulum stress suppression, as well as prevention of synaptic dysregulation in the striatum, excessive neuroinflammation, and glutamate-induced excitotoxicity. Underlying signaling pathways involve phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin, extracellular signal-regulated kinase 1/2, and inositol requiring kinase 1α/XBP1/C/EBP homologous protein. Herein, we discuss the role of apelin/APJ axis and associated molecular mechanisms on the pathogenesis of PD in vitro and in vivo and provide evidence for its challenging therapeutic potential.

Group 1 Metabotropic Glutamate Receptors in Neurological and Psychiatric Diseases: Mechanisms and Prospective

Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors that are activated by glutamate in the central nervous system (CNS). Basically, mGluRs contribute to fine-tuning of synaptic efficacy and control the accuracy and sharpness of neurotransmission. Among eight subtypes, mGluR1 and mGluR5 belong to group 1 (Gp1) family, and are implicated in multiple CNS disorders, such as Alzheimer’s disease, autism, Parkinson’s disease, and so on. In the present review, we systematically discussed underlying mechanisms and prospective of Gp1 mGluRs in a group of neurological and psychiatric diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder, epilepsy, Huntington’s disease, intellectual disability, Down’s syndrome, Rett syndrome, attention-deficit hyperactivity disorder, addiction, anxiety, nociception, schizophrenia, and depression, in order to provide more insights into the therapeutic potential of Gp1 mGluRs.

Pharmacological interactions between adenosine A2A receptor antagonists and different neurotransmitter systems

While Parkinson's disease (PD) is traditionally characterized by dopaminergic neuron degeneration, several neurotransmitters and neuromodulators besides dopamine are also involved in the onset and progression of the disease and its symptoms. The other principal neurotransmitters/neuromodulators known to control basal ganglia functions and, in particular, motor functions, are GABA, glutamate, serotonin (5-HT), noradrenaline, acetylcholine, adenosine and endocannabinoids. Among these, adenosine is the most relevant, acting through its adenosine A_2A receptor. Work in experimental models of PD has established the effects of A_2A receptor antagonists, including the alleviation of disrupted dopamine functions and improved efficacy of dopamine replacement therapy. Moreover, positive interactions between A_2A receptor antagonists and both D₂ and D₁ receptor agonists have been described in vitro at the receptor-receptor level or in more complex in vivo models of PD, respectively. In addition, the interactions between A_2A receptor antagonists and glutamate ionotropic GluN_2B-containing N-Methyl-d-aspartic acid receptors, or metabotropic glutamate (mGlu) receptors, including both mGlu₅ receptor inhibitors and mGlu₄ receptor activators, have been reported in both in vitro and in vivo animal models of PD, as have positive interactions between A_2A and endocannabinoid CB₁ receptor antagonists. At the same time, a combination of A_2A receptor antagonists and 5-HT_1A-5-HT_1B receptor agonists have been described to modulate the expression of dyskinesia induced by chronic dopamine replacement therapy.

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Recent advances in dopaminergic strategies for the treatment of Parkinson's disease

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease worldwide. However, there is no available therapy reversing the neurodegenerative process of PD. Based on the loss of dopamine or dopaminergic dysfunction in PD patients, most of the current therapies focus on symptomatic relief to improve patient quality of life. As dopamine replacement treatment remains the most effective symptomatic pharmacotherapy for PD, herein we provide an overview of the current pharmacotherapies, summarize the clinical development status of novel dopaminergic agents, and highlight the challenge and opportunity of emerging preclinical dopaminergic approaches aimed at managing the features and progression of PD.

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

Study on Effect of Striatal mGluR2/3 in Alleviating Motor Dysfunction in Rat PD Model Treated by Exercise Therapy

Background: Exercise therapy has been widely applied in clinical rehabilitation as an important practical and side effect-free adjuvant therapy, with a significant effect in alleviating motor dysfunction of patients with Parkinson's disease (PD) or animal PD models. This study focuses on the effect of exercise therapy in reducing the concentration of extracellular glutamate (Glu) in the striatum in a rat PD model by upregulating the expression of group II metabotropic Glu receptor (mGluR2/3), so as to alleviate motor dysfunction in the rat PD model. Methods: Neurotoxin 6-hydroxydopamine (6-OHDA) was injected into the right medial forebrain bundle (MFB) of the rats to establish the semi-lateral cerebral damage PD model. The sham-operated group was given an equal amount of normal saline at the same site and taken as the control group. The apomorphine (APO)-induced rotational behavior test combined with immunohistochemical staining with tyrosine hydroxylase (TH) in the substantia nigra (SNc) and striatum was performed to assess the reliability of the model. The exercise group was given treadmill exercise intervention for 4 weeks (11 m/min, 30 min/day, 5 days/week) 1 week after the operation. The open field test (OFT) was performed to assess the locomotor activity of the rats; the Western blot technique was used to detect SNc TH and striatal mGluR2/3 protein expressions; real-time polymerase chain reaction (RT-PCR) was applied to detect striatal mGluR2 and mGluR3 mRNA expressions; the microdialysis-high-performance liquid chromatography (HPLC) method was adopted to detect the concentration of extracellular Glu in striatal neurons. Results: Compared with the control group, the number of rotations of each model group at the first week was significantly increased (P < 0.01); compared with the PD group, the number of rotations of the PD + exercise group at the third week and the fifth week was significantly decreased (P < 0.05, P < 0.01). Compared with the control group, the total movement distance, the total movement time, and the mean velocity of each model group at the first week were significantly reduced (P < 0.05); compared with the PD group, the total movement distance, the total movement time, and the mean velocity of the PD + exercise group at the third week and the fifth week were significantly increased (P < 0.01). Compared with the control group, the count of immunopositive cells and protein expression of SNc TH, and the content of immunopositive fiber terminals in the striatal TH of each model group significantly declined (P < 0.01). Compared with the PD group, the striatal mGluR2/3 protein expression of the PD + exercise group significantly rose (P < 0.01). Compared with the control group, the concentration of extracellular Glu in striatal neurons of each model group at the first week significantly grew (P < 0.05); compared with the PD group, the concentration of extracellular Glu in striatal neurons of the PD + exercise group at the third week and the fifth week was significantly decreased (P < 0.01); compared with the PD + exercise group, the concentration of extracellular Glu in striatal neurons of the group injected with mGluR2/3 antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) into the striatum at the third week and the fifth week was significantly increased (P < 0.05, P < 0.01). Compared with the control group, the striatal mGluR2/3 protein expression of the PD group was significantly downregulated (P < 0.01); compared with the PD group, the striatal mGluR2/3 protein expression of the PD + exercise group was significantly upregulated (P < 0.05); compared with the control group, the striatal mGluR3 mRNA expression of the PD group was significantly downregulated (P < 0.01); compared with the PD group, the striatal mGluR3 mRNA expression of the PD + exercise group was significantly upregulated (P < 0.01); 6-OHDA damage and exercise intervention had no significant effect on the striatal mGluR2 mRNA expression (P > 0.05). Compared with the PD + exercise group, the total movement distance, the total movement time, and the mean velocity of the PD + exercise + APICA group were significantly decreased (P < 0.05); compared with the PD group, the PD + exercise + APICA group had no significant change in the total movement distance, the total movement time, and the mean velocity (P > 0.05). Conclusion: These data collectively demonstrate that the mGluR2/3-mediated glutamatergic transmission in the striatum is sensitive to dopamine (DA) depletion and may serve as a target of exercise intervention for mediating the therapeutic effect of exercise intervention in a rat model of PD.

Group I metabotropic glutamate receptors in the primate motor thalamus: subsynaptic association with cortical and sub-cortical glutamatergic afferents

Preclinical evidence indicates that mGluR5 is a potential therapeutic target for Parkinson's disease and L-DOPA-induced dyskinesia. However, the mechanisms through which these therapeutic benefits are mediated remain poorly understood. Although the regulatory role of mGluR5 on glutamatergic transmission has been examined in various basal ganglia nuclei, very little is known about the localization and function of mGluR5 in the ventral motor and intralaminar thalamic nuclei, the main targets of basal ganglia output in mammals. Thus, we used immuno-electron microscopy to map the cellular and subcellular localization of group I mGluRs (mGluR1a and mGluR5) in the ventral motor and caudal intralaminar thalamic nuclei in rhesus monkeys. Furthermore, using double immuno-electron microscopy, we examined the subsynaptic localization of mGluR5 in relation to cortical and sub-cortical glutamatergic afferents. Four major conclusions can be drawn from these data. First, mGluR1a and mGluR5 are expressed postsynaptically on the plasma membrane of dendrites of projection neurons and GABAergic interneurons in the basal ganglia- and cerebellar-receiving regions of the ventral motor thalamus and in CM. Second, the plasma membrane-bound mGluR5 immunoreactivity is preferentially expressed perisynaptically at the edges of cortical and sub-cortical glutamatergic afferents. Third, the mGluR5 immunoreactivity is more strongly expressed in the lateral than the medial tiers of CM, suggesting a preferential association with thalamocortical over thalamostriatal neurons in the primate CM. Overall, mGluR5 is located to subserve powerful modulatory role of cortical and subcortical glutamatergic transmission in the primate ventral motor thalamus and CM.

Comprehensive review on the interaction between natural compounds and brain receptors: Benefits and toxicity

Given their therapeutic activity, natural products have been used in traditional medicines throughout the centuries. The growing interest of the scientific community in phytopharmaceuticals, and more recently in marine products, has resulted in a significant number of research efforts towards understanding their effect in the treatment of neurodegenerative diseases, such as Alzheimer's (AD), Parkinson (PD) and Huntington (HD). Several studies have shown that many of the primary and secondary metabolites of plants, marine organisms and others, have high affinities for various brain receptors and may play a crucial role in the treatment of diseases affecting the central nervous system (CNS) in mammalians. Actually, such compounds may act on the brain receptors either by agonism, antagonism, allosteric modulation or other type of activity aimed at enhancing a certain effect. The current manuscript comprehensively reviews the state of the art on the interactions between natural compounds and brain receptors. This information is of foremost importance when it is intended to investigate and develop cutting-edge drugs, more effective and with alternative mechanisms of action to the conventional drugs presently used for the treatment of neurodegenerative diseases. Thus, we reviewed the effect of 173 natural products on neurotransmitter receptors, diabetes related receptors, neurotrophic factor related receptors, immune system related receptors, oxidative stress related receptors, transcription factors regulating gene expression related receptors and blood-brain barrier receptors.

Perinatal Exposure to Arsenic in Drinking Water Alters Glutamatergic Neurotransmission in the Striatum of C57BL/6 Mice

Although exposure to arsenic (As) induces developmental neurotoxicity, there is a lack of data regarding its specific effects on glutamatergic neurotransmission in offspring from dams exposed to As during gestation and lactation. In this study, the body weight, glutamate content, and expression of vesicular glutamate transporter 2 (VGLUT2) and metabotropic glutamate receptors mGluR2 and mGluR3 was examined in the striatum of offspring following treatment of the dams with As (10 or 100 mg/L NaAsO₂ in drinking water). At postnatal day 21, body weight was decreased significantly, whereas the glutamate content in the striatum of offspring in the 100-mg/L As group were not significantly different from those in the control group. Although mGluR3 expression was not significantly different, VGLUT2 and mGluR2 expression was significantly lower in the striatum of offspring of As-exposed dams. These data indicate that altered glutamatergic neurotransmission may contribute to As-induced developmental neurotoxic effects.

Comprehensive review of mechanisms of pathogenesis involved in Alzheimer's disease and potential therapeutic strategies

AD is a progressive neurodegenerative disorder and a leading cause of dementia in an aging population worldwide. The enormous challenge which AD possesses to global healthcare makes it as urgent as ever for the researchers to develop innovative treatment strategies to fight this disease. An in-depth analysis of the extensive available data associated with the AD is needed for a more comprehensive understanding of underlying molecular mechanisms and pathophysiological pathways associated with the onset and progression of the AD. The currently understood pathological and biochemical manifestations include cholinergic, Aβ, tau, excitotoxicity, oxidative stress, ApoE, CREB signaling pathways, insulin resistance, etc. However, these hypotheses have been criticized with several conflicting reports for their involvement in the disease progression. Several issues need to be addressed such as benefits to cost ratio with cholinesterase therapy, the dilemma of AChE selectivity over BChE, BBB permeability of peptidic BACE-1 inhibitors, hurdles related to the implementation of vaccination and immunization therapy, and clinical failure of candidates related to newly available targets. The present review provides an insight to the different molecular mechanisms involved in the development and progression of the AD and potential therapeutic strategies, enlightening perceptions into structural information of conventional and novel targets along with the successful applications of computational approaches for the design of target-specific inhibitors.

Potential Therapeutic Drugs for Parkinson's Disease Based on Data Mining and Bioinformatics Analysis

The objective is to search potential therapeutic drugs for Parkinson's disease based on data mining and bioinformatics analysis and providing new ideas for research studies on “new application of conventional drugs.” Method differential gene candidates were obtained based on data mining of genes of PD brain tissue, original gene data analysis, differential gene crossover, pathway enrichment analysis, and protein interaction, and potential therapeutic drugs for Parkinson's disease were obtained through drug-gene relationship. Result. 250 common differential genes were obtained from 3 research studies, and 31 differential gene candidates were obtained through gene enrichment analysis and protein interaction. 10 drugs such as metformin hydrochloride were directly or indirectly correlated to differential gene candidates. Conclusion. Potential therapeutic drugs that may be used for prevention and treatment of Parkinson's disease were discovered through data mining and bioinformatics analysis, which provided new ideas for research and development of drugs. Results showed that metformin hydrochloride and other drugs had certain therapeutical effect on Parkinson's disease, and melbine (DMBG) can be used for treatment of Parkinson's disease and type 2 diabetes patients.

Therapeutic strategies for Parkinson disease: beyond dopaminergic drugs

Existing therapeutic strategies for managing Parkinson disease (PD), which focus on addressing the loss of dopamine and dopaminergic function linked with degeneration of dopaminergic neurons, are limited by side effects and lack of long-term efficacy. In recent decades, research into PD pathophysiology and pharmacology has focused on understanding and tackling the neurodegenerative processes and symptomology of PD. In this Review, we discuss the challenges associated with the development of novel therapies for PD, highlighting emerging agents that aim to target cell death, as well as new targets offering a symptomatic approach to managing features and progression of the disease.

An mGlu4-Positive Allosteric Modulator Alleviates Parkinsonism in Primates

BACKGROUND

Levodopa remains the gold-standard treatment for PD. However, it becomes less effective as the disease progresses and produces debilitating side effects, such as motor fluctuations and l-dopa-induced dyskinesia. Modulation of metabotropic glutamate receptor 4 represents a promising antiparkinsonian approach in combination with l-dopa, but it has not been demonstrated in primates.

OBJECTIVE

We studied whether a novel positive allosteric modulator of the metabotropic glutamate receptor 4, PXT002331 (foliglurax), could reduce parkinsonism in primate models.

METHODS

We assessed the therapeutic potential of PXT002331 in three models of MPTP-induced parkinsonism in macaques. These models represent three different stages of disease evolution: early stage and advanced stage with and without l-dopa-induced dyskinesia.

RESULTS

As an adjunct to l-dopa, PXT002331 induced a robust and dose-dependent reversal of parkinsonian motor symptoms in macaques, including bradykinesia, tremor, posture, and mobility. Moreover, PXT002331 strongly decreased dyskinesia severity, thus having therapeutic efficacy on both parkinsonian motor impairment and l-dopa-induced dyskinesia. PXT002331 brain penetration was also assessed using PET imaging in macaques, and pharmacodynamic analyses support target engagement in the therapeutic effects of PXT002331.

CONCLUSIONS

This work provides a demonstration that a positive allosteric modulator of metabotropic glutamate receptor 4 can alleviate the motor symptoms of PD and the motor complications induced by l-dopa in primates. PXT002331 is the first compound of its class to enter phase IIa clinical trials. © 2018 International Parkinson and Movement Disorder Society.

The Impact and Mechanism of Methylated Metabotropic Glutamate Receptors 1 and 5 in the Hippocampus on Depression-Like Behavior in Prenatal Stress Offspring Rats

An increasing number of epidemiological investigations and animal models research suggest that prenatal stress (PS) could cause depression-like behavior in the offspring, which is sex specific. However, the underlying mechanisms remain to be elucidated. This study is to investigate the promoter methylation of metabotropic glutamate receptor 1 (mGluR1) and metabotropic Glutamate Receptor 5 (mGluR5) gene modification on PS induced depression-like behavior in offspring rats (OR). PS models were established, with or without 5-aza-2′-deoxycytidine (5-azaD, decitabine) treatment. Animal behavior was assessed by the sucrose preference test (SPT), forced swimming test (FST), and open field test (OFT). The mRNA and protein expression levels of mGluR1 and mGluR5 in the hippocampus of offspring were detected with quantitative real-time PCR and Western blot analysis, respectively. The promoter methylation in the hippocampus of mGluR1 and mGluR5 OR were also analyzed. SPT showed significantly reduced sucrose preference in PS induced OR. FST showed significantly prolonged immobility time in PS induced OR. OFT showed significantly reduced central residence time in PS induced OR and no significantly influence in rearing as well as in frequency of micturition. Moreover, the mRNA, protein expression levels, and gene promoter methylation level of mGluR1 and mGluR5 in the hippocampus were significantly increased in the PS induced male OR, while no significantly influence in the PS induced female OR. Furthermore, the PS induced effects in male OR could be reversed by the microinjection of 5-azaD. In conclusion, our results showed that the promoter methylation of mGluR1 and mGluR5 gene modification is only involved in PS induced depression-like behavior in male OR in a sex-specific manner. These findings might contribute to the understanding of the disease pathogenesis and clinical treatment in future.

Current Nondopaminergic Therapeutic Options for Motor Symptoms of Parkinson's Disease

Objective:

The aim of this study was to summarize recent studies on nondopaminergic options for the treatment of motor symptoms in Parkinson's disease (PD).

Data Sources:

Papers in English published in PubMed, Cochrane, and Ovid Nursing databases between January 1988 and November 2016 were searched using the following keywords: PD, nondopaminergic therapy, adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator. We also reviewed the ongoing clinical trials in the website of clinicaltrials.gov.

Study Selection:

Articles related to the nondopaminergic treatment of motor symptoms in PD were selected for this review.

Results:

PD is conventionally treated with dopamine replacement strategies, which are effective in the early stages of PD. Long-term use of levodopa could result in motor complications. Recent studies revealed that nondopaminergic systems such as adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator pathways could include potential therapeutic targets for motor symptoms, including motor fluctuations, levodopa-induced dyskinesia, and gait disorders. Some nondopaminergic drugs, such as istradefylline and amantadine, are currently used clinically, while most such drugs are in preclinical testing stages. Transitioning of these agents into clinically beneficial strategies requires reliable evaluation since several agents have failed to show consistent results despite positive findings at the preclinical level.

Conclusions:

Targeting nondopaminergic transmission could improve some motor symptoms in PD, especially the discomfort of dyskinesia. Although nondopaminergic treatments show great potential in PD treatment as an adjunct therapy to levodopa, further investigation is required to ensure their success.

mGlu4 allosteric modulation for treating Parkinson's disease

2017 is the 200th anniversary of the first published description of Parkinson's disease (PD). Fifty years ago, the clinical benefit of levodopa was first documented, representing the most important advance in the treatment of PD so far. Among the novel targets identified in the last decade, positive allosteric modulators (PAM) of mGlu₄ receptors show great promise, with the potential to change the paradigm of the PD treatment approach. mGlu₄ PAMs have shown consistent efficacy in various preclinical models of PD, and entered clinical trials for the first time in 2017. This review synthesizes the rationale for mGlu₄ PAM development for PD and progress to date, reporting the key achievements from preclinical studies to the first-in-class compound assessment in man.

Psychosis in Parkinson's Disease: Epidemiology, Pathophysiology, and Management

Psychotic symptoms are common in Parkinson's disease (PD) and are associated with poorer quality of life and increased caregiver burden. PD psychosis is correlated with several factors, such as more advanced disease, cognitive impairment, depression, and sleep disorders. The underlying causes of psychosis in PD thus involve a complex interplay between exogenous (e.g., drugs, intercurrent illnesses) and endogenous (e.g., PD disease pathology) factors. Current theories of the pathophysiology of PD psychosis have come from several neuropathological and neuroimaging studies that implicate pathways involving visual processing and executive function, including temporo-limbic structures and neocortical gray matter with altered neurotransmitter functioning (e.g., dopamine, serotonin, and acetylcholine). Treatment of PD psychosis requires a step-wise process, including initial careful investigation of treatable triggering conditions and a comprehensive evaluation with adjustment of PD medications and/or initiation of specific antipsychotic therapies. Clozapine remains the only recommended drug for the treatment of PD psychosis; however, because of regular blood monitoring, quetiapine is usually first-line therapy, although less efficacious. Emerging studies have focused on agents involving other neurotransmitters, including the serotonin 5-HT2A receptor inverse agonist pimavanserin, cholinesterase inhibitors, and antidepressants and anxiolytics.

Recent advances in the medicinal chemistry of group II and group III mGlu receptors

Metabotropic glutamate receptors (mGlu receptors) belong to the G-protein-coupled receptors superfamily. They are divided into three groups, in which group II and group III belong to presynaptic receptors that negatively modulate glutamate and γ-aminobutyric acid (GABA) release when activated. In this review, we introduce not only the functions of mGlu receptors, but also the group II and group III allosteric modulators and agonists/antagonists reported over the past five years according to a classification of their structures, with a specific focus on their biological activity and selectivity. In particular, the structure of these compounds and the future directions of ideal candidates are highlighted.

Nondopaminergic treatments for Parkinson's disease: current and future prospects

Parkinson's disease is primarily caused by dysfunction of dopaminergic neurons, however, nondopaminergic (ND) systems are also involved. ND targets are potentially useful to reduce doses of levodopa or to treat nonlevodopa-responsive symptoms. Recent studies have investigated the role of ND drugs for motor and nonmotor symptoms. Adenosine A2A receptor antagonists, mixed inhibitors of sodium/calcium channels and monoamine oxidase-B have recently been found to improve motor fluctuations. N-methyl-d-aspartate receptor antagonists and serotonin 5HT1B receptor agonists demonstrated benefit in levodopa-induced dyskinesia. Conversely, studies using antiepileptic drugs and adrenoreceptor antagonist had conflicting results. Moreover, metabotropic glutamate receptor antagonists also failed to improve symptoms. The current review summarizes the most recent findings on ND drugs over the last 2 years.

Animal models of Parkinson's disease: An updated overview

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology, besides a minority of genetic cases, is still largely unknown. Animal models have contributed to elucidate PD etiology and pathogenesis, as well as its cellular and molecular mechanisms, leading to the general hypothesis that this neurological disorder is due to complex interactions between environmental and genetic factors. However, the full understanding of PD is still very far from being achieved, and new potential treatments need to be tested to further improve patients' quality of life and, possibly, slow down the neurodegenerative process. In this context, animal models of PD are required to address all these issues. "Classic" models are based on neurotoxins that selectively target catecholaminergic neurons (such as 6-hydroxydopamine, 1-methyl-1,2,3,6-tetrahydropiridine, agricultural pesticides, etc.), while more recent models employ genetic manipulations that either introduce mutations similar to those find in familial cases of PD (α-synuclein, DJ-1, PINK1, Parkin, etc.) or selectively disrupt nigrostriatal neurons (MitoPark, Pitx3, Nurr1, etc.). Each one of these models has its own advantages and limitations, thus some are better suited for studying PD pathogenesis, while others are more pertinent to test therapeutic treatments. Here, we provide a critical and updated review of the most used PD models.

Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.