Metabotropic glutamate receptor 5 mediates the potentiation of N-methyl-D-aspartate responses in medium spiny striatal neurons

The excitatory-inhibitory balance as a target for the development of novel drugs to treat schizophrenia

The intricate balance between excitation and inhibition (E/I) in the brain plays a crucial role in normative information processing. Dysfunctions in the E/I balance have been implicated in various psychiatric disorders, including schizophrenia (SCZ). In particular, abnormalities in GABAergic signaling, specifically in parvalbumin (PV)-containing interneurons, have been consistently observed in SCZ pathophysiology. PV interneuron function is vital for maintaining an ideal E/I balance, and alterations in PV interneuron-mediated inhibition contribute to circuit deficits observed in SCZ, including hippocampus hyperactivity and midbrain dopamine system overdrive. While current antipsychotic medications primarily target D2 dopamine receptors and are effective primarily in treating positive symptoms, novel therapeutic strategies aiming to restore the E/I balance could potentially mitigate not only positive symptoms but also negative symptoms and cognitive deficits. This could involve, for instance, increasing the inhibitory drive onto excitatory neurons or decreasing the putative enhanced pyramidal neuron activity due to functional loss of PV interneurons. Compounds targeting the glycine site at glutamate NMDA receptors and muscarinic acetylcholine receptors on PV interneurons that can increase PV interneuron drive, as well as drugs that increase the postsynaptic action of GABA, such as positive allosteric modulators of α5-GABA-A receptors, and decrease glutamatergic output, such as mGluR2/3 agonists, represent promising approaches. Preventive strategies aiming at E/I balance also represent a path to reduce the risk of transitioning to SCZ in high-risk individuals. Therefore, compounds with novel mechanisms targeting E/I balance provide optimism for more effective and tailored interventions in the management of SCZ.

FOXP1 regulates the development of excitatory synaptic inputs onto striatal neurons and induces phenotypic reversal with reinstatement

Long-range glutamatergic inputs originating from the cortex and thalamus are indispensable for striatal development, providing the foundation for motor and cognitive functions. Despite their significance, transcriptional regulation governing these inputs remains largely unknown. We investigated the role of a transcription factor encoded by a high-risk autism-associated gene, FOXP1, in sculpting glutamatergic inputs onto spiny projection neurons (SPNs) within the striatum. We find a neuron subtype-specific role of FOXP1 in strengthening and maturing glutamatergic inputs onto dopamine receptor 2-expressing SPNs (D2 SPNs). We also find that FOXP1 promotes synaptically driven excitability in these neurons. Using single-nuclei RNA sequencing, we identify candidate genes that mediate these cell-autonomous processes through postnatal FOXP1 function at the post-synapse. Last, we demonstrate that postnatal FOXP1 reinstatement rescues electrophysiological deficits, cell type-specific gene expression changes, and behavioral phenotypes. Together, this study enhances our understanding of striatal circuit development and provides proof of concept for a therapeutic approach for FOXP1 syndrome and other neurodevelopmental disorders.

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Synaptic plasticity in human thalamocortical assembloids

Synaptic plasticities, such as long-term potentiation (LTP) and depression (LTD), tune synaptic efficacy and are essential for learning and memory. Current studies of synaptic plasticity in humans are limited by a lack of adequate human models. Here, we modeled the thalamocortical system by fusing human induced pluripotent stem cell-derived thalamic and cortical organoids. Single-nucleus RNA-sequencing revealed that most cells in mature thalamic organoids were glutamatergic neurons. When fused to form thalamocortical assembloids, thalamic and cortical organoids formed reciprocal long-range axonal projections and reciprocal synapses detectable by light and electron microscopy, respectively. Using whole-cell patch-clamp electrophysiology and two-photon imaging, we characterized glutamatergic synaptic transmission. Thalamocortical and corticothalamic synapses displayed short-term plasticity analogous to that in animal models. LTP and LTD were reliably induced at both synapses; however, their mechanisms differed from those previously described in rodents. Thus, thalamocortical assembloids provide a model system for exploring synaptic plasticity in human circuits.

Neuroplasticity in levodopa-induced dyskinesias: An overview on pathophysiology and therapeutic targets

Levodopa-induced dyskinesias (LIDs) are a common complication in patients with Parkinson's disease (PD). A complex cascade of electrophysiological and molecular events that induce aberrant plasticity in the cortico-basal ganglia system plays a key role in the pathophysiology of LIDs. In the striatum, multiple neurotransmitters regulate the different forms of physiological synaptic plasticity to provide it in a bidirectional and Hebbian manner. In PD, impairment of both long-term potentiation (LTP) and long-term depression (LTD) progresses with disease and dopaminergic denervation of striatum. The altered balance between LTP and LTD processes leads to unidirectional changes in plasticity that cause network dysregulation and the development of involuntary movements. These alterations have been documented, in both experimental models and PD patients, not only in deep brain structures but also at motor cortex. Invasive and non-invasive neuromodulation treatments, as deep brain stimulation, transcranial magnetic stimulation, or transcranial direct current stimulation, may provide strategies to modulate the aberrant plasticity in the cortico-basal ganglia network of patients affected by LIDs, thus restoring normal neurophysiological functioning and treating dyskinesias. In this review, we discuss the evidence for neuroplasticity impairment in experimental PD models and in patients affected by LIDs, and potential neuromodulation strategies that may modulate aberrant plasticity.

Targeting Striatal Glutamate and Phosphodiesterases to Control L-DOPA-Induced Dyskinesia

A large body of work during the past several decades has been focused on therapeutic strategies to control L-DOPA-induced dyskinesias (LIDs), common motor complications of long-term L-DOPA therapy in Parkinson's disease (PD). Yet, LIDs remain a clinical challenge for the management of patients with advanced disease. Glutamatergic dysregulation of striatal projection neurons (SPNs) appears to be a key contributor to altered motor responses to L-DOPA. Targeting striatal hyperactivity at the glutamatergic neurotransmission level led to significant preclinical and clinical trials of a variety of antiglutamatergic agents. In fact, the only FDA-approved treatment for LIDs is amantadine, a drug with NMDAR antagonistic actions. Still, novel agents with improved pharmacological profiles are needed for LID therapy. Recently other therapeutic targets to reduce dysregulated SPN activity at the signal transduction level have emerged. In particular, mechanisms regulating the levels of cyclic nucleotides play a major role in the transduction of dopamine signals in SPNs. The phosphodiesterases (PDEs), a large family of enzymes that degrade cyclic nucleotides in a specific manner, are of special interest. We will review the research for antiglutamatergic and PDE inhibition strategies in view of the future development of novel LID therapies.

FOXP1 regulates the development of excitatory synaptic inputs onto striatal neurons and induces phenotypic reversal with reinstatement

Long-range glutamatergic inputs from the cortex and thalamus are critical for motor and cognitive processing in the striatum. Transcription factors that orchestrate the development of these inputs are largely unknown. We investigated the role of a transcription factor and high-risk autism-associated gene, FOXP1, in the development of glutamatergic inputs onto spiny projection neurons (SPNs) in the striatum. We find that FOXP1 robustly drives the strengthening and maturation of glutamatergic input onto dopamine receptor 2-expressing SPNs (D2 SPNs) but has a comparatively milder effect on D1 SPNs. This process is cell-autonomous and is likely mediated through postnatal FOXP1 function at the postsynapse. We identified postsynaptic FOXP1-regulated transcripts as potential candidates for mediating these effects. Postnatal reinstatement of FOXP1 rescues electrophysiological deficits, reverses gene expression alterations resulting from embryonic deletion, and mitigates behavioral phenotypes. These results provide support for a possible therapeutic approach for individuals with FOXP1 syndrome.

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.

Targeting Shank3 deficiency and paresthesia in autism spectrum disorder: A brief review

Autism spectrum disorder (ASD) includes a group of multifactorial neurodevelopmental disorders characterized by impaired social communication, social interaction, and repetitive behaviors. Several studies have shown an association between cases of ASD and mutations in the genes of SH3 and multiple ankyrin repeat domain protein 3 (SHANK3). These genes encode many cell adhesion molecules, scaffold proteins, and proteins involved in synaptic transcription, protein synthesis, and degradation. They have a profound impact on all aspects of synaptic transmission and plasticity, including synapse formation and degeneration, suggesting that the pathogenesis of ASD may be partially attributable to synaptic dysfunction. In this review, we summarize the mechanism of synapses related to Shank3 in ASD. We also discuss the molecular, cellular, and functional studies of experimental models of ASD and current autism treatment methods targeting related proteins.

The metabotropic glutamate receptor 5 as a biomarker for psychiatric disorders

The role of glutamate system in the etiology and pathophysiology of psychiatric disorders has gained considerable attention in the past two decades, including dysregulation of the metabotropic glutamatergic receptor subtype 5 (mGlu5). Thus, mGlu5 may represent a promising therapeutic target for psychiatric conditions, particularly stress-related disorders. Here, we describe mGlu5 findings in mood disorders, anxiety, and trauma disorders, as well as substance use (specifically nicotine, cannabis, and alcohol use). We highlight insights gained from positron emission tomography (PET) studies, where possible, and discuss findings from treatment trials, when available, to explore the role of mGlu5 in these psychiatric disorders. Through the research evidence reviewed in this chapter, we make the argument that, not only is dysregulation of mGlu5 evident in numerous psychiatric disorders, potentially functioning as a disease "biomarker," the normalization of glutamate neurotransmission via changes in mGlu5 expression and/or modulation of mGlu5 signaling may be a needed component in treating some psychiatric disorders or symptoms. Finally, we hope to demonstrate the utility of PET as an important tool for investigating mGlu5 in disease mechanisms and treatment response.

Altered Expression and In Vivo Activity of mGlu5 Variant a Receptors in the Striatum of BTBR Mice: Novel Insights Into the Pathophysiology of Adult Idiopathic Forms of Autism Spectrum Disorders

BACKGROUND

mGlu5 metabotropic glutamate receptors are considered as candidate drug targets in the treatment of "monogenic" forms of autism spectrum disorders (ASD), such as Fragile- X syndrome (FXS). However, despite promising preclinical data, clinical trials using mGlu5 receptor antagonists to treat FXS showed no beneficial effects.

OBJECTIVE

Here, we studied the expression and function of mGlu5 receptors in the striatum of adult BTBR mice, which model idiopathic forms of ASD, and behavioral phenotype.

METHODS

Behavioral tests were associated with biochemistry analysis including qPCR and western blot for mRNA and protein expression. In vivo analysis of polyphosphoinositides hydrolysis was performed to study the mGlu5-mediated intracellular signaling in the striatum of adult BTBR mice under basal conditions and after MTEP exposure.

RESULTS

Expression of mGlu5 receptors and mGlu5 receptor-mediated polyphosphoinositides hydrolysis were considerably high in the striatum of BTBR mice, sensitive to MTEP treatment. Changes in the expression of genes encoding for proteins involved in excitatory and inhibitory neurotransmission and synaptic plasticity, including Fmr1, Dlg4, Shank3, Brd4, bdnf-exon IX, Mef2c, and Arc, GriA2, Glun1, Nr2A, and Grm1, Grm2, GriA1, and Gad1 were also found. Behaviorally, BTBR mice showed high repetitive stereotypical behaviors, including self-grooming and deficits in social interactions. Acute or repeated injections with MTEP reversed the stereotyped behavior and the social interaction deficit. Similar effects were observed with the NMDA receptor blockers MK-801 or ketamine.

CONCLUSION

These findings support a pivotal role of mGlu5 receptor abnormal expression and function in idiopathic ASD adult forms and unveil novel potential targets for therapy.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

Evaluation of the effects of the mGlu2/3 antagonist LY341495 on dyskinesia and psychosis-like behaviours in the MPTP-lesioned marmoset

BACKGROUND

We have previously demonstrated that the metabotropic glutamate 2 and 3 (mGlu_2/3) antagonist LY341495 reverses the anti-dyskinetic and anti-psychotic benefits conferred by mGlu₂ activation and serotonin 2A (5-HT_2A) antagonism. Here, we hypothesised that a higher dose of LY341495, associated with a higher antagonistic effect at mGlu₃ receptors, would result in a reduction of the reversal of mGlu₂ activation and 5-HT_2A blockade on dyskinesia, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset.

METHODS

After induction of parkinsonism with MPTP, marmosets entered 3 streams of experiments, in which the following treatments were administered, in combination with l-3,4-dihydroxyphenylalanine (L-DOPA), after which dyskinesia, psychosis-like behaviours (PLBs) and parkinsonism were rated: 1. vehicle/vehicle, LY354740 (mGlu_2/3 orthosteric agonist)/vehicle, LY354740/LY341495 1 mg/kg and LY354740/LY341495 3 mg/kg; 2. vehicle/vehicle, LY487379 (mGlu₂ positive allosteric modulator)/vehicle, LY487379/LY341495 1 mg/kg and LY487379/LY341495 3 mg/kg; 3. vehicle/vehicle, EMD-281,014 (5-HT_2A antagonist)/vehicle, EMD-281,014/LY341495 1 mg/kg and EMD-281,014/LY341495 3 mg/kg.

RESULTS

Each of LY354740, LY487379 and EMD-281,014 reduced the severity of L-DOPA-induced dyskinesia, by 55%, 39% and 40%, respectively (all p < 0.001), as well as the severity of PLBs, by 48%, 36% and 41%, respectively (all p < 0.001). Adding LY341495 1 and 3 mg/kg to each of LY354740, LY487379 and EMD-281,014 resulted in a dose-dependent reversal of their anti-dyskinetic and anti-psychotic actions. No effect on the anti-parkinsonian action of L-DOPA was noted with any treatment combination.

CONCLUSION

These results suggest that an antagonistic effect at mGlu₃ receptors may not be sufficient to overcome the deleterious effect of mGlu₂ blockade on dyskinesia in PD. It remains to be seen whether similar effects would have been obtained with a selective mGlu₃ antagonist.

Glutamate modulation for the treatment of levodopa induced dyskinesia: a brief review of the drugs tested in the clinic

Levodopa is the standard treatment for Parkinson's disease, but its use is marred by the emergence of dyskinesia, for which treatment options remain limited. Here, we review the glutamatergic modulators that were assessed for their antidyskinetic potential in clinical trials, including N-methyl-D-aspartate (NMDA) antagonists, agonists at the glycine-binding site on NMDA receptors, metabotropic glutamate (mGlu) 4 agonists, mGlu₅ antagonists, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonists and glutamate release inhibitors. Several agents that were investigated are not selective for their targets, raising uncertainty about the extent to which glutamatergic modulation contributed to their effects. Except for amantadine, the use of glutamatergic modulators for the treatment of dyskinesia in Parkinson's disease remains largely investigational, with promising results obtained with mGlu₅ negative allosteric modulation.

Plasticity, genetics, and epigenetics in l-dopa-induced dyskinesias

l-Dopa-induced dyskinesias (LIDs) are a frequent complication in l-dopa-treated patients affected by Parkinson's disease (PD). In the last years, several progresses in the knowledge of LIDs mechanisms have led to the identification of several molecular and electrophysiologic events. A complex cascade of intracellular events underlies the pathophysiology of LIDs, and, among these, aberrant plasticity in the cortico-basal ganglia system, at striatal and cortical level, plays a key role. Furthermore, several recent studies have investigated genetic susceptibility and epigenetic modifications in LIDs pathophysiology that might have future relevance in clinical practice and pharmacologic research. These progresses might lead to the development of specific strategies not only to treat, but also to prevent or delay the development of LIDs in PD.

Upregulated mGluR5 induces ER stress and DNA damage by regulating the NMDA receptor subunit NR2B

Dysfunction caused by mGluR5 expression or activation is an important mechanism in the development of Parkinson's disease (PD). Early clinical studies on mGluR5 negative allosteric modulators have shown some limitations. It is therefore necessary to find a more specific approach to block mGluR5-mediated neurotoxicity. Here, we determined the role of NMDA receptor subunit NR2B in mGluR5-mediated ER stress and DNA damage. In vitro study, rotenone-induced ER stress and DNA damage were accompanied by an increase in mGluR5 expression, and overexpressed or activated mGluR5 with agonist CHPG induced ER stress and DNA damage, while blocking mGluR5 with antagonist MPEP alleviated the effect. Furthermore, the damage caused by CHPG was blocked by NMDA receptor antagonist MK-801. Additionally, rotenone or CHPG increased the p-Src and p-NR2B, which was inhibited by MPEP. Blocking p-Src or NR2B with PP2 or CP101,606 alleviated CHPG-induced ER stress and DNA damage. Overactivation of mGluR5 accompanied with the increase of p-Src and p-NR2B in the ER stress and DNA damage was found in rotenone-induced PD rat model. These findings suggest a new mechanism wherein mGluR5 induces ER stress and DNA damage through the NMDA receptor and propose NR2B as the molecular target for therapeutic strategy for PD.

Effects of two inhibitors of metabolic glutamate receptor 5 on expression of endogenous homer scaffold protein 1 in the auditory cortex of mice with tinnitus

Tinnitus is deemed as the result of abnormal neural activities in the brain, and Homer proteins are expressed in the brain that convey nociception. The expression of Homer in tinnitus has not been studied. We hypothesized that expression of Homer in the auditory cortex was altered after tinnitus treatment. Mice were injected with sodium salicylate to induce tinnitus. Expression of Homer was detected by quantitative real-time polymerase chain reaction, western blotting, and immunohistochemistry assays. We found that Homer1 expression was upregulated in the auditory cortex of mice with tinnitus, while expression of Homer2 or Homer3 exhibited no significant alteration. Effects of two inhibitors of metabolic glutamate receptor 5 (mGluR5), noncompetitive 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) and competitive α-methyl-4-carboxyphenylglycine (MCPG), on the tinnitus scores of the mice and on Homer1 expression were detected. MPEP significantly reduced tinnitus scores and suppressed Homer1 expression in a concentration dependent manner. MCPG had no significant effects on tinnitus scores or Homer1 expression. In conclusion, Homer1 expression was upregulated in the auditory cortex of mice after tinnitus, and was suppressed by noncompetitive mGluR5 inhibitor MPEP, but not competitive mGluR5 inhibitor MCPG.

Partial mGlu5 Negative Allosteric Modulator M-5MPEP Demonstrates Antidepressant-Like Effects on Sleep Without Affecting Cognition or Quantitative EEG

Selective negative allosteric modulators (NAMs) targeting the metabotropic glutamate receptor subtype 5 (mGlu5) demonstrate anxiolytic-like and antidepressant-like effects yet concern regarding adverse effect liability remains. Functional coupling of mGlu5 with ionotropic N-methyl-D-aspartate receptors (NMDARs) represents a potential mechanism through which full inhibition leads to adverse effects, as NMDAR inhibition can induce cognitive impairments and psychotomimetic-like effects. Recent development of "partial" mGlu5 NAMs, characterized by submaximal but saturable levels of blockade, may represent a novel development approach to broaden the therapeutic index of mGlu5 NAMs. This study compared the partial mGlu5 NAM, M-5MPEP, with the full mGlu5 NAM, VU0424238 on sleep, cognition, and brain function alone and in combination with a subthreshold dose of the NMDAR antagonist, MK-801, using a paired-associates learning (PAL) cognition task and electroencephalography (EEG) in rats. M-5MPEP and VU0424238 decreased rapid eye movement (REM) sleep and increased REM sleep latency, both putative biomarkers of antidepressant-like activity. Neither compound alone affected accuracy, but 30 mg/kg VU0424238 combined with MK-801 decreased accuracy on the PAL task. Using quantitative EEG, VU0424238, but not M-5MPEP, prolonged arousal-related elevations in high gamma power, and, in combination, VU0424238 potentiated effects of MK-801 on high gamma power. Together, these studies further support a functional interaction between mGlu5 and NMDARs that may correspond with cognitive impairments. Present data support further development of partial mGlu5 NAMs given their potentially broader therapeutic index than full mGlu5 NAMs and use of EEG as a translational biomarker to titrate doses aligning with therapeutic versus adverse effects.

Neonatal maternal separation affects metabotropic glutamate receptor 5 expression and anxiety-related behavior of adult rats

Exposure to early life stress leads to long-term neurochemical and behavioral alterations. Stress-induced psychiatric disorders, such as depression, have recently been linked to dysregulation of glutamate signaling, mainly via its postsynaptic receptors. The role of metabotropic glutamate receptor 5 (mGluR5) in stress-induced psychopathology has been the target of several studies in humans. In rodents, blockade of mGluR5 produces antidepressant-like actions, whereas mice lacking mGluR5 exhibit altered anxiety-like behaviors and learning. In this study, we used well-known rodent models of early life stress based on mother-infant separation during the first 3 weeks of life in order to examine the effects of neonatal maternal separation on mGluR5 expression and on anxiety-related behavior in adulthood. We observed that brief (15 min) neonatal maternal separation, but not prolonged (3 h), induced increases in mGluR5 mRNA and protein expression levels in medial prefrontal cortex and mGluR5 protein levels in dorsal, but not ventral, hippocampus of adult rat brain. Behavioral testing using the open-field and the elevated-plus maze tasks showed that brief maternal separations resulted in increased exploratory and decreased anxiety-related behavior, whereas prolonged maternal separations resulted in increased anxiety-related behavior in adulthood. The data indicate that the long-lasting effects of neonatal mother-offspring separation on anxiety-like behavior and mGluR5 expression depend on the duration of maternal separation and suggest that the increased mGluR5 receptors in medial prefrontal cortex and hippocampus of adult rats exposed to brief neonatal maternal separations may underlie their heightened ability to cope with stress.

Selective inhibition of glycogen synthase kinase 3α corrects pathophysiology in a mouse model of fragile X syndrome

Fragile X syndrome is caused by FMR1 gene silencing and loss of the encoded fragile X mental retardation protein (FMRP), which binds to mRNA and regulates translation. Studies in the Fmr1^-/y mouse model of fragile X syndrome indicate that aberrant cerebral protein synthesis downstream of metabotropic glutamate receptor 5 (mGluR5) signaling contributes to disease pathogenesis, but clinical trials using mGluR5 inhibitors were not successful. Animal studies suggested that treatment with lithium might be an alternative approach. Targets of lithium include paralogs of glycogen synthase kinase 3 (GSK3), and nonselective small-molecule inhibitors of these enzymes improved disease phenotypes in a fragile X syndrome mouse model. However, the potential therapeutic use of GSK3 inhibitors has been hampered by toxicity arising from inhibition of both α and β paralogs. Recently, we developed GSK3 inhibitors with sufficient paralog selectivity to avoid a known toxic consequence of dual inhibition, that is, increased β-catenin stabilization. We show here that inhibition of GSK3α, but not GSK3β, corrected aberrant protein synthesis, audiogenic seizures, and sensory cortex hyperexcitability in Fmr1^-/y mice. Although inhibiting either paralog prevented induction of NMDA receptor-dependent long-term depression (LTD) in the hippocampus, only inhibition of GSK3α impaired mGluR5-dependent and protein synthesis-dependent LTD. Inhibition of GSK3α additionally corrected deficits in learning and memory in Fmr1^-/y mice; unlike mGluR5 inhibitors, there was no evidence of tachyphylaxis or enhanced psychotomimetic-induced hyperlocomotion. GSK3α selective inhibitors may have potential as a therapeutic approach for treating fragile X syndrome.

Genetic disruption of Grm5 causes complex alterations in motor activity, anxiety and social behaviors

Autism is a neurodevelopmental disorder characterized by impaired social interactions and restricted and repetitive behaviors. Although group 1 metabotropic glutamate receptors (mGluRs), and in particular mGluR5, have been extensively proposed as potential targets for intervention in autism and other neurodevelopmental disorders, there has not been a comprehensive analysis of the effect of mGluR5 loss on behaviors typically assessed in autism mouse models thought to be correlates of behavioral symptoms of human disorders. Here we present a behavioral characterization of mice with complete or partial loss of mGluR5 (homozygous or heterozygous null mutations in Grm5 gene). We tested several autism related behaviors including social interaction, repetitive grooming, digging and locomotor behaviors. We found that digging and marble burying behaviors were almost completely abolished in mGluR5 ko mice, although self-grooming was not altered. Social interaction was impaired in ko but not in heterozygote (het) mice. In tests of locomotor activity and anxiety related behaviors, mGluR5 ko mice exhibited hyperactivity and reduced anxiety in the open field test but unexpectedly, showed hypoactivity in the elevated zero-maze test. There was no impairment in motor learning in the accelerating rotarod in both ko and het mutant. Together these results provide support for the importance of mGluR5 in motor and social behaviors that are specifically affected in autism disorders.

Rescue of striatal long-term depression by chronic mGlu5 receptor negative allosteric modulation in distinct dystonia models

An impairment of long-term synaptic plasticity is considered as a peculiar endophenotype of distinct forms of dystonia, a common, disabling movement disorder. Among the few therapeutic options, broad-spectrum antimuscarinic drugs are utilized, aimed at counteracting abnormal striatal acetylcholine-mediated transmission, which plays a crucial role in dystonia pathophysiology. We previously demonstrated a complete loss of long-term synaptic depression (LTD) at corticostriatal synapses in rodent models of two distinct forms of isolated dystonia, resulting from mutations in the TOR1A (DYT1), and GNAL (DYT25) genes. In addition to anticholinergic agents, the aberrant excitability of striatal cholinergic cells can be modulated by group I metabotropic glutamate receptor subtypes (mGlu1 and 5). Here, we tested the efficacy of the negative allosteric modulator (NAM) of metabotropic glutamate 5 (mGlu) receptor, dipraglurant (ADX48621) on striatal LTD. We show that, whereas acute treatment failed to rescue LTD, chronic dipraglurant rescued this form of synaptic plasticity both in DYT1 mice and GNAL rats. Our analysis of the pharmacokinetic profile of dipraglurant revealed a relatively short half-life, which led us to uncover a peculiar time-course of recovery based on the timing from last dipraglurant injection. Indeed, striatal spiny projection neurons (SPNs) recorded within 2 h from last administration showed full expression of synaptic plasticity, whilst the extent of recovery progressively diminished when SPNs were recorded 4-6 h after treatment. Our findings suggest that distinct dystonia genes may share common signaling pathway dysfunction. More importantly, they indicate that dipraglurant might be a potential novel therapeutic agent for this disabling disorder.

Developmental up-regulation of NMDA receptors in the prefrontal cortex and hippocampus of mGlu5 receptor knock-out mice

mGlu5 metabotropic glutamate receptors are highly expressed and functional in the early postnatal life, and are known to positively modulate NMDA receptor function. Here, we examined the expression of NMDA receptor subunits and interneuron-related genes in the prefrontal cortex and hippocampus of mGlu5-/- mice and wild-type littermates at three developmental time points (PND9, - 21, and - 75). We were surprised to find that expression of all NMDA receptor subunits was greatly enhanced in mGlu5-/- mice at PND21. In contrast, at PND9, expression of the GluN2B subunit was enhanced, whereas expression of GluN2A and GluN2D subunits was reduced in both regions. These modifications were transient and disappeared in the adult life (PND75). Changes in the transcripts of interneuron-related genes (encoding parvalbumin, somatostatin, vasoactive intestinal peptide, reelin, and the two isoforms of glutamate decarboxylase) were also observed in mGlu5-/- mice across postnatal development. For example, the transcript encoding parvalbumin was up-regulated in the prefrontal cortex of mGlu5-/- mice at PND9 and PND21, whereas it was significantly reduced at PND75. These findings suggest that in mGlu5-/- mice a transient overexpression of NMDA receptor subunits may compensate for the lack of the NMDA receptor partner, mGlu5. Interestingly, in mGlu5-/- mice the behavioral response to the NMDA channel blocker, MK-801, was significantly increased at PND21, and largely reduced at PND75. The impact of adaptive changes in the expression of NMDA receptor subunits should be taken into account when mGlu5-/- mice are used for developmental studies.

Pharmacological activation of mGlu5 receptors with the positive allosteric modulator VU0360172, modulates thalamic GABAergic transmission

Previous studies have shown that injection of the mGlu5 receptor positive allosteric modulator (PAM) VU0360172 into either the thalamus or somatosensory cortex markedly reduces the frequency of spike-and-wave discharges (SWDs) in the WAG/Rij model of absence epilepsy. Here we have investigated the effects of VU0360172 on GABA transport in the thalamus and somatosensory cortex, as possible modes of action underlying the suppression of SWDs. Systemic VU0360172 injections increase GABA uptake in thalamic synaptosomes from epileptic WAG/Rij rats. Consistent with this observation, VU0360172 could also enhance thalamic GAT-1 protein expression, depending on the dosing regimen. This increase in GAT-1 expression was also observed in the thalamus from non-epileptic rats (presymptomatic WAG/Rij and Wistar) and appeared to occur selectively in neurons. The tonic GABA_A receptor current present in ventrobasal thalamocortical neurons was significantly reduced by VU0360172 consistent with changes in GAT-1 and GABA uptake. The in vivo effects of VU0360172 (reduction in tonic GABA current and increase in GAT-1 expression) could be reproduced in vitro by treating thalamic slices with VU0360172 for at least 1 h and appeared to be dependent on the activation of PLC. Thus, the effects of VU0360172 do not require an intact thalamocortical circuit. In the somatosensory cortex, VU0360172 reduced GABA uptake but did not cause significant changes in GAT-1 protein levels. These findings reveal a novel mechanism of regulation mediated by mGlu5 receptors, which could underlie the powerful anti-absence effect of mGlu5 receptor enhancers in animal models.

NMDAR PAMs: Multiple Chemotypes for Multiple Binding Sites

The N-methyl-D-aspartate receptor (NMDAR) is a member of the ionotropic glutamate receptor (iGluR) family that plays a crucial role in brain signalling and development. NMDARs are nonselective cation channels that are involved with the propagation of excitatory neurotransmission signals with important effects on synaptic plasticity. NMDARs are functionally and structurally complex receptors, they exist as a family of subtypes each with its own unique pharmacological properties. Their implication in a variety of neurological and psychiatric conditions means they have been a focus of research for many decades. Disruption of NMDAR-related signalling is known to adversely affect higherorder cognitive functions (e.g. learning and memory) and the search for molecules that can recover (or even enhance) receptor output is a current strategy for CNS drug discovery. A number of positive allosteric modulators (PAMs) that specifically attempt to overcome NMDAR hypofunction have been discovered. They include various chemotypes that have been found to bind to several different binding sites within the receptor. The heterogeneity of chemotype, binding site and NMDAR subtype provide a broad landscape of ongoing opportunities to uncover new features of NMDAR pharmacology. Research on NMDARs continues to provide novel mechanistic insights into receptor activation and this review will provide a high-level overview of the research area and discuss the various chemical classes of PAMs discovered so far.

A Review of Recent Advances and Research on Drug Target Identification Methods

BACKGROUND

From a therapeutic viewpoint, understanding how drugs bind and regulate the functions of their target proteins to protect against disease is crucial. The identification of drug targets plays a significant role in drug discovery and studying the mechanisms of diseases. Therefore the development of methods to identify drug targets has become a popular issue.

METHODS

We systematically review the recent work on identifying drug targets from the view of data and method. We compiled several databases that collect data more comprehensively and introduced several commonly used databases. Then divided the methods into two categories: biological experiments and machine learning, each of which is subdivided into different subclasses and described in detail.

RESULTS

Machine learning algorithms are the majority of new methods. Generally, an optimal set of features is chosen to predict successful new drug targets with similar properties. The most widely used features include sequence properties, network topological features, structural properties, and subcellular locations. Since various machine learning methods exist, improving their performance requires combining a better subset of features and choosing the appropriate model for the various datasets involved.

CONCLUSION

The application of experimental and computational methods in protein drug target identification has become increasingly popular in recent years. Current biological and computational methods still have many limitations due to unbalanced and incomplete datasets or imperfect feature selection methods.

Discovery and Development of Non-Dopaminergic Agents for the Treatment of Schizophrenia: Overview of the Preclinical and Early Clinical Studies

Schizophrenia is a chronic psychiatric disorder that affects about 1 in 100 people around the world and results in persistent emotional and cognitive impairments. Untreated schizophrenia leads to deterioration in quality of life and premature death. Although the clinical efficacy of dopamine D2 receptor antagonists against positive symptoms of schizophrenia supports the dopamine hypothesis of the disease, the resistance of negative and cognitive symptoms to these drugs implicates other systems in its pathophysiology. Many studies suggest that abnormalities in glutamate homeostasis may contribute to all three groups of schizophrenia symptoms. Scientific considerations also include disorders of gamma-aminobutyric acid-ergic and serotonergic neurotransmissions as well as the role of the immune system. The purpose of this review is to update the most recent reports on the discovery and development of non-dopaminergic agents that may reduce positive, negative, and cognitive symptoms of schizophrenia, and may be alternative to currently used antipsychotics. This review collects the chemical structures of representative compounds targeting metabotropic glutamate receptor, gamma-aminobutyric acid type A receptor, alpha 7 nicotinic acetylcholine receptor, glycine transporter type 1 and glycogen synthase kinase 3 as well as results of in vitro and in vivo studies indicating their efficacy in schizophrenia. Results of clinical trials assessing the safety and efficacy of the tested compounds have also been presented. Finally, attention has been paid to multifunctional ligands with serotonin receptor affinity or phosphodiesterase inhibitory activity as novel strategies in the search for dedicated medicines for patients with schizophrenia.

d-Cycloserine reverses scopolamine-induced object and place memory deficits in a spontaneous recognition paradigm in rats

d-Cycloserine (DCS) is a partial agonist of the glutamatergic N-methyl-d-aspartate (NMDA) receptor-associated glycine site, and it prevents the amnesic effects of the muscarinic receptor antagonist scopolamine in various memory tests in rodents. In the present study, we tested the hypothesis that DCS has anti-amnesic effects in scopolamine-induced deficits using spontaneous object recognition and place recognition tests. In both tests, scopolamine (0.5 mg/kg, i.p.) was systemically administered 60 min prior to testing, while DCS (7.5, 15, 30 mg/kg, i.p.) was administered 30 min before testing, which consisted of a sample phase (5 min), a delay interval (15 min) and a test phase (2 min). DCS treatment reversed scopolamine-induced deficits in discriminatory behavior during the test phase. However, DCS did not affect decreased object exploration itself or increased thigmotaxis in the open-field arena induced by scopolamine. These results support our hypothesis and suggest differential contributions of glutamatergic-cholinergic system interactions to recognition memory and non-mnemonic exploratory behaviors.

Family C G-Protein-Coupled Receptors in Alzheimer's Disease and Therapeutic Implications

Alzheimer’s disease (AD), particularly its sporadic or late-onset form (SAD/LOAD), is the most prevalent (96–98% of cases) neurodegenerative dementia in aged people. AD’s neuropathology hallmarks are intrabrain accumulation of amyloid-β peptides (Aβs) and of hyperphosphorylated Tau (p-Tau) proteins, diffuse neuroinflammation, and progressive death of neurons and oligodendrocytes. Mounting evidences suggest that family C G-protein-coupled receptors (GPCRs), which include γ-aminobutyric acid B receptors (GABA_BRs), metabotropic glutamate receptors (mGluR1-8), and the calcium-sensing receptor (CaSR), are involved in many neurotransmitter systems that dysfunction in AD. This review updates the available knowledge about the roles of GPCRs, particularly but not exclusively those expressed by brain astrocytes, in SAD/LOAD onset and progression, taking stock of their respective mechanisms of action and of their potential as anti-AD therapeutic targets. In particular, GABA_BRs prevent Aβs synthesis and neuronal hyperexcitability and group I mGluRs play important pathogenetic roles in transgenic AD-model animals. Moreover, the specific binding of Aβs to the CaSRs of human cortical astrocytes and neurons cultured in vitro engenders a pathological signaling that crucially promotes the surplus synthesis and release of Aβs and hyperphosphorylated Tau proteins, and also of nitric oxide, vascular endothelial growth factor-A, and proinflammatory agents. Concurrently, Aβs•CaSR signaling hinders the release of soluble (s)APP-α peptide, a neurotrophic agent and GABA_BR1a agonist. Altogether these effects progressively kill human cortical neurons in vitro and likely also in vivo. Several CaSR’s negative allosteric modulators suppress all the noxious effects elicited by Aβs•CaSR signaling in human cortical astrocytes and neurons thus safeguarding neurons’ viability in vitro and raising hopes about their potential therapeutic benefits in AD patients. Further basic and clinical investigations on these hot topics are needed taking always heed that activation of the several brain family C GPCRs may elicit divergent upshots according to the models studied.

Potential synaptic plasticity-based Shenzhiling oral liquid for a SAD Mouse Model

BACKGROUND

Synaptic plasticity is the basis of memory formation. The pathological manifestations of abnormal glucose metabolism in the nervous system of sporadic Alzheimer's disease (SAD) may affect synaptic plasticity, thus causing memory damage. As a traditional Chinese medicine compound preparation, the mechanism by which Shenzhiling (SZL) oral liquid can alleviate the cognitive impairment of SAD by improving synaptic plasticity remains unclear.

OBJECTIVE

This article mainly discusses whether SZL can exert a protective synaptic effect as mediated by glutamate receptors and glycogen synthesis kinase 3β (GSK3β); further, it discusses whether SZL can improve cognitive function in SAD.

METHODS

C57BL/6 mice were used as a SAD model after injection with streptozotocin (STZ) into the bilateral lateral ventricles; mice of the same background were injected with artificial cerebrospinal fluid into bilateral ventricles and were used as a control group. After 3 months of exposure to the intervention, the step-down test was carried out. Western blot was used to detect the levels of NMDAR2B, p-NMDAR2B, mGlu5, GSK3β, and p-GSK3β in the hippocampus of mice. Immunohistochemical analysis was used to observe the number of GSK3β-positive cells in the CA1 region of mouse hippocampus.

RESULTS

The memory retention ability of mice was significantly improved after 3 months of SZL treatment, and the expression levels of NMDAR2B, mGlu5, and GSK3β were significantly changed.

CONCLUSION

Shenzhiling provides a potential for the treatment for SAD with traditional Chinese medicine.

NMDAR Hypofunction Animal Models of Schizophrenia

The N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis has been proposed to help understand the etiology and pathophysiology of schizophrenia. This hypothesis was based on early observations that NMDAR antagonists could induce a full range of symptoms of schizophrenia in normal human subjects. Accumulating evidence in humans and animal studies points to NMDAR hypofunctionality as a convergence point for various symptoms of schizophrenia. Here we review animal models of NMDAR hypofunction generated by pharmacological and genetic approaches, and how they relate to the pathophysiology of schizophrenia. In addition, we discuss the limitations of animal models of NMDAR hypofunction and their potential utility for therapeutic applications.

The neurobiological basis for novel experimental therapeutics in dystonia

Dystonia is a movement disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures that may affect one or multiple body regions. Dystonia is the third most common movement disorder after Parkinson's disease and essential tremor. Despite its relative frequency, small molecule therapeutics for dystonia are limited. Development of new therapeutics is further hampered by the heterogeneity of both clinical symptoms and etiologies in dystonia. Recent advances in both animal and cell-based models have helped clarify divergent etiologies in dystonia and have facilitated the identification of new therapeutic targets. Advances in medicinal chemistry have also made available novel compounds for testing in biochemical, physiological, and behavioral models of dystonia. Here, we briefly review motor circuit anatomy and the anatomical and functional abnormalities in dystonia. We then discuss recently identified therapeutic targets in dystonia based on recent preclinical animal studies and clinical trials investigating novel therapeutics.

Targeting of Metabotropic Glutamate Receptors for the Development of Novel Antidepressants

Since discovering that ketamine has robust antidepressant effects, the glutamatergic system has been proposed as an attractive target for the development of novel antidepressants. Among the glutamatergic system, metabotropic glutamate (mGlu) receptors are of interest because mGlu receptors play modulatory roles in glutamatergic transmission, consequently, agents acting on mGlu receptors might not exert the adverse effects associated with ketamine. mGlu receptors have eight subtypes that are classified into three groups, and the roles of each mGlu receptor subtype in depression are being investigated. To date, the potential use of mGlu5 receptor antagonists and mGlu2/3 receptor antagonists as antidepressants has been actively investigated, and the mechanisms underlying these antidepressant effects are being delineated. Although the outcomes of clinical trials using an mGlu5 receptor negative allosteric modulator and an mGlu2/3 receptor negative allosteric modulator have not been encouraging, these trials have been inconclusive, and additional trials using other compounds with more appropriate profiles are needed. In contrast, the roles of group III mGlu receptors have not yet been fully elucidated because of a lack of suitable pharmacological tools. Nonetheless, investigations of the use of mGlu4 and mGlu7 receptors as drug targets for the development of antidepressants have been ongoing, and some interesting evidence has been obtained.

Targeting mGlu Receptors for Optimization of Antipsychotic Activity and Disease-Modifying Effect in Schizophrenia

Metabotropic glutamate (mGlu) receptors are considered as candidate drug targets for the treatment of schizophrenia. These receptors form a family of eight subtypes (mGlu1 to -8), of which mGlu1 and -5 are coupled to G_q/11, and all other subtypes are coupled to G_i/o. Here, we discuss the possibility that selective ligands of individual mGlu receptor subtypes may be effective in controlling the core symptoms of schizophrenia, and, in some cases, may impact mechanisms underlying the progression of the disorder. Recent evidence indicates that activation of mGlu1 receptors inhibits dopamine release in the meso-striatal system. Hence, selective positive allosteric modulators (PAMs) of mGlu1 receptors hold promise for the treatment of positive symptoms of schizophrenia. mGlu5 receptors are widely expressed in the CNS and regulate the activity of cells that are involved in the pathophysiology of schizophrenia, such as cortical GABAergic interneurons and microglial cells. mGlu5 receptor PAMs are under development for the treatment of schizophrenia and cater the potential to act as disease modifiers by restraining neuroinflammation. mGlu2 receptors have attracted considerable interest because they negatively modulate 5-HT_2A serotonin receptor signaling in the cerebral cortex. Both mGlu2 receptor PAMs and orthosteric mGlu2/3 receptor agonists display antipsychotic-like activity in animal models, and the latter drugs are inactive in mice lacking mGlu2 receptors. So far, mGlu3 receptors have been left apart as drug targets for schizophrenia. However, activation of mGlu3 receptors boosts mGlu5 receptor signaling, supports neuronal survival, and drives microglial cells toward an antiinflammatory phenotype. This strongly encourages research of mGlu3 receptors in schizophrenia. Finally, preclical studies suggest that mGlu4 receptors might be targeted by novel antipsychotic drugs, whereas studies of mGlu7 and mGlu8 receptors in animal models of psychosis are still at their infancy.

Metaplasticity at the addicted tetrapartite synapse: A common denominator of drug induced adaptations and potential treatment target for addiction

In light of the current worldwide addiction epidemic, the need for successful therapies is more urgent than ever. Although we made substantial progress in our basic understanding of addiction, reliable therapies are lacking. Since 40-60% of patients treated for substance use disorder return to active substance use within a year following treatment discharge, alleviating the vulnerability to relapse is regarded as the most promising avenue for addiction therapy. Preclinical addiction research often focuses on maladaptive synaptic plasticity within the reward pathway. However, drug induced neuroadaptations do not only lead to a strengthening of distinct drug associated cues and drug conditioned behaviors, but also seem to increase plasticity thresholds for environmental stimuli that are not associated with the drug. This form of higher order plasticity, or synaptic metaplasticity, is not expressed as a change in the efficacy of synaptic transmission but as a change in the direction or degree of plasticity induced by a distinct stimulation pattern. Experimental addiction research has demonstrated metaplasticity after exposure to multiple classes of addictive drugs. In this review we will focus on the concept of synaptic metaplasticity in the context of preclinical addiction research. We will take a closer look at the tetrapartite glutamatergic synapse and outline forms of metaplasticity that have been described at the addicted synapse. Finally we will discuss the different potential avenues for pharmacotherapies that target glutamatergic synaptic plasticity and metaplasticity. Here we will argue that aberrant metaplasticity renders the reward seeking circuitry more rigid and hence less able to adapt to changing environmental contingencies. An understanding of the molecular mechanisms that underlie this metaplasticity is crucial for the development of new strategies for addiction therapy. The correction of drug-induced metaplasticity could be used to support behavioral and pharmacotherapies for the treatment of addiction.

In Vivo Non-radioactive Assessment of mGlu5 Receptor-Activated Polyphosphoinositide Hydrolysis in Response to Systemic Administration of a Positive Allosteric Modulator

mGlu5 receptor-mediated polyphosphoinositide (PI) hydrolysis is classically measured by determining the amount of radioactivity incorporated in inositolmonophosphate (InsP) after labeling of membrane phospholipids with radioactive inositol. Although this method is historically linked to the study of mGlu receptors, it is inappropriate for the assessment of mGlu5 receptor signaling in vivo. Using a new ELISA kit we showed that systemic treatment with the selective positive allosteric modulator (PAM) of mGlu5 receptors VU0360172 enhanced InsP formation in different brain regions of CD1 or C57Black mice. The action of VU0360172 was sensitive to the mGlu5 receptor, negative allosteric modulator (NAM), MTEP, and was abolished in mice lacking mGlu5 receptors. In addition, we could demonstrate that endogenous activation of mGlu5 receptors largely accounted for the basal PI hydrolysis particularly in the prefrontal cortex. This method offers opportunity for investigation of mGlu5 receptor signaling in physiology and pathology, and could be used for the functional screening of mGlu5 receptor PAMs in living animals.

Metabotropic Glutamatergic Receptor 5 and Stress Disorders: Knowledge Gained From Receptor Imaging Studies

The metabotropic glutamatergic receptor subtype 5 (mGluR5) may represent a promising therapeutic target for stress-related psychiatric disorders. Here, we describe mGluR5 findings in stress disorders, particularly major depressive disorder (MDD), highlighting insights from positron emission tomography studies. Positron emission tomography studies report either no differences or lower mGluR5 in MDD, potentially reflecting MDD heterogeneity. Unlike the rapidly acting glutamatergic agent ketamine, mGluR5-specific modulation has not yet shown antidepressant efficacy in MDD and bipolar disorder. Although we recently showed that ketamine may work, in part, through significant mGluR5 modulation, the specific role of mGluR5 downregulation in ketamine's antidepressant response is unclear. In contrast to MDD, there has been much less investigation of mGluR5 in bipolar disorder, yet initial studies indicate that mGluR5-specific treatments may aid in both depressed and manic mood states. The direction of modulation needed may be state dependent, however, limiting clinical feasibility. There has been relatively little study of posttraumatic stress disorder or obsessive-compulsive disorder to date, although there is evidence for the upregulation of mGluR5 in these disorders. However, while antagonism of mGluR5 may reduce fear conditioning, it may also reduce fear extinction. Therefore, studies are needed to determine the role mGluR5 modulation might play in the treatment of these conditions. Further challenges in modulating this prevalent neurotransmitter system include potential induction of significant side effects. As such, more research is needed to identify level and type (positive/negative allosteric modulation or full antagonism) of mGluR5 modulation required to translate existing knowledge into improved therapies.

Chronic Adolescent CDPPB Treatment Alters Short-Term, but not Long-Term, Glutamatergic Receptor Expression

Dysfunction of the glutamatergic system is believed to underlie many neurodevelopmental disorders including autism, Rett syndrome and schizophrenia. Metabotropic glutamate receptor (mGluR5) positive allosteric modulators (PAM) potentiate glutamatergic signaling, particularly indirectly via the NMDA receptor. Preclinical studies report mGluR5 PAMs can improve schizophrenia-relevant behaviours. Furthermore, adolescent administration has shown to prevent cognitive induced deficits in adult rodents. However, there is limited understanding of the short- and long-term neurochemical effects of mGluR5 PAMs, which may underlie their therapeutic effects. We examined the effect of 7-day adolescent (PN28-34) treatment with the mGluR5 PAM, CDDPB (30 mg/kg), on glutamatergic receptor expression at adolescence (PN35) and adulthood (PN96). Immunoblot analysis revealed that 7-day adolescent CDPPB treatment increased protein expression of glutamatergic receptors including the NMDA receptor subunits, NR1 and NR2A and the AMPA subunits (GluA1 and GluA2) in the adolescent hippocampus, changes that did not extend to adulthood. In contrast, there were no changes in the adolescent frontal cortex, however elevated mGluR5 protein expression was observed at adulthood following adolescent CDPPB treatment. The present study indicates adolescent CDPPB treatment may cause brain region dependent effects on the glutamatergic system, which do not persist into adulthood. These findings may have implications for the preclinical development of mGluR5 PAMs for the treatment of neurodevelopmental disorders.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Glutamate heteroreceptor complexes in the brain

The existence of mGluR, NMDAR, AMPAR and putative KAR heteroreceptor complexes in synaptic and extrasynaptic regions of brain glutamate synapses represents a major integrative mechanism. Our aim in the current article is to analyze if the formation of the different types glutamate hetereceptor complexes involves the contribution of triplet amino acid homologies (protriplets) in a postulated receptor interface based on the triplet puzzle theory. Seven main sets (lists) of receptor pairs in databases were used containing various sets (lists) of human receptor heteromers and nonheteromers obtained from the available scientific publications including the publically available GPCR-hetnet database. Brain mGluR1-mGluR5 and mGluR2-mGluR4 isoreceptor complexes were demonstrated with a predominant extrasynaptic localization at a post- and prejunctional localization. The existence of putative mGluR4-mGluR7 heteroreceptor complexes in the basal ganglia is proposed. Metabotropic glutamate receptor subtypes also participated in the formation of a large number of heteroreceptor complexes like mGluR1-A1R, mGluR5-A2AR, mGluR5-D2R and D2R-A2AR-mGluR5, located in relation to glutamate synapses, especially in the basal ganglia. A putative mGluR1-GABAB1/2 heterocomplex may also exist. NMDAR heteroreceptor complexes were also demonstrated as a fundamental integrative mechanism in the glutamate synapse and its extrasynaptic membranes. It represented fundamental work on inter alia NMDAR-mGluR5, NMDAR-D1R and NMDAR-D2R heteroreceptor complexes involving both antagonistic and facilitatory allosteric receptor-receptor interactions. As to AMPA receptors, a heterocomplex was found for the interaction between IFNgR1 and the AMPAR mediated via the subunit GluA1 which may be of relevance for neuroinflammation. AMPAR-D2R heteroreceptor complexes were also demonstrated. Besides glutamate heteroreceptor complexes and their allosteric receptor-receptor interactions, a significant mechanism for the functional crosstalk can also be phosphorylation and/or reorganization of adapter proteins with dynamic binding to the two receptors modulating the allosteric receptor mechanism.

A postmortem analysis of NMDA ionotropic and group 1 metabotropic glutamate receptors in the nucleus accumbens in schizophrenia

BACKGROUND

The nucleus accumbens (NAcc) has been implicated in the pathology and treatment of schizophrenia. Recent postmortem evidence suggests a hyperglutamatergic state in the NAcc. With the present study we aimed to explore possible glutamatergic alterations in the NAcc of a large schizophrenia cohort.

METHODS

We performed immunoblots on postmortem NAcc samples from 30 individuals who had schizophrenia and 30 matched controls. We examined the protein expression of primary glutamatergic receptors, including the N-methyl-D-aspartate (NMDA) receptor (NR1, NR2A and NR2B subunits) and the group 1 metabotropic glutamate receptor (mGluR1 and mGluR5; dimeric and monomeric forms). In addition, we measured the group 1 mGluR endogenous regulators, neurochondrin and Homer1b/c, which have recently been implicated in the pathophysiology of schizophrenia.

RESULTS

Protein levels of glutamatergic receptors and endogenous regulators were not significantly different between the controls and individuals who had schizophrenia. Furthermore, mGluR5, but not mGluR1, showed a positive association with NMDA receptor subunits, suggesting differential interactions between these receptors in this brain region.

LIMITATIONS

Investigation of these proteins in antipsychotic-naive individuals, in addition to the subregions of the NAcc and subcellular fractions, will strengthen future studies.

CONCLUSION

The present study does not provide evidence for glutamatergic abnormalities within the NAcc of individuals with schizophrenia. Taken together with the results of previous studies, these findings suggest NMDA receptors and group 1 mGluRs are altered in a brain region-dependent manner in individuals with schizophrenia. The differential associations between mGluR1, mGluR5 and NMDA receptors observed in this study warrant further research into the interactions of these proteins and the implications for the therapeutic and adverse effect profile of glutamatergic-based novel therapeutics.

mGluR5 antagonism inhibits cocaine reinforcement and relapse by elevation of extracellular glutamate in the nucleus accumbens via a CB1 receptor mechanism

Metabotropic glutamate receptor 5 (mGluR5) antagonism inhibits cocaine self-administration and reinstatement of drug-seeking behavior. However, the cellular and molecular mechanisms underlying this action are poorly understood. Here we report a presynaptic glutamate/cannabinoid mechanism that may underlie this action. Systemic or intra-nucleus accumbens (NAc) administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) dose-dependently reduced cocaine (and sucrose) self-administration and cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-taking and cocaine-seeking was associated with a reduction in cocaine-enhanced extracellular glutamate, but not cocaine-enhanced extracellular dopamine (DA) in the NAc. MPEP alone, when administered systemically or locally into the NAc, elevated extracellular glutamate, but not DA. Similarly, the cannabinoid CB1 receptor antagonist, rimonabant, elevated NAc glutamate, not DA. mGluR5s were found mainly in striatal medium-spiny neurons, not in astrocytes, and MPEP-enhanced extracellular glutamate was blocked by a NAc CB1 receptor antagonist or N-type Ca⁺⁺ channel blocker, suggesting that a retrograde endocannabinoid-signaling mechanism underlies MPEP-induced glutamate release. This interpretation was further supported by our findings that genetic deletion of CB1 receptors in CB1-knockout mice blocked both MPEP-enhanced extracellular glutamate and MPEP-induced reductions in cocaine self-administration. Together, these results indicate that the therapeutic anti-cocaine effects of mGluR5 antagonists are mediated by elevation of extracellular glutamate in the NAc via an endocannabinoid-CB1 receptor disinhibition mechanism.

Altered expression of schizophrenia-related genes in mice lacking mGlu5 receptors

The evidence underlying the so-called glutamatergic hypothesis ranges from NMDA receptor hypofunction to an imbalance between excitatory and inhibitory circuits in specific brain structures. Among all glutamatergic system components, metabotropic receptors play a main role in regulating neuronal excitability and synaptic plasticity. Here, we investigated, using qRT-PCR and western blot, consequences in the hippocampus and prefrontal/frontal cortex (PFC/FC) of mice with a genetic deletion of the metabotropic glutamate receptor 5 (mGlu5), addressing key components of the GABAergic and glutamatergic systems. We found that mGlu5 knockout (KO) mice showed a significant reduction of reelin, GAD65, GAD67 and parvalbumin mRNA levels, which is specific for the PFC/FC, and that is paralleled by a significant reduction of protein levels in male KO mice. We next analyzed the main NMDA and AMPA receptor subunits, namely GluN1, GluN2A, GluN2B and GluA1, and we found that mGlu5 deletion determined a significant reduction of their mRNA levels, also within the hippocampus, with differences between the two genders. Our data suggest that neurochemical abnormalities impinging the glutamatergic and GABAergic systems may be responsible for the behavioral phenotype associated with mGlu5 KO animals and point to the close interaction of these molecular players for the development of neuropsychiatric disorders such as schizophrenia. These data could contribute to a better understanding of the involvement of mGlu5 alterations in the molecular imbalance between excitation and inhibition underlying the emergence of a schizophrenic-like phenotype and to understand the potential of mGlu5 modulators in reversing the deficits characterizing the schizophrenic pathology.

Glutamatergic regulation of cognition and functional brain connectivity: insights from pharmacological, genetic and translational schizophrenia research

The pharmacological modulation of glutamatergic neurotransmission to improve cognitive function has been a focus of intensive research, particularly in relation to the cognitive deficits seen in schizophrenia. Despite this effort, there has been little success in the clinical use of glutamatergic compounds as procognitive drugs. Here, we review a selection of the drugs used to modulate glutamatergic signalling and how they impact on cognitive function in rodents and humans. We highlight how glutamatergic dysfunction, and NMDA receptor hypofunction in particular, is a key mechanism contributing to the cognitive deficits observed in schizophrenia and outline some of the glutamatergic targets that have been tested as putative procognitive targets for this disorder. Using translational research in this area as a leading exemplar, namely, models of NMDA receptor hypofunction, we discuss how the study of functional brain network connectivity can provide new insight into how the glutamatergic system impacts on cognitive function. Future studies characterizing functional brain network connectivity will increase our understanding of how glutamatergic compounds regulate cognition and could contribute to the future success of glutamatergic drug validation. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

Pharmacological enhancement of mGlu5 receptors rescues behavioral deficits in SHANK3 knock-out mice

SHANK3 (also called PROSAP2) genetic haploinsufficiency is thought to be the major cause of neuropsychiatric symptoms in Phelan-McDermid syndrome (PMS). PMS is a rare genetic disorder that causes a severe form of intellectual disability (ID), expressive language delays and other autistic features. Furthermore, a significant number of SHANK3 mutations have been identified in patients with autism spectrum disorders (ASD), and SHANK3 truncating mutations are associated with moderate to profound ID. The Shank3 protein is a scaffold protein that is located in the postsynaptic density (PSD) of excitatory synapses and is crucial for synapse development and plasticity. In this study, we investigated the molecular mechanisms associated with the ASD-like behaviors observed in Shank3Δ11^-/- mice, in which exon 11 has been deleted. Our results indicate that Shank3 is essential to mediating metabotropic glutamate receptor 5 (mGlu5)-receptor signaling by recruiting Homer1b/c to the PSD, specifically in the striatum and cortex. Moreover, augmenting mGlu5-receptor activity by administering 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide ameliorated the functional and behavioral defects that were observed in Shank3Δ11^-/- mice, suggesting that pharmaceutical treatments that increase mGlu5 activity may represent a new approach for treating patients that are affected by PMS and SHANK3 mutations.

Molecular mechanisms of group I metabotropic glutamate receptor mediated LTP and LTD in basolateral amygdala in vitro

The roles of group I metabotropic glutamate receptors, metabotropic glutamate receptor 1 (mGluR1) and mGluR5, in regulating synaptic plasticity and metaplasticity in the basolateral amygdala (BLA) remain unclear. The present study examined mGluR1- and mGluR5-mediated synaptic plasticity in the BLA and their respective signaling mechanisms. Bath application of the group I mGluR agonist, 3,5-dihydroxyphenylglycine (DHPG) (20 μM), directly suppressed basal fEPSPs (84.5 ± 6.3% of the baseline). The suppressive effect persisted for at least 30 min after washout; it was abolished by the mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) but was unaffected by the mGluR5 antagonist 2-methyl-6- (phenylethynyl)-pyridine (MPEP). Interestingly, application of DHPG (at both 2 and 20 μM), regardless of the presence of CPCCOEt, could transform single theta burst stimulation (TBS)-induced short-term synaptic potentiation into a long-term potentiation (LTP). Such a facilitating effect could be blocked by the mGluR5 antagonist MPEP. Blockade of phospholipase C (PLC), the downstream enzyme of group I mGluR, with U73122, prevented both mGluR1- and mGluR5-mediated effects on synaptic plasticity. Nevertheless, blockade of protein kinase C (PKC), the downstream enzyme of PLC, with chelerythrine (5 μM) only prevented the transforming effect of DHPG on TBS-induced LTP and did not affect DHPG-induced long-term depression (LTD). These results suggest that mGluR1 activation induced LTD via a PLC-dependent and PKC-independent mechanism, while the priming action of mGluR5 receptor on the BLA LTP is both PLC and PKC dependent. The BLA metaplasticity mediated by mGluR1 and mGluR5 may provide signal switching mechanisms mediating learning and memory with emotional significance.