Novel allosteric antagonists shed light on mglu(5) receptors and CNS disorders

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Neurotransmission systems in Parkinson’s disease

Parkinson’s disease (PD) is histologically characterized by the accumulation of α-synuclein particles, known as Lewy bodies. The second most common neurodegenerative disorder, PD is widely known because of the typical motor manifestations of active tremor, rigidity, and postural instability, while several prodromal non-motor symptoms including REM sleep behavior disorders, depression, autonomic disturbances, and cognitive decline are being more extensively recognized. Motor symptoms most commonly arise from synucleinopathy of nigrostriatal pathway. Glutamatergic, γ-aminobutyric acid (GABA)ergic, cholinergic, serotoninergic, and endocannabinoid neurotransmission systems are not spared from the global cerebral neurodegenerative assault. Wide intrabasal and extrabasal of the basal ganglia provide enough justification to evaluate network circuits disturbance of these neurotransmission systems in PD. In this comprehensive review, English literature in PubMed, Science direct, EMBASE, and Web of Science databases were perused. Characteristics of dopaminergic and non-dopaminergic systems, disturbance of these neurotransmitter systems in the pathophysiology of PD, and their treatment applications are discussed.

Group 1 mGlu-family proteins promote neuroadaptation to ethanol and withdrawal-associated hippocampal damage

BACKGROUND

Group 1 mGlu-family proteins (i.e., mGlu) consist of mGlu1 and mGlu5 and their activity may influence voluntary ethanol intake. The present studies sought to examine the influence of these receptors on the development of ethanol dependence using in vitro and in vivo models of chronic, intermittent ethanol (CIE).

METHODS

Rat hippocampal explants were exposed to CIE with or without the addition of mGlu1 antagonist (7-hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt; 0.5, 1, and 3μM) or mGlu5 antagonist (E)-2-methyl-6-styryl-pyridine (SIB-1893; 20, 100, and 200μM) to assess sparing of withdrawal-induced cytotoxicity. In a separate study, adult male rats were administered CIE with or without the addition of oral administration of group 1 mGlu antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 3mg/kg). Blood ethanol levels (BELs) were determined at 0930h on Day 2 of Weeks 1, 2, and 3. Withdrawal behavior was monitored during Day 6 of the third consecutive withdrawal.

RESULTS

CIE produced significant hippocampal cytotoxicity. These effects were attenuated by co-exposure to CPCCOEt (3μM) with ethanol in the CA3. By contrast, these effects were blocked by SIB-1893 (20μM) in each primary cell layer. Oral administration of MPEP with ethanol significantly attenuated behavioral effects of subsequent withdrawal and reduced BELs.

CONCLUSIONS

These data demonstrate that ethanol activates group 1 mGlu-family proteins to promote withdrawal-associated cytotoxicity in vitro and physical dependence in vivo. These findings suggest that group 1 mGlu-family proteins may be therapeutic targets for treatment of alcohol use disorders.

Identification of alpha-substituted acylamines as novel, potent, and orally active mGluR5 negative allosteric modulators

This Letter describes the identification of a series of novel non-acetylenic mGluR5 negative allosteric modulators based on the alpha-substituted acylamine structure. An initial structure-activity relationship study suggested that (R)-19b and (R)-19j might have good in vitro activity. When administered orally, these compounds were found to have an anxiolytic-like effect in a mouse model of stress-induced hyperthermia.

Transcriptome assembly, gene annotation and tissue gene expression atlas of the rainbow trout

Efforts to obtain a comprehensive genome sequence for rainbow trout are ongoing and will be complemented by transcriptome information that will enhance genome assembly and annotation. Previously, transcriptome reference sequences were reported using data from different sources. Although the previous work added a great wealth of sequences, a complete and well-annotated transcriptome is still needed. In addition, gene expression in different tissues was not completely addressed in the previous studies. In this study, non-normalized cDNA libraries were sequenced from 13 different tissues of a single doubled haploid rainbow trout from the same source used for the rainbow trout genome sequence. A total of ~1.167 billion paired-end reads were de novo assembled using the Trinity RNA-Seq assembler yielding 474,524 contigs > 500 base-pairs. Of them, 287,593 had homologies to the NCBI non-redundant protein database. The longest contig of each cluster was selected as a reference, yielding 44,990 representative contigs. A total of 4,146 contigs (9.2%), including 710 full-length sequences, did not match any mRNA sequences in the current rainbow trout genome reference. Mapping reads to the reference genome identified an additional 11,843 transcripts not annotated in the genome. A digital gene expression atlas revealed 7,678 housekeeping and 4,021 tissue-specific genes. Expression of about 16,000–32,000 genes (35–71% of the identified genes) accounted for basic and specialized functions of each tissue. White muscle and stomach had the least complex transcriptomes, with high percentages of their total mRNA contributed by a small number of genes. Brain, testis and intestine, in contrast, had complex transcriptomes, with a large numbers of genes involved in their expression patterns. This study provides comprehensive de novo transcriptome information that is suitable for functional and comparative genomics studies in rainbow trout, including annotation of the genome.

Protein kinase A directly phosphorylates metabotropic glutamate receptor 5 to modulate its function

Metabotropic glutamate receptor 5 (mGluR5) regulates excitatory post-synaptic signaling in the central nervous system (CNS) and is implicated in various CNS disorders. Protein kinase A (PKA) signaling is known to play a critical role in neuropsychiatric disorders such as Parkinson's disease, schizophrenia, and addiction. Dopamine signaling is known to modulate the properties of mGluR5 in a cAMP- and PKA-dependent manner, suggesting that mGluR5 may be a direct target for PKA. Our study identifies mGluR5 at Ser870 as a direct substrate for PKA phosphorylation and demonstrates that this phosphorylation plays a critical role in the PKA-mediated modulation of mGluR5 functions such as extracellular signal-regulated kinase phosphorylation and intracellular Ca(2+) oscillations. The identification of the molecular mechanism by which PKA signaling modulates mGluR5-mediated cellular responses contributes to the understanding of the interaction between dopaminergic and glutamatergic neuronal signaling. We identified serine residue 870 (S870) in metabotropic glutamate receptor 5 (mGluR5) as a direct substrate for protein kinase A (PKA). The phosphorylation of this site regulates the ability of mGluR5 to induce extracellular signal-regulated kinase (ERK) phosphorylation and intracellular Ca(2+) oscillations. This study provides a direct molecular mechanism by which PKA signaling interacts with glutamate neurotransmission.

Pharmacological Profiles of Oligomerized μ-Opioid Receptors

Opioids are widely prescribed pain relievers with multiple side effects and potential complications. They produce analgesia via G-protein-protein coupled receptors: μ-, δ-, κ-opioid and opioid receptor-like 1 receptors. Bivalent ligands targeted to the oligomerized opioid receptors might be the key to developing analgesics without undesired side effects and obtaining effective treatment for opioid addicts. In this review we will update the biological effects of μ-opioids on homo- or hetero-oligomerized μ-opioid receptor and discuss potential mechanisms through which bivalent ligands exert beneficial effects, including adenylate cyclase regulation and receptor-mediated signaling pathways.

Palladium mediated ¹¹C-cyanation and characterization in the non-human primate brain of the novel mGluR5 radioligand [¹¹C]AZD9272

INTRODUCTION

The aims of the present positron emission tomography (PET) study were to set up a system for (11)C-cyanation labeling of the selective mGluR5-antagonist [(11)C]AZD9272 and to perform the first in vivo characterization of [(11)C]AZD9272 binding in cynomolgus monkeys.

METHODS

[(11)C]AZD9272 was labeled using palladium mediated (11)C-cyanation. Altogether seven PET measurements were performed in three cynomolgus monkeys including baseline and co-injection experiments with unlabelled AZD9272 (0.04 and 0.4 mg/kg). Radiometabolites in plasma were measured using HPLC.

RESULTS

[(11)C]AZD9272 was prepared in over 50% incorporation yield from hydrogen [(11)C]cyanide in a total synthesis time of 45-50 min. The radiochemical purity of the radioligand in its final formulation was high (>99%) and the mean specific radioactivity was 47 GBq/ μmol (1278 Ci/mmol, n=7) calculated at end of bombardment (EOB). In the baseline measurements 10% of the total injected radioactivity was present in monkey brain at five minutes after i.v. injection. The radioactivity concentration was high in the caudate, cingulate gyrus and thalamus whereas it was moderate in the temporal cortex and lower for the cerebellum. After co-injection with cold AZD9272 the binding of [(11)C]AZD9272 was reduced in a dose-dependent fashion. Analysis of radiometabolites showed relatively slow metabolism and resulted only in hydrophilic radiometabolites.

CONCLUSION

A fast and efficient method was developed to label AZD9272 with (11)C. PET-examination in Cynomolgus monkeys showed that [(11)C]AZD9272 entered the brain to a high extent, that binding was saturable and that the regional radioactivity pattern was in accordance with the known distribution of mGluR5. The results support further examination of [(11)C]AZD9272 binding in human subjects.

Synthesis and characterization of two PET radioligands for the metabotropic glutamate 1 (mGlu1) receptor

The metabotropic glutamate 1 receptor (mGlu1) is an important protein in the regulation of glutamate transmission in the brain, and believed to be involved in disorders such as ischemia, epilepsy, neuropathic pain, anxiety, and schizophrenia. The goal of this study was to evaluate two selective mGlu1 antagonists [(11) C]3 and [(18) F]4 as potential PET radioligands for the in vivo imaging of the mGlu1 receptor. Biodistribution studies in rats indicated high uptake of [(11) C]3 and [(18) F]4 in the brain. The highest activity level was found in the cerebellum, followed by striatum, hippocampus, frontal cortex, and medulla, in a pattern consistent with the distribution of mGlu1 receptor in rat. At 30 min postinjection, the activity ratio of cerebellum to medulla was 4.5 for [(11) C]3, indicating a high degree of specific binding, while specific binding was lower for [(18) F]4 (cerebellum to medulla activity ratio of 2.0). Moreover, binding of the radioligands [(11) C]3 and [(18) F]4 in mGlu1 receptor-rich region such as cerebellum was blocked by pretreatment of the rats with their respective unlabeled compound or the selective mGlu1 antagonist (compound 5, 2 mg/kg each), but not by the selective mGlu2 antagonist LY341495, or the selective mGlu5 antagonist MPEP (2 mg/kg), thus indicating the binding specificity and selectivity of [(11) C]3 and [(18) F]4 to the mGlu1 receptor. However, in imaging experiments in baboons [(11) C]3 displayed a small specific binding signal only in the cerebellum, while the specific binding of [(18) F]4 was difficult to detect. Species differences in receptor density and affinity of the radioligands in large part account for the differences in the behavior of [(11) C]3 and [(18) F]4 in rats and baboons. Radioligands with higher affinity and/or lower lipophilicity are needed to successfully image the mGlu1 receptor in humans.

Differential effects of diazepam and MPEP on habituation and neuro-behavioural processes in inbred mice

Background

Previous studies have demonstrated a profound lack of habituation in 129P3 mice compared to the habituating, but initially more anxious, BALB/c mice. The present study investigated whether this non-adaptive phenotype of 129P3 mice is primarily based on anxiety-related characteristics.

Methods

To test this hypothesis and extend our knowledge on the behavioural profile of 129P3 mice, the effects of the anxiolyticdiazepam (1, 3 and 5 mg/kg) and the putative anxiolytic metabotropic glutamate receptor 5 (mGlu5R) antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP, 3, 10 and 30 mg/kg) treatment on within-trial (intrasession) habituation, object recognition (diazepam: 1 mg/kg; MPEP 10 mg/kg) and on the central-nervous expression of the immediate early gene c-Fos (diazepam: 1 mg/kg; MPEP 10 mg/kg) were investigated.

Results

Behavioural findings validated the initially high, but habituating phenotype of BALB/c mice, while 129P3 mice were characterized by impaired intrasession habituation. Diazepam had an anxiolytic effect in BALB/c mice, while in higher doses caused behavioural inactivity in 129P3 mice. MPEP revealed almost no anxiolytic effects on behaviour in both strains, but reduced stress-induced corticosterone responses only in 129P3 mice. These results were complemented by reduced expression of c-Fos after MPEP treatment in brain areas related to emotional processes, and increased c-Fos expression in higher integrating brain areas such as the prelimbic cortex compared to vehicle-treated 129P3 mice.

Conclusions

These results suggest that the strain differences observed in (non)adaptive anxiety behaviour are at least in part mediated by differences in gamma-aminobutyric acid- A and mGluR5 mediated transmission.

Differential Modulation of Thresholds for Intracranial Self-Stimulation by mGlu5 Positive and Negative Allosteric Modulators: Implications for Effects on Drug Self-Administration

Pharmacological manipulation of the type 5 metabotropic glutamate (mGlu5) receptor alters various addiction related behaviors such as drug self-administration and the extinction and reinstatement of drug-seeking behavior. However, the effects of pharmacological modulation of mGlu5 receptors on brain reward function have not been widely investigated. We examined the effects of acute administration of positive and negative allosteric modulators (PAMs and NAMs, respectively) on brain reward function by assessing thresholds for intracranial self-stimulation (ICSS). In addition, when acute effects were observed, we examined changes in ICSS thresholds following repeated administration. Male Sprague-Dawley rats were implanted with bipolar electrodes into the medial forebrain bundle and trained to respond for ICSS, followed by assessment of effects of mGlu5 ligands on ICSS thresholds using a discrete trials current–intensity threshold determination procedure. Acute administration of the selective mGlu5 NAMs MTEP (0, 0.3, 1, or 3 mg/kg) and fenobam (0, 3, 10, or 30 mg/kg) dose-dependently increased ICSS thresholds (∼70% at the highest dose tested), suggesting a deficit in brain reward function. Acute administration of the mGlu5 PAMs CDPPB (0, 10, 30, and 60 mg/kg) or ADX47273 (0, 10, 30, and 60 mg/kg) was without effect at any dose tested. When administered once daily for five consecutive days, the development of tolerance to the ability of threshold-elevating doses of MTEP and fenobam to increase ICSS thresholds was observed. We conclude that mGlu5 PAMs and NAMs differentially affect brain reward function, and that tolerance to the ability of mGlu5 NAMs to reduce brain reward function develops with repeated administration. These brain reward deficits should be taken into consideration when interpreting acute effects of mGlu5 NAMs on drug self-administration, and repeated administration of these ligands may be an effective method to reduce these deficits.

The acute antiparkinsonian and antidyskinetic effect of AFQ056, a novel metabotropic glutamate receptor type 5 antagonist, in L-Dopa-treated parkinsonian monkeys

Overactivity of glutamatergic transmission has been implicated in Parkinson's disease (PD) and levodopa (L-Dopa)-induced dyskinesias. Striatal metabotropic glutamate receptors type 5 (mGluR5) are abundant and provide specific targets to modulate glutamatergic activity. This study investigated the acute effects of the novel mGluR5 antagonist AFQ056 on motor behavior in L-Dopa-treated monkeys with a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion to model PD. Six Macaca fascicularis MPTP monkeys were treated repeatedly with L-Dopa; this treatment increased their locomotion and reduced their parkinsonian scores, but also induced dyskinesias. When AFQ056 (doses of 5, 25, 125 or 250mg/kg) was administered one hour prior to a high dose of L-Dopa, the antiparkinsonian activity of L-Dopa was maintained as measured with locomotion and antiparkinsonian scores, whereas dyskinesias were significantly reduced at 25, 125 and 250mg/kg AFQ056 for peak dyskinesia score and at 125 and 250mg/kg for the 1h peak period of dyskinesia score. Administration of AFQ056 one hour before L-Dopa led to peak or elevated plasma AFQ056 concentrations occurring close to L-Dopa peak-dose dyskinesias. We next investigated AFQ056 25mg/kg combined with a low dose of L-Dopa. The antiparkinsonian activity of L-Dopa was increased as measured with locomotion, while dyskinesias remained low at these doses. Our results show a beneficial motor effect of AFQ056 with L-Dopa in MPTP monkeys. This supports the therapeutic use of an mGluR5 antagonist to restore normal glutamatergic neurotransmission in PD and decrease dyskinesias.

Chronic, systemic treatment with a metabotropic glutamate receptor 5 antagonist produces anxiolytic-like effects and reverses abnormal firing activity of projection neurons in the basolateral nucleus of the amygdala in rats with bilateral 6-OHDA lesions

Although 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective metabotropic glutamate receptor 5 antagonist, improves the motor symptoms of Parkinson's disease (PD), the effects of MPEP on the psychiatric symptom of PD and the mechanism involved are still unclear. In the present study, we examined the effects of MPEP in anxiolytic-like behavior and firing activity of projection neurons in the basolateral nucleus of the amygdala (BLA) in rats with 6-hydroxydopamine (6-OHDA) injected bilaterally into dorsal striatum. Rats were divided into three groups, sham-operated group, 6-OHDA lesion with vehicle treatment group and 6-OHDA lesion with MPEP treatment group. Injection of 6-OHDA (10.5 μg) into the dorsal striatum produced 31.5% loss of tyrosine hydroxylase immunoreactive (TH-ir) neurons in the SNpc. The 6-OHDA-lesioned rats showed anxiety behavior and the firing rate of BLA projection neurons decreased significantly compared with sham-operated rats, and no difference was found in the firing pattern of these neurons. Whereas chronic, systemic treatment of MPEP (3 mg/kg/day, i.p.; 14 days) attenuated loss of TH-ir neurons, produced anxiolytic-like effect and normalized the abnormal firing rate of projection neurons of the BLA in rats with the bilateral lesions. Systemic administration of cumulative apomorphine (10-160 μg/kg, i.v.) inhibited the firing rate of BLA projection neurons in sham-operated, 6-OHDA lesion with vehicle-treated and MPEP-treated rats, but the 6-OHDA lesion decreased the response of BLA projection neurons to apomorphine stimulation, while MPEP reversed the reactivity of these neurons. These data demonstrate that the partial lesion of the nigrostriatal pathway causes anxiety symptom and decreases firing rate of BLA projection neurons in the rat. Furthermore, chronic, systemic MPEP treatment has the neuroprotective and anxiolytic-like effects, and reverses the abnormal firing rate of BLA projection neurons, suggesting that MPEP has important implication for the treatment of PD.

Molecular imaging in neuroscience research with small-animal PET in rodents

Cognitive neuroscience, which studies the biological basis of mental processes, widely uses neuroimaging technologies like functional magnetic resonance imaging and positron emission tomography (PET) to study the human brain. Small laboratory animals, like rodents, are commonly used in brain research and provide abundant models of human brain diseases. The development of high-resolution small-animal PET and various radiotracers together with sophisticated methods for analyzing functional brain imaging data have accelerated research on brain function and neurotransmitter release during behavioral tasks in rodents. In this review, we first summarize advances in the methodology of cognitive research brought about by the development of sophisticated methods for whole-brain imaging analysis and improvements in neuroimaging protocols. Then, we discuss basic mechanisms related to metabolic changes and the expression of neurotransmitters in various brain areas during task-induced neural activity. In particular, we discuss glucose metabolism imaging and brain receptor imaging for various receptor systems. Finally, we discuss the current status and future perspectives. Mechanisms of neurotransmitter expression will probably become an increasingly important field of study in the future, leading to more collaboration between investigators in fields such as computational and theoretical neuroscience.

Potential therapeutic interventions for fragile X syndrome

Fragile X syndrome (FXS) is caused by a lack of the fragile X mental retardation protein (FMRP); FMRP deficiency in neurons of patients with FXS causes intellectual disability (IQ<70) and several behavioural problems, including hyperactivity and autistic-like features. In the brain, no gross morphological malformations have been found, although subtle spine abnormalities have been reported. FXS has been linked to altered group I metabotropic glutamate receptor (mGluR)-dependent and independent forms of synaptic plasticity. Here, we discuss potential targeted therapeutic strategies developed to specifically correct disturbances in the excitatory mGluR and the inhibitory gamma-aminobutyric (GABA) receptor pathways that have been tested in animal models and/or in clinical trials with patients with FXS.

Fragile X Syndrome and Alzheimer's Disease: Another story about APP and beta-amyloid

As the mechanisms underlying neuronal development and degeneration become clarified, a number of common effectors and signaling pathways are becoming apparent. Here we describe the identification of Abeta, long considered a pathologic mediator of Alzheimers Disease and Down Syndrome, as similarly over-expressed in the neurodevelopmental disease, Fragile X Syndrome. We also show that mGluR5 inhibitors, currently employed for the treatment of Fragile X, reduce Abeta production in rodent models of Fragile X and AD as well as reduce disease phenotypes including seizures. Thus seemingly disparate neurologic diseases may share a common pathologic instigator and be treatable with a common, currently available class of therapeutics.

Neurochemistry of the nucleus accumbens and its relevance to depression and antidepressant action in rodents

There is accumulating evidence that the nucleus accumbens (NAc) plays an important role in the pathophysiology of depression. Given that clinical depression is marked by anhedonia (diminished interest or pleasure), dysfunction of the brain reward pathway has been suggested as contributing to the pathophysiology of depression.Since the NAc is the center of reward and learning, it is hypothesized that anhedonia might be produced by hampering the function of the NAc. Indeed, it has been reported that stress, drug exposure and drug withdrawal, all of which produce a depressive-phenotype, alter various functions within the NAc, leading to inhibited dopaminergic activity in the NAc.In this review, we describe various factors as possible candidates within the NAc for the initiation of depressive symptoms. First, we discuss the roles of several neurotransmitters and neuropeptides in the functioning of the NAc, including dopamine, glutamate, gamma-aminobutyric acid (GABA), acetylcholine, serotonin, dynorphin, enkephaline, brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART). Second, based on previous studies, we propose hypothetical relationships among these substances and the shell and core subregions of the NAc.

Hit-to-lead optimization of disubstituted oxadiazoles and tetrazoles as mGluR5 NAMs

Here we report the discovery and early SAR of a series of mGluR5 negative allosteric modulators (NAMs). Starting from a moderately active HTS hit we synthesized 3,5-disubstituted-oxadiazoles and tetrazoles as mGluR5 NAMs. Based on the analysis of ligand efficiency and lipophilic efficiency metrics we identified a promising lead candidate as a starting point for further optimization.

Cognitive effects of Group I metabotropic glutamate receptor ligands in the context of drug addiction

Glutamate plays a pivotal role in regulating drug self-administration and drug-seeking behavior, and the past decade has witnessed a substantial surge of interest in the role of Group I metabotropic glutamate receptors (mGlu(1) and mGlu(5) receptors) in mediating these behaviors. As will be reviewed here, Group I mGlu receptors are involved in normal and drug-induced synaptic plasticity, drug reward, reinforcement and relapse-like behaviors, and addiction-related cognitive processes such as maladaptive learning and memory, behavioral inflexibility, and extinction learning. Animal models of addiction have revealed that antagonists of Group I mGlu receptors, particularly the mGlu(5) receptor, reduce self-administration of virtually all drugs of abuse. Since inhibitors of mGlu5 receptor function have now entered clinical trials for other medical conditions and appear to be well-tolerated, a key question that remains unanswered is - what changes in cognition are produced by these compounds that result in reduced drug intake and drug-seeking behavior? Finally, in contrast to mGlu(5) receptor antagonists, recent studies have indicated that positive allosteric modulation of mGlu(5) receptors actually enhances synaptic plasticity and improves various aspects of cognition, including spatial learning, behavioral flexibility, and extinction of drug-seeking behavior. Thus, while inhibition of Group I mGlu receptor function may reduce drug reward, reinforcement, and relapse-related behaviors, positive allosteric modulation of the mGlu5 receptor subtype may actually enhance cognition and potentially reverse some of the cognitive deficits associated with chronic drug use.

Effect of the metabotropic glutamate receptor type 5 antagonists MPEP and MTEP in parkinsonian monkeys

Brain glutamate overactivity is well documented in Parkinson's disease (PD) and antiglutamatergic drugs have been proposed to relieve PD symptoms and decrease dyskinesias. Metabotropic glutamate receptors are topics of recent interest in PD. This study investigated the effects of the metabotropic glutamate receptors type 5 (mGluR5) antagonists MPEP and MTEP on motor behavior in monkeys with a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion to model PD and treated with L-Dopa the gold standard therapy. Six Macaca fascicularis MPTP monkeys were initially treated repeatedly with L-Dopa; this treatment increased their locomotion and reduced their parkinsonian scores but also induced dyskinesias. Then, a dose-response of MPEP and MTEP (1.5-30 mg/kg) administered 15 and 30 min respectively prior to L-Dopa, showed that the antiparkinsonian activity of L-Dopa was generally maintained as measured with locomotion and antiparkinsonian scores as well as the onset and duration of the L-Dopa response. Interestingly the mean dyskinesia score during all the duration of the L-Dopa motor effect, the 1 h peak period dyskinesias scores as well as the maximal dyskinesias scores were dose-dependently reduced with both drugs reaching statistical significance at 10 and 30 mg/kg. Our results showed a beneficial antidyskinetic effect of blocking mGluR5 in L-Dopa-treated MPTP monkeys. This supports the therapeutic use of an mGluR5 antagonist to restore normal brain glutamate neurotransmission in PD and decrease dyskinesias.

Piperidyl amides as novel, potent and orally active mGlu5 receptor antagonists with anxiolytic-like activity

High throughput screening led to the identification of nicotinamide derivative 2 as a structurally novel mGluR5 antagonist. Optimization of the modular scaffold led to the discovery of 16m, a compound with high affinity for mGluR5 and excellent selectivity over other glutamate receptors. Compound 16m exhibits a favorable PK profile in rats, robust anxiolytic-like effects in three different animal models of fear and anxiety, as well as a good PK/PD correlation.

Metabotropic glutamate receptor ligands as potential therapeutics for addiction

There is now compelling evidence that the excitatory amino acid neurotransmitter glutamate plays a pivotal role in drug addiction and alcoholism. As a result, there has been increasing interest in developing glutamate-based therapies for the treatment of addictive disorders. Receptors for glutamate are primarily divided into two classes: ionotropic glutamate receptors (iGluRs) that mediate fast excitatory glutamate transmission, and metabotropic glutamate receptors (mGluRs), which are G-protein coupled receptors that mediate slower, modulatory glutamate transmission. Most iGluR antagonists, while showing some efficacy in animal models of addiction, exhibit serious side effects when tested in humans. mGluR ligands, on the other hand, which have been advanced to testing in clinical trials for various medical conditions, have demonstrated the ability to reduce drug reward, reinforcement, and relapse-like behaviors in animal studies. mGluR ligands that have been shown to be primarily effective are Group I (mGluR1 and mGluR5) negative allosteric modulators and Group II (mGluR2 and mGluR3) orthosteric presynaptic autoreceptor agonists. In this review, we will summarize findings from animal studies suggesting that these mGluR ligands may be of potential benefit in reducing on-going drug self-administration and may aid in the prevention of relapse. The neuroanatomical distribution of mGluR1, mGluR2/3, and mGluR5 receptors and the pharmacological properties of Group I negative allosteric modulators and Group II agonists will also be overviewed. Finally, we will discuss the current status of mGluR ligands in human clinical trials.

Effect of the metabotropic glutamate antagonist MPEP on striatal expression of the Homer family proteins in levodopa-treated hemiparkinsonian rats

RATIONALE

Striatal glutamatergic hyperactivity through the metabotropic receptors and their intracellular signaling pathways is considered critical in the development of levodopa-induced dyskinesias in Parkinson's disease and in experimental parkinsonism.

OBJECTIVE

We investigated whether the administration of the metabotropic glutamate antagonist, MPEP, modifies striatal expression of Homer family proteins which are involved in the intracellular mechanisms mediated by these receptors.

MATERIALS AND METHODS

Sprague-Dawley rats were unilaterally lesioned in the nigrostriatal pathway with 6-hydroxydopamine (8 microg) and treated with: levodopa (12 mg/kg, i.p.) plus vehicle (n=10) divided in two daily injections; levodopa plus MPEP (1.5 and 3 mg/kg, i.p.; n=6-13) divided in two daily injections; or saline (n=7) for 10 consecutive days. Axial, limb, and orolingual dyskinesias were evaluated. Striatal expression of tyrosine hydroxylase (TH), Homer 1a, 1b/c, and deltaFosB were measured by Western Blot.

RESULTS

Animals treated with levodopa showed an increase of dyskinesia score (p<0.01) that was attenuated by the administration of MPEP (p<0.01). In the ipsilateral side of the lesion, striatal TH expression was decreased (p<0.01). No significant differences in striatal Homer 1a or b/c expression were observed between the groups of treatment. Striatal deltaFosB expression increased in the animals treated with levodopa (p<0.05) being attenuated after MPEP administration (p<0.05). MPEP effect was not paralleled by any modification of striatal Homer proteins expression.

CONCLUSIONS

These results suggest that Homer protein family is not causally involved in the development of dyskinetic movements induced by levodopa treatment in this animal model of parkinsonism.

Allosteric modulation of metabotropic glutamate receptor 5 affects phosphorylation, internalization, and desensitization of the micro-opioid receptor

Recent evidence suggests that opioid analgesia and tolerance can be modulated by metabotropic glutamate receptors. Therefore, we studied the functional coupling and desensitization of the micro-opioid receptor (MOR) in human embryonic kidney (HEK) 293 cells which co-express metabotropic glutamate receptor 5 (mGluR5). As demonstrated by the D-Ala2,N-MePhe4,Gl-ol5-enkephalin (DAMGO)-induced inhibition of intracellular cAMP level and by binding studies, the co-expression of mGluR5 had no substantial effect on the agonist binding sites and functional coupling of the MOR. However, in MOR/ mGluR5 co-expressing cells, the non-competitive mGluR5 antagonist MPEP (2-methyl-6-(phenyl-ethynyl)-pyridine) decreases the DAMGO-induced MOR phosphorylation, internalization, and desensitization, whereas non-selective competitive mGluR antagonists or agonists had no effects. These findings indicate that an allosteric modulation of mGluR5 can affect the agonist-induced MOR signalling and regulation. As a mechanistic basis for the observed effects we suggested an interaction/heterodimerization of MOR and mGluR5, which is supported by the DAMGO-induced co-internalization of MOR and mGluR5 and by the increase of MPEP binding sites (Bmax) and a change of the binding affinity (K(D)) of mGluR5 receptors after the co-expression of MOR. In addition, co-immunoprecipitation experiments revealed evidence for an interaction between MOR and mGluR5 which is facilitated by MPEP treatment.

Discovery of a potent and brain penetrant mGluR5 positive allosteric modulator

This Letter describes the discovery of a novel series of mGluR5 positive allosteric modulators (PAMs). The lead compound, 11c, exhibits excellent potency (EC(50)=30 nM) in vitro, and reaches high brain levels in both rats and mice after oral administration.

The fragile X mental retardation protein in circadian rhythmicity and memory consolidation

The control of new protein synthesis provides a means to locally regulate the availability of synaptic components necessary for dynamic neuronal processes. The fragile X mental retardation protein (FMRP), an RNA-binding translational regulator, is a key player mediating appropriate synaptic protein synthesis in response to neuronal activity levels. Loss of FMRP causes fragile X syndrome (FraX), the most commonly inherited form of mental retardation and autism spectrum disorders. FraX-associated translational dysregulation causes wide-ranging neurological deficits including severe impairments of biological rhythms, learning processes, and memory consolidation. Dysfunction in cytoskeletal regulation and synaptic scaffolding disrupts neuronal architecture and functional synaptic connectivity. The understanding of this devastating disease and the implementation of meaningful treatment strategies require a thorough exploration of the temporal and spatial requirements for FMRP in establishing and maintaining neural circuit function.

Antagonists at metabotropic glutamate receptor subtype 5: structure activity relationships and therapeutic potential for addiction

As a result of intensive investigation, particularly in the pharmaceutical industry, a number of potent and selective metabotropic glutamate receptor subtype 5 (mGluR5) antagonists have been discovered. The structure activity relationship studies that led to the discovery of these mGluR5 antagonists are presented in this review. Results from studies on selected mGluR5 antagonists in animal models that simulate drug reward, reinforcement, and relapse appear promising. The comorbidity between drug abuse and anxiety and depression make drugs active in these disorders of great interest. Clinical studies showed that the mGluR5 antagonist fenobam was an active anxiolytic drug. Several new mGluR5 antagonists produced anxiolytic and antidepressant-like effects in animal models of these disorders. The results from the clinical and animal studies provide information for new approaches to finding mechanistically distinct pharmacotherapies to help patients achieve and maintain abstinence from cocaine, methamphetamine, opiates, ethanol, and nicotine (smoking).

In vivo metabotropic glutamate receptor 5 (mGluR5) antagonism prevents cocaine-induced disruption of postsynaptically maintained mGluR5-dependent long-term depression

Metabotropic glutamate receptor 5 (mGluR5) plays a critical role in psychostimulant-induced behavior, yet it is unclear whether mGluR5 is activated by psychostimulant administration, or whether its role is constitutive. We previously reported that activation of mGluR5 with the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) can induce a long-term depression (DHPG-LTD) of glutamatergic transmission in the bed nucleus of the stria terminalis (BNST), and that ex vivo induction of this LTD is disrupted by repeated in vivo administration of cocaine. Here we demonstrate that DHPG-LTD is not maintained by alterations in glutamate release, and that postsynaptic endocytosis is necessary. Furthermore, we find that a single administration of cocaine produces a transient disruption of DHPG-LTD, and the duration of this disruption was increased by repeated days of cocaine administration. The disruption produced by cocaine was not permanent, because DHPG-LTD could be induced 10 d after cocaine administration. To test the role of mGluR5 in vivo in the cocaine-induced disruption of DHPG-LTD, we injected mice with the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine before cocaine. mGluR5 antagonism during in vivo cocaine administration rescued subsequent ex vivo induction of DHPG-LTD. The effects of in vivo cocaine could be mimicked by application of cocaine to BNST-containing slices, suggesting that the actions of cocaine are local. Thus, using a novel strategy of in vivo antagonist-induced rescue of ex vivo agonist effects for the same receptor, we provide evidence suggesting that mGluR5 activation is actively recruited by in vivo cocaine.

Attenuation of cocaine self-administration in squirrel monkeys following repeated administration of the mGluR5 antagonist MPEP: comparison with dizocilpine

RATIONALE

The mGluR5 antagonist MPEP has effects that suggest potential as a pharmacotherapy for cocaine addiction. MPEP can attenuate self-administration of cocaine in animals; however, studies usually involved only acute treatment with MPEP and a single dose of self-administered cocaine. Cocaine addicts use varied amounts of cocaine over long periods of time, and an effective pharmacotherapy would almost certainly require more chronic treatment.

OBJECTIVES

The present study (1) compared the effects of repeated treatment with MPEP or the NMDA receptor antagonist dizocilpine on the reinforcing effects of a range of doses of cocaine and (2) determined the pharmacological specificity of the effects of the drugs in attenuating cocaine self-administration compared to food-reinforced behavior. An effective pharmacotherapy should selectively reduce cocaine self-administration.

MATERIALS AND METHODS

Groups of monkeys responded under a fixed-ratio schedule of i.v. cocaine self-administration or food-pellet delivery. The effects of daily treatment with MPEP and dizocilpine were determined under both the schedule of i.v. cocaine injection and food delivery.

RESULTS

Treatment with MPEP and dizocilpine significantly reduced cocaine self-administration, producing rightward and downward shifts in the ascending limb of the cocaine dose-response function. MPEP and dizocilpine selectively and significantly attenuated self-administration of a low reinforcing dose of cocaine compared to food without evidence of tolerance.

CONCLUSIONS

Both MPEP and dizocilpine functioned as partially surmountable antagonists of the reinforcing effects of cocaine. The similar effects of the two drugs raises the possibility that MPEP attenuated the reinforcing effects of cocaine, at least in part, via mGluR5-mediated inhibition of NMDA receptor activity.

Role of metabotropic glutamate receptor 5 in the procholinergic effects of neuropsychotherapeutic compounds

We investigated the participation of the metabotropic glutamate receptor type 5 (mGluR5) in mediating increases in cortical acetylcholine (ACh) efflux elicited by established or putative neuropsychotherapeutic compounds, using in vivo microdialysis in rats. The norepinephrine transporter inhibitor atomoxetine, the cannabinoid CB1 receptor antagonist SR141716A, the dopamine D1 receptor agonist dihydrexidine, and the atypical antipsychotic clozapine increased cortical ACh (by about 2-3 fold), whereas the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) by itself had no effect. The stimulatory effects of atomoxetine, SR141716A and dihydrexidine on cortical ACh were abolished by pretreatment with MPEP. MPEP also attenuated the stimulatory effect of clozapine on ACh efflux. Thus, mGluR5 activation appears to be involved in the procholinergic effects of compounds that exhibit therapeutic properties or potential in neuropsychiatry.

Calmodulin dynamically regulates the trafficking of the metabotropic glutamate receptor mGluR5

Metabotropic glutamate receptors (mGluRs) 1-8 are G protein-coupled receptors (GPCRs) that modulate excitatory neurotransmission, neurotransmitter release, and synaptic plasticity. PKC regulates many aspects of mGluR function, including protein-protein interactions, Ca(2+) signaling, and receptor desensitization. However, the mechanisms by which PKC regulates mGluR function are poorly understood. We have now identified calmodulin (CaM) as a dynamic regulator of mGluR5 trafficking. We show that the major PKC phosphorylation site on the intracellular C terminus of mGluR5 is serine 901 (S901), and phosphorylation of this residue is up-regulated in response to both receptor and PKC activation. In addition, S901 phosphorylation inhibits mGluR5 binding to CaM, decreasing mGluR5 surface expression. Furthermore, blocking PKC phosphorylation of mGluR5 on S901 dramatically affects mGluR5 signaling by prolonging Ca(2+) oscillations. Thus, our data demonstrate that mGluR5 activation triggers phosphorylation of S901, thereby directly linking PKC phosphorylation, CaM binding, receptor trafficking, and downstream signaling.

Metabotropic glutamate receptor modulation, translational methods, and biomarkers: relationships with anxiety

RATIONALE

The increasing awareness of the need to align clinical and preclinical research to facilitate rapid development of new drug therapies is reflected in the recent introduction of the term "translational medicine". This review examines the implications of translational medicine for psychiatric disorders, focusing on metabotropic glutamate (mGlu) receptor biology in anxiety disorders and on anxiety-related biomarkers.

OBJECTIVES

This review aims to (1) examine recent progress in translational medicine, emphasizing the role that translational research has played in understanding of the potential of mGlu receptor agonists and antagonists as anxiolytics, (2) identify lacunas where animal and human research have yet to be connected, and (3) suggest areas where translational research can be further developed.

RESULTS

Current data show that animal and human mGlu(5) binding can be directly compared in experiments using the PET ligand (11)C-ABP688. Testing of the mGlu(2/3) receptor agonist LY354740 in the fear-potentiated startle paradigm allows direct functional comparisons between animals and humans. LY354740 has been tested in panic models, but in different models in rats and humans, hindering efforts at translation. Other potentially translatable methods, such as stress-induced hyperthermia and HPA-axis measures, either have been underexploited or are associated with technical difficulties. New techniques such as quantitative trait loci (QTL) analysis may be useful for generating novel biomarkers of anxiety.

CONCLUSIONS

Translational medicine approaches can be valuable to the development of anxiolytics, but the amount of cross-fertilization between clinical and pre-clinical departments will need to be expanded to realize the full potential of these approaches.

Challenges for translational psychopharmacology research--some basic principles

We introduce below several principles that recur in the discussion of translating preclinical findings to clinical applications, and conversely, developing animal models of human disorders: 1. The translation of preclinical data to clinical concerns is more successful when the scope of experimental models is restricted to a core symptom of a psychiatric disorder. 2. Preclinical experimental models gain in clinical relevance if they incorporate conditions that induce maladaptive behavioral or physiological changes that have some correspondence with species-normative behavioral adaptations. 3. Preclinical data are more readily translated to the clinical situation when they are based on converging evidence from several experimental procedures, each capturing cardinal features of the disorder. 4. The more closely a model approximates significant clinical symptoms, the more likely it is to generate data that will yield clinical benefits. 5. The choice of environmental, genetic, and/or physiological manipulations that induce a cardinal symptom or cluster of behavioral symptoms reveals the theoretical approach used to construct the model. 6. Preclinical experimental preparations that are validated by predicting treatment success with a prototypic agent are only able to detect alternative treatments that are based on the same mechanism as the existing treatment that was used to validate the screen. 7. The degree to which an experimental model fulfills the criteria of high construct validity relative to face or predictive validity depends on the purpose of the model. 8. Psychological processes pertinent to affect and cognition can only be studied in preclinical models if they are defined in behavioral and neural terms.

Nonselective suppression of operant ethanol and sucrose self-administration by the mGluR7 positive allosteric modulator AMN082

Emerging evidence indicates that specific metabotropic glutamate receptors (mGluRs) modulate ethanol self-administration. In general, inhibition of glutamate transmission through blockade of postsynaptic mGluRs, or activation of presynaptic mGluRs, inhibits ethanol self-administration. The goal of this preclinical study was to further characterize mGluR regulation of ethanol self-administration by examining effects of AMN082, an allosteric positive modulator of presynaptic mGluR7 activity. Separate groups of C57BL/6J male mice were trained to self-administer ethanol or sucrose on a fixed-ratio 4 schedule of reinforcement during 1 h sessions. On test days, mice were pretreated with AMN082 (0, 1.0, 3.0, 5.6, or 10 mg/kg) 30 min prior to self-administration sessions. Functional specificity and activity was examined by testing the effects of AMN082 (0-10 mg/kg) on open-field locomotor activity and HPA axis function as measured by plasma corticosterone levels. AMN082 (10 mg/kg) produced a significant reduction in ethanol and sucrose reinforced responding, and inhibited locomotor activity. Plasma corticosterone levels were significantly increased following AMN082 (5.6 and 10 mg/kg) suggesting a dose-dependent dissociation between the behavioral and hormonal effects of the compound. These data suggest that activation of mGluR7 by AMNO82 produces nonspecific reductions in motivated behavior that are associated with negative effects on motor activity.