Activation of group III metabotropic glutamate receptors inhibits basal and amphetamine-stimulated dopamine release in rat dorsal striatum: an in vivo microdialysis study

Novel pharmacological targets for GABAergic dysfunction in ADHD

Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopment disorder that affects approximately 5% of the population. The disorder is characterized by impulsivity, hyperactivity, and deficits in attention and cognition, although symptoms vary across patients due to the heterogenous and polygenic nature of the disorder. Stimulant medications are the standard of care treatment for ADHD patients, and their effectiveness has led to the dopaminergic hypothesis of ADHD in which deficits in dopaminergic signaling, especially in cortical brain regions, mechanistically underly ADHD pathophysiology. Despite their effectiveness in many individuals, almost one-third of patients do not respond to stimulant treatments and the long-term negative side effects of these medications remain unclear. Emerging clinical evidence is beginning to highlight an important role of dysregulated excitatory/inhibitory (E/I) balance in ADHD. These deficits in E/I balance are related to functional abnormalities in glutamate and Gamma-Aminobutyric Acid (GABA) signaling in the brain, with increasing emphasis placed on GABAergic interneurons driving specific aspects of ADHD pathophysiology. Recent genome-wide association studies (GWAS) have also highlighted how genes associated with GABA function are mutated in human populations with ADHD, resulting in the generation of several new genetic mouse models of ADHD. This review will discuss how GABAergic dysfunction underlies ADHD pathophysiology, and how specific receptors/proteins related to GABAergic interneuron dysfunction may be pharmacologically targeted to treat ADHD in subpopulations with specific comorbidities and symptom domains. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

The role of metabotropic glutamate receptors in neurobehavioral effects associated with methamphetamine use

Metabotropic glutamate (mGlu) receptors are expressed throughout the central nervous system and act as important regulators of drug-induced neuroplasticity and behavior. Preclinical research suggests that mGlu receptors play a critical role in a spectrum of neural and behavioral consequences arising from methamphetamine (meth) exposure. However, an overview of mGlu-dependent mechanisms linked to neurochemical, synaptic, and behavioral changes produced by meth has been lacking. This chapter provides a comprehensive review of the role of mGlu receptor subtypes (mGlu1-8) in meth-induced neural effects, such as neurotoxicity, as well as meth-associated behaviors, such as psychomotor activation, reward, reinforcement, and meth-seeking. Additionally, evidence linking altered mGlu receptor function to post-meth learning and cognitive deficits is critically evaluated. The chapter also considers the role of receptor-receptor interactions involving mGlu receptors and other neurotransmitter receptors in meth-induced neural and behavioral changes. Taken together, the literature indicates that mGlu5 regulates the neurotoxic effects of meth by attenuating hyperthermia and possibly through altering meth-induced phosphorylation of the dopamine transporter. A cohesive body of work also shows that mGlu5 antagonism (and mGlu2/3 agonism) reduce meth-seeking, though some mGlu5-blocking drugs also attenuate food-seeking. Further, evidence suggests that mGlu5 plays an important role in extinction of meth-seeking behavior. In the context of a history of meth intake, mGlu5 also co-regulates aspects of episodic memory, with mGlu5 stimulation restoring impaired memory. Based on these findings, we propose several avenues for the development of novel pharmacotherapies for Methamphetamine Use Disorder based on the selective modulation mGlu receptor subtype activity.

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.

AMN082, a metabotropic glutamate receptor 7 allosteric agonist, attenuates locomotor sensitization and cross-sensitization induced by cocaine and morphine in mice

Previous studies have indicated that metabotropic glutamate receptors 7 (mGluR7s) are involved in drug addiction. However, the role of these receptors in drug-induced behavioral sensitization is unknown. The aim of the present study was to determine whether systemic injection of AMN082, a selective mGluR7 allosteric agonist, reduces the cocaine- and morphine-induced hyperactivity and the development and expression of locomotor sensitization, and also affects the reciprocal cross-sensitization to the stimulant effect of cocaine and morphine in mice. AMN082 (1.25-10.0 mg/kg, i.p.) did not have an impact on locomotion of naive mice and did not affect the acute cocaine- or morphine-induced hyperactivity, except the dose of 10 mg/kg that suppressed the locomotor effect of both drugs. Repeated exposure to cocaine or morphine (10 mg/kg, 5× every 3 days) gradually increased locomotion during induction of sensitization and after 4 (cocaine) or 7 day (morphine) withdrawal phase when challenged with cocaine (10 mg/kg, i.p.) or morphine (10 mg/kg, i.p.) on day 17 or 20, respectively. Pretreatment of animals with the lower doses of AMN082 (1.25-5.0 mg/kg, i.p.), 30 min before every cocaine or morphine injection during repeated drug administration or before cocaine or morphine challenge, dose-dependently attenuated the development, as well as the expression of cocaine or morphine locomotor sensitization. AMN082 also inhibited the reciprocal cross-sensitization between these drugs. Prior to administration of MMPIP (10 mg/kg, i.p.), a selective mGluR7 antagonist reversed the inhibitory effect of AMN082 on the development or expression of cocaine or morphine sensitization. These data indicate that AMN082 attenuated the development and expression of cocaine and morphine sensitization, and the reciprocal cross-sensitization via a mechanism that involves mGluR7s. Thus, AMN082 might have therapeutic implications not only in the treatment of cocaine or opioid addiction but also in the treatment of cocaine/opioid polydrug-abusers.

Group III metabotropic glutamate receptors and drug addiction

Neuroadaptations of glutamatergic transmission in the limbic reward circuitry are linked to persistent drug addiction. Accumulating data have demonstrated roles of ionotropic glutamate receptors and group I and II metabotropic glutamate receptors (mGluRs) in this event. Emerging evidence also identifies Gαi/o-coupled group III mGluRs (mGluR4/7/8 subtypes enriched in the limbic system) as direct substrates of drugs of abuse and active regulators of drug action. Auto- and heteroreceptors of mGluR4/7/8 reside predominantly on nerve terminals of glutamatergic corticostriatal and GABAergic striatopallidal pathways, respectively. These presynaptic receptors regulate basal and/or phasic release of respective transmitters to maintain basal ganglia homeostasis. In response to operant administration of common addictive drugs, such as psychostimulants (cocaine and amphetamine), alcohol and opiates, limbic group III mGluRs undergo drastic adaptations to contribute to the enduring remodeling of excitatory synapses and to usually suppress drug seeking behavior. As a result, a loss-of-function mutation (knockout) of individual group III receptor subtypes often promotes drug seeking. This review summarizes the data from recent studies on three group III receptor subtypes (mGluR4/7/8) expressed in the basal ganglia and analyzes their roles in the regulation of dopamine and glutamate signaling in the striatum and their participation in the addictive properties of three major classes of drugs (psychostimulants, alcohol, and opiates).

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Inhibitory actions of mGlu4 receptor ligands on cocaine-, but not nicotine-, induced sensitizing and conditioning locomotor responses in rats

BACKGROUND

Male Wistar rats were used to verify the hypothesis that metabotropic glutamate 4 (mGlu4) receptor ligands may modulate the locomotor effects evoked by cocaine or nicotine.

METHODS

The preferential mGlu4 receptor orthosteric agonist (2S)-2-amino-4-[hydroxy[hydroxy(4-hydroxy-3-methoxy-5-nitrophenyl)methyl]phosphoryl]butanoic acid (LSP1-2111) and the mGlu4 receptor positive allosteric modulator (+)-cis-N(1)-(3,4-dichlorophenyl)cyclohexane-1,2-dicarboxamide (Lu AF21934) were used in the study. Rats were given repeated pairings of a test environment with cocaine (10mg/kg), nicotine (0.4 mg/kg) or the respective vehicles for 5 days. On day 10, animals were challenged with cocaine (10mg/kg, cocaine sensitization), nicotine (0.4 mg/kg, nicotine sensitization) or vehicle (conditioned hyperlocomotion) in experimental cages.

RESULTS

Given on day 10, LSP1-2111 (3mg/kg) as well as Lu AF21934 (2.5-5mg/kg) decreased the expression of cocaine sensitization. In another set of experiments, LSP1-2111 (3mg/kg) and Lu AF21934 (5mg/kg) administered on day 10 attenuated the conditioned hyperlocomotion in rats treated repeatedly with cocaine. Neither LSP1-2111 (1-3mg/kg) nor Lu AF21934 (2.5-5mg/kg) changed the expression of nicotine sensitization and conditioned hyperlocomotion in rats treated repeatedly with nicotine. None of the mGlu4 receptor agonist/modulator altered the basal locomotor activity or acute hyperactivity to cocaine or nicotine.

CONCLUSIONS

The present data indicate that pharmacological stimulation of mGlu4 receptors reduces the cocaine-induced expression of sensitization as well as conditioned hyperactivity. In contrast, mGlu4 receptor activation seems to be devoid of any effect on the locomotor effects of nicotine.

Metabotropic glutamate 7 (mGlu7) receptor: a target for medication development for the treatment of cocaine dependence

Brain glutamate has been shown to play an important role in reinstatement to drug seeking, a behavior considered to be of relevance to relapse to drug taking in humans. Therefore, glutamate receptors, in particular metabotropic glutamate (mGlu) receptors, have become important targets for medication development for the treatment of drug dependence. In this review article, we focus on the mGlu7 receptor subtype, and discuss recent findings with AMN082, a selective mGlu7 receptor allosteric agonist, in animal models with relevance to drug dependence. Systemic or local administration of AMN082 into the nucleus accumbens (NAc), a critical brain region involved in reward and drug dependence processes, inhibited the reinforcing and motivational effects of cocaine, heroin and ethanol, as assessed by the intravenous drug self-administration procedure. In addition, AMN082 inhibited the reward-enhancing effects induced by cocaine, as assessed in the intracranial self-stimulation procedure, and cocaine- or cue-induced reinstatement of drug-seeking behavior. In vivo microdialysis studies indicated that systemic or intra-NAc administration of AMN082 significantly decreased extracellular γ-aminobutyric acid (GABA) and elevated extracellular glutamate, but had no effect on extracellular dopamine in the NAc, suggesting that a non-dopaminergic mechanism underlies the effects of AMN082 on the actions of cocaine. Further, data indicated that AMN082-induced changes in glutamate were the net effect of two actions: one is the direct inhibition of glutamate release by activation of mGlu7 receptors on glutamatergic neurons; another is the indirect increases of glutamate release mediated by decreases in GABA transmission. These increases in extracellular glutamate functionally antagonized cocaine-induced inhibition of NAc-ventral pallidum GABAergic neurotransmission, and therefore, the rewarding effects of cocaine. In addition, elevated extracellular glutamate activated presynaptic mGlu2/3 autoreceptors which in turn inhibited cocaine priming- or cue-induced enhancement of glutamate release and reinstatement of drug-seeking behavior. Taken together, these findings suggest that the mGlu7 receptor is an important target for medication development for the treatment of drug dependence. AMN082 or other mGlu7 receptor allosteric agonists may have potential as novel pharmacotherapies for cocaine addiction. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Metabotropic glutamate 7 receptor subtype modulates motor symptoms in rodent models of Parkinson's disease

Metabotropic glutamate (mGlu) receptors modulate synaptic transmission in the central nervous system and represent promising therapeutic targets for symptomatic treatment of Parkinson's disease (PD). Among the eight mGlu receptor subtypes, mGlu7 receptor is prominently expressed in the basal ganglia, but its role in restoring motor function in animal models of PD is not known. The effects of N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), the first selective allosteric activator of mGlu7 receptors, were thus tested in different rodent models of PD. Here, we show that oral (5 mg/kg) or intrastriatal administration (0.1 and 0.5 nmol) of AMN082 reverses haloperidol-induced catalepsy in rats. AMN082 (2.5 and 5 mg/kg) reduces apomorphine-induced rotations in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. In a more complex task commonly used to evaluate major akinetic symptoms of PD patients, 5 mg/kg AMN082 reverses the increased reaction time to respond to a cue of bilateral 6-OHDA-lesioned rats. In addition, AMN082 reduces the duration of haloperidol-induced catalepsy in a mGlu7 receptor-dependent manner in wild-type but not mGlu7 receptor knockout mice. Higher doses of AMN082 (10 and 20 mg/kg p.o.) have no effect on the same models of PD. Overall these findings suggest that mGlu7 receptor activation can reverse motor dysfunction associated with reduced dopamine activity. Selective ligands of mGlu7 receptor subtypes may thus be considered as promising compounds for the development of antiparkinsonian therapeutic strategies.

Hypothesis-driven medication discovery for the treatment of psychostimulant addiction

Psychostimulant abuse is a serious social and health problem, for which no effective treatments currently exist. A number of review articles have described predominantly 'clinic'-based pharmacotherapies for the treatment of psychostimulant addiction, but none have yet been shown to be definitively effective for use in humans. In the present article, we review various 'hypothesis'- or 'mechanism'-based pharmacological agents that have been studied at the preclinical level and evaluate their potential use in the treatment of psychostimulant addiction in humans. These compounds target brain neurotransmitter or neuromodulator systems, including dopamine (DA), gamma-aminobutyric acid (GABA), endocannabinoid, glutamate, opioid and serotonin, which have been shown to be critically involved in drug reward and addiction. For drugs in each category, we first briefly review the role of each neurotransmitter system in psychostimulant actions, and then discuss the mechanistic rationale for each drug's potential anti-addiction efficacy, major findings with each drug in animal models of psychostimulant addiction, abuse liability and potential problems, and future research directions. We conclude that hypothesis-based medication development strategies could significantly promote medication discovery for the effective treatment of psychostimulant addiction.

Metabotropic glutamate receptor 7 modulates the rewarding effects of cocaine in rats: involvement of a ventral pallidal GABAergic mechanism

The metabotropic glutamate receptor 7 (mGluR7) has received much attention as a potential target for the treatment of epilepsy, major depression, and anxiety. In this study, we investigated the possible involvement of mGluR7 in cocaine reward in animal models of drug addiction. Pretreatment with the selective mGluR7 allosteric agonist N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082; 1-20 mg/kg, i.p.) dose-dependently inhibited cocaine-induced enhancement of electrical brain-stimulation reward and intravenous cocaine self-administration under both fixed-ratio and progressive-ratio reinforcement conditions, but failed to alter either basal or cocaine-enhanced locomotion or oral sucrose self-administration, suggesting a specific inhibition of cocaine reward. Microinjections of AMN082 (1-5 microg/microl per side) into the nucleus accumbens (NAc) or ventral pallidum (VP), but not dorsal striatum, also inhibited cocaine self-administration in a dose-dependent manner. Intra-NAc or intra-VP co-administration of 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP, 5 microg/microl per side), a selective mGluR7 allosteric antagonist, significantly blocked AMN082's action, suggesting an effect mediated by mGluR7 in these brain regions. In vivo microdialysis demonstrated that cocaine (10 mg/kg, i.p.) priming significantly elevated extracellular DA in the NAc or VP, while decreasing extracellular GABA in VP (but not in NAc). AMN082 pretreatment selectively blocked cocaine-induced changes in extracellular GABA, but not in DA, in both naive rats and cocaine self-administration rats. These data suggest: (1) mGluR7 is critically involved in cocaine's acute reinforcement; (2) GABA-, but not DA-, dependent mechanisms in the ventral striatopallidal pathway appear to underlie AMN082's actions; and (3) AMN082 or other mGluR7-selective agonists may be useful in the treatment of cocaine addiction.

Acute administration of cocaine reduces metabotropic glutamate receptor 8 protein expression in the rat striatum in vivo

Metabotropic glutamate receptors (mGluRs) are densely expressed in the limbic system of the mammalian brain. Increasing evidence suggests a critical role of mGluRs in the pathogenesis of various mental illnesses, including drug abuse and addiction. In this study, we investigated the effect of psychostimulant, cocaine, on protein expression of a specific mGluR subtype, mGluR8, in the rat forebrain in vivo. A rabbit antibody against the extracellular N-terminus of mGluR8 was developed to detect changes in mGluR8 proteins in immunoblot assays. With this antibody, we found that acute systemic injection of cocaine reduced mGluR8 protein levels in the striatum. The reduction of mGluR8 proteins was rapid and transient as it was induced 25min after cocaine injection and returned to the normal level by 6h. No significant change in mGluR8 protein levels in the prefrontal cortex and the hippocampus was observed following cocaine administration. These data demonstrate that protein expression of mGluR8 is subject to the modulation by dopamine stimulation. Acute exposure to cocaine results in a dynamic and region-specific downregulation of mGluR8 expression in the striatum.

Modulation by group I mGLU receptor activation and group III mGLU receptor blockade of locomotor responses induced by D1-like and D2-like receptor agonists in the nucleus accumbens

Evidence for functional motor interactions between group I and group III metabotropic glutamatergic (mGlu) receptors and dopamine neurotransmission is now clearly established [David, H.N., Abraini, J.H., 2001a. The group I metabotropic glutamate receptor antagonist S-4-CPG modulates the locomotor response produced by the activation of D1-like, but not D2-like, dopamine receptors in the rat nucleus accumbens. Eur. J. Neurosci. 15, 2157-2164, David, H.N., Abraini, J.H., 2002. Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity. Eur. J. Neurosci. 15, 869-875]. Nevertheless, whether or not and how, activation of group I and blockade of group III mGlu receptors modulate the motor responses induced by the activation of dopaminergic receptors in the NAcc still remains unknown. Answering this question needs to be assessed since functional interactions between neurotransmitters in the NAcc are well known to depend upon the level of activation of glutamatergic and/or dopaminergic receptors and because the effects of glutamatergic receptor agonists and antagonists on dopaminergic receptor-mediated locomotor responses are not always reciprocal as shown in previous studies. Our results show that activation of group I mGlu receptors by DHPG in the NAcc potentiated the locomotor response induced by intra-NAcc activation of D1-like receptors and blocked those induced by D2-like presynaptic or postsynaptic receptors. Alternatively, blockade of group III mGlu receptors by MPPG in the NAcc potentiated the locomotor responses mediated by D1-like receptors and by D2-like postsynaptic receptors and inhibited that induced by D2-like presynaptic receptors. These results compiled with previous data demonstrate that group I mGlu receptors and group III mGlu receptors can modulate the locomotor responses produced by D1-like and/or D2-like receptor agonists in a complex phasic and tonic fashion.

Upregulation of metabotropic glutamate receptor 8 mRNA expression in the rat forebrain after repeated amphetamine administration

Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors and are densely expressed in the forebrain of adult rats. Accumulative evidence suggests a critical role of mGluRs in the regulation of normal physiological activity of neurons and pathogenesis of mental illnesses such as schizophrenia, depression, and substance addiction. In this study, we investigated alterations in mGluR8 subtype mRNA expression in the rat forebrain in response to repeated intraperitoneal administration of amphetamine (twice daily for 12 days, 5mg/kg per injection) using quantitative in situ hybridization. We found that mGluR8 mRNA levels were profoundly increased in the dorsal (caudate putamen) and ventral (nucleus accumbens) striatum 1 day after the discontinuation of amphetamine treatments. Such increases were sustained up to 21 days of withdrawal. Increases in mGluR8 mRNAs were also found in the cerebral cortex, including the cingulate and sensory cortex but not the piriform cortex, at 1 and 21 days. These data demonstrate a positive response of mGluR8 in mRNA abundance in most forebrain regions to repeated stimulant exposure.

The metabotropic glutamate receptor 7 (mGluR7) allosteric agonist AMN082 modulates nucleus accumbens GABA and glutamate, but not dopamine, in rats

The group III metabotropic glutamate receptor 7 (mGluR7) has been implicated in many neurological and psychiatric diseases, including drug addiction. However, it is unclear whether and how mGluR7 modulates nucleus accumbens (NAc) dopamine (DA), L-glutamate or gamma-aminobutyric acid (GABA), important neurotransmitters believed to be involved in such neuropsychiatric diseases. In the present study, we found that systemic or intra-NAc administration of the mGluR7 allosteric agonist N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) dose-dependently lowered NAc extracellular GABA and increased extracellular glutamate, but had no effect on extracellular DA levels. Such effects were blocked by (R,S)-alpha-methylserine-O-phosphate (MSOP), a group III mGluR antagonist. Intra-NAc perfusion of tetrodotoxin (TTX) blocked the AMN082-induced increases in glutamate, but failed to block the AMN082-induced reduction in GABA, suggesting vesicular glutamate and non-vesicular GABA origins for these effects. In addition, blockade of NAc GABAB receptors by 2-hydroxy-saclofen itself elevated NAc extracellular glutamate. Intra-NAc perfusion of 2-hydroxy-saclofen not only abolished the enhanced extracellular glutamate normally produced by AMN082, but also decreased extracellular glutamate in a TTX-resistant manner. We interpret these findings to suggest that the increase in glutamate is secondary to the decrease in GABA, which overcomes mGluR7 activation-induced inhibition of non-vesicular glutamate release. In contrast to its modulatory effect on GABA and glutamate, the mGluR7 receptor does not appear to modulate NAc DA release.

Glutamate receptor metabotropic 7 is cis-regulated in the mouse brain and modulates alcohol drinking

Alcoholism is a heritable disease that afflicts about 8% of the adult population. Its development and symptoms, such as craving, loss of control, physical dependence, and tolerance, have been linked to changes in mesolimbic, mesocortical neurotransmitter systems utilizing biogenic amines, GABA, and glutamate. Identification of genes predisposing to alcoholism, or to alcohol-related behaviors in animal models, has been elusive because of variable interactions of multiple genes with relatively small individual effect size and sensitivity of the predisposing genotype to lifestyle and environmental factors. Here, using near-isogenic advanced animal models with reduced genetic background interactions, we integrate gene mapping and gene mRNA expression data in segregating and congenic mice and identify glutamate receptor metabotropic 7 (Grm7) as a cis-regulated gene for alcohol consumption. Traditionally, the mesoaccumbal dopamine reward hypothesis of addiction and the role of the ionotropic glutamate receptors have been emphasized. Our results lend support to an emerging direction of research on the role of metabotropic glutamate receptors in alcoholism and drug addiction. These data suggest for the first time that Grm7 is a risk factor for alcohol drinking and a new target in addiction therapy.

Modulation of amphetamine-induced dopamine release by group II metabotropic glutamate receptor agonist LY354740 in non-human primates studied with positron emission tomography

Pharmacological evidence suggests that schizophrenia is associated with increased stimulation of dopamine (DA) D2 receptors. Recently, several groups have demonstrated that amphetamine-induced DA release is increased in schizophrenia, providing direct evidence for dysregulation of DA systems in this condition. In healthy volunteers, pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine increases amphetamine-induced DA release to levels similar to those observed in patients with schizophrenia. Therefore, the dysregulation of DA function observed in schizophrenia might be secondary to NMDA hypofunction. In this study, the regulation of this response by glutamate (GLU) transmission was further characterized by using a metabotropic glutamate (mGlu) receptor group II agonist to inhibit GLU transmission. The amphetamine- (0.5 mg/kg intravenously (i.v.)) induced decrease in [11C]raclopride equilibrium-specific binding (V3'') was measured under control conditions and following pretreatment with the mGlu2/3 receptor agonist LY354740 (20 mg/kg i.v.) in four baboons. Amphetamine reduced [11C]raclopride V3'' by 28+/-7% under control conditions. Following LY354740 pretreatment, amphetamine-induced reduction in [11C]raclopride V3'' was significantly enhanced (35+/-7%, p=0.002). The enhancement of the amphetamine-induced reduction in [11C]raclopride V3'' by LY354740 was not a simple additive effect, as LY354740 alone did not reduce [11C]raclopride V3''. In conclusion, the results of this study further document the involvement of GLU transmission in regulating the effect of amphetamine-induced DA release, and provide additional support to the hypothesis that the dysregulation of DA function revealed by the amphetamine challenge in schizophrenia might stem from a deficit in GLU transmission.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Mice lacking metabotropic glutamate receptor 4 do not show the motor stimulatory effect of ethanol

Group III metabotropic glutamate receptors (mGluRs), specifically receptors 4, 6, 7, and 8 (i.e., mGluR4, mGluR6, mGluR7, mGluR8), play an important role in the generation of locomotion as well as in the behavioral effects of some psychostimulants. Because the arousing or stimulant effects of ethanol seem to be relevant behavioral traits associated with its rewarding properties and genetic susceptibility to alcoholism, we addressed the role of mGluR4 by studying behavioral actions of ethanol in mutant mice lacking mGluR4. Null mutant mice showed higher motor response to novelty than did wild-type mice. Ethanol (1.0-2.5 g/kg) stimulated motor activity of wild-type mice, but not of null mutant mice. There were no significant differences between wild-type and knockout strains in ethanol consumption or preference in two-bottle paradigm, severity of ethanol-induced acute withdrawal, or duration of loss of righting reflex. These results show that mGluR4 may play a role in locomotor activity in general and also display specificity for mediation of the motor stimulant effect of ethanol. Consistent with findings of other studies, these results confirm the lack of correlation between ethanol-induced motor stimulation and consumption of ethanol measured in a self-administration paradigm in mice.

Inhibition of non-vesicular glutamate release by group III metabotropic glutamate receptors in the nucleus accumbens

Previous in vitro studies have shown that group III metabotropic glutamate receptors (mGluRs) regulate synaptic glutamate release. The present study used microdialysis to characterize this regulation in vivo in rat nucleus accumbens. Reverse dialysis of the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (L-AP4) decreased, whereas the antagonist (R,S)-alpha-methylserine-O-phosphate (MSOP) increased the extracellular level of glutamate. The decrease by L-AP4 or the increase by MSOP was antagonized by co-administration of MSOP or L-AP4, respectively. Activation of mGluR4a by (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid or mGluR6 by 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid had no effect on extracellular glutamate. (R,S)-4-Phosphonophenylglycine (PPG), another group III agonist with high affinity for mGluR4/6/8, reduced extracellular glutamate only at high concentrations capable of binding to mGluR7. The increase in extracellular glutamate by MSOP was tetrodotoxin-independent, and resistant to both the L-type and N-type Ca2+ channel blockers. L-AP4 failed to block 30 mm K+-induced vesicular glutamate release. Blockade of glutamate uptake by d,l-threo-beta-benzyloxyaspartate caused a Ca2+-independent elevation in extracellular glutamate that was reversed by L-AP4. Finally, (S)-4-carboxyphenylglycine, an inhibitor of cystine-glutamate antiporters, attenuated the L-AP4-induced reduction in extracellular glutamate. Together, these data indicate that group III mGluRs regulate in vivo extracellular glutamate in the nucleus accumbens by inhibiting non-vesicular glutamate release.

Blockade of the locomotor stimulant effects of amphetamine by group I, group II, and group III metabotropic glutamate receptor ligands in the rat nucleus accumbens: possible interactions with dopamine receptors

Previous investigations have shown that mGlu receptors would be involved in the amphetamine-induced motor response. However, data are somewhat controversial across studies where methodological protocols vary. The aim of the present study was to determine the involvement of mGlu receptors in the NAcc in the locomotor-activating properties of amphetamine in rats well habituated to their experimental environment, a condition known to modulate the motor response to amphetamine. Focal infusion of the group I mGlu receptor antagonist S-4-CPG, which has no effect on basal motor activity, virtually suppressed the locomotor response to amphetamine, while infusion of the group II mGlu receptor antagonist LY 341495 or the group III mGlu receptor agonist AP4, at the minimal dose that produces locomotor activation, reduced it by approximately a half. These effects were blocked by the group I mGlu receptor agonist DHPG, the group II mGlu receptor agonist APDC, and the group III mGlu receptor antagonist MPPG, respectively. These data confirm that mGlu receptors in the NAcc contribute to the psychostimulant motor effect of amphetamine. Results are discussed from the view of recent neuropharmacological studies that have defined the effects of these mGlu receptor ligands on basal motor activity and DA receptor agonists-induced locomotor responses in rats exposed to similar experimental procedures (Eur J Neuroscience 13 (2001) 2157; Neuropharmacology 41 (2001) 454; Eur J Neuroscience 13 (2001) 869). It is suggested that the contribution of mGlu receptors to the amphetamine-induced motor response may result mainly from their functional, either direct or indirect, interactions with D1-like receptors in the NAcc.

Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity

Evidence for functional interactions between metabotropic glutamate (mGlu) receptors and dopamine (DA) neurotransmission is now clearly established. In the present study, we investigated interactions between group III mGlu receptors and D1- and D2-like receptors in the nucleus accumbens (NAcc). Administration, into the NAcc, of the selective group III mGlu receptor agonist, AP4, resulted in an increase in locomotor activity, which was blocked by pretreatment with the group III mGlu receptor antagonist, MPPG. In addition, pretreatment with AP4 further blocked the increase in motor activity induced by the D1-like receptor agonist, SKF 38393, but potentiated the locomotor responses induced by either the D2-like receptor agonist, quinpirole, or coinfusion of SKF 38393 and quinpirole. MPPG reversed the effects of AP4 on the motor responses induced by D1-like and/or D2-like receptor activation. These results confirm that glutamate transmission may control DA-dependent locomotor function through mGlu receptors and further indicate that group III mGlu receptors oppose the behavioural response produced by D1-like receptor activation and favour those produced by D2-like receptor activation.

Differentially altered mGluR1 and mGluR5 mRNA expression in rat caudate nucleus and nucleus accumbens in the development and expression of behavioral sensitization to repeated amphetamine administration

Altered glutamatergic transmission in the striatum may be implicated in behavioral sensitization to repeated amphetamine (AMPH) administration. Quantitative in situ hybridization histochemistry was performed to define the effects of acute and chronic AMPH exposures on mRNA expression of Group I metabotropic glutamate receptors (mGluRs) in the striatum. Behavioral ratings indicated that the motor activity of rats was significantly higher after the final of five daily AMPH injections (4 mg/kg, i.p.) than that after the first of five daily AMPH, indicative of the development of behavioral sensitization. Moreover, the motor activity of rats treated with five daily AMPH was significantly greater than that of rats treated with five daily saline in response to a 2 mg/kg challenge dose of AMPH 7, 14, 28, and 60 days after the discontinuation of drug treatments, indicative of the persistent expression of behavioral sensitization. Three hours after acute administration of AMPH to naive rats, mGluR1 and mGluR5 mRNA expression in the dorsal (caudatoputamen) and ventral (nucleus accumbens) striatum showed no change as compared to acute saline injection. In rats that developed behavioral sensitization to repeated AMPH, mGluR1 levels in the dorsal and ventral striatum were increased by 53% and 43%, respectively, 3 h after the final AMPH treatment. However, this change did not persist during withdrawal since it was not observed 7, 14, and 28 days after the discontinuation of AMPH treatment. Conversely, mGluR5 levels were markedly reduced 3 h after the final of five daily AMPH treatments in the entire striatum of sensitized rats (34% and 77% of controls in the dorsal and ventral striatum, respectively). The reduction persisted at 7, 14, and 28 days of withdrawal. These results reveal a close linkage between striatal Group I mGluR gene expression and behavioral sensitization to AMPH. This may indicate functional implications of the two subtypes of Group I mGluRs in the regulation of behavioral sensitization to the dopamine stimulant.

Selective activation of group I metabotropic glutamate receptors upregulates preprodynorphin, substance P, and preproenkephalin mRNA expression in rat dorsal striatum

Group I metabotropic glutamate receptors (mGluRs) are positively coupled to phosphoinositide hydrolysis through G-proteins and are densely expressed in the medium-sized spiny neurons of striatum. Activation of this group of mGluRs in the striatum produces long-lasting stimulation of behavioral activity. In this study, the role of group I mGluRs in the modulation of neuropeptide mRNA expression in striatal neurons was investigated using a Group I-selective agonist, 3,5-dihydroxyphenylglycine (DHPG) in chronically cannulated rats. Unilateral injections of DHPG into the dorsal striatum (caudoputamen) at behaviorally active doses of 20, 40, and 80 nmol elevated basal levels of preprodynorphin (PPD), substance P (SP), and preproenkephalin (PPE) mRNAs in the injected dorsal striatum as revealed by quantitative in situ hybridization. The elevation of all three mRNAs was dose-dependent and the responsiveness of opioid peptide mRNAs (PPD and PPE) to acute injection of DHPG at each dose surveyed was greater than that of SP mRNA. Induction of the mRNAs was delayed and prolonged as increases in hybridization signal became evident at 2 (SP and PPE) or 3 (PPD) h, reached a peak between 3 and 6 h, and returned to normal levels 24 h after DHPG injection. Coadministration of a Group I-selective antagonist, n-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carbo xamide (PHCCC, 10 nmol), with DHPG markedly attenuated DHPG-stimulated PPD, PPE, and, to a lesser extent, SP expression. Administration of PHCCC alone had no significant effect on basal levels of three mRNA expression in the striatum. This study provides a detailed description of the dose- and time-related alterations in striatonigral PPD/SP and striatopallidal PPE mRNA expression in response to a single injection of the Group I agonist DHPG. Data obtained demonstrate a facilitatory, dynamic regulation of constitutive expression of PPD, SP, and PPE mRNAs by local enhancement of glutamatergic tone on DHPG- and PHCCC-sensitive Group I mGluRs.