Impact of metabotropic glutamate 2/3 receptor stimulation on activated dopamine release and locomotion

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

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

G protein-coupled receptor modulation of striatal dopamine transmission: Implications for psychoactive drug effects

Dopamine transmission in the striatum is a critical mediator of the rewarding and reinforcing effects of commonly misused psychoactive drugs. G protein-coupled receptors (GPCRs) that bind a variety of neuromodulators including dopamine, endocannabinoids, acetylcholine and endogenous opioid peptides regulate dopamine release by acting on several components of dopaminergic circuitry. Striatal dopamine release can be driven by both somatic action potential firing and local mechanisms that depend on acetylcholine released from striatal cholinergic interneurons. GPCRs that primarily regulate somatic firing of dopamine neurons via direct effects or modulation of synaptic inputs are likely to affect distinct aspects of behaviour and psychoactive drug actions compared with those GPCRs that primarily regulate local acetylcholine-dependent dopamine release in striatal regions. This review will highlight mechanisms by which GPCRs modulate dopaminergic transmission and the relevance of these findings to psychoactive drug effects on physiology and behaviour.

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.

Local modulation by presynaptic receptors controls neuronal communication and behaviour

Central nervous system neurons communicate via fast synaptic transmission mediated by ligand-gated ion channel (LGIC) receptors and slower neuromodulation mediated by G protein-coupled receptors (GPCRs). These receptors influence many neuronal functions, including presynaptic neurotransmitter release. Presynaptic LGIC and GPCR activation by locally released neurotransmitters influences neuronal communication in ways that modify effects of somatic action potentials. Although much is known about presynaptic receptors and their mechanisms of action, less is known about when and where these receptor actions alter release, especially in vivo. This Review focuses on emerging evidence for important local presynaptic receptor actions and ideas for future studies in this area.

Adolescent Dopamine Neurons Represent Reward Differently during Action and State Guided Learning

Neuronal underpinning of learning cause-and-effect associations in the adolescent brain remains poorly understood. Two fundamental forms of associative learning are Pavlovian (classical) conditioning, where a stimulus is followed by an outcome, and operant (instrumental) conditioning, where outcome is contingent on action execution. Both forms of learning, when associated with a rewarding outcome, rely on midbrain dopamine neurons in the ventral tegmental area (VTA) and substantia nigra (SN). We find that, in adolescent male rats, reward-guided associative learning is encoded differently by midbrain dopamine neurons in each conditioning paradigm. Whereas simultaneously recorded VTA and SN adult neurons have a similar phasic response to reward delivery during both forms of conditioning, adolescent neurons display a muted reward response during operant but a profoundly larger reward response during Pavlovian conditioning. These results suggest that adolescent neurons assign a different value to reward when it is not gated by action. The learning rate of adolescents and adults during both forms of conditioning was similar, supporting the notion that differences in reward response in each paradigm may be because of differences in motivation and independent of state versus action value learning. Static characteristics of dopamine neurons, such as dopamine cell number and size, were similar in the VTA and SN of both ages, but there were age-related differences in stimulated dopamine release and correlated spike activity, suggesting that differences in reward responsiveness by adolescent dopamine neurons are not because of differences in intrinsic properties of these neurons but engagement of different dopaminergic networks.SIGNIFICANCE STATEMENT Reckless behavior and impulsive decision-making by adolescents suggest that motivated behavioral states are encoded differently by the adolescent brain. Motivated behavior, which is dependent on the function of the dopamine system, follows learning of cause-and-effect associations in the environment. We find that dopamine neurons in adolescents encode reward differently depending on the cause-and-effect relationship of the means to receive that reward. Compared with adults, reward contingent on action led to a muted response, whereas reward that followed a cue but was not gated by action produced an augmented phasic response. These data demonstrate an age-related difference in dopamine neuron response to reward that is not uniform and is guided by processes that differentiate between state and action values.

Beyond Dopamine Receptor Antagonism: New Targets for Schizophrenia Treatment and Prevention

Treatment of schizophrenia (SCZ) historically relies on the use of antipsychotic drugs to treat psychosis, with all of the currently available antipsychotics acting through the antagonism of dopamine D2 receptors. Although antipsychotics reduce psychotic symptoms in many patients, they induce numerous undesirable effects and are not effective against negative and cognitive symptoms. These highlight the need to develop new drugs to treat SCZ. An advanced understanding of the circuitry of SCZ has pointed to pathological origins in the excitation/inhibition balance in regions such as the hippocampus, and restoring function in this region, particularly as a means to compensate for parvalbumin (PV) interneuron loss and resultant hippocampal hyperactivity, may be a more efficacious approach to relieve a broad range of SCZ symptoms. Other targets, such as cholinergic receptors and the trace amine-associated receptor 1 (TAAR1), have also shown some promise for the treatment of SCZ. Importantly, assessing efficacy of novel compounds must take into consideration treatment history of the patient, as preclinical studies suggest prior antipsychotic treatment may interfere with the efficacy of these novel agents. However, while novel therapeutic targets may be more effective in treating SCZ, a more effective approach would be to prevent the transition to SCZ in susceptible individuals. A focus on stress, which has been shown to be a predisposing factor in risk for SCZ, is a possible avenue that has shown promise in preclinical studies. Therefore, therapeutic approaches based on our current understanding of the circuitry of SCZ and its etiology are likely to enable development of more effective therapeutic interventions for this complex disorder.

The G Protein-Coupled Glutamate Receptors as Novel Molecular Targets in Schizophrenia Treatment-A Narrative Review

Schizophrenia is a severe neuropsychiatric disease with an unknown etiology. The research into the neurobiology of this disease led to several models aimed at explaining the link between perturbations in brain function and the manifestation of psychotic symptoms. The glutamatergic hypothesis postulates that disrupted glutamate neurotransmission may mediate cognitive and psychosocial impairments by affecting the connections between the cortex and the thalamus. In this regard, the greatest attention has been given to ionotropic NMDA receptor hypofunction. However, converging data indicates metabotropic glutamate receptors as crucial for cognitive and psychomotor function. The distribution of these receptors in the brain regions related to schizophrenia and their regulatory role in glutamate release make them promising molecular targets for novel antipsychotics. This article reviews the progress in the research on the role of metabotropic glutamate receptors in schizophrenia etiopathology.

Emerging therapeutic targets for schizophrenia: a framework for novel treatment strategies for psychosis

Introduction: Antipsychotic drugs are central to the treatment of schizophrenia, but their limitations necessitate improved treatment strategies. Multiple lines of research have implicated glutamatergic dysfunction in the hippocampus as an early source of pathophysiology in schizophrenia. Novel compounds have been designed to treat glutamatergic dysfunction, but they have produced inconsistent results in clinical trials. Areas covered: This review discusses how the hippocampus is thought to drive psychotic symptoms through its influence on the dopamine system. It offers the reader an evaluation of proposed treatment strategies including direct modulation of GABA or glutamate neurotransmission or reducing the deleterious impact of stress on circuit development. Finally, we offer a perspective on aspects of future research that will advance our knowledge and may create new therapeutic opportunities. PubMed was searched for relevant literature between 2010 and 2020 and related studies. Expert opinion: Targeting aberrant excitatory-inhibitory neurotransmission in the hippocampus and its related circuits has the potential to alleviate symptoms and reduce the risk of transition to psychosis if implemented as an early intervention. Longitudinal multimodal brain imaging combined with mechanistic theories generated from animal models can be used to better understand the progression of hippocampal-dopamine circuit dysfunction and heterogeneity in treatment response.

The mGluR2/3 agonist pomaglumetad methionil normalizes aberrant dopamine neuron activity via action in the ventral hippocampus

The group 2 metabotropic glutamate receptor (mGluR2/3) agonist, pomaglumetad methionil (POM), showed promise as a novel antipsychotic in preclinical research but failed to show efficacy in clinical trials, though it has been suggested that it may be effective in certain patient populations. Although previous studies have shown that mGluR2/3 agonists have no effect on dopamine (DA) in wild type rats, we used the methylzoxymethanol acetate (MAM) model to determine whether POM may indirectly normalize DA neuron activity in a model representative of the hyperdopaminergic state thought to underlie psychosis, compared to SAL rats, using in vivo, anesthetized, electrophysiological recordings. POM dose-dependently reduced the number of spontaneously active DA neurons in the VTA of MAM rats to control levels without affecting DA firing in SAL rats, which persisted following 14d repeated treatment with POM. In female MAM rats, POM significantly reduced DA neuron population activity only during proestrous and estrous stages. MAM rats also demonstrated dose-dependent improvement in novel object recognition following acute POM, which was not observed in SAL rats. Similar to the MAM rats, DA neuron population activity was increased in a hippocampal-dependent manner following acute restraint stress. Administration of POM prior to 2 h restraint stress prevented the restraint-induced increase in DA neuron population activity, and this effect was blocked by pretreatment with an mGluR2/3 antagonist. Thus, the ability of POM to reduce the hyperdopaminergic activity in both MAM rats and in wild type rats following restraint stress suggests that it can indirectly regulate DA neuron activity, which may underlie its potential therapeutic effects.

Operant self-stimulation of thalamic terminals in the dorsomedial striatum is constrained by metabotropic glutamate receptor 2

Dorsal striatal manipulations including stimulation of dopamine release and activation of medium spiny neurons (MSNs) are sufficient to drive reinforcement-based learning. Glutamatergic innervation of the striatum by the cortex and thalamus is a critical determinant of MSN activity and local regulation of dopamine release. However, the relationship between striatal glutamatergic afferents and behavioral reinforcement is not well understood. We evaluated the reinforcing properties of optogenetic stimulation of thalamostriatal terminals, which are associated with vesicular glutamate transporter 2 (Vglut2) expression, in the dorsomedial striatum (DMS), a region implicated in goal-directed behaviors. In mice expressing channelrhodopsin-2 (ChR2) under control of the Vglut2 promoter, optical stimulation of the DMS reinforced operant lever-pressing behavior. Mice also acquired operant self-stimulation of thalamostriatal terminals when ChR2 expression was virally targeted to the intralaminar thalamus. Stimulation trains that supported operant responding evoked dopamine release in the DMS and excitatory postsynaptic currents in DMS MSNs. Our previous work demonstrated that the presynaptic G protein-coupled receptor metabotropic glutamate receptor 2 (mGlu₂) robustly inhibits glutamate and dopamine release induced by activation of thalamostriatal afferents. Thus, we examined the regulation of thalamostriatal self-stimulation by mGlu₂. Administration of an mGlu_2/3 agonist or an mGlu₂-selective positive allosteric modulator reduced self-stimulation. Conversely, blockade of these receptors increased thalamostriatal self-stimulation, suggesting that endogenous activation of these receptors negatively modulates the reinforcing properties of thalamostriatal activity. These findings demonstrate that stimulation of thalamic terminals in the DMS is sufficient to reinforce a self-initiated action, and that thalamostriatal reinforcement is constrained by mGlu₂ activation.

Insights on current and novel antipsychotic mechanisms from the MAM model of schizophrenia

Current antipsychotic drugs (APDs) act on D₂ receptors, and preclinical studies demonstrate that repeated D₂ antagonist administration downregulates spontaneously active DA neurons by producing overexcitation-induced inactivation of firing (depolarization block). Animal models of schizophrenia based on the gestational MAM administration produces offspring with adult phenotypes consistent with schizophrenia, including ventral hippocampal hyperactivity and a DA neuron overactivity. The MAM model reveals that APDs act differently in a hyperdopamineregic system compared to a normal one, including rapid onset of depolarization block in response to acute D₂ antagonist administration and downregulation of DA neuron population activity following acute and repeated D₂ partial agonist administration, none of which are observed in normal rats. Novel target compounds have been developed based on the theory that glutamatergic dysfunction is central to schizophrenia pathology. Despite showing promise in preclinical research, none of the novel drugs succeeded in clinical trials. However, preclinical research is generally performed in normal, drug-naïve rats, whereas models with disease-relevant pathology and prior APD exposure may improve the predictive validity of preclinical research. Indeed, in MAM rats, chronic D₂ antagonist treatment leads to persistent DA supersensitivity that interferes with the response to drugs that target upstream pathology. Moreover, MAM rats revealed that the peri-pubertal period is a stress-sensitive window that can be targeted to prevent the development of MAM pathology in adulthood. Neurodevelopmental models, such as the MAM model, can thus be used to test potential pharmacotherapies that may be able to treat schizophrenia in early stages of the disease. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Acupuncture inhibition of methamphetamine-induced behaviors, dopamine release and hyperthermia in the nucleus accumbens: mediation of group II mGluR

Methamphetamine (METH) increases metabolic neuronal activity in the mesolimbic dopamine (DA) system and mediates the reinforcing effect. To explore the underlying mechanism of acupuncture intervention in reducing METH-induced behaviors, we investigated the effect of acupuncture on locomotor activity, ultrasonic vocalizations, extracellular DA release in the nucleus accumbens (NAcs) using fast-scan cyclic voltammetry and alterations of brain temperature (an indicator of local brain metabolic activity) produced by METH administration. When acupuncture was applied to HT7, but not TE4, both locomotor activity and 50-kHz ultrasonic vocalizations were suppressed in METH-treated rats. Acupuncture at HT7 attenuated the enhancement of electrically stimulated DA release in the NAc of METH-treated rats. Systemic injection of METH produced a sustained increase in NAc temperature, which was reversed by the DA D1 receptor antagonist SCH 23390 or acupuncture at HT7. Acupuncture inhibition of METH-induced NAc temperature was prevented by pre-treatment with a group II metabotropic glutamate receptors (mGluR2/3) antagonist EGLU into the NAc or mimicked by injection of an mGluR2/3 agonist DCG-IV into the NAc. These results suggest that acupuncture reduces extracellular DA release and metabolic neuronal activity in the NAc through activation of mGluR2/3 and suppresses METH-induced affective states and locomotor behavior.

Effects of Clavulanic Acid Treatment on Reinstatement to Methamphetamine, Glial Glutamate Transporters, and mGluR 2/3 Expression in P Rats Exposed to Ethanol

Evidence demonstrated that the glutamatergic system is implicated in mediating relapse to several drugs of abuse, including methamphetamine (METH). Glutamate homeostasis is maintained by a number of glutamate transporters, such as glutamate transporter type 1 (GLT-1), cystine/glutamate transporter (xCT), and glutamate aspartate transporter (GLAST). In addition, group II metabotropic glutamate receptors (mGluR2/3) were found to be implicated in relapse-seeking behavior. Ample evidence showed that β-lactam antibiotics are effective in upregulating GLT-1 and xCT expression, thus improving glutamate homeostasis and attenuating relapse to drugs of abuse. In this study, we investigated the reinstatement of METH using conditioned place preference (CPP) in male alcohol-preferring (P) rats exposed to home-cage free choice ethanol drinking. Here, we tested the effect of clavulanic acid (CA), a β-lactam, on the reinstatement of METH-seeking and ethanol drinking. In addition, we examined the expression of GLT-1, xCT, and GLAST as well as metabotropic glutamate receptor (mGluR2/3) in the nucleus accumbens (NAc) shell, NAc core, and dorsomedial prefrontal cortex (dmPFC). A priming i.p. injection of METH reinstated preference in METH-paired chamber following extinction. Chronic exposure to ethanol decreased the expression of GLT-1 and xCT in the NAc shell, but not in the NAc core or dmPFC. CA treatment blocked the reinstatement of METH-seeking, decreased ethanol intake, and restored the expression of GLT-1 and xCT in the NAc shell. In addition, the expression of mGluR2/3 was increased by CA treatment in the NAc shell and dmPFC. These findings suggest that these glutamate transporters and mGluR2/3 might be potential therapeutic targets for the attenuation of reinstatement to METH-seeking.

The group II metabotropic glutamate receptor agonist LY354740 and the D2 receptor antagonist haloperidol reduce locomotor hyperactivity but fail to rescue spatial working memory in GluA1 knockout mice

Group II metabotropic glutamate receptor agonists have been suggested as potential anti‐psychotics, at least in part, based on the observation that the agonist LY354740 appeared to rescue the cognitive deficits caused by non‐competitive N‐methyl‐d‐aspartate receptor (NMDAR) antagonists, including spatial working memory deficits in rodents. Here, we tested the ability of LY354740 to rescue spatial working memory performance in mice that lack the GluA1 subunit of the AMPA glutamate receptor, encoded by Gria1, a gene recently implicated in schizophrenia by genome‐wide association studies. We found that LY354740 failed to rescue the spatial working memory deficit in Gria1^−/− mice during rewarded alternation performance in the T‐maze. In contrast, LY354740 did reduce the locomotor hyperactivity in these animals to a level that was similar to controls. A similar pattern was found with the dopamine receptor antagonist haloperidol, with no amelioration of the spatial working memory deficit in Gria1^−/− mice, even though the same dose of haloperidol reduced their locomotor hyperactivity. These results with LY354740 contrast with the rescue of spatial working memory in models of glutamatergic hypofunction using non‐competitive NMDAR antagonists. Future studies should determine whether group II mGluR agonists can rescue spatial working memory deficits with other NMDAR manipulations, including genetic models and other pharmacological manipulations of NMDAR function.

Metabotropic glutamate receptor 2 inhibits thalamically-driven glutamate and dopamine release in the dorsal striatum

The striatum plays critical roles in action control and cognition, and activity of striatal neurons is driven by glutamatergic input. Inhibition of glutamatergic inputs to projection neurons and interneurons of the striatum by presynaptic G protein-coupled receptors (GPCRs) stands to modulate striatal output and striatum-dependent behaviors. Despite knowledge that a substantial number of glutamatergic inputs to striatal neurons originate in the thalamus, most electrophysiological studies assessing GPCR modulation do not differentiate between effects on corticostriatal and thalamostriatal transmission, and synaptic inhibition is frequently assumed to be mediated by activation of GPCRs on corticostriatal terminals. We used optogenetic techniques and recently-discovered pharmacological tools to dissect the effects of a prominent presynaptic GPCR, metabotropic glutamate receptor 2 (mGlu₂), on corticostriatal vs. thalamostriatal transmission. We found that an agonist of mGlu₂ and mGlu₃ induces long-term depression (LTD) at synapses onto MSNs from both the cortex and the thalamus. Thalamostriatal LTD is selectively blocked by an mGlu₂-selective negative allosteric modulator and reversed by application of an antagonist following LTD induction. Activation of mGlu_2/3 also induces LTD of thalamostriatal transmission in striatal cholinergic interneurons (CINs), and pharmacological activation of mGlu_2/3 or selective activation of mGlu₂ inhibits CIN-mediated dopamine release evoked by selective stimulation of thalamostriatal inputs. Thus, mGlu₂ activation exerts effects on striatal physiology that extend beyond modulation of corticostriatal synapses, and has the potential to influence cognition and striatum-related disorders via inhibition of thalamus-derived glutamate and dopamine release.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Lower [3H]LY341495 binding to mGlu2/3 receptors in the anterior cingulate of subjects with major depressive disorder but not bipolar disorder or schizophrenia

INTRODUCTION

The glutamatergic system has recently been implicated in the pathogenesis and treatment of major depressive disorders(MDD) and mGlu2/3 receptors play an important role in regulating glutamatergic tone. We therefore measured cortical levels of mGlu2/3 to determine if they were changed in MDD.

METHODS

Binding parameters for [(3)H]LY341495 (mGlu2/3 antagonist) were determined to allow optimized in situ binding with autoradiography to be completed using a number of CNS regions. Subsequently, density of [(3)H]LY341495 binding was measured in BA24(anterior cingulate cortex), BA17(visual cortex) and BA46(dorsolateral prefrontal cortex) from subjects with MDD, Bipolar Disorder(BPD), Schizophrenia(SCZ), and controls, as well as rats treated with imipramine (20mg/kg), fluoxetine (10mg/kg), or vehicle.

RESULTS

mGlu2/3 are widely expressed throughout the brain with high levels observed in cortex. [(3)H]LY341495 binding was significantly lower in BA24 from subjects with MDD (mean ± SEM=141.3 ± 14.65 fmol/ETE) relative to controls (184.9 ± 7.76 fmol/ETE; Cohen's d=1.005, p<0.05). There were no other differences with diagnoses, and chronic antidepressant treatment in rats had minimal effect on binding.

LIMITATIONS

Using this approach we are unable to determine whether the change represents fluctuations in mGlu2, mGlu3, or both. Moreover, using postmortem tissue we are unable to dissociate the irrevocable confound of suicidality upon binding levels.

CONCLUSION

We have demonstrated lower [(3)H]LY341495 binding levels in MDD in BA24-a brain region implicated in depression. Moreover we show that the lower levels are unlikely to be the result of antidepressant treatment. These data suggest that levels of either mGlu2 and/or mGlu3 are affected in the aetiology of MDD.

Testosterone attenuates and the selective estrogen receptor modulator, raloxifene, potentiates amphetamine-induced locomotion in male rats

Although sex steroids are known to modulate brain dopamine, it is still unclear how testosterone modifies locomotor behaviour controlled, at least in part, by striatal dopamine in adolescent males. Our previous work suggests that increasing testosterone during adolescence may bias midbrain neurons to synthesise more dopamine. We hypothesised that baseline and amphetamine-induced locomotion would differ in adult males depending on testosterone exposure during adolescence. We hypothesised that concomitant stimulation of estrogen receptor signaling, through a selective estrogen receptor modulator (SERM), raloxifene, can counter testosterone effects on locomotion. Male Sprague-Dawley rats at postnatal day 45 were gonadectomised (G) or sham-operated (S) prior to the typical adolescent testosterone increase. Gonadectomised rats were either given testosterone replacement (T) or blank implants (B) for six weeks and sham-operated (i.e. intact or endogenous testosterone group) were given blank implants. Subgroups of sham-operated, gonadectomised and gonadectomised/testosterone-replaced rats were treated with raloxifene (R, 5mg/kg) or vehicle (V), daily for the final four weeks. There were six groups (SBV, GBV, GTV, SBR, GBR, GTR). Saline and amphetamine-induced (1.25mg/kg) locomotion in the open field was measured at PND85. Gonadectomy increased amphetamine-induced locomotion compared to rats with endogenous or with exogenous testosterone. Raloxifene increased amphetamine-induced locomotion in rats with either endogenous or exogenous testosterone. Amphetamine-induced locomotion was negatively correlated with testosterone and this relationship was abolished by raloxifene. Lack of testosterone during adolescence potentiates and testosterone exposure during adolescence attenuates amphetamine-induced locomotion. Treatment with raloxifene appears to potentiate amphetamine-induced locomotion and to have an opposite effect to that of testosterone in male rats.

The [18F]FDG μPET readout of a brain activation model to evaluate the metabotropic glutamate receptor 2 positive allosteric modulator JNJ-42153605

Using [(18)F]fluorodeoxyglucose μ-positron emission tomography ([(18)F]FDG μPET), we compared subanesthetic doses of memantine and ketamine on their potential to induce increases in brain activation. We also studied the reversal effect of the well-known metabotropic glutamate receptor (mGluR)-2/3 agonist LY404039 [(-)-(1R,4S,5S,6S)-4-amino-2-sulfonylbicyclo[3.1.0]hexane-4,6-dicarboxylic acid] and the novel mGluR2 positive allosteric modulator (PAM) JNJ-42153605 [3-cylcopropylmethyl-7-(4-phenylpiperidin-1-yl)-8-trifluoromethyl [1,2,4] triazolo[4,3-a]pyridine]. First, rats (n = 12) were subjected to LY404039 (10 mg/kg s.c.) or vehicle, 30 minutes prior to saline, ketamine (30 mg/kg i.p.), or memantine (20 mg/kg i.p.). Second, rats (n = 12) were subjected to 2.5 mg/kg or 10 mg/kg mGluR2 PAM JNJ-42153605 or vehicle (s.c.), 30 minutes prior to memantine (20 mg/kg i.p.) or saline. Fifteen minutes later, [(18)F]FDG was injected (37 MBq i.v.) followed by a μPET/computed tomography scan. The increase due to memantine is significant for all relevant brain areas, whereas for ketamine this is not the case. Standard uptake values (SUVs) of the LY404039 pretreated and memantine-challenged group display a full reversal. Pretreatment with JNJ-42153605 also dose-dependently decreases SUV with a full reversal as well (for 10 mg/kg). Moreover, specificity of JNJ-42153605 is reached at this dose. In conclusion, this μPET experiment clearly indicates that subanesthetic doses of memantine induce significant increases of [(18)F]FDG SUVs in discrete brain areas and that the novel mGluR2 PAM has the capacity to dose-dependently and specifically reverse memantine-induced brain activation.

Quinoline- and isoquinoline-sulfonamide analogs of aripiprazole: novel antipsychotic agents?

The introduction of typical antipsychotics over six decades ago signaled an important milestone in psychiatry. However, second-generation antipsychotics ameliorated the positive symptoms of schizophrenia but displayed limited effectiveness for the negative and cognitive symptoms. In addition, while the newer antipsychotics produced fewer motor side effects, the atypical antipsychotics still induced weight gain and endocrinopathies. In recent years, a third generation of antipsychotics was identified. Aripiprazole was the first approved drug acting as a D2 partial agonist/functionally selective ligand. This review presents the state of the development of novel antipsychotic dopaminergic and non-dopaminergic agents, supported by an overview of the compounds evaluated under advanced preclinical and clinical development (e.g., cariprazine and brexpiprazole). In line with the recent trends in the development of modern atypical antipsychotics, we present our strategic development of long-chain arylpiperazine-derived quinoline- and isoquinoline-sulfonamide displaying a multireceptor binding profile and partial D2 receptor agonism.

Decreased striatal dopamine in group II metabotropic glutamate receptor (mGlu2/mGlu3) double knockout mice

BACKGROUND

Group II metabotropic glutamate receptors (mGlu2 and mGlu3, encoded by Grm2 and Grm3) have been the focus of attention as treatment targets for a number of psychiatric conditions. Double knockout mice lacking mGlu2 and mGlu3 (mGlu2/3-/-) show a subtle behavioural phenotype, being hypoactive under basal conditions and in response to amphetamine, and with a spatial memory deficit that depends on the arousal properties of the task. The neurochemical correlates of this profile are unknown. Here, we measured tissue levels of dopamine, 5-HT, noradrenaline and their metabolites in the striatum and frontal cortex of mGlu2/3-/- double knockout mice, using high performance liquid chromatography. We also measured the same parameters in mGlu2-/- and mGlu3-/- single knockout mice.

RESULTS

mGlu2/3-/-mice had reduced dopamine levels in the striatum but not in frontal cortex, compared to wild-types. In a separate cohort we replicated this deficit and, using tissue punches, found it was more prominent in the nucleus accumbens than in dorsolateral striatum. Noradrenaline, 5-HT and their metabolites were not altered in the striatum of mGlu2/3-/- mice, although the noradrenaline metabolite MHPG was increased in the cortex. In mGlu2-/- and mGlu3-/- single knockout mice we found no difference in any monoamine or metabolite, in either brain region, compared to their wild-type littermates.

CONCLUSIONS

Group II metabotropic glutamate receptors impact upon striatal dopamine. The effect may contribute to the behavioural phenotype of mGlu2/3-/- mice. The lack of dopaminergic alterations in mGlu2-/- and mGlu3-/- single knockout mice reveals a degree of redundancy between the two receptors. The findings support the possibility that interactions between mGlu2/3 and dopamine may be relevant to the pathophysiology and therapy of schizophrenia and other disorders.

Reduced presynaptic dopamine activity in adolescent dorsal striatum

Adolescence coincides with symptomatic onset of several psychiatric illnesses including schizophrenia and addiction. Excess limbic dopamine activity has been implicated in these vulnerabilities. We combined molecular and dynamic indices of dopamine neurotransmission to assess dopamine function in adolescent rats in two functionally distinct striatal subregions: nucleus accumbens (NAc) and dorsal striatum (DS). In adolescents, we find an overall reduction in dopamine availability selective to the DS. Dopamine release in the DS, but not in the NAc, was less responsive to amphetamine in adolescents compared to adults. The dopamine transporter (DAT) inhibitor, nomifensine, similarly inhibited basal and amphetamine-induced dopamine release in either regions of both the age groups, suggesting that the reduced effectiveness of amphetamine is not due to differences in DAT function. Furthermore, DAT and vesicular monoamine transporter-2 expressions were similar in the DS and NAc of adolescent rats. In contrast, expression of tyrosine hydroxylase (TH) was reduced in the DS, but not in the NAc, of adolescents compared to adults. Behaviorally, adolescents were less sensitive to amphetamine but more sensitive to a TH inhibitor. These data indicate that, in contrast to the general notion that dopamine is hyperactive in adolescents, there is diminished presynaptic dopamine activity in adolescents that is selective to the DS and may result from attenuated TH activity. Given recent reports of altered dopamine activity in associative/dorsal striatum of individuals at a clinically high risk of psychosis, our data further support the idea that dorsal, as opposed to ventral, regions of the striatum are a locus of vulnerability for psychosis.

Effects of N-acetylaspartylglutamate (NAAG) peptidase inhibition on release of glutamate and dopamine in prefrontal cortex and nucleus accumbens in phencyclidine model of schizophrenia

The "glutamate" theory of schizophrenia emerged from the observation that phencyclidine (PCP), an open channel antagonist of the NMDA subtype of glutamate receptor, induces schizophrenia-like behaviors in humans. PCP also induces a complex set of behaviors in animal models of this disorder. PCP also increases glutamate and dopamine release in the medial prefrontal cortex and nucleus accumbens, brain regions associated with expression of psychosis. Increased motor activation is among the PCP-induced behaviors that have been widely validated as models for the characterization of new antipsychotic drugs. The peptide transmitter N-acetylaspartylglutamate (NAAG) activates a group II metabotropic receptor, mGluR3. Polymorphisms in this receptor have been associated with schizophrenia. Inhibitors of glutamate carboxypeptidase II, an enzyme that inactivates NAAG following synaptic release, reduce several behaviors induced by PCP in animal models. This research tested the hypothesis that two structurally distinct NAAG peptidase inhibitors, ZJ43 and 2-(phosphonomethyl)pentane-1,5-dioic acid, would elevate levels of synaptically released NAAG and reduce PCP-induced increases in glutamate and dopamine levels in the medial prefrontal cortex and nucleus accumbens. NAAG-like immunoreactivity was found in neurons and presumptive synaptic endings in both regions. These peptidase inhibitors reduced the motor activation effects of PCP while elevating extracellular NAAG levels. They also blocked PCP-induced increases in glutamate but not dopamine or its metabolites. The mGluR2/3 antagonist LY341495 blocked these behavioral and neurochemical effects of the peptidase inhibitors. The data reported here provide a foundation for assessment of the neurochemical mechanism through which NAAG achieves its antipsychotic-like behavioral effects and support the conclusion NAAG peptidase inhibitors warrant further study as a novel antipsychotic therapy aimed at mGluR3.

Metabotropic glutamate receptors as therapeutic targets for schizophrenia

Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking in current therapies for this disorder. Compounds targeting the metabotropic glutamate (mGlu) receptors hold promise as a more comprehensive therapeutic alternative to typical and atypical antipsychotics and may avoid the occurrence of extrapyramidal side effects that accompany these treatments. Activation of the group II mGlu receptors (mGlu(2) and mGlu(3)) and the group I mGlu(5) are hypothesized to normalize the disruption of thalamocortical glutamatergic circuitry that results in abnormal glutamaterigic signaling in the prefrontal cortex (PFC). Agonists of mGlu(2) and mGlu(3) have demonstrated efficacy for the positive symptom group in both animal models and clinical trials with mGlu(2) being the subtype most likely responsible for the therapeutic effect. Limitations in the chemical space tolerated by the orthosteric site of the mGlu receptors has led to the pursuit of compounds that potentiate the receptor's response to glutamate by acting at less highly conserved allosteric sites. Several series of selective positive allosteric modulators (PAMs) for mGlu(2) and mGlu(5) have demonstrated efficacy in animal models used for the evaluation of antipsychotic agents. In addition, evidence from animal studies indicates that mGlu(5) PAMs hold promise for the treatment of cognitive deficits that occur in schizophrenia. Hopefully, further optimization of allosteric modulators of mGlu receptors will yield clinical candidates that will allow full evaluation of the potential efficacy of these compounds in the treatment of multiple symptom domains in schizophrenia patients in the near future.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

Fractionation of spatial memory in GRM2/3 (mGlu2/mGlu3) double knockout mice reveals a role for group II metabotropic glutamate receptors at the interface between arousal and cognition

Group II metabotropic glutamate receptors (mGluR2 and mGluR3, encoded by GRM2 and GRM3) are implicated in hippocampal function and cognition, and in the pathophysiology and treatment of schizophrenia and other psychiatric disorders. However, pharmacological and behavioral studies with group II mGluR agonists and antagonists have produced complex results. Here, we studied hippocampus-dependent memory in GRM2/3 double knockout (GRM2/3(-/-)) mice in an iterative sequence of experiments. We found that they were impaired on appetitively motivated spatial reference and working memory tasks, and on a spatial novelty preference task that relies on animals' exploratory drive, but were unimpaired on aversively motivated spatial memory paradigms. GRM2/3(-/-) mice also performed normally on an appetitively motivated, non-spatial, visual discrimination task. These results likely reflect an interaction between GRM2/3 genotype and the arousal-inducing properties of the experimental paradigm. The deficit seen on appetitive and exploratory spatial memory tasks may be absent in aversive tasks because the latter induce higher levels of arousal, which rescue spatial learning. Consistent with an altered arousal-cognition relationship in GRM2/3(-/-) mice, injection stress worsened appetitively motivated, spatial working memory in wild-types, but enhanced performance in GRM2/3(-/-) mice. GRM2/3(-/-) mice were also hypoactive in response to amphetamine. This fractionation of hippocampus-dependent memory depending on the appetitive-aversive context is to our knowledge unique, and suggests a role for group II mGluRs at the interface of arousal and cognition. These arousal-dependent effects may explain apparently conflicting data from previous studies, and have translational relevance for the involvement of these receptors in schizophrenia and other disorders.

Activation of metabotropic glutamate 2/3 receptors attenuates methamphetamine-induced hyperlocomotion and increase in prefrontal serotonergic neurotransmission

RATIONALE

Metabotropic glutamate (mGlu) 2/3 receptor agonists inhibit amphetamine- and phencyclidine-induced hyperlocomotion. The mechanism for the antipsychotic effect of mGlu2/3 receptor agonists was studied in a hypoglutamatergic model, but not a hyperdopaminergic model.

OBJECTIVES

To study the mechanism for the antipsychotic effect of the agonist in the hyperdopaminergic model, this study examined the effects of the selective mGlu2/3 receptor agonist MGS0028 on methamphetamine-induced hyperlocomotion and the increases in extracellular levels of serotonin, dopamine, noradrenaline, and glutamate in the prefrontal cortex and nucleus accumbens of mice.

RESULTS

Systemic administration of MGS0028 attenuated methamphetamine-induced hyperlocomotion in a dose-dependent manner. Microdialysis studies showed that MGS0028 significantly inhibited methamphetamine-induced increases in the extracellular serotonin, but not dopamine and noradrenaline, levels in the prefrontal cortex, and it did not affect methamphetamine-induced increases in the extracellular amine levels in the nucleus accumbens. Methamphetamine did not affect the glutamate release in the prefrontal cortex and nucleus accumbens. Local application of MGS0028 into the prefrontal cortex also attenuated methamphetamine-induced hyperlocomotion and increases in the extracellular serotonin levels in the prefrontal cortex. Moreover, MGS0028 did not affect methamphetamine-induced hyperlocomotion in the mice pretreated with p-chlorophenylalanine, a serotonin synthesis inhibitor.

CONCLUSIONS

Activation of prefrontal mGlu2/3 receptors inhibits the psychomotor stimulant effect of methamphetamine in mice, and the prefrontal serotonergic system may be involved in this effect. The finding provides evidence that prefrontal mGlu2/3 receptors are functionally coupled with the serotonergic system.

Targeting glutamate synapses in schizophrenia

Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M(1) and M(4), and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing the promise for a multifaceted pharmacological approach to treat schizophrenia.

Interaction of mGlu2/3 agonism with clozapine and lurasidone to restore novel object recognition in subchronic phencyclidine-treated rats

RATIONALE

Subchronic administration to rodents of the N-methyl-D-aspartate non-competitive antagonist, phencyclidine (PCP), impairs novel object recognition (NOR). Atypical antipsychotic drugs (APDs) reverse the effects of subchronic PCP on NOR. The effect of metabotropic glutamate₂/₃ receptor (mGlu₂/₃) agonists upon NOR is unknown.

OBJECTIVES AND METHODS

We tested the hypotheses that the mGlu₂/₃ agonist, LY379268, by itself, or in combination with APDs or pimavanserin, a 5-HT(2A) inverse agonist, would reverse the deficit in NOR induced by subchronic treatment with PCP (2 mg/kg, b.i.d., for 7 days).

RESULTS

The mGlu₂/₃ agonist LY379268 (1 or 3 mg/kg) did not attenuate the PCP-induced NOR deficit. However, together with sub-effective dose of the atypical APDs, clozapine (0.1 mg/kg) or lurasidone (0.03 mg/kg), but not the typical APD, haloperidol (0.1 mg/kg), or pimavanserin (3 mg/kg), LY379268, 1 mg/kg, significantly reversed the PCP-induced NOR deficit. Moreover, the effect of clozapine was blocked by the mGlu₂/₃ antagonist, LY341495 (1 mg/kg).

CONCLUSIONS

These results indicate that mGlu₂/₃ agonism can potentiate the ability of atypical, but not typical APDs, to ameliorate the effect of subchronic PCP on NOR, that mGlu₂/₃ agonism may contribute to the ability of atypical APDs to acutely reverse the effect of subchronic PCP on NOR, but that by itself, mGlu₂/₃ agonism, is ineffective in this model of cognitive impairment in schizophrenia. These results suggest that mGlu₂/₃ receptor agonism should be investigated as an adjunctive treatment of cognitive impairment in schizophrenia rather than as monotherapy, which may be effective for control of psychosis, but not for cognitive impairment.

The role of metabotropic glutamate receptors in addiction: evidence from preclinical models

Addiction is a chronic disorder characterised by repeated bouts of drug taking, abstinence and relapse. The addicted state may be in part due to drug-induced neuroadaptations in the mesocorticolimbic and corticostriatal pathways. Recently focus has been on the role of aberrant glutamate transmission and its contribution to the hierarchical control over these systems. This review will expand our current knowledge of the most recent advances that have been made in preclinical animal models that provide evidence that implicate metabotropic glutamate receptors (mGluRs) in contributing to the neuroadaptations pertinent to addiction, as well as the role of Homer proteins in regulating these responses. The recent discovery of receptor mosaics will be discussed which add an additional dimension to the complexity of understanding the mechanism of glutamate mediated behaviours. Finally this review introduces a new area related to glutamatergic responses, namely microRNAs, that may become pivotal in directing our future understanding of how to best target intervention strategies to prevent addictive behaviours.

T-type calcium channel antagonism produces antipsychotic-like effects and reduces stimulant-induced glutamate release in the nucleus accumbens of rats

T-type calcium channels are important in burst firing and expressed in brain regions implicated in schizophrenia. Therefore, we examined the effects of novel selective T-type calcium channel antagonists in preclinical assays predictive of antipsychotic-like activity. TTA-A2 blocked the psychostimulant effects of amphetamine and MK-801 and decreased conditioned avoidance responding. These effects appeared mechanism based, rather than compound specific, as two structurally dissimilar T-type antagonists also reduced amphetamine-induced psychomotor activity. Importantly, the ability to reduce amphetamine's effects was maintained following 20 days pre-treatment with TTA-A2. To explore the neural substrates mediating the observed behavioral effects, we examined the influence of TTA-A2 on amphetamine-induced c-fos expression as well as basal and stimulant-evoked dopamine and glutamate release in the nucleus accumbens. TTA-A2 decreased amphetamine-induced c-fos expression as well as MK-801-induced, but not basal, glutamate levels in the nucleus accumbens. Basal, amphetamine- and MK-801-induced dopamine efflux was altered. These findings suggest that T-type calcium channel antagonism could represent a novel mechanism for treating schizophrenia.