The mGlu2/3 receptor agonist LY379268 blocks the expression of locomotor sensitization by amphetamine

Acute and chronic effects by nicotine on striatal neurotransmission and synaptic plasticity in the female rat brain

Introduction

Tobacco use is in part a gendered activity, yet neurobiological studies outlining the effect by nicotine on the female brain are scarce. The aim of this study was to outline acute and sub-chronic effects by nicotine on the female rat brain, with special emphasis on neurotransmission and synaptic plasticity in the dorsolateral striatum (DLS), a key brain region with respect to the formation of habits.

Methods

In vivo microdialysis and ex vivo electrophysiology were performed in nicotine naïve female Wistar rats, and following sub-chronic nicotine exposure (0.36 mg/kg free base, 15 injections). Locomotor behavior was assessed at the first and last drug-exposure.

Results

Acute exposure to nicotine ex vivo depresses excitatory neurotransmission by reducing the probability of transmitter release. Bath applied nicotine furthermore facilitated long-term synaptic depression induced by high frequency stimulation (HFS-LTD). The cannabinoid 1 receptor (CB1R) agonist WIN55,212-2 produced a robust synaptic depression of evoked potentials, and HFS-LTD was blocked by the CB1R antagonist AM251, suggesting that HFS-LTD in the female rat DLS is endocannabinoid mediated. Sub-chronic exposure to nicotine in vivo produced behavioral sensitization and electrophysiological recordings performed after 2-8 days abstinence revealed a sustained depression of evoked population spike amplitudes in the DLS, with no concomitant change in paired pulse ratio. Rats receiving sub-chronic nicotine exposure further demonstrated an increased neurophysiological responsiveness to nicotine with respect to both dopaminergic- and glutamatergic signaling. However, a tolerance towards the plasticity facilitating property of bath applied nicotine was developed during sub-chronic nicotine exposure in vivo. In addition, the dopamine D2 receptor agonist quinpirole selectively facilitate HFS-LTD in slices from nicotine naïve rats, suggesting that the tolerance may be associated with changes in dopaminergic signaling.

Conclusion

Nicotine produces acute and sustained effects on striatal neurotransmission and synaptic plasticity in the female rat brain, which may contribute to the establishment of persistent nicotine taking habits.

Sustained inhibitory transmission but dysfunctional dopamine D2 receptor signaling in dorsal striatal subregions following protracted abstinence from amphetamine

Behavioral sensitization to amphetamine is a complex phenomenon that engages several neurotransmitter systems and brain regions. While dysregulated signaling in the mesolimbic dopamine system repeatedly has been linked to behavioral sensitization, later research has implicated dorsal striatal circuits and GABAergic neurotransmission in contributing to behavioral transformation elicited by amphetamine. The aim of this study was thus to determine if repeated amphetamine exposure followed by abstinence would alter inhibitory neurotransmission in dorsal striatal subregions. To this end, male Wistar rats received amphetamine (2.0 mg/kg) in an intermittent manner for a total of five days. Behavioral sensitization to amphetamine was measured in locomotor-activity boxes, while neuroadaptations were recorded in the dorsolateral (DLS) and dorsomedial striatum (DMS) using ex vivo electrophysiology at different timepoints of amphetamine abstinence (2 weeks, 4-5 weeks, 10-11 weeks). Data show that repeated drug-exposure produces behavioral sensitization to the locomotor-stimulatory properties of amphetamine, which sustains for at least ten weeks. Electrophysiological recordings demonstrated a long-lasting suppression of evoked population spikes in both striatal subregions. Furthermore, following ten weeks of abstinence, the responsiveness to a dopamine D2 receptor agonist was significantly impaired in brain slices from rats previously receiving amphetamine. However, neither the frequency nor the amplitude of spontaneous inhibitory currents was affected by treatment at any of the time points analyzed. In conclusion, passive administration of amphetamine initiates long-lasting neuroadaptations in brain regions associated with goal-directed behavior and habitual performance, but these transformations do not appear to be driven by changes in GABAergic neurotransmission.

Synthesis, docking studies, and pharmacological evaluation of 2-hydroxypropyl-4-arylpiperazine derivatives as serotoninergic ligands

A new series of norbornene and exo-N-hydroxy-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboximide derivatives was prepared, and their affinities to the 5-HT_1A , 5-HT_2A , and 5-HT_2C receptors were evaluated and compared with a previously synthesized series of derivatives characterized by the same nuclei, to identify selective ligands for the subtype receptors. Arylpiperazines represent one of the most important classes of 5-HT_1A R ligands, and the research of new derivatives has been focused on the modification of one or more portions of this pharmacophore. The combination of structural elements (heterocyclic nucleus, hydroxyalkyl chain, and 4-substituted piperazine), known to be critical for the affinity to 5-HT_1A receptors, and the proper selection of substituents resulted in compounds with high specificity and affinity toward serotoninergic receptors. The most active compounds were selected for further in vivo assays to determine their functional activity. Finally, to rationalize the obtained results, molecular docking studies were performed. The results of the pharmacological studies showed that 3e, 4j, and 4n were the most active and promising derivatives for the serotonin receptor considered in this study.

Metabotropic glutamate receptor modulation of dopamine release in the nucleus accumbens shell is unaffected by phencyclidine pretreatment: In vitro assessment using fast-scan cyclic voltammetry rat brain slices

The non-competitive glutamate antagonist, phencyclidine is used in rodents to model behavioural deficits see in schizophrenia. Importantly, these deficits endure long after the cessation of short-term chronic treatment (sub-chronic), indicating that the drug treatment causes long-term changes in the physiology and/or chemistry of the brain. There is evidence that this may occur through glutamatergic modulation of mesolimbic dopamine release, perhaps involving metabotropic glutamate receptors (mGluR). This study sought to investigate the effect of sub-chronic phencyclidine pretreatment on modulation of dopamine neurotransmission by metabotropic glutamate receptors 2 and 5 (mGluR2 and mGluR5) in the nucleus accumbens shell in vitro, with the hypothesis that phencyclidine pretreatment would disrupt the mGluR-mediated modulation of dopamine release. We showed that the orthosteric mGluR2 agonist LY379268 (0.1 µM, 1 µM and 10 µM) and mGluR5 positive allosteric modulator CDPPB (1 µM and 10 µM) both attenuated potassium-evoked dopamine release, underscoring their role in modulating dopamine neurotransmission in the nucleus accumbens. Sub-chronic PCP treatment, which caused cognitive deficits measured by performance in the novel object recognition task, modelling aspects of behavioral deficits seen in schizophrenia, induced neurobiological changes that enhanced dopamine release in the nucleus accumbens, but had no effect on mGluR2 or mGluR5 mediated changes in dopamine release. Therefore it is unlikely that schizophrenia-related behavioural changes seen after sub-chronic phencyclidine pre-treatment are mediated through mGluR modulation of 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).

Blockage of acquisition and expression of morphine-induced conditioned place preference in rats due to activation of glutamate receptors type II/III in nucleus accumbens

Numerous studies have shown that glutamate in the nucleus accumbens (NAc) is an essential neurotransmitter for the extension of morphine-induced place preference. mGlu2/3 glutamate receptors in the NAc have important roles in the reward pathway. However, less is known about the role of this glutamate receptor subtype in morphine-induced conditioned place preference (CPP). In this study, we examined the effects of bilateral intra-accumbal administration of LY379268, an mGlu2/3 receptor agonist on the acquisition and expression of morphine-induced CPP in rats. Adult male Wistar rats (n=136; 220-250g) were evaluated in a CPP paradigm. Doses of LY379268 (0.3, 1 and 3μg/0.5μL saline per side) were administered into the NAc on both sides during the 3days of the conditioning (acquisition) or post-conditioning (expression) phase. The results show that bilateral intra-accumbal administration of LY379268 (0.3, 1 and 3μg) markedly decreased the acquisition of morphine-induced CPP in a dose-dependent manner. In a second series of experiments, we determined that injection of LY379268 into the NAc considerably attenuated the expression of morphine CPP only at the highest dose (3μg). Our findings suggest that activation of mGlu2/3 receptors in the NAc dose-dependently blocked both the establishment and the maintenance of morphine-induced CPP and confirmed the role of this system as a potential therapeutic target for addiction.

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.

Effects of repeated cocaine exposure on habit learning and reversal by N-acetylcysteine

Exposure to drugs of abuse can result in a loss of control over both drug- and nondrug-related actions by accelerating the transition from goal-directed to habitual control, an effect argued to reflect changes in glutamate homeostasis. Here we examined whether exposure to cocaine accelerates habit learning and used in vitro electrophysiology to investigate its effects on measures of synaptic plasticity in the dorsomedial (DMS) and dorsolateral (DLS) striatum, areas critical for actions and habits, respectively. We then administered N-acetylcysteine (NAC) in an attempt to normalize glutamate homeostasis and hence reverse the cellular and behavioral effects of cocaine exposure. Rats received daily injections of cocaine (30 mg/kg) for 6 days and were then trained to lever press for a food reward. We used outcome devaluation and whole-cell patch-clamp electrophysiology to assess the behavioral and cellular effects of cocaine exposure. We then examined the ability of NAC to reverse the effects of cocaine exposure on these measures. Cocaine treatment produced a deficit in goal-directed action, as assessed by outcome devaluation, and increased the frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) in the DMS but not in the DLS. Importantly, NAC treatment both normalized EPSC frequency and promoted goal-directed control in cocaine-treated rats. The promotion of goal-directed control has the potential to improve treatment outcomes in human cocaine addicts.

The effects of mGluR2/3 activation on acute and repeated amphetamine-induced locomotor activity in differentially reared male rats

Environmental stimuli play a key role in affecting the likelihood to abuse drugs. Environmental enrichment can reduce that likelihood. The neurotransmitter glutamate contributes to both drug reward and rearing-induced changes in the brain. The current study investigated the effects of the Group-2 metabotropic glutamate receptor (mGluR2/3) agonist, LY-379268 (0.5, 1.0 mg/kg), on acute and repeated amphetamine-induced locomotor activity in differentially reared male rats. Male Sprague-Dawley rats were randomly assigned to one of 3 environmental conditions postweaning: enriched (EC), isolated (IC), or standard (SC), where they reared for 30 days. The effect of LY-379268 on acute amphetamine-induced locomotor activity was assessed. Rats were injected with either LY-379268 (0.5, 1.0 mg/kg) or saline prior to an amphetamine (0.5 mg/kg) or saline challenge injection. Rats were also administered amphetamine (0.5 mg/kg) or saline injections prior to 5 locomotor sessions. Following a rest period of 14-15 days, the effects of repeated amphetamine exposure were evaluated using LY-379268 (0.5, 1.0 mg/kg) or saline injections 30 min prior to receiving amphetamine (0.5 mg/kg). Results showed that LY-379268 administration dose-dependently attenuated acute amphetamine-induced locomotor activity, with EC rats generally displaying less attenuation than IC or SC rats. After repeated amphetamine administrations, the ability of LY-379268 to attenuate the final expression of amphetamine-induced locomotor activity in differentially reared rats was dose-dependent. The differing effect of LY-379268 observed in EC rats suggests enrichment-induced glutamatergic alterations that may protect against sensitivity to psychostimulants.

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.

The group II metabotropic glutamate receptor agonist, LY379268, decreases methamphetamine self-administration in rats

BACKGROUND

Given the problems associated with the escalation in methamphetamine (METH) use, the identification of more effective treatment strategies is essential. Group II metabotropic glutamate receptors (mGluRs) have been suggested to be a novel therapeutic target for psychostimulant addiction. We sought to test the ability of the selective group II mGluR agonist LY379268 to reduce METH self-administration in rats.

METHODS

Rats were trained to self-administer METH on a progressive ratio (PR) schedule. Animals were then switched to fixed ratio responding and given daily extended access (6 h/day) to METH self-administration for 14 days. Rats were then re-tested on the PR schedule. The effect of LY379268 on METH-reinforced PR responding was determined before and after 14 days of extended access. To test for non-specific effects, a separate group of animals received LY379268 prior to a sucrose pellet-reinforced PR schedule.

RESULTS

Animals escalated their daily intake of METH during extended access. PR responding did not change as a function of extended access. LY379268 significantly attenuated METH reinforced responding, both before and after extended access. The degree of attenuation did not change as a function of extended access. LY379268 had no effect on sucrose pellet-reinforced responding at any dose.

CONCLUSIONS

LY379268 selectively reduced the motivation to self-administer METH. In contrast to data with other compounds, the sensitivity to the effects of LY379268 did not change following extended access to METH self-administration. Group II mGluR agonists, therefore, may represent a relatively new class of compounds for the development of pharmacotherapies for METH addiction.

The influence of ionotropic and metabotropic glutamate receptor ligands on anxiety-like effect of amphetamine withdrawal in rats

Chronic amphetamine use results in anxiety-like states after drug cessation. The aim of the study was to determine a role of ionotropic and metabotropic glutamate receptor ligands in amphetamine-evoked withdrawal anxiety in the elevated plus-maze test in rats. In our study memantine (8 and 12 mg/kg), a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist did not reduce amphetamine withdrawal anxiety. Acamprosate (NMDA and metabotropic glutamate 5 receptor (mGluR5) antagonist) at the dose 200 and 400mg/kg showed anxiolytic-like effect, thus increasing the percent of time spent in open arms and a number of open arm entries. mGluR5 selective antagonist, MTEP (3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine hydrochloride) and mGluR2/3 agonist, LY354740 (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid), caused effects similar to acamprosate at doses 1.25-5mg/kg and 2.5-5mg/kg, respectively. None of the glutamate ligands influenced locomotor activity of rats when given to the saline-treated group. Taking into account the positive correlation between amphetamine withdrawal-induced anxiety and relapse to amphetamine taking, our results suggest that modulation of mGluRs may prevent relapse to amphetamine and might pose a new direction in amphetamine abuse therapy.

Animal models of schizophrenia

Developing reliable, predictive animal models for complex psychiatric disorders, such as schizophrenia, is essential to increase our understanding of the neurobiological basis of the disorder and for the development of novel drugs with improved therapeutic efficacy. All available animal models of schizophrenia fit into four different induction categories: developmental, drug-induced, lesion or genetic manipulation, and the best characterized examples of each type are reviewed herein. Most rodent models have behavioural phenotype changes that resemble 'positive-like' symptoms of schizophrenia, probably reflecting altered mesolimbic dopamine function, but fewer models also show altered social interaction, and learning and memory impairment, analogous to negative and cognitive symptoms of schizophrenia respectively. The negative and cognitive impairments in schizophrenia are resistant to treatment with current antipsychotics, even after remission of the psychosis, which limits their therapeutic efficacy. The MATRICS initiative developed a consensus on the core cognitive deficits of schizophrenic patients, and recommended a standardized test battery to evaluate them. More recently, work has begun to identify specific rodent behavioural tasks with translational relevance to specific cognitive domains affected in schizophrenia, and where available this review focuses on reporting the effect of current and potential antipsychotics on these tasks. The review also highlights the need to develop more comprehensive animal models that more adequately replicate deficits in negative and cognitive symptoms. Increasing information on the neurochemical and structural CNS changes accompanying each model will also help assess treatments that prevent the development of schizophrenia rather than treating the symptoms, another pivotal change required to enable new more effective therapeutic strategies to be developed.

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

RATIONALE

Activation of metabotropic glutamate (mGlu) 2/3 receptors may provide a novel strategy for treating schizophrenia. This effect is thought to be mediated through dopamine-independent mechanisms because mGlu2/3-receptor agonists have no considerable affinity for dopamine receptors. These agonists, however, reduce amphetamine-induced hyperlocomotion suggesting that they influence dopamine neurotransmission.

OBJECTIVE

We evaluated whether the inhibitory effect of mGlu2/3-receptor activation on amphetamine-induced hyperlocomotion correlates with attenuated dopamine release. We also assessed whether mGlu 2/3 receptor activation has inhibitory effects on activity-dependent vesicular release of dopamine in behaving animals.

METHODS

Microdialysis was used to measure extracellular levels of dopamine in the dorsal striatum (DStr) and nucleus accumbens (NAc) of freely moving rats. The effect of the mGlu2/3-receptor agonist LY354740 on dopamine release and locomotion elicited by amphetamine, electrical stimulation of the ventral tegmental area, or L-dopa was assessed.

RESULTS

We find that the inhibitory effect of mGlu2/3 activation on amphetamine-induced hyperlocomotion correlates with an attenuated increase in dopamine release in the NAc and DStr. However, when dopamine levels were increased by electrical stimulation of dopamine neurons or by administration of the dopamine precursor L-dopa, activation of mGlu2/3 receptors had no effect on dopamine release or on behavior.

CONCLUSIONS

Activation of mGlu2/3 receptors attenuates amphetamine-induced dopamine release through a mechanism that does not affect activity dependent vesicular release, reuptake or synthesis of dopamine.

Group II metabotropic glutamate receptors (mGlu2/3) in drug addiction

Drug addiction is characterized by maladaptive decision-making and dysfunctional brain circuitry regulating motivated behaviors, resulting in loss of the behavioral flexibility needed to abstain from drug seeking. Hence, addicts face high risk of relapse even after prolonged periods of abstinence from drug use. This is thought to result from long-lasting drug-induced neuroadaptations of glutamate and dopaminergic transmission in the mesocorticolimbic and cortico-striatal circuits where group II metabotropic glutamate receptors (mGlu(2/3) receptors) are densely expressed. mGlu(2/3) receptors presynaptically control glutamate as well as dopamine release throughout the mesocorticolimbic structures involved in reward processing and drug seeking, and their function is reduced after prolonged exposure to drugs of abuse. In pre-clinical models, mGlu(2/3) receptors have been shown to regulate both reward processing and drug seeking, in part through the capacity to control release of dopamine and glutamate respectively. Specifically, mGlu(2/3) receptor agonists administered systemically or locally into certain brain structures reduce the rewarding value of commonly abused drugs and inhibit the reinstatement of drug seeking. Given the ability of mGlu(2/3) receptor agonists to compensate for and possibly reverse drug-induced neuroadaptations in mesocorticolimbic circuitry, this class of receptors emerges as a new therapeutic target for reducing relapse in drug addiction.

Combined administration of an mGlu2/3 receptor agonist and a 5-HT 2A receptor antagonist markedly attenuate the psychomotor-activating and neurochemical effects of psychostimulants

RATIONALE

It was recently reported that administration of the metabotropic glutamate 2 and 3 (mGlu2/3) receptor agonist prodrug LY2140023 to schizophrenic patients decreased positive symptoms. However, at the single, potentially suboptimal, dose that was tested, LY2140023 trended towards being inferior to olanzapine on several indices of efficacy within the Positive and Negative Syndrome Scale.

OBJECTIVES

In this study, we examined whether the antipsychotic potential of mGlu2/3 receptor agonism can be enhanced with 5-HT(2A) receptor antagonism.

MATERIALS AND METHODS

Specifically, we characterized the effects of co-administering submaximally effective doses of the 5-HT(2A) receptor antagonist M100907 (0.2 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg) on amphetamine-induced and MK-801-induced psychomotor activity in rats, an assay sensitive to antipsychotics. We also determined the effects of co-administering these two compounds on MK-801-induced dopamine and norepinephrine efflux in the nucleus accumbens (NAc).

RESULTS

At the submaximally effective doses tested, the effects of M100907 and LY379268 on amphetamine-induced and MK-801-induced psychomotor activity were significantly greater when given together than when given separately. Furthermore, coadministration of these doses of M100907 and LY379268 reduced MK-801-induced dopamine efflux in the NAc. This effect on dopamine release was not observed with the administration of either compound alone, even at higher doses that attenuated MK-801-induced psychomotor activity.

CONCLUSIONS

Our results suggest that a single compound having both mGlu2/3 receptor agonist and 5-HT(2A) receptor antagonist activity, or coadministration of two compounds selective for these receptors, could be superior in terms of efficacy and/or reduced side-effect liability relative to an mGlu2/3 receptor agonist alone.

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.

Repeated administration of amphetamine induces a shift of the prefrontal cortex and basolateral amygdala motor function

The role of the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) in the expression of behavioural locomotor sensitization to amphetamine (Amph) has been poorly studied. In the present study, we investigated how lidocaine infused in the mPFC or BLA modulated motor responses to acute and repeated (sensitization) Amph administration. We showed that reversible blockade of mPFC or BLA by lidocaine increased both locomotor and rearing responses to acute Amph, but blocked the expression of behavioural sensitization to Amph. These findings indicate that under free-lidocaine conditions repeated administration of Amph would produce a shift of mPFC and BLA motor function from an inhibitory to a facilitatory role in response to Amph. We propose that this phenomenon may be of major critical importance in the development of drug dependence.

Effects of chronic buprenorphine treatment on levels of nucleus accumbens glutamate and on the expression of cocaine-induced behavioral sensitization in rats

RATIONALE

Chronic treatment with the mu-opioid receptor agonist, buprenorphine, reduces cocaine-induced behaviors in rats with a history of cocaine self-administration. The mechanisms underlying these actions of buprenorphine remain unclear.

OBJECTIVES

The objective of this study is to investigate the effects of chronic buprenorphine treatment on cocaine-induced activity and levels of glutamate and dopamine (DA) in the nucleus accumbens (NAc) in rats that were preexposed to cocaine or drug-naïve.

MATERIALS AND METHODS

In experiment 1, basal levels of NAc glutamate were assessed using in vivo microdialysis in cocaine-naïve rats that were treated chronically with buprenorphine (3.0 mg/kg per day) via osmotic minipumps or that underwent sham surgery. In experiment 2, rats were preexposed to seven daily injections of cocaine or saline. After a 12-16-day drug-free period, extracellular levels of NAc glutamate and DA and locomotor activity were assessed simultaneously, before and after an acute injection of cocaine (15 mg/kg, intraperitoneal), in rats under sham and chronic buprenorphine (3.0 mg/kg per day) treatment.

RESULTS

Chronic buprenorphine treatment increased basal levels of glutamate in drug-naïve and cocaine-preexposed rats, blocked the expression of locomotor sensitization to cocaine, and potentiated the NAc DA response to acute cocaine in cocaine-preexposed rats.

CONCLUSIONS

These findings suggest that buprenorphine may block the expression of cocaine sensitization and other cocaine-related behaviors by increasing basal levels of glutamate in the NAc, which would serve to decrease the effectiveness of cocaine or cocaine-associated cues.

Blockade of group II metabotropic glutamate receptors produces hyper-locomotion in cocaine pre-exposed rats by interactions with dopamine receptors

It was previously reported that blockade of group II metabotropic glutamate receptors (mGluRs) produces hyper-locomotion in rats previously exposed to amphetamine, indicating that group II mGluRs are well positioned to modulate the expression of behavioral sensitization by amphetamine. The present study further examined the locomotor activating effects of specific blockade of these receptors after cocaine pre-exposures. First, rats were pre-exposed to seven daily injections of cocaine (15mg/kg, IP). When challenged the next day with an injection of either saline or the group II mGluR antagonist LY341495 (0.5, 1.0 or 2.5mg/kg, IP), they produced hyper-locomotor activity, measured by infrared beam interruptions, to LY341495 compared to saline in a dose-dependent manner. Second, rats were pre-exposed to either saline or seven daily injections of cocaine (15mg/kg, IP). Three weeks later, when they were challenged with an injection of either saline or LY341495 (1.0mg/kg, IP), only rats pre-exposed to cocaine produced hyper-locomotor activity to LY341495 compared to saline. These effects, however, were not present when dopamine D1 (SCH23390; 5 or 10microg/kg), but not D2 (eticlopride; 10 or 50microg/kg), receptor antagonist was pre-injected, indicating that this cocaine-induced hyper-locomotor activity to LY341495 may be mediated in dopamine D1 receptor-dependent manner. These results suggest that group II mGluRs may be adapted to interact with dopaminergic neuronal signaling in mediating the sensitized locomotor activity produced by repeated cocaine pre-exposures.

The preclinical properties of a novel group II metabotropic glutamate receptor agonist LY379268

Activation of group II metabotropic glutamate (mGlu2/3) receptors reduces excessive glutamate release that is often associated with neurodegenerative and psychiatric disorders. This finding encouraged the search for potent and selective agonists as potential therapeutic agents. The search led to the discovery of LY379268 {(-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid}, which is a highly potent and systemically available mGlu2/3 receptor agonist. LY379268 was effective in several animal models of stroke, epilepsy, drug abuse, schizophrenia, and pain. Suppression of motor activity is the major side effect of LY379268. Upon repeated administration tolerance develops to this side effect, while the therapeutic effects of LY379268 remain. To date, no clinical data with LY379268 are available. This review article summarizes the preclinical pharmacology of LY379268.

Environmental enrichment increases amphetamine-induced glutamate neurotransmission in the nucleus accumbens: a neurochemical study

In addition to dopamine (DA), evidence indicates that glutamatergic regulation of the mesolimbic reward pathway is involved in mediating the abuse-related effects of psychostimulants, including amphetamine. Since rats raised in an enrichment condition (EC) during development are more sensitive to the locomotor stimulant effects of acute amphetamine compared to rats raised in an impoverished condition (IC), the present study examined amphetamine-induced extracellullar glutamate and aspartate levels in the nucleus accumbens (NAcc) of EC and IC rats using in vivo microdialysis coupled with HPLC-electrochemical detection. Basal extracellular levels of glutamate or aspartate were not significantly different between EC and IC rats. Acute systemic amphetamine (0.5 or 2.0 mg/kg, sc) increased extracellular glutamate levels in NAcc of EC rats (137% or 305% of basal) and IC rats (120% or 187% of basal). Similarly, acute systemic amphetamine (0.5 or 2.0 mg/kg, sc) elevated aspartate levels in NAcc of EC rats (148% or 237% of basal) and IC rats (115% or 170% of basal). Glutamate levels were elevated by amphetamine to a greater extent in EC rats than in IC rats. Pretreatment with systemic MK-801 (0.25 mg/kg, ip), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, prevented the acute amphetamine-induced increase in extracellular glutamate and aspartate levels in NAcc. Overall, these results suggest that alterations in glutamate in the NAcc may be involved in the environment-dependent effects of amphetamine.

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.

Higher locomotor response to cocaine in female (vs. male) rats selectively bred for high (HiS) and low (LoS) saccharin intake

Rats selectively bred for high saccharin consumption (HiS) self-administer more oral ethanol and i.v. cocaine than those selectively bred for low saccharin consumption (LoS). Male and female drug-seeking-prone (HiS) and -resistant (LoS) rats were used in the present experiment to test the prediction that cocaine-induced locomotor activity and sensitization varied with sex and their selective breeding status (HiS and LoS). All rats were intermittently exposed over 2 weeks to pairs of sequential saline and cocaine injections, separated by 45 min. The first 5 pairs of injections, each separated by 2-3 days (10-12 days total), were given to examine the development of cocaine-induced locomotor activity and the development of locomotor sensitization, which was determined by comparing the effects of cocaine injection 1 with injection 6 (given 2 weeks after the 5 pairs of intermittent injections). Results indicated that after the first injection pair (saline, cocaine) the HiS and LoS groups did not differ (saline vs. cocaine) in locomotor activity; however, after cocaine injection pairs 1, 5, and 6, HiS females were more active than HiS males and LoS females. There were also significant phenotype differences (HiS>LoS) in locomotor activity after cocaine injections 5 and 6. There was a weak sensitization effect in cocaine-induced locomotor activity in HiS females after cocaine injection 5 (compared to 1); however it was not present after injection 6 or in other groups. The lack of a strong sensitization effect under these temporal and dose conditions was inconsistent with previous reports. However, the results showing HiS>LoS and females>males on cocaine-induced activity measures are consistent with several measures of cocaine-seeking behavior (acquisition, maintenance, escalation, extinction, and reinstatement), and they suggest that cocaine-induced locomotor activity and sensitization are behavioral markers of drug-seeking phenotypes.

Systemic and central amygdala injections of the mGluR2/3 agonist LY379268 attenuate the expression of incubation of sucrose craving in rats

We previously reported that systemic or central amygdala injections of the mGluR(2/3) agonist LY379268 (which decreases glutamate release) prevented enhanced cue-induced cocaine seeking in extinction tests after prolonged withdrawal (incubation of cocaine craving). Here, we report that systemic and central amygdala injections of LY379268 also prevented the enhanced cue-induced sucrose seeking in extinction tests after prolonged sucrose-free period (incubation of sucrose craving). These findings suggest that central amygdala glutamate plays an important role in the incubation of reward craving after withdrawal from both drug and non-drug rewards.

Involvement of group II metabotropic glutamate receptors in stress-induced behavioural sensitization

RATIONALE

A short session of repeated foot shocks in rats causes long-lasting sensitization of behavioural, hormonal and autonomic responses to novel stressful challenges. The behavioural sensitization can be reduced by anxiolytics and mimics aspects of stress-induced changes in patients with post-traumatic stress disorder.

OBJECTIVES

The aim of this study was to test the efficacy of a group II metabotropic glutamate receptor (mGluR) agonist and assess altered brain mGluR receptor expression in shock-sensitized rats.

MATERIALS AND METHODS

Male Wistar rats were exposed to a 15-min session with ten 6-s foot shocks (preshocked). One and 2 weeks later, rats were intraperitoneally injected with the group II metabotropic glutamate receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or vehicle, and 30 min later exposed to 5 min of 85 dB noise. For in situ hybridization with probes for mGluR1, mGluR2, mGluR3 and mGluR5, preshocked and control rats were killed under basal conditions 2 weeks after foot shocks and their brains cryosectioned.

RESULTS

APDC had no clear effect in controls, but dose-dependently reduced high immobility and increased low locomotion and rearing seen in preshocked rats to the levels of controls. mGluR3 expression was increased in the basolateral nucleus of the amygdala, and mGluR2 expression was increased in the agranular insular cortex of preshocked rats compared to controls.

CONCLUSIONS

Shock-induced behavioural sensitization in rats is reduced by acute treatment with a group II metabotropic glutamate receptor agonist. This effect may depend on the increased expression of amygdala mGluR3, which could be hypothesized as an endogenous mechanism to counteract stress-induced neuronal sensitization.

Systemic and central amygdala injections of the mGluR(2/3) agonist LY379268 attenuate the expression of incubation of cocaine craving

BACKGROUND

We and others reported time-dependent increases in cue-induced cocaine seeking after withdrawal, suggesting that craving incubates over time. Recently, we found that central amygdala extracellular signal-regulated kinases (ERK) and glutamate are involved in this incubation. Here, we further explored the role of central amygdala glutamate in the incubation of cocaine craving by determining the effect of systemic or central amygdala injections of the mGluR2/3 agonist LY379268 (which decreases glutamate release) on cue-induced cocaine seeking during early and late withdrawal.

METHODS

Rats were trained to self-administer cocaine for 10 days (6 hours/day); infusions were paired with a tone-light cue. Cocaine seeking and craving after systemic or central amygdala injections of LY379268 were then assessed in extinction tests in the presence of the cocaine-associated cues during early (day 3) or late (day 21) withdrawal.

RESULTS

Systemic (1.5 or 3 mg/kg) or central amygdala (.5 or 1.0 microg/side) injections of LY379268 attenuated enhanced extinction responding on day 21 but had no effect on lower extinction responding on day 3.

CONCLUSIONS

Results confirm our previous findings on the role of central amygdala glutamate in the incubation of cocaine craving and together with previous reports suggest that mGluR(2/3) agonists should be considered in the treatment of drug relapse.

Activation of group II metabotropic glutamate receptors in the nucleus accumbens shell attenuates context-induced relapse to heroin seeking

Using a rat relapse model, we previously reported that re-exposing rats to a drug-associated context, following extinction of operant responding in a different context, reinstates heroin seeking. In an initial pharmacological characterization, we found that the mGluR2/3 agonist LY379268, which acts centrally to reduce evoked glutamate release, attenuates context-induced reinstatement of heroin seeking when injected systemically or into the ventral tegmental area, the cell body region of the mesolimbic dopamine system. Here, we tested whether injections of LY379268 into the nucleus accumbens (NAc), a terminal region of the mesolimbic dopamine system, would also attenuate context-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin; drug infusions were paired with a discrete tone-light cue. Subsequently, lever pressing was extinguished in the presence of the discrete cue in a context that differed from the drug self-administration context in terms of visual, auditory, tactile, and circadian cues. After extinction of responding, LY379268 was injected to different groups of rats into the NAc core or shell or into the caudate-putamen, a terminal region of the nigrastriatal dopamine system. Injections of LY379268 into the NAc shell (0.3 or 1.0 microg) dose-dependently attenuated context-induced reinstatement of heroin seeking. Injections of 1.0 microg of LY379268 into the NAc core had no effect, while a higher dose (3.0 microg) decreased this reinstatement. Injections of LY379268 (3.0 microg) 1.5 mm dorsal from the NAc core into the caudate-putamen were ineffective. Results suggest an important role of glutamate transmission in the NAc shell in context-induced reinstatement of heroin seeking.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Blockade of group II metabotropic glutamate receptors in the nucleus accumbens produces hyperlocomotion in rats previously exposed to amphetamine

The neurotransmitter glutamate is known to participate in both the induction and expression of locomotor sensitization by psychostimulant drugs like amphetamine. Previously, it was reported that subtype nonselective blockade of metabotropic glutamate receptors (mGluRs) in the nucleus accumbens (NAcc) produces hyperlocomotion in rats previously exposed to amphetamine. The present experiments examined whether group II mGluRs may contribute to this effect. Rats in different groups were administered five injections of either saline or amphetamine (1.0 mg/kg, i.p.), one injection given every third day. Two weeks later, they were tested for 2 h following an injection of either saline or the group II mGluR antagonist LY341495. In one experiment, test injections were administered systemically (saline or LY341495, 1.0 mg/kg, i.p.). Rats previously exposed to amphetamine showed a greater locomotor response to LY341495 on the test compared to controls previously exposed to saline. This hyperlocomotor response was absent in rats tested with a combination of LY341495 and the group II mGluR agonist LY379268 (1.0 mg/kg, i.p.). In a second experiment, different rats were tested following microinjections into the NAcc (saline or LY341495, 0.1, 10 or 100 microg/0.5 microl/side). Again, rats previously exposed to amphetamine showed a greater dose-dependent locomotor response to LY341495 on the test relative to saline-exposed controls. Locomotor activity in saline-exposed rats challenged with LY341495 did not differ from that observed in rats previously exposed and tested with saline in either experiment. These results indicate that group II mGluRs, particularly those found in the NAcc, are well positioned to modulate the expression of locomotor sensitization by amphetamine.

Pharmacological stimulation of group ii metabotropic glutamate receptors reduces cocaine self-administration and cocaine-induced reinstatement of drug seeking in squirrel monkeys

Group II metabotropic glutamate receptors (mGluRs) have been implicated in regulating the psychopharmacologic effects of cocaine and other drugs of abuse. The present study investigated the interactions between the group II mGluR agonist LY379268 [(-)-2-oxa-4-aminobicyclo [3.1.0] hexane-4,6-dicarboxylate] and cocaine in squirrel monkeys whose operant behavior was maintained under a second order schedule of i.v. cocaine self-administration with or without presentations of a cocaine-paired visual stimulus, extinguished and subsequently reinstated by priming injections of cocaine with or without presentations of a cocaine-paired stimulus, and controlled by cocaine trained as a discriminative stimulus. Antagonism studies with the group II mGluR antagonist LY341495 [2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl) propanoic acid] investigated the extent to which the cocaine-modulating effects of LY379268 could be reversed by blocking group II mGluRs. Quantitative observational studies investigated the effects of LY379268 and LY341495 on species-typical behaviors, balance, and muscle resistance. Pretreatment with LY379268 reduced cocaine self-administration and cocaine-induced reinstatement of drug seeking in a dose-dependent, LY341495-reversible manner. Significant effects of LY379268 were observed both in the presence and absence of the cocaine-paired stimulus. LY379268 did not alter the discriminative stimulus effects of cocaine, nor did it markedly affect observed behavior, with the exception of an increase in visual scanning. Emesis frequently was observed after the highest dose of LY379268 (1.0 mg/kg). The results suggest that LY379268, by stimulating group II mGluRs, can attenuate the reinforcing and priming effects of cocaine at doses that do not alter its perceptibility or markedly suppress other behaviors.

Novel pharmacotherapeutic targets for the management of drug addiction

Despite individual variation in the liability to the abuse of psychoactive substances, there is substantial commonality shared by drugs of abuse. The knowledge of these common mechanisms together with the continued elucidation of the neurobiological underpinnings of withdrawal symptoms, drug intake, craving, relapse, and co-morbid psychiatric associations are critically important for the development of new therapeutic strategies. The present review will focus on recent advances in the development of innovative pharmacotherapeutic agents, which should promote higher efficacy (abstinence, prevention of relapse, long-term recovery) and patient compliance, as well as improved safety profiles.

A role of ventral tegmental area glutamate in contextual cue-induced relapse to heroin seeking

The environmental context previously associated with opiate use plays an important role in human relapse, but the neuronal mechanisms involved in context-induced drug relapse are not known. Using a rat relapse model, we determined the effect of a group II metabotropic glutamate receptor agonist [LY379268 ((-)-2-oxa-4-aminobicylco hexane-4,6-dicarboxylic acid)] on contextual cue-induced reinstatement of heroin seeking. LY379268, which acts centrally to reduce evoked glutamate release, was injected systemically or directly into the ventral tegmental area (VTA), a brain area involved in opiate reward and conditioned drug effects. Rats were trained to self-administer intravenous heroin for 12 d; drug infusions were paired with a discrete tone-light cue. Subsequently, lever pressing was extinguished in the presence of the discrete cue in a context that differed from the drug self-administration context in terms of visual, auditory, tactile, and circadian cues. After extinction of lever responding, LY379268 was injected systemically or into the VTA, and nonreinforced responding was determined in the extinction context or the drug context. Exposure to the heroin-associated context induced robust reinstatement of drug seeking, and this effect was attenuated by systemic or intra-VTA injections of LY379268. Results indicate that glutamate transmission in the VTA plays an important role in contextual cue-induced relapse to heroin seeking.

Metabotropic glutamate 2 receptor potentiators: receptor modulation, frequency-dependent synaptic activity, and efficacy in preclinical anxiety and psychosis model(s)

RATIONALE

To increase subtype selectivity and provide a novel means to alter receptor function, we discovered and characterization potentiators for the metabotropic glutamate 2 receptor (mGlu2).

METHODS AND RESULTS

A class of 3-pyridylmethylsulfonamides (e.g., 3-MPPTS; 2,2,2-trifluoro-N-[3-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide) were found to be potent, subtype-selective potentiators of human and rat mGlu2. The sulfonamides increased agonist potency in functional assays but did not displace orthosteric radiolabeled antagonist or agonist binding to cloned mGlu2 receptors. Rather, the modulators increased the affinity of most of the orthosteric agonists including glutamate, DCG-IV (2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine), and LY354740 (1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicaroxylate monohydrate). In striatal brain slices, LY354740 inhibited evoked excitatory postsynaptic potentials (EPSPs) equally well following either a low- (0.06 Hz) or high (4 Hz)-frequency stimulation of corticostriatal afferents. In contrast, the mGlu2 potentiator cyPPTS (2,2,2-trifluoro-N-[3-(cyclopentyloxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide) inhibited striatal EPSPs only at higher frequencies of stimulation (2 and 4 Hz). Several sulfonamides including 4-MPPTS, 4-APPES (N-[4-(4-carboxamidophenoxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide hydrochloride monohydrate) and/or CBiPES N-[4'-cyano-biphenyl-3-yl)-N-(3-pyridinylmethyl)-ethanesulfonamide hydrochloride) were tested in mGlu2/3 agonist-sensitive rodent model(s) of anxiety and psychosis. As seen with LY354740, both 4-MPPTS and 4-APPES were efficacious in a rat fear-potentiated startle paradigm. Likewise in mice, CBiPES attenuated a stress-induced hyperthermia and PCP-induced hyperlocomotor activity. Furthermore, CBiPES mediated alteration in PCP-induced hyperlocomotor activity was sensitive to mGlu2/3 antagonist pretreatment.

CONCLUSIONS

Taken together, the data indicate mGlu2 receptor potentiators have a unique use-dependent effect on presynaptic glutamate release, and show efficacy in several mGlu2/3-sensitive animal models of psychiatric disorders.

Activation of group II mGlu receptors blocks the enhanced drug taking induced by previous exposure to amphetamine

Repeated exposure to amphetamine (AMPH) leads to the development of behavioural sensitization that can be demonstrated in rats as enhanced locomotor responding to and self-administration of the drug. Glutamate systems are known to participate in the induction and expression of sensitization by psychostimulants. Group II metabotropic glutamate receptors (mGluRs), because they negatively regulate both vesicular and nonvesicular glutamate release, are thus well positioned to gate its expression. Here we report that the expression of locomotor sensitization by AMPH is completely prevented by a systemic injection of the selective group II mGluR agonist LY379268 at a dose that produced no effects when administered alone. The activation of group II mGluRs in AMPH-sensitized rats also reduced the enhanced overflow of both dopamine and glutamate normally observed in the nucleus accumbens, a brain region critical for the generation of locomotor and drug self-administration behaviours. To directly determine the effect of group II mGluR activation on enhanced drug self-administration, AMPH-sensitized rats were allowed to self-administer a mixture of LY379268 and AMPH. These rats continued to self-administer but did not exhibit the enhanced work output and drug intake observed in AMPH-sensitized rats self-administering AMPH alone. Thus, activating group II mGluRs prevents the expression of different manifestations of AMPH sensitization including enhanced self-administration of the drug. These receptors may represent a potentially important target for therapeutic intervention directed at drugs of abuse.

Enhanced cocaine responsiveness and impaired motor coordination in metabotropic glutamate receptor subtype 2 knockout mice

Extensive pharmacological studies have recently emerged indicating that group 2 metabotropic glutamate receptors (mGluRs) comprising mGluR2 and mGluR3 subtypes are associated with several neurological and psychiatric disorders. mGluR2 is widely distributed both presynaptically and postsynaptically in a variety of neuronal cells, but the physiological role of mGluR2 in brain function is poorly understood. This investigation involves a comprehensive behavioral analysis of mGluR2-/- knockout (KO) mice to explore the physiological role of mGluR2 in brain function. Although, under general observation, mGluR2-/- KO mice appeared to have no behavioral abnormalities, they exhibited several lines of behavioral alterations in the enforcing and defined behavioral tests. They showed a significant increase in locomotor sensitization and conditioned place preference in association with repeated cocaine administration, indicating that mGluR2 contributes to behavioral responses implicated in reinforcement and addiction of cocaine. Upon in vivo microdialysis analysis after cocaine administration, not only did extracellular levels of dopamine increase but also the response pattern of glutamate release markedly changed in the nucleus accumbens of mGluR2-/- KO mice. The mGluR2-/- KO mice also showed significant impairment in motor coordination in the accelerating rota-rod test and exhibited hyperlocomotion in novel environmental and stressful conditions, when assessed by the open-field and forced-swim tests. These results indicate that the inhibitory mGluR2 plays a pivotal role in synaptic regulation of glutamatergic transmission in the neural network.

Preferential effects of the metabotropic glutamate 2/3 receptor agonist LY379268 on conditioned reinstatement versus primary reinforcement: comparison between cocaine and a potent conventional reinforcer

Metabotropic glutamate receptors (mGluRs) have been implicated in regulating anxiety, stress responses, and the neurobehavioral effects of psychostimulants. The present study sought to determine whether group II mGluR activation by the potent mGlu2/3 receptor agonist, (-)-2-oxa-4-aminobicylco hexane-4,6-dicarboxylic acid (LY379268), antagonizes reinstatement of cocaine-seeking induced by cocaine-related stimuli and whether this effect extends to behavior induced by stimuli conditioned to a potent conventional reinforcer, sweetened condensed milk (SCM). Also, we tested whether the suppressant effects of LY379268 on conditioned reinstatement extend to the primary reinforcing effects of cocaine or SCM. Rats were trained to associate discriminative stimuli (S(D)) with the availability of cocaine or SCM versus non-reward and then subjected to repeated extinction sessions during which the respective reinforcers and S(D) were withheld. Subsequent reexposure to the cocaine or SCM S(D), but not the non-reward S(D), produced recovery of responding at the previously active lever. LY379268 (0.3-3.0 mg/kg, s.c.) dose-dependently attenuated recovery of cocaine seeking but reduced conditioned reinstatement by the SCM S(D) only at the highest dose. LY379268 did not alter responding reinforced directly by SCM, and only the highest LY379268 dose reduced cocaine self-administration. The results suggest that the effects of LY379268 are selective for behavior maintained by cocaine as opposed to palatable conventional reinforcers. More importantly, the results show that LY379268 suppresses behavior motivated by stimuli conditioned to cocaine or SCM more effectively than consummatory behavior maintained by the unconditioned effects of these substances. As such, the results identify group II mGluRs as a pharmacotherapeutic target for craving and relapse prevention associated with cocaine cue exposure.

The ups and downs of addiction: role of metabotropic glutamate receptors

Drug addiction is characterized by drug-induced positive affect, followed by withdrawal-associated negative affect. Such drug-induced positive and negative affective states provide crucial sources of motivation that drive compulsive drug consumption. Metabotropic glutamate (mGlu) receptors, which are responsible for slow glutamate-mediated neurotransmission, are located throughout limbic and cortical brain regions that are implicated in drug addiction. Emerging evidence indicates that mGlu receptors regulate many behavioral actions of addictive drugs. In particular, group I mGlu receptors play an important role in regulating the reinforcing effects of drugs of abuse. Furthermore, group II mGlu receptors have been implicated in the synaptic adaptations that occur in response to chronic drug exposure and contribute to the aversive behavioral syndrome observed during withdrawal. These findings increase our understanding of the pathological processes that are associated with the development of drug addiction, and might ultimately lead to new therapies for the treatment of this disorder.

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.