Amphetamine-induced glutamate efflux in the rat ventral tegmental area is prevented by MK-801, SCH 23390, and ibotenic acid lesions of the prefrontal cortex

Troriluzole inhibits methamphetamine place preference in rats and normalizes methamphetamine-evoked glutamate carboxypeptidase II (GCPII) protein levels in the mesolimbic pathway

Riluzole, approved to manage amyotrophic lateral sclerosis, is mechanistically unique among glutamate-based therapeutics because it reduces glutamate transmission through a dual mechanism (i.e., reduces glutamate release and enhances glutamate reuptake). The profile of riluzole is favorable for normalizing glutamatergic dysregulation that perpetuates methamphetamine (METH) dependence, but pharmacokinetic and metabolic liabilities hinder repurposing. To mitigate these limitations, we synthesized troriluzole (TRLZ), a third-generation prodrug of riluzole, and tested the hypothesis that TRLZ inhibits METH hyperlocomotion and conditioned place preference (CPP) and normalizes METH-induced changes in mesolimbic glutamate biomarkers. TRLZ (8, 16 mg/kg) reduced hyperlocomotion caused by METH (1 mg/kg) without affecting spontaneous activity. TRLZ (1, 4, 8, 16 mg/kg) administered during METH conditioning (0.5 mg/kg x 4 d) inhibited development of METH place preference, and TRLZ (16 mg/kg) administered after METH conditioning reduced expression of CPP. In rats with established METH place preference, TRLZ (16 mg/kg) accelerated extinction of CPP. In cellular studies, chronic METH enhanced mRNA levels of glutamate carboxypeptidase II (GCPII) in the ventral tegmental area (VTA) and prefrontal cortex (PFC). Repeated METH also caused enhancement of GCPII protein levels in the VTA that was prevented by TRLZ (16 mg/kg). TRLZ (16 mg/kg) administered during chronic METH did not affect brain or plasma levels of METH. These results indicate that TRLZ, already in clinical trials for cerebellar ataxia, reduces development, expression and maintenance of METH CPP. Moreover, normalization of METH-induced GCPII levels in mesolimbic substrates by TRLZ points toward studying GCPII as a therapeutic target of TRLZ.

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

Enhanced psychostimulant response, but not social avoidance, depends on GluA1 AMPA receptors in VTA dopamine neurons following intermittent social defeat stress in rats

Evidence from both human and animal studies demonstrates the importance of social stress in the development of addiction‐related behaviour. In rats, intermittent social defeat stress causes long‐lasting psychostimulant cross‐sensitization. Our recent data reveal heightened expression of AMPA receptor (AMPAR) GluA1 subunit in rat ventral tegmental area (VTA), which occurs concurrently with social stress‐induced amphetamine (AMPH) cross‐sensitization. In addition, social stress in rats induced social avoidance behaviour. The present study evaluated the effects of intermittent social defeat stress on GluA1 expression in VTA dopamine (DA) neurons, then utilized Cre‐dependent virus‐mediated gene transfer to determine the functional role of homomeric GluA1‐AMPARs in these neurons. Social defeat stress exposure induced GluA1 expression in VTA DA neurons, as demonstrated by a greater density of GluA1/tyrosine hydroxylase (TH) double‐labelling in VTA neurons in stressed rats. Additionally, functional inactivation of VTA GluA1 AMPARs in DA neurons prevented stress‐induced cross‐sensitization, or augmented locomotor response to low dose AMPH challenge (1.0 mg/kg, i.p.), but had no effect on social stress‐induced social avoidance behaviour. Furthermore, wild‐type overexpression of GluA1 in VTA DA neurons had the opposite effect; locomotor‐activating effects of AMPH were significantly augmented, even in the absence of stress. Taken together, these results suggest that stress‐induced GluA1 expression in VTA DA neurons is necessary for psychostimulant cross‐sensitization, but not for social avoidance. This differential effect suggests that different neural pathways are implicated in these behaviours. These findings could lead to novel pharmacotherapies to help prevent stress‐induced susceptibility to substance abuse.

Interference with AMPA receptor endocytosis: effects on behavioural and neurochemical correlates of amphetamine sensitization in male rats

BACKGROUND

Behavioural sensitization has been linked to drug craving in both clinical and preclinical studies of addiction. Increased motor activity is accompanied by enhanced dopamine (DA) release, particularly in the nucleus accumbens (NAcc). The neural bases of sensitization are linked to alterations in synaptic connections that also underlie learning and memory. The present study uses an "interference" peptide, Tat-GluA2(3Y), that blocks long-term depression (LTD) at glutamatergic synapses by disrupting the endocytosis of α- amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs), to explore the role of this form of synaptic plasticity in the induction and maintenance of sensitization.

METHODS

Rats were given 5 injections of d-amphetamine (d-AMPH, 1.0 mg/kg, intraperitoneal) every second day. Tat-GluA2(3Y), was administered by 2 different routes (intravenously and intracerebrally to the ventral tegmental area [VTA] or to the NAcc) before each injection of d-AMPH. After a 14-day drug-free period, expression of behavioural sensitization was evoked by a challenge injection of d-AMPH (0.5 mg/kg, intraperitoneal). Dopamine efflux in the NAcc was measured by high-pressure liquid chromatography with electrochemical detection analyses of brain dialysates on days 1, 9 and 24 of the intravenous peptide experiment.

RESULTS

Systemic administration of Tat-GluA2(3Y) during the induction phase blocked maintenance of behavioural sensitization and attenuated the maintenance of neurochemical sensitization. Intra-VTA infusion of Tat-GluA2(3Y) before each administration of d-AMPH did not affect induction, but inhibited maintenance and subsequent expression of sensitization, whereas intra-NAcc infusion of the peptide did not affect induction or maintenance of sensitization.

LIMITATIONS

The relevance of behavioural sensitization in rodents is related to the development of craving and does not provide direct measures of drug reinforcement.

CONCLUSION

These findings confirm that drug-induced neuroplasticity is labile and may be subject to disruption at a time when long-lasting associations between drug reward and contextual stimuli are formed. Furthermore, the unique ability of Tat-GluA2(3Y) to block maintenance of behavioural sensitization implicates LTD in the consolidation of essential associative memories. Tat-GluA2(3Y) has the unique ability to disrupt functional neuroadaptations triggered by repeated psychostimulant exposure and therefore may protect against the development of craving and drug seeking behaviours.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

Interaction between the mGlu receptors 5 antagonist, MPEP, and amphetamine on memory and motor functions in mice

RATIONALE

Metabotropic glutamate mGlu receptors 5 (mGluR5) receptors are abundant in corticolimbic circuitry where they modulate glutamate and dopamine signal transduction.

OBJECTIVES

In this study, we explored the hypothesis that mGluR5 antagonist, (2-methyl-6-(phenylethynyl)pyridine hydrochloride) (MPEP), facilitates dopamine-dependent effects on memory and motor functions.

METHODS

To this aim, we examined the effects of different doses (from 0 to 24 mg/kg) of the mGluR5 antagonist, MPEP, on the modulation of amphetamine-dependent behaviors, namely passive avoidance, locomotor activity, and rotation behavior in intact and dopamine-depleted CD1 male mice.

RESULTS

We demonstrated that a low dose (3 mg/kg) of MPEP, which is void of behavioral effects on its own, facilitates amphetamine-induced effects independently on the behavior measured both in naïve and in dopamine-lesioned mice; this synergistic effect is lost when higher doses of MPEP are used.

CONCLUSION

The results are discussed in terms of possible balance between dopamine and glutamate activity in regulating the proper fine tuning of information processing.

Impact of mGluR5 during amphetamine-induced hyperactivity and conditioned hyperactivity in differentially reared rats

RATIONALE

3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP) is a metabotropic glutamate receptor 5 (mGluR5) antagonist that may alter drug sensitivity in differentially reared rats due to its involvement in the psychostimulant reward pathway and plasticity.

OBJECTIVES

The purpose of this study was to assess the effects of MTEP on acute amphetamine-induced hyperactivity, conditioned hyperactivity, and sensitization.

METHODS

Rats were reared in an enriched (EC), isolated (IC), or standard (SC) condition after which rats were either administered MTEP (1.0 mg/kg, ip) or saline prior to an acute (0.5 or 1.0 mg/kg, sc) or repeated (0.3 mg/kg, sc) amphetamine exposure. Rats undergoing repeated amphetamine exposure were administered MTEP prior to conditioned hyperactivity and sensitization tests.

RESULTS

EC and SC rats administered with MTEP prior to acute amphetamine demonstrated attenuated amphetamine-induced locomotor activity compared to controls, while IC rats administered MTEP following repeated amphetamine exposure demonstrated attenuated amphetamine-induced locomotor activity. Interestingly, MTEP treatment only altered conditioned hyperactivity in EC rats, as MTEP pretreatment resulted in conditioned hyperactivity in EC rats while conditioned hyperactivity was not observed in EC rats pretreated with saline.

CONCLUSIONS

Glutamatergic pathways are altered during differential rearing, which differentially alters the role of mGluR5 in EC, IC, and SC rats when administered psychostimulant acutely versus repeatedly. These findings suggest that differential rearing alters glutamatergic function, which reduces sensitivity to psychostimulants.

Repeated exposure to cocaine alters medial prefrontal cortex dopamine D₂-like receptor modulation of glutamate and dopamine neurotransmission within the mesocorticolimbic system

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Previous studies have demonstrated that sensitization to cocaine is associated with a decrease in dopamine D₂ receptor function in the medial prefrontal cortex. The present report tested the hypothesis that reduced medial prefrontal cortex D₂ receptor function as a result of repeated cocaine exposure results in augmented excitatory transmission to the nucleus accumbens and ventral tegmental area, possibly as a partial result of enhanced inhibition of local dopamine release. Dual probe microdialysis experiments were conducted in male Sprague-Dawley rats 1, 7 or 30 days following the last of four daily injections of saline (1.0 mL/kg) or cocaine (15 mg/kg). Infusion of quinpirole (0.01, 1.0 and 100 μM), a D₂-like receptor agonist, into the medial prefrontal cortex produced a dose-dependent decrease in cortical, nucleus accumbens and ventral tegmental area extracellular glutamate levels in control but not sensitized animals. Quinpirole also reduced basal dopamine levels in the medial prefrontal cortex in sensitized animals following 1 day of withdrawal from cocaine. Following 30 days of withdrawal, quinpirole also reduced dopamine levels in sensitized animals relative to saline controls, but not relative to baseline levels. These findings indicate that the expression of sensitization to cocaine is associated with altered modulation of mesocorticolimbic glutamatergic transmission at the level of the medial prefrontal cortex.

Association study of serine racemase gene with methamphetamine psychosis

Experimental studies have demonstrated that not only dopaminergic signaling but also glutamatergic/NMDA receptor signaling play indispensable roles in the development of methamphetamine psychosis. Our recent genetic studies provided evidence that genetic variants of glutamate-related genes such as DTNBP1, GLYT1, and G72, which are involved in glutamate release and regulation of co-agonists for NMDA receptors, conferred susceptibility to methamphetamine psychosis. Serine racemase converts l-serine to d-serine, which is an endogenous co-agonist for NMDA receptors. Three single nucleotide polymorphisms (SNPs) in the promoter region of the serine racemase gene (SRR), rs224770, rs3760229, and rs408067, were proven to affect the transcription activity of SRR. Therefore, we examined these SNPs in 225 patients with methamphetamine psychosis and 291 age- and sex-matched controls. There was no significant association between methamphetamine psychosis and any SNP examined or between the disorder and haplotypes comprising the three SNPs. However, rs408067 was significantly associated with the prognosis for methamphetamine psychosis and multi-substance abuse status. The patients with C-positive genotypes (CC or CG) of rs408067 showed better prognosis of psychosis after therapy and less abuse of multiple substances than the patients with GG genotypes. Because the C allele of rs408067 reduces the expression of SRR, a lower d-serine level or reduced NMDA receptor activation may affect the prognosis of methamphetamine psychosis and multiple substance abuse. Our sample size is, however, not large enough to eliminate the possibility of a type I error, our findings must be confirmed by replicate studies with larger samples.

Drug wanting: behavioral sensitization and relapse to drug-seeking behavior

Repeated exposure to drugs of abuse enhances the motor-stimulant response to these drugs, a phenomenon termed behavioral sensitization. Animals that are extinguished from self-administration training readily relapse to drug, conditioned cue, or stress priming. The involvement of sensitization in reinstated drug-seeking behavior remains controversial. This review describes sensitization and reinstated drug seeking as behavioral events, and the neural circuitry, neurochemistry, and neuropharmacology underlying both behavioral models will be described, compared, and contrasted. It seems that although sensitization and reinstatement involve overlapping circuitry and neurotransmitter and receptor systems, the role of sensitization in reinstatement remains ill-defined. Nevertheless, it is argued that sensitization remains a useful model for determining the neural basis of addiction, and an example is provided in which data from sensitization studies led to potential pharmacotherapies that have been tested in animal models of relapse and in human addicts.

Antagonism at serotonin 5-HT(2A) receptors modulates functional activity of frontohippocampal circuit

RATIONALE

Several second-generation antipsychotics are characterised by a significant antagonistic effect at serotonin 5-HT(2A) receptors (5-HT(2A)R), a feature that has been associated with lower incidence of extra-pyramidal symptoms and a putative amelioration of positive and negative symptoms experienced by schizophrenic patients. However, the neurofunctional substrate of 5-HT(2A) antagonism and its exact contribution to the complex pharmacological profile of these drugs remain to be elucidated.

OBJECTIVES

Here, we used pharmacological magnetic resonance imaging to map the modulatory effects of the selective 5-HT(2A)R antagonist Ml00907 on the spatiotemporal patterns of brain activity elicited by acute phencyclidine (PCP) challenge in the rat. PCP is a non-competitive NMDA receptor antagonist that induces dysregulation of corticolimbic glutamatergic neurotransmission and produces cognitive impairment and psychotic-like symptoms reminiscent of those observed in schizophrenia.

RESULTS

Pre-administration of M100907 produced focal and region-dependent attenuation of PCP-induced response in frontoseptohippocampal areas. As early studies highlighted a permissive role of 5-HT(2A)R on frontal dopamine release, the role of post-synaptic dopamine D(1) receptors on PCP-induced response was examined by using the potent antagonist SCH23390. Interestingly, SCH23390 did not affect PCP's response in any of the regions examined. This finding rules out a significant contribution of dopamine in the functional changes mapped and, indirectly, the inhibitory effect of M100907, in favour of a glutamatergic origin.

CONCLUSIONS

Our data expand recent evidence suggesting a key role of 5-HT(2A)R in modulating glutamate-mediated cognitive performance in the prefrontal cortex and highlight the whole frontoseptohippocampal circuit as a key functional substrate of 5-HT(2A)R antagonism in normal and disease states.

Neuroplasticity as a target for the pharmacotherapy of anxiety disorders, mood disorders, and schizophrenia

Current treatments for psychiatric disorders were developed with the aim of providing symptomatic relief rather than reversing underlying abnormalities in neuroplasticity or neurodevelopment that might contribute to psychiatric disorders. This review considers the possibility that psychiatric treatments might be developed that target neuroplasticity deficits or that manipulate neuroplasticity in novel ways. These treatments might not provide direct symptomatic relief. However, they might complement or enhance current pharmacotherapies and psychotherapies aimed at the prevention and treatment of psychiatric disorders. In considering neuroplasticity as a target for the treatment of psychiatric disorders, we build on exciting new findings in the areas of anxiety disorders, mood disorders, and schizophrenia.

Unique antipsychotic activities of the selective metabotropic glutamate receptor 1 allosteric antagonist 2-cyclopropyl-5-[1-(2-fluoro-3-pyridinyl)-5-methyl-1H-1,2,3-triazol-4-yl]-2,3-dihydro-1H-isoindol-1-one

A newly discovered metabotropic glutamate receptor (mGluR) 1 allosteric antagonist, 2-cyclopropyl-5-[1-(2-fluoro-3-pyridinyl)-5-methyl-1H-1,2,3-triazol-4-yl]-2,3-dihydro-1H-isoindol-1-one (CFMTI), was tested both in vitro and in vivo for its pharmacological effects. CFMTI demonstrated potent and selective antagonistic activity on mGluR1 in vitro and in vivo after oral administration. CFMTI inhibited L-glutamate-induced intracellular Ca(2+) mobilization in Chinese hamster ovary cells expressing human and rat mGluR1a, with IC(50) values of 2.6 and 2.3 nM, respectively. The selectivity of CFMTI to mGluR1 over mGluR5 was >2000-fold, and CFMTI at 10 microM showed no agonistic or antagonistic activities toward other mGluR subtypes and other receptors. It antagonized face-washing behavior in mice induced by (S)-3,5-dihidroxyphenylglycine at a dose range of 3 to 30 mg/kg, for which receptor occupancy was 73 to 94%. As with the classical neuroleptic haloperidol and an atypical antipsychotic, clozapine, orally administered CFMTI reduced methamphetamine-induced hyperlocomotion and disruption of prepulse inhibition (PPI) at the same dose range as required to antagonize the face-washing behavior. CFMTI and clozapine improved ketamine-induced hyperlocomotion, PPI disruption and (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801)-induced social withdrawal without any cataleptogenic activities, whereas haloperidol only improved ketamine-induced hyperlocomotion. CFMTI, unlike clozapine, caused neither hypolocomotion nor motor incoordination at therapeutic doses. In c-fos expression studies, CFMTI and clozapine increased the number of fos-positive neurons in the nucleus accumbens and medial prefrontal cortex but not in the dorsolateral striatum. These results suggest that the antipsychotic activities of mGluR1 antagonists are more similar to those of atypical antipsychotics than those of typical antipsychotics.

Neonatal status epilepticus alters prefrontal-striatal circuitry and enhances methamphetamine-induced behavioral sensitization in adolescence

Neonatal seizures may alter the developing neurocircuitry and cause behavioral abnormalities in adulthood. We found that rats previously subjected to lithium-pilocarpine (LiPC)-induced neonatal status epilepticus (NeoSE) exhibited enhanced behavioral sensitization to methamphetamine (MA) in adolescence. Neurochemically, dopamine (DA) and metabolites were markedly decreased in prefrontal cortex (PFC) and insignificantly changed in striatum by NeoSE, but were increased in both PFC and striatum by NeoSE+MA. Glutamate levels were increased in both PFC and striatum in the NeoSE+MA group. DA turnover, an index of utilization and activity, was increased by NeoSE but reversed by MA in PFC. Gene expression of the regulator of G-protein signaling 4 (RGS4) was downregulated in PFC and striatum by NeoSE and further suppressed by MA. These findings suggest NeoSE affects both dopaminergic and glutamatergic systems in the prefrontal-striatal circuitry that manifests as enhanced behavioral sensitization to MA in adolescence.

Pharmacological effects of the metabotropic glutamate receptor 1 antagonist compared with those of the metabotropic glutamate receptor 5 antagonist and metabotropic glutamate receptor 2/3 agonist in rodents: detailed investigations with a selective allosteric metabotropic glutamate receptor 1 antagonist, FTIDC [4-[1-(2-fluoropyridine-3-yl)-5-methyl-1H-1,2,3-triazol-4-yl]-N-isopropyl-N-methyl-3,6-dihydropyridine-1(2H)-carboxamide]

The functional roles of metabotropic glutamate receptor (mGluR) 1 in integrative brain functions were investigated using a potent and selective mGluR1 allosteric antagonist, FTIDC [4-[1-(2-fluoropyridine-3-yl)-5-methyl-1H-1,2,3-triazol-4-yl]-N-isopropyl-N-methyl-3,6-dihydropyridine-1(2H)-carboxamide], in comparison with the mGluR5 allosteric antagonist and the mGluR2/3 orthosteric agonist in rodents. FTIDC reduced maternal separation-induced ultrasonic vocalization and stress-induced hyperthermia without affecting behaviors in the elevated plus maze. An mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and an mGluR2/3 agonist, LY379268 [(1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid], showed anxiolytic activities in these models, suggesting involvement of postsynaptic mGluR1 in stress-related responses comparable with mGluR5 and mGluR2/3. Analgesic effects of FTIDC were seen in the formalin test but not in the tail immersion test. FTIDC selectively blocked methamphetamine-induced hyperlocomotion and disruption of prepulse inhibition, whereas MPEP and LY379268 did not alter those behaviors, suggesting that pharmacological blockade of mGluR1 could result in antipsychotic-like effects. FTIDC did not elicit catalepsy or impair motor functions at 10 times higher dose than doses showing antipsychotic-like action. In conclusion, blockade of mGluR1 showed antipsychotic-like effects without impairing motor functions, whereas blockade of mGluR5 and activation of mGluR2/3 did not display such activities.

Cellular and molecular mechanisms involved in the neurotoxicity of opioid and psychostimulant drugs

Substance abuse and addiction are the most costly of all the neuropsychiatric disorders. In the last decades, much progress has been achieved in understanding the effects of the drugs of abuse in the brain. However, efficient treatments that prevent relapse have not been developed. Drug addiction is now considered a brain disease, because the abuse of drugs affects several brain functions. Neurological impairments observed in drug addicts may reflect drug-induced neuronal dysfunction and neurotoxicity. The drugs of abuse directly or indirectly affect neurotransmitter systems, particularly dopaminergic and glutamatergic neurons. This review explores the literature reporting cellular and molecular alterations reflecting the cytotoxicity induced by amphetamines, cocaine and opiates in neuronal systems. The neurotoxic effects of drugs of abuse are often associated with oxidative stress, mitochondrial dysfunction, apoptosis and inhibition of neurogenesis, among other mechanisms. Understanding the mechanisms that underlie brain dysfunction observed in drug-addicted individuals may contribute to improve the treatment of drug addiction, which may have social and economic consequences.

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.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Regulation of netrin-1 receptors by amphetamine in the adult brain

Netrin-1 is a guidance cue molecule fundamental to the organization of neuronal connectivity during development. Netrin-1 and its receptors, deleted in colorectal cancer (DCC) and UNC-5 homologues (UNC-5), continue to be expressed in the adult brain, although neither their function nor the kinds of events that activate their expression are known. Two lines of evidence suggest a role for netrin-1 in amphetamine-induced dopamine plasticity in the adult. First, DCC is highly expressed by adult dopamine neurons. Second, adult mice with reduced DCC levels do not develop amphetamine-induced behavioral sensitization. To explore the role of netrin-1 in amphetamine-induced plasticity, we examined the effects of sensitizing treatment regimens of amphetamine on DCC and/or UNC-5 protein expression in the adult rat. These treatments produced striking and enduring increases in DCC and UNC-5 expression in the cell body, but not terminal regions, of the mesocorticolimbic dopamine system. Notably, neuroadaptations in the cell body region of mesocorticolimbic dopamine neurons underlie the development of sensitization to the effects of amphetamine. Furthermore, these localized amphetamine-induced changes were prevented by co-treatment with an N-methyl-d-aspartate receptor antagonist, a treatment known to block the development of amphetamine-induced sensitization of behavioral activation, dopamine release and motivated behavior. Using immunohistochemistry, we showed that both DCC and UNC-5 receptors are highly expressed by adult mesocorticolimbic dopamine neurons. These results provide the first evidence that repeated exposure to a stimulant drug such as amphetamine affects netrin-1 receptor expression in the adult brain. Taken together, our findings suggest that changes in netrin-1 receptor expression may play a role in the lasting effects of exposure to amphetamine and other stimulant drugs.

Tuning the engine of cognition: A focus on NMDA/D1 receptor interactions in prefrontal cortex

The prefrontal cortex of the primate frontal lobes provides the capacity for judgment which can constantly adapt behavior in order to optimize its outcome. Adjudicating between long-term memory programs and prepotent responses, this capacity reviews all incoming information and provides an interpretation dependent on the events that have just occurred, the events that are predicted to happen, and the alternative response strategies that are available in the given situation. It has been theorized that this function requires two essential integrated components, a central executive which guides selective attention based on mechanisms of associative memory, as well as the second component, working memory buffers, in which information is held online, abstracted, and translated on a mental sketchpad of work in progress. In this review, we critically outline the evidence that the integration of these processes and, in particular, the induction and maintenance of persistent activity in prefrontal cortex and related networks, is dependent upon the interaction of dopamine D1 and glutamate NMDA receptor signaling at critical nodes within local circuits and distributed networks. We argue that this interaction is not only essential for representational memory, but also core to mechanisms of neuroadaptation and learning. Understanding its functional significance promises to reveal major new insights into prefrontal dysfunction in schizophrenia and, hence, to target a new generation of drugs designed to ameliorate the debilitating working memory deficits in this disorder.

The laterodorsal tegmentum contributes to behavioral sensitization to amphetamine

A critical event in the development of behavioral sensitization is a transient increase in excitatory drive to dopamine neurons of the ventral tegmental area (VTA). This is likely to be due, in part, to the ability of drugs of abuse to produce long-term potentiation, expressed as increased AMPA receptor transmission, at excitatory synapses onto VTA dopamine neurons. We investigated the role of the laterodorsal tegmentum (LDT) in behavioral sensitization because LDT neurons provide an important source of excitatory drive to VTA dopamine neurons, through mixed glutamate and cholinergic inputs. To test the role of the LDT in amphetamine sensitization, ibotenic acid or sham lesions of the LDT were performed 1 week before the first of six daily amphetamine injections. When challenged with amphetamine 13 days after the last injection, sham rats expressed sensitization of stereotypy and post-stereotypy locomotor hyperactivity, whereas the latter was attenuated by ibotenic acid lesions of the LDT. To determine whether plasticity occurs in the LDT during amphetamine sensitization, we used a previously developed microdialysis assay in which increased ability of AMPA to activate a pathway serves as a marker for long-term potentiation. Two days after discontinuing repeated saline or amphetamine injections, the responsiveness of LDT-VTA neurons to AMPA was determined by microinjecting AMPA (0.4 nmol) into the LDT and measuring glutamate efflux in the ipsilateral VTA. Glutamate efflux was transiently increased in both groups but a delayed group difference was apparent with relatively higher glutamate efflux in amphetamine rats 30-60 min after AMPA injection. In parallel experiments, dopamine efflux in the nucleus accumbens (NAc) following intra-LDT AMPA declined in saline rats but remained relatively stable in amphetamine rats. Both results suggest relatively greater excitability of the LDT-VTA-NAc pathway after repeated amphetamine treatment. Our results provide the first evidence that neuronal plasticity in the LDT contributes to behavioral sensitization.

Identification of an excitatory amino acid-mediated component of the ventral tegmental area local field potential response to medial prefrontal cortex stimulation: effect of acute d-amphetamine

The induction of sensitisation to the behavioural effects of d-amphetamine - a model of drug addiction - involves the potentiation of exctiatory amino acid (EAA)-ergic synapses on dopaminergic neurons in the ventral tegmental area (VTA). Such potentiation has been reported as early as 2 hr post-injection, however earlier time points have not been assessed. Consequently, we examined the effects of systemic d-amphetamine on an EAA-mediated component of the VTA local field potential response to stimulation of the medial prefrontal cortex, an EAAergic afferent critical for sensitisation, over the immediate 2 hr post-injection period. D-amphetamine and saline both depressed the amplitude of this component to a similar extent throughout the recording session. It is concluded that overt aspects of EAA-mediated potentiation appear to be delayed with respect to drug administration, which may have implications for sensitisation's putative role in linking drug-related environmental stimuli and the central effects of the drug.

Relationship between development of cross-sensitization to MK-801 and delayed increases in glutamate levels in the nucleus accumbens induced by a high dose of methamphetamine

RATIONALE

The present study hypothesized that delayed increases in extracellular glutamate (Glu) levels in the nucleus accumbens (NAC), induced by a high dose of methamphetamine (METH), can result in some functional changes of excitatory amino acid receptors, developing behavioral cross-sensitization to a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801.

OBJECTIVES

The present study aims to examine whether two different doses of METH (2.5 and 1.0 mg/kg) induce different effects on the development of cross-sensitization to MK-801. To clarify the mechanisms for development and expression of cross-sensitization to MK-801, we measured extracellular Glu and dopamine (DA) levels in the NAC at METH injections in a treatment period and at MK-801 injection after a 12-day withdrawal period.

MATERIALS AND METHODS

METH- or MK-801-induced changes in Glu and DA levels and in locomotion were measured using in vivo microdialysis and infrared sensor, respectively.

RESULTS

METH, at only 2.5 mg/kg, produced delayed increases in Glu levels and developed behavioral cross-sensitization to MK-801 (0.2 mg/kg). MK-801 (0.2 mg/kg) induced delayed increases in Glu levels in the NAC, but this time course was not completely consistent with MK-801-induced enhanced hyperlocomotion. During this time course, MK-801 (0.2 mg/kg) did not induce any changes in DA levels.

CONCLUSIONS

These results suggest that METH-induced, at 2.5 mg/kg, delayed increases in Glu levels are necessary for development of behavioral cross-sensitization to MK-801, but not METH. The enhanced locomotion-inducing effect of MK-801 might be related to some functional changes in excitatory amino acid receptors such as NMDA and DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid in the NAC.

Impact of basic FGF expression in astrocytes on dopamine neuron synaptic function and development

Behavioural sensitization to amphetamine (AMPH) requires action of the drug in the ventral midbrain where dopamine (DA) neurons are located. In vivo studies suggest that AMPH sensitization requires enhanced expression of basic fibroblast growth factor (bFGF) in the nucleus of midbrain astrocytes. One idea is that the AMPH-induced increase in bFGF expression in astrocytes leads to enhanced secretion of this peptide and to long-term plasticity in DA neurons. To study directly the effects of astrocytic expression of bFGF on DA neurons, we established a cell-culture model of mesencephalic astrocytes and DA neurons. Immunolabelling showed that even in the absence of a pharmacological stimulus, the majority of mesencephalic astrocytes in culture express bFGF at a nuclear level. Arguing against the idea that bFGF was secreted, bFGF was undetectable in the extracellular medium (below 10 pg/mL). However, supplementing culture medium with exogenous bFGF at standard concentrations (20 ng/mL) led to a dramatic change in the morphology of astrocytes, increased spontaneous DA release, and inhibited synapse formation by individual DA neurons. RNA interference (siRNA) against bFGF mRNA, caused a reduction in DA release but produced no change in synaptic development. Together these data demonstrate that under basal conditions (in the absence of a pharmacological stimulus such as amphetamine) bFGF is not secreted even though there is abundant nuclear expression in astrocytes. The effects of bFGF seen here on DA neurons are thus likely to be mediated through more indirect glial-neuronal interactions, leading to enhanced DA release without a necessary change in synapse number.

Local injection of a glutamate uptake inhibitor into the ventral tegmental area produces sensitization to the behavioural effects of d-amphetamine

Circumstantial evidence suggests that sensitization to the behavioral effects of d-amphetamine is mediated by increased glutamate levels in the ventral tegmental area. To test this directly, the present study examined whether increasing glutamate levels in the ventral tegmental area with a glutamate uptake inhibitor is sufficient, in the absence of d-amphetamine administration, to elicit sensitization to a subsequent d-amphetamine challenge. Rats were treated bilaterally once a day for 2 days with either intra-ventral tegmental area L-trans-pyrollidine-2,4-dicarboxylic acid (50 nmol), saline, L-trans-pyrollidine-2,4-dicarboxylic acid coadministered with the competitive N-methyl-d-aspartate antagonist (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid; CPP, 0.5 nmol), or CPP alone (0.5 nmol; all 1.0 microl/side). Following a 2 day withdrawal period, all rats were administered systemic d-amphetamine (1 mg/kg, i.p.). Repeated intra-ventral tegmental area injection of L-trans-pyrollidine-2,4-dicarboxylic acid sensitized animals to the behavioral effects of a systemic d-amphetamine challenge, an action which was blocked by co-administration of CPP. The results directly implicate ventral tegmental area glutamate in the process of sensitization to d-amphetamine. Furthermore, they demonstrate that inhibition of glutamate uptake produces the neuroadaptations necessary to induce sensitization, adding support to the contention that d-amphetamine sensitizes by modulating glutamate uptake.

Rapid synaptic plasticity of glutamatergic synapses on dopamine neurons in the ventral tegmental area in response to acute amphetamine injection

Drugs of abuse activate the reward circuitry of the mesocorticolimbic system, and it has been hypothesized that drug exposure triggers synaptic plasticity of glutamatergic synapses onto dopamine (DA) neurons of the ventral tegmental area. Here, we show that just a 2 h in vivo exposure to amphetamine is sufficient to potentiate these synapses, measured as an increase in the synaptic AMPAR/NMDAR ratio. We tested the prediction that an increase in GluR1-containing AMPA receptors would result in an increase in GluR1 homomeric receptors at synapses, but were unable to observe any evidence of the predicted rectification in DA neurons from animals treated with amphetamine. We also examined the possibility of increased AMPA receptor insertion in the membrane, but did not detect a significant increase in biotinylated surface GluR1. We conclude that amphetamine induces rapid changes in synaptic AMPAR/NMDAR ratios, suggesting that potentiation of glutamatergic synapses is a relatively early event in the series of neuroadaptations in response to drugs of abuse.

Suppressing calcium/calmodulin-dependent protein kinase II activity in the ventral tegmental area enhances the acute behavioural response to cocaine but attenuates the initiation of cocaine-induced behavioural sensitization in rats

In the present experiments we administered an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist (CNQX), N-methyl-D-aspartate (NMDA) receptor antagonist (AP-5), or l-type calcium channel blocker (diltiazem) directly into the ventral tegmental area (VTA) before each of four daily systemic cocaine injections in order to assess their influence on the initiation phase of behavioural sensitization. Results indicated that pretreatment with CNQX or AP-5 impaired the initiation of cocaine-induced behavioural sensitization. Intra-VTA administration of diltiazem significantly increased the behavioural activation induced by an acute cocaine injection, but impaired the development of cocaine-induced behavioural sensitization. Because AMPA and NMDA receptors, as well as l-type calcium channels are calcium permeable, we also investigated the role of the calcium-activated second messenger calcium/calmodulin-dependent protein kinase II (CaM-KII). Similar to the results obtained with diltiazem, administration of the CaM-KII inhibitor KN-93 into the VTA enhanced the acute behavioural response to cocaine but prevented the augmentation of cocaine-induced behavioural hyperactivity following repeated injections. Consistent with this finding, the behavioural hyperactivity produced by cocaine was markedly enhanced among homozygous alpha-CaM-KII knockout mice but the initiation of behavioural sensitization to cocaine was attenuated relative to wild-type mice. Separate experiments performed in rats demonstrated an increase in total protein levels of CaM-KII in the VTA 24 h after the last of seven daily injections of cocaine. Taken together, these results indicate that blocking l-type calcium channels or impairing CaM-KII activity in the VTA augments the acute behavioural hyperactivity induced by cocaine. The present findings also suggest that increased calcium influx through AMPA receptors, NMDA receptors and l-type calcium channels on dopaminergic neurons in the VTA contributes significantly to the initiation of behavioural sensitization by amplifying calcium signalling through CaM-KII.

Brain neurotransmitter turnover rates during rat intravenous cocaine self-administration

The turnover rates of dopamine, norepinephrine, serotonin, aspartate, glutamate and GABA were measured in 27 brain regions of rats self-administering cocaine and in yoked cocaine- and yoked vehicle-infused controls using radioactive pulse-labeling procedures to identify brain neuronal systems underlying self-administration. Changes in the activity of heretofore unrecognized dopamine, norepinephrine, serotonin, glutamate and GABA innervations of the forebrain specific to cocaine self-administration were found. This included innervations of the nucleus accumbens, ventral pallidum, lateral hypothalamus and the anterior and posterior cingulate, entorhinal-subicular and visual cortices. Turnover rates also were calculated using metabolite/neurotransmitter ratios which were inconsistent with the pulse-label technologies indicating that ratio procedures are not accurate measures of neurotransmitter utilization. Results with the pulse-label technique provide evidence of the involvement of neuronal systems in cocaine self-administration not previously known, some of which may have a broader role in brain reinforcement processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are thought to produce reinforcing effects by modulating activity in these endogenous systems.

Previous exposure to VTA amphetamine enhances cocaine self-administration under a progressive ratio schedule in an NMDA, AMPA/kainate, and metabotropic glutamate receptor-dependent manner

Previous exposure to amphetamine (AMPH) in the ventral tegmental area (VTA) enhances cocaine self-administration in a D(1) dopamine receptor-dependent manner. The present study examined the contribution of VTA NMDA, AMPA/kainate, and metabotropic glutamate (mGlu) receptors to this effect. Rats in different groups received three intra-VTA injections, one every third day, of either saline (0.5 microl/side), AMPH (2.5 microg/0.5 microl/side), AMPH+CPP (NMDA receptor antagonist; 10 microM or 100 microM/0.5 microl/side), AMPH+CNQX (AMPA/kainate receptor antagonist; 0.3 mM or 1 mM/0.5 microl/side), AMPH+MCPG (mGlu receptor antagonist; 0.5 mM or 50 mM/0.5 microl/side), or the glutamate receptor antagonists alone. Starting 7-10 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.3 mg/kg/infusion) and then tested under a progressive ratio (PR) schedule of reinforcement for 6 consecutive days. As reported previously, VTA AMPH pre-exposed rats worked more and obtained more infusions of cocaine than saline pre-exposed animals. Coadministration of CPP, CNQX, or MCPG with AMPH during pre-exposure dose-dependently blocked this enhancement of cocaine self-administration. Rats pre-exposed to the glutamate receptor antagonists alone did not differ on the test days from the saline pre-exposed controls. These results indicate that, in a manner paralleling the induction of sensitization of the locomotor stimulating effects of AMPH, activation of NMDA, AMPA/kainate, and mGlu receptors during pre-exposure to AMPH in the VTA is necessary for the enhancement of cocaine self-administration to develop.

The roles of calcium/calmodulin-dependent and Ras/mitogen-activated protein kinases in the development of psychostimulant-induced behavioral sensitization

Although the development of behavioral sensitization to psychostimulants such as cocaine and amphetamine is confined mainly to one nucleus in the brain, the ventral tegmental area (VTA), this process is nonetheless complex, involving a complicated interplay between neurotransmitters, neuropeptides and trophic factors. In the present review we present the hypothesis that calcium-stimulated second messengers, including the calcium/calmodulin-dependent protein kinases and the Ras/mitogen-activated protein kinases, represent the major biochemical pathways whereby converging extracellular signals are integrated and amplified, resulting in the biochemical and molecular changes in dopaminergic neurons in the VTA that represent the critical neuronal correlates of the development of behavioral sensitization to psychostimulants. Moreover, given the important role of calcium-stimulated second messengers in the expression of behavioral sensitization, these signal transduction systems may represent the biochemical substrate through which the transient neurochemical changes associated with the development of behavioral sensitization are translated into the persistent neurochemical, biochemical and molecular alterations in neuronal function that underlie the long-term expression of psychostimulant-induced behavioral sensitization.

Expression of amphetamine-induced behavioral sensitization after short- and long-term withdrawal periods: participation of mu- and delta-opioid receptors

Repeated amphetamine administration results in behavioral sensitization, an enduring behavioral transformation expressed after short and long periods of withdrawal. To investigate the participation of the opioid system in amphetamine-induced behavioral sensitization, we studied the effect of naloxone, an opioid receptor antagonist, on the expression of behavioral sensitization tested after short- (2 days) and long-term (14 days) withdrawal periods. In addition, using quantitative competitive RT-PCR, we examined the levels of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) mRNA in the nucleus accumbens shell (NAcSh) and ventral tegmental area (VTA) of behaviorally sensitized rats, at these two withdrawal times. This study showed that whereas naloxone did not modify the expression of behavioral sensitization tested after 2 days of withdrawal, it completely blocked the expression when tested after 14 days of withdrawal. DOR and MOR mRNA levels were not modified in the NAcSh of rats expressing behavioral sensitization after 2 or 14 days of withdrawal. Conversely, DOR and MOR mRNA levels were elevated in the VTA of animals expressing behavioral sensitization after 2 days of withdrawal. However, whereas DOR mRNA returned to control levels, MOR mRNA levels remained elevated in animals expressing behavioral sensitization after 14 days of withdrawal. These results indicate a striking difference between the role played by opioid receptors in the expression of amphetamine-induced behavioral sensitization, when tested after short- or long-term withdrawal periods. In addition, our results support the notion that repeated amphetamine-induced changes in opioid receptor expression may contribute to the perpetuation of psychostimulant abuse and/or relapse.

Effects of acute toxic doses of psychostimulants on extracellular levels of excitatory amino acids and taurine in rats: comparison of d-amphetamine and sydnocarb

We used microdialysis to study how acute toxic doses of d-amphetamine and sydnocarb [3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine], an original Russian psychostimulant, affect extracellular levels of glutamate, aspartate, and taurine in the neostriatum of halothane-anesthetized male Sprague-Dawley rats. The administration of d-amphetamine (5.0 mg/kg x 4 i.p.) caused gradual fivefold increases in the extracellular glutamate and taurine levels and moderate increases in the extracellular aspartate level. Sydnocarb administration (23.8 mg/kg x 4 i.p., a dose equimolar to 5.0 mg/kg d-amphetamine) elicited a marked increase in the extracellular aspartate level and a small increase in the extracellular level of glutamate. The extracellular taurine level increased only after the last (fourth) injection. We conclude that a massive increase in extracellular taurine reflects hyperactivation of glutamatergic neurotransmission elicited by acute toxic dose of d-amphetamine. Sydnocarb seems to be less neurotoxic than d-amphetamine, because it elicits lesser changes in the extracellular levels of glutamate and taurine.

In vivo modulation of ventral tegmental area dopamine and glutamate efflux by local GABA(B) receptors is altered after repeated amphetamine treatment

The activity of dopamine neurons in the ventral tegmental area is modulated by excitatory (glutamatergic) and inhibitory (GABAergic) afferents. GABA, released by intrinsic neurons and by projection neurons originating in the nucleus accumbens and other regions, inhibits dopamine neurons via activation of GABA(A) and GABA(B) receptor subtypes. Using in vivo microdialysis in freely moving rats, we investigated the role of ventral tegmental area GABA(B) receptors in modulating levels of dopamine and glutamate within the ventral tegmental area, both in naive rats and in rats treated repeatedly with saline or amphetamine (5 mg/kg i.p., for 5 days). In naive rats, administration of a potent and selective GABA(B) receptor antagonist (CGP 55845A) into the ventral tegmental area elicited a concentration-dependent increase in dopamine levels, but did not alter glutamate levels. In rats tested 3 days after discontinuing repeated amphetamine administration, 50 microM CGP 55845A increased dopamine levels to a greater extent than in saline controls. This difference was no longer present in rats tested 10-14 days after discontinuing repeated amphetamine injections. CGP 55845A (50 microM) had no effect on glutamate levels in the ventral tegmental area of saline-treated rats. However, it produced a robust increase in glutamate levels in rats tested 3 days, but not 10-14 days, after discontinuing repeated amphetamine injections. These results suggest that somatodendritic dopamine release is normally under strong tonic inhibitory control by GABA(B) receptors. Repeated amphetamine administration enhances GABA(B) receptor transmission in the ventral tegmental area during the early withdrawal period, increasing inhibitory tone on both dopamine and glutamate levels. This is the first demonstration, in an intact animal, that drugs of abuse alter GABA(B) receptor transmission in the ventral tegmental area.

Sensitization to cocaine after a single intra-cerebral injection of orphanin FQ/nociceptin

Orphanin FQ/nociceptin (OFQ/N) has been shown to modulate mesolimbic dopaminergic neurotransmission. Repeated administration of OFQ/N into the ventral tegmental area results in a sensitized locomotor response to subsequent peripheral cocaine administration. The aim of the present study was to examine the potential for OFQ/N to produce a sensitized locomotor response to cocaine after a single intra-VTA administration and to determine if this effect of OFQ/N extrapolates to other points along the mesolimbic or nigrostriatal dopaminergic axes. Bilateral administration of OFQ/N (30 microg/side) into the VTA on day 1 to male Sprague--Dawley rats resulted in an enhanced locomotor response to cocaine (10 mg/kg i.p) administered on day 2. However, OFQ/N (3, 10 and 30 microg per side) administered on day 2, 5 mins prior to the administration of cocaine (10 mg/kg i.p), in animals treated with aCSF or OFQ/N on day 1, similarly blocked the action of cocaine, suggesting that the sensitized response was not due to tolerance to the effect of endogenously released OFQ/N. The administration of OFQ/N into the substantia nigra or nucleus accumbens failed to produce a significant sensitized response to a cocaine challenge 24 h later. A significant increase in cocaine stimulated locomotor response on day 2 was observed after injection of OFQ/N into the striatum on day 1. These results demonstrate the ability of a single intra-VTA or intra-striatal administration of OFQ/N to produce increases in the sensitivity to cocaine and may indicate a role for endogenous OFQ/N systems in regulating responses to psychostimulant drugs.

The effects of acute or repeated cocaine administration on nerve terminal glutamate within the rat mesolimbic system

Cocaine administration alters glutamate function within several brain regions. Using quantitative electron microscopic immunocytochemistry, the present study investigated the effect of repeated intermittent cocaine (resulting in behavioral sensitization) or acute cocaine administration on the density of glutamate immunogold labeling within nerve terminals. Rats were treated daily with saline or cocaine for 7 days. Following a 14-day withdrawal animals were challenged with saline or cocaine. On the challenge day, most (75%) animals that received cocaine repeatedly showed a heightened locomotor response to cocaine compared to the first day of cocaine administration, and were considered behaviorally sensitized.Three days after the challenge, glutamate immunogold labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens, ventral tegmental area and medial prefrontal cortex. There was a decrease in such labeling in the nucleus accumbens in the group receiving acute cocaine. Locomotor activity was positively correlated with glutamate immunolabeling within nerve terminals in the nucleus accumbens core only for the cocaine-sensitized group. Nerve terminal glutamate immunolabeling in the nucleus accumbens core, but not the shell, was increased in the non-sensitized compared to the cocaine-sensitized group. In the ventral tegmental area, glutamate immunolabeling was significantly higher in the cocaine-sensitized compared to the acute cocaine group. In the prefrontal cortex, there were no significant differences in glutamate immunogold labeling between treatment groups. This study indicates that acute cocaine administration significantly decreases nerve terminal glutamate immunoreactivity in the nucleus accumbens. We suggest that sensitization results in differential changes in the nucleus accumbens core versus the shell, and may alter presynaptic mechanisms regulating glutamate release or re-uptake in the core.

Alterations in behaviour and glutamate transmission following presentation of stimuli previously associated with cocaine exposure

To study the role of glutamate in cocaine-conditioned responses, we developed a rat model in which conditioned locomotion is produced by repeated pairing of cocaine with discrete stimuli (flashing light and metronome). "Paired" subjects received cocaine (15 mg/kg) prior to six exposures to stimuli for 30 min in the test environment. "Unpaired" subjects received equivalent presentations of the stimuli yet received cocaine in home cages. Tests with the stimuli alone demonstrated that the conditioned locomotion displayed by Paired subjects was evident at 3 or 10 days post-training and resistant to two sessions of testing. The degree of conditioned locomotion was not correlated with the subjects' response to novelty or cocaine. Administration of the noncompetitive AMPA receptor antagonist GYKI 52466 (2.5 mg/kg, a dose without effect on spontaneous activity) attenuated the expression of conditioned activity. In vivo microdialysis revealed that Paired subjects had significantly lower basal glutamate levels in the nucleus accumbens (NAc) than did Unpaired subjects when no stimuli were presented. Presentation of the conditioned stimuli resulted in significant increases in glutamate levels in the NAc in the Paired group whilst glutamate levels in the Unpaired group remained unchanged. The associative control of glutamate levels in the NAc by stimuli formerly paired with a drug of abuse is an unprecedented finding. It is likely to reflect the convergence of excitatory inputs that the NAc receives from limbic structures.

Effects of sydnocarb and D-amphetamine on the extracellular levels of amino acids in the rat caudate-putamen

The neurotoxic effects of psychostimulants at high dosages limit their clinical applicability but the mechanism of neurotoxicity is still unsettled. We now studied by microdialysis how acute and subchronic (four times at 2-h intervals) administrations of D-amphetamine and sydnocarb [3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine], an original novel Russian psychostimulant, affected the extracellular levels of amino acids in the caudate-putamen of halothane-anesthetized male Sprague-Dawley rats. Acute D-amphetamine administration (5.0 mg/kg, i.p.) produced a moderate accumulation of extracellular glutamate and aspartate. Sydnocarb (23.8 mg/kg, i.p., a dose equimolar to 5.0 mg/kg D-amphetamine) also increased extracellular glutamate and alanine. Subchronic D-amphetamine administration (5.0 mg/kg x 4, i.p.) caused gradual fivefold increases in the glutamate and taurine levels and moderate increases in the aspartate and alanine levels. Subchronic sydnocarb administration (23.8 mg/kg x 4, i.p.) elicited a marked increase in the aspartate level and a small increase in the level of glutamate. The alanine level increased temporarily after each administration of sydnocarb, while the taurine level increased only after the last injection. We conclude that the mode of action of sydnocarb differs from that of D-amphetamine. Sydnocarb also seems to be less neurotoxic than D-amphetamine, since it elicits lesser changes in the extracellular level of glutamate.

Amphetamine-induced plasticity of AMPA receptors in the ventral tegmental area: effects on extracellular levels of dopamine and glutamate in freely moving rats

Previous electrophysiological studies suggested that the initiation of behavioral sensitization to cocaine and amphetamine involves a transient increase in AMPA receptor responsiveness in the ventral tegmental area (VTA). To test this, we used in vivo microdialysis to examine the effects of intra-VTA administration of AMPA (10 microm) and NMDA (100 microm) on dopamine (DA) and glutamate efflux in the VTA and the nucleus accumbens (NAC), an important target of VTA DA neurons. We compared rats treated for 5 d with saline or 5 mg/kg amphetamine and withdrawn for 3 or 10-14 d. After 3 d of withdrawal, intra-VTA AMPA increased both NAC and VTA DA levels to a greater extent in the amphetamine group, whereas NMDA produced similar effects in the saline and amphetamine groups. This enhanced responsiveness to AMPA was no longer evident in rats tested 10-14 d after the last injection. In addition, intra-VTA AMPA but not NMDA increased both VTA and NAC glutamate levels in rats tested 3 d after the last injection of amphetamine but not in saline controls. After 10-14 d, the responsiveness of glutamate levels to AMPA was no longer evident in the NAC but persisted in the VTA. Additional studies indicated that the glutamate effect in the NAC may involve increased responsiveness of DA receptors within the NAC. These findings establish an in vivo animal model with which to explore the consequences of repeated drug administration for AMPA receptor plasticity in the VTA. They also indicate that repeated amphetamine leads to potentiated interactions between DA and glutamate transmission.

Amphetamine administration does not alter protein levels of the GLT-1 and EAAC1 glutamate transporter subtypes in rat midbrain, nucleus accumbens, striatum, or prefrontal cortex

Our laboratory and others have previously shown that glutamate transmission is required for chronic amphetamine-induced neuroadaptations, and that glutamate transmission itself is altered by chronic amphetamine administration. For example, N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor subunit expression are altered in a region- and withdrawal-specific manner. The goal of this study was to determine whether repeated amphetamine administration influences the expression of two glutamate transporter subtypes, GLT-1 and EAAC1. Rats were treated with saline or 5 mg/kg amphetamine for 5 days (chronic saline and amphetamine groups, respectively), or saline for 4 days and 5 mg/kg amphetamine on day 5 (acute amphetamine group), and decapitated 24 h after the last injection. Tissue was dissected from brain regions involved in the psychomotor effects of amphetamine (nucleus accumbens, striatum, prefrontal cortex, ventral tegmental area, and substantia nigra). Levels of GLT-1 and EAAC1 were quantified by Western blotting and normalized to actin levels. We found no significant change in levels of GLT-1 or EAAC1 in response to either acute or chronic amphetamine treatment. These findings suggest that the transporter component of the glutamate system might not play a significant role in the alterations in glutamate transmission observed following repeated amphetamine administration.

Sensitized increase of period gene expression in the mouse caudate/putamen caused by repeated injection of methamphetamine

Methamphetamine (MAP) causes the sensitization phenomena not only in MAP-induced locomotor activity, dopamine release, and Fos expression, but also in MAP-induced circadian rhythm. Cocaine-induced sensitization is reportedly impaired in Drosophila melanogaster mutant for the Period (Per) gene. Thus, sensitization may be related to induction of the Per gene. A rapid induction of mPer1 and/or mPer2 in the suprachiasmatic nucleus after light exposure is believed to be necessary for light-induced behavioral phase shifting. Although the caudate/putamen (CPu) expresses mPer1 and/or mPer2 mRNA, the function of these genes in this nucleus has not yet been elucidated. Therefore, we examined whether MAP affects the expression of mPer1 and/or mPer2 mRNA in the mouse CPu. Injection of MAP augmented the expression of mPer1 but not mPer2 or mPer3 in the CPu, and this MAP-induced increase in mPer1 expression lasted for 2 h. Also, the MAP-induced increase of mPer1 mRNA was strongly antagonized by pretreatment with a dopamine D1 receptor and N-methyl-D-aspartate (NMDA) receptor antagonist, but not by a D2 receptor antagonist. Interestingly, application of either the D1 or the D2 agonist alone did not cause mPer1 expression. The present results demonstrate that activation of both NMDA and D1 receptors is necessary to produce MAP-induced mPer1 expression in the CPu. Repeated injection of MAP caused a sensitization in not only the locomotor activity but also mPer1 expression in the CPu without affecting the level of mPer2, mPer3, or mTim mRNA. Thus, these results suggest that MAP-induced mPer1 gene expression may be related to the mechanism for MAP-induced sensitization in the mouse.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Amphetamine increases glutamate efflux in the rat ventral tegmental area by a mechanism involving glutamate transporters and reactive oxygen species

We have shown that amphetamine produces a delayed and sustained increase in glutamate levels in the ventral tegmental area, a region containing dopamine cell bodies important in acute and chronic effects of amphetamine administration. The present study characterized the mechanism underlying amphetamine-induced glutamate efflux. It was abolished by the glutamate uptake inhibitor dihydrokainate, but unaffected by perfusion with a low Ca(2+)/high Mg(2+) solution, implicating glutamate transporters. Because reactive oxygen species inhibit glutamate uptake, we examined the effect of amphetamine on hydroxyl radical formation by perfusing with D-phenylalanine (5 mM) and monitoring p-tyrosine production. Although no increase in hydroxyl radical formation was detected, D-phenylalanine completely prevented the amphetamine-induced increase in glutamate efflux, as did systemic injection of another trapping agent, alpha-phenyl-N-tert-butyl nitrone (60 mg/kg). Thus, amphetamine-induced glutamate efflux may involve reactive oxygen species. In other studies, we found that repeated coadministration of alpha-phenyl-N-tert-butyl nitrone with amphetamine attenuated the development of behavioral sensitization. This supports prior results indicating that the increase in glutamate efflux produced by each amphetamine injection in a chronic regimen is important in triggering drug-induced adaptations in ventral tegmental area dopamine neurons, and that such adaptations may in part represent a response to metabolic and oxidative stress

Changes in astrocytic basic fibroblast growth factor expression during and after prolonged exposure to escalating doses of amphetamine

We have shown that brief exposure to amphetamine leads to sustained glutamate-dependent increases in expression of the neurotrophic, neuroprotective factor, basic fibroblast growth factor, in astrocytes in dopaminergic cell body regions and that blockade of basic fibroblast growth factor in this region prevents the development of behavioral sensitization to amphetamine. Here we examine the effects of prolonged exposure to an escalating-dose regimen of amphetamine known to induce long-lasting sensitization to amphetamine and leading to increases in neuronal dendritic length and spine density in nucleus accumbens and prefrontal cortex and to decreases in spine density in occipital cortex. Astrocytic basic fibroblast growth factor immunoreactivity was increased in both dopaminergic cell body and terminal regions one week after termination of a two-week amphetamine treatment (1-4mg/kg). These effects were not evident one week after a five-week treatment (1-9mg/kg) and, in fact, one month later basic fibroblast growth factor levels in cell body regions were decreased. In the occipital cortex, basic fibroblast growth factor immunoreactivity was decreased one week after the two-week amphetamine treatment, but was not different from that seen in saline-treated animals after the five-week treatment. Increased astrocytic basic fibroblast growth factor expression appears to be an early, but relatively prolonged, response to amphetamine exposure and seems to parallel structural changes induced by repeated drug exposure.These findings suggest that basic fibroblast growth factor may participate in the development of structural changes brought about by amphetamine. The fact that the basic fibroblast growth factor response is not maintained after prolonged intense exposure to amphetamine suggests that the factors that initially induce basic fibroblast growth factor expression are self-regulating.

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.