Glutamate transmission in addiction

The mGlu5 receptor antagonist MTEP attenuates opiate self-administration and cue-induced opiate-seeking behaviour in mice

The mGlu5 receptor (mGluR5) has been implicated in the rewarding effect of various drugs of abuse and drug-seeking behaviour. In the present study we investigated the impact of antagonism of mGluR5 with the selective negative allosteric, modulator 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) on operant self-administration of morphine as well as cue-induced drug-seeking in adult CD1 mice. Administration of MTEP (20 mg/kg, i.p.) attenuated operant responding for morphine (0.1 mg/kg/infusion) and cue-induced morphine-seeking after a period of forced abstinence. Collectively, these data implicate mGluR5 in the reinforcing effects of opiates and support the proposition that mGluR5 is a potential therapeutic target for treatment of drug addiction.

Effects of alcohol on the membrane excitability and synaptic transmission of medium spiny neurons in the nucleus accumbens

Chronic and excessive alcohol drinking lead to alcohol dependence and loss of control over alcohol consumption, with serious detrimental health consequences. Chronic alcohol exposure followed by protracted withdrawal causes profound alterations in the brain reward system that leads to marked changes in reinforcement mechanisms and motivational state. These long-lasting neuroadaptations are thought to contribute to the development of cravings and relapse. The nucleus accumbens (NAcc), a central component of the brain reward system, plays a critical role in alcohol-induced neuroadaptive changes underlying alcohol-seeking behaviors. Here we review the findings that chronic alcohol exposure produces long-lasting neuroadaptive changes in various ion channels that govern intrinsic membrane properties and neuronal excitability, as well as excitatory and inhibitory synaptic transmission in the NAcc that underlie alcohol-seeking behavior during protracted withdrawal.

Attenuation of methamphetamine seeking by the mGluR2/3 agonist LY379268 in rats with histories of restricted and escalated self-administration

Recent findings implicate group II metabotropic glutamate receptors (mGluR(2/3)) in the reinforcing effects of psychostimulants and have identified these receptors as potential treatment targets for drug addiction. Here, we investigated the effects of mGluR(2/3) stimulation on cue- and drug-primed reinstatement in rats with different histories of methamphetamine (METH) self-administration training, under two conditions: 16 daily sessions of short access (90 min/day, ShA), or 8 daily sessions of short access followed by 8 sessions of long access (6 h/day, LgA). Following self-administration and subsequent extinction training, rats were pretreated with the selective mGluR(2/3) agonist LY379268 (variable dose, 0-3 mg/kg), exposed to METH-paired cues or a priming injection of METH (1 mg/kg), and tested for reinstatement of METH-seeking behavior. LgA rats self-administered greater amounts of METH during the second half of training, but when pretreated with vehicle, ShA and LgA rats showed cue- and drug-primed reinstatement at equivalent response rates. However, LgA rats demonstrated greater sensitivity to mGluR(2/3) stimulation with attenuated responding during cue-induced reinstatement after 0.3 mg/kg and higher doses of LY379268, whereas ShA rats decreased cue-induced reinstatement behavior following 1.0 mg/kg and 3.0 mg/kg LY379268. Additionally, both LgA and ShA rats exhibited decreased METH-primed reinstatement behavior following 0.3 mg/kg and higher doses of LY379268. A separate group of control rats was trained to self-administer sucrose pellets, and demonstrated attenuated cue-induced sucrose-seeking behavior following 1.0 and 3.0 mg/kg LY379268. Together, the results indicate that LY379268 has differential attenuating effects on cue-induced reinstatement behavior in rats with different histories of METH intake. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Epigenetics of nicotine: another nail in the coughing

In a mouse model, chronic nicotine exposure before cocaine use exacerbated the epigenetic, gene-expression, electrophysiological, and behavioral effects that occur during the transition from acute to chronic responses to cocaine that have been linked with the addictive process. Nicotine enhancement of the effects can be mimicked with an inhibitor of chromatin-modifying enzymes (class I and II histone deacetylases). These findings may spur the discovery of therapeutics for the treatment of addiction.

Minocycline: therapeutic potential in psychiatry

Pharmacological interventions to treat psychiatric illness have previously focused on modifying dysfunctional neurotransmitter systems to improve symptoms. However, imperfect understanding of the aetiology of these heterogeneous syndromes has been associated with poor treatment outcomes for many individuals. Growing evidence suggests that oxidative stress, inflammation, changes in glutamatergic pathways and neurotrophins play important roles in many psychiatric illnesses including mood disorders, schizophrenia and addiction. These novel insights into pathophysiology allow new treatment targets to be explored. Minocycline is an antibiotic that can modulate glutamate-induced excitotoxicity, and has antioxidant, anti-inflammatory and neuroprotective effects. Given that these mechanisms overlap with the newly understood pathophysiological pathways, minocycline has potential as an adjunctive treatment in psychiatry. To date there have been promising clinical indications that minocycline may be a useful treatment in psychiatry, albeit from small trials most of which were not placebo controlled. Case reports of individuals with schizophrenia, psychotic symptoms and bipolar depression have shown serendipitous benefits of minocycline treatment on psychiatric symptoms. Minocycline has been trialled in open-label or small randomized controlled trials in psychiatry. Results vary, with findings supporting use in schizophrenia, but showing less benefit for nicotine dependence and obsessive-compulsive disorder. Given the limited data from rigorous clinical trials, further research is required. However, taken together, the current evidence suggests minocycline may be a promising novel therapy in psychiatry.

NMDA receptor glycine modulatory site in the ventral tegmental area regulates the acquisition, retrieval, and reconsolidation of cocaine reward memory

RATIONALE AND OBJECTIVES

Accumulating clinical and preclinical studies have shown that the memories of the rewarding effects of drugs and their paired cues may contribute to relapse and persistent cocaine use. Glutaminergic actions in the ventral tegmental area (VTA) have been shown to regulate the rewarding effect of drugs and conditioned responses to drug-associated cues, but the role of the VTA in the acquisition, retrieval, and reconsolidation of cocaine cues is not yet known.

METHODS

In the present study, we used 7-chlorothiokynurenic acid (7-CTKA), an N-methyl-D-aspartate (NMDA) receptor glycine modulatory site antagonist with no rewarding effects, to examine the role of the NMDA receptor glycine modulatory site in the acquisition, retrieval, and reconsolidation of cocaine-related reward memory using the conditioned place preference (CPP) paradigm.

RESULTS

Separate groups of Sprague-Dawley rats were trained to acquire cocaine-induced CPP. Vehicle or 7-CTKA was microinjected into the VTA or substantia nigra (SN) (5 μg/μl) at different time points: 10 min before each CPP training session (acquisition), 10 min before the reactivation of CPP (retrieval), and immediately after the reactivation of CPP (reconsolidation). Cocaine-induced CPP was retested 24 h and 1 and 2 weeks after 7-CTKA administration. 7-CTKA microinjected into the VTA, but not SN, significantly impaired the acquisition, retrieval, and reconsolidation of cocaine-induced CPP without affecting cocaine-induced locomotion.

CONCLUSIONS

Our findings suggest that the NMDA receptor glycine modulatory site in the VTA plays a major role in cocaine reward memory, and NMDA receptor glycine site antagonists may be potential pharmacotherapies for the management of relapse.

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

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

Differential effects of cocaine access and withdrawal on glutamate type 1 transporter expression in rat nucleus accumbens core and shell

Cocaine addiction is characterized by compulsive drug seeking, including relapse after a period of withdrawal. The relapse response requires increased glutamate transmission in the nucleus accumbens (NAc). Consistent with this view, glutamate type 1 transporter (GLT1), the transporter responsible for >90% of glutamate uptake, is downregulated in NAc after several days of withdrawal in rats previously trained to self-administer cocaine under limited access conditions (1-2 h/d). Human addiction, however, appears to be better modeled by extending daily drug access (6-8 h/d) and introducing long periods of withdrawal. Here, we determined the combined effects of manipulating cocaine access and withdrawal on GLT1 expression in NAc core and shell. Rats were trained to self-administer cocaine (0.25 mg per intravenous infusion) in daily limited or extended access sessions for 11 days followed by a period of short (1 day) or long (40-45 days) withdrawal. We found that although cocaine withdrawal decreases GLT1 expression in both core and shell, only in core is GLT1 downregulation sensitive to both access and withdrawal. In fact, after long withdrawal, GLT1 in core is downregulated more than in shell in either the limited or extended access condition. Thus, glutamate regulation in core appears to be a critical factor in the drug-seeking behavior that follows relatively long periods of cocaine withdrawal.

Regulation of cocaine-induced reinstatement by group II metabotropic glutamate receptors in the ventral tegmental area

RATIONALE

A high rate of relapse is a daunting challenge facing clinical treatment of cocaine addiction. Recent studies have shown that drugs of abuse enhance glutamate neurotransmission in dopamine neurons in the ventral tegmental area (VTA) and such enhancement may contribute to the risk of relapse.

OBJECTIVES

Given the important role of group II metabotropic glutamate receptors (mGluR2/3s) in regulating glutamate release from the glutamatergic terminals, this study aimed to test whether activation of mGluR2/3s in the VTA can inhibit cocaine-induced reinstatement of cocaine-seeking behavior, a model of relapse to drug-seeking behavior.

METHODS

Rats were trained to self-administer intravenous cocaine (0.25 mg/infusion) under a modified fixed-ratio 5 schedule. After rats reached the training criteria, they went through extinction training to extinguish cocaine-seeking behavior. Then the dose-response effects of a selective mGluR2/3 agonist LY 379268 microinjected into the VTA on cocaine-induced reinstatement of cocaine-seeking behavior were assessed.

RESULTS

LY 379268 (0.032-0.1 μg/side) dose-dependently decreased cocaine-induced reinstatement. The effect could not be fully attributed to diffusion of the drug to the neighboring substantia nigra or to motor impairment. Interestingly, LY 379268 has a less potent effect on cocaine-induced reinstatement than on sucrose-induced reinstatement of sucrose-seeking behavior.

CONCLUSIONS

Our data support the idea that glutamate release in the VTA is critically involved in cocaine-induced reinstatement and indicate that loss of mGluR2/3-mediated regulation of glutamate release in the VTA may critically contribute to the risk of relapse.

Inactivation of the central nucleus of the amygdala reduces the effect of punishment on cocaine self-administration in rats

Continued cocaine use despite the negative consequences is a hallmark of cocaine addiction. One such consequence is punishment, which is often used by society to curb cocaine use. Unfortunately, we know little about the mechanism involved in regulation by punishment of cocaine use. The fact that cocaine addicts continue to use cocaine despite potentially severe punishment suggests that the mechanism may be impaired. Such impairment is expected to critically contribute to compulsive cocaine use. This study was aimed at testing the hypothesis that the central nucleus of the amygdala (CeN) plays a critical role in such regulation. To this end, rats were trained to press a lever to self-administer cocaine under a chained schedule: a response on one lever (cocaine-seeking lever) led to access to the other lever (cocaine-taking lever), on which a response was reinforced by cocaine and cues. Thereafter, responses on the seeking lever were punished by footshock with a probability of 0.5. Cocaine self-administration (SA) was significantly suppressed by punishment in an intensity-dependent manner. Interestingly, rats trained with daily 6-h (extended access) but not 2-h (limited access) sessions showed resistance to the lower intensity of punishment. Inactivation of the CeN induced a robust anti-punishment effect in both groups. These data provided evidence that the CeN is a critical neural substrate involved in regulation by punishment of cocaine SA. Rats with a history of extended cocaine SA appeared to be less sensitive to punishment. The decreased sensitivity could result from the neuroplastic changes induced by extended cocaine SA in the CeN.

Pathological Gambling: Neuropsychopharmacology and Treatment

Pathological gambling (PG) affects about 0.2-2% of adults and the impact extends to family members, employers and society as a whole. Recent research has identified similarities in the pathophysiologies of PG and substance use disorders (SUDs). As such, findings regarding SUDs provide a framework for investigating PG. The aims of the manuscript are two-fold. First, we will briefly revivew neural systems implicated in PG. Cortico-limbic circuitry involving the ventral striatum, ventromedial prefrontal cortex, anterior cingulate cortex, and dorsolateral prefrontal cortex are discussed as are the neurotransmitters norepinephrine, serotonin, dopamine, opioids, glutamate, and gamma-aminobutyric acid (GABA). This background will provide a framework for reviewing the psychopharmacological treatments that have been tested for efficacy and safety in treating PG. Of medications, the strongest data suggest the efficacy and tolerability of opioid antagonists in the treatment of PG, and other agents have varying degree of empirical support. As behavioral therapies have also shown efficacy, they will be briefly considered as well. Future research is needed to understand how treatments work in PG and for whom specific treatments might work best.

Cocaine modulates both glutaminase gene expression and glutaminase activity in the brain of cocaine-sensitized mice

RATIONALE

Glutaminase is considered the main glutamate (Glu)-producing enzyme. Two isoforms, liver (LGA)- and kidney (KGA)-type glutaminases, have been identified in neurons. The role of both enzymes in psychopharmacological responses to cocaine remains unknown.

OBJECTIVES

We examined both mRNA and protein expression of KGA and LGA in the brain of mice sensitized to cocaine. Additionally, total glutaminase activity was also measured.

METHODS

Total glutaminase activity and mRNA and protein expression of KGA and LGA were measured on the dorsal striatum, prefrontal cortex, hippocampus and cerebellum of cocaine-sensitized mice.

RESULTS

Cocaine-sensitized animals (20 mg/kg × 5 days, followed by 5 drug-free days) exhibited a decrease of total glutaminase activity in both the dorsal striatum and the prefrontal cortex. This was associated with an increase in KGA mRNA expression in both brain areas that was not observed when protein KGA levels were measured by western blot. LGA mRNA expression was increased as results of acute cocaine administration in sensitized animals, although protein levels were only enhanced in the prefrontal cortex of sensitized mice. These findings suggest that chronic cocaine administration modulates glutamate production through the regulation of glutaminase expression and activity. These actions are mainly observed in the prefrontal cortex-dorsal striatum circuit, the neuroanatomical target for the psychostimulant sensitization properties of cocaine.

CONCLUSIONS

The present results indicate that glutaminase enzymes (mainly KGA) are modulated by cocaine in both the prefrontal cortex and the dorsal striatum, as part of the neuroadaptions associated with behavioural sensitization to this drug of abuse.

mGluR5 receptors in the basolateral amygdala and nucleus accumbens regulate cue-induced reinstatement of ethanol-seeking behavior

Pharmacological blockade of the type 5 metabotropic glutamate receptor (mGluR5) attenuates cue-induced reinstatement of ethanol-seeking behavior, yet the brain regions involved in these effects are not yet known. The purpose of the present study was to determine if local blockade of mGluR5 receptors in the basolateral amygdala (BLA) and/or the nucleus accumbens (NAc), two brain regions known to be involved in stimulus-reward associations, attenuate the reinstatement of ethanol-seeking behavior induced by ethanol-paired cues. As a control for possible non-specific effects, the effects of mGluR5 blockade in these regions on cue-induced reinstatement of sucrose-seeking were also assessed. Male Wistar rats were implanted with bilateral microinjection cannulae aimed at the BLA or NAc. Following recovery, animals were trained to self-administer ethanol (10% w/v) or 45 mg sucrose pellets on an FR1 schedule of reinforcement in 30 min daily sessions using a sucrose fading procedure. Following stabilization of responding, animals underwent extinction training. Next, animals received infusions of vehicle or the selective mGluR5 antagonist MTEP (3 μg/μl) into the BLA or NAc prior to cue-induced reinstatement testing sessions. mGluR5 blockade eliminated cue-induced reinstatement of alcohol - but not sucrose-seeking behavior. Results from this study indicate that mGluR5 receptors in the BLA and NAc mediate cue-induced reinstatement of ethanol-seeking behavior, and provide two potential neuroanatomical sites of action where systemically administered mGluR5 antagonists attenuate cue-induced reinstatement. These data are consistent with previous findings that cue-induced reinstatement of ethanol-seeking increases neuronal activity and glutamatergic transmission in these two regions.

Nucleus accumbens neuronal activity in freely behaving rats is modulated following acute and chronic methylphenidate administration

Methylphenidate (MPD) is a psychostimulant that enhances dopaminergic neurotransmission in the central nervous system by using mechanisms similar to cocaine and amphetamine. The mode of action of brain circuitry responsible for an animal's neuronal response to MPD is not fully understood. The nucleus accumbens (NAc) has been implicated in regulating the rewarding effects of psychostimulants. The present study used permanently implanted microelectrodes to investigate the acute and chronic effects of MPD on the firing rates of NAc neuronal units in freely behaving rats. On experimental day 1 (ED1), following a saline injection (control), a 30 min baseline neuronal recording was obtained immediately followed by a 2.5 mg/kg i.p. MPD injection and subsequent 60 min neuronal recording. Daily 2.5 mg/kg MPD injections were given on ED2 through ED6 followed by 3 washout days (ED7 to ED9). On ED10, neuronal recordings were resumed from the same animal after a saline and MPD (rechallenge) injection exactly as obtained on ED1. Sixty-seven NAc neuronal units exhibited similar wave shape, form and amplitude on ED1 and ED10 and their firing rates were used for analysis. MPD administration on ED1 elicited firing rate increases and decreases in 54% of NAc units when compared to their baselines. Six consecutive MPD administrations altered the neuronal baseline firing rates of 85% of NAc units. MPD rechallenge on ED10 elicited significant changes in 63% of NAc units. These alterations in firing rates are hypothesized to be through mechanisms that include D1 and D2-like DA receptor induced cellular adaptation and homeostatic adaptations/deregulation caused by acute and chronic MPD administration.

Insights to drug addiction derived from ultrastructural views of the mesocorticolimbic system

Drugs of abuse increase the release of dopamine from mesocorticolimbic neurons in the ventral tegmental area. Thus, insights into the cytoarchitecture and the synaptic circuitry affecting the activity of dopaminergic neurons in this area are fundamental for understanding the commonalities produced by mechanistically distinct drugs of abuse. Electron microscopic immunolabeling has provided these insights and also shown the critical relationships between the dopaminergic axon terminals and their targeted neurons in the prefrontal cortex and in the both the dorsal and ventral striatum. These brain regions are among those where dopamine and associated neurotransmitters are most implicated in the transition from recreational to compulsive consumption of reinforcing drugs. Thus, the synaptic circuitry and drug-induced plasticity occurring in the ventral tegmental area and in dopamine-targeted regions are reviewed, as both are essential for understanding the long-lasting changes produced by addictive substances.

Natural rewards, neuroplasticity, and non-drug addictions

There is a high degree of overlap between brain regions involved in processing natural rewards and drugs of abuse. "Non-drug" or "behavioral" addictions have become increasingly documented in the clinic, and pathologies include compulsive activities such as shopping, eating, exercising, sexual behavior, and gambling. Like drug addiction, non-drug addictions manifest in symptoms including craving, impaired control over the behavior, tolerance, withdrawal, and high rates of relapse. These alterations in behavior suggest that plasticity may be occurring in brain regions associated with drug addiction. In this review, I summarize data demonstrating that exposure to non-drug rewards can alter neural plasticity in regions of the brain that are affected by drugs of abuse. Research suggests that there are several similarities between neuroplasticity induced by natural and drug rewards and that, depending on the reward, repeated exposure to natural rewards might induce neuroplasticity that either promotes or counteracts addictive behavior.

Investigating Methamphetamine Craving Using the Extinction-Reinstatement Model in the Rat

Like all other drugs of abuse, the primary therapeutic objective for treating methamphetamine addiction research is the maintenance of abstinence and prevention of relapse to habitual drug-taking. Compounds with the potential to prevent relapse are often investigated in rats that are trained to self-administer intravenous methamphetamine, subjected to extinction training where responding is no longer reinforced, and then given tests for reinstatement of drug-seeking behavior triggered by methamphetamine injections or re-exposure to drug-paired cues. Experimental compounds are administered to the animals prior to the reinstatement tests to evaluate their potential for attenuating or preventing drug-seeking behavior. This article describes the common procedures of the extinction-reinstatement model in studies of this type, and identifies areas of discrepancy. This is followed by a comprehensive overview of the currently published anti-reinstatement effects of pharmacological compounds, classified by the most relevant neurological systems associated with these compounds. The article concludes with a brief discussion of how the study of anti-reinstatement effects can be expanded to further verify existing positive results or to find novel neurobiological targets.

Lesions and reversible inactivation of the dorsolateral caudate-putamen impair cocaine-primed reinstatement to cocaine-seeking in rats

Recent evidence suggests that cocaine addiction may involve progressive drug-induced neuroplasticity of the dorsal striatum. Here, we examined the effects of a) dorsolateral caudate putamen (dlCPu) lesions on cocaine self-administration, extinction of responding, and subsequent reinstatement to cocaine-seeking, and b) reversible inactivation of the dlCPu with GABA receptor agonists (baclofen and muscimol) immediately prior to reinstatement testing. Male, Sprague-Dawley rats self-administered cocaine (0.2mg/50μl infusion, i.v.) along an FR1 schedule in daily 2h sessions for 10days, whereby lever presses resulted in cocaine infusions and presentation of a paired light-tone stimulus complex. After 14days of abstinence, animals were returned to the self-administration chamber and lever responding was recorded, but had no programmed consequences (relapse test). Animals then underwent daily extinction, followed by reinstatement tests in the presence of the conditioned cues, after a cocaine priming injection (10mg/kg), or cues+cocaine prime. Lesions of the dlCPu failed to affect responding during self-administration, extinction, relapse, or cued-induced reinstatement. However, lesioned animals showed reduced cocaine-seeking during cocaine-primed reinstatement as compared to sham controls. Furthermore, reversible inactivation of the dlCPu significantly impaired both cocaine-primed and cocaine-primed+cue-induced reinstatement. These results demonstrate the critical involvement of the dlCPu in cocaine-primed reinstatement, perhaps via chronic drug-induced changes in the interoceptive effects of cocaine that impact drug-seeking.

Chronic cocaine exposure induces putamen glutamate and glutamine metabolite abnormalities in squirrel monkeys

RATIONALE

Chronic cocaine exposure has been associated with progressive brain structural and functional changes. Clarifying mechanisms underlying cocaine's progressive brain effects may help in the development of effective cocaine abuse treatments.

OBJECTIVES

We used a controlled squirrel monkey model of chronic cocaine exposure (45 mg/kg/week for 9 months) combined with ultra-high magnetic field (9.4 T) proton magnetic resonance spectroscopy to prospectively measure putamen metabolite changes.

METHODS

Proton metabolites were measured with a STEAM sequence, quantified with LCModel using a simulated basis set, and expressed as metabolite/total creatine (tCr) ratios.

RESULTS

We found cocaine-induced time-dependent changes in putamen glutamate/tCr and glutamine/tCr metabolite ratios suggestive of altered glutamate compartmentalization, neurotransmission, and metabolism. By contrast, saline-treated monkeys exhibited no metabolite changes over time. The time course of cocaine-induced metabolite abnormalities we detected is consistent with the apparent time course of glutamate abnormalities identified in a cross-sectional study in human cocaine users, as well as with microdialysis findings in rodent models of repeated cocaine exposure.

CONCLUSIONS

Together, these findings suggests that this squirrel monkey model may be useful for characterizing glutamatergic changes associated with cocaine exposure and for determining efficacies of treatments designed to mitigate cocaine-induced glutamatergic system dysfunction.

Nucleus accumbens dopamine/glutamate interaction switches modes to generate desire versus dread: D(1) alone for appetitive eating but D(1) and D(2) together for fear

The medial shell of nucleus accumbens (NAc) and its mesolimbic dopamine inputs mediate forms of fearful as well as of incentive motivation. For example, either appetitive and/or actively fearful behaviors are generated in a keyboard pattern by localized glutamate disruptions in NAc (via microinjection of the AMPA receptor antagonist DNQX) at different anatomical locations along a rostrocaudal gradient within the medial shell of rats. Rostral glutamate disruptions produce intense increases in eating, but more caudally placed disruptions produce increasingly fearful behaviors: distress vocalizations and escape attempts to human touch, and a spontaneous and directed antipredator response called defensive treading/burying. Local endogenous dopamine is required for either intense motivation to be generated by AMPA disruptions. Here we report that only endogenous local signaling at D(1) dopamine receptors is needed for rostral generation of excessive eating, potentially implicating a direct output pathway contribution. In contrast, fear generation at caudal sites requires both D(1) and D(2) signaling simultaneously, potentially implicating an indirect output pathway contribution. Finally, when motivation valence generated by AMPA disruptions at intermediate sites was flipped by manipulating environmental ambience, from mostly appetitive in a comfortable home environment to mostly fearful in a stressful environment, the roles of local D(1) and D(2) signaling in dopamine/glutamate interaction at microinjection sites also switched dynamically to match the motivation valence generated at the moment. Thus, NAc D(1) and D(2) receptors, and their associated neuronal circuits, play different and dynamic roles in enabling desire and dread to be generated by localized NAc glutamate disruptions in medial shell.

A silent synapse-based mechanism for cocaine-induced locomotor sensitization

Locomotor sensitization is a common and robust behavioral alteration in rodents whereby following exposure to abused drugs such as cocaine, the animal becomes significantly more hyperactive in response to an acute drug challenge. Here, we further analyzed the role of cocaine-induced silent synapses in the nucleus accumbens (NAc) shell and their contribution to the development of locomotor sensitization. Using a combination of viral vector-mediated genetic manipulations, biochemistry, and electrophysiology in a locomotor sensitization paradigm with repeated, daily, noncontingent cocaine (15 mg/kg) injections, we show that dominant-negative cAMP-element binding protein (CREB) prevents cocaine-induced generation of silent synapses of young (30 d old) rats, whereas constitutively active CREB is sufficient to increase the number of NR2B-containing NMDA receptors (NMDARs) at synapses and to generate silent synapses. We further show that occupancy of CREB at the NR2B promoter increases and is causally related to the increase in synaptic NR2B levels. Blockade of NR2B-containing NMDARs by administration of the NR2B-selective antagonist Ro256981 directly into the NAc, under conditions that inhibit cocaine-induced silent synapses, prevents the development of cocaine-elicited locomotor sensitization. Our data are consistent with a cellular cascade whereby cocaine-induced activation of CREB promotes CREB-dependent transcription of NR2B and synaptic incorporation of NR2B-containing NMDARs, which generates new silent synapses within the NAc. We propose that cocaine-induced activation of CREB and generation of new silent synapses may serve as key cellular events mediating cocaine-induced locomotor sensitization. These findings provide a novel cellular mechanism that may contribute to cocaine-induced behavioral alterations.

Glutamatergic medications for the treatment of drug and behavioral addictions

Historically, most pharmacological approaches to the treatment of addictive disorders have utilized either substitution-based methods (i.e., nicotine replacement or opioid maintenance) or have targeted monoaminergic or endogenous opioidergic neurotransmitter systems. However, substantial evidence has accumulated indicating that ligands acting on glutamatergic transmission are also of potential utility in the treatment of drug addiction, as well as various behavioral addictions such as pathological gambling. The purpose of this review is to summarize the pharmacological mechanisms of action and general clinical efficacy of glutamatergic medications that are currently approved or are being investigated for approval for the treatment of addictive disorders. Medications with effects on glutamatergic transmission that will be discussed include acamprosate, N-acetylcysteine, d-cycloserine, gabapentin, lamotrigine, memantine, modafinil, and topiramate. We conclude that manipulation of glutamatergic neurotransmission is a relatively young but promising avenue for the development of improved therapeutic agents for the treatment of drug and behavioral addictions.

Addiction and brain reward and antireward pathways

Addictive drugs have in common that they are voluntarily self-administered by laboratory animals (usually avidly), and that they enhance the functioning of the reward circuitry of the brain (producing the 'high' that the drug user seeks). The core reward circuitry consists of an 'in-series' circuit linking the ventral tegmental area, nucleus accumbens and ventral pallidum via the medial forebrain bundle. Although originally believed to simply encode the set point of hedonic tone, these circuits are now believed to be functionally far more complex, also encoding attention, expectancy of reward, disconfirmation of reward expectancy, and incentive motivation. 'Hedonic dysregulation' within these circuits may lead to addiction. The 'second-stage' dopaminergic component in this reward circuitry is the crucial addictive-drug-sensitive component. All addictive drugs have in common that they enhance (directly or indirectly or even transsynaptically) dop-aminergic reward synaptic function in the nucleus accumbens. Drug self-administration is regulated by nucleus accumbens dopamine levels, and is done to keep nucleus accumbens dopamine within a specific elevated range (to maintain a desired hedonic level). For some classes of addictive drugs (e.g. opiates), tolerance to the euphoric effects develops with chronic use. Postuse dysphoria then comes to dominate reward circuit hedonic tone, and addicts no longer use drugs to get high, but simply to get back to normal ('get straight'). The brain circuits mediating the pleasurable effects of addictive drugs are anatomically, neurophysiologically and neurochemically different from those mediating physical dependence, and from those mediating craving and relapse. There are important genetic variations in vulnerability to drug addiction, yet environmental factors such as stress and social defeat also alter brain-reward mechanisms in such a manner as to impart vulnerability to addiction. In short, the 'bio-psycho-social' model of etiology holds very well for addiction. Addiction appears to correlate with a hypodopaminergic dysfunctional state within the reward circuitry of the brain. Neuroimaging studies in humans add credence to this hypothesis. Credible evidence also implicates serotonergic, opioid, endocannabinoid, GABAergic and glutamatergic mechanisms in addiction. Critically, drug addiction progresses from occasional recreational use to impulsive use to habitual compulsive use. This correlates with a progression from reward-driven to habit-driven drug-seeking behavior. This behavioral progression correlates with a neuroanatomical progression from ventral striatal (nucleus accumbens) to dorsal striatal control over drug-seeking behavior. The three classical sets of craving and relapse triggers are (a) reexposure to addictive drugs, (b) stress, and (c) reexposure to environmental cues (people, places, things) previously associated with drug-taking behavior. Drug-triggered relapse involves the nucleus accumbens and the neurotransmitter dopamine. Stress-triggered relapse involves (a) the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and the neurotransmitter corticotrophin-releasing factor, and (b) the lateral tegmental noradrenergic nuclei of the brain stem and the neurotransmitter norepinephrine. Cue-triggered relapse involves the basolateral nucleus of the amygdala, the hippocampus and the neurotransmitter glutamate. Knowledge of the neuroanatomy, neurophysiology, neurochemistry and neuropharmacology of addictive drug action in the brain is currently producing a variety of strategies for pharmacotherapeutic treatment of drug addiction, some of which appear promising.

Harnessing neuroplasticity for clinical applications

Neuroplasticity can be defined as the ability of the nervous system to respond to intrinsic or extrinsic stimuli by reorganizing its structure, function and connections. Major advances in the understanding of neuroplasticity have to date yielded few established interventions. To advance the translation of neuroplasticity research towards clinical applications, the National Institutes of Health Blueprint for Neuroscience Research sponsored a workshop in 2009. Basic and clinical researchers in disciplines from central nervous system injury/stroke, mental/addictive disorders, paediatric/developmental disorders and neurodegeneration/ageing identified cardinal examples of neuroplasticity, underlying mechanisms, therapeutic implications and common denominators. Promising therapies that may enhance training-induced cognitive and motor learning, such as brain stimulation and neuropharmacological interventions, were identified, along with questions of how best to use this body of information to reduce human disability. Improved understanding of adaptive mechanisms at every level, from molecules to synapses, to networks, to behaviour, can be gained from iterative collaborations between basic and clinical researchers. Lessons can be gleaned from studying fields related to plasticity, such as development, critical periods, learning and response to disease. Improved means of assessing neuroplasticity in humans, including biomarkers for predicting and monitoring treatment response, are needed. Neuroplasticity occurs with many variations, in many forms, and in many contexts. However, common themes in plasticity that emerge across diverse central nervous system conditions include experience dependence, time sensitivity and the importance of motivation and attention. Integration of information across disciplines should enhance opportunities for the translation of neuroplasticity and circuit retraining research into effective clinical therapies.

Nicotine up-regulates alpha4beta2 nicotinic receptors and ER exit sites via stoichiometry-dependent chaperoning

The up-regulation of α4β2* nicotinic acetylcholine receptors (nAChRs) by chronic nicotine is a cell-delimited process and may be necessary and sufficient for the initial events of nicotine dependence. Clinical literature documents an inverse relationship between a person’s history of tobacco use and his or her susceptibility to Parkinson’s disease; this may also result from up-regulation. This study visualizes and quantifies the subcellular mechanisms involved in nicotine-induced nAChR up-regulation by using transfected fluorescent protein (FP)-tagged α4 nAChR subunits and an FP-tagged Sec24D endoplasmic reticulum (ER) exit site marker. Total internal reflection fluorescence microscopy shows that nicotine (0.1 µM for 48 h) up-regulates α4β2 nAChRs at the plasma membrane (PM), despite increasing the fraction of α4β2 nAChRs that remain in near-PM ER. Pixel-resolved normalized Förster resonance energy transfer microscopy between α4-FP subunits shows that nicotine stabilizes the (α4)₂(β2)₃ stoichiometry before the nAChRs reach the trans-Golgi apparatus. Nicotine also induces the formation of additional ER exit sites (ERES). To aid in the mechanistic analysis of these phenomena, we generated a β2_{enhanced-ER-export} mutant subunit that mimics two regions of the β4 subunit sequence: the presence of an ER export motif and the absence of an ER retention/retrieval motif. The α4β2_{enhanced-ER-export} nAChR resembles nicotine-exposed nAChRs with regard to stoichiometry, intracellular mobility, ERES enhancement, and PM localization. Nicotine produces only small additional PM up-regulation of α4β2_{enhanced-ER-export} receptors. The experimental data are simulated with a model incorporating two mechanisms: (1) nicotine acts as a stabilizing pharmacological chaperone for nascent α4β2 nAChRs in the ER, eventually increasing PM receptors despite a bottleneck(s) in ER export; and (2) removal of the bottleneck (e.g., by expression of the β2_{enhanced-ER-export} subunit) is sufficient to increase PM nAChR numbers, even without nicotine. The data also suggest that pharmacological chaperoning of nAChRs by nicotine can alter the physiology of ER processes.

Activation of D₂-like receptors in rat ventral tegmental area inhibits cocaine-reinstated drug-seeking behavior

Relapse is a hallmark of cocaine addiction. Cocaine-induced neuroplastic changes in the mesocorticolimbic circuits critically contribute to this phenomenon. Pre-clinical evidence indicates that relapse to cocaine-seeking behavior depends on activation of dopamine neurons in the ventral tegmental area. Thus, blocking such activation may inhibit relapse. Because the activity of dopamine neurons is regulated by D₂-like autoreceptors expressed on somatodendritic sites, this study, using the reinstatement model, aimed to determine whether activation of D₂-like receptors in the ventral tegmental area can inhibit cocaine-induced reinstatement of extinguished cocaine-seeking behavior. Rats were trained to self-administer i.v. cocaine (0.25 mg/infusion) under a modified fixed-ratio 5 schedule. After such behavior was well learned, rats went through extinction training to extinguish cocaine-seeking behavior. The effect of quinpirole, a selective D₂-like receptor agonist microinjected into the ventral tegmental area, on cocaine-induced reinstatement was then assessed. Quinpirole (0-3.2 μg/side) dose-dependently decreased cocaine-induced reinstatement and such effects were reversed by the selective D₂-like receptor antagonist eticlopride when co-microinjected with quinpirole into the ventral tegmental area. The effect appeared to be specific to the ventral tegmental area because quinpirole microinjected into the substantia nigra had no effect. Because D₂-like receptors are expressed on rat ventral tegmental area dopamine neurons projecting to the pre-frontal cortex and nucleus accumbens, our data suggest that these dopamine circuits may play a critical role in cocaine-induced reinstatement. The role of potential changes in D₂-like receptors and related signaling molecules of dopamine neurons in the vulnerability to relapse was discussed.

Unraveling glutamate-opioid receptor interactions using high-resolution electron microscopy: implications for addiction-related processes

Adaptive responses in glutamate and opioid receptor systems in limbic circuits are emerging as a critical component of the neural plasticity induced by chronic use of abused substances. The present commentary reviews findings from neuroanatomical studies, with superior spatial resolution, that support a cellular basis for prominent interactions of glutamate and opioid receptor systems in preclinical models of drug addiction. The review begins by highlighting the advantages of high-resolution electron microscopic immunohistochemistry for unraveling receptor interactions at the synapse. With an emphasis on a recent publication describing the anatomical relationship between the μ-opioid receptor (MOR) and the AMPA-GluR2 subunit (Beckerman, M. A., and Glass, M. J., 2011. Ultrastructural relationship between the AMPA-GluR2 receptor subunit and the mu-opioid receptor in the mouse central nucleus of the amygdala. Exp Neurol), we review the anatomical evidence for opioid-induced neural plasticity of glutamate receptors in selected brain circuits that are key integrative substrates in the brain's motivational system. The findings stress the importance of glutamate-opioid interactions as important neural mediators of adaptations to chronic use of abused drugs, particularly within the amygdaloid complex.

Cocaine-induced plasticity in the nucleus accumbens is cell specific and develops without prolonged withdrawal

Cocaine induces plasticity at glutamatergic synapses in the nucleus accumbens (NAc). Withdrawal was suggested to play an important role in the development of this plasticity by studies showing that some changes only appear several weeks after the final cocaine exposure. In this study, the requirement for prolonged withdrawal was evaluated by comparing the changes in glutamatergic transmission induced by two different noncontingent cocaine treatments: a short treatment followed by prolonged withdrawal, and a longer treatment without prolonged withdrawal. Recordings were performed from mouse medium spiny neurons (MSNs) in the NAc at the same time after the first cocaine injection under both treatments. A similar increase in the frequency of glutamate-mediated miniature EPSCs was observed in D(1)-expressing MSNs after both cocaine treatments, demonstrating that prolonged withdrawal was not required. Furthermore, larger AMPA receptor-to-NMDA receptor ratios, higher spine density, and enlarged spine heads were observed in the absence of withdrawal after a long cocaine treatment. These synaptic adaptations expressed in D(1)-containing MSNs of the NAc core were not further enhanced by protracted withdrawal. In conclusion, a few repeated cocaine injections are enough to trigger adaptations at glutamatergic synapses in D(1)-expressing MSNs, which, although they take time to develop, do not require prolonged cocaine withdrawal.

Ceftriaxone, a beta-lactam antibiotic, reduces ethanol consumption in alcohol-preferring rats

AIMS

Changes in glutamatergic transmission affect many aspects of neuroplasticity associated with ethanol and drug addiction. For instance, ethanol- and drug-seeking behavior is promoted by increased glutamate transmission in key regions of the motive circuit. We hypothesized that because glutamate transporter 1 (GLT1) is responsible for the removal of most extracellular glutamate, up-regulation or activation of GLT1 would attenuate ethanol consumption.

METHODS

Alcohol-preferring (P) rats were given 24 h/day concurrent access to 15 and 30% ethanol, water and food for 7 weeks. During Week 6, P rats received either 25, 50, 100 or 200 mg/kg ceftriaxone (CEF, i.p.), a β-lactam antibiotic known to elevate GLT1 expression, or a saline vehicle for five consecutive days. Water intake, ethanol consumption and body weight were measured daily for 15 days starting on Day 1 of injections. We also tested the effects of CEF (100 and 200 mg/kg, i.p.) on daily sucrose (10%) consumption as a control for motivated behavioral drinking.

RESULTS

Statistical analyses revealed a significant reduction in daily ethanol, but not sucrose, consumption following CEF treatment. During the post treatment period, there was a recovery of ethanol intake across days. Dose-dependent increases in water intake were manifest concurrent with the CEF-induced decreases in ethanol intake. Nevertheless, CEF did not affect body weight. An examination of a subset of the CEF-treated ethanol-drinking rats, on the third day post CEF treatment, revealed increases in GTL1 expression levels within the prefrontal cortex and nucleus accumbens.

CONCLUSIONS

These results indicate that CEF effectively reduces ethanol intake, possibly through activation of GLT1, and may be a potential therapeutic drug for alcohol addiction treatment.

Ciproxifan, a histamine H₃-receptor antagonist / inverse agonist, modulates methamphetamine-induced sensitization in mice

The role of histamine neurons in schizophrenia and psychostimulant abuse remains unclear. Behavioural sensitization to psychostimulants is a cardinal feature of these disorders. Here, we have explored the ability of imetit and ciproxifan (CPX), a reference H₃-receptor agonist and inverse agonist, respectively, to modulate locomotor sensitization induced in mice by methamphetamine (MET). Mice received saline, CPX (3 mg/kg) or imetit (3 mg/kg) 2 h before MET (2 mg/kg), once daily for 12 days, and were killed after a 2-day wash out. Imetit had no effect, but CPX induced a decrease of MET-induced locomotor activity, which became significant at Day 5, and even more at Day 10. Quantitative polymerase chain reaction was used in the sensitized mice to quantify brain-derived neurotrophic factor (BDNF) and N-methyl-D-aspartate (NMDA)-receptor subunit 1 (NR1) mRNAs, two factors that are altered in both schizophrenia and drug abuse. Imetit and CPX used alone had no effect on any marker. Sensitization by MET decreased BDNF mRNAs by 40% in the hippocampus. This decrease was reversed by CPX. Sensitization by MET also induced strong decreases of NR1 mRNAs in the cerebral cortex, hippocampus and striatum, but not hypothalamus. These decreases were also reversed by CPX. The strong modulator effect of CPX in mice sensitized to MET may result from its modulator effect on NR1 mRNAs in the cerebral cortex and striatum. The reversal by CPX of BDNF and NR1 mRNAs in the hippocampus of sensitized animals further strengthens the interest of H₃-receptor inverse agonists for the long-term treatment of cognitive deficits of patients with schizophrenia.

N-acetylcysteine in psychiatry: current therapeutic evidence and potential mechanisms of action

There is an expanding field of research investigating the benefits of alternatives to current pharmacological therapies in psychiatry. N-acetylcysteine (NAC) is emerging as a useful agent in the treatment of psychiatric disorders. Like many therapies, the clinical origins of NAC are far removed from its current use in psychiatry. Whereas the mechanisms of NAC are only beginning to be understood, it is likely that NAC is exerting benefits beyond being a precursor to the antioxidant, glutathione, modulating glutamatergic, neurotropic and inflammatory pathways. This review outlines the current literature regarding the use of NAC in disorders including addiction, compulsive and grooming disorders, schizophrenia and bipolar disorder. N-acetylcysteine has shown promising results in populations with these disorders, including those in whom treatment efficacy has previously been limited. The therapeutic potential of this acetylated amino acid is beginning to emerge in the field of psychiatric research.

Drug-evoked synaptic plasticity in addiction: from molecular changes to circuit remodeling

Addictive drugs have in common that they target the mesocorticolimbic dopamine (DA) system. This system originates in the ventral tegmental area (VTA) and projects mainly to the nucleus accumbens (NAc) and prefrontal cortex (PFC). Here, we review the effects that such drugs leave on glutamatergic and GABAergic synaptic transmission in these three brain areas. We refer to these changes as drug-evoked synaptic plasticity, which outlasts the presence of the drug in the brain and contributes to the reorganization of neural circuits. While in most cases these early changes are not sufficient to induce the disease, with repetitive drug exposure, they may add up and contribute to addictive behavior.

Prenatal Stress Alters Progestogens to Mediate Susceptibility to Sex-Typical, Stress-Sensitive Disorders, such as Drug Abuse: A Review

Maternal-offspring interactions begin prior to birth. Experiences of the mother during gestation play a powerful role in determining the developmental programming of the central nervous system. In particular, stress during gestation alters developmental programming of the offspring resulting in susceptibility to sex-typical and stress-sensitive neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. However, neither these effects, nor the underlying mechanisms, are well understood. Our hypothesis is that allopregnanolone, during gestation, plays a particularly vital role in mitigating effects of stress on the developing fetus and may mediate, in part, alterations apparent throughout the lifespan. Specifically, altered balance between glucocorticoids and progestogens during critical periods of development (stemming from psychological, immunological, and/or endocrinological stressors during gestation) may permanently influence behavior, brain morphology, and/or neuroendocrine-sensitive processes. 5α-reduced progestogens are integral in the developmental programming of sex-typical, stress-sensitive, and/or disorder-relevant phenotypes. Prenatal stress (PNS) may alter these responses and dysregulate allopregnanolone and its normative effects on stress axis function. As an example of a neurodevelopmental, neuropsychiatric, and/or neurodegenerative process, this review focuses on responsiveness to drugs of abuse, which is sensitive to PNS and progestogen milieu. This review explores the notion that allopregnanolone may effect, or be influenced by, PNS, with consequences for neurodevelopmental-, neuropsychiatric-, and/or neurodegenerative- relevant processes, such as addiction.

Acamprosate in the treatment of binge eating disorder: a placebo-controlled trial

OBJECTIVE

To assess preliminarily the effectiveness of acamprosate in binge eating disorder (BED).

METHOD

In this 10-week, randomized, placebo-controlled, flexible dose trial, 40 outpatients with BED received acamprosate (N = 20) or placebo (N = 20). The primary outcome measure was binge eating episode frequency.

RESULTS

While acamprosate was not associated with a significantly greater rate of reduction in binge eating episode frequency or any other measure in the primary longitudinal analysis, in the endpoint analysis it was associated with statistically significant improvements in binge day frequency and measures of obsessive-compulsiveness of binge eating, food craving, and quality of life. Among completers, weight and BMI decreased slightly in the acamprosate group but increased in the placebo group.

DISCUSSION

Although acamprosate did not separate from placebo on any outcome variable in the longitudinal analysis, results of the endpoint and completer analyses suggest the drug may have some utility in BED.

Normal and abnormal functions of adenosine receptors in the central nervous system revealed by genetic knockout studies

Endogenous adenosine is a widely distributed upstream regulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways that converge to contribute to the expression of an array of important brain functions. Over the past decade, the generation and characterization of genetic knockout models for all four G-protein coupled adenosine receptors, the A1 and A2A receptors in particular, has confirmed and extended the neuromodulatory and integrated role of adenosine receptors in the control of a broad spectrum of normal and abnormal brain functions. After a brief introduction of the available adenosine receptor knockout models, this review focuses on findings from the genetic knockout approach, placing particular emphasis on the most recent findings. This review is organized into two sections to separately address (i) the role of adenosine receptors in normal brain processes including neuroplasticity, sleep-wake cycle, motor function, cognition, and emotion-related behaviors; and (ii) their role in the response to various pathologic insults to brain such as ischemic stroke, neurodegeneration, or brain dysfunction/disorders. We largely limit our overview to the prominent adenosine receptor subtypes in brain-the A1 and A2A receptors-for which numerous genetic knockout studies on brain function are available. A1 and A2A receptor knockouts have provided significant new insights into adenosine's control of complex physiologic (e.g., cognition) and pathologic (e.g., neuroinflammation) phenomena. These findings extend and strengthen the support for A1 and A2A receptors in brain as therapeutic targets in several neurologic and psychiatric diseases. However, they also emphasize the importance of considering the disease context-dependent effect when developing adenosine receptor-based therapeutic strategies.

Allosteric Inhibitors of NMDA Receptor Functions

NMDA receptors are glutamate-activated ion-channels involved in many essential brain functions including learning, memory, cognition, and behavior. Given this broad range of function it is not surprising that the initial attempts to correct NMDA receptor-mediated pathologies with en-mass receptor blockade were derailed by unacceptable side effects. Recent successes with milder or more targeted pharmaceuticals and increasing knowledge of how these receptors operate offer new incentives for rational development of effective NMDA receptor-targeted therapies. In this article we review evidence that l-alanine, a glycine-site partial agonist and pregnanolone sulfate, a use-dependent allosteric inhibitor, while attenuating NMDA receptor activity to similar levels elicit remarkably dissimilar functional outcomes. We suggest that detailed understanding of NMDA receptor activation mechanisms and of structural correlates of function will help better match modulator with function and neurological condition and may unleash the yet untapped potential of NMDA receptor pharmaceutics.

Orphanin FQ/nociceptin not only blocks but also reverses behavioral adaptive changes induced by repeated cocaine in mice

BACKGROUND

Orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the opioid receptor-like (ORL1) receptor, blocks cocaine sensitization in rats. In this study, we tested whether OFQ/N would block sensitization to the motor stimulatory and conditioned rewarding actions of cocaine in mice. We also examined whether OFQ/N, given to cocaine-sensitized mice, would reverse the sensitized response and whether it would prevent the amplified sensitized response induced by a second cocaine-sensitizing regimen in sensitized mice.

METHODS

ORL1 knockout and wild-type mice were treated with saline or OFQ/N before saline or cocaine on Days 1-3 and tested for sensitization on Day 8. Additionally, wild-type mice were treated similarly but tested for the conditioned rewarding action of cocaine, in which mice were tested for place preference before and after single conditioning with cocaine. Furthermore, mice were rendered sensitized, treated with saline or OFQ/N before saline or cocaine on Days 13-15, and received cocaine on Day 20 to test whether OFQ/N would reverse sensitization or block the amplified sensitized response induced by a second cocaine-sensitizing regimen in sensitized mice.

RESULTS

OFQ/N blocked cocaine-induced psychomotor sensitization in wild-type but not knockout mice. It also blocked sensitization to the conditioned rewarding action of cocaine and reversed a preexisting locomotor sensitized response. Furthermore, OFQ/N prevented the amplified sensitized response that developed following a second cocaine sensitizing regimen given to sensitized mice.

CONCLUSIONS

These results illustrate that OFQ/N not only blocks but also reverses maladaptive behavioral changes induced by repeated cocaine treatment in mice.

Extinction training after cocaine self-administration induces glutamatergic plasticity to inhibit cocaine seeking

Learning to inhibit drug seeking can be an important strategy for inhibiting relapse, and this can be modeled by extinguishing drug seeking in response to a drug-paired context. Rats were either extinguished or withdrawn without extinction training (abstinence) from cocaine self-administration, and measurements of postsynaptic density proteins in the core and shell subcompartments of the nucleus accumbens were compared with yoked-saline controls. Only extinguished rats had elevations of PSD-95, Homer1b/c, and Narp in the postsynaptic density of the core, whereas no proteins measured were altered in the postsynaptic density of the shell in either extinguished or abstinent rats. Using a biotinylation strategy, it was found that surface expression of mGluR5 was reduced only in the core of extinguished animals. Although both extinguished and abstinent animals showed a reduction in long-term potentiation elicited in the core by stimulating prefrontal cortex, blunted long-term depression was observed only in extinguished rats. These data indicate that the elevation in Homer1b/c in the core may have sequestered mGluR5 away from the membrane surface and that the loss of surface mGluR5 inhibits long-term depression. Accordingly, when Homer1c was overexpressed in the core of cocaine-naive rats with an adenoassociated virus, long-term depression was inhibited. This mechanism may contribute to the inhibition of cocaine seeking by extinction training because overexpression of Homer1c in the core also inhibited cue-induced reinstatement of cocaine seeking. These data identify a cellular mechanism that may contribute to extinction-induced inhibition of cocaine seeking.

Metabolite levels in the brain reward pathway discriminate those who remain abstinent from those who resume hazardous alcohol consumption after treatment for alcohol dependence

OBJECTIVE

This study compared baseline metabolite levels in components of the brain reward system among individuals who remained abstinent and those who resumed hazardous alcohol consumption after treatment for alcohol dependence.

METHOD

Fifty-one treatment-seeking alcohol-dependent individuals (abstinent for approximately 7 days [SD = 3]) and 26 light-drinking nonsmoking controls completed 1.5-T proton magnetic resonance spectroscopic imaging, yielding regional concentrations of N-acetylaspartate, choline-containing compounds, creatine-containing compounds, and myoinositol. Metabolite levels were obtained in the following component of the brain reward system: dorsolateral prefrontal cortex, anterior cingulate cortex, insula, superior corona radiata, and cerebellar vermis. Alcohol-dependent participants were followed over a 12-month period after baseline study (i.e., at 7 days of abstinence [SD = 3]) and were classified as abstainers (no alcohol consumption; n = 18) and resumers (any alcohol consumption; n = 33) at follow-up. Baseline metabolite levels in abstainers and resumers and light-drinking nonsmoking controls were compared in the above regions of interest.

RESULTS

Resumers demonstrated significantly lower baseline N-acetylaspartate concentrations than light-drinking nonsmoking controls and abstainers in all regions of interest. Resumers also exhibited lower creatine-containing-compound concentrations than abstainers in the dorsolateral prefrontal cortex, superior corona radiata, and cerebellar vermis. Abstainers did not differ from light-drinking nonsmoking controls on baseline metabolite concentrations in any region of interest.

CONCLUSIONS

The significantly decreased N-acetylaspartate and creatine-containing-compound concentrations in resumers suggest compromised neuronal integrity and abnormalities in cellular bioenergetics in major neocortical components and white-matter interconnectivity of the brain reward pathway. The lack of metabolite differences between abstainers and light-drinking nonsmoking controls suggests premorbid factors potentially contributed to the baseline brain metabolite abnormalities observed in resumers.

Mixing pleasures: review of the effects of drugs on sex behavior in humans and animal models

Drugs of abuse act on the brain circuits mediating motivation and reward associated with natural behaviors. There is ample evidence that drugs of abuse impact male and female sexual behavior. First, the current review discusses the effect of drugs of abuse on sexual motivation and performance in male and female humans. In particular, we discuss the effects of commonly abused drugs including psychostimulants, opiates, marijuana/THC, and alcohol. In general, drug use affects sexual motivation, arousal, and performance and is commonly associated with increased sexual risk behaviors. Second, studies on effects of systemic administration of drugs of abuse on sexual behavior in animals are reviewed. These studies analyze the effects on sexual performance and motivation but do not investigate the effects of drugs on risk-taking behavior, creating a disconnect between human and animal studies. For this reason, we discuss two studies that focus on the effects of alcohol and methamphetamine on inhibition of maladaptive sex-seeking behaviors in rodents. Third, this review discusses potential brain areas where drugs of abuse may be exerting their effect on sexual behavior with a focus on the mesolimbic system as the site of action. Finally, we discuss recent studies that have brought to light that sexual experience in turn can affect drug responsiveness, including a sensitized locomotor response to amphetamine in female and male rodents as well as enhanced drug reward in male rats.

Cortical perfusion in alcohol-dependent individuals during short-term abstinence: relationships to resumption of hazardous drinking after treatment

Relapse to hazardous levels of alcohol consumption after treatment for alcohol use disorders is common. Investigation of the neurobiological correlates of resumption of hazardous drinking is necessary to clarify the mechanisms contributing to relapse. Fifty-seven treatment-seeking alcohol-dependent participants (ALC) completed arterial spin labeling perfusion MRI of the frontal and parietal gray matter (GM) at 7+/-3 days of abstinence (baseline). ALC participants were restudied after 35+/-11 days of abstinence (assessment point 2: AP2). Twenty-eight nonsmoking, light-drinking control participants (nsLD) from the community were studied with perfusion MRI. ALC participants were followed over 12 months after baseline study and were classified as abstainers (no alcohol consumption; n=19) and resumers (any alcohol consumption; n=38) at follow-up. Cross-sectional and longitudinal perfusion was compared in abstainers, resumers, and nsLD. At baseline, resumers demonstrated significantly lower frontal and parietal GM perfusion than nsLD and abstainers. Abstainers and nsLD were not different on frontal or parietal GM perfusion. No significant longitudinal perfusion changes were observed in abstainers and resumers. At AP2, resumers showed significantly lower frontal GM perfusion than nsLD and abstainers, whereas no group differences were observed for parietal GM. Abstainers and nsLD were not different on frontal GM perfusion. The significantly decreased frontal GM perfusion in resumers compared with both abstainers and nsLD across the assessment interval suggests premorbid and/or acquired neurobiological abnormalities of the frontal GM in resumers.

Glial dysfunction in abstinent methamphetamine abusers

Persistent neurochemical abnormalities in frontal brain structures are believed to result from methamphetamine use. We developed a localized (13)C magnetic resonance spectroscopy (MRS) assay on a conventional MR scanner, to quantify selectively glial metabolic flux rate in frontal brain of normal subjects and a cohort of recovering abstinent methamphetamine abusers. Steady-state bicarbonate concentrations were similar, between 11 and 15 mmol/L in mixed gray-white matter of frontal brain of normal volunteers and recovering methamphetamine-abusing subjects (P>0.1). However, glial (13)C-bicarbonate production rate from [1-(13)C]acetate, equating with glial tricarboxylic acid (TCA) cycle rate, was significantly reduced in frontal brain of abstinent methamphetamine-addicted women (methamphetamine 0.04 micromol/g per min (N=5) versus controls 0.11 micromol/g per min (N=5), P=0.001). This is equivalent to 36% of the normal glial TCA cycle rate. Severe reduction in glial TCA cycle rate that normally comprises 10% of total cerebral metabolic rate may impact operation of the neuronal glial glutamate cycle and result in accumulation of frontal brain glutamate, as observed in these recovering methamphetamine abusers. Although these are the first studies to define directly an abnormality in glial metabolism in human methamphetamine abuse, sequential studies using analogous (13)C MRS methods may determine 'cause and effect' between glial failure and neuronal injury.

Effects of the mGlu2/3 agonist LY379268 and the mGlu5 antagonist MTEP on ethanol seeking and reinforcement are differentially altered in rats with a history of ethanol dependence

BACKGROUND

Growing evidence supports a role of metabotropic glutamate receptors (mGluRs) in ethanol reinforcement, ethanol seeking, and ethanol withdrawal. To extend the understanding of the role of mGluRs in the addiction-relevant effects of ethanol as well as of the treatment target potential of these receptors for alcohol abuse, the effects of a selective mGlu2/3 agonist (LY379268) and a selective mGlu5 antagonist (MTEP) were tested on two processes central to alcohol addiction: ethanol reinforcement and stress-induced reinstatement of ethanol seeking in rats with a history of ethanol dependence.

METHODS

Following operant ethanol self-administration training, male Wistar rats were made dependent by intragastric ethanol intubations. Ethanol dependence was confirmed by the presence of somatic withdrawal signs. Following 2 weeks of withdrawal, stable ethanol self-administration was reestablished, and the effects of LY379268 (0-3 mg/kg subcutaneous) and MTEP (0-3 mg/kg, intraperitoneal) on ethanol self-administration were determined in both nondependent and postdependent rats. A second set of rats underwent extinction training and then was tested for the effects of LY379268 or MTEP on reinstatement of ethanol seeking induced by footshock stress.

RESULTS

LY379268 and MTEP dose-dependently reduced both ethanol self-administration and reinstatement of ethanol seeking induced by footshock stress. Additionally, LY379268 was more effective than MTEP in inhibiting both behaviors in postdependent than in nondependent animals.

CONCLUSIONS

These findings suggest that neuroadaptation associated with chronic ethanol exposure or withdrawal alters the sensitivity of mGlu2/3 receptors, with implications for the understanding of the neural basis of alcohol dependence and the treatment target potential of these receptors.

Ultrastructural relationships between cortical, thalamic, and amygdala glutamatergic inputs and group I metabotropic glutamate receptors in the rat accumbens

Changes in glutamatergic transmission in the nucleus accumbens play a key role in mediating reward-related behaviors and addiction to psychostimulants. Glutamatergic inputs to the accumbens originate from multiple sources, including the prefrontal cortex, basolateral amygdala, and midline thalamus. The group I metabotropic glutamate receptors (mGluRs) are found throughout the core and shell of the nucleus accumbens, but their localization and function at specific glutamatergic synapses remain unknown. To further characterize the substrate that underlies group I mGluR functions in the accumbens, we combined anterograde tract tracing method with electron microscopy immunocytochemistry to study the ultrastructural relationships between specific glutamatergic afferents and mGluR1a- or mGluR5-containing neurons in the rat nucleus accumbens. Although cortical, thalamic, and amygdala glutamatergic terminals contact both mGluR1a- and mGluR5-immunoreactive dendrites and spines in the shell and core of the accumbens, they do so to varying degrees. Overall, glutamatergic terminals contact mGluR1a-positive spines about 30% of the time, whereas they form synapses twice as frequently with mGluR5-labeled spines. At the subsynaptic level, mGluR5 is more frequently expressed perisynaptically and closer to the edges of glutamatergic axospinous synapses than mGluR1a, suggesting a differential degree of activation of the two group I mGluRs by transmitter spillover from glutamatergic synapses in the rat accumbens. These results lay the foundation for a deeper understanding of group I mGluR-mediated effects in the ventral striatum, and their potential therapeutic benefits in drug addiction and other neuropsychiatric changes in reward-related behaviors.