N-acetylcysteine reduces extinction responding and induces enduring reductions in cue- and heroin-induced drug-seeking

β-lactams modulate astroglial glutamate transporters and attenuate dependence to CP 55,940, a CB1 receptor agonist, in rat model

Studies on cannabinoids have reported contradictory findings, showing both aversion and rewarding outcomes in conditioned place preference (CPP). Various possibilities have been suggested to explain the aversive properties of cannabinoids, including the pharmacokinetics profile and dose selection. In this study, we have established a CPP method to investigate the effects of modulating astroglial glutamate transporters in cannabinoid dependence using a cannabinoid receptor 1 (CB1R) agonist, CP 55,940 (CP). Previous reports using CPP paradigm demonstrated the involvement of glutamatergic system in seeking behavior of several drugs of abuse such as cocaine, heroin and nicotine. Glutamate homeostasis is maintained by several astroglial glutamate transporters, such as glutamate transporter 1 (GLT-1), cystine/glutamate transporter (xCT) and glutamate aspartate transporter (GLAST). In this study, we investigated the effects of Ampicillin/Sulbactam, β-lactam compounds known to upregulate GLT-1 and xCT, on cannabinoid seeking behavior using CP. We found first that one prime dose of CP induced CP reinstatement; this effect was associated, in part, with significant downregulation of xCT expression in the nucleus accumbens, dorsomedial prefrontal cortex and amygdala. Moreover, GLT-1 expression was downregulated in the amygdala. Importantly, Ampicillin/Sulbactam treatment during the extinction phase attenuated CP-induced reinstatement and restored the expression of GLT-1 and xCT in mesocorticolimbic brain regions. These findings suggest that β-lactams may play a potential therapeutic role in attenuating dependence to cannabinoids, in part, through upregulation of GLT-1 and xCT.

Chronic treatment with N-acetylcysteine decreases extinction responding and reduces cue-induced nicotine-seeking

N-acetylcysteine (NAC), a promising glutamatergic therapeutic agent, has shown some clinical efficacy in reducing nicotine use in humans and has been shown to reverse drug-induced changes in glutamatergic neurophysiology. In rats, nicotine-seeking behavior is associated with alterations in glutamatergic plasticity within the nucleus accumbens core (NAcore). Specifically, cue-induced nicotine-seeking is associated with rapid, transient synaptic plasticity (t-SP) in glutamatergic synapses on NAcore medium spiny neurons. The goal of the present study was to determine if NAC reduces nicotine-seeking behavior and reverses reinstatement-associated NAcore glutamatergic alterations. Rats were extinguished from nicotine self-administration, followed by subchronic NAC administration (0 or 100 mg/kg/d) for 4 days prior to cue-induced reinstatement. NAcore synaptic potentiation was measured via dendritic spine morphology and mRNA and protein of relevant glutamatergic genes were quantified. Nicotine-seeking behavior was not reduced by subchronic NAC treatment. Also, NAcore transcript and protein expression of multiple glutamatergic genes, as well as spine morphological measures, were unaffected by subchronic NAC. Finally, chronic NAC treatment (15 days total) during extinction and prior to reinstatement significantly decreased extinction responding and reduced reinstatement of nicotine-seeking compared to vehicle. Together, these results suggest that chronic NAC treatment is necessary for its therapeutic efficacy as a treatment strategy for nicotine addiction and relapse.

Glial mechanisms underlying substance use disorders

Addiction is a devastating disorder that produces persistent maladaptive changes to the central nervous system, including glial cells. Although there is an extensive body of literature examining the neuronal mechanisms of substance use disorders, effective therapies remain elusive. Glia, particularly microglia and astrocytes, have an emerging and meaningful role in a variety of processes beyond inflammation and immune surveillance, and may represent a promising therapeutic target. Indeed, glia actively modulate neurotransmission, synaptic connectivity and neural circuit function, and are critically poised to contribute to addictive-like brain states and behaviors. In this review, we argue that glia influence the cellular, molecular, and synaptic changes that occur in neurons following drug exposure, and that this cellular relationship is critically modified following drug exposure. We discuss direct actions of abused drugs on glial function through immune receptors, such as Toll-like receptor 4, as well as other mechanisms. We highlight how drugs of abuse affect glia-neural communication, and the profound effects that glial-derived factors have on neuronal excitability, structure, and function. Recent research demonstrates that glia have brain region-specific functions, and glia in different brain regions have distinct contributions to drug-associated behaviors. We will also evaluate the evidence demonstrating that glial activation is essential for drug reward and drug-induced dopamine release, and highlight clinical evidence showing that glial mechanisms contribute to drug abuse liability. In this review, we synthesize the extensive evidence that glia have a unique, pivotal, and underappreciated role in the development and maintenance of addiction.

A Model of Δ9-Tetrahydrocannabinol Self-administration and Reinstatement That Alters Synaptic Plasticity in Nucleus Accumbens

BACKGROUND

Cannabis is the most widely used illicit drug, but knowledge of the neurological consequences of cannabis use is deficient. Two primary components of cannabis are Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). We established a THC+CBD model of self-administration and reinstated drug seeking to determine if, similar to other addictive drugs, cannabis produces enduring synaptic changes in nucleus accumbens core (NAcore) thought to contribute vulnerability to drug reinstatement.

METHODS

Sprague Dawley rats were trained to self-administer THC+CBD (n = 165) or were used as vehicle self-administering control animals (n = 24). Reinstatement was initiated by context, cues, drug priming, and stress (yohimbine injection). Enduring neuroadaptations produced by THC+CBD self-administration were assayed using four measures: dendritic spine morphology, long-term depression, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate ratios, and behavioral pharmacology.

RESULTS

We described a novel rodent model of cannabis relapse involving intravenous THC+CBD self-administration and drug seeking induced by conditioned context, cues, and stress. Cued reinstatement of THC+CBD seeking depended on a sequence of events implicated in relapse to other addictive drugs, as reinstatement was prevented by daily treatment with N-acetylcysteine or acute intra-NAcore pretreatment with a neuronal nitric oxide synthase or matrix metalloprotease-9 inhibitor, all of which normalize impaired glutamate homeostasis. The capacity to induce N-methyl-D-aspartate long-term depression in NAcore medium spiny neurons was abolished and dendritic spine density was reduced, but alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate ratio was unaltered in THC+CBD-trained animals, akin to opioids, but not to psychostimulants.

CONCLUSIONS

We report enduring consequences of THC+CBD use on critical relapse circuitry and synaptic physiology in NAcore following rat self-administration and provide the first report of cue- and stress-induced reinstatement with this model.

Geranylgeranylacetone blocks the reinstatement of morphine-conditioned place preference

Morphine is widely used for clinical pain management and induces the dependence. Addiction to morphine is a major public health issue. Geranylgeranylacetone (GGA) is widely used in clinic for treating ulcer. GGA induces expression of thioredoxin-1 (Trx-1) extensively. Trx-1 is a redox regulating protein and plays protecting roles in nervous system. GGA prevents mice against morphine-induced hyperlocomotion, rewarding effect, and withdrawal syndrome. However, whether GGA blocks morphine-conditioned place preference (CPP) reinstatement is still unknown. In the present study, we found that GGA administration blocked the reinstatement of morphine-CPP. The expressions of Trx-1, N-methyl d-aspartate receptor 2B subunit (NR2B), phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (p-CaMKII), phosphorylated extracellular signaling regulated kinases (p-ERK), and phosphorylated cAMP-response element binding protein (p-CREB) were induced in nucleus accumbens (NAc) and hippocampus by morphine or GGA, whereas these proteins were not changed by morphine in GGA-treated mice. Our results indicate that GGA may prevent the reinstatement of morphine-CPP through strengthening the expression of Trx-1 and regulating NR2B/ERK pathway. Thus, we suggest that GGA may be a promising therapeutic candidate for morphine-induced relapse.

Normal glutathione levels in autopsied brain of chronic users of heroin and of cocaine

BACKGROUND

Animal studies suggest that exposure to either of the two widely used drugs of abuse, heroin or cocaine, causes depletion of the antioxidant, reduced glutathione, a hallmark of oxidative stress, in the brain. However, the relevance of the animal findings to the human is uncertain and clinical trials with the antioxidant GSH precursor n-acetylcysteine have produced mixed results in cocaine dependence.

METHODS

Our major objective was to compare glutathione levels, determined by an HPLC-coulometric procedure, in autopsied brain of chronic heroin (n = 11) and cocaine users (n = 9), who were positive for the drugs in the brain, to those of matched controls (n = 16). Six brain regions were examined, including caudate, hippocampus, thalamus and frontal, temporal and insular cortices.

RESULTS

In contrast to experimental animal findings, we found no statistically significant difference between mean levels of reduced or oxidized glutathione in the drug user vs. control groups. Moreover, no correlation was found between levels of drugs in the brain and those of glutathione.

CONCLUSIONS

Acknowledging the many generic limitations of an autopsied human brain study and the preliminary nature of the findings, our data nevertheless suggest that any oxidative stress caused by heroin or cocaine in chronic users of the drugs might not be sufficient to cause substantial loss of stores of glutathione in the human brain, at least during early withdrawal. These findings, requiring replication, might also have some relevance to future clinical trials employing glutathione supplement therapy as an anti-oxidative strategy in chronic users of the two abused drugs.

Effects of ceftriaxone on hydrocodone seeking behavior and glial glutamate transporters in P rats

Hydrocodone (HYD) is one of the most widely prescribed opioid analgesic drugs. Several neurotransmitters are involved in opioids relapse. Among these neurotransmitters, glutamate is suggested to be involved in opioid dependence and relapse. Glutamate is regulated by several glutamate transporters, including glutamate transporter 1 (GLT-1) and cystine/glutamate transporter (xCT). In this study, we investigated the effects of ceftriaxone (CEF) (200 mg/kg, i.p.), known to upregulate GLT-1 and xCT, on reinstatement to HYD (5 mg/kg, i.p.) using the conditioned place preference (CPP) paradigm in alcohol-preferring (P) rats. Animals were divided into three groups: 1) saline-saline group (SAL-SAL); 2) HYD-SAL group; and 3) HYD-CEF group. The CPP was conducted as follows: habituation phase, conditioning phase with HYD (i.p.) injections every other day for four sessions, extinction phase with CEF (i.p.) injections every other day for four sessions, and reinstatement phase with one priming dose of HYD. Time spent in the HYD-paired chamber after conditioning training was increased as compared to pre-conditioning. There was an increase in time spent in the HYD-paired chamber with one priming dose of HYD in the reinstatement test. HYD exposure downregulated xCT expression in the nucleus accumbens and hippocampus, but no effects were observed in the dorsomedial prefrontal cortex and amygdala. Importantly, CEF treatment attenuated the reinstatement effect of HYD and normalized xCT expression in the affected brain regions. These findings demonstrate that the attenuating effect of HYD reinstatement with CEF might be mediated through xCT.

The role of glutamate signaling in incentive salience: second-by-second glutamate recordings in awake Sprague-Dawley rats

The attribution of incentive salience to reward-predictive stimuli has been shown to be associated with substance abuse-like behavior such as increased drug taking. Evidence suggests that glutamate neurotransmission and sequential N-methyl-D-aspartate (NMDA) activation are involved in the attribution of incentive salience. Here, we further explore the role of second-by-second glutamate neurotransmission in the attribution of incentive salience to reward-predictive stimuli by measuring sign-tracking behavior during a Pavlovian conditioned approach procedure using ceramic-based microelectrode arrays configured for sensitive measures of extracellular glutamate in awake behaving Sprague-Dawley rats. Specifically, we show that there is an increase in extracellular glutamate levels in the prelimbic cortex (PrL) and the nucleus accumbens core (NAcC) during sign-tracking behavior to a food-predictive conditioned stimulus (CS+) compared to the presentation of a non-predictive conditioned stimulus (CS-). Furthermore, the results indicate greater increases in extracellular glutamate levels in the PrL compared to NAcC in response to the CS+, including differences in glutamate release and signal decay. Taken together, the present research suggests that there is differential glutamate signaling in the NAcC and PrL during sign-tracking behavior to a food-predictive CS+.

Heroin seeking becomes dependent on dorsal striatal dopaminergic mechanisms and can be decreased by N-acetylcysteine

The alarming increase in heroin overdoses in the USA is a reminder of the need for efficacious and novel treatments for opiate addiction. This may reflect the relatively poor understanding of the neural basis of heroin, as compared to cocaine, seeking behaviour. While cocaine reinforcement depends on the mesolimbic system, well‐established cocaine seeking is dependent on dorsolateral striatum (aDLS) dopamine‐dependent mechanisms which are disrupted by N‐acetylcysteine, through normalisation of corticostriatal glutamate homeostasis. However, it is unknown whether a functional recruitment of aDLS dopamine‐dependent control over instrumental responding also occurs for heroin seeking, even though heroin reinforcement does not depend on the mesolimbic dopamine system. Lister Hooded rats acquired heroin self‐administration and were subsequently trained to seek heroin daily over prolonged periods of time under the control of drug‐paired cues, as measured under a second‐order schedule of reinforcement. At different stages of training, that is, early on and when heroin seeking behaviour was well established, we measured the sensitivity of drug‐seeking responses to either bilateral aDLS infusions of the dopamine receptor antagonist α‐flupenthixol (5, 10 and 15 μg/side) or systemic administration of N‐acetylcysteine (30, 60 and 90 mg/kg). The results demonstrate that control over heroin seeking behaviour devolves to aDLS dopamine‐dependent mechanisms after extended training. Further aDLS‐dependent well‐established, cue‐controlled heroin seeking was disrupted by N‐acetylcysteine. Comparison with previous data on cocaine suggests that the development of drug seeking habits and the alteration of corticostriatal glutamate homeostasis, which is restored by N‐acetylcysteine, are quantitatively similar between heroin and cocaine.

Dorsal-CA1 Hippocampal Neuronal Ensembles Encode Nicotine-Reward Contextual Associations

Natural and drug rewards increase the motivational valence of stimuli in the environment that, through Pavlovian learning mechanisms, become conditioned stimuli that directly motivate behavior in the absence of the original unconditioned stimulus. While the hippocampus has received extensive attention for its role in learning and memory processes, less is known regarding its role in drug-reward associations. We used in vivo Ca²⁺ imaging in freely moving mice during the formation of nicotine preference behavior to examine the role of the dorsal-CA1 region of the hippocampus in encoding contextual reward-seeking behavior. We show the development of specific neuronal ensembles whose activity encodes nicotine-reward contextual memories and that are necessary for the expression of place preference. Our findings increase our understanding of CA1 hippocampal function in general and as it relates to reward processing by identifying a critical role for CA1 neuronal ensembles in nicotine place preference.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Opioid and Psychostimulant Plasticity: Targeting Overlap in Nucleus Accumbens Glutamate Signaling

Commonalities in addictive behavior, such as craving, stimuli-driven drug seeking, and a high propensity for relapse following abstinence, have pushed for a unified theory of addiction that encompasses most abused substances. This unitary theory has recently been challenged - citing distinctions in structural neural plasticity, biochemical signaling, and neural circuitry to argue that addiction to opioids and psychostimulants is behaviorally and neurobiologically distinct. Recent more selective examination of drug-induced plasticity has highlighted that these two drug classes promote an overall reward circuitry signaling overlap through modifying excitatory synapses in the nucleus accumbens - a key constituent of the reward system. We discuss adaptations in presynaptic/postsynaptic and extrasynaptic glutamate signaling produced by opioids and psychostimulants, and their relevance to circuit remodeling and addiction-related behavior - arguing that these core neural adaptations are important targets for developing pharmacotherapies to treat addiction to multiple drugs.

Regulation of glutamate transporter 1 (GLT-1) gene expression by cocaine self-administration and withdrawal

Downregulation of the astroglial glutamate transporter GLT-1 is observed in the nucleus accumbens (NAc) following administration of multiple drugs of abuse. The decrease in GLT-1 protein expression following cocaine self-administration is dependent on both the amount of cocaine self-administered and the length of withdrawal, with longer access to cocaine and longer withdrawal periods leading to greater decreases in GLT-1 protein. However, the mechanism(s) by which cocaine downregulates GLT-1 protein remains unknown. We used qRT-PCR to examine gene expression of GLT-1 splice isoforms (GLT-1A, GLT-1B) in the NAc, prelimbic cortex (PL) and basolateral amygdala (BLA) of rats, following two widely used models of cocaine self-administration: short-access (ShA) self-administration, and the long-access (LgA) self-administration/incubation model. While downregulation of GLT-1 protein is observed following ShA cocaine self-administration and extinction, this model did not lead to a change in GLT-1A or GLT-1B gene expression in any brain region examined. Forced abstinence following ShA cocaine self-administration also was without effect. In contrast, LgA cocaine self-administration and prolonged abstinence significantly decreased GLT-1A gene expression in the NAc and BLA, and significantly decreased GLT-1B gene expression in the PL. No change was observed in NAc GLT-1A gene expression one day after LgA cocaine self-administration, indicating withdrawal-induced decreases in GLT-1A mRNA. In addition, LgA cocaine self-administration and withdrawal induced hypermethylation of the GLT-1 gene in the NAc. These results indicate that a decrease in NAc GLT-1 mRNA is only observed after extended access to cocaine combined with protracted abstinence, and that epigenetic mechanisms likely contribute to this effect.

mGluR2/3 mediates short-term control of nicotine-seeking by acute systemic N-acetylcysteine

Chronic self-administration of nicotine induces maladaptive changes in the cortico-accumbal glutamate (Glu) network. Consequently, re-exposure to nicotine-associated cues raises extracellular Glu in the nucleus accumbens reinstating drug-seeking. Restoring basal concentrations of extracellular Glu, thereby increasing tonic activation of the presynaptic group II metabotropic Glu receptors (mGluR2/3) with N-acetylcysteine (N-AC), might offer a valid therapeutic approach for maintaining smoking abstinence. Although N-AC modulates nicotine-seeking behavior by drug-associated stimuli in abstinent rats, it is still unclear whether it occurs through activation of mGluR2/3. Male Wistar rats were trained to associate discriminative stimuli (S^D s) with the availability of intravenous nicotine (0.03 mg/kg/65 µl/2-second/infusion) or oral saccharin (100 µl of 50 mg/l) self-administration versus non-reward. Reinforced response was followed by a cue signaling 20-second time-out (CSs). Once the training criterion was met, rats underwent lever press extinction, without reinforcers, S^D s and CSs. Re-exposure to nicotine or saccharin S^D+ /CS⁺ , but not non-reward S^D- /CS^- , revived responding on the previously reinforced lever. Acute N-AC, 100 but not 60 or 30 mg/kg i.p., reduced cue-induced nicotine-seeking. N-AC 100 mg/kg did not modify cue-induced saccharin-seeking behavior or influenced locomotor activity. Blocking mGluR2/3 with the selective antagonist LY341495, 1 mg/kg i.p., completely prevented the antirelapse activity of N-AC. The finding that N-AC prevents cue-induced nicotine-seeking by stimulating mGluR2/3 might indicate a therapeutic opportunity for acute cue-controlled nicotine-seeking. Future studies could evaluate the persistent effects of chronic N-AC in promoting enduring suppression of nicotine-cue conditioned responding.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Corticostriatal plasticity, neuronal ensembles, and regulation of drug-seeking behavior

The idea that interconnected neuronal ensembles code for specific behaviors has been around for decades; however, recent technical improvements allow studying these networks and their causal role in initiating and maintaining behavior. In particular, the role of ensembles in drug-seeking behaviors in the context of addiction is being actively investigated. Concurrent with breakthroughs in quantifying ensembles, research has identified a role for synaptic glutamate spillover during relapse. In particular, the transient relapse-associated changes in glutamatergic synapses on accumbens neurons, as well as in adjacent astroglia and extracellular matrix, are key elements of the synaptic plasticity encoded by drug use and the metaplasticity induced by drug-associated cues that precipitate drug-seeking behaviors. Here, we briefly review the recent discoveries related to ensembles in the addiction field and then endeavor to link these discoveries with drug-induced striatal plasticity and cue-induced metaplasticity toward deeper neurobiological understandings of drug seeking.

Glutamate Transport: A New Bench to Bedside Mechanism for Treating Drug Abuse

Drug addiction has often been described as a "hijacking" of the brain circuits involved in learning and memory. Glutamate is the principal excitatory neurotransmitter in the brain, and its contribution to synaptic plasticity and learning processes is well established in animal models. Likewise, over the past 20 years the addiction field has ascribed a critical role for glutamatergic transmission in the development of addiction. Chronic drug use produces enduring neuroadaptations in corticostriatal projections that are believed to contribute to a maladaptive deficit in inhibitory control over behavior. Much of this research focuses on the role played by ionotropic glutamate receptors directly involved in long-term potentiation and depression or metabotropic receptors indirectly modulating synaptic plasticity. Importantly, the balance between glutamate release and clearance tightly regulates the patterned activation of these glutamate receptors, emphasizing an important role for glutamate transporters in maintaining extracellular glutamate levels. Five excitatory amino acid transporters participate in active glutamate reuptake. Recent evidence suggests that these glutamate transporters can be modulated by chronic drug use at a variety of levels. In this review, we synopsize the evidence and mechanisms associated with drug-induced dysregulation of glutamate transport. We then summarize the preclinical and clinical data suggesting that glutamate transporters offer an effective target for the treatment of drug addiction. In particular, we focus on the role that altered glutamate transporters have in causing drug cues and contexts to develop an intrusive quality that guides maladaptive drug seeking behaviors.

N-acetylaspartylglutamate Inhibits Heroin Self-Administration and Heroin-Seeking Behaviors Induced by Cue or Priming in Rats

Activation of presynaptic group II metabotropic glutamate receptors (mGluR2/3) inhibits drug reward and drug-seeking behavior, but the role of N-acetylaspartylglutamate (NAAG), an agonist of endogenous mGluR2/3, in heroin reward and heroin-seeking behavior remained unclear. Here, we aimed to explore the effects of exogenous NAAG on heroin self-administration and heroin-seeking behavior. First, rats were trained to self-administer heroin under a fixed ratio 1 (FR1) schedule for 10 days, then received NAAG (50 or 100 μg/10 μL in each nostril) in the absence or presence of LY341495 (1 mg/kg, i.p.), an antagonist of mGluR2/3, on day 11 and the effects of NAAG on heroin self-administration under FR1 were recorded for 3 consecutive days. Motivation was assessed in heroin self-administration under a progressive ratio schedule on day 11 in another 5 groups with the same doses of NAAG. Additional rats were withdrawn for 14 days after 14 days of heroin self-administration, then received the same pharmacological pretreatment and were tested for heroin-seeking behaviors induced by heroin priming or cues. The results showed that intranasal administration of NAAG significantly decreased intravenous heroin self-administration on day 12, but not on day 11. Pretreatment with LY341495 prior to testing on day 12 prevented the inhibitory effect of NAAG on heroin reinforcement. The break-point for reward motivation was significantly reduced by NAAG. Moreover, NAAG also significantly inhibited the heroin-seeking behaviors induced by heroin priming or cues and these were restored by pretreatment with LY341495. These results demonstrated that NAAG, via activation of presynaptic mGluR2/3, attenuated the heroin reinforcement, heroin motivational value, and heroin-seeking behavior, suggesting that it may be used as an adjunct treatment for heroin addiction.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Acquisition, Maintenance and Relapse-Like Alcohol Drinking: Lessons from the UChB Rat Line

This review article addresses the biological factors that influence: (i) the acquisition of alcohol intake; (ii) the maintenance of chronic alcohol intake; and (iii) alcohol relapse-like drinking behavior in animals bred for their high-ethanol intake. Data from several rat strains/lines strongly suggest that catalase-mediated brain oxidation of ethanol into acetaldehyde is an absolute requirement (up 80%–95%) for rats to display ethanol’s reinforcing effects and to initiate chronic ethanol intake. Acetaldehyde binds non-enzymatically to dopamine forming salsolinol, a compound that is self-administered. In UChB rats, salsolinol: (a) generates marked sensitization to the motivational effects of ethanol; and (b) strongly promotes binge-like drinking. The specificity of salsolinol actions is shown by the finding that only the R-salsolinol enantiomer but not S-salsolinol accounted for the latter effects. Inhibition of brain acetaldehyde synthesis does not influence the maintenance of chronic ethanol intake. However, a prolonged ethanol withdrawal partly returns the requirement for acetaldehyde synthesis/levels both on chronic ethanol intake and on alcohol relapse-like drinking. Chronic ethanol intake, involving the action of lipopolysaccharide diffusing from the gut, and likely oxygen radical generated upon catechol/salsolinol oxidation, leads to oxidative stress and neuro-inflammation, known to potentiate each other. Data show that the administration of N-acetyl cysteine (NAC) a strong antioxidant inhibits chronic ethanol maintenance by 60%–70%, without inhibiting its initial intake. Intra-cerebroventricular administration of mesenchymal stem cells (MSCs), known to release anti-inflammatory cytokines, to elevate superoxide dismutase levels and to reverse ethanol-induced hippocampal injury and cognitive deficits, also inhibited chronic ethanol maintenance; further, relapse-like ethanol drinking was inhibited up to 85% for 40 days following intracerebral stem cell administration. Thus: (i) ethanol must be metabolized intracerebrally into acetaldehyde, and further into salsolinol, which appear responsible for promoting the acquisition of the early reinforcing effects of ethanol; (ii) acetaldehyde is not responsible for the maintenance of chronic ethanol intake, while other mechanisms are indicated; (iii) the systemic administration of NAC, a strong antioxidant markedly inhibits the maintenance of chronic ethanol intake; and (iv) the intra-cerebroventricular administration of anti-inflammatory and antioxidant MSCs inhibit both the maintenance of chronic ethanol intake and relapse-like drinking.

Similarities and Differences in Neurobiology

Substance addiction is a chronic, relapsing brain disease characterized by compulsive drug seeking and use despite harmful consequences. Non-substance addiction is defined recently that people may compulsively engage in an activity despite any negative consequences to their lives. Despite differences with respect to their addictive object, substance addiction and non-substance addiction may share similarities with respect to biological, epidemiological, clinical, genetic and other features. Here we review the similarities and differences in neurobiology between these two addictions with a focus on dopamine, serotonin, opioid, glutamate and norepinephrine systems. Studies suggest the involvement of all these systems in both substance addiction and non-substance addiction while differences may exist with respect to their contributions.

Recent developments in the behavioural and pharmacological enhancement of extinction of drug seeking

One of the principal barriers to overcoming addiction is the propensity to relapse, even after months or years of abstinence. Relapse can be precipitated by cues and contexts associated with drug use; thus, decreasing the conditioned properties of these cues and contexts may assist in preventing relapse. The predictive power of drug cues and contexts can be reduced by repeatedly presenting them in the absence of the drug reinforcer, a process known as extinction. The potential of extinction to limit relapse has generated considerable interest and research over the past few decades. While pre-clinical animal models suggest extinction learning assists relapse prevention, treatment efficacy is often lacking when extinction learning principles are translated into clinical trials. Conklin and Tiffany (Addiction, 2002) suggest the lack of efficacy in clinical practice may be due to limited translation of procedures demonstrated through animal research and propose several methodological improvements to enhance extinction learning for drug addiction. This review will examine recent advances in the behavioural and pharmacological manipulation of extinction learning, based on research from pre-clinical models. In addition, the translation of pre-clinical findings-both those suggested by Conklin and Tiffany () and novel demonstrations from the past 13 years-into clinical trials and the efficacy of these methods in reducing craving and relapse, where available, will be discussed. Finally, we highlight areas where promising pre-clinical models have not yet been integrated into current clinical practice but, if applied, could improve upon existing behavioural and pharmacological methods.

Cocaine sensitization increases subthreshold activity in dopamine neurons from the ventral tegmental area

The progressive escalation of psychomotor responses that results from repeated cocaine administration is termed sensitization. This phenomenon alters the intrinsic properties of dopamine (DA) neurons from the ventral tegmental area (VTA), leading to enhanced dopaminergic transmission in the mesocorticolimbic network. The mechanisms underlying this augmented excitation are nonetheless poorly understood. DA neurons display the hyperpolarization-activated, nonselective cation current, dubbed I_h We recently demonstrated that I_h and membrane capacitance are substantially reduced in VTA DA cells from cocaine-sensitized rats. The present study shows that 7 days of cocaine withdrawal did not normalize I_h and capacitance. In cells from cocaine-sensitized animals, the amplitude of excitatory synaptic potentials, at -70 mV, was ∼39% larger in contrast to controls. Raise and decay phases of the synaptic signal were faster under cocaine, a result associated with a reduced membrane time constant. Synaptic summation was paradoxically elevated by cocaine exposure, as it consisted of a significantly reduced summation indexed but a considerably increased depolarization. These effects are at least a consequence of the reduced capacitance. I_h attenuation is unlikely to explain such observations, since at -70 mV, no statistical differences exist in I_h or input resistance. The neuronal shrinkage associated with a diminished capacitance may help to understand two fundamental elements of drug addiction: incentive sensitization and negative emotional states. A reduced cell size may lead to substantial enhancement of cue-triggered bursting, which underlies drug craving and reward anticipation, whereas it could also result in DA depletion, as smaller neurons might express low levels of tyrosine hydroxylase.

NEW & NOTEWORTHY

This work uses a new approach that directly extracts important biophysical parameters from alpha function-evoked synaptic potentials. Two of these parameters are the cell membrane capacitance (C_m) and rate at any time point of the synaptic waveform. The use of such methodology shows that cocaine sensitization reduces C_m and increases the speed of synaptic signaling. Paradoxically, although synaptic potentials show a faster decay under cocaine their temporal summation is substantially elevated.

Effects of histone deacetylase inhibitor sodium butyrate on heroin seeking behavior in the nucleus accumbens in rats

Histone acetylation and other modifications of the chromatin are important regulators of gene expression and may contribute to drug-induced behaviors and neuroplasticity. Inhibition of histone deacetylases (HDAC) activity results in the change of some drug-induced behaviors，however, relatively little is known about the effects of HDAC inhibitors on heroin-seeking behavior. In the present study, male rats were trained to self-administer heroin under a FR1 schedule for consecutive 14 days, followed by 14 daily 2h extinction session in the operant chamber. After training, the heroin priming (250μg/kg) was introduced for the reinstatement of heroin-seeking behavior. Pretreatment with sodium butyrate (NaB) (200 or 400mg/kg, i.p.), an inhibitor of HDAC, failed to affect heroin self-administration. Additionally，systemic administration of NaB (400mg/kg, i.p.)increased significantly the reinstatement of heroin-seeking induced by heroin priming when NaB administered 12h, but not 6h before the reinstatement test. The same effect was observed after the intracerebroventricular injection of NaB (5μL, 100μg/μL). Moreover, the levels of histone H3 acetylation at lysine 18（H3K18）and H4 acetylation at lysine 5 or lysine 8（H4K5 or H4K8）in the accumbens nucleus core and shell were remarkably increased during the reinstatement and were further strengthened after intracerebroventricular injection of NaB. These results demonstrated that activation of histone acetylation may be involved in the heroin-seeking behavior, and identifying these epigenetic changes will be critical in proposing a novel pharmacological strategy for treating heroin addiction.

N-acetylcysteine amide (AD4) reduces cocaine-induced reinstatement

RATIONALE

Chronic exposure to drugs of abuse changes glutamatergic transmission in human addicts and animal models. N-acetylcysteine (NAC) is a cysteine prodrug that indirectly activates cysteine-glutamate antiporters. In the extrasynaptic space, NAC restores basal glutamate levels during drug abstinence and normalizes increased glutamatergic tone in rats during reinstatement to drugs of abuse. In initial clinical trials, repeated NAC administration seems to be promising for reduced craving in cocaine addicts.

OBJECTIVE

In this study, NAC-amide, called AD4 or NACA, was examined in intravenous cocaine self-administration and extinction/reinstatement procedures in rats. We investigated the behavioral effects of AD4 in the olfactory bulbectomized (OBX) rats, considered an animal model of depression. Finally, we tested rats injected with AD4 or NAC during 10-daily extinction training sessions to examine subsequent cocaine seeking.

RESULTS

AD4 (25-75 mg kg(-1)) given acutely did not alter the rewarding effects of cocaine in OBX rats and sham-operated controls. However, at 6.25-50 mg kg(-1), AD4 decreased dose-dependently cocaine seeking and relapse triggered by cocaine priming or drug-associated conditioned cues in both phenotypes. Furthermore, repeated treatment with AD4 (25 mg kg(-1)) or NAC (100 mg kg(-1)) during daily extinction trials reduced reinstatement of drug-seeking behavior in sham-operated controls. In the OBX rats only, AD4 effectively blocked cocaine-seeking behavior.

CONCLUSIONS

Our results demonstrate that AD4 is effective at blocking cocaine-seeking behavior, highlighting its potential clinical use toward cocaine use disorder.

Acute inhibition of mGluR5 disrupts behavioral flexibility

Conditioned cues can sometimes elicit maladaptive responses as seen in the post-traumatic stress disorder (PTSD). Lack of effective fear extinction, which involves a process of inhibitory learning, is hypothesized to associate with PTSD. In this study, we tested the effect of acute pharmacological inhibition of mGluR5 activity on the extinction of fear memory and spatial memory. Intraperitoneal injection of the mGluR5 (metabotropic glutamate receptor 5) antagonist MPEP [2-Methyl-6-(phenylethynyl) pyridine hydrochloride] allowed the retrieval but prevented the extinction of contextual fear memory in mice. Without altering locomotor activity, MPEP inhibited the acquisition but not the consolidation of contextual fear memory. Further, administration of MPEP blocked the extinction of spatial memory in the Morris water maze paradigm. Our data suggest a necessary role of mGluR5 in regulating certain aspects of behavioral flexibility.

Glutamate Transporter GLT-1 as a Therapeutic Target for Substance Use Disorders

The development of new treatments for substance use disorders requires identification of targetable molecular mechanisms. Pathology in glutamatergic neurotransmission system in brain reward circuitry has been implicated in relapse to multiple classes of drugs. Glutamate transporter 1 (GLT-1) crucially regulates glutamatergic signaling by removing excess glutamate from the extrasynaptic space. The purpose of this review is to highlight the effects of addictive drug use on GLT-1 and glutamate uptake, and using GLT-1 as a target in addiction pharmacotherapy. Cocaine, opioids, ethanol, nicotine, amphetamines, and cannabinoids each affect GLT-1 expression and glutamate uptake, and restoring GLT-1 expression with N-acetylcysteine or ceftriaxone shows promise in correcting pre-clinical and clinical manifestations of drug addiction.

N-acetylcysteine decreased nicotine reward-like properties and withdrawal in mice

RATIONALE

N-acetylcysteine can increase extrasynaptic glutamate and reduce nicotine self-administration in rats and smoking rates in humans.

OBJECTIVES

The aim of this study was to determine if N-acetylcysteine modulates the development of nicotine place conditioning and withdrawal in mice.

METHODS

N-acetylcysteine was given to nicotine-treated male ICR mice. Experiment 1: reward-like behavior. N-acetylcysteine (0, 5, 15, 30, or 60 mg/kg, i.p.) was given 15 min before nicotine (0.5 mg/kg, s.c.) or saline (10 ml/kg, s.c.) in an unbiased conditioned place preference (CPP) paradigm. Conditioning for highly palatable food served as control. Experiment 2: spontaneous withdrawal. The effect of N-acetylcysteine (0, 15, 30, 120 mg/kg, i.p.) on anxiety-like behavior, somatic signs, and hyperalgesia was measured 18-24 h after continuous nicotine (24 mg/kg/day, 14 days). Experiment 3: mecamylamine-precipitated, withdrawal-induced aversion. The effect of N-acetylcysteine (0, 15, 30, 120 mg/kg, i.p.) on mecamylamine (3.5 mg/kg, i.p.)-precipitated withdrawal was determined after continuous nicotine (24 mg/kg, i.p., 28 days) using the conditioned place aversion (CPA) paradigm.

RESULTS

Dose-related reductions in the development of nicotine CPP, somatic withdrawal signs, hyperalgesia, and CPA were observed after N-acetylcysteine pretreatment. No effect of N-acetylcysteine was found on palatable food CPP, anxiety-like behavior, or motoric capacity (crosses between plus maze arms). Finally, N-acetylcysteine did not affect any measure in saline-treated mice at doses effective in nicotine-treated mice.

CONCLUSIONS

These are the first data suggesting that N-acetylcysteine blocks specific mouse behaviors associated with nicotine reward and withdrawal, which adds to the growing appreciation that N-acetylcysteine may have high clinical utility in combating nicotine dependence.

Toward early estimation and treatment of addiction vulnerability: radial arm maze and N-acetyl cysteine before cocaine sensitization or nicotine self-administration in neonatal ventral hippocampal lesion rats

RATIONAL

Prefrontal cortical (PFC)-hippocampal-striatal circuits, interconnected via glutamatergic signaling, are dysfunctional in mental illnesses that involve addiction vulnerability.

OBJECTIVES

In healthy and neurodevelopmentally altered rats, we examined how Radial Arm Maze (RAM) performance estimates addiction vulnerability, and how starting a glutamatergic modulating agent, N-acetyl cysteine (NAC) in adolescence alters adult mental illness and/or addiction phenotypes.

METHODS

Rats with neonatal ventral hippocampal lesions (NVHL) vs. SHAM-operated controls were randomized to NAC vs. saline in adolescence followed by cognitive testing (RAM) in early adulthood and then cocaine behavioral sensitization (experiment 1; n = 80) or nicotine self-administration (experiment 2; n = 12).

RESULTS

In experiment 1, NVHL rats showed over-consumption of food (Froot-Loops (FL)) baiting the RAM with poor working memory (low-arm entries to repeat (ETR)), producing an elevated FL to ETR ratio ("FLETR"; p < 0.001). FLETR was the best linear estimator (compared to FL or ETR) of magnitude of long-term cocaine sensitization (R ² = 0.14, p < 0.001). NAC treatment did not alter FL, ETR, FLETR, or cocaine sensitization. In experiment 2, FLETR also significantly and uniquely correlated with subsequent drug seeking during nicotine-induced reinstatement after extinction of nicotine self-administration (R ² = 0.47, p < 0.01). NAC did not alter RAM performance, but significantly reversed NVHL-induced increases in nicotine seeking during extinction and reinstatement.

CONCLUSIONS

These findings demonstrate the utility of animal models of mental illness with addiction vulnerability for developing novel diagnostic measures of PFC-hippocampal-striatal circuit dysfunction that may reflect addiction risk. Such tests may direct pharmacological treatments prior to adulthood and addictive drug exposure, to prevent or treat adult addictions.

Fronto-striatal circuits in response-inhibition: Relevance to addiction

Disruptions to inhibitory control are believed to contribute to multiple aspects of drug abuse, from preexisting vulnerability in at-risk individuals, through escalation to dependence, to promotion of relapse in chronic users. Paradigms investigating the suppression of actions have been investigated in animal and human research on drug addiction. Rodent research has focused largely on impulsive behaviors, often gauged by premature responding, as a viable model highlighting the relevant role of dopamine and other neurotransmitters primarily in the striatum. Human research on action inhibition in stimulant dependence has highlighted impaired performance and largely prefrontal cortical abnormalities as part of a broader pattern of cognitive abnormalities. Animal and human research implicate inhibitory difficulties mediated by fronto-striatal circuitry both preceding and as a result of excessive stimulus use. In this regard, response-inhibition has proven a useful cognitive function to gauge the integrity of fronto-striatal systems and their role in contributing to impulsive and compulsive features of drug dependence.

Activation of AMPA receptor in the infralimbic cortex facilitates extinction and attenuates the heroin-seeking behavior in rats

Infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction, but whether the IL regulates the extinction and reinstatement of heroin-seeking behavior is unknown. To address this issue, the male SD rats were trained to self-administer heroin under a FR1 schedule for consecutive 14 days, then the rats underwent 7 daily 2h extinction session in the operant chamber. The activation of IL by microinjection PEPA, an allosteric AMPA receptor potentiator into IL before each of extinction session facilitated the extinction responding after heroin self-administration, but did not alter the locomotor activity in an open field testing environment. Other rats were first trained under a FR1 schedule for heroin self-administration for 14 days, followed by 14 days of extinction training, and reinstatement of heroin-seeking induced by cues was measured for 2h. Intra-IL microinjecting of PEPA at 15min prior to test inhibited the reinstatement of heroin-seeking induced by cues. Moreover, the expression of GluR1 in the IL and NAc remarkably increased after treatment with PEPA during the reinstatement. These finding suggested that activation of glutamatergic projection from IL to NAc shell may be involved in the extinction and reinstatement of heroin-seeking.

S-Glutathionylation and Redox Protein Signaling in Drug Addiction

Drug addiction is a chronic relapsing disorder that comes at a high cost to individuals and society. Therefore understanding the mechanisms by which drugs exert their effects is of prime importance. Drugs of abuse increase the production of reactive oxygen and nitrogen species resulting in oxidative stress. This change in redox homeostasis increases the conjugation of glutathione to protein cysteine residues; a process called S-glutathionylation. Although traditionally regarded as a protective mechanism against irreversible protein oxidation, accumulated evidence suggests a more nuanced role for S-glutathionylation, namely as a mediator in redox-sensitive protein signaling. The reversible modification of protein thiols leading to alteration in function under different physiologic/pathologic conditions provides a mechanism whereby change in redox status can be translated into a functional response. As such, S-glutathionylation represents an understudied means of post-translational protein modification that may be important in the mechanisms underlying drug addiction. This review will discuss the evidence for S-glutathionylation as a redox-sensing mechanism and how this may be involved in the response to drug-induced oxidative stress. The function of S-glutathionylated proteins involved in neurotransmission, dendritic spine structure, and drug-induced behavioral outputs will be reviewed with specific reference to alcohol, cocaine, and heroin.

Neuroscience of learning and memory for addiction medicine: from habit formation to memory reconsolidation

Identifying effective pharmacological treatments for addictive disorders has remained an elusive goal. Many different classes of drugs have shown some efficacy in preclinical models, but the number of effective clinical therapeutics has remained stubbornly low. The persistence of drug use and the high frequency of relapse is at least partly attributable to the enduring ability of environmental stimuli associated with drug use to maintain behavioral patterns of drug use and induce craving during abstinence. We propose that stimuli associated with drug use exert such powerful control over behavior through the development of abnormally strong memories, and their ability to initiate subconscious sequences of motor actions (habits) that promote uncontrolled drug use. In this chapter, we will review the evidence suggesting that drugs of abuse strengthen associations with cues in the environment and facilitate habit formation. We will also discuss potential mechanisms for disrupting memories associated with drug use to help improve treatments for addiction.

N-acetylcysteine prevents behavioral and biochemical changes induced by alcohol cessation in rats

N-acetylcysteine (NAC), a glutamate-modulating agent with antioxidant and anti-inflammatory properties, has been considered as a potential anti-addictive drug. Beneficial effects were reported for cocaine, cannabis, and tobacco addicts, but the effect of NAC in alcoholics or in alcohol animal models is unknown. The aggravation of alcohol withdrawal symptoms, such as anxiety, has been associated with increased levels of serum corticosterone and leptin. Thus, the aim of this study was to assess the effects of NAC on anxiety, as well as corticosterone and leptin serum levels, after cessation of chronic alcohol treatment in rats. Male Wistar rats were treated with 2 g/kg ethanol, twice daily, by gavage for 30 days; control animals received an appropriate dose of glucose to balance caloric intake. Rats were treated for 4 days with NAC (60 and 90 mg/kg, intra-peritoneally [i.p.]) or saline after alcohol cessation. Twenty-four hours after the last treatment, rats were exposed to a 5-min session in the open-field test (OF). Corticosterone and leptin serum levels were determined by ELISA in samples collected within 30 min after the OF. Results showed that rats were hypoactive (decreased rearing, peripheral, and total crossings), and that corticosterone and leptin levels were increased 5 days after alcohol cessation. Four days of NAC prevented the behavioral and biochemical changes brought about by alcohol cessation. We suggest that, in addition to the anti-addictive properties reported for other drugs of abuse, NAC is potentially useful in the management of alcohol withdrawal.

Taking the fuel out of the fire: evidence for the use of anti-inflammatory agents in the treatment of bipolar disorders

BACKGROUND

Inflammation has emerged as a potentially important factor - and thus putative pharmacological target - in the pathology of bipolar disorders. However to date no systematic evaluations of the efficacy of add on anti-inflammatory treatment for the depressive and manic episodes have been carried out.

METHODS

Sixteen articles were ultimately identified - by computer searches of databases (including PsycINFO, MEDLINE, and EMBASE), supplemented by hand searches and personal communication - as meeting study inclusion criteria.

RESULTS

Anti-manic effects were evaluated in two trials, one of adjunctive n-acetyl cysteine (NAC), one of omega-3 fatty acids (O3FA), and significant improvements only emerged for NAC. Celecoxib had a rapid but short-lived antidepressant effect. Despite limited effects of O3FA on symptoms, imaging data demonstrated alterations in neuronal functioning that might have longer-term therapeutic effects. Evidence was strongest for adjunctive NAC in bipolar depression though conclusions are limited by small sample sizes.

LIMITATIONS

Definitive conclusions are limited by the paucity of data, small study sizes, and the variability in methodology used.

CONCLUSIONS

Current evidence for aspirin or celecoxib is insufficient though further investigation of the potential of celecoxib in early illness onset is warranted. Variable evidence exists for add-on O3FA though an indication of short-term treatment effects on membrane fluidity and neuronal activity suggest longer follow-up assessment is needed. The strongest evidence emerged for NAC in depression and future studies must address the role of illness duration and patients׳ baseline medications on outcomes. Careful consideration of lithium toxicity in the elderly and renal impaired is essential.

Ceftriaxone and cefazolin attenuate the cue-primed reinstatement of alcohol-seeking

Alcohol consumption and the reinstatement of alcohol-seeking rely on glutamate and GABA transmission. Modulating these neurotransmitters may be a viable treatment strategy to prevent alcohol relapse. N-acetylcysteine (NAC) and the antibiotic ceftriaxone (CEF) alter the glial reuptake and release of glutamate while the antibiotic cefazolin (CEFAZ) modulates GABA signaling without affecting glutamate. Here, we used the extinction-reinstatement model of relapse to test the ability of these compounds to attenuate the reinstatement of alcohol-seeking. Male Sprague-Dawley rats were trained to self-administer 20% (v/v) alcohol in the home cage using an intermittent schedule (24 h on, 24 h off) for 12 sessions. Subsequently, animals self-administered alcohol during daily 45-min operant sessions for 26 sessions, followed by extinction training. We tested whether chronic administration of NAC, CEF, or CEFAZ attenuated the cue-primed reinstatement of alcohol-seeking. CEF and CEFAZ attenuated cue-primed reinstatement of alcohol-seeking while NAC had no effect. We subsequently investigated whether CEF and CEFAZ alter the self-administration of sucrose and chow pellets and if CEFAZ attenuates the reinstatement of cocaine-seeking. The operant self-administration of regular chow and sucrose was not altered by either CEF or CEFAZ. CEFAZ had no effect on cocaine reinstatement, a behavior that has been strongly tied to altered glutamate homeostasis in the nucleus accumbens. Thus the ability of CEFAZ to attenuate alcohol reinstatement likely does not involve the glial modulation of glutamate levels. The dampening of GABA transmission may be a common mechanism of action of cefazolin and ceftriaxone.

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

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

N-acetylcysteine for therapy-resistant tobacco use disorder: a pilot study

INTRODUCTION

N-Acetylcysteine (NAC) may have efficacy in treating tobacco use disorder (TUD) by reducing craving and smoking reward. This study examines whether treatment with NAC may have a clinical efficacy in the treatment of TUD.

METHODS

A 12-week double blind randomized controlled trial was conducted to compare the clinical efficacy of NAC 3 g/day versus placebo. We recruited 34 outpatients with therapy resistant TUD concurrently treated with smoking-focused group behavioral therapy. Participants had assessments of daily cigarette use (primary outcome), exhaled carbon monoxide (CO(EXH)) (secondary outcome), and quit rates as defined by CO(EXH) < 6 ppm. Depression was measured with the Hamilton Depression Rating Scale (HDRS). Data were analyzed using conventional and modified intention-to-treat endpoint analyses.

RESULTS

NAC treatment significantly reduced the daily number of cigarettes used (Δ mean ± SD = -10.9 ± 7.9 in the NAC-treated versus -3.2 ± 6.1 in the placebo group) and CO(EXH) (Δ mean ± SD = -10.4 ± 8.6 ppm in the NAC-treated versus -1.5 ± 4.5 ppm in the placebo group); 47.1% of those treated with NAC versus 21.4% of placebo-treated patients were able to quit smoking as defined by CO(EXH) < 6 ppm. NAC treatment significantly reduced the HDRS score in patients with tobacco use disorder.

CONCLUSIONS

These data show that treatment with NAC may have a clinical efficacy in TUD. NAC combined with appropriate psychotherapy appears to be an efficient treatment option for TUD.

Glutamate transporter GLT-1 mediates N-acetylcysteine inhibition of cocaine reinstatement

Both pre-clinical and clinical studies indicate that N-acetylcysteine (NAC) may be useful in treating relapse to addictive drug use. Cocaine self-administration in rats reduces both cystine-glutamate exchange and glutamate transport via GLT-1 in the nucleus accumbens, and NAC treatment normalizes these two glial processes critical for maintaining glutamate homeostasis. However, it is not known if one or both of these actions by NAC is needed to inhibit relapse to cocaine seeking. To determine whether the restoration of GLT-1 and/or cystine-glutamate exchange is required for NAC to inhibit cue-induced reinstatement of cocaine seeking, we utilized the rat self-administration/extinction/reinstatement model of cocaine relapse. Rats were pre-treated in the nucleus accumbens with vivo-morpholino antisense oligomers targeting either GLT-1 or xCT (catalytic subunit of the cystine-glutamate exchanger) overlapping with daily NAC administration during extinction (100 mg/kg, i.p. for the last 5 days). Rats then underwent cue-induced reinstatement of active lever pressing in the absence of NAC, to determine if preventing NAC-induced restoration of one or the other protein was sufficient to block the capacity of chronic NAC to inhibit reinstatement. The vivo-morpholino suppression of xCT reduced cystine-glutamate exchange but did not affect NAC-induced reduction of reinstated cocaine seeking. In contrast, suppressing NAC-induced restoration of GLT-1 not only prevented NAC from inhibiting reinstatement, but augmented the capacity of cues to reinstate cocaine seeking. We hypothesized that the increased reinstatement after inhibiting NAC induction of GLT-1 resulted from increased extracellular glutamate, and show that augmented reinstatement is prevented by blocking mGluR5. Restoring GLT-1, not cystine-glutamate exchange, is a key mechanism whereby daily NAC reduces cue-induced cocaine reinstatement.

Regulation of hippocampal synaptic strength by glial xCT

Most extracellular glutamate in the brain is released by xCT, a glial antiporter that exports glutamate and imports cystine. The function of xCT, and extracellular glutamate in general, remains unclear. Several lines of evidence suggest that glutamate from xCT could act in a paracrine fashion to suppress glutamatergic synapse strength by triggering removal of postsynaptic glutamate receptors. To test this idea, we used whole-cell patch-clamp electrophysiology and immunohistochemistry to quantify receptor number and synapse function in xCT knock-out mouse hippocampal CA3-CA1 synapses. Consistent with the hypothesis that xCT suppresses glutamate receptor number and synapse strength, xCT knock-out synapses showed increased AMPA receptor abundance with concomitant large enhancements of spontaneous and evoked synaptic transmission. We saw no evidence for changes in GABA receptor abundance or the overall number of glutamatergic synapses. The xCT knock-out phenotype was replicated by incubating slices in the xCT inhibitor (S)-4-carboxyphenylglycine, and consistent with the idea that xCT works by regulating extracellular glutamate, the xCT knock-out phenotype could be reproduced in controls by incubating the slices in glutamate-free aCSF. We conclude that glutamate secreted via xCT suppresses glutamatergic synapse strength by triggering removal of postsynaptic AMPA receptors.