Nucleus accumbens D2/3 receptors predict trait impulsivity and cocaine reinforcement

Pathological gambling in Parkinson's disease--a review of the literature

The prevalence of pathological gambling is 3.4% to 6% in treated Parkinson's disease, which is higher than the background population rate. In this review we discuss current evidence to indicate that dopamine agonists are much more likely to trigger this behavior than either L-dopa or selective monoamine oxidase B inhibitor monotherapy. New insights from recent behavioral and functional imaging studies and possible treatment approaches are also covered. A PubMed literature search using the terms "gambling" and "Parkinson's disease," "impulse control disorder," "impulsive compulsive behaviour," "dopamine agonist," of individual dopamine agonists, and of ongoing drug trials, using http://www.clinicaltrials.gov, was carried out for the period up to January 2011.

Cognitive and neurobiological mechanisms of alcohol-related aggression

Alcohol-related violence is a serious and common social problem. Moreover, violent behaviour is much more common in alcohol-dependent individuals. Animal experiments and human studies have provided insights into the acute effect of alcohol on aggressive behaviour and into common factors underlying acute and chronic alcohol intake and aggression. These studies have shown that environmental factors, such as early-life stress, interact with genetic variations in serotonin-related genes that affect serotonergic and GABAergic neurotransmission. This leads to increased amygdala activity and impaired prefrontal function that, together, predispose to both increased alcohol intake and impulsive aggression. In addition, acute and chronic alcohol intake can further impair executive control and thereby facilitate aggressive behaviour.

Strain differences in the dose-response relationship for morphine self-administration and impulsive choice between Lewis and Fischer 344 rats

Dose-response studies are thought to be a valuable tool to predict the most genetically drug-vulnerable individuals. However, dose-response curves for morphine self-administration have not yet been examined and nor strain differences might be evident. Therefore, this study aimed to define the dose-response curve for morphine self-administration (0.25, 0.5, 1 and 2 mg/kg) in Lewis (LEW) rats and their histocompatible Fischer-344 (F344) rats. In addition, impulsivity has been suggested as one of the genetic factors contributing most to the initiation of drug use. Therefore, the impulsive choice of both rat strains in the presence or absence of the same morphine doses was also analysed. LEW rats self-administered significantly more morphine whatever the dose tested and they exhibited greater basal impulsive choice compared with F344 rats. The F344 strain showed a preference for the dose of 0.5 mg/kg, while any of the doses used had a differential reinforcing effect in the LEW strain. The basal pattern of strain differences in impulsive choice was not affected by morphine administration. These data suggest that the LEW strain has a highly drug-vulnerable phenotype and they point to the strength of impulsivity as a pre-existing behavioural trait that might make this rat strain more vulnerable to the reinforcing effects of drugs and, therefore, to develop addiction.

Genetic dissection of behavioral flexibility: reversal learning in mice

BACKGROUND

Behavioral inflexibility is a feature of schizophrenia, attention-deficit/hyperactivity disorder, and behavior addictions that likely results from heritable deficits in the inhibitory control over behavior. Here, we investigate the genetic basis of individual differences in flexibility, measured using an operant reversal learning task.

METHODS

We quantified discrimination acquisition and subsequent reversal learning in a cohort of 51 BXD strains of mice (2-5 mice/strain, n = 176) for which we have matched data on sequence, gene expression in key central nervous system regions, and neuroreceptor levels.

RESULTS

Strain variation in trials to criterion on acquisition and reversal was high, with moderate heritability (∼.3). Acquisition and reversal learning phenotypes did not covary at the strain level, suggesting that these traits are effectively under independent genetic control. Reversal performance did covary with dopamine D2 receptor levels in the ventral midbrain, consistent with a similar observed relationship between impulsivity and D2 receptors in humans. Reversal, but not acquisition, is linked to a locus on mouse chromosome 10 with a peak likelihood ratio statistic at 86.2 megabase (p < .05 genome-wide). Variance in messenger RNA levels of select transcripts expressed in neocortex, hippocampus, and striatum correlated with the reversal learning phenotype, including Syn3, Nt5dc3, and Hcfc2.

CONCLUSIONS

This work demonstrates the clear trait independence between, and genetic control of, discrimination acquisition and reversal and illustrates how globally coherent data sets for a single panel of highly related strains can be interrogated and integrated to uncover genetic sources and molecular and neuropharmacological candidates of complex behavioral traits relevant to human psychopathology.

The role of dopamine transporter (SLC6A3) and dopamine D2 receptor/ankyrin repeat and kinase domain containing 1 (DRD2/ANKK1) gene polymorphisms in personality traits

Variations in personality traits are caused by interactions between multiple genes of small effect and environmental factors. To date, gender- and ethnicity-specific variations in personality have been established. In the present study, we aimed to test: (1) the effects of four polymorphisms of dopamine system genes: ANKK1/DRD2 Taq1A, DRD2 rs6275, SLC6A3 40-bp VNTR and rs27072, on personality traits; (2) whether these effects differ between men and women and between Russians and Tatars. A sample of 652 healthy individuals (222 men and 430 women) of Caucasian origin (233 Russians and 419 Tatars) from Russia was subjected to personality traits assessment with Eysenck Personality Inventory (EPI) and Temperament and Character Inventory-125 (TCI-125). The associations between each personality trait and polymorphisms were assessed with regression models adjusted for gender and ethnicity. There were significant effects of ANKK1/DRD2 Taq1A on Neuroticism (p=0.016) and of SLC6A3 rs27072 on Persistence (p=0.021) in both genders. The association between ANKK1/DRD2 Taq1A A2/A2-genotype and higher Novelty Seeking and lower Reward Dependence was shown in men only (p for gender interaction=0.018). In women only, there was a significant association between SLC6A3 10R*G-haplotype and higher Persistence (p=0.002). Our findings provide evidence for a modifying effect of gender on the associations between dopamine system genes and approach-related traits (in men) and Persistence (in women).

Behavioral and emotional regulation and adolescent substance use problems: a test of moderation effects in a dual-process model

In a structural model, we tested how relations of predictors to level of adolescent substance use (tobacco, alcohol, marijuana), and to substance-related impaired-control and behavior problems, are moderated by good self-control and poor regulation in behavioral and emotional domains. The participants were a sample of 1,116 public high-school students. In a multiple-group analysis for good self-control, the paths from negative life events to substance use level and from level to behavior problems were lower among persons scoring higher on good behavioral self-control. In a multiple-group analysis for poor regulation, the paths from negative life events and peer use to level of substance use were greater among persons scoring higher on poor behavioral (but not emotional) regulation; an inverse path from academic competence to level was greater among persons scoring higher on both aspects of poor regulation. Paths from level to impaired-control and behavior problems were greater among persons scoring higher on both poor behavioral and poor emotional regulation. Theoretical implications concerning the role of behavioral and emotional regulation in moderation effects are discussed.

The role of impulsivity in the aetiology of drug dependence: reward sensitivity versus automaticity

RATIONALE

Impulsivity has long been known as a risk factor for drug dependence, but the mechanisms underpinning this association are unclear. Impulsivity may confer hypersensitivity to drug reinforcement which establishes higher rates of instrumental drug-seeking and drug-taking behaviour, or may confer a propensity for automatic (non-intentional) control over drug-seeking/taking and thus intransigence to clinical intervention.

METHOD

The current study sought to distinguish these two accounts by measuring Barratt Impulsivity and craving to smoke in 100 smokers prior to their completion of an instrumental concurrent choice task for tobacco (to measure the rate of drug-seeking) and an ad libitum smoking test (to measure the rate of drug-taking-number of puffs consumed).

RESULTS

The results showed that impulsivity was not associated with higher rates of drug-seeking/taking, but individual differences in smoking uptake and craving were. Rather, nonplanning impulsivity moderated (decreased) the relationship between craving and drug-taking, but not drug-seeking.

CONCLUSIONS

These data suggest that whereas the uptake of drug use is mediated by hypervaluation of the drug as an instrumental goal, the orthogonal trait nonplanning impulsivity confers a propensity for automatic control over well-practiced drug-taking behaviour.

High impulsivity predicting vulnerability to cocaine addiction in rats: some relationship with novelty preference but not novelty reactivity, anxiety or stress

RATIONALE

Impulsivity is a vulnerability marker for drug addiction in which other behavioural traits such as anxiety and novelty seeking ('sensation seeking') are also widely present. However, inter-relationships between impulsivity, novelty seeking and anxiety traits are poorly understood.

OBJECTIVE

The objective of this paper was to investigate the contribution of novelty seeking and anxiety traits to the expression of behavioural impulsivity in rats.

METHODS

Rats were screened on the five-choice serial reaction time task (5-CSRTT) for spontaneously high impulsivity (SHI) and low impulsivity (SLI) and subsequently tested for novelty reactivity and preference, assessed by open-field locomotor activity (OF), novelty place preference (NPP), and novel object recognition (OR). Anxiety was assessed on the elevated plus maze (EPM) both prior to and following the administration of the anxiolytic drug diazepam, and by blood corticosterone levels following forced novelty exposure. Finally, the effects of diazepam on impulsivity and visual attention were assessed in SHI and SLI rats.

RESULTS

SHI rats were significantly faster to enter an open arm on the EPM and exhibited preference for novelty in the OR and NPP tests, unlike SLI rats. However, there was no dimensional relationship between impulsivity and either novelty-seeking behaviour, anxiety levels, OF activity or novelty-induced changes in blood corticosterone levels. By contrast, diazepam (0.3-3 mg/kg), whilst not significantly increasing or decreasing impulsivity in SHI and SLI rats, did reduce the contrast in impulsivity between these two groups of animals.

CONCLUSIONS

This investigation indicates that behavioural impulsivity in rats on the 5-CSRTT, which predicts vulnerability for cocaine addiction, is distinct from anxiety, novelty reactivity and novelty-induced stress responses, and thus has relevance for the aetiology of drug addiction.

Food reward, hyperphagia, and obesity

Given the unabated obesity problem, there is increasing appreciation of expressions like "my eyes are bigger than my stomach," and recent studies in rodents and humans suggest that dysregulated brain reward pathways may be contributing not only to drug addiction but also to increased intake of palatable foods and ultimately obesity. After describing recent progress in revealing the neural pathways and mechanisms underlying food reward and the attribution of incentive salience by internal state signals, we analyze the potentially circular relationship between palatable food intake, hyperphagia, and obesity. Are there preexisting individual differences in reward functions at an early age, and could they be responsible for development of obesity later in life? Does repeated exposure to palatable foods set off a cascade of sensitization as in drug and alcohol addiction? Are reward functions altered by secondary effects of the obese state, such as increased signaling through inflammatory, oxidative, and mitochondrial stress pathways? Answering these questions will significantly impact prevention and treatment of obesity and its ensuing comorbidities as well as eating disorders and drug and alcohol addiction.

Tridimensional personalities and polymorphism of dopamine D2 receptor among heroin addicts

BACKGROUND

Drug addiction is a multifactorial disorder. Researchers have posited that an individual's inherited behavioral propensity or temperament contributes to the disorder by shaping a personality strongly linked with the risk of drug abuse. Further, they hypothesize that the polymorphism of dopamine D2 receptor increases the susceptibility to and severity of addiction. We, therefore, investigated possible associations between dopamine D2 receptor (DRD2) and personality traits among intravenous heroin addicts.

METHODS

We assessed 93 intravenous heroin addicts and controls using Diagnostic and Statistical Manual, Fourth Edition (DSM-IV) and the Tridimensional Personality Questionnaire (TPQ). We confirmed drug-dependence status using a questionnaire based on DSM-IV criteria. We extracted DNA from the subjects' whole blood and genotyped it for DRD2 allelic variants.

RESULTS

Genotype analysis showed a significantly higher frequency for the TaqIA polymorphism among the addicts (69.9%) compared to control subjects (42.6%; Fisher's exact χ(2), p < .05). We observed no significant differences for other variants between the addicts and controls. The addicts had higher scores for novelty seeking (NS) and harm avoidance (HA) personality traits but lower scores for reward dependence (RD) when compared to control subjects.

LIMITATION

The environmental cues are different for the addicts, and the healthy university students we used as controls. We recommend that researchers employ a gene-environment interaction approach to study factors associated with addictive behaviors in future studies.

CONCLUSION

Taq1A may be implicated for an increased vulnerability to addiction. Screening of this marker might be useful for identifying individuals at risk of addiction.

Contrasting roles for dopamine D1 and D2 receptor subtypes in the dorsomedial striatum but not the nucleus accumbens core during behavioral inhibition in the stop-signal task in rats

Dopamine and dopamine-receptor function are often implicated in behavioral inhibition, and deficiencies within behavioral inhibition processes linked to attention deficit/hyperactivity disorder (ADHD), schizophrenia, obsessive-compulsive disorder, and drug addiction. In the stop-signal task, which measures the speed of the process of inhibition [stop-signal reaction time (SSRT)], psychostimulant-related improvement of SSRT in ADHD is linked with dopamine function. However, the precise nature of dopaminergic control over SSRT remains unclear. This study examined region- and receptor-specific modulation of SSRT in the rat using direct infusions of the dopamine D1 receptor (DRD1) antagonist SCH 23390 or dopamine D2 receptor (DRD2) antagonist sulpiride into the dorsomedial striatum (DMStr) or nucleus accumbens core (NAcbC). DRD1 and DRD2 antagonists had contrasting effects on SSRT that were specific to the DMStr. SCH 23390 decreased SSRT with little effect on the go response. Conversely, sulpiride increased SSRT but also increased go-trial reaction time and reduced trial completion at the highest doses. These results suggest that DRD1 and DRD2 function within the DMStr, but not the NAcbC, may act to balance behavioral inhibition in a manner that is independent of behavioral activation.

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.

Impulsiveness and venturesomeness in German smokers

INTRODUCTION

Cigarette smoking is a behavior, which is influenced by genetic, demographic, and psychological factors. A large body of research has examined the association of cigarette smoking variables with individual differences in personality traits. The aim of the current study was to replicate the findings of higher self-reported impulsivity in smokers compared with never-smokers in a German sample using Eysenck´s construct of impulsivity. Furthermore, it was intended to further the knowledge about associations between different self-reported impulsivity components and different smoking variables.

METHODS

We used the Impulsiveness-Venturesomeness-Empathy questionnaire (I7) to measure self-reported impulsiveness and venturesomeness and the Temperament and Character Inventory (TCI) to measure novelty seeking (NS) in a sample of 82 nicotine-dependent smokers and 119 never-smokers.

RESULTS

Smokers scored higher on impulsiveness, venturesomeness, and NS than never-smokers independent of age, gender, and years of education. We found a significant association between venturesomeness, impulsiveness and smoking status in daily smokers.

CONCLUSIONS

In summary, this study provides evidence that impulsiveness and venturesomeness as well as the novelty-seeking subscale extravagance are significantly associated with smoking status in a German sample of female and male smokers compared with never-smokers.

Rats prone to attribute incentive salience to reward cues are also prone to impulsive action

Animals vary considerably in the degree to which they attribute incentive salience to cues predictive of reward. When a discrete cue (conditional stimulus) is repeatedly paired with delivery of a food reward (unconditional stimulus) only some rats ("sign-trackers"; STs) come to find the cue itself an attractive and desirable incentive stimulus. For other rats ("goal-trackers"; GTs) the cue is an effective conditional stimulus - it evokes a conditional response - but it is less attractive and less desirable. Given that STs have particular difficulty resisting reward cues, and are thought to have poor inhibitory control over their behavior, we hypothesized that they may also be more impulsive. There are, however, multiple forms of impulsivity; therefore, we compared STs and GTs on two tests of so-called impulsive action - a 2-choice serial reaction time task and a differential reinforcement of low rates of responding task, and one test of impulsive choice - a delay discounting choice procedure. We found that relative to GTs, STs were more impulsive on the two tests of impulsive action, but not on the test of impulsive choice. We speculate that when these two traits combine, that is, when an individual is not only prone to attribute incentive salience to reward cues but also prone to impulsive action, they may be especially susceptible to impulse control disorders, including addiction.

Impulsivity, compulsivity, and top-down cognitive control

Impulsivity is the tendency to act prematurely without foresight. Behavioral and neurobiological analysis of this construct, with evidence from both animal and human studies, defines several dissociable forms depending on distinct cortico-striatal substrates. One form of impulsivity depends on the temporal discounting of reward, another on motor or response disinhibition. Impulsivity is commonly associated with addiction to drugs from different pharmacological classes, but its causal role in human addiction is unclear. We characterize in neurobehavioral and neurochemical terms a rodent model of impulsivity based on premature responding in an attentional task. Evidence is surveyed that high impulsivity on this task precedes the escalation subsequently of cocaine self-administration behavior, and also a tendency toward compulsive cocaine-seeking and to relapse. These results indicate that the vulnerability to stimulant addiction may depend on an impulsivity endophenotype. Implications of these findings for the etiology, development, and treatment of drug addiction are considered.

Contributions of serotonin in addiction vulnerability

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.

Reversal of cocaine-conditioned place preference and mesocorticolimbic Zif268 expression by social interaction in rats

Little is known how social interaction, if offered as an alternative to drug consumption, affects neural circuits involved in drug reinforcement and substance dependence. Conditioned place preference (CPP) for cocaine (15 mg/kg i.p.) or social interaction (15 minutes) as an alternative stimulus was investigated in male Sprague-Dawley rats. Four social interaction episodes with a male adult conspecific completely reversed cocaine CPP and were even able to prevent reacquisition of cocaine CPP. Social interaction also reversed cocaine CPP-induced expression of the immediate-early gene zif268 in the nucleus accumbens shell, the central and basolateral amygdala and the ventral tegmental area. These findings suggest that social interaction, if offered in a context that is clearly distinct from the previously drug-associated ones, may profoundly decrease the incentive salience of drug-associated contextual stimuli. The novel experimental design facilitates the neurobiological investigation of this phenomenon which may be beneficial for human drug users in treatment.

Whisker: a client-server high-performance multimedia research control system

We describe an original client-server approach to behavioral research control and the Whisker system, a specific implementation of this design. The server process controls several types of hardware, including digital input/output devices, multiple graphical monitors and touchscreens, keyboards, mice, and sound cards. It provides a way to access this hardware for client programs, communicating with them via a simple text-based network protocol based on the standard Internet protocol. Clients to implement behavioral tasks may be written in any network-capable programming language. Applications to date have been in experimental psychology and behavioral and cognitive neuroscience, using rodents, humans, nonhuman primates, dogs, pigs, and birds. This system is flexible and reliable, although there are potential disadvantages in terms of complexity. Its design, features, and performance are described.

Neuroscience of behavioral and pharmacological treatments for addictions

Although substantial advances have been made in behavioral and pharmacological treatments for addictions, moving treatment development to the next stage may require novel ways of approaching addictions, particularly ways based on new findings regarding the neurobiological underpinnings of addictions that also assimilate and incorporate relevant information from earlier approaches. In this review, we first briefly review theoretical and biological models of addiction and then describe existing behavioral and pharmacologic therapies for the addictions within this framework. We then propose new directions for treatment development and targets that are informed by recent evidence regarding the heterogeneity of addictions and the neurobiological contributions to these disorders.

Mutant DISC1 affects methamphetamine-induced sensitization and conditioned place preference: a comorbidity model

Genetic factors involved in neuroplasticity have been implicated in major psychiatric illnesses such as schizophrenia, depression, and substance abuse. Given its extended interactome, variants in the Disrupted-In-Schizophrenia-1 (DISC1) gene could contribute to drug addiction and psychiatric diseases. Thus, we evaluated how dominant-negative mutant DISC1 influenced the neurobehavioral and molecular effects of methamphetamine (METH). Control and mutant DISC1 mice were studied before or after treatment with non-toxic escalating dose (ED) of METH. In naïve mice, we assessed METH-induced conditioned place preference (CPP), dopamine (DA) D2 receptor density and the basal and METH-induced activity of DISC1 partners, AKT and GSK-3β in the ventral striatum. In ED-treated mice, 4 weeks after METH treatment, we evaluated fear conditioning, depression-like responses in forced swim test, and the basal and METH-induced activity of AKT and GSK-3β in the ventral striatum. We found impairment in METH-induced CPP, decreased DA D2 receptor density and altered METH-induced phosphorylation of AKT and GSK-3β in naïve DISC1 female mice. The ED regimen was not neurotoxic as evidenced by unaltered brain regional monoamine tissue content. Mutant DISC1 significantly delayed METH ED-produced sensitization and affected drug-induced phosphorylation of AKT and GSK-3β in female mice. Our results suggest that perturbations in DISC1 functions in the ventral striatum may impact the molecular mechanisms of reward and sensitization, contributing to comorbidity between drug abuse and major mental diseases.

Impulsive choice, as measured in a delay discounting paradigm, remains stable after chronic heroin administration

Heroin addicts display poorer impulse control than non-addicts, however it is not known if high impulsivity is a function of chronic heroin intake or a pre-disposing vulnerability for heroin addiction. Using animal models, relatively few studies have examined changes in impulsive choice as a function of chronic drug. The objective of this study was to measure alterations in impulsive choice through a delay discounting paradigm, as a function of chronic heroin administration. Animals were trained on a series of delay discounting sessions. Each session contained 5 blocks of trials. Blocks started with 2 forced, followed by 6 free choice trials. Pressing one lever resulted in the delivery of a small immediate (1 food pellet) reward and another lever in a large delayed (5 pellets) reward. Sessions consisted of the 3 ascending delay sequences in seconds. On the terminal sequence (0, 10, 20, 40, and 60s) animals exhibited a reversal of reward choice pattern of responding that allowed for the calculation of an indifference point (IP). After animals showed stable IPs they were treated with either heroin or saline for 12 days. Three days after the last injection animals were again placed in operant chambers and experienced the terminal delay discounting sequence at which time IPs were reassessed. Heroin-treated animals exhibited significant progressive increases in locomotor activity. Groups did not differ in IPs or performance across delay conditions during either before or after chronic treatment periods. These results indicate that chronic heroin intake does not impact later impulsive responding for natural (food) reward.

Behavioral characterization of adult male and female rhesus monkeys exposed to cocaine throughout gestation

RATIONALE

In utero cocaine exposure has been associated with alterations in the dopamine (DA) system in monkeys. However, the behavioral outcomes of prenatal cocaine exposure in adulthood are poorly understood.

OBJECTIVES

To assess several behavioral measures in 14-year-old rhesus monkeys exposed to cocaine in utero and controls (n = 10 per group).

MATERIALS AND METHODS

For these studies, two unconditioned behavioral tasks, novel object reactivity and locomotor activity, and two conditioned behavioral tasks, response extinction and delay discounting, were examined. In addition, cerebrospinal fluid (CSF) samples were analyzed for concentrations of the monoamine metabolites homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA).

RESULTS

No differences in CSF concentrations of 5-HIAA and HVA, latencies to touch a novel object or locomotor activity measures were observed between groups or sexes. However, prenatally cocaine-exposed monkeys required a significantly greater number of sessions to reach criteria for extinction of food-reinforced behavior than control monkeys. On the delay-discounting task, male prenatally cocaine-exposed monkeys switched preference from the larger reinforcer to the smaller one at shorter delay values than male control monkeys; no differences were observed in females.

CONCLUSIONS

These findings suggest that prenatal cocaine exposure results in long-term neurobehavioral deficits that are influenced by sex of the individual.

Selective norepinephrine reuptake inhibition by atomoxetine prevents cue-induced heroin and cocaine seeking

BACKGROUND

Preventing relapse to drug use is a major challenge for drug addiction treatment. We have recently shown that impulsivity predating drug-taking increases the susceptibility to relapse to cocaine seeking and that treatment with the anti-impulsivity drug atomoxetine (ATO), a selective norepinephrine re-uptake inhibitor (norepinephrine transporter), prevents relapse. Here, we investigated further the effects of ATO on cue-maintained heroin and cocaine seeking and relapse and compared these effects with those of the anti-impulsivity stimulant drug methylphenidate (MPH).

METHODS

Rats were trained to seek and self-administer cocaine or heroin under a second-order schedule of reinforcement. After acquisition of stable responding, groups of rats (n = 10-12) were treated, in a within-subject design, with either ATO or MPH (.3-3.0 mg/kg IP), and the effects on cocaine and heroin seeking were measured. The effects of ATO (.3-1.0 mg/kg) on cue-induced relapse to cocaine seeking after a 1-week period of abstinence were also studied.

RESULTS

Atomoxetine significantly decreased both cue-controlled cocaine and heroin seeking, whereas MPH had no significant effect. Atomoxetine also significantly attenuated cue-induced relapse to cocaine seeking after abstinence. The effects of ATO were selective for cue-controlled drug-seeking, because it did not affect responding in the absence of the drug-paired cue; nor did it alter responding for oral sucrose, except minimally at the highest dose, or locomotor activity.

CONCLUSIONS

Selective norepinephrine transporter inhibition by ATO might be an effective treatment for the prevention of relapse to both stimulant and opiate addiction.

Insulin modulates cocaine-sensitive monoamine transporter function and impulsive behavior

Because insulin acutely enhances the function of dopamine transporters, the tyrosine kinase receptors activated by this hormone may modulate transporter-dependent neurochemical and behavioral effects of psychoactive drugs. In this respect, we examined the effects of insulin on exocytotic monoamine release and the efficacy of the monoamine transporter blocker cocaine in rat nucleus accumbens. Whereas insulin reduced electrically evoked exocytotic [(3)H]dopamine release in nucleus accumbens slices, the hormone potentiated the release-enhancing effect of cocaine thereon. The phosphatidylinositol 3-kinase inhibitor LY294002 abolished these effects, indicating the involvement of insulin receptors. Similar insulin effects were observed on the release of [(3)H]norepinephrine in nucleus accumbens slices, but not on that of [(3)H]serotonin, and were also apparent in medial prefrontal cortex slices. As might then be expected, insulin also potentiated the dopamine and norepinephrine release-enhancing effects of the selective monoamine uptake inhibitors GBR12909 and desmethylimipramine, respectively. In subsequent behavioral experiments, we investigated the role of insulin in motor impulsivity that depends on monoamine neurotransmission in the nucleus accumbens. Intracranial administration of insulin in the nucleus accumbens alone reduced premature responses in the five-choice serial reaction time task and enhanced the stimulatory effect of peripheral cocaine administration on impulsivity, resembling the observed neurochemical effects of the hormone. In contrast, cocaine-induced locomotor activity remained unchanged by intra-accumbal insulin application. These data reveal that insulin presynaptically regulates cocaine-sensitive monoamine transporter function in the nucleus accumbens and, as a consequence, impulsivity. Therefore, insulin signaling proteins may represent targets for the treatment of inhibitory control deficits such as addictive behaviors.

Translating cognition from animals to humans

Many clinical disorders, whether neurological (e.g. Alzheimer's disease) or neuropsychiatric (e.g. schizophrenia and depression), exhibit cognitive symptoms that require pharmacological treatment. Cognition is multi-faceted and includes processes of perception, attention, working memory, long-term memory, executive function, language and social cognition. This article reviews how it is feasible to model many aspects of human cognition with the use of appropriate animal models and associated techniques, including the use of computer controlled tests (e.g. touch-screens), for optimising translation of experimental research to the clinic. When investigating clinical disorders, test batteries should aim to profile cognitive function in order to determine which aspects are impaired and which are preserved. In this review we have paid particular attention to the validation of translational methods; this may be done through the application of common theoretical principles, by comparing the effects of psychological manipulations and, wherever feasible, with the demonstration of homologous neural circuitry or equivalent pharmacological actions in the animal and human paradigms. Of particular importance is the use of 'back-translation' to ensure that the animal model has validity, for example, in predicting the effects of therapeutic drugs already found in human studies. It is made clear that the choice of appropriate behavioral tests is an important element of animal models of neuropsychiatric or neurological disorder; however, of course it is also important to select appropriate manipulations, whether genetic, neurodevelopmental, neurotoxic, or pharmacological, for simulating the neural substrates relevant to the disorders that lead to predictable behavioral and cognitive impairments, for optimising the testing of candidate compounds.

Social neuroscience: challenges and opportunities in the study of complex behavior

Social species are so characterized because they form organizations that extend beyond the individual. The goal of social neuroscience is to investigate the biological mechanisms that underlie these social structures, processes, and behavior and the influences between social and neural structures and processes. Such an endeavor is challenging because it necessitates the integration of multiple levels. Mapping across systems and levels (from genome to social groups and cultures) requires interdisciplinary expertise, comparative studies, innovative methods, and integrative conceptual analysis. Examples of how social neuroscience is contributing to our understanding of the functions of the brain and nervous system are described, and societal implications of social neuroscience are considered.

Advances in animal models of drug addiction

Drug addiction is a syndrome of impaired response inhibition and salience attribution, which involves a complex neurocircuitry underlying drug reinforcement, drug craving, and compulsive drug-seeking and drug-taking behaviors despite adverse consequences. The concept of disease stages with transitions from acute rewarding effects to early- and end-stage addiction has had an important impact on the design of nonclinical animal models. This chapter reviews the main advances in nonclinical paradigms that aim to at model (1) positive and negative reinforcing effects of addictive drugs; (2) relapse to drug-seeking behavior; (3) reconsolidation of drug cue memories, and (4) compulsive/impulsive drug intake. In addition, recent small animal neuroimaging studies and invertebrate models will be briefly discussed (see also Bifone and Gozzi, Animal models of ADHD, 2011). Continuous improvement in modeling drug intake, craving, withdrawal symptoms, relapse, and comorbid psychiatric associations is a necessary step to better understand the etiology of the disease and to ultimately foster the discovery, validation and optimization of new efficacious pharmacotherapeutic approaches. The modeling of specific subprocesses or constructs that address clinically defined criteria will ultimately increase our understanding of the disease as a whole. Future research will have to address the questions of whether some of these constructs can be reliably used as outcome measures to assess the effects of a treatment in clinical settings, whether changes in those measures can be a target of therapeutic efforts, and whether they relate to biological markers of traits such as impulsivity, which contribute to increased drug-seeking and may predict binge-like patterns of drug intake.

Human cognitive flexibility depends on dopamine D2 receptor signaling

RATIONALE

Accumulating evidence indicates that the cognitive effects of dopamine depend on the subtype of dopamine receptor that is activated. In particular, recent work with animals as well as current theorizing has suggested that cognitive flexibility depends on dopamine D2 receptor signaling. However, there is no evidence for similar mechanisms in humans.

OBJECTIVES

We aim to demonstrate that optimal dopamine D2 receptor signaling is critical for human cognitive flexibility.

METHODS

To this end, a pharmacological pretreatment design was employed. This enabled us to investigate whether effects of the dopamine receptor agonist bromocriptine on task-set switching were abolished by pretreatment with the D2 receptor antagonist sulpiride. To account for individual (genetic) differences in baseline levels of dopamine, we made use of a common variable number of tandem repeat (VNTR) polymorphism in the 3'-untranslated region of the dopamine transporter gene, DAT1.

RESULTS

Bromocriptine improved cognitive flexibility relative to placebo, but only in subjects with genetically determined low levels of dopamine (n = 27). This beneficial effect of bromocriptine on cognitive flexibility was blocked by pretreatment with the selective dopamine D2 receptor antagonist sulpiride (n = 14).

CONCLUSIONS

These results provide strong evidence in favor of the hypothesis that human cognitive flexibility implicates dopamine D2 receptor signaling.

The roles of dopamine and serotonin in decision making: evidence from pharmacological experiments in humans

Neurophysiological experiments in primates, alongside neuropsychological and functional magnetic resonance investigations in humans, have significantly enhanced our understanding of the neural architecture of decision making. In this review, I consider the more limited database of experiments that have investigated how dopamine and serotonin activity influences the choices of human adults. These include those experiments that have involved the administration of drugs to healthy controls, experiments that have tested genotypic influences upon dopamine and serotonin function, and, finally, some of those experiments that have examined the effects of drugs on the decision making of clinical samples. Pharmacological experiments in humans are few in number and face considerable methodological challenges in terms of drug specificity, uncertainties about pre- vs post-synaptic modes of action, and interactions with baseline cognitive performance. However, the available data are broadly consistent with current computational models of dopamine function in decision making and highlight the dissociable roles of dopamine receptor systems in the learning about outcomes that underpins value-based decision making. Moreover, genotypic influences on (interacting) prefrontal and striatal dopamine activity are associated with changes in choice behavior that might be relevant to understanding exploratory behaviors and vulnerability to addictive disorders. Manipulations of serotonin in laboratory tests of decision making in human participants have provided less consistent results, but the information gathered to date indicates a role for serotonin in learning about bad decision outcomes, non-normative aspects of risk-seeking behavior, and social choices involving affiliation and notions of fairness. Finally, I suggest that the role played by serotonin in the regulation of cognitive biases, and representation of context in learning, point toward a role in the cortically mediated cognitive appraisal of reinforcers when selecting between actions, potentially accounting for its influence upon the processing salient aversive outcomes and social choice.

The effects of priming restrained versus disinhibited behaviour on alcohol-seeking in social drinkers

BACKGROUND

Deficient response inhibition (disinhibition) may play a causal role in alcohol abuse, with impaired inhibition occurring prior to, and acting as a risk factor for, subsequent alcohol problems. We experimentally primed either disinhibited or restrained behaviour while participants completed a Stop-Signal task, before examining the effects on alcohol-seeking behaviour.

METHODS

Fifty three social drinkers completed a Stop-Signal task following instructions that either emphasised rapid responding at the expense of successful inhibition (Disinhibition group) or vice versa (Restrained group). Subsequent ad lib alcohol-seeking was measured with a bogus taste test.

RESULTS

As predicted, participants in the Disinhibition group consumed more beer during the taste test compared to participants in the Restrained group. Furthermore, within the Restrained group only, correlations indicated that those participants who responded more cautiously during the Stop-Signal task subsequently consumed less beer.

CONCLUSIONS

An experimental manipulation of response set during a response inhibition task, emphasising either restrained or disinhibited responding, has a causal influence on alcohol-seeking behaviour in social drinkers.

Animal models of ADHD

Studies employing animal models of attention-deficit/hyperactivity disorder (ADHD) present clear inherent advantages over human studies. Animal models are invaluable tools for the study of underlying neurochemical, neuropathological and genetic alterations that cause ADHD, because they allow relatively fast, rigorous hypothesis testing and invasive manipulations as well as selective breeding. Moreover, especially for ADHD, animal models with good predictive validity would allow the assessment of potential new therapeutics. In this chapter, we describe and comment on the most frequently used animal models of ADHD that have been created by genetic, neurochemical and physical alterations in rodents. We then discuss that an emerging and promising direction of the field is the analysis of individual behavioural differences among a normal population of animals. Subjects presenting extreme characteristics related to ADHD can be studied, thereby avoiding some of the problems that are found in other models, such as functional recovery and unnecessary assumptions about aetiology. This approach is justified by the theoretical need to consider human ADHD as the extreme part of a spectrum of characteristics that are distributed normally in the general population, as opposed to the predominant view of ADHD as a separate pathological category.

Neurogenetics and pharmacology of learning, motivation, and cognition

Many of the individual differences in cognition, motivation, and learning-and the disruption of these processes in neurological conditions-are influenced by genetic factors. We provide an integrative synthesis across human and animal studies, focusing on a recent spate of evidence implicating a role for genes controlling dopaminergic function in frontostriatal circuitry, including COMT, DARPP-32, DAT1, DRD2, and DRD4. These genetic effects are interpreted within theoretical frameworks developed in the context of the broader cognitive and computational neuroscience literature, constrained by data from pharmacological, neuroimaging, electrophysiological, and patient studies. In this framework, genes modulate the efficacy of particular neural computations, and effects of genetic variation are revealed by assays designed to be maximally sensitive to these computations. We discuss the merits and caveats of this approach and outline a number of novel candidate genes of interest for future study.

Addiction: decreased reward sensitivity and increased expectation sensitivity conspire to overwhelm the brain's control circuit

Based on brain imaging findings, we present a model according to which addiction emerges as an imbalance in the information processing and integration among various brain circuits and functions. The dysfunctions reflect (a) decreased sensitivity of reward circuits, (b) enhanced sensitivity of memory circuits to conditioned expectations to drugs and drug cues, stress reactivity, and (c) negative mood, and a weakened control circuit. Although initial experimentation with a drug of abuse is largely a voluntary behavior, continued drug use can eventually impair neuronal circuits in the brain that are involved in free will, turning drug use into an automatic compulsive behavior. The ability of addictive drugs to co-opt neurotransmitter signals between neurons (including dopamine, glutamate, and GABA) modifies the function of different neuronal circuits, which begin to falter at different stages of an addiction trajectory. Upon exposure to the drug, drug cues or stress this results in unrestrained hyperactivation of the motivation/drive circuit that results in the compulsive drug intake that characterizes addiction.

The IMAGEN study: reinforcement-related behaviour in normal brain function and psychopathology

A fundamental function of the brain is to evaluate the emotional and motivational significance of stimuli and to adapt behaviour accordingly. The IMAGEN study is the first multicentre genetic-neuroimaging study aimed at identifying the genetic and neurobiological basis of individual variability in impulsivity, reinforcer sensitivity and emotional reactivity, and determining their predictive value for the development of frequent psychiatric disorders. Comprehensive behavioural and neuropsychological characterization, functional and structural neuroimaging and genome-wide association analyses of 2000 14-year-old adolescents are combined with functional genetics in animal and human models. Results will be validated in 1000 adolescents from the Canadian Saguenay Youth Study. The sample will be followed up longitudinally at the age of 16 years to investigate the predictive value of genetics and intermediate phenotypes for the development of frequent psychiatric disorders. This review describes the strategies the IMAGEN consortium used to meet the challenges posed by large-scale multicentre imaging-genomics investigations. We provide detailed methods and Standard Operating Procedures that we hope will be helpful for the design of future studies. These include standardization of the clinical, psychometric and neuroimaging-acquisition protocols, development of a central database for efficient analyses of large multimodal data sets and new analytic approaches to large-scale genetic neuroimaging analyses.

Trait-like impulsivity does not predict escalation of heroin self-administration in the rat

RATIONALE

The neural and psychological mechanisms underlying vulnerability to drug addiction are poorly understood. Although a number of animal models have been developed to investigate vulnerability to stimulant addiction, few have considered how vulnerability traits such as impulsivity predict hallmark features of heroin addiction including the escalation of drug intake and increased propensity for relapse following protracted abstinence.

OBJECTIVE

The aim of this study was to investigate whether high impulsivity in rats predicts the propensity to escalate intravenous heroin self-administration and to relapse following an extended withdrawal period from heroin.

METHODS

High (HI)- and low (LI)-impulsive rats, defined by the extent of premature responding on the 5-choice serial reaction time test (5-CSRTT), were catheterized and allowed to self-administer heroin (40 μg/100 μl/infusion). After 5 days of short access (1 h/day) to heroin, rats were then given extended (6 h/day) access to heroin for 18 consecutive days.

RESULTS

High impulsivity predicted neither a greater tendency to acquire heroin SA nor an increased escalation of heroin self-administration. Moreover, high impulsivity was not associated with an increased propensity to relapse after protracted withdrawal from heroin. Nevertheless, marked inter-individual differences in the escalation of heroin self-administration were observed.

CONCLUSIONS

Although high impulsivity on the 5-CSRTT has been shown to predict loss of control over cocaine intake, this does not generalize to a loss of control over heroin self-administration. These findings suggest important distinctions in vulnerability mechanisms underlying cocaine and heroin addiction with trait-like impulsivity playing a role in stimulant but not opiate addiction.

Khat use and neurobehavioral functions: suggestions for future studies

Although there is a rich body of research available regarding the effect of acute and chronic khat dosing in animal models, research on the behavioral and cognitive effects of khat in human subjects is not extensive and several of the available studies have been done only in the context of observational and single-case studies. In light of the absence of a substantial literature on the neurobehavioral deficits associated with khat use and to provide a context that could be used to identify themes for future research we review previous research that has focused on other stimulant drugs. This review highlights multiple areas of neurocognitive deficit that have been identified in previous studies of individuals who have been chronic users of stimulants, such as amphetamines and methamphetamines. The review highlights a substantial body of evidence demonstrating a wide range of learning and memory impairments including deficits that persist during abstinence from active drug use. This review does not imply a similar khat effect, but due to some similarities pharmacologically between the active components of khat (cathinone and cathine) and amphetamines, future studies examining these same domains of cognitive functioning in chronic khat users and abstinent khat users appears to be warranted, if possible using some of the same or similar laboratory measures.

Frontostriatal involvement in task switching depends on genetic differences in d2 receptor density

Recent studies suggest an association of dopamine D2 receptor (DRD2) availability with flexibility in reward-based learning. We extend these results by demonstrating an association of genetically based differences in DRD2 density with the ability to intentionally switch between nonrewarded tasks: noncarriers of the A1 allele of the DRD2/ANKK1-TaqIa polymorphism, associated with higher DRD2 density, show increased task-switching costs, increased prefrontal switching activity in the inferior frontal junction area, and increased functional connectivity in dorsal frontostriatal circuits, relative to A1 allele carriers. A DRD2 haplotype analysis in the same sample confirmed these results, indicating an association between high D2 density and increased task-switching effort. Our results provide evidence that converges with that from association studies relating increased D2 density to deficits in cognitive flexibility in schizophrenia. We suggest that individual differences in striatal D2 signaling in healthy humans modulate goal-directed gating to prefrontal cortex, thus leading to individual differences in switching intentionally to newly relevant behaviors.

Response inhibition and reward response bias mediate the predictive relationships between impulsivity and sensation seeking and common and unique variance in conduct disorder and substance misuse

BACKGROUND

Disinhibited traits, assessed both at the self-report and at the cognitive/behavioral levels, have been frequently implicated in externalizing behaviors, such as conduct disorder (CD), binge drinking, and drug use. However, self-report measures of disinhibition, such as impulsivity (IMP) and sensation seeking (SS), and cognitive measures of disinhibition are not often studied together in the same participants. Thus, it is still unclear how cognitive measures such as response inhibition and reward response bias relate to self-report measures of IMP and SS, and whether they can explain some of the association found between these self-report measures and specific facets of externalizing problems.

METHODS

The aim of this study was to assess whether cognitive measures of disinhibition relate to self-report measures of disinhibition and can mediate the specific relationships between self-report measures of disinhibition and CD symptoms, binge drinking and drug use in adolescence. Seventy-six adolescents were assessed on personality, substance use, and conduct problems every 6 months from 14 to 16 years of age and completed a test battery that included a Stop task, rewarded go/no-go task, digit span, and intelligence quotient tests at 16 years of age.

RESULTS

Multiple regression analyses showed that self-report IMP at 14 and deficits in response inhibition were associated with a 2-year average CD symptoms score and that deficits in response inhibition partially mediated the association between self-report IMP and CD symptoms (ab = 0.018 CI: 0.00002 to 0.04827). In contrast, SS and reward response bias were significantly associated with the unique variance in binge drinking, and that part of the overlap between SS and binge drinking was mediated by reward response bias (ab = 0.019, CI: 0.00131 to 0.04662).

CONCLUSIONS

Findings show a dissociation between inhibitory measures associated with CD symptoms and those associated with binge drinking, with "cool" inhibitory and executive functions being associated with CD but "hot," reward-related disinhibition measures being specific mediators between SS and binge drinking. The findings support the theoretical conceptualization for dual cognitive/motivational pathways of disinhibition, in this case IMP and SS, and their unique association with externalizing behavior in adolescence.

Dopamine release in ventral striatum of pathological gamblers losing money

OBJECTIVE

To investigate dopaminergic neurotransmission in relation to monetary reward and punishment in pathological gambling. Pathological gamblers (PG) often continue gambling despite losses, known as 'chasing one's losses'. We therefore hypothesized that losing money would be associated with increased dopamine release in the ventral striatum of PG compared with healthy controls (HC).

METHOD

We used Positron Emission Tomography (PET) with [(11)C]raclopride to measure dopamine release in the ventral striatum of 16 PG and 15 HC playing the Iowa Gambling Task (IGT).

RESULTS

PG who lost money had significantly increased dopamine release in the left ventral striatum compared with HC. PG and HC who won money did not differ in dopamine release.

CONCLUSION

Our findings suggest a dopaminergic basis of monetary losses in pathological gambling, which might explain loss-chasing behavior. The findings may have implications for the understanding of dopamine dysfunctions and impaired decision-making in pathological gambling and substance-related addictions.

Associations with substance abuse treatment completion among drug court participants

Subjects in the study included all participants (N = 573) in drug treatment court in amid-sized U.S. city from 1996 through 2004. Administrative data from the drug court included measures of demographics and socioeconomics, substance use, and criminal justice history. Stepwise multivariate logistic regression yielded a final model of failure to complete drug treatment. Unemployment, lower educational attainment, and cocaine use disorders were associated with failure to complete treatment. The limitations of administrative data should be considered in the interpretation of results.