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

Altered neuronal activity in the ventromedial prefrontal cortex drives nicotine intake escalation

Nicotine addiction develops after prolonged drug use and escalation of drug intake. However, because of difficulties in demonstrating escalation of nicotine use in rats, its underlying neuroadaptations still remain poorly understood. Here we report that access to unusually high doses of nicotine (i.e., from 30 µg to 240 µg/kg/injection) for self-administration precipitated a rapid and robust escalation of nicotine intake and increased the motivation for the drug in rats. This nicotine intake escalation also induced long-lasting changes in vmPFC neuronal activity both before and during nicotine self-administration. Specifically, after escalation of nicotine intake, basal vmPFC neuronal activity increased above pre-escalation and control activity levels, while ongoing nicotine self-administration restored these neuronal changes. Finally, simulation of the restoring effects of nicotine with in vivo optogenetic inhibition of vmPFC neurons caused a selective de-escalation of nicotine self-administration.

Common and distinct fronto-striatal volumetric changes in heroin and cocaine use disorders

Different drugs of abuse impact the morphology of fronto-striatal dopaminergic targets in both common and unique ways. While dorsal striatal volume tracks with addiction severity across drug classes, opiates impact ventromedial prefrontal cortex (vmPFC) and nucleus accumbens (NAcc) neuroplasticity in preclinical models, and psychostimulants alter inhibitory control, rooted in cortical regions such as the inferior frontal gyrus (IFG). We hypothesized parallel grey matter volume changes associated with human heroin or cocaine use disorder: lower grey matter volume of vmPFC/NAcc in heroin use disorder and IFG in cocaine use disorder, and putamen grey matter volume to be associated with addiction severity measures (including craving) across both. In this cross-sectional study, we quantified grey matter volume (P < 0.05-corrected) in age/sex/IQ-matched individuals with heroin use disorder (n = 32, seven females), cocaine use disorder (n = 32, six females) and healthy controls (n = 32, six females) and compared fronto-striatal volume between groups using voxel-wise general linear models and non-parametric permutation-based tests. Overall, individuals with heroin use disorder had smaller vmPFC and NAcc/putamen volumes than healthy controls. Bilateral lower IFG grey matter volume patterns were specifically evident in cocaine versus heroin use disorders. Correlations between addiction severity measures and putamen grey matter volume did not reach nominal significance level in this sample. These results indicate alterations in dopamine-innervated regions (in the vmPFC and NAcc) in heroin addiction. For the first time we demonstrate lower IFG grey matter volume specifically in cocaine compared with heroin use disorder, suggesting a signature of reduced inhibitory control, which remains to be tested directly using select behavioural measures. Overall, results suggest substance-specific volumetric changes in human psychostimulant or opiate addiction, with implications for fine-tuning biomarker and treatment identification by primary drug of abuse.

Role of preexisting inhibitory control deficits vs. drug use history in mediating insensitivity to aversive consequences in a rat model of polysubstance use

RATIONALE

The nature and predictors of insensitivity to aversive consequences of heroin + cocaine polysubstance use are not well characterized.

OBJECTIVES

Translational methods incorporating a tightly controlled animal model of drug self-administration and measures of inhibitory control and avoidance behavior might be helpful for clarifying this issue.

METHODS

The key approach for distinguishing potential contributions of pre-existing inhibitory control deficits vs. drug use history in meditating insensitivity to aversive consequences was comparison of two rat strains: Wistar (WIS/Crl), an outbred strain, and the spontaneously hypertensive rat (SHR/NCrl), an inbred strain shown previously to exhibit heightened cocaine and heroin self-administration and poor inhibitory control relative to WIS/Crl.

RESULTS

In separate tasks, SHR/NCrl displayed greater impulsive action and compulsive-like behavior than WIS/Crl prior to drug exposure. Under two different schedules of drug delivery, SHR/NCrl self-administered more cocaine than WIS/Crl, but self-administered a similar amount of heroin + cocaine as WIS/Crl. When half the session cycles were punished by random foot shock, SHR/NCrl initially were less sensitive to punishment than WIS/Crl when self-administering cocaine, but were similarly insensitive to punishment when self-administering heroin + cocaine. Based on correlation analyses, only trait impulsivity predicted avoidance capacity in rats self-administering cocaine and receiving yoked-saline. In contrast, only amount of drug use predicted avoidance capacity in rats self-administering heroin + cocaine. Additionally, baseline drug seeking and taking predicted punishment insensitivity in rats self-administering cocaine or heroin + cocaine.

CONCLUSIONS

Based on the findings revealed in this animal model, human laboratory research concerning the nature and predictors of insensitivity to aversive consequences in heroin and cocaine polysubstance vs. monosubstance users is warranted.

Altered Accumbal Dopamine Terminal Dynamics Following Chronic Heroin Self-Administration

Administration of heroin results in the engagement of multiple brain regions and the rewarding and addictive effects are mediated, at least partially, through activation of the mesolimbic dopamine system. However, less is known about dopamine system function following chronic exposure to heroin. Withdrawal from chronic heroin exposure is likely to drive a state of low dopamine in the nucleus accumbens (NAc), as previously observed during withdrawal from other drug classes. Thus, we aimed to investigate alterations in NAc dopamine terminal function following chronic heroin self-administration to identify a mechanism for dopaminergic adaptations. Adult male Long Evans rats were trained to self-administer heroin (0.05 mg/kg/inf, IV) and then placed on a long access (FR1, 6-h, unlimited inf, 0.05 mg/kg/inf) protocol to induce escalation of intake. Following heroin self-administration, rats had decreased basal extracellular levels of dopamine and blunted dopamine response following a heroin challenge (0.1 mg/kg/inf, IV) in the NAc compared to saline controls. FSCV revealed that heroin-exposed rats exhibited reduced stimulated dopamine release during tonic-like, single-pulse stimulations, but increased phasic-like dopamine release during multi-pulse stimulation trains (5 pulses, 5–100 Hz) in addition to an altered dynamic range of release stimulation intensities when compared to controls. Further, we found that presynaptic D3 autoreceptor and kappa-opioid receptor agonist responsivity were increased following heroin self-administration. These results reveal a marked low dopamine state following heroin exposure and suggest the combination of altered dopamine release dynamics may contribute to increased heroin seeking.

Enhanced heroin self-administration and distinct dopamine adaptations in female rats

Increasing evidence suggests that females are more vulnerable to the harmful effects of drugs of abuse, including opioids. Additionally, rates of heroin-related deaths substantially increased in females from 1999 to 2017 [1], underscoring the need to evaluate sex differences in heroin vulnerability. Moreover, the neurobiological substrates underlying sexually dimorphic responding to heroin are not fully defined. Thus, we evaluated male and female Long Evans rats on acquisition, dose-responsiveness, and seeking for heroin self-administration (SA) as well as using a long access model to assess escalation of intake at low and high doses of heroin, 0.025 and 0.1 mg/kg/inf, respectively. We paired this with ex vivo fast-scan cyclic voltammetry (FSCV) in the medial nucleus accumbens (NAc) shell and quantification of mu-opioid receptor (MOR) protein in the ventral tegmental area (VTA) and NAc. While males and females had similar heroin SA acquisition rates, females displayed increased responding and intake across doses, seeking for heroin, and escalation on long access. However, we found that males and females had similar expression levels of MORs in the VTA and NAc, regardless of heroin exposure. FSCV results revealed that heroin exposure did not change single-pulse elicited dopamine release, but caused an increase in dopamine transporter activity in both males and females compared to their naïve counterparts. Phasic-like stimulations elicited robust increases in dopamine release in heroin-exposed females compared to heroin-naïve females, with no differences seen in males. Together, our results suggest that differential adaptations of dopamine terminals may underlie the increased heroin SA behaviors seen in females.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

Behavioral predictors of individual differences in opioid addiction vulnerability as measured using i.v. self-administration in rats

BACKGROUND

Like other forms of psychopathology, vulnerability to opioid addiction is subject to wide individual differences. Animal behavioral models are valuable in advancing our understanding of mechanisms underlying vulnerability to the disorder's development and amenability to treatment.

METHODS

This review provides an overview of preclinical work on behavioral predictors of opioid addiction vulnerability as measured using the intravenous (i.v.) self-administration (SA) model in rats. We also highlight several new approaches to studying individual differences in opioid addiction vulnerability in preclinical models that could have greater sensitivity and lead to more clinically relevant findings.

RESULTS AND CONCLUSIONS

Evidence for the relationship between various behavioral traits and opioid SA in the preclinical literature is limited. With the possible exceptions of sensitivity to opioid agonist/withdrawal effects and stress reactivity, predictors of individual differences in SA of other drugs of abuse (e.g. sensation-seeking, impulsivity) do not predict vulnerability to opioid SA in rats. Refinement of SA measures and the use of multivariate designs and statistics could help identify predictors of opioid SA and lead to more clinically relevant studies on opioid addiction vulnerability.

Consolidating the Circuit Model for Addiction

Addiction is a disease characterized by compulsive drug seeking and consumption observed in 20-30% of users. An addicted individual will favor drug reward over natural rewards, despite major negative consequences. Mechanistic research on rodents modeling core components of the disease has identified altered synaptic transmission as the functional substrate of pathological behavior. While the initial version of a circuit model for addiction focused on early drug adaptive behaviors observed in all individuals, it fell short of accounting for the stochastic nature of the transition to compulsion. The model builds on the initial pharmacological effect common to all addictive drugs-an increase in dopamine levels in the mesolimbic system. Here, we consolidate this early model by integrating circuits underlying compulsion and negative reinforcement. We discuss the genetic and epigenetic correlates of individual vulnerability. Many recent data converge on a gain-of-function explanation for circuit remodeling, revealing blueprints for novel addiction therapies.

Pleasure, addiction, and hypocretin (orexin)

The hypocretins/orexins were discovered in 1998. Within 2 years, this led to the discovery of the cause of human narcolepsy, a 90% loss of hypothalamic neurons containing these peptides. Further work demonstrated that these neurons were not simply linked to waking. Rather these neurons were active during pleasurable behaviors in waking and were silenced by aversive stimulation. This was seen in wild-type mice, rats, cats, and dogs. It was also evident in humans, with increased Hcrt release during pleasurable activities and decreased release, to the levels seen in sleep, during pain. We found that human heroin addicts have, on average, an increase of 54% in the number of detectable Hcrt neurons compared to "control" human brains and that these Hcrt neurons are substantially smaller than those in control brains. We found that in mice, chronic morphine administration induced the same changes in Hcrt neuron number and size. Our studies in the mouse allowed us to determine the specificity, dose response relations, time course of the change in the number of Hcrt neurons, and that the increased number of Hcrt neurons after opiates was not due to neurogenesis. Furthermore, we found that it took a month or longer for these anatomical changes in the mouse brain to return to baseline. Human narcoleptics, despite their prescribed use of several commonly addictive drugs, do not show significant evidence of dose escalation or substance use disorder. Similarly, mice in which the peptide has been eliminated are resistant to addiction. These findings are consistent with the concept that an increased number of Hcrt neurons may underlie and maintain opioid or cocaine use disorders.

Higher anhedonia during withdrawal from initial opioid exposure is protective against subsequent opioid self-administration in rats

RATIONALE

Understanding factors contributing to individual differences in vulnerability to opioid addiction is essential for developing more effective preventions and treatments, yet few reliable behavioral predictors of subsequent opioid self-administration have been identified in rodents. Sensitivity to the acute effects of initial drug exposure predicts later addiction vulnerability in both humans and animals, but the relationship between sensitivity to withdrawal from initial drug exposure and later drug use vulnerability is unclear.

OBJECTIVE

The goal of the current study was to evaluate whether the degree of anhedonia experienced during withdrawal from early opioid exposure predicts subsequent vulnerability to opioid self-administration.

METHODS

Rats were first tested for withdrawal sensitivity following acute injections of morphine (i.e., "acute dependence"), measured as elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior) during naloxone-precipitated and spontaneous withdrawal. Rats were then tested for addiction-like behavior using various measures of i.v. morphine self-administration (MSA) including acquisition, demand, extinction, and reinstatement induced by morphine, stress, and/or drug-associated cues.

RESULTS

Greater naloxone-precipitated withdrawal across repeated morphine injections and greater peak spontaneous withdrawal severity following a single morphine injection were associated with lower addiction-like behavior on multiple MSA measures. Withdrawal-induced anhedonia predicted a wider range of MSA measures than did any individual measure of MSA itself.

CONCLUSIONS

Our data establish WIA as one of the first behavioral measures to predict individual differences in opioid SA in rodents. This model promises to be useful for furthering our understanding of behavioral and neurobiological mechanisms underlying vulnerability to opioid addiction.

The role of impulsivity as predisposing behavioural trait in different aspects of alcohol self-administration in rats

BACKGROUND

Therapeutic interventions to promote abstinence and prevent relapse in alcohol use disorder (AUD) are limitedly available. Therefore, targeting risk factors in the onset and maintenance of AUD could pose an interesting alternative treatment strategy. In this regard, over the last decade trait impulsivity has received considerable attention as such a risk factor predisposing substance dependence both in clinical populations and preclinical rodent studies. This study investigated whether different forms of impulsivity (action versus choice) predict distinct stages of instrumental alcohol self-administration, extinction and cue-induced relapse.

METHODS

Two cohorts of n = 48 rats each were trained in an operant tasks for either impulsive action or impulsive choice. Subsequently, high and low impulsive rats were then tested in an alcohol self-administration and relapse model and following this retested in the impulsivity tasks to evaluate possible changes in impulsivity levels.

RESULTS

The current data show that neither impulsive action, nor impulsive choice predict the extent to which rats consume alcohol and the extent to which rats are motivated to self-administer alcohol. Moreover, extinction of responding for alcohol and cue-induced relapse was not predicted by impulsivity. Interestingly, rats and most prominently low impulsive rats became more impulsive after the alcohol self-administration procedure. Although due to employed experimental design it is not clear whether this resulted from alcohol consumption or alcohol abstinence.

CONCLUSION

Together, these findings lend further support for the notion of a unidirectional relationship between self-administration of the depressant drug alcohol and impulsivity.

Personality driven alcohol and drug abuse: New mechanisms revealed

While the majority of the regular consumers of alcohol controls their consumption well over life span and even takes instrumentalization benefits from it, a minority, but yet high total number of users develops an alcohol addiction. It has long been known that particular personality types are more addiction prone than others. Here we review recent progress in the understanding of neurobiological pathways that determine personality and facilitate drug abuse. Novel approaches to characterize personality traits leading to addiction proneness in social settings in mice are discussed. A common genetic and neurobiological base for the behavioural traits of sensation seeking or a depressed phenotype and escalating alcohol consumption are reviewed. Furthermore, recent progress on how social and cognitive factors, including impulsivity and decision making, act at brain level to make an individual more vulnerable to alcohol abuse, are discussed. Altogether, this review provides an update on brain mechanisms underlying a broad spectrum of personality traits that make an individual more prone to alcohol and drug abuse and addiction.

Escalation of drug use in persons dually diagnosed with opioid and cocaine dependence: Gender comparison and dimensional predictors

BACKGROUND

Persons dually diagnosed with opioid and cocaine dependence (OD + CD) present a clinical challenge and are at risk of morbidity and mortality. The time of escalation of heroin and cocaine exposure in persons with OD + CD remain understudied, and the influence of gender and other variables have not been examined. This observational study focused on the time of escalation of heroin and cocaine in volunteers with OD + CD, examining gender and exposure to other drugs (e.g., cannabis or alcohol) as predictors. Ages of first use and of onset of heaviest use of each drug were collected (in whole years). Time of escalation was defined as the interval between age of first use and onset of heaviest use.

VOLUNTEERS

sequentially ascertained adult volunteers recruited from the New York Metropolitan area, of which n = 297 were diagnosed with OD + CD.

METHODS

Instruments administered were the SCID-I diagnostic interview (DSM-IV criteria), BIS-11 impulsiveness scale, and KMSK scales, dimensional measures of maximal exposure to specific drugs.

RESULTS

In volunteers with OD + CD, ages of onset of heaviest use of cannabis (median age = 15) and alcohol (median age = 19) were in adolescence or emerging adulthood and preceded those for heroin and cocaine (median ages = 26 and 25, respectively). Maximal levels of cannabis and alcohol exposure were high, in volunteers with OD + CD. In adjusted Cox regressions, gender was not a significant predictor of time of heroin or cocaine escalation. However, more rapid time of alcohol escalation was a predictor of more rapid time of escalation of both heroin and cocaine, in volunteers with OD + CD.

Individual Vulnerability to Stress Is Associated With Increased Demand for Intravenous Heroin Self-administration in Rats

Opioid use is a widespread epidemic, and traumatic stress exposure is a critical risk factor in opioid use and relapse. There is a significant gap in our understanding of how stress contributes to heroin use, and there are limited studies investigating individual differences underlying stress reactivity and subsequent stress-induced heroin self-administration. We hypothesized that greater individual vulnerability to stress would predict higher demand for heroin self-administration in a within-subjects rodent model of stress and heroin use comorbidity. Male rats were exposed to inescapable intermittent swim stress (ISS) and individual biological (corticosterone) or behavioral [open field, social exploration, and forced swim tests (FSTs)] measures were assessed before and after the stress episode. Individual demand for self-administered heroin (0.05 mg/kg/infusion; 12-h sessions) was assessed using a behavioral economics approach followed by extinction and reinstatement tests triggered by stress re-exposure, non-contingent cue presentations, and yohimbine (0, 1.0, or 2.5 mg/kg). We found that behavioral, biological, and a combination of behavioral and biological markers sampled prior to and after the stress episode that occurred weeks before the access to heroin self-administration predicted the magnitude of individual demand for heroin. Non-contingent presentation of cues, that were previously associated with heroin, reinstated heroin seeking in extinction. For the first time, we show that individual biological response to an ecologically relevant stressor in combination with associated behavioral markers can be used to predict subsequent economic demand for heroin.

Tropisetron Facilitates Footshock Suppression of Compulsive Cocaine Seeking

BACKGROUND

The hallmark characteristics of the murine model of drug addiction include the escalation of cocaine consumption and compulsive punishment-resistant drug seeking. In this study, we evaluated the motivation for drug seeking in cocaine self-administering rats exposed to an escalated dosing regimen that endeavored to mimic the characteristic of escalating drug intake in human addicts. Tropisetron is a 5-HT3 receptor antagonist and α7-nicotinic receptor partial agonist. Utilizing rats trained on the escalated-dosing regimen, we examined the effects of tropisetron on control over compulsive drug-seeking behavior that was defined as footshock-resistant lever pressing.

METHODS

Rats were trained to self-administer cocaine with incremental-infusion doses (from 0.6 to 2.4 mg/kg/infusion) across training sessions (3 h/session) or with a long-access paradigm (i.e., 0.6 mg/kg/infusion, 6 h/d training session). The drug-seeking motivations of 2 groups were estimated by the patterns of drug intake and progressive-ratio schedule. The compulsivity for drug seeking of the group with an escalated dose was further evaluated using the footshock-associated seeking-taking chain task.

RESULTS

The rats trained on the dose-escalated protocol achieved the same levels of motivated drug seeking as those subjected to a long-access paradigm, as indicated by cocaine intake per training session and breakpoints on a progressive ratio schedule. Tropisetron attenuated compulsive behavior of rats when pressing of the seeking lever potentially led to footshock. Intriguingly, tropisetron did not change the motivation to seek cocaine when footshock was absent. Tropisetron had no effect on locomotor activities or saccharin self-administration.

CONCLUSIONS

These results demonstrate that tropisetron restored control over compulsive cocaine seeking, and they indicate that 5-HT3/α7-nicotinic receptors may be potential therapeutic targets for relieving compulsive drug seeking.

Modelling Differential Vulnerability to Substance Use Disorder in Rodents: Neurobiological Mechanisms

Despite the prevalence of drug use within society, only a subset of individuals actively taking addictive drugs lose control over their intake and develop compulsive drug-seeking and intake that typifies substance use disorder (SUD). Although research in this field continues to be an important and dynamic discipline, the specific neuroadaptations that drive compulsive behaviour in humans addicted to drugs and the neurobiological mechanisms that underlie an individual's innate susceptibility to SUD remain surprisingly poorly understood. Nonetheless, it is clear from research within the clinical domain that some behavioural traits are recurrently co-expressed in individuals with SUD, thereby inviting the hypothesis that certain behavioural endophenotypes may be predictive, or at least act in some way, to modify an individual's probability for developing this disorder. The analysis of such endophenotypes and their catalytic relationship to the expression of addiction-related behaviours has been greatly augmented by experimental approaches in rodents that attempt to capture diagnostically relevant aspects of this progressive brain disorder. This work has evolved from an early focus on aberrant drug reinforcement mechanisms to a now much richer account of the putatively impaired cognitive control processes that ultimately determine individual trajectories to compulsive drug-related behaviours. In this chapter we discuss the utility of experimental approaches in rodents designed to elucidate the neurobiological and genetic underpinnings of so-called risk traits and how these innate vulnerabilities collectively contribute to the pathogenesis of SUD.

Early life adversity potentiates expression of addiction-related traits

Many individuals sporadically and circumstantially sample addictive drugs, yet few become addicted. The individual vulnerabilities underlying the development of addiction are not well understood. Correlational findings show that early life adversity is associated with a greater propensity to develop drug addiction. However, the mechanisms by which early life adversity increases addiction vulnerability are unknown. Separate lines of research have found that several traits are associated with addiction. Here, we examined the effects of early life adversity on addiction-related traits in adulthood. We weaned male and female Sprague-Dawley rats (postnatal day - PND21) and randomly assigned them to either a non-adversity group (N-ADV) or an adversity group (ADV). ADV rats experienced adversity from PND 21-35, they were: a) singly housed, b) food restricted for 12h/day, c) subjected to forced-swim sessions, and d) restrained and exposed to predator odour (1h). As adults, rats were tested for impulsivity, anxiety-like behaviour, novelty preference, and attribution of incentive salience to a reward cue. ADV rats showed enhanced novelty preference and attributed greater incentive value to a reward cue. Compared to N-ADV rats, a greater proportion of ADV rats expressed multiple addiction risk traits. Furthermore, fewer ADV rats expressed no addiction risk traits. This effect was most evident in female ADV rats.

Expression of immediate early genes in brain reward circuitries: Differential regulation by psychostimulant and opioid drugs

Although some of the clinical manifestations of substance use disorders might be superficially similar, it is highly likely that different classes of abused drugs including opioids (heroin, morphine, and oxycodone, other opioids) and psychostimulants (cocaine and amphetamines) cause different neuroadaptations in various brain regions dependent in the distribution and concentration of their biochemical sites of actions. In fact, different molecular networks are indeed impacted by acute and chronic administration of addictive substances. Some of the genes whose expression is influenced by the administration of these substances are immediate-early genes (IEGs). IEGs include classes of low expression genes that can become very highly induced within seconds or minutes of activation by endogenous or exogenous stimuli. These IEGs might play important roles in activating target genes that regulate adaptations implicated in the behavioral manifestations diagnosed as addiction. Therefore, the purpose of this review is to provide an overview of recent data on the effects of psychostimulants and opioids on IEG expression in the brain. The review documents some contrasting effects of these classes of drugs on gene expression and indicates that further studies are necessary to identify the specific effects of each drug class when trying to predict clinical responses to therapeutic agents.

Impulsive choice does not predict binge-like eating in rats

Binge eating disorder is an addiction-like disorder characterized by recurrent, excessive food consumption within discrete periods of time, and it has been linked to increased trait impulsivity. Within impulsivity components, while impulsive action was shown to predict binge-like and addictive-like eating, the role of impulsive choice is instead unknown. The goal of this study was to determine if impulsive choice predicted, or was altered by binge-like eating of a sugary, highly palatable diet. We utilized a modified adjusting delay task procedure in free-fed rats to assess impulsive choice behavior, that is. the tendency to respond for a larger, delayed reward over a lesser, immediate reward. We found that baseline impulsive choice was not a predictor of binge-like eating in 1-h sessions of palatable diet operant self-administration. Furthermore, binge-like eating of the same palatable diet had no effect on later impulsive choice behavior. Thus, our data suggest that, unlike impulsive action, impulsive choice behavior does not predict binge-like eating in rats.

Locomotor activity does not predict individual differences in morphine self-administration in rats

Understanding factors contributing to individual differences in opioid addiction vulnerability is essential for developing more effective preventions and treatments. Sensation seeking has been implicated in addiction to several drugs of abuse, yet its relationship with individual differences in opioid addiction vulnerability has not been well established. The primary goal of this study was to evaluate the relationship between locomotor activity in a novel environment, a preclinical model of sensation-seeking, and individual differences in acquisition of i.v. morphine self-administration (SA) in rats. A secondary goal was to evaluate the relationship between activity and elasticity of demand (reinforcing efficacy) for morphine measured using a behavioral economic approach. Following an initial locomotor activity screen, animals were allowed to acquire morphine SA at a unit dose of 0.5 mg/kg/infusion in 4 hour/day sessions (Experiment 1) or 0.2 mg/kg/infusion in 2 hour/day sessions (Experiment 2) until infusion rates were stable. Unit price was subsequently manipulated via progressive reductions in unit dose (Experiment 1) or increases in response requirement per infusion (Experiment 2). Activity levels were not correlated with acquisition of morphine SA in either experiment. Morphine consumption was generally well described by an exponential demand function in both experiments (R² values > 0.95 for rats as a group), but activity did not correlate with behavioral economic measures. Locomotor activity in a novel environment did not predict individual differences in acquisition of morphine SA. These data complement findings from some human studies and suggest that the role of sensation seeking in individual differences in opioid addiction vulnerability may be limited.

Decrease of cocaine, but not heroin, self-administration and relapse by the tyrosine kinase inhibitor masitinib in male Sprague Dawley rats

RATIONALE

Accumulating evidence shows that cocaine, and also heroin, influence several tyrosine kinases, expressed in neurons and in non-neuronal populations such as microglia, astrocytes and mast-cells. Drug-induced activation of mast cells both triggers inflammatory processes in the brain mediated by the glial cells they activate, and facilitates histamine release which may directly influence the dopamine system. Thus, by triggering the activation and degranulation of mast cells dependent on the tyrosine kinase c-kit and Fyn, the latter being also involved in NMDA-dependent synaptic plasticity, cocaine and heroin may indirectly influence the neural mechanisms that mediate their reinforcing properties. Masitinib, a novel tyrosine kinase inhibitor with high selectivity for c-Kit, Fyn and Lyn, may alter the aberrant consequences of the activation of these tyrosine kinases by cocaine and heroin.

OBJECTIVE

We investigated in rats the effect of a chronic oral treatment with masitinib (20 mg/kg) on the reinforcing and motivational properties of self-administered cocaine (250 μg/infusion) and heroin (40 μg/infusion).

METHODS

Three different cohorts of rats were trained instrumentally to respond for cocaine, heroin or food under continuous reinforcement. In each group, we assessed the influence of chronic daily treatment with masitinib on the maintenance of instrumental responding and intake and the motivation for the reinforcer. Thus, masitinib and vehicle-treated rats were challenged to adapt to high behavioural demand, to respond under a progressive ratio schedule of reinforcement and to reinstate instrumental responding after extinction and/or abstinence.

RESULTS

Masitinib selectively decreased cocaine intake, the motivation for cocaine and the subsequent propensity to respond for cocaine under extinction, while having no effect on instrumental responding for heroin or food.

CONCLUSION

The present findings suggest masitinib, a drug with proven efficacy in CNS disorders, could represent a novel treatment for cocaine addiction provided its influence on the reinforcing and incentive properties of the drug is confirmed.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

The relationship between the striatal dopamine transporter and novelty seeking and cognitive flexibility in opioid dependence

Novelty seeking (NS) is a core personality trait that primes the susceptibility to drug addiction. Striatal dopamine activity contributes to cognitive flexibility, an important cognitive strategy to inhibit impulsivity and compulsive drug-seeking behavior. Evidence supports the association between dopamine and NS. Opioid-dependent patients show higher levels of NS, and repeated opioid exposure can cause cognitive deficits including poor cognitive flexibility and impaired impulse control. However, in opioid-dependent patients, the link between NS, striatal dopamine activity, and cognitive flexibility is still unclear. We recruited 22 opioid-dependent individuals and 30 age- and sex-matched healthy controls. Single-photon emission computed tomography with [^99mTc]TRODAT-1 as a ligand was used to measure the striatal dopamine transporter (DAT) availability. The Trail Making Test (TMT) was performed to assess cognitive flexibility. Cloninger's Tridimensional Personality Questionnaire (TPQ) was used to measure NS. We found that in opioid-dependent patients, the striatal DAT availability was lower and negatively associated with TMT Part B÷Part A. Moreover, an inverted-U shape significantly matched the scores of NS as a function of the striatal DAT availability, with maximum NS potential in the midrange of the DAT availability. An extra sum-of-squares F test was conducted, indicating that a quadratic model fitted the association between the DAT and NS better than a linear model did. In brief, in opioid-dependent patients, the striatal DAT availability is nonlinearly linked to NS and linearly linked to cognitive flexibility. The role of the striatal DAT in the transition from controlled to compulsive opioid use warrants further research.

Bidirectional regulation over the development and expression of loss of control over cocaine intake by the anterior insula

RATIONALE

Increasing evidence suggests that the anterior insular cortex (AIC) plays a major role in cocaine addiction, being implicated in both impaired insight and associated decision-making and also craving and relapse. However, the nature of the involvement of the insula in the development and maintenance of cocaine addiction remains unknown, thereby limiting our understanding of its causal role in addiction. We therefore investigated whether pre- and post-training bilateral lesions of the AIC differentially influenced the development and the expression of the escalation of cocaine self-administration during extended access to the drug.

METHODS

In a series of experiments, Sprague Dawley rats received bilateral excitotoxic lesions of the AIC either prior to, or after 3 weeks of training under 12-h extended self-administration conditions, which are known to promote a robust escalation of intake. We also investigated the influence of AIC lesions on anxiety, as measured in an elevated plus maze and sensitivity to conditioned stimuli (CS)- or drug-induced reinstatement of an extinguished instrumental response.

RESULTS

Whereas, post-escalation lesions of the AIC, as anticipated, restored control over cocaine intake and prevented drug-induced reinstatement, pre-training lesions resulted in a facilitation of the development of loss of control with no influence over the acquisition of cocaine self-administration or anxiety.

CONCLUSIONS

AIC lesions differentially affect the development and maintenance of the loss of control over cocaine intake, suggesting that the nature of the contribution of cocaine-associated interoceptive mechanisms changes over the course of escalation and may represent an important component of addiction.

Drug Addiction: Updating Actions to Habits to Compulsions Ten Years On

A decade ago, we hypothesized that drug addiction can be viewed as a transition from voluntary, recreational drug use to compulsive drug-seeking habits, neurally underpinned by a transition from prefrontal cortical to striatal control over drug seeking and taking as well as a progression from the ventral to the dorsal striatum. Here, in the light of burgeoning, supportive evidence, we reconsider and elaborate this hypothesis, in particular the refinements in our understanding of ventral and dorsal striatal mechanisms underlying goal-directed and habitual drug seeking, the influence of drug-associated Pavlovian-conditioned stimuli on drug seeking and relapse, and evidence for impairments in top-down prefrontal cortical inhibitory control over this behavior. We further review animal and human studies that have begun to define etiological factors and individual differences in the propensity to become addicted to drugs, leading to the description of addiction endophenotypes, especially for cocaine addiction. We consider the prospect of novel treatments for addiction that promote abstinence from and relapse to drug use.

Premature responses in the five-choice serial reaction time task reflect rodents' temporal strategies: evidence from no-light and pharmacological challenges

RATIONALE

The five-choice serial reaction time task (5-CSRTT) is regularly used to study attention and impulsivity. In the 5-CSRTT, rodents initiate a trial, then after an inter-trial interval (ITI), a light appears in one of five holes. Responding in the lit vs. unlit hole reflects attention (accuracy), while responding prematurely before a light appears is suggested to reflect impulsivity/response disinhibition. Comparison of rat and mouse 5-CSRTT performance has raised questions on the validity of premature responses as measuring impulsivity/response inhibition. To minimize effort, rodents may use a temporal strategy, enabling their "timing" of the ITI, minimizing the need to attend during this delay. Greater reliance on this strategy could result in premature responses due to "guesses" if their timing was poor/altered.

OBJECTIVES

To assess the degree to which rats and/or mice utilize a temporal strategy, we challenged performance using infrequent no-light trials during 5-CSRTT performance.

RESULTS

Even when no light appeared when one was expected, rats responded ~60 % compared to ~40 % in mice, indicating a greater reliance on a temporal strategy by rats than by mice. Consistent with this hypothesis, rats made more premature responses than mice. Additional studies using a temporal discrimination task and a 5-CSRTT variant demonstrated that delta-9-tetrahydrocannabinol, the active ingredient in cannabis, slowed temporal perception and reduced premature responses.

CONCLUSIONS

These data provide behavioral and pharmacological evidence indicating that premature responses are heavily influenced by temporal perception. Hence, they may reflect an aspect of waiting impulsivity, but not response disinhibition, an important distinction for translational clinical research.

Low Impulsive Action, but not Impulsive Choice, Predicts Greater Conditioned Reinforcer Salience and Augmented Nucleus Accumbens Dopamine Release

Poor impulse control is associated with an increased propensity to develop an addiction and may contribute to relapse as high impulsive subjects appear to attribute greater salience toward drug-paired stimuli. In these studies, we determined whether trait impulsivity also predicts the desire to obtain natural reward-paired stimuli. Rats trained on the 5-choice serial reaction time task to measure impulsive action (Experiment 1) or a delay-discounting task to measure impulsive choice (Experiment 2) were separated into low, intermediate, or high impulsive action (L-IA, I-IA, H-IA) or choice (L-IC, I-IC, H-IC) groups. The motivation to obtain a conditioned stimulus (CS) paired with water-reward was subsequently determined by measuring responding for the CS as a conditioned reinforcer (CRf). Dopamine release in the nucleus accumbens was also measured using in vivo microdialysis. The effects of amphetamine were assessed on all tests. In Experiment 1, amphetamine increased impulsive action in all groups. L-IA rats initially demonstrated the highest responding for the CRf. Amphetamine increased responding for the CRf and this effect was augmented in L-IA rats. Dopamine release following amphetamine was greatest in L-IA subjects. In Experiment 2, amphetamine increased impulsive choice for L-IC and I-IC rats. However, all groups responded similarly for the CRf and dopamine release was moderately greater in L-IC rats. In conclusion, impulsive choice was unrelated to responding for a CRf. L-IA subjects initially attributed enhanced salience to a CS and exhibited greater dopamine release. Lower dopamine release in H-IA rats could result in reduced reinforcing properties of the CRf.

Machine-learning identifies substance-specific behavioral markers for opiate and stimulant dependence

BACKGROUND

Recent animal and human studies reveal distinct cognitive and neurobiological differences between opiate and stimulant addictions; however, our understanding of the common and specific effects of these two classes of drugs remains limited due to the high rates of polysubstance-dependence among drug users.

METHODS

The goal of the current study was to identify multivariate substance-specific markers classifying heroin dependence (HD) and amphetamine dependence (AD), by using machine-learning approaches. Participants included 39 amphetamine mono-dependent, 44 heroin mono-dependent, 58 polysubstance dependent, and 81 non-substance dependent individuals. The majority of substance dependent participants were in protracted abstinence. We used demographic, personality (trait impulsivity, trait psychopathy, aggression, sensation seeking), psychiatric (attention deficit hyperactivity disorder, conduct disorder, antisocial personality disorder, psychopathy, anxiety, depression), and neurocognitive impulsivity measures (Delay Discounting, Go/No-Go, Stop Signal, Immediate Memory, Balloon Analogue Risk, Cambridge Gambling, and Iowa Gambling tasks) as predictors in a machine-learning algorithm.

RESULTS

The machine-learning approach revealed substance-specific multivariate profiles that classified HD and AD in new samples with high degree of accuracy. Out of 54 predictors, psychopathy was the only classifier common to both types of addiction. Important dissociations emerged between factors classifying HD and AD, which often showed opposite patterns among individuals with HD and AD.

CONCLUSIONS

These results suggest that different mechanisms may underlie HD and AD, challenging the unitary account of drug addiction. This line of work may shed light on the development of standardized and cost-efficient clinical diagnostic tests and facilitate the development of individualized prevention and intervention programs for HD and AD.

Neuroligin-2 Expression in the Prefrontal Cortex is Involved in Attention Deficits Induced by Peripubertal Stress

Emerging evidence indicates that attention deficits, which are frequently observed as core symptoms of neuropsychiatric disorders, may be elicited by early life stress. However, the mechanisms mediating these stress effects remain unknown. The prefrontal cortex (PFC) has been implicated in the regulation of attention, including dysfunctions in GABAergic transmission, and it is highly sensitive to stress. Here, we investigated the involvement of neuroligin-2 (NLGN-2), a synaptic cell adhesion molecule involved in the stabilization and maturation of GABAergic synapses, in the PFC in the link between stress and attention deficits. First, we established that exposure of rats to stress during the peripubertal period impairs attention in the five-choice serial reaction time task and results in reductions in the GABA-synthesizing enzyme glutamic acid decarboxylase in different PFC subregions (ie, prelimbic (PL), infralimbic, and medial and ventral orbitofrontal (OFC) cortex) and in NLGN-2 in the PL cortex. In peripubertally stressed animals, NLGN-2 expression in the PL and OFC cortex correlated with attention measurements. Subsequently, we found that adeno-associated virus-induced rescue of NLGN-2 in the PFC reverses the stress-induced attention deficits regarding omitted trials. Therefore, our findings highlight peripuberty as a period that is highly vulnerable to stress, leading to the development of attention deficits and a dysfunction in the PFC GABAergic system and NLGN-2 expression. Furthermore, NLGN-2 is underscored as a promising target to treat stress-induced cognitive alterations, and in particular attentional deficits as manifested by augmented omissions in a continuous performance task.

How Preclinical Models Evolved to Resemble the Diagnostic Criteria of Drug Addiction

Drug addiction is a complex neuropsychiatric disorder that affects a subset of the individuals who take drugs. It is characterized by maladaptive drug-seeking habits that are maintained despite adverse consequences and intense drug craving. The pathophysiology and etiology of addiction is only partially understood despite extensive research because of the gap between current preclinical models of addiction and the clinical criteria of the disorder. This review presents a brief overview, based on selected methodologies, of how behavioral models have evolved over the last 50 years to the development of recent preclinical models of addiction that more closely mimic diagnostic criteria of addiction. It is hoped that these new models will increase our understanding of the complex neurobiological mechanisms whereby some individuals switch from controlled drug use to compulsive drug-seeking habits and relapse to these maladaptive habits. Additionally, by paving the way to bridge the gap that exists between biobehavioral research on addiction and the human situation, these models may provide new perspectives for the development of novel and effective therapeutic strategies for drug addiction.

Alcohol-Preferring Rats Show Goal Oriented Behaviour to Food Incentives but Are Neither Sign-Trackers Nor Impulsive

Drug addiction is often associated with impulsivity and altered behavioural responses to both primary and conditioned rewards. Here we investigated whether selectively bred alcohol-preferring (P) and alcohol-nonpreferring (NP) rats show differential levels of impulsivity and conditioned behavioural responses to food incentives. P and NP rats were assessed for impulsivity in the 5-choice serial reaction time task (5-CSRTT), a widely used translational task in humans and other animals, as well as Pavlovian conditioned approach to measure sign- and goal-tracking behaviour. Drug-naïve P and NP rats showed similar levels of impulsivity on the 5-CSRTT, assessed by the number of premature, anticipatory responses, even when the waiting interval to respond was increased. However, unlike NP rats, P rats were faster to enter the food magazine and spent more time in this area. In addition, P rats showed higher levels of goal-tracking responses than NP rats, as measured by the number of magazine nose-pokes during the presentation of a food conditioned stimulus. By contrast, NP showed higher levels of sign-tracking behaviour than P rats. Following a 4-week exposure to intermittent alcohol we confirmed that P rats had a marked preference for, and consumed more alcohol than, NP rats, but were not more impulsive when re-tested in the 5-CSRTT. These findings indicate that high alcohol preferring and drinking P rats are neither intrinsically impulsive nor do they exhibit impulsivity after exposure to alcohol. However, P rats do show increased goal-directed behaviour to food incentives and this may be associated with their strong preference for alcohol.

The dopamine theory of addiction: 40 years of highs and lows

For several decades, addiction has come to be viewed as a disorder of the dopamine neurotransmitter system; however, this view has not led to new treatments. In this Opinion article, we review the origins of the dopamine theory of addiction and discuss the ability of addictive drugs to elicit the release of dopamine in the human striatum. There is robust evidence that stimulants increase striatal dopamine levels and some evidence that alcohol may have such an effect, but little evidence, if any, that cannabis and opiates increase dopamine levels. Moreover, there is good evidence that striatal dopamine receptor availability and dopamine release are diminished in individuals with stimulant or alcohol dependence but not in individuals with opiate, nicotine or cannabis dependence. These observations have implications for understanding reward and treatment responses in various addictions.

Investigating expectation and reward in human opioid addiction with [(11) C]raclopride PET

The rewarding properties of some abused drugs are thought to reside in their ability to increase striatal dopamine levels. Similar increases have been shown in response to expectation of a positive drug effect. The actions of opioid drugs on striatal dopamine release are less well characterized. We examined whether heroin and the expectation of heroin reward increases striatal dopamine levels in human opioid addiction. Ten opioid-dependent participants maintained on either methadone or buprenorphine underwent [¹¹C]raclopride positron emission tomography imaging. Opioid-dependent participants were scanned three times, receiving reward from 50-mg intravenous heroin (diamorphine; pharmaceutical heroin) during the first scan to generate expectation of the same reward at the second scan, during which they only received 0.1-mg intravenous heroin. There was no heroin injection during the third scan. Intravenous 50-mg heroin during the first scan induced pronounced effects leading to high levels of expectation at the second scan. There was no detectable increase in striatal dopamine levels to either heroin reward or expectation of reward. We believe this is the first human study to examine whether expectation of heroin reward increases striatal dopamine levels in opioid addiction. The absence of detectable increased dopamine levels to both the expectation and delivery of a heroin-related reward may have been due to the impact of substitute medication. It does however contrast with the changes seen in abstinent stimulant users, suggesting that striatal dopamine release alone may not play such a pivotal role in opioid-maintained individuals.

Drug specificity in drug versus food choice in male rats

Although different classes of drug differ in their mechanisms of reinforcement and effects on behavior, little research has focused on differences in self-administration behaviors maintained by users of these drugs. Persistent drug choice despite available reinforcement alternatives has been proposed to model behavior relevant to addiction. The present study used a within-subjects procedure, where male rats (Long-Evans, N = 16) were given a choice between cocaine (1.0 mg/kg/infusion) and food (a single 45-mg grain pellet) or between heroin (0.02 mg/kg/infusion) and food in separate phases (drug order counterbalanced). All rats were initially trained to self-administer each drug, and the doses used were based on previous studies showing that small subsets of rats tend to prefer drug over food reinforcement. The goal of the present study was to determine whether rats that prefer cocaine would also prefer heroin. Choice sessions consisted of 2 forced-choice trials with each reinforcer, followed by 14 free-choice trials (all trials separated by 10-min intertrial interval). Replicating previous results, small subsets of rats preferred either cocaine (5 of the 16 rats) or heroin (2 of the 16 rats) to the food alternative. Although 1 of the 16 rats demonstrated a preference for both cocaine and heroin to the food alternative, there was no relationship between degree of cocaine and heroin preference in individual rats. The substance-specific pattern of drug preference observed suggests that at least in this animal model, the tendencies to prefer cocaine or heroin in preference to a nondrug alternative are distinct behavioral phenomena.

Using cognitive modelling to investigate the psychological processes of the Go/NoGo discrimination task in male abstinent heroin misusers

AIMS

To use cognitive modelling to investigate psychological processes underlying decision-making in male abstinent heroin misusers (AHMs).

DESIGN

A case-control study design.

SETTING

A drug misuse treatment centre in Taiwan.

PARTICIPANTS

Eighty-eight male AHMs and 48 male controls.

MEASUREMENTS

Four parameters representing the attention to wins, learning rate, response sensitivity and incentive of heroin-related stimuli from the modified Go/NoGo discrimination task.

FINDINGS

A modified cue-dependent learning (CD) model with four parameters representing attention to wins, learning rate, response sensitivity and incentive of heroin-related stimuli had a lower value of the sum of Bayesian information criterion (showing a better fit) than the original CD model (9555.50 versus 11,192.22, P < 0.001). The AHM group had a higher value of the heroin-incentive parameter than the control group (0.26 versus -1.66, P < 0.05). The attention to wins and heroin-incentive parameters were associated positively with total commission rate and negatively with total omission rate in the AHM group (P < 0.001).

CONCLUSIONS

Male abstinent heroin misusers appear to be more influenced by heroin-related stimuli during decision-making than males with no history of heroin misuse.

Increased impulsivity retards the transition to dorsolateral striatal dopamine control of cocaine seeking

BACKGROUND

Development of maladaptive drug-seeking habits occurs in conjunction with a ventral-to-dorsal striatal shift in dopaminergic control over behavior. Although these habits readily develop as drug use continues, high impulsivity predicts loss of control over drug seeking and taking. However, whether impulsivity facilitates the transition to dorsolateral striatum (DLS) dopamine-dependent cocaine-seeking habits or whether impulsivity and cocaine-induced intrastriatal shifts are additive processes is unknown.

METHODS

High- and low-impulsive rats identified in the five-choice serial reaction-time task were trained to self-administer cocaine (.25 mg/infusion) with infusions occurring in the presence of a cue-light conditioned stimulus. Dopamine transmission was blocked in the DLS after three stages of training: early, transition, and late-stage, by bilateral intracranial infusions of α-flupenthixol (0, 5, 10, or 15 μg/side) during 15-min cocaine-seeking test sessions in which each response was reinforced by a cocaine-associated conditioned stimulus presentation.

RESULTS

In early-stage tests, neither group was affected by DLS dopamine receptor blockade. In transition-stage tests, low-impulsive rats showed a significant dose-dependent reduction in cocaine seeking, whereas high-impulsive rats were still unaffected by α-flupenthixol infusions. In the final, late-stage seeking test, both groups showed dose-dependent sensitivity to dopamine receptor blockade.

CONCLUSIONS

The results demonstrate that high impulsivity is associated with a delayed transition to DLS-dopamine-dependent control over cocaine seeking. This suggests that, if impulsivity confers an increased propensity to addiction, it is not simply through a more rapid development of habits but instead through interacting corticostriatal and striato-striatal processes that result ultimately in maladaptive drug-seeking habits.

Neural and psychological mechanisms underlying compulsive drug seeking habits and drug memories--indications for novel treatments of addiction

This review discusses the evidence for the hypothesis that the development of drug addiction can be understood in terms of interactions between Pavlovian and instrumental learning and memory mechanisms in the brain that underlie the seeking and taking of drugs. It is argued that these behaviours initially are goal-directed, but increasingly become elicited as stimulus-response habits by drug-associated conditioned stimuli that are established by Pavlovian conditioning. It is further argued that compulsive drug use emerges as the result of a loss of prefrontal cortical inhibitory control over drug seeking habits. Data are reviewed that indicate these transitions from use to abuse to addiction depend upon shifts from ventral to dorsal striatal control over behaviour, mediated in part by serial connectivity between the striatum and midbrain dopamine systems. Only some individuals lose control over their drug use, and the importance of behavioural impulsivity as a vulnerability trait predicting stimulant abuse and addiction in animals and humans, together with consideration of an emerging neuroendophenotype for addiction are discussed. Finally, the potential for developing treatments for addiction is considered in light of the neuropsychological advances that are reviewed, including the possibility of targeting drug memory reconsolidation and extinction to reduce Pavlovian influences on drug seeking as a means of promoting abstinence and preventing relapse.

Different relations between schedule-induced polydipsia and impulsive behaviour in the Spontaneously Hypertensive Rat and in high impulsive Wistar rats: questioning the role of impulsivity in adjunctive behaviour

Rats belonging to three different strains (15 Wistar, 8 Spontaneously Hypertensive - SHR- and 8 Wistar Kyoto - WKY-) were used to evaluate the possible relationship between different levels of impulsivity and development of schedule-induced polydipsia (SIP). We first measured the rats' levels of impulsivity by means of delay-discounting and indifference-point procedures. Secondly, development of SIP was studied under a series of fixed time 15, 30, 60 and 120s food schedules, which were counterbalanced by means of a Latin-square design. Finally, we re-assessed the rats' levels of impulsivity by replicating the delay-discounting test. The findings showed that, starting from equivalent levels of impulsivity, development of SIP differed among the groups of rats. In comparison with the rest of the animals, the SHRs were observed to attain elevated drinking rates under SIP. On the other hand, the Wistar rats which had initial high impulsivity levels similar to those of the SHRs, displayed the lowest rates of induced drinking. Moreover, low levels of impulsivity in Wistar rats prior to SIP acquisition were reflected into high drinking rates. Relation of SIP and impulsivity is questioned by present results, which gives ground to the understanding of the behavioural mechanisms involved in adjunctive behaviour and its usefulness as an animal model of excessive behaviour.

Atomoxetine decreases vulnerability to develop compulsivity in high impulsive rats

BACKGROUND

The factors contributing to the development and severity of obsessive-compulsive spectrum disorders such as obsessive-compulsive disorder, Tourette's syndrome, pathological gambling, and addictions remain poorly understood, limiting the development of therapeutic and preventive strategies. Recent evidence indicates that impulse-control deficits may contribute to the severity of compulsivity in several of these disorders. This suggests that impulsivity may be a transnosological endophenotype of vulnerability to compulsivity. However, the precise nature of the link between impulsivity and compulsivity in anxiety-related compulsive disorders remains unknown.

METHODS

We investigated the relationship between impulsivity and the development of a compulsive behavior in rats, which captures the hallmarks of compulsivity as defined in the DSM-IV--namely, that it is maladaptive, excessive, repetitive, and anxiolytic.

RESULTS

We demonstrate that a high-impulsivity trait, as measured in the five-choice serial reaction time task, predicts an increased propensity to develop compulsivity as measured in a schedule-induced polydipsia procedure. Trait impulsivity and compulsivity were nonlinearly related. This impulsivity-compulsivity relationship was lost after the development of compulsivity or under chronic treatment with atomoxetine, a noradrenergic reuptake inhibitor used to treat attention-deficit/hyperactivity disorder. Atomoxetine treatment both decreased impulsivity and prevented the development of compulsivity in high-impulsive animals.

CONCLUSIONS

These observations provide insight into the reciprocal influence of impulsivity and compulsivity in compulsive disorders and suggest that atomoxetine may be a useful treatment for patients suffering from obsessive-compulsive spectrum disorders with high impulsivity.

Heroin and amphetamine users display opposite relationships between trait and neurobehavioral dimensions of impulsivity

The multidimensional construct of impulsivity is implicated in all phases of the addiction cycle. Substance dependent individuals (SDIs) demonstrate elevated impulsivity on both trait and laboratory tests of neurobehavioral impulsivity; however our understanding of the relationship between these different aspects of impulsivity in users of different classes of drugs remains rudimentary. The goal of this study was to assess for commonalities and differences in the relationships between trait and neurobehavioral impulsivity in heroin and amphetamine addicts. Participants included 58 amphetamine dependent (ADIs) and 74 heroin dependent individuals (HDIs) in protracted abstinence. We conducted Principal Component Analyses (PCA) on two self-report trait and six neurobehavioral measures of impulsivity, which resulted in two trait impulsivity (action, planning) and four neurobehavioral impulsivity composites (discriminability, response inhibition efficiency, decision-making efficiency, quality of decision-making). Multiple regression analyses were used to determine whether neurobehavioral impulsivity is predicted by trait impulsivity and drug type. The analyses revealed a significant interaction between drug type and trait action impulsivity on response inhibition efficiency, which showed opposite relationships for ADIs and HDIs. Specifically, increased trait action impulsivity was associated with worse response inhibition efficiency in ADIs, but with better efficiency in HDIs. These results challenge the unitary account of drug addiction and contribute to a growing body of literature that reveals important behavioral, cognitive, and neurobiological differences between users of different classes of drugs.

New developments in human neurocognition: clinical, genetic, and brain imaging correlates of impulsivity and compulsivity

Impulsivity and compulsivity represent useful conceptualizations that involve dissociable cognitive functions, which are mediated by neuroanatomically and neurochemically distinct components of cortico-subcortical circuitry. The constructs were historically viewed as diametrically opposed, with impulsivity being associated with risk-seeking and compulsivity with harm-avoidance. However, they are increasingly recognized to be linked by shared neuropsychological mechanisms involving dysfunctional inhibition of thoughts and behaviors. In this article, we selectively review new developments in the investigation of the neurocognition of impulsivity and compulsivity in humans, in order to advance our understanding of the pathophysiology of impulsive, compulsive, and addictive disorders and indicate new directions for research.

Addictions and Personality Traits: Impulsivity and Related Constructs

Behavioral tendencies that might be captured through self-report measures may provide insight into personality features that are associated with substance addictions. Recently, impulsivity and related constructs, such as sensation-seeking, have been examined to help better understand their relationships with addictions. Here, we review recent findings that show links over developmental epochs between addictive behaviors and impulsivity, sensation-seeking, and other constructs that are theoretically linked. These findings have significant implications for generating improved treatments and interventions aimed at preventing the development of addictive disorders.

A phenotypic structure and neural correlates of compulsive behaviors in adolescents

BACKGROUND

A compulsivity spectrum has been hypothesized to exist across Obsessive-Compulsive disorder (OCD), Eating Disorders (ED), substance abuse (SA) and binge-drinking (BD). The objective was to examine the validity of this compulsivity spectrum, and differentiate it from an externalizing behaviors dimension, but also to look at hypothesized personality and neural correlates.

METHOD

A community-sample of adolescents (N=1938; mean age 14.5 years), and their parents were recruited via high-schools in 8 European study sites. Data on adolescents' psychiatric symptoms, DSM diagnoses (DAWBA) and substance use behaviors (AUDIT and ESPAD) were collected through adolescent- and parent-reported questionnaires and interviews. The phenotypic structure of compulsive behaviors was then tested using structural equation modeling. The model was validated using personality variables (NEO-FFI and TCI), and Voxel-Based Morphometry (VBM) analysis.

RESULTS

Compulsivity symptoms best fit a higher-order two factor model, with ED and OCD loading onto a compulsivity factor, and BD and SA loading onto an externalizing factor, composed also of ADHD and conduct disorder symptoms. The compulsivity construct correlated with neuroticism (r=0.638; p ≤ 0.001), conscientiousness (r=0.171; p ≤ 0.001), and brain gray matter volume in left and right orbitofrontal cortex, right ventral striatum and right dorsolateral prefrontal cortex. The externalizing factor correlated with extraversion (r=0.201; p ≤ 0.001), novelty-seeking (r=0.451; p ≤ 0.001), and negatively with gray matter volume in the left inferior and middle frontal gyri.

CONCLUSIONS

Results suggest that a compulsivity spectrum exists in an adolescent, preclinical sample and accounts for variance in both OCD and ED, but not substance-related behaviors, and can be differentiated from an externalizing spectrum.

Contribution of impulsivity and novelty-seeking to the acquisition and maintenance of MDMA self-administration

It has been suggested that the response to novelty and impulsivity predict the latency to acquisition and maintenance of drug self-administration, respectively. The aim of this study was to examine the relationship between these two traits and (1) the latency to acquisition and (2) maintenance (drug-seeking) of 3,4-methylenedioxymethamphetamine (MDMA) self -administration. Impulsivity, measured as premature responding on the five-choice serial reaction time task (5-CSRTT), and novelty-seeking, measured as the locomotor response in a novel environment, were measured prior to self-administration. Latency to acquisition was determined as the number of test sessions required to self-administer an initial criterion of 90 infusions of 1.0 mg/kg/infusion, as well as an additional 150 infusions of 0.5 mg/kg/infusion MDMA. For some rats, the ability of MDMA [0, 5.0 or 10.0 mg/kg, intraperitoneal (IP)] to produce drug-seeking was subsequently measured, and for others, impulsivity was again measured following self-administration. Novelty-seeking was not significantly correlated with either the acquisition or drug-seeking measures of MDMA self-administration. Impulsivity was not significantly correlated with the latency to acquire self-administration of MDMA, but was significantly and positively correlated with the magnitude of MDMA-produced drug-seeking. Furthermore, MDMA self-administration produced a number of notable, but transient, deficits in the 5-CSRTT; there was an increase in omission rate and a delayed increase in premature responses in particular. These findings suggest that impulsivity, but not sensation seeking, might be a risk factor for the development of compulsive drug-seeking following withdrawal from MDMA self-administration.