Impulsive choice and impulsive action predict vulnerability to distinct stages of nicotine seeking in rats

Bidirectional relationship between attentional deficits and escalation of nicotine intake in male rats

RATIONALE

People with tobacco addiction have deficits in cognition, in particular deficits in attention. It is not clear however, whether deficits are a cause or a consequence, or both, of chronic nicotine use. Here we set out a series of experiments in rats to address this question and, more specifically, to assess the effects of exposure to and withdrawal from chronic nicotine self-administration on attentional performance.

METHODS

Animals were trained in a 5-choice serial reaction time task to probe individual attentional performance and, then, were given access to a fixed versus increasing dose of intravenous nicotine for self-administration, a differential dose procedure known to induce two between-session patterns of nicotine intake: a stable versus escalation pattern. Attentional performance was measured daily before, during and also 24-h after chronic access to the differential dose procedure of nicotine self-administration.

CONCLUSIONS

We found that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake. Moreover, while chronic nicotine self-administration increases attention, withdrawal from nicotine intake escalation induces attentional deficits, a withdrawal effect that is dose-dependently reversed by acute nicotine. Together, these results suggest that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake, and that part of the motivation for using nicotine during escalation might be to alleviate withdrawal-induced attentional deficits.

The interplay of childhood trauma, oxytocin, and impulsivity in predicting the onset of methamphetamine use

BACKGROUND

Childhood trauma is associated with substance use disorders, including methamphetamine use disorder (MUD). Oxytocin, involved in social bonding, stress regulation, and reward processing, may influence addiction vulnerability and impulsivity in individuals with a history of childhood trauma.

OBJECTIVE

To investigate the relationships among childhood trauma, oxytocin levels, impulsivity, and the age of first methamphetamine use in individuals with MUD.

PARTICIPANTS AND SETTING

The study included 298 male participants (148 individuals with MUD and 150 healthy controls) from both probation offices and psychiatric clinics.

METHODS

Childhood trauma was assessed using the Childhood Trauma Questionnaire-Short Form (CTQ-SF), impulsivity with the Barratt Impulsiveness Scale-11 (BIS-11), and plasma oxytocin levels were obtained.

RESULTS

Individuals with MUD exhibited higher levels of childhood trauma, impulsivity, and lower plasma oxytocin levels compared to healthy controls. Childhood trauma was associated with a younger age of first methamphetamine use, higher impulsivity, and lower oxytocin levels among individuals with MUD. Plasma oxytocin levels partially mediated the relationship between childhood trauma and both the age of first methamphetamine use and impulsivity. Serial mediation analysis demonstrated that oxytocin levels and impulsivity sequentially mediated the relationship between childhood trauma and the age of first methamphetamine use.

CONCLUSIONS

The findings reveal the complex interplay among childhood trauma, oxytocin, impulsivity, and methamphetamine use, emphasizing the importance of considering these factors in prevention and intervention strategies for MUD. Future research should explore oxytocin and impulsivity-focused interventions to mitigate the effects of childhood trauma and reduce MUD development risk.

Relationship between two forms of impulsivity in mice at baseline and under acute and sub-chronic atomoxetine treatment

RATIONALE

Impulsivity is a symptom of various mental disorders, including attention deficit hyperactivity disorder (ADHD), bipolar disorder, and addiction. Impulsivity is not a unitary construct, but is present in different forms, yet only a few rodent studies have explored the relationship between these forms within individual subjects.

OBJECTIVES

In this study, we compared behaviors representing two impulsivity forms, delay discounting (choice impulsivity) and premature responding (waiting impulsivity), within the same mice.

METHODS

C57BL/6J male mice were concurrently trained and tested in the delay discounting task and the rodent continuous performance test in a counterbalanced design. The effects of the ADHD medication atomoxetine were tested in both tasks, after both acute (0.3-5.0 mg/kg) and sub-chronic (0.3 mg/kg twice daily for seven days) administration.

RESULTS

There was no correlation between the two impulsivity forms at baseline. Acute atomoxetine treatment (1, 3, and 5 mg/kg) significantly reduced premature responding. Furthermore, sub-chronic treatment with 0.3 mg/kg of atomoxetine caused a stable decrease in premature responding. Atomoxetine had no significant effect on delay discounting after acute or sub-chronic administration, although the acute administration of 1 mg/kg showed a trend towards increasing delay discounting.

CONCLUSIONS

The present results support that delay discounting and premature responding represent two different forms of impulsivity that show dissimilar responses to atomoxetine treatment. The consistency with findings in humans lends support to the translatability of the results in mice.

Rats have low motivation to self-administer oral methamphetamine across increasing response requirements

Methamphetamine (METH) is a psychostimulant drug that has become increasingly popular in recent years, with overdose deaths more than doubling during the second half of the 2010s. As methamphetamine use disorder rates continue to increase, finding effective treatment strategies to decrease METH dependence is important. Animal studies are well-suited for studying the neurobiological mechanisms underlying addiction-like behaviors. Although individuals can ingest METH orally, few studies have examined oral METH self-administration in animals. Mice show decreased responding for oral METH as the response requirement increases across sessions. The purpose of the current study was to determine if rats show a similar decrease in motivation to earn oral METH across increasing response requirements. Sixteen Sprague Dawley rats were trained to emit a response in an aperture to receive a 0.1-ml METH solution (40 mg/l) according to an FR 1 schedule. The FR requirement increased across sessions to a terminal FR 10. Responses for METH decreased significantly when an FR 10 schedule was used. These results suggest that rats, similarly to mice, have low motivation to self-administer oral METH.

Poor inhibitory control predicts sex-specific vulnerability to nicotine rewarding properties in mice

RATIONALE

The risk of becoming addicted to tobacco varies greatly from individual to individual, raising the possibility of behavioural biomarkers capable of predicting sensitivity to nicotine reward, a crucial step in the development of nicotine addiction. Amongst all of nicotine's pharmacological properties, one of central importance is the enhancement of cognitive performances, which depend on the balance between attentional processes and inhibitory control. However, whether the cognitive enhancement effects of nicotine are predictive of sensitivity to its rewarding properties is still unknown.

OBJECTIVE

Using male and female mice, we investigated whether the effects of nicotine on cognitive performances are predictive of sensitivity to the rewarding properties of nicotine and, if so, whether this relationship is sex dependent.

METHODS

Naïve male and female mice were first assessed for their performances in both baseline conditions and following nicotine injection (0.15 and 0.30 mg/kg) in a cued-Fixed Consecutive Number task (FCNcue) measuring both optimal (attention) and premature (inhibitory control) responding. Next, all mice underwent nicotine-induced conditioned place preference (CPP) in order to evaluate inter-individual differences in response to nicotine reward (0.30 mg/kg).

RESULTS

Results showed that males and females benefited from the effect of nicotine as a cognitive enhancer in the FCNcue task. However, only those males displaying poor inhibitory control, namely high-impulsive animals, subsequently displayed sensitivity to nicotine reward. In females, sensitivity to nicotine reward was independent of FCNcue performances, in both basal and nicotine conditions.

CONCLUSION

Thus, our study suggests that poor inhibitory control and its modulation by nicotine may be a behavioural biomarker for sensitivity to nicotine reward and consequent vulnerability to nicotine addiction in males but not females.

Repeated Cocaine Intake Differentially Impacts Striatal D2/3 Receptor Availability, Psychostimulant-Induced Dopamine Release, and Trait Behavioral Markers of Drug Abuse

Current research indicates that altered dopamine (DA) transmission in the striatum contributes to impulsivity and novelty-seeking, and it may mediate a link concerning a higher susceptibility to drug abuse. Whether increased susceptibility to drug abuse results from a hyperdopaminergic or hypodopaminergic state is still debated. Here, we simultaneously tracked changes in DA D2/3 receptor (D2/3R) availability and amphetamine-(AMPH)-induced DA release in relation to impulsivity and novelty-seeking prior to, and following, cocaine self-administration (SA) in Roman high- (RHA) and low- (RLA) avoidance rats. We found that high-impulsive/high novelty-seeking RHA rats exhibited lower D2/3R availabilities and higher AMPH-induced DA release in the striatum that predicted higher levels of cocaine intake compared with RLAs. Cocaine SA did not alter striatal D2/3R availability or impulsivity in RHA or RLA rats. Critically, cocaine exposure led to a baseline-dependent blunting of stimulated DA release in high-impulsive/high novelty-seeking RHA rats only, and to a baseline-dependent increase in novelty-seeking in low-impulsive/low novelty-seeking RLA rats only. Altogether, we propose that susceptibility to drug abuse results from an innate hyper-responsive DA system, promoting impulsive action and novelty-seeking, and producing stronger initial drug-reinforcing effects that contribute to the initiation and perpetuation of drug use. However, with repeated cocaine use, a tolerance to drug-induced striatal DA elevations develops, leading to a compensatory increase in drug consumption to overcome the reduced reward effects.

Motor impulsivity but not risk-related impulsive choice is associated to drug intake and drug-primed relapse

Introduction

Motor impulsivity and risk-related impulsive choice have been proposed as vulnerability factors for drug abuse, due to their high prevalence in drug abusers. However, how these two facets of impulsivity are associated to drug abuse remains unclear. Here, we investigated the predictive value of both motor impulsivity and risk-related impulsive choice on characteristics of drug abuse including initiation and maintenance of drug use, motivation for the drug, extinction of drug-seeking behavior following drug discontinuation and, finally, propensity to relapse.

Methods

We used the Roman High- (RHA) and Low- Avoidance (RLA) rat lines, which display innate phenotypical differences in motor impulsivity, risk-related impulsive choice, and propensity to self-administer drugs. Individual levels of motor impulsivity and risk-related impulsive choice were measured using the rat Gambling task. Then, rats were allowed to self-administer cocaine (0.3 mg/kg/infusion; 14 days) to evaluate acquisition and maintenance of cocaine self-administration, after which motivation for cocaine was assessed using a progressive ratio schedule of reinforcement. Subsequently, rats were tested for their resistance to extinction, followed by cue-induced and drug-primed reinstatement sessions to evaluate relapse. Finally, we evaluated the effect of the dopamine stabilizer aripiprazole on reinstatement of drug-seeking behaviors.

Results

We found that motor impulsivity and risk-related impulsive choice were positively correlated at baseline. Furthermore, innate high levels of motor impulsivity were associated with higher drug use and increased vulnerability to cocaine-primed reinstatement of drug-seeking. However, no relationships were observed between motor impulsivity and the motivation for the drug, extinction or cue-induced reinstatement of drug-seeking. High levels of risk-related impulsive choice were not associated to any aspects of drug abuse measured in our study. Additionally, aripiprazole similarly blocked cocaine-primed reinstatement of drug-seeking in both high- and low-impulsive animals, suggesting that aripiprazole acts as a D_2/3R antagonist to prevent relapse independently of the levels of impulsivity and propensity to self-administer drugs.

Discussion

Altogether, our study highlights motor impulsivity as an important predictive factor for drug abuse and drug-primed relapse. On the other hand, the involvement of risk-related impulsive choice as a risk factor for drug abuse appears to be limited.

Blockade of the orexin 1-receptors and 'cocaine- and amphetamine-regulated transcript' in the nucleus accumbens shell alters impulse control in rats

Impulsivity is a multifaceted construct and alterations in impulsiveness are often associated with psychiatric diseases, including drug addiction and binge eating disorder. Impulse control involves several brain regions. The present study assessed the role of the orexigenic, appetite stimulating neuropeptide orexin (OX) and the anorexigenic, appetite reducing neuropeptide cocaine- and amphetamine-regulated transcript (CART) within the nucleus accumbens shell (NAcSh) in impulse control in rats. The animals were ranked for their trait impulsivity based on a screening in the 5-choice serial reaction time task (5-CSRTT). The rats' performances were analysed after bilateral infusions of the OX 1-receptor antagonist SB-334867 (SB) and CART-antibodies (CART-ABs) into the NAcSh. After SB infusions, there was no change in premature responses observed on average. Further analysis revealed a negative linear correlation between the effect of intra-NAcSh SB infusions on premature responses and trait impulsivity. The effect of SB ranged from an increase, no change to a decrease in premature responses in the individual animals with increasing trait impulsivity. Infusions of CART-ABs led to consistently enhanced impulse control with fewer irrelevant actions, independent of trait impulsivity. These data suggest that both OX, especially OX A, and CART in the NAcSh, can be considered endogenous regulators of impulsive action, dependent on underlying impulsivity in the case of OX and independent from trait impulsivity in the case of CART.

Optogenetic and pharmacological interventions link hypocretin neurons to impulsivity in mice

Neurons in the lateral hypothalamus expressing the neuropeptide Hypocretin, also known as orexin, are known critical modulators of arousal stability. However, their role in the different components of the arousal construct such as attention and decision making is poorly understood. Here we study Hypocretin neuronal circuit dynamics during stop action impulsivity in a Go/NoGo task in mice. We show that Hypocretin neuronal activity correlates with anticipation of reward. We then assessed the causal role of Hypocretin neuronal activity using optogenetics in a Go/NoGo task. We show that stimulation of Hypocretin neurons during the cue period dramatically increases the number of premature responses. These effects are mimicked by amphetamine, reduced by atomoxetine, a norepinephrine uptake inhibitor, and blocked by a Hypocretin receptor 1 selective antagonist. We conclude that Hypocretin neurons have a key role in the integration of salient stimuli during wakefulness to produce appropriate and timely responses to rewarding and aversive cues.

Effects of quinpirole in the ventral tegmental area on impulsive behaviour during performance on the five-choice serial reaction time task

Impulsive behaviour on the five-choice serial reaction time task (5CSRTT), a task measuring attention and impulsivity in rodents, is known to depend on dopamine (DA) neurotransmission in the mesolimbic DA pathway. Previous research in our lab reported that systemic administration of the D2/3 agonist quinpirole, which decreases DA release in the striatum, reduced premature responses in rats performing the 5CSRTT. It is unclear, however, whether this effect is mediated by the activation of inhibitory somatodendritic receptors in the ventral tegmental area (VTA), which in turn leads to a reduction in DA release in the nucleus accumbens, a major terminal region of the mesolimbic DA pathway. In the present study, we investigated this possibility by infusing quinpirole directly into the VTA of rats during performance on the 5CSRTT. We found that quinpirole, at the highest dose, significantly reduced the frequency of premature responses on the 5CSRTT. Thus, the effects of quinpirole and other D2/3 receptor agonists to reduce this form of impulsive behaviour appear to depend on the activation of somatodendritic D2/3 receptors in the VTA.

Investigating Temporal Memory Strength and Time-Based Impulse Control Using the DRL Task

Differential reinforcement of low rate (DRL) responding is a schedule-controlled behavior sometimes used in timing research, but also received critics of not providing a pure measure of timing due to the influence of the subject’s motivation or inhibitory control. Nevertheless, we argue that the DRL task provides a unique approach to study how timing and emotion interact with each other. Here, we review evidence showing that male rats prenatally treated with choline supplementation had difficulty in acquiring longer criterion times in the DRL task. This was possibly due to the stronger memory strength of their previously learned shorter criterion times. Female rats, in contrast, performed better than male rats in the same task, but those receiving prenatal choline supplementation were the best performers in this task with longer criterion times because they required less training. Like all female rats, male rats treated with prenatal choline supplementation made very few burst responses, suggesting that the treatment improved male rats’ emotional regulation when facing ‘frustrating’ outcomes. The differential impulse control plus different memory strength of the rats trained in the DRL task revealed the potential interaction of sex hormones and prenatal choline supplementation, a rare combination in a single animal study on timing and time perception. In summary, although the DRL task is certainly not the best timing task, it may be useful in assisting us in better understanding how time perception participates in emotional regulation, especially relevant when the emotion is triggered by a failure in timing.

The effects of early life stress on impulsivity

Elevated impulsivity is a symptom shared by various psychiatric disorders such as substance use disorder, bipolar disorder, and attention-deficit/hyperactivity disorder. However, impulsivity is not a unitary construct and impulsive behaviors fall into two subcategories: impulsive action and impulsive choice. Impulsive choice refers to the tendency to prefer immediate, small rewards over delayed, large rewards, whereas impulsive action involves difficulty inhibiting rash, premature, or mistimed behaviors. These behaviors are mediated by the mesocorticolimbic dopamine (DA) system, which consists of projections from the ventral tegmental area to the nucleus accumbens and prefrontal cortex. Early life stress (ELS) alters both impulsive choice and impulsive action in rodents. ELS also changes DA receptor expression, transmission, and activity within the mesocorticolimbic system. This review integrates the dopamine, impulsivity, and ELS literature to provide evidence that ELS alters impulsivity via inducing changes in the mesocorticolimbic DA system. Understanding how ELS affects brain circuits associated with impulsivity can help advance treatments aimed towards reducing impulsivity symptoms in a variety of psychiatric disorders.

Interactions between impulsivity and MDPV self-administration in rats

Synthetic cathinones, such as 3,4-methylenedioxypyrovalerone (MDPV), are recreational drugs of abuse often identified in 'bath salts' preparations. Humans report compulsive patterns of bath salts use, and previous work suggests that a subset of rats develop unusually high levels of MDPV self-administration. This study aims to test the hypothesis that high levels of impulsivity (e.g., inability to withhold responding for a sucrose reward) will predispose rats to high levels of MDPV self-administration relative to rats with lower levels of impulsivity. The 1-choice serial reaction time task (1-CSRTT) was used to assess impulsivity (i.e., premature responding) in 10 female and 10 male Sprague Dawley rats. Rats were then allowed to self-administer 0.032 mg/kg/inf MDPV or 0.32 mg/kg/inf cocaine, after which full dose-response curves for MDPV (0.001-0.1 mg/kg/inf) or cocaine (0.01-1 mg/kg/inf) were generated under a FR5 schedule of reinforcement. After a history of self-administering MDPV or cocaine, impulsivity was reassessed under the 1-CSRTT, prior to evaluating the acute effects of MDPV (0.032-0.32 mg/kg) or cocaine (0.1-1 mg/kg) on impulsivity. Level of impulsivity was not correlated with subsequent levels of either MDPV or cocaine self-administration, and level of drug self-administration was also not correlated with subsequent levels of impulsivity, although acute administration of MDPV and cocaine did increase premature responding. In failing to find direct relationships between either impulsivity and subsequent drug-taking behaviour, or drug-taking behaviour and subsequent assessments of impulsivity, these findings highlight the complexity inherent in the associations between impulsive behaviour and drug-taking behaviour in both animal models and humans.

Testing delay of gratification in rats using a within-session increasing-delay task

In delay discounting, preference reversals refer to shifts in preference from a larger-later reward to a smaller-sooner reward. Steep hyperbolic discounting predicts a preference reversal when a smaller-sooner and larger-later reward both become temporally proximal; prior research is consistent with this prediction. Hyperbolic discounting does not predict a preference reversal, however, after an individual chooses a larger-later reward over a smaller-immediate reward; prior research is inconsistent with this prediction. We sought to replicate and extend these findings using a delay of gratification task in rats. The task included a defection response which allowed rats to reverse their preference after choosing a larger-later sucrose reinforcer to instead obtain a smaller-immediate sucrose reinforcer. In Experiment 1, we found that rats would defect on their choice of the larger-later reinforcer, systematically replicating prior research. We also found that experience on the delay of gratification task led to decreases in defection responses. In Experiment 2, we found that prior experience on an intertemporal choice task, with no opportunity to defect, also led to few defection responses on the delay of gratification task. We discuss our findings in the context of whether inhibitory control or temporal learning could be involved in delay of gratification.

The Effects of Drug Treatments for ADHD in Measures of Cognitive Performance

Based on core symptoms of inattention and deficient impulse control, and the identification of effective pharmacotherapies such as amphetamine (AMP; Adderall^®), methylphenidate (MPH; Ritalin^®), and atomoxetine (ATX; Strattera^®), ADHD is a clinical condition which provides opportunity for translational research. Neuropsychological tests such as the 5-Choice and Continuous Performance Tasks, which measure aspects of attention and impulse control in animals and humans, provide scope for both forward (animal to human) and reverse (human to animal) translation. Rodent studies support pro-attentive effects of AMP and MPH and effectiveness in controlling some forms of impulsive behavior. In contrast, any pro-attentive effects of ATX appear to be less consistent, the most reliable effects of ATX are recorded in tests of impulsivity. These differences may account for AMP and MPH being recognized as first-line treatments for ADHD with a higher efficacy relative to ATX. DSM-5 classifies three "presentations" of ADHD: predominantly inattentive type (ADHD-I), predominantly hyperactive/impulsive type (ADHD-HI), or combined (ADHD-C). Presently, it is unclear whether AMP, MPH, or ATX has differential levels of efficacy across these presentation types. Nonetheless, these studies encourage confidence for the forward translation of NCEs in efforts to identify newer pharmacotherapies for ADHD.

Dissociating reward sensitivity and negative urgency effects on impulsivity in the five-choice serial reaction time task

Negative urgency describes the tendency for rash and impulsive behaviour during negative emotional states and has been linked to a number of psychiatric disorders. However, there has been limited research on negative urgency as an explanatory mechanism for impulsivity in experimental animals. Such research has important implications for elucidating the neurobiology of negative urgency and thereby the development of future therapeutic interventions. In this study, we investigated the effects of negative urgency using a partial reinforcement schedule to increase the frequency of non-rewarded (i.e. frustrative) trials in the five-choice serial reaction time task, a widely used task to assess visual attention and impulsivity. Using a Markov chain model to analyse trial-by-trial outcomes we found that premature (i.e. impulsive) responses in the five-choice serial reaction time task were more likely to occur after a non-rewarded trial, and mostly after a previous premature trial. However, contrary to the frustration hypothesis of negative urgency, increasing the probability of reinforcement (p(R)) from p(R) = 0.5 to p(R) = 1 increased the number of premature responses in each session. Micro and macro levels of analyses revealed that impulsivity in the five-choice serial reaction time task is governed by at least two processes, one dependent on the overall level of reinforcement hypothesised to determine the state of behavioural activation, the second dependent on trial-by-trial outcomes consistent with negative urgency effects. These processes may depend on distinct neurobiological mechanisms and have relevance for neuropsychiatric disorders that implicate impulsive behaviours dependent on positive and negative affective states.

Reward-Related Decision-Making in Current and Past Disordered Gambling: Implications for Impulsive Choice and Risk Preference in the Maintenance of Gambling Disorder

Impulsive reward-related decision-making (RRDM) is robustly associated with gambling disorder (GD), although its role in the development and perpetuation of GD is still being investigated. This project sought to examine the possible roles of impulsive and risky choice, two aspects of RRDM, in the perpetuation of GD. Additionally, the potential moderating role of comorbid substance misuse was considered. A total of 434 participants with symptoms of current GD and symptoms of concurrent substance use disorder (SUD; n = 105), current GD with past SUD (n = 98), past GD with current SUD (n = 53), or past GD with past substance use disorder (SUD; n = 92), and 96 healthy controls were recruited through MTurk. Participants completed a randomly adjusting delay discounting (a measure of impulsive choice) and probabilistic discounting (a measure of risky choice) task and self-report questionnaires of gambling participation, GD and SUD symptomology, and trait impulsivity. Although control participants showed significantly greater delay discounting compared to individuals with a current or history of GD, no significant group differences emerged between individuals with current GD or a history of GD. Individuals with current GD showed significantly less probabilistic discounting compared to individuals with a history of GD and control participants showed the greatest rates of probabilistic discounting. These effects remained after controlling for lifetime gambling symptom severity and trait impulsivity. Overall, these findings suggest a potential maintaining role of risky choice in gambling disorder, but do not support a maintaining role for impulsive choice.

Adaptive aspects of impulsivity and interactions with effects of catecholaminergic agents in the 5-choice serial reaction time task: implications for ADHD

BACKGROUND

Work in humans has shown that impulsivity can be advantageous in certain settings. However, evidence for so-called functional impulsivity is lacking in experimental animals.

AIMS

This study investigated the contexts in which high impulsive (HI) rats show an advantage in performance compared with mid- (MI) and low impulsive (LI) rats. We also assessed the effects of dopaminergic and noradrenergic agents to investigate underlying neurotransmitter mechanisms.

METHODS

We tested rats on a variable inter-trial interval (ITI) version of the 5-choice serial reaction time task (5CSRTT). Rats received systemic injections of methylphenidate (MPH, 1 mg/kg and 3 mg/kg), atomoxetine (ATO, 0.3 mg/kg and 1 mg/kg), amphetamine (AMPH, 0.2 mg/kg), the alpha-2a adrenoceptor antagonist atipamezole (ATI, 0.3 mg/kg) and the alpha-1 adrenoceptor agonist phenylephrine (PHEN, 1 mg/kg) prior to behavioural testing.

RESULTS

Unlike LI rats, HI rats exhibited superior performance, earning more reinforcers, on short ITI trials, when the task required rapid responding. MPH, AMPH and ATI improved performance on short ITI trials and increased impulsivity in long ITI trials, recapitulating the behavioural profile of HI. In contrast, ATO and PHEN impaired performance on short ITI trials and decreased impulsivity, thus mimicking the behavioural profile of LI rats. The effects of ATO were greater on MI rats and LI rats.

CONCLUSIONS

These findings indicate that impulsivity can be advantageous when rapid focusing and actions are required, an effect that may depend on increased dopamine neurotransmission. Conversely, activation of the noradrenergic system, with ATO and PHEN, led to a general inhibition of responding.

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.

Transition to substance use disorders: impulsivity for reward and learning from reward

Substance dependence constitutes a profound societal burden. Although large numbers of individuals use licit or illicit substances, few transition to dependence. The specific factors influencing this transition are not well understood. Substance-dependent individuals tend to be swayed by the immediate rewards of drug taking, but are often insensitive to delayed negative consequences of their behavior. Dependence is consequently associated with impulsivity for reward and atypical learning from feedback. Behavioral impulsivity is indexed using tasks measuring spontaneous decision-making and capacity to control impulses. While evidence indicates drug taking exacerbates behavioral impulsivity for reward, animal and human studies of drug naïve populations demonstrate it might precede any drug-related problems. Research suggests dependent individuals are also more likely to learn from rewarding (relative to punishing) feedback. This may partly explain why substance-dependent individuals fail to modify their behavior in response to negative outcomes. This enhanced learning from reward may constitute a further pre-existing risk factor for substance dependence. Although impulsivity for reward and preferential learning from rewarding feedback are both underpinned by a compromised dopaminergic system, few studies have examined the relationship between these two mechanisms. The interplay of these processes may help enrich understanding of why some individuals transition to substance dependence.

Neurochemical substrates linked to impulsive and compulsive phenotypes in addiction: A preclinical perspective

Drug compulsion manifests in some but not all individuals and implicates multifaceted processes including failures in top-down cognitive control as drivers for the hazardous pursuit of drug use in some individuals. As a closely related construct, impulsivity encompasses rash or risky behaviour without foresight and underlies most forms of drug taking behaviour, including drug use during adverse emotional states (i.e., negative urgency). While impulsive behavioural dimensions emerge from drug-induced brain plasticity, burgeoning evidence suggests that impulsivity also predates the emergence of compulsive drug use. Although the neural substrates underlying the apparently causal relationship between trait impulsivity and drug compulsion are poorly understood, significant advances have come from the interrogation of defined limbic cortico-striatal circuits involved in motivated behaviour and response inhibition, together with chemical neuromodulatory influences from the ascending neurotransmitter systems. We review what is presently known about the neurochemical mediation of impulsivity, in its various forms, and ask whether commonalities exist in the neurochemistry of compulsive drug-motivated behaviours that might explain individual risk for addiction.

Translational Research in Nicotine Addiction

While commendable strides have been made in reducing smoking initiation and improving smoking cessation rates, current available smoking cessation treatment options are still only mildly efficacious and show substantial interindividual variability in their therapeutic responses. Therefore, the primary goal of preclinical research has been to further the understanding of the neural substrates and genetic influences involved in nicotine's effects and reassess potential drug targets. Pronounced advances have been made by investing in new translational approaches and placing more emphasis on bridging the gap between human and rodent models of dependence. Functional neuroimaging studies have identified key brain structures involved with nicotine-dependence phenotypes such as craving, impulsivity, withdrawal symptoms, and smoking cessation outcomes. Following up with these findings, rodent-modeling techniques have made it possible to dissect the neural circuits involved in these motivated behaviors and ascertain mechanisms underlying nicotine's interactive effects on brain structure and function. Likewise, translational studies investigating single-nucleotide polymorphisms (SNPs) within the cholinergic, dopaminergic, and opioid systems have found high levels of involvement of these neurotransmitter systems in regulating the reinforcing aspects of nicotine in both humans and mouse models. These findings and coordinated efforts between human and rodent studies pave the way for future work determining gene by drug interactions and tailoring treatment options to each individual smoker.

Temporally Specific Roles of Ventral Tegmental Area Projections to the Nucleus Accumbens and Prefrontal Cortex in Attention and Impulse Control

Deficits in impulse control and attention are prominent in the symptomatology of mental disorders such as attention deficit hyperactivity disorder (ADHD), substance addiction, schizophrenia, and bipolar disorder, yet the underlying mechanisms are incompletely understood. Frontostriatal structures, such as the nucleus accumbens (NAcb), the medial prefrontal cortex (mPFC), and their dopaminergic innervation from the ventral tegmental area (VTA) have been implicated in impulse control and attention. What remains unclear is how the temporal pattern of activity of these VTA projections contributes to these processes. Here, we optogenetically stimulated VTA dopamine (DA) cells, as well as VTA projections to the NAcb core (NAcbC), NAcb shell (NAcbS), and the mPFC in male rats performing the 5-choice serial reaction time task (5-CSRTT). Our data show that stimulation of VTA DA neurons, and VTA projections to the NAcbC and the mPFC immediately before presentation of the stimulus cue, impaired attention but spared impulse control. Importantly, in addition to reducing attention, activation of VTA-NAcbS also increased impulsivity when tested under a longer intertrial interval (ITI), to provoke impulsive behavior. Optogenetic stimulation at the beginning of the ITI only partially replicated these effects. In sum, our data show how attention and impulsivity are modulated by neuronal activity in distinct ascending output pathways from the VTA in a temporally specific manner. These findings increase our understanding of the intricate mechanisms by which mesocorticolimbic circuits contribute to cognition.SIGNIFICANCE STATEMENT Deficits in impulse control and attention are prominent in the symptomatology of several mental disorders, yet the brain mechanisms involved are incompletely understood. Since frontostriatal circuits have been implicated in impulse control and attention, we here examined the role of ascending projections from the midbrain ventral tegmental area (VTA) to the nucleus accumbens (NAcb) and prefrontal cortex (PFC). Using optogenetics to individually stimulate these projections with time-locked precision, we distinguished the role that each of these projections plays, in both impulse control and attention. As such, our study enhances our understanding of the neuronal circuitry that drives impulsive and attentive behavior.

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.

Social isolation at adolescence: a systematic review on behaviour related to cocaine, amphetamine and nicotine use in rats and mice

Adolescence is known for its high level of risk-taking, and neurobiological alterations during this period may predispose to psychoactive drug initiation and progression into more severe use patterns. Stress is a risk factor for drug consumption, and post-weaning social isolation increases drug self-administration in rodents. This review aimed to provide an overview of the effects of adolescent social isolation on cocaine, amphetamine and nicotine use-related behaviours, highlighting the specific period when animals were submitted to stress and these drugs. We wondered if there was a specific period during adolescence that isolation stress would increase drug use vulnerability. A total of 323 publications from the Scopus, Web of Science and PubMed (Medline) electronic databases were identified using the words "social isolation" and "adolescence" and "drug" or "cocaine" or "amphetamine" or "nicotine", resulting in 24 articles after analyses criteria following the PRISMA statement. The main points raised were social isolation during adolescence increased cocaine self-administration, amphetamine and nicotine locomotor activity. We did not observe a pattern of a specific moment during the adolescent period that could lead to an increased vulnerability to drug use. The precise conditions under which adolescent social stress alters drug use parameters are complex and likely depend on several factors.

High impulsive choice is accompanied by an increase in dopamine release in rat dorsolateral striatum

Dopamine neurotransmission has been consistently associated with individual differences in impulsive choice. Clinical and preclinical evidence suggests that low striatal dopamine D₂ signaling predisposes to engage in impulsive behaviors. Although dopamine D₂ signaling controls dopamine (DA) extracellular levels, the relationship between striatal dopamine extracellular levels and impulsive choice remains poorly understood. Using quantitative microdialysis, we investigated whether extracellular DA levels in rat dorsolateral striatum (DLS) correlates with preference for an immediate small reward or for a delayed larger reward. Rats were tested in a delay-discounting task and classified as high impulsive (HI) or low impulsive (LI) according to the area under the discounting curve (AUC). No-net flux microdialysis experiments, assessing basal DA release, DA-uptake, and DA extracellular concentration (DA C_ext), were carried out in dorsolateral striatum (DLS) of urethane-anesthetized rats. Rats classified as HI showed a higher DA release compared with LI rats. Differences in DLS DA-uptake and DA C_ext were non-significant. Importantly, a significant negative correlation was observed between AUC and DA release, indicating that the lower the AUC, the higher the DLS DA release. This finding shows that DA release is augmented in the DLS of rats classified as HI, suggesting that a hyper-activated nigro-striatal pathway contributes to impulsive choice.

Effects of a high-fat diet on impulsive choice in rats

INTRODUCTION

Obesity and binge eating disorder are associated with high levels of impulsivity, but the causal role of eating and palatable food in these associations is unclear. Studies in rodents show that a high-fat diet can increase one aspect of impulsivity (impulsive action); it is less clear, however, whether a dissociable aspect of impulsivity (impulsive choice) is similarly affected. Hence, the aim of this study was to ascertain whether chronic exposure to a high-fat diet would alter impulsive choice.

METHODS

Male rats were maintained on either a high-fat or control chow diet for two weeks ad libitum. They then underwent equi-caloric food restriction for the duration of the experiment, with each group maintained on their respective diet. To measure impulsive choice, rats were trained on a delay discounting task (DDT) in which they made discrete choices between a lever that delivered a small food reward immediately and a lever that delivered a large food reward accompanied by systematically increasing delays. Upon reaching stable performance on the DDT, rats were given acute systemic injections of amphetamine prior to testing in the DDT to determine whether increased monoamine transmission affected impulsive choice differently in the two diet groups. Lastly, subjects were tested on a progressive ratio schedule of reinforcement to assess motivation for a sucrose reward.

RESULTS

There was no significant effect of the high-fat diet on impulsive choice. Further, amphetamine decreased choice of the large, delayed reward (increased impulsive choice) to the same extent in both groups. Exposure to the high-fat diet did, however, increase motivation to obtain a sucrose reward.

CONCLUSIONS

These experiments reveal that, under conditions that do not promote weight gain, a chronic high-fat diet does not affect impulsive choice in a delay discounting task. The data are surprising in light of findings showing that this same diet alters impulsive action, and highlight the necessity of further research to elucidate relationships between palatable food consumption and impulsivity.

5-HT2A and 5-HT2C receptors as potential targets for the treatment of nicotine use and dependence

Nicotine use and dependence, typically achieved through cigarette smoking, but increasingly through vape products, is the leading cause of preventable death today. Despite a recognition that many current smokers would like to quit, the success rate at doing so is low and indicative of the persistent nature of nicotine dependence and the high urge to relapse. There are currently three main forms of pharmacotherapy approved as aids to treat nicotine dependence: a variety of nicotine replacement products (NRT's), the mixed NA/DA reuptake inhibitor bupropion (Zyban®), and the preferential nicotinic α4β2 receptor agonist drug, varenicline (Chantix®); the latter being generally recognized to be the most effective. However, each of these approaches afford only limited efficacy, and various other pharmacological approaches are being explored. This chapter focusses on approaches targeted to the serotonin (5-HT) system, namely, selective serotonin reuptake inhibitors (SSRI's) which served a pioneer role in the investigation of serotoninergic modulators in human smoking cessation trials; and secondly drugs selectively interacting with the 5-HT2A and 5-HT2C receptor systems. From an efficacy perspective, measured as smoking abstinence, the 5-HT2A agonist psychedelics, namely psilocybin, seem to show the most promise; although as the article highlights, these findings are both preliminary and there are significant challenges to the route to approval, and therapeutic use of this class should they reach approval status. Additional avenues include 5-HT2C receptor agonists, which until recently was pioneered by lorcaserin, and 5-HT2A receptor antagonists represented by pimavanserin. Each of these approaches has distinct profiles across preclinical tests of nicotine dependence, and may have therapeutic potential. It is anticipated as diagnostic and predictive biomarkers emerge, they may provide opportunities for subject stratification and opportunities for personalizing smoking cessation treatment. The clinical assessment of SSRI, 5-HT2A and/or 5-HT2C receptor-based treatments may be best served by this process.

Early environmental enrichment and impoverishment differentially affect addiction-related behavioral traits, cocaine-taking, and dopamine D2/3 receptor signaling in a rat model of vulnerability to drug abuse

RATIONALE

Risk factors for drug addiction include genetics, environment, and behavioral traits such as impulsivity and novelty preference (NP), which have been related to deficits in striatal dopamine (DA) D_2/3-receptors (D_2/3R) and heightened amphetamine (AMPH)-induced DA release. However, the influence of the early rearing environment on these behavioral and neurochemical variables is not clear.

OBJECTIVES

We investigated the influence of early rearing environment on striatal D_2/3R availabilities and AMPH-induced DA release in relation to impulsivity, NP, and propensity to drug self-administration (SA) in "addiction-prone" Roman high- (RHA) and "addiction-resistant" Roman low-avoidance (RLA) rats.

METHODS

Animals were reared post-weaning in either environmental enrichment (EE) or impoverishment (EI) and were assessed at adulthood for impulsivity, NP, and propensity to cocaine SA. EE and EI rats were also scanned using single-photon emission computed tomography to concurrently measure in vivo striatal D_2/3R availability and AMPH-induced DA release.

RESULTS

EE vs. EI was associated with heightened impulsivity and a lack of NP in both rat lines. Higher dorsal striatal D_2/3R densities were found in RHA EE and higher AMPH-induced DA release in RLA EE. Both impulsivity and NP were negatively correlated to dorsal striatal D_2/3R availabilities and positively correlated with AMPH-induced DA release in EI but not in EE. EE vs. EI was related to a faster rate of cocaine intake and elevated active timeout responses in RHAs.

CONCLUSION

Our results suggest non-monotonic, environment-dependent, relationships between impulsivity, NP, and D_2/3R-mediated signaling, and suggest that EI vs. EE may decrease the reinforcing effects of psychostimulants in predisposed individuals.

Effects of Psychopathy on Neurocognitive Domains of Impulsivity in Abstinent Opiate and Stimulant Users

Background: Psychopathy and substance use disorders (SUDs) are both characterized by neurocognitive impairments reflecting higher levels of impulsivity such as reward-driven decision-making and deficient inhibitory control. Previous studies suggest that psychopathy may exacerbate decision-making deficits, but it may be unrelated to other neurocognitive impairments among substance dependent individuals (SDIs). The aim of the present study was to examine the role of psychopathy and its interpersonal-affective and impulsive-antisocial dimensions in moderating the relationships between dependence on different classes of drugs and neurocognitive domains of impulsivity. Method: We tested 693 participants (112 heroin mono-dependent individuals, 71 heroin polysubstance dependent individuals, 115 amphetamine mono-dependent individuals, 76 amphetamine polysubstance dependent individuals, and 319 non-substance dependent control individuals). Participants were administered the Psychopathy Checklist: Screening Version (PCL:SV) and seven neurocognitive tasks measuring impulsive choice/decision-making (Iowa Gambling Task; Cambridge Gambling Task; Kirby Delay Discounting Task; Balloon Analog Risk Task), and impulsive action/response inhibition (Go/No-Go Task, Immediate Memory Task, and Stop Signal Task). Results: A series of hierarchical multiple regressions revealed that the interpersonal-affective dimension of psychopathy moderated the association between decision-making, response inhibition and both amphetamine and heroin dependence, albeit differently. For amphetamine users, low levels of interpersonal-affective traits predicted poor decision-making on the Iowa Gambling Task and better response inhibition on the Stop Signal task. In contrast, in heroin users high interpersonal-affective psychopathy traits predicted lower risk taking on the Cambridge Gambling Task and better response inhibition on the Go/No-Go task. The impulsive-antisocial dimension of psychopathy predicted poor response inhibition in both amphetamine and heroin users. Conclusions: Our findings reveal that psychopathy and its dimensions had both common and unique effects on neurocognitive function in heroin and amphetamine dependent individuals. Our results suggest that the specific interactions between psychopathy dimensions and dependence on different classes of drugs may lead to either deficient or superior decision-making and response inhibition performance in SDIs, suggesting that psychopathy may paradoxically play a protective role for some neurocognitive functions in specific subtypes of substance users.

A role for reward valuation in the serotonergic modulation of impulsivity

RATIONALE

Impulsive behavior is a deleterious component of a number of mental health disorders but has few targeted pharmacotherapies. One contributing factor to the difficulty in understanding the neural substrates of disordered impulsivity is the diverse presentations of impulsive behavior. Defining the behavioral and cognitive processes which contribute to different subtypes of impulsivity is important for understanding the neural underpinnings of dysregulated impulsive behavior.

METHODS

Using a mouse model for disordered impulsivity, our goal was to identify behavioral and cognitive processes that are associated with increased impulsivity. Specifically, we were interested in the facets of impulsivity modulated by serotonin signaling. We used mice lacking the serotonin 1B receptor (5-HT_1BR) and measured different types of impulsivity as well as goal-directed responding, extinction, habitual-like behavior, cue reactivity, and reward reactivity.

RESULTS

Mice lacking expression of 5-HT_1BR had increased levels of impulsive action, goal-directed responding, and motivation, with no differences seen in rate of extinction, development of habitual behavior, delay discounting, or effort-based discounting. Interestingly, mice lacking 5-HT_1BR expression also showed an overall increase in the choice of higher value rewards, increased hedonic responses to sweet rewards, and responded more for cues that predict reward. We developed a novel paradigm to demonstrate that increasing anticipated reward value could directly increase impulsive action. Furthermore, we found that 5-HT_1BR KO-induced impulsivity could be ameliorated by decreasing the reward value relative to controls, suggesting that the increased 5-HT_1BR-associated impulsive action may be a result of increased reward valuation.

CONCLUSIONS

Taken together, these data show that the effects of serotonin on impulsive action are mediated through the modulation of hedonic value, which may alter the reward representations that motivate action. Overall, this data supports a role for reward value as an important substrate in impulsive action which may drive clinically relevant increases in impulsivity.

Brain-Derived neurotrophic factor Val66Met induces female-specific changes in impulsive behaviour and alcohol self-administration in mice

Substance use disorders are a debilitating neuropsychiatric condition, however it remains unclear why some individuals are at greater risk of substance use disorders than others and what genetic factors determine such individual differences. Impulsivity appears a promising candidate endophenotype to bridge the gap between genetic risk and addiction. Brain-derived neurotrophic factor (BDNF), and in particular the BDNF^Val66Met polymorphism, has been suggested to be involved in both impulsivity and substance use disorders, however results so far have been inconsistent. To investigate the role of BDNF, and more specifically the BDNF^Val66Met polymorphism, in both impulsivity and operant alcohol self-administration using the same animal model. Separate cohorts of humanized Val66Met transgenic mice were assessed for either trait impulsivity in the 5-choice serial reaction time (5-CSRT) touchscreen task, or propensity towards obtaining ethanol in an operant paradigm. It was found that female hBDNF^Val/Val mice exhibited both greater impulsivity compared to hBDNF^Met/Met mice of the same sex as shown by a higher number of premature responses at one of three increased inter-trial intervals tested in the 5-CSRT task, and a greater propensity toward stable ethanol self-administration relative to male mice of the same genotype in the operant paradigm. By contrast, male mice showed no difference between genotypes in impulsivity or stable ethanol self-administration. The hBDNF^Met/Met genotype appears to sex-specifically alter aspects of both impulsive behaviour and addiction propensity. These results suggest that impulse behaviour may be a possible predictor of addiction risk.

The acute and repeated effects of cigarette smoking and smoking-related cues on impulsivity

INTRODUCTION AND AIMS

Impulsivity may be a risk factor that increases vulnerability to nicotine dependence. However, nicotine exposure itself may directly increase impulsivity. This is a secondary analysis of the first study in a controlled laboratory setting, which assessed the effects of nicotine administration (acute and repeated) and exposure to smoking cues on behavioural impulsivity in humans (ClinicalTrials.gov Identifier: NCT01395797).

DESIGN AND METHODS

Twenty-seven smokers completed three tasks to assess behavioural impulsivity (the Immediate Memory Task and the Delayed Memory Task assessing response initiation, and the GoStop Task assessing response inhibition) following: (i) 4 days of cigarette smoking (nicotinised or denicotinised cigarette); (ii) acute cigarette smoking (nicotinised); and (iii) exposure to smoking-related cues.

RESULTS

Four days of nicotinised cigarette smoking (vs. denicotinised) did not significantly increase Immediate Memory Task, Delayed Memory Task and GoStop scores. However, acute cigarette smoking increased GoStop impulsivity, but only following 4 days of smoking nicotinised cigarettes (P < 0.05). Exposure to smoking-related cues had no statistically significant effect on impulsivity.

DISCUSSION AND CONCLUSIONS

Our results suggest that repeated nicotine exposure may sensitise subsequent acute nicotine effects on behavioural impulsivity in heavy smokers.

Increased vulnerability to impulsive behavior after streptococcal antigen exposure and antibiotic treatment in rats

RATIONALE

The inflammation induced by Group A Streptococcus (GAS) infection has been viewed as a vulnerability factor in mental disorders characterized by inhibitory control deficits, such as attention-deficit/hyperactivity disorder or obsessive-compulsive disorder. Antibiotic treatment reduces GAS symptoms; however, its effects on impulsivity have not been fully assessed.

OBJECTIVES

We investigated whether GAS exposure during early adolescence might be a vulnerability factor for adult impulsivity, if antibiotic treatment acts as a protective factor, and whether these differences are accompanied by changes in the inflammatory cytokine frontostriatal regions.

METHODS

Male Wistar rats were exposed to the GAS antigen or to vehicle plus adjuvants at postnatal day (PND) 35 (with two boosts), and they received either ampicillin (supplemented in the drinking water) or water alone from PND35 to PND70. Adult impulsivity was assessed using two different models, the 5-choice serial reaction time task (5-CSRT task) and the delay discounting task (DDT). The levels of interleukin-6 (IL-6) and IL-17 were measured in the prefrontal cortex (PFc), and the tumor necrosis factor α levels (TNFα) were measured in the PFc and nucleus accumbens (NAcc).

RESULTS

GAS exposure and ampicillin treatment increased the waiting impulsivity by a higher number of premature responses when the animals were challenged by a long intertrial interval during the 5-CSRT task. The GAS exposure revealed higher impulsive choices at the highest delay (40 s) when tested by DDT, while coadministration with ampicillin prevented the impulsive choice. GAS exposure and ampicillin reduced the IL-6 and IL-17 levels in the PFc, and ampicillin treatment increased the TNFα levels in the NAcc. A regression analysis revealed a significant contribution of GAS exposure and TNFα levels to the observed effects.

CONCLUSIONS

GAS exposure and ampicillin treatment induced an inhibitory control deficit in a different manner depending on the form of impulsivity measured here, with inflammatory long-term changes in the PFc and NAcc that might increase the vulnerability to impulsivity-related neuropsychiatric disorders.

Sex differences in impulsivity in adult rats are mediated by organizational actions of neonatal gonadal hormones and not by hormones acting at puberty or in adulthood

Males as compared to females display increased impulsivity and inefficient inhibitory control and are more frequently diagnosed with disorders characterized by impulsivity. We previously demonstrated male rats make more impulsive action responses (i.e. premature responding) than females on the 5-choice serial reaction time task (5-CSRTT). Furthermore, pre-pubertal male rats make more impulsive choice responses (i.e. choosing an immediate small reward over a delayed larger reward) than females on a delayed-based reward T-maze task. The goal of the current work was to determine if gonadal hormones impact sex differences in impulsivity in adult rats. In an initial experiment, male and female rats underwent sham surgeries or were gonadectomized either pre-pubertally or during adulthood and tested on the 5-CSRTT in adulthood. Males displayed more impulsive action responses than females regardless of hormone status. In a second experiment, females received testosterone or vehicle injections on postnatal days 1 and 2. Males received vehicle injections. All rats were gonadectomized prior to puberty and tested on the 5-CSRTT in adulthood. Females treated neonatally with testosterone and control males made more impulsive action responses than control females. In another set of experiments, manipulation of gonadal hormones led to no differences in performance on the delayed-based reward T-maze task in males and females. Results indicate that no sex difference is apparent in impulsive choice on a delayed-base reward task in adult rats. They also reveal that adult sex differences on a task of impulsive action is mediated by organizational effects of gonadal hormones acting during the neonatal period and not impacted by hormones acting during puberty or adulthood.

Evaluation of the relationship between the gene expression level of orexin-1 receptor in the rat blood and prefrontal cortex, novelty-seeking, and proneness to methamphetamine dependence: A candidate biomarker

BACKGROUND

previous studies have suggested that methamphetamine (METH) abuse may affect orexin regulation. However, the data regarding the relationship between the current level of orexin and the vulnerability to METH abuse are minimal. Here, we have investigated the correlation between the gene expression level of the orexin-1 receptor (OX1R) in the rat prefrontal cortex (PFC) and blood lymphocytes and susceptibility to METH dependence and its impact on novelty-seeking behavior.

METHODS

male Wistar rats were first examined for novelty-seeking behavior by the novel object recognition test, and the expression level of OX1R in their blood lymphocytes was evaluated by real-time PCR. Then, the susceptibility to METH abuse was investigated by voluntary METH oral consumption test. According to the amounts of METH consumption, the animals were divided into two groups of METH preferring and non-preferring. Half of the rats in each group were sacrificed, and the level of OX1R in their blood lymphocytes and PFC tissue was measured. The other half were sacrificed for the same reason after two weeks of drug abstinence.

RESULTS

The indexes of novelty-seeking behavior were significantly higher in the METH- preferring group compared to the non-preferring animals. Furthermore, the expression level of OX1R in the blood lymphocytes and PFC in the preferring group was considerably higher than the non-preferring group.

CONCLUSION

Up-regulation of the mRNA expression level of OX1R in the lymphocytes and PFC may predict vulnerability to the METH consumption and novelty-seeking, which may serve as a potential biomarker for METH abuse.

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.

Reducing impulsive choice VII: effects of duration of delay-exposure training

Impulsive choice is related to substance use disorders, obesity, and other behaviors that negatively impact human health. Reducing impulsive choice may prove beneficial in ameliorating these maladaptive behaviors. Preclinical research in rats indicates that one reliable method for producing large and lasting reductions in impulsive choice is delay-exposure (DE) training. In all six of the prior DE-training experiments, rats were given extensive experience (~ 120 training sessions) with a delayed reinforcement contingency. The present experiment evaluated if similar large and lasting reductions in impulsive choice could be achieved with less training. The duration of DE training between groups of male Wistar rats was 0 sessions (training ended after a lever-pressing acquisition criterion was met), 30, 60, or 120 sessions. Comparison groups were given the same durations of training with immediate reinforcement. A post-training assessment of impulsive choice was completed using an increasing-delay procedure. For rats assigned to the 60-session condition, impulsive choice was reassessed at a 120-day follow-up. DE training reduced impulsive choice but, contrary to expectation, reductions in impulsive choice did not increase with DE-training duration (no significant training-duration by group interaction). Importantly, 60 sessions of DE training produced reductions in impulsive choice that were comparable to prior published findings and this effect remained significant at the 120-day follow-up. Procedural elements that may be responsible for the DE-training effect, and how they could be improved in future experiments, are discussed.

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.

Diminished Myoinositol in Ventromedial Prefrontal Cortex Modulates the Endophenotype of Impulsivity

Maladaptive impulsivity manifests in a variety of disorders, including attention-deficit hyperactivity disorder (ADHD), depression, and substance use disorder. However, the etiological mechanisms of impulsivity remain poorly understood. In the present study, we used in-vivo proton magnetic resonance spectroscopy (1H-MRS) to investigate neurometabolite content in the prefrontal cortex (PFC) and striatum of rats exhibiting low- versus high-impulsive (LI, HI) behavior on a visual attentional task. We validated our 1H-MRS findings using regionally resolved ex-vivo mass spectroscopy, transcriptomics, and site-directed RNA interference in the ventromedial PFC. We report a significant reduction in myoinositol levels in the PFC but not the striatum of HI rats compared with LI rats. Reduced myoinositol content was localized to the infralimbic (IL) cortex, where significant reductions in transcript levels of key proteins involved in the synthesis and recycling of myoinositol (IMPase1) were also present. Knockdown of IMPase1in the IL cortex increased impulsivity in nonimpulsive rats when the demand on inhibitory response control was increased. We conclude that diminished myoinositol levels in ventromedial PFC causally mediate a specific form of impulsivity linked to vulnerability for stimulant addiction in rodents. Myoinositol and related signaling substrates may thus offer novel opportunities for treating neuropsychiatric disorders comorbid with impulsive symptomology.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Impulsivity is a heritable trait in rodents and associated with a novel quantitative trait locus on chromosome 1

Impulsivity describes the tendency to act prematurely without appropriate foresight and is symptomatic of a number of neuropsychiatric disorders. Although a number of genes for impulsivity have been identified, no study to date has carried out an unbiased, genome-wide approach to identify genetic markers associated with impulsivity in experimental animals. Herein we report a linkage study of a six-generational pedigree of adult rats phenotyped for one dimension of impulsivity, namely premature responding on the five-choice serial reaction time task, combined with genome wide sequencing and transcriptome analysis to identify candidate genes associated with the expression of the impulsivity trait. Premature responding was found to be heritable (h² = 13-16%), with significant linkage (LOD 5.2) identified on chromosome 1. Fine mapping of this locus identified a number of polymorphic candidate genes, however only one, beta haemoglobin, was differentially expressed in both the founder strain and F6 generation. These findings provide novel insights into the genetic substrates and putative neurobiological mechanisms of impulsivity with broader translational relevance for impulsivity-related disorders in humans.

Are suicide attempters more impulsive than suicide ideators?

OBJECTIVE

For over 100 years impulsiveness has been cited as a key factor in why some people that think about killing themselves go on to attempt suicide. Yet prior studies are limited by not using experimental groups that can test this hypothesis and by treating impulsiveness as a unidimensional construct. To overcome these limitations, we compared suicide ideators and suicide attempters on several dimensions of impulsiveness.

METHOD

In Study 1 we compared inpatient suicide attempters who made an attempt within the prior two weeks (n = 30), current inpatient suicide ideators (n = 31), and community controls (n = 34) on several dimensions of impulsiveness using self-report and behavioral measures. In Study 2 (n = 346), we compared three similar groups based on lifetime and past year suicidal behaviors on several of the measures in Study 1.

RESULTS

In Study 1, we found only that negative urgency was clearly elevated among attempters compared with ideators. In Study 2, there were no significant differences on any impulsiveness constructs, including negative urgency.

CONCLUSIONS

Results from the two studies suggest that attempters may not have significantly elevated trait impulsiveness, compared to ideators; however, attempters may have higher impulsiveness when in a negative state.

Neural circuitry and mechanisms of waiting impulsivity: relevance to addiction

Impatience-the failure to wait or tolerate delayed rewards (e.g. food, drug and monetary incentives)-is a common behavioural tendency in humans. However, when rigidly and rapidly expressed with limited regard for future, often negative consequences, impatient or impulsive actions underlie and confer susceptibility for such diverse brain disorders as drug addiction, attention-deficit hyperactivity disorder (ADHD) and major depressive disorder. Consequently, 'waiting' impulsivity has emerged as a candidate endophenotype to inform translational research on underlying neurobiological mechanisms and biomarker discovery for many of the so-called impulse-control disorders. Indeed, as reviewed in this article, this research enterprise has revealed a number of unexpected targets and mechanisms for intervention. However, in the context of drug addiction, impulsive decisions that maximize short-term gains (e.g. acute drug consumption) over longer-term punishment (e.g. unemployment, homelessness, personal harm) defines one aspect of impulsivity, which may or may not be related to rapid, unrestrained actions over shorter timescales. We discuss the relevance of this distinction in impulsivity subtypes for drug addiction with reference to translational research in humans and other animals. This article is part of the theme issue 'Risk taking and impulsive behaviour: fundamental discoveries, theoretical perspectives and clinical implications'.

The Neuropharmacology of Impulsive Behaviour, an Update

Neuropharmacological interventions in preclinical translational models of impulsivity have tremendously contributed to a better understanding of the neurochemistry and neural basis of impulsive behaviour. In this regard, much progress has been made over the last years, also due to the introduction of novel techniques in behavioural neuroscience such as optogenetics and chemogenetics. In this chapter, we will provide an update of how the behavioural pharmacology field has progressed and built upon existing data since an earlier review we wrote in 2008. To this aim, we will first give a brief background on preclinical translational models of impulsivity. Next, recent interesting evidence of monoaminergic modulation of impulsivity will be highlighted with a focus on the neurotransmitters dopamine and noradrenaline. Finally, we will close the chapter by discussing some novel directions and drug leads in the neuropharmacological modulation of impulsivity.

High trait impulsivity potentiates the effects of chronic pain on impulsive behavior

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We explored a potential relation between trait and chronic pain-induced impulsivity.

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Low trait impulsivity rats with neuropathic pain perform similarly to controls.

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High trait impulsivity rats are delay intolerant in chronic pain conditions.

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Trait characteristics influence chronic pain comorbid manifestations.

Preclinical studies on impulsive decision-making in chronic pain conditions are sparse and often contradictory. Outbred rat populations are heterogeneous regarding trait impulsivity manifestations and therefore we hypothesized that chronic pain-related alterations depend on individual traits. To test this hypothesis, we used male Wistar-Han rats in two independent experiments. Firstly, we tested the impact of spared nerve injury (SNI) in impulsive behavior evaluated by the variable delay-to-signal test (VDS). In the second experiment, SNI impact on impulsivity was again tested, but in groups previously categorized as high (HI) and low (LI) trait impulsivity in the VDS.

Results showed that in an heterogenous population SNI-related impact on motor impulsivity and delay intolerance cannot be detected. However, when baseline impulsivity was considered, HI showed a significantly higher delay intolerance than the respective controls more prevalent in left-lesioned animals and appearing to result from a response correction on prematurity from VDS I to VDS II, which was present in Sham and right-lesioned animals.

In conclusion, baseline differences should be more often considered when analyzing chronic pain impact. While this study pertained to impulsive behavior, other reports indicate that this can be generalized to other behavioral dimensions and that trait differences can influence not only the manifestation of comorbid behaviors but also pain itself in a complex and not totally understood manner.