Alcohol-induced impulsivity in rats: an effect of cue salience?

The Continuing Challenges of Studying Parallel Behaviours in Humans and Animal Models

The use of animal models continues to be essential for carrying out research into clinical phenomena, including addiction. However, the complexity of the clinical condition inevitably means that even the best animal models are inadequate, and this may go some way to account for the apparent failures of discoveries from animal models, including the identification of potential novel therapies, to translate to the clinic. We argue here that it is overambitious and misguided in the first place to attempt to model complex, multifacetted human disorders such as addiction in animals, and especially in rodents, and that all too frequently "validity" of such models is limited to superficial similarities, referred to as "face validity", that reflect quite different underlying phenomena and biological processes from the clinical situation. Instead, a more profitable approach is to identify (a) well-defined intermediate human behavioural phenotypes that reflect defined, limited aspects of, or contributors to, the human clinical disorder, and (b) to develop animal models that are homologous with those discrete human behavioural phenotypes in terms of psychological processes, and underlying neurobiological mechanisms. Examples of past and continuing weaknesses and suggestions for more limited approaches that may allow better homology between the test animal and human condition are made.

Guanfacine treatment improves ADHD phenotypes of impulsivity and hyperactivity in a neurofibromatosis type 1 mouse model

Background

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder with a mutation in one copy of the neurofibromin gene (NF1^+/−). Even though approximately 40–60% of children with NF1 meet the criteria for attention deficit hyperactivity disorder (ADHD), very few preclinical studies, if any, have investigated alterations in impulsivity and risk-taking behavior. Mice with deletion of a single NF1 gene (Nf1^+/−) recapitulate many of the phenotypes of NF1 patients.

Methods

We compared wild-type (WT) and Nf1^+/− mouse strains to investigate differences in impulsivity and hyperactivity using the delay discounting task (DDT), cliff avoidance reaction (CAR) test, and open field. We also investigated whether treatment with the clinically effective alpha-2A adrenergic receptor agonist, guanfacine (0.3 mg/kg, i.p.), would reverse deficits observed in behavioral inhibition.

Results

Nf1^+/− mice chose a higher percentage of smaller rewards when both 10- and 20-s delays were administered compared to WT mice, suggesting Nf1^+/− mice are more impulsive. When treated with guanfacine (0.3 mg/kg, i.p.), Nf1^+/− mice exhibited decreased impulsive choice by waiting for the larger, delayed reward. Nf1^+/− mice also exhibited deficits in behavioral inhibition compared to WT mice in the CAR test by repetitively entering the outer edge of the platform where they risk falling. Treatment with guanfacine ameliorated these deficits. In addition, Nf1^+/− mice exhibited hyperactivity as increased distance was traveled compared to WT controls in the open field. This hyperactivity in Nf1^+/− mice was reduced with guanfacine pre-treatment.

Conclusions

Overall, our study confirms that Nf1^+/− mice exhibit deficits in behavioral inhibition in multiple contexts, a key feature of ADHD, and can be used as a model system to identify alterations in neural circuitry associated with symptoms of ADHD in children with NF1.

Ethanol pre-exposure does not increase delay discounting in P rats, but does impair the ability to dynamically adapt behavioral allocation to changing reinforcer contingencies

Increased subjective discounting of delayed rewards is associated with substance abuse, and individuals tend to discount their drug of choice at a greater rate compared to monetary rewards. While there is evidence indicating that increased delay discounting (DD) is a risk factor for substance abuse, some results suggest that exposure to drugs of abuse also increases DD, but effects are mixed. The current study examined whether ethanol pre-exposure increases DD and if an ethanol reinforcer would be discounted at a greater rate than sucrose. Alcohol preferring (P) rats were pre-exposed to either ethanol or sucrose using an intermittent access protocol (IAP) for 8 weeks. Then animals completed an operant fixed choice procedure where each pre-exposure group was split into either an ethanol or sucrose reinforcer group. Afterwards, animals completed an adjusting delay DD task using the same groups as the fixed choice task. Animals that received access to ethanol in the IAP showed increased delayed reward preference in a delay and session dependent manner. Specifically, ethanol pre-exposed animals took more sessions to decrease their preference for the delayed reward at longer delays. In the adjusting delay task, no differences in mean adjusting delays were seen, but ethanol pre-exposure impaired animals' ability to reach stability criteria. The observed results are not consistent with ethanol pre-exposure causing a change in DD. Rather they indicate ethanol pre-exposure impaired animals' ability to reallocate their behavior in response to a change in reinforcer contingencies. The current findings extend prior results showing alcohol naïve P rats exhibit both increased DD and decreased response inhibition (Beckwith and Czachowski 2014, 2016) by demonstrating that after alcohol exposure they exhibit a form of behavioral inflexibility. Hence, a "two-hit" genetic vulnerability/environmental acceleration of addictive behavior is supported.

Facets of impulsivity and alcohol use: What role do emotions play?

Alcohol misuse is a major public concern. Impulsivity has been recognised as a significant risk factor predisposing for the initiation of alcohol use, continuation and excessive alcohol use. Evidence suggests that impulsivity is also a result of both acute alcohol intoxication and long-term alcohol abuse. The multifaceted character of impulsivity and the various ways of assessing it in humans and animal models, hampers the full understanding of how impulsivity relates to alcohol use and misuse. Therefore, in this review we evaluate recent developments in the field, trying to disentangle the contribution of different impulsivity subtypes as causes and effects of alcohol use. Moreover, we review a growing body of evidence, including brain imaging, suggesting the importance of emotional states in engaging in alcohol consumption, particularly in highly impulsive individuals. We also present recent insights into how emotional processing is manifested in alcoholism and binge drinking and suggest novel approaches to treatment and prevention opportunities which target emotional-regulation as well as emotional perception and insight.

Individual differences in voluntary alcohol intake in rats: relationship with impulsivity, decision making and Pavlovian conditioned approach

RATIONALE

Alcohol use disorder (AUD) has been associated with suboptimal decision making, exaggerated impulsivity, and aberrant responses to reward-paired cues, but the relationship between AUD and these behaviors is incompletely understood.

OBJECTIVES

This study aims to assess decision making, impulsivity, and Pavlovian-conditioned approach in rats that voluntarily consume low (LD) or high (HD) amounts of alcohol.

METHODS

LD and HD were tested in the rat gambling task (rGT) or the delayed reward task (DRT). Next, the effect of alcohol (0-1.0 g/kg) was tested in these tasks. Pavlovian-conditioned approach (PCA) was assessed both prior to and after intermittent alcohol access (IAA). Principal component analyses were performed to identify relationships between the most important behavioral parameters.

RESULTS

HD showed more optimal decision making in the rGT. In the DRT, HD transiently showed reduced impulsive choice. In both LD and HD, alcohol treatment increased optimal decision making in the rGT and increased impulsive choice in the DRT. PCA prior to and after IAA was comparable for LD and HD. When PCA was tested after IAA only, HD showed a more sign-tracking behavior. The principal component analyses indicated dimensional relationships between alcohol intake, impulsivity, and sign-tracking behavior in the PCA task after IAA.

CONCLUSIONS

HD showed a more efficient performance in the rGT and DRT. Moreover, alcohol consumption enhanced approach behavior to reward-predictive cues, but sign-tracking did not predict the level of alcohol consumption. Taken together, these findings suggest that high levels of voluntary alcohol intake are associated with enhanced cue- and reward-driven behavior.

Acute ethanol effects on neural encoding of reward size and delay in the nucleus accumbens

Acute ethanol administration can cause impulsivity, resulting in increased preference for immediately available rewards over delayed but more valuable alternatives. The manner in which reward size and delay are represented in neural firing is not fully understood, and very little is known about ethanol effects on this encoding. To address this issue, we used in vivo electrophysiology to characterize neural firing in the core of the nucleus accumbens (NAcc) in rats responding for rewards that varied in size or delay after vehicle or ethanol administration. The NAcc is a central element in the circuit that governs decision-making and importantly, promotes choice of delayed rewards. We found that NAcc firing in response to reward-predictive cues encoded anticipated reward value after vehicle administration, but ethanol administration disrupted this encoding, resulting in a loss of discrimination between immediate and delayed rewards in cue-evoked neural responses. In addition, NAcc firing occurring at the time of the operant response (lever pressing) was inversely correlated with behavioral response latency, such that increased firing rates were associated with decreased latencies to lever press. Ethanol administration selectively attenuated this lever press-evoked firing when delayed but not immediate rewards were expected. These effects on neural firing were accompanied by increased behavioral latencies to respond for delayed rewards. Our results suggest that ethanol effects on NAcc cue- and lever press-evoked encoding may contribute to ethanol-induced impulsivity.

Repeated ethanol exposure during early and late adolescence: double dissociation of effects on waiting and choice impulsivity

BACKGROUND

A strong association exists between impulsivity and binge drinking, and between adolescent alcohol exposure and alcohol abuse in humans. To understand the extent to which early-life alcohol exposure contributes to increased impulsivity, we developed an animal model of binge drinking using 2 strains of mice, C57BL/6J (B6) and DBA2/J (D2), that differ in both motor impulsivity and alcohol drinking.

METHODS

Mice were treated with 2 g/kg ethanol (EtOH) during their early (intermittent ethanol exposure [IEE]_Early; postnatal day [PND]30 to 45) or late (IEE_Late; PND45 to 60) adolescence or with saline (control group [CON]) throughout the adolescence period. To determine the consequences IEE on waiting impulsivity and attentional function, the number of premature responses and omissions, respectively, were evaluated in adulthood using the 5-choice serial reaction time task (5-CSRTT). To examine the effects of IEE on choice impulsivity, risky decision making was assessed in adulthood using a mouse version of the Iowa Gambling Task (mIGT). Additionally, the acute effects of EtOH in adulthood on waiting impulsivity and choice preference were investigated.

RESULTS

We provide experimental evidence that IEE during late, but not early, adolescence disrupts waiting impulsivity and attentional abilities in the 5-CSRTT. In contrast, IEE during early, but not late, adolescence altered risky decision making in the mIGT. D2 mice consistently showed lower premature responding than B6 mice in both the mIGT and the 5-CSRTT, but greater risky decision making on the mIGT. IEE and CON mice showed similar responsiveness to the acute EtOH effects on premature responding, but increased risky choices only in B6_IEE_Early mice.

CONCLUSIONS

Our observations suggest a direct effect of IEE during adolescence on waiting and choice impulsivity and attention later in life.

A within-subject between-apparatus comparison of impulsive choice: T-maze and two-lever chamber

Whereas intertemporal choice procedures are a common method for examining impulsive choice in nonhuman subjects, the apparatus used to implement this procedure varies across studies. The purpose of the present study was to compare impulsive choice between a two-lever chamber and a T-maze. In Experiment 1, rats chose between a smaller, immediate reinforcer and a larger, delayed reinforcer, first in a two-lever chamber and then in a T-maze. Delay to the larger reinforcer changed in an ascending and descending order (0-32 s) across sessions. Experiment 2 examined the same between-apparatus comparison but under steady-state conditions with the delay fixed at 32 s. In Experiment 1, choice for the larger, delayed reinforcer was generally higher in the T-maze than in the two-lever chamber. Similarly in Experiment 2, steady-state choice for the larger, delayed reinforcer was higher in the T-maze. Choice for the 32-s delayed reinforcer was also greater in Experiment 2 than in Experiment 1, suggesting that extended exposure to the delay is required for the T-maze to yield reliable impulsive choice data. While the reasons for the between-apparatus discrepancies are at present unknown, results from both experiments clearly demonstrate that the apparatus matters when assessing overall level and reliability of impulsive choice data.

Altered performance in a rat gambling task after acute and repeated alcohol exposure

RATIONALE

A bidirectional relationship between alcohol use disorder (AUD) and deficits in impulse control and decision making has been suggested. However, the mechanisms by which neurocognitive impairments predispose to, or result from AUD remain incompletely understood.

OBJECTIVES

The aim of this study is to gain more insight in the effects of alcohol exposure on decision making and impulse control. We used two modified versions of the rat gambling task (rGT) that differ in the net gain and the punishment magnitude associated with the different response options.

METHODS

In experiment 1, we assessed the effects of acute alcohol treatment (0-0.8 g/kg) on rGT performance. In experiment 2, we determined the effects of alcohol on rGT acquisition (15 sessions, 0.6 g/kg). Next, these animals were challenged with alcohol (0-1.0 g/kg) prior to rGT sessions.

RESULTS

Acute alcohol treatment suppressed baseline performance in both rGT versions but only modestly altered decision making. Treatment with alcohol during acquisition increased risky choices in the rGT version that involved larger punishment and blunted the reduction in win-shift behavior during acquisition in both rGT versions. Moreover, rats treated with alcohol during acquisition showed an increase in premature and perseverative responding upon subsequent alcohol challenges (0-1.0 g/kg) and were less sensitive to the behavioral suppressant effects of alcohol.

CONCLUSIONS

Our results show that repeated alcohol exposure alters decision making during rGT acquisition and reduces the ability to adjust choice behavior on the basis of feedback. In addition, repeated alcohol exposure unmasks its behavioral disinhibitory effects in the rGT. Impaired responsiveness to choice feedback and behavioral disinhibition may contribute to the development of AUD.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Alpha-synuclein deletion decreases motor impulsivity but does not affect risky decision making in a mouse Gambling Task

RATIONALE

There is evidence to support the role of alpha-synuclein in motor impulsivity, but the extrapolation of this finding to other types of impulsivity remains to be elucidated.

OBJECTIVE

This study aims to investigate the role of alpha-synuclein in choice impulsivity/risky decision-making by means of a mouse version of the Iowa Gambling Task (mIGT).

METHODS

Two strains of mice that differ in the expression of the alpha-synuclein gene, the C57BL/6JOlaHsd (HA) and C57BL/6J (CR), were tested in the mIGT. HA mice differ from their CR ancestors in possessing a chromosomal deletion resulting in the loss of two genes: snca, encoding alpha-synuclein and mmrn1, encoding multimerin-1. Mice were trained in the mIGT until a stable pattern of responding was achieved and then the acute effects of ethanol and cocaine in choice preference were investigated.

RESULTS

No differences between the strains were evident in risky decision-making in any of the experiments, but HA mice showed consistently reduced levels of premature responding in comparison with CR mice, confirming the reduced motor impulsivity found in a previous study. Ethanol did not modify the percentage of advantageous choices in either strain, while cocaine increased the risky choice behaviour by increasing the percentage of disadvantageous choices in both strains.

CONCLUSIONS

We provide further evidence for the involvement of alpha-synuclein in motor impulsivity and suggest that alpha-synuclein does not play a role in risky decision-making as evaluated in the mIGT.

Sex-dependent changes in ADHD-like behaviors in juvenile rats following cortical dopamine depletion

Reduced cortical dopamine levels have been observed in individuals with attention deficit hyperactivity disorder (ADHD). Global dopamine depletions by 6-hydroxydopamine (6-OHDA; with noradrenergic protection) in neonatal rats produces locomotor hyperactivity, with less known about how cortical depletion modulates risky behaviors. Here, we determined the effect of a medial prefrontal cortex (PFC) 6-OHDA depletions (30-60%) or sham microinjection at postnatal day 11 on behavior in male and female juvenile rats. Separate groups were studied for delay discounting (impulsive choice), novelty-preference, and preferences for cues and environments associated with cocaine (10, 20, and 40 mg/kg), their extinction, and reinstatement with place conditioning. Because PFC D1 receptors play a role in these behaviors, confocal microscopy was used to measure D1-immunoreactive projections to the nucleus accumbens core. Both 6-OHDA males and females increased delay discounting relative to sham controls, although only 6-OHDA females increased novelty preferences. Preferences for cocaine-associated environments, their extinction, and reinstatement with a priming dose of cocaine were reduced in 6-OHDA subjects overall. However, impulsive choice at 5s positively correlated with preferences for cocaine-associated environments in 6-OHDA subjects, but not sham controls. As possible compensation for low dopamine levels, D1-immunoreactivity on traced neurons increased in 6-OHDA females; dopamine levels did not remain low in adolescent 6-OHDA males and D1 did not change. We believe that these modest depletions restricted to the PFC demonstrate the role of dopamine, and not norepinephrine, in understanding these behaviors in other animal models where cortical dopamine is reduced during development.

Viral over-expression of D1 dopamine receptors in the prefrontal cortex increase high-risk behaviors in adults: comparison with adolescents

RATIONALE

Adolescents are often described as "lacking brakes" resulting in an increase in several behaviors associated with risk for addiction. Prefrontal cortex dopamine and cortico-limbic interaction play an important role in addiction, and we have previously shown that the dopamine D1 receptor is elevated on prelimbic prefrontal output neurons in adolescent rats. We hypothesized that a constellation of risk-related behaviors is mediated by prefrontal output neuron expression of D1.

OBJECTIVES

We aimed to determine the role of the dopamine D1 receptor in behavioral and neural correlates of risk for addiction that are often observed in adolescents. Therefore, high-risk behaviors as well as subcortical D2 receptor expression were investigated in adult animals with experimentally elevated D1 on prefrontal glutamatergic neurons.

METHODS

A lentiviral vector that selectively expressed the D1 receptor within glutamate neurons was injected in the prelimbic prefrontal cortex of adult male rats. Place conditioning to cocaine, alcohol, and nicotine, as well as delay discounting, novelty preferences, anxiety, cocaine self-administration, and sucrose preferences were assessed.

RESULTS

Virally mediated D1 over-expression in adults leads to stronger drug-cue associations and greater consumption of sweet solutions, elevates bias towards immediate satisfaction rather than delaying gratification, decreases anxiety, and causes rats to work harder for and take more cocaine. Furthermore, elevated cortical D1 reduces D2 receptors in the accumbens (a putative risk marker).

CONCLUSIONS

Together, these data suggest a common mechanism for increased motivational drive to seek and consume substances with hedonic value, consistent with adolescent addictive processes.

Impulsive choice and anxiety-like behavior in adult rats exposed to chronic intermittent ethanol during adolescence and adulthood

Binge drinking during adolescence and adulthood may have differential long-term effects on the brain. We investigated the long-term effects of chronic intermittent ethanol (CIE) exposure during adolescence and adulthood on impulsivity and anxiety-like behavior. Adolescent (adolescent-exposed) and adult (adult-exposed) rats were exposed to CIE/water on postnatal days (PND) 28-53 and PND146-171, respectively, and a 4-day ethanol/water binge on PND181-184 and PND271-274, respectively. During withdrawal from CIE and 4-day binge exposures, anxiety-like behavior and arousal were measured in the light-potentiated startle (LPS) and acoustic startle (ASR) procedures, respectively. Impulsive choice was evaluated in the delay discounting task (DDT) at baseline and after ethanol challenges. Independent of age, ASR and LPS were decreased during withdrawal from CIE exposure. In contrast, LPS was increased in adult-exposed, but not adolescent-exposed, rats during withdrawal from the 4-day ethanol binge. CIE exposure had no effect on preference for the large delayed reward at baseline, independent of age. During DDT acquisition, CIE-exposed, compared with water-exposed rats, omitted more responses, independent of age, suggesting the CIE-induced disruption of cognitive processes. Ethanol challenges decreased preference for the large reward in younger adolescent-exposed rats but had no effect in older adult-exposed rats, independent of previous CIE/water exposure. Taken together, the present studies demonstrate that CIE withdrawal-induced decreases in anxiety and arousal were not age-specific. CIE exposure had no long-term effects on baseline impulsive choice. Subsequent ethanol exposure produced age-dependent effects on impulsivity (increased impulsivity in younger adolescent-exposed rats) and anxiety-like behavior (increased anxiety-like behavior in older adult-exposed rats).

Sensitivity to reinforcer delay predicts ethanol's suppressant effects, but itself is unaffected by ethanol

BACKGROUND

Relative preference for smaller, sooner rewards over larger, later rewards ("delay discounting") is increased by acute ethanol. Additionally, drug-naïve levels of delay discounting can predict subsequent ethanol consumption. However, it is unknown whether these phenomena are driven by a difference in sensitivity to the reinforcer delay or a difference in sensitivity to the reinforcer magnitude, because typical delay discounting tasks manipulate both parameters simultaneously.

METHODS

To disambiguate these factors, two tasks were developed in which animals chose between levers with either different delay contingencies (adjusting delay task) or different magnitude contingencies (adjusting magnitude task). When task performance was stable, rats received ethanol (0, 0.6, and 0.9 g/kg, i.p.).

RESULTS

Ethanol did not affect sensitivity to delay or sensitivity to magnitude. However, responding was suppressed at the highest dose of ethanol (0.9 g/kg). Less suppression was found in animals exhibiting high levels of drug-naïve sensitivity to delay.

CONCLUSION

Thus, this study suggests that ethanol's effect on standard delay discounting tasks is not due to an alteration in sensitivity to delay or magnitude. Additionally, these data show that animals with high sensitivity to delay are resistant to the behaviorally suppressant effects of ethanol, which suggests that low tolerance for delayed rewards and low sensitivity to the behaviorally suppressant effects of ethanol may partly be driven by the same underlying mechanism.

Early and prolonged exposure to reward delay: effects on impulsive choice and alcohol self-administration in male rats

Naturally occurring impulsive choice has been found to positively predict alcohol consumption in rats. However, the extent to which experimental manipulation of impulsive choice may modify alcohol consumption remains unclear. In the present study, we sought to: (a) train low levels of impulsive choice in rats using early, prolonged exposure to reward delay, and (b) determine the effects of this manipulation on subsequent alcohol consumption. During a prolonged training regimen, three groups of male, adolescent Long-Evans rats (21-22 days old at intake) responded on a single lever for food rewards delivered after either a progressively increasing delay, a fixed delay, or no delay. Posttests of impulsive choice were conducted, as was an evaluation of alcohol consumption using a limited-access, two-bottle test. Following delay-exposure training, both groups of delay-exposed rats made significantly fewer impulsive choices than did rats in the no-delay group. In addition, fixed-delay rats consumed significantly more alcohol during daily, 30-min sessions than no-delay rats. Possible mechanisms of these effects are discussed, as is the significance of these findings to nonhuman models of addiction.

Impulsivity, risky behaviors and accidents in alcohol-dependent patients

Impulsivity and alcohol drinking are both considered as important predictors of unintentional as well as intentional injuries. However, relationships of impulsivity with risky behaviors and a history of accidents have not been investigated in alcohol dependence. The aim of this study was to analyze relationships between the frequency of risky behaviors and level of behavioral as well as cognitive impulsivity in alcohol-dependent patients. By means of Barratt's Impulsiveness Scale (BIS) and stop-signal task, the levels of cognitive and behavioral impulsivity among 304 alcohol-dependent patients were measured. Also, patients were asked to answer questions from the Short Inventory of Problems applying to risky behaviors and accidents after alcohol drinking. In addition participants completed a questionnaire to assess frequency of other behaviors from the analyzed spectrum (use of other drugs, driving or aggressive behavior after alcohol drinking). The statistical analysis revealed a significant association between impulsivity and frequency of risky behaviors in alcohol-dependent patients. Individuals with higher scores in BIS behaved more frequently in a risky way and had significantly more accidents after alcohol drinking. The association with risky behaviors was strongest for non-planning and attentional impulsivity subscales, whereas frequency of accidents was particularly associated with motor impulsivity. A multivariate analysis revealed that impulsivity was the most important predictor of risky behaviors, but did not significantly predict a history of accidents. Our study confirms that impulsivity is an important correlate of risky behaviors in alcohol-dependent individuals, along with global psychopathology and severity of alcohol dependence.

Acute ethanol does not always affect delay discounting in rats selected to prefer or avoid ethanol

AIMS

The purpose of this study was to determine whether animals predisposed to prefer alcohol possess an altered acute response to alcohol on a delay discounting task relative to animals predisposed to avoid alcohol.

METHODS

We used rats selected to prefer or avoid alcohol to assess whether genotype moderates changes in delay discounting induced by acute ethanol exposure. Selectively bred rat lines of Sardinian alcohol-preferring (sP; n = 8) and non-preferring (sNP; n = 8) rats, and alko alcohol (AA, n = 8) and alko non-alcohol (ANA, n = 8) rats were trained in an adjusting amount task to assess delay discounting.

RESULTS

There were no significant effects of line on baseline discounting; however, both lines of alcohol-preferring rats exhibit slowed reaction times. Acute ethanol (0, 0.25, 0.5 g/kg) treatment also had no effect on delay discounting in any of the selectively bred rat lines.

CONCLUSION

Our data indicate that in these lines of animals, alcohol preference or avoidance has no impact on delay discounting following acute ethanol exposure. It is possible that other genetic models or lines may be differentially affected by alcohol and exhibit qualitatively and quantitatively different responses in delay discounting tasks.

Characterization of a semi-rapid method for assessing delay discounting in rodents

Delay discounting is a key component of many psychiatric disorders, including drug addiction, compulsive gambling, ADHD, and obesity. However, its underlying mechanisms are not yet fully characterized. One impediment to full characterization of such mechanisms is the fact that rodent models of the task are often complicated and involve extended training of subjects, often requiring more than a month before a stable baseline is obtained. We have therefore characterized a version of the rodent delay discounting task which generates data more quickly than most other published versions. In this version of the task, learning of the operant response is established prior to introduction of the delay component, and delay is tested across subsequent daily sessions with a single delay length per day. We demonstrate here that this version generates a delay discounting curve similar to many published tasks, and is sensitive to changes in reward magnitude and to chronic treatment with cocaine. Furthermore, we present a detailed description of the within-session patterns of behavior in the task, which provides evidence of within-session learning and establishment of stable response patterns. This faster version of the delay discounting task will facilitate future studies involving pharmacological, electrophysiological, and other mechanistic studies of the underlying basis of this important disease process.

Attention, impulsivity, and cognitive flexibility in adult male rats exposed to ethanol binge during adolescence as measured in the five-choice serial reaction time task: the effects of task and ethanol challenges

RATIONALE

Alcohol abuse is prevalent in adolescent humans, but the long-term behavioral consequences of binge alcohol drinking are unknown.

OBJECTIVES

This study investigated the long-term effects of adolescent intermittent ethanol (AIE) exposure on attention and impulsivity.

METHODS

Adolescent male rats were exposed to 5 g/kg of 25% (v/w) ethanol every 8 h for 4 days. During adulthood, rats were tested in the five-choice serial reaction time task (5-CSRTT) assessing attention, impulsivity and cognitive flexibility.

RESULTS

There was no metabolic tolerance to ethanol in adolescent rats during AIE exposure. In the 5-CSRTT under baseline conditions, there were no differences between AIE-exposed and control rats in accuracy, omissions, or premature responses, although AIE-exposed rats tended to make more timeout responses than control rats. The short-duration stimulus challenge decreased accuracy and increased omissions and timeout responses in both AIE-exposed and control rats. The long intertrial interval challenge increased premature responses in all rats. An ethanol challenge decreased correct responses, and increased omissions in control, but not in AIE-exposed, rats. Control, but not AIE-exposed, rats exhibited decreased premature and timeout responses after ethanol administration. Response latencies were not affected in AIE-exposed or control rats indicating no sedative effects of ethanol challenge.

CONCLUSIONS

The results indicate that ethanol binge exposure during adolescence has long-lasting neurobehavioral consequences, which persist into adulthood and can be revealed after re-exposure to ethanol. AIE-induced diminished responses to the disruptive effects of ethanol on attention, impulsivity and cognitive flexibility may lead to increased alcohol drinking and other maladaptive behaviors in adulthood.

Critical thoughts on current rodent models for evaluating potential treatments of alcohol addiction and withdrawal

Despite years of neurobiological research that have helped to identify potential therapeutic targets, we do not have a reliable pharmacological treatment for alcoholism. There are a range of possible explanations for this failure, including arguments that alcoholism is a spectrum disorder and that different population subtypes may respond to different treatments. This view is supported by categorisations such as early- and late-onset alcoholism, whilst multifactorial genetic factors may also alter responsivity to pharmacological agents. Furthermore, experience of alcohol withdrawal may play a role in future drinking in a way that may distinguish alcoholism from other forms of addiction. Additionally, our neurobiological models, based largely upon results from rodent studies, may not mimic specific aspects of the human condition and may reflect different underlying phenomena and biological processes from the clinical pattern. As a result, potential treatments may be targeting inappropriate aspects of alcohol-related behaviours. Instead, we suggest a more profitable approach is (a) to identify well-defined intermediate behavioural phenotypes in human experimental models that reflect defined aspects of the human clinical disorder and (b) to develop animal models that are homologous with those phenotypes in terms of psychological processes and underlying neurobiological mechanisms. This review describes an array of animal models currently used in the addiction field and what they tell us about alcoholism. We will then examine how established pharmacological agents have been developed using only a limited number of these models, before describing some alternative novel approaches to achieving homology between animal and human experimental measures.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Ethanol modulation of synaptic plasticity

Synaptic plasticity in the most general terms represents the flexibility of neurotransmission in response to neuronal activity. Synaptic plasticity is essential both for the moment-by-moment modulation of neural activity in response to dynamic environmental cues and for long-term learning and memory formation. These temporal characteristics are served by an array of pre- and post-synaptic mechanisms that are frequently modulated by ethanol exposure. This modulation likely makes significant contributions to both alcohol abuse and dependence. In this review, I discuss the modulation of both short-term and long-term synaptic plasticity in the context of specific ethanol-sensitive cellular substrates. A general discussion of the available preclinical, animal-model based neurophysiology literature provides a comparison between results from in vitro and in vivo studies. Finally, in the context of alcohol abuse and dependence, the review proposes potential behavioral contributions by ethanol modulation of plasticity.

Insight into the relationship between impulsivity and substance abuse from studies using animal models

Drug use disorders are often accompanied by deficits in the capacity to efficiently process reward-related information and to monitor, suppress, or override reward-controlled behavior when goals are in conflict with aversive or immediate outcomes. This emerging deficit in behavioral flexibility and impulse control may be a central component of the progression to addiction, as behavior becomes increasingly driven by drugs and drug-associated cues at the expense of more advantageous activities. Understanding how neural mechanisms implicated in impulse control are affected by addictive drugs may therefore prove a useful strategy in the search for new treatment options. Animal models of impulsivity and addiction could make a significant contribution to this endeavor. Here, some of the more common behavioral paradigms used to measure different aspects of impulsivity across species are outlined, and the importance of the response to reward-paired cues in such paradigms is discussed. Naturally occurring differences in forms of impulsivity have been found to be predictive of future drug self-administration, but drug exposure can also increase impulsive responding. Such data are in keeping with the suggestion that impulsivity may contribute to multiple stages within the spiral of addiction. From a neurobiological perspective, converging evidence from rat, monkey, and human studies suggest that compromised functioning within the orbitofrontal cortex may critically contribute to the cognitive sequelae of drug abuse. Changes in gene transcription and protein expression within this region may provide insight into the mechanism underlying drug-induced cortical hypofunction, reflecting new molecular targets for the treatment of uncontrolled drug-seeking and drug-taking behavior.

Understanding the construct of impulsivity and its relationship to alcohol use disorders

There are well-established links between impulsivity and alcohol use in humans and other model organisms; however, the etiological nature of these associations remains unclear. This is likely due, in part, to the heterogeneous nature of the construct of impulsivity. Many different measures of impulsivity have been employed in human studies, using both questionnaire and laboratory-based tasks. Animal studies also use multiple tasks to assess the construct of impulsivity. In both human and animal studies, different measures of impulsivity often show little correlation and are differentially related to outcome, suggesting that the impulsivity construct may actually consist of a number of more homogeneous (and potentially more meaningful) subfacets. Here, we provide an overview of the different measures of impulsivity used across human and animal studies, evidence that the construct of impulsivity may be better studied in the context of more meaningful subfacets, and recommendations for how research in this direction may provide for better consilience between human and animal studies of the connection between impulsivity and alcohol use.

Managing the pandemic of obesity: siding with the fox or the hedgehog?

Obesity is a major public health problem of pandemic proportion. Despite its high prevalence and widespread distribution (consistent with a common underlying etiology), clinical psychologists and primary care physicians routinely approach the problem with individualized but often ineffective treatments like psychotherapy and pharmacotherapy, or propose alterations to specific components of the 'toxic environment', cultural influences, and psychosocial factors purported to cause overeating. This paper presents an alternative perspective and proposes a potential framework for assisting health professionals in developing rational approaches to education about and preventive treatment of obesity based on the role of factors in early life that contribute to personality and behavior and which over time lead to obesity and its maintenance.

Effects of chronic administration of drugs of abuse on impulsive choice (delay discounting) in animal models

Drug-addicted individuals show high levels of impulsive choice, characterized by preference for small immediate over larger but delayed rewards. Although the causal relationship between chronic drug use and elevated impulsive choice in humans has been unclear, a small but growing body of literature over the past decade has shown that chronic drug administration in animal models can cause increases in impulsive choice, suggesting that a similar causal relationship may exist in human drug users. This article reviews this literature, with a particular focus on the effects of chronic cocaine administration, which have been most thoroughly characterized. The potential mechanisms of these effects are described in terms of drug-induced neural alterations in ventral striatal and prefrontal cortical brain systems. Some implications of this research for pharmacological treatment of drug-induced increases in impulsive choice are discussed, along with suggestions for future research in this area.

Cognitive processes in alcohol binges: a review and research agenda

Alcohol abuse is associated with a cluster of long-term changes in cognitive processes, as predicted by contemporary models of addiction. In this paper we review evidence which suggests that similar changes may occur during an alcohol binge, and as such they may play an important role in explaining the loss of control over alcohol consumption that occurs during alcohol binges. As a consequence of both acute alcohol intoxication (alcohol 'priming' effects) and exposure to environmental alcohol-related cues, we suggest that a number of changes in cognitive processes are likely. These include increased subjective craving for alcohol, increased positive and arousing outcome expectancies and implicit associations for alcohol use, increased attentional bias for alcohol-related cues, increased action tendencies to approach alcohol, increased impulsive decision-making, and impaired inhibitory control over drives and behaviour. Potential reciprocal relationships between these different aspects of cognition during an alcohol binge are discussed. Finally, we discuss the relationship between the current model and existing models of cognitive processes in substance abuse, and we speculate on the implications of the model for the reduction binge drinking and its consequences.

Ethanol effects on impulsivity in two mouse strains: similarities to diazepam and ketamine

RATIONALE

The effects of ethanol on attention and impulsivity have been contradictory.

OBJECTIVES

The aim of the present investigation is to study the effects of acute ethanol administration in measures of attention and response control in the five-choice serial reaction time task (5-CSRTT) in two strains of mice, C57BL/6JOlaHsd and CD1.

MATERIALS AND METHODS

Mice were trained in the 5-CSRTT and then were injected intraperitoneally (i.p.) with 0, 0.5, 1 and 2 g/kg ethanol before testing under standard parameters and in a long inter-trial interval (ITI) session, which promotes the emergence of premature responses, a measure of poor inhibitory control. To examine if the effects of ethanol in the 5-CSRTT were due to its actions at GABA(A) receptors or at NMDA receptors, the GABA(A) receptor agonist diazepam (1 and 2 mg/kg, i.p.) and the non-competitive NMDA antagonist ketamine (10 and 20 mg/kg, i.p.) were tested in long ITI sessions.

RESULTS

Ethanol did not affect attention or impulsivity in the standard procedure, but increased premature responding in long ITI sessions. The effects of ethanol were mimicked by diazepam in both strains of mice, whereas ketamine increased premature responding only in the CD1 strain.

CONCLUSIONS

Ethanol's ability to increase impulsivity in the 5-CSRTT is mediated by both common and different neurotransmitter systems in the two strains of mice and is dependent on the task's parameters. Furthermore, ethanol did not decrease response accuracy, suggesting that attentional mechanisms are preserved after acute ethanol in mice and that the increases in impulsive behaviour are independent of attentional performance.