Imagine before you leap: Episodic future thinking combined with transcranial direct current stimulation training for impulsive choice in repetitive negative thinking

Background

Immediate reward preference in repetitive negative thinking (RNT) has a high clinical correlation with a variety of maladaptive behaviors, whereas episodic future thinking (EFT) may be conducive to dealing with non-adaptive thinking and decision-making.

Objectives

This study aimed to evaluate the efficacy of EFT training combined with transcranial direct current stimulation (tDCS) stimulation over the ventromedial PFC (vmPFC) in inhibiting impulsive choice of RNT individuals.

Method

Study 1 explored the effects of EFT on immediate reward preference of participants with high and low RNT (N = 48). Study 2 conducted a randomized controlled trial (RCT) to examine the treatment effect of the EFT-neural training on impulsive choice of high RNT individuals (N = 103).

Results

In study 1, individuals with high RNT were more likely to choose smaller and sooner (SS) rewards, however, there were no significant differences between the high-RNT group and the low-RNT group under the positive EFT condition. In study 2, a significant decrease was shown in the proportion of choosing SS rewards under the 8-week EFT-neural training, and the effect was maintained at 1 month follow-up.

Conclusion

RNT is a vulnerability factor for short-sighted behaviors, and EFT-neural training could be suitable for reducing RNT and improving immediate reward preference.

Self-Control Training: A Scoping Review

We conducted a scoping review of the behavior analytic self-control training (SCT) literature. To identify included articles, we searched key terms in six databases for articles published between 1988 and 2021. We included empirical articles that used a behavioral approach to self-control training with human participants for whom increasing self-control choice was a clinically significant goal and measured self-control and impulsive choice as dependent variables. Twenty-five experiments from 24 articles with a total of 79 participants were included in the review. This review aims to summarize the characteristics of SCT procedures and outcomes, provide recommendations for future research directions, and offer practical suggestions to clinicians incorporating SCT into practice. We examined similarities across studies regarding the independent variables manipulated in SCT, dependent variables measured, metrics of successful interventions, and assessment of generalization and maintenance of self-control choice. Twenty-one experiments arranged concurrent self-control- and impulsive-choice options with positive reinforcement, and four experiments arranged self-control training with negative-reinforcement contingencies. Variations of SCT included progressively increasing delays, intervening activities, signaled delays, antecedent rules, and commitment responses. Providing an intervening activity during the delay was largely successful at increasing self-control choice. Maintenance and generalization of increased self-control choice were assessed in two and three experiments, respectively. Future research should focus on improving the generality of SCT procedures in clinical settings by increasing terminal delays, fading out intervening activities, including probabilistic outcomes, and combining appetitive and aversive outcomes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40617-023-00885-y.

Active and passive waiting in impulsive choice: Effects of fixed-interval and fixed-time delays

Behavioral interventions to improve self-control, preference for a larger-later (LL) reward over a smaller-sooner (SS) reward, involve experience with delayed rewards. Whether they involve timing processes remains controversial. In rats, there have been inconsistent results on whether timing processes may be involved in intervention-induced improvements in self-control. Interventions that improved self-control with corresponding timing improvements used fixed-interval (FI) delays, whereas interventions that failed to find corresponding timing improvements used fixed-time (FT) delays. The FI schedule includes a response contingency (active waiting), whereas the FT schedule delivers reward automatically (passive waiting). The present study compared the effects of FI and FT schedules in interventions and impulsive choice tasks to evaluate effects on self-control and timing behavior. The impulsive choice task evaluated preference for an SS option (one pellet after 10-, 15-, 20-, 25-, and 30-s delays) versus an LL option (two pellets after a 30-s delay). The intervention task included forced-choice SS (one pellet after 10 s) and LL (two pellets after 30 s) sessions under FI or FT schedules. FI schedules produced greater sensitivity to SS delay in the impulsive choice task. Both FI and FT interventions increased LL choices. Following choice testing, temporal bisection and peak interval tasks revealed better timing precision for rats with an FI delay experience. Overall, the FI choice contingency was associated with improved temporal attention and timing precision.

Mechanisms of impulsive choice: Experiments to explore and models to map the empirical terrain

Impulsive choice is preference for a smaller-sooner (SS) outcome over a larger-later (LL) outcome when LL choices result in greater reinforcement maximization. Delay discounting is a model of impulsive choice that describes the decaying value of a reinforcer over time, with impulsive choice evident when the empirical choice-delay function is steep. Steep discounting is correlated with multiple diseases and disorders. Thus, understanding the processes underlying impulsive choice is a popular topic for investigation. Experimental research has explored the conditions that moderate impulsive choice, and quantitative models of impulsive choice have been developed that elegantly represent the underlying processes. This review spotlights experimental research in impulsive choice covering human and nonhuman animals across the domains of learning, motivation, and cognition. Contemporary models of delay discounting designed to explain the underlying mechanisms of impulsive choice are discussed. These models focus on potential candidate mechanisms, which include perception, delay and/or reinforcer sensitivity, reinforcement maximization, motivation, and cognitive systems. Although the models collectively explain multiple mechanistic phenomena, there are several cognitive processes, such as attention and working memory, that are overlooked. Future research and model development should focus on bridging the gap between quantitative models and empirical phenomena.

Effects of the estrous cycle on impulsive choice and interval timing in female rats

Research in humans and animals shows differences in impulsive choice, which is a failure to wait for larger, delayed rewards, when comparing males and females. It is possible that fluctuations in sex hormones (estradiol and progesterone) across the reproductive cycle contribute to sex differences in impulsive choice. The current study delivered an impulsive choice task with peak interval trials to female rats while estrous cycles, the rodent reproductive cycle, were tracked over the course of the task. Female rats were more sensitive to changes in delay in the proestrus phase of the estrous cycle and made more larger-later choices when in estrus, particularly when the delay to the smaller reward was short. Estradiol increases dramatically during proestrus while progesterone peaks during estrus, suggesting that estradiol and progesterone may affect impulsive choice through mechanisms such as delay discounting, delay aversion, and/or timing processes. Analyses of timing of the choice task delays showed inconsistent effects of the estrous cycle across delays, suggesting that reward-timing interactions may have complicated how hormone fluctuations affected interval timing. Further research is needed to determine the mechanism underlying increased larger-later choices during the estrus phase, increased delay sensitivity during the proestrus phase, and variability in interval timing across delays and estrous cycle stages.

Concurrent measures of impulsive action and choice are partially related and differentially modulated by dopamine D1- and D2-like receptors in a rat model of impulsivity

Impulsivity is a multidimensional construct, but the relationships between its constructs and their respective underlying dopaminergic underpinnings in the general population remain unclear. A cohort of Roman high- (RHA) and low- (RLA) avoidance rats were tested for impulsive action and risky decision-making in the rat gambling task, and then for delay discounting in the delay-discounting task to concurrently measure the relationships among the three constructs of impulsivity using a within-subject design. Then, we evaluated the effects of dopaminergic drugs on the three constructs of impulsivity, considering innate differences in impulsive behaviors at baseline. Risky decision-making and delay-discounting were positively correlated, indicating that both constructs of impulsive choice are related. Impulsive action positively correlated with risky decision-making but not with delay discounting, suggesting partial overlap between impulsive action and impulsive choice. RHAs showed a more impulsive phenotype in the three constructs of impulsivity compared to RLAs, demonstrating the comorbid nature of impulsivity in a population of rats. Amphetamine increased impulsive action and had no effect on risky decision-making regardless of baseline levels of impulsivity, but it decreased delay discounting only in high impulsive RHAs. In contrast, while D₁R and D₃R agonism as well as D_2/3R partial agonism decreased impulsive action regardless of baseline levels of impulsivity, D_2/3R agonism decreased impulsive action exclusively in high impulsive RHAs. Irrespective of baseline levels of impulsivity, risky decision-making was increased by D₁R and D_2/3R agonism but not by D₃R agonism or D_2/3R partial agonism. Finally, while D₁R and D₃R agonism, D_2/3R partial agonism and D₂R blockade increased delay discounting irrespective of baseline levels of impulsivity, D_2/3R agonism decreased it in low impulsive RLAs only. These findings indicate that the acute effects of dopamine drugs were partially overlapping across dimensions of impulsivity, and that only D_2/3R agonism showed baseline-dependent effects on impulsive action and impulsive choice.

Reducing Impulsive Choice: VIII. Effects of Delay-Exposure Training in Female Rats

Impulsive choice may play an important role in serious health-related decisions, like addiction tendencies. Thus, there is merit in exploring interventions that reduce impulsive choice. Delay-exposure training involves extended experience with delayed reinforcement. Following training, delay-exposed rats make fewer impulsive choices than control rats. The reducing effects of delay exposure training on impulsive choice have been replicated in male rats seven times. For the first time, this study evaluated the effects of delay exposure training in female rats. Thirty-six rats were randomly assigned to either delay-exposure or immediacy-exposure training. Then, rats underwent two impulsive choice assessments in which they chose between one immediate pellet or three delayed pellets. In the first assessment, delays increased within-sessions, across trial blocks from 0, 8, 16, to 32s. In the second assessment, delays to the larger reward increased between-sessions, from 8, 16, 32, to 4s. Unlike findings with male rats, delay-exposure training produced a reduction in impulsive choice only in the initial five sessions in female rats. Possible reasons for the lack of lasting effect in female rats are discussed and future research directions are identified.

Generalizability of time-based interventions: Effects of choice procedure and smaller-sooner delay

Interventions exposing rats to delayed-reward contingencies attenuate suboptimal impulsive choices, a preference for a smaller-sooner (SS) over a larger-later (LL) reward. Interventions may potentially improve delay-tolerance, timing of delays, and/or discrimination of reward magnitudes. Generalization from the intervention to impulsive choice under different procedures can provide insights into the processes that underlie the intervention effects. Experiment 1 tested intervention effects on systematic-delay (SYS) and adjusting-delay (ADJ) procedures, predicting that intervention effects would be more effective on the SYS procedure with predictable delays. The ADJ procedure did not benefit significantly from intervention, but the SYS procedure, unexpectedly, showed greater impulsive choices following intervention. Experiment 2 tested whether short (5 s) SS intervention delays may have promoted greater impulsivity in the SYS impulsive choice procedure in Experiment 1. Short SS delays in choice and intervention procedures increased impulsive choices in comparison to longer (10 s) delays. Incongruent SS delays in the intervention/choice procedures resulted in negative intervention effects. The results suggest that short SS delays are detrimental to self-control and that specific temporal information generalizes from the intervention to the SYS choice task, but not the ADJ choice task.

GABAB receptors in prelimbic cortex and basolateral amygdala differentially influence intertemporal decision making and decline with age

The ability to decide adaptively between immediate vs. delayed gratification (intertemporal choice) is critical for well-being and is associated with a range of factors that influence quality of life. In contrast to young adults, many older adults show enhanced preference for delayed gratification; however, the neural mechanisms underlying this age difference in intertemporal choice are largely un-studied. Changes in signaling through GABA_B receptors (GABA_BRs) mediate several age-associated differences in cognitive processes linked to intertemporal choice. The current study used a rat model to determine how GABA_BRs in two brain regions known to regulate intertemporal choice (prelimbic cortex; PrL and basolateral amygdala; BLA) contribute to age differences in this form of decision making in male rats. As in humans, aged rats showed enhanced preference for large, delayed over small, immediate rewards during performance in an intertemporal choice task in operant test chambers. Activation of PrL GABA_BRs via microinfusion of the agonist baclofen increased choice of large, delayed rewards in young adult rats but did not influence choice in aged rats. Conversely, infusion of baclofen into the BLA strongly reduced choice of large, delayed rewards in both young adult and aged rats. Aged rats further showed a significant reduction in expression of GABA_BR1 subunit isoforms in the prefrontal cortex, a discovery that is consonant with the null effect of intra-PrL baclofen on intertemporal choice in aged rats. In contrast, expression of GABA_BR subunits was generally conserved with age in the BLA. Jointly, these findings elucidate a role for GABA_BRs in intertemporal choice and identify fundamental features of brain maturation and aging that mediate an improved ability to delay gratification.

The relationship between dietary fat intake, impulsive choice, and metabolic health

Unhealthful foods are convenient, ubiquitous, and inexpensive. Overconsumption of unhealthful foods can result in disease states such as obesity and Type 2 diabetes. In addition to the physiological consequences of unhealthful foods, research in rats has shown that diets high in processed fat and sugar induce impulsive choice behavior. Research in humans has demonstrated a link between metabolic health and impulsive choice, but most investigations have not included diet. We investigated how dietary fat intake interacts with body fat percentage, fasting glucose, insulin response, and systemic inflammation levels to predict impulsive choices in humans. Participants were split into either Control (<35% calories from fat) or High-Fat (≥40% calories from fat) groups based on self-reported dietary intake, completed an impulsive choice task, and underwent testing to determine their body fat, glucose, insulin response, and inflammation levels. High-fat diets were not predictive of impulsive choices, but added sugar was predictive. Body fat percentage was associated with impulsive choices only in the group who reported consuming high-fat diets. In addition, fasting glucose was associated with impulsive choices in the control group. Therefore, metabolic health and dietary fat intake interacted to predict impulsive choices. These findings indicate that knowledge of dietary patterns coupled with metabolic health markers may help us better understand impulsive choices, thereby improving our ability to target individuals who could benefit from interventions to reduce impulsive choice behavior, with the goal of promoting more self-controlled food choices.

Effects of effort training on effort-based impulsive choice

Because impulsive decision-making is correlated with many maladaptive tendencies, researchers have increasingly studied methods for reducing impulsive choice. These efforts have primarily focused on increasing choices of larger, more-delayed rewards. A second type of impulsive choice is selecting a smaller, less-effortful reward over a larger, more-effortful one. Little nonhuman research has examined experimental methods for reducing effort-based impulsive choice. Within the realm of delay-based impulsive choice, extended exposure to reinforcer delays has proven effective in reducing impulsive choices in rats. The current study took a similar tack by evaluating if reductions in effort-based impulsive choice could be achieved by providing rats with extended exposure to high-effort training, that is, reinforcement contingencies requiring a large number of responses. Male rats were randomly assigned to either a high-effort training (fixed-ratio 50) or low-effort training (fixed-ratio 1) group. Following training, both groups completed a test of effort-based impulsive choice. High-effort training produced a temporary reduction in effort-based impulsive choice. In the same test phase, groups also differed in response run durations (shorter following high-effort training), with larger differences in the initial sessions, which may have contributed to the short-term effect of high-effort training in effort-based impulsive choice.

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.

Impulsivity traits and neurocognitive mechanisms conferring vulnerability to substance use disorders

Impulsivity - the tendency to act without sufficient consideration of potential consequences in pursuit of short-term rewards - is a vulnerability marker for substance use disorders (SUD). Since impulsivity is a multifaceted construct, which encompasses trait-related characteristics and neurocognitive mechanisms, it is important to ascertain which of these aspects are significant contributors to SUD susceptibility. In this review, we discuss how different trait facets, cognitive processes and neuroimaging indices underpinning impulsivity contribute to the vulnerability to SUD. We reviewed studies that applied three different approaches that can shed light on the role of impulsivity as a precursor of substance use related problems (versus a consequence of drug effects): (1) longitudinal studies, (2) endophenotype studies including non-affected relatives of people with SUD, and (3) clinical reference groups-based comparisons, i.e., between substance use and behavioural addictive disorders. We found that, across different methodologies, the traits of non-planning impulsivity and affect-based impulsivity and the cognitive processes involved in reward-related valuation are consistent predictors of SUD vulnerability. These aspects are associated with the structure and function of the medial orbitofrontal-striatal system and hyperexcitability of dopamine receptors in this network. The field still needs more theory-driven, comprehensive studies that simultaneously assess the different aspects of impulsivity in relation to harmonised SUD-related outcomes. Furthermore, future studies should investigate the impact of impulsivity-related vulnerabilities on novel patterns of substance use such as new tobacco and cannabinoid products, and the moderating impact of changes in social norms and lifestyles on the link between impulsivity and SUD. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

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.

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 paradoxical performance by different species on the ephemeral reward task

The ephemeral reward task consists of giving an animal a choice between two distinctive stimuli, A and B (e.g., black and white), on each of which is placed a bit of food. If the animal chooses the food on A, it gets that reinforcer, but the other stimulus, B, is removed, and the trial is over. If it chooses the food on B, however, it gets that food and the stimulus A remains, so it can have that food as well. Thus, choice of stimulus B gives the animal two reinforcers rather than one. Wrasse (cleaner fish) easily learn to choose optimally, whereas surprisingly, most non-human primates do not. Parrots, however, appear to learn this task as easily as the fish. To test the hypothesis that animals that choose with their mouth can learn it, we tested pigeons and found that they show no evidence of optimal learning with this task (with either the manual presentation of the stimuli or the operant presentation of the stimuli). Similarly, rats show no evidence of optimal learning. However, if a 20-s delay (fixed-interval schedule) is inserted between stimulus choice and reinforcement, both pigeons and rats learn to perform optimally. The ephemeral reward task appears to promote impulsive choice in several species, but with this task (and others), inserting a delay between choice and reinforcement promotes more careful choice, leading to optimal performance.

Cognitive deficits in methamphetamine addiction: Independent contributions of dependence and intelligence

BACKGROUND

Methamphetamine's effects on brain function have been associated with cognitive deficits, which have a negative impact on clinical outcomes. However, it remains unclear if cognitive deficits relate to methamphetamine dependence (potentially amenable to abstinence and retraining) or background characteristics, mental health and other drug use. We tested the association between methamphetamine dependence and cognitive performance, while factoring in the impact of background characteristics, depressive symptoms and tobacco, alcohol and cannabis use.

METHOD

The sample comprised 108 treatment-seeking participants who met the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV TR) criteria for methamphetamine dependence and 50 socio-demographically matched controls. We administered a comprehensive neuropsychological test battery (delay discounting, decision making, disinhibition, episodic and working memory) and examined cognitive deficits in methamphetamine users after taking into account socio-demographic characteristics, tobacco, alcohol and cannabis use, and depressive symptoms.

RESULTS

Hierarchical multiple regression analyses showed that methamphetamine dependence was associated with poorer performance in decision-making and disinhibition over and above other predictors, while IQ better explained performance in episodic and working memory. Although duration of methamphetamine use was linked to disinhibition, other patterns of methamphetamine use (including dose and frequency) were not consistently related to performance.

CONCLUSIONS

Methamphetamine dependence impacts inhibitory control and decision-making, whereas lower IQ associates with memory/working memory deficits among methamphetamine users. Findings suggest the need to target disinhibition and impulsive decision-making as part of methamphetamine dependence treatment, while buffering the impact of IQ on memory systems.

A time-based intervention to treat impulsivity in male and female rats

Time-based interventions have emerged as promising treatments for disorders associated with impulsivity. These interventions can be implemented to test their efficacy in preventing or treating impulsive choice in animal models of diseases related to impulsivity such as drug abuse. Impulsive choice is typically defined as choosing a smaller-sooner (SS) reward over a larger-later (LL) reward when the LL is relatively more optimal. Previous research has shown that these interventions promote LL choices in males and females, but sex differences have not been assessed. Because sex differences can complicate the application of therapies, it is critical to compare the effects of the intervention in males and females. The intervention group received exposure to 10-s and 30-s interval schedules, and the control rats received no delay to reward. Different impulsive choice tasks were used to assess the intervention efficacy across the two experiments. Following the intervention, reductions in impulsive choice were found in male and female rats, but the degree of improvement was inconsistent across sex and task. Bayesian analyses that combined the results revealed robust evidence of an overall intervention effect with the intervention group showing greater self-control, but there was no evidence for the intervention affecting males and females differently. Taken together, these results suggest that time-based interventions are effective tools to treat impulsivity in both males and females and offer promising translational capability to humans.

Effects of d-amphetamine and MK-801 on impulsive choice: Modulation by schedule of reinforcement and delay length

Procedural modifications can modulate drug effects in delay discounting, such as signaling the delay to reinforcement and altering the order in which delays are presented. Although the schedule of reinforcement can alter the rate at which animals discount a reinforcer, research has not determined if animals trained on different schedules of reinforcement are differentially affected by pharmacological manipulations. Similarly, research has not determined if using different delays to reinforcement can modulate drug effects in delay discounting. Male Sprague Dawley rats (n = 36) were split into four groups and were trained in a delay-discounting procedure. The schedule of reinforcement (fixed ratio [FR] 1 vs. FR 10) and delays to reinforcement (0, 5, 10, 20, and 50 s vs. 0, 10, 30, 60, 100 s) were manipulated for each group. Following behavioral training, rats were treated with d-amphetamine (0, 0.25, 0.5, and 1.0 mg/kg) and MK-801 (0, 0.03, and 0.06 mg/kg). Results showed that amphetamine decreased impulsive choice when a FR 1 schedule was used, but only when the short delay sequence was used. Conversely, amphetamine decreased impulsive choice when a FR 10 schedule was used, but only when rats were trained on the long delay sequence. MK-801 decreased impulsive choice in rats trained on a FR 1 schedule, regardless of delay sequence, but did not alter choice in rats trained on a FR 10 schedule. These results show that schedule of reinforcement and delay length can modulate drug effects in delay discounting.

Brain structure and trait impulsivity: A comparative VBM study contrasting neural correlates of traditional and alternative concepts in healthy subjects

Impulsivity as a trait modulates a range of cognitive functions, e.g. planning, decision-making, or response inhibition. Recent behavioural and psychometric findings challenge both the neurobiological models as well as the conceptualisation of psychometric measures of impulsivity. In the present study, we aimed to test the association of brain structure with the Barratt Impulsiveness Scale (BIS-11), a commonly applied self-rating instrument for impulsivity, using both the classical three-factor-model for impulsive behaviour (motor (IM), attentional (IA) and non-planning impulsivity (INP)), as well as the recently proposed alternative model contrasting inability to wait for reward (IWR) as an index of impulsive choice and rapid response style (RRS) as an index of impulsive action. We analysed brain structural data in a community sample of 85 healthy individuals, who completed the BIS-11, using voxel-based morphometry (CAT12: Computational Anatomy Toolbox 12). Regional volumes were correlated with the three traditional BIS-11 subscales, as well as IWR and RRS. BIS-11 total score was positively correlated with right inferior parietal, postcentral, and supramarginal grey matter (p < 0.05, FWE cluster-level corrected). Attentional impulsivity (IA) was also positively correlated with right inferior and superior parietal and supramarginal gyri. Comparison of the other scales did show some divergence, but most correlations did not survive correction for multiple comparisons. Our findings suggest that difference facets of trait impulsivity might be related to different brain areas, and might thus dissociate along distinct but overlapping neural networks. In contrast to lesion or patient studies, these analyses delineate physiological variance, and can thus help to conceptualise network models in the absence of pathology.

Dietary effects on the determinants of food choice: Impulsive choice, discrimination, incentive motivation, preference, and liking in male rats

The current study sought to understand how long-term exposure to diets high in saturated fat and refined sugar affected impulsive choice behavior, discrimination abilities, incentive motivation, food preferences, and liking of fat and sugar in male rats. The results showed that 8 weeks of dietary exposure impaired impulsive choice behavior; rats exposed to diets high in processed fat or sugar were more sensitive to changes in delay, a marker of impulsivity. For the high-fat group, these deficits in impulsive choice may stem from poor time discrimination, as their performance was impaired on a temporal discrimination task. The high-fat group also showed reduced magnitude sensitivity in the impulsive choice task, and they earned fewer rewards during lever press training indicating potentially reduced incentive motivation. The high-fat group also developed a preference for high-fat foods compared to the chow and high-sugar group who both preferred sugar. In contrast, dietary exposure did not alter the liking of fat or sugar as measured by a taste reactivity task. Together, the results suggest that the alterations in impulsive choice, time discrimination, incentive motivation, and food preferences induced by consumption of a high-fat diet could make individuals vulnerable to overeating, and thus obesity.

Juvenile exposure to methylphenidate and guanfacine in rats: effects on early delay discounting and later cocaine-taking behavior

RATIONALE

Both methylphenidate (MPH), a catecholamine reuptake blocker, and guanfacine, an alpha2A agonist, are used to treat attention-deficit hyperactivity disorder (ADHD). Childhood impulsivity, including delay discounting, is associated with increased substance use during adolescence. These effects can be mitigated by juvenile exposure to MPH, but less is known about the long-term effects of developmental exposure to guanfacine in males and females.

OBJECTIVE

This study aims to determine sex differences and dose-dependent effects of juvenile exposure to MPH or guanfacine on delay-discounting and later cocaine self-administration.

METHODS

The dose-dependent effects of vehicle, MPH (0.5, 1, and 2 mg/kg p.o.) or guanfacine (0.003, 0.03, and 0.3 mg/kg, i.p.) on discounting were determined in male and female Sprague-Dawley rats beginning at postnatal day (P)20. At P90, the amount, motivation, and sensitivity to cocaine following early drug exposure were determined with self-administration.

RESULTS

Guanfacine, but not MPH, significantly reduced weight by 22.9 ± 4.6% in females. MPH dose dependently decreased delay discounting in both juvenile males and females, while guanfacine was only effective in males. Discounting was associated with cocaine self-administration in vehicle males (R² = -0.4, P < 0.05) and self-administration was reduced by guanfacine treatment (0.3 mg/kg). Guanfacine significantly decreased cocaine sensitivity in both sexes.

CONCLUSIONS

These data suggest that MPH is effective in reducing delay discounting in both sexes. Due to both weight loss and ineffectiveness on discounting in females, guanfacine should be used only in males to reduce delay discounting and later cocaine use.

Interventions aimed at changing impulsive choice in rats: Effects of immediate and relatively long delay to reward training

A relatively strong preference for smaller-sooner rewards (SSR) over larger-later rewards (LLR) is associated with a host of maladaptive behavioral patterns. As such, the clinical implications for increasing preference for LLR are profound. There is a growing body of literature that suggests extended exposure to delayed reward may increase preference for LLR in rats. However, questions remain about the underlying mechanism driving this effect and the extent to which extended exposure to immediate rewards may decrease LLR choice. In Experiment 1, we tested effects of a differential-reinforcement-of-low-rates schedule (DRL) to increase LLR choice using a pretest/posttest design with Wistar rats as subjects. We compared this group to a group of rats exposed to a differential-reinforcement-of-high-rates schedule (DRH). The DRH intervention has never been employed in this research context, but explicitly programs an immediate response-reinforcement requirement. In Experiment 2, we tested effects of an intervention with a delay longer than those used in the delay discounting pretest and posttest. No previous research has tested effects of an intervention delay this long, relative to the delay discounting task. We compared this group to a group exposed to a delay that was part of the delay discounting pretest and posttest and to a group exposed to a traditional no-delay, fixed-ratio (FR) 2 control intervention. In both experiments, we found that exposure to delayed rewards in the intervention phase significantly increased LLR choice relative to pretest performance. These findings replicate and extend a growing body of literature showing that delay exposure increases preference for LLR. We also found significant decreases in LLR choice from pretest to posttest in the DRH and no-delay intervention groups in Experiments 1 and 2, respectively. This is the first report of such an effect and has implications for understanding and interpreting effects of delay exposure training in past and future research. Our results also suggested no relationship between improved temporal tracking of reward and increases in LLR choice as a result of delay exposure training.

Neurocognitive Pathways in Attention-Deficit/Hyperactivity Disorder and White Matter Microstructure

BACKGROUND

This study sought to identify attention-deficit/hyperactivity disorder (ADHD) abnormalities in relationships between brain white matter structure and individual differences in several types of impulsive behavior.

METHODS

Adolescents, n = 67 with ADHD combined subtype and n = 68 without ADHD, were given neuropsychological tests and underwent diffusion tensor imaging scans. Principal component analysis reduced test scores into factors representing different types of impulsive behavior. Tract-based spatial statistics quantified white matter integrity in relationship to components of impulsive behavior. ADHD versus non-ADHD differences in the strength and nature of linear relationships between regional white matter and three impulsivity components were examined using multiple regression.

RESULTS

Principal component analysis found three separate impulsivity-related factors that were interpreted as motor response inhibition, impulsive choice, and delay aversion. Relationships between regional fractional anisotropy and response inhibition or impulsive choice did not differ between ADHD and non-ADHD groups. There was a significant interaction between diagnostic group and delay aversion test performance relationships with regional fractional anisotropy. For youths without ADHD, greater anisotropy in numerous tracts predicted better delay aversion test performance. In contrast, anisotropy in regions including the corpus callosum, corona radiata, internal capsule, and corticospinal tracts had either a negative or no relationship with delay aversion test performance in ADHD.

CONCLUSIONS

The results provide additional support that different proposed etiological pathways to ADHD have discretely different neurobiological features. Large disorganization of white matter microstructure appears to contribute to reward-based ADHD pathways rather than motor inhibition.

Effects of N-methyl-d-aspartate receptor (NMDAr) uncompetitive antagonists in a delay discounting paradigm using a concurrent-chains procedure

Impulsive choice is often assessed in rodents using a delay discounting (DD) paradigm in which the delay to a large reinforcer (LR) increases across the session. This procedure allows one to test the effects of pharmacological manipulations within a single session. Because discounting is influenced by sensitivity to reinforcer magnitude (SRM) and sensitivity to delayed reinforcement (SDR), applying quantitative analyses (e.g., fitting hyperbolic function) is important for determining the precise behavioral mechanisms being altered following drug administration. One caveat to this approach is that observing increases in SMR/SDR can be difficult (e.g., most rats choose the LR when its delivery is immediate, whereas some rats may show exclusive preference for the small reinforcer [SR] when a delay on the LR is imposed). We utilized a variant of a concurrent-chains procedure in which rats (n = 8) could not show exclusive preference for either reinforcer, thus allowing one to observe increases/decreases in responding at each delay. The NMDAr antagonists MK-801 (0, 0.003, 0.01, 0.03 mg/kg), ketamine (0, 1.0, 5.0, 10.0 mg/kg), and memantine (0, 2.5, 5.0, 7.5 mg/kg) were administered following baseline training because this receptor has recently been implicated in DD. MK-801 (0.03 mg/kg) decreased SRM and SDR. Memantine (7.5 mg/kg) decreased SRM only. These results show that this variant of the concurrent-chains procedure can be used to study the effects of pharmacological manipulations on distinct aspects of DD.

5-HT3 antagonists decrease discounting rate without affecting sensitivity to reward magnitude in the delay discounting task in mice

RATIONALE

Impulsive choice has often been evaluated in rodents according to the proportion of choices for the delayed large magnitude reinforcer (%large choice) in a delay-discounting task (DDT). However, because %large choice is influenced by both sensitivity to reinforcer magnitude and sensitivity to delayed reinforcement (i.e., discounting rate), distinctively evaluating such discounting parameters represents a critical issue demanding methods to determine each parameter in rats. The serotonin (5-HT) system is well known to be involved in impulsive choice; nevertheless, only a few studies have distinguished discounting parameters and investigated how 5-HT modulators affect discounting rate.

OBJECTIVE

Here, we performed a discounting parameter analysis in mice and examined the effects of various 5-HT modulators on discounting rate.

METHODS

We set up DDTs with different delay schedules to determine which schedule could address delay-discounting rates in mice. We examined the effect of the following drugs on impulsive choice: a 5-HT reuptake inhibitor (paroxetine), a 5-HT_1A receptor agonist (8-OH-DPAT), and two 5-HT₃ receptor antagonists (granisetron and ondansetron).

RESULTS

Mice showed typical delay discounting at the shorter delay schedules (up to 4 s delay). The %large choice under shorter, but not longer, schedules followed an exponential function and allowed us to derive discounting rates. We selected a DDT with a 4-s delay schedule for further experiments. Granisetron and ondansetron, but not paroxetine or 8-OH-DPAT, decreased discounting rates without affecting sensitivity to reinforcer magnitude.

CONCLUSION

We found that a method to calculate discounting rates in rats is also applicable to mouse models. We also provided evidence that 5-HT₃ antagonism controls impulsive choice in mice.

Females in the forefront: time-based intervention effects on impulsive choice and interval timing in female rats

Impulsive choice has been implicated in substance abuse, gambling, obesity, and other maladaptive behaviors. Deficits in interval timing may increase impulsive choices, and therefore, could serve as an avenue through which suboptimal impulsive choices can be moderated. Temporal interventions have successfully attenuated impulsive choices in male rats, but the efficacy of a temporal intervention has yet to be assessed in female rats. As such, this experiment examined timing and choice behavior in female rats, and evaluated the ability of a temporal intervention to mitigate impulsive choice behavior. The temporal intervention administered in this study was successful in reducing impulsive choices compared to a control group. Results of a temporal bisection task indicated that the temporal intervention increased long responses at the shorter durations. Further, results from the peak trials within the choice task combined with the progressive interval task suggest that the intervention increased sensitivity to delay and enhanced timing confidence. Overall, these results indicate that a temporal intervention can be a successful avenue for reducing impulsive choice behavior in female rats, and could contribute to the development of behavioral interventions to prevent impulsive choice and maladaptive behaviors that can be applied to both sexes.

Momentary assessment of impulsive choice and impulsive action: Reliability, stability, and correlates

Impulsivity is associated with substance use, including tobacco use. The degree to which impulsivity fluctuates over time within persons, and the degree to which such intra-individual changes can be measured reliably and validly in ambulatory assessments is not known, however. The current study evaluated two novel ambulatory measures of impulsive choice and impulsive action. Impulsive choice was measured with an eight-item delay discounting task designed to estimate the subjective value of delayed monetary rewards. Impulsive action was measured with a two-minute performance test to assess behavioral disinhibition (the inability to inhibit a motor response when signaled that such a response will not be rewarded). Valid data on impulsive choice were collected at 70% of scheduled reports and valid data on impulsive action were collected on 55% of scheduled reports, on average. Impulsive choice and action data were not normally distributed, but models of relations of these measures with within- and between-person covariates were robust across distributional assumptions. Intra-class correlations were substantial for both impulsive choice and action measures. Between persons, random intercepts in impulsive choice and action were significantly related to laboratory levels of their respective facets of impulsivity, but not self-reported or other facets of impulsivity. Validity of the ambulatory measures is supported by associations between abstinence from smoking and increased impulsivity, but challenged by an association between strong temptations to smoke and reduced impulsive choice. Results suggest that meaningful variance in impulsive choice and action can be captured using ambulatory methods, but that additional measure refinement is needed.

Reducing impulsive choice: V. The role of timing in delay-exposure training

Impulsive decision-making is common in addiction-related disorders, with some research suggesting it plays a causal role in their development. Therefore, reducing impulsive decision-making may prevent or reduce addiction-related behaviors. Recent research shows that prolonged experience with response-contingent delayed reward (delay exposure [DE] training) reduces impulsive choice in rats, but it is unclear what behavioral mechanisms underlie this effect. The present study evaluated whether improvements in interval timing mediate the effects of DE training on impulsive choice. Thirty-nine Long-Evans rats were randomly assigned to groups completing DE, immediacy-, or no-exposure training, followed by impulsive-choice and timing tasks (temporal bisection). Despite replicating the DE effect on impulsive choice, timing accuracy and precision were unaffected by DE training and unrelated to impulsive choice. The present findings did not replicate previous reports that timing precision and impulsive choice are related, which may be due to between-laboratory differences in impulsive choice tasks. Continued research to identify candidate behavioral mechanisms of DE training may assist in improving training efficacy and facilitating translational efforts.

Effects of the NMDA receptor antagonists dizocilpine and Ro 63-1908 on delay-discounting and risky decision-making in a gambling task

Previous studies demonstrated that NMDA receptor antagonists such as dizocilpine (MK801) and the GluN2B NMDA antagonist Ro 63-1908 promote impulsive action (motor impulsivity). The effects of these treatments on impulsive choice and decision-making is less well characterized. Two experiments were undertaken. In the first experiment, given evidence for delay order as a factor in choice selection, the effect of dizocilpine was examined in a delay discounting task with separate groups of male Long-Evans rats trained to a schedule of either ascending (i.e. 0-40 s), or descending delays (i.e. 40-0 s). Under the ascending-delay schedule, dizocilpine (0.03-0.06 mg/kg SC) reduced discounting, yet on the descending-delay schedule discounting was increased. Subgrouping rats according to discounting rate under vehicle pretreatment were consistent with a treatment-induced choice perseveration. In a second experiment, male Long-Evans rats were trained to a gambling task (rGT). Neither dizocilpine (0.01-0.06 mg/kg SC) nor Ro 63-1908 (0.1-1 mg/kg SC) shifted choice from the advantageous to the disadvantageous options. However dizocilpine, and marginally Ro 63-1908, increased choice of the least risky, but suboptimal option. This effect was most evident in rats that initially preferred the disadvantageous options. Consistent with previous studies, both treatments increased measures of motor impulsivity. These results demonstrate that dizocilpine has effects on discounting dependent on delay order and likely reflective of perseveration. On the rGT task, neither dizocilpine nor Ro 63-1908 promoted risky choice, yet both NMDA receptor antagonists increased impulsive action.

Durability and generalizability of time-based intervention effects on impulsive choice in rats

Impulsive choice involves choosing a smaller-sooner (SS) reward over a larger-later (LL) reward. Due to the importance of timing processes in impulsive choice, time-based interventions have been developed to decrease impulsive choice. The present set of experiments assessed the durability and generalizability of time-based interventions. Experiment 1 assessed fixed interval (FI) or variable interval (VI) intervention efficacy over 9 months. The FI intervention decreased impulsive choice, and this effect persisted over time, but the VI intervention effects were only apparent when tested immediately after the intervention. Experiment 2 examined the generalizability of the FI and VI interventions on choice tasks manipulating the SS delay, LL delay, or LL magnitude. The FI intervention decreased sensitivity to delay, promoting LL choices in both delay tasks, but the VI intervention only altered choices when manipulating the SS delay. Experiment 3 further examined the FI intervention effects on tasks that manipulated the LL delay or magnitude immediately following the intervention. The intervention decreased sensitivity to both delay and magnitude. The experiments indicate that the FI intervention is effective at decreasing impulsive choice behavior for an extended period across changing delays and magnitudes, suggesting a relatively broad effect on choice behavior.

Dissecting drug effects in preclinical models of impulsive choice: emphasis on glutamatergic compounds

RATIONALE

Impulsive choice is often measured with delay discounting paradigms. Because there are multiple discounting procedures, as well as different statistical analyses that can be applied to data generated from these paradigms, there are some inconsistencies in the literature regarding drug effects on impulsive choice.

OBJECTIVES

The goal of the current paper is to review the methodological and analytic approaches used to measure discounting and to discuss how these differences can account for differential drug effects observed across studies.

RESULTS

Because some procedures/analyses use a single data point as the dependent variable, changes in this value following pharmacological treatment may be interpreted as alterations in sensitivity to delayed reinforcement, but when other procedures/analyses are used, no changes in behavior are observed. Even when multiple data points are included, some studies show that the statistical analysis (e.g., ANOVA on raw proportion of responses vs. using hyperbolic/exponential functions) can lead to different interpretations. Finally, procedural differences (e.g., delay presentation order, signaling the delay to reinforcement, etc.) in the same discounting paradigm can alter how drugs affect sensitivity to delayed reinforcement.

CONCLUSIONS

Future studies should utilize paradigms that allow one to observe alterations in responding at each delay (e.g., concurrent-chains schedules). Concerning statistical analyses, using parameter estimates derived from nonlinear functions or incorporating the generalized matching law can allow one to determine if drugs affect sensitivity to delayed reinforcement or impair discrimination of the large and small magnitude reinforcers. Using these approaches can help further our understanding of the neurochemical underpinnings of delay discounting.

Pair Housing Alters Delay Discounting in Lewis and Fischer 344 Rats

Impulsive choice underlies several psychological disorders and can be assessed in laboratory rats using delay-discounting tasks, in which choice is for either one food pellet immediately or three food pellets after a delay. Choice for the smaller, immediate reinforcer is considered the impulsive choice. Lewis (LEW) and Fischer 344 (F344) rats differ in the number of impulsive choices made during this task when singly housed, with LEW choosing the impulsive option more often. Due to increasing recommendations to provide environmental enrichment as a component of animal-husbandry practices, a systematic replication of two previous studies was conducted using pair-housed LEW and F344. Delay discounting was assessed with pair-housed LEW and F344 and compared to previous data from singly housed LEW and F344 collected from the same laboratory. Results showed that differences in impulsive choice between the two strains were attenuated with pair housing. The main result driving this change appears to be an increase in impulsive choice in pair-housed F344 relative to singly housed F344.

Nucleus accumbens core lesions induce sub-optimal choice and reduce sensitivity to magnitude and delay in impulsive choice tasks

The nucleus accumbens core (NAc) has long been recognized as an important contributor to the computation of reward value that is critical for impulsive choice behavior. Impulsive choice refers to choosing a smaller-sooner (SS) over a larger-later (LL) reward when the LL is more optimal in terms of the rate of reward delivery. Two experiments examined the role of the NAc in impulsive choice and its component processes of delay and magnitude processing. Experiment 1 delivered an impulsive choice task with manipulations of LL reward magnitude, followed by a reward magnitude discrimination task. Experiment 2 tested impulsive choice under manipulations of LL delay, followed by temporal bisection and progressive interval tasks. NAc lesions, in comparison to sham control lesions, produced suboptimal preferences that resulted in lower reward earning rates, and led to reduced sensitivity to magnitude and delay within the impulsive choice task. The secondary tasks revealed intact reward magnitude and delay discrimination abilities, but the lesion rats persisted in responding more as the progressive interval increased during the session. The results suggest that the NAc is most critical for demonstrating good sensitivity to magnitude and delay, and adjusting behavior accordingly. Ultimately, the NAc lesions induced suboptimal choice behavior rather than simply promoting impulsive choice, suggesting that an intact NAc is necessary for optimal decision making.

Telomere length in alcohol dependence: A role for impulsive choice and childhood maltreatment

Telomere shortening, a marker of cellular aging, has been considered to be linked with psychosocial stress as well as with chronic alcohol consumption, possibly mediated by oxidative stress and inflammatory response. Recent findings suggested that early life adversity on telomere dynamics may be related to impulsive choice. To further our understanding of the association of impulsive choice and childhood trauma on telomere length, we examined whether delayed discounting and childhood trauma or their interaction is related to leukocyte telomere length, while controlling for multiple potential confounding variables, in patients with alcohol dependence who are considered to have higher impulsive choice and shorter telomere length. We recruited 253 male patients with chronic alcohol dependence. All participants performed the delay discounting task, and the area under curve was used as a measure of delay discounting. Steeper delay discounting represents more impulsive choices. The modified Parent-Child Conflict Tactics Scale was used to measure childhood maltreatment. In addition, confounding factors, including socio-demographic characteristics, the Alcohol Use Disorders Identification Test, the Buss-Perry Aggression Questionnaire, the Resilience Quotient, the Beck Depression Inventory, and the Beck Anxiety Inventory, were also assessed. Hierarchical regression analyses showed a significant main effect of delay discounting (β=0.161, t=2.640, p=0.009), and an interaction effect between delay discounting and childhood maltreatment on leukocyte telomere length (β=0.173, t=2.138, p=0.034). In subsequent analyses stratified by childhood maltreatment, patients with alcohol dependence and high childhood trauma showed a significant relationship between delay discounting and leukocyte telomere length (β=0.279, t=3.183, p=0.002), while those with low trauma showed no association between them. Our findings suggest that higher impulsive choice is associated with shorter telomere length, and childhood trauma may exert a moderating effect in the relationship between impulsive choice and telomere length.

Differential effects of social and novelty enrichment on individual differences in impulsivity and behavioral flexibility

Early life experience profoundly impacts behavior and cognitive functions in rats. The present study investigated how the presence of conspecifics and/or novel objects, could independently influence individual differences in impulsivity and behavioral flexibility. Twenty-four rats were reared in an isolated condition, an isolated condition with a novel object, a pair-housed social condition, or a pair-housed social condition with a novel object. The rats were then tested on an impulsive choice task, a behavioral flexibility task, and an impulsive action task. Novelty enrichment produced an overall increase in impulsive choice, while social enrichment decreased impulsive choice in the absence of novelty enrichment and also produced an overall increase in impulsive action. In the behavioral flexibility task, social enrichment increased regressive errors, whereas both social and novelty enrichment reduced never-reinforced errors. Individual differences analyses indicated a significant relationship between performance in the behavioral flexibility and impulsive action tasks, which may reflect a common psychological correlate of action inhibition. Moreover, there was a relationship between delay sensitivity in the impulsive choice task and performance on the DRL and behavioral flexibility tasks, suggesting a dual role for timing and inhibitory processes in driving the interrelationship between these tasks. Overall, these results indicate that social and novelty enrichment produce distinct effects on impulsivity and adaptability, suggesting the need to parse out the different elements of enrichment in future studies. Further research is warranted to better understand how individual differences in sensitivity to enrichment affect individuals' interactions with and the resulting consequences of the rearing environment.

A working memory task reveals different patterns of impulsivity in male and female college students

Impulsivity is an important personality trait that affects people's lives every day. Because of the complicated structures and various measurements of impulsivity, the conclusion that whether there were gender differences on impulsivity remained controversial. In our study, we used delay discounting and probability discounting to measure impulsive choice and employed stop signal reaction time task (SSRT) to measure impulsive action within the same subjects. No inherent gender differences were found, either on impulsive choice or on impulsive action. However, after adding a working memory (WM) task, we found an interaction between gender and WM: males made more impulsive choices in the delay discounting task, but females remained no change, and this only occurred when the reward amount was large; in the SSRT, the males showed better inhibitory control under the WM load condition, but females did not. These results demonstrate that gender difference does not exist on impulsivity biologically, but the increased working memory load could affect the gender's sense of delay gratification and the ability of inhibitory control differently. These findings can contribute to the studies of gender differences on impulsivity and draw attention to the need for further research that gender factors should be considered more carefully when exploring the effects of working memory.

Effects of N-methyl-D-aspartate receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure

RATIONALE

The N-methyl-D-aspartate (NMDA) receptor has been recently identified as an important mediator of impulsive choice, as assessed in delay discounting. Although discounting is independently influenced by sensitivity to reinforcer magnitude and delayed reinforcement, few studies have examined how NMDA receptor ligands differentially affect these parameters.

OBJECTIVES

The current study examined the effects of various NMDA receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure.

METHODS

Following behavioral training, rats received treatments of the following NMDA receptor ligands: the uncompetitive antagonists ketamine (0, 1.0, 5.0, or 10.0 mg/kg; i.p.), MK-801 (0, 0.003, 0.01, or 0.03 mg/kg; s.c.), and memantine (0, 2.5, 5.0, or 10.0 mg/kg; i.p.), the competitive antagonist CGS 19755 (0, 5.0, 10.0, or 20.0 mg/kg; s.c.), the non-competitive NR2B subunit-selective antagonist ifenprodil (0, 1.0, 3.0, or 10.0 mg/kg; i.p), and the partial agonist D-cycloserine (0, 3.25, 15.0, or 30.0 mg/kg; s.c.).

RESULTS

When an exponential model was used to describe discounting, CGS 19755 (5.0 mg/kg) increased impulsive choice without altering sensitivity to reinforcer magnitude. Conversely, ketamine (10.0 mg/kg), memantine (5.0 mg/kg), and ifenprodil (10.0 mg/kg) decreased sensitivity to reinforcer magnitude without altering impulsive choice. MK-801 and D-cycloserine did not alter delay-discounting performance, although two-way ANOVA analyses indicated D-cycloserine (15.0 mg/kg) decreased impulsive choice.

CONCLUSIONS

The behavioral changes observed in delay discounting following administration of NMDA receptor antagonists do not always reflect an alteration in impulsive choice. These results emphasize the utility in employing quantitative methods to assess drug effects in delay discounting.

Effects of Group I metabotropic glutamate receptor antagonists on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting task in rats: Contribution of delay presentation order

Metabotropic glutamate receptor 1 (mGluR₁) blockade has been shown to decrease impulsive choice, as measured in delay discounting. However, several variables are known to influence an animal's discounting, including sensitivity to delayed reinforcement and sensitivity to reinforcer magnitude. The goal of this experiment was to determine the effects of mGluR₁, as well as mGluR₅, antagonism on these parameters. Forty Sprague Dawley rats were trained in delay discounting, in which consistently choosing a small, immediate reward reflects impulsive choice. For half of the rats, the delay to the large reinforcer increased across blocks of trials, whereas the delay decreased across the session for half of the rats. Following training, half of the rats received injections of the mGluR₁ antagonist JNJ 16259685 (JNJ; 0, 0.1, 0.3, or 1.0mg/kg; i.p), and half received injections of the mGluR₅ antagonist MPEP (0, 1.0, 3.0, or 10.0mg/kg; i.p.). Administration of JNJ increased sensitivity to delayed reinforcement (i.e., promoted impulsive choice), regardless of which schedule was used. However, the order in which delays were presented modulated the effects of JNJ on sensitivity to reinforcer magnitude. Specifically, JNJ decreased sensitivity to reinforcer magnitude in rats trained on the descending schedule only. MPEP did not alter sensitivity to reinforcer magnitude or sensitivity to delayed reinforcement. These results show that mGluR₁ is an important mediator of impulsive choice, and they provide further evidence that delay order presentation is an important variable that influences drug effects in delay discounting.

Effects of nucleus accumbens amphetamine administration on performance in a delay discounting task

Chronic administration of cocaine can cause pronounced and enduring cognitive alterations such as increases in impulsive choice. Chronic cocaine can also result in enhanced dopamine (DA) release in the nucleus accumbens (NAc) in response to reward-related cues. It is possible that this enhanced DA release in the NAc is a mechanism by which cocaine increases impulsive choice. To date, however, the specific role of DA in the NAc in impulsive choice is unclear. To begin to address this, rats received acute microinjections of the indirect DA agonist amphetamine directly into the NAc prior to testing in a delay discounting task in which rats chose between a small, immediate and a large, delayed food reward. When delays to the large reward increased within test sessions, amphetamine increased choice of the large reward. When delays decreased within test sessions, however, amphetamine decreased choice of the large reward. These findings suggest that, rather than specifically mediating impulsive choice, DA neurotransmission in the NAc is necessary for flexible adaptation of choice strategies in the presence of shifting reward contingencies. These results further indicate that enhancements in NAc DA release likely do not account for lasting increases in impulsive choice caused by chronic cocaine.

Adolescent methylmercury exposure affects choice and delay discounting in mice

The developing fetus is vulnerable to low-level exposure to methylmercury (MeHg), an environmental neurotoxicant, but the consequences of exposure during the adolescent period remain virtually unknown. The current experiments were designed to assess the effects of low-level MeHg exposure during adolescence on delay discounting, preference for small, immediate reinforcers over large, delayed ones, using a mouse model. Thirty-six male C57BL/6n mice were exposed to 0, 0.3, or 3.0ppm mercury (as MeHg) via drinking water from postnatal day 21 through 59, encompassing the murine adolescent period. As adults, mice lever pressed for a 0.01-cc droplet of milk solution delivered immediately or four 0.01-cc droplets delivered after a delay. Delays ranged from 1.26 to 70.79s, and all were presented within a session. A model based on the Generalized Matching Law indicated that sensitivity to reinforcer magnitude was lower for MeHg-exposed mice relative to controls, indicating that responding in MeHg-exposed mice was relatively indifferent to the larger reinforcer. Sensitivity to reinforcer delay was reduced (delay discounting was decreased) in the 0.3-ppm group, but not in the 3.0-ppm group, compared to controls. Adolescence is a developmental period during which the brain and behavior may be vulnerable to MeHg exposure. As with gestational MeHg exposure, the effects are reflected in the impact of reinforcing stimuli.

Preventative treatment in an animal model of ADHD: Behavioral and biochemical effects of methylphenidate and its interactions with ovarian hormones in female rats

Clinical and preclinical studies on attention deficit hyperactivity disorder (ADHD) show that juvenile males that are exposed to methylphenidate (MPH) show reduced risk for substance use later in life. In contrast, little is known about whether females have the same enduring treatment response to stimulants and how gonadal hormones influence their behavior later in life. Females received either a sham or 6-hydroxydopamine (6-OHDA) microinjection in the prefrontal cortex (PFC) at postnatal day (P)10. Subjects were then treated with Vehicle or MPH (2mg/kg, p.o.) between P20-35 and tested during late adolescence/young adulthood (P60); half of these subjects underwent ovariectomy at P55 to determine hormonal influences. Females with 6-OHDA were depleted of PFC dopamine by 61% and demonstrated increased impulsive choice (delayed discounting) and preferences for cocaine-associated environments relative to control females. Both MPH and ovariectomy reduced impulsive choice and cocaine preferences in 6-OHDA females, but had no enduring effect in Sham females. Ovariectomy itself did not significantly affect impulsivity. Juvenile MPH interacted strongly with 6-OHDA to increase D4, D5, Alpha-1A, Alpha-2A, and 5-HT-1A mRNA receptor expression in the PFC. MPH alone effected D1 mRNA, while 6-OHDA increased BDNF; all markers were decreased by ovariectomy. Together, these data suggest that 6-OHDA changes in dopamine are not only relevant for ADHD-like behaviors, but their long-term modulation by treatment and the influence of cyclical differences in menstrual cycle.

Sex differences in the reduction of impulsive choice (delay discounting) for cocaine in rats with atomoxetine and progesterone

RATIONALE

Impulsive choice, or an inability to delay immediate gratification, has been strongly linked to the development and persistence of drug abuse. Indeed, delaying drug use itself may underlie drug addiction and relapse. Thus, employing treatments that are efficacious in reducing impulsive choice (atomoxetine; ATO) or drug-seeking behavior (progesterone; PRO) may be an effective means of treating drug addiction.

OBJECTIVE

The current study assessed sex differences in the effects of PRO, ATO, and their combination in a delay discounting paradigm for cocaine and for sucrose pellets.

METHOD

Male and female rats chose between a small-immediate or a large-delayed (0, 7.5, 15, 30, 60 s) outcome in an impulsive choice procedure for sucrose pellets (1 vs. 3 pellets) or for iv cocaine infusions (0.3 vs. 0.9 mg/kg). Following baseline assessment of impulsive choice, rats received daily treatment of vehicle (VEH), PRO (0.5 mg/kg), ATO (1.5 mg/kg), or a combination (PRO + ATO) until a second assessment of impulsive choice was determined.

RESULTS

Compared to the VEH group, females were less impulsive for cocaine following PRO or the PRO + ATO combined treatment, whereas males were less impulsive for cocaine following ATO. No treatment effects were observed on impulsive choice for sucrose pellets.

CONCLUSIONS

The present results indicate that impulsive choice for cocaine is reduced by PRO in females and by ATO in males. These findings suggest both treatments may be an effective intervention in treating cocaine abuse, but that their effectiveness differs by sex.

Impulsive choice and pre-exposure to delays: III. Four-month test-retest outcomes in male wistar rats

Delay discounting describes the tendency for organisms to devalue outcomes because they are delayed. Robust, positive correlations exist between excessive delay discounting and many maladaptive behaviors (e.g., substance abuse, obesity). Several studies have demonstrated that delay discounting can be reduced and this may hold promise for improving treatment outcomes. One method of reducing delay discounting provides rats with extended training with delayed reinforcement (i.e., delay-exposure training) and this significantly reduces impulsive choices, relative to rats trained with an equal number of immediate-reinforcement sessions (i.e., immediate-exposure training). To evaluate the stability of this effect, 12 weanling male Wistar rats were randomly assigned to receive either delay-exposure or immediate-exposure training for 120 sessions. Impulsive choice was assessed using an increasing-delay procedure immediately following training and 120 days after completion of the initial assessment. Delay-exposed rats discounted delayed food rewards significantly less than immediate-exposed rats in the initial assessment and the reassessment conducted 120 days later. These results are encouraging as they suggest that the effects of delay-exposure training are robust to the passage of time and intervening experience.

The effects of physical activity on impulsive choice: Influence of sensitivity to reinforcement amount and delay

Impulsive choice is a diagnostic feature and/or complicating factor for several psychological disorders and may be examined in the laboratory using delay-discounting procedures. Recent investigators have proposed using quantitative measures of analysis to examine the behavioral processes contributing to impulsive choice. The purpose of this study was to examine the effects of physical activity (i.e., wheel running) on impulsive choice in a single-response, discrete-trial procedure using two quantitative methods of analysis. To this end, rats were assigned to physical activity or sedentary groups and trained to respond in a delay-discounting procedure. In this procedure, one lever always produced one food pellet immediately, whereas a second lever produced three food pellets after a 0, 10, 20, 40, or 80-s delay. Estimates of sensitivity to reinforcement amount and sensitivity to reinforcement delay were determined using (1) a simple linear analysis and (2) an analysis of logarithmically transformed response ratios. Both analyses revealed that physical activity decreased sensitivity to reinforcement amount and sensitivity to reinforcement delay. These findings indicate that (1) physical activity has significant but functionally opposing effects on the behavioral processes that contribute to impulsive choice and (2) both quantitative methods of analysis are appropriate for use in single-response, discrete-trial procedures.

Differential effects of co-administration of oxotremorine with SCH 23390 on impulsive choice in high-impulsive rats and low-impulsive rats

The effect of acetylcholine on impulsive choice is thought to be due to interactions between cholinergic and dopaminergic systems, but this hypothesis has not been proven. This study investigated whether D1-like receptors were involved in the effects of the muscarinic cholinergic agonist oxotremorine on impulsive choice in high-impulsive rats (HI rats, n=8) and low-impulsive rats (LI rats, n=8) characterized by basal levels of impulsive choice in a delay-discounting task. The results revealed that oxotremorine (0.05mg/kg) significantly increased the choice of the large reinforcer in HI rats, whereas decreased the choice of the large reinforcer in LI rats. The D1-like antagonist SCH 23390 produced significant reductions in the large-reinforcer choice in HI rats (0.01mg/kg) and LI rats (0.005, 0.0075, and 0.01mg/kg). SCH 23390 significantly inhibited the increase in the choice of the large reinforcer induced by oxotremorine (0.05mg/kg) in HI rats at doses of 0.005 and 0.0075mg/kg, but enhanced the effect of oxotremorine in LI rats only at the dose of 0.0075mg/kg. These findings suggested that D1-like receptors might be involved in the differential effects of oxotremorine on impulsive choice between HI rats and LI rats.

Differential effects of the pharmacological stressor yohimbine on impulsive decision making and response inhibition

RATIONALE

High levels of impulsivity have been associated with psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and substance abuse. In addition, acute stress is known to exacerbate many psychiatric symptoms in impulse control disorders.

OBJECTIVES

The purpose of the current study was to investigate the acute effects of the pharmacological stressor yohimbine on response inhibition and impulsive choice.

METHODS

A group of male rats (n = 12) was trained in the delayed reward task (DRT) to assess impulsive choice. A separate group (n = 10) was trained in the stop-signal task (SST) to measure response inhibition. Upon stable responding, the effects of yohimbine (0, 1.25, 2.5, and 5 mg/kg i.p.) were tested in a Latin square design.

RESULTS

Acute yohimbine significantly increased the preference for the large and delayed reinforcer in the DRT, indicating a decrease in impulsive choice. On the contrary, the effect size of 1.25 mg/kg yohimbine on stop-signal reaction times correlated negatively with baseline performance, suggesting a baseline-dependent effect on response inhibition as measured in the SST.

CONCLUSIONS

The current data suggest that the effects of the pharmacological stressor yohimbine on impulse control strongly depend on the type of impulsive behavior. Pharmacological stress decreased impulsive decision making, an observation that is in line with previously published rodent studies. By contrast, the lowest dose of yohimbine revealed a baseline-dependent effect on response inhibition. As such, the effects of yohimbine are largely comparable to the effects of psychostimulants on impulsivity and may support the notion of cross sensitization of stress and psychostimulants.

Sex differences in impulsive action and impulsive choice

Here, we review the evidence for sex differences in behavioral measures of impulsivity for both humans and laboratory animals. We focus on two specific components of impulsivity: impulsive action (i.e., difficulty inhibiting a prepotent response) and impulsive choice (i.e., difficulty delaying gratification). Sex differences appear to exist on these measures, but the direction and magnitude of the differences vary. In laboratory animals, impulsive action is typically greater in males than females, whereas impulsive choice is typically greater in females. In humans, women discount more steeply than men, but sex differences on measures of impulsive action depend on tasks and subject samples. We discuss implications of these findings as they relate to drug addiction. We also point out the major gaps in this research to date, including the lack of studies designed specifically to examine sex differences in behavioral impulsivity, and the lack of consideration of menstrual or estrous phase or sex hormone levels in the studies.

Recent Translational Findings on Impulsivity in Relation to Drug Abuse

Impulsive behavior is strongly implicated in drug abuse, as both a cause and a consequence of drug use. To understand how impulsive behaviors lead to and result from drug use, translational evidence from both human and non-human animal studies is needed. Here, we review recent (2009 or later) studies that have investigated two major components of impulsive behavior, inhibitory control and impulsive choice, across preclinical and clinical studies. We concentrate on the stop-signal task as the measure of inhibitory control and delay discounting as the measure of impulsive choice. Consistent with previous reports, recent studies show greater impulsive behavior in drug users compared with non-users. Additionally, new evidence supports the prospective role of impulsive behavior in drug abuse, and has begun to identify the neurobiological mechanisms underlying impulsive behavior. We focus on the commonalities and differences in findings between preclinical and clinical studies, and suggest future directions for translational research.

Partial inactivation of nucleus accumbens core decreases delay discounting in rats without affecting sensitivity to delay or magnitude

Increased preference for smaller, sooner rewards (delay discounting) is associated with several behavioral disorders, including ADHD and substance use disorders. However, delay discounting is a complex cognitive process and the relationship is unclear between the pathophysiology of the disorders and the component processes underlying delay discounting, including sensitivity to reinforcer delay and sensitivity to reinforcer magnitude. To investigate these processes, male Long Evans rats were trained in one of three tasks measuring sensitivity to delay, sensitivity to magnitude, or both (typical delay discounting task). After learning the task, animals were implanted with bilateral cannulae into either the nucleus accumbens core (AcbC) or the lateral orbitofrontal cortex (lOFC), both of which have been implicated in delay discounting. Upon recovering from the surgery, a baclofen/muscimol cocktail was infused to temporarily inactivate each of these two regions and task performance was assessed. Unlike previous studies showing that lesions of the AcbC increased delay discounting, partial inactivation of the AcbC decreased delay discounting, although it had no effects on the tasks independently assessing either sensitivity to delay or magnitude. The effects of AcbC inactivation were larger in animals that had low levels of delay discounting at baseline. Inactivation of the lOFC had no effects on behavior in any task. These findings suggest that the AcbC may act to promote impulsive choice in individuals with low impulsivity. Furthermore, the data suggest that the AcbC is able to modulate delay and magnitude sensitivity together, but not either of the two in isolation.