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

Methylphenidate, but not citalopram, decreases impulsive choice in rats performing a temporal discounting task

Introduction

Drugs targeting monoamine systems remain the most common treatment for disorders with impulse control impairments. There is a body of literature suggesting that drugs affecting serotonin reuptake and dopamine reuptake can modulate distinct aspects of impulsivity - though such tests are often performed using distinct behavioral tasks prohibiting easy comparisons.

Methods

Here, we directly compare pharmacologic agents that affect dopamine (methylphenidate) vs serotonin (citalopram) manipulations on choice impulsivity in a temporal discounting task where rats could choose between a small, immediate reward or a large reward delayed at either 2 or 10s. In control conditions, rats preferred the large reward at a small (2s) delay and discounted the large reward at a long (10s) delay.

Results

Methylphenidate, a dopamine transport inhibitor that blocks reuptake of dopamine, dose-dependently increased large reward preference in the long delay (10s) block. Citalopram, a selective serotonin reuptake inhibitor, had no effect on temporal discounting behavior. Impulsive behavior on the temporal discounting task was at least partially mediated by the nucleus accumbens shell. Bilateral lesions to the nucleus accumbens shell reduced choice impulsivity during the long delay (10s) block. Following lesions, methylphenidate did not impact impulsivity.

Discussion

Our results suggest that striatal dopaminergic systems modulate choice impulsivity via actions within the nucleus accumbens shell, whereas serotonin systems may regulate different aspects of behavioral inhibition/impulsivity.

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.

Behavioral mechanisms of oxycodone's effects in female and male rats: Reinforcement delay and impulsive choice

μ-Opioid agonists (e.g., morphine) typically increase impulsive choice, which has been interpreted as an opioid-induced increase in sensitivity to reinforcement delay. Relatively little research has been done with opioids other than morphine (e.g., oxycodone), or on sex differences in opioid effects, on impulsive choice. The present study investigated the effects of acute (0.1-1.0 mg/kg) and chronic (1.0 mg/kg twice/day) administration of oxycodone on choice controlled by reinforcement delay, a primary mechanism implicated in impulsive choice, in female and male rats. Rats responded under a concurrent-chains procedure designed to quantify the effects of reinforcement delay on choice within each session. For both sexes, choice was sensitive to delay under this procedure. Sensitivity to delay under baseline was slightly higher for males than females, suggesting more impulsive choice with males. When given acutely, intermediate and higher doses of oxycodone decreased sensitivity to delay; this effect was larger and more reliable in males than females. When given chronically, sex differences were also observed: tolerance developed to the sensitivity-decreasing effects in females, whereas sensitization developed in males. These data suggest that reinforcement delay may play an important role in sex differences in impulsive choice, as well as in the effects of acute and chronic administration of opioids in impulsive choice. However, drug-induced changes in impulsive choice could be related to at least two potential behavioral mechanisms: reinforcement delay and/or reinforcement magnitude. Effects of oxycodone on sensitivity to reinforcement magnitude remain to be fully characterized. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Selective chemogenetic inactivation of corticoaccumbal projections disrupts trait choice impulsivity

Impulsive choice has enduring trait-like characteristics and is defined by preference for small immediate rewards over larger delayed ones. Importantly, it is a determining factor in the development and persistence of substance use disorder (SUD). Emerging evidence from human and animal studies suggests frontal cortical regions exert influence over striatal reward processing areas during decision-making in impulsive choice or delay discounting (DD) tasks. The goal of this study was to examine how these circuits are involved in decision-making in animals with defined trait impulsivity. To this end, we trained adolescent male rats to stable behavior on a DD procedure and then re-trained them in adulthood to assess trait-like, conserved impulsive choice across development. We then used chemogenetic tools to selectively and reversibly target corticostriatal projections during performance of the DD task. The prelimbic region of the medial prefrontal cortex (mPFC) was injected with a viral vector expressing inhibitory designer receptors exclusively activated by designer drugs (Gi-DREADD), and then mPFC projections to the nucleus accumbens core (NAc) were selectively suppressed by intra-NAc administration of the Gi-DREADD actuator clozapine-n-oxide (CNO). Inactivation of the mPFC-NAc projection elicited a robust increase in impulsive choice in rats with lower vs. higher baseline impulsivity. This demonstrates a fundamental role for mPFC afferents to the NAc during choice impulsivity and suggests that maladaptive hypofrontality may underlie decreased executive control in animals with higher levels of choice impulsivity. Results such as these may have important implications for the pathophysiology and treatment of impulse control, SUDs, and related psychiatric disorders.

Reward maximization assessed using a sequential patch depletion task in a large sample of heterogeneous stock rats

Choice behavior requires animals to evaluate both short- and long-term advantages and disadvantages of all potential alternatives. Impulsive choice is traditionally measured in laboratory tasks by utilizing delay discounting (DD), a paradigm that offers a choice between a smaller immediate reward, or a larger more delayed reward. This study tested a large sample of Heterogeneous Stock (HS) male (n = 896) and female (n = 898) rats, part of a larger genetic study, to investigate whether measures of reward maximization overlapped with traditional models of delay discounting via the patch depletion model using a Sequential Patch Depletion procedure. In this task, rats were offered a concurrent choice between two water "patches" and could elect to "stay" in the current patch or "leave" for an alternative patch. Staying in the current patch resulted in decreasing subsequent reward magnitudes, whereas the choice to leave a patch was followed by a delay and a resetting to the maximum reward magnitude. Based on the delay in a given session, different visit durations were necessary to obtain the maximum number of rewards. Visit duration may be analogous to an indifference point in traditional DD tasks. Males and females did not significantly differ on traditional measures of DD (e.g. delay gradient; AUC). When examining measures of patch utilization, females made fewer patch changes at all delays and spent more time in the patch before leaving for the alternative patch compared to males. Consistent with this, there was some evidence that females deviated from reward maximization more than males. However, when controlling for body weight, females had a higher normalized rate of reinforcement than males. Measures of reward maximization were only weakly associated with traditional DD measures and may represent distinctive underlying processes. Taken together, females performance differed from males with regard to reward maximization that were not observed utilizing traditional measures of DD, suggesting that the patch depletion model was more sensitive to modest sex differences when compared to traditional DD measures in a large sample of HS rats.

Cognitive mechanisms underlying decision making involving risk of explicit punishment in male and female rats

Individuals engage in the process of risk-based decision making on a daily basis to navigate various aspects of life. There are, however, individual differences in this form of decision making, with some individuals exhibiting preference for riskier choices (risk taking) and others exhibiting preference for safer choices (risk aversion). Recent work has shown that extremes in risk taking (e.g., excessive risk taking or risk aversion) are not only cognitive features of neuropsychiatric diseases, but may in fact predispose individuals to the development of such diseases. To better understand individual differences in risk taking, and thus the mechanisms by which they confer disease vulnerability, the current study investigated the cognitive contributions to risk taking in both males and females. Rats were first behaviorally characterized in a decision-making task involving risk of footshock punishment and then tested on a battery of cognitive behavioral assays. Individual variability in risk taking was compared with performance on these tasks. Consistent with prior work, females were more risk averse than males. With the exception of the Set-shifting Task, there were no sex differences in performance on other cognitive assays. There were, however, sex-dependent associations between risk taking and specific cognitive measures. Greater risk taking was associated with better cognitive flexibility in males whereas greater risk aversion was associated with better working memory in females. Collectively, these findings reveal that distinct cognitive mechanisms are associated with risk taking in males and females, which may account for sex differences in this form of decision making.

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.

Reward Maximization Assessed Using a Sequential Patch Depletion Task in a Large Sample of Heterogeneous Stock Rats

Choice behavior requires animals to evaluate both short- and long-term advantages and disadvantages of all potential alternatives. Impulsive choice is traditionally measured in laboratory tasks by utilizing delay discounting (DD), a paradigm that offers a choice between a smaller immediate reward, or a larger more delayed reward. This study tested a large sample of Heterogeneous Stock (HS) male (n = 896) and female (n = 898) rats, part of a larger genetic study, to investigate whether measures of reward maximization overlapped with traditional models of delay discounting via the patch depletion model using a Sequential Patch Depletion procedure. In this task, rats were offered a concurrent choice between two water "patches" and could elect to "stay" in the current patch or "leave" for an alternative patch. Staying in the current patch resulted in decreasing subsequent reward magnitudes, whereas the choice to leave a patch was followed by a delay and a resetting to the maximum reward magnitude. Based on the delay in a given session, different visit durations were necessary to obtain the maximum number of rewards. Visit duration may be analogous to an indifference point in traditional DD tasks. While differences in traditional DD measures (e.g., delay gradient) have been detected between males and females, these effects were small and inconsistent. However, when examining measures of reward maximization, females made fewer patch changes at all delays and spent more time in the patch before leaving for the alternative patch compared to males. This pattern of choice resulted in males having a higher rate of reinforcement than females. Consistent with this, there was some evidence that females deviated from the optimal more, leading to less reward. Measures of reward maximization were only weakly associated with traditional DD measures and may represent distinctive underlying processes. Taken together, females performance differed from males with regard to reward maximization that were not observed utilizing traditional measures of DD, suggesting that the patch depletion model was more sensitive to modest sex differences when compared to traditional DD measures in a large sample of HS rats.

Temporal expectations in delay of gratification

We examined how temporal expectations influence preference reversals in a delay of gratification task for rats based on a hypothesis of Rachlin (2000), who suggested that preference for a larger-later reward may shift in favor of a smaller-immediate reward as a result of changes in when that larger reward is expected. To explore Rachlin's hypothesis, we preexposed two groups of rats to the delays associated with a larger-later reinforcer from a delay of gratification task. One group experienced the delays as a function of their choices in an intertemporal choice task and the other group experienced delays yoked from the first group (independent of their behavior) in an exposure training procedure. In addition, we included a third group of rats that were not exposed to delays during preexposure training as a comparison to the other two groups. Overall, the two groups of rats that experienced delays during preexposure training tended to make fewer defection responses than the comparison group during the delay of gratification task. Consistent with Rachlin's hypothesis, our results suggest that temporal learning may influence preference reversals in a delay of gratification task, providing a number of future directions for research in this area.

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 32 s. In the second assessment, delays to the larger reward increased between-sessions, from 8, 16, 32, to 4 s. 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.

Effects of immediate-reinforcement training on delay discounting behavior in rats

Chronic exposure to delayed reinforcement has been shown to increase choice for larger, later reinforcement in a subsequent delay discounting task. In the 3 experiments presented in this paper, the opposite was tested: effects of chronic exposure to immediate reinforcement on choice in a subsequent delay discounting task. In Experiment 1, larger, later reinforcement choice was significantly reduced as a result of exposure to immediate reinforcement, compared to a maturation/handle control group, in experienced, male Lewis rats. In Experiment 2, with naive male and female Wistar rats, and Experiment 3, with naïve male Long Evans rats, the impact of exposure to immediate reinforcement was less robust, but directionally consistent with results from Experiment 1. These results align with some previous work reporting that exposure to immediate reinforcement may decrease choice for larger, later reinforcement in a delay discounting task, and/or blunt maturational increases in choice for larger, later reinforcement. These findings have implications for future research investigating experience-based interventions to manipulate delay discounting behavior. They also have clinical implications for understanding and treating disorders involving maladaptive choice.

Hazard function effects on promoting self-control in variable interval time-based interventions in rats

The present experiments investigated properties of time-based interventions used to increase self-control. Rats received impulsive-choice assessments before and after interventions that consisted of different distributions of delays to reinforcement. In Experiment 1, rats received an intervention with an increasing hazard function where delays were more evenly distributed, a decreasing hazard function where delays were mostly short, or a constant hazard function where delays were exponentially distributed. Surprisingly, rats that received the decreasing hazard function made the most self-controlled choices. Response rates during intervention trials showed that rats anticipated reinforcement based on the shape of the distributions they received. In Experiment 2, rats received an intervention with a decreasing hazard function with a steep slope or a shallow slope. Both time-based interventions increased self-control and produced similar response-rate patterns, indicating that the slope of the decreasing hazard function may not play a strong role in intervention efficacy. While this research aligns with previous literature showing that time-based interventions improved self-control, exposure to short delays produced the biggest improvements. Ultimately, exposure to short delays may increase the subjective value of the larger-later choice while occasional long delays may promote the ability to wait, which may have important implications for translational applications.

Quantifying the contribution of individual variation in timing to delay-discounting

Delay-discounting studies in neuroscience, psychology, and economics have been mostly focused on concepts of self-control, reward evaluation, and discounting. Another important relationship to consider is the link between intertemporal choice and time perception. We presented 50 college students with timing tasks on the range of seconds to minutes and intertemporal-choice tasks on both the time-scale of seconds and of days. We hypothesized that individual differences in time perception would influence decisions about short experienced delays but not long delays. While we found some evidence that individual differences in internal clock speed account for some unexplained variance between choices across time-horizons, overall our findings suggest a nominal contribution of the altered sense of time in intertemporal choice.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Testicular hormones mediate robust sex differences in impulsive choice in rats

Impairments in choosing optimally between immediate and delayed rewards are associated with numerous psychiatric disorders. Such ‘intertemporal’ choice is influenced by genetic and experiential factors; however, the contributions of biological sex are understudied and data to date are largely inconclusive. Rats were used to determine how sex and gonadal hormones influence choices between small, immediate and large, delayed rewards. Females showed markedly greater preference than males for small, immediate over large, delayed rewards (greater impulsive choice). This difference was neither due to differences in food motivation or reward magnitude perception, nor was it affected by estrous cycle. Ovariectomies did not affect choice in females, whereas orchiectomies increased impulsive choice in males. These data show that male rats exhibit less impulsive choice than females and that this difference is at least partly maintained by testicular hormones. These differences in impulsive choice could be linked to gender differences across multiple neuropsychiatric conditions.