Risk-preferring rats make worse decisions and show increased incubation of craving after cocaine self-administration

Increased risky choice during forced abstinence from fentanyl on the cued rat gambling task

RATIONALE

The use of illicit opioids has arguably never been more risky; street drug potency can be dangerously high, is often unknown to the consumer, and results in multiple daily fatalities worldwide. Furthermore, substance use disorder (SUD) is associated with increased maladaptive, risky decisions in laboratory-based gambling tasks. Animal studies can help determine whether this decision-making deficit is a cause or consequence of drug use. However, most experiments have only assessed psychostimulant drugs.

OBJECTIVES

To assess differences in decision-making strategies both before, during, and after self-administration of fentanyl in male and female Long Evans rats.

METHODS

Male and female Long Evans rats were trained to perform the rat gambling task (rGT), loosely based on the Iowa Gambling Task (IGT) used clinically, and/or self-administer fentanyl. We used the cued version of the rGT, in which sound and light stimuli signal sugar pellet rewards, as cocaine self-administration has the greatest effects on decision making in this task variant.

RESULTS

After training on the cued rGT, female rats self-administered fentanyl more readily, an effect that was most apparent in optimal decision-makers. Contrary to previous reports using cocaine self-administration, decision-making was unaffected during fentanyl self-administration training in either sex. However, risky decision-making increased throughout forced abstinence from fentanyl in males.

CONCLUSIONS

These findings complement those from human subjects, in whom preference for uncertain outcomes increased before relapse. These data highlight an abstinence-induced change in cognition that is unique to opiates as compared to psychostimulants, and which may critically contribute to the maintenance of addiction and relapse.

Methamphetamine increases risky choice in rats, but only when magnitude and probability of reinforcement are manipulated within a session

Risky choice is associated with maladaptive behaviors, particularly substance use disorders. Current animal models of risky choice are often confounded by other constructs like behavioral flexibility and suboptimal choice. The purpose of the current experiment was to determine if the psychostimulant methamphetamine, a drug whose popularity has increased in recent years, increases risky choice in an equivalent expected value (EEV) task. In the EEV task, rats are given a choice between two reinforcer alternatives that differ in magnitude and probability of delivery, but have equivalent expected value. Forty-eight Sprague Dawley rats were tested in three versions of the EEV task. In the first version of the EEV task, both reinforcer magnitude and probability were adjusted across blocks of trials for both alternatives. In the second and the third versions of the EEV task, reinforcer magnitude was held constant across each block of trials (either 1 vs. 2 pellets or 4 vs. 5 pellets). We found that male rats preferred the "riskier" option, except when reinforcer magnitudes were held constant at 4 and 5 pellets across each block of trials. Methamphetamine (0.5 mg/kg) increased preference for the risky option in both males and females, but only when both reinforcer magnitude and probability were manipulated across blocks of trials for each alternative. The current results demonstrate that both magnitude of reinforcement and probability of reinforcement interact to influence risky choice. Overall, this study provides additional support for using reinforcers with expected value to measure risky choice.

Lateral Orbitofrontal Cortex Encodes Presence of Risk and Subjective Risk Preference During Decision-Making

Adaptive decision-making requires consideration of objective risks and rewards associated with each option, as well as subjective preference for risky/safe alternatives. Inaccurate risk/reward estimations can engender excessive risk-taking, a central trait in many psychiatric disorders. The lateral orbitofrontal cortex (lOFC) has been linked to many disorders associated with excessively risky behavior and is ideally situated to mediate risky decision-making. Here, we used single-unit electrophysiology to measure neuronal activity from lOFC of freely moving rats performing in a punishment-based risky decision-making task. Subjects chose between a small, safe reward and a large reward associated with either 0% or 50% risk of concurrent punishment. lOFC activity repeatedly encoded current risk in the environment throughout the decision-making sequence, signaling risk before, during, and after a choice. In addition, lOFC encoded reward magnitude, although this information was only evident during action selection. A Random Forest classifier successfully used neural data accurately to predict the risk of punishment in any given trial, and the ability to predict choice via lOFC activity differentiated between and risk-preferring and risk-averse rats. Finally, risk preferring subjects demonstrated reduced lOFC encoding of risk and increased encoding of reward magnitude. These findings suggest lOFC may serve as a central decision-making hub in which external, environmental information converges with internal, subjective information to guide decision-making in the face of punishment risk.

Decoupling Dopamine Synthesis from Impulsive Action, Risk-Related Decision-Making, and Propensity to Cocaine Intake: A Longitudinal [18F]-FDOPA PET Study in Roman High- and Low-Avoidance Rats

Impulsive action and risk-related decision-making (RDM) are two facets of impulsivity linked to a hyperdopaminergic release in the striatum and an increased propensity to cocaine intake. We previously showed that with repeated cocaine exposure, this initial hyperdopaminergic release is blunted in impulsive animals, potentially signaling drug-induced tolerance. Whether such dopaminergic dynamics involve changes in dopamine (DA) synthesis as a function of impulsivity is currently unknown. Here, we investigated the predictive value of DA synthesis for impulsive action, RDM, and the propensity to take cocaine in a rat model of vulnerability to cocaine abuse. Additionally, we assessed the effects of cocaine intake on these variables. Rats were tested sequentially in the rat Gambling Task (rGT) and were scanned with positron emission tomography and [18F]-FDOPA to respectively assess both impulsivity facets and striatal DA synthesis before and after cocaine self-administration (SA). Our results revealed that baseline striatal levels of DA synthesis did not significantly predict impulsive action, RDM, or a greater propensity to cocaine SA in impulsive animals. Besides, we showed that impulsive action, but not RDM, predicted higher rates of cocaine taking. However, chronic cocaine exposure had no impact on DA synthesis, nor affected impulsive action and RDM. These findings indicate that the hyper-responsive DA system associated with impulsivity and a propensity for cocaine consumption, along with the reduction in this hyper-responsive DA state in impulsive animals with a history of cocaine use, might not be mediated by dynamic changes in DA synthesis.

Cocaine intake correlates with risk-taking behavior and affects estrous cycling in female Sprague-Dawley rats

Navigating complex decisions and considering their relative risks and rewards is an important cognitive ability necessary for survival. However, use of and dependence on illicit drugs can result in long-lasting changes to this risk/reward calculus in individuals with substance use disorder. Recent work has shown that chronic exposure to cocaine causes long-lasting increases in risk taking in male and female rats, but there are still significant gaps in our understanding of the relationship between cocaine use and changes in risk taking. For example, it is unclear whether the magnitude of cocaine intake dictates the extent to which risk taking is altered. To address this, male and female Sprague-Dawley rats underwent cocaine (or sucrose) self-administration and, following a period of abstinence, were trained and tested in a rodent model of risky decision making. In this behavioral task, rats made discrete-trial choices between a lever associated with a small food reward (i.e., "safe" option) and a lever associated with a larger food reward accompanied by a variable risk of footshock delivery (i.e., "risky" option). Surprisingly, and in contrast to prior work in Long-Evans rats, there were no effects of cocaine self-administration on choice of the large, risky reward (i.e., risk taking) during abstinence in males or females. There was, however, a significant relationship between cocaine intake and risk taking in female rats, with greater intake associated with greater preference for the large, risky reward. Relative to their sucrose counterparts, female rats in the cocaine group also exhibited irregular estrous cycles, characterized by prolonged estrus and/or diestrus phases. Collectively, these data suggest that there may be strain differences in the effects of cocaine on risk taking and highlight the impact that chronic cocaine exposure has on hormonal cyclicity in females. Future work will focus on understanding the neural mechanisms underlying cocaine's intake-dependent effects on risk taking in females, and whether this is directly related to cocaine-induced alterations in neuroendocrine function.

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

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

Whole-brain resting-state connectivity underlying impaired inhibitory control during early versus longer-term abstinence in cocaine addiction

Lapses in inhibitory control have been linked to relapse in human drug addiction. Evidence suggests differences in inhibitory control depending on abstinence duration, but the underlying neural mechanisms remain unknown. We hypothesized that early abstinence (2-5 days) would be characterized by the strongest impairments of inhibitory control and most wide-spread deviations in resting-state functional connectivity of brain networks, while longer-term abstinence (>30 days) would be characterized by weaker impairments as compared to healthy controls. In this laboratory-based cross-sectional study, we compared individuals with Cocaine Use Disorder (iCUD) during early (cocaine urine-positive: N = 19, iCUD+; 32% female; mean age: 46.8 years) and longer-term abstinence (cocaine urine-negative: N = 29, iCUD-; 15% female; mean age: 46.6 years) to healthy controls (N = 33; 24% female; mean age: 40.9 years). We compared the groups on inhibitory control performance (Stop-Signal Task) and, using a whole-brain graph theory analysis (638 region parcellation) of functional magnetic resonance imaging (fMRI) data, we tested for group differences in resting-state brain function (local/global efficiency). We characterized how resting-state brain function was associated with inhibitory control performance within iCUD. Inhibitory control performance was worst in the early abstinence group, and intermediate in the longer-term abstinence group, as compared to the healthy control group (P < 0.01). More recent use of cocaine (CUD+ > CUD- > healthy controls) was characterized by decreased efficiency in fronto-temporal and subcortical networks (primarily in the salience, semantic, and basal ganglia networks) and increased efficiency in visual networks. Importantly, a similar functional connectivity pattern characterized impaired inhibitory control performance within iCUD (all brain analyses P < 0.05, FWE-corrected). Together, we demonstrated that a similar pattern of systematic and widespread deviations in resting-state brain efficiency, extending beyond the networks commonly investigated in human drug addiction, is linked to both abstinence duration and inhibitory control deficits in iCUD.

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

Introduction

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

Methods

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

Results

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

Discussion

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

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.

Exploring decision-making strategies in the Iowa gambling task and rat gambling task

Decision-making requires that individuals perceive the probabilities and risks associated with different options. Experimental human and animal laboratory testing provide complimentary insights on the psychobiological underpinnings of decision-making. The Iowa gambling task (IGT) is a widely used instrument that assesses decision-making under uncertainty and risk. In the task participants are faced with a choice conflict between cards with varying monetary reinforcer/loss contingencies. The rat gambling task (rGT) is a pre-clinical version using palatable reinforcers as wins and timeouts mimicking losses. However, interspecies studies elaborating on human and rat behavior in these tasks are lacking. This study explores decision-making strategies among young adults (N = 270) performing a computerized version of the IGT, and adult outbred male Lister Hooded rats (N = 72) performing the rGT. Both group and individual data were explored by normative scoring approaches and subgroup formations based on individual choices were investigated. Overall results showed that most humans and rats learned to favor the advantageous choices, but to a widely different extent. Human performance was characterized by both exploration and learning as the task progressed, while rats showed relatively consistent pronounced preferences for the advantageous choices throughout the task. Nevertheless, humans and rats showed similar variability in individual choice preferences during end performance. Procedural differences impacting on the performance in both tasks and their potential to study different aspects of decision-making are discussed. This is a first attempt to increase the understanding of similarities and differences regarding decision-making processes in the IGT and rGT from an explorative perspective.

Regulation of sex differences in risk-based decision making by gonadal hormones: Insights from rodent models

Men and women differ in their ability to evaluate options that vary in their rewards and the risks that are associated with these outcomes. Most studies have shown that women are more risk averse than men and that gonadal hormones significantly contribute to this sex difference. Gonadal hormones can influence risk-based decision making (i.e., risk taking) by modulating the neurobiological substrates underlying this cognitive process. Indeed, estradiol, progesterone and testosterone modulate activity in the prefrontal cortex, amygdala and nucleus accumbens associated with reward and risk-related information. The use of animal models of decision making has advanced our understanding of the intersection between the behavioral, neural and hormonal mechanisms underlying sex differences in risk taking. This review will outline the current state of this literature, identify the current gaps in knowledge and suggest the neurobiological mechanisms by which hormones regulate risky decision making. Collectively, this knowledge can be used to understand the potential consequences of significant hormonal changes, whether endogenously or exogenously induced, on risk-based decision making as well as the neuroendocrinological basis of neuropsychiatric diseases that are characterized by impaired risk taking, such as substance use disorder and schizophrenia.

Does Traumatic Brain Injury Cause Risky Substance Use or Substance Use Disorder?

There is a high co-occurrence of risky substance use among adults with traumatic brain injury (TBI), although it is unknown if the neurologic sequelae of TBI can promote this behavior. We propose that to conclude that TBI can cause risky substance use, it must be determined that TBI precedes risky substance use, that confounders with the potential to increase the likelihood of both TBI and risky substance use must be ruled out, and that there must be a plausible mechanism of action. In this review, we address these factors by providing an overview of key clinical and preclinical studies and list plausible mechanisms by which TBI could increase risky substance use. Human and animal studies have identified an association between TBI and risky substance use, although the strength of this association varies. Factors that may limit detection of this relationship include differential variability due to substance, sex, age of injury, and confounders that may influence the likelihood of both TBI and risky substance use. We propose possible mechanisms by which TBI could increase substance use that include damage-associated neuroplasticity, chronic changes in neuroimmune signaling, and TBI-associated alterations in brain networks.

Serotonin 2C Antagonism in the Lateral Orbitofrontal Cortex Ameliorates Cue-Enhanced Risk Preference and Restores Sensitivity to Reinforcer Devaluation in Male Rats

Previous research has indicated that reward-paired cues can enhance disadvantageous risky choice in both humans and rodents. Systemic administration of a serotonin 2C receptor antagonist can attenuate this cue-induced risk preference in rats. However, the neurocognitive mechanisms mediating this effect are currently unknown. We therefore assessed whether the serotonin 2C receptor antagonist RS 102221 is able to attenuate cue-enhanced risk preference via its actions in the lateral orbitofrontal cortex (lOFC) or prelimbic (PrL) area of the medial prefrontal cortex (mPFC). A total of 32 male Long–Evans rats were trained on the cued version of the rat gambling task (rGT), a rodent analog of the human Iowa gambling task, and bilateral guide cannulae were implanted into the lOFC or PrL. Intra-lOFC infusions of the 5-HT_2C antagonist RS 102221 reduced risky choice in animals that showed a preference for the risky options of the rGT at baseline. This effect was not observed in optimal decision-makers, nor those that received infusions targeting the PrL. Given prior data showing that 5-HT_2C antagonists also improve reversal learning through the same neural locus, we hypothesized that reward-concurrent cues may amplify risky decision-making through cognitive inflexibility. We therefore devalued the sugar pellet rewards used in the cued rGT (crGT) through satiation and observed that decision-making patterns did not shift unless animals also received intra-lOFC RS 102221. Collectively, these data suggest that the lOFC is one critical site through which reward-concurrent cues promote risky choice patterns that are insensitive to reinforcer devaluation, and that 5-HT_2C antagonism may optimize choice by facilitating exploration.

Dopamine neurons gate the intersection of cocaine use, decision making, and impulsivity

Gambling and substance use disorders are highly comorbid. Both clinical populations are impulsive and exhibit risky decision-making. Drug-associated cues have long been known to facilitate habitual drug-seeking, and the salient audiovisual cues embedded within modern gambling products may likewise encourage problem gambling. The dopamine neurons of the ventral tegmental area (VTA) are exquisitely sensitive to drugs of abuse, uncertain rewards, and reward-paired cues and may therefore be the common neural substrate mediating synergistic features of both disorders. To test this hypothesis, we first gained specific inhibitory control over VTA dopamine neurons by transducing a floxed inhibitory DREADD (AAV5-hSyn-DIO-hM4D(Gi)-mCherry) in rats expressing Cre recombinase in tyrosine hydroxylase neurons. We then trained rats in our cued rat gambling task (crGT), inhibiting dopamine neurons throughout task acquisition and performance, before allowing them to self-administer cocaine in the same diurnal period as crGT sessions. The trajectories of addiction differ in women and men, and the dopamine system may differ functionally across the sexes; therefore, we used male and female rats here. We found that inhibition of VTA dopamine neurons decreased cue-induced risky choice and reduced motor impulsivity in males, but surprisingly, enhanced risky decision making in females. Inhibiting VTA dopamine neurons also prevented cocaine-induced changes in decision making in both sexes, but nevertheless drove all animals to consume more cocaine. These findings show that chronic dampening of dopamine signalling can have both protective and deleterious effects on addiction-relevant behaviours, depending on biological sex and dependent variable of interest.

The association between risky decision making and cocaine conditioned place preference is moderated by sex

BACKGROUND

Excessive risk taking is a characteristic trait of several psychiatric conditions, including substance use disorders. High risk-taking (HiR) rats self-administer more cocaine compared to low risk-taking (LoR) rats. However, research has not determined if risk taking is associated with enhanced cocaine conditioned place preference (CPP).

METHODS

Male and female Sprague Dawley rats (n = 48 each sex) were first tested in the risky decision task (RDT), in which a response on one lever resulted in safe delivery of one food pellet, and a response on a different lever resulted in delivery of two pellets and probabilistic delivery of foot shock. Following RDT training, rats were tested for cocaine CPP. The first session was a pretest that measured rats' preference for three compartments that provided different visual and tactile cues. Rats then learned to associate one compartment with cocaine (either 10.0 mg/kg or 20.0 mg/kg; i.p.) and one compartment with saline (1.0 ml/kg; i.p.) across eight conditioning sessions. Finally, rats explored all three compartments in a drug-free state.

RESULTS

Sex significantly moderated the association between risky decision making and cocaine CPP. While increased risk aversion was somewhat positively associated with cocaine CPP in males, increased risk taking was positively correlated with cocaine CPP in females.

CONCLUSIONS

These results highlight the moderating role of sex on the relationship between risky decision making and cocaine reward.

Sign tracking predicts suboptimal behavior in a rodent gambling task

RATIONALE

Reward-associated cues can promote maladaptive behavior, including risky decision-making in a gambling setting. A propensity for sign tracking over goal tracking-i.e., interaction with a reward-predictive cue rather than the site of reward-demonstrates an individual's tendency to transfer motivational value to a cue. However, the relationship of sign tracking to risky decision-making remains unclear.

OBJECTIVES

To determine whether sign tracking predicts risky choice, we used a Pavlovian conditioned approach task to evaluate the tendency of male rats to sign track to a lever cue and then trained rats on a rodent gambling task (rGT) with win-associated cues. We also tested the effects of D-amphetamine, quinpirole (a D₂/D₃ receptor agonist), and PD128907 (a D₃ receptor agonist) on gambling behavior in sign tracker and goal tracker individuals.

RESULTS

Increased sign tracking relative to goal tracking was associated with suboptimal performance on the rGT, including decreased selection of the optimal choice, increased selection of a high-risk/high-reward option, and increased impulsive premature choices. Amphetamine increased choices of a low-risk/low-reward option at the expense of optimal and high-risk choices, whereas quinpirole and PD128907 had little effect on choice allocation, but reduced impulsivity. Drug effects were similar across sign tracker and goal tracker individuals.

CONCLUSIONS

Cue reactivity, as measured by sign tracking, is predictive and may be an important driver of risky and impulsive choices in a gambling setting laden with salient audiovisual cues. Evaluating an individual's sign tracking behavior may be an avenue to predict vulnerability to pathological gambling and the efficacy of treatments.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

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

Choose your path: Divergent basolateral amygdala efferents differentially mediate incentive motivation, flexibility and decision-making

To survive in a complex environment, individuals form associations between environmental stimuli and rewards to organize and optimize reward seeking behaviors. The basolateral amygdala (BLA) uses these learned associations to inform decision-making processes. In this review, we describe functional projections between BLA and its cortical and striatal targets that promote learning and motivational processes central to decision-making. Specifically, we compare and contrast divergent projections from the BLA to the orbitofrontal (OFC) and to the nucleus accumbens (NAc) and examine the roles of these pathways in associative learning, value-guided decision-making, choice behaviors, as well as cue and context-driven drug seeking. Finally, we consider how these projections are involved in disorders of motivation, with a focus on Substance Use Disorder.

Pharmacological evidence of a cholinergic contribution to elevated impulsivity and risky decision-making caused by adding win-paired cues to a rat gambling task

BACKGROUND

Pairing rewards with sensory stimulation, in the form of auditory and visual cues, increases risky decision-making in both rats and humans. Understanding the neurobiological basis of this effect could help explain why electronic gambling machines are so addictive, and inform treatment development for compulsive gambling and gaming. Numerous studies implicate the dopamine system in mediating the motivational influence of reward-paired cues; recent data suggest the cholinergic system also plays a critical role. Previous work also indicates that cholinergic drugs alter decision-making under uncertainty.

AIMS

We investigated whether the addition of reward-concurrent cues to the rat gambling task (crGT) altered the effects of peripherally administered cholinergic compounds.

METHODS

Muscarinic and nicotinic agonists and antagonists were administered to 16 male, Long-Evans rats trained on the crGT. Measures of optimal/risky decision-making and motor impulsivity were the main dependent variables of interest.

RESULTS

The muscarinic receptor antagonist scopolamine improved decision-making overall, decreasing selection of one of the risky options while increasing choice of the more advantageous options. The muscarinic agonist oxotremorine increased choice latency but did not significantly affect option preference. Neither the nicotinic antagonist mecamylamine nor the agonist nicotine affected choice patterns, but mecamylamine decreased premature responding, an index of motor impulsivity.

CONCLUSIONS

These results contrast sharply from those obtained previously using the uncued rGT, and suggest that the deleterious effects of win-paired cues on decision-making and impulse control may result from elevated cholinergic tone.

Advances in understanding meso-cortico-limbic-striatal systems mediating risky reward seeking

The risk of an aversive consequence occurring as the result of a reward-seeking action can have a profound effect on subsequent behavior. Such aversive events can be described as punishers, as they decrease the probability that the same action will be produced again in the future and increase the exploration of less risky alternatives. Punishment can involve the omission of an expected rewarding event ("negative" punishment) or the addition of an unpleasant event ("positive" punishment). Although many individuals adaptively navigate situations associated with the risk of negative or positive punishment, those suffering from substance use disorders or behavioral addictions tend to be less able to curtail addictive behaviors despite the aversive consequences associated with them. Here, we discuss the psychological processes underpinning reward seeking despite the risk of negative and positive punishment and consider how behavioral assays in animals have been employed to provide insights into the neural mechanisms underlying addictive disorders. We then review the critical contributions of dopamine signaling to punishment learning and risky reward seeking, and address the roles of interconnected ventral striatal, cortical, and amygdala regions to these processes. We conclude by discussing the ample opportunities for future study to clarify critical gaps in the literature, particularly as related to delineating neural contributions to distinct phases of the risky decision-making process.

Differential effects of glutamate N-methyl-D-aspartate receptor antagonists on risky choice as assessed in the risky decision task

RATIONALE

Risky choice can be measured using the risky decision task (RDT). In the RDT, animals choose between a large, risky option that is paired with probabilistic foot shock and a small, safe option that is never paired with shock. To date, studies examining the neurochemical basis of decision-making in the RDT have focused primarily on the dopaminergic system but have not focused on the glutamatergic system, which has been implicated in risky decision-making.

OBJECTIVES

Because glutamate is known to play a critical role in decision-making, we wanted to determine the contribution of the glutamatergic system to performance in the RDT.

METHODS

In the experiment, 32 rats (16 male; 16 female) were tested in the RDT. The probability of receiving a foot shock increased across the session (ascending schedule) for half of the rats but decreased across the session (descending schedule) for half of the rats. Following training, rats received injections of the N-methyl-D-aspartate (NMDA) receptor competitive antagonist CGS 19755 (0, 1.0, 2.5, 5.0 mg/kg; s.c.) and the GluN2B-selective antagonist Ro 63-1908 (0, 0.1, 0.3, 1.0 mg/kg; s.c.).

RESULTS

CGS 19755 (2.5 and 5.0 mg/kg) increased risky choice in males and females trained on the ascending schedule. Ro 63-1908 (1.0 mg/kg) decreased risky choice, but only in male rats trained on the ascending schedule.

CONCLUSIONS

Although NMDA receptor antagonists differentially alter risky choice in the RDT, the current results show that NMDA receptors are an important mediator of decision-making involving probabilistic delivery of positive punishment.

Traumatic brain injury substantially reduces the conditioned reinforcing effects of environmental cues in rats

Traumatic brain injury affects millions of people each year and is an established risk factor for addiction. Recent animal studies have causally demonstrated that injuries can increase drug self-administration across a variety of substances. One potential behavioral mediator for this finding is an increased responsivity to drug-associated cues. This endophenotype can be identified by profiling non-drug-related behaviors. The current study evaluated several paradigms (conditioned approach, conditioned reinforcement, extinction from variable interval responding, conditioned facilitation) to determine how rats with a frontal TBI differed in their response to Pavlovian conditioning in response to food-paired cues. Surprisingly, rats with a TBI demonstrated increased goal-tracking in a conditioned approach paradigm and exerted less effort for a conditioned reinforcer. Moreover, they had slightly facilitated extinction (as demonstrated by significantly larger interresponse times) in the face of reinforcer-associated cues. Despite these effects, TBI rats still demonstrated conditioned facilitation to an auditory stimulus. Together, these effects suggest a phenotype in the opposite direction of what might be anticipated. Cues still served a strong discriminative function and altered behavior; however, they did not function as strong conditioned reinforcers for TBI animals. One potential reason for this is that substantial changes to the dopamine system after TBI may reduce the conditioned reinforcing effects of cues, but sensitize the brain to potent drugs of abuse. More research will be needed to determine whether this is the case.

Impaired decision making following escalation of cocaine self-administration predicts vulnerability to relapse in rats

Impairments in cost‐benefit decision making represent a cardinal feature of drug addiction. However, whether these alterations predate drug exposure, thereby contributing to facilitating loss of control over drug intake, or alternatively arise as a result of drug use and subsequently confer vulnerability to relapse has yet to be determined. Male Sprague‐Dawley rats were trained to self‐administer (SA) cocaine during 19 daily long‐access (12‐h) sessions; conditions reliably shown to promote escalation. One week after cocaine SA, rats underwent an extinction/relapse test immediately followed by conditioned stimuli–, stress‐, and drug‐primed reinstatement challenges. The influence of escalated cocaine intake on decision making was measured over time by four test sessions of a rodent analogue of the Iowa Gambling Task (rGT), once prior to cocaine exposure and then 1 day, 1 week, and 1 month after the last SA session. Substantial individual variability was observed in the influence of escalated cocaine SA on decision‐making performance. A subset of rats displayed pronounced deficits, while others showed unaffected or even improved performance on the rat Gambling Task (rGT) 24 hours after the last SA session. When challenged with a relapse test after 1 week of forced abstinence, animals that showed impaired decision making following SA displayed an increased propensity to respond for cocaine under extinction. These data suggest that decision‐making deficits in individuals with drug addiction are not antecedent to—but arise as a consequence of—drug exposure. Moreover, these data indicate that susceptibility to the deleterious effects of drugs on decision making confers vulnerability toward relapse.

Translating concepts of risk and loss in rodent models of gambling and the limitations for clinical applications

Gambling involves placing something of value at risk in exchange for the opportunity to potentially gain something of greater value in return. A variety of gambling paradigms have been designed to study the maladaptive decision-making that underlies problematic gambling. Central to these gambling models are the definitions of "risk" and "loss", especially when translating the results from rodent studies to clinical applications. Risk and loss are not mutually exclusive but rather share some overlap. With careful interpretation and consideration of the limitations of these behavioral paradigms, results from rodent models may provide insights into the neurobiology of risky decision-making that leads to problematic gambling in humans.

Distinct relationships between risky decision making and cocaine self-administration under short- and long-access conditions

Substance use is strongly associated with impaired decision making, with cocaine use particularly linked to elevated risky and impulsive choice. It is not clear, however, whether such maladaptive decision making is a consequence of cocaine use or instead precedes and predisposes individuals to cocaine use. The current study was designed to specifically address the latter possibility with respect to risky choice in both male and female rats. Rats were first trained in a "Risky Decision-making Task" (RDT), in which they made discrete choices between a small, "safe" food reward and a large, "risky" food reward accompanied by increasing probabilities of mild footshock punishment. After reaching stable performance, rats underwent jugular catheter surgery followed by either short-access cocaine self-administration sessions (2 h, 0.5 mg/kg/infusion) for 5 days or long-access cocaine self-administration sessions (6 h, 0.5 mg/kg/infusion) for 14 days. Under short-access conditions, there was no relationship between risk preference and changes in cocaine intake over time, but greater risk aversion in females predicted greater overall cocaine intake. Under long-access conditions, heightened risk taking predicted greater escalation of cocaine intake over the course of self-administration, supporting the notion that pre-existing risk-taking behavior predicts cocaine intake. Collectively, results from these experiments have implications for understanding and identifying pre-existing vulnerabilities to substance use, which may lead to strategies to prevent development of substance use disorders.

The Mouse Gambling Task: Assessing Individual Decision-making Strategies in Mice

Decision-making is a complex cognitive process which consists of choosing one option among several alternatives. In humans, this process is featured in the Iowa gambling task (IGT), a decision-making task that mimics real life situations by reproducing uncertain conditions based on probabilistic rewards or penalties (see Background). Several authors wanted to adapt the IGT in rodents with subtle differences in protocols that match various aspects of the human task. Here we propose, for the first time in mice, a protocol that contains the most important characteristics of the IGT: 4 different options, choices based on 4 ambiguous outcomes with immediate and long term rewards, a total of 100 trials, no learning of the contingency before the task, and presence of both a certain reward and a probable penalty. During this task, mice have to choose between options more or less advantageous in the short and long term by developing a decision-making strategy that differs between individuals. Therefore, the strength of this protocol is that it is one of the first to enable the study of decision-making in a complex situation, and demonstrates inter-individual differences regarding decision-making strategies in mice.

Clinical Trials for Stimulant Use Disorders: Addressing Heterogeneities That May Undermine Treatment Outcomes

In recent years, use of cocaine and amphetamines and deaths associated with stimulants have been on the rise, and there are still no FDA-approved medications for stimulant use disorders. One contributing factor may involve heterogeneity. At the neurobiological level, dual dopamine dysfunction may be undermining medication efficacy, suggesting a need for combination pharmacotherapies. At the population level, individual variability is expressed in a number of ways and, if left unaddressed, may interfere with medication efficacy. This chapter reviews studies investigating medications to address dopamine dysfunction, and it also identifies several prominent heterogeneities associated with stimulant (and other substance) use disorders. The chapter has implications for improving interventions to treat stimulant use disorders, and the theme of individual heterogeneity may have broader application across substance use disorders.

Challenges and Opportunities in Animal Models of Gambling-like Behavior

In the past several years, there has been an explosion of interest in animal models of risk-based decision-making, a fundamental process associated with gambling disorder. While early work focused on establishing various tasks for assaying decision-making, current studies are determining the (subtle and not-so-subtle) influence of cues in driving risky decisions to better understand problem gambling. In addition, these decision-making paradigms are now being used to investigate comorbid conditions such as substance dependence or brain injury and replicating observations from human patients. These animal models have now developed to a point where therapeutic interventions may be assessed for not just gambling disorder, but also a number of other conditions which engender risky decision-making.

Using rodent models to understand interactions between gambling and substance use

Substance use is strongly associated with gambling, but the nature of this association can be difficult to determine. Rodents offer the opportunity to test causal models of these relationships through isolation of individual variables of interest. This review describes recent research in rodents showing: a) predisposing factors for both gambling-like behavior and substance use; b) exposure to drugs of abuse increasing gambling-like behavior; c) experience with gambling-like behavior increasing substance use; and d) links between gambling-like behavior and substance use in models of Parkinson's disease therapies. These findings reveal novel relationships between gambling and substance use, and highlight the utility of rodent models for future work in this area.

Enhanced methamphetamine-induced conditioned place preference in risk-taking rats

Patients with psychiatric disorders, such as gambling and substance use, tend to exhibit maladaptive decision-making. In this study, we assessed individual differences in risk-taking behaviors using a rat gambling task (GT) and investigated the relationship between risk-taking behaviors and vulnerability to drug dependence using methamphetamine (METH)-induced conditioned place preference (CPP). In the GT using a radial arm maze, male Long-Evans rats were trained to choose one of three choice arms (a low-risk/low reward (L-L), a high-risk/high reward (H-H), and an empty arm) in 16 trials per day for 14 days. METH-induced CPP consisted of 6 sessions: habituation, conditioning, preference test (Test I), extinction, extinction test (Test II), and reinstatement test (Test III). Results demonstrated that the percentage of choosing the H-H arm was significantly positively correlated with the percentage of time spent in the METH-paired compartment in the preference test, but not with the extinction and reinstatement tests, suggesting that risk-taking rats are more vulnerable to drug dependence.

Exposure to uncertainty mediates the effects of traumatic brain injury on probabilistic decision-making in rats

Primary Objective: Traumatic brain injury (TBI) is associated with numerous psychiatric comorbidities, and subclinical psychiatric symptoms. While many symptoms have been replicated in animal models of brain injury, a vast majority of studies utilize naïve rats as subjects, which fail to mimic the complex learning history of human patients.Methods and Procedures: In the current study, we evaluated the effects of a brain injury in animals with early exposure to uncertainty on post-injury decision-making in a probabilistic task, the rodent gambling task (RGT).Main Outcomes and Results: Exposure to uncertainty resulted in a heterogeneous sample relative to prior publications, and brain-injured rats showed no deficits in choice behavior compared to shams which contrasts with large, pervasive deficits in previously published work. However, TBI increased impulsivity and caused transient changes in behavioral variables indicative of initial motivational deficits (pellets earned, omitted responses). Notably, effects of amphetamine were similar on this heterogeneous sample of rats relative to a number of other published reports, suggesting consistent effects of gross monoaminergic manipulations on choice behavior, independent of experience.Conclusions: Going forward, translational studies need to consider the heterogeneity that exists at the clinical level and account for these problems when modeling diseases in animals.

Increased motor impulsivity in a rat gambling task during chronic ropinirole treatment: potentiation by win-paired audiovisual cues

RATIONALE

Chronic administration of D_2/3 receptor agonists ropinirole or pramipexole can increase the choice of uncertain rewards in rats, theoretically approximating iatrogenic gambling disorder (iGD).

OBJECTIVES

We aimed to assess the effect of chronic ropinirole in animal models that attempt to capture critical aspects of commercial gambling, including the risk of losing rather than failing to gain, and the use of win-paired sensory stimuli heavily featured in electronic gambling machines (EGMs).

METHODS

Male Long-Evans rats learned the rat gambling task (rGT; n = 24), in which animals sample between four options that differ in the magnitude and probability of rewards and time-out punishments. In the cued rGT (n = 40), reward-concurrent audiovisual cues were added that scaled in complexity with win size. Rats were then implanted with an osmotic pump delivering ropinirole (5 mg/kg/day) or saline for 28 days.

RESULTS

Chronic ropinirole did not unequivocally increase preference for more uncertain outcomes in either the cued or uncued rGT. Ropinirole transiently increased premature responses, a measure of motor impulsivity, and this change was larger and more long-lasting in the cued task.

CONCLUSIONS

These data suggest that explicitly signaling loss prevents the increase in preference for uncertain rewards caused by D_2/3 receptor agonists observed previously. The ability of win-paired cues to amplify ropinirole-induced increases in motor impulsivity may explain why compulsive use of EGMs is particularly common in iGD. These data offer valuable insight into the cognitive-behavioral mechanisms through which chronic dopamine agonist treatments may induce iGD and related impulse control disorders.

Deconstructing value-based decision making via temporally selective manipulation of neural activity: Insights from rodent models

The ability to choose among options that differ in their rewards and costs (value-based decision making) has long been a topic of interest for neuroscientists, psychologists, and economists alike. This is likely because this is a cognitive process in which all animals (including humans) engage on a daily basis, be it routine (which road to take to work) or consequential (which graduate school to attend). Studies of value-based decision making (particularly at the preclinical level) often treat it as a uniform process. The results of such studies have been invaluable for our understanding of the brain substrates and neurochemical systems that contribute to decision making involving a range of different rewards and costs. Value-based decision making is not a unitary process, however, but is instead composed of distinct cognitive operations that function in concert to guide choice behavior. Within this conceptual framework, it is therefore important to consider that the known neural substrates supporting decision making may contribute to temporally distinct and dissociable components of the decision process. This review will describe this approach for investigating decision making, drawing from published studies that have used techniques that allow temporal dissection of the decision process, with an emphasis on the literature in animal models. The review will conclude with a discussion of the implications of this work for understanding pathological conditions that are characterized by impaired decision making.

Cued for risk: Evidence for an incentive sensitization framework to explain the interplay between stress and anxiety, substance abuse, and reward uncertainty in disordered gambling behavior

Gambling disorder is an impairing condition confounded by psychiatric co-morbidity, particularly with substance use and anxiety disorders. Yet, our knowledge of the mechanisms that cause these disorders to coalesce remains limited. The Incentive Sensitization Theory suggests that sensitization of neural "wanting" pathways, which attribute incentive salience to rewards and their cues, is responsible for the excessive desire for drugs and cue-triggered craving. The resulting hyper-reactivity of the "wanting' system is believed to heavily influence compulsive drug use and relapse. Notably, evidence for sensitization of the mesolimbic dopamine pathway has been seen across gambling and substance use, as well as anxiety and stress-related pathology, with stress playing a major role in relapse. Together, this evidence highlights a phenomenon known as cross-sensitization, whereby sensitization to stress, drugs, or gambling behaviors enhance the sensitivity and dopaminergic response to any of those stimuli. Here, we review the literature on how cue attraction and reward uncertainty may underlie gambling pathology, and examine how this framework may advance our understanding of co-mordidity with substance-use disorders (e.g., alcohol, nicotine) and anxiety disorders. We argue that reward uncertainty, as seen in slot machines and games of chance, increases dopaminergic activity in the mesolimbic pathway and enhances the incentive value of reward cues. We propose that incentive sensitization by reward uncertainty may interact with and predispose individuals to drug abuse and stress, creating a mechanism through which co-mordidity of these disorders may emerge.

Prior Exposure to Salient Win-Paired Cues in a Rat Gambling Task Increases Sensitivity to Cocaine Self-Administration and Suppresses Dopamine Efflux in Nucleus Accumbens: Support for the Reward Deficiency Hypothesis of Addiction

Rats trained to perform a version of the rat gambling task (rGT) in which salient audiovisual cues accompany reward delivery, similar to commercial gambling products, show greater preference for risky options. Given previous demonstrations that probabilistic reinforcement schedules can enhance psychostimulant-induced increases in accumbal DA and locomotor activity, we theorized that performing this cued task could perpetuate a proaddiction phenotype. Significantly more rats developed a preference for the risky options in the cued versus uncued rGT at baseline, and this bias was further exacerbated by cocaine self-administration, whereas the choice pattern of optimal decision-makers was unaffected. The addition of reward-paired cues therefore increased the proportion of rats exhibiting a maladaptive cognitive response to cocaine self-administration. Risky choice was not associated with responding for conditioned reinforcement or a marker of goal/sign-tracking, suggesting that reward-concurrent cues precipitate maladaptive choice via a unique mechanism unrelated to simple approach toward, or responding for, conditioned stimuli. Although "protected" from any resulting decision-making impairment, optimal decision-makers trained on the cued rGT nevertheless self-administered more cocaine than those trained on the uncued task. Collectively, these data suggest that repeated engagement with heavily cued probabilistic reward schedules can drive addiction vulnerability through multiple behavioral mechanisms. Rats trained on the cued rGT also exhibited blunted locomotor sensitization and lower basal accumbal DA levels, yet greater cocaine-induced increases in accumbal DA efflux. Gambling in the presence of salient cues may therefore result in an adaptive downregulation of the mesolimbic DA system, rendering individuals more sensitive to the deleterious effects of taking cocaine.SIGNIFICANCE STATEMENT Impaired cost/benefit decision making, exemplified by preference for the risky, disadvantageous options on the Iowa Gambling Task, is associated with greater risk of relapse and treatment failure in substance use disorder. Understanding factors that enhance preference for risk may help elucidate the neurobiological mechanisms underlying maladaptive decision making in addiction, thereby improving treatment outcomes. Problem gambling is also highly comorbid with substance use disorder, and many commercial gambling products incorporate salient win-paired cues. Here we show that adding reward-concurrent cues to a rat analog of the IGT precipitates a hypodopaminergic state, characterized by blunted accumbal DA efflux and attenuated locomotor sensitization, which may contribute to the enhanced responsivity to uncertain rewards or the reinforcing effects of cocaine we observed.

Rats' optimal choice behavior in a gambling-like task

Among the different procedures that model gambling behavior in non-human animals, the "suboptimal choice procedure" has been extensively employed for analyzing the impact of environmental cues on choice behavior. It has been repeatedly demonstrated that pigeons prefer an alternative that infrequently presents a stimulus that signals a larger amount of reinforcement, than another alternative that always presents a stimulus associated with a smaller amount of reinforcement, even though the net rate of reinforcement is lower in the former. In the present study, we tested rats in the magnitude version of the suboptimal choice procedure. Eight rats were given a choice between two alternatives: a) one in which a stimulus predicting the delivery of ten pellets was presented with probability (p) = 0.2 and a stimulus predicting zero pellets was presented with p = 0.8, and b) one in which either of two stimuli predicted the delivery of three pellets with p = 1.0. Contrary to the consistent and robust suboptimal behavior of pigeons, rats preferred the optimal alternative. This effect occurred despite the high index of discrimination of the stimuli associated with the different outcomes shown by the rats. The relevance of this result to the development of animal models of gambling behavior is discussed.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Cocaine self-administration is increased after frontal traumatic brain injury and associated with neuroinflammation

Traumatic brain injury (TBI) has been linked to the development of numerous psychiatric diseases, including substance use disorder. However, it can be difficult to ascertain from clinical data whether the TBI is cause or consequence of increased addiction vulnerability. Surprisingly few studies have taken advantage of animal models to investigate the causal nature of this relationship. In terms of a plausible neurobiological mechanism through which TBI could magnify the risk of substance dependence, numerous studies indicate that TBI can cause widespread disruption to monoaminergic signaling in striatal regions, and also increases neuroinflammation. In the current study, male Long-Evans rats received either a mild or severe TBI centered over the frontal cortex via controlled cortical impact, and were subsequently trained to self-administer cocaine over 10 6-hour sessions. At the end of the study, markers of striatal dopaminergic function, and levels of inflammatory cytokine levels in the frontal lobes, were assessed via western blot and multiplex ELISA, respectively. There was significantly higher cocaine intake in a subset of animals with either mild or severe TBI. However, many animals within both TBI groups failed to acquire self-administration. Principal components analysis suggested that both dopaminergic and neuroinflammatory proteins were associated with overall cocaine intake, yet only an inflammatory component was associated with acquisition of self-administration, suggesting neuroinflammation may make a more substantial contribution to the likelihood of drug-taking. Should neuroinflammation play a causal role in mediating TBI-induced addiction risk, anti-inflammatory therapy may reduce the likelihood of substance abuse in TBI populations.

Animal models in addiction research: A dimensional approach

Drug addiction affects approximately 10% of the population and these numbers are rising. Treatment and prevention of addiction are impeded by current diagnostic systems, such as DSM-5, which are based on outcomes rather than processes. Here, we review the importance of adopting a dimensional framework, specifically the Research Domain Criteria (RDoC), to identify protective and vulnerability mechanisms in addiction. We discuss how preclinical researchers should work within this framework to develop animal models based on domains of function. We highlight RDoC paradigms related to addiction and discuss how these can be used to investigate the biological underpinnings of an addiction cycle (i.e., binge/intoxication, negative affect, and craving). Using this information, we then outline the critical role of animal research in ongoing revisions to the RDoC matrix (specifically the functional significance of domains, constructs and subconstructs) and its contribution to the development and refinement of addiction theories. We conclude with an overview of the contribution that animal research has made to the development of pharmacological and behavioural treatments for addiction.

Examination of the effects of cannabinoid ligands on decision making in a rat gambling task

Although exposure to delta-9-tetrahydrocannabinol (THC) is perceived to be relatively harmless, mounting evidence has begun to show that it is associated with a variety of cognitive deficits, including poor decision making. THC-induced impairments in decision making are thought to be the result of cannabinoid CB1 receptor activation, and although clinical literature suggests that chronic activation via THC contributes to perturbations in decision making, acute CB1 receptor modulation has yielded mixed results. Using an animal model to examine how CB1-specific ligands impact choice biases would provide significant insight as to how recruitment of the endocannabinoid system may influence decision making. Here, we used the rat gambling task (rGT), a validated analogue of the human Iowa Gambling Task, to assess baseline decision making preferences in male Wistar rats. After acquisition rGT performance was measured. Animals were challenged with the CB1 receptor antagonist rimonabant, the partial agonist THC, and the synthetic agonist WIN55,212-2. Animals were also treated acutely with the fatty acid amide hydrolase (FAAH) inhibitor URB597 to selectively upregulate the endocannabinoid anandamide. Blockade of the CB1 receptor produced a trend improvement in decision making in animals who preferred the advantageous task options, yet left choice unaffected in risk-prone rats. Neither CB1 receptor agonist had strong effects on decision making, but a high dose THC decreased premature responses, whereas WIN55,212-2 did the opposite. URB597 did not affect task performance. These results indicate that although chronic CB1 receptor activation may be associated with impaired decision making, acute modulation has modest effects on choice and instead may play a substantive role in regulating impulsive responding.

MeCP2 Expression in a Rat Model of Risky Decision Making

Many neuropsychiatric disorders are associated with abnormal decision making involving risk of punishment, but the underlying molecular basis remains poorly understood. Methyl CpG-binding protein 2 (MeCP2) is an epigenetic factor that regulates transcription by directly binding to methylated DNA. Here, we evaluated MeCP2 expression in the context of risk-taking behaviors using the Risky Decision-making Task (RDT), in which rats make discrete choices between a small "safe" food reward and a large "risky" food reward accompanied by varying probabilities of punishment. In Experiment 1, expression of MeCP2 as assessed by immunoblotting in the medial prefrontal cortex (mPFC), but not the striatum, was inversely correlated with the degree of preference for the large, risky reward (risk taking) seven days after the last RDT test. In Experiment 2, MeCP2 expression 90 min after RDT testing, assessed using immunohistochemistry, was suppressed in both the dorsal mPFC (dmPFC) and nucleus accumbens compared to home cage controls, indicating that MeCP2 expression is modulated by RDT performance. Additional experiments revealed that RDT performance increased expression of MeCP2 phosphorylated at Ser421 (associated with neuronal activity and activation of gene expression) in dmPFC principal neurons. Finally, as in Experiment 1, lower expression of MeCP2 in the ventral mPFC was associated with greater risk taking under baseline conditions. Together, these findings indicate a complex regulatory role of MeCP2 in risky decision making, and suggest that epigenetic factors may be an important component of the molecular mechanisms underlying such decision-making processes.

Pharmacological evidence that 5-HT2C receptor blockade selectively improves decision making when rewards are paired with audiovisual cues in a rat gambling task

RATIONALE

Adding reward-concurrent cues to a rat gambling task (rGT) increases risky choice. This cued version of the task may reflect an "addiction-like" cognitive process, more similar to human gambling than the uncued task. Serotonergic drugs that target 5-HT₂ receptors alter mechanisms linked to impulse control. However, relatively little is known regarding the impact of such agents on either risky decision making, or the ability of conditioned stimuli to bias the choice process, despite potential relevance to addiction development and treatment.

OBJECTIVES

The aim of this study was to determine the effects of SB 242,084 and M100907, selective antagonists at the 5-HT_2C and 5-HT_2A receptors respectively, as well as the selective 5-HT_2C receptor agonist Ro-60-0175, on performance of both cued and uncued versions of the rGT.

RESULTS

SB 242,084 significantly and dose-dependently increased choice of the most optimal option in the cued rGT only, despite concurrently increasing impulsive responses made prematurely on both the cued and uncued rGT. M100907 and Ro-60-0175 did not alter risky decision making, but nevertheless produced the expected decrease in premature responses on both task variants.

CONCLUSIONS

These findings demonstrate that the 5-HT₂ receptor-mediated regulation of risky decision making and motor impulsivity can be pharmacologically dissociated and further show that the presence of highly salient reward-paired cues critically alters the neurochemical regulation of the choice process. Importantly, these results suggest that 5-HT_2C receptor antagonists may be of use in disrupting maladaptive patterns of decision making.

Interaction between trait and housing condition produces differential decision-making toward risk choice in a rat gambling task

Poor decision-making is a core problem in psychiatric disorders such as pathological gambling and substance abuse. Both trait and environmental factors are considerably important to affect decision-making. However, it has not yet been systematically shown how they interact to affect risk preference in animal models evaluating decision-making. Here, we trained rats, housed in pairs or in isolation, in a touch screen chamber to detect the association between four different light signals on the screen and accompanied reward and punishment outcomes arranged with different schedules. Then, the rats were allowed to freely choose from 4 different light signals. Once animals showed a stabilized pattern of preference (risk-averse or risk-seeking), they were injected with saline or cocaine (a single injection per day for 7 days) followed by 2 weeks of withdrawal. Then, their preference of choice was re-tested in the touch screen chamber while they were cocaine challenged. All rats significantly changed their preference toward more risky choices when they were exposed to and challenged with cocaine, except those in the risk-averse/isolated housing group. These results indicate that the pre-existing trait toward risk and the housing condition interact to affect the quality of decision-making, and cocaine may help to aggravate this process.

Deep-Brain Stimulation of the Subthalamic Nucleus Selectively Decreases Risky Choice in Risk-Preferring Rats

Deep brain stimulation of the subthalamic nucleus (STN-DBS) can improve the motor symptoms of Parkinson's disease (PD) and negate the problematic side effects of dopamine replacement therapy. Although there is concern that STN-DBS may enhance the development of gambling disorder and other impulse control disorders in this patient group, recent data suggest that STN-DBS may actually reduce iatrogenic impulse control disorders, and alleviate obsessive-compulsive disorder (OCD). Here, we sought to determine whether STN-DBS was beneficial or detrimental to performance of the rat gambling task (rGT), a rodent analogue of the Iowa Gambling Task (IGT) used to assess risky decision making clinically. Rats chose between four options associated with different amounts and probabilities of sugar pellet rewards versus timeout punishments. As in the IGT, the optimal approach was to favor options associated with smaller per-trial gains but lower timeout penalties. Once a stable behavioral baseline was established, electrodes were implanted bilaterally into the STN, and the effects of STN-DBS assessed on-task over 10 consecutive sessions using an A-B-A design. STN-DBS did not affect choice in optimal decision makers that correctly favored options associated with smaller per-trial gains but also lower penalties. However, a minority (∼25%) preferred the maladaptive "high-risk, high-reward" options at baseline. STN-DBS significantly and progressively improved choice in these risk-preferring rats. These data support the hypothesis that STN-DBS may be beneficial in ameliorating maladaptive decision making associated with compulsive and addiction disorders.

Modeling the development of drug addiction in male and female animals

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