A Neuronal Ensemble in the Rostral Agranular Insula Tracks Cocaine-Induced Devaluation of Natural Reward and Predicts Cocaine Seeking

Still a "hidden island"? The rodent insular cortex in drug seeking, reward, and risk

The insular cortex (IC) is implicated in risky decision making and drug-seeking behaviors, in a manner dissociable from natural reward seeking. However, evidence from rodent studies of motivated behaviors suggests that the role of the IC is not always consistent across procedures. Moreover, there is evidence of dissociation of function between posterior (pIC) and anterior (aIC) subregions in these behaviors. Under which circumstances, and by which mechanisms, these IC subregions are recruited to regulate motivated behaviors remains unclear. Here, we discuss evidence of rodent pIC and aIC function across drug-related behaviors, natural reward seeking, and decision making under risk and highlight procedural differences that may account for seemingly conflicting findings. Although gaps in the literature persist, we hypothesize that IC activity is broadly important for selection of appropriate behaviors based on learned action-outcome contingencies and that associated risk is sufficient, but not necessary, to recruit the aIC in reward seeking without involving the pIC.

Prelimbic Cortex Activity during a Distress Tolerance Task Predicts Cocaine-Seeking Behavior in Male, But Not Female Rats

Distress tolerance (DT) is defined as the ability to persist in challenging goal-directed behavior in the face of stress, and individuals with low DT exhibit heightened drug-seeking behavior. However, no preclinical studies have examined the neurobiology underlying this phenomenon. To assess this, in vivo electrophysiology was used in Long Evans male and female rats during a DT task to record neural activity in the prelimbic cortex (PrL), a brain region implicated in drug-seeking. Rats were first assessed for DT, defined as the amount of time elapsed before rats quit seeking reward in an increasingly difficult operant task. Subsequently, rats underwent 2 weeks of self-administration for either water/saline or cocaine for 6 h/day. Animals then began a 1 month period of experimenter-imposed abstinence to induce heightened drug-seeking behavior. On day 28 of abstinence, DT and neural activity were reassessed; and on day 30, cocaine-seeking behavior was examined under extinction. Males had significantly higher DT than females and exhibited significantly more phasic PrL activity during the DT task. Furthermore, in male rats with a history of cocaine, PrL activity shifted to track DT; and this change in activity significantly correlated with the change in DT. Additionally, male (but not female) rats with low DT after 28 d of abstinence had significantly heightened drug-seeking behavior. Finally, PrL activity during the DT task predicted cocaine-seeking behavior. Collectively, these data demonstrate an important role for the PrL in DT in males, and link this neural activity and behavior to drug-seeking, particularly in males.SIGNIFICANCE STATEMENT Distress tolerance (DT) is defined as the ability to persist in challenging goal-directed behavior in the face of stress, and individuals with low DT exhibit heightened drug-seeking. Here, we investigated the role of the prelimbic cortex (PrL) in DT and its relationship to cocaine-seeking in male and female rats. We found that males had significantly higher DT than females and exhibited significantly more PrL activity during the DT task. Furthermore, male (but not female) rats with low DT after 28 d of abstinence had significantly heightened drug-seeking behavior. Finally, PrL activity during the DT task predicted cocaine-seeking. These data demonstrate an important role for the PrL in DT and link this neural activity and behavior to drug-seeking in males.

Transforming experiences: Neurobiology of memory updating/editing

Long-term memory is achieved through a consolidation process where structural and molecular changes integrate information into a stable memory. However, environmental conditions constantly change, and organisms must adapt their behavior by updating their memories, providing dynamic flexibility for adaptive responses. Consequently, novel stimulation/experiences can be integrated during memory retrieval; where consolidated memories are updated by a dynamic process after the appearance of a prediction error or by the exposure to new information, generating edited memories. This review will discuss the neurobiological systems involved in memory updating including recognition memory and emotional memories. In this regard, we will review the salient and emotional experiences that promote the gradual shifting from displeasure to pleasure (or vice versa), leading to hedonic or aversive responses, throughout memory updating. Finally, we will discuss evidence regarding memory updating and its potential clinical implication in drug addiction, phobias, and post-traumatic stress disorder.

GABAA receptor subtypes and benzodiazepine use, misuse, and abuse

Benzodiazepines have been in use for over half a century. While they remain highly prescribed, their unfavorable side-effect profile and abuse liability motivated a search for alternatives. Most of these efforts focused on the development of benzodiazepine-like drugs that are selective for specific GABA_A receptor subtypes. While there is ample evidence that subtype-selective GABA_A receptor ligands have great potential for providing symptom relief without typical benzodiazepine side-effects, it is less clear whether subtype-selective targeting strategies can also reduce misuse and abuse potential. This review focuses on the three benzodiazepine properties that are relevant to the DSM-5-TR criteria for Sedative, Hypnotic, or Anxiolytic Use Disorder, namely, reinforcing properties of benzodiazepines, maladaptive behaviors related to benzodiazepine use, and benzodiazepine tolerance and dependence. We review existing evidence regarding the involvement of different GABA_A receptor subtypes in each of these areas. The reviewed studies suggest that α1-containing GABA_A receptors play an integral role in benzodiazepine-induced plasticity in reward-related brain areas and might be involved in the development of tolerance and dependence to benzodiazepines. However, a systematic comparison of the contributions of all benzodiazepine-sensitive GABA_A receptors to these processes, a mechanistic understanding of how the positive modulation of each receptor subtype might contribute to the brain mechanisms underlying each of these processes, and a definitive answer to the question of whether specific chronic modulation of any given subtype would result in some or all of the benzodiazepine effects are currently lacking from the literature. Moreover, how non-selective benzodiazepines might lead to the maladaptive behaviors listed in DSM and how different GABA_A receptor subtypes might be involved in the development of these behaviors remains unexplored. Considering the increasing burden of benzodiazepine abuse, the common practice of benzodiazepine misuse that leads to severe dependence, and the current efforts to generate side-effect free benzodiazepine alternatives, there is an urgent need for systematic, mechanistic research that provides a better understanding of the brain mechanisms of benzodiazepine misuse and abuse, including the involvement of specific GABA_A receptor subtypes in these processes, to establish an informed foundation for preclinical and clinical efforts.

Impaired leptin responsiveness in the nucleus accumbens of leptin-overexpressing transgenic mice with dysregulated sucrose and lipid preference independent of obesity

While hypothalamic leptin resistance can occur prior to establishment of obesity, clarification is needed as to whether the impaired response to leptin in the reward-related nuclei occurs independently of obesity. To answer this question, we attempted to dissociate the normally coexisting leptin resistance from obesity. We investigated phenotypes of leptin-overexpressing transgenic mice fed for 1 week with 60 % high-fat diet (HFD) (LepTg-HFD1W mice). After 1 week, we observed that LepTg-HFD1W mice weighed as same as wild type (WT) mice fed standard chow diet (CD) for 1 week (WT-CD1W mice). However, compared to WT-CD1W mice, LepTg-HFD1W mice exhibited attenuated leptin-induced anorexia, decreased leptin-induced c-fos immunostaining in nucleus accumbens (NAc), one of important site of reward system, decreased leptin-stimulated pSTAT3 immunostaining in hypothalamus. Furthermore, neither sucrose nor lipid preference was suppressed by leptin in LepTg-HFD1W mice. On the contrary, leptin significantly suppressed both preferences in WT mice fed HFD (WT-HFD1 W mice). These results indicate that leptin responsiveness decreases in NAc independently of obesity. Additionally, in this situation, suppressive effect of leptin on the hedonic feeding results in impaired regulation. Such findings suggest the impaired leptin responsiveness in NAc partially contributes to dysregulated hedonic feeding behavior independently of obesity.

Better living through understanding the insula: Why subregions can make all the difference

Insula function is considered critical for many motivated behaviors, with proposed functions ranging from attention, behavioral control, emotional regulation, goal-directed and aversion-resistant responding. Further, the insula is implicated in many neuropsychiatric conditions including substance abuse. More recently, multiple insula subregions have been distinguished based on anatomy, connectivity, and functional contributions. Generally, posterior insula is thought to encode more somatosensory inputs, which integrate with limbic/emotional information in middle insula, that in turn integrate with cognitive processes in anterior insula. Together, these regions provide rapid interoceptive information about the current or predicted situation, facilitating autonomic recruitment and quick, flexible action. Here, we seek to create a robust foundation from which to understand potential subregion differences, and provide direction for future studies. We address subregion differences across humans and rodents, so that the latter's mechanistic interventions can best mesh with clinical relevance of human conditions. We first consider the insula's suggested roles in humans, then compare subregional studies, and finally describe rodent work. One primary goal is to encourage precision in describing insula subregions, since imprecision (e.g. including both posterior and anterior studies when describing insula work) does a disservice to a larger understanding of insula contributions. Additionally, we note that specific task details can greatly impact recruitment of various subregions, requiring care and nuance in design and interpretation of studies. Nonetheless, the central ethological importance of the insula makes continued research to uncover mechanistic, mood, and behavioral contributions of paramount importance and interest. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

A sex-dependent role for the prelimbic cortex in impulsive action both before and following early cocaine abstinence

Although impulsive action is strongly associated with addiction, the neural underpinnings of this relationship and how they are influenced by sex have not been well characterized. Here, we used a titrating reaction time task to assess differences in impulsive action in male and female Long Evans rats both before and after short (4-6 days) or long (25-27 days) abstinence from 2 weeks of cocaine or water/saline self-administration (6 h daily access). Neural activity in the prelimbic cortex (PrL) and nucleus accumbens (NAc) core was assessed at each time point. We found that a history of cocaine self-administration increased impulsivity in all rats following short, but not long, abstinence. Furthermore, male rats with an increased ratio of excited to inhibited neurons in the PrL at the start of each trial in the task exhibited higher impulsivity in the naïve state (before self-administration). Following short abstinence from cocaine, PrL activity in males became more inhibited, and this change in activity predicted the shift in impulsivity. However, PrL activity did not track impulsivity in female rats. Additionally, although the NAc core tracked several aspects of behavior in the task, it did not track impulsivity in either sex. Together, these findings demonstrate a sex-dependent role for the PrL in impulsivity both before and after a history of cocaine.

Circuit selectivity in drug versus natural reward seeking behaviors

Substance use disorder (SUD) is characterized, in part by behavior biased toward drug use and away from natural sources of reward (e.g., social interaction, food, sex). The neurobiological underpinnings of SUDs reveal distinct brain regions where neuronal activity is necessary for the manifestation of SUD-characteristic behaviors. Studies that specifically examine how these regions are involved in behaviors motivated by drug versus natural reward allow determinations of which regions are necessary for regulating seeking of both reward types, and appraisals of novel SUD therapies for off-target effects on behaviors motivated by natural reward. Here, we evaluate studies directly comparing regulatory roles for specific brain regions in drug versus natural reward. While it is clear that many regions drive behaviors motivated by all reward types, based on the literature reviewed we propose a set of interconnected regions that become necessary for behaviors motivated by drug, but not natural rewards. The circuitry is selectively necessary for drug seeking includes an Action/Reward subcircuit, comprising nucleus accumbens, ventral pallidum, and ventral tegmental area, a Prefrontal subcircuit comprising prelimbic, infralimbic, and insular cortices, a Stress subcircuit comprising the central nucleus of the amygdala and the bed nucleus of the stria terminalis, and a Diencephalon circuit including lateral hypothalamus. Evidence was mixed for nucleus accumbens shell, insular cortex, and ventral pallidum. Studies for all other brain nuclei reviewed supported a necessary role in regulating both drug and natural reward seeking. Finally, we discuss emerging strategies to further disambiguate the necessity of brain regions in drug- versus natural reward-associated behaviors.

Prior Cocaine Exposure Increases Firing to Immediate Reward While Attenuating Cue and Context Signals Related to Reward Value in the Insula

The insula contributes to behavioral control and is disrupted by substance abuse, yet we know little about the neural signals underlying these functions or how they are disrupted after chronic drug self-administration. Here, male and female rats self-administered either cocaine (experimental group) or sucrose (control) for 12 consecutive days. After a 1 month withdrawal period, we recorded from insula while rats performed a previously learned reward-guided decision-making task. Cocaine-exposed rats were more sensitive to value manipulations and were faster to respond. These behavioral changes were accompanied by elevated counts of neurons in the insula that increased firing to reward. These neurons also fired more strongly at the start of long-delay trials, when a more immediate reward would be expected, and fired less strongly in anticipation of the actual delivery of delayed rewards. Although reward-related firing to immediate reward was enhanced after cocaine self-administration, reward-predicting cue and context signals were attenuated. In addition to revealing novel firing patterns unique to insula, our data suggest changes in such neural activity likely contribute to impaired decision making observed after drug use.SIGNIFICANCE STATEMENT The insula plays a clear role in drug addiction and drug-induced impairments of decision making, yet there is little understanding of its underlying neural signals. We found that chronic cocaine self-administration reduces cue and context encoding in insula while enhancing signals related to immediate reward. These changes in neural activity likely contribute to impaired decision making and impulsivity observed after drug use.

Stimulation in the Rat Anterior Insula and Anterior Cingulate During an Effortful Weightlifting Task

When performing tasks, animals must continually assess how much effort is being expended, and gage this against ever-changing physiological states. As effort costs mount, persisting in the task may be unwise. The anterior cingulate cortex (ACC) and the anterior insular cortex are implicated in this process of cost-benefit decision-making, yet their precise contributions toward driving effortful persistence are not well understood. Here we investigated whether electrical stimulation of the ACC or insular cortex would alter effortful persistence in a novel weightlifting task (WLT). In the WLT an animal is challenged to pull a rope 30 cm to trigger food reward dispensing. To make the action increasingly effortful, 45 g of weight is progressively added to the rope after every 10 successful pulls. The animal can quit the task at any point - with the rope weight at the time of quitting taken as the "break weight." Ten male Sprague-Dawley rats were implanted with stimulating electrodes in either the ACC [cingulate cortex area 1 (Cg1) in rodent] or anterior insula and then assessed in the WLT during stimulation. Low-frequency (10 Hz), high-frequency (130 Hz), and sham stimulations were performed. We predicted that low-frequency stimulation (LFS) of Cg1 in particular would increase persistence in the WLT. Contrary to our predictions, LFS of Cg1 resulted in shorter session duration, lower break weights, and fewer attempts on the break weight. High-frequency stimulation of Cg1 led to an increase in time spent off-task. LFS of the anterior insula was associated with a marginal increase in attempts on the break weight. Taken together our data suggest that stimulation of the rodent Cg1 during an effortful task alters certain aspects of effortful behavior, while insula stimulation has little effect.

Novelty preferences and cocaine-associated cues influence regions associated with the salience network in juvenile female rats

Preferences for novel environments (novelty-seeking) is a risk factor for addiction, with little known about its underlying circuitry. Exposure to drug cues facilitates addiction maintenance, leading us to hypothesize that exposure to a novel environment activates a shared neural circuitry. Stimulation of the D1 receptor in the prelimbic cortex increases responsivity to drug-associated environments. Here, we use D1 receptor overexpression in the prelimbic cortex to probe brain responses to novelty-preferences (in a free-choice paradigm) and cocaine-associated odors following place conditioning. These same cocaine-conditioned odors were used to study neural circuitry with Blood Oxygen Level Dependent (BOLD) activity. D1 overexpressing females had deactivated BOLD signals related to novelty-preferences within the insula cortex and amygdala and activation in the frontal cortex and dopamine cell bodies. BOLD responses to cocaine cues were also sensitive to D1. Control females demonstrated a place preference for cocaine environments with no significant BOLD response, while D1 overexpressing females demonstrated a place aversion and weak BOLD responses to cocaine-conditioned odor cues within the insula cortex. For comparison, we provide data from an earlier study with juvenile males overexpressing D1 that show a strong preference for cocaine and elevated BOLD responses. The results support the use of a pharmacological manipulation (e.g., D1 overexpression) to probe the neural circuitry downstream from the prelimbic cortex.

Systemic nicotine enhances opioid self-administration and modulates the formation of opioid-associated memories partly through actions within the insular cortex

Habitual use of nicotine containing products increases propensity to misuse prescription opioids and its prevalence is substantially increased in individuals currently involved in opioid-treatment programs. Nicotine enhances self-administration of many classes of drugs in rodents, though evidence for direct effects on opioids is lacking. We sought to measure the effects of nicotine pretreatment on the reinforcing efficacy of opioids in both self-administration and contextual conditioning paradigms. First, we measured the effect of systemic nicotine pretreatment on self-administration of two opioids. Additionally, we measured the degree to which systemic nicotine pretreatment impacts the formation of morphine-associated contextual memories in conditioned taste avoidance and place preference paradigms. Given the involvement of the insula in the maintenance of substance abuse, its importance in nicotine addiction, and findings that insular inactivation impairs contextual drug conditioning, we examined whether nicotine administered directly to the insula could recapitulate the effects of systemic nicotine. We demonstrate that systemic nicotine pretreatment significantly enhances opioid self-administration and alters contextual conditioning. Furthermore, intra-insula nicotine similarly altered morphine contextual conditioning by blocking the formation of taste avoidance at all three morphine doses tested (5.0, 10, and 20 mg/kg), while shifting the dose–response curve of morphine in the place preference paradigm rightward. In conclusion, these data demonstrate that nicotine facilitates opioid intake and is partly acting within the insular cortex to obfuscate aversive opiate memories while potentiating approach to morphine-associated stimuli at higher doses.

Limited versus extended cocaine intravenous self-administration: Behavioral effects and electrophysiological changes in insular cortex

Aims

Limited vs extended drug exposure has been proposed as one of the key factors in determining the risk of relapse, which is the primary characteristic of addiction behaviors. The current studies were designed to explore the related behavioral effects and neuronal alterations in the insular cortex (IC), an important brain region involved in addiction.

Methods

Experiments started with rats at the age of 35 days, a typical adolescent stage when initial drug exposure occurs often in humans. The drug‐seeking/taking behaviors, and membrane properties and intrinsic excitability of IC pyramidal neurons were measured on withdrawal day (WD) 1 and WD 45‐48 after limited vs extended cocaine intravenous self‐administration (IVSA).

Results

We found higher cocaine‐taking behaviors at the late withdrawal period after limited vs extended cocaine IVSA. We also found minor but significant effects of limited but not extended cocaine exposure on the kinetics and amplitude of action potentials on WD 45, in IC pyramidal neurons.

Conclusion

Our results indicate potential high risks of relapse in young rats with limited but not extended drug exposure, although the adaptations detected in the IC may not be sufficient to explain the neural changes of higher drug‐taking behaviors induced by limited cocaine IVSA.

Regional Activity in the Rat Anterior Cingulate Cortex and Insula during Persistence and Quitting in a Physical-Effort Task

As animals carry out behaviors, particularly costly ones, they must constantly assess whether or not to persist in the behavior or quit. The anterior cingulate cortex (ACC) has been shown to assess the value of behaviors and to be especially sensitive to physical effort costs. Complimentary to these functions, the insula is thought to represent the internal state of the animal including factors such as hunger, thirst, and fatigue. Using a novel weight-lifting task for rats, we characterized the local field potential (LFP) activity of the ACC and anterior insula (AI) during effort expenditure. In the task, male rats are challenged to work for sucrose reward, which costs progressively more effort over time to obtain. Rats are able to quit the task at any point. We found modest shifts in LFP theta (7-9 Hz) activity as the task got progressively more difficult in terms of absolute effort expenditure. However, when the LFP data were analyzed based on the relative progress of the rat toward quitting the task, substantial shifts in LFP power in the theta and gamma (55-100 Hz) frequency bands were observed in ACC and AI. Both ACC and AI theta power decreased as the rats got closer to quitting, while ACC and AI gamma power increased. Furthermore, coherency between ACC and AI in the delta (2-4 Hz) range shifted alongside the performance state of the rat. Overall, we show that ACC and AI LFP activity changes correlate to the relative performance state of rats in an effort-based task.

Postretrieval Microinjection of Baclofen Into the Agranular Insular Cortex Inhibits Morphine-Induced CPP by Disrupting Reconsolidation

Environmental cues associated with drug abuse are powerful mediators of drug craving and relapse in substance-abuse disorders. Consequently, attenuating the strength of cue-drug memories could reduce the number of factors that cause drug craving and relapse. Interestingly, impairing cue-drug memory reconsolidation is a generally accepted strategy aimed at reducing the intensity of cues that trigger drug-seeking and drug-taking behaviors. In addition, the agranular insular cortex (AI) is an important component of the neural circuits underlying drug-related memory reconsolidation. GABA_B receptors (GABA_BRs) are potential targets for the treatment of addiction, and baclofen (BLF) is the only prototypical GABA_B agonist available for application in clinical addiction treatment. Furthermore, ΔFosB is considered a biomarker for the evaluation of potential therapeutic interventions for addiction. Here, we used the morphine-induced conditioned place preference (CPP) paradigm to investigate whether postretrieval microinjections of BLF into the AI could affect reconsolidation of drug-reward memory, reinstatement of CPP, and the level of ΔFosB in mice. Our results showed that BLF infused into the AI immediately following morphine CPP memory retrieval, but not 6 h postretrieval or following nonretrieval, could eliminate the expression of a morphine CPP memory. This effect persisted in a morphine-priming–induced reinstatement test, suggesting that BLF in the AI was capable of preventing the reconsolidation of the morphine CPP memory. Our results also showed that the elimination of morphine CPP memory was associated with reduced morphine-associated ΔFosB expression in the longer term. Taken together, the results of our research provide evidence to support that GABA_BRs in the AI have an important role in drug-cue memory reconsolidation and further our understanding of the role of the AI in drug-related learning and memory.

The kappa opioid receptor modulates GABA neuron excitability and synaptic transmission in midbrainprojections from the insular cortex

As an integrative hub, the insular cortex (IC) translates external cues into interoceptive states that generate complex physiological, affective, and behavioral responses. However, the precise circuit and signaling mechanisms in the IC that modulate these processes are unknown. Here, we describe a midbrain-projecting microcircuit in the medial aspect of the agranular IC that signals through the Gαi/o-coupled kappa opioid receptor (KOR) and its endogenous ligand dynorphin (Dyn). Within this microcircuit, Dyn is robustly expressed in layer 2/3, while KOR is localized to deep layer 5, which sends a long-range projection to the substantia nigra (SN). Using ex vivo electrophysiology, we evaluated the functional impact of KOR signaling in layer 5 of the IC. We found that bath application of dynorphin decreased GABA release and increased glutamate release on IC-SN neurons, but did not alter their excitability. Conversely, dynorphin decreased the excitability of GABA neurons without altering synaptic transmission. Pretreatment with the KOR antagonist nor-BNI blocked the effects of dynorphin in IC-SN neurons and GABA neurons, indicating that the changes in synaptic transmission and excitability were selectively mediated through KOR. Selective inhibition of IC GABA neurons using a KOR-derived DREADD recapitulated these effects. This work provides insight into IC microcircuitry and indicates that Dyn/KOR signaling may act to directly reduce activity of layer 5 GABA neurons. In turn, KOR-driven inhibition of GABA promotes disinhibition of IC-SN neurons, which can modulate downstream circuits. Our findings present a potential mechanism whereby chronic upregulation of IC Dyn/KOR signaling can lead to altered subcortical function and downstream activity.

The presence of a social stimulus reduces cocaine-seeking in a place preference conditioning paradigm

BACKGROUND

One challenge in the treatment of substance use disorders is to re-engage the interest toward non-drug-related activities. Among these activities, social interaction has had a prominent role due to its positive influence on treatment outcome.

AIMS AND METHODS

Our aim was to study whether the presence of a social stimulus during the cocaine-induced conditioned place preference test was able to reduce the time spent in the drug-paired compartment. For that purpose, mice were trained for four days on a conditioned place preference task with one compartment paired with cocaine and the opposite with saline. On the test day, we introduced an unfamiliar juvenile male mouse into the saline-conditioned compartment (inside a pencil cup) to analyse the animal preference towards the two rewarding stimuli (cocaine vs mouse). Additionally, to discard the possible effect of novelty, as well as the housing condition (social isolation) on social preference, we decided to include a novel object during the test session, as well as perform the same conditioned place preference protocol with a group of animals in social housing conditions.

RESULTS

The social stimulus was able to reduce the preference for cocaine and enhance the active interaction with the juvenile mouse (sniffing) compared to the empty pencil cup paired with the drug. The introduction of a novel object during the test session did not reduce the preference for the cocaine-paired compartment, and interestingly, the preference for the social stimulus was independent of the housing condition. c-Fos immunohistochemistry revealed a different pattern of activation based on cocaine-paired conditioning or the presence of social stimulus.

CONCLUSIONS

These results suggest that social interaction could constitute a valuable component in the treatment of substance use disorders by reducing the salience of the drug.

Prelimbic Cortical Neurons Track Preferred Reward Value and Reflect Impulsive Choice during Delay Discounting Behavior

In delay discounting, individuals discount the value of a reward based on the delay to its receipt. The prelimbic cortex (PrL) is heavily interconnected with several brain regions implicated in delay discounting, but the specific contributions of the PrL to delay discounting are unknown. Here, we used multineuron electrophysiological recording methods in Long-Evans male (n = 10) and female (n = 9) rats to characterize the firing dynamics of PrL neurons during discrete cue and lever press events in a delay discounting task. Rats' initial preference for the large reward decreased as delays for that outcome increased across blocks, reflecting classic discounting behavior. Electrophysiological recordings revealed that subgroups of neurons exhibited phasic responses to cue presentations and lever presses. These phasic neurons were found to respond to either large/delay, small/immediate, or both trial types and the percentage of these neurons shifted across blocks as the expected value of the reward changed. Critically, this shift was only seen during trials in which animals could choose their preferred option (free choice trials) and not during trials where animals could choose only one option (forced choice trials). Further, this shift was dependent on rats' inherent impulsivity because high impulsive rats demonstrated a greater percentage of small/immediate-responsive neurons as the task progressed. Collectively, these findings suggest a unique role for the PrL in encoding reward value during delay discounting that is influenced by individual differences in impulsivity.SIGNIFICANCE STATEMENT In delay discounting, individuals discount the value of a reward based on the delay to its receipt. Here, we used electrophysiology to investigate the role of the prelimbic cortex (PrL) in this process. We found that subsets of neurons shifted activity as a function of the changing expected delay and reward magnitude, but this shift was only evident during trials in which animals could choose their preferred option. Further, this dynamic neural activity depended on rats' inherent impulsivity, with impulsive rats exhibiting a stronger neural shift toward the immediate reward as the task progressed. These findings suggest a role for the PrL in encoding reward value during delay discounting that is influenced by goal-directed context and individual differences in impulsivity.