Less is more: prolonged intermittent access cocaine self-administration produces incentive-sensitization and addiction-like behavior

Intermittent nicotine access is as effective as continuous access in promoting nicotine seeking and taking in rats

RATIONALE

Nicotine is a principal psychoactive agent in tobacco, contributing to tobacco's addictive potential. Preclinical studies on the effects of voluntary nicotine intake typically use self-administration procedures that provide continuous nicotine access during each self-administration session. However, many smokers consume cigarettes intermittently rather than continuously throughout each day. For drugs including cocaine and opioids, research in laboratory rats shows that intermittent intake can be more effective than continuous intake in producing patterns of drug use relevant to addiction.

OBJECTIVE

We determined how intermittent versus continuous nicotine self-administration influences nicotine seeking and taking behaviours.

METHODS

Female and male rats had continuous (i.e., Long Access; LgA, 6 h/day) or intermittent (IntA; 12 min ON, 60 min OFF, for 6 h/day) access to intravenous nicotine (15 µg/kg/infusion), for 12 daily sessions. We then assessed intake, responding for nicotine under a progressive ratio schedule of drug reinforcement and cue- and nicotine-induced reinstatement of drug seeking. We also estimated nicotine pharmacokinetic parameters during LgA and IntA self-administration.

RESULTS

Overall, LgA rats took twice more nicotine than did IntA rats, yielding more sustained increases in estimated brain concentrations of the drug. However, the two groups showed similar motivation to seek and take nicotine, as measured using reinstatement and progressive ratio procedures, respectively.

CONCLUSIONS

Intermittent nicotine use is just as effective as continuous use in producing addiction-relevant behaviours, despite significantly less nicotine exposure. This has implications for modeling nicotine self-administration patterns in human smokers and resulting effects on brain and behaviour.

Punishment resistance for cocaine is associated with inflexible habits in rats

Addiction is characterized by continued drug use despite negative consequences. In an animal model, a subset of rats continues to self-administer cocaine despite footshock consequences, showing punishment resistance. We sought to test the hypothesis that punishment resistance arises from failure to exert goal-directed control over habitual cocaine seeking. While habits are not inherently permanent or maladaptive, continued use of habits under conditions that should encourage goal-directed control makes them maladaptive and inflexible. We trained male and female Sprague Dawley rats on a seeking-taking chained schedule of cocaine self-administration. We then exposed them to four days of punishment testing in which footshock was delivered randomly on one-third of trials. Before and after punishment testing (four days pre-punishment and ≥ four days post-punishment), we assessed whether cocaine seeking was goal-directed or habitual using outcome devaluation via cocaine satiety. We found that punishment resistance was associated with continued use of habits, whereas punishment sensitivity was associated with increased goal-directed control. Although punishment resistance for cocaine was not predicted by habitual responding pre-punishment, it was associated with habitual responding post-punishment. In parallel studies of food self-administration, we similarly observed that punishment resistance was associated with habitual responding post-punishment but not pre-punishment in males, although it was associated with habitual responding both pre- and post-punishment in females, indicating that punishment resistance was predicted by habitual responding in food-seeking females. These findings indicate that punishment resistance is related to habits that have become inflexible and persist under conditions that should encourage a transition to goal-directed behavior.

Ultra-Processed Food Addiction: A Research Update

PURPOSE OF REVIEW

Detail recent advancements in the science on ultra-processed food (UPF) addiction, focusing on estimated prevalence rates and emerging health disparities; progress towards identifying biological underpinnings and behavioral mechanisms; and implications for weight management.

RECENT FINDINGS

Notable developments in the field have included: (1) estimating the global prevalence of UPF addiction at 14% of adults and 15% of youths; (2) revealing health disparities for persons of color and those with food insecurity; (3) observing altered functioning across the brain-gut-microbiome axis; (4) providing early evidence for UPF withdrawal; and (5) elucidating poorer weight management outcomes among persons with UPF addiction. The breadth of recent work on UPF addiction illustrates continued scientific and public interest in the construct and its implications for understanding and treating overeating behaviors and obesity. One pressing gap is the lack of targeted interventions for UPF addiction, which may result in more optimal clinical outcomes for this underserved population.

Relapse after intermittent access to cocaine: Discriminative cues more effectively trigger drug seeking than do conditioned cues

RATIONALE

When people with drug addiction encounter cues associated with drug use, this can trigger cravings and relapse. These cues can include conditioned stimuli (CSs) signaling drug delivery and discriminative stimuli (DSs) signaling drug availability. Compared to CS effects, DS effects are less explored in preclinical studies on cue-induced relapse.

OBJECTIVE

We compared CS and DS effects on reward seeking following abstinence from intermittent-access cocaine (or sucrose) self-administration.

METHODS

During 15-20 intermittent-access sessions, rats self-administered i.v. cocaine or sucrose pellets paired with a light-tone CS. Cocaine/sucrose was available for 5-min (signalled by a light; DS+) and unavailable for 25 min (signalled by different lighting conditions; DS-), and this cycled for 4 h/session. Following abstinence, we measured cocaine/sucrose seeking under extinction triggered by CS and DS presentation, and instrumental responding reinforced by these cues.

RESULTS

Across intermittent-access sessions, rats increased lever pressing for cocaine or sucrose during DS+ periods and decreased responding during DS- periods. On days 2 and 21 of abstinence, only presentation of the DS+ or DS+ and CS combined elicited increased cocaine/sucrose-seeking behaviour (i.e., increased active lever presses). Presenting the DS- alongside the DS+ suppressed the increased cocaine-seeking behaviour otherwise produced by the DS+ . Finally, on day 21 of abstinence, rats showed equivalent levels of lever pressing reinforced by the DS+ , CS and by the DS+ and CS combined, suggesting comparable conditioned reinforcing value.

CONCLUSIONS

After intermittent self-administration, cocaine-associated DSs and CSs acquire similar conditioned reinforcing properties, but DSs more effectively trigger increases in drug seeking.

Effects of access condition on substance use disorder-like phenotypes in male and female rats self-administering MDPV or cocaine

Substance use disorder (SUD) is a heterogeneous disorder, where severity, symptoms, and patterns of substance use vary across individuals. Yet, when rats are allowed to self-administer drugs such as cocaine under short-access conditions, their behavior tends to be well-regulated and homogeneous in nature; though individual differences can emerge when rats are provided long- or intermittent-access to cocaine. In contrast to cocaine, significant individual differences emerge when rats are allowed to self-administer 3,4-methylenedioxypyrovalerone (MDPV), even under short-access conditions, wherein ~30% of rats rapidly transition to high levels of drug-taking. This study assessed the SUD-like phenotypes of male and female Sprague Dawley rats self-administering MDPV (0.032 mg/kg/infusion) or cocaine (0.32 mg/kg/infusion) by comparing level of drug intake, responding during periods of signaled drug unavailability, and sensitivity to footshock punishment to test the hypotheses that: (1) under short-access conditions, rats that self-administer MDPV will exhibit a more robust SUD-like phenotype than rats that self-administered cocaine; (2) female rats will have a more severe phenotype than male rats; and (3) compared to short-access, long- and intermittent-access to MDPV or cocaine self-administration will result in a more robust SUD-like phenotype. After short-access, rats that self-administered MDPV exhibited a more severe phenotype than rats that self-administered cocaine. Though long- and intermittent-access to cocaine and MDPV self-administration altered drug-taking patterns, manipulating access conditions did not systematically alter their SUD-like phenotype. Evidence from behavioral and quantitative autoradiography studies suggest that these differences are unlikely due to changes in expression levels of dopamine transporter, dopamine D2 or D3 receptors, or 5-HT1B, 5-HT2A, or 5-HT2C receptors, though these possibilities cannot be ruled out. These results show that the phenotype exhibited by rats self-administering MDPV differs from that observed for rats self-administering cocaine, and suggests that individuals that use MDPV and/or related cathinones may be at greater risk for developing a SUD, and that short-access MDPV self-administration may provide a useful method to understand the factors that mediate the transition to problematic or disordered substance use in humans.

DREADD activation of the lateral orbitofrontal increases cocaine-taking and cocaine-seeking in male and female rats during intermittent access self-administration under risky conditions

Addiction is a disorder that can be characterized in part as the constant pursuit of a particular substance despite negative consequences. Although the orbitofrontal cortex (OFC) is known to regulate risk-taking more generally and be critical to the development of addiction, its role in regulating drug use under risk-taking conditions is unknown. To address this, we examined drug-taking and drug-seeking in male and female rats under conditions where cocaine infusions were paired with foot shock punishment 50% of the time and combined this paradigm with cFos immunohistochemistry. We found that rats that showed higher levels of drug-taking and drug-seeking prior to punishment showed decreased responding during self-administration sessions under risky conditions and lower levels of c-Fos expression in the lateral but not medial OFC. However, despite these initial differences in responses to infusions paired with foot shocks, all rats showed decreased responding with additional punishment sessions. We then used chemogenetic viral approaches to examine how altering activity of the lateral OFC affects drug-taking and drug-seeking during punished drug use. Although there was no effect of Gi/o DREADD-mediated inhibition of the lateral OFC on these behaviors, Gq DREADD-mediated activation increased drug-taking and drug-seeking when drug use was associated with foot shock 50% of the time. Interestingly, this manipulation had no effect on non-risky self-administration behavior. These results suggest that the involvement of lateral OFC in cocaine use is context-sensitive and influences decision-making based on negative outcomes.

Separating desire from prediction of outcome value

Individuals typically want what they expect to like, often based on memories of previous positive experiences. However, in some situations desire can decouple completely from memories and from learned predictions of outcome value. The potential for desire to separate from prediction arises from independent operating rules that control motivational incentive salience. Incentive salience, or 'wanting', is a type of mesolimbic desire that evolved for adaptive goals, but can also generate maladaptive addictions. Two proof-of-principle examples are presented here to show how motivational 'wanting' can soar above memory-based predictions of outcome value: (i) 'wanting what is remembered to be disgusting', and (ii) 'wanting what is predicted to hurt'. Consequently, even outcomes remembered and predicted to be negatively aversive can become positively 'wanted'. Similarly, in human addictions, people may experience powerful cue-triggered cravings for outcomes that are not predicted to be enjoyable.

Unpredictable, intermittent access to sucrose or water promotes increased reward pursuit in rats

Reward uncertainty can sensitize reward pathways, promoting increased reward-seeking and -taking behaviours. This is relevant to human conditions such as pathological gambling, eating disorders and drug addiction. In the context of addiction, preclinical self-administration procedures have been developed to model the intermittency of human drug use. These intermittent-access (IntA) procedures involve intermittent but predictable access to drug during self-administration sessions. However, human drug use typically involves intermittent and unpredictable drug access. We introduce a new procedure modeling unpredictable, intermittent access (UIntA) to a reinforcer, and we compare it to procedures that provide predictable reinforcer availability; continuous (ContA) or intermittent (IntA) access. Female rats self-administered water or liquid sucrose in daily hour-long sessions. IntA and ContA rats had access to a fixed volume of water or sucrose (0.1 ml), under a fixed ratio 3 schedule of reinforcement. IntA rats had predictable 5-min reinforcer ON and 25-min reinforcer OFF periods. ContA rats had 60 min of reinforcer access during each session. For UIntA rats, variation in the length of ON and OFF periods (1, 5 or 9 min/period), response requirement (variable ratio 3 schedule of reinforcement), reinforcer probability (50%) and quantity (0, 0.1 or 0.2 ml) introduced reward uncertainty. Following 14 daily self-administration sessions, UIntA rats showed the highest levels of responding for water or sucrose under progressive ratio conditions, responding under extinction conditions, and cue-induced reinstatement of sucrose seeking. Thus, unpredictable, intermittent reward access promotes increased reward pursuit. This has implications for modeling addiction and other disorders of increased reward seeking.

Punishment resistance for cocaine is associated with inflexible habits in rats

Addiction is characterized by continued drug use despite negative consequences. In an animal model, a subset of rats continues to self-administer cocaine despite footshock consequences, showing punishment resistance. We sought to test the hypothesis that punishment resistance arises from failure to exert goal-directed control over habitual cocaine seeking. While habits are not inherently permanent or maladaptive, continued use of habits under conditions that should encourage goal-directed control makes them maladaptive and inflexible. We trained male and female Sprague Dawley rats on a seeking-taking chained schedule of cocaine self-administration (2 h/day). We then exposed them to 4 days of punishment testing, in which footshock (0.4 mA, 0.3 s) was delivered randomly on one-third of trials, immediately following completion of seeking and prior to extension of the taking lever. Before and after punishment testing (4 days pre-punishment and ≥4 days post-punishment), we assessed whether cocaine seeking was goal-directed or habitual using outcome devaluation via cocaine satiety. We found that punishment resistance was associated with continued use of habits, whereas punishment sensitivity was associated with increased goal-directed control. Although punishment resistance was not predicted by habitual responding pre-punishment, it was associated with habitual responding post-punishment. In parallel studies of food self-administration, we similarly observed that punishment resistance was associated with habitual responding post-punishment but not pre-punishment. These findings indicate that punishment resistance is related to habits that have become inflexible and persist under conditions that should encourage a transition to goal-directed behavior.

Intermittent-access operant alcohol self-administration promotes binge-like drinking and drinking despite negative consequences in male and female heterogeneous stock rats

The study of Alcohol Use Disorders (AUD) in preclinical models is hampered by difficulty in training rodents to voluntarily consume high levels of alcohol. The intermittency of alcohol access/exposure is well known to modulate alcohol consumption (e.g., alcohol deprivation effect, intermittent-access two-bottle-choice) and recently, intermittent access operant self-administration procedures have been used to produce more intense and binge-like self-administration of intravenous psychostimulant and opioid drugs. In the present study, we sought to systematically manipulate the intermittency of operant self-administered alcohol access to determine the feasibility of promoting more intensified, binge-like alcohol consumption. To this end, 24 male and 23 female NIH Heterogeneous Stock rats were trained to self-administer 10% w/v ethanol, before being split into three different-access groups. Short Access (ShA) rats continued receiving 30-min training sessions, Long Access (LgA) rats received 16-h sessions, and Intermittent Access (IntA) rats received 16-h sessions, wherein the hourly alcohol-access periods were shortened over sessions, down to 2 min. IntA rats demonstrated an increasingly binge-like pattern of alcohol drinking in response to restriction of alcohol access, while ShA and LgA rats maintained stable intake. All groups were tested on orthogonal measures of alcohol-seeking and quinine-punished alcohol drinking. The IntA rats displayed the most punishment-resistant drinking. In a separate experiment, we replicated our main finding, that intermittent access promotes a more binge-like pattern of alcohol self-administration using 8 male and 8 female Wistar rats. In conclusion, intermittent access to self-administered alcohol promotes more intensified self-administration. This approach may be useful in developing preclinical models of binge-like alcohol consumption in AUD.

Metabotropic glutamate group II receptor activation in the ventrolateral dorsal striatum suppresses incentive motivation for cocaine in rats

RATIONALE

After a history of intermittent cocaine intake, rats develop patterns of drug use characteristic of substance use disorder. The dorsal striatum is involved in the increased pursuit of cocaine after intermittent drug self-administration experience. Within the dorsal striatum, chronic cocaine use changes metabotropic glutamate type II receptor (mGlu_2/3) density and function.

OBJECTIVES

We examined the extent to which activity at Glu_2/3 receptors mediates responding for cocaine after intermittent cocaine use.

METHODS

Male (n = 11) and female (n = 10) Wistar rats self-administered 0.25 mg/kg/infusion cocaine during 10 daily intermittent access (IntA) sessions (5 min ON/25 min OFF, for 5 h/session). We then examined the effects of microinjections of the mGlu_2/3 receptor agonist LY379268 (0, 1, and 3 µg/hemisphere) into the ventrolateral part of the dorsal striatum on cocaine self-administration under a progressive ratio schedule of reinforcement.

RESULTS

Across 10 IntA sessions, the sexes showed similar levels of cocaine intake. In females only, locomotion significantly increased over sessions, suggesting that female rats developed psychomotor sensitization to self-administered cocaine. After 10 IntA sessions, intra-dorsal striatum LY379268 significantly reduced breakpoints achieved for cocaine, active lever presses, and cocaine infusions earned under progressive ratio. LY379268 had no effects on locomotion or inactive lever presses, indicating no motor effects.

CONCLUSIONS

These results suggest that mGlu_2/3 receptor activation in the ventrolateral dorsal striatum suppresses incentive motivation for cocaine, and this holds promise for new treatments to manage substance use disorder.

Intermittent access cocaine self-administration produces context-specific escalation and increased motivation

The intermittent-access (IntA) self-administration procedure has been reported to produce intensified addiction-like behavior compared to continuous-access (ContA) procedures. In a common variation of the IntA procedure, cocaine is available for 5 min at the beginning of each half hour of a 6-h session. In contrast, during ContA procedures, cocaine is available continuously throughout a session, typically lasting one or more hours. Previous studies comparing procedures have used between-subjects designs, where separate groups of rats self-administer cocaine on either IntA or ContA procedures. The present study used a within-subjects design where subjects self-administered cocaine on the IntA procedure in one context and self-administered cocaine on the continuous short-access (ShA) procedure in another context during separate sessions. Across sessions, rats escalated cocaine intake in the IntA, but not ShA, context. Following sessions eight and 11, rats were administered a progressive ratio test in each context to monitor the change in cocaine motivation. Rats obtained more cocaine infusions on the progressive ratio test in the IntA context than in the ShA context following 11 sessions. These results suggest that addiction-like behaviors following IntA self-administration may be influenced by context-specific learning factors.

Cocaine-induced plasticity, motivation, and cue responsivity do not differ in obesity-prone vs obesity-resistant rats; implications for food addiction

RATIONALE

Compared to obesity-resistant rats, obesity-prone rats consume more food, work harder to obtain food, show greater motivational responses to food-cues, and show greater striatal plasticity in response to eating sugary/fatty foods. Therefore, it is possible that obesity-prone rats may also be more sensitive to the motivational properties of cocaine and cocaine-paired cues, and to plasticity induced by cocaine.

OBJECTIVE

To examine baseline differences in motivation for cocaine and effects of intermittent access (IntA) cocaine self-administration on cocaine motivation, neurobehavioral responsivity to cocaine-paired cues, and locomotor sensitization in male obesity-prone vs obesity-resistant rats.

METHODS

Intravenous cocaine self-administration was used to examine drug-taking and drug-seeking in males. Motivation for cocaine was measured using a within session threshold procedure. Cue-induced c-Fos expression in mesocorticolimbic regions was measured.

RESULTS

Drug-taking and drug-seeking, cue-induced c-Fos, locomotor sensitization, and preferred level of cocaine consumption (Q0) were similar between obesity-prone and obesity-resistant groups. Maximal responding during demand testing (Rmax) was lower in obesity-prone rats. IntA experience enhanced motivation for cocaine (Pmax) in obesity-prone rats.

CONCLUSIONS

The results do not support robust inherent differences in motivation for cocaine, cue-induced cocaine seeking, or neurobehavioral plasticity induced by IntA in obesity-prone vs obesity-resistant rats. This contrasts with previously established differences seen for food and food cues in these populations and shows that inherent enhancements in motivation for food and food-paired cues do not necessarily transfer to drugs and drug-paired cues.

Sex chromosome and gonadal hormone contributions to binge-like and aversion-resistant ethanol drinking behaviors in Four Core Genotypes mice

Introduction

While substantial research has focused on the contribution of sex hormones to driving elevated levels of alcohol drinking in female rodents, fewer studies have investigated how genetic influences may underlie sex differences in this behavior.

Methods

We used the Four Core Genotypes (FCG) mouse model to explore the contribution of sex chromosome complement (XX/XY) and gonad type [ovaries (Sry-)/testes (Sry+)] to ethanol (EtOH) consumption and quinine-resistant drinking across two voluntary self-administration tasks: limited access consumption in the home cage and an operant response task.

Results

For limited access drinking in the dark, XY/Sry + (vs. XX/Sry +) mice consumed more 15% EtOH across sessions while preference for 15% EtOH vs. water was higher in XY vs. XX mice regardless of gonad type. XY chromosomes promoted quinine-resistant drinking in mice with ovaries (Sry-) and the estrous cycle did not affect the results. In the operant response task, responding for EtOH was concentration dependent in all genotypes except XX/Sry + mice, which maintained consistent response levels across all concentrations (5-20%) of EtOH. When increasing concentrations of quinine (100-500 μM) were added to the solution, FCG mice were insensitive to quinine-punished EtOH responding, regardless of sex chromosome complement. Sry + mice were further found to be insensitive to quinine when presented in water. Importantly, these effects were not influenced by sensitivity to EtOH's sedative effect, as no differences were observed in the time to lose the righting reflex or the time to regain the righting reflex between genotypes. Additionally, no differences in EtOH concentration in the blood were observed between any of the genotypes once the righting reflex was regained.

Discussion

These results provide evidence that sex chromosome complement regulates EtOH consumption, preference, and aversion resistance and add to a growing body of literature suggesting that chromosomal sex may be an important contributor to alcohol drinking behaviors. Examination of sex-specific genetic differences may uncover promising new therapeutic targets for high-risk drinking.

Repeated dosing with cocaine produces strain-dependent effects on responding for conditioned reinforcement in Collaborative Cross mice

RATIONALE

Cocaine use disorder (CUD) is a highly heritable form of substance use disorder, with genetic variation accounting for a substantial proportion of the risk for transitioning from recreational use to a clinically impairing addiction. With repeated exposures to cocaine, psychomotor and incentive sensitization are observed in rodents. These phenomena are thought to model behavioral changes elicited by the drug that contribute to the progression into addiction, but little is known about how genetic variation may moderate these consequences.

OBJECTIVES

Here, we describe the use of two Collaborative Cross (CC) recombinant inbred mouse strains that either exhibit high (CC018/UncJ) or no (CC027/GeniUncJ) psychomotor sensitization in response to cocaine to measure phenotypes related to incentive sensitization after repeated cocaine exposures; given the relationship of incentive motivation to nucleus accumbens core (NAc) dopamine release and reuptake, we also assessed these neurochemical mechanisms.

METHODS

Adult male and female CC018/UncJ and CC027/GeniUncJ mice underwent Pavlovian conditioning to associate a visual cue with presentation of a palatable food reward, then received five, every-other-day injections of cocaine or vehicle. Following Pavlovian re-training, they underwent testing acquisition of a new operant response for the visual cue, now serving as a conditioned reinforcer. Subsequently, electrically evoked dopamine release was assessed using fast-scan cyclic voltammetry from acute brain slices containing the NAc.

RESULTS

While both strains acquired the Pavlovian association, only CC018/UncJ mice showed conditioned reinforcement and incentive sensitization in response to cocaine, while CC027/GeniUncJ mice did not. Voltammetry data revealed that CC018/UncJ, compared to CC027/GeniUnc, mice exhibited higher baseline dopamine release and uptake. Moreover, chronic cocaine exposure blunted tonic and phasic dopamine release in CC018/UncJ, but not CC027/GeniUncJ, mice.

CONCLUSIONS

Genetic background is a moderator of cocaine-induced neuroadaptations in mesolimbic dopamine signaling, which may contribute to both psychomotor and incentive sensitization and indicate a shared biological mechanism of variation.

Reward-Mediated, Model-Free Reinforcement-Learning Mechanisms in Pavlovian and Instrumental Tasks Are Related

Model-free and model-based computations are argued to distinctly update action values that guide decision-making processes. It is not known, however, if these model-free and model-based reinforcement learning mechanisms recruited in operationally based instrumental tasks parallel those engaged by pavlovian-based behavioral procedures. Recently, computational work has suggested that individual differences in the attribution of incentive salience to reward predictive cues, that is, sign- and goal-tracking behaviors, are also governed by variations in model-free and model-based value representations that guide behavior. Moreover, it is not appreciated if these systems that are characterized computationally using model-free and model-based algorithms are conserved across tasks for individual animals. In the current study, we used a within-subject design to assess sign-tracking and goal-tracking behaviors using a pavlovian conditioned approach task and then characterized behavior using an instrumental multistage decision-making (MSDM) task in male rats. We hypothesized that both pavlovian and instrumental learning processes may be driven by common reinforcement-learning mechanisms. Our data confirm that sign-tracking behavior was associated with greater reward-mediated, model-free reinforcement learning and that it was also linked to model-free reinforcement learning in the MSDM task. Computational analyses revealed that pavlovian model-free updating was correlated with model-free reinforcement learning in the MSDM task. These data provide key insights into the computational mechanisms mediating associative learning that could have important implications for normal and abnormal states.SIGNIFICANCE STATEMENT Model-free and model-based computations that guide instrumental decision-making processes may also be recruited in pavlovian-based behavioral procedures. Here, we used a within-subject design to test the hypothesis that both pavlovian and instrumental learning processes were driven by common reinforcement-learning mechanisms. Sign-tracking and goal-tracking behaviors were assessed in rats using a pavlovian conditioned approach task, and then instrumental behavior was characterized using an MSDM task. We report that sign-tracking behavior was associated with greater model-free, but not model-based, learning in the MSDM task. These data suggest that pavlovian and instrumental behaviors may be driven by conserved reinforcement-learning mechanisms.

Establishment of multi-stage intravenous self-administration paradigms in mice

Genetically tractable animal models provide needed strategies to resolve the biological basis of drug addiction. Intravenous self-administration (IVSA) is the gold standard for modeling psychostimulant and opioid addiction in animals, but technical limitations have precluded the widespread use of IVSA in mice. Here, we describe IVSA paradigms for mice that capture the multi-stage nature of the disorder and permit predictive modeling. In these paradigms, C57BL/6J mice with long-standing indwelling jugular catheters engaged in cocaine- or remifentanil-associated lever responding that was fixed ratio-dependent, dose-dependent, extinguished by withholding the drug, and reinstated by the presentation of drug-paired cues. The application of multivariate analysis suggested that drug taking in both paradigms was a function of two latent variables we termed incentive motivation and discriminative control. Machine learning revealed that vulnerability to drug seeking and relapse were predicted by a mouse's a priori response to novelty, sensitivity to drug-induced locomotion, and drug-taking behavior. The application of these behavioral and statistical-analysis approaches to genetically-engineered mice will facilitate the identification of neural circuits driving addiction susceptibility and relapse and focused therapeutic development.

Intermittent access training produces greater motivation for a non-drug reinforcer than long access training

It has recently been proposed that the intermittent access (IntA) drug self-administration procedure better produces behavioral changes relevant to addiction than the long access (LgA) procedure. In this version of the IntA procedure, the drug is made available for a 5-min period during each half hour of a 6-h session. In contrast, on the LgA procedure, the drug is available continuously for 6 h. Previous studies have found that IntA drug self-administration produces greater drug motivation, measured by increased progressive ratio breakpoints, than LgA self-administration. It has been hypothesized that this effect is due to the rapid, "spiking" brain levels of the drug, and consequent neuroadaptations, experienced by rats during IntA sessions. However, no study has compared the effects of IntA versus LgA training on reinforcer motivation when using a non-drug reinforcer. The present study compared motivation for a saccharin reinforcer after IntA or LgA training. In Experiment 1, separate groups of rats lever-pressed for saccharin on the IntA or LgA procedures. In Experiment 2, a within-subjects design was used where rats pressed one lever on the IntA procedure and another lever on the LgA procedure for saccharin. In both experiments, IntA training produced greater breakpoints than LgA training. As no drug was used here, spiking drug levels could not have been responsible for the increased saccharin motivation observed after IntA training. Instead, it is proposed that differences in stimulus-reinforcer associations learned during IntA versus LgA training may be responsible for the effect. Future research is needed to determine the extent to which such learning factors may contribute to the increased motivation observed after IntA training with drug reinforcers.

D-amphetamine maintenance therapy reduces cocaine use in female rats

RATIONALE

D-amphetamine maintenance therapy is a promising strategy to reduce drug use in cocaine use disorder (addiction). In both male rats and human cocaine users, d-amphetamine treatment reduces cocaine-taking and -seeking. However, this has not been examined systematically in female animals, even though cocaine addiction afflicts both sexes, and the sexes can differ in their response to cocaine.

OBJECTIVES

We determined how d-amphetamine maintenance therapy during cocaine self-administration influences cocaine use in female rats.

METHODS

In experiment 1, two groups of female rats received 14 intermittent access (IntA) cocaine self-administration sessions. One group received concomitant d-amphetamine maintenance treatment (COC + A rats; 5 mg/kg/day, via minipump), the other group did not (COC rats). After discontinuing d-amphetamine treatment, we measured responding for cocaine under a progressive ratio schedule, responding under extinction, and cocaine-primed reinstatement of drug-seeking. In experiment 2, we assessed the effects of d-amphetamine maintenance on these measures in already IntA cocaine-experienced rats. Thus, rats first received 14 IntA cocaine self-administration sessions without d-amphetamine. They then received 14 more IntA sessions, now either with (COC/COC + A rats) or without (COC/COC rats) concomitant d-amphetamine treatment.

RESULTS

In both experiments, d-amphetamine treatment did not significantly influence ongoing cocaine self-administration behaviour. After d-amphetamine treatment cessation, cocaine-primed reinstatement of cocaine-seeking was also unchanged. However, after d-amphetamine treatment cessation, rats responded less for cocaine both under progressive ratio and extinction conditions.

CONCLUSIONS

D-amphetamine treatment can both prevent and reverse increases in the motivation to take and seek cocaine in female animals.

Association of Food Insecurity and Food Addiction Symptoms: A Secondary Analysis of Two Samples of Low-Income Female Adults

BACKGROUND

Household food insecurity persists in the United States and has important implications for health and well-being. Food insecurity in female-identified caregivers is particularly concerning, given its association with their mental health and adverse health outcomes for their children. Food insecurity is associated with disordered eating but, to our knowledge, no prior studies have examined an association between food insecurity and food addiction.

OBJECTIVE

Our aim was to examine whether food insecurity is associated with higher food addiction symptom endorsement in low-income female adults.

DESIGN

Secondary analysis of baseline data from a quasi-experimental study of a mindfulness-based intervention on gestational weight gain among low-income pregnant individuals and an observational study of low-income families.

PARTICIPANTS/SETTING

Participants in study 1 (n = 208) were English-speaking, low-income pregnant individuals with overweight or obesity, recruited in California from 2011 to 2013. Participants in study 2 (n = 181) were English-speaking, low-income female caregivers for children aged 8 through 10 years, recruited in Michigan from 2018 to 2019. Both studies recruited participants from community health clinics, social service agencies, and online advertisements.

MAIN OUTCOME MEASURES

The primary outcome measure was food addiction symptoms, assessed by the Yale Food Addiction Scale.

STATISTICAL ANALYSIS

Multivariate Poisson regression was used to examine the association between household food insecurity and food addiction symptoms in each sample, adjusted for sociodemographic characteristics.

RESULTS

In study 1, pregnant individuals in food-insecure households reported 21% higher food addiction symptoms than pregnant individuals in food-secure households (incidence rate ratio 1.21; 95% CI 1.00 to 1.47; P = .047). In study 2, caregivers in food-insecure households had 56% higher food addiction symptoms than caregivers in food-secure households (incidence rate ratio 1.56; 95% CI 1.01 to 2.40; P = .045).

CONCLUSIONS

These findings provide preliminary support for a relationship between household food insecurity and food addiction. Future research should examine potential mechanisms and whether interventions to reduce food insecurity lower risk of food addiction.

Investigating discriminative stimulus modulation of opioid seeking after conflict-induced abstinence in sign- and goal-tracking rats

RATIONALE

Discriminative stimuli (DS) are cues that predict reward availability. DS are resistant to extinction and motivate drug seeking even after long periods of abstinence. Previous studies have demonstrated that sign-tracking (ST) and goal-tracking (GT) differences in Pavlovian approach predict distinct cue-modulated vulnerabilities to cocaine reinstatement. GT rats show heightened reinstatement to contextual and DS, while ST rats show heightened reinstatement to discrete stimuli. Here we examine whether DS modulate reinstatement after electric barrier-induced abstinence and whether tracking-related relapse vulnerabilities generalize to opioid relapse.

OBJECTIVES

We examine whether DS-modulated reinstatement to fentanyl seeking persists in the presence of reduced adverse consequences after electric barrier-induced abstinence. We also examine whether tracking differences predict the magnitude of DS-modulated reinstatement of fentanyl seeking after electric barrier-induced abstinence.

METHODS

We used Pavlovian lever autoshaping (PLA) training to determine sign-, goal-, and intermediate tracking groups in male and female Sprague Dawley rats. We then trained rats in a DS model of intermittent fentanyl self-administration, and extinguished drug seeking by imposing an electric barrier of increasing intensity. We then measured the level of DS-modulated reinstatement in the presence of a reduced electric barrier intensity.

RESULTS

We report that DS strongly modulate fentanyl seeking after electric barrier-induced abstinence. DS-modulation of fentanyl acquisition, electric barrier-induced abstinence, and reinstatement was similar for sign- and goal-tracking groups.

CONCLUSIONS

Discriminative stimuli powerfully motivate opioid seeking, despite continued aversive consequences. Pavlovian approach differences do not predict the level of DS-modulated reinstatement to fentanyl seeking after conflict-induced abstinence.

Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice

RATIONALE

Drug-induced potentiation of ventral tegmental area (VTA) glutamate signaling contributes critically to the induction of sensitization - an enhancement in responding to a drug following exposure which is thought to reflect neural changes underlying drug addiction. The laterodorsal tegmental nucleus (LDTg) provides one of several sources of glutamate input to the VTA.

OBJECTIVE

We used optogenetic techniques to test either the role of LDTg glutamate cells or their VTA afferents in the development of cocaine sensitization in male VGluT2::Cre mice. These were inhibited using halorhodopsin during each of five daily cocaine exposure injections. The expression of locomotor sensitization was assessed following a cocaine challenge injection 1-week later.

RESULTS

The locomotor sensitization seen in control mice was absent in male mice subjected to inhibition of LDTg-VTA glutamatergic circuitry during cocaine exposure. As sensitization of nucleus accumbens (NAcc) dopamine (DA) overflow is also induced by this drug exposure regimen, we used microdialysis to measure NAcc DA overflow on the test for sensitization. Consistent with the locomotor sensitization results, inhibition of LDTg glutamate afferents to the VTA during cocaine exposure prevented the sensitization of NAcc DA overflow observed in control mice.

CONCLUSIONS

These data identify the LDTg as the source of VTA glutamate critical for the development of cocaine sensitization in male mice. Accordingly, the LDTg may give rise to the synapses in the VTA at which glutamatergic plasticity, known to contribute to the enhancement of addictive behaviors, occurs.

Incubation of cocaine craving coincides with changes in dopamine terminal neurotransmission

Relapse to drug use is one of the major challenges in treating substance use disorders. Exposure to drug-related cues and contexts triggers drug craving, which drives cocaine seeking, and increases the probability of relapse. Clinical and animal studies have shown a progressive intensification of cocaine seeking and craving that develops over the course of abstinence, a phenomenon commonly referred to as incubation of cocaine craving. Although the neurobiology underlying incubation of cocaine craving has been examined – particularly within the context of glutamate plasticity– the extent to which increased cocaine craving engenders mesolimbic dopamine (DA) changes has received relatively little attention. To assess whether incubation of cocaine craving is associated with alterations in DA terminal neurotransmission in the nucleus accumbens core (NAc), we used ex vivo fast scan cyclic voltammetry in female and male rats to assess DA dynamics following short access, long access, or intermittent access to cocaine self-administration followed by 28 days of abstinence. Results indicated that both long access and intermittent access to cocaine produced robust incubation of cocaine craving, which was associated with increases in cocaine potency. In addition, intermittent access self-administration also produced a robust increase in DA uptake rate at baseline. In contrast, short access to cocaine did not engender incubation of cocaine craving, nor produce changes in DA neurotransmission. Together these observations indicate that incubation of cocaine craving coincides with changes in DA transmission, suggesting that underlying changes in mesolimbic DA signaling may contribute to the progressive intensification of drug craving that occurs across periods of abstinence.

Utility of 'substance use disorder' as a heuristic for understanding overeating and obesity

Rates of obesity and obesity-associated diseases have increased dramatically in countries with developed economies. Substance use disorders (SUDs) are characterized by the persistent use of the substance despite negative consequences. It has been hypothesized that overconsumption of palatable energy dense food can elicit SUD-like maladaptive behaviors that contribute to persistent caloric intake beyond homeostatic need even in the face of negative consequences. Palatable food and drugs of abuse act on many of the same motivation-related circuits in the brain, and can induce, at least superficially, similar molecular, cellular, and physiological adaptations on these circuits. As such, applying knowledge about the neurobiological mechanisms of SUDs may serve as useful heuristic to better understand the persistent overconsumption of palatable food that contributes to obesity. However, many important differences exist between the actions of drugs of abuse and palatable food in the brain. This warrants caution when attributing weight gain and obesity to the manifestation of a putative SUD-related behavioral disorder. Here, we describe similarities and differences between compulsive drug use in SUDs and overconsumption in obesity and consider the merit of the concept of "food addiction".

Relapse-like behavior and nAChR sensitization following intermittent access nicotine self-administration

Many tobacco smokers consume nicotine intermittently, but the underlying mechanisms and neurobiological changes associated with intermittent nicotine intake are unclear. Understanding intermittent nicotine intake is a high priority, as it could promote therapeutic strategies to attenuate tobacco consumption. We examined nicotine intake behavior and neurobiological changes in male rats that were trained to self-administer nicotine during brief (5 min) trials interspersed with longer (15 min) drug-free periods. Rats readily adapted to intermittent access (IntA) SA following acquisition on a continuous access (ContA) schedule. Probabilistic analysis of IntA nicotine SA suggested reduced nicotine loading behavior compared to ContA, and nicotine pharmacokinetic modeling revealed that rats taking nicotine intermittently may have increased intake to maintain blood levels of nicotine that are comparable to ContA SA. After IntA nicotine SA, rats exhibited an increase in unreinforced responses for nicotine-associated cues (incubation of craving) and specific alterations in the striatal proteome after 7 days without nicotine. IntA nicotine SA also induced nAChR functional upregulation in the interpeduncular nucleus (IPN), and it enhanced nicotine binding in the brain as determined via [¹¹C]nicotine positron emission tomography. Reducing the saliency of the cue conditions during the 5 min access periods attenuated nicotine intake, but incubation of craving was preserved. Together, these results indicate that IntA conditions promote nicotine SA and nicotine seeking after a nicotine-free period.

PPL-138 (BU10038): A bifunctional NOP/mu partial agonist that reduces cocaine self-administration in rats

The search for new and effective treatments for cocaine use disorder (CUD) is a priority. We determined whether PPL-138 (BU10038), a compound with partial agonist activity at both nociceptin opioid peptide (NOP) and mu-opioid receptors, reduces cocaine consumption, reinstatement, and whether the compound itself produces reinforcing effects in rats. Using an intermittent access (IntA) cocaine self-administration procedure, we found that PPL-138 (0.1 and 0.3 mg/kg) effectively decreased the total number of cocaine infusions and burst-like cocaine intake in both male and female rats. Responses for food in an IntA model of food self-administration were not altered for either sex, although locomotor activity was increased in female but not male rats. Blockade of NOP receptors with the selective antagonist J-113397 (5 mg/kg) did not prevent the PPL-138-induced suppression of cocaine self-administration, whereas blockade of mu-opioid receptors by naltrexone (1 mg/kg) reversed such effect. Consistently, treatment with morphine (1, 3, and 10 mg/kg) dose-dependently reduced IntA cocaine self-administration measures. PPL-138 also reduced reinstatement of cocaine seeking at all doses examined. Although an initial treatment with PPL-138 (2.5, 10, and 40 μg/kg/infusion) appeared rewarding, the compound did not maintain self-administration behavior. Animals treated with PPL-138 showed initial suppression of cocaine self-administration, which was eliminated following repeated daily dosing. However, suppression of cocaine self-administration was retained when subsequent PPL-138 treatments were administered 48 h apart. These findings demonstrate that the approach of combining partial NOP/mu-opioid activation successfully reduces cocaine use, but properties of PPL-138 seem to depend on the timing of drug administration.

Novelty preference does not predict trait cocaine behaviors in male rats

Heightened novelty seeking is a risk factor for the initiation of drug use and development of substance use disorders. In rats, novelty seeking can be examined by assessing preference for a novel environment. Some evidence indicates that high novelty preferring (HNP) rats have higher drug intake compared to low novelty preferring (LNP) rats, although these data are mixed. Moreover, the extent to which the HNP phenotype can predict other initial drug behaviors, including economic demand for cocaine, has not been tested. Here, we screened a cohort (n=60) of male rats for novelty preference and several subsequent cocaine behaviors, including locomotor reactivity to a cocaine priming injection, acquisition of cocaine self-administration, as well as cocaine demand using a within-session behavioral economics procedure. Novelty preference did not correlate with cocaine behaviors, nor were there any differences between HNP and LNP rats identified using a median split strategy. Moreover, regression analyses indicated that novelty preference did not have predictive utility for any of the cocaine behaviors tested. Thus, the extent to which the novelty preference trait can predict initial cocaine-related behaviors in male rats may be limited. This is in contrast to the novel locomotor reactivity phenotype, which is strongly linked with initial cocaine intake, indicating that these traits are distinct and differentially predict cocaine behaviors in rats.

Knockout of Dopamine D3 Receptor Gene Blocked Methamphetamine-Induced Distinct Changes of Dopaminergic and Glutamatergic Synapse in the Nucleus Accumbens Shell of Mice

Structural plasticity changes in the brain are thought to underlie, at least partially, drug-induced persistent changes in behavior. Our previous study reported that increased synaptic density in the nucleus accumbens shell (NAcsh) correlates with and may contribute to behavioral sensitization induced by methamphetamine (METH). However, the distinct changes of dopaminergic and glutamatergic synapses and the modulating effects of dopamine D3 receptor remain unclear. In the current study, we used immunohistochemistry electron-microscopy and immunofluorescence to detect the changes of dopamine D1, D2, and glutamate NR2B-positive synapses and cells in the NAcsh of METH-sensitized wild type (WT) and knockout of dopamine D3 receptor gene (D3^–/–) mice. We found that METH induced long-term behavioral sensitization in WT mice, which was accompanied by an increased number and rate of dopamine D1 receptor-positive synapses and cells, as well as glutamate NR2B-positive synapses and cells. In contrast, the number and rate of dopamine D2 receptor-positive synapses and cells were significantly decreased in the NAcsh of METH-sensitized WT mice. D3^–/– mice exhibited attenuated acute locomotor responses and behavioral sensitization to METH compared with WT mice. Moreover, the knockout of dopamine D3 receptor gene inhibited METH-induced changes of dopaminergic and glutamatergic synapses in the NAcsh of METH-sensitized mice. Taken together, our results suggest that METH induced distinct changes of dopaminergic and glutamatergic synapses and cells in the NAcsh of mice, which was blocked by the knockout of dopamine D3 receptor gene, and may contribute to, at least partially, METH-induced behavior sensitization as well as the modulating effect of the dopamine D3 receptor.

Genetic characterization of outbred Sprague Dawley rats and utility for genome-wide association studies

Sprague Dawley (SD) rats are among the most widely used outbred laboratory rat populations. Despite this, the genetic characteristics of SD rats have not been clearly described, and SD rats are rarely used for experiments aimed at exploring genotype-phenotype relationships. In order to use SD rats to perform a genome-wide association study (GWAS), we collected behavioral data from 4,625 SD rats that were predominantly obtained from two commercial vendors, Charles River Laboratories and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we obtained dense, high-quality genotypes at 291,438 SNPs across 4,061 rats. This genetic data allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that the two populations are highly diverged (F_ST > 0.4). Furthermore, even for rats obtained from the same vendor, there was strong population structure across breeding facilities and even between rooms at the same facility. We performed multiple separate GWAS by fitting a linear mixed model that accounted for population structure and using meta-analysis to jointly analyze all cohorts. Our study examined Pavlovian conditioned approach (PavCA) behavior, which assesses the propensity for rats to attribute incentive salience to reward-associated cues. We identified 46 significant associations for the various metrics used to define PavCA. The surprising degree of population structure among SD rats from different sources has important implications for their use in both genetic and non-genetic studies.

Outbred Sprague Dawley rats are among the most commonly used rats for neuroscience, physiology and pharmacological research; in the year 2020, 4,188 publications contained the keyword “Sprague Dawley”. Rats identified as “Sprague Dawley” are sold by several commercial vendors, including Charles River Laboratories and Harlan Sprague Dawley Inc. (now Envigo). Despite their widespread use, little is known about the genetic diversity of SD. We genotyped more than 4,000 SD rats, which we used for a genome-wide association study (GWAS) and to characterize genetic differences between SD rats from Charles River Laboratories and Harlan. Our analysis revealed extensive population structure both between and within vendors. The GWAS for Pavlovian conditioned approach (PavCA) identified a number of genome-wide significant loci for that complex behavioral trait. Our results demonstrate that, despite sharing an identical name, SD rats that are obtained from different vendors are very different. Future studies should carefully define the exact source of SD rats being used and may exploit their genetic diversity for genetic studies of complex traits.

Inhibition of a cortico-thalamic circuit attenuates cue-induced reinstatement of drug-seeking behavior in "relapse prone" male rats

RATIONALE

Relapse often occurs when individuals are exposed to stimuli or cues previously associated with the drug-taking experience. The ability of drug cues to trigger relapse is believed to be a consequence of incentive salience attribution, a process by which the incentive value of reward is transferred to the reward-paired cue. Sign-tracker (ST) rats that attribute enhanced incentive value to reward cues are more prone to relapse compared to goal-tracker (GT) rats that primarily attribute predictive value to such cues.

OBJECTIVES

The neurobiological mechanisms underlying this individual variation in relapse propensity remains largely unexplored. The paraventricular nucleus of the thalamus (PVT) has been identified as a critical node in the regulation of cue-elicited behaviors in STs and GTs, including cue-induced reinstatement of drug-seeking behavior. Here we used a chemogenetic approach to assess whether "top-down" cortical input from the prelimbic cortex (PrL) to the PVT plays a role in mediating individual differences in relapse propensity.

RESULTS

Chemogenetic inhibition of the PrL-PVT pathway selectively decreased cue-induced reinstatement of drug-seeking behavior in STs, without affecting behavior in GTs. In contrast, cocaine-primed drug-seeking behavior was not affected in either phenotype. Furthermore, when rats were characterized based on a different behavioral phenotype-locomotor response to novelty-inhibition of the PrL-PVT pathway had no effect on either cue- or drug-induced reinstatement.

CONCLUSIONS

These results highlight an important role for the PrL-PVT pathway in vulnerability to relapse that is consequent to individual differences in the propensity to attribute incentive salience to discrete reward cues.

Dissociation Between Incubation of Cocaine Craving and Anxiety-Related Behaviors After Continuous and Intermittent Access Self-Administration

Studies using either continuous or intermittent access cocaine self-administration procedures showed that cocaine seeking increases during abstinence (incubation of cocaine craving), and that this effect is higher after intermittent cocaine access. Other studies showed that cocaine abstinence is characterized by the emergence of stress- and anxiety-related states which were hypothesized to increase relapse vulnerability. We examined whether incubation of cocaine craving and anxiety-related behaviors are correlated and whether intermittent cocaine self-administration would potentiate these behaviors during abstinence. Male rats self-administered cocaine either continuously (6 h/day) or intermittently (5 min ON, 25 min OFF × 12) for 14 days, followed by relapse tests after 1 or 21 abstinence days. A group of rats that self-administered saline served as a control. Anxiety-related behaviors were measured on the same abstinence days, using the novelty induced-hypophagia test. Finally, motivation for cocaine was measured using a progressive ratio reinforcement schedule. Lever-presses after 21 abstinence days were higher than after 1 day and this incubation effect was higher in the intermittent access group. Progressive ratio responding was also higher after intermittent cocaine access. Intermittent and continuous cocaine access did not induce anxiety-like responses in the novelty-induced hypophagia test after 1 or 21 abstinence days. Independent of the access condition, incubation of cocaine seeking was not correlated with the novelty-induced hypophagia measures. Results suggest that cocaine-induced anxiety-related states during protracted abstinence do not contribute to incubation of cocaine craving. However, this conclusion is tentative because we used a single anxiety-related measure and did not test female rats.

New directions in modelling dysregulated reward seeking for food and drugs

Behavioral models are central to behavioral neuroscience. To study the neural mechanisms of maladaptive behaviors (including binge eating and drug addiction), it is essential to develop and utilize appropriate animal models that specifically focus on dysregulated reward seeking. Both food and cocaine are typically consumed in a regulated manner by rodents, motivated by reward and homeostatic mechanisms. However, both food and cocaine seeking can become dysregulated, resulting in binge-like consumption and compulsive patterns of intake. The speakers in this symposium for the 2021 International Behavioral Neuroscience Meeting utilize behavioral models of dysregulated reward-seeking to investigate the neural mechanisms of binge-like consumption, enhanced cue-driven reward seeking, excessive motivation, and continued use despite negative consequences. In this review, we outline examples of maladaptive patterns of intake and explore recent animal models that drive behavior to become dysregulated, including stress exposure and intermittent access to rewards. Lastly, we explore select behavioral and neural mechanisms underlying dysregulated reward-seeking for both food and drugs.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

The effect of adolescent social isolation on vulnerability for methamphetamine addiction behaviours in female rats

RATIONALE

Stress exposure during adolescence contributes to developing a methamphetamine (METH) use disorder. However, most of the studies investigating addiction-related behaviours include only male rodents, despite METH addiction rates being higher in females. Furthermore, animal studies investigating the effects of stress on methamphetamine addiction have used only basic self-administration models which may not be sensitive to the effects of stress.

OBJECTIVES

This project explored whether adolescent isolation stress exposure increases the incidence of four key addiction-related behaviours in female rats.

METHODS

Thirty-two female rat pups were caged in groups of four or individually during adolescence from postnatal (PND) day 22, with the latter being re-socialised in groups of four on PND 43. In adulthood, rats were tested for addiction-like behaviours in a METH self-administration paradigm modelling motivation to take METH, persistence in drug-seeking behaviour when METH was not available, resistance to extinction, and propensity to reinstate after a period of withdrawal.

RESULTS

Adolescent social isolation resulted in lower METH intake during acquisition; however, the paradigm modelling drug-seeking when the drug was unavailable engendered intermittent METH bingeing in all rats, abolishing the group differences in intake during this phase. Adolescent social isolation also accelerated extinction of non-reinforced lever pressing, and increased stress-primed reinstatement, compared to the group-housed rats.

CONCLUSIONS

Adolescent social isolation stress alters various methamphetamine addiction-like behaviours in female rats.

Dopamine Circuit Mechanisms of Addiction-Like Behaviors

Addiction is a complex disease that impacts millions of people around the world. Clinically, addiction is formalized as substance use disorder (SUD), with three primary symptom categories: exaggerated substance use, social or lifestyle impairment, and risky substance use. Considerable efforts have been made to model features of these criteria in non-human animal research subjects, for insight into the underlying neurobiological mechanisms. Here we review evidence from rodent models of SUD-inspired criteria, focusing on the role of the striatal dopamine system. We identify distinct mesostriatal and nigrostriatal dopamine circuit functions in behavioral outcomes that are relevant to addictions and SUDs. This work suggests that striatal dopamine is essential for not only positive symptom features of SUDs, such as elevated intake and craving, but also for impairments in decision making that underlie compulsive behavior, reduced sociality, and risk taking. Understanding the functional heterogeneity of the dopamine system and related networks can offer insight into this complex symptomatology and may lead to more targeted treatments.

Sex and heredity are determinants of drug intake in a novel model of rat oral oxycodone self-administration

The steady rise in prescription opioids such as oxycodone has led to a virulent epidemic of widespread abuse and deaths in the United States; approximately 80% of affected individuals initiate the habitual use of oxycodone by using prescription oral oxycodone. Given the importance of drug pharmacokinetics in determining abuse potential, we designed an oral operant oxycodone self-administration (SA) procedure in rats to model drug intake by most human users/abusers of oxycodone. Key aspects of the model include limited initial drug intake followed by increasing drug concentrations during extended 4-h sessions on alternating days. Sex and genetic predisposition are major determinants of human opiate abuse. Therefore, we studied females in seven inbred strains (WLI, WMI, LEW, DSS, F344, BN and SHR) and both sexes in three of these strains. All strains increased intake across serially increasing doses (0.025-0.2 mg/ml; p < 0.001): the range of intakes at the final concentration of oxycodone was 0.72 ± 0.17-4.84 ± 1.42 mg/kg (mean ± SEM) - a 6.7-fold difference across strains. In LEW, WLI and WMI strains, oxycodone intake increased significantly across all sessions in both sexes. However, in LEW and WMI male rats but not WLI, daily oxycodone intake was significantly lower across all 4-h sessions than females (p < 0.005). The estimated heritability in oxycodone intake was in the range of 0.21-0.41. In summary, our novel operant oral oxycodone SA model captures the strong abuse potential of oral oxycodone and shows dose, sex and strain-specific drug intake that is significantly dependent on heredity.

Circuit Investigation of Social Interaction and Substance Use Disorder Using Miniscopes

Substance use disorder (SUD) is comorbid with devastating health issues, social withdrawal, and isolation. Successful clinical treatments for SUD have used social interventions. Neurons can encode drug cues, and drug cues can trigger relapse. It is important to study how the activity in circuits and embedded cell types that encode drug cues develop in SUD. Exploring shared neurobiology between social interaction (SI) and SUD may explain why humans with access to social treatments still experience relapse. However, circuitry remains poorly characterized due to technical challenges in studying the complicated nature of SI and SUD. To understand the neural correlates of SI and SUD, it is important to: (1) identify cell types and circuits associated with SI and SUD, (2) record and manipulate neural activity encoding drug and social rewards over time, (3) monitor unrestrained animal behavior that allows reliable drug self-administration (SA) and SI. Miniaturized fluorescence microscopes (miniscopes) are ideally suited to meet these requirements. They can be used with gradient index (GRIN) lenses to image from deep brain structures implicated in SUD. Miniscopes can be combined with genetically encoded reporters to extract cell-type specific information. In this mini-review, we explore how miniscopes can be leveraged to uncover neural components of SI and SUD and advance potential therapeutic interventions.

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.

Sign- and goal-tracking score does not correlate with addiction-like behavior following prolonged cocaine self-administration

RATIONALE

In classical conditioning, sign-tracking reflects behavior directed toward a conditioned stimulus (CS) in expectation of a reward (unconditioned stimulus, US); in contrast, goal-tracking describes behavior directed toward the location of delivery of a US. As cues previously paired with drugs of abuse promote drug-seeking and drug-taking behavior in both animals and humans and thus contribute to the severity of substance abuse, sign-tracking may represent a maladaptive cue-focused behavior that may increase addiction vulnerability as compared to goal-tracking. Recent studies do, in fact, support this possibility. Previous work in this area has focused primarily on paradigms using relatively limited exposure to drug rather than extended drug intake.

OBJECTIVES

Here, we used the DSM-IV-based 3-criteria (3-CRIT) model and examined whether a relationship exists between sign- or goal-tracking phenotypes and the prevalence of criteria associated with addiction-like behavior following extended cocaine self-administration as measured in this model.

METHODS

Forty-six male Sprague Dawley rats underwent a Pavlovian conditioned approach (PCA) procedure and were characterized along a continuum as goal-trackers (GTs), intermediates (INTs), or sign-trackers (STs). The animals were subsequently trained to intravenous self-administer cocaine during 45 self-administration (SA) sessions and characterized for the 3 criteria outlined in the model: persistence of drug-seeking, motivation for cocaine-taking, and resistance to punishment.

RESULTS

We performed correlational analyses on the traits measured, finding no relationships between PCA score and addiction-like characteristics measured using the 3-CRIT model of addiction. However, STs showed significantly greater resistance to punishment than GTs.

CONCLUSIONS

Phenotyping along a continuum of PCA scores may not be a valid predictor for identifying vulnerability to the addiction-like behaviors examined using the 3-CRIT model. However, PCA phenotype may predict a single feature of the 3-CRIT model, resistance to punishment, among those rats classified as either STs or GTs.

Extinction vs. Abstinence: A Review of the Molecular and Circuit Consequences of Different Post-Cocaine Experiences

The intravenous cocaine self-administration model is widely used to characterize the neurobiology of cocaine seeking. When studies are aimed at understanding relapse to cocaine-seeking, a post-cocaine abstinence period is imposed, followed by “relapse” tests to assess the ability of drug-related stimuli (“primes”) to evoke the resumption of the instrumental response previously made to obtain cocaine. Here, we review the literature on the impact of post-cocaine abstinence procedures on neurobiology, finding that the prelimbic and infralimbic regions of the prefrontal cortex are recruited by extinction training, and are not part of the relapse circuitry when extinction training does not occur. Pairing cocaine infusions with discrete cues recruits the involvement of the NA, which together with the dorsal striatum, is a key part of the relapse circuit regardless of abstinence procedures. Differences in molecular adaptations in the NA core include increased expression of GluN1 and glutamate receptor signaling partners after extinction training. AMPA receptors and glutamate transporters are similarly affected by abstinence and extinction. Glutamate receptor antagonists show efficacy at reducing relapse following extinction and abstinence, with a modest increase in efficacy of compounds that restore glutamate homeostasis after extinction training. Imaging studies in humans reveal cocaine-induced adaptations that are similar to those produced after extinction training. Thus, while instrumental extinction training does not have face validity, its use does not produce adaptations distinct from human cocaine users.

Male Goal-Tracker and Sign-Tracker Rats Do Not Differ in Neuroendocrine or Behavioral Measures of Stress Reactivity

Environmental cues attain the ability to guide behavior via learned associations. As predictors, cues can elicit adaptive behavior and lead to valuable resources (e.g., food). For some individuals, however, cues are transformed into incentive stimuli and elicit motivational states that can be maladaptive. The goal-tracker (GT)/sign-tracker (ST) animal model captures individual differences in cue-motivated behaviors, with reward-associated cues serving as predictors of reward for both phenotypes but becoming incentive stimuli to a greater degree for STs. While these distinct phenotypes are characterized based on Pavlovian conditioned approach (PavCA) behavior, they exhibit differences on a number of behaviors relevant to psychopathology. To further characterize the neurobehavioral endophenotype associated with individual differences in cue-reward learning, neuroendocrine and behavioral profiles associated with stress and anxiety were investigated in male GT, ST, and intermediate responder (IR) rats. It was revealed that baseline corticosterone (CORT) increases with Pavlovian learning, but to the same degree, regardless of phenotype. No significant differences in behavior were observed between GTs and STs during an elevated plus maze (EPM) or open field test (OFT), nor were there differences in CORT response to the OFT or physiological restraint. Upon examination of central markers associated with stress reactivity, we found that STs have greater glucocorticoid receptor (GR) mRNA expression in the ventral hippocampus, with no phenotypic differences in the dorsal hippocampus or prelimbic cortex (PrL). These findings demonstrate that GTs and STs do not differ on stress-related and anxiety-related behaviors, and suggest that differences in neuroendocrine measures between these phenotypes can be attributed to distinct cue-reward learning styles.

Effects of 5-HT2A receptor stimulation on economic demand for fentanyl after intermittent and continuous access self-administration in male rats

The relative value of and motivation for abused drugs often increases with drug experience and differs based on drug availability. Here, we determined how different intake patterns of fentanyl, a μ-opioid agonist, alter economic demand for fentanyl and how 5-HT_2A receptor stimulation affects economic demand for fentanyl. We used a within-session demand threshold procedure to characterize changes in economic demand for fentanyl before and after intermittent or continuous access schedules. We subsequently tested the acute effects of 5-HT_2A receptor stimulation with psychedelic 2,5-dimethoxy-4-iodoamphetamine (DOI) on economic demand for fentanyl. Extended fentanyl experience with both intermittent and continuous schedules increased fentanyl consumption at low cost (Q₀ ), increased total fentanyl consumption, and decreased demand elasticity (α), indicating both schedules elevated motivation to self-administer fentanyl. Overall, the two schedules produced similar alterations in economic demand for fentanyl, although low-cost consumption (Q₀ ) increased more in the continuous access group. Systemic injections of DOI (0.0-0.4 mg/kg, i.p.) dose-dependently produced economic demand changes in the opposite direction produced by fentanyl experience. DOI decreased motivation (increased "α"), decreased Q₀ , and decreased total fentanyl consumption. The selective 5-HT_2A antagonist, M100907 (0.3 mg/kg, i.p.), blocked the effects of DOI, indicating that DOI is acting through 5-HT_2A receptors to alter economic demand for fentanyl. In an economic food demand experiment, DOI (0.4 mg/kg) also increased demand elasticity and reduced food consumption. These results demonstrate that both intermittent and continuous fentanyl experience raise the economic demand for fentanyl, and acute 5-HT_2A receptor activation reduces economic demand for fentanyl and food.

Intermittent self-administration of fentanyl induces a multifaceted addiction state associated with persistent changes in the orexin system

The orexin (hypocretin) system plays a critical role in motivated drug taking. Cocaine self-administration with the intermittent access (IntA) procedure produces a robust addiction-like state that is orexin-dependent. Here, we sought to determine the role of the orexin system in opioid addiction using IntA self-administration of fentanyl. Different groups of male rats were either given continuous access in 1-h period (short access [ShA]), 6-h period (long access [LgA]), or IntA (5 min of access separated by 25 min of no access for 6 h) to fentanyl for 14 days. IntA produced a greater escalation of fentanyl intake, increased motivation for fentanyl on a behavioral economics task, persistent drug seeking during abstinence, and stronger cue-induced reinstatement compared with rats given ShA or LgA. We found that addiction behaviors induced by IntA to fentanyl were reversed by the orexin-1 receptor antagonist SB-334867. IntA to fentanyl was also associated with a persistent increase in the number of orexin neurons. Together, these results indicate that the IntA model is a useful tool in the study of opioid addiction and that the orexin system is critical for the maintenance of addiction behaviors induced by IntA self-administration of fentanyl.

Dopamine 'ups and downs' in addiction revisited

Repeated drug use can change dopamine (DA) function in ways that promote the development and persistence of addiction, but in what direction? By one view, drug use blunts DA neurotransmission, producing a hypodopaminergic state that fosters further drug use to overcome a DA deficiency. Another view is that drug use enhances DA neurotransmission, producing a sensitized, hyperdopaminergic reaction to drugs and drug cues. According to this second view, continued drug use is motivated by sensitization of drug 'wanting'. Here we discuss recent evidence supporting the latter view, both from preclinical studies using intermittent cocaine self-administration procedures that mimic human patterns of use and from related human neuroimaging studies. These studies have implications for the modeling of addiction in the laboratory and for treatment.

The sensation seeking trait confers a dormant susceptibility to addiction that is revealed by intermittent cocaine self-administration in rats

Heightened sensation seeking is associated with an increased risk of substance use disorder in clinical populations. In rats, sensation seeking is often examined by measuring locomotor reactivity to a novel environment. So-called high responders (HR) acquire self-administration of psychostimulants more quickly and consume higher amounts of drug compared to low responder (LR) rats, indicating that the HR trait might confer a stronger addiction propensity. However, studies of addiction-like behaviors in HR vs LR rats have typically utilized self-administration paradigms that do not dissociate individual differences in the hedonic/reinforcing and motivational properties of a drug. Moreover, little attention has been given to whether HR rats are more susceptible to drug-access conditions that promote a state-dependent addiction phenotype. We report that on a behavioral economics task, HR rats have higher preferred brain-cocaine levels compared to LR rats but do not differ with respect to their demand elasticity for cocaine. In contrast, when tested on an intermittent access schedule of cocaine self-administration, which has been shown to promote several addiction-related endophenotypes, HR rats exhibit greater escalation of intake and more drastic reductions in cocaine demand elasticity. Together, these data indicate that the HR trait does not confer higher extant addiction behavior, but rather that this phenotype is associated with a propensity for addiction that remains dormant until it is actuated by intermittent drug intake. These findings reveal a 'trait' (HR) by 'state' (intermittent drug intake) interaction that produces a strong addiction-like phenotype. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Sign Tracking in an Enriched Environment: A Potential Ecologically Relevant Animal Model of Adaptive Behavior Change

When an object conditioned stimulus (CS) is paired with a food unconditioned stimulus (US), anticipatory goal-directed action directed at the US location (goal tracking) is accompanied by behavior directed at the object CS (sign tracking). Sign-tracking behavior appears to be compulsive and habit-like and predicts increased vulnerability to the addictive potential of drugs in animal models. A large body of the literature also suggests that environmental enrichment protects against the development of addiction-prone phenotypes. Thus, we investigated whether compulsive-like sign tracking develops in environmentally enriched rats trained directly in their enriched home environment. We demonstrate that adolescent enriched-housed male Sprague-Dawley rats readily sign track a 5% ethanol bottle CS in their home environment and at a rate higher than adolescent standard-housed rats. We also show that enriched adolescent rats sign track less than enriched adult-trained rats and that acute isolation stress affects sign- and goal-tracking performance of adolescents and adults differently. Sign tracking increased more in the adult than the adolescent rats. Whereas the younger rats showed a decrease in goal tacking after the final stressor manipulation, the adults showed increased goal tracking. Our results are consistent with recent studies, which suggest that although sign tracking performance is compulsive-like, it is not as inflexible and habit-like as previously assumed. Testing in an enriched home environment with object CSs having greater affordance than "neutral" lever CSs may provide greater ecological relevance for investigating the development and expression of adaptive and compulsive-like behaviors in translational research.

Bidirectional role of acupuncture in the treatment of drug addiction

Drug addiction is a chronically relapsing disorder, affecting people from all walks of life. Studies of acupuncture effects on drug addiction are intriguing in light of the fact that acupuncture can be used as a convenient therapeutic intervention for treating drug addiction by direct activation of brain pathway. The current review aims to discuss the neurobiological mechanisms underlying acupuncture's effectiveness in the treatment of drug addiction, on the basis of two different theories (the incentive sensitization theory and the opponent process theory) that have seemingly opposite view on the role of the mesolimbic reward pathways in mediating compulsive drug-seeking behavior. This review provides evidence that acupuncture may reduce relapse to drug-seeking behavior by regulating neurotransmitters involved in drug craving modulation via somatosensory afferent mechanisms. Also, acupuncture normalizes hyper-reactivity or hypoactivity of the mesolimbic dopamine system in these opposed processes in drug addiction, suggesting bidirectional role of acupuncture in regulation of drug addiction. This proposes that acupuncture may reduce drug craving by correcting both dysfunctions of the mesolimbic dopamine pathway.

Neurochemical mechanisms and neurocircuitry underlying the contribution of stress to cocaine seeking

In individuals with substance use disorders, stress is a critical determinant of relapse susceptibility. In some cases, stressors directly trigger cocaine use. In others, stressors interact with other stimuli to promote drug seeking, thereby setting the stage for relapse. Here, we review the mechanisms and neurocircuitry that mediate stress-triggered and stress-potentiated cocaine seeking. Stressors trigger cocaine seeking by activating noradrenergic projections originating in the lateral tegmentum that innervate the bed nucleus of the stria terminalis to produce beta adrenergic receptor-dependent regulation of neurons that release corticotropin releasing factor (CRF) into the ventral tegmental area (VTA). CRF promotes the activation of VTA dopamine neurons that innervate the prelimbic prefrontal cortex resulting in D1 receptor-dependent excitation of a pathway to the nucleus accumbens core that mediates cocaine seeking. The stage-setting effects of stress require glucocorticoids, which exert rapid non-canonical effects at several sites within the mesocorticolimbic system. In the nucleus accumbens, corticosterone attenuates dopamine clearance via the organic cation transporter 3 to promote dopamine signaling. In the prelimbic cortex, corticosterone mobilizes the endocannabinoid, 2-arachidonoylglycerol (2-AG), which produces CB1 receptor-dependent reductions in inhibitory transmission, thereby increasing excitability of neurons which comprise output pathways responsible for cocaine seeking. Factors that influence the role of stress in cocaine seeking, including prior history of drug use, biological sex, chronic stress/co-morbid stress-related disorders, adolescence, social variables, and genetics are discussed. Better understanding when and how stress contributes to drug seeking should guide the development of more effective interventions, particularly for those whose drug use is stress related.