Autoshaping i.v. cocaine self-administration in rats: effects of nondrug alternative reinforcers on acquisition

Can we enhance the clinical efficacy of cognitive and psychological approaches to treat substance use disorders through understanding their neurobiological mechanisms?

Despite decades of research in the field of addiction, relapse rates for substance use disorders remain high. Consequently, there has been growing focus on providing evidence-based treatments for substance use disorders, resulting in the increased development and use of cognitive and psychological interventions. Such treatment approaches, including contingency management, community-reinforcement approach, and cognitive bias modification, have shown promising clinical efficacy in reducing substance use and promoting abstinence during treatment. However, these interventions are still somewhat limited in achieving sustained periods of abstinence post-treatment. The neurobiological mechanisms underpinning these treatment approaches remain largely unknown and under-studied, in part, due to a lack of translational animal models. The adoption of a reverse translational approach may assist in development of more representative models that can facilitate elucidation of the mechanisms behind these clinically relevant interventions. This review examines our current understanding of addiction neurobiology from clinical, preclinical research and existing animal models, and considers how the efficacy of such behavioral-oriented interventions alone, or in combination with pharmacotherapy, may be enhanced to improve treatment outcomes.

Sex differences in inflammatory cytokine levels following synthetic cathinone self-administration in rats

Synthetic cathinones are used as stimulants of abuse. Many abused drugs, including stimulants, activate nuclear factor-κB (NF-κB) transcription leading to increases in NF-κB-regulated pro-inflammatory cytokines, and the level of inflammation appears to correlate with length of abuse. The purpose of this study was to measure the profile of IL-1α, IL-1β, IL-6, CCL2 and TNF-α in brain and plasma to examine if drug exposure alters inflammatory markers. Male and female Sprague-Dawley rats were trained to self-administer α-pyrrolidinopentiophenone (α-PVP) (0.1 mg/kg/infusion), 4-methylmethcathinone (4MMC) (0.5 mg/kg/infusion), or saline through autoshaping, and then self-administered for 21 days during 1 h (short access; ShA) or 6 h (long access; LgA) sessions. Separate rats were assigned to a naïve control group. Cytokine levels were examined in amygdala, hippocampus, hypothalamus, prefrontal cortex, striatum, thalamus, and plasma. Rats acquired synthetic cathinone self-administration, and there were no sex differences in drug intake. Synthetic cathinone self-administration produced sex differences in IL-1α, IL-1β, IL-6, CCL2 and TNF-α levels. There were widespread increases in inflammatory cytokines in the brains of male rats compared to females, particularly for 4MMC, whereas females were more likely to show increased inflammatory cytokines in plasma compared to saline groups than males. Furthermore, these sex differences in cytokine levels were more common after LgA access to synthetic cathinones than ShA. These results suggest that synthetic cathinone use likely produces sex-selective patterns of neuroinflammation during the transition from use to abuse. Consequently, treatment need may differ depending on the progression of synthetic cathinone abuse and based on sex.

Paternal alcohol exposure reduces acquisition of operant alcohol self-administration and affects Bdnf DNA methylation in male and female offspring

Familial transmission of alcohol use disorder reflects genetic and environmental factors. Paternal alcohol exposure may affect rodent offspring via epigenetic modifications transmitted through the male germ line. While such exposure alters alcohol sensitivity in mouse offspring, no studies examined if it impacts the development of operant alcohol self-administration in rats. We exposed male (sires) Wistar rats to chronic intermittent ethanol in vapour chambers (16 h/day; 5 days/week) or to air for 6 weeks. Eight weeks later, rats were mated with alcohol-naive females. Adult alcohol- and control-sired F1 offspring were assessed in acquisition of alcohol self-administration in which increasing alcohol concentrations (2.5%, 5% and 10%, v/v) were delivered after one lever press (fixed ratio 1 or FR1). Prior to alcohol sessions, rats were trained to lever press for food delivery under an FR1 schedule of reinforcement. DNA methylation levels of the brain derived neurotrophic factor (Bdnf) gene were measured in sperm, nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) in sires and in offspring. Alcohol-exposed sires had lower Bdnf DNA methylation levels in NAc and greater methylation levels in mPFC. Although this pattern was not recapitulated in offspring, alcohol-sired offspring of both sexes did show aberrant Bdnf DNA methylation patterns compared to control-sired offspring. Alcohol-sired offspring self-administered less alcohol (5% and 10%) with no group differences in food responding. Results indicate that paternal alcohol exposure prior to conception protects against alcohol's initial reinforcing effects but the pattern of dysregulated Bdnf methylation in reward-related circuitry did not mimic changes seen in sires.

Alpha-pyrrolidinopentiophenone and mephedrone self-administration produce differential neurochemical changes following short- or long-access conditions in rats

Stimulant-induced neurochemical changes may occur at different times for different brain regions or neurotransmitter systems. This study sought to examine the behavioral and neurochemical effects of extended access to α-pyrrolidinopentiophenone (α-PVP) and 4-methylmethcathinone (4MMC). Male and female Sprague-Dawley rats were trained to self-administer α-PVP (0.1 mg/kg/infusion) or 4MMC (0.5 mg/kg/infusion) through autoshaping, and then self-administered for 21 days during 1 h (short access; ShA) or 6 h (long access; LgA) sessions. Separate rats were assigned to a naïve control group. Amygdala, hippocampus, hypothalamus, prefrontal cortex (PFC), striatum, and thalamus were extracted, and tissue was analyzed with electrochemical detection and liquid chromatography mass spectrometry. Rats acquired self-administration of α-PVP and 4MMC, and LgA rats showed more escalation of self-administration than ShA rats. Synthetic cathinone administration produced several effects on neurotransmitters. LgA self-administration of α-PVP increased 5-HIAA levels in all brain regions, compared to control. In contrast, both LgA and ShA 4MMC self-administration decreased 5-HT and 5-HIAA levels in most brain regions. LgA exposure to both synthetic cathinones increased DOPAC levels in hypothalamus and striatum, and increased HVA levels in striatum compared to control. LgA self-administration of either synthetic cathinone produced region-specific increases in NE levels, whereas ShA self-administration lowered NE levels in select locations compared to control. These alterations in neurotransmitter levels indicate that synthetic cathinone use may produce differential neurochemical changes during the transition from use to abuse, and that 21 days of self-administration only models the beginning stages of dysregulated drug intake.

Drug Sensitization, Substance Abuse, and Chemical Sensitivity

Substance abuse, involving drugs such as cocaine, heroin, alcohol, marijuana, nicotine, barbiturates, etc., is by far by the most prevalent psychiatric disorder. Much has been learned about the abuse of these substances that may be useful to consider in designing and analyzing research concerning multiple chemical sensitivities (MCS). We review the central role of sensitization in this literature, including its definition, measurement, and expression in animals and human volunteers. Common factors among abused drugs, including sensitization, are discussed. Finally, empirical studies are delineated or proposed to test some of the notions presented in this paper.

Presence of a social peer enhances acquisition of remifentanil self-administration in male rats

BACKGROUND

Social peers influence human drug use at every stage of addiction. Using a dual-compartment apparatus that allows for limited social contact, recent work has shown that cocaine self-administration is enhanced when two rats are trained to self-administer at the same time compared to rats trained alone or trained in the presence of a saline self-administration control peer. The current study measured social influence on self-administration of the short-acting opioid remifentanil using a dual-compartment operant conditioning chamber.

METHODS

Adult male rats were placed in one of five groups: (1) REMI-REMI group, in which both rats self-administered remifentanil; (2) REMI-SAL group, in which rats self-administered remifentanil in the presence of a peer that self-administered saline; (3) SAL-REMI group, in which rats self-administered saline in the presence of a peer that self-administered remifentanil; and (4) REMI ALONE and (5) SAL ALONE groups, in which rats administered their respective drugs alone (no peer). Self-administration was measured using a 2-lever procedure during acquisition, maintenance, increasing fixed-ratio, and dose-response phases.

RESULTS

The presence of a social peer enhanced drug intake during acquisition, regardless of the drug exposure of their peer. Additionally, active lever position significantly affected remifentanil intake during acquisition and maintenance, with the greatest influence occurring when the active lever was close to the peer.

CONCLUSION

The presence of a social peer in the drug-taking context potentiates remifentanil self-administration, regardless of the peer's drug access. Future studies utilizing the dual-compartment apparatus will help elucidate the neural mechanisms underlying social influence on opioid abuse.

Addiction is driven by excessive goal-directed drug choice under negative affect: translational critique of habit and compulsion theory

Drug addiction may be a goal-directed choice driven by excessive drug value in negative affective states, a habit driven by strong stimulus-response associations, or a compulsion driven by insensitivity to costs imposed on drug seeking. Laboratory animal and human evidence for these three theories is evaluated. Excessive goal theory is supported by dependence severity being associated with greater drug choice/economic demand. Drug choice is demonstrably goal-directed (driven by the expected value of the drug) and can be augmented by stress/negative mood induction and withdrawal-effects amplified in those with psychiatric symptoms and drug use coping motives. Furthermore, psychiatric symptoms confer risk of dependence, and coping motives mediate this risk. Habit theory of addiction has weaker support. Habitual behaviour seen in drug-exposed animals often does not occur in complex decision scenarios, or where responding is rewarded, so habit is unlikely to explain most human addictive behaviour where these conditions apply. Furthermore, most human studies have not found greater propensity to habitual behaviour in drug users or as a function of dependence severity, and the minority that have can be explained by task disengagement producing impaired explicit contingency knowledge. Compulsion theory of addiction also has weak support. The persistence of punished drug seeking in animals is better explained by greater drug value (evinced by the association with economic demand) than by insensitivity to costs. Furthermore, human studies have provided weak evidence that propensity to discount cost imposed on drug seeking is associated with dependence severity. These data suggest that human addiction is primarily driven by excessive goal-directed drug choice under negative affect, and less by habit or compulsion. Addiction is pathological because negative states powerfully increase expected drug value acutely outweighing abstinence goals.

The effects of combined intravenous cocaine and ethanol self-administration on the behavioral and amino acid profile of young adult rats

Under paradigms of combined intravenous cocaine and ethanol self-administration, the effects on behavior have been poorly explored. Numerous studies have found sex differences in amino acids profile and behavioral responses to each drug, yet few have focused on the interactions between cocaine and ethanol. The main objective of this work was to explore the acquisition and maintenance of intravenous self-administration behavior with a combination of cocaine and ethanol in male and female young adult rats. Likewise, the amino acids profile in blood plasma was quantified 48 hours after the last self-administration session. Male and female 52 days old Wistar rats were randomly assigned to one of 3 groups: i) saline control, ii) cocaine (1 mg/kg bodyweight/injection) and iii) cocaine and ethanol (1 mg + 133 mg/kg bodyweight/ injection). After 24 self-administration sessions carried out on a fixed-ratio-1 schedule, with a limit of 15 doses per session, 14 plasma amino acids were quantified by mean Capillary Electrophoresis technique. The curve of cocaine and ethanol combined self-administration was similar to that associated with cocaine administration alone, with females acquiring self-administration criterion before males. The self-administration of cocaine and ethanol altered the plasma concentration and relative ratios of the amino acid L-Tyrosine. In our intravenous self-administration model, females appeared more vulnerable to acquire abusive consumption of the cocaine and ethanol combination, which altered plasma L-Tyrosine levels.

Synthetic cathinone self-administration in female rats modulates neurotransmitter levels in addiction-related brain regions

Synthetic cathinones are used for their stimulant-like properties. Stimulant-induced neurochemical changes are thought to occur at different times in different brain regions and neurotransmitter systems. This study sought to examine the behavioral and neurochemical effects of α-pyrrolidinopentiophenone (α-PVP) and mephedrone (4MMC) in female rats. Methods probed the chronology of effects of synthetic cathinone exposure. Female rats were trained to self-administer α-PVP, 4MMC, or saline. Drug exposure ceased after 7 days of autoshaping for half of each drug group; the other half self-administered for another 21 days. Amygdala, hippocampus, hypothalamus, PFC, striatum, and thalamus were extracted, and tissue was analyzed with electrochemical detection and liquid chromatography mass spectrometry. Responding was minimal during autoshaping; thus, most infusions were delivered noncontingently in the autoshaping phase. Rats acquired self-administration of α-PVP and 4MMC. Synthetic cathinone administration, and duration of exposure produced several effects on neurotransmitters. α-PVP primarily increased serotonin, 5-hydroxy-3-acetic acid (5-HIAA), norepinephrine, and glutamate in hypothalamus. In contrast, 4MMC decreased serotonin and 5-HIAA in several brain regions. Longer durations of exposure to both synthetic cathinones increased 5-HIAA, norepinephrine, and glutamate in multiple brain regions compared to the short exposure during autoshaping. Notably, both α-PVP and 4MMC produced minimal changes in dopamine levels, suggesting that the dopaminergic effects of these synthetic cathinones are transient. These alterations in neurotransmitter levels indicate that synthetic cathinone use may produce differential neurochemical changes during the transition from use to abuse.

Sex and Individual Differences in Alcohol Intake Are Associated with Differences in Ketamine Self-Administration Behaviors and Nucleus Accumbens Dendritic Spine Density

Clinical and preclinical studies have shown that ketamine, an NMDA receptor antagonist, has promising therapeutic value for the treatment of alcohol use disorder (AUD). However, the maintenance of remission will ultimately require repeated infusions of ketamine, which may lead to abuse potential and may hinder its therapeutic benefits. It is therefore crucial to assess the effects of repeated treatments with ketamine on alcohol intake. Accordingly, this study aimed to examine in both sexes how individual differences in alcohol intake alter ketamine self-administration and how ketamine self-administration will alter subsequent alcohol-drinking behaviors. Male and female rats intermittently drank alcohol or water for 10 weeks and were divided into high- or low-alcohol intake groups prior to ketamine self-administration. Rats self-administered ketamine under fixed and progressive ratio schedules of reinforcement from week 4 to 7, and the incubation of ketamine craving was examined from week 8 to 10. To investigate structural plasticity in a brain region involved in reward, nucleus accumbens dendritic spine morphology was examined. Our results show that high alcohol intake in male rats attenuated ketamine self-administration, whereas in female rats high alcohol intake enhanced motivation to self-administer ketamine. Ketamine reduced alcohol intake in high-alcohol male rats but increased it in low-alcohol female rats. Incubation of ketamine craving developed in all groups except low-alcohol females. Three weeks of abstinence from ketamine was associated with increased mushroom spines in all groups except the high-alcohol male group. Overall, these data suggest that ketamine as a treatment for AUD may benefit male subjects, but not female subjects, and warrants further investigation before use as a therapeutic agent.

Effects of methamphetamine isomers on d-methamphetamine self-administration and food-maintained responding in male rats

RATIONALE

Methamphetamine (METH) abuse is generally attributed to the d-isomer. Self-administration of l-METH has been examined only in rhesus monkeys with a history of cocaine self-administration or drug-naïve rats using high toxic doses.

OBJECTIVES

In this study, the ability of l-METH and, for comparison, d-METH to engender self-administration in experimentally naïve rats, as well as to decrease d-METH self-administration and food-maintained responding, was examined.

METHODS

Male Sprague-Dawley rats were used in 3 separate experiments. In experiment 1, the acquisition of l- or d-METH self-administration followed by dose-response determinations was studied. In experiment 2, rats were trained to self-administer d-METH (0.05 mg/kg/infusion) and, then, various doses of l- or d-METH were given acutely prior to the session; the effect of repeated l-METH (30 mg/kg) also was examined. In experiment 3, rats were trained to respond for food reinforcement and, then, various doses of l- or d-METH were given acutely prior to the session; the effect of repeated l-METH (3 mg/kg) also was examined.

RESULTS

Reliable acquisition of l- and d-METH self-administration was obtained at unit doses of 0.5 and 0.05 mg/kg/infusion respectively. The dose-response function for l-METH self-administration was flattened and shifted rightward compared with d-METH self-administration, with peak responding for l- and d-METH occurring at unit doses of 0.17 and 0.025 respectively. l-METH also was approximately 10-fold less potent than d-METH in decreasing d-METH self-administration and 2-fold lower in decreasing food-maintained responding. Tolerance did not occur to repeated l-METH pretreatments on either measure.

CONCLUSIONS

As a potential pharmacotherapeutic, l-METH has less abuse liability than d-METH and its efficacy in decreasing d-METH self-administration and food-maintained responding is sustained with repeated treatment.

Analysis of neurotransmitter levels in addiction-related brain regions during synthetic cathinone self-administration in male Sprague-Dawley rats

RATIONALE

Synthetic cathinones are used as stimulants of abuse. Different stimulants may induce distinct rates of disease progression, yielding neurochemical changes that may vary across brain regions or neurotransmitter systems.

OBJECTIVES

This research sought to behaviorally and chemically differentiate stages of synthetic cathinone abuse through rodent self-administration and measurement of the neurotransmitter profile in multiple brain regions.

METHODS

Male rats were trained to self-administer α-PVP, mephedrone (4MMC), or saline. Half of each drug group stopped self-administering after autoshaping; the other half self-administered for another 21 days. Brain tissue from amygdala, hippocampus, hypothalamus, PFC, striatum, and thalamus was profiled with electrochemical detection to assess neurotransmitter levels.

RESULTS

During autoshaping, the majority of infusions were delivered noncontingently. In the self-administration phase, rats responded more for α-PVP and 4MMC than for saline, demonstrating that both synthetic cathinones were reinforcing. Longer durations of exposure elevated 5-HIAA in hypothalamus, PFC, and hippocampus, indicating that learning may produce changes in addiction-related brain regions. Both synthetic cathinones decreased norepinephrine in hippocampus, while α-PVP decreased glutamate in hippocampus and PFC, and 4MMC decreased glutamate in thalamus. Furthermore, α-PVP increased dopaminergic metabolites in striatum, whereas 4MMC decreased serotonin in the amygdala, hippocampus, and PFC. Interestingly, neither synthetic cathinone affected dopamine levels despite their functional effects on the dopaminergic system.

CONCLUSIONS

In summary, the neurotransmitter changes observed here suggest that synthetic cathinone use likely produces sequential neurochemical changes during the transition from use to abuse. Consequently, treatment need may differ depending on the progression of synthetic cathinone abuse.

Beyond simple tests of value: measuring addiction as a heterogeneous disease of computation-specific valuation processes

Addiction is considered to be a neurobiological disorder of learning and memory because addiction is capable of producing lasting changes in the brain. Recovering addicts chronically struggle with making poor decisions that ultimately lead to relapse, suggesting a view of addiction also as a neurobiological disorder of decision-making information processing. How the brain makes decisions depends on how decision-making processes access information stored as memories in the brain. Advancements in circuit-dissection tools and recent theories in neuroeconomics suggest that neurally dissociable valuation processes access distinct memories differently, and thus are uniquely susceptible as the brain changes during addiction. If addiction is to be considered a neurobiological disorder of memory, and thus decision-making, the heterogeneity with which information is both stored and processed must be taken into account in addiction studies. Addiction etiology can vary widely from person to person. We propose that addiction is not a single disease, nor simply a disorder of learning and memory, but rather a collection of symptoms of heterogeneous neurobiological diseases of distinct circuit-computation-specific decision-making processes.

Alcohol use disorder symptoms are associated with greater relative value ascribed to alcohol, but not greater discounting of costs imposed on alcohol

RATIONALE

Alcohol dependence is characterised by persistent drinking despite health, social and economic costs. Behavioural economics has proposed two explanations for the persistence of alcohol use despite costs. Dependent individuals may (a) ascribe excessively high value to alcohol, such that costs associated with alcohol are exceeded, and/or (b) they may discount (neglect) the costs associated with alcohol.

METHODS

To test these predictions, the current study recruited 127 student drinkers who reported varied alcohol use disorder symptom severity in the Alcohol Use Disorders Inventory Test (AUDIT; mean = 11.17, 69% above the hazardous cutoff). Participants made concurrent forced choices between alcohol and food points under conditions that manipulated the magnitude of points (1, 2 or 3) and the delay to receive points (0 or 3 s). Alcohol value was indexed by preferential choice of alcohol versus food points, whereas sensitivity to costs was indexed by the decrease in alcohol choice when food points were of greater magnitude (sensitivity to opportunity costs) and when alcohol points were delayed (sensitivity to delay costs).

RESULTS

Percent choice of alcohol over food varied consistently with the relative magnitude of reward points offered (p < .001) and with time delays imposed on reward points (p < .001). AUDIT scores were associated with greater alcohol versus food choice across all conditions (p = .001). As alcohol use disorder symptom severity increased, the sensitivity of alcohol choice to the relative magnitude of points (p = .29) and time delays (p = .62) remained unchanged, suggesting no differential discounting of opportunity or delay costs imposed on alcohol. In contrasts of AUDIT categories, there was comparable sensitivity to costs across groups defined as low-risk (N = 39), hazardous (n = 57), harmful (n = 20) and possible dependent drinkers (n = 11).

CONCLUSIONS

Alcohol use disorder symptom severity is associated with greater relative value ascribed to alcohol, but not with greater discounting of opportunity or delay costs imposed on alcohol. Despite limitations of the current study, it may be concluded that cost discounting plays a lesser role in dependence than previously thought.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

The ability for cocaine and cocaine-associated cues to compete for attention

In humans, reward cues, including drug cues in individuals experiencing addiction, are especially effective in biasing attention towards them, so much so they can disrupt ongoing task performance. It is not known, however, whether this happens in rats. To address this question, we developed a behavioral paradigm to assess the capacity of an auditory drug (cocaine) cue to evoke cocaine-seeking behavior, thus distracting thirsty rats from performing a well-learned sustained attention task (SAT) to obtain a water reward. First, it was determined that an auditory cocaine cue (tone-CS) reinstated drug-seeking equally in sign-trackers (STs) and goal-trackers (GTs), which otherwise vary in the propensity to attribute incentive salience to a localizable drug cue. Next, we tested the ability of an auditory cocaine cue to disrupt performance on the SAT in STs and GTs. Rats were trained to self-administer cocaine intravenously using an Intermittent Access self-administration procedure known to produce a progressive increase in motivation for cocaine, escalation of intake, and strong discriminative stimulus control over drug-seeking behavior. When presented alone, the auditory discriminative stimulus elicited cocaine-seeking behavior while rats were performing the SAT, but it was not sufficiently disruptive to impair SAT performance. In contrast, if cocaine was available in the presence of the cue, or when administered non-contingently, SAT performance was severely disrupted. We suggest that performance on a relatively automatic, stimulus-driven task, such as the basic version of the SAT used here, may be difficult to disrupt with a drug cue alone. A task that requires more top-down cognitive control may be needed.

Exposure to sucrose during periods of withdrawal does not reduce cocaine-seeking behavior in rats

Concomitant access to drugs of abuse and alternative rewards such as sucrose has been shown to decrease addiction-related behaviors in animals. Here we investigated whether access to sucrose during abstinence in contexts that are temporally and physically distinct from drug-related contexts could reduce subsequent drug seeking. In addition, we investigated whether a history of cocaine self-administration would alter the rewarding effects of sucrose. Rats self-administered cocaine for ten sessions, while yoked-saline rats received only saline injections, and then we subjected them to a 30-day withdrawal period during which they had access to water and sucrose continuously or intermittently according to a schedule that induces binge-drinking behavior. At the end of the withdrawal period, rats were tested for cocaine seeking behavior during a single 6 h session. We found that exposure to cocaine increased sucrose consumption only when rats had intermittent access to sucrose, but exposure to sucrose did not alter drug seeking regardless of the schedule of access. These results suggest that exposure to cocaine cross-sensitizes to the rewarding effects of sucrose, but exposure to sucrose during abstinence, temporally and physically distinct from drug-related environments, does not to reduce drug seeking.

Choosing Under the Influence: A Drug-Specific Mechanism by Which the Setting Controls Drug Choices in Rats

Ample evidence shows that the setting can control drug choices in both humans and animals. Here we reveal in rats that a major mechanism of this control involves a regulation of the drug influence on other competing options at the time of choice. Briefly, rats were offered a choice between a drug dose (cocaine or heroin) and a brief access to water sweetened with saccharin in two different settings. In one setting, choosing under the influence was not possible and rats largely preferred saccharin over either cocaine or heroin. In contrast, when the same rats were shifted to a setting where choosing under the influence was possible, they chose the drug either nonexclusively or exclusively depending on whether the drug enhanced or suppressed sweet reward, respectively. Thus, when rats were under the orexigenic influence of heroin at the time of choice, they more frequently chose saccharin in alternation with heroin. In contrast, when rats were under the anorexic influence of cocaine, they stopped choosing saccharin and continued taking cocaine exclusively. These setting- and drug-specific changes in preference were rapid and reversible, and could be induced by passively administering cocaine or heroin before choice. Finally, rats behaved as if they were oblivious to the drug influence on their choices. This behavior could explain why rats are vulnerable to harm themselves, sometimes to the point of death, in settings where choices are made under the drug influence, notably if this influence excludes other important options or, conversely, enhances harmful ones.

Amphetamine self-administration and dopamine function: assessment of gene × environment interactions in Lewis and Fischer 344 rats

RATIONALE

Previous research suggests both genetic and environmental influences on substance abuse vulnerability.

OBJECTIVES

The current work sought to investigate the interaction of genes and environment on the acquisition of amphetamine self-administration as well as amphetamine-stimulated dopamine (DA) release in nucleus accumbens shell using in vivo microdialysis.

METHODS

Inbred Lewis (LEW) and Fischer (F344) rat strains were raised in either an enriched condition (EC), social condition (SC), or isolated condition (IC). Acquisition of amphetamine self-administration (0.1 mg/kg/infusion) was determined across an incrementing daily fixed ratio (FR) schedule. In a separate cohort of rats, extracellular DA and the metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in the nucleus accumbens shell following an acute amphetamine injection (1 mg/kg).

RESULTS

"Addiction-prone" LEW rats had greater acquisition of amphetamine self-administration on a FR1 schedule compared to "addiction-resistant" F344 rats when raised in the SC environment. These genetic differences were negated in both the EC and IC environments, with enrichment buffering against self-administration and isolation enhancing self-administration in both strains. On a FR5 schedule, the isolation-induced increase in amphetamine self-administration was greater in F344 than LEW rats. While no group differences were obtained in extracellular DA, gene × environment differences were obtained in extracellular levels of the metabolite DOPAC. In IC rats only, LEW rats showed attenuation in the amphetamine-induced decrease in DOPAC compared to F344 rats. IC LEW rats also had an attenuated DOPAC response to amphetamine compared to EC LEW rats.

CONCLUSIONS

The current results demonstrate gene × environment interactions in amphetamine self-administration and amphetamine-induced changes in extracellular DOPAC in nucleus accumbens (NAc) shell. However, the behavioral and neurochemical differences were not related directly, indicating that mechanisms independent of DA metabolism in NAc shell likely mediate the gene × environment effects in amphetamine self-administration.

Binge-like acquisition of 3,4-methylenedioxypyrovalerone (MDPV) self-administration and wheel activity in rats

RATIONALE

Lack of access to conventional sources of reinforcement has been proposed as a risk factor for substance abuse in lower socioeconomic populations. There is laboratory evidence that behavioral alternatives (enrichment or exercise) and alternative reinforcers (e.g., sweetened solutions) can reduce self-administration of a variety of drugs.

OBJECTIVES

The objective of this study is to determine if drug self-administration could devalue wheel activity in an animal model.

METHODS

Male Wistar rats were trained to self-administer 3,4-methylenedioxypyrovalerone (MDPV; "bath salts"), 0.05 mg/kg/infusion, i.v., with concurrent access to a running wheel that was either locked (LW) or unlocked (UW).

RESULTS

MDPV intake steadily increased across the 20-session acquisition interval but did not differ significantly between UW and LW groups. Mean wheel rotations declined significantly across the acquisition interval in the UW group. Of the rats that acquired self-administration, 60 % engaged in a binge-like behavior at the initiation of acquisition; intake was limited only by post-reinforcement time-out. The binge rats had higher post-acquisition levels of drug intake (even after excluding the binge session), and the UW binge rats showed a precipitous post-acquisition drop in wheel activity that was not observed in the UW no-binge rats.

CONCLUSIONS

These data confirm that MDPV is a powerful reward/reinforcer and show that a relatively high rate of intake at the onset of drug taking can devalue natural rewards (wheel activity) and can predict higher subsequent drug intake levels. Thus, limiting the intensity of initial drug exposure may attenuate subsequent drug abuse/addiction by preventing the devaluation of natural alternative rewards/reinforcers.

Effects of the combination of wheel running and atomoxetine on cue- and cocaine-primed reinstatement in rats selected for high or low impulsivity

BACKGROUND

Aerobic exercise and the attention-deficit/hyperactivity disorder medication, atomoxetine (ATO), are two monotherapies that have been shown to suppress reinstatement of cocaine-seeking in an animal model of relapse. The present study investigated the effects of combining wheel running and ATO versus each treatment alone on cocaine-seeking precipitated by cocaine and cocaine-paired cues in rats with differing susceptibility to drug abuse (i.e., high vs. low impulsive).

METHODS

Rats were screened for high (HiI) or low impulsivity (LoI) based on their performance on a delay-discounting task and then trained to self-administer cocaine (0.4 mg/kg/inf) for 10 days. Following 14 days of extinction, both groups were tested for reinstatement of cocaine-seeking precipitated by cocaine or cocaine-paired cues in the presence of concurrent running wheel access (W), pretreatment with ATO, or both (W+ATO).

RESULTS

HiI rats acquired cocaine self-administration more quickly than LoI rats. While both individual treatments and W+ATO significantly attenuated cue-induced cocaine seeking in HiI and LoI rats, only W+ATO was effective in reducing cocaine-induced reinstatement compared with vehicle treatment. There were dose-dependent and phenotype-specific effects of ATO with HiI rats responsive to the low but not high ATO dose. Floor effects of ATO and W on cue-induced reinstatement prevented the assessment of combined treatment effects.

CONCLUSIONS

These findings demonstrated greater attenuation of cue- versus cocaine-induced reinstatement by ATO and W alone and recapitulate impulsivity phenotype differences in both acquisition of cocaine self-administration and receptivity to treatment.

Menthol facilitates the intravenous self-administration of nicotine in rats

Menthol is preferred by ~25% of smokers and is the most common flavoring additive in tobacco and electronic cigarettes. Although some clinical studies have suggested that menthol facilitates the initiation of smoking and enhances the dependence on nicotine, many controversies remain. Using licking as the operant behavior, we found that adolescent rats self-administering nicotine (30μg/kg/infusion, free base, i.v.) with contingent oral menthol (60μl, 0.01% w/v) obtained significantly more infusions than rats receiving a vehicle cue or rats self-administering i.v. saline with a menthol cue. Rats yoked to their menthol-nicotine masters emitted significantly fewer licks on the active spouts, indicating that contingent pairing between nicotine and menthol is required for sustained nicotine intake. Rats that self-administered nicotine with a menthol cue also exhibited a long-lasting extinction burst and robust reinstatement behavior, neither of which were observed in rats that self-administered saline with a menthol cue. The cooling sensation of menthol is induced by activating the transient receptor potential M8 (TRPM8) channel. When WS-23, an odorless agonist of the TRPM8 channel, was used as a contingent cue for nicotine, the rats obtained a similar number of nicotine infusions as the rats that were provided a menthol cue and exhibited a strong preference for the active spout. In contrast, highly appetitive taste and odor cues failed to support nicotine self-administration. These data indicated that menthol, likely by inducing a cooling sensation, becomes a potent conditioned reinforcer when it is contingently delivered with nicotine. Together, these results provide a key behavioral mechanism by which menthol promotes the use of tobacco products or electronic cigarettes.

The effects of social learning on the acquisition of cocaine self-administration

BACKGROUND

Social learning models of substance use propose that drug-use behaviors are learned by observing and mimicking the behavior of others. The aim of this study was to examine the acquisition of cocaine self-administration in three groups of experimentally naïve rats: rats that were tested in isolation, rats that were tested in the presence of another rat that had access to cocaine and had previously been trained to self-administer cocaine, and rats that were tested in the presence of another rat that did not have access to cocaine.

METHODS

Male rats were reared in isolated or pair-housed conditions and implanted with intravenous catheters. Pair-housed rats were then assigned to drug-experienced or drug-naïve conditions. In the drug-experienced condition, one rat of each pair was trained to self-administer cocaine in isolation before the reintroduction of its partner. In the drug-naïve condition, one rat of each pair did not have access to cocaine for the duration of the study. For each group, the acquisition of cocaine self-administration was measured over 15 days in rats with access to cocaine but no prior operant training.

RESULTS

Rats tested with a drug-experienced partner were faster to acquire cocaine self-administration and emitted more active lever presses than rats tested with a cocaine-naïve partner. Data for the isolated control group fell between the other two groups on these measures.

CONCLUSION

These data indicate that the acquisition of cocaine self-administration can either be facilitated or inhibited by social contact. Collectively, these results support a social learning model of substance use.

Environmental enrichment reduces methamphetamine cue-induced reinstatement but does not alter methamphetamine reward or VMAT2 function

Environmental factors influence a variety of health-related outcomes. In general, being raised in an environment possessing social, sensory, and motor enrichment reduces the rewarding effects of various drugs, thus protecting against abuse vulnerability. However, in the case of methamphetamine (METH), which acts at the vesicular monoamine transporter 2 (VMAT2) to enhance dopamine release from the cytosol, previous evidence suggests that METH reward may not be altered by environmental enrichment. This study examined the influence of an enriched environment on measures of METH reward, METH seeking, and VMAT2 function. Rats were raised from weaning to adulthood in either an enriched environment (presence of social cohorts and novel objects) or an isolated environment (no cohorts or novel objects). Rats in these two conditions were subsequently tested for their acquisition of conditioned place preference (CPP), METH self-administration, maintenance of self-administration at various unit doses of METH (0.001-0.5mg/kg/infusion), and cue-induced reinstatement. VMAT2 function in striatum from these two groups also was assessed. No significant environment effects were found in CPP or METH self-administration, which paralleled a lack of effect in VMAT2 function between groups. However, cue-induced reinstatement was reduced by environmental enrichment. Together, these results suggest that environmental enrichment does not alter VMAT2 function involved in METH reward. However, the enrichment-induced decrease in cue-induced reinstatement indicates that enrichment may have a beneficial effect against relapse following a period of extinction via a neural mechanism other than striatal VMAT2 function.

On the motivational properties of reward cues: Individual differences

Cues associated with rewards, such as food or drugs of abuse, can themselves acquire motivational properties. Acting as incentive stimuli, such cues can exert powerful control over motivated behavior, and in the case of cues associated with drugs, they can goad continued drug-seeking behavior and relapse. However, recent studies reviewed here suggest that there are large individual differences in the extent to which food and drug cues are attributed with incentive salience. Rats prone to approach reward cues (sign-trackers) attribute greater motivational value to discrete localizable cues and interoceptive cues than do rats less prone to approach reward cues (goal-trackers). In contrast, contextual cues appear to exert greater control over motivated behavior in goal-trackers than sign-trackers. It is possible to predict, therefore, before any experience with drugs, in which animals specific classes of drug cues will most likely reinstate drug-seeking behavior. The finding that different individuals may be sensitive to different triggers capable of motivating behavior and producing relapse suggests there may be different pathways to addiction, and has implications for thinking about individualized treatment. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Compared with DBA/2J mice, C57BL/6J mice demonstrate greater preference for saccharin and less avoidance of a cocaine-paired saccharin cue

Rats avoid intake of a saccharin cue when paired with a drug of abuse. While this is true for most subjects, the degree of avoidance of the drug-paired cue depends upon many factors including an individual rat's preference for rewards. That said, the direction of this effect is complex. For example, reward-preferring Lewis rats exhibit greater cocaine-induced avoidance of a saccharin cue relative to Fischer 344 rats; while reward-preferring mice that overexpress ΔFosB (NSE-tTA × TetOp-ΔFosB) exhibit less avoidance of the drug-paired taste cue compared to controls. The aim here was to use two strains of commonly used mice, C57BL/6J and DBA/2J, to determine whether known differences in sensitivity to rewards will facilitate or attenuate drug-induced suppression of intake of a drug-paired taste cue. The results of Experiment 1 demonstrate that C57BL/6J mice, compared with DBA/2J mice, exhibit attenuated suppression of saccharin intake when it is paired with cocaine. The results of Experiment 2 demonstrate that strain differences in impulsivity are not likely to account for these differences. It is proposed that, while the C57BL/6J mice typically are more responsive to drug, they also are more responsive to natural rewards (in this case saccharin), and the stronger preference for saccharin serves to militate against drug.

Animal studies of addictive behavior

It is increasingly recognized that studying drug taking in laboratory animals does not equate to studying genuine addiction, characterized by loss of control over drug use. This has inspired recent work aimed at capturing genuine addiction-like behavior in animals. In this work, we summarize empirical evidence for the occurrence of several DSM-IV-like symptoms of addiction in animals after extended drug use. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, increased motivation for drugs, preference for drugs over nondrug rewards, and resistance to punishment. The fact that addiction-like behavior can occur and be studied in animals gives us the exciting opportunity to investigate the neural and genetic background of drug addiction, which we hope will ultimately lead to the development of more effective treatments for this devastating disorder.

Neurobiology of addiction versus drug use driven by lack of choice

Research on the neurobiology of addiction often involves nonhuman animals that are given ready access to drugs for self-administration but without other choices. Here we argue using cocaine as an example that this standard setting may no longer be sufficient and can even lead to the formulation of unrealistic views about the neurobiology of addiction. Addiction as a psychiatric disorder is defined as resulting from brain dysfunctions that affect normal choice-making, not as an expectable response to lack of alternative choices. We encourage neurobiologists involved in addiction research to increase animals' choice during drug access, preferably by supplying alternative rewarding pursuits. Only animals that continue to take and prefer drugs despite and at the expense of other available choices may be considered as having developed an addiction-like behavior in comparison to those that remain able to stop drug use for other pursuits, even after extended drug use. The systematic comparison of these two individual behaviors should reveal new insights about the neurobiology of drug choice and addiction. More generally, this research should also shed a unique light on how the brain 'chooses' among qualitatively different kinds of pursuits.

Strain differences in self-administration of methylphenidate and sucrose pellets in a rat model of attention-deficit hyperactivity disorder

Despite its abuse potential, methylphenidate (MPH) is widely prescribed for treatment of attention-deficit hyperactivity disorder (ADHD). The purpose of the present study was to examine MPH self-administration in a rat model of ADHD. Experiment 1 examined the acquisition of MPH self-administration and assessed the MPH dose-effect curve in spontaneously hypertensive rats (SHR), an inbred rat model of ADHD, Wistar Kyotos (WKY), the progenitor strain for SHR, and Sprague-Dawley (SD), an outbred control strain. Experiment 2 replicated Experiment 1, but replaced MPH infusions with sucrose pellets. Initial acquisition of MPH self-administration was greater in SHR and SD than WKY. However, with extended training using an incrementing fixed ratio schedule SHR and WKY did not differ in responding for MPH using the training dose (0.3 mg/kg/infusion) or other unit doses, except that SHR showed greater responding than WKY at 0.1 mg/kg/infusion. SHR also showed greater acquisition and maintenance of sucrose-reinforced responding compared with both WKY and SD. Greater initial acquisition of MPH self-administration in SHR than WKY may not be due to a strain-specific difference in sensitivity to the reinforcing effect of MPH.

Environmental enrichment during development decreases intravenous self-administration of methylphenidate at low unit doses in rats

Despite the efficacy and widespread use of methylphenidate (MPH) as a treatment modality for attention deficit hyperactivity disorder, clinical and preclinical findings indicate that it has abuse potential. Environmental enrichment reduces susceptibility to cocaine and amphetamine self-administration and decreases impulsive behavior, but its effects on MPH self-administration are unknown. The present experiments sought to determine the influence of environmental enrichment on MPH self-administration. Male rats were raised in an enriched condition (EC) or isolated condition (IC). They were trained to self-administer MPH (0.3 mg/kg/infusion) and then exposed to varying doses of MPH on either a fixed-ratio (experiment 1) or a progressive-ratio (experiment 2) schedule of reinforcement. EC rats earned significantly fewer infusions of MPH at low doses (0.03 and 0.056 mg/kg/infusion) compared with IC rats under both schedules; however, no differences were observed at high unit doses (0.1-1.0 mg/kg/infusion). During saline substitution at the end of MPH self-administration, EC rats also responded less for saline compared with IC rats, indicative of more rapid extinction. As with other stimulant drugs with different mechanisms of action, environmental enrichment during development protects against self-administration of MPH at low unit doses but not at high unit doses.

The effect of VMAT2 inhibitor GZ-793A on the reinstatement of methamphetamine-seeking in rats

RATIONALE

The vesicular monoamine transporter 2 (VMAT2) has been identified as a potential target for the treatment of methamphetamine (METH) abuse. GZ-793A is a potent and selective VMAT2 inhibitor that has been shown to block the primary and conditioned reinforcing effects of METH, while demonstrating no abuse liability when given alone.

OBJECTIVES

The aim of the current study was to determine if GZ-793A attenuates METH- or cue-induced reinstatement of METH-seeking after a period of extinction. The effect of acute GZ-793A on locomotor activity also was assessed.

METHODS

After a period of extinction, rats were administered GZ-793A (15 mg/kg, s.c.) 15 min prior to a priming injection of METH or re-exposure to cues associated with METH infusions. GZ-793A also was administered 20 min prior to an injection of METH (0.5 mg/kg, s.c.) or saline to determine its effect on locomotor behavior.

RESULTS

Pretreatment with GZ-793A (15 mg/kg) decreased cue-induced reinstatement, without demonstrating any response suppressive effects when administered in the absence of reinstating stimuli. GZ-793A also decreased methamphetamine-induced reinstatement; however, response suppressant effects of GZ-793A were obtained when the compound was presented alone. In this latter experiment, GZ-793A may have reduced responding for the conditioned reinforcing effects of the contingently available cues rather than having nonspecific effects on baseline responding. GZ-793A had no effect on locomotor activity when administered alone or with METH.

CONCLUSIONS

GZ-793A and related VMAT2 inhibitors may be promising leads for reducing the risk of relapse to METH use following exposure to drug-associated cues.

Considering gender in cannabinoid research: a step towards personalized treatment of marijuana addicts

Addiction is a complex disorder with interacting factors, including environmental factors, drug-induced neurobiological changes, comorbidity, personality traits and stress responsivity. Numerous genetic variants that affect these factors may work in concert to affect vulnerability and severity of addiction. Traditionally, abuse of illicit drugs, including cannabis, was considered to be primarily a problem specific to men but the recent focus on drug addiction in women has brought attention to numerous sex differences in the central effects of these drugs, epidemiology of abuse-related disorders, etiologic considerations and psychiatric comorbidity. Gender is now recognised as a major factor in the modulation of the pharmacological effects of drugs of abuse, and sex differences have been reported in various phases of the addiction cycle in both humans and animals. Recently, important gender-dependent differences have been detected in the rates of initiation of marijuana smoking and in the frequency of use. Several animal studies, and in particular self-administration studies, confirmed the crucial role played by sex and gonadal hormones in determining higher sensitivity to marijuana's rewarding properties and vulnerability to cannabis addiction in females than in males. In general, women also show higher rate of relapse to drug use than men, likely due to divergent withdrawal experiences and increased reactivity to internal (emotional) and external (drug-associated) cues. According with this, craving for marijuana smoking and propensity to re-use cannabis after abstinence may also develop differently between the sexes and require distinct treatment approaches, thus highlighting the urgent need for gender-tailored prevention strategies and detoxification treatments.

Drug treatments in criminal justice settings

The available evidence suggests that drug treatment can lead to modest, but real, reductions in criminal offending for drug-using criminal offenders. Considering the scope of the problem of drug-related crime and the expense of dealing with these issues, even marginal improvements can lead to important aggregate savings in both economic and humanitarian terms. More randomized, controlled trials of drug treatment in criminal justice programs will lead to a more sophisticated understanding of what kind of treatment works best for this group.

Environmental enrichment protects against the acquisition of cocaine self-administration in adult male rats, but does not eliminate avoidance of a drug-associated saccharin cue

One of the most menacing consequences of drug addiction is the devaluation of natural rewards (e.g. food, sex, work, money, caring for one's offspring). However, evidence also suggests that natural rewards, such as an enriched environment, can devalue drugs of abuse. Thus, this study used a rodent model to test whether exposure to an enriched environment could protect adult rats from acquiring cocaine self-administration and from the resultant drug-induced devaluation of a natural saccharin reward cue. Adult male Sprague-Dawley rats were implanted with intravenous jugular catheters. Rats were then separated into two housing conditions: an enriched condition, including social companions(four/cage) and novel objects (e.g. balls, polyethylene tubes, paper, etc.), and a nonenriched condition where the rats were singly housed with no novel objects. During testing, the rats were given 5-min access to 0.15% saccharin, followed by 1 h to self-administer saline or cocaine (0.167 mg/infusion) on fixed ratio and progressive ratio schedules of reinforcement. The results showed that rats that were singly housed in the nonenriched environment fell into two groups: low drug-takers (n=34) and high drug-takers (n=12). In comparison, only one out of the 22 rats housed in the enriched environment was a high drug-taker. Thus, all rats in the enriched environment, except one, behaved like low drug-takers under the nonenriched condition. As such, these rats self-administered almost no drug on either the fixed ratio or the progressive ratio schedule of reinforcement and were extremely slow to self-administer their first cocaine infusion. Interestingly, despite their very low levels of drug self-administration, low-drug-taking rats housed in the enriched environment continued to avoid intake of the drug-associated saccharin cue. Taken together, these data suggest that the enriched environment itself served as a salient natural reward that reduced cocaine seeking and cocaine taking, but had little impact on avoidance of the cocaine-paired taste cue. The protective effects of the enriched environment were robust and, as such, have important implications for the methods used in the study of drug addiction in animal models and for the prevention, and possibly the treatment, of the disease in adult humans.

Diazepam promotes choice of abstinence in cocaine self-administering rats

When facing a choice between cocaine and a potent, albeit inessential, non-drug alternative (i.e. water sweetened with saccharin), most cocaine self-administering rats abstain from cocaine in favor of the non-drug pursuit, regardless of the dose available and even after extended drug use. Only a minority continues to take the drug despite the opportunity of making a different choice and increasing stakes. This pattern of individual variation could suggest that the majority of rats are resilient to addiction, taking cocaine by default of other options. Only a minority would be vulnerable to addiction. This study tested the hypothesis that rats choose to refrain from cocaine self-administration because cocaine would be conflictual, having both rewarding and anxiogenic properties. Contrary to this hypothesis, however, we report here that diazepam-a broad-spectrum benzodiazepine anxiolytic-did not decrease, but instead, further increased cocaine abstinence. Interestingly, although diazepam decreased locomotion, rats adapted to this effect by spending more time near the lever associated with the preferred reward, a behavior that minimized the need for locomotion at the moment of choice. When responding for cocaine or saccharin was analyzed separately, we found that diazepam decreased responding for cocaine without affecting responding for saccharin. Finally, the abstinence-promoting effects of diazepam were also induced in cocaine-preferring rats treated chronically with diazepam. Overall, this study demonstrates that abstinence from cocaine cannot be explained away by the anxiogenic effects of cocaine, thereby reinforcing the notion of resilience to addiction. It also supports the use of benzodiazepines in the treatment of cocaine addiction.

A history of bingeing on fat enhances cocaine seeking and taking

Binge eating and substance dependence are disorders characterized by a loss of control over consummatory behaviors. Given the common characteristics of these two types of disorders, it is not surprising that the comorbidity between eating disorders and substance abuse disorders is high (20-40%; Conason et al., 2006). It is unknown, however, whether loss of control in one disorder predisposes an individual to loss of control in the other. The present study, therefore, used a rodent model to test whether a history of binge eating would augment subsequent responding for cocaine. Using the limited access protocol described by Corwin et al. (1998), 45 adult male Sprague-Dawley rats were maintained on one of four dietary protocols for a period of six weeks: chow only (Chow; n = 9), continuous access to an optional source of dietary fat (Ad Lib; n = 12), 1-h access to an optional source of dietary fat daily (Daily; n = 12), or 1-h access to an optional source of dietary fat on Monday, Wednesday, and Friday (MWF; n = 12). All four groups also had unrestricted access to a nutritionally complete diet of chow and water. Fat-bingeing behaviors developed in the MWF rats, the group with the most restricted access to the optional fat. Thereafter, cocaine-seeking and -taking behaviors were assessed in all rats using a self-administration protocol modified from that described by Deroche-Gamonet et al. (2004), which focused on the motivation for and preoccupation with obtaining and consuming drug (assessed using a progressive ratio [PR] schedule of reinforcement) and persistence in responding for drug during periods of signaled drug non-availability (SNA). Rats with the MWF history tended to take more cocaine late in fixed ratio (FR) training, they persisted in their efforts to obtain cocaine in the face of signaled non-availability, worked harder for cocaine on a PR schedule of reinforcement, and exhibited more goal-directed behavior toward the cocaine-associated operandum. These results demonstrate a link between binge-type intake of fat and the development of drug-seeking and -taking behaviors, suggesting that a history of fat bingeing may predispose individuals to exhibit more robust "addiction-like" behaviors toward a substance of abuse. Thus, it appears that conditions promoting excessive behavior toward one substance (e.g., a palatable fatty food) beget excessive behavior toward another (e.g., cocaine).

Access to a running wheel inhibits the acquisition of cocaine self-administration

Physical activity decreases cocaine self-administration in laboratory animals and is associated with positive outcomes in substance abuse treatment programs; however, less is known about its efficacy in preventing the establishment of regular patterns of substance use in drug-naive individuals. The purpose of the present study was to examine the effects of access to a running wheel on the acquisition of cocaine self-administration in experimentally naive rats. Male, Long-Evans rats were obtained at weaning and assigned to sedentary (no wheel) or exercising (access to wheel) conditions immediately upon arrival. After six weeks, rats were surgically implanted with intravenous catheters and placed in operant conditioning chambers for 2 h/day for 15 consecutive days. Each session began with a noncontingent priming infusion of cocaine, followed by a free-operant period in which each response on the active lever produced an infusion of cocaine on a fixed ratio (FR1) schedule of reinforcement. For days 1-5, responding was reinforced with 0.25 mg/kg/infusion cocaine; for days 6-15, responding was reinforced with 0.75 mg/kg/infusion cocaine. In addition, all rats were calorically restricted during days 11-15 to 85% to 95% of their free-feeding body weight. Compared to sedentary rats, exercising rats acquired cocaine self-administration at a significantly slower rate and emitted significantly fewer active lever presses during the 15 days of behavioral testing. These data indicate that access to a running wheel inhibits the acquisition of cocaine self-administration, and that physical activity may be an effective intervention in substance abuse prevention programs.

The science of making drug-addicted animals

Research involving animal models of drug addiction can be viewed as a sort of reverse psychiatry. Contrary to clinicians who seek to treat addicted people to become and remain abstinent, researchers seek to make drug-naïve animals addicted to a drug with known addictive properties in humans. The goals of this research are to better understand the neuroscience of drug addiction and, ultimately, to translate this knowledge into effective treatments for people with addiction. The present review will not cover the vast literature that has accumulated over the past 50 years on animal models of drug addiction. It is instead more modestly devoted to recent research spanning the past decade on drug self-administration-based models of addiction in the rat (the animal species most frequently used in the field), with a special focus on current efforts to model compulsive cocaine use as opposed to nonaddictive use. Surprisingly, it turns out that modeling compulsive cocaine use in rats is possible but more difficult than previously thought. In fact, it appears that resilience to cocaine addiction is the norm in rats. As in human cocaine users, only few individual rats would be vulnerable. This conclusion has several important implications for future research on the neuroscience of cocaine addiction and on preclinical medication development.

Effect of environmental enrichment on escalation of cocaine self-administration in rats

BACKGROUND

Previous studies found that environmental enrichment protects against the initiation of stimulant self-administration in rats, but it is unclear if enrichment also protects against the escalation of stimulant use with long-term exposure.

OBJECTIVE

The current study examined the effects of environmental enrichment on escalation of cocaine self-administration using an extended access procedure.

METHODS

Rats were raised from 21 days in an enriched condition (EC) with social cohorts and novel objects, a social condition with only social cohorts (SC), a novelty condition (NC) with novel objects in isolated cages, or an isolated condition (IC) without social cohorts or novel objects. In young adulthood, EC, SC, NC, and IC rats were separated into short access (ShA) or long access (LgA) groups that received either 1 or 6 h, respectively, of daily cocaine self-administration (0.1 mg/kg/infusion) for 14 days. In a second experiment, EC and IC rats were used to assess differences in acquisition and escalation of cocaine self-administration at a 0.5 mg/kg/infusion unit dose.

RESULTS

With ShA sessions, EC rats acquired cocaine self-administration at a slower rate than IC rats at both unit doses; however, with extended training, both groups eventually reached similar rates. At the 0.1 mg/kg/infusion dose, only NC and IC rats escalated in amount of intake when switched to the LgA sessions. At the 0.5 mg/kg/infusion dose, rates of cocaine self-administration escalated in LgA groups over 14 days regardless of EC or IC rearing condition; however, EC rats escalated at a faster rate, eventually reaching the same level of intake observed in IC rats.

CONCLUSIONS

Although environmental enrichment protects against escalation of a low unit dose of cocaine, it may not protect against escalation with a higher unit dose. In addition, at a lower unit dose, this protective mechanism appears to be due to the presence of social cohorts rather than novel objects.

Genetics of novelty seeking, amphetamine self-administration and reinstatement using inbred rats

Previous research using outbred rats indicates that individual differences in activity in a novel environment predict sensitivity to the reinforcing effect of psychostimulant drugs. The current study examined if the link between responses related to novelty and amphetamine self-administration is heritable. Twelve inbred rat strains were assessed for locomotor activity in a novel environment, preference for a novel environment, and intravenous amphetamine self-administration (acquisition, extinction and amphetamine-induced reinstatement). Strain differences were observed in activity in a novel environment, novelty preference and amphetamine self-administration, indicating a genetic influence for each of these behaviors. While there was no relation between activity in an inescapable novel environment and amphetamine self-administration, strain-dependent differences in novelty preference were positively correlated with the amount of amphetamine self-administered. There was also a positive correlation between the dose-dependent rate of amphetamine self-administration and magnitude of reinstatement. These results show that the activity in an inescapable novel environment and the preference for a novel environment are different genetically, and thus likely to reflect different behavioral constructs. Moreover, these results implicate a genetic influence on the relation between novelty seeking and stimulant self-administration, as well as on the relation between stimulant reward and reinstatement.

Cocaine is low on the value ladder of rats: possible evidence for resilience to addiction

Background

Assessing the relative value of cocaine and how it changes with chronic drug use represents a long-standing goal in addiction research. Surprisingly, recent experiments in rats – by far the most frequently used animal model in this field – suggest that the value of cocaine is lower than previously thought.

Methodology/Principal Findings

Here we report a series of choice experiments that better define the relative position of cocaine on the value ladder of rats (i.e., preference rank-ordering of different rewards). Rats were allowed to choose either taking cocaine or drinking water sweetened with saccharin – a nondrug alternative that is not biologically essential. By systematically varying the cost and concentration of sweet water, we found that cocaine is low on the value ladder of the large majority of rats, near the lowest concentrations of sweet water. In addition, a retrospective analysis of all experiments over the past 5 years revealed that no matter how heavy was past cocaine use most rats readily give up cocaine use in favor of the nondrug alternative. Only a minority, fewer than 15% at the heaviest level of past cocaine use, continued to take cocaine, even when hungry and offered a natural sugar that could relieve their need of calories.

Conclusions/Significance

This pattern of results (cocaine abstinence in most rats; cocaine preference in few rats) maps well onto the epidemiology of human cocaine addiction and suggests that only a minority of rats would be vulnerable to cocaine addiction while the large majority would be resilient despite extensive drug use. Resilience to drug addiction has long been suspected in humans but could not be firmly established, mostly because it is difficult to control retrospectively for differences in drug self-exposure and/or availability in human drug users. This conclusion has important implications for preclinical research on the neurobiology of cocaine addiction and for future medication development.

Methylphenidate as a reinforcer for rats: contingent delivery and intake escalation

Methylphenidate (MPH) is one of the most widely prescribed drugs for treating attention-deficit hyperactivity disorder. Previous research suggested that MPH is a reinforcer for rats, but not all of the manipulations to show that lever pressing is controlled by the contingency to obtain MPH have been examined. In Experiment 1, responding for MPH on a progressive ratio (PR) schedule was assessed. Rats self-administered varying doses of MPH (0.056-1.0 mg/kg/infusion) on a PR schedule of reinforcement, and self-administered more MPH than saline, with maximal responding occurring at a unit dose of 0.56 mg/kg/infusion. Experiment 2 examined if there were differences in responding between contingent and noncontingent MPH (0.56 mg/kg/infusion) on a fixed ratio schedule of reinforcement. Results showed that rats responded for contingent MPH, and that responding was not maintained when MPH was delivered noncontingently. Experiment 3 examined self-administration of MPH (0.1 or 0.3 mg/kg/infusion) during long access (6 hr) compared to short access sessions (1 hr). Results showed that rats given long access to MPH showed an escalation of intake across sessions, with this escalation being more pronounced at the lower unit dose (0.1 mg/kg/infusion); in contrast, rats given short access to MPH did not show an increase in MPH self-administration across sessions at either MPH dose tested. Taken together, these results indicate that MPH is an effective intravenous reinforcer for rats and that, similar to other stimulants such as cocaine, amphetamine and methamphetamine, MPH is subject to abuse as reflected by dysregulated intake across repeated long access sessions.

Differences in impulsivity on a delay-discounting task predict self-administration of a low unit dose of methylphenidate in rats

There is controversy about the abuse liability of methylphenidate (MPH) in humans, and MPH has yet to be established fully as a reinforcer in rats. The present experiment examined whether intravenous MPH served as a reinforcer in rats, and how individual differences in impulsivity impacted MPH self-administration. Rats were exposed to a delay-discounting procedure, and then were implanted with an intravenous catheter to assess self-administration of 0.56 mg/kg/infusion MPH at different fixed ratio values. Self-administration rates of different MPH doses (0.03-1.0 mg/kg/infusion) were also examined. Both high and low impulsive rats acquired MPH self-administration at the same rate. All rats pressed more on the active lever than the inactive lever regardless of MPH dose, and pressed more for MPH than for saline. High impulsive rats self-administered more MPH than low impulsive rats at a low unit dose (0.1 mg/kg/infusion), though not at higher doses, indicating that individual differences in impulsive choice influence the dose-dependent reinforcing effects of MPH.

Toward a nonhuman model of contingency management: effects of reinforcing abstinence from nicotine self-administration in rats with an alternative nondrug reinforcer

RATIONALE

Reinforcing abstinence from drug use with alternative nondrug reinforcers (e.g., contingency management) is one of the most effective interventions for drug abuse. While nonhuman studies have also shown that access to alternative nondrug reinforcers reduces drug self-administration, this effect has not been examined in nicotine self-administration models. Moreover, abstinence contingencies per se under free-operant conditions have not been examined.

OBJECTIVE

The objective of this experiment was to begin development of a model of contingency management interventions by employing a differential-reinforcement-of-alternative-behavior (DRA) schedule of alternative nondrug reinforcement in rats self-administering nicotine.

METHODS

Two groups of rats were trained to self-administer nicotine under a multiple schedule of nicotine and sucrose delivery. The DRA-group was then exposed to an interlocking FR3 nicotine DRA t-sec sucrose schedule. Under this schedule, nicotine continued to be available under the FR schedule while a sucrose pellet was made available contingent upon every pause in self-administration responding (DRA interval) of 40, 80, or 160 s. The FT-group was exposed to noncontingent delivery of sucrose under fixed time (FT) schedules at an average rate equal to that obtained under the DRA schedule in the DRA-group.

RESULTS

The DRA schedule significantly reduced NSA by 73, 69, and 59% at the DRA 40, 80, and 160 s intervals, respectively, compared to baseline, while noncontingent sucrose had no significant effect. The effect of the DRA schedule was apparent throughout the NSA sessions.

CONCLUSIONS

The present assay approximates the abstinence contingencies arranged in contingency management interventions for drug abuse and provides a preliminary nonhuman model of such interventions.