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

Does compulsion explain addiction?

One of the leading drug addiction theories states that habits and the underlying neural process of a ventral to dorsal striatal shift are the building blocks of compulsive drug-seeking behaviour and that compulsion is the maladaptive persistence of responding despite adverse consequences. Here we discuss that compulsive behaviour as defined primarily from the perspective of animal experimentation falls short of the clinical phenomena and their neurobiological correlates. Thus for the human condition, the concept of compulsive habbits should be critically addressed and potentially revised.

Environmental enrichment alleviates cognitive and psychomotor alterations and increases adult hippocampal neurogenesis in cocaine withdrawn mice

Cocaine is a widely used psychostimulant drug whose repeated exposure induces persistent cognitive/emotional dysregulation, which could be a predictor of relapse in users. However, there is scarce evidence on effective treatments to alleviate these symptoms. Environmental enrichment (EE) has been shown to be associated with improved synaptic function and cellular plasticity changes related to adult hippocampal neurogenesis (AHN), resulting in cognitive enhancement. Therefore, EE could mitigate the negative impact of chronic administration of cocaine in mice and reduce the emotional and cognitive symptoms present during cocaine abstinence. In this study, mice were chronically administered with cocaine for 14 days, and control mice received saline. After the last cocaine or saline dose, mice were submitted to control or EE housing conditions, and they stayed undisturbed for 28 days. Subsequently, mice were evaluated with a battery of behavioural tests for exploratory activity, emotional behaviour, and cognitive performance. EE attenuated hyperlocomotion, induced anxiolytic-like behaviour and alleviated cognitive impairment in spatial memory in the cocaine-abstinent mice. The EE protocol notably upregulated AHN in both control and cocaine-treated mice, though cocaine slightly reduced the number of immature neurons. Altogether, these results demonstrate that EE could enhance hippocampal neuroplasticity ameliorating the behavioural and cognitive consequences of repeated administration of cocaine. Therefore, environmental stimulation may be a useful strategy in the treatment cocaine addiction.

Towards a machine-learning assisted diagnosis of psychiatric disorders and their operationalization in preclinical research: Evidence from studies on addiction-like behaviour in individual rats

Over the last few decades, there has been a progressive transition from a categorical to a dimensional approach to psychiatric disorders. Especially in the case of substance use disorders, interest in the individual vulnerability to transition from controlled to compulsive drug taking warrants the development of novel dimension-based objective stratification tools. Here we drew on a multidimensional preclinical model of addiction, namely the 3-criteria model, previously developed to identify the neurobehavioural basis of the individual's vulnerability to switch from controlled to compulsive drug taking, to test a machine-learning assisted classifier objectively to identify individual subjects as vulnerable/resistant to addiction. Datasets from our previous studies on addiction-like behaviour for cocaine or alcohol were fed into a variety of machine-learning algorithms to develop a classifier that identifies resilient and vulnerable rats with high precision and reproducibility irrespective of the cohort to which they belong. A classifier based on K-median or K-mean-clustering (for cocaine or alcohol, respectively) followed by artificial neural networks emerged as a highly reliable and accurate tool to predict if a single rat is vulnerable/resilient to addiction. Thus, each rat previously characterized as displaying 0-criterion (i.e., resilient) or 3-criteria (i.e., vulnerable) in individual cohorts was correctly labelled by this classifier. The present machine-learning-based classifier objectively labels single individuals as resilient or vulnerable to developing addiction-like behaviour in a multisymptomatic preclinical model of addiction-like behaviour in rats. This novel dimension-based classifier increases the heuristic value of these preclinical models while providing proof of principle to deploy similar tools for the future of diagnosis of psychiatric disorders.

Neurobehavioral effects of environmental enrichment and drug abuse vulnerability: An updated review

Environmental enrichment consisting of social peers and novel objects is known to alter neurobiological functioning and have an influence on the behavioral effects of drugs of abuse in preclinical rodent models. An earlier review from our laboratory (Stairs and Bardo, 2009) provided an overview of enrichment-specific changes in addiction-like behaviors and neurobiology. The current review updates the literature in this extensive field. Key findings from this updated review indicate that enrichment produces positive outcomes in drug abuse vulnerability beyond just psychostimulants. Additionally, recent studies indicate that enrichment activates key genes involved in cell proliferation and protein synthesis in nucleus accumbens and enhances growth factors in hippocampus and neurotransmitter signaling pathways in prefrontal cortex, amygdala, and hypothalamus. Remaining gaps in the literature and future directions for environmental enrichment and drug abuse research are identified.

Dose titration with the glucagon-like peptide-1 agonist, liraglutide, reduces cue- and drug-induced heroin seeking in high drug-taking rats

Opioid use disorder (OUD), like other substance use disorders (SUDs), is widely understood to be a disorder of persistent relapse. Despite the use of three FDA-approved medications for OUD, typically in conjunction with behavioral treatments, relapse rates remain unacceptably high. Whereas medication assisted therapy (MAT) reduces the risk of opioid overdose mortality, the benefits of MAT are negated when people discontinue the medications. Currently approved medications present barriers to efficient use, including daily visits to a treatment center or work restrictions. With spiking increases in opioid relapse and death, it is imperative to identify new treatments that can reduce the risk of relapse. Recent evidence suggests that glucagon-like peptide-1 receptor agonists (GLP-1RAs), currently FDA-approved to treat obesity and type two diabetes, may be promising candidates to reduce relapse. GLP-1RAs have been shown to reduce relapse in rats, whether elicited by cues, drug, and/or stress. However, GLP-1RAs also can cause gastrointestinal malaise, and therefore, in humans, the medication typically is titrated up to full dose when initiating treatment. Here, we used a rodent model to test whether cue- and drug-induced heroin seeking can be reduced by the GLP-1RA, liraglutide, when the dose is titrated across the abstinence period and prior to test. The results show this titration regimen is effective in reducing both cue-induced heroin seeking and drug-induced reinstatement of heroin seeking, particularly in rats with a history of high drug-taking. Importantly, this treatment regimen had no effect on either circulating glucose or insulin. GLP-1RAs, then, appear strong candidates for the non-opioid prevention of relapse to opioids.

Enhancement of the GluN2B subunit of glutamatergic NMDA receptors in rat brain areas after cocaine abstinence

BACKGROUND

Cocaine use disorder is associated with compulsive drug-seeking and drug-taking, whereas relapse may be induced by several factors, including stress, drug-related places, people, and cues. Recent observations strongly support the involvement of the N-methyl-D-aspartate (NMDA) receptors in cocaine use disorders and abstinence, whereas withdrawal in different environments may affect the intensification of relapse.

METHODS

The aim of this study was to examine the GluN2B subunit expression and its association with the postsynaptic density protein 95 (PSD95) in several brain structures in rats with a history of cocaine self-administration and housed either in an enriched environment or in an isolated condition. Furthermore, a selective antagonist of the GluN2B subunit-CP 101,606 (10 and 20 mg/kg) administered during exposure to cocaine or a drug-associated conditional stimulus (a cue) was used to evaluate seeking behavior in rats.

RESULTS

In rats previously self-administering cocaine, we observed an increase in the GluN2B expression in the total homogenate from the dorsal hippocampus under both enriched environment and isolation. Cocaine abstinence under isolation conditions increased the GluN2B and GluN2B/PSD95 complex levels in the PSD fraction of the prelimbic cortex in rats previously self-administering cocaine. Administration of CP 101,606 attenuated cue-induced cocaine-seeking behavior only in isolation-housed rats.

CONCLUSION

In summary, in this study we showed region-specific changes in both the expression of GluN2B subunit and NMDA receptor trafficking during cocaine abstinence under different housing conditions. Furthermore, we showed that the pharmacological blockade of the GluN2B subunit may be useful in attenuating cocaine-seeking behavior.

Are Virginia opossums really ecological traps for ticks? Groundtruthing laboratory observations

Virginia opossums (Didelphis virginiana) are a common synanthrope in North America, and serve as host to many species of ectoparasites. Research on captive Virginia opossums estimated that opossums eat, on average, 5500 larval ticks (Acari: Ixodidae) per week. To investigate this apparent preference exhibited by opossums for ingesting ticks, we comprehensively analyzed stomach contents of 32 Virginia opossums from central Illinois. Using a dissecting microscope, we searched the contents exhaustively for ticks and tick body parts, without sieving or pre-rinsing the stomach contents. We did not locate any ticks or tick parts in the stomach contents of Virginia opossums. We also performed a vigorous literature search for corroborating evidence of tick ingestion. Our search revealed 23 manuscripts that describe diet analyses of Virginia opossums, 19 of which were conducted on stomach or digestive tract contents and four of which were scat-based analyses. None of the studies identified ticks in their analyses of diet items. We conclude that ticks are not a preferred diet item for Virginia opossums. Considering that wildlife unconditioned to laboratory conditions may exhibit non-typical behaviors, we recommend that lab-based studies of wildlife behavior be groundtruthed with studies based in natural conditions.

Early environmental enrichment and impoverishment differentially affect addiction-related behavioral traits, cocaine-taking, and dopamine D2/3 receptor signaling in a rat model of vulnerability to drug abuse

RATIONALE

Risk factors for drug addiction include genetics, environment, and behavioral traits such as impulsivity and novelty preference (NP), which have been related to deficits in striatal dopamine (DA) D_2/3-receptors (D_2/3R) and heightened amphetamine (AMPH)-induced DA release. However, the influence of the early rearing environment on these behavioral and neurochemical variables is not clear.

OBJECTIVES

We investigated the influence of early rearing environment on striatal D_2/3R availabilities and AMPH-induced DA release in relation to impulsivity, NP, and propensity to drug self-administration (SA) in "addiction-prone" Roman high- (RHA) and "addiction-resistant" Roman low-avoidance (RLA) rats.

METHODS

Animals were reared post-weaning in either environmental enrichment (EE) or impoverishment (EI) and were assessed at adulthood for impulsivity, NP, and propensity to cocaine SA. EE and EI rats were also scanned using single-photon emission computed tomography to concurrently measure in vivo striatal D_2/3R availability and AMPH-induced DA release.

RESULTS

EE vs. EI was associated with heightened impulsivity and a lack of NP in both rat lines. Higher dorsal striatal D_2/3R densities were found in RHA EE and higher AMPH-induced DA release in RLA EE. Both impulsivity and NP were negatively correlated to dorsal striatal D_2/3R availabilities and positively correlated with AMPH-induced DA release in EI but not in EE. EE vs. EI was related to a faster rate of cocaine intake and elevated active timeout responses in RHAs.

CONCLUSION

Our results suggest non-monotonic, environment-dependent, relationships between impulsivity, NP, and D_2/3R-mediated signaling, and suggest that EI vs. EE may decrease the reinforcing effects of psychostimulants in predisposed individuals.

Environment-dependent behavioral traits and experiential factors shape addiction vulnerability

The transition from controlled drug use to drug addiction depends on an interaction between a vulnerable individual, their environment and a drug. Here we tested the hypothesis that conditions under which individuals live influence behavioral vulnerability traits and experiential factors operating in the drug taking environment to determine the vulnerability to addiction. The role of behavioral vulnerability traits in mediating the influence of housing conditions on the tendency to acquire cocaine self-administration was characterized in 48 rats housed in either an enriched (EE) or a standard (SE) environment. Then, the influence of these housing conditions on the individual vulnerability to develop addiction-like behavior for cocaine or alcohol was measured in 72 EE or SE rats after several months of cocaine self-administration or intermittent alcohol drinking, respectively. The determining role of negative experiential factors in the drug taking context was further investigated in 48 SE rats that acquired alcohol drinking to self-medicate distress in a schedule-induced polydipsia procedure. The environment influenced the acquisition of drug intake through its effect on behavioral markers of resilience to addiction. In contrast, the initiation of drug taking as a coping strategy or in a negative state occasioned by the contrast between enriched housing conditions and a relatively impoverished drug taking setting, facilitated the development of compulsive cocaine and alcohol intake. These data indicate that addiction vulnerability depends on environmentally determined experiential factors, and suggest that initiating drug use through negative reinforcement-based self-medication facilitates the development of addiction in vulnerable individuals. SIGNIFICANCE STATEMENT: The factors that underlie an individual's vulnerability to switch from controlled, recreational drug use to addiction are not well understood. We showed that in individuals housed in enriched conditions, the experience of drugs in the relative social and sensory impoverishment of the drug taking context, and the associated change in behavioral traits of resilience to addiction, exacerbate the vulnerability to develop compulsive drug intake. We further demonstrated that the acquisition of alcohol drinking as a mechanism to cope with distress increases the vulnerability to develop compulsive alcohol intake. Together these results demonstrate that experiential factors in the drug taking context shape the vulnerability to addiction.

Sex differences in the effects of a combined behavioral and pharmacological treatment strategy for cocaine relapse prevention in an animal model of cue exposure therapy

Brief interventions of environmental enrichment (EE) or the glycine transporter-1 inhibitor Org24598 administered with cocaine-cue extinction training were shown previously to inhibit reacquisition of cocaine self-administration in male rats trained to self-administer a moderate 0.3 mg/kg dose of cocaine. Determining how EE and Org24598 synergize in combination in an animal model of cue exposure therapy is novel. Important changes made in this investigation were increasing the cocaine training dose to 1.0 mg/kg and determining sex differences. Adult male and female rats self-administering 1.0 mg/kg cocaine for 35-40 daily sessions exhibited an addiction-like phenotype under a second-order schedule of cocaine delivery and cue presentation. Rats next underwent 6 weekly extinction training sessions for which treatments consisted of EE or NoEE and Vehicle or Org24598 (3.0 mg/kg in males; 3.0 or 7.5 mg/kg in females). Rats then were tested for reacquisition of cocaine self-administration for 15 daily sessions. In males, the combined EE +3.0 mg/kg Org24598 treatment facilitated extinction learning and inhibited reacquisition of cocaine self-administration to a greater extent than no treatment and to individual EE or 3.0 mg/kg Org24598 treatments. In females, EE +7.5 mg/kg Org24598 facilitated extinction learning, but did not inhibit reacquisition of cocaine self-administration. Thus, there were sex differences in the ability of EE + Org24598 administered in conjunction with extinction training to inhibit cocaine relapse in rats exhibiting an addiction-like phenotype. These findings suggest that this multimodal treatment approach might be a feasible option during cue exposure therapy in cocaine-dependent men, but not women.

Cocaine Self-Administration and Abstinence Modulate NMDA Receptor Subunits and Active Zone Proteins in the Rat Nucleus Accumbens

Cocaine-induced plasticity in the glutamatergic transmission and its N-methyl-d-aspartate (NMDA) receptors are critically involved in the development of substance use disorder. The presynaptic active zone proteins control structural synaptic plasticity; however, we are still far from understanding the molecular determinants important for cocaine seeking behavior. The aim of this study was to investigate the effect of cocaine self-administration and different conditions of cocaine forced abstinence on the composition of the NMDA receptor subunits and on the levels of active zone proteins, i.e., Ras-related protein 3A (Rab3A), Rab3 interacting molecules 1 (RIM1) and mammalian uncoordinated protein 13 (Munc13) in the rat nucleus accumbens. We found an up-regulation of the accumbal levels of GluN1 and GluN2A following cocaine self-administration that was paralleled by an increase of Munc13 and RIM1 levels. At the same time, we also demonstrated that different conditions of cocaine abstinence abolished changes in NMDA receptor subunits (except for higher GluN1 levels after cocaine abstinence with extinction training), while an increase in the Munc13 concentration was shown in rats housed in an enriched environment. In conclusion, cocaine self-administration is associated with the specific up-regulation of the NMDA receptor subunit composition and is related with new presynaptic targets controlling neurotransmitter release. Moreover, changes observed in cocaine abstinence with extinction training and in an enriched environment in the levels of NMDA receptor subunit and in the active zone protein, respectively, may represent a potential regulatory step in cocaine-seeking behavior.

Environmental enrichment during forced abstinence from cocaine self-administration opposes gene network expression changes associated with the incubation effect

Environmental enrichment (EE) is a robust intervention for reducing cocaine-seeking behaviors in animals when given during forced abstinence. However, the mechanisms that underlie these effects are not well-established. We investigated the adult male rat transcriptome using RNA-sequencing (RNA-seq) following differential housing during forced abstinence from cocaine self-administration for either 1 or 21 days. Enriched, 21-day forced abstinence rats displayed a significant reduction in cocaine-seeking behavior compared to rats housed in isolation. RNA-seq of the nucleus accumbens shell revealed hundreds of differentially regulated transcripts between rats of different forced abstinence length and housing environment, as well as within specific contrasts such as enrichment (isolated 21 days vs. enriched 21 days) or incubation (isolated 1 day vs. isolated 21 days). Ingenuity Pathway Analysis affirmed several pathways as differentially enriched based on housing condition and forced abstinence length including RELN, the Eif2 signaling pathway, synaptogenesis and neurogenesis pathways. Numerous pathways showed upregulation with incubation, but downregulation with EE, suggesting that EE may prevent or reverse changes in gene expression associated with protracted forced abstinence. The findings reveal novel candidate mechanisms involved in the protective effects of EE against cocaine seeking, which may inform efforts to develop pharmacological and gene therapies for treating cocaine use disorders. Furthermore, the finding that EE opposes multiple pathway changes associated with incubation of cocaine seeking strongly supports EE as a therapeutic intervention and suggests EE is capable of preventing or reversing the widespread dysregulation of signaling pathways that occurs during cocaine forced abstinence.

Therapeutic efficacy of environmental enrichment for substance use disorders

Addiction to drug and alcohol is regarded as a major health problem worldwide for which available treatments show limited effectiveness. The biggest challenge remains to enhance the capacities of interventions to reduce craving, prevent relapse and promote long-term recovery. New strategies to meet these challenges are being explored. Findings from preclinical work suggest that environmental enrichment (EE) holds therapeutic potential for the treatment of substance use disorders, as demonstrated in a number of animal models of drug abuse. The EE intervention introduced after drug exposure leads to attenuation of compulsive drug taking, attenuation of the rewarding (and reinforcing) effects of drugs, reductions in control of behavior by drug cues, and, very importantly, relapse prevention. Clinical work also suggests that multidimensional EE interventions (involving physical activity, social interaction, vocational training, recreational and community involvement) might produce similar therapeutic effects, if implemented continuously and rigorously. In this review we survey preclinical and clinical studies assessing the efficacy of EE as a behavioral intervention for substance use disorders and address related challenges. We also review work providing empirical evidence for EE-induced neuroplasticity within the mesocorticolimbic system that is believed to contribute to the seemingly therapeutic effects of EE on drug and alcohol-related behaviors.

Effects of single parenthood on mothers' behavior, morphology, and endocrine function in the biparental California mouse

Motherhood is energetically costly for mammals and is associated with pronounced changes in mothers' physiology, morphology and behavior. In ~5% of mammals, fathers assist their mates with rearing offspring and can enhance offspring survival and development. Although these beneficial consequences of paternal care can be mediated by direct effects on offspring, they might also be mediated indirectly, through beneficial effects on mothers. We tested the hypothesis that fathers in the monogamous, biparental California mouse (Peromyscus californicus) reduce the burden of parental care on their mates, and therefore, that females rearing offspring with and without assistance from their mates will show differences in endocrinology, morphology and behavior, as well as in the survival and development of their pups. We found that pups' survival and development in the lab did not differ between those raised by a single mother and those reared by both mother and father. Single mothers spent more time in feeding behaviors than paired mothers. Both single and paired mothers had higher lean mass and/or lower fat mass and showed more anxiety-like behavior in open-field tests and tail-suspension tests, compared to non-breeding females. Single mothers had higher body-mass-corrected liver and heart masses, but lower ovarian and uterine masses, than paired mothers and/or non-breeding females. Mass of the gastrointestinal tract did not differ between single and paired mothers, but single mothers had heavier gastrointestinal tract compared to non-breeding females. Single motherhood also induced a flattened diel corticosterone rhythm and a blunted corticosterone response to stress, compared to non-breeding conditions. These findings suggest that the absence of a mate induces morphological and endocrine changes in mothers, which might result from increased energetic demands of pup care and could potentially help maintain normal survival and development of pups.

Alternation in dopamine D2-like and metabotropic glutamate type 5 receptor density caused by differing housing conditions during abstinence from cocaine self-administration in rats

BACKGROUND

Environmental conditions have an important function in substance use disorder, increasing or decreasing the risks of relapse. Several studies strongly support the role of the dopamine D₂-like and metabotropic glutamate type 5 receptors in maladaptive neurobiological responses to cocaine reward and relapse.

AIMS

The present study employed cocaine self-administration with yoked-triad procedure in rats to explore whether drug abstinence in different housing conditions affects the drug-seeking behaviour and the dopamine D₂-like and metabotropic glutamate type 5 receptor density and affinity in several regions of the animal brain.

METHODS

Rats were trained to self-administer cocaine and later they were forced to abstain either in: (a) enriched environment or (b) isolation cage conditions to evaluate the effect of housing conditions on the drug-seeking behaviour and to assess changes concerning receptors in animals brain.

RESULTS

Our results show that exposure to enriched environment conditions strongly reduced active lever presses during cue-induced drug-seeking. At the neurochemical level, we demonstrated a significant increase in the dopamine D₂-like receptor density in the prefrontal cortex in animals following drug abstinence in isolation cage or enriched environment conditions, and the reduction in their density in the dorsal striatum provoked by isolation cage conditions. The metabotropic glutamate type 5 receptor density decreased only in the prefrontal cortex after isolation cage and enriched environment abstinence.

CONCLUSIONS

This study shows the different impacts caused by the type of housing conditions during abstinence from cocaine self-administration on drug-seeking behaviour in rats. The observed changes in the dopamine D₂-like and metabotropic glutamate type 5 receptor B_max and/or K_d values were brain-region specific and related to either pharmacological and/or motivational features of cocaine.

Environmental enrichment and drug value: a behavioral economic analysis in male rats

Rats raised in an enriched condition (EC) show decreased stimulant self-administration relative to rats reared in an isolated condition (IC). However, few studies have examined the behavioral mechanisms underlying this environment-induced difference in self-administration. Because economic demand for drugs of abuse predicts addiction-like behavior in both humans and animals, we applied a behavioral economic analysis to cocaine self-administration data in EC and IC rats. During cocaine self-administration, the dose decreased across blocks of trials (0.75-0.003 mg/kg/inf), which allowed for a determination of demand intensity and demand elasticity. Demand intensity did not differ between EC and IC rats; however, cocaine was more elastic in EC rats relative to IC rats (i.e. EC rats were less willing to respond for cocaine as the unit price increased). When EC rats were placed in an isolated condition, demand elasticity decreased, whereas elasticity increased for IC rats placed in an enriched condition. Additionally, we applied behavioral economic analyses to previously published self-administration data and found that our results replicate past findings with cocaine and methylphenidate. To determine if differences in demand elasticity are specific to drug reinforcement, a separate group of rats was tested in sucrose or saccharin self-administration. Results showed that sucrose and saccharin were more elastic in EC rats relative to IC rats, and demand intensity was lower for saccharin in EC rats relative to IC rats. Overall, drug and nondrug reinforcers are more elastic in EC rats, which may account for the protective effects of environmental enrichment against stimulant self-administration.

Enriched Environment Exposure Enhances Social Interactions and Oxytocin Responsiveness in Male Long-Evans Rats

Both social and physical stimuli contribute to the complexity of an animal’s environment, influencing biobehavioral responses to subsequent challenges. In the current study, male Long-Evans rats were randomly assigned to an isolate (ISO), social control (SC) or social enriched (SE) group (n = 8 per group). The SC and SE conditions were group housed with the SE group exposed to physical enrichment stimuli that were natural as opposed to manufactured (e.g., hollowed out log instead of plastic hiding place). On three occasions during their 40-day enriched environment exposure, night/dark phase videos were obtained for 1 h during the early part of the dark phase. During this time, the SE animals exhibited significantly more social grooming with no differences between the SE and SC in the frequency of play or self-grooming bouts. Subsequently, all animals were assessed in social interaction and problem-solving escape tasks during the last week of the enriched environment exposure. SE rats exhibited increased digging bouts toward the restrained conspecific in the social interaction task whereas the other groups exhibited more escape responses. In the problem-solving task, SE animals exhibited a decreased latency to cross the barrier to escape from the predator odor (i.e., cat urine and fur). Neural analyses indicated increased oxytocin-immunoreactive (OT-ir) tissue in the SE supraoptic and paraventricular nuclei of the hypothalamus compared to the other groups. Interestingly, blood samples indicated lower peripheral corticosterone (CORT) and higher OT levels in the ISO animals when compared to the SC and SE animals, an effect retrospectively attributed to separation anxiety in the SE and SC animals in preparation for histology procedures. When the behavioral, neural and endocrine data were visualized as a multifaceted dataset via a multidimensional scaling analysis, however, an association between social enrichment and higher OT involvement was observed in the SE animals, as well as heightened stress responsivity in the ISO and SC groups. In sum, the SE animals exhibited a facilitation of social responses, problem-solving ability and OT immunoreactive responsiveness. These findings provide new information about the influences of both physical and social stimuli in dynamic and enriched environments.

Exposure to environmental enrichment attenuates addiction-like behavior and alters molecular effects of heroin self-administration in rats

Environmental factors profoundly affect the addictive potential of drugs of abuse and may also modulate the neuro-anatomical/neuro-chemical impacts of uncontrolled drug use and relapse propensity. This study examined the impact of environmental enrichment on heroin self-administration, addiction-related behaviors, and molecular processes proposed to underlie these behaviors. Male Sprague-Dawley rats in standard and enriched housing conditions intravenously self-administered similar amounts of heroin over 14 days. However, environmental enrichment attenuated progressive ratio, extinction, and reinstatement session responding after 14 days of enforced abstinence. Molecular mechanisms, namely DNA methylation and gene expression, are proposed to underlie abstinence-persistent behaviors. A global reduction in methylation is reported to coincide with addiction, but no differences in total genomic methylation or repeat element methylation were observed in CpG or non-CpG (CH) contexts across the mesolimbic circuitry as assessed by multiple methods including whole genome bisulfite sequencing. Immediate early gene expression associated with drug seeking, taking, and abstinence also were examined. EGR1 and EGR2 were suppressed in mesolimbic regions with heroin-taking and environmental enrichment. Site-specific methylation analysis of EGR1 and EGR2 promoter regions using bisulfite amplicon sequencing (BSAS) revealed hypo-methylation in the EGR2 promoter region and EGR1 intragenic CpG sites with heroin-taking and environmental enrichment that was associated with decreased mRNA expression. Taken together, these findings illuminate the impact of drug taking and environment on the epigenome in a locus and gene-specific manner and highlight the need for positive, alternative rewards in the treatment and prevention of drug addiction.

Animal Models of the Impact of Social Stress on Cocaine Use Disorders

Cocaine use disorders are strongly influenced by the social conditions prior, during, and after exposure to cocaine. In this chapter, we discuss how social factors such as early life stress, social rank stress, and environmental stress impact vulnerability and resilience to cocaine. The discussion of each animal model begins with a brief review of examples from the human literature, which provide the psychosocial background these models attempt to capture. We then discuss preclinical findings from use of each model, with emphasis on how social factors influence cocaine-related behaviors and how sex and age influence the behaviors and neurobiology. Models discussed include (1) early life social stress, such as maternal separation and neonatal isolation, (2) social defeat stress, (3) social hierarchies, and (4) social isolation and environmental enrichment. The cocaine-related behaviors reviewed for each of these animal models include cocaine-induced conditioned place preference, behavioral sensitization, and self-administration. Together, our review suggests that the degree of psychosocial stress experienced yields robust effects on cocaine-related behaviors and neurobiology, and these preclinical findings have translational impact for the future of cocaine use disorder treatment.

Environmental enrichment facilitates cocaine abstinence in an animal conflict model

In this study, we sought to discover if housing in an enriched environment (EE) is an efficacious intervention for encouraging abstinence from cocaine seeking in an animal "conflict" model of abstinence. Sixteen Long-Evans rats were trained in 3-h daily sessions to self-administer a cocaine solution (1 mg/kg/infusion) until each demonstrated a stable pattern of drug-seeking. Afterward, half were placed in EE cages equipped with toys, obstacles, and a running wheel, while the other half were given clean, standard laboratory housing. All rats then completed daily 30-min sessions during which the 2/3 of flooring closest to the self-administration levers was electrified, causing discomfort should they approach the levers; current strength (mA) was increased after every day of drug seeking until the rat ceased activity on the active lever for 3 consecutive sessions (abstinence). Rats housed in EE abstained after fewer days and at lower current strengths than rats in standard housing. These results support the idea that EE administered after the development of a cocaine-taking habit may be an effective strategy to facilitate abstinence.

Relatively enriched housing conditions delay binge onset but do not attenuate binge size

Housing and enrichment conditions are essential factors to consider when using animal models of behavior, as they can alter the behavior that is under investigation. The goal of this study was to determine the impact of the relatively enriched environment recommended by current animal care guidelines on development and maintenance of binge-type behavior in rats, using the limited access (LA) binge model. Non-food-deprived rats were divided into two groups, enriched and nonenriched, with all rats housed in shoebox cages. Bedding, nesting material, toys, and a solid floor were provided only to the enriched group to create a state of relative enrichment, or RE, compared to the nonenriched conditions historically used in the LA model. Enriched and nonenriched groups were further divided into control and experimental groups. Control rats received access to an optional source of fat (vegetable shortening) for 30min each day (daily access) while experimental rats received 30-min optional fat access on Monday, Wednesday, and Friday only (intermittent access). The four groups were designated C-E (Control-Enriched), C-NE (Control-Nonenriched), I-E (Intermittent-Enriched), and I-NE (Intermittent-Nonenriched). Bingeing in the LA model is established when a group with intermittent access (i.e., the I-E or I-NE group) consumes significantly more vegetable shortening during the limited access period than a group with daily access (i.e., the C-E or C-NE group). Access sessions continued for 8weeks under these conditions, at which time the housing conditions of the I-E and I-NE groups were reversed for an additional 8weeks of access sessions. Intakes of the C-E and C-NE groups were similar and data from these two groups were combined. Relative to this Combined Control Group (CCG), the I-NE group began bingeing in week 3 while the I-E group binged during weeks 6 and 8. Following the reversal at the beginning of week 9, the newly enriched I-NE group ceased bingeing in week 9 but resumed bingeing in weeks 10-16. The newly nonenriched I-E group continued bingeing through the remainder of the study. Intakes of the I-E and I-NE groups were not significantly different at any time during the study. These results indicate that RE delays binge onset; that is, RE increases the time between the first fat access session and the first occurrence of bingeing. However, RE does not significantly alter the amount of fat consumed during binge sessions. Furthermore, addition of RE to a nonenriched group of animals (I-NE) does not reverse established binge behavior. Thus it appears that regardless of enrichment condition, intermittent access to vegetable shortening induces greater consumption of fat than does daily access. However, it is clear that a certain level of austerity in housing conditions is required for rapid development of lasting binge-type eating to occur. In addition, results suggest that it is unlikely that enrichment, to the degree provided in this study, can prevent or reverse binge-type eating in rats.

Modified single prolonged stress reduces cocaine self-administration during acquisition regardless of rearing environment

Until recently, there were few rodent models available to study the interaction of post-traumatic stress disorder (PTSD) and drug taking. Like PTSD, single prolonged stress (SPS) produces hypothalamic-pituitary-adrenal (HPA) axis dysfunction and alters psychostimulant self-administration. Other stressors, such as isolation stress, also alter psychostimulant self-administration. However, it is currently unknown if isolation housing combined with SPS can alter the acquisition or maintenance of cocaine self-administration. The current study applied modified SPS (modSPS; two hours restraint immediately followed by cold swim stress) to rats raised in an isolation condition (Iso), enrichment condition (Enr), or standard condition (Std) to measure changes in cocaine self-administration and HPA markers. Regardless of rearing condition, rats exposed to modSPS had greater corticosterone (CORT) release and reduced cocaine self-administration during initial acquisition compared to non-stressed controls. In addition, during initial acquisition, rats that received both Iso rearing and modSPS showed a more rapid increase in cocaine self-administration across sessions compared to Enr and Std rats exposed to modSPS. Following initial acquisition, a dose response analysis showed that Iso rats were overall most sensitive to changes in cocaine unit dose; however, modSPS had no effect on the cocaine dose response curve. Further, there was no effect of either modSPS or differential rearing on expression of glucocorticoid receptor (GR) in hypothalamus, medial prefrontal cortex, amygdala, or nucleus accumbens. By using modSPS in combination with Iso housing, this study identified unique contributions of each stressor to acquisition of cocaine self-administration.

Blockade of α2-adrenergic receptors in prelimbic cortex: impact on cocaine self-administration in adult spontaneously hypertensive rats following adolescent atomoxetine treatment

RATIONALE

Research with the spontaneously hypertensive rat (SHR) model of attention deficit/hyperactivity disorder demonstrated that chronic methylphenidate treatment during adolescence increased cocaine self-administration established during adulthood under a progressive ratio (PR) schedule. Compared to vehicle, chronic atomoxetine treatment during adolescence failed to increase cocaine self-administration under a PR schedule in adult SHR.

OBJECTIVES

We determined if enhanced noradrenergic transmission at α2-adrenergic receptors within prefrontal cortex contributes to this neutral effect of adolescent atomoxetine treatment in adult SHR.

METHODS

Following treatment from postnatal days 28-55 with atomoxetine (0.3 mg/kg) or vehicle, adult male SHR and control rats from Wistar-Kyoto (WKY) and Wistar (WIS) strains were trained to self-administer 0.3 mg/kg cocaine. Self-administration performance was evaluated under a PR schedule of cocaine delivery following infusion of the α2-adrenergic receptor antagonist idazoxan (0 and 10-56 μg/side) directly into prelimbic cortex.

RESULTS

Adult SHR attained higher PR break points and had greater numbers of active lever responses and infusions than WKY and WIS. Idazoxan dose-dependently increased PR break points and active lever responses in SHR following adolescent atomoxetine vs. vehicle treatment. Behavioral changes were negligible after idazoxan pretreatment in SHR following adolescent vehicle or in WKY and WIS following adolescent atomoxetine or vehicle.

CONCLUSIONS

α2-Adrenergic receptor blockade in prelimbic cortex of SHR masked the expected neutral effect of adolescent atomoxetine on adult cocaine self-administration behavior. Moreover, greater efficacy of acute idazoxan challenge in adult SHR after adolescent atomoxetine relative to vehicle is consistent with the idea that chronic atomoxetine may downregulate presynaptic α2A-adrenergic autoreceptors in SHR.

Once is too much: Early development of the opponent process in taste reactivity behavior is associated with later escalation of cocaine self-administration in rats

Evidence suggests that the addiction process may begin immediately in some vulnerable subjects. Specifically, some rats have been shown to exhibit aversive taste reactivity (gapes) following the intraoral delivery of a cocaine-predictive taste cue after as few as 1-2 taste-drug pairings. After only 3-4 trials, the number of gapes becomes a reliable predictor of later cocaine self-administration. Given that escalation of drug-taking behavior over time is recognized as a key feature of substance use disorder (SUD) and addiction, the present study examined the relationship between early aversion to the cocaine-predictive flavor cue and later escalation of cocaine self-administration in an extended-access paradigm. The data show that rats who exhibit the greatest conditioned aversion early in training to the intraorally delivered cocaine-paired cue exhibit the greatest escalation of cocaine self-administration over 15 extended-access trials. This finding suggests that early onset of the conditioned opponent process (i.e., the near immediate shift from ingestion to rejection of the drug-paired cue) is a reliable predictor of future vulnerability and resilience to cocaine addiction-like behavior. Future studies must determine the underlying neural mechanisms associated with this early transition and, hence, with early vulnerability to the later development of SUD and addiction. In so doing, we shall be in position to discover novel diagnostics and novel avenues of prevention and treatment.

Behavioral and physiological consequences of enrichment loss in rats

Significant loss produces the highest degree of stress and compromised well-being in humans. Current rodent models of stress involve the application of physically or psychologically aversive stimuli, but do not address the concept of loss. We developed a rodent model for significant loss, involving removal of long-term access to a rewarding enriched environment. Our results indicate that removal from environmental enrichment produces a profound behavioral and physiological phenotype with depression-like qualities, including helplessness behavior, hypothalamo-pituitary-adrenocortical axis dysregulation and overeating. Importantly, this enrichment removal phenotype was prevented by antidepressant treatment. Furthermore, the effects of enrichment removal do not occur following relief from chronic stress and are not duplicated by loss of exercise or social contact.

Environmental Enrichment Blunts Ethanol Consumption after Restraint Stress in C57BL/6 Mice

Elevated alcohol intake after abstinence is a key feature of the addiction process. Some studies have shown that environmental enrichment (EE) affects ethanol intake and other reinforcing effects. However, different EE protocols may vary in their ability to influence alcohol consumption and stress-induced intake. The present study evaluated whether short (3 h) or continuous (24 h) EE protocols affect ethanol consumption after periods of withdrawal. Mice were challenged with stressful stimuli (24 h isolation and restraint stress) to evaluate the effects of stress on drinking. Male C57BL/6 mice were subjected to a two-bottle choice drinking-in-the-dark paradigm for 15 days (20% ethanol and water, 2 h/day, acquisition phase). Control mice were housed under standard conditions (SC). In the first experiment, one group of mice was housed under EE conditions 24 h/day (EE24h). In the second experiment, the exposure to EE was reduced to 3 h/day (EE3h). After the acquisition phase, the animals were deprived of ethanol for 6 days, followed by 2 h ethanol access once a week. Animals were tested in the elevated plus maze (EPM) during ethanol withdrawal. During the last 2 weeks, the mice were exposed to 24 h ethanol access. A 1-h restraint stress test was performed immediately before the last ethanol exposure. EE24h but not EE3h increased anxiety-like behavior during withdrawal compared to controls. Neither EE24h nor EE3h affected ethanol consumption during the 2 h weekly exposure periods. However, EE24h and EE3h mice that were exposed to acute restraint stress consumed less ethanol than controls during a 24 h ethanol access. These results showed that EE reduces alcohol intake after an acute restraint stress.

Deficits in Access to Reward Are Associated with College Student Alcohol Use Disorder

BACKGROUND

Reward deprivation has been implicated in major depressive disorder and severe substance abuse, but its potential relation to alcohol use disorder (AUD) symptoms in non-treatment-seeking young adult drinkers is less clear. Depression is often comorbid with alcohol misuse, so relations of AUD with reward deprivation might be due in part to the presence of depressive symptoms in young adults. Behavioral economic theory views addiction as a state that is related in part to deficits in drug-free rewards, and therefore requires an investigation into whether reward deprivation has a direct relation to alcohol misuse that is, at least partially, independent of mood.

METHODS

This study evaluates the contribution of 2 facets of reward deprivation (reward availability and experience) to alcohol use, AUD symptoms, and depression in a sample of young adult heavy episodic drinkers. Data were collected from 392 undergraduates (60.4% female, 85.1% Caucasian) who reported recent heavy drinking (83.7% with at least 1 AUD symptom).

RESULTS

Low reward availability (environmental suppression) was significantly associated with both DSM-5 AUD symptoms and alcohol-related problems after controlling for age, gender, depressive symptomatology, and drinking level.

CONCLUSIONS

This study provides support for behavioral economic models that emphasize reward deprivation as a unique risk factor for AUD that is independent of mood and drinking level. Limited access to natural rewards may be a risk and/or maintaining factor for AUD symptoms in college student drinkers.

Greater avoidance of a heroin-paired taste cue is associated with greater escalation of heroin self-administration in rats

Heroin addiction is a disease of chronic relapse affecting over half of its users. Therefore, modeling individual differences in addiction-like behavior is needed to better reflect the human condition. In a rodent model, avoidance of a cocaine-paired saccharin cue is associated with greater cocaine seeking and taking. Here, we tested whether rats would avoid a saccharin cue when paired with the opportunity to self-administer heroin and whether the rats that most greatly avoid the heroin-paired taste cue would exhibit the greatest drug escalation over time, the greatest willingness to work for drug, and the greatest heroin-induced relapse. Adult male Sprague-Dawley rats received 5 min access to a 0.15% saccharin solution followed by the opportunity to self-administer either saline or heroin for 3 hr (short access) or 6 hr (extended access). Following 16 to 18 pairings, terminal saccharin intake was used to categorize the rats into small (>200 licks/5min) or large (<200 licks/5min) suppressors and responding for drug was examined accordingly. Only 5% of the short access rats reached the criteria for large suppressors. This large suppressor did not differ from the small suppressors in drug-taking behavior. On the other hand, 50% of the extended access saccharin-heroin rats were large suppressors and showed the largest escalation of drug intake, drug-loading behavior, and the greatest relapse-like behaviors. Extended access small suppressors displayed drug-taking behaviors that were similar to rats in the short access heroin condition. Avoidance of a heroin-paired taste cue reliably identifies individual differences in addiction-like behavior for heroin using extended drug access.

Early avoidance of a heroin-paired taste-cue and subsequent addiction-like behavior in rats

The ability to predict individual vulnerability to substance abuse would allow for a better understanding of the progression of the disease and development of better methods for prevention and/or early intervention. Here we use drug-induced devaluation of a saccharin cue in an effort to predict later addiction-like behavior in a model akin to that used by Deroche-Gamonet et al. (2004) and seek to link such vulnerability to changes in expression of various mu opioid receptor and D2 receptor-interacting proteins in brain. The results show that the greatest heroin-induced suppression of intake of a saccharin cue is associated with the greatest vulnerability to later addiction-like behavior and to differences in the expression of WLS, β-catenin, and NCS-1 in brain compared to rats that exhibited the least suppression of intake of the heroin-paired cue and/or saline controls. Finally, because the self-administration model employed produced no significant differences in drug intake between groups, overall, the resultant changes in protein expression can be more closely linked to individual differences in motivation for drug.

Reversal of the sleep-wake cycle by heroin self-administration in rats

The goal of this study was to examine how heroin self-administration, abstinence, and extinction/reinstatement affect circadian sleep-wake cycles and the associated sleep architecture. We used electroencephalography (EEG) and electromyography (EMG) to measure sleep patterns in male Sprague-Dawley rats over 16 trials of heroin self-administration (acquisition), 14 days of abstinence, and a single day of extinction and drug-induced reinstatement. Rats self-administering heroin showed evidence of reversed (diurnal) patterns of wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep throughout acquisition. During abstinence, their wake and NREM sleep patterns were immediately restored to the normal nocturnal distribution. REM patterns remained inverted for the first 3-6 days of abstinence in heroin self-administering rats. The single extinction/reinstatement test was without effect. These data suggest that heroin may have the ability to affect circadian distribution of sleep and wakefulness, either indirectly, where animals shift their sleep-wake cycle to allow for drug taking, or directly, through wake-promoting actions or actions at circadian oscillators in the brain.

One day access to a running wheel reduces self-administration of D-methamphetamine, MDMA and methylone

BACKGROUND

Exercise influences drug craving and consumption in humans and drug self-administration in laboratory animals, but the effects can be variable. Improved understanding of how exercise affects drug intake or craving would enhance applications of exercise programs to human drug users attempting cessation.

METHODS

Rats were trained in the intravenous self-administration (IVSA) of D-methamphetamine (METH; 0.05 mg/kg/inf), 3,4-methylenedioxymethamphetamine (MDMA; 0.5 mg/kg/inf) or methylone (0.5 mg/kg/inf). Once IVSA was established, the effect of ∼ 22 h of wheel access in the home cage on subsequent drug taking was assessed in a two cohort crossover design.

RESULTS

Provision of home cage wheel access during the day prior to IVSA sessions significantly decreased the self-administration of METH, MDMA and methylone. At the individual level, there was no correlation between the amount a rat used the wheel and the size of the individual's decrease in drug intake.

CONCLUSIONS

Wheel access can reduce self-administration of a variety of psychomotor stimulants. It does so immediately, i.e., without a need for weeks of exercise prior to drug access. This study therefore indicates that future mechanistic investigations should focus on acute effects of exercise. In sum, the results predict that exercise programs can be used to decrease stimulant drug use in individuals even with no exercise history and an established drug taking pattern.

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.

Food reward system: current perspectives and future research needs

This article reviews current research and cross-disciplinary perspectives on the neuroscience of food reward in animals and humans, examines the scientific hypothesis of food addiction, discusses methodological and terminology challenges, and identifies knowledge gaps and future research needs. Topics addressed herein include the role of reward and hedonic aspects in the regulation of food intake, neuroanatomy and neurobiology of the reward system in animals and humans, responsivity of the brain reward system to palatable foods and drugs, translation of craving versus addiction, and cognitive control of food reward. The content is based on a workshop held in 2013 by the North American Branch of the International Life Sciences Institute.

Conditioned Aversion for a Cocaine-Predictive Cue is Associated with Cocaine Seeking and Taking in Rats

Rats emit aversive taste reactivity (TR) behavior (i.e., gapes) following intraoral delivery of a cocaine-paired taste cue, and greater conditioned aversive TR in well-trained rats predicts greater drug-taking. Here, we used a between-groups design and tracked the development of this conditioned aversive TR behavior on a trial by trial basis in an effort to determine when the change in behavior occurs and at what point individual differences in cue reactivity become predictive of cocaine-seeking and cocaine-taking. The results demonstrate that conditioned aversive TR to a cocaine-predictive flavor cue appears very early in training (i.e., following as few as 1 to 2 taste-drug pairings), stabilizes quickly, and becomes predictive of terminal self-administration within 3 to 4 trials. Indeed, rats exhibiting high conditioned aversive TR to the cocaine-paired cue also exhibited greater goal-directed behavior, were faster to take drug, self-administered more cocaine, and exhibited greater seeking during periods of drug non-availability. High conditioned aversive TR, then, develops quickly and is associated with a greater motivation for drug.

Inoculation stress hypothesis of environmental enrichment

One hallmark of psychiatric conditions is the vast continuum of individual differences in susceptibility vs. resilience resulting from the interaction of genetic and environmental factors. The environmental enrichment paradigm is an animal model that is useful for studying a range of psychiatric conditions, including protective phenotypes in addiction and depression models. The major question is how environmental enrichment, a non-drug and non-surgical manipulation, can produce such robust individual differences in such a wide range of behaviors. This paper draws from a variety of published sources to outline a coherent hypothesis of inoculation stress as a factor producing the protective enrichment phenotypes. The basic tenet suggests that chronic mild stress from living in a complex environment and interacting non-aggressively with conspecifics can inoculate enriched rats against subsequent stressors and/or drugs of abuse. This paper reviews the enrichment phenotypes, mulls the fundamental nature of environmental enrichment vs. isolation, discusses the most appropriate control for environmental enrichment, and challenges the idea that cortisol/corticosterone equals stress. The intent of the inoculation stress hypothesis of environmental enrichment is to provide a scaffold with which to build testable hypotheses for the elucidation of the molecular mechanisms underlying these protective phenotypes and thus provide new therapeutic targets to treat psychiatric/neurological conditions.

Once is too much: conditioned aversion develops immediately and predicts future cocaine self-administration behavior in rats

Rats emit aversive taste reactivity (TR) behavior (i.e., gapes) following intraoral delivery of a cocaine-paired taste cue and greater conditioned aversive TR at the end of training predicts greater drug-seeking and taking. Here, we examined the development of this conditioned aversive TR behavior on a trial-by-trial basis in an effort to determine when the change in behavior occurs and whether early changes in this behavior can be used to predict later drug taking. The results show that conditioned aversive TR to a cocaine-paired cue occurs very early in training (i.e., following as few as 1-2 taste-drug pairings) and, importantly, that it can be used to predict later drug seeking and drug taking in rats.

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.

Individual differences in cocaine addiction: maladaptive behavioural traits

Cocaine use leads to addiction in only a subset of individuals. Understanding the mechanisms underlying these individual differences in the transition from cocaine use to cocaine abuse is important to develop treatment strategies. There is agreement that specific behavioural traits increase the risk for addiction. As such, both high impulsivity and high anxiety have been reported to predict (compulsive) cocaine self-administration behaviour. Here, we set out a new view explaining how these two behavioural traits may affect addictive behaviour. According to psychological and psychiatric evolutionary views, organisms flourish well when they fit (match) their environment by trait and genotype. However, under non-fit conditions, the need to compensate the failure to deal with this environment increases, and, as a consequence, the functional use of rewarding drugs like cocaine may also increase. It suggests that neither impulsivity nor anxiety are bad per se, but that the increased risk to develop cocaine addiction is dependent on whether behavioural traits are adaptive or maladaptive in the environment to which the animals are exposed. This 'behavioural (mal)adaptation view' on individual differences in vulnerability to cocaine addiction may help to improve therapies for addiction.

Do motivation deficits in schizophrenia-spectrum disorders promote cannabis use? An investigation of behavioural response to natural rewards and drug cues

Deficits in incentive motivation are often present in both Schizophrenia Spectrum Disorders (SSD) and substance-use disorders. The current study aims to test whether the presence of such deficits confers vulnerability to cannabis use in individuals with SSD. SSD patients (n=35) and healthy controls (n=35) were each divided into a group with (n=20) and a group without (n=15) current cannabis use disorder. Subjects performed a behavioural task designed for schizophrenia patients in which they could seek exposure to pleasant and cannabis visual stimuli on the basis of internal representations of these stimuli. Intensity of cannabis use was assessed by self-report. SSD patients were significantly less likely than controls to exert effort to try to re-view pleasant stimuli but were not significantly less likely to work to avoid unpleasant stimuli. Lack of response to re-view pleasant stimuli significantly predicted higher subsequent cannabis self-administration in patients but not controls, after controlling for degree of prior exposure to cannabis. Deficits in incentive motivation may be an aspect of SSD which promotes cannabis use in this population.

The effects of rearing environment and chronic methylphenidate administration on behavior and dopamine receptors in adolescent rats

Rearing young rodents in socially isolated or environmentally enriched conditions has been shown to affect numerous components of the dopamine system as well as behavior. Methylphenidate (MPH), a commonly used dopaminergic agent, may affect animals differently based on rearing environment. Here we examined the interaction between environment and chronic MPH treatment at clinically relevant doses, administered via osmotic minipump. Young Sprague Dawley rats (PND 21) were assigned to environmentally enriched, pair-housed, or socially isolated rearing conditions, and treated with either 0, 2, 4, or 8 mg/kg/day MPH for 3 weeks. At the end of the treatment period, animals were tested for locomotor activity and anxiety-like behavior. The densities of D1-like and D2-like receptors were measured in the striatum using in vitro receptor autoradiography. Locomotor activity and anxiety-like behavior were increased in isolated animals compared to pair-housed and enriched animals. The density of D1-like receptors was greater in isolated animals, but there were no differences between groups in D2-like receptor density. Finally, there were no effects of MPH administration on any reported measure. This study provides evidence for an effect of early rearing environment on the dopamine system and behavior, and also suggests that MPH administration may not have long-term consequences.

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.

A novel model of chronic sleep restriction reveals an increase in the perceived incentive reward value of cocaine in high drug-taking rats

Substance abuse and sleep deprivation are major problems in our society. Clinical studies suggest that measures of poor sleep quality effectively predict relapse to substance abuse. Previously, our laboratory has shown that acute sleep deprivation increases the rate and efficiency (i.e., the goal-directed nature of responding) of cocaine self-administration using a progressive ratio (PR) schedule of reinforcement. However, the problem of sleep deprivation in our nation is largely one of chronicity. Therefore, the current study used a rodent model of chronic sleep restriction more akin to that experienced by humans (approximately 25% reduction in baseline sleep over the course of 8 days) to assess the impact of chronic sleep deprivation on cocaine-seeking and cocaine-taking behaviors in rats early during acquisition of self-administration. While low drug-taking rats were unaffected by chronic sleep restriction, high drug-takers in the chronic sleep restriction (CSR) group exhibited enhanced fixed ratio (FR) responding by the fourth day of FR training and significantly higher PR breakpoints than their non-sleep restriction (NSR) counterparts. This study is the first to directly assess the impact of chronic sleep deprivation on drug self-administration. These results show that chronic sleep deprivation early during acquisition of self-administration has a significant effect on the perceived incentive reward value of cocaine in high drug-takers, as indicated by both increased FR responding and an increased willingness to work for drug. Thus, it is important to be mindful of such factors in clinical settings designed for treatment of addiction and relapse prevention.

Alteration of synaptic activity-regulating genes underlying functional improvement by long-term exposure to an enriched environment in the adult brain

BACKGROUND

Housing animals in an enriched environment (EE) enhances behavioral function. However, the mechanism underlying this EE-mediated functional improvement and the resultant changes in gene expression have yet to be elucidated.

OBJECTIVES

We attempted to investigate the underlying mechanisms associated with long-term exposure to an EE by evaluating gene expression patterns.

METHODS

We housed 6-week-old CD-1 (ICR) mice in standard cages or an EE comprising a running wheel, novel objects, and social interaction for 2 months. Motor and cognitive performances were evaluated using the rotarod test and passive avoidance test, and gene expression profile was investigated in the cerebral hemispheres using microarray and gene set enrichment analysis (GSEA).

RESULTS

In behavioral assessment, an EE significantly enhanced rotarod performance and short-term working memory. Microarray analysis revealed that genes associated with neuronal activity were significantly altered by an EE. GSEA showed that genes involved in synaptic transmission and postsynaptic signal transduction were globally upregulated, whereas those associated with reuptake by presynaptic neurotransmitter transporters were downregulated. In particular, both microarray and GSEA demonstrated that EE exposure increased opioid signaling, acetylcholine release cycle, and postsynaptic neurotransmitter receptors but decreased Na+ / Cl- -dependent neurotransmitter transporters, including dopamine transporter Slc6a3 in the brain. Western blotting confirmed that SLC6A3, DARPP32 (PPP1R1B), and P2RY12 were largely altered in a region-specific manner.

CONCLUSION

An EE enhanced motor and cognitive function through the alteration of synaptic activity-regulating genes, improving the efficient use of neurotransmitters and synaptic plasticity by the upregulation of genes associated with postsynaptic receptor activity and downregulation of presynaptic reuptake by neurotransmitter transporters.

Brief exposure to novel or enriched environments reduces sucrose cue-reactivity and consumption in rats after 1 or 30 days of forced abstinence from self-administration

Environmental enrichment (EE) reduces drug and sucrose cue-reactivity in rats. In a previous study we reported that 1 month of EE (large cage, toys, and social cohorts) significantly reduced sucrose cue-reactivity. In the present study, we examined whether overnight (22 h) EE would be as effective. We also examined whether social enrichment (SE), enrichment alone (SoloEE), or exposure to an alternative environment (AEnv) might account for the EE effect. Rats self-administered 10% sucrose (.2 mL/delivery) in 10 daily 2-h sessions. Sucrose delivery was accompanied by a tone+light cue. Rats were then exposed to enrichment or alternative environment conditions overnight (acute) or for 29 days (chronic). Sucrose cue-reactivity was measured after this period of forced abstinence in a session identical to training, but no sucrose was delivered with the cue. All acute conditions markedly reduced sucrose cue-reactivity after 1 day of forced abstinence compared to single-housed rats in standard vivarium housing (CON). Sucrose consumption was also significantly reduced in all groups but SoloEE in a next-day test. All acute conditions but SE significantly reduced sucrose cue-reactivity when administered just prior to Day 30 of forced abstinence; all reduced sucrose consumption in a next-day test. All chronic conditions except for SE and AEnv significantly reduced sucrose cue-reactivity on the Day 30 test and sucrose consumption in a next day test. For both acute and chronic comparisons, EE manipulations were the most effective at reducing sucrose cue-reactivity and consumption. SoloEE and EE were equally effective at reducing sucrose cue-reactivity and similarly effective at reducing sucrose consumption. This indicates that social interaction is not a necessary condition for reducing sucrose-motivated behaviors. These results may be useful in the development of anti-relapse strategies for drug and food addictions.