Differential effects of an early housing manipulation on cocaine-induced activity and self-administration in laboratory rats

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Early social isolation stress increases addiction vulnerability to heroin and alters c-Fos expression in the mesocorticolimbic system

RATIONALE

Adverse psychosocial factors during early childhood or adolescence compromise neural structure and brain function, inducing susceptibility for many psychiatric disorders such as substance use disorder. Nevertheless, the mechanisms underlying early life stress-induced addiction vulnerability is still unclear, especially for opioids.

OBJECTIVES

To address this, we used a mouse heroin self-administration model to examine how chronic early social isolation (ESI) stress (5 weeks, beginning at weaning) affects the behavioral and neural responses to heroin during adulthood.

RESULTS

We found that ESI stress did not alter the acquisition for sucrose or heroin self-administration, nor change the motivation for sucrose on a progressive ratio schedule. However, ESI stress induced an upward shift of heroin dose-response curve in female mice and increased motivation and seeking for heroin in both sexes. Furthermore, we examined the neuronal activity (measured by c-Fos expression) within the key brain regions of the mesocorticolimbic system, including the prelimbic cortex (PrL), infralimbic cortex (IL), nucleus accumbens (NAc) core and shell, caudate putamen, and ventral tegmental area (VTA). We found that ESI stress dampened c-Fos expression in the PrL, IL, and VTA after 14-day forced abstinence, while augmented the neuronal responses to heroin-predictive context and cue in the IL and NAc core. Moreover, ESI stress disrupted the association between c-Fos expression and attempted infusions during heroin-seeking test in the PrL.

CONCLUSIONS

These data indicate that ESI stress leads to increased seeking and motivation for heroin, and this may be associated with distinct changes in neuronal activities in different subregions of the mesocorticolimbic system.

Friend of the Devil: Negative Social Influences Driving Substance Use Disorders

Substance use disorders in humans have significant social influences, both positive and negative. While prosocial behaviors promote group cooperation and are naturally rewarding, distressing social encounters, such as aggression exhibited by a conspecific, are aversive and can enhance the sensitivity to rewarding substances, promote the acquisition of drug-taking, and reinstate drug-seeking. On the other hand, withdrawal and prolonged abstinence from drugs of abuse can promote social avoidance and suppress social motivation, accentuating drug cravings and facilitating relapse. Understanding how complex social states and experiences modulate drug-seeking behaviors as well as the underlying circuit dynamics, such as those interacting with mesolimbic reward systems, will greatly facilitate progress on understanding triggers of drug use, drug relapse and the chronicity of substance use disorders. Here we discuss some of the common circuit mechanisms underlying social and addictive behaviors that may underlie their antagonistic functions. We also highlight key neurochemicals involved in social influences over addiction that are frequently identified in comorbid psychiatric conditions. Finally, we integrate these data with recent findings on (±)3,4-methylenedioxymethamphetamine (MDMA) that suggest functional segregation and convergence of social and reward circuits that may be relevant to substance use disorder treatment through the competitive nature of these two types of reward. More studies focused on the relationship between social behavior and addictive behavior we hope will spur the development of treatment strategies aimed at breaking vicious addiction cycles.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Long-Term Impacts of Post-weaning Social Isolation on Nucleus Accumbens Function

Adolescence is a period of incredible change, especially within the brain's reward circuitry. Stress, including social isolation, during this time has profound effects on behaviors associated with reward and other neuropsychiatric disorders. Because the Nucleus Accumbens (NAc), is crucial to the integration of rewarding stimuli, the NAc is especially sensitive to disruptions by adolescent social isolation stress. This review highlights the long-term behavioral consequences of adolescent social isolation rearing on the NAc. It will discuss the cellular and molecular changes within the NAc that might underlie the long-term effects on behavior. When available sex-specific effects are discussed. Finally by mining publicly available data we identify, for the first time, key transcriptional profiles induced by adolescence social isolation in genes associated with dopamine receptor 1 and 2 medium spiny neurons and genes associated with cocaine self-administration. Together, this review provides a comprehensive discussion of the wide-ranging long-term impacts of adolescent social isolation on the dopaminergic system from molecules through behavior.

Role of adolescent-formed, context-drug-associations on reinstatement of drug-seeking behavior in rats

RATIONALE

Drug use during adolescence results in a lifelong risk to develop substance-use disorders. Adolescent rats are less reactive to cocaine-associated cues compared with adults; however, the contribution of adolescent-formed, context-drug-associations to elicit relapse-like behavior is underexplored. Although it is known that social isolation can impact drug-seeking behavior, the effects of housing conditions on context-induced, cocaine-seeking during adolescence vs adulthood are unknown.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations under different housing conditions (pair vs single) on cocaine-seeking behavior during adolescence or adulthood. This objective was accomplished using operant cocaine self-administration (Coc-SA) under a standard, non-abbreviated (Non-ABRV) or modified abbreviated (ABRV) paradigm.

METHODS

In experiment 1, adolescent and adult rats received Non-ABRV Coc-SA in a distinct context (2 h, 1×/day, 10 days), and extinction training (EXT) in a second context (1 h, 1×/day, 8 days) with reinstatement test (TEST) during adulthood in the cocaine-paired context. In experiments 2 and 3, rats received all behavioral phases during adolescence or adulthood: ABRV Coc-SA (2 h, 2×/day, 5 days), EXT (1 h, 4×/day, 2 days) with TEST in a cocaine-paired or novel, unpaired context. All experiments included pair and single-housing conditions.

RESULTS AND CONCLUSIONS

Age at cocaine exposure did not influence behavior in Non-ABRV or ABRV paradigms. Under Non-ABRV conditions, adolescent and adult single-housed rats had higher seeking behavior than pair housed. These data suggest that social isolation influences context-induced, cocaine-seeking regardless of age at drug exposure and provides a condensed, ABRV paradigm to investigate context-induced, cocaine-seeking behavior during adolescence.

Social interaction reward: A resilience approach to overcome vulnerability to drugs of abuse

Drug addiction is a multifactorial disorder resulting from the complex interaction between biological, environmental and drug-induced effects. Generally, stress is a well-known risk factor for the development of drug addiction and relapse. While most of the research focuses on risk factors that increase the vulnerability to drugs of abuse, recent studies are focusing on the areas of strength/positive coping approaches that can increase resistance to drugs of abuse. In this review, we concentrate on resilience, seen as a dynamic process, which can allow individuals to positively adapt within the context of a specific risk for psychiatric illness. Here, we discuss the effects of social stress in animal models on drug use, particularly cocaine. In contrast, we suggest social interaction reward when available as an alternative to drug use as an approach contracting negative stress effects and increasing resistance to drug use. Indeed, interventions, which aim at enhancing resilience to stress through the facilitation of social interaction and the enhancement of social support, could be particularly effective in helping people cope with stress and preventing drug use problems or relapse. Finally, understanding the neurobiological mechanisms underlying protective factors such as social interaction reward should provide the basis for future evidence-based interventions targeting substance abuse and stress-related pathologies.

Incentive and dopamine sensitization produced by intermittent but not long access cocaine self-administration

The temporal pattern of drug use (pharmacokinetics) has a profound effect on the ability of self-administered cocaine to produce addiction-like behavior in rodents, and to change the brain. To further address this issue, we compared the effects of long access (LgA) cocaine self-administration, which is widely used to model the transition to addiction, with intermittent access (IntA), which is thought to better reflect the pattern of drug use in humans, on the ability of a single, self-administered injection of cocaine to increase dopamine (DA) overflow in the core of the nucleus accumbens (using in vivo microdialysis), and to produce addiction-like behavior. IntA experience was more effective than LgA in producing addiction-like behavior-a drug experience-dependent increase in motivation for cocaine assessed using behavioral economic procedures, and cue-induced reinstatement-despite much less total drug consumption. There were no group differences in basal levels of DA in dialysate [DA], but a single self-administered IV injection of cocaine increased [DA] in the core of the nucleus accumbens to a greater extent in rats with prior IntA experience than those with LgA or limited access experience, and the latter two groups did not differ. Furthermore, high motivation for cocaine was associated with a high [DA] response. Thus, IntA, but not LgA, produced both incentive and DA sensitization. This is consistent with the notion that a hyper-responsive dopaminergic system may contribute to the transition from casual patterns of drug use to the problematic patterns that define addiction.

Long-Term Behavioral Effects of Post-weaning Social Isolation in Males and Females

Adolescence is a developmental period associated with vast neural and behavioral changes which are accompanied by altered sensitivity to stimuli, both stressful and rewarding. Perturbations, especially stressful stimuli, during this period have been shown to alter behavior in adulthood. Social isolation rearing is one such perturbation. This review highlights the long-term behavioral consequences of adolescent social isolation rearing in rodents with a specific focus on anxiety- and addiction-related behaviors. Sex-specific effects are discussed where data are available. We then consider changes in monoaminergic neurotransmission as one possible mechanism for the behavioral effects described. This research on both normative and perturbed adolescent development is crucial to understanding and treating the increased vulnerability to psychiatric disorders seen in humans during this life stage.

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.

Negative consequences of early-life adversity on substance use as mediated by corticotropin-releasing factor modulation of serotonin activity

Early-life adversity is associated with increased risk for substance abuse in later life, with women more likely to report past and current stress as a mediating factor in their substance use and relapse as compared to men. Preclinical models of neonatal and peri-adolescent (early through late adolescence) stress all support a direct relationship between experiences of early-life adversity and adult substance-related behaviors, and provide valuable information regarding the underlying neurobiology. This review will provide an overview of these animal models and how these paradigms alter drug and alcohol consumption and/or seeking in male and female adults. An introduction to the corticotropin-releasing factor (CRF) and serotonin systems, their development and their interactions at the level of the dorsal raphe will be provided, illustrating how this particular stress system is sexually dimorphic, and is well positioned to be affected by stressors early in development and throughout maturation. A model for CRF-serotonin interactions in the dorsal raphe and how these influence dopaminergic activity within the nucleus accumbens and subsequent reward-associated behaviors will be provided, and alterations to the activity of this system following early-life adversity will be identified. Overall, converging findings suggest that early-life adversity has long-term effects on the functioning of the CRF-serotonin system, highlighting a potentially important and targetable mediator linking stress to addiction. Future work should focus on identifying the exact mechanisms that promote long-term changes to the expression and activity of CRF receptors in the dorsal raphe. Moreover, it is important to clarify whether similar neurobiological mechanisms exist for males and females, given the sexual dimorphism both in CRF receptors and serotonin indices in the dorsal raphe and in the behavioral outcomes of early-life adversity.

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Early life stress increases risk for substance abuse in adulthood.

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Stress and drugs increase CRF which alters serotonin release in the brain.

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CRF₂ receptor expression in the dorsal raphe is altered by early life stress.

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Resultant changes to serotonin output facilitates dopamine in the accumbens.

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CRF₂-sertotonin-dopamine interactions may link early life stress with substance abuse.

Animal models of drug addiction: Where do we go from here?

Compulsion and impulsivity are both primary features of drug addiction. Based on decades of animal research, we have a detailed understanding of the factors (both environmental and physiological) that influence compulsive drug use, but still know relatively little about the impulsive aspects of drug addiction. This review outlines our current knowledge of the relationship between impulsivity and drug addiction, focusing on cognitive and motor impulsivity, which are particularly relevant to this disorder. Topics to be discussed include the influence of chronic drug administration on impulsivity, the mechanisms that may explain drug-induced impulsivity, and the role of individual differences in the development of impulsive drug use. In addition, the manner in which contemporary theories of drug addiction conceptualize the relationship between impulsivity and compulsion is examined. Most importantly, this review emphasizes a critical role for animal research in understanding the role of impulsivity in the development and maintenance of drug addiction.

Effects of environmental enrichment on self-administration of the short-acting opioid remifentanil in male rats

BACKGROUND

Opioid abuse is a major problem around the world. Identifying environmental factors that contribute to opioid abuse and addiction is necessary for decreasing this epidemic. In rodents, environmental enrichment protects against the development of low dose stimulant self-administration, but studies examining the effect of enrichment and isolation (compared to standard housing) on the development of intravenous opioid self-administration have not been conducted. The present study investigated the role of environmental enrichment on self-administration of the short-acting μ-opioid remifentanil.

METHODS

Rats were raised in an enriched condition (Enr), standard condition (Std), or isolated condition (Iso) beginning at 21 days of age and were trained to lever press for 1 or 3 μg/kg/infusion remifentanil in young adulthood. Acquisition of self-administration and responding during increasing fixed ratio requirements were assessed, and a dose-response curve was generated.

RESULTS

In all phases, Enr rats lever pressed significantly less than Std and Iso rats, with Enr rats pressing between 9 and 40% the amount of Iso rats. Enr rats did not acquire remifentanil self-administration when trained with 1 μg/kg/infusion, did not increase responding over increasing FR when trained at either dose, and their dose-response curves were flattened compared to Std and Iso rats. When expressed as economic demand curves, Enr rats displayed a decrease in both essential value (higher α) and reinforcer intensity (Q 0) compared to Std and Iso rats at the 1 μg/kg/infusion training dose.

CONCLUSION

Environmental enrichment reduced remifentanil intake, suggesting that social and environmental novelty may protect against opioid abuse.

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.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Adolescent D-amphetamine treatment in a rodent model of ADHD: Pro-cognitive effects in adolescence without an impact on cocaine cue reactivity in adulthood

Attention-deficit/hyperactivity disorder (ADHD) is comorbid with cocaine abuse. Whereas initiating ADHD medication in childhood does not alter later cocaine abuse risk, initiating medication during adolescence may increase risk. Preclinical work in the Spontaneously Hypertensive Rat (SHR) model of ADHD found that adolescent methylphenidate increased cocaine self-administration in adulthood, suggesting a need to identify alternatively efficacious medications for teens with ADHD. We examined effects of adolescent d-amphetamine treatment on strategy set shifting performance during adolescence and on cocaine self-administration and reinstatement of cocaine-seeking behavior (cue reactivity) during adulthood in male SHR, Wistar-Kyoto (inbred control), and Wistar (outbred control) rats. During the set shift phase, adolescent SHR needed more trials and had a longer latency to reach criterion, made more regressive errors and trial omissions, and exhibited slower and more variable lever press reaction times. d-Amphetamine improved performance only in SHR by increasing choice accuracy and decreasing errors and latency to criterion. In adulthood, SHR self-administered more cocaine, made more cocaine-seeking responses, and took longer to extinguish lever responding than control strains. Adolescent d-amphetamine did not alter cocaine self-administration in adult rats of any strain, but reduced cocaine seeking during the first of seven reinstatement test sessions in adult SHR. These findings highlight utility of SHR in modeling cognitive dysfunction and comorbid cocaine abuse in ADHD. Unlike methylphenidate, d-amphetamine improved several aspects of flexible learning in adolescent SHR and did not increase cocaine intake or cue reactivity in adult SHR. Thus, adolescent d-amphetamine was superior to methylphenidate in this ADHD model.

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.

On the interaction between drugs of abuse and adolescent social behavior

RATIONALE

Social factors influence drug abuse. Conversely, drugs of abuse alter social behavior. This is especially pertinent during post-weaning development, when there are profound changes in the social repertoire, and the sensitivity to the positive and negative effects of drugs of abuse is altered.

OBJECTIVES

This study aimed to provide an overview of our current understanding of the interaction between drugs of abuse and juvenile/adolescent social behavior.

METHODS

We first provide evidence that a characteristic form of juvenile and adolescent social behavior, i.e., social play behavior, has reinforcing properties and is affected by drugs of abuse. Next, social risk factors for drug use and addiction are described, including antisocial personality traits and early social insults. Last, we discuss research that investigates social influences on drug use, as well as the consequences of perinatal drug exposure on later social interactions.

RESULTS

Social play behavior is highly rewarding in laboratory animals, and it is affected by low doses of opioids, cannabinoids, ethanol, nicotine, and psychostimulants. In humans, antisocial personality traits, most prominently in the form of conduct disorder, are a prominent risk factor for drug addiction. Preclinical studies have consistently shown altered sensitivity to drugs as a result of social isolation during post-weaning development. The social environment of an individual has a profound, but complex, influence on drug use, and perinatal drug exposure markedly alters later social interactions.

CONCLUSIONS

The studies reviewed here provide a framework to understand the interaction between drugs of abuse and adolescent social interaction, at the preclinical and the clinical level.

Disrupted social development enhances the motivation for cocaine in rats

RATIONALE

Early social experiences are of major importance for behavioural development. In particular, social play behaviour during post-weaning development is thought to facilitate the attainment of social, emotional and cognitive capacities. Conversely, social insults during development can cause long-lasting behavioural impairments and increase the vulnerability for psychiatric disorders, such as drug addiction.

OBJECTIVES

The aim of this study was to investigate whether a lack of social experiences during the juvenile and early adolescent stage, when social play behaviour is highly abundant, alters cocaine self-administration in rats.

METHODS

Rats were socially isolated from postnatal days 21 to 42 followed by re-socialization until adulthood. Cocaine self-administration was then assessed under a fixed ratio and progressive ratio schedule of reinforcement. Next, cue, cocaine and stress-induced reinstatement of cocaine seeking was determined following extinction of self-administration.

RESULTS

Early social isolation resulted in an enhanced acquisition of self-administration of a low dose (0.083 mg/infusion) of cocaine, but the sensitivity to cocaine reinforcement, assessed using a dose-response analysis, was not altered in isolated rats. Moreover, isolated rats displayed an increased motivation for cocaine under a progressive ratio schedule of reinforcement. Extinction and reinstatement of cocaine seeking was not affected by early social isolation.

CONCLUSIONS

Early social isolation causes a long-lasting increase in the motivation to self-administer cocaine. Thus, aberrations in post-weaning social development, such as the absence of social play, enhance the vulnerability for drug addiction later in life.

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.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Emotional valence and context of social influences on drug abuse-related behavior in animal models of social stress and prosocial interaction

RATIONALE

Social factors are important determinants of drug dependence and relapse.

OBJECTIVES

We reviewed pre-clinical literature examining the role of social experiences from early life through the development of drug dependence and relapse, emphasizing two aspects of these experiences: (1) whether the social interaction is appetitive or aversive and (2) whether the social interaction occurs within or outside of the drug-taking context.

METHODS

The models reviewed include neonatal care, isolation, social defeat, chronic subordination, and prosocial interactions. We review results from these models in regard to effects on self-administration and conditioned place preference established with alcohol, psychostimulants, and opiates.

RESULTS

We suggest that in general, when the interactions occur outside of the drug-taking context, prosocial interactions are protective against drug abuse-related behaviors, whereas social stressors facilitate these behaviors. By contrast, positive or negative social interactions occurring within the drug-taking context may interact with other risk factors to enhance or inhibit these behaviors.

CONCLUSIONS

Despite differences in the nature and complexity of human social behavior compared to other species, the evolving animal literature provides useful models for understanding social influences on drug abuse-related behavior that will allow for research on the behavioral and biological mechanisms involved. The models have contributed to understanding social influences on initiation and maintenance of drug use, but more research is needed to understand social influences on drug relapse.

Sex differences in the effects of social and physical environment on novelty-induced exploratory behavior and cocaine-stimulated locomotor activity in adolescent rats

Many factors influence the rewarding effects of drugs such as cocaine. The present study was done to determine whether social and environmental factors alter behavior in adolescent male and female rats. On postnatal day (PND) 23, rats were housed in one of several same-sex conditions. Both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone (1 rat/cage) in an environment that either was impoverished (with no toys; II) or enriched (with toys; IE). Standard housing for these studies was social and impoverished, which was 2 rats/cage with no toys (SI2). Other rats were housed 2/cage with toys (SE2), or 3/cage with (SE3) or without (SI3) toys. On PND 37, novelty-induced locomotor activity was measured for 30min. On PND 44-46, locomotor activity in response to an injection of 5mg/kg cocaine was measured for 60min each day. For male rats, only social conditions altered novelty-induced activity. Males housed in groups of three had the most activity, compared to pair-housed and isolated rats. For females, social and environmental enrichment interacted to alter novelty-induced activity. In contrast to males, isolated females had increased activity, compared to group-housed females. Further, isolated females in impoverished environments had more activity than isolated females in enriched environments and group-housed females in impoverished environments. The effect of environmental enrichment on cocaine-stimulated locomotor activity was altered depending upon the number of rats living in a cage for males. For females, only social conditions altered cocaine-stimulated behavior, with activity increasing with the number of rats in the cage, regardless of environmental enrichment. These data show that social and environmental enrichment differentially alter novelty-induced and cocaine-stimulated locomotor activity in adolescent male and female rats.

Social experiences affect reinstatement of cocaine-induced place preference in mice

RATIONALE

Drug addiction is a multifactorial disorder resulting from an interaction between genetic and environmental factors, and negative and positive environmental conditions may increase or reduce, respectively, vulnerability to drug addiction.

OBJECTIVES

The influence of different social experiences on the acquisition, extinction, and reinstatement of a cocaine-induced conditioned place preference (CPP) was evaluated.

METHODS

In experiment 1, adolescent and adult male OF1 mice housed under four different conditions (grouped, isolated, crowded, and cohabitating with a female) were conditioned with 50, 12.5, or 3.125 mg/kg of cocaine. All mice underwent extinction sessions until the CPP was extinguished. The effects of cocaine priming injections on the reinstatement of CPP were then evaluated. In experiment 2, the effect of different social experiences on the maintenance and reinstatement of cocaine-CPP in adult mice was studied.

RESULTS

Although housing conditions did not affect the acquisition of cocaine-CPP, it did modify reinstatement after extinction. Adolescent mice living in crowded conditions or cohabitating with a female did not present reinstatement after cocaine priming. Similarly, neither isolated adult mice nor adults cohabitating with a female presented reinstatement. In grouped adult mice, isolation after acquisition of the CPP and social defeat before reinstatement increased the vulnerability to reinstatement induced by cocaine priming. Conversely, both exposure to females and a brief social interaction undermined cocaine-induced reinstatement.

CONCLUSIONS

Social experiences modify vulnerability to reinstatement, acting as prevention or risk factors in the development of drug addiction.

Social and physical environment alter cocaine conditioned place preference and dopaminergic markers in adolescent male rats

This study was done to determine whether social and environmental factors alter cocaine reward and proteins implicated in mediating drug reward in rats during early adolescence. On postnatal day (PND) 23, rats were housed under conditions where both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone impoverished with no toys (II) or enriched with toys (IE). Social rats were housed two rats/cage with no toys (SI2) or with toys (SE2), or three/cage with (SE3) or without (SI3) toys. On PND 43, cocaine conditioned place preference (CPP) sessions began with the post-test done on PND 47. Cocaine CPP was established in response to 5 or 10 mg/kg cocaine in II rats, and CPP was decreased with the addition of cage mates or toys. No CPP was seen to any dose in SI3 or SE3 rats. Enriched housing (SE3) increased dopamine transporter (DAT) protein in the nucleus accumbens compared to II. There also were differential effects of cocaine on tyrosine hydroxylase and DAT depending on housing, with both increased by cocaine in II but not SE3 rats. DARPP-32 was unchanged by housing or cocaine, while phospho-Thr(34)-DARPP-32 was increased by cocaine treatment across conditions. Thus, both social and environmental enrichment decrease cocaine CPP during adolescence and different housing alters proteins that regulate dopaminergic neurotransmission in a manner that may account for the observed differences in cocaine-induced reward.

Effects of environmental enrichment on sensitivity to cocaine in female rats: importance of control rates of behavior

Environmental enrichment produces functional changes in mesolimbic dopamine transmission and alters sensitivity to psychomotor stimulants. These manipulations also alter the control rate of many behaviors that are sensitive to stimulant administration, which can make comparison of drug effects between isolated and enriched subjects difficult. The purpose of this study was to examine the effects of environmental enrichment on control rates of behavior and on sensitivity to cocaine in tests of locomotor activity, drug self-administration, conditioned place preference, and toxicity. In the locomotor activity test, isolated rats exhibited greater activity after the administration of cocaine, but also had higher control rates of activity. When locomotor activity was expressed as a percentage of saline control values, enriched rats exhibited a greater increase relative to their own control than isolated rats. In the drug self-administration procedure, isolated rats had higher breakpoints on a progressive-ratio schedule of reinforcement when responding was maintained by cocaine; however, isolated rats also had higher breakpoints in saline substitution tests and higher rates of inactive lever responding. When the self-administration data were expressed as a percentage of these control values, enriched rats exhibited a greater increase in responding relative to their own control rates than isolated rats. No differences were observed between isolated and enriched rats under control conditions in the place preference and toxicity studies. In both of these procedures, enriched rats were more sensitive than isolated rats to all the doses of cocaine tested. These data emphasize the importance of considering control rates of behavior in studies examining environmental enrichment and drug sensitivity, and suggest that environmental enrichment increases sensitivity to cocaine across a range of dependent measures when differences in control rates of behavior are taken into account.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

Repeated Catha edulis oral administration enhances the baseline aggressive behavior in isolated rats

The effects of repeated oral administration of the psychostimulant plant, Catha edulis and its active principle, cathinone on rats were studied using isolation induced aggression paradigm. The behavioral responses were videotaped and scored later by offline data analyses. Rats were decapitated at the end of the behavioral experiment and in the relevant brain regions, monoamines were assessed. The results demonstrate that isolation of male rats produces a baseline aggression. Treatments with the psychostimulant plant, Catha edulis or commercial S-(--)-cathinone enhanced significantly: The locomotor activities and the baseline aggression behaviors compared with vehicle treated rats. Neurochemical correlates revealed a significant depletion of serotonin (5-HT) and its corresponding metabolites (5-HIAA) in both the anterior and posterior striatum. There was also a reduction in the level of homovanillic acid (HVA) in the hippocampus. Additionally, elevation of dopamine level was observed in the nucleus accumbens, especially, in those rats treated with Catha edulis extract. Cathinone, on the other hand, increased the level of HVA in the posterior striatum and decreased HVA in the nucleus accumbens. In conclusion, the present data demonstrate that repeated administration of Catha edulis or S-(--)-cathinone enhances aggression in rats, presumably by decreasing the level of serotonin and its corresponding metabolites. Besides, the data obtained do not rule out the involvement of dopamine in aggression behavior.

Effect of environmental stressors on opiate and psychostimulant reinforcement, reinstatement and discrimination in rats: a review

Studies in humans suggest that exposure to life stressors is correlated with compulsive drug abuse and relapse to drugs during periods of abstinence. The behavioral and neurobiological mechanisms involved in the effect of stress on drug abuse, however, are not known. Here, we review data from studies using preclinical models in rats on the effect of environmental stressors on opiate and psychostimulant reinforcement, as measured by the intravenous drug self-administration and conditioned place preference procedures, on relapse to these drugs, as measured by the reinstatement procedure, and on the subjective effects of these drugs, as measured by the drug discrimination procedure. The results of the studies reviewed here suggest that while stressors are important modulators of the behavioral effects of opiate and psychostimulant drugs, the effect of stress on behavior in these animal models is stressor-specific, and to some degree, procedure- and drug-class-specific. The review of studies on the neurobiological mechanisms underlying stress-drug interactions in these animal models indicate that central noradrenaline and extrahypothalamic corticotropin-releasing factor mediate the effect of one form of stress (intermittent footshock) on reinstatement of opiate and psychostimulant seeking after prolonged drug-free periods. At present, however, little is known about the neuronal events that mediate the effect of environmental stressors on opiate and psychostimulant reinforcement or discrimination. The broader implications of the data reviewed here for future research and for the treatment of opiate and psychostimulant addiction are briefly discussed.

Effects of the social conditions of housing through testing on cocaine-induced contextual sensitisation and conditioned locomotion in C57BL/6J mice

The potential differential effects of isolated and collective housing through the testing phase on sensitisation to cocaine-induced locomotion, the subsequent conditioned locomotion and the context-dependent expression of sensitisation were examined in C57Bl/6J male mice. Sensitisation was first generated in mice receiving seven once-daily subcutaneous injections of either saline or 10 mg/kg cocaine, before being placed in a testing chamber (singly) or in their home cage. On Day 8, mice were tested for conditioned locomotion (under saline). On Day 12, after three daily sessions of reinstatement of sensitisation, they were tested for contextual sensitisation (under cocaine). Whereas little or no effect of housing on the development of sensitisation was found, postsensitisation conditioned activity was significantly greater in isolation-kept mice (in comparison with the group-kept animals) and the likelihood of inducing a context-dependent expression of sensitisation was greater in grouped-housed mice. The results indicate that some of the aspects of contextual sensitisation can be influenced by the social conditions of lodging, not only when these conditions start several weeks before or during previous developmental periods of animals life (as previously published), but also when social isolation or social grouping are initiated after sexual maturity and applied through the period of treatments and testing. Moreover, the differential effect of social conditions of lodging on conditioned activity and contextual expression of sensitisation disagrees with the excitatory conditioning account of contextual sensitisation.

The HPA axis in cocaine use: implications for pharmacotherapy

The role of the hypothalamic-pituitary-adrenal (HPA) axis in cocaine use is reviewed within the context of two approaches to developing pharmacological treatments in humans: (1) reducing the reinforcing effects of cocaine and (2) reducing cocaine addict's susceptibility to stress-induced relapse. This review suggests that HPA-axis-suppressing medications are unlikely to block cocaine's reinforcing effects completely but may be useful in decreasing the frequency of use by increasing the addicted individual's resistance to stress-induced relapse. Implications for designing inpatient studies to test the safety and efficacy of candidate pharmacological agents are discussed.

Social dominance in monkeys: dopamine D2 receptors and cocaine self-administration

Disruption of the dopaminergic system has been implicated in the etiology of many pathological conditions, including drug addiction. Here we used positron emission tomography (PET) imaging to study brain dopaminergic function in individually housed and in socially housed cynomolgus macaques (n = 20). Whereas the monkeys did not differ during individual housing, social housing increased the amount or availability of dopamine D2 receptors in dominant monkeys and produced no change in subordinate monkeys. These neurobiological changes had an important behavioral influence as demonstrated by the finding that cocaine functioned as a reinforcer in subordinate but not dominant monkeys. These data demonstrate that alterations in an organism's environment can produce profound biological changes that have important behavioral associations, including vulnerability to cocaine addiction.

Early social isolation, but not maternal separation, affects behavioral sensitization to amphetamine in male and female adult rats

Early life stressful manipulations, such as maternal separation (MS) or social isolation (SI), can influence the neurobiological development of rats and alter the response of adult animals to drugs of abuse. The present study examined the acute and sensitized behavioral responses (locomotor activity (LMA) and stereotypy) induced by amphetamine after MS or SI in male and female rats. In addition, the hypothesis that the combination of SI and MS could lead to additional effects on the behavioral response to amphetamine was tested. After the repetitive, intermittent administration of 1.5 mg/kg D-amphetamine over five consecutive days, the behavioral expression of sensitization to a challenge injection was assessed following a 2-day withdrawal period. In both sexes, MS and SI did not alter the acute locomotor activating effects of amphetamine as measured in the open-field environment after the first administration of the drug. Whereas SI altered the expression of sensitization to amphetamine in both sexes, MS did not affect it. Finally, in none of the behavioral variables measured did MS and SI interact to further modify the behavioral profile of the animals. The present results suggest that a postweaning manipulation of the environment (SI) is more effective than a preweaning manipulation (MS) in modifying the expression of sensitization to amphetamine.

Locomotor-stimulating effects of indirect dopamine agonists are attenuated in Fawn hooded rats independent of postweaning social experience

The effects of the indirect dopamine (DA) agonists cocaine and D-amphetamine on locomotor activity were examined in Fawn hooded (FH) rats and Wistar rats. The effect of isolation rearing was also examined to determine if it might have different effects in these two strains. Contrary to previous findings in other rat strains, only small increases in locomotor-stimulating responses to low doses of cocaine were observed in the present study as a result of isolation rearing. However, at higher cocaine doses, locomotor activity was substantially attenuated in FH rats relative to Wistar rats. A similar pattern of effects was observed for amphetamine in FH rats but only at the intermediate dose. The effects of strain and rearing were independent. There was no evidence for interactions between these factors.

Behavioral responses to repeated cocaine exposure in mice selectively bred for differential sensitivity to pentobarbital

Mice from the 20th generation of three lines divergently selected for response to pentobarbital-induced sedation times [long-sedation time (LST), short sedation time (SST), and randomly bred control (RBC)] were used to study cocaine-induced behavioral sensitization. These lines showed variable degrees of locomotor activities in response to cocaine. At a low cocaine dose and long withdrawal period (10 mg/kg, twice a day for 5 days followed by a 14-day withdrawal), the LST mice showed tolerance development. In response to cocaine, the locomotor activities of the SST were not significantly different from the RBC group. At a higher dose and a shorter withdrawal period (20 mg/kg, daily for 7 days followed by a 3-day withdrawal), the SST mice showed behavioral sensitization similar to the RBC mice, but the LST mice did not develop sensitization. The different responses in locomotor activity induced by cocaine suggest that genetic factors may play a role in determining the magnitude of response to this drug. Dopamine (DA) levels did not differ significantly in either striatum (STR) or nucleus accumbens (NAC) for the cocaine-treated animals to their corresponding saline-treated controls. The affinity (Kd) of D2 in the NAC decreased significantly, without changes in density (Bmax), in the cocaine-treated SST and RBC mice. On the other hand, the density of D2 binding sites in the SST and the RBC mice in the STR was significantly increased in cocaine-treated groups without change in Kd. The LST mice did not show any changes in the Kd and Bmax in either the STR or the NAC. Taken together, these findings suggest that the changes in the Kd of D2 in the NAC and the Bmax of D2 in the STR may contribute to the differences in locomotor responses to cocaine exposure in these mouse lines.

Preclinical research on cocaine self-administration: environmental determinants and their interaction with pharmacological treatment

It has been asserted that any comprehensive understanding of cocaine abuse and its treatment will require attention to both behavioral and pharmacological variables. Although the preclinical literature evaluating the effects of pharmacological variables on cocaine self-administration has been extensively reviewed, no comprehensive review of the effects of environmental variables on cocaine self-administration has been published. The present review summarizes and critiques the preclinical findings on environmental determinants of cocaine self-administration. The influence of environmental variables on the effects of pharmacological interventions on cocaine self-administration are also described. Several environmental variables have been shown to affect cocaine self-administration, including unit dose, schedule of cocaine delivery, schedules of nondrug stimuli, behavioral history, conditioned stimuli, food deprivation, exposure to stress, and rearing environment. Among these variables, unit dose, schedule of cocaine delivery, availability of alternative nondrug reinforcers, food deprivation, and rearing environment have also been shown to alter pharmacological treatment effects on cocaine self-administration. Thus, drug effects on cocaine self-administration are malleable and dependent upon the environmental context within which they occur. Suggestions for future research on the effects of these and other environmental variables on cocaine self-administration and its pharmacological treatment are presented.

The developmental psychobiology of behavioural plasticity in mice: the role of social experiences in the family unit

Small perturbations of young animals' sensory experience or hormonal milieu have been shown to alter ontogenetic pathways and to potentially produce huge effects on CNS functioning and behaviour later in life. From a social point of view, variables such as the expression of affiliative bonding and of playful interactions among littermates, the quantity/quality of maternal care, or episodes of maternal or sibling deprivation during critical phases in development, seem to interfere as epigenetic factors with the rigidly ordered temporal sequences of events that occur during the ontogenesis of CNS. This leads to the onset of adaptive neurodevelopmental changes, which are observable within a continuum that encompasses both "normal" individual variability and potential behavioural disorganisation, which in turn will probably be related to profound alteration in the establishment of adult social competence. The present review summarises the more recent work in mice dealing with short-term, as well as long-term modifications, in naturally occurring species-typical social and non-social responses as a function of the early manipulation of social characteristics of the family unit (such as litter gender composition and time of weaning). These analyses were carried out on infant animals, i.e. during the ontogenetic stage of the establishment of social bonding, as well as on pre-pubertal and adult mice and on lactating adult females. Critical issues, such as the respective roles of sibling-sibling and dam-offspring interactions in the shaping of "sibling effects", are also addressed. Overall, these studies indicate that, within their natural range of variation, early patterns of social stimulation are powerful determinants of subsequent behaviour of developing altricial rodents, and confirm that early social life events warrant attention because they can strongly affect neurobehavioural development. Evidence of a relationship between social events occurring during early rearing (i.e. when dramatic transitions in neuroendocrine and neurochemical CNS systems occur) and individual behavioural variability in the infant and adult response to the effects of psychostimulants abused by humans is presented. A better understanding of the mechanisms that mediate such remarkable plasticity might have great psychobiological as well as clinical importance, especially when considering the issue of vulnerability to drug abuse.

The role of stress in drug self-administration

Environmental experiences have an important effect on the sensitivity of an individual to drugs of abuse. Studies of drug self-administration in laboratory animals have shown that both physical and psychological stressors facilitate the acquisition of drug self-administration, probably by increasing the reinforcing efficacy of drugs of abuse. Stressors also facilitate the reinstatement of drug taking even after prolonged periods of withdrawal. The adrenal hormones, glucocorticoids, which increase the sensitivity of mesencephalic dopaminergic neurones to drugs, seem to be one of the biological substrates of the effects of stress on the propensity to develop drug intake. In this review, Pier Vincenzo Piazza and Michel Le Moal discuss theories of drug abuse, the influence of different stressful experiences on drug self-administration and their possible mechanisms of action.

Effect of differential rearing environments on morphine-induced behaviors, opioid receptors and dopamine synthesis

Rats were raised from weaning (21 days old) to young adulthood (50-60 days old) in either an enriched or impoverished stimulus environment. In the enriched condition (EC), rats were group-housed with various novel objects that were re-arranged daily. In the impoverished condition (IC), rats were housed individually without any objects. As adults, a four-trial conditioned place preference (CPP) test was used to assess locomotor activity and reward produced by morphine (0, 0.1, 1 or 10 mg/kg). On morphine conditioning day 1, both EC and IC rats displayed an inverted U-shaped dose-effect curve for locomotor activity and the locomotor stimulant effect of acute morphine was greater in IC than EC rats. Across morphine conditioning days 1-4, both EC and IC rats displayed locomotor sensitization; the locomotor sensitization following repeated morphine injections was greater in IC than EC rats. In contrast to the enhanced locomotor stimulant effect of morphine observed in IC rats, morphine-induced CPP was attenuated in IC rats relative to EC rats, indicating that the locomotor and rewarding effects of opioids depend upon different neural substrates. Measurement of mu opioid receptor density and rates of morphine-stimulated dopamine synthesis in the mesolimbic and nigrostriatal systems of EC and IC rats revealed no reliable differences between groups. Therefore, the ability of mu opioid receptors to modulate mesolimbic dopamine neurotransmission does not account for the differential behavioral effects of morphine in EC and IC rats.

Effects of the kappa-opioid receptor agonist U-50,488 on morphine-induced place preference conditioning in the developing rat

The ability of the kappa-opioid receptor agonist trans-(+/-)- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10- and 17-day-old) and periadolescent (35-day-old) rats using the conditioned place preference paradigm. Conditioning and testing were conducted in a three compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond or lemon). An abbreviated conditioned place preference procedure was used in which rats received two saline-odor pairings on the first conditioning day, and two saline- or morphine-odor pairings on the second day. In some experiments, rats were given U-50,488 (2-10 mg/kg, s.c.) 30 min prior to being conditioned with morphine (0.1-8 mg/kg, i.p.). On the third day, rats were allowed free access to the entire chamber for 900 s and compartment preferences were determined. Similar to adult rats, morphine (0.5 mg/kg) was consistently able to induce conditioned place preferences in the two preweanling age groups. This effect was attenuated by kappa-opioid receptor agonist pretreatment, as U-50,488 not only enhanced the locomotor activity of 10- and 17-day-old rats, but it blocked the morphine-induced place preference conditioning of these younger animals. In contrast, periadolescent (35-day-old) rats did not exhibit morphine-induced place preferences, nor did they show enhanced locomotor activity after U-50,488 treatment; however, using the same procedure, a different group of similarly aged rats showed conditioned preference produced by 20 mg/kg cocaine (i.p.). Therefore, these results suggest that reward processes are functionally mature in the preweanling rat (at least by 10 days of age), but that periadolescent rats are generally unresponsive to mu- and kappa-opioid drugs.

Enhancement of morphine self-administration in drug naive, inbred strains of mice by acute emotional stress

The primary reinforcing effect of morphine was compared in two genetically inbred strains of mice (C57BL/6 and DBA/2) using the intravenous self-administration procedure in drug naive animals. The morphine self-administration differed between the mouse strains. DBA/2 but not C57BL/6 acquired self-administration of morphine with a bell-shaped unit dose-response curve. Acute physical stress induced by electrical footshocks did not significantly affect the self-administration in both strains. Acute emotional stress induced by forcing mice to witness another mouse being subjected to acute physical stress caused a shift of the bell-shaped unit dose-response curve of morphine self-administration to the left in the DBA/2 mice. The C57BL/6 mice, which initially failed to demonstrate stable self-administration, started to self-administer morphine after emotional but not physical stress. Emotional distress may increase the individual sensitivity to the rewarding effects of morphine and may render an individual more susceptible to development of drug dependence.

Environmental enrichment attenuates locomotor sensitization, but not in vitro dopamine release, induced by amphetamine

Rats were raised from weanling until young adulthood in either an enriched condition (EC) or isolated condition (IC). Following this, the locomotor and rewarding effects of amphetamine were determined using the conditioned place preference (CPP) paradigm. EC rats were more sensitive to the acute locomotor stimulant effect and rewarding effect of amphetamine relative to IC rats. In contrast, EC rats were less sensitive than IC rats to the locomotor sensitization effect obtained across repeated amphetamine injections. To determine the effect of environmental enrichment on alteration of brain dopamine (DA) function induced by amphetamine, the effect of amphetamine on electrically evoked release of DA and dihydroxyphenylacetic acid (DOPAC) was determined in vitro using tissue slices from the nucleus accumbens and striatum of EC and IC rats. No differences between EC and IC rats in release of DA or DOPAC were evident, suggesting that the environmentally induced difference in sensitivity to the behavioral effects of amphetamine involves a neural mechanism extrinsic to the mesolimbic and nigrostriatal terminal field regions.

Social deprivation enhances the vulnerability of male Wistar rats to stressor- and amphetamine-induced behavioral sensitization

The aim of the present study was to assess the influence of experiential factors on the vulnerability of rats to develop amphetamine (AMPH)- and stressor-induced behavioral sensitization. Young male Wistar rats with previous social experience were isolated from their peers for 2 weeks. 1) The effect of this short-lasting social deprivation were: a) a reduced tendency to explore a fearful environment; b) a prolonged exploratory activity in response to a novel but little fearful environment; and c) a dose-dependent increase in the psychomotor stimulation induced by systemic AMPH injection. 2) After repeated AMPH injections (injection every other day for 10 days), isolated rats exhibited behavioral sensitization at lower doses (0.5 and 0.75 mg/kg) than those required for group-housed rats (1 mg/kg). 3) After being submitted to a repeated stressor (3, 7 or 14 footshock sessions, with 2 days between sessions), the isolated rats exhibited a greater increase in the behavioral responsivity to a subsequent AMPH challenge (1 mg/kg) than did the group-housed rats regardless of the number of stress sessions. In conclusion, these results suggest that experiential factors such as privation of contact with peers (social isolation) may make rats more vulnerable to the long-term repercussions of chronic environmental and pharmacological challenges.

Isolation rearing impairs the reinforcing efficacy of intravenous cocaine or intra-accumbens d-amphetamine: impaired response to intra-accumbens D1 and D2/D3 dopamine receptor antagonists

Male Lister hooded rats were raised from weaning either alone (isolation reared) or in groups of five (socially reared controls). At 5 months of age, bilateral guide cannulae were implanted within the nucleus accumbens, and experiments began. The effect of isolation rearing upon the reinforcing efficacy of the intravenous self-administration of cocaine (experiment 1), or the bilateral intra-accumbens self-administration of d-amphetamine (experiment 2) was assessed. Self-administration was made contingent upon the acquisition of a novel lever-pressing response. Two identical levers were available within each operant chamber. Responding on one lever resulted in the delivery of drug (experiment 1: cocaine, 1.5 mg/kg per infusion; experiment 2: d-amphetamine, 0.25 micrograms/side), responding on the second, control lever was recorded but had no programmed consequences. Animals were not "primed" with noncontingent infusions at any time. For experiment 1, animals received intra-accumbens infusions of the D1 dopamine receptor antagonist SCH-23390, or the D2 dopamine receptor antagonist sulpiride over two test sessions. Within each session, animals received a cumulative series of doses of each dopamine receptor antagonist. A validation group received doses of each antagonist according to more conventional methods (one dose per session). In either case, intra-accumbens infusions of SCH-23390 or sulpiride enhanced the rate of the self-administration of cocaine in socially reared controls. However, isolation rearing impaired this response to intra-accumbens infusions of the dopamine receptor antagonists. Experiment 2a examined the acquisition of the intra-accumbens self-administration of d-amphetamine. Socially reared controls acquired readily a selective response upon the drug lever. However, isolation reared animals acquired a selective response at a greatly retarded rate. In experiment 2b, a full d-amphetamine dose-response function was examined. Isolation rearing impaired the response to a range of doses of d-amphetamine. In experiment 2c, the infusate (1 microgram d-amphetamine per infusion) was adulterated with either SCH-23390 or sulpiride. Adulteration with either dopamine receptor antagonist enhanced the rate of response by socially reared controls. Isolation rearing impaired this response to SCH-23390, and blocked the response to sulpiride. These data are discussed in relation to the functioning of cortico-limbic-striatal systems, with particular reference to the mesoaccumbens dopamine projection.