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

A bibliometric analysis of studies on environmental enrichment spanning 1967-2024: patterns and trends over the years

Environmental Enrichment (EE) has received considerable attention for its potential to enhance cognitive and neurobiological outcomes in animal models. This bibliometric analysis offers a comprehensive evaluation of the EE research spanning from 1967 to 2024, utilizing data extracted from Scopus and analyzed through R and VOSviewer. The volume of publications, citation patterns, and collaborations were systematically reviewed, highlighting important contributions and emerging trends within the field of animal research. Core concepts of EE research are mapped, revealing key themes such as neuroplasticity, cognitive function, and behavioral outcomes. A significant increase in EE research is demonstrated, particularly after the year 2000, reflecting growing scientific and public interest in EE paradigms. This analysis provides insights into the global contributions and collaborative networks that have shaped EE studies over time. The role of EE in advancing the understanding of neurobiological, neurodevelopmental, and neurodegenerative processes is underscored. Influential contributors, leading countries, and high-impact journals in the field of EE are identified, offering a valuable resource for researchers seeking to understand or extend the current knowledge base. The strategic selection of keywords and rigorous data curation methods ensure that the findings accurately reflect the most impactful aspects of EE research in animals. This study serves as an essential reference for future explorations and applications of EE across disciplines. By providing a clear and structured overview of the field, this paper aims to serve as a foundation for ongoing and future research initiatives, encouraging more robust investigations and applications of EE to enhance cognitive and neurological health globally.

Differential rearing alters Fos in the accumbens core and ventral palidum following reinstatement of cocaine seeking in male Sprague-Dawley rats

Rearing rats in environmental enrichment produces a protective effect when exposed to stimulants, as enriched rats display attenuated cocaine seeking during reinstatement. However, less is known about what changes in the brain are responsible for this protective effect. The current study investigated differences in Fos protein expression following reinstatement of cocaine seeking in differentially reared rats. Rats were reared in either enriched (EC) or impoverished (IC) conditions for 30 days, after which rats self-administered cocaine in 2-h sessions. Following self-administration, rats underwent extinction and cue-induced or cocaine-primed reinstatement of cocaine seeking, brains were extracted, and Fos immunohistochemistry was performed. IC rats sought cocaine significantly more than EC rats during cue-induced reinstatement, and cocaine seeking was positively correlated with Fos expression in the nucleus accumbens core and ventral pallidum. IC rats displayed greater Fos expression than EC rats in the accumbens and ventral pallidum, suggesting a role of these areas in the enrichment-induced protective effect.

Environmental enrichment promotes adaptive responding during tests of behavioral regulation in male heterogeneous stock rats

Organisms must regulate their behavior flexibly in the face of environmental challenges. Failure can lead to a host of maladaptive behavioral traits associated with a range of neuropsychiatric disorders, including attention deficit hyperactivity disorder, autism, and substance use disorders. This maladaptive dysregulation of behavior is influenced by genetic and environmental factors. For example, environmental enrichment produces beneficial neurobehavioral effects in animal models of such disorders. The present study determined the effects of environmental enrichment on a range of measures related to behavioral regulation using a large cohort of male, outbred heterogeneous stock (HS) rats as subjects. Subjects were reared from late adolescence onwards either in pairs in standard housing with minimal enrichment (n = 200) or in groups of 16 in a highly enriched environment consisting of a large multi-level cage filled with toys, running wheels, and shelters (n = 64). Rats were subjected to a battery of tests, including: (i) locomotor response to novelty, (ii) light reinforcement, (iii) social reinforcement, (iv) reaction time, (v) a patch-depletion foraging test, (vi) Pavlovian conditioned approach, (vii) conditioned reinforcement, and (viii) cocaine conditioned cue preference. Results indicated that rats housed in the enriched environment were able to filter out irrelevant stimuli more effectively and thereby regulate their behavior more efficiently than standard-housing rats. The dramatic impact of environmental enrichment suggests that behavioral studies using standard housing conditions may not generalize to more complex environments that may be more ethologically relevant.

Environmental enrichment promotes adaptive responding during tests of behavioral regulation in male heterogeneous stock rats

Organisms must regulate their behavior flexibly in the face of environmental challenges. Failure can lead to a host of maladaptive behavioral traits associated with a range of neuropsychiatric disorders, including attention deficit hyperactivity disorder, autism, and substance use disorders. This maladaptive dysregulation of behavior is influenced by genetic and environmental factors. For example, environmental enrichment produces beneficial neurobehavioral effects in animal models of such disorders. The present study determined the effects of environmental enrichment on a range of measures related to behavioral regulation using a large cohort of male, outbred heterogeneous stock (HS) rats as subjects to mimic the genetic variability found in the human population. Subjects were reared from late adolescence onwards either in pairs in standard housing with minimal enrichment (n=200) or in groups of 16 in a highly enriched environment consisting of a large multi-level cage filled with toys, running wheels, and shelters (n=64). Rats were subjected to a battery of tests, including: (i) locomotor response to novelty, (iI) light reinforcement, (iii) social reinforcement, (iv) reaction time, (v) a patch-depletion foraging test, (vi) Pavlovian conditioned approach, (vii) conditioned reinforcement, and (viii) cocaine conditioned cue preference. Results indicated that rats housed in the enriched environment were able to filter out irrelevant stimuli more effectively and thereby regulate their behavior more efficiently than standard-housing rats. The dramatic impact of environmental enrichment suggests that behavioral studies using standard housing conditions may not generalize to more complex environments that may be more ethologically relevant.

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.

Social isolation at adolescence: a systematic review on behaviour related to cocaine, amphetamine and nicotine use in rats and mice

Adolescence is known for its high level of risk-taking, and neurobiological alterations during this period may predispose to psychoactive drug initiation and progression into more severe use patterns. Stress is a risk factor for drug consumption, and post-weaning social isolation increases drug self-administration in rodents. This review aimed to provide an overview of the effects of adolescent social isolation on cocaine, amphetamine and nicotine use-related behaviours, highlighting the specific period when animals were submitted to stress and these drugs. We wondered if there was a specific period during adolescence that isolation stress would increase drug use vulnerability. A total of 323 publications from the Scopus, Web of Science and PubMed (Medline) electronic databases were identified using the words "social isolation" and "adolescence" and "drug" or "cocaine" or "amphetamine" or "nicotine", resulting in 24 articles after analyses criteria following the PRISMA statement. The main points raised were social isolation during adolescence increased cocaine self-administration, amphetamine and nicotine locomotor activity. We did not observe a pattern of a specific moment during the adolescent period that could lead to an increased vulnerability to drug use. The precise conditions under which adolescent social stress alters drug use parameters are complex and likely depend on several factors.

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.

Enriched environment effect on lipopolysaccharide-induced spatial learning, memory impairment and hippocampal inflammatory cytokine levels in male rats

Neuro-inflammation is responsible for cognitive impairments and neurodegenerative diseases such as Alzheimer's disease. In this study, we aimed to investigate the enriched environment (EE) effect on learning and memory impairment as well as on pro-inflammatory cytokines changes induced by lipopolysaccharide (LPS). LPS injection (1 mg/kg/i.p, days 1, 3, 5, and 7) was used to develop the animal model of neuro-inflammation. Twenty-eight male Wistar rats were used in the experiment and randomly divided into 4 groups: 1) sham (S), 2) sham + enriched environment (SE), 3) LPS (L), and 4) LPS + EE (LE). Two different housing conditions, including standard environment (SE) and enriched environment, were used. The Morris Water Maze (MWM) test was used to examine animals learning and memory. IL-1β, IL-10, and TNF-α levels were measured in the brain using ELISA. We found that LPS significantly impaired learning and memory (p < 0.05) in the MWM task, but EE could significantly improve learning and memory impairment (p < 0.05). IL-1 and IL-10 levels dramatically increased in the LPS group (P < 0.05), whereas EE could decrease and increase IL-1β and IL-10 values in the LPS + EE group (P < 0.05), respectively. TNF-α levels were traced but had not detectable values in the hippocampus. Thus, we can conclude that EE has healing effects on LPS induced neuro-inflammation and can improve learning and memory deficit; however, further studies are needed to support the findings of our study.

Differential effects of voluntary exercise on development and expression of methamphetamine conditioned hyperactivity and sensitization in mice

The present experiments determined the effects of voluntary home-cage wheel running on the development (Experiments 1 and 2a) and expression (Experiment 2b) of conditioned hyperactivity and long-term sensitization in male, Swiss-Webster mice. Mice experienced 3 weeks of wheel running (exercise) or not (sedentary) either beginning prior to (Experiments 1 and 2a), or immediately following (Experiment 2b), the acquisition phase. During the acquisition phase, mice (n = 12-15/group) received injections (subcutaneous) of either vehicle (saline) or methamphetamine (0.5 or 1.0 mg/kg, Experiment 1; 1.0 mg/kg, Experiments 2a and 2b) immediately prior to 5 locomotor-activity sessions. The extinction phase began 48 hours (h) (Experiment 1) or 3 weeks (Experiments 2a and 2b) after acquisition and all mice received vehicle injections prior to 4 locomotor-activity sessions. Tests of long-term sensitization occurred 72 h after the last extinction session and involved an escalating, methamphetamine-dose regimen (0.25 ➔ 1.0 mg/kg), 1 dose/session for 3 sessions. While pre-acquisition wheel running failed to alter development of conditioned hyperactivity after training with the 0.5 mg/kg methamphetamine dose, it blunted the development of conditioned hyperactivity, and blocked (Experiment 1) or attenuated (Experiment 2a) induction of long-term sensitization after training with the 1.0 mg/kg methamphetamine dose. Furthermore, while post-acquisition wheel running retarded extinction of conditioned hyperactivity, it did not alter expression of conditioned hyperactivity or long-term sensitization (Experiment 2b). Collectively, the results suggest that the impact of voluntary exercise on context-drug associations and long-term sensitization is critically dependent on the timing of exercise relative to drug conditioning.

Lysophosphatidic acid-induced increase in adult hippocampal neurogenesis facilitates the forgetting of cocaine-contextual memory

Erasing memories of cocaine-stimuli associations might have important clinical implications for addiction therapy. Stimulating hippocampal plasticity by enhancing adult hippocampal neurogenesis (AHN) is a promising strategy because the addition of new neurons may not only facilitate new learning but also modify previous connections and weaken retrograde memories. To investigate whether increasing AHN prompted the forgetting of previous contextual cocaine associations, mice trained in a cocaine-induced conditioned place preference (CPP) paradigm were administered chronic intracerebroventricular infusions of lysophosphatidic acid (LPA, an endogenous lysophospholipid with pro-neurogenic actions), ki16425 (an LPA_1/3 receptor antagonist) or a vehicle solution, and they were tested 23 days later for CPP retention and extinction. The results of immunohistochemical experiments showed that the LPA-treated mice exhibited reduced long-term CPP retention and an approximately twofold increase in the number of adult-born hippocampal cells that differentiated into mature neurons. Importantly, mediation analyses confirmed a causal role of AHN in reducing CPP maintenance. In contrast, the ki16425-treated mice displayed aberrant responses, with initially decreased CPP retention that progressively increased across the extinction sessions, leading to no effect on AHN. The pharmacological treatments did not affect locomotion or general exploratory or anxiety-like responses. In a second experiment, normal and LPA₁ -receptor-deficient mice were acutely infused with LPA, which revealed that LPA₁ -mediated signaling was required for LPA-induced proliferative actions. These results suggest that the LPA/LPA₁ pathway acts as a potent in vivo modulator of AHN and highlight the potential usefulness of pro-AHN strategies to treat aberrant cognition in those addicted to cocaine.

Enrichment-induced differences in methamphetamine drug discrimination in male rats

Rats raised in an enriched environment show a decrease in sensitivity to the subjective effects of the psychostimulant d-amphetamine. The purpose of the present study was to determine if environmental enrichment during development alters the subjective effects of the more commonly abused drug methamphetamine. Male Sprague-Dawley rats were raised in either an enriched (EC) or an isolated condition (IC). EC and IC rats were trained on a two-lever operant procedure to discriminate 1.0 mg/kg (i.p.) methamphetamine from saline. Following acquisition of the discrimination a methamphetamine generalization curve (0.1-1.0 mg/kg) was determined. The antagonistic effects of dopamine D₁ receptor antagonist SCH23390 (0.0075-0.06 mg/kg) and the dopamine D₂ receptor antagonist eticlopride (0.01-0.3 mg/kg) were also tested. Finally, the ability of nicotine (0.05-0.5 mg/kg) to generalize and the ability of the nicotinic receptor antagonist mecamylamine (0.125-0.5 mg/kg) to antagonize the discriminative stimulus effects of methamphetamine were determined. EC rats were less sensitive to discriminative stimulus effects of methamphetamine compared to IC rats at a low 0.3 mg/kg dose and showed full antagonism of methamphetamine discrimination following SCH23390 compared to IC rats. There were no environmentally-induced differences in the effects of eticlopride. Nicotine only partially generalized to the effects of methamphetamine in both EC and IC rats. While mecamylamine failed to antagonize the effects of methamphetamine in either EC or IC rats. These results suggest that environmental enrichment decreases sensitivity to the discriminative effects of methamphetamine and the differences may be mediated through changes in the D₁ dopamine receptor.

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.

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.

Effects of environmental enrichment on d-amphetamine self-administration following nicotine exposure

Adolescent nicotine exposure has been shown to lead to further psychostimulant use in adulthood. Previous preclinical research in rats has shown that environmental enrichment may protect against drug abuse vulnerability. The current study was designed to examine whether environmental enrichment can block the ability of adolescent nicotine exposure to increase d-amphetamine self-administration in adulthood. Male Sprague-Dawley rats were raised in either enriched conditions (ECs) or isolated conditions (ICs) and then injected with saline or nicotine (0.4 mg/kg, sc) for 7 days during adolescence. In adulthood rats were allowed to self-administer d-amphetamine under a fixed ratio (FR; 0, 0.006, 0.01, 0.02, 0.06, and 0.1 mg/kg/infusion) and progressive ratio (PR; 0, 0.006, 0.06, and 0.1 mg/kg/infusion) schedule of reinforcement. Nicotine-treated IC rats self-administered more d-amphetamine at 0.006, 0.01, and 0.02 mg/kg/infusion doses compared with their saline-treated IC counterparts regardless of the schedule maintaining behavior. This effect of nicotine was reversed in EC rats on a fixed ratio schedule. These findings indicate that environmental enrichment can limit the ability of adolescent nicotine exposure to increase vulnerability to other psychostimulant drugs, such as d-amphetamine. (PsycINFO Database Record

Differential housing and novelty response: Protection and risk from locomotor sensitization

High novelty seeking increases the risk for drug experimentation and locomotor sensitization. Locomotor sensitization to psychostimulants is thought to reflect neurological adaptations that promote the transition to compulsive drug taking. Rats reared in enrichment (EC) show less locomotor sensitization when compared to rats reared in isolation (IC) or standard conditions (SC). The current research study was designed to test if novelty response contributed locomotor sensitization and more importantly, if the different housing environments could change the novelty response to protect against the development of locomotor sensitization in both adolescence and adulthood. Experiment 1: rats were tested for their response to novelty using the inescapable novelty test (IEN) and pseudorandomly assigned to enriched (EC), isolated (IC), or standard (SC) housing conditions for 30days. After housing, they were tested with IEN. Rats were then administered amphetamine (0.5mg/kg) or saline and locomotor activity was measured followed by a sensitization test 14days later. Experiment 2: rats were tested in the IEN test early adulthood and given five administrations of amphetamine (0.3mg/kg) or saline and then either stayed in or switched housing environments for 30days. Rats were then re-tested in the IEN test in late adulthood and administered five more injections of their respective treatments and tested for locomotor sensitization. Results indicate that IC and SC increased the response to novelty. EC housing decreased locomotor response to amphetamine and saline, and SC housing increased the locomotor response to amphetamine. Mediation results indicated that the late adult novelty response fully mediates the locomotor response to amphetamine and saline, while the early adulthood novelty response did not.

CONCLUSIONS

Differential housing changes novelty and amphetamine locomotor response. Novelty response is altered into adulthood and provides evidence that enrichment can be used to reduce drug vulnerability.

Effects of environmental enrichment on behavioral and spatial cognitive deficits in morphine-dependent and -withdrawn rats

This study was designed to examine the effect of environmental enrichment during morphine dependence and withdrawal on morphine-induced behavioral and spatial cognitive disorders in morphine-withdrawn rats. Adult male Wistar rats (190 ± 20 g) were injected with bi-daily doses (10 mg/kg, 12 h intervals) of morphine for 14 days. Rats were reared in SE or EE during the development of dependence on morphine and withdrawal. Then, rats were tested for spatial learning and memory (the water maze), spontaneous withdrawal signs, and grooming behavior. We found that the EE blocked chronic morphine-induced partial impairments of spatial memory retention. Moreover, the EE diminished the occurrence of spontaneous morphine withdrawal signs as mild and the self-grooming behavior. Our findings showed that EE ameliorates chronic morphine-induced partial deficits of spatial cognition, obsessive-like behavior, and the overall severity of the morphine withdrawal. Thus, environmental enrichment may be a potential therapeutic strategy for spatial memory and behavioral deficits in morphine-dependent individuals.

Effect of environmental enrichment on physical and psychological dependence signs and voluntary morphine consumption in morphine-dependent and morphine-withdrawn rats

This study was designed to examine the effect of environmental enrichment during morphine dependency and withdrawal on the severity of naloxone-precipitated withdrawal signs, anxiety, and depressive-like behaviors and voluntary morphine consumption in morphine-dependent rats. The rats were injected with bi-daily doses (10 mg/kg, 12 h intervals) of morphine for 14 days following rearing in a standard environment (SE) or enriched environment (EE) during the development of morphine dependence and withdrawal. Then, rats were tested for withdrawal signs after naloxone injection, anxiety (the elevated plus maze) and depression-related behavior (sucrose preference test), and voluntary consumption of morphine using a two-bottle choice paradigm, in morphine-dependent and morphine-withdrawn rats. The results showed that EE decreased naloxone-precipitated withdrawal signs, but not anxiety or sucrose preference during dependence on morphine. The EE-withdrawn rats showed an increase in the elevated plus maze open arm time and entries and higher levels of sucrose preference than SE rats. Voluntary consumption of morphine was lower in the EE-withdrawn rats than in the SE groups in the second period of drug intake. Thus, exposure to EE reduced the severity of morphine dependence and voluntary consumption of morphine, alongside reductions in anxiety and depression-related behavior in morphine-withdrawn rats.

Pharmacological manipulation of glucocorticoid receptors differentially affects cocaine self-administration in environmentally enriched and isolated rats

Social isolation rearing (isolated condition, IC) is used as a model of early life stress in rodents. Rats raised in this condition are often compared to rats raised in an environmentally enriched condition (EC). However, EC rats are repeatedly exposed to forced novelty, another classic stressor in rodents. These studies explored the relationship between cocaine self-administration and glucocorticoid receptor (GR) activation and measured total levels of GR protein in reward-related brain regions (medial prefrontal cortex, orbitofrontal cortex, nucleus accumbens, amygdala) in rats chronically exposed to these conditions. For experiment 1, rats were housed in EC or IC and were then trained to self-administer cocaine. Rats raised in these housing conditions were tested for their cocaine responding after pretreatment with the GR antagonist, RU486, or the GR agonist, corticosterone (CORT). For experiment 2, levels of GR from EC and IC rats were measured in brain regions implicated in drug abuse using Western blot analysis. Pretreatment with RU486 (20 mg/kg) decreased responding for a low unit dose of cocaine (0.03 mg/kg/infusion) in EC rats only. IC rats were unaffected by RU486 pretreatment, but earned significantly more cocaine than EC rats after pretreatment with CORT (10 mg/kg). No difference in GR expression was found between EC and IC rats in any brain area examined. These results, along with previous literature, suggest that enrichment enhances responsivity of the HPA axis related to cocaine reinforcement, but this effect is unlikely due simply to differential baseline GR expression in areas implicated in drug abuse.

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.

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.

Effects of cocaine combined with a social cue on conditioned place preference and nucleus accumbens monoamines after isolation rearing in rats

RATIONALE

Social interaction during drug exposure can potentiate cocaine reward. Isolation rearing (ISO) during adolescence increases social interaction and may amplify this potentiation.

OBJECTIVES

The objectives of this study are to determine whether ISO alters conditioned place preference (CPP) for cocaine when combined with a social cue and to determine whether ISO alters the effects of cocaine when combined with social cue on nucleus accumbens shell (NAcS) dopamine (DA) and serotonin (5-HT).

METHODS

Male and female rats were either ISO or group (GRP) reared for 4 weeks during adolescence. CPP was performed using a low dose of cocaine (2 mg/kg or saline) with or without exposure to a novel same-sex conspecific during conditioning. In vivo microdialysis was performed using the same parameters.

RESULTS

ISO rats engaged in more social and aggressive behaviors during conditioning relative to GRP. Cocaine reduced social and aggressive behaviors in all rats. CPP was not influenced by rearing condition. Cocaine produced significant CPP, and a social cue produced CPP only in males. In contrast, the interaction of cocaine and a social cue on NAcS DA and 5-HT differed depending upon rearing condition. In isolates, cocaine-induced DA was attenuated, while cocaine plus a social cue produced potentiated DA and 5-HT.

CONCLUSIONS

Exposure to a low dose of cocaine in the presence of a social cue produced additive effects on CPP while producing synergistic effects on DA and 5-HT in the NAcS of ISO rats. The aversive effects of this compound stimulus may negate the rewarding effects in isolates.

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

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

Chronic wheel running reduces maladaptive patterns of methamphetamine intake: regulation by attenuation of methamphetamine-induced neuronal nitric oxide synthase

We investigated whether prior exposure to chronic wheel running (WR) alters maladaptive patterns of excessive and escalating methamphetamine intake under extended access conditions, and intravenous methamphetamine self-administration-induced neurotoxicity. Adult rats were given access to WR or no wheel (sedentary) in their home cage for 6 weeks. A set of WR rats were injected with 5-bromo-2'-deoxyuridine (BrdU) to determine WR-induced changes in proliferation (2-h old) and survival (28-day old) of hippocampal progenitors. Another set of WR rats were withdrawn (WRw) or continued (WRc) to have access to running wheels in their home cages during self-administration days. Following self-administration [6 h/day], rats were tested on the progressive ratio (PR) schedule. Following PR, BrdU was injected to determine levels of proliferating progenitors (2-h old). WRc rats self-administered significantly less methamphetamine than sedentary rats during acquisition and escalation sessions, and demonstrated reduced motivation for methamphetamine seeking. Methamphetamine reduced daily running activity of WRc rats compared with that of pre-methamphetamine days. WRw rats self-administered significantly more methamphetamine than sedentary rats during acquisition, an effect that was not observed during escalation and PR sessions. WR-induced beneficial effects on methamphetamine self-administration were not attributable to neuroplasticity effects in the hippocampus and medial prefrontal cortex, but were attributable to WR-induced inhibition of methamphetamine-induced increases in the number of neuronal nitric oxide synthase expressing neurons and apoptosis in the nucleus accumbens shell. Our results demonstrate that WR prevents methamphetamine-induced damage to forebrain neurons to provide a beneficial effect on drug-taking behavior. Importantly, WR-induced neuroprotective effects are transient and continued WR activity is necessary to prevent compulsive methamphetamine intake.

Wheel-running mitigates psychomotor sensitization initiation but not post-sensitization conditioned activity and conditioned place preference induced by cocaine in mice

Previous literature suggests that physical exercise allowed by an unlimited access to a running wheel for several weeks can mitigate chronic neurobehavioral responsiveness to several addictive drugs in rodents. Here, the potential preventive effects of unlimited wheel-running on the initiation of psychomotor sensitization and the acquisition and extinction of conditioned place preference (CPP) induced by 10 mg/kg cocaine in C56BL/6J mice were assessed in two independent experiments. To this end, half of the mice were singly housed with a running wheel at 28 days of age for 10 weeks prior to psychopharmacological tests, during which housing conditions did not change, and the other half of mice were housed without running wheel. In Experiment 1, prior to initiating sensitization, psychomotor activity on the two first drug-free once-daily sessions was not affected by wheel-running. This was also found for the acute psychomotor-activating effect of cocaine on the first sensitization session. Psychomotor sensitization readily developed over the 9 following once-daily sessions in mice housed without wheel, whereas it was inhibited in mice housed with a wheel. However, that difference did not transfer to post-sensitization conditioned activity. In contrast with the sensitization results, mice housed with a wheel still expressed a clear-cut CPP which did not extinguish differently from that of the other group, a result in disaccord with previous studies reporting either an attenuating or an increasing effect of wheel-running on cocaine-induced conditioned reward. The available results together indicate that interactions between wheel-running and cocaine effects are far from being satisfactorily characterized.

Effects of environmental enrichment on nicotine-induced sensitization and cross-sensitization to d-amphetamine in rats

INTRODUCTION

Research indicates that adolescent nicotine exposure may predispose individuals to use other psychostimulants later in adulthood, offering support for the incentive-sensitization theory of addiction. Preclinical studies testing the incentive-sensitization theory show that repeated nicotine exposure in adolescent rats can lead to an increased sensitivity to the motor stimulant effects of nicotine and other psychostimulants in adulthood. Although previous nicotine exposure can increase sensitivity to stimulant drugs, rats raised in enriched conditions (EC) show, decreased sensitivity to psychostimulant drugs compared to rats raised in isolation conditions (IC).

METHODS

We examined whether nicotine sensitization or cross-sensitization to d-amphetamine induced by adolescent nicotine exposure is altered by exposure to environmental enrichment. Adolescent EC and IC male rats received subcutaneous (s.c.) injections of saline or 0.4mg/kg of nicotine once daily for seven days. Thirty-five days following the last nicotine injection EC and IC animals were challenged with saline, nicotine (0.2 or 0.4mg/kg) or d-amphetamine (0.5 or 1.0mg/kg).

RESULTS

EC rats failed to show nicotine sensitization at either nicotine dose tested while IC rats showed nicotine sensitization following the 0.4mg/kg nicotine dose. EC rats also failed to show nicotine-induced cross-sensitization to the 0.5mg/kg dose of d-amphetamine while IC rats displayed cross-sensitization. However, EC rats did exhibit nicotine-induced cross-sensitization to the 1.0mg/kg dose of d-amphetamine.

CONCLUSION

These findings indicate that environmental enrichment can alter the ability of adolescent nicotine exposure to induce sensitization and cross-sensitization in adulthood and may be used as a protectant factor against adolescent nicotine exposure.

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.

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.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.

The effects of rearing condition on methamphetamine self-administration and cue-induced drug seeking

BACKGROUND

Little is known about the effect of different rearing conditions on the effects of methamphetamine and whether the introduction of enriched rearing conditions at different stages of development could produce different behavioral outcomes.

METHODS

In Experiment 1, rats were reared in either enriched (EE) or isolated environments (IE) from PND 21 to 60. In Experiment 2, two groups of animals were handled in the same fashion as those in Experiment 1. Additional two groups were housed in IE during the first 20 or 30 days and then housed under EE for the remaining 20 or 10 days respectively. Locomotor activity and Morris Water Maze were tested. The effects of rearing conditions on methamphetamine (METH) self-administration were investigated.

RESULTS

IE animals exhibited higher levels of locomotion than EE animals, but EE animals showed enhanced Morris water maze performance. Animals reared in IE for 30 and 40 days more readily acquired METH self-administration, compared to those reared in IE for 20 and in EE for 40 days respectively. However, the effect of rearing conditions was only seen at the lowest dose tested under FR schedule and breakpoints obtained from PR schedule were not significantly affected. Those reared in IE for 20 and EE for 40 days animals produced significantly fewer responses during the extinction and cue-induced reinstatement of METH self-administration, compared with animals reared in IE for 30 and 40 days, respectively.

CONCLUSION

Rearing condition plays a significant role in locomotor activity, spatial memory and behavioral effects of METH.

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.

Access to a running wheel decreases cocaine-primed and cue-induced reinstatement in male and female rats

BACKGROUND

Relapse to drug use after a period of abstinence is a persistent problem in the treatment of cocaine dependence. Physical activity decreases cocaine self-administration in laboratory animals and is associated with a positive prognosis in human substance-abusing populations. The purpose of this study was to examine the effects of long-term access to a running wheel on drug-primed and cue-induced reinstatement of cocaine-seeking behavior in male and female rats. methods: Long-Evans rats were obtained at weaning and assigned to sedentary (no wheel) and exercising (access to wheel) groups for the duration of the study. After 6 weeks, rats were implanted with intravenous catheters and trained to self-administer cocaine for 14 days. After training, saline was substituted for cocaine and responding was allowed to extinguish, after which cocaine-primed reinstatement was examined in both groups. Following this test, cocaine self-administration was re-established in both groups for a 5-day period. Next, a second period of abstinence occurred in which both cocaine and the cocaine-associated cues were withheld. After 5 days of abstinence, cue-induced reinstatement was examined in both groups.

RESULTS

Sedentary and exercising rats exhibited similar levels of cocaine self-administration, but exercising rats responded less than sedentary rats during extinction. In tests of cocaine-primed and cue-induced reinstatement, exercising rats responded less than sedentary rats, and this effect was apparent in both males and females.

CONCLUSIONS

These data indicate that long-term access to a running wheel decreases drug-primed and cue-induced reinstatement, and that physical activity may be effective at preventing relapse in substance-abusing populations.

Assessing locomotor-stimulating effects of cocaine in rodents

Locomotor activity procedures are useful for characterizing the behavioral effects of a drug, the influence of pharmacological, neurobiological, and environmental manipulations on drug sensitivity, and changes in activity following repeated administration (e.g., tolerance or sensitization) are thought to be related to the development of an addiction-like behavioral phenotype. The effects of cocaine on locomotor activity have been relatively extensively characterized. Many of the published studies use between-subject experimental designs, even though changes in sensitivity within a particular individual due to experimental manipulations, or behavioral and pharmacological histories is potentially the most important outcome as these changes may relate to differential development of an addiction-like phenotype in some, but not all, animals (including humans). The two behavioral protocols described herein allow extensive within-subject analyses. The first protocol uses daily locomotor activity levels as a stable baseline to assess the effects of experimental manipulations, and the second uses a pre- versus post-session experimental design to demonstrate the importance of drug-environment interactions in determining the behavioral effects of cocaine.

The effects of aerobic exercise on cocaine self-administration in male and female rats

RATIONALE

In drug self-administration procedures, extended-access test sessions allow researchers to model maladaptive patterns of excessive and escalating drug intake that are characteristic of human substance-abusing populations.

OBJECTIVES

The purpose of the present study was to examine the ability of aerobic exercise to decrease excessive and escalating patterns of drug intake in male and female rats responding under extended-access conditions.

METHODS

Male and female Long-Evans rats were obtained at weaning and divided into sedentary (no running wheel) and exercising (running wheel) groups immediately upon arrival. After 6 weeks, rats were surgically implanted with intravenous catheters and allowed to self-administer cocaine under positive reinforcement contingencies. In experiment 1, cocaine self-administration was examined during 23-h test sessions that occurred every 4 days. In experiment 2, the escalation of cocaine intake was examined during daily 6-h test sessions over 14 consecutive days.

RESULTS

In experiment 1, sedentary rats self-administered significantly more cocaine than exercising rats during uninterrupted 23-h test sessions, and this effect was apparent in both males and females. In experiment 2, sedentary rats escalated their cocaine intake to a significantly greater degree than exercising rats over the 14 days of testing. Although females escalated their cocaine intake to a greater extent than males, exercise effectively attenuated the escalation of cocaine intake in both sexes.

CONCLUSIONS

These data indicate that aerobic exercise decreases maladaptive patterns of excessive and escalating cocaine intake under extended-access conditions.

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

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

Genotype-dependent effects of adolescent nicotine exposure on dopamine functional dynamics in the nucleus accumbens shell in male and female mice: a potential mechanism underlying the gateway effect of nicotine

RATIONALE

The tendency to use cocaine is determined by genetic and environmental effects across the lifespan. One critical environmental effect is early drug exposure, which is both driven by and interacts with genetic background. The mesoaccumbens dopamine system, which is critically involved in the rewarding properties of drugs of abuse, undergoes significant development during adolescence, and thus may be at particular risk to repeated nicotine exposure during this period, thereby establishing vulnerability for subsequent adult psychostimulant use.

OBJECTIVES

We tested the hypotheses that adolescent nicotine exposure results in attenuation of the enhancing effects of cocaine on medial forebrain bundle (MFB) electrical stimulation-evoked dopamine release in the nucleus accumbens shell (AcbSh) in adulthood and that this effect is significantly influenced by genotype.

METHODS

Mice from the progenitor strains C57BL/6J and DBA/2J and those from the BXD20/TyJ and BXD86/RwwJ recombinant inbred lines were exposed to nicotine via osmotic minipumps from postnatal day (P) 28 to P56. When mice reached P70, dopamine functional dynamics in AcbSh was evaluated by means of in vivo fixed potential amperometry in combination with electrical stimulation of mesoaccumbens dopaminergic axons in the MFB.

RESULTS

Adolescent exposure to nicotine in all strains dose-dependently reduced the ability of a fixed-dose challenge injection of cocaine (10 mg/kg, i.p.) to enhance MFB electrical stimulation-evoked dopamine release in AcbSh in adults. The magnitude of this effect was genotype-dependent.

CONCLUSIONS

These results suggest a genotype-dependent mechanism by which nicotine exposure during adolescence causes persistent changes in the sensitivity to "hard" stimulants such as cocaine.

The impact of environmental enrichment in laboratory rats--behavioural and neurochemical aspects

The provision of environmental enrichment (EE) for laboratory rats is recommended in European guidelines governing laboratory animal welfare. It is believed the EE implementation can improve animals' well-being and EE has been used to demonstrate learning and plasticity of the brain in response to the environment. This review suggests that the definition and duration of EE varies considerably across laboratories. Notwithstanding this, some EE protocols have revealed profound effects on brain neurochemistry and resulting behaviour, suggesting that EE can have the potential to significantly modify these parameters in rats. For this review, a literature search was conducted using PubMed and the search terms "Environmental Enrichment" and "rats". From the results of this search the most important variables for consideration in the implementation of EE are identified and summarised, and include cage size and housing density; rat age, sex and strain; duration of EE; the EE protocol and enrichment items employed; and the use of appropriate controls. The effects of EE in a number of behavioural tests and its effects on neurotransmitters, neurotrophic factors, stress hormones and neurogenesis and proliferation are outlined. The findings summarised in the present review show the range of EE protocols employed and their effects in tests of activity, learning and affect, as well neurochemical effects which mediate enhanced plasticity in the brain. EE, as is provided in many laboratories, may be of benefit to the animals, however it is important that future work aims to provide a better understanding of EE effects on research outcomes.

Local cerebral glucose utilization in rats exposed to an enriched environment: a comparison to impoverishment

Environmental enrichment and environmental impoverishment have been shown to differentially alter brain function. Here, we investigate the effects of enrichment vs. impoverishment on cerebral use of glucose in rodents. Rats were housed from postnatal day 28 to day 58 in either a socially and environmentally enriched environment or an impoverished environment devoid of other rats or environmental stimuli. Locomotor activity was measured at the end of the enrichment/impoverishment period. Following the duration of the exposure to these environments, cerebral metabolic rate of glucose utilization was determined using quantitative 2-[(14)C]deoxyglucose autoradiography in 37 brain regions in the cerebral cortex, forebrain, brain stem and thalamus. There were no differences in locomotor activity between the conditions. The nucleus accumbens core and shell had significantly higher rates of glucose utilization in enriched compared to impoverished animals. These data suggest that environment has a significant effect on brain function which may help to explain the beneficial and protective effects of enrichment against drug abuse and addiction.