Modeling risk factors for nicotine and other drug abuse in the preclinical laboratory

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.

Symptoms of depression and PTSD are associated with elevated alcohol demand

BACKGROUND

Behavioral economic demand curves measure individual differences in motivation for alcohol and have been associated with problematic patterns of alcohol use, but little is known about the variables that may contribute to elevated demand. Negative visceral states have been theorized to increase demand for alcohol and to contribute to excessive drinking patterns, but little empirical research has evaluated this possibility. The present study tested the hypothesis that symptoms of depression and PTSD would be uniquely associated with elevated alcohol demand even after taking into account differences in typical drinking levels.

METHOD

An Alcohol Purchase Task (APT) was used to generate a demand curve measure of alcohol reinforcement in a sample of 133 college students (50.4% male, 64.4% Caucasian, 29.5% African-American) who reported at least one heavy drinking episode (5/4 or more drinks in one occasion for a man/woman) in the past month. Participants also completed standard measures of alcohol consumption and symptoms of depression and PTSD.

RESULTS

Regression analyses indicated that symptoms of depression were associated with higher demand intensity (alcohol consumption when price=0; ΔR(2)=.05, p=.002) and lower elasticity (ΔR(2)=.04, p=.03), and that PTSD symptoms were associated with all five demand curve metrics (ΔR(2)=.04-.07, ps<.05).

CONCLUSIONS

These findings provide support for behavioral economic models of addiction that highlight the role of aversive visceral states in increasing the reward value of alcohol and provide an additional theoretical model to explain the association between negative affect and problematic drinking patterns.

Exercise to reduce the escalation of cocaine self-administration in adolescent and adult rats

RATIONALE

Concurrent access to an exercise wheel decreases cocaine self-administration under short access (5 h/day for 5 days) conditions and suppresses cocaine-primed reinstatement in adult rats.

OBJECTIVE

The effect of exercise (wheel running) on the escalation of cocaine intake during long access (LgA, 6 h/day for 26 days) conditions was evaluated.

METHODS

Adolescent and adult female rats acquired wheel running, and behavior was allowed to stabilize for 3 days. They were then implanted with an iv catheter and allowed to self-administer cocaine (0.4 mg/kg, iv) during 6-h daily sessions for 16 days with concurrent access to either an unlocked or a locked running wheel. Subsequently, for ten additional sessions, wheel access conditions during cocaine self-administration sessions were reversed (i.e., locked wheels became unlocked and vice versa).

RESULTS

In the adolescents, concurrent access to the unlocked exercise wheel decreased responding for cocaine and attenuated escalation of cocaine intake irrespective of whether the locked or unlocked condition came first. However, cocaine intake increased when the wheel was subsequently locked for the adolescents that had initial access to an unlocked wheel. Concurrent wheel access either before or after the locked wheel access did not reduce cocaine intake in adults.

CONCLUSIONS

Wheel running reduced cocaine intake during LgA conditions in adolescent but not adult rats, and concurrent access to the running wheel was necessary. These results suggest that exercise prevents cocaine seeking and that this effect is more pronounced in adolescents than adults.

A randomized controlled trial of a behavioral economic supplement to brief motivational interventions for college drinking

OBJECTIVE

Behavioral economic theory suggests that a reduction in substance use is most likely when there is an increase in rewarding substance-free activities. The goal of this randomized controlled clinical trial was to evaluate the incremental efficacy of a novel behavioral economic supplement (Substance-Free Activity Session [SFAS]) to a standard alcohol brief motivational interviewing (BMI) session for heavy-drinking college students.

METHOD

Participants were 82 first-year college students (50% female; 81.7% White/European American; M age = 18.5 years, SD = 0.71) who reported 2 or more past-month heavy drinking episodes. After completing a baseline assessment and an individual alcohol-focused BMI, participants were randomized to either the SFAS or to a Relaxation Training (RT) control session. The SFAS was delivered in an MI style and attempted to increase the salience of delayed academic and career rewards and the patterns of behavior leading to those rewards.

RESULTS

The combination of an alcohol BMI plus the SFAS was associated with significantly greater reductions in alcohol problems compared with an alcohol BMI plus RT at the 1-month and 6-month follow-up assessments (p = .015, ηp² = .07), an effect that was partially mediated by increases in protective behavioral strategies. BMI + SFAS was also associated with greater reductions in heavy drinking among participants who at baseline reported low levels of substance-free reinforcement or symptoms of depression.

CONCLUSION

These results are consistent with behavioral economic theory and suggest that a single session focused on increasing engagement in alternatives to drinking can enhance the effects of brief alcohol interventions.

Loss of environmental enrichment increases vulnerability to cocaine addiction

Life experiences, especially during critical periods of maturation, such as adolescence, can dramatically affect vulnerability to diseases at adulthood. Early exposure to positive environmental conditions such as environmental enrichment (EE) has been shown to reduce the occurrence and the intensity of neurological and psychiatric disorders including drug addiction. However, whether or not exposure to EE during early stages of life would protect from addiction when, at adulthood, individuals may find themselves in non-enriched conditions has not been investigated. Here we show that switching mice from EE to non-enriched standard environments not only results in the loss of the preventive effects of EE but also increases the rewarding effects of cocaine. This enhanced vulnerability is associated with emotional distress and with increased levels in the mRNA levels of corticotropin releasing factor (CRF) in the bed nucleus of the stria terminalis (BNST), as well as with increases in CREB phosphorylation in the BNST and in the shell of the nucleus accumbens. The increased sensitivity to the rewarding effects of cocaine is completely blocked by the CRF antagonist antalarmin, confirming a major role of the CRF system in the negative consequences of this environmental switch. These results indicate that positive life conditions during early stages of life, if they are not maintained at adulthood, may have negative emotional consequences and increase the risks to develop drug addiction.

Cocaine-induced c-Fos expression in rats selectively bred for high or low saccharin intake and in rats selected for high or low impulsivity

Sweet preference and impulsivity are predictors of cocaine self-administration; however, no research has been conducted to investigate neuronal activation in key brain reward areas after first time exposure to cocaine in rats that differ in their propensity for cocaine-seeking and -taking behavior. In this study we used rats that had been selectively bred for high vs. low saccharin intake and rats selected for high vs. low impulsivity for food. The goal of this study was to investigate whether there are differences of c-Fos reactivity between high and low phenotypes and determine whether these differences are similar between the two animal models. A group of rats was bred for high or low saccharin intake. Another group of rats was selected as high or low impulsive based on performance in a delay-discounting task. Subsequently, rats were given an acute injection of cocaine or saline and then c-Fos expression was observed and analyzed in several brain regions. The low reward-seeking phenotypes showed higher cocaine-induced c-Fos expression in several of these regions. Low saccharin preferring rats showed higher cocaine-induced c-Fos expression in the nucleus accumbens shell, and low impulsive rats showed higher cocaine-induced c-Fos expression in the orbitofrontal cortex and cingulate gyrus 1 area. In addition, both low impulsive and low saccharin rats had higher cocaine-induced c-Fos in the dorsal medial and dorsal lateral caudate putamen. The results indicate that individual differences in neuronal reactivity exist prior to chronic exposure to drugs of abuse. Furthermore, similar differences between the two animal models may be indicative of a common mechanism underlying vulnerability to drugs of abuse.

Monetary-based consequences for drug abstinence: methods of implementation and some considerations about the allocation of finances in substance abusers

Conceptualizing drug abuse within the framework of behavioral theories of choice highlights the relevance of environmental variables in shifting behavior away from drug-related purchases. Choosing to use drugs results in immediate and certain consequences (e.g., drug high and relief from withdrawal), whereas choosing abstinence typically results in delayed, and often uncertain, consequences (e.g., improved health, interpersonal relationships, money).

METHODS

This is a selective review of the literature on Contingency management (CM).

RESULTS

We highlight a variety of methods to deliver CM in practical, effective, and sustainable ways. We consider a number of parameters that are critical to the success of monetary-based CM, and the role of the context in influencing CM's effects. To illustrate the broad range of applications of CM, we also review different methods for arranging contingencies to promote abstinence and other relevant behavior. Finally, we discuss some considerations about how drug-dependent individuals allocate their finances in the context of CM interventions.

CONCLUSIONS

Contingency management (CM) increases choice for drug abstinence via the availability of immediate, financial-based gains, contingent on objective evidence of abstinence.

A behavioral economic supplement to brief motivational interventions for college drinking

Basic behavioral and neurobiological research has demonstrated that deficiencies in naturally occurring substance-free rewards are both a cause and a consequence of substance abuse that are due in part to the systematic discounting of delayed substance-free rewards. Existing brief motivational interventions (BMIs) for alcohol abuse do not target this mechanism of change. The goal of this uncontrolled pilot study was to evaluate a behavioral economic Substance-Free Activity Session (SFAS) to traditional alcohol BMIs. Participants were 13 college freshmen who reported two or more heavy drinking episodes (>5/4 drinks in an occasion for men/ women) in the past month. All participants completed a baseline assessment and a BMI that addressed alcohol use. In addition, participants received the SFAS, a 50-min individual session that attempts to increase engagement in constructive alternatives to drinking by enhancing the salience of delayed rewards (academic and career success) and the patterns of behavior (academic and extracurricular engagement) leading to these outcomes. At the 1-month follow-up assessment, participants reported significant reductions in heavy drinking, and moderate to large effect size reductions in weekly drinking and peak blood alcohol levels. The results of this pilot study provide preliminary support for the efficacy of this behavioral economic intervention session as a supplement to traditional alcohol BMIs.

Responding during signaled availability and nonavailability of iv cocaine and food in rats: age and sex differences

RATIONALE

Research suggests that age and sex are vulnerability factors for drug abuse. However, few studies have systematically examined their interaction.

OBJECTIVE

The purpose of the present study was to examine male and female, adult and adolescent rats under a procedure that measures responding during periods of signaled availability and nonavailability of iv cocaine and food reinforcers.

METHODS

Adolescent and adult rats lever pressed for iv infusions of cocaine or food pellets under a procedure with three components of signaled availability of the reinforcer alternating with two components of signaled nonavailability. Adolescent rats were removed and then later retested under the same conditions as adults, and a group of adult rats was also removed and retested after a similar number of days. A subset of rats earning cocaine infusions under the initial test was later retested with food pellets under the same behavioral task to assess the influence of prior cocaine exposure on subsequent responding for a nondrug reinforcer.

RESULTS

Adolescents (vs. adults) made more responses during periods of signaled iv cocaine availability and nonavailabiltiy, and adult females responded more than adult males during these periods. Responding during periods of signaled nonavailability of iv cocaine and food did not differ between the initial and subsequent retest conditions in adult rats. Further, adult males and females exposed to cocaine during adolescence responded more during periods of food availability compared to cocaine-naïve adults.

CONCLUSION

These results indicate that sex and age are vulnerability factors in cocaine abuse, and cocaine exposure during critical developmental stages can have long-lasting effects.

Contributions of serotonin in addiction vulnerability

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.

Early exposure to environmental enrichment alters the expression of genes of the endocannabinoid system

Early environmental enrichment (EE) produces several changes in gene expression in the brain and confers protection against the behavioral, neurochemical and molecular effects of repeated administration of drugs of abuse. Because the endogenous cannabinoid system (ECS) is known to play an important role in the rewarding effects of drugs, we investigated whether the positive effects of early exposure to EE are associated with changes in the expression of genes encoding for proteins that belong to the ECS in C57 mice. Using in situ hybridization, we compared the expression of the cannabinoid receptor CB1, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL) enzymes in brain regions involved in drug addiction in mice reared in either EE or standard environments (SE) from weaning until adulthood. We found that EE increases CB1 mRNA levels in the hypothalamus and in the basolateral amygdala but decreased them in the basomedial amygdala. Similarly, we found that FAAH mRNA levels are higher in the hypothalamus and the basolateral amygdala of EE mice compared to SE mice, with no change in the basomedial amygdala. In contrast, MGL mRNA levels were not affected by EE in any of the areas analyzed. The regional selectivity of EE-induced changes may indicate that early exposure to EE induces changes in the ECS that could result in reduced responses to stress, as confirmed in EE mice in a novelty-induced suppression of feeding test, and, ultimately, in resistance to addiction.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Validity of a demand curve measure of nicotine reinforcement with adolescent smokers

High or inelastic demand for drugs is central to many laboratory and theoretical models of drug abuse, but it has not been widely measured with human substance abusers. The authors used a simulated cigarette purchase task to generate a demand curve measure of nicotine reinforcement in a sample of 138 adolescent smokers. Participants reported the number of cigarettes they would purchase and smoke in a hypothetical day across a range of prices, and their responses were well-described by a regression equation that has been used to construct demand curves in drug self-administration studies. Several demand curve measures were generated, including breakpoint, intensity, elasticity, P(max), and O(max). Although simulated cigarette smoking was price sensitive, smoking levels were high (8+ cigarettes/day) at prices up to 50¢ per cigarette, and the majority of the sample reported that they would purchase at least 1 cigarette at prices as high as $2.50 per cigarette. Higher scores on the demand indices O(max) (maximum cigarette purchase expenditure), intensity (reported smoking level when cigarettes were free), and breakpoint (the first price to completely suppress consumption), and lower elasticity (sensitivity of cigarette consumption to increases in cost), were associated with greater levels of naturalistic smoking and nicotine dependence. Greater demand intensity was associated with lower motivation to change smoking. These results provide initial support for the validity of a self-report cigarette purchase task as a measure of economic demand for nicotine with adolescent smokers.

Females are more vulnerable to drug abuse than males: evidence from preclinical studies and the role of ovarian hormones

Human and animal research indicates the presence of sex differences in drug abuse. These data suggest that females, compared to males, are more vulnerable to key phases of the addiction process that mark transitions in drug use such as initiation, drug bingeing, and relapse. Recent data indicate that the female gonadal hormone estrogen may facilitate drug abuse in women. For example, phases of the menstrual cycle when estrogen levels are high are associated with enhanced positive subjective measures following cocaine and amphetamine administration in women. Furthermore, in animal research, the administration of estrogen increases drug taking and facilitates the acquisition, escalation, and reinstatement of cocaine-seeking behavior. Neurobiological data suggest that estrogen may facilitate drug taking by interacting with reward- and stress-related systems. This chapter discusses sex differences in and hormonal effects on drug-seeking behaviors in animal models of drug abuse. The neurobiological basis of these differences and effects are also discussed.

Pharmacological enhancement of drug cue extinction learning: translational challenges

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may constitute a rational strategy for the clinical management of drug relapse. While certain success has been reported for this form of therapy in anxiety disorders, in this paper we highlight several obstacles that may undermine the efficacy of exposure therapy for substance use disorders. We also review translational studies that have evaluated the facilitative effects of the cognitive enhancer D-cycloserine on extinction targeting drug-related cues. Finally, important considerations for the design and implementation of future studies evaluating exposure therapy combined with pharmacotherapy for substance use disorders are discussed.

Environmental living conditions introduced during forced abstinence alter cocaine-seeking behavior and Fos protein expression

Environmental enrichment (EE) introduced during abstinence from cocaine self-administration is protective in reducing cue-elicited incentive motivation for cocaine in rats. This study examined neural activation associated with this protective effect of EE using Fos protein expression as a marker. Rats were trained to press a lever reinforced by cocaine (0.75 mg/kg/0.1 mL infusion) and light and tone cues across 15 consecutive days during which they were all housed in isolated conditions (IC). Rats were then assigned to either remain in IC, or to live in pair-housed conditions (PC) or EE for 30 days of forced abstinence from cocaine. Subsequently, cocaine-seeking behavior (lever presses without cocaine reinforcement) elicited by response-contingent cue presentations was assessed for 90 min, after which the rats' brains were immediately harvested for Fos protein immunohistochemistry. EE attenuated, whereas IC enhanced, cue-elicited cocaine-seeking behavior relative to PC. Also, within the prelimbic and orbitofrontal cortices and basolateral amygdala, IC enhanced, whereas EE reduced, Fos expression relative to PC. Furthermore, EE attenuated Fos expression in the infralimbic and anterior cingulate cortices, the nucleus accumbens (core and shell), bed nucleus of the stria terminalis, and ventral tegmental area, evident as a reduction relative to both PC and IC. In contrast, IC enhanced Fos expression in the dorsal caudate putamen, substantia nigra, and central amygdala, evident as an increase relative to both PC and EE. These results suggest that EE blunts neural activation throughout the mesocorticolimbic circuitry involved in cue-elicited incentive motivation for cocaine, whereas IC enhances activation primarily within the nigrostriatal dopamine pathway. These findings have important implications for understanding and treating drug-conditioned craving in humans.

The interactive effects of environmental enrichment and extinction interventions in attenuating cue-elicited cocaine-seeking behavior in rats

Cues associated with cocaine can elicit craving and relapse. Attempts have been made to employ extinction therapy, which is aimed at attenuating the incentive motivational effects of cocaine cues, as a treatment for cocaine addiction; however, this approach has been largely unsuccessful perhaps due to the inability to extinguish all cues associated with cocaine use while in a clinic. Recently, environmental enrichment (EE) during abstinence has been proposed as a strategy to attenuate cue-elicited cocaine craving. The present study used an animal model to examine whether the utility of extinction toward attenuating cue-elicited cocaine-seeking behavior could be enhanced by also providing EE. All rats were trained to self-administer cocaine while housed in isolated conditions and then subsequently underwent 17 days of forced abstinence, during which they were either housed in pairs or under EE and they either received daily 1-h extinction sessions or similar handling without exposure to the self-administration environment. Following this intervention period, all rats were tested for cue-elicited cocaine-seeking behavior. To examine whether effects of these interventions persist, all rats were subsequently single-housed for an additional 7-day forced abstinence period, followed by a second test for cue-elicited cocaine-seeking behavior. We found that although daily extinction training and EE each attenuated subsequent cue-elicited cocaine-seeking behavior, the combined treatment of extinction training+EE completely prevented it. However, once these interventions were discontinued, their protective effects diminished. These findings suggest that combining behavioral therapy approaches may improve outcomes; however, future work is needed to improve the longevity of these strategies beyond their implementation.

Staging perspectives in neurodevelopmental aspects of neuropsychiatry: agents, phases and ages at expression

Neurodevelopmental risk factors have assumed a critical role in prevailing notions concerning the etiopathogenesis of neuropsychiatric disorders. Staging, diagnostic elements at which phase of disease is determined, provides a means of conceptualizing the degree and extent of factors affecting brain development trajectories, but is concurrently specified through the particular interactions of genes and environment unique to each individual case. For present purposes, staging perspectives in neurodevelopmental aspects of the disease processes are considered from conditions giving rise to neurodevelopmental staging in affective states, adolescence, dopamine disease states, and autism spectrum disorders. Three major aspects influencing the eventual course of individual developmental trajectories appear to possess an essential determinant influence upon outcome: (i) the type of agent that interferes with brain development, whether chemical, immune system activating or absent (anoxia/hypoxia), (ii) the phase of brain development at which the agent exerts disruption, whether prenatal, postnatal, or adolescent, and (iii) the age of expression of structural and functional abnormalities. Clinical staging may be assumed at any or each developmental phase. The present perspective offers both a challenge to bring further order to diagnosis, intervention, and prognosis and a statement regarding the extreme complexities and interwoven intricacies of epigenetic factors, biomarkers, and neurobehavioral entities that aggravate currents notions of the neuropsychiatric disorders.

Reduction of extinction and reinstatement of cocaine seeking by wheel running in female rats

RATIONALE AND OBJECTIVES

Previous work has shown that wheel running reduced the maintenance of cocaine self-administration in rats. In the present study, the effect of wheel running on extinction and reinstatement of cocaine seeking was examined. Female rats were trained to run in a wheel during 6-h sessions, and they were then catheterized and placed in an operant conditioning chamber where they did not have access to the wheel but were allowed to self-administer iv cocaine. Subsequently, rats were divided into four groups and were tested on the extinction and reinstatement of cocaine seeking while they had varying access to a wheel in an adjoining compartment. The four groups were assigned to the following wheel access conditions: (1) wheel running during extinction and reinstatement (WER), (2) wheel running during extinction and a locked wheel during reinstatement (WE), (3) locked wheel during extinction and wheel running during reinstatement (WR), and (4) locked wheel during extinction and reinstatement (WL). WE and WR were retested later to examine the effect of one session of wheel access on cocaine-primed reinstatement.

RESULTS

There were no group differences in wheel revolutions, in rate of acquisition of cocaine self-administration, or in responding during maintenance when there was no wheel access. However, during extinction, WE and WER responded less than WR and WL. WR and WER had lower cocaine-primed reinstatement than WE and WL. One session of wheel exposure in WE also suppressed cocaine-primed reinstatement.

CONCLUSIONS

Wheel running immediately and effectively reduced cocaine-seeking behavior, but concurrent access to running was necessary. Thus, exercise is a useful and self-sustaining intervention to reduce cocaine-seeking behavior.

Environmental enrichment does not reduce the rewarding and neurotoxic effects of methamphetamine

Abuse of amphetamine analogues, such as methamphetamine (METH), represents an important health problem because of their powerful addictive and neurotoxic effects. Abuse of METH induces dopamine neuron terminals loss and cell death in the striatum similar to what is found in other neurodegenerative processes. Exposing mice and rats to enriched environments (EE) has been shown to produce significant protective effects against drug-induced reward as well as against neurodegenerative processes. Here, we investigated whether exposure to EE could reduce the METH-induced reward and neurotoxicity. For this, we reared mice for 2 months during early stages of life in standard environments or EE and then, at adulthood, we tested the ability of METH to induce conditioned place preference and neurotoxicity. We found that, contrary to what we found with other drugs such as cocaine and heroin, EE was unable to reduce the rewarding effects of METH. In addition, contrary to what we found with other toxins such as MPTP, EE did not diminish the striatal neurotoxicity induced by METH (4 x 10 mg/kg) as measured by dopamine content, tyrosine hydroxylase protein levels and apoptosis. Our results demonstrate that the rewarding and neurotoxic effects of METH are not reduced by EE and highlight the great risks associated with the increased popularity of this drug amongst the young population.

Reinstatement of cocaine seeking induced by drugs, cues, and stress in adolescent and adult rats

RATIONALE

In human and animal studies, adolescence marks a period of increased vulnerability to the initiation and subsequent abuse of drugs. Adolescents may be especially vulnerable to relapse, and a critical aspect of drug abuse is that it is a chronically relapsing disorder. However, little is known of how vulnerability factors such as adolescence are related to conditions that induce relapse, triggered by the drug itself, drug-associated cues, or stress.

OBJECTIVE

The purpose of this study was to compare adolescent and adult rats on drug-, cue-, and stress-induced reinstatement of cocaine-seeking behavior.

METHODS

On postnatal days 23 (adolescents) and 90 (adults), rats were implanted with intravenous catheters and trained to lever press for i.v. infusions of cocaine (0.4 mg/kg) during two daily 2-h sessions. The rats then self-administered i.v. cocaine for ten additional sessions. Subsequently, visual and auditory stimuli that signaled drug delivery were unplugged, and rats were allowed to extinguish lever pressing for 20 sessions. Rats were then tested on cocaine-, cue-, and yohimbine (stress)-induced cocaine seeking using a within-subject multicomponent reinstatement procedure.

RESULTS

Results indicated that adolescents had heightened cocaine seeking during maintenance and extinction compared to adults. During reinstatement, adolescents (vs adults) responded more following cocaine- and yohimbine injections, while adults (vs adolescents) showed greater responding following presentations of drug-associated cues.

CONCLUSION

These results demonstrated that adolescents and adults differed across several measures of drug-seeking behavior, and adolescents may be especially vulnerable to relapse precipitated by drugs and stress.

Environmental enrichment reduces cocaine seeking and reinstatement induced by cues and stress but not by cocaine

Whereas earlier studies have focused on the preventive effects of enriched environments (EE) in drug addiction, in a recent study we suggested that EE can also have 'curative' effects. In fact, we found that cocaine addiction-related behaviors can be eliminated by housing cocaine-treated mice in EE during periods of forced abstinence. However, those results were obtained with two simple models of addiction, conditioned place preference (CPP), and behavioral sensitization. In this study, we used intravenous drug self-administration procedures in rats to further investigate the beneficial effects of EE on cocaine addiction in a reinstatement model of relapse. Singly housed rats learned to self-administer cocaine during 10 consecutive daily sessions (0.6 mg/injection, 6 h/day). They were then housed three per cage in either standard environments (SE) or EE and were kept abstinent in the animal facility until testing for extinction and reinstatement. We found that 30 days of EE significantly and consistently reduced cocaine seeking during a 6-h extinction session. In addition, EE significantly reduced cue- and stress-induced reinstatement. Surprisingly, given our earlier results in mice with CPP, EE did not reduce cocaine-induced reinstatement regardless of the level of exposure to cocaine and the duration of the period of abstinence and exposure to EE. Altogether, these results support the hypothesis that EE can reduce cocaine-induced craving and highlight the importance of positive life conditions in facilitating abstinence and preventing relapse to cocaine addiction.