Strain and housing affect cocaine self-selection and open-field locomotor activity in mice

Sex Differences in Cocaine Sensitization Vary by Mouse Strain

INTRODUCTION

Preclinical literature, frequently utilizing rats, suggests females display a more rapid advancement of substance abuse and a greater risk of relapse following drug abstinence. In clinical populations, it is less clear as to what extent biological sex is a defining variable in the acquisition and maintenance of substance use. Even without considering environmental experiences, genetic factors are presumed to critically influence the vulnerability to addiction. Genetically diverse mouse models provide a robust tool to examine the interactions between genetic background and sex differences in substance abuse.

METHODS

We explored mouse strain variability in male versus female behavioral sensitization to cocaine. Locomotor sensitization was observed following 5 consecutive days of subcutaneous cocaine across three genetically different mice strains: C57BL/6J, B6129SF2/J, and Diversity Outbred (DO/J).

RESULTS

Sex differences in cocaine locomotor sensitization were dependent on mouse strain. Specifically, we observed opposing sex differences in locomotor sensitization, with male C57BL/6J and female B6129SF2/J mice displaying heightened activity compared to their opposite sex counterparts. Conversely, no sex differences were observed in the DO/J mice. Acute cocaine administration resulted in locomotor differences across strains in male, but not female, mice. The magnitude of sensitization (or lack thereof) also varied by genetic background.

CONCLUSIONS

While sex differences in drug addiction may be observed, these effects can be mitigated, or even reversed, depending on genetic background. The clinical implications are that in the absence of understanding the genetic variables underlying vulnerability to addiction, sex provides little information regarding the predisposition of an individual to drug abuse.

Cocaine self-administration induces sex-dependent protein expression in the nucleus accumbens

Substance use disorder (SUD) is a chronic neuropsychiatric condition characterized by long-lasting alterations in the neural circuitry regulating reward and motivation. Substantial work has focused on characterizing the molecular substrates that underlie these persistent changes in neural function and behavior. However, this work has overwhelmingly focused on male subjects, despite mounting clinical and preclinical evidence that females demonstrate dissimilar progression to SUD and responsivity to stimulant drugs of abuse, such as cocaine. Here, we show that sex is a critical biological variable that defines drug-induced plasticity in the nucleus accumbens (NAc). Using quantitative mass spectrometry, we assessed the protein expression patterns induced by cocaine self-administration and demonstrated unique molecular profiles between males and females. We show that 1. Cocaine self-administration induces non-overlapping protein expression patterns in significantly regulated proteins in males and females and 2. Critically, cocaine-induced protein regulation differentially interacts with sex to eliminate basal sexual dimorphisms in the proteome. Finally, eliminating these baseline differences in the proteome is concomitant with the elimination of sex differences in behavior for non-drug rewards. Together, these data suggest that cocaine administration is capable of rewriting basal proteomic function and reward-associated behaviors.

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

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

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

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

Implementation of the mouse frailty index

Frailty is considered a state of high vulnerability for adverse health outcomes for people of the same age. Those who are frail have higher mortality, worse health outcomes, and use more health care services than those who are not frail. Despite this, little is known about the biology of frailty, the effect of frailty on pharmacological or surgical outcomes, and potential interventions to attenuate frailty. In humans, frailty can be quantified using a frailty index (FI) based on the principle of deficit accumulation. The recent development of an FI in naturally ageing mice provides an opportunity to conduct frailty research in a validated preclinical model. The mouse FI has been successfully used across a wide range of applications; however, there are some factors that should be considered in implementing this tool. This review summarises the current literature, presents some original data, and suggests areas for future research on the current applications of the mouse FI, inter-rater reliability of the FI, the effect of observer characteristics and environmental factors on mouse FI scores, and the individual items that make up the FI assessment. The implementation of this tool into preclinical frailty research should greatly accelerate translational research in this important field.

Sex differences in the acquisition and maintenance of cocaine and nicotine self-administration in rats

RATIONALE

Consistent sex differences are observed in human drug addiction, with females often exceeding males on drug intake. However, there is still a need for animal models for some aspects of addiction such as acquisition of drug self-administration and the subsequent development of drug-seeking.

OBJECTIVES

The present study examined sex differences in the acquisition and maintenance of self-administration of two widely used stimulants, cocaine and nicotine.

METHODS

Male and female rats self-administered cocaine (0.4 mg/kg/infusion) or nicotine (0.03 mg/kg/infusion) daily under a fixed-ratio 1 (FR 1) schedule until acquisition criteria were met (maximum of 30 sessions). The self-administration criterion for cocaine was ≥20 infusions in a 2-h session and ≥5 infusions in a 1-h session for nicotine. Sex differences were assessed by examining the percentage of rats that met acquisition criteria, the number of sessions to meet criteria, and the number of infusions earned during the maintenance phase.

RESULTS

A significantly higher percentage of male rats acquired both cocaine and nicotine self-administration than females, and males met acquisition criteria in fewer sessions. However, after criteria were met, females self-administered more cocaine than males during the first 5 days of maintenance. There were no sex differences in nicotine infusions post-acquisition.

CONCLUSIONS

Differences in acquisition amongst sexes can reveal factors that are integral to initiation of drug use, an often overlooked phase of drug addiction.

Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice

RATIONALE

Several findings on sex differences in cocaine response suggest a role for hormonal milieu in modulating the subjective effects of cocaine. Nitric oxide (NO) has been involved in the neurochemical, hormonal, and behavioral changes related to stress and anxiety. Within the brain, the anteroventral subdivision of the medial amygdala (MeAV) is an important area involved in processing emotional responses such as anxiety and a high density of NO-producing neurons is observed in this area.

OBJECTIVES

In this study, we hypothesize the possibility of sex/hormonal differences in response to cocaine and that these differences may reflect a change in the MeAV nitrergic system. We have examined cocaine's acute effects on nicotinamide adenine dinucleotide phosphate diaphorase (nadph-d) expression, as well as its effect on motor activity and anxiety in male and estrus and diestrus females.

RESULTS

Our results show that acute cocaine administration produces an increase in both anxiety behaviors and nadph-d expression in the MeAV. Male and diestrus female mice were more susceptible to these effects of cocaine than estrus female mice in which no differences were detected. In addition, we examined individual differences in male and female mice responding to intravenous cocaine reinforcement in a self-administration paradigm. Female mice acquired cocaine self-administration at a faster rate than males and showed a higher motivation to self-administer cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Our data suggest a complex interaction between hormonal milieu and the behavioral and reinforcing effects of cocaine.

Crack cocaine use impairs anterior cingulate and prefrontal cortex function in women with HIV infection

Crack cocaine use is associated with impaired verbal memory in HIV-infected women more than uninfected women. To understand the neural basis for this impairment, this study examined the effects of crack cocaine use on activation of the prefrontal cortex (PFC) and strategic encoding during a verbal memory task in HIV-infected women. Three groups of HIV-infected women from the Chicago Consortium of the Women's Interagency HIV Study were compared: current users of crack cocaine (n = 10), former users of cocaine (n = 11), and women who had never used cocaine (n = 9). Participants underwent functional magnetic resonance imaging during a verbal memory task and completed a neuropsychological test of verbal memory. On the neuropsychological test, current crack users performed significantly worse than other groups on semantic clustering, a measure of strategic encoding, p < 0.05. During encoding, activation in left anterior cingulate cortex (ACC) was lower in current and former cocaine users compared to never users. During recognition, activation in bilateral PFC, specifically left dorsal medial PFC and bilateral dorsolateral PFC, was lower in current and former users compared to women who had never used cocaine. Lower activation in left dorsolateral PFC was correlated with worse performance on the recognition task, p < 0.05. The verbal learning and memory deficits associated with cocaine use in women with HIV may be partially accounted for by alterations in ACC and PFC function.

Sex differences in novelty- and psychostimulant-induced behaviors of C57BL/6 mice

RATIONALE

Women are more sensitive than men to psychostimulants and progress from initial use to drug addiction more quickly. The mouse has been an under-utilized model to study sex differences in psychostimulant action. Mice could serve as an ideal genetically tractable model for mechanistic studies into sex and hormone effects on psychostimulant behavior.

OBJECTIVES

The objective of this study was to characterize psychostimulant effects in male and female mice with a combination of automated data collection and behavioral observation.

METHODS

Male and female C57BL/6 mice (Charles River) were given a single dose or sequential ascending binge doses of D-amphetamine (AMPH) or cocaine (COC). Behavior was assessed in open field chambers using both automated photobeam interruptions and behavioral observations. Brain psychostimulant concentrations were determined at the time of maximum behavioral stimulation.

RESULTS

Psychostimulants induced behavioral activation in mice including both increased locomotion as detected with an automated system and a sequence of behaviors progressing from stereotyped sniffing at low doses to patterned locomotion and rearing at high doses. Females exhibited more patterned locomotion and a shift towards higher behavior scores after either psychostimulant despite having lower AMPH and equivalent COC brain levels as males.

CONCLUSIONS

Female C57BL/6 mice exhibit enhanced psychostimulant-induced behavior compared to males, similar to reports in rats. The combination of automated behavioral measures and behavioral observation was essential for verifying the existence of these differences. These results indicate the importance of testing both sexes when characterizing genetically manipulated mice to control for potential sex-specific effects.

Chronic escalating cocaine exposure, abstinence/withdrawal, and chronic re-exposure: effects on striatal dopamine and opioid systems in C57BL/6J mice

Cocaine addiction is a chronic relapsing disease with periods of chronic escalating self-exposure, separated by periods of abstinence/withdrawal of varying duration. Few studies compare such cycles in preclinical models. This study models an "addiction-like cycle" in mice to determine neurochemical/molecular alterations that underlie the chronic, relapsing nature of this disease. Groups of male C57BL/6J mice received acute cocaine exposure (14-day saline/14-day withdrawal/13-day saline + 1-day cocaine), chronic cocaine exposure (14 day cocaine) or chronic re-exposure (14-day cocaine/14-day withdrawal/14-day cocaine). Escalating-dose binge cocaine (15-30 mg/kg/injection × 3/day, i.p. at hourly intervals) or saline (14-day saline) was administered, modeling initial exposure. In "re-exposure" groups, after a 14-day injection-free period (modeling abstinence/withdrawal), mice that had received cocaine were re-injected with 14-day escalating-dose binge cocaine, whereas controls received saline. Microdialysis was conducted on the 14th day of exposure or re-exposure to determine striatal dopamine content. Messenger RNA levels of preprodynorphin (Pdyn), dopamine D1 (Drd1) and D2 (Drd2) in the caudate putamen were determined by real-time PCR. Basal striatal dopamine levels were lower in mice after 14-day escalating exposure or re-exposure than in those in the acute cocaine group and controls. Pdyn mRNA levels were higher in the cocaine groups than in controls. Long-term adaptation was observed across the stages of this addiction-like cycle, in that the effects of cocaine on dopamine levels were increased after re-exposure compared to exposure. Changes in striatal dopaminergic responses across chronic escalating cocaine exposure and re-exposure are a central feature of the neurobiology of relapsing addictive states.

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

RATIONALE

Social factors are important determinants of drug dependence and relapse.

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Psychomotor stimulant effects of cocaine in rats and 15 mouse strains

Relative to intravenous drug self-administration, locomotor activity is easier to measure with high throughput, particularly in mice. Therefore its potential to predict differences in self-administration between genotypes (e.g., targeted mutations, recombinant inbred strains) is appealing, but such predictive value is unverified. The main goal of this study was to evaluate the utility of the locomotor assay for accurately predicting differences in cocaine self-administration. A second goal was to evaluate any correlation between activity in a novel environment, and cocaine-induced hyperactivity, between strains. We evaluated locomotor activity in male and female Sprague-Dawley rats and 15 mouse strains (129S1/SvImJ, 129S6/SvEvTac, 129X1/SvJ, A/J, BALB/cByJ, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, SJL/J, SPRET/EiJ, and outbred Swiss Webster and CD-1/ICR), as well as cocaine self-administration in BALB substrains. All but BALB/cJ mice showed locomotor habituation and significant cocaine-induced hyperactivity. BALB/cJ mice also failed to self-administer cocaine. BALB/cByJ mice showed modest locomotor habituation, cocaine-induced locomotion, and cocaine self-administration. As previously reported, female rats showed greater cocaine-induced locomotion than males, but this was only observed in one of 15 mouse strains (FVB/NJ), and the reverse was observed in two strains (129X1/SvJ, BALB/cByJ). The intriguing phenotype of the BALB/cJ strain may indicate some correlation between all-or-none locomotion in a novel environment, and stimulant and reinforcing effects of cocaine. However, neither novelty- nor cocaine-induced activity offered a clear prediction of relative reinforcing effects among strains. Additionally, these results should aid in selecting mouse strains for future studies in which relative locomotor responsiveness to psychostimulants is a necessary consideration.

Acute locomotor responses to cocaine in adolescents vs. adults from four divergent inbred mouse strains

Growing evidence suggests that adolescent mice display differential sensitivity to the acute locomotor activating effects of cocaine as compared to adults, but the direction of the difference varies across studies and the reasons are not clear. Few studies have directly examined genetic contributions to age differences in locomotor stimulation from cocaine. The goal of this study was to determine the extent to which reduced stimulation in C57BL/6J adolescents as compared to adults generalizes to other strains. Therefore, we examined male and female mice from four genetically divergent inbred stains (BALB/cByJ, C57BL/6J, DBA/2J and FVB/NJ) at two ages, postnatal day 30 and postnatal day 65. Mice received either saline or cocaine (15 or 30 mg/kg), and then immediately were placed back into their home cages. Locomotor activity was recorded continuously in the home cage by video tracking. Adolescents displayed reduced stimulation as compared to adults for C57BL/6J, BALB/cByJ and female FVB/NJ mice. No age differences were observed for DBA/2J or male FVB/NJ. No main effects of sex were observed. Strain differences in pharmacokinetics, neural development or physiology could contribute to the observed differences between ages across strains. Future comparative studies could discover biological differences between strains that explain age differences in cocaine sensitivity.

Comparison of toluene-induced locomotor activity in four mouse strains

The mechanisms by which abused inhalants exert their neurobehavioral effects are only partially understood. In research with other drugs of abuse, specific inbred mouse strains have been useful in exploring genetic loci important to variation in behavioral reactions to these drugs. In the present investigation, mice from three inbred strains (Balb/cByj, C57BL/6J and DBA/2J) and one outbred strain (Swiss Webster) were studied for their acute and chronic sensitivity to toluene-induced changes in locomotor activity. Mice were exposed to toluene (0, 100, 2000, 8000, and 10,000 ppm) for 30 min in static exposure chambers equipped with activity monitors. In the acute condition, concentrations of toluene <8000 ppm increased ambulatory distance while the concentrations of > or =8000 ppm induced temporally biphasic effects with initial increases in activity followed by hypoactivity. Between-group differences in absolute locomotor activity levels were evident. The inbred Balb/cByj and DBA/2J strains as well as the outbred Swiss Webster strain of mice showed greater increases in activity after an acute challenge exposure to 2000 ppm than the inbred C57BL/6J strain. The same animals were then exposed 30 min/day to 8000 ppm toluene for 14 consecutive days. Re-determination of responses to 2000-ppm challenge exposures revealed that sensitization developed in locomotor activity and that the DBA/2J strain showed the greatest increase in sensitivity. These baseline differences in acute sensitivity and the differential shifts in sensitivity after repeated exposures among the inbred mouse strains suggest a genetic basis for the behavioral effects to toluene. The results support the notion that like for other drugs of abuse, using various strains of mice may be useful for investigating mechanisms that underlie risk for inhalant abuse.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Are DBA/2 mice associated with schizophrenia-like endophenotypes? A behavioural contrast with C57BL/6 mice

RATIONALE

Due to its intrinsic deficiency in prepulse inhibition (PPI), the inbred DBA/2 mouse strain has been considered as an animal model for evaluating antipsychotic drugs. However, the PPI impairment observed in DBA/2 mice relative to the common C57BL/6 strain is confounded by a concomitant reduction in baseline startle reactivity. In this study, we examined the robustness of the PPI deficit when this confound is fully taken into account.

MATERIALS AND METHODS

Male DBA/2 and C57BL/6 mice were compared in a PPI experiment using multiple pulse stimulus intensities, allowing the possible matching of startle reactivity prior to examination of PPI. The known PPI-enhancing effect of the antipsychotic, clozapine, was then evaluated in half of the animals, whilst the other half was subjected to two additional schizophrenia-relevant behavioural tests: latent inhibition (LI) and locomotor reaction to the psychostimulants-amphetamine and phencyclidine.

RESULTS

PPI deficiency in DBA/2 relative to C57BL/6 mice was essentially independent of the strain difference in baseline startle reactivity. Yet, there was no evidence that DBA/2 mice were superior in detecting the PPI-facilitating effect of clozapine when startle difference was balanced. Compared with C57BL/6 mice, DBA/2 mice also showed impaired LI and a different temporal profile in their responses to amphetamine and phencyclidine.

CONCLUSION

Relative to the C57BL/6 strain, DBA/2 mice displayed multiple behavioural traits relevant to schizophrenia psycho- and physiopathology, indicative of both dopaminergic and glutamatergic/N-methyl-D: -aspartic acid receptor dysfunctions. Further examination of their underlying neurobiological differences is therefore warranted in order to enhance the power of this specific inter-strain comparison as a model of schizophrenia.

Sex differences in drug addiction: a review of animal and human studies

Addiction research has historically neglected research on women, and most studies have been conducted on men only, with the concluding results generalized to the female population. The role of sex differences in vulnerability to drug abuse, their repercussions on prevention and treatment strategies all require detailed studies, as does the progression from recreational drug use to dependence. This review synthesizes evidence of gender differences in drug addiction, with particular emphasis on women's health and implications. We first reviewed behavioral studies showing sex differences in the preference for and self-administration of licit (i.e., alcohol and nicotine) and illicit (i.e., cocaine, amphetamine, heroin and cannabis) substances as revealed by animal models of addiction. Clinical studies demonstrating differences between men and women in craving, drug use, abstinence and relapse will then be examined. For both animal and human studies, the effects of hormones and estrous/menstrual cycle will be reviewed. Finally, neurobiological factors underlying gender differences in vulnerability to drug addiction (i.e., brain morphology and neurotransmission) and need for gender-specific detoxification treatments will be discussed.

Modeling the role of environment in addiction

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

Enhanced acquisition of cocaine self-administration by increasing percentages of C57BL/6J genes in mice with a nonpreferring outbred background

RATIONALE

Individual differences in the propensity to acquire drug self-administration may have a substantial genetic basis.

OBJECTIVES

To study the genetic contribution to cocaine self-administration by comparing hybrids of cocaine preferring (C57BL/6J) and nonpreferring (ICR) mice.

METHODS

ICR and C57BL/6J parental strains were compared to hybrids with 75% ICR:25% C57BL/6J, 50% ICR:50% C57BL/6J, and 25% ICR:75% C57BL/6J genetic backgrounds for acquisition of sucrose pellet and intravenous cocaine self-administration in 1-h test sessions. Mice that acquired cocaine self-administration were subsequently tested in a between-session self-administration dose-response procedure.

RESULTS

Increasing presence of C57BL/6J genes increased the percentage of mice that acquired sucrose pellet self-administration in the first test session. In lever-trained mice, only 19% of ICR mice met acquisition criteria for cocaine self-administration after 15 sessions, whereas 76% of C57BL/6J mice met acquisition criteria, although both strains initially sampled a similar number of cocaine injections. Increasing the percentage of C57BL/6J genes in the nonpreferring ICR background to 50 and 75% led to increasing percentages of mice that met acquisition criteria to 31 and 52%, respectively. In mice that acquired self-administration, only mice with 75% C57BL/6J genes showed a typical inverted U-shaped self-administration dose-response curve, whereas the curve was flat across doses for mice with < or = 50 and 100% C57BL/6J genes.

CONCLUSIONS

The findings are consistent with a genetically based dose-dependent enhancement of cocaine reinforcement by C57BL/6J genes. These results suggest that heritable traits impart a substantial genetic load that facilitates the propensity for cocaine addiction among individuals in outbred populations.

The influence of sex on extracellular dopamine and locomotor activity in C57BL/6J mice before and after acute cocaine challenge

C57BL6/J (C57) mice serve as a useful animal model of cocaine abuse because they self-administer cocaine, exhibit place conditioning to cocaine, discriminate the interoceptive cues of cocaine, and are used for backcrossing strains of genetically modified mice. The present study was to examine the influence of sex on extracellular DA and locomotor activity in C57 mice in response to acute cocaine challenge. In the first experiment, male and female mice were implanted with guide cannulae aimed at the dorsal striatum. Microdialysates were collected in three consecutive phases: baseline, post-saline injection, and post-cocaine injection. Sex did not influence DA measurements during baseline or after intraperitoneal (i.p.) saline injection. Cocaine (20 mg/kg) injections increased peak extracellular DA of both sexes, and the increase was greater for males (278%+/-14.0%) than females (182.5%+/-10.8%) (P<0.05). In the second experiment, under conditions similar to the microdialysis experiment, locomotor activity of male and female mice was assessed during baseline, after saline injection, and after cocaine injection (5, 10, 20, or 40 mg/kg). Cocaine dose-dependently increased activity; however, sex did not influence locomotor activity during baseline, after saline, or after any cocaine dose. Results of the experiments established that cocaine (20 mg/kg) increased extracellular DA in the dorsal striatum to a greater extent in male than in female mice; however, when cocaine was administered under similar experimental conditions, sex did not influence cocaine stimulation of locomotor activity over a wide range of doses.

Sex differences in the escalation of intravenous cocaine intake following long- or short-access to cocaine self-administration

Preclinical data have indicated that extended access to cocaine self-administration (e.g., 6-12 h/day) facilitates an escalation in daily cocaine intake that is not seen when rats are given shorter (e.g., 1-2 h/day) access to cocaine for self-administration. Data from studies with rats have shown that females self-administer more cocaine than males during all phases of drug abuse (e.g., acquisition, maintenance, and reinstatement). The purpose of this study was to examine potential differences between males and females in the escalation of intravenous cocaine intake following a differential access (e.g., 1 vs. 6 h) period of cocaine self-administration. Four groups of rats were compared: (1) long-access (LgA; 6 h) females; (2) LgA males; (3) short-access (ShA; 1 h) females; and (4) ShA males. Animals were given LgA or ShA to intravenous cocaine (0.5 mg/kg/infusion) self-administration under an Fr 1 schedule for 21 days. Subsequently, access conditions were made equal (3 h) across groups, and dose-response curves for cocaine were compared. Results revealed that the LgA groups' dose-response curves were significantly elevated above those of ShA groups. Additionally, the dose-response curve of LgA female rats was significantly elevated above that of LgA male rats. These results suggest that female rats are more sensitive than male rats to factors that contribute to the escalation of cocaine intake (e.g., extended access conditions).

Influence of estrogen in the acquisition of intravenously self-administered heroin in female rats

Previous research indicates that female rats acquire cocaine and heroin self-administration at a faster rate than male rats, and female rats with endogenous estrogen, or ovariectomized (OVX) rats with estrogen replacement acquire cocaine self-administration more rapidly than female rats with estrogen either surgically or chemically blocked. The purpose of this investigation was to extend the above findings to the acquisition of heroin (0.0075 mg/kg) self-administration in female rats. An automated autoshaping procedure was used to train rats to self-administer heroin. Three groups of female rats were compared: (1) OVX+estradiol benzoate (OVX+EB), (2) OVX+vehicle (OVX+VEH), and (3) sham-operated+vehicle (SH+VEH). Results revealed that OVX+EB rats acquired heroin self-administration in significantly fewer days compared to OVX+VEH rats. Additionally, OVX+EB rats that met the acquisition criteria self-administered a significantly greater number of heroin infusions during the last 5 days of the acquisition period compared to OVX+VEH rats. These results indicate that OVX+EB rats initiate heroin use sooner than OVX+VEH rats and consume greater amounts of heroin during the last 5 days of acquisition compared to OVX+VEH female rats.

Sex differences in the effects of baclofen on the acquisition of intravenous cocaine self-administration in rats

Baclofen, a GABA(B) agonist, decreases both the maintenance and reinstatement of i.v. cocaine-reinforced responding in rats. In the present experiment the effects of baclofen were extended to a comparison of male and female rats during the acquisition of i.v. cocaine self-administration. Four groups of rats were trained to self-administer i.v. cocaine (0.2 mg/kg) under a fixed-ratio 1 (FR 1) schedule using an autoshaping procedure. The criterion for acquisition was a 5-day period during which a mean of 100 cocaine infusions were administered. Rats were given 30 days to reach this criterion. Male and female groups (n=10-13) were pretreated with i.p. injections of baclofen (2.5 mg/kg) or vehicle 30-min prior to the sessions. A subset of rats (N=5) that did not acquire cocaine self-administration continued to be exposed to the acquisition procedure after baclofen treatment ended. Pretreatment with baclofen decreased both the rate of acquisition of cocaine self-administration and the percentage of rats meeting the acquisition criterion to a greater extent in females than in males. Female rats that did not meet the acquisition criterion with baclofen treatment, acquired within a few days after treatment ended. The findings confirm previous reports of enhanced acquisition of cocaine self-administration in females versus males, and they indicate that baclofen suppressed the acquisition of cocaine self-administration significantly more in females than in males.

Differential effects of cocaine on local cerebral glucose utilization in the mouse and in the rat

The aim of the study was to determine whether cocaine causes a different pattern of functional changes in the rat as compared to the mouse. The [(14)C]2-deoxyglucose method, for measuring local cerebral metabolic rates for glucose, was carried out in Sprague-Dawley rats and in two strains of mice, C57BL/6 and DBA/2, following a single intravenous administration of cocaine. Cocaine, according to previous reports, increased glucose utilization in the nucleus accumbens of the rat, while the drug decreased metabolic rates in most of brain areas of both strains of mice. The post-hoc analysis, however, suggests that the pattern of metabolic changes differ in the two strains. In particular, the effect on the shell of the nucleus accumbens was present in the C57, but not in the DBA mice. As the C57 mice are more likely than DBA to initiate cocaine self-administration, the effect on the nucleus accumbens support the role of the mesolimbic pathway in mediating the motivational properties of psychostimulants.

Behavioral despair in mice after prenatal stress

Maternal stress during pregnancy produced behavioral alterations in both sexes with regard to sexual behavior, aggressive, maternal, lateralization and depression. In the present paper, sex differences for depression in mice was studied. No sex differences between female and male mice groups were observed either in swimming-induced immobility or in the open-field test (ambulation, rearing and boluses). Prenatal stress produced: 1) an increase of immobility time in female mice for swimming-induced immobility, but not in male mice; 2) an increase of ambulation in female mice for open-field test, but not in male mice; 3) there were no significant differences in rearing and boluses between stress and control groups either for female or male mice. Prenatal stress increases the risk of depression and locomotor activity in adult female mice.

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

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

Ethanol modulates cocaine-induced behavioral change in inbred mice

We recently conducted a study of the behavioral effects of combined cocaine and ethanol in genetically defined mice. Male and female C57BL/6 (B6) and DBA/2 (D2) were tested in an automated activity monitor on 2 consecutive days. On day 1, all animals received an IP injection of sterile saline and were placed into the activity monitor for 30 min. Behaviors measured were total distance traveled, stereotypy, nosepokes, and wall-seeking. On day 2, all animals were tested again for 15 min following injection of one of the following: saline, 10% v/v ethanol at 2.0 g kg(-1) or 2.0 g kg(-1) ethanol plus 5, 15, or 30 mg kg(-1) cocaine. Cocaine alone at the same doses was injected into separate groups of animals. For the B6 strain, the overall effect of ethanol was to reduce cocaine-induced locomotor stimulation; no consistent effect of ethanol on cocaine-induced locomotion was observed in D2 mice. Cocaine-induced inhibition of nosepokes in both strains and sexes was partially reversed by ethanol. Ethanol also partially reversed cocaine-elevated stereotypy in both strains and both sexes. In B6 mice, cocaine-increased wall seeking tended to be reversed by coadministration of ethanol, whereas no consistent pattern was observed in the D2s. Results from this study suggest that the several measures affected by cocaine (locomotor activity, stereotypy, exploration, thigmotaxis) were, in turn, differentially affected by concurrent treatment with ethanol. Furthermore, our results point to genetic-based differences in ethanol's effects on cocaine-related behaviors. We address the implications for combined ethanol and cocaine use in humans.

Distribution and clearance of cocaine in brain is influenced by genetics

The purpose of this study was to examine the pharmacokinetics of cocaine in two inbred mouse strains, C57BL/6 (B6) and DBA/2 (D2). Male and female mice were administered 30 mg kg(-1) cocaine IP and killed after 5, 15, 30, or 60 minutes postinjection. Brains were removed quickly and assayed for total brain cocaine concentration. Quantification of cocaine was conducted using gas chromatography and mass spectrometry. The results of this study revealed a strain difference in total brain cocaine kinetics. Specifically, we observed that at 5 min onward, B6 mice cleared cocaine from the brain with a t1/2 estimated at 22.3 min, while distribution in D2 mice appeared to be incomplete until 15 min with a subsequent t1/2 estimated at 11.2 min. These results show that despite faster clearance by D2 mice, the prolonged time to distribution in this strain may help explain why D2 mice show initial greater locomotor activation by cocaine, compared to B6s.

Inbred mouse strains vary in oral self-selection of nicotine

Inbred mouse strains differ in sensitivity to a first dose of nicotine and in the development of tolerance to nicotine. The experiments reported here used six inbred mouse strains (A, BUB, C3H, C57BL/6, DBA/2, ST/b) that differ in sensitivity to an acute challenge dose of nicotine to determine whether differences in oral self-selection of nicotine exist. Animals were presented with solutions containing nicotine or vehicle (water or 0.2% saccharin) and their daily intake of the two fluids was measured for 4 days starting with a 10 micrograms/ml nicotine solution. This was followed by sequential 4-day testing with 20, 35, 50, 65, 80, 100, 125, 160 and 200 micrograms/ml nicotine solutions. The strains differed dramatically in their self-selection of nicotine and in maximal daily dose (mg/kg); the rank order of the strains was C57BL/6 > DBA > BUB > A > or = C3H > or = ST/b for both the tap water and 0.2% saccharin choice experiments. Correlations between nicotine consumption and sensitivity to nicotine, as measured by a battery of behavioral and physiological responses, were also calculated. Strain differences in nicotine intake were highly correlated with sensitivity to nicotine-induced seizures. As sensitivity to nicotine-induced seizures increases, oral self-selection of nicotine decreases. This finding may suggest that this toxic action of nicotine serves to limit intake.

Behavioral responses to low doses of cocaine are affected by genetics and experimental history

We recently conducted a set of two experiments to investigate the possible co-operation between genetics and exposure to novelty on the putative locomotor inhibiting effects of low doses of cocaine in male and female C57BL/6 and DBA/2 mice. Experiment one examined the effects of three low doses of cocaine (0.1, 0.5, and 1.0 mg/kg) on locomotion, exploration, stereotypy and wall-seeking in an automated activity monitor. Testing occurred on two consecutive days, with subjects receiving an IP injection of saline on day one, and one dose of cocaine on day 2 (S-C). Immediately following injection, subjects were placed into automated activity monitors, where four behaviors were recorded; total distance, nosepokes, stereotypy and margin time. Using this S-C injection regimen, we found significant decreases in measures of total distance and stereotypy when compared to saline in both male and female C57 mice. Experiment two was designed to determine if the observed decrease in locomotor activity was the result of low-dose cocaine or pre-exposure to the test procedure and apparatus. All conditions and procedures were identical to those in experiment one, with the exception of the injection regimen. In this experiment, we injected all subjects IP with 0.1 mg/kg cocaine on day one, followed by saline on day two (C-S). Additionally, a group of subjects receiving saline on both days (S-S) served as the control. In contrast to experiment one results, cocaine produced locomotor activation. Furthermore, significant sex and strain differences were found in both experiments. The results of our experiments suggest that the behavioral effects of low doses of cocaine are markedly influenced by both the genetic constitution of the experimental animal and by familiarity with the test apparatus.

Ethanol, nicotine, amphetamine, and aspartame consumption and preferences in C57BL/6 and DBA/2 mice

Using a two-bottle choice paradigm, adult C57BL/6 and DBA/2 mice (11 males an 10 females per strain) were given access to tapwater and an ascending series of concentrations of ethanol, nicotine, amphetamine, and th artificial sweetener, aspartame. The C57 mice consumed more ethanol, nicotine, and amphetamine, and showed greater preferences for these substances, than did the DBA/2 mice. In contrast, DBAs consumed more and showed greater preference for aspartame than C57s. However, measures of drug and aspartame consumption and preference were moderately intercorrelated when the effects of gender and strain were controlled for. This pattern of results suggests that factors modulating differences between C57BL/6 and DBA/2 mice in ethanol consumption and preference also modulate differences in consumption of nicotine and amphetamine.