Effects of a non-drug reinforcer, saccharin, on oral self-administration of phencyclidine in male and female rhesus monkeys

Discriminating goal-directed and habitual cocaine seeking in rats using a novel outcome devaluation procedure

Habits are theorized to play a key role in compulsive cocaine seeking, yet there is limited methodology for assessing habitual responding for intravenous (IV) cocaine. We developed a novel outcome devaluation procedure to discriminate goal-directed from habitual responding in cocaine-seeking rats. This procedure elicits devaluation temporarily and requires no additional training, allowing repeated testing at different time points. After training male rats to self-administer IV cocaine, we devalued the drug outcome via experimenter-administered IV cocaine (a "satiety" procedure) prior to a 10-min extinction test. Many rats were sensitive to outcome devaluation, a hallmark of goal-directed responding. These animals reduced responding when given a dose of experimenter-administered cocaine that matched or exceeded satiety levels during self-administration. However, other rats were insensitive to experimenter-administered cocaine, suggesting their responding was habitual. Importantly, reinforcement schedules and neural manipulations that produce goal-directed responding (i.e., ratio schedules or dorsolateral striatum lesions) caused sensitivity to outcome devaluation, whereas reinforcement schedules and neural manipulations that produce habitual responding (i.e., interval schedules or dorsomedial striatum lesions) caused insensitivity. Satiety-based outcome devaluation is an innovative new tool to dissect the neural and behavioral mechanisms underlying IV cocaine-seeking behavior.

Psychometric evaluation of the Reward Probability Index in emerging adult drinkers

OBJECTIVE

Diminished access to environmental rewards is an established risk factor for addiction and a focus of many effective treatment approaches. Nevertheless, there is inconsistency in measurement approaches and a need for a psychometrically sound measure. The Reward Probability Index (RPI; Carvalho, Behavior Therapy, 42, 2011, pp. 249-262) is a 20-item self-report rating scale that measures access to and ability to experience psychosocial reward.

METHOD

The current studies sought to evaluate the psychometric properties of the RPI in 2 samples of emerging adult heavy drinkers.

RESULTS

In Study 1, exploratory factor analysis in a sample of 393 college student drinkers supported a 2-factor model of the RPI (Reward Probability and Environmental Suppressors) after removal of redundant items, and corresponding subscales demonstrated good internal consistency. In Study 2, confirmatory factor analysis with 602 emerging adult drinkers recruited from the community supported the 2-factor model as best fitting after removal of one poor indicator, although absolute fit was only adequate. This 2-factor model demonstrated configural, metric, and scalar invariance across non-college and college subgroups as well as Black and White subgroups. Study 2 also demonstrated that the revised RPI subscales showed significant associations with measures of substance-free activity participation and enjoyment, anhedonia, and depressive symptoms. Furthermore, the study revealed the RPI Environmental Suppressors subscale predicted alcohol-related problems (β = .25, p < .001) beyond demographic covariates, weekly drinking, and depressive symptoms.

CONCLUSIONS

These studies provide evidence for the validity of the RPI as an efficient measure of access to reward among emerging adult heavy drinkers. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Behavioral economic demand as a unifying language for addiction science: Promoting collaboration and integration of animal and human models

The intersection of pharmacological, psychological, and economic theory within behavioral economics has helped advance an understanding of substance use disorder. A notable contribution of this approach is the conceptualization of reinforcement from a behavioral economic demand perspective. Demand analyses provide a multidimensional view of reinforcement in which distinct behavioral mechanisms are measured that impact decision making and drug consumption. This review describes the state of research on behavioral economic demand as a common language for addiction science researchers across varied model systems and stages of a translational continuum. We first provide an overview of the theoretical concepts and procedures used to evaluate demand in animal and human models. The potential for demand to serve as a common language for diverse research groups in psychopharmacology and addiction science (e.g., those evaluating neurobehavioral outcomes, medications development, clinical practice) is then described. An overview is also provided of existing empirical studies that, while small in number, suggest good linguistic and conceptual overlap between animal and human demand models when studying biological, environmental, and pharmacological individual difference vulnerabilities underlying drug-taking behavior. Refinement of methodological procedures and incorporation of more nuanced environmental features should help improve correspondence between animal and human demand studies as well as clinical translation of such findings. Our hope is that this review and commentary ultimately serves as inspiration for new collaborative efforts involving behavioral economic demand between animal and human researchers who share a common goal of improving substance use treatment outcomes and broader psychological wellbeing. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

The therapeutic potential of exercise for neuropsychiatric diseases: A review

Exercise is known to have a myriad of health benefits. There is much to be learned from the effects of exercise and its potential for prevention, attenuation and treatment of multiple neuropsychiatric diseases and behavioral disorders. Furthermore, recent data and research on exercise benefits with respect to major health crises, such as, that of opioid and general substance use disorders, make it very important to better understand and review the mechanisms of exercise and how it could be utilized for effective treatments or adjunct treatments for these diseases. In addition, mechanisms, epigenetics and sex differences are examined and discussed in terms of future research implications.

How to study sex differences in addiction using animal models

The importance of studying sex as a biological variable in biomedical research is becoming increasingly apparent. There is a particular need in preclinical studies of addiction to include both sexes, as female animals are often excluded from studies, leaving large gaps in our knowledge of not only sex differences and potential prevention and treatment strategies but also with regard to the basic neurobiology of addiction. This review focuses on methodology that has been developed in preclinical studies to examine sex differences in the behavioral aspects and neurobiological mechanisms related to addiction across the full range of the addiction process, including initiation (acquisition), maintenance, escalation, withdrawal, relapse to drug seeking and treatment. This review also discusses strategic and technical issues that need to be considered when comparing females and males, including the role of ovarian hormones and how sex differences interact with other major vulnerability factors in addiction, such as impulsivity, compulsivity and age (adolescent versus adult). Novel treatments for addiction are also discussed, such as competing non-drug rewards, repurposed medications such as progesterone and treatment combinations. Practical aspects of conducting research comparing female and male animals are also considered. Making sex differences a point of examination requires additional effort and consideration; however, such studies are necessary given mounting evidence demonstrating that the addiction process occurs differently in males and females. These studies should lead to a better understanding of individual differences in the development of addiction and effective treatments for males and females.

Sex and menstrual cycle effects on chronic oral cocaine self-administration in rhesus monkeys: Effects of a nondrug alternative reward

BACKGROUND

In previous studies, female monkeys self-administered more oral phencyclidine (PCP) than males, and PCP intake differed by phase of menstrual cycle.

OBJECTIVES

The purpose of this study was to examine sex and hormonal influences on oral cocaine self-administration in male and female rhesus monkeys in the follicular vs. luteal phases of the menstrual cycle, with concurrent access to an alternative nondrug reward, saccharin (SACC) vs. water.

MATERIALS AND METHODS

Concurrent access to cocaine (0.2, 0.4, and 0.8 mg/ml) and SACC or water was available from two drinking spouts under concurrent fixed-ratio (FR) 2, 4, and 8 schedules during daily 3-h sessions.

RESULTS

Cocaine deliveries were similar in males and females in the females' luteal phase, but cocaine deliveries were higher in females during the follicular phase than the luteal phase and compared to males. When SACC was available, cocaine deliveries were reduced in females in the follicular phase of the cycle, and cocaine intake (mg/kg) was reduced in males and in females' follicular and luteal phases.

CONCLUSIONS

Access to concurrent SACC (vs. water) reduced cocaine intake (mg/kg) in males and in females during both menstrual phases, and the magnitude of the reduction in cocaine intake was greatest during the females' follicular phase. Thus, a nondrug alternative reward, SACC, is a viable alternative treatment for reducing cocaine's rewarding effects on male and female monkeys, and reductions in cocaine seeking were optimal in the females' luteal phase.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

Comparison of dehydroepiandrosterone (DHEA) and pregnanolone with existing pharmacotherapies for alcohol abuse on ethanol- and food-maintained responding in male rats

The present study compared two putative pharmacotherapies for alcohol abuse and dependence, dehydroepiandrosterone (DHEA) and pregnanolone, with two Food and Drug Administration (FDA)-approved pharmacotherapies, naltrexone and acamprosate. Experiment 1 assessed the effects of different doses of DHEA, pregnanolone, naltrexone, and acamprosate on both ethanol- and food-maintained responding under a multiple fixed-ratio (FR)-10 FR-20 schedule, respectively. Experiment 2 assessed the effects of different mean intervals of food presentation on responding for ethanol under a FR-10 variable-interval (VI) schedule, whereas Experiment 3 assessed the effects of a single dose of each drug under a FR-10 VI-80 schedule. In Experiment 1, all four drugs dose-dependently decreased response rate for both food and ethanol, although differences in the rate-decreasing effects were apparent among the drugs. DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding, whereas the reverse was true for naltrexone. Acamprosate decreased responding for both reinforcers with equal potency. In Experiment 2, different mean intervals of food presentation significantly affected the number of food reinforcers obtained per session; however, changes in the number of food reinforcements did not significantly affect responding for ethanol. Under the FR-10 VI-80 schedule in Experiment 3, only naltrexone significantly decreased both the dose of alcohol presented and blood ethanol concentration (BEC). Acamprosate and pregnanolone had no significant effects on any of the dependent measures, whereas DHEA significantly decreased BEC, but did not significantly decrease response rate or the dose presented. In summary, DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding under a multiple FR-10 FR-20 schedule, and were more selective for decreasing ethanol self-administration than either naltrexone or acamprosate under that schedule. Experiment 2 showed that ethanol intake was relatively independent of the interval of reinforcement in the food-maintained component, and Experiment 3 showed that naltrexone was the most effective drug at the doses tested when the interval for food reinforcement was low and maintained under a variable-interval schedule.

The behavioral economics of substance use disorders: reinforcement pathologies and their repair

The field of behavioral economics has made important inroads into the understanding of substance use disorders through the concept of reinforcer pathology. Reinforcer pathology refers to the joint effects of (a) the persistently high valuation of a reinforcer, broadly defined to include tangible commodities and experiences, and/or (b) the excessive preference for the immediate acquisition or consumption of a commodity despite long-term negative outcomes. From this perspective, reinforcer pathology results from the recursive interactions of endogenous person-level variables and exogenous environment-level factors. The current review describes the basic principles of behavioral economics that are central to reinforcer pathology, the processes that engender reinforcer pathology, and the approaches and procedures that can repair reinforcement pathologies. The overall goal of this review is to present a new understanding of substance use disorders as viewed by recent advances in behavioral economics.

Cocaine-, caffeine-, and stress-evoked cocaine reinstatement in high vs. low impulsive rats: treatment with allopregnanolone

BACKGROUND

Previous research indicates that individual differences in traits such as impulsivity, avidity for sweets, and novelty reactivity are predictors of several aspects of drug addiction. Specifically, rats that rank high on these behavioral measures are more likely than their low drug-seeking counterparts to exhibit several characteristics of drug-seeking behavior. In contrast, initial work suggests that the low drug-seeking animals are more reactive to negative events (e.g., punishment and anxiogenic stimuli). The goal of this study was to compare high and low impulsive rats on reinstatement of cocaine-seeking behavior elicited by cocaine (COC) and by negative stimuli such as the stress-inducing agent yohimbine (YOH) or a high dose of caffeine (CAFF). An additional goal was to determine whether treatment with allopregnanolone (ALLO) would reduce reinstatement (or relapse) of cocaine-seeking behavior under these priming conditions.

METHODS

Female rats were selected as high (HiI) or low (LoI) impulsive using a delay-discounting task. After selection, they were allowed to self-administer cocaine for 12 days. Cocaine was then replaced with saline, and rats extinguished lever responding over 16 days. Subsequently, rats were pretreated with either vehicle control or ALLO, and cocaine seeking was reinstated by injections of COC, CAFF, or YOH.

RESULTS

While there were no phenotype differences in maintenance and extinction of cocaine self-administration or reinstatement under control treatment conditions, ALLO attenuated COC- and CAFF-primed reinstatement in LoI but not HiI rats.

CONCLUSIONS

Overall, the present findings suggest that individual differences in impulsive behavior may influence efficacy of interventions aimed to reduce drug-seeking behavior.

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.

Exercise as a novel treatment for drug addiction: a neurobiological and stage-dependent hypothesis

Physical activity, and specifically exercise, has been suggested as a potential treatment for drug addiction. In this review, we discuss clinical and preclinical evidence for the efficacy of exercise at different phases of the addiction process. Potential neurobiological mechanisms are also discussed focusing on interactions with dopaminergic and glutamatergic signaling and chromatin remodeling in the reward pathway. While exercise generally produces an efficacious response, certain exercise conditions may be either ineffective or lead to detrimental effects depending on the level/type/timing of exercise exposure, the stage of addiction, the drug involved, and the subject population. During drug use initiation and withdrawal, its efficacy may be related to its ability to facilitate dopaminergic transmission, and once addiction develops, its efficacy may be related to its ability to normalize glutamatergic and dopaminergic signaling and reverse drug-induced changes in chromatin via epigenetic interactions with brain-derived neurotrophic factor (BDNF) in the reward pathway. We conclude with future directions, including the development of exercise-based interventions alone or as an adjunct to other strategies for treating drug addiction.

Cocaine self-administration punished by i.v. histamine in rat models of high and low drug abuse vulnerability: effects of saccharin preference, impulsivity, and sex

A key feature of substance use disorders is continued drug consumption despite aversive consequences. This has been modeled in the animal laboratory by pairing drug self-administration with electric shock, thereby punishing drug intake (Deroche-Gamonet et al. 2004). In the present experiments, we examined the effects of punishment on i.v. cocaine self-administration by adding histamine to the cocaine solution with three different animal models of high and low vulnerability to drug abuse: rats selectively bred for high (HiS) and low (LoS) saccharin consumption, rats selected for high (HiI) and low (LoI) impulsivity, and sex differences. Animals were allowed to self-administer cocaine (0.4 mg/kg/infusion) to establish a baseline of operant responding. Histamine (4.0mg/kg/infusion) was then added directly into the cocaine solution and its consequent effects on self-administration were compared to baseline. The histamine+cocaine solution was then replaced with a cocaine-only solution, and the rats' operant responding was again compared to baseline. Concurrent histamine exposure was effective in reducing cocaine consumption in all groups of rats; however, LoS and female rats took longer to return to baseline levels of cocaine consumption after histamine was removed compared to HiS and male rats. These data suggest that the reduction of drug self-administration by aversive consequences may differ in groups that vary in drug use vulnerability . Such results may inform pharmacological strategies that enhance the negative aspects of drug consumption.

Sex differences in selecting between food and cocaine reinforcement are mediated by estrogen

Cocaine-dependent women, relative to their male counterparts, report shorter cocaine-free periods and report transiting faster from first use to entering treatment for addiction. Similarly, preclinical studies indicate that female rats, particularly those in the estrus phase of their reproductive cycle, show increased operant responding for cocaine under a wide variety of schedules. Making maladaptive choices is a component of drug dependence, and concurrent reinforcement schedules that examine cocaine choice offers an animal model of the conditions of human drug use; therefore, the examination of sex differences in decision-making may be critical to understanding why women display a more severe profile of cocaine addiction than men. Accordingly, we assessed sex and estrous cycle differences in choice between food (45 mg grain pellets) and intravenous cocaine (0.4 or 1.0 mg/kg per infusion) reinforcement in male, female (freely cycling), and ovariectomized (OVX) females treated with either estrogen benzoate (EB; 5 μg per day) or vehicle. At both cocaine doses, intact female rats choose cocaine over food significantly more than male rats. However, the estrous cycle did not impact the level of cocaine choice in intact females. Nevertheless, OVX females treated with vehicle exhibited a substantially lower cocaine choice compared with those receiving daily EB or to intact females. These results demonstrate that intact females have a greater preference for cocaine over food compared with males. Furthermore, this higher preference is estrogen-dependent, but does not vary across the female reproductive cycle, suggesting that ovarian hormones regulate cocaine choice. The present findings indicate that there is a biological predisposition for females to forgo food reinforcement to obtain cocaine reinforcement, which may substantially contribute to women experiencing a more severe profile of cocaine addiction than men.

Effects of progesterone on escalation of intravenous cocaine self-administration in rats selectively bred for high or low saccharin intake

Progesterone decreases cocaine self-administration in women and in female rats. In a previous study using rats selectively bred for high (HiS) or low (LoS) saccharin intake, HiS rats escalated their cocaine intake compared with LoS rats. Our goal was to examine the effects of progesterone on the escalation of cocaine self-administration in HiS and LoS rats. Four groups of female rats were compared: HiS P (progesterone treated), LoS P, HiS VEH (vehicle treated), and LoS VEH. Rats were trained to self-administer 0.8 mg/kg cocaine intravenously under a fixed-ratio 1 schedule during daily short-access (ShA) 2-h sessions. Rats then self-administered three randomly-presented doses of cocaine (0.2, 0.4, and 1.6 mg/kg), and then had daily 6-h long-access (LgA) sessions with 0.4 mg/kg of cocaine for 21 days. Cocaine intake was then reassessed with the four doses under the ShA condition. Throughout the experiment, rats were treated with daily subcutaneous injections of progesterone (0.5 mg/kg) or an equal volume of vehicle 30 min before each session. During the initial ShA condition, HiS rats earned more cocaine infusions than LoS rats at all doses, and during the subsequent LgA condition, HiS rats escalated cocaine intake, whereas the LoS rats maintained a steady rate. Progesterone treatment potentiated escalation of cocaine intake in the HiS rats but had an opposite effect on LoS rats, attenuating their cocaine self-administration. Results from the post-LgA dose-response ShA condition indicated that both LoS and HiS vehicle-treated and progesterone-treated rats earned more infusions than pre-LgA, but mainly at low doses. These results suggest that genetic differences in drug abuse vulnerability contribute differentially to treatment outcomes during escalation, a critical phase of the drug abuse process.

Cocaine dose and self-administration history, but not initial cocaine locomotor responsiveness, affects sensitization to the motivational effects of cocaine in rats

Cocaine addiction is a significant and complex disease. Part of this complexity is caused by the variability of the drug experience early in drug use (initial responsiveness, amount of use, etc.). In rats, individual differences in initial cocaine responsiveness and cocaine self-administration history both predict the development of cocaine sensitization, a putative mechanism contributing to the development of cocaine addiction. Here, we sought to determine the role of these factors and cocaine dose on the development of sensitization to cocaine's motivational effects during the earliest stages of self-administration. Rats were classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced locomotor activity (10 mg/kg i.p.) before learning to self-administer cocaine (0.6 mg/kg/infusion i.v.) under a fixed ratio 1 (FR1) schedule of reinforcement. After acquisition, rats self-administered cocaine (0.6 or 1.2 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement either immediately or after an additional five FR1 sessions (0.6 or 1.2 mg/kg/infusion). No LCR/HCR differences in sensitization were observed. However, regardless of LCR/HCR classification, exposure to the higher dose of cocaine produced sensitization to cocaine's motivational effects on the PR schedule (i.e., increased break points) and an escalation of consumption on the FR schedule. Thus, our results reveal a novel model for studying escalation and sensitization very early after acquisition and suggest that sensitization may be important in the earliest stages of the cocaine addiction process.

Impact of Sex and Gonadal Hormones on Cocaine and Food Reinforcement Paradigms

Men and women express sexually dimorphic patterns of cocaine abuse, such that women progress faster from initially trying cocaine to becoming dependent upon the drug and display a greater incidence of relapse. Sex differences in response to cocaine are also seen in the laboratory in both humans and animal models. In this review, animal models of cocaine abuse that have reported sex differences in appetitive reinforcement are discussed. In both human and animal studies, sex differences in the subjective and behavioral effects of cocaine are often related to the female reproductive cycle and ovarian hormones. As a comparison, food reinforcement studies have shown the opposite profile of sex differences and the impact of sex steroids on food intake and response rate. In contrast, limited attention has been given to "choice" models in rodents of either sex, however, our recent studies have indicated a role of sex and estrogen in cocaine choice over food with intact females, and OVX females treated with estrogen, choosing cocaine significantly more than males. Interestingly, estrous cycle phase does not seem to impact cocaine choice as it does response rate in single-reinforcer studies, suggesting that genomic rather than neurosteroid effects of estrogen modulate sex differences in this model. Future studies should more fully explore the impact of sex hormones on concurrent reinforcement and discrete choice models of addiction.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

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.

Impulsivity as a behavioral measure of withdrawal of orally delivered PCP and nondrug rewards in male and female monkeys

BACKGROUND

Withdrawal of phencyclidine (PCP), ethanol (ETOH), and other drugs reduces operant responding maintained by food.

OBJECTIVES

Experiment 1 examined the effects of withdrawing daily short access (2 h) to drug on impulsivity for saccharin (SACC) using a delay discounting task and comparing male and female rhesus monkeys. Experiment 2 examined the effects of withdrawing a nondrug substance (e.g., food or SACC) on impulsivity for PCP.

MATERIALS AND METHODS

In experiment 1, either PCP or ETOH was available daily with water for 2 h under a fixed ratio 16 (FR 16) or FR 8 schedule, respectively. In a second component, SACC was available for 45 min under a delay discounting schedule. Next, water was substituted, and drug access was then restored. In experiment 2, PCP was available under a delay discounting schedule during food satiation or restriction or during concurrent SACC vs water access.

RESULTS

In experiment 1, withdrawal of 0.5 mg/ml PCP increased impulsivity for SACC, but not SACC intake, in males and females. During 16% ETOH access, impulsivity for SACC was elevated compared to baseline water access, and it returned to baseline levels during ETOH withdrawal. In experiment 2, food restriction resulted in increased PCP intake in males and females and increases in impulsivity for PCP that were greater in males than in females. SACC withdrawal had no effect on impulsivity for PCP or PCP intake.

CONCLUSIONS

Withdrawal of PCP and reduced food access increased impulsivity for SACC or PCP, respectively. Impulsivity is a sensitive indicator of drug dependence.

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

Risk factors that predict vulnerability for nicotine and other drug abuse have been identified using preclinical models, and there is close agreement with clinical and epidemiological studies. The major risk factors to be discussed are age, sex/hormonal status, impulsivity, sweet-liking, novelty reactivity, proclivity for exercise, and environmental impoverishment (vs. enrichment). This discussion will focus on factors that preclinical research has determined are strong and translatable predictors of nicotine and other drug abuse. An advantage of using preclinical models is that prospective, longitudinal studies and within-subject designs can be used to reveal risk factors that are diverse yet maintain unique characteristics. The many interrelationships among these factors lead to an additive vulnerability that increases the predictability that drug abuse will occur. A feature that these risk factors have in common is that they consistently predict vulnerability to drug abuse over critical transition phases of addiction that are difficult to examine prospectively in humans, such as acquisition, escalation, and reinstatement of drug-seeking after abstinence (relapse). The models offer valuable information that has been transferred to effective prevention and treatment strategies for smoking and other drug abuse in humans.

Novelty-induced locomotion is positively associated with cocaine ingestion in adolescent rats; anxiety is correlated in adults

The present studies assessed the roles of sex, age, novelty-seeking and plus-maze behavior on cocaine drinking in rats. Cocaine/saccharin solution was available in three daily, 5-hour sessions then a saccharin-only solution was also available in following sessions. In the one-bottle drinking phase, early and late adolescent males, post-natal day 28 (PN28) and PN42, consumed more cocaine/saccharin solution than young adults (PN65), but females did not exhibit significant age differences. Adolescents of both sexes consumed more cocaine/saccharin than adults during choice drinking. Saccharin availability in the two-bottle trials decreased cocaine/saccharin consumption in PN28 and PN65 rats. After a drug-free period, cocaine-stimulated locomotion was lower in cocaine/saccharin drinking than saccharin-only males, indicating tolerance. We tested the hypothesis that individual differences in pre-screened behavioral traits would correlate with cocaine/saccharin consumption in PN28 and PN65 male rats. High locomotor responses to novelty were associated with greater cocaine/saccharin drinking in adults in one-bottle sessions. In the subsequent choice drinking phase, correlations were age-specific. Adolescents with high novelty-induced locomotion and adults that spent less time on open arms of the elevated plus-maze drank more cocaine/saccharin. Thus, behavioral phenotypes correlated with individual differences in cocaine/saccharin consumption in an age-related manner.

Supply of a nondrug substitute reduces escalated heroin consumption

Escalation of drug consumption-a hallmark of addiction-has been hypothesized to be associated with a relative devaluation of alternative nondrug rewards and thus with a decrease in their ability to compete with or to substitute for the drug. In a behavioral economic framework, decreased substitutability of nondrug rewards for drug would explain why drug consumption is behaviorally dominant and relatively resistant to change (eg price-inelastic) in drug-addicted individuals. The goal of the present study was to test this hypothesis using a validated rat model of heroin intake escalation. Escalation was precipitated by long (6 h, long access (LgA)), but not short (1 h, short access (ShA)), daily access to i.v. heroin self-administration. After escalation, the effects of price (ie fixed-ratio value) on heroin consumption were assessed under two alternative reward conditions: in the presence or absence of a nondrug substitute for heroin (ie four freely available chow pellets). As expected, escalated heroin consumption by LgA rats was less sensitive to price than heroin consumption by ShA rats, showing that heroin had acquired greater reinforcing strength during escalation. However, supplying a substitute during access to heroin was sufficient to reverse this post-escalation increase in the reinforcing effectiveness of heroin. Thus, escalated heroin consumption is not associated with a decreased sensitivity to competing nondrug rewards. Escalated drug use may therefore persist, not so much because of a relative devaluation of nondrug substitutes, but because of a loss or reduction of their availability.

Morphine deprivation increases self-administration of the fast- and short-acting mu-opioid receptor agonist remifentanil in the rat

Opiate dependence and withdrawal have long been hypothesized to enhance the reinforcing effects of opiates; however, opiate agonist self-administration in these states has yet to be systematically assessed. To address this issue, the reinforcing property of the short-acting mu-opioid agonist, remifentanil, was assessed in morphine-dependent (MD), morphine-dependent and -withdrawn (MW), and nondependent, control (C) rats. Dependence was established by twice daily administration of increasing doses of morphine for 4 days (10, 20, 30, and 40 mg/kg s.c.) and then maintained with a daily injection of the large dose. Morphine deprivation-induced withdrawal (defined by weight loss and hyperalgesia) was apparent 24, but not 12, h after morphine treatment. Remifentanil self-administration (0.4, 0.8, 1.6, 3.2, or 6.4 mug/kg/infusion) was assessed over 20 successive, daily, 1-h sessions, either 12 or 24 h after the maintenance dose of morphine. Compared with the control group, the MD group demonstrated suppressed remifentanil self-administration, whereas the MW group exhibited enhanced responding for every dose of remifentanil. The increased responding observed in the MW group compared with the control and MD groups resulted in an upward shift in the remifentanil dose-response curve, an effect that was expressed only after repeated exposure to the contingency, demonstrating that morphine withdrawal ultimately enhances the reinforcing effects of remifentanil.

Social stimuli enhance phencyclidine (PCP) self-administration in rhesus monkeys

Environmental factors, including social interaction, can alter the effects of drugs of abuse on behavior. The present study was conducted to examine the effects of social stimuli on oral phencyclidine (PCP) self-administration by rhesus monkeys. Ten adult rhesus monkeys (M. mulatta) were housed side by side in modular cages that could be configured to provide visual, auditory, and olfactory stimuli provided by another monkey located in the other side of a paired unit. During the first experiment, monkeys self-administered PCP (0.25 mg/ml) and water under concurrent fixed ratio (FR) 16 schedules of reinforcement with either a solid or a grid (social) partition separating each pair of monkeys. In the second experiment, a PCP concentration-response relationship was determined under concurrent progressive ratio (PR) schedules of reinforcement during both the solid and grid partition conditions. Under the concurrent FR 16 schedules, PCP and water self-administration were significantly higher during exposure to a cage mate through a grid partition than when a solid partition separated the monkeys. The relative reinforcing strength of PCP, as measured by PR break points, was greater during the grid partition condition compared to the solid partition condition indicated by an upward shift in the concentration-response curve. To determine whether the social stimuli provided by another monkey led to activation of the hypothalamic-pituitary-adrenal (HPA) axis, which may have evoked the increase of PCP self-administration during the grid partition condition, a third experiment was conducted to examine cortisol levels under the two housing conditions. A modest, but nonsignificant increase in cortisol levels was found upon switching from the solid to the grid partition condition. The results suggest that social stimulation among monkeys in adjoining cages leads to enhanced reinforcing strength of PCP.

Effects of menstrual cycle phase on the reinforcing effects of phencyclidine (PCP) in rhesus monkeys

Substantive evidence indicates that there are sex differences in the reinforcing effects of drugs, and gonadal steroid hormones, such as estrogen and progesterone, likely contribute to these differences. Among females, subjective effects of drugs differ as a function of menstrual cycle phase. The purpose of the present study was to compare oral self-administration of phencyclidine (PCP) in female rhesus monkeys (Macaca mulatta) across different phases of the menstrual cycle. Since the 28-day menstrual cycle of non-human primates is similar to that of humans, this model could provide important evidence supporting the implication that changes in the levels of gonadal hormones across menstrual phase can alter a drug's reinforcing effects. Oral self-administration of several concentrations of PCP (0.125, 0.25, and 0.5 mg/ml) was examined in three sexually mature female monkeys during 3-h experimental sessions. Menstrual cycle phase was determined by onset of menses and verified by examining vaginal cytology. PCP self-administration was greater during the luteal phase at the 0.125 and 0.25 mg/ml concentrations, which is normally characterized by high levels of progesterone and moderate levels of estrogen, than during the follicular phase, when levels of estrogen are increasing and progesterone levels are low. When examined within each phase, numbers of PCP deliveries were highest during the mid-luteal phase, compared to the early and mid-follicular phases. No differences in self-administration were observed between early and mid-follicular phases, but a significant difference in PCP deliveries was found between mid- and late luteal phases at the lowest concentration of PCP tested. The results from this study suggest that PCP's reinforcing effects in female monkeys differ as a function of menstrual cycle phase.

Chronic food restriction: enhancing effects on drug reward and striatal cell signaling

Chronic food restriction (FR) increases behavioral sensitivity to drugs of abuse in animal models and is associated with binge eating, which shares comorbidity with drug abuse, in clinical populations. Behavioral, biochemical and molecular studies conducted in this laboratory to elucidate the functional and mechanistic bases of these phenomena are briefly reviewed. Results obtained to date indicate that FR increases the reward magnitude and locomotor-activating effects of abused drugs, and direct dopamine (DA) receptor agonists, as a result of neuroadaptations rather than changes in drug disposition. Changes in striatal DA dynamics, and postsynaptic cell signaling and gene expression in response to D-1 DA receptor stimulation have been observed. Of particular interest is an upregulation of NMDA receptor-dependent MAP kinase and CaM Kinase II signaling, CREB phosphorylation, and immediate-early and neuropeptide gene expression in nucleus accumbens (NAc) which may facilitate reward-related learning, but also play a role in the genesis of maladaptive goal-directed behaviors. Covariation of altered drug reward sensitivity with body weight loss and recovery suggests a triggering role for one of the endocrine adiposity hormones. However, neither acute nor chronic central infusions of leptin or the melanocortin 3/4 receptor agonist, MTII, have attenuated d-amphetamine reward or locomotor activation in FR rats. Interestingly, chronic intracerebroventricular leptin infusion in ad libitum fed (AL) rats produced a sustained decrease in food intake and body weight that was accompanied by a reversible potentiation of rewarding and locomotor-activating effects of d-amphetamine. This raises the interesting possibility that rapid progressive weight loss is sufficient to increase behavioral sensitivity to drugs of abuse. Whether weight loss produced by leptin infusion produces the same neuroadaptations as experimenter-imposed FR, and whether any of the observed neuroadaptations are necessary for expression of increased behavioral responsiveness to acute drug challenge remain to be investigated.

Effect of short- vs. long-term estrogen on reinstatement of cocaine-seeking behavior in female rats

Estrogen effects on cocaine-induced reinstatement of lever responding were examined in sham-operated, vehicle-treated (SH+VEH), ovariectomized (OVX+VEH), and OVX female Wistar rats with estrogen replacement (OVX+EB). The effect of long- (64+/-1.56 days) and short-term (9 days) EB treatment on reinstatement of cocaine-seeking behavior was compared in Experiment 1 and 2, respectively, in order to compare the effect of EB when it was present during the development vs. expression of reinstatement of cocaine-seeking behavior. Rats were trained to self-administer 0.4 mg/kg/inf cocaine. After the acquisition criteria were met, rats continued to respond for cocaine for 2 h/day for a 14-day maintenance period. Cocaine was then replaced with saline and the 21-day extinction period commenced. Subsequently, rats were tested for reinstatement of lever responding on the previously drug-paired lever after alternating daily injections of saline or cocaine. In both experiments, there were no differences between groups in self-administration behavior during training, maintenance, or extinction. In Experiment 1, SH+VEH and chronically treated OVX+EB rats had greater cocaine-induced reinstatement than OVX+VEH rats. In Experiment 2, short-term treated OVX+EB rats also showed enhanced cocaine-induced reinstatement compared to OVX+VEH rats. The results indicate that EB-mediated enhancement of cocaine-induced reinstatement is dependent on EB presence during the expression of reinstatement but not during the formation of stimulus-reward associations during the development of cocaine-reinforced behavior.

Sex differences in the contribution of nicotine and nonpharmacological stimuli to nicotine self-administration in rats

RATIONALE

Sex differences have been reported for the impact of nicotine and nonpharmacological cues on smoking. While nonpharmacological environmental stimuli have also been shown to influence nicotine self-administration in rats, there have been no attempts to examine the impact of sex differences in the contributions of nicotine and nondrug stimuli to this behavior.

OBJECTIVES

This experiment investigated sex differences in operant responding for nicotine in rats when drug infusions were delivered either in the absence of, or in combination with, a nonpharmacological stimulus.

METHODS

Initially, male and female rats acquired self-administration for nicotine alone across a range of doses (0.03, 0.06, and 0.15 mg kg(-1) inf(-1), freebase). After stable acquisition, nicotine infusions were combined with a weakly reinforcing, compound visual stimulus.

RESULTS

While there was no overall effect of dose on active lever responding for nicotine in the absence of the visual stimulus, female rats responded more on the reinforced lever than males at 0.06 and 0.15 mg kg(-1) inf(-1) on an FR5 schedule. However, they also showed increased responding on the nonreinforced lever compared to males at the same doses. Combining nicotine infusions with the visual stimulus doubled responding compared to nicotine alone at 0.03 and 0.06, but not at 0.15 mg kg(-1) inf(-1): this effect was significantly greater for female rats.

CONCLUSIONS

These data highlight the prominent contribution of nonpharmacological stimuli to nicotine-reinforced behavior across a range of doses in both male and female rats. They also reveal sex differences in operant responding for nicotine under conditions where a nonpharmacological stimulus is either absent, or combined with drug delivery.

Sex differences in the escalation of oral phencyclidine (PCP) self-administration under FR and PR schedules in rhesus monkeys

RATIONALE

Studies with male rats indicate that long access (LgA) vs short access (ShA) to i.v. cocaine and heroin self-administration leads to an escalation of drug intake and a subsequent upward shift of the dose-response function.

OBJECTIVE

The purpose of this experiment was to extend these results to male and female rhesus monkeys and oral phencyclidine (PCP) self-administration under fixed-ratio (FR) and progressive-ratio (PR) schedules.

METHODS

Adult rhesus monkeys (seven females and nine males) orally self-administered PCP (0.25 mg/ml) and water under concurrent FR 16 FR 16 schedules during daily ShA 3-h sessions. Since females weighed less than males, each liquid delivery (0.6 ml) represented a higher unit dose mg/kg for females than males, but drug concentration mg/ml remained constant. Concurrent PR PR schedules were then used to obtain a concentration-response function (0.125, 0.25, 0.5, and 1.0 mg/ml). Next, PCP and water were available during LgA 6-h sessions under concurrent FR 16 FR 16 schedules for 21 days. The monkeys were then retested under the concurrent FR 16 FR 16 and PR PR conditions during ShA sessions.

RESULTS

Under the initial ShA concurrent FR 16 FR 16 schedules, females and males did not differ on PCP deliveries or intake (mg/kg); however, during LgA, males and females had more PCP deliveries compared with ShA. During LgA, males exceeded females in PCP deliveries, but females were higher than males in mg/kg PCP intake. Also, PCP (but not water) deliveries and mg/kg PCP intake significantly increased from the first 3 days to the last 3 days of the 21-day LgA period in both males and females. The subsequent ShA FR 16 FR 16 performance did not differ by sex, but it was significantly elevated above the first ShA period in both sexes. The concentration-response function for PCP break point under the PR PR schedules and PCP intake (mg/kg) were significantly shifted upward during the second (vs first) ShA period, and females' mg/kg intake significantly exceeded males'.

CONCLUSIONS

Male and female rhesus monkeys both showed escalation of PCP self-administration during LgA to PCP and during ShA that occurred after (vs before) LgA. Both showed vertical upward shifts in the concentration x intake (mg/kg) function under the PR schedule, and females exceeded males on this measure. These findings with PCP and monkeys are consistent with vertical upward shifts of cocaine dose-response functions in previous escalation studies in male rats and reports of sex differences (F>M) during several other phases of drug abuse.

Ethanol-maintained responding of rats is more resistant to change in a context with added non-drug reinforcement

Alternative non-drug reinforcers reliably decrease drug-maintained responding in self-administration procedures. Studies of the resistance to change of food-maintained behavior, however, have found that responding in the presence of a stimulus associated with an alternative reinforcer is more resistant to disruption. This increase in persistence occurs despite lower response rates when the alternative reinforcer is present. The present experiment examined if, in addition to decreasing response rates, an alternative non-drug reinforcer also increases the persistence of drug-maintained responding. Rats self-administered oral ethanol in a multiple schedule of reinforcement in which responding was reinforced in two components signaled by different stimuli. In one component, response-independent food was delivered in addition to the earned ethanol. The effects of the alternative food reinforcer on response rates and resistance to extinction in the two components were examined. As in previous experiments on the resistance to change of food-maintained operant behavior, response rates were lower, but more resistant to extinction in the presence of the stimulus associated with the alternative reinforcer. These findings suggest that all the reinforcers obtained in a context in which drugs are consumed may contribute to the persistence of drug seeking in that context. This increase in persistence may occur even if the alternative reinforcers interfere with drug seeking.

Sex and estrogen influence drug abuse

Evidence is accumulating that the etiology, epidemiology, consequences and mechanisms that underlie drug abuse are different in males and females. In this review, we present examples of sex differences in all phases of drug abuse, including acquisition, steady-state maintenance, escalation, dysregulation, withdrawal, relapse and treatment. Most reported findings are based on laboratory research in animals, but there are corroborating reports from human clinical and epidemiological studies. In all phases of drug abuse, females seem to be more sensitive to the rewarding effects of drugs than males, and estrogen is a major factor that underlies these sex differences.

Differential effects of bremazocine on oral phencyclidine (PCP) self-administration in male and female rhesus monkeys

Sex differences exist in many phases of drug abuse, but few studies have focused on sex differences in drug abuse treatment. In this study, the effects of bremazocine, a kappa-opioid receptor agonist, were compared in age-matched male and female rhesus monkeys self-administering orally delivered phencyclidine (PCP). Bremazocine (0.00032. 0.001, and 0.0025 mg/kg, intramuscular) was administered for 5 consecutive days. 15 min prior to daily 3-hr sessions when PCP (0.25 mg/ml) and water were available under concurrent fixed-ratio schedules. Bremazocine dose-dependently decreased PCP-maintained responding and consumption (mg/kg) in males and females, and these measures were suppressed at a lower bremazocine dose in females than in males. The percentage reduction in PCP-maintained responding and intake (mg/kg) was significantly greater in females than it was in males at the low and middle doses of bremazocine, suggesting that females may be more responsive to kappa agonist treatment than males.