Binge self-administration and deprivation produces sensitization to the reinforcing effects of cocaine in rats

Environmental influence on the preclinical evaluation of substance use disorder therapeutics

Substance use disorders (SUD) develop as a result of complex interactions between the environment, the subject, and the drug of abuse. Preclinical basic research investigating each of these tripartite components of SUD individually has resulted in advancements in our fundamental knowledge regarding the progression from drug abuse to SUD and severe drug addiction and the underlying behavioral and neurobiological mechanisms. How these complex interactions between the environment, the subject, and the drug of abuse impact the effectiveness of candidate or clinically used medications for SUD has not been as extensively investigated. The focus of this chapter will address the current state of our knowledge how these environmental, subject, and pharmacological variables have been shown to impact candidate or clinical SUD medication evaluation in preclinical research using drug self-administration procedures as the primary dependent measure. The results discussed in this chapter highlight the importance of considering environmental variables such as the schedule of reinforcement, concurrent availability of alternative nondrug reinforcers, and experimental housing conditions in the context of SUD therapeutic evaluation. The thesis of this chapter is that improved understanding of environmental variables in the context of SUD research will facilitate the utility of preclinical drug self-administration studies in the evaluation and development of candidate SUD therapeutics.

Shifts in the neurobiological mechanisms motivating cocaine use with the development of an addiction-like phenotype in male rats

RATIONALE

The development of addiction is accompanied by a shift in the mechanisms motivating cocaine use from nucleus accumbens (NAc) dopamine D₁ receptor (D₁R) signaling to glutamate AMPA-kainate receptor (AMPA-R) signaling.

OBJECTIVE

Here, we determined whether similar shifts occur for NAc-D₂R signaling and following systemic manipulation of D₁R, D₂R, and AMPA-R signaling.

METHODS

Male rats were given short-access (20 infusions/day) or extended-access to cocaine (24 h/day, 96 infusions/day, 10 days). Motivation for cocaine was assessed following 14 days of abstinence using a progressive-ratio schedule. Once responding stabilized, the effects of NAc-D₂R antagonism (eticlopride; 0-10.0 μg/side) and systemic D₁R (SCH-23390; 0-1.0 mg/kg), D₂R (eticlopride; 0-0.1 mg/kg), and AMPA-R (CNQX; 0-1.5 mg/kg) antagonism, and NAc-dopamine-R gene expression (Drd1/2/3) were examined.

RESULTS

Motivation for cocaine was markedly higher in the extended- versus short-access group confirming the development of an addiction-like phenotype in the extended-access group. NAc-infused eticlopride decreased motivation for cocaine in both the short- and extended-access groups although low doses (0.1-0.3 μg) were more effective in the short-access group and high doses (3-10 μg/side) tended to be more effective in the extended-access group. Systemic administration of eticlopride (0.1 mg/kg) was more effective in the extended-access group, and systemic administration of CNQX was effective in the extended- but not short-access group. NAc-Drd2 expression was decreased in both the short- and extended-access groups.

CONCLUSION

These findings indicate that in contrast to NAc-D₁R, D₂R remain critical for motivating cocaine use with the development of an addiction-like phenotype. These findings also indicate that shifts in the mechanisms motivating cocaine use impact the response to both site-specific and systemic pharmacological treatment.

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

Different functional domains measured by cocaine self-administration under the progressive-ratio and punishment schedules in male Wistar rats

BACKGROUND

Current diagnosis of drug addiction like other mental disorders is based on clinical symptoms not on neural pathophysiology and consequently, does not provide useful information on the underlying pathophysiology and may impede the efforts to identify the underlying mechanisms. Identifying the functional deficits that are relevant to addiction and can be traced to the neural systems will greatly facilitate our understanding of the heterogeneity of the condition and improve future diagnosis and treatment. Cocaine addiction is characterized by the continued use despite the dire consequences, and the deficit in inhibitory control may play a key role in this process. This study aimed to develop a paradigm to measure the punishment-induced inhibitory regulation of reward-seeking behavior.

METHODS

Rats were first trained to self-administer sucrose pellets under a chained schedule and then the breaking points (BPs) under the progressive-ratio schedule, and the intensity-response effects of footshock punishment on sucrose SA were measured. Subsequently, the rats went on to self-administer intravenous cocaine, and the BPs and the punishment intensity-response effects were similarly determined.

RESULTS

The areas under the punishment intensity-response curves (AUCs) were calculated and used as an indicator of the sensitivity of the inhibitory system. The BPs for cocaine were not correlated with the AUCs. Furthermore, the change in the BPs for cocaine induced by changing cocaine dose did not predict the change in the AUCs.

CONCLUSION

The intensity-response effects of punishment can be used to measure the function or sensitivity of the inhibitory system independent of the motivational state.

High and escalating levels of cocaine intake are dissociable from subsequent incentive motivation for the drug in rats

RATIONALE

Taking high and increasing amounts of cocaine is thought to be necessary for the development of addiction. Consequently, a widely used animal model of drug self-administration involves giving animals continuous drug access during long sessions (LgA), as this produces high and escalating levels of intake. However, human cocaine addicts likely use the drug with an intermittent rather than continuous pattern, producing spiking brain cocaine levels.

OBJECTIVES

Using an intermittent-access (IntA) cocaine self-administration procedure in rats, we studied the relationship between escalation of cocaine intake and later incentive motivation for the drug, as measured by responding under a progressive ratio schedule of cocaine reinforcement.

RESULTS

First, under IntA, rats escalated their cocaine use both within and between sessions. However, escalation did not predict later incentive motivation for the drug. Second, incentive motivation for cocaine was similar in IntA-rats limited to low- and non-escalating levels of drug intake (IntA-Lim) and in IntA-rats that took high and escalating levels of drug. Finally, IntA-Lim rats took much less cocaine than rats given continuous drug access during each self-administration session (LgA-rats). However, IntA-Lim rats later responded more for cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Taking large and escalating quantities of cocaine does not appear necessary to increase incentive motivation for the drug. Taking cocaine in an intermittent pattern-even in small amounts-is more effective in producing this addiction-relevant change. Thus, beyond the amount of drug taken, the temporal kinetics of drug use predict change in drug use over time.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

Exposure of adolescent mice to 3,4-methylenedioxypyrovalerone increases the psychostimulant, rewarding and reinforcing effects of cocaine in adulthood

BACKGROUND AND PURPOSE

3,4-Methylenedioxypyrovalerone (MDPV) is a synthetic cathinone with powerful psychostimulant effects. It selectively inhibits the dopamine transporter (DAT) and is 10-50-fold more potent as a DAT blocker than cocaine, suggesting a high abuse liability. The main objective of the present study was to assess the consequences of an early (adolescence) MDPV exposure on the psychostimulant, rewarding and reinforcing effects induced by cocaine in adult mice.

EXPERIMENTAL APPROACH

Twenty-one days after MDPV pretreatment (1.5 mg·kg^-1 , s.c., twice daily for 7 days), adult mice were tested with cocaine, using locomotor activity, conditioned place preference and self-administration (SA) paradigms. In parallel, dopamine D₂ receptor density and the expression of c-Fos and ΔFosB in the striatum were determined.

KEY RESULTS

MDPV treatment enhanced the psychostimulant and conditioning effects of cocaine. Acquisition of cocaine SA was unchanged in mice pretreated with MDPV, whereas the breaking point achieved under a progressive ratio programme and reinstatement after extinction were higher in this group of mice. MDPV decreased D₂ receptor density but increased ΔFosB expression three-fold. As expected, acute cocaine increased c-Fos expression, but MDPV pretreatment negatively influenced its expression. ΔFosB accumulation declined during MDPV withdrawal, although it remained elevated in adult mice when tested for cocaine effects.

CONCLUSION AND IMPLICATIONS

MDPV exposure during adolescence induced long-lasting adaptive changes related to enhanced responsiveness to cocaine in the adult mice that seems to lead to a higher vulnerability to cocaine abuse. This particular behaviour correlated with increased expression of ΔFosB.

Sugar addiction: the state of the science

PURPOSE

As obesity rates continue to climb, the notion that overconsumption reflects an underlying 'food addiction' (FA) has become increasingly influential. An increasingly popular theory is that sugar acts as an addictive agent, eliciting neurobiological changes similar to those seen in drug addiction. In this paper, we review the evidence in support of sugar addiction.

METHODS

We reviewed the literature on food and sugar addiction and considered the evidence suggesting the addictiveness of highly processed foods, particularly those with high sugar content. We then examined the addictive potential of sugar by contrasting evidence from the animal and human neuroscience literature on drug and sugar addiction.

RESULTS

We find little evidence to support sugar addiction in humans, and findings from the animal literature suggest that addiction-like behaviours, such as bingeing, occur only in the context of intermittent access to sugar. These behaviours likely arise from intermittent access to sweet tasting or highly palatable foods, not the neurochemical effects of sugar.

CONCLUSION

Given the lack of evidence supporting it, we argue against a premature incorporation of sugar addiction into the scientific literature and public policy recommendations.

Effects of the dopamine/norepinephrine releaser phenmetrazine on cocaine self-administration and cocaine-primed reinstatement in rats

RATIONALE

Like other monoamine releasers such as D-amphetamine, chronic treatment with phenmetrazine can attenuate cocaine self-administration in monkeys.

OBJECTIVES

The present studies extended this finding to rodents and to cocaine-primed reinstatement, a putative laboratory animal model of relapse.

METHODS

In experiment 1, rats self-administered food pellets or injections of 0.19 mg/kg cocaine (i.v.) under a progressive-ratio schedule. When responding was stable, subcutaneous osmotic pumps were implanted containing saline or (+)-phenmetrazine (25 or 50 mg/kg per day). In experiment 2, rats self-administered injections of 0.75 mg/kg cocaine under a fixed-ratio 1 schedule in daily 6-h sessions. When responding was stable, rats were removed from the self-administration environment for 7 days and treated continuously with saline, 5 mg/kg per day D-amphetamine or phenmetrazine (25 or 50 mg/kg per day) via osmotic pumps. Rats were then returned to the self-administration context while treatment continued, and responding was extinguished by removing response-contingent stimulus changes and cocaine injections. Once responding was extinguished, reinstatement tests were conducted using cocaine injections (10 mg/kg i.p.).

RESULTS

Phenmetrazine decreased self-administration of cocaine, but not food pellets, during the 14-day treatment period; effects persisted for several days after treatment was discontinued. Moreover, cocaine-induced increases in responding during the reinstatement test were attenuated by D-amphetamine and both phenmetrazine doses.

CONCLUSIONS

These results extend the study of the effects of phenmetrazine on cocaine self-administration to a rodent model, and provide further support for the use of monoamine releasers as agonist medications for cocaine abuse.

Binge-like acquisition of 3,4-methylenedioxypyrovalerone (MDPV) self-administration and wheel activity in rats

RATIONALE

Lack of access to conventional sources of reinforcement has been proposed as a risk factor for substance abuse in lower socioeconomic populations. There is laboratory evidence that behavioral alternatives (enrichment or exercise) and alternative reinforcers (e.g., sweetened solutions) can reduce self-administration of a variety of drugs.

OBJECTIVES

The objective of this study is to determine if drug self-administration could devalue wheel activity in an animal model.

METHODS

Male Wistar rats were trained to self-administer 3,4-methylenedioxypyrovalerone (MDPV; "bath salts"), 0.05 mg/kg/infusion, i.v., with concurrent access to a running wheel that was either locked (LW) or unlocked (UW).

RESULTS

MDPV intake steadily increased across the 20-session acquisition interval but did not differ significantly between UW and LW groups. Mean wheel rotations declined significantly across the acquisition interval in the UW group. Of the rats that acquired self-administration, 60 % engaged in a binge-like behavior at the initiation of acquisition; intake was limited only by post-reinforcement time-out. The binge rats had higher post-acquisition levels of drug intake (even after excluding the binge session), and the UW binge rats showed a precipitous post-acquisition drop in wheel activity that was not observed in the UW no-binge rats.

CONCLUSIONS

These data confirm that MDPV is a powerful reward/reinforcer and show that a relatively high rate of intake at the onset of drug taking can devalue natural rewards (wheel activity) and can predict higher subsequent drug intake levels. Thus, limiting the intensity of initial drug exposure may attenuate subsequent drug abuse/addiction by preventing the devaluation of natural alternative rewards/reinforcers.

Single prolonged stress effects on sensitization to cocaine and cocaine self-administration in rats

Posttraumatic stress disorder (PTSD) is often comorbid with substance use disorders (SUD). Single prolonged stress (SPS) is a well-validated rat model of PTSD that provides a framework to investigate drug-induced behaviors as a preclinical model of the comorbidity. We hypothesized that cocaine sensitization and self-administration would be increased following exposure to SPS. Male Sprague-Dawley rats were exposed to SPS or control treatment. After SPS, cocaine (0, 10 or 20 mg/kg, i.p.) was administered for 5 consecutive days and locomotor activity was measured. Another cohort was assessed for cocaine self-administration (0.1 or 0.32 mg/kg/i.v.) after SPS. Rats were tested for acquisition, extinction and cue-induced reinstatement behaviors. Control animals showed a dose-dependent increase in cocaine-induced locomotor activity after acute cocaine whereas SPS rats did not. Using a sub-threshold sensitization paradigm, control rats did not exhibit enhanced locomotor activity at Day 5 and therefore did not develop behavioral sensitization, as expected. However, compared to control rats on Day 5 the locomotor response to 20mg/kg repeated cocaine was greatly enhanced in SPS-treated rats, which exhibited enhanced cocaine locomotor sensitization. The effect of SPS on locomotor activity was unique in that SPS did not modify cocaine self-administration behaviors under a simple schedule of reinforcement. These data show that SPS differentially affects cocaine-mediated behaviors causing no effect to cocaine self-administration, under a simple schedule of reinforcement, but significantly augmenting cocaine locomotor sensitization. These results suggest that SPS shares common neurocircuitry with stimulant-induced plasticity, but dissociable from that underlying psychostimulant-induced reinforcement.

Long-term inhibition of ethanol intake by the administration of an aldehyde dehydrogenase-2 (ALDH2)-coding lentiviral vector into the ventral tegmental area of rats

Previous studies suggest that acetaldehyde generated from ethanol in the brain is reinforcing. The present studies tested the feasibility of achieving a long-term reduction of chronic and post-deprivation binge ethanol drinking by a single administration into the brain ventral tegmental area (VTA) of a lentiviral vector that codes for aldehyde dehydrogenase-2 (ALDH2), which degrades acetaldehyde. The ALDH2 gene coding vector or a control lentiviral vector were microinjected into the VTA of rats bred for their alcohol preference. In the chronic alcohol administration model, naïve animals administered the control vector and subsequently offered 10% ethanol and water ingested 8-9 g ethanol/kg body weight/day. The single administration of the ALDH2-coding vector prior to allowing ethanol availability reduced ethanol drinking by 85-90% (P < 0.001) for the 45 days tested. In the post-deprivation binge-drinking model, animals that had previously consumed ethanol chronically for 81 days were administered the lentiviral vector and were thereafter deprived of ethanol for three 7-day periods, each interrupted by a single 60-minute ethanol re-access after the last day of each deprivation period. Upon ethanol re-access, control vector-treated animals consumed intoxicating 'binge' amounts of ethanol, reaching intakes of 2.7 g ethanol/kg body weight in 60 minutes. The administration of the ALDH2-coding vector reduced re-access binge drinking by 75-80% (P < 0.001). This study shows that endowing the ventral tegmental with an increased ability to degrade acetaldehyde greatly reduces chronic alcohol consumption and post-deprivation binge drinking for prolonged periods and supports the hypothesis that brain-generated acetaldehyde promotes alcohol drinking.

Appropriate experimental approaches for predicting abuse potential and addictive qualities in preclinical drug discovery

INTRODUCTION

Drug abuse is an increasing social and public health issue, putting the onus on drug developers and regulatory agencies to ensure that the abuse potential of novel drugs is adequately assessed prior to product launch.

AREAS COVERED

This review summarizes the core preclinical data that frequently contribute to building an understanding of abuse potential for a new molecular entity, in addition to highlighting models that can provide increased resolution regarding the level of risk. Second, an important distinction between abuse potential and addiction potential is drawn, with comments on how preclinical models can inform on each.

EXPERT OPINION

While the currently adopted preclinical models possess strong predictive validity, there are areas for future refinement and research. These areas include a more refined use of self-administration models to assess relative reinforcement; and the need for open innovation in pursuing improvements. There is also the need for careful scientifically driven application of models rather than a standardization of methodologies, and the need to explore the opportunities that may exist for enhancing the value of physical dependence and withdrawal studies by focusing on withdrawal-induced drug seeking, rather than broad symptomology.

Dose-dependent effects of wheel running on cocaine-seeking and prefrontal cortex Bdnf exon IV expression in rats

RATIONALE

Physical activity, and specifically exercise, has shown promise as an intervention for drug addiction; however, the exercise conditions that produce the most efficacious response, as well as its underlying mechanism, are unknown.

OBJECTIVE

In this study, we examined the dose-dependent effects of wheel running, an animal model of exercise, during abstinence on subsequent cocaine-seeking and associated changes in prefrontal cortex (PFC) brain-derived neurotrophic factor (Bdnf) exon IV expression, a marker of epigenetic regulation implicated in cocaine relapse and known to be regulated by exercise.

METHODS

Cocaine-seeking was assessed under a within-session extinction/cue-induced reinstatement procedure following extended access cocaine or saline self-administration (24-h/day, 4 discrete trials/h, 10 days, 1.5 mg/kg/infusion) and a 14-day abstinence period. During abstinence, rats had either locked or unlocked running wheel access for 1, 2, or 6 h/day. Bdnf exon IV expression was assessed using quantitative real-time polymerase chain reaction.

RESULTS

Cocaine-seeking was highest under the locked wheel condition, and wheel running dose dependently attenuated this effect. Cocaine increased Bdnf exon IV expression, and wheel running dose dependently attenuated this increase, with complete blockade in rats given 6-h/day access. Notably, the efficacy of exercise was inversely associated with Bdnf exon IV expression, and both its efficacy and its effects on Bdnf exon IV expression were mimicked by treatment during abstinence with sodium butyrate, a histone deacetylase inhibitor that, like exercise, modulates gene transcription, including Bdnf exon IV expression.

CONCLUSION

Taken together, these results indicate that the efficacy of exercise is dose dependent and likely mediated through epigenetic regulation of PFC Bdnf.

Animal studies of addictive behavior

It is increasingly recognized that studying drug taking in laboratory animals does not equate to studying genuine addiction, characterized by loss of control over drug use. This has inspired recent work aimed at capturing genuine addiction-like behavior in animals. In this work, we summarize empirical evidence for the occurrence of several DSM-IV-like symptoms of addiction in animals after extended drug use. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, increased motivation for drugs, preference for drugs over nondrug rewards, and resistance to punishment. The fact that addiction-like behavior can occur and be studied in animals gives us the exciting opportunity to investigate the neural and genetic background of drug addiction, which we hope will ultimately lead to the development of more effective treatments for this devastating disorder.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effects of sensitization on the detection of an instrumental contingency

While prior exposure to drugs of abuse permanently changes many behaviors, the underlying psychological mechanisms are relatively obscure. Here, the effects of sensitization on the detection of an action-outcome relationship were assessed, using a particularly stringent contingency degradation procedure. Rats were trained to leverpress until the probability of reinforcement for a response on one lever, or alternative reinforcement for a response on a second lever was reduced to 0.05 per second. Sensitization was then carried out (1mg/kg d-amphetamine/day for 7 days). Then, one reinforcer was also made available for a lack of response on either lever (p=0.05/s), maintaining its contiguity with the original response but eliminating its contingent relationship. Sensitized animals were more active, particularly early in the contingency degradation phase, but reduced responding directed at the degraded action-outcome contingency at a similar rate as controls. However, controls also reduced responding directed at the nondegraded contingency until very late in training, while sensitized animals maintained nondegraded responding at baseline levels. It was suggested that the relatively specific response shown by sensitized animals may reflect either improved action-outcome utilization or discrimination of relevant task features.

Gene expression changes in the medial prefrontal cortex and nucleus accumbens following abstinence from cocaine self-administration

Background

Many studies of cocaine-responsive gene expression have focused on changes occurring during cocaine exposure, but few studies have examined the persistence of these changes with cocaine abstinence. Persistent changes in gene expression, as well as alterations induced during abstinence may underlie long-lasting drug craving and relapse liability.

Results

Whole-genome expression analysis was conducted on a rat cocaine binge-abstinence model that has previously been demonstrated to engender increased drug seeking and taking with abstinence. Gene expression changes in two mesolimbic terminal fields (mPFC and NAc) were identified in a comparison of cocaine-naïve rats with rats after 10 days of cocaine self-administration followed by 1, 10, or 100 days of enforced abstinence (n = 6-11 per group). A total of 1,461 genes in the mPFC and 414 genes in the NAc were altered between at least two time points (ANOVA, p < 0.05; ± 1.4 fold-change). These genes can be subdivided into: 1) changes with cocaine self-administration that do not persist into periods of abstinence, 2) changes with cocaine self-administration that persist with abstinence, 3) and those not changed with cocaine self-administration, but changed during enforced abstinence. qPCR analysis was conducted to confirm gene expression changes observed in the microarray analysis.

Conclusions

Together, these changes help to illuminate processes and networks involved in abstinence-induced behaviors, including synaptic plasticity, MAPK signaling, and TNF signaling.

Effects of extended cocaine access and cocaine withdrawal on choice between cocaine and food in rhesus monkeys

Chronic drug use may lead to sufficient drug intake to produce dependence and the emergence of abstinence signs during withdrawal. Although withdrawal can increase the reinforcing effects of some drugs (eg opioids), the impact of withdrawal on the reinforcing effects of stimulants like cocaine is less clear. This study used a novel cocaine vs food choice procedure to examine the relative reinforcing strength of cocaine before, during, and after exposure to graded levels of extended cocaine access. Responding in four rhesus monkeys was maintained by cocaine (0-0.1 mg/kg/injection) and food delivery under a concurrent-choice schedule during daily 2-h sessions. Under baseline conditions, cocaine maintained a dose-dependent increase in cocaine choice. Subsequently, subjects were exposed to and withdrawn from periods of extended cocaine access, which was accomplished by implementing daily 21-h supplemental sessions of cocaine self-administration in addition to daily choice sessions. During supplemental sessions, cocaine (0.1 mg/kg/injection) was available under a fixed-ratio 10/time-out X schedule, and the duration of the time-out was varied from 30 to 7.5 min. Cocaine intake increased 10-fold to >11 mg/kg/day during exposure to supplemental sessions with the shortest post-injection time-out. However, parameters of cocaine choice were not significantly affected either during or after extended cocaine access. These results do not support the hypothesis that cocaine withdrawal increases the reinforcing strength of cocaine. This differs from results with the opioid agonist heroin and suggests that withdrawal may have different functions in the maintenance of opioid and stimulant abuse.

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

Pattern of intake and drug craving predict the development of cocaine addiction-like behavior in rats

BACKGROUND

Clinical observations suggest that cocaine addiction often emerges with new patterns of use. Whether these changes are a cause of addiction or its consequence is unknown. We investigated whether the development of an addiction-like behavior in the rat is associated with the pattern of cocaine intake and with cocaine craving, a major feature of cocaine addiction.

METHODS

To determine whether changes in the pattern of cocaine use and enhanced craving precede or parallel the onset of addiction, we used a rat addiction model that incorporates core features of human addiction. For this purpose, the pattern of inter-infusion intervals (a measure of pattern of cocaine intake), sensitivity to cocaine-induced reinstatement (a measure of cocaine craving), and addiction-like behaviour were assessed over several months of intravenous cocaine self-administration.

RESULTS

We found that, even at early stages of cocaine self-administration, both the pattern of cocaine intake and the intensity of drug-induced reinstatement predict the severity of cocaine use, measured after 75 days of self-administration.

CONCLUSIONS

Our results identify key predictors of cocaine addiction-intensified pattern of drug use and high drug-induced craving-that may help in the identification of subjects at risk for subsequent development of severe cocaine addiction.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

Escalation of food-maintained responding and sensitivity to the locomotor stimulant effects of cocaine in mice

Escalation of drug self-administration is a consequence of extended drug access and is thought to be specifically related to addiction, but few studies have investigated whether intake of non-drug reinforcers may also escalate with extended-access. The goal of these studies was to determine the effects of limited and extended-access to food reinforcers on behavioral and pharmacological endpoints in mice. In distinct groups, responding on a lever was maintained by liquid reinforcement, or nose-poke responses were maintained by sucrose pellets or liquid food in sessions lasting 1 h (limited-access) or 10 h (extended-access). The reinforcing strength of each food, as well as reinforcer-associated cues, was tested before and after extended-access using a progressive ratio (PR) schedule, and locomotor activity in response to novelty and increasing doses of cocaine was assessed in an open field setting in all animals after access to food reinforcers. Escalation of lever-pressing behavior reinforced by liquid food, but not nose-poke behavior reinforced by liquid food or sucrose pellets, was observed across successive extended-access sessions. A concomitant increase in the reinforcing strength of liquid food and its associated cues was apparent in mice that escalated their responding, but not in mice that did not escalate. Finally, extended reinforcer access leading to behavioral escalation was accompanied by an increased sensitivity to the psychostimulant effects of cocaine compared to limited-access. These findings indicate that behavioral escalation can develop as a consequence of extended-access to a non-drug reinforcer, although both the nature of the reinforcer (liquid versus solid food) and the topography of the operant response (lever versus nose-poke) modulate its development. These data also suggest that some of the behavioral and pharmacological corrolaries of behavioral escalation observed following extended-access to drug self-administration may not be due to drug exposure, but rather, may result from basic behavioral processes which underlie operant responding maintained by appetitive stimuli.

Parsing the Addiction Phenomenon: Self-Administration Procedures Modeling Enhanced Motivation for Drug and Escalation of Drug Intake

Investigators who study drug addiction are fortunate to have access to excellent animal models. Such models will be invaluable in the assessment of factors involved in the progression of drug addiction. The relevance of these findings, however, will depend on the general understanding of how each model is related to drug addiction. The present review focuses on several procedures that were designed to model the addiction process and questions whether these models are tapping into the same underlying process or whether each is addressing a unique feature. Furthermore, various factors (e.g., rate of drug onset, dose magnitude, early drug history, periods of abstinence) influencing the progression of these addiction-like changes in behavior are discussed.

Persistent alterations in mesolimbic gene expression with abstinence from cocaine self-administration

Cocaine-responsive gene expression changes have been described after either no drug abstinence or short periods of abstinence. Little data exist on the persistence of these changes after long-term abstinence. Previously, we reported that after discrete-trial cocaine self-administration and 10 days of forced abstinence, incubation of cocaine reinforcement was observable by a progressive ratio schedule. The present study used rat discrete-trial cocaine self-administration and long-term forced abstinence to examine extinction responding, mRNA abundance of known cocaine-responsive genes, and chromatin remodeling. At 30 and 100 days of abstinence, extinction responding increased compared to 3-day abstinent rats. Decreases in both medial prefrontal cortex (mPFC) and nucleus accumbens c-fos, Nr4a1, Arc, and EGR1 mRNA were observed, and in most cases persisted, for 100 days of abstinence. The signaling peptides CART and neuropeptide Y (NPY) transiently increased in the mPFC, but returned to baseline levels following 10 days of abstinence. To investigate a potential regulatory mechanism for these persistent mRNA changes, levels of histone H3 acetylation at promoters for genes with altered mRNA expression were examined. In the mPFC, histone H3 acetylation decreased after 1 and 10 days of abstinence at the promoter for EGR1. H3 acetylation increased for NPY after 1 day of abstinence and returned to control levels by 10 days of abstinence. Behaviorally, these results demonstrate incubation after discrete-trial cocaine self-administration and prolonged forced abstinence. This incubation is accompanied by changes in gene expression that persist long after cessation of drug administration and may be regulated by chromatin remodeling.

How to make a rat addicted to cocaine

Procedures have been developed which provide extremely stable patterns of cocaine self-administration in rats and these have been useful in lesion and drug pretreatment studies aimed at understanding the neurobiology of cocaine reinforcement. The issue now is whether studying the neurobiology of reinforcement is the same as studying the neurobiology of addiction. If the goal is to understand a progressive and deteriorating disorder, then the self-administration procedures should model specific aspects of the progressive stages of the addiction process. Here we review theoretical strategies for modeling the addiction process and present data from a series of experiments from our laboratory showing conditions which produce a progressive change in the motivation to self-administer cocaine in rats. This phenomenon is revealed by an escalation in breakpoints on a progressive ratio schedule. The effect, which is robust and persistent, depends on dose and speed of injection. Interestingly, high drug intake can retard the development of this effect, which we argue indicates that the addiction process has a developmental sequence. Finally, we suggest that specific parameters (dose, price and availability) can be used to examine the transition from recreational use to binge-like intake.

Explaining the escalation of drug use in substance dependence: models and appropriate animal laboratory tests

Escalation of drug use, a hallmark of drug dependence, has traditionally been interpreted as reflecting the development of tolerance to the drug's effects. However, on the basis of animal behavioral data, several groups have recently proposed alternative explanations, i.e. that such an escalation of drug use might not be based on (1) tolerance, but rather be indicative of (2) sensitization to the drug's reinforcing effect, (3) reward allostasis, (4) an increase in the incentive salience of drug-associated stimuli, (5) an increase in the reinforcing strength of the drug reinforcer relative to alternative reinforcers, or (6) habit formation. From the pharmacological perspective, models 1-3 allow predictions about the change in the shape of drug dose-effect curves that are based on mathematically defined models governing receptor-ligand interaction and signal transduction. These predictions are tested in the present review, which also describes the other currently championed models for drug use escalation and other components of apparent 'reinforcement' (in its original meaning, like 'tolerance' or 'sensitization', a purely descriptive term). It evaluates the animal experimental approaches employed to support or prove the existence of each of the models and reinforcement components, and recapitulates the clinical evidence, which strongly suggests that escalation of drug use is predominantly based on an increase in the frequency of intoxication events rather than an increase in the dose taken at each intoxication event. Two apparent discrepancies in animal experiments are that (a) sensitization to overall reinforcement has been found more often for psychostimulants than for opioids, and that (b) tolerance to the reinforcing and other effects has been observed more often for opioids than for cocaine. These discrepancies are resolved by the finding that cocaine levels seem to be more tightly regulated at submaximum reinforcing levels than opioid levels are. Consequently, animals self-administering opioids are more likely to expose themselves to higher above-threshold doses than animals self-administering psychostimulants, rendering the development of tolerance to opioids more likely than tolerance to psychostimulants. The review concludes by making suggestions on how to improve the current behavioral experimental approaches.

Biphasic alterations in Serotonin-1B (5-HT1B) receptor function during abstinence from extended cocaine self-administration

Alterations in 5-HT1B receptor function during cocaine abstinence were evaluated in rats given either limited- or extended access (LA and EA, respectively) to cocaine self-administration. The locomotor response to the 5-HT1B/1A agonist RU24969 was significantly reduced in cocaine-experienced animals relative to cocaine-naïve controls following 6 h of abstinence but became sensitized over the subsequent 14 days of abstinence. Both the early phase subsensitivity and later phase supersensivity to RU 24969-induced activity were greater in EA versus LA animals. Intra-nucleus accumbens administration of the 5-HT1B agonist CP 93, 129 produced significantly greater increases in dialysate dopamine levels in EA versus control animals following 14 days of abstinence. However, there was no difference between EA and cocaine-naïve control animals in the augmentation of cocaine-induced increases in nucleus accumbens DA produced by intra-VTA CP 93, 129. Collectively these findings demonstrate that 5-HT1B receptor function is persistently altered by cocaine self-administration.

Influence of abstinence and conditions of cocaine access on the reinforcing strength of cocaine in nonhuman primates

The development of addiction is marked by a transition from recreational to uncontrolled drug use. Investigators modeling this phenomenon in rodents observed increases in cocaine self-administration when conditions of drug access were altered as well as after abstinence. The present studies were designed to extend this research to nonhuman primates by examining whether the reinforcing strength of cocaine could be altered by changing conditions of cocaine availability or by introducing abstinence periods. Rhesus monkeys self-administered cocaine (0.03-0.3 mg/kg per injection) under a progressive-ratio (PR) schedule of reinforcement in evening sessions, with the number of injections earned serving as a measure of reinforcing strength. Alterations in the reinforcing strength of cocaine were assessed after additional access to cocaine under a fixed-ratio (FR) schedule was provided in morning sessions and following various periods of abstinence (3, 7 and 14 days) from regimens of self-administration that resulted in a range of cocaine intakes. Under baseline PR conditions, the maximum number of cocaine injections increased dose-dependently, peaking when 0.3 mg/kg per injection cocaine was available. No increases in the reinforcing strength of cocaine were observed under any condition. In contrast, a statistically significant decrease in the reinforcing strength of cocaine was observed following 14 days of abstinence under one condition. These results fail to support the views that increasing access to cocaine or abstinence enhances the reinforcing strength of cocaine.

Discrete-trials heroin self-administration produces sensitization to the reinforcing effects of cocaine in rats

RATIONALE

The prevalence of cocaine use in opioid-dependent individuals is reportedly high, and the associated negative health and social consequences are severe and well documented. Sensitization of the reinforcing effects of cocaine has been demonstrated following noncontingent opioid exposure in animals; however, no preclinical studies have investigated the impact of opioid self-administration on cocaine's reinforcing effects.

OBJECTIVE

Experiments were designed to investigate whether access to heroin self-administration altered the subsequent reinforcing effects of cocaine.

METHODS

Baseline responding for cocaine under a progressive ratio schedule was first established. Heroin was then self-administered under a 24-h discrete-trials procedure (DT5; access to heroin five times per hour). Subsequently, cocaine-maintained responding was reassessed.

RESULTS

Here we demonstrate that 10 days of DT5 heroin self-administration (50 microg/kg per infusion) resulted in an increase in cocaine's reinforcing effects at several doses across the cocaine dose-effect curve (0.38-3.0 mg/kg per infusion). These increases were relatively long lasting, exceeding the time course of a mild withdrawal syndrome.

CONCLUSIONS

The DT5x10-day history of heroin self-administration resulted in an upward shift in the cocaine dose-effect curve, suggesting that DT5 heroin self-administration produced an increase in potency and sensitization of the maximal effectiveness with which cocaine functions as a reinforcer. The present experiments contribute to a growing amount of preclinical evidence suggesting an impact of opioid exposure on the reinforcing effects of cocaine, which may partially explain the high incidence of cocaine use in opioid-dependent individuals.

Rapid and persistent sensitization to the reinforcing effects of cocaine

The development of drug addiction involves a transition from recreational use to compulsive drug seeking and taking, and this progression can occur rapidly with cocaine use. These data highlight the importance of early drug exposure and the development of drug dependence; however, little experimental attention has been paid to this phenomenon in animal models of drug abuse. The present experiments demonstrate a progressive and rapid sensitization to the reinforcing strength of cocaine assessed using a progressive ratio (PR) schedule in rats. The first experiment found that rats show increased breakpoints over a 2-week period following acquisition. Subsequent experiments examined the role of total cocaine intake during the initial exposure period and found that low intakes (20 mg/kg/day x 5 days) resulted in sensitization, whereas relatively higher intake (60 or 100 mg/kg/day x 5 days) suppressed the development of sensitization. In contrast, this higher level of intake (60 mg/kg/day x 5 days) only transiently suppressed the expression of sensitization. Examination of breakpoints maintained by various doses of cocaine revealed an upward and leftward displacement of the cocaine dose-effect curve, relative to nonsensitized animals. These studies describe a form of sensitization that occurs rapidly to the reinforcing effects of cocaine, and provide a model to study the potential impact of initial experience on the development of drug dependence.

Behavioral and neural mechanisms of compulsive drug seeking

Not the mere procurement and use of drugs, but the fact that patterns of seeking and taking become compulsive after prolonged drug use is a defining characteristic of drug addiction. Development of a therapy that targets the compulsive aspects of drug use and thus addresses addiction at its core would therefore be very desirable. In the present review, we will discuss animal studies that attempt to model loss of control over drug use. Furthermore, we will try to put these studies in a theoretical perspective, and discuss the hypothesized underlying neural and behavioral mechanisms.

Intense cocaine self-administration after episodic social defeat stress, but not after aggressive behavior: dissociation from corticosterone activation

RATIONALE

An intense stress response characterizes both the dominant and submissive individuals during an aggressive confrontation, and these stress responses have enduring neural and behavioral consequences.

OBJECTIVES

In spite of similar glucocorticoid and corticolimbic dopamine activation, dominant and defeated individuals appear to diverge in terms of their drug taking. Do rats that are intermittently subjected to defeat stress become more sensitized to cocaine taking relative to rats that engage in aggressive bouts?

METHODS

Separate groups of male Long-Evans rats were investigated after an initial 10-day period with four brief episodes of social defeat (intruders) or aggressive behavior (residents): (1) the corticosterone responses to the very first and the last confrontations were measured; (2) the locomotor response to an amphetamine (1 mg/kg) challenge 10 days after the last stress exposure served as an index of behavioral sensitization; (3) intravenous self-administration sessions assessed the reinforcing effects of 0.75 mg/kg/infusion cocaine when available after every fifth response (fixed ratio), when delivered after completing progressively more demanding response requirements (progressive ratio; 0.3 mg/kg/infusion), and when available during a 24-h binge of continuous access (0.3 mg/kg/infusion).

RESULTS

Both social defeat of the intruder rat and attack behavior by the resident rat rapidly increased plasma levels of corticosterone after the first and last aggressive confrontation, indicating no habituation to these types of stress. Intermittent social defeat engenders a sensitized locomotor response to a 1 mg/kg amphetamine challenge and increases cocaine self-administration as indicated by more behavioral effort to obtain cocaine infusions and by accumulating more cocaine during 24 h of continuous access (binge). By contrast, experiences with aggressive behavior do not impact on the motorically activating and reinforcing effects of stimulant administrations.

CONCLUSIONS

The closely similar corticosterone activation in dominant and subordinate rats, followed by divergent patterns of cocaine self-administration indicates that different forms of social stress have dissociable effects on cocaine taking.

Extended cocaine self-administration and deprivation produces region-specific and time-dependent changes in connexin36 expression in rat brain

Cocaine addiction is a disease that develops over time, and it is thought that drug-induced neuro-adaptations underlie the changes in behavior seen across the addictive process. While a number of alterations in synaptic transmission have been identified, little is currently known regarding cocaine's effects on gap junctional communication between neurons. Here we examine the effects of a cocaine self-administration regimen, previously shown to increase the reinforcing efficacy of cocaine, on the expression of the neuron-specific gap junction-forming protein connexin36 (C x 36). Using real-time RT-PCR and immunoblotting, we show that binge cocaine self-administration produces region-specific and time-dependent changes in C x 36 mRNA and protein expression in the nucleus accumbens, prefrontal cortex, and hippocampus. A number of changes in C x 36 were present 1 day and 7 days following self-administration, and C x 36 mRNA and protein appeared to be differentially regulated in a region-specific manner. C x 36 protein was significantly decreased in the prefrontal cortex 7 days following self-administration, a time point when behavioral sensitization to the reinforcing effects of cocaine is observed. These results suggest that changes in neuronal gap junction expression may be one mechanism by which cocaine self-administration produces enduring changes in behavior.

Sensitization of the reinforcing effects of self-administered cocaine in rats: effects of dose and intravenous injection speed

Speed of drug onset is assumed to be an important determinant of the abuse liability of a drug. Studies in human and non-human primates suggest that the subjective and reinforcing effects of cocaine can be influenced by route of administration and/or speed of intravenous injection. Sensitization to the reinforcing effects of cocaine was studied in rats and the effects of various injection durations (i.e. speed of injection) on the development of sensitization was examined using a progressive ratio schedule. In addition, the effects of cocaine dose on sensitization and the effects of injection duration on the acute reinforcing effects of cocaine were examined. The initial study demonstrated that the development of sensitization (i.e. progressive increases in breakpoints) was dose-dependent. A robust sensitization of the reinforcing effects of cocaine was replicated in animals receiving cocaine at the highest rate (i.e. shortest duration; 5 s), but not in animals receiving the same dose over 25 or 50 s. Subsequent testing revealed that injection duration did not have profound effects on the acute reinforcing effects of cocaine (assessed by breakpoints or rate of responding on a fixed ratio schedule). These findings are similar to recent studies demonstrating that the development of sensitization, but not the acute responsivity, to cocaine's locomotor-activating effects are influenced by rate of intravenous injection. Taking these findings together, we hypothesize that the process of drug addiction involves both the acute reinforcing effects and the development of sensitization.

Effects of extended-access self-administration and deprivation on breakpoints maintained by cocaine in rats

RATIONALE

Animal models that identify the effects of self-administration histories on subsequent patterns, levels of intake, and other aspects of reinforcement will help clarify the controlling variables of human drug use.

OBJECTIVE

Identify the effects of extended-access to cocaine and 1 or 7 days of deprivation on cocaine-maintained breakpoints on a progressive ratio (PR) schedule of reinforcement.

METHODS

Male, Sprague-Dawley rats were trained to self-administer intravenous cocaine (expt 1: 1.5 mg/kg per infusion; expt 2: 0.75 mg/kg per infusion), and then given various histories of self-administration and deprivation. Breakpoints, the number of infusions self-administered on a PR schedule, were assessed following the deprivation period.

RESULTS

Rates of cocaine intake increased when access to cocaine was extended to 6 h/day. From day 1 to day 14, daily intake increased from 92 (+/-2.5) to 101 (+/-2.8) mg/kg in expt 1, and from 55 (+/-4) to 78 (+/-2.2) mg/kg in expt 2. Total intake across this 2-week period was approximately 1260 and 970 mg/kg in expts 1 and 2. Breakpoints were not different following this escalation period. The introduction of a 7-day deprivation period failed to alter breakpoints.

CONCLUSIONS

There is dissociation between changes in rate of cocaine intake (or consumption) and breakpoints maintained on a PR schedule. Extended-access to cocaine produced increases in rate of intake without altering breakpoints. Depending on the experimental question, extended-access conditions may prove useful for studying changes in certain aspects of reinforcement, such as consumption, but not others, such as the strength of a drug as a reinforcer.