Comparison of the reinforcing effects of cocaine and cocaine/heroin combinations under progressive ratio and choice schedules in rats

Explaining the high mortality among opioid-cocaine co-users compared to opioid-only users. A systematic review

RATIONALE

The opioid crisis in North America has recently seen a fourth wave, which is dominated by drug-related deaths due to the combined use of illicitly manufactured fentanyl [IMF] and stimulants such as cocaine and methamphetamine.

OBJECTIVES

A systematic review addressing the question why drug users combine opioids and stimulants and why the combination results in such a high overdose mortality: from specific and dangerous pharmacokinetic or pharmacodynamic interactions or from accidental poisoning?

RESULTS

Motives for the combined use include a more intensive high or rush when used at the same time, and some users have the unfounded and dangerous belief that co-use of stimulants will counteract opioid-induced respiratory depression. Overdose deaths due to combined (intravenous) use of opioids and stimulants are not likely to be caused by specific pharmacokinetic or pharmacodynamic interactions between the two drugs and it is unlikely that the main cause of overdose deaths is due to accidental poisoning.

CONCLUSION

The unexpectedly high overdose rates in this population could not be attributed to accidental overdosing or pharmacokinetic/pharmacodynamic interactions. The most likely explanation for the high rate of drug-related deaths in opioid-cocaine co-users is careless overdosing with either cocaine, opioid(s) or both, probably facilitated by the high level of preexisting impulsivity in these co-users and a further acute increase in impulsivity following cocaine use. The primary corollary is that cocaine users should avoid IMF use in the same time window. In addition, IMF users should refrain from cocaine use to avoid impulsive IMF overdosing.

Prelimbic Ensembles Mediate Cocaine Seeking After Behavioral Acquisition and Once Rats Are Well-Trained

Substance use disorder (SUD) is a chronic relapsing condition characterized by continued use of drugs despite negative consequences. SUD is thought to involve disordered learning and memory wherein drug-paired cues gain increased salience, and ultimately drive craving and relapse. These types of associations are thought to be encoded within sparsely distributed sets of neurons, called neuronal ensembles, that drive encoded behaviors through synchronous activity of the participant neurons. We have previously found that Fos-expressing neuronal ensembles within the prefrontal cortex are required for well-trained cocaine seeking. However, less is known about how quickly cortical neuronal ensembles form during the initiation of cocaine seeking behavior. Here, we seek to further elucidate the role of Fos-expressing neuronal ensembles within the prelimbic cortex (PL) after the initial acquisition of cocaine self-administration (SA), or, after 10 days of additional SA training (well-trained). We trained Fos-LacZ transgenic rats to lever press for cocaine under an FR1 schedule of reinforcement. Once rats met acquisition criteria for cocaine self-administration, we ablated Fos-expressing neuronal ensembles in the PL using the Daun02 inactivation method, either 1 or 10 days after the rats met the acquisition criteria. Targeted ablation of Fos-expressing neuronal ensembles in the PL attenuated active lever pressing both 1 day and 10 days after rats acquired cocaine self-administration. Together, this suggests that Fos-expressing neuronal ensembles rapidly form in the PL and continue to mediate maintained cocaine seeking behavior.

Role of preexisting inhibitory control deficits vs. drug use history in mediating insensitivity to aversive consequences in a rat model of polysubstance use

RATIONALE

The nature and predictors of insensitivity to aversive consequences of heroin + cocaine polysubstance use are not well characterized.

OBJECTIVES

Translational methods incorporating a tightly controlled animal model of drug self-administration and measures of inhibitory control and avoidance behavior might be helpful for clarifying this issue.

METHODS

The key approach for distinguishing potential contributions of pre-existing inhibitory control deficits vs. drug use history in meditating insensitivity to aversive consequences was comparison of two rat strains: Wistar (WIS/Crl), an outbred strain, and the spontaneously hypertensive rat (SHR/NCrl), an inbred strain shown previously to exhibit heightened cocaine and heroin self-administration and poor inhibitory control relative to WIS/Crl.

RESULTS

In separate tasks, SHR/NCrl displayed greater impulsive action and compulsive-like behavior than WIS/Crl prior to drug exposure. Under two different schedules of drug delivery, SHR/NCrl self-administered more cocaine than WIS/Crl, but self-administered a similar amount of heroin + cocaine as WIS/Crl. When half the session cycles were punished by random foot shock, SHR/NCrl initially were less sensitive to punishment than WIS/Crl when self-administering cocaine, but were similarly insensitive to punishment when self-administering heroin + cocaine. Based on correlation analyses, only trait impulsivity predicted avoidance capacity in rats self-administering cocaine and receiving yoked-saline. In contrast, only amount of drug use predicted avoidance capacity in rats self-administering heroin + cocaine. Additionally, baseline drug seeking and taking predicted punishment insensitivity in rats self-administering cocaine or heroin + cocaine.

CONCLUSIONS

Based on the findings revealed in this animal model, human laboratory research concerning the nature and predictors of insensitivity to aversive consequences in heroin and cocaine polysubstance vs. monosubstance users is warranted.

Interactions between opioids and stimulants: Behavioral pharmacology of abuse-related effects

Opioid abuse continues to be a significant public health challenge, with rates of opioid-related overdose deaths increasing continuously over the last two decades. There also has been a sharp increase in overdose deaths involving stimulant drugs, primarily cocaine and methamphetamine. Recent estimates indicate a high prevalence of co-use of opioids and stimulants, which is a particularly complex problem. Behavioral pharmacology research over the last few decades has characterized interactions between opioids and stimulants as well as evaluated potential treatments. This chapter describes interactions between opioids and stimulants, with a focus on pre-clinical studies of abuse-related behavioral effects using self-administration, reinstatement, drug discrimination, place conditioning, and intracranial self-stimulation paradigms in laboratory animals. In general, the literature provides substantial evidence of mutual enhancement between opioids and stimulants for abuse-related effects, although such results are not ubiquitous. Enhanced abuse-related effects could manifest in many ways including engaging in drug seeking and taking behaviors with greater persistence, effort, and motivation and/or increased likelihood of relapse. Moreover, studies on opioid/stimulant combinations set the stage for evaluating potential treatments for polysubstance use. Behavioral pharmacology research has proven invaluable for elucidating these relationships using rigorous experimental designs and quantitative analyses of pharmacological and behavioral data.

The influence of reinforcement schedule on experience-dependent changes in motivation

The progressive ratio procedure is used across fields to assess motivation for different reinforcers, define the effects of experimental interventions on motivation, and determine experience-dependent changes in motivation. However, less is known about how operant training schedules affect performance on this widely utilized task. Here we designed an experiment to examine the effect of variable ratio versus fixed ratio training schedules of reinforcement on progressive ratio performance while holding other performance variables constant between groups. We found a robust increase in maximum ratio completed between the pretest and posttraining test highlighting a robust training effect on progressive ratio performance. However, it did not matter if the training was under a fixed or variable ratio schedule. Additionally, we show that neither individual rates during training nor extinction responding correlated with maximum ratio achieved during the sessions. Finally, we show that rates during the training sessions do correlate with extinction performance, suggesting that these variables measure a different aspect of performance that does not predict motivation.

Relative reinforcing effects of cocaine and 3,4-methylenedioxypyrovalerone (MDPV) under a concurrent access self-administration procedure in rats

BACKGROUND

Recent evidence suggesting that polysubstance use is the norm rather than the exception highlights the need for a better understanding of interactions amongst the abuse-related effects of commonly co-abused drugs. Synthetic cathinones remain one of the most popular families of novel psychoactive substances and are typically used in preparations containing multiple stimulants. Evaluating the reinforcing effects of drugs under both single-operant procedures and procedures in which alternatives are available can provide a more complete characterization of their reinforcing effects and economic interactions.

METHODS

These studies utilized a drug-versus-drug choice procedure in 18 male Sprague-Dawley rats to evaluate economic interactions between the synthetic cathinone, MDPV, and cocaine in addition to how a history of concurrent access impacts reinstatement behavior.

RESULTS

When equi-effective doses of MDPV and cocaine were made concurrently available, approximately half of the subjects responded exclusively on the MDPV-reinforced lever whereas the other half responded exclusively on the cocaine-reinforced lever. Allocation of responding was reversed when the cost of the preferred drug increased, or the cost of the non-preferred drug decreased. Drug-paired cues and MDPV, cocaine, and methamphetamine pretreatments reinstated responding on both drug levers, regardless of preference.

CONCLUSION

These data demonstrate that MDPV and cocaine act as economic substitutes and suggest that measures of reinforcing effectiveness determined under a progressive ratio schedule of reinforcement can predict drug choice. These data also suggest that environmental stimuli associated with a particular drug might stimulate class-specific drug-seeking, however, further studies are needed to test the generality of this claim.

Application of dose-addition analyses to characterize the abuse-related effects of drug mixtures

Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.

Modeling Stimulant and Opioid Co-use in Rats Provided Concurrent Access to Methamphetamine and Fentanyl

Concurrent use of stimulants (e.g., methamphetamine) and opioids (e.g., fentanyl) has become increasingly common in recent years and continues to pose an enormous health burden, worldwide. Despite the prevalence, relatively little is known about interactions between the reinforcing effects of stimulants and opioids in this pattern of polysubstance use. The goals of the current study were to evaluate the relative reinforcing and relapse-related effects of methamphetamine and fentanyl using a concurrent access, drug-vs.-drug choice procedure. Male Sprague-Dawley rats were first allowed to acquire self-administration for either 0.1 mg/kg/infusion methamphetamine or 0.0032 mg/kg/infusion fentanyl, independently, after which concurrent access to both drugs was provided. When training doses of methamphetamine and fentanyl were concurrently available, a subset of rats self-administered both drugs, either within a session or alternating across sessions, whereas the remaining rats responded exclusively for one drug. When the cost of the preferred drug was increased (i.e., unit dose reduced), or the cost of the non-preferred drug was decreased (i.e., unit dose increased), choice was largely allocated toward the cheaper alternative. Following extinction of responding, methamphetamine- and fentanyl-paired cues reinstated responding on both levers. Responding reinstated by a priming injection of methamphetamine or fentanyl allocated more responding to the lever previously reinforced by the priming drug. The current studies suggest that choice of methamphetamine and fentanyl is largely allocated to the cheaper alternative, although more co-use was observed than would be expected for economic substitutes. Moreover, they lay the groundwork for more fully evaluating interactions between commonly co-abused drugs (e.g., stimulants and opioids) in order to better understand the determinants of polysubstance use and develop effective treatment strategies for individuals suffering from a polysubstance use disorder.

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

Interactions Between Opioids and Dextroamphetamine on Locomotor Activity: Influence of an Opioid's Relative Efficacy at the Mu Receptor

Opioids and stimulants are often used in combination for both recreational and non-recreational purposes. High-efficacy mu opioid agonists generally increase the behavioral effects of stimulants, whereas opioid receptor antagonists generally attenuate the behavioral effects of stimulants; however, less is known regarding the interactions between stimulants and opioids possessing low to intermediate efficacy at the mu receptor. The purpose of this study was to examine the role of an opioid's relative efficacy at the mu receptor in altering the behavioral effects of dextro(d-)amphetamine. To this end, opioids possessing a range of relative efficacy at the mu receptor were examined alone and in combination with cumulative doses of d-amphetamine on a test of open-field, locomotor activity in male rats. Levorphanol, buprenorphine, butorphanol, nalbuphine, (-)-pentazocine, (-)-metazocine, (-)-cyclazocine, (-)-NANM, and nalorphine increased the locomotor effects of d-amphetamine in either an additive or greater-than-additive manner according to an effect-additive model. Only the selective, high-efficacy kappa agonist, spiradoline, and the non-selective opioid receptor antagonist, naloxone, failed to increase the effects of d-amphetamine under the conditions examined. These data indicate that opioids possessing a large range of relative efficacy at the mu receptor, including those possessing very low relative efficacy, significantly increase the locomotor effects of d-amphetamine.

The impact of cocaine and heroin drug history on motivation and cue sensitivity in a rat model of polydrug abuse

RATIONALE

Comorbid use of heroin and cocaine is highly prevalent among drug users and can greatly increase addiction risk. Nonetheless, little is known regarding how a multi-drug history impacts motivation and cue responsivity to individual drugs.

OBJECTIVE

We used behavioral-economic procedures to examine motivation to maintain drug consumption and tests of drug-seeking to drug-associated cues to assess sensitivity to heroin and cocaine-associated cues in rats that had a self-administration history of heroin, cocaine, or both drugs.

RESULTS

Unexpectedly, we found that groups with a polydrug history of heroin and cocaine did not have higher levels of motivation or cue-induced reinstatement of drug-seeking for either cocaine or heroin compared to single drug groups. Nonetheless, we did find drug-specific differences in both economic price and cue sensitivity. Specifically, demand elasticity was lower for cocaine compared to heroin in animals with a single drug history, but not with polydrug groups. In addition, cocaine demand was predictive of the degree of cue-induced reinstatement of drug-seeking for cocaine following extinction, whereas heroin demand was predictive of the degree of reactivity to a heroin-associated cue. Furthermore, although cue reactivity following the initial self-administration phase did not differ across cues and drug history, reactivity to both heroin and cocaine cues was greater during subsequent heroin use compared to cocaine use, and this enhanced reactivity to heroin cues persisted during forced abstinence.

CONCLUSIONS

These results indicate that there is a greater motivation to maintain cocaine consumption, but higher sensitivity to drug-associated cues with a history of heroin use, suggesting that cocaine and heroin may drive continued drug use through different behavioral processes.

Conditioned taste avoidance induced by the combination of heroin and cocaine: Implications for the use of speedball

Speedball (heroin + cocaine) is a prevalent drug combination among intravenous drug users. Although its use is generally discussed to be a function of changes in the rewarding effects of either or both drugs, changes in the aversive effects of either drug may also be impacted (weakened) by the combination. To address this latter possibility and its potential role in the use of speedball, the present studies examined the interaction of cocaine and heroin in taste avoidance conditioning. In Experiment 1, male Sprague-Dawley rats were given access to a novel saccharin solution and then injected with either vehicle or heroin (3.2 mg/kg, IP) followed immediately by various doses of cocaine (10, 18 or 32 mg/kg, SC). At the two lowest doses of cocaine, only animals injected with the drug combination (H + C) displayed a taste avoidance relative to control subjects (taste avoidance was induced with both the combination and the high dose of cocaine). At no dose did animals injected with the combination of heroin and cocaine drink more than animals injected with cocaine alone. In Experiment 2, male Sprague-Dawley rats were similarly treated but injected with vehicle or cocaine (10 mg/kg) followed by injections of various doses of heroin (1.8, 3.2, 5.6 or 10 mg/kg). At the three highest doses of heroin, only animals injected with the drug combination (C + H) displayed significant avoidance relative to control subjects (no avoidance was evident with the combination of cocaine and the low dose of heroin). At no dose did animals injected with the combination of cocaine and heroin drink more than animals injected with heroin alone. Together, these results suggest that the aversive effects of heroin and cocaine are not attenuated by co-administration by cocaine and heroin, respectively. The importance of this for the use of speedball was discussed.

Deletion of the type 2 metabotropic glutamate receptor increases heroin abuse vulnerability in transgenic rats

Opioid abuse is a rapidly growing public health crisis in the USA. Despite extensive research in the past decades, little is known about the etiology of opioid addiction or the neurobiological risk factors that increase vulnerability to opioid use and abuse. Recent studies suggest that the type 2 metabotropic glutamate receptor (mGluR2) is critically involved in substance abuse and addiction. In the present study, we evaluated whether low-mGluR2 expression may represent a risk factor for the development of opioid abuse and addiction using transgenic mGluR2-knockout (mGluR2-KO) rats. Compared to wild-type controls, mGluR2-KO rats exhibited higher nucleus accumbens (NAc) dopamine (DA) and locomotor responses to heroin, higher heroin self-administration and heroin intake, more potent morphine-induced analgesia and more severe naloxone-precipitated withdrawal symptoms. In contrast, mGluR2-KO rats displayed lower motivation for heroin self-administration under high price progressive-ratio (PR) reinforcement conditions. Taken together, these findings suggest that mGluR2 may play an inhibitory role in opioid action, such that deletion of this receptor results in an increase in brain DA responses to heroin and in acute opioid reward and analgesia. Low-mGluR2 expression in the brain may therefore be a risk factor for the initial development of opioid abuse and addiction.

Status and Future Directions of Preclinical Behavioral Pharmacology in Tobacco Regulatory Science

Behavioral pharmacology is a branch of the experimental analysis of behavior that has had great influence in drug addiction research and policy. This paper provides an overview of recent behavioral pharmacology research in the field of tobacco regulatory science, which provides the scientific foundation for the Food and Drug Administration Center for Tobacco Products (FDA CTP) to set tobacco control policies. The rationale and aims of tobacco regulatory science are provided, including the types of preclinical operant behavioral models it deems important for assessing the abuse liability of tobacco products and their constituents. We then review literature relevant to key regulatory actions being considered by the FDA CTP, including regulations over nicotine and menthol content of cigarettes, and conclude with suggesting some directions for future research. The current era of tobacco regulatory science provides great opportunities for behavioral pharmacologists to address the leading cause of preventable death and disease worldwide.

Substitutability of nicotine alone and an electronic cigarette liquid using a concurrent choice assay in rats: A behavioral economic analysis

BACKGROUND

For the Food and Drug Administration to effectively regulate tobacco products, the contribution of non-nicotine tobacco constituents to the abuse liability of tobacco must be well understood. Our previous work compared the abuse liability of electronic cigarette refill liquids (EC liquids) and nicotine (Nic) alone when each was available in isolation and found no difference in abuse liability (i.e., demand elasticity). Another, and potentially more sensitive measure, would be to examine abuse liability in a choice context, which also provides a better model of the tobacco marketplace.

METHODS

Demand elasticity for Nic alone and an EC liquid were measured when only one formulation was available (alone-price demand) and when both formulations were concurrently available (own-price demand), allowing an assessment of the degree to which each formulation served as a substitute (cross-price demand) when available at a low fixed-price.

RESULTS

Own-price demand for both formulations were more elastic compared to alone-price demand, indicating that availability of a substitute increased demand elasticity. During concurrent access, consumption of the fixed-price formulation increased as the unit-price of the other formulation increased. The rate of increase was similar between formulations, indicating that they served as symmetrical substitutes.

CONCLUSION

The cross-price model reliably quantified the substitutability of both nicotine formulations and indicated that the direct CNS effects of non-nicotine constituents in EC liquid did not alter its abuse liability compared to Nic. These data highlight the sensitivity of this model and its potential utility for examining the relative abuse liability and substitutability of tobacco products.

Development of an opioid self-administration assay to study drug seeking in zebrafish

The zebrafish (Danio rerio) has become an excellent tool to study mental health disorders, due to its physiological and genetic similarity to humans, ease of genetic manipulation, and feasibility of small molecule screening. Zebrafish have been shown to exhibit characteristics of addiction to drugs of abuse in non-contingent assays, including conditioned place preference, but contingent assays have been limited to a single assay for alcohol consumption. Using inexpensive electronic, mechanical, and optical components, we developed an automated opioid self-administration assay for zebrafish, enabling us to measure drug seeking and gain insight into the underlying biological pathways. Zebrafish trained in the assay for five days exhibited robust self-administration, which was dependent on the function of the μ-opioid receptor. In addition, a progressive ratio protocol was used to test conditioned animals for motivation. Furthermore, conditioned fish continued to seek the drug despite an adverse consequence and showed signs of stress and anxiety upon withdrawal of the drug. Finally, we validated our assay by confirming that self-administration in zebrafish is dependent on several of the same molecular pathways as in other animal models. Given the ease and throughput of this assay, it will enable identification of important biological pathways regulating drug seeking and could lead to the development of new therapeutic molecules to treat addiction.

Preference for an Opioid/Benzodiazepine Mixture over an Opioid Alone Using a Concurrent Choice Procedure in Rhesus Monkeys

Increased abuse of opioids is contributing to an escalation in overdose deaths. Benzodiazepines are frequently abused with opioids, possibly because they increase the potency and/or effectiveness of opioids to produce reinforcing effects. This study used a concurrent-choice procedure to determine whether monkeys would choose to self-administer a mixture of the opioid remifentanil and the benzodiazepine midazolam over remifentanil alone. Initially, three monkeys could respond on one lever for saline and on a second lever for either remifentanil alone or midazolam alone. Thereafter, monkeys chose between a dose of remifentanil (0.32 µg/kg/infusion) that did not change and a dose of remifentanil that varied across sessions; for some sessions, midazolam was combined with varying doses of remifentanil. All monkeys received more infusions of remifentanil (0.0032-0.32 µg/kg/infusion) than saline, whereas only two monkeys responded more for midazolam than for saline. When 0.32 µg/kg/infusion remifentanil was available on one lever and a dose of remifentanil that varied across sessions (0.1-1 µg/kg/infusion) was available on the other lever, monkeys chose the larger dose. Combining 3.2 µg/kg/infusion midazolam with 0.32 µg/kg/infusion remifentanil increased responding for the mixture over 0.32 µg/kg/infusion remifentanil alone, although monkeys chose remifentanil alone over mixtures containing smaller doses of remifentanil. When 10 µg/kg/infusion midazolam was combined with 0.1 µg/kg/infusion remifentanil, monkeys chose the mixture over 0.32 µg/kg/infusion remifentanil alone. Thus, monkeys prefer some opioid/benzodiazepine mixtures to larger doses of the opioid alone, suggesting that opioid/benzodiazepine coabuse might be due to increased potency (and possibly effectiveness) of opioids to produce reinforcing effects.

Critical needs in drug discovery for cessation of alcohol and nicotine polysubstance abuse

Polysubstance abuse of alcohol and nicotine has been overlooked in our understanding of the neurobiology of addiction and especially in the development of novel therapeutics for its treatment. Estimates show that as many as 92% of people with alcohol use disorders also smoke tobacco. The health risks associated with both excessive alcohol consumption and tobacco smoking create an urgent biomedical need for the discovery of effective cessation treatments, as opposed to current approaches that attempt to independently treat each abused agent. The lack of treatment approaches for alcohol and nicotine abuse/dependence mirrors a similar lack of research in the neurobiology of polysubstance abuse. This review discusses three critical needs in medications development for alcohol and nicotine co-abuse: (1) the need for a better understanding of the clinical condition (i.e. alcohol and nicotine polysubstance abuse), (2) the need to better understand how these drugs interact in order to identify new targets for therapeutic development and (3) the need for animal models that better mimic this human condition. Current and emerging treatments available for the cessation of each drug and their mechanisms of action are discussed within this context followed by what is known about the pharmacological interactions of alcohol and nicotine. Much has been and will continue to be gained from studying comorbid alcohol and nicotine exposure.

Self-administration of cocaine and remifentanil by monkeys under concurrent-access conditions

RATIONALE

Cocaine and opioids are often co-abused. Laboratory research has focused largely on the reinforcing effects of mixtures of drugs relative to the drugs alone. Less research has examined drug mixing by the subject under concurrent-access conditions.

OBJECTIVE

Self-administration of various doses of cocaine and remifentanil was examined under concurrent-access conditions. It was hypothesized that if cocaine and opioid combinations were more effective reinforcers than the single drugs, subjects would mix the two drugs by adjusting their responding to cocaine and an opioid alternative to maintain an optimal ratio of cocaine/remifentanil intake.

METHOD

Three male rhesus monkeys were allowed to self-administer cocaine (0.05-0.2 mg/kg/inj) or saline on one lever and remifentanil (0.05-0.4 μg/kg/inj) or saline on the other lever under concurrent fixed-ratio (FR) 10 schedules. Daily sessions lasted 2 h, and there was a 1-s timeout after every 10-s injection.

RESULTS

When saline and drug were concurrently available, responding on the saline-associated lever was low relative to the drug alternative. When cocaine and remifentanil were concurrently available, both drugs were self-administered above saline levels. Cocaine intake decreased, and remifentanil intake increased as a function of the remifentanil dose that was available. Conversely, cocaine intake and remifentanil intake did not change systematically as a function of the cocaine dose that was available.

CONCLUSION

Monkeys will mix cocaine and an opioid when the two drugs are available concurrently. However, there was no indication that monkeys titrated drug intake to maintain an optimal ratio of intake of the two compounds.

Exercise decreases speedball self-administration

AIMS

Epidemiological studies report that individuals who exercise are less likely to abuse drugs. Preclinical studies report that exercise, in the form of treadmill or wheel running, reliably decreases the self-administration of psychomotor stimulants and opioids. To date, preclinical studies have only examined the effects of exercise on responding maintained by individual drugs and not by combinations of multiple drugs. This limits the translational appeal of these studies because polydrug abuse is common among substance abusing populations. The purpose of this study was to examine the effects of exercise on the self-administration of speedball, a combination of cocaine and heroin that is frequently encountered in intravenous drug abusing populations.

MAIN METHODS

Female rats were obtained at weaning and assigned to sedentary or exercising conditions. Sedentary rats were housed in standard cages that permitted no exercise beyond normal cage ambulation; exercising rats were housed in similar cages with an activity wheel. After 6weeks, rats were implanted with intravenous catheters and trained to self-administer cocaine, heroin, and dose combinations of cocaine and heroin (i.e., speedball) on a progressive ratio schedule of reinforcement.

KEY FINDINGS

Doses of speedball maintained greater levels of responding than corresponding doses of cocaine and heroin alone. Importantly, responding maintained by cocaine, heroin, and speedball was lower in exercising rats than sedentary rats.

SIGNIFICANCE

These data indicate that exercise decreases the self-administration of speedball and suggest that exercise may reduce the abuse of drug combinations that have traditionally been resistant to treatment.

Drug specificity in drug versus food choice in male rats

Although different classes of drug differ in their mechanisms of reinforcement and effects on behavior, little research has focused on differences in self-administration behaviors maintained by users of these drugs. Persistent drug choice despite available reinforcement alternatives has been proposed to model behavior relevant to addiction. The present study used a within-subjects procedure, where male rats (Long-Evans, N = 16) were given a choice between cocaine (1.0 mg/kg/infusion) and food (a single 45-mg grain pellet) or between heroin (0.02 mg/kg/infusion) and food in separate phases (drug order counterbalanced). All rats were initially trained to self-administer each drug, and the doses used were based on previous studies showing that small subsets of rats tend to prefer drug over food reinforcement. The goal of the present study was to determine whether rats that prefer cocaine would also prefer heroin. Choice sessions consisted of 2 forced-choice trials with each reinforcer, followed by 14 free-choice trials (all trials separated by 10-min intertrial interval). Replicating previous results, small subsets of rats preferred either cocaine (5 of the 16 rats) or heroin (2 of the 16 rats) to the food alternative. Although 1 of the 16 rats demonstrated a preference for both cocaine and heroin to the food alternative, there was no relationship between degree of cocaine and heroin preference in individual rats. The substance-specific pattern of drug preference observed suggests that at least in this animal model, the tendencies to prefer cocaine or heroin in preference to a nondrug alternative are distinct behavioral phenomena.

Ethanol self-administration in rats responding under concurrent schedules for milk or ethanol plus milk

The relative reinforcing strength of drugs can be characterized by the distribution of operant behavior during the availability of other reinforcing stimuli. 'Choice' procedures are not widely used in rats, with the exception of ethanol self-administration in which there often is a choice between ethanol and water, which typically does not maintain much responding. A procedure was developed to evaluate the relative reinforcing strength of ethanol in rats when a similar appetitive reinforcer is concurrently available. Rats were trained to respond on two levers under concurrent fixed-ratio schedules of reinforcement with milk (1-50%) or ethanol+milk (4-32% ethanol+5-10% milk). Daily 60-min sessions began with a forced sample of each reinforcer, followed by the concurrent schedules. Under this schedule, rats preferentially allocated their responding to the ethanol-associated lever under conditions of ethanol+5% milk versus 5% milk, but neither preferred nor avoided ethanol when ethanol+10% milk versus 10% milk was available. When 8% ethanol+5% milk was available, 85±6% of responses were directed toward the ethanol-associated lever and the mean ethanol intake was 1.55±0.10 g/kg. The response rate decreased monotonically with the concentration of ethanol. Naltrexone injections did not affect the distribution of responding, but slightly decreased ethanol intake. It is concluded that stable behavior can be maintained under concurrent fixed-ratio schedules of ethanol and milk presentation in rats, resulting in intake of behaviorally active amounts of ethanol.

Extended heroin access increases heroin choices over a potent nondrug alternative

Epidemiological research shows that the proportion of drug users who become addicted to heroin is higher than to cocaine. Here we tested whether this difference could be due to a difference in the addiction liability between the two drugs. Addiction liability was assessed under a discrete-trials choice procedure by measuring the proportion of rats that prefer the drug over a potent alternative reward (ie, water sweetened with saccharin). Previous research on choice between self-administration of i.v. cocaine or sweet water showed that the proportion of cocaine-preferring rats remains relatively low and invariable (ie, 15%), even after extended drug access and regardless of past drug consumption (ie, total drug use before choice testing). By contrast, the present study shows that under similar choice conditions, the proportion of heroin-preferring rats considerably increases with extended heroin access (6-9 h per day for several weeks) and with past heroin consumption, from 11 to 51% at the highest past drug consumption level. At this level, the proportion of drug-preferring rats was about three times higher with heroin than with cocaine (51% vs 15%). This increase in the rate of heroin preference after extended heroin access persisted even after recovery from acute heroin withdrawal. Overall, these findings show that choice procedures are uniquely sensitive to different drugs and suggest that heroin is more addictive than cocaine. This higher addiction liability may contribute to explain why more drug users become addicted to heroin than to cocaine in epidemiological studies.

Conflation of cocaine seeking and cocaine taking responses in IV self-administration experiments in rats: methodological and interpretational considerations

IV drug self-administration is a special case of an operant task. In most operant experiments, the instrumental response that completes the schedule requirement is separate and distinct from the consumptive response (e.g. eating or drinking) that follows the delivery of the reinforcing stimulus. In most IV self-administration studies drug seeking and drug taking responses are conflated. The instrumental lever press or nose poke is also a consumptive response. The conflation of these two response classes has important implications for interpretation of the data as they are differentially regulated by dose and price. The types of pharmacological pretreatments that affect appetitive responses are not necessarily the same as those that affect consumptive responses suggesting that the neurobiology of the two response classes are to some extent controlled by different mechanisms. This review discusses how schedules of reinforcement and behavioral economic analyses can be used to assess the regulation of drug seeking and drug taking separately. New methods are described that allow the examination of appetitive or consumptive responding in isolation and provide subjects with greater control over the self-administered dose. These procedures provide novel insights into the regulation of drug intake. Cocaine intake patterns that result in large, intermittent spikes in cocaine levels are shown to produce increases in appetitive responding (i.e. drug seeking). The mechanisms that control drug intake should be considered distinct from appetitive and motivational processes and should be taken into consideration in future IV self-administration studies.

Cocaine versus food choice procedure in rats: environmental manipulations and effects of amphetamine

We have adapted a nonhuman primate model of cocaine versus food choice to the rat species. To evaluate the procedure, we tested cocaine versus food choice under a variety of environmental manipulations as well as pharmacological pretreatments. Complete cocaine-choice dose-effect curves (0-1.0 mg/kg/infusion) were obtained for each condition under concurrent fixed ratio schedules of reinforcement. Percentage of responding emitted on the cocaine-reinforced lever was not affected significantly by removal of cocaine-associated visual or auditory cues, but it was decreased after removal of response-contingent or response-independent cocaine infusions. Cocaine choice was sensitive to the magnitude and fixed ratio requirement of both the cocaine and food reinforcers. We also tested the effects of acute (0.32, 0.56, 1.0, 1.8 mg/kg) and chronic (0.1, 0.32 mg/kg/hr) d-amphetamine treatment on cocaine choice. Acute and chronic d-amphetamine had opposite effects, with acute increasing and chronic decreasing cocaine choice, similar to observations in humans and in nonhuman primates. The results suggest feasibility and utility of the choice procedure in rats and support its comparability to similar procedures used in humans and monkeys.

Examination of behavioral strategies regulating cocaine intake in rats

RATIONALE

It has long been observed that rats self-administer psychostimulants in a highly regular pattern. The inverse relationship between dose and rate of drug intake has been interpreted as a titration phenomenon wherein brain-cocaine levels are maintained within a range. Most studies examining this phenomenon have used fixed, unit doses in which case the only titration strategy available to the animal is to adjust inter-infusion intervals.

OBJECTIVES

In this study, we examined whether selection of dose size could also be a factor in regulation of intake. We used a schedule of reinforcement, under which the dose can vary through a wide range and is determined by the behavior of the animal.

METHODS

Rats self-administered cocaine using a behaviorally dependent dosing schedule of reinforcement, under which the size of each dose was determined by the length of time the lever was held down. The concentration of cocaine was changed across sessions.

RESULTS

Total pump-time self-administered decreased by 56 % following each doubling of the concentration, which led to an average 11 % increase in total intake. Similarly, estimated brain levels of cocaine increased by 12 % for each doubling of concentration. These adjustments were the result of manipulation of both the size and spacing of infusions.

CONCLUSIONS

In agreement with previous studies, the regular pattern of intake appears to be the result of a titration mechanism in which animals maintain brain levels of cocaine above some threshold. Compensatory regulation appeared to involve both the selection of dose size and inter-infusion intervals.

Preclinical Determinants of Drug Choice under Concurrent Schedules of Drug Self-Administration

Drug self-administration procedures have played a critical role in the experimental analysis of psychoactive compounds, such as cocaine, for over 50 years. While there are numerous permutations of this procedure, this paper will specifically focus on choice procedures using concurrent schedules of intravenous drug self-administration. The aims of this paper are to first highlight the evolution of drug choice procedures and then review the subsequent preclinical body of literature utilizing these choice procedures to understand the environmental, pharmacological, and biological determinants of the reinforcing stimulus effects of drugs. A main rationale for this paper is our proposition that choice schedules are underutilized in investigating the reinforcing effects of drugs in assays of drug self-administration. Moreover, we will conclude with potential future directions and unexplored scientific space for the use of drug choice procedures.

Drug specificity in extended access cocaine and heroin self-administration

Increased drug availability can precipitate a rapid escalation of drug consumption in both vulnerable humans and laboratory animals. Drug intake escalation is observed across a broad spectrum of drugs of abuse, including stimulants, opiates, ethanol and phencyclidine. Whether and to what extent the processes underlying escalated levels of drug intake vary across different substances is poorly understood. The present study sought to address this question in rats self-administering both cocaine and heroin-two addictive drugs with both common and different neurobiological effects. In experiment 1, we determined how cocaine intake is initially related to heroin intake in non-escalated rats with a limited access to both drugs. In experiment 2, two groups of rats were initially allowed to self-administer either cocaine or heroin for 1 hour per day and then after behavioral stabilization, for 6 hours per day to precipitate drug intake escalation. In each group, dose-injection functions for cocaine and heroin self-administration were generated. In experiment 1, regardless of the dose, rats with a high intake of one drug did not necessarily have a high intake of the alternate drug. In experiment 2, escalated levels of heroin or cocaine self-administration did not generalize to the other drug. This outcome was confirmed in a third drug substitution experiment following different access lengths to cocaine self-administration (i.e. 1, 4 and 8 hours). The processes underlying spontaneous and escalated drug overconsumption appear thus to vary across different drugs of abuse. More research should be devoted in the future to these differences.

The behavioral pharmacology of effort-related choice behavior: dopamine, adenosine and beyond

For many years, it has been suggested that drugs that interfere with dopamine (DA) transmission alter the "rewarding" impact of primary reinforcers such as food. Research and theory related to the functions of mesolimbic DA are undergoing a substantial conceptual restructuring, with the traditional emphasis on hedonia and primary reward yielding to other concepts and lines of inquiry. The present review is focused upon the involvement of nucleus accumbens DA in effort-related choice behavior. Viewed from the framework of behavioral economics, the effects of accumbens DA depletions and antagonism on food-reinforced behavior are highly dependent upon the work requirements of the instrumental task, and DA-depleted rats show a heightened sensitivity to response costs, especially ratio requirements. Moreover, interference with accumbens DA transmission exerts a powerful influence over effort-related choice behavior. Rats with accumbens DA depletions or antagonism reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and show increased selection of low reinforcement/low cost options. Nucleus accumbens DA and adenosine interact in the regulation of effort-related functions, and other brain structures (anterior cingulate cortex, amygdala, ventral pallidum) also are involved. Studies of the brain systems regulating effort-based processes may have implications for understanding drug abuse, as well as symptoms such as psychomotor slowing, fatigue or anergia in depression and other neurological disorders.

Cocaine self-administration in rats: hold-down procedures

For decades, researchers have used animal self-administration models to examine the effects drugs of abuse have on physiology and behavior. Sophisticated self-administration procedures have been developed to model many different aspects of drug addiction. The hold-down procedure provides animals with control over the amount of each injection. Holding the lever down turns the syringe pump on and subsequently releasing the lever turns the pump off. In this way, animals can hold the lever down for any duration of time thereby self-administering any dose on a continuous spectrum. This procedure eliminates some of the ambiguity in translating results from effects only observed at one unit dose and allows examination of which dose the animal "prefers" at different times.

Seeking-taking chain schedules of cocaine and sucrose self-administration: effects of reward size, reward omission, and α-flupenthixol

RATIONALE

In heterogeneous seeking-taking (ST) chain schedules of self-administration, seeking rewards and taking rewards are distinct actions, giving animals explicit control over their intake of the reward. However, the neurobehavioral characteristics of ST chain schedules are relatively unexplored.

OBJECTIVES

This study was made to evaluate two variants of ST chain schedules of self-administration to measure seeking and taking of sucrose and cocaine in rats.

METHODS

Rats had to respond on one lever (seeking lever) under a random interval (RI) or under a progressive ratio (PR) schedule, to gain access to a second lever (taking lever), responding on which under a fixed-ratio 1 (FR-1) schedule of reinforcement delivered the reward. We assessed the effects of reward size, reward omission, and administration of the dopamine receptor antagonist α-flupenthixol. The effects of α-flupenthixol on responding for cocaine or sucrose under an FR-1 schedule of reinforcement were also assessed.

RESULTS

Cocaine seeking under both schedules was reduced by decreasing reward size, reward omission, and α-flupenthixol treatment. Cocaine taking was decreased by α-flupenthixol treatment and reward omission, but not by altering reward size. Sucrose seeking was not affected by reward size, but was reduced by α-flupenthixol and reward omission. Sucrose taking was diminished by reward omission only. α-Flupenthixol increased cocaine but not sucrose intake under an FR-1 schedule of reinforcement.

CONCLUSIONS

Both ST(PR) and ST(RI) schedules can be used to assess seeking and taking of sucrose and cocaine. Dopaminergic neurotransmission mediates the positive subjective properties of cocaine but not sucrose and the motivational properties of both sucrose and cocaine.

Brain-cocaine concentrations determine the dose self-administered by rats on a novel behaviorally dependent dosing schedule

A novel behaviorally dependent dosing (BDD) schedule was used to examine the relationship between doses of cocaine self-administered by rats and brain drug levels within a session. The BDD schedule used a hold-down response to activate a syringe pump. The length of time the lever was held down determined the duration that the syringe pump was activated. In the first experiment, rats self-administered cocaine for daily 3 h sessions and brain levels of cocaine were modeled using well-established parameters. Although analysis revealed that rats self-administered doses within a predicted range, one extremely large dose was consistently observed at the beginning of each session when brain levels of cocaine were low. In the second experiment, we introduced a range of timeout periods (10-25 min) in order to produce variability in brain-cocaine concentrations. Animals self-administered larger doses immediately following each timeout period and the dose size was inversely correlated with the length of the timeout. These results show that the dose of cocaine that rats self-administer within a session is inversely related to the amount of drug on board.

Self-administration of cocaine and remifentanil by monkeys: choice between single drugs and mixtures

RATIONALE

Cocaine and opioids are often co-abused. As yet, however, there is no clear evidence that the drugs interact to make the mixture a more effective reinforcer.

OBJECTIVE

The present study examined the relative reinforcing potency and maximum effectiveness of the cocaine-opioid combination in monkeys given a choice between cocaine-opioid mixtures and the single-component drugs.

METHOD

Rhesus monkeys were allowed to choose between injections of cocaine (100 μg/kg/inj) and other doses of cocaine (10-560 μg/kg/inj) or remifentanil (0.03-3.0 μg/kg/inj). A dose-addition model was used to select dose combinations for mixtures of cocaine and remifentanil predicted to be equivalent to 100 μg/kg/inj of cocaine in reinforcing effect if the drugs were additive. The monkeys were then allowed to choose between (a) cocaine and mixtures predicted to be equivalent to 100 μg/kg/inj of cocaine, (b) increasing doses of the mixtures and the single-component drugs, and (c) cocaine or remifentanil at doses that were in the highest safe range.

RESULTS

Generally, monkeys preferred the mixtures over 100 μg/kg/inj of cocaine, evidence for superadditivity. However, preferences for the mixture ceased when relatively high doses of single-component drugs were offered as alternatives. When doses within the mixture were raised and offered with relatively high doses of the single drugs, there was no clear preference for either option. The highest dose of remifentanil was chosen over the highest dose of cocaine by all monkeys.

CONCLUSION

The current results indicate that cocaine-opioid combinations can be super-additive in terms of potency, but are not, at maximum, more effective than the single-component drugs.

Social defeat stress in rats: escalation of cocaine and "speedball" binge self-administration, but not heroin

RATIONALE

Exposure to intermittent episodes of social defeat stress can increase drug seeking and leads to intense drug taking in rats.

OBJECTIVES

This study investigated the consequences of repeated, intermittent social defeat stress on patterns of drug self-administration in rats with access to heroin, cocaine, or a heroin-cocaine combination ("speedball").

METHODS

Male Long-Evans rats were either handled (controls) or subjected to 25-min social defeat stress episodes on days 1, 4, 7, and 10 during confrontations with an aggressive resident. Ten days following the last defeat, rats were assessed for locomotor cross-sensitization in response to heroin or cocaine. Animals were then prepared with intrajugular catheters for drug self-administration. Separate groups of controls and defeated rats were examined for self-administration of heroin (experiment 1), a heroin-cocaine combination (experiment 2), or cocaine (experiment 3). Drug self-administration patterns were evaluated using fixed or progressive ratio schedules of reinforcement during limited access sessions or a 24-h unlimited access binge.

RESULTS

Rats with a history of intermittent social defeat stress showed sensitized locomotor behavior when challenged with heroin or cocaine relative to controls. During the 24-h binge session, defeated rats escalated cocaine-taking behavior (ca. 110 mg/kg vs. 66 mg/kg in controls), persisted in self-administering cocaine or the heroin-cocaine mixture for more hours, and showed a tendency for increased heroin-cocaine intake, but no effects on heroin taking.

CONCLUSIONS

A history of social defeat stress seems to preferentially promote escalated intake of cocaine but not heroin, unless a heroin-cocaine combination is available.

Comparison of self-administration behavior and responsiveness to drug-paired cues in rats running an alley for intravenous heroin and cocaine

RATIONALE

Evidence suggests that responsiveness to a drug-paired cue is predicted by the reinforcing magnitude of the drug during prior self-administration. It remains unclear, however, if this principle holds true when comparisons are made across drug reinforcers.

OBJECTIVE

The current study was therefore devised to test the hypothesis that differences in the animals' responsiveness to a cocaine- or heroin-paired cue presented during extinction would reflect differences in the patterns of prior cocaine and heroin runway self-administration.

METHODS

Rats ran a straight alley for single intravenous injections of either heroin (0.1 mg/kg/inj) or cocaine (1.0 mg/kg/inj) each paired with a distinct olfactory cue. Animals experienced 15 trials with each drug reinforcer in a counterbalanced manner. Start latencies, run times, and retreat behaviors (a form of approach-avoidance conflict) provided behavioral indices of the subjects' motivation to seek the reinforcer on each trial. Responsiveness to each drug-paired cue was assessed after 7, 14, or 21 days of non-reinforced extinction trials. Other animals underwent conditioned place preference (CPP) testing to ensure that the two drug reinforcers were capable of producing drug-cue associations.

RESULTS

While both drugs produced comparable CPPs, heroin served as a stronger incentive stimulus in the runway as evidenced by faster start and run times and fewer retreats. In contrast, cocaine- but not heroin-paired cues produced increases in drug-seeking behavior during subsequent extinction trials.

CONCLUSIONS

The subjects' responsiveness to drug-paired cues during extinction was not predicted by differences in the motivation to seek heroin versus cocaine during prior drug self-administration.

Cocaine self-administration on a hold-down schedule of reinforcement in rats

RATIONALE AND OBJECTIVE

Although many contingencies operating in the natural environment include continuous dimensions of responses and reinforcers, previous studies of drug self-administration have almost exclusively used discrete dimensions of responses (e.g., a lever press) and reinforcers (e.g., 1.0 mg/kg/injection cocaine). Therefore, the present study provides an initial examination under experimental conditions with both responses and reinforcers measured along continuous dimensions.

MATERIALS AND METHODS

Cocaine-maintained responding was studied in rats under a novel, hold-down schedule of reinforcement wherein the duration of the response was directly related to the magnitude of the reinforcer. These conditions were established by activating the syringe pump when the lever was pressed down and turning the pump off when the lever was released. The concentration of cocaine available in the syringe was varied across sessions.

RESULTS

Cocaine self-administration was readily maintained under these conditions and remained stable across sessions. Responding was concentration dependent, with the number of responses and total duration of the response inversely related to concentration, and overall session intake of cocaine was stable across concentrations. In general, the duration of the responses were less than 0.5 s and did not vary as a function of concentration.

CONCLUSIONS

Stability of responding under these schedule conditions was acquired quickly. This schedule of reinforcement may be useful for comparing across drug classes, can be extended for use with other types of responses and reinforcers, and may be more representative of the natural world where response-reinforcer contingencies are more likely to be experienced along continuous, rather than discrete, dimensions.

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.

Cocaine self-administration reinforced on a progressive ratio schedule decreases with continuous D-amphetamine treatment in rats

RATIONALE

To date, there is no medication specifically approved for cocaine addiction. Agonist medications are used clinically in the treatment of other addictions, which suggests that this method of drug therapy could potentially be successful in treating cocaine addiction as well.

OBJECTIVE

The objective of this study was to determine the effect of extended D-amphetamine treatment on responding on a progressive ratio (PR) schedule reinforced by cocaine.

MATERIALS AND METHODS

Rats were trained to self-administer cocaine (0.19, 0.38, 0.75, or 1.5 mg/kg/injection) or food on a PR schedule. After stable baseline breakpoints (the number of reinforcers earned in one session) were established over 3 days, animals were implanted with osmotic mini-pumps that continuously delivered D-amphetamine (5 mg/kg/day) for a duration of either 7 or 14 days. Breakpoints were then determined during and/or after this treatment period.

RESULTS

Rats demonstrated dose-dependent decreases in cocaine-reinforced responding over the D-amphetamine treatment period. Breakpoints for doses of 0.75 mg/kg/injection and below decreased significantly when compared to baseline and remained decreased for up to 14 days after mini-pump removal, whereas those for the highest dose of cocaine remained unchanged. Additionally, D-amphetamine treatment during a 14-day abstinence period from cocaine self-administration had no effect on breakpoints when tested the day after mini-pump removal.

CONCLUSIONS

These data suggest that the reduction in cocaine-reinforced responding after continuous D-amphetamine treatment cannot be accounted for by tolerance alone. Instead, the roles of learning and the interaction between cocaine and D-amphetamine must be considered and examined in future studies.

Self-administration of cocaine-remifentanil mixtures by monkeys: an isobolographic analysis

RATIONALE

Abuse of mixtures of stimulants and opioids ("speedball") is common. Although this combination has been studied in the laboratory, conclusions about the nature of the cocaine/opioid interaction have been mixed.

OBJECTIVES

The objectives of the present experiment were to allow monkeys to self-administer mixtures of cocaine and the mu opioid agonist remifentanil and to quantify the interaction using the isobolographic approach. Our hypothesis was that the drugs would be super-additive in their reinforcing effects.

MATERIALS AND METHODS

Rhesus monkeys (n = 5) prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule. When responding was stable, doses of cocaine or remifentanil were made available in test sessions. Next, mixtures of doses of the drugs were tested over a range of doses in 1:1, 1:2, and 2:1 ratios of their ED(50)s. Results were analyzed using isobolographic techniques.

RESULTS

Both drugs alone and all drug mixtures functioned as positive reinforcers in a dose-related manner. Cocaine maintained more responding at maximum than did remifentanil, i.e., was a stronger reinforcer. The experimentally determined equi-effective dose for the 1:1 and 1:2 cocaine/remifentanil mixtures tended toward super-additivity, but the difference from additivity did not achieve statistical significance. The 2:1 mixture was super-additive. Maximum responding maintained by the mixtures was higher than that maintained by remifentanil but not different from cocaine.

CONCLUSIONS

Combinations of cocaine and remifentanil can be additive or super-additive as positive reinforcers, depending on proportions of each. Interactions between stimulants and opioids may contribute to the abuse of these mixtures.

Effect of drug-paired exteroceptive stimulus presentations on methamphetamine reinstatement in rats

The purpose of the present study was to examine the impact of drug-paired cues on methamphetamine reinstatement. Three groups of rats were trained to self-administer 0.1 mg/kg/infusion methamphetamine. Each methamphetamine infusion was accompanied by a 6 s flashing light+tone stimulus (cues). After training, the groups were then given 12, daily extinction sessions either with or without response-contingent drug-paired cues and then tested for 1 mg/kg i.p. methamphetamine priming-induced reinstatement either with or without cues. Methamphetamine priming significantly reinstated drug-appropriate responding regardless of whether response-contingent cues were omitted during both extinction and testing, presented during both extinction and testing, or omitted during extinction but presented during reinstatement testing. The group in which cues were omitted during extinction and presented during reinstatement exhibited significantly greater reinstatement than did the other two groups. A separate group of rats was also tested demonstrating that response-contingent presentation of previously methamphetamine-paired cues alone, without methamphetamine priming, significantly reinstated drug-appropriate responding. These data show that methamphetamine priming produces a robust reinstatement effect which can be influenced by drug-paired cues.

Self-administration of drug mixtures by monkeys: combining drugs with comparable mechanisms of action

RATIONALE

Abuse of drug mixtures is common. Drug interactions that are super-additive in terms of reinforcing effects may contribute to this phenomenon. Although quantitative methods for assessing drug interactions have been developed, they have not been widely applied to the analysis of reinforcing effects.

OBJECTIVES

The present experiment was designed to study self-administration of mixtures of drugs with comparable pharmacological mechanisms of action. Our hypothesis was that the drugs would be dose-additive.

MATERIALS AND METHODS

Rhesus monkeys prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule in baseline sessions. When responding was stable, two mu opioid agonists, alfentanil and remifentanil, were tested alone in one group (n = 5). Two dopamine (DA) uptake blockers, cocaine and RTI-117 were tested in the other group (n = 6). Next, mixtures of doses of the two opioids or the two DA uptake blockers were tested in approximate 1:1, 1:2, and 2:1 ratios of their ED50s. Results were analyzed using isobolographic techniques.

RESULTS

All drugs alone and drug mixtures functioned as positive reinforcers in a dose-related manner. There was no difference between experimentally determined ED50 values and predicted additive ED50 values for any mixture. Maximum responding maintained by mixtures, a measure of reinforcing strength, did not differ from that for single drugs.

CONCLUSIONS

Mixtures of various proportions of two drugs with comparable mechanisms of action were additive, i.e., they did not interact. This result will serve as the basis for comparison to studies of mixtures of drugs with various mechanisms of action.

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.

A behavioral economic analysis of cocaine and remifentanil self-administration in rhesus monkeys

RATIONALE

Behavioral economics can be used to evaluate the relative reinforcing effectiveness of drugs and the economic interaction between drugs, information which may help to explain patterns of polydrug abuse in humans.

OBJECTIVES

In phase 1, the reinforcing effectiveness of the opiate remifentanil and the stimulant cocaine was compared using a demand-curve analysis. In phase 2, the economic relation between these drugs was determined.

MATERIALS AND METHODS

Rhesus monkeys pressed levers according to fixed-ratio schedules for intravenous drug infusions. A demand-curve analysis was conducted (phase 1) in which drug consumption was measured as the response requirement, or price, was increased, and the rate at which consumption decreased with increases in price (demand elasticity) provided an index of the reinforcing effectiveness of each drug. Cocaine and remifentanil were then available concurrently (phase 2), and the price of one drug was increased (the manipulated-price alternative) while the price of the other drug was held constant (the fixed-price alternative). Consumption of the fixed-price alternative was measured as a function of increases in the price of the manipulated-price alternative, and demand for the manipulated-price alternative was assessed.

RESULTS

The reinforcing effectiveness of cocaine and remifentanil did not significantly differ, and these drugs functioned as economic substitutes. As the price of the manipulated-price alternative increased, consumption of the fixed-price alternative increased. In addition, demand for the manipulated-price alternative became more elastic with the concurrent availability of the fixed-price alternative.

CONCLUSION

Polydrug use involving stimulants and opiates may occur because these drugs are highly substitutable.

Effects of d-amphetamine and buprenorphine combinations on speedball (cocaine+heroin) self-administration by rhesus monkeys

The simultaneous i.v. administration of heroin and cocaine, called a 'speedball,' is often reported clinically, and identification of effective pharmacotherapies is a continuing challenge. We hypothesized that treatment with combinations of a monoamine releaser d-amphetamine, and a mu partial agonist, buprenorphine, might reduce speedball self-administration by rhesus monkeys. Speedballs (0.01 mg/kg/inj cocaine+0.0032 mg/kg/inj heroin) and food (1 g banana-flavored pellets) were available during four daily sessions on a second-order schedule of reinforcement (fixed ratio (FR)2 (variable ratio (VR)16:S)). Monkeys were treated for 10 days with saline or ascending doses of d-amphetamine (0.0032-0.032 mg/kg/h)+buprenorphine (0.075 or 0.237 mg/kg/day) in combination. d-Amphetamine+both doses of buprenorphine produced an amphetamine dose-dependent decrease in speedball self-administration in comparison to the saline treatment baseline (P<0.01-0.001), but food-maintained responding did not change significantly. d-Amphetamine alone (0.032 mg/kg/h) significantly decreased both food (P<0.01) and speedball-maintained responding (P<0.05). During saline control treatment, speedball unit doses of 0.0032 mg/kg/inj cocaine+0.001 mg/kg/inj heroin were at the peak of the speedball dose-effect curve. Daily treatment with 0.01 mg/kg/h d-amphetamine+0.237 mg/kg/day buprenorphine produced a significant downward and rightward shift in the speedball dose-effect curve (P<0.01) and no significant effect on food-maintained responding. A significant decrease in speedball self-administration was sustained over 10 days of treatment. These findings are consistent with our previous reports and suggest that medication mixtures designed to target both the stimulant and the opioid component of the speedball may be an effective approach to polydrug abuse treatment.

Spectroscopic and electrochemical studies of cocaine–opioid interactions

The drugs of abuse cocaine (C), heroin (H), and morphine (M) have been studied to enable understanding of the occurrence of cocaine-opioid interactions at a molecular level. Electrochemical, Raman, and NMR studies of the free drugs and their mixtures were used to study drug-drug interactions. The results were analyzed using data obtained from quantum-mechanical calculations. For the cocaine-morphine mixture (C-MH), formation of a binary complex was detected; this involved the 3-phenolic group and the heterocyclic oxygen of morphine and the carbonyl oxygen and the methyl protons of cocaine's methyl ester group. NMR studies conducted simultaneously also revealed C-MH binding geometry consistent with theoretical predictions and with electrochemical and vibrational spectroscopy results. These results provide evidence for the occurrence of a cocaine-morphine interaction, both in the solid state and in solution, particularly for the hydrochloride form. A slight interaction, in solution, was also detected by NMR for the cocaine-heroin mixture.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

The effects of chronic buprenorphine on intake of heroin and cocaine in rats and its effects on nucleus accumbens dopamine levels during self-administration

RATIONALE

Buprenorphine reduces both heroin and cocaine intake in opioid addicts, but the mechanisms remain unclear.

OBJECTIVES

To determine the effects of chronic buprenorphine treatment on intake of heroin and/or cocaine and measure nucleus accumbens (NAc) dopamine (DA) levels during self-administration.

METHODS

In experiment 1, plasma levels of buprenorphine were determined in rats with buprenorphine osmotic minipumps (3.0 mg/kg/day) using an ELISA. In experiment 2, rats self-administered (FR1) one dose of heroin [(0.025, 0.05, or 0.1 mg/kg/infusion (inf)] and one dose of cocaine (0.25, 0.5, or 1.0 mg/kg/inf) before and under sham or chronic buprenorphine treatment (1.5 or 3.0 mg/kg/day). In experiment 3, the effect of sham or chronic buprenorphine treatment (3.0) on heroin (0.05 mg/kg/inf) or cocaine (0.5 mg/kg/inf) self-administration under FR5 and progressive ratio (PR) schedules was evaluated. In experiment 4, in vivo microdialysis sampling from the NAc was carried out during heroin (0.05 mg/kg/inf) or cocaine (0.5 mg/kg/inf) self-administration (FR1) under sham or buprenorphine treatment (3.0).

RESULTS

Buprenorphine levels in plasma were stable over time. Buprenorphine treatment had no effect on total heroin intake at any dose or under any schedule, whereas it suppressed cocaine intake at all doses and under all schedules. Buprenorphine enhanced basal levels of DA, attenuated the NAc DA response to heroin, and enhanced the DA response to cocaine. It is interesting to note that buprenorphine increased the latency to respond to drug-associated cues at the start of self-administration sessions.

CONCLUSIONS

Chronic buprenorphine reduces cocaine, but not heroin, intake and possibly reduces drug seeking by reducing the salience of the drug-associated cues.

Self-administration of heroin, cocaine and their combination under a discrete trial schedule of reinforcement in rats

Several self-administration models have been used to study the interactions between cocaine and heroin, and the schedule of reinforcement used is an important consideration for these studies. Behavior maintained by cocaine, heroin or their combination was studied using a discrete trial schedule that has been described for cocaine self-administration previously. This schedule permits 24 h access to drug without mortality associated with unlimited access to cocaine, and provides unique measures related to the circadian pattern of drug self-administration. Cocaine and heroin combined maintained higher rates of responding compared to either drug alone when a maximum of three infusions were available each hour (DT3), and decreased food intake compared to cocaine alone. There were significantly greater numbers of hours in both the light and dark cycles during which animals self-administered heroin or the combination of cocaine/heroin compared to cocaine alone. When the FR was increased to 4 under the DT3 access conditions, responding maintained by cocaine or heroin extinguished to levels not different than those maintained by saline while food reinforcement remained intact. The combination of cocaine and heroin maintained robust responding under these conditions. This schedule of reinforcement appears to elucidate behavioral interactions between cocaine and heroin that are more complex than rate of drug consumption and may provide a procedure to address some of the issues related to co-abuse of these drugs.