Effects of acute and chronic aripiprazole treatment on choice between cocaine self-administration and food under a concurrent schedule of reinforcement in rats

Effects of naltrexone on amphetamine choice in rhesus monkeys and rats

Clinical amphetamine use is constrained by high abuse potential, and amphetamine use disorder is a persistent clinical problem with no approved medications for its treatment. The opioid antagonist naltrexone has been reported to reduce some abuse-related effects of amphetamine. This study used an amphetamine-versus-food choice procedure in rhesus monkeys and rats to test the hypothesis that naltrexone might serve as either (a) a maintenance medication for amphetamine use disorder treatment or (b) an "abuse-deterrent" adjunct to clinical amphetamine formulations. Male rhesus monkeys and male and female rats were trained to choose between increasing unit doses of intravenous amphetamine and an alternative food reinforcer during daily behavioral sessions. Experiment 1 evaluated effectiveness of continuous naltrexone maintenance to reduce amphetamine-versus-food choice in both monkeys and rats. Experiment 2 combined naltrexone with amphetamine in fixed-proportion amphetamine + naltrexone mixtures to evaluate the effectiveness of naltrexone in both species to reduce mixture choice relative to amphetamine-alone choice. Amphetamine maintained a dose-dependent increase in amphetamine choice in both monkeys and rats. Naltrexone maintenance did not significantly decrease amphetamine choice in either species. Addition of naltrexone to amphetamine reduced amphetamine choices per session in monkeys, but behavior was not reallocated to food choice, and in rats, the addition of naltrexone only decreased food choice without significantly affecting amphetamine choice. These results argue against the use of naltrexone as either (a) a maintenance medication for treatment of amphetamine use disorder or (b) an "abuse-deterrent" adjunct to amphetamine for clinical applications. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

The Effects of the Dopamine Transporter Ligands JJC8-088 and JJC8-091 on Cocaine versus Food Choice in Rhesus Monkeys

Although there are no Food and Drug Administration-approved treatments for cocaine use disorder, several modafinil analogs have demonstrated promise in reducing cocaine self-administration and reinstatement in rats. Furthermore, the range of dopamine transporter (DAT) compounds provides an opportunity to develop pharmacotherapeutics without abuse liability. This study extended the comparison of JJC8-088 and JJC8-091, the former compound having higher DAT affinity and predicted abuse liability, to rhesus monkeys using a concurrent cocaine versus food schedule of reinforcement. First, binding to striatal DAT was examined in cocaine-naïve monkey tissue. Next, intravenous pharmacokinetics of both JJC compounds were evaluated in cocaine-experienced male monkeys (n = 3/drug). In behavioral studies, acute and chronic administration of both compounds were evaluated in these same monkeys responding under a concurrent food versus cocaine (0 and 0.003-0.1 mg/kg per injection) schedule of reinforcement. In nonhuman primate striatum, JJC8-088 had higher DAT affinity compared with JJC8-091 (14.4 ± 9 versus 2730 ± 1270 nM, respectively). Both JJC compounds had favorable plasma pharmacokinetics for behavioral assessments, with half-lives of 1.1 hours and 3.5 hours for JJC8-088 (0.7 mg/kg, i.v.) and JJC8-091 (1.9 mg/kg, i.v.), respectively. Acute treatment with both compounds shifted the cocaine dose-response curve to the left. Chronic treatment with JJC8-088 decreased cocaine choice in two of the three monkeys, whereas JJC8-091 only modestly reduced cocaine allocation in one monkey. Differences in affinities of JJC8-091 DAT binding in monkeys compared with rats may account for the poor rodent-to-monkey translation. Future studies should evaluate atypical DAT blockers in combination with behavioral interventions that may further decrease cocaine choice. SIGNIFICANCE STATEMENT: Cocaine use disorder (CUD) remains a significant public health problem with no Food and Drug Administration-approved treatments. The ability of drugs that act in the brain in a similar manner to cocaine, but with lower abuse liability, has clinical implications for a treatment of CUD.

Choosing between cocaine and sucrose under the influence: testing the effect of cocaine tolerance

RATIONALE

Cocaine use not only depends on the reinforcing properties of the drug, but also on its pharmacological effects on alternative nondrug activities. In animal models investigating choice between cocaine and alternative sweet rewards, the latter influence can have a dramatic impact on choice outcomes. When choosing under cocaine influence is prevented by imposing sufficiently long intervals between choice trials, animals typically prefer the sweet reward. However, when choosing under the drug influence is permitted, animals shift their preference in favor of cocaine.

OBJECTIVES

We previously hypothesized that this preference shift is mainly due to a direct suppression of responding for sweet reward by cocaine pharmacological effects. Here we tested this hypothesis by making rats tolerant to this drug-induced behavioral suppression.

RESULTS

Contrary to our expectation, tolerance did not prevent rats from shifting their preference to cocaine when choosing under the influence.

CONCLUSION

Thus, other mechanisms must be invoked to explain the influence of cocaine intoxication on choice outcomes.

Effects of dopamine D1-like receptor ligands on food-cocaine choice in socially housed male cynomolgus monkeys

Although dopamine is known to play a prominent role in mediating the abuse-related effects of cocaine, the specific roles of dopamine receptor subtypes are not fully understood. Whereas the effects of drugs acting at dopamine D2-like receptors (D2R) have been characterized, less is known about dopamine D1-like receptors (D1R). The present experiments examined the effect of drugs with varying intrinsic efficacy at D1R on the relative reinforcing strength of cocaine in male cynomolgus monkeys. Use of socially housed monkeys permitted the assessment of whether social status influenced the behavioral effects of D1R-acting drugs. The high-efficacy D1R agonist SKF 81297, low-efficacy D1R agonist SKF 38393 and D1R antagonist SCH 23390 were administered acutely to monkeys self-administering cocaine under a food-cocaine choice procedure in which a cocaine choice dose-effect curve was determined daily. To assess selectivity of behavioral effects on cocaine choice, effects of doses that did not disrupt responding (indicated by a {greater than or equal to}35% decrease in total reinforcers delivered) were analyzed. Neither SKF 81297 nor SCH 23390 affected cocaine choice in dominant or subordinate monkeys. However, the low-efficacy agonist SKF 38393 selectively decreased cocaine choice; this effect was larger and only reached statistical significance in subordinate monkeys. Increasing the pretreatment time did not affect these results. The results indicate that, like D2R-acting drugs, the behavioral effects of D1R-acting drugs on cocaine choice can differ according to intrinsic efficacy and social status. Moreover, they demonstrate that D1R-acting drugs affect behavior under a narrower range of conditions than D2R-acting drugs. Significance Statement Cocaine use disorder remains a significant public health problem with no FDA-approved treatments. Although dopamine receptors have been strongly implicated in mediating the abuse-related effects of cocaine, the roles of dopamine receptor subtypes are incompletely understood. The present study in nonhuman primates found that cocaine choice was decreased only by a low-efficacy D1R agonist, and this effect was dependent on the social status of the monkey.

A drug-vs-food "choice" self-administration procedure in rats to investigate pharmacological and environmental mechanisms of substance use disorders

BACKGROUND

Preclinical drug self-administration procedures are commonly used to investigate expression, mechanisms, and treatment of substance use disorders.

NEW METHOD

The aims were to back-translate an intravenous drug-vs-food choice procedure primarily utilized in monkeys to male and female rats and to develop a surgical method for sustained intravenous catheter patency suitable for long-term drug-choice studies.

RESULTS

The surgical protocol resulted in a median intravenous jugular catheter patency in male and female rats of 126 days (range: 25-365 days). Drug-vs-food choice was established with opioids (fentanyl and heroin), psychostimulants (cocaine, methamphetamine, and amphetamine), and an opioid/psychostimulant mixture (fentanyl + methamphetamine). The average time from catheter implantation to stable choice behavior across all drugs was 27 sessions (range: 16-44 sessions). Choice behavior stabilized more quickly for cocaine and fentanyl than for other drugs. Manipulations of both environmental variables (e.g., response requirement or food reinforcer magnitude) and pharmacological variables (e.g., extended access drug self-administration or continuous buprenorphine treatment via osmotic pump) significantly shifted opioid-vs-food choice consistent with previous monkey studies.

COMPARISON WITH EXISTING METHODS

Duration of intravenous catheter patency in rats was suitable for long-term, within-subject drug choice studies. Effects of environmental and pharmacological manipulations in rats confirmed and extended previous results from monkeys.

CONCLUSIONS

The concordance of behavioral results between rats and monkeys using the present drug-vs-food choice procedure supports its utility to improve our basic understanding of the expression and mechanisms of substance use disorders towards to development of more effective therapeutics.

Effects of the monoamine stabilizer, (-)-OSU6162, on cocaine-induced locomotion and conditioned place preference in mice

Cocaine addiction is a severe mental disorder for which few treatment options are available. The underlying mechanisms include facilitation of monoamine-neurotransmission, particularly dopamine. Here, we tested the hypothesis that the monoamine stabilizers, (-)-OSU6162 ((3S)-3-(3-methylsulfonylphenyl)-1-propylpiperidine) and aripiprazole (7-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butoxy]-3,4-dihydro-1H-quinolin-2-one), prevent cocaine-induced behaviors. Male Swiss mice received injections of (-)-OSU6162 or aripiprazole and cocaine and were tested for cocaine-induced hyperlocomotion, locomotor sensitization, and acquisition and expression of conditioned place preference (CPP). The increase in the distance traveled induced by cocaine (20 mg/kg) was prevented by pretreatment with aripiprazole (1 and 10 mg/kg), whereas (-)-OSU6162 (3 mg/kg) exerted a minor effect. Aripiprazole, however, also impaired spontaneous locomotion. Neither (-)-OSU6162 nor aripiprazole interfered with the locomotor sensitization and expression of CPP induced by cocaine (15 mg/kg). (-)-OSU6162 (3 mg/kg), but not aripiprazole, prevented the acquisition of CPP induced by cocaine (15 mg/kg). (-)-OSU6162 exerts a minor effect in reducing cocaine-induced stimulatory activity and context-related memories, which are responsible for triggering drug seeking. Further studies are required to establish whether (-)-OSU6162 could be a candidate drug for the treatment of cocaine addiction.

Effects of muscarinic M1 receptor stimulation on reinforcing and neurochemical effects of cocaine in rats

Cocaine addiction is a chronic illness characterized by maladaptive drug-induced neuroplastic changes that confer lasting vulnerability to relapse. Over several weeks we observed the effects of the M₁ receptor-selective agonist VU0364572 in adult male rats that self-administer cocaine in a cocaine vs. food choice procedure. The drug showed unusual long-lasting effects, as rats gradually stopped self-administering cocaine, reallocating behavior towards the food reinforcer. The effect lasted as long as tested and at least 4 weeks. To begin to elucidate how VU0364572 modulates cocaine self-administration, we then examined its long-term effects using dual-probe in vivo dopamine and glutamate microdialysis in nucleus accumbens and medial prefrontal cortex, and ex vivo striatal dopamine reuptake. Microdialysis revealed marked decreases in cocaine-induced dopamine and glutamate outflow 4 weeks after VU0364572 treatment, without significant changes in dopamine uptake function. These lasting and marked effects of M₁ receptor stimulation reinforce our interest in this target as potential treatment of cocaine addiction. M₁ receptors are known to modulate medium spiny neuron responses to corticostriatal glutamatergic signaling acutely, and we hypothesize that VU0364572 may oppose the addiction-related effects of cocaine by causing lasting changes in this system.

Improving translation of animal models of addiction and relapse by reverse translation

Critical features of human addiction are increasingly being incorporated into complementary animal models, including escalation of drug intake, punished drug seeking and taking, intermittent drug access, choice between drug and non-drug rewards, and assessment of individual differences based on criteria in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Combined with new technologies, these models advanced our understanding of brain mechanisms of drug self-administration and relapse, but these mechanistic gains have not led to improvements in addiction treatment. This problem is not unique to addiction neuroscience, but it is an increasing source of disappointment and calls to regroup. Here we first summarize behavioural and neurobiological results from the animal models mentioned above. We then propose a reverse translational approach, whose goal is to develop models that mimic successful treatments: opioid agonist maintenance, contingency management and the community-reinforcement approach. These reverse-translated 'treatments' may provide an ecologically relevant platform from which to discover new circuits, test new medications and improve translation.

Trying to make sense of rodents' drug choice behavior

Since the first experimental hint for the existence of "an actual desire or striving for the drug" in nonhuman animals by Sidney Spragg in the late 1930s, much effort has been expended by lab researchers to try to model in a valid manner the key behavioral aspects and signs of addiction in animals, typically in rodents (i.e., mainly rats and, to a lesser extent, mice). Despite much advances, there still remains a lingering doubt about the disordered status of drug use in rodents. This is mainly because drug use occurs in a particular setting where animals have access to a drug for self-administration but without access to other valuable behavioral options that could compete with and divert from drug use. Here I review evidence showing that enriching the drug setting with other behavioral options can dramatically influence the pattern of drug choices in rodents. Overall, access to other options during drug access can divert the vast majority of rats from continued drug use. Only few individuals continue to engage in drug use despite access to and at the expense of other options. However, there exist certain high-risk settings in which virtually all animals are vulnerable to develop a harmful pattern of exclusive drug use that can even become fatal in the long run if not discontinued by an outside intervention. Paradoxically, it appears that the behavioral trait that is hypothesized to uniquely render rodents vulnerable to the latter settings (i.e., a narrow focus on the local, current choice, with no consideration of the global pattern of choice) would also protect most of them from using drugs in other choice settings. I conclude with an attempt to make sense of this peculiar setting-specific behavior and with some general propositions for future research.

Aripiprazole for cocaine abstinence: a randomized-controlled trial with ecological momentary assessment

Aripiprazole blocks psychostimulant seeking in a rat model of relapse. However, in humans, it may increase ongoing use. We tested aripiprazole specifically for relapse prevention. Methadone-maintained outpatients who were abstinent from cocaine in weeks 11-12 were randomized to double-blind aripiprazole (15 mg daily) or placebo in weeks 13-27 after 12 weeks of contingency management. Participants reported craving through ecological momentary assessment. We stopped the trial because very few (18/41) participants fulfilled the abstinence criterion. The results suggested that aripiprazole delayed lapse [hazard ratio (HR)=0.45, 95% confidence interval (CI)=0.14-1.42, P=0.17] and relapse (HR=0.31, 95% CI=0.07-1.27, P=0.10), but the effects did not reach statistical significance. Unexpectedly, the proportion of participants reporting cocaine craving was higher in the aripiprazole group (Fisher's exact P=0.026), although the frequency of craving was similar in the aripiprazole and placebo groups (1.89 vs. 1.16%, reffect=0.43, 95% CI=-0.08-0.76). The results suggest that in recently abstinent cocaine users, aripiprazole might delay relapse, but might also slightly increase craving. Difficulty in trial implementation underscores the fact that initial abstinence from cocaine is not a trivial hurdle.

Late Reduction of Cocaine Cravings in a Randomized, Double-Blind Trial of Aripiprazole vs Perphenazine in Schizophrenia and Comorbid Cocaine Dependence

PURPOSE

Co-occurring schizophrenia spectrum disorder and International Statistical Classification of Diseases, 10th Revision cocaine dependence present a particularly destructive constellation that is often difficult to treat. Both conditions raise dopamine transmission effects in the brain. Traditional neuroleptics block dopamine receptors, whereas aripiprazole modulates dopamine activity as an agonist/antagonist. We tested whether dopamine modulation is superior to dopamine blocking in dual-diagnosis patients.

METHODS

In a randomized, double-blind, comparison design, cocaine-dependent schizophrenic subjects actively using cocaine received either aripiprazole or perphenazine in an 8-week trial. Primary outcome targeted cocaine-free urine sample proportions, whereas cocaine craving scores were a secondary variable.

RESULTS

Subjects (N = 44) randomized (n = 22 per group) did not differ at baseline. The proportion of cocaine-free urine samples did not differ by medication group. Contrasting weeks 3 to 5 vs 6 to 8 revealed significant late reductions in craving with aripiprazole. On the respective 5-point subscales, craving intensity decreased by 1.53 ± 0.43 (P < 0.0005) points, craving frequency by 1.4 ± 0.40 (P > 0.0004) points, and craving duration by 1.76 ± 0.44 (P > 0.0001) points.

CONCLUSIONS

A drug effect of aripiprazole on craving items appeared at week 6 of treatment, on average, and was not seen before that length of drug exposure. The data suggest that dopamine modulation reduces cocaine cravings but requires an acclimation period. To understand the mechanism of action better, a trial of depot aripiprazole may be useful. Clinically, a reduction in craving potentially offers a clearer focus for ongoing behavioral treatment. It may also offer a longer-term treatment effect with respect to the severity of relapse.

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.

Effects of Acute and Chronic Treatments with Dopamine D2 and D3 Receptor Ligands on Cocaine versus Food Choice in Rats

Dopamine D₃ receptor ligands are potential medications for psychostimulant addiction. Medication assessment may benefit from preclinical studies that evaluate chronic medication effects on choice between an abused drug and an alternative, nondrug reinforcer. This study compared acute and chronic effects of dopamine D₂- and D₃-preferring ligands on choice between intravenous cocaine and palatable food in rats. Under baseline conditions, cocaine maintained dose-dependent increases in cocaine choice and reciprocal decreases in food choice. Acutely, the D₂ agonist R-(-)-norpropylapomorphine (NPA) and antagonist L-741,626 [3-[[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole] produced leftward and rightward shifts in cocaine dose-effect curves, respectively, whereas the partial agonist terguride had no effect. All three drugs dose-dependently decreased food-maintained responding. Chronically, the effects of R-(-)-norpropylapomorphine and L-741,626 on cocaine self-administration showed marked tolerance, whereas suppression of food-reinforced behavior persisted. Acute effects of the D₃ ligands were less systematic and most consistent with nonselective decreases in cocaine- and food-maintained responding. Chronically, the D₃ agonist PF-592,379 [5-[(2R,5S)-5-methyl-4-propylmorpholin-2-yl]pyridin-2-amine] increased cocaine choice, whereas an intermediate dose of the D₃ antagonist PG01037 [N-[(E)-4-[4-(2,3-dichlorophenyl)piperazin-1-yl]but-2-enyl]-4-pyridin-2-ylbenzamide] produced a therapeutically desirable decrease in cocaine choice early in treatment; however, tolerance to this effect developed, and lower and higher doses were ineffective. D₃ ligands failed to significantly modify total cocaine intake but caused persistent decreases in food intake. Thus, D₂-and D₃-preferring ligands showed distinct profiles, consistent with different pharmacological actions. In addition, these results highlight the role of acute versus chronic treatment as a determinant of test drug effects. With the possible exception of the D₃ antagonist PG01037, no ligand was promising in terms of cocaine addiction treatment.

Role of Dopamine 2 Receptor in Impaired Drug-Cue Extinction in Adolescent Rats

Adolescent drug users display resistance to treatment such as cue exposure therapy (CET), as well as increased liability to relapse. The basis of CET is extinction learning, which involves dopamine signaling in the medial prefrontal cortex (mPFC). This system undergoes dramatic alterations during adolescence. Therefore, we investigated extinction of a cocaine-associated cue in adolescent and adult rats. While cocaine self-administration and lever-alone extinction were not different between the two ages, we observed that cue extinction reduced cue-induced reinstatement in adult but not adolescent rats. Infusion of the selective dopamine 2 receptor (D2R)-like agonist quinpirole into the infralimbic cortex (IL) of the mPFC prior to cue extinction significantly reduced cue-induced reinstatement in adolescents. This effect was replicated by acute systemic treatment with the atypical antipsychotic aripiprazole (Abilify), a partial D2R-like agonist. These data suggest that adolescents may be more susceptible to relapse due to a deficit in cue extinction learning, and highlight the significance of D2R signaling in the IL for cue extinction during adolescence. These findings inspire new tactics for improving adolescent CET, with aripiprazole representing an exciting potential pharmacological adjunct for behavioral therapy.

Opposing effects of dopamine D1- and D2-like agonists on intracranial self-stimulation in male rats

Dopamine acts through dopamine Type I receptors (comprising D1 and D5 subtypes) and dopamine Type II receptors (comprising D2, D3, and D4 subtypes). Intracranial self-stimulation (ICSS) is 1 experimental procedure that can be used to evaluate abuse-related effects of drugs targeting dopamine receptors. This study evaluated effects of dopamine receptor ligands on ICSS in rats using experimental procedures that have been used previously to examine abused indirect dopamine agonists such as cocaine and amphetamine. Male Sprague-Dawley rats responded under a fixed-ratio 1 schedule for electrical stimulation of the medial forebrain bundle, and frequency of stimulation varied from 56-158 Hz in 0.05 log increments during each experimental session. Drug potency and time course were determined for the D1 ligands A77636, SKF82958, SKF38393, fenoldopam, and SCH39166 and the D2/3 ligands sumanirole, apomorphine, quinpirole, PD128907, pramipexole, aripiprazole, eticlopride, and PG01037. The high-efficacy D1 agonists A77636 and SKF82958 produced dose-dependent, time-dependent, and abuse-related facilitation of ICSS. Lower efficacy D1 ligands and all D2/3 ligands failed to facilitate ICSS at any dose or pretreatment time. A mixture of SKF82958 and quinpirole produced a mixture of effects produced by each drug alone. Quinpirole also failed to facilitate ICSS after regimens of repeated treatment with either quinpirole or cocaine. These studies provide more evidence for divergent effects of dopamine D1- and D2-family agonists on ICSS procedure in rats and suggest that ICSS may be a useful complement to other approaches for preclinical abuse potential assessment, in part because of the reproducibility of results. (PsycINFO Database Record

Choosing Under the Influence: A Drug-Specific Mechanism by Which the Setting Controls Drug Choices in Rats

Ample evidence shows that the setting can control drug choices in both humans and animals. Here we reveal in rats that a major mechanism of this control involves a regulation of the drug influence on other competing options at the time of choice. Briefly, rats were offered a choice between a drug dose (cocaine or heroin) and a brief access to water sweetened with saccharin in two different settings. In one setting, choosing under the influence was not possible and rats largely preferred saccharin over either cocaine or heroin. In contrast, when the same rats were shifted to a setting where choosing under the influence was possible, they chose the drug either nonexclusively or exclusively depending on whether the drug enhanced or suppressed sweet reward, respectively. Thus, when rats were under the orexigenic influence of heroin at the time of choice, they more frequently chose saccharin in alternation with heroin. In contrast, when rats were under the anorexic influence of cocaine, they stopped choosing saccharin and continued taking cocaine exclusively. These setting- and drug-specific changes in preference were rapid and reversible, and could be induced by passively administering cocaine or heroin before choice. Finally, rats behaved as if they were oblivious to the drug influence on their choices. This behavior could explain why rats are vulnerable to harm themselves, sometimes to the point of death, in settings where choices are made under the drug influence, notably if this influence excludes other important options or, conversely, enhances harmful ones.

Effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in male rhesus monkeys

One complicating factor in cocaine addiction may be concurrent exposure and potential dependence on nicotine. The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in rhesus monkeys (Macaca mulatta). For comparison, we also determined effects of the nicotinic receptor antagonist mecamylamine on cocaine versus food choice during continuous saline and nicotine treatment. Rhesus monkeys (N = 3) responded under a concurrent schedule of food pellet (1 g) and intravenous cocaine (0-0.1 mg/kg/injection) availability. Saline and ascending nicotine doses (0.1-1.0 mg/kg/hr, intravenous) were continuously infused for 7-day treatment periods and separated by 24-hr saline treatment periods. Acute effects of mecamylamine (0.32-1.8 mg/kg, intramuscular, 15 min pretreatment) were determined during continuous saline and 0.32-mg/kg/hr nicotine treatments. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Nicotine treatment did not alter cocaine versus food choice. In contrast, preference of 0.032 mg/kg/injection cocaine was attenuated 24 hr following termination of 0.32-mg/kg/hr nicotine treatment, despite no somatic abstinence signs being observed. Acute mecamylamine enhanced cocaine choice during saline treatment and mainly suppressed rates of behavior during nicotine treatment. Overall, continuous nicotine exposure, up to 1 mg/kg/hr, does not enhance cocaine choice and does not produce nicotine dependence, as demonstrated by the lack of abstinence signs.

Cocaine choice procedures in animals, humans, and treatment-seekers: Can we bridge the divide?

Individuals with cocaine use disorder chronically self-administer cocaine to the detriment of other rewarding activities, a phenomenon best modeled in laboratory drug-choice procedures. These procedures can evaluate the reinforcing effects of drugs versus comparably valuable alternatives under multiple behavioral arrangements and schedules of reinforcement. However, assessing drug-choice in treatment-seeking or abstaining humans poses unique challenges: for ethical reasons, these populations typically cannot receive active drugs during research studies. Researchers have thus needed to rely on alternative approaches that approximate drug-choice behavior or assess more general forms of decision-making, but whether these alternatives have relevance to real-world drug-taking that can inform clinical trials is not well-understood. In this mini-review, we (A) summarize several important modulatory variables that influence cocaine choice in nonhuman animals and non-treatment seeking humans; (B) discuss some of the ethical considerations that could arise if treatment-seekers are enrolled in drug-choice studies; (C) consider the efficacy of alternative procedures, including non-drug-related decision-making and 'simulated' drug-choice (a choice is made, but no drug is administered) to approximate drug choice; and (D) suggest opportunities for new translational work to bridge the current divide between preclinical and clinical research.

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.

Use of Preclinical Drug vs. Food Choice Procedures to Evaluate Candidate Medications for Cocaine Addiction

Drug addiction is a disease that manifests as an inappropriate allocation of behavior towards the procurement and use of the abused substance and away from other behaviors that produce more adaptive reinforcers (e.g. exercise, work, family and social relationships). The goal of treating drug addiction is not only to decrease drug-maintained behaviors, but also to promote a reallocation of behavior towards alternative, nondrug reinforcers. Experimental procedures that offer concurrent access to both a drug reinforcer and an alternative, nondrug reinforcer provide a research tool for assessment of medication effects on drug choice and behavioral allocation. Choice procedures are currently the standard in human laboratory research on medications development. Preclinical choice procedures have been utilized in biomedical research since the early 1940's, and during the last 10-15 years, their use for evaluation of medications to treat drug addiction has increased. We propose here that parallel use of choice procedures in preclinical and clinical studies will facilitate translational research on development of medications to treat cocaine addiction. In support of this proposition, a review of the literature suggests strong concordance between preclinical effectiveness of candidate medications to modify cocaine choice in nonhuman primates and rodents and clinical effectiveness of these medications to modify either cocaine choice in human laboratory studies or metrics of cocaine abuse in patients with cocaine use disorder. The strongest evidence for medication effectiveness in preclinical choice studies has been obtained with maintenance on the monoamine releaser d-amphetamine, a candidate agonist medication for cocaine use analogous to use of methadone to treat heroin abuse or nicotine formulations to treat tobacco dependence.

Effects of oral and intravenous administration of buspirone on food-cocaine choice in socially housed male cynomolgus monkeys

Drugs acting at D3 dopamine receptors have been suggested as medications for cocaine dependence. These experiments examined the effects of intravenously and orally administered buspirone, a D2-like receptor antagonist with high affinity for D3 and D4 receptors, on the relative reinforcing strength of cocaine in group-housed male cynomolgus monkeys. Use of socially housed monkeys permitted the assessment of whether social status, known to influence D2-like receptor availability, modulates the behavioral effects of buspirone. Buspirone was administered acutely to monkeys self-administering cocaine under a food-drug choice procedure in which a cocaine self-administration dose-effect curve was determined daily. When administered by either route, buspirone significantly decreased cocaine choice in dominant-ranked monkeys. In subordinate monkeys, however, i.v. buspirone was ineffective on average, and oral buspirone increased choice of lower cocaine doses. The effects of buspirone only differed according to route of administration in subordinate monkeys. Moreover, it is noteworthy that the effects of buspirone were similar to those of the D3 receptor-selective antagonist PG01037 and qualitatively different than those of less selective drugs that act at D2-like or serotonin (5-HT)1A receptors, suggesting a D3 and possibly D4 receptor mechanism of action for buspirone. Taken together, the data support the utility of drugs targeting D3/D4 receptors as potential treatments for cocaine addiction, particularly in combination with enriching environmental manipulations.

Identifying Medication Targets for Psychostimulant Addiction: Unraveling the Dopamine D3 Receptor Hypothesis

The dopamine D3 receptor (D3R) is a target for developing medications to treat substance use disorders. D3R-selective compounds with high affinity and varying efficacies have been discovered, providing critical research tools for cell-based studies that have been translated to in vivo models of drug abuse. D3R antagonists and partial agonists have shown especially promising results in rodent models of relapse-like behavior, including stress-, drug-, and cue-induced reinstatement of drug seeking. However, to date, translation to human studies has been limited. Herein, we present an overview and illustrate some of the pitfalls and challenges of developing novel D3R-selective compounds toward clinical utility, especially for treatment of cocaine abuse. Future research and development of D3R-selective antagonists and partial agonists for substance abuse remains critically important but will also require further evaluation and development of translational animal models to determine the best time in the addiction cycle to target D3Rs for optimal therapeutic efficacy.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Sign-tracking predicts increased choice of cocaine over food in rats

The purpose of this study was to determine whether the tendency to sign-track to a food cue was predictive of rats' choice of cocaine over food. First, rats were trained on a procedure where insertion of a retractable lever was paired with food. A sub-group of rats - sign-trackers - primarily approached and contacted the lever, while another sub-group - goal-trackers - approached the site of food delivery. Rats were then trained on a choice task where they could choose between an infusion of cocaine (1.0 mg/kg) and a food pellet (45 mg). Sign-trackers chose cocaine over food significantly more often than did goal-trackers. These results support the incentive-salience theory of addiction and add to a growing number of studies which suggest that sign-trackers may model an addiction-prone phenotype.

Acute and chronic effects of the M1/M4-preferring muscarinic agonist xanomeline on cocaine vs. food choice in rats

RATIONALE

We previously showed that the M1/M4-preferring muscarinic agonist xanomeline can acutely attenuate or eliminate cocaine self-administration in mice.

OBJECTIVE

Medications used to treat addictions will arguably be administered in (sub)chronic or repeated regimens. Tests of acute effects often fail to predict chronic effects, highlighting the need for chronic testing of candidate medications.

METHODS

Rats were trained to lever press under a concurrent FR5 FR5 schedule of intravenous cocaine and food reinforcement. Once baseline behavior stabilized, the effects of 7 days once-daily injections of xanomeline were evaluated.

RESULTS

Xanomeline pretreatment dose-dependently (1.8-10 mg/kg/day) shifted the dose-effect curve for cocaine rightward (up to 5.6-fold increase in A 50), with reallocation of behavior to the food-reinforced lever. There was no indication of tolerance, rather effects grew over days. The suppression of cocaine choice appeared surmountable at high cocaine doses, and xanomeline treatment did not significantly decrease total-session cocaine or food intake.

CONCLUSIONS

In terms of xanomeline's potential for promoting abstinence from cocaine in humans, the findings were mixed. Xanomeline did produce reallocation of behavior from cocaine to food with a robust increase in food reinforcers earned at some cocaine/xanomeline dose combinations. However, effects appeared surmountable, and food-maintained behavior was also decreased at some xanomeline/cocaine dose combinations, suggesting clinical usefulness may be limited. These data nevertheless support the notion that chronic muscarinic receptor stimulation can reduce cocaine self-administration. Future studies should show whether ligands with higher selectivity for M1 or M1/M4 subtypes would be less limited by undesired effects and can achieve higher efficacy.

Effects of phendimetrazine treatment on cocaine vs food choice and extended-access cocaine consumption in rhesus monkeys

There is currently no Food and Drug Administration-approved pharmacotherapy for cocaine addiction. Monoamine releasers such as d-amphetamine constitute one class of candidate medications, but clinical use and acceptance are hindered by their own high-abuse liability. Phendimetrazine (PDM) is a schedule III anorectic agent that functions as both a low-potency monoamine-uptake inhibitor and as a prodrug for the monoamine-releaser phenmetrazine (PM), and it may serve as a clinically available, effective, and safer alternative to d-amphetamine. This study determined efficacy of chronic PDM to reduce cocaine self-administration by rhesus monkeys (N=4) using a novel procedure that featured both daily assessments of cocaine vs food choice (to assess medication efficacy to reallocate behavior away from cocaine choice and toward choice of an alternative reinforcer) and 20 h/day cocaine access (to allow high-cocaine intake). Continuous 21-day treatment with ramping PDM doses (days 1-7: 0.32 mg/kg/h; days 8-21: 1.0 mg/kg/h) reduced cocaine choices, increased food choices, and nearly eliminated extended-access cocaine self-administration without affecting body weight. There was a trend for plasma PDM and PM levels to correlate with efficacy to decrease cocaine choice such that the monkey with the highest plasma PDM and PM levels also demonstrated the greatest reductions in cocaine choice. These results support further consideration of PDM as a candidate anti-cocaine addiction pharmacotherapy. Moreover, PDM may represent a novel pharmacotherapeutic approach for cocaine addiction because it may simultaneously function as both a monoamine-uptake inhibitor (via the parent drug PDM) and as a monoamine releaser (via the active metabolite PM).

Drug self-administration studies: a novel reinforcement schedule enhances choice

Relative reinforcing effects of different ethanol and different cocaine doses were studied under concurrent independent fixed-ratio (FR) schedules and concurrent nonindependent FR schedules with rhesus monkeys. Nonindependent FR schedules differed from independent FR schedules in that responses on either side counted towards the FR requirements of two concurrently presented choices. Thus, responses on the right operandum counted toward completion of both right and left FR schedules and, symmetrically, responses on the left did the same. Nonindependent schedules allow the number of responses per drug delivery to vary considerably, unlike independent schedules, thereby making the number of responses per delivery a sensitive dependent variable. In contrast, standard independent schedules do not allow responses per drug delivery to vary; the required number of responses is an independent variable. Three rhesus monkeys were subjects, and choices between different doses of ethanol or cocaine were studied. Larger doses maintained higher response rates than smaller doses - consistent with previous choice studies. By using nonindependent schedules, however, graded responses per drug delivery and increased switching between sides were obtained, providing additional data and useful measures of choice.

Effects of repeated treatment with the dopamine D2/D3 receptor partial agonist aripiprazole on striatal D2/D3 receptor availability in monkeys

RATIONALE

Chronic treatment with dopamine (DA) receptor agonists and antagonists can differentially affect measures of DA D2/D3 receptor number and function, but the effects of chronic treatment with a partial D2/D3 receptor agonist are not clear.

OBJECTIVE

We used a within-subjects design in male cynomolgus monkeys to determine the effects of repeated (17-day) treatment with the D2/D3 receptor partial agonist aripiprazole (ARI; 0.03 mg/kg and 0.1 mg/kg i.m.) on food-reinforced behavior (n = 5) and on D2/D3 receptor availability as measured with positron emission tomography (PET; n = 9).

METHODS

Five monkeys responded under a fixed-ratio 50 schedule of food reinforcement and D2/D3 receptor availability was measured before and 4 days after ARI treatment using PET and the D2/D3 receptor-selective radioligand [18F]fluoroclebopride (FCP). Four additional monkeys were studied using [11C]raclopride and treated sequentially with each dose of ARI for 17 days.

RESULTS

ARI decreased food-maintained responding with minimal evidence of tolerance. Repeated ARI administration increased FCP and raclopride distribution volume ratios (DVRs) in the caudate nucleus and putamen in most monkeys, but decreases were observed in monkeys with the highest baseline DVRs.

CONCLUSIONS

The results indicate that repeated treatment with a low-efficacy DA receptor partial agonist produces effects on brain D2/D3 receptor availability that are qualitatively different from those of both high-efficacy receptor agonists and antagonists, and suggest that the observed individual differences in response to ARI treatment may reflect its partial agonist activity.

Reduction of cocaine self-administration and D3 receptor-mediated behavior by two novel dopamine D3 receptor-selective partial agonists, OS-3-106 and WW-III-55

Dopamine D3 receptor (D3R)-selective compounds may be useful medications for cocaine dependence. In this study, we identified two novel arylamide phenylpiperazines, OS-3-106 and WW-III-55, as partial agonists at the D3R in the adenylyl cyclase inhibition assay. OS-3-106 and WW-III-55 have 115- and 862-fold D3R:D2 receptor (D2R) binding selectivity, respectively. We investigated their effects (0, 3, 5.6, or 10 mg/kg) on operant responding by using a multiple variable-interval (VI) 60-second schedule that alternated components with sucrose reinforcement and components with intravenous cocaine reinforcement (0.375 mg/kg). Additionally, we evaluated the effect of OS-3-106 (10 mg/kg) on the dose-response function of cocaine self-administration and the effect of WW-III-55 (0-5.6 mg/kg) on a progressive ratio schedule with either cocaine or sucrose reinforcement. Both compounds were also examined for effects on locomotion and yawning induced by a D3R agonist. OS-3-106 decreased cocaine and sucrose reinforcement rates, increased latency to first response for cocaine but not sucrose, and downshifted the cocaine self-administration dose-response function. WW-III-55 did not affect cocaine self-administration on the multiple-variable interval schedule, but it reduced cocaine and sucrose intake on the progressive ratio schedule. Both compounds reduced locomotion at doses that reduced responding, and both compounds attenuated yawning induced by low doses of 7-OH-DPAT (a D3R-mediated behavior), but neither affected yawning on the descending limb of the 7-OH-DPAT dose-response function (a D2R-mediated behavior). Therefore, both compounds blocked a D3R-mediated behavior. However, OS-3-106 was more effective in reducing cocaine self-administration. These findings support D3Rs, and possibly D2Rs, as targets for medications aimed at reducing the motivation to seek cocaine.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Effects of monoamine releasers with varying selectivity for releasing dopamine/norepinephrine versus serotonin on choice between cocaine and food in rhesus monkeys

Monoamine releasers constitute one class of candidate medications for the treatment of cocaine abuse, and concurrent cocaine-versus-food choice procedures are potentially valuable as experimental tools to evaluate the efficacy and safety of candidate medications. This study assessed the choice between cocaine and food by rhesus monkeys during treatment with five monoamine releasers that varied in selectivity to promote the release of dopamine and norepinephrine versus serotonin (5HT) [m-fluoroamphetamine, (+)-phenmetrazine, (+)-methamphetamine, napthylisopropylamine and (±)-fenfluramine]. Rhesus monkeys (n=8) responded under a concurrent-choice schedule of food delivery (1-g pellets, fixed ratio 100 schedule) and cocaine injections (0-0.1 mg/kg/injection, fixed ratio 10 schedule). Cocaine choice dose-effect curves were determined daily during continuous 7-day treatment with saline or with each test compound dose. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice, and the highest cocaine doses (0.032-0.1 mg/kg/injection) maintained almost exclusive cocaine choice. Efficacy of monoamine releasers to decrease cocaine choice corresponded to their pharmacological selectivity to release dopamine and norepinephrine versus 5HT. None of the releasers reduced cocaine choice or promoted reallocation of responding to food choice to the same extent as when saline was substituted for cocaine. These results extend the range of conditions across which dopamine and norepinephrine-selective releasers have been shown to reduce cocaine self-administration.

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.

Effects of aripiprazole, an atypical antipsychotic, on the motor alterations induced by acute ethanol administration in mice

Aripiprazole is an antipsychotic that acts as a partial agonist at dopamine receptors. As the effects of most drugs of abuse converge to enhance dopamine-mediated neurotransmission, the present study was designed to test the hypothesis that aripiprazole would inhibit the acute effects of ethanol, a widely abused substance. Male Swiss mice received acute injections and were evaluated for motor activity in three distinct tests. In the open field, ethanol (1.5, 2.5 and 3.5 g/kg) induced an increase in locomotion in a U-shaped dose-related fashion, whereas aripiprazole (0.1, 1 and 10 mg/kg) did not affect this parameter. All the doses of the antipsychotic were able to prevent the stimulant effects of 2.5 g/kg of ethanol. In the rotarod test, ethanol (2.5 and 3.5 g/kg) reduced the latency to fall from the apparatus, an effect also observed with the higher dose of aripiprazole. Contrary to what was observed in the open field, this antipsychotic did not interfere with the effects of ethanol in motor balance. Finally, we tested animals in the wire hang test, in which ethanol, but not aripiprazole, reduced latency to fall at all doses. In this test, aripiprazole did not change ethanol effects. The present data lead to the conclusion that aripiprazole prevents the stimulant effects of ethanol on locomotion, without interfering with the motor impairment induced by this drug.

Effects of dopamine D2/D3 receptor ligands on food-cocaine choice in socially housed male cynomolgus monkeys

Dopamine D2/D3 receptor partial agonists have been suggested as medications for cocaine dependence. The present experiments examined the effect of acute and repeated administration of drugs with varying intrinsic efficacy at D2/D3 receptors on the relative reinforcing strength of cocaine. Use of socially housed cynomolgus monkeys permitted the assessment of whether social status, known to alter D2/D3 receptor availability, influenced the behavioral effects of D2/D3 receptor compounds. The high-efficacy agonist R(-)-norpropylapomorphine [(-)-NPA], low-efficacy agonist aripiprazole (ARI), and antagonist eticlopride (ETIC) were administered acutely to monkeys self-administering cocaine under a food-cocaine choice procedure in which a cocaine self-administration dose-effect curve was determined daily. The effects of 5-day treatment with ARI and (-)-NPA were characterized under conditions in which monkeys did (ARI) or did not [ARI and (-)-NPA] self-administer cocaine during treatment. When administered acutely, ARI and ETIC increased the choice of low cocaine doses, and only (-)-NPA decreased the choice of higher cocaine doses and cocaine intake; effects were similar across social ranks. When administered repeatedly while self administration occurred only on days 1 and 5 of treatment, ARI, but not (-)-NPA, decreased cocaine choice in dominant monkeys, whereas (-)-NPA, but not ARI, did so in subordinates. When dominant monkeys self-administered cocaine on all five days of ARI treatment, however, these effects were not observed. The results indicate that the behavioral effects of D2/D3 receptor agonists can differ according to intrinsic efficacy and subject characteristics. Moreover, these results suggest that exposure to cocaine during treatment can counteract treatment-induced reductions in the reinforcing effects of cocaine.

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.

Phenylpiperazine derivatives with selectivity for dopamine D3 receptors modulate cocaine self-administration in rats

This study examined cocaine self-administration after pretreatments with three structurally related compounds that bind selectively to dopamine D3 receptors (D3Rs) relative to the D2 receptor subtype (D2Rs) and exhibit varying intrinsic activities in the forskolin-stimulated adenylyl cyclase assay. The compounds are: a) WC10, a D3R weak partial agonist/antagonist with 42-fold D3R:D2R selectivity, b) WC26, a 51-fold selective D3R partial agonist, c) WC44, a 23-fold selective D3R agonist. Rats were stabilized on a multiple variable-interval 60-s (VI60) schedule with alternating components of sucrose (45 mg pellets) or cocaine reinforcement (0.375 mg/kg, IV) and then tested for effects of the WC compounds (0.0, 1.0, 3.0, 5.6, or 10.0 mg/kg, IP). Another cohort was trained to self-administer cocaine (0.75 mg/kg, IV) on a VI60 schedule then tested with various doses of cocaine available (0.0-1.5 mg/kg, IV) following pretreatment with WC10 (5.6 or 10.0 mg/kg) or WC44 (10.0 mg/kg). WC10 and WC26 decreased both cocaine and sucrose reinforcement rates at the 10.0 mg/kg dose, whereas WC44 decreased only cocaine reinforcement rate at this dose. Furthermore, WC26 and WC44 increased response latency for cocaine but not sucrose. In the cocaine dose-response experiment, WC10 and WC44 flattened the dose-effect function of cocaine reinforcement rate. All compounds decreased spontaneous locomotion. WC10 and WC26 also reduced cocaine-induced locomotion. These results support the targeting of D3Rs for treatments for cocaine dependence. WC26 and WC44, in particular, show promise as they increased the latency to respond for cocaine but not sucrose, suggesting selective reduction of the motivation for cocaine.

Alternative reinforcer response cost impacts cocaine choice in humans

Cocaine use disorders are an unrelenting public health concern. Behavioral treatments reduce cocaine use by providing non-drug alternative reinforcers. The purpose of this human laboratory experiment was to determine how response cost for non-drug alternative reinforcers influenced cocaine choice. Seven cocaine-using, non-treatment-seeking subjects completed a crossover, double-blind protocol in which they first sampled doses of intranasal cocaine (5, 10, 20 or 30 mg) and completed a battery of subject-rated and physiological measures. Subjects then made eight discrete choices between the sampled dose and an alternative reinforcer (US$0.25). The response cost to earn a cocaine dose was always a fixed ratio (FR) of 100 responses. The response cost for the alternative reinforcer varied across sessions (FR1, FR10, FR100, FR1000). Dose-related increases were observed for cocaine choice. Subjects made fewer drug choices when the FR requirements for the alternative reinforcers were lower than that for drug relative to when the FR requirements were equal to or higher than that for drug. Intranasal cocaine also produced prototypical stimulant-like subject-rated and physiological effects (e.g., increased ratings of Like Drug; elevated blood pressure). These data demonstrate that making alternative reinforcers easier to earn reduces cocaine self-administration, which has implications for treatment efforts.

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

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

Discriminative stimulus, subject-rated and cardiovascular effects of cocaine alone and in combination with aripiprazole in humans

Aripiprazole is a dopamine D(2) receptor partial agonist undergoing evaluation as a pharmacotherapy for stimulant-use disorders. Acutely administered aripiprazole attenuates the discriminative stimulus and other behavioral effects of d-amphetamine in humans; however, whether aripiprazole attenuates the effects of more commonly abused stimulants is unknown. The aim of this experiment was to assess the discriminative stimulus, subject-rated and cardiovascular effects of oral cocaine alone and following acute administration of aripiprazole in humans. Eight cocaine-dependent subjects learned to discriminate 150 mg cocaine from placebo. After acquiring the discrimination, the effects of cocaine (0, 25, 50, 100 and 200 mg) administered alone and in combination with aripiprazole (15 mg) were determined. Significant effects of cocaine were observed for the drug discrimination task, stimulant-like subject-rated effects and heart rate. Limited effects of aripiprazole were revealed. However, for most measures, fewer doses of cocaine were significantly greater than placebo when combined with aripiprazole, suggesting a reduction in the discriminative stimulus, self-reported and cardiovascular effects of cocaine. These data are consistent with previous studies that have tested acutely administered aripiprazole in combination with d-amphetamine and suggest that the ability of aripiprazole to modify stimulant effects is a function of the duration of treatment (acute vs. chronic).

Aripiprazole maintenance increases smoked cocaine self-administration in humans

RATIONALE

Partial dopamine receptor agonists have been proposed as candidate pharmacotherapies for cocaine dependence.

OBJECTIVE

This 42-day, within-subject, human laboratory study assessed how maintenance on aripiprazole, a partial D(2) receptor agonist, influenced smoked cocaine self-administration, cardiovascular measures, subjective effects, and cocaine craving in nontreatment-seeking, cocaine-dependent volunteers.

METHODS

In order to achieve steady-state concentrations, participants (n = 8 men) were administered placebo and aripiprazole (15 mg/day) capsules in counter-balanced order for 21 days. A smoked cocaine dose-response curve (0, 12, 25, 50 mg) was determined twice under placebo and aripiprazole maintenance. Sessions comprised a "sample" trial, when participants smoked the cocaine dose available that session, and five choice trials, when they responded on a progressive-ratio schedule of reinforcement to receive the cocaine dose or receive $5.00.

RESULTS

Cocaine's reinforcing, subjective, and cardiovascular effects were dose-dependent. Aripiprazole significantly increased cocaine (12, 25 mg) self-administration. Following a single administration of cocaine (25 mg), aripiprazole decreased ratings of how much participants would pay for that dose. Following repeated cocaine (50 mg) self-administration, aripiprazole decreased ratings of cocaine quality, craving, and good drug effect as compared to placebo.

CONCLUSIONS

These data suggest that aripiprazole may have increased self-administration to compensate for a blunted subjective cocaine effect. Overall, the findings do not suggest aripiprazole would be useful for treating cocaine dependence.

Prolonged attenuation of the reinforcing strength of cocaine by chronic d-amphetamine in rhesus monkeys

Chronic treatment with the indirect dopamine agonist d-amphetamine can reduce cocaine use in clinical trials and, in preclinical studies in laboratory animals, attenuates daily cocaine self-administration. The present study extended previous results to conditions designed to reflect a more clinically relevant experience of cocaine exposure and d-amphetamine treatment. Each morning, monkeys pressed a lever to receive food pellets under a 50-response fixed-ratio schedule of reinforcement. After determining a dose-response curve for cocaine (0.003-0.56 mg/kg per injection, i.v.) under a progressive-ratio (PR) schedule of reinforcement in the evening, cocaine self-administration sessions were suspended and d-amphetamine (0.01-0.056 mg/kg/h, i.v.) was administered continuously for at least 24 days, except during cocaine self-administration sessions, which were conducted using the PR schedule once every 8 days. When a persistent decrease in self-administration was observed, the cocaine dose-effect curve was redetermined. Cocaine- and food-maintained responding were also examined after discontinuation of d-amphetamine. Although individual differences in sensitivity were observed, d-amphetamine produced selective, qualitatively similar decreases in the reinforcing strength of cocaine in all monkeys that persisted at least 4 weeks. Moreover, cocaine dose-effect curves were shifted downward and/or to the right. For 2 weeks following discontinuation of d-amphetamine treatment, the reinforcing strength of cocaine varied within and across individuals, however, on the whole no increased sensitivity was apparent. These data provide further support for the use of agonist medications for cocaine abuse, and extend the conditions under which such treatment is successful to those that incorporate clinically relevant patterns of cocaine use and drug treatment.

Attenuation of reinforcing and psychomotor stimulant effects of amphetamine by aripiprazole

Partial dopamine agonists are potential medications for the treatment of amphetamine addiction. They have been hypothesized to stabilize the dopamine system by acting as antagonists during high dopaminergic tone resulting from amphetamine use and as agonists during withdrawal. Aripiprazole is an atypical antipsychotic that acts as a partial D2 dopamine and a serotonin 5-HT₁(A) agonist and a serotonin 5-HT₂(A) antagonist. The aim of the present study was to examine the effects of aripiprazole on behaviors induced and maintained by d-amphetamine. To this end, intravenous d-amphetamine self-administration [fixed ratio 3 (FR3) schedule, 0.02 mg/infusion] and d-amphetamine-induced (0, 1.5 mg/kg intraperitoneally) locomotor activity, as well as spontaneous locomotor activity and sucrose pellet selfadministration (FR3 schedule) were studied in male Wistar rats after aripiprazole (0, 0.3, 1, 3 mg/kg i.p.) administration. Aripiprazole pre-treatment resulted in bidirectional effects on amphetamine self-administration. The 1 mg/kg dose increased, and the highest dose decreased the number of amphetamine infusions. In the locomotor activity experiments, aripiprazole attenuated amphetamine-induced activity dose-dependently and tended to suppress spontaneous activity. The highest aripiprazole doses decreased also sucrose pellet self-administration. The increase in amphetamine self-administration with the intermediate aripiprazole dose, as well as the decrease in amphetamine-induced locomotor activity, suggests that aripiprazole acted as a dopamine antagonist. Suppression of amphetamine and sucrose self-administration by the highest aripiprazole dose was probably caused by non-specific effects. Together, these results indicate that under conditions of dopaminergic stimulation, aripiprazole attenuates the reinforcing and psychomotor stimulant effects of d-amphetamine, but the dose range for this effect is rather limited.

Efficacy of the atypical antipsychotic aripiprazole in d-amphetamine-based preclinical models of mania

The atypical antipsychotic aripiprazole has been demonstrated to reduce symptoms of bipolar mania. To further profile the antimanic-like properties of aripiprazole in relevant preclinical models, we examined its efficacy in d-amphetamine-based behavioural models of acute mania in rats. The effects of acute and repeated administration of aripiprazole were assessed in the facilitation of intracranial self-stimulation (ICSS) and hyperlocomotion after acute d-amphetamine, and in the sensitized facilitation of ICSS function and hyperlocomotion after repeated d-amphetamine. Acutely, aripiprazole (0.75, 1.5 and 2.5 mg/kg i.p.) increased ICSS thresholds, attenuated the reward-facilitating effects of d-amphetamine (0.5 mg/kg i.p.), decreased motor activity and prevented d-amphetamine-induced hyperlocomotion. Co-administration of aripiprazole and d-amphetamine for 7 d resulted in aripiprazole counteracting the d-amphetamine-induced sensitization in facilitation of brain reward function and hyperlocomotion. These results indicate the efficacy of aripiprazole in d-amphetamine-based preclinical models of acute mania that are characterized by increased motivational drive and/or hyperfunction of brain reward.