Development of a translational model to screen medications for cocaine use disorder I: Choice between cocaine and food in rhesus monkeys

Cocaine abstinence during the "critical period" of a contingency management trial predicts future abstinence in people with cocaine use disorder

BACKGROUND

Contingency Management (CM) is being piloted as a treatment for stimulant use disorder in several US states, highlighting the need for treatment optimization. One important goal of optimization is decreasing drug use during the early stages of treatment, which has predicted success in other interventions. However, this "critical period" has not been reported in CM trials. The purpose of this analysis was to determine if, after accounting for baseline abstinence and incentive condition, abstinence in a CM trial for people with Cocaine Use Disorder (CUD) could be predicted by cocaine use during a first-week critical period.

METHODS

Eighty-seven participants with CUD were randomized to receive contingent high or low value incentives for cocaine abstinence or were in a non-contingent control group. Generalized estimating equations (GEE) were used to analyze urine test results over 36 timepoints during the 12-week intervention. To assess for a critical period, the first three visits were included in the GEE as a covariate for remaining urine test results.

RESULTS

Participants who provided more negative samples during the critical period were significantly more likely to produce a negative urine sample during the remainder of the trial, though some effects of group remained after controlling for the critical period.

CONCLUSIONS

These results indicate that a critical period exists for CM trials, and it can explain a substantial amount of future performance. Early contact with an abstinence-contingent high magnitude alternative reinforcer may explain additional performance beyond the critical period, further justifying the use of high magnitude alternative reinforcers.

Effects of environmental and pharmacological manipulations on cocaine-vs-negative reinforcer choice in male and female rats

RATIONALE

The adverse consequences of human addictive drug use could be the result of either addictive drug consumption resulting in punishment (e.g., incarceration) or failure to engage in negative-reinforced behaviors that might compete with drug-maintained behaviors (e.g., contingency management strategies that reset payment amounts for drug free urines).

OBJECTIVE

The goal of the present study was to establish a discrete-trial cocaine-vs-negative reinforcer (SNR) choice procedure where rats were presented with a simplified model of this conflict: choose negative reinforcement (i.e., escape or avoid foot shock) or choose an intravenous (IV) cocaine infusion followed by an inescapable shock.

METHODS

Responding was maintained in male and female rats by IV cocaine infusions (0.32-1.8 mg/kg/inf) and a SNR (0.1-0.7 mA shock) under a discrete-trial concurrent "choice" schedule during daily sessions. Following parametric reinforcer magnitude and response requirement experiments, the effects of 12 h extended access cocaine self-administration and acute diazepam (0.32-10 mg/kg, IP) pretreatment were determined on cocaine-vs-SNR choice.

RESULTS

Negative reinforcement was chosen over all cocaine doses. Lowering shock magnitude or increasing SNR response requirement failed to promote behavioral reallocation towards cocaine. Extended access cocaine self-administration sessions resulted in high daily cocaine intakes but failed to significantly increase cocaine choice in all (19) but one rat. Acute diazepam pretreatment also did not alter choice behavior up to doses that produced behavioral depression.

CONCLUSIONS

These results suggest that SNRs may be a source of reinforcement that effectively compete with and mitigate maladaptive addictive drug-maintained behaviors in the general population.

A concurrently available negative reinforcer robustly decreases cocaine self-administration in male and female rats

Continued drug-taking despite adverse consequences is hypothesized to be an insidious behavioral hallmark of drug addiction. Although most preclinical research has focused on drug self-administration in the presence of positive punishment, another source of potential adverse consequences is behavioral allocation away from negative reinforcers (i.e., escape/avoid electric shock) and towards drug reinforcers. The goals of the present study were to establish a discrete-trial cocaine-vs-negative reinforcer choice procedure in male and female rats and determine sensitivity of choice behavior to environmental and pharmacological manipulations. Rats could make up to nine discrete choices between an intravenous cocaine infusion (0.32 - 1.8 mg/kg/inf) under a fixed-ratio (FR) 3 schedule and a negative reinforcer (escape or avoidance of electric shock, 0.1 - 0.7 mA) under an FR1 schedule. The negative reinforcer was consistently chosen over all cocaine doses. Lowering shock magnitude decreased negative reinforcer trials, increased omitted trials, and failed to promote behavioral reallocation towards cocaine. Increasing the negative reinforcement response requirement between sessions only increased omitted trials. Introduction of 12-hr extended access cocaine self-administration sessions across two weeks resulted in high daily cocaine intakes but failed to significantly increase cocaine choice. Acute diazepam pretreatment also did not impact choice behavior up to doses that produced behavioral depression. Overall, the lack of behavioral allocation between cocaine infusions and a negative reinforcer suggests these two reinforcers may be economic independents. Additionally, the failure of extended cocaine access to increase cocaine choice highlights the importance of alternative reinforcers and environmental context in preclinical models of drug addiction.

Effects of environmental manipulations on cocaine-vs-social choice in male and female rats

Cocaine use disorder occurs in an environment where cocaine and other nondrug commodities are concurrently available. Preclinical drug-vs-nondrug choice procedures are one simplified method of modeling this complex clinical environment. The present study established a discrete-trial cocaine-vs-social interaction choice procedure in male and female rats and determined sensitivity of choice behavior to manipulations of reinforcer magnitude and non-contingent "sample" reinforcer presentation. Rats could make up to nine discrete choices between an intravenous cocaine infusion (0.1-1.0 mg/kg/inf) and social interaction with a same-sex social "Partner" rat. Cocaine infusions were available under a progressive-ratio (PR) schedule of reinforcement, and social interaction was available under a fixed-ratio (FR) 3 schedule. Social interaction was chosen over no or small cocaine doses (saline, 0.01 mg/kg/inf) and behavior was reallocated away from social and towards cocaine at larger cocaine doses (1.0 mg/kg/inf). Manipulating social interaction time as one method to alter social reinforcer magnitude did not significantly alter cocaine-vs-social choice. Removing the non-contingent reinforcer presentations before the discrete choice trials also failed to affect cocaine-vs-social choice, suggesting the time interval was sufficient to minimize any potential influence of the non-contingent cocaine infusions on subsequent choice behavior. Overall, the present results were consistent with previous drug-vs-social choice studies and extend our knowledge of environmental factors impacting drug-vs-social choice. Future studies determining the pharmacological sensitivity of cocaine-vs-social choice will be important in expanding the preclinical utility of these procedures for candidate medication drug development.

Logical fallacies and misinterpretations that hinder progress in translational addiction neuroscience

Substance use disorders (SUDs) are heterogeneous and complex, making the development of translationally predictive rodent and nonhuman primate models to uncover their neurobehavioral underpinnings difficult. Neuroscience-focused outcomes have become highly prevalent, and with this, the notion that SUDs are disorders of the brain embraced as a dominant theoretical orientation to understand SUD etiology and treatment. These efforts, however, have led to few efficacious pharmacotherapies, and in some cases (as with cocaine or methamphetamine), no pharmacotherapies have translated from preclinical models for clinical use. In this theoretical commentary, we first describe the development of animal models of substance use behaviors from a historical perspective. We then define and discuss three logical fallacies including 1) circular explanation, 2) affirming the consequent, and 3) reification that can apply to developed models. We then provide three case examples in which conceptual or logical issues exist in common methods (i.e., behavioral economic demand, escalation, and reinstatement). Alternative strategies to refocus behavioral models are suggested for the field to better bridge the translational divide between animal models, the clinical condition of SUDs, and current and future regulatory pathways for intervention development.

Contextual extinction of drug-associated discriminative stimuli fails to attenuate drug-vs-food choice in rhesus monkeys

Relapse within the context of a substance use disorder can be triggered by cues that function as discriminative stimuli to signal contingencies of drug availability and promote drug-taking behavior. Extinction procedures can weaken this association between drug-associated cues and drug-taking behavior and may reduce the probability of relapse. This study evaluated a regimen of extinction training on cocaine and heroin self-administration in rhesus monkeys under a drug-vs-food choice procedure. Behavior was initially maintained under a concurrent schedule of food (1-g food pellets; fixed-ratio 100 schedule) and cocaine injections (0-0.1 mg/kg/injection; fixed-ratio 10) (n = 4 males) or heroin injections (0-0.01 mg/kg/injection; fixed-ratio 10) (n = 3 females and 1 male) during daily 2-hr choice sessions. Subsequently, choice sessions were supplemented by daily 20-hr saline self-administration sessions for 14 consecutive days. During saline self-administration sessions, only drug-associated discriminative stimuli were presented and responding produced saline injections. Drug continued to be available during choice sessions. Prior to extinction training, both cocaine and heroin maintained dose-dependent increases in drug-vs-food choice. Exposure to 14 saline self-administration sessions failed to significantly decrease drug choice and increase food choice. These preclinical results do not support the effectiveness of extinguishing drug-associated discriminative stimuli as a nonpharmacological treatment strategy for reducing drug choice.

Open-label pilot study of lisdexamfetamine for cocaine use disorder

Background: Cocaine use disorder (CUD) is a substantial public health problem with no FDA-approved medication treatments. Psychostimulants have shown promise as pharmacotherapy for CUD. Lisdexamfetamine, a novel prodrug psychostimulant, is roughly 40-50% as potent as dextroamphetamine.Objectives: To evaluate the safety, tolerability, and optimal dosing of lisdexamfetamine for treating CUD.Methods: Open-label, 8-week trial of 17 CUD adults. Participants were titrated to the maximum tolerated dose of 140 mg over 2-week period and maintained for 4 weeks, followed by a two-week taper period. The primary outcome measures were the maximum daily dose achieved during the study period and tolerability as measured by medication-related study drop-out.Results: Among the 16 participants with post-enrollment data, the mean dose of lisdexamfetamine achieved was 118.1 mg (standard deviation (SD) = 40.4), mean retention was 6.5 weeks (SD = 2.0), and no participants discontinued study medication due to adverse effects. Four participants had dose reductions due to adverse effects and continued in the trial. Six participants (37.5%) were abstinent for the last 3 weeks of their study participation. Mean dollars of cocaine spent per day significantly decreased from $19.72 at baseline to $7.57 during the last 3 weeks of study participation (t₁₅ = 3.60, p = .003). The mean percent of using days significantly decreased from 25% at baseline to 12% during the last 3 weeks of study participation (t₁₅ = 3.33, p = .005).Conclusion: The use of lisdexamfetamine for CUD in doses ranging to 140 mg daily was safe and generally well tolerated.

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.

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.

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

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

Pharmacological validation of a translational model of cocaine use disorder: Effects of d-amphetamine maintenance on choice between intravenous cocaine and a nondrug alternative in humans and rhesus monkeys

Drug self-administration procedures are the gold standard for laboratory research to study mechanisms of drug use disorders and evaluate candidate medications. However, preclinical-to-clinical translation has been hampered by a lack of coordination. To address this limitation, we previously developed homologous intravenous (IV) cocaine choice self-administration procedures in rhesus monkeys and humans, and then demonstrated their functional equivalence. The present studies sought to determine the sensitivity of these procedures to d-amphetamine maintenance. Three (N = 3) rhesus monkeys with histories of cocaine self-administration and 16 (N = 16) humans with cocaine use disorder completed the studies. Monkeys were maintained on IV d-amphetamine (0, 0.019, 0.037 and 0.074 mg/kg/h), and then completed 7 sessions during each condition in which they completed 9 choice trials to receive 0.14 mg/kg/injection IV cocaine (corresponding to 10 mg/70 kg in humans) or 10 food pellets under independent, concurrent progressive-ratio schedules. Humans were maintained on oral extended release d-amphetamine (0, 30 and 60 mg/day, corresponding to the lowest 3 doses in monkeys) and participated in 12 sessions in which they chose money ($6.00) or IV cocaine (0, 3, 10 and 30 mg/70 kg). Blood samples were taken to compare d-amphetamine plasma levels across species. In monkeys and humans, d-amphetamine reduced the number of cocaine choices and produced comparable blood levels at equivalent daily doses. d-Amphetamine had similar efficacy, though lower potency, at reducing choice for an equivalent cocaine dose in monkeys relative to humans. These coordinated studies support the utility of these procedures as a translational model for cocaine use disorder. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Sex differences in opioid reinforcement under a fentanyl vs. food choice procedure in rats

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure^® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure^® reinforcement in Sprague-Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure^® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure^® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

Utility of Nonhuman Primates in Substance Use Disorders Research

Substance use disorders (i.e., drug addiction) constitute a global and insidious public health issue. Preclinical biomedical research has been invaluable in elucidating the environmental, biological, and pharmacological determinants of drug abuse and in the process of developing innovative pharmacological and behavioral treatment strategies. For more than 70 years, nonhuman primates have been utilized as research subjects in biomedical research related to drug addiction. There are already several excellent published reviews highlighting species differences in both pharmacodynamics and pharmacokinetics between rodents and nonhuman primates in preclinical substance abuse research. Therefore, the aim of this review is to highlight three advantages of nonhuman primates as preclinical substance abuse research subjects. First, nonhuman primates offer technical advantages in experimental design compared to other laboratory animals that afford unique opportunities to promote preclinical-to-clinical translational research. Second, these technical advantages, coupled with the relatively long lifespan of nonhuman primates, allows for pairing longitudinal drug self-administration studies and noninvasive imaging technologies to elucidate the biological consequences of chronic drug exposure. Lastly, nonhuman primates offer advantages in the patterns of intravenous drug self-administration that have potential theoretical implications for both the neurobiological mechanisms of substance use disorder etiology and in the drug development process of pharmacotherapies for substance use disorders. We conclude with potential future research directions in which nonhuman primates would provide unique and valuable insights into the abuse of and addiction to novel psychoactive substances.

Medications for substance use disorders (SUD): emerging approaches

INTRODUCTION

Substance use disorders are a group of chronic relapsing disorders of the brain, which have massive public health and societal impact. In some disorders (e.g., heroin/prescription opioid addictions) approved medications have a major long-term benefit. For other substances (e.g., cocaine, amphetamines and cannabis) there are no approved medications, and for alcohol there are approved treatments, which are not in wide usage. Approved treatments for tobacco use disorders are available, and novel medications are also under study. Areas covered: Medication-based approaches which are in advanced preclinical stages, or which have reached proof-of concept clinical laboratory studies, as well as clinical trials. Expert opinion: Current challenges involve optimizing translation between preclinical and clinical development, and between clinical laboratory studies to therapeutic clinical trials. Comorbidities including depression or anxiety are challenges for study design and analysis. Improved pharmacogenomics, biomarker and phenotyping approaches are areas of interest. Pharmacological mechanisms currently under investigation include modulation of glutamatergic, GABA, vasopressin and κ-receptor function, as well as inhibition of monoamine re-uptake. Other factors that affect potential market size for emerging medications include stigma, availability of treatment settings, adoption by clinicians, and the prevalence of persons with SUD who are not actively treatment-seeking.

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 d-amphetamine and d-methamphetamine as active metabolites of benzphetamine: Evidence from drug discrimination and pharmacokinetic studies in male rhesus monkeys

Benzphetamine is a Schedule III anorectic agent that is a prodrug for d-amphetamine and d-methamphetamine and may have utility as an "agonist" medication for cocaine use disorder treatment. This study evaluated the pharmacokinetic-pharmacodynamic profile of benzphetamine using a drug discrimination procedure in rhesus monkeys. The potency and time course of cocaine-like discriminative stimulus effects were compared for benzphetamine (10-18mg/kg, intramuscular (IM)) and d-amphetamine (0.032-0.32mg/kg, IM) in monkeys (n=3-4) trained to discriminate IM cocaine (0.32mg/kg) from saline in a two-key food-reinforced discrimination procedure. Parallel pharmacokinetic studies in the same monkeys determined plasma benzphetamine, d-methamphetamine and/or d-amphetamine levels for correlation with behavioral effects. d-Amphetamine produced dose-dependent, time-dependent, and full cocaine-like effects, i.e. ≥90% cocaine-appropriate responding, in all monkeys without altering response rates. The time course of d-amphetamine's cocaine-like discriminative stimulus effects correlated with plasma d-amphetamine levels. Benzphetamine was 180-fold less potent than d-amphetamine and produced full cocaine-like effects in only 2 of 4 monkeys while significantly decreasing response rates. Benzphetamine administration increased plasma d-methamphetamine (peak at 100min) and d-amphetamine (peak at 24h) levels, but the time course of behavioral effects did not correlate with increased levels of benzphetamine, d-methamphetamine or d-amphetamine. These results suggest that benzphetamine yields d-amphetamine and d-methamphetamine as active metabolites in rhesus monkeys, but generation of these metabolites is not sufficient to account for benzphetamine behavioral effects. The incomplete cocaine substitution profile and protracted d-amphetamine plasma levels suggest that benzphetamine may still warrant further evaluation as a candidate pharmacotherapy for cocaine use disorder treatment.

Insights from Preclinical Choice Models on Treating Drug Addiction

Substance-use disorders are a global public health problem that arises from behavioral misallocation between drug use and more adaptive behaviors maintained by nondrug alternatives (e.g., food or money). Preclinical drug self-administration procedures that incorporate a concurrently available nondrug reinforcer (e.g., food) provide translationally relevant and distinct dependent measures of behavioral allocation (i.e., to assess the relative reinforcing efficacy of the drug) and behavioral rate (i.e., to assess motor competence). In particular, preclinical drug versus food 'choice' procedures have produced increasingly concordant results with both human laboratory drug self-administration studies and double-blind placebo-controlled clinical trials. Accordingly, here we provide a heuristic framework of substance-use disorders based on a behavioral-centric perspective and recent insights from these preclinical choice procedures.

Utility of preclinical drug versus food choice procedures to evaluate candidate medications for methamphetamine use disorder

Substance use disorders are diagnosed as a manifestation of inappropriate behavioral allocation toward abused drugs and away from other behaviors maintained by more adaptive nondrug reinforcers (e.g., money and social relationships). Substance use disorder treatment goals include not only decreasing drug-maintained behavior but also promoting behavioral reallocation toward these socially adaptive alternative reinforcers. Preclinical drug self-administration procedures that offer concurrent access to both drug and nondrug reinforcers provide a translationally relevant dependent measure of behavioral allocation that may be useful for candidate medication evaluation. In contrast to other abused drugs, such as heroin or cocaine, preclinical methamphetamine versus food choice procedures have been a more recent development. We hypothesize that preclinical to clinical translatability would be improved by the evaluation of repeated pharmacological treatment effects on methamphetamine self-administration under a methamphetamine versus food choice procedure. In support of this hypothesis, a literature review suggests strong concordance between preclinical pharmacological treatment effects on methamphetamine versus food choice in nonhuman primates and clinical medication treatment effects on methamphetamine self-administration in human laboratory studies or methamphetamine abuse metrics in clinical trials. In conclusion, this literature suggests preclinical methamphetamine versus food choice procedures may be useful in developing innovative pharmacotherapies for methamphetamine use disorder.

Development of a translational model to screen medications for cocaine use disorder II: Choice between intravenous cocaine and money in humans

BACKGROUND

A medication for treating cocaine use disorder has yet to be approved. Laboratory-based evaluation of candidate medications in animals and humans is a valuable means to demonstrate safety, tolerability and initial efficacy of potential medications. However, animal-to-human translation has been hampered by a lack of coordination. Therefore, we designed homologous cocaine self-administration studies in rhesus monkeys (see companion article) and human subjects in an attempt to develop linked, functionally equivalent procedures for research on candidate medications for cocaine use disorder.

METHODS

Eight (N=8) subjects with cocaine use disorder completed 12 experimental sessions in which they responded to receive money ($0.01, $1.00 and $3.00) or intravenous cocaine (0, 3, 10 and 30mg/70kg) under independent, concurrent progressive-ratio schedules. Prior to the completion of 9 choice trials, subjects sampled the cocaine dose available during that session and were informed of the monetary alternative value.

RESULTS

The allocation of behavior varied systematically as a function of cocaine dose and money value. Moreover, a similar pattern of cocaine choice was demonstrated in rhesus monkeys and humans across different cocaine doses and magnitudes of the species-specific alternative reinforcers. The subjective and cardiovascular responses to IV cocaine were an orderly function of dose, although heart rate and blood pressure remained within safe limits.

CONCLUSIONS

These coordinated studies successfully established drug versus non-drug choice procedures in humans and rhesus monkeys that yielded similar cocaine choice behavior across species. This translational research platform will be used in future research to enhance the efficiency of developing interventions to reduce cocaine use.