Acute d-amphetamine pretreatment does not alter stimulant self-administration in humans

Reinforcing effects of d-amphetamine: influence of novel ratios on a progressive-ratio schedule

Progressive-ratio schedules are useful for studying the reinforcing effects of drugs. Earlier human laboratory studies showed that d-amphetamine significantly increased break points relative to placebo. However, the magnitude of the increase was modest, which may be attributable to rather high levels of placebo responding. We used novel response requirements in a modified progressive-ratio procedure and hypothesized that the altered range of response requirements would decrease responding for placebo and increase responding for d-amphetamine. Eight participants completed the study. The participants first sampled oral doses of d-amphetamine (0, 8, 16, and 24 mg). In subsequent sessions, the participants were offered the opportunity to work for the sampled dose on a modified progressive-ratio procedure with response requirements ranging from 400 to 1800 mouse clicks. A battery of participant-rated drug-effect questionnaires, a performance measure, and cardiovascular measures were included to more fully characterize the effects of d-amphetamine. Placebo maintained low levels of responding. The intermediate dose of d-amphetamine increased responding significantly above placebo levels. d-Amphetamine produced prototypical subject-rated effects that were an orderly function of dose. These data suggest that the modified response requirements resulted in lower levels of placebo taking and a larger separation between the number of placebo and d-amphetamine capsules earned.

Agonist replacement for stimulant dependence: a review of clinical research

Stimulant use disorders are an unrelenting public health concern worldwide. Agonist replacement therapy is among the most effective strategies for managing substance use disorders including nicotine and opioid dependence. The present paper reviewed clinical data from human laboratory self-administration studies and clinical trials to determine whether agonist replacement therapy is a viable strategy for managing cocaine and/or amphetamine use disorders. The extant literature suggests that agonist replacement therapy may be effective for managing stimulant use disorders, however, the clinical selection of an agonist replacement medication likely needs to be based on the pharmacological mechanism of the medication and the stimulant abused by patients. Specifically, dopamine releasers appear most effective for reducing cocaine use whereas dopamine reuptake inhibitors appear most effective for reducing amphetamine use.

Separate and combined impact of acute naltrexone and alprazolam on subjective and physiological effects of oral d-amphetamine in stimulant users

RATIONALE

Opioid antagonists (e.g., naltrexone) and positive modulators of γ-aminobutyric-acidA (GABAA) receptors (e.g., alprazolam) modestly attenuate the abuse-related effects of stimulants like amphetamine. The use of higher doses to achieve greater efficacy is precluded by side effects. Combining naltrexone and alprazolam might safely maximize efficacy while avoiding the untoward effects of the constituent compounds.

OBJECTIVES

The present pilot study tested the hypothesis that acute pretreatment with the combination of naltrexone and alprazolam would not produce clinically problematic physiological effects or negative subjective effects and would reduce the positive subjective effects of d-amphetamine to a greater extent than the constituent drugs alone.

METHODS

Eight nontreatment-seeking, stimulant-using individuals completed an outpatient experiment in which oral d-amphetamine (0, 15, and 30 mg) was administered following acute pretreatment with naltrexone (0 and 50 mg) and alprazolam (0 and 0.5 mg). Subjective effects, psychomotor task performance, and physiological measures were collected.

RESULTS

Oral d-amphetamine produced prototypical physiological and stimulant-like positive subjective effects (e.g., VAS ratings of Active/Alert/Energetic, Good Effect, and High). Pretreatment with naltrexone, alprazolam, and their combination did not produce clinically problematic acute physiological effects or negative subjective effects. Naltrexone and alprazolam each significantly attenuated some of the subjective effects of d-amphetamine. The combination attenuated a greater number of subjective effects than the constituent drugs alone.

CONCLUSIONS

The present results support the continued evaluation of an opioid receptor antagonist combined with a GABAA-positive modulator using more clinically relevant experimental conditions like examining the effect of chronic dosing with these drugs on methamphetamine self-administration.

Influence of aripiprazole pretreatment on the reinforcing effects of methamphetamine in humans

Methamphetamine use disorders remain a significant public health concern. Methamphetamine produces its behavioral effects by facilitating release of monoamines like dopamine (DA) and serotonin (5-HT). Results from animal studies show that acute pretreatment with DA and 5-HT antagonists attenuates the effects of methamphetamine, but this area remains largely unexplored in humans. This study sought to assess whether aripiprazole, a partial agonist at D2/5-HT1A receptors and an antagonist at 5-HT2A receptors, would attenuate the reinforcing and subject-rated effects of oral methamphetamine. Seven subjects with histories of recreational stimulant use completed a placebo-controlled, crossover, double-blind protocol in which they first sampled doses of oral methamphetamine (0, 4, 8 or 16 mg) following acute pretreatment with aripiprazole (0 and 15 mg). During each Sampling Session, subjects also completed a battery of subject-rated, cardiovascular, and other performance measures. In subsequent Self-Administration Sessions, subjects were provided the opportunity to earn the previously sampled methamphetamine dose on a progressive-ratio procedure. Methamphetamine functioned as a reinforcer, and produced prototypical stimulant-like subject-rated and cardiovascular effects (e.g., increased ratings of Stimulated; elevated blood pressure). Aripiprazole reduced methamphetamine self-administration and attenuated some of the positive subject-rated effects of methamphetamine (e.g., ratings of Like Drug). These results indicate that acute aripiprazole pretreatment attenuates the abuse-related effects of methamphetamine.

Effects of acute oral naltrexone on the subjective and physiological effects of oral D-amphetamine and smoked cocaine in cocaine abusers

Despite the prevalent worldwide abuse of stimulants, such as amphetamines and cocaine, no medications are currently approved for treating this serious public health problem. Both preclinical and clinical studies suggest that the opioid antagonist naltrexone (NTX) is effective in reducing the abuse liability of amphetamine, raising the question of whether similar positive findings would be obtained for cocaine. The purpose of this study was to evaluate the ability of oral NTX to alter the cardiovascular and subjective effects of D-amphetamine (D-AMPH) and cocaine (COC). Non-treatment-seeking COC users (N=12) completed this 3-week inpatient, randomized, crossover study. Participants received 0, 12.5, or 50 mg oral NTX 60 min before active or placebo stimulant administration during 10 separate laboratory sessions. Oral AMPH (0, 10, and 20 mg; or all placebo) was administered in ascending order within a laboratory session using a 60-min interdose interval. Smoked COC (0, 12.5, 25, and 50 mg; or all placebo) was administered in ascending order within a laboratory session using a 14-min interdose interval. Active COC and AMPH produced dose-related increases in cardiovascular function that were of comparable magnitude. In contrast, COC, but not AMPH, produced dose-related increases in several subjective measures of positive drug effect (eg, high, liking, and willingness to pay for the drug). NTX did not alter the cardiovascular effects of AMPH or COC. NTX also did not alter positive subjective ratings after COC administration, but it did significantly reduce ratings of craving for COC and tobacco during COC sessions. These results show that (1) oral AMPH produces minimal abuse-related subjective responses in COC smokers, and (2) NTX reduces craving for COC and tobacco during COC sessions. Future studies should continue to evaluate NTX as a potential anti-craving medication for COC dependence.

Relationship between oral D-amphetamine self-administration and ratings of subjective effects: do subjective-effects ratings correspond with a progressive-ratio measure of drug-taking behavior?

The abuse potential of drugs has traditionally been determined in humans using subjective ratings of drug effects. However, drug self-administration procedures also provide valuable information about the reinforcing effects of drugs that may contribute to their potential for abuse. Although ratings of subjective effects and drug self-administration data are generally concordant, some divergent findings have been reported. Therefore, the aim of the present analysis was to directly investigate the relationship between the subjective-effects profile and self-administration of oral D-amphetamine in healthy volunteers with a history of stimulant use or abuse, using Pearson's correlational analyses. The results indicated that positive subjective and reinforcing effects significantly increased as a function of D-amphetamine dose. Further, significant, but modest, correlations were observed between ratings of six of 17 total items (Any Effect, High, Like Drug, Good Effects, Willing to Pay For, and Willing to Take Again) and D-amphetamine self-administration under a progressive-ratio schedule of reinforcement. The current findings suggest that, at least under the current set of conditions with oral D-amphetamine, subjective-effects measures and drug self-administration data likely provide different but complimentary information about abuse potential. The most informative findings will thus be obtained from studies that use ratings of subjective effects and drug self-administration methods.

An Exploration of Responses to Drug Conditioned Stimuli during Treatment for Substance Dependence

Although it is well established that drug conditioned stimuli produce a variety of conditioned responses, it is not known whether such stimuli can also reinforce an arbitrary operant response and thus serve as conditioned reinforcers. Volunteers (n = 39) recruited from a residential treatment center for substance dependence were tested on a task in which presses on computer keys activated images of drugs/drug paraphernalia on a progressive ratio schedule of reinforcement. They also completed a personalized craving questionnaire and a personalized Implicit Association Test. A significant bias in responding was found for images of preferred drugs/route of drug administration. Craving, however, was low and the images generated negative evaluative reactions. Two additional studies were performed to ascertain the generalizability of the effects to a different population of drug-using individuals (i.e., students who drink) and to incentive stimuli of a different nature (i.e., sexual). The additional studies partially replicated and extended the central findings of the main study. Therefore, although these data should be considered preliminary in light of small group sizes, it is concluded that cue specificity and availability of the unconditioned stimuli (drugs and sex) plays a role in modulating responding maintained by conditioned reinforcers.

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.

Amphetamine self-administration in light and moderate drinkers

BACKGROUND

Light and moderate drinkers respond differently to the effects of abused drugs, including stimulants such as amphetamine. The purpose of this study was to determine whether light and moderate drinkers differ in their sensitivity to the reinforcing and subjective effects of d-amphetamine. We hypothesized that moderate drinkers (i.e., participants that reported consuming at least seven alcohol-containing beverages per week) would be more sensitive to the reinforcing and positive subject-rated effects of d-amphetamine than light drinkers.

METHODS

Data from four studies that employed similar d-amphetamine self-administration procedures and subject-rated drug-effect measures were included in the analysis. Light (n = 17) and moderate (n = 16) drinkers sampled placebo, low (8 to 10 mg), and high (16 to 20 mg) doses of oral d-amphetamine administered in eight capsules. Following sampling sessions, participants worked for a maximum of eight capsules, each containing 12.5% of the previously sampled dose, on a modified progressive-ratio schedule of reinforcement.

RESULTS

Both active doses of d-amphetamine functioned as a reinforcer in the moderate drinkers, while only the high dose did so in the light drinkers. The moderate drinkers worked for significantly more capsules that contained the high dose of d-amphetamine than did the light drinkers. d-Amphetamine produced prototypical stimulant-like subjective effects (e.g., dose-dependent increases in ratings of Good Effects; Like Drug and Willing to Take Again). Moderate drinkers reported significantly greater subjective effects than the light drinkers.

CONCLUSION

These results are consistent with those from previous laboratory experiments and suggest that moderate alcohol consumption may increase vulnerability to the abuse-related effects of stimulants.

Addiction: decreased reward sensitivity and increased expectation sensitivity conspire to overwhelm the brain's control circuit

Based on brain imaging findings, we present a model according to which addiction emerges as an imbalance in the information processing and integration among various brain circuits and functions. The dysfunctions reflect (a) decreased sensitivity of reward circuits, (b) enhanced sensitivity of memory circuits to conditioned expectations to drugs and drug cues, stress reactivity, and (c) negative mood, and a weakened control circuit. Although initial experimentation with a drug of abuse is largely a voluntary behavior, continued drug use can eventually impair neuronal circuits in the brain that are involved in free will, turning drug use into an automatic compulsive behavior. The ability of addictive drugs to co-opt neurotransmitter signals between neurons (including dopamine, glutamate, and GABA) modifies the function of different neuronal circuits, which begin to falter at different stages of an addiction trajectory. Upon exposure to the drug, drug cues or stress this results in unrestrained hyperactivation of the motivation/drive circuit that results in the compulsive drug intake that characterizes addiction.

Human sex differences in d-amphetamine self-administration

Women and men may respond differently to the effects of stimulants such as amphetamine and cocaine.

AIM

In order to assess potential sex differences in the reinforcing effects of d-amphetamine, a retrospective-analysis was conducted on data collected from three studies that employed similar d-amphetamine self-administration procedures and used identical subject-rated drug-effect measures.

METHODS

Data from 10 women and 15 men were included in the analysis. In all studies, participants sampled placebo, low (8-10 mg) or high (16-20 mg) dose oral d-amphetamine. Following sampling sessions, participants worked for capsules containing one eighth of the previously sampled dose on a modified progressive-ratio schedule of reinforcement. We hypothesized that women and men would be differentially sensitive to the reinforcing effects of d-amphetamine. A two-way mixed-model analysis of variance (sex and dose) and planned comparisons were used in the statistical analyses.

RESULTS

The low dose of d-amphetamine functioned as a reinforcer in women, but not men, whereas the high dose of d-amphetamine functioned as a reinforcer in men, but not women. Men self-administered significantly more capsules under the high dose condition than women.

CONCLUSION

The results of this study suggest that men are more sensitive to the reinforcing effects of a high dose of d-amphetamine than women. Future research is needed that determines prospectively the reinforcing effects of weight-adjusted doses of d-amphetamine in women and men while controlling for menstrual cycle phase.

Reward sensitivity: issues of measurement, and achieving consilience between human and animal phenotypes

Reward is a concept fundamental to discussions of drug abuse and addiction. The idea that altered sensitivity to either drug-reward, or to rewards in general, contributes to, or results from, drug-taking is a common theme in several theories of addiction. However, the concept of reward is problematic in that it is used to refer to apparently different behavioural phenomena, and even to diverse neurobiological processes (reward pathways). Whether these different phenomena are different behavioural expressions of a common underlying process is not established, and much research suggests that there may be only loose relationships among different aspects of reward. Measures of rewarding effects of drugs in humans often depend upon subjective reports. In animal studies, such insights are not available, and behavioural measures must be relied upon to infer rewarding effects of drugs or other events. In such animal studies, but also in many human methods established to objectify measures of reward, many other factors contribute to the behaviour being studied. For that reason, studying the biological (including genetic) bases of performance of tasks that ostensibly measure reward cannot provide unequivocal answers. The current overview outlines the strengths and weaknesses of current approaches that hinder the conciliation of cross-species studies of the genetics of reward sensitivity and the dysregulation of reward processes by drugs of abuse. Some suggestions are made as to how human and animal studies may be made to address more closely homologous behaviours, even if those processes are only partly able to isolate 'reward' from other factors contributing to behavioural output.

Reinforcing effects of stimulants in humans: sensitivity of progressive-ratio schedules

Drug self-administration methodologies have been developed for use in humans to model naturalistic stimulant drug-taking behaviors. These methodologies use a number of schedules of reinforcement, including progressive-ratio schedules. As the name implies, in a progressive-ratio schedule, the response requirement for each subsequent delivery of drug increases, and the primary outcome variable is often the break point (i.e., the last ratio completed to receive a drug delivery). These schedules have been used in a number of human laboratory studies evaluating the reinforcing effects of stimulants. The results of these studies have demonstrated that progressive-ratio schedules are sensitive to manipulation of a pharmacological variable, dose, and to nonpharmacological variables contributing to stimulant drug effects. In addition, findings with progressive-ratio schedules are largely concordant with clinical findings, suggesting that drug self-administration under these schedules has predictive validity in terms of drug abuse and dependence. Future research is necessary, however, to understand better how pharmacological factors like route of administration, onset of effects, and pretreatment influence the reinforcing effects of stimulants under progressive-ratio schedules.

The role of human drug self-administration procedures in the development of medications

The purpose of this review is to illustrate the utility and value of employing human self-administration procedures in medication development, including abuse liability assessments of novel medications and evaluation of potential pharmacotherapies for substance use disorders. Traditionally, human abuse liability testing has relied primarily on subjective reports describing drug action by use of questionnaires; similarly, drug interactions between putative treatment agents and the drugs of abuse have relied on these measures. Subjective reports are highly valued because they provide qualitative and quantitative information about the characteristics of central and peripheral pharmacodynamic effects as well as safety and tolerability. However, self-administration procedures directly examine the behavior of interest-that is, drug taking. The present paper (1) reviews the most commonly used human self-administration procedures, (2) discusses the concordance of subjective reports and self-administration within the context of medications development for substance use disorders, focusing primarily on illustrative examples from development efforts with opioid and cocaine dependence, and (3) explores the utility of applying self-administration procedures to assess the abuse liability of novel compounds, including "abuse-deterrent" formulations (ADFs). The review will focus on opioid and cocaine dependence because a rich database from both clinical laboratory and clinical trial research exists for these two drug classes. The data reviewed suggest that drug-induced changes in self-administration and subjective effects are not always concordant. Therefore, assessment of self-administration in combination with subjective effects provides a more comprehensive picture that may have improved predictive validity for translating to the clinical setting.

Imaging dopamine's role in drug abuse and addiction

Dopamine is involved in drug reinforcement but its role in addiction is less clear. Here we describe PET imaging studies that investigate dopamine's involvement in drug abuse in the human brain. In humans the reinforcing effects of drugs are associated with large and fast increases in extracellular dopamine, which mimic those induced by physiological dopamine cell firing but are more intense and protracted. Since dopamine cells fire in response to salient stimuli, supraphysiological activation by drugs is experienced as highly salient (driving attention, arousal, conditioned learning and motivation) and with repeated drug use may raise the thresholds required for dopamine cell activation and signaling. Indeed, imaging studies show that drug abusers have marked decreases in dopamine D2 receptors and in dopamine release. This decrease in dopamine function is associated with reduced regional activity in orbitofrontal cortex (involved in salience attribution; its disruption results in compulsive behaviors), cingulate gyrus (involved in inhibitory control; its disruption results in impulsivity) and dorsolateral prefrontal cortex (involved in executive function; its disruption results in impaired regulation of intentional actions). In parallel, conditioning triggered by drugs leads to enhanced dopamine signaling when exposed to conditioned cues, which then drives the motivation to procure the drug in part by activation of prefrontal and striatal regions. These findings implicate deficits in dopamine activity-inked with prefrontal and striatal deregulation-in the loss of control and compulsive drug intake that results when the addicted person takes the drugs or is exposed to conditioned cues. The decreased dopamine function in addicted individuals also reduces their sensitivity to natural reinforcers. Therapeutic interventions aimed at restoring brain dopaminergic tone and activity of cortical projection regions could improve prefrontal function, enhance inhibitory control and interfere with impulsivity and compulsive drug administration while helping to motivate the addicted person to engage in non-drug related behaviors.

Effects of potential agonist-replacement therapies for stimulant dependence on inhibitory control in cocaine abusers

Two experiments were conducted to determine whether methylphenidate or modafinil, two potential pharmacotherapies for stimulant dependence, would impair inhibitory behavior in cocaine users. Eleven cocaine abusers were administered methylphenidate (0, 15, 30, and 45 mg) or modafinil (0, 150, 300, and 450 mg) across four experimental sessions. A cued go-no-go task was used to measure response execution and inhibition. Subjective and cardiovascular measures were collected. Neither methylphenidate nor modafinil impaired inhibitory control, but produced prototypical subject-rated and cardiovascular effects. The results of these studies may have implications for the use of these drugs as agonist-replacement therapies for stimulant dependence.