Discriminative stimulus and self-reported effects of methylphenidate, d-amphetamine, and triazolam in methylphenidate-trained humans

A Brief Introduction to Human Behavioral Pharmacology: Methods, Design Considerations and Ethics

Human behavioral pharmacology methods have been used to rigorously evaluate the effects of a range of centrally acting drugs in humans under controlled conditions for decades. Methods like drug self-administration and drug discrimination have been adapted from nonhuman laboratory animal models. Because humans have the capacity to communicate verbally, self-report methods are also commonly used to understand drug effects. This perspective article provides an overview of these traditional human behavioral pharmacology methods and introduces some novel methodologies that have more recently been adapted for use in the field. Design (e.g., using placebo controls, testing multiple doses) and ethical (e.g., avoiding enrollment of individuals seeking treatment, determining capacity to consent) considerations that must be addressed when conducting these types of studies are also described.

An update on the clinical pharmacology of methylphenidate: therapeutic efficacy, abuse potential and future considerations

INTRODUCTION

Methylphenidate remains a first-line medication for treating ADHD in children and adults. However, its behavioral pharmacological similarities to methamphetamine and cocaine have historically created concern for its potential as a drug of abuse. In September 2019, the FDA published a docket requesting comments for the development of abuse deterrent formulations for CNS stimulants, emphasizing the abuse of methylphenidate as a public health concern.

AREAS COVERED

We conducted a narrative review of research on the clinical pharmacology, therapeutic efficacy, and abuse potential of methylphenidate.

EXPERT OPINION

Several studies indicate that methylphenidate has at least some abuse potential. Methylphenidate, amphetamine, methamphetamine, and cocaine overlap in their subjective, reinforcing, and discriminative stimulus effects. Regardless, methylphenidate remains an efficacious treatment for ADHD in children and adults when properly adhered to, especially when paired with non-pharmacological treatments. The development of abuse deterrent formulations of methylphenidate is warranted.

Abuse Potential of Oral Phendimetrazine in Cocaine-dependent Individuals: Implications for Agonist-like Replacement Therapy

OBJECTIVES

Phendimetrazine is a prodrug for the monoamine releaser phenmetrazine-a drug with known abuse potential. Preclinical studies suggest that phendimetrazine has limited abuse potential and may have promise as an agonist-like replacement therapy for cocaine dependence. This study evaluated the abuse potential of phendimetrazine in humans.

METHODS

Nine cocaine-dependent individuals (N = 9) were enrolled to investigate the abuse potential of phendimetrazine and d-amphetamine, using a double-blind, placebo-controlled, within-subject design. Subjective and cardiovascular effects of oral phendimetrazine (35, 70, and 105 mg), d-amphetamine (10, 20, and 30 mg), and placebo were assessed in quasi-random order across 8 sessions lasting for approximately 8 hours each.

RESULTS

d-Amphetamine (20 and 30 mg) significantly increased cardiovascular measures in a time and dose-related manner, but phendimetrazine did not systematically alter cardiovascular measures. Although d-amphetamine and phendimetrazine significantly increased ratings indicative of abuse potential (eg, drug liking) and stimulant-like effects relative to placebo, these increases were generally small in magnitude, with phendimetrazine producing significant effects on fewer abuse-related measures and at fewer time points than d-amphetamine.

CONCLUSIONS

These preliminary findings suggest that oral phendimetrazine and d-amphetamine may have limited abuse potential in cocaine-dependent individuals. These findings collectively emphasize that the clinical utility of medications to treat cocaine-use disorders should be weighed carefully against their potential for abuse and diversion, with careful attention paid to evaluating abuse potential in a clinically relevant population of interest. Future studies are needed to further elucidate the potential utility of phendimetrazine as an agonist-like replacement therapy for cocaine dependence.

Human drug discrimination: A primer and methodological review

Drug-discrimination procedures empirically evaluate the control that internal drug states exert over behavior. They provide a highly selective method to investigate the neuropharmacological underpinnings of the interoceptive effects of drugs. Historically, drug discrimination has been one of the most widely used assays in the field of behavioral pharmacology. Drug-discrimination procedures have also been adapted for use with humans and are conceptually similar to preclinical drug-discrimination techniques in that a behavior is differentially reinforced contingent on the presence or absence of a specific interoceptive drug stimulus. This review gives some general history and background concerning the major theoretical concepts and principles of drug-discrimination research as well as its relevance to substance-use disorders. This article also provides a procedural overview and discusses key methodological issues that must be considered when designing and conducting a human drug-discrimination study. Although drug discrimination is unequivocally one of the most sophisticated and useful behavioral assays to investigate the underlying neuropharmacology of drugs in vivo, enthusiasm for its use has steadily declined in the last decade and a half. We conclude by commenting on the current state of drug-discrimination research and suggest potential avenues for future drug-discrimination research. (PsycINFO Database Record

Assessment of human abuse potential of dasotraline compared to methylphenidate and placebo in recreational stimulant users

AIMS

The aim of this study was to evaluate the abuse potential of dasotraline, a novel dopamine and norepinephrine reuptake inhibitor with slow absorption (tmax, 10-12h) and elimination (t1/2=47-77 h) that is in development for the treatment of attention deficit hyperactivity disorder (ADHD).

METHODS

Recreational stimulant users (N=48) who had specific experience with cocaine, and who were able to distinguish methylphenidate (60 mg) versus placebo in a qualification session, were randomized, in a 6-period, double-blind, crossover design, to receive single doses of dasotraline 8 mg, 16 mg, and 36 mg, methylphenidate (MPH) 40 mg and 80 mg, and placebo. The primary endpoint was the Drug Liking Visual Analog Scale (VAS) score at the time of peak effect (Emax).

RESULTS

There were no significant differences between the 3 doses of dasotraline and placebo on the drug liking VAS at Emax, and on most secondary endpoints. Both doses of MPH had significantly higher VAS-drug liking scores at Emax relative to both placebo (P<0.001 for all comparisons) and dasotraline 8 mg (P<0.001), 16 mg (P<0.001) and 36 mg (P<0.01). The increase in heart rate for MPH and dasotraline 36 mg showed a time-course that closely matched subject-rated measures such as Any Effects VAS.

CONCLUSIONS

In this study, dasotraline was found to have low potential for abuse, which may be, in part, related to its established pharmacokinetics (PK) profile, which is characterized by slow absorption and gradual elimination.

Influence of sensitization on the discriminative stimulus effects of methylphenidate in mice

Methylphenidate (MPH) remains an important therapy for attention-deficit hyperactivity disorder, but aspects of its pharmacology remain unclear. In the present study, we used a regimen of MPH (8 mg/kg daily×14 days) in C57BL/6J mice to determine whether establishing locomotor sensitization to MPH influenced the acquisition and the dose-response function of MPH in a classic drug discrimination procedure. MPH-sensitized mice (SENS group) showed enhanced locomotor activity to the 8 mg/kg exposure dose as well as a 2 mg/kg dose before discrimination training. However, the SENS mice did not acquire discrimination of either a low dose (2 mg/kg) or a higher dose (4 mg/kg) of MPH any more rapidly than the CTRL mice. Further, during generalization testing, the dose-response functions for the SENS and CTRL mice were identical. Therefore, we did not find that previous exposure to MPH, which produced a sensitized locomotor response, facilitated MPH discrimination.

Mu opioid mediated discriminative-stimulus effects of tramadol: an individual subjects analysis

Drug discrimination procedures use dose-dependent generalization, substitution, and pretreatment with selective agonists and antagonists to evaluate receptor systems mediating interoceptive effects of drugs. Despite the extensive use of these techniques in the nonhuman animal literature, few studies have used human participants. Specifically, human studies have not routinely used antagonist administration as a pharmacological tool to elucidate the mechanisms mediating the discriminative stimulus effects of drugs. This study evaluated the discriminative-stimulus effects of tramadol, an atypical analgesic with monoamine and mu opioid activity. Three human participants first learned to discriminate 100 mg tramadol from placebo. A range of tramadol doses (25 to 150 mg) and hydromorphone (4 mg) with and without naltrexone pretreatment (50 mg) were then administered to participants after they acquired the discrimination. Tramadol produced dose-dependent increases in drug-appropriate responding and hydromorphone partially or fully substituted for tramadol in all participants. These effects were attenuated by naltrexone. Individual participant records indicated a relationship between mu opioid activity (i.e., miosis) and drug discrimination performance. Our findings indicate that mu opioid activity may mediate the discriminative-stimulus effects of tramadol in humans. The correspondence of generalization, substitution, and pretreatment findings with the animal literature supports the neuropharmacological specificity of the drug discrimination procedure.

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.

Differential substitution for the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine and methylphenidate in rats

Previous studies have demonstrated that methylphenidate, MDMA (3,4-methylenedioxymethamphetamine), and other psychostimulants exert stimulant-like subjective effects in humans. Furthermore, MDMA and methylphenidate substitute for the discriminative stimulus effects of psychostimulants, such as amphetamine and cocaine, in animals, which suggests that MDMA and methylphenidate may produce similar discriminative stimulus effects in rats. However, there is no evidence regarding the similarities between the discriminative stimulus effects of MDMA and methylphenidate. To explore this issue, cross-substitution, substitution, and combination tests were conducted in rats that had been trained to discriminate between MDMA (2.5 mg/kg) or methylphenidate (5.0 mg/kg) and saline. In the cross-substitution tests, MDMA and methylphenidate did not cross-substitute for each other. In the substitution test, methamphetamine substituted for the discriminative stimulus effects of methylphenidate, but not for those of MDMA. Furthermore, ephedrine and bupropion, which activate dopaminergic and noradrenergic systems, substituted for the discriminative stimulus effects of methylphenidate. On the other hand, serotonin (5-HT) receptor agonists 5-HT1A and 5-HT2 fully substituted for the discriminative stimulus effects of MDMA. These results suggest that activation of the noradrenergic and dopaminergic systems is important for the discriminative stimulus effects of methylphenidate, whereas activation of the serotonergic system is crucial for the discriminative stimulus effects of MDMA. Even though MDMA, like psychostimulants, exerts stimulant-like effects, our findings clearly indicate that the discriminative stimulus effects of MDMA are distinctly different from those of other psychostimulants in rats.

Methylphenidate increases cigarette smoking in participants with ADHD

RATIONALE

Methylphenidate (Ritalin®) is commonly prescribed for behavioral problems associated with attention deficit/hyperactivity disorder (ADHD). The results of previous studies suggest that methylphenidate increases cigarette smoking in participants without psychiatric diagnoses. Whether methylphenidate increases cigarette smoking in participants diagnosed with ADHD is unknown.

OBJECTIVE

In this within-subjects, repeated measures experiment, the acute effects of a range of doses of methylphenidate (10, 20, and 40 mg) and placebo were assessed in nine cigarette smokers who were not attempting to quit and met diagnostic criteria for ADHD but no other Axis I psychiatric disorders other than nicotine dependence.

METHODS

Each dose of methylphenidate was tested once while placebo was tested twice. One hour after ingesting drug, participants were allowed to smoke ad libitum for 4 h. Measures of smoking included total cigarettes smoked, total puffs, and carbon monoxide levels. Snacks and decaffeinated drinks were available ad libitum; caloric intake during the 4-h smoking session was calculated.

RESULTS

Methylphenidate increased the total number of cigarettes smoked, total number of puffs, and carbon monoxide levels. Methylphenidate decreased the number of food items consumed and caloric intake.

CONCLUSIONS

The results of this experiment suggest that acutely administered methylphenidate increases cigarette smoking in participants with ADHD, which is concordant with findings from previous studies that tested healthy young adults. These data indicate that clinicians may need to consider non-stimulant options or counsel their patients before starting methylphenidate when managing ADHD-diagnosed individuals who smoke.

Application of the conditioned taste aversion paradigm to assess discriminative stimulus properties of psychostimulants in rats

BACKGROUND

The conditioned taste aversion (CTA) paradigm is one of the reliable methods to evaluate the discriminative stimulus properties of drugs and is characterized by a short conditioning period and no need for special equipment. This method, however, has not yet been fully investigated for psychostimulants such as cocaine and methamphetamine.

METHODS

In the present study, rats were trained to discriminate between cocaine and a vehicle using CTA and substitution tests with various psychostimulants were conducted to evaluate the usefulness of the method for assessing the discriminative stimulus properties of this pharmacological class. Male rats received an intraperitoneal (i.p.) injection of cocaine (10mg/kg) 10 min prior to access saccharin for 20-min, and immediately after the saccharin access they received an i.p. dose of LiCl (1.8 mEq; n=8, Group CL) or the vehicle (n=8, Group CW) on the day of conditioning; on the other days (2 or 3 days between the cocaine conditioning days), they were injected with saline prior to access to saccharin without the LiCl or vehicle injection after the access.

RESULTS

By the fifteenth cocaine conditioning trial, all animals acquired discrimination. In the substitution test, cocaine dose dependently decreased saccharin consumption. The psychostimulants, methamphetamine, methylphenidate, bupropion and sibutramine, substituted for cocaine, whereas the opioid μ agonist morphine and the cannabinoid agonist, Δ9-tetrahydrocannabinol, did not substitute for cocaine. Mazindol did not substitute for cocaine although it has CNS stimulant activities.

CONCLUSION

These results suggest that discriminative stimulus properties of psychostimulants can be evaluated using the CTA paradigm.

Discriminative stimulus effects of tramadol in humans

Tramadol is an unscheduled atypical analgesic that acts as an agonist at μ-opioid receptors and inhibits monoamine reuptake. Tramadol can suppress opioid withdrawal, and chronic administration can produce opioid physical dependence; however, diversion and abuse of tramadol is low. The present study further characterized tramadol in a three-choice discrimination procedure. Nondependent volunteers with active stimulant and opioid use (n = 8) participated in this residential laboratory study. Subjects were trained to discriminate between placebo, hydromorphone (8 mg), and methylphenidate (60 mg), and tests of acquisition confirmed that all volunteers could discriminate between the training drugs. The following drug conditions were then tested during discrimination test sessions: placebo, hydromorphone (4 and 8 mg), methylphenidate (30 and 60 mg), and tramadol (50, 100, 200, and 400 mg). In addition to discrimination measures, which included discrete choice, point distribution, and operant responding, subjective and physiological effects were measured for each test condition. Both doses of hydromorphone and methylphenidate were identified as hydromorphone- and methylphenidate-like, respectively. Lower doses of tramadol were generally identified as placebo, with higher doses (200 and 400 mg) identified as hydromorphone, or opioid-like. The highest dose of tramadol increased ratings on the stimulant scale, but was not significantly identified as methylphenidate-like. Tramadol did not significantly increase subjective ratings associated with reinforcement. Taken together, these results extend previous work with tramadol as a potential medication for the treatment of opioid dependence and withdrawal, showing acute doses of tramadol exhibit a profile of effects similar to opioid agonists and may have abuse liability in certain populations.

Subjective and physiological effects of acute intranasal methamphetamine during d-amphetamine maintenance

RATIONALE

Methamphetamine abuse and dependence are significant public-health concerns. Behavioral therapies are effective for reducing methamphetamine use. However, many patients enrolled in behavioral therapies are unable to achieve significant periods of abstinence, suggesting other strategies like pharmacotherapy are needed.

OBJECTIVES

This experiment determined the subjective and physiological effects of intranasal methamphetamine during D: -amphetamine maintenance in eight non-treatment-seeking stimulant-dependent participants. We predicted D: -amphetamine maintenance would attenuate the acute subjective effects of intranasal methamphetamine. We also predicted intranasal methamphetamine would be well tolerated during D: -amphetamine maintenance.

METHODS

After at least 7 days of maintenance on sustained-release D: -amphetamine (0 and 45 mg/day), participants were administered ascending doses of intranasal methamphetamine (0, 2.5, 5, 10, and 20 mg) across two experimental sessions. Intranasal methamphetamine doses were separated by 90 min.

RESULTS

Intranasal methamphetamine produced prototypical subjective and physiological effects (e.g., increased ratings of Like Drug; increased heart rate, blood pressure, and body temperature). The acute effects of intranasal methamphetamine were significantly diminished during D: -amphetamine maintenance relative to placebo maintenance.

CONCLUSIONS

These results are concordant with those of clinical trials and provide further support for the use of agonist replacement therapy to manage methamphetamine dependence. Additional research in humans is needed to determine the effectiveness of D: -amphetamine under different experimental conditions that more closely reflect use in the natural environment (e.g., higher methamphetamine doses) and behavioral arrangements that are predictive of pharmacotherapy effectiveness (e.g., drug self-administration).

Methylphenidate increases choice of cigarettes over money

INTRODUCTION

Stimulants increase cigarette smoking in the naturalistic environment and laboratory. The effects of methylphenidate on a 9-trial, discrete cigarette versus money ($0.25) choice task were tested to elucidate the mechanisms underlying stimulant-induced increases in smoking.

METHODS

Eleven participants who reported smoking 10-20 cigarettes/day completed the study. Four doses of methylphenidate (0, 10, 20, and 40 mg) were administered across 5 experimental sessions, with placebo administered twice. One hour following medication administration and at 30-min intervals thereafter, participants chose between smoking a cigarette and receiving US$0.25. The primary behavioral outcome measure was number of cigarette choices.

RESULTS

Methylphenidate increased the number of cigarette choices over money. Puffs per session and carbon monoxide levels increased significantly and caloric intake decreased significantly following methylphenidate administration relative to placebo.

CONCLUSIONS

The results of this study suggest that methylphenidate increases the relative reinforcing efficacy of cigarette smoking. Stimulant use may thus be an important consideration for individuals attempting to quit smoking.

Comparison of the behavioral and cardiovascular effects of intranasal and oral d-amphetamine in healthy human subjects

Recent reports indicate an increase in intranasal use of prescription oral stimulant medication. However, there do not appear to be any published clinical studies that have characterized the behavioral and cardiovascular effects of intranasally administered d-amphetamine, which is commonly prescribed for Attention Deficit Hyperactivity Disorder. In this study, a range of d-amphetamine doses (0, 16, 24, and 32 mg/70 kg) were administered as an intranasal solution delivered using a mucosal atomization device. Equal oral doses were included for comparison. Assessments were conducted before and at regular intervals for 3 hours following drug administration and included self-reported drug-effect questionnaires, cardiovascular indices, a performance task, and 2 measures of impulsivity. d-Amphetamine produced prototypical stimulant effects (eg, increased subject ratings of Stimulated and Like Drug, elevated heart rate and blood pressure, and improved rate and accuracy on the digit symbol substitution task) irrespective of dose, but the onset of these effects was generally earlier following intranasal administration, with significant effects emerging 15 to 30 minutes after intranasal dosing and 45 to 60 minutes after oral dosing. These results demonstrate that intranasal administration of d-amphetamine results in a more rapid onset compared to oral dosing, which could be associated with the popularity of intranasal prescription stimulant use and an enhanced potential for abuse.

Behavioral effects of d-amphetamine in humans: influence of subclinical levels of inattention and hyperactivity

OBJECTIVE

Several studies suggest a link between stimulant abuse and attention-deficit hyperactivity disorder (ADHD) symptoms (e.g., inattention and hyperactivity). To further assess the nature of this relationship, the present study examined the association between subclinical symptoms of inattention and hyperactivity and the behavioral effects of d-amphetamine.

METHODS

Participants were classified into a High- (n = 8) or Low-Score (n = 9) group based on their responses on a rating scale that assessed inattention and hyperactivity symptoms.

RESULTS

The participants did not differ across the High-Score and Low-Score groups in their ability to discriminate d-amphetamine. The participants in the High-Score group were significantly more sensitive to the positive participant-rated effects of d-amphetamine (e.g., Good Effects, Like Drug), but less sensitive to drug-induced increases in blood pressure and heart rate.

CONCLUSION

The selective increase in positive subjective effects of d-amphetamine suggests that individuals with subclinical inattention and hyperactivity symptoms may have increased vulnerability to stimulant abuse.

Substitution profile of Delta9-tetrahydrocannabinol, triazolam, hydromorphone, and methylphenidate in humans discriminating Delta9-tetrahydrocannabinol

RATIONALE

Preclinical evidence suggests that non-cannabinoid neurotransmitter systems are involved in the behavioral and physiological effects of cannabinoids, but relatively little research has been conducted in humans.

OBJECTIVES

The aims of this study were to assess whether oral Delta(9)-tetrahydrocannabinol (Delta(9)-THC) would function as a discriminative stimulus in humans and to examine the substitution profile of drugs acting at opioid, GABA, and dopamine systems.

METHODS

Healthy subjects who reported moderate cannabis use were enrolled. Subjects learned to identify when they received oral 25 mg Delta(9)-THC or placebo under double-blind conditions. Once subjects acquired the discrimination (i.e., > or =80% drug-appropriate responding for four consecutive sessions), multiple doses of Delta(9)-THC, the GABA(A) positive modulator triazolam, the micro-opioid agonist hydromorphone and the dopamine reuptake inhibitor methylphenidate were tested to determine if they shared discriminative-stimulus effects with the training dose of Delta(9)-THC.

RESULTS

Eight subjects (N = 8) accurately discriminated Delta(9)-THC and completed the study. The training dose of Delta(9)-THC functioned as a discriminative stimulus and produced prototypical subject-rated drug effects. All of the drugs tested produced significant effects on the self-report questionnaires, but only Delta(9)-THC substituted for the training dose.

CONCLUSION

These results suggest that the discriminative-stimulus effects of Delta(9)-THC in humans are not directly mediated through central neurotransmitter systems acted upon by the drugs tested in this study.

Potential adverse effects of amphetamine treatment on brain and behavior: a review

Amphetamine stimulants have been used medically since early in the twentieth century, but they have a high abuse potential and can be neurotoxic. Although they have long been used effectively to treat attention deficit hyperactivity disorder (ADHD) in children and adolescents, amphetamines are now being prescribed increasingly as maintenance therapy for ADHD and narcolepsy in adults, considerably extending the period of potential exposure. Effects of prolonged stimulant treatment have not been fully explored, and understanding such effects is a research priority. Because the pharmacokinetics of amphetamines differ between children and adults, reevaluation of the potential for adverse effects of chronic treatment of adults is essential. Despite information on the effects of stimulants in laboratory animals, profound species differences in susceptibility to stimulant-induced neurotoxicity underscore the need for systematic studies of prolonged human exposure. Early amphetamine treatment has been linked to slowing in height and weight growth in some children. Because the number of prescriptions for amphetamines has increased several fold over the past decade, an amphetamine-containing formulation is the most commonly prescribed stimulant in North America, and it is noteworthy that amphetamines are also the most abused prescription medications. Although early treatment does not increase risk for substance abuse, few studies have tracked the compliance and usage profiles of individuals who began amphetamine treatment as adults. Overall, there is concern about risk for slowed growth in young patients who are dosed continuously, and for substance abuse in patients first medicated in late adolescence or adulthood. Although most adult patients also use amphetamines effectively and safely, occasional case reports indicate that prescription use can produce marked psychological adverse events, including stimulant-induced psychosis. Assessments of central toxicity and adverse psychological effects during late adulthood and senescence of adults who receive prolonged courses of amphetamine treatment are warranted. Finally, identification of the biological factors that confer risk and those that offer protection is also needed to better specify the parameters of safe, long-term, therapeutic administration of amphetamines to adults.

Discriminative stimulus and subject-rated effects of methamphetamine, d-amphetamine, methylphenidate, and triazolam in methamphetamine-trained humans

Methamphetamine abuse is a significant public health concern. Although widely studied in laboratory animals, little is known about the abuse-related behavioral effects of methamphetamine relative to other abused stimulants in controlled laboratory settings in humans. The aim of this study was to examine the discriminative stimulus, subject-rated, performance, and cardiovascular effects of methamphetamine in humans. In the present study, subjects first learned to discriminate 10 mg of oral methamphetamine from placebo. After acquiring the discrimination (> or = 80% drug-appropriate responding on four consecutive sessions), a range of oral doses of methamphetamine (2.5-15 mg), d-amphetamine (2.5-15 mg), methylphenidate (5-30 mg), and triazolam (0.0625-0.375 mg) was tested. Methamphetamine functioned as a discriminative stimulus and produced prototypical stimulant-like subject-rated effects. d-Amphetamine and methylphenidate produced dose-related increases in methamphetamine-appropriate responding, whereas triazolam did not. d-Amphetamine and methylphenidate produced stimulant-like behavioral effects, whereas triazolam produced sedative-like effects. Methamphetamine, but no other drug, increased heart rate, systolic pressure, and diastolic pressure significantly above placebo levels. Performance in the Digit-Symbol Substitution Test was not affected by any of the drugs tested. Overall, these results demonstrate that the acute behavioral effects of methamphetamine, d-amphetamine, and methylphenidate overlap extensively in humans, which is concordant with findings from preclinical studies. Future studies should assess whether the similarity in the behavioral effects of methamphetamine and related stimulants can be extended to other behavioral assays, such as measures of reinforcement, in humans.

Abuse liability assessment of atomoxetine in a drug-abusing population

BACKGROUND

Atomoxetine is a non-amphetamine medication approved to treat ADHD in children, adolescents, and adults. Previous studies demonstrated low abuse potential for atomoxetine in recreational drug users. This study assessed the abuse potential of atomoxetine in stimulant-preferring drug abusers compared to methylphenidate and phentermine as positive controls and desipramine and placebo as negative controls.

METHODS

Forty male and female, 32-53 years old stimulant-preferring drug abusers completed this balanced Latin-square designed study. Subjects received acute, double-blind doses of placebo, desipramine (100 and 200 mg), methylphenidate (90 mg), phentermine (60 mg), and atomoxetine (45, 90, and 180 mg). Subjective and physiological effects were collected for 24 h following each drug treatment.

RESULTS

Methylphenidate and phentermine were liked significantly more than placebo, atomoxetine, or desipramine. No atomoxetine dose was liked significantly more than placebo and liking scores for atomoxetine were similar to, or significantly lower than, desipramine, as assessed by the Drug Rating Questionnaire-Subject. While atomoxetine 45 and 180 mg did not significantly change any Addiction Research Center Inventory (ARCI) scores, atomoxetine 90 mg significantly increased A and BG stimulant scores of the ARCI and both methylphenidate and phentermine produced greater A and BG increases than any atomoxetine dose and also increased MBG (euphoria) scores relative to placebo.

CONCLUSIONS

Atomoxetine has significantly less abuse liability than methylphenidate or phentermine and no greater abuse liability than desipramine.

Modulation of the discriminative stimulus effects of triazolam across the menstrual cycle phase in healthy pre-menopausal women

Pre-clinical studies indicate that changes in progesterone levels across menstrual cycle phases modulate the behavioral effects of sedative drugs acting at GABA(A) receptor sites. In this study, seven healthy women learned to discriminate triazolam (0.25 mg/70 kg) from placebo. After acquiring the discrimination, a range of triazolam doses (0.00, 0.06, 0.12 and 0.25 mg/70 kg) was tested during the early follicular and mid-luteal menstrual cycle phases. During the mid-luteal phase, when progesterone levels were elevated, 0.12 mg/70 kg triazolam was identified as the active triazolam training dose (0.25 mg/70 kg), whereas 0.12 mg/70 kg triazolam was identified as placebo during the early follicular phase, when progesterone levels were low. Triazolam engendered prototypical sedative effects on subjective effect, performance and cardiovascular measures that were generally independent of cycle phase. These results suggest that the discriminative stimulus effects of the positive GABA(A) modulator, triazolam, are sensitive to menstrual cycle phase in healthy adult women.

A pharmacological analysis of stimulant-induced increases in smoking

RATIONALE

Stimulants increase tobacco smoking in healthy adults under controlled laboratory conditions. The mechanisms that mediate stimulant-induced increases in smoking are not known.

OBJECTIVE

The purpose of the present experiment was to characterize the pharmacological specificity of stimulant-induced increases in smoking. We tested the effects of methylphenidate and atomoxetine on smoking behavior. Atomoxetine is a norepinephrine transport inhibitor that does not increase dopamine levels in the nucleus accumbens or striatum. If stimulant-induced increases in smoking result from an additive or synergistic effect of these drugs and nicotine on dopamine levels in the nucleus accumbens or striatum, methylphenidate but not atomoxetine should increase smoking.

MATERIALS AND METHODS

Doses of methylphenidate (10, 20, and 40 mg) and atomoxetine (20, 40, and 80 mg) were tested once while placebo was tested twice in 12 cigarette smokers. One hour after ingesting drug, participants smoked ad libitum for 4 h. Measures of smoking included total cigarettes, total puffs, and carbon monoxide levels. Snacks and decaffeinated drinks were available ad libitum, and food intake was calculated.

RESULTS

Methylphenidate but not atomoxetine dose-dependently increased the number of cigarettes, puffs, and carbon monoxide levels. Methylphenidate and atomoxetine decreased food intake.

CONCLUSIONS

The results of this experiment are consistent with the notion that stimulant-induced increases in smoking may result from an additive or synergistic effect of these drugs and nicotine on dopamine levels in the nucleus accumbens or striatum. Additional research is needed to more fully understand the pharmacological mechanisms that mediate the relationship between stimulant use and smoking.

Discriminative-stimulus, self-reported, performance, and cardiovascular effects of atomoxetine in methylphenidate-trained humans

Atomoxetine is marketed as a nonstimulant medication indicated for the treatment of attention-deficit/hyperactivity disorder in adults. Previous laboratory research suggests that atomoxetine has limited abuse potential but that some of its behavioral effects might overlap with traditional psychomotor stimulants like methylphenidate and d-amphetamine. A drug with this profile might be useful for the treatment of stimulant dependence. The aim of this experiment was to compare the discriminative-stimulus and self-reported effects of atomoxetine with methylphenidate, damphetamine, and triazolam in humans who had acquired a methylphenidate (30 mg) discrimination. Six healthy subjects with a recent history of nontherapeutic stimulant use were enrolled in this outpatient study. After subjects acquired the methylphenidate discrimination, a range of doses of methylphenidate (5-30 mg), atomoxetine (15-90 mg), d-amphetamine (2.5-15 mg), triazolam (0.06-0.375 mg), and placebo were tested. To more fully characterize the behavioral effects of atomoxetine, a battery of self-reported drug-effect questionnaires, a performance task, and cardiovascular assessments were also included. Methylphenidate and d-amphetamine increased drug-appropriate responding and produced typical stimulant-like effects (e.g., increased ratings of "Active, Alert, Energetic"). Atomoxetine partially substituted for methylphenidate (i.e., 33%-50%) and produced some dose-dependent, stimulant-like, subject-rated drug effects, although the magnitude of these effects was less than d-amphetamine and methylphenidate and generally did not attain statistical significance. These data suggest that the behavioral effects of atomoxetine overlap to a small degree with psychomotor stimulants and that it has low abuse potential.

Methylphenidate increases cigarette smoking

RATIONALE

Methylphenidate (Ritalin) and d-amphetamine (Dexedrine), stimulants commonly prescribed for behavioral problems associated with attention deficit hyperactivity disorder (ADHD), produce a similar constellation of behavioral effects. The results of previous studies suggest that d-amphetamine increases rates of smoking and the reinforcing effects of smoking. The effects of methylphenidate on smoking have not been assessed although it is the most commonly prescribed pharmacotherapy for ADHD and individuals with ADHD are at increased risk for smoking.

OBJECTIVE

In this experiment the acute effects of a range of doses of methylphenidate (5, 10, 20, and 40 mg) and placebo were assessed in ten cigarette smokers who were not attempting to quit and were without ADHD or other Axis I psychiatric disorders.

METHODS

Each dose of methylphenidate was tested once, whereas placebo was tested twice. One hour after ingesting drug, participants were allowed to smoke ad libitum for 4 h. Measures of smoking included total cigarettes smoked, total puffs, latency to the first cigarette, and carbon monoxide levels. Snacks and decaffeinated drinks were available ad libitum, and caloric intake during the 4-h smoking session was calculated.

RESULTS

Methylphenidate dose dependently increased the total number of cigarettes smoked, number of puffs, and carbon monoxide levels. As expected, methylphenidate dose dependently decreased the number of food items consumed and caloric intake.

CONCLUSIONS

The results of this experiment suggest that methylphenidate, like d-amphetamine, increases rates of cigarette smoking.

Oxazepam does not modulate the behavioral effects of d-amphetamine in humans

Benzodiazepines, which are gamma-aminobutyric acid-A (GABA(A)) receptor positive modulators, can block the behavioral effects of psychomotor stimulants. In the present study, the ability of oxazepam, which may have less abuse potential compared to some other benzodiazepines, to attenuate the discriminative-stimulus, subject-rated and psychomotor performance effects of d-amphetamine in humans was determined. Six healthy participants (2 female, 4 male) learned to discriminate 15 mg oral d-amphetamine. After acquiring the discrimination (i.e., > or = 80% correct responding on 4 consecutive days), the effects of d-amphetamine (0, 2.5, 5, 10 and 15 mg), alone and in combination with acutely administered oxazepam (0 and 20 mg) were assessed. d-Amphetamine alone functioned as a discriminative stimulus, produced stimulant-like subject-rated effects (e.g., increased ratings of Stimulated on a Drug-Effect Questionnaire) and enhanced psychomotor performance. Oxazepam alone increased subject ratings of sedation (e.g., increased ratings of Sluggish, Fatigued and Lazy on a Drug-Effect Questionnaire) and impaired psychomotor performance. Oxazepam alone did not occasion d-amphetamine-like discriminative-stimulus effects, and had no effect on the discriminative-stimulus or subject-rated effects of d-amphetamine when given in combination. The results of this experiment are discordant with previous research and suggest that benzodiazepines differ in their ability to modulate the behavioral effects of d-amphetamine.