Stimulus properties of 1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDA) analogs

Experimental strategies to discover and develop the next generation of psychedelics and entactogens as medicines

Research on classical psychedelics (psilocybin, LSD and DMT) and entactogen, MDMA, has produced a renaissance in the search for more effective drugs to treat psychiatric, neurological and various peripheral disorders. Psychedelics and entactogens act though interaction with 5-HT_2A and other serotonergic receptors and/or monoamine reuptake transporters. 5-HT, which serves as a neurotransmitter and hormone, is ubiquitously distributed in the brain and peripheral organs, tissues and cells where it has vasoconstrictor, pro-inflammatory and pro-nociceptive actions. Serotonergic psychedelics and entactogens have known safety and toxicity risks. For these drugs, the risks been extensively researched and empirically assessed through human experience. However, novel drug-candidates require thorough non-clinical testing not only to predict clinical efficacy, but also to address the risks they pose during clinical development and later after approval as prescription medicines. We have defined the challenges researchers will encounter when developing novel serotonergic psychedelics and entactogens. We describe screening techniques to predict clinical efficacy and address the safety/toxicity risks emerging from our knowledge of the existing drugs: 1) An early-stage, non-clinical screening cascade to pharmacologically characterise novel drug-candidates. 2) Models to detect hallucinogenic activity. 3) Models to differentiate hallucinogens from entactogens. 4) Non-clinical preclinical lead optimisation technology (PLOT) screening to select drug-candidates. 5) Modified animal models to evaluate the abuse and dependence risks of novel psychedelics in Safety Pharmacology testing. Our intention has been to design non-clinical screening strategies that will reset the balance between benefits and harms to deliver more effective and safer novel psychedelics for clinical use. This article is part of the Special Issue on 'National Institutes of Health Psilocybin Research Speaker Series'.

Acute effects of 3,4-methylenedioxymethamphetamine (MDMA) and R(-) MDMA on actigraphy-based daytime activity and sleep parameters in rhesus monkeys

3,4-Methylenedioxymethamphetamine (MDMA) affects monoaminergic pathways that play a critical role in sleep-wake cycles. Dopaminergic mechanisms are thought to mediate the sleep-disrupting effects of stimulant drugs. However, the mechanisms underlying the effects of MDMA on sleep-wake cycles and the effects of R(-) MDMA, a stereoisomer that lacks dopaminergic activity, on sleep remain unknown. The aim of the present study was to investigate the effects of racemic MDMA and R(-) MDMA on daytime activity and sleep-like parameters evaluated with actigraphy in adult rhesus macaques (Macaca mulatta, n = 6). Actiwatch monitors were attached to the monkeys' collars and actigraphy recording was conducted during baseline conditions and after the administration of acute intramuscular injections of saline (vehicle), racemic MDMA (0.3, 1.0, or 1.7 mg/kg), or R(-) MDMA (0.3, 1.0, or 1.7 mg/kg) at 9 or 16 h (3 h before "lights off"). Morning treatments had no effects on sleep-like parameters. Racemic MDMA decreased general daytime activity during the first hour after injection and increased daytime activity at 3 hr posttreatment. Although afternoon administration of racemic MDMA increased sleep latency, it improved other sleep parameters, decreasing wake time after sleep onset (WASO) and increasing sleep efficiency for subjects with low baseline sleep efficiency. Afternoon treatment with R(-) MDMA improved sleep measures, increasing sleep efficiency and decreasing sleep latency and WASO, while having no effects on daytime activity. The stimulant and sleep-disrupting effects of racemic MDMA are likely mediated by dopaminergic and noradrenergic mechanisms, while serotonergic pathways appear to be involved in the sleep-promoting effects of MDMA. (PsycINFO Database Record

Pharmacology of MDMA- and Amphetamine-Like New Psychoactive Substances

New psychoactive substances (NPS) with amphetamine-, aminoindan-, and benzofuran basic chemical structures have recently emerged for recreational drug use. Detailed information about their psychotropic effects and health risks is often limited. At the same time, it emerged that the pharmacological profiles of these NPS resemble those of amphetamine or 3,4-methylenedioxymethamphetamine (MDMA). Amphetamine-like NPS induce psychostimulation and euphoria mediated predominantly by norepinephrine (NE) and dopamine (DA) transporter (NET and DAT) inhibition and transporter-mediated release of NE and DA, thus showing a more catecholamine-selective profile. MDMA-like NPS frequently induce well-being, empathy, and prosocial effects and have only moderate psychostimulant properties. These MDMA-like substances primarily act by inhibiting the serotonin (5-HT) transporter (SERT) and NET, also inducing 5-HT and NE release. Monoamine receptor interactions vary considerably among amphetamine- and MDMA-like NPS. Clinically, amphetamine- and MDMA-like NPS can induce sympathomimetic toxicity. The aim of this chapter is to review the state of knowledge regarding these substances with a focus on the description of the in vitro pharmacology of selected amphetamine- and MDMA-like NPS. In addition, it is aimed to provide links between pharmacological profiles and in vivo effects and toxicity, which leads to the conclusion that abuse liability for amphetamine-like NPS may be higher than for MDMA-like NPS, but that the risk for developing the life-threatening serotonin syndrome may be increased for MDMA-like NPS.

Hallucinogens in Drug Discrimination

Hallucinogens comprise a diverse collection of chemicals with multifarious receptor actions in the central nervous system. Preclinical drug screening methods have proven invaluable in the evaluation and characterization of hallucinogen psychopharmacology. Used in concert with structural chemistry and receptor pharmacology methods, preclinical drug discrimination research has informed our current understanding of hallucinogens and the neurochemical receptor mechanisms responsible for their interoceptive stimulus effects. This chapter summarizes the strengths and limitations of drug discrimination as an in vivo drug detection method and offers a brief review of historical and contemporary drug discrimination research with classical hallucinogens.

Interspecies In Vitro Evaluation of Stereoselective Protein Binding for 3,4-Methylenedioxymethamphetamine

Abuse of 3,4-methylenedioxymethamphetamine (MDMA) is becoming more common worldwide. To date, there is no information available on stereoselectivity of MDMA protein binding in humans, rats, and mice. Since stereoselectivity plays an important role in MDMA’s pharmacokinetics and pharmacodynamics, in this study we investigated its stereoselectivity in protein binding. The stereoselective protein binding ofrac-MDMA was investigated using two different concentrations (20 and 200 ng/mL) in human plasma and mouse and rat sera using an ultrafiltration technique. No significant stereoselectivity in protein binding was observed in both human plasma and rat serum; however, a significant stereoselective binding (p<0.05) was observed in mouse serum. Since the protein binding of MDMA in mouse serum is considerably lower than in humans and rats, caution should be exercised when using mice for in vitro studies involving MDMA.

Impact of Cytochrome P450 2D6 Function on the Chiral Blood Plasma Pharmacokinetics of 3,4-Methylenedioxymethamphetamine (MDMA) and Its Phase I and II Metabolites in Humans

3,4-methylenedioxymethamphetamine (MDMA; ecstasy) metabolism is known to be stereoselective, with preference for S-stereoisomers. Its major metabolic step involves CYP2D6-catalyzed demethylenation to 3,4-dihydroxymethamphetamine (DHMA), followed by methylation and conjugation. Alterations in CYP2D6 genotype and/or phenotype have been associated with higher toxicity. Therefore, the impact of CYP2D6 function on the plasma pharmacokinetics of MDMA and its phase I and II metabolites was tested by comparing extensive metabolizers (EMs), intermediate metabolizers (IMs), and EMs that were pretreated with bupropion as a metabolic inhibitor in a controlled MDMA administration study. Blood plasma samples were collected from 16 healthy participants (13 EMs and three IMs) up to 24 h after MDMA administration in a double-blind, placebo-controlled, four-period, cross-over design, with subjects receiving 1 week placebo or bupropion pretreatment followed by a single placebo or MDMA (125 mg) dose. Bupropion pretreatment increased the maximum plasma concentration (C_max) and area under the plasma concentration-time curve from 0 to 24 h (AUC₂₄) of R-MDMA (9% and 25%, respectively) and S-MDMA (16% and 38%, respectively). Bupropion reduced the C_max and AUC₂₄ of the CYP2D6-dependently formed metabolite stereoisomers of DHMA 3-sulfate, DHMA 4-sulfate, and 4-hydroxy-3-methoxymethamphetamine (HMMA sulfate and HMMA glucuronide) by approximately 40%. The changes that were observed in IMs were generally comparable to bupropion-pretreated EMs. Although changes in stereoselectivity based on CYP2D6 activity were observed, these likely have low clinical relevance. Bupropion and hydroxybupropion stereoisomer pharmacokinetics were unaltered by MDMA co-administration. The present data might aid further interpretations of toxicity based on CYP2D6-dependent MDMA metabolism.

Differences in the locomotor-activating effects of indirect serotonin agonists in habituated and non-habituated rats

The indirect serotonin (5-HT) agonist 3,4-methylenedioxymethamphetamine (MDMA) produces a distinct behavioral profile in rats consisting of locomotor hyperactivity, thigmotaxis, and decreased exploration. The indirect 5-HT agonist α-ethyltryptamine (AET) produces a similar behavioral profile. Using the Behavioral Pattern Monitor (BPM), the present investigation examined whether the effects of MDMA and AET are dependent on the novelty of the testing environment. These experiments were conducted in Sprague-Dawley rats housed on a reversed light cycle and tested during the dark phase of the light/dark cycle. We found that racemic MDMA (RS-MDMA; 3 mg/kg, SC) increased locomotor activity in rats tested in novel BPM chambers, but had no effect on locomotor activity in rats habituated to the BPM chambers immediately prior to testing. Likewise, AET (5 mg/kg, SC) increased locomotor activity in non-habituated animals but not in animals habituated to the test chambers. These results were unexpected because previous reports indicate that MDMA has robust locomotor-activating effects in habituated animals. To further examine the influence of habituation on MDMA-induced locomotor activity, we conducted parametric studies with S-(+)-MDMA (the more active enantiomer) in habituated and non-habituated rats housed on a standard or reversed light cycle. Light cycle was included as a variable due to reported differences in sensitivity to serotonergic ligands during the dark and light phases. In confirmation of our initial studies, rats tested during the dark phase and habituated to the BPM did not show an S-(+)-MDMA (3 mg/kg, SC)-induced increase in locomotor activity, whereas non-habituated rats did. By contrast, in rats tested during the light phase, S-(+)-MDMA increased locomotor activity in both non-habituated and habituated rats, although the response in habituated animals was attenuated. The finding that habituation and light cycle interact to influence MDMA- and AET-induced hyperactivity demonstrates that there are previously unrecognized complexities associated with the behavioral effects of these drugs.

The neuropharmacology of prolactin secretion elicited by 3,4-methylenedioxymethamphetamine ("ecstasy"): a concurrent microdialysis and plasma analysis study

3,4-methylenedioxymethamphetamine (MDMA) is a substituted phenethylamine that is widely abused as the street drug "ecstasy". Racemic MDMA (S,R(+/-)-MDMA) and its stereoisomers elicit complex spectrums of psychobiological, neurochemical, and hormonal effects. In this regard, recent findings demonstrated that S,R(+/-)-MDMA and its stereoisomer R(-)-MDMA elicit increases in striatal extracellular serotonin levels and plasma levels of the hormone prolactin in rhesus monkeys. In the present mechanistic study, we evaluated the role of the serotonin transporter and the 5-HT(2A) receptor in S,R(+/-)-MDMA- and R(-)-MDMA-elicited prolactin secretion in rhesus monkeys through concurrent microdialysis and plasma analysis determinations and drug interaction experiments. Concurrent neurochemical and hormone determinations showed a strong positive temporal correlation between serotonin release and prolactin secretion. Consistent with their distinct mechanisms of action and previous studies showing that the serotonin transporter inhibitor fluoxetine attenuates the behavioral and neurochemical effects of S,R(+/-)-MDMA, pretreatment with fluoxetine attenuated serotonin release elicited by either S,R(+/-)-MDMA or R(-)-MDMA. As hypothesized, at a dose that had no significant effects on circulating prolactin levels when administered alone, fluoxetine also attenuated prolactin secretion elicited by S,R(+/-)-MDMA. In contrast, combined pretreatment with both fluoxetine and the selective 5-HT(2A) receptor antagonist M100907 was required to attenuate prolactin secretion elicited by R(-)-MDMA, suggesting that this stereoisomer of S,R(+/-)-MDMA elicits prolactin secretion through both serotonin release and direct agonism of 5-HT(2A) receptors. Accordingly, these findings inform our understanding of the neuropharmacology of both S,R(+/-)-MDMA and R(-)-MDMA and the regulation of prolactin secretion.

Endocrine and neurochemical effects of 3,4-methylenedioxymethamphetamine and its stereoisomers in rhesus monkeys

3,4-Methylenedioxymethamphetamine (MDMA) is an amphetamine derivative that elicits complex biological effects in humans. One plausible mechanism for this phenomenon is that racemic MDMA is composed of two stereoisomers that exhibit qualitatively different pharmacological effects. In support of this, studies have shown that R(-)-MDMA tends to have hallucinogen-like effects, whereas S(+)-MDMA tends to have psychomotor stimulant-like effects. However, relatively little is known about whether these stereoisomers engender different endocrine and neurochemical effects. In the present study, the endocrine and neurochemical effects of each stereoisomer and the racemate were assessed in four rhesus monkeys after intravenous delivery at doses (1-3 mg/kg) that approximated voluntary self-administration by rhesus monkeys and human recreational users. Specifically, fluorescence-based enzyme-linked immunosorbent assay was used to assess plasma prolactin concentrations, and in vivo microdialysis was used to assess extracellular dopamine and serotonin concentrations in the dorsal striatum. R(-)-MDMA, but not S(+)-MDMA, significantly increased plasma prolactin levels and the effects of S,R(+/-)-MDMA were intermediate to each of its component stereoisomers. Although S(+)-MDMA did not alter prolactin levels, it did significantly increase extracellular serotonin concentrations. In addition, S(+)-MDMA, but not R(-)-MDMA, significantly increased dopamine concentrations. Furthermore, as in the prolactin experiment, the effects of the racemate were intermediate to each of the stereoisomers. These studies demonstrate the stereoisomers of MDMA engender qualitatively different endocrine and neurochemical effects, strengthening the inference that differences in these stereoisomers might be the mechanism producing the complex biological effects of the racemic mixture of MDMA in humans.

Reinstatement of extinguished amphetamine self-administration by 3,4-methylenedioxymethamphetamine (MDMA) and its enantiomers in rhesus monkeys

RATIONALE

The effectiveness of MDMA and its enantiomers to reinstate responding previously maintained by drug self-administration has not been thoroughly investigated.

OBJECTIVES

The present study was designed to compare the reinstatement effects of amphetamine, the piperazine-analog BZP, SR(+/-)-MDMA, S(+)-MDMA, R(-)-MDMA, and fenfluramine on behavior maintained under a second-order schedule of intravenous amphetamine self-administration in rhesus monkeys (n=4).

METHODS

Following saline substitution and extinction, a range of doses of amphetamine, BZP, SR(+/-)-MDMA, S(+)-MDMA, R(-)-MDMA, and fenfluramine were administered i.v. as non-contingent priming injections in order to characterize their effectiveness to reinstate responding previously maintained by amphetamine self-administration.

RESULTS

Priming injections of amphetamine, BZP, SR(+/-)-MDMA, and S(+)-MDMA induced significant reinstatement effects. In contrast, neither R(-)-MDMA nor fenfluramine effectively reinstated behavior. Pretreatment with the selective serotonin transporter inhibitor, fluoxetine, attenuated the reinstatement effects of SR(+/-)-MDMA, S(+)-MDMA, and BZP but had no significant effect on amphetamine-primed reinstatement.

CONCLUSIONS

Given the profile of neurochemical effects published previously, these findings suggest that the reinstatement effects of MDMA are mediated primarily by dopamine release; however, the attenuation of MDMA-induced reinstatement by fluoxetine supports previous research demonstrating the complex behavioral pharmacology of MDMA-like drugs and that the reinstatement effects of MDMA are at least partially mediated by serotonergic mechanisms.

Discriminative stimulus effects of psychostimulants and hallucinogens in S(+)-3,4-methylenedioxymethamphetamine (MDMA) and R(-)-MDMA trained mice

3,4-Methylenedioxymethamphetamine (MDMA) is a substituted phenethylamine more commonly known as the drug of abuse "ecstasy." The acute and persistent neurochemical effects of MDMA in the mice are distinct from those in other species. MDMA shares biological effects with both amphetamine-type stimulants and mescaline-type hallucinogens, which may be attributable to distinct effects of its two enantiomers, both of which are active in vivo. In this regard, among the substituted phenethylamines, R(-)-enantiomers tend to have hallucinogen-like effects, whereas S(+)-enantiomers tend to have stimulant-like effects. In the present study, mice were trained to discriminate S(+)- or R(-)-MDMA from vehicle. Drug substitution tests were then undertaken with the structurally similar phenethylamine dopamine/norepinephrine releaser S(+)-amphetamine, the structurally dissimilar tropane nonselective monoamine reuptake inhibitor cocaine, the structurally similar phenethylamine 5-hydroxytryptamine (5-HT)(2A) agonist 2,5-dimethoxy-4-(n)-propylthiophenethylamine (2C-T-7), and the structurally dissimilar mixed action tryptamine 5-HT(2A) agonist/monoamine reuptake inhibitor N,N-dipropyltryptamine (DPT). S(+)-amphetamine fully substituted in the S(+)-MDMA-treated animals but did not substitute for the R(-)-MDMA cue. 2C-T-7 fully substituted in the R(-)-MDMA-trained animals but did not substitute for the S(+)-MDMA cue. Cocaine and DPT substituted for both training drugs, but whereas cocaine was more potent in S(+)-MDMA-trained mice, DPT was more potent in R(-)-MDMA-trained mice. These data suggest that qualitative differences in the discriminative stimulus effects of each stereoisomer of MDMA exist in mice and further our understanding of the complex nature of the interoceptive effects of MDMA.

Pharmacological aspects of the combined use of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and gamma-hydroxybutyric acid (GHB): a review of the literature

Epidemiological studies show that the use of club drugs is on the rise. Furthermore, the last few decades have seen a rise in patterns of polydrug use. One of the combinations frequently used is ecstasy (MDMA) with gammahydroxybutyrate (GHB). For effective prevention it is important to be aware of this phenomenon and of the pharmacology of these drugs. The effects of the combination extend to different neurotransmitter systems, including serotonin, dopamine and noradrenaline. Studies investigating the effects of combinations of psychoactive substances are limited. In this review we describe the subjective effects of the MDMA/GHB combination. Furthermore, we review the individual actions of MDMA on serotonin, dopamine and noradrenaline systems. In addition, actions of GHB on these systems are discussed as a possible pharmacological basis for the interaction of both drugs. It is postulated that GHB attenuates the unpleasant or dysphoric effects of MDMA by its effect on the central dopaminergic system.

MDMA (N-methyl-3,4-methylenedioxyamphetamine) and its stereoisomers: Similarities and differences in behavioral effects in an automated activity apparatus in mice

Racemic MDMA (0.3-30 mg/kg), S(+)-MDMA (0.3-30 mg/kg), R(-)-MDMA (0.3-50 mg/kg) and saline vehicle (10 ml/kg) were comprehensively evaluated in fully automated and computer-integrated activity chambers, which were designed for mice, and provided a detailed analysis of the frequency, location, and/or duration of 18 different activities. The results indicated that MDMA and its isomers produced stimulation of motor actions, with S(+)-MDMA and (+/-)-MDMA usually being more potent than R(-)-MDMA in measures such as movement (time, distance, velocity), margin distance, rotation (clockwise and counterclockwise), and retraced activities. Interestingly, racemic MDMA appeared to exert a greater than expected potency and/or an enhanced effect on measures such as movement episodes, center actions (entries and distance), clockwise rotations, and jumps; actions that might be explained by additive or synergistic (i.e. potentiation) effects of the stereoisomers. In other measures, the enantiomers displayed different effects: S(+)-MDMA produced a preference to induce counterclockwise (versus clockwise) rotations, and each isomer exerted a different profile of effect on vertical activities and jumps. Furthermore, each isomer of MDMA appeared to attenuate the effect of its opposite enantiomer on some behaviors; antagonism effects that were surmised from a lack of expected activities by racemic MDMA. S(+)-MDMA (but not R(-)-MDMA), for example, produced an increase in vertical entries (rearing) and a preference to increase counterclockwise (versus clockwise) rotations; (+/-)-MDMA also should have induced such effects but did not. Apparently, R(-)-MDMA, when combined with S(+)-MDMA to form (+/-)-MDMA, prevented the appearance of those increases (from control) in activities. Similarly, R(-)-MDMA (but not S(+)-MDMA) produced increases in episodes (i.e. jumps) and vertical distance that racemic MDMA also should have, but were not, exhibited. Evidently, the presence of S(+)-MDMA in the racemic mixture inhibited the appearance of those increases (from control) in behavior. Taken together, the various and complex effects of MDMA and its stereoisomers are noted and a strategy is suggested for future studies that stresses the importance of steric effects and interplay, probable interaction(s) with various neurotransmitters, and interaction(s) with the particular behavioral or biological event (or action) being measured.

N-Methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA) and N-Methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) produce non-identical discriminative stimuli in rats

N-Methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (methylenedioxymethamphetamine, MDMA, Ecstasy) and its structurally abbreviated congener N-methyl-1-(4-methoxyphenyl)-2-aminopropane (para-methoxymethamphetamine, PMMA) are chemically related designer drugs, and PMMA is sometimes sold on the clandestine market as a substitute for MDMA. Prior drug discrimination studies have found that MDMA and PMMA substitute for one another suggesting that they produce similar discriminative stimulus effects in rats. However, there also are some indications that the two agents produce distinct stimulus effects. In this study, further comparisons were made between the stimulus effects of these two agents. Sprague-Dawley rats were trained to discriminate either 1.25 mg/kg of PMMA or 1.5 mg/kg of MDMA from saline vehicle in a two-lever operant paradigm. A structure-activity comparison revealed that MDMA and PMMA behave similarly upon homologation of their terminal amine substituents. In contrast, the PMMA stimulus, unlike an MDMA stimulus, failed to generalize completely to the psychostimulant cocaine, 8-hydroxy-2-(N,N-di-n-propylamino)tetralin (8-OH DPAT), and R(-)-1-(3-methoxyphenyl)-2-aminopropane [R(-)MMA]. In an additional group of animals, a (+)amphetamine stimulus partially generalized to R(-)MMA. Taken together, the results argue and re-emphasize the conclusion that the stimulus effects produced by MDMA and PMMA are similar, but non-identical, and that PMMA is the less "stimulant-like" of the two.

The effects of fluoxetine on the subjective and physiological effects of 3,4-methylenedioxymethamphetamine (MDMA) in humans

OBJECTIVE

The purpose of this study was to investigate the role of serotonin (5-HT) in the effects of oral 3,4-methylenedioxymethamphetamine (MDMA) in humans.

MATERIALS AND METHODS

The subjective and physiological effects of 1.5 mg/kg MDMA were evaluated after 20 mg fluoxetine in eight recreational MDMA users in a double-blind, placebo-controlled study. During phase 1, participants were maintained on placebo for at least 5 days and tested with MDMA and placebo on separate sessions. In phase 2, the procedure was the same except fluoxetine was administered daily for at least 5 days. During sessions, placebo or fluoxetine was given 1 h before the session drug and effects were measured over the next 7 h.

RESULTS

MDMA increased positive-like subjective effects on all the Addiction Research Center Inventory scales; Arousal, Elation, Positive Mood, and Vigor on the Profile of Mood States; Drug Liking, Friendly, Good Drug Effect, High, Stimulated, and Talkative on the Visual Analog Scale; and End-of-Session Liking and Crossover Point on the Multiple Choice Procedure. MDMA also increased measures of anxiety. On the Hallucinogenic Rating Scale, all scales except Volition were increased. MDMA also increased blood pressure and heart rate. Fluoxetine treatment attenuated most of the positive-like subjective effects including the Affect and Soma scales of the Hallucinogen Rating Scale. In addition, heart rate but not blood pressure increases were reduced.

CONCLUSIONS

These results suggest that blockade of 5-HT reuptake by fluoxetine can dampen the effects of MDMA and further supports the role of 5-HT in its behavioral effects in humans.

Reinforcing effects of methylenedioxy amphetamine congeners in rhesus monkeys: are intravenous self-administration experiments relevant to MDMA neurotoxicity?

RATIONALE

Many animal models relevant to the persistent effects of drugs of abuse necessitate the application of interspecies dose scaling procedures to approximate drug administration regimens in humans, but drug self-administration procedures differ in that they allow animal subjects to control their own drug intake.

OBJECTIVES

This report reviews the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA), its enantiomers, and several structural analogs in rhesus monkeys, paying particular attention to the pharmacological mechanisms of such reinforcing effects, the development of structure activity relationships among these compounds, the stability of MDMA self-administration behavior over time, and the persistent effects of self-administered MDMA on monoamines.

RESULTS

The methylenedioxy amphetamine congeners MDMA, 3,4-methylenedioxyamphetamine, N-ethyl-3,4-methylenedioxyamphetamine, and N-methyl-1-(3,4-methylenedioxyphenyl)-2-butanamine function as reinforcers in rhesus monkeys, maintaining self-administration behavior greater than that engendered by contingent saline but less than that engendered by traditional psychostimulants. These findings are remarkable as structurally distinct serotonergic hallucinogen-like drugs do not maintain reliable self-administration in laboratory animals. During prolonged MDMA self-administration, MDMA-maintained responding progressively weakens, and MDMA eventually fails to maintain significant self-administration. The neurochemical correlates of this effect have not yet been identified.

CONCLUSIONS

Procedures in which MDMA and related compounds are self-administered can be established in rhesus monkeys. These techniques can be used to engender contingent MDMA exposure without resorting to controversial methods of interspecies dose scaling. As such, further application of self-administration methods may provide important new insights into the persistent effects of MDMA on brain and behavior in nonhuman primates.

S(+)- and R(-)N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) as discriminative stimuli: effect of cocaine

Racemic N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (methylenedioxymethamphetamine, MDMA), a central stimulant and empathogenic agent, and cocaine are drugs of abuse that function as training drugs in drug discrimination studies. In tests of stimulus generalization (substitution), asymmetric generalization occurs between the two agents: a (+/-)MDMA stimulus generalized to cocaine, but a cocaine stimulus did not generalize to (+/-)MDMA. A possible explanation may be found, at least in part, in the stimulus effects of the optical isomers of MDMA. In the present study, groups of male Sprague-Dawley rats were trained to discriminate either S(+)MDMA (training dose=1.5 mg/kg, i.p.; n=10; ED50=0.6 mg/kg) or R(-)MDMA (training dose=1.75 mg/kg, i.p.; n=7; ED50=0.4 mg/kg) from saline vehicle using a VI-15s schedule of reinforcement. Tests of stimulus generalization with cocaine were conducted in each of the two groups. Cocaine only partially substituted for the S(+)MDMA stimulus (maximum=39% drug-appropriate responding), and various doses of cocaine did not enhance the percent drug-appropriate responding produced by a low dose (0.5 mg/kg) of S(+)MDMA. In contrast, the R(-)MDMA stimulus generalized completely to cocaine (ED50=1.3 mg/kg). Taken together with an earlier report that a (+/-)MDMA stimulus generalizes to cocaine, it would seem that the stimulus actions of cocaine might share greater similarity with R(-)MDMA than with S(+)MDMA.

3,4-Methylenedioxymethamphetamine counteracts akinesia enantioselectively in rat rotational behavior and catalepsy

We have shown recently that 3,4-methylenedioxymethamphetamine (MDMA) has symptomatic antiparkinsonian activity in rodent models of Parkinson's disease. In search of its mechanism of action, we further investigated the enantiomers of MDMA in the rotational behavioral model and catalepsy test. Catalepsy testing was done in drug-naive unlesioned animals. The parkinsonian symptoms rigor and akinesia (i.e., catalepsy) were induced by intraperitoneal administration of haloperidol 0.5 mg/kg and measured repeatedly as descent latency from a horizontal bar and a vertical grid. MDMA and both its enantiomers were effective in counteracting haloperidol-induced catalepsy, but if given as racemic, the effects were more pronounced than with the enantiomers. For testing of rotational behavior, male Sprague Dawley rats were lesioned unilaterally with 6-hydroxydopamine (6-OHDA) at the medial forebrain bundle. Administration of S-MDMA (5 mg/kg) produced ipsilateral rotations. R-MDMA was far less effective in inducing ipsilateral rotations in 6-OHDA unilaterally lesioned rats, but when S-MDMA was given additionally rotations immediately increased. Regarding their overall antiparkinsonian effects, the S-enantiomer of MDMA was more effective than its R-congener. R-MDMA was able to increase the actions of S-MDMA.

The Neuropsychopharmacology and Toxicology of 3,4-methylenedioxy-N-ethyl-amphetamine (MDEA)

This paper reviews the pharmacology and toxicology of 3,4-methylenedioxy-N-ethylamphetamine (MDEA, "eve"). MDEA is a ring-substituted amphetamine (RSA) like MDMA, its well known N-methyl analog. Both have become very popular substances of abuse in the techno- and house-music scene. They can evoke psychomotor stimulation, mild alterations of perception, sensations of closeness and a positive emotional state as well as sympathomimetic physical effects. At present, the name "ecstasy" is no longer used only for MDMA, but for the whole group of RSAs (MDA, MDMA, MDEA and MBDB) as they are chemically and pharmacologically nearly identical; moreover, many ecstasy pills contain mixtures of the RSAs. Hence, for a selective review on MDEA, it is crucial to strictly differentiate between: 1) street and chemical names, and 2) studies with or without chemically defined substances. In order to present MDEA-specific information, the pharmacodynamics and kinetics are described on the basis of MDEA challenge studies in animals and humans. In the toxicology section, we present a collection of case reports on fatalities where MDEA was toxicologically confirmed. On the question of serotonergic neurotoxicity and possible long-term consequences, however, MDEA-specific information is available from animal studies only. The neurotoxic potential of MDEA in humans is difficult to estimate, as ecstasy users do not consume pure substances. For future research, challenge studies in animals using dosing regimens adapted to human consumption patterns are needed. Such challenge studies should directly compare individual RSAs. They will represent the most viable and fruitful approach to the resolution of the highly controversial issues of serotonergic neurotoxicity and its functional consequences.

Comparison of the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine: asymmetric generalization

Evidence suggests that +/- 3,4-methylenedioxymethamphetamine (MDMA) and psychostimulants produce similar but non-identical stimulus effects in animals. To examine this hypothesis, groups of rats were trained to discriminate either MDMA (1.5 mg/kg) or cocaine (8 mg/kg) from saline vehicle using a two-lever operant procedure under a variable interval (VI) 15 s schedule of reinforcement. Once the animals were trained, tests of stimulus generalization were conducted with +/- MDMA, cocaine, S+ MDMA, and R- MDMA. As previously demonstrated, both S+ MDMA and R- MDMA (ED50 = 0.8 and 1.2 mg/kg, respectively) substituted for +/- MDMA. Stimulus generalization also occurred upon administration of cocaine (ED50 = 4.6 mg/kg) to the +/- MDMA-trained animals. In the cocaine-trained animals, however, stimulus generalization did not occur to +/- MDMA, S+ MDMA nor R- MDMA. Receptor binding profiles for MDMA and cocaine were compared in an effort to identify any novel and common receptor-based mechanism(s) to explain stimulus generalization of MDMA-trained animals to the effects of cocaine, but only their actions on neurotransmitter transporters seem applicable. Taken together, the results indicate that stimulus substitution between MDMA and cocaine is asymmetric and suggest that although similarities exist between the stimulus actions of MDMA and cocaine, differences might be explained by their differential effects on increasing synaptic concentrations of serotonin (5-HT), dopamine (DA), and/or norepinephrine (NE).

The pre-clinical behavioural pharmacology of 3,4-methylenedioxymethamphetamine (MDMA)

3,4-Methylenedioxymethamphetamine (MDMA) is a relatively novel drug of abuse and as such little is currently known of its behavioural pharmacology. This review aims to examine whether MDMA represents a novel class of abused drug. MDMA is known as a selective serotonergic neurotoxin in a variety of animal species but acutely it is a potent releaser and/or reuptake inhibitor of presynaptic serotonin, dopamine, noradrenaline, and acetylcholine. Interaction of these effects contributes to its behavioural pharmacology, in particular its effects on body temperature. Drug discrimination studies indicate that MDMA and related drugs produce unique interoceptive effects which have led to their classification as entactogens. This is supported by results from other behavioural paradigms although there is evidence for dose dependency of MDMA-specific effects. MDMA also produces conditioned place preference but is not a potent reinforcer in self-administration studies. These unique behavioural effects probably underlie its current popularity. MDMA is found in the street drug ecstasy but it may not be appropriate to equate the two as other drugs are routinely found in ecstasy tablets

Effect of PMA optical isomers and 4-MTA in PMMA-trained rats

1-(4-Methoxyphenyl)-2-aminopropane (PMA) and its sulfur analog, 1-(4-methylthiophenyl)-2-aminopropane (4-MTA), have been misrepresented as the controlled substance analog, N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA; "Ecstasy"). Because MDMA has been shown to produce both amphetamine-like and N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA)-like stimulus effects in rats, we examined S(+)PMA, R(-)PMA and 4-MTA in rats trained to discriminate either PMMA (1.25 mg/kg) or (+)amphetamine (1.0 mg/kg) from saline vehicle. The sulfur analog of PMMA (i.e., 4-MTMA) was also examined. The PMMA stimulus generalized to R(-)PMA (ED50=0.4 mg/kg), whereas S(+)PMA produced a maximum of 72% PMMA-appropriate responding. 4-MTA (ED50=0.3 mg/kg) also substituted for PMMA, but 4-MTMA produced a maximum of only 36% PMMA-appropriate responding. None of the four agents substituted for (+)amphetamine. Hence, like MDMA, R(-)PMA and 4-MTA are capable of producing PMMA stimulus effects in rats, but unlike MDMA, neither agent substituted for (+)amphetamine.

Further characterization of the stimulus properties of 5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline

This investigation is based on the premise that conformational restriction of abused phenylalkylamines in a tetrahydroisoquinoline conformation alters their pharmacology in such a manner that their original action is lost and that a new action emerges. TDIQ or 5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline, is a conformationally constrained phenylalkylamine that serves as a discriminative stimulus in animals. Although TDIQ bears structural resemblance to phenylalkylamine stimulants (e.g., amphetamine), hallucinogens (e.g., 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane [DOM]), and designer drugs (e.g., N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane [MDMA], N-methyl-1-(4-methoxyphenyl)-2-aminopropane [PMMA]), the TDIQ stimulus failed to generalize to (+)amphetamine or MDMA. In the present investigation, further evaluations were made of the stimulus nature of TDIQ. Specifically, the stimulus similarities of TDIQ, PMMA, and DOM were examined. In no case was stimulus generalization (substitution) observed. The results confirm that TDIQ produces stimulus effects distinct from those of the abovementioned phenylalkylamines. We also examined the structure-activity relationships of a series of TDIQ analogs, including several that might be viewed as conformationally restricted (CR) analogs of phenylalkylamine hallucinogens, stimulants, and designer drugs. These agents were examined in rats trained to discriminate either DOM (1.0 mg/kg), (+)amphetamine (1.0 mg/kg), MDMA (1.5 mg/kg), or TDIQ (5.0 mg/kg) from saline vehicle. Whereas we have demonstrated that none of these agents retains their respective phenylalkylamine stimulus actions, several of these agents were found to substitute for TDIQ. N-Methylation abolished TDIQ-stimulus action. These results, coupled with previous findings, imply that TDIQ derivatives represent a novel class of phenylalkylamines analogs with unique stimulus properties. Preliminary radioligand binding studies suggest that an alpha2-adrenergic mechanism might underlie the stimulus effects produced by TDIQ.

The stimulus effect of 5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline is similar to that of cocaine but different from that of amphetamine

5,6,7,8-Tetrahydro-1,3-dioxolo[4,5-g]isoquinoline (TDIQ) is a conformationally restricted phenylalkylamine related in structure to amphetamine and N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) that does not act as a locomotor stimulant. To further evaluate this agent, a group of six rats was trained to discriminate 5.0 mg/kg of TDIQ from vehicle and tests of stimulus generalization were conducted to define the stimulus. The TDIQ stimulus (ED(50)=0.9 mg/kg) failed to generalize to the central stimulants (+)amphetamine, methylphenidate or (-)ephedrine but, curiously, generalized to cocaine (ED(50)=1.5 mg/kg). When administered to rats (n=5) trained to discriminate 1.0 mg/kg of (+)amphetamine from vehicle, TDIQ produced a maximum of 7% (+)amphetamine-appropriate responding, whereas when administered to rats (n=7) trained to discriminate 4.0 mg/kg of (-)ephedrine from vehicle, TDIQ produced a maximum of 57% drug-appropriate responding. Administration of MDMA to TDIQ-trained animals resulted in 76% TDIQ-appropriate responding. Tests of stimulus generalization were also conducted with fenfluramine, nisoxetine, clenbuterol, imipramine and buspirone, and tests of antagonism were conducted with haloperidol and R(+)SCH-23390 using the TDIQ-trained animals. Results were inconclusive in that these agents either failed to completely substitute for or failed to completely antagonize the TDIQ stimulus. Nevertheless, the generalization seen with cocaine, the partial generalization seen with (-)ephedrine, MDMA, nisoxetine, clenbuterol and buspirone and the partial antagonism seen with haloperidol suggest that TDIQ might be acting through a mixed mechanism that involves adrenergic, dopaminergic and/or serotonergic systems. Given that TDIQ is an agent that seems to differentiate among the stimuli produced by amphetamine, methylphenidate, ephedrine and cocaine, it is proposed that further tests be undertaken, using animal models of cocaine abuse, to evaluate the potential usefulness of TDIQ as pharmacotherapy in cocaine dependence.

Discriminative stimulus properties of alpha-ethyltryptamine optical isomers

alpha-Ethyltryptamine (alpha-ET) possesses central stimulant and hallucinogenic activity. Also, in tests of stimulus generalization using rats trained to discriminate the controlled substance analog (i.e., designer drug) N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) from vehicle, alpha-ET substituted for MDMA. These previous studies employed racemic alpha-ET. Because psychoactive phenylalkylamines with abuse potential can produce one or more of three distinct stimulus effects (i.e., amphetamine-, DOM- and/or PMMA-like effects) in animals trained to discriminate either the stimulant (+)amphetamine, the hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), or N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA) from vehicle, and because these effects can be stereoselective, the individual optical isomers of alpha-ET were examined in groups of animals trained to discriminate (+)amphetamine, DOM, PMMA and MDMA from saline vehicle. (-)alpha-ET (ED(50)=7.8 mg/kg), but not (+)alpha-ET (maximum of 53% drug-appropriate responding), substituted for (+)amphetamine, whereas (+)alpha-ET (ED(50)=2.7 mg/kg), but not (-)alpha-ET (maximum of 33% drug-appropriate responding), substituted for DOM. Both optical isomers of alpha-ET substituted for PMMA and MDMA with ED(50) values of 1.6 and 1.4 mg/kg (PMMA-trained animals) and 1.3 and 2.0 mg/kg (MDMA-trained animals) for (-)alpha-ET and (+)alpha-ET, respectively. The results of this investigation suggest that both optical isomers of alpha-ET are capable of producing an MDMA/PMMA-like effect at nearly comparable doses, and that the stimulant or amphetamine-like nature of alpha-ET resides primarily with its (-)isomer whereas hallucinogenic or DOM-like character resides primarily with the (+)enantiomer.

PMMA-stimulus generalization to the optical isomers of MBDB and 3,4-DMA

Psychoactive phenylisopropylamines can produce one or more of several different stimulus effects in animals. These effects are typified by the hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), the central stimulant amphetamine, and by N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA), an agent whose actions are not yet well understood. The optical isomers of two phenylisopropylamines known to lack DOM and amphetamine-stimulus character, that is N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminobutane (MBDB) and 1-(3,4-dimethoxyphenyl)-2-aminopropane (3,4-DMA), were examined in rats trained to discriminate 1.25 mg/kg of PMMA from vehicle. The PMMA stimulus (ED(50)=0.4 mg/kg) generalized to all four agents: S(+)-MBDB (ED(50)=0.8 mg/kg), R(-)-MBDB (ED(50)=2.0 mg/kg), S(+)-3,4-DMA (ED(50)=2.6 mg/kg) and R(-)-3,4-DMA (ED(50)=3.9 mg/kg). The results show that these agents produce stimulus effects similar to those produced by PMMA. Both isomers of MBDB have been previously demonstrated to substitute for N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) in rats trained to discriminate MDMA from vehicle, but MBDB-trained animals failed to recognize DOM or amphetamine. Similar results were obtained with the 3,4-DMA optical isomers in the present investigation using rats trained to discriminate MDMA, DOM or (+)-amphetamine from vehicle; both isomers of 3,4-DMA substituted for an MDMA stimulus, but not for a DOM or amphetamine stimulus. Taken together, the evidence suggests that PMMA, S(+)-MBDB, R(-)-MBDB, S(+)-3,4-DMA, R(-)-3,4-DMA, and S(+)-MDMA can produce common stimulus effects in rats. The present findings also better define the PMMA stimulus and the structural requirements necessary to produce this type of stimulus effect.

Cathinone: an investigation of several N-alkyl and methylenedioxy-substituted analogs

Structurally, methcathinone is to cathinone what methamphetamine is to amphetamine. Due to increased interest in the abuse of such agents we wished to determine if certain derivatives of cathinone would behave in a manner consistent with what is known about their amphetamine counterparts; that is, can amphetamine structure-activity relationships be extrapolated to cathinone analogs? As expected on the basis of known structure-activity relationships for amphetaminergic agents, both N-monoethylcathinone and N-mono-n-propylcathinone (N-Et CAT and N-Pr CAT; ED50 = 0.77 and 2.03 mg/kg, respectively) produced amphetamine-like stimulus effects in rats trained to discriminate 1 mg/kg of (+)amphetamine from vehicle and were somewhat less potent than racemic methcathinone. In contrast, (-)N,N-dimethylcathinone or (-)Di Me CAT (ED50 = 0.44 mg/kg) was more potent than expected; although (+)N,N-dimethylamphetamine is sevenfold less potent than (+)methamphetamine, (-)Di Me CAT is only about 1.6-fold less potent than (-)methcathinone, and is essentially equipotent with (-)cathinone. In addition, although it has been previously demonstrated that 1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDA) results in stimulus generalization in rats trained to discriminate (+)amphetamine or DOM from vehicle, the cathinone counterpart of MDA (i.e., MDC) resulted in partial (maximum: 58%) generalization in (+)amphetamine-trained animals, and failed to produce >7% DOM-appropriate responding in rats trained to discriminate DOM from vehicle. On the other hand, the N-methyl analog of MDC (i.e., MDMC) behaved in a manner similar to that of the N-methyl analog of MDA (i.e., MDMA); that is, a (+)amphetamine stimulus (MDMC: ED50 = 2.36 mg/kg) but not a DOM stimulus generalized to MDMC. In MDMA-trained rats, stimulus generalization occured both to MDC and MDMC (ED50 = 1.64 and 1.60 mg/kg, respectively). Although this and previous studies have demonstrated that significant parallelisms exist between the structure-activity relationships of amphetamine analogs and cathinone analogs, we now report several unexpected qualitative and/or quantitative differences. It is suggested that caution be used in attempting to draw conclusions or make predictions about the activity and potency of novel cathinone analogs by analogy to the structure-activity relationships derived from amphetamine-related agents; it would appear that each new cathinone analog will require individual investigation.

Time course analysis of the discriminative stimulus effects of the optical isomers of 3,4-methylenedioxymethamphetamine (MDMA)

The present study examined the discriminative stimulus effects of the MDMA optical isomers administered at different presession injection intervals. In the first experiment, male Sprague-Dawley rats were trained in a two-lever, food-reinforced operant procedure to discriminate either (+)-MDMA (1.25 mg/kg) or (-)-MDMA (3.50 mg kg) at either 20 or 90 min following injection. Animals administered (+)-MDMA or saline 90 min before training sessions failed to attain the discrimination criteria after 73 training sessions, whereas (-)-MDMA successfully established discriminative stimulus control at both the 20 min and the 90 min postinjection intervals. (+)-Amphetamine did not substitute for either isomer, although a significant amount of drug-appropriate responding occurred in animals trained to discriminate (+)-MDMA at 20 min and (-)-MDMA at 90 min. Sch 39166 partially reduced the discrimination of (+)-MDMA at 20 min and (-)-MDMA at 90 min, although this effect was not dose dependent. Sch 39166 had no effect on animals trained to discriminate (-)-MDMA at 20 min. Haloperidol did not alter the discrimination of (+)-MDMA at 20 min but partially reduced the discriminative stimulus control of (-)-MDMA at 20 min and (-)-MDMA at 90 min. Fenfluramine substituted for both isomers of MDMA. Pirenpirone completely blocked the discriminative stimulus effects of (-)-MDMA at 20 min, although (+)-MDMA at 20 min and (-)-MDMA at 90 min were only partly blocked. WAY 100,135 had little effect on drug-appropriate responding; however, the discrimination of (+)-MDMA at 20 min was partly reduced by this 5-HT1A antagonist. In a second experiment, rats trained to discriminate (+)-MDMA (1.5 mg/kg) or (-)-MDMA (3.0 mg/kg) from saline were administered substitution tests with both isomers 20, 60, 90 and 120 min after injection. Results confirmed those of the first experiment that (+)-MDMA appears to have a shorter duration of action than (-)-MDMA. These results are discussed in light of the training doses employed.

Assessment of the MDA and MDMA optical isomers in a stimulant-hallucinogen discrimination

The phenylisopropylamine derivatives 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyamphetamine (MDA) have been compared to both psychostimulants and hallucinogens in drug discrimination investigations. The stereoisomers of these compounds, in particular those of MDA, appear to produce differential effects. Previous studies have demonstrated that animals trained to discriminate amphetamine from vehicle generalize to the S(+)-isomers but not the R(-)-isomers of MDA and MDMA while animals trained to discriminate LSD from saline generalize to R(-)-MDA and neither isomer of MDMA. However, animals trained to discriminate mescaline from vehicle generalize to both stereoisomers of these phenylisopropylamine derivatives. The present study consisted of two experiments in which a three-choice drug discrimination procedure was employed to compare the stereoisomers of MDA and MDMA to both amphetamine and either mescaline (experiment one) or LSD (experiment two). Sixteen male Sprague-Dawley rats were trained to discriminate S(+)-amphetamine (1.0 mg/kg) and mescaline (12.5 mg/kg) and eight rats were trained to discriminate S(+)-amphetamine (1.0 mg/kg) and LSD (0.08 mg/kg) from saline in three-choice, food reinforced drug discrimination procedures. Substitution tests were administered with the isomers of MDA and MDMA. In the second experiment, substitution tests were also administered with lower doses of each training compound and with the stimulant cocaine and the hallucinogen 2,5-dimethoxy-4-methylphenylisopropylamine (DOM). In both experiments, all of the isomers produced very few responses on the S(+)-amphetamine lever. In the first experiment, R(-)-MDA and R(-)-MDMA produced nearly complete substitution for mescaline. The results of the second experiment revealed partial substitution for LSD with both isomers of MDMA and S(+)-MDA, and nearly complete substitution with R(-)MDA for LSD. The present findings do not support previous reports that S(+)-MDMA and S(+)-MDMA substitute for S(+)-amphetamine. The three-lever drug discrimination procedure may provide a more sensitive behavioral assay in which to examine the discriminative stimulus effects of drugs with compound stimulus properties.

Initial characterization of PMMA as a discriminative stimulus

The phenylisopropylamine PMMA or N-methyl-1-(4-methoxyphenyl)-2-aminopropane, a structural hybrid of paramethoxyamphetamine (PMA) and methamphetamine, has been previously shown to unexpectedly lack amphetamine-like or hallucinogen-like stimulus properties in animals. For example, in tests of stimulus generalization, neither a (+)amphetamine stimulus nor a DOM stimulus generalized to PMMA. It has also been shown, however, that stimulus generalization does occur in animals trained to discriminate the designer drug MDMA ("Ecstasy" or N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane) from vehicle. In order to further characterize this unique agent, we trained a group of six Sprague-Dawley rats to discriminate 1.25 mg/kg of PMMA (ED50 = 0.44 mg/kg) from saline vehicle. The PMMA stimulus failed to generalize to the phenylisopropylamine stimulant (+)amphetamine, or to the phenylisopropylamine hallucinogen DOM. Stimulus generalization occurred to (+/-)MDMA (ED50 = 1.32 mg/kg) and S(+)MDMA (ED50 = 0.48 mg/kg). Partial generalization occurred with R(+)MDMA, PMA, 3.4-DMA, and fenfluramine. The PMMA stimulus also generalized to the alpha-ethyl homolog of PMMA (EH/PMMA, ED50 = 1.29 mg/kg). Taken together, the results of these studies suggest that PMMA is an MDMA-like agent that lacks the amphetamine-like stimulant character of MDMA. These findings support our previous suggestion that PMMA be considered the structural parent of the MDMA-like family of designer drugs.

Examination of the action of 3,4-methylenedioxymethamphetamine on rat A10 dopamine neurons

Extracellular single cell recording was used to examine the effect of intravenous administration of (-), (+), and (+/-)-3,4-methylenedioxymethamphetamine (MDMA) on A10 dopamine (DA) neurons in chloral hydrate anesthetized male rats. Both (+/-)-MDMA and (+)-MDMA inhibited the firing rate of most (79%) A10 DA cells. By contrast, (-)-MDMA induced either no effect or a slight increase in the firing rate of these cells. Analysis of the effects of (+/-)-MDMA on the firing pattern of the DA cells revealed on overall decrease in the percentage of spikes in bursts but both increases and decreases were seen in the coefficient of variation of interspike intervals. To determine the contribution of 5-HT and DA to the (+/-)-MDMA-induced inhibition of A10 DA cells rats were pretreated either with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) or the DA synthesis inhibitor alpha-methyl-p-tyrosine (AMPT). Pretreatment of rats with PCPA did not reduce the ability of (+/-)-MDMA to inhibit the DA cells. However, in rats pretreated with AMPT, the (+/-)-MDMA-induced inhibition was blocked and some cells (44%) showed instead an increase in firing rate following administration of (+/-)-MDMA. The administration of l-3,4-dihydroxyphenylalanine (L-DOPA) to AMPT-treated rats rapidly restored the inhibition of cell firing by (+/-)-MDMA. In conclusion, the results reported here demonstrate that MDMA has an overall inhibitory effect on A10 DA cells. Despite MDMA's greater potency in releasing 5-HT compared to DA, the inhibitory effect of this drug on A10 DA cells appears to be mediated by the latter transmitter.

Effects of (+)-fenfluramine on 3,4-methylenedioxymethamphetamine (MDMA) discrimination in rats

This study examined the effects of a presumed neurotoxic dose regimen of (+)-fenfluramine on the discrimination of MDMA and (+)-amphetamine in male Sprague-Dawley rats trained to discriminate 1.5 mg/kg MDMA from saline in a two-choice operant task. Substitution tests were conducted with saline, several doses of MDMA (0.19-1.5 mg/kg), and (+)-amphetamine (0.125-1.0 mg/kg) prior to and again following the administration of (+)-fenfluramine (4.0 mg/kg twice a day for 4 days; n = 11) or a similar pattern of saline injections (n = 10). During pretreatment substitution tests, lower doses of MDMA elicited drug-appropriate responding in a dose-dependent manner, although none of these doses substituted for the training dose. Likewise, no dose of (+)-amphetamine substituted for the training drug during pretreatment substitution tests. The discrimination of MDMA was disrupted in some animals following (+)-fenfluramine treatment, but with subsequent training, discrimination criteria were met. In posttreatment substitution tests, the lowest dose of MDMA produced significantly higher drug-appropriate responding in (+)-fenfluramine treated animals but not in saline-treated animals. The amount of drug-appropriate responding during posttreatment substitution tests with (+)-amphetamine varied little from pretreatment substitution tests in saline-treated animals, but was greater at all doses in (+)-fenfluramine-treated animals; the highest dose of (+)-amphetamine substituted for MDMA subsequent to (+)-fenfluramine treatment. These results support previous findings that the long-lasting serotonergic effects of fenfluramine may have functional consequences that can be detected using a drug discrimination procedure. Specifically, serotonin depletion may unmask or strengthen the stimulant-like effects of MDMA.

3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy"): pharmacology and toxicology in animals and humans

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy"), a ring-substituted amphetamine derivative first synthesized in 1914, has emerged as a popular recreational drug of abuse over the last decade. Pharmacological studies indicate that MDMA produces a mixture of central stimulant and psychedelic effects, many of which appear to be mediated by brain monoamines, particularly serotonin and dopamine. In addition to its pharmacologic actions, MDMA has been found to possess toxic activity toward brain serotonin neurones. Serotonergic neurotoxicity after MDMA has been demonstrated in a variety of experimental animals (including non-human primates). In monkeys, the neurotoxic dose of MDMA closely approaches that used by humans. While the possibility that MDMA is also neurotoxic in humans is under investigation, other adverse effects of MDMA in humans have been documented, including various systemic complications and a number of untoward neuropsychiatric sequelae. Notably, many of the adverse neuropsychiatric consequences noted after MDMA involve behavioral domains putatively influenced by brain serotonin (e.g., mood, cognition and anxiety). Given the restricted status of MDMA use, retrospective clinical observations from suspecting clinicians will probably continue to be a primary source of information regarding MDMA's effects in humans. As such, this article is intended to familiarize the reader with the behavioral pharmacology and toxicology of MDMA, with the hope that improved recognition of MDMA-related syndromes will provide insight into the function of serotonin in the human brain, in health as well as disease.

Stimulant effects of 3,4-methylenedioxymethamphetamine in the nucleus accumbens of rat

This study examined the behavioral effects in rats of intracerebral administration of S(+)-3,4-methylenedioxymethamphetamine (S-MDMA) using an automated holeboard and open-field apparatus. Administration of S-MDMA into the nucleus accumbens septi produced locomotor hyperactivity. Although the stimulant effects of S-MDMA administered systemically are antagonized by fluoxetine pretreatment, the activating effects of S-MDMA administered into the nucleus accumbens were not antagonized by fluoxetine. A similar increase in locomotor activity was observed after S-amphetamine administration into the nucleus accumbens. In contrast, the selective 5-HT-releasing drug and S-MDMA congener N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (MBDB) produces MDMA-like locomotor hyperactivity when administered systemically but did not alter locomotor activity when injected into the nucleus accumbens. These data indicate that S-MDMA actions in the nucleus accumbens are pharmacologically distinct from the primary effects of systemically administered S-MDMA. Behavioral effects of S-MDMA in the nucleus accumbens may result from the catecholamine-releasing properties that S-MDMA shares with S-amphetamine and not via the 5-HT-releasing properties that it shares with MBDB.

A pharmacokinetic analysis of 3,4-methylenedioxymethamphetamine effects on monoamine concentrations in brain dialysates

Interpretation of the in vivo actions of 3,4-methylenedioxymethamphetamine (MDMA) is complicated by the formation of the active metabolite, 3,4-methylenedioxyamphetamine (MDA). This study evaluates the role of MDA in the dopamine releasing actions of (+)-MDMA. In the study, rats were given subcutaneous doses of (+)-MDMA and concentrations of monoamines and their metabolites in striatal dialysate were measured at 15 min intervals. In parallel experiments, plasma concentrations of (+)- and (-)-MDMA and MDA were determined by GC/MS procedures. The time course of MDMA levels was comparable for the two isomers as were their bioavailabilities. In contrast, the plasma levels of MDA were about three times higher after (+)-MDMA. (+)-MDMA caused a rapid increase in striatal dialysate levels of dopamine and decreased extracellular levels of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). There was a significant correlation between dopamine concentration in striatal dialysate and plasma MDMA concentration, but not with plasma MDA. These results indicate that MDMA itself has stereoselective actions on dopamine neurons. However, the higher plasma MDA levels after (+)-MDMA may account for part of the enantiomeric differences in the behavioral and neurotoxicological effects of MDMA.

Stimulus properties of ring-methyl amphetamine analogs

There are three possible ring-substituted methyl amphetamines (or tolylaminopropanes; TAPs): oTAP, mTAP and pTAP. These agents are positional isomers of methamphetamine. Although all three isomers have been previously reported to possess amphetamine-like character, few studies have examined all three agents in comparison with (+)amphetamine. Using rats trained to discriminate 1 mg/kg of (+)amphetamine from saline under a variable-interval 15-sec schedule of reinforcement, tests of stimulus generalization were conducted with the three positional isomers. Only oTAP (ED50 dose = 4.1 mg/kg) completely substituted for (+)amphetamine. mTAP and pTAP resulted only in partial (ca. 50% amphetamine-appropriate responding) generalization. It is concluded that oTAP is capable of producing amphetamine-like stimulus effects and that it is approximately one-tenth as potent as (+)amphetamine; however, because the partial generalization produced by mTAP and pTAP was followed by disruption of behavior at slightly higher doses, it cannot be reliably stated that these latter two isomers lack amphetamine-like character.

Stimulus properties of a new designer drug: 4-methylaminorex ("U4Euh")

Like other phenylisopropylamine derivatives, 4-methylaminorex is a central stimulant. The cis isomer of 4-methylaminorex ("U4Euh"; "ICE") has appeared on the clandestine market as a novel designer drug and was recently classified as a Schedule I substance. In the present investigation, the stimulus properties of racemic cis, racemic trans, and all four individual optical isomers of 4-methylaminorex were examined in rats trained to discriminate 1 mg/kg of S(+)amphetamine sulfate from saline. The S(+)amphetamine stimulus generalized to all of the agents investigated and the relative potencies of the optical isomers (followed by ED50 values) were as follows: trans(4S,5S) (0.25 mg/kg) greater than cis(4S,5R) (1.2 mg/kg) = cis(4R,5S) (1.5 mg/kg) greater than trans(4R,5R). The trans(4R,5R) isomer did not completely substitute for S(+)amphetamine unless a longer (i.e., 60-min) presession injection interval was used, suggesting that it has a longer duration of onset than the other isomers of 4-methylaminorex. The results, which are consistent with established structure-activity relationships, suggest that the trans(4S,5S) isomer (which has not been scheduled) is similar in potency to (+)amphetamine (ED50 = 0.4 mg/kg) and is more potent than either of the cis isomers.

Enantiomeric differences in the effects of 3,4-methylenedioxymethamphetamine on extracellular monoamines and metabolites in the striatum of freely-moving rats: an in vivo microdialysis study

The effects of (+) and (-) 3,4-methylenedioxymethamphetamine (MDMA) and racemic p-chloroamphetamine (PCA) on extracellular dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the metabolite of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), were determined in dialysates of the striatum conscious rats by using intracerebral dialysis and high performance liquid chromatography with electrochemical detection (HPLC-EC). The (+) and (-)MDMA isomers (5, 10 mg/kg, s.c.) and PCA (2.5, 5 mg/kg, s.c.) caused a rapid increase of extracellular levels of dopamine and decreased extracellular levels of DOPAC and HVA immediately after administration in dialysates of striatum. The order of potency for this effect was PCA greater than (+)MDMA greater than (-)MDMA. The levels of 5-HIAA also decreased after the administration of drugs, but the effect had a slower time course than DOPAC and HVA and did not exhibit an enantiomeric difference. The data indicate that, although these drugs are thought to affect the 5-HT neuronal system preferentially, they also affect dopamine systems and by a mechanism in which the (+) isomer was more potent than the (-).

Stereoselective pharmacokinetics of 3,4-methylenedioxymethamphetamine in the rat

Studies to characterize the pharmacokinetics of the enantiomers of MDMA were conducted in rats using the iliac arterial cannulation. Two routes of administration, intravenous and subcutaneous, were evaluated at two dose levels for each route [20 and 40 mg/kg (+/-)-MDMA for subcutaneous, 10 and 20 mg/kg (+/-)-MDMA for intravenous administrations]. The average half-life (+/- SD) for all dosing groups was 2.5 +/- 0.8 h for (-)-(R)-MDMA and 2.2 +/- 0.8 h for (+)-(S)-MDMA. The more rapid clearance of (+)-(S)-MDMA compared with (-)-(R)-MDMA is consistent with the area under the curve (AUC) data of the parent drug and its primary metabolite MDA. The mean (+/- SD) AUC S/R ratios of MDMA and MDA were 0.70 +/- 0.05 and 3.1 +/- 0.8, respectively. Following a 20 mg/kg dose of racemic MDMA iv the mean (+/- SD) of the percent dose excreted as (-)-(R)-MDMA, (+)-(S)-MDMA, (-)-(R)-MDA, and (+)-(S)-MDA were 20 +/- 10, 12 +/- 6, 3 +/- 1, and 6 +/- 2, respectively.

MDMA (3,4-methylenedioxymethamphetamine) inhibits the firing of dorsal raphe neurons in brain slices via release of serotonin

The effects of MDMA (3,4-methylenedioxymethamphetamine) on the activity of serotonin (5-HT)-containing dorsal raphe neurons were characterized using extracellular single-unit recording and microdialysis techniques in the in vitro midbrain slice preparation. Addition of (+)-MDMA, (-)-MDMA or p-chloroamphetamine (PCA) to the superfusate (final concentration 3-100 microM) produced a concentration-dependent inhibition of 5-HT cell firing which was reversible and reproducible. Based upon IC50 values, (+)-MDMA was 2- to 3-fold more potent than (-)-MDMA. Pretreatment with the selective 5-HT uptake inhibitor fluoxetine, at a concentration which had no effect on baseline firing (20 microM), blocked the inhibitory effect of (+)-MDMA and PCA on dorsal raphe neurons. The selective norepinephrine uptake inhibitor desipramine (20 microM) was ineffective. In a parallel series of experiments, microdialysis probes resting on the brain slice surface provided a means to estimate 5-HT release from the dorsal raphe nucleus. (+)-MDMA (100 microM) caused the release of measureable quantities of 5-HT with a time course which corresponded to the change in dorsal raphe cell firing rate. Taken together, these data suggest that MDMA acts indirectly to inhibit dorsal raphe neurons through release of endogenous 5-HT.

Prepulse inhibition of the acoustic startle response is disrupted by N-ethyl-3,4-methylenedioxyamphetamine (MDEA) in the rat

N-Ethyl-3,4-methylenedioxyamphetamine (MDEA) is a derivative of methylenedioxyamphetamine (MDA), a substituted amphetamine with demonstrated abuse liability. MDA, MDEA and a third substituted amphetamine, methylenedioxymethamphetamine (MDMA), all produce a destructive action on central serotonin neurons and appear to induce some similar behavioral effects. The present study investigated the effects of racemic MDEA and its stereoisomers on prepulse inhibition of the acoustic startle response, a behavioral model of sensorimotor gating that is sensitive to psychostimulant drugs. Rats were subjected to 122 dB[A] acoustic noises, some of which were preceded by a weak 80 dB[A] prepulse noise. In vehicle-injected control rats, the prepulse induced a significant decrease in startle amplitude when compared to trials in which startle stimuli were not preceded by prepulses. Administration of racemic MDEA (0.3-10.0 mg/kg) and (+) MDEA (0.1-3.0 mg/kg) induced a significant attenuation in prepulse inhibition, while (-) MDEA (0.3-10.0 mg/kg) did not. Racemic MDMA (0.3-10.0 mg/kg) produced similar though not significant effects. These results confirm a stimulant-like behavioral effect of MDEA despite its relatively modest effects on dopamine markers, and support findings that the (+) stereoisomers of substituted amphetamines are more potent than tha (-) stereoisomers in producing psychostimulant-like biochemical and behavioral effects.

Stimulus effects of N-monoethyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDE) and N-hydroxy-1-(3,4-methylenedioxyphenyl)-2-aminopropane (N-OH MDA) in rats trained to discriminate MDMA from saline

Tests of stimulus generalization were conducted using rats trained to discriminate 1.5 mg/kg of N-monomethyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane HCl (MDMA) from saline in order to determine if two structurally related analogs (MDE and N-OH MDA) would produce similar stimulus effects. The MDMA-stimulus (MDMA, ED50 value = 0.76 mg/kg) generalized both to MDE (ED50 value = 0.73 mg/kg) and N-OH MDA (ED50 value = 0.47 mg/kg). Administration of (+)amphetamine resulted in partial generalization (maximum of 49% MDMA-appropriate responding) in the MDMA-trained animals. Taken together with our previous studies showing that MDMA substitutes for the phenylisopropylamine stimulant (+)amphetamine, but that neither MDE nor N-OH MDA substitute for (+)amphetamine or for the phenylisopropylamine hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), the present results [i.e., MDMA-stimulus generalization to MDE, N-OH MDA, but not to (+)amphetamine] suggest that 1) MDMA produces effects other than those that may be considered amphetamine-like, and 2) MDE and N-OH MDA are MDMA-like agents with even less of an amphetamine-like component of action than MDMA itself.

Stereochemistry of the metabolism of MDMA to MDA

The chiral derivatizing reagent N-trifluoroacetyl-L-prolyl chloride (LTPC) was used to form diastereomers of 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyamphetamine (MDA) which were resolved on an achiral gas chromatographic column using a mass spectrometer as a detector. Rats were subcutaneously dosed with 40 mg/kg of (+/-) MDMA.HCl and blood was obtained by decapitation four hours after dosing. Plasma was separated and extracted. The extract was derivatized on-column with LTPC. In addition to the two MDMA isomers, the demethylated metabolites, S(+) and R(-)-MDA were identified. In all experimental groups (male rats, food deprived male rats, female rats, post partum female rats, and mice) dosed with racemic MDMA, higher levels of the S(+) isomer of MDA relative to the R(-) MDA isomer were observed. This may be significant since it has been shown that the S(+) isomer of MDMA is the more neurotoxic isomer of the racemic drug of abuse MDMA.

A role for the mesolimbic dopamine system in the psychostimulant actions of MDMA

Methylenedioxymethamphetamine (MDMA) is a phenylethylamine with a chemical structure that resembles both the amphetamines and mescaline and has both stimulant and perception altering properties. The stimulant properties of MDMA were assessed in photocell cages designed to measure locomotor activity in rats. MDMA, over a range of doses (2.5-10.0 mg/kg, SC) produced locomotor hyperactivity which lasted up to 4 h. Further studies examined the role of the mesolimbic dopamine system in the hyperactivity induced by MDMA. 6-Hydroxydopamine lesions of the Nucleus accumbens attenuated the locomotor response produced by MDMA. The well characterized attenuation of the locomotor response produced by amphetamine was also demonstrated in the same rats. The present study demonstrates similarities in the stimulant properties of MDMA and amphetamine, and also suggests that as with amphetamine, the locomotor activation associated with MDMA may involve the presynaptic release of dopamine in the region of the Nucleus accumbens. However, MDMA may have a more unusual pharmacological profile because of its longer duration of action, neurotoxic potential, and differences in the qualitative aspects of its psychoactive effects.

A preliminary behavioral investigation of PMMA, the 4-methoxy analog of methamphetamine

The controlled-substance analog N-monomethyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA) may be viewed as being either the 4-methoxy analog of methamphetamine or the N-methyl analog of 1-(4-methoxyphenyl)-2-aminopropane (PMA). Because of its abuse potential, PMMA was examined with regard to (a) its stimulus properties in rats trained to discriminate either 1.0 mg/kg of (+)amphetamine or (+/-)DOM from saline, (b) its toxicity (isolated and aggregated) in mice relative to (+/-)PMA, and (c) its locomotor stimulant activity in mice relative to (+/-)amphetamine, (+/-)methamphetamine, and (+/-)PMA. Racemic PMMA produced neither DOM-like nor, unlike PMA, amphetamine-like stimulus effects. There was no significant difference between the 24-hr isolated (LD50 = 63 mg/kg) and aggregated (LD50 = 53 mg/kg) toxicity, and PMMA did not produce significant locomotor stimulation at doses of up to 30 mg/kg. The present results suggest that while PMMA may produce central effects it does not appear to behave as a simple amphetamine-like agent.

A preliminary investigation of the psychoactive agent 4-bromo-2,5-dimethoxyphenethylamine: a potential drug of abuse

4-Bromo-2,5-dimethoxyphenethylamine (alpha-desMe DOB) is a psychoactive agent that may possess significant abuse potential. Because of its structural similarity to the established hallucinogen 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB), and because almost no pharmacological data are available on this agent, we undertook this preliminary investigation. alpha-DesMe DOB (Ki = 1 nM), like DOB itself (Ki = 0.79 nM), displays a high affinity for [3H]DOB-labeled central 5-HT2 serotonin receptors. However, unlike DOB, the alpha-desmethyl derivative also binds with significant affinity to 5-HT1A, 5-HT1B, and 5-HT1C serotonin receptors and, as such, is less selective than DOB. In drug discrimination studies using rats trained to discriminate either DOM (i.e., the 4-methyl analog of DOB) or R(-)DOB from saline, stimulus generalization occurred in both groups of animals. However, stimulus generalization was associated with extensive disruption of behavior, alpha-DesMe DOB may produce stimulus effects similar, but not identical, to those of DOM and R(-)DOB; in addition, this agent may be capable of producing other, as yet undefined, central effects at comparable doses. These other effects may be reflective of the lack of selectivity of alpha-desMe DOB for 5-HT2 serotonin receptors. Because other hallucinogenic agents display high affinity for 5-HT2 serotonin receptors and result in stimulus generalization in DOM- and/or DOB-trained animals, it is tentatively concluded that alpha-desMe DOB is a psychoactive agent with at least some hallucinogenic or DOB-like properties.