Study of the behavioural responses related to the potential addictive properties of MDMA in mice

Modulation of Gut Microbiome in Ecstasy/MDMA-Induced Behavioral and Biochemical Impairment in Rats and Potential of Post-Treatment with Anacyclus pyrethrum L. Aqueous Extract to Mitigate Adverse Effects

The use of illicit substances continues to pose a substantial threat to global health, affecting millions of individuals annually. Evidence suggests the existence of a 'brain-gut axis' as the involving connection between the central nervous system and gut microbiome (GM). Dysbiosis of the GM has been associated with the pathogenesis of various chronic diseases, including metabolic, malignant, and inflammatory conditions. However, little is currently known about the involvement of this axis in modulating the GM in response to psychoactive substances. In this study, we investigated the effect of MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy")-dependence on the behavioral and biochemical responses, and the diversity and abundance of the gut microbiome in rats post-treated (or not) with aqueous extract of Anacyclus pyrethrum (AEAP), which has been reported to exhibit anticonvulsant activity. The dependency was validated using the conditioned place preference (CPP) paradigm, behavioral, and biochemical tests, while the gut microbiota was identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The CPP and behavioral tests confirmed the presence of MDMA withdrawal syndrome. Interestingly, treatment with AEAP led to a compositional shift in the GM compared to the MDMA-treated rats. Specifically, the AEAP group yielded a higher relative abundance of Lactobacillus and Bifidobacter, while animals receiving MDMA had higher levels of E. coli. These findings suggest that A. pyrethrum therapy may directly modulate the gut microbiome, highlighting a potential target for regulating and treating substance use disorders.

MDMA and memory, addiction, and depression: dose-effect analysis

RATIONALE

±3,4-Methylenedioxymethamphetamine (MDMA) is a recreational drug that shows substantial promise as a psychotherapeutic agent. Still, there is some concern regarding its behavioral toxicity, and its dose-effect relationship is poorly understood. We previously explored the role of dose in the cognitive effects of MDMA in a systematic review of existing literature and found no evidence in animals that MDMA impairs memory at low doses (< 3 mg/kg) but mixed results at high doses (≥ 3 mg/kg). Since this review comprised mostly of single-dose studies and an assortment of methodologies, an empirical dose-ranging study on this topic is warranted.

OBJECTIVES

The current study aims to evaluate the conclusion from our systematic review that 3 mg/kg may be the threshold for MDMA-induced amnesia, and to further understand the dose-effect relationship of MDMA on behavioral assays of memory, addiction, and depression.

METHODS

We systematically examined the effects of 0.01 to 10 mg/kg MDMA on Pavlovian fear conditioning; behavioral sensitization, conditioned place preference, and conditioned responding; and the Porsolt forced swim test in mice.

RESULTS

High doses of MDMA (≥ 3 mg/kg) produced amnesia of fear conditioning memory, some evidence of an addictive potential, and antidepressant effects, while low doses of MDMA (≤ 1 mg/kg) had no effect on these behaviors.

CONCLUSIONS

The present dose-ranging study provides further evidence that 3 mg/kg is the threshold for MDMA-induced amnesia. These findings, in addition to our systematic review, demonstrate that careful selection of MDMA dose is critical. High doses (≥ 3 mg/kg) should likely be avoided due to evidence that they can produce amnesia and addiction. Conversely, there is little evidence to suggest that low doses, which are usually administered in clinical studies (approximately 1-2 mg/kg), will lead to these same adverse effects. Ultra-low doses (< 1 mg/kg) are likely even safer and should be investigated for therapeutic effects in future studies.

Use and abuse of dissociative and psychedelic drugs in adolescence

Adolescence is a period of profound developmental changes, which run the gamut from behavioral and neural to physiological and hormonal. It is also a time at which there is an increased propensity to engage in risk-taking and impulsive behaviors like drug use. This review examines the human and preclinical literature on adolescent drug use and its consequences, with a focus on dissociatives (PCP, ketamine, DXM), classic psychedelics (LSD, psilocybin), and MDMA. It is the case for all the substances reviewed here that very little is known about their effects in adolescent populations. An emerging aspect of the literature is that dissociatives and MDMA produce mixed reinforcing and aversive effects and that the balance between reinforcement and aversion may differ between adolescents and adults, with consequences for drug use and addiction. However, many studies have failed to directly compare adults and adolescents, which precludes definitive conclusions about these consequences. Other important areas that are largely unexplored are sex differences during adolescence and the long-term consequences of adolescent use of these substances. We provide suggestions for future work to address the gaps we identified in the literature. Given the widespread use of these drugs among adolescent users, and the potential for therapeutic use, this work will be crucial to understanding abuse potential and consequences of use in this developmental stage.

Of mice and men on MDMA: A translational comparison of the neuropsychobiological effects of 3,4-methylenedioxymethamphetamine ('Ecstasy')

MDMA (3,4-methylendioxymethamphetamine), also known as Ecstasy, is a stimulant drug recreationally used by young adults usually in dance clubs and raves. Acute MDMA administration increases serotonin, dopamine and noradrenaline by reversing the action of the monoamine transporters. In this work, we review the studies carried out over the last 30 years on the neuropsychobiological effects of MDMA in humans and mice and summarise the current knowledge. The two species differ with respect to the neurochemical consequences of chronic MDMA, since it preferentially induces serotonergic dysfunction in humans and dopaminergic neurotoxicity in mice. However, MDMA alters brain structure and function and induces hormonal, psychomotor, neurocognitive, psychosocial and psychiatric outcomes in both species, as well as physically damaging and teratogen effects. Pharmacological and genetic studies in mice have increased our knowledge of the neurochemical substrate of the multiple effects of MDMA. Future work in this area may contribute to developing pharmacological treatments for MDMA-related disorders.

Neurochemical substrates of the rewarding effects of MDMA: implications for the development of pharmacotherapies to MDMA dependence

In recent years, studies with animal models of reward, such as the intracranial self-stimulation, self-administration, and conditioned place preference paradigms, have increased our knowledge on the neurochemical substrates of the rewarding effects of 3,4-methylenedioxymetamphetamine (MDMA) in rodents. However, pharmacological and neuroimaging studies with human participants are scarce. Serotonin [5-hydroxytryptamine (5-HT)], dopamine (DA), endocannabinoids, and endogenous opiates are the main neurotransmitter systems involved in the rewarding effects of MDMA in rodents, but other neurotransmitters such as glutamate, acetylcholine, adenosine, and neurotensin are also involved. The most important finding of recent research is the demonstration of differential involvement of specific neurotransmitter receptor subtypes (5-HT2, 5-HT3, DA D1, DA D2, CB1, μ and δ opioid, etc.) and extracellular proteins (DA and 5-HT transporters) in the acquisition, expression, extinction, and reinstatement of MDMA self-administration and conditioned place preference. It is important to extend the research on the effects of different compounds acting on these receptors/transporters in animal models of reward, especially in priming-induced, cue-induced, and stress-induced reinstatement. Increase in knowledge of the neurochemical substrates of the rewarding effects of MDMA may contribute to the design of new pharmacological treatments for individuals who develop MDMA dependence.

The Involvement of Norepinephrine in Behaviors Related to Psychostimulant Addiction

Although it is generally accepted that the abuse-related effects of amphetamines and cocaine result from the activation of the brain dopaminergic (DA) system, the psychostimulants also alter other neurotransmitter systems. In particular, they increase extracellular levels of norepinephrine (NE) and serotonin by inhibiting respective plasma membrane transporters and/or inducing release. The present review will discuss the preclinical findings on the effects of the NE system modulation (lesions, pharmacological and genetic approaches) on behaviors (locomotor hyperactivity, behavioral sensitization, modification of intracranial self-stimulation, conditioned place preference, drug self-administration, extinction/reinstatement of drug seeking behavior) related to the psychostimulant addiction.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

Assessment of the abuse potential of MDMA in the conditioned place preference paradigm: role of CB1 receptors

Numerous reports have highlighted the role of the endocannabinoid system in the addictive potential of MDMA (3,4-methylenedioxy-methamphetamine). A previous report showed that CB1 knockout (KOCB1) mice do not acquire MDMA self-administration, despite developing conditioned place preference (CPP). This contradiction could be due to the particular procedure of place conditioning used. The present work compares MDMA-induced CPP in KOCB1 mice using unbiased and biased procedures of place conditioning. In the unbiased procedure, MDMA induced CPP and reinstatement of the extinguished preference in wild type (WT) mice, but not in KOCB1 mice. In contrast, in a biased protocol of CPP, MDMA produced preference in both types of mice. The anxiolytic response induced by MDMA in the elevated plus maze (EPM) was observed only in KOCB1 mice and may have been responsible, at least partially, for the CPP in the biased procedure. A neurochemical analysis revealed that KOCB1 mice presented higher striatal DA and DOPAC levels in response to MDMA, but no alterations in their levels of monoamine transporters. In line with previous self-administration studies, our data suggest that CB1 receptors play an important role in the reinforcing effects of MDMA, and that the experimental procedure of CPP employed should be taken into account when drawing conclusions.

Abuse liability of novel 'legal high' designer stimulants: evidence from animal models

In the last few years, the variety and recreational use of 'legal high' designer stimulants has increased to unprecedented levels. Since their rapid emergence in drug markets, numerous adverse physical and psychological effects have been extensively reported. However, less is understood about the potential for compulsive use of and addiction to these drugs. Recently, a small collection of scientific studies assessing the abuse liability of these drugs has emerged. This new knowledge has been derived primarily from animal studies using behaviorally based procedures which include intravenous self-administration, conditioned place preference, intracranial self-stimulation, and drug discrimination. In this review we present a brief history of the recent rise in designer stimulant use followed by a short methodological description of the aforementioned procedures. We then review neurochemical and abuse liability studies on designer stimulants that have been examined to date. Finally, we conclude with a discussion of these collective findings, our current understanding of the abuse liability of these drugs in relation to each other and the illicit drugs they are designed to mimic, and recommend future research directions.

Heteromeric nicotinic receptors are involved in the sensitization and addictive properties of MDMA in mice

We have investigated the effect of nicotinic receptor ligands in the behavioral sensitization (hyperlocomotion) and rewarding properties (conditioned place preference paradigm, CPP) of 3,4-methylenedioxy-methamphetamine (MDMA) in mice. Each animal received intraperitoneal pretreatment with either saline, dihydro-β-erythroidine (DHβE, 1 mg/kg) or varenicline (VAR, 0.3 mg/kg), 15 min prior to subcutaneous saline or MDMA (5 mg/kg), for 10 consecutive days. On day 1, both DHβE and VAR inhibited the MDMA-induced hyperlocomotion. After 10 days of treatment, MDMA induced a hyperlocomotion that was not reduced (rather enhanced) in antagonist-pretreated animals. This early hyperlocomotion was accompanied by a significant increase in heteromeric nicotinic receptors in cortex that was not blocked by DHβE or VAR. Behavioral sensitization to MDMA was highest 2 weeks after the discontinuation of MDMA treatment. This additional increase in sensitivity was prevented in animals pretreated with DHβE or VAR. At this time, MDMA-treated mice showed a significant increase in heteromeric receptors in cortex that was prevented by DHβE and VAR. An involvement of α7 nicotinic receptors in this effect is ruled out. MDMA (10 mg/kg) induced positive CPP that was abolished by DHβE (2 mg/kg) and VAR (2 mg/kg). Moreover, chronic nicotine pretreatment (2 mg/kg, ip, b.i.d., for 14 days) caused MDMA, administered at a low dose (3 mg/kg), to induce CPP, which would otherwise not occur. Finally, present results point out that heteromeric nicotinic receptors are involved in locomotor sensitization and addictive potential induced by MDMA. Thus, varenicline might be a useful drug to treat both tobacco and MDMA abuse at once.

The Reinforcing and Rewarding Effects of Methylone, a Synthetic Cathinone Commonly Found in "Bath Salts"

Methylone is a member of the designer drug class known as synthetic cathinones which have become increasingly popular drugs of abuse in recent years. Commonly referred to as "bath salts", these amphetamine-like compounds are sold as "legal" alternatives to illicit drugs such as cocaine, methamphetamine, and 3,4-methylenedioxymethamphetamine (MDMA, ecstasy). Following their dramatic rise in popularity along with numerous reports of toxicity and death, several of these drugs were classified as Schedule I drugs in the United States in 2012. Despite these bans, these drugs and other new structurally similar analogues continue to be abused. Currently, however, it is unknown whether these compounds possess the potential for compulsive use and addiction. The present study sought to determine the relative abuse liability of methylone by employing intravenous self-administration (IVSA) and intracranial self-stimulation (ICSS) paradigms in rats. We demonstrate that methylone (0.05, 0.1, 0.2, and 0.5 mg/kg/infusion) dose-dependently functions as a reinforcer, and that there is a significant positive relationship between methylone dose and reinforcer efficacy. Furthermore, responding during short access sessions (ShA, 2 hr/day) appeared more robust than previous IVSA studies with MDMA. However, unlike previous findings with abused stimulants such as cocaine or methamphetamine, long access sessions (LgA, 6 hr/day) did not lead to escalated drug intake or increased reinforcer efficacy. Finally, methylone produced a dose-dependent, but statistically non-significant, trend towards reductions in ICSS thresholds. Together these results reveal that methylone may possess an addiction potential similar to or greater than MDMA, yet patterns of self-administration and effects on brain reward function suggest that this drug may have a lower potential for abuse and compulsive use than prototypical psychostimulants.

The nucleus accumbens 5-HTR₄-CART pathway ties anorexia to hyperactivity

In mental diseases, the brain does not systematically adjust motor activity to feeding. Probably, the most outlined example is the association between hyperactivity and anorexia in Anorexia nervosa. The neural underpinnings of this 'paradox', however, are poorly elucidated. Although anorexia and hyperactivity prevail over self-preservation, both symptoms rarely exist independently, suggesting commonalities in neural pathways, most likely in the reward system. We previously discovered an addictive molecular facet of anorexia, involving production, in the nucleus accumbens (NAc), of the same transcripts stimulated in response to cocaine and amphetamine (CART) upon stimulation of the 5-HT(4) receptors (5-HTR(4)) or MDMA (ecstasy). Here, we tested whether this pathway predisposes not only to anorexia but also to hyperactivity. Following food restriction, mice are expected to overeat. However, selecting hyperactive and addiction-related animal models, we observed that mice lacking 5-HTR(1B) self-imposed food restriction after deprivation and still displayed anorexia and hyperactivity after ecstasy. Decryption of the mechanisms showed a gain-of-function of 5-HTR(4) in the absence of 5-HTR(1B), associated with CART surplus in the NAc and not in other brain areas. NAc-5-HTR(4) overexpression upregulated NAc-CART, provoked anorexia and hyperactivity. NAc-5-HTR(4) knockdown or blockade reduced ecstasy-induced hyperactivity. Finally, NAc-CART knockdown suppressed hyperactivity upon stimulation of the NAc-5-HTR(4). Additionally, inactivating NAc-5-HTR(4) suppressed ecstasy's preference, strengthening the rewarding facet of anorexia. In conclusion, the NAc-5-HTR(4)/CART pathway establishes a 'tight-junction' between anorexia and hyperactivity, suggesting the existence of a primary functional unit susceptible to limit overeating associated with resting following homeostasis rules.

Long-term cognitive and neurochemical effects of "bath salt" designer drugs methylone and mephedrone

INTRODUCTION/AIMS

The use of cathinone-derivative designer drugs methylone and mephedrone has increased rapidly in recent years. Our aim was to investigate the possible long-term effects of these drugs on a range of behavioral tests in mice. Further, we investigated the long-term effects of these drugs on brain neurochemistry in both rats and mice.

METHODS

We treated animals with a binge-like regimen of methylone or mephedrone (30 mg/kg, twice daily for 4 days) and, starting 2 weeks later, we performed behavioral tests of memory, anxiety and depression and measured brain levels of dopamine (DA), serotonin (5-HT), their metabolites and norepinephrine (NE). 5-HT and DA transporter (5-HTT and DAT) levels were also measured in rats by [(3)H]paroxetine and [(3)H]mazindol binding.

RESULTS

Mephedrone reduced working memory performance in the T-maze spontaneous alternation task but did not affect neurotransmitter levels aside from a 22% decrease in striatal homovanillic acid (HVA) levels in mice. Methylone had little effect on behavior or neurotransmitter levels in mice but produced a widespread depletion of 5-HT and 5-HTT levels in rats.

CONCLUSIONS

Both methylone and mephedrone appeared to have a long-term effect on either behavioral or biochemical gauges of neurotoxicity in rodents.

Role of the dopaminergic system in the acquisition, expression and reinstatement of MDMA-induced conditioned place preference in adolescent mice

Background

The rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in conditioned place preference (CPP) procedures, but the involvement of the dopaminergic system in MDMA-induced CPP and reinstatement is poorly understood.

Methodology/Principal Findings

In this study, the effects of the DA D1 antagonist SCH 23390 (0.125 and 0.250 mg/kg), the DA D2 antagonist Haloperidol (0.1 and 0.2 mg/kg), the D2 antagonist Raclopride (0.3 and 0.6 mg/kg) and the dopamine release inhibitor CGS 10746B (3 and 10 mg/kg) on the acquisition, expression and reinstatement of a CPP induced by 10 mg/kg of MDMA were evaluated in adolescent mice. As expected, MDMA significantly increased the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, and a priming dose of 5 mg/kg reinstated the extinguished preference. The higher doses of Haloperidol, Raclopride and CGS 10746B and both doses of SCH 23390 blocked acquisition of the MDMA-induced CPP. However, only Haloperidol blocked expression of the CPP. Reinstatement of the extinguished preference was not affected by any of the drugs studied. Analysis of brain monoamines revealed that the blockade of CPP acquisition was accompanied by an increase in DA concentration in the striatum, with a concomitant decrease in DOPAC and HVA levels. Administration of haloperidol during the Post-C test produced increases in striatal serotonin, DOPAC and HVA concentrations. In mice treated with the higher doses of haloperidol and CGS an increase in SERT concentration in the striatum was detected during acquisition of the CPP, but no changes in DAT were observed.

Conclusions/Significance

These results demonstrate that, in adolescent mice, the dopaminergic system is involved in the acquisition and expression of MDMA-induced CPP, but not in its reinstatement.

The ugly side of amphetamines: short- and long-term toxicity of 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy'), methamphetamine and D-amphetamine

Amphetamine ('Speed'), methamphetamine ('Ice') and its congener 3,4-methylenedioxymethamphetamine (MDMA; 'Ecstasy') are illicit drugs abused worldwide for their euphoric and stimulant effects. Despite compelling evidence for chronic MDMA neurotoxicity in animal models, the physiological consequences of such toxicity in humans remain unclear. In addition, distinct differences in the metabolism and pharmacokinetics of MDMA between species and different strains of animals prevent the rationalisation of realistic human dose paradigms in animal studies. Here, we attempt to review amphetamine toxicity and in particular MDMA toxicity in the pathogenesis of exemplary human pathologies, independently of confounding environmental factors such as poly-drug use and drug purity.

Lack of genotype effect on D1, D2 receptors and dopamine transporter binding in triple MOP-, DOP-, and KOP-opioid receptor knockout mice of three different genetic backgrounds

We investigated D1, D2 receptors and dopamine transporter (DAT) binding levels in mice lacking all three opioid receptors and wild-type (WT) mice on three different genetic backgrounds. Quantitative autoradiography was used to determine the level of radioligand binding to the D1 and D2 receptors and DAT labeled with [(3)H]SCH23390, [(3)H]raclopride, and [(3)H]mazindol, respectively in triple-opioid receptor knockout (KO) and WT maintained on C57BL/6 (B6) and 129/SvEvTac (129) as well as C57BL/6 x 129/SvPas (B6 x 129) strains. No significant genotype effect was observed in D1, D2 receptors and DAT binding in any regions analyzed in any of the strains studied, suggesting that a lack of all three opioid receptors does not influence D1, D2 receptors and DAT expression, irrespective of their genetic strain background. However, strain differences were observed in D1 binding between the three strains of mice studied. Lower levels of D1 binding were observed in the substantia nigra of B6 x 129 WT mice compared with the 129 WT mice and in the olfactory tubercle of B6 x 129 WT compared with B6 WT and 129 WT mice. Lower levels of D1 binding were observed in the caudate putamen of B6 x 129 KO mice compared with 129 KO mice. In contrast, no significant strain differences were observed in D2 and DAT binding between the three strains of mice in any regions analyzed. Overall, these results indicate a lack of modulation of the dopaminergic system by the deletion of all three opioid receptors regardless of different background strains.

Is ecstasy a drug of dependence?

This paper examines the evidence for an MDMA or "ecstasy" dependence syndrome. Animal evidence suggests that MDMA may be a less potent reinforcer than other drugs, but that it does have dependence potential. This suggests that (a) ecstasy dependence might be less likely than dependence upon other drugs; and (b) factors related to the behavioural and psychological aspects of reward and dependence may make a relatively greater contribution for ecstasy than for other drugs, where physically centred (and better understood) features of dependence may be more salient. Human evidence supports this proposition. Some people report problems with their use, but the literature suggests that physical features play a more limited role than psychological ones. Tolerance is apparent, and withdrawal is self-reported, but it is unclear whether these reports distinguish sub-acute effects of ecstasy intoxication from symptoms reflective of neuroadaptive processes underlying a "true" withdrawal syndrome. Studies examining the structure of dependence upon ecstasy suggest it may be different from drugs such as alcohol, methamphetamine and opioids. Consistent with studies of hallucinogens, a two-factor structure has been identified with factors suggestive of "compulsive use" and "escalating use". Regardless of the nature of any dependence syndrome, however, there is evidence to suggest that a minority of ecstasy users become concerned about their use and seek treatment. Further controlled studies are required to investigate this phenomenon.

Test-re-test reliability of DSM-IV adopted criteria for 3,4-methylenedioxymethamphetamine (MDMA) abuse and dependence: a cross-national study

AIMS

This study evaluated the prevalence and reliability of DSM-IV adopted criteria for 3,4-methylenedioxymethamphetamine (MDMA) abuse and dependence with a purpose to determine whether it is best conceptualized within the category of hallucinogens, amphetamines or its own category.

DESIGN

Test-re-test study.

PARTICIPANTS

MDMA users (life-time use >5 times) were recruited in St Louis, Miami and Sydney (n=593). The median life-time MDMA consumption was 50 pills at the baseline.

MEASUREMENTS

The computerized Substance Abuse Module for Club Drug (CD-SAM) was used to assess MDMA abuse and dependence. The Discrepancy Interview Protocol (DIP) was used to determine the reasons for the discrepant responses between the two interviews. Reliability of diagnoses, individual diagnostic criteria and withdrawal symptoms was examined using the kappa coefficient (κ). findings for baseline data, 15% and 59% met MDMA abuse and dependence, respectively. Substantial test-re-test reliability of the diagnoses was observed consistently across cities (κ=0.69). 'Continued use despite knowledge of physical/psychological problems' (87%) and 'withdrawal' (68%) were the two most prevalent dependence criteria. 'Physically hazardous use' was the most prevalent abuse criterion. Six dependence criteria and all abuse criteria were reported reliably across cities (κ: 0.53-0.77). Seventeen of 19 withdrawal symptoms showed consistency in the reliability across cities. The most commonly reported reason for discrepant responses was 'interpretation of question changed'. Only a small proportion of the total discrepancies were attributed to lying or social desirability.

CONCLUSION

The adopted DSM-IV diagnostic classification for MDMA abuse and dependence was moderately reliable across cities. findings on MDMA withdrawal support the argument that MDMA should be separated from other hallucinogens in DSM.

Acquisition and reinstatement of MDMA-induced conditioned place preference in mice pre-treated with MDMA or cocaine during adolescence

Those who take ecstasy are more likely to consume other drugs than non-users with cocaine abuse being reported by 75.5% of high school student MDMA (+/- 3,4-methylenedioxymetamphetamine hydrochloride) users. The aim of this work was to evaluate the effects of exposure during adolescence to MDMA, cocaine or to both drugs on the MDMA-induced conditioned place preference (CPP) in adult mice. Animals received two daily administrations of saline, 10 mg/kg of MDMA, 25 mg/kg of cocaine or 10 mg/kg of MDMA plus 25 mg/kg of cocaine over 3 days (from PD28 to 30). Three weeks after pre-treatment, the MDMA-induced CPP procedure was initiated (PD52). Acquisition of CPP was induced with a sub-threshold dose of MDMA (1.25 mg/kg) only in animals treated during adolescence with MDMA alone. Preference was established in all the groups after conditioning with 10 mg/kg of MDMA, while the time required to achieve extinction was longer in those pre-treated with cocaine or MDMA alone (46 and 28 sessions, respectively). Moreover, preference was reinstated with progressively lower priming doses of MDMA in mice pre-treated with MDMA or cocaine alone. These results demonstrate that early exposure to MDMA or cocaine induces long-lasting changes that last until adulthood and modify the response of animals to MDMA.

MDMA reinstates cocaine-seeking behaviour in mice

BACKGROUND

MDMA effects are mediated by monoaminergic systems, which seem to play a central role in cocaine craving and relapse.

METHODS

CD1 mice trained to self-administer cocaine (1 mg/kg/infusion) underwent an extinction procedure in which the cues contingent with drug self-administration remained present. Mice achieving extinction were injected with MDMA (10 mg/kg), d-amphetamine (1 and 2 mg/kg) or saline and tested for reinstatement.

RESULTS

Acute MDMA, but not d-amphetamine or saline reinstated cocaine-seeking behaviour in mice in which cocaine self-administration and contingent cues were previously extinguished.

CONCLUSIONS

Acute MDMA can reinstate cocaine-seeking behaviour in mice.

MDMA, methamphetamine and their combination: possible lessons for party drug users from recent preclinical research

The substituted amphetamines 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') and methamphetamine (METH, 'ice', 'speed') are increasingly popular drugs amongst party-drug users. Studies with humans have investigated the acute and possible long-term adverse effects of these drugs, yet outcomes of such studies are often ambiguous due to a variety of confounding factors. Studies employing animal models have value in determining the acute and long-term effects of MDMA and METH on brain and behaviour. Self-administration studies show that intravenous METH is a particularly potent reinforcer in rats and other species. In contrast, MDMA appears to have powerful effects in enhancing social behaviour in laboratory animals. Brief exposure to MDMA or METH may produce long-term reductions in dopamine, serotonin and noradrenaline in the brain and alterations in the density of various receptor and transporter proteins. However it is still unclear, particularly in the case of MDMA, whether this reflects a 'neurotoxic' effect of the drug. Lasting alterations in social behaviour, anxiety, depressive symptoms and memory have been demonstrated in laboratory rats given MDMA or METH and this matches long-term changes reported in some human studies. Recent laboratory studies suggest that MDMA/METH combinations may produce greater adverse neurochemical and behavioural effects than either drug alone. This is of some concern given recent evidence that party drug users may be frequently exposed to this combination of drugs.

CB1 cannabinoid receptor modulates 3,4-methylenedioxymethamphetamine acute responses and reinforcement

BACKGROUND

3,4-Methylenedioxymethamphetamine (MDMA) is a popular recreational drug widely abused by young people. The endocannabinoid system is involved in the addictive processes induced by different drugs of abuse. However, the role of this system in the pharmacological effects of MDMA has not yet been clarified.

METHODS

Locomotion, body temperature, and anxiogenic-like responses were evaluated after acute MDMA administration in CB(1) cannabinoid receptor 1 knockout mice. Additionally, MDMA rewarding properties were investigated in the place conditioning and the intravenous self-administration paradigms. Extracellular levels of dopamine (DA) in the nucleus accumbens were also analyzed after a single administration of MDMA by in vivo microdialysis.

RESULTS

Acute MDMA administration increased locomotor activity, body temperature, and anxiogenic-like responses in wild-type mice, but these responses were lower or abolished in knockout animals. 3,4-Methylenedioxymethamphetamine produced similar conditioned place preference and increased dopamine extracellular levels in the nucleus accumbens in both genotypes. Nevertheless, CB(1) knockout mice failed to self-administer MDMA at any of the doses used.

CONCLUSIONS

These results indicate that CB(1) cannabinoid receptors play an important role in the acute prototypical effects of MDMA and are essential in the acquisition of an operant behavior to self-administer this drug.

MDMA modifies active avoidance learning and recall in mice

RATIONALE

Several studies have suggested the existence of cognitive deficits after repeated or high doses of 3,4-methylenedioxymethamphetamine (MDMA) in humans and experimental animals. However, the extent of the impairments observed in learning or memory tasks remains unclear.

OBJECTIVE

The objective of this study was to evaluate the effects of different dosing regimens of MDMA on the ability of mice to learn and recall an active avoidance task.

MATERIALS AND METHODS

Animals were treated with MDMA (0, 1, 3, 10 and 30 mg/kg) under four different experimental conditions, and active avoidance acquisition and recall were evaluated. In experiments 1 and 2, MDMA was administered 1 h before different active avoidance training sessions. In experiments 3 and 4, mice received a repeated treatment with MDMA before or after active avoidance training, respectively. Changes in presynaptic striatal dopamine transporter (DAT) binding sites were evaluated at two different time points in animals receiving a high dose of MDMA (30 mg/kg) or saline twice a day over 4 days.

RESULTS

MDMA administered before the active avoidance sessions interfered with the acquisition and the execution of a previously learned task. A repeated treatment with high doses of MDMA administered before training reduced acquisition of active avoidance in mice, while pre-treatment with both high and low doses of MDMA impaired recall of this task. A reduction in DAT binding was observed 4 days but not 23 days after the last MDMA administration.

CONCLUSIONS

Acute MDMA modifies the acquisition and execution of active avoidance in mice, while repeated pre-treatment with MDMA impairs acquisition and recall of this task.

Behavioural and neurochemical effects of combined MDMA and THC administration in mice

RATIONALE

Cannabis is the most widely consumed drug associated with 3,4-methylenedioxymethamphetamine (MDMA) use.

OBJECTIVES

This study examines whether low doses of MDMA and delta-9-tetrahydrocannabinol (THC) produce synergistic rewarding/reinforcing effects in mice using the conditioned place preference (CPP) and operant self-administration paradigms. Changes in dopamine (DA) outflow were monitored in the nucleus accumbens (NAC) after single or combined administration of these compounds.

RESULTS

MDMA induced a significant CPP at the dose of 10 mg/kg but not at the dose of 3 mg/kg. THC (0.3 mg/kg) by itself was also ineffective in this paradigm. The combined administration of the low dose of MDMA (3 mg/kg) and THC (0.3 mg/kg) produced CPP, whereas the combination of MDMA (10 mg/kg) and THC (0.3 mg/kg) significantly decreased CPP. Animals treated with THC self-administered a sub-threshold dose of MDMA (0.06 mg/kg per infusion), while animals receiving vehicle did not. However, THC did not modify the self-administration of an effective dose of MDMA (0.125 mg/kg per infusion). In microdialysis studies, a low dose of THC significantly increased DA outflow in the NAC, while a low dose of MDMA did not. When MDMA was administered before THC, DA levels decreased with respect to THC. However, when THC was administered before MDMA, DA levels were not significantly modified with respect to THC.

CONCLUSIONS

These results demonstrate that a low dose of THC modifies in different ways (increases and decreases) the sensitivity of animals to the behavioural effects of MDMA and that THC and MDMA converge at a common mechanism modulating DA outflow in the NAC of mice.

3,4-methylenedioxymethamphetamine self-administration is abolished in serotonin transporter knockout mice

BACKGROUND

The neurobiological mechanism underlying the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA) remains unclear. The aim of the present study was to determine the contribution of the serotonin transporter (SERT) in MDMA self-administration behavior by using knockout (KO) mice deficient in SERT.

METHODS

Knockout mice and wild-type (WT) littermates were trained to acquire intravenous self-administration of MDMA (0, .03, .06, .125, and .25 mg/kg/infusion) on a fixed ratio 1 (FR1) schedule of reinforcement. Additional groups of mice were trained to obtain food and water to rule out operant responding impairments. Microdialysis studies were performed to evaluate dopamine (DA) and serotonin (5-HT) extracellular levels in the nucleus accumbens (NAC) and prefrontal cortex (PFC), respectively, after acute MDMA (10 mg/kg).

RESULTS

None of the MDMA doses tested maintained intravenous self-administration in KO animals, whereas WT mice acquired responding for MDMA. Acquisition of operant responding for food and water was delayed in KO mice, but no differences between genotypes were observed on the last day of training. MDMA increased DA extracellular levels to a similar extent in the NAC of WT and KO mice. Conversely, extracellular concentrations of 5-HT in the PFC were increased following MDMA only in WT mice.

CONCLUSIONS

These findings provide evidence for the specific involvement of SERT in MDMA reinforcing properties.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Rewarding effects and reinstatement of MDMA-induced CPP in adolescent mice

Although the rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in self-administration and conditioned place preference (CPP) procedures, its addictive potential (ie, the vulnerability to relapse, measured by its ability to induce reinstatement of an extinguished response), remains poorly understood. In this study, the effects of MDMA (5, 10, and 20 mg/kg) on the acquisition, extinction and reinstatement of CPP were evaluated in mice, using two different protocols during acquisition of CPP. In the first experiment, animals were trained using a two-session/day schedule (MDMA and saline for 4 consecutive days), whereas in the second experiment, they were trained using an alternating day schedule (MDMA and saline each 48 h). After extinction, the ability of drug priming to reinstate CPP was evaluated. In Experiment 1, MDMA did not significantly increase the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, although the preference was evident a week afterwards, lasting between 2 and 21 weeks. No reinstatement was observed after MDMA priming. In Experiment 2, all doses produced CPP in Post-C, which lasted between 1 and 4 weeks. MDMA induces reinstatement at doses up to 4 times lower than those used in conditioning. The analyses of brain monoamines revealed that the daily schedule of treatment induces a non-dose-dependent decrease in dopamine and serotonin (5-HT) in the striatum, whereas the alternating schedule produces a dose-dependent decrease of 5-HT in the cortex. These results demonstrate that MDMA produces long-lasting rewarding effects and reinstatement after extinction, suggesting the susceptibility of this drug to induce addiction.

MDMA attenuates THC withdrawal syndrome in mice

INTRODUCTION

3, 4-Methylenedioxymethamphetamine (MDMA) and cannabis are widely abused illicit drugs that are frequently consumed in combination. Interactions between these two drugs have been reported in several pharmacological responses observed in animals, such as body temperature, anxiety, cognition, and reward. However, the interaction between MDMA and cannabis in addictive processes such as physical dependence has not been elucidated yet.

DISCUSSION

In this study, the effects of acute and chronic MDMA were evaluated on the behavioral manifestations of Delta(9)-tetrahydrocannabinol (THC) abstinence in mice. THC withdrawal syndrome was precipitated by injecting the cannabinoid antagonist rimonabant (10 mg/kg, i.p.) in mice chronically treated with THC and receiving MDMA (2.5, 5 and 10 mg/kg i.p.) or saline just before the withdrawal induction or chronically after the THC administration.

RESULTS

Both chronic and acute MDMA decreased in a dose-dependent manner the severity of THC withdrawal. In vivo microdialysis experiments showed that acute MDMA (5 mg/kg, i.p.) administration increased extracellular serotonin levels in the prefrontal cortex, but not dopamine levels in the nucleus accumbens. Our results also indicate that the attenuation of THC abstinence symptoms was not due to a direct interaction between rimonabant and MDMA nor to the result of the locomotor stimulating effects of MDMA.

CONCLUSION

The modulation of the cannabinoid withdrawal syndrome by acute or chronic MDMA suggests a possible mechanism to explain the associated consumption of these two drugs in humans.

Rewarding effects of 3,4-methylenedioxymethamphetamine ("Ecstasy") in dominant and subordinate OF-1 mice in the place preference conditioning paradigm

We tested the ability of 3,4-methylenedioxymethamphetamine (MDMA) to induce conditioned place preference (CPP) in dominant and subordinate OF-1 mice subjected to cohabitation and repeated sessions of agonistic confrontation, as well as in non-confronted mice. We selected doses of MDMA (2, 6, 10 mg/kg) previously reported to induce CPP in mice and we measured expression of c-Fos evoked by the treatments in non-confronted mice. MDMA induced c-Fos protein in several corticolimbic regions involved in drug-induced reward. Mice were exposed to brief sessions of agonistic confrontation on 5 consecutive days. Determinations of circulating hormones and drug conditioning tests were carried out on completion of the encounters. The results of hormone assays indicated that dominant mice had higher serum concentrations of testosterone, but lower levels of corticosterone, than submissive mice. Post-conditioning tests after drug conditioning (4 injections of MDMA or saline on alternate days) showed that MDMA significantly produced CPP at doses of 2 and 6 mg/kg, but not at 10 mg/kg, an inverted U-shaped pattern of conditioning that was invariable in non-confronted, dominant and subordinate mice. These results demonstrate that the endocrine and behavioural correlates linked to social status and social stress in mice are not paralleled by significant changes in the rewarding efficacy of MDMA in the CPP paradigm under the specific conditions tested.

Long-term voluntary consumption of MDMA and THC in rats is modified by individual and situational factors

Factors influencing long-term voluntary intake of 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) and Delta9-tetrahydrocannabinol (THC; cannabis) were studied in a rat model. Sixty-four male Wistar rats were given free choice for 49 weeks between (1) water and MDMA solution, (2) water and THC solution or (3) water, MDMA solution and THC solution. After the first experiences with the drugs, animals of both monodrug and polydrug group developed an individually stable pattern of MDMA consumption, whereas the individual predictability of THC consumption remained poor. While THC consumption was maintained for the whole experimental period, MDMA consumption decreased with time and was nearly ceased after 3-7 months. Intake of both drugs was adapted to social changes, with THC consumption being more sensitive to social changes than MDMA consumption. In the re-test after 4 months of abstinence, all animals ceased drug consumption when the drug solutions were adulterated with bitter tasting quinine. The results show that the rats had maintained a flexible mode of drug consumption and had not become addicted. Response to novelty of the rats in test trials before the start of drug supply correlated with later MDMA intake. In conclusion, although very low amounts of MDMA and/or THC were consumed, the findings that drug-experienced animals responded differentially to a stressor and that housing conditions influenced drug intake suggests that MDMA and THC can induce psychopharmacological effects in our long-term voluntary consumption paradigm.

Role of nitrergic system in behavioral and neurotoxic effects of amphetamine analogs

Several amphetamine analogs are potent psychostimulants and major drugs of abuse. In animal models, the psychomotor and reinforcing effects of amphetamine, methamphetamine (METH), 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy), and methylphenidate (MPD; Ritalin) are thought to be dependent on increased extracellular levels of dopamine (DA) in mesocorticolimbic and mesostriatal pathways. However, amphetamine analogs that increase primarily serotonergic transmission, such as p-chloroamphetamine (PCA) and fenfluramine (FEN), have no potential for abuse. High doses of METH, MDMA, PCA, and FEN produce depletions of dopaminergic and serotonergic nerve terminal markers and are considered as potential neurotoxicants. The first part of this review briefly summarizes the behavioral and neurotoxic effects of amphetamines that have a different spectrum of activity on dopaminergic and serotonergic systems. The second part discusses evidence supporting involvement of the nitrergic system in dopamine-mediated effects of amphetamines. The nitrergic system in this context corresponds to nitric oxide (NO) produced from neuronal nitric oxide synthase (nNOS) that has roles in nonsynaptic interneuronal communication and excitotoxic neuronal injury. Increasing evidence now suggests cross talk between dopamine, glutamate, and NO. Results from our laboratory indicate that dopamine-dependent psychomotor, reinforcing, and neurotoxic effects of amphetamines are diminished by pharmacological blockade of nNOS or deletion of the nNOS gene. These findings, and evidence supporting the role of NO in synaptic plasticity and neurotoxic insults, suggest that NO functions as a neuronal messenger and a neurotoxicant subsequent to exposure to amphetamine-like psychostimulants.

A reliable model of intravenous MDMA self-administration in naïve mice

RATIONALE

MDMA is one of the most widely consumed recreational drugs in Europe. However, the mechanisms involved in the reinforcing properties of MDMA are still unclear. In this sense, the establishment of a reliable model of MDMA self-administration in mice could represent an important approach to study the neuronal substrates associated with MDMA reward by using genetically modified mice.

OBJECTIVES

To develop a reliable model of operant intravenous MDMA self-administration in drug-naïve mice.

MATERIALS AND METHODS

Mice were trained to acquire intravenous self-administration of MDMA at different doses (0, 0.06, 0.125, 0.25, 0.5 and 1.0 mg/kg/infusion) on a FR1 schedule of reinforcement for 15 consecutive days. The motivational value of different doses of MDMA (0.125, 0.25 and 0.5 mg/kg/infusion) was then tested using a progressive ratio paradigm. Finally, [3H]-mazindol autoradiographic studies were carried out in order to quantitatively assess presynaptic dopamine transporter (DAT) binding sites in the striatum of mice trained to self-administer MDMA (0 and 1.0 mg/kg/infusion) during 15 days.

RESULTS

The latency for discrimination between the active and inactive holes, as well as the number of animals acquiring stability criteria, varied as a function of the dose of MDMA. The mice responding for intermediate doses (0.125, 0.25 and 0.5 mg/kg/infusion) discriminated earlier than those responding for low (0.06 mg/kg/infusion) or high (1.0 mg/kg/infusion) doses. The percentage of animals achieving stability criteria increased with days of testing and was inversely proportional to the dose of MDMA. The breaking points achieved for doses of 0.125 and 0.25 mg/kg/infusion were significantly higher than for a dose of 0.5 mg/kg/infusion. No significant DAT neurotoxicity was observed in the striatum of animals self-administering MDMA at a dose of 1 mg/kg/infusion.

CONCLUSIONS

The present results show that MDMA can be reliably self-administered by drug-naïve mice.

Analysis of transcriptional responses in the mouse dorsal striatum following acute 3,4-methylenedioxymethamphetamine (ecstasy): identification of extracellular signal-regulated kinase-controlled genes

3,4-Methylenedioxymethamphetamine (ecstasy), a widely used recreational drug with psychoactive properties, induces both serotonin and dopamine release in the brain. However, little is known about its intracellular effects. We previously showed that 3,4-methylenedioxymethamphetamine rewarding effects in mice were dependent upon extracellular signal-regulated kinase activation and that dorsal striatum was a critical region for mediating extracellular signal-regulated kinase-dependent Egr1 3,4-methylenedioxymethamphetamine-induced transcription. Here, we extend these findings by showing that 3,4-methylenedioxymethamphetamine is indeed able to activate extracellular signal-regulated kinase within this structure. To identify genes regulated by acute 3,4-methylenedioxymethamphetamine in the mice dorsal striatum, and selectively controlled by this kinase, we performed microarray experiments by using a selective inhibitor of extracellular signal-regulated kinase activation, SL327. Of the approximately 24,000 genes from the microarray, 27 showed altered expression after exposure to 3,4-methylenedioxymethamphetamine, and among these, 59% were partially or totally inhibited by SL327 pretreatment. Our results showed that the genes regulated by 3,4-methylenedioxymethamphetamine encode proteins that belong to transcription factors family, signaling pathways (phosphatases, cytoskeleton regulation), and synaptic functions. These early changes, and especially those controlled by extracellular signal-regulated kinase activation might play significant roles in the expression of many of the behaviors that occur following 3,4-methylenedioxymethamphetamine taking.

Chronic and acute effects of 3,4-methylenedioxy-N-methylamphetamine ('Ecstasy') administration on the dynorphinergic system in the rat brain

The prodynorphin system is implicated in the neurochemical mechanism of psychostimulants. Exposure to different drugs of abuse can induce neuroadaptations in the brain and affect opioid gene expression. The present study aims to examine the possibility of a common neurobiological substrate in drug addiction processes. We studied the effects of single and repeated 3,4-methylenedioxy-N-methylamphetamine ('Ecstasy') on the gene expression of the opioid precursor prodynorphin, and on the levels of peptide dynorphin A in the rat brain. Acute (8 mg/kg, intraperitoneally) 3,4-methylenedioxy-N-methylamphetamine markedly raised, two hours later, prodynorphin mRNA levels in the prefrontal cortex, and in the caudate putamen, whereas it decreased gene expression in the ventral tegmental area. Chronic (8 mg/kg, intraperitoneally, twice a day for 7 days) 3,4-methylenedioxy-N-methylamphetamine increased prodynorphin mRNA in the nucleus accumbens, hypothalamus and caudate putamen and decreased it in the ventral tegmental area. Dynorphin A levels increased after chronic treatment in the ventral tegmental area and decreased after acute treatment in the nucleus accumbens, prefrontal cortex and hypothalamus. These findings confirm the role of the dynorphinergic system in mediating the effects of drugs of abuse, such as 3,4-methylenedioxy-N-methylamphetamine, in various regions of the rat brain, which may be important sites for the opioidergic mechanisms activated by addictive drugs.

Effects of the plant-derived hallucinogen salvinorin A on basal dopamine levels in the caudate putamen and in a conditioned place aversion assay in mice: agonist actions at kappa opioid receptors

RATIONALE

Salvinorin A is a naturally occurring hallucinogen derived from the plant Salvia divinorum. Salvinorin A is also a potent and selective kappa opioid receptor agonist in vitro. It has been shown that kappa agonists decrease dopamine levels in the caudate putamen and nucleus accumbens and cause conditioned place aversion in rodents.

OBJECTIVES

To study the effects of salvinorin A on basal dopamine levels in the caudate putamen and nucleus accumbens, and to determine whether salvinorin A induces conditioned place preference or aversion and changes in locomotor activity in the mouse.

METHODS

In the first experiment, changes in dopamine levels in these brain regions after administration of salvinorin A were measured with in vivo microdialysis. In the second experiment, we examined whether salvinorin A led to conditioned place preference or aversion, and changes in locomotor activity during conditioning sessions.

RESULTS

The higher doses of salvinorin A studied (1.0 mg/kg and 3.2 mg/kg, i.p.) significantly decreased dopamine levels in the caudate putamen, but not in the nucleus accumbens, and this effect was completely blocked by pre-injection with 10 mg/kg of the kappa opioid receptor antagonist nor-binaltorphimine. The same doses of salvinorin A caused conditioned place aversion and decreased locomotor activity.

CONCLUSIONS

The inhibitory effect of salvinorin A on striatal dopamine levels may contribute to its induction of conditioned place aversion and decreases in locomotion in mice. These findings are consistent with the in vitro characterization of salvinorin A as a kappa opioid receptor agonist. It is of interest that a compound such as salvinorin A, that lowers striatal dopamine levels and leads to conditioned place aversion in rodents, is self-administered by humans under certain conditions.

The rewarding properties of MDMA are preserved in mice lacking mu-opioid receptors

The involvement of mu-opioid receptors in the rewarding properties of MDMA was explored in mu-opioid receptor knockout mice using the conditioning place preference paradigm. The associated release of dopamine in the nucleus accumbens was investigated by in vivo microdialysis. A significant rewarding effect of MDMA (10 mg/kg, i.p.) was observed in both wild-type and mu-opioid receptor knockout mice. MDMA (10 mg/kg, i.p.) also induced similar increases in dopamine and decreases in 3,4-dihydroxyphenylacetic acid and homovanillic acid in the nucleus accumbens dialysates of both wild-type and mu-opioid receptor knockout mice. No significant differences in basal levels of dopamine, 3,4-dihydroxyphenylacetic or homovanillic acids between wild-type and mu-opioid receptor knockout mice were observed. In summary, the present results suggest that, in contrast to what has been reported for other drugs of abuse such as opioids, ethanol, nicotine and Delta(9)-tetrahydrocannabinol, mu-opioid receptors do not play a major role in the rewarding properties of MDMA. These differences could be due to distinct mechanisms controlling dopamine release in the nucleus accumbens and suggest that the effects of MDMA on dopaminergic neurons are independent of micro -opioid receptors.