Acquisition of MDMA self-administration: pharmacokinetic factors and MDMA-induced serotonin release

Behavioral metabolomics: how behavioral data can guide metabolomics research on neuropsychiatric disorders

INTRODUCTION

Metabolomics produces vast quantities of data but determining which metabolites are the most relevant to the disease or disorder of interest can be challenging.

OBJECTIVES

This study sought to demonstrate how behavioral models of psychiatric disorders can be combined with metabolomics research to overcome this limitation.

METHODS

We designed a preclinical, untargeted metabolomics procedure, that focuses on the determination of central metabolites relevant to substance use disorders that are (a) associated with changes in behavior produced by acute drug exposure and (b) impacted by repeated drug exposure. Untargeted metabolomics analysis was carried out on liquid chromatography-mass spectrometry data obtained from 336 microdialysis samples. Samples were collected from the medial striatum of male Sprague-Dawley (N = 21) rats whilst behavioral data were simultaneously collected as part of a (±)-3,4-methylenedioxymethamphetamine (MDMA)-induced behavioral sensitization experiment. Analysis was conducted by orthogonal partial least squares, where the Y variable was the behavioral data, and the X variables were the relative concentrations of the 737 detected features.

RESULTS

MDMA and its derivatives, serotonin, and several dopamine/norepinephrine metabolites were the greatest predictors of acute MDMA-produced behavior. Subsequent univariate analyses showed that repeated MDMA exposure produced significant changes in MDMA metabolism, which may contribute to the increased abuse liability of the drug as a function of repeated exposure.

CONCLUSION

These findings highlight how the inclusion of behavioral data can guide metabolomics data analysis and increase the relevance of the results to the phenotype of interest.

Self-Administration of Entactogen Psychostimulants Dysregulates Gamma-Aminobutyric Acid (GABA) and Kappa Opioid Receptor Signaling in the Central Nucleus of the Amygdala of Female Wistar Rats

Male rats escalate intravenous self-administration of entactogen psychostimulants, 3,4-methylenedioxymethcathinone (methylone) and 3,4-methylenedioxymethamphetamine (MDMA) under extended access conditions, as with typical psychostimulants. Here, we investigated whether female rats escalate self-administration of methylone, 3,4-methylenedioxypentedrone (pentylone), and MDMA and then studied consequences of MDMA and pentylone self-administration on GABAA receptor and kappa opioid receptor (KOR) signaling in the central nucleus of the amygdala (CeA), a brain area critically dysregulated by extended access self-administration of alcohol or cocaine. Adult female Wistar rats were trained to self-administer methylone, pentylone, MDMA (0.5 mg/kg/infusion), or saline-vehicle using a fixed-ratio 1 response contingency in 6-h sessions (long-access: LgA) followed by progressive ratio (PR) dose-response testing. The effects of pentylone-LgA, MDMA-LgA and saline on basal GABAergic transmission (miniature post-synaptic inhibitory currents, mIPSCs) and the modulatory role of KOR at CeA GABAergic synapses were determined in acute brain slices using whole-cell patch-clamp. Methylone-LgA and pentylone-LgA rats similarly escalated their drug intake (both obtained more infusions compared to MDMA-LgA rats), however, pentylone-LgA rats reached higher breakpoints in PR tests. At the cellular level, baseline CeA GABA transmission was markedly elevated in pentylone-LgA and MDMA-LgA rats compared to saline-vehicle. Specifically, pentylone-LgA was associated with increased CeA mIPSC frequency (GABA release) and amplitude (post-synaptic GABAA receptor function), while mIPSC amplitudes (but not frequency) was larger in MDMA-LgA rats compared to saline rats. In addition, pentylone-LgA and MDMA-LgA profoundly disrupted CeA KOR signaling such as both KOR agonism (1 mM U50488) and KOR antagonism (200 nM nor-binaltorphimine) decreased mIPSC frequency suggesting recruitment of non-canonical KOR signaling pathways. This study confirms escalated self-administration of entactogen psychostimulants under LgA conditions in female rats which is accompanied by increased CeA GABAergic inhibition and altered KOR signaling. Collectively, our study suggests that CeA GABA and KOR mechanisms play a critical role in entactogen self-administration like those observed with escalation of alcohol or cocaine self-administration.

Lack of evidence for positive reinforcing and prosocial effects of MDMA in pair-housed male and female rats

3,4-Methylenedioxymethamphetamine (MDMA) is classified as an entactogen, producing feelings of emotional openness and relatedness. One unique feature of MDMA is that people tend to selectively take this drug in social and/or intimate situations. Although MDMA is recognized as having abuse liability, preclinical studies report that it has weak reinforcing effects in animals. The objective of this study was to characterize the positive reinforcing and prosocial effects of MDMA in a translational model of the social environment in which two rats have simultaneous and contingent access to MDMA in close physical proximity. To this end, MDMA self-administration was examined on both fixed and progressive ratio schedules of reinforcement in six groups of rats: (1) isolated males, (2) isolated females, (3) male-male dyads, (4) female-female dyads, (5) male-female dyads, and (6) female-male dyads. For pair-housed rats, data from both rats were analyzed. Next, social preferences were examined in a partner preference test. MDMA failed to produce positive reinforcing effects under all conditions examined. Across a 30-fold dose range (0.01-1.0 mg/kg/infusion), MDMA did not maintain higher responding than saline on both schedules of reinforcement and in all groups tested. In partner preference tests, a history of shared exposure to MDMA did not establish a social preference, and acute administration of MDMA failed to establish a preference for another MDMA-treated rat. These data suggest that social contact does not increase the positive reinforcing effects of MDMA in rats, and that neither contingent nor noncontingent MDMA administration establishes a social preference in rats.

Methylenedioxymethamphetamine (MDMA): Serotonergic and dopaminergic mechanisms related to its use and misuse

Methylenedioxymethamphetamine (MDMA) is an amphetamine analogue that preferentially stimulates the release of serotonin (5HT) and results in relatively small increases in synaptic dopamine (DA). The ratio of drug-stimulated increases in synaptic DA, relative to 5HT, predicts the abuse liability; drugs with higher DA:5HT ratios are more likely to be abused. Nonetheless, MDMA is a drug that is misused. Clinical and preclinical studies have suggested that repeated MDMA exposure produces neuroadaptive responses in both 5HT and DA neurotransmission that might explain the development and maintenance of MDMA self-administration in some laboratory animals and the development of a substance use disorder in some humans. In this paper, we describe the research that has demonstrated an inhibitory effect of 5HT on the acquisition of MDMA self-administration and the critical role of DA in the maintenance of MDMA self-administration in laboratory animals. We then describe the circuitry and 5HT receptors that are positioned to modulate DA activity and review the limited research on the effects of MDMA exposure on these receptor mechanisms.

Reinforcing effects of synthetic cathinones in rhesus monkeys: Dose-response and behavioral economic analyses

The abuse of synthetic cathinones ("bath salts") with psychomotor stimulant and/or entactogenic properties emerged as a public health concern when they were introduced as "legal" alternatives to drugs of abuse such as cocaine or MDMA. In this study, experiments were conducted in nonhuman primates to examine how differences in transporter selectivity might impact the reinforcing effects of synthetic cathinones. Rhesus monkeys (N = 5) were trained to respond for intravenous injections under a fixed-ratio (FR) 30, timeout 60-s schedule of reinforcement. The reinforcing effects of selected cathinones (e.g., MDPV, αPVP, MCAT, and methylone) with a range of pharmacological effects at dopamine and serotonin transporters were compared to cocaine and MDMA using dose-response analysis under a simple FR schedule and behavioral economic procedures that generated demand curves for two doses of each drug. Results show that one or more doses of all drugs were readily self-administered in each subject and, excepting MDMA (21 injections/session), peak levels of self-administration were similar across drugs (between 30 and 40 injections/session). Demand elasticity for the peak and the peak + 1/2-log dose of each drug did not significantly differ, and when data for the two doses were averaged for each drug, the following rank-order of reinforcing strength emerged: cocaine > MCAT = MDPV = methylone > αPVP = MDMA. These results indicate that the reinforcing strength of synthetic cathinones are not related to their selectivity in binding dopamine or serotonin transporter sites.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

5-HT1A and α2 adrenergic receptor levels are associated with high anxiety-like patterns and impulsivity in selectively bred Long Evans rats

Impulsivity and anxiety are psychological traits involved in many aspects of the drug addiction cycle. However, few preclinical models exist for examining both impulsive and anxiety patterns. In the current study, we investigated whether 6^th generation rats selectively bred for high anxiety (HAn)-like behavior would display amphetamine (AMPH) hyperactivity. In the same generational line, we also determined if HAn animals would display impulsivity in an operant task. Filial 5 male Long Evans rats phenotyped as HAn and low anxiety (LAn) were tested on the elevated plus maze (EPM) and in locomotor chambers following a low dose of AMPH (0.5 mg/kg, IP). Next, a separate group of F5 animals was exposed to a differential reinforcement of low rate of responding (DRL: 30 s) operant schedule to assess impulsivity. Postmortem, 5-HT1A and α2 adrenergic receptor protein levels were measured in the medial prefrontal cortex (mPFC), nucleus accumbens (NAc) core and shell, and α2 adrenergic counts were assessed in the locus coeruleus (LC), and the paraventricular nucleus (PVN) of the hypothalamus. F5 outbred HAn rats had decreased percent open arm time and entries on the EPM and elevated AMPH-induced locomotion. In the DRL, HAn rats displayed an impulsive profile, they attained fewer total rewards, had more inter-response times, and showed greater burst ratios. We found that HAn rats had a higher number of 5-HT1A receptor immunostained cells in the mPFC but were not different than LAn in NAc core or shell. By contrast, levels of the α2 adrenergic receptor protein were no different in the mPFC while HAn rats had greater levels in the LC and lower levels in the PVN. Overall, these data further validate our outbred trait anxiety rats: HAn males show anxiety-like behavior, AMPH hypersensitivity, greater impulsivity, and varying levels of limbic and midbrain 5-HT1A and α2 adrenergic receptor proteins.

REBUS and the Anarchic Brain: Toward a Unified Model of the Brain Action of Psychedelics

This paper formulates the action of psychedelics by integrating the free-energy principle and entropic brain hypothesis. We call this formulation relaxed beliefs under psychedelics (REBUS) and the anarchic brain, founded on the principle that-via their entropic effect on spontaneous cortical activity-psychedelics work to relax the precision of high-level priors or beliefs, thereby liberating bottom-up information flow, particularly via intrinsic sources such as the limbic system. We assemble evidence for this model and show how it can explain a broad range of phenomena associated with the psychedelic experience. With regard to their potential therapeutic use, we propose that psychedelics work to relax the precision weighting of pathologically overweighted priors underpinning various expressions of mental illness. We propose that this process entails an increased sensitization of high-level priors to bottom-up signaling (stemming from intrinsic sources), and that this heightened sensitivity enables the potential revision and deweighting of overweighted priors. We end by discussing further implications of the model, such as that psychedelics can bring about the revision of other heavily weighted high-level priors, not directly related to mental health, such as those underlying partisan and/or overly-confident political, religious, and/or philosophical perspectives. SIGNIFICANCE STATEMENT: Psychedelics are capturing interest, with efforts underway to bring psilocybin therapy to marketing authorisation and legal access within a decade, spearheaded by the findings of a series of phase 2 trials. In this climate, a compelling unified model of how psychedelics alter brain function to alter consciousness would have appeal. Towards this end, we have sought to integrate a leading model of global brain function, hierarchical predictive coding, with an often-cited model of the acute action of psychedelics, the entropic brain hypothesis. The resulting synthesis states that psychedelics work to relax high-level priors, sensitising them to liberated bottom-up information flow, which, with the right intention, care provision and context, can help guide and cultivate the revision of entrenched pathological priors.

Receptor-Enriched Analysis of functional connectivity by targets (REACT): A novel, multimodal analytical approach informed by PET to study the pharmacodynamic response of the brain under MDMA

One of the main limitations of pharmacological fMRI is its inability to provide a molecular insight into the main effect of compounds, leaving an open question about the relationship between drug effects and haemodynamic response. The aim of this study is to investigate the acute effects of 3,4-methylenedioxymethamphetamine (MDMA) on functional connectivity (FC) using a novel multimodal method (Receptor-Enriched Analysis of functional Connectivity by Targets - REACT). This approach enriches the resting state (rs-)fMRI analysis with the molecular information about the distribution density of serotonin receptors in the brain, given the serotonergic action of MDMA.

Twenty healthy subjects participated in this double-blind, placebo-controlled, crossover study. A high-resolution in vivo atlas of four serotonin receptors (5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₄) and its transporter (5-HTT) was used as a template in a two-step multivariate regression analysis to estimate the spatial maps reflecting the whole-brain connectivity behaviour related to each target under placebo and MDMA.

Results showed that the networks exhibiting significant changes after MDMA administration are the ones informed by the 5-HTT and 5-HT_1A distribution density maps, which are the main targets of this compound. Changes in the 5-HT_1A-enriched functional maps were also associated with the pharmacokinetic levels of MDMA and MDMA-induced FC changes in the 5-HT_2A-enriched maps correlated with the spiritual experience subscale of the Altered States of Consciousness Questionnaire.

By enriching the rs-fMRI analysis with molecular data of voxel-wise distribution of the serotonin receptors across the brain, we showed that MDMA effects on FC can be understood through the distribution of its main targets. This result supports the ability of this method to characterise the specificity of the functional response of the brain to MDMA binding to serotonergic receptors, paving the way to the definition of a new fingerprint in the characterization of new compounds and potentially to a further understanding to the response to treatment.

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MDMA connectivity effects understood through the distribution of 5-HT_1A and 5-HTT.

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Direct link between PK levels of MDMA and 5-HT_1A-enriched functional connectivity maps.

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Ability to link receptor targets to functional mechanisms underlying behaviour.

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Mapping pharmacodynamic effects onto the drug's molecular targets.

Effects of repeated treatment with methcathinone, mephedrone, and fenfluramine on intracranial self-stimulation in rats

RATIONALE

Synthetic cathinones constitute a class of abused drugs that can act at dopamine, norepinephrine, and serotonin transporters (DAT, NET, and SERT, respectively). Intracranial self-stimulation (ICSS) is a preclinical procedure that can be used to evaluate abuse potential of drugs, and prior studies have indicated that abuse-related ICSS effects of monoamine-transporter substrates, including some synthetic cathinones, are positively correlated with drug selectivity for DAT vs. SERT. Abuse potential of drugs can also be influenced by regimens of repeated drug exposure, but the role of repeated exposure on abuse-related ICSS effects of synthetic cathinones has not been examined.

OBJECTIVES

This study used ICSS to evaluate effects of repeated treatment with the DAT>SERT substrate methcathinone, the DAT<SERT substrate fenfluramine, and the DAT≈SERT substrate mephedrone.

METHODS

Male Sprague-Dawley rats were trained in a frequency-rate ICSS procedure, and different groups were used to evaluate effects of methcathinone, mephedrone, and fenfluramine before, during, and after regimens of repeated treatment with the designated drug.

RESULTS

Before repeated treatment, methcathinone produced dose-dependent and abuse-related ICSS facilitation, fenfluramine produced dose-dependent ICSS depression, and mephedrone produced mixed effects that included both facilitation and depression. Chronic treatment produced no change in effects of methcathinone, but complete tolerance to effects of fenfluramine. For mephedrone, chronic treatment produced partial tolerance to ICSS depression and enhanced expression of ICSS facilitation.

CONCLUSIONS

Repeated exposure to mixed-action DAT≈SERT substrates such as mephedrone can result in increased abuse potential due to sustained expression of DAT-mediated abuse-related effects and tolerance to SERT-mediated abuse-limiting effects.

Effects of cocaine on the discriminative stimulus and reinforcing effects of mephedrone in male rats

RATIONALE

Abuse of cathinones has been a worldwide health concern for some time. Their chemical structures and wide variation in pharmacodynamic effects have led to clinical and preclinical effects that can be both similar to and different from other psychoactive substances such as methylenedioxymethamphetamine (MDMA), methamphetamine, and cocaine.

OBJECTIVE

The present study examined the discriminative stimulus and reinforcing effects of mephedrone to further characterize the behavioral and pharmacological profile of this first-generation substituted methcathinone.

METHODS

Rats were trained to discriminate mephedrone (3.2 mg/kg) from saline under a fixed-ratio 20 (FR-20) schedule of food presentation. After establishing dose-effect curves for increasing cumulative doses of mephedrone, substitution tests were conducted with bupropion (5.6-32 mg/kg), cocaine (1.8-18 mg/kg), morphine (0.56-10 mg/kg), and amitriptyline (3.2-32 mg/kg). In addition, cocaine (3.2-18 mg/kg) and the serotonin type-2 (5-HT2) receptor antagonist ritanserin (1, 3.2, and 10 mg/kg) were administered prior to the cumulative doses of mephedrone. Lastly, varying infusion doses of cocaine were substituted for mephedrone in subjects trained to self-administer mephedrone, and varying infusion doses of mephedrone were substituted for cocaine in subjects trained to self-administer cocaine to assess the importance of drug history on the reinforcing effects of mephedrone.

RESULTS

Of the drugs tested, cocaine had the highest level of mephedrone-lever responding when administered alone (73.5%). In combination with mephedrone, cocaine shifted the mephedrone dose-effect curve upwards in an infra-additive manner. Ritanserin had a small, but non-significant, effect on mephedrone's discriminative stimulus effects. An extensive history (baseline) of cocaine self-administration increased mephedrone self-administration compared to that obtained in mephedrone-trained subjects, whereas a baseline of mephedrone self-administration decreased cocaine self-administration compared to that obtained in cocaine-trained subjects.

CONCLUSION

The similarity between the discriminative stimulus effects of cocaine and mephedrone in male rats suggests an important overlap and the relative importance of the dopamine (DAT) and serotonin (SERT) transporters. The self-administration data suggest that mephedrone is less reinforcing than cocaine, but that a history of responding for cocaine can increase the reinforcing effects of mephedrone.

Comparison of the effects of abstinence on MDMA and cocaine self-administration in rats

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA) preferentially increases synaptic serotonin (5HT). This response was attenuated following repeated exposure but there was recovery as a result of abstinence. Effects of abstinence on self-administration of many drugs have been documented but the impact on MDMA self-administration is unknown.

OBJECTIVE

This study compared the effects of abstinence on MDMA and cocaine self-administration.

METHODS

Six-hour daily MDMA or cocaine sessions were conducted until a total of 350 mg/kg had been self-administered. Following this, rats were randomly assigned to either a 0- or 14-day abstinence group. Self-administration testing then continued for an additional 7 days.

RESULTS

The latency to self-administer 350 mg/kg was shorter for rats that self-administered cocaine. The temporal distribution of responding within each test session also differed; MDMA self-administration was high during the first hour of each session, and decreased during subsequent hours, whereas cocaine self-administration was evenly distributed throughout each hour of the session. Abstinence decreased MDMA but not cocaine self-administration.

CONCLUSIONS

The selective reduction of MDMA self-administration following abstinence is consistent with the idea that MDMA-stimulated 5-HT release is inhibitory to MDMA self-administration.

Methoxetamine: A foe or friend?

Methoxetamine (MXE) is an N-methyl-D-aspartate (NMDA) receptor antagonist that is chemically and pharmacologically similar to other dissociative substances, such as ketamine and phencyclidine. There are reports on the misuse of MXE, which sometimes resulted in adverse consequences and death. Studies have also shown that MXE has abuse liability and stimulates dopamine neurotransmission in the mesolimbic reward pathway in the brain. These findings have contributed to the negative impression on MXE. However, recent preclinical studies have identified the antidepressant properties of MXE, which are attributed to its ability to affect the glutamatergic and serotonergic systems. MXE is also reported to have analgesic effects. These findings show some of the "redeeming qualities" of MXE and indicate its possible therapeutic uses. In this paper, we have reviewed the findings that provide insights into the adverse and potential therapeutic effects of MXE. We compiled studies on the toxicity, psychotomimetic effects, and abuse liability of MXE, as well as its promising antidepressant and analgesic properties. We also have discussed the mechanism of action that might mediate the somewhat paradoxical effects observed. Importantly, this review provides valuable information on MXE for future research and will enable a better understanding of its psychopharmacological properties and the mechanisms responsible for its unique effects.

(±)-MDMA and its enantiomers: potential therapeutic advantages of R(-)-MDMA

The use of (±)-3,4-methylenedioxymethamphetamine ((±)-MDMA) as an adjunct to psychotherapy in the treatment of psychiatric and behavioral disorders dates back over 50 years. Only in recent years have controlled and peer-reviewed preclinical and clinical studies lent support to (±)-MDMA's hypothesized clinical utility. However, the clinical utility of (±)-MDMA is potentially mitigated by a range of demonstrated adverse effects. One potential solution could lie in the individual S(+) and R(-) enantiomers that comprise (±)-MDMA. Individual enantiomers of racemic compounds have been employed in psychiatry to improve a drug's therapeutic index. Although no research has explored the individual effects of either S(+)-MDMA or R(-)-MDMA in humans in a controlled manner, preclinical research has examined similarities and differences between the two molecules and the racemic compound. This review addresses information related to the pharmacodynamics, neurotoxicity, physiological effects, and behavioral effects of S(+)-MDMA and R(-)-MDMA that might guide preclinical and clinical research. The current preclinical evidence suggests that R(-)-MDMA may provide an improved therapeutic index, maintaining the therapeutic effects of (±)-MDMA with a reduced side effect profile, and that future investigations should investigate the therapeutic potential of R(-)-MDMA.

An Update on the Role of Serotonin and its Interplay with Dopamine for Reward

The specific role of serotonin and its interplay with dopamine (DA) in adaptive, reward guided behavior as well as drug dependance, still remains elusive. Recently, novel methods allowed cell type specific anatomical, functional and interventional analyses of serotonergic and dopaminergic circuits, promising significant advancement in understanding their functional roles. Furthermore, it is increasingly recognized that co-release of neurotransmitters is functionally relevant, understanding of which is required in order to interpret results of pharmacological studies and their relationship to neural recordings. Here, we review recent animal studies employing such techniques with the aim to connect their results to effects observed in human pharmacological studies and subjective effects of drugs. It appears that the additive effect of serotonin and DA conveys significant reward related information and is subjectively highly euphorizing. Neither DA nor serotonin alone have such an effect. This coincides with optogenetically targeted recordings in mice, where the dopaminergic system codes reward prediction errors (PE), and the serotonergic system mainly unsigned PE. Overall, this pattern of results indicates that joint activity between both systems carries essential reward information and invites parallel investigation of both neurotransmitter systems.

Altered Insula Connectivity under MDMA

Recent work with noninvasive human brain imaging has started to investigate the effects of 3,4-methylenedioxymethamphetamine (MDMA) on large-scale patterns of brain activity. MDMA, a potent monoamine-releaser with particularly pronounced serotonin- releasing properties, has unique subjective effects that include: marked positive mood, pleasant/unusual bodily sensations and pro-social, empathic feelings. However, the neurobiological basis for these effects is not properly understood, and the present analysis sought to address this knowledge gap. To do this, we administered MDMA-HCl (100 mg p.o.) and, separately, placebo (ascorbic acid) in a randomized, double-blind, repeated-measures design with twenty-five healthy volunteers undergoing fMRI scanning. We then employed a measure of global resting-state functional brain connectivity and follow-up seed-to-voxel analysis to the fMRI data we acquired. Results revealed decreased right insula/salience network functional connectivity under MDMA. Furthermore, these decreases in right insula/salience network connectivity correlated with baseline trait anxiety and acute experiences of altered bodily sensations under MDMA. The present findings highlight insular disintegration (ie, compromised salience network membership) as a neurobiological signature of the MDMA experience, and relate this brain effect to trait anxiety and acutely altered bodily sensations-both of which are known to be associated with insular functioning.

Repeated MDMA administration increases MDMA-produced locomotor activity and facilitates the acquisition of MDMA self-administration: role of dopamine D2 receptor mechanisms

RATIONALE

Repeated exposure to ±3, 4-methylenedioxymethamphetamine (MDMA) produces sensitization to MDMA-produced hyperactivity, but the mechanisms underlying the development of this sensitized response or the relationship to the reinforcing effects of MDMA is unknown.

OBJECTIVES

This study determined the effect of a sensitizing regimen of MDMA exposure on the acquisition of MDMA self-administration and investigated the role of dopamine D₂ receptor mechanisms.

METHODS

Rats received the selective D₂ antagonist, eticlopride (0.0 or 0.3 mg/kg, i.p.) and MDMA (0.0 or 10.0 mg/kg, i.p.) during a five-day pretreatment regimen. Two days following the final session, the locomotor activating effects of MDMA (5 mg/kg, i.p.) and the latency to acquisition of MDMA self-administration were determined.

RESULTS

Pretreatment with MDMA enhanced the locomotor activating effects of MDMA and facilitated the acquisition of MDMA self-administration. Administration of eticlopride during MDMA pretreatment completely blocked the development of sensitization to MDMA-produced hyperactivity but failed to significantly alter the facilitated acquisition of MDMA self-administration. Pretreatment with eticlopride alone facilitated the acquisition of self-administration.

CONCLUSIONS

These data suggest that repeated MDMA exposure sensitized both the locomotor activating and reinforcing effects of MDMA. Activation of D₂ receptors during MDMA pretreatment appears critical for the development of sensitization to MDMA-produced hyperactivity. The role of D₂ receptor mechanisms in the development of sensitization to the reinforcing effects of MDMA is equivocal.

Contribution of Impulsivity and Serotonin Receptor Neuroadaptations to the Development of an MDMA ('Ecstasy') Substance Use Disorder

As is the case with other drugs of abuse, a proportion of ecstasy users develop symptoms consistent with a substance use disorder (SUD). In this paper, we propose that the pharmacology of MDMA, the primary psychoactive component of ecstasy tablets, changes markedly with repeated exposure and that neuroadaptations in dopamine and serotonin brain systems underlie the shift from MDMA use to MDMA misuse in susceptible subjects. Data from both the human and laboratory animal literature are synthesized to support the idea that (1) MDMA becomes a less efficacious serotonin releaser and a more efficacious dopamine releaser with the development of behaviour consistent with an SUD and (2) that upregulated serotonin receptor mechanisms contribute to the development of the MDMA SUD via dysregulated inhibitory control associated with the trait of impulsivity.

Predicting the Abuse Liability of Entactogen-Class, New and Emerging Psychoactive Substances via Preclinical Models of Drug Self-administration

Animal models of drug self-administration are currently the gold standard for making predictions regarding the relative likelihood that a recreational drug substance will lead to continued use and addiction. Such models have been found to have high predictive accuracy and discriminative validity for a number of drug classes including ethanol, nicotine, opioids, and psychostimulants such as cocaine and methamphetamine. Members of the entactogen class of psychostimulants (drugs that produce an "open mind state" including feelings of interpersonal closeness, intimacy and empathy) have been less frequently studied in self-administration models. The prototypical entactogen 3,4-methylenedioxymethamphetamine (MDMA; "Ecstasy") supports self-administration but not with the same consistency nor with the same efficacy as structurally related drugs amphetamine or methamphetamine. Consistent with these observations, MDMA use is more episodic in the majority of those who use it frequently. Nevertheless, substantial numbers of MDMA users will meet the criteria for substance dependence at some point in their use history. This review examines the currently available evidence from rodent self-administration studies of MDMA and two of the new and emerging psychoactive substances (NPS) that produce entactogen type neuropharmacological responses - mephedrone (4-methylmethcathinone; 4MMC; "meow meow") and methylone (3,4-methylenedioxymethcathinone). Overall, the current evidence predicts that these NPS entactogens have enhanced abuse liability compared with MDMA.

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.

Reinforcing and neurochemical effects of the "bath salts" constituents 3,4-methylenedioxypyrovalerone (MDPV) and 3,4-methylenedioxy-N-methylcathinone (methylone) in male rats

RATIONALE

3,4-Methylenedioxypyrovalerone (MDPV) and 3,4-methylenedioxy-N-methylcathinone (methylone) are synthetic drugs found in so-called "bath salts" products. Both drugs exert their effects by interacting with monoamine transporter proteins. MDPV is a potent uptake blocker at transporters for dopamine and norepinephrine while methylone is a non-selective releaser at transporters for dopamine, norepinephrine, and serotonin (5-HT).

OBJECTIVES

We hypothesized that prominent 5-HT-releasing actions of methylone would render this drug less reinforcing than MDPV.

METHODS

To test this hypothesis, we compared behavioral effects of MDPV and methylone using intravenous (i.v.) self-administration on a fixed-ratio 1 schedule in male rats. Additionally, neurochemical effects of the drugs were examined using in vivo microdialysis in nucleus accumbens, in a separate cohort of rats.

RESULTS

MDPV self-administration (0.03 mg/kg/inj) was acquired rapidly and reached 40 infusions per session, similar to the effects of cocaine (0.5 mg/kg/inj), by the end of training. In contrast, methylone self-administration (0.3 and 0.5 mg/kg/inj) was acquired slowly, and response rates only reached 20 infusions per session by the end of training. In dose substitution studies, MDPV and cocaine displayed typical inverted U-shaped dose-effect functions, but methylone did not. In vivo microdialysis revealed that i.v. MDPV (0.1 and 0.3 mg/kg) increased extracellular dopamine while i.v. methylone (1 and 3 mg/kg) increased extracellular dopamine and 5-HT.

CONCLUSIONS

Our findings support the hypothesis that elevations in extracellular 5-HT in the brain can dampen positive reinforcing effects of cathinone-type drugs. Nevertheless, MDPV and methylone are both self-administered by rats, suggesting these drugs possess significant abuse liability in humans.

Repeated administration of the 5-HT₁B/₁A agonist, RU 24969, facilitates the acquisition of MDMA self-administration: role of 5-HT₁A and 5-HT₁B receptor mechanisms

RATIONALE

3,4 Methylenedioxymethamphetamine (MDMA) preferentially stimulates the release of serotonin (5-HT) that subsequently produces behavioral responses by activation of post-synaptic receptor mechanisms. The 5-HT1A and 5-HT1B receptors are both well localized to regulate dopamine (DA) release, and have been implicated in modulating the reinforcing effects of many drugs of abuse, but a role in acquisition of self-administration has not been determined.

OBJECTIVES

This study was designed to determine the effect of pharmacological manipulation of 5-HT1A and 5-HT1B receptor mechanisms on the acquisition of MDMA self-administration.

METHODS

The 5-HT1B/1A receptor agonist, RU 24969 (0.0 or 3.0 mg/kg, bid), was administered for 3 days in order to down-regulate both 5-HT1A and 5-HT1B receptors. Following the pretreatment phase, latency to acquisition of MDMA self-administration was measured.

RESULTS

Repeated administration of RU 24969 significantly decreased the latency to acquisition and increased the proportion of animals that acquired MDMA self-administration. Dose-effect curves for the 5-HT1A-mediated hyperactivity produced by the 5-HT1A agonist, 8-OH-DPAT, and the 5-HT1B-mediated adipsic response produced by RU 24969 were shifted rightward, suggesting a desensitization of 5-HT1A and 5-HT1B receptor mechanisms.

CONCLUSIONS

These data suggest that the initial reinforcing effects of MDMA are modulated by 5-HT1A and/or 5-HT1B receptor mechanisms. The potential impact of these changes on the DAergic response relevant to self-administration and a possible role in conditioned reinforcement pertaining to acquisition of self-administration are discussed.

The Effects of Acutely Administered 3,4-Methylenedioxymethamphetamine on Spontaneous Brain Function in Healthy Volunteers Measured with Arterial Spin Labeling and Blood Oxygen Level-Dependent Resting State Functional Connectivity

BACKGROUND

The compound 3,4-methylenedioxymethamphetamine (MDMA) is a potent monoamine releaser that produces an acute euphoria in most individuals.

METHODS

In a double-blind, placebo-controlled, balanced-order study, MDMA was orally administered to 25 physically and mentally healthy individuals. Arterial spin labeling and seed-based resting state functional connectivity (RSFC) were used to produce spatial maps displaying changes in cerebral blood flow (CBF) and RSFC after MDMA administration. Participants underwent two arterial spin labeling and two blood oxygen level-dependent scans in a 90-minute scan session; MDMA and placebo study days were separated by 1 week.

RESULTS

Marked increases in positive mood were produced by MDMA. Decreased CBF only was observed after MDMA, and this was localized to the right medial temporal lobe (MTL), thalamus, inferior visual cortex, and the somatosensory cortex. Decreased CBF in the right amygdala and hippocampus correlated with ratings of the intensity of global subjective effects of MDMA. The RSFC results complemented the CBF results, with decreases in RSFC between midline cortical regions, the medial prefrontal cortex, and MTL regions, and increases between the amygdala and hippocampus. There were trend-level correlations between these effects and ratings of intense and positive subjective effects.

CONCLUSIONS

The MTLs appear to be specifically implicated in the mechanism of action of MDMA, but further work is required to elucidate how the drug's characteristic subjective effects arise from its modulation of spontaneous brain activity.

Role of 5-HT₂C receptors in effects of monoamine releasers on intracranial self-stimulation in rats

RATIONALE

Many monoamine releasers are abused by humans and produce abuse-related facilitation of intracranial self-stimulation (ICSS) in rats. Facilitation of ICSS in rats can be limited by monoamine releaser-induced serotonin (5-HT) release, but receptors that mediate 5-HT effects of monoamine releasers are unknown.

OBJECTIVES

The aim of this study is to investigate whether 5-HT2C receptor activation is necessary for rate-decreasing effects produced in an ICSS procedure in rats by the 5-HT-selective monoamine releaser fenfluramine and the non-selective releasers napthylisopropylamine (PAL-287) and (+)-3,4-methylenedioxymethamphetamine ((+)-MDMA).

METHODS

Adult male Sprague-Dawley rats with electrodes implanted in the medial forebrain bundle were trained to lever press for brain stimulation under a "frequency-rate" ICSS procedure. Effectiveness of the 5-HT2C antagonist SB 242,084 was evaluated to block rate-decreasing effects produced by (1) the 5-HT2C agonist Ro 60-0175, (2) the 5-HT-selective releaser fenfluramine, and (3) the mixed-action dopamine (DA)/norepinephrine (NE)/5-HT releasers PAL-287 (1.0-5.6 mg/kg) and (+)-MDMA (1.0-3.2 mg/kg). For comparison, effectiveness of SB 242,084 to alter rate-decreasing effects of the kappa-opioid receptor agonist U69,593 and rate-increasing effects of the DA>5-HT releaser amphetamine was also examined.

RESULTS

SB 242,084 pretreatment blocked rate-decreasing effects of Ro 60-0175 and fenfluramine, but not the rate-decreasing effects of U69,593 or the rate-increasing effects of amphetamine. SB 242,084 blunted the rate-decreasing effects and enhanced expression of rate-increasing effects of PAL-287 and (+)-MDMA.

CONCLUSIONS

These data suggest that 5-HT2C receptor activation contributes to rate-decreasing effects that are produced by selective and mixed-action 5-HT releasers in rats and that may oppose and limit the expression of abuse-related ICSS facilitation by these compounds.

Intravenous self-administration of entactogen-class stimulants in male rats

The intravenous self-administration (IVSA) of 3,4-methylenedioxymethamphetamine (MDMA) is inconsistent in rats, with up to half of subjects failing to acquire reliable drug intake. It is unknown if this changes under long-access conditions (6 h sessions) under which the IVSA of cocaine and methamphetamine escalates. The entactogen class cathinone stimulants which exhibit MDMA-like monoamine effects in the nucleus accumbens, mephedrone (4-methylmethcathinone) and methylone (3,4-methylenedioxymethcathinone), may support more reliable IVSA but results have been mixed. This study was designed to directly compare the IVSA of these three compounds. Groups of male Wistar rats were trained to self-administer mephedrone, methylone or MDMA (0.5 mg/kg/inf) under a Fixed-Ratio (FR) 1 schedule of reinforcement for 14 sessions. Following the acquisition interval, animals were evaluated in FR (0.0, 0.125, 0.25, 0.5, 1.0, 2.5 mg/kg/inf) and Progressive Ratio (PR; 0.125, 1.0 mg/kg/inf) dose-substitution procedures. Long access conditions escalated MDMA intake over the 6 h session but not in the first 2 h. In short access, drug intake was significantly higher in mephedrone-trained rats compared with either the methylone-trained or MDMA-trained groups during acquisition. Mephedrone resulted in the highest intakes during FR and PR dose-substitution in MDMA- and mephedrone-trained groups. Overall it was found that mephedrone is a more effective reinforcer than methylone or MDMA and represents a higher risk for compulsive use.

Binge-like acquisition of 3,4-methylenedioxypyrovalerone (MDPV) self-administration and wheel activity in rats

RATIONALE

Lack of access to conventional sources of reinforcement has been proposed as a risk factor for substance abuse in lower socioeconomic populations. There is laboratory evidence that behavioral alternatives (enrichment or exercise) and alternative reinforcers (e.g., sweetened solutions) can reduce self-administration of a variety of drugs.

OBJECTIVES

The objective of this study is to determine if drug self-administration could devalue wheel activity in an animal model.

METHODS

Male Wistar rats were trained to self-administer 3,4-methylenedioxypyrovalerone (MDPV; "bath salts"), 0.05 mg/kg/infusion, i.v., with concurrent access to a running wheel that was either locked (LW) or unlocked (UW).

RESULTS

MDPV intake steadily increased across the 20-session acquisition interval but did not differ significantly between UW and LW groups. Mean wheel rotations declined significantly across the acquisition interval in the UW group. Of the rats that acquired self-administration, 60 % engaged in a binge-like behavior at the initiation of acquisition; intake was limited only by post-reinforcement time-out. The binge rats had higher post-acquisition levels of drug intake (even after excluding the binge session), and the UW binge rats showed a precipitous post-acquisition drop in wheel activity that was not observed in the UW no-binge rats.

CONCLUSIONS

These data confirm that MDPV is a powerful reward/reinforcer and show that a relatively high rate of intake at the onset of drug taking can devalue natural rewards (wheel activity) and can predict higher subsequent drug intake levels. Thus, limiting the intensity of initial drug exposure may attenuate subsequent drug abuse/addiction by preventing the devaluation of natural alternative rewards/reinforcers.

Reduced cocaine-induced serotonin, but not dopamine and noradrenaline, release in rats with a genetic deletion of serotonin transporters

It has recently been proposed that the increased reinforcing properties of cocaine and ecstasy observed in rats with a genetic deletion of serotonin transporters are the result of a reduction in the psychostimulant-induced release of serotonin. Here we provide the neurochemical evidence in favor of this hypothesis and show that changes in synaptic levels of dopamine or noradrenaline are not very likely to play an important role in the previously reported enhanced psychostimulant intake of these serotonin transporter knockout rats. The results may very well explain why human subjects displaying a reduced expression of serotonin transporters have an increased risk to develop addiction.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

3,4-methylenedioxymethamphetamine (MDMA): current perspectives

Ecstasy is a widely used recreational drug that usually consists primarily of 3,4-methylenedioxymethamphetamine (MDMA). Most ecstasy users consume other substances as well, which complicates the interpretation of research in this field. The positively rated effects of MDMA consumption include euphoria, arousal, enhanced mood, increased sociability, and heightened perceptions; some common adverse reactions are nausea, headache, tachycardia, bruxism, and trismus. Lowering of mood is an aftereffect that is sometimes reported from 2 to 5 days after a session of ecstasy use. The acute effects of MDMA in ecstasy users have been attributed primarily to increased release and inhibited reuptake of serotonin (5-HT) and norepinephrine, along with possible release of the neuropeptide oxytocin. Repeated or high-dose MDMA/ecstasy use has been associated with tolerance, depressive symptomatology, and persisting cognitive deficits, particularly in memory tests. Animal studies have demonstrated that high doses of MDMA can lead to long-term decreases in forebrain 5-HT concentrations, tryptophan hydroxylase activity, serotonin transporter (SERT) expression, and visualization of axons immunoreactive for 5-HT or SERT. These neurotoxic effects may reflect either a drug-induced degeneration of serotonergic fibers or a long-lasting downregulation in 5-HT and SERT biosynthesis. Possible neurotoxicity in heavy ecstasy users has been revealed by neuroimaging studies showing reduced SERT binding and increased 5-HT_2A receptor binding in several cortical and/or subcortical areas. MDMA overdose or use with certain other drugs can also cause severe morbidity and even death. Repeated use of MDMA may lead to dose escalation and the development of dependence, although such dependence is usually not as profound as is seen with many other drugs of abuse. MDMA/ecstasy-dependent patients are treated with standard addiction programs, since there are no specific programs for this substance and no proven medications. Finally, even though MDMA is listed as a Schedule I compound by the Drug Enforcement Agency, MDMA-assisted psychotherapy for patients with chronic, treatment-resistant posttraumatic stress disorder is currently under investigation. Initial results show efficacy for this treatment approach, although considerably more research must be performed to confirm such efficacy and to ensure that the benefits of MDMA-assisted therapy outweigh the risks to the patients.