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

5-HT2C receptors in the nucleus accumbens constrain the rewarding effects of MDMA

MDMA is a promising adjunct to psychotherapy and has well-known abuse liability, although less than other amphetamine analogs. While the reinforcing dopamine (DA)-releasing properties of MDMA are on par with methamphetamine (METH), MDMA is a far more potent serotonin (5-HT) releaser, via the 5-HT transporter (SERT). MDMA-mediated 5-HT release in a major reward center, the nucleus accumbens (NAc), drives prosocial behaviors via 5-HT1BR activation. We hypothesized that this prosocial mechanism contributes to the reduced reinforcing properties of MDMA compared to METH and used a platform of assays to predict the balance of prosocial and abuse-linked effects of (R)-MDMA, a novel entactogen in clinical development. NAc DA release, measured by GRAB-DA photometry in vivo, increased in proportion to MDMA (7.5 and 15 mg/kg, i.p.) and METH (2 mg/kg i.p.)-conditioned place preference (CPP). Using conditional knockouts (cKOs) for DAT and SERT, microdialysis, and photometry, we found that MDMA-released 5-HT limited MDMA-released DA through actions in the NAc, rather than at ventral tegmental area DAergic cell bodies. SERT cKO reduced the MDMA dose required for CPP three-fold. This enhanced MDMA-CPP and increased DA release were replicated by intra-NAc infusion of either a 5-HT reuptake inhibitor (escitalopram) to prevent MDMA interaction with SERT, or a 5-HT2CR antagonist (SB242084), but not by the 5-HT1BR antagonist NAS-181. These data support separate mechanisms for the low abuse potential versus prosocial effect of MDMA. Using this platform of assays, (R)-MDMA is predicted to have prosocial effects and low abuse potential.

The dopamine D1 receptor antagonist SCH-23390 blocks the acquisition, but not expression of mitragynine-induced conditioned place preference in rats

Mitragynine (MG) is the primary active constituent of Mitragyna speciosa Korth (kratom), a psychoactive Southeast Asian plant with potential therapeutic use. Numerous studies support roles of dopaminergic system in drug reward. However, the involvement of the dopaminergic system in mediating MG reward and drug-seeking is poorly understood. Using conditioned place preference (CPP) paradigm, the present study aims to evaluate the roles of the dopamine (DA) D1 receptor in the acquisition and expression of MG-induced CPP in rats. The effects of SCH-23390, a selective DA D1 receptor antagonist, on the acquisition of MG-induced CPP were first investigated. Rats were pre-treated systemically with SCH-23390 (0, 0.1 and 0.3 mg/kg, i.p.) prior to MG (10 mg/kg) conditioning sessions. Next, we tested the effects of the DA D1 receptor antagonist on the expression of MG-induced CPP. Furthermore, the effects of a MG-priming dose (5 mg/kg) on the reinstatement of extinguished CPP were tested. The results showed that SCH-23390 dose-dependently suppressed the acquisition of a MG-induced CPP. In contrast, SCH-23390 had no effect on the expression of a MG-induced CPP. The findings of this study suggested a crucial role of the DA D1 receptor in the acquisition, but not the expression of the rewarding effects of MG in a CPP test. Furthermore, blockade of the D1-like receptor during conditioning did not prevent MG priming effects on CPP reinstatement test, suggesting no role for the DA D1 receptor in reinstatement sensitivity.

A proposed mechanism for the MDMA-mediated extinction of traumatic memories in PTSD patients treated with MDMA-assisted therapy

Post-traumatic stress disorder (PTSD) is a devastating psychiatric disorder afflicting millions of people around the world. Characterized by severe anxiety, intrusive thoughts, pervasive nightmares, an assortment of somatic symptoms, associations with severe long-term health problems, and an elevated risk of suicide, as much as 40-70% of patients suffer from refractory disease. 3,4-Methylenedioxy-methamphetamine (MDMA), like classic psychedelics such as psilocybin, have been used to enhance the efficacy of psychotherapy almost since their discovery, but due to their perceived potential for abuse and inclusion on USFDA (United States Food and Drug Administration) schedule 1, research into the mechanism by which they produce improvements in PTSD symptomology has been limited. Nevertheless, several compelling rationales have been explored, with the pro-social effects of MDMA thought to enhance therapeutic alliance and thus facilitate therapist-assisted trauma processing. This may be insufficient to fully explain the efficacy of MDMA in the treatment of psychiatric illness. Molecular mechanisms such as the MDMA mediated increase of brain-derived neurotrophic factor (BDNF) availability in the fear memory learning pathways combined with MDMA's pro-social effects may provide a more nuanced explanation for the therapeutic actions of MDMA.

Systemic enhancement of serotonin signaling reverses social deficits in multiple mouse models for ASD

Autism spectrum disorder (ASD) is a common set of heterogeneous neurodevelopmental disorders resulting from a variety of genetic and environmental risk factors. A core feature of ASD is impairment in prosocial interactions. Current treatment options for individuals diagnosed with ASD are limited, with no current FDA-approved medications that effectively treat its core symptoms. We recently demonstrated that enhanced serotonin (5-HT) activity in the nucleus accumbens (NAc), via optogenetic activation of 5-HTergic inputs or direct infusion of a specific 5-HT1b receptor agonist, reverses social deficits in a genetic mouse model for ASD based on 16p11.2 copy number variation. Furthermore, the recreational drug MDMA, which is currently being evaluated in clinical trials, promotes sociability in mice due to its 5-HT releasing properties in the NAc. Here, we systematically evaluated the ability of MDMA and a selective 5-HT1b receptor agonist to rescue sociability deficits in multiple different mouse models for ASD. We find that MDMA administration enhances sociability in control mice and reverses sociability deficits in all four ASD mouse models examined, whereas administration of a 5-HT1b receptor agonist selectively rescued the sociability deficits in all six mouse models for ASD. These preclinical findings suggest that pharmacological enhancement of 5-HT release or direct 5-HT1b receptor activation may be therapeutically efficacious in ameliorating some of the core sociability deficits present across etiologically distinct presentations of ASD.

Insular cortex dopamine 1 and 2 receptors in methamphetamine conditioned place preference and aversion: Age and sex differences

Rodent studies have proposed that adolescent susceptibility to substance use is at least partly due to adolescents experiencing reduced aversive effects of drugs compared to adults. We thus investigated methamphetamine (meth) conditioned place preference/aversion (CPP/CPA) in adolescent and adult mice in both sexes using a high dose of meth (3 mg/kg) or saline as controls. Mice tagged with green-fluorescent protein (GFP) at Drd1a or Drd2 were used so that dopamine receptor 1 (D1) and 2 (D2) expression within the insular cortex (insula) could be quantified. There are sex differences in how the density of D1+ and D2+ cells in the insula changes across adolescence that may be related to drug-seeking behaviors. Immunohistochemistry followed by stereology were used to quantify the density of cells with c-Fos and/or GFP in the insula. Unexpectedly, mice showed huge variability in behaviors including CPA, CPP, or no preference or aversion. Females were less likely to show CPP compared to males, but no age differences in behavior were observed. Conditioning with meth increased the number of D2 + cells co-labelled with c-Fos in adults but not in adolescents. D1:D2 ratio also sex- and age-dependently changed due to meth compared to saline. These findings suggest that reduced aversion to meth is unlikely an explanation for adolescent vulnerability to meth use. Sex- and age-specific expressions of insula D1 and D2 are changed by meth injections, which has implications for subsequent meth use.

The neurochemistry of social reward during development: What have we learned from rodent models?

Social rewards are fundamental to survival and overall health. Several studies suggest that adequate social stimuli during early life are critical for developing appropriate socioemotional and cognitive skills, whereas adverse social experiences negatively affect the proper development of brain and behavior, by increasing the susceptibility to develop neuropsychiatric conditions. Therefore, a better understanding of the neural mechanisms underlying social interactions, and their rewarding components in particular, is an important challenge of current neuroscience research. In this context, preclinical research has a crucial role: Animal models allow to investigate the neurobiological aspects of social reward in order to shed light on possible neurochemical alterations causing aberrant social reward processing in neuropsychiatric diseases, and they allow to test the validity and safety of innovative therapeutic strategies. Here, we discuss preclinical research that has investigated the rewarding properties of two forms of social interaction that occur in different phases of the lifespan of mammals, that is, mother-infant interaction and social interactions with peers, by focusing on the main neurotransmitter systems mediating their rewarding components. Together, the research performed so far helped to elucidate the mechanisms of social reward and its psychobiological components throughout development, thus increasing our understanding of the neurobiological substrates sustaining social functioning in health conditions and social dysfunction in major psychiatric disorders.

Cannabidiol attenuated the maintenance and reinstatement of extinguished methylphenidate-induced conditioned place preference in rats

Methylphenidate (MPH) is a mild CNS stimulant that has been used in hyperactive children, and patients with neurodegenerative and major depressive disorders. Exposure to MPH-associated cues enhances craving and arousal in drug users. On the other hand, cannabidiol (CBD) has antipsychotic potential that might be useful in alleviating symptoms of drug addiction. The aim of this study was to investigate the effect of CBD administration on extinction and reinstatement of MPH-induced conditioning place preference (CPP) in rats. Male rats received MPH (1, 2.5 or 5 mg/kg, i.p) or morphine (5 or 10 mg/kg, s.c.) during the conditioning phase. Following the establishment of CPP, during extinction training, 60 min prior to every CPP session, animals were given daily ICV CBD (10 or 50 μg/5 μL), vehicle alone (DMSO) 10 % or were treatment-naïve. On the reinstatement day animals after receiving the initial dose of MPH, 0.5 mg/kg, and were placed into the CPP box to evaluate the CPP scoring for 10-min. Our findings indicated that morphine (5 and 10 mg/kg; s.c.) and MPH (1 and 2.5 mg/kg; i.p.) induced a CPP. The ICV administration of both doses of CBD (10 and 50 μg/5 μL) prevented the reinstatement of MPH-induced CPP, which displayed shorter extinction latency compared to treatment-naïve or DMSO 10 % groups. Therefore, CBD's site of action is a potential target for reducing the risk of MPH relapse; however, more investigation is required.

Two newly-emerging substituted phenethylamines MAL and BOD induce differential psychopharmacological effects in rodents

BACKGROUND

Recently, the recreational use of substituted phenethylamines has grown rapidly. Among these are 2-(3,5-dimethoxy-4-((2-methylallyl)oxy)phenyl)ethanamine (MAL) and 2-(2,5-dimethoxy-4-methylphenyl)-2-methoxyethan-1-amine (BOD). However, studies characterizing their abuse potential are still lacking.

AIM

The purpose of this study was to investigate the abuse potential of MAL and BOD.

METHODS

The psychostimulant, reinforcing, and rewarding properties of MAL and BOD were analyzed using locomotor sensitization, self-administration, and conditioned place preference tests. Dopamine antagonists (i.e. SCH23390, haloperidol) were administered during conditioned place preference to evaluate the involvement of the mesolimbic dopamine system. Furthermore, dopamine-related protein expression in the nucleus accumbens and the ventral tegmental area was measured along with dopamine concentrations in the nucleus accumbens. Electroencephalography was conducted to determine effects of MAL and BOD on brain wave activity.

RESULTS

MAL induced psychostimulant effects and sensitization, while BOD induced locomotor depression in mice. Only MAL was self-administered by rats. Both drugs induced conditioned place preference in mice at different doses; dopamine receptor antagonists blocked MAL- and BOD-induced conditioned place preference. Both the compounds altered the expression of dopamine receptor D₁ and D₂ proteins in the nucleus accumbens and tyrosine hydroxylase (TH) and dopamine transporter in the ventral tegmental area, enhanced dopamine levels in the nucleus accumbens, and increased delta and gamma wave activities in the brain.

CONCLUSIONS

MAL may induce abuse potential via the mesolimbic dopaminergic system and possibly accompanied by alterations in brain wave activity. Moreover, the lack of rewarding and reinforcing effects in BOD suggest that this drug may have little to no capability to engender compulsive behavior, though having found to induce alterations in dopaminergic system and brain wave activities.

Mephedrone and MDMA: A comparative review

Mephedrone and MDMA are both constituents of party drugs, with mephedrone being relatively new compared to MDMA. This review compares current knowledge regarding the patterns of usage and neuropsychobiological effects of both mephedrone and MDMA. Both drugs share common psychoactive effects, the duration of which is significantly shorter with mephedrone use, attributing towards a pattern of binge use among users. Both drugs have also been associated with adverse health, psychiatric, and neurocognitive problems. Whilst there is extensive research into the psychobiological problems induced by MDMA, the evidence for mephedrone is comparatively limited. The adverse effect profile of mephedrone appears to be less severe than that of MDMA. Users often believe it to be safer, although both drugs have been associated with overdoses. The neurotoxic potential of mephedrone appears to be low, whereas MDMA can cause long-term damage to the serotonergic system, although this needs further investigation. The abuse liability of mephedrone is significantly greater than that of MDMA, raising concerns regarding the impact of lifetime usage on users. Given that mephedrone is relatively new, the effects of long-term exposure are yet to be documented. Future research focused on lifetime users may highlight more severe neuropsychobiological effects from the drug.

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.

Distinct neural mechanisms for the prosocial and rewarding properties of MDMA

The extensively abused recreational drug (±)3,4-methylenedioxymethamphetamine (MDMA) has shown promise as an adjunct to psychotherapy for treatment-resistant psychiatric disease. It is unknown, however, whether the mechanisms underlying its prosocial therapeutic effects and abuse potential are distinct. We modeled both the prosocial and nonsocial drug reward of MDMA in mice and investigated the mechanism of these processes using brain region-specific pharmacology, transgenic manipulations, electrophysiology, and in vivo calcium imaging. We demonstrate in mice that MDMA acting at the serotonin transporter within the nucleus accumbens is necessary and sufficient for MDMA's prosocial effect. MDMA's acute rewarding properties, in contrast, require dopaminergic signaling. MDMA's prosocial effect requires 5-HT1b receptor activation and is mimicked by d-fenfluramine, a selective serotonin-releasing compound. By dissociating the mechanisms of MDMA's prosocial effects from its addictive properties, we provide evidence for a conserved neuronal pathway, which can be leveraged to develop novel therapeutics with limited abuse liability.

Dark Classics in Chemical Neuroscience: 3,4-Methylenedioxymethamphetamine

Better known as "ecstasy", 3,4-methylenedioxymethamphetamine (MDMA) is a small molecule that has played a prominent role in defining the ethos of today's teenagers and young adults, much like lysergic acid diethylamide (LSD) did in the 1960s. Though MDMA possesses structural similarities to compounds like amphetamine and mescaline, it produces subjective effects that are unlike any of the classical psychostimulants or hallucinogens and is one of the few compounds capable of reliably producing prosocial behavioral states. As a result, MDMA has captured the attention of recreational users, the media, artists, psychiatrists, and neuropharmacologists alike. Here, we detail the synthesis of MDMA as well as its pharmacology, metabolism, adverse effects, and potential use in medicine. Finally, we discuss its history and why it is perhaps the most important compound for the future of psychedelic science-having the potential to either facilitate new psychedelic research initiatives, or to usher in a second Dark Age for the field.

SHANK3 Downregulation in the Ventral Tegmental Area Accelerates the Extinction of Contextual Associations Induced by Juvenile Non-familiar Conspecific Interaction

Haploinsufficiency of the SHANK3 gene, encoding for a scaffolding protein located in the postsynaptic density of glutamatergic synapse, has been linked to forms of autism spectrum disorders (ASDs). It has been shown that SHANK3 controls the maturation of social reward circuits in the ventral tegmental area (VTA). Whether the impairments in associative learning observed in ASD relate to SHANK3 insufficiency restricted to the reward system is still an open question. Here, we first characterize a social-conditioned place preference (CPP) paradigm based on the direct and free interaction with a juvenile and non-familiar conspecific. In both group- and single-housed C57Bl6/j late adolescence male mice, this CPP protocol promotes the formation of social-induced contextual associations that undergo extinction. Interestingly, the downregulation of Shank3 expression in the VTA altered the habituation to a non-familiar conspecific during conditioning and accelerated the extinction of social-induced conditioned responses. Thus, inspired by the literature on drugs of abuse-induced contextual learning, we propose that acquisition and extinction of CPP might be used as behavioral assays to assess social-induced contextual association and “social-seeking” dysfunctions in animal models of psychiatric disorders.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

The involvement of brain-derived neurotrophic factor in 3,4-methylenedioxymethamphetamine-induced place preference and behavioral sensitization

3,4-Methylenedioxymethamphetamine (MDMA) is known to induce dependence and psychosis in humans. Brain-derived neurotrophic factor (BDNF) is involved in the synaptic plasticity and neurotrophy in midbrain dopaminergic neurons. This study aimed to investigate the role of BDNF in MDMA-induced dependence and psychosis. A single dose of MDMA (10mg/kg) induced BDNF mRNA expression in the prefrontal cortex, nucleus accumbens, and amygdala, but not in the striatum or the hippocampus. However, repeated MDMA administration for 7 days induced BDNF mRNA expression in the striatum and hippocampus. Both precursor and mature BDNF protein expression increased in the nucleus accumbens, mainly in the neurons. Additionally, rapidly increased extracellular serotonin levels and gradually and modestly increased extracellular dopamine levels were noted within the nucleus accumbens of mice after repeated MDMA administration. Dopamine receptor antagonists attenuated the effect of repeated MDMA administration on BDNF mRNA expression in the nucleus accumbens. To examine the role of endogenous BDNF in the behavioral and neurochemical effects of MDMA, we used mice with heterozygous deletions of the BDNF gene. MDMA-induced place preference, behavioral sensitization, and an increase in the levels of extracellular serotonin and dopamine within the nucleus accumbens, were attenuated in BDNF heterozygous knockout mice. These results suggest that BDNF is implicated in MDMA-induced dependence and psychosis by activating the midbrain serotonergic and dopaminergic neurons.

Dopamine D2 receptors mediate the increase in reinstatement of the conditioned rewarding effects of cocaine induced by acute social defeat

Social stress modifies the activity of brain areas involved in the rewarding effects of psychostimulants, inducing neuroadaptations in the dopaminergic mesolimbic system and modifying the sensitivity of dopamine receptors. In the present study we evaluated the effect of the dopamine D₁- and D₂-like receptor antagonists (SCH23390 and raclopride, respectively) on the short-time effects of acute social defeat (ASD). Male OF1 mice were socially defeated before each conditioning session of the conditioned place preference (CPP) induced by 1mg/kg or 25mg/kg of cocaine plus the corresponding dopamine antagonist. A final experiment was designed to evaluate the effect of the dopamine antagonists on the CPP induced by 3mg/kg of cocaine with or without a stress experience. Mice exposed to ASD showed an increase in reinstatement of the conditioned reinforcing effects of cocaine that was blocked by all of the dopamine receptor antagonists. Blockade of dopamine D₂-like receptors with raclopride specifically prevented the effects of stress without affecting the rewarding properties of cocaine. However, SCH23390 inhibited cocaine-induced preference in the control groups and even induced aversion in defeated mice conditioned with the lower dose of cocaine. Moreover, the lowest dose of SCH23390 blocked the rewarding effects of 3mg/kg of cocaine-induced CPP. Our results confirm that the dopamine D₂ receptor is involved in the short-term effects of ASD on the rewarding effects of cocaine. The dopamine D₁ receptor is clearly involved in the rewarding effects of cocaine, but its role in the effects of ASD remains to be demonstrated.

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.

Role of dopamine neurotransmission in the long-term effects of repeated social defeat on the conditioned rewarding effects of cocaine

Numerous studies report that social defeat stress alters dopamine (DA) neurotransmission in several areas of the brain. Alterations of the mesolimbic dopaminergic pathway are believed to be responsible for the increased vulnerability to drug use observed as a result of social stress. In the present study, we evaluated the influence of DA receptors on the long-term effect of repeated social defeat (RSD) on the conditioned rewarding and reinstating effects of cocaine. For this purpose, the D1R antagonist SCH 23390 and the D1R antagonist raclopride were administered 30min before each social defeat and a cocaine-induced CPP procedure was initiated three weeks later. The expression of the D1R and D2R was also measured in the cortex and hippocampus throughout the entire procedure. Mice exposed to RSD showed an increase in the conditioned rewarding effects of cocaine that was blocked by both DA receptors antagonists when a subthreshold dose of cocaine was employed. However, while the vulnerability to reinstatement of the preference induced by 25mg/kg cocaine-induced CPP was abolished by the D1R antagonist, it was practically unaffected by raclopride. Increases in D2R receptor levels were observed in the cortex of defeated animals after the first and fourth social defeats and in the hippocampus 3weeks later. Nevertheless, D1R receptor levels in the hippocampus decreased only after the last social defeat. Our results confirm that RSD enhances the conditioned rewarding effects of cocaine and that both DA receptors are involved in this enduring effect of social stress.

Abuse potential of methylenedioxymethamphetamine (MDMA) and its derivatives in zebrafish: role of serotonin 5HT2-type receptors

RATIONALE

The synthetic phenethylamines are recreational drugs known to produce psychostimulant effects. However, their abuse potential has not been widely studied.

OBJECTIVES

Here, we investigated the rewarding and the hallucinatory effects of 2,5-dimetoxy-4-bromo-amphetamine hydrobromide (DOB) and para-methoxyamphetamine (PMA) in comparison with the classical 3,4-methylenedioxymethamphetamine (MDMA). In addition, the role of serotonin 5-HT2-like receptor on the abovementioned effects was evaluated.

METHODS

Zebrafish were intramuscularly (i.m.) treated with a wide range of doses of DOB (0.1-20 mg/kg), PMA (0.0005-2 mg/kg), or MDMA (0.5-160 mg/kg). Animals were submitted to a conditioned place preference (CPP) task, to investigation of the rewarding properties, and to the evaluation of hallucinatory behavior in terms of appearance of a trance-like behavior. The serotonin 5-HT2 subtype receptor antagonist ritanserin (0.025-2.5 mg/kg) in association with the maximal effective dose of MDMA, DOB, and PMA was given i.m., and the effect on CPP or hallucinatory behavior was evaluated.

RESULTS

MDMA and its derivatives exhibited CPP in a biphasic fashion, being PMA the most potent. This effect was accompanied, for DOB (2 mg/kg) and PMA (0.1 mg/kg), by a trance-like hallucinatory behavior. MDMA at a high dose as 160 mg/kg did not induce any hallucinatory behavior. Ritanserin significantly blocked the rewarding and hallucinatory effects suggesting the involvement of serotonin 5HT2 subtype receptor.

CONCLUSION

Collectively, these findings demonstrate for the first time that the rewarding properties of DOB and PMA are accompanied by hallucinatory behavior through a serotonergic system and reinforce zebrafish as an emerging experimental model for screening new hallucinogens.

Dorsal hippocampal NMDA receptors mediate the interactive effects of arachidonylcyclopropylamide and MDMA/ecstasy on memory retrieval in rats

A combination of cannabis and ecstasy may change the cognitive functions more than either drug alone. The present study was designed to investigate the possible involvement of dorsal hippocampal NMDA receptors in the interactive effects of arachidonylcyclopropylamide (ACPA) and ecstasy/MDMA on memory retrieval. Adult male Wistar rats were cannulated into the CA1 regions of the dorsal hippocampus (intra-CA1) and memory retrieval was examined using the step-through type of passive avoidance task. Intra-CA1 microinjection of a selective CB1 receptor agonist, ACPA (0.5-4ng/rat) immediately before the testing phase (pre-test), but not after the training phase (post-training), impaired memory retrieval. In addition, pre-test intra-CA1 microinjection of MDMA (0.5-1μg/rat) dose-dependently decreased step-through latency, indicating an amnesic effect of the drug by itself. Interestingly, pre-test microinjection of a higher dose of MDMA into the CA1 regions significantly improved ACPA-induced memory impairment. Moreover, pre-test intra-CA1 microinjection of a selective NMDA receptor antagonist, D-AP5 (1 and 2μg/rat) inhibited the reversal effect of MDMA on the impairment of memory retrieval induced by ACPA. Pre-test intra-CA1 microinjection of the same doses of D-AP5 had no effect on memory retrieval alone. These findings suggest that ACPA or MDMA consumption can induce memory retrieval impairment, while their co-administration improves this amnesic effect through interacting with hippocampal glutamatergic-NMDA receptor mechanism. Thus, it seems that the tendency to abuse cannabis with ecstasy may be for avoiding cognitive dysfunction.

Neuronal changes and oxidative stress in adolescent rats after repeated exposure to mephedrone

Mephedrone is a new designer drug of abuse. We have investigated the neurochemical/enzymatic changes after mephedrone administration to adolescent rats (3×25 mg/kg, s.c. in a day, with a 2 h interval between doses, for two days) at high ambient temperature (26±2 °C), a schedule that intends to model human recreational abuse. In addition, we have studied the effect of mephedrone in spatial learning and memory. The drug caused a transient decrease in weight gain. After the first dose, animals showed hypothermia but, after the subsequent doses, temperature raised over the values of saline-treated group. We observed the development of tolerance to these thermoregulatory effects of mephedrone. Mephedrone induced a reduction of the densities of dopamine (30% in the frontal cortex) and serotonin (40% in the frontal cortex and the hippocampus and 48% in the striatum) transporters without microgliosis. These deficits were also accompanied by a parallel decrease in the expression of tyrosine hydroxylase and tryptophan hydroxylase 2. These changes matched with a down-regulation of D2 dopamine receptors in the striatum. Mephedrone also induced an oxidative stress evidenced by an increase of lipid peroxidation in the frontal cortex, and accompanied by a rise in glutathione peroxidase levels in all studied brain areas. Drug-treated animals displayed an impairment of the reference memory in the Morris water maze one week beyond the cessation of drug exposure, while the spatial learning process seems to be preserved. These findings raise concerns about the neuronal long-term effects of mephedrone.

The novelty-seeking phenotype modulates the long-lasting effects of adolescent MDMA exposure

Exposure to drugs such as ethanol or cocaine during adolescence induces alterations in the central nervous system that are modulated by the novelty-seeking trait. Our aim was to evaluate the influence of this trait on the long-term effects of MDMA administration during adolescence on spontaneous behavior and conditioned rewarding effects in adulthood. Adolescent mice were classified as high or low novelty seekers (HNS or LNS) according to the hole-board test and received either MDMA (0, 10 or 20mg/kg PND 33-42) or saline. Three weeks later, having entered adulthood (PND>68), one set of mice performed the elevated plus maze and social interaction tests, while another set performed the conditioning place preference (CPP) test induced by cocaine-(1mg/kg) or MDMA-(1mg/kg). Only HNS mice treated with MDMA during adolescence acquired CPP in adulthood with a non-effective dose of cocaine or MDMA. Although it did not produce changes in motor activity, exposure to MDMA during adolescence was associated with more aggressive behaviors (threat and attack) and increased social contacts in HNS mice, while an anxiolytic effect was noted in LNS mice pre-treated with the highest dose of MDMA (20mg/kg). Administration of MDMA (10 or 20mg/kg) induced a decrease in DA levels in the striatum in LNS mice only and lower striatal serotonin levels in mice treated with the highest MDMA dose. Our findings show that adolescent MDMA exposure results in higher sensitivity to the conditioned reinforcing properties of MDMA and cocaine in adult HNS mice, which suggests that the relationship between exposure to MDMA in adolescence and a higher probability of substance is a feature of high novelty seekers only.

Dose and time-dependent selective neurotoxicity induced by mephedrone in mice

Mephedrone is a drug of abuse marketed as 'bath salts". There are discrepancies concerning its long-term effects. We have investigated the neurotoxicity of mephedrone in mice following different exposition schedules. Schedule 1: four doses of 50 mg/kg. Schedule 2: four doses of 25 mg/kg. Schedule 3: three daily doses of 25 mg/kg, for two consecutive days. All schedules induced, in some animals, an aggressive behavior and hyperthermia as well as a decrease in weight gain. Mephedrone (schedule 1) induced dopaminergic and serotoninergic neurotoxicity that persisted 7 days after exposition. At a lower dose (schedule 2) only a transient dopaminergic injury was found. In the weekend consumption pattern (schedule 3), mephedrone induced dopamine and serotonin transporter loss that was accompanied by a decrease in tyrosine hydroxylase and tryptophan hydroxylase 2 expression one week after exposition. Also, mephedrone induced a depressive-like behavior, as well as a reduction in striatal D2 density, suggesting higher susceptibility to addictive drugs. In cultured cortical neurons, mephedrone induced a concentration-dependent cytotoxic effect. Using repeated doses for 2 days in an elevated ambient temperature we evidenced a loss of frontal cortex dopaminergic and hippocampal serotoninergic neuronal markers that suggest injuries at nerve endings.

Using conditioned place preference to identify relapse prevention medications

Stimuli, including contexts, which predict the availability or onset of a drug effect, can acquire conditioned incentive motivational properties. These conditioned properties endure after withdrawal, and can promote drug-seeking which may result in relapse. Conditioned place preference (CPP) assesses the associations between drugs and the context in which they are experienced. Here, we review the potential utility of CPP procedures in rodents and humans to evaluate medications that target conditioned drug-seeking responses. We discuss the translational potential of the CPP procedure from rodents to humans, and review findings with FDA-approved treatments that support the use of CPP to develop relapse-reduction medications. We also discuss challenges and methodological questions in applying the CPP procedure to this purpose. We argue that an efficient and valid CPP procedure in humans may reduce the burden of full clinical trials with drug-abusing patients that are currently required for testing promising treatments.

Olanzapine treatment of adolescent rats alters adult reward behaviour and nucleus accumbens function

Antipsychotic drugs are increasingly used in children and adolescents to treat a variety of psychiatric disorders. However, little is known about the long-term effects of early life antipsychotic drug (APD) treatment. Most APDs are potent antagonists or partial agonists of dopamine (DA) D₂ receptors; atypical APDs also have multiple serotonergic activities. DA and serotonin regulate many neurodevelopmental processes. Thus, early life APD treatment can, potentially, perturb these processes, causing long-term behavioural and neurobiological sequelae. We treated adolescent, male rats with olanzapine (Ola) on post-natal days 28-49, under dosing conditions that approximate those employed therapeutically in humans. As adults, they exhibited enhanced conditioned place preference for amphetamine, as compared to vehicle-treated rats. In the nucleus accumbens core, DA D₁ receptor binding was reduced, D₂ binding was increased and DA release evoked by electrical stimulation of the ventral tegmental area was reduced. Thus, adolescent Ola treatment enduringly alters a key behavioural response to rewarding stimuli and modifies DAergic neurotransmission in the nucleus accumbens. The persistence of these changes suggests that even limited periods of early life Ola treatment may induce enduring changes in other reward-related behaviours and in behavioural and neurobiological responses to therapeutic and illicit psychotropic drugs. These results underscore the importance of improved understanding of the enduring sequelae of paediatric APD treatment as a basis for weighing the benefits and risks of adolescent APD therapy, especially prophylactic treatment in high-risk, asymptomatic patients.

Effects of risperidone on the acquisition and reinstatement of the conditioned place preference induced by MDMA

Some users of 3,4-methylenedioxymethylamphetamine (MDMA or ecstasy) abuse this drug and/or become concerned about their use. These individuals would benefit greatly from the development of pharmacological strategies to reduce MDMA consumption. We have previously observed that antipsychotics block acquisition and expression of the conditioned place preference (CPP) induced by MDMA, though they do not modify priming-induced reinstatement of MDMA-induced CPP after extinction. In the present study we have evaluated the capacity of the mixed serotonin (5-HT2A)/dopamine (DA D2) antagonist risperidone to block acquisition and reinstatement of MDMA induced-CPP. Adolescent male mice conditioned with 10mg/kg of MDMA were treated with 0.1 or 0.3mg/kg of risperidone during acquisition of conditioning (experiment 1) or before the reinstatement test (experiment 2). Risperidone was devoid of motivational effects in the CPP paradigm, but the higher dose blocked acquisition of the MDMA-induced CPP. This behavioural effect was accompanied by an increase in the level of dopamine transporters in the striatum. However, risperidone had no effects on reinstatement of the CPP induced by a priming of MDMA. Our results suggest that risperidone induces the same effects as other antipsychotics, in which case its efficacy for treating MDMA abuse is limited.

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.