Initial deficit and recovery of function after MDMA preexposure in rats

Generalization of serotonin and dopamine ligands to the discriminative stimulus effects of different doses of ±3,4-methylenedioxymethamphetamine

Studies that have attributed the discriminative stimulus effects of ±3,4-methylenedioxymethamphetamine (MDMA) to serotonergic mechanisms typically use a relatively low training dose of 1.5 mg/kg. The role of serotonin in the discriminative stimulus effects of higher doses of MDMA is, however, unknown. Separate groups of rats were trained to discriminate MDMA (1.5 or 3.0 mg/kg) from saline using a two-lever, food-reinforced drug-discrimination procedure. Generalization tests were carried out with a range of serotonin and dopamine ligands. Fluoxetine (0.3-3 mg/kg), clomipramine (1-10 mg/kg) and meta-chlorophenylpiperazine (0.3-2 mg/kg) dose-dependently substituted for the 1.5 mg/kg MDMA stimulus, but not the 3.0 mg/kg MDMA stimulus. 8-OH-DPAT (0.03-0.3 mg/kg) and RU-24969 (0.3-3 mg/kg) substituted for both the low-dose and the high-dose MDMA stimulus. The generalization dose-effect curve produced by 2,5-dimethoxy-4-iodoamphetamine (0.3-3 mg/kg) was shifted to the right for the 3.0 mg/kg MDMA-trained group. Amphetamine (0.25 and 0.5 mg/kg) and apomorphine (0.125 and 0.25 mg/kg) substituted for the 3.0 mg/kg, but not the 1.5 mg/kg MDMA stimulus. The results suggest some differences in the role of serotonin and dopamine in the discriminative stimulus effects of a low versus a higher dose of MDMA.

Opioid gene expression changes and post-translational histone modifications at promoter regions in the rat nucleus accumbens after acute and repeated 3,4-methylenedioxy-methamphetamine (MDMA) exposure

The recreational drug of abuse 3,4-methylenedioxymethamphetamine (MDMA) has been shown to produce neurotoxic damage and long-lasting changes in several brain areas. In addition to the involvement of serotoninergic and dopaminergic systems, little information exists about the contribution of nociceptin/orphaninFQ (N/OFQ)-NOP and dynorphin (DYN)-KOP systems in neuronal adaptations evoked by MDMA. Here we investigated the behavioral and molecular effects induced by acute (8mg/kg) or repeated (8mg/kg twice daily for seven days) MDMA exposure. MDMA exposure affected body weight gain and induced hyperlocomotion; this latter effect progressively decreased after repeated administration. Gene expression analysis indicated a down-regulation of the N/OFQ system and an up-regulation of the DYN system in the nucleus accumbens (NAc), highlighting an opposite systems regulation in response to MDMA exposure. Since histone modifications have been strongly associated to the addiction-related maladaptive changes, we examined two permissive (acH3K9 and me3H3K4) and two repressive transcription marks (me3H3K27 and me2H3K9) at the pertinent opioid gene promoter regions. Chromatin immunoprecipitation assays revealed that acute MDMA increased me3H3K4 at the pN/OFQ, pDYN and NOP promoters. Following acute and repeated treatment a significant decrease of acH3K9 at the pN/OFQ promoter was observed, which correlated with gene expression results. Acute treatment caused an acH3K9 increase and a me2H3K9 decrease at the pDYN promoter which matched its mRNA up-regulation. Our data indicate that the activation of the DYNergic stress system together with the inactivation of the N/OFQergic anti-stress system contribute to the neuroadaptive actions of MDMA and offer novel epigenetic information associated with MDMA abuse.

Ecstasy exposure & gender: examining components of verbal memory functioning

OBJECTIVE

Studies have demonstrated verbal memory deficits associated with past year ecstasy use, although specific underlying components of these deficits are less understood. Further, prior research suggests potential gender differences in ecstasy-induced serotonergic changes. Therefore, the current study investigated whether gender moderated the relationship between ecstasy exposure and components of verbal memory after controlling for polydrug use and confounding variables.

METHOD

Data were collected from 65 polydrug users with a wide range of ecstasy exposure (ages 18-35; 48 ecstasy and 17 marijuana users; 0-2310 ecstasy tablets). Participants completed a verbal learning and memory task, psychological questionnaires, and a drug use interview.

RESULTS

Increased past year ecstasy exposure predicted poorer short and long delayed free and cued recalls, retention, and recall discrimination. Male ecstasy users were more susceptible to dose-dependent deficits in retention than female users.

CONCLUSION

Past year ecstasy consumption was associated with verbal memory retrieval, retention, and discrimination deficits in a dose-dependent manner in a sample of healthy young adult polydrug users. Male ecstasy users were at particular risk for deficits in retention following a long delay. Gender difference may be reflective of different patterns of polydrug use as well as increased hippocampal sensitivity. Future research examining neuronal correlates of verbal memory deficits in ecstasy users are needed.

Prior MDMA exposure inhibits learning and produces both tolerance and sensitization in the radial-arm maze

Recent research using the partially-baited radial arm maze with rats has shown that acute or single-session high-dose exposure to +/-3,4-methylenedioxymethaphemtamine (MDMA) impairs reference memory processes more so than working memory processes. The current study examined the effect that an initial binge exposure of MDMA had on the ability to learn this task and whether such exposure produced tolerance or sensitization to subsequent low dose acute MDMA exposure. Following either an initial binge dose (4×10mg/kg) of MDMA or saline, rats received regular weekly injections of either MDMA (4.0mg/kg) or saline during task acquisition training. Although eventually able to acquire the task at a level comparable to controls, initial binge MDMA exposure significantly impaired task acquisition. Binge-treated rats displayed evidence of a tolerance effect to subsequent acute injections of MDMA; although with continued training this tolerance effect was reduced. However, initial binge treatment also produced a sensitized response to subsequent acute MDMA exposure that manifested as a prolonged period of impairment up to 24h following administration. Consistent with the effects seen following other regimes of exposure to MDMA, the pattern of errors made was best described as an impairment to reference memory processes as opposed to working memory processes.

Electroencephalographic and convulsive effects of binge doses of (+)-methamphetamine, 5-methoxydiisopropyltryptamine, and (±)-3,4-methylenedioxymethamphetamine in rats

The abuse of drugs such as methamphetamine (MA), 3,4-methylenedioxymethamphetamine (Ecstasy, MDMA), and 5-methoxydiisopropyltryptamine (5-MeO-DIPT; Foxy) is global. Symptoms from taking these drugs include tachycardia, agitation, hyperpyrexia, and sometimes seizures. We compared the EEG effects of these drugs in male Sprague-Dawley rats (~300 g) implanted with cortical electroencephalographic (EEG) electrodes prior to testing. Animals received four subcutaneous injections of MA, MDMA, or Foxy (10 mg/kg each as freebase, administered every 2 h), or saline as these doses produce lasting effects on learning, memory, and monoamines. EEG tracings were recorded before, during, and after treatment. Animals receiving MDMA showed no significant EEG abnormalities or myoclonus. MA treatment resulted in myoclonic activity and in brief (<10 s) EEG epileptiform activity in ~50% of the rats. Longer seizure activity (10 s to 5 min) was recorded in some MA-treated rats following the third and fourth doses. The onset of myoclonic activity following Foxy treatment occurred shortly after the first dose. All rats receiving Foxy showed seizures by the second dose and this continued throughout the treatment regimen. The results show that binge doses of MA and MDMA, which mimic the neurochemical changes seen in chronic users, increase EEG abnormalities after MA but not after MDMA. While the neurochemical effects of Foxy are not known in humans, this drug causes severe EEG abnormalities and overt seizures in 100% of tested animals.

Rats preexposed to MDMA display attenuated responses to its aversive effects in the absence of persistent monoamine depletions

RATIONALE

The abuse potential of a given drug may be mediated by both its rewarding and aversive effects, the latter of which are often far less characterized.

OBJECTIVES

Using the conditioned taste-aversion (CTA) preparation, the present experiments examined changes in the aversive effects of the commonly used recreational drug MDMA following repeated drug exposures.

METHODS

Experiment 1 used three varying doses of MDMA (1.0, 1.8, and 3.2 mg/kg) to determine a dose that produced taste aversions of intermediate strength. Experiments 2 and 3 characterized the effects of repeated preexposures to MDMA (1.8 or 3.2 mg/kg) on taste aversions induced by MDMA (1.8 mg/kg). Additionally, levels of several monoamines and metabolites were analyzed in frontal cortex and caudate-putamen from subjects in Experiment 3 to assess for persistent monoamine depletions.

RESULTS

MDMA induced dose-dependent taste aversions. Preexposure to MDMA (at both doses) resulted in an attenuation of MDMA-induced taste aversions. These effects were not likely due to persistent monoamine depletions, as subjects preexposed to the higher MDMA dose did not differ from controls in levels of monoamines or metabolites in either brain region examined.

CONCLUSIONS

Prior MDMA experience weakened the ability of MDMA to induce taste aversions. This attenuation of MDMA's aversive effects may occur with low doses that do not persistently alter monoamine levels.

MDMA ("ecstasy") abuse as an example of dopamine neuroplasticity

A number of reviews have focused on the short- and long-term effects of MDMA and, in particular, on the persistent deficits in serotonin neurotransmission that accompany some exposure regimens. The mechanisms underlying the serotonin deficits and their relevance to various behavioral and cognitive consequences of MDMA use are still being debated. It has become clear, however, that some individuals develop compulsive and uncontrolled drug-taking that is consistent with abuse. For other drugs of abuse, this transition has been attributed to neuroadaptations in central dopamine mechanisms that occur as a function of repeated drug exposure. A question remains as to whether similar neuroadaptations occur as a function of exposure to MDMA and the impact of serotonin neurotoxicity in the transition from use to abuse. This review focuses specifically on this issue by first providing an overview of human studies and then reviewing the animal literature with specific emphasis on paradigms that measure subjective effects of drugs and self-administration as indices of abuse liability. It is suggested that serotonin deficits resulting from repeated exposure to MDMA self-administration lead to a sensitized dopaminergic response to the drug and that this sensitized response renders MDMA comparable to other drugs of abuse.

Tolerance to 3,4-methylenedioxymethamphetamine is associated with impaired serotonin release

Tolerance to the behavioural effects of 3,4-methylenedioxymethamphetamine (MDMA) following high dose exposure has been attributed to alterations in serotonergic systems. The present study aimed to determine whether decreased 5-HT release and/or 5-HT(2A/C) receptor desensitization might play a role in tolerance by measuring the response to selective ligands following MDMA exposure. To this end, the latency to nose poke and emerge from a hide box to an open field arena following administration of various ligands to MDMA pre-treated and control rats was measured. Acute exposure to MDMA (0.0-3.3 mg/kg), the 5-HT releasing stimulant fenfluramine (0.0-2.0 mg/kg) and the 5-HT(2) receptor agonist m-CPP (0.0-1.25 mg/kg) increased nose poke and emergence latency. Following administration of doses that produce 5-HT(2A) receptor-mediated behaviours, the 5-HT(2) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane failed to alter nose poke and emergence latency, suggesting a limited role of this receptor subtype in these behaviours. Activation of 5-HT(2C) receptors was implicated in the behavioural response to both MDMA and m-CPP since the increased emergence latency was dose-dependently attenuated by pre-treatment with the selective 5-HT(2C) receptor antagonist RS102221 (0.0-1.0 mg/kg). Tolerance to the behavioural effect of MDMA and fenfluramine but not m-CPP was produced by prior exposure to MDMA (10 mg/kg administered at two-hour intervals, total 40 mg/kg), and tissue levels of 5-HT and 5-HIAA were decreased. These findings suggest that tolerance to the increased nose poke and emergence latency produced by MDMA is due to impaired 5-HT release.

Effects of priming injections of MDMA and cocaine on reinstatement of MDMA- and cocaine-seeking in rats

Exposure to self-administered drugs is sufficient to produce drug-seeking in animal models. In many cases priming injections of drugs that share discriminative stimulus properties with the self-administered drug also can lead to drug-seeking, suggesting that exposure might precipitate relapse. The present investigation examined the ability of MDMA or cocaine priming injections to reinstate extinguished drug-seeking in rats. Priming injections of cocaine (0-20.0 mg/kg) and MDMA (0.0-10.0 mg/kg) reinstated extinguished drug-taking for both the cocaine- and MDMA-trained rats. In a separate group of cocaine-trained rats that received repeated exposure to 10.0 mg/kg MDMA, the initial exposure to MDMA (10.0 mg/kg, i.p.) failed to reinstate extinguished responding but MDMA became an effective prime for reinstatement of extinguished cocaine-taking behavior with repeated exposure. Effects of MDMA in MDMA-trained rats was greater than the effect in cocaine-trained rats suggesting that extensive experience with MDMA self-administration might have sensitized rats to this effect. These findings show that extinguished MDMA self-administration, like self-administration of other drugs of abuse, can be reinstated by exposure to psychostimulants thereby precipitating relapse.

Tolerance to 3,4-methylenedioxymethamphetamine in rats exposed to single high-dose binges

3,4-Methylenedioxymethamphetamine (MDMA or ecstasy) stimulates the transporter-mediated release of monoamines, including 5-HT. High-dose exposure to MDMA causes persistent 5-HT deficits (e.g. depletion of brain 5-HT) in animals, yet the functional and clinical relevance of such deficits are poorly defined. Here we examine functional consequences of MDMA-induced 5-HT depletions in rats. Male rats received binges of three i.p. injections of MDMA or saline, one injection every 2 h; MDMA was given at a threshold pharmacological dose (1.5 mg/kgx3, low dose) or at a fivefold higher amount (7.5 mg/kgx3, high dose). One week later, jugular catheters and intracerebral guide cannulae were implanted. Two weeks after binges, rats received acute i.v. challenge injections of 1 and 3 mg/kg MDMA. Neuroendocrine effects evoked by i.v. MDMA (prolactin and corticosterone secretion) were assessed via serial blood sampling, while neurochemical effects (5-HT and dopamine release) were assessed via microdialysis in brain. MDMA binges elevated core temperatures only in the high-dose group, with these same rats exhibiting approximately 50% loss of forebrain 5-HT 2 weeks later. Prior exposure to MDMA did not alter baseline plasma hormones or dialysate monoamines, and effects of i.v. MDMA were similar in saline and low-dose groups. By contrast, rats pretreated with high-dose MDMA displayed significant reductions in evoked hormone secretion and 5-HT release when challenged with i.v. MDMA. As tolerance developed only in rats exposed to high-dose binges, hyperthermia and 5-HT depletion are implicated in this phenomenon. Our results suggest that MDMA tolerance in humans may reflect 5-HT deficits which could contribute to further dose escalation.

Interactions between ethanol and cocaine, amphetamine, or MDMA in the rat: thermoregulatory and locomotor effects

RATIONALE

(+/-)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is often taken recreationally with ethanol (EtOH). In rats, EtOH may potentiate MDMA-induced hyperactivity, but attenuate hyperthermia.

OBJECTIVE

Experiment 1 compared the interactions between EtOH (1.5 g/kg) and MDMA (6.6 mg/kg) with EtOH + cocaine (COCA; 10 mg/kg) and EtOH + amphetamine (AMPH; 1 mg/kg) on locomotor activity and thermoregulation. Experiment 2 used a weaker dose of MDMA (3.3 mg/kg) and larger doses of COCA (20 mg/kg) and AMPH (2 mg/kg).

MATERIALS AND METHODS

Drug treatments were administered on four occasions (2, 5, and 2 days apart, respectively; experiment 1) or two (2 days apart; experiment 2).

RESULTS

All psychostimulants increased activity, and EtOH markedly increased the effect of MDMA. AMPH alone-related hyperactivity showed modest sensitization across treatment days, while MDMA + EtOH activity showed marked sensitization. AMPH, COCA, and MDMA induced hyperthermia of comparable amplitude (+1 to +1.5 degrees C). Co-treatment with EtOH and AMPH (1 mg/kg) or COCA (10 mg/kg) produced hypothermia greater than that produced by EtOH alone. Conversely, EtOH attenuated MDMA-related hyperthermia, an effect increasing across treatment days. These results demonstrate that the interaction between MDMA and EtOH may be different from the interaction between EtOH and AMPH or COCA.

CONCLUSION

Because of potential health-related consequences of such polydrug misuse, it is worth identifying the mechanisms underlying these interactions, especially between EtOH and MDMA. Given the different affinity profiles of the three drugs for serotonin, dopamine, and norepinephrine transporters, our results appear compatible with the possibility of an important role of serotonin in at least the EtOH-induced potentiation of MDMA-induced hyperlocomotion.

Neurochemical and behavioral consequences of striatal injection of 5,7-dihydroxytryptamine

It is known that central serotonin (5HT) is involved in anxiety, but the behavioral results of many studies have been inconsistent. A prevalent research approach is to destroy 5HT neurotoxically. Such lesions were mostly generated by injecting 5,7-dihydroxytryptamine into ventricles or raphé nuclei, leading to rather global losses of 5HT in the brain. However, there is evidence for differential effects of 5HT in different brain structures regarding anxiety. Therefore, we decided to study the effects of injecting 5,7-dihydroxytryptamine into the forebrain. We chose the ventral striatum as the site of injection, since there is evidence that 5HT may be involved in anxiety there. We administered the neurotoxin bilaterally in adult rats, and analyzed neurochemical and behavioral consequences in three experiments. The first one showed that the toxin dose-dependently (10-50 microg) depleted 5HT in the ventral striatum, neostriatum, frontal cortex, and amygdala. Besides 5HT, dopamine was also partly depleted there. This dopaminergic lesion was prevented in a second experiment, where rats were pre-treated systemically with the dopamine reuptake inhibitor nomifensine. In the final experiment, the functional consequences of such 5HT lesions were tested, which yielded moderate anxiogenic effects in the elevated plus maze and in the open field. Also, there were lesion effects on aversively motivated ultrasonic vocalization during an active avoidance test. In contrast, active avoidance performance itself and general activity in the open field were not affected. Lesion effects became discernible there when challenging rats with MDMA. The psycho-stimulatory effectiveness of this drug, which acts largely via the availability of 5HT in the brain, was reduced to degrees that depended on the size of 5HT lesion. These results are discussed with respect to factors such as severity of lesion, anatomical specificity, and the role of 5HT in anxiety.