Inhibition of 3,4-methylenedioxymethamphetamine metabolism leads to marked decrease in 3,4-dihydroxymethamphetamine formation but no change in serotonin neurotoxicity: implications for mechanisms of neurotoxicity

Neurotoxicity of MDMA: Main effects and mechanisms

Preclinical and clinical studies indicate that 3,4-methylenedioxymethamphetamine (MDMA; 'ecstasy'), in addition to having abuse potential, may elicit acute and persistent abnormalities of varying severity at the central level. Importantly, neurotoxic effects of MDMA have been demonstrated in experimental animals. Accordingly, central toxicity induced by MDMA may pose a serious harm for health, since MDMA is among the substances that are used for recreational purposes by young and adult people. This review provides a concise overview of recent findings from preclinical and clinical studies that evaluated the central effects of MDMA, and the mechanisms involved in the neurotoxicity induced by this amphetamine-related drug.

Bucladesine Attenuates Spatial Learning and Hippocampal Mitochondrial Impairments Induced by 3, 4-Methylenedioxymethamphetamine (MDMA)

Neurotoxic effects of systemic administration of 3, 4- methylenedioxymethamphetamine (MDMA) has been attributed to MDMA and its metabolites. However, the role of the parent compound in MDMA-induced mitochondrial and memory impairment has not yet been investigated. Moreover, it is not yet studied that analogs of 3', 5'-cyclic adenosine monophosphate (cAMP) could decrease these neurotoxic effects of MDMA. We wished to investigate the effects of the central administration of MDMA on spatial memory and mitochondrial function as well as the effects of bucladesine, a membrane-permeable analog of cAMP, on these effects of MDMA. We assessed the effects of pre-training bilateral intrahippocampal infusion of MDMA (0.01, 0.1, 0.5, and 1 μg/side), bucladesine (10 and 100 μM) or combination of them on spatial memory, and different parameters of hippocampal mitochondrial function including the level of reactive oxygen species (ROS) production, mitochondrial membrane potential (MMP), mitochondrial swelling, mitochondrial outer membrane damage, the amount of cytochrome c release as well as hippocampal ADP/ATP ratio. The results showed that MDMA caused spatial memory impairments as well as mitochondrial dysfunction as evidenced by the marked increase in hippocampal ADP/ATP ratio, ROS level, the collapse of MMP, mitochondrial swelling, and mitochondrial outer membrane damage leading to cytochrome c release from the mitochondria. The current study also found that bucladesine markedly reduced the destructive effects of MDMA. These results provide evidence of the role of the parent compound (MDMA) in MDMA-induced memory impairments through mitochondrial dysfunction. This study highlights the role of cAMP/PKA signaling in MDMA-induced memory and mitochondrial defects.

Differential effects of MDMA and cocaine on inhibitory avoidance and object recognition tests in rodents

INTRODUCTION

Drug addiction continues being a major public problem faced by modern societies with different social, health and legal consequences for the consumers. Consumption of psychostimulants, like cocaine or MDMA (known as ecstasy) are highly prevalent and cognitive and memory impairments have been related with the abuse of these drugs.

AIM

The aim of this work was to review the most important data of the literature in the last 10 years about the effects of cocaine and MDMA on inhibitory avoidance and object recognition tests in rodents.

DEVELOPMENT

The object recognition and the inhibitory avoidance tests are popular procedures used to assess different types of memory. We compare the effects of cocaine and MDMA administration in these tests, taking in consideration different factors such as the period of life development of the animals (prenatal, adolescence and adult age), the presence of polydrug consumption or the role of environmental variables. Brain structures involved in the effects of cocaine and MDMA on memory are also described.

CONCLUSIONS

Cocaine and MDMA induced similar impairing effects on the object recognition test during critical periods of lifetime or after abstinence of prolonged consumption in adulthood. Deficits of inhibitory avoidance memory are observed only in adult rodents exposed to MDMA. Psychostimulant abuse is a potential factor to induce memory impairments and could facilitate the development of future neurodegenerative disorders.

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

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

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

The current study aimed to elucidate the role of pharmacokinetic (PK) parameters and neurotransmitter efflux in explaining variability in (±) 3, 4-methylenedioxymethamphetamine (MDMA) self-administration in rats. PK profiles of MDMA and its major metabolites were determined after the administration of 1.0 mg/kg MDMA (iv) prior to, and following, the acquisition of MDMA self-administration. Synaptic levels of 5-hydroxytryptamine (5HT) and dopamine (DA) in the nucleus accumbens were measured following administration of MDMA (1.0 and 3.0 mg/kg, iv) using in vivo microdialysis and compared for rats that acquired or failed to acquire MDMA self-administration. Effects of the 5HT neurotoxin, 5,7 dihydroxytryptamine (5, 7-DHT), on the acquisition of MDMA and cocaine self-administration were also determined. In keeping with previous findings, approximately 50% of rats failed to meet a criterion for acquisition of MDMA self-administration. The PK profiles of MDMA and its metabolites did not differ between rats that acquired or failed to acquire MDMA self-administration. MDMA produced more overflow of 5HT than DA. The MDMA-induced 5HT overflow was lower in rats that acquired MDMA self-administration compared with those that did not acquire self-administration. In contrast, MDMA-induced DA overflow was comparable for the two groups. Prior 5,7-DHT lesions reduced tissue levels of 5HT and markedly increased the percentage of rats that acquired MDMA self-administration and also decreased the latency to acquisition of cocaine self-administration. These data suggest that 5HT limits the initial sensitivity to the positively reinforcing effects of MDMA and delays the acquisition of reliable self-administration.

Catechol-o-methyltransferase and 3,4-({+/-})-methylenedioxymethamphetamine toxicity

Metabolism of 3,4-(±)-methylenedioxymethamphetamine (MDMA) is necessary to elicit its neurotoxic effects. Perturbations in phase I and phase II hepatic enzymes can alter the neurotoxic profile of systemically administered MDMA. In particular, catechol-O-methyltransferase (COMT) plays a critical role in determining the fraction of MDMA that is converted to potentially neurotoxic metabolites. Thus, cytochrome P450 mediated demethylenation of MDMA, or its N-demethylated metabolite, 3,4-(±)-methylenedioxyamphetamine, give rise to the catechols, N-methyl-α-methyldopamine and α-methyldopamine, respectively. Methylation of these catechols by COMT limits their oxidation and conjugation to glutathione, a process that ultimately gives rise to neurotoxic metabolites. We therefore determined the effects of modulating COMT, a critical enzyme involved in determining the fraction of MDMA that is converted to potentially neurotoxic metabolites, on MDMA-induced toxicity. Pharmacological inhibition of COMT in the rat potentiated MDMA-induced serotonin deficits and exacerbated the acute MDMA-induced hyperthermic response. Using a genetic mouse model of COMT deficiency, in which mice lack a functional COMT gene, such mice displayed greater reductions in dopamine concentrations relative to their wild-type (WT) counterparts. Neither WT nor COMT deficient mice were susceptible to MDMA-induced decreases in serotonin concentrations. Interestingly, mice devoid of COMT were far more susceptible to the acute hyperthermic effects of MDMA, exhibiting greater increases in body temperature that ultimately resulted in death. Our findings support the view that COMT plays a pivotal role in determining the toxic response to MDMA.

Illicit stimulant use in humans is associated with a long-term increase in tremor

Use of illicit stimulants such as methamphetamine, cocaine, and ecstasy is a significant health problem. The United Nations Office on Drugs and Crime estimates that 14-57 million people use stimulants each year. Chronic use of illicit stimulants can cause neurotoxicity in animals and humans but the long-term functional consequences are not well understood. Stimulant users self-report problems with tremor whilst abstinent. Thus, the aim of the current study was to investigate the long-term effect of stimulant use on human tremor during rest and movement. We hypothesized that individuals with a history of stimulant use would exhibit abnormally large tremor during rest and movement. Tremor was assessed in abstinent ecstasy users (n = 9; 22 ± 3 yrs) and abstinent users of amphetamine-like drugs (n = 7; 33 ± 9 yrs) and in two control groups: non-drug users (n = 23; 27 ± 8 yrs) and cannabis users (n = 12; 24 ± 7 yrs). Tremor was measured with an accelerometer attached to the index finger at rest (30 s) and during flexion and extension of the index finger (30 s). Acceleration traces were analyzed with fast-Fourier transform. During movement, tremor amplitude was significantly greater in ecstasy users than in non-drug users (frequency range 3.9-13.3 Hz; P<0.05), but was unaffected in cannabis users or users of amphetamine-like drugs. The peak frequency of tremor did not significantly differ between groups nor did resting tremor. In conclusion, abstinent ecstasy users exhibit an abnormally large tremor during movement. Further work is required to determine if the abnormality translates to increased risk of movement disorders in this population.

Studies of (±)-3,4-methylenedioxymethamphetamine (MDMA) metabolism and disposition in rats and mice: relationship to neuroprotection and neurotoxicity profile

The neurotoxicity of (±)-3,4-methylenedioxymethamphetamine (MDMA; "Ecstasy") is influenced by temperature and varies according to species. The mechanisms underlying these two features of MDMA neurotoxicity are unknown, but differences in MDMA metabolism have recently been implicated in both. The present study was designed to 1) assess the effect of hypothermia on MDMA metabolism, 2) determine whether the neuroprotective effect of hypothermia is related to inhibition of MDMA metabolism, and 3) determine if different neurotoxicity profiles in mice and rats are related to differences in MDMA metabolism and/or disposition in the two species. Rats and mice received single neurotoxic oral doses of MDMA at 25°C and 4°C, and body temperature, pharmacokinetic parameters, and serotonergic and dopaminergic neuronal markers were measured. Hypothermia did not alter MDMA metabolism in rats and only modestly inhibited MDMA metabolism in mice; however, it afforded complete neuroprotection in both species. Rats and mice metabolized MDMA in a similar pattern, with 3,4-methylenedioxyamphetamine being the major metabolite, followed by 4-hydroxy-3-methoxymethamphetamine and 3,4-dihydroxymethamphetamine, respectively. Differences between MDMA pharmacokinetics in rats and mice, including faster elimination in mice, did not account for the different profile of MDMA neurotoxicity in the two species. Taken together, the results of these studies indicate that inhibition of MDMA metabolism is not responsible for the neuroprotective effect of hypothermia in rodents, and that different neurotoxicity profiles in rats and mice are not readily explained by differences in MDMA metabolism or disposition.