Short- and long-term effects of MDMA (“ecstasy”) on synaptosomal and vesicular uptake of neurotransmitters in vitro and ex vivo
Introduction
3,4-Methylenedioxymethamphetamine (MDMA), popularly known as ecstasy, is an amphetamine derivative widely used as a recreational drug. According to the Norwegian Institute for Alcohol and Drug Research, 5.7% of youths in Oslo (aged 15–20 years) took MDMA at least once in 2000. The mechanisms underlying the pharmacological and toxicological effects of MDMA are not clear at present, but there is considerable evidence that MDMA acts by increasing the extracellular concentrations of the monoamine neurotransmitters serotonin, noradrenaline and dopamine (Iravani et al., 2000, Kalant, 2001). The increase in monoamine concentrations in the synaptic cleft is caused by the ability of MDMA to stimulate neurotransmitter release, inhibit neurotransmitter uptake or block neurotransmitter metabolism (Sanchez et al., 2001). The relative importance of these factors is still not elucidated. However, recent studies have favored that MDMA causes inhibition of the reuptake transporter rather than releasing neurotransmitters directly, unlike the effect of the related compound (+)-amphetamine (Iravani et al., 2000, Kalant, 2001).
MDMA is considered to be potentially toxic to serotonergic axons and nerve terminals (Shankaran et al., 1999, Falk et al., 2002, Meyer and Ali, 2002, O’Shea et al., 2002). The exact mechanism of MDMAs neurotoxicity is not known, but oxidative stress, hyperthermia, and increased level of dopamine are all possible factors suggested to be involved (Shankaran et al., 1999, Sanchez et al., 2001). The mechanism leading to MDMA-induced generation of reactive oxygen species (ROS) is unknown. Sprague et al. (1998) postulated that dopamine released after exposure to MDMA, is subsequently taken up by serotonergic nerve endings where it undergoes intraneuronal oxidation through monoamine oxidase B, and gives rise to ROS. In contrast, based on the finding that animals with near total depletions of brain dopamine still are susceptible to MDMA neurotoxicity, others report that MDMA neurotoxicity is not dependent upon endogenous dopamine (Yuan et al., 2002). A study showing that MDMA-induced ROS formation is absent in rats in which serotonergic terminals have been depleted by fenfluramine, supports the importance of serotonin itself in the mechanism of MDMA-induced neurotoxicity (Shankaran et al., 1999).
MDMA induces acute behavioral changes and hyperthermia in both rats and humans. The increase in body temperature is caused by increased muscle activity together with a direct action on the thermoregulatory system in the brain (Kalant, 2001, O’Loinsigh et al., 2001, Mechan et al., 2002). Production of a hyperthermic response is thought to be critical for MDMA-induced neurotoxicity, since neurodegeneration can be attenuated if body temperature is kept low by using low ambient temperatures or drugs that produce hypothermia (Malberg et al., 1996, Huether et al., 1997, O’Loinsigh et al., 2001).
There is growing concern about the long-term effects of repeated use of MDMA, partly because of neurotoxic effects in animals at doses similar to those used recreationally by humans (O’Loinsigh et al., 2001), and partly because of the severe character of the long-term effects seen, like cognitive deficits, panic disorder and psychotic episodes (Mayerhofer et al., 2001, Montoya et al., 2002). The structural and functional similarities of MDMA and the antidepressant paroxetine are interesting. Both compounds possess a 3,4-methylenedioxyphenyl group, for which a specific mechanism is suggested to exist in the brain (Hashimoto et al., 1993). In the present study the effect of paroxetine, which is an extremely potent inhibitor of the serotonin transporter (Hajos-Korcsok et al., 2000, Yamane et al., 2001), is compared with the effect of MDMA.
Despite the functional and structural similarities discussed above, MDMA is considered to be a potentially neurotoxic compound, while paroxetine is a widely used medical drug. The aim of our study was to compare the effects of MDMA and paroxetine on synaptosomal and vesicular uptake of neurotransmitters in order to throw light on their differences in neurotoxicity. Possible cellular mechanisms for the toxicity of MDMA are discussed. In addition, short- and long-term effects of MDMA on rats ex vivo were investigated.
Section snippets
Materials
[2,5,6-]dopamine (5.9 or 6.5 Ci/mmol) and 5-hydroxy []tryptamine trifluoroacetate (84 or 106 Ci/mmol) were purchased from Amersham Pharmacia Biotech (UK). l-[2,3,4-]glutamic acid (60 Ci/mmol) and [2,3-]aminobutyric acid were purchased from American Radiolabeled Chemicals Inc. (St. Louis, USA). Filter-Count was purchased from Packard Instrument Co. (Meriden, USA). Paroxetine hydrochloride hemihydrate was a gift from SmithKlineBeecham Pharmaceuticals (Beecham, UK). MDMA (purity > 98%) was a
Effects of MDMA on neurotransmitter uptake in vitro
The uptake of serotonin and dopamine in synaptosomes was significantly reduced at all MDMA concentrations (0.5–20 μM) tested (Fig. 1a). IC50 values were found to be approximately 1.5 and 3 μM for serotonin and dopamine, respectively. In contrast, MDMA exposure produced no change in the uptake of glutamate or GABA.
MDMA also produced a significant reduction in the uptake of serotonin and dopamine in synaptic vesicles (Fig. 1b), with IC50 values at approximately 9 and 6 μM, respectively. As for
Discussion
The present study examined the effects of MDMA and paroxetine on cerebral synaptosomes and synaptic vesicles in vitro and ex vivo. Our results show that MDMA reduces the synaptosomal and vesicular uptake of serotonin and dopamine in a dose dependent manner in vitro. In contrast, neither the uptake of glutamate nor GABA was affected by in vitro exposure to MDMA.
Short-term studies of MDMA (3 × 15 mg/kg; 2-h intervals) ex vivo showed a 37% inhibition of the synaptosomal uptake of dopamine when
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