On the role of tyrosine and peripheral metabolism in 3,4-methylenedioxymethamphetamine-induced serotonin neurotoxicity in rats
Introduction
3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is an amphetamine derivative, which has become a very popular drug among young adults despite physical or psychiatric complications reported in recreational MDMA users (for reviews see Cole and Sumnall, 2003, Green et al., 2003). It is known that single or repeated injections of MDMA cause different changes in neurochemical and histological markers of the serotonergic function in the brain of rodents (Ricaurte et al., 2000), primates (Hatzidimitriou et al., 1999) and, possibly, in humans (Kish et al., 2000, McCann et al., 1998). Such neurotoxicity is evidenced by the decline in the activity of tryptophan hydroxylase (Stone et al., 1988), a decrease in serotonin (5-hydroxytryptamine, 5-HT) content, reduced levels of the type 2 vesicular monoamine transporter (Ricaurte et al., 2000), a lower density of [3H]paroxetine-labeled 5-HT transporters in several regions of the brain (Aguirre et al., 1995, Aguirre et al., 1999, Hervias et al., 2000) and an impairment of central 5-HT function (Barrionuevo et al., 2000, Hatzidimitriou et al., 1999). This constellation of findings, coupled with neuroanatomical observations using different techniques such as silver impregnation methods (Commins et al., 1987), immunohistochemistry (Molliver et al., 1990, O'Hearn et al., 1988), and Fluoro-Jade B staining for neuronal degeneration (Schmued, 2003) strongly suggest that MDMA is a selective 5-HT neurotoxin in rats.
There is a substantial body of evidence indicating that increased free radical formation is responsible for MDMA-induced neurotoxicity (e.g. Aguirre et al., 1999, Shankaran et al., 2001). However, during the last few years the issue of whether tyrosine/dopamine (DA) or toxic MDMA metabolites are responsible for MDMA toxicity has been a matter of debate. Taking into account that central administration of the drug does not cause any change in the serotonergic system, it appears reasonable to think that MDMA must be peripherally metabolized in order to cause neurotoxicity (Esteban et al., 2001, Goñi-Allo et al., 2007, Nixdorf et al., 2001). It has been proposed that quinone thioether adducts of MDMA catechol type metabolites, 3,4-dihydroxymethaphetamine (HHMA) and 3,4-dihydroxyamphetamine (HHA), are responsible for the free radical formation which may lead to neurotoxicity (Bai et al., 1999, Bai et al., 2001, Jones et al., 2004, Miller et al., 1997) (Fig. 1). Support for this contention comes from a recent study in which glutathione (GSH) and N-acetylcysteine conjugates of HHMA have been identified in the brain of rats administered with MDMA systemically (Jones et al., 2005). Other authors, however, have suggested that free radical formation is dependent upon the prolonged and excessive release of DA elicited by MDMA (Goñi-Allo et al., 2006, Sprague et al., 1998). Recent support for this contention comes from a report by Breier et al. (2006). In this work the authors showed that MDMA increases the concentration of tyrosine in the brain to cause a long-term depletion of 5-HT via the non-enzymatic, tyrosine hydroxylase-independent, hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and subsequently to DA via aromatic amino acid decarboxylase within 5-HT terminals.
Although at first sight both hypotheses appear to contend with each other, we believe that they are complementary. Therefore, this study was undertaken to try to reconcile both hypotheses and to address the dilemma of the causality of MDMA neurotoxicity. That is, the metabolism of MDMA into putative toxic compounds or increased concentrations of tyrosine.
Section snippets
Drugs and chemicals
(+/−)-MDMA–HCl was purchased from Lipomed (Arlesheim, Switzerland), or was provided by the “Servicio de Restricción de Estupefacientes” (Spanish regulatory body on psychotropic drugs). 5-HT creatinine sulfate, 5-hydroxyindole acetic acid (5-HIAA), DA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), l-tyrosine methyl ester–HCl, and acivicin were from Sigma (UK). Entacapone was obtained from Comtan® tablets (Novartis). 4-Hydroxy-3-methoxymethamphetamine (HMMA),
Comparative study of the effects of a toxic vs. a non-toxic dose of MDMA on core body temperature, plasma tyrosine concentrations, long-term 5-HT depletion and MDMA disposition
Statistical analysis of AUC derived from plasma concentrations of MDMA and its main metabolites revealed significant differences between both doses of MDMA [MDMA: t(14) = 3.738, p < 0.01; MDA: t(14) = 15.294, p < 0.001; HMMA: t(14) = 4.829, p < 0.001]. Plasma concentrations of HMA were below the limit of detection after the non-toxic dose of MDMA (Fig. 2). Noteworthy, the dose of MDMA (15 mg/kg) markedly increased blood MDA levels, but not MDMA levels, when compared to the non-toxic dose. In fact, doubling
Discussion
In the present study, we investigated the contribution of l-tyrosine or MDMA metabolism to the mechanisms underlying MDMA-induced brain 5-HT depletions. For this, we first administered a non-toxic dose of MDMA (7.5 mg/kg i.p.) alone or in combination with l-tyrosine. Secondly, using the same dosage regimen of MDMA, we interfered with MDMA metabolic disposition. Our data suggest that while l-tyrosine can contribute to 5-HT toxicity, MDMA metabolism appears to be the limiting step in the
Acknowledgments
The authors would like to thank “Fundación para la Investigación Médica Aplicada (FIMA)” and Ministerio de Educación y Ciencia for a fellowship to B.G.-A. and E.P., respectively. This work was supported by grants from the Ministerio de Educación y Ciencia (SAF2005-07919-C02-02), Ministerio de Sanidad y Consumo (PNSD), the Spanish Networks of Excellence (ISCIII, Red de Trastornos Adictivos, and Red CIEN) and Generalitat de Catalunya (2005SGR00032).
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These authors contributed equally to this work.