Role of Tyrosine in MDMA Toxicity

描述（由申请人提供）：广泛滥用的安非他明类似物，34 -亚甲基二氧基甲基苯丙胺（MDMA，摇头丸）选择性地损害大脑5-HT神经元的轴突末端。几条线索的证据表明多巴胺（DA）有助于这种毒性。虽然DA明显产生氧化副产物，如活性氧和醌，但仍不清楚DA衍生的氧化物质如何对5HT末端产生选择性损伤，5HT末端是mdma诱导的神经毒性的标志。更具体地说，DA如何在5-HT末端积累并产生选择性损伤的机制尚不清楚。目前提出的总体假设是L-酪氨酸，DA的氨基酸前体，有助于神经退行性过程。这一假设是基于我们最近的初步数据，表明MDMA后体内测量的酪氨酸细胞外浓度增加了5倍。与DA不同的是，酪氨酸被主动地从外周转运到大脑和神经元。虽然酪氨酸是DA神经元内DA合成的天然前体，但5HT神经元中高浓度的酪氨酸可能会产生有害的后果。该提议的假设框架是MDMA和高温在5-HT神经元中产生的氧化环境导致酪氨酸非酶氧化为DA前体多巴。5-HT末端的芳香氨基酸脱羧酶（Aromatic amino acid decarboxylase， AADC）将多巴脱羧为DA，导致DA的积累，从而导致DA衍生的活性氧和5HT末端的氧化损伤。酪氨酸作为mdma诱导毒性介质的假设作用是一种新的机制，它有效地综合了当前现有的假设，在某种程度上，不同的观察结果和明显的警告，形成了一个有凝聚力的，理论的和可测试的框架。

DESCRIPTION (provided by applicant): The widely abused amphetamine analogue, 3 4-methylenedioxymethamphetamine (MDMA, Ecstasy) selectively damages the axon terminals of 5-HT neurons in the brain. Several lines of evidence suggest that dopamine (DA) contributes to this toxicity. Although DA clearly produces oxidative by-products such as reactive oxygen species and quinones, it remains unclear as to how DA-derived oxidative species produce selective damage to 5HT terminals, a hallmark of MDMA-induced neurotoxicity. More specifically, the mechanism as to how DA accumulates within 5-HT terminals to produce its selective damage is unknown. The overaraching hypothesis of the current proposal is that L- tyrosine, the amino acid precursor of DA, contributes to the neurodegenerative process. This hypothesis is based on our recent preliminary data indicating a 5-fold increase in the extracellular concentration of tyrosine measured in vivo after MDMA. Unlike DA, tyrosine is actively transported from the periphery and into the brain and neurons. While tyrosine is the natural precursor for DA synthesis within DA neurons, high concentrations of tyrosine in 5HT neurons may have deleterious consequences. The hypothetical framework of this proposal is that the oxidative environment produced by MDMA and hyperthermia in 5-HT neurons causes the non-enzymatic oxidation of tyrosine to the DA precursor, DOPA. Aromatic amino acid decarboxylase (AADC), within the 5-HT terminal then would decarboxylate DOPA to DA, leading to an accumulation of DA and consequently, DA-derived reactive oxygen species and oxidative damage within 5HT terminals. This hypothesized role of tyrosine as a mediator of MDMA-induced toxicity is a novel mechanism that effectively synthesizes current existing hypotheses and to some extent, discrepant observations and apparent caveats into a cohesive, theoretical and testable framework. The Specific Aims are (1) to demonstrate the non-enzymatic oxidation of tyrosine to DOPA and DA within 5HT neurons, (2) to characterize the contributions of L-tyrosine and tyramine to MDMA and hyperthermia-induced oxidative stress and (3) to assess subsequent neuronal damage. The use of cultured RN46A 5HT cells is a novel approach that is uniquely suited to address the hypothesized mechanism of MDMA-induced damage. These experiments will directly measure the individual and relative effects of MDMA, tyrosine, hyperthermia, and the formation of intracellular dopamine on oxidative processes within 5-HT neurons and how these variables affect cell viability. The testing of this model in conjunction with in vivo microdialysis studies, provides a unique and powerful approach to address enigmatic issues previously related to MDMA-induced neurodegeneration of 5HT systems