L-DOPA OVERLOAD: MECHANISMS FOR THE SIDE EFFECTS.

说明(申请人提供)：L多巴是多巴胺(DA)的前体，是治疗帕金森病(PD)症状最有效的药物，但由于连续使用会出现严重的副作用，限制了其使用。原因尚不清楚，但在L-多巴治疗过程中，组织中L-多巴的水平大大超过了正常情况下几乎检测不到的水平，因此L-多巴的超负荷对儿茶酚胺系统进行了异常调节。我们的假说认为，以高水平的L-多巴和多巴胺为底物的酶，如L芳香族氨基酸脱羧酶、多巴胺(DA)-β羟基酶和儿茶酚-O-甲基转移酶，以及催化合成辅因子S-腺苷甲硫氨酸的蛋氨酸腺苷转移酶(MAT)，是L-多巴和多巴胺代谢的酶。COMT和MAT的诱导会增加L-多巴和多巴胺的代谢，并产生干扰的甲基代谢物。DBH的诱导会产生异位去甲肾上腺素(NE)，这可能会稀释DA在黑质纹状体通路中的作用。诱导的LAAD将加速L-多巴对DA的催化作用，引起DA峰。DA反过来将导致对LAAD的反馈抑制，从而产生可能有助于引起开关效应的搏动性DA供应。我们的假说建立在初步结果的基础上，初步结果表明，L-多巴诱导COMT和MAT，这两种酶反过来又代谢L-多巴和多巴胺。L-多巴还可诱导脑LAAD。多巴胺的甲基代谢产物3-甲氧基酪胺(3-MT)和3，4-二甲氧基苯乙胺(DIMPEA)可使注射L多巴的帕金森病模型大鼠和L多巴治疗的帕金森病模型大鼠的运动活动减少和增加，并使DA受体结合和3-O-甲基多巴(3-OMD)水平升高。3-OMD降低L-多巴的药效和DA的转换率。研究还发现，服用L多巴的帕金森病患者的心律失常是由NE升高引起的。具体研究目的是：(1)进一步研究L-多巴对玉米的COMT、MAT以及LAAD和DBH的诱导作用。(#2)确定LAAD的诱导是否导致DA激增，而DA反过来抑制LAAD，从而导致DA的脉动性供应，这可能与开关效应有关。(3)定位黑质纹状体神经元的DBH诱导和去甲肾上腺素的产生，并确定纹状体组织中去甲肾上腺素是否与多巴胺共同释放。(4)评价3-O-甲基多巴、3-MT和DIMPEA的行为和受体效应，以了解它们是否参与了L-多巴的副作用。研究重点将放在基底节的变化上。正在研究的生化途径是药物作用的关键，结果可以很容易地转化为治疗，因此，该项目的前景是找到在L-多巴的益处不受影响的情况下，能够针对特定的酶和代谢途径的药物。

DESCRIPTION (provided by applicant):  L-dopa, the precursor of dopamine (DA), is the most effective drug used for treating the symptoms of Parkinson's disease (PD), but its use is limited by serious side effects that occur after continuous use. The causes are unknown, but during L-dopa treatments tissue L-dopa greatly exceeds the barely detectable levels that normally exist, therefore the overload of L-dopa dys-regulates the catecholamine system. Our hypothesis proposes that enzymes that sense the high levels of L-dopa and DA as substrates, such as L- aromatic amino acid decarboxylase (LAAD), dopamine (DA)-beta hydroxylase (DBH) and catechol-O- methyltransferase (COMT), as well as methionine adenosyltransferase (MAT) that catalyses the synthesis of the cofactor, S-adenosylmethionine, for the metabolism of L-dopa and DA, are induced. The induction of COMT and MAT will increase the metabolism of L-dopa and DA and will generate interfering methyl metabolites. The induction of DBH will produce ectopic norepinephrine (NE) that may dilute the efficacy of DA in the nigrostriatal pathway. The induced LAAD will accelerate the catalysis of L-dopa to DA, causing DA surge. DA in turn will cause feedback inhibition of LAAD, generating a pulsatile supply of DA that may help to cause the on-off effects. "We base our hypothesis on preliminary results showing that L-dopa induced COMT and MAT, enzymes that, in turn, metabolize L-dopa and DA. L-dopa also induced brain LAAD. The methyl metabolites of DA, 3-methoxytyramine (3-MT) and 3,4-dimethoxyl-phenylethylamine (DIMPEA), respectively, decreased and increased motor activities and DA receptor binding and 3-O-methyldopa (3- OMD) occurred in high levels in rat models of PD injected with L-dopa and in L-dopa-treated PD patients. 3- OMD decreased the efficacy of L-dopa and the turnover of DA. Studies also found that cardiac arrhythmia in L-dopa-treated PD patients was caused by increased NE.  The specific aims are: (# 1) To further study the induction of COMT, MAT as well as LAAD and DBH by L-dopa. (# 2) Determine if the induction of LAAD causes DA surges and that DA in turn counter inhibits LAAD causing a pulsatile supply of DA that may be related to the on-off effects. (# 3) Localize DBH induction and NE production in nigrostriatal neurons and determine if NE is co-released with DA from striatal tissues. (#4) Evaluate the behavioral and receptor effects of 3-O-methyldopa, 3-MT and DIMPEA, to know if they contribute to the side effects of L-dopa. "The studies will focus on changes in the basal ganglia. The biochemical pathways being studied are at the crux of drug actions and the results can be readily translated into therapy, so, the outlook for the project is to find agents that will target specific enzymes and metabolic pathways at times when the beneficial effects of L-dopa will not be compromised.