Sirinathsinghji D J, Heavens R P, McBride C S
Department of Neuroendocrinology, AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge, U.K.
Brain Res. 1988 Mar 8;443(1-2):101-16. doi: 10.1016/0006-8993(88)91603-4.
This study examined the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+) on the levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in push-pull perfusates of the striatum in chloral hydrate-anaesthetized rats. In control animals the levels of DA and DOPAC remained stable for at least 6 h and responded rapidly to a depolarizing stimulus of 25 mM K+. This K+-induced DA release was Ca2+-dependent since no stimulation was observed when the striatal sites were perfused with high K+ in a Ca2+-free medium containing 2 mM EGTA thus verifying that the striatal sites were functionally active. MPTP (0.025 and 0.05 microgram/microliter) stimulated DA release and inhibited DOPAC output in a dose-related manner. MPP+ (0.01, 0.025 and 0.05 microgram/microliter) produced a more robust dose-dependent increase in DA levels in the perfusates; however, the level of suppression of DOPAC was similar to that in response to MPTP. The effect of MPP+ on DA release was attenuated by 10(-6) M benztropine, the DA re-uptake blocker and completely inhibited by 10 micrograms/kg i.p. benztropine and 10(-4) M ouabain, the Na+, K+-ATPase (Na pump) inhibitor. However, although these substances prevented the MPP+-induced release of DA, the levels of DOPAC in the perfusates did not recover and remained completely suppressed suggesting that MPP+ may inhibit extraneuronal rather than intraneuronal monoamine oxidase (MAO). Perfusion of the striatal sites with a Ca2+-free medium containing 2 mM EGTA did not prevent the MPP+-induced DA release indicating that MPP+ does not release DA from the striatal DA terminals by the Ca2+-dependent process of exocytosis. The responses of DA and DOPAC to 25 mM K+ were markedly suppressed in animals treated with MPTP and MPP+, these effects being most severe with the highest dose of MPP+. Moreover, this suppression of the K+-induced responses persisted in animals perfused with MPP+ in the presence of benztropine or ouabain, thus suggesting that MPP+ may have potent deleterious membrane effects. These studies have provided the first direct in vivo demonstration of the action of MPTP and MPP+ and the neuropharmacological basis of this action on DA metabolism in the rat striatum. The results show that the elevated levels of DA in the striatal perfusates are due to a direct action of MPTP and MPP+ on the nigrostriatal DA terminals and cannot be fully accounted for solely by their inhibition of MAO activity and/or inhibition of DA re-uptake.(ABSTRACT TRUNCATED AT 250 WORDS)
本研究检测了1-甲基-4-苯基-1,2,3,6-四氢吡啶（MPTP）及其代谢产物1-甲基-4-苯基吡啶（MPP+）对水合氯醛麻醉大鼠纹状体推挽灌注液中多巴胺（DA）和3,4-二羟基苯乙酸（DOPAC）水平的影响。在对照动物中，DA和DOPAC水平至少6小时保持稳定，并对25 mM K+的去极化刺激迅速产生反应。这种K+诱导的DA释放是Ca2+依赖性的，因为当纹状体部位在含有2 mM EGTA的无Ca2+培养基中用高K+灌注时未观察到刺激，从而证实纹状体部位功能活跃。MPTP（0.025和0.05微克/微升）以剂量相关的方式刺激DA释放并抑制DOPAC输出。MPP+（0.01、0.025和0.05微克/微升）使灌注液中DA水平产生更强的剂量依赖性升高；然而，DOPAC的抑制水平与对MPTP的反应相似。MPP+对DA释放的作用被DA再摄取阻滞剂10(-6) M苯海索减弱，并被10微克/千克腹腔注射苯海索和Na+，K+-ATP酶（钠泵）抑制剂10(-4) M哇巴因完全抑制。然而，尽管这些物质阻止了MPP+诱导的DA释放，但灌注液中DOPAC水平并未恢复且仍被完全抑制，这表明MPP+可能抑制细胞外而非细胞内单胺氧化酶（MAO）。用含有2 mM EGTA的无Ca2+培养基灌注纹状体部位并不能阻止MPP+诱导的DA释放，这表明MPP+不是通过Ca2+依赖性胞吐过程从纹状体DA终末释放DA。在用MPTP和MPP+处理的动物中，DA和DOPAC对25 mM K+的反应明显受到抑制，这些作用在MPP+最高剂量时最为严重。此外，在苯海索或哇巴因存在下用MPP+灌注的动物中，这种对K+诱导反应的抑制持续存在，因此表明MPP+可能具有强大的有害膜效应。这些研究首次在体内直接证明了MPTP和MPP+的作用以及这种作用对大鼠纹状体DA代谢的神经药理学基础。结果表明，纹状体灌注液中DA水平升高是由于MPTP和MPP+对黑质纹状体DA终末的直接作用，不能仅通过它们对MAO活性的抑制和/或对DA再摄取的抑制来完全解释。（摘要截短于250字）
