Functional Change of Glutamate Transporter GLT-1 during Ischemia

谷氨酸转运是脑内谷氨酸从细胞外液中排出的唯一机制，因此，谷氨酸转运体功能障碍或功能变化可能参与了缺血时细胞外谷氨酸的增加过程。在本研究中，我们对遗传缺乏GLT-1的小鼠进行了体内脑微透析实验，并检测了GLT-1在缺血期间细胞外谷氨酸增加中的作用。将微透析探针放置在野生型小鼠和GLT-1缺失小鼠的海马CA1区，并在缺血5分钟和20分钟时测定谷氨酸水平。同时进行组织学检查。缺乏GLT-1的小鼠脑缺血5分钟的谷氨酸水平显著高于野生型小鼠相应的谷氨酸水平。在缺乏GLT-1的小鼠的CA1区可诱导迟发性神经元死亡，而在野生型小鼠的CA1区则无此现象。野生型小鼠在缺血20分钟的最后12.5分钟内的谷氨酸水平显著高于缺乏GLT-1的小鼠相应的谷氨酸水平。GLT-1缺失小鼠的CA1区和野生型小鼠的CA1区均可诱导急性神经元死亡。这些结果表明，GLT-1在缺血期间发生了功能变化：在5分钟的缺血期间，它清除了细胞外的谷氨酸以保护神经元，并在20分钟的最后12.5分钟期间释放了谷氨酸，引发了神经元的急性死亡。

The glutamate transport is the only mechanism for removal of glutamate from the extracellular fluid in the brain, therefore, dysfunction or functional changes in glutamate transporters may be involved in the process of increase in extracellular glutamate during ischemia. In the present study, we performed in vivo brain microdialysis experiments in the mice genetically lacking GLT-1, and examined the role of GLT-1 in increase in the extracellular glutamate during ischemia. A microdialysis probe was placed in the hippocampal CA1 of wild type mice and mice lacking GLT-1, and glutamate levels were measured during 5- and 20-min ischemia. Histological examinations were also performed. The glutamate levels during 5-min ischemia in mice lacking GLT-1 were significantly higher than the corresponding glutamate levels in wild type mice. Delayed neuronal death following 5-min ischemia was induced in the CA1 of the mice lacking GLT-1 but not in the CA1 of the wild type mice. The glutamate levels during the last 12.5 minutes of 20-min ischemia in wild type mice were significantly higher than the corresponding glutamate levels in mice lacking GLT-1. Acute neuronal death was induced in the CA1 of mice lacking GLT-1 and also in the CA1 of the wild type mice. These results suggest that GLT-1 changes in function during ischemia : It removes extracellular glutamate to protect neurons during 5-min ischemia and releases glutamate, triggering acute neuronal death, during the last 12.5 minutes of 20-min ischemia.