CEREBRAL GLUTAMATE CONTROL BY IN VIVO 15N MR

我们建议重新探讨这一规定 大脑谷氨酰胺、谷氨酸和γ-氨基丁酸(GABA) 代谢，以阐明神经毒性的机制。 肝性脑病。 新的推动力是酶的非侵入性检测的发展 活体大鼠脑内的活动和代谢物流动 15N磁共振(MR)技术和更灵敏的方法 1H-15N异核多量子相干磁共振波谱 (夫人)。这两种方法都依赖于新陈代谢池中 稳定同位素15N。 具体地说，谷氨酰胺合成酶(GS)的体内活性，以及 星形胶质细胞标志酶和标志谷氨酸脱羧酶(GAD) 将在大鼠的大脑中测量突触前神经末梢的酶 高氨大鼠作为HE动物模型及硫代乙酰胺治疗的实验研究 大鼠作为暴发性肝功能衰竭模型。一般事务人员活动将 表示体内氨掺入谷氨酰胺的速率和 星形胶质细胞的谷氨酸耗尽。GAD活动将反映 神经末梢谷氨酸浓度和GABA比率 综合。最后，全脑的补充率 谷氨酸池将从转氨率确定 富含15N的必需氨基酸，很容易通过血脑 障碍。这些实验将阐明高氨血症对 神经元和星形胶质细胞谷氨酸和GABA的池，并有助于 关于神经毒性机制的三种主要假说的评价 在HE中：1)氨本身具有神经毒性；2)谷氨酰胺蓄积 3)高氨血症通过以下途径引起HE 谷氨酸的神经递质池耗尽。此外， 苯二氮类药物和GABA在脑部病因学中的相对重要性 暴发性肝功能衰竭的功能障碍将被探讨。这些 实验将揭示一组日益重要的 神经系统疾病，包括肝性脑病。

We propose to re-explore the regulation of cerebral glutamine, glutamate and gamma-aminobutyric acid (GABA) metabolism in order to elucidate the mechanisms of neurotoxicity in hepatic encephalopathy (HE). The new impetus is the development of non-invasive assays of enzyme activity and metabolite flux in the brain of living rats, using novel techniques of 15N magnetic resonance (MR) and the more sensitive method of 1H-15N heteronuclear multiple quantum coherence (HMQC) MR spectroscopy (MRS). Both methods depend upon enrichment of metabolic pools with the stable isotope 15N. Specifically, in vivo activities of glutamine synthetase (GS), an astrocyte marker enzyme, and of glutamate decarboxylase (GAD), a marker enzyme of pre-synaptic nerve-terminals, will be measured in the brain of hyperammonemic rats as an animal model of HE, and in thioacetamide-treated rats as a model of fulminant hepatic failure. The GS activity will indicate the in vivo rate of ammonia incorporation into glutamine and of glutamate depletion from the astrocytes. The GAD activity will reflect glutamate concentration in nerve terminals and indicate the rate of GABA synthesis. Finally, the rate of replenishment of the total cerebral glutamate pool will be determined from the rate of transamination from 15N-enriched essential amino acids that readily cross the blood-brain barrier. These experiments will clarify the effect of hyperammonemia on neuronal vs astrocytic pools of glutamate and GABA and contribute to evaluation of three leading hypotheses on the mechanism of neurotoxicity in HE: 1) ammonia per se is neurotoxic; 2) glutamine accumulation contributes to the pathogenesis; and 3) hyperammonemia causes HE through depletion of the neurotransmitter pool of glutamate. In addition, the relative importance of benzodiazepines and GABA in the etiology of brain dysfunction in fulminant hepatic failure will be explored. These experiments will shed light on an increasingly important group of neurological diseases, including hepatic encephalopathy.