Mechanisms for Extracellular Glutamate Formation in Brai

据推测，间质间的谷氨酸水平维持在低水平以防止兴奋性毒性。这是至关重要的，因为发育中的大脑由于兴奋性氨基酸受体的上调而增加了对兴奋性毒性的敏感性。围产期缺氧/缺血时细胞外谷氨酸升高仅增加约3倍，但却是死亡率和发病率的主要原因。幸存者通常会留下严重的永久性神经损伤。然而，我们假设在间质空间中，低水平的谷氨酸是必要的。谷氨酸的基线水平已被证明与代谢受体相互作用。本项目将研究细胞外谷氨酸在围产期和成人大脑中正常和创伤性围产期大脑缺氧/缺血后产生的机制。初步结果表明，大鼠和小鼠输注谷氨酰胺后

It has been postulated that glutamate levels in the interstitial space are maintained at low levels to prevent excitotoxicity. This is critical since the developing brain has increased sensitivity to excitotoxicity due to up regulation of excitatory amino acid receptors. Elevated extracellular glutamate in perinatal hypoxia/ischemia increases only about 3 fold, yet is a major cause of mortality and morbidity. Survivors are often left with severe, permanent neurological impairment. However, we postulate that a low level of glutamate is necessary in the interstitial space. Baseline levels of glutamate have been shown to interact with metabotropic receptors. This project will examine mechanisms by which extracellular glutamate is generated in both perinatal and adult brain in the normal and in the traumatized perinatal brain following an episode of hypoxia/ischemia. Preliminary results indicate that infusion of glutamine into the rat and mouse brain by microdialysis markedly increases the concentration of extracellular glutamate to excitotoxic levels and that it is synthesized in the extracellular space from glutamine. The concentration of extracellular glutamine in brain is normally 200 -300 mu M providing a ready precursor. We hypothesize that under normal conditions the primary mechanism for extracellular glutamate synthesis is maleate activated glutaminase (MAG), a side reaction of the ectoenzyme gamma-glutamyl transpeptidase (gamma-GTP). We hypothesize a second mechanism predominates in trauma. We propose that intramitochondrial phosphate dependent glutaminase (PDG) is released (or exposed) from damaged cells and converts glutamine to glutamate resulting in excitotoxicity and cell death days after the initial traumatic event. These enzymes may also play a role in conditions such as elevated ammonia. We will determine the enzymatic mechanism for extracellular glutamate formation in normal and 7-day old hypoxic-ischemic rat brain, adult rats under conditions of ammonia toxicity, mice deficient in the enzyme gamma-GTP, and in neuronal cell cultures. Studies will utilize microdialysis and cell culture in the presence and absence of effectors of the enzymes involved. It is anticipated that these studies will lead to new approaches in the care of infants that have experienced hypoxic or ischemic injury during infancy.