Compartmentation of Brain Glutamate Metabolism in Neurons and Glia

• Published in: The Journal of nutrition Vol. 130; no. 4; pp. 1026S - 1031S
• Main Authors: Daikhin, Yevgeny, Yudkoff, Marc
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Bethesda, MD Elsevier Inc 01.04.2000; American Society for Nutritional Sciences; American Institute of Nutrition
• Subjects: Amino acids; Animals; Biochemistry and metabolism; Biological and medical sciences; Brain; Brain - cytology; Brain - metabolism; branched-chain amino acids; Central nervous system; Fundamental and applied biological sciences. Psychology; glia; Glucose; glutamate; Glutamic Acid - metabolism; glutamine; Glutamine - metabolism; human nutrition; Metabolism; Neuroglia - metabolism; neuron; Neurons; Neurons - metabolism; Synaptic Transmission - physiology; transport; Vertebrates: nervous system and sense organs; glucose; AMPA; neuron; NMDA; CNS; metabolism; transport; glutamine; branched-chain amino acids; brain; glia; glutamate; Glial cell; Neuroglia; Biological transport; Central nervous system; Review; Glutamate; Metabolism; Neuron; Excitatory aminoacid; Neurotransmitter; Carrier protein; Brain (vertebrata); Glutamine
• ISSN: 0022-3166; 1541-6100
• DOI: 10.1093/jn/130.4.1026S

Intrasynaptic [glutamate] must be kept low in order to maximize the signal-to-noise ratio after the release of transmitter glutamate. This is accomplished by rapid uptake of glutamate into astrocytes, which convert glutamate into glutamine. The latter then is released to neurons, which, via mitochondrial glutaminase, form the glutamate that is used for neurotransmission. This pattern of metabolic compartmentation is the “glutamate-glutamine cycle.” This model is subject to the following two important qualifications: 1) brain avidly oxidizes glutamate via aspartate aminotransferase; and 2) because almost no glutamate crosses from blood to brain, it must be synthesized in the central nervous system (CNS). The primary source of glutamate carbon is glucose, and a major source of glutamate nitrogen is the branched-chain amino acids, which are transported rapidly into the CNS. This arrangement accomplishes the following: 1) maintenance of low external [glutamate], thereby maximizing signal-to-noise ratio upon depolarization; 2) the replenishing of the neuronal glutamate pool; 3) the “trafficking” of glutamate through the extracellular fluid in a nonneuroactive form (glutamine); 4) the importation of amino groups from blood, thus maintaining brain nitrogen homeostasis; and 5) the oxidation of glutamate/glutamine, a process that confers an additional level of control in terms of the regulation of brain glutamate, aspartate and γ-aminobutyric acid.