Currency system of the glutamate period
Answers
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 “glutamateglutamine 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 g-aminobutyric acid.
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