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Effective Mechanism for Synthesis of Neurotransmitter Glutamate and its Loading into Synaptic Vesicles
Authors:Kouji Takeda  Tetsufumi Ueda
Institution:1.Molecular and Behavioral Neuroscience Institute, Medical School,The University of Michigan,Ann Arbor,USA;2.Department of Pharmacology, Medical School,The University of Michigan,Ann Arbor,USA;3.Department of Psychiatry, Medical School,The University of Michigan,Ann Arbor,USA;4.Department of Education,Tokyo University of Agriculture,Tokyo,Japan
Abstract:Glutamate accumulation into synaptic vesicles is a pivotal step in glutamate transmission. This process is achieved by a vesicular glutamate transporter (VGLUT) coupled to v-type proton ATPase. Normal synaptic transmission, in particular during intensive neuronal firing, would demand rapid transmitter re-filling of emptied synaptic vesicles. We have previously shown that isolated synaptic vesicles are capable of synthesizing glutamate from α-ketoglutarate (not from glutamine) by vesicle-bound aspartate aminotransferase for immediate uptake, in addition to ATP required for uptake by vesicle-bound glycolytic enzymes. This suggests that local synthesis of these substances, essential for glutamate transmission, could occur at the synaptic vesicle. Here we provide evidence that synaptosomes (pinched-off nerve terminals) also accumulate α-ketoglutarate-derived glutamate into synaptic vesicles within, at the expense of ATP generated through glycolysis. Glutamine-derived glutamate is also accumulated into synaptic vesicles in synaptosomes. The underlying mechanism is discussed. It is suggested that local synthesis of both glutamate and ATP at the presynaptic synaptic vesicle would represent an efficient mechanism for swift glutamate loading into synaptic vesicles, supporting maintenance of normal synaptic transmission.
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