Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-C]Glucose Metabolism in Anesthetized Rats.

The C turnover of neurotransmitter amino acids (glutamate, GABA and aspartate) were determined from extracts of forebrain nerve terminals and brain homogenate, and fronto-parietal cortex from anesthetized rats undergoing timed infusions of [1,6-C]glucose or [2-C]acetate. Nerve terminal C fractional labeling of glutamate and aspartate was lower than those in whole cortical tissue at all times measured (up to 120 min), suggesting either the presence of a constant dilution flux from an unlabeled substrate or an unlabeled (effectively non-communicating on the measurement timescale) glutamate pool in the nerve terminals. Half times of C labeling from [1,6-C]glucose, as estimated by least squares exponential fitting to the time course data, were longer for nerve terminals (Glu, 21.8 min; GABA 21.0 min) compared to cortical tissue (Glu, 12.4 min; GABA, 14.5 min), except for Asp, which was similar (26.5 vs. 27.0 min). The slower turnover of glutamate in the nerve terminals (but not GABA) compared to the cortex may reflect selective effects of anesthesia on activity-dependent glucose use, which might be more pronounced in the terminals. The C labeling ratio for glutamate-C4 from [2-C]acetate over that of C-glucose was twice as large in nerve terminals compared to cortex, suggesting that astroglial glutamine under the C glucose infusion was the likely source of much of the nerve terminal dilution. The net replenishment of most of the nerve terminal amino acid pools occurs directly via trafficking of astroglial glutamine.