Metabolic compartmentation in cortical synaptosomes: influence of glucose and preferential incorporation of endogenous glutamate into GABA.

Metabolism of glutamine was determined under a variety of conditions to study compartmentation in cortical synaptosomes. The combined intracellular and extracellular amounts of [U-13C] GABA, [U-13C]glutamate and [U-13C]glutamine were the same in synaptosomes incubated with U-13C]glutamine in the presence and absence of glucose. However, the concentration of these amino acids was decreased in the latter group, demonstrating the requirement for glucose to maintain the size of neurotransmitter pools. In hypoglycemic synaptosomes more [U-13C]glutamine was converted to [U-13C]aspartate, and less glutamate was re-synthesized from the tricarboxylic acid (TCA) cycle, suggesting use of the partial TCA cycle from alpha-ketoglutarate to oxaloacetate for energy. Compartmentation was studied in synaptosomes incubated with glucose plus labeled and unlabeled glutamine and glutamate. Incubation with [U-13C]glutamine plus unlabeled glutamate gave rise to [U-13C]GABA but not labeled aspartate; however, incubation with [U-13C]glutamate plus unlabeled glutamine gave rise to [U-13C]aspartate, but not labeled GABA. Thus the endogenous glutamate formed via glutaminase in synaptic terminals is preferentially used for GABA synthesis, and is metabolized differently than glutamate taken up from the extracellular milieu.