Glial uptake of neurotransmitter glutamate from the extracellular fluid studied in vivo by microdialysis and 13 C NMR

Glial uptake of neurotransmitter glutamate (GLU) from the extracellular fluid was studied in vivo in rat brain by 13 C NMR and microdialysis combined with gas‐chromatography/mass‐spectrometry. Brain GLU C5 was 13 C enriched by intravenous [2,5‐ 13 C]glucose infusion, followed by [ 12 C]glucose infusion to chase 13 C from the small glial GLU pool. This leaves [5‐ 13 C]GLU mainly in the large neuronal metabolic pool and the vesicular neurotransmitter pool. During the chase, the 13 C enrichment of whole‐brain GLU C5 was significantly lower than that of extracellular GLU (GLU ECF ) derived from exocytosis of vesicular GLU. Glial uptake of neurotransmitter [5‐ 13 C]GLU ECF was monitored in vivo through the formation of [5‐ 13 C, 15 N]GLN during 15 NH 4 Ac infusion. From the rate of [5‐ 13 C, 15 N]GLN synthesis (1.7 ± 0.03 µmol/g/h), the mean 13 C enrichment of extracellular GLU (0.304 ± 0.011) and the 15 N enrichment of precursor NH 3 (0.87 ± 0.014), the rate of synthesis of GLN (V′ GLN ), derived from neurotransmitter GLU ECF , was determined to be 6.4 ± 0.44 µmol/g/h. Comparison with V GLN measured previously by an independent method showed that the neurotransmitter provides 80–90% of the substrate GLU pool for GLN synthesis. Hence, under our experimental conditions, the rate of 6.4 ± 0.44 µmol/g/h also represents a reasonable estimate for the rate of glial uptake of GLU ECF , a process that is crucial for protecting the brain from GLU excitotoxicity.