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

Glial uptake of neurotransmitter glutamate (GLU) from the extracellular fluid was studied in vivo in rat brain by 13C NMR and microdialysis combined with gas-chromatography/mass-spectrometry. Brain GLU C5 was 13C enriched by intravenous [2,5-13C]glucose infusion, followed by [12C]glucose infusion to chase 13C from the small glial GLU pool. This leaves [5-13C]GLU mainly in the large neuronal metabolic pool and the vesicular neurotransmitter pool. During the chase, the 13C enrichment of whole-brain GLU C5 was significantly lower than that of extracellular GLU (GLUECF) derived from exocytosis of vesicular GLU. Glial uptake of neurotransmitter [5-13C]GLUECF was monitored in vivo through the formation of [5-13C,15N]GLN during 15NH4Ac infusion. From the rate of [5-13C,15N]GLN synthesis (1.7 ± 0.03 µmol/g/h), the mean 13C enrichment of extracellular GLU (0.304 ± 0.011) and the 15N enrichment of precursor NH3 (0.87 ± 0.014), the rate of synthesis of GLN (V′GLN), derived from neurotransmitter GLUECF, was determined to be 6.4 ± 0.44 µmol/g/h. Comparison with VGLN 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 GLUECF, a process that is crucial for protecting the brain from GLU excitotoxicity.