Glial Uptake Of Neurotransmitter Glutamate From The Extracellular Fluid Studied In Vivo By Microdialysis And C-13 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) NH4 Ac infusion. From the rate of [5-(13) C,(15) N]GLN synthesis (1.7 +/- 0.03 mumol/g/h), the mean (13) C enrichment of extracellular GLU (0.304 +/- 0.011) and the (15) N enrichment of precursor NH3 (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 mumol/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 mumol/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.