title: Glial uptake of neurotransmitter glutamate from the extracellular fluid studied in vivo by microdialysis and (13)C NMR.
abstract: 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 micromol/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 micromol/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 micromol/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.
authors.affiliation: Magnetic Resonance Spectroscopy Laboratory, Huntington Medical Research Institutes, 660 South Fair Oaks Avenue, Pasadena, CA 91105, USA. mrs@hmri.org
authors.forename: Keiko
authors.initials: K
authors.lastname: Kanamori
authors.forename: Brian D
authors.initials: BD
authors.lastname: Ross
authors.forename: Richard W
authors.initials: RW
authors.lastname: Kondrat
mesh_terms: D000085:Acetates / Q000008:administration & dosage / Q000378:metabolism; D000641:Ammonia / Q000032:analysis / Q000097:blood / Q000378:metabolism; D000758:Anesthesia; D000818:Animals; D001825:Body Fluid Compartments / Q000502:physiology; D001921:Brain / Q000166:cytology / Q000378:metabolism*; D001923:Brain Chemistry; D002247:Carbon Isotopes; D005110:Extracellular Space / Q000737:chemistry / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D005947:Glucose / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism* / Q000493:pharmacokinetics; D005973:Glutamine / Q000032:analysis / Q000378:metabolism; D007262:Infusions, Intravenous; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D017551:Microdialysis; D009457:Neuroglia / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism* / Q000493:pharmacokinetics; D051381:Rats; D017208:Rats, Wistar; D014851:Wakefulness
chemical_list: D000085:Acetates; D002247:Carbon Isotopes; D018377:Neurotransmitter Agents; D005973:Glutamine; D018698:Glutamic Acid; D000641:Ammonia; D005947:Glucose; C018824:ammonium acetate