In vivo monitoring of extracellular 13N-glutamine derived from blood-borne 13N-ammonia in rat striatum using microdialysis with radio-LC method

Glutamine synthetase (GS) is selectively localized in astrocytes and has important roles in the central nervous system (CNS). Cerebral extracellular excess ammonia and glutamate are taken up by astrocytes and converted to glutamine via GS to protect the CNS against neurotoxicity. In this study, we monitored cerebral extracellular N-13-glutamine derived from N-13-ammonia as a potential marker for astroglial metabolism using in vivo microdialysis combined with ultra performance liquid chromatography-radiometric detection. This method allowed rapid and highly sensitive radiometric analysis of N-13-ammonia and its metabolite, N-13-glutamine, in striatal extracellular fluid with good time resolution. Inhibition of GS with methionine sulfoximine resulted in a decrease of extracellular N-13-glutamine accompanied by an increase of N-13-ammonia as compared with control. Fluorocitrate, a selective inhibitor of glial metabolism, also decreased N-13-glutamine production and increased unmetabolized N-13-ammonia. In contrast, N-13-glutamine was increased with 5 mmol/kg of ammonium acetate without significant changes in N-13-ammonia as compared with control. These results suggest that the concentration of extracellular N-13-glutamine strongly reflects the biological changes in the metabolic activity of astroglial cells. (C) 2009 Elsevier B.V. All rights reserved.