The first in vivo observation of (13)C-(15)N coupling in mammalian brain.

5-(13)C,(15)N]Glutamine, with (1)J((13)C-(15)N) of 16 Hz, was observed in vivo in the brain of spontaneously breathing rats by (13)C MRS at 4.7 T. The brain 5-(13)C]glutamine peak consisted of the doublet from 5-(13)C,(15)N]glutamine and the center 5-(13)C,(14)N]glutamine peak, resulting in an apparent triplet with a separation of 8 Hz. The time course of formation of brain 5-(13)C,(15)N]glutamine was monitored in vivo with a time resolution of 20-35 min. This 5-(13)C,(15)N]glutamine was formed by glial uptake of released neurotransmitter 5-(13)C]glutamate and its reaction with (15)NH(3) catalyzed by the glia-specific glutamine synthetase. The neurotransmitter glutamate C5 was selectively (13)C-enriched by intravenous 2,5-(13)C]glucose infusion to (13)C-label whole-brain glutamate C5, followed by (12)C]glucose infusion to chase (13)C from the small and rapidly turning-over glial glutamate pool, leaving (13)C mainly in the neurotransmitter 5-(13)C]glutamate pool, which is sequestered in vesicles until release. Hence, the observed 5-(13)C,(15)N]glutamine arises from a coupling between (13)C of neuronal origin and (15)N of glial origin. Measurement of the rate of brain 5-(13)C,(15)N]glutamine formation provides a novel noninvasive method of studying the kinetics of neurotransmitter uptake into glia in vivo, a process that is crucial for protecting the brain from glutamate excitotoxicity.