To clarify the unique characteristics of
amino acid metabolism derived from
glucose in the central nervous system (CNS), we injected [1-13C]
glucose intraperitoneally to the rat, and extracted the free
amino acids from several kinds of tissues and measured the amount of incorporation of 13C derived from [1-13C]
glucose into each
amino acid using 13C-magnetic resonance spectroscopy (NMR). In the adult rat brain, the intensities of resonances from 13C-amino
acids were observed in the following order:
glutamate,
glutamine,
aspartate, gamma-aminobutyrate (
GABA) and
alanine. There seemed no regional difference on this labeling pattern in the brain. However, only in the striatum and thalamus, the intensities of resonances from [2-13C]
GABA were larger than that from [2,3-13C]
aspartate. In the other tissues, such as heart, kidney, liver, spleen, muscle, lung and small intestine, the resonances from
GABA were not detected and every intensity of resonances from 13C-amino
acids, except 13C-alanine, was much smaller than those in the brain and spinal cord. In the serum, 13C-amino
acid was not detected at all. When the rats were decapitated, in the brain, the resonances from [1-13C]
glucose greatly reduced and the intensities of resonances from [3-13C]
lactate, [3-13C]
alanine, [2, 3, 4-13C]
GABA and [2-13C]
glutamine became larger as compared with those in the case that the rats were sacrificed with microwave. In other tissues, the resonances from [1-13C]
glucose were clearly detected even after the
decapitation. In the
glioma induced by
nitrosoethylurea in the spinal cord, the large resonances from
glutamine and
alanine were observed; however, the intensities of resonances from
glutamate were considerably reduced and the resonances from
GABA and
aspartate were not detected. These results show that the pattern of 13C label incorporation into
amino acids is unique in the central nervous tissues and also suggest that the metabolic compartmentalization could exist in the CNS through the metabolic trafficking between neurons and astroglia.