Landmarks in the application of 13C-magnetic resonance spectroscopy to studies of neuronal/glial relationships

The development of the use of carbon isotopes as metabolic tracers is briefly described. C-13-labelled precursors ((CO2)-C-13, (CH4)-C-13) first became available in 1940 and were studied in microorganisms, but their use was limited by very low enrichments and lack of suitable analytical equipment. More success was achieved with C-11 and especially C-14, as these radioactive tracers did not need to be highly enriched. Although the stable C-13 isotope can be used at a low percentage enrichment in mass spectrometry, its application to magnetic resonance spectroscopy (MRS) requires very highly enriched precursors, due to its low natural abundance and low sensitivity. Despite such limitations, however, the great advantage of C-13-MRS lies in its exquisite chemical specificity, in that labelling of different carbon atoms can be distinguished within the same molecule. Effective exploitation became feasible in the early 1970s with the advent of stable instruments, Fourier transform C-13-MRS, and the availability of highly enriched precursors. Reports of its use in brain research began to appear in the mid-1980s. The applications of C-13 isotopomer analysis to research on neuronal/glial relationships are reviewed. The presence of neighbouring C-13-labelled atoms affects the appearance of the resonances (splitting due to C-C coupling), and so allows for unique quantification of rates through different and possibly competing pathways. Isotopomer patterns in resonances labelled from a combination of [1-C-13]glucose and [1,2-C-13(2)]lacetate have revealed aspects of neuronal/glial metabolic trafficking on depolarization and under hypoxic conditions in vitro. This approach has now been applied to in vivo studies on inhibition of glial metabolism using fluoroacetate. The results confirm the glial specificity of the toxin and demonstrate that it does not affect entry of acetate. When the glial TCA cycle is inhibited, the ability of the glia to participate in the glutamate/glutamine cycle remains unimpaired, in that labelling of glutamine, which can only be derived from neuronal metabolism of glucose, persists. The results also confirmed earlier evidence that part of the GABA transmitter pool is derived from glial glutamine.