In vivo NMR Studies of the Glutamate Neurotransmitter Flux and Neuroenergetics: Implications for Brain Function

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Abstract
▪ Abstract Until very recently, non-invasive measurement of the glutamate-glutamine cycle in the intact mammalian brain had not been possible. In this review, we describe some studies that have led to quantitative assessment of the glutamate-glutamine cycle (Vcyc), as well as other important metabolic fluxes (e.g., glucose oxidation, CMRglc(ox)), with 13C magnetic resonance spectroscopy (MRS) in vivo. These 13C MRS studies clearly demonstrate that glutamate released from presynaptic neurons is taken up by the astrocyte for subsequent glutamine synthesis. Contrary to the earlier concept of a small, metabolically inactive neurotransmitter pool, in vivo 13C MRS studies demonstrate that glutamate release and recycling is a major metabolic pathway that cannot be distinguished from its actions of neurotransmission. Furthermore, the in vivo 13C MRS studies demonstrate in the rat cerebral cortex that increases in Vcyc and neuronal CMRglc(ox) are linearly related with a close to 1:1 slope. Measurements in human ce...

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