Glycogen is the reservoir for
glucose in the brain. Beyond the general agreement that
glycogen serves as an energy source in the central nervous system, its exact role in brain energy metabolism has yet to be elucidated. Experiments performed in cell and tissue culture and animals have shown that
glycogen content is affected by several factors, including
glucose,
insulin,
neurotransmitters, and neuronal activation. The study of in vivo
glycogen metabolism has been hindered by the inability to measure
glycogen noninvasively, but, in the past several years, the development of a noninvasive localized (13) C nuclear magnetic resonance (NMR) spectroscopy method has allowed the study of
glycogen metabolism in the conscious human. With this technique, (13) C-
glucose is administered intravenously, and its incorporation into and washout from brain
glycogen is tracked. One application of this method has been to the study of brain
glycogen metabolism in humans during
hypoglycemia: data have shown that mobilization of brain
glycogen is augmented during
hypoglycemia, and, after a single episode of
hypoglycemia,
glycogen synthesis rate is increased, suggesting that
glycogen stores rebound to levels greater than baseline. Such studies suggest that
glycogen may serve as a potential energy reservoir in
hypoglycemia and may participate in the brain's adaptation to recurrent
hypoglycemia and eventual development of
hypoglycemia unawareness. Beyond this focused area of study, (13) C NMR spectroscopy has a broad potential for application in the study of brain
glycogen metabolism and carries the promise of a better understanding of the role of brain
glycogen in diabetes and other conditions.