It is assumed that the brain has homeostatic mechanisms to prevent the depletion of cellular energy, required for all cellular activities. For example, the blood flow increases, and oxygen and glucose are actively delivered in the brain region in which neural firing activity occurs. Besides, the cerebral blood flow and glucose uptake into the cells fluctuate accompanying the variations of cellular activities in the brain across the sleep-wake states of animals. Under these brain energy homeostatic mechanisms, it is assumed that the cellular energy status in the brain could be maintained constant in all physiological conditions including across the sleep-wake states of animals. However, this has not been experimentally proven.
To investigate whether the cellular energy status in the brain of living animals is always constant or variated, the researchers measured the neuronal intracellular concentration of adenosine 5′-triphosphate (ATP), the major cellular energy metabolite, using a fluorescent sensor in the