Stimulated changes in localized cerebral energy consumption under anesthesia

Focal changes in the cerebral metabolic rate of glucose utilization (CMR(glc)) are small (10–40%) during sensory activation in awake humans, as well as in awake rodents and primates (20–50%). They are significantly larger (50–250%) in sensory activation studies of anesthetized rats and cats. Our data, in agreement with literature values, show that in the resting anesthetized state values of CMR(glc) are lower than in the resting nonanesthetized state whereas the final state values, reached upon activation, are similar for the anesthetized and nonanesthetized animals. The lower resting anesthetized state values of CMR(glc) explain why the increments upon activation from anesthesia are larger than when starting from the nonanesthetized conditions. Recent (13)C NMR measurements in our laboratory have established a quantitative relationship between the energetics of glucose oxidation, CMR(glc) (oxidative), and the flux of the glutamate/γ-aminobutyric acid/glutamine neurotransmitter cycle, V(cycle). In both the resting awake value of CMR(glc)(oxidative), and its increment upon stimulation, a large majority (≈80%) of the brain energy consumption is devoted to V(cycle). In the differencing methods of functional imaging, it is assumed that the incremental change in the measured signal represents the modular activity that supports the functional response. However, the same amount of activity must be present during the response to stimulation, irrespective of the initial basal state of the cortex. Thus, whereas the incremental signals of ΔCMR(glc) can localize neurotransmitter activity, the magnitude of such activity during the response is represented by the total localized CMR(glc), not the increment.