INCREASED TRICARBOXYLIC-ACID CYCLE FLUX IN RAT-BRAIN DURING FOREPAW STIMULATION DETECTED WITH H-1 [C-13] NMR

NMR spectroscopy was used to test recent proposals that the additional energy required for brain activation is provided through nonoxidative glycolysis. Using localized NMR spectroscopic methods, the rate of C4 -glutamate isotopic turnover from infused [1-C-13]glucose was measured in the somatosensory cortex of rat brain both at rest and during fore paw stimulation, Analysis of the glutamate turnover data using a mathe matical model of cerebral glucose metabolism showed that the tricarbox ylic acid cycle flux (V-TCA) increased from 0.49 +/- 0.03 at rest to 1 .48 +/- 0.82 mu mol/g/min during stimulation (P < 0.01), The minimum f raction of C4-glutamate derived from C1-glucose was approximate to 75% and this fraction was found in both the resting and stimulated rats, Hence, the percentage increase in oxidative cerebral metabolic rate of glucose use (CMR(gIc)) equals the percentage increases in V-TCA and c erebral metabolic rate of oxygen consumption (CMR(o2)). Comparison wit h previous work for the same rat model, which measured total CMR(gIc) [Ueki, M., Linn, F. & Hossman, K. A. (1988) J. Cereb. Blood Flow Metab . 8, 486-494], indicates that oxidative CMR(gIc) supplies the majority of energy during sustained brain activation.