The Contribution of Ketone Bodies to Basal and Activity-Dependent Neuronal Oxidation in Vivo

Abstract

The capacity of ketone bodies to replace glucose in support of neuronal function is unresolved. Here, we determined the contributions of glucose and ketone bodies to neocortical oxidative metabolism over a large range of brain activity in rats fasted 36 hours and infused intravenously with [2,4-13 C 2 ]-D-b-hydroxybutyrate (BHB). Three animal groups and conditions were studied: awake ex vivo, pentobarbital-induced isoelectricity ex vivo, and halothane-anesthetized in vivo, the latter data reanalyzed from a recent study. Rates of neuronal acetyl-CoA oxidation from ketone bodies (V acCoA-kbN ) and pyruvate (V pdhN ), and the glutamateglutamine cycle (V cyc ) were determined by metabolic modeling of 13 C label trapped in major brain amino acid pools. V acCoA-kbN increased gradually with increasing activity, as compared with the steeper change in tricarboxylic acid (TCA) cycle rate (V tcaN ), supporting a decreasing percentage of neuronal ketone oxidation: B100% (isoelectricity), 56% (halothane anesthesia), 36% (awake) with the BHB plasma levels achieved in our experiments (6 to 13 mM). In awake animals ketone oxidation reached saturation for blood levels 417 mM, accounting for 62% of neuronal substrate oxidation, the remainder (38%) provided by glucose. We conclude that ketone bodies present at sufficient concentration to saturate metabolism provides full support of basal (housekeeping) energy needs and up to approximately half of the activity-dependent oxidative needs of neurons. Keywords: 13 C isotopes; glucose utilization; glutamate/glutamine cycle; ketone body utilization; neuroenergetics; nuclear magnetic resonance spectroscopy INTRODUCTIONThe adult mammalian brain shows a remarkably high utilization of glucose compared with other potential fuel substrates, such as lactate or ketone bodies. Brain consumption of alternate substrates is limited by transport from the blood, and factors which alter membrane transport alter the rate of consumption. Under fasting conditions, starvation, or diets high in fats, blood ketone body levels rise, increasing their cerebral rate of consumption. The extent to which ketone bodies can support the different aspects of brain function, once transport is no longer limiting, has not been clearly established. Ketone bodies are metabolized by neurons and astrocytes in vitro and in vivo, 1-3 and like glucose, neuronal (glutamatergic) oxidation dominates quantitatively in vivo, 3,4 reflecting the relatively larger volume fraction of neurons compared with glial cells. Despite extensive study to ascertain the role of ketone bodies in neural function, basic unanswered questions persist about their involvement, and that of glucose, in the energy requirements associated with neurotransmission.Previous studies from our laboratory using 13 C magnetic resonance spectroscopy (MRS) with [1-13 C]glucose reported a linear relationship between neuronal oxidation of glucose in the tricarboxylic acid (TCA) cycle and glutamate (Glu)-glutamine (Gln) cycling flux over a large range of brain activity, 5-7...

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats

“…This relationship is consistent with calculations of the energetics of specific ion flows associated with glutamatergic neurotransmission [42, 43]. Different molecular-mediated coupling mechanisms have been proposed to explain this relationship [10, 44–49]. The in vivo 13 C NMR spectroscopic measurements of fluxes reflect whole tissue averages of large populations of neurons and astrocytes.…”

“…Therefore, halothane anesthesia may have had a greater effect in suppressing glucose oxidation in synaptic terminals/dendrites compared to the neuronal soma, effectively reducing 13 C enrichment and fluxes in nerve terminals relative to neuronal metabolic flux in the tissue as a whole. This can be seen quantitatively [27, 29] by considering the relationship between neurotransmitter glutamate-glutamine cycling and neuronal TCA cycle flux determined over a large range of neural activity (deep anesthesia to awake state) and described by the equation, V TCA(n) = 1.80V cyc + 0.19 [49]. This relation provides an estimate of the fraction of total neuronal TCA cycle flux (presynaptic terminals and dendrites, axons and soma) coupled to glutamate-glutamine cycling through the nerve terminals.…”

Insights from Neuroenergetics into the Interpretation of Functional Neuroimaging: An Alternative Empirical Model for Studying the Brain's Support of Behavior

“…1,20 13 C Magnetic resonance spectroscopy measurements of the relationship between the glutamate/glutamine cycle and neuronal glucose oxidation To determine the energetic cost of brain function 13 C MRS studies have been performed in rats over a wide range of activities from isoelectric pentobarbital anesthesia under which there is no cortical signaling to awake states with high levels of cortical signaling. The original results by Sibson et al 24 have been reproduced in several laboratories, 25-38 demonstrating a tight correlation between V cycle and CMR glc(ox),N (Figure 2; see Supplementary Table 1 for both rat [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] and human 23,40-52 results). This neurometabolic coupling in the rat somatosensory cortex spanning awake to deeply anesthetized conditions has several consequences.…”

Caloric restriction preserves memory and reduces anxiety of aging mice with early enhancement of neurovascular functions

“…Basal CBF is tightly coupled with cerebral metabolic rate of oxygen [45] and utilization of ketone bodies significantly elevate the oxygen utilization in mitochondria through beta-oxidation of fatty acid [40, 46]. This is supported by evidence from isolated mitochondria, showing CR enhances mitochondrial function and induces bioenergetic efficiency (7,19).…”