An increase in lactate output by brain tissue serves to meet the energy needs of glutamate-activated neurons

Abstract

Aerobic energy metabolism uses glucose and oxygen to produce all the energy needs of the brain. Several studies published over the last 13 years challenged the assumption that the activated brain increases its oxidative glucose metabolism to meet the increased energy demands. Neuronal function in rat hippocampal slices supplied with 4 mM glucose could tolerate a 15 min activation by a 5 mM concentration of the excitatory neurotransmitter glutamate (Glu), whereas slices supplied with 10 mM glucose could tolerate a 15 min activation by 20 mM Glu. However, in slices in which neuronal lactate use was inhibited by the lactate transporter inhibitor a-cyano-4-hydroxycinnamate (4-CIN), activation by Glu elicited a permanent loss of neuronal function, with a twofold to threefold increase in tissue lactate content. Inhibition of glycolysis with the glucose analog 2-deoxy-D-glucose (2DG) during the period of exposure to Glu diminished normal neuronal function in the majority of slices and significantly reduced the number of slices that exhibited neuronal function after activation. However, when lactate was added with 2DG, the majority of the slices were neuronally functional after activation by Glu. NMDA, a nontransportable Glu analog by the glial glutamate transporter, could not induce a significant increase in slice lactate level when administered in the presence of 4-CIN. It is suggested that the heightened energy demands of activated neurons are met through increased glial glycolytic flux. The lactate thus formed is a crucial aerobic energy substrate that enables neurons to endure activation.

Fig. 1.

Tissue content of lactate before (Baseline) and during (shaded area) exposure to Glu and during Glu washout (Washout) in control (open symbols) and 4-CIN-treated (filled symbols) rat hippocampal slices perfused with either 4 (A) or 10 mm(B) glucose–aCSF. *p < 0.03; **p = 0.01; ***p < 0.004, significantly different from control.

Fig. 2.

Tissue content of lactate before (Baseline) and during (shaded area) exposure to NMDA and during NMDA washout (Washout) in control (open symbols) and 4-CIN-treated (filled symbols) rat hippocampal slices perfused with 4 mm glucose–aCSF. Asterisks indicate that assay was done in duplicates only.

Fig. 3.

Schematic diagram of the two main pathways of energy metabolism, glycolysis, and oxidative phosphorylation in two brain tissue compartments, neuronal and glial, during resting state (left) and during a state of activation (right). For more details, see Results.