Lactic Acid - Brain Metabolism

Brain Metabolism

Although glucose is usually assumed to be the main energy source for living tissues, there are some indications that it is lactate, and not glucose, that is preferentially metabolized by neurons in the brain of several mammals species (the notable ones being mice, rats, and humans). According to the lactate-shuttling hypothesis, glial cells are responsible for transforming glucose into lactate, and for providing lactate to the neurons. Because of this local metabolic activity of glial cells, the extracellular fluid immediately surrounding neurons strongly differs in composition from the blood or cerebro-spinal fluid, being much richer with lactate, as it was found in microdialysis studies.

The role of lactate for brain metabolism seems to be even more important at early stages of development (prenatal and early postnatal), with lactate at these stages having higher concentrations in body liquids, and being utilized by the brain even more preferentially over glucose. It was also hypothesized that lactate may exert a strong action over GABAergic networks in the developing brain, making them more inhibitory than it was previously assumed, acting either through better support of metabolites, or alterations in base intracellular pH levels, or both.

A more recent paper by Zilberter's group looked directly at the energy metabolism features in brain slices of mice and showed that beta-hydroxybutyrate, lactate and pyruvate acted as oxidative energy substrates causing an increase in the NAD(P)H oxidation phase, that glucose was insufficient as an energy carrier during intense synaptic activity and finally, that lactate can be an efficient energy substrate capable of sustaining and enhancing brain aerobic energy metabolism in vitro. The paper was positively commented by Kasischke: "The study by Ivanov et al. (2011) also provides novel data on biphasic NAD(P)H fluorescence transients, an important physiological response to neural activation that has been reproduced in many studies and that is believed to originate predominately from activity-induced concentration changes to the cellular NADH pools."