Ex Vivo Analysis of Lactate and Glucose Metabolism in the Rat Brain under Different States of Depressed Activity

Abstract

Brain metabolism of glucose and lactate was analyzed by ex vivo NMR spectroscopy in rats presenting different cerebral activities induced after the administration of pentobarbital, ␣-chloralose, or morphine. The animals were infused with a solution of either [1-13 C]glucose plus lactate or glucose plus [3-13 C]lactate for 20 min. Brain metabolite contents and enrichments were determined from analyses of brain tissue perchloric acid extracts according to their post-mortem evolution kinetics. When amino acid enrichments were compared, both the brain metabolic activity and the contribution of blood glucose relative to that of blood lactate to brain metabolism were linked with cerebral activity. The data also indicated the production in the brain of lactate from glycolysis in a compartment other than the neurons, presumably the astrocytes, and its subsequent oxidative metabolism in neurons. Therefore, a brain electrical activity-dependent increase in the relative contribution of blood glucose to brain metabolism occurred via the increase in the metabolism of lactate generated from brain glycolysis at the expense of that of blood lactate. This result strengthens the hypothesis that brain lactate is involved in the coupling between neuronal activation and metabolism.In the last decade, the idea of the involvement of lactate in the coupling between neuronal activation and energy metabolism has arisen from the results of various experimental investigations. Briefly, in vitro studies have evidenced lactate release from astrocytes after glycolysis stimulation by glutamate uptake (1, 2) and, in the particular case of the mammalian retina, the use of lactate from Mü ller cells as an energy substrate for photoreceptors (3). In vivo studies on brain have demonstrated the uncoupling of oxygen and glucose utilization (4) and the release of lactate into the extracellular fluid in response to neuronal activation (5, 6). On the other hand, brain lactate has been demonstrated to efficiently protect neurons against delayed ischemic damage (7). The metabolic fate of exogenous lactate in whole brain has been investigated by ex vivo NMR spectroscopy. In this way, it has been demonstrated that blood lactate enters the brain and is more specifically metabolized in neurons (8 -10). The proposed astrocyte-neuron lactate shuttle hypothesis (ANLSH) 1 (1) as the coupling model between neuronal activity and energy metabolism requires the involvement of different components, the occurrence and localization of which have been investigated analytically at the molecular level, i.e. glutamate transporter and Na ϩ ,K ϩ -ATPase (11), lactate dehydrogenase isoenzymes (12), and monocarboxylate transporters (13,14). Although brain lactate production and lactate use by neurons as an energy source are widely admitted, the relevance of the coupling model, particularly the neuronal utilization of glia-produced lactate, is a topical issue (15, 16).Brain lactate mostly derives from glycolysis. Therefore, the comparative analysis of the contribution of brain l...

Close Coupling between Astrocytic and Neuronal Metabolisms to Fulfill Anaplerotic and Energy Needs in the Rat Brain

“…In addition, it suggested that relatively more pyruvate than acetate was available to fuel the astrocytic TCA cycle in awake rats, due to the overall increase in metabolic activity. This latter suggestion would be in agreement with the Glu uptake stimulation of astrocytic glycolysis (Pellerin and Magistretti, 1994), which leads to a higher production of lactate/ pyruvate in the astrocytic compartment (Serres et al, 2004). Therefore, in this case, the extra threecarbon units produced from glucose would be used to fuel not only the neuronal TCA cycle (via the astrocyte-neuron lactate shuttle mechanism) (Magistretti et al, 1999;Serres et al, 2004) but also the astrocytic TCA cycle to fulfill the energy needs of the glial cells during activation (as discussed recently) (Hertz et al, 2007).…”

“…Rats under pentobarbital were kept under a glow lamp to avoid hypothermia. Awake rats under morphine were immobilized in a restrainer (Serres et al, 2004).…”

Cerebral activation by fasting induces lactate accumulation in the hypothalamus

“…13 C HR-MAS proved to be sensitive enough to analyze these small biopsies. Our results showed that fasting stimuli promote hypothalamic [3-13 C]lactate (Lac) accumulation, in a similar way to that of other sensorial or motor paradigms resulting in lactate accumulation during glutamatergic or GABAergic neurotransmissions (7)(8)(9)14). We also report on the hypothalamic accumulation of [2-13 C]GABA under fasting stimulation.…”

“…In vivo and in vitro 1 H and 13 C NMR approaches have been successful in providing a wealth of information on cerebral metabolism and neuroglial interactions during sensory or motor activation (7)(8)(9)(10). However, the relatively large voxel sizes involved in the acquisition of in vivo NMR spectra preclude its use in studying the relatively small hypothalamic area, particularly in small rodents.…”

Simultaneous recording of hippocampal oxygen and glucose in real time using constant potential amperometry in the freely-moving rat

“…Support for this comes from the growing body of evidence to suggest that the brain switches between glucose and lactate under different levels of activity (Dienel and Hertz, 2001). The utilisation of glucose by the brain has been suggested to be dependent on neuronal activity (Serres et al, 2005) and it has been shown that there is increased utilisation of lactate during chloral hydrate anaesthesia, suggesting that the brain switches to lactate as an energy source during this (Yamada et al, 2009) and other anaesthetic conditions (Serres et al, 2004;Horn and Klein, 2010).…”