Astrocytes play a critical role in the regulation of brain metabolic responses to activity. One detailed mechanism proposed to describe the role of astrocytes in some of these responses has come to be known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). Although controversial, the original concept of a coupling mechanism between neuronal activity and glucose utilization that involves an activation of aerobic glycolysis in astrocytes and lactate consumption by neurons provides a heuristically valid framework for experimental studies. In this context, it is necessary to provide a survey of recent developments and data pertaining to this model. Thus, here, we review very recent experimental evidence as well as theoretical arguments strongly supporting the original model and in some cases extending it. Aspects revisited include the existence of glutamate-induced glycolysis in astrocytes in vitro, ex vivo, and in vivo, lactate as a preferential oxidative substrate for neurons, and the notion of net lactate transfer between astrocytes and neurons in vivo. Inclusion of a role for glycogen in the ANLSH is discussed in the light of a possible extension of the astrocyte-neuron lactate shuttle (ANLS) concept rather than as a competing hypothesis. New perspectives offered by the application of this concept include a better understanding of the basis of signals used in functional brain imaging, a role for neuron-glia metabolic interactions in glucose sensing and diabetes, as well as novel strategies to develop therapies against neurodegenerative diseases based upon improving astrocyte-neuron coupled energetics.

Summary:The authors investigated concomitant lactate and glucose metabolism in primary neuronal cultures using 13 Cand 1 H-NMR spectroscopy. Neurons were incubated in a medium containing either [1- 13C]glucose and different unlabeled lactate concentrations, or unlabeled glucose and different [3-13 C]lactate concentrations. Overall, 13 C-NMR spectra of cellular extracts showed that more 13 C was incorporated into glutamate when lactate was the enriched substrate. Glutamate 13 C-enrichment was also found to be much higher in lactatelabeled than in glucose-labeled conditions. When glucose and lactate concentrations were identical (5.5 mmol/L), relative contributions of glucose and lactate to neuronal oxidative metabolism amounted to 21% and 79%, respectively. Results clearly indicate that when neurons are in the presence of both glucose and lactate, they preferentially use lactate as their main oxidative substrate. Key Words: Energy metabolismBrain-NMR spectroscopy-TCA cycle-MonocarboxylateGlutamate.

S and A.A. performed and supervised in vitro experiments in cell/astrocyte cultures and ex vivo analysis of brain tissue; A.B.G, C.I. and P.G.S. performed behavioral experiments and surgical procedures in mice; E.R. and M.G. provided some CB1-KO mice to the group of J.P.B.; D.A and A.P. performed electrophysiological experiments not shown in the manuscript; M.V. and F.J.K performed mouse perfusion and immunohistochemistry experiments; A.C. and L.B. produced some of the viral constructs used (e.g. Syn-mitoCAT); I.B.R, N.P., S.A. and P.G. performed and supervised electron microscopy experiments; M.L.L.R. provided pharmacological tools (HU-Biot); C.J., N.D and L.P provided specific viral constructs to modulate the MCT-2 transporter; C.J. and G.B. provided data and viral vectors regarding mouse retro-orbital injections; B.L and P.V.P. provided important conceptual ideas; A.K.B.S performed in vivo NMR experiments.

Competition between glucose and lactate as oxidative energy substrates was investigated in both primary cultures of astrocytes and neurons using physiological concentrations (1.1 mm for each). Glucose metabolism was distinguished from lactate metabolism by using alternatively labelled substrates in the medium ([1-13C]glucose + lactate or glucose + [3-13C]lactate). After 4 h of incubation, 1H and 13C-NMR spectra were realized on perchloric acid extracts of both cells and culture media. For astrocytic cultures, spectra showed that amino acids (glutamine and alanine) were more labelled in the glucose-labelled condition, indicating that glucose is a better substrate to support oxidative metabolism in these cells. The opposite was observed on spectra from neuronal cultures, glutamate being much more labelled in the lactate-labelled condition, confirming that neurons consume lactate preferentially as an oxidative energy substrate. Analysis of glutamine and glutamate peaks (singlets or multiplets) also suggests that astrocytes have a less active oxidative metabolism than neurons. In contrast, they exhibit a stronger glycolytic metabolism than neurons as indicated by their high lactate production yield. Using a mathematical model, we have estimated the relative contribution of exogenous glucose and lactate to neuronal oxidative metabolism. Under the aforementioned conditions, it represents 25% for glucose and 75% for lactate. Altogether, these results obtained on separate astrocytic and neuronal cultures support the idea that lactate, predominantly produced by astrocytes, is used as a supplementary fuel by neurons in vivo already under resting physiological conditions.