The energy requirements of the brain are very high, and tight regulatory mechanisms operate to ensure adequate spatial and temporal delivery of energy substrates in register with neuronal activity. Astrocytes-a type of glial cell-have emerged as active players in brain energy delivery, production, utilization, and storage. Our understanding of neuroenergetics is rapidly evolving from a "neurocentric" view to a more integrated picture involving an intense cooperativity between astrocytes and neurons. This review focuses on the cellular aspects of brain energy metabolism, with a particular emphasis on the metabolic interactions between neurons and astrocytes.

The energy demands of the brain are high: they account for at least 20% of the body's energy consumption. Evolutionary studies indicate that the emergence of higher cognitive functions in humans is associated with an increased glucose utilization and expression of energy metabolism genes. Functional brain imaging techniques such as fMRI and PET, which are widely used in human neuroscience studies, detect signals that monitor energy delivery and use in register with neuronal activity. Recent technological advances in metabolic studies with cellular resolution have afforded decisive insights into the understanding of the cellular and molecular bases of the coupling between neuronal activity and energy metabolism and point at a key role of neuron-astrocyte metabolic interactions. This article reviews some of the most salient features emerging from recent studies and aims at providing an integration of brain energy metabolism across resolution scales.

Lactate in the brain has long been associated with ischaemia; however, more recent evidence shows that it can be found there under physiological conditions. In the brain, lactate is formed predominantly in astrocytes from glucose or glycogen in response to neuronal activity signals. Thus, neurons and astrocytes show tight metabolic coupling. Lactate is transferred from astrocytes to neurons to match the neuronal energetic needs, and to provide signals that modulate neuronal functions, including excitability, plasticity and memory consolidation. In addition, lactate affects several homeostatic functions. Overall, lactate ensures adequate energy supply, modulates neuronal excitability levels and regulates adaptive functions in order to set the 'homeostatic tone' of the nervous system.

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L-lactate is a product of aerobic glycolysis that can be used by neurons as an energy substrate. Here we report that in neurons L-lactate stimulates the expression of synaptic plasticity-related genes such as Arc, c-Fos, and Zif268 through a mechanism involving NMDA receptor activity and its downstream signaling cascade Erk1/2. L-lactate potentiates NMDA receptor-mediated currents and the ensuing increase in intracellular calcium. In parallel to this, L-lactate increases intracellular levels of NADH, thereby modulating the redox state of neurons. NADH mimics all of the effects of L-lactate on NMDA signaling, pointing to NADH increase as a primary mediator of L-lactate effects. The induction of plasticity genes is observed both in mouse primary neurons in culture and in vivo in the mouse sensory-motor cortex. These results provide insights for the understanding of the molecular mechanisms underlying the critical role of astrocyte-derived L-lactate in long-term memory and long-term potentiation in vivo. This set of data reveals a previously unidentified action of L-lactate as a signaling molecule for neuronal plasticity.brain energy metabolism | learning and memory | astrocyte-neuron interaction | astrocyte-neuron lactate shuttle T he transfer of L-lactate from astrocytes to neurons was recently shown to be necessary for the establishment of longterm memory (LTM) in an inhibitory avoidance (IA) paradigm and for the maintenance of in vivo long-term potentiation (LTP) in the rodent hippocampus (1). This key role of L-lactate in neuronal plasticity mechanisms was demonstrated in experiments in which specific pharmacological and gene expression down-regulation interventions were implemented to prevent the production of L-lactate from glycogen-which is exclusively localized in astrocytes-and its release from these cells in the hippocampus during behavioral training (1). Such interventions completely prevented the establishment of LTM and their effect was fully reversed by the intrahippocampal administration of L-lactate during the training session. The fact that glucose at equicaloric concentrations only marginally mimicked the rescuing effect of L-lactate was taken as an unexpected indication that the primary mechanism of action of L-lactate on plasticity mechanisms was independent of its ability to act as an energy substrate. A role of L-lactate in memory processes was also recently shown in other behavioral paradigms (2, 3). We therefore set out to investigate the molecular mechanisms at the basis of the function of L-lactate on neuronal plasticity.Molecular mechanisms underlying both LTM and long-term plasticity include the induction of expression of a group of immediate early genes (IEGs) such as early growth response 1 (Zif268 or Egr1), CCAAT/enhancer binding protein (C/EBP), and proto-oncogene c-Fos (c-Fos) as well as activity-regulated cytoskeletal-associated protein (Arc or Arg3.1) as a direct effector protein at the synapse, which all participate to different physiological processes associated with neuronal p...