While the ultimate dependence of brain function on its energy supply is evident, how basic neuronal parameters and network activity respond to energy metabolism deviations is unresolved. The resting membrane potential (E m ) and reversal potential of GABA-induced anionic currents (E GABA ) are among the most fundamental parameters controlling neuronal excitability. However, alterations of E m and E GABA under conditions of metabolic stress are not sufficiently documented, although it is well known that metabolic crisis may lead to neuronal hyper-excitability and aberrant neuronal network activities. In this work, we show that in slices, availability of energy substrates determines whether GABA signaling displays an inhibitory or excitatory mode, both in neonatal neocortex and hippocampus. We demonstrate that in the neonatal brain, E m and E GABA strongly depend on composition of the energy substrate pool. Complementing glucose with ketone bodies, pyruvate or lactate resulted in a significant hyperpolarization of both E m and E GABA , and induced a radical shift in the mode of GABAergic synaptic transmission towards network inhibition. Generation of giant depolarizing potentials, currently regarded as the hallmark of spontaneous neonatal network activity in vitro, was strongly inhibited both in neocortex and hippocampus in the energy substrate enriched solution. Based on these results we suggest the composition of the artificial cerebrospinal fluid, which bears a closer resemblance to the in vivo energy substrate pool. Our results suggest that energy deficits induce unfavorable changes in E m and E GABA , leading to neuronal hyperactivity that may initiate a cascade of pathological events. Keywords: brain slices, cortex, energy substrates, GABA, hippocampus, network oscillations. mechanism provides all the necessary energy and cofactors for normal fetal development. At birth this transplacental supply of nutrients ends and crucial changes in the energy supply occur. Following a brief pre-suckling period (postnatal starvation) there is an adaptation to a fat-rich diet (Girard et al. 1992;Medina and Tabernero 2005;Ward Platt and Deshpande 2005). Immediately after birth but before suckling, KBs are not available and lactate is the main energy substrate to the newborn (Girard et al. 1992;Medina et al. 1996;Medina and Tabernero 2005;Ward Platt and Deshpande 2005). The rate of lactate utilization by neurons in the early neonatal rat brain is significantly higher than that of glucose or beta-hydroxybutyrate [BHB, the predominant ketone body in the blood (Bough and Rho 2007)] ( Arizmendi and Medina 1983;Fernandez and Medina 1986;Vicario et al. 1991) and recent results showed the importance of lactate as a cerebral oxidative energy substrate (Schurr and Payne 2007;Bak et al. 2009;Castro et al. 2009).In the postnatal developing rat brain, blood glucose levels are close to those in adults (Pereira de Vasconcelos and Nehlig 1987;Nehlig and Pereira de Vasconcelos 1993). However, glucose utilization is limited and is o...

Network activation triggers a significant energy metabolism increase in both neurons and astrocytes. Questions of the primary neuronal energy substrate (e.g., glucose vs. lactate) as well as the relative contributions of glycolysis and oxidative phosphorylation and their cellular origin (neurons vs. astrocytes) are still a matter of debates. Using simultaneous measurements of electrophysiological and metabolic parameters during synaptic stimulation in hippocampal slices from mature mice, we show that neurons and astrocytes use both glycolysis and oxidative phosphorylation to meet their energy demands. Supplementation or replacement of glucose in artificial cerebrospinal fluid (ACSF) with pyruvate or lactate strongly modifies parameters related to network activity-triggered energy metabolism. These effects are not induced by changes in ATP content, pH i , [Ca 2 þ ] i or accumulation of reactive oxygen species. Our results suggest that during network activation, a significant fraction of NAD(P)H response (its overshoot phase) corresponds to glycolysis and the changes in cytosolic NAD(P)H and mitochondrial FAD are coupled. Our data do not support the hypothesis of a preferential utilization of astrocyte-released lactate by neurons during network activation in slices-instead, we show that during such activity glucose is an effective energy substrate for both neurons and astrocytes. Keywords: astrocytes; energy metabolism; glycolysis; lactate; network activity; neurons INTRODUCTION High cellular energy demands during network activation are met by upregulation of cytosolic glycolysis and mitochondrial oxidative phosphorylation. Mitochondrial metabolism provides most of the ATP but glycolysis is also enhanced and may contribute to the energy production. Journal of Cerebral Blood1-4 For elucidation of the cellular basis of neuroenergetics, measurements of metabolic signals including the oxygen utilization and NAD(P)H/FAD autofluorescence provide valuable information for connecting energy metabolism with neuronal activity. NADH (reduced form) is fluorescent when excited with UV light whereas NAD þ is not, leading to a decrease in observed fluorescence as a result of NADH oxidation. In contrast, FAD (oxidized form) is fluorescent, so the oxidation of FADH 2 to FAD causes an increase in fluorescence. The fluorescence of NADH cannot be separated from that of NADPH and their emission is measured in concert (NAD(P)H). NAD(P)H fluorescence represents a 'mixed' signal since this cofactor can be produced by both glycolysis and mitochondria, whereas FAD fluorescence is entirely mitochondrial. 5,6 Measurements of these parameters in combination with electrophysiological recordings have been used in many studies to monitor the energy status during neuronal activity in brain tissues.Typically, NAD(P)H transients induced by synaptic stimulation have a characteristic biphasic waveform: the initial short dip is followed by a long-lasting overshoot. While there exists a common