Energy substrate availability as a determinant of neuronal resting potential, GABA signaling and spontaneous network activity in the neonatal cortex in vitro

Abstract

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...

Non–cell-autonomous factor induces the transition from excitatory to inhibitory GABA signaling in retina independent of activity

“…S1A), in agreement with previously reported data (22). This early depolarizing action of GABA is not prevented by addition of the alternate energy substrate pyruvate (Table S1), in contrast to recent observations in both neocortex (11) and hippocampus (26).…”

GABA Depolarizes Immature Neocortical Neurons in the Presence of the Ketone Body β-Hydroxybutyrate

“…GABAmediated excitation during early development is a central component of this concept because GABAergic synapses are formed before glutamatergic contacts (Ben-Ari et al, 2007) showing that an interference with intracellular Cl Ϫ accumulation at an immature stage perturbs both the proper morphological maturation of neurons and their functional synapse development (Cancedda et al, 2007;Pfeffer et al, 2009). Therefore, an absence of GABA-mediated excitation (Rheims et al, 2009;Holmgren et al, 2010) (Yuste and Katz, 1991;Owens et al, 1996;Yamada et al, 2004). This seems to be plausible, since a broad-spectrum antagonist of VGCCs abolished the responses almost completely (Fig.…”

“…Sagittal slices (300 m) comprising the occipital cortex were cut on a vibratome and stored for at least 1 h before their use at room temperature in artificial CSF (ACSF) containing the following (in mM): 125 NaCl, 4 KCl, 10 glucose, 1.25 NaH 2 PO 4 , 25 NaHCO 3 , 2 CaCl 2 , and 1 MgCl 2 , bubbled with 5% CO 2 /95% O 2 , pH 7.4. In subsets of experiments, an energy substrate-enriched ACSF (eACSF) was used containing (in mM): 126 NaCl, 3.5 KCl, 1.2 NaH 2 PO 4 , 25 NaHCO 3 , 1.3 MgCl 2 , 2 CaCl 2 , 5 choline chloride, 5 glucose, 2 DL-sodium ␤-hydroxybutyrate, 5 sodium pyruvate, pH 7.4 (Holmgren et al, 2010).…”

“…Despite the wealth of data supporting the hypothesis of a depolarizing/excitatory mode of GABA action during early development, this concept has been seriously questioned. Recently, it was reported that GABA-mediated depolarization in immature neocortical pyramidal cells could be due to an inadequate energy supply of the in vitro preparation (Rheims et al, 2009;Holmgren et al, 2010). More precisely, Rheims et al (2009) found that addition of the ketone body DL-␤-hydroxybutyrate to the extracellular medium abolished excitatory effects of GABA.…”

GABA depolarizes immature neurons and inhibits network activity in the neonatal neocortex in vivo

“…Although criticism [26][27][28] concerning the concept of depolarizing GABA has been largely invalidated in recent years [29][30][31][32][33] , supportive evidence from the intact mammalian brain is still lacking. In favour of a depolarizing GABA action in nonmammalian species in vivo, iontophoretic application of GABA or glycine was found to evoke somatic Ca 2 þ transients (CaTs) in spinal neurons of zebrafish larvae 34 .…”