No abstract

To investigate excitatory and inhibitory GABA actions in cortical neuronal networks, we present a novel optogenetic approach using a mouse knock-in line with conditional expression of channelrhodopsin-2 (ChR2) in GABAergic interneurons. During whole-cell recordings from hippocampal and neocortical slices from postnatal day (P) 2-P15 mice, photostimulation caused depolarization and excitation of interneurons and evoked barrages of postsynaptic GABAergic currents. Excitatory/inhibitory GABA actions on pyramidal cells were assessed by monitoring the alteration in the frequency of EPSCs during photostimulation of interneurons. We found that in slices from P2-P8 mice, photostimulation evoked an increase in EPSC frequency, whereas in P9 -P15 mice the response switched to a reduction in EPSC frequency, indicating a developmental excitatory-to-inhibitory switch in GABA actions on glutamatergic neurons. Using a similar approach in urethane-anesthetized animals in vivo, we found that photostimulation of interneurons reduces EPSC frequency at ages P3-P9. Thus, expression of ChR2 in GABAergic interneurons of mice enables selective photostimulation of interneurons during the early postnatal period, and these mice display a developmental excitatory-to-inhibitory switch in GABA action in cortical slices in vitro, but so far show mainly inhibitory GABA actions on spontaneous EPSCs in the immature hippocampus and neocortex in vivo.

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...