Another Look at Early GABAergic Neurotransmission: Maybe It's Not So Exciting After All!

GABA Action in Immature Neocortical Neurons Directly Depends on the Availability of Ketone Bodies .  Rheims S, Holmgren CD, Chazal G, Mulder J, Harkany T, Zilberter T, Zilberter Y. J Neurochem 2009;110(4):1330–1338.  In the early postnatal period, energy metabolism in the suckling rodent brain relies to a large extent on metabolic pathways alternate to glucose such as the utilization of ketone bodies (KBs). However, how KBs affect neuronal excitability is not known. Using recordings of single NMDA and GABA‐activated channels in neocortical pyramidal cells we studied the effects of KBs on the resting membrane potential ( E m ) and reversal potential of GABA‐induced anionic currents ( E GABA ), respectively. We show that during postnatal development (P3–P19) if neocortical brain slices are adequately supplied with KBs, E m and E GABA are both maintained at negative levels of about −83 and −80 mV, respectively. Conversely, a KB deficiency causes a significant depolarization of both E m (>5 mV) and E GABA (>15 mV). The KB‐mediated shift in E GABA is largely determined by the interaction of the NKCC1 cotransporter and Cl − /HCO 3 transporter(s). Therefore, by inducing a hyperpolarizing shift in E m and modulating GABA signaling mode, KBs can efficiently control the excitability of neonatal cortical neurons. Energy Substrate Availability as a Determinant of Neuronal Resting Potential, GABA Signaling and Spontaneous Network Activity in the Neonatal Cortex In Vitro .  Holmgren CD, Mukhtarov M, Malkov AE, Popova IY, Bregestovski P, Zilberter Y. J Neurochem 2010;112(4):900–912.  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.