There is no doubt that slices are only pale reflectors of the in situ situation, and investigators continue to make suggestions to improve this compromised situation. One of the most limiting factors in establishing in situ–like conditions in slices is the lack of metabolic regulations and feedback loops that are instrumental in our reactions to the environment. For example, gluconeogenesis is heavily controlled by several organs, and increasing ketone bodies or sugar will alter general metabolic supply by directly altering glucose availability; and these complex loops cannot be reproduced in slices. However, slices from brains of 6- to 10-day-old rats perfused with medium containing 10 mm glucose are clearly not “energy-starved” and remain the most appropriate preparation to determine cellular and molecular mechanisms of brain operation.
When discovered more than two decades ago, the GABA developmental shift was considered a curious observation without fundamental implications. Studies performed in the last two decades have shown that this “curiosity” is a fundamental property of the developing brain. First, the GABA shift is but one of the many developmental sequences that enables immature neurons to act very differently from adult neurons. Almost every voltage- and transmitter-gated current has been shown to differ in young and adult neurons, in terms of subunit composition and operation. This developmental shift acts to adapt brain activity to the different functions of immature and adult networks. Immature neurons must have long-lasting “sloppy” currents to enable neurons to fire and wire together despite their large fate and birth heterogeneity. For example, in primates, cortical neurons divide over a period of up to 100 days, so some cells may have—at a given time point—no or few synapses, whereas adjacent cells will have thousands of functional synapses. Secondly, the actions of depolarizing GABA are made possible by the activation by GABA of other voltage-gated currents (notably the persistent sodium current—see Valeeva et al., 2010), and of NMDA currents (Leinekugel et al., 1997; Medina et al., 1999) facilitated by the presence of immature forms of NMDA-R channels endowed with less voltage dependence and longer kinetics (Pollard et al., 1993; Monyer et al., 1994). This interdependence underlies the well-recognized trophic role of GABA signals that when blocked retards neuritic development and neuronal migration. Higher chloride concentrations and/or accumulation not only makes the actions of GABA, or glycine, depolarizing but also artificially prolongs the time course of the synaptic currents (Pitt et al., 2008; Houston et al., 2009). Thirdly, the developmental sequence of chloride cotransporters and the alterations of intracellular chloride concentrations attest to the importance of chloride in brain development. Fourth, at least in the hippocampus and other subcortical brain structures, GABAergic signaling develop before glutamatergic ones and conveys most if not all the activity at early developmental stages (Tyzio et al., 1999). Fifth, the developmental sequence of cortical networks involves initial intrinsic nonsynapse driven connectivity followed by synapse-driven patterns that entrain large neuronal populations; GABA signals play a crucial role in these developments, strongly suggesting that GABA plays different roles in immature and adult neurons. This is substantiated by both the observation that GABAergic interneurons follow a different migration route than excitatory/glutamatergic neurons, and by the demonstration that the radial migration of glutamatergic neurons requires operative GABA currents (Manent et al., 2005; Crepel et al., 2007; Bortone & Polleux, 2009). The biologic relevance of the GABA shift is illustrated by our recent discovery that oxytocin, which triggers delivery during pregnancy, also dramatically and abruptly reduces the intracellular levels of chloride in the brain of the newborn (Tyzio et al., 2006). This alteration transiently shifts GABA from excitation to inhibition and exerts a neuroprotective action on neurons by augmenting their resistance to anoxia during this vulnerable phase. Interestingly, following the observation that pain sensitivity was higher in C-section than in vaginally delivered newborns (Bergqvist et al., 2009), we reported an analgesic action of oxytocin and bumetanide in pups mediated by a direct reduction of intracellular chloride levels in pain pathways (Mazzuca et al., 2011), again reflecting the putative wide range applications of the developmental sequence of GABA actions. In a parallel study, using a dynamic two-photon imaging technique to determine the activity of several hundreds neurons, GABAergic interneurons were found to orchestrate the generation of GDPs, further confirming the crucial role of GABAergic signals (Bonifazi et al., 2009). Therefore, the developmental sequence of the polarity of GABA actions is not a curiosity made with in an artificial in vitro preparation, but a convergence of a wide range of observations that bears relevance to a fundamental developmental mechanism.