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Excitatory GABA in Rodent Developing Neocortex In Vitro

¹Institut de Neurobiologie de la Mediterranee, Institut National de la Santé et de la Recherche Médicale U901, Université de la Méditerranée, Marseille, France; and ²Department of Neurology, Dartmouth Medical School, Lebanon, New Hampshire

Submitted 26 March 2008; accepted in final form 3 May 2008

GABA depolarizes immature cortical neurons. However, whether GABA excites immature neocortical neurons and drives network oscillations as in other brain structures remains controversial. Excitatory actions of GABA depend on three fundamental parameters: the resting membrane potential (E_m), reversal potential of GABA (E_GABA), and threshold of action potential generation (V_thr). We have shown recently that conventional invasive recording techniques provide an erroneous estimation of these parameters in immature neurons. In this study, we used noninvasive single N-methyl-D-aspartate and GABA channel recordings in rodent brain slices to measure both E_m and E_GABA in the same neuron. We show that GABA strongly depolarizes pyramidal neurons and interneurons in both deep and superficial layers of the immature neocortex (P2–P10). However, GABA generates action potentials in layer 5/6 (L5/6) but not L2/3 pyramidal cells, since L5/6 pyramidal cells have more depolarized resting potentials and more hyperpolarized V_thr. The excitatory GABA transiently drives oscillations generated by L5/6 pyramidal cells and interneurons during development (P5–P12). The NKCC1 co-transporter antagonist bumetanide strongly reduces [Cl^–]_i, GABA-induced depolarization, and network oscillations, confirming the importance of GABA signaling. Thus a strong GABA excitatory drive coupled with high intrinsic excitability of L5/6 pyramidal neurons and interneurons provide a powerful mechanism of synapse-driven oscillatory activity in the rodent neocortex in vitro. In the companion paper, we show that the excitatory GABA drives layer-specific seizures in the immature neocortex.