Interneurons set the tune of developing networks.

Despite a rather long migratory journey, interneurons are functional before vertically migrating pyramidal neurons and they constitute the source and target of the first functional synapses in the developing hippocampus. Interneuron-driven network patterns are already present in utero while principal cells are mostly quiescent. At that early stage, GABAergic synapses--which are formed before glutamatergic ones--are excitatory, suggesting that GABA is a pioneer, much like the neurons from which it is released. This review discusses this sequence of events, its functional significance and the role that interneurons might play in the construction of cortical networks.