Editorial: Plasticity of GABAergic synapses

For long time, plasticity of brain circuits has been hypothesized to mainly rely on the flexibility of glutamatergic excitatory synapses, whereas inhibitory synapses have been assumed to be relatively invariant. Based on this view, inhibition should be exclusively modulated by the differential glutamatergic-driven activation of a highly diverse population of inhibitory interneurons displaying specific temporal dynamics and selective innervation patterns. However, it has been demonstrated that inhibitory synapses undergo several forms of plasticity, thus providing an additional source of versatility to the regulation of the neuronal network and the emergence of complex brain states. The cellular and molecular mechanisms occurring at inhibitory synapses during the induction/expression of inhibitory short-and long-term synaptic plasticity are now beginning to be unraveled. At the presynaptic side, retrograde synaptic messengers modulate GABA release In addition, acute or chronic alterations of intracellular chloride concentration modulate the driving force of GABAergic currents and the subunit The 14 articles presented in this ebook (including hypothesis and theory, minireviews, reviews, and original research articles) cover the mechanisms of inhibitory synaptic plasticity, at the molecular and microcircuit levels. Zacchi et al. (2014) focus on the signaling pathways controlling the phosphorylation state of gephyrin, a key scaffold protein at inhibitory synapses responsible for the synaptic clustering of both glycine and GABAA receptors. By considering the synapse as a highly dynamic element, Petrini and Barberis (2014), review the recent literature addressing the role of protein diffusion in the reorganization of the inhibitory postsynaptic density during inhibitory synaptic plasticity. A similar conceptual approach, based on the analysis of receptor dynamics, has been adopted by Muir and Kittler (2014) to investigate inhibitory plasticity in relation to GABAA receptor diffusion at inhibitory synapses located in the axon initial segment. This original research article reports that chronic depolarization increases the lateral mobility of GABAA receptors and reduces the size of post-synaptic GABAA receptor clusters, thus critically interfering with neuronal excitability. Hirano and Kawaguchi (2014) review another form of postsynaptic inhibitory plasticity observed at cerebellar synapses formed by stellate cells onto Purkinje cells. This inhibitory long-term potentiation involves the CaMKII-dependent increase of GABAA receptor signaling through direct GABAA receptor phosphorylation and/or promoted surface delivery via a GABARAP-dependent mechanism. In their original article, Gao et al. (2014) further address the molecular mechanisms of the aforementioned long-term inhibitory plasticity at cerebellar Purkinje cells. They report that the pathway of iLTP induction critically depends on the coordinated action of both …