, 2006, Lefort et al., 2009 and Yoshimura et al., 2005). The connectivity between interneurons and principal cells has also been explored especially in the neocortex, where the large diversity of interneuron types suggests functional diversity. These studies generally report a cell-type-specific organization between cortical layers ( Jiang et al., 2013, Kätzel et al., 2011, Yoshimura and Callaway, 2005 and Yoshimura et al., 2005), but a dense nonspecific local connectivity ( Fino and Yuste, 2011 and Packer

and Yuste, 2011). The connectivity from excitatory to inhibitory cells ( Bock et al., 2011 and Hofer et al., 2011) suggests that cortical interneurons sample their excitatory inputs randomly. The available results thus indicate that interconnectivity of principal cells is structured, whereas connectivity of interneurons is unstructured. However, an important BAY 73-4506 in vitro element remains to be probed in more detail: the higher-order connectivity among interneurons. Recently, the interaction between the

different types of cortical interneurons and its functional implications have attracted interest ( Jiang et al., 2013, Letzkus et al., 2011 and Pi et al., 2013). Interneuron networks are known to share electrical and/or chemical synapses in various brain areas ( Bartos et al., 2002, Galarreta and Hestrin, 1999, Galarreta and Hestrin, 2002, Gibson et al., 1999, Landisman et al., 2002 and Tamás et al., Vorinostat 2000), including in a cell-type-specific manner ( Blatow et al., 2003, Gibson et al., 1999, Jiang et al., 2013 and Koós and Tepper, 1999) and are thought to underlie important features of network dynamics, such as synchronization and oscillations ( Bartos et al., 2007 and Whittington and Traub, 2003). However, quantitative information about the connectivity motifs and network architecture of interneuron-interneuron connections,

in particular among interneurons of the same cell type, is still lacking and is essential in order to fully understand their operation ( Buzsáki et al., 2004). Molecular layer interneurons in the cerebellum play an important role in regulating cerebellar output and motor learning (Jörntell et al., 2010). They are interconnected by GABAergic chemical synapses (Häusser and Clark, 1997 and Llano and Gerschenfeld, 1993) and by electrical synapses CYTH4 (Alcami and Marty, 2013 and Mann-Metzer and Yarom, 1999). The connections between molecular layer interneurons have important functional roles: the electrical connections can promote synchrony (Mann-Metzer and Yarom, 1999), whereas the chemical synapses can delay action potentials and affect the precision of spike timing in postsynaptic interneurons (Häusser and Clark, 1997 and Mittmann et al., 2005). However, the level of overlap between the chemical and electrical networks and their higher-level organization remain unclear.