Cortical GABAergic neurons can be parsed into several non-overlapping populations and, in a few cases, bona-fide types based on developmental origin, innervation targets, and molecular markers (Kepecs and Fishell, 2014). The embryonic medial and caudal ganglionic eminences (MGE and CGE) give rise to two broad groups, the former is divided into parvalbumin (PV) and somatostatin (SST) populations and the latter is marked by HTR3a (Kepecs and Fishell, 2014) (Figure 1A–B). The PV population includes fast-spiking basket cells (PVBC) that innervate the perisomatic region (Hu et al., 2014) and chandelier cells (CHC) that target the axon initial segment (AIS) (Taniguchi et al., 2013). The SST population includes Martinotti cells (MNC) that target distal dendrites (Silberberg and Markram, 2007), long projection cells (LPC) (Kilduff et al., 2011) and multiple other types. The HTR3a group includes the Vassoactive intestinal peptide (VIP) and Reelin populations, and the VIP population comprises interneuron-selective dis-inhibitory cells (ISC) (Staiger et al., 2004), Cholecystokinin (CCK) small basket cells (CCKC) (Armstrong and Soltesz, 2012) and likely additional types. Accumulated anatomical, physiological, and molecular evidence indicate that these are non-overlapping subpopulations, and CHC, LPC and PVBC are considered cardinal types (He et al., 2016).
