By combining an inducible genetic fate mapping strategy with electrophysiological analysis, we have systematically characterized the populations of cortical GABAergic interneurons that originate from the caudal ganglionic eminence (CGE). Interestingly, in comparison to medial ganglionic eminence (MGE)-derived cortical interneuron populations, the initiation (E12.5) and peak production (E16.5) of interneurons from this embryonic structure occurs three days later in development. Moreover, unlike either pyramidal cells or MGE-derived cortical interneurons, CGE-derived interneurons do not integrate into the cortex in an inside-out manner but preferentially (75%) occupy superficial cortical layers independent of birthdate. In contrast to previous estimates, CGE-derived interneurons are both considerably greater in number (around 30% of all cortical interneurons) and diversity (comprised by at least nine distinct subtypes). Furthermore, we have found that a large proportion of CGE-derived interneurons, including the neurogliaform subtype, express the glycoprotein Reelin. In fact, most CGE-derived cortical interneurons express either Reelin or vasoactive intestinal polypeptide (VIP). Thus, in conjunction with previous studies, we have now determined the spatial and temporal origins of the vast majority of cortical interneuron subtypes.
Keywords: cortical interneurons, development, genetics, fate mapping, electrophysiology, Reelin, neurogliaform, late-spiking, adapting, Mash1, Ascl1