Common Origins of Hippocampal Ivy and Nitric Oxide Synthase Expressing Neurogliaform Cells

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Abstract
GABAergic interneurons critically regulate cortical computation through exquisite spatiotemporal control over excitatory networks. Precision of this inhibitory control requires a remarkable diversity within interneuron populations that is largely specified during embryogenesis. Although interneurons expressing the neuronal isoform of nitric oxide synthase (nNOS) constitute the largest hippocampal interneuron cohort their origin and specification remain unknown. Thus, as neurogliaform cells (NGC) and Ivy cells (IvC) represent the main nNOS+interneurons, we investigated their developmental origins. Although considered distinct interneuron subtypes, NGCs and IvCs exhibited similar neurochemical and electrophysiological signatures, including NPY expression and late spiking. Moreover, lineage analyses, including loss-of-function experiments and inducible fate-mapping, indicated that nNOS+IvCs and NGCs are both derived from medial ganglionic eminence (MGE) progenitors under control of the transcription factor Nkx2-1. Surprisingly, a subset of NGCs lacking nNOS arises from caudal ganglionic eminence (CGE) progenitors. Thus, while nNOS+NGCs and IvCs arise from MGE progenitors, a CGE origin distinguishes a discrete population of nNOS−NGCs.