Laminar fate of cortical GABAergic interneurons is dependent on both birthdate and phenotype.

Pioneering work indicates that the final position of neurons in specific layers of the mammalian cerebral cortex is determined primarily by birthdate. Glutamatergic projection neurons are born in the cortical proliferative zones of the dorsal telencephalon, and follow an "inside-out" neurogenesis gradient: later-born cohorts migrate radially past earlier-born neurons to populate more superficial layers. GABAergic interneurons, the major source of cortical inhibition, comprise a heterogeneous population and are produced in proliferative zones of the ventral telencephalon. Mechanisms by which interneuron subclasses find appropriate layer-specific cortical addresses remain largely unexplored. Major cortical interneuron subclasses can be identified based on expression of distinct calcium-binding proteins including parvalbumin, calretinin, or calbindin. We determined whether cortical layer-patterning of interneurons is dependent on phenotype. Parvalbumin-positive interneurons populate cortical layers with an inside-out gradient, and birthdate is isochronous to projection neurons in the same layers. In contrast, another major GABAergic subtype, labeled using calretinin, populates the cerebral cortex using an opposite "outside-in" gradient, heterochronous to neighboring neurons. In addition to birthdate, phenotype is also a determinant of cortical patterning. Discovery of a cortical subpopulation that does not follow the well-established inside-out gradient has important implications for mechanisms of layer formation in the cerebral cortex.