Early Excitatory Activity-Dependent Maturation of Somatostatin Interneurons in Cortical Layer 2/3 of Mice.

GABAergic interneurons perform distinct functions during cortical development in the mouse brain. Among the diverse GABAergic neurons present in the brain, early-born somatostatin (SST)-expressing inhibitory interneurons, which are innervated by other interneurons and local pyramidal cells (PCs), act in a neural computational role in circuitry regulation. The synapses between the SST+ interneurons and other cells form gradually during development. Here, we traced the developmental course of the electrophysiological properties of SST+ interneurons at layer 2/3 of the neocortical secondary motor area (M2) in mouse, and the synaptic connectivity between SST+ interneurons and PCs. Also, we used toxin-mediated and genetic method to suppress the activities of PCs, and demonstrate that decreasing excitatory input at early stage (before P1) rather than late stage (after P8) would delay the functional maturation of SST+ interneurons. In conclusion, our results indicate that early functional activity of PCs is crucial for the intrinsic maturation of SST+ interneurons, following which these interneurons participate in local circuitry.