Translaminar synchronous neuronal activity is required for columnar synaptic strengthening in the mouse neocortex

Synchronous neuronal activity is a hallmark of the developing mouse primary somatosensory cortex. While the patterns of synchronous neuronal activity in cortical layer 2/3 have been well described, the source of the robust layer 2/3 activity is still unknown. Using a novel microprism preparation and in vivo 2-photon imaging in neonatal mice, we show that synchronous neuronal activity is organized in barrel columns across layers. Monosynaptic rabies tracing and slice electrophysiology experiments reveal that layer 2/3 pyramidal neurons receive significant layer 5 inputs during the first postnatal week, and silencing layer 5 synaptic outputs results in a significant reduction in spontaneous activity, abnormal sensory-evoked activity and disrupted layer 4-layer 2/3 connectivity. Our results demonstrate that translaminar layer 5-layer 2/3 connectivity plays an important role in synchronizing the developing barrel column to ensure the strengthening of layer 4-layer 2/3 connections, supporting the formation of the canonical cortical organization in barrel cortex.