Transient coupling between subplate and subgranular layers to L1 neurons before and during the critical period

Cortical layer 1 (L1) contains a diverse population of interneurons which can modulate processing in superficial cortical layers but the intracortical sources of synaptic input to these neurons and how these inputs change over development is unknown. We here investigated the changing intracortical connectivity to L1 in primary auditory cortex (A1) in slices of mouse A1 across development using laser-scanning photostimulation. Before P10 L1 cells receive most excitatory input from within L1, L2/3, L4 and L5/6 as well as the subplate. Excitatory inputs from all layers increase and peak during P10-P16, the peak of the critical period. Inhibitory inputs followed a similar pattern. Functional circuit diversity in L1 emerges after P16. In adult, L1 neurons receive ascending inputs from superficial L2/3 and subgranular L5/6, but only few inputs from L4. A subtype of L1 neurons, NDNF+ neurons, follow a similar pattern, suggesting that transient hyperconnectivity is a universal feature of developing cortical circuits. Our results demonstrate that deep excitatory and superficial inhibitory circuits are tightly linked in early development and might provide a functional scaffold for the layers in between. These results suggest that early thalamic driven spontaneous and sensory activity in subplate can be relayed to L1 from the earliest ages on, that the critical period is characterized by high transient columnar hyperconnectivity, and that in particular circuits originating in L5/6 and subplate might play a key role.