Cell-type-selective synaptogenesis during the development of layer 6 corticothalamic neuron connectivity in the mammalian neocortex

The function of the mammalian neocortex relies on the timing of axon extension and establishment of cell-type-biased patterns of excitatory synaptic connections. A subtype of excitatory neurons, layer 6 corticothalamic neurons (L6CThNs), ultimately exhibits a marked preference for synapsing onto parvalbumin-positive (PV) inhibitory interneurons over more common excitatory cells in layers 6 and 4 (L6, L4). We show that the intracortical axons of L6CThNs develop in phases, first elongating within L6, then pausing before extending translaminar branches into L4. Decreasing L6CThN excitability selectively enhances axon growth in L6 but not the later elaboration in L4. For both layers, we test whether preferential synaptogenesis onto rarer PV interneurons, or non-selective synapse formation followed by selective pruning, generates adult connectivity. We find that L6CThNs form functional AMPA-receptor-containing synapses preferentially onto PV interneurons. Silent L6CThN synapses are not detected. Our findings show that cell-type-biased synaptogenesis underlies the formation of functional cell-type-specific excitatory connections in the neocortex.