Ipsi- and contralateral corticocortical projection-dependent subcircuits in layer 2 of the rat frontal cortex

In the neocortex, both layer 2/3 and layer 5 contain corticocortical pyramidal cells projecting to other cortices. We previously found that among L5 pyramidal cells of the secondary motor cortex (M2), not only intratelencephalic projection cells but also pyramidal tract cells innervate ipsilateral cortices and that the two subtypes are different in corticocortical projection diversity and axonal laminar distributions. Layer 2/3 houses intratelencephalically projecting pyramidal cells that also innervate multiple ipsilateral and contralateral cortices. However, it remained unclear whether layer 2/3 pyramidal cells can be divided into projection subtypes each with distinct innervation to specific targets. In the present study we show that layer 2 pyramidal cells are organized into subcircuits on the basis of corticocortical projection targets. Layer 2 corticocortical cells of the same projection subtype were monosynaptically connected. Between the contralaterally and ipsilaterally projecting corticocortical cells, the monosynaptic connection was more common from the former to the latter. We also found that ipsilaterally and contralaterally projecting corticocortical cell subtypes differed in their morphological and physiological characteristics. Our results suggest that layer 2 transfers separate outputs from M2 to individual cortices and that its subcircuits are hierarchically organized to form the discrete corticocortical outputs.NEW & NOTEWORTHY Pyramidal cell subtypes and their dependent subcircuits are well characterized in cortical layer 5, but much less is understood for layer 2/3. We demonstrate that in layer 2 of the rat secondary motor cortex, ipsilaterally and contralaterally projecting corticocortical cells are largely segregated. These layer 2 cell subtypes differ in dendrite morphological and intrinsic electrophysiological properties, and form subtype-dependent connections. Our results suggest that layer 2 pyramidal cells form distinct subcircuits to provide discrete corticocortical outputs.