Specific Functional Connectivity of Molecular Subtypes of Subplate and Layer 6b Neurons

Subplate neurons (SpNs) are among the earliest generated cortical neurons that form functional cortical synapses and aid in cortical circuit development. A fraction of SpNs survive and form Layer (L)6b in the adult cortex. While SpNs exhibit a large variety of molecular identities, it is unclear if molecular identity correlates with their functional circuits and if different SpNs have similar developmental trajectories. To resolve these questions, we here characterize the functional intracortical circuits to molecularly identified subpopulations of SpNs with SpN-specific Cre-lines [connective tissue growth factor (CTGF)-dgCre and dopamine receptor D1 (Drd1)-Cre] in in vitro brain slices of the primary auditory cortex using whole-cell patch–clamp recordings and laser-scanning photostimulation. We targeted three age groups: before [Postnatal Day (P)7–P9] and after (P14–P20) the ear canal opening and when circuits are mature (P60–P80). While the excitatory intracortical circuits impinging on both subtypes were similar, inhibitory circuits differed, particularly those from subplate/L6b. At P7–P9, Drd1 neurons received stronger inhibition from the subplate compared with CTGF neurons. The functional circuits on SpNs prune with age. By P60–P80, the inhibitory connections from L6b on CTGF neurons increased and became significantly abundant than those on Drd1 neurons. However, the inhibition strength between the two subtypes remained unchanged. Thus, SpNs exhibit diverse neuronal morphologies and intracortical input patterns, independent of molecular expression. Thus, although the subplate comprises distinct molecular neural classes, molecular expression is not clearly correlated with neural morphologies and functional circuits throughout development.