Specific functional connectivity of molecular subtypes of subplate and layer 6b neurons

Abstract

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 6b in the adult cortex. While SpNs exhibit a large variety of molecular identities, it is unclear if molecular identity correlates with functional or connectomic identities 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 (CTGF-dgCre and 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 opens and when circuits are mature (P60-P80). The excitatory intracortical circuits impinging on both subtypes revealed similar patterns, but not the inhibitory circuits, particularly those from subplate/layer 6b. At P7-P9, Drd1 neurons received stronger inhibition from the subplate compared to CTGF neurons. The functional circuits on SpNs prune with age. By P60-P80, the inhibitory connections from layer 6b on CTGF neurons increased and became significantly abundant than those on Drd1 neurons. However, the inhibition strength between the two subtypes remained unchanged, suggesting that inhibition on CTGF was generally weaker at each stimulation site. Thus, SpNs exhibit diverse neuronal morphologies and intracortical input patterns, independent of molecular expression. Thus, although the subplate comprises distinct molecular classes of neurons, their molecular expression is not clearly correlated with morphologies and functional circuits throughout development.Significance statement Subplate neurons pioneer cortical circuit formation and shape its maturation. Subplate neurons can be categorized into different subpopulation based on their molecular identities. However, the relationship between functional circuitry and molecular identities was unclear. Our study demonstrated that excitatory inputs on different molecular classes of subplate neurons develop similarly but not the inhibitory inputs, particularly those from within subplate/layer6b. Moreover, subplate neurons within the same molecular classes exhibit diverse patterns of intracortical circuit connections and neuronal morphologies. This diversity becomes more pronounced in adulthood. Therefore, distinguishing the functional connectivity between the two subtypes based solely on their molecular identities is impossible. Overall, the molecular expression of subplate neurons is not clearly correlated with their morphologies and functional connectivity pattern.