Supplementary Material for: Subtype- and Layer-Specific Developmental Gradients of Postnatal GABAergic Neurons in the Rodent Barrel Cortex

Introduction: The developing brain exhibits rostro-caudal gradients that align with the maturation of functionally organized circuits. The barrel cortex, a spatially precise sensory structure, serves as an ideal model to examine such gradients within a confined functional domain. GABAergic interneurons, characterized by subtype-specific developmental trajectories and pivotal roles in early cortical dynamics, provide a strategic cellular entry point for this investigation. Methods: To explore cellular and functional gradients in the developing barrel cortex, we combined transgenic mouse lines, immunohistochemical analyses with in vivo electrophysiological recordings of whisker-evoked activity during early postnatal stages. We also employed a model of ethanol exposure to assess potential differences in apoptotic vulnerability along the rostro-caudal axis. Results: Immunohistochemistry revealed distinct layer- and subtype-specific gradients of GABAergic neurons. Notably, we observed a widespread rostro-caudal gradient in 5-HT3AR-expressing cells and a localized gradient of somatostatin-positive (SST+) interneurons in the deep layers. These gradients diminished in a subtype-specific manner from postnatal day (P)5 to P10, indicating transient developmental features. In vivo recordings showed that caudal whisker stimulation elicited stronger responses, while rostral stimulation produced a broader spatial spread of activity, suggesting region-specific functional properties. Furthermore, rostral regions exhibited higher expression of the maturation marker KCC2, supporting the notion of more advanced maturation in the rostral barrel cortex. Ethanol exposure induced greater apoptosis in caudal layer 5 compared to its rostral counterpart, revealing layer- and region-specific vulnerabilities. Conclusions: These findings highlight spatially regulated trajectories of cortical maturation and underscore how regional differences in development may influence sensory processing and contribute to the heterogeneity of symptoms observed in neurodevelopmental disorders.