Sncg, Mybpc1, and Parm1 Classify subpopulations of VIP-expressing interneurons in layers 2/3 of the somatosensory cortex.

Neocortical vasoactive intestinal polypeptide-expressing (VIP+) interneurons display highly diverse morpho-electrophysiological and molecular properties. To begin to understand the function of VIP+ interneurons in cortical circuits, they must be clearly and comprehensively classified into distinct subpopulations based on specific molecular markers. Here, we utilized patch-clamp RT-PCR (Patch-PCR) to simultaneously obtain the morpho-electric properties and mRNA profiles of 155 VIP+ interneurons in layers 2 and 3 (L2/3) of the mouse somatosensory cortex. Using an unsupervised clustering method, we identified 3 electrophysiological types (E-types) and 2 morphological types (M-types) of VIP+ interneurons. Joint clustering based on the combined electrophysiological and morphological features resulted in 3 morpho-electric types (ME-types). More importantly, we found these 3 ME-types expressed distinct marker genes: ~94% of Sncg+ cells were ME-type 1, 100% of Mybpc1+ cells were ME-type 2, and ~78% of Parm1+ were ME-type 3. By clarifying the properties of subpopulations of cortical L2/3 VIP+ interneurons, this study establishes a basis for future investigations aiming to elucidate their physiological roles.

Correlation Analysis of Molecularly-Defined Cortical Interneuron Populations with Morpho-Electric Properties in Layer V of Mouse Neocortex.

Cortical interneurons can be categorized into distinct populations based on multiple modalities, including molecular signatures and morpho-electrical (M/E) properties. Recently, many transcriptomic signatures based on single-cell RNA-seq have been identified in cortical interneurons. However, whether different interneuron populations defined by transcriptomic signature expressions correspond to distinct M/E subtypes is still unknown. Here, we applied the Patch-PCR approach to simultaneously obtain the M/E properties and messenger RNA (mRNA) expression of >600 interneurons in layer V of the mouse somatosensory cortex (S1). Subsequently, we identified 11 M/E subtypes, 9 neurochemical cell populations (NCs), and 20 transcriptomic cell populations (TCs) in this cortical lamina. Further analysis revealed that cells in many NCs and TCs comprised several M/E types and were difficult to clearly distinguish morpho-electrically. A similar analysis of layer V interneurons of mouse primary visual cortex (V1) and motor cortex (M1) gave results largely comparable to S1. Comparison between S1, V1, and M1 suggested that, compared to V1, S1 interneurons were morpho-electrically more similar to M1. Our study reveals the presence of substantial M/E variations in cortical interneuron populations defined by molecular expression.

Early-generated interneurons regulate neuronal circuit formation during early postnatal development.

A small subset of interneurons that are generated earliest as pioneer neurons are the first cohort of neurons that enter the neocortex. However, it remains largely unclear whether these early-generated interneurons (EGIns) predominantly regulate neocortical circuit formation. Using inducible genetic fate mapping to selectively label EGIns and pseudo-random interneurons (pRIns), we found that EGIns exhibited more mature electrophysiological and morphological properties and higher synaptic connectivity than pRIns in the somatosensory cortex at early postnatal stages. In addition, when stimulating one cell, the proportion of EGIns that influence spontaneous network synchronization is significantly higher than that of pRIns. Importantly, toxin-mediated ablation of EGIns after birth significantly reduce spontaneous network synchronization and decrease inhibitory synaptic formation during the first postnatal week. These results suggest that EGIns can shape developing networks and may contribute to the refinement of neuronal connectivity before the establishment of the adult neuronal circuit.