Multimodal classification of neurons in the lateral septum

The lateral septum (LS) is a nucleus in the ventral forebrain that modulates complex social and affective behaviors. Several distinct neuronal types have been described in the LS; however, the full extent of this cellular and molecular diversity remains unclear. We address this gap by profiling the transcriptional identity of mature LS neurons originating from two progenitor lineages defined by their anatomical location and expression of the transcription factor Nkx2.1. We describe 12 molecularly distinct subtypes of LS neurons that fall into two main groups: those with a history of Nkx2.1 expression and those without. We discovered that LS neurons from the Nkx2.1 lineage share an enrichment of select cell adhesion and communication molecules. Despite this, we found that LS neurons that have distinct developmental origins can exhibit significant transcriptional similarities. We then examined the spatial relationships among neurons in the LS, revealing that each subtype occupies a discrete anatomical domain. These anatomical domains are defined by graded patterns of gene expression that correlate with the molecular taxonomy of LS neuron subtypes and encode proteins involved in synaptic signaling. Lastly, we genetically labeled non-overlapping subgroups of LS neurons, and detailed their connective, morphological, and electrophysiological properties. Our findings offer a deeper understanding of neuronal heterogeneity in the LS, paving the way for future studies into how these neuronal types contribute to regulating emotional and motivated behaviors.