Multimodal profiling and intersectional genetics reveal CA1 border inhibitory neuron diversity and postnatal emergence of retrosplenial connectivity

Inhibitory neurons (INs) at the stratum radiatum/stratum lacunosum-moleculare border of hippocampal CA1 comprise diverse populations, including long-range-projecting neurons that innervate the retrosplenial cortex (RSCtx). However, their molecular organization and developmental connectivity remain poorly understood. Here, we combine multimodal transcriptomic profiling, intersectional genetics, and retrograde tracing to characterize CA1 border INs. We identified two major caudal ganglionic eminence-derived, Cxcl14-enriched populations distinguished by Syt6/Cck-related and Ntng1-expressing transcriptional profiles. We then generated a Cxcl14-Cre mouse line that, when combined with an IN-specific viral enhancer, provides genetic access to CA1 border INs. Retrograde labeling established that CA1 INs projecting to the RSCtx belong to the Ntng1-expressing population, and that this long-range projection emerges during postnatal mouse development, between postnatal days 3 and 5. Finally, analysis of genes encoding cell-surface proteins, key regulators of connectivity, revealed distinct expression profiles in RSCtx-projecting and SYT6-positive INs. Together, these findings establish a molecular and developmental framework for CA1 border IN diversity and provide a genetic strategy for investigating the assembly and function of these inhibitory circuits.