Circuit and molecular architecture of a ventral hippocampal network.

The ventral hippocampus (vHPC) is a critical hub in networks that process emotional information. While recent studies have indicated that ventral CA1 (vCA1) projection neurons are functionally dissociable, the basic principles of how the inputs and outputs of vCA1 are organized remain unclear. Here, we used viral and sequencing approaches to define the logic of the extended vCA1 circuit. Using high-throughput sequencing of genetically barcoded neurons (MAPseq) to map the axonal projections of thousands of vCA1 neurons, we identify a population of neurons that simultaneously broadcast information to multiple areas known to regulate the stress axis and approach–avoidance behavior. Through molecular profiling and viral input–output tracing of vCA1 projection neurons, we show how neurons with distinct projection targets may differ in their inputs and transcriptional signatures. These studies reveal new organizational principles of vCA1 that may underlie its functional heterogeneity. The ventral hippocampus is central in the processing of emotional information. Here, a combination of viral and sequencing approaches defines the organizational logic of the extended ventral CA1 circuit.