Transsynaptic neural circuit mapping of ventral hippocampus motivational control systems.

The ventral hippocampus contributes to food intake regulation and a range of motivational and memory processes, and its dysfunction is associated with several cognitive and behavioral disorders. However, its circuit-level organization remains incompletely understood. Ventral CA1 (CA1v) neurons send projections to several regions involved in motivational control. Here we focus on three major forebrain targets: the nucleus accumbens shell (ACBsh), medial prefrontal cortex (mPFC), and lateral hypothalamic area (LHA). We mapped the upstream and downstream circuitry of CA1v neurons defined by their projections to these target regions in rats using complementary transsynaptic anterograde and retrograde viral tracing approaches. Monosynaptic outputs to ACBsh, mPFC, and LHA were targeted using ATLAS, a novel transsynaptic anterograde viral approach that drives Cre recombinase in neurons receiving synaptic transmission from the CA1v. Second-order projections arising from these defined pathways were then mapped using a Cre-dependent anterograde viral tracing strategy. In parallel, upstream inputs to CA1v neurons projecting to each downstream target were mapped using a conditional retrograde glycoprotein-deleted rabies viral approach. Anterograde tracing revealed both shared and pathway-specific second-order targets, including bidirectional CA1v projections. Retrograde tracing confirmed expected inputs (e.g., CA3) and uncovered previously unrecognized cortical sources that differed across downstream projection-defined CA1v subpopulations. Together, these findings delineate pathway-specific, multi-node circuits linking CA1v neurons to key motivational systems that may inform future therapeutic strategies for disorders involving ventral hippocampal dysfunction.