Kawamura, T.; Nair, R.; Tsutsui, K.-I.; Ohara, S.

The hippocampus and its projection targets constitute hippocampal output circuits that are essential for memory, navigation, and emotional regulation. Although cortical and subcortical regions receiving hippocampal projections have been well characterized, it remains unclear how hippocampal signals are processed within these projection target regions. To precisely understand information processing in hippocampal output circuits, it is therefore important to gain genetic access to neuronal subpopulations receiving direct hippocampal inputs for structural and functional analyses. Anterograde transsynaptic transduction using adeno-associated virus (AAV) serotype 1 has emerged as a powerful approach for targeting postsynaptic neurons, but its application is limited by unintended retrograde transport, which leads to false-positive labeling in reciprocally connected circuits. Here, we developed a direction-selective anterograde transsynaptic transduction by combining AAV vectors with distinct infection properties and an intersectional gene expression system. Using the hippocampal-entorhinal circuit, we identified an optimal viral combination that enables predominantly anterograde transsynaptic labeling. We further applied this method to map hippocampal projections to reciprocally connected regions, including the amygdala, providing a robust approach for dissecting complex hippocampal output circuits.