New non-canonical hippocampal circuit connections: substantial inputs from ventral hippocampal CA1 and subicular complex to dorsal hippocampal CA3
The hippocampal formation (HF) is traditionally viewed as having a feedforward, unidirectional circuit organization. Within this framework, the subiculum is viewed as a major output region of the hippocampus proper, and hippocampal CA3 projects strongly to CA1. However, new advances in virology and genetic technology offer powerful tools for discovering new hippocampal circuits while complementing more traditional approaches. Our recent studies (Sun et al., 2014; 2018; 2019) using genetically targeted rabies tracing show a significant back-projection pathway from SUB and the subicular complex to hippocampal CA1 in the mouse as previously suggested in other mammalian species using less strict mapping methods. We refer to this circuit as ‘non-canonical’ in that it runs opposite the prominent pathway leading from CA1 to SUB. Furthermore, theta rhythms generated in the rat SUB are reported to flow backward to actively modulate spike timing and local network rhythms in CA1 and CA3 (Jackson et al., 2014). The large extent of SUB activity back-propagation suggests an even larger, non-canonical circuit network involving the subicular complex, hippocampal CA1 and CA3. To test this hypothesis, we used monosynaptic retrograde viral tracing technologies including genetically modified canine adenovirus type 2 (CAV2) and rabies virus (RV) to map local and long-range synaptic inputs to dorsal hippocampal CA3 subfields (proximal CA3b, and more distal CA3b and CA3a). Both approaches produced robust mapping results. The inputs regions include intrinsic CA3 connections, and contralateral CA3, dentate gyrus, medial septum, entorhinal cortex, retromammalilary, and median raphe. Our new findings demonstrate non-canonical synaptic inputs to hippocampal CA3 from ventral CA1, perirhinal cortex, and subicular complex (including ventral SUB and subiculum-transition area (STr)). Thus, ventral CA1 inputs to dorsal CA3 run opposite the trisynaptic pathway and opposite the direction of flow the theta frequency oscillation along the longitudinal pathway. Further, our quantitative analysis of connectivity strengths indicates that the non-canonical input connectivity varies with dorsal CA3 subfield topographic locations. Our results with retrograde viral tracers were confirmed by anterograde-directed herpes simplex virus (H129) mapping of projections from input mapped regions to hippocampal CA3. Together, our data support the proposed hypothesis of a large, non-canonical circuit network, and lay an anatomical foundation to consider the largely unexplored role of these non-canonical hippocampal circuit connections in hippocampal dynamics and behavior.