COMPREHENSIVE MAPPING OF NEURONAL INPUTS TO DORSAL CA1 USING RETROGRADE TRACING TO ELUCIDATE MEMORY MECHANISM

The trisynaptic pathway, involving the entorhinal cortex, dentate gyrus, CA3, and CA1 regions of the hippocampus, is a key theory explaining memory mechanisms, but it is far from capturing the full complexity of the process. Emerging research indicates that various cortical regions significantly contribute to memory, highlighting the need for detailed neural mapping. This study aims to provide a comprehensive mapping of the neuronal inputs to the CA1 region to gain deeper insights into memory coordination. The study utilized rabies viral vectors to trace excitatory neurons connected to the hippocampal CA1. Brain tissues were sectioned and imaged to map neuronal input regions. QuPath and ABBA software were used to quantify labelled cells, and specific laminar layers and AP coordinates that transmit signals to CA1 were identified. Our analysis revealed that over 50% of neuronal inputs to CA1 originate locally within the hippocampus. the CA1 also receive major projection from cortical regions including the entorhinal cortex, with both medial and lateral parts showing distinct connections. Other regions such as the retrosplenial and perirhinal cortices exhibit significant ipsilateral connectivity with CA1. We extended our investigation to the perirhinal cortex, employing the same tracing technique to disynaptically map the upstream regions of the perirhinal cortex that project to the CA1 region. Our findings indicate that PERI-CA1 neurons receive inputs from various sensory association cortices, including olfactory, visual, and auditory cortices. The study quantified the regions providing input to CA1 and identified specific laminar layers potentially involved in spatial learning and memory.