Pre-synaptic and post-synaptic pathways from the hippocampus to medial prefrontal cortex in Rhesus monkeys
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The hippocampal to medial prefrontal cortex (HPC-mPFC) pathway has a role in mnemonic processing. A key function of the hippocampus (HPC) is to organize contextual memories by how they were experienced, and the prefrontal cortex (PFC) retrieves contextual memories by sorting and suppressing irrelevant memories for the task at hand. Studies have highlighted the HPC-mPFC connection in rodents, however, there is a relative paucity of primate studies. The present study addressed this issue by investigating the connection from the HPC to anterior cingulate cortex (ACC; areas 24a, 25 and 32) of the mPFC in rhesus monkeys (Macaca mulatta). The distribution of hippocampal axons and terminals (boutons) was largest in area 25. Bouton diameter was largest in the deep layers of area 25, suggesting an efficient transmission system from the HPC. The robust projections from the HPC terminated most densely in the superficial layers of area 25. The HPC pathway also innervated some inhibitory neurons, labeled for the calcium binding proteins calbindin or calretinin in the superficial layers of the ACC, whereas axons innervated parvalbumin inhibitory neurons in the deep layers of the ACC. The findings suggest that area 25 may be a fundamental pathway from the HPC for memory processing and can be a focal point in therapeutic interventions in neurological and psychiatric diseases.