Functional organization of medial entorhinal cortex layer VI

The deep layers (V/VI) of the entorhinal cortex constitute a major relay station for routing neuronal activity from the hippocampus to downstream networks. To identify contributing pathways, we recently investigated the structural and functional organization of hippocampal projections to layer V (LV) of the medial entorhinal cortex (MEC) (Ohara, Rannap et al., Cell Rep. 2023). Here, we performed whole-cell patch-clamp recordings from MEC-LVI neurons combined with field potential recordings from hippocampal area CA1 in horizontal mouse brain slices. We found that a spontaneously occurring hippocampal activity pattern, the sharp-wave-ripple complex (SPW-R), propagates efficiently to LVI neurons. Over a third of recorded LVI cells (6/16) showed a high level of temporal coupling between postsynaptic currents and upstream ripple oscillations in CA1. To map hippocampal-MEC-LVI projections, we injected AAV-hSyn-EGFP or AAV-hChR2-EYFP into the dorsal or ventral hippocampus (CA1/subiculum) and analyzed the resulting axonal projections in MEC-LVI. Dorsal hippocampal projections to layer VI were confined to the dorsal MEC and their optogenetic activation elicited weak monosynaptic responses in approximately half of recorded LVI neurons (9/20). Ventral hippocampal projections, in turn, innervated ventral MEC and elicited stronger monosynaptic responses in almost all recorded neurons (6/7). Finally, we investigated local connectivity between neurons in LVI and LVa/LVb using paired recordings. While interlaminar connections between LVI and LVa/LVb excitatory neurons were minimal (~1%), interlaminar connectivity between excitatory cells and fast-spiking interneurons was markedly higher (~25-30%). Our findings reveal a specific functional architecture of MEC-LVI and suggest a critical involvement of MEC-LVI in hippocampal-entorhinal signal propagation.