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Group led by Tohoku University reviews 30 years of established theory and discovers neural network in the ventral hippocampus


A research group led by Assistant Professor Shinya Ohara of the Graduate School of Science and Faculty of Science at Tohoku University, in collaboration with a research group led by Professor Menno Witter of the Norwegian University of Science and Technology and Dr. Alexei Egorov of Heidelberg University in Germany, investigated the structure of neural circuits in the hippocampus and medial entorhinal cortex in rats and mice, which have a deep connection with memories. This structure has long been studied as a memory network and has long been believed to be similar in the ventral and dorsal parts of the hippocampus, but research has revealed that the ventral part of the hippocampus constitutes a different network than the dorsal part, transmitting information to neurons in the Va layer of the medial entorhinal cortex.

A: Neural circuits were examined by labeling hippocampal neurons. Dorsal hippocampal neurons send their axons to layer Vb of the dorsal medial entorhinal cortex (red). In contrast, ventral hippocampal neurons send their axons to layer Va of both dorsal and ventral medial entorhinal cortex (cyan, yellow arrowhead). B: Updated circuit diagram of the circuit from the hippocampus to the medial entorhinal cortex. We found that the ventral hippocampus efficiently send out the information to the neocortex via the medial entorhinal cortex (blue arrows).
Provided by Tohoku University

To understand how memories are formed, it is important to know how the neural networks in the brain regions responsible for memory are organized, with the entorhinal cortex located next to the hippocampus being crucial for memory along with the hippocampus. The ventral hippocampal network was coincidentally labelled while investigating this entorhinal cortex circuit. The research started when the group realized that the network structure deviated from the general understanding of the hippocampal network that has been standard for the past 30 years.

The research group comprehensively re‐examined the structure of the neural network of the hippocampus‐medial entorhinal cortex, with a particular focus on the ventral hippocampus. They found that the dorsal and ventral parts of the hippocampus connect to different groups of neurons in the medial olfactory entorhinal cortex, creating distinct neural networks. The dorsal hippocampus conveys information to Vb layer neurons in the medial olfactory entorhinal cortex, whereas the ventral hippocampus acts mainly on Va layer neurons.

The ventral hippocampus was shown to convey information not only to the ventral part of the medial entorhinal cortex, as previously reported, but also to Va layer neurons in the dorsal part. Va layer neurons of the medial entorhinal cortex, which send information to the neocortex, the final repository of memory, are very important for the formation of long‐term memories. The dense network with Va layer neurons suggests that the ventral hippocampus regulates the transfer of information from the hippocampus to the neocortex and is closely involved in the formation of long‐term memories.

"Moving forward, we will find out how the discovered neural network works," said Ohara. "We will investigate whether mice can remember places where they had pleasant or unpleasant experiences when the transmission of information from the ventral hippocampus to the medial entorhinal cortex is inhibited. We want to understand the mechanisms of how we remember events when we are happy or sad."

Journal Information
Publication: Cell Reports
Title: Hippocampal‐medial entorhinal circuit is differently organized along the dorsoventral axis in rodents
DOI: 10.1016/j.celrep.2023.112001

This article has been translated by JST with permission from The Science News Ltd. ( Unauthorized reproduction of the article and photographs is prohibited.

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