Partial EC outputs by degraded cues are amplified in hippocampal CA3 circuits for retrieving stored patterns

PLOS ONE RESEARCH ARTICLE Partial EC outputs by degraded cues are amplified in hippocampal CA3 circuits for retrieving stored patterns Kisang Eom ID* Department of Physiology, School of Medicine, Keimyung University, Dalseogu District, Daegu Metropolitan City, Republic of Korea * Abstract a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Eom K (2023) Partial EC outputs by degraded cues are amplified in hippocampal CA3 circuits for retrieving stored patterns. PLoS ONE 18(4): e0281458. https://doi.org/10.1371/journal. pone.0281458 Hippocampus is known to be important for episodic memories. Measuring of hippocampal neural ensembles is therefore important for observing hippocampal cognitive processes such as pattern completion. Previous studies on pattern completion had a limitation because the activities of CA3 were not simultaneously observed with the activities of the entorhinal cortex that project to the CA3. In addition, in previous research and modelling, distinct concepts such as pattern completion and pattern convergence have not been considered separately. Here, I used a molecular analysis technique that enables comparison of neural ensembles that evoked two successive events and evaluated neural ensembles in the hippocampal CA3 region and entorhinal cortex. By comparing neural ensembles in hippocampus and entorhinal cortex, I could obtain evidence that suggests pattern completion occurring in the CA3 region was induced by the partial input from EC. Use of the molecularbased ensemble measurement allows measuring two or more brain regions simultaneously, which can lead to insights into the cognitive functions of neural circuits. Editor: Giuseppe Biagini, University of Modena and Reggio Emilia, ITALY Received: August 30, 2022 Accepted: January 24, 2023 Published: April 19, 2023 Copyright: © 2023 Kisang Eom. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: The authors received no specific funding for this work. Competing interests: The authors declared that they have no competing interests. Introduction The hippocampus has an important role in the encoding and retrieval of episodic memory [1]. For the encoding and retrieval of episodic memory, two distinct process would be important: pattern separation and pattern completion. Pattern separation is a process in neural networks that causes redundant or similar inputs to be converted into less similar outputs, which is known that the dentate gyrus (DG) of hippocampus that constitutes the sparse network contributes this phenomenon. Pattern completion is another process in neural network that reconstructs of complete stored representations from partial inputs that are part of stored representation [2]. Since previous literatures had suggested that auto-associational networks of hippocampal CA3 regions would contribute to pattern completion and associative memory [3–6], many studies for pattern completion have been conducted [7–10]. Previous studies revealed that the layer II of entorhinal cortex (EC) sends axon to distal dendrites of CA3 pyramidal cells (CA3-PCs) and dendrites of granule cells of dentate gyrus (DG) [4]. Recently, a PLOS ONE | https://doi.org/10.1371/journal.pone.0281458 April 19, 2023 1 / 18 PLOS ONE Completion of CA3 ensembles by EC outputs study revealed that the hippocampus converts dynamic activities from the EC into stable ones [11]. Therefore, in order to study the activities of the hippocampus related to behavioral tasks, it would be necessary to simultaneously consider the activities of the hippocampus and EC. Previously, EC ensembles or hippocampal CA3 ensembles have been measured using various methods, including in vivo electrophysiology or live imaging [11–13]. However, studies to observe the activity of the hippocampus and EC at the same time have only recently begun [11]. In previous studies, pattern completion is regard as a situation in which the similarity between output ensembles is greater than that between input ensembles [9, 13, 14]. However, this situation corresponds to pattern convergence, not pattern completion [15]. Considering the previous models of pattern completion [6, 15] and the influence of EC neural activity on CA3 neural activity [11, 16, 17], it would be important to compare the EC activities evoked by either the partial external input or whole external input with the activities of CA3 corresponding to each of the two activities. However, in previous experiments on pattern completion, the activity of the EC responsible for projecting axons to CA3 via the PP [18, 19] has been often overlooked. Here, I applied H1a/Arc catFISH (cellular analysis of temporal activity of fluorescence of in situ hybridization with Arc and H1a riboprobes) [20, 21] to measure neural ensembles in EC and CA3 and found that activation of a subset of CA3 ensembles which would be evoked by partial activation of EC ensembles [6, 18, 22] effectively activates the remaining CA3 ensembles. The IEG-based imaging method is a convenience method that can easily measure and compare neural ensembles in multiple regions [20, 23]. With this method, I was able to confirm that pattern completion of CA3 circuits could be triggered by direct cortical input and the storage capacity of CA3 could contribute to the storage and retrieval of stored patterns in neural circuits during rapid contextual learning. Materials and methods Animals and maintenance procedure For using animals in experiments, 3-month-old male C57/BL6J mice were maintained in standard environmental conditions (temperature: 25 ± 2˚C, humidity: 60 ± 5%, dark/light cycle: 8:00 p.m.–8:00 a.m. of next day/8:00 a.m.–8:00 p.m.) and monitored under the veterinary supervision. All mice used in the experiment were housed alone for 1 week before the experiment, during which time they were acclimatized to routine handling. All animal procedures were approved by the Animal Care Committee of Keimyung University (KM-2021-07). Pre-exposure mediated contextual fear conditioning The Pre-exposure mediated contextual fear conditioning (PECFC) tasks of animals are based on the context pre-exposure facilitation effect (CPFE), i.e., enhanced contextual fear conditioning due to pre-exposure to the context before a separate brief context shock episode [24]. Previous studies have shown that this phenomenon is dependent on hippocampus-dependent conjunctive representation [25] and impaired by ablation of CA3 output [7]. In the evaluation of pattern completion, it will be important to evaluate the degree of degradation to retrieval cues and the degree of the corresponding response of the neural networks [2]. Because pattern completion h (...truncated)