Conspecific Presence Improves Episodic-Like Memory in Rats

Abstract

A number of studies have provided evidence that animals, including rats, remember past episodes. However, few experiments have addressed episodic-like memory from a social perspective. In the present study, we evaluated Wistar rats in the WWWhen/ELM task as single setups and in dyads, applying a long retention interval. We also investigated behaviors that could subserve the emergence of this type of memory. We found that only rats tested in the social setting were able to recollect an integrated episodic-like memory that lasted 24 h. Additionally, rats in dyads presented higher levels of exploration during the task. When exposed to the testing environment, the dyads exhibited affiliative behavior toward each other and presented fewer anxiety-like responses. Our findings indicate that the presence of a conspecific could act as a facilitating factor in memory evaluations based on spontaneous exploration of objects and provide empirical support for applying more naturalistic settings in investigations of episodic-like memory in rats.

Keywords: Wistar rats; anxiety-like behavior; episodic-like memory; exploration; social environment.

FIGURE 1

Experimental design. Three groups of rats performed habituation to the apparatus, followed by the WWWhen/ELM task. The control group went through habituation sessions and the WWWhen/ELM task always in single setups. The Co-Hab group performed the habituation sessions in dyads but conducted the WWWhen/ELM task in single setups. The Co-Test group performed both habituation and the WWWhen/ELM task in dyads. The WWWhen/ELM task consists of two sample trials and a test session of object exploration. Compared to samples, objects could be stationary (A1 and B1) or in different locations (A2 and B2). Animals tend to explore A1 > B1 (“temporal pattern”), B2 > B1 (“spatial pattern”) and A1 > A2 (“integrative pattern”). A1: old stationary; A2: old displaced; B1: recent stationary; B2: recent displaced.

FIGURE 2

The effects of the social context on the WWWhen/ELM task. Discrimination indexes for (A) Temporal, (B) Spatial, and (C) Integration components of ELM for Control, Co-Hab and Co-Test experimental groups. Only the rats in the Co-Test group presented episodic-like memory (i.e.,: only the Co-Test group recollected an integrative memory for “what,” “where,” and “when,” since it showed positive discrimination indexes for the spatial, temporal, and integration elements of ELM) (*p < 0.05 comparison against zero chance level using one-sample t-test). Data represent mean ± SEM.

FIGURE 3

The effects of the social context on the WWWhen/ELM task. Exploration time ratio of each object – A1, A2, B1, and B2 – for Control, Co-Hab and Co-Test groups during test session. The Co-Test group exhibited the exploratiordsn pattern that defines WWWhen/ELM, i.e.,: they explored the stationary object more than the displaced one if they were presented earlier, while simultaneously exploring the displaced object more than the stationary one if they were presented recently. However, the Control and Co-Hab groups did not show this pattern. *p < 0.05 for comparison between biased exploratory pattern A1 > A2, A1 > B1, and B2 > B1 of the WWWhen/ELM task (Wilcoxon signed-rank test, Bonferroni corrected). The graphs represent the median ± interquartile range (IQR).