Wistar rats show episodic-like memory for unique experiences

Human episodic memory refers to the recollection of an unique past experience in terms of its details, its locale, and temporal occurrence. Episodic memory, even in principle, has been difficult to demonstrate in non-verbal mammals. Previously, we provided evidence that mice are able to form an integrated memory for "what," "where," and "when" aspects of single experiences by combining different versions of the novelty-preference paradigm, i.e., object recognition memory, the memory for locations in which objects were explored, and the temporal order memory for objects presented at distinct time points. In the present series of experiments we evaluated whether this paradigm, with minor modifications, also works with rats. We found that rats spent more time exploring an "old familiar" object relative to a "recent familiar" object, suggesting that they recognized objects previously explored during separate trials and remembered their order of presentation. Concurrently, the rats responded differentially to spatial object displacement dependent on whether an "old familiar" or "recent familiar" object was shifted to a location, where it was not encountered previously. These results provide strong evidence that the rats established an integrated memory for "what," "where," and "when." We also found that acute stress impaired the animal's performance in the episodic-like memory task, which, however, could be partially reversed by the N-Methyl-D-aspartate-receptors agonist D-cycloserine.