Developmental trajectory of episodic-like memory in rats

Abstract

IntroductionEpisodic memory formation requires the binding of multiple associations to a coherent episodic representation, with rich detail of times, places, and contextual information. During postnatal development, the ability to recall episodic memories emerges later than other types of memory such as object recognition. However, the precise developmental trajectory of episodic memory, from weaning to adulthood has not yet been established in rats. Spontaneous object exploration tasks do not require training, and allow repeated testing of subjects, provided novel objects are used on each trial. Therefore, these tasks are ideally suited for the study of the ontogeny of episodic memory and its constituents (e.g., object, spatial, and contextual memory).MethodsIn the present study, we used four spontaneous short-term object exploration tasks over two days: object (OR), object-context (OCR), object-place (OPR), and object-place-context (OPCR) recognition to characterise the ontogeny of episodic-like memory and its components in three commonly used outbred rat strains (Lister Hooded, Long Evans Hooded, and Sprague Dawley).ResultsIn longitudinal studies starting at 3–4 weeks of age, we observed that short term memory for objects was already present at the earliest time point we tested, indicating that it is established before the end of the third week of life (consistent with several other reports). Object-context memory developed during the fifth week of life, while both object-in-place and the episodic-like object-place-context memory developed around the seventh postnatal week. To control for the effects of previous experience in the development of associative memory, we confirmed these developmental trajectories using a cross-sectional protocol.DiscussionOur work provides robust evidence for different developmental trajectories of recognition memory in rats depending on the content and/or complexity of the associations and emphasises the utility of spontaneous object exploration tasks to assess the ontogeny of memory systems with high temporal resolution.

Episodic memory development: Bridging animal and human research

“… 58 However, these results are at odds with Asiminas et al., who only found novelty preferences on the OCR task in pups in the fifth post-natal week. 60 Although the reason for these discrepant results remains to be ascertained, there were notable methodological differences between the two studies. For Ramsaran et al.…”

“…Context in these paradigms has been argued to represent a form of temporal encoding. 60 Two studies have attempted to chart its ontogenetic emergence. Asiminas and colleagues found this encoding triad in the rat emerged at 7 weeks of age, while Ramsaran and colleagues observed an inflection point in 5th post-natal week.…”

“…OCR, OPC, OPCR, and VPC) (e.g., Campbell and Campbell, 55 Guskjolen et al., 57 Ramsaran et al., 59 and Asiminas et al. 60 ). However, extensive training is also common—for example, in watermaze studies, 69 , 70 even though one-session paradigms exist for adults 209 and could be optimized for use in developing animals.…”