Sleep-dependency of episodic-like memory consolidation in rats

Episodic memory refers to the recollection of a representation that binds together into a unique past experience "what" happened, "where" and "when". Sleep has been identified as a state that optimizes the consolidation of newly acquired memory. To determine if sleep is important for the consolidation of episodic-like memory, we tested rats on an episodic-like memory task requiring the binding of an object memory into a spatio-temporal context, as well as retention of its individual components, using separate tests of novel-object recognition ("what"), object-place recognition ("where") and temporal memory ("when"), respectively. The 80-min retention interval between encoding of the task and retrieval testing covered either a period of regular morning sleep or sleep deprivation or a period of evening wakefulness. Sleep during the retention interval, compared with the other two retention conditions, significantly enhanced retrieval in the episodic-like memory task as well as in the object-place recognition and temporal memory tasks. In fact, when the rats stayed awake during the retention interval, there was no significant memory left at retrieval testing for the learnt object place and temporal memory. Sleep did not benefit novel-object recognition memory which unlike the other components of episodic-like memory is considered not to critically rely on the hippocampus. In an additional delayed sleep condition, episodic-like memory in rats which had stayed awake during the first 80-min interval after encoding, was not recovered when they were allowed to sleep during a subsequent 80-min interval. Our results suggest that sleep specifically supports the aspects in episodic memory most closely linked to hippocampal function, i.e., the binding of an event into spatio-temporal context as well as the spatio-temporal context itself. Sleep is particularly effective when it occurs shortly after encoding.