Experimental sleep fragmentation impairs spatial reference but not working memory in Fischer/Brown Norway rats

Abstract

Sleep fragmentation is a common symptom in sleep disorders and other medical complaints resulting in excessive daytime sleepiness. The present study seeks to explore the effects of sleep fragmentation on learning and memory in a spatial reference memory task and a spatial working memory (WM) task. Fischer/Brown Norway rats lived in custom treadmills designed to induce locomotor activity every 2 min throughout a 24-h period. Separate rats were either on a treadmill schedule that allowed for consolidated sleep or experienced no locomotor activation. Rats were tested in one of two water maze-based tests of learning and memory immediately following 24 h of sleep interruption. Rats tested in a spatial reference memory task (eight massed acquisition trials) with a 24-h follow-up probe trial to assess memory retention showed no differences in acquisition performance but were impaired on the 24 h retention of the platform location. In contrast, the performance of rats tested in a spatial WM task (delayed matching to position task) was not impaired. Therefore, sleep fragmentation prior to testing impairs the ability to retain spatial reference memories but does not impair spatial reference memory acquisition or spatial WM in Fischer-Norway rats.

1

The Diagram represents the nine novel platform location used in the delayed matching to position task to asses spatial working memory (adapted from Steele and Morris 1999).

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The recall of the platform location was impaired by 24 hours of SI. A: Mean (±SEM) latency (left) and path length (right) to reach the hidden platform over 8 consecutive acquisition trials. No significant differences in learning were observed among the 3 different groups of rats. B. Mean (+SEM) percent time (left) and distance (right) during probe trial that rats spent searching in the target quadrant that formerly contained the hidden platform. SI rats spent significantly less time in the target quadrant than did CC, but EC was not significantly different from CC. * p < 0.05

3

The delayed matching to position task tested spatial working memory in rats. Mean (±SEM) latency (top) and distance (bottom) for CC rats to find the visible platform in the first trial compared to finding the hidden platform in the second trial. Rats spent significantly longer periods of time finding the previously indicated platform location as delays increased from 1 to 5 to 10 minutes. * comparison between visible and hidden platform p < 0.05; + comparison between delays p < 0.05

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24 hours of SI prior to performance did not impact performance on the spatial working memory task. Mean (±SEM) latency (top) and distance (bottom) for rats to find the hidden platform for CC versus SI rats. No significant differences were observed between groups.