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Sleep Deprivation and the Hippocampus February 10, 2007

Posted by Johan in Neuroscience, Sleep.
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It is quite striking that we currently have no strong account of the purpose of an activity that we all spend somewhere between a quarter and a third of our lives engaging in: sleep. However, it’s not fair to say that we don’t know why we sleep: sleep has been found to affect a range of behaviours. These various purposes that sleep serves may be exactly the reason why a singular account that explains sleep has not been forthcoming. It’s possible that sleep is necessary for several more-or-less independent processes.

Some “creative” functions of sleep such as physical restoration (implausible, as measurable effects of deprivation only appear after extremely prolonged sleep deprivation) and adaptation (in this view, animals are better off sleeping away down-time when they don’t need to feed, as this keeps them out of harm’s way) have been proposed, but the main functions that appear to have tangible effects in experiments concern development and memory.

According to developmental accounts, REM sleep deprivation in developing organisms results in behavioral problems, decreased brain mass, and neuronal cell death. Researchers that focus on the role of sleep in memory suggest that sleep is essential for consolidating information in long-term memory, possibly with relation to emotional content: A recent paper reported on an experiment where participants read texts with emotional or neutral content, followed by three hours of sleep or waking. When testing recognition four years later (!), the participants who had slept immediately after encoding showed a recognition advantage for the emotional stories, but not for the neutral ones (Wagner et al, 2006). Participants who had been awake following encoding showed no such recognition advantage.

Results obtained by Mirescu et al (2006) exemplifies the possibility that developmental and memory accounts are related. In this study, rats were deprived of sleep while their levels of the hormone corticosterone were either held constant or unmanipulated. Mirescu et al (2006) reported that rats who are deprived of sleep in this manner showed reduced cell proliferation in the hippocampus (which appears to be a critical area for encoding events into long-term memory), but this effect was mediated by corticosterone: rats who had their corticosterone levels held constant did not show inhibited cell proliferation. This suggests that sleep deprivation affects normal functioning of the hippocampus, and that corticosterone may be the causative factor… Although this only pushes the question back to what causes corticosterone levels to increase during sleep deprivation. It would be quite interesting to replicate this study in a developmental framework, with memory tests. Sleep deprived rats with constant corticosterone levels may not show the memory impairments that normally appear in rats who have been subjected to extended sleep deprivation.

By the way, it’s worth emphasising that these rats had had no sleep for 72 hours: no effects appeared for rats that had been deprived of sleep for 24 hours. So if you’re wondering if these results suggest that you should start getting more sleep, the answer is probably going to be no, at least not for this reason.

References

Mirescu, C., Peters, J.D., Noiman, L., & Gould, E. (2006). Sleep Deprivation Inhibits Adult Neurogenesis in the Hippocampus by Elevating Glucocorticoids. Proceedings of the National Academy of Sciences of the United States of America, 103, 19170-19175.

Wagner, U., Hallschmid, M., Rasch, B., & Born, J. (2006). Brief Sleep after Learning Keeps Emotional Memories Alive for Years. Biological Psychiatry, 60, 788-790

Wikipedia: Theories Regarding the Function of Sleep.

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