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Sleep suppresses brain rebalancing

sleeping

Why humans and other animals sleep is one of the remaining deep mysteries of physiology. One prominent theory in neuroscience is that sleep is when the brain replays memories “offline” to better encode them (“memory consolidation“). A prominent and competing theory is that sleep is important for rebalancing activity in brain networks that have been perturbed during learning while awake.

Now, a new study shows that these homeostatic mechanisms are indeed gated by sleep and wake, but in the opposite direction from that theorized previously: homeostatic brain rebalancing occurs exclusively when animals are awake, and is suppressed by sleep.

These findings raise the intriguing possibility that different forms of brain plasticity – for example, those involved in memory consolidation and those involved in homeostatic rebalancing – must be temporally segregated from each other to prevent interference.

The requirement that neurons carefully maintain an average firing rate, much like the thermostat in a house senses and maintains temperature, has long been suggested by computational work. Without homeostatic (“thermostat-like”) control of firing rates, models of neural networks cannot learn and drift into states of epilepsy-like saturation or complete quiescence.

In 2013, researchers provided the first in vivo evidence for firing rate homeostasis in the mammalian brain. The team recorded the activity of individual neurons in the visual cortex of freely behaving rat pups for 8 hours per day across a nine-day period during which vision through one eye was occluded.

The activity of neurons initially dropped, but over the next four days, firing rates came back to basal levels despite the visual occlusion. In essence, these experiments confirmed what had long been suspected – the activity of neurons in intact brains is indeed homeostatically governed.

Due to the unique opportunity to study a fundamental mechanism of brain plasticity in an unrestrained animal, the lab has been probing the possibility of an intersection between an animal’s behavior and homeostatic plasticity. In order to truly evaluate possible circadian and behavioral influences on neuronal homeostasis, it was necessary to capture the entire 9-day experiment, rather than evaluate snapshots of each day.

For this work, researchers had to find creative computational solutions to recording many terabytes of data necessary to follow the activity of single neurons without interruption for more than 200 hours.

Ultimately, these data revealed that the homeostatic regulation of neuronal activity in the cortex is gated by sleep and wake states. In a surprising and unpredicted twist, the homeostatic recovery of activity occurred almost exclusively during periods of activity and was inhibited during sleep. Prior predictions either assumed no role for the behavioral state or that sleeping would account for homeostasis.

Finally, the team established evidence for a causal role for active waking by artificially enhancing natural waking periods during the homeostatic rebound. When animals were kept awake, homeostatic plasticity was further enhanced.

This finding opens doors onto a new field of understanding the behavioral, environmental, and circadian influences on homeostatic plasticity mechanisms in the brain. Some of the key questions that immediately beg to be answered include:

What it is about sleep that precludes the expression of homeostatic plasticity?

How is it possible that mechanisms requiring complex patterns of transcription, translation, trafficking, and modification can be modulated on the short timescales of behavioral state-transitions in rodents?

And finally, how generalizable is this finding? As homeostasis is bidirectional, does a shift in the opposite direction similarly require wake or does the change in sign allow for new rules in expression?

Sources:
Hengen, K., Torrado Pacheco, A., McGregor, J., Van Hooser, S., & Turrigiano, G. (2016). Neuronal Firing Rate Homeostasis Is Inhibited by Sleep and Promoted by Wake Cell DOI: 10.1016/j.cell.2016.01.046

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6 responses

  1. Sleepy Head

    Shouldn’t you credit the Brandeis blog post you’ve reproduced here?
    http://blogs.brandeis.edu/science/2016/03/18/sleep-suppresses-brain-rebalancing/

    March 22, 2016 at 11:34 pm

    • It is a press release, so we link to the study, not to what appears to be the copy of the press release on the group’s website itself.

      March 22, 2016 at 11:42 pm

  2. Marinda

    By testing the theories of how sleep affects our cortex and minds, the data gathered suggests that we (mammals actually suppress our regulating body functions) Considering the testing was done on rats, I think it was a very interesting concept it does cause a few questions to arise from the article, such as if our mind is also restoring and replying the many aspects of our memories. It would be very interesting to test this study on a human, to see if the mind functions differently and if we truly suppress our natural homeostatic tendencies. It would be quite interesting if our suppressant of memories and functionality caused us to see images and such that form our dreams. Thank you for the information on how sleep affects a mammals mind and body.

    March 23, 2016 at 1:19 pm

    • I agree a study on humans is in order. However, because the human brain is so complex it is hard not to be forced to use reductive methods to parse out something to help us make sense of… well us. Thanks for taking the time to comment and I really hope we see further studies in the future as well!

      March 23, 2016 at 3:11 pm

  3. jacob

    This post is very interesting to me and the experiment done on the rats. I think the outcome may have been different with the rats then it would with humans. I’d really like to see the experiment done with multiple humans and see if they get the same results. Sleep is the most important part of our everyday lives and we all have dreams if we know it or not. In the beginning of the article they say “One prominent theory in neuroscience is that sleep is when the brain replays memories “offline” to better encode them (“memory consolidation“).” (Loonylabs.org) i’d like to see them test this theory and see what the outcome might be.

    April 1, 2016 at 5:13 pm

    • I agree, I hope we can see human studies. It would be handy to determine exactly what function sleep has for us, but it may be a while before we get a definitive answer.

      April 2, 2016 at 11:22 am

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