Populations of neurons in the brains of sleep-deprived rats go to sleep though the animals remain awake
Sleeping neurons may be at the root of the most common symptoms of sleep deprivation, including attention lapses, poor judgment, and frequent mistakes in cognitive tasks. Specifically, populations of neurons in the brains of rats forced to stay awake briefly go "off-line," into a sleep-like state, according to a study published today (April 27) in linkurl:Nature.
| Sleep-deprived rat during sugar pellet test|
Courtesy of Yuval Nir, University of Wisconsin-Madison
"We've all have this experience where we're forcing ourselves to stay awake later than we usually do," said linkurl:Christopher Colwell,;http://faculty.bri.ucla.edu/institution/personnel?personnel_id=45975 who studies sleep and circadian rhythms at the University of California, Los Angeles, and was not involved in the study. "We can still perform -- we're still obviously awake -- but...a person's performance is not as good," he said. "So, what's going on during that time?"
Neuroscientist linkurl:Giulio Tononi,;http://tononi.psychiatry.wisc.edu/People/GiulioTononi.html along with Vlad Vyazovskiy and a team at the University of Wisconsin-Madison, set out to see what happens to neurons after they have been pushed beyond a normal waking period into a state of sleep deprivation. The researchers kept rats, a nocturnal species, awake for four extra hours in the morning, and attached microwires to neurons in the motor and parietal cortices to monitor neuronal activity.
They found that though rats were active and had an overall "awake" electroencephalogram (EEG) -- low-voltage, high-frequency waves -- populations of neurons in one or the other cortex at times seemed to deviate briefly from this pattern, for about 50 milliseconds at a time, demonstrating a local slow wave EEG pattern indicative of non-REM sleep, the most common type of sleep. Even within the same cortical area, some neurons went offline while others stayed on. And the longer rats were kept awake, the more often populations of neurons went offline into the sleep-like pattern of activity.
The team assessed the rats' behavioral performance while neurons were offline. Tasked with reaching a sugar pellet, rats' poor performance was correlated with the occurrence of offline periods of neurons in the motor cortex, a region typically engaged during the task. And performance worsened with prolonged sleep deprivation when such periods occurred more frequently.
The finding stands in contrast with a long-standing belief that sleep is a global phenomenon, one that affects the whole brain. "Sleep and wake are still by and large global behaviors," said Tononi. "But there is more and more evidence now, and very strong evidence, that these things can happen locally." Colwell agrees, "It's been causing me, as I look at the literature, to change the way I'm thinking about [sleep]."
The finding that neurons are locally online or offline is not entirely surprising, says Tononi. Human sleepwalking, for example, is evidence that some parts of the brain can be awake while other sleep. Marine mammals and birds also show a special ability to keep one hemisphere of the brain awake -- to keep the animal moving continuously through the water to breathe, for example -- while the other half sleeps.
"The implications are somewhat obvious if [the new findings] were also to apply to humans," said Tononi. "Many of the symptoms and signs of sleep deprivation, including bad judgment, irritability, and various other things that are well known, might actually be due to neurons going offline randomly in your brain."
But whether similar events are indeed happening in humans remains to be seen, said Colwell, author of an accompanying News & Views article in Nature.
In addition, it would be valuable to prove not just a correlation between offline neurons and performance, but a direct cause and effect demonstration. "It's an important first step and an exciting study," said Colwell, "but the next thing is to show you can manipulate these same parameters and get the predicted effect."
Vyazovskiy, V., et al., "Local sleep in awake rats," Nature, 472:443-7, 2011.
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