Written by Chrystal Moulton, Staff Writer. Researchers found that restorative function of sleep is due to an increase in interstitial space, which enables removal of waste products in the brain accumulated during waking hours.
Every one of us knows sleep is an integral part of our survival. Besides giving us the energy for a new day, have you ever wondered what exactly sleep does for your body. Decades of research demonstrate that lack of sleep diminishes learning and performance, prolongs reaction time, and in some people can cause seizures. Furthermore, prolonged sleep deprivation leads to dementia and even death. (1,2)
During our waking hours, our brain consumes much of the body’s fuel and, as a result of its own chemical processes, releases as waste products linked to brain diseases.(3) These include Beta Amyloid Protein, alpha-synuclein, and tau. All of these proteins have been linked to degenerative diseases such as Alzheimer and dementia. (3,4) However, the brain does not remove these degenerative proteins as soon as they are formed. They simply build up in the fluid environment outside the brain cells (neurons). In humans and rodents β-amyloid protein concentration is higher during waking hours than during sleep, so it could be assumed that wakefulness is associated with increase in β-amyloid.
According to the researchers, there is a process they call the glymphatic system, which removes these damaging proteins. According to them, cerebrospinal fluid (CSF) exchange is made through the convection of fluid across the neuronal membrane and the removal of degenerative proteins like β-amyloid is dependent on the aquaporin-4 water channels in the system. They hypothesized that sleep is associated with a removal of β-amyloid protein and the wake and sleep cycle may regulate the glymphatic system. (2)
In order to evaluate the removal of degenerative proteins in the cerebrospinal fluid and fluid between cells in the brain (interstitial fluid), researchers decided to run a series of tests on mice.
In the first test, researchers wanted to know if there was an influx of cerebrospinal fluid in the brain tissue during sleep versus waking hours. They injected a fluorescent tracer into the CSF of six sleeping mice to track the movement of CSF into brain tissue. Electrocorticography and electromyography was used to monitor the brain state for each animal during the test. Since mice sleep mostly during the day, they were given the tracer at midday and images of the movement of the tracer were taken. Imaging showed a huge influx of CSF tracer while the mice were sleeping.
When mice were awakened, researchers once more injected the mice with the CSF tracer. They found that influx was reduced by approximately 95% in awake versus sleeping mice. To verify that the state of sleep itself was the cause of this influx, another set of six mice awake in the evening were injected with the CSF tracer. All of the same procedures were followed as in the previous experiment only after 30 mins of imaging, the mice were anesthetized. Images CSF tracer movement and brain activity were taken of the mice during anesthesia. They found once again a significant influx of CSF into brain tissue in anesthetized versus awake mice (p<0.05). In other words, the sleep state is marked by a significant influx of CSF into brain tissue in mice.
Realizing that sleep is associated with an influx of CSF, researchers wanted to see what exactly may be causing the influx of CSF into the brain tissue. They decided to test if there was any change in the space between cells (interstitial space) in awake versus sleeping mice. Six mice were observed during wake and sleep states using a specialized method that allows researchers to evaluate and record the space between cells in the brain of each mouse. The mouse heads were fixed to allow for the readings to take place. Recordings taken from sleeping mice, showed an average interstitial volume of 23.4±1.9% for all mice. However, the volume between cells while the mice were awake dropped to 14.1 ±1.8% (n=4, p<0.01). In order to validate findings, they were anesthetized during normal waking periods to see, once again, if sleep is affecting the increase in interstitial space. Their results demonstrated a more than 60% increase in interstitial space from the waking period (13.6±1.6%) to the anesthetized period of sleep in the same mice (22.7±1.3%, p<0.01). Therefore, influx of CSF tracer into brain tissue was a result of an increase in the space between cells during sleep in mice.
Now it was time to test based on previous observations, whether sleep was associated with the clearance of β-amyloid, a waste product of brain processes strongly associated with damage to brain functioning. Radiolabeled β-amyloid was injected into three groups of mice: freely behaving awake mice, sleeping mice, and anesthetized mice. Brains were harvested 10-240 min later for analysis. Researchers found that β-amyloid was removed from the brain two times faster in both anesthetized and sleeping mice, compared to awake mice. Through this observation, they believed that increase in interstitial space may have contributed to faster removal of β-amyloid in sleeping mice versus awake mice.
In their final test, they wanted to determine the signaling system in the brain that is responsible for the increase in interstitial volume. They hypothesized that inhibition of adrenergic signaling, which uses norepinephrine to arouse an animal from sleep, may play an important role in increasing space between cells. They applied an inhibitive molecule directly to the surface of the brains of awake mice. Results showed an increase in interstitial space from 14.3±5.2% to 22.6±1.2% (p<0.01). In other words, compared to their awake litter-mates, interstitial volumes of inhibited mice were significantly higher, but also comparable to interstitial volumes in sleeping or anesthetized mice.
All in all, they believed that the restorative function of sleep could be due to the increase in interstitial space, which allows for efficient removal of waste products in the brain accumulated during waking hours.
Source: Xie, Lulu, et al. “Sleep drives metabolite clearance from the adult brain.” science 342.6156 (2013): 373-377.
Copyright © 2013 by the American Association for the Advancement of Science; all rights reserved.
Posted December 6, 2013.
Chrystal Moulton BA, PMP, is a 2008 graduate of the University of Illinois at Chicago. She graduated with a bachelor’s in psychology with a focus on premedical studies and is a licensed project manager. She currently resides in Indianapolis, IN.
References:
- Neurobiological consequences of sleep deprivation. Alkadhi K, Zagaar M, Alhaider I, et al. Curr Neuropharmacol. 2013 May; 11(3):231-49.
- Sleep Drives Metabolite Clearance from the Adult Brain. Lulu Xie et al. Science 342, 373 (2013).
- Brain fuel metabolism, aging, and Alzheimer’s disease. Cunnane S, Nugent S, Roy M, et al. Nutrition. 2011 Jan; 27(1):3-20.
- Alzheimer’s disease. Querfurth HW, LaFerla FM. N Engl J Med. 2010 Jan 28; 362(4):329-44.
