Published: Oct 18, 2013 | Updated: Oct 21, 2013
By John Gever

Full Story:  http://www.medpagetoday.com/PrimaryCare/SleepDisorders/42365

Studies in mice indicate that sleep’s critical function is to allow metabolic waste products to be cleared from the brain, which apparently cannot occur during waking hours, researchers said.

Real-time imaging in live mice — awake, sleeping normally, and under anesthesia — showed that both the natural and artificially induced sleep states were associated with 60% increases in interstitial space within the animals’ brains, with large corresponding increases in convective exchange of cerebrospinal fluid (CSF) with interstitial fluid, according toMaiken Nedergaard, MD, DMSc, of the University of Rochester in New York state, and colleagues.

These convective currents, which Nedergaard and colleagues have dubbed the “glymphatic system,” were themselves associated with accelerated clearance of beta-amyloid proteins from the brain during the sleeping states, the researchers reported in the Oct. 18 issue of Science.

“An extension of the findings reported here is that the restorative function of sleep may be due to the switching of the brain into a functional state that facilitates the clearance of degradation products of neural activity that accumulate during wakefulness,” Nedergaard and colleagues wrote.

The purpose of sleep has been among the biggest puzzles in the history of science. Until now, even the most advanced technologies have been unable to determine firmly why people die when deprived of sleep for long enough.

Nedergaard and colleagues had previously studied the glymphatic system — the vasculature that brings CSF into the brain where it mixes with interstitial fluid that then exits the brain. A key marker of this exchange is beta-amyloid protein, they explained in the Science report.

Earlier studies have shown that beta-amyloid levels in interstitial fluid are higher in rodents and humans when they are awake than when they are asleep, which had prompted speculation that wakefulness promotes beta-amyloid release.

“We tested the alternative hypothesis that [beta-amyloid] clearance is increased during sleep and that the sleep-wake cycle regulates glymphatic clearance,” Nedergaard and colleagues wrote.

In the current study, the researchers used two-photon in vivo imaging to track CSF and interstitial fluid movement in real time within the brains of mice. The animals were also wired up for electrocorticography and electromyography for continuous monitoring of brain activity.

Waking the animals led to sharp drops in periarterial (reflecting CSF channels) and parenchymal fluid influx into the animal’s brains, the researchers found. In a separate experiment, putting mice to sleep with anesthesia caused increases in fluid movement through CSF channels.

“Thus, glymphatic CSF influx is sharply suppressed in conscious alert mice as compared with naturally sleeping or anesthetized littermates,” Nedergaard and colleagues wrote.

That still left open the question of how this effect is controlled. CSF channels sit near cerebral arteries, the pulsing of which provide a motive force. But the researchers considered it unlikely that sleep-wake variations in arterial pulsation could be responsible for the diurnal difference in CSF flow.

Instead, they considered an alternative mechanism, whereby reduction in interstitial space within the brain during waking time would impede CSF influx.

Measurement of the interstitial space, using an iontophoretic tetramethyl ammonium tracer, in sleeping mice showed a volume fraction of 23.4% (SD 1.9%), compared with 14.1% (SD 1.8%, P<0.01) in awake mice.

The studies of beta-amyloid clearance were conducted by injection of a radiolabeled version of the protein into the brains of mice while sleeping, under anesthesia, and awake.

Clearance rates were doubled in the naturally sleeping mice compared with awake animals; there was no difference in beta-amyloid clearance between the sleeping and anesthetized mice.

Clearance of an inert tracer, radiolabeled inulin, was also substantially lower in awake mice than in sleeping or anesthetized mice.

Other experiments suggested that adrenergic signalling was critical to the sleep-wake differences in fluid movement and amyloid clearance, “modulating not only cortical neuronal activity but also the volume of the interstitial space,” Nedergaard and colleagues wrote.

Still to be determined, though, is the identity of specific cell types that expand and shrink the interstitial space, the researchers noted.