Our bodies rely on their lymphatic system to drain excessive fluids and remove waste from tissues, feeding those back into the bloodstream. It’s a complex yet efficient cleaning mechanism that works in every organ except the brain. “When cells are active, they produce waste metabolites, which also happens in the brain. Since there are no lymphatic vessels in the brain, the question was what was it that cleaned the brain,” Natalie Hauglund, a neuroscientist at Oxford University who led a recent study on the brain-clearing mechanism, told Ars.

Earlier studies on mice discovered that the brain has a system that flushes its tissues with cerebrospinal fluid, which carries away waste products in a process called glymphatic clearance. Scientists noticed that this only happened during sleep, but it was unknown what about sleep initiated this cleaning process,” Hauglund explains.

Her study found that glymphatic clearance is mediated by a hormone called norepinephrine. It occurs almost exclusively during the NREM sleep phase but only works when sleep is natural. Anesthesia and sleeping pills shut this process down nearly completely.

Taking it slowly

Dr. Maiken Nedergaard, a Danish neuroscientist and coauthor of Hauglund’s paper, discovered the glymphatic system in the brain in 2013. Since then, numerous studies have been conducted to understand how it works, but most have one problem: they were conducted on anesthetized mice.

“What makes anesthesia useful is that you can have a very controlled setting,” Hauglund says.

However, looking into the brain of a mouse that runs around and wiggles during sleep wasn’t easy. The team pulled it off using flow fiber photometry, which imagines fluids tagged with fluorescent markers using a probe implanted in the brain. So, the mice got the optical fibers implanted in their brains. Once that was done, the team put fluorescent tags in the mice’s blood, cerebrospinal fluid, and the norepinephrine hormone. “Fluorescent molecules in the cerebrospinal fluid had one wavelength, blood had another wavelength, and norepinephrine had yet another wavelength,” HauHaglundys.

This way, her team could get a reasonably precise idea of brain fluid dynamics when mice were awake and asleep. The glymphatic system turned brain tissues into a slowly moving pump.

Pumping up

“Norepinephrine is released from a small area of the brain in the brain stem,” Hauglund says. “It is mainly known as a response to stressful situations. For example, you see norepinephrine levels increasing in fight or flight scenarios.” Its main effect is causing blood vessels to contract. Still, in more recent research, people found that norepinephrine is released in slow waves that roll over the brain roughly once a minute during sleep. This oscillatory norepinephrine release proved crucial to the operation of the glymphatic system.

“When we used the flow fiber photometry method to look into the brains of mice, we saw these slow waves of norepinephrine, but we also saw how it works synchronously with fluctuation in the blood volume,” Haglund says.

Every time the norepinephrine level went up, it caused the contraction of the blood vessels in the brain, and the blood volume went down. At the same time, the contraction increased the volume of the perivascular spaces around the blood vessels, which were immediately filled with the cerebrospinal fluid.

When the norepinephrine level went down, the process reversed: the blood vessels dilated, letting the blood in and pushing the cerebrospinal fluid out. “We found that norepinephrine worked a little bit like an orchestra conductor and makes the blood and cerebrospinal fluid move in synchrony in these slow waves,” Haglund says.

Because The study was designed to monitor this process in freely moving, undisturbed mice, the team learned precisely when all this was happening. When mice were awake, their norepinephrine levels were much higher but relatively steady. The team observed the opposite during the REM sleep phase when they were consistently low. The oscillatory behavior was present exclusively during the NREM sleep phase.

So, the team wanted to check how glymphatic clearance would work when they gave the mice zolpidem, a sleeping drug proven to increase NREM sleep time. Theoretically, zolpidem should have boosted brain-clearing, but it turned it off.

Non-sleeping pills

“When we looked at the mice after giving them zolpidem, we saw they all fell asleep quickly. That was expected—we take zolpidem because it makes it easier for us to sleep,” Hauglund says. “But then we saw those slow fluctuations in norepinephrine, blood volume, and cerebrospinal fluid almost completely stopped.”