Scientists have discovered that rhythmic oscillations of a specific neurotransmitter play a vital role in clearing toxic proteins from the brain during non-rapid eye movement (non-REM) sleep. These oscillations drive the glymphatic system by powering the coordinated movement of cerebrospinal fluid and blood. However, the commonly prescribed sleep aid zolpidem (commonly marketed as Ambien) disrupts this process, potentially impairing the brain’s ability to clear waste. The findings have been published in the journal Cell.
The glymphatic system is a network in the brain responsible for clearing waste products, such as amyloid and tau proteins, which are associated with neurodegenerative diseases like Alzheimer’s. Unlike other organs, the brain lacks traditional lymphatic vessels for waste removal. Instead, it relies on cerebrospinal fluid (CSF) to flush out toxins through specialized spaces surrounding blood vessels.
“When we started this study, we already knew that the glymphatic system is vital for cleaning the brain, that it relies on brain fluid (CSF) flushing through the brain, and that it is activated during sleep. However, we did not know how sleep was driving the removal of waste from the brain,” explained Natalie Hauglund, the first author of the study and currently a postdoctoral fellow at the University of Oxford.
The researchers conducted a series of experiments on mice to observe the glymphatic system in action during sleep.
The first set of experiments focused on how norepinephrine, a neurotransmitter that regulates arousal and blood vessel constriction, interacts with cerebral blood flow and CSF movement during sleep. Using “flow fiber photometry,” a technique that allows real-time tracking of norepinephrine levels, blood flow, and CSF dynamics, the researchers found that norepinephrine levels exhibited slow, rhythmic oscillations during non-REM sleep.
These oscillations coincided with synchronized cycles of blood vessel constriction and relaxation (vasomotion), which created a pumping mechanism to drive CSF through the brain. Importantly, these oscillations were absent during wakefulness and disrupted during REM sleep, suggesting that non-REM sleep provides a unique state for optimal glymphatic activity.
To confirm that norepinephrine oscillations were directly driving vasomotion, Hauglund and her colleagues used optogenetics to manipulate the locus coeruleus, a brain region responsible for norepinephrine release. By stimulating or inhibiting this region, they demonstrated that norepinephrine release tightly controlled blood vessel dynamics and, by extension, CSF flow.
To directly test whether vasomotion acts as a “pump” for CSF flow, the researchers conducted optogenetic experiments targeting smooth muscle cells in blood vessels. By using light to stimulate rhythmic constriction and relaxation of these vessels in naturally sleeping mice, they artificially increased the frequency of vasomotion.
The stimulation enhanced CSF flow and glymphatic clearance in brain regions near the site of vascular manipulation, providing direct evidence that cycles of arterial constriction and dilation drive glymphatic activity. This experiment confirmed that vasomotion plays a central role in the glymphatic system.
“We discovered that what drives the CSF flow through the brain, and thereby the brain cleaning during sleep, is a slow pumping mechanism created by synchronous constriction and dilation of the blood vessels in the brain,” Hauglund told PsyPost. “This is controlled by a signaling molecule called norepinephrine, which is released in the brain roughly every 50 seconds, creating slow oscillations in norepinephrine levels during sleep.”
Another experiment investigated how natural sleep microarchitecture, particularly the frequency of brief awakenings called micro-arousals, influenced glymphatic activity. Using EEG and EMG recordings to monitor brain activity and sleep states, the researchers correlated the frequency of micro-arousals with glymphatic clearance efficiency.
Mice with more frequent micro-arousals during non-REM sleep exhibited greater glymphatic clearance of tracer molecules. This finding supported the idea that norepinephrine oscillations and the associated vascular dynamics, which often coincide with micro-arousals, play a pivotal role in driving CSF flow.
Interestingly, while micro-arousals were associated with increased glymphatic activity, they were not the sole determinant of clearance. The researchers concluded that the oscillatory release of norepinephrine during non-REM sleep acts as the primary driver of CSF flow, with micro-arousals serving as a secondary, parallel process.
“Our study showed that the frequency of micro-arousals, tiny awakenings that happen throughout the night without being perceived by the sleeper, correlates positively with glymphatic flow,” Hauglund explained. “This may seem surprising, as micro-arousals are often viewed as a sign of fragmented sleep.”
“However, more and more evidence indicates that micro-arousals are a natural part of healthy sleep and may have important functions for the beneficial effects of sleep. The reason for the correlation between glymphatic flow and micro-arousals is that the norepinephrine waves that control the ‘pump’ driving the CSF flow also induce micro-arousals.”
To investigate the effects of zolpidem on sleep architecture and glymphatic activity, the researchers administered the drug to a group of mice and monitored norepinephrine levels, blood vessel dynamics, and CSF flow. Although zolpidem helped the mice fall asleep more quickly, it significantly disrupted the infraslow oscillations in norepinephrine levels and blood vessel vasomotion that are critical for glymphatic clearance.
Using EEG recordings, the researchers also observed that zolpidem-treated mice had more frequent micro-arousals but with diminished norepinephrine peaks. As a result, the natural synchronization of blood flow and CSF movement was impaired.
When the researchers measured glymphatic clearance by injecting a fluorescent tracer into the CSF, they found that zolpidem-treated mice exhibited reduced tracer inflow and clearance compared to control mice. This indicated that while zolpidem induced sleep, it interfered with the restorative processes of natural sleep, specifically the brain’s ability to clear harmful waste products through the glymphatic system.
“We found that the sleep aid zolpidem disrupted the norepinephrine oscillations and thereby reduced the fluid flow,” Hauglund told PsyPost. “This suggests that the sleep you get while using sleep medication is not as beneficial as regular sleep in terms of restorative processes, such as brain cleaning.”
The experiments were conducted in mice, which, while biologically similar in some respects, do not fully replicate human sleep architecture or physiology. However, Hauglund noted that “results from human studies indicate that the same mechanism exists. For example, MRI scans of people sleeping inside a scanner have shown that slow oscillations in blood volume and CSF volume are present in the brain during sleep.”
Future research could investigate how factors such as aging, vascular health, and neurodegenerative diseases impact the system’s efficiency. Exploring potential interventions to enhance glymphatic clearance—whether through pharmacological agents, lifestyle modifications, or non-invasive therapies—would also be valuable.
“Many questions are still waiting to be answered,” Hauglund said. “For example, is will be important to see how different disease states affect the CSF pumping, and if there are ways to enhance the ‘pump’ in order to boost the removal of waste from the brain.”
The study, “Norepinephrine-mediated slow vasomotion drives glymphatic clearance during sleep,” was authored by Natalie L. Hauglund, Mie Andersen, Klaudia Tokarska, Tessa Radovanovic, Celia Kjaerby, Frederikke L. Sørensen, Zuzanna Bojarowska, Verena Untiet, Sheyla B. Ballestero, Mie G. Kolmos, Pia Weikop, Hajime Hirase, and Maiken Nedergaard.
