​​After the defense, the Department of Neurology will host a reception at the NRU, Rigshospitalet 8057, Inge Lehmanns Vej 8, 5th floor.

Assessment Committee
Prof. Adam Espe Hansen, Department of Clinical Medicine, University of Copenhagen, Denmark
Prof. Rune Enger, GliaLab and Letten Centre, Division of Anatomy, Department of Molecular Medicine, Institute of Basic Medical Sciences, University of Oslo, Norway
Ass. Prof. Swati Rane Levendovszky, Department of Radiology, University of Washington Medical Center, USA

Academic Supervisors
Prof. Gitte M. Knudsen, Neurobiology Research Unit, Copenhagen, Denmark
Dr. Sebastian Camillo Holst, Neurobiology Research Unit, Copenhagen, Denmark

THESIS SUMMARY
Sleep promotes cerebrospinal fluid (CSF) flow into and through the brain, which is essential for clearing metabolic waste. Preclinical studies describe how this sleep-dependent influx of CSF, also known as ‘the glymphatic system’, is facilitated by the water channel aquaporin-4 (AQP4) and driven by cerebrovascular oscillations and pressure waves generated by respiratory and cardiac cycles. While growing human evidence supports sleep's role in brain-fluid flow and waste removal, the mechanisms by which sleep regulates this flow remain largely unknown.
The aim of this thesis was to improve our understanding of the interplay between homeostatic sleep mechanisms and brain-fluid dynamics in humans. In study I, we first evaluated whether AQP4 water channel function modulates sleep-wake regulation by examining nocturnal sleep recordings and responses to sleep loss in individuals genotyped for a common AQP4 haplotype. In the subsequent two studies, we used the ultrafast brain imaging techniques Magnetic Resonance Encephalography (MREG) to measure physiological brain oscillations, thought to reflect brain fluid motion. Study II is a descriptive study, validating the physiological origin of MREG-detected brain oscillations through breath-holding and the Valsalva maneuver (known to influence cerebral vasomotion and cardiac activity). Study III examined how sleep deprivation, slow-wave-rich sleep, and pharmacological modulation of cerebrovascular pulsatility impact the strength of physiological brain oscillations in a circadian-controlled study, including a randomized, double-blind, placebo-controlled crossover administration of the α1 and β-adrenergic antagonist carvedilol. Lastly, in study IV, we extended our investigations of cerebrovascular oscillations to examine their relationship with blood norepinephrine levels, using Multiband echo planar imaging (MB). This method offers higher spatial resolution, but slower imaging acquisition than MREG (4.6 images per second versus 10) and was collected alternately with MREG in study III.

Study I demonstrated that carriers of a low-AQP4-expressing variant of an AQP4 haplotype exhibit heightened slow-wave energy during nocturnal sleep and experience increased global alertness and less sleepiness during extended wakefulness.

Study II showed a causal link between both MREG-detected LFOs and cerebral vasomotion and between MREG-detected brain pulsations in respiration- and cardiac frequency ranges and their corresponding physiological processes.

Study III found that sleep deprivation promotes LFOs, while slow-wave-rich sleep (stages N2 and N3) enhances respiration- and cardiac-driven brain pulsations in gray- and white matter, with their strength correlating with sleep depth and EEG delta power. Carvedilol also dampened LFOs, supporting that these reflect cerebrovascular oscillations.

Study IV showed that LFOs are modulated by norepinephrine in rested wakefulness and sleep, and further confirmed the observed effects of sleep deprivation on LFOs and slow-wave-rich sleep on respiration-driven brain pulsations.
In conclusion, this work presents the first human evidence linking brain fluid dynamics to sleep slow waves, heightened sleep pressure and norepinephrine. It also disentangles the effects of sleep deprivation and autonomic arousal from those of sleep itself and suggests that sleep may not be the sole driver of glymphatic flow, as cerebrovascular oscillations are regulated by heightened sleep need and norepinephrine levels