Proceedings of The Physiological Society

Sleep Sleep and Circadian Rhythms (London, UK) (2018) Proc Physiol Soc 42, C01

Poster Communications

Interrelationship between sleep stability and glymphatic function

N. L. Hauglund1

1. Center for Translational Neuromidicine, University of Copenhagen, Copenhagen N, Denmark.

  • Figure 1. EEG analysis of sleep in control mice (saline injections or sham surgery) and mice with impaired glymphatic function (acetazolamide injections (20 mg/kg), CM puncture, AQP4 KO). (A) average duration/hour of wake, nREM, and REM sleep. (B) average number of bouts/hour for each state. (C) average bout length for each state. Two-way Anova and Tukey's test, n=5. (D) normalized power spectrum of nREM sleep during the first 4 hours of the light period following glymphatic manipulations. Two-way Anova and Dunnet's test, n=4-5. (E) these results indicate that there is a one-way relationship between sleep and glymphatic flux, as sleep drives glymphatic clearance but impaired glymphatic flux does not acutely decrease sleep stability. Error bars are mean +/- SEM. Significance is shown as * = p<0.05, ** = p<0.01 and *** = p<0.001. CM = cisterna magna, AQP4 = aquaporin-4, KO = knockout, ns = not significant.

It was recently discovered that waste products, such as amyloid-b, are cleared from the brain by cerebrospinal fluid via what was named the glymphatic system, and that this cleaning of the brain is only active during sleep [1], [2]. Neurodegenerative diseases, such as Alzheimer's disease, are often comorbid with sleep disturbances [3], and impaired glymphatic clearance has been proposed as a possible risk factor of amyloid build-up [1]. Therefore, a better understanding of the link between the glymphatic system and sleep could have important implications for treatment of diseases that share this common link. In this study, we explored the possibility that glymphatic flux might not only be controlled by sleep, but in itself participate in stabilization of the sleep state. To test this, sleep was measured in mouse models of impaired glymphatic flux previously used in our laboratory, namely aquaporin 4 knockout mice (AQP4 KO), mice subjected to cisterna magna puncture (CM puncture, performed under anesthesia with a mixture of ketamine (100 mg/kg) and xylazine (10 mg/kg) intraperitoneally (i.p.)), and mice subjected to acetazolamide treatment (20 mg/kg i.p. every 6 hour for a total of 4 injections) [4]. To investigate whether glymphatic impairment leads to decreased sleep stability, sleep was monitored using EEG and EMG electrodes that had been implanted under isoflurane anesthesia and i.p. injections of carprofen (5 mg/kg) and buprenorphine (0,05 mg/kg) 2 weeks prior to recording. Furthermore, sleep characteristics of a second group of mice were monitored non-invasively using immobility-defined sleep analysis. Non-invasive sleep analysis and EEG did not show any overall differences in sleep pattern between mice treated with saline, acetazolamide, CM puncture or sham surgery (figure 1A-C). However, AQP4 KO mice were found to have shorter nREM bouts than all other groups, and shorter REM bouts compared to saline-treated mice. Spectral analysis of nREM sleep during the first 4 hours of the light period after glymphatic manipulations did not show any differences between mice treated with acetazolamide, saline or sham surgery (figure 1D). However, AQP4 KO mice displayed a right shift in delta power with significantly higher power in the high end of the delta spectrum. Furthermore, mice subjected to CM puncture displayed increased power in the low delta frequencies. Our study did not show a causative relationship between glymphatic flux and sleep stability. However, it does not rule out that water transport through AQP4 water channels could have implications for sleep, as AQP4 KO mice were found to have fewer bouts of nREM sleep and REM sleep, and displayed a right shift in nREM delta frequency. In conclusion, this study supports a model where sleep drives glymphatic clearance, but glymphatic flux does not have a direct and acute effect and sleep stability or sleep quality (figure 1E).

Where applicable, experiments conform with Society ethical requirements