The glymphatic system and "brain washing" (metabolite clearance) during mammalian sleep:
Xie et al. (2013) Sleep Drives Metabolite Clearance from the Adult Brain (also here) has been cited over 3,000 times. It describes in-vivo brain imaging and radio-labeled β-amyloid techniques to document sleep-correlated modulation of interstitial space and β-amyloid clearance rate. If I understand correctly the Glymphatic system was first named in this work by corresponding author Maiken Nedergaard.
Abstract:
The conservation of sleep across all animal species suggests that sleep serves a vital function. We here report that sleep has a critical function in ensuring metabolic homeostasis. Using real-time assessments of tetramethylammonium diffusion and two-photon imaging in live mice, we show that natural sleep or anesthesia are associated with a 60% increase in the interstitial space, resulting in a striking increase in convective exchange of cerebrospinal fluid with interstitial fluid. In turn, convective fluxes of interstitial fluid increased the rate of β-amyloid clearance during sleep. Thus, the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system.
From Wikipedia's Glymphatic system:
The glymphatic system (or glymphatic clearance pathway, or paravascular system) was described and named in 2013 as a system for waste clearance in the central nervous system (CNS) of vertebrates. According to this model, cerebrospinal fluid (CSF) flows into the paravascular space around cerebral arteries, combining with interstitial fluid (ISF) and parenchymal solutes, and exiting down venous paravascular spaces.1 The pathway consists of a para-arterial influx route for CSF to enter the brain parenchyma, coupled to a clearance mechanism for the removal of interstitial fluid (ISF) and extracellular solutes from the interstitial compartments of the brain and spinal cord. Exchange of solutes between CSF and ISF is driven primarily by arterial pulsation2 and regulated during sleep by the expansion and contraction of brain extracellular space. Clearance of soluble proteins, waste products, and excess extracellular fluid is accomplished through convective bulk flow of ISF, facilitated by astrocytic aquaporin 4 (AQP4) water channels.
The name "glymphatic system" was coined by the Danish neuroscientist Maiken Nedergaard in recognition of its dependence upon glial cells and the similarity of its functions to those of the peripheral lymphatic system.
1Bacyinski A, Xu M, Wang W, Hu J (2017). "The Paravascular Pathway for Brain Waste Clearance: Current Understanding, Significance and Controversy". Frontiers in Neuroanatomy. 11: 101.
2Kiviniemi V, Wang X, Korhonen V, Keinänen T, Tuovinen T, Autio J, et al. (June 2016). "Ultra-fast magnetic resonance encephalography of physiological brain activity - Glymphatic pulsation mechanisms?". Journal of Cerebral Blood Flow and Metabolism. 36 (6): 1033–45.
3Konnikova M (11 January 2014). "Goodnight. Sleep Clean". The New York Times. Retrieved 18 February 2014. She called it the glymphatic system, a nod to its dependence on glial cells"
Cetacean sleep and conscious breathing:
Wikipedia's Dolpin; Sleeping and Whales; Sleep indicate many species of each type of mammal are conscious breathers i.e. many are not thought to be able to go somewhere safe and undisturbed and enter extended periods of unconscious sleep.
Question:
How widespread among mammals is the glymphatic system's metabolite clearance in the brain during sleep currently known to be?
I've mentioned conscious breathing in cetacean sleep as an example of both how challenging it may be to investigate some mammals and of where it might be less likely to be found. Since Xie et al. (2013) there must have been a substantial investigation in humans not only to address the "Why we absolutely must sleep?" question but due to the significant place that Amyloid beta holds in the investigations of human disease, including Alzheimer's disease.
Potentially helpful:
- This extensive answer to What is happening physically when I get sleepy and when I wake up?
- This answer to Do all mammals have a Ventricular system? says "The ventricular system is common to vertebrates." and links to Lowery and Sive (2009) Totally Tubular: The Mystery behind Function and Origin of the Brain Ventricular System
- Jessen et al. (2015) The Glymphatic System: A Beginner’s Guide (also here)
