Academic journal article Genetics

Sleep-Active Neurons: Conserved Motors of Sleep

Academic journal article Genetics

Sleep-Active Neurons: Conserved Motors of Sleep

Article excerpt

Sleep Is a Conserved Behavioral and Physiological State

LACK of sleep or low sleep quality causes tiredness, lowers productivity, and decreases mood. Sleeping problems are widespread in human societies across the globe with severe health and economic consequences (Colten and Altevogt 2006). Because of its vital role, this behavior can be found in most animals that have a nervous system and that have been studied carefully (Campbell and Tobler 1984; Cirelli and Tononi 2008). To fulfill its essential functions, sleep is exquisitely controlled by the brain and other organs to ensure that enough of this behavioral and physiological state takes place (Saper et al. 2010). It is necessary to understand how sleep is controlled if we are to develop strategies to intervene with sleeping disorders and to harness the power of sleep to improve human health.

Sleep is defined and can be identified by behavioral criteria such as reversible behavioral quiescence with a reduced responsiveness to sensory stimulation and homeostatic control (Campbell and Tobler 1984). Using these behavioral criteria, sleep has been detected in various species including mammals and birds, and also in all other major animal model systems, such as the zebrafish Danio rerio, the fruit fly Drosophila melanogaster, and the nematode Caenorhabditis elegans (Hendricks et al. 2000; Shaw et al. 2000; Zhdanova et al. 2001; Raizen et al. 2008; Kayser and Biron 2016; Trojanowski and Raizen 2016; Artiushin and Sehgal 2017; Oikonomou and Prober 2017). Sleep has even been detected in basal metazoans that have a nervous system such as cnidarians, suggesting that sleep evolved together with a nervous system before its centralization (Figure 1) (Nath etal. 2017).

Electrophysiological parameters that match behavioral characteristics are often used to detect and characterize sleep. For example, in the mammalian and avian brain, electrical activity measurements called electroencephalograms (EEGs) are typically used to classify and quantify different sleep stages. The EEG measures local field potentials of the cortex (Jackson and Bolger 2014). Electrophysiological measurements in humans led to the discovery of two types of sleep: rapid eye movement (REM) sleep, which is also called active sleep, and non-REM sleep, which is also called quiet sleep. REM sleep is characterized by asynchronous brain activity that is similar to cerebral firing patterns during wakefulness. Body muscles are paralyzed, with the notable exception of certain muscles required for respiration as well as muscles required for eye movement, allowing the continuation of breathing and causing complex eye movements (Aserinsky and Kleitman 1953; Orem et al. 2000; Peever and Fuller 2017). REM sleep is present in mammals and birds, and evidence for REM sleep also exists in nonavian reptiles, suggesting an evolutionary emergence of REM sleep at the level of the reptiles (Libourel and Herrel 2016; Shein-Idelson et al. 2016). Non-REM sleep in mammals and birds is characterized by strongly reduced muscle tone and brain activity, as well as slow oscillatory EEG patterns called slow waves. The extent of slow waves, called slow-wave activity, was found to correlate with the difficulty of arousing a subject during sleep across many conditions and is thus used as a proxy for sleep depth (Dijk 2009; Vorster and Born 2015). While EEG slowwave activity is an established marker for non-REM sleep in mammals and birds, such electrical activity is a product of specific brain architecture and thus cannot necessarily be detected in other species. Similar patterns can still be observed in amphibians and reptiles (Libourel and Herrel 2016; SheinIdelson et al. 2016). However, in zebrafish, Drosophila, and C. elegans, slow-wave activities have not been described. Nevertheless, even invertebrates show brain activity characteristic for quiet sleep, such as an overall reduction of neural activity (Nitz et al. 2002; Schwarz et al. …

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