Review of neurotransmitters and neuromodulators qualitative distribution across sleep stages: hypothesis to their role in generating specific sleep oscillations

(1)

1

Review of neurotransmitters and neuromodulators qualitative distribution across sleep stages:

hypothesis to their role in generating specific sleep oscillations

Anjali Rajagopal

Supervisor: Dr Charlotte Boccara

Project thesis, Faculty of Medicine University of Oslo

2022

(2)

2

Abstract

BACKGROUND

Sleep is a fundamental part of all living beings. The electrophysiological changes during sleep are seen as different brain oscillations, and these oscillations have for a long time been the hallmark of the different sleep stages. Although these brain oscillations have been registered for several decades ago, their functions seem to be not so well understood even today.

Therefore, I wanted to do a literature study on what we know currently on the relation between brain oscillations, neurotransmitters and neuromodulators, this with the aim to get a better understanding of sleep.

OBJECTIVE

The aim with this project thesis was to see if specific brain oscillations during sleep were generated and regulated by neurotransmitters and neuromodulator, and if these processes were a local or global phenomenon.

METHODS

I conducted a non-systematic search in PubMed.

RESULTS

The results show that brain oscillations during sleep are strongly linked to specific neurotransmitters and neuromodulators, but there are still some uncertainties in terms of their function and why they occur at a specific time in the sleep-wake-cycle. In addition, several publications support the relatively new perception of sleep being a local phenomenon rather than a global phenomenon.

CONCLUSION

Neurotransmitters and neuromodulators have an essential role in generating specific brain oscillations during sleep at a local level.

(5)

5

Introduction

One of the most essential part of our lives is sleep, but surprisingly it is still an unresolved mystery. It is seen in all species, and living a life without sleep seems impossible (Scammell, Arrigoni, & Lipton, 2017). Even though new technology and ground-breaking research have brought us closer to an answer, we still have a long way to go to get a better understanding of sleep, both in terms of its physiology and its functions.

Sleep can be characterized in different ways, but the two classically hallmarks have been behavioural changes and electrophysiological changes (Keene & Duboue, 2018). The behavioral definition of sleep is that it is a naturally-occurring, rapidly reversible and recurring state of reduced responsiveness, consciousness, motor activity and metabolism (Boccara, 2021). Physiologically, sleep can also be defined by its typical brain oscillations (Scammell et al., 2017). These oscillations can be recorded with different techniques, such as electroencephalogram (EEG) and local field potential (LFP), in addition to muscle activity (EMG). EEG is a technique by which one measure in a non-invasively manner the electrical activity from dipoles at the level of the scalps. An equivalent to EEG is LFP, which is an invasive method to record oscillations directly from the extracellular space resulting from the firing of localized groups of neurons (Adamantidis, Gutierrez Herrera, & Gent, 2019).

It has been proposed that sleep serve many different functions such as energy-saving, repairing of cell tissues, thermoregulation, metabolic regulation, hormone regulation, adaptive immune regulation, neural detoxification, reconstruction of biological tissues, coping with emotions and stress, growth and development, and cognitive and memory processes (Boccara, 2021).

Disturbances in sleep alters cognition and performance in a wide variety of behavioural domains, leading to association with many medical and psychiatric conditions (Ritchie E.

Brown, Basheer, McKenna, Strecker, & McCarley, 2012). Hence, a better understanding of the function and mechanism of sleep will be of scientific interest and clinical importance.

The last decades, there has been lots of progress to elucidate the neural circuits underlying regulation of wake and sleep stages (Saper & Fuller, 2017). There are good evidence that neural pathways in several brain regions play an important role of regulating the sleep-wake cycle, and that different neurotransmitters and neuromodulators generate the timing of these states (Scammell et al., 2017). Despite extensive use of EEG for elucidating sleep-wake states, the mechanisms that generate, underlie and regulate brain oscillations during sleep still remain unknown (Kanda, Ohyama, Muramoto, Kitajima, & Sekiya, 2017).

(6)

6 Sleep and wakefulness are usually treated as two mutually exclusive behavioural states, with hardly any grey zone in between, and this “all-or-none” view of sleep has dominated sleep research for a long time (Siclari & Tononi, 2017). Though sleep has long been treated as a global brain phenomenon, the recent concept of sleep being a local process of any viable neuronal network could be of a greater explanatory value (James M. Krueger, Nguyen, Dykstra- Aiello, & Taishi, 2019). Since the idea of local sleep is not well covered in the current literature, I wanted to research further this concept by doing a non-systematic literature review. My main goal with this project was to see how different brain oscillations have been associated to the different sleep stages, and how brain region, neurotransmitters and neuromodulators could support such brain oscillations during sleep, especially at a local level. This information will be essential for my supervisor’s lab to study sleep and investigate its functions. I will outline some of these applications in the last section.

Hereafter, I will first present a review of the current literature focusing on brain oscillations, brain regions, neurotransmitters and neuromodulators that are important in the regulation of the sleep-wake cycle. In the discussion section, I will answer my main question – what role do neurotransmitters and neuromodulators have in generating specific brain oscillations – in addition to present the phenomena of local sleep. At the end, I will outline why generating specific brain oscillations can be important for our understanding of the function of sleep, as well as briefly explain how my supervisor’s lab could use such knowledge.

1 Method

In this literature study, I have been using the database PubMed to find relevant publications. As a starting point my supervisor recommended some publication that could be relevant for my thesis. Then I searched for systematic reviews and original articles, using self-defined keyword combined with “sleep”, either with AND or OR. Some of the keywords I used was “sleep- wake”, “regulation”, “stage”, “brain oscillation”, “brain region”, “mechanism”, “function,”

“neurotransmitter”, “neuromodulator”, “local sleep”. For some of the sub-theme, such as “brain oscillation” and “neurotransmitter/neuromodulator”, I also used more specific keywords, which was combined with the keyword “sleep”, e.g. “Sleep” AND “Theta waves”, “Sleep” AND

“Serotonin”.

My main inclusion criteria were 1) English language, 2) focus on brain oscillations during sleep and 3) peer-reviewed publication. To limit the selection of papers, I had to exclude some publications. Since my project thesis is mainly about normal physiology of sleep, I excluded

(7)

7 publications that were focusing on pathophysiology and sleep related diseases. Yet, I included some of these publications when there was relevancy for my project or when there was no other publication to choose. Some publications from the reference lists of the selected reviews were also included when they matched my search terms and inclusion criteria.

2 Results

2.1 Classification of sleep stages

Sleep has classically been characterized by either electrophysiological or behavioural changes associated with sleep-like states (Keene & Duboue, 2018). Electrophysiologically, sleep is characterized by changes in brain wave activity, as measured by the electroencephalogram (EEG) (Keene & Duboue, 2018). In addition, muscle activity recorded as electromyogram (EMG) and eye movements recorded as electrooculogram (EOG) have been used to define sleep/wake states (Ritchie E. Brown et al., 2012).

The original Rechtschaffen and Kales sleep scoring system used in polysomnography studies divided NREM sleep in humans into four stages, S1-S4, based on changes in neural oscillatory activity (Hori et al., 2001). In 2007, American Academy of Sleep Medicine simplified this system to three stages of NREM, N1-N3 (Iber, Ancoli-Israel, Chesson, & Quan, 2007). As homeostatic sleep pressure dissipates across the night, NREM-sleep become lighter and REM-sleep become longer (Scammell et al., 2017).

As mentioned, the different sleep stages can be distinguished with EEG and EMG. Wakefulness with eyes closed is typically associated with alpha waves (Le Bon, 2020). In brief, slow waves and sleep spindles characterize NREM sleep, while REM sleep is characterized by gamma and theta waves accompanied with muscle atonia (Adamantidis et al., 2019). For my project, the characterization of the different sleep states by brain oscillations was of main interest, and I will present them in more details in the next section.

The classification of sleep stages in human have for a long time been suggested to be different compared to other mammals, but this opinion have recently been disproved. Specifically, one

The figure shows a hypnogram – a graph that represent normal sleep cycles in a human subject across one night. One cycle consists of NREM and REM sleep, and repeats approximately every 90 minutes. Scammell, 2017

(8)

8 study have suggested that there are three NREM stages in rodents analogous to human (Lacroix et al., 2018). The obstacle with classifying sleep stages in rodents could partly be caused by the technical difficulty of precisely detecting specific oscillations in the EEG of small animals (Ritchie E. Brown et al., 2012). It is also important to mention that most mammalian species are polyphasic sleepers, but humans are monophasic sleepers (Le Bon, 2020). While REM and NREM sleep in humans recur with 90 min cycles, in most other mammals it recurs more rapidly (Schwartz & Kilduff, 2015). For instance, rodents present a mean duration of sleep cycles of 10 minutes (Trachsel, Tobler, Achermann, & Borbély, 1991). Despite some differences, there seem to be some well conserved features that all animals share. For example, all animals typically enter REM sleep after NREM sleep, and not directly from wakefulness (Weber &

Dan, 2016).

2.2 Brain oscillations during sleep

Brain activity during sleep is characterized by circuit-specific oscillations (Adamantidis et al., 2019; Wright, Badia, & Wauquier, 1995)(Adamantidis et al., 2019) and scalp EEG allow us to describe the morphological and topographical aspect of these oscillations (Dang-Vu, 2012).

There are two types of oscillations: on one hand the persistent oscillations such as delta, alpha, theta and gamma waves, on the other hand the transient oscillations such as spindle waves, K- complexes, sharp wave ripples (SWR), infra-slow oscillations (ISO) and ponto-geniculo- occipital (PGO) waves. Transient waves are isolated waveforms or complexes that are distinguishable from the persistent activity in the background (Nayak & Anilkumar, 2022).

These brain rhythms are thought to provide a framework for higher-order brain function such as attention, memory formation and conscious awareness by binding the firing of neurons within cortical areas (Ritchie E. Brown et al., 2012). Improved understanding of these brain oscillations can shed light on their implication for both sleep regulation and sleep function.

Furthermore, it may allow us to call attention to early pathophysiological changes in sleep oscillations associated with epilepsy, schizophrenia, major depression and dementia (Adamantidis et al., 2019). The approach in this section will be a short report of each brain oscillation involved in sleep and a summarizing table at the end. I focused on oscillations recorded in human and rodents, and therefore one should note that frequency of oscillations may vary between species and brain regions.

(9)

9 Alpha waves

Alpha waves are brain oscillations in the frequency range of 8–13 Hz in humans (S. W. Hughes

& Crunelli, 2005). Occipital alpha rhythms were one of the first described EEG rhythms by Hans Berger (Ritchie E. Brown et al., 2012). They mark the transitions from quiet wakefulness to eyes closed in humans (Adamantidis et al., 2019). They result from an interaction of thalamic and neocortical circuitries with a moderate level of brain stem cholinergic inputs, and can be recorded anywhere in the cortex, commonly in the parietal and occipital cortex areas (Lopes Da Silva & Storm Van Leeuwen, 1977). Normally, they are observed during quiet wakefulness and or while resting with eyes closed. They can also be seen in reversible coma. A study showed that alpha rhythms may play an important role in internally directed thoughts processes since they are strengthened during tasks requiring mental arithmetic and visual imagery (Ray & Cole, 1985).

Delta waves and slow waves

Slow wave activity is in the frequency of 0.5– 4.5 Hz, but includes both slow oscillations in the frequency of < 1 Hz and delta oscillations in the frequency of 1–4.5 Hz (Steriade, Nuñez, &

Amzica, 1993). They most often originate from frontal derivations (Dang-Vu, 2012).

(A) Shows the different neural oscillations and their frequency band, while (B) shows the characteristic brain oscillations during the two sleep states – NREM sleep and REM sleep. Byron, 2021

(10)

10 Derivation refers to the EEG-recording from electrode pair on the scalp, detecting source activity (Basser, 1981). Slow wave activity is believed to originate from premotor cortex and primary motor cortex, but can be recorded from almost any cortical area (Dang-Vu, 2012).

These oscillations reflect the variation in the resting membrane potential of thalamic and cortical neurons, which switch between depolarization “UP” and hyperpolarization “DOWN”

states (Poulet & Petersen, 2008). During down states, neural and synaptic activities are very low, while up states are rich in high frequency and spindle activity (Halász, 2016). The level of NREM delta power is highly dependent on inputs from lateral hypothalamus to the thalamocortical networks (Herrera et al., 2016). In addition, one should consider that the prior history of sleep and wakefulness is important for delta activity – e.g. sleep recorded after a prolonged period of wakefulness tend to show increased delta waves, vice-versa daytime naps would tend to increase their power in subsequent sleep (Schwartz & Kilduff, 2015). In young adults, slow waves are highest in the first NREM cycle of the night and then exponentially decline in intensity across successive NREM sleep cycles, strengthening their bond with homeostatic dissipation of sleep pressure (Mander, Winer, & Walker, 2017). Furthermore, large waves of cerebrospinal fluid flow (CFS) were shown to precede (by a few seconds) neural slow wave activity during NREM (Bojarskaite et al., 2020). This indicates that intrinsic neural dynamics of sleep are linked to CFS fluid flow (Lewis, 2021).

Theta waves

Theta waves are brain oscillations in the frequency of 4–7 Hz in humans and 5–8 Hz in animals (Uchida, Maehara, Hirai, Okubo, & Shimizu, 2001). It can also correspond to a frequency between 8–12 Hz in the hippocampus. Theta waves are prominent in wake and in REM-sleep often coupled to gamma oscillations (Clemens et al., 2009). These waves have been hypothesised to be either locally generated in the hippocampus (Bland, 1986) or inherited from the septum cholinergic neurons. Indeed, cholinergic neurons have a firing rate in the theta range (Holst & Landolt, 2018). They are mainly active during wakefulness and REM sleep. Theta has been observed mainly in the prefrontal cortex (Courtin et al., 2014), the amygdala (Popa, Duvarci, Popescu, Léna, & Paré, 2010) and the medial septum (Adamantidis et al., 2019). One of their supposed roles is to support neural communication between brain areas with the hippocampus, in connection with memory processes and executive functions (Jafari, Kolb, &

Mohajerani, 2020). Furthermore, activation of the dorsolateral pons during REM-sleep is seen to regulate the theta activity (Schwartz & Kilduff, 2015).

(11)

11 Gamma waves

Gamma waves occur as short transitional bursts in the frequency range of 30 – 50 Hz. They are typically seen during wakefulness and REM-sleep, associated with theta waves (Montgomery, Sirota, & Buzsáki, 2008). They are generated locally by the hippocampus or other cortical neurons, and can be recorded in the neocortex, hippocampus, amygdala, striatum, thalamus and hypothalamus (György Buzsáki & Wang, 2012). Some studies subdivide gamma rhythms into two frequency-bands: low gamma (30 – 70 Hz) and high gamma (7 – 120 Hz), and suggest that they arise in different cortical layers and have different pharmacological modulation properties (Oke et al., 2010). Serotonin inhibits gamma and theta rhythms (Khateb, Fort, Alonso, Jones,

& Mühlethaler, 1993), while basal forebrain ACh neurons discharge at maximal rates in association with these waves (Lee, Hassani, Alonso, & Jones, 2005) . The role of gamma waves is unclear, but they increase when there is less cortical activity, as well as during tonic REM- sleep (Wehrle et al., 2007). Therefore, gamma waves during sleep may play an important role in stabilizing REM-sleep and in preventing awakening (Adamantidis et al., 2019). In addition, gamma waves have been related to processing of information, active maintenance of memory contents and conscious perception (Jafari et al., 2020).

Spindles/Sigma waves

Spindles or sigma waves are transient burst of oscillations in the frequency of 11 – 15 Hz. They are generated by interaction of the reticular nucleus of the thalamus with thalamocortical neurons (Piantoni, Halgren, & Cash, 2016). They are mostly recorded over frontal lobe regions (Mander et al., 2017), but can also be recorded in the basal ganglia (Dejean, Gross, Bioulac, &

Boraud, 2007). They appear in NREM and are a hallmark of the N2 stage (Fernandez & Lüthi, 2019). Spindles are single event that can be associated with the UP state of slow waves (Mölle, Bergmann, Marshall, & Born, 2011). They tend to decline during deep sleep and reappear just prior to the transition to REM sleep (Ritchie E. Brown et al., 2012). It is believed that there is a relationship between spindle activity, memory processing and consolidation of skills during sleep (Rasch & Born, 2013).

K-complexes

K-complexes are high-voltage slow transient components, greater than 100 μV, and of a duration of approximately 1 s, often followed by a burst of spindle waves. They can occur spontaneously or be evoked by an external stimuli like sound or touch of the skin (Adamantidis et al., 2019). They are mainly generated in the thalamic circuits (Bastien & Campbell, 1992).

They are topographically distributed in the frontal or fronto-central area of the brain (Colrain,

(12)

12 2005). K-complexes are mainly seen in stage N2 in human and in the transition from stage 2 to stage 3 (Halász, 2016). They are less well described in rodents (Adamantidis et al., 2019). They are reflective of a brain state that is conductive to sleep (Colrain, 2005) and compensate the disturbing effect of incoming stimuli by producing slow wave activity (Halász, 2016). Since K- complexes mirror other slow waves in NREM-sleep, they are suggested to be fundamental promoters of sleep (Wauquier, Aloe, & Declerck, 1995).

Sharp wave ripples (SWR)

SWRs are 150 – 200 Hz oscillations with a duration of 50 – 100 ms that originate in the hippocampus (Ylinen et al., 1995). They can be recorded in the hippocampus and associated area during quiet wakefulness and NREM sleep in both rodents (G. Buzsáki, 1986) and humans (Bragin, Engel, Wilson, Fried, & Buzsáki, 1999).

The generation and propagation of SWR in the hippocampus remains undetermined, but it is thought to be generated by an interplay between CA1 and CA3 neuronal circuits (Csicsvari, Hirase, Mamiya, & Buzsáki, 2000). Interestingly, SWRs are only coupled with delta waves and spindles during sleep, which allows active system consolidation and local cortical process (Tang & Jadhav, 2019). Furthermore, there is abundant evidence that SWRs are important for the consolidation of hippocampus-dependent memories during sleep (Rasch & Born, 2013).

Infra-slow oscillations

Infra-slow oscillations are oscillations < 0.1 Hz that appear in different brain regions, including hippocampus, basal ganglia, locus coeruleus and thalamus, in both human and rodents (Stuart W. Hughes, Lőrincz, Parri, & Crunelli, 2011). Though these oscillations have been recorded in many brain regions, the mechanisms and specific neuronal circuits by which they might be generated remain poorly understood (Stuart W. Hughes et al., 2011). The long-lasting hyperpolarizing potential is believed to be regulated by the G-protein coupled inwardly- rectifying potassium channel (GIRK) adenosine A1 receptor (Stuart W. Hughes et al., 2011). It is assumed that these oscillations play a role in the maintenance or stability of NREM sleep (Lecci et al., 2017).

Klinzing, 2019

(13)

13 Ponto-geniculo-occipital (PGO) waves

PGO waves are large phasic waves, lasting 60 – 120 ms with an amplitude of 200 – 300 μV, that occur in clusters during REM sleep (Fernández-Mendoza et al., 2009). These waves are generated by the pons, which receives inhibitory serotonergic projections from the raphe nuclei, before they move to the lateral geniculate nucleus residing in the thalamus, and finally end up in the primary visual cortex in the occipital lobe (Gott, Liley, & Hobson, 2017). They have been implicated in nervous system maturation, memory consolidation and visual perception during dreaming (Gott et al., 2017). PGO-waves are considered by some to be the source of dreaming and visual imagery during REM since they

occur simultaneously with rapid eye movements associated with gaze direction in dream imagery (Dement & Kleitman, 1957). The inactivity of serotonin during sleep allows PGO to propagate from the pons to the thalamus and cortex, releasing associated eye movements and twitches (Siegel, 2004).

PGO-waves in cats. The activity correlates with a triggering neuron transition from tonic to high frequency activity during REM-sleep. Datta and Hobson, 1994

(14)

14 A schematic overview of the brain oscillations involved in sleep

OSCILLATION FREQUENCY

(in human)

ORIGIN RECORDED

IN

SLEEP

STAGE FUNCTION

Alpha-waves 8 – 12 Hz

Interaction between thalamic

neurons and neocortical

neurons

Cortex - commonly in

parietal and occipital cortex

areas

Transitions from wakefulness

to eyes closed

Relaxation, mental arithmetic

and visual imagery

Delta-waves and slow waves

0.5 – 4.5 Hz

Thalamic and cortical neurons

The whole brain

NREM – N3 Homeostatic dissipation of sleep pressure Precede CFS

fluid flow

Theta-waves

4 – 7 Hz

Hippocampus, septum

Prefrontal cortex, amygdala, medial septum

REM Wakefulness

Memory/learning, executive functions, locomotion

Gamma waves 30 – 150 Hz

Hippocampus, cortex

Neocortex, hippocampus,

amygdala, striatum, thalamus and hypothalamus

REM Wakefulness

Prevent awakening,

process information,

memory consolidation.

Spindle waves 11 - 15 Hz

Reticular nucleus of thalamus and

thalamocortical neurons

Frontal lobe, basal ganglia

NREM – N2

Associated with memory processing and consolidation of

skills.

K-complexes 0.8 – 4.0 Hz Thalamic neurons

Frontal brain area

NREM – N2 Reflect a brain conductive to

sleep Sharp wave ripples

(SWR)

100 – 200 Hz Hippocampus Hippocampus NREM Wakefulness

Consolidation of memories

Infra-slow oscillations < 0.1 Hz Unknown Thalamus, hippocampus, basal ganglia, locus coeruleus

NREM Maintenance of NREM-sleep

Ponto-geniculo-occipital waves (PGO)

1 – 4 Hz Pons Lateral

geniculate, occipital cortex

REM

Visual perception during dreaming,

memory extinction

(15)

15

2.3 Brain regions involved in sleep

Though sleep has mainly been considered as a global phenomenon, the recent better understanding of sleep regulation has contributed to promote the idea that sleep is probably generated in local networks spread across different brain regions before it involves large and widespread numbers of cortical regions (J. M. Krueger et al., 2008). The brain regions involved in the promotion of the waking state are predominantly located in the brain stem, hypothalamus and basal forebrain (Monti, 2011). While the regulation of NREM-sleep is mainly related to the preoptic area of the hypothalamus, the regulation of REM-sleep is associated with the pons, the medulla oblongata and the hypothalamus (Boccara, 2021).Since this section may overlap with the section about neurotransmitters and neuromodulators involved in sleep (see 2.5), I will only briefly describe the most important brain regions involved in generating oscillations during sleep and in the regulation of the different sleep stages.

The basal forebrain can be defined by fields of cholinergic neurons extending from the medial septum back to the substantia innominate, as well as neurons producing GABA and glutamate, which all directly innervate the cortex (Xu et al., 2015).All the ascending projections into the forebrain follow a dorsal and a ventral route. The dorsal route terminates in nonspecific thalamic nuclei, which in turn project to the cerebral cortex. The ventral route passes through the hypothalamus and continues into the basal forebrain, where cells in turn project to the cerebral cortex and the hippocampus (Monti, 2011). Most of the neurons in the basal forebrain are wake- and REM-sleep-active, like the cholinergic neurons (Weber & Dan, 2016), but some of their neurons are mainly active during NREM sleep (Hassani, Lee, Henny, & Jones, 2009).

The NREM-sleep inducing basal forebrain neurons are not well covered in the literature, but some appear to be GABAergic neurons that produce somatostatin (Scammell et al., 2017), which inhibit local, wake-active basal forebrain neurons (Xu et al., 2015).

The thalamus resides in the midbrain and is composed of a collection of distinct nuclei that predominantly project to the cortex and other subcortical structures like the striatum, the amygdala and the hippocampus (Gent, Bassetti, & Adamantidis, 2018). In turn, it receives important inputs from the subcortical nuclei, including the lateral hypothalamus, the brainstems and the amygdala (Gent et al., 2018). The thalamus have been strongly suggested to have a role in initiating cortical slow waves in NREM-sleep (Gent et al., 2018).

The hypothalamus contains both sleep-promoting neurons, wake-promoting neurons and the circadian pacemaker cells (Schwartz & Kilduff, 2015). The preoptic area, which contains

(16)

16 neurons that promote sleep, is located in the most rostral part of the hypothalamus (McGinty &

Sterman, 1968). Orexin cells of the posterior and lateral hypothalamus are critical for maintaining wakefulness (Schwartz & Kilduff, 2015), and chronic loss of these neurons result in poor REM-sleep regulation, such as seen in narcolepsy (Scammell et al., 2017). The other wake-promoting neurons in the posterior site of hypothalamus are histaminergic neurons in the tuberomammillary nucleus and glutamatergic neurons in the supramammillary area (Saper, Scammell, & Lu, 2005). Additionally, the circadian timekeeping cells responsible for the regulation of sleep-wake-states are located in the suprachiasmatic nucleus in the hypothalamus (Marcheva et al., 2013).

The brainstems contain several wake-promoting nuclei like the locus coeruleus, the dorsal and medians raphe nuclei and the substantia nigra (Menon et al., 2019). The pons is critical for generating multiple components that characterize REM-sleep, like EEG synchronization, eye movements and muscle atonia (Schwartz & Kilduff, 2015). This role was historically demonstrated by inactivating cats’ brain stems. This experiment resulted in the loss of both cortical activation during wake and REM sleep (Webster, Friedman, & Jones, 1986).

The wake-promoting pathways

The illustration shows the neurons and regions responsible for promoting wakefulness. The light green circles are the monoaminergic neurons and the dark green circles are the parabrachial nucleus and cholinergic regions. Scammell, 2017

(17)

17 The NREM-sleep-promoting pathways

The REM-sleep-promoting pathways

The blue circles are the NREM-promoting nuclei. GABAergic neurons promote sleep by inhibiting wake-promoting neurons, while the basal forebrain and cortex contain sleep-active neurons. Scammell, 2017

The blue circles are the REM-sleep promoting nuclei and the green circles are the REM-sleep suppressing nuclei. Scammell, 2017

(18)

18

2.4 The sleep-wake regulation

The time, depth and duration of sleep are controlled by the interaction of circadian rhythms and sleep homeostatic processes such as sleep pressure (Borbély, 1982) (Ritchie E. Brown et al., 2012). Sleep pressure refers to the urge to sleep after extended periods of wakefulness, also called sleep deprivation. Accumulation of sleep pressure increases the amount of somnogens (Weber & Dan, 2016).This eventually leads to sleep rebound, which is an increased amount of sleep a person receives after sleep deprivation (Adamantidis et al., 2019). According to the flip- flop switch model, sleep and wake are under the control of two antagonizing systems, a wake- inducing arousal system and a sleep-inducing system (Bringmann, 2019). This model avoids transitional states, and might explain why wake-sleep transitions often are relatively abrupt (Saper, 2005 #68). Both wake- and sleep-promoting neurons are homeostatically regulated in part through changes in receptors at their membrane (Jones, 2020). Interestingly, if either side of the flip-flop model is weakened, the transition time between states increases. This may result in the inability to fall back to sleep after waking up and/or chronic tiredness leading to frequent somnolence (Lu, Greco, Shiromani, & Saper, 2000).

Another model to consider is the thalamocortical loop model. According to this model, sleep is initiated locally at the neuronal level, as a consequence of previous activity, and is only afterwards consolidated by central mechanisms. Thus, the homeostatic regulation of sleep occurs in multiple brain circuits in response to prior cellular or network activity (Vyazovskiy et al., 2011). There is strong evidence that support this theory of sleep being initiated at a local network level in the circuits most active during prior waking, and the whole organism sleep is posited to emerge with the synchronization of local networks (Jewett & Krueger, 2012).

In addition, it is important to consider the fact that sleep is coupled to the circadian clock (Marcheva et al., 2013). The circadian clock can be described as the periodic oscillation of

An illustration of the flip-flop model. (A) Shows that during wake, the arousal system dominates and suppresses the sleep.

(B) shows that during sleep, the sleep-inducing system dominates and suppresses wake

(19)

19 physiological and behavioural phenomena across a 24-hour periodicity (Adamantidis et al., 2019). The circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (Scheer

& Czeisler, 2005 71) drives this biological clock.

2.5 Neuromodulators and neurotransmitters

Sleep is accompanied by a significant change in activity levels of different neuromodulators and neurotransmitters (Scammell et al., 2017). I have chosen to focus here on the most prominent neurotransmitters and neuromodulators involved in sleep – where they originate from, which receptors they bind to, which brain areas they project to and their role in the sleep- wake regulation. At the end, I will present a schematic overview in a table that briefly summarizes my literature findings.

Acetylcholine

Acetylcholine (Ach) is a neurotransmitter that activates ionotropic nicotinic receptors, expressed in presynaptic and postsynaptic membranes, and metabotropic muscarinic receptors, expressed in the cortex, the striatum, as well as other brain regions (Holst & Landolt, 2018).

Acetylcholine-releasing nuclei, in the peduncular-pontine and lateral-dorsal-tegmental nuclei of the pons, project primarily to the basal forebrain, as well as to the thalamic relays and to the reticulum (Holst & Landolt, 2018). These neurons have a “REM-on” profile with their highest discharge rate occurring during REM sleep (Schwartz & Kilduff, 2015). A second cluster of cholinergic neurons is located in the basal forebrain, which projects widely to the cortex (Holst

& Landolt, 2018). At the cortical level, ACh has been shown to elicit fast cortical activity and prevent slow wave activity through the activation of muscarinic receptors (Jones, 2020). Studies have suggested that cholinergic neurons in the brainstems are more involved in regulating the transition into REM-sleep, whereas the cell of the basal forebrain are more linked to general arousal in connection with cognitive activity and sustained attention (Holst & Landolt, 2018).

Dopamine

Dopamine is a monoamine produced primarily by neurons in the substantia nigra (SN), the ventral tegmental area (VTA) and the ventral periaqueductal grey (vPAG) (Radwan, Liu, &

Chaudhury, 2019). In 2006, researchers founded that lesions in dopamine neurons of vPAG promote sleep, and that these neurons together with the other monoamines, belong to the arousal pathway (Lu, Jhou, & Saper, 2006). The nigrostriatal pathway projecting from the SN to the dorsal striatum is involved in motor planning and movement execution, while the mesocortical pathway – from the VTA to the nucleus accumbens, hippocampus and amygdala – regulates

(20)

20 mood and motivation (Radwan et al., 2019). There are five types of dopaminergic G-protein coupled receptors, D1 – 5. While D1 and D5 receptors are coupled to Gs protein and are mainly stimulatory, D2 – 4 receptors are coupled to inhibitory Gi protein (Holst & Landolt, 2018).

Dopamine’s capacity to positively drive arousal is clear from the potent effects of drugs that increase or decrease dopamine signalling – e.g. amphetamine strongly promotes wake by increasing synaptic concentration of dopamine – whereas dopamine antagonists, such as antipsychotics, are quite sedating (Scammell et al., 2017). Interestingly, D1 and D2 receptors form functional heteromers with adenosine A1 and A2A receptors, so that the binding of adenosine results in reducing dopaminergic signalling (Holst & Landolt, 2018).

Norepinephrine

Norepinephrine (or noradrenaline) cells are localized in the locus coeruleus of the pons (Siegel, 2004). They are different from other neurons of the brainstem because they give rise to highly diffused projections that reach multiple targets through the brainstems, the spinal cord, the thalamus, the hypothalamus, the basal forebrain and the entire cortical mantle (Jones, 2020).

Norepinephrine can activate three types of α-adrenergic receptors and one type of β-adrenergic receptors, which are all widely expressed in the CNS (Holst & Landolt, 2018). The norepinephrine cells are inactive during REM-sleep (Carter et al., 2010) and promote arousal in general, for instance required when responding to salient stimuli and stressors (Scammell et al., 2017). They modulate both slow waves and vessel diameters during sleep (Lewis, 2021). In addition, they are believed to be associated with the regulation of muscle tone, e.g. during cataplexy norepinephrine cells become inactive (Siegel, 2004).

Serotonin

Serotonin (5-HT) neurons are clustered in several nuclei along the midline of the brain stems in the raphe nuclei (Jacobs & Azmitia, 1992). They have their effect on the 5-HT receptors, which can be classified into at least seven classes and several subtypes (Monti, 2011). Except the ligand-gated 5-HT3 ion channel, all serotonin-receptors belong to the GPCR superfamily (Holst & Landolt, 2018). Serotoninergic neurons are mostly inactive during sleep, especially during REM-sleep. They have a role in maintaining arousal, regulating muscle tone and regulating some of the phasic events of REM sleep (Siegel, 2004). Besides their sleep regulating functions, they are also implicated in regulating mood, reward, patience and response to salient cues (Scammell et al., 2017). Serotonergic suppression of NREM sleep is suggested to be a consequence of 5-HT1A receptor mediating postsynaptic inhibition of sleep active VLPO neurons (Gallopin et al., 2000). On the other hand, the inhibition of REM sleep is due to a

(21)

21 postsynaptic inhibition of REM-on cholinergic neurons in the brain stem (Boutrel, Monaca, Hen, Hamon, & Adrien, 2002).

Histamine

The tuberomammillary nucleus in the posterior hypothalamus is the sole neuronal source of histamine in the brain (Scammell et al., 2017). The histaminergic neurons show a projection pattern that is similar to noradrenergic and serotonergic neurons (R. E. Brown, Stevens, & Haas, 2001), including diffuse projections to the forebrain and cerebral cortex (Jones, 2020). There are three types of histamine-receptors, H1-H3, that are coupled to Gq, Gs and Gi proteins (R.

E. Brown et al., 2001). The activity in the histaminergic cells appears to be tightly linked to wakefulness, notably in connection with attention and fast response rate (White & Rumbold, 1988). Their inactivity, caused by GABAergic cells, appear to be linked to sleepiness (Siegel, 2004). This sedative effect is also seen in patients using antihistaminergic drugs, which are mainly H1 antagonists (White & Rumbold, 1988). More recent studies have shown that histamine not only promotes wake, but have a wake-on, NREM-slow and REM-off firing pattern (Takahashi, Lin, & Sakai, 2006).

GABA

GABA is an inhibitory neurotransmitter distributed in the whole central nervous system, particularly densely in the suprachiasmatic nucleus of the hypothalamus (Wagner, Castel, Gainer, & Yarom, 1997). In contrast to the modulatory neurotransmitters such as monoamines and Ach, GABA and glutamate are fast neurotransmitters with a global effect on sleep (Saper

& Fuller, 2017). GABA is involved in generating sleep-wake state, but its role depend upon its discharge profile, the receptors it bears and its specific projections, which can be to local or distant specific target neurons (Jones, 2020). The action of GABA are mediated via ligand- gated ion channel referred to as GABA-A and GABA-C receptors, as well as the G-protein coupled GABA-B receptor (Holst & Landolt, 2018). It is the GABA-A receptor that is the target for the clinically widely used sleep-inducing medications, benzodiazepines and Z-drugs, and is responsible for the regulation of slow wave oscillations during sleep (Saper & Fuller, 2017).

The GABAergic neurons can turn off all kind of cells, including wake-promoting neurons like cholinergic and histaminergic cells (Siegel, 2004). In the sleep context, their main function seem to be regulating inhibitory inputs to the ascending reticular activating system, promoting the transition from wakefulness to sleep and maintenance of NREM-sleep (Holst & Landolt, 2018).

(22)

22 Glutamate

Since glutamate cannot penetrate the blood-brain barrier, some reports have suggested that glutamate in the brain is synthesized de novo in astrocytes or is indirectly converted from glucose by enzymes (He, Zhang, & Qu, 2019). The amino acid glutamate is primarily an excitatory neurotransmitter supporting neural communications throughout the brain. It has an important role in promoting arousal (Holst & Landolt, 2018). Many anaesthetics antagonize glutamate NMDA receptors which can be located upon neurons at every level of the brainstem and ascending reticular activating system, including the thalamus, hypothalamus, basal forebrain and cortex (Jones, 2020). Though glutamatergic neurons can be founded in many brain regions, neurons of the medial parabrachial nucleus may be especially important for wake as this region heavily innervates the basal forebrain. The destruction or local deletion of vesicular glutamate transporter in this region reduces wake and increases delta waves during NREM sleep (Scammell et al., 2017). The activity of glutamate and GABA are homeostatically regulated, so that prolonged activity of GABA is followed by increased inhibitory activity and decreased glutamate-receptors, and vice versa (Turrigiano, 1999). In addition, some studies have suggested that the excitation-inhibition-balance, caused by both GABA and glutamate, is strongly influenced by the circadian clock (Chellappa et al., 2016).

Orexin

Orexin, also called hypocretin, is the most recently discovered peptide neurotransmitter related to sleep and is produced by a group of hypothalamic cells, situated between the rostral region where there are sleep-active neurons, and the caudal regions where there are histamine-wake neurons (de Lecea et al., 1998). These neurons project to all arousal pathway, especially to the locus coereleus, the raphe nuclei and the tuberomamillary nuclei (Holst & Landolt, 2018). They bind to two subtypes of GPCRs, referred to as Hcrt-1 and Hcrt-2 receptors (Sakurai et al., 1998).

Orexins promote wakefulness (Piper, Upton, Smith, & Hunter, 2000), suppress REM-sleep and enhance wakefulness in periods of starvation (Yamanaka et al., 2003). Orexin has both been shown to regulate appetite and sleep. Its implication in sleep regulation was evidence when it was discovered that mice lacking the gene for the neuropeptide orexin and dogs lacking the gene for the receptors to orexin manifested the inability to maintain wake with muscle tone, which characterizes the disorder of narcolepsy with cataplexy (Jones, 2020). The optogenetic activation of orexin neurons trigger brief awakenings from both REM-sleep and NREM-sleep, an effect that is diminished with increasing sleep pressure (Holst & Landolt, 2018).

Furthermore, orexin can cause the release of the excitatory amino acid glutamate. In this

(23)

23 contest, orexin is important for eliciting and maintaining a waking state with muscle tone (Li &

de Lecea, 2020).

Somnogens

Somnogens are paracrine mediators of NREM-sleep (Scammell et al., 2017). The main NREM- sleep promoting substances are adenosine, prostaglandine D2 and cytokines such as interleukin (IL) 1 and tumour necrosis factor (TNF) α (James M. Krueger, Obál Jr, Fang, Kubota, & Taishi, 2001).

Adenosine have long been suggested to be important for sleep-wake regulation based on the knowledge that the world’s most consumed psychostimulant, caffeine, antagonizes adenosine receptors (Lazarus, Chen, Huang, Urade, & Fredholm, 2019). It is a neuromodulator that promotes sleep by binding to A1- and A2A-receptors (Oishi & Lazarus, 2017). Furthermore, it is the breakdown product of ATP and has therefore been linked to restoration of intracellular energy stores during sleep (Schwartz & Kilduff, 2015). Interestingly, astrocytes are a major source of extracellular adenosine (Scammell et al., 2017). Levels of adenosine are decreased by the enzyme adenosine deaminase, which is predominantly localized in the tuberomammillary nucleus of the brain. There, we can also find enriched histamine neurons containing A1- receptors, thereby suggesting that the histaminergic arousal system is regulated by adenosine (Lazarus et al., 2019). A2A-receptors are believed to mediate their effects in the nucleus accumbens (Lazarus et al., 2019). Extracellular adenosine increases with time spent awake and declines during recovery sleep mainly in the basal forebrain and cortex, and less in other brain regions (Schwartz & Kilduff, 2015).

Prostaglandin D2 is the most potent endogenous sleep-promoting substance produced by lipocalin-type PGD synthase localized in the leptomeninges, the choroid plexus and the oligodendrocytes in the brain, and secreted into the cerebrospinal fluid (Urade & Hayaishi, 2011). The site of action for PGD2 is the ventral surface of the rostral basal forebrain (Schwartz

& Kilduff, 2015). Two distinct subtypes of receptors for PGD2 have been identified: the G- protein coupled receptor DP1 and the chemo-attractant receptor DP2 (Urade & Hayaishi, 2011).

Prostaglandin induces sleep, especially slow wave sleep, in rats and monkey (Schwartz &

Kilduff, 2015). In addition to being a somnogen, PGD2 is also an important inflammatory mediator and could be important for sleep induction observed in infectious or inflammatory diseases (Urade & Hayaishi, 2011). Moreover, the activation of prostaglandin receptors also increases adenosine levels (Weber & Dan, 2016). This PGD2-induced increase of adenosine is

(24)

24 absent in DP1 receptor KO mice, indicating that DP1 receptors are required for this mechanism (Urade & Hayaishi, 2011).

Cytokines are molecules that have historically been characterized as components of the peripheral immune system. However, some of these cytokines regulate sleep under physiological conditions, also in the absence of infection or immune challenges (Opp, 2005).

IL-1 (interleukin) and TNF-α (tumor necrosis factor) are two well-known cytokines that promote NREM-sleep (Scammell et al., 2017). Receptors for IL-1 and TNF-α are widely distributed in the brain, including the hypothalamus (Opp, 2005). They may regulate physiological sleep through direct receptor-mediated modulation of the hypothalamus and the serotonergic raphe nuclei (Schwartz & Kilduff, 2015). In human, plasma levels of IL-1 are highest at the onset of sleep and circulating levels of TNF-α correlate with EEG slow-wave activity (James M. Krueger et al., 2001).

(25)

25 A table of the neurotransmitters and neuromodulators involved in sleep

Neurotransmitter, Neuromodulator

Originate from Projects to Receptors Function in sleep

Norepinephrine Locus coereleus in pons Brainstem, spinal cord, thalamus, hypothalamus, basal forebrain and the entire cortical mantle

α1-3

β Promote wakefulness Inactive during REM Regulating muscle tone

Serotonin Dorsal and medial raphe nuclei. Brainstem, spinal cord, thalamus, hypothalamus, basal forebrain, cortex

5HT1A Promote wakefulness Inactive during REM

Dopamine Ventral tegmental area and substantia nigra, venral periaqueductal gray matter

Striatum, nucleus

accumbens, hippocampus, amygdala, cortex

D1-D5 Promote wakefulness

Histamine The tuberomammilary nucleus in hypothalamus

Brainstem, spinal cord, thalamus, hypothalamus, basal forebrain, cortex

H1-H3 Promote wakefulness Low level induces NREM Inactive during REM Acetylcholine Pons and basal forebrain Cortex, striatum, basal

forebrain, thalamus

Nicotinic, muscarinic

Promote REM

Active during wakefulness Glutamate Basal forebrain, hypothalamus Whole brain NMDA,

AMPA

Fires in wake and sleep Maintain muscle tone during wakefulness

GABA Basal forebrain, the medullary brainstem, ventrolateral preoptic area and dorsomedial

hypothalamus

Whole brain GABA-A

GABA-B GABA-C

Mainly promotes sleep Can promote NREM

Orexin (Hypocretin)

Lateral hypothalamus and

dorsomedial hypothalamic neurons

All arousal pathway, especially to locus coerelus, raphe nuclei, ventral periaqueductal gray matter and the tuberomamillary nuclei

Hcrt 1 Hcrt 2

Promote wakefulness Inactive during sleep Promote wakefulness when starving

Somnogens (NREM promoting substances)

Adenosine

All cells, especially astrocytes

Basal forebrain, cortex, hippocampus, nucleus accumbens

A1 A2A

Promote NREM

Prostaglandin D2

Leptomeninges, choroid plexus and oligodendrocytes in the brain

Basal forebrain

DP1 DP2

Promote NREM

Cytokines (IL-1 and TNFα) Peripheral nerve tissue

Whole brain, especially hypothalamus and raphe nuclei

IL-1 receptor TNFα receptor

Promote NREM sleep Can induce symptoms associated with sleep such as sleepiness, fatigue and poor cognition (Jewett & Krueger, 2012)

(26)

26

3 Discussion – applications

3.1 Neurotransmitters and neuromodulators role in generating specific brain oscillation during sleep

There is much evidence that neurotransmitters and neuromodulators are important in the regulation of sleep-wake cycle. Like brain oscillations, they can be emblematic of different sleep stages. In addition, it seems that most brain oscillations during sleep are generated and regulated by neurotransmitters and neuromodulators. However, these processes seem to be more complex than one neurotransmitter generating one specific brain oscillation at a specific time.

The work I have reviewed in my thesis shows that brain oscillations during sleep result from the synchronisation of neuronal activity linked to multiple interacting neurotransmitters and neuromodulators. The neurotransmitters and neuromodulators can either activate or inhibit neurons in different brain regions, resulting in the promotion/inhibition of specific brain oscillations. For instance, ACh was shown to elicit fast cortical activity and to prevent slow wave activity through the activation of muscarinic receptors. Therefore, the interplay between different brain oscillations during sleep seems to be dependent on the activity of neurotransmitters and neuromodulators. Additionally, loss of a specific neurotransmitter, such as orexin, and drugs that act on receptors of a neurotransmitter, such as antihistamines, alter a natural occurring sleep-wake-cycle.

While these observations support the fact that neurotransmitters and neuromodulators play an important role in generating brain oscillations during sleep, little is known about (i) how these brain oscillations suddenly arise from a silent neuronal network during sleep, and (Sakurai et al.) what role the relevant neurotransmitters and neuromodulators play in modulating these oscillations. In my opinion, further research should address such questions. How do oscillations persist for so long during a sleep stage? Through which mechanisms neuromodulation interplays with specific rhythmicity?

(27)

27

The figure shows a typical sleep profile in a human with related brain oscillations, neurotransmitters and neuromodulators.

Rasch, 2013

3.2 The therapeutic use of generating specific brain oscillation during sleep

As a medical student and upcoming doctor, I know that I will be prescribing drugs to treat patients with sleep problems. Sleep-medications are among the most widely prescribed medicines (Ritchie E. Brown et al., 2012). The pharmacological agents mainly act upon GABA- receptors (Jones, 2020). Another sleep-inducing drug is based on the endogenous hormone melatonin, which entrain the circadian rhythm, and is normally high during nigh and low during the day (Scheer & Czeisler, 2005). While these medications are helpful for inducing sleep, they do not “repay” sleep debt after a sleep deprivation and cannot exactly mimic natural sleep

(28)

28 (Franks & Wisden, 2021). Therefore, developing new and better drugs to treat sleep problems could improve the daily life of many.

My most surprising observation in the clinics related to sleep problems, was the fact that so many diseases could affect a normal sleep-cycle, not only the typical sleep disorders. In addition, the diseases that affect the sleep-cycle appear to have a pathological feature in one or more sleep stages (Lacroix et al., 2018). For instance, NREM stage 1 sleep is increased in fibromyalgia (Wu, Chang, Lee, Fang, & Tsai, 2017) and narcolepsy with cataplexy (Dauvilliers, Arnulf, & Mignot, 2007), and slow wave sleep is heavily reduced in patients with schizophrenia (Chan, Chung, Yung, & Yeung, 2017). There are limited studies on which brain oscillations are affected in diseases associated with sleep problems. Further investigations are needed to be done to increase our knowledge related to the pathophysiology behind these diseases. Such findings could lead to therapeutical advances. Since personalized medicine have been a hot topic the recent years, I think that developing new drugs that selectively act on sleep pathways and specific brain oscillation, could be a ground-breaking benefit for many patients.

3.3 The function of specific brain oscillations during sleep

Despite the differences between all the brain oscillations, both in term of origin and modulation, they all seem to be precisely orchestrated during the sleep-wake cycle to ensure good and functional sleep. Loss of one type of oscillation may affect one or several sleep stages, and thereby affect the total sleep. Though still very little is known on the function of brain oscillations during sleep, several studies point to a link between brain oscillations and cognitive processes (e.g. memory consolidation, see below).

Since brain oscillations reflect the synchronous activity of neuronal networks, their function is likely related to ongoing neuronal processes. For instance, spindle activity is increased during NREM sleep after learning declarative tasks and motor skills (Rasch & Born, 2013). Also, spindles are associated with slow waves, and are mainly active just before the transitions to REM sleep. We already know that delta, theta and gamma waves are important for memory consolidation (Fell & Axmacher, 2011), so in combination with spindles, these oscillations may be functionally important for memorizing newly learned tasks and skills.

We know that many sleep disorders are characterized by abnormal sleep pattern, yet a major knowledge gap is to understand which brain oscillations or sleep stages can be affected, and why this happens. If we could connect a sleep disorder with decreased power of a specific brain oscillation, we could get closer to an understanding of their function, as well as possible

(29)

29 therapeutical applications. Another interesting question is whether these oscillations change throughout the lifespan, and if some of these oscillations are more important in childhood than in adulthood. This is one of the prime interests of my host laboratory.

3.4 Local vs. global sleep

Though sleep have historically been described as a paradigm that affects the whole brain, many sleep phenomena do not fit with that explanation, such as unilateral sleep, sleep-walking and poor performance after sleep deprivation (James M. Krueger et al., 2019). In my thesis, I have found that several sleep-promoting and wake-promoting neurons regulates the sleep-wake cycle. However, an unanswered question is whether these cell groups are organized hierarchically, in recurrent loops, or whether they work in parallel to control different aspects of sleep (Weber & Dan, 2016). Therefore, introducing the term “local sleep” could be of major interest for further research.

Local sleep can be defined as a complex physiological phenomenon occurring either within anatomically discrete brain locations in vivo or in cultures neuronal networks (James M. Krueger et al., 2019). The most remarkable example of local sleep is observed in bottlenose dolphins, in which slow wave sleep can occur in only one hemisphere during swimming, while the other hemisphere displays low-voltage fast desynchronized EEG activity typical of wakefulness (Siclari &

Tononi, 2017). Although unihemispheric sleep does not occur in humans, it was documented that low frequency neural dynamics like slow waves can occur locally in the awake brain, locally or globally in NREM, and in restricted cortical regions in REM (Lewis, 2021).

Furthermore, it has been demonstrated in freely behaving rats that, after a long period in an awake state, cortical neurons can briefly go

“offline” as in sleep, this accompanied by slow waves in local neural oscillations and impaired cognitive performances (Vyazovskiy et al., 2011). These findings support the idea of local sleep being an important aspect of sleep and may suggest the need to refine the characterization of sleep stages and even maybe the definition of sleep.

Though local sleep gives us new perspectives on sleep and may provide answers as to the pathophysiology of some sleep disorders, it seems that animals and humans cannot live without

Figure showing that slow waves could occur local and global, depending on the sleep state.

Lewis, 2021.

(30)

30 global sleep (Franks & Wisden, 2021). A study for sleep deprivation showed that after a period of intense cortical activity, local delta oscillations broke out of the neocortex and were seen in the whole brain, even though the animal was behaving as if it was awake (Vyazovskiy et al., 2011). This supports the fact that even if we tend to use “all of our energy” to suppress sleep, we eventually must shut down and enter global sleep. Therefore, sleep seems to encompass both local and global aspects.

3.5 The next step in understanding sleep – what can my host laboratory do?

I have reviewed some of the current literature on neurotransmitters, neuromodulators and brain regions that are important for generating and modulating brain oscillations during sleep, but there are still many open questions. Further investigations must be done and my supervisor’s lab could be a suitable place for some relevant experiments.

Over the past decades, several new techniques have become widely available to measure and control the activity of specific cell types and dissecting their synaptic connections, including optogenetics, pharmacogenetics, imaging with genetically encoded calcium indicators and virus-mediated circuit tracing (Weber & Dan, 2016). My host laboratory is planning to use

The illustration shows a closed-loop experiment. Brain signals are recorded in real time to detect the sleep patterns. While doing this, one can use invasive or non-invasive methods to manipulate local neuronal circuits. Girardeau, 2021.

(31)

31 optogenetic and chemogenetics methods to reversibly silence and activated different neurons at different sleep stages. Their goal is to activate and silence specific brain oscillations while recording it in real time, using close-loop systems. Such experiments could shed light on the phenomenon of local sleep for example. It would also be interesting to investigate whether the inactivation of sleep-regulatory regions or circuits can link the lack of specific brain oscillations and impairments in cognitive functions supposedly supported during sleep.

4 Conclusion

In conclusion, brain oscillations during sleep are strongly linked to specific neuronal circuits that seem to be regulated by neurotransmitters and neuromodulators. New sleep research suggests that sleep is initiated locally, and then distributes through the whole brain, giving rise to global sleep states. Further experiments could provide us with a better understanding of neurotransmitters and neuromodulators role in generating brain oscillations during sleep, and may provide answers as to their functional role. Such information would be extremely valuable for developing new therapeutics aimed at sleep disorders.

5 Literature

1. Adamantidis, A. R., Gutierrez Herrera, C., & Gent, T. C. (2019). Oscillating circuitries in the sleeping brain. Nature Reviews Neuroscience, 20(12), 746-762. doi:10.1038/s41583-019- 0223-4

2. Basser, L. S. (1981). The method of source derivation for the EEG. Clin Exp Neurol, 17, 85- 91.

3. Bastien, C., & Campbell, K. (1992). The evoked K-complex: all-or-none phenomenon? Sleep, 15(3), 236-245. doi:10.1093/sleep/15.3.236

4. Bland, B. H. (1986). The physiology and pharmacology of hippocampal formation theta rhythms. Prog Neurobiol, 26(1), 1-54. doi:10.1016/0301-0082(86)90019-5

5. Boccara, C. (2021, 30.03.21). Søvn. Retrieved from https://sml.snl.no/s%C3%B8vn 6. Bojarskaite, L., Bjørnstad, D. M., Pettersen, K. H., Cunen, C., Hermansen, G. H.,

Åbjørsbråten, K. S., . . . Nagelhus, E. A. (2020). Astrocytic Ca(2+) signaling is reduced during sleep and is involved in the regulation of slow wave sleep. Nat Commun, 11(1), 3240.

doi:10.1038/s41467-020-17062-2

7. Borbély, A. A. (1982). A two process model of sleep regulation. Hum Neurobiol, 1(3), 195- 204.

8. Boutrel, B., Monaca, C., Hen, R., Hamon, M., & Adrien, J. (2002). Involvement of 5-HT1A receptors in homeostatic and stress-induced adaptive regulations of paradoxical sleep: studies in 5-HT1A knock-out mice. J Neurosci, 22(11), 4686-4692. doi:10.1523/jneurosci.22-11- 04686.2002

9. Bragin, A., Engel, J., Jr., Wilson, C. L., Fried, I., & Buzsáki, G. (1999). High-frequency oscillations in human brain. Hippocampus, 9(2), 137-142. doi:10.1002/(sici)1098- 1063(1999)9:2<137::Aid-hipo5>3.0.Co;2-0

10. Bringmann, H. (2019). Genetic sleep deprivation: using sleep mutants to study sleep functions. EMBO Rep, 20(3). doi:10.15252/embr.201846807

11. Brown, R. E., Basheer, R., McKenna, J. T., Strecker, R. E., & McCarley, R. W. (2012).

Control of sleep and wakefulness. In (pp. 1087-1187). Baltimore, Md.

Review of neurotransmitters and neuromodulators qualitative distribution across sleep stages: hypothesis to their role in generating specific sleep oscillations