Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

Abstract

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Figure 1

Typical polygraphic appearance of wakefulness (left column), slow-wave sleep (middle column), and REM sleep (right column) in the adult mouse, rat, cat, and human, showing similarities in behavioral state-specific physiological signs across species. During wakefulness, cortical electroencephalogram (EEG) presents low amplitude, high frequency waves (sign of activated cortex) and electromyogram (EMG) shows presence of muscles tone in the antigravity muscles. During slow-wave sleep (SWS, part of NREM sleep), low amplitude, high frequency EEG waves are replaced with high amplitude, low frequency waves and muscle tone is still present (EMG), but slightly reduced compared to wakefulness. During REM sleep, cortical EEG is as activated (low amplitude, high frequency waves) as during wakefulness, but the EMG shows marked reduction or absence of muscle tone (atonia). During REM sleep, electrooculogram (EOG) records the presence of rapid eye movements. Due to its smaller size, the EOG recodings are normally not utilized to identify the sleep-wake states in the mouse. Two additional physiological signs of REM sleep are the rhythmic theta-wave activity (Theta) in the hippocampal EEG (HPC, shown only in the recordings of rat) and the phasic pontine-wave (P-wave)/Ponto-geniculo-occipital wave (PGO-wave). Occasionally, hippocampal theta wave is also present during wakefulness, but theta wave epochs are short lasting and are not as rhythmic as during REM sleep. Theta wave is completely absent during the period of SWS. The P-wave is represented by spiky waves shown in the rat pontine EEG (PON) and are the physiological equivalent to PGO-waves (LGB) shown only in the cat. Normally, P/PGO-waves are absent during wakefulness and SWS. Time bar (for all species and all states) = 5 sec. Polygraphic recordings of mouse, rat and cat are from Datta Laboratory. Human records are kindly provided by Dr. E. F. Pace-Schott.

Figure 2

Schematics of the sagittal view of human brain depicting the location of brain regions and their neurotransmitters, as well as pathways that are involved in the generation and maintenance of wakefulness. Ascending projections originating from the brainstem cholinergic cells (synthesize neurotransmitter acetylcholine, Ach) in the pedunculopontine tegmentum (PPT) travel dorsally to the thalamus to activate thalamo-cortical network. Projections from PPT cholinergic cells also travel ventrally to the basal forebrain (BF, to both cholinergic and GABAergic cells) to relay activating signals to the cortex. Ascending brainstem aminergic projections originating from the noradrenergic cells (synthesize neurotransmitter norepinephrine, NE) in the locus coeruleus (LC) and from the serotonergic cells (synthesize neurotransmitter serotonin, 5-HT) in the raphe nucleus (RN) travel both dorsally to the thalamus to activate thalamo-cortical network and ventrally to the hypothalamus and basal forebrain to activate hypothalamo-cortical and basalo-cortical networks. Projections from these aminergic cell groups also travel directly to the cortex. Projections from the brainstem glutamatergic cells (synthesize neurotransmitter glutamate, Glut) in the mesencephalic reticular formation (MRF) travel dorsally to the thalamus to activate thalamo-cortical and ventrally to the hypothalamus to activate hypothalamo-cortical network. Projections from the dopaminergic cells (synthesize neurotransmitter dopamine, DA) in the ventral tegmental area (VTA) and substantia nigra compacta (SNc) also travel to the thalamus, hypothalamus, and basal forebrain to activate thalamo-cortical, hypothalamo-cortical, and basalo-cortical network. The projections from these dopaminergic cells also travel directly to the cortex. Histaminergic cells (synthesize neurotransmitter histamine, HA) in the posterior hypothalamic tuberomammillary nucleus (PH-TMN) and cholinergic cells (Ach) in the basal forebrain (BF) project directly to the cortex. Activation of cells in the suprachiasmatic nucleus (SCN; both glutamatergic and neuropeptide-Y cells), lateral hypothalamus (LH-Hcrt; hypocretinergic/orexinergic cells synthesize hypocretin/orexin), and medial prefrontal cortex (mPFC; glutamatergic cells) could also directly activate the entire cortex. Activation of these brain regions both directly and/or indirectly through the thalamo-cortical, hypothalamo-cortical, and/or basalo-cortical ascending pathways causes activation of cortex as well as global activation of the brain to promote and maintain behavioral states of wakefulness. Although, at this time, we understand very little about different states of wakefulness, it is likely that those various states of wakefulness are the result of differential activation of specific wake-promoting regions in the brain. The synthesis and accumulation of brain metabolites (shown as Metabolites (+), green) are directly proportional to the intensity and duration of activation of those wake-promoting areas and global activation of the brain.

Figure 3

Metabolite Homeostatic model of physiological mechanisms for the initiation of sleep. Throughout wakefulness metabolites accumulate in the brain as a result of increased neuronal activity in wake-promoting structures and increased global neuronal activity. When these metabolites accumulate to a critically high level the brain responds by decreasing neuronal activities in the wake-promoting regions and ultimately global neuronal activities. This decreased neuronal activity causes withdrawal of wakefulness that ultimately initiates consolidated sleep (green). Thus, the initiation of sleep is a passive process caused by the withdrawal of wakefulness. Conversely, during consolidated sleep, metabolites reach a critically low level which results in a disinhibition of neuronal activity in the wake-promoting regions of the brain (red). This disinhibition is the direct predecessor to the increased neuronal activity in the wake-promoting regions of the brain and behavioral states of wakefulness.

Figure 4

The schematic of human brain depicting regions that are involved in the generation and maintenance slow-wave sleep (SWS). Following initiation of sleep, suppressed neuronal activity in the wake-promoting regions of the brain allow for the activation of γ-aminobutyric acid (GABA) and Galanin (Gal) synthesizing cells in the anterior hypothalamus/preoptic area (AH/POA). Increased activity of GABA and Galanin synthesizing cells in the AH/POA (red) ultimately generates slow wave sleep (SWS). Those GABA and Galanin synthesizing cells innervate the major wake-promoting cell groups of the brain including: histaminergic cells (HA) in the tuberomammilary nucleus (PH-TMN), hypocretin/orexinergic cells (Hcrt) in the lateral hypothalamus (LH), noradrenergic cells (NE) in the locus coeruleus (LC), serotonergic cells (5-HT) in the raphe nucleus (RN) and cholinergic cells (Ach) in the pedunculopontine tegmentum (PPT). The continued inhibition of these wake-promoting structures, via GABA and Galanin release, maintains episodes of SWS. Due to increased GABA and Galanin release, neuronal activity remains suppressed in other wake-promoting regions including: the medial prefrontal cortex (mPFC), suprachiasmatic nucleus (SCN), basal forebrain (BF), ventral tegmental area (VTA)/substantis nigra compacta (SNc) and medial reticular formation (MRF). In addition to AH/POA GABA and Galanin synthesizing cells, increased activity of GABAergic cells in layers I and II of the cortex and cells in the solitary nucleus also contribute to the maintenance of SWS.

Figure 5

Cellular-Molecular-Network model of physiological mechanisms for the generation of REM sleep. Each of the individual signs of REM sleep (right column, polygraphic signs) is executed by the increased activation of distinct cell groups in the brainstem (red-colored oval shapes). For example, cortical EEG activation sign of REM sleep is executed jointly by the activation of neurons in the mesencephalic reticular formation (MRF) and rostrally-projecting bulbar reticular formation (medullary magnocellular nucleus, MN), muscle atonia is executed by the neurons in the locus coeruleus alpha (LCα), rapid eye movements is executed by the neurons in the peri-abducens reticular formation (PAb), PGO/P-waves are executed by the neurons in the caudo-lateral peribrachial area (C-PBL) of predator mammals and in the dorsal part of the nucleus subcoeruleus (Sub C) of prey mammals, hippocampal theta rhythm is executed by the neurons in the pontis oralis (PO) and increased brain temperature and cardio-respiratory fluctuations are executed by the neurons in the parabrachial nucleus (PBN). These REM sleep sign generating executive neurons are excited by the increased release of cholinergic neurotransmitter while the releases of aminergic neurotransmitter are reduced and/or absent. The sources of the cholinergic neurotransmitter (red) are the cholinergic neurons in the pedunculopontine tegmentum (PPT) and lateral dorsal tegmentum (LDT). The sources of the aminergic neurotransmitters (blue) are the noradrenergic neurons in the locus coeruleus (LC) and serotonergic neurons in the raphe nucleus (RN). For the initiation of REM sleep, kainate receptors on the cholinergic cells are activated by the increased release of glutamate (yellow) that ultimately activates cholinergic cells and increase release of acetylcholine in each of the REM sleep sign-generators and in the cholinoceptive REM sleep inducing site in the medial pontine reticular formation (mPRF). While, PPT/LDT cholinergic cells are activated, local GABAergic cells (green) in the LC and RN are also activated. Activation of these local GABAergic cells actively inhibits aminergic cells in the LC and RN. Active inhibition of those aminergic cells reduces and/or stops releasing aminergic neurotransmitters to those REM sleep sign-generators. For the maintenance of REM sleep episodes, increased acetylcholine release in the mPRF activates glutamatergic cells that continue to release glutamate in the PPT/LDT to maintain activity of cholinergic cells. Thus, PPT/LDT cholinergic cells and mPRF glutamatergic cells create a positive feedback loop to maintain REM sleep. Activation of mPRF glutamatergic cells also releases glutamate in the LC and RN. This glutamate could also activate both aminergic cells and local GABAergic cells in the LC and RN. Activation of GABAergic cells intensifies their inhibitory response to those aminergic cells. But, the possibility of mPRF glutamate activating these aminergic cells is eliminated by the increased release of GABA in the LC and RN and also auto-inhibition.

Figure 6

Molecular mechanisms of the Cellular-Molecular-Network model based on recent findings. REM-on and Wake-REM-on cells in the cholinergic cell compartment of the PPT contain both glutamate and γ-aminobutyric acid (GABA) receptors. When endogenously released or exongenously applied glutamate binds to a kainate receptor, intracellular Ca2+ increases through activation of ion channels. Kainate receptor activation-mediated increase in intracellular Ca2+ activated adenylate cyclase (AC). Activation of AC increases synthesis of cAMP, increasing the amount of substrate available to activate protein kainase A (PKA). The phosphorylation of intracellular proteins by PKA elicits the physiological response (REM sleep) via numerous possible intermediate processes (some of which are shown boxed). Similarly, endogenously released or exongenously applied GABA binds to the GABA-B receptor. Activation of GABA-B receptor inhibits AC via G proteins (Gi/GO). The GABA-B receptor mediated inhibition of AC blocks the production of cAMP, limiting the amount of substrate available to activate PKA. By reducing the production of cAMP/PKA, the GABA-B receptor in the PPT cell inhibits REM sleep.

Figure 7

Cellular-Molecular-Network model of cholinergic and aminergic activity for the expression of physiological REM sleep. For the activation of an individual REM sleep sign generator, an increase in acetylcholine (Ach) is accompanied by a decrease in norepinephrine (NE) and serotonin (5-HT) within each generator. An increase in Ach (red) is the result of increased glutamate release in the pedunculopontine tegmentum (PPT) and the lateral dorsal tegmentum (LDT). This increased glutamate activates kainate receptors on the PPT/LDT and signals the release of Ach. The increased Ach in the sleep sign generators also activate medial pontine reticular formation (mPRF) cells that release glutamate (green) to the PPT/LDT and facilitate the continued release of Ach during REM sleep (maintenance of REM sleep episode). A decrease in NE and 5-HT (blue) is the result of increased γ-aminobutyric acid (GABA) release in the locus coeruleus (LC) and raphe nucleus (RN), respectively. The increased GABA reduces the presence of NE and 5-HT by inhibiting the cellular activity of the LC and RN. The combination of decreased NE and 5-HT and increased Ach in each individual REM sleep sign generator ultimately generates the expression of physiological REM sleep.