Sleep Neurobiology from a Clinical Perspective

Abstract

Many neurochemical systems interact to generate wakefulness and sleep. Wakefulness is promoted by neurons in the pons, midbrain, and posterior hypothalamus that produce acetylcholine, norepinephrine, dopamine, serotonin, histamine, and orexin/hypocretin. Most of these ascending arousal systems diffusely activate the cortex and other forebrain targets. NREM sleep is mainly driven by neurons in the preoptic area that inhibit the ascending arousal systems, while REM sleep is regulated primarily by neurons in the pons, with additional influence arising in the hypothalamus. Mutual inhibition between these wake- and sleep-regulating regions likely helps generate full wakefulness and sleep with rapid transitions between states. This up-to-date review of these systems should allow clinicians and researchers to better understand the effects of drugs, lesions, and neurologic disease on sleep and wakefulness.

Citation:

España RA; Scammell TE. Sleep neurobiology from a clinical perspective. SLEEP 2011;34(7):845-858.

INTRODUCTION

Sleep medicine physicians often encounter questions that require an understanding of the neurobiology of sleep: How do certain brain injuries produce coma or hypersomnolence? Why do antidepressants often reduce REM sleep? Why do people with narcolepsy have trouble staying awake? How do amphetamines improve alertness and wakefulness? To help with these and similar questions, this paper provides an overview of the basic circuits that control sleep and wakefulness. This paper has evolved from one we wrote several years ago¹ and has been updated to include many of the latest discoveries on the circuits and neurochemistry of sleep, more information on drugs that are used in clinical practice, and some thoughts on medications that are now in clinical trials. We hope this will provide the reader with useful perspectives on sleep disorders, how drugs influence sleep and wakefulness, and how injuries in different brain regions may affect sleep.

Almost 100 years ago, clinicians and pioneer neuroscientists began to identify the general brain regions that regulate sleep and wakefulness. After an epidemic of encephalitis lethargica around 1915–1920, Baron Constantin von Economo found that patients with encephalitis of the posterior hypothalamus and rostral midbrain often had crushing sleepiness, whereas those with injury to the preoptic area usually had severe insomnia.² He thus hypothesized that the preoptic area and adjacent anterior hypothalamus contained neurons that promoted sleep, whereas neurons in the posterior hypothalamus and rostral midbrain promoted wakefulness. In the 1940s, Moruzzi and Magoun found that stimulation of the rostral reticular formation caused the EEG of an anesthetized animal to switch from slow waves to the low-voltage desynchronized pattern typical of wakefulness, suggesting that this general region is capable of promoting arousal.³ Soon after the discovery of REM sleep in the mid-1950s, Jouvet and others established that this state is driven by circuitry in the pons.^4–6 Over the last few decades, the latest generations of researchers and clinicians have built on these ideas and identified many distinct systems, each of which contributes to specific aspects of sleep-wake behavior.

The Reticular Formation

The reticular formation is a heterogeneous region that runs through the core of the brainstem from the medulla up to the midbrain and into the posterior hypothalamus. Soon after Moruzzi and Magoun showed that the rostral reticular formation can activate the cortex, experimental lesions in animals and clinical observations in patients with strokes or tumors confirmed that the rostral reticular formation is necessary for generating wakefulness, as these injuries often produce hypersomnolence or coma.^7,8 Thus, many researchers hypothesized that the reticular formation received inputs from a number of sensory systems and promoted wakefulness via excitatory projections to the thalamus, hypothalamus, and basal forebrain. More recently, researchers have reconsidered the idea of a monolithic reticular formation and instead attribute its functions to the activity of specific systems that promote arousal using acetylcholine, glutamate, or monoamine neurotransmitters (e.g., norepinephrine, histamine, serotonin, and dopamine). These neurotransmitters are generally considered to produce arousal through widespread, often excitatory effects on target neurons. In addition, they can act as neuromodulators to enhance other excitatory or inhibitory inputs to these cells. This modulation can thus amplify neuronal signals over much of the brain to recruit the many systems necessary for waking behaviors. Thus, the term reticular formation is helpful anatomically, but more insight can be gained from understanding the specific cells and pathways contained within this general region.

Wake-Promoting Neurochemical Systems

Acetylcholine (ACh)

The basal forebrain (BF) and brainstem contain large groups of cholinergic neurons that promote wakefulness and REM sleep and also participate in learning, memory, and cognition. The BF is a region surrounding the front of the hypothalamus that includes the medial septum, magnocellular preoptic nucleus, diagonal band of Broca, and substantia innominata (Figure 1). Most BF cholinergic neurons are active during wakefulness and REM sleep, and they directly promote fast EEG rhythms via projections to the cortex and hippocampus (Table 1).^9–12 The BF also contains a large population of neurons that produce the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and these likely activate the cortex by reducing activity in inhibitory cortical interneurons.^13,14

Figure 1.

A variety of neurochemical systems promote arousal via projections to the forebrain. Cortical and subcortical regions are excited by monoaminergic neurotransmitters including norepinephrine (NE) from the locus coeruleus (LC), serotonin (5-HT) from the dorsal and median raphe nuclei, histamine (HA) from the tuberomammillary nucleus (TMN); and dopamine (DA) from the substantia nigra, ventral tegmental area, and ventral periaqueductal gray (SN/VTA/vPAG). Neurons of the basal forebrain (BF) promote cortical activation using acetylcholine (ACh) and γ-aminobutyric acid (GABA). Neurons in the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT) release ACh to excite neurons in the thalamus, hypothalamus, and brainstem.

Table 1.

Activity profiles of neurotransmitter systems across sleep/wakefulness

Neurotransmitter	Wakefulness	NREM sleep	REM sleep
Acetylcholine	↑↑	—	↑↑
Monoamines	↑↑	↑	—
Orexin/Hypocretin	↑↑	—	—
MCH	—	—	↑↑
VLPO/MNPO	—	↑↑	↑↑

Neuronal activity: ↑↑, rapid firing rate; ↑, slower firing rate; —, little or no firing.

A second major group of cholinergic neurons is found in the pons within the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT). In contrast to the BF, LDT/PPT neurons primarily project to subcortical regions including the thalamus, lateral hypothalamus, and BF.^15,16 Like most BF neurons, cholinergic neurons in the LDT/PPT are mainly active during wakefulness and REM sleep, and promote cortical activation by releasing ACh into the thalamus.^17,18

Pharmacological studies that manipulate ACh neurotransmission offer further evidence for its importance in the control of sleep and wakefulness. ACh, nicotine, and muscarinic receptor agonists such as pilocarpine produce desynchronized cortical activity and increases wakefulness.^19,20 Similar effects occur with physostigmine which blocks enzymatic degradation of ACh. In contrast, agents that reduce ACh signaling, including the muscarinic antagonists scopolamine and atropine, produce immobility and EEG slow waves.^21,22

Norepinephrine (NE)

NE is produced by several brainstem nuclei and may help generate arousal during conditions that require high attention or activation of the sympathetic nervous system. The major source of NE to the forebrain is the locus coeruleus (LC), an elongated nucleus just beneath the floor of the fourth ventricle. LC neurons fire most rapidly during wakefulness, are much less active during NREM sleep, and are nearly silent during REM sleep.^23,24 Extracellular levels of NE are linearly related to LC neuronal activity, with the highest rates of release observed during wakefulness.²⁵ NE is also made by neurons in the ventral medulla that mediate autonomic responses, and though much less studied, these cells may also promote arousal.²⁶

Pharmacological studies provide some of the strongest evidence that NE regulates wakefulness and sleep. For example, infusion of NE or the noradrenergic agonists isoproterenol and phenylephrine into the lateral ventricle or BF increases behavioral and EEG indices of wakefulness.²⁷ LC neurons are normally inhibited by NE via α₂ receptors, and blockade of this negative feedback with yohimbine increases LC activity and also increases wakefulness.^28,29 Conversely, bilateral inactivation of the LC with the α₂-agonist clonidine, or co-administration of both α₁ and β noradrenergic antagonists (prazosin and timolol) increases NREM sleep.^30,31

The NE system may be especially important in promoting arousal under conditions that require responding to a behaviorally important stimulus, a cognitive challenge, or stress. In broad terms, an animal may be drowsy and inattentive if LC activity is too low, distractible and anxious if LC activity is too high, but optimally attentive and aroused with intermediate levels of activity. NE tone is clearly linked to cognition as LC neurons in monkeys fire phasically in response to a salient stimulus that signals a reward such as food, but these cells do not respond to a distracting stimulus.³² Integrating these ideas, Aston-Jones and colleagues have proposed that activity in the LC may promote arousal in a way that optimizes attention and task performance.³³ In addition, the LC and NE neurons in the ventral medulla are active during stress.^23,34 The necessity of this system is clear in mice lacking NE because after exposure to a mild stressor, they fall asleep more rapidly than control mice.³⁵ Similarly, rats with lesions of the LC show considerably less behavioral arousal and cortical activation when confronted with novel stimuli.³⁶ On the other hand, excessive NE tone with anxiety might contribute to insomnia, and the α₁ antagonist prazosin can reduce the vivid nightmares and nighttime arousal of PTSD.³⁷

Histamine (HA)

HA plays an essential role in promoting wakefulness, yet little is known about which aspects of arousal it governs.³⁸ The tuberomammillary nucleus (TMN) is a small cluster of cells adjacent to the mammillary body at the base of the posterior hypothalamus. Though few in number, these cells innervate much of the forebrain and brainstem and are the sole source of HA in the brain. Similar to the pattern seen in the LC and other monoaminergic nuclei, TMN firing rates and HA release are highest during wakefulness, lower during NREM sleep and lowest during REM sleep.^39,40 Administration of HA or an H₁-receptor agonist increases cortical activation and wakefulness while reducing NREM and REM sleep.^41,42 In contrast, drugs that reduce HA signaling, including classical antihistamines, such as the H₁ receptor antagonists diphenhydramine, pyrilamine, and low dose doxepin increase NREM and REM sleep (Table 2).^41–46

Table 2.

Effects of commonly used drugs on sleep and waking

Drug Type	Examples	Pharmacologic Effect	Neurobiologic Mechanism	Clinical Effects
Selective serotonin reuptake inhibitors (SSRIs)	Fluoxetine	Increase extracellular levels of 5-HT	5-HT inhibits REM sleep-producing cells	Decreased REM sleep
	Fluvoxamine
	Citalopram
Tricyclic antidepressants	Amitriptyline	Increase extracellular levels of 5-HT and NE	5-HT and NE inhibit REM sleep-producing cells	Decreased REM sleep
	Nortriptyline
	Clomipramine
	Desipramine
Traditional, amphetamine-like stimulants	Amphetamine	Increase extracellular levels of DA and NE	Increased DA and NE signaling	Increased wakefulness
	Dextroamphetamine
	Methylphenidate
Wake-promoting, non-traditional stimulants	Modafinil	Increase extracellular levels of DA	Increased DA signaling	Increased wakefulness
	Armodafinil
Benzodiazepines	Diazepam	Enhance GABA signaling via GABAA receptors	GABA inhibits the arousal systems	Increased sleep
	Clonazepam
	Lorazepam
	Triazolam
Non-benzodiazepine sedative hypnotics	Zolpidem	Enhance GABA signaling via GABAA receptors	GABA inhibits the arousal systems	Increased sleep
	Zaleplon
	Zopiclone
Classic antihistamines	Diphenhydramine	Block HA H1 receptors	Reduced HA signaling	Increased sleep
	Triprolidine
Typical antipsychotics	Haloperidol	Block DA receptors	Reduced DA signaling	Increased sleep
	Chlorpromazine

Over the last several years, a new class of wake-promoting drugs has been developed to target the autoinhibitory histamine H₃ receptors.^47,48 The clinical rationale for these agents appears strong, as many people with narcolepsy or idiopathic hypersomnia have reduced HA levels,⁴⁹ and blockade of H₃ receptors should increase HA signaling. For example, H₃ antagonists (or reverse agonists) such as ciproxifan or tiprolisant promote wakefulness and EEG desynchrony and improve the excessive daytime sleepiness observed with narcolepsy.^50–54 This wake-promoting effect is likely mediated by increased HA tone as the response to an H₃ antagonist is absent in mice lacking H₁ receptors.⁵⁵

Which aspects of arousal are mediated by the HA system remains unclear. HA improves attention and psychomotor performance,⁵⁶ and it may promote motivated behaviors such as food seeking.⁵⁷ In addition, mice lacking HA have less wakefulness at the beginning of their active period,⁵⁸ suggesting that HA may be especially important for initiating arousal. As sleep inertia upon awakening is common in many patients with idiopathic hypersomnia, it is possible that low HA signaling is a contributing factor.

Serotonin (5-HT)

Understanding how 5-HT promotes arousal is challenging because: there are many sources of 5-HT; 5-HT binds to at least 15 different receptors with varied effects, and 5-HT has been shown to influence many other aspects of behavior including mood, anxiety, aggression, and appetite. 5-HT is produced by neurons in the dorsal raphe nucleus and other raphe nuclei scattered along the midline of the brainstem, and together these neurons innervate many brain regions that can influence sleep/wake behavior, including the preoptic area, basal forebrain, hypothalamus, and thalamus. Early studies suggested that 5-HT might help produce NREM and possibly REM sleep, but more recent work indicates 5-HT generally promotes wakefulness and suppresses REM sleep. The firing rates of dorsal raphe neurons and extracellular 5-HT levels are highest during wakefulness, much lower during NREM sleep, and lowest during REM sleep—a pattern very similar to that of the NE and HA systems.^59,60 In support of this wake-promoting role, agonists of the 5-HT_1A, 5-HT_1B, 5-HT₂, or 5-HT₃ receptors increase wakefulness.^61–65 Of clinical relevance, similar effects occur with selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine and citalopram that increase wakefulness and reduce REM sleep in both people and rodents.^66–70 In addition, drugs that block 5-HT₂ receptors such as ritanserin or agomelatine are thought to promote NREM sleep and thus are under development as treatments for insomnia.^{62,64,71–73}

Dopamine (DA)

DA has been implicated in the regulation of a variety of behavioral and physiological processes including motor function, motivation, reward, and learning. Additionally, DA exerts potent wake-promoting effects that are of great clinical relevance. For example, sleepiness is common with DA antagonists such as haloperidol or chlorpromazine or in patients with Parkinson's disease who have a loss of DA-producing neurons.^74–76 Additionally, D₂ agonists like ropinirole can produce sleepiness via activation of autoinhibitory D₂ receptors that reduce DA signaling.^77,78

However, it is unclear which DA neurons actually promote arousal. DA-producing neurons are most abundant in the substantia nigra and ventral tegmental area, yet cells in these regions fire in relation to movement or reward but, in general, have not been found to alter their rates of firing across sleep and wakefulness.^79–82 Nevertheless, extracellular levels of DA are high during periods of wakefulness and lower during NREM sleep, suggesting that some DA neurons must be wake-active.⁸¹ One candidate population sits in the ventral periaqueductal gray of the pons, and lesions of these wake-active DA neurons produce moderate reductions in wakefulness.⁸³ The conditions under which these or other DA wake-promoting neurons fire are unknown, but in general, DA may naturally promote arousal when an individual is highly motivated or physically active.

Drugs that increase DA signaling are used frequently to improve excessive daytime sleepiness. Classical stimulants such as methylphenidate and amphetamine increase extracellular levels of DA by disrupting the function of the DA transporter (DAT), thereby increasing extracellular levels of DA (Figure 2).⁸⁴ These drugs are usually very effective, but because they enhance DA signaling in reward and motor pathways, they have high abuse potential and can elicit tics or other movement disorders. At higher doses, these stimulants can also block the reuptake of NE and 5-HT which can result in tachycardia, arrhythmias, mania, and psychosis.

Figure 2.

A prototypical dopamine synapse. Under normal conditions, action potentials in a DA nerve terminal cause DA-filled vesicles to fuse with the presynaptic membrane, and DA is released into the synaptic cleft where it can bind to postsynaptic DA receptors. DA is removed from the synaptic cleft primarily by the DA transporter (DAT). Once DA is back inside the presynaptic terminal, it is repackaged into synaptic vesicles for future release via the vesicular monoamine transporter (VMAT). Amphetamines increase the synaptic concentration of DA through two main mechanisms: Amphetamines interfere with the reuptake of DA through the DAT, and they disrupt vesicular packaging of DA which increases cytosolic levels of DA which can then leak out through the DAT via reverse transport.

Modafinil is frequently prescribed for treating the sleepiness of narcolepsy and some other disorders. Clinically, it promotes wakefulness effectively, usually with fewer side effects than encountered with classical stimulants. Like amphetamines, modafinil disrupts DAT function in humans and rodents,^85,86 and this is a necessary part of its wake-promoting mechanism, as mice lacking the DAT show no increase in wakefulness with modafinil,⁸⁷ and D₁ and D₂ receptor antagonists can block modafinil-induced wakefulness.⁸⁸ Still, if modafinil acts via the DAT, it seems surprising that it has less abuse potential than amphetamines. One possible explanation is that amphetamines produce a dramatic efflux of DA into the synapse via reverse transport through the DAT, and this may be very reinforcing. In contrast, modafinil may simply block reuptake of DA through the DAT, leading to more modest rises in DA that are not as reinforcing. A better understanding of these mechanisms could drive the discovery of even better wake-promoting medications.

Orexin/Hypocretin

The excitatory neuropeptides orexin-A and -B (also known as hypocretin-1 and −2) are synthesized by neurons in the lateral and posterior hypothalamus and play essential roles in the regulation of wakefulness and sleep.^89,90 The orexin neurons project widely and heavily innervate all the arousal regions described above, with particularly dense innervation of the LC and TMN (Figure 3).^89,91 Orexins excite target neurons through the OX1 and OX2 receptors. Like most other wake-promoting neurons, orexin neurons fire mainly during wakefulness, especially during active exploration, and are silent during NREM and REM sleep.^92,93 Orexin levels are highest during wakefulness,⁹⁴ and when injected into the brain, orexins increase arousal and behavioral activity while suppressing NREM and REM sleep.^95–97 Consistent with this, selective optogenetic activation of orexin neurons can trigger brief awakenings from sleep.^98,99 Additionally, orexin receptor antagonists such as almorexant reduce sleep latency and increase the amounts of REM and NREM sleep.^100–102

Figure 3.

Orexin/hypocretin-producing neurons in the lateral hypothalamus innervate all of the ascending arousal systems, as well as the cerebral cortex.

The most compelling evidence that orexins are necessary for the regulation of wakefulness and sleep was the discovery that narcolepsy with cataplexy is associated with a loss of orexin signaling.^103–107 Dogs with a mutation of the OX2 receptor gene display many of the classic symptoms of narcolepsy, including cataplexy when presented with palatable foods.¹⁰⁴ Further, mice lacking the orexin peptides or the orexin-producing neurons have severe sleepiness and cataplexy.^{103,108–111} Most importantly, people with narcolepsy with cataplexy have a severe (85% to 95%) loss of the orexin neurons and very low CSF levels of orexin-A.^106,107,112 Less severe loss (20% to 60%) of the orexin neurons also occurs in other disorders that can cause sleepiness such as Parkinson disease, multiple system atrophy, and traumatic brain injury.^113–117

In just the last 10 years, much has been learned about the ways in which orexins promote arousal. In general, it may be best to think of this as a system for sustaining wakefulness as people and mice with narcolepsy have approximately normal amounts of wakefulness, but have great difficulty maintaining long periods of wakefulness.¹¹⁰ Orexins may also stabilize sleep as people with narcolepsy often have fragmented sleep, and orexins certainly regulate REM sleep as discussed below. In addition, orexins promote arousal responses to homeostatic challenges and drive motivated behaviors such as seeking food. Orexins directly excite neurons of the mesolimbic reward pathways, and orexin antagonists can reduce the motivation to seek drugs of abuse.^118–121 The orexin neurons are also activated by humoral indicators of hunger such as low glucose or high levels of ghrelin,^122,123 and while normal mice have a clear increase in arousal when deprived of food, mice lacking the orexin neurons show little response.¹²⁴ Thus, one can view the orexin system as helping sustain wakefulness across much of the day, and increasing arousal in motivating conditions.

Cortical and Thalamic Activity across Sleep and Wakefulness

All the arousal systems we have discussed thus far are located in the BF, hypothalamus, or brainstem and exert diffuse effects on the cortex and many other target regions. These subcortical systems are essential for the generation of sleep/wake states and for the regulation of the transitions between these states. However, patterns of EEG activity and consciousness itself arise from interactions between these subcortical systems, the thalamus, and the cortex.

Thalamic neurons relay information to and from the cortex and have intrinsic electrical characteristics that help generate some of the cortical rhythms seen in NREM sleep.^125,126 The thalamus contains two major types of neurons, glutamatergic thalamocortical projection neurons that relay sensory, motor, and limbic information to the cortex, and GABAergic neurons in the reticular nucleus of the thalamus that are innervated by the projection neurons and cortex and in turn inhibit the projection neurons. These reciprocal connections are thought to drive some cortical rhythms, including sleep spindles.¹²⁷ Thalamic neurons are hyperpolarized during NREM sleep, promoting a pattern of burst firing and reducing their responsiveness to incoming sensory stimuli.¹²⁸ During wakefulness and REM sleep, ACh depolarizes thalamic neurons to suppress spindles and slow waves and promote the transmission of single spikes that efficiently transmit information to the cortex and drive desynchronized cortical activity.¹²⁹ During wakefulness, monoamines bolster this effect.¹¹⁹ Extensive damage to the thalamus severely impairs consciousness and the ability to interact with the environment, but the general patterns of wakefulness, NREM, and REM sleep persist, suggesting that the thalamus is not required for the basic generation of sleep states.^130–133

The cortex contains a wide variety of neurons, and much less is known about their activity in relation to sleep/wake states. The EEG reflects broad patterns of excitatory and inhibitory post-synaptic potentials, mainly arising from the dendrites of pyramidal neurons. During wakefulness and REM sleep, these potentials are desynchronized, resulting in low-amplitude fast activity, but during NREM sleep these signals are synchronized, resulting in high-amplitude slow activity. Release of ACh and monoamines during wakefulness generally excites cortical neurons and increases their responsiveness to incoming sensory stimuli. Delta waves likely arise from interactions amongst cortical neurons and may also be influenced by the BF and other subcortical sites. Recent work has identified a population of widely projecting GABAergic neurons within the cortex that are uniquely active during NREM sleep, suggesting that these cells may broadly inhibit other cortical neurons, helping generate slow waves during NREM sleep.¹³⁴ In addition, the intensity of cortical slow waves may reflect prior local activity and changes in synaptic strength, as slow waves during NREM sleep are increased over supplementary motor cortex after learning a motor task but decreased with arm immobilization.^135–137

The Arousal Network: Interactions among Wake-Promoting Neurotransmitter Systems

Each of the arousal systems presented above is independently capable of promoting wakefulness, yet these systems work together to generate behavioral arousal. Anatomically, there are many interconnections between the systems. For instance, ACh and 5-HT fibers innervate and excite LC neurons, and nearly all wake-promoting neurons respond to HA, NE, and orexin. In addition, these neurotransmitters often produce similar effects on their targets. For example, all the arousal systems excite thalamic and cortical neurons. These interconnections and parallel effects may explain why injury to any one of the arousal systems often produces little lasting effect on wakefulness. Functionally, this is adaptive, as it helps ensure that wakefulness will still occur after injury to any one of the arousal systems. In fact, there are only a few brain regions in which lesions produce lasting reductions in arousal. One is the rostral reticular formation in the midbrain and posterior hypothalamus in which lesions from strokes or tumors can produce severe hypersomnolence or even coma, probably from damage to many of the ascending monoaminergic and cholinergic pathways.

Wakefulness is a complex and dynamic state, arising from networks of neurons driven by homeostatic, affective, cognitive, and motivational processes. Thus, it is likely that each arousal system helps promote specific aspects of behavioral arousal so that individuals can detect sensory and internal stimuli and generate appropriate motor and affective responses.¹³⁸ For example, NE, HA, and ACh may be particularly important for enhancing attention and responding to novel, stressful, or salient stimuli. Similarly, through its limbic and striatal projections, DA may promote arousal especially when an individual is motivated or physically active. The orexin peptides help sustain wakefulness and also may help drive goal-oriented behaviors and locomotion. So, while lesions of some arousal systems appear to have little effect on the amounts of wakefulness, deficits in arousal may be best revealed by carefully examining the response to specific circumstances and challenges.

NREM Sleep-Promoting Systems

Preoptic area

In the early 20th century, most researchers thought that sleep was a passive consequence of inactivity in the arousal systems, but many experiments have now shown that specific neurons actively promote sleep. Baron von Economo first observed that insomnia was common in patients with encephalitis injuring the preoptic area (the rostral end of the hypothalamus, just above the optic chiasm) and the adjacent BF.² This observation suggested that this region might contain neurons that promote sleep, and subsequent research in animals identified sleep-active neurons in the ventrolateral preoptic area (VLPO) and median preoptic area (MNPO).^139,140 Many neurons in these nuclei fire most frequently during NREM sleep and to some degree during REM sleep but are virtually silent during wakefulness.^141–143 Interestingly, these sleep active VLPO neurons show particularly high rates of firing during deep NREM sleep, and MNPO neurons often begin firing just before NREM sleep. Lesions of the preoptic area and specifically of the VLPO markedly reduce sleep, and the sleep that does occur is light and fragmented.^144,145 Collectively, these observations suggest that MNPO neurons may help initiate sleep, whereas VLPO neurons may be necessary for the maintenance of sleep.

Anatomically, the VLPO and MNPO are well positioned to promote sleep. The neurons in these nuclei contain the inhibitory neurotransmitter GABA and the inhibitory neuropeptide galanin,^146,147 and they innervate all the arousal-promoting regions, including the LDT/PPT, LC, DR, TMN, and also the orexin neurons (Figure 4). Thus, the VLPO and MNPO are hypothesized to promote sleep by coordinating the inhibition of arousal regions during NREM and REM sleep.^146,148,149

Figure 4.

NREM sleep pathways. Ventrolateral preoptic area (VLPO) neurons are active during NREM sleep and reduce activity in the ascending arousal systems using GABA and galanin. A subset of VLPO neurons is also active during REM sleep.

Other brain regions contain neurons active in NREM sleep, but these populations are less well understood. For example, parts of the BF and lateral hypothalamus contain scattered GABAergic neurons that are active during NREM sleep.^150–152 Some of these cells may directly innervate the cortex, and it is possible that they modulate cortical networks to promote slow wave activity.

Many of the medications now used to treat insomnia do so by promoting GABA signaling. Benzodiazepines, (e.g., diazepam), barbiturates (e.g., pentobarbital), and the newer non-benzodiazepine agents (e.g., zolpidem) all bind to GABA-A receptors to enhance the effects of GABA.^153,154 Gamma hydroxybutyrate (sodium oxybate) promotes very deep sleep, most likely by binding to GABA-B receptors.¹⁵⁵ These drugs may promote sleep by boosting signaling by the VLPO and other NREM sleep-active populations, but GABA can inhibit neurons throughout the brain, and the precise pathways through which these drugs work remain unclear.

REM Sleep-Promoting Systems

Soon after the discovery of REM sleep in the mid-1950s,^4,5 researchers learned that the pons plays an essential role in the generation of REM sleep.⁶ After several decades of work, much has been learned, but the specific pathways that generate this state are still debated (Figure 5).

Figure 5.

Pathways that control REM sleep. (A) A classic perspective on REM sleep control involves interactions between the cholinergic and aminergic systems. REM sleep-active cholinergic neurons in the LDT/PPT activate thalamo-cortical signaling and drive atonia by exciting neurons in the ventromedial medulla that inhibit motor neurons. During REM sleep, monoaminergic neurons including the LC, DR, and TMN become silent, which disinhibits the LDT/PPT and lessens the excitation of motor neurons by NE and 5-HT. (B) Recent observations have expanded on the classic view of REM sleep control. In this model, mutual inhibition between REM sleep-on neurons of the sublaterodorsal nucleus (SLD) and REM sleep-off neurons of the ventrolateral periaqueductal gray and lateral pontine tegmentum (vlPAG/LPT) is thought to regulate transitions into and out of REM sleep. During REM sleep, SLD neurons activate GABA/glycine neurons in the ventromedial medulla and spinal cord that inhibit motor neurons. At most times, the vlPAG/LPT inhibits the SLD, but during REM sleep, the vlPAG/LPT may be inhibited by neurons making melanin concentrating hormone (MCH) and other neurotransmitters. Solid lines depict pathways active during REM sleep, while dashed lines are pathways inactive during REM sleep.

Acetylcholine

Many researchers have hypothesized that REM sleep is controlled by cholinergic neurons located in the LDT/PPT. These are the same nuclei that contain wake-promoting cells, but a subpopulation of these cholinergic neurons are active in both wakefulness and REM sleep or are selectively active in REM sleep.^9,156–158 When injected into the lateral pontine tegmentum (LPT; a heterogeneous region extending rostrally from the PPT that is lateral to the periaqueductal gray), drugs that enhance ACh signaling such as the cholinergic agonist carbachol or the acetylcholinesterase blocker, neostigmine, elicit intense and long-lasting REM sleep.^159–161 Conversely, cholinergic antagonists reduce the duration of REM sleep bouts.^162,163 Furthermore, large lesions that include the LDT/PPT produce significant reductions in REM sleep,^164,165 suggesting that the LDT/PPT is necessary for REM sleep.

Neurons in the LDT/PPT may help generate the cortical activation and atonia of REM sleep. The LDT/PPT is the main source of ACh to the thalamus, and ACh depolarizes thalamic neurons to promote the transmission of information through the thalamus, driving the cortical activation that is probably required for the complex dreams of REM sleep. The LDT/PPT neurons may also activate atonia-promoting neurons in the ventromedial medulla.^158,166 These medullary cells release GABA and another inhibitory neurotransmitter glycine onto spinal and brainstem motor neurons during REM sleep, producing hyperpolarization and inhibition.¹⁶⁷ This descending inhibition is clearly important for atonia as drugs that block glycine signaling such as strychnine can markedly increase muscle tone in REM sleep and wakefulness.^168,169

Monoamines

Monoamines such as NE and 5-HT increase muscle tone by directly exciting motor neurons.^170–173 In the genioglossus muscle, withdrawal of this excitatory tone contributes more to atonia than the inhibitory effects of GABA and glycine.^174,175 Whether this applies to most muscles is unknown, but it is clear that atonia during REM sleep is probably due to a combination of inhibition (GABA and glycine) and a loss of excitation (NE and 5-HT).

Monoamines also inhibit REM sleep itself. During wakefulness, and to some degree during NREM sleep, the REM-active cholinergic neurons are inhibited by 5-HT, NE, and HA.¹⁷⁶ This interaction between cholinergic and monoaminergic populations forms the foundation of the classic model explaining the alternation of NREM and REM sleep across the night (Figure 5A).¹⁷⁷

These monoaminergic effects on motor tone and REM sleep may account for many phenomena commonly seen by sleep clinicians. NE and 5-HT reuptake inhibitors often increase muscle tone during sleep and can unmask REM sleep behavior disorder (RBD) and worsen periodic limb movements of sleep.¹⁷⁸ These drugs and other antidepressants also strongly suppress REM sleep, and thus can markedly reduce REM sleep during overnight polysomnograms or during the MSLT.¹⁷⁹

GABA

Over the last few years, new observations have expanded on the classic model of REM sleep control (Figure 5B). One region that has received significant attention is the sublaterodorsal nucleus (SLD; also termed the subcoeruleus, or LCα), which is a small cluster of cells ventral to the LC that produce GABA or glutamate.¹⁸⁰ Many neurons in the SLD are active during REM sleep,^{158,181–183} and they project to the ventromedial medulla and ventral horn of the spinal cord, providing pathways through which they may inhibit motor neurons. Activation of the SLD region elicits atonia and REM sleep-like EEG activity,¹⁵⁸ while inhibition of the SLD promotes wakefulness and reduces REM sleep. Most importantly, lesions of the SLD region disrupt REM sleep atonia and reduce REM sleep.^{165,180,182,184,185} Neuronal loss near the SLD has been reported in some patients with RBD,¹⁸⁶ suggesting that injury to the SLD may contribute to the inadequate atonia of RBD.

Another new perspective on the classic view of REM sleep is that the SLD neurons may be strongly inhibited by REM sleep-suppressing neurons in the mid-pons.^149,182,187 These GABAergic cells are scattered from the ventral part of the periaqueductal gray out into the lateral pontine tegmentum (vlPAG/LPT) and lesions of this region substantially increase REM sleep.¹⁸⁸ The vlPAG/LPT inhibits the SLD, and the SLD may in turn inhibit the vlPAG/LPT, giving rise to a mutually inhibitory circuit that may regulate transitions between NREM and REM sleep.^149,182

Melanin-concentrating hormone (MCH)

Mixed in with the orexin neurons of the lateral hypothalamus are a large number of REM sleep-active neurons that produce both MCH and GABA.^189–191 These cells innervate nearly all the same target regions as the orexin neurons including the DR and LC,^192,193 yet in contrast to the excitatory effects of orexins, both MCH and GABA are inhibitory. Electrophysiological recordings demonstrate that MCH neurons fire at a high rate during REM sleep, with much less firing during NREM sleep and complete inactivity during wakefulness.¹⁵² The amount of REM sleep is increased by infusions of MCH into the lateral ventricles and decreased by a MCH antagonist.^191,194 Consistent with these observations, mice lacking MCH spend less time in NREM and REM sleep.¹⁹⁵ Thus, it seems likely that the MCH neurons promote REM sleep by inhibiting the arousal regions. This pattern is strikingly opposite to that of the orexin neurons and much remains to be learned about how the activity of these intertwined systems is organized.

Mechanisms that Regulate the Transitions between Sleep and Wakefulness

The systems that promote wakefulness, NREM, and REM sleep dynamically interact in a variety of ways to ensure rapid and complete transitions between sleep/wake states.^148,149 The VLPO and other sleep-promoting preoptic neurons inhibit monoaminergic and cholinergic wake-promoting neurons, and the preoptic neurons themselves are inhibited by NE, 5-HT, and ACh.^196,197 During wakefulness, high monoaminergic and cholinergic tone should thoroughly silence the VLPO, thus disinhibiting the arousal regions and helping ensure the production of complete wakefulness. Conversely, during sleep, preoptic neurons become active and inhibit the arousal regions, thus disinhibiting their own firing. This mutual inhibition should produce stable wakefulness and sleep while facilitating rapid transitions between sleep and wakefulness and minimizing time in drowsy, intermediate states. Similar mutually inhibitory circuits may regulate REM sleep as REM sleep-active neurons in the SLD inhibit and are inhibited by neurons in the vlPAG/LPT that are inactive in REM sleep.¹⁸²

The orexin neuropeptides probably reinforce these mutually inhibitory systems. Orexins may stabilize wakefulness by enhancing activity in the arousal systems, ensuring full alertness and long periods of wakefulness despite rising homeostatic pressure across the day.⁹⁴ During wakefulness and perhaps to a lesser degree in NREM sleep, orexins may excite a variety of neurons that inhibit REM sleep, including monoaminergic neurons, the vlPAG/LPT, and GABAergic inputs to the SLD.^96,198–201 However, loss of the orexin neurons in narcolepsy with cataplexy results in persistent sleepiness, frequent transitions between states, and odd states such as cataplexy and hypnagogic hallucinations in which it seems that elements of REM sleep mix into wakefulness. Collectively, these symptoms may be best thought of as behavioral state instability, a phenomena that is likely caused by loss of the stabilizing effects of orexins on the mutually inhibitory circuits that regulate wakefulness, NREM, and REM sleep.^110,202

Somnogens

Most of the preceding text describes neural pathways that regulate sleep/wake states, but these states can also be influenced by diffusible or circulating factors that act upon many brain regions to promote sleep. In fact, more than 100 years ago, researchers found that the CSF of sleep deprived dogs contained somnogens, substances that promote sleep.^203,204 Much evidence now suggests that adenosine, cytokines, prostaglandins, and probably additional substances serve as natural sleep-generating signals.²⁰⁵

Adenosine

During wakefulness, brain metabolic activity is high, and adenosine may promote sleep in response to this metabolic challenge.^206–208 When cells have ample energy, nearly all adenosine is phosphorylated to ATP and adenosine levels are low. However, when cells are fatigued, ATP production is lower, adenosine levels rise, and then adenosine acts as an inhibitory neuromodulator. For example, adenosine reduces the activity of most wake-promoting neurons, but disinhibits VLPO neurons. With prolonged wakefulness, adenosine levels rise in the basal forebrain and other regions, and levels then fall during recovery sleep.²⁰⁹ Most likely, the extracellular levels of adenosine are governed by the activity of astrocytes, the support cells of the brain, because manipulations of astrocytes reduce the usual increases in sleep and delta power after sleep deprivation.^207,209,210 Furthermore, adenosine receptor agonists increase sleep and NREM delta power, while caffeine and other drugs that block adenosine receptors promote wakefulness.^{207,211–213} As caffeine promotes arousal after sleep deprivation,²¹⁴ it seems likely that adenosine is an important mediator of everyday sleepiness.

Cytokines

Cytokines are intercellular signaling peptides released by immune cells, neurons, and astrocytes, and several cytokines, including interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), promote sleep.²¹⁵ Administration of IL-1β into the preoptic area of rats reduces firing rates of wake-active neurons and promotes NREM sleep.²¹⁶ Similarly, TNF-α infusions into the preoptic area also promote NREM sleep.²¹⁷ During infections, bacterial cell wall products such as lipopolysaccharide and muramyl dipeptide may trigger production of these cytokines that then increase NREM sleep and reduce REM sleep.²¹⁸ In addition, these cytokines may promote spontaneous, physiological sleep as IL-1β and TNF-α mRNA and protein levels are highest around sleep onset, and blockade of IL-1β and TNF-α signaling with antagonists, antibodies, or deletion of their receptors reduces spontaneous NREM sleep.^219–221

Prostaglandin D2

Prostaglandin D2 (PGD2) is a lipid derived from fatty acids that potently promotes NREM sleep.²¹¹ Unlike adenosine or cytokines which are made in the brain parenchyma, PGD2 is likely synthesized in the basal meninges just below the hypothalamus.²²² PGD2 levels in cerebrospinal fluid are highest during the sleep period,²²³ and PDG2 levels increase with sleep deprivation.²²⁴ Infusions of PGD2 just below or within the preoptic area activate neurons in the VLPO and increase NREM and REM sleep, perhaps by increasing local concentrations of adenosine.^225–229 Like cytokines, PGD2 may contribute to the sleepiness seen with inflammation as patients with African sleeping sickness display increased CSF levels of PGD2.²³⁰

Process C and Process S

The two-process model provides a useful macroscopic perspective on the dynamic control of sleep and wakefulness. It is likely that a homeostatic factor (process S) accumulates during wakefulness and declines during sleep, and this factor interacts with a circadian process (process C) that helps regulate the timing of wakefulness and REM sleep.^231–233 After a period of wakefulness, delta power in NREM sleep is thought to be a good indicator of Process S,^234,235 and somnogens such as adenosine may be the neurobiologic equivalent of Process S as disruption of adenosine signaling can blunt the usual increase in NREM sleep and the intense EEG delta power seen after sleep deprivation.²¹⁰

Process C is driven by the suprachiasmatic nucleus (SCN), the master pacemaker that regulates the circadian rhythms of sleep, wakefulness, and most other physiologic rhythms.²³⁶ The activity of individual SCN neurons is strikingly rhythmic, especially when coupled with other SCN neurons.²³⁷ This rhythmicity arises from positive and negative feedback loops in the transcription and translation of several genes.²³⁸ To synchronize its activity with the environmental light-dark cycle, the SCN uses luminance information from photosensitive retinal ganglion cells that contain the photopigment melanopsin.²³⁹ SCN neurons then relay these timing signals to the adjacent subparaventricular zone, using neuropeptides such as prokineticin 2 or transforming growth factor-α.^240,241 This signal is then passed through the dorsomedial nucleus of the hypothalamus and on to brain regions that regulate sleep and wakefulness such as the LC, VLPO, and lateral hypothalamus.²⁴² The SCN also regulates the daily rhythm of body temperature, and through these cycles in temperature, the SCN can entrain circadian activity in cells throughout the body.²⁴³ Circadian rhythms are closely linked to metabolism, and a breakdown in this coordination of central and peripheral rhythms may contribute to the obesity and glucose intolerance that is common in people with shift work sleep disorder or insufficient sleep.²⁴⁴

CONCLUSIONS

Since the days of von Economo and then Moruzzi and Magoun, much has been learned about the neurobiology of sleep and wakefulness. We now know that neurons producing ACh and monoamines such as NE, 5-HT, DA, and HA promote various aspects of wakefulness. In addition, orexins/hypocretins help sustain long periods of wakefulness while suppressing REM sleep. NREM sleep is mainly regulated by neural pathways originating in the VLPO and other preoptic regions, yet it is also influenced by diffusible somnogens such as adenosine. REM sleep is driven by neurons in the pons that make ACh and GABA. These discoveries provide a useful framework to better understand sleep disorders and the effects of medications on sleep.

Nevertheless, despite these advances, many questions of clinical importance remain unanswered. What goes wrong in these circuits to cause parasomnias such as sleepwalking and periodic limb movements of sleep? Under what conditions are specific wake- and sleep-promoting systems especially necessary? How is sleep restorative? What are the functions of NREM and REM sleep? Undoubtedly, future sleep research will provide helpful insights into the underlying causes of sleep disorders and lead to new and more powerful therapeutics to treat them.

DISCLOSURE STATEMENT

This was not an industry supported study. Dr. Scammell has consulted for Merck and Cephalon. Dr. España has indicated no financial conflicts of interest.