Neuroscience of Sleep
Alexander Levit

Wide differences in neural activity are observed throughout many brain regions when an organism transitions between sleep and wakefulness, making it challenging to isolate the neural circuitry that moderates sleep behaviour. By integrating behavioural studies, neuroimaging, histology, drug studies and genetic manipulations, scientists have been able to identify brain regions and neural circuits that play major roles in transitioning between sleep and wakefulness. Neural mechanisms of sleep are often analyzed in the context of general arousal which relies on the integration of multiple circuits. Hence, at least in the human brain, there is no sole system that controls sleep behaviour.


2.1 Arousing Diffuse Modulatory Systems

Sleep is thought to be largely moderated through diffuse modulatory systems (DMS). Despite much structural variation in their projections, nuclei of the DMS are well defined and mostly found in the brainstem and basal forebrain. The anatomical components of the DMS were historically categorized as part of the reticular formation. However, as anatomists began to better discern the intricacies of the brain stem, the term reticular formation has fallen out of use and specific nuclei are now identified when speaking of DMS[1] . These nuclei project onto many disperse regions of the brain through volume transmission, modulating the excitability of the thalamus, cerebral cortex and other large brain regions that are often associated with wakefulness. It is not uncommon for a single neuron in the DMS to communicate with 100,000 postsynaptic neurons.
Whereas most of the CNS relies on glutamate and GABA neurotransmitters for signal transduction across synapses, most DMS neurons release acetylcholine (ACh), histamine and monoamines (Figure 1). For convenience, researchers classify DMS by their respective neurotransmitter types or by the nuclei from which the system's projections originate[2] . It is important to note that the DMS described here play a role in many other functions in addition to sleep including general arousal, mood, learning and reward. No single DMS system has been identified as necessary for transitioning between sleep and wakeful states. Rather, it appears that the different DMS must work cooperatively in regulating sleep transitioning and wakefulness (Table 1).


Noradrenergic System

DMSmap.jpg
Figure 1 – Schematic anatomy of arousing diffuse modulatory systems (DMS) from a medial sagittal view; nuclei, respective neurotransmitters and projections. Please click on figure to see full caption.
The locus coeruleus (LC) is the major source of norepinephrine (NE) in the brain. This nucleus appears bilaterally near the midline as a short column just underneath the floor of the fourth ventricle and can be identified by its blue tint in unstained tissue. A pair of human LC nuclei is estimated to have around 24,000 neurons and each individual neuron might stimulate as many as 250,000 postsynaptic neurons, ranking LC as one of the most widely connected cluster of neurons [1]. Regions known to be targeted by LC projections include the thalamus, cerebral cortex, cerebellar cortex, striatum, limbic system and the spinal cord. It was found in early animal model studies that LC neurons fired most rapidly in a wakeful state, less frequently during NREM sleep and were inactive during REM sleep[3] . Bursts of LC activity were also found to precede interruptions of sleep and occurred spontaneously or in response to waking sensory stimulation [4] . This last feature of LC activity has captivated the interests of many researchers. In one recent study, the locus coeruleus was selectively stimulated or inhibited through optogenetic control. Inhibition of the region resulted in impaired wakefulness and excitation of the region consistently induced a full sleep to wake transition within 5 seconds of stimulation, demonstrating the crucial role that this sole region plays in regulating sleep[5] . The diffuse release of NE in the brain is mostly attributed to the neurons of the LC, though there is no evidence to suggest that other nuclei do not provide significant contributions to NE levels.
The ventral medulla, though far less studied, is also thought to contribute at least some NE release in the brain and might play a role in arousal as well[6] . NE levels in the brain, in addition to awakening, are important in regulating alertness and attention levels. Excessive NE release may cause anxiety disorders and insomnia while inadequate release might lower wakefulness and hasten the onset of sleep. This has been observed both in behaviour studies and in EEG studies of the cortex, both of which are found to be depressed in transgenic mice that are NE deficient[7] . Conversely, baseline wakefulness was not altered by LC lesioning suggesting that when it comes to transitioning from sleep to wakefulness, either other sources of NE compensate for the loss of LC neurons or that NE, although strongly correlated, does not play a necessary role in wakefulness[8] .

Serotenergic System

Unlike NE, serotonin (5-HT) is known to be released significantly from multiple nuclei in the brain. Most of these nuclei belong to a cluster known as the raphe nuclei which extend throughout the brainstem just on either side of the midline. The more caudal raphe nuclei innervate the cerebellum and spinal cord while the rostral raphe nuclei project onto structures of the forebrain as diverse as those targeted by the noradrenergic DMS. To add to the complexity of the serotonergic DMS, 5-HT binds to more than 15 different receptors and will therefore have different effects depending on the receptors present on the postsynaptic neuron [2] .
Like the noradrenergic DMS, firing rates of the serotonergic DMS are greatest during wakefulness, decreased during NREM sleep and lowest during REM sleep[9] . This may explain why individuals treated with selective serotonin reuptake inhibitors (SSRIs) might experience decreased REM sleep. Also of clinical relevance, 5-HT antagonists are now being evaluated as a treatment for insomnia[10] [11] . By attempting to mimic the reduction of 5-HT observed during sleep, these therapies might be more effective in restoring the quality of asleep than existing medications such as benzodiazepines.

Dopaminergic System

Dopamine (DA) releasing neurons are found in several regions of the brain but are released diffusely only from the ventral tegmental area (VTA) and the substantia nigra (SN), both of which are found in the midbrain. Though more often implicated in motor function, learning and reward, the dopaminergic DMS is important for maintaining wakefulness and alertness as well. This was first suggested when it was observed that patients taking DA antagonists as antipsychotics experienced excessive drowsiness[12] . Accordingly, individuals with Parkinson’s disease also experience increased sleepiness as the neurodegenerative disease and associated loss of DA neurons progresses[13] . Correspondingly, the DA transporter inhibitor, Modafinil, is used as a treatment for narcolepsy because of its ability to raise circulating DA levels in the brain in mechanisms similar to amphetamine[14] [15] . Despite all this evidence based on neurochemistry, there have been no electrophysiological observations that the VTA and SN firing rates change during transitions between sleep and wakefulness[2] . Firing rates of these two regions are typically greater during wakeful states but this might be confounded by motor movement which naturally occurs at a lower frequency during sleep[16] .
More recently, the ventral periaqueductal grey (vPAG) has been identified as contributing to the dopaminergic DMS as well and has also been linked to promoting wakefulness[17] . The injection of agonists at this region promote motor movement and decrease sleep, while antagonists will produce opposite effects[18] . The firing rates of neurons belonging to the vPAG in relation to sleep/wake transitioning have not been assessed and will be difficult to isolate because of their intimate connections with serotonergic cells of the raphe nuclei[19] .

Cholinergic System

orexin_map.jpg
Figure 2 – Schematic anatomy of the orexinergic diffuse modulatory system (DMS) from a medial sagittal view; orexin releasing neurons located in the lateral and posterior hypothalamus and their projections. Please click on figure for full caption.
The basal forebrain (BF), found anterior to the hypothalamus, releases acetylcholine (ACh) from its projections to the cortex and hippocampus. The cholinergic neurons of the BF are scattered amongst a complex of nuclei. The medial septal nuclei and the basal nucleus of Meynert are the easiest of cholinergic nuclei to visually identify, projecting to the cortex and hippocampus respectively[1] . Most of these neurons fire both during wakefulness and REM sleep and their activation promotes fast EEG rhythms
[20] . To a lesser extent, the BF also releases γ-aminobutyric acid (GABA) which is thought to disinhibit the cortex by lowering the excitability of inhibitory cortical interneurons[21] . In Alzheimer’s disease, one of the first cell types to degenerate are cholinergic cells of the BF. No specific relation has been realized between cholinergic cells and the disease aside from their early degeneration[22] . This might occur simply because the long axonal projections throughout the cerebrum could more rapidly accumulate neurofibrillary tangles (note that no other prominent DMS have nuclei in the cerebrum, one of the most affected regions of the brain in Alzheimer's disease). Another set of cholinergic neurons are found in the rostral pons, known as the laterodorsal and pendunculopontine tegmental nuclei (LDT/PPT). Neurons of these nuclei are one of the few DMS neurons without axonal projections into the cerebrum but only onto the diencephalon. Even so, they show the same firing patterns as BF cholinergic neurons (active during wakefulness and REM sleep)[23] .

Histaminergic System

A DMS that is relatively poorly understood in relation to any function is the histaminergic system. The tuberomammillary nucleus (TMN) contains histamine (HA) releasing neurons located near the mammillary bodies of the hypothalamus. These are known to be the sole source of histamine in the brain, innervating many regions of the forebrain and brainstem[2] . Like the firing rates of monoaminergic systems, the TMN fires most rapidly during wakefulness, less rapidly during NREM sleep and is inactive during REM sleep[24] . In agreement, the experimental use of doxepin, a H1 receptor antagonist, has been found to be effective in treating insomnia[25] . On the other hand of the clinical spectrum, H3 autoreceptor antagonists are being used to reduce the negative feedback of HA release to promote wakefulness in narcoleptics[26] .

Orexinergic System

A relatively new pair of neuropeptides, orexin-A and -B may play a crucial role in coordinating the different DMS (they are also referred to as hypocretin-1 and -2, respectively). Orexin releasing neurons are few in number, are located in the lateral and posterior hypothalamus and their projections extend throughout the entire brain (Figure 2)[2]. These neurons fire during wakefulness but are silent during NREM and REM sleep[27] . Orexin release is considered to be most important for sustaining wakefulness and stabilizing sleep states. This was suggested by the finding that narcoleptics often lack orexin receptors, lack orexin producing neurons, have low levels of circulating orexin or have some other form of impairment in orexin signalling [28] . Milder loss of orexin signalling also occurs in other neurological disorders that cause excessive sleepiness such as Parkinson’s disease and tramatic brain injury[29] [30] [31] . What makes this DMS unique is that it heavily innervates the nuclei of other DMS (more than the cholinergic system from the LDT/PPT).


VLPO_map2.jpg
Figure 3 – Schematic anatomy of the sleep promoting ventral lateral preoptic area (VLPO) from a medial sagittal view; nucleus and its projections. Please click on figure for full caption.

2.2 Sleep Promoting Diffuse Modulatory Systems

Less is known about regions of the brain that promote sleep, hence it used to be hypothesized that sleep would occur at the absence of arousal [2] . As clinical observations were made linking lesions of the preoptic area to insomnia, researchers drew their attention to this area towards the end of the 20th century. The ventrolateral preoptic area (VLPO) and median preoptic area (MNPO) have been observed to have diffuse-like inhibitory projections onto the arousing DMS (Figure 3) (releasing GABA and galanin) and fire most frequently during NREM sleep and are silent during wakefulness[32] [33] . When these regions are lesioned, the result is a notable reduction in sleep quantity and quality[34] . This circuit may pertain to the efficacy of today’s most common medications that are used to treat insomnia, all of which promote GABA signalling: benzodiazepines, barbiturates and non-benzodiazepine agents[35] [36] .
Located amongst orexinergic neurons and similarly innervating the arousing DMS are melanin-concentrating hormone (MHC) and GABA releasing neurons[37] [38] . MHC, along with GABA, inhibits these target regions including the LC. What is unique to these neurons is that they fire at their greatest rate during REM sleep, less frequently during NREM sleep and are inactive during wakefulness[39] . These neurons fit the puzzle very nicely by inhibiting arousing DMS during sleep states and will likely be studied extensively by sleep researchers in coming years. Other chemicals are also found to be circulating at greater levels during sleep including somnogens, adenosine, cytokines, prostaglandin D2, process C and process S [2] . These likely serve as chemical messengers of fatigue.


2.3 Other Functional Regions

To generalize, the forebrain relays information from the environment to help regulate the internal sleep drive, as with other primitive drives. While the important functions of consciousness and cognitions greatly rely on processing by forebrain structures such as the thalamus and cortex, the DMS are thought to act as the driving forces or “switches” for sleep & wakefulness transitioning. With this rationale, sleep researchers today are still more interested in these regions of the brain that act as sleep switches. Nonetheless, the large amounts of electrical activity produced by these brain structures are important for EEG studies.

Forebrain Structures

Neurons of the thalamus are inhibited during NREM sleep so that they are less responsive to sensory stimuli. During wakefulness and REM sleep, excitation from the cholinergic system suppresses thalamic activity patterns that are associated with NREM sleep (slow wave EEG rhythms)[40] . In the case that the thalamus is damaged, though consciousness might be severely impaired, transitions between wakefulness, REM sleep and NREM sleep will still persist[41] . The cortex provides a similar contribution in the regulation of sleep; like the thalamus, it acts more as the effector of wakefulness, as regulated by general excitation or inhibition by the DMS. All things considered, the forebrain is mostly valued for the generation of observable electrical activity when it comes to sleep research. This is crucial for assessing the quality of sleep and wakefulness.

Pineal Gland

Melatonin is primarily synthesized in and released from the pineal gland, a midline structure found just posterior to the thalamus. This is preserved in all vertebrates which has encouraged the use of animal models such as various primates, rats, cats and zebrafish for studying the role of melatonin in sleep[42] . A tryptophan derivative, melatonin is synthesized during dark conditions and its production is inhibited by bright conditions. Since this pathway is closely linked to day/night cycles, melatonin release from the pineal gland is inherently central to the study of circadian rhythms. Lighting conditions are relayed to the pineal gland in an indirect pathway. Some of the signals from retinal photoreceptors are sent to the suprachiasmatic nuclei (SCN), relayed to the paraventricular nuclei that innervate the sympathetic superior cervical ganglia which then finally relay brightness information to the pineal gland. Melatonin receptors are found mostly on neurons belonging to this very same pathway, particularly select neurons of the SCN and retina, suggesting that a closely regulated feedback mechanism is involved[43] . Though studied since the late 1950’s, there is little consensus on how melatonin affects sleep in humans. Due to the many differences in sleep patterns amongst animal models that have been studied, it has been difficult to reconcile the physiological studies of the pineal gland. Similarly, clinical studies on the use of melatonin as a therapeutic agent for sleep disorders such as insomnia have reported mixed results[44] [42] . The use of melatonin agonists and analogs such as agomelatine appear to have been more effective[45] [46] [47] [48] .


2.4 Integration of Multiple Systems

A recurrent theme amongst all of the brain circuits and regions relevant to sleep/wake control is that not one of them is independently sufficient for transitioning between sleep and wakefulness. Though some of the sleep promoting diffuse modulatory systems appear to provide pivotal contributions to inducing sleep (such as the VLPO), it has yet to be established that these systems are unaccompanied by complimentary mechanisms. Only lesions of the posterior hypothalamus and the rostral region of the midbrain alone are known to selectively inhibit wakefulness, indicating that these regions might be necessary for waking[2]. There is a lot of redundancy in these systems, all having parallel excitatory effects on the thalamus and cerebrum. Futhermore, the DMS are also reciprocally interconnected. For instance, the cholinergic system and in particular, the orexinergic system, provide strong excitation of the LC neurons[2]. This can be hypothesized to help coordinate the different systems but makes it even harder to tease apart the functions of individual neurotransmitter systems. Theoretically, this redundancy is functionally beneficial as wakefulness can be maintained even if one of the arousing systems might be damaged.

DMS
Wakefulness
NREM sleep
REM Sleep
Norepinephrine
++
+
-
Serotonin
++
+
-
Dopamine
No difference
Acetylcholine
++
-
++
Histamine
++
+
-
Orexin/Hypocretin
++
-
-
MCH
-
+
++
VLPO/MNPO
-
++
++
Table 1 - (adapted from [2] España & Scamell, 2011). Firing rates of diffuse modulatory system neurons (DMS)
under wakefulness, NREM sleep and REM sleep as classified by neurotransmitter type/nuclei of origin.
(++) indicates greatest firing rate, (+) indicate reduced firing rate and (-) indicates absent firing rate.

External Links

Music to listen to while reading this page
Changes in Neurotransmitter Systems
Modern EEG Read
The physiology behind sleep & dreams
Wikipedia - Neuromodulation
Wikipedia - Sleep


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