Evie Summers
Orexin, also known as hypocretin, is a peptide produced by neurons in the lateral hypothalamus. It is a key modulator of the sleep-wake cycle, promoting wakefulness and inhibiting sleep. Orexin activates orexin neurons, monoaminergic and cholinergic neurons in the hypothalamus/brainstem regions, maintaining a long, consolidated awake period. The role of orexin in regulating the sleep-wake cycle was first discovered in relation to narcolepsy, a sleep disorder characterized by a primary disorganization of sleep/wakefulness cycles. Orexin deficiency is associated with narcolepsy, and people with the condition experience fragmented sleep and excessive daytime sleepiness.
| Characteristics | Values |
|---|---|
| Role in sleep-wake cycle | Orexin is a key modulator of the sleep-wake cycle and stimulates the wake-promoting systems. |
| Orexin neurons | Orexin-containing neurons regulate wakefulness and inhibit sleep. |
| Narcolepsy | Loss of orexin produces narcolepsy. |
| Alzheimer's disease | Orexin is dysregulated in Alzheimer's patients, and increased orexin in the cerebrospinal fluid is associated with decreased sleep efficiency and REM sleep. |
| Homeostasis | Orexin regulates homeostasis by controlling feeding and energy balance. |
| Insomnia | Possible mechanisms of insomnia are diminished GABA and elevated orexin levels. |
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What You'll Learn
Orexin is a key modulator of the sleep-wake cycle
Orexin, also known as hypocretin, is a key modulator of the sleep-wake cycle. It is a peptide produced by neurons in the lateral hypothalamus, which projects to both cortical and subcortical targets. Orexin is one of the wake-promoting systems that inhibit the sleep-promoting systems, with the balance between the two determining whether one is awake or asleep.
Orexin-producing neurons project their axons throughout the brain, which suggests that their functions are varied. These neurons are observed in the serotonergic dorsal raphe nucleus (DR), noradrenergic locus coeruleus (LC), and histaminergic tuberomammillary nucleus (TMN). All of these nuclei are involved in promoting arousal. Orexin neurons are also regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose concentration.
The role of orexin in regulating the sleep-wake cycle was first discovered in relation to narcolepsy. People with narcolepsy experience chronic, sometimes severe, daytime sleepiness, sleep attacks, and state instability. Studies have shown that 90% of people with narcolepsy have orexin deficiency, with reduced orexin neurons and decreased orexin peptide levels in the cerebrospinal fluid.
Orexin neurons contribute to arousal maintenance through the indirect inhibition of sleep-promoting neurons of the ventrolateral preoptic nucleus. Activation of this subcortical circuit rapidly drives wakefulness from sleep by modulating the activity of ventrolateral preoptic neurons. This results in increased awake time and decreased REM and non-REM sleep time.
Orexin has also been found to be dysregulated in Alzheimer's disease (AD) patients, with increased orexin in the cerebrospinal fluid associated with decreased sleep efficiency and REM sleep, as well as cognitive impairment. A better understanding of the mechanisms linking the orexin system, sleep disturbance, and AD could make orexin receptor antagonists a promising target for the prevention or treatment of the disease.
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Orexin neurons inhibit sleep to promote arousal
Orexin, also known as hypocretin, is a peptide produced by neurons in the lateral hypothalamus. It is a key modulator of the sleep-wake cycle and plays a critical role in regulating sleep and wakefulness. Orexin-producing neurons project their axons throughout the brain, influencing various homeostatic functions.
Orexin neurons inhibit sleep by promoting arousal and consolidating wakefulness. They achieve this by silencing sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO). Specifically, orexin neurons increase the activity of aminergic arousal regions, including the locus coeruleus (LC) and raphe nuclei, leading to increased wakefulness and suppressed REM sleep.
The role of orexin in regulating the sleep-wake cycle was first discovered in relation to narcolepsy. People with narcolepsy experience chronic daytime sleepiness and sudden sleep attacks. Research has found that narcoleptic patients have reduced numbers of orexin neurons and decreased levels of orexin peptides in the cerebrospinal fluid.
Orexin knockout mice, orexin receptor knockout mice, and orexin neuron-ablated transgenic mice all exhibit fragmented sleep cycles and increased sleep fragmentation, further highlighting the importance of orexin in regulating sleep and wakefulness.
Additionally, orexin has been implicated in Alzheimer's disease (AD). Increased levels of orexin in the cerebrospinal fluid of AD patients are associated with decreased sleep efficiency, reduced REM sleep, and cognitive impairment. A better understanding of the link between the orexin system, sleep disturbance, and AD may lead to the development of orexin receptor antagonists as a potential treatment for the disease.
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Orexin is implicated in the stimulation of the wake-promoting systems
Orexin, also known as hypocretin, is a peptide produced by neurons in the lateral hypothalamus. It is implicated in the stimulation of the wake-promoting systems and the stabilisation of the sleep-wake cycle. Orexin-producing neurons project their axons throughout the brain, suggesting that their functions are varied.
Orexin is a key modulator of the sleep-wake cycle and has been found to be dysregulated in patients with Alzheimer's disease. Increased levels of orexin in the cerebrospinal fluid are associated with decreased sleep efficiency and REM sleep, as well as cognitive impairment in these patients. The orexin system has profuse projections to brain regions that are implicated in arousal and cognition and has been found to participate in the progression of Alzheimer's disease pathology.
Orexin-containing neurons regulate wakefulness, and loss of orexin produces narcolepsy. In fact, most people with narcolepsy (90%) are orexin-deficient. Their sleep-wake cycle is destabilized such that, although they sleep the same amount of time as others, sleep happens in short bouts throughout the day and wakefulness occurs more than usual.
Orexin neurons drive arousal by silencing sleep-promoting neurons in the ventrolateral preoptic nucleus. They also have a strong, direct, excitatory effect on cholinergic neurons of the basal forebrain, which is hypothesized to play an important role in arousal. Cholinergic neurons of the pedunculopontine tegmental nucleus (PPN) and the laterodorsal tegmental nucleus (LDT) play a pivotal role in the regulation of REM sleep and wakefulness. These areas are also strongly innervated by orexin neurons.
Orexin increases the activity of aminergic arousal regions, including the locus coeruleus (LC) and raphe nuclei, which then increase wakefulness and suppress REM sleep.
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Orexin regulates sleep and arousal
Orexin, also known as hypocretin, is a peptide produced by neurons in the lateral hypothalamus. It is a key modulator of the sleep-wake cycle, promoting wakefulness and inhibiting sleep. Orexin-producing neurons project their axons throughout the brain, activating orexin neurons, monoaminergic and cholinergic neurons in the hypothalamus/brainstem regions, and maintaining a long, consolidated awake period.
The role of orexin in regulating the sleep-wake cycle was discovered through its relationship to narcolepsy. Narcolepsy is a sleep disorder characterized by a primary disorganization of sleep/wakefulness cycles. People with narcolepsy experience excessive daytime sleepiness, sleep attacks, and state instability. Research has found that 90% of people with narcolepsy have a deficiency in orexin, with decreased levels of orexin peptide in the cerebrospinal fluid.
Orexin neurons drive arousal by silencing sleep-promoting neurons in the ventrolateral preoptic nucleus. They increase the activity of aminergic arousal regions, including the locus coeruleus (LC) and raphe nuclei, which then increase wakefulness and suppress REM sleep. Orexin also has a strong, direct, excitatory effect on cholinergic neurons of the basal forebrain, which is hypothesized to play a role in arousal.
Orexin's role in regulating sleep and arousal has been demonstrated in studies using ICV injection of orexin A or orexin B. During the light period, increased levels of orexin led to more time spent awake and less time in REM and non-REM sleep. Similarly, microinjection of orexin A into the laterodorsal tegmental nucleus (LDT) in cats increased their awake time and decreased their REM sleep time.
Orexin is also involved in the coordination of emotion, energy homeostasis, the reward system, and arousal. For example, orexin neurons are regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose concentration.
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Orexin is a peptide produced by neurons in the lateral hypothalamus
Orexin, also known as hypocretin, is a peptide produced by neurons in the lateral hypothalamus. These orexin-producing neurons are also referred to as orexin neurons. Orexin neurons have been identified as key modulators of the sleep/wakefulness cycle. They play a critical role in regulating sleep and wakefulness, promoting arousal and inhibiting sleep.
Orexin neurons project their axons throughout the brain, influencing various homeostatic functions. They are observed in several nuclei that are involved in promoting arousal, including the serotonergic dorsal raphe nucleus (DR), noradrenergic locus coeruleus (LC), and histaminergic tuberomammillary nucleus (TMN). These nuclei increase wakefulness and suppress REM sleep. The activation of orexin neurons leads to increased activity in these arousal regions, stabilising wakefulness and consolidating it into sustained episodes.
The role of orexin in regulating the sleep-wake cycle was discovered through its association with narcolepsy. Narcolepsy is a sleep disorder characterised by a disorganised sleep/wakefulness cycle, with patients experiencing excessive daytime sleepiness, sleep attacks, and state instability. Studies have found that individuals with narcolepsy have reduced numbers of orexin neurons and decreased levels of orexin peptides in the cerebrospinal fluid.
Orexin neurons contribute to the maintenance of arousal by inhibiting sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO). They achieve this through indirect inhibition, possibly involving synaptic GABA signaling. By inhibiting sleep-promoting neurons, orexin neurons stabilise wakefulness and prevent the onset of sleep. This mechanism highlights the role of orexin in the regulation of the sleep-wake cycle, as the balance between orexin and sleep-promoting systems determines an individual's state of wakefulness or sleep.
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Frequently asked questions
The sleep-wake cycle is a highly regulated system that is driven by two underlying processes: the homeostatic, which regulates the amount of sleep, and the circadian, which regulates the timing of sleep.
Orexin is a key modulator of the sleep-wake cycle. It is a peptide produced by neurons in the lateral hypothalamus. Orexin inhibits sleep by activating orexin neurons, monoaminergic and cholinergic neurons in the hypothalamus/brainstem regions, to maintain a long, consolidated awake period.
Insomnia may be caused by elevated orexin levels. On the other hand, narcolepsy is associated with diminished orexin levels.
