Index
There are only 10,000-20,000 orexin-producing neurons in the human brain, predominantly located in the perifornical area and in the lateral hypothalamus.
They project widely throughout the central nervous system , regulating wakefulness , eating, and other behaviors.
There are two types of orexin peptide and two types of orexin receptor.
The terms orexin and hypocretin are synonymous, it is a neuropeptide that regulates arousal, wakefulness and appetite.
The most common form of narcolepsy , in which the patient experiences brief loss of muscle tone (cataplexy), is caused by a lack of orexin in the brain due to the destruction of the cells that produce it.
Hypocretins (orexins) are recently described as hypothalamic neuropeptides that are believed to have an important role in the regulation of sleep and arousal states. Its discovery was reported independently by two groups using different techniques.
Orexin was discovered in 1998 around the same time by two independent groups of researchers working on the rat brain.
One group called it orexin, from orexis, which means “appetite” in Greek; the other group called it hypocretin, because it is produced in the hypothalamus and bears a weak resemblance to secretin, another peptide.
Lecea et al. identified the pro-hormone pre-prohypocretin and its peptide products hypocretin-1 (Hcrt-1) and hypocretin-2 (Hcrt-2), by nucleotide sequencing.
The discovery of orexins, orexin-A (Orx-A) and orexin-B (Orx-B), was reported almost simultaneously by Sakurai et al. who used the orphan receptor cloning technique.
The use of both terms is now a practical necessity, as hypocretin is used to refer to gene products and orexin is used to refer to protein products.
There is a great affinity between the orexin system in the rat brain and that of the human brain.
The finding that the cerebrospinal fluid (CSF) levels of these peptides were abnormal in narcolepsy patients has stimulated research on the potential role of these peptides in human disease.
Discovery
In 1998, reports of the orexin / hypocretin discovery were published almost simultaneously.
Luis de Lecea, Thomas Kilduff and their colleagues reported the discovery of the hypocretin system at the same time as Takeshi Sakurai from Masashi Yanagisawa’s laboratory at the University of Texas Southwestern Medical Center in Dallas.
He reported the discovery of orexins to reflect the orexigenic (appetite stimulating) activity of these peptides.
The two groups also took different approaches to their discovery. One team was interested in finding new genes that were expressed in the hypothalamus.
In 1996, scientists at the Scripps Research Institute reported the discovery of several genes in the rat brain, including one they called “clone 35.”
They extracted selective DNA found in the lateral hypothalamus. They cloned this DNA and studied it using electron microscopy.
The neurotransmitters found in this area were strangely similar to the gut hormone secretin, a member of the incretin family, which is why they named hypocretin to represent a hypothalamic member of the incretin family.
At first, these cells were thought to reside and function only within the lateral hypothalamus area, but immunocytochemistry tactics revealed the various projections this area actually had with other parts of the brain.
Most of these projections reached the limbic system and its associated structures (including the amygdala, septum, and basal forebrain area).
On the other hand, Sakurai and his colleagues were studying the orexin system as orphan receptors. To this end, they used transgenic cell lines that expressed individual orphan receptors and then exposed them to different potential ligands.
They found that orexin peptides activated cells expressing orexin receptors and went on to find orexin peptide expression specifically in the hypothalamus.
Furthermore, when the peptide orexin was administered to rats, it stimulated feeding, giving rise to the name ‘orexin’.
The orexin / hypocretin system nomenclature now acknowledges the history of its discovery. “Hypocretin” refers to the gene (s) and “orexin” refers to the protein, reflecting the different approaches that led to their discovery.
The use of both terms is also a practical necessity because “HCRT” is the standard genetic symbol in databases such as GenBank and “OX” is used to refer to the pharmacology of the peptide system by the International Union of Basic and Clinical Pharmacology.
Isoforms
There are two types of orexin: orexin-A and orexin-B (hypocretin-1 and -2). They are excitatory neuropeptides with approximately 50% sequence identity, produced by the cleavage of a single precursor protein.
orexin-A is 33 amino acids long and has two intrachain disulfide bonds; orexin-B is a linear peptide of 28 amino acids.
Orexins are strongly conserved peptides, found in all major classes of vertebrates.
Neurochemical actions of hypocretins
Hypocretins are believed to act primarily as excitatory neurotransmitters.
Systemic and intracerebroventricular administration of hypocretins directly stimulates cells of the noradrenergic locus coeruleus (LC) system in rats and monkeys
Suggesting a role for hypocretins in various central nervous functions related to noradrenergic innervation, including vigilance, attention, learning, and memory.
Its actions on the neurotransmission of serotonin, histamine, acetylcholine, and dopamine are thought to be excitatory and a facilitating role in gamma-aminobutyric acid (GABA) and glutamate-mediated neurotransmission is suggested.
In particular, intravenous administration of orexin-A in rats produces a differential release of gamma-aminobutyric acid and glutamate in the hypocretin-dense amygdala compared to the cerebellum, suggesting that modulation of these neurotransmitters is dependent on innervation of the hypocretin.
Function
Initially it was suggested that the orexin system was primarily involved in stimulating food intake, based on the finding that central administration of orexin-A and orexin-B increased food intake.
In addition, it stimulates wakefulness, regulates energy expenditure and modulates visceral function. In addition to their primary role in controlling sleep and arousal, hypocretins have been implicated in multiple functions, including:
- Food and energy regulation.
- Neuroendocrine regulation.
- Control of the gastrointestinal system.
- Control of the cardiovascular system.
- Regulation of water balance.
- Pain modulation.
A role in behavior is also postulated. The cell bodies responsible for the synthesis of hypocretin are located in the tuberal part of the hypothalamus, the so-called feeding center.
The observation that orexin-A increases the metabolic rate and the demonstration that insulin-induced hypoglycemia activates up to one third of hypocretin-containing neurons has led to the suggestion that hypocretins are mediators of energy metabolism.
The neuroendocrine effects of hypocretins include a decrease in plasma prolactin and growth hormone and an increase in levels of corticotropin and cortisol, insulin, and luteinizing hormone.
Central administration of hypocretins increases water intake, stimulates gastric acid secretion, and increases intestinal motility. Hypocretins increase mean arterial pressure and heart rate.
The localization of long descending axonal projections containing hypocretin at all levels of the spinal cord suggests a role in the modulation of sensation and pain.
The strong innervation of the caudal region of the sacral cord suggests a role in the regulation of sympathetic and parasympathetic functions.
Activation of brown fat
Obesity in orexin knockout mice is the result of the inability of brown preadipocytes to differentiate into brown adipose tissue (BAT), which in turn reduces the thermogenesis of brown adipose tissue.
Differentiation of brown adipose tissue can be restored in these knockout mice by injections of orexin. Orexin deficiency has also been linked to narcolepsy, a sleep disorder.
Also, narcoleptic people are more likely to be obese. Therefore, obesity in narcoleptic patients may be due to an orexin deficiency leading to impaired thermogenesis and energy expenditure.
Wakefulness
Orexin appears to promote wakefulness. The discovery that an orexin receptor mutation causes the canine narcolepsy sleep disorder in Doberman Pinschers subsequently indicated an important role for this system in regulating sleep.
Genetically inactivated mice lacking the orexin gene have also been reported to have narcolepsy.
In fact, narcoleptic patients with orexin deficiency have increased obesity rather than decreased Body Mass Index, as would be expected if orexin were primarily an appetite-stimulating peptide.
Another indication that orexin deficits cause narcolepsy is that depriving monkeys of sleep for 30-36 hours and then injecting them with the neurochemical alleviates the cognitive deficits normally seen with such an amount of sleep loss.
Furthermore, genome-wide analysis shows that, in addition to the human leukocyte antigen variant, narcoleptic humans also exhibit a specific gene mutation at the T-cell alpha receptor locus.
Hypocretin in narcolepsy
Narcolepsy is a primary alertness disorder with an estimated prevalence of 0.03-0.05%. It can develop at any age, but the peak of onset is in adolescence with a secondary peak in the fourth decade.
The initial symptom is usually excessive daytime sleepiness, with irresistible attacks of sleep during the day.
Other symptoms of this syndrome are cataplexy, brief episodes of muscle weakness or paralysis precipitated by strong emotions, such as laughter or surprise.
Sleep paralysis, which is a symptom due to persistent atony of rapid eye movement (REM) sleep on awakening and hypnogogic hallucinations or dream images, which characteristically occur at the onset of sleep.
Short periods of automatic behavior can also occur, a reflection of brief intrusions of sleep (“micro-dreams”) in the drowsy state.
Hypocretin studies in narcolepsy
In 1999, Lin et al. demonstrated a mutation in the hypocretin receptor 2 gene in canine narcolepsy.
The subsequent finding that mice lacking hypocretin receptors show behavioral arrests similar to the symptoms of narcolepsy (eg, cataplexy) direct transitions from wakefulness to REM sleep.
Disturbance of preceding gait and rocking activity during behavioral arrest episodes led to a recognition of the potential importance of hypocretins in sleep, arousal, and arousal.
Animal models of narcolepsy show some variability in the defect that causes the narcolepsy-like syndrome.
In the murine model, the interruption of both types of hypocretin receptor pathways, orexin-A and orexin-B, is necessary to produce narcoleptic findings, whereas in the canine model of narcolepsy the predominant defect is in the orexin receptor. B.
Intravenous administration of orexin-A to narcoleptic dogs (Dobermans) reduces cataplexy and normalizes sleep and the duration of wakefulness.
Food intake
Orexin increases the desire to eat and correlates with the function of the substances that promote its production. Orexin also increases meal size by suppressing postingestive inhibitory feedback.
However, some studies suggest that the stimulating effects of orexin in diet may be due to general arousal without necessarily increasing total food intake.
The findings of the review suggest that hyperglycemia that occurs in mice due to a habitual high-fat diet leads to a reduction in orexin receptor 2 signaling, and that orexin receptors may be a future therapeutic target.
Lack of sleep leads to a lack of energy.
To make up for this lack of energy, many people eat high-carbohydrate, high-fat foods that can ultimately lead to poor health and weight gain.
Other dietary nutrients, amino acids, can also activate orexin neurons, and can suppress the glucose response of orexin neurons at physiological concentrations, causing the energy balance that orexin maintains outside of its normal cycle.
Addiction
Studies on the implication of orexin in nicotine addiction have had mixed results.
For example, blocking the orexin-1 receptor with the selective orexin antagonist SB-334.867 reduced nicotine self-administration in rats and smokers who suffered damage to the insula, a region of the brain that regulates cravings and contains orexin receptors. -1, lost the desire to smoke.
However, other studies in rats using the dual orexin receptor antagonist TCS 1102 have not found similar effects.
lipid metabolism
Recently, orexin-A (OXA) has been shown to have a direct effect on one aspect of lipid metabolism. OXA stimulates glucose absorption in 3T3-L1 adipocytes and that increased energy absorption is stored as lipids (triacylglycerol). OXA increases lipogenesis.
It also inhibits lipolysis and stimulates adiponectin secretion. These effects are believed to be confirmed primarily through the PI3K pathway because this pathway inhibitor (LY294002) completely blocks the effects of OXA on adipocytes.
The link between OXA and lipid metabolism is new and is currently under further investigation.
Obesity in orexin knockout mice is associated with impaired brown adipose tissue thermogenesis.
Mood
High levels of orexin-A have been associated with happiness in human subjects, while low levels have been associated with sadness.
The finding suggests that increased orexin-A levels could elevate mood in humans, making it a possible future treatment for disorders such as depression.
Hypocretin in cerebrospinal fluid
There have been several studies of hypocretin in human cerebrospinal fluid. The work published to date has tested the presence of orexin-A only and not orexin-B. cerebrospinal fluid orexin-A levels in healthy adults are within a narrow range (250-280 pg / ml).
A recent study did not indicate significant differences in hypocretin levels with respect to gender or age, and concluded that very low or undetectable hypocretin levels in cerebrospinal fluid are an abnormal finding at any age.
The initial study by Nishino et al. found that 7 of 9 patients with narcolepsy-cataplexy had undetectable levels of hypocretin in the cerebrospinal fluid. Of the 2 patients with detectable hypocretin, one was within the control range and the other had elevated levels.
Both patients were indistinguishable from the other narcolepsy patients. The authors suggested that these patients might have a hypocretin receptor defect rather than a hypocretin production deficiency.
Ripley et al. have reported undetectable levels of hypocretin in the cerebrospinal fluid of 32 of 36 patients evaluated. In the remaining 4, hypocretin levels were below the control range.
There have been two studies examining postmortem hypocretin cells in the brains of narcolepsy patients. They both found a surprising reduction, to around 10% of the normal number of hypocretin neurons, in narcoleptic brains.
In the initial study there was cell loss without gliosis or signs of inflammation. However, in the other study there was evidence of gliosis in the hypocretin cell region, implying that a degenerative process was the cause of hypocretin cell loss in narcolepsy.
Another support for the degenerative hypothesis is their finding of a greater number of astrocytes in the hypothalamus of narcoleptic patients than in controls.
The absence of hypocretin neurons can be explained by mechanisms that include neurodegeneration, developmental failure, reduced synthesis or release of hypocretins, or some mutation in the DNA sequence encoding hypocretin (although only 1 of the 74 narcoleptic patients examined showed a mutation) .
Orexin neurons
Neurotransmitters
Orexinergic neurons have been shown to be sensitive to:
- Group III metabotropic glutamate receptor entries.
- Cannabinoid receptors 1.
- Heterodimers of the CB1-OX1 receptor.
- Adenosine A1 receptors.
- M3 muscarinic receptors.
- Serotonin 5-HT1A receptors.
- Neuropeptide Y receptors.
- Cholecystokinin A receptors.
- Catecholamines.
As well as ghrelin, leptin and glucose. The retinergic neurons themselves regulate the release of acetylcholine, serotonin, and norepinephrine.
Orexinergic neurons can be differentiated into two groups based on connectivity and functionality.
Orexinergic neurons in the lateral hypothalamic group are closely related to reward-related functions, such as conditioned place preference.
These neurons preferentially innervate the ventral tegmental area and the ventromedial prefrontal cortex. In contrast to the lateral hypothalamic neurons, the perifornian dorsal group of orexinergic neurons are involved in functions related to arousal and autonomic response.
These neurons project between the hypothalamus, as well as the brain stem, where orexin release modulates various autonomous processes.
Clinical uses
The orexin / hypocretin system is the target of the insomnia drug sumorexant, which works by blocking both orexin receptors.
Suvorexant has undergone three phase III trials and was approved in 2014 by the U.S. Food and Drug Administration (FDA) after being denied approval the previous year. It is marketed as Belsomra.
In 2016, the University of Texas Health Sciences Center registered a clinical trial for the use of suvorexant in people with cocaine dependence. They plan to measure the reactivity of anxiety and stress.
Other potential uses
Intranasal orexin is capable of increasing cognition in primates, especially in sleep deprivation situations, which may provide an opportunity for the treatment of excessive daytime sleepiness.
Hypocretin in neurological and psychiatric disorders
The role of hypocretin in other neurological diseases has not yet been established.
A recent study found that hypocretin levels in cerebrospinal fluid did not differ significantly between two groups, one with neuroimmune disease and the other with non-neuroimmune disease and normal controls.
In a subgroup analysis, the researchers found that 4 of 10 patients with Guillain-Barré syndrome had significantly lower levels of orexin-A than controls.
Another study has shown low levels of hypocretin in the cerebrospinal fluid in patients with subarachnoid hemorrhage, acoustic schwannoma and head trauma, perhaps explained by damage and / or dysfunction of the hypothalamus.
The dense hypocretin projections to the noradrenergic, serotonergic, dopaminergic, cholinergic, and gamma-aminobutyric acid / glutamate areas of the brain suggest a possible role in psychiatric and neuropsychiatric disorders.
The hypocretin system can be important in affective disorders, such as major depression and bipolar affective disorder.
The monoamine hypothesis (biogenic amine hypothesis) of depression suggests that dysfunctional or deficient neurotransmission of norepinephrine and / or serotonin underlies the symptoms of depression.
More recently, the emphasis has shifted to the possible roles of neuropeptides in the etiology and treatment of depression. The involvement of the hypocretin system in depression is suggested for neuroanatomical and pharmacological reasons.
The only substance known to innervate all relevant brain areas involved in the neurobiology of depression is hypocretin and excitatory innervation of the locus coeruleus and the dorsal raphe region, stimulation of dopamine and acetylcholine, and prohistaminergic actions point to an antidepressant effect.
These therapeutic possibilities remain to be clarified by appropriate studies.
