Orexin (/ɒˈrɛksɪn/), also known as hypocretin, is a
neuropeptide that regulates
arousal,
wakefulness, and
appetite.[5] It exists in the forms of
orexin-A and
orexin-B. The most common form of
narcolepsy, type 1, in which the individual experiences brief losses of muscle tone ("drop attacks" or
cataplexy), is caused by a lack of orexin in the brain due to destruction of the cells that produce it.[6][7]
There are 50,000–80,000 orexin-producing
neurons in the human brain,[8] located predominantly in the perifornical area and
lateral hypothalamus.[5][9] They project widely throughout the central nervous system, regulating wakefulness, feeding, and other behaviours.[5] There are two types of orexin
peptide and two types of orexin
receptor.[10][9]
Orexin was discovered in 1998 almost simultaneously by two independent groups of researchers working on the
ratbrain.[11][12][13] One group named it orexin, from orexis, meaning "appetite" in Greek; the other group named it hypocretin, because it is produced in the hypothalamus and bears a weak resemblance to secretin, another
peptide.[6] Officially, hypocretin (HCRT) is used to refer to the
genes and
transcripts, while orexin is used to refer to the encoded
peptides.[14] There is considerable similarity between the orexin system in the rat brain and that in the human brain.[10]
Discovery
In 1998, reports of the discovery of orexin/hypocretin were published nearly simultaneously. Luis de Lecea,
Thomas Kilduff, and colleagues reported the discovery of the hypocretin system at the same time as Takeshi Sakurai from
Masashi Yanagisawa's lab at the
University of Texas Southwestern Medical Center at Dallas reported the discovery of the orexins to reflect the orexigenic (appetite-stimulating) activity of these peptides. In their 1998 paper describing these neuropeptides, they also reported discovery of two orexin receptors, dubbed OX1R and OX2R.[11] Masashi Yanagisawa and Emmanuel Mignot were awarded the Breakthrough Prize in 2022 for this discovery.
The two groups also took different approaches towards their discovery. One team was interested in finding new genes that were expressed in the hypothalamus. In 1996, scientists from the
Scripps Research Institute reported the discovery of several genes in the rat brain, including one they dubbed "clone 35." Their work showed that clone 35 expression was limited to the lateral hypothalamus.[15] They extracted selective DNA found in the lateral hypothalamus. They cloned this DNA and studied it using electron microscopy. Neurotransmitters found in this area were oddly similar to the gut hormone, secretin, a member of the
incretin family, so they named hypocretin to stand for a hypothalamic member of the incretin family.[16] These cells were first thought to reside and work only within the lateral hypothalamus area, but immunocytochemistry techniques revealed the various projections this area truly had to other parts of the brain. A majority of these projections reached the limbic system and structures associated with it (including the amygdala, septum, and basal forebrain area).
On the other hand, Sakurai and 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 the orexin peptides activated the cells expressing the orexin receptors and went on to find orexin peptide expression specifically in the hypothalamus. Additionally, when either orexin peptide was administered to rats it stimulated feeding, giving rise to the name 'orexin'.[11]
The nomenclature of the orexin/hypocretin system now recognizes the history of its discovery. "Hypocretin" refers to the gene or genetic products and "orexin" refers to the protein, reflecting the differing approaches that resulted in its discovery.[13] The use of both terms is also a practical necessity because "HCRT" is the standard gene symbol in databases like
GenBank and "OX" is used to refer to the pharmacology of the peptide system by the
International Union of Basic and Clinical Pharmacology.[14]
Isoforms
There are two types of orexin:
orexin-A and
orexin-B (hypocretin-1 and hypocretin-2).[17][18] They are excitatory
neuropeptides with approximately 50% sequence identity, produced by cleavage of a single precursor protein.[17] This precursor protein is known as prepro-orexin (or preprohypocretin) and is a 130
amino acidpre-pro-peptide encoded by the
geneHRCT and located on
chromosome 17 (17q21).[19] Orexin-A is 33 amino acid residues long and has two intrachain
disulfide bonds; orexin-B is a linear 28 amino acid residue peptide.[17] Although these peptides are produced by a very small population of cells in the lateral and posterior
hypothalamus, they send projections throughout the brain. The orexin peptides bind to the two
G-protein coupledorexin receptors,
OX1 and
OX2, with orexin-A binding to both OX1 and OX2 with approximately equal affinity while orexin-B binds mainly to OX2 and is 5 times less potent at OX1.[20][21]
The orexins are strongly conserved peptides, found in all major classes of vertebrates.[22]
The orexin system was initially suggested to be primarily involved in the stimulation of food intake, based on the finding that central administration of orexin-A and -B increased food intake. In addition, it stimulates wakefulness, regulates
energy expenditure, and modulates visceral function. The orexin system has been hypothesized function by exciting other neurons that produce neurotransmitters (such as the locus coeruleus), as well as by inhibiting neurons in the ventrolateral preoptic nucleus, which is a region of the brain whose neuronal activity is imperative to proper sleep function.[23]
Brown fat activation
Many studies support that the orexin neurons regulate
brown adipose tissue (BAT) activity via the sympathetic nervous system to enhance energy expenditure.[24][25]
Although orexin
knockout mice were reported to show maldevelopment of
brown adipose tissue (BAT),[26] subsequent report has shown normal development of BAT.[27]
Wakefulness
Orexin seems to promote
wakefulness. Studies indicate that a major role of the orexin system is to integrate metabolic, circadian and
sleep debt influences to determine whether an animal should be asleep, or awake and active. Orexin neurons strongly excite various brain nuclei with important roles in wakefulness including the
dopamine,
norepinephrine,
histamine and
acetylcholine systems[28][29] and appear to play an important role in stabilizing wakefulness and sleep.
The discovery that an orexin receptor mutation causes the
sleep disorder canine
narcolepsy[30] in
Doberman Pinschers subsequently indicated a major role for this system in
sleep regulation. Genetic knockout mice lacking the gene for orexin were also reported to exhibit narcolepsy.[31] Transitioning frequently and rapidly between sleep and wakefulness, these mice display many of the symptoms of narcolepsy. Researchers are using this animal model of narcolepsy to study the disease.[32] Narcolepsy results in
excessive daytime sleepiness, inability to consolidate wakefulness in the day (and sleep at night), and
cataplexy, which is the loss of muscle tone in response to strong, usually positive, emotions. Dogs that lack a functional receptor for orexin have narcolepsy, while animals and people lacking the orexin neuropeptide itself also have narcolepsy. Organisms with narcolepsy were also found to experience REM sleep at any time of day, suggesting an alteration of function of REM sleep which can lead to hypnagogic hallucinations (hallucinations that occur as an organism goes to sleep).[33]
Central administration of orexin-A strongly promotes wakefulness, increases body temperature and locomotion, and elicits a strong increase in energy expenditure.
Sleep deprivation also increases orexin-A transmission. The orexin system may thus be more important in the regulation of energy expenditure than in the regulation of food intake. In fact, orexin-deficient people with narcolepsy have increased obesity rather than decreased
BMI, as would be expected if orexin were primarily an appetite stimulating peptide. Another indication that deficits of orexin cause narcolepsy is that depriving monkeys of sleep for 30–36 hours and then injecting them with the neurochemical alleviates the cognitive deficiencies normally seen with such amount of sleep loss.[34][35]
In humans, narcolepsy is associated with a specific variant of the
human leukocyte antigen (HLA) complex.[36] Furthermore, genome-wide analysis shows that, in addition to the HLA variant, people with narcolepsy also exhibit a specific genetic
mutation in the
T-cellreceptor alpha locus.[37] In conjunction, these genetic anomalies cause the immune system to attack and kill the critical orexin neurons. Hence the absence of orexin-producing
neurons in people with narcolepsy may be the result of an
autoimmune disorder.[38]
Food intake
Orexin increases the craving for food, and correlates with the function of the substances that promote its production. Orexin is also shown to increase meal size by suppressing inhibitory postingestive feedback.[39] However, some studies suggest that the stimulatory effects of orexin on feeding may be due to general arousal without necessarily increasing overall food intake.[40]
Review findings suggest that
hyperglycemia that occurs in mice due to a habitual high-fat diet leads to a reduction in signalling by orexin receptor-2, and that orexin receptors may be a future therapeutic target.[41]Leptin is a hormone produced by fat cells and acts as a long-term internal measure of energy state.
Ghrelin is a short-term factor secreted by the stomach just before an expected meal, and strongly promotes food intake. Orexin-producing cells have been shown to be inhibited by leptin (through the leptin receptor pathway), but are activated by ghrelin and
hypoglycemia (
glucose inhibits orexin production). Orexin, as of 2007, is claimed to be a very important link between metabolism and sleep regulation.[42][43] Such a relationship has been long suspected, based on the observation that long-term sleep deprivation in rodents dramatically increases food intake and energy metabolism, i.e.,
catabolism, with lethal consequences on a long-term basis. Sleep deprivation then leads to a lack of energy. In order to make up for this lack of energy, many people use high-carbohydrate and high-fat foods that ultimately can lead to poor health and weight gain. Other dietary nutrients, amino acids, also can activate orexin neurons, and they can suppress the glucose response of orexin neurons at physiological concentration, causing the energy balance that orexin maintains to be thrown off its normal cycle.[44]
Addiction
Preliminary research shows potential for orexin blockers in the treatment of cocaine, opioid, and
alcohol addiction.[45][46][47] For example,
lab rats given drugs which targeted the orexin system lost interest in alcohol despite being given free access in experiments.[48][49] Wild type mice that were treated with morphine were found to be at a higher risk of developing addiction when compared to mice that did not produce orexin.[50]
Studies of orexin involvement in nicotine addiction have had mixed results. For example, blocking the orexin-1 receptor with the selective
orexin antagonistSB-334,867 reduced nicotine
self-administration in rats and that smokers who sustained damage to the
insula, a brain region that regulates cravings and contains orexin-1 receptors, lost the desire to smoke.[51] However, other studies in rats using the dual orexin receptor antagonist TCS 1102 have not found similar effects.[52]
Lipid metabolism
Orexin-A (OXA) has been demonstrated to have a direct effect on an aspect of
lipid metabolism. OXA stimulates
glucose uptake in 3T3-L1
adipocytes and that increased energy uptake is stored as lipids (
triacylglycerol). OXA thus increases
lipogenesis. It also inhibits
lipolysis and stimulates the secretion of
adiponectin. These effects are thought to be mostly conferred via the
PI3K pathway because this pathway inhibitor (LY294002) completely blocks OXA effects in adipocytes.[53] The link between OXA and lipid metabolism is currently being studied.
Mood
High levels of orexin-A have been associated with happiness in human subjects, while low levels have been associated with sadness.[54] The finding suggests that boosting levels of orexin-A could elevate mood in humans, being thus a possible future treatment for disorders like depression. Orexins have also been hypothesized to aid in the development of resilience to the stress response, as their activity in the
ventral pallidum was found to decrease depressive symptoms by activating GABAergic neurons at that site.[55]
It has been observed that orexin, while implicated in addiction and depression, is also involved in the display of anhedonia in ADHD. Proper functioning of orexin has been shown to have a large degree of control over behaviors that are motivated by a need to survive, such as searching for food when an organism is starving. When orexin does not function as intended, it impairs an organism's ability to feel pleasure from strongly motivated actions.[56]
Orexinergic neurons can be differentiated into two groups based on connectivity and functionality. Orexinergic neurons in the
lateral hypothalamic group are closely associated with reward related functions, such as
conditioned place preference. These neurons preferentially innervate the
ventral tegmental area and the
ventromedial prefrontal cortex. The neurons found in the ventral tegmental area, the ventromedial prefrontal cortex, and the nucleus accumbens shell are strongly implicated in addiction and the sensitization of neurons to stimulating drugs (such as amphetamines). Orexin producing neurons in these areas have been found to be primarily indicated in seeking behavior when externally stimulated by environmental signals such as stress.[72] These neurons are in contrast to the lateral hypothalamic neurons, the perifornical-dorsal group of orexinergic neurons are involved in functions related to arousal and autonomic response. These neurons project inter-hypothalamically, as well as to the brainstem, where the release of orexin modulates various autonomic processes.[73][74]
Orexin system dysfunction
Orexin/hypocretin system dysfunction might be associated with a variety of disorders and medical conditions.[75][76]
Takotsubo syndrome
Orexin/hypocretin system dysfunction has been proposed as a novel pathophysiological model of
Takotsubo syndrome (acute failure syndrome).[77]
ESSENCE
ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations) is an umbrella term covering a wide range of neurodevelopmental disorders and difficulties (ADHD, developmental coordination disorder, autism spectrum disorder) as well as ESSENCE-associated conditions (behavioural phenotype syndromes, some neurological conditions and disorders, and severe early-onset mental disorders). Orexin/hypocretin system dysfunction might be associated with many symptoms in a variety of ESSENCE.[78]
Clinical uses
The orexin/hypocretin system is the target of the insomnia medication
suvorexant (Belsomra), which works by blocking both orexin receptors.[79] Suvorexant has undergone three phase III trials and was approved in 2014 by the
US Food and Drug Administration (FDA) after being denied approval the year before.[80] The other FDA-approved
orexin antagonists are
lemborexant (Dayvigo)[81] and
daridorexant (Quviviq).[82]
Intranasal orexin is able to increase cognition in primates, especially under sleep deprived situations,[85] which may provide an opportunity for the treatment of excessive daytime sleepiness.[86]
A study has reported that transplantation of orexin neurons into the pontine
reticular formation in rats is feasible, indicating the development of alternative therapeutic strategies in addition to pharmacological interventions to treat narcolepsy.[87]
Orexins are also thought to have potential implications in learning and aiding in fending off diseases such as dementia and other disorders that impair cognition.[72]
^Wong KK, Ng SY, Lee LT, Ng HK, Chow BK (April 2011). "Orexins and their receptors from fish to mammals: a comparative approach". General and Comparative Endocrinology. 171 (2): 124–130.
doi:
10.1016/j.ygcen.2011.01.001.
PMID21216246.
^Klein J, Sato A (September 2000). "The HLA system. Second of two parts". The New England Journal of Medicine. 343 (11): 782–786.
doi:
10.1056/NEJM200009143431106.
PMID10984567.
^Ida T, Nakahara K, Katayama T, Murakami N, Nakazato M (March 1999). "Effect of lateral cerebroventricular injection of the appetite-stimulating neuropeptide, orexin and neuropeptide Y, on the various behavioral activities of rats". Brain Research. 821 (2): 526–529.
doi:
10.1016/S0006-8993(99)01131-2.
PMID10064841.
S2CID39775146.
^Thompson MD, Xhaard H, Sakurai T, Rainero I, Kukkonen JP (2014).
"OX1 and OX2 orexin/hypocretin receptor pharmacogenetics". Frontiers in Neuroscience. 8: 57.
doi:10.3389/fnins.2014.00057.
PMC4018553.
PMID24834023. OX1–CB1 dimerization was suggested to strongly potentiate orexin receptor signaling, but a likely explanation for the signal potentiation is, instead, offered by the ability of OX1 receptor signaling to produce 2-arachidonoyl glycerol, a CB1 receptor ligand, and a subsequent co-signaling of the receptors (Haj-Dahmane and Shen, 2005; Turunen et al., 2012; Jäntti et al., 2013). However, this does not preclude dimerization.
^Jäntti MH, Mandrika I, Kukkonen JP (March 2014). "Human orexin/hypocretin receptors form constitutive homo- and heteromeric complexes with each other and with human CB1 cannabinoid receptors". Biochemical and Biophysical Research Communications. 445 (2): 486–490.
doi:
10.1016/j.bbrc.2014.02.026.
PMID24530395. Orexin receptor subtypes readily formed homo- and hetero(di)mers, as suggested by significant BRET signals. CB1 receptors formed homodimers, and they also heterodimerized with both orexin receptors. ... In conclusion, orexin receptors have a significant propensity to make homo- and heterodi-/oligomeric complexes. However, it is unclear whether this affects their signaling. As orexin receptors efficiently signal via endocannabinoid production to CB1 receptors, dimerization could be an effective way of forming signal complexes with optimal cannabinoid concentrations available for cannabinoid receptors.
^Yamanaka A, Muraki Y, Ichiki K, Tsujino N, Kilduff TS, Goto K, Sakurai T (July 2006). "Orexin neurons are directly and indirectly regulated by catecholamines in a complex manner". Journal of Neurophysiology. 96 (1): 284–298.
CiteSeerX10.1.1.320.260.
doi:
10.1152/jn.01361.2005.
PMID16611835.
^Ohno K, Sakurai T (January 2008). "Orexin neuronal circuitry: role in the regulation of sleep and wakefulness". Frontiers in Neuroendocrinology. 29 (1): 70–87.
doi:
10.1016/j.yfrne.2007.08.001.
PMID17910982.
S2CID7888110.
^Grimaldi D, Silvani A, Benarroch EE, Cortelli P (January 2014). "Orexin/hypocretin system and autonomic control: new insights and clinical correlations". Neurology. 82 (3): 271–278.
doi:
10.1212/WNL.0000000000000045.
PMID24363130.
S2CID9209963.
^López JM, Sanz-Morello B, González A (November 2014). "Organization of the orexin/hypocretin system in the brain of two basal actinopterygian fishes, the cladistians Polypterus senegalus and Erpetoichthys calabaricus". Peptides. 61: 23–37.
doi:
10.1016/j.peptides.2014.08.011.
PMID25169954.
S2CID34333729.