Orexin-B
Also known as: Orexin B, Hypocretin-2, Hcrt-2, OXB, Hypocretin 2
Not approved as a drug anywhere; research peptide only. The small-molecule OX2R agonist oveporexton (TAK-861) was FDA-approved in August 2026 for narcolepsy type 1, but it is not Orexin-B.
5 notable studies · 7 references
TLDR
Orexin-B (hypocretin-2) is a 28-amino-acid hypothalamic neuropeptide made alongside Orexin-A that mainly activates the OX2 receptor, the branch of the orexin system most tied to stable wakefulness, but it has almost no direct administration research and has never been tested as a treatment in controlled human studies, even though the OX2R-targeting small molecule oveporexton was FDA-approved for narcolepsy type 1 in August 2026.
At a Glance
Classification: Endogenous 28-amino-acid linear hypothalamic neuropeptide; prepro-orexin (HCRT gene) product; orexin receptor 2 (OX2R)-preferring agonist neuropeptide
Orexin-B is the second of the two orexin neuropeptides, cut from the same precursor protein (prepro-orexin) as Orexin-A. It is also called hypocretin-2. It is a 28-amino-acid linear peptide with an amidated C-terminus. Unlike Orexin-A, it has no disulfide bridges, which makes it less stable. The key difference between the two peptides is receptor preference: • Orexin-A activates both OX1R and OX2R strongly. • Orexin-B activates OX2R about as well as Orexin-A does, but is roughly 10- to 100-fold weaker at OX1R. That matters because OX2R is the receptor most closely tied to stable wakefulness. Narcolepsy in dogs is caused by OX2R mutations, and OX2R-knockout mice show disrupted wakefulness. So Orexin-B sits right on the most clinically relevant branch of the orexin system. Even so, Orexin-B itself has very little direct research as an administered compound. Most of what is known comes from receptor pharmacology and genetic models, not from giving Orexin-B to animals or people.
What Is Orexin-B?
Orexin-B is an endogenous neuropeptide made by orexin neurons in the lateral hypothalamus. HCRT gene ↓ prepro-orexin ↓ Orexin-A (33 aa) + Orexin-B (28 aa). Both peptides are released from the same neurons. Orexin-B is studied far less than Orexin-A, partly because it is less stable and is harder to measure in cerebrospinal fluid. Clinical narcolepsy testing measures Orexin-A (hypocretin-1), not Orexin-B.
What Are Researchers Studying?
OX2R selectivity
Cell studies comparing the two peptides show that Orexin-B matches Orexin-A at OX2R but is much weaker at OX1R. That makes Orexin-B a useful lab tool for separating the roles of the two receptors.
Wakefulness and narcolepsy
The OX2R pathway that Orexin-B prefers is the one most strongly linked to narcolepsy in animal genetics. Canine narcolepsy comes from an OX2R gene mutation, and OX2R-knockout mice show disrupted wakefulness with milder cataplexy than mice lacking orexin entirely. This supports the receptor target. It does not show that giving Orexin-B treats narcolepsy.
Feeding
The 1998 discovery paper reported that both orexins increased food intake when injected into the rat brain. Later Orexin-B feeding studies in other species have given mixed results.
Peripheral tissues
Orexin receptors are also found outside the brain, including in the gut, adrenal gland and pituitary. Their roles there are less clearly defined.
How Does It Work?
Orexin-B ↓ preferentially activates OX2R (HCRTR2), a G protein-coupled receptor ↓ excitation of arousal-promoting neurons, notably histamine neurons of the tuberomammillary nucleus, which rely heavily on OX2R ↓ consolidation of wakefulness and regulation of transitions between wake and non-REM sleep. Genetic studies in mice suggest OX2R is critical for keeping wake and non-REM sleep separate. Losing all orexin signaling also causes severe REM-related problems such as cataplexy-like attacks, which involve OX1R as well. That receptor split is why drug developers targeted OX2R. Oveporexton (TAK-861), a small-molecule OX2R agonist, was FDA-approved in August 2026 for narcolepsy type 1. It is not Orexin-B, but it is aimed at the same receptor.
What Does the Research Show?
Human research
Biology: MODERATE (indirect) Human narcolepsy brains show a global loss of hypocretin peptides, which includes Orexin-B. Clinical testing relies on Orexin-A in cerebrospinal fluid. Administration of Orexin-B: NOT ESTABLISHED We found no published controlled study giving Orexin-B to humans.
Animal research
MODERATE (mostly indirect) The strongest animal evidence concerns its receptor: OX2R mutations cause canine narcolepsy, and OX2R-knockout mice have fragmented wakefulness. Direct Orexin-B administration studies exist (brain injections in rodents and livestock, mostly in feeding research) but are fewer and less consistent than for Orexin-A.
In-vitro research
MODERATE–HIGH Receptor-binding and signaling studies clearly define Orexin-B's OX2R preference.
Notable Studies
Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior
in-vitroJournal: Cell. Discovery of orexin-A and orexin-B and their two receptors. The paper reported the peptides' structures, hypothalamic expression and receptor pharmacology, and found that both orexins increased food intake after central administration in rats.
Limitations: Foundational characterization. The feeding data come from brain injection in rodents and do not translate directly to any human use.
View publication →The sleep disorder canine narcolepsy is caused by a mutation in the hypocretin (orexin) receptor 2 gene
animalJournal: Cell. Positional cloning in narcoleptic Doberman pinschers and Labrador retrievers identified disrupting mutations in the OX2R (Hcrtr2) gene as the cause of inherited canine narcolepsy.
Limitations: Concerns the receptor, not Orexin-B administration. Human narcolepsy is usually caused by loss of orexin neurons, not receptor mutations.
View publication →A mutation in a case of early onset narcolepsy and a generalized absence of hypocretin peptides in human narcoleptic brains
humanJournal: Nature Medicine. Post-mortem study of six narcolepsy brains plus genetic screening of 74 patients. Hypocretin (orexin) peptides and their mRNA were globally lost in narcolepsy brains, without signs of inflammation. One early-onset case carried a mutation in the hypocretin gene itself.
Limitations: Small post-mortem sample. Shows loss of the endogenous peptides, not any effect of giving Orexin-B.
View publication →Distinct recognition of OX1 and OX2 receptors by orexin peptides
in-vitroJournal: Journal of Pharmacology and Experimental Therapeutics. Compared orexin-A, orexin-B and modified orexin-A fragments at cloned OX1 and OX2 receptors. The work helps explain why orexin-B matches orexin-A at OX2R while having 10- to 100-fold lower affinity and potency at OX1R.
Limitations: Cell-based receptor pharmacology only.
View publication →Distinct narcolepsy syndromes in Orexin receptor-2 and Orexin null mice: molecular genetic dissection of Non-REM and REM sleep regulatory processes
animalJournal: Neuron. OX2R-knockout and orexin-knockout mice both showed similarly disrupted wakefulness, but OX2R-knockout mice had only mild cataplexy-like attacks. The authors concluded that wake/non-REM transitions depend critically on OX2R, while REM-sleep control involves more than OX2R.
Limitations: Genetic knockout model. It defines the receptor's role, not the effects of administered Orexin-B.
View publication →What We Still Don't Know
• Whether administered Orexin-B has any meaningful effect in humans; no controlled human administration studies were identified • Its stability and half-life after peripheral or intranasal administration • Whether it reaches the brain in useful amounts by any non-invasive route • The separate physiological role of endogenous Orexin-B versus Orexin-A • Reliable measurement of Orexin-B in human cerebrospinal fluid and blood • Long-term safety of OX2R stimulation by a peptide • The roles of peripheral orexin receptors. The literature on Orexin-B as an administered compound is thin. Most evidence on this page concerns its receptor (OX2R) or the orexin system as a whole.
Related Compounds
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Sources & References
- Sakurai T, Amemiya A, Ishii M, et al. Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior. Cell. 1998;92(4):573–585.
- de Lecea L, Kilduff TS, Peyron C, et al. The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity. Proc Natl Acad Sci U S A. 1998;95(1):322–327.
- Lin L, Faraco J, Li R, et al. The sleep disorder canine narcolepsy is caused by a mutation in the hypocretin (orexin) receptor 2 gene. Cell. 1999;98(3):365–376.
- Peyron C, Faraco J, Rogers W, et al. A mutation in a case of early onset narcolepsy and a generalized absence of hypocretin peptides in human narcoleptic brains. Nat Med. 2000;6(9):991–997.
- Ammoun S, Holmqvist T, Shariatmadari R, et al. Distinct recognition of OX1 and OX2 receptors by orexin peptides. J Pharmacol Exp Ther. 2003;305(2):507–514.
- Willie JT, Chemelli RM, Sinton CM, et al. Distinct narcolepsy syndromes in Orexin receptor-2 and Orexin null mice: molecular genetic dissection of Non-REM and REM sleep regulatory processes. Neuron. 2003;38(5):715–730.
- U.S. Food and Drug Administration. FDA Approves First Drug to Treat the Full Range of Narcolepsy Type 1 Symptoms (oveporexton). 2026.
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