Orexin A

Overview

Orexin A (also known as hypocretin-1) is a 33-amino acid neuropeptide discovered in 1998 by Sakurai and colleagues. Expressed exclusively by approximately 10,000 neurons in the lateral hypothalamus, orexin A plays a critical role in stabilizing wakefulness, regulating energy homeostasis, and modulating reward circuits. Loss of orexin-producing neurons causes narcolepsy type 1, a disorder characterized by excessive daytime sleepiness and cataplexy.

Structure and Biosynthesis

Orexin A is derived from a 131-amino acid precursor (prepro-orexin) that is cleaved to yield two mature peptides: orexin A (33 residues) and orexin B (28 residues). Orexin A contains two intramolecular disulfide bonds (Cys6-Cys12 and Cys7-Cys17), creating a compact tertiary structure that confers resistance to proteolytic degradation. The peptide lacks homology to other neuropeptides and is uniquely conserved across vertebrate species.

Receptors and Signaling

Orexin A binds with high affinity to the orexin-1 receptor (OX1R, formerly HCRTR1) and with lower affinity to the orexin-2 receptor (OX2R, formerly HCRTR2). Both are Gq-coupled receptors that activate phospholipase C, elevate intracellular calcium, and stimulate MAPK signaling. OX1R is preferentially expressed in limbic regions, while OX2R predominates in the hypothalamus and brainstem arousal centers. The differential receptor distribution accounts for the distinct roles of each receptor subtype.

Physiological Functions

Orexin A promotes and stabilizes wakefulness through activation of the tuberomammillary nucleus (histamine), locus coeruleus (norepinephrine), dorsal raphe (serotonin), and pedunculopontine tegmentum (acetylcholine). The peptide stimulates appetite and food intake, increases energy expenditure, and enhances sympathetic tone. Orexin A also modulates reward processing, reinforcing the drive to eat palatable foods and influencing drug-seeking behavior.

Clinical Significance

Autoimmune destruction of orexin neurons causes narcolepsy type 1, affecting approximately 0.02-0.05% of the population. The dual orexin receptor antagonist (DORA) suvorexant was the first approved therapy targeting the orexin system for insomnia. Selective OX1R antagonists (e.g., garenoxant) are under investigation for substance use disorders. Orexin-based therapies may also benefit conditions involving excessive sleepiness, such as shift work disorder and obstructive sleep apnea.