Orexin-B (Hypocretin-2)

Orexin-B (also known as hypocretin-2) is a 28-amino acid neuropeptide produced alongside orexin-A from the same prepro-orexin precursor in lateral hypothalamic neurons. While both orexins were co-discovered in 1998, orexin-B has received comparatively less individual attention than orexin-A, though it has distinct receptor binding properties and physiological significance. Orexin-B acts selectively at the OX2 receptor with approximately equal affinity to orexin-A, but has significantly lower affinity for the OX1 receptor.

The OX2 receptor selectivity of orexin-B is particularly relevant because the OX2 receptor appears to be more important than OX1R for maintaining wakefulness. Studies in receptor knockout mice showed that OX2R-deficient mice exhibit narcolepsy-like symptoms, while OX1R knockout mice show relatively mild sleep phenotypes. This suggests that the wake-promoting effects of the orexin system are primarily mediated through OX2R, and by extension, that orexin-B signaling at OX2R is a critical component of arousal maintenance.

Orexin-B differs from orexin-A structurally in several important ways. It lacks the two disulfide bonds present in orexin-A, making it a linear peptide rather than a constrained structure. It also lacks the N-terminal pyroglutamate residue of orexin-A. These structural differences give orexin-B a shorter half-life than orexin-A, as it is more susceptible to enzymatic degradation. Despite this, orexin-B is functionally important in the regulation of sleep-wake states, feeding behavior, and energy metabolism.

Research on orexin-B has contributed to the understanding of narcolepsy pathophysiology and has informed the development of OX2R-selective agonists for narcolepsy treatment. The selectivity of orexin-B for OX2R provides a template for designing molecules that promote wakefulness without the broader effects mediated through OX1R, which include appetite stimulation, reward processing, and autonomic activation.