The orexin receptors, also known as the hypocretin receptors, are a type of G-protein-coupled receptors that bind the neuropeptide orexin. Like other neuropeptide receptors, the orexin receptors have 7-transmembrane domains. The orexin peptides bind selectively to the OX1 and OX2 receptors, which are strongly conserved in mammals, with 94% identity in the amino acid sequences between rats and humans. However, despite the structural similarities to other neuropeptide receptors, neither OX1R nor OX2R has any significant affinity for neuropeptide Y, secretin, or similar peptides. The orexin receptors are produced in the hypothalamus and share similarities with the incretin peptide family. It has become clear that although the orexin peptides have only modest effects on feeding and appetite, their effects on arousal and sleep are profound. In fact, narcolepsy is one of the most common causes of sleepiness due to the loss of orexin-producing neurons, and this has led to a strong interest in the development of orexin antagonists as a new method for promoting sleep and treating insomnia.
Fig.1 Orexin signaling mechanisms. (Scammell, 2011)
OX1R (HCRTR1) and OX2R (HCRTR2) mRNA is widely expressed in the brain in a pattern similar to that of orexin nerve terminals. Most studies focus on the brain, but small amounts of orexins and their receptors can be produced elsewhere. The testes contain moderate amounts of prepro-orexin mRNA, but the receptor expression appears to be low. Orexin and OX2R mRNA are detected in the adrenal cortex. Orexin-producing neurons have also been described in the submucosal and myenteric plexuses of the stomach and small intestine.
Here show two of the orexin receptors, OX1R, and OX2R, also known as HCRTR1 and HCRTR2, which are encoded by different genes (HCRTR1 and HCRTR2).
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