Orexins and the treatment of obesity.

Orexin-A and -B are two peptides derived by proteolytic cleavage from a 130-amino acid precursor, prepro-orexin, which were recently isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by two amino acids. These peptides bind to two Gq-coupled receptors, termed orexin-1 and orexin-2. The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at orexin-1 and orexin-2 receptors, whilst orexin-B displays moderate (∼10 fold) selectivity for orexin-2 receptors. The distribution and pharmacology of the orexin peptides and their receptors indicate that they play a role in various regulatory systems including energy homeostasis and the regulation of feeding, the evidence for which is reviewed here.