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Orexin receptors are involved in many processes including energy homeostasis, wake/sleep cycle, metabolism, and reward. Development of potent and selective ligands is an essential step for defining the mechanism(s) underlying such critical processes. The goal of this study was to further investigate the structure-activity relationships of these peptides and to identify the truncated form of the orexin peptides active at OX1. Truncation studies have led to OXA (17-33) as the shortest active peptide known to date with a 23 fold selectivity for OX1 over OX2. Alanine, D-amino acid, and proline scans have highlighted the particular importance of Tyr(17), Leu(20), Asn(25), and His(26) for agonist properties of OXA(17-33). The conformation of the C-terminus might also be a defining factor in agonist activity and selectivity of the orexin peptides for the OX1 receptor
