Oligopeptides as Neuropeptide Y Y₄ Receptor Ligands: Identification of a High-Affinity Tetrapeptide Agonist and a Hexapeptide Antagonist

Within the family of neuropeptide Y (NPY) receptors, the Y4 receptor (Y4R) is unique as it prefers pancreatic polypeptide over NPY and peptide YY. Today, low-molecular-weight Y4R ligands are lacking, in particular antagonists. We synthesized a series of peptidic NPY Y4R ligands, derived from the hexapeptide acetyl-Arg-Tyr-Arg-Leu-Arg-Tyr-NH2 (1), reported to be a Y4R partial agonist with high affinity (pKi Y4R: 8.43). Peptide 1 was N-terminally extended as well as truncated and subjected to a d-amino acid scan, and Leu was replaced by different amino acids. Compounds were characterized by radioligand competition binding and functional studies (Cai2+ mobilization and β-arrestin 1/2 recruitment). N-terminal truncation of 1 resulted in a tetrapeptide (Arg-Leu-Arg-Tyr-NH2), being a Y4R partial agonist with unchanged Y4R affinity (pKi: 8.47). Remarkably, replacement of Leu in 1 and in derivatives of 1 by Trp turned Y4R agonism to antagonism, giving Y4R antagonists with pKi values ≤7.57.