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 (pK_i 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 (CA_i²⁺ 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 (pK_i: 8.47). Remarkably, replacement of Leu in 1 and in derivatives of 1 by Trp turned Y₄R agonism to antagonism, giving Y₄R antagonists with pK_i values ≤7.57.