Synthesis and biological activity of nociceptin/orphanin FQ(1–13)NH2 analogues modified in 9 and/or 13 position

The purpose of the present study was the synthesis and the biological screening of new analogues of N/OFQ(1–13)NH2, the minimal sequence maintaining the same activity as the natural peptide nociceptin. In order to investigate the role of Lys, we substituted Lys at positions 9 and/or 13 by Orn, Dab (diaminobutanoic acid) or Dap (diaminopropanoic acid). The new N/OFQ(1–13)NH2 analogues exerted strong and naloxone-resistant inhibition of electrically evoked contractions of rat vas deferens. Lys replacement with Orn maintained or even enhanced the inhibitory activity, while replacements with Dab and Dap decreased inhibitory activity.