Endomorphin 1[ψ] and endomorphin 2[ψ], endomorphins analogues containing a reduced (CH2NH) amide bond between Tyr1 and Pro2, display partial agonist potency but significant antinociception

Endomorphin 1 (EM1) and endomorphin 2 (EM2) are highly potent and selective μ-opioid receptor agonists and have significant antinociceptive action. In the μ-selective pocket of endomorphins (EMs), Pro2 residue is a spacer and directs the Tyr1 and Trp3/Phe3 side chains into the required orientation. The present work was designed to substitute the peptide bond between Tyr1 and Pro2 of EMs with a reduced (CH2NH) bond and study the agonist potency and antinociception of EM1[ψ] (Tyr[ψ(CH2NH)]Pro-Trp-Phe-NH2) and EM2[ψ] (Tyr[ψ(CH2NH)]Pro-Phe-Phe-NH2). Both EM1[ψ] and EM2[ψ] are partial μ opioid receptor agonists showing significant loss of agonist potency in GPI assay. However, EMs[ψ] exhibited potent supraspinal antinociceptive action in vivo. In the mice tail-flick test, EMs[ψ] (1, 5, 10 nmol/mouse, i.c.v.) produced potent and short-lasting antinociception in a dose-dependent and naloxone (1 mg/kg) reversed manner. At the highest dose of 10 nmol, the effect of EM2[ψ] was prolonged and more significant than that of EM2. In the rat model of formalin injection induced inflammatory pain, EMs[ψ] (0.1, 1, 10 nmol/rat, i.c.v.), like EMs, exerted transient but not dose-dependent antinociception. These results suggested that in the μ-selective pocket of EMs, the rigid conformation induced by the peptide bond between Tyr1 and Pro2 is essential to regulate their agonist properties at the μ opioid receptors. However, the increased conformational flexibility induced by the reduced (CH2NH) bond made less influence on their antinociception.