Antinociceptive mechanisms of [D-Arg2]-dermorphin tripeptide analogs

These studies examined the antinociceptive effects and mechanisms of opioid activity of synthetic dermorphin tripeptide analogs containing D-Arg as the second amino acid, H-Tyr-D-Arg-Phe-NHCH3 and H-Tyr-D-Arg-Phe-N(CH3)2. Both tripeptides, administered i.c.v., i.t. and s.c. in mice, produced a potent and long-lasting antinociceptive activity as compared with morphine. In the guinea pig isolated ileum (GPI) assay, the IC50 value of both peptides was lower than that of morphine on the electrically induced contractions of the GPI. In radioligand binding studies with rat brain membrane synaptosomes, both tripeptides bound with high affinity to [3H]DAMGO-labeled mu-type sites, whereas they bound with no or negligible affinity to [3H]DPDPE-labeled delta sites and [3H]U-69593-labeled kappa sites. In the enzymatic degradation using high-performance liquid chromatography, both tripeptides showed good enzymatic stability after 25 hr of incubation with solubilized enzymes of mouse brain or spinal cord, in contrast to a rapid degradation of Met-enkephalin. The in vitro and in vivo pharmacological profile of [D-Arg2]-dermorphin tripeptide analogs demonstrates that they are potent and selective agonists at the mu opioid receptor. A high resistance of the tripeptides to enkephalin-degrading enzymes may largely contribute to their prolonged opioid activity.