Antinociceptive potencies of beta-casomorphin analogs as compared to their affinities towards mu and delta opiate receptor sites in brain and periphery.
beta-Casomorphins and their analogs were tested for their opioid activities in the myenteric plexus longitudinal muscle preparation of the guinea pig ileum ( GPI), the isolated mouse vas deferens (MVD), and for their affinities to mu- delta- and kappa- binding sites in rat brain membranes. C-terminal amidation of beta-casomorphin-4 and (-5) increased opioid potency in both organ preparations ( GPI, MVD) and affinity to mu-binding sites in brain whereas binding to delta-sites was diminished. These beta-casomorphin-amides displayed a 2-3 times greater naloxone reversible antinociceptive effect than natural beta-casomorphins. Introduction of D-alanine at position 2 in the beta-casomorphin-amides increased potency in the GPI whereas activity in the MVD was only slightly changed. These compounds, however, showed a remarkable increase in binding to delta-sites in brain with an unaffected or slightly increased binding to mu-sites and decreased binding to kappa-sites. D-Ala2-beta-casomorphin-4 and (-5) amides were 10 times more potent antinociceptive agents than corresponding beta-casomorphin-amides. These results suggest firstly, that peripheral delta-receptors in the MVD are not as closely related to delta-binding sites at rat brain membranes as is the case with mu-receptors in the GPI and mu-binding sites, and secondly, in addition to mu-receptors, delta-receptors may be of importance in mediating antinociception.[1]References
- Antinociceptive potencies of beta-casomorphin analogs as compared to their affinities towards mu and delta opiate receptor sites in brain and periphery. Brantl, V., Pfeiffer, A., Herz, A., Henschen, A., Lottspeich, F. Peptides (1982) [Pubmed]
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