Fragments of the N terminal part of adrenocorticotrophic hormone (ACTH) inhibited the electrically evoked contractions of the mouse vas deferens. This inhibition could be antagonized by naloxone.
The same fragments displaced radiolabelled morphine from morphine antiserum.
Structure‐activity relationship studies showed that in both assay systems the active core is located within the sequence ACTH 7–10.
It is postulated that the Trp9 residue and the peptide bond between Trp9 and Gly10 are particularly important for interaction of ACTH fragments with morphine receptors.
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