ACTH1-24 counteracts the prolactin-releasing effect of an opioid.

The administration of the synthetic and stable opioid peptide[D-Ala2,MePhe4,Met(O)5o1]enkephalin (FK 33-824) at the dose of 0.2 mg/kg i.v. induced a rise in plasma levels of prolactin in the rat, an effect which was prevented by 1 mg/kg i.v. of the opiate antagonist naloxone. A simultaneous i.v. injection of 20 microgram/kg i.v. of ACTH1-24 significantly reduced the prolactin releasing effect of FK 33-824, therefore giving further evidence of an in vivo interaction between ACTH-related peptides and opioid peptides.