Human placental opioid peptides: correlation to the route of delivery.

The levels of opioid peptides in placental extracts were determined by means of a radioreceptor assay with two prototypical opioid ligands, 3H-ethylketocyclazocine and 3H-D-ala2-enkephalinamide. Opioid peptide levels were significantly higher in the placentas obtained by the vaginal route than in those by the abdominal route. The different peptides in the extract were detected by reverse phase high-performance liquid chromatography and a radioreceptor assay for all fractions. The presence of dynorphin-like peptides in several fractions suggests that one or more could be the "natural" ligand of the opioid receptors in the human placenta. A role for these peptides in parturition is a viable possibility.