Characterization of human placental opioid receptors by 3H-ethylketocyclazocine and 3H-naloxone binding.

The human placenta contains membrane bound opioid receptors. The binding of the antagonist naloxone and the fairly kappa-selective agonist EKC were studied in the microvillous membrane fraction. In both cases high affinity binding sites were detected with Kd values in the nanomolar range. A series of kappa-selective ligand (PD-117302, EKC, and U-50, 488H) were tested in displacement experiments, and found to be potent inhibitors of agonist and antagonist binding. It was confirmed that large percentage of the binding is associated with the kappa sub-type which is of current interest, in that it shows distinctive pharmacology and distribution and is selective for the natural opioid polypeptide, dynorphin.