[Kappa-type opioid receptor in human placental membrane]

Since many opioid receptor preparations are heterogeneous systems containing multiple types of receptor, characterization of each type of the receptor is influenced by contamination with other types of receptor. Demonstrating kappa receptor proved more difficult owing to the ability of kappa ligands to interact with a number of receptor classes and to the lack of the homogeneous preparation. It has been reported that kappa ligands selectively bind to human placental membrane. To establish homogeneous kappa receptor preparation, the kappa binding to human placenta was characterized. The portion which was predominantly villus tissue was removed from freshly collected placenta, and the P3-membrane fraction was prepared. Kappa opioid agonist, such as 3H-Dynorphin A, 3H-ethylketocyclazocine and 3H-U-69593, bound to the human placental membrane fraction in a manner of single class of binding site (Bmax: approximately 40 fmol.mg-1 protein). In contrast, 3H-dihydromorphine, 3H-DAGO (mu agonist), 3H-DADLE (delta agonist) or 3H-SKF-10047 (sigma agonist) showed no binding activity. The specific binding of 3H-diprenorphine was displaced only by kappa-selective ligand (U-50488H, dynorphin, butorphanol). These results suggest that opioid binding in human placental membrane is specific for kappa opioid.