Characteristics of opioid receptors in guinea-pig cerebellum and human placenta.

Characteristics of opioid receptors were examined in guinea-pig cerebellum and human placental membrane preparations. Two binding sites for [3H]-ethylketocyclazocine ([3H]-EKCZ) were found in guinea-pig cerebellum membranes whereas only one binding site was detected in human placental membranes. The potencies of kappa opioid agonists were decreased in competing with the binding of [3H]-naloxone to guinea-pig cerebellum membranes or human placental membranes by 5'-guanylylimidodiphosphate (Gpp(NH)P) and sodium. These results suggest that there may be true kappa receptors in human placenta and that kappa receptors in both preparations may couple with the adenylate cyclase system.