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Abstract
Human placental opioid receptors were assayed using the radioactive opioid agonist, etorphine, to determine the number of binding sites in villous tissue membrane preparations. Significant differences in receptor concentration per milligram of protein of tissue were found between placentas obtained following vaginal or abdominal delivery (P < 0.002). Labor itself did not alter apparent receptor numbers. In patients with maternal narcotic abuse during pregnancy, no opioid binding could be detected regardless of the mode of delivery, suggesting possible receptor down-regulation.