Opiate receptors on lymphocytes and platelets in man

Receptors for opiates, opiate-like substances, and their antagonists, such as naloxone (a close chemical and conformational congener of morphine), on brain cell homogenates and neuroblastoma X glioma hybrid cells in tissue culture have been reported. The present study on the binding of [3H]naloxone to lymphocytes and platelets freshly isolated from the peripheral blood of 39 healthy adult human volunteers showed that (1) [3H]naloxone bound to lymphocytes and platelets at 4 degrees C, reaching equilibrium in 30 min, and was not removed by washing (three times) with the suspending medium; (2) the binding of [3H]naloxone to cells decreased in the presence of increasing amounts of unlabeled naloxone, approaching a plateau; (3) significant amounts of the radioligand remained bound in the presence of micromolar quantities of the unlabeled ligand; and (4) in the absence of Na+ ions, 1 to 10 nmol of morphine hydrochloride for 10(6) lymphocytes, and 1 to 25 nmol of morphine hydrochloride for 10(8) platelets, decreased the binding of [3H]naloxone by 43 to 57%. It is concluded that at least some of the [3H]naloxone binding sites on human lymphocytes and platelets are specific opioid receptor sites of the mu type (Enkephalins define the delta sites.) The observations on the binding of naloxone to cells do not appear to be artifacts. Opioid receptor sites on lymphocyte and platelet membranes may have properties similar to those on nerve cell membranes.