Potentiation of opiate action in neuroblastoma N18TG2 cells by lipid incorporation

The effect of cerebroside sulfate, phosphatidylserine, and other phospholipids on opiate receptor function in neuroblastoma N18TG2 cells was studied by incorporation of lipids into the membrane bilayer of viable cells. A concentration- and time-dependent incorporation of sulfatide by N18TG2 cells was observed. The incorporated lipid was not metabolized during the incubation period of up to 48 hr at 37° . Optimal conditions for lipid incorporation were determined to be 4 days after the cell seeding and in 1% fetal calf serum. The incorporated lipid was established to be associated with the plasma membrane fraction of the crude cell homogenate. Furthermore, increases in V(max) but not K(m) values of the adenylate cyclase for Mg²⁺, ATP, and prostaglandin E₁ were observed in neuroblastoma N18TG2 cells exposed to cerebroside sulfate for 4-6 hr. The incorporation of cerebroside sulfate or phosphatidylserine by N18TG2 cells did not increase the number of opiate binding sites in this cell line as determined by [³H]naloxone, [³H]etorphine, or ³H-labeled D-Ala²-Met⁵-enkephalinamide binding. Although there was an increase in the affinity of [³H]naloxone binding, linear correlation between the amount of cerebroside sulfate incorporated and the quantity of binding increase was not observed. However, augmentation of both the potencies and the efficacies (maximal inhibitory level) of morphine and enkephalin to regulate adenylate cyclase activity was observed after sulfatide incorporation. At the maximal concentration of cerebroside sulfate used (67 μM) the opiate receptor activity in N18TG2 cells approached that of NG108-15 cells. Identical treatment of N18TG2 cells with cerebroside or psychosine sulfate did not produce any potentiation of the opiate inhibition of adenylate cyclase. Of all of the phospholipids tested - phosphatidylserine, phosphatidylinositol, and phosphatidylcholine - only phosphatidylcholine produced a potentiation of the opiate effect. Both synthetic dipalmitoyl phosphatidylcholine or brain phosphatidylcholine could elicit the potentiation.