Effect of Phospholipases on Chronic Opiate Action in Neuroblastoma × Glioma NG108‐15 Hybrid Cells

Abstract

Chronic treatment of neuroblastoma × glioma NG108‐15 hybrid cells with opiate agonist resulted in loss of the acute opiate inhibition of adenylate cyclase activity with a concomitant increase in the enzymatic activity observable on addition of the antagonist naloxone. The role of membrane lipids in the cellular expression of these chronic opiate effects was investigated by the hydrolysis of phospho‐lipids with various lipases. Treatment with phospholipase C from Clostridium welchii produced an enzyme concentration‐dependent decrease of prostaglandin E1‐stimulated adenylate cyclase activity in control or etorphine‐treated (1 μM for 4 h) hybrid cells. In addition, incubation of hybrid cells with phospholipase C concentrations of >0.5 U/ml completely abolished the compensatory increase in adenylate cyclase activity after chronic opiate treatment. This attenuation of the increase in adenylate cyclase activity by phospholipase C could be prevented by inclusion of phosphatidylcholine but not of phosphatidic acid during the enzymatic incubations. The specificity of the phospholipids involved in expression of the chronic opiate effect could be demonstrated further by the absence of effect exhibited by phospholipase C from Bacillus cereus and phospholipase D. Hydrolysis of the acyl side chains of phospholipids with phospholipase A2 did not alter the chronic opiate effect after removal of lysophosphatides with bovine serum albumin. Because the guanylylimidodiphosphate‐ and NaF‐sensitive adenylate cyclase activities were not affected by these phospholipase treatments, the expression of the compensatory increase in adenylate cyclase activity is mediated via an increase in the coupling between hormonal receptor and adenylate cyclase with the participation of the polar head groups of the phospholipids and not the hydrophobic side chains.