Chronic opioid treatment of the mouse thymoma cell lines R1.G1 and R1EGO leads to down-regulation of the kappa opioid receptor without desensitization of adenylyl cyclase activity

Abstract

Kappa opioid agonists alter some immune functions of macrophages, and T- and B-lymphocytes. The mouse thymoma cell lines R1.G1 and R1EGO express only κ-opioid receptors and these κ-opioid receptors are coupled to an inhibitory GTP-binding regulatory protein. Binding of κ-opioid agonists to the opioid receptor leads to the inhibition of adenylyl cyclase activity in these cells. In this study, an acute (15 min) and chronic (24 h) treatment of R1.G1 and R1EGO cell with a potent κ-opioid agonist (−)U50,488 (100 nM) was studied to determine if a κ-opioid agonist altered receptor number and/or desensitization of adenylyl cyclase activity in these two cell lines. Chronic treatment of both R1.G1 and R1EGO cells with (−)U50,488 lead to down-regulation of the κ-opioid receptor, measured as a decrease of approximately 50% in the B max value for the binding of [ 3H]U69,593. The binding affinity ( K d value) was not affected after chronic treatment either in R1.G1 or R1EGO cells. There was no difference in the magnitude of inhibition of adenylyl cyclase activity by (−)U50,488 between the acute (15 min) and chronic (24-h) treatment in both cell lines R1.G1 and R1EGO. This study indicates that chronic opioid treatment of mouse thymoma R1.G1 and R1EGO cell lines leads to down-regulation of the receptor, without desensitization. This phenomenon was observed in R1.1 parent mouse thymoma cell line and recently in CHO cells expressing κ-opioid receptor. This study demonstrates that unlike some neuronal preparations, chronic opioid treatment of the thymoma cell lines resulted in receptor down-regulation without desensitization.