Basal phosphorylation of μ opioid receptor is agonist modulated and Ca2+-dependent

The μ opioid receptor was shown to be phosphorylated at a basal rate in the absence of agonist, measured in permeabilized HEK293 cells transfected with an epitope tagged μ receptor (EE-μ) [Arden, J., Segredo, V., Wang, Z., Lameh, J. and Sadee, W. (1995) J. Neurochem. 65, 1636–1645]. In the present study, basal phosphorylation was found to be Ca2+ dependent; however, several inhibitors of protein kinase C and Ca2+-cahnodulin dependent kinases failed to affect basal μ receptor phosphorylation. Thus, the basal μ receptor phosphorylating activity differed from the main kinases involved in receptor regulation. The general kinase inhibitor H7 (100 μM) suppressed basal μ receptor phosphorylation. Pretreatment with the agonist morphine, followed by drug removal, resulted in a sustained increase of basal It receptor phosphorylation. The gradual agonist dependent modulation of basal μ receptor phosphorylation suggests a novel regulatory mechanism which may play a role in narcotic tolerance and dependence.