Tolerance to morphine at the μ-opioid receptor differentially induced by cAMP-dependent protein kinase activation and morphine

Abstract

Human neuroblastoma SH-SY5Y cells express endogenous μ-opioid receptor and develop cellular tolerance to morphine after prolonged (≥4 h) treatment with morphine. Treatment with forskolin (25 μM, 12 h), an adenylyl cyclase activator, also desensitized μ-opioid receptor response to morphine (10 μM) by 38% ( P<0.001), which was reversed by the cyclic AMP (cAMP) dependent kinase inhibitor N-(2-aminoethyl)-5-isoquinolinesulfonamide (H8) (100 μM). Treatment with both morphine and forskolin appeared to cause an additive effect in desensitizing μ-opioid receptor. In μ-opioid receptor stably transfected human embryonic kidney 293 (HEK-μ) cells, morphine treatment produced cAMP upregulation, yet failed to induce μ-opioid receptor tolerance. However, treatment with forskolin (25 μM) or 8-bromo-cAMP (1mM) led to profound μ-opioid receptor tolerance, which was reversed by H8. These results demonstrate that cAMP-dependent kinase activation causes μ-opioid receptor tolerance. However, morphine-induced μ-opioid receptor tolerance in SH-SY5Y cells is not mediated by cAMP-dependent kinase activation. In addition, our results indicate that cAMP-upregulation does not necessarily lead to μ-opioid receptor tolerance.