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

Human neuroblastoma SH-SY5Y cells express endogenous mu-opioid receptor and develop cellular tolerance to morphine after prolonged (greater than or equal to 4 h) treatment with morphine. Treatment with forskolin (25 mu M, 12 h), an adenylyl cyclase activator, also desensitized mu-opioid receptor response to morphine (10 mu M) by 38% (P < 0.001), which was reversed by the cyclic AMP (cAMP) dependent kinase inhibitor N-(2-aminoethyl)-5-isoquinolinesulfonamide (H8) (100 mu M). Treatment with both morphine and forskolin appeared to cause an additive effect in desensitizing mu-opioid receptor. In mu-opioid receptor stably transfected human embryonic kidney 293 (HEK-mu) cells, morphine treatment produced cAMP upregulation, yet failed to induce mu-opioid receptor tolerance. However, treatment with forskolin (25 mu M) or 8-bromo-cAMP (1 mM) led to profound mu-opioid receptor tolerance, which was reversed by H8. These results demonstrate that cAMP-dependent kinase activation causes mu-opioid receptor tolerance. However, morphine-induced mu-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 mu-opioid receptor tolerance. (C) 2000 Elsevier Science B.V. All rights reserved.