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Desensitization of the μ‐opioid activation of phospholipase C in SH‐SY5Y cells: the role of protein kinases C and A and Ca 2+ ‐activated K + currents
Author(s) -
Smart D.,
Lambert D.G.
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb17222.x
Subject(s) - desensitization (medicine) , phospholipase c , protein kinase c , kinase , chemistry , microbiology and biotechnology , signal transduction , endocrinology , biology , biochemistry , receptor
1 . In SH‐SY5Y cells, μ‐opioids cause a rapidly desensitizing activation of phospholipase C (PLC), that appears secondary to Ca 2+ influx via L‐type voltage‐sensitive Ca 2+ channels (VSCCs). The aim of the present study was to characterize the mechanisms of desensitization of the μ‐opioid‐induced inositol (1,4,5) triphosphate (Ins(1,4,5)P 3 ) response, by use of a stereospecific radioreceptor mass assay. 2 . (R +)‐Bay K 8644 (1 n m ‐10μ m ) dose‐dependently inhibited fentanyl‐induced Ins(1,4,5)P 3 formation, with an IC 50 of 28.5 n m , confirming our earlier observations that μ‐opioids open L‐type VSCCs, thus allowing Ca 2+ influx to activate PLC. 3 . Ro 31–8220 (0.1 n m ‐10μm), a protein kinase C inhibitor, dose‐dependently enhanced fentanyl‐induced Ins(1,4,5)P 3 formation (EC 50 = 20.0 n m ), whilst acute phorbol 12,13‐dibutrate (1 μ m ) abolished the response. 4 . H‐89 (1 n m ‐10 μm), a protein kinase A inhibitor, also dose‐dependently enhanced fentanyl‐induced Ins(1,4,5)P 3 formation (EC 50 = 93 n m ), whilst dibutryl cyclic AMP (0.5 μ m ) abolished the response. 5 . lockade of Ca 2+ ‐activated K + currents with 4‐aminopyridine (2 μ m ) or iberiotoxin (10 n m ) had no effect on fentanyl‐induced Ins(1,4,5)P 3 formation but further increased the Ro 31–8220‐enhanced response. 6 . All three mechanisms had additive, or even supra‐additive, effects, but only at later (120–300 s) time points. In addition, fentanyl‐induced Ins(1,4,5)P 3 formation, even if enhanced by H‐89, Ro 31–8220 and/ or 4‐aminopyridine, was inhibited by nifedipine (1 n m ‐10 μ m ). 7 . In conclusion, desensitization of the μ‐opioid‐induced activation of PLC is multifactorial, involving protein kinases C and A and Ca 2+ ‐activated K + efflux, but the L‐type VSCC is of critical importance and may be a possible common site of action.