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μ‐Opioid Receptor Stimulation of Inositol (1,4,5)Trisphosphate Formation via a Pertussis Toxin‐Sensitive G Protein
Author(s) -
Smart D.,
Smith G.,
Lambert D. G.
Publication year - 1994
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1994.62031009.x
Subject(s) - naltrindole , pertussis toxin , g protein , phospholipase c , endocrinology , medicine , (+) naloxone , opioid receptor , chemistry , stimulation , inositol , receptor , opioid , pharmacology , biology
The cellular mechanisms underlying opioid action remain to be fully determined, although there is now growing indirect evidence that some opioid receptors may be coupled to phospholipase C. Using SH‐SY5Y human neuroblastoma cells (expressing both μ‐and δ‐opioid receptors), we demonstrated that fentanyl, a μ‐preferring opioid, caused a dose‐dependent (EC 50 = 16 n M ) monophasic increase in inositol (1,4,5)trisphosphate mass formation that peaked at 15 s and returned to basal within 1–2 min. This response was of similar magnitude (25.4 ± 0.8 pmol/mg of protein for 0.1 μ M fentanyl) to that found in the plateau phase (5 min) following stimulation with 1 m M carbachol (18.3 ± 1.4 pmol/mg of protein), and was naloxone‐, but not naltrindole‐(a δ antagonist), reversible. Further studies using [ d ‐Ala 2 , MePhe 4 , Gly(ol) 5 ]enkephalin and [ d ‐Pen 2,5 ]enkephalin confirmed that the response was specific for the μ receptor. Incubation with Ni 2+ (2.5 m M ) or in Ca 2+ ‐free buffer abolished the response, as did pretreatment (100 ng/ml for 24 h) with pertussis toxin (control plus 0.1 μ M fentanyl, 26.9 ± 1.5 pmol/mg of protein; pertussis‐treated plus 0.1 μ M fentanyl, 5.1 ± 1.3 pmol/mg of protein). In summary, we have demonstrated a μ‐opioid receptor‐mediated activation of phospholipase C, via a pertussis toxin‐sensitive G protein, that is Ca 2+ ‐dependent. This stimulatory effect of opioids on phospholipase C, and the potential inositol (1,4,5)trisphosphate‐mediated rises in intracellular Ca 2+ , could play a part in the cellular mechanisms of opioid action.