Distinct Differences Between Morphine- and [d-Ala2,N-MePhe4,Gly-ol5]-Enkephalin- μ-Opioid Receptor Complexes Demonstrated by Cyclic AMP-Dependent Protein Kinase Phosphorylation |
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Authors: | Sumita Chakrabarti Ping-Yee Law Horace H Loh |
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Institution: | Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota, U.S.A. |
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Abstract: | Abstract: The present study demonstrates a conditional, agonist-dependent phosphorylation of the μ-opioid receptor (MOR-1) by cyclic AMP-dependent protein kinase (PKA) in membrane preparations of MOR-1-transfected neuroblastoma Neuro2A cells. Opioid agonist-dependent phosphorylation occurs in a time- and concentration-dependent manner (EC50~40 n M ) and can be abolished by the receptor antagonist naloxone. Stoichiometric analysis indicates incorporation of a maximum of 6 mol of phosphate/mol of receptor in the presence of 1 µ M morphine and 6 n M PKA. Although morphine and related alkaloids as well as some peptide agonists (PLO17 and β-endorphin) stimulated phosphorylation of MOR-1 by PKA, the potent μ-opioid-selective peptide d -Ala2, N -MePhe4,Gly-ol5]-enkephalin (DAMGO) or other enkephalin analogues such as d -Ala2]-Met5-enkephalinamide (DALA), d -Ala2, d -Leu5]-enkephalin (DADLE), and Met5-enkephalin had no effect. The lack of the effect of DAMGO on MOR-1 phosphorylation state was evident also after chronic pretreatment. These results suggest the existence of different agonist-dependent conformations of MOR-1. Furthermore, phosphorylation may be a useful parameter with which to identify different agonist-receptor conformations. |
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Keywords: | μ-Opioid receptor Morphine [d-Ala2 N-MePhe4 Gly-ol5]-enkephalin Cyclic AMP-dependent protein kinase Opioid Phosphorylation |
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