Distinct differences between morphine- and [D-Ala², N-MePhe⁴,Gly- ol⁵]-enkephalin- μ-opioid receptor complexes demonstrated by cyclic AMP- dependent protein kinase phosphorylation

The present study demonstrates a conditional, agonist-dependent phosphorylation of the max-opioid receptor (MOR-1) by cyclic AMP-dependent protein kinase (PKA) in membrane preparations of MOR-l-transfected neuroblastoma Neuro2A cells. Opioid agonist-dependent phosphorylation occurs in a time- and concentration-dependent manner (EC₅₀ ~40 nM) 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 μM morphine and 6 nM 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.