Homologous (A) and heterologous (B) phosphorylation of MOP receptor Ser-377. Representative Western blots show the levels of Ser-377 phosphorylation induced by treatment with 1 μm DAMGO, 1 μm DAMGO + 1 μm 1DMe, 1 μm 1DMe, or 10 μm clonidine for 30 min in (SH2-D9)MOP-YFP cells. Samples were immunoblotted with anti-Ser(P)-377 antibody (P-MOP) followed by anti-actin antibody (actin) for normalization. Upper panels show representative results from at least three independent experiments. The P-MOP band migrated between the 95- and 130-kDa molecular mass markers. The actin band was detected at the level of the 43-kDa molecular mass marker. Lower panels show means ± S.E. of Ser(P)-377 quantification normalized to actin from at least three independent experiments performed in duplicate. c indicates control cells. *, p < 0.05; ***, p < 0.001; one-way ANOVA followed by Bonferroni post-tests.