Fig. 5.

Methadone but not morphine caused endocytosis of flag-tagged MOR in HEK293 cells. A, Representative images from endocytosis experiments. Fluorescence stains internalized receptor after 30 min exposure to 30 μm methadone (left), morphine (middle), and M6G (right). B, Summarized data from several endocytosis experiments testing exposure to alkaloid opioids for 30 min at 37°C. Normorphine, methadone, and etorphine caused internalization that was dependent on the concentration. Morphine and M6G did not cause MOR endocytosis above the constitutive level (dotted lines), even at high concentrations. ∗∗p < 0.05; one-way ANOVA; Tukey's multiple comparison test.