MOP internalization after agonist treatment. HEK 293T cells expressing esMOP were plated in serum-free DMEM and treated with saline (control), Met-enkephalin (Met-enk;1 μM), etorphine (E; 1 μM), or morphine (M; 1 μM) for 1 hr (top row). A separate group was treated with naloxone (Nal; 1 μM) 15 min prior to agonist treatment (bottom row). Met-enkephalin and etorphine treatment caused robust MOP internalization (MOP-Flag = green, DAPI = blue) that was blocked by naloxone pretreatment. Morphine, a noninternalizing opioid agonist, failed to stimulate MOP internalization in these cells. Scale bar = 75 μm