Protein expression levels of MOR (89.55±3.42%, p<0.005; F2,43=6.73) and β-arrestin2 (87.11±4.26%, p<0.05; F3,41=8.1), which regulates MOR desensitization and internalization,(43) were significantly reduced in heroin abusers compared to controls (Fig 1A–B), indicating that the tone of MOR-signaling cascades may be altered. One of the key intracellular pathways activated upon MOR stimulation is the MAPK pathway, depicted in Figure 1C. We focused on the core components of the pathway including MAPK ERK kinases (MEK)1/2 and ERK1/2 because they regulate phosphorylation status and activity of transcription factors. Levels of both MEK1 (91.28±2.55%, p<0.05; F1,41=4.33) and its substrate ERK1 (84.78 ± 3.83%, p<0.05; F1,46= 4.40) were significantly reduced. We observed an opposing increase in the protein levels of ERK2 (111.78±2.32%. p<0.005; F1,44=9.33). The levels of phosphorylated ERK1 (105.67 ± 13.01%) and phosphorylated ERK2 (120.1 ± 11.4%) were not different from controls due to the strong confound of brain pH which significantly impacted pERK1 (p<0.0005) and pERK2 (p< 0.00001) levels (data not shown), suggesting that phosphorylation of ERK1/2 is sensitive to the perimortem state (data not shown).
