MAPK3 and opioid use disorder: Levels of phosphor-Mapk3 (Erk1) were significantly downregulated in the prefrontal cortex of postmortem tissue from patients with opioid use disorder, although expression of Mapk3 (Erk1) has also been reported to be upregulated in the locus coeruleus and striatum of rats treated with morphine [45,46].