To assess further downstream events of MAPK signaling, we examined expression of transcription factors ELK1 and CREB, which are nuclear targets for ERK1/2.(44, 45) CREB is activated in response to ERK1/2 through signaling by kinases such as p90RSK.(46–48) Both p90RSK (82.01±10.2% control) and CREB (79.07±4.69% control) were reduced in the putamen, but this was confounded by sensitivity to pH (p<0.0001). However, heroin abusers had significantly higher expression of ELK1 (133.26±8.21%, p<0.01; F1,46=7.38). In order to gain entry into the nucleus, ELK1 must be phosphorylated by ERK1/2(49) and the levels of activated pELK1 were significantly reduced (86.92±3.35%, p<0.05; F1,41=5.21) in heroin subjects. Furthermore, the ratio of pELK1 to total ELK1 was significantly decreased in heroin abusers (66.37±5.46, p<.001; F1,46=12.01; Fig 1D), indicating that repeated heroin use leads to an imbalance in total versus phosphorylated ELK1 protein.
