Prolonged Activation of C-fos and Optimal Activation of Pro-opiomelanocortin MRNA After Repeated Morphine Exposure in SH-SY5Y Cells

The time course of change in the mRNA concentrations of the proto-oncogene c-fos and pro-opiomelanocortin after two different methods of morphine treatment was examined in SH-SY5Y human neuroblastoma cells. In a repeated treatment design, SH-SY5Y cells exposed to morphine sulfate (MS) for 12 h or more were periodically given fresh morphine (10 or 1 muM). In a single-dose design, 10 muM MS was added to the flasks at designated times without changing the medium. Slot-blotting hybridization analysis of total cellular RNA using a [(32)P]-fos cDNA probe revealed that repeated morphine treatment caused both an early transient induction of c-fos and a later prolonged increase in c-fos. Single treatment with morphine caused only a transient and rapid induction of c-fos. Slot-blotting hybridization with a [(32)P]-POMC cRNA probe revealed that POMC mRNA was significantly activated at 6 h and remained significantly elevated up to 7 days in the cells with repeated morphine treatment. In the single-dose experiments, however, the POMC mRNA was not significantly elevated at 2 days or less. It was significantly activated at 6 days, but at a much lower level than that seen in the repeated-dose design. These results indicate that repeated exposure to morphine induces a prolonged activation of c-fos mRNA which may be functionally related to the significant activation of POMC mRNA in SH-SY5Y cells.