Regional expression of extracellular signal-regulated kinase 1 and 2 mRNA in a morphine-induced conditioned place preference model

Chronic morphine administration has been shown to change the expression of extracellular signal-regulated kinase (ERK), which is a molecule known to play an important role in homeostatic adaptations caused by addictive drugs. In the present study, we investigated the expression of ERK messenger ribonucleic acid (mRNA) of the prefrontal cortex (PFC), nucleus accumbens (NAc), hippocampus, and caudate putamen (CPu) in morphine-induced conditioned place preference (CPP) by real-time reverse transcriptase polymerase chain reaction (real-time PCR). CPP was established by alternate morphine (10 mg/kg) injections, extinguished after a 10-day extinction training, and reinstated by a priming injection of morphine (10 mg/kg). During three phases of morphine-induced CPP, the expression levels of ERK1 and ERK2 mRNA were altered in various brain regions. In the PFC, the expression levels of ERK1 and ERK2 mRNA were increased after chronic morphine injection (p=0.003, p=0.000), and did not return to the basal level after extinction training (p=0.025, p=0.000), but decreased after a priming injection (p=0.000, p=0.000). In the CPu, ERK1 mRNA had an abrupt increase following a priming injection (p=0.000). Different from other brain regions, the expression levels of ERK1 and ERK2 mRNA were decreased in three phases of morphine-induced CPP in the hippocampus (ERK1: p=0.000, p=0.040, p=0.000; ERK2: p=0.000, p=0.000, p=0.000, respectively). These results suggest region-specific changes of ERK1 and ERK2 mRNA expression during morphine-induced CPP.