Differential effects of acute and repeated morphine treatment on κ-opioid receptor mRNA levels in mesocorticolimbic system

BackgroundMorphine induces adaptive changes in gene expression throughout the reward circuitry of the brain. Here, we investigated the acute and chronic effects of morphine on mRNA levels of κ-opioid receptor in the rat mesocorticolimbic system.MethodsThree groups of rats received ip injection of saline, acute morphine (8.0 mg/kg) and repeated morphine (8.0 mg/kg, once daily for 5 consecutive days) and the κ-receptor mRNA expression was examined using real-time quantitative PCR method.ResultsWe found that κ-receptor mRNA in medial prefrontal cortex (mPFC) increased after acute and repeated morphine treatment. However, the mRNA levels in nucleus accumbens (NAc) and ventral tegmental area (VTA) were upregulated after acute morphine treatment and returned to basal levels after repeated morphine treatment.ConclusionsThese results suggest that morphine dependence was associated with regionally specific changes in mRNA levels of κ-opioid receptor in mesocorticolimbic system.