In earlier studies using morphine pellets [e.g., 17] or heroin administration by experimenters [18], it was found that POMC mRNA in the hypothalamus was decreased after chronic opiate exposure in rats. In the current study, we examined POMC gene expression in two brain regions (the medial hypothalamus and Me/BLA) after 9 days of abstinence from 7 days of heroin self-administration, and found a significant decrease in the POMC mRNA levels in the medial hypothalamus only. These novel results extend the above earlier findings to the heroin abstinence after active intravenous heroin self-administration. Different from the AVP in the Me/BLA, similar decreases in POMC mRNA levels of the hypothalamus were found in both H-RI and L-RI groups, showing a dis-association of the POMC decrease with stress-related heroin reinstatement.
