Expression Levels of the Tyrosine Hydroxylase Gene and Histone Modifications Around its Promoter in the Locus Coeruleus and Ventral Tegmental Area of Rats during Forced Abstinence from Morphine.

BACKGROUND: Epigenetic mechanisms such as histone modifications may be involved in the structural and behavioral changes associated with addiction. We studied whether morphine-induced changes in mRNA levels of the catecholamine biosynthesis enzyme, tyrosine hydroxylase (TH), are associated with histone modifications around the promoter of this gene in the locus coeruleus (LC) and ventral tegmental area (VTA) of rats.

METHODS: Dependence was induced in rats by intraperitoneal injections of morphine for 11 days. The animals were killed 2 h (chronic morphine), 24 h and 7 days (spontaneous withdrawal) after the last injection of morphine.

RESULTS: Analysis of our real-time quantitative reverse transcription PCR results by 1-way ANOVA showed significant upregulation (5.13 ± 0.39 folds) of LC levels of the TH transcript 24 h after the last injection of morphine to rats, when compared with 2 h and 7 days time points. Chronic morphine and morphine abstinence failed to cause any significant changes in the levels of TH mRNA in the VTA after cessation of morphine. Consistently, chromatin immunoprecipitation real-time quantitative PCR assays revealed that 24 h after the last injection of morphine, levels of H3 acetylation were significantly increased (4.12 ± 0.38 folds) at the promoter of the TH gene in the LC but not in the VTA. Our data also showed that histone H3 trimethylation failed to change around the TH gene promoter either in the VTA or in the LC after morphine abstinence.

CONCLUSIONS: Results of the present study, for the first time, demonstrate the involvement of histone H3 acetylation in the regulation of TH gene expression in the LC of rats during forced abstinence from morphine.