Histone Methyltransferase G9a Plays an Essential Role on Nicotine Preference in Zebrafish

Abstract

AbstractPsychostimulants regulate behavioral responses in zebrafish via epigenetic mechanisms. We have previously shown that DNAmethylation and histone deacetylase (HDAC) inhibition abolish nicotine-induced conditioned place preference (CPP) but littleis known about the role of histone methylation in addictive-like behaviors. To assess the influence of histone methylation onnicotine-CPP, zebrafish were treated with a histone (H3) lysine-9 (K9) dimethyltransferase G9a/GLP inhibitor, BIX-01294(BIX), which was administered before conditioning sessions. We observed a dual effect of the inhibitor BIX: at high dosesinhibited while at low doses potentiated nicotine reward. Transcriptional expression of α6 and α7 subunits of the nicotinicacetylcholine receptor and of G9a, DNA methyl transferase-3, and HDAC-1 was upregulated in zebrafish with positive scoresfor nicotine-CPP. Changes in relative levels of these mRNA molecules reflected the effects of BIX on nicotine reward. BIXtreatment per sé did not affect transcriptional levels of epigenetic enzymes that regulate trimethylation or demethylation ofH3. BIX reduced H3K9me2 protein levels in a dose-dependent manner in key structures of the reward pathway. Thus, ourfindings indicated that different doses of BIX differentially affect nicotine CPP via strong or weak inhibition of G9a/GLPactivity. Additionally, we found that the lysine demethylase inhibitor daminozide abolished nicotine-CPP and drug seeking.Our data demonstrate that H3 methylation catalyzed by G9a/GLP is involved in nicotine-CPP induction. Dimethylation ofK9 at H3 is an important epigenetic modification that should be considered as a potential therapeutic target to treat nicotinereward and perhaps other drug addictions.