Pre-Exposure to Nicotine with Nocturnal Abstinence Induces Epigenetic Changes that Potentiate Nicotine Preference

Prior exposure to drugs of abuse may facilitate addiction. It has been described that pre-exposure to nicotine can increase or, contrarily, prevent conditioned place preference (CPP). Here, we evaluated the effect of nicotine pre-exposure on CPP performance using an original protocol mimicking smokers' behaviour in zebrafish. We simulated nicotine withdrawal at sleep time by exposing zebrafish to nicotine during daylight but not at night (D/N) for 14 days and then performed nicotine-CPP in zebrafish. D/N-nicotine-treated zebrafish obtained the highest CPP score, whereas zebrafish pre-exposed continuously to nicotine did not show nicotine-CPP. Evaluation of locomotor activity, seeking and anxiety-like behaviours supported the CPP findings. Nicotinic receptor subunit gene expression showed significant increases in the brain of zebrafish exposed to nicotine. Zebrafish exposed to D/N-nicotine showed further increases of α6- and α7-subunit expression after CPP establishment. Inhibition of histone acetylation by phenylbutyrate prevented nicotine-CPP. Transcriptional expression of epigenetic enzymes controlling histone acetylation/deacetylation and DNA methylation/demethylation was widely modified in brain portions containing reward areas of zebrafish exposed to D/N-nicotine after CPP. Zebrafish exposed to D/N-nicotine showed high levels of acetylated histone 3 and pCREB immunoreactivity differentially found in nuclei of the dopaminergic reward circuit in zebrafish homologous to the ventral tegmental area, nucleus accumbens and dorsal habenula. Our findings demonstrated that repetitive abstinent periods are risky factors for drug abuse that potentiate nicotine-environment associations and seeking. Brain modifications can persist long after nicotine use and are likely due to changes in the transcriptional expression of enzymes regulating drug reward-related gene expression via epigenetic modifications.