Overexpression of the Histone Dimethyltransferase G9a in Nucleus Accumbens Shell Increases Cocaine Self-Administration, Stress-Induced Reinstatement, and Anxiety

Repeated exposure to cocaine induces lasting epigenetic changes in neurons that promote the development and persistence of addiction. One epigenetic alteration involves reductions in levels of the histone dimethyltransferase G9a in nucleus accumbens (NAc) after chronic cocaine administration. This reduction in G9a may enhance cocaine reward because overexpressing G9a in the NAc decreases cocaine-conditioned place preference. Therefore, we hypothesized that HSV-mediated G9a overexpression in the NAc shell (NAcSh) would attenuate cocaine self-administration (SA) and cocaine-seeking behavior. Instead, we found that G9a overexpression, and the resulting increase in histone 3 lysine 9 dimethylation (H3K9me2), increases sensitivity to cocaine reinforcement and enhances motivation for cocaine in self-administering male rats. Moreover, when G9a overexpression is limited to the initial 15 d of cocaine SA training, it produces an enduring postexpression enhancement in cocaine SA and prolonged (over 5 weeks) increases in reinstatement of cocaine seeking induced by foot-shock stress, but in the absence of continued global elevations in H3K9me2. The increase in stress-induced reinstatement is paralleled by heightened anxiety measures, suggesting that countering the cocaine-induced decreases in endogenous G9a with ectopic G9a overexpression leads to lasting anxiogenic effects. Finally, we found an enduring reduction in phosphorylated cAMP-response element binding protein levels in the NAcSh that could account for the increased anxiety. These data demonstrate a novel role for G9a in promoting comorbid cocaine addiction and anxiety and suggest that increased epigenetic repression of transcription through H3K9 during cocaine use can have long-lasting and unexpected negative consequences on behavior.

SIGNIFICANCE STATEMENT Cocaine addiction is a neuropsychiatric disorder that is detrimental to society and currently has no effective treatments. The difficulty in treating drug addiction is compounded by the high comorbidity with other psychiatric illnesses, including anxiety disorders. Here, we demonstrate that G9a, an epigenetic repressor of gene expression, acting in the nucleus accumbens, a brain reward region, is capable of increasing both addiction- and anxiety-like behaviors in rats. These findings are intriguing because repeated cocaine exposure decreases G9a in this region and thereby enhances expression of certain addiction-promoting genes. However, our results suggest that countering this cocaine-induced decrease in G9a activity actually exacerbates addiction and sensitivity to relapse under stressful situations.