Knockdown of CREB was verified immunohistochemically and by counting CREB+ cells in NAc of AAV-Cre-GFP injected Crebfl/fl mice ( Figures 7B and 7C). Consistent with our hypothesis that
G9a induction mediates behavioral resilience to chronic social stress in part through downregulation of CREB activity in NAc, AAV-Cre-GFP expressing Crebfl/fl mice displayed consistent antidepressant-like behavioral responses in the social defeat paradigm (i.e., decreased social avoidance), forced swim test (i.e., decreased immobility), and Sirolimus order sucrose preference test (i.e., increased sucrose preference), compared to AAV-GFP expressing animals ( Figures 7D–7F). Here, we demonstrate that repeated cocaine increases the severity of depressive-like responses to social stress in mice—a phenomenon that parallels high comorbid rates of substance abuse and mood disorders in humans. Furthermore, our data reveal a critical role for repressive histone methylation in NAc in mediating this cocaine-induced vulnerability to social stress. We show that repeated cocaine reduces global levels of H3K9me2 in this
brain region, and its associated writer enzymes, G9a and GLP, which enhance susceptibility to subsequent social defeat stress. We demonstrate similar reductions in H3K9me2 and G9a/GLP levels in NAc of depressed humans and Sorafenib order of mice subjected to chronic social defeat stress, but only in those animals susceptible to the negative consequences of repeated stress. We then establish that such downregulation of G9a and H3K9me2 in NAc mediates cocaine enhancement of stress vulnerability by demonstrating that local knockout of G9a in this brain region is sufficient to enhance an animal’s vulnerability to social stress, while overexpression of G9a in NAc blocks the ability of chronic cocaine to increase stress susceptibility. An important role for repressive H3K9me2 modifications in the regulation of both cocaine and stress responses comes from recent ChIP-chip studies that Thiamine-diphosphate kinase characterized altered H3K9me2 binding in NAc, genome wide, in response to repeated cocaine or chronic social defeat stress (Renthal
et al., 2009 and Wilkinson et al., 2009). Interestingly, we found that a majority of changes in H3K9me2 binding observed in NAc of susceptible mice were reversed by chronic treatment with standard antidepressant treatments and were not observed in unsusceptible animals (Wilkinson et al., 2009). Although chronic cocaine and chronic social defeat stress similarly regulate repressive histone methylation in NAc, development of therapeutics to target enzymes regulating these processes would be difficult, given the ubiquitous nature of histone methyltransferases and demethylases. Therefore, it will be important to identify downstream proteins affected by such alterations in histone methylation, with the hopes that such targets may be more suitable for future drug interventions.