Overlapping Neural Substrates of Alcohol- and Anxiety-Related Behavior in the Rat

Abstract: Alcohol use is a leading cause of death and disease worldwide. A large part of this disease burden is associated with alcohol use disorder (AUD), a diagnostic category characterized by excessive use in spite of negative consequences ("compulsive use"), a loss of control over intake, and choice of alcohol over natural rewards. These behavioral symptoms are believed to reflect the emergence of persistent neuroadaptations in key brain regions that exert control over motivated behavior. A major challenge to addressing the treatment needs of patients with AUD is the high prevalence of co-occurring psychiatric disorders, of which anxiety disorders are the most common. Both AUD and anxiety disorders are characterized by broad changes in gene expression within brain regions that include the prelimbic cortex (PL) and the amygdala complex. Although the risk for AUD has a substantial genetic component, heavy alcohol use and stress also contribute to disease risk. Our lab previously identified DNA hypermethylation as a mechanism behind alcohol-induced downregulation of prelimbic Syt1 and Prdm2. In a subsequent study, our lab demonstrated a functional role of Prdm2 in alcohol-associated behaviors. In the work that constitutes this thesis, we have further investigated the behavioral consequences of Syt1 and Prdm2 downregulation. We found that Syt1 knock-down in the PL of non-dependent rats is sufficient to promote several behaviors that model critical aspects of AUD. We further identified the PL-basolateral amygdala (BLA) projection as a key brain circuit within which Syt1 knock-down promotes compulsive-like alcohol intake. In another study, we showed that Prdm2 knock-down in the PL increases the expression of fear memory, a central feature of anxiety disorders. Knock-down after memory formation (consolidation) did not increase the fear expression, indicating that Prdm2 regulates fear memory consolidation. We further showed that knock-down of Prdm2 in the PL-BLA projection was sufficient to promote the increased fear expression. Transcriptome analysis specifically in neurons projecting from the PL to the BLA showed a marked up-regulation of genes involved in synaptogenesis, suggesting that Prdm2 downregulation leads to excessive fear by strengthening fear memory consolidation in the PL-BLA circuit. In a third study, we used a model of social defeat- and witness stress to investigate mechanisms of co-occurring escalated alcohol intake and increased anxiety-like behavior ("comorbidity"). We recapitulated the broad range of individual stress responses observed in human populations. With gene expression analysis, we identified a marked upregulation of Avp in the amygdala of rats with "co-morbid" characteristics, and this upregulation correlated with the magnitude of the comorbidity. Together, our findings highlight the contribution of epigenetic mechanisms in regulating the behavioral consequences of alcohol-dependence, and identify specific downstream target genes whose expression is influenced by alcohol-induced epigenetic reprogramming to mediate long-term behavioral consequences. Our work also identifies amygdala Avp as a possible neurobiological substrate of individual susceptibility for stress-induced alcohol- and anxiety-related behaviors.