Stress is a critical factor in the onset and progression of alcohol dependence, a chronic condition characterized by loss of control over alcohol drinking. Stress is also a major risk factor for other psychiatric disorders, such as anxiety disorders. Alcohol dependence and anxiety disorders share overlapping neuroadaptations, in part mediated by epigenetic mechanisms. We have previously demonstrated that chronic alcohol exposure induces DNA hypermethylation in the dorsomedial prefrontal cortex (dmPFC) in rats. In a follow-up study, our group reported that alcohol-induced hypermethylation reduces the expression of the histone methyltransferase PRDM2, which in turn promotes stress-induced reinstatement of alcohol. These findings point to undiscovered contributions of PRDM2 to stress responses.

This thesis aimed to further understand the role of PRDM2 in stress-induced maladaptive behaviors. Study 1 showed that decreased Prdm2 expression in the dmPFC-basolateral amygdala (BLA) projection resulted in an over-consolidation of fear memory through upregulated glutamatergic signaling. Study 2 showed that decreased PRDM2 activity promotes footshock-induced relapse-like behavior in both male and female rats. Together, these findings suggest that PRDM2 may be a shared molecular substrate between alcohol dependence and anxiety disorder. Downregulation of Prdm2 in the dmPFC-BLA projection did not impact stress-induced reinstatement, while decreased Prdm2 in the dmPFC-nucleus accumbens core projection promoted reinstatement. Consistent with our preclinical findings, Study 2 also found that people with alcohol dependence exhibited reduced PRDM2 expression and increased methylation at its promoter in the prefrontal cortex. This suggests that PRDM2 and its regulatory mechanisms have a mechanistic role in stress responses, and serve as epigenetic markers for alcohol dependence across species.

In both Study 1 and Study 2, we observed a broad range of defensive behaviors in rats. Therefore, in Study 3, an ethological analysis was performed to identify multiple behaviors in response to threat-associated cues. We also assessed the utility of a machine learning pipeline to perform ethological analysis. The approach from Study 3 was implemented in Study 4, in which patterns of behaviors were characterized among rats observing a conspecific undergoing fear conditioning. Heightened fear in the observer rats associated with increased neural activation in several areas, including the dmPFC. However, the increased dmPFC activity and the heightened vicarious fear were not mediated by decreased Prdm2 expression. This suggests that direct and observational fear learning in part rely on distinct molecular mechanisms. Collectively, this thesis highlights the importance of PRDM2 and its behavioral- and circuit-specific effect in mediating fear- and alcohol-associated behaviors.