Psychological stressors such as social stress and bullying are prevalent in today’s society. Disorders such as PTSD, depression and social anxiety disorder can be either caused or exacerbated by social stress and treatment options are not always effective in providing relief for these disorders. Our laboratory studies a form of social stress termed conditioned defeat, whereby a defeated Syrian hamster no longer displays species-typical territorial aggression but instead is submissive and defensive toward an intruder in its own cage. We hypothesized that the nucleus accumbens is a necessary component of the circuit mediating the acquisition and expression of conditioned defeat and that dopamine is necessary within the nucleus accumbens for inducing memory processes as well as expression of behavioral responses to stressful situations. We also hypothesized that defeat activates dopaminergic and/or nondopaminergic neurons in the ventral tegmental area (VTA) and that dopamine released by neurons projecting from the VTA to the nucleus accumbens and basolateral amygdala (BLA) increases neuronal activation of these structures during defeat. We found that dopamine, but not GABA, modulates memory of social defeat within the nucleus accumbens. However, GABA does affect the expression of behavioral responses to social defeat. Defeat also increased Fos activation of non-dopaminergic neurons, but it did not increase activation of dopaminergic neurons. Baclofen infusion into the VTA prior to defeat, which was hypothesized to specifically inhibit dopaminergic neurons, did not affect Fos activation within the nucleus accumbens and the basolateral amygdala. These experiments determined that dopamine does modulate memory of social defeat within the nucleus accumbens, but it is currently unclear what the source of this dopamine is. Future experiments are planned to determine this source of dopamine that could be a target of treatment for disorders that are caused or exacerbated by social stress.

Social stress is the most common stressor experienced by humans and exposure to social stress is thought to cause or exacerbate neuropsychiatric illness. Social stress also leads to behavioral and physiological responses in many animal models that closely mirror the symptoms of fear and anxiety in humans. Our laboratory uses Syrian hamsters to study behavioral responses to social stress. Hamsters are highly territorial, but after losing an agonistic encounter, hamsters exhibit a striking behavioral change, abandoning all territorial aggression and instead becoming highly submissive. This behavioral shift is termed conditioned defeat. Epigenetic modifications, such as changes in histone acetylation, are a possible molecular mechanism underlying such behavioral shifts. Histone deacetylase (HDAC) inhibitors have been shown to enhance fear learning and conditioned place preference for drugs of abuse, while suppressing histone acetylation with histone acetyltransferase (HAT) inhibitors impairs long-term memory formation. The first goal of this study was to test the hypothesis that histone acetylation is a molecular mechanism underlying conditioned defeat. We found that animals given an HDAC inhibitor systemically before social defeat later exhibited increased conditioned defeat. This treatment also suppressed defeat-induced immediate-early gene activity in the infralimbic cortex but not the basolateral amygdala. Next, we demonstrated that administration of an HDAC inhibitor in the infralimbic cortex before defeat enhanced stress-induced behavioral responses while HAT inhibition blocked these behavioral changes. Although both males and females exhibit conditioned defeat, the behavioral expression is more pronounced in males. We next used transcriptomic analysis to investigate potential genetic mechanisms leading to this sexually dimorphic expression and to further delineate the role of acetylation in stress-induced behavioral changes. We sequenced the whole brain transcriptome of male and female hamsters as well as the transcriptome of basolateral amygdala, a nucleus necessary for the acquisition and expression of conditioned defeat, of dominant, subordinate, and control animals. Our analysis revealed that numerous genes relating to histone acetylation, including several HDACs, were differentially expressed in animals of different social status and between sexes. Together, these data support the hypotheses that histone modifications underlie behavioral responses to social stress and that some of these modifications are sexually dimorphic.

Previous research indicates that serotonin (5-HT) enhances the acquisition of stress-induced changes in behavior; although it is unclear which serotonin receptors mediate this enhancement. 5-HT2 receptors are potential candidates because activation at these receptors is associated with increased fear and anxiety. In this study we investigated whether pharmacological treatments targeting 5-HT2A and 5-HT2C receptors modulated the acquisition and expression of conditioned defeat. Conditioned defeat is a social defeat model in Syrian hamsters (Mesocricetus auratus) that is characterized by increased submissive and defensive behavior and a loss of territorial aggression following social defeat. In experiment 1, we injected the 5-HT2C receptor agonist mCPP (0.3, 1.0, or 3.0 mg/kg) or vehicle prior to social defeat and tested subjects for conditioned defeat behavior in a social interaction test 24 hours later. In experiment 2, subjects received a social defeat, and 24 hours later we injected mCPP (0.3, 1.0, or 3.0 mg/kg) or vehicle prior to a social interaction test. We found that injection of mCPP increased the expression, but not acquisition, of conditioned defeat. In experiment 3, we injected the 5-HT2A receptor antagonist MDL 11,939 (0.5 or 2.0 mg/kg) or vehicle prior to a social defeat and tested subjects for conditioned defeat behavior. In experiment 4, subjects received a social defeat, and 24 hours later we injected MDL 11,939 (0.5 or 2.0 mg/kg) or vehicle prior to a social interaction test. We found that injection of MDL 11,939 significantly decreased the acquisition, but not expression, of conditioned defeat. These data suggest that pharmacological activation of 5-HT2C receptors enhances the expression of conditioned defeat, while pharmacological blockade of 5-HT2A receptors impairs the acquisition of conditioned defeat. These data extend other studies indicating that 5-HT signaling at 5-HT2A receptors facilitate memories for aversive events and 5-HT signaling at 5-HT2C receptors enhance stress-induced anxiety.

Social conflict is a part of everyday life, and it can be a potent stressor for both humans and other animals. In the laboratory, when two Syrian hamsters (Mesocricetus auratus) compete for territory, a dominance hierarchy is quickly formed. Becoming subordinate is a significant stressor resulting in increased release of adrenocorticotropic hormone, β-endorphin, and cortisol. Defeated hamsters will also subsequently fail to display territorial aggression in future social encounters and will instead display increased submissive behavior, even in the presence of a smaller, non-aggressive intruder. This change in behavior is consistent and long-lasting and has been termed conditioned defeat (CD). Corticotropin releasing factor (CRF) is an important neuropeptide in the control of the hypothalamo-pituitary-adrenal (HPA) axis response to stress. It is also involved in a number of behaviors such as anxiety, stress responding, food intake, learning, and memory. The widespread distribution of CRF, CRF-like peptides, and CRF receptors, particularly in brain sites related to anxiety, fear, and stress responses, suggests a role for CRF and CRF-like peptides in modulating emotional responses other than via HPA axis activity. It has also been shown that CRF may have a role in the acquisition and expression of CD. Non-specific and CRF type 2-specific CRF antagonists reduce the acquisition and expression of CD in male hamsters while injection of a CRF type 1-specific antagonist does not. Therefore, the goal of this dissertation was to investigate the role of CRF type 1 and 2 receptors in CD in hamsters and to identify neuroanatomical locations where CRF may be acting. It was found that non-specific or CRF type 1 receptor specific agonists enhance the expression, but not acquisition, of CD. Further, these agonists appear to enhance aggressive behavior in animals that were not previously defeated, suggesting a modulatory role for CRF type 1 receptors in agonistic behavior that depends on an animal’s previous social experience. Further, localization of CRF receptors was determined in hamster brain in sites thought important for CD and agonistic behavior, but changes in receptor binding following defeat were not observed. Implications of these results and future directions are discussed.

Social stress is a prevalent factor in society that can cause or exacerbate neuropsychiatric disorders including depression and posttraumatic stress disorder. According to the National Institutes of Health, 6.9% of adults in this country currently suffer from depression, and 4.1% suffer from an anxiety disorder. Unfortunately, current treatments are ineffective in reducing or alleviating symptoms in a majority of these patients. Thus, it is critical to understand how social stress changes in brain and behavior so that we might develop alternative treatments. Brain derived neurotrophic factor (BDNF), which binds to tyrosine kinase B (TrkB) receptors, plays a role in fear learning and in behavioral responses to stress, although we do not currently know whether BDNF promotes or prevents these responses. The purpose of this project was to understand how BDNF alters brain and behavior in response to social stress using a model of social stress in Syrian hamsters, termed conditioned defeat (CD). CD refers to the marked increase in submissive and defensive behavior following social defeat. Specific Aim (SA) 1 tested the hypothesis that BDNF, via TrkB receptors, promotes CD learning. Instead, we found that BDNF and a selective TrkB receptor agonist reduced CD and that a TrkB receptor antagonist enhanced CD. SA 2 tested the hypothesis that the behavioral response observed following systemic administration of TrkB-active drugs is mediated via their action in specific nodes of the neural circuit underlying CD. Unfortunately, the vehicle in which these drugs are dissolved independently activates immediate early gene expression making interpretation of these data impossible. Finally, SA 3 tested the hypothesis that BDNF alters defeat-induced neural activation at least in part by acting in the medial prefrontal cortex (mPFC). We demonstrated that BNDF microinjected into the mPFC site-specifically altered defeat-induced neural activation in the CD neural circuit supporting this hypothesis. Overall, these data suggest that BDNF acts to prevent social stress-induced changes in behavior, at least in part via the basolateral amygdala and the mPFC, and that BDNF-active drugs might be a useful avenue to pursue to discover new treatments for patients that suffer from stress-related neuropsychiatric disorders.