Untangling the mesh of social stress: Extracellular matrix remodeling in preclinical models of psychosocial stress

Major depressive disorder (MDD) is among the most prevalent psychiatric disorders, with an immense socioeconomic burden, yet with unknown etiology. Although the cause(s) of MDD remain unknown, psychosocial stress is a major predisposing factor for MDD. While MDD is well-known for its mood-related symptoms, cognitive dysfunction is a common symptom of MDD. Yet, deficits of the cognitive domain in MDD remain less studied and the underlying mechanisms are largely unexplored. In my PhD thesis, I aimed to unravel the lasting effects of psychosocial stress, with an emphasis on its effects on cognition. Furthermore, I aimed to characterize the neurobiological mechanisms that underlie these long-lasting stress-induced cognitive deficits. To gain access to the molecular and cellular mechanisms driving chronic stress effects, I used social defeat, an animal model of psychosocial stress, both in rats and mice. In chapter 2, our unbiased proteomic study identified extracellular matrix (ECM) proteins to be regulated in SDPS (social-defeat induced persistent stress)-exposed rats during a depressive-like state. This was accompanied with an increase in the number of perineuronal nets (PNNs) surrounding parvalbumin expressing (PV)-interneurons at the hippocampal CA1 subfield. Moreover, maintenance of LTP (long-term potentiation), together with reduced inhibitory neurotransmission was present during the depressive-like state. Importantly, enzyme-mediated normalization of hippocampal ECM levels reversed the SDPS-induced physiological and cognitive deficits. Taken together, these findings identified hippocampal ECM as a novel substrate for hippocampal dysfunction during the sustained depressive-like state. In chapter 3, I characterized how the depressive-like state develops by assessing behavioral and ECM changes over the weeks and months after social defeat stress. Temporal profiling of behavioral disturbances revealed major differences between early stress effects and the late-emerging depressive-like state. Cognitive function was disturbed immediately after stress, but this deficit subsided in the weeks after stress, and re-emerged long after stress. Conversely, affective deficit took weeks to develop. Temporal profiling of hippocampal ECM remodeling revealed a co-occurrence with cognitive dysfunction. These findings underscore a dichotomy in the effects of stress on the affective and cognitive domains and highlights time-dependence of stress effects. Yet, the mechanisms driving these dynamic stress-induced changes on hippocampal ECM remain unknown. Therefore, in chapter 4 I investigated the upstream mechanisms that could propel stress-induced ECM remodeling. Specifically, I assessed the potential role of metalloproteinase mediated (MMP) regulation of ECM in driving the initial effects of stress. I showed that systemic administration of IPR-179, a gelatinase inhibitor, before each daily defeat prevented the early, defeat-induced memory impairment, suggesting that MMP-mediated mechanisms play a role in stress-induced cognitive deficits. In chapter 5, I aimed to set up a social defeat-based model for depression in mice. For this, I developed a novel stress paradigm, in which physical social defeat is combined with vicarious defeat. Behavioral profiling of the effects of PVDPS (physical and vicarious defeat-induced persistent stress) demonstrated a temporal discrepancy in the development of cognitive and affective dysfunction. Moreover, increased build-up of the ECM, together with aberrant inhibitory transmission at the CA1, coincided with late hippocampal memory deficit. Together, these findings emphasize a temporal discrepancy in stress-induced cognitive and affective disturbances in mice. Furthermore, this study further supports the important role of the hippocampal ECM in mediating persistent stress-induced memory deficits. Together, my studies highlight the cross-species SDPS model as valuable tool to study depression-related pathology, as it recapitulates individual subdomains of depression. Importantly, temporal characterization of stress effects demonstrates stress recency as a crucial factor in determining the final stress-triggered outcome, both at the molecular and behavioral levels. Furthermore, my data suggest an important role for hippocampal ECM in mediating stress-induced memory impairments. Thus, ECM-targeted interventions offer promising therapeutic strategies against MDD

Autonomic and Redox Imbalance Correlates With T-Lymphocyte Inflammation in a Model of Chronic Social Defeat Stress

Patients diagnosed with post-traumatic stress disorder (PTSD) are at a significantly elevated risk of developing comorbid inflammatory conditions, but the mechanisms underlying this predilection remain unclear. Our previous work has shown that T-lymphocytes exposed to elevated levels of norepinephrine (NE) displayed a pro-inflammatory signature reminiscent of an autoreactive phenotype. With this, we hypothesized that the increased sympathetic tone observed during psychological trauma may be promoting pro-inflammatory T-lymphocytes, which causes a predisposition to comorbid inflammatory conditions. Here, we examined the consequences of psychological trauma on splenic T-lymphocytes using a mouse model of repeated social defeat stress. Social defeat led to anxiety-like and depression-like behavior as has been previously described. The spleens of socially-defeated mice showed significant elevations of NE, tyrosine hydroxylase (TH), and acetylcholinesterase (ACHE) levels, which appeared to be due in part to increased expression within T-lymphocytes. Additionally, T-lymphocytes from stressed animals showed higher levels of pro-inflammatory cytokines and mitochondrial superoxide. Interestingly, in this model system, close associations exist within splenic T-lymphocytes amid the autonomic, inflammatory, and redox environments, but these only weakly correlate with individual behavioral differences among animals suggesting the psychological and physiological manifestations of trauma may not be tightly coupled. Last, we describe, for the first time, elevations in calprotectin levels within T-lymphocytes and in circulation of psychologically stressed animals. Calprotectin correlated with both behavioral and physiological changes after social defeat, suggesting the potential for a new biological marker and/or therapeutic target for psychological trauma and its inflammatory comorbidities

Gut Microbiota, Inflammation, and Behavioral Expression Following Social Defeat

Social stress exacerbates symptoms of mood and anxiety disorders in humans. Here, we tested the hypothesis that social stress increases anxiety- and depression-like responses via changes in gut microbiota and inflammation. We used a social defeat model in Syrian hamsters to determine whether exposure to social stress alters the gut microbial community. We then tested whether alterations in the gut microbial community impacts susceptibility to social stress, and, if so, whether it might do so via immunological pathways. In Aim 1, the gut microbial community of hamsters was assessed by 16S mRNA Illumina sequencing after one and repeated agonistic encounters. Both dominant and subordinate hamsters exhibited alterations in the gut microbial community and reductions in species richness following social stress. LEfSE analysis revealed that some microbial taxa correlated with achieving dominant or subordinate status in a future agonistic encounter. In Aim 2, hamsters were treated with either a probiotic for 2 weeks or an emulsifier for 12 weeks to test whether manipulating gut microbiota impacts behavioral susceptibility to social defeat. Probiotics are thought to promote a healthy microbial composition and emulsifiers have been shown to disrupt the gut microbial community. Probiotic treatment increased avoidance behavior and decreased social interaction following defeat. Probiotic treatment also altered the gut microbial community and serum cytokines following defeat. Emulsifier treatment had no effect on behavior. In Aim 3, neuroinflammation was assessed following social defeat. There was no increase in microglial activation in brain following defeat suggesting that exposure to mild social stress in hamsters does not induce robust neuroinflammation. As a positive control, we examined microglial activation following administration of lipopolysaccharide, a bacterial endotoxin, and were able to demonstrate a robust inflammatory response in hamster brain. Thus, the experiments in Aim 3 suggest that neuroinflammation is not necessary for behavioral responses to social stress in hamsters. Collectively, these data demonstrate that exposure to social stress can alter gut microbiota and that the microbiota can alter susceptibility to social stress. Future studies will be necessary to determine the mechanisms underlying this two-way relationship

The adverse effects of chronic social stress on learning and the role of serotonin quantified by a binary logistic regression model in individual crickets (Gryllus bimaculatus)

The ability to learn and change future behaviour based on past experiences is crucial for the life and survival of animals. For various behaviours exhibited by animals it is clear that in a seemingly homogeneous population not all individuals behave the same way, even in invertebrates. In crickets (Gryllus bimaculatus), a model system for the mechanisms of intra-specific aggression, agonistic experiences with the underlying impact of neuromodulators have been identified as a cause of inter-individual differences. For mammals and humans, the experience of adversity and stress can have detrimental effects on cognitive abilities and chronic defeat stress is used as a model for depression. In crickets the equivalent, the chronic social defeat stress paradigm, has been established. This thesis first sets out to construct a new model for measuring a conditioned response from multiple behavioural aspects and quantify learning in individual crickets. Video tracking of responses revealed behavioural variables that were included in a binary logistic regression analysis, whereas the resulting multi-variable model proves to be superior to other models constructed and can give the probability of an individual exhibiting a conditioned response. With this, learning indices can be calculated for each individual trained in a differential appetitive olfactory paradigm. With the method at hand, this thesis reveals that the experience of chronic social stress impairs learning in crickets, susceptible and resilient to defeat stress alike. The experience of multiple wins, however, does neither improve nor decrease learning abilities, but a long-term winner effect on aggression could be shown. Although inter-individual differences in learning are present, the aggressive state of crickets is not correlated to the learning indices. The application of serotonergic drugs that block receptors or act as re-uptake inhibitors reveal the influence of serotonin on learning within this paradigm. In addition to maintaining reduced aggressiveness, serotonin promotes the impairment of learning after the experience of chronic social defeat stress.:1 INTRODUCTION.....................................................................................................1 2 MATERIALS AND METHODS............................................................................... 8 2.1 Experimental animals...................................................................................... 8 2.2 Appetitive olfactory conditioning..................................................................... 8 2.2.1 Odour application and rewarding....................................................... 8 2.2.2 Absolute conditioning paradigm........................................................ 10 2.2.3 Differential conditioning paradigm.................................................... 11 2.3 Experimental setup for video-tracking.............................................................. 12 2.4 Binary logistic regression model....................................................................... 13 2.4.1 Binary groups for model building...................................................... 13 2.4.2 Variables of a behavioural response................................................... 14 2.4.3 Calculating a conditioned odour response probability (Presp) ............ 15 2.5 Evaluation of learning with the binary logistic model...................................... 17 2.6 Evaluation of aggression with a standardised fight.......................................... 18 2.7 Multiple agonistic experiences......................................................................... 19 2.7.1 Chronic social defeat stress................................................................ 19 2.7.2 Multiple wins..................................................................................... 20 2.8 Serotonin......................................................................................................... 20 2.8.1 Pharmacological treatments............................................................... 20 2.8.2 Methiothepin and ketanserin.............................................................. 21 2.8.3 Fluoxetine with non-chronic defeat................................................... 21 2.9 Additional data analysis and statistic................................................................ 22 3 RESULTS............................................................................................................ 23 3.1 Binary logistic regression model for quantifying learning............................... 23 3.1.1 Behavioural variables of a conditioned odour response.................... 23 3.1.2 Model building and selection............................................................. 29 3.1.3 Odour response probabilities (Presp)................................................... 31 3.1.4 Application of the regression model to assess the quantification of learning.................................................................................................... 34 3.2 The influence of agonistic experiences on aggression and learning................. 39 3.2.1 Chronic social defeat stress................................................................ 39 3.2.2 Multiple experiences of winning........................................................ 46 3.2.3 Correlation of aggression and learning............................................... 48 3.2.4 Summary of learning capacities – multiple experiences.................... 50 3.3 The influence of serotonergic drugs on learning after chronic defeat.............. 51 3.3.1 Methiothepin and ketanserin.............................................................. 51 3.3.2 Fluoxetine........................................................................................... 57 3.3.3 Summary of learning capacities – chronic defeat and serotonin........ 60 4 DISCUSSION....................................................................................................... 63 4.1 The semi-automated measurement of olfactory learning in individually assayed crickets................................................................................................................... 64 4.2 The influence of multiple agonistic experiences on learning........................... 71 4.3 The role of serotonin in chronic social defeat influenced learning................... 77 4.4 Overall conclusion and outlook........................................................................ 80 5 SUMMARY........................................................................................................... 82 6 ZUSAMMENFASSUNG........................................................................................ 87 7 REFERENCES.................................................................................................... 93 8 APPENDIX................................................................................................... 106 8.1 Figures and tables................................................................................... 106 8.2 Publications and published abstracts....................................................... 108 8.3 Curriculum vitae....................................................................................... 109 8.4 Acknowledgements.................................................................................. 11

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens.

Despite abundant expression of DNA methyltransferases (Dnmts) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We found that Dnmt3a expression was regulated in mouse nucleus accumbens (NAc) by chronic cocaine use and chronic social defeat stress. Moreover, NAc-specific manipulations that block DNA methylation potentiated cocaine reward and exerted antidepressant-like effects, whereas NAc-specific Dnmt3a overexpression attenuated cocaine reward and was pro-depressant. On a cellular level, we found that chronic cocaine use selectively increased thin dendritic spines on NAc neurons and that DNA methylation was both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli

Sensory Contact Model: Protocol, Control, Applications

Among the models that become more and more popular in behavioral neuroscience are biosocial models, which allow studying the consequences of chronic social conflicts and social stress in animals. The sensory contact model appears to represent one of such models. Repeated experience of aggression or social defeats in daily agonistic interactions in male mice of different strains leads to the formation of opposing kinds of social behavior: one attributable to winners (aggressors) and another attributable to losers (defeated males, victims of aggression). A large variety of behavioral pathologies which develop in male mice in these conditions (anxious depression, catalepsy, social withdrawal, pronounced aggression, anxiety, hyperactivity, cognitive disturbances, anhedonia etc.), which are accompanied by somatic changes (reduced gonad function, psychogenic immune deficiency etc), suggest that this approach could be used for different aims of biomedical studies. Putative mechanisms of release and maintenance of aggressive and submissive behaviors in male mice under the sensory contact model, criteria of correct application, basic experimental setups and problem of the control, methodical capabilities and potentials of the sensory contact model applications are discussed in this paper

Effect of different agonistic experiences on behavioural seizures in fully amygdala kindled rats

Fully amygdala kindled rats were exposed to two different inter-male agonistic experiences in order to study the interaction between epilepsy and acute social stress. Victory experience did not influence the severity of seizure behaviour, whereas a single acute defeat modified both ictal and postictal seizure manifestations. Defeat resulted in less severe and shorter lasting motor seizures, and the accompanied postictal inhibition or behavioural depression was of shorter duration in comparison with pre-stress values. The ability of acute defeat to trigger anticonvulsant activity as implied by the weakened convulsive response is discussed.

Anxious to see you: Neuroendocrine mechanisms of social vigilance and anxiety during adolescence.

Social vigilance is a behavioral strategy commonly used in adverse or changing social environments. In animals, a combination of avoidance and vigilance allows an individual to evade potentially dangerous confrontations while monitoring the social environment to identify favorable changes. However, prolonged use of this behavioral strategy in humans is associated with increased risk of anxiety disorders, a major burden for human health. Elucidating the mechanisms of social vigilance in animals could provide important clues for new treatment strategies for social anxiety. Importantly, during adolescence the prevalence of social anxiety increases significantly. We hypothesize that many of the actions typically characterized as anxiety behaviors begin to emerge during this time as strategies for navigating more complex social structures. Here, we consider how the social environment and the pubertal transition shape neural circuits that modulate social vigilance, focusing on the bed nucleus of the stria terminalis and prefrontal cortex. The emergence of gonadal hormone secretion during adolescence has important effects on the function and structure of these circuits, and may play a role in the emergence of a notable sex difference in anxiety rates across adolescence. However, the significance of these changes in the context of anxiety is still uncertain, as not enough studies are sufficiently powered to evaluate sex as a biological variable. We conclude that greater integration between human and animal models will aid the development of more effective strategies for treating social anxiety

The relationship between entrapment and suicidal behavior through the lens of the integrated motivational-volitional model of suicidal behavior

Suicide and suicidal behavior are major public health concerns. As a result, a number of psychological models have been developed to better understand the emergence of suicidal ideation and suicide attempts. One such model is the integrated motivational–volitional model, a tri-partite model of suicidal behavior, which posits that entrapment is central to the final common pathway to suicide. In this review, we summarize the extant research evidence for the relationship between entrapment and suicidal ideation and behavior. Although there is robust evidence for the relationship between entrapment and suicidal ideation and behavior, there are gaps in our knowledge. We discuss the clinical implications and suggest key directions for future research