Structural plasticity of the social brain
Patel, Deepika
DOI:
10.33612/diss.133472775
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Publication date: 2020
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Patel, D. (2020). Structural plasticity of the social brain: Social stress-induced adaptations in dendritic remodeling and behavior. University of Groningen. https://doi.org/10.33612/diss.133472775
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General Discussion
Concluding Remarks
Numerous studies in humans and animals have reported stress-induced changes in the hippocampus, amygdala and prefrontal cortical brain regions at different levels of neural organization. In this thesis, along with determining social stress-induced structural changes in these brain regions in rats (chapter 2-4, 6 and 7), we also tried to examine risk factors underlying the individual differentiation in the susceptibility to develop stress-related psychopathologies. For instance, we studied the role of coping style or animal personality in the vulnerability to stress (chapter 5 and 6). A colony-intruder paradigm, an alternative to the well-known resident-intruder paradigm, was introduced to increase the lasting impact of social stress in resilient individuals (chapter 8). Neurobiological alterations in the different brain areas were investigated for cellular plasticity (neurogenesis and cell proliferation; chapter 3), structural remodeling as reflected in dendritic arborization (chapter 2) and dendritic spines (chapter 2, 4 and 5) as well as molecular changes involved in this structural plasticity (ratio between phosphorylated cofilin and cofilin; chapter 6 and 7). The studies were performed using different animal strains (Wistar and wild-type Groningen rats), housing conditions (social and non-social) and social stress paradigms (stress of social defeat in resident-intruder, colony-intruder and sensory contact model, and social stress of chronic subordination in the visible burrow system).
This thesis focused on the structural plasticity of hippocampus, amygdala and prefrontal cortex of the rats introduced to various social stimuli. The work emphasizes on the structural plasticity in some key nodes of the social brain, the dissociation of susceptible and resilient individuals and the usage of various stress models. In the following sections I discuss the results I obtained and their implications.
1. Structural plasticity of the social brain
External stimuli, both aversive and rewarding, usually lead to experience-dependent plasticity in individuals at multiple levels of neural organization [323]. Remarkably, excessive and prolonged stress alters the structural architecture of the developing as well as adult brain [306]. How does this remodeling happen?
Much attention has been focused on the hippocampus in stress studies, both because of its role in learning and memory abilities, and its impressive degree of structural plasticity. Aversive experiences, such as social defeat or predator odor exposure, tend to decrease the production of new neurons, whereas more rewarding experiences, such as physical activity or mating, tend to increase the production of new neurons [324,325]. However, in chapter 3, we could not confirm that a single episode of a social as well as a non-social stressor affects cell production or proliferation in Wistar and WTG rats. This was rather surprising considering the fact that both strains responded with a large acute corticosterone response to both stressors. Particularly surprising was the finding that the Wistar rats we used showed no inhibition of neurogenesis because in the past they clearly demonstrated to be relatively vulnerable to stress exposure. We hypothesized that the individual differences in stress-susceptibility of Wistar rats are based on
their source of supplier. Another surprising finding was the overall very low neuron proliferation rate in WTG rats.
Dendritic spines also undergo stress-induced structural plasticity. Data from this thesis have shown experience-dependent alterations in spine density. In agreement with previously published studies, we see a robust reduction of spine density in the CA1 region of the hippocampus in response to repeated social stress (chapter 2; [212]). Interestingly, similar kind of dendritic spine plasticity was also observed in all the brain regions studied of the winning WTG rats. This raises the possibility that corticosterone, which increases in response to both losing (aversive) and winning (rewarding) social interactions [196], could be responsible for mediating the spine remodeling in the winners and losers. Behaviorally, however, winners and losers respond completely differently. Although the spine density changes are similar, changes in spine morphology could also contribute to differential behavioral response [234]. Due to time constraints, spine morphology was not studied in depth for winners and losers. A detailed explanation of this opposite behavioral outcome warrants further investigation of the circuitry involved in the behavioral response.
Along with dendritic spine density analysis, studying dendrite branching is also essential as it is also heavily influenced by trauma and diseases [326]. We showed that repeated social stress induces dendritic atrophy and hypertrophy in the basal dendrites of CA1 hippocampal and BLA neurons, respectively (Chapter 2; [230]). Although not much is known about the synaptic inputs in BLA, the proximal apical and basal dendrites of CA1 are known to receive inputs primarily from CA3 neurons [171]. Kole et al. 2004 showed that both a double defeat and repetitive defeat experiences induce apical dendritic atrophy in CA3 pyramidal neurons. Interestingly, three weeks after a double defeat experience the basal dendrites of CA3 neurons expand dramatically [67]. This dynamic reorganization and de novo growth of dendrite branches in the CA3 neurons might influence structural remodeling observed in the CA1 pyramidal neurons. Future studies could elucidate the possible mechanisms responsible for CA3-CA1 dendritic remodeling due to social stress.
The dendritic and spine morphological analysis was performed using Golgi-Cox staining. Golgi-Cox staining, although a very popular and robust technique, stains the neurons stochastically. Recent advances in morphological analysis tools, such as targeted dye filling, are much quicker and could help future studies in elucidating the detailed mechanisms, specific to neuronal connectivity and plasticity.
Studies have shown that activity from presynaptic neurons can influence structural remodeling of the brain by modulating actin dynamics of the postsynaptic neurons [327,328]. Changes in actin dynamics are regulated by actin binding proteins such as cofilin. Increase in cofilin activity causes shrinkage and loss of dendritic spines through the depolymerization and breakdown of actin filaments [277,329] and this function of cofilin is inhibited by phosphorylation of cofilin at serine 3 site [283,330]. Hence, p-cofilin to cofilin ratio could be used as a good proxy for spine dynamics.
In our studies, defeated proactive rats displayed increased p-cofilin to cofilin ratio in amygdala and reduced ratio in the dorsal hippocampus of the defeated reactive rats which is expected to lead to an increased number of spines in the amygdala and a reduced number of spines in the dorsal hippocampus. The association of facilitation of emotionality in the defeated proactive and impaired cognitive functioning in defeated reactive rats could be studied to validate the correlation of cofilin levels with behavioral responses.
Similar amygdalar plasticity was seen in aggressive VBS housed dominant rats. In addition, higher spinogenesis was also found in the medial prefrontal cortex region of the subordinates. This was surprising as we had hypothesized that stress leads to a reduction of prefrontal cortex-dependent structure and functional plasticity [50]. However, for the subordinates, living in a social colony setting may become a life threatening situation in colonies where hierarchies are very steep with dominant males that are not controlling or reducing their aggressive behavior. Hence, they would need a compensatory coping mechanism to adapt to varying degrees of threat experienced in the VBS. Therefore, future investigations could focus on understanding the connection of spinogenesis in mPFC to enhanced behavioral flexibility [306] which in turn could favor optimal coping and therefore help an individual to prevent development of pathology or to increase resilience [219,244,331].
In sum, it remains to be determined whether the social stress-induced structural modifications are beneficial or detrimental to an individual’s successful coping with its environment. Future studies may unlock the mysteries and move closer to a detailed understanding of such crucial temporal dynamic process of pyramidal neurons due to stress. Eventually, this can help to identify novel prevention and treatment strategies for stress-related disorders [250].
2. Dissociation of susceptibility and resilient
It is well-known that there exists a large variability within a population in susceptibility to stress-related pathologies. In our studies we focused on various behavioral and physiological indicators to characterize individual Wistar and WTG rats for susceptibility and resilience to social stress. In comparing the two strains we demonstrated inter-population variation, however, and not as much intra-population variation. The differences between different strains nevertheless can help in the search for biomarkers for susceptibility and resilience. In order to study particularly intra-population variability in general many more animals are needed per group allowing proper selection of vulnerable and resilient individuals. In the WTG strain we noticed that individuals susceptible to the long-term negative consequences of social stress are relatively rare, which hampers the search for mechanisms behind intra-population variability. The first indication of stress vulnerability can be the change in an individual’s behavior [332]. In a social avoidance behavioral test, Wistar rats generally showed clear social avoidance behavior after repeated social stress experiences. WTG rats, however, showed no anxiety-like behavior when tested on both social avoidance test and EPM. We speculate that to observe a differential behavioral response to stress in WTG rats, we need more sensitive behavioral paradigms. However, it’s also possible that the WTG rats show high behavioral resilience
despite a strong acute physiological response to the social stress (chapter 8) and as also indicated in the neuroendocrine response to social and non-social stress in chapter 3.
In another set of experiments, we measured general anxiety levels using a single episode of stress. In this experiment, neither Wistar nor WTG rats showed any intra-population differences when tested on the elevated plus maze. Although Wistar rats are known to respond to single episode of stress [92], different breeder/supplier source could be the reason we failed to see any behavioral or structural phenotype (Sabina Spijker, oral communication). WTG rats, which are known to be more resilient [168], might not show lasting effects to a single episode of stress at all. A previous study from Vidal et al, had also reported differential response by Wistar and WTG rats on social avoidance behavior post social stress during adolescence. Defeated Wistar rats clearly avoided social stimuli which reflects that they are behaviorally more susceptible to stress than WTG rats. Whereas WTG rats proved to be more social and resilient animals [168]. Together with data from our experiments, we can conclude that WTG rats can be a good model to study stress-resilience markers, whereas Wistar rats can be used to study stress susceptibility markers but also here individual differences may exist based upon the source of the animals (Envigo Wistar Unilever rats proofed to be vulnerable in our experience).
Apart from having diagnostic factors responsible for evoking divergent consequences to social stress, studying predictive markers, such as social status and coping styles, can be crucial in understanding of the manifestation of stress vulnerability. Rank in social hierarchies and colony dynamics have also been identified as a major determining factors for vulnerability to chronic stress [251]. Obtaining a higher status in the social hierarchy makes the dominant mice vulnerable to stress ([251], poster from Sabanovic et al, from Dipesh Chaudhury’s group at FENS 2018, Berlin). This can be explained by the fact that losing higher status can be very stressful for an individual and makes them more susceptible. Also, it is observed, in our experiments as well as by other groups, that subordinate individuals, which spend significantly more time hiding in the dark burrows, experience more stress in the VBS.
For using coping style as a predictive marker for vulnerability, we tested WTG rats using the Sensory Contact Model (SCM). However, being resilient to stress, WTG rats did not show a robust response to stress making it not possible to dissociate rats in their stress response based upon coping style. Hence, future investigations, on establishing coping styles, but also social status, as predictive markers of stress vulnerability in WTG rats, could use a behavioral paradigm with a stronger social stressor.
The data from above experiments touch upon the use of behavioral and physiological factors for the determination and prediction of vulnerability. However, further systematic investigation would be needed for both predicting as well as treating stress-induced psychopathologies.
3. Comparative analysis of social stress models used
In order to understand the aetiology and treatment of stress pathology, an important question is whether the stress model induces sufficient amounts of stress? If yes, what is the nature of the stressor? Is it ethologically relevant and can it be translated to humans?
First, to validate the use of social stress models over physical stress models, behavioral and structural effects of immobilization and social defeat stress were compared. Although we did not see any effect on behavior or morphology due to single episode of stress, anxiety, as measured on the EPM, was higher in Wistar rats that experienced a single episode of social defeat than the ones that experienced immobilization stress. This indicates that social stress has higher impact than immobilization stress on the Wistar rats.
Another reason of using social stress models for studying stress pathology is that it enhances the translational value of the models. Social environment is not only essential for survival but also forms in itself an important source of stress. Considering factors like controllability and predictability [319], social stressors acts as a better stress model than non-social stressors such as immobilization stress. Social stressors usually are both uncontrollable and unpredictable for the recipient individual whereas there may occur an increase in predictability of time and duration of restraint or immobility. Although with many repeats, the Resident-Intruder (RI) paradigm and SCM, may also introduce habituation and increase the sense of controllability. This warrants use of better social stress paradigms. Also, to keep the experiment relevant to the human scenario, studying animals in a social hierarchy could be more useful. We therefore decided to use the visible burrow system, also a chronic social stress model, where social hierarchy is known to be established [300].
An important question to ask is whether the formation of social hierarchy among the animals is good or bad for the individual? A wealth of evidence shows that social hierarchies are endemic, innate, and most likely, evolved to support survival within a group-living context [287,333]. But, this does not mean that living in a hierarchy is pleasant for each individual and the level of stress experienced could depend on various circumstances such as steepness of the hierarchy, if it is egalitarian or not, or if it is stable or dynamic. In a hierarchy, the higher-ranking members possess more power, influence, and advantages than the lower-higher-ranking members [334,335] and it can be stable or dynamic requiring maintenance or (re)establishment of the colony. In our VBS experiments, 3 out of 8 colonies were dynamic and in all colonies, both dominant and subordinate rats seem to experience a similar level of stress. Compared with other studies [257], this is somewhat unexpected, because although dominant and subordinate animals showed similar stress-related physiological phenotypes higher neurogenesis was also observed in the dentate gyrus of the dominant males. This points towards the role of variables other than stress in adult neurogenesis [336] and hence, indicating that maintaining and adapting to the new hierarchical ranking can also be stressful. Thus, existence of social hierarchy is necessary and important for the survival of the species but can be stressful for an individual.
In the VBS, introducing intruders in the colony can quickly identify dominance and also the intruder rat experiences heavy defeats by the residents in the colony [320]. Hence, we introduced a novel social paradigm, which we call a intruder paradigm. In the colony-intruder paradigm, even highly resilient proactive WTG rats can be successfully defeated by the VBS resident WTG rats, as observed by the lasting suppression of circadian amplitudes of temperature and cardiovascular regulation. This is due to that the intruders in the VBS have less control and less predictability because of many potential aggressors. In future, characterizing the behavioral and structural phenotypes in both residents and intruders in this new paradigm would be useful. In addition, individual differences to susceptibility to stress can also be studied using social status as a factor of vulnerability. The colony-intruder paradigm not only mitigates the issues of RI and SCM such as habituation, but also opens up many avenues for studying social stress and susceptibility. Thus, it could serve as the authoritative paradigm for future studies on social stress.
Conclusion and future perspectives
Social environment not only plays an important role in the survival of an individual by favoring cooperative and affiliative behaviors, social interactions are often the main source of insidious stress that negatively impacts physical and mental health of certain individuals. Controllability and predictability of a social situation and importantly individual’s coping response to a social challenge, are critical factors for individual’s vulnerability to develop stress related diseases [319]. Some of the common and consistent features experienced by conspecific individuals living in social groups are bullying, domestic violence and subordination. These are successfully modelled in laboratory settings and have helped to investigate behavioral, neural and endocrine correlates of social stress.
From our experiments, we consider the VBS as a good model to induce chronic social stress among proactive aggressive-experienced rats and both chronic and episodic social stress for intruder rats. Molecular markers involved in structural plasticity such as cofilin, BDNF, spinophilin, arc, can be studied with respect to social stress of dominance and subordination. Accordingly, targeted infusion studies can be designed to uncover rescue mechanisms aiming at restoring structural and eventually behavioral dysfunction that may occur due to social stress. Furthermore, other important forms of plasticity such as presynaptic, axon and glial plasticity [337–339] can also be explored while studying social hierarchy formation.
In this thesis, the study was limited to male rats. However, since gender is considered a strong risk factor for major depression [340,341], there is an essential need of considering females in stress studies. Schmidt et al. have used a model based on the disruption of the social network in a group-housed situation in male and female mice. This model creates an unpredictable social environment which apparently induces chronic social stress in female mice. Further, this model was very well characterized behaviorally, physiologically and neurobiologically [342]. Similarly, chronic stress could be induced in females by changing their VBS housing at regular intervals. In our experience, females usually don’t show any signs of stress, such as body
weight gain, location preference or social interactions, when housed in one colony throughout the housing period. Future studies can be conducted to address many questions, including:
1. Since female rats don’t show any signs of stress during VBS housing, can it be considered as an enriched environment for female rats?
2. Is a social hierarchy also formed among the female WTG rats or can they outrank males?
3. If yes, then which female experiences the most stress, the dominant or subordinate one? 4. What do the interactions between sexes mean for social behavior and health of the
individual?
5. Do social interactions between male and female rats rewire the brain and influence social and affiliative behaviors like aggression?
6. Will dominance hierarchy among males or females be affected if the females reproduce in the colonies?
In the end, social stress can be detrimental for an individual, irrespective of its social status in the hierarchy. However, the detrimental effects of social stress can be buffered. Extensive studies on effect of microbiome, daily moderate exercise and living in an enriched environment show that these factors can help cope with the stress [343–345]. It would be interesting to test the influence of the above mentioned factors on the social brain to help overcome physical, mental and emotional stress. Understanding the complex relationships between these factors with animal sociability may help in identifying avenues for treating social stress disorders in humans.
