Research reportMorphological changes in the basolateral amygdala and behavioral disruptions associated with social isolation
Introduction
In a modern industrialized society where the advancement of technology can connect people from opposite sides of the globe with only a few clicks, a surprising number of people report objective social isolation, feelings of loneliness, or disconnection from others [1], [2]. Further, perceptions of loneliness and objective social isolation have become front and center during the global COVID-19 pandemic [3], [4], [5]. Feelings of loneliness are associated with affective symptoms such as depression and anxiety [6], [7], as well as health issues including heart disease, metabolic syndrome, and immune disorders [8], [9], [10]. Similar to loneliness, objective social isolation contributes to psychological and physiological disturbances and reduced mortality [11]. For example, relocation due to work or education oftentimes requires physical seperation between loved ones (e.g., military deployment) or between children and parents (e.g., first year university students living on their own); these physical seperations from familiar social partners and environments are associated with increased mental and physical health issues [12], [13].
In searching for an animal model to investigate the neurobiological and behavioral effects of chronic social stressors, the prairie vole (Microtus ochrogaster) offers a unique perspective. The prairie vole is a rodent species that exhibits a similar social structure to humans but unlike other laboratory rodents, including forming long-term pair-bonds with partners, displaying biparental care of offspring, and living in large family units in the wild [14], [15]. Exposure to social stressors in this species—including both long-term social isolation from family members and the disruption of established pair-bonds between males and females—elicits several behavioral and physiological disruptions similar to humans. For example, prairie voles that are socially isolated from a same-sex sibling for at least 4 weeks or that are removed from a pair-bonded partner of the opposite sex for at least 5 days display maladaptive behaviors such as poor stress-coping strategies, anhedonic, and anxiogenic behaviors [16], [17], [18], [19], [20], [21], [22]. Neuroendocrine changes accompany these behaviors, including hyperactive and reactive hypothalamic-pituitary-adrenal (HPA) processes and altered oxytocin (OT) function [16], [17], [20], [23]. These findings demonstrate that the prairie vole is an ideal translational animal model to investigate behavioral and neuroendocrine changes as a function of social isolation and the disruption of established social bonds.
Previous research from laboratory rodents, including prairie voles, indicates that prolonged social isolation is associated with morphological and functional changes in the HPA axis [16], [17], [18], [20]. The HPA axis works intimately with other systems and brain regions to maintain biological homeostasis [24]. Therefore, an activation of these HPA-related regions can alter HPA function [25], [26], and dysfunction in these regions can have downstream effects on the HPA axis [27]. For example, the neuropeptides OT and arginine vasopressin (AVP) interact with HPA-related processes. OT and AVP are both synthesized primarily by the magnocellular neurons of the hypothalamic paraventricular nucleus (PVN), which is an important region for HPA signaling [28]. Further, central and peripheral release of OT and AVP mediate stress responses by interacting directly and indirectly with the HPA axis [29]. Prairie voles that were allowed to recover from a restraint stressor with a pair-bonded partner displayed elevated OT levels in the PVN, with attenuated plasma corticosterone levels, compared to animals that recovered alone [30]. Moreover, this HPA-buffering effect by the pair-bonded partner was rendered ineffective following administration of an OT receptor antagonist, suggesting that partner-mediated release of central OT had a downregulating effect on the HPA axis [30]. Lastly, central release of OT and AVP mediate prosocial and reproductive behaviors in socially monogamous rodent species [28], [31], and separation from a pair-bonded partner or a same-sex sibling can result in altered OT, AVP, and HPA function [16], [32], [33], [34]. For example, male prairie voles that were socially isolated from a pair-bonded partner for 4 weeks displayed elevated levels of OT-, AVP-, and corticotropin-releasing hormone-immunoreactivity in the PVN [23]. These previous data indicate that social isolation and social buffering can influence many neuroendocrine systems. The interactions among these systems are complex, and long-term health effects are not fully understood.
In addition to these peptide systems, the amygdala—which mediates emotional processing, memory consolidation, and fear conditioning [35]—plays an important role within the HPA-related network. The amygdala can disinhibit the PVN indirectly from the controls of the bed nucleus of stria terminalis and from the nucleus of the solitary tract in the brain stem through GABA-ergic projections [26], [27]. Additionally, the amygdala is rich in OT receptors [36], [37], and binding of OT in this region may mediate fear conditioning and fear-related behavior [38], [39]. Furthermore, the amygdala also integrates cognitive, emotional, and autonomic information from the prefrontal cortex, hippocampus, and brain stem [40]. Therefore, dysfunction of the amygdala can result in alterations of emotional processing and neuroendocrine signaling, which may produce long-term health effects [41], [42].
Chronic stress influences the structure and function of the amygdala. In particular, the basolateral region of the amygdala (BLA) has received attention in recent years due to its association with social stressors and environmental cues [43], [44]. For example, increased dendritic arborization and spine density in the BLA are observed in mice exposed to repeated social defeat stress [45], [46]. Interestingly, repeated corticosterone administration does not alter BLA spine density or morphology in male rats [47], [48], but a single dose of corticosterone appears to induce dendritic hypertrophy in the BLA, suggesting a fast neuronal reorganization and adaptation in this region [47]. There are limited data on BLA arborization and morphology in socially isolated rodents, with existing findings showing conflicting results. For example, post-weaning isolation in male rats was associated with increased dendritic arborization in the BLA, with shorter dendritic lengths [49]. However, adolescent social isolation did not augment dendritic branching or morphology in the BLA of mice [50]. Furthermore, prolonged social isolation leads to abnormal neuronal firing in the amygdala of male rats [51], and is associated with decreased cell proliferation, survival, differentiation, and increased FosB/ΔFosB-immunoreactivity in the BLA of female prairie voles [18], [52]. These changes in the BLA may contribute to hyperactivated HPA function as well as depressive- and anxiety-related behaviors often observed in lonely individuals and socially isolated animals [25], [43].
Given the importance of the social environment in mediating several aspects of behavior and neurobiological functioning, it is important to gain a better understanding of stress circuitry, HPA axis reactivity, and behavior in the context of social stress. The current study was designed to examine the effects of prolonged social isolation on the development of depressive- and anxiety-like behaviors, as well as plasma corticosterone, OT, and AVP responses to an acute social stressor in male prairie voles. The present study also evaluated amygdaloid morphology in the prairie vole model. We predicted that social isolation (relative to control conditions) would be associated with (a) depressive- and anxiety-like phenotypes; (b) altered corticosterone, OT, and AVP reactivity following a brief social stressor; and (c) greater branching and altered neuronal connectivity in the BLA.
Section snippets
Animals
Thirty-two adult male prairie voles, with a mean (±standard error of the mean; SEM) age of 120 ± 7 days and a mean (±SEM) body weight of 52 ± 1 g, were used for the present experiments. Animals were descendants of a wild stock originating from Champaign, Illinois. Animals were housed on a 14/10 h light/dark cycle with lights on at 6:30AM. The temperature of the housing environment was maintained at 25 ± 2 °C and a relative humidity of 40 ± 5%. Animals were allowed food (Purina rabbit chow) and
Body weight
Body weight was analyzed using a mixed-design ANOVA. Body weight did not differ between paired and isolated groups at either the baseline period (53 ± 2 g in paired group vs. 52 ± 2 g in isolated group) or following 4 weeks of the housing period (51 ± 2 g in paired group vs. 51 ± 3 g in isolated group). There were no significant main effects of condition or time, and no condition by time interaction (p > 0.05 for all analyses).
Fluid intake
Sucrose and water intake were analyzed using mixed-design ANOVAs (
Discussion
The current study investigated the effects of prolonged social isolation in male prairie voles on maladaptive emotion-related behaviors, neuroendocrine markers of stress reactivity, and structural BLA changes. Four weeks of social isolation was associated with increased anxiolytic and anhedonic behaviors, as well as elevated plasma OT reactivity following the resident-intruder test. Social isolation also was associated with decreased amount of dendritic material in the BLA, both in branching
Conclusions
In conclusion, the current study investigated the associations among chronic social isolation, affective behaviors, endocrine reactivity, and structural changes in the BLA of male prairie voles. Prolonged social isolation induced anxiety-related and anhedonic behaviors. These data indicate that prolonged social isolation increases affective behaviors, potentially influencing the development of depression and/or anxiety disorders. Social isolation also was associated with increased behavioral
Role of the funding source
Financial support for this work was provided by National Institutes of Health [grant numbers MH077581, HL112350, HL147179]. The sponsor had no role in the study design, data collection, data analysis and interpretation, writing of the report, or the decision to submit the article for publication.
CRediT authorship contribution statement
Michael J Hylin: Conceptualization, Methodology, Validation, Formal Analysis, Investigation, Resources, Writing – Original Draft, Writing – Review and Editing, Visualization, Supervision.W. Tang Watanasriyakul: Writing – Original Draft, Writing – Review and Editing, Visualization Natalee Hite: Formal Analysis, Writing – Review and Editing, Visualization Neal McNeal: Conceptualization, Methodology, Validation, Formal Analysis, Investigation, Resources, Writing – Review and Editing, Visualization
Declarations of Interest
None.
Acknowledgements
The authors would like to thank Ashley Dagner for valuable technical assistance.
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