Chronic stress is one of the major precursors to numerous neuropsychiatric disorders, such as depression. Women are twice as likely to be affected by mood disorders and respond differently than men to current available treatments. Nonetheless, many preclinical studies are conducted only in male rodents. Investigating the sex-specific responses to stress is critical to identifying mechanisms underlying mood disorders and moving towards developing treatments suitable for sex differences (Zhang et al. 2024).
Chronic stress is associated with increased inflammation in the brain. The astrocyte is a type of cell in the nervous system that regulates inflammation in the brain. Astrocytes are important for keeping neurons alive, maintaining homeostasis, and secreting cytokines that regulate proinflammatory factors in the brain. The morphological changes of reactive astrocytes can tell us about the inflammation in the brain. These changes include the number of branches the astrocyte has; the more branches it has, the more reactive the astrocyte is, which is a sign of increased stress and inflammation in the brain. Chronic stress can lead to hyperactivation of astrocytes, impairing their ability to control and limit the spread of inflammation. Remodeling of astrocytes through changes in their cellular branching (more cellular branching) has been observed in suicide victims and preclinical chronic stress models. A recent study at Rowan University investigated sex-specific astrocyte responses to chronic stress in brain regions associated with mood disorders.
Figure 1. Non-reactive vs. Reactive astrocytes. Reactive astrocytes have thicker cell bodies and processes. Astrocytes become reactive in response to injury, as well as to chronic stress. Adapted from: Pekny M, Wilhelmsson U, Pekna M. 2014. The dual role of astrocyte activation and reactive gliosis. Neuroscience Letters. 565:30–38.
The study used the unpredictable chronic mild stress (UCMS) model to model chronic stress or a lipopolysaccharide (LPS) injection to model systemic inflammation. The UCMS model in rodents induces behavioral symptoms commonly associated with clinical depression as well as physiological and neurological changes that are associated with depression, such as hypertension and learned helplessness. The protocol involves randomized, daily exposures to different stressors, such as removal or bedding, social stresses, or predator sounds/smells. This model allows for an in-depth investigation of changes associated with chronic-stress induced depression (Frisbee et al. 2015). The LPS injection leads to neuroinflammation, sickness behavior, and cognitive impairment; it is a model often used to study neuroinflammation-associated diseases in mice. LPS causes inflammation because it activates microglia, which are the immune cells in the nervous system and play a large role in neuroinflammation (Zhao et al. 2019).
Male and female mice were randomly assigned to the control group (saline), a 4 hour LPS injection (4LPS), a 24 LPS injection (24LPS), no stress (NS), or stress (UCMS). They measured GFAP (a biological marker of astrocyte reactivity) fluorescent intensity for astrocyte expression and quantified branch points to assess astrocyte complexity in different brain regions.
The astrocyte complexity was investigated in the hippocampus, amygdala, and hypothalamus. The hippocampus and amygdala are critical brain regions in regulating physiological and behavioral stress processes, which can be useful in the short-term but detrimental in the long-term. In the short term, they help the body respond to stressors in order to maintain homeostasis. However, these stress mechanisms can lead to long-term dysregulation of this process as they promote maladaptive damage on the body and brain under chronically stressful conditions, which compromises resiliency and health. The amygdala is involved in detecting and responding to threats in the environment; the hippocampus is important for memory. These regions work together to make emotional and salient memories strong and long-lasting. Inflammation may hinder learning and memory through structural remodeling of the hippocampus (McEwen and Gianaros 2010). The hypothalamus, which is part of the hypothalamic-pituitary-adrenal (HPA) axis, is essential for mediating the stress response, primarily through the release of stress hormones (Bao et al. 2008).
Figure 2. The hypothalamus, hippocampus, and amygdala. Adapted from: https://www.brainframe-kids.com/emotions/facts-brain.htm.
The study found that chronic stress-induced morphological changes in astrocytes occurred in all brain regions that were looked at, and that the effects of chronic stress were both region and sex specific. Females had greater stress or inflammation-induced astrocyte activation in the hypothalamus, hippocampus, and amygdala than males. This indicates that chronic stress induces astrocyte activation that could drive sex-specific differences, which may contribute to the sex differences of mood disorders and disease.
To better assess astrocyte reactivity, they used the ramification index, which is the ratio between the total number of primary branches and the branch maximum. It indicates how ramified the astrocytes are, as more branches (more ramified) indicates more reactive astrocytes. The ramification index indicated that astrocytes were significantly ramified due to chronic stress or LPS injection. They also conducted analysis of morphological changes, which provides the strongest evidence of astrocyte reactivity. The following results demonstrate the sex differences due to stress in branch point and terminal point morphology measurements.
In the chronic-stress induced inflammatory environment (UCMS), there was higher astrocyte activation in the female hippocampus and hypothalamus, as demonstrated by increased branch points (Figure 3).
Figure 3. UCMS and LPS activate astrocytes in the hypothalamus by inducing morphological changes. A. Representative images of female astrocytes in the hypothalamus in each condition (saline, no stress, 4 hours post-LPS, 24 hours post-LPS, and UCMS). D. Branching points in the hypothalamus. There were significant differences between treatment and between sex. There were significantly more branch points for females in the UCMS condition than for males. Adapted from: Zhang AY, Elias E, Manners MT. 2024. Sex-dependent astrocyte reactivity: Unveiling chronic stress-induced morphological changes across multiple brain regions. Neurobiology of Disease. 200:106610.
In the amygdala and hippocampus in the 24LPS condition, there was increased astrocyte reactivity in females compared to males. This indicates that females are more susceptible to chronic systemic inflammation than males in these brain regions (Figure 4).
Figure 4. UCMS and LPS activate astrocytes in the amygdala by inducing morphological changes. A. Representative images of female astrocytes in the amygdala in the saline, no stress, 4LPS, 24LPS, and UCMS groups. D. Analysis of branch points in the amygdala. There were significant differences between treatment and between sex. Females had significantly more branching points in the 24LPS condition than males. Adapted from: Zhang AY, Elias E, Manners MT. 2024. Sex-dependent astrocyte reactivity: Unveiling chronic stress-induced morphological changes across multiple brain regions. Neurobiology of Disease. 200:106610.
Astrocytes work in tandem with other cells in the nervous system, including microglia, to regulate various processes. Microglia, the cells in the nervous system important for immune response, are also important in the inflammatory response in the brain. Reactive microglia can activate astrocytes by secreting cytokines. Blocking microglia may be able to decrease the number of reactive astrocytes and apply a protective effect against inflammation in the brain. Future studies can look at the activation of microglia and how microglia and astrocytes interact in the chronic stress model.
Overall, it is important that this study looked at sex differences since there is such a disparity among mental health disorders and treatment in women and men. Understanding the mechanisms behind the sex differences can improve the development of new medications for stress-related disorders so that both men and women can be correctly treated.
Moreover, female susceptibility to chronic stress may mediate the increased risk for Alzheimer’s Disease. The different biochemical responses to stress, such as activity of the hypothalamic-pituitary-adrenal (HPA) axis and female-biased increases in molecules associated with AD, between females and males could be a sex-dependent risk factor for AD. Female-specific alterations in inflammation and microglial function are proposed to be one reason, but this needs more investigation (Yan et al. 2018). Understanding sex-specific disease mechanisms is essential for the development of personalized medicine, which is the use of an individual’s genetic profile to prevent, diagnose, and treat disease. Differences in mechanisms of disease between sexes will likely require different drugs for men and women to treat a variety of psychiatric and neurological disorders (Bangasser and Wicks 2017).
References.
Bangasser DA, Wicks B. 2017. Sex-specific mechanisms for responding to stress. Journal of Neuroscience Research. 95(1–2):75–82.
Bao A-M, Meynen G, Swaab DF. 2008. The stress system in depression and neurodegeneration: Focus on the human hypothalamus. Brain Research Reviews. 57(2):531–553.
Emotion Facts: Emotions in the Brain. [accessed 2024 Oct 29]. https://www.brainframe-kids.com/emotions/facts-brain.htm.
Frisbee JC, Brooks SD, Stanley SC, d’Audiffret AC. 2015. An Unpredictable Chronic Mild Stress Protocol for Instigating Depressive Symptoms, Behavioral Changes and Negative Health Outcomes in Rodents. J Vis Exp.(106):53109.
McEwen BS, Gianaros PJ. 2010. Central role of the brain in stress and adaptation: Links to socioeconomic status, health, and disease. Annals of the New York Academy of Sciences. 1186:190.
Pekny M, Wilhelmsson U, Pekna M. 2014. The dual role of astrocyte activation and reactive gliosis. Neuroscience Letters. 565:30–38.
Tynan RJ, Naicker S, Hinwood M, Nalivaiko E, Buller KM, Pow DV, Day TA, Walker FR. 2010. Chronic stress alters the density and morphology of microglia in a subset of stress-responsive brain regions. Brain, Behavior, and Immunity. 24(7):1058–1068.
Yan Y, Dominguez S, Fisher DW, Dong H. 2018. Sex differences in chronic stress responses and Alzheimer’s disease. Neurobiology of Stress. 8:120–126.
Zhang AY, Elias E, Manners MT. 2024. Sex-dependent astrocyte reactivity: Unveiling chronic stress-induced morphological changes across multiple brain regions. Neurobiology of Disease. 200:106610.
Zhao J, Bi W, Xiao S, Lan X, Cheng X, Zhang J, Lu D, Wei W, Wang Y, Li H, et al. 2019. Neuroinflammation induced by lipopolysaccharide causes cognitive impairment in mice. Sci Rep. 9(1):5790.