- Title
- The impact of stress on glia: a characterisation of the chronic stress-induced density and morphological alterations observed in astrocytes and microglia
- Creator
- Tynan, Ross
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Stress is a ubiquitous sensation that everyone has experienced at some stage in their lives. Whilst in the short term, our response to stress is often essential and indeed sometimes even beneficial, excessive or prolonged exposure can be quite damaging. It is therefore not surprising that stress has been continuously associated with the development and/or exacerbation of a number of both pathological and psychopathological conditions. For instance, stress has frequently been implicated in the development of depression. Whilst the precise aetiology of depression remains elusive, a number of neurobiological models have been devised in an attempt to better our understanding. One of the most prominent models in recent literature is the neuroinflammatory model, which suggests that depression is associated with a dysregulation of inflammatory processes. Within the brain, the primary cells responsible for both responding to and synthesising pro-inflammatory molecules are a class of cells known as glia. The primary focus of the work presented within this thesis is to empirically investigate whether chronic stress, a common antecedent to depression, can induce significant perturbation to two of the primary subtypes of glial cells, astrocytes and microglia. Initially (Chapter Two), it was necessary to characterise both the behavioural and physiological response to the stressor used in the current investigations, chronic restraint stress. Animals exposed to chronic restraint stress exhibited a number of physiological responses to stress, including a sustained reduction in weight gain and a significant elevation in core body temperature. Furthermore, they manifested a number of behaviours typically used to indicate depression in animal models, such as a significant stress induced anhedonia (as indicated by a decrease in sucrose preference), and also a learned helplessness like behaviour (as indicated by a significant reduction in struggling behaviour during restraint). The results of these preliminary investigations verified the effectiveness of the restraint protocol used to induce behavioural changes consistent with what is commonly reported within the literature. The experiments described in Chapter Two also demonstrate, using immunohistochemistry, that chronic stress could induce significant modulation of microglia. Specifically, it was shown for the first time, that ionized calcium adaptor protein-1 (Iba1), a protein marker constitutively expressed by microglia, was significantly increased following exposure to chronic stress. The observed increase occurred in a number of brain regions that have been previously identified as crucial in regulating the stress response, such as the medial prefrontal cortex, the hippocampus, periaqueductal grey, bed nucleus of the stria terminalis and the amygdala. While this increase in cell density is consistent with a shift in the activation status of the cell, we could find no appreciable change in the levels of major histocompatibility complex II, a protein marker often associated with microglial activation. Nevertheless, these findings provide a novel insight into the changes associated with chronic stress, clearly indicating that microglia are perturbed, and may suggest that the cell plays a vital role in controlling and/or adapting to stress. In Chapter Three, we further investigated microglia, by evaluating changes in their activity when treated with commercially available antidepressants. Specifically, we sought to establish the relative anti-inflammatory potency of both selective serotonin and serotonin norepinephrine reuptake inhibitors (SSRI; SNRI). To do this, the study assessed the capacity of five SSRIs (fluoxetine, paroxetine, sertraline, citalopram, fluvoxamine) and one SNRI (venlafaxine) to suppress microglial activity in response to the inflammatory stimulant lipopolysaccharide (LPS). To compare the ability of antidepressants to suppress microglial activity, we measured the microglial production of the two pro-inflammatory molecules tumour necrosis factor-α (TNF-α) and nitric oxide (NO), at both 4 and 24 hours post LPS stimulation. Our results indicated that whilst the SNRI venlafaxine showed negligible anti-inflammatory potency, all SSRIs significantly attenuated both TNF-α and NO production. In terms of an underlying mechanism, we found evidence to suggest that cAMP signalling may be involved in regulating the observed anti-inflammatory response. These findings highlight the sensitivity of microglia to antidepressant treatment, and question whether antidepressants owe at least some of their therapeutic effectiveness to their ability to suppress microglial activity. In addition to the observed changes in microglia, we found that chronic stress could also induce substantial alterations to astrocytes, one of the primary glial cell subtypes within the CNS. Using three dimensional structural remodelling of glial fibrillary acidic protein positive (GFAP+) astrocytes, our results showed that chronic stress significantly decreased astrocyte cell volume, process length and the complexity of the branching of the cells processes. These changes were observed with coinciding decreases in the density of GFAP+ immunoreactive material and the number of GFAP+ cell counts. However, as there were no corresponding decreases observed in the total number of cells (as indicated by Nissl stain), or in the number of S100β positive cells, it is more likely that the observed decrease in GFAP+ cell counts reflects a loss of cell phenotype, rather than loss of the cell. Despite this, the observed decrease in the size of the cell could have profound implications, particularly given many diverse functional roles recently discovered in astrocytes. Taken together, the experiments presented within this thesis clearly demonstrate that chronic stress can cause significant perturbation to glial cells. Specifically, the findings reported show that chronic stress can induce profound alterations to glial cell structural morphology, density, and their expression of cellular phenotype. In addition, evidence is presented to demonstrate that glial functioning can be substantially altered with antidepressant treatment. The relationship between stress and glia has a number of vast implications. For instance, they may indicate that the cells have vital functional roles in both responding to and adapting to stress exposure. In addition, the evidence presented highlight glia as a potential target for the development of future medications designed for the treatment of stress and stress related conditions.
- Subject
- stress; microglia; astrocyte; antidepressant; depression; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1043429
- Identifier
- uon:14186
- Rights
- Copyright 2014 Ross Tynan
- Language
- eng
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| View Details Download | ATTACHMENT02 | Thesis | 11 MB | Adobe Acrobat PDF | View Details Download |