Ross J. Tynan

Chronic stress alters the density and morphology of microglia in a subset of stress responsive brain regions

The current study, in parallel experiments, evaluated the impact of chronic psychological stress on physiological and behavioural measures, and on the activation status of microglia in 15 stress-responsive brain regions. Rats were subjected, for 14 days, to two 30 min sessions of restraint per day, applied at random times each day. In one experiment the effects of stress on sucrose preference, weight gain, core body temperature, and struggling behaviour during restraint, were determined. In the second experiment we used immunohistochemistry to investigate stress-induced changes in ionized calcium-binding adaptor molecule-1 (Iba1), a marker constitutively expressed by microglia, and major histocompatibility complex-II (MHC-II), a marker often expressed on activated microglia, in a total of 15 stress-responsive nuclei. We also investigated cellular proliferation in these regions using Ki67 immunolabelling, to check for the possibility of microglial proliferation. Collectively, the results we obtained showed that chronic stress induced a significant increase in anhedonia, a decrease in weight gain across the entire observation period, a significant elevation in core body temperature during restraint, and a progressive decrease in struggling behaviour within and over sessions. With regard to microglial activation, chronic stress induced a significant increase in the density of Iba1 immunolabelling (nine of 15 regions) and the number of Iba1-positive cells (eight of 15 regions). Within the regions that exhibited an increased number of Iba1-positive cells after chronic stress, we found no evidence of a between group difference in the number of MHC-II or Ki67 positive cells. In summary, these results clearly demonstrate that chronic stress selectively increases the number of microglia in certain stress-sensitive brain regions, and also causes a marked transition of microglia from a ramified-resting state to a non-resting state. These findings are consistent with the view that microglial activation could play an important role in controlling and/or adapting to stress.

A comparative examination of the anti-inflammatory effects of SSRI and SNRI antidepressants on LPS stimulated microglia

Selective serotonin and serotonin norepinephrine reuptake inhibitors (SSRI; SNRI) are the first choice pharmacological treatment options for major depression. It has long been assumed that the primary therapeutic mechanism of action of these drugs involves the modulation of monoaminergic systems. However, contemporary investigations have revealed that depression is linked with inflammation, and that SSRI/SNRIs possess significant anti-inflammatory actions. While these anti-inflammatory properties initially only related to work undertaken on cells of the peripheral immune system, it has recently become apparent that these drugs also exert anti-inflammatory effects on microglia, the principal cells within the CNS that regulate and respond to inflammatory factors. The aim of the current study was to compare SSRI/SNRIs in terms of their anti-inflammatory potency, and to determine the specific mechanisms through which these effects are mediated. Accordingly, the current study evaluated the ability of five different SSRIs (fluoxetine, sertraline, paroxetine, fluvoxamine and citalopram) and one SNRI (venlafaxine) to suppress microglial responses to an inflammatory stimulus. Specifically, we examined their ability to alter tumour necrosis factor-α (TNF-α) and nitric oxide (NO) production after 4 and 24 h stimulation with lipopolysaccharide. Our results indicated that the SSRIs potently inhibited microglial TNF-α and NO production. We then investigated whether these effects might involve either β-adrenoceptor or cAMP signalling. Using the protein kinase A inhibitor Rp-CAMPs, we found evidence to suggest that cAMP signalling is involved in regulating the anti-inflammatory response. These findings suggest that antidepressants may owe at least some of their therapeutic effectiveness to their anti-inflammatory properties.

Chronic Stress Induced Remodeling of the Prefrontal Cortex: Structural Re-Organization of Microglia and the Inhibitory Effect of Minocycline

Recently, it has been discovered that the working memory deficits induced by exposure to chronic stress can be prevented by treating stressed animals with minocycline, a putative inhibitor of microglial activity. One of the pressing issues that now requires clarification is exactly how exposure to chronic stress modifies microglial morphology, this being a significant issue as microglial morphology is tightly coupled with their function. To examine how chronic stress alters microglial morphology, we digitally reconstructed microglia within the rat medial prefrontal cortex. Our analysis revealed that stress increased the internal complexity of microglia, enhancing ramification (i.e. branching) without altering the overall area occupied by the cell and that this effect was more pronounced in larger cells. We subsequently determined that minocycline treatment largely abolished the pro-ramifying effects of stress. With respect to mechanisms, we could not find any evidence of increased inflammation or neurodegeneration (interleukin-1β, MHC-II, CD68, terminal deoxynucleotidyl transferase dUTP nick end labeling, and activated caspase-3). We did, however, find that chronic stress markedly increased the expression of β1-integrin (CD29), a protein previously implicated in microglial ramification. Together, these findings highlight that increased ramification of microglia may represent an important neurobiological mechanism through which microglia mediate the behavioral effects of chronic psychological stress.

Chronic stress-induced disruption of the astrocyte network is driven by structural atrophy and not loss of astrocytes

Chronic stress is well recognized to decrease the number of GFAP+ astrocytes within the prefrontal cortex (PFC). Recent research, however, has suggested that our understanding of how stress alters astrocytes may be incomplete. Specifically, chronic stress has been shown to induce a unique form of microglial remodelling, but it is not yet clear whether astrocytes also undergo similar structural modifications. Such alterations may be significant given the role of astrocytes in modulating synaptic function. Accordingly, in the current study we have examined changes in astrocyte morphology following exposure to chronic stress in adult rats, using three-dimensional digital reconstructions of astrocytes. Our analysis indicated that chronic stress produced profound atrophy of astrocyte process length, branching and volume. We additionally examined changes in astrocyte-specific S100β, which are both a putative astrocyte marker and a protein whose expression is associated with astrocyte distress. While we found that S100β levels were increased by stress, this increase was not correlated with atrophy. We further established that while chronic stress was associated with a decrease in astrocyte numbers when GFAP labelling was used as a marker, we could find no evidence of a decrease in the total number of cells, based on Nissl staining, or in the number of S100β+ cells. This finding suggests that chronic stress may not actually reduce astrocyte numbers and may instead selectively decrease GFAP expression. The results of the current study are significant as they indicate stress-induced astrocyte-mediated disturbances may not be due to a loss of cells but rather due to significant remodeling of the astrocyte network.

Increased microglial activation in the rat brain following neonatal exposure to a bacterial mimetic

Neonatal lipopolysaccharide (LPS) exposure increases anxiety-like behaviour in adulthood. Our current aim was to examine whether neonatal LPS exposure is associated with changes in microglial activation, and whether these alterations correspond with alterations in behaviour. In adulthood, LPS-treated animals exhibited significantly increased anxiety-like behaviour and hippocampal microglial activation. The efficacy of the LPS challenge was confirmed by increased neonatal plasma corticosterone and tyrosine hydroxylase (TH) phosphorylation in the adrenal medulla. These findings suggest a neuroimmune pathway which may underpin the long-term behavioural and neuroendocrine changes following neonatal infection.

124. A comparative examination of the anti-inflammatory effects of SSRI and SNRI antidepressants on microglia

Selective serotonin and serotonin norepinephrine reuptake inhibitors (SSRI; SNRI) are the frontline pharmacological treatment options for major depression. While these drugs have long been assumed to exert their antidepressant effects because of their ability to alter central monoamine levels, they have also been shown to exert anti-inflammatory effects. Recently it has become apparent that these drugs can also exert anti-inflammatory effects on microglia, the principal cells within the CNS that regulate and respond to inflammatory factors. From a pharmacological standpoint, much critical information remains unknown. In particular, the relative efficacy of these drugs in modulating microglial responses is yet to be determined. To address these issues, we evaluated the ability of different SSRIs (fluoxetine, sertraline, paroxetine, fluvoxamine and citalopram) and one SNRI (venlafaxine) to suppress microglial responses to an inflammatory stimulus. Specifically, we examined their ability to alter tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) production after stimulation with lipopolysaccharide. Our results indicated that the SSRIs potently inhibited microglial TNF-alpha and NO production. We then investigated whether these effects might involve either beta-adrenoceptor or cAMP signalling. Using the protein kinase A inhibitor Rp-CAMPs, we found evidence to suggest that cAMP signalling may be partially involved in regulating the anti-inflammatory response. Findings such as these could suggest that antidepressants may owe at least some of their therapeutic effectiveness to their anti-inflammatory properties.

70. Chronic stress induces profound structural atrophy of astrocytes within the prefrontal cortex: An emerging story in glial remodeling in response to stress

Astrocytes play a critical role in maintaining neuronal homeostasis. It is for this reason, that there has been considerable interest in understanding how exposure to chronic stress alters the function of these cells. In the current study we wished to extend this research by examining whether astrocytes, which form vast inter-connected networks, undergo some form of structural remodelling in response to stress. This involved creating high resolution three-dimensional digital reconstructions of astrocytes from adult rats exposed to chronic stress. We additionally, assessed changes in cell numbers using multiple techniques, including unbiased stereology, using GFAP and S100b labelling (both astrocyte specific makers), and the non-specific cell marker, Nissl stain. Our analysis indicated that chronic stress produced profound structural atrophy of astrocytes within the prefrontal cortex ( p

126. What have microglia got to do with it? New directions in the neurobiology of depression

biomarkers of stress and inflammation, and poor pregnancy outcome. Two hundred women were recruited from Denver Health Medical Center during their first trimester of pregnancy and followed until delivery. Early and late in pregnancy, women completed a series of psychosocial assessments and provided a blood sample. Assessments included measures of pregnancy-specific distress and support, overall stress, major life events, and self-efficacy. Serum levels of TNF-a, IL-6, IL-10, and CRP were measured via ELISA at each timepoint. Data on complications, delivery, and infant outcome were obtained through chart extraction. Elevated proinflammatory markers were related to lower social support, higher pregnancy-specific distress, and lower self-efficacy. Early elevations in TNF-a, IL-6, and CRP were predictive of preterm birth and increased frequency of complications overall. Similarly, higher distress and lower support were also related to more complications and preterm birth. This work provides some of the first data showing that stress and psychosocial factors are related to increases in stress and inflammation-related biomarkers, and that these increases, in turn, are associated with increased likelihood of pregnancy complications and premature delivery.