Subfield-specific Effects of Chronic Mild Unpredictable Stress on Hippocampal Astrocytes

Abstract

Major Depressive Disorder (MDD) is a debilitating neuropsychiatric illness affecting over 20% of the population worldwide. Despite its prevalence, our understanding of its pathophysiology is severely limited, thus hampering the development of novel therapeutic strategies. Recent advances have clearly established astrocytes as major players in the pathophysiology, and plausibly pathogenesis, of major depression. In particular, astrocyte density in the hippocampus is severely diminished in MDD patients and correlates strongly with the disease outcome. Moreover, astrocyte densities from different subfields of the hippocampus show varying trends in terms of their correlation to the disease outcome. Given the central role that hippocampus plays in the pathophysiology of depression and in the action of antidepressant drugs, changes in hippocampal astrocyte density and physiology may have a significant effect on behavioral symptoms of MDD. In this study, we used Chronic Mild Unpredictable Stress (CMUS) in mice, which induces a depressive-like state, and examined its effects on astrocytes from different subfields of the hippocampus. We used S100β immunostaining to estimate the number of astrocytes per mm 2 from various hippocampal subfields. Furthermore, using confocal images of fluorescently labeled GFAPimmunopositive hippocampal astrocytes, we quantified various morphology-related parameters and performed Sholl analysis. We found that CMUS exerts differential effects on astrocyte cell density, ramification, cell radius, surface area, and process width of hippocampal astrocytes from different hippocampal subfields. Taken together, our study reveals that chronic stress doesn't uniformly affect all hippocampal astrocytes; but exerts its effects differentially on different astrocytic subpopulations within the hippocampus.

Automated Morphometric Analysis Reveals Plasticity Induced by Chronic Antidepressant Treatment in Hippocampal Astrocytes

“…3 H). Previously, it has been shown that chronic mild unpredictable stress (CMUS) results in a significant increase in the branch width, albeit in the hilar astrocytes 27 . These results show that chronic DMI treatment and CMUS may have opposing effects on the thickness of astrocytic processes.…”

“…Stimuli that influence the development or the onset or progression of neurodegenerative disorders robustly alter the morphology of neuronal and glial cells 18,20,42 . On the other hand, subtler changes in morphology can be induced by physiologically relevant stimuli [3][4][5]15,22,27,28 . In the comparison studies, SMorph took 40 seconds to complete the analysis and it was estimated to be 1080 times faster than the analysis in ImageJ, when the faster of the two Sholl analysis methods was considered.…”

“…Immunohistochemistry and imaging: Immunohistochemistry was performed as described before 27 . In brief, sections were washed 3 times with PB and blocked with 10% normal donkey serum + 3% Bovine Serum Albumin (BSA) + 0.3% TritonX100 in 0.1M PB at room temperature for 1 hour.…”

“…Studies in depressed patients consistently show reduced astrocyte density in key limbic brain regions [23][24][25] . Additionally several studies in animal models of depression have shown reduced glial density and morphological complexity [26][27][28][29][30] , which may be reversed by chronic antidepressant treatment 26 . Interestingly, depletion of prefrontal astrocytes has been shown to be sufficient to induce depressive symptoms in experimental rats 31 , thus suggesting a causal link between glial loss and behavioral symptoms of major depressive disorder (MDD).…”

“…Studies in animal models have also shown that chronic stress results in reduced GFAPimmunoreactivity as well as reduced morphological complexity [27][28][29][30] . It has been hypothesized that such degenerative changes in astrocytes as a result of chronic stress may play a causative role in the behavioral symptoms of depression.…”