Effect of Chronic Stress on SST+ GABAergic Interneurons

Restricted (Penn State Only)
Author:
Bhatt, Shreya
Area of Honors:
Biology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Bernhard Luscher, Thesis Supervisor
  • Timothy J Jegla, Honors Advisor
Keywords:
  • Somatostatin
  • Major Depressive Disorder
  • Chronic Stress
Abstract:
Chronic psychological stress increases the risk of developing Major Depressive Disorder (MDD) and can precipitate depressive episodes in MDD patients. The stress paradigm, Unpredictable Chronic Mild Stress (UCMS), appears to have long-lasting detrimental behavioral effects in part by reducing the number or functionality of somatostatin (SST)+ GABAergic interneurons. The aim of my thesis was to determine whether predicted stress induced down-regulation in the density of somatostatin-expressing interneurons in the medial prefrontal cortex and hippocampus of mice subjected to UCMS reflected neuronal death, or down-regulation of somatostatin protein expression, and whether downregulation of SST cells is reversible after recovery from stress. To label SST cells genetically and independent of continued expression of SST, I worked with SSTCre:γ2f/+:Ai9 mice in which an Ai9 reporter gene is tagged during mouse development by means of Cre mediated recombination of the Ai9 gene selectively in SST expressing cells. In these mice, SST expressing cells are permanently tagged by expression of the Ai9 reporter protein tdTomato, which enables monitoring of SST cells independent of continued or altered SST expression. To control for regional variability in the density of SST immunoreactive and tdTomato-positive cells, I further stained brain sections for the pan-neuronal marker, NeuN. I compared three groups of mice: non-stressed controls, mice subjected to six weeks of UCMS and mice subjected to six weeks of UCMS followed by six weeks of recovery from stress. Focusing on brain sections of the medial prefrontal cortex and dorsal hippocampus, I found that the immuno-fluorescent staining for SST was highly variable. Moreover, only a fraction of tdTomato-positive cells was immuno-positive in my hands. These limitations of immunostaining were mostly specific for SST, as immunostaining for NeuN was efficient and more reproducible. Due to the high variability of SST immunostaining and low sample size (2-3 mice per group), my experiments could not conclusively determine any UCMS-induced changes in the density of SST immunoreactive cells. In the absence of measurable UCMS effects, I was also unable to address whether UCMS-induced changes in SST immunoreactive cells were reversible. As expected, the density of genetically labeled tdTomato-positive cells was unaffected by UCMS and following recovery, although once again these experiments were too underpowered to be reliably conclusive. For a more definite interpretation, my experiments will need to be repeated with larger group sizes and under conditions in which SST immunostaining is more effective and reproducible.