Determing Brain Regions Underlying an Antidepressant-like Phenotype in Mice

Open Access
Author:
Morris, Christian James
Area of Honors:
Biochemistry and Molecular Biology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Bernhard Luscher, Thesis Supervisor
  • Ming Tien, Honors Advisor
Keywords:
  • major depressive disorder
  • immunohistochemistry
  • somatostatin
  • Fos
  • stress
  • fluorescence microscopy
Abstract:
Major Depressive Disorder (MDD) is a highly phenotypically diverse and disabling psychiatric disorder that affects up to 17% of the U.S. population at least once in their lives (Kessler et al., 2003). Studies of human patients and animal models suggest that imbalances between neurotransmission by the main inhibitory and excitatory neurotransmitters, GABA and glutamate, respectively play a key role in MDD. Somatostatin-positive (SST+) neurons are one of the three major subtypes of GABAergic inhibitory neurons of the brain. A recent study in our lab investigated the emotion-related behavior of SSTCre:γ2f/f mice with γ2f/f control mice. These mice feature a conditional γ2 subunit deletion in only SST cells. SST GABAergic interneurons have been implicated in mood disorders, making them a cell population of interest. The γ2 subunit plays a critical role in postsynaptic accumulation of GABAARs, and deletion of this subunit directly causes functionally impaired GABAergic inhibition. Electrophysiological analyses of SST+ cells of layers 2 and 3 of the anterior cingulate cortex and CA1 of the ventral hippocampus have shown these cells to be constitutively hyperexcitable when compared to γ2f/+ controls. The aim of this experiment was to establish an immunohistochemical technique that allows monitoring of the activity of neurons globally while also elucidating which brain regions and cell populations show increased SST+ cell activity. I investigated the activity of SST+ cells of the medial prefrontal cortex (mPFC) and CA1 stratum oriens (S. oriens) using immunofluorescent staining for the c-Fos protein, which is established to reliably detect stress-induced neuronal activity in rodents. After inducing the expression of c-Fos through an acute stressor, the mice are perfused with fixative, their brains sectioned and stained and subjected to confocal imaging, and then the images of these sections quantified. I found that in S. oriens of SSTCre:γ2f/f mice there was a strong statistical trend indicating an increased percentage of Fos-expressing SST+ cells in this region following an acute stressor. This finding indicates the successful optimization of a protocol for identifying SST+ cells that are activated following a stressor and can be applied to other brain regions and SST+ cell populations of interest.