Chemogenetically Induced Activation of Cortical Somatostatin Interneurons Results in Enhanced Pyramidal Neuron Activity in Rodent Models
Restricted (Penn State Only)
- Author:
- Hutsell, Alexander
- Area of Honors:
- Biochemistry and Molecular Biology
- Degree:
- Bachelor of Arts
- Document Type:
- Thesis
- Thesis Supervisors:
- Bernhard Luscher, Thesis Supervisor
Timothy Charles Meredith, Thesis Honors Advisor - Keywords:
- Chronic Stress
Chemogenetic Manipulation
Prelimbic Cortex
medial prefrontal cortex
Somatostatin Interneurons
Pyramidal Cells
Inhibition and Excitation in the brain - Abstract:
- Chronic stress is regarded as a key contributor to several of the most common severe and debilitating neuropsychiatric disorders and is labeled by the World Health Organization as the “health epidemic of the 21st century” in recognition of its deleterious effects. Ample evidence indicates that exposure to prolonged stress leads to disruptions and long-lasting changes to the neural circuitry of the medial prefrontal cortex (mPFC). This characterization makes the mPFC a pivotal target for studying the effects of chronic stress on the brain. Preclinical studies by the Lüscher lab have shown that mice with genetically enhanced GABAergic inhibition throughout the brain exhibit a stress-resilient behavioral phenotype. This was achieved by conditional deletion of γ2 subunit containing GABA-A receptors selectively from somatostatin neurons (SSTNs) a class of GABAergic interneurons that preferentially innervates and inhibits pyramidal cells at apical dendrites. A similar stress-resilient phenotype can be induced by selectively increasing the activity of SSTNs in the medial prefrontal cortex using a chemogenetic approach. Chemogenetic activation of SSTNs thereby mimics the effects of antidepressant drug treatment which is known to similarly increase stress resilience. A large body of evidence suggests that antidepressants act by increasing the activity of PNs. This raises the question of whether SSTN-mediated inhibition of PN dendrites increases or reduces the activity of PNs. Accordingly, the objective of my thesis was to determine whether chemogenetic activation of SSTNs in chronically stressed mice results in increased or reduced activity of PNs. To test this, I made use of c-Fos as an endogenous reporter of neuronal activity in vivo. Chronically stressed SSTCre transgenic mice that had been injected stereotaxically with a Cre-dependent recombinant adeno-associated virus (AAV) expressing the Designer Receptor Exclusively Activated by Designer Drugs (DREADD) hM3Dq and equipped with minipumps that steadily release the DREADD agonist clozapine-N-Oxide (CNO) were regarding as chemogenetically induced mice. These mice were compared with control mice expressing an empty AAV and otherwise had been treated identically. I used immunohistochemical staining and confocal imaging of brain sections from these two groups of mice to quantitate the density of SSTNs and PN in the prelimbic cortex that express c-Fos. Interestingly and as predicted, I found that chronic chemogenetic activation of SSTNs in the PLC leads to a significant increase in the density of PNs that express c-Fos, indicating that SSTN activity-mediated reversal of chronic stress effects involves SSTN-mediated activation of PNs. These findings are consistent with current understanding of antidepressant drug mechanisms, which are thought to involve increased activity of prefrontal PNs in both animals and patients. Therefore, even though SSTNs are known to hyperpolarize and thus inhibit the dendrites of PNs, the end effect of this GABAergic input over time in stressed mice is to increase the activity of PNs. Curiously, however, the density of SSTNs that expressed c-Fos was unaltered in AAV-hM3Dq vs control virus-injected mice, pointing to a more complex relationship between SSTN activity and PN c-Fos expression. Possible explanations and planned future experiments for further exploration of this mechanism are discussed.