Author ORCID Identifier

https://orcid.org/0000-0002-4185-6468

Semester

Fall

Date of Graduation

2023

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Not Listed

Committee Chair

Charles Anderson

Committee Member

Ariel Agmon

Committee Member

Bernard Schreurs

Committee Member

Martin Hruska

Committee Member

Anne-Marie Oswald

Abstract

The neocortex is the most evolutionarily advanced part of the mammalian brain and is responsible for a multitude of important tasks, such as sensory processing, movement, memory and learning and, in humans, cognition, and language. Within the neocortex, there are intricate circuits of neurons that are responsible for these tasks. These circuits are comprised of a delicate balance of excitatory and inhibitory neurons. Inhibitory interneurons have the crucial role of constraining and controlling the amount of excitation in the brain; disruptions in this balance can lead to a number of neuropsychiatric diseases and disorders. Our studies focused on the subpopulation of somatostatin-containing (SOM) inhibitory interneurons, which are known to be important for sensory processing.

This dissertation explores the diversity of SOM interneurons in the somatosensory (barrel) cortex of the mouse. Chapter 1 provides an introductory overview of the barrel cortex and thalamocortical circuitry. In Chapter 2 (published as Hostetler et al., 2023), we describe the genetic, laminar distributions, laminar axonal targeting, protein marker expression, and electrophysiological properties of four novel triple-transgenic mouse models to identify nonoverlapping subtypes of SOM cells. To generate these models, we crossed four Cre driver lines of interest (Calb2-Cre, Chrna2-Cre, Calb1-Cre, Pdyn-Cre) with a SOM-Flp line, and a dual reporter line (RC::FLTG) that induced GFP expression in double positive Cre+/Flp+ cells, and tdTomato expression in Cre-/Flp+ cells. In these models, we were able to characterize and compare subgroups of SOM cells (GFP+, co-expressing Cre) to the remaining SOM cell population (TdTomato+, not co-expressing Cre). We concluded that layer 5 of the barrel cortex contains at least three nonoverlapping SOM subtypes: Calb2 SOM, Chrna2 SOM, and X94 SOM, and we tested genetic strategies to target these subtypes for future studies. These three groups differ in their axonal laminar targeting, genetic labeling, electrophysiological properties, and marker protein expression, and combined they account for > 50% of the SOM population in layer 5 and > 40% of the total SOM population in all layers. In Chapter 3 of this dissertation, X94 SOM cells were studied further for their synaptic inputs. Published and unpublished electrophysiological results from our lab demonstrate that X94 SOM cells receive monosynaptic thalamocortical synapses from the somatosensory thalamus (ventroposterior medial nucleus), in contrast to several other studies showing that thalamocortical synapses on SOM cells are weak and/or nonexistent. In Chapter 3, we demonstrate likely thalamocortical synapses on X94 SOM cells anatomically, using high-resolution confocal microscopy and super-resolution Stimulated Emission Depletion (STED) microscopy and map the somatodendritic distribution of these synapses. Chapter 4 describes preliminary results on the morphological characteristics and postsynaptic targeting of SOM subtypes identified in Chapter 2 and pilot ultrastructural studies of the thalamocortical synapses identified in Chapter 3 using Correlative Light-Electron Microscopy (CLEM). Together, these results provide novel information on the diversity of subtypes within the SOM population, differences in their pre- and post-synaptic connectivities, and novel approaches to genetically target these subtypes for future studies.

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