Student Theses and Dissertations
Date of Award
2019
Document Type
Thesis
Degree Name
Doctor of Philosophy (PhD)
Thesis Advisor
Donald W. Pfaff
Keywords
generalized arousal, nucleus gigantocellularis, Chx10+ neurons, reticulospinal circuit, motor activity, circadian transitions
Abstract
Generalized arousal (GA) is a fundamental force in the nervous system that alerts an individual to abrupt changes in its environment. A state of high GA is operationally defined by increases in an animal's a.) locomotor output, b.) responsiveness to sensory stimuli, and c.) emotional reactivity. Previous studies have identified the nucleus gigantocellularis (NGC), a small group of large-bodied neurons in the hindbrain reticular formation, as a potential neuronal substrate for GA. These neurons are responsive to a wide range of sensory modalities and have diverse projections that target both forebrain areas and motor effectors directly within the spinal cord, thereby facilitating rapid responses to sensory stimulation. Here, we used three different approaches to study the role of GA in driving and modulating mammalian motor activity: in silico modeling of GA circuits, in vitro culture of a reticulospinal circuit, and in vivo behavioral assays of circadian transitions in GA. In our in silico study, we constructed a variety of computational models of the generalized arousal circuit and asked how modifying specific aspects of the NGC and its connectivity would influence the responsiveness of motor effectors in the circuit to arousing sensory stimuli. These models reveal that an NGC with a homogeneous microstructure that integrates all inputs equally and bifurcating projections that simultaneously target limbic and spinal areas is most effective at transducing an arousing sensory signal. We then chose to focus specifically on hindbrain Chx10+ neurons, a population of spinally projecting neurons localized to the NGC, and developed an in vitro system to culture these neurons both as an isolated population and together with spinal motor neurons. Under these conditions, Chx10+ neurons develop a cell-type specific pattern of robust network bursts that they can impose on otherwise irregularly spiking motor neurons, thereby generating a functional reticulospinal connection. The activity of Chx10+ neurons was inhibited AMPAR blockers, indicating that their bursts are generated by a synaptic mechanism. Furthermore, we identified a subset of Chx10+ neurons that respond to the arousal neuromodulators orexin and norepinephrine, highlighting these neurons' role in communicating arousal signals to the spinal cord. As these two studies have demonstrated the intimate link between GA and motor output, we then used an in vivo behavioral assay of voluntary motor activity to study the dynamics of circadian transition in arousal levels in mice. We found that despite the intrinsic noisiness and variability of mouse behavior, these transitions follow a remarkably lawful sigmoidal curve that could be robustly fit to a logistic equation with only three parameters and shows time reversibility between the low-to-high and high-to-low arousal transitions. In addition to demonstrating how complex behavior can be reduced to a relatively simple mathematical form, this new curve fitting paradigm allowed us to quantify how different behavioral conditions affect arousal transitions in greater detail than ever before.
License and Reuse Information
This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License.
Recommended Citation
Bubnys, Adele, "In Vitro, In Vivo, and In Silico Studies of Reticulospinal Circuits and Generalized Arousal" (2019). Student Theses and Dissertations. 500.
https://digitalcommons.rockefeller.edu/student_theses_and_dissertations/500
Comments
A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy