Dissertation
The effect of time-of-day and circadian phase on breathing and vulnerability to seizure-induced respiratory arrest and death
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2020
DOI: 10.17077/etd.005642
Abstract
Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related death in patients with refractory epilepsy. Convergent lines of evidence suggest that SUDEP occurs due to seizure-induced perturbation of respiratory, cardiac, and electrocerebral function. It is consistently observed that SUDEP happens more during the night, but the reason for this is unknown. The increased rate of SUDEP during the night is likely multifactorial involving both situational factors, such as being alone and in bed, and physiological changes such as those that occur during sleep. The primary goal of my doctoral research was to examine how circadian rhythmicity might alter vulnerability to seizure-induced death independent of sleep and situational factors. Because of the integral role of respiratory disruption in SUDEP etiology the first step was to characterize how circadian rhythms alter breathing. The effect of time-of-day on breathing has been previously characterized; however, an endogenous free-running respiratory rhythm in mammals has not previously been described. Furthermore, it is assumed that circadian rhythms in breathing are dependent on the hypothalamic suprachiasmatic nucleus (SCN), the home of the mammalian central circadian oscillator, but this has not been shown experimentally. The breathing of mice was monitored during wakefulness using whole body plethysmography at six times of day while housed under light-dark conditions and at six circadian phases while housed under constant darkness. Respiratory frequency and minute ventilation, but not tidal volume, were significantly higher during the active phase in both entrained and free-running conditions. To determine whether circadian regulation of breathing requires the SCN, in separate sets of animals this structure was electrolytically lesioned bilaterally or a sham surgery was performed, and breathing was measured at six different time points. Time-dependent oscillations in breathing were lost in SCN-lesioned animals, but not those subjected to sham surgery. These results suggest that breathing is subject to circadian regulation via the SCN.
Having examined the effect of circadian rhythms on breathing, the next step was to determine whether circadian rhythms alter vulnerability to seizure-induced respiratory arrest. Using two mouse models of seizure-induced death we demonstrate that time-of-day and circadian rhythms alter vulnerability to seizure-induced death independent of other factors such as sleep. We exposed normally entrained DBA/1 mice to a potentially seizure-inducing acoustic stimulus at different times of day and compared the characteristics and outcomes of the seizures. Time-of-day did not alter the likelihood of a seizure, but it did alter the likelihood of seizure-induced death. To determine whether circadian rhythms alter vulnerability to seizure-induced death we induced maximal electroshock seizures in free-running C57BL/6J mice at different circadian time points while measuring breathing via whole body plethysmography. Circadian phase did not affect seizure severity, but it did alter postictal respiratory outcomes and the likelihood of seizure-induced death. Contrary to our expectations, in normally entrained and free-running mice, vulnerability to seizure-induced death was greatest during the night and subjective night, respectively. These findings suggest that circadian rhythmicity may be responsible for the increased nighttime prevalence of SUDEP and that the underlying mechanism is phase conserved between nocturnal and diurnal mammals.
The purine metabolite and neuromodulator adenosine is regulated in a circadian fashion and has been implicated in the pathophysiology of SUDEP. Adenosine inhibits neuronal activity and seizure-induced upregulation of adenosine signaling is critical to seizure termination; however, seizure-induced adenosine surging may potentiate SUDEP by excessive inhibition of brain areas necessary for life such as respiratory nuclei in the brainstem. I hypothesized that (1) by administering adenosine antagonists the potentially deleterious effects of adenosine surging on breathing and electrocerebral activity could be alleviated and (2) by inhibiting the metabolic clearance of adenosine the potentially deleterious effects of adenosine surging on breathing and electrocerebral activity could be exacerbated. I found mixed support for these hypotheses. High doses of adenosine antagonists caused some seizures to result in seizure-induced death; however, paradoxically, animals in these conditions that did not experience seizure-induced death had little or no postictal EEG suppression. Inhibitors of adenosine metabolism had no effect on postictal breathing but, did worsen some postictal electrocerebral outcomes. The unexpected effect of adenosine antagonists on seizure-induced death observed in this study may be due to the vasoconstriction or phosphodiesterase inhibition caused by these drugs at high doses.
Together, the findings described in this dissertation demonstrate that both breathing and vulnerability to seizure-induced respiratory arrest are subject to circadian regulation. The observation that both nocturnal (i.e. rodents) and diurnal (i.e. humans) mammals appear to be more vulnerable to seizure-induced death during the night might have important implications for the underlying mechanism. Understanding why SUDEP happens more during the night may inform future preventative countermeasures.
Details
- Title: Subtitle
- The effect of time-of-day and circadian phase on breathing and vulnerability to seizure-induced respiratory arrest and death
- Creators
- Benton Scott Purnell
- Contributors
- Gordon F Buchanan (Advisor)Mark S Blumberg (Committee Member)Mark W Chapleau (Committee Member)Brian K Gehlbach (Committee Member)George B Richerson (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Neuroscience
- Date degree season
- Autumn 2020
- DOI
- 10.17077/etd.005642
- Publisher
- University of Iowa
- Number of pages
- xvi, 146 pages
- Copyright
- Copyright 2020 Benton Scott Purnell
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 121-146).
- Public Abstract (ETD)
Epilepsy is a disease characterized by unchecked electrical activity in the brain called seizures. Some seizures cause significant disruption in breathing. In most cases, breathing recovers normally when the seizure is over; however, sometimes breathing does not recover and the person dies in a phenomenon called sudden unexpended death in epilepsy (SUDEP). SUDEP is the leading cause of premature death in the 70 million people living with epilepsy. SUDEP typically happens after a seizure during the night. There are a variety of reasons why SUDEP might happen more during the night, one of which is circadian rhythms. Circadian rhythms are daily patterns that change many aspects of how the body works, potentially in ways that might make seizure-induced death more likely. The purpose of the experiments described in this thesis was to study how circadian rhythms alter breathing, determine if circadian rhythms alter vulnerability to seizure-induced death, and investigate how a signaling molecule in the brain called adenosine, which is affected by circadian rhythms, might alter vulnerability to seizure-induced death. I found that circadian rhythms alter breathing and vulnerability to seizure-indued death; however, my results indicate that circadian differences in vulnerability to seizure-induced death are not likely the result of circadian changes in breathing. I also observed that drugs that affect adenosine signaling change the outcome of seizures. Future experimentation should focus on determining the underlying mechanism responsible for circadian differences in vulnerability to seizure-induced death.
- Academic Unit
- Interdisciplinary Graduate Program in Neuroscience
- Record Identifier
- 9984035795602771
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