- Title
- Examining changes in the neural control of respiration following early life inflammation
- Creator
- Hedley, Kateleen E.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2024
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Early life inflammation within the critical windows of development are known to have wide-reaching impacts, on not only the periphery but also on the central nervous system (CNS). However, neuroinflammation and functional consequences arising in central respiratory networks following a neonatal inflammatory challenge have not been fully explored. This thesis aims to characterise how brainstem respiratory centres respond to neonatal peripheral inflammation throughout the life span, and this work is described over 7 chapters as detailed below. Chapter 1 presents the background to this thesis, with a systematic review providing a thorough exploration of the literature available at the time of publication. This systematic review highlighted significant gaps in the literature, with exploration of inflammatory mediators limited to a select common few and functional neuronal studies almost non-existent. Study design was also limited, as very rarely were males and females investigated independently, with most studies only including one sex, and only acute timepoints were examined, regardless if it was a neonatal or adult infection, with no data available on long-term consequences. Based upon the findings in my systematic review, my thesis aims to fully explore the neuroimmune environment of the respiratory control centres within the brainstem and evaluate the anatomical, molecular and functional changes to these networks. These comparisons will occur across multiple timepoints and in both sexes to fully elucidate the long-term and sex-specific consequences of early life inflammatory events. Additional background information, as well as rationale, hypotheses and aims are included after the systematic review to introduce the thesis as a whole. Chapter 2 explored the acute and long-term effects of systemic neonatal lipopolysaccharide (LPS) exposure on the autonomic regions of the brainstem in rats. One aspect the systematic review identified, was that LPS exposure was a recurring laboratory model used to induce early life inflammation. The neonatal LPS model is commonly used to explore behavioural disorders, such as anxiety, resulting from early life inflammation but the impacts on the autonomic centres have not been explored. The work in this chapter examines the expression of a suite of inflammatory mediators and the morphological characteristics of microglia in the brainstem at both acute and long-term timepoints. I observed a distinct sex-specific response of all inflammatory mediators at both timepoints, as well as significant neonatal differences in inflammatory mediators within saline groups, indicating fundamental differences between the sexes. At both ages, microglial morphology showed significant modulation in response to LPS exposure. These data highlight the strong sex-specific response of neonates to LPS exposure and the significant life-long impact on the autonomic region of the brainstem. Chapter 3 examined the acute effects of a neonatal respiratory infection on the inflammatory environment in the lungs, as well as the respiratory control centres in the medulla oblongata, utilising the well characterised model of a Chlamydia muridarum (CMU) neonatal respiratory infection. A CMU infection was chosen as acute respiratory infections are one of the most frequent causes of morbidity in neonates and the Chlamydia genus is one of the most common bacterial pathogens to cause respiratory infections. Peripheral infection was confirmed through increased expression of bacterial genes as well as multiple key inflammatory mediators. In the medulla oblongata, there was a strong sex-dependent response with females demonstrating an upregulation of anti-inflammatory cytokines and males showing very few changes. Microglia within the medulla oblongata also demonstrated sex-specificity with females showing decreases in number of microglia and lower mean fluorescent intensity of labelling, while males showed increases. In contrast, astrocytes appeared to show very minimal changes in response to infection. These data highlight the strong sex-specific impacts peripheral inflammation can have on the respiratory control centres within the CNS in the acute phase of infection-induced inflammation. Chapter 4 illustrated the functional consequences to respiratory physiology and the modulation of the neuroinflammatory profile of the medulla oblongata that persisted into adolescence following a neonatal CMU respiratory infection. Adolescence is known to be a time of great developmental change and many conditions of both the periphery and CNS, such as asthma and some behavioural disorders, arise at this time. When measured in adolescence, lung function revealed significantly reduced capacity for diffusion of gas and increased airway hyperresponsiveness in both sexes following neonatal CMU infection. In the medulla there was significant upregulation of a suite of key inflammatory mediators in both sexes. Glia however, showed sex-dependent responses, with microglia demonstrating a more amoeboid-like morphology in females and a primed phenotype evident in males. In astrocytes, only females showed significant increases in the fluorescent intensity of labelling. Overall, the impact of the neonatal CMU infection persisted into adolescence, disrupting respiratory function and potentially impacting the ongoing neural development during this stage. Chapter 5 built upon the work in Chapters 3 and 4 by examining the peripheral and central responses in adulthood following a neonatal CMU respiratory infection. Adulthood is the final timepoint of this thesis, as although the CNS remains plastic throughout life, priming of the environment by early life events could alter this plasticity, and therefore exploring this timepoint revealed any life-long alterations. I showed significant respiratory deficits in both sexes, with increased airway hyperresponsiveness. The expression of inflammatory mediators in the medulla oblongata was more drastically impacted in males, with significant upregulation in a number of key mediators. Importantly, there were also significant differences between the sexes in uninfected groups, indicating fundamental sex-differences in the expression of inflammatory mediators in the brainstem. Similar to the earlier ages, glia demonstrated sex-specific differences, with females showing changes to astrocytes and demonstrating a classical ‘primed’ microglia morphology and males showing an increase in the number of microglia. These data highlight the considerable long-term changes to both the respiratory and central systems brought upon by a neonatal respiratory infection. Chapter 6 characterised the functional changes to neurons within two important respiratory nuclei, the dorsal motor nucleus of the vagus (DMX) and the nucleus tractus solitarii (NTS), in adulthood following a neonatal CMU respiratory infection. Given the finding of my systematic review, that very few studies present functional data on neuronal physiology, I used whole-cell patch clamp recordings to examine both the intrinsic and extrinsic properties of the two nuclei to determine any modifications to individual neurons or network connectivity. Overall, in the DMX, the majority of the changes occurred in spontaneous excitatory postsynaptic currents (sEPSCs), indicating modulation of the overall network. Females demonstrated smaller and faster events likely driven by alterations in the dendritic tree and synapse density, while males showed increased excitatory drive, driven by increases in the frequency of sEPSCs. There were fewer changes within the NTS, with hyperpolarised resting membrane potentials that were independent of sex, but only infected males required a larger rheobase current to evoke a response. Network excitability of the NTS (i.e., sEPSCs) were modulated in a sex-specific manner, with infected females having faster events and infected males having slower events. Overall, modulation to both nuclei in adulthood following neonatal respiratory infection was apparent, however, more complex physiological studies would need to be carried out to identify the underlying synaptic mechanisms. Changes to these processes could lead to disruption to homeostasis and have a flow on effects to the immediate neuronal environment and the respiratory system as a whole. Chapter 7 presents overall conclusions and future directions summarising the above findings and further experiments to build upon this work. Comparisons have been made between the two inflammatory models utilised in this thesis, and age progressions are presented to clearly demonstrate the life-long impact of a neonatal CMU infection. Given the substantial findings in two separate models of early life inflammation, across multiple ages and in both sexes, this clearly highlights the importance of the fundamental exploration into the neuroimmune consequences of early life inflammation in critical control centres of respiration.
- Subject
- brainstem respiratory centres; neonatal; Chlamydia muridarum; central nervous system
- Identifier
- http://hdl.handle.net/1959.13/1510694
- Identifier
- uon:56442
- Rights
- Copyright 2024 Kateleen E. Hedley
- Language
- eng
- Full Text
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