Thomas E. Dick - US grants

respiratory hypoxia; hypoxia; pulmonary respiration; neural information processing; pons; hypoventilation; glutamine; respiration regulatory center; brain electrical activity; genetically modified animals; laboratory rat; laboratory mouse;

DESCRIPTION (Applicant's abstract): Sleep apnea syndrome is prevalent (3-5 percent of the adult population) and associated with significant morbidity including hypertension. The increased blood pressure appears to result from an up-regulation of basal sympathetic nerve activity (SNA). Treatment reverses this up-regulation. We theorize that the up-regulation of SNA results from both short- and long-term sequelae of episodic hypoxemia associated with sleep apnea. The respiratory and cardiovascular systems are coordinated in the maintenance of homeostasis. Respiratory modulation of SNA is an aspect of this coordination. Not only is SNA modulated with the respiratory cycle but also this modulation increases during and following brief periods of hypoxemia. The neural substrate for this coordination is undefined. Recent studies have focused on neuronal interaction between medullary respiratory-modulated and pre-motor sympathetic neurons of the rostral ventrolateral medulla (RVLM). However, neurons in the dorsolateral (dl) pontine Kolliker-Fuse (KF) nucleus are the only brainstem neurons other than the those in the NTS that project to the RVLM and that are activated by hypoxia. We hypothesize that a direct pontomedullary interaction between respiratory-modulated neurons of the KF nucleus and neurons in RVLM contributes to the respiratory modulation of sympathetic activity and that this interaction underlies the enhanced respiratory modulation of sympathetic activity during and following hypoxia. To test this hypothesis, we propose a series of neurophysiologic experiments addressing the following specific aims: 1) to determine if inhibition of dl pontine activity blocks the transient and sustained increases in respiratory modulation of SNA during and following hypoxia, 2) to determine if activation of dl pons enhances respiratory modulation of SNA, and 3) to determine if respiratory-modulated KF neurons project to and excite RVLM neurons and if these KF neurons are activated during and after hypoxia. We will investigate the neural substrate controlling respiration and blood pressure, the transient and sustained consequences of brief hypoxemia on this control, and the modulation of this control by morbidity, i.e., the development of hypertension. We propose experiments in normo- and hypertensive rats as well as in cats to evaluate the dl pontine influence on SNA, the changes in this influence with transient hypoxemia and hypertension. These proposed studies examine the mechanism of up-regulation of sympathetic nerve activity that is associated with sleep apnea.

DESCRIPTION (provided by applicant): The ventrolateral pons plays a key role in the short-term depression (STD) in respiratory frequency following hypoxia; and serotonin, as well as nitric oxide, play a part in the development of long-term facilitation (LTF) following repetitive bouts of hypoxia. GABAa-receptor sub-units in the pontomedullary circuits controlling sympathetic nerve activation (SNA) and phrenic nerve activity (PNA) show differential expression of mRNA following conditioning with hypoxia. Our data indicate that respiratory modulation of SNA not only occurs in a reduced preparation but also quantitatively (increases) and quantitatively changes its activation pattern within a breathing cycle during and following brief periods of hypoxia. Thus, we hypothesize that the up-regulation of SNA results from plasticity in the respiratory neural systems associated with repetitive hypoxic events, a study design called conditioning. To test this hypothesis, we propose a series of neurophysiologic and molecular biologic experiments addressing the following specific aims: 1) to differentiate central versus peripheral mechanisms underlying the increases in SNA following conditioning, 2) to correlate the short-term potentiation and depression (STD) as well as long-term facilitation (LTF) evident in phrenic nerve activity (PNA) with changes in SNA, and 3) to characterize the temporal expression of subunits of GABAa receptors in the brainstem nuclei controlling SNA and PNA in this study design, and 4) to compare the effects of CIH conditioning in two rodent strains with different responses to hypoxia and to nitric oxide synthetase inhibitors. This approach provides an opportunity to determine whether these animals develop increased SNA in proportion to the hypoxic response, and the interrelationship of sympathetic and respiratory control plasticity. These proposed studies examine neurophysiologic and molecular mechanisms relevant to the up-regulation of SNA activity which seems to occur in clinical conditions associated with repetitive hypoxia; e.g., sleep apnea and congestive heart failure.

DESCRIPTION (provided by applicant): Heart failure (HF) is a leading cause of mortality and is associated with a waxing and waning breathing pattern as well as enhanced sensitivity to blood gases but decreased sensitivity to blood pressure. Patients treated with continuous positive airway pressure not only have improved ventilation but also, unexpectedly, myocardial function. Our long-term goal is to understand the coordinated control of cardiovascular and respiratory function (cardiorespiratory coupling). With HF, the loss of pontine conditioning of pulmonary, baro- and chemo-sensory feedback may underlie the sympatho-respiratory co-morbidities. Our Specific Aims are: 1) To determine the influence of activating pulmonary stretch-, baro-and chemo-receptors, on sympathetic and respiratory motor patterns and coupling, and to determine how these influences are altered in HF, 2) To determine the interaction between these sensory inputs in shaping the magnitude and consistency of respiratory-and pulse-correlated activities of ventrolateral medullary and dorsolateral pontine neurons, and 3) To determine the role of pontine GABAergic and NMDA receptors in mediating the effect of these afferents on cardiorespiratory coupling in normal and HF animals. To address these Aims, we propose a collaborative approach that offers novel capabilities and synergisms. The PI will study neural mechanisms of sympatho-respiratory coupling in normal and a rodent model of HF. The HF rodent colony will be maintained by Dr. Hoit, who will also assess cardiac function in HF rats (Aims 1 & 3). Dr. Siegel will characterize pontine GABAA and NMDA receptor subunit mRNA expression, subtype formation and localization (Aim 3). With Dr. Morris at USF, the PI will analyze brainstem neurons and their responses to chemo- and baro-afferent stimulation before and after vagotomy (Aim 2). These studies will demonstrate the role of pontine conditioning of afferent input in determining the effect pulmonary, baro-and chemo-sensory information on sympatho-respiratory rhythm in health and in HF.

[unreadable] DESCRIPTION (provided by applicant): Ventilatory arrhythmia plays a pathogenic role in many common respiratory disorders ranging from sleep apnea, and acute lung injury to ventilatory support in the setting of chronic lung disease. Brainstem neural circuits that control cardiopulmonary functions generate oscillatory patterns that drive respiratory as well as sympathetic motor activities. These patterns exhibit highly structured variability and patients with various chronic diseases exhibit aberrations of these patterns and their variabilities. Analytic tools for quantifying ventilatory arrhythmia and for stratification of severity or prognosis are unavailable, representing a major barrier to defining its pathogenic contribution to disease, or to developing novel non-invasive or therapeutic markers. The long-term objectives of this exploratory project are these targets by determining the neurophysiologic mechanisms for ventilatory arrhythmia, specifically the physiological balance between central (pontomedullary) and afferent (pulmonary and baro) feedback mechanisms in the control of respiratory phase switching and pattern stabilization. The applicants hypothesize that alterations in this balance are evident in the pathology of the pulmonary conditions, but lie dormant due to lack of quantitative understanding of the dynamic properties of the respiratory control system. This hypothesis will be tested by analyzing breathing patterns in: 1) a mouse model of Rett syndrome, in which ventilatory arrhythmia originates primarily from central deficits and 2) in humans with lung disease and a rat model of lung injury, in which ventilatory arrhythmia originates primarily from altered afferent feedback. The central aim is to develop analytical methods that incorporate new characteristics of breathing pattern variability, and a computational model that accurately predicts respiratory rhythm variability resulting from internal (e.g. network modulation of feedback gain, neuromodulator interactions etc.) and external factors (peripheral chemoreceptor function, lung mechanics). An interdisciplinary research team that includes four experienced groups at different Universities will collaborate closely to perform this project. The specific aims are: 1) to expand a computational model of the brainstem respiratory network to include not only the ponto-medullary circuits but pulmonary and baro-feedback and their interactions (Rybak); 2) to test novel tools permitting the identification of disturbed breathing patterns (Loparo/Wilson); 3) to elucidate the cellular mechanisms involved in reciprocal ponto-vagal interactions by synaptic inputs to pontine and medullary respiratory neurons elicited by vagal afferent activation, including an influence of brain derived neurotrophic factor on the balance of pontine-vagal control of phase duration (Dutschmann); 4) to determine how the network interactions are altered by activation of vagal or dorsolateral pontine neurons in normal and disease states (Dick/Jacono); and 5) to describe the relative role of heritable vagal mechanisms in generating breathing pattern variability in adult twins; and the impact of ventilatory coupling to cardiac activation (cardioventilatory coupling) on breathing variability in twins and patients with lung disease (Strohl/ Jacono). The quantitative tools and insights created from this unique collaboration will permit insight into new diagnostic, prognostic and therapeutic avenues to promote stable breathing and improve patient outcomes in acute and chronic lung injury. (End of Abstract) [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Respiration in mammals is a primal homeostatic process, regulating levels of oxygen (O2) and carbon dioxide (CO2) in blood and tissues and is crucial for life. Rhythmic respiratory movements must occur continuously throughout life and originate from neural activity generated by specially organized circuits in the brain stem constituting the respiratory central pattern generator (CPG). The respiratory CPG generates rhythmic patterns of motor activity that produce coordinated movements of the respiratory pump (diaphragm, thorax, and abdomen), controlling lung inflation and deflation, and upper airway muscles, controlling airflow. These coordinated rhythmic movements drive exchange and transport of O2 and CO2 that maintain physiological homeostasis of the brain and body. Uncovering complex multilevel and multiscale mechanisms operating in the respiratory system, leading to mechanistic understanding of breathing, including breathing in different disease states requires a Physiome-type approach that relies on the development and explicit implementation of multiscale computational models of particular organs and physiological functions. The specific aims of this multi-institutional project are: (1) develop a Physiome-type, predictive, multiscale computational model of neural control of breathing that links multiple physiological mechanisms and processes involved in the vital function of breathing but operating at different scales of functional and structural organization, (2) validate this model in a series of complementary experimental investigations and (3) use the model as a computational framework for formulating predictions about possible sources and mechanisms of respiratory pattern alteration associated with heart failure. The project brings together a multidisciplinary team of scientists with long standing collaboration and complementary expertise in respiration physiology, neuroscience and translational medical studies (Thomas E. Dick, Case Western Reserve University; Julian F.R. Paton, University of Bristol, UK; Robert F. Rogers, Drexel University; Jeffrey C. Smith, NINDS, NIH, intramural), mathematics, system analysis and bioengineering (Alona Ben-Tal, Massey University, NZ), and computational neuroscience and neural control (Ilya A. Rybak, Drexel University). The end result of our proposed cross-disciplinary modeling and experimental studies will be the development and implementation of a new, fully operational, multiscale model of the integrated neurophysiological control system for breathing based on the current state of physiological knowledge. This model can then be used as a computational framework for formulating predictions about possible neural mechanisms of respiratory diseases and suggesting possible treatments.

Sepsis is systemic infection accompanied by an uncontrolled inflammatory response; a condition that can deteriorate rapidly. Early diagnosis is critical for survival. Heart rate variability (HRV), a proposed biomarker for sepsis, predicts its prognosis but is too nonspecific to make a diagnosis. Often HRV is quantified by its power spectra, its variability in the frequency domain; the `high-frequency' component reflects respiratory modulation of vagal nerve activity. Computational deterministic models of the brainstem cardiorespiratory control networks have proposed plausible neural mechanisms for the vago-respiratory coupling. In contrast to HRV, Dynamic Network Analysis (DyNA) and Dynamic Bayesian Network (DyBN) models are highly specific and successful in identifying a `tipping point' in sepsis, i.e. when a controlled inflammatory response becomes uncontrolled but its many variables are hard to measure. Recently, we identified that the brainstem becomes inflamed in endotoxemia. We hypothesize that progressive inflammation is a critical factor in losing HRV, ventilatory pattern variability (VPV), and cardiorespiratory coupling (CRC) associated with sepsis. We propose to build on the strengths of agent-based and computational modeling approaches and perform model-driven experiments to determine how alterations of brainstem neurophysiology in sepsis limit physiologic pattern variability. Our preliminary data show that endotoxemic rats lose CRC progressively in association with proinflammatory cytokines expression first in the nucleus tractus solitarius (nTS) then in the nucleus Ambiguus. Further, consistent with a progressive loss of CRC focal IL-1? microinjections in the nTS uncouples the arterial pulse pressure's influence on respiration leaving RSA intact. The Specific Aims are: 1) to develop DyNa and DyBN models of cytokine expression in brainstem cardiorespiratory control nuclei during septicemia to determine if central and peripheral inflammation patterns, 2) to adapt these models to critically-ill humans at risk for sepsis and probe the robustness of the model by applying therapeutic interventions in rats, and 3) to apply our control model to propose plausible and testable mechanisms for the effects of cytokines on the function of cardiorespiratory control circuitry. Our computational model of the neural control of cardiorespiratory coupling as well as the models defining the interactions among cytokines in tissue inflammation have been applied successfully to other conditions (sympatho-respiratory coupling) or to peripheral tissues (cytokine expression and interaction). Integrating these models will provide cross-scale mechanistic explanations for the loss of RSA and CVC observed during sepsis, identify critical cytokines for therapeutic intervention, and will establish a scientific rationale for using CRC and variability measures as complementary and sensitive biomarkers of sepsis.