Donald C. Bolser - US grants

DESCRIPTION (provided by applicant): Cough is the most common reason why sick patients visit physicians in the US. The long-range goal of this research is to delineate the neurogenic mechanisms by which cough is produced and regulated. The central hypothesis of this research is that the cough motor pattern is produced by an assembly of components that includes a novel regulatory element responsible for controlling the behavior of a reconfigured respiratory pattern generator. Moreover, the novel regulatory components that control laryngeal and tracheobronchial cough are not identical. The rationale for the proposed research is that once the functional organization of the brainstem cough pattern generation system is established, the mechanisms responsible for the production of pathological cough can be identified. The specific aims of the project are: 1) Determine the role of modulation of the expiratory phase in the regulation of the tracheobronchial and laryngeal cough motor patterns, 2) Determine the functional organization of the central regulatory system for tracheobronchial and laryngeal cough, 3) Determine the role of brainstem expiratory motor pathways in the antitussive-sensitive regulatory system for tracheobronchial cough, 4) Determine the role of spinal expiratory motor pathways in the antitussive-sensitive regulatory system for cough. In the first aim, key regulatory mechanisms controlling the frequency and magnitude of repetitive tracheobronchial and laryngeal cough will be determined by altering of the excitability of each. Our preliminary data suggest a) that the frequency of repetitive coughing is primarily controlled by modulation of the duration of the latter part of the cough expiratory phase, and b) that separate regulatory mechanisms are responsible for the control of the frequency and intensity of repetitive coughing. In support of the second aim, preliminary findings suggest differential sensitivity of tracheobronchial and laryngeal cough to antitussive drugs and thus divergent central regulatory mechanisms for each. In the third aim, we test a model that predicts the presence of a tracheobronchial cough gating mechanism that is presynaptic to medullary and spinal expiratory motor pathways. This gating mechanism is sensitive to antitussive drugs. The sensitivity of rostral and caudal medullary expiratory neurons to antitussive drugs will be determined during breathing and cough to differentiate between inhibition or disfacilitation of these neurons by these compounds. In the fourth aim, antitussive drugs will be delivered intrathecally while monitoring expiratory motor drive during cough to determine the role of spinal pathways in the suppression of expiratory motor discharge. The results of these experiments will provide an important test of the proposed functional organization of the cough pattern generator. Furthermore, this project will test the proposed roles of medullary and spinal cellular elements that contribute to the generation and control of expiratory motor discharge during cough.

DESCRIPTION (provided by applicant): The long range goal of this research is to delineate the brainstem mechanisms by which cough is produced and regulated. The central hypothesis of this research is that the core respiratory network is controlled to produce cough by neuronal assemblies dynamically organized into regulatory elements required for the expression of airway defensive behaviors. These behavioral control assemblies (BCA) are composed of neurons that operate cooperatively in circuits and are transiently configured to process and store information related to the regulation of a given behavior. We propose that BCAs for cough are composed of neurons (raphe neurons and a novel medullary population) that are not currently considered to be part of the central respiratory pattern generator (CPG). BCAs exert a critical controlling function of the respiratory CPG, allowing it to a) reconfigure to generate widely variant motor patterns associated with different respiratory behaviors such as cough, and b) impart novel regulatory characteristics to the system such that each behavior can be controlled by afferent systems in a manner that is functionally appropriate. Our overall approach will be to expand and test the current model to account for the known regulatory features of the cough reflex. The rationale for the proposed research is that once the organization and regulation of the brainstem cough pattern generator are established, the mechanisms responsible for the production of pathological cough can be identified. The Specific Aims of the project are: 1) Identify the functional relevance of raphe and caudal medial column neurons in the neurogenesis of cough, 2) Develop a predictive model that accounts for known regulatory features of cough as well as the proposed roles of raphe and caudal medial column neurons in the neurogenesis of this behavior, and 3) Identify the role of raphe and caudal medial column neurons in cough hyperresponsiveness induced by laryngeal inflammation. In the first aim, multiple raphe, caudal medial medullary, and ventral respiratory column (VRC) neurons will be recorded simultaneously during cough. Advanced spike train analysis and metrics of synchrony will be used to determine cooperative discharge patterns among these neurons specific to cough. Our preliminary data support an important role of these populations of neurons in assemblies that control coughing. In aim 2, we will test a revised model of the cough network using network simulation tools that allow both discrete "integrate and fire" (IF) populations and "hybrid" populations that incorporate Hodgkin-Huxley style equations for subthreshold currents. We will also iteratively incorporate inferred functional interactions among specific brainstem neuronal populations identified from analyses of spike trains simultaneously recorded with multiple electrode arrays. In aim 3, metrics of synchrony and neuronal population dynamics will be applied to data from a model of acute laryngeal inflammation to identify cooperative discharge patterns that contribute to enhanced coughing. The results of these experiments will significantly advance our understanding of neural mechanisms for cough. Cough is responsible for over 25 million visits to physicians annually in this country. Patients often suffer from chronic debilitating cough for years before they are successfully treated, largely because of our lack of understanding of the basic mechanisms that produce this behavior in health and disease. (End of Abstract)

DESCRIPTION (provided by applicant): A variety of neuromuscular diseases result in impaired cough (dystussia) and/or impaired swallow (dysphagia). The long-term goal of this project is to determine brainstem mechanisms that control and coordinate cough and swallow. Our central hypothesis is that a core respiratory network is reconfigured by neuronal assemblies dynamically organized into regulatory elements (BCAs-behavioral control assemblies) necessary for the expression of airway protective behaviors. The pharyngeal phase of swallow has an important airway protective component, and this mechanism along with airflows generated by cough combine to prevent aspiration and to eject materials that penetrate the airway. The operational features, identity, and specific neural mechanisms which regulate and coordinate cough and swallow to optimize airway protection are unknown. The currently accepted model for cough proposes that the central pattern generating network for breathing is rapidly reconfigured to produce the cough motor pattern. There are no published models that explain how cough is coordinated with swallow to protect the airway from aspiration. BCAs exert a critical controlling function of the respiratory CPG, allowing it to a) reconfigure to generate widely variant motor patterns associated with cough and swallow, and b) impart novel regulatory characteristics to the system such that each behavior can be controlled by afferent systems in a manner that is functionally appropriate. The rationale for the proposed research is that once the organization and regulation of the brainstem airway protection system is established, the mechanisms responsible for aspiration in neurologic disease can be identified. The Specific Aims of this project are: 1) Identify the operational principles that govern the coordination of the cough and swallow motor patterns to protect the airway from aspiration. 2) Determine the functional role of caudal medial column neurons in the neurogenesis of the cough and swallow motor patterns. 3) Develop a predictive computational distributed network model with known regulatory mechanisms in the neurogenesis of cough and swallow. The project is expected to yield the following outcomes. First, the role of a newly identified population of neurons in the caudal medial medulla in the neurogenesis of airway protection will become known. This information will allow us to test a unified model of airway protection and elucidate the functional organization of this system. Second, this organization will be studied during challenges that promote the simultaneous expression of breathing, cough and swallow. In doing so, we also will enhance our understanding of the central mechanisms responsible for behavior selection. Third, the resultant model of the airway protection network will allow us to predict elements of the network that may be affected neurologic disease, resulting in dystussia and/or dysphagia. These outcomes will define the central mechanisms responsible for the regulation of airway protection and provide fundamental new information that will advance our understanding of the central organization of breathing, cough, and swallow. PUBLIC HEALTH RELEVANCE: A variety of neuromuscular diseases result in impaired cough (dystussia) and/or impaired swallow function (dysphagia). Impairment of these airway protective behaviors results in an increase in pulmonary infections due to aspiration. Pulmonary complications related to inadequate airway defense are the leading cause of death in patients with spinal cord injuries and Parkinson's Disease.

PROJECT SUMMARY Various neuromuscular diseases result in impaired cough (dystussia). Disorders of these airway protective behaviors increase pulmonary infection due to aspiration, the leading cause of death in neuromuscular disease. Mortality rates of aspiration pneumonia - present in over half of long-term care residents - can approach 40%. Defense of the airway is achieved through coordination of multiple protective behaviors by brain circuits that remain incompletely understood. A contemporary data-driven computational model incorporating the brainstem network for breathing can rapidly reconfigure to produce the three phases a cough motor pattern: inspiration, compression, and expulsion. However, critical elements of airway protection cannot be explained. Based on motivating preliminary data and network simulations, we propose that a circuit in the nucleus of the solitary tract (NTS) and dorsal medulla regulates phase timing and respiratory muscle drive during paroxysmal coughs and exerts a command function over the brainstem respiratory control system to coordinate coughing and breathing. The project has 3 Specific Aims: (1) Determine dynamic behavior- dependent organization of NTS circuits during the expression of airway protective behaviors. (2) Determine functional connectivity between NTS and VRC neurons during expression of coughing. (3) Reconstruct our respiratory system model to incorporate regulation of both airway protective reflexes and breathing. Our unique approach, building upon experimental interrogation of the NTS region, incorporates multi-array recording technologies in an animal model system that generates defensive behaviors in response to physiologically relevant airway perturbations. We anticipate that the project will lead to: a) a new, predictive model of airway protection will be produced, b) we will understand functional relationships between conditionally active cells and t-E NTS neurons in producing cough, and c) we will identify critical NTS to parafacial/VRC functional relationships that regulate cough and breathing. This new knowledge will provide a critical step in understanding the neurogenesis of cough and how this behavior is controlled to protect the airway.

PROJECT SUMMARY Many of the 54,000 opioid-related deaths in 2017 were due to respiratory depression. The specific mechanisms by which opioids depress breathing are not fully understood. Current hypotheses focus on single areas within the respiratory network, such as the pons or pre-Bötzinger complex. However, perturbation of these areas has not fully predicted the actions of opioids to depress breathing. Based on preliminary data and model simulations we have developed the following hypothesis: opioid administration induces reconfiguration of the respiratory network in a dose-dependent manner. This reconfiguration involves alterations in the pattern of discharge of some neurons (discharge identity), loss of synchrony among interconnected inspiratory neuron networks in the ventral respiratory column (VRC) and altered functional connectivity within and between respiratory areas in the brainstem. This project has three Specific Aims: 1) Determine the influence of opioids on functional network interactions between neurons in the nucleus tractus solitarius (NTS), raphe, pontine, and VRC that regulate motor output to respiratory muscles, 2) Determine the role of opioids in modulating synchrony within inspiratory neuron circuits in the VRC, 3) Generate a network-scale model of the brainstem circuits that control breathing in the presence of opioids. We anticipate this project will lead to: a) identification of elements of the respiratory control network that participate in opioid-mediated reconfiguration, b) delineation of functional relationships between E-T/NBM in the pons, raphe, NTS area and VRC neurons that contribute to depression of breathing by opioids, c) the role of impaired synchrony in inspiratory neuron networks in reduced motor drive produced by opioids, and d) new predictive model will be produced featuring the network-scale mechanisms that contribute to opioid depression of breathing. This new knowledge will provide a critical step in understanding the network scale mechanisms of action of opioids to depress breathing.