Supraesophageal complications are a major component of gastroesophageal reflux disease (GERD). GERD, for example, is a leading cause of chronic cough. GERD is also a frequently observed co-morbidity of asthma and COPD that worsens the symptoms associated with these pulmonary disorders. We hypothesize that refluxate initiates coughing and other airway defensive reflexes by two disparate mechanisms. First, aspiration of refluxate activates an airway afferent nerve subtype we have recently identified leading directly to coughing. Terminating beneath the epithelium of the extrapulmonary airways, the "cough receptors" are exquisitely sensitive to punctuate mechanical stimuli and acid and ideally situated and responsive to regulate coughing upon aspiration. Second, we propose that pulmonary reflexes such as cough may be amplified by reflux, independent of aspiration. We have recently identified 3 subtypes of nociceptors innervating the esophagus that are activated by noxious mechanical and chemical stimuli including acid. We hypothesize that transmitters released from the central terminals of these nociceptors will act to sensitize reflexes initiated by activation of airway vagal afferent nerves (i.e. central sensitization regulated by esophageal and airway afferent nerves). We have shown this directly, as capsaicin selectively administered to the esophagus does not evoke cough but markedly sensitizes the cough reflex evoked by cough receptor stimulation. In Aims 1 and 2 of this proposal, we will further characterize the electrophysiological and neurochemical properties of the vagal afferents innervating the esophagus and assess the relative capacity of these subtypes to regulate the cough reflex. In Aim 3, retrograde neuronal tracing and pharmacological analyses using microinjection will be used to define the pathways, transmitters and mechanisms by which esophageal afferent nerve activation sensitizes the cough reflex. Finally, in Aim 4, we will use patch clamp and extracellular recording techniques along with in vivo pharmacological analyses to determine the ion channels regulating cough receptor activation by acid and refluxate. The multidisciplinary approach of the planned experimentation should provide important insights into the mechanisms underlying the supraesophageal consequences of GERD. This research may also help identify novel therapeutic strategies for treating both GERD and chronic cough.
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