CONTROL OF BREATHING DURING PHYSIOLOGIC CONDITIONS

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The role of the ventrolateral medulla (VLM) and diaphragmatic afferents (DA) in the control of breathing under anesthesia remains controversial, and their role in unanesthetized conditions is even more speculative. Thus our objective is to test controversial hypotheses regarding the VLM and DA during physiologic conditions. For the VLM, we will study awake, anesthetized, and asleep goats using: 1) chronically implanted thermodes to cool (20 degrees) and thereby create reversible neuronal dysfunction at sites near the VLM surface, and 2) microinjections of kainic acid to create permanent neuronal dysfunction at discrete VLM sites. With cooling, dysfunction effects are not obscured by compensatory changes that may occur between studies before and after permanent lesions. With permanent lesions the nuclei underlying functions can be identified. Key hypotheses are: 1) Bilateral dysfunction of an area that includes the retrotrapezoid nucleus (RTN area) will cause apnea or hypoventilation during eucapnia; during several hyperpneic conditions it will not cause apnea but will uniformly attenuate breathing; 2) Bilateral dysfunction more caudally in the intermediate VLM (IVLM) will cause a general attenuation of breathing and respiratory muscle activity, but the attenuation will be greater during elevated CO2-H+ stimulation than during exercise, hypoxia, NaCN infusion, and ventilatory loading; and 3) Simultaneous dysfunction of both the RTN area and the IVLM after carotid chemoreceptor denervation will eliminate the CO2-H+ hyperpnea and cause prolonged apnea in anesthesia and NREM sleep. Testing these hypotheses will suggest whether: 1) the RTN is critical for generation of respiratory rhythm and whether it facilitates more dorsal medullary respiratory neurons, 2) the IVLM integrates or processes intracranial chemoreception and peripheral reflexes, 3) in all states, these VLM sites are critical for CO2-H+ ventilatory sensitivity, and 4) in anesthesia and NREM sleep breathing is critically dependent on the CO2-H+ stimulus and functional RTN and IVLM areas. For DA related objectives, we will study awake ponies. We hypothesize diaphragmatic deafferentation will: 1) reduce the increased stimulation of the diaphragm which occurs in a normal pony as that of reflexes known as operational length compensation and ventilatory load compensation; 2) eliminate this increased stimulation in a lung denervated pony. These findings will support the concept that DA contribute to these reflexes. Our studies will provide unique insights into the control of breathing during physiologic conditions and diseases such as Sudden Infant Death Syndrome, Ondine's Curse, sleep apnea, and COPD.