Rat triple perfused in situ preparation for studying the interaction of carotid body, brainstem and spinal cord chemoreceptors
Authors: Wilson Richard J.A. , Roy Arijit , Dutschmann Mathias , Harris Michael B 
Published: November 2023

The primary roles of the cardiorespiratory system are to oxygenate tissue and remove carbon dioxide. When blood oxygenation falls, breathing accelerates and the sympathetic nervous system kicks into high gear, increasing heart rate and blood pressure and redirecting oxygenated blood to vital organs including the brain. Without carotid body and aortic chemoreceptors the respiratory response to acute hypoxia is obliterated ‐‐ demonstrating that arterial chemoreceptors are responsible for the lion's share of the breathing responses ‐‐ but much of the cardiovascular response remains.Recently, using in vivo and in situ preparations, we discovered high fidelity oxygen chemoreceptors in the thoracic spinal cord that promote sympathetic activity and peripheral vasoconstriction (countering local hypoxia‐mediated vasodilation) during mild acute hypoxia. Remarkably, these spinal oxygen sensors (SOS) can respond to small increases or decreases in blood oxygen saturation without inputs from the aortic and carotid body chemoreceptors, vagus or brainstem.To determine how the SOS interact with carotid body and brainstem chemoreceptors, we have developed a novel triple‐perfused in situ preparation that allows separate perfusion of the carotid bodies, brainstem and thoracic spinal cord. This preparation is a descendent of the working heart brainstem preparation, being oxygenated with artificial saline and decerebrate, and thus limiting the use of anaesthetics to dissection. The carotid bodies and brainstem are perfused in the anterograde direction via cannula placed in the common carotid arteries and aortic arch, respectively. The descending aorta is ligated at its rostral end and cumulated at the level of the diaphragm, and perfused in a retrograde direction. Perfusate is equilibrated by gas mixtures produced by mass flow controllers, delivered via peristaltic pumps (perfusate to the brainstem and spinal cord are pressure feedback controlled), exits the preparation via cut blood vessels and is recirculated. Output variables are respiratory activity (recorded from phrenic nerve) and sympathetic activity (recorded from the sympathetic chain or the greater splanchnic nerve).To demonstrate the utility of this preparation and build on our previous research exploring the interaction of carotid body and brainstem respiratory chemoreflexes, we have tested whether SOS activation has an influence on respiratory responses to carotid body hypoxia via the sympathetic chain. Preliminary results suggest that spinal cord hypoxia does not affect respiratory responses to carotid body hypoxia, at least not via the superior cervical ganglia of the sympathetic chain. Future experiments will test how SOS, carotid body and brainstem chemoreceptors interact to mediate sympathetic responses to systemic hypoxia and CO2.Support or Funding InformationThis works was supported by operating grants from CIHR, Calgary SIDS Society (RJAW) and NMRCA (MD) and salary support from AIHS (RJAW) and ARC (MD).