Distribution and role of peripheral arterial chemoreceptors in cardio-respiratory control of the South American rattlesnake (Crotalus durissus)

Peripheral arterial chemoreceptors monitor the levels of arterial blood gases and adjust ventilation and perfusion to meet metabolic demands. These chemoreceptors are present in all vertebrates studied to date but have not been described fully in reptiles other than turtles. The goals of this study were to (1) identify functional chemosensory areas in the South American rattlesnake (Crotalus durissus), (2) determine the neurochemical content of putative chemosensory cells in these areas and (3) determine the role each area plays in ventilatory and cardiovascular control. To this end, rattlesnakes were instrumented with transonic flow probes, arterial catheters and subcutaneous impedance electrodes to measure shunt fraction, heart rate, blood pressure and ventilation. The catheters were placed at three putative chemosensory sites, the bases of the aortic arch and pulmonary artery, and the carotid bifurcation, for site-specific activation with sodium cyanide (NaCN). These same sites were subsequently examined using immunohistochemical markers for acetylcholine, tyrosine hydroxylase (the rate-limiting enzyme in catecholamine synthesis) and serotonin to identify putative oxygen-sensing cells. All three sites were chemosensory and stimulating each led to cardiovascular (shunt fraction and heart rate) and respiratory adjustments although not in an identical fashion. All three chemosensory areas contained cells positive for serotonin; however, cells positive for vesicular acetylcholine transporter (VAChT) were found only in the aorta and pulmonary artery. We found no labelling for tyrosine hydroxylase at any site.