| Summary: | Peripheral arterial chemoreceptors show anatomical and functional similarities and differences among vertebrate groups. Fishes have widely distributed neuroepithelial cells containing serotonin in all gill arches and extrabranchial sites, while mammals have clustered glomus cells in the carotid bifurcation and aortic arch that contain a number of neurotransmitters. However, we do not know how peripheral arterial chemoreceptors of amphibians and reptiles compare to other groups. My thesis established the location, distribution, neurochemical content, reflex roles and plasticity of peripheral arterial chemoreceptors in representative amphibians and reptiles (snakes (Crotalus durissus); turtles (Trachemys scripta elegans) and frogs (Rana catesbeiana)). I found functional chemosensory areas in the carotid bifurcation, aorta and pulmonary artery of rattlesnakes, the same locations where peripheral chemoreceptors are found in amphibians, turtles and tortoises. I used immunohistochemistry and tract tracing to identify putative O₂-sensing cells in snakes, turtles and frogs, and determined their neurochemical content and anatomical relation to branches of the glossopharyngeal and vagus nerves. Although the structure and innervation pattern of these cells is clearly maintained among vertebrate taxa, the types of neurochemicals involved in oxygen chemotransduction seem to have increased in number progressing throughout the vertebrate taxa. While serotonin is found in all vertebrates, the presence of other neurotransmitters varied among species. Catecholamines were found in the chemosensory areas of amphibians, while turtles and snakes contained acetylcholine. While the serotonergic and catecholamine containing cells were organized singly or in small clusters in amphibians and reptiles, cholinergic cells in reptiles were always arranged in large clusters. Unlike mammals, anatomically distinct chemoreceptor groups in snakes did not differ in their reflex response. All chemosensory areas regulated the respiratory and cardiovascular systems, the latter through adjustments in heart rate and the cardiac shunt. Furthermore, changes in the breathing pattern of turtles resulted from daily changes in the sensitivity of chemoreceptors independent of metabolism, indicating that biological rhythms play a role in respiratory control in reptiles. My findings suggest that while O₂-sensing structures are essential among vertebrate groups, considerable plasticity exists for the specifics of location and neurochemicals, which is likely related to differing needs to match oxygen supply and demand. === Science, Faculty of === Zoology, Department of === Graduate
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