[Amphibians as a model system for the investigation of respiratory control development]. / Un modèle amphibien pour l'étude du développement du contrôle de la respiration.

Recent medical advances have made it possible for babies to survive premature birth at increasingly earlier developmental stages. This population requires costly and sophisticated medical care to address the problems associated with immaturity of the respiratory system. In addition to pulmonary complications, respiratory instability and apnea reflecting immaturity of the respiratory control system are major causes of hospitalization and morbidity in this highly vulnerable population. These medical concerns, combined with the curiosity of physiologists, have contributed to the expansion of research in respiratory neurobiology. While most researchers working in this field commonly use rodents as an animal model, recent research using in vitro brainstem preparation from bullfrogs (Rana catesbeiana) have revealed the technical advantages of this animal model, and shown that the basic principles underlying respiratory control and its ontogeny are very similar between these two groups of vertebrates. The present review highlights the recent advances in the area of research with a focus on intermittent (episodic) breathing and the role of serotonergic and GABAergic modulation of respiratory activity during development.

Intermittent hypoxia and plasticity of respiratory chemoreflexes in metamorphic bullfrog tadpoles.

We tested the hypothesis that intermittent hypoxia elicits plasticity in respiratory chemoreflexes in bullfrog tadpoles. Metamorphic tadpoles (Taylor-Kollros stages XVI-XX) were subjected to intermittent hypoxia (PW(O(2))=45 Torr; 12 h/day) or constant normoxia (PW(O(2))=156 Torr) for 2 weeks before ventilatory responses to hypoxia and hypercarbia were measured. Buccal pressure changes were used to quantify the frequency and amplitude of movements associated with gill and lung ventilation. Morphometric assessment showed that intermittent hypoxia delayed development in comparison with controls. Oxygen consumption was enhanced in tadpoles subjected to intermittent hypoxia; however, this increase was not sufficient to affect basal ventilatory activity or the hypoxic ventilatory response. During acute hypercarbic exposure, tadpoles subjected to intermittent hypoxia showed (1) a greater decrease in gill ventilation frequency and (2) a greater increase in lung ventilation frequency than tadpoles maintained under control conditions. We conclude that intermittent hypoxia augments the responsiveness to hypercarbia, thereby promoting lung ventilation when animals face this stimulus. This manifestation of respiratory plasticity may reflect uncoupling between physiological and morphological development in the bi-modally breathing bullfrog tadpole.