Ontogeny of central chemoreception during fictive gill and lung ventilation in an in vitro brainstem preparation of Rana catesbeiana.

An isolated brainstem preparation of the bullfrog tadpole, Rana catesbeiana, displays coordinated rhythmic bursting activities in cranial nerves V, VII and X in vitro. In decerebrate, spontaneously breathing tadpoles, we have previously shown that these bursts correspond to fluctuations in buccal and lung pressures and to bursts of activity in the buccal levator muscle H3a. This demonstrates that the rhythmic bursting activities recorded in vitro represent fictive gill and lung ventilation. To investigate the ontogeny of central respiratory chemoreception during the transition from gill to lung ventilation, we superfused the isolated brainstems of four larval stage groups with oxygenated artificial cerebrospinal fluid at various levels of PCO2. We measured shifts in the pattern of fictive respiratory output and the response to central hypercapnic stimulation throughout development. At normal PCO2 (2.3 kPa), stage 3­9 tadpoles displayed rhythmic neural bursts associated with gill ventilation, while stages 10­14 and 15­19 tadpoles produced oscillating bursting activity associated with both gill and lung respiration, and tadpoles at stages 20­25 displayed neural activity predominantly associated with lung ventilation. In stage 3­9 tadpoles, variations in PCO2 of the superfusate (0.5­6.0 kPa) caused almost no change in fictive gill or lung ventilation. By contrast, stage 10­14 tadpoles showed a significant hypercapnic response (P<0.05) in the amplitude and frequency of fictive gill ventilation, which was accompanied by a significant increase (P<0.05) in the burst amplitude and respiratory output of cranial nerve X over that occurring at all other stages. The amplitude and frequency of fictive gill ventilation in stages 15­19 increased significantly (P<0.05) in response to pH reduction, but became insensitive to hypercapnia at stages 20­25. The frequency of fictive lung ventilation was unresponsive to hypercapnia in stage 10­14, increased significantly by stage 15­19 (P<0.05) and became maximal (P<0.05) in stages 20­25. Overall, we describe the ontological development of central respiratory chemoreceptors driving respiratory output in the larval amphibian, demonstrating transfer in central chemoreceptive influence from gill to lung regulation during metamorphic stages. In addition, we provide novel evidence for the stimulatory influence of central chemoreceptors on fictive gill ventilation in response to CO2.