Is the Vertebrate Respiratory Central Pattern Generator Conserved? In: Poon C, Kazemi H, editors. Frontiers in Modeling and Control of Breathing. Boston: Springer US; 2001. pp. 127-32. [DOI: 10.1007/978-1-4615-1375-9_20]

Lung and Buccal Ventilation in the Frog: Uncoupling Coupled Oscillators

The frog, with two distinct ventilatory acts, provides a useful model to investigate the prospective interaction of two oscillators in generating the respiratory rhythm. Building on evidence supporting the existence of separate oscillators generating buccal and lung ventilation, we have attempted to uncouple the two rhythms in the isolated brain stem preparation. Opioid preferentially inhibits the lung rhythm, suggesting an uncoupling of the lung from the buccal oscillator. Reduction of the superfusate chloride concentration alters both the buccal and the lung rhythms. Joint application of opioid and reduced-chloride superfusate leads to an increase in the variability of the buccal burst-to-lung burst intervals. This increase in variability suggests that chloride-mediated mechanisms are involved in coupling the buccal oscillator to the lung oscillator. Given the results from these interventions, we propose a simple schematic model of the frog respiratory rhythm generator, outlining the coupling of the lung and buccal oscillators.

Chemoreceptor and pulmonary stretch receptor interactions within amphibian respiratory control systems

The hypercapnic drive to breathe in amphibians is generally greater than hypoxic ventilatory drive and a variety of interdependent control systems function to regulate both the hypoxic and hypercapnic ventilatory responses. During exposure to hypercapnic conditions, breathing increases in response to input from central chemoreceptors (sensitive to CSF pH/CO(2) levels) and peripheral chemoreceptors (sensitive to arterial blood O(2) and CO(2)). On the other hand, olfactory CO(2) receptors in the nasal epithelium inhibit breathing during exposure to acute hypercapnia. Further complexity arises from the CO(2)-sensitive nature of the pulmonary stretch receptors (PSR) which provide both tonic (stimulates lung inflation at low lung volumes; deflation at higher volumes) and phasic (generally excitatory) feedback. This review focuses on interactions between the various populations of chemoreceptors and interactions between chemoreceptors and PSR. Differences between various levels of experimental reduction (i.e., in vitro; in situ; in vivo) are highlighted as are the effects of chronic respiratory challenges on acute hypoxic and hypercapnic chemoreflexes.

Ancient gill and lung oscillators may generate the respiratory rhythm of frogs and rats

Though the mechanics of breathing differ fundamentally between amniotes and "lower" vertebrates, homologous rhythm generators may drive air breathing in all lunged vertebrates. In both frogs and rats, two coupled oscillators, one active during the inspiratory (I) phase and the other active during the preinspiratory (PreI) phase, have been hypothesized to generate the respiratory rhythm. We used opioids to uncouple these oscillators. In the intact rat, complete arrest of the external rhythm by opioid-induced suppression of the putative I oscillator, that is, pre-Bötzinger complex (PBC) oscillator, did not arrest the putative PreI oscillator. In the unanesthetized frog, the comparable PreI oscillator, that is, the putative buccal/gill oscillator, was refractory to opioids even though the comparable I oscillator, the putative lung oscillator, was arrested. Studies in en bloc brainstem preparations derived from both juvenile frogs and metamorphic tadpoles confirmed these results and suggested that opioids may play a role in the clustering of lung bursts into episodes. As the frog and rat respiratory circuitry produce functionally equivalent motor outputs during lung inflation, these data argue for a close homology between the frog and rat oscillators. We suggest that the respiratory rhythm of all lunged vertebrates is generated by paired coupled oscillators. These may have originated from the gill and lung oscillators of the earliest air breathers.

Gap junction blockade with carbenoxolone differentially affects fictive breathing in larval and adult bullfrogs

This study examined the role of gap junctional communication in the modulation of respiratory related motor output using in vitro brainstem preparations of larval (N=14) and adult (N=14) bullfrogs (Rana catesbeiana). Superfusion of the isolated brainstem for at least 1 h with the gap junction blocker carbenoxolone (CBX; 100 microM and 1 mM) dissolved in artificial cerebrospinal fluid (aCSF) elicited significant changes in respiratory-related burst frequency in both larval and adult preparations. In tadpole preparations, both concentrations of CBX significantly decreased gill and lung burst frequency over 20-40 min, with 1 mM CBX producing complete cessation of gill and lung burst activity by 40 min in all preparations. There was little or no change in other burst characteristics such as burst amplitude or duration. By contrast, superfusion of the adult brainstem preparation with CBX significantly increased lung burst frequency over 10-20 min, and caused cessation of lung burst activity with 100 microM CBX (five of seven preparations) and with 1 mM CBX (seven of seven preparations). Adult preparations that ceased activity with 100 microM CBX recovered in control aCSF, but those in 1 mM did not recover, despite up to 3 h superfusion with control aCSF. In two additional adult preparations, 1 h exposure to hypercapnic aCSF (7-10% CO2) following the cessation of fictive breathing with 1 mM CBX failed to evoke respiratory activity. The inhibition of fictive breathing in tadpoles suggests that gap junctional communication may be important for respiratory rhythmogenesis prior to the development of central CO2 chemosensitivity. Following metamorphosis to the terrestrial adult, however, gap junctional communication may contribute to regulation of respiratory frequency and possibly the transduction of central CO2 chemosensitivity.