Naofumi Kimura

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.

Spatiotemporal organization of frog respiratory neurons visualized on the ventral medullary surface

We visualized the spatiotemporal activity of respiratory-related neurons in the frog using the isolated brainstem spinal cord preparation. We recorded optical signals from the ventral surface of the medulla using a voltage-sensitive dye, and calculated cross-correlations with the integrated respiratory activity of the trigeminal nerve. Lung burst-related depolarizing optical signals were observed bilaterally as longitudinal columns in the ventrolateral medulla between the levels of trigeminal and hypoglossal rootlets, mostly caudal to the vagal rootlet. However, we could not differentiate between neurons involved in rhythm generation and motoneurons. The dye weakened the buccal rhythm and slowed the lung rhythm, which might have influenced the results. Extracellular recording of respiratory neurons verified the optically identified area. Strychnine disrupted the spatiotemporal organization of optical signals, although trigeminal periodic bursts persisted. We conclude that the pattern generator but not the rhythm generator of lung burst in the frog involves glycinergic mechanisms and lies as longitudinal columns in the reticular formation of the ventrolateral medulla.

Chemosensitive neuronal network organization in the ventral medulla analyzed by dynamic voltage-imaging.

Central chemosensitivity is critically important to maintain homeostasis. e have successfully applied a voltage-imaging technique to medullary slice and isolated brainstem-spinal cord preparations and analyzed chemosensitive neuronal network organization in the rat ventral medulla. Our results indicate that neurons in the superficial ventral medullary chemosensitive regions and deeply located medullary respiratory neuronal network are interconnected. We propose a neuronal network organization model for central chemoreceptors.

Fictive Lung Ventilation in the Isolated Brainstem Preparation of the Aquatic Frog, Xenopus Laevis

Aquatic pipid frogs, unlike other anurans, never show sole buccal ventilation cycle, and exhale air from the lung before aspirating air into the buccal cavity. To study the mechanism that pipid frogs lack the buccal cycle, respiratory motor activities were recorded from the isolated brainstem-spinal cord preparation of Xenopus laevis. Brainstem preparations of Xenopus exhibited the intermittent burst complex (lung bursts) similar to the lung ventilation cycle in vivo. Lung bursts spontaneously occurred in the cranial nerve V, IX and X, and in the hypoglossal and third spinal nerves. Small bursts with regular cycle similar to buccal oscillation in ranid frogs, were observed in the cranial nerve V and X but not in the hypoglossal and the third spinal nerve. These results suggest that Xenopus is capable of oscillating buccal rhythm within the brainstem.