Evidence for rhombomeric organization of multiple respiratory oscillators in the bullfrog brainstem

Abstract

The anuran brainstem is segmentally organized into rhombomeres (r) and this segmental organization is uniquely preserved throughout development. We hypothesized that rhombomeres associated with cranial nerves (CN) also contain neural networks that are capable of producing respiratory rhythm in isolation. We used in vitro brainstem preparations from larval tadpoles and juvenile bullfrogs to determine if rhombomeric organization of respiratory oscillators is present throughout development. Brainstems were superfused with oxygenated artificial CSF (20–22 C) and respiratory output was measured with suction electrodes attached to CN V, X and XII. Brainstems were cut into segments that contained at least one CN and one or more rhombomeres. Lung bursts in an anterior segment containing CN V (r0–r5) occurred in 57% of tadpoles and 100% of frogs. Lung bursts in a middle segment containing CN X (r5–r6) occurred in 29% of tadpoles and 38% of frogs, while a caudal segment containing CN XII (r8) produced lung bursts in 50% of tadpoles and 63% of frogs. Each segment was capable of producing independent respiratory rhythm, but there were changes in respiratory frequency and whole nerve amplitude following isolation of specific segments. These data support the hypothesis that there are multiple respiratory oscillators in the anuran brainstem associated with specific groups of rhombomeres. Supported by NIH SO6 GM48135.