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

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 oscillators that are capable of producing rhythmic motor output (bursts) in isolation. We used in vitro brainstem preparations from pre- and post-metamorphic bullfrogs (Lithobates catesbeianus) to determine if rhombomeric organization of oscillators is present throughout development. Brainstems were transected into segments containing one or more rhombomeres and motor output was measured with suction electrodes attached to CN V, X and XII. Rhythmic motor output was observed in 85% of tadpoles and 91% of frogs in an anterior segment (r0-r5), 27% of tadpoles and 18% of frogs in the middle segment (r6-r7), and 77% of tadpoles and 55% of frogs in the caudal segment (r8). There were significant reductions in burst frequency and whole nerve amplitude following transections. These data support the hypothesis that brainstem oscillators associated with specific groups of rhombomeres are present throughout development in anurans.