Regulating breathing and calling in an aquatic frog: Neuronal networks in the Xenopus laevis hindbrain

The aquatic frog, Xenopus laevis, possesses a complex vocal repertoire. Unlike most tetrapods, calls are produced independent of respiration using a single muscle pair. Thus, the separation of respiration and vocalization offers an important opportunity to investigate how two tightly coordinated circuits in terrestrial animals evolved to function separately in the transition to an aquatic environment. Previous studies identified vocal nuclei in the hindbrain: cranial motor nucleus (n.) IX-X, the adjacent reticular formation (Ri), and the dorsal tegmental area of the medulla (DTAM). Nucleus IX-X contains respiratory (glottal) and vocal (laryngeal) motor neurons. It is believed that reciprocal connections between these nuclei are important for generating vocal patterns, however, specific premotor neuron populations were not known. I used anatomical tracers to identify premotor vocal and respiratory neurons and their axonal targets. Findings revealed robust connections between n.IX-X and DTAM as well as strong bilateral projections. Two populations of neurons in DTAM and n.IX-X project directly to glottal and respiratory motor neurons. Electrical stimulation of these populations produced short-latency coordinated spiking of laryngeal, but not glottal, motor neurons. However, DTAM stimulation also produced a long-latency burst of glottal activity. Intracellular recordings revealed excitatory inputs from DTAM to laryngeal motor neurons, but mixed excitatory and inhibitory inputs to glottal motor neurons. Thus, DTAM circuitry may be heterogeneous. Taken together, results suggest that DTAM controls respiration and vocalization, perhaps acting as a switch between the two.