Evidence for a widespread brain stem escape network in larval zebrafish

Zebrafish escape behaviors, which typically consist of a C bend, a counter-turn, and a bout of rapid swimming, are initiated by firing of the Mauthner cell and two segmental homologs. However, after laser-ablation of the Mauthner cell and its homologs, escape-like behaviors still occur, albeit at a much longer latency. This might suggest that additional neurons contribute to this behavior. We therefore recorded the activity of other descending neurons in the brain stem using confocal imaging of cells retrogradely labeled with fluorescent calcium indicators. A large majority of identified descending neurons present in the larval zebrafish, including both ipsilaterally and contralaterally projecting reticulospinal neurons, as well as neurons from the nucleus of the medial longitudinal fasciculus, showed short-latency calcium responses after gentle taps to the head of the larva-a stimulus that reliably evokes an escape behavior. Previous studies had associated such in vivo calcium responses with the firing of action potentials, and because all responding cells have axons projecting into to spinal cord, this suggests that these cells are relaying escape-related information to spinal cord. Other identified neurons failed to show consistent calcium responses to escape-eliciting stimuli. In conjunction with previous lesion studies, these results indicate that the neural control systems for turning and swimming behaviors are widely distributed in the larval zebrafish brain stem. The degree of robustness or redundancy of this system has implications for the descending control of vertebrate locomotion.