In vivo imaging of zebrafish reveals differences in the spinal networks for escape and swimming movements

Most studies of spinal interneurons in vertebrate motor circuits have focus ed on the activity of interneurons in a single motor behavior. As a result, relatively little is known about the extent to which particular classes of spinal interneurons participate in different behaviors. Similarities betwe en the morphology and connections of interneurons activated in swimming and escape movements in different fish and amphibians led to the hypothesis th at spinal interneurons might be shared by these behaviors. To test this hyp othesis, we took advantage of the optical transparency of zebrafish larvae and developed a new preparation in which we could use confocal calcium imag ing to monitor the activity of individual identified interneurons noninvasi vely, while we simultaneously filmed the movements of the fish with a high- speed digital camera. With this approach, we could directly examine the inv olvement of individual interneurons in different motor behaviors. Our work revealed unexpected differences in the interneurons activated in swimming a nd escape behaviors. The observations lead to predictions of different beha vioral roles for particular classes of spinal interneurons that can eventua lly be tested directly in zebrafish by using laser ablations or mutant line s with interneuronal deficits.