VISUALIZATION OF ACTIVE NEURAL CIRCUITRY IN THE SPINAL-CORD OF INTACTZEBRAFISH

1. One of the major obstacles in studying vertebrate neural networks i s the difficulty in simultaneously monitoring activity in a population of neurons. To take advantage of the transparency of larval zebrafish , we used confocal microscopy to look into the spinal cord of immobili zed fish to monitor neural responses during an escape behavior. 2. Pop ulations of identified neurons were labeled with a calcium indicator a nd neural activity was monitored on a millisecond time scale. The calc ium dependent nature of the fluorescent signals was confirmed by monit oring the accumulation, diffusion, and removal of calcium that was int roduced by electrical and sensory stimulation. 3. Zebrafish, like most swimming vertebrates, have two major classes of motoneurons; large pr imary motoneurons thought to be used primarily for rapid movements and smaller secondary motoneurons implicated in slower movements. Our opt ical approach allowed us to ask how these groups of primary and second ary motoneurons respond during the escape behavior-one of the fastest and most forceful motor behaviors produced by vertebrates. 4. We demon strate a previously unknown synchrony in the response of populations o f primary and secondary motoneurons. This synchrony can account for th e massive activation of the axial musculature during powerful escapes. Detection of this synchrony depended on the rapid in vivo imaging of activity in this neuronal population. This optical approach will allow functional studies of neuronal populations in the brain and spinal co rd of normal and mutant lines of zebrafish.