LIAO, James C.*; FETCHO, Joseph R.; Cornell University: Identification of sensory spinal interneurons by using optical, genetic, and electrophysiological techniques in larval zebrafish

Sensory feedback is critical to move successfully in complex environments, yet little is known about the pattern of cellular activity in response to sensory inputs to the spinal cord. Here we use optical, genetic, and electrophysiological techniques to understand how sensory inputs shape different locomotory behaviors in larval zebrafish. An initial step in this process is to identify which neurons are active in response to stimuli. To this end we adopt two complementary techniques. First, we injected a green fluorescent calcium sensor into the cord to visualize cellular activity across several body segments in response to electrical skin stimuli. Experiments were performed on larvae (3-5 dpf) raised from embryos that were injected with DNA containing a general neuronal promoter linked to a red fluorescent protein (DS-red). DS-red clearly reveals soma and axon morphology and thus facilitates subsequent identification of active cells. Second, we recorded ventral root motor activity while simultaneously patch clamping individual neurons. Our data indicate that localized skin stimuli elicit action potentials in glutamatergic Rohon Beard cells and commissural primary ascending interneurons (CoPAs). In contrast, commissural secondary ascending interneurons (CoSAs), which have hindbrain contacts like CoPAs but have smaller somata, only display sub-threshold post-synaptic potentials. GABAergic dorsal ipsilateral interneurons (DoLAs) also fire action potentials in response to skin stimuli, perhaps to modulate sensory activity. The post-synaptic targets of these cells, along with their patterns of recruitment, may determine how different motor behaviors are initiated and modulated by sensory inputs.