Xenopus laevis has a lateral line mechanosensory system throughout its full life cycle and a previous study on pre-feeding stage tadpoles revealed that it may play a role in motor responses to both water suction and water jets. Here, we investigated the physiology of the anterior lateral line system in newly hatched tadpoles and the motor outputs induced by its activation in response to brief suction stimuli. High-speed videoing showed tadpoles tended to turn and swim away when strong suction was applied close to the head. The lateral line neuromasts were revealed by using DASPEI staining, and their inactivation with neomycin eliminated tadpole motor responses to suction. In immobilised preparations, suction or electrically stimulating the anterior lateral line nerve reliably initiated swimming but the motor nerve discharges implicating turning was observed only occasionally. The same stimulation applied during ongoing fictive swimming produced a halting response. The anterior lateral line nerve showed spontaneous afferent discharges at rest and increased activity during stimulation. Efferent activities were only recorded during tadpole fictive swimming and were largely synchronous with the ipsilateral motor nerve discharges. Finally, calcium imaging identified neurons with fluorescence increase time-locked with suction stimulation in the hindbrain and midbrain. A cluster of neurons at the entry point of the anterior lateral line nerve in the dorsolateral hindbrain had the shortest latency in their responses, supporting their potential sensory interneuron identity. Future studies need to reveal how the lateral line sensory information is processed by the central circuit to determine tadpole motor behaviour.

Xenopus laevis has a lateral line mechanosensory system throughout its full life cycle. Previous studies of the tadpole lateral line system revealed that it may play a role in escape behaviour. In this study, we used DASPEI staining to reveal the location of tadpole lateral line neuromasts. Destroying these neuromasts with neomycin resulted in loss of escape responses in tadpoles. We then studied the physiology of anterior lateral line in immobilised tadpoles. Activating the neuromasts behind one eye could evoke asymmetrical motor nerve discharges when the tadpole was resting, suggestive of turning/escape, followed by fictive swimming. When the tadpole was already producing fictive swimming however, anterior lateral line activation reliably led to the termination of swimming. The anterior lateral line had spontaneous afferent discharges at rest, and when activated showed typical adaptation. There were also efferent activities during tadpole swimming, the activity of which was loosely in phase with ipsilateral motor nerve discharges, implying modulation by the motor circuit from the same side. Calcium imaging experiments located sensory interneurons in the primary anterior lateral line nucleus in the hindbrain. Future studies are needed to reveal how sensory information is processed by the central circuit to determine tadpole motor behaviour.Summary statementActivating tadpole anterior lateral line evokes escape responses followed by swimming and halts ongoing swimming. The afferent and efferent activities and sensory interneuron locations in the hindbrain are reported.