Initiation of locomotion in adult zebrafish.

Motor behavior is generated by specific neural circuits. Those producing locomotion are located in the spinal cord, and their activation depends on descending inputs from the brain or on sensory inputs. In this study, we have used an in vitro brainstem-spinal cord preparation from adult zebrafish to localize a region where stimulation of descending inputs can induce sustained locomotor activity. We show that a brief stimulation of descending inputs at the junction between the brainstem and spinal cord induces long-lasting swimming activity. The swimming frequencies induced are remarkably similar to those observed in freely moving adult fish, arguing that the induced locomotor episode is highly physiological. The motor pattern is mediated by activation of ionotropic glutamate and glycine receptors in the spinal cord and is not the result of synaptic interactions between neurons at the site of the stimulation in the brainstem. We also compared the activity of motoneurons during locomotor activity induced by electrical stimulation of descending inputs and by exogenously applied NMDA. Prolonged NMDA application changes the shape of the synaptic drive and action potentials in motoneurons. When escape activity occurs, the swimming activity in the intact zebrafish was interrupted and some of the motoneurons involved became inhibited in vitro. Thus, the descending inputs seem to act as a switch to turn on the activity of the spinal locomotor network in the caudal spinal cord. We propose that recurrent synaptic activity within the spinal locomotor circuits can transform a brief input into a well coordinated and long-lasting swimming pattern.