Responses of cerebellar Purkinje cells during fictive optomotor behavior in larval zebrafish.

Although most studies of the cerebellum have been conducted in mammals, cerebellar circuitry is highly conserved across vertebrates, suggesting that studies of simpler systems may be useful for understanding cerebellar function. The larval zebrafish is particularly promising in this regard because of its accessibility to optical monitoring and manipulations of neural activity. Although several studies suggest that the cerebellum plays a role in behavior at larval stages, little is known about the signals conveyed by particular classes of cerebellar neurons. Here we use electrophysiological recordings to characterize subthreshold, simple spike, and climbing fiber responses in larval zebrafish Purkinje cells in the context of the fictive optomotor response (OMR)-a paradigm in which fish adjust motor output to stabilize their virtual position relative to a visual stimulus. Although visual responses were prominent in Purkinje cells, they lacked the direction or velocity sensitivity that would be expected for controlling the OMR. On the other hand, Purkinje cells exhibited strong responses during fictive swim bouts. Temporal characteristics of these responses are suggestive of a general role for the larval zebrafish cerebellum in controlling swimming. Climbing fibers encoded both visual and motor signals but did not appear to encode signals that could be used to adjust OMR gain, such as retinal slip. Finally, the observation of diverse relationships between simple spikes and climbing fiber responses in individual Purkinje cells highlights the importance of distinguishing between these two types of activity in calcium imaging experiments.