Minicerebellum, now available for reductionists' functional study

Diversity of cellular physiology and morphology of Purkinje cells in the adult zebrafish cerebellum

This study was designed to explore the functional circuitry of the adult zebrafish cerebellum, focusing on its Purkinje cells and using whole-cell patch recordings and single cell labeling in slice preparations. Following physiological characterizations, the recorded single cells were labeled for morphological identification. It was found that the zebrafish Purkinje cells are surprisingly diverse. Based on their physiology and morphology, they can be classified into at least three subtypes: Type I, a narrow spike cell, which fires only narrow Na⁺ spikes (<3 ms in duration), and has a single primary dendrite with an arbor restricted to the distal molecular layer; Type II, a broad spike cell, which fires broad Ca²⁺ spikes (5-7 ms in duration) and has a primary dendrite with limited branching in the inner molecular layer and then further radiates throughout the molecular layer; and Type III, a very broad spike cell, which fires very broad Ca²⁺ spikes (≥10 ms in duration) and has a dense proximal dendritic arbor that is either restricted to the inner molecular layer (Type IIIa), or radiates throughout the entire molecular layer (Type IIIb). The graded paired-pulse facilitation of these Purkinje cells' responses to parallel fiber activations and the all-or-none, paired-pulse depression of climbing fiber activation are largely similar to those reported for mammals. The labeled axon terminals of these Purkinje cells end locally, as reported for larval zebrafish. The present study provides evidence that the corresponding functional circuitry and information processing differ from what has been well-established in the mammalian cerebellum.

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.