Characterisation of Cerebellar Efferents in Larval Zebrafish

Abstract

The cerebellum is known to be the major centre for motor learning in vertebrates. It has a well-conserved circuit architecture, where a layer of inhibitory neurons, called Purkinje neurons, receives a copy of the executive signals from the higher motor areas and error feedback from lower neurons, computes the ‘error’ and tunes the motor execution. Interestingly, not much is known about how cerebellar output executes motor error correction. In mammals, the output from the cerebellum is carried by the axons of the deep cerebellar nuclear cells (DCNs), which project to diverse brain regions, including the motor thalamus and the reticular formation. In this work, we have looked at the circuitry encoding cerebellar efferent signals using larval zebrafish as our model. In fish, Purkinje neurons project onto a group of glutamatergic neurons called the Eurydendroid cells (ECs), which are thought to be the equivalent of the mammalian DCNs. ECs have been shown to also project widely within the brain of the fish and thus are likely to play a critical role in conveying motor error correction information to their targets. However, not much is known about the inherent activity patterns of these neurons. We performed spontaneous in-vivo loose patch recordings from ECs of paralyzed zebrafish larvae. We describe two distinct populations of ECs in terms of their electrophysiological activity i.e. chatty (consistent/high spiking) and stuttering(irregularly spiking/low spiking). With pharmacological treatments, we also show that these neurons are inherently spiking. We also tried to look at the spatial distribution of the two kinds of ECs as a population. Hence, we performed two-photon calcium imaging and tried to classify the neurons based on their calcium activity. With further experiments, we hope to dissect the mechanisms underlying the distinct activity patterns of the two populations and what their roles might be in terms of cerebellar function. Thereafter, we hope to answer more general questions about the fundamental nature of the error correction implemented by the cerebellum.