Involvement of cerebellar neural circuits in active avoidance conditioning in zebrafish

When animals repeatedly receive a combination of neutral conditional stimulus (CS) and aversive unconditional stimulus (US), they learn the relationship between CS and US, and show conditioned fear responses after CS. They show passive responses such as freezing or panic movements (classical or Pavlovian fear conditioning), or active behavioral responses to avoid aversive stimuli (active avoidance). Previous studies suggested the roles of the cerebellum in classical fear conditioning but it remains elusive whether the cerebellum is involved in active avoidance conditioning. In this study, we analyzed the roles of cerebellar neural circuits during active avoidance in adult zebrafish,. When pairs of CS (light) and US (electric shock) were administered to wild-type zebrafish, about half of them displayed active avoidance. The expression of botulinum toxin, which inhibits the release of neurotransmitters, in cerebellar granule cells or Purkinje cells did not affect conditioning-independent swimming behaviors, but did inhibit active avoidance conditioning. Nitroreductase-mediated ablation of Purkinje cells in adult zebrafish also impaired active avoidance. Furthermore, the inhibited transmission of granule cells or Purkinje cells resulted in reduced fear-conditioned Pavlovian fear responses. Our findings suggest that the zebrafish cerebellum plays an active role in active avoidance conditioning.Significance StatementAn animal can associate a neutral conditioned stimulus and an aversive unconditioned stimulus, and escape to avoid an aversive stimulus. This is called active avoidance conditioning and is essential for an animal's survival. Although the amygdala and habenula nucleus are reportedly involved in active avoidance conditioning, the roles of other brain regions are largely unknown. We describe the roles of the cerebellum during active avoidance in adult zebrafish. The neurotoxin botulinum toxin-mediated inhibition of granule cells or Purkinje cells, or the ablation of Purkinje cells, suppressed active avoidance conditioning. Our findings indicate that the cerebellum plays a positive role in active avoidance conditioning.