Correspondence between behavioural, physiological and anatomical measurements of visual function in inhibitory-neuron-ablated zebrafish

PURPOSE. To compare the effects of reduced inhibitory neuron function in the retina across
behavioral, physiological, and anatomical levels.
METHODS. Inhibitory neurons were ablated in larval zebrafish retina. The Ptf1a gene, which
determines inhibitory neuron fate in developing vertebrates, was used to express
nitroreductase. By exposing larvae to the prodrug metronidazole, cytotoxicity was selectively
induced in inhibitory neurons. Visual phenotypes were characterized at behavioral,
physiological, and anatomical levels using an optomotor response (OMR) assay, electroretinography (ERG), and routine histology, respectively. Nonvisual locomotion was also assessed
to reveal any general behavioral effects due to ablation of other nonvisual neurons that also
express Ptf1a.
RESULTS. Injured larvae showed severely reduced OMR relative to controls. Locomotor
assessment showed unaltered swimming ability, indicating that reduced OMR was due to
visual deficits. For ERG, injured larvae manifested either reduced (type-I) or absent (type-II) bwave signals originating from bipolar interneurons in the retina. Histologic analysis showed
altered retinal morphology in injured larvae, with reductions in synaptic inner plexiform layer
(IPL) thickness and synaptic density more pronounced in type-II than type-I larvae; type-II
larvae also had smaller retinae overall.
CONCLUSIONS. The consequences of inhibitory neuron ablation corresponded closely across
behavioral, physiological, and anatomical levels. Inhibitory neuron loss likely increases the
ratio of neural excitation to inhibition, leading to hyperexcitability. In addition to modulating
visual signals, inhibitory neurons may be critical for maintaining retinal structure and
organization. This study highlights the utility of a multidisciplinary approach and provides a template for characterizing other zebrafish models of neurological disease.