20 – 23 In the retina, inhibitory neurons include horizontal and amacrine interneurons. 19 Inhibitory neuron deficits in the human brain are implicated in neurodevelopmental disorders including autism spectrum disorders, schizophrenia, and epilepsy. For example, reduced inhibitory synaptic transmission in mice leads to an imbalanced ratio of neural excitation to inhibition (E/I), and thus hippocampal hyperexcitability. Inhibitory neurons play vital roles throughout the CNS. 11 – 15 Together, these well-established tools allow multilevel analysis of the visual system. The ERG is an important tool for assessing visual function, color vision, circadian rhythms, normal development, and drug response. 18 The most commonly analyzed ERG components are the a-wave and b-wave, generated by light-sensing photoreceptors and bipolar cells, respectively. When visual input is detected and processed in the retina, electrical currents are generated and transferred to the central corneal surface, where they can be detected using the ERG. 7, 8, 16, 17 Unlike the OMR, which captures visual function across the central nervous system (CNS) as a whole, the ERG assesses neural function specifically in the retina. 7 – 14 The OMR is an innate, visually guided behavior that can be elicited as early as 4 days post fertilization (dpf), 15 and is often exploited in the lab to screen for visual deficits, measure motion perception, or characterize chromatic and spatiotemporal response properties of the visual system. In addition to well-described histologic techniques, the behavioral optomotor response (OMR) and functional electroretinogram (ERG) are also increasingly used in studies of zebrafish vision.
The visual system is arguably the best characterized and most easily probed neural system, as it can be readily assessed at multiple levels including behavior, physiology, and histology. This study highlights the utility of a multidisciplinary approach and provides a template for characterizing other zebrafish models of neurological disease. In addition to modulating visual signals, inhibitory neurons may be critical for maintaining retinal structure and organization. Inhibitory neuron loss likely increases the ratio of neural excitation to inhibition, leading to hyperexcitability. The consequences of inhibitory neuron ablation corresponded closely across behavioral, physiological, and anatomical levels. 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. For ERG, injured larvae manifested either reduced (type-I) or absent (type-II) b-wave signals originating from bipolar interneurons in the retina. Locomotor assessment showed unaltered swimming ability, indicating that reduced OMR was due to visual deficits. Injured larvae showed severely reduced OMR relative to controls. Nonvisual locomotion was also assessed to reveal any general behavioral effects due to ablation of other nonvisual neurons that also express Ptf1a. Visual phenotypes were characterized at behavioral, physiological, and anatomical levels using an optomotor response (OMR) assay, electroretinography (ERG), and routine histology, respectively. By exposing larvae to the prodrug metronidazole, cytotoxicity was selectively induced in inhibitory neurons. The Ptf1a gene, which determines inhibitory neuron fate in developing vertebrates, was used to express nitroreductase. Inhibitory neurons were ablated in larval zebrafish retina.
To compare the effects of reduced inhibitory neuron function in the retina across behavioral, physiological, and anatomical levels.