Eyes Wide Open: Assessing Early Visual Behavior in Zebrafish Larvae

Abstract

Early diagnosis is critical for the effective management of neurodegenerative disorders, and retinal alterations have emerged as promising early biomarkers due to the retina's close developmental and functional link to the brain. The zebrafish (Danio rerio), with its rapid development, transparent embryos, and evolutionarily conserved visual system, represents a powerful and versatile model for studying retinal degeneration. This review discusses a range of behavioral assays-including visual adaptation, motion detection, and color discrimination-that are employed to evaluate retinal function in zebrafish. These methods enable the detection of subtle visual deficits that may precede overt anatomical damage, providing a non-invasive, efficient strategy for early diagnosis and high-throughput drug screening. Importantly, these behavioral tests also serve as sensitive functional readouts to evaluate the efficacy of pharmacological treatments over time. Compared to traditional murine models, zebrafish offer advantages such as lower maintenance costs, faster development, optical transparency for live imaging, and ethical benefits due to reduced use of higher vertebrates. However, variability in experimental protocols highlights the need for standardization to ensure reliability and reproducibility.

Keywords: Danio rerio; behavior; neurodegeneration; neurodevelopment; retina.

Figure 1

The retina is a window to brain degeneration. Shared degenerative mechanisms between the retina and the brain highlight their common developmental and functional origin as components of the CNS. This illustration points to converging pathological features—oxidative stress, protein aggregation, mitochondrial damage, axonal degeneration, microglial activation, and vascular dysfunction—observed in both retinal and cerebral neurodegeneration. Given the retina’s accessibility, it is a valuable model for investigating early neurodegenerative events, offering critical insights into disease mechanisms. The image also depicts structural similarities between the human brain and the zebrafish brain, further supporting the use of zebrafish as a translational model in neurodegenerative research.

Figure 2

Visual pathway from retina to brain. (a) Superior view of the visual circuit in the human brain. Signals from the nasal retina cross at the optic chiasm, while those from the temporal retina remain ipsilateral. These projections synapse at the lateral geniculate nucleus and then relay to the primary visual cortex. Additional projections reach the pretectal nucleus. (b) Lateral view of the visual circuit in the human brain. (c) Schematic zebrafish visual circuit: The optic tectum is the principal visual processing center in zebrafish, mediating sensorimotor information for visually guided behaviors. The pretectal nucleus, which receives input from the tectum, sends inhibitory projections to the nucleus isthmi, modulating visuomotor reflexes like the optokinetic response. The nucleus isthmi forms reciprocal connections with the optic tectum and contributes to visual attention and stimulus selection.

Figure 3

Development of visual circuits and related behavior assays in larval zebrafish. Schematic timeline of visual network development in zebrafish with functional behavioral assays available at specific timepoints to elicit precise visual behavioral responses. On the top left side is a close-up schematic view of retinal structure and cellular organization in zebrafish retina, highlighting the layered organization fully developed at 5 days post fertilization (dpf). Retinal layers include, as in humans, the ganglion cell layer, inner and outer plexiform layers (synaptic layers), inner and outer nuclear layers, and the outer photoreceptor layer, which contains rods and cones. Each layer is pivotal in processing visual information from photoreception to signal transmission to the brain.