Stephan C.F. Neuhauss

An inexpensive device for non-invasive electroretinography in small aquatic vertebrates.

Electroretinographic (ERG) method records a sum field potential of the retina in response to light. It mainly arises in the outer retina and is used as a non-invasive measure in both animal experiments and the clinic. Since it is a comprehensive method to assess outer retinal function, it is becoming increasingly useful in genetic studies of vision. Here we present a simple in-house built setup to measure ERGs of aquatic vertebrates. We have used this setup to efficiently and reliably measure intact larvae of zebrafish (Danio rerio), Medaka fish (Oryzias latipes), and Xenopus laevis tadpoles. By slight modification of the setup, we were also able to measure adult zebrafish and Medaka, demonstrating the general versatility of the setup. We picked these organisms since they are increasingly used to study visual function with genetic means. This setup is easily built and will be particularly useful for laboratories setting up ERG measurements as a complement to their genetic studies.

Nyctalopin is essential for synaptic transmission in the cone dominated zebrafish retina

The first synapse in the vertebrate visual system is the photoreceptor synapse between rod and cone photoreceptors and the second‐order bipolar cells. Although mutations in the nyctalopin gene (NYX) in humans lead to congenital stationary night blindness (CSNB1), affecting synaptic transmission between both types of photoreceptors and ON‐bipolar cells, the function of nyctalopin in cone‐dominant animal models has not been studied. Because the larval zebrafish retina is cone‐dominant, we isolated the zebrafish nyx ortholog and raised a polyclonal antibody against the protein. Nyctalopin is expressed postsynaptically in both synaptic layers of the retina. Functional disruption via morpholino antisense injection leads to characteristic defects in the electroretinogram and defects in visual contrast sensitivity. We therefore demonstrated that nyctalopin plays a similar role in retinal synapse function in the cone pathway as in the rod pathway, thereby creating a genetic model for CSNB1 and its effects on cone vision.

Retinal function and morphology in two zebrafish models of oculo‐renal syndromes

We characterized visual system defects in two recessive zebrafish mutants oval and elipsa. These mutants share the syndromic phenotype of outer retinal dystrophy in conjunction with cystic renal disorder. We tested the function of the larval visual system in a behavioural assay, eliciting optokinetic eye movements by high‐contrast motion stimulation while recording eye movements in parallel. Visual stimulation did not elicit eye movements in mutant larvae, while spontaneous eye movements could be observed. The retina proved to be unresponsive to light using electroretinography, indicative of a defect in the outer retina. Histological analysis of mutant retinas revealed progressive degeneration of photoreceptors, initiated in central retinal locations and spreading to more peripheral regions with increasing age. The inner retina remains unaffected by the mutation. Photoreceptors display cell type‐specific immunoreactivity prior to apoptotic cell death, arguing for a dystrophic defect. Genomic mapping employing simple sequence‐length polymorphisms located both mutations on different regions of zebrafish linkage group 9. These mutants may serve as accessible animal models of human outer retinal dystrophies, including oculo‐renal diseases, and show the general usefulness of a behavioural genetic approach to study visual system development in the model vertebrate zebrafish.

Retinal defects in the zebrafish bleached mutant

The recessive zebrafish mutant bleached has, apart from its defects in pigmentation, a heritable defect leading to larval blindness. Here, we analyze the retina of homozygous bleached larvae, employing morphological and electrophysiological methods. Electroretinography revealed a complete lack of electrical signals in response to light. Histological analysis of mutant retinae showed a severely affected outer retina with a hypopigmented pigment epithelium and a disorganized outer nuclear layer containing few or no intact photoreceptors. Using the TUNEL assay for cell death detection, we noticed a strong increase of apoptotic cells in all retinal cell layers, starting in young larvae even before retinal support of visual function. At later stages cell death is most pronounced at the marginal zone, where new cells are constantly added to the retina. At early stages increased apoptosis is mainly confined to the retina, while at later stages elevated cell death is also apparent in extra-retinal tissues, particularly in the brain. Hence, the lack of visual responses in homozygous bleached larvae can be attributed to a severe defect of the outer retina, preceded by increased levels of apoptotic cell death in all retinal cell layers.