What Can a Zebrafish See with Only an Off-Pathway and Other Fish Stories?

Zebrafish have a marvellous visual system for study. Their retinas have abundant cones of four spectral types, i.e. red-, green-, blue-and ultraviolet–sensitive cones. The animals are exceptionally responsive behaviourally; robust visual responses can be measured just four to five days after egg fertilization 1. My group has been carrying out forward genetic studies on zebrafish for about fifteen years, seeking mutants with altered retinal neurons and/or circuitry such that we can advance our understanding of visual mechanisms. In addition, we study the functional organization of the retina in normal zebrafish. Herein, I describe two studies, one on a mutant that is providing new information on photoreceptor synapses and how certain visual responses may be mediated by the animal, and the other on an interesting phenomenon that occurs in young, larval zebrafish. I began with the mutant designated nrc. To isolate behavioral mutants we screen mutagenized fish by observing the optokinetic reflex (OKR) 2. This is simply performed by observing five to six-day-old fish placed in a petri-dish inside a rotating drum lined with black and white stripes. We observe whether a fish can move its eyes in response to the moving stripes. With this test we have found completely blind, partially blind, color blind and even movement deficient fish 1. The nrc fish was discovered because it completely failed in the OKR test, under all tested conditions. The fish appeared entirely normal, even the retinal histology looked fine. Close examination at the light microscope level showed a thin outer plexiform layer but little else. Electron microscopy, on the other hand, revealed, that the photoreceptor terminals were strikingly abnormal. 3 Most of the processes from bipolar and horizontal cells failed to penetrate into the terminals and consequently ribbon synapses in the terminals did not form. We did observe basal synaptic contacts, although they were displaced to the base of the terminals (Fig.1). Ribbon synapses drive ON-bipolar cells, whereas basal synapses drive OFF-bipolar cells, suggesting there are no ON-responding bipolar cells in the nrc retina, but perhaps some OFF-responding bipolar cells. This supposition was tested and confirmed by recording ERG's from nrc retinas. The b-wave of the ERG reflects mainly ON-bipolar cell activity while the d-wave corresponds to OFF-bipolar cell responses. Although there was no or minimal b-wave activity recorded from these fish, d-waves were seen consistently. ERG d-waves suggest that OFF-ganglion cells may be activated in nrc retinas and this was …