Visuomotor Behaviors in Larval Zebrafish after GFP-Guided Laser Ablation of the Optic Tectum

Abstract

The optic tectum is the largest visual center in most vertebrates and the main target for retinal ganglion cells (RGCs) conveying visual information from the eye to the brain. The retinotectal projection has served as an important model in many areas of developmental neuroscience. However, knowledge of the function of the tectum is limited. We began to address this issue using laser ablations and subsequent behavioral testing in zebrafish. We used a transgenic zebrafish line that expresses green-fluorescent protein in RGCs projecting to the tectum. By aiming a laser beam at the labeled retinal fibers demarcating the tectal neuropil, the larval tectum could be selectively destroyed. We tested whether tectum-ablated zebrafish larvae, when presented with large-field movements in their surroundings, displayed optokinetic responses (OKR) or optomotor responses (OMR), two distinct visuomotor behaviors that compensate for self-motion. Neither OKR nor OMR were found to be dependent on intact retinotectal connections. Also, visual acuity remained unaffected. Tectum ablation, however, slowed down the OKR by reducing the frequency of saccades but left tracking velocity, gain, and saccade amplitude unaffected. Removal of the tectum had no effect on the processing of second-order motion, to which zebrafish show both OKR and OMR, suggesting that the tectum is not an integral part of the circuit that extracts higher-order cues in the motion pathway.

Zebrafish: A model system for the study of eye genetics

“…The movement of the stripes initiated eye movement but did not influence the rate of the pursuit, and, in contrast to wild-type larvae, the amplitude of the movement of the stimulated eye was less than the amplitude of movement of the opposite eye. Although the optic tectum does not mediate the OKR in zebrafish, the level of misrouting to the tectum correlated well with the altered behavior and may therefore reflect the degree of misrouting of ganglion cell projects to other nuclei, including the pretectal nuclei thought to be involved in mediating the OKR (Roeser and Baier, 2003). Cloning of the mutant loci identified a point mutation in the gene encoding the lim domain transcription factor Lhx2 (Seth, et al, 2006); neither gene function nor expression data provide an immediate clue to the molecular mechanism leading to the behavioral mutant phenotype.…”

Visual Prey Capture in Larval Zebrafish Is Controlled by Identified Reticulospinal Neurons Downstream of the Tectum

“…Furthermore, a Shh:GFP transgenic line (Neumann and NuessleinVolhard, 2000) was used to visualize RGC axons simultaneously with backfilled RS neurons. Shh:GFP labels all RGC types, in addition to subpopulations of neurons in the ventral thalamus (Roeser and Baier, 2003). Depending on the experiment, GFP was imaged either with a confocal laser-scanning microscope (MRC-1024; Bio-Rad, Hercules, CA), a compound fluorescence microscope (Axioskop II; Zeiss, Oberkochen, Germany), or an epifluorescence-equipped dissecting microscopem (MZ FLIII; Leica, Nussloch, Germany).…”

“…The OKR was tested in larvae with unilateral nMLF lesions, using previously published procedures (Roeser and Baier, 2003;Orger et al, 2004). Briefly, a computer-generated image of a rotating black and white grating was projected onto the inner walls of a white drum (height, 50 mm; inner diameter, 56 mm).…”

“…Retinal ganglion cells (RGCs) project to 10 distinct areas in the brain (Burrill and Easter, 1994), consistent with functional segregation of behavioral pathways. Laser ablation of the optic tectum, the largest retinorecipient structure, leaves optomotor responses (OMRs) and optokinetic responses (OKRs) unaffected, two visuomotor reflexes that compensate for self-motion (Roeser and Baier, 2003). This result suggests that the individual visual pathways in these larval fish are already dedicated to processing behavior-specific information.…”

Analysis of Optokinetic Response in Zebrafish by Computer-Based Eye Tracking

“…Using linear projection, only monocular stimulation is possible ( 11 ) . When the projector is placed below the subject, the gratings are projected via a mirror to the whole drum enabling binocular stimulation ( 12,13 ) . In order to avoid visible light from the projector in fl uencing the recording, the animal is illuminated from below with infrared-emitting diodes.…”