Optokinetic behavior is reversed in achiasmatic mutant zebrafish larvae

Abstract

The vertebrate optokinetic nystagmus (OKN) is a compensatory oculomotor behavior that is evoked by movement of the visual environment. It functions to stabilize visual images on the retina. The OKN can be experimentally evoked by rotating a drum fitted with stripes around the animal and has been studied extensively in many vertebrate species, including teleosts. This simple behavior has earlier been used to screen for mutations affecting visual system development in the vertebrate model organism zebrafish. In such a screen, we have found a significant number of homozygous belladonna (bel) mutant larvae to be defective in the correct execution of the OKN [1]. We now show that about 40% of homozygous bel larvae display a curious reversal of the OKN upon visual stimulation. Monocular stimulation leads to primary activation of ipsilateral eye movements in larvae that behave like the wild type. In contrast, affected larvae display contralateral activation of eye movements upon monocular stimulation. Anatomical analysis of retinal ganglion cell axon projections reveal a morphological basis for the observed behavioral defect. All animals with OKN reversal are achiasmatic. Further behavioral examination of affected larvae show that OKN-reversed animals execute this behavior in a stimulus-velocity-independent manner. Our data support a parsimonious model of optokinetic reversal by the opening of a controlling feedback loop at the level of the optic chiasm that is solely responsible for the observed behavioral abnormality in mutant belladonna larvae.

Behavioral genetic approaches to visual system development and function in zebrafish

“…About 50% of homozygous bel larvae show a reversal of eye movements, so that a clockwise movement of the stimulating drum evokes a counterclockwise response of the eyes. The velocity of eye rotation is stimulus independent and not influenced by the speed of the rotating drum (Rick et al, 2000).…”

“…Neuhauss et al (1999) used the OKR and the OMR Only those alleles are listed that have been used in behavioral assays. References: 1, (Brockerhoff et al, 1995); 2, ; 3, (Brockerhoff et al, 1998); 4, (Gnuegge et al, 2001); 5, (Allwardt et al, 2001); 6, (Li and Dowling, 1997); 7, (Li and Dowling, 2000); 8, (Neuhauss et al, 1999); 9, (Neuhauss et al, 2002); 10, (Rick et al, 2000); 11, (Van Epps et al, 2001) to screen through a collection of about 450 mutant strains being kept in the Tübingen zebrafish stock center (Neuhauss et al, 1999). Some strains could not be tested in both assays due to early embryonic lethality or an inability to swim straight in the OMR test chamber.…”

“…Indeed, a combined behavioral and anatomical analysis revealed a defect in optic chiasm formation , Rick et al, 2000. Wild-type zebrafish have a completely contralateral retinotectal projection, so that axons from one eye all cross the midline and synapse in the contralateral brain.…”

Zebrafish: A model system for the study of eye genetics

“…The belladonna mutation (bel), so named for a pigmentation defect of the eye resulting in the appearance of a dilated pupil, also was found to display a misrouting of ganglion cell axons (Karlstrom et al, 1996;Trowe et al, 1996;Neuhauss et al, 1999;Rick et al, 2000). Whereas in wild-type larvae contralateral projections from ganglion cells to the tectum are the norm, in bel mutants ganglion cell axons project to the ipsilateral tectum ( Figure 5).…”

“…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. Rather, based upon careful scrutiny of the behavioral deficit (Huang, et al, 2006;Rick, et al, 2000) it was proposed that: in the wild-type fish, visual stimulation of one eye drives movement of that eye through projections to a contralateral OKRmediated nucleus and integrator nucleus that ultimately controls the ipsilateral motor nuclei and the ocular muscles of the stimulated eye ( Figure 5; (Rick et al, 2000)). In this model, the neural basis of the altered behavior in bel mutants can be attributed to the singular defect in the projection of ganglion cell axons to the ipsilateral OKR-mediated nucleus.…”