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F. Emran, J. Rihel, A. Adolph, J. E. Dowling; Can Off-Ganglion Cell Activity Signal Movement in Zebrafish?. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1941.
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© ARVO (1962-2015); The Authors (2016-present)
The photoreceptor synaptic terminals are abnormal in the nrc zebrafish mutant. Horizontal and bipolar cell processes fail to invaginate into the terminals, and few ribbon synapses are seen. However, basal synaptic contacts are observed. The animals appear to be totally blind when tested behaviorally using the optokinetic reflex (OKR), but the ERG consistently shows a d-wave, indicative of OFF-bipolar cell responses. Last year we reported that OFF-ganglion cell activity can be recorded in these animals. We have extended these studies and examined the visual behavior of nrc mutants using a novel visual-motor behavioral assay.
Ganglion cell responses were recorded as described (ARVO 2006). For the behavioral studies, wild type (n=40) and nrc fish (n=40) are placed in small, individual compartments and their movements are monitored by a video camera and measured in terms of pixel displacement. The animals are dark-adapted, followed by 4-6 periods of light (10-30 min) and dark (10-30 min).
In wild type larvae (n=121) the majority of ganglion cells (55%) give ON-OFF responses to 1 second light flashes. Sustained and transient ON cells (32%) and OFF-cells (12%) are also observed. In nrc mutants (n=41) the great majority of ganglion cells (70%) gave only OFF responses. Some ON-OFF cells (30%) were found, but no ON cells. To test whether nrc mutants respond to any visual stimuli, we examined their motor behavior to the onset and offset of a prolonged light stimulus. Virtually all wild type fish respond vigorously to both the on-set and off-set of light. Nrc fish consistently respond vigorously to the off-set of light like wild type fish, but their response to light-onset is variable, delayed or even absent.
Mutant nrc fish consistently respond strongly to the off-set of a steady light, but less well to light-onset. The failure of the nrc mutants to respond behaviorally using the OKR may relate to the fact that the OKR requires the detection of moving stimuli, and the ganglion cells remaining in the nrc mutant may not be movement sensitive.
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