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F. Emran, A. Adolph, K.Y. Wong, J.E. Dowling; Analysis of the Inner Retina in Zebrafish Mutants Using Retinal Ganglion Cell Recordings . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5896.
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We have developed a technique that allows us to record the action potentials of retinal ganglion cells (RGCs) from 5–6 dpf zebrafish to study the inner retinal activity of mutant fish. The no optokinetic response c (nrc) mutant shows no evidence of an optokinetic reflex (OKR) under any conditions tested and appears to be completely blind. The nrc mutant is characterized by abnormal photoreceptor terminals and an unusual ERG consisting often of multiple a– and b–wave like potentials. The b–wave–like activity in the mutants suggests that bipolar cells are responding to the light stimulus. Histological analysis of the inner retina revealed no abnormalities.
Materials and Methods: :
This technique employs the isolated eye preparation described by Wong et al., Zebrafish 2004. A glass electrode with a 2 µm tip, filled with Ringer’s solution, is placed either into the optic nerve or on the surface of the retina. The responses are recorded conventionally and last for over 20 minutes.
When the action potentials of RGCs from wild type larvae were recorded in response to a 1 second full–field flash of white light, the vast majority of the cells responded to the onset and offset of light, i.e. were ON– and OFF cells. Other types of RGC spike activity were also observed: sustained ON cells, transient ON cells, and OFF cells. In nrc mutants, spiking cells, thought to be ganglion cells, were recorded, but the great majority of these cells responded only to the offset of the light stimulus, i. e. were OFF cells. Occasionally they responded to both the onset and offset of light, but no pure ON cells were recorded. Moreover, the frequency of recorded spike activity of the nrc mutant was lower compared to the wild type zebrafish eye.
The action potentials elicited from the nrc mutant are likely to be from ganglion cells, as they are recorded from the retinal surface. The observation that most spiking cells in the mutant generate only OFF responses may relate to the morphology of the photoreceptor terminals in these animals. Whereas there are few invaginating ribbon synapses observed in the terminals, there are clear flat or basal contacts seen, although they are misplaced as compared to the normal terminals. OFF bipolar cell responses are believed to be generated at basal contacts, whereas ON responses are believed to be generated at invaginating synapses. A surprise is that the NRC animals fail to show any optokinetic response even though they have active ganglion cells of the OFF type.
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