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I. A. Mills, A. R. Adolph, J. E. Dowling; Rods are Functional in 5-6 Day Old Larval Zebrafish. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4566.
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© ARVO (1962-2015); The Authors (2016-present)
Rod photoreceptors are present anatomically in the zebrafish retina as early as 3 days postfertilization (dpf), but previous experiments did not observe rod behavioral (optomotor) responses or rod electrophysiological (ERG) responses until 12-15 dpf. We have utilized the no optokinetic response f (nof) fish, a mutant that lacks functional cones, to determine the temporal course of rod photoreceptor functionality in zebrafish. To test for rod responsiveness, we employed the optokinetic reflex (OKR) behavioral test and ERG recordings.
All larvae were dark adapted for 2 hours and prepared for behavioral testing under dim long wavelength light. The OKR was evoked utilizing a rotating drum (3 - 4 rpm) lined with vertical black and white stripes. To obtain dark-adapted ERG recordings, we surgically removed the larval eyes and placed the electrode under the cornea using infrared light intensifiers. ERGs were recorded using 200 ms full-field light flashes of varying intensities.
Both dark-adapted nof larvae and their non-mutant siblings demonstrated OKR responses at 5 log units below the maximum test light intensity (log I = 0 is equal to 20 µW/cm2). This result suggests that behavioral responses under low light conditions are due to rods. As the light intensity increased, the OKR of nof larvae did not increase in robustness unlike control fish, and at the highest light intensities many nof larvae failed to show any OKR, suggesting their rods had saturated. At 6 dpf, ERG recordings of both nof and control larvae gave b-wave amplitudes of about 30 µV at the lowest light intensity, suggesting that these responses too were due to rod function. As the light increased, the control fish b-wave amplitudes increased to as much as 900 µV whereas the nof fish responses remained in the range of 30-70 µV. To determine if an increase in rod responsiveness is observed as nof larvae aged, ERGs of 22 dpf nof fish were recorded. A comparison of 6 and 22 dpf nof responses showed a 1.5 - 2.5 fold increase in b-wave amplitudes at all stimulus intensities in the older fish.
Contrary to earlier findings, we observe rod function in larval zebrafish as early as 6 dpf. As expected, we also find that rod function increases substantially between 6 and 22 dpf.
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