Department of Biological Sciences, Purdue University, West Lafayette, Indiana, United States.
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, United States.
Invest Ophthalmol Vis Sci. 2020 Oct 1;61(12):11. doi: 10.1167/iovs.61.12.11.
Although zebrafish rods begin to develop as early as 2 days postfertilization (dpf), they are not deemed anatomically mature and functional until 15 to 21 dpf. A recent study detected a small electroretinogram (ERG) from rods in a cone mutant called no optokinetic response f (nof) at 5 dpf, suggesting that young rods are functional. Whether they can mediate behavioral responses in larvae is unknown.
We first confirmed rod function by measuring nof ERGs under photopic and scotopic illumination at 6 dpf. We evaluated the role of rods in visual behaviors using two different assays: the visual-motor response (VMR) and optokinetic response (OKR). We measured responses from wild-type (WT) larvae and nof mutants under photopic and scotopic illuminations at 6 dpf.
Nof mutants lacked a photopic ERG. However, after prolonged dark adaptation, they displayed scotopic ERGs. Compared with WT larvae, the nof mutants displayed reduced VMRs. The VMR difference during light onset gradually diminished with decreased illumination and became nearly identical at lower light intensities. Additionally, light-adapted nof mutants did not display an OKR, whereas dark-adapted nof mutants displayed scotopic OKRs.
Because the nof mutants lacked a photopic ERG but displayed scotopic ERGs after dark adaptation, the mutants clearly had functional rods. WT larvae and the nof mutants displayed comparable scotopic light-On VMRs and scotopic OKRs after dark adaptation, suggesting that these responses were driven primarily by rods. Together, these observations indicate that rods contribute to zebrafish visual behaviors as early as 6 dpf.
尽管斑马鱼的视杆早在受精后 2 天（dpf）就开始发育，但直到 15 至 21 dpf 才被认为在解剖上成熟和功能完善。最近的一项研究在一种名为无光感受器运动反应（nof）的锥体突变体的视杆中检测到了一个小的视网膜电图（ERG），这表明年幼的视杆具有功能。但它们是否能够在幼虫中介导行为反应尚不清楚。
我们首先通过在 6 dpf 时在明、暗两种光照条件下测量 nof ERG，证实了视杆的功能。我们使用两种不同的测定方法评估了视杆在视觉行为中的作用：视觉运动反应（VMR）和光运动反应（OKR）。我们在明、暗两种光照条件下测量了野生型（WT）幼虫和 nof 突变体的反应。
nof 突变体缺乏明视 ERG，但在长时间暗适应后，它们显示出暗视 ERG。与 WT 幼虫相比，nof 突变体的 VMR 降低。在光照开始时，VMR 差异随着光照强度的降低而逐渐减小，在较低的光照强度下几乎相同。此外，适应光照的 nof 突变体没有表现出 OKR，而适应黑暗的 nof 突变体则表现出暗视 OKR。
由于 nof 突变体缺乏明视 ERG，但在暗适应后显示出暗视 ERG，因此突变体显然具有功能性视杆。WT 幼虫和 nof 突变体在暗适应后显示出相似的暗视光-On VMR 和暗视 OKR，表明这些反应主要由视杆驱动。综上所述，这些观察结果表明，视杆在 6 dpf 时就开始对斑马鱼的视觉行为产生影响。
