This Research Insight covers a collaborative study pioneered by the Yoshimatsu Lab, which elucidates a rod photoreceptor-driven pathway in the zebrafish retina that supports dim-light vision and is shared across species. Whereas the rod pathway has been speculated to be unique to the mammalian retina, this study presents the first converging evidence that a homologous pathway exists in the zebrafish, extending the utility of the zebrafish as a model system to investigate and develop treatments for night vision-related diseases.
In this study published in Nature Ecology & Evolution, Takeshi Yoshimatsu, PhD, assistant professor of ophthalmology at WashU Medicine, and his collaborators across research institutions present transcriptomic, anatomical, and functional evidence of two distinct rod-driven pathways in the zebrafish retina. Of these two pathways, one is comparable to the rod pathway present in the mammalian retina—such as the mouse and human—and which is required for dim-light vision. The second pathway appears unique to zebrafish and warrants additional study.
Together, their results demonstrate that the zebrafish may be a viable model system to study night blindness and other challenges to dim-light vision. These findings underscore the importance of understanding neural circuits across species to expand the range of models and tools available to study human disease.
Visual circuits to support night vision
The wide range of light levels an organism experiences daily—ranging from near-darkness to bright light—present a challenge to the visual system. One way that the retina—the light-sensitive neural tissue that lines the back of the eye—addresses this challenge is through its light detector units, photoreceptors. Populations of rod and cone photoreceptors have evolved different cellular properties to process dim and bright light, respectively.
All dim-light, night vision is initiated by the detection of light by rods. In the mammalian retina, the light signals generated by rods are communicated to a specialized group of bipolar cells—rod bipolar cells (RBCs)—which amplify dim-light signaling and integrate rod-mediated signaling with cone-driven pathways via connections to A17 and A2 amacrine cells, respectively.
This designated rod-driven pathway is thought to be specific to mammals. How non-mammalian species—such as zebrafish—process dim-light information mediated by rods has remained uncertain. Here, Yoshimatsu and colleagues analyzed zebrafish retinas to identify neural pathways capable of processing this information, based on knowledge established by studies of the mouse retina.
Identifying correlates of mammalian RBCs in the zebrafish retina
The authors sought a dim-light circuit in the zebrafish retina by screening for genetic signatures consistent with those of the mouse retina. They performed a single-cell RNA sequencing study of zebrafish bipolar cells, which isolated two putative populations of zebrafish RBCs—RBC1 and RBC2—that expressed key genes required by mouse RBCs to receive signals from rods and relay them to downstream pathways via intermediary amacrine cells.
To confirm the identity of these putative RBC types in the zebrafish retina, the authors identified two strains of zebrafish in which RBC1 and RBC2 could be visualized specifically using fluorescent labels. The authors found that the anatomy and structural arrangement of RBC1 and RBC2 resembled those of RBCs in the mouse retina. Further, they showed that the arbors of RBC1 and RBC2 each independently cover the surface of the retina, suggesting that they constitute two distinct cell types.
Zebrafish RBCs process rod-mediated signaling
The authors analyzed the inputs to RBC1 and RBC2. They showed anatomically that the arbors of both RBC1 and RBC2 contact with most of the rods and red-light-sensitive cones present within their dendritic field, consistent with the RBCs in the mouse retina.
They then performed electrophysiological recordings from RBC1 and RBC2 to determine whether these contacts with photoreceptors constituted functional synapses. As expected, the onset of a spot of red light drove a robust light response in both RBC1 and RBC2. Likewise, the onset of dim blue light, which exclusively activates rods, triggered a response in RBC1. The authors determined that the RBC1 light response was sensitive to drugs that prevented signaling via mGluR6, a key component of the functional RBC synapse in the mouse retina.
The transcriptomic, anatomical, and functional evidence uncovered suggest that RBC1 and RBC2 are two distinct bipolar cell types in the retina. Their inputs reveal that both cell types receive dim-light signals communicated predominantly from rods, enabling them to independently and effectively sample this information across the visual field, similar to RBCs in the mouse retina.
Zebrafish RBCs participate in distinct rod-mediated visual pathways
Finally, the authors analyzed the cellular pathways in which RBC1 and RBC2 participate to understand how these two cell types might process dim-light information. Here, their results between the two cell types diverged, indicating that RBC1 and RBC2 in the zebrafish retina participate in distinct circuits and communicate dim-light signals to different populations of amacrine cells for further processing.
Their anatomical results suggest that RBC1—but not RBC2—participates in a neural circuit like that observed in the mouse retina. Most of the amacrine cells that the terminals of RBC1 contact share structural similarities with the A17 and A2 amacrine cells that communicate extensively with mouse RBCs. Specifically, RBC1 formed reciprocal synapses with an A17-like cell and formed extensive synapses with a bi-stratified, A2-like amacrine cell that also forms extensive contacts with ON- and OFF-responsive cone bipolar cells.
The observed connectivity pattern of the RBC1 directly parallels the connectivity of mouse RBCs to their postsynaptic partners to accomplish gain control and signal integration. In contrast, the RBC2 cell contacted a morphologically distinct amacrine cell, which remains unidentified and warrants further exploration.
Night vision in the zebrafish retina: An emerging model to study human disease
Together, the evidence presented in this study dispel the belief that the rod-mediated signaling pathway emerged specifically in the mammalian retina. The parallels between the zebrafish RBC1 and mouse RBC connectivity highlight the potential for previously underappreciated model organisms—such as the zebrafish and the genetic tools available in the fish, but not mammals—to be used to advance our understanding of human vision and disease.