Retinal inputs that drive optomotor responses of mice under mesopic conditions

Optomotor responses are a popular way to assess sub-cortical visual responses in mice. We studied photoreceptor inputs into optomotor circuits using genetically-modified mice lacking the exocytotic calcium sensors synaptotagmin 1 (Syt1) and 7 (Syt7) in rods or cones. We also tested mice that in which cone transducin, GNAT2, had been eliminated. We studied spatial frequency sensitivity under mesopic conditions by varying the spatial frequency of a grating rotating at 12 deg/s and contrast sensitivity by varying luminance contrast of 0.2c/deg gratings. We found that eliminating Syt1 from rods reduced responses to a low spatial frequency grating (0.05c/deg) consistent with low resolution in this pathway. Conversely, eliminating the ability of cones to respond to light (by eliminating GNAT2) or transmit light responses (by selectively eliminating Syt1) showed weaker responses to a high spatial frequency grating (3c/deg). Eliminating Syt7 from the entire optomotor pathway in a global knockout had no significant effect on optomotor responses. We isolated the secondary rod pathway involving transmission of rod responses to cones via gap junctions by simultaneously eliminating Syt1 from rods and GNAT2 from cones. We found that the secondary rod pathway is sufficient to drive robust optomotor responses under mesopic conditions. Finally, eliminating Syt1 from both rods and cones almost completely abolished optomotor responses, but we detected weak responses to large, bright rotating gratings that are likely driven by input from intrinsically photosensitive retinal ganglion cells.