Phototactic T-maze Behavioral Assay for Comparing the Functionality of Color-sensitive Photoreceptor Subtypes in the Visual System.

The retina contains light-sensitive photoreceptors (R cells) with distinct spectral sensitivities that allow them to distinguish light by its spectral composition. R7 and R8 photoreceptors are important for color vision, and can be further classified into pale (p) or yellow (y) subtypes depending on the rhodopsin expressed. While both R7y and R7p are sensitive to UV light, R8y and R8p detect light in the green and blue spectrum, respectively. The ability of R7 and R8 photoreceptors to distinguish different spectral sensitivities and the natural preference for towards light sources (phototaxis), allow for the development of a phototactic T-maze assay that compares the functionality of different R7 and R8 subtypes. A "UV vs. blue" choice can compare the functionalities of R7p and R8p photoreceptors, while a "UV vs. green" choice can compare the functionalities of R7y and R8y photoreceptors. Additionally, a "blue vs. green" choice could be used to compare R8p and R8y photoreceptors, while a "dark vs. light" choice could be used to determine overall vision functionality. Although electrophysiological recordings and calcium imaging have been used to examine functionality of R7 and R8 photoreceptors, these approaches require expensive equipment and are technically challenging. The phototactic T-maze assay we present here is a robust, straight-forward and an inexpensive method to study genetic and developmental factors that contribute to the individual functionality of R7 and R8 photoreceptors, and is especially useful when performing large-scale genetic screens.