Researchers have uncovered how a solitary light receptor in the pineal gland of zebrafish perceives colors.
The ability to see colors stems from photoreceptor cones in our eyes, which pick up light waves that correspond to red, green, and blue, while we perceive brightness and darkness through photoreceptor rods. In contrast, many non-mammalian vertebrates, including fish, detect both color and brightness via the pineal gland, an integral part of the brain. A research team from Osaka Metropolitan University has shed more light on the functioning of fish’s pineal organs in this regard.
In earlier studies, a research team led by Professor Akihisa Terakita and Professor Mitsumasa Koyanagi from the Graduate School of Science discovered that the pineal gland in bony fish utilizes a different process to detect colors compared to their eyes. Specifically, a type of pineal photoreceptor cell that contains a protein known as parapinopsin 1 (PP1) is responsible for color detection.
These PP1 cells adapt their activity levels based on the wavelength of light they encounter, becoming more active with long waves (like red) and less so with short ones (like blue). The inactivation of PP1 is crucial for discerning colors, and researchers have recently unraveled the underlying mechanism.
In their study with zebrafish, the team examined arrestin proteins within the pineal gland that are involved in deactivating photoreceptor proteins like PP1. Among the seven arrestin types found in zebrafish, they identified Sagb and Arr3a as key players in the interaction with PP1.
Arr3a quickly deactivates PP1 in low light, whereas Sagb takes over when the light becomes brighter. The researchers found that this unique dual function of the single photoreceptor relies on the switching of these arrestins based on varying light intensities.
“There are multiple types of arrestins within a single cell. Our findings imply that the roles of arrestins may change depending on the strength of the stimulus, marking a significant insight into these genes,” stated Professor Koyanagi.
Professor Terakita added, “Understanding the mechanism behind color discrimination through a single photoreceptor protein is anticipated to advance the field of optogenetics, which leverages parapinopsin to manipulate cells using light color.”
