Using zebrafish, researchers from Osaka Metropolitan University (OMU) have identified the tegmentum region in the fish midbrain as the area where light input from both the fish’s eyes and the pineal organ—the ‘third eye’—is integrated. Their findings suggest that fish use the integrated light signals in this region to swim up or down in response to differences in the wavelength of light.

In the aquatic world, light changes depending on depth, water conditions, and differences such as sunlight and shade. Differences in the levels of visible and UV light enable fish to infer these factors, which they may use to make survival decisions.

To understand the related processes taking place in the brain, an OMU research team led by Professors Akihisa Terakita and Mitsumasa Koyanagi with Dr. Seiji Wada of the Graduate School of Science looked at the opsin parapinopsin 1 (PP1). Opsins are specialized proteins that respond to light. They are typically found in the eyes, but in some species, opsins like PP1 are also found in the pineal organ. Using calcium imaging, the team investigated how color-detection signals produced by PP1 in the pineal photoreceptor cells are passed to the brain by nerve cells.

“We decided to study zebrafish, as their larvae are transparent,” Professor Koyanagi said. “This transparency means that changes in calcium levels within nerve cells can be observed as changes in the fluorescence intensity of the calcium indicator, allowing us to measure the strength of neural activity.”

PP1 exhibits opposite responses to UV and visible light. Using calcium imaging, the group traced these responses to light from the pineal organ to the tegmentum via ganglion cells.

“Our study showed that the tegmentum integrates visual information from the eyes that is combined with color information detected by the pineal organ. These integrated signals then contribute to the fish’s up and down swimming behavior,” Dr. Wada, the first author of the paper, said.

When they raised fish without the PP1 gene, they did not show the typical responses to changes in the wavelength of light.

“These findings shed light on how animals process visual information, advance the analysis of neural circuits using light, and expand research into behavioral control,” Professor Terakita said. “In the future, these findings may contribute to applications in neuroscience and biomedicine, such as the identification of neural circuits using PP1-based optogenetics.”

The study was published in Proceedings of the National Academy of Sciences.

Declaration of interests
The authors declare no competing interests.

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