LOTUS-V Brain Imaging Technique Shows Promise In Neuroscience

Researchers at Osaka University have developed a method to record brain activity in multiple freely moving mice simultaneously. The method is based on a recent bioluminescence-based indicator of membrane voltage.

Researchers applied the indicator (called LOTUS-V) to cells in the primary visual cortex, which is known to respond to locomotor activity and visual stimulations. The bioluminescent probe is genetically encoded — delivered to target cells noninvasively via a common gene expression system (the adeno-associated virus). Its signal is derived from cell membrane voltage changes, which reflect brain activity in response to visual stimuli and locomotor activity, including interactions with other mice.

Bioluminescence from the primary visual cortex (green) of four mice was observed at the same time (left). Pseudo-colored locomotion trajectories, indicating velocity (center) and brain activity (right) of four mice freely interacting with each other in the same cage. Courtesy of professor Takeharu Nagai.

"The LOTUS-V method reported brain activity in freely moving mice with a good sensitivity and without motion artifacts," said corresponding author Takeharu Nagai. "More importantly, it could measure dynamically changing brain activity in the primary visual cortex during social interactions."

Existing electrophysiological and fluorescence-based brain imaging techniques in mice are generally invasive, require head fixes or cables, and are not suitable for long-term recordings. While there have been recent advancements in imaging methods in freely moving animals, there are still major limitations for researchers interested in the brain correlates of social behaviors.

“The cable disturbs natural animal behavior and social interaction,” Nagai told Photonics Media. If more than one mouse is placed in the cage, he said, cables attached to their heads would be tangled, and their behaviors and locomotion would immediately or eventually be severely restricted. "Only a cable-free imaging method can avoid such disturbance, and allows the simultaneous detection of brain activities from multiple freely moving animals.” Even for a single animal, the cable is thought to cause stress, Nagai said.

The LOTUS-V demonstrated that neural activity was significantly higher when a mouse approached others. Importantly, the signal was not affected by leaky signals emitted from other nearby mice, which means that it faithfully reflected in vivo brain activity.

"Our method successfully detected activity of the superficial layer of the primary visual cortex — this is about 300 μm deep," said Shigenori Inagaki, first author of the study. "It will be important to test its applicability to recording in deeper brain regions."

While the temporal resolution of the LOTUS-V method was sufficient to investigate the dynamics of brain activity triggered by specific events, it is not yet superior to that of the fiber-based method.

"These results could be really exciting for social neurobiologists," Nagai said. "It is minimally invasive, doesn't require cables or head fixes, and is suitable for long-term recordings in freely moving animals, meaning it could be useful in a broad range of other research fields, too."

Nagai told Photonics Media that the new method will be used to discover new brain regions or activity patterns critical to social interaction, which could lead to new targets for therapies. Brains in all animals need to be well-tuned in order to regulate social interaction, he said.

“Researchers can apply this method to perform various types of new tests,” Nagai said, “including drug screenings or new types of therapies — for example, environmental enrichment, food control, sleep/circadian control, and so on — simultaneously with monitoring brain activity of interest and ongoing animal interactions."

While the researchers can’t yet determine how powerful the technique will be, Nagai expects the information collected from the tests will lead to improvements in the technology.

The research was published in Scientific Reports (https://doi.org/10.1038/s41598-019-43897-x).