Successful fiber photometry in mazes and other complex environments

Fiber photometry is most commonly used to monitor activity and chemical signaling at a specific location in the brains of freely moving animals. The technique uses fluorescent sensor proteins to report on the changing local concentration of calcium and neurotransmitters in real-time. To capture the fluorescent signal, an optical fiber is implanted in the brain with the tip just above the region of interest.

Fiber photometry: Wireless vs patch cord

Traditionally, a fiber optic patch cord (patch cable) is used to both deliver the excitation light to the animal and return the fluorescent signal to a fluorescence cube and a detector outside the cage. However, patch cords can create severe limitations for researchers. Sometimes the cord can directly affect the behavior of the animals: mice become confused, tilt their head, gnaw on the cord, or otherwise react to its presence. The patch cord can also run into objects in the environment, limiting the animal’s ability to move or carry out tasks. Patch cords can also introduce noise and artifacts into the signal when used with fiber photometry. Patch cords also complicate scoring of freezing behavior in fear conditioning experiments. Even after an animal freezes (stops moving), the patch cord can keep swaying, and some video tracking software will confuse this with the continued movement of the animal.

Many mazes, for example, elevated zero mazes and elevated plus mazes, have high walls which can trap the cable as a mouse or rat moves from an open area to an enclosed area.

Elevated Plus Maze

Combining fiber photometry with behavioral measures for anxiety

Rodents typically prefer enclosed locations, and become anxious in unenclosed, wide-open spaces will limit their time spent exploring wide-open spaces. Anxiety levels play a strong role in how willing mice are to explore the unenclosed areas of a maze, so the zero maze is frequently used to study if a drug or other intervention is anxiolytic or anxiogenic. But a patch cord catching on the end of a wall could prevent a mouse from returning to the enclosed alley, so the researcher may have to be present to manipulate the cord.

Patch cords hanging from above a maze can get caught in doorways and at the ends of alleys.

In contrast, users have found mice fitted with wireless headstages such as those used with Amuza fiber photometry, optogenetics, and EEG have no trouble navigating mazes, and users report that the animals behave naturally.

For example, the Dimitrov lab at Rosalind Franklin University studies how stress and pain pathways interact in the brain. They used fiber photometry to monitor calcium fluorescence in the mPFC (medial prefrontal cortex) of mice traversing an elevated Zero maze while subject to inflammatory pain. They simultaneously monitored anxiety-like behavior by using video tracking to determine how much time the mice spent in exposed vs enclosed areas of the maze. They found that calcium fluorescence increased in male mice placed in the maze while subject to inflammatory pain, but there was no change in female mice under those conditions. Combined with other results, this finding suggests that sex-linked differences in the neural circuit between the locus coeruleus and mPFC are related to the differences in behavior and cognition displayed by male and female mice when subjected to inflammatory pain.

Cardenas A, Papadogiannis A, Dimitrov E. The role of medial prefrontal cortex projections to locus ceruleus in mediating the sex differences in behavior in mice with inflammatory pain. FASEB J. 2021 Jul;35(7):e21747.
https://pubmed.ncbi.nlm.nih.gov/34151467/

Animals: male and female mice
Sensor: GCaMP6f (calcium)
Vector: pAAV5.Syn.GCaMP6f, pAAV5.Syn.GCaMP6f.WPRE.SV40
Target Region: right mPFC (medial prefrontal cortex)
Coordinates: 1.8, ±0.4, and −2.2 mm in respect to bregma
Fiber: fiber core 400 μm NA 0.39, length 3 mm
Behavior test: Elevated O-maze
Fiber photometry data analysis: Amuza TeleFipho software
Model: Injection of complete Freund’s adjuvant (CFA) as a model for inflammatory pain.
Results: Inflammatory pain altered the calcium fluorescence signal from the mPFC of male mice placed on an elevated 0-maze, but did not alter the signal in female mice.

Other mazes which require a mouse to pass through or a doorway or tunnel, such as the puzzle box maze, Light/Dark box, and many social interaction tests, can also present difficulties when using patch cords. Yunlei Yang’s lab used Amuza fiber photometry while mice explored a light/dark box to help characterize an anxiogenic circuit between septal OXTr neurons and the HDB, and identified a possible cause of OXT therapy side effects.

Huang, Tuanjie, Fangxia Guan, Julio Licinio, Ma-Li Wong, and Yunlei Yang. “Activation of septal OXTr neurons induces anxiety-but not depressive-like behaviors.” Molecular Psychiatry (2021): 1-10.
https://pubmed.ncbi.nlm.nih.gov/34489531/

Animals: Male and female C57BL/6 J and Oxtr-Cre mice.
Sensor: GCaMP6s (calcium)
Vectors: AAV1-hSyn1-GCaMP6s-P2A-nls-dTomato, AAV1-hSyn1-axon-GCaMP6s
Target Region: Fiber: vHPC (ventral hippocampus) Vector: lateral septum.

Fiber: fiber core 400 μm diameter, NA 0.39
Behavior test: light-dark box, Elevated Plus Maze
Fiber photometry data analysis: Amuza TeleFipho software
Results: Anxiogenic conditions activate the vHPC and vHPC projections to the lateral septum, as shown by increased calcium levels.

Problems with tethers also apply when using optogenetics and EEG, which is why we recommend using wireless optogenetics and EEG with freely moving animals.