Recording Neural Activity in Freely Moving Mice Using Fiber Photometry

Start with a freely moving mouse with an implanted optic fiber cannula.

Connect one end of a fiber-optic patch cord to the cannula and the other end to an excitation light source.

The mouse expresses a green fluorophore-tagged genetically encoded calcium indicator, or GECI, in the lateral hypothalamic area, or LHA neurons.

The implanted optic fiber contacts the lateral habenula, or LHb, region, where the LHA axons terminate.

Excite the GECI at its optimal excitation wavelength and record the baseline signal.

Now, apply repeated air puffs to the mouse to evoke an aversive stress response.

This stimulus triggers action potentials in LHA axon terminals, opening voltage-gated calcium channels and allowing calcium influx.

Intracellular calcium ions bind to the GECI, inducing a conformational change that increases green fluorescence emission.

Visualize the optical signal.

Increased fluorescence in response to air puffs in the mouse confirms the role of the LHA-LHb pathway in transmitting aversive signals.

Begin by inspecting the distal end of the fibers of the patch cord by eye, and with a mini fiber microscope. If the surface of the fibers is scratched, repolish the fibers using fiber polishing film with fine grit. Then, clean the distal ends of the patch cord with 70% ethanol and a cotton-tip applicator. Clean the fiber optic cannulas using 70% ethanol and a cotton-tip applicator.

Connect the ferrule end of the patch cord to the implanted fiber using a ceramic split sleeve covered with a black shrink tube. During the connection, make sure that the sleeve is tight. Allow the animal to recover for a few minutes, then, start recording the optical signal and run the experiment. While recording, keep a careful eye on the live trace to ensure quality recordings, and watch for any jumps in signal indicating the sleeve is not tight enough.