Fiber photometry is an optical method that is based on the principle of measuring the light emitted from fluorescent molecules via time-correlated single-photon counting (TCSPC)- based fiber optics. From this principle, we know that the realization of this experiment requires probe tools that can express fluorescence in the tissue, as well as a set of equipment that can transmit fluorescence and detect it.
Then we look at the fluorescence tools first. Currently the most commonly used fluorescence tools are divided into calcium indicators, neurotransmitter indicators, voltage indicators and so on. Because there are many kinds of animal models for experimental applications, and also we need to study specific brain regions or even specific kinds of cells, gene editing techniques have been widely used for probe development.
Common genetically encoded calcium indicators (GECIs) are fluorescent proteins derived from green fluorescent protein (GFP, etc.) and its variants (e.g., circularly arranged GFP, YFP, CFP, RFP, etc.) fused to calmodulin (CaM) and the M13 domain of myosin light chain kinase. When Ca2+ is present, it binds to CaM, leading to interaction between the M13 and CaM structural domains, triggering a structural rearrangement of cpEGFP, which enhances the green/red fluorescence signal.
The more widely used GECIs are: GCaMP, Pericams, Cameleons, TN-XXL and Twitch, among which GCaMP6, GCaMP7 are now widely used in in vivo calcium imaging studies due to its superior sensitivity. The following is a summary of the types of the common GECIs.