AAV5-CaMKII-GCaMP6m

Loading calcium, neurotransmitter, and voltage-sensitive fluorescent reporters into AAVs enables neuroscientists to sample neuronal firing in both in vitro and in vivo models. Given that calcium is an important secondary messenger in many signaling cascades and is dynamically and rapidly released during neuronal activation, these sensors serve as proxies for neuronal activity that can be visualized using light-trapping devices such as optical cannulas paired with high-magnification microscopy such as two-photon and stimulated emission depletion (STED) microscopy. This allows researchers to express these sensors specifically in neurons of interest to study nodal activity in freely behaving rodents and non-human primates, or to modulate other inputs to the circuit using optogenetics or chemogenetics and image outputs elsewhere in the node. Studies using these methods enabled by AAVs can interpret true measurements of input/output changes within the circuit. Using these techniques, researchers were able to view real-time calcium imaging data that demonstrated how dopamine neurons influence reward, reward prediction error, aversion, and skill learning tasks. Additionally, monitoring of ventral tegmental area (VTA) dopaminergic neurons allows for mathematical modeling of how different inputs (e.g., motor, sensory, and cognitive variables) modulate dopamine release. In addition to calcium, there are now sensors for glutamate (iGluSnFR), gamma-aminobutyric acid (GABA) (iGABASnFR), dopamine (dLight), and many others are in development. AAV-driven, genetically encoded fluorescent voltage sensors allow for direct visualization of membrane voltage dynamics within AAV-transduced neurons. Neuroscientists can now not only visualize changes in neuronal firing, but also determine what is being released from these nodes and how the diversity of these signals modulate specific circuits.