(A) The principle behind our proposed GPCR Activation-Based (GRAB) sensors (left) for detecting transmitters, with the corresponding theoretical performance index (right). (B) A similar GRAB strategy was successfully used for in vivo monitoring of acetylcholine (ACh), dopamine (DA), and norepinephrine (NE) in behaving animals. (C) A proposed toolbox of GRAB sensors for monitoring various purinergic transmitters; the structure of each transmitter is shown at the top. Data shown in (B) adapted from (Jing et al., 2018), (Sun et al., 2018), (Patriarchi et al., 2018), and (Feng et al., 2019).