The ability to directly measure acetylcholine (ACh) release is an essential step towards understanding its physiological function. Here, we optimized the GRAB ACh ( G PC R - A ctivation‒ B ased- ACh ) sensor to achieve substantially improved sensitivity in ACh detection, as well as reduced downstream coupling to intracellular pathways. The improved version of the ACh sensor retains the sub-second response kinetics, physiological-relevant affinity and precise molecular specificity to ACh of its predecessor. Using this sensor, we revealed compartmental ACh signals in the olfactory center of transgenic flies in response to external stimuli including odor and body shock. Using fiber photometry recording and two-photon imaging, our ACh sensor also enabled sensitive detection of single-trial ACh dynamics in multiple brain regions in mice in a variety of behaviors.

Serotonin (5-HT) is a phylogenetically conserved monoamine neurotransmitter modulating important processes in the brain. To directly visualize the release of 5-HT, we developed a genetically encoded G PC R - A ctivation- B ased 5-HT (GRAB 5-HT ) sensor with high sensitivity, selectivity, subsecond kinetics, and subcellular resolution. GRAB 5-HT detects 5-HT release in multiple physiological and pathological conditions in both flies and mice, and provides new insights into the dynamics and mechanisms of 5-HT signaling.

The ability to directly measure acetylcholine (ACh) release is an essential first step towards understanding its physiological function. Here we optimized the GRABACh (GPCR-Activation-Based-ACh) sensor with significantly improved sensitivity and minimal downstream coupling. Using this sensor, we measured in-vivo cholinergic activity in both Drosophila and mice, revealing compartmental ACh signals in fly olfactory center and single-trial ACh dynamics in multiple regions of the mice brain under a variety of different behaviors 2 / 37 Cholinergic signals mediated by the neurotransmitter ACh are involved in a wide range of physiological processes, including muscle contraction, cardiovascular function, neural plasticity, attention and memory 1-3 .Previously, cholinergic activity was mainly measured using either electrophysiology to record nicotinic receptormediated currents 4, 5 or microdialysis followed by biochemical purification and identification 6 . However, these methods generally lack both cell-type specificity and the spatial-temporal resolution needed to precisely dissect cholinergic signals in vivo. Combining the type 3 muscarinic ACh receptor (M3R) with the conformationalsensitive circular permutated GFP (cpGFP), we recently developed GACh2.0 (short as ACh2.0), a genetically encoded GRAB (GPCR-Activation Based) ACh sensor that can convert the ACh-induced conformational change on M3R into a sensitive fluorescence response 7 . The ACh2.0 sensor responds selectively to physiological concentration of ACh with an EC50 of 2 μM and has been used in several model organisms to detect the endogenous release and regulation of cholinergic signals. Here, we optimized the GRABACh sensor using sitedirected mutagenesis and cell-based screening to further increase the sensitivity.To improve the performance of the GRABACh sensor, we focused on the interface between M3R and cpGFP, including the receptor's third intracellular loop (ICL3) and linker peptides, as well as critical residues in cpGFP that contribute to its fluorescence intensity (Figs. 1A and S1A-D). Our initial screening based on mediumthroughput imaging identified several variants with improved performance; these variants were subsequently verified using confocal microscopy (see Methods for details). The sensor with the largest ACh-induced fluorescence response was selected for further study and is named as GRABACh3.0 or ACh3.0 (Fig. 1A). We also generated a ligand-insensitive form of ACh3.0 by introducing the W200A mutation into the receptor 8 (Figs. 1A and S1E). When expressed in HEK293T cells or cultured neurons, the ACh3.0 sensor localized to the plasma membrane of the soma, and trafficked to dendrites and axons in neurons ( Fig. 1B-D). Moreover, compared to ACh2.0, the ACh3.0 sensor had a significantly larger fluorescence change (ΔF/F0~280%) in response to 100 μM ACh (Figs. 1B-D and S2A-E); in contrast, the ligand-insensitive ACh3.0-mut sensor had no detectable 4 coverslips for ACh2.0, ACh3.0, and ACh3.0-mut, respectively, with an average of >20 cells per co...