sDarken: Next generation genetically encoded fluorescent sensors for serotonin

We developed a new family of genetically encoded serotonin (5-HT) sensors ( sDarken ) on the basis of the native 5-HT1A receptor and circularly permuted GFP. sDarken 5-HT sensors are bright in the unbound state and diminish their fluorescence upon binding of 5-HT. Sensor variants with different affinities for serotonin were engineered to increase the versatility in imaging of serotonin dynamics. Experiments in vitro and in vivo showed the feasibility of imaging serotonin dynamics with high temporal and spatial resolution. As demonstrated here, the designed sensors showed excellent membrane expression, have high specificity, a superior signal-to-noise ratio, detect the endogenous release of serotonin and are suitable for two-photon in vivo imaging. ### Competing Interest Statement The authors have declared no competing interest.