Understanding how neural networks generate complex behaviour is one of the major goals of neuroscience. Neurotransmitters and neuromodulators are crucial for the flow of information between neurons, and understanding their dynamics is key to unravelling their role in behaviour. We have recently developed a new family of genetically encoded serotonin (5–HT) sensors (sDarken) based on the native 5–HT1A receptor and circularly permuted GFP. sDarken 5–HT sensors are bright in the
unbound state and decrease their fluorescence upon binding of 5–HT. Sensor variants with different affinities for serotonin have been developed to increase the versatility of imaging serotonin dynamics. Experiments in vitro and in vivo demonstrated the feasibility of imaging serotonin dynamics with high temporal and spatial resolution. The designed sensors have excellent membrane expression, high specificity and signal–to–noise ratio, detect endogenous serotonin release and are suitable for in vivo imaging. Furthermore, a new and improved red–shifted calcium sensor based on cpmScarlet (PinkyCaMP) is presented.