Genetically encoded sensors for measuring histamine release both in vitro and in vivo

Histamine (HA) is a key biogenic monoamine involved in a wide range of physiological and pathological processes in both the central nervous system and the periphery. Because the ability to directly measure extracellular HA in real-time will provide important insights into the functional role of HA in complex circuits under a variety of conditions, we developed a series of genetically encoded G protein-coupled receptor activation-based (GRAB) HA (GRABHA) sensors. These sensors produce a robust increase in fluorescence upon HA application, with good photostability, sub-second kinetics, nanomolar affinity, and high specificity. Using these GRABHA sensors, we measured electrical stimulation-evoked HA release in acute brain slices with high spatiotemporal resolution. Moreover, we recorded HA release in the preoptic area of the hypothalamus and in the medial prefrontal cortex during the sleep-wake cycle in freely moving mice, finding distinct patterns of HA release in these specific brain regions. Together, these in vitro and in vivo results show that our GRABHA sensors have high sensitivity and specificity for measuring extracellular HA, thus providing a robust new set of tools for examining the role of HA signaling in both health and disease.