Deciphering the molecular pharmacology of the histamine H3 receptor

The histamine H3 receptor (H3R) is a G protein-coupled receptor predominantly expressed in the central nervous system, where it regulates the release of histamine and other neurotransmitters involved in cognition, arousal, and sleep-wake regulation. Although H3R has been validated as a therapeutic target, the limited clinical success of H3R-directed drugs highlights the need for improved pharmacological tools and a deeper understanding of H3R signaling complexity. This thesis investigates H3R pharmacology through the development of novel photopharmacological tools, analysis of ligand-biased signaling, and characterization of seven alternatively spliced H3R isoforms. Photopharmacological strategies were applied to create a light-activated H3R agonist, enabling spatiotemporal control of receptor activation. Pharmacological profiling of human H3R isoforms revealed differences in ligand binding affinities depending on their level of constitutive activity. In addition, several synthetic agonists and receptor isoforms preferentially recruit mini-Gi protein while minimally engaging β-arrestin1 or β-arrestin2, as revealed by receptor-proximal biosensor assays. Together, these findings demonstrate that H3R signalling is shaped by ligand properties, optical control, and receptor isoforms, providing new tools and insights that may support the development of pathway-selective and isoform-aware strategies for targeting the H3R.