Pharmacological characterization of seven human histamine H ₃ receptor isoforms

The histamine H ₃ receptor (H ₃ R) regulates as a presynaptic G protein-coupled receptor the release of histamine and other neurotransmitters in the brain, and is consequently a potential therapeutic target for neuronal disorders. The human H ₃ R encodes for seven splice variants that vary in the length of intracellular loop 3 and/or the C-terminal tail but are all able to induce heterotrimeric G _i protein signaling. The last two decades H ₃ R drug discovery and lead optimization has been exclusively focused on the 445 amino acids-long reference isoform H ₃ R-445.

In this study, we pharmacologically characterized for the first time all seven H ₃ R isoforms by determining their binding affinities for reference histamine H ₃ receptor agonists and inverse agonists. The H ₃ R-453, H ₃ R-415, and H ₃ R-413 isoforms display similar binding affinities for all ligands as the H ₃ R-445. However, increased agonist binding affinities were observed for the three shorter isoforms H ₃ R-329, H ₃ R-365, and H ₃ R-373, whereas inverse agonists such as the approved anti-narcolepsy drug pitolisant (Wakix®) displayed significantly decreased binding affinities for the latter two isoforms. This opposite change in binding affinity of agonist versus inverse agonists on H ₃ R-365 and H ₃ R-373 is associated with their higher constitutive activity in a cAMP biosensor assay as compared to the other 5 isoforms. The observed differences in pharmacology between longer and shorter H ₃ R isoforms should be considered in future drug discovery programs.