Decoding ligand recognition and constitutive activation of histamine H3 and H4 receptors.

Histamine H3 receptor (H3R) and H4 receptor (H4R) are key members of the histamine receptor family, with H3R as a potential target for narcolepsy treatments and H4R as a candidate for next-generation antihistamines for inflammatory and allergic diseases. Although progress has been made in understanding the structure of histamine receptors, the detailed mechanisms of ligand recognition and receptor antagonism for H3R and H4R remain unclear. In this study, using cryo-electron microscopy, we present an inactive structure of H4R bound to a selective antagonist, adriforant, and two Gi-coupled structures of H3R and H4R in complex with histamine. Our structural and mutagenesis analyses provide insights into the selective binding of adriforant to H4R and the recognition of histamine across histamine receptors. Our findings also uncovered distinct antagonistic mechanisms for H3R and H4R and identified the role of aromatic amino acids on extracellular loop 2 in modulating the constitutive activity of H3R and H4R. These findings advance our knowledge of the functional modulation of histamine receptors, providing a foundation for the development of targeted therapeutics for neurological and immune-related disorders.