No abstract

The human histamine H 1 receptor (H 1 R) is an important, well characterized target for the development of antagonists to treat allergic conditions. Many neuropsychiatric drugs are known to potently antagonize the H 1 R, thereby producing some of their side effects. In contrast, the tolerability and potential therapeutic utility of H 1 R agonism is currently unclear. We have used a cell-based functional assay to evaluate known therapeutics and reference drugs for H 1 R agonist activity. Our initial functional screen identified three ergot-based compounds possessing heretofore-unknown H 1 R agonist activity. 8R-lisuride demonstrated potent agonist activity in various assays including receptor selection and amplification technology, inositol phosphate accumulation, and activation of nuclear factor-B with pEC 50 values of 8.1, 7.9, and 7.9, respectively, and with varying degrees of efficacy. Based on these assays, 8R-lisuride is the most potent stereospecific partial agonist for the human H 1 R yet reported. Investigation of the residues involved in histamine and lisuride binding, using H 1 R mutants and molecular modeling, have revealed that although these ligands are structurally different, the lisuride-binding pocket in the H 1 R closely corresponds to the histamine-binding pocket.

The human histamine H 1 receptor (H 1 R) is a prototypical G protein-coupled receptor and an important, well characterized target for the development of antagonists to treat allergic conditions. Many neuropsychiatric drugs are also known to potently antagonize this receptor, underlying aspects of their side effect profiles. We have used the cell-based receptor selection and amplification technology assay to further define the clinical pharmacology of the human H 1 R by evaluating Ͼ130 therapeutic and reference drugs for functional receptor activity. Based on this screen, we have reported on the identification of 8R-lisuride as a potent stereospecific partial H 1 R agonist (Mol Pharmacol 65: 538 -549, 2004). In contrast, herein we report on a large number of varied clinical and chemical classes of drugs that are active in the central nervous system that display potent H 1 R inverse agonist activity. Absolute and rank order of functional potency of these clinically relevant brain-penetrating drugs may possibly be used to predict aspects of their clinical profiles, including propensity for sedation.

The aim of this study was to create and characterize constitutively active mutant (CAM) histamine H 1 receptors (H 1 R) using random mutagenesis methods to further investigate the activation process of the rhodopsin-like family of G protein-coupled receptors (GPCRs). This approach identified position 6.40 in TM 6 as a "hot spot" because mutation of Ile6.40 420 either to Glu, Gly, Ala, Arg, Lys, or Ser resulted in highly active CAM H 1 Rs, for which almost no histamine-induced receptor activation response could be detected. The highly conserved hydrophobic amino acid at position 6.40 defines, in a computational model of the H 1 R, the asparagine cage motif that restrains the side chain of Asn7.49 of the NPxxY motif toward transmembrane domain (TM 6) in the inactive state of the receptor. Mutation of the asparagine cage into Ala or Gly, removing the interfering bulky constraints, increases the constitutive activity of the receptor. The fact that the Ile6.40 420 Arg/Lys/Glu mutant receptors are highly active CAM H 1 Rs leads us to suggest that a positively charged residue, presumably the highly conserved Arg3.50 from the DRY motif, interacts in a direct or an indirect (through other side chains or/and internal water molecules) manner with the acidic Asp2.50⅐⅐Asn7.49 pair for receptor activation.