Abstract

The autacoid and neurotransmitter histamine activates the H₁ G protein-coupled receptor (GPCR) to stimulate predominantly phospholipase C (PLC)/inositol phosphate (IP) signaling and, to a lesser extent, adenylyl cyclase (AC)/cAMP signaling in a variety of mammalian cells and tissues, as well as H₁-transfected clonal cell lines. This study reports that two novel H₁ receptor ligands developed in our laboratory, (-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene (trans-PAT) and (±)-cis-5-phenyl-7-dimethylamino-5,6,7,8-tetrahydro-9H-benzocycloheptane (cis-PAB), activate H₁ receptors to selectively stimulate AC/cAMP formation and PLC/IP formation, respectively, in Chinese hamster ovary cells transfected with guinea pig H₁ receptor cDNA. trans-PAT and cis-PAB also are shown to be functionally selective antagonists of H₁-linked PLC/IP and AC/cAMP signaling, respectively. Whereas cis-PAB H₁ receptor activity is shown to be typically competitive, trans-PAT displays a complex interaction with the H₁ receptor that is not competitive regarding antagonism of saturation binding by the standard H₁ antagonist radioligand [³H]mepyramine or H₁/PLC/IP functional activation by histamine. trans-PAT, however, does competitively block H₁/PLC/IP functional activation by cis-PAB. Molecular determinants for trans-PAT versus cis-PAB differential binding to H₁ receptors, which presumably leads to differential activation of AC/cAMP versus PLC/IP signaling, likely involves stereochemical factors as well as more subtle steric influences. Results suggest the trans-PAT and cis-PAB probes will be useful to study molecular mechanisms of ligand-directed GPCR multifunctional signaling. Moreover, because most untoward cardiovascular-, respiratory-, and gastrointestinal H₁ receptor-mediated effects proceed via the PLC/IP pathway, PAT-type agonists that selectively enhance H₁-mediated AC/cAMP signaling provide a mechanistic basis for exploiting H₁ receptor activation for drug design purposes.