A novel phenylaminotetralin radioligand reveals a subpopulation of histamine H1 receptors

Previously, (-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene ([-]-trans-H-2-PAT) was shown to activate stereospecifically histamine H-1 receptors coupled to modulation of tyrosine hydroxylase activity in guinea pig and rat forebrain in vitro and in vivo. Furthermore, the novel radioligand [H-3](-)-trans-H-2-PAT was shown to label selectively H-1 receptors in guinea pig and rat brain with high affinity (K-D, similar to0.1 and 0.5 nM, respectively) and a B-max about 50 and 15%, respectively, of that observed for the H-1 antagonist radioligand [H-3]mepyramine. In the current study, [H-3](-)-trans-H-2-PAT-labeled cloned guinea pig and human H-1 receptors in Chinese hamster ovary (CHO) cell membranes with high affinity (K-D, similar to0.08 and 0.23 nM, respectively) and a B-max about 15% of that observed for [H-3]mepyramine. The binding of H-2-PAT to H-1 receptors in both CHO-H-1 cell lines was stereoselective with the (-)-trans-isomer having affinity (K-i, similar to1.5 nM) about 4-, 20-, and 50- times higher than the (-)-cis-, (+)-trans-, and (+)-cis-isomers, respectively; the affinity of (-)-trans-H-2-PAT was unaffected by excess GTP. In functional assays, (-)-trans-H-2-PAT was a full antagonist of histamine H-1-mediated stimulation of phospholipase C (PLC) and [H-3] inositol phosphates (IP) formation in CHO-H-1 cells, a full inverse agonist of constitutively active H-1 receptors in COS-7-H-1 cells, and a full competitive antagonist (pA(2) = 9.2) of histamine H-1-mediated contraction of guinea pig ileum. It is concluded that (-)-trans- H-2-PAT is an antagonist at H-1 receptors coupled to PLC/IP formation and smooth muscle contraction. Meanwhile, the observation that [H-3](-)-trans-H-2-PAT labels only a subpopulation of H-1 receptors and that (-)-trans-H-2-PAT activates H-1 receptors coupled to modulation of tyrosine hydroxylase suggests that there may be post-translational H-1 receptor heterogeneity.