ciproxifan and thioperamide

The existence of mouse H3-receptor isoforms was investigated by PCR analysis and cDNA cloning. Splicing mechanisms previously reported in various species are conserved in the mouse. The retention/deletion of a fragment in the third intracellular loop of the mouse receptor leads to the existence of three isoforms designated mH(3(445)), mH(3(413)) and mH(3(397)) according to the length of their deduced amino acid sequence. PCR analysis showed that mouse H3-receptor isoforms display different expression patterns in the brain. Following expression in Cos-1 cells, [125I]iodoproxyfan binding indicated similar pharmacological profiles of the mH(3(445)), mH(3(413)) and mH(3(397)) isoforms. The pharmacological profile of the mouse H3 receptor is more similar to the rat receptor than to the human receptor, although some differences were also observed between the mouse and rat receptors. For example, the potency of thioperamide and ciproxifan is slightly higher at the mouse receptor than at the rat receptor but 40-100-fold higher than at the human receptor. In situ hybridization histochemistry showed that the distribution of H3-receptor mRNAs in the mouse brain is rather similar to that previously reported in the rat brain. However, the autoradiographic and cellular expression patterns observed in several brain areas such as the thalamus or hippocampus reveal important differences between the two species.

Topics: Animals; Blotting, Northern; Brain; Chlorocebus aethiops; Cloning, Molecular; Competitive Bidding; COS Cells; Gene Expression; Histamine; Histamine Agonists; Histamine Antagonists; Imidazoles; In Situ Hybridization; Iodine Radioisotopes; Isoenzymes; Mice; Piperidines; Radioligand Assay; Rats; Receptors, Histamine H3; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Thiourea; Transfection

Starting from the sequence of the human histamine H(3) receptor (hH(3)R) cDNA, we have cloned the corresponding rat cDNA. Whereas the two deduced proteins show 93.5% overall homology and differ only by five amino acid residues at the level of the transmembrane domains (TMs), some ligands displayed distinct affinities. Thioperamide and ciproxifan were about 10 fold more potent at the rat than at the human receptor, whereas FUB 349 displayed a reverse preference. Histamine, (R)alpha-methylhistamine, proxyfan or clobenpropit were nearly equipotent at H(3) receptors of both species. The inverse discrimination patterns of ciproxifan and FUB 349 were partially changed by mutation of one amino acid (V122A), and fully abolished by mutation of two amino acids (A119T and V122A), in TM3 of the rH(3)R located in the vicinity of Asp(114) purported to salt-link the ammonium group of histamine. Therefore, these two residues appear to be responsible for the distinct pharmacology of the H(3)R in the two species.

Topics: Amino Acid Sequence; Amino Acid Substitution; Amino Acids; Animals; Binding, Competitive; COS Cells; DNA, Complementary; Dose-Response Relationship, Drug; Histamine Antagonists; Humans; Imidazoles; Membrane Proteins; Molecular Sequence Data; Mutagenesis, Site-Directed; Mutation; Piperidines; Protein Structure, Tertiary; Radioligand Assay; Rats; Receptors, Histamine H3; Sequence Alignment; Sequence Homology, Amino Acid; Tritium