piperidines and iodoproxyfan

Histamine H3 receptor inverse agonists are known to enhance the activity of histaminergic neurons in brain and thereby promote vigilance and cognition. 1-{3-[3-(4-Chlorophenyl)propoxy]propyl}piperidine, hydrochloride (BF2.649) is a novel, potent, and selective nonimidazole inverse agonist at the recombinant human H3 receptor. On the stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate binding to this receptor, BF2.649 behaved as a competitive antagonist with a Ki value of 0.16 nM and as an inverse agonist with an EC50 value of 1.5 nM and an intrinsic activity approximately 50% higher than that of ciproxifan. Its in vitro potency was approximately 6 times lower at the rodent receptor. In mice, the oral bioavailability coefficient, i.e., the ratio of plasma areas under the curve after oral and i.v. administrations, respectively, was 84%. BF2.649 dose dependently enhanced tele-methylhistamine levels in mouse brain, an index of histaminergic neuron activity, with an ED50 value of 1.6 mg/kg p.o., a response that persisted after repeated administrations for 17 days. In rats, the drug enhanced dopamine and acetylcholine levels in microdialysates of the prefrontal cortex. In cats, it markedly enhanced wakefulness at the expense of sleep states and also enhanced fast cortical rhythms of the electroencephalogram, known to be associated with improved vigilance. On the two-trial object recognition test in mice, a promnesiant effect was shown regarding either scopolamine-induced or natural forgetting. These preclinical data suggest that BF2.649 is a valuable drug candidate to be developed in wakefulness or memory deficits and other cognitive disorders.

Topics: Acetylcholine; Animals; Cats; Dopamine; Electroencephalography; Guinea Pigs; Histamine Agonists; Histamine Antagonists; Histamine Release; Humans; Imidazoles; Male; Methylhistamines; Mice; Mice, Inbred C57BL; Piperidines; Prefrontal Cortex; Receptors, Histamine H3; Scopolamine

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

We determined the affinities of 16 newly synthesized H3 receptor antagonists in an H3 receptor binding assay and the potencies of 12 of these compounds at functional H3 receptors in the mouse brain cortex and guinea-pig ileum. The compounds differ from histamine in that the C-C-N side chain is replaced by a chain of the structure C-C-C-O. The two major aims of the study were (1) to investigate whether the two functional H3 receptors are pharmacologically different and (2) to derive structure-activity relationships. The specific binding of 3H-Na-methylhistamine to rat brain cortex membranes was monophasically displaced by each of the 16 compounds at pKi values ranging from 7.30 to 9.48. In superfused mouse brain cortex slices preincubated with 3H-noradrenaline, the electrically evoked tritium overflow was slightly decreased by iodoproxyfan and its deiodo analogue; this effect was counteracted by the H3 receptor antagonist clobenpropit. The other compounds did not affect the evoked tritium overflow by themselves. The concentration-response curve of histamine for its inhibitory effect on the electrically evoked tritium overflow was shifted to the right by the 12 compounds with apparent pA2 values ranging from 7.02 to 9.00. The 12 compounds also shifted to the right the concentration-response curve of R-a-methylhistamine for its inhibitory effect on the electrically induced contraction in guinea-pig ileum strips; the apparent pA2 values ranged from 5.97 to 9.00. Iodoproxyfan decreased the electrically induced contraction by itself and this effect was counteracted by the H3 receptor antagonist thioperamide. The apparent pA2 values in the two functional H3 receptor models showed a highly significant correlation (r = 0.882; P < 0.001). Highly significant correlations were also obtained when the pKi values of the compounds in the binding assay were compared to their apparent pA2 values in the mouse brain (r = 0.799; P < 0.004) and in the guinea-pig ileum (r = 0.851; P < 0.001). In each of the three experimental models, iodoproxyfan was the most potent compound; its deiodo analogue was less potent by more than 1.1 log units. The present results show that the compounds under study possess moderate to high affinity and/or (partial) H3 receptor antagonist potency. The two functional H3 receptors in the mouse brain cortex and the guinea-pig ileum may be slightly different; further studies are necessary to clarify whether this difference is due to H3 receptor heterogen

Topics: Animals; Binding, Competitive; Cerebral Cortex; Dose-Response Relationship, Drug; Female; Guinea Pigs; Histamine Antagonists; Histamine H2 Antagonists; Imidazoles; In Vitro Techniques; Intestine, Small; Iodine Radioisotopes; Male; Methylhistamines; Mice; Piperidines; Rats; Rats, Wistar; Structure-Activity Relationship