proxyfan and ciproxifan

The putative partial H1-receptor agonism of some H3-receptor antagonists belonging to the proxifan series was characterized in a functional in-vitro assay using guinea-pig ileum.. Whole segments of guinea-pig ileum were mounted in Tyrode's solution under isotonic conditions in the presence of atropine (10(-7) M) and were cumulatively treated with histamine as an internal reference. After washout, the putative H1-receptor agonists were added cumulatively to determine agonist potency (pEC50) and intrinsic activity (Emax) relative to histamine. Maximal or supramaximal concentrations of partial agonists, or sufficient concentrations of H1-receptor antagonists were incubated for 3-15 min prior to construction of a second concentration-effect curve to histamine in order to calculate partial agonist or antagonist affinity for the H1 receptor (pKP or pA2 value, respectively).. Several analogues of FUB 372 displayed low H1-receptor affinities (pA2 or pKP 4.2-5.5) except for a methyl benzoate derivative (pA2 = 6.81, Schild plot slope unity). FUB 372, four ortho-substituted derivatives (R = F, CH3, OCH3, CF3), and ciproxifan were weak contractile agents (Emax 9-38%, pEC50 4.73-5.68, histamine: 6.70) susceptible to antagonism by the H1-antihistaminergic drug mepyramine (2.10(-9)-10(-7) M). Agonist potency and H1-receptor affinity of these compounds did not correlate with the data of a set of H1-histaminergic 2-phenylhistamines bearing the same substituents.. A specific subset of proxifans related to FUB 372 and ciproxifan represent a unique type of H1-receptor agonists lacking a basic side chain.

Topics: Animals; Guinea Pigs; Histamine Agonists; Histamine Antagonists; Ileum; Imidazoles; Molecular Structure; Muscle Contraction; Receptors, Histamine H1; Receptors, Histamine H3

Different histamine H3-receptor antagonists have been tested in displacement studies at human and rat H3 receptors in stably transfected cells. Based on an actual rhodopsin structure, models for receptor antagonist interaction were developed for receptors of both species. Similarities and discrepancies in binding profiles can be explained, but not quantified by hydrophilic interactions with Asp114 and an important lipophilic binding pocket modified by two nearby amino acids.

Topics: Animals; CHO Cells; Cricetinae; Female; Histamine Antagonists; Humans; Imidazoles; Models, Molecular; Protein Binding; Rats; Receptors, Histamine H3; Rhodopsin; Species Specificity

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

Some G-protein-coupled receptors display 'constitutive activity', that is, spontaneous activity in the absence of agonist. This means that a proportion of the receptor population spontaneously undergoes an allosteric transition, leading to a conformation that can bind G proteins. The process has been shown to occur with recombinant receptors expressed at high density, and/or mutated, but also non-mutated recombinant receptors expressed at physiological concentrations. Transgenic mice that express a constitutively active mutant of the beta2-adrenergic receptor display cardiac anomalies; and spontaneous receptor mutations leading to constitutive activity are at the origin of some human diseases. Nevertheless, this process has not previously been found to occur in animals expressing normal levels of receptor. Here we show that two isoforms of the recombinant rat H3 receptor display high constitutive activity. Using drugs that abrogate this activity ('inverse agonists') and a drug that opposes both agonists and inverse agonists ('neutral antagonist'), we show that constitutive activity of native H3 receptors is present in rodent brain and that it controls histaminergic neuron activity in vivo. Inverse agonists may therefore find therapeutic applications, even in the case of diseases involving non-mutated receptors expressed at normal levels.

Topics: Amino Acid Sequence; Animals; Brain; CHO Cells; Cloning, Molecular; Corpus Striatum; Cricetinae; Guanosine 5'-O-(3-Thiotriphosphate); Histamine; Histamine Antagonists; Imidazoles; Ligands; Molecular Sequence Data; Neurons; Protein Isoforms; Rats; Receptors, Histamine H3; Recombinant Proteins; Reverse Transcriptase Polymerase Chain Reaction; Transfection