P.1.31 Constitutive dimerization of the histamineH3 receptor in living cells

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Localization of the Histamine H2-Receptor and Gene Transcripts in Rat Stomach: Back to Parietal Cells

International audienceIn contrast with many physiological studies suggesting that histamine H2 receptors are present on acid-secreting parietal cells of the gastric epithelium, it was recently shown that immune cells in the lamina propria are the only cells expressing H2-receptor mRNAs (Mezey and Palkovits, Science, 1992, 258, 1662-1665). We have reinvestigated the cellular localization of H2 receptors in the rat stomach by visualizing both the H2 receptor mRNA and the H2-receptor protein itself. In situ hybridization histochemistry performed with an antisense riboprobe for the rat H2 receptor, and autoradiographic distribution of 125I-aminopotentidine binding sites, a highly selective H2-receptor ligand, did not show any labeling of the lamina propria. Signals were clearly and solely detected in the gastric epithelium, the strongest being observed in the upper part of the glands where the H2 receptor gene transcripts were only detected within parietal cells. In situ hybridization performed with an antisense riboprobe for L-histidine decarboxylase mRNA confirmed the basal localization of the histamine-synthetizing cells in the rat gastric gland, at some distance from parietal histamine-sensitive cells

A novel rat serotonin (5-HT6) receptor: molecular cloning, localization and stimulation of cAMP accumulation.

International audienceUsing a strategy based upon nucleotide sequence homology and starting from the sequence of the rat histamine H2 receptor (Ruat et al., Biochem. Biophys. Res. Commun. 1991, 179, 1470-1478), we have cloned a rat cDNA encoding a functional serotonin receptor (5-HT6). Its coding sequence corresponds to a glycoprotein of 436 amino acids displaying significant homology with other cloned monoaminergic receptors, e.g., various serotonin receptors. Genomic analysis of its gene indicated the presence of at least one intron. The major transcript of the 5-HT6 receptor gene has a size of approximately 4.1 kb but another minor 3.2 kb transcript was also evidenced. The highest expression, detected by Northern blot analysis as well as by in situ hybridization occurs in various serotoninergic areas of rat or guinea pig brain such as striatum, olfactory tubercle, nucleus accumbens and hippocampus, but a faint expression is also detectable in rat stomach. When transiently expressed in transfected COS-7 cells the 5-HT6 receptor appears to be positively coupled to cyclic AMP production

Three histamine receptors (H1, H2 and H3) visualized in the brain of human and non-human primates

International audienceThe distribution of histamine H1, H2 and H3 receptors in postmortem human and rhesus monkey brain was examined using receptor autoradiography. [125I]Iodobolpyramine, [125I]iodoaminopotentine and [3H](R) alpha-methylhistamine were used as ligands to label H1, H2 and H3 receptors respectively. The 3 receptor subtypes were identified in the human and monkey brains. Each receptor presented comparable distribution in the two primate brains. H1 and H2 receptors were particularly enriched in the caudate and putamen and observed in other brain areas such as the neocortex and hippocampus. H3-receptors were found to predominate in the basal ganglia where the highest densities were localized in the two segments of the globus pallidus. They were also observed in the hippocampus and cortical areas. The distribution of these 3 histamine receptors in the primate brain suggests the involvement of histaminergic mechanism in the functions of many brain areas. In particular, H2 and H3 receptors could play a role in the regulation of the basal ganglia functions in primates

Constitutive activity of the recombinant and native histamine H3 receptor

International audienceAlthough constitutive activity was shown to occur with many recombinant and/or mutated G-protein-coupled receptors, the physiological relevance of the process has remained debated. We have further explored this important issue with the histamine H3 receptor (H3R), a presynaptic receptor regulating histamine neuron activity in the brain. Constitutive activity of the recombinant receptor was studied using [3H]arachidonic acid release, [35S]GTPγ[S] binding and inhibition of cAMP accumulation. Evidence for constructive activity was obtained in these three functional assays with two isoforms of the rat H3 receptor, as well as with the human H3 receptor, expressed at physiological densities. Several standard H3-receptor antagonists, such as thioperamide and ciproxifan, were in fact acting as potent inverse agonists. Proxyfan opposed both agonists and inverse agonists and was therefore identified as a neutral antagonist. Using these drugs, we show high constitutive activity of native receptors. [35S]GTPγ[S] binding demonstrated constitutive activity of H3 receptors expressed at a normal level in mouse or rat brain. Constitutive activity of presynaptic H3 autoreceptors modulates histamine release from cortical synaptosomes in vitro and controls histamine neuron activity in vivo. This implies that inverse agonists rather than neutral antagonists may find therapeutic applications

S.16.04 Histamine H3-receptor ligands: Effects of inverse agonists and protean ligands

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