General construction pattern of histamine H3-receptor antagonists: change of a paradigm

Novel omega-phenyl substituted and unsubstituted alkyl and alkenyl imidazole derivatives were prepared and tested for their antagonist activity in vitro and in vivo at histamine H3-receptors. Some compounds showed high in vitro and in vivo H3-receptor activity despite their structure bearing no polar moiety in the centre of the molecule which is a common structural feature of all other antagonists known. Quite probably there are further in vivo effects for some compounds resulting from other receptor interactions.

Development of FUB 181, a selective histamine H3-receptor antagonist of high oral in vivo potency with 4-(omega-(arylalkyloxy)alkyl)-1H-imidazole structure

A series of 4-(omega-(arylalkyloxy)alkyl)-1H-imidazoles and related sulphur-containing compounds have been prepared and evaluated for their histamine H3-autoreceptor antagonist in vitro potency in an assay on synaptosomes of rat cerebral cortex. In addition, the in vivo potency has been determined from the changes in N tau-methylhistamine levels in brain after p.o. administration to mice. Compounds with different alkyl chains and various aryl moities have been synthesized and tested to explore structure-activity relationships. Within this series of novel antagonists, (1H-imid-azol-4-yl)methyl and 2-(1H-imidazol-4-yl)ethyl ether derivatives showed low to moderate H3-receptor antagonist potency, whereas the corresponding allyl and propyl derivatives were compounds with high antagonist in vitro potency. Corresponding thioether or sulphoxide derivatives also showed antagonists activity. Additionally, some ether derivatives possessed high in vivo potency as well. The most active ether derivatives under in vivo conditions were 4-(3-(3-(4-fluorophenyl)propyloxy)propyl)-1H-imidazole (11b) and the corresponding chloro compound 11c (FUB 181) with ED50 values of 0.76 and 0.80 mg/kg, respectively. On the other hand, all compounds tested showed weak activity at histamine H1 or H2 receptors. Furthermore, the most promising ether FUB 181 exhibited low activity at adrenergic alpha 1, beta 1/2, serotonergic 5-HT2A, 5-HT3, and muscarinic M3 receptors. Time-course investigations of FUB 181 in mice showed a rapid mode of action with the highest value 3 h after p.o. application. Thus, FUB 181 appears to block histamine H3 receptors potently and selectively.

Search for novel leads for histamine H3-receptor antagonists: oxygen-containing derivatives

This study was performed in order to develop new leads for antagonists of the histamine H3-receptor subtype. omega-(1 H-Imidazol-4-yl)alkyl derivatives with ester, ketone or alcohol functionality in the side chain were synthesized and tested concerning their H3-receptor antagonist activity on synaptosomes of rat cerebral cortex. The novel compounds, which possess no nitrogen-containing polar group in the side chain of the imidazole moiety, presented moderate to high antagonist potency in vitro. In this series 3-(1 H-imidazol-4-yl)propyl-3-cyclopentylpropanoate (4) was the most potent compound in vitro with -log Ki = 8.5. Unfortunately, no central antagonist H3-receptor activity was detectable for ester derivatives in the in vitro H3-receptor assay based upon measurement of brain N tau-methylhistamine levels after p.o. administration to mice. Some of these novel antagonists are useful tools for investigations on ligand-receptor interaction because of their distinct receptor activities. On the other hand, the ketone derivative 1-(1 H-imidazol-4-yl)-7-phenyl-4-heptanone (9) in vitro presented an ED50-value of 3.5 +/- 1.5 mg/kg p.o. thus proving to be a new lead for further drug investigations. The most potent compounds in vitro and in vivo also showed high H3-receptor selectivity when tested at other histamine receptor subtypes.

Search for novel leads for histamine H3-receptor antagonists: amine derivatives

In search for novel leads for histamine H3-receptor antagonists a number of amine derivatives of different (1 H-imidazol-4-yl)anilines and omega-(1 H-imidazol-4-yl)alkanamines were prepared. Pharmacological in vitro H3-receptor investigations of the prepared urea, amide, inverse amide, thioamide, and thiourea derivatives on synaptosomes of rat cerebral cortex proved that the aniline derivatives are inactive at H3-receptors, whereas derivatives of the omega-(1 H-imidazol-4-yl)-alkanamines showed moderate to good activity. Some compounds are active in the nanomolar concentration range. The most potent compounds in this series are the thioamide derivative 7 and the urea derivatives 11, 12 of 3-(1 H-imidazol-4-yl) propanamine. Therefore, the urea derivatives were tested in vitro on isolated organs of the guinea pig for their activity on the other two histamine receptor subtypes proving their high selectivity. In vivo studies of the effects of the urea derivatives 11 and 12 on brain Nt-methylhistamine levels, a test of central H3-receptor activity after peroral application to mice, showed no detectable activity. Thus, the urea type antagonists are useful potent and selective H3-receptor tools for in vitro studies and for investigations of ligand-receptor interactions.

Cloning and selective expression in brain and kidney of ARNT2 homologous to the Ah receptor nuclear translocator (ARNT)

Arnt2, a new member of the basic-helix-loop-helix transcription factor family, was cloned from rat brain cDNAs. Its deduced 712 amino acid sequence displays 63% identity with that of the aryl hydrocarbon receptor nuclear translocator (Arnt1) that was completely established. Whereas Arnt2 gene expression, established by Northern blotting and in situ hybridization histochemistry, occurred selectively in brain and kidney, that of Arnt1 was ubiquitous, suggesting that the two proteins play distinct roles, presumably via dimerization and DNA binding with different partners.

Novel histamine H3-receptor antagonists with benzyl ether structure or related moieties: synthesis and structure-activity relationships

In search of new histamine H3-receptor ligands sixteen ether derivatives of 3-(1H-imidazol-4-yl)propanol with benzylic partial structure or related moieties were prepared and investigated as H3-receptor antagonists. The new compounds belong to a general construction pattern developed by other histamine H3-receptor antagonists. Structural modifications were introduced in an attempt to optimize in vitro as well as in vivo activity. Structure-activity relationships of the new histamine H3-receptor antagonists are discussed. All ether derivatives showed in vitro activities in the nanomolar concentration range, but only compounds with bulky lipophilic residues were also active under in vivo conditions. The most active compound within this series was 3-(1H-imidazol-4-yl)propyl 1-naphthylmethyl ether (4n) presenting an ED50 of 3.2 +/- 1.9 mg/kg regarding enhancement of endogenous histamine in brain after p.o. administration to mice. Furthermore, comparison of the H3-receptor activities measured on synaptosomes of rat cerebral cortex and on guinea pig ileum gave a good correlation indicating homogeneity of central and peripheral H3-receptor test models. The most interesting compounds were also evaluated in functional in vitro assays with regard to their activities at histamine H1-, H2-, and muscarinic M3-receptors. The tested compounds showed very weak activities at these receptor subtypes demonstrating their H3-receptor selectivity.

Novel carbamates as potent histamine H3 receptor antagonists with high in vitro and oral in vivo activity

The known histamine H3 receptor antagonists burimamide, thioperamide, clobenpropit, and a related homohistamine thioamide derivative were taken as templates in search for new leads. Novel histamine H3 receptor antagonists structurally described as carbamate derivatives of 3-(1H-imidazol-4-yl)propanol were prepared in high yields by treatment of the alcohol with corresponding isocyanates or carbamoyl chlorides and investigated for their H3 receptor antagonist activity. Different chain lengths and various substituents possessing different electronic and steric parameters were introduced and structure-activity relationships established. In different functional tests, the new antagonists showed high H3 receptor antagonist potencies in vitro (-log Ki values of 6.4-8.4) at synaptosomes of rat cerebral cortex and low activities at histamine H1 and H2 receptor subtypes. They were also screened for their central in vivo activity in mice after peroral administration. The most promising compounds (2, 16, 19) showed ED(50) values of about 1-2 mg/kg and thus are potential drugs for the therapy of H3 receptor dependent diseases. Some of the novel carbamate derivatives are H3 receptor selective compounds with high in vitro and in vivo activity.

Arachidonate 5-lipoxygenase and its activating protein: prominent hippocampal expression and role in somatostatin signaling

5-Lipoxygenase-activating protein (FLAP) is an 18-kDa integral membrane protein required, in peripheral cells, for the activation of 5-lipoxygenase (5-LO) and for the resulting synthesis of leukotrienes from arachidonic acid. In the brain, the leukotrienes have been implicated in several pathophysiological events and in the electrophysiological effect of somatostatin, yet the cellular origin and role of these messenger molecules are still poorly understood. In the present study, we used reverse transcriptase-polymerase chain reaction, in situ hybridization, and immunohistochemistry to demonstrate that 5-LO and FLAP are expressed in various regions of the rat brain, including hippocampus, cerebellum, primary olfactory cortex, superficial neocortex, thalamus, hypothalamus, and brainstem. Highest levels of expression were observed in cerebellum and hippocampus. In the latter we demonstrate the colocalization of 5-LO and FLAP in CA1 pyramidal neurons. Moreover, electrophysiological experiments show that selective inhibition of FLAP with the compound MK-886 (0.25-1 microM) prevents the somatostatin-induced augmentation of the hippocampal K+ M-current. Our results provide necessary evidence for the presence and signaling role of 5-LO and FLAP in central neurons and strongly support their proposed participation in somatostatin-receptor transmembrane signaling.

Design of potent non-thiourea H3-receptor histamine antagonists

Starting from thioperamide, the first potent and selective H3-receptor histamine antagonist, analogues have been synthesized and tested in vitro on rat cerebral cortex to explore structure-activity relationships. The aim has been to design potent compounds which do not possess the thiourea group of thioperamide and which may have improved brain penetration. In a short series of open chain thiourea analogues, the optimum chain length for H3-antagonist potency was found to be (CH2)3. Compounds derived from histamine and possessing an aromatic nitrogen-containing heterocycle on the side chain amino group in place of thiourea show H3-antagonist activity. Furthermore, when the heterocycle is 2-pyridyl, electron-withdrawing substituents (e.g. NO2, CF3, CO2Me) in the pyridine 5-position increased potency. The synthesis of 4-[4(5)-imidazolyl]piperidine and its conversion into the (trifluoromethyl)pyridyl analogue 5b of thioperamide is described; however, 5b is not as potent as thioperamide. Replacing imidazole by pyridine or substituting imidazole on the remote N considerably reduced potency. Replacing the side-chain NH by S increased potency still further and the most potent compound is 2-([2-[4(5)-imidazolyl]ethyl]thio)-5-nitropyridine (UCL 1199) which has Ki = 4.8 nM.

Characterization of histamine H3 receptors regulating acetylcholine release in rat entorhinal cortex

1. The pharmacological properties and location of H3 receptors modulating acetylcholine release have been investigated in non-superfused slices and synaptosomes of rat entorhinal cortex preloaded with [3H]-choline. 2. (R)alpha-methylhistamine, an H3-receptor agonist, potently inhibited the K(+)-evoked tritium release from slices, an effect antagonized by thioperamide, an H3-receptor antagonist, with nanomolar potency. 3. The K(+)-evoked tritium release from synaptosomes remained unaltered in the presence of the potent and selective H3-receptor agonists, imetit and (R)alpha-methylhistamine, suggesting that H3 receptors modulating acetylcholine release are not presynaptically located on cholinergic nerve terminals. 4. Phenylbutanoylhistamine and phenylpropylhistamine, two H3-receptor antagonists of moderate potency, failed to antagonize the inhibitory effects of (R)alpha-methylhistamine observed in slices. Unexpectedly, both compounds when used alone, inhibited tritium release from slices and synaptosomes with micromolar potency and to the same extent (by approximately 50% when added at a final concentration of 200 microM). This inhibitory effect did not involve H1, H2 or H3 receptors and was not mediated by an unknown histamine receptor site, since histamine used at a high concentration neither reproduced nor antagonized the effect of phenylbutanoylhistamine. It remained unaltered in the presence of scopolamine and was neither mimicked nor antagonized by vasoactive intestinal peptide, previously shown to be colocalized with acetylcholine in some neurones. 5. It is concluded that acetylcholine release in rat entorhinal cortex is modulated by H3 receptors presumably not located on cholinergic axon terminals. It remains to be established whether these H3 receptors belong to a receptor subtype different from those previously described since the potency ofphenylbutanoylhistamine and phenylpropylhistamine as H3-receptor antagonists was probably greatly underestimated by additional properties of both drugs.

Structure-activity studies with histamine H3-receptor ligands

Starting from thioperamide, the first potent and selective H3-receptor histamine antagonist, analogues have been synthesised and tested in vitro on rat cerebral cortex to explore structure-activity relationships. The aim has been to design compounds which do not possess the thiourea group of thioperamide and which may have improved brain penetration. Some compounds derived from histamine and possessing an aromatic nitrogen-containing heterocycle on the side-chain amino group show strong H3-antagonist activity. These have served as leads to provide aryloxyethyl- and aryloxy-propylimidazoles which are potent H3 antagonists of histamine. Structure-activity studies of the very potent and selective agonist, imetit (S-[2-imidazol-4-yl)ethyl]isothiourea), have explored the the transition between agonist, partial agonist and antagonist. The isosteric isourea is also a potent agonist. N,N'-Dibutyl-[S-[3-(imidazol-4-yl)propyl]isothiourea is a very potent antagonist having Ki = 1.5 nM.

Chromosomal localization of the human histamine H1-receptor gene

We have assigned the human histamine H1-receptor gene to chromosome 3 by Southern blot analysis of a chromosome mapping panel constructed from human-hamster somatic cell hybrids. This assignment was confirmed by in situ hybridization on metaphase chromosomes and involved bands 3p14-p21.

Pharmacological properties of histamine receptor subtypes

Histamine, a ubiquitous cell-to-cell messenger, exerts its numerous actions through interaction with three pharmacologically distinct receptor subtypes, termed H1, H2 and H3. The design of selective agonists and antagonists has allowed to establish their respective pharmacological profile. Radioligand binding studies and, very recently, molecular biological studies have shown that they all belong to the superfamily of G-protein coupled receptors. H1 and H2-receptor antagonists have been successfully used for a long time in the treatment of allergy and ulcer, respectively. Some of them have been designed as highly potent and selective radioligands and have allowed to analyze the precise distribution of H1 and H2 receptors in various tissues including the brain. Recently, H1- and H2-receptor genes have been cloned in various animal species. Transfection of mammalian cells with these intronless genes has confirmed the respective coupling of H1 and H2 receptors with phospholipase C and adenylylcyclase. However, other known or unknown intracellular signals, could also be triggered by the stimulation in a transfected cell of a single H1 or H2 receptor through coupling to different G-proteins. A third histamine receptor subtype, the H3 receptor was evidenced in rodent and human brain by the inhibition of histamine release and synthesis it mediates in various areas. Thus, H3 receptors were considered as autoreceptors localized on histaminergic terminals. With the design of several potent and selective H3-receptor agonists and of an antagonist thioperamide, the critical role of H3 receptors in the control of histaminergic neurons in vivo was established.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of the histamine H2 receptor and gene transcripts in rat stomach: back to parietal cells

In 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.

Guinea pig histamine H1 receptor. II. Stable expression in Chinese hamster ovary cells reveals the interaction with three major signal transduction pathways

A cDNA encoding a guinea pig histamine H1 receptor was stably expressed in Chinese hamster ovary (CHO) cells. In one resulting clone, named CHO(H1), the H1 receptor was found to be coupled to several major signal transduction pathways. In each case the involvement of a Gi/Go protein with pertussis toxin (PTX) was assessed, as well as the influence of extracellular Ca2+ and of protein kinase C activation by phorbol 12-myristate 13-acetate (PMA). Histamine induced, in a PTX- and PMA-insensitive manner, a biphasic increase in the intracellular Ca2+ level of which only the second sustained phase was dependent on the extracellular Ca2+ level. Histamine also caused a threefold elevation of inositol phosphate production, which was PTX-insensitive, but slightly inhibited by PMA and reduced by 75% in the absence of extracellular Ca2+. Histamine also caused a massive release of arachidonic acid, which occurred in a Ca(2+)- and PMA-sensitive manner, probably through the activation of a cytosolic phospholipase A2, which partly involves coupling to a PTX-sensitive G protein. In comparison, in HeLa cells endowed with a native H1 receptor, the histamine-induced arachidonic acid release was also Ca(2+)- and PMA-sensitive, but totally PTX-insensitive. Finally, in CHO(H1) cells, histamine in very low concentrations potentiated the cyclic AMP accumulation induced by forskolin. This response appeared to be insensitive to PTX, extracellular Ca2+, and PMA.(ABSTRACT TRUNCATED AT 250 WORDS)

Guinea pig histamine H1 receptor. I. Gene cloning, characterization, and tissue expression revealed by in situ hybridization

An intronless DNA encoding the guinea pig H1 receptor was cloned from a genomic library using probes derived from the bovine H1 receptor. It encodes a protein of 488 amino acids with a calculated molecular mass of 55,619 daltons compared with a size of 56-68 kDa for the photoaffinity-labeled receptor as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. The protein displays a 66% homology with the bovine receptor. Stable expression of the H1 receptor, characterized by the appearance of [3H]mepyramine binding sites with a pharmacology similar to that of the native H1 receptor, was obtained following transfection of Chinese hamster ovary cells. Southern blot analysis, using a variety of restriction enzymes, did not provide any evidence of multiple H1 isoreceptors. Northern blot analysis of a variety of guinea pig peripheral or cerebral tissues identified, in most cases, a single transcript of 3.3 kb, but also, in some tissues, a second transcript of 3.7 kb, possibly generated by the use of different promoter or polyadenylation sites or corresponding to a transcript from a distinct gene. In situ hybridization studies showed the highly contrasted cerebral expression of H1-receptor gene transcripts, which was compared with autoradiographic receptor localization. This allowed the identification of some major cell populations expressing the H1 receptor, e.g., Purkinje cells in cerebellum or pyramidal cells in the hippocampal complex.

Molecular cloning, characterization, and localization of a high-affinity serotonin receptor (5-HT7) activating cAMP formation

By using a strategy based on nucleotide sequence homology, we have cloned a cDNA encoding a functional serotonin (5-HT) receptor. The deduced amino acid sequence of the 5-HT7 receptor displays limited homology with that of other 5-HT receptors. In addition to the seven stretches of hydrophobic amino acids that characterize the superfamily of receptors interacting with guanine nucleotide-binding proteins, the 448-aa sequence of the 5-HT7 receptor contains a hydrophobic domain located at its N-terminal end. Genomic analysis indicated the presence of introns interrupting the coding sequence. The 5-HT7 receptor, stably expressed in transfected CHO cells, bound [3H]5-HT with high affinity (Kd = 1 nM), like receptors of the 5-HT1 subfamily from which, however, it was clearly distinguished by its pharmacology. 5-HT in nanomolar concentrations stimulated cAMP accumulation in these CHO cells by approximately 10-fold, whereas lysergic acid diethylamide displayed low intrinsic agonist activity. These various properties differentiate the 5-HT7 receptor from the four other subfamilies of mammalian 5-HT receptors (i.e., the 5-HT1-, 5-HT2-, 5-HT3-, and 5-HT4-like subfamilies) and, therefore, appear to define another receptor subfamily. Northern blot and in situ hybridization analyses showed the 5-HT7 transcripts to be expressed in discrete areas of the limbic brain (e.g., pyramidal hippocampus cells, tenia tecta, amygdaloid, or mammillary nuclei), suggesting that the receptor mediates serotoninergic controls in functions like mood, learning, or neuroendocrine and vegetative behaviors.

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

Using 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.

A detailed autoradiographic mapping of histamine H3 receptors in rat brain areas

[3H](R)alpha-methylhistamine, a selective histamine H3-receptor ligand, was used to perform binding studies with membranes and generate light microscopic autoradiograms in sections of the rat brain. High densities of H3 receptors were found in membranes from the anterior part of the cerebral cortex, the accumbens nucleus, the striatum, the olfactory tubercles and the substantia nigra. Autoradiography of sagittal and frontal sections evidenced specific labelling in a number of gray matter areas over a very low background, as determined using thioperamide, a selective H3-receptor antagonist, as competing drug. Labelled areas were identified by comparison with adjacent Nissl-stained sections and their labelling was rated visually. H3 receptors are heterogeneously distributed among areas known to receive histaminergic projections. In the cerebral cortex, H3 receptors are present in all areas and layers, with a rostrocaudal gradient and a higher density in deep layers (laminae IV-VI). In the hippocampal formation, H3 receptors are the most abundant in the dentate gyrus and the subiculum. In the amygdaloid complex, the highest densities are found in the central, lateral and basolateral groups of nuclei. In the basal forebrain, the accumbens nucleus, the striatum, the olfactory tubercles and the globus pallidus are highly labelled. In the thalamus in which histaminergic fibres are scarce, H3 receptors are present in a rather high density, particularly in the midline, median and intralaminar groups of nuclei. In the hypothalamus, where the densest network of histaminergic fibres is found, H3 receptors occur in moderate density, being slightly more abundant in the anterior and medial part. They are also present at the level of the tuberomammillary nuclei where they may reside on histaminergic perikarya. In mesencephalon and lower brainstem, H3 receptors are abundant in the reticular part of the substantia nigra and central gray. They are present in low density in areas of noradrenergic and serotoninergic perikarya and in the spinal cord, where a faint specific labelling is detected in the gray matter, particularly in the external layers of the dorsal horn. In the cerebellum and pituitary gland, H3 receptors are scarce. Kainic acid infusions into the striatum were followed by marked local decreases in H3 receptors evidenced in both membrane binding and autoradiographic studies. Unilateral interruption of the ascending histaminergic pathways via electrocoagulation of the lateral hypothalamic area was followed by ipsilateral increase in striatal [3H](R)alpha-methylhistamine binding, a process consistent with denervation up regulation of postsynaptic H3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. II. An in vivo study

In a companion paper (Serafin et al. 1992) we have demonstrated in vitro that histamine depolarizes three previously described medial vestibular nucleus neuron (MVNn) types (Serafin et al. 1991a, b). It has also been shown that this effect was exclusively mediated through postsynaptic H2 receptors. All the same, the eventual contribution of presynaptic H3 receptors to the physiological response of the MVNn to histamine remained an open question since, during the slicing procedure, any histaminergic axons projecting to the vestibular nuclei would have been interrupted. This rendered our study of H3-mediated effects of histamine difficult. Hence, in the present in vivo study our aim was three-fold: (1) to investigate the presence of H3 receptors at the vestibular nuclei level; (2) to evaluate the functional importance of MVNn H2 receptors; and (3) to explore whether H3 ligands, when injected intraperitoneally (i.p.), could modulate dynamic vestibular functions. In order to address the first two questions, we investigated postural changes induced by perfusion of the guinea-pig's vestibular nuclear complex with specific ligands of the H2 and H3 receptors. Our data extend the conclusions of our in vitro study and suggest that lateral vestibular nuclei neurons and the MVNn are endowed with both H2 and H3 receptors. Our results indicate furthermore that histamine can modulate, quite effectively, static vestibular reflexes. Finally, the present study demonstrates that i.p. injection of thioperamide, an H3 antagonist, induces a significant decrease in the horizontal vestibular-ocular reflex gain and, by contrast to most of the clinically used antihistaminics, has no detrimental effect on the alertness level. Our results may thus lead to clinical testing and use of H3 antagonists as antivertigo or anti motion-sickness drugs.

Synthesis, absolute configuration, stereoselectivity, and receptor selectivity of (alpha R, beta S)-alpha,beta-dimethylhistamine, a novel high potent histamine H3 receptor agonist

Depending on the selected synthetic pathway, structural variations of the neurotransmitter histamine led to mixtures of alpha,beta-dimethylhistamines as well as to the corresponding pure optical isomers. One of these isomers, namely (alpha R,beta S)-alpha,beta-dimethylhistamine, proved to be a highly potent H3 receptor agonist with exceptional receptor selectivity. The absolute configuration of the compound was determined by X-ray structure analysis of its dihydrobromide using the anomalous dispersion of bromine. The optical purity of both enantiomers of erythro-alpha,beta-dimethylhistamine was checked by 1HNMR investigations after acylation of the amines with (R)-2-methoxy-2-phenylacetyl chloride. As expected H3 receptors distinguish in a very strong way between the title compound and its alpha S,beta R-configured enantiomer. The agonistic potency of the latter is 2 orders of magnitude lower than the potency of (alpha R,beta S)-alpha,beta-dimethylhistamine.

S-[2-(4-imidazolyl)ethyl]isothiourea, a highly specific and potent histamine H3 receptor agonist

The effects of a new agonist of histamine (HA) H3 receptors, Imetit (S-[2-(4-(imidazolyl)ethyl]isothiourea) were investigated in vitro and in vivo and compared to those of (R)-alpha-methylhistamine [(R)-alpha-MeHA], a prototypic drug. Imetit inhibited the binding of [3H](R-alpha-MeHA to rat brain membranes with a Ki value of 0.1 +/- 0.01 nM. The release of endogenously synthesized [3H]HA induced by K(+)-depolarization from rat brain slices and synaptosomes was inhibited by Imetit with EC50 values of 1.0 +/- 0.3 and 2.8 +/- 0.7 nM, respectively. Imetit behaved as a full agonist and was about 4 times more potent than (R)-alpha-MeHA and 60 times more potent than HA. Thioperamide, a selective H3 receptor antagonist, elicited a parallel rightward shift of the concentration-response curve for Imetit with an apparent Ki value of 5.6 +/- 1.4 nM. Imetit potencies relative to HA were less than 0.1% and only 0.6% at HA H1 and H2 receptor reference systems, respectively. Imetit was found not to be a substrate or an inhibitor of HMT. After p.o. administration to mice or rats, Imetit decreased (by approximately 50%) the tele-MeHA level in the cerebral cortex with ED50 values of 1.0 +/- 0.3 and 1.6 +/- 0.3 mg/kg, respectively. This effect was still maximal after 6 hr. The in vivo potency and duration of action of Imetit were in the same range as those of (R)-alpha-MeHA. It is therefore concluded that Imetit represents a new potent and selective HA H3 receptor agonist.