Characterization of the binding of the first selective radiolabelled histamine H3-receptor antagonist, [125I]-iodophenpropit, to rat brain

Chemical Probes for Histamine Receptor Subtypes

Ligands with different properties and different selectivity are highly needed for in vitro and in vivo studies on the (patho)physiological influence of the chemical mediator histamine and its receptor subtypes. A selection of well-described ligands for the different receptor subtypes and different studies is shown with a particular focus on affinity and selectivity. In addition, compounds with radioactive or fluorescence elements will be presented with their beneficial use for other species or different investigations.

Bioluminescence Resonance Energy Transfer Based G Protein-Activation Assay to Probe Duration of Antagonism at the Histamine H3 Receptor

Duration of receptor antagonism, measured as the recovery of agonist responsiveness, is gaining attention as a method to evaluate the 'effective' target-residence for antagonists. These functional assays might be a good alternative for kinetic binding assays in competition with radiolabeled or fluorescent ligands, as they are performed on intact cells and better reflect consequences of dynamic cellular processes on duration of receptor antagonism. Here, we used a bioluminescence resonance energy transfer (BRET)-based assay that monitors heterotrimeric G protein activation via scavenging of released Venus-Gβ₁γ₂ by NanoLuc (Nluc)-tagged membrane-associated-C-terminal fragment of G protein-coupled receptor kinase 3 (masGRK3ct-Nluc) as a tool to probe duration of G protein-coupled receptor (GPCR) antagonism. The Gα_i-coupled histamine H₃ receptor (H₃R) was used in this study as prolonged antagonism is associated with adverse events (e.g., insomnia) and consequently, short-residence time ligands might be preferred. Due to its fast and prolonged response, this assay can be used to determine the duration of functional antagonism by measuring the recovery of agonist responsiveness upon washout of pre-bound antagonist, and to assess antagonist re-equilibration time via Schild-plot analysis. Re-equilibration of pre-incubated antagonist with agonist and receptor could be followed in time to monitor the transition from insurmountable to surmountable antagonism. The BRET-based G protein activation assay can detect differences in the recovery of H₃R responsiveness and re-equilibration of pre-bound antagonists between the tested H₃R antagonists. Fast dissociation kinetics were observed for marketed drug pitolisant (Wakix^®) in this assay, which suggests that short residence time might be beneficial for therapeutic targeting of the H₃R.

Homogeneous, Real-Time NanoBRET Binding Assays for the Histamine H3 and H4 Receptors on Living Cells

Receptor-binding affinity and ligand-receptor residence time are key parameters for the selection of drug candidates and are routinely determined using radioligand competition-binding assays. Recently, a novel bioluminescence resonance energy transfer (BRET) method utilizing a NanoLuc-fused receptor was introduced to detect fluorescent ligand binding. Moreover, this NanoBRET method gives the opportunity to follow fluorescent ligand binding on intact cells in real time, and therefore, results might better reflect in vivo conditions as compared with the routinely used cell homogenates or purified membrane fractions. In this study, a real-time NanoBRET-based binding assay was established and validated to detect binding of unlabeled ligands to the histamine H₃ receptor (H₃R) and histamine H₄ receptor on intact cells. Obtained residence times of clinically tested H₃R antagonists were reflected by their duration of H₃R antagonism in a functional receptor recovery assay.

Role of the Histamine H3 Receptor in the Central Nervous System

The G_i/o protein-coupled histamine H₃ receptor is distributed throughout the central nervous system including areas like cerebral cortex, hippocampus and striatum with the density being highest in the posterior hypothalamus, i.e. the area in which the histaminergic cell bodies are located. In contrast to the other histamine receptor subtypes (H₁, H₂ and H₄), the H₃ receptor is located presynaptically and shows a constitutive activity. In detail, H₃ receptors are involved in the inhibition of histamine release (presynaptic autoreceptor), impulse flow along the histaminergic neurones (somadendritic autoreceptor) and histamine synthesis. Moreover, they occur as inhibitory presynaptic heteroreceptors on serotoninergic, noradrenergic, dopaminergic, glutamatergic, GABAergic and perhaps cholinergic neurones. This review shows for four functions of the brain that the H₃ receptor represents a brake against the wake-promoting, anticonvulsant and anorectic effect of histamine (via postsynaptic H₁ receptors) and its procognitive activity (via postsynaptic H₁ and H₂ receptors). Indeed, H₁ agonists and H₃ inverse agonists elicit essentially the same effects, at least in rodents; these effects are opposite in direction to those elicited by brain-penetrating H₁ receptor antagonists in humans. Although the benefit for H₃ inverse agonists for the symptomatic treatment of dementias is inconclusive, several members of this group have shown a marked potential for the treatment of disorders associated with excessive daytime sleepiness. In March 2016, the European Commission granted a marketing authorisation for pitolisant (Wakix^R) (as the first representative of the H₃ inverse agonists) for the treatment of narcolepsy.

Assessment of [125I]WYE-230949 as a novel histamine H3 receptor radiopharmaceutical

Histamine H3 receptor therapeutics have been proposed for several diseases such as schizophrenia, attention deficit hyperactivity disorder, Alzheimer's disease and obesity. We set out to evaluate the novel compound, [125I]WYE-230949, as a potential radionuclide imaging agent for the histamine H3 receptor in brain. [125I]WYE-230949 had a high in vitro affinity for the rat histamine H3 receptor (Kd of 6.9 nM). The regional distribution of [125I]WYE-230949 binding sites in rat brain, demonstrated by in vitro autoradiography, was consistent with the known distribution of the histamine H3 receptor. Rat brain uptake of intravenously injected [125I]WYE-230949 was low (0.11 %ID/g) and the ratio of specific: non-specific binding was less than 1.4, as determined by ex vivo autoradiography. In plasma, metabolism of [125I]WYE-230949 into a less lipophilic species occurred, such that less than 38% of the parent compound remained 30 minutes after injection. Brain uptake and metabolism of [125I]WYE-230949 were increased and specific binding was reduced in anaesthetised compared to conscious rats. [125I]WYE230949 is not a potential radiotracer for imaging rat histamine H3 receptors in vivo due to low brain uptake, in vivo metabolism of the parent compound and low specific binding.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Serotonergic neuron regulation informed by in vivo single-cell transcriptomics

Despite the recognized importance of the dorsal raphe (DR) serotonergic (5-HT) nuclei in the pathophysiology of depression and anxiety, the molecular components/putative drug targets expressed by these neurons are poorly characterized. Utilizing the promoter of an ETS domain transcription factor that is a stable marker of 5-HT neurons (Pet-1) to drive 5-HT neuronal expression of YFP, we identified 5-HT neurons in live acute slices. We isolated RNA from single 5-HT neurons in the ventromedial and lateral wings of the DR and performed single-cell RNA-Seq analysis identifying >500 G-protein coupled receptors (GPCRs) including receptors for classical transmitters, lipid signals, and peptides as well as dozens of orphan-GPCRs. Using these data to inform our selection of receptors to assess, we found that oxytocin and lysophosphatidic acid 1 receptors are translated and active in costimulating, with the α1-adrenergic receptor, the firing of DR 5-HT neurons, while the effects of histamine are inhibitory and exerted at H3 histamine receptors. The inhibitory histamine response provides evidence for tonic in vivo histamine inhibition of 5-HT neurons. This study illustrates that unbiased single-cell transcriptomics coupled with functional analyses provides novel insights into how neurons and neuronal systems are regulated.

Mapping pharmaceuticals in rat brain sections using MALDI imaging mass spectrometry

Matrix-assisted laser desorption/ionization-tandem mass spectrometric method (MALDI-MS/MS) has proven to be a reliable tool for direct measurement of the disposition of small molecules in animal tissue sections. As example, MALDI-MS/MS imaging system was employed for visualizing the spatial distribution of astemizole and its primary metabolite in rat brain tissues. Astemizole is a second-generation antihistamine, a block peripheral H1 receptor, which was introduced to provide comparable therapeutic benefit but was withdrawn in most countries due to toxicity risks. Astemizole was observed to be heterogeneously distributed to most parts of brain tissue slices including cortex, hippocampus, hypothalamic, thalamus, and ventricle regions while its major metabolite, desmethylastemizole, was only found around ventricle sites. We have shown that astemizole alone is likely to be responsible for the central nervous system (CNS) side effects when its exposures became elevated.

MALDI–tandem mass spectrometry imaging of astemizole and its primary metabolite in rat brain sections

Background: Matrix-assisted laser desorption/ionization (MALDI)–tandem mass spectrometry (MS)/MS is a proven reliable tool for visualizing the spatial distribution of dosed drugs and their primary metabolites in animal tissue sections. Materials & methods: The rat brain tissue sections coated with dihydroxybenzoic acid as matrix, were analyzed by MALDI–MS/MS imaging experiments. The potential metabolites of astemizole in rat brain homogenate selected for MALDI–MS/MS imaging experiments were first identified by high-performance liquid chromatography coupled to an electrospray ionization source and a hybrid-quadrupole–linear-ion-trap mass spectrometer. Results: Astemizole was observed to be heterogeneously distributed to most parts of the brain tissue slices including the cortex, hippocampus, hypothalamic, thalamus and ventricle regions, while its major metabolite, desmethylastemizole, was only found around ventricle sites. Conclusion: The results indicated that the dosed compound alone might be responsible for the CNS side-effects when drug exposures became elevated.

An 80-amino acid deletion in the third intracellular loop of a naturally occurring human histamine H3 isoform confers pharmacological differences and constitutive activity

In this article, we pharmacologically characterized two naturally occurring human histamine H3 receptor (hH3R) isoforms, hH3R(445) and hH3R(365). These abundantly expressed splice variants differ by a deletion of 80 amino acids in the intracellular loop 3. In this report, we show that the hH3R(365) is differentially expressed compared with the hH3R(445) and has a higher affinity and potency for H3R agonists and conversely a lower potency and affinity for H3R inverse agonists. Furthermore, we show a higher constitutive signaling of the hH3R(365) compared with the hH3R(445) in both guanosine-5'-O-(3-[35S]thio) triphosphate binding and cAMP assays, likely explaining the observed differences in hH3R pharmacology of the two isoforms. Because H3R ligands are beneficial in animal models of obesity, epilepsy, and cognitive diseases such as Alzheimer's disease and attention deficit hyperactivity disorder and currently entered clinical trails, these differences in H3R pharmacology of these two isoforms are of great importance for a detailed understanding of the action of H3R ligands.

Comparison of the binding distribution of agonist and antagonist ligands for histamine H3 receptors in pig brain by quantitative autoradiography

The relationship between the abundances of agonist and antagonist-binding sites for monoamine receptors is poorly established. Therefore, we used quantitative autoradiography to investigate the distribution and concentration of binding sites for histamine H(3) receptor ligands in cryostat sections of pig brain. As in other species, binding of the histamine H(3) receptor agonist [(3)H]N(alpha)-methylhistamine was highly heterogeneous in the pig brain, with highest B(max) in the substantia nigra, followed by the nucleus accumbens and caudate, intermediate binding in frontal cortex, diencephalon, and mesencephalon, and absent specific binding in cerebellum: the affinity of [(3)H]N(alpha)-methylhistamine was close to 1 nM in all regions of pig brain. Thus, the saturation binding parameters for this H(3) receptor agonist in pig brain were similar to the earlier reports in rat, guinea pig, and human. The distribution of histamine H(3) receptors labeled with the receptor antagonist [(125)I]iodophenpropit in adjacent cryostat sections from the same group of pigs was very similar to that of [(3)H]N(alpha)-methylhistamine. However, the B(max) of the receptor antagonist was 40% higher in the basal ganglia than was the B(max) of the receptor agonist. The K(d) for the receptor antagonist ligand was close to 0.9 nM in all regions. These results suggest that histamine H(3) receptor agonist-binding sites, i.e. those linked to intracellular G-protein, comprise a subset of the total receptor antagonist-binding sites in the basal ganglia, as has been reported for dopamine D(2) receptors.

Molecular aspects of the histamine H3 receptor

The cloning of the histamine H(3) receptor (H(3)R) cDNA in 1999 by Lovenberg et al. [10] allowed detailed studies of its molecular aspects and indicated that the H(3)R can activate several signal transduction pathways including G(i/o)-dependent inhibition of adenylyl cyclase, activation of phospholipase A(2), Akt and the mitogen activated kinase as well as the inhibition of the Na(+)/H(+) exchanger and inhibition of K(+)-induced Ca(2+) mobilization. Moreover, cloning of the H(3)R has led to the discovery several H(3)R isoforms generated through alternative splicing of the H(3)R mRNA. The H(3)R has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders like obesity, myocardial ischemia, migraine, inflammatory diseases and several CNS disorders like Alzheimer's disease, attention-deficit hyperactivity disorder and schizophrenia. In this paper, we review various molecular aspects of the hH(3)R including its signal transduction, dimerization and the occurrence of different H(3)R isoforms.

Use of an inverse agonist radioligand [3H]A-317920 reveals distinct pharmacological profiles of the rat histamine H3 receptor

Selective radioligands for histamine H(3) receptors have been used to characterize H(3) receptor pharmacology by radioligand binding assays and to determine H(3) receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [(3)H]A-317920 (furan-2-carboxylic acid(2-[4-[3-([3,5-(3)H]4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H(3) receptor. The binding of [(3)H]A-317920 to rat cortical and cloned H(3) receptors revealed fast on- and slower off-rate kinetics with calculated K(d) values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [(3)H]A-317920 with the agonist [(3)H](N)-alpha-methylhistamine ([(3)H]NalphaMH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [(3)H]NalphaMH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [(3)H]A-317920 displacement by agonists was best fit by two-site binding models, and the potencies of the high affinity, GDP-sensitive sites correlated with the potencies defined in [(3)H]NalphaMH binding. Unlike [(125)I]iodoproxyfan, [(3)H]A-317920 exhibits potent and selective binding to rat H(3) receptors with low binding to non-H(3) sites, including cytochrome P450. These findings show that [(3)H]A-317920 is a potent rat H(3) receptor antagonist radioligand and has utility for studying H(3) receptor pharmacology.

Genetic and pharmacological aspects of histamine H3 receptor heterogeneity

Histaminergic H3 receptors modulate the release of neurotransmitters within the CNS and periphery. Ligands for these receptors have potential clinical utility in a variety of disease states. However, the pharmacological characteristics of these receptors have been enigmatic for more than a decade because of the diversity of pharmacological effects observed with the limited number of heretofore-available compounds. Recent cloning of the H3 receptor has revealed interspecies differences in the protein sequences in key regions, the existence of splice variants that differ in composition between species, and potential differences in signal transduction processes between either different tissues and/or species. This review attempts to summarize these findings within the context of the molecular biological and pharmacological data accumulated to date. Also, we suggest a nomenclature strategy to reduce potential confusion that has arisen from different naming systems used by various investigators. While some facets of this genetic and pharmacological diversity help to rationalize various aspects of H3 receptor heterogeneity, there remains an insufficient repertoire of selective ligands, assays, or other measures to completely resolve all components of this diversity. The promise of newly available tools to further explore H3 receptor function may provide the insight to bring the promised clinical potential of H3 receptor ligands to realization.

H3 agonist immepip markedly reduces cortical histamine release, but only weakly promotes sleep in the rat

Presynaptic H3 receptors exert negative control on brain histamine synthesis and release and may thereby play a key role in the control of the sleep/wake cycle. This suggests that pharmacological stimulation by H3 receptor agonists may potentially decrease wakefulness and induce sleep. This study reports the effect of a potent and selective H3 agonist, immepip, on EEG assessed sleep/wake phases in Sprague-Dawley rats at doses that significantly modulate brain histamine release. Immepip injected intraperitoneally (i.p.) at 5 or 10 mg kg(-1) induced a sustained decrease in cortical histamine efflux as measured by in vivo microdialysis. In a separate experiment, rats were prepared for EEG/EMG recording and evaluated during the dark phase of their light/dark cycle. The results showed that the same i.p. doses of 5 and 10 mg kg(-1) of immepip was devoid of any significant impact on the sleep/wake phases (active awake, drowsiness and slow wave sleep), except for a slight, albeit significant, decrease in sleep onset latency. These results reveal that a marked H3 receptor agonist-mediated reduction in cortical histamine release is not corroborated by a significant sleep promoting effect and therefore question the hypnotic potential of H3 agonists.

A detailed mapping of the histamine H(3) receptor and its gene transcripts in rat brain

The detailed distribution of histamine H(3) receptor mRNAs in rat brain was analyzed by in situ hybridization using a 33P-labelled riboprobe and was combined for the first time with the detailed autoradiographic distribution of the receptor determined in the same animals with [(125)I]iodoproxyfan, a selective radioligand. The signals generated on adjacent brain sections by each probe were quantified and/or rated and were compared in order to identify neuronal populations expressing the receptor. In addition, the cellular localization of the transcripts within various brain structures was analyzed in sections dipped in a photographic emulsion. In the cerebral cortex, the strong mRNA expression in intermediate and deep layers indicates the presence of H(3) receptors on several types of neurons. The binding is dense except in layer V, suggesting that H(3) receptors are located on granule cells and apical dendrites of pyramidal cells. In addition to their localization on monoaminergic afferents, the dense binding in layer IV and strong mRNA expression in thalamic nuclei suggest the presence of heteroreceptors on thalamocortical projections. In the hippocampus, the strong mRNA expression but low binding in pyramidal layers of the CA1 and ventral CA3 fields suggest that H(3) receptors are abundant on efferent projections of pyramidal cells. In the dentate gyrus, some binding sites in the molecular layer may correspond to H(3) receptors synthesized in granule cells and coexpressed with H(1) and H(2) receptors in their dendrites. In the basal ganglia, H(3) receptors are highly expressed in the striatal complex and olfactory tubercles but not in islands of Calleja. Some of the striatal binding sites may correspond to presynaptic receptors present on afferents. The mRNAs in cortical layer V may encode for heteroreceptors on corticostriatal neurons. The presence of mRNAs in the substantia nigra pars compacta suggests that H(3) receptors are located upon nigrostriatal afferents. However, the absence of any signal in the ventral tegmental area indicates that some but not all dopaminergic neurons express H(3) receptors. In addition, the homogeneous mRNA expression within the caudate putamen and nucleus accumbens suggests that many striatal H(3) receptors are present on medium-sized, spiny projection neurons of both the direct and indirect movement pathways. In agreement, a dense binding, but low mRNA expression, is observed in external and internal pallidum and in substantia nigra pars reticulata. In the amygdala, the dense binding and mRNA expression indicate the presence of receptors on both afferents and projections. In the thalamus, the binding in some association nuclei may correspond to receptors present on neurons emanating from the deep cortical layers that strongly express the mRNAs, as well as receptors on the visual systems. However, the low binding and high mRNA expression in most nuclei indicate that many receptors are present upon thalamic projections. In the hypothalamus, the mRNA expression parallels the density of binding sites and is the highest in the tuberomammillary nucleus. Further investigation is needed to know if the dense binding and mRNA expression observed in other nuclei such as the paraventricular, ventromedial and medial tuberal nuclei correspond to pre- and/or postsynaptic receptors. mRNAs are also observed in several areas projecting to the tuberomammillary nucleus, such as the ventrolateral preoptic nucleus. In the lower brainstem, the high mRNA expression and very low binding in the locus coeruleus and raphe nuclei indicate that presynaptic rather than somatodendritic receptors regulate noradrenaline and serotonin release, respectively. A similar pattern in vestibular nuclei suggests that receptors located on projections account for the anti-vertigo properties of H(3) receptor antagonists. In the cerebellum, binding is hardly detectable but a strong mRNA expression is found in most, if not all, Purkinje cells as well as in several central cerebellar nuclei, suggesting the presence of H(3) receptors on efferent projections. The present study reports the first detailed quantification and/or rating of H(3) receptor mRNAs in the brain. The comparison, performed in the same animals, with the distribution of the H(3) receptor protein provides evidence for the presence of H(3) receptors on many neuronal perikarya, dendrites and projections. Although some localizations, mainly as auto- or heteroreceptors, are consistent with previous functional studies, the physiological role, if any, of most of these presynaptic or postsynaptic receptors remains to be established.

Histamine H(3) receptor-mediated inhibition of depolarization-induced, dopamine D(1) receptor-dependent release of [(3)H]-gamma-aminobutryic acid from rat striatal slices

1. A study was made of the regulation of [(3)H]-gamma-aminobutyric acid ([(3)H]-GABA) release from slices of rat striatum by endogenous dopamine and exogenous histamine and a histamine H(3)-agonist. Depolarization-induced release of [(3)H]-GABA was Ca(2+)-dependent and was increased in the presence of the dopamine D(2) receptor family antagonist, sulpiride (10 microM). The sulpiride-potentiated release of [(3)H]-GABA was strongly inhibited by the dopamine D(1) receptor family antagonist, SCH 23390 (1 microM). Neither antagonist altered basal release. 2. The 15 mM K(+)-induced release of [(3)H]-GABA in the presence of sulpiride was inhibited by 100 microM histamine (mean inhibition 78+/-3%) and by the histamine H(3) receptor-selective agonist, immepip, 1 microM (mean inhibition 81+/-5%). The IC(50) values for histamine and immepip were 1.3+/-0.2 microM and 16+/-2 nM, respectively. The inhibitory effects of histamine and immepip were reversed by the H(3) receptor antagonist, thioperamide, 1 microM. 3. The inhibition of 15 mM K(+)-induced [(3)H]-GABA release by immepip was reversed by the H(3) receptor antagonist, clobenpropit, K(d) 0.11+/-0.04 nM. Clobenpropit alone had no effect on basal or stimulated release of [(3)H]-GABA. 4. Elevated K(+) caused little release of [(3)H]-GABA from striatal slices from reserpinized rats, unless the D(1) partial agonist, R(+)-SKF 38393, 1 microM, was also present. The stimulated release in the presence of SKF 38393 was reduced by 1 microM immepip to the level obtained in the absence of SKF 38393. 5. These observations demonstrate that histamine H(3) receptor activation strongly inhibits the dopamine D(1) receptor-dependent release of [(3)H]-GABA from rat striatum; primarily through an interaction at the terminals of GABA neurones.

Role of histamine H(3) receptors in the control of gastrointestinal motility. An overview

Over the last few years, the biochemical and functional characterization of H(3) receptors has been a matter for extensive investigation, culminating in the cloning of the human, guinea pig and rat receptor protein from brain tissues. This discovery contributed to determine the distribution of receptors in the body and to define the molecular mechanisms which follow activation. The major breakthrough in the histamine H(3) receptor field came with the synthesis of selective and potent agonists and antagonists, which unravelled the function of this receptor subtype in the different tissues. As expected from the ubiquitous location of histamine in the body, histamine H(3) receptors have also been identified in virtually every tissue, although they are quantitatively less abundant than H(1) and H(2) receptors. Concerning the gastrointestinal tract, this new receptor subtype seems to have multiple cellular locations, which include neurons, enteric ganglia, paracrine and immune cells and, in some tissues, also smooth muscle cells. Therefore it might be regarded as a general regulatory system of different digestive functions, including motility. The effects mediated by histamine H(3)-receptors mainly reflect the presynaptic inhibition of the release of either excitatory or inhibitory neurotransmitters from the myenteric plexus. The molecular mechanism of presynaptic inhibition seems to involve a restriction of calcium entry into the nerve endings, but other mechanisms (reduction of cAMP), possibly associated to different H(3) receptor subtypes, may be involved. Despite the widespread distribution and the well defined inhibitory effects evoked in the majority of in vitro models of intestinal motility, no clear cut evidence of its involvement in the control of peristalsis could be provided. In vivo models of gastrointestinal transit, indeed, did not reveal a defined effect of histamine H(3) receptor ligands, even though the possibility of a central inhibition was pointed out in several studies. Therefore, it is not clear at the present what is the physiological meaning of the histamine H(3) receptor in the control of gastrointestinal motility and whether it could represent a potential target for novel therapeutic interventions in deranged motility, taking into account that human gastrointestinal tissues are apparently devoid of this receptor.

Histamine H(3) receptor-mediated inhibition of endogenous acetylcholine release from the isolated, vascularly perfused rat stomach

We studied the effects of histamine H(3) receptor ligands on the release of endogenous acetylcholine from the isolated, vascularly perfused rat stomach. The stomach was perfused via the celiac artery with modified Krebs-Ringer solution containing physostigmine. Released acetylcholine from the portal vein was electrochemically measured using high-performance liquid chromatography and an enzyme system. Vagus nerves were electrically stimulated twice for 2 min (0.5 or 2.5 Hz). Acetylcholine release evoked at 2.5 Hz was slightly inhibited by histamine and effectively potentiated by thioperamide, a histamine H(3) receptor antagonist. Acetylcholine release evoked at 0.5 Hz in the presence of atropine was not influenced by thioperamide, but effectively inhibited by histamine, R-alpha-methylhistamine or imetit, histamine H(3) receptor agonists. These inhibitory effects were abolished by thioperamide or pertussis toxin. These results suggest that histamine attenuates acetylcholine release from vagus nerves through histamine H(3) receptor-mediated and pertussis toxin-sensitive mechanisms in the rat stomach.

Antinociceptive activity of derivatives of improgran and burimamide

Improgan, a compound related to H2 and H3 antagonists, induces antinociception in rodents after intraventricular administration. Characteristics of improgan and its congeners include: (a) morphine-like antinociception on thermal and mechanical tests in two species; (b) no impairment of motor coordination or locomotor activity; (c) evidence for a novel, nonopioid mechanism that is independent of known histamine receptors; (d) lack of tolerance with daily dosing; and (e) unique structure-activity relationships (SARs). Presently, the antinociceptive activity of several new derivatives of improgan was investigated in rats. Among compounds similar to burimamide, VUF4577 (possessing a two-carbon side chain) and VUF4582 (an N-phenyl derivative of VUF4577) induced complete, dose- and time-dependent antinociception on the hot-plate and tail-flick tests with no behavioral side effects. These compounds (with ED50 values of 71-117 nmol) were approximately twice as potent as burimamide itself (a four-carbon derivative). Two other derivatives in which the thiourea group (C=S, known to cause human toxicity) was replaced by either nitroethene (C=CH-NO2, VUF5405) or urea (C=O, VUF5407) also showed effective, potent antinociception on both assays. The latter compound is the most potent improgan-like drug discovered to date (ED50 = 71 nmol). Furthermore, positional isomers of antinociceptive compounds either lacked activity (VUF5394) or induced toxicity (VUF5393), revealing a high degree of pharmacological specificity. Although the mechanism of improgan antinociception remains unknown, the present results show promise for the further development of safe, effective, and potent pain-relieving compounds.

Evidence that histamine homologues discriminate between H3-receptors in guinea-pig cerebral cortex and ileum longitudinal muscle myenteric plexus

1. The binding of the selective histamine H3-receptor agonist ([3H]-R-alpha-methylhistamine) to sites in guinea-pig cerebral cortex and ileum longitudinal muscle myenteric plexus has been characterized and a comparison made of the apparent affinities of a series of H3-receptor ligands. 2. Saturation analysis suggested that [3H]-R-alpha-methylhistamine labelled a homogeneous population of histamine H3-receptors in guinea-pig cerebral cortex (pKD=9.91+/-0. 07; nH=1.07+/-0.03; n=5) and ileum longitudinal muscle myenteric plexus (pKD=9.75+/-0.21; nH=0.97+/-0.02; n=5). There was no significant difference in the estimated affinity of [3H]-R-alpha-methylhistamine in the two tissues. The cerebral cortex had a significantly higher receptor density (3.91+/-0.37 fmol mg-1 tissue) than the ileum longitudinal muscle myenteric plexus (0. 39+/-0.11 fmol mg-1). 3. Overall, the apparent affinities of compounds, classified as H3-receptor ligands, in cerebral cortex and ileum longitudinal muscle myenteric plexus were well correlated (r=0. 91, P<0.0001) and consistent with the cerebral cortex and ileum longitudinal muscle myenteric plexus expressing histamine H3-receptor population(s) that are pharmacologically indistinguishable by the majority of histamine H3-receptor ligands. However, it was evident that the homologues of histamine within this group of compounds could discriminate between the receptor populations in the two tissues. Thus, the estimated affinity of five imidazole unbranched alkylamines (histamine, homohistamine, VUF4701, VUF4732 and impentamine) were significantly higher in the guinea-pig cerebral cortex than in the ileum longitudinal muscle myenteric plexus assay.

Evaluation of [18F]VUF 5000 as a potential PET ligand for brain imaging of the histamine H3 receptor

[18F]VUF 5000 was evaluated as a potential PET ligand for the histamine H3 receptor. In the rat a high uptake of [18F]VUF 5000 was observed in liver, lung and kidney and a low uptake in the brain. In order to explain these findings we determined the LogD(oct,7.2) of [18F]VUF 5000, studied the biodistribution in the presence of carrier VUF 5000, modified [18F]VUF 5000 chemically and studied the binding of [18F]VUF 5000 to human serum albumin. From the results of these experiments it was concluded that [18F]VUF 5000 is not suitable as a PET ligand for brain imaging of the histamine H3 receptor, since [18F]VUF 5000 hardly penetrates into the brain.

The profiles of human and primate [3H]Nalpha-methylhistamine binding differ from that of rodents

Characterization of the histamine H3 receptor in rodent species has been extensive but limited characterization has been done with primate or human tissue. We have characterized the binding of [3H]Nalpha-methylhistamine to cynomolgus monkey and human brain membranes to determine whether there are any significant differences among species' pharmacology. In monkey, [3H]Nalpha-methylhistamine bound, in a guanine nucleotide-sensitive fashion, to an apparently homogeneous class of sites at equilibrium (K(D) = 1.4 nM, Bmax = 34 fmol/mg protein). The profile of binding was broadly similar to that of rodents, with a couple of significant differences. Most notably, the potency of the histamine H3-receptor-specific antagonist thioperamide (Ki = 240 nM) was substantially less than reported for rodents and under assay conditions that yield a two-site curve fit in rodents only a single class of thioperamide binding sites was detected in monkey. Burimamide, however, yielded a two-site curve fit (KiH = 6.7 nM, KiL = 1100 nM) independent of the presence of sodium in the assay, as it does in rodents. Characterization of the human brain histamine H3 receptor showed that it was similar to the monkey and not rodent receptor. Our findings indicate that differences between primate and rodent histamine H3 receptors of potentially serious importance for the discovery of antagonists active in humans do exist.

In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application

We investigated the effect of the new potent and selective histamine H3 receptor agonist, immepip, and the histamine H3 receptor antagonist, clobenpropit, on in vivo neuronal histamine release from the anterior hypothalamic area of urethane-anesthetized rats, using microdialysis. Intrahypothalamic perfusion with immepip at concentrations of 1 and 10 nM reduced histamine release to 75% and 35% of its basal level, respectively. Peripheral injection of immepip (5 mg/kg) caused a sustained decrease in histamine release of 50%. Clobenpropit potently increased histamine release after intrahypothalamic perfusion. The maximal increase in histamine release was 2-fold, observed at a concentration of 10 nM clobenpropit. Peripheral injection of clobenpropit (5-15 mg/kg) increased histamine release to about 150% of the basal value. A more marked increase in histamine release was found after injection of the histamine H3 receptor antagonist, thioperamide (5 mg/kg). In conclusion, intrahypothalamic perfusion of the histamine H3 receptor agonist, immepip and the histamine H3 receptor antagonist, clobenpropit, potently and oppositely modulated in vivo histamine release from the anterior hypothalamic area. The decreased histamine release after peripheral injection of immepip indicates that this novel agonist readily crosses the blood-brain barrier, making it a potential candidate for in vivo histamine H3 receptor studies. The differential increase in histamine release after peripheral injection of clobenpropit and thioperamide is discussed.

Pharmacological characterisation of the histamine H3 receptor in the rat hippocampus

The purpose of this report was to pharmacologically characterise the histamine H3 in the rat hippocampus using radioligand binding studies with the H3 receptor antagonist [125I]iodophenpropit and the H3 receptor mediated inhibition of [3H]noradrenaline release. A dissociation constant of 0.33 nM and a maximal number of binding sites of 125 fmol/mg protein were found for [125I]iodophenpropit. Competition studies showed stereoselectivity for the (R) and (S) enantiomers of alpha-methylhistamine and 10 microM of GTPgammaS shifted the curve of (R)-alpha-methylhistamine rightwards. Up to 1 microM, (R)-alpha-methylhistamine displaced only 30% whereas the tested H3-antagonists displaced 50-60% of the total [125I]iodophenpropit bound. This indicates the presence of an additional non-H3 receptor binding site(s) for [125I]iodophenpropit in the rat hippocampus. This secondary site shows low affinity for H3 agonists, but high affinity for the tested H3 antagonists. Electrically evoked [3H]acetylcholine release was shown in slices of rat hippocampus. No H3 receptor modulation of [3H]acetylcholine release from hippocampal slices was detectable. However, H3 receptor activation inhibited 42% of the electrically-evoked [3H]noradrenaline release in rat hippocampal slices. The inhibition of [3H]noradrenaline release was effectively antagonized by the H3 antagonists thioperamide and burimamide. We describe the pharmacological identification of the histamine H3 receptor in the rat hippocampus and its similarities and differences from the cortical H3 receptor. These studies enable us to investigate changes in density and functionality of the hippocampal H3 receptor under (patho)physiological conditions.

H3-receptor antagonists: synthesis and structure-activity relationships of para- and meta-substituted 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles

We report the synthesis, octanol/water partition coefficient (log P), dissociation constants (pKa), H3-receptor affinity (pKi in rat brain membranes, [3H]-N alpha-methylhistamine), and H3-antagonist potency (pA2 in guinea ileum, (R)-alpha-methylhistamine) of novel H3-receptor antagonists obtained by introducing a para or meta substituent on the phenyl ring of the lead compound 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazole (3a). The substituents were chosen to obtain broad and uncorrelated variation in their lipophilic, electronic, and steric properties. The log P values of the neutral species cover almost 3 orders of magnitude (from 1.40 to 4.11). The pKa,2 values (protonation of the 2-thioimidazole fragment) vary from 3.13 to 4.34, indicating that this fragment, which incorporates the so-called polar group common to many H3-receptor antagonists, is neutral at physiological pH. The compounds had pKi values in a range too narrow (from 7.28 to 8.03) to derive QSAR equations. In one case (3g), a biphasic displacement curve was observed (pKi,1 = 8.53; pKi,2 = 6.90). The pA2 values ranged 2 orders of magnitude (from 6.83 to 8.87) and yielded a QSAR model (PLS) indicating that antagonist potency depends parabolically on lipophilicity and is decreased by bulky para substituents. The compounds of this series, therefore, maintain a fair-to-good affinity for rat brain H3-receptor and a fair-to-good H3-antagonist potency on guinea pig ileum, although varying markedly in their lipophilicity. The series thus appears as a good candidate for pharmacokinetic optimization leading to brain-penetrating H3-receptor antagonists.

Brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit in rat and mouse, determined with ex vivo [125I]iodophenpropit binding

We investigated the brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit using ex vivo [125I]iodophenpropit binding. Homogenates of the rat cortex, striatum and mouse whole brain were prepared 1 h after subcutaneous injection of the H3 antagonists and incubated with [125I]iodophenpropit, a radiolabeled H3 receptor antagonist, to determine the H3 receptor occupancy. Specific [125I]iodophenpropit binding to the rat cortex and striatum was inhibited by thioperamide with IC30 values of 1.0 and 1.5 mg/kg, respectively. Clobenpropit also inhibited [125I]iodophenpropit binding, but was less potent (IC30: 18 and 19 mg/kg in the rat cortex and striatum, respectively) than thioperamide. Similar results were obtained in experiments with mouse whole brain (3.5 and 13 mg/kg for thioperamide and clobenpropit), indicating that there is no important species differences in the brain penetration of these drugs between rats and mice. These findings suggest that after peripheral injection both in rat and mouse thioperamide penetrates the blood-brain barrier more efficiently compared to clobenpropit.

Characterisation of the specific binding of the histamine H3 receptor antagonist radioligand [3H]GR168320

We have examined the specific binding of the tritiated derivative of the potent histamine H3 receptor antagonist, [3,4-3H2]-cyclohex-yl-¿[4-(3H-imidazol-4-yl)-piperidin-l-yl] iminomethyl¿- amine ([3H]GR168320), to homogenates of rat cerebral cortex. Specific binding of [3H]GR168320 at 37 degrees C associated and dissociated rapidly. Binding was saturable (Bmax 412 +/- 89 fmol/mg protein) and of high affinity (Kd 0.12 +/- 0.11 nM). Saturation studies suggested the involvement of a single site. Histamine H3 receptor agonists and antagonists inhibited [3H]GR168320 binding with high affinity. Agonist and antagonist affinities correlated when compared with affinities obtained using the tritiated histamine H3 agonist radioligand N alpha-methylhistamine.

[3H]-thioperamide as a radioligand for the histamine H3 receptor in rat cerebral cortex

1. The purpose of the present study was to characterize the binding of the histamine H3 receptor antagonist, [3H]-thioperamide, to rat cerebral cortical membranes. 2. The binding of [3H]-thioperamide to rat cerebral cortical membranes reached equilibrium after incubation with [3H]-thioperamide after 8-10 h at 4 degrees C. Equilibrium was maintained for up to 18 h of incubation. Addition of 1 microM (R)-alpha-methylhistamine rapidly dissociated [3H]-thioperamide from its binding sites. From these kinetic experiments a dissociation constant of 0.3 nM was obtained for [3H]-thioperamide. 3. Saturation experiments with [3H]-thioperamide using 1 microM (R)-alpha-methylhistamine to define nonspecific binding were best analysed according to a single site model. A dissociation constant (KD) of 0.80 +/- 0.06 nM (n = 3) and a maximal number of binding sites (Bmax) of 73 +/- 20 fmol mg-1 protein (n = 3) were obtained for the binding of [3H]-thioperamide to rat cerebral cortical membranes. 4. Saturation experiments with [3H]-thioperamide using 0.3 microM iodophenpropit to define nonspecific binding were best analysed according to a two site model. For the high affinity [3H]-thioperamide site a KD value of 1.1 +/- 0.3 nM (n = 3) and Bmax value of 162 +/- 108 fmol mg-1 protein (n = 3) were obtained whereas KD and Bmax values for the low affinity site were 96 +/- 19 nM and 4346 +/- 3092 fmol mg-1 protein (n = 3), respectively. 5. Using 5 nM [3H]-thioperamide, the binding was hardly displaced by H3 agonists within concentration-ranges expected to bind to the histamine H3 receptor. Under these conditions, [3H]-thioperamide binding was fully displaced by various H3-antagonists, yet most H3 antagonists showed Ki values different from those expected for the histamine H3 receptor. 6. Using 0.3 nM [3H]-thioperamide, 50-60% of the total binding was potently displaced by the H3 agonists histamine, (R)-alpha-methylhistamine, (S)-alpha-methylhistamine, imetit and immepip. Displacement of the binding of 0.3 nM [3H]-thioperamide binding exhibited clear stereoselectivity for the R and S isomers of alpha-methylhistamine. 7. Binding of 0.3 nM [3H]-thioperamide was completely displaced by several H3 antagonists (thioperamide, iodophenpropit, iodoproxyfan, and burimamide) and biphasic displacement curves were obtained; the Ki values for the high affinity site corresponded well with the expected values for the H3 receptor. Antagonists fully displaced the binding of 5 nM [3H]-thioperamide with affinities comparable to the low affinity site found with 0.3 nM [3H]-thioperamide. 8. Ondansetron and haloperidol did not displace binding of 5 nM [3H]-thioperamide at concentrations at which the former are known to bind to 5-HT3 or sigma receptors, respectively. On the other hand, nonselective cytochrome P450 inhibitors displaced the binding of 5 nM [3H]-thioperamide from both rat cerebral cortical membranes and rat liver microsomes. 9. It is concluded that the histamine H3 antagonist, [3H]-thioperamide, can be used as a radioligand to study the histamine H3 receptor in rat brain, provided that subnanomolar concentrations are used in displacement studies. Moreover, the specific binding should be defined with an H3 agonist, since most H3 antagonists share with [3H]-thioperamide a low affinity, high density, non-H3 receptor binding site(s) in rat brain. The latter is probably due to binding to cytochrome P450 isoenzymes.

Binding of clozapine metabolites and analogues to the histamine H3 receptor in rat brain cortex

Following up the finding that the non-imidazole drug clozapine shows a considerable histamine H3 receptor antagonistic activity, a series of analogues and metabolites (clozapine-N-oxide, and N-desmethylclozapine) were tested for their affinity towards the H3 receptor using the radiolabelled H3 antagonist [125I]-iodophenpropit. Qualitative structure affinity relationships are derived for the tested compounds. In the clozapine molecule four structurally different moieties may be considered. In comparison with the affinity for the H3 receptor shown by clozapine, the following main conclusions can be drawn: The 4-piperazinyl region does not allow substituents longer than a CH3 or electronegative atoms such as an O (as in clozapine-N-oxide); the lack of the CH3 group (as in N-desmethylclozapine) also reduces the affinity for H3 receptors. Substitutions at the 5-diazepine position do not drastically alter the affinity for the H3 receptor, although a basic nitrogen is favoured over CH2, O, or S. The 8 position in ring I is an important modulatory site for H3 affinity; electronegative substituents such as chloro and fluoro in this aromatic ring increase the affinity. When these substituents are, however, present at position X2 in the ring, they disable binding to the H3 receptor. The two major clozapine metabolites (clozapine-N-oxide, and N-desmethylclozapine) will not be responsible for a possible contribution of the H3 receptor antagonism to the clinical profile of clozapine.

Potencies of antagonists chemically related to iodoproxyfan at histamine H3 receptors in mouse brain cortex and guinea-pig ileum: evidence for H3 receptor heterogeneity?

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 heterogeneity, species variants or differences in the efficiency of receptor coupling. The marked difference in the affinity/potency between iodoproxyfan and its deiodo analogue may suggest that a highly lipophilic residue in that part of the molecule favours a high affinity/antagonistic potency at H3 receptors.

[125I]iodoproxyfan and related compounds: a reversible radioligand and novel classes of antagonists with high affinity and selectivity for the histamine H3 receptor

The synthesis and biological evaluation of new histamine H3 receptor antagonists with an iodinated aryl partial structure are described as part of an extensive research program to find model compounds for the development of a new radioligand with high H3 receptor affinity and specific activity. All compounds were tested for their H3 receptor antagonist activity in a [3H]-histamine-release assay with synaptosomes from rat cerebral cortex. The new leads with potent H3 receptor antagonist activity belong to a series of derivatives of 3-(1H-imidazol-4-yl)propanol with carbamate (4-7), ester (8-16), and ether (17-22) as functional groups. Structure-activity relationships are discussed. The most active compound in the functional test (-log Ki = 8.3) and in binding studies with [3H]-(R)-alpha-methylhistamine on rat cerebral cortex (-log Ki = 9.0) in vitro was 3-(1H-imidazol-4-yl)propyl (4-iodophenyl)methyl ether (iodoproxyfan, 19) exhibiting no central H3 receptor antagonist activity in vivo. The potency of iodoproxyfan is more than 300 times lower at H1, H2, alpha1, alpha2, beta1, 5-HT2A, 5-HT3, and M3 receptors than at histamine H3 receptors. Because of the high potency and selectivity of 19, this compound has also been prepared in the [125I]-iodinated form by a nucleophilic halogen exchange reaction using the corresponding bromo derivative 22 as a precursor. The newly prepared [125I]iodoproxyfan (23) possesses advantageous pharmacological properties and fulfills all criteria of a useful radioligand.

Evaluation of the receptor selectivity of the H3 receptor antagonists, iodophenpropit and thioperamide: an interaction with the 5-HT3 receptor revealed

1. In the present study we evaluated the receptor selectivity of the potent histamine H3 receptor antagonist, iodophenpropit (IPP) in comparison with the prototype antagonist, thioperamide. 2. IPP proved to be a potent competitive H3 receptor antagonist as measured against (R)-alpha-methylhistamine-induced inhibition of electrically-evoked contractions of the guinea-pig jejunum (pA2 = 9.12 +/- 0.06, Schild slope: 1.0 +/- 0.1, n = 8). In the same assay, thioperamide was slightly less potent (pA2 = 8.9 +/- 0.2). 3. In radioligand binding studies, IPP showed a high affinity for the H3 receptor. Displacement of [125I]-IPP binding to rat cortex membranes by unlabelled IPP resulted in a Ki value of 0.97 +/- 0.06 nM (n = 3). In contrast, IPP showed only a weak affinity for the histamine H1- and H2 receptor. Displacement of [3H]-mepyramine and [125I]-iodoaminopotentidine binding to respectively guinea-pig H1- and human H2 receptors by IPP resulted in Ki values of 1.71 +/- 0.32 microM (n = 3) and 2.28 +/- 0.81 microM (n = 3). For thioperamide the affinities for the H1-, H2- and H3 receptor were respectively > 10 microM, > 10 microM and 4.3 +/- 1.6 nM (n = 7). 4. Testing IPP and thioperamide in 39 different receptor binding assays revealed that IPP showed relatively high affinity for the 5-hydroxytryptamine 5-HT3 receptor (Ki = 11 +/- 1 nM, n = 3), the alpha 2-adrenoceptor (Ki = 120 +/- 5 nM, n = 3) and the sigma receptor (Ki = 170 +/- 70 nM, n = 3). Thioperamide showed relatively high affinity for the 5-HT3 receptor (Ki = 120 +/- 30 nM, n = 3) and the sigma receptor (Ki = 180 +/- 90 nM, n = 3). 5. Due to the low density of histamine H3 receptors in the brain, the interaction of IPP with the 5-HT3-, the alpha 2- and the sigma receptor might interfere with [125I]-IPP binding to rat cortex membranes. Yet, in this preparation [125I]-IPP binding was not influenced by ondansetron, yohimbine or haloperidol. The interaction with the 5-HT3 receptor was not restricted to IPP or thioperamide, but was alsofound with other H3 receptor antagonists. The potent H3 receptor agonist imetit, a compound belongingto the same chemical class of IPP, also interacted with the 5-HT3 receptor (Ki = 240 +/- 40 nM). In contrast,histamine or the H3 receptor agonist, (R)-a-methylhistamine showed no affinity for the 5-HT3 receptor.7 In the guinea-pig isolated ileum, imetit evoked concentration-dependent contractions, resulting in apD2 value of 4.72 +/- 0.03 (n = 9). The contractions were antagonized by ondansetron, yielding a pA2 valueof 7.1 +/- 0.1 (n = 9). Similarly ondansetron antagonized the contractions evoked by the 5-HT3 receptoragonist, 2-methyl-5-HT with a pA2 value of 7.3 +/- 0.1 (n = 4). IPP and thioperamide did not mimic 2-methyl-5-HT but non-competitively inhibited the 2-methyl-5-HT-induced contractions of thispreparation.8 In an in vivo model for 5-HT3 activity, the Von Bezold Jarisch reflex, thioperamide showedantagonism in low dosages, which correlated well with the affinity for the 5-HT3 receptor site. Yet, athigher dosages no further 5-HT3 receptor antagonism was observed. For IPP no 5-HT3 receptor activitycould be observed in vivo.9 In the present study we showed that many H3 receptor compounds, that are regarded as highlyselective (including the prototype drug, thioperamide), also interact with the 5-HT3 receptor, albeit athigher drug concentrations.Keywords.: Histamine H3-receptor; iodophenpropit; thioperamide; receptor selectivity; 5-hydroxytryptamine 5-HT3 receptor;guinea-pig intestine; rat brain; Von Bezold Jarisch reflex

An update on histamine H3 receptors and gastrointestinal functions

The distribution and functions of histamine H3 receptors in the gastrointestinal tract is reviewed with particular reference to the effects on gastric acid secretion, mucosal protection, and intestinal motility. Histamine H3 receptor activation has negative effects on acid secretion induced by indirect secretagogues in cats, dogs, and rabbits; less clear effects were found in rats. An inhibitory effect on histamine release induced by different stimuli was observed in rats, rabbits, and dogs after H3 receptor agonists, thus supporting the idea that H3 receptors occur in ECL cells. (R)-alpha-methylhistamine has a marked protective effect against gastric lesions induced by ethanol in rats, being slightly less effective against aspirin and stress. H3 receptor activation decreases the intestinal motility induced by electrical stimulation in a variety of gut preparations, reducing both cholinergic and NANC neurotransmitter release. In this tissue the inhibitory effects mediated by histamine H3 receptors seem to be coupled, via a G protein, to a restriction of Ca2+ access into the nerve terminal; other mechanisms, however, have been suggested in the gastric mucosa. Histamine H3 receptors have already been subdivided into two receptor subtypes, H3A and H3B, the former being the subtype predominant in the gastrointestinal tissue. The increasing availability of selective agonists and antagonists of H3 receptors will unravel possible novel actions and physiological roles of histamine.

Interaction of clozapine with the histamine H3 receptor in rat brain

We examined possible interactions between neuroleptics and the histamine H3 receptor and found an interaction of clozapine with this receptor. In competition binding experiments, using the H3 antagonist, [125I]-iodophenpropit, we observed a Ki of 236 +/- 87 nM. Functionally, clozapine was studied on the H3-mediated inhibition of [3H]-5-hydroxytryptamine ([3H]-5-HT) release from rat brain cortex slices. Clozapine acts as an antagonist with an apparent KB value of 79.5 nM.