H3 receptor antagonist, thioperamide, inhibits adrenal steroidogenesis and histamine binding to adrenocortical microsomes and binds to cytochrome P450

Histamine H3 receptor antagonists - Roles in neurological and endocrine diseases and diabetes mellitus

Human histamine H3 receptor (H3R) was initially described in the brain of rat in 1983 and cloned in 1999. It can be found in the human brain and functions as a regulator of histamine synthesis and release. H3 receptors are predominantly resident in the presynaptic region of neurons containing histamine, where they modulate the synthesis and release of histamine (autoreceptor) or other neurotransmitters such as dopamine, norepinephrine, gamma-aminobutyric acid (GABA), glutamate, acetylcholine and serotonin (all heteroreceptors). The human histamine H3 receptor has twenty isoforms of which eight are functional. H3 receptor expression is seen in the cerebral cortex, neurons of the basal ganglia and hippocampus, which are important for process of cognition, sleep and homoeostatic regulation. In addition, histamine H3R antagonists stimulate insulin release, through inducing the release of acetylcholine and cause significant reduction in total body weight and triglycerides in obese subjects by causing a feeling of satiety in the hypothalamus. The ability of histamine H3R antagonist to reduce diabetes-induced hyperglycaemia is comparable to that of metformin. It is reasonable therefore, to claim that H3 receptor antagonists may play an important role in the therapy of disorders of cognition, the ability to sleep, oxidative stress, inflammation and anomaly of glucose homoeostasis. A large number of H3R antagonists are being developed by pharmaceutical companies and university research centres. As examples of these new drugs, this review will discuss a number of drugs, including the first histamine H3R receptor antagonist produced.

A subset of histamine receptor ligands improve thermotolerance of the yeast Saccharomyces cerevisiae

AIMS

Histamine interacts with the stress response in eukaryotes. This study investigated the effects of antihistamines on the heat shock (HS) response in yeast, thereby exploring their functions in a well-established histamine receptor (H(x) R)-free model.

METHODS AND RESULTS

Stress response was evaluated by determining growth and viability of postlogarithmic phase grown yeast cultures after HS at 53°C for 30 min. The effects of H(x) R ligands were investigated following short- and long-term administration. The H(1) R antagonist dimethindene exerted dose-related antifungal actions, whereas the H(2) R antagonist ranitidine failed to elicit any effect. In contrast, the H(3/4) R and H(4) R ligands, thioperamide and JNJ7777120, respectively, induced the thermotolerant phenotype. The circumvention of thermotolerance by cycloheximide and the induction of Hsp70 and Hsp104 expression indicated the contribution of de novo protein synthesis in the adaptive process, likely directed towards alterations in Hsp expression.

CONCLUSIONS

The data provide evidence for the differential function of H(x) R ligands in thermotolerance induction in yeast.

SIGNIFICANCE AND IMPACT OF THE STUDY

First demonstration of the action of antihistamines in the HS response in yeast. The work supports the potential H(x) R-independent functions of histaminergic compounds in fungal adaptation and stimulates research on the prospect of their exploitation in eukaryotic (patho)physiology.

Effects of L-histidine and histamine H3 receptor modulators on ethanol-induced sedation in mice

Recent studies suggest that the brain histaminergic system and especially the H3 receptors are involved in the regulation of alcohol consumption and alcohol-induced behaviors. Part of this effect might be due to a modulation of ethanol-induced sedation by central histamine. The aim of the present study was to investigate the effects of several histaminergic drugs on ethanol-induced sedation using the loss of righting reflex experimental protocol in female Swiss mice. A pretreatment with L-histidine, the histamine precursor, significantly reduced ethanol-induced sedation, suggesting that brain histamine protects against the sedative effects of ethanol. In a second set of experiments, several H3 receptor agonists (immepip or imetit) and inverse agonists/antagonists (thioperamide, A331440, or BF2.649) were tested. Surprisingly, both H3 receptor agonists and antagonists potentiated the sedative effects of ethanol. This paradoxical effect might be due to the subtle regulatory actions related to the H3 heteroreceptor function.

Autoregulation of McA-RH7777 hepatoma cell proliferation by histamine H3 receptors

Previous studies have suggested that histamine (HA) acts as an autocrine growth factor. We have explored the modulation of cell proliferation by HA using McA-RH7777 hepatoma cells. High L-histidine decarboxylase (HDC) expression and HA synthesis were found in McA-RH7777 cells. Whereas extracellular HA reached submicromolar concentrations, intracellular levels were very low, indicating that HA was secreted by the cells. McA-RH7777 cells also express H3-receptor (H3R) transcripts and proteins. Reverse transcriptase-polymerase chain reaction analysis detected only transcripts for the long isoform. Immunocytochemistry performed with a selective H3R antibody showed that most cells were immunoreactive. H3R binding sites (Bmax approximately 30 fmol/mg protein) were identified when [125I] iodoproxyfan binding was displaced by the agonist imetit. High-affinity binding also occurred at cytochrome P450 enzymes. This binding was not inhibited by HA, H3R agonists, or by a nonimidazole H3R antagonist but was displaced by imidazole H3R antagonists or by ketoconazole, a imidazole-containing cytochrome inhibitor. HA inhibited proliferation of McA-RH7777 hepatoma cells. The absence of uptake system, its much higher potency at H3Rs, and its low intracellular levels suggested that HA interacted with H3Rs rather than cytochromes. In agreement, both imidazole H3R antagonists, a nonimidazole H3R antagonist, and the HDC inhibitor alpha-monofluoromethyl histidine increased cell proliferation (up to approximately 60%), revealing a H3R-mediated inhibition by endogenous HA. Moreover, exogenous HA inhibited the increase induced by alpha-FMH or H3R antagonists with a nanomolar potency. In conclusion, our findings show that HA regulates proliferation of McA-RH7777 hepatoma cells by interacting with autoinhibitory H3Rs.

Highly Potent Fluorescence-Tagged Nonimidazole Histamine H3 Receptor Ligands

Different (3-phenoxypropyl)piperidine derivatives have been coupled to fluorescent moieties (5-dimethylaminonaphthalene-1-sulfonyl, carbazol-9-ylcarbonyl, 2-cyanoisoindol-1-yl, 2-cyanobenzo[f]isoindol-1-yl, 2,4-dinitrobenzen-1-yl, 2,4-diaminophenyl, 7-nitrobenzofurazan-4-yl, 7-aminosulfonylbenzofurazan-4-yl, 4-methylcoumarin-6-yl) as novel histamine H(3) receptor ligands. They have been synthesised starting from piperidine in a few steps. The compounds display good to excellent histamine hH(3) receptor affinities with K(i) values ranging from 13.4 to 0.048 nM. Some of the new compounds belong to the most potent ligands known so far and may act as tools for identification and understanding of the binding site on the histamine H(3) receptor. In vivo screening on selected derivatives of Sanger's reagent showed antagonist potencies with ED(50) values from 7.9 to 0.39 mg kg(-1), p.o.

Detection of multiple H3 receptor affinity states utilizing [3H]A-349821, a novel, selective, non-imidazole histamine H3 receptor inverse agonist radioligand

1. A-349821 is a selective histamine H3 receptor antagonist/inverse agonist. Herein, binding of the novel non-imidazole H3 receptor radioligand [3H]A-349821 to membranes expressing native or recombinant H3 receptors from rat or human sources was characterized and compared with the binding of the agonist [3H]N--methylhistamine ([3H]NMH). 2. [3H]A-349821 bound with high affinity and specificity to an apparent single class of saturable sites and recognized human H3 receptors with 10-fold higher affinity compared to rat H3 receptors. [3H]A-349821 detected larger populations of receptors compared to [3H]NMH. 3. Displacement of [3H]A-349821 binding by H3 receptor antagonists/inverse agonists was monophasic, suggesting recognition of a single binding site, while that of H3 receptor agonists was biphasic, suggesting recognition of both high- and low-affinity H3 receptor sites. 4. pKi values of high-affinity binding sites for H3 receptor competitors utilizing [3H]A-349821 were highly correlated with pKi values obtained with [3H]NalphaMH, consistent with labelling of H3 receptors by [3H]A-349821. 5. Unlike assays utilizing [3H]NMH, addition of GDP had no effect on saturation parameters measured with [3H]A-349821, while displacement of [3H]A-349821 binding by the H3 receptor agonist histamine was sensitive to GDP. 6. In conclusion, [3H]A-349821 labels interconvertible high- and low-affinity states of the H3 receptor, and displays improved selectivity over imidazole-containing H3 receptor antagonist radioligands. [3H]A-349821 competition studies showed significant differences in the proportions and potencies of high- and low-affinity sites across species, providing new information about the fundamental pharmacological nature of H3 receptors.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Histamine-cytokine connection in immunity and hematopoiesis

A number of recent studies have led to a reappraisal of the functional capacities of histamine in immunity and hematopoiesis. This change of perspective was provided by the following findings: (1) the evidence for multiple cellular sources of histamine, differing from mature basophils and mast cells by their ability to newly synthesize and liberate the mediator without prior storage, (2) the discovery of a novel histamine receptor (H4R), preferentially expressed on hematopoietic and immunocompetent cells, (3) the potential intracellular activity of histamine through cytochrome P450 and (4) the demonstration of a histamine-cytokine cross-talk. Indeed, cytokines not only modulate the degranulation process of histamine but also control its neosynthesis by the histamine-forming enzyme, histidine decarboxylase (HDC), at transcriptional and post-transcriptional levels. In turn, histamine intervenes in the intricate cytokine network, regulating cytokine production by immune cells through distinct receptors signaling distinct biological effects. This type of regulation is particularly relevant in the context of TH1/TH2 differentiation, autoimmunity and tumor immunotherapy.

Pharmacological Properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile]: II. Neurophysiological Characterization and Broad Preclinical Efficacy in Cognition and Schizophrenia of a Potent and Selective Histamine H3 Receptor Antagonist

Acute pharmacological blockade of central histamine H3 receptors (H3Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H3R antagonist with high affinity for rat (pK(i) = 8.9) and human (pK(i) = 9.5) H3Rs. Acute functional blockade of central H3Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-alpha-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1-1.0 mg/kg), a 10- to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)(cyclopropyl) methanone], A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl) phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxoethyl)-2-furamide], and A-349821 [(4'-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this model was maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01-0.3 mg/kg) and aged (0.3-1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0-3.0 mg/kg) and N40 (1.0-10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamine-induced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1-3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimer's disease and schizophrenia.

4-(2-[2-(2(R)-Methylpyrrolidin-1-yl)ethyl]benzofuran-5-yl)benzonitrile and Related 2-Aminoethylbenzofuran H3 Receptor Antagonists Potently Enhance Cognition and Attention

H(3) receptor antagonists based on a 2-aminoethylbenzofuran skeleton have been discovered, which are potent in vitro at human and rat H(3) receptors, with K(i) values of 0.1-5.8 nM. Analogues were discovered with potent (0.01-1 mg/kg) cognition and attention enhancing properties in animal models. One compound in particular, 4-(2-[2-(2(R)-methylpyrrolidin-1-yl)ethyl]benzofuran-5-yl)benzonitrile (ABT-239), combined potent and selective H(3) receptor antagonism and excellent pharmacokinetic and metabolic properties across species, with full efficacy in two behavioral models: a five-trial inhibitory avoidance acquisition model in rat pups at 0.1 mg/kg and a social recognition memory model in adult rats at 0.01 mg/kg. Furthermore, this compound did not stimulate locomotor activity and showed high selectivity for the induction of behavioral efficacy versus central nervous system based side effects. The potency and selectivity of this compound and of analogues from this class support the potential of H(3) receptor antagonists for the treatment of cognitive dysfunction.

Pharmacological and behavioral properties of A-349821, a selective and potent human histamine H3 receptor antagonist

Histamine H3 receptors regulate the release of a variety of central neurotransmitters involved in cognitive processes. A-349821 ((4'-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone) is a novel, non-imidazole H3 receptor ligand, displaying high affinity for recombinant rat and human H3 receptors, with pKi values of 9.4 and 8.8, respectively, and high selectivity for the H3 receptor versus H1, H2, and H4 histamine receptors. A-349821 is a potent H3 receptor antagonist in a variety of models using recombinant human and rat receptors, reversing agonist induced changes in cyclic AMP formation (pKb= 8.2 and pKb= 8.1, respectively), [35S]-GTPgammaS binding (pKb= 9.3 and pKb= 8.6, respectively) and calcium levels (human pKb= 8.3). In native systems, A-349821 competitively reversed agonist induced inhibition of electric field stimulated guinea-pig ileum (pA2= 9.5) and histamine-mediated inhibition of [3H]-histamine release from rat brain cortical synaptosomes (pKb= 9.2). Additionally, A-349821 inhibited constitutive GTPgammaS binding at both rat and human H3 receptors with respective pEC50 values of 9.1 and 8.6, demonstrating potent inverse agonist properties. In behavioral studies, A-349821 (0.4 mg/kg-4 mg/kg) potently blocked (R)-alpha-methylhistamine-induced dipsogenia in mice. The compound also enhanced cognitive activity in a five-trial inhibitory avoidance model in spontaneously hypertensive rat (SHR) pups at doses of 1-10mg/kg, with the 1mg/kg dose showing comparable efficacy to a fully efficacious dose of ciproxifan (3mg/kg). These doses of A-349821 were without effect on spontaneous locomotor activity. Thus, A-349821 is a novel, selective non-imidazole H3 antagonist/inverse agonist with balanced high potency across species and favorable cognition enhancing effects in rats.

Two novel and selective nonimidazole histamine H3 receptor antagonists A-304121 and A-317920: I. In vitro pharmacological effects

Histamine H3 receptor (H3R) antagonists enhance neurotransmitter release and are being developed for the treatment of a variety of neurological and cognitive disorders. Many potent histamine H3R antagonists contain an imidazole moiety that limits receptor selectivity and the tolerability of this class of compounds. Here we present the in vitro pharmacological data for two novel piperazine amide ligands, A-304121 [4-(3-((2R)-2-aminopropanoyl-1-piperazinyl)propoxy)phenyl)cyclopropylmethanone] and A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl)phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxo-ethyl-)-2-furamide], and compare them with the imidazole H3R antagonists ciproxifan, clobenpropit, and thioperamide. Both A-304121 and A-317920 bind potently to recombinant full-length rat H3R(pKi values = 8.6 and 9.2, respectively) but have lower potencies for binding the full-length human H3R (pKi values = 6.1 and 7.0, respectively). A-304121 and A-317920 are potent antagonists at rat H3R in reversing R-alpha-methylhistamine [(R)-alpha-MeHA] inhibition of forskolin-stimulated cAMP formation (pKb values = 8.0 and 9.1) but weak antagonists at human H3Rs in cyclase (pKb values = 6.0 and 6.3) and calcium mobilization (pKb values = 6.0 and 7.3) assays in cells co-expressing Galphaqi5-protein. Both compounds potently antagonize native H3Rs by blocking histamine inhibition of potassium-evoked [3H]histamine release from rat brain cortical synaptosomes (pKb values = 8.6 and 9.3) and (R)-alpha-MeHA reversal of electric field-stimulated guinea pig ileum contractions (pA2 values = 7.1 and 8.3). A-304121 and A-317920 are also more efficacious inverse agonists in reversing basal guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding at the human H3R (pEC50 values = 5.7 and 7.0) than are the imidazole antagonists. These novel and selective piperazine amides represent useful leads for the development of H3R antagonist therapeutic agents.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Interaction of histamine and other bioamines with cytochromes P450: implications for cell growth modulation and chemopotentiation by drugs

We have characterized microsomal and nuclear histamine sites, designated H(IC), through which this amine acts as an intracellular mediator of platelet aggregation and lymphocyte mitogenesis. A major proportion, at least, of the microsomal H(IC) sites are on cytochromes P450, an important family of microsomal enzymes that are present in all cells, but most abundant in the liver. These enzymes are involved in the metabolism of xenobiotics and natural substrates, including lipid hormones that modulate gene function and cell growth. We have shown that polyamines, hormones (including estrogen, testosterone and progesterone), antihormones (including tamoxifen and flutamide) and various antidepressants and antihistamines, all inhibit histamine binding to P450; we have postulated that, through binding to the heme moiety, intracellular histamine regulates cell function by modulating the catalytic activity of P450 enzymes, an action that may be perturbed by endogenous and exogenous substances. We now demonstrate that, in addition to histamine, melatonin and the biogenic amines dopamine, serotonin and noradrenaline bind to P450 isozymes and to cytochrome C. Thus, heme enzymes in general may represent common targets where multiple bioamines, hormones and drugs interact to influence cell function and growth.

Characterization of the binding of [3H]-clobenpropit to histamine H3-receptors in guinea-pig cerebral cortex membranes

1 We have investigated the binding of a novel histamine H3-receptor antagonist radioligand, [3H]- clobenpropit ([3H]-VUF9153), to guinea-pig cerebral cortex membranes. 2 Saturation isotherms for [3H]-clobenpropit appeared biphasic. Scatchard plots were curvilinear and Hill plot slopes were significantly less than unity (0.63+/-0.03; n = 12+/-s.e.mean). The radioligand appeared to label two sites in guinea-pig cerebral cortex membranes with apparent affinities (pKD') of 10.91+/-0.12 (Bmax = 5.34+/-0.85 fmol mg(-1) original wet weight) and 9.17+/-0.16 (Bmax = 23.20+/-6.70 fmol mg(-1)). 3 In the presence of metyrapone (3 mM) or sodium chloride (100 mM), [3H]-clobenpropit appeared to label a homogeneous receptor population (Bmax=3.41+/-0.46 fmol mg-1 and 3.49+/-0.44 fmol mg(-1), pKD' = 10.59+/-0.17 and 10.77+/-0.02, respectively). Scatchard plots were linear and Hill slopes were not significantly different from unity (0.91+/-0.04 and 0.99+/-0.02, respectively). Granisetron (1 microM), rilmenidine (3 microM), idazoxan (0.3 microM), pentazocine (3 microM) and 1,3-di-(2-tolyl)guanidine (0.3 microM) had no effect on the binding of [3H]-clobenpropit. 4 The specific binding of [3H]-clobenpropit appeared to reach equilibrium after 25 min at 21+/-3 degrees C and remained constant for >180 min. The estimated pKD' (10.27+/-0.27; n = 3+/-s.e.mean) was not significantly different from that estimated by saturation analysis in the presence of metyrapone. 5 A series of histamine H3-receptor ligands expressed affinity values for sites labelled with [3H]-clobenpropit which were not significantly different from those estimated when [3H]-R-alpha-MH was used to label histamine H3-receptors in guinea-pig cerebral cortex membranes.

Potent interaction of histamine and polyamines at microsomal cytochrome P450, nuclei, and chromatin from rat hepatocytes

Histamine and polyamines have been implicated in the mediation of cell proliferation. Our previous work linked the growth-modulatory effects of histamine with its binding to intracellular sites in microsomes and nuclei of various tissues. In this study, we identify cytochrome P450 enzymes as a major component of microsomal intracellular sites in hepatocytes and demonstrate that polyamines compete with high affinity for histamine binding to them. Spectral measurement of histamine binding to P450 in liver microsomes resolved high and intermediate affinity binding sites (Ks1 = 2.4 +/- 1.6 microM; Ks2 = 90 +/- 17 microM) that corresponded to microsomal binding sites (Kd1 = 1.0 +/- 0.9 microM; Kd2 = 57 +/- 13 microM) resolved by 3H-histamine binding; additional low affinity (Kd3 approximately 3 mM), and probably physiologically irrelevant, sites were resolved only by 3H-histamine radioligand studies. As determined spectrally, treatment of microsomes with NADPH/carbon monoxide decreased histamine binding to P450 by about 90% and, as determined by 3H-histamine binding, abolished the high affinity sites and reduced by 85% the number of intermediate sites. Spermine competed potently for 3H-histamine binding: in microsomes, Ki = 9.8 +/- 5.8 microM; in nuclei, Ki = 13.7 +/- 3.1 microM; in chromatin, Ki = 46 +/- 33 nM. Polyamines inhibited the P450/histamine absorbance complex with the rank order of potency: spermine > spermidine >> putrescine. In contrast, histamine did not compete for 3H-spermidine binding in nuclei or microsomes, suggesting that polyamines modulate histamine binding allosterically. We propose that certain P450 isozymes that modulate gene function by controlling the level of oxygenated lipids, represent at least one common intracellular target of growth-regulatory endogenous bioamines and, as shown previously, of exogenous growth-modulatory drugs including antiestrogens, antiandrogens, and certain antidepressants and antihistamines.

From oocyte to neuron: do neurotransmitters function in the same way throughout development?

1. Classical neurotransmitters (such as acetylcholine, biogenic amines, and GABA) are functionally active throughout ontogenesis. 2. Based on accumulated evidence, reviewed herein, we present an hypothetical scheme describing developmental changes in this functional activity, from the stage of maturing oocytes through neuronal differentiation. This scheme reflects not only the spatio-temporal sequence of these changes, but also the genesis of neurotransmitter functions, from "protosynapses" in oocytes and cleaving embryos to the development of functional neuronal synapses. 3. Thus, it appears that neurotransmitters participate in various forms of intra- and intercellular signalling throughout all stages of ontogenesis.

Histamine selectively enhances human immunoglobulin E (IgE) and IgG4 production induced by anti-CD58 monoclonal antibody

We studied the effects of histamine on human immunoglobulin (IgE) and IgG4 production. Histamine selectively enhanced IgE and IgG4 production in purified surface IgE and IgG4 negative (sIgE-sIgG4-) B cells from normal donors stimulated with interleukin (IL)-4 plus anti-CD58 or IL-13 plus anti-CD58 monoclonal antibody (mAb) without affecting production of IgG1, IgG2, IgG3, IgM, IgA1, or IgA2. In cultures with IL-4 plus anti-CD58 mAb, histamine-induced enhancement of IgE and IgG4 production was specifically blocked by thioperamide (H3 receptor antagonist), and was inhibited by anti-IL-10 antibody (Ab). In contrast, in cultures with IL-13 plus anti-CD58 mAb, histamine-induced enhancement was blocked by dimaprit (H1 receptor antagonist), and was inhibited by anti-IL-6 mAb. Histamine also enhanced IgE and IgG4 production by in vivo-generated sIgE+ and sIgG4+ B cells, respectively, from atopic patients; enhancement was blocked by dimaprit and thioperamide, and was inhibited by anti-IL-6 mAb and anti-IL-10 Ab. In sIgE-sIgG4- B cells, IL-4 plus anti-CD58 mAb induced IL-10 production and IL-10 receptor expression, whereas IL-13 plus anti-CD58 mAb induced IL-6 production and IL-6 receptor expression. Histamine increased IL-10 and IL-6 production without affecting IL-10 and IL-6 receptor expression, in cultures with IL-4 plus anti-CD58 mAb and with IL-13 plus anti-CD58 mAb, respectively, which was blocked by thioperamide and dimaprit, respectively. In contrast, sIgE+ and sIgG4+ B cells spontaneously produced both IL-6 and IL-10 and constitutively expressed IL-6 and IL-10 receptors, and histamine increased IL-6 and IL-10 production without affecting IL-6 or IL-10 receptor expression, which was blocked by thioperamide and dimaprit. These results indicate that histamine enhanced IgE and IgG4 production by increasing endogenous IL-6 and IL-10 production via H1 and H3 receptors, respectively.

[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.

Enhancement of tumor growth by drugs with some common molecular actions

A group of structurally related drugs representing diverse therapeutic classes share, among a number of pharmacological properties, enhancement of tumor growth in several rodent models of malignancy. One common action, the inhibition of histamine binding to and catalytic activity of cytochrome P450 monooxygenases, is highly correlated with potency to enhance tumor growth. Among members of this drug ensemble, the antiestrogen tamoxifen has been shown in controlled clinical studies to increase the incidence of uterine and gastrointestinal cancer and to accelerate the course of gastric cancer, and the tamoxifen analogue clomiphene has been linked to neuroblastoma and the tricyclic group of antidepressants to ovarian cancer. The determination of drug affinities for protein modulators of cell growth, proliferation, and transformation suggests a strategy for identifying at least some classes of chemicals that impart oncologic risks to humans.

Can the clinical course of cancer be influenced by non-antineoplastic drugs?

Laboratory and anecdotal clinical evidence suggests that some common non-antineoplastic drugs may affect the course of cancer. The authors present two cases that appear to be consistent with such a possibility: that of a 63-year-old woman in whom a high-grade angiosarcoma of the forehead improved after discontinuation of lithium therapy and then progressed rapidly when treatment with carbamazepine was started, and that of a 74-year-old woman with metastatic adenocarcinoma of the colon that regressed when self-treatment with a nonprescription decongestant preparation containing antihistamine was discontinued. The authors suggest that epidemiologic studies are needed to investigate a possible association between non-antineoplastic drugs and the clinical course of cancer and that consideration should be given to discontinuing all nonessential medications for patients with cancer.

Anaphylactic shock: mechanisms and treatment

This paper reviews the mechanisms of anaphylactic shock in terms of the immunoglobulin and non-immunoglobulin triggering events, and the cellular events based on the rise in intracellular cyclic AMP and calcium that release preformed granule-associated mediators and the rapidly formed, newly synthesized mediators predominantly based on arachidonic acid metabolism. These primary mediators recruit other cells with the release of secondary mediators that either potentiate or ultimately curtail the anaphylactic reaction. The roles of these mediators in the various causes of cardiovascular collapse are examined. The treatment of anaphylactic shock involves oxygen, adrenaline and fluids. The importance and safety of intravenous adrenaline are discussed. Combined H1 and H2 blocking antihistamines and steroids have a limited role. Glucagon and other adrenergic drugs are occasionally used, and several new experimental drugs are being developed.

High affinity histamine-binding and antihistaminic activity of the salivary nitric oxide-carrying heme protein (nitrophorin) of Rhodnius prolixus

The salivary glands of Rhodnius prolixus contain a nitrosyl-heme protein, named nitrophorin, that releases the vasodilatory and antiplatelet compound nitric oxide (NO). Because imidazole compounds such as histamine can interact with Fe(III) heme proteins, we investigated whether such substances could interact with Rhodnius nitrophorins. Both imidazole and histamine, but not histidine can produce full of the difference spectra of the Soret band in the 1-3 microM concentration range (at a heme protein concentration of 0.4 microM). The apparent K0.5 for the binding of histamine with the heme protein is below 1 microM. Furthermore, the complex histamine-heme protein does not dissociate after molecular sieving chromatography. To investigate whether histamine could displace NO from the native nitrosyl nitrophorins, histamine was added to the native heme proteins, leading to displacement of the bound NO as observed by changes in the absorption spectra as well as by the production of nitrite. Finally, the antihistamine effect of the heme protein was demonstrated by its inhibition of the histamine-provoked contractures of the guinea pig ileum. It is concluded that histamine, a common autacoid found at the site of injury and exposure to antigenic substances such as the site of feeding by hematophagous arthropods, can be scavenged by the nitrosyl nitrophorin of R. prolixus, which, in return, will release the vasodilatory and platelet inhibiting NO to counteract the host hemostatic response.

More questions than answers: antihistamines' link to cancer

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Comparison of the cardiovascular and neural activity of endothelin-1, -2, -3 and respective proendothelins: effects of phosphoramidon and thiorphan

1. In the anaesthetized, ganglion-blocked rat, intravenous boluses of endothelin-1, endothelin-2 and endothelin-3 induced a transient hypotensive effect followed by a potent long lasting pressor response (ED50 mmHg: 0.72 +/- 0.05, 1.8 +/- 0.2 and 2.7 +/- 0.3 nmol kg-1, respectively). The maximal effect for the three peptides was of a similar order of magnitude (delta MAP: 84 to 89 mmHg). Neither of these effects was influenced by phosphoramidon or thiorphan (10 mg kg-1, i.v.). 2. Intravenously administered big-endothelin-1 and -2 induced a transient (1-2 min) hypotension followed by a potent long lasting (> 25 min) vasopressor effect (ED50 mmHg: 1.8 +/- 0.2 and 6.7 +/- 0.4 nmol kg-1, respectively), with a similar maximal activity (delta MAP: 85 +/- 4 and 81 +/- 2.4 mmHg, respectively). The onset of the big-endothelin-1 vasopressor effect was more rapid (5-6 min) than that of big-endothelin-2 (10-13 min). Big-endothelin-3 was found to induce only a potent, long lasting (> 35 min) hypertension, with a maximal effect of 75 +/- 4.6 mmHg at 10 nmol kg-1 and an ED50 mmHg of 6.5 +/- 0.4 nmol kg-1. The onset of this effect was much slower (20-25 min) than that of the other proendothelins. Pressor responses induced by big-endothelin-1, -2 and -3 (3, 15 and 10 nmol kg-1, respectively) were markedly reduced (60, 80 and 100%) in the presence of phosphoramidon (10 mg kg-1, i.v.). Thiorphan (10 mg kg-1, i.v.) did not inhibit the effects of big-endothelin-1, -2 and -3. 3. In the electrically stimulated rat vas deferens, endothelin-I and -2 were found to be equipotent enhancers of the twitch response (EC100%: 4.0 +/- 0.4 nm and 7.9 +/- 4.8 nm, respectively), both about 3-4 fold as active as endothelin-3 (EC100%: 19 +/- 2.5 nM). Endothelin-1 and -3 showed a comparable maximalstimulatory effect (Emax: 296 +/- 30 and 262 +/- 24%) while endothelin-2 was less active (Emax: 194 +/- 30%).4. Big-endothelin-l and -2 were potent enhancers of the twitch response too (EC 100,%: 10.0 +/- 2.6 nM and 21.6 +/- 3.2 nM, respectively), with a comparable maximal stimulatory effect (Emax: 254 +/- 22 and 264 +/-24%). Big-endothelin-3 was found to be less potent (EC,100%: 275 +/- 21 nM), but retained a marked potentiating effect (Emax: 200 +/- 38%). Phosphoramidon, but not thiorphan, concentration-dependently(10 and 100 microM) reduced big-endothelin-1 (58 and 86% respectively) and big-endothelin-2 (21 and 56%)-mediated responses. Conversely, the big-endothelin-3 effect was reduced by phosphoramidon only at 100 microM (-70%), while thiorphan acts concentration-dependently (31 and 71% at 10 and 100 microM respectively); thus, in the rat vas deferens, big-endothelin-I and -2 were as potent as their corresponding endothelins, while big-endothelin-3 was about 20 times less potent than endothelin-3.5. The increasing effect of endothelin-2 (194 +/- 30% over baseline) was significantly enhanced by either 10 microM phosphoramidon (277 +/- 42%) or thiorphan (318 +/- 15%). The endothelin-I and endothelin-3-mediated twitch enhancement was not affected by the two protease inhibitors (10 microM).6. These results suggest that in vivo big-endothelin-1, -2 and -3, are processed through a similar phosphoramid on-sensitive enzymatic pathway although with different apparent affinity. This enzymatic process is probably attributable to a neutral endoprotease, distinct from neutral-endopeptidase 24.11(NEP). On the other hand, a NEP-like enzymatic activity may be involved, in the rat vas deferens, in the activation of big-endothelin-3 to endothelin-3 and in the metabolism of endothelin-2, but not of endothelin-I or endothelin-3.