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Background: Autism spectrum disorders (ASDs) are a group of neurodevelopmental conditions defined by behavioral deficits in social communication and interactions, mental inflexibility and repetitive behaviors. Converging evidence from observational and preclinical studies suggest that excessive repetitive behaviors in people with ASD may be due to elevated histaminergic H3 receptor signaling in the striatum. We hypothesized that systemic administration of pharmacological histamine H3 receptor antagonists would attenuate the expression of repetitive behaviors in the BTBR T+Itpr3tf/J (BTBR) mouse inbred strain, an established mouse model presenting autism-like repetitive behaviors and novelty-induced hyperactivity. We further aimed to investigate whether agonism of the histamine H3 receptor would be sufficient to induce repetitive behaviors in the C57BL/6J control mouse strain., Methods: Different doses of H3 receptor agonists (i.e., (R)-α-methylhistamine and immethridine) and H3 receptor antagonists/inverse agonists (i.e., ciproxifan and pitolisant) were administered via intraperitoneal (i.p.) injection in male mice to characterize the acute effects of these compounds on ASD-related behavioral readouts., Results: The highly selective H3 receptor agonist immethridine significantly increased the time spent in stereotypic patterns in C57BL/6J mice, but this effect appeared to be driven by general sedative properties of the compound. High doses of pitolisant significantly decreased locomotor hyperactivity in novel environments in BTBR mice, without significant effects on repetitive behaviors., Conclusions: Based on our findings, we conclude that acute H3 receptor manipulation mainly affected general motor activity levels in novel environments. Small changes in stereotyped behaviors were observed but appeared to be driven by altered general activity levels., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

During the past decades, histamine H 3 receptors have received widespread attention in pharmaceutical research due to their involvement in pathophysiology of several diseases such as neurodegenerative disorders. In this context, blocking of these receptors is of paramount importance in progression of such diseases. In the current investigation, novel histamine H 3 receptor ligands were designed by exploiting scaffold-hopping drug-design strategy. We inspected the designed molecules in terms of ADME properties, drug-likeness, as well as toxicity profiles. Additionally molecular docking and dynamics simulation studies were performed to predict binding mode and binding free energy calculations, respectively. Among the designed structures, we selected compound d2 and its demethylated derivative as examples for synthesis and affinity measurement. In vitro binding assays of the synthesized molecules demonstrated that d2 has lower binding affinity (K i  = 2.61 μM) in radioligand displacement assay to hH 3 R than that of demethylated form (K i  = 12.53 μM). The newly designed compounds avoid of any toxicity predictors resulted from extended in silico and experimental studies, can offer another scaffold for histamine H 3 R antagonists for further structure-activity relationship studies., (Copyright © 2021. Published by Elsevier Inc.)

Oxaliplatin is a widely used chemotherapeutic drug and represents the cornerstone of colorectal cancer therapy, in combination with 5-fluorouracil and folinic acid. As with many chemotherapeutic agents, its use is associated with a number of side effects, ranging from hypersensitivity reactions to haematological dyscrasias. Oxaliplatin also induces acute and chronic peripheral neuropathy. While it is likely that the haematological side effects are associated with its anti-proliferative effects and with the ability to form DNA adducts, the molecular mechanisms underlying peripheral neuropathy and hypersensitivity reactions are poorly understood, and therefore the choice of adequate supportive therapies is largely empirical. Here we show that an acute low dose oxaliplatin application on DRG neurons is able to induce an increase in intracellular calcium that is dependent on the Histamine 1 receptor (H1). Oxaliplatin-induced intracellular calcium rises are blocked by two selective H1 antagonist, as well as by U73122, a PLC inhibitor, and by 2-APB, a non-specific IP 3 receptor blocker. Moreover, expression of the H1 receptor on HEK293 t cells unmasks an oxaliplatin-induced Ca 2+ -rise. Last, activation of H1 via either histamine or oxaliplatin activates TRPV1 receptors, a mechanism that has been associated with itch. These data, together with literature data that has shown that anti-histamine agents reduce the incidence of oxaliplatin-induced hypersensitivity, may provide a molecular mechanism of this side effect in oncological patients., Competing Interests: Declaration of Competing Interest The authors declares no conflict of interest., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: A method that promotes the retrieval of lost long-term memories has not been well established. Histamine in the central nervous system is implicated in learning and memory, and treatment with antihistamines impairs learning and memory. Because histamine H 3 receptor inverse agonists upregulate histamine release, the inverse agonists may enhance learning and memory. However, whether the inverse agonists promote the retrieval of forgotten long-term memory has not yet been determined., Methods: Here, we employed multidisciplinary methods, including mouse behavior, calcium imaging, and chemogenetic manipulation, to examine whether and how the histamine H 3 receptor inverse agonists, thioperamide and betahistine, promote the retrieval of a forgotten long-term object memory in mice. In addition, we conducted a randomized double-blind, placebo-controlled crossover trial in healthy adult participants to investigate whether betahistine treatment promotes memory retrieval in humans., Results: The treatment of H 3 receptor inverse agonists induced the recall of forgotten memories even 1 week and 1 month after training in mice. The memory recovery was mediated by the disinhibition of histamine release in the perirhinal cortex, which activated the histamine H 2 receptor. Histamine depolarized perirhinal cortex neurons, enhanced their spontaneous activity, and facilitated the reactivation of behaviorally activated neuronal ensembles. A human clinical trial revealed that treatment of H 3 receptor inverse agonists is specifically more effective for items that are more difficult to remember and subjects with poorer performance., Conclusions: These results highlight a novel interaction between the central histamine signaling and memory engrams., (Copyright © 2018 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.)

Aims: Betahistine is a histamine analogue that is used for the treatment of Menière's disease. Animal studies showed that it increases local blood flow in the stria vascularis. In terms of its mode of action, recent studies have prompted discussion of whether betahistine actively affects cochlear microcirculation by dilations of pericytes or of precapillary arterioles or by mere downstream effects. Hence, we investigated the effects of betahistine on cochlear capillary pericytes and precapillary arterioles., Main Methods: The stria vascularis was visualized in 12 guinea pigs by in vivo fluorescence microscopy. In these, 152 pericytes were stained and local diameter at sites of pericyte somas and downstream controls as well as intravascular blood flow were measured before and after betahistine application. Moreover, in two guinea pigs the precapillary arterioles were visualized by 2-photon-microscopy before and after betahistine application., Key Findings: There was no significant change in capillary diameter at sites of pericyte somas after betahistine application compared to controls, baseline or downstream controls, even though cochlear blood flow increased significantly. The two-photon measurements indicated an active dilation of precapillary arterioles., Significance: Since we found no evidence that betahistine affects cochlear microcirculation by cochlear pericytes, its main mode of action is evidently active dilation of pre-capillary arterioles. These findings are in line with similar effects reported in the central nervous system and indicate an active effect on cochlear microcirculation., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Recently we found that synthetic compounds containing amino group linked to hydrophobic or aromatic moiety are potent modulators of the proton-gated channels (ASICs). These structures have clear similarity with ligands of histamine receptors. We have also demonstrated that histamine potentiates homomeric ASIC1a by shifting its activation dependence to less acidic conditions. In the present work the action of a series of histamine receptors ligands on recombinant ASIC1a and ASIC2a was characterized. Two types of action were found for ASIC1a. 1-methylhistamine, N-alpha-methylhistamine, dimaprit and thioperamide caused significant potentiation, which was pH-dependent and voltage-independent. The H4R antagonist A943931 caused inhibition, which is likely due to voltage-dependent pore block. ASIC2a were virtually insensitive to the drugs tested. We conclude that ligands of histamine receptors should also be considered as ASIC modulators., (Copyright © 2017. Published by Elsevier Inc.)

Brain histamine acts as a neurotransmitter and regulates various physiological functions, such as learning and memory, sleep-wake cycles, and appetite regulation. We have recently shown that histamine H3 receptor (H3R) is expressed in primary mouse microglia and has a strong influence on critical functions in microglia, including chemotaxis, phagocytosis, and cytokine secretion in vitro. However, the importance of H3R in microglial activity in vivo remains unknown. Here, we examined the effects of JNJ10181457 (JNJ), a selective and potent H3R inverse agonist, on microglial functions ex vivo and in vivo. First, we injected ATP, which is a typical chemoattractant, into hippocampal slices to investigate the effect of JNJ on chemotaxis. ATP-induced microglial migration toward the injected site was significantly suppressed by JNJ treatment. Next, we examined whether JNJ affected microglial phagocytosis in hippocampal slices and in the prefrontal cortex. Microglial engulfment of dead neurons induced by N-methyl- d -aspartate was inhibited in the presence of JNJ. The increase in zymosan particle uptake by activated microglia in the prefrontal cortex was prevented by JNJ administration. Finally, we determined the importance of JNJ in a lipopolysaccharide (LPS)-induced depression model. JNJ reduced the LPS-induced upregulation of microglial pro-inflammatory cytokines and improved depression-like behaviour in the tail-suspension test. These results demonstrate the inhibitory effects of JNJ on chemotaxis, phagocytosis, and cytokine production in microglia inside the brain, and highlight the importance of microglial H3R for brain homeostasis., (Copyright © 2017 Elsevier Inc. All rights reserved.)

A set of histamine H 1 receptor (H 1 R) agonists and antagonists was characterized in functional assays, using dynamic mass redistribution (DMR), electric cell-substrate impedance sensing (ECIS) and various signaling pathway specific readouts (Fura-2 and aequorin calcium assays, arrestin recruitment (luciferase fragment complementation) assay, luciferase gene reporter assay). Data were gained from genetically engineered HEK293T cells and compared with reference data from GTPase assays and radioligand binding. Histamine and the other H 1 R agonists gave different assay-related pEC 50 values, however, the order of potency was maintained. In the luciferase fragment complementation assay, the H 1 R preferred β-arrestin2 over β-arrestin1. The calcium and the impedimetric assay depended on G q coupling of the H 1 R, as demonstrated by complete inhibition of the histamine-induced signals in the presence of the G q inhibitor FR900359 (UBO-QIC). Whereas partial inhibition by FR900359 was observed in DMR and the gene reporter assay, pertussis toxin substantially decreased the response in DMR, but increased the luciferase signal, reflecting the contribution of both, G q and G i , to signaling in these assays. For antagonists, the results from DMR were essentially compatible with those from conventional readouts, whereas the impedance-based data revealed a trend towards higher pK b values. ECIS and calcium assays apparently only reflect G q signaling, whereas DMR and gene reporter assays appear to integrate both, G q and G i mediated signaling. The results confirm the value of the label-free methods, DMR and ECIS, for the characterization of H 1 R ligands. Both noninvasive techniques are complementary to each other, but cannot fully replace reductionist signaling pathway focused assays., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Ligand residence time is thought to be a critical parameter for optimizing the in vivo efficacy of drug candidates. For the histamine H1 receptor (H1R) and other G protein-coupled receptors, the kinetics of ligand binding are typically measured by low throughput radioligand binding experiments using homogenized cell membranes expressing the target receptor. In this study, a real-time proximity assay between H1R and β-arrestin2 in living cells was established to investigate the dynamics of antihistamine binding to the H1R. No receptor reserve was found for the histamine-induced recruitment of β-arrestin2 to the H1R and the transiently recruited β-arrestin2 therefore reflected occupancy of the receptor by histamine. Antihistamines displayed similar kinetic signatures on antagonizing histamine-induced β-arrestin2 recruitment as compared to displacing radioligand binding from the H1R. This homogeneous functional method unambiguously determined the fifty-fold difference in the dissociation rate constant between mepyramine and the long residence time antihistamines levocetirizine and desloratadine., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Second generation antipsychotic drugs (SGAs) cause substantial body weight gain/obesity and other metabolic side-effects such as dyslipidaemia. Their antagonistic affinity to the histaminergic H1 receptor (H1R) has been identified as one of the main contributors to weight gain/obesity side-effects. The effects and mechanisms of betahistine (a histaminergic H1R agonist and H3 receptor antagonist) have been investigated for ameliorating SGA-induced weight gain/obesity in both animal models and clinical trials. It has been demonstrated that co-treatment with betahistine is effective in reducing weight gain, associated with olanzapine in drug-naïve patients with schizophrenia, as well as in the animal models of both drug-naïve rats and rats with chronic, repeated exposure to olanzapine. Betahistine co-treatment can reduce food intake and increase the effect of thermogenesis in brown adipose tissue by modulating hypothalamic H1R-NPY-AMPKα (NPY: neuropeptide Y; AMPKα: AMP-activated protein kinase α) pathways, and ameliorate olanzapine-induced dyslipidaemia through modulation of AMPKα-SREBP-1-PPARα-dependent pathways (SREBP-1: Sterol regulatory element binding protein 1; PPARα: Peroxisome proliferator-activated receptor-α) in the liver. Although reduced locomotor activity was observed from antipsychotic treatment in rats, betahistine did not affect locomotor activity. Importantly, betahistine co-treatment did not influence the effects of antipsychotics on serotonergic receptors in the key brain regions for antipsychotic therapeutic efficacy. However, betahistine co-treatment reverses the upregulated dopamine D2 binding caused by chronic olanzapine administration, which may be beneficial in reducing D2 supersensitivity often observed in chronic antipsychotic treatment. Therefore, these results provide solid evidence supporting further clinical trials in treating antipsychotics-induced weight gain using betahistine in patients with schizophrenia and other mental disorders., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The antipsychotic properties of haloperidol are primarily attributed to its ability to block dopamine D2 receptors. Histaminergic transmission modulates some of the behavioral effects of haloperidol. Hence, the present study investigated the contribution of central histaminergic transmission in the cataleptic and neuroleptic effect of haloperidol respectively, using bar test and conditioned avoidance response (CAR) in a two-way shuttle box. The studies revealed that haloperidol (0.50 or 1 mg/kg, i.p.) exhibited cataleptic behavior and inhibited conditioned avoidance response (CAR) in the doses 0.25 or 0.50 mg in rats. The rats, pretreated centrally (i.c.v.) with histamine precursor, L-histidine (1, 2.5 μg) or histamine neuronal inducer (H3 receptor antagonist), thioperamide (20, 50 μg/rat), showed an enhanced cataleptic effect with sub-maximal dose of haloperidol (0.5 mg/kg, i.p.). Similarly, the neuroleptic effect of haloperidol (0.25 mg/kg, i.p.) in CAR was also potentiated in the rats pretreated with L-histidine (2.5 μg) or thioperamide (50 μg/rat). Further, the cataleptic effect of haloperidol (1 mg/kg, i.p.) was attenuated in rats pretreated with the H1 receptor antagonist, chlorpheniramine (60, 80 μg/rat, i.c.v.) or H2 receptor antagonist, ranitidine (60 μg/rat, i.c.v.). However, the neuroleptic effect of haloperidol (0.5 mg/kg, i.p.) was completely reversed by pretreatment with ranitidine (60 μg/rat, i.c.v.), and partially attenuated by chlorpheniramine (80 μg/rat, i.c.v.). These findings suggest the possible involvement of histaminergic transmission in the cataleptic and neuroleptic effects of haloperidol probably via H1 or H2 receptor stimulation., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Histamine 4 receptor (H4R) is a novel target for the pharmacological modulation of histamine-mediated immune signals during inflammatory diseases. The purpose of this study was to assess the effects of the H4R agonist 4-methylhistamine dihydrochloride (4-MeH) and antagonist JNJ7777120 (JNJ) in the inflamed rat knee. Animals were fasted for 18h before a single dose of 4-MeH or JNJ (30mg/kg) was administered intraperitoneally (i.p.), both followed by intra-articular (i.a.) injection of LPS 2h later. Blood and synovial fluid were collected after a short incubation period and TNF-α, NF-κB, and IkB-α levels were measured via flow cytometry. Additionally, we assessed the effects of H4R engagement on the expression of IL-1β, TNF-α, and NF-κB mRNAs and the protein levels of TNF-α, NF-κB, JAK-1, and STAT-3 in the inflamed knee tissue. These results revealed increased TNF-α and NF-κB expression and decreased IkB-α levels in both the LPS alone and 4-MeH treated groups in whole blood and synovial fluid. Further, IL-1β, TNF-α, and NF-κB mRNA levels were significantly increased and western blot analysis confirmed increased expression of TNF-α, NF-κB, JAK-1, and STAT-3 in both LPS and 4-MeH treatment groups. Furthermore, these increases were completely inhibited in the inflamed knee tissue of the JNJ-treated group. Thus, the inhibition of inflammatory mediators and signaling pathways by the H4R antagonist JNJ suggests the anti-arthritic importance of this molecule., (Copyright © 2015 Elsevier GmbH. All rights reserved.)

Alterations to the immune system caused by stress have been considered to markedly increase the risk for immune-related diseases such as cancer and autoimmune disorders. We investigated the potential anti-stress effects of the histamine 4 receptor (H4R) agonist, 4-methylhistamine (4-MeH), in a murine stress model. Mice were placed in 50ml conical centrifuge tubes for 12h followed by a 12h rest. The effects of treatment with 4-MeH (30mg/kg, i.p., twice daily) for 2 days were assessed. At 2 days after physical restraint, mice were sacrificed and tissues harvested. We evaluated the effects of 4-MeH treatment on CD4(+) T cell production, and intracellular IFN-γ and IL-4 expression in these cells. We also assessed IL-1β, IFN-γ, TNF-α, and IL-4 mRNA expression as well as IFN-γ, TNF-α, GITR, Ox40 and IL-4 protein expression in the spleen. The results showed that 4-MeH treatment of stressed mice results in a substantial increase in the CD4(+) T cells as well as in IFN-γ production by these cells. Compared to both untreated and stressed controls. In contrast, IL-4 expression decreased significantly following 4-MeH treatment of mice. Moreover, stimulation of the H4R resulted in up-regulated expression of IL-1β, IFN-γ and TNF-α mRNAs and decreased the expression of IL-4. Western blot analysis confirmed decreased protein expression of IFN-γ, TNF-α, GITR, Ox40 and increased IL-4 in the SC group and treatment of mice with 4-MeH reversed these effects. Our results confirm the significant impact of chronic stress on T cell function and production of Th1/Th2 mediators H4R., (Copyright © 2014 Elsevier GmbH. All rights reserved.)

Aims: The diverse physiological functions of histamine are mediated through distinct histamine receptors. In this study we investigated the role of H2R and H4R in the effects of histamine on the production of reactive oxygen species by phagocytes in whole blood., Main Methods: Changes in reactive oxygen species (ROS) production by whole blood phagocytes after treatment with histamine, H4R agonists (4-methylhistamine, VUF8430), H2R agonist (dimaprit) and their combinations with H4R antagonist (JNJ10191584) and H2R antagonist (ranitidine) were determined using the chemiluminescence (CL) assay. To exclude the direct scavenging effects of the studied compounds on the CL response, the antioxidant properties of all compounds were measured using several methods (TRAP, ORAC, and luminol-HRP-H2O2 based CL)., Key Findings: Histamine, 4-methylhistamine, VUF8430 and dimaprit inhibited the spontaneous and OZP-activated whole blood CL in a dose-dependent manner. On the other hand, only VUF8430 was able to inhibit PMA-activated whole blood CL. Ranitidine, but not JNJ10191584, completely reduced the effects of histamine, 4-methylhistamine and dimaprit. The direct scavenging ability of tested compounds was negligible., Significance: Our results demonstrate that the inhibitory effects of histamine on ROS production in whole blood phagocytes were caused by H2R. Our results also suggest that H4R agonists in concentrations higher than 10(-6)M may also influence ROS production via binding to H2R., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Vitiligo is a progressive depigmenting disorder. Histamine has been shown to induce melanogenesis via histamine receptor 2, suggesting the possibility of histamine as a repigmenting agent for the treatment of vitiligo. However, the role and signaling mechanism of histamine are still unclear in melanogenesis, especially in relation to growth-differentiation factor-15, which is a protein belonging to transforming growth factor beta and found to be overexpressed in metastatic or malignant melanoma. We found that histamine induces growth-differentiation factor-15 in melanoma cell lines such as SK-MEL-2, B16F10, and melan-a cells. Therefore, in the present study, the role of growth-differentiation factor-15 in histamine-induced melanogenesis was investigated using gene silencing or overexpression of growth-differentiation factor-15 and histamine related compounds such as histamine, amthamine, and cimetidine. Gene silencing of growth-differentiation factor-15 suppressed histamine-induced proliferation, melanin production, tyrosinase activity, and chemotactic migration of SK-MEL-2 cells. Histamine-induced expression of tyrosinase, tyrosinase-related protein 1, and tyrosinase-related protein 2 was also suppressed by growth-differentiation factor-15 gene silencing. On the other hand, overexpression of growth-differentiation factor-15 using a plasmid containing growth-differentiation factor-15 in SK-MEL-2 cells increased melanin production and chemotactic migration. Amthamine induced expression of growth-differentiation factor-15 in a time and concentration dependent manner. Amthamine-induced expression of growth-differentiation factor-15 was suppressed by cimetidine. Our results suggest that growth-differentiation factor-15 is a new player in histamine-induced melanogenesis, which can help researchers to extend the knowledge of the role of the transforming growth factor beta family in melanogenesis and in skin pigment disorders such as vitiligo., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Objective: Ischemic preconditioning and some drugs can protect tissues from injury by preserving microcirculation. This study evaluated vascular permeability in a hamster cheek pouch preparation using either short ischemic periods or bradykinin as preconditioning stimuli followed by 30 min of ischemia/reperfusion., Method: Sixty-six male hamsters were divided into 11 groups: five combinations of different ischemic frequencies and durations (one, three or five shorts periods of ischemia, separated by one or five minutes) with 10 min intervals between the ischemic periods, followed by 30 min ischemia/reperfusion; three or five 1 min ischemic periods with 10 min intervals between them followed by the topical application of histamine (2 µM); bradykinin (400 nM) followed by 30 min of ischemia/reperfusion; and three control groups (30 min of ischemia/reperfusion or histamine or bradykinin by themselves). Macromolecular permeability was assessed by injection of fluorescein-labeled dextran (FITC-dextran, MW= 150 kDa; 250 mg/Kg body weight), and the number of leaks/cm2 was counted using an intravital microscope and fluorescent light in the cheek pouch., Results: Plasma leakage (number of leaks/cm²) was significantly reduced by preconditioning with three and five 1 min ischemic periods, one and three 5 min ischemic periods and by bradykinin. Histamine-induced macromolecular permeability was also reduced after three periods of 5 min of ischemia., Conclusion: Short ischemic periods and bradykinin can function as preconditioning stimuli of the ischemia/reperfusion response in the hamster cheek pouch microcirculation. Short ischemic periods also reduced histamineinduced macromolecular permeability.

Cholestasis is associated with changes including analgesia. The histaminergic system regulates pain perception. The involvement of histamine H(3) receptors in modulation of nociception in a model of elevated endogenous opioid tone, cholestasis, was investigated in this study using immepip and thioperamide as selective H(3) receptor agonist and antagonist respectively. Cholestasis was induced by ligation of main bile duct using two ligatures and transsection the duct between them. Cholestatic rats had increased tail-flick latencies (TFLs) compared to non-cholestatics. Administration of immepip (5 and 30mg/kg) and thioperamide (10 and 20mg/kg) to the cholestatic groups significantly increased and decreased TFLs compared to the saline treated cholestatic group. Immepip antinociception in cholestatic animals was attenuated by co-administration of naloxone. Immepip and thioperamide injections into non-cholestatic animals did not alter TFLs. At the doses used here, none of the drugs impaired motor coordination, as revealed by the rotarod test. The present data show that the histamine H(3) receptor system may be involved in the regulation of nociception during cholestasis in rats., (Copyright 2010 Elsevier Inc. All rights reserved.)

The role of histamine and its receptors in basal ganglia neurocircuitry was assessed in apomorphine-induced turning behavior. Rats with unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta and medial forebrain bundle were administered histaminergic agents, and apomorphine-induced turning behavior was tested on Days 7 and 14 post-lesion. Compared with saline-treated rats, histidine (500 mg/kg, i.p.), a precursor of histamine, increased turning behavior (p<0.05), while alpha-fluoromethylhistidine (alpha-FMH, 25 microg, i.c.v.), an irreversible inhibitor of histidine decarboxylase, decreased turning behavior (p<0.05) but only on Day 14 post-lesion. Both the histamine H(1) receptor antagonist pyrilamine (10 and 50 microg, i.c.v.) and the H(2) receptor antagonist cimetidine (10 and 50 microg, i.c.v.) significantly decreased turning behavior on Days 7 and 14 post-lesion. The histamine H(3) receptor agonist immepip (10 microg, i.c.v.) decreased turning behavior (p<0.05) on Day 14 post-lesion. The present findings indicate the complex interactions of histamine on basal ganglia function.

Our previous study demonstrated the co-localization of histamine with norepinephrine (NE) within superior cervical ganglia (SCG), and the release of histamine from sympathetic nerve endings of guinea pig evoked by stimulations. We have now further investigated that whether the histamine can be synthesized, stored and released from the sympathetic nerve systems of Macaca mulatto monkey, and investigated the modulation of the sympathetic endogenous histamine release through histamine H(3) receptor in the monkey cardiac sympathetic nerve system. Double-labeled immunofluorescence technique was applied to investigate co-localization of histamine and NE in SCG of Macaca mulatto monkey. The cardiac sympathetic nerve terminals (synaptosomes) of Macaca mulatto monkey was prepared and depolarized with 50 mmol/L K(+). Histamine released from synaptosomes was detected by spectrofluorometer and regulations of histamine release through Ca(2+), Ca(2+)-channel blockers, H(3)-receptor agonist (R)-alpha-methylhistamine and histamine H(3)-receptor antagonist, thioperamide were observed. Co-localization of histamine and NE was identified within the same neuron of SCG. Release of histamine was Ca(2+)-dependent and inhibited by N-type Ca(2+)-channel blocker omega-conotoxin, but not affected by the L-type Ca(2+)-channel blocker lacidipine. Compound 48/80, a mast cell releaser, did not affect cardiac synaptosome histamine exocytosis. Cardiac synaptosome histamine release was augmented by the enhanced synthesis of histamine or the inhibition of histamine metabolism. Histamine H(3)-receptor activation by (R)-alpha-methylhistamine inhibited high K(+)-evoked histamine release and thioperamide blocked the effects of (R)-alpha-methylhistamine. These results firstly showed that histamine co-existed with NE within sympathetic neurons of monkey and the exocytosis of histamine from sympathetic terminals could be regulated by presynaptic histamine H(3) receptors. Sympathetic histamine may act as a neurotransmitter to modulate sympathetic neurotransmission.

Pharmacological activation of histamine H3 receptors is known to reduce the release of inflammatory peptides, thereby reducing pain and inflammation, but the site(s) and mechanism(s) of these effects are currently unknown. The present study addressed these questions by examining the effects of the H3 agonist immepip and the H3 antagonist thioperamide on nociceptive behaviors and swelling produced during the rat formalin test. Systemic administration of immepip (5 and 30 mg/kg, s.c.) significantly attenuated formalin-induced flinching but not licking responses during both phases. This attenuation was reversed by either systemic (15 mg/kg, i.p.) or intrathecal (20 or 50 microg) administration of thioperamide. Furthermore, immepip (30 mg/kg, s.c.) significantly inhibited formalin-induced swelling, an action which was completely reversed by systemic (15 mg/kg, i.p.), but not intrathecal (50 microg) thioperamide. Also consistent with this pattern, intrathecal immepip (50 microg) reduced flinching responses, but had no effect on formalin-induced paw swelling. The present findings suggest that activation of H3 receptors located on peripheral and spinal terminals of deep dermal fibers attenuates formalin-induced swelling and flinching, respectively. Pharmacological stimulation of H3 receptors could be an important therapeutic approach for many disorders related to deep dermal or inflammatory pain.

Blockade of presynaptic histamine H(3) receptors with potent and selective ligands improves cognitive function in rodents and there is significant interest in developing such drugs for long-term symptomatic treatment of CNS disorders such as attention deficit hyperactivity disorder (ADHD). Unfortunately, little is known about the effects of repeated exposure to H(3) receptor antagonists/inverse agonists. We therefore investigated the effects of acute and repeated daily administration of two potent, brain penetrating H(3) receptor antagonists/inverse agonists, ciproxifan and A-304121, on rat body weight, food and water intake, core temperature and locomotor activity, as well as H(3) receptor density and gene expression levels. Methylphenidate, used clinically for the treatment of ADHD, was included as an additional comparator. Ciproxifan, an imidazole-based compound, decreased food intake over the first 10 days and locomotor activity acutely, but these effects were lost after further repeated administration. The ex vivo binding studies revealed increased H(3) receptor density in rats following repeated administration of ciproxifan for 10 or 15 days; however, H(3) receptor gene expression was not changed. In contrast, rats treated with the non-imidazole, A-304121, did not differ from controls on any measure during the observation period, while rats treated with methylphenidate exhibited hyperthermia and hyperactivity. The implications for potential long-term treatment with H(3) receptor antagonists in CNS disorders such as ADHD are discussed.

Previous studies have revealed distribution of histaminergic fibers and presence of histamine receptors in globus pallidus (GP). In this study, the brain slice preparation of adult rats was used to examine the effect of histamine on the spontaneous unitary discharge of GP neurons and the underlying receptor mechanism. Ninety-five GP neurons were extracellularly recorded from 42 slices containing the GP, of which 87 (91.6%) were excited by the stimulation of histamine. The histamine-induced excitation was concentration-dependent and persisted in low Ca2+/high Mg2+ medium (n = 9), demonstrating that the action of histamine on the GP neurons was postsynaptic. The excitatory effect of histamine on the GP neurons was not blocked by selective histamine H1 receptor antagonist triprolidine (n = 16) or chlorpheniramine (n = 6), but was effectively suppressed by ranitidine, a highly selective histamine H2 receptor antagonist (n = 21). On the other hand, highly selective histamine H2 receptor agonist dimaprit mimicked the excitatory effect of histamine on the GP neurons (n = 23), while histamine H1 receptor agonists, including 2-pyridylethylamine (n = 22), 2-thiazolyethylamine (n = 9) and betahistine (n = 9), did not cause GP neurons any response. The dimaprit-induced GP neuronal excitation was effectively antagonized by selective histamine H2 receptor antagonist ranitidine (n = 14) but not influenced by selective histamine H1 receptor antagonist triprolidine (n = 12). Moreover, adenylate cyclase (AC) activator forskolin (n = 7) was observed to evoke GP neurons an excitatory response, whereas the histamine-induced excitation was effectively reduced by H-89 (n = 9), a selective and potent inhibitor of protein kinase A (PK(A)). Finally, it was noted that neurons of both subdivisions of the GP, the internal (GPi, n = 35) and external (GPe, n = 60) segment, showed no differences in their responses to stimulations of the tested histaminergic reagents. These results demonstrated that histamine excited GP (including GPi and GPe) neurons via histamine H2 receptors and H2 receptors linked intracellular G-protein-AC-PK(A) signaling pathway, suggesting that the hypothalamic histaminergic afferent fibers innervating GP may play an important modulatory role in motor control through its excitatory effect on GP neurons.

Recently a series of H3-antagonists related to Imoproxifan was realised (I); in these products the oxime substructure of the lead was constrained in NO-donor furoxan systems and in the corresponding furazan derivatives. In this paper, a new series of compounds derived from I by substituting the imidazole ring with the ethoxycarbonylpiperazino moiety present in the non-imidazole H3-ligand A-923 is described. For all the products synthesis and preliminary pharmacological characterisation, as well as their hydrophilic-lipophilic balance, are reported. The imidazole ring replacement generally results in a decreased H3-antagonist activity with respect to the analogues of series I and, in some cases, induces relaxing effects on the electrically contracted guinea-pig ileum, probably due to increased affinity for other receptor systems.

The present pharmacological study was conducted to investigate a possible role of the brain histaminergic system in vocalization induced in guinea pig pups by maternal separation and isolation in an unfamiliar environment. The effects of drugs acting at histamine receptors were determined after intraperitoneal injection, comprising hydroxyzine and chlorpheniramine, both histamine H1 antagonists, and the H3 agonist, immepip. A range of psychoactive drugs known to be active in this paradigm was also tested for comparison. Hydroxyzine, 4.3 to 14.3 mg/kg, dose-dependently suppressed vocalization but neither chlorpheniramine, 2 to 16 mg/kg, nor immepip, 5 to 20 mg/kg, was active. All reference drugs, fluoxetine, 5 and 10 mg/kg, imipramine, 16 and 32 mg/kg, and chlordiazepoxide, 5 and 10 mg/kg, were shown to be active. The present data indicate that, consistent with known anxiolytic effects in man, the antihistamine hydroxyzine proved effective in suppressing maternal-separation-induced vocalization in guinea pig pups. However, in view of the lack of effect of either chlorpheniramine or immepip, it is proposed that additional nonhistaminergic effects are involved in the tranquilizing action of hydroxyzine.

The specific binding of [(11)C]doxepin, which has been used as a radioligand for mapping histamine H(1) receptors in human brain by positron emission tomography, was evaluated in five animal species. In mice the [(11)C]doxepin uptake was reduced by treatment with cold doxepin and two H(1) receptor antagonists, but not with H(2)/H(3) antagonists. The specific binding evaluated with treatment with (+)-chlorpheniramine (H(1) antagonist) was in the range of 10-30% in mouse, rat, rabbit, and monkey, but was not detected in guinea pig.

It was hypothesized that fish possess functioning H1 histamine receptors that have the ability to bind agonists and antagonists specific to the H1 histamine receptor subtype. For these experiments, a combination of a novel, in vivo 2-deoxyglucose method and a standard in vitro autoradiography procedure was utilized. A regional, statistically significant dose response in neurological functioning was observed when fish were exposed to histaminergic agents (i.e., H1 agonists and antagonists), which created the first neurological profile for the H1 histamine receptor in fish brain. The H1 histamine receptor was chosen as a characterization receptor in fish because histamine has been linked to a variety of neurological functions such as the control of arousal, attention, sensory processing, and cognition. Histamine also plays a role in pituitary hormone secretion, appetite control, and, potentially, regulation of vestigular reactivity. In addition, the fish brain is well characterized structurally, and the existence of an H3-like receptor has been documented recently in zebrafish. However, to date there is little detailed information about specific localization and functioning of the H1 histamine receptor in fish.

Despite histamine being a potent endogenous vasoactive agent released in increasing amounts postburn, its role in postburn oedema formation has been controversial and its effect on burn circulation poorly investigated. The present study investigated the involvement of H(1), H(2) and H(3) receptors in postburn edema in rats exposed to skin and muscle burns and their influence on skin circulation postburn. We used the selective antagonists clemastine (H(1)), ranitidine (H(2)), thioperamide (H(3)) and the selective H(3) receptor agonist, imetit. Results showed that none of the antagonists or the H(3) agonist had significant effect on postburn edema measured by quantitative spectrophotometric analysis of extravasated Evans blue-albumin in the full-thickness burned skin or muscle. Clemastine and thioperamide failed to induce significant effect on blood flow in the partial- or full-thickness skin burn injury as measured by laser Doppler flowmetry, while ranitidine significantly (P<0.01) reduced blood flow in the full-thickness burn. In contrast, the H(3) receptor agonist, imetit, significantly increased blood flow, both in the partial-thickness burn injury (P<0.05) and in the full-thickness burn (P<0.01). Moreover, imetit significantly (P<0.01) increased mean arterial pressure while thioperamide significantly (P<0.01) reduced systemic pressure. In conclusion, H(1), H(2) and H(3) receptors are not important actors in the regulation of vascular patency permeability, whereas H(3) receptors play an important role by increasing skin circulation postburn, presumably by relaxation of vascular smooth muscle and/or by interacting with other inflammatory neurotransmitters. Data also suggest that H(2) receptor blockers may not be best choice for stress ulcer prophylaxis in burn patients.

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.

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.

Electrical and chemical stimulation of the dorsal periaqueductal gray matter (dPAG) and the inferior colliculus (IC) induces escape behavior, usually accompanied by autonomic responses and antinociception. Recently, we presented evidence for a tonic inhibitory control exerted by H(2) histamine receptors on defensive behaviors generated in these midbrain tectum sites. Since treatments of these areas that elicit the defensive behavior repertoire frequently also have anxiogenic effects, we here used the elevated plus-maze (EPM) test for assessing the effects of microinjections of histamine (5-40 nmol), dimaprit (5-10 nmol) and ranitidine (10-30 nmol) into either dPAG or IC, which have a relative abundance of histamine-containing cells and histaminergic receptors. Dimaprit is an agonist and ranitidine is an antagonist of H(2) histamine receptors. Immediately after the injections, the animals were submitted to the EPM test. Whereas dPAG injections of dimaprit had no behavioral effects, histamine (40 nmol) caused a significant reduction in exploratory activity. On the other hand, ranitidine alone or following saline had aversive-like effects in both structures, i.e. reduced open arm, but not closed arm, entries. This pattern is usually interpreted as representing an anxiogenic effect. These effects were more pronounced after injection into dPAG than into IC. Freezing, the most prominent effect produced by ranitidine, was significantly inhibited by histamine as well as dimaprit. Thus, H(2) receptor blockade has fear-like action in the midbrain tectum with predominance in the dPAG. Such an action can be understood as a concomitant of defensive behavior, which has been shown to be a consequence of H(2) receptor antagonism in both dPAG and IC. The functional significance of the different effects of H(2) receptor blockade in dPAG and IC is discussed in the light of the probable distinct roles of these structures in the organization of defensive behavior.

The selective H(3) receptor agonist (R)-alpha-methylhistamine [(R)-alpha-MeHA] stimulates drinking in the adult rat. In the present study, we investigated the role of the H(3) receptor in mediating this behavior in a new dipsogenia model using the CD-1 mouse. In addition, the putative inverse agonists ciproxifan, thioperamide and clobenpropit; the reported antagonist (1R,2R)-4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]imidazole (GT-2331); and the putative neutral antagonist/weak partial agonist proxyfan were evaluated for possible differences in pharmacological activity in this new model. Water intake increased over baseline in a dose-related manner following intraperitoneal administration of 80, 160 or 240 micromol/kg (R)-alpha-MeHA, but this effect was dependent on age (P30

The effects of different histamine receptor agonists and antagonists on the nociceptive threshold were investigated in mice by two different kinds of noxious stimuli: thermal (hot plate) and chemical (acetic acid-induced abdominal writhing). Intracerebroventricular (icv) injection of the histamine H(1) receptor agonist, HTMT (6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl) heptanecarboxamide) (50 microg/mouse), produced a hypernociception in the hot plate and writhing tests. Conversely, intraperitoneal (ip) injection of dexchlorpheniramine (30 and 40 mg/kg) and diphenhydramine (20 and 40 mg/kg) increased the pain threshold in both tests. The histamine H(2) receptor agonist, dimaprit (50 and 100 microg/mouse icv), or antagonist, ranitidine (50 and 100 microg/mouse icv), raised the pain threshold in both hot plate and writhing tests. In the mouse hot plate test, the histamine H(3) receptor agonist, imetit (50 mg/kg ip), reduced the pain threshold, while the histamine H(3) receptor antagonist, thioperamide (10 and 20 mg/kg ip), produced an antinociception. The hypernociceptive effects of HTMT and imetit were antagonized by dexchlorpheniramine (20 mg/kg ip) and thioperamide (5 mg/kg ip), respectively. The results suggest that histaminergic mechanisms may be involved in the modulation of nociceptive stimuli.

We have studied the Ca(2+)-dependence and wortmannin-sensitivity of the initial inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) response induced by activation of either histamine or muscarinic receptors in smooth muscle from guinea pig urinary bladder. Activation of H(1) receptors with histamine (100 microM) produced a significant elevation in Ins(1,4,5)P(3) levels with only 5s stimulation and in the presence of external Ca(2+). However, this response was abolished fully by either the prolonged absence of external Ca(2+) or the depletion of internal Ca(2+) stores with thapsigargin (100nM) or ryanodine (10 microM). In contrast, the same conditions only slightly reduced the initial Ins(1,4,5)P(3) response induced by carbachol. The prolonged incubation of smooth muscle in 10 microM wortmannin to inhibit type III PI 4-kinase abolished both the early histamine-evoked Ins(1,4,5)P(3) and Ca(2+) responses. Conversely, wortmannin did not alter Ca(2+) release induced by carbachol, despite a partial reduction of its Ins(1,4,5)P(3) response. Collectively, these data indicate that the detectable histamine-induced increase in Ins(1,4,5)P(3) is more the consequence of Ca(2+) release from internal stores than a direct activation of phospholipase C by H(1) receptors. In addition, the effect of wortmannin implies the existence of a Ca(2+)-dependent amplification loop for the histamine-induced Ins(1,4,5)P(3) response in smooth muscle., (Copyright 2002, Elsevier Science Ltd. All rights reserved.)

Effects of substances that are able to alter the histamine level, a histamine H(1)-receptor agonist and antagonist, and a histamine H(2)-receptor agonist were investigated in an anxiety-like state in mice by means of the light/dark box test. Diazepam was used as positive control. The histamine H(3)-receptor antagonist, thioperamide (2, 5, and 20 mg/kg s.c.), showed an anxiogenic-like effect that reached a maximum with the dosage of 5 mg/kg. The histamine-N-methyltransferase (HMT) inhibitor, metoprine (5 and 20 mg/kg s.c.), also decreased the time in the light at the highest dose used and, likewise, the highly selective histamine H(1)-receptor agonist, 2-(3-trifluoromethylphenyl)histamine (FMPH) (2.65 and 6.5 microg/mouse, i.c.v.). On the contrary, the histamine H(2)-receptor agonist, impromidine (3, 10, 20, and 30 microg/mouse, i.c.v.), dose-dependently showed an anxiolytic-like effect. The selective histamine H(1) antagonist, pyrilamine (20 mg/kg i.p.) was able to prevent the anxiogenic-like effect of FMPH significantly, and that of thioperamide partially, while the effect caused by metoprine remained unvaried. It is suggested that the histaminergic system modulates anxiety-like states via the activation of both postsynaptic receptors in a contrasting manner: activation of the H(1) receptor causes an anxiogenic-like effect, while that of the H(2) receptors reduces anxiousness. However, on the basis of effects observed with the substances capable of releasing endogenous histamine, it seems likely that the anxiogenic-like effect is prevalent.

Histamine is important in mediating peripheral sensory information such as inflammation, allergic hypersensitivity, and itch. In the present study, using video-enhanced microscopy, we investigated the effect of histamine on axonal transport in cultured dorsal root ganglion (DRG) neurons of the mouse. Application of histamine (100 microM) reversibly reduced the number of particles transported within neurites in both anterograde and retrograde directions. The histamine H(1)-receptor agonist 2-thiazolylethylamine (100 microM) and the H(3)-receptor agonist R-alpha-methylhistamine (100 microM) also reduced anterograde and retrograde axonal transport, whereas the histamine H(2)-receptor agonist dimaprit (100-1000 microM) had no effect. The effect of histamine was partially blocked by pretreatment with H(1)-receptor antagonist pyrilamine (1 microM) or the H(3)-receptor antagonist thioperamide (1 microM). Pretreatment with a combination of pyrilamine (1 microM) and thioperamide (1 microM) completely blocked the response to histamine. The H(2)-receptor antagonist cimetidine (1 microM) was ineffective. These results suggest that histamine inhibits axonal transport of cultured mouse DRG neurons via the activation of H(1)- and H(3)-receptors.

Effects of histaminergic agonists and antagonists were investigated on withdrawal signs in lorazepam-dependent rats. Physical dependence was developed by giving lorazepam admixed with the food in the following dose schedule (in mg/kg given daily x days): 10 x 4, 20 x 4, 40 x 4, 80 x 4, and 120 x 7. The parameters observed during the periods of administration of lorazepam and after its withdrawal were spontaneous locomotor activity (SLA), reaction time to pain, foot shock aggression (FSA), and audiogenic seizures. During the withdrawal period, the rats were divided into groups of 10 each. Control-withdrawal group did not receive any drug. The drugs (in mg/kg administered intramuscularly)--L-histidine (50), histamine-N-methyl (2), promethazine (10), pheniramine (10), astemizole (10), and thioperamide (1)--were given separately in other groups daily during the withdrawal period. The withdrawal signs in control group were hyperkinesia, hyperaggression, and audiogenic seizures. L-Histidine, precursor of histamine, and thioperamide, antagonist of H3 receptor, potentiated hyperkinesia, hyperaggression, and audiogenic seizures. Histamine-N-methyl, agonist of H3 receptor, and H1 receptor antagonists, promethazine and pheniramine, blocked all the withdrawal signs. Astemizole, a peripheral antagonist of H1 receptor, could not affect any withdrawal sign. It may be concluded that histamine H1 receptors are facilitatory and H3 receptors are inhibitory for benzodiazepine (BZD) withdrawal syndrome.

The interaction of selective histamine H3-receptor agonist R(alpha)-methyl-histamine (RAMH) and antagonist thioperamide (THP) with some antiepileptic drugs [AED; phenytoin (PHT), carbamazepine (CBZ), sodium valproate (SVP), and gabapentin (GBP)] was studied on seizures induced by maximal electroshock (MES) and pentylenetetrazole (PTZ) in mice. It was found that subeffective dose of THP in combination with the subeffective doses of PHT and GBP provided protection against MES and/or PTZ-induced seizures. Further, RAMH reversed the protection afforded by either PHT or GBP on MES and/or PTZ seizures. In another set of experiments, the histamine content was measured in the whole brain and in different brain regions including cerebral cortex, hypothalamus, brain stem and cerebellum following convulsant (MES and PTZ) and AED treatment. It was seen that while MES exhibited a tendency to enhance brain histamine levels, PTZ showed the opposite effect. AEDs either increased (PHT and GBP) or decreased (SVP) brain histamine content in different regions to varying degrees. The results indicate a role for histamine in seizures and in the action of AEDs and suggest that selective H3-receptor antagonists may prove to be of value as adjuncts to conventional AEDs.

Two new analogues of lidocaine were synthesized at the College of Pharmacy, King Saud University: compound I (Methyl-2-[2-(N,N-diethylamino) acetamido]-3-cyano-4,5-dimethylbenzoate) and compound II (Methyl-2-[2-(piperidino) acetamido]-3-cyano-4,5-dimethylbenzoate). Their influence on the arterial blood pressure and the heart rate of urethane-anaesthetized rats was studied and compared with the actions of lidocaine. Compounds I, II and lidocaine induced significant dose-dependent decreases in the arterial blood pressure and heart rate, which usually returned to basal values within 3-5 min. There were significant differences in the potency of the three compounds in producing their effects on blood pressure and heart rate (P< 0.0001, ANOVA). Compound II was 14 and 6 times more potent in reducing blood pressure and 8 and 2 times more capable of reducing the heart rate than lidocaine and compound I, respectively. The results of this study also indicated the ineffectiveness of antagonists of autonomic, histaminergic and 5-HT receptor, and various vasodilators in blocking the actions of the three compounds on blood pressure and heart rate. Pretreatment with CaCl(2)significantly reduced the hypotension and bradycardia induced by the three compounds, suggesting the involvement of calcium channels, probably of the L type. Several possible mechanisms are postulated. In conclusion, the results direct attention to the capability of the two new compounds to decrease blood pressure and heart rate; affects that may have clinical potential., (Copyright 2001 Academic Press.)

A new histamine receptor, HH4R, was cloned from human leukocyte cDNA. The deduced amino acid sequence showed about 40% identity to that of the human histamine H3 receptor, HH3R. HH4R-expressing cells responded to histamine, inhibiting forskolin-induced cAMP accumulation. An H3 agonist, N-alpha-methylhistamine (NAMHA), bound specifically to HH4R, while another H3 agonist, R(-)-alpha-methylhistamine (RAMHA), and the H3 antagonist, thioperamide, competed with this binding. RAMHA, NAMHA, and imetit inhibited forskolin-induced cAMP accumulation in HH4R-expressing cells. However, the binding affinities and agonistic activities of H3 agonists to HH4R were weaker than those to HH3R. Low expression of HH4R was detected in a wide variety of peripheral tissues by RT-PCR; however, in contrast with HH3R, expression was not detected in the brain. These observations indicate that the clone is a distinct histamine receptor from HH3R, and thus is named HH4R., (Copyright 2000 Academic Press.)

The effect of histamine (10(-9)-10(-3) M) on horse penile dorsal artery was evaluated. Precontracted vessels showed a biphasic response (relaxation-contraction) to histamine, while at basal tone, histamine only induced a contractile effect. The H1 receptor agonist, 2-pyridylethylamine (PEA) (10(-9)-10(-3) M), induced concentration-dependent relaxation in precontracted rings and provoked vasoconstriction at basal tone. Mepyramine (10(-9)-10(8) M), an H1 receptor antagonist, competitively antagonized the relaxant response to histamine (pA2 = 9.7) and PEA (pA2 = 9.2). At basal tone, mepyramine (10(-10)-10(-8) M) also caused a rightward shift in the histamine contraction curve (pA2 = 10.1). Mepyramine (10(-9)-10(-8) M)/PEA Schild plots for resting vessels yielded a pA2 value of 9.4. A regulatory role for H2 and H3 receptors was precluded since there was no response to their agonists (dimaprit (10(-9)-10(-3) M), (R)-alpha-methylhistamine (10(-10)- 3 x 10(-4) M)), and antagonists (cimetidine (10(-5) M), thioperamide (10(-6) M)) did not affect control curves. Removal of the endothelium abolished the relaxant component causing a leftward shift in the contractile component in precontracted rings, with no effect on maximum contraction. Inhibitors of nitric oxide (NO) synthesis, L-NAME (3 x 10(-4) M) and L-NOARG (3 x 10(-4) M), modified the relaxant response while contraction was unaffected. L-Arginine (3 x 10(-4) M) potentiated maximum relaxation but did not affect contraction in precontracted rings. Effects of a prostanoid and K+ channels were ruled out. The biphasic response of precontracted vessels persisted in the presence of indomethacin (3 x 10(-6) M), tetraethylammonium (10(-3) M) and gliblenclamide (10(-5) M). L-NAME plus indomethacin, or this combination plus TEA or glibenclamide produced similar effects as isolated treatments. In resting vessels, histamine contraction was also unaffected by the lack of endothelium, or L-NAME, L-arginine or indomethacin pretreatment. The biphasic response to histamine is probably mediated by H1 receptors with a partial role for NO in the relaxant response in precontracted vessels. In the absence of tone, the contractile effect may be mediated by direct action on smooth muscle.

Hyperpigmentation frequently accompanies chronic or acute inflammation. A number of inflammatory mediators have been shown to stimulate melanin synthesis in human melanocytes. Although histamine is ubiquitous as an inflammatory factor, its involvement in pigmentation remains obscure. In this work, we examined the effects of histamine on cultured human melanocytes. Treatment of human melanocytes with 0.1-10 microM histamine evoked morphologic changes and increases in tyrosinase activity. The concomitant increases in melanin content of the histamine-treated melanocytes indicated an elevation of melanin synthesis by tyrosinase activation. These stimulatory effects of histamine were completely inhibited by an H2 antagonist, famotidine, whereas H1 and H3 antagonists had no inhibitory effect whatsoever. In addition, an H2 agonist, dimaprit, induced the same degree of melanogenesis as histamine at concentrations of 0.1-10 microM. We observed an increase in the intracellular cAMP contents of human melanocytes induced by histamine via the H2 receptors. We know that this cAMP accumulation and subsequent protein kinase A activation plays a critical role in histamine-induced melanogenesis, because a specific protein kinase A inhibitor, H-89, completely suppressed these stimulatory effects of histamine, and because dibutylic cAMP, a specific protein kinase A activator, stimulated human melanocytes as potently as histamine. Taken together, we show here that histamine induces melanogenesis of human cultured melanocytes by protein kinase A activation via H2 receptors.

Recent neuroanatomical studies have revealed a direct hypothalamocerebellar histaminergic pathway. However, the functional significance of the histaminergic fibers in the cerebellum is not yet clear. In this study, the effects of histamine on the firing of cerebellar Purkinje cells (PCs) were investigated in vitro. Histamine predominantly produced excitatory (106/111, 95.5%) and in a few cases inhibitory (5/111, 4.5%) responses in PCs. The histamine-induced excitation was not blocked by perfusing the slice with low Ca2+ high/Mg2+ medium (n = 8), supporting a direct postsynaptic action of histamine. The histamine H2 receptor antagonist ranitidine effectively blocked the excitatory response of PCs to histamine (n = 20), but triprolidine, an H1 receptor antagonist, could not significantly block the histamine-induced excitation, or only very slightly decreased the excitatory effect of histamine on the cells (n = 13). On the other hand, the highly selective H2 receptor agonist dimaprit mimicked the excitatory effect of histamine on PCs and this dimaprit-induced excitation was also blocked by ranitidine (n = 20), but not triprolidine (n = 8). However, the H1 receptor agonists betahistine and 2-thiazolylethylamine did not show any effect on the PCs (n = 9 and 14). These results reveal that histamine excites cerebellar PCs via H2 receptors and suggest that the hypothalamocerebellar histaminergic fibers may play an important role in functional activities of the cerebellum.

A series of 2-substituted thiazol-4-ylethanamines have been synthesized and tested for their histaminergic H1-receptor activities. The compounds with 2-phenyl substitution, regardless of the different physicochemical properties of the meta-substituents at the phenyl ring, showed weak H1-agonistic activity with pD2 values ranging from 4.35 to 5.36. When the phenyl group was replaced by a benzyl group, the resulting compounds all exhibited weak H1-antagonistic activity (pA2: 4.14-4.82).

The brain neuromodulator histamine induces antinociception when administered directly into the rodent CNS. However, several compounds derived from H2 and H3 antagonists also produce antinociception after central administration. Pharmacological studies have shown that a prototype of these agents, improgan, induces analgesia that is not mediated by actions on known histamine receptors. Presently, the antinociceptive properties of a compound that chemically resembles both improgan and histamine were investigated in rats. Intraventricular (i.v.t.) administration of impentamine (4-imidazolylpentylamine) induced reversible, near-maximal antinociception on the hot plate and tail flick tests (15 microg, 98 nmol). The dose-response function was extremely steep, however, since other doses showed either no effect or behavioral toxicity. On the tail flick test, impentamine antinociception was resistant to antagonism by blockers of H1, H2, or H3 receptors, similar to characteristics previously found for improgan. In contrast, histamine antinociception was highly attenuated by H1 and H2 antagonists. These findings suggest that: 1) the histamine congener impentamine may induce antinociception by a mechanism similar to that produced by improgan, and 2) additional histamine receptors may be discovered that are linked to pain-attenuating processes.

The time- and dose-related effects of exogenous histamine on spontaneous motor activity and receptors involved were evaluated in male rats. Intracerebroventricular administration of histamine (5.4 and 54.3 nmol) produced a biphasic effect with initial transitory hypoactivity and later hyperactivity expressed by locomotion frequency in an open-field. The rearing frequencies were only reduced by all doses of histamine used. The histamine-induced hypoactivity was inhibited by the H3-antagonist thioperamide and was also induced by the H3-agonist N-alpha-methylhistamine. The histamine-induced hyperactivity phase was blocked by the H1-antagonist mepyramine. The H2-antagonist ranitidine increased locomotion and rearing frequencies. The participation of other neurotransmitters in the persistent hypokinetic effect induced by 135.8 nmol of histamine was determined by HPLC in the striatum and hypothalamus as counter-proof. A decreased DOPAC/DA ratio was observed only in the striatum. In the hypothalamus, low levels of 5HT were detected, probably not correlated with motor activity. In conclusion, the present results suggest that the exogenous histamine-induced hypoactivity response is probably due to activation of H3-receptors as heteroreceptors reducing the activity of the striatal dopaminergic system. This effect can partially overlap with the expression of the hyperactivity induced by H1-receptor activation. The participation of H2-receptors requires further investigation.

The influence of pretreatment with (R)-alpha-methylhistamine, selective agonist of histamine H3 receptors, has been investigated on gastric mucosal lesions at different time intervals, from 5 to 60 minutes, after administration of absolute ethanol in the rat. The amount and depth of lesions were quantitatively evaluated by light microscopy. In rats pretreated with (R)-alpha-methylhistamine, the integrity of the mucosa was preserved in approximately 80% of the total mucosal length measured despite ethanol challenge. Prevention of lesion formation was as great at 5 min after ethanol administration as at 60 min. When present, damage involved mainly superficial mucosa and lesions extending deeply into the gland region were evident in 1-2.5% of the total mucosa. Present findings indicate that mechanisms by which (R)-alpha-methylhistamine operates enable the mucosa to counteract damage just from the moment of exposure to ethanol and that protection is exerted both on surface and pit cells and on gastric glands.

To characterize the mechanism of the analgesic action of H1 antihistaminics the effects of a new, highly selective agonist, 2-(3-trifluoromethylphenyl)histamine dihydrogenmaleate (FMPH), and of the better known H1 agonist, 2-thiazolylethylamine (2-TEA), were studied on pain threshold by means of three different kinds of tests for nociception (mouse hot plate and abdominal constriction, and rat paw pressure tests). Low doses of both substances (2.65 and 6.5 microg/animal i.c.v. for FMPH in the hot plate and paw pressure tests, and 0.3 microg/rat i.c.v. for 2-TEA in the paw pressure test) were slightly but significantly hypernociceptive. The selective H1 receptor antagonist, pyrilamine maleate (10-30 mg/kg s.c.), induced a dose-dependent antinociception in all three tests, and both FMPH and 2-TEA prevented its effect, but not that of morphine, thus indicating action on H1 receptors. The same low doses of FMPH were also able to enhance animals' spontaneous motility and curiosity. High doses of FMPH (13.23-132.3 microg/mouse i.c.v.) raised the pain threshold, but due to the severe motor impairment evidenced by the rota rod test, this cannot be considered as real antinociception. An increase in the pain threshold lacking any motor impairment was observed for tenfold higher doses of 2-TEA (3 and 10 microg/mouse i.c.v.). This may be due to action on H2 receptors, with the reported relative potency of 2-TEA for H1 and H2 receptors being about 12:1. It is therefore suggested that H1 receptor activation increases sensitivity to noxious stimuli.

Activation of histamine H3 receptors by histamine (0.1 to 10 microM), (R)alpha-methylhistamine and N(alpha)-methylhistamine (0.01 to 0.3 microM) was shown to inhibit cholinergic nerve transmission in the guinea-pig ileum. Iodoaminopotentidine (IAP 300 nM), a potent H2 receptor antagonist, was found to decrease this effect but had no significant effect (P>0.05) on contractile responses produced by exogenous acetylcholine (0.2 microM). Dimaprit (0.1 to 10 microM) an H2 receptor agonist/H3 receptor antagonist, produced no significant effect (P>0.05) on the response to cholinergic nerve stimulation but reduced the effect of N(alpha)-methylhistamine. Furthermore, ranitidine (10 microM) an H2 receptor antagonist did not modify the inhibitory effect of histamine. These results suggest that IAP may inhibit H3 receptors in the ileum at similar concentrations reported to inhibit H2 receptors in functional studies.