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This study attempted to clarify the role of histamine H 1 receptors in epilepsy by exploring the effects of agonists and inverse agonists on the rundown of the current induced by iterative applications of NMDA or GABA in primary neuronal culture. Mepyramine, a classical H 1 -receptor antagonist/inverse agonist, increased the NMDA current by about 40% during the first minutes of recording. This effect was concentration-dependent, maximal at 10 nM, and mimicked by triprolidine, another antagonist/inverse agonist. No endogenous histamine was detected in the cultures by a selective immunoassay; both compounds were acting as inverse agonists. Indicating a high constitutive activity of the H 1 receptor in this system, histamine did not affect the NMDA rundown, including its settlement, but significantly reversed the effect of mepyramine. A similar pattern was obtained with 2,3 bromophenyl histamine, a selective H 1 -receptor agonist. The initial increase induced by the two inverse agonists was followed by the same rundown as in controls. H 1 - and NMDA receptors are colocalized in most cultured neuronal cells. Mepyramine and histamine did not affect the GABA rundown. Our findings suggest an interaction between H 1 - and NMDA receptors. Inactivation of the H 1 -receptor by its inverse agonists delays the settlement of the NMDA rundown, which may underlie their proconvulsant effect reported in clinics. Therefore, H 1 -receptor constitutive activity and the effect of histamine revealed in its absence, tend to facilitate the initiation of the rundown, which is consistent with the anticonvulsant properties of histamine via activation of H 1 -receptors reported in many studies., (© 2024 The Author(s). Pharmacology Research & Perspectives published by British Pharmacological Society and American Society for Pharmacology and Experimental Therapeutics and John Wiley & Sons Ltd.)

Despite numerous studies investigating histamine and its receptors, the impact of histamine protonation states on binding to the histamine H1-receptor (H1R) has remained elusive. Therefore, we assessed the influence of different histamine tautomers (τ-tautomer, π-tautomer) and charge states (mono- vs. dicationic) on the interaction with the ternary histamine-H1R-G q complex. In atomistic molecular dynamics simulations, the τ-tautomer formed stable interactions with the receptor, while the π-tautomer induced a rotation of the histamine ring by 180° and formed only weaker hydrogen bonding interactions. This suggests that the τ-tautomer is more relevant for stabilization of the active ternary histamine-H1R-G q complex. In addition to the two monocationic tautomers, the binding of dicationic histamine was investigated, whose interaction with the H1R had been observed in a previous experimental study. Our simulations showed that the dication is less compatible with the ternary histamine-H1R-G q complex and rather induces an inactive conformation in the absence of the G q protein. Our data thus indicate that the charge state of histamine critically affects its interactions with the H1R. Ultimately these findings might have implications for the future development of new ligands that stabilize distinct H1R activation states.

Histamine has been implicated in urinary bladder dysfunction as an inflammatory mediator driving sensory nerve hypersensitivity. However, the direct influence of histamine on smooth muscle has not been thoroughly investigated. We hypothesized that histamine directly contracts urinary bladder smooth muscle (UBSM) independent of effects on nerves. Single cell quantitative RT-PCR determined that only histamine H 1 and H 2 receptors were expressed on UBSM cells. In isolated tissue bath experiments, histamine (200 µM) caused a highly variable and rapidly desensitizing contraction that was completely abolished by the H 1 receptor antagonist fexofenadine (5 µM) and the G q/11 inhibitor YM254890 (1 µM). Neither the muscarinic receptor antagonist atropine (1 µM), the Na + channel blocker tetrodotoxin (1 µM), nor the transient receptor potential vanilloid type 1 antagonist capsazepine (10 µM) altered responses to histamine, suggesting that nerve activation was not involved. UBSM desensitization to histamine was not due to receptor internalization, as neither the cholesterol-depleting agent methyl-β-cyclodextrin (10 mM), the dynamin-mediated endocytosis inhibitor dynasore (100 µM), nor the clathrin-mediated endocytosis inhibitor pitstop2 (15 µM) augmented or prolonged histamine contractions. Buffer from desensitized tissues still contracted histamine-naïve tissues, revealing that histamine was not metabolized. Prolonged exposure to histamine also had no effect on contractions due to electrical field stimulation, suggesting that both efferent nerve and UBSM excitability were unchanged. Together, these data suggest that histamine, although able to transiently contract UBSM, does not have a lasting effect on UBSM excitability or responses to efferent nerve input. Thus, any acute effects of histamine directly on UBSM contractility are unlikely to alter urinary bladder function. NEW & NOTEWORTHY Histamine is commonly associated with inflammatory bladder pathologies. We sought to investigate the role of histamine on urinary bladder contractility. Histamine contracts the bladder, but this response is highly variable and desensitizes completely in minutes. This desensitization is not due to internalization of the receptor or metabolism of histamine. Because nerve-evoked contractions are also not increased in the presence of histamine, our findings suggest that histamine is not directly acting to change contractility.

A decreased H 1 receptor activity is observed in the anterior cingulate cortex (aCgCx) of depressed patients. The role of this abnormality in the development of depression-related processes is unstudied. We examined the influence of a decreased brain H 1 receptor activity on rat behavior in the sucrose preference test. The H 1 receptor deficit was simulated by injection of an H 1 antagonist into the aCgCx; also, two aCgCx projection areas, lateral and medial entorhinal cortices were examined. A blockade of H 1 -receptors in the aCgCx and lateral entorhinal cortex (LEntCx) significantly reduced sucrose preference. These findings suggest the existence of H 1 receptor-mediated aCgCx-LEntCx circuitry mechanism regulating anhedonic-like behavior in rats. The presented data suggest that H 1 receptor-mediated processes might be a therapeutic target in depressive disorders., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Histamine H 1 receptor ligands used clinically as antiallergics rank among the most widely prescribed and over-the-counter drugs in the world. They exert the therapeutic actions by blocking the effects of histamine, due to null or negative efficacy towards Gα q -phospholipase C (PLC)-inositol triphosphates (IP 3 )-Ca 2+ and nuclear factor-kappa B cascades. However, there is no information regarding their ability to modulate other receptor responses. The aim of the present study was to investigate whether histamine H 1 receptor ligands could display positive efficacy concerning receptor desensitization, internalization, signaling through Gα q independent pathways or even transcriptional regulation of proinflammatory genes. While diphenhydramine, triprolidine and chlorpheniramine activate ERK1/2 (extracellular signal-regulated kinase 1/2) pathway in A549 cells, pre-treatment with chlorpheniramine or triprolidine completely desensitize histamine H 1 receptor mediated Ca 2+ response, and both diphenhydramine and triprolidine lead to receptor internalization. Unlike histamine, histamine H 1 receptor desensitization and internalization induced by antihistamines prove to be independent of G protein-coupled receptor kinase 2 (GRK2) phosphorylation. Also, unlike the reference agonist, the recovery of the number of cell-surface histamine H 1 receptors is a consequence of de novo synthesis. On the other hand, all of the ligands lack efficacy regarding cyclooxygenase-2 (COX-2) and interleukin-8 (IL-8) mRNA regulation. However, a prolonged exposure with each of the antihistamines impaires the increase in COX-2 and IL-8 mRNA levels induced by histamine, even after ligand removal. Altogether, these findings demonstrate the biased nature of histamine H 1 receptor ligands contributing to a more accurate classification, and providing evidence for a more rational and safe use of them., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The inflammatory reaction influences the deposition of collagen within wound granulation tissue. The aim of the present study is to determine whether histamine acting directly on myofibroblasts derived from wound granulation tissue may influence collagen deposition. It also identifies the histamine receptor involved in this process. The experiments were carried out on cells isolated from the granulation tissue of a wound model (a polypropylene net inserted subcutaneously to rats) or intact rat skin. Collagen content was measured following the addition of different concentrations of histamine and treatment with histamine receptor antagonists (ketotifen - H1 inhibitor, ranitidine - H2 inhibitor) and a histamine receptor H1 agonist (2-pyridylethylamine dihydrochloride).The cells were identified as myofibroblasts: alpha-smooth muscle actin, vimentin, and desmin positive in all experimental conditions. Histamine increased the collagen level within both cell cultures, i.e., those isolated from granulation tissue or intact skin. It did not, however, influence the expression of either the collagen type I or III genes within the cultured myofibroblasts. Histamine activity was reduced by ketotifen (the H1 receptor inhibitor) and increased by the H1 receptor agonist, as demonstrated by changes in the levels of collagen in the myofibroblast culture. Histamine increased collagen content within the cultures, acting directly on myofibroblasts via H1 receptor stimulation.

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: Stroke is associated with cerebral ischemia/reperfusion (I/R) injury. Ischemic postconditioning (IPoC) reduces cerebral ischemic injury in rats and offers neuroprotection. The central histaminergic pathway possesses a crucial role in the pathogenesis of cerebral I/R, but its neuroprotective role in IPoC is still unidentified., Objective: This research explored the role of the histaminergic in IPoC during cerebral I/R injury in the rat., Methods: Global cerebral ischemia/reperfusion (GCI/R) injury in Wistar albino rats was induced by occluding the bilateral carotid arteries for 10 minutes, followed by reperfusion. IPoC was provided by giving three episodes of I/R post GCI (10 min), after which of reperfusion was permitted. Inclined- beam-walk, hanging-wire, lateral-push, and rota-rod tests were employed to assess motor functions, and Morris water maze (MWM) was used to assess spatial learning as well as memory in animals. Cerebral oxidative markers (thiobarbituric acid reactive species-TBARS, reduced glutathione- GSH), inflammatory markers (myeloperoxidase-MPO), acetylcholinesterase activity- AChE, infarct size, and histopathological changes were also assessed. L-histidine and chlorpheniramine were used as histaminergic agonists and antagonists., Results: I/R animals showed a reduction in memory and motor function, and an increase in cerebral oxidative stress, inflammation, AChE activity, infarct size and histopathological changes. Episodes of IPoC post-ischemia attenuated the deleterious effects of I/R injury. Pretreatment (30 min before cerebral ischemia) with L-histidine mimicked the neuroprotective effects of IPoC. However, neuroprotection produced by IPoC was abolished by pretreatment with chlorpheniramine (histaminergic- H 1 receptor antagonist)., Conclusion: IPoC may provide neuroprotection against cerebral I/R induced brain injury by modulating the histaminergic-H1-receptor pathway., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

The mechanisms underlying bladder contractile disorders such as overactive bladder are not fully understood, and there is limited understanding of the receptor systems modulating spontaneous bladder contractions. We investigated the potential for histamine to have a role in mediating contractility of the urothelium with lamina propria (U&LP) or detrusor via the H1-H4 histamine receptor subtypes. Isolated strips of porcine U&LP or detrusor smooth muscle were mounted in gassed Krebs-bicarbonate solution and responses to histamine obtained in the absence and presence of selective receptor antagonists. The presence of histamine increases the frequency of U&LP spontaneous phasic contractions and baseline tensions. In response to histamine, H1-antagonists pyrilamine, fexofenadine and cyproheptadine were effective at inhibiting contractile responses. Cimetidine (H2-antagonist) enhanced increases in baseline tension in response histamine, whereas amthamine (H2-agonist) induced relaxation. Although thioperamide (H3/H4-antagonist) increased baseline tension responses to histamine, selective H1/H2-receptor antagonism revealed no influence of these receptors. In detrusor preparations, pyrilamine, fexofenadine and cyproheptadine were effective at inhibiting baseline tension increases in response to histamine. Our findings provide evidence that histamine produces contractile responses both in the U&LP and detrusor via the H1-receptor, and this response is significantly inhibited by activation of the H2-receptor in the U&LP but not the detrusor.

The ventral pallidum (VP) is a crucial component of the limbic loop of the basal ganglia and participates in the regulation of reward, motivation, and emotion. Although the VP receives afferent inputs from the central histaminergic system, little is known about the effect of histamine on the VP and the underlying receptor mechanism. Here, we showed that histamine, a hypothalamic-derived neuromodulator, directly depolarized and excited the GABAergic VP neurons which comprise a major cell type in the VP and are responsible for encoding cues of incentive salience and reward hedonics. Both postsynaptic histamine H1 and H2 receptors were found to be expressed in the GABAergic VP neurons and co-mediate the excitatory effect of histamine. These results suggested that the central histaminergic system may actively participate in VP-mediated motivational and emotional behaviors via direct modulation of the GABAergic VP neurons. Our findings also have implications for the role of histamine and the central histaminergic system in psychiatric disorders.

The present study elucidated the role of histamine H 1 receptor in the caffeine induced locomotor sensitization. Intermittent administration of caffeine (15 mg/kg, i.p.) on alternate days (induction phase) i.e. 1 st , 3 rd , 5 th , 7 th , 9 th , 11 th and 13 th resulted in the development of locomotor sensitization. In addition, challenge with sub-stimulant dose of caffeine (10 mg/kg, i.p.) directly on 17 th day to induction group animals resulted in expression to locomotor sensitization to caffeine. I.c.v. injection of histaminergic agents concomitantly with caffeine during induction phase i.e. histamine H 1 receptor agonist, FMPH (6.5 μg/mouse) significantly potentiated while H 1 receptor antagonist, cetirizine (0.1 μg/mouse) attenuated the locomotor sensitization induced by caffeine (15 mg/kg, i.p.). In addition, challenge with caffeine (10 mg/kg, i.p.) on the expression day (17 th ) to the induction group mice on FMPH + caffeine treatment showed enhanced, while those on cetirizine + caffeine treatment exhibited lesser expression to locomotor sensitization. Therefore, a possible contributory role of the central histaminergic system via H 1 receptor stimulation or up-regulation in the caffeine-induced locomotor sensitizing effect is proposed., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The affinity of ligands for G-protein-coupled receptors (GPCRs) is allosterically regulated by Na + via a highly conserved aspartate residue (Asp 2.50 ) in the second transmembrane domain of GPCRs. In the present study, we examined the Na + -mediated regulation of the affinity of ligands for G q/11 -protein-coupled human histamine H 1 receptors in Chinese hamster ovary cells. The affinities of 3 agonists and 20 antihistamines were evaluated by their displacement curves against the binding of [ 3 H]-mepyramine to membrane preparations in the presence or absence of 100mM NaCl. The affinities of most drugs including histamine, an agonist, and d-chlorpheniramine, a first-generation antihistamine, were reduced by NaCl, with the extent of NaCl-mediated changes varying widely between drugs. In contrast, the affinities of some second-generation antihistamines such as fexofenadine were increased by NaCl. These changes were retained in intact cells. The mutation of Asp 2.50 (Asp73) to asparagine abrogated NaCl-induced reductions in affinities for histamine and d-chlorpheniramine, but not NaCl-induced increases in the affinity for fexofenadine. Quantitative structure-activity relationship (QSAR) analyses showed that these Na + -mediated changes were explained and predicted by a combination of the molecular energies and implicit solvation energies of the compounds. These results suggest that Na + diversely regulates the affinity of ligands for H 1 receptors from the extracellular sites of receptors via Asp73-dependent and -independent mechanisms in a manner that depends on the physicochemical properties of ligands. These results may contribute to a deeper understanding of the fundamental mechanisms by which the affinity of ligands for their receptors is allosterically regulated by Na + ., (Copyright © 2016 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.)

It has been suggested that the agonist-induced internalization of G-protein-coupled receptors from the cell surface into intracellular compartments regulates cellular responsiveness. We previously reported that G q/11 -protein-coupled human histamine H 1 receptors internalized via clathrin-dependent mechanisms upon stimulation with histamine. However, the molecular determinants of H 1 receptors responsible for agonist-induced internalization remain unclear. In this study, we evaluated the roles of the intracellular C-terminal of human histamine H 1 receptors tagged with hemagglutinin (HA) at the N-terminal in histamine-induced internalization in Chinese hamster ovary cells. The histamine-induced internalization was evaluated by the receptor binding assay with [ 3 H]mepyramine and confocal immunofluorescence microscopy with an anti-HA antibody. We found that histamine-induced internalization was inhibited under hypertonic conditions or by pitstop, a clathrin terminal domain inhibitor, but not by filipin or nystatin, disruptors of the caveolar structure and function. The histamine-induced internalization was also inhibited by truncation of a single amino acid, Ser487, located at the end of the intracellular C-terminal of H 1 receptors, but not by its mutation to alanine. In contrast, the receptor-G-protein coupling, which was evaluated by histamine-induced accumulation of [ 3 H]inositol phosphates, was potentiated by truncation of Ser487, but was lost by its mutation to alanine. These results suggest that the intracellular C-terminal of human H 1 receptors, which only comprises 17 amino acids (Cys471-Ser487), plays crucial roles in both clathrin-dependent internalization of H 1 receptors and G-protein signaling, in which truncation of Ser487 and its mutation to alanine are revealed to result in biased signaling toward activation of G-proteins and clathrin-mediated internalization, respectively., (© 2016 International Society for Neurochemistry.)

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.)

Histamine regulates release of neurotransmitters such as dopamine, serotonin, gamma-aminobutyric acid (GABA), glutamate and also is involved in several functions in central nervous system (CNS). It has been shown that histamine participates in disorders like seizure. It has been well documented that morphine dose-dependently induces anti or proconvulsant effects. In the current study, we firstly showed that morphine (1mg/kg) exerts anticonvulsant effects which significantly reversed by naltrexone administration. Secondly, we determined seizure threshold for H1 and H3 receptors agonists and antagonists in mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. Our results showed that activation of H1 receptors by 2-(2-Pyridyl)-ethylamine exerts anticonvulsant properties while inhibition of H1 receptors by pyrilamine maleate induced proconvulsant effects. Furthermore, we showed that immepip dihydrobromide, a H3 receptor agonist, increased seizure susceptibility to PTZ whereas thioperamide, a H3 receptor antagonist increased seizure threshold. We also revealed that pretreatment with morphine potently reversed the effects of histaminergic system on seizure threshold suggesting the involvement of opioid system in alteration of seizure threshold by histaminergic drugs., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Histamine and opioid systems are involved in supraspinal modulation of pain. In this study, we investigated the effects of separate and combined microinjections of agonists and antagonists of histamine H1 and H2 and opioid receptors into the thalamic submedius (Sm) nucleus on the formalin-induced orofacial pain. Two guide cannulas were implanted into the right and left sides of the Sm in ketamine- and xylazine-anesthetized rats. Orofacial formalin pain was induced by subcutaneous injection of a diluted formalin solution (50 μl, 1.5%) into the vibrissa pad. Face rubbing durations were recorded at 3-min blocks for 45 min. Formalin produced a biphasic pain response (first phase: 0-3 min and second phase: 15-33 min). Separate and combined microinjections of histamine H1 and H2 receptor agonists, 2-pyridylethylamine (2-PEA) and dimaprit, respectively, and opioid receptor agonist, morphine, attenuated the second phase of pain. The analgesic effects induced by 2-PEA, dimaprit, and morphine were blocked by prior microinjections of fexofenadine (a histamine H1 receptor antagonist), famotidine (a histamine H2 receptor antagonist), and naloxone (an opioid receptor antagonist), respectively. Naloxone also prevented 2-PEA- and dimaprit-induced antinociception, and the analgesic effect induced by morphine was inhibited by fexofenadine and famotidine. These results showed the involvement of histamine H1 and H2 and opioid receptors in the Sm modulation of orofacial pain. Opioid receptor might be involved in analgesia induced by activation of histamine H1 and H2 receptors and vice versa.

Aim: The nucleus tractus solitarii (NTS) is a central brainstem structure that plays an important role in regulating cardiovascular homeostasis. In this study, we examined whether H1 receptors in the NTS can control the baroreceptor reflex function by modulating synaptic transmission., Methods: Cardiac baroreceptor reflex function was assessed before and after the microinjection of 2-pyridylethylamine (10-25 nmol), a histamine H1 receptor-specific agonist, into the NTS of urethane-anaesthetized Wistar rats. The cardiovascular responses induced by l-glutamate microinjection into the NTS were also examined before and after the NTS administration of 2-pyridylethylamine., Results: Nucleus tractus solitarii microinjections of 2-pyridylethylamine significantly inhibited the gain of the cardiac baroreceptor reflex and bradycardiac/depressor responses induced by l-glutamate microinjection into the NTS. These findings suggest that histamine H1 receptors regulate the cardiac baroreceptor reflex in a post-synaptic manner to inhibit barosensitive NTS neurons., Conclusion: Taken together with our previous findings, the present results provide further evidence that histamine may play a role within the NTS in regulating cardiovascular homeostasis., (© 2014 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)

Background and Purpose: Histamine H1 receptors are highly expressed in hypothalamic neurons and mediate histaminergic modulation of several brain-controlled physiological functions, such as sleep, feeding and thermoregulation. In spite of the fact that the mouse is used as an experimental model for studying histaminergic signalling, the pharmacological characteristics of mouse H1 receptors have not been studied. In particular, selective and potent H1 receptor agonists have not been identified., Experimental Approach: Ca(2+) imaging using fura-2 fluorescence signals and whole-cell patch-clamp recordings were carried out in mouse preoptic/anterior hypothalamic neurons in culture., Key Results: The H1 receptor antagonists mepyramine and trans-triprolidine potently antagonized the activation by histamine of these receptors with IC50 values of 0.02 and 0.2 μM respectively. All H1 receptor agonists studied had relatively low potency at the H1 receptors expressed by these neurons. Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine had full-agonist activity with potencies similar to that of histamine. In contrast, 2-pyridylethylamine and betahistine showed only partial agonist activity and lower potency than histamine. The histamine receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl)heptanecarboxamide (HTMT) had no agonist activity at the H1 receptors H1 receptors expressed by mouse preoptic/anterior hypothalamic neurons but displayed antagonist activity., Conclusions and Implications: Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine were identified as full agonists of mouse H1 receptors. These results also indicated that histamine H1 receptors in mice exhibited a pharmacological profile in terms of agonism, significantly different from those of H1 receptors expressed in other species., (© 2013 The British Pharmacological Society.)

G protein-coupled receptor signaling does not result from sequential activation of a linear pathway of proteins/enzymes, but rather from complex interactions of multiple, branched signaling routes, i.e., signaling networks. In this work we present an exhaustive study of the cross-talk between H1 and H2 histamine receptors (H1R and H2R) in U937 cells and Chinese hamster ovary-transfected cells. By desensitization assays we demonstrated the existence of a crossdesensitization between both receptors independent of protein kinase A or C. H1R-agonist stimulation inhibited cell proliferation and induced apoptosis in U937 cells following treatment of 48 hours. H1R-induced antiproliferative and apoptotic response was inhibited by an H2R agonist suggesting that the cross-talk between both receptors modifies their function. Binding and confocal microscopy studies revealed cointernalization of both receptors upon treatment with the agonists. To evaluate potential heterodimerization of the receptors, sensitized emission fluorescence resonance energy transfer experiments were performed in human embryonic kidney 293T cells using H1R-cyan fluorescent protein and H2R-yellow fluorescent protein. To our knowledge these findings may represent the first demonstration of agonist-induced heterodimerization of the H1R and H2R. In addition, we also show that the inhibition of the internalization process did not prevent receptor crossdesensitization, which was mediated by G protein-coupled receptor kinase 2. Our study provides new insights into the complex signaling network mediated by histamine and further knowledge for the rational use of its ligands.

By using brain slice preparations and extracellular recordings, the effect of histamine on spontaneous firing activities of neurons in the inferior vestibular nucleus (IVN), a key structure responsible for integration of vestibular, multisensory, and cerebellar inputs, in rats was investigated. Perfusing slices with histamine (1-10μM) elicited an excitatory response on IVN neurons. The responses were not blocked by low Ca(2+)/high Mg(2+) medium, indicating a direct postsynaptic effect of the amine. Furthermore, the histamine-induced excitation was partially blocked by selective histamine H1 receptor antagonist mepyramine (1μM) and H2 receptor antagonist ranitidine (1μM), respectively. Co-application of mepyramine and ranitidine nearly totally antagonized the histamine-induced excitation. Additionally, both selective H1 receptor agonist 2-pyridylethylamine (30-300μM) and H2 receptor agonist dimaprit (10-100μM) effectively mimicked the excitatory action of histamine on IVN neurons. Moreover, selective H4 antagonist JNJ7777120 (10μM) and agonist VUF8430 (30-300μM) had no effect on IVN neurons. These results demonstrate that histamine excites IVN neurons via postsynaptic H1 and H2 rather than H4 receptors, and suggest that the central histaminergic system actively modulate all four major vestibular nuclei including the IVN and may subsequently influence the vestibular nuclei-related reflexes and functions., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)

Antihistamines are thought to antagonize histamine and prevent it from binding to the histamine H1 receptor (H1R). However, recent studies indicate that antihistamines are classified into two groups, i.e., inverse agonists and neutral antagonists on the basis of their ability to down-regulate the constitutive activity of H1R. As H1R is an allergy-sensitive gene whose expression influences the severity of allergic symptoms, inverse agonists should more potently alleviate allergic symptoms than neutral antagonists by inhibiting H1R constitutive activity. Therefore, it is important to assess inverse agonistic activity of antihistamines. Here we report a novel assay method using HeLa cells expressing H1R endogenously for evaluation of inverse agonistic activity of antihistamines. Pretreatment with inverse agonists down-regulated H1R gene expression below to its basal level. On the other hand, basal H1R mRNA expression was unchanged by neutral antagonist pretreatment. Both inverse agonists and neutral antagonists suppressed histamine-induced H1R mRNA elevation. Classification of antihistamines on the basis of their suppressive activity of basal H1R gene expression was consistent with that of inositol phosphate accumulation in H1R-overexpressed cells. Our data suggest that the assay method using HeLa cells is more convenient and useful than the existing methods and may contribute to develop new antihistamines with inverse agonistic activity., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Background: Histamine exerts diverse effects on immune regulation through four types of histamine receptors (HRs). Among them, type 1 receptor (H1R) plays an important role in allergic inflammation. Dendritic cells (DCs), which express at least three types of HRs, are professional antigen-presenting cells controlling the development of allergic inflammation. However, the molecular mechanisms involved in H1R-mediated NF-ĸB signaling of DCs remain poorly defined., Methods: Bone-marrow (BM)-derived DCs (BM-DCs) were treated with H1R inverse agonists to interrupt basal H1R-mediated signaling. The crosstalk of H1R-mediated signaling and the NF-ĸB pathway was examined by NF-ĸB cellular activity using a luciferase reporter assay, NF-ĸB subunit analysis using Western blotting and TNF-α promoter activity using chromatin immunoprecipitation., Results: Blockage of H1R signaling by inverse agonists significantly inhibited TNF-α and IL-6 production of BM-DCs. H1R-specific agonists were able to enhance TNF-α production, but this overexpression was significantly inhibited by NF-ĸB inhibitor. The H1R inverse agonist ketotifen also suppressed cellular NF-ĸB activity, suggesting crosstalk between H1R and NF-ĸB signaling in DCs. After comprehensive analysis of NF-ĸB subunits, c-Rel protein expression was significantly down-regulated in ketotifen-treated BM-DCs, which led to inhibition of the promoter activity of TNF-α. Finally, adoptive transfer of the ketotifen-treated BM-DCs did not induce significant allergic airway inflammation compared to that of control cells in vivo., Conclusions: Our results suggest that c-Rel controls H1R-mediated proinflammatory cytokine production in DCs. This study provides a potential mechanism of H1R-mediated signaling and NF-ĸB pathway crosstalk in allergic inflammation., (Copyright © 2012 S. Karger AG, Basel.)

The superior vestibular nucleus (SVN), which holds a key position in vestibulo-ocular reflexes and nystagmus, receives direct hypothalamic histaminergic innervations. By using rat brainstem slice preparations and extracellular unitary recordings, we investigated the effect of histamine on SVN neurons and the underlying receptor mechanisms. Bath application of histamine evoked an excitatory response of the SVN neurons, which was not blocked by the low-Ca(2+)/high-Mg(2+) medium, indicating a direct postsynaptic effect of the amine. Selective histamine H1 receptor agonist 2-pyridylethylamine and H2 receptor agonist dimaprit, rather than VUF8430, a selective H4 receptor agonist, mimicked the excitation of histamine on SVN neurons. In addition, selective H1 receptor antagonist mepyramine and H2 receptor antagonist ranitidine, but not JNJ7777120, a selective H4 receptor antagonist, partially blocked the excitatory response of SVN neurons to histamine. Moreover, mepyramine together with ranitidine nearly totally blocked the histamine-induced excitation. Immunostainings further showed that histamine H1 and H2 instead of H4 receptors existed in the SVN. These results demonstrate that histamine excites the SVN neurons via postsynaptic histamine H1 and H2 receptors, and suggest that the central histaminergic innervation from the hypothalamus may actively bias the SVN neuronal activity and subsequently modulate the SVN-mediated vestibular functions and gaze control., (Copyright © 2012 S. Karger AG, Basel.)

Purpose: The present work was carried out to reveal the involvement of histamine receptors at the neuro-melanophore junction of teleost, Oreochromis mossambicus., Methods: The isolated scale melanophores were assayed using the mean melanophore size index and their responses were recorded in presence of various concentrations of histamine along with H(1) and H(2) receptor specific agonists and antagonist and potentiator compound 48/80., Results: Melanophores showed high sensitivity to histamine and its specific agonists. Histamine caused a dose-dependent pigment aggregation, whereas 2-(2-Pyridyl) ethylamine (PEA), a specific H(1)R agonist also caused aggregation in a similar manner. Conversely, amthamine, a specific H(2)R agonist resulted in pigment dispersion. The effects were antagonized by mepyramine; specific H(1)R antagonist and ranitidine a specific H(2)R antagonist., Conclusion: It is concluded that O. mossambicus melanophores have both H(1) and H(2) receptors which mediate melanophore aggregation and dispersion respectively. Compound 48/80 augmented the melanin-aggregating and dispersing effects of PEA and amthamine. It is suggested that the effect of histamine is directly mediated through H1 and H2 receptors, whereas H1Rs may be predominantly involved in the aggregatory responses.

Conclusion: The data suggest that histamine up-regulates MUC2 gene regulation and mucin production in airway epithelial cells through histamine 1 receptor (H1R). Histamine appears to play an important role in the early phase of mucin regulation, which might be effectively blocked by an H1R antagonist., Objective: Histamine is an important inflammatory mediator during the early phase of allergic response and antihistamine is known to have an ability to reduce mucus secretion in inflamed airways. The goal of the present study was to determine the effects of histamine on MUC2 gene expression and mucin secretion and to investigate the response to histamine 1 receptor (H1R) blocker in NCI-H292 cells and HM3-MUC2 cells., Methods: NCI-H292 cells, a human pulmonary mucoepidermoid carcinoma cell line, and HM3-MUC2 cells transfected with MUC2 promoter (-2,864/+19) pGL2 luciferase construct were used in the study. MUC2 mRNA expression was analyzed by RT-PCR for NCI-H292 cells and by luciferase assays for HM3-MUC2 cells. MUC2 protein production was determined by immunoassay and immunofluorescent stain in NCI-H292 cells., Results: Histamine increased MUC2 gene expression in a dose- and time-dependent manner. Peak response was reached at 12 h after histamine administration. MUC2 protein production was also dose-dependently increased, while it decreased with time in NCI-H292 cells. Pretreatment with histamine at a concentration of 1 mM induced MUC2 mRNAand protein production, which was equivalent to that caused by 10 µg/ml LPS, but less than that of 0.5 µM PMA. Histamine-induced MUC2 mRNA expression and mucin secretion were significantly suppressed by pretreatment with H1R antagonist.

The central histaminergic nervous system, originating from the tuberomammillary nucleus of the hypothalamus, widely innervates almost the whole brain as well as the spinal cord. However, the effect of histamine on spinal motoneurons, the final common path for motor control, is still unknown. By using 8-14-day-old rat spinal slice preparations and intracellular recordings, the effect of histamine on motoneurons in lumbar spinal cord and the underlying mechanisms were studied. Bath application of histamine (30-300 μM) induced a membrane depolarization in the majority of recorded spinal motoneurons (78/90, 86%). Perfusing slices with tetrodotoxin or low-Ca(2+) /high-Mg(2+) medium did not block the histamine-induced excitation, indicating a direct postsynaptic action of histamine on motoneurons. Separate application of the selective histamine H(1) receptor antagonist mepyramine or the selective histamine H(2) receptor antagonist ranitidine partially suppressed the histamine-induced excitation, whereas a combination of ranitidine and mepyramine totally blocked the excitatory effect of histamine on motoneurons. On the other hand, both the selective histamine H(1) receptor agonist 2-pyridylethylamine and the selective histamine H(2) receptor agonist dimaprit mimicked the excitation of histamine on spinal motoneurons. These agonist-induced excitations were also blocked by mepyramine or ranitidine. Furthermore, histamine affected membrane input resistance and potentiated repetitive firing behavior of spinal motoneurons. These results demonstrate that histamine excites rat spinal motoneurons via the histamine H(1) and H(2) receptors and increases their excitability, suggesting that the hypothalamospinal histaminergic fibers may directly modulate final motor outputs and actively regulate ongoing motor execution andspinal motor reflexes., (Copyright © 2011 Wiley Periodicals, Inc.)

Purpose: Previously, retinopetal axons containing histamine and dopaminergic neurons expressing histamine H(1)-receptor had been localized in mouse retinas using anatomic techniques. The goal of these experiments was to demonstrate that these receptors are functional., Methods: Dopaminergic cells were acutely isolated from retinas of transgenic mice expressing red fluorescent protein under control of the tyrosine hydroxylase promoter and loaded with the calcium indicator Fura-2., Results: Under control conditions, there were spontaneous oscillations in the levels of free intracellular calcium in dopaminergic cells. These oscillations were abolished in nominally calcium-free extracellular medium and in 1 μM tetrodotoxin, findings suggesting that the oscillations were mediated by calcium entry across the plasma membrane in response to sodium-dependent action potentials. Histamine increased the mean free intracellular calcium in the dopaminergic cells by increasing the frequency and/or amplitude of the calcium oscillations. The effects of histamine were dose-dependent and reached maximum at 5 μM. With this dose, there was a 65% increase in the mean free intracellular calcium concentration. The histamine H(1)-receptor antagonist, pyrilamine, blocked the effects of 5 μM histamine when applied at 50 μM. The selective histamine H(1)-receptor agonists, 2-(3-trifluoromethylphenyl) histamine and methylhistaprodifen significantly increased mean free intracellular calcium when applied at 5 μM., Conclusions: Histamine released from retinopetal axons in the mouse retina can elevate intracellular calcium levels in the perikarya of dopaminergic cells via the activation of histamine H(1)-receptors.

The preoptic area/anterior hypothalamus, a region that contains neurons that control thermoregulation, is the main locus at which histamine affects body temperature. Here we report that histamine reduced the spontaneous firing rate of GABAergic preoptic neurons by activating H3 subtype histamine receptors. This effect involved a decrease in the level of phosphorylation of the extracellular signal-regulated kinase and was not dependent on synaptic activity. Furthermore, a population of non-GABAergic neurons was depolarized, and their firing rate was enhanced by histamine acting at H1 subtype receptors. In our experiments, activation of the H1R receptors was linked to the PLC pathway and Ca(2+) release from intracellular stores. This depolarization persisted in TTX or when fast synaptic potentials were blocked, indicating that it represents a postsynaptic effect. Single-cell reverse transcription-PCR analysis revealed expression of H3 receptors in a population of GABAergic neurons, while H1 receptors were expressed in non-GABAergic cells. Histamine applied in the median preoptic nucleus induced a robust, long-lasting hyperthermia effect that was mimicked by either H1 or H3 histamine receptor subtype-specific agonists. Our data indicate that histamine modulates the core body temperature by acting at two distinct populations of preoptic neurons that express H1 and H3 receptor subtypes, respectively.

Although pulp fibroblasts are a major cell type in dental pulp, their roles in microbial recognition and pulpal inflammation are not well-understood. Considering the pivotal role of Toll-like receptors (TLRs) in the recognition of micro-organisms, we hypothesized that TLRs on pulp fibroblasts may induce inflammatory signals in dental pulp. In human pulp fibroblasts, TLR2, 3, 4, and 5 were constitutively expressed. Stimulation of TLR2 and 3 induced the expression of pro-inflammatory genes such as CXCL10, CCL5, and/or Cox-2 in pulp fibroblasts. Interestingly, histamine synergistically activated TLR2-mediated Cox-2 expression and PGE(2) production. The synergistic effect of histamine is mediated by histamine receptor-1 (H1R). Studies on the intra-cellular signaling pathways revealed that p38 activation is required for the synergistic activation of Cox-2 by TLR2 and histamine. Analysis of these data suggests that TLR2 on pulp fibroblasts, in concert with H1R, can induce an inflammatory response during microbial infection in dental pulp.

The receptor type mediating the positive inotropic effect of histamine was examined in left atria from neonatal guinea pigs. The positive inotropic effect of histamine, as well as its action potential prolonging effect, was antagonized by ranitidine, but not by chlorpheniramine or thiperamide. The positive inotropic effect was enhanced by isobutylmethylxantine. Receptor binding studies revealed the presence of both H₁ and H₂ receptor types. These results suggest that the positive inotropic effect of histamine in the neonatal guinea-pig atrium is mediated by H₂ receptors.

Histamine (HA) is an important neuro-modulator, contributing to a variety of physiological responses in the mammalian central nervous system (CNS). However there is little information about the cell/signaling mechanism underlying its role. In the present study, we characterized HA responses in single large neostriatal neurons acutely dissociated from wild type (WT) and HA receptor knock-out (KO) mice, with a particular emphasis on identifying the role of HA receptor subtypes. HA (10 microM) and a selective H(2) receptor agonist dimaprit (1 microM) both evoked an inward current in H(1)-KO mice, and HA and a selective H(1) receptor agonist HTMT (10 microM) both evoked an inward current in H(2)-KO mice. In the H(1) and H(2) double (H(1/2)) KO mice, there was no response to either the application of HA or the selective H(1), H(2) receptor agonists. Hence we have confirmed that the targeted genes were indeed absent in these KO mice and that both receptor subtypes contribute to HA's excitatory actions. Furthermore the HA-induced inward currents were mediated by a decrease in current through K(+) channels. In addition, we observed the effects of methamphetamine (METH) on the locomotor activity of WT and HA receptor KO mice, and found that METH-induced behavioral sensitization is evident in H(1/2)-KO mice, but not in H(1)- or H(2)-KO mice. These observations suggest that suppressive roles of HA on methamphetamine-induced behavioral sensitization would be mediated through both H(1) and H(2) receptors in the CNS including neostriatum.

Previous studies revealed that phenylhistamines and histaprodifens possess higher potency and affinity at guinea pig histamine H(1)-receptor (gpH(1)R) than at human histamine H(1)-receptor (hH(1)R). However, we recently identified an imidazolylpropylguanidine [N(1)-(3-cyclohexylbutanoyl)-N(2)-[3-(1H-imidazol-4-yl)-propyl]guanidine (UR-AK57)] with higher potency and efficacy at hH(1)R compared with gpH(1)R. The aim of this study was to reveal the molecular basis for the species differences of synthetic ligands. We studied 11 novel phenylhistamines and phenoprodifens. H(1)R species isoforms were expressed in Sf9 insect cells, and [(3)H]mepyramine competition binding and GTPase assays were performed. We identified bulky phenylhistamines with higher potency and affinity at hH(1)R compared with gpH(1)R. Molecular dynamics simulations of ligand-H(1)R interactions revealed four potential binding modes for phenylhistamines possessing an additional histamine moiety; the terminal histamine moiety showed a high flexibility in the binding pocket. There are striking similarities in ligand properties in bulky phenylhistamines and UR-AK57. Comparison of bulky phenylhistamine binding mode with binding mode of UR-AK57 suggests that only one of these four binding modes should be established. The higher potency is explained by more effective van der Waals interaction of the compounds with Asn(2.61) (hH(1)R) relative to Ser(2.61) (gpH(1)R). In addition, two stable binding modes for phenoprodifens with different orientations in the binding-pocket were identified. Depending on phenoprodifen orientation, the highly conserved Trp(6.48), part of the toggle switch involved in receptor activation, was found in an inactive or active conformation, respectively. We identified the first phenylhistamines with higher potency at hH(1)R than at gpH(1)R and obtained insight into the binding mode of bulky phenylhistamines and imidazolylpropylguanidines.

Cholangiopathies are characterized by the heterogeneous proliferation of different-sized cholangiocytes. Large cholangiocytes proliferate by a cAMP-dependent mechanism. The function of small cholangiocytes may depend on the activation of inositol trisphosphate (IP(3))/Ca(2+)-dependent signaling pathways; however, data supporting this speculation are lacking. Four histamine receptors exist (HRH1, HRH2, HRH3, and HRH4). In several cells: 1) activation of HRH1 increases intracellular Ca(2+) concentration levels; and 2) increased [Ca(2+)](i) levels are coupled with calmodulin-dependent stimulation of calmodulin-dependent protein kinase (CaMK) and activation of cAMP-response element binding protein (CREB). HRH1 agonists modulate small cholangiocyte proliferation by activation of IP(3)/Ca(2+)-dependent CaMK/CREB. We evaluated HRH1 expression in cholangiocytes. Small and large cholangiocytes were stimulated with histamine trifluoromethyl toluidide (HTMT dimaleate; HRH1 agonist) for 24-48 h with/without terfenadine, BAPTA/AM, or W7 before measuring proliferation. Expression of CaMK I, II, and IV was evaluated in small and large cholangiocytes. We measured IP(3), Ca(2+) and cAMP levels, phosphorylation of CaMK I, and activation of CREB (in the absence/presence of W7) in small cholangiocytes treated with HTMT dimaleate. CaMK I knockdown was performed in small cholangiocytes stimulated with HTMT dimaleate before measurement of proliferation and CREB activity. Small and large cholangiocytes express HRH1, CaMK I, and CaMK II. Small (but not large) cholangiocytes proliferate in response to HTMT dimaleate and are blocked by terfenadine (HRH1 antagonist), BAPTA/AM, and W7. In small cholangiocytes, HTMT dimaleate increased IP(3)/Ca(2+) levels, CaMK I phosphorylation, and CREB activity. Gene knockdown of CaMK I ablated the effects of HTMT dimaleate on small cholangiocyte proliferation and CREB activation. The IP(3)/Ca(2+)/CaMK I/CREB pathway is important in the regulation of small cholangiocyte function.

The present study evaluated the effects of histamine 10(-2) M on longitudinal preparations of rat portal vein. It was observed that histamine 10(-2) M induced relaxation of rat portal vein preparations pre-contracted with phenylephrine 10(-4) M. On the other hand, no pharmacological effects were observed in preparations not pre-contracted. The observed histamine-induced relaxing effect was absent in preparations pre-contracted with KCl (120 mM) or in the presence of depolarizing nutritive solution. However, the histamine-induced relaxation was still present in the endothelium-removed preparations. The histamine-induced relaxation also was not prevented by astemizole (10(-6) M, 10(-5) M and 10(-4) M), cimetidine (10(-5) M, 10(-4) M and 10(-3) M) or thioperamide (10(-6) M, 10(-5) M and 10(-4) M), selective antagonists H(1), H(2) and H(3), respectively. The presence of L-NAME 10(-4) M or L-NAME 10(-4) M plus indomethacin 10(-5) M also did not prevent the histamine-induced relaxation observed in rat portal vein. Thus, the histamine-induced relaxation observed in rat portal vein appears to involve a non-endothelial hyperpolarizing mechanism independent of H(1), H(2) and H(3) receptors.

Histamine H1 antagonists impaired the spatial memory performance. On the other hand, it is well recognized that the hippocampal theta rhythm plays a critical role in spatial memory. However, little work has been done the effect of H1 antagonists on the hippocampal theta rhythm which was associated with the memory performance. We investigated the effect of pyrilamine, a selective H1 receptor antagonist, on spatial memory performance as well as hippocampal theta rhythm during the memory task in rats. Effect of pyrilamine on spatial memory was measured using eight-arm radial maze with four arms baited. Hippocampal theta rhythm during the radial maze task was recorded with a polygraph system with a telemetric technique. Intraperitoneal injection of pyrilamine resulted in impairments of both reference and working memory on the radial maze task. The working memory deficit induced by pyrilamine was antagonized by the intrahippocampal injection of histamine and 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl)heptanecarboxamide (HTMT), a histamine H1 agonist. Intraperitoneal injection of pyrilamine decreased the hippocampal theta power at a dose that impaired reference and working memory. This effect was antagonized by the intrahippocampal injection of histamine and HTMT at a dose that ameliorated the working memory deficit. Intrahippocampal injection of pyrilamine impaired working memory and simultaneously decreased the hippocampal theta power. These results suggest that: (i) the hippocampal H1 receptors play an important role in the working memory processes on the radial maze performance and (ii) the decrease in the hippocampal theta power is associated with the working memory deficit induced by the blocking of H1 receptors.

The human histamine H(1) receptor (H(1)R) is a prototypical G protein-coupled receptor and an important, well characterized target for the development of antagonists to treat allergic conditions. Many neuropsychiatric drugs are also known to potently antagonize this receptor, underlying aspects of their side effect profiles. We have used the cell-based receptor selection and amplification technology assay to further define the clinical pharmacology of the human H(1)R by evaluating >130 therapeutic and reference drugs for functional receptor activity. Based on this screen, we have reported on the identification of 8R-lisuride as a potent stereospecific partial H(1)R agonist (Mol Pharmacol 65:538-549, 2004). In contrast, herein we report on a large number of varied clinical and chemical classes of drugs that are active in the central nervous system that display potent H(1)R inverse agonist activity. Absolute and rank order of functional potency of these clinically relevant brain-penetrating drugs may possibly be used to predict aspects of their clinical profiles, including propensity for sedation.

Neuronal histamine regulates several functions in the vertebrate brain. The zebrafish brain contains a widespread histaminergic system and H(3) receptor ligand binding has been reported. In this study we provide evidence for the existence of histamine H(1), H(2) and H(3) receptor genes in zebrafish. Single copies of putative histamine H(1), H(2) and H(3) receptors were identified and cloned from the zebrafish brain. Expression analysis suggested that they are expressed in the brain and a few other tissues. Widespread distribution of zebrafish H(2) receptor binding sites was detected with [(125)I]iodoaminopotentidine in brain sections. Zebrafish larvae were exposed to 1, 10 or 100 microM of the H(1) ligand pyrilamine, the H(2) ligand cimetidine and the H(3) ligands thioperamide and immepip for 5 days. Significant decreases in swimming distance were observed with the highest dose of all ligands, whereas cimetidine gave a significant decrease also with 1 and 10 microM doses. These results provide the first molecular biological evidence for the presence of histamine receptors in zebrafish. These histamine receptors resemble those of higher vertebrates and they provide a useful model for pharmacological and behavioral studies for characterizing the functions of histamine in more detail.

G protein coupled receptors (GPCRs) represent a major superfamily of transmembrane receptor proteins that are crucial in cellular signaling and are major pharmacological targets. While the activity of GPCRs can be modulated by agonist binding, the mechanisms that link agonist binding to G protein coupling are poorly understood. Here we present a method to accurately examine the activity of ligands in their bound state, even at low affinity, by solid-state NMR dipolar correlation spectroscopy and confront this method with the human H1 receptor. The analysis reveals two different charge states of the bound agonist, dicationic with a charged imidazole ring and monocationic with a neutral imidazole ring, with the same overall conformation. The combination of charge difference and pronounced heterogeneity agrees with converging evidence that the active and inactive states of the GPCR represent a dynamic equilibrium of substates and that proton transfer between agonist and protein side chains can shift this equilibrium by stabilizing the active receptor population relative to the inactive one. In fact, the data suggest a global functional analogy between H1 receptor activation and the meta I/meta II charge/discharge equilibrium in rhodopsin (GPCR). This corroborates current ideas on unifying principles in GPCR structure and function.

Imidazolylpropylguanidines derived from impromidine and arpromidine are more potent and efficacious agonists at the guinea pig histamine H2 receptor (gpH2R) than at the human H2R (hH2R) in the GTPase assay. Additionally, such guanidines are histamine H1 receptor (H1R) antagonists with preference for the human relative to the guinea pig receptor. The purpose of this study was to examine structure-activity relationships of guanidines at human and guinea pig H1R and H2R species isoforms expressed in Sf9 insect cells. Three impromidine analogues and six arpromidine analogues exhibited agonistic activity at H2R and antagonistic activity at H1R as assessed in the steady-state GTPase assay. Species selectivity of derivatives was similar as compared with the parent compounds. None of the structural modifications examined (different aromatic ring systems and different ring substituents) was superior in terms of H2R potency and efficacy relative to impromidine and arpromidine, respectively. These data point to substantial structural constraints at the agonist binding site of H2R. Guanidines exhibited distinct structure-activity relationships for H1R antagonism in a radioligand competition binding assay and the GTPase assay and for H1R inverse agonism. Our data indicate that it is difficult to obtain guanidine-type agonists with high potency and high efficacy for hH2R, but those compounds may be useful tools for exploring the antagonist binding site and constitutive activity of H1R.

Phospholipase Cbeta (PLCbeta) isozymes represent a family of molecules that link G protein-coupled receptors (GPCRs) to an intracellular signaling network. Here, we investigated the function of PLCbeta isozymes in sensory neurons by using mutant mice deficient for specific PLCbeta family members. Expression analysis indicated that PLCbeta3, one of the four isoforms, is predominantly expressed in a subpopulation of C-fiber nociceptors. A subset of these neurons expressed the histamine H1 receptor. Ca(2+) imaging studies revealed that PLCbeta3 specifically mediates histamine-induced calcium responses through the histamine H1 receptor in cultured sensory neurons. In line with this, we found that PLCbeta3(-/-) mice showed significant defects in scratching behavior induced by histamine; histamine-trifluoromethyl-toluidine (HTMT), a selective H1 agonist; and compound 48/80, a mast cell activator. These results demonstrate that PLCbeta3 is required to mediate "itch" sensation in response to histamine acting on the histamine H1 receptor in C-fiber nociceptive neurons.

Activation of the histamine H1 receptor stimulates tyrosine hydroxylase (TH) to increase catecholamine neurotransmitter synthesis in mammalian brain and adrenal tissues. Histamine non-selectively activates both H1-linked phospholipase (PL) C/inositol phosphates (IP)/diacylglycerol (DAG) signaling and adenylyl cyclase (AC)/adenosine 3',5'-cyclic monophosphate (cAMP) signaling, confounding determination of signaling events involved in H(1)-mediated TH activation. This research uses two new functionally-selective H1 agonists, cis-PAB and trans-PAT, that selectively activate H1/PLC/IP/DAG and H1/AC/cAMP signaling, respectively, to characterize H(1)-mediated activation of TH in rat striatum and bovine adrenal chromaffin (BAC) cells. Histamine, cis-PAB, and trans-PAT produced a two-fold maximal TH activation by an H1 receptor mechanism in rat striatum and BAC cells. Histamine is more potent and efficacious in BAC cells (EC50 approximately 0.2 microM, Emax approximately 200% basal) versus rat striatum (EC50 approximately 0.4 microM; Emax approximately 150%). Cis-PAB and trans-PAT are more potent in rat striatum (EC50 approximately 0.1 microM for both agonists) versus BAC cells (EC50 approximately 1.0 microM for both), with similar efficacy in both preparations (Emax approximately 160% for both agonists). Signaling studies in BAC cells revealed that protein kinase (PK) A but not PKC is involved in H1 -mediated TH activation by trans-PAT and histamine, while, both PKA and PKC are involved for cis-PAB. Results for cis-PAB suggest H1/PLC/IP/DAG/PKC signaling activates PKA, downstream of cAMP formation, indicating apparent direct activation of PKA by PKC.

This study investigated the involvement of H(1) and H(2) histaminegic receptors on the acquisition of a new task in Carassius auratus by using an inhibitory avoidance paradigm in which the animals had to learn to avoid an aversive stimulus. Before training, the fish received injections of H(2) antagonist zolantidine at a dose of 20 mg/kg, or H(1) antagonist chlorpheniramine at a dose of 4 or 16 mg/kg. Control animals were injected with distilled water. A facilitatory effect of chlorpheniramine was observed at the dose of 16 mg/kg. On the other hand, the administration of 20 mg/kg of zolantidine inhibited acquisition. Place preference conditioning was used to observe the aversive or reinforcing effects of the drugs, which could interfere with the inhibitory avoidance procedure; however, no effects were observed. Thus, it can be suggested that both receptors, H(1) and H(2), are involved in the acquisition of a new task in this species.

The aim of the present study was to investigate the effect of histamine, a product of e.g. mast cells, on short-circuit current (I(sc)) across rat distal colon. Histamine concentration-dependently stimulated an increase in I(sc), which often was preceded by a transient negative current. Neither a release of neurotransmitters nor a release of prostaglandins contributed to the histamine response. The histamine-induced increase in I(sc) was blocked by the histamine H(1) antagonist, pyrilamine, but was resistant against the histamine H(2) antagonist, cimetidine. Conversely, the histamine H(1) agonist, TMPH (2-(3-trifluoromethylphenyl)histamine), exclusively evoked an increase in I(sc), whereas the histamine H(2) agonist, amthamine, evoked only a decrease in I(sc) suggesting that stimulation of different types of histamine receptors is responsible for the two phases of the response evoked by native histamine. Histamine induces the opening of glibenclamide-sensitive Cl(-) channels and of charybdotoxin-sensitive K(+) channels in the apical membrane as demonstrated by experiments at basolaterally depolarized epithelia. A further action site is the basolateral membrane, because histamine stimulates a charybdotoxin- and tetrapentylammonium-sensitive K(+) conductance in this membrane as observed in tissues, in which the apical membrane was permeabilized with an ionophore, nystatin. The increase in I(sc) evoked by histamine was blocked after depletion of intracellular Ca(2+) stores with cyclopiazonic acid and after blockade of inositol 1,4,5-trisphosphate (IP(3)) receptors, suggesting a release of stored Ca(2+). This was confirmed by the observation that the histamine H(1) agonist TMPH induced an increase in the fura-2 ratio signal of epithelial cells within isolated colonic crypts. Consequently, the mediator histamine seems to stimulate both histamine H(1) and H(2) receptors, from which the former seems to be prominently involved in the induction of epithelial chloride secretion.

Both the histamine H1-receptor (H1R) and H2-receptor (H2R) exhibit pronounced species selectivity in their pharmacological properties; i.e., bulky agonists possess higher potencies and efficacies at guinea pig (gp) than at the corresponding human (h) receptor isoforms. In this study, we examined the effects of NG-acylated imidazolylpropylguanidines substituted with a single phenyl or cyclohexyl substituent on H1R and H2R species isoforms expressed in Sf9 insect cells. N1-(3-Cyclohexylbutanoyl)-N2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-AK57) turned out to be the most potent hH2R agonist identified so far (EC50 of 23 nM in the GTPase assay at the hH2R-Gsalpha fusion protein expressed in Sf9 insect cells). UR-AK57 was almost a full-hH2R agonist and only slightly less potent and efficacious than at gpH2R-Gsalpha. Several NG-acylated imidazolylpropylguanidines showed similar potency at hH2R and gpH2R. Most unexpectedly, UR-AK57 exhibited moderately strong partial hH1R agonism with a potency similar to that of histamine, whereas at gpH1R, UR-AK57 was only a very weak partial agonist. Structure/activity relationship studies revealed that both the alkanoyl chain connecting the aromatic or alicyclic substituent with the guanidine moiety and the nature of the carbocycle (cyclohexyl versus phenyl ring) critically determine the pharmacological properties of this class of compounds. Collectively, our data show that gpH1R and gpH R do not necessarily exhibit preference for bulky agonists (2) compared with hH1R and hH2R, respectively, and that UR-AK57 is a promising starting point for the development of both potent and efficacious hH1R and hH2R agonists.

The role of histamine in regulating excitability of sympathetic preganglionic neurons (SPNs) and the expression of histamine receptor mRNA in SPNs was investigated using whole-cell patch-clamp electrophysiological recording techniques combined with single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in transverse neonatal rat spinal cord slices. Bath application of histamine (100 microM) or the H1 receptor agonist histamine trifluoromethyl toluidide dimaleate (HTMT; 10 microM) induced membrane depolarization associated with a decrease in membrane conductance in the majority (70%) of SPNs tested, via activation of postsynaptic H1 receptors negatively coupled to one or more unidentified K+ conductances. Histamine and HTMT application also induced or increased the amplitude and/or frequency of membrane potential oscillations in electrotonically coupled SPNs. The H2 receptor agonist dimaprit (10 microM) or the H3 receptor agonist imetit (100 nM) were without significant effect on the membrane properties of SPNs. Histamine responses were sensitive to the H1 receptor antagonist triprolidine (10 microM) and the nonselective potassium channel blocker barium (1 mM) but were unaffected by the H2 receptor antagonist tiotidine (10 microM) and the H3 receptor antagonist, clobenpropit (5 microM). Single cell RT-PCR revealed mRNA expression for H1 receptors in 75% of SPNs tested, with no expression of mRNA for H2, H3, or H4 receptors. These data represent the first demonstration of H1 receptor expression in SPNs and suggest that histamine acts to regulate excitability of these neurons via a direct postsynaptic effect on H1 receptors.

Histamine shifts TH1/TH2 cytokine balance from TH1 to TH2 cytokines and regulates the function of lymphocytes after binding to histamine receptors. The phosphorylation of STAT factors and the translocation to the nucleus are important steps in the regulation of TH1/TH2 cytokine balance. This study was designed to investigate the effects of histamine on the phosphorylation of STAT4. C57BL/6 splenocytes were isolated and treated with histamine (10(-4) to 10(-9) M) after activation with either PMA (phorbol 12 myristate 13-acetate) plus ionomycin or IL-12. The phosphorylated STAT4 levels were analyzed by Western Blot Analysis. Unstimulated splenocytes expressed both STAT4 and phosphorylated STAT4. However, phosphorylated STAT4 gradually declined within 24 h. Histamine increased the phosphorylation of STAT4 at lower concentrations (10(-6) to 10(-9) M), and had no effect at higher concentrations (10(-4) and 10(-5) M) after the cells were stimulated with PMA + ionomycin. Histamine did not affect IL-12-induced phosphorylation of STAT4. To characterize the histamine receptor subtypes involved in the up-regulation of STAT4 phosphorylation, various H1, H2 and H3/H4 receptor antagonists and/or agonists were employed. H1 receptor agonist (betahistine), but not H2 receptor agonist (amthamine), induced phosphorylation of STAT4. H1 receptor antagonist (pyrilamine) inhibited histamine-mediated phosphorylation of STAT4. However, H2 receptor antagonist (ranitidine) and H3/H4 receptor antagonist (thioperamide) did not alter this effect. Tyrosine kinase inhibitor (tyrphostin) failed to block histamine-mediated phosphorylation of STAT4. These observations suggest that histamine up-regulated the phosphorylation of STAT4 via H1 receptors, and that the Ca2+-PKC pathway, but not the tyrosine kinase pathway, was involved in this effect.