Profiling of histamine H4 receptor agonists in native human monocytes

Background and Purpose

Since the identification of the histamine H₄ receptor, several ligands activating this receptor have been described and more compounds are in development. These ligands are well characterized in pharmacological assays, including radioligand competition binding studies, GTPγS and GTPase assays. In most cases, these experiments are performed in transfected cell lines, expressing unnaturally high levels of target receptors and G-protein signalling components. In this study we investigated the specific properties of H₄ receptor ligands in native cells.

Experimental Approach

Histamine and five different H₄ receptor agonists – 4-methylhistamine, UR-PI376, clobenpropit, VUF8430 and ST-1006 – were characterized in freshly isolated human monocytes. The ligands (10 nM–10 μM) were tested as inhibitors of IL-12p70 secretion from human monocytes and the effects of the H₂ receptor antagonist ranitidine and the H₄ receptor antagonist JNJ7777120 on their action was investigated.

Key Results

Histamine and all the tested agonists reduced IL-12p70 secretion into monocyte supernatants by 40–70%. The potencies varied with pEC₅₀ values ranging from 5.7 to 6.9, depending on the agonist used. All potencies were lower than those determined in the original investigations of the compounds. Pretreatment of monocytes with H₂ or H₄ receptor antagonists showed that some H₄ receptor ligands also had low activity at the H₂ receptor.

Conclusions and Implications

Our study demonstrates discrepancies between the potencies obtained from assays in transfected cell lines and assays in native human cells, indicating the importance of evaluating H₄ receptor ligands in native cells.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

Functional pharmacology of H1 histamine receptors expressed in mouse preoptic/anterior hypothalamic neurons

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

Ca²⁺ 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.     

Design and pharmacological characterization of VUF14480, a covalent partial agonist that interacts with cysteine 983.36 of the human histamine H4 receptor

Background and Purpose

The recently proposed binding mode of 2-aminopyrimidines to the human (h) histamine H₄ receptor suggests that the 2-amino group of these ligands interacts with glutamic acid residue E182^5.46 in the transmembrane (TM) helix 5 of this receptor. Interestingly, substituents at the 2-position of this pyrimidine are also in close proximity to the cysteine residue C98^3.36 in TM3. We hypothesized that an ethenyl group at this position will form a covalent bond with C98^3.36 by functioning as a Michael acceptor. A covalent pyrimidine analogue will not only prove this proposed binding mode, but will also provide a valuable tool for H₄ receptor research.

Experimental Approach

We designed and synthesized VUF14480, and pharmacologically characterized this compound in hH₄ receptor radioligand binding, G protein activation and β-arrestin2 recruitment experiments. The ability of VUF14480 to act as a covalent binder was assessed both chemically and pharmacologically.

Key Results

VUF14480 was shown to be a partial agonist of hH₄ receptor-mediated G protein signalling and β-arrestin2 recruitment. VUF14480 bound covalently to the hH₄ receptor with submicromolar affinity. Serine substitution of C98^3.36 prevented this covalent interaction.

Conclusion and Implications

VUF14480 is thought to bind covalently to the hH₄ receptor-C98^3.36 residue and partially induce hH₄ receptor-mediated G protein activation and β-arrestin2 recruitment. Moreover, these observations confirm our previously proposed binding mode of 2-aminopyrimidines. VUF14480 will be a useful tool to stabilize the receptor into an active confirmation and further investigate the structure of the active hH₄ receptor.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

The histamine H4 receptor is functionally expressed on neurons in the mammalian CNS

Background and purpose:

The histamine H₄ receptor is the most recently identified of the G protein-coupled histamine receptor family and binds several neuroactive drugs, including amitriptyline and clozapine. So far, H₄ receptors have been found only on haematopoietic cells, highlighting its importance in inflammatory conditions. Here we investigated the possibility that H₄ receptors may be expressed in both the human and mouse CNS.

Methods:

Immunological and pharmacological studies were performed using a novel anti-H₄ receptor antibody in both human and mouse brains, and electrophysiological techniques in the mouse brain respectively. Pharmacological tools, selective for the H₄ receptor and patch clamp electrophysiology, were utilized to confirm functional properties of the H₄ receptor in layer IV of the mouse somatosensory cortex.

Results:

Histamine H₄ receptors were prominently expressed in distinct deep laminae, particularly layer VI, in the human cortex, and mouse thalamus, hippocampal CA4 stratum lucidum and layer IV of the cerebral cortex. In layer IV of the mouse somatosensory cortex, the H₄ receptor agonist 4-methyl histamine (20 µmol·L⁻¹) directly hyperpolarized neurons, an effect that was blocked by the selective H₄ receptor antagonist JNJ 10191584, and promoted outwardly rectifying currents in these cells. Monosynaptic thalamocortical CNQX-sensitive excitatory postsynaptic potentials were not altered by 4-methyl histamine (20 µmol·L⁻¹) suggesting that H₄ receptors did not act as hetero-receptors on thalamocortical glutamatergic terminals.

Conclusions and implications:

This is the first demonstration that histamine H₄ receptors are functionally expressed on neurons, which has major implications for the therapeutic potential of these receptors in neurology and psychiatry.     

In vitro and in vivo characterization of A-940894: a potent histamine H4 receptor antagonist with anti-inflammatory properties

Background and purpose:

The histamine H₄ receptor is widely expressed in cells of immune origin and has been shown to play a role in a variety of inflammatory processes mediated by histamine. In this report, we describe the in vitro and in vivo anti-inflammatory properties of a potent histamine H₄ receptor antagonist, A-940894 (4-piperazin-1-yl-6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-d]pyrimidin-2-ylamine).

Experimental approach:

We have analysed the pharmacological profile of A-940894 at mouse native, rat recombinant and human recombinant and native, histamine H₄ receptors by radioligand binding, calcium mobilization, mast cell shape change, eosinophil chemotaxis assays and in the mouse model of zymosan-induced peritonitis.

Key results:

A-940894 potently binds to both human and rat histamine H₄ receptors and exhibits considerably lower affinity for the human histamine H₁, H₂ or H₃ receptors. It potently blocked histamine-evoked calcium mobilization in the fluorometric imaging plate reader assays and inhibited histamine-induced shape change of mouse bone marrow-derived mast cells and chemotaxis of human eosinophils in vitro. In a mouse mast cell-dependent model of zymosan-induced peritonitis, A-940894 significantly blocked neutrophil influx and reduced intraperitoneal prostaglandin D₂ levels. Finally, A-940894 has good pharmacokinetic properties, including half-life and oral bioavailability in rats and mice.

Conclusions and Implications:

These data suggest that A-940894 is a potent and selective histamine H₄ receptor antagonist with pharmacokinetic properties suitable for long-term in vivo testing and could serve as a useful tool for the further characterization of histamine H₄ receptor pharmacology.     

The expression and function of histamine H3 receptors in pancreatic beta cells

BACKGROUND AND PURPOSE

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

EXPERIMENTAL APPROACH

The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H₃ receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H₃ receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting.

KEY RESULTS

Histamine H₃ receptors were expressed in pancreatic beta cells. A selective H₃ receptor agonist, imetit, and a selective inverse H₃ receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H₃ receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H₃ receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Histamine H₃ receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.     

Histamine inhibits adhesion molecule expression in human monocytes, induced by advanced glycation end products, during the mixed lymphocyte reaction

Background and purpose:

Post-transplant diabetes mellitus is a frequent complication among transplant recipients. Ligation of advanced glycation end products (AGEs) with their receptor on monocytes/macrophages plays important roles in the genesis of diabetic complications. The enhancement of adhesion molecule expression on monocytes/macrophages activates T-cells, reducing allograft survival. Out of four distinct AGE subtypes (AGE-2, AGE-3, AGE-4 and AGE-5), only AGE-2 and AGE-3 induced expression of intercellular adhesion molecules (ICAMs), output of cytokines and proliferation of lymphocytes, during the mixed lymphocyte reaction (MLR). Here we have assessed the role of histamine in the actions of AGEs during the MLR.

Experimental approach:

Human peripheral blood cells were used in these experiments. Flow cytometry was used to examine the expression of the ICAM-1, B7.1, B7.2 and CD40. Production of the cytokine interferon-γ, and levels of cAMP were determined by elisa. Lymphocyte proliferation was determined by [³H]-thymidine uptake.

Key results:

Histamine concentration dependently inhibited the action of AGE-2 and AGE-3. The actions of histamine were antagonized by an H₂-receptor antagonist, famotidine, and mimicked by H₂/H₄-receptor agonists, dimaprit and 4-methylhistamine. The effects of histamine were reversed by a protein kinase A (PKA) inhibitor, H89, and mimicked by dibutyryl cAMP and an adenylate cyclase activator, forskolin.

Conclusions and implications:

Histamine down-regulated AGE-2- and AGE-3-induced expression of adhesion molecules, cytokine production and lymphocyte proliferation via histamine H₂ receptors and the cAMP/PKA pathway.     

Arrestins differentially regulate histamine- and oxytocin-evoked phospholipase C and mitogen-activated protein kinase signalling in myometrial cells

BACKGROUND AND PURPOSE

The uterotonins oxytocin and histamine, mediate contractile signals through specific G protein-coupled receptors, a process which is tightly controlled during gestation to prevent preterm labour. We previously identified G protein-coupled receptor kinase (GRK)2 and GRK6 as respective cardinal negative regulators of histamine H₁ and oxytocin receptor signalling. GRK-mediated phosphorylation promotes arrestin recruitment, not only desensitizing receptors but activating an increasing number of diverse signalling pathways. Here we investigate potential roles that arrestins play in the regulation of myometrial oxytocin/histamine H₁ receptor signalling.

EXPERIMENTAL APPROACH

Endogenous arrestins2 and 3 were specifically depleted using RNA-interference in a human myometrial cell line and the consequences of this for G protein-coupled receptor-mediated signalling were assessed using Ca²⁺/inositol 1,4,5-trisphophate imaging and standard mitogen-activated protein kinase (MAPK) assays.

KEY RESULTS

Depletion of arrestin3, but not arrestin2 enhanced and prolonged H₁ receptor-stimulated Ca²⁺ responses, whilst depletion of either arrestin increased oxytocin receptor responses. Arrestin3 depletion decreased H₁ receptor desensitization, whilst removal of either arrestin isoform was equally effective in preventing oxytocin receptor desensitization. Following arrestin3 depletion oxytocin-induced phospho-extracellular signal-regulated kinase1/2 signals were diminished and histamine-stimulated signals virtually absent, whereas depletion of arrestin2 augmented extracellular signal-regulated kinase1/2 responses to each agonist. Conversely, depletion of arrestin3 enhanced p38 signals to each agonist, whilst arrestin2 suppression increased oxytocin-, but not histamine-induced p38 MAPK responses.

CONCLUSIONS AND IMPLICATIONS

Arrestin proteins are key regulators of H₁ and oxytocin receptor desensitization, and play integral roles mediating uterotonin-stimulated MAPK-signalling. These data provide insights into the in situ regulation of these receptor subtypes and may inform pathophysiological functioning in preterm labour.     

Urethane,but not pentobarbitone,attenuates presynaptic receptor function in rats: a contribution to the choice of anaesthetic

Background and purpose:

We examined whether cannabinoid CB₁ and histamine H₃ receptors resemble α₂-adrenoceptors in that their presynaptically mediated cardiovascular effects are less marked in urethane- than in pentobarbitone-anaesthetized pithed rats.

Experimental approach:

Effects of the cannabinoid agonist CP-55,940 and the H₃ receptor agonist imetit on electrically induced tachycardic and vasopressor responses, respectively, was compared in pithed rats anaesthetized with urethane or pentobarbitone. The affinity of urethane for the three receptors was measured by radioligand binding studies in rat brain cortex membranes and its potency assessed in superfused mouse tissues preincubated with ³H-noradrenaline.

Key results:

The neurogenic tachycardic response was less markedly inhibited by CP-55,940 in urethane- than in pentobarbitone-anaesthetized pithed rats. Imetit inhibited the neurogenic vasopressor response after pentobarbitone but not after urethane. The catecholamine-induced tachycardic and vasopressor response did not differ between rats anaesthetized with either compound. Urethane 10 mM (plasma concentration reached under anaesthesia) did not affect binding to CB₁ or H₃ receptors and α₂ adrenoceptors, nor did it alter the inhibitory effect of agonists at the three receptors on electrically evoked ³H-noradrenaline release.

Conclusions and implications:

Urethane, but not pentobarbitone, abolished the H₃ receptor-mediated vascular response in pithed rats and attenuated the CB₁ receptor-mediated cardiac response much more than pentobarbitone. The weaker effects of CB₁, H₃ and α₂ receptor agonists cannot be explained by antagonism by urethane at the three receptors in vitro. Pentobarbitone, but not urethane, is suitable as an anaesthetic for investigations of inhibitory presynaptic receptor function in pithed and anaesthetized rats.     

Pharmacological characterization of GSK1004723, a novel, long-acting antagonist at histamine H(1) and H(3) receptors

BACKGROUND AND PURPOSE

Preclinical pharmacological characterization of GSK1004723, a novel, dual histamine H₁ and H₃ receptor antagonist.

EXPERIMENTAL APPROACH

GSK1004723 was characterized in vitro and in vivo using methods that included radioligand binding, intracellular calcium mobilization, cAMP production, GTPγS binding, superfused human bronchus and guinea pig whole body plethysmography.

KEY RESULTS

In cell membranes over-expressing human recombinant H₁ and H₃ receptors, GSK1004723 displayed high affinity, competitive binding (H₁ pKi = 10.2; H₃ pKi = 10.6). In addition, GSK1004723 demonstrated slow dissociation from both receptors with a t_1/2 of 1.2 and 1.5 h for H₁ and H₃ respectively. GSK1004723 specifically antagonized H₁ receptor mediated increases in intracellular calcium and H₃ receptor mediated increases in GTPγS binding. The antagonism exerted was retained after cell washing, consistent with slow dissociation from H₁ and H₃ receptors. Duration of action was further evaluated using superfused human bronchus preparations. GSK1004723 (100 nmol·L⁻¹) reversed an established contractile response to histamine. When GSK1004723 was removed from the perfusate, only 20% recovery of the histamine response was observed over 10 h. Moreover, 21 h post-exposure to GSK1004723 there remained almost complete antagonism of responses to histamine. In vivo pharmacology was studied in conscious guinea pigs in which nasal congestion induced by intranasal histamine was measured indirectly (plethysmography). GSK1004723 (0.1 and 1 mg·mL⁻¹ intranasal) antagonized the histamine-induced response with a duration of up to 72 h.

CONCLUSIONS AND IMPLICATIONS

GSK1004723 is a potent and selective histamine H₁ and H₃ receptor antagonist with a long duration of action and represents a potential novel therapy for allergic rhinitis.     

Therapeutic potential of histamine H4 receptor agonists in triple-negative human breast cancer experimental model

Background and Purpose

The presence of the histamine H₄ receptor (H₄R) was previously reported in benign and malignant lesions and cell lines derived from the human mammary gland. The aim of this work was to evaluate the effects of H₄R ligands on the survival, tumour growth rate and metastatic capacity of breast cancer in an experimental model.

Experimental Approach

Xenograft tumours of the highly invasive human breast cancer cell line MDA-MB-231 were established in immune deficient nude mice. The following H₄R agonists were employed: histamine (5 mg kg⁻¹), clozapine (1 mg kg⁻¹) and the experimental compound JNJ28610244 (10 mg kg⁻¹).

Results

Data indicate that developed tumours were highly undifferentiated, expressed H₄R and exhibited high levels of histamine content and proliferation marker (PCNA) while displaying low apoptosis. Mice of the untreated group displayed a median survival of 60 days and a tumour doubling time of 7.4 ± 0.6 days. A significant decrease in tumour growth evidenced by an augment of the tumour doubling time was observed in the H₄R agonist groups (13.1 ± 1.2, P < 0.01 in histamine group; 15.1 ± 1.1, P < 0.001 in clozapine group; 10.8 ± 0.7, P < 0.01 in JNJ28610244 group). This effect was associated with a decrease in the PCNA expression levels, and also reduced intratumoural vessels in histamine and clozapine treated mice. Histamine significantly increased median survival (78 days; Log rank Mantel-Cox Test, P = 0.0025; Gehan-Breslow-Wilcoxon Test, P = 0.0158) and tumoural apoptosis.

Conclusions and Implications

Histamine through the H₄R exhibits a crucial role in tumour progression. Therefore, H₄R ligands offer a novel therapeutic potential as adjuvants for breast cancer treatment.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

Histamine modulates γδ-T lymphocyte migration and cytotoxicity, via Gi and Gs protein-coupled signalling pathways

Background and purpose:

The biogenic amine, histamine plays a pathophysiological regulatory role in cellular processes of a variety of immune cells. This work analyses the actions of histamine on γδ-T lymphocytes, isolated from human peripheral blood, which are critically involved in immunological surveillance of tumours.

Experimental approach:

We have analysed effects of histamine on the intracellular calcium, actin reorganization, migratory response and the interaction of human γδ T cells with tumour cells such as the A2058 human melanoma cell line, the human Burkitt''s Non-Hodgkin lymphoma cell line Raji, the T-lymphoblastic lymphoma cell line Jurkat and the natural killer cell-sensitive erythroleukaemia cell line, K562.

Key results:

γδ T lymphocytes express mRNA for different histamine receptor subtypes. In human peripheral blood γδ T cells, histamine stimulated Pertussis toxin-sensitive intracellular calcium increase, actin polymerization and chemotaxis. However, histamine inhibited the spontaneous cytolytic activity of γδ T cells towards several tumour cell lines in a cholera toxin-sensitive manner. A histamine H₄ receptor antagonist abolished the histamine induced γδ T cell migratory response. A histamine H₂ receptor agonist inhibited γδ T cell-mediated cytotoxicity.

Conclusions and implications:

Histamine activated signalling pathways typical of chemotaxis (G_i protein-dependent actin reorganization, increase of intracellular calcium) and induced migratory responses in γδ T lymphocytes, via the H₄ receptor, whereas it down-regulated γδ T cell mediated cytotoxicity through H₂ receptors and G_s protein-coupled signalling. Our data suggest that histamine activated γδ T cells could modulate immunological surveillance of tumour tissue.     

MDA7: a novel selective agonist for CB2 receptors that prevents allodynia in rat neuropathic pain models

Background and purpose:

There is growing interest in using cannabinoid type 2 (CB₂) receptor agonists for the treatment of neuropathic pain. In this report, we describe the pharmacological characteristics of MDA7 (1-[(3-benzyl-3-methyl-2,3-dihydro-1-benzofuran-6-yl)carbonyl]piperidine), a novel CB₂ receptor agonist.

Experimental approach:

We characterized the pharmacological profile of MDA7 by using radioligand-binding assays and in vitro functional assays at human cannabinoid type 1 (CB₁) and CB₂ receptors. In vitro functional assays were performed at rat CB₁ and CB₂ receptors. The effects of MDA7 in reversing neuropathic pain were assessed in spinal nerve ligation and paclitaxel-induced neuropathy models in rats.

Key results:

MDA7 exhibited selectivity and agonist affinity at human and rat CB₂ receptors. MDA7 treatment attenuated tactile allodynia produced by spinal nerve ligation or by paclitaxel in a dose-related manner. These effects were selectively antagonized by a CB₂ receptor antagonist but not by CB₁ or opioid receptor antagonists. MDA7 did not affect rat locomotor activity.

Conclusion and implications:

MDA7, a novel selective CB₂ agonist, was effective in suppressing neuropathic nociception in two rat models without affecting locomotor behaviour. These results confirm the potential for CB₂ agonists in the treatment of neuropathic pain.     

A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors

BACKGROUND AND PURPOSE

5-HT₃ receptors are composed of 5-HT₃A subunits (homomeric receptors), or combinations of 5-HT₃A and other 5-HT₃ receptor subunits (heteromeric receptors, the best studied of which are 5-HT₃AB receptors). Here we explore the effects of partial agonists at 5-HT₃A and 5-HT₃AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.

EXPERIMENTAL APPROACH

Wild type and mutant 5-HT₃A and 5-HT₃AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [³H]granisetron binding.

KEY RESULTS

Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT₃A and 5-HT₃AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT₃A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT₃AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6''S substitution in the 5-HT₃A subunit decreased EC₅₀ and increased R_max of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT₃A_T6''SB receptors, where it became a full agonist (EC₅₀ = 7 nM) with an EC₅₀ 58-fold less than 5-HT (EC₅₀ = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.

CONCLUSIONS AND IMPLICATIONS

These results indicate that a mutation in the pore lining domain in both 5-HT₃A and 5-HT₃AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.     

Altered histamine H3 receptor radioligand binding in post-mortem brain samples from subjects with psychiatric diseases

Background and purpose:

Histamine is a modulatory neurotransmitter in the brain. Auto- and hetero-histamine H₃ receptors are present in human brain and are potential targets of antipsychotics. These receptors may also display disease-related abnormalities in psychiatric disorders. Here we have assessed how histamine H₃ receptors in human brain may be affected in schizophrenia, bipolar disorder, major depression.

Experimental approach:

Histamine H₃ receptor radioligand binding assays were applied to frozen post-mortem prefrontal and temporal cortical sections and anterior hippocampal sections from subjects with schizophrenia, bipolar disorder, major depression and matched controls.

Key results:

Compared with the controls, increased H₃ receptor radioligand binding was found in dorsolateral prefrontal cortex of schizophrenic subjects (especially the ones who were treated with atypical antipsychotics), and bipolar subjects with psychotic symptoms. No differences in H₃ receptor radioligand binding were found in the temporal cortex. In hippocampal formation of control subjects, H₃ receptor radioligand binding was prominent in dentate gyrus, subiculum, entorhinal cortex and parasubiculum. Decreased H₃ binding was found in the CA4 area of bipolar subjects. Decreased H₃ binding in CA2 and presubiculum of medication-free bipolar subjects was also seen.

Conclusions and implications:

The results suggest that histamine H₃ receptors in the prefrontal cortex take part in the modulation of cognition, which is impaired in schizophrenic subjects and bipolar subjects with psychotic symptoms. Histamine H₃ receptors probably regulate connections between hippocampus and various cortical and subcortical regions and could also be involved in the neuropathology of schizophrenia and bipolar disorder.     

Histamine is required for H3 receptor-mediated alcohol reward inhibition,but not for alcohol consumption or stimulation

Background and Purpose

Conflicting data have been published on whether histamine is inhibitory to the rewarding effects of abused drugs. The purpose of this study was to clarify the role of neuronal histamine and, in particular, H₃ receptors in alcohol dependence-related behaviours, which represent the addictive effects of alcohol.

Experimental Approach

Alcohol-induced conditioned place preference (alcohol-CPP) was used to measure alcohol reward. Alcohol-induced locomotor stimulation, alcohol consumption and kinetics were also assessed. mRNA levels were quantified using radioactive in situ hybridization.

Key Results

Low doses of H₃ receptor antagonists, JNJ-10181457 and JNJ-39220675, inhibited alcohol reward in wild-type (WT) mice. However, these H₃ receptor antagonists did not inhibit alcohol reward in histidine decarboxylase knock-out (HDC KO) mice and a lack of histamine did not alter alcohol consumption. Thus H₃ receptor antagonists inhibited alcohol reward in a histamine-dependent manner. Furthermore, WT and HDC KO mice were similarly stimulated by alcohol. The expression levels of dopamine D₁ and D₂ receptors, STEP61 and DARPP-32 mRNA in striatal subregions were unaltered in HDC KO mice. No differences were seen in alcohol kinetics in HDC KO compared to WT control animals. In addition, JNJ-39220675 had no effect on alcohol kinetics in WT mice.

Conclusions and Implications

These data suggest that histamine is required for the H₃ receptor-mediated inhibition of alcohol-CPP and support the hypothesis that the brain histaminergic system has an inhibitory role in alcohol reward. Increasing neuronal histamine release via H₃ receptor blockade could therefore be a novel way of treating alcohol dependence.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

The Modulatory Role of Spinally Located Histamine Receptors in the Regulation of the Blood Glucose Level in D-Glucose-Fed Mice

The possible roles of spinal histamine receptors in the regulation of the blood glucose level were studied in ICR mice. Mice were intrathecally (i.t.) treated with histamine 1 (H1) receptor agonist (2-pyridylethylamine) or antagonist (cetirizine), histamine 2 (H2) receptor agonist (dimaprit) or antagonist (ranitidine), histamine 3 (H3) receptor agonist (α-methylhistamine) or antagonist (carcinine) and histamine 4 (H4) receptor agonist (VUF 8430) or antagonist (JNJ 7777120), and the blood glucose level was measured at 30, 60 and 120 min after i.t. administration. The i.t. injection with α-methylhistamine, but not carcinine slightly caused an elevation of the blood glucose level. In addition, histamine H1, H2, and H4 receptor agonists and antagonists did not affect the blood glucose level. In D-glucose-fed model, i.t. pretreatment with cetirizine enhanced the blood glucose level, whereas 2-pyridylethylamine did not affect. The i.t. pretreatment with dimaprit, but not ranitidine, enhanced the blood glucose level in D-glucose-fed model. In addition, α-methylhistamine, but not carcinine, slightly but significantly enhanced the blood glucose level D-glucose-fed model. Finally, i.t. pretreatment with JNJ 7777120, but not VUF 8430, slightly but significantly increased the blood glucose level. Although histamine receptors themselves located at the spinal cord do not exert any effect on the regulation of the blood glucose level, our results suggest that the activation of spinal histamine H2 receptors and the blockade of spinal histamine H1 or H3 receptors may play modulatory roles for up-regulation and down-regulation, respectively, of the blood glucose level in D-glucose fed model.     

Proxyfan acts as a neutral antagonist of histamine H3 receptors in the feeding-related hypothalamic ventromedial nucleus

BACKGROUND AND PURPOSE

Centrally acting histamine H₃ receptor ligands are proposed as potential treatments for obesity, although the value of inverse agonists at these receptors is still debated. Functional inhibition of H₃ autoreceptors activates neurones in a hypothalamic ‘satiety’ centre. The H₃ receptor antagonist, proxyfan was used as a tool to assess the action of histaminergic compounds in this model.

EXPERIMENTAL APPROACH

We compared the actions of histamine on feeding with those of an H₃ receptor agonist (imetit) and inverse agonist (thioperamide) in rats and mice. Sites of action were identified by immunohistochemistry and the hypothalamic ventromedial nucleus (VMN) was investigated using electrophysiological techniques.

KEY RESULTS

Central histamine or thioperamide decreased fast-induced feeding, whereas imetit increased feeding. Systemic thioperamide entered the brain to activate hypothalamic feeding centres and to reduce feeding without causing any adverse behaviours. Thioperamide activated neurones in the VMN through an action on histamine autoreceptors, whilst imetit had the opposite effect. Proxyfan administered alone did not affect either feeding or electrical activity. However, it blocked the actions of both thioperamide and imetit, acting as a neutral antagonist in this system.

CONCLUSIONS AND IMPLICATIONS

The H₃ receptor inverse agonist, thioperamide, potently reduced appetite without adverse behavioural effects. This action was blocked by proxyfan, acting as a neutral antagonist in this model and, therefore, this compound is useful in determining the selectivity of H₃ receptor-directed drugs. A major action of thioperamide is through presynaptic autoreceptors, inducing stimulation by endogenous histamine of postsynaptic H₁ receptors on anorectic hypothalamic neurones.