Discovery of a novel member of the histamine receptor family

The Diverse Network of Brain Histamine in Feeding: Dissect its Functions in a Circuit-Specific Way

Feeding is an intrinsic and important behavior regulated by complex molecular, cellular and circuit-level mechanisms, one of which is the brain histaminergic network. In the past decades, many studies have provided a foundation of knowledge about the relationship between feeding and histamine receptors, which are deemed to have therapeutic potential but are not successful in treating feeding- related diseases. Indeed, the histaminergic circuits underlying feeding are poorly understood and characterized. This review describes current knowledge of histamine in feeding at the receptor level. Further, we provide insight into putative histamine-involved feeding circuits based on the classic feeding circuits. Understanding the histaminergic network in a circuit-specific way may be therapeutically relevant for increasing the drug specificity and precise treatment in feeding-related diseases.

Structural insights into the agonists binding and receptor selectivity of human histamine H4 receptor

Histamine is a biogenic amine that participates in allergic and inflammatory processes by stimulating histamine receptors. The histamine H4 receptor (H4R) is a potential therapeutic target for chronic inflammatory diseases such as asthma and atopic dermatitis. Here, we show the cryo-electron microscopy structures of the H4R-Gq complex bound with an endogenous agonist histamine or the selective agonist imetit bound in the orthosteric binding pocket. The structures demonstrate binding mode of histamine agonists and that the subtype-selective agonist binding causes conformational changes in Phe3447.39, which, in turn, form the "aromatic slot". The results provide insights into the molecular underpinnings of the agonism of H4R and subtype selectivity of histamine receptors, and show that the H4R structures may be valuable in rational drug design of drugs targeting the H4R.

Global Research Trends and Hotspots Analysis of the Scientific Production of Amitriptyline: A Bibliometric Approach

Amitriptyline was first introduced as a medication to treat depression. Over time, this substance has been used to treat other conditions, such as gastrointestinal disorders, fibromyalgia, neuropathic pain, and analgesia, among others. However, there are no published studies that provide a broad view of the possible motivations that have led to changes in the use of amitriptyline. In this study, we have identified the landscape of use for amitriptyline based on knowledge mapping of the 100 most-cited articles about this drug. We searched Web of Science Core Collection without time and language restrictions. We obtained 14,446 results, but we only used the 100 most-cited articles that had amitriptyline as the object of study. We collected the following information from each article: authors, country of the corresponding authors, year of publication, citation count, citation density (number of citations per year), and keywords. In addition, we seek to map in the chosen articles study design and research findings. We found that since 1980, the use of amitriptyline has expanded beyond depression, moving to off-label use to treat a variety of diseases and conditions, including post-herpetic neuralgia, neuropathic pain, primary fibrosis, fibromyalgia, and migraine, can be considered a drug with more clinical applicability than its original clinical indication.

Histamine and Its Receptors in the Mammalian Inner Ear: A Scoping Review

BACKGROUND

Histamine is a widely distributed biogenic amine with multiple biological functions mediated by specific receptors that determine the local effects of histamine. This review aims to summarize the published findings on the expression and functional roles of histamine receptors in the inner ear and to identify potential research hotspots and gaps.

METHODS

A search of the electronic databases PubMed, Web of Science, and OVID EMBASE was performed using the keywords histamine, cochlea*, and inner ear. Of the 181 studies identified, 18 eligible publications were included in the full-text analysis.

RESULTS

All four types of histamine receptors were identified in the mammalian inner ear. The functional studies of histamine in the inner ear were mainly in vitro. Clinical evidence suggests that histamine and its receptors may play a role in Ménière's disease, but the exact mechanism is not fully understood. The effects of histamine on hearing development remain unclear.

CONCLUSIONS

Existing studies have successfully determined the expression of all four histamine receptors in the mammalian inner ear. However, further functional studies are needed to explore the potential of histamine receptors as targets for the treatment of hearing and balance disorders.

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

Effects of the Antidepressant Amitriptyline on Juvenile Brown Trout and Their Modulation by Microplastics

Pharmaceuticals such as antidepressants are designed to be bioactive at low concentrations. According to their mode of action, they can also influence non-target organisms due to the phylogenetic conservation of molecular targets. In addition to the pollution by environmental chemicals, the topic of microplastics (MP) in the aquatic environment came into the focus of scientific and public interest. The aim of the present study was to investigate the influence of the antidepressant amitriptyline in the presence and absence of irregularly shaped polystyrene MP as well as the effects of MP alone on juvenile brown trout (Salmo trutta f. fario). Fish were exposed to different concentrations of amitriptyline (nominal concentrations between 1 and 1000 µg/L) and two concentrations of MP (104 and 105 particles/L; <50 µm) for three weeks. Tissue cortisol concentration, oxidative stress, and the activity of two carboxylesterases and of acetylcholinesterase were assessed. Furthermore, the swimming behavior was analyzed in situations with different stress levels. Exposure to amitriptyline altered the behavior and increased the activity of acetylcholinesterase. Moreover, nominal amitriptyline concentrations above 300 µg/L caused severe acute adverse effects in fish. MP alone did not affect any of the investigated endpoints. Co-exposure caused largely similar effects such as the exposure to solely amitriptyline. However, the effect of amitriptyline on the swimming behavior during the experiment was alleviated by the higher MP concentration.

Mechanism of cholangiocellular damage and repair during cholestasis

The mechanism of damage of the biliary epithelium remains partially unexplored. However, recently many works have offered new evidence regarding the cholangiocytes' damage process, which is the main target in a broad spectrum of pathologies ranging from acute cholestasis, cholangiopathies to cholangiocarcinoma. This is encouraging since some works addressed this epithelium's relevance in health and disease until a few years ago. The biliary tree in the liver, comprised of cholangiocytes, is a pipeline for bile flow and regulates key hepatic processes such as proliferation, regeneration, immune response, and signaling. This review aimed to compile the most recent advances on the mechanisms of cholangiocellular damage during cholestasis, which, although it is present in many cholangiopathies, is not necessarily a common or conserved process in all of them, having a relevant role cAMP and PKA during obstructive cholestasis, as well as Ca²⁺-dependent PKC in functional cholestasis. Cholangiocellular damage could vary according to the type of cholestasis, the aggressor, or the bile ducts' location where it develops and what kind of damage can favor cholangiocellular carcinoma development.

Histamine in cancer immunology and immunotherapy. Current status and new perspectives

Cancer is the second leading cause of death globally and its incidence and mortality are rapidly increasing worldwide. The dynamic interaction of immune cells and tumor cells determines the clinical outcome of cancer. Immunotherapy comes to the forefront of cancer treatments, resulting in impressive and durable responses but only in a fraction of patients. Thus, understanding the characteristics and profiles of immune cells in the tumor microenvironment (TME) is a necessary step to move forward in the design of new immunomodulatory strategies that can boost the immune system to fight cancer. Histamine produces a complex and fine-tuned regulation of the phenotype and functions of the different immune cells, participating in multiple regulatory responses of the innate and adaptive immunity. Considering the important actions of histamine-producing immune cells in the TME, in this review we first address the most important immunomodulatory roles of histamine and histamine receptors in the context of cancer development and progression. In addition, this review highlights the current progress and foundational developments in the field of cancer immunotherapy in combination with histamine and pharmacological compounds targeting histamine receptors.

Inhibition of Astrocytic Histamine N-Methyltransferase as a Possible Target for the Treatment of Alzheimer's Disease

Alzheimer’s disease (AD) represents the principal cause of dementia among the elderly. Great efforts have been established to understand the physiopathology of AD. Changes in neurotransmitter systems in patients with AD, including cholinergic, GABAergic, serotoninergic, noradrenergic, and histaminergic changes have been reported. Interestingly, changes in the histaminergic system have been related to cognitive impairment in AD patients. The principal pathological changes in the brains of AD patients, related to the histaminergic system, are neurofibrillary degeneration of the tuberomammillary nucleus, the main source of histamine in the brain, low histamine levels, and altered signaling of its receptors. The increase of histamine levels can be achieved by inhibiting its degrading enzyme, histamine N-methyltransferase (HNMT), a cytoplasmatic enzyme located in astrocytes. Thus, increasing histamine levels could be employed in AD patients as co-therapy due to their effects on cognitive functions, neuroplasticity, neuronal survival, neurogenesis, and the degradation of amyloid beta (Aβ) peptides. In this sense, the evaluation of the impact of HNMT inhibitors on animal models of AD would be interesting, consequently highlighting its relevance.

Specific Engineered G Protein Coupling to Histamine Receptors Revealed from Cellular Assay Experiments and Accelerated Molecular Dynamics Simulations

G protein-coupled receptors (GPCRs) are targets of extracellular stimuli and hence occupy a key position in drug discovery. By specific and not yet fully elucidated coupling profiles with α subunits of distinct G protein families, they regulate cellular responses. The histamine H2 and H4 receptors (H2R and H4R) are prominent members of Gs- and Gi-coupled GPCRs. Nevertheless, promiscuous G protein and selective Gi signaling have been reported for the H2R and H4R, respectively, the molecular mechanism of which remained unclear. Using a combination of cellular experimental assays and Gaussian accelerated molecular dynamics (GaMD) simulations, we investigated the coupling profiles of the H2R and H4R to engineered mini-G proteins (mG). We obtained coupling profiles of the mGs, mGsi, or mGsq proteins to the H2R and H4R from the mini-G protein recruitment assays using HEK293T cells. Compared to H2R-mGs expressing cells, histamine responses were weaker (pEC50, Emax) for H2R-mGsi and -mGsq. By contrast, the H4R selectively bound to mGsi. Similarly, in all-atom GaMD simulations, we observed a preferential binding of H2R to mGs and H4R to mGsi revealed by the structural flexibility and free energy landscapes of the complexes. Although the mG α5 helices were consistently located within the HR binding cavity, alternative binding orientations were detected in the complexes. Due to the specific residue interactions, all mG α5 helices of the H2R complexes adopted the Gs-like orientation toward the receptor transmembrane (TM) 6 domain, whereas in H4R complexes, only mGsi was in the Gi-like orientation toward TM2, which was in agreement with Gs- and Gi-coupled GPCRs structures resolved by X-ray/cryo-EM. These cellular and molecular insights support (patho)physiological profiles of the histamine receptors, especially the hitherto little studied H2R function in the brain, as well as of the pharmacological potential of H4R selective drugs.

Histamine-4 Receptor: Emerging Target for the Treatment of Neurological Diseases

A major challenge in the field of the biogenic amine histamine is the search for new-generation histamine receptor specific drugs. Daniel Bovet and Sir James Black received their Nobel Prizes for Medicine for their work on histamine-1 receptor (H₁R) and H₂R antagonists to treat allergies and gastrointestinal disorders. The first H₃R-targeting drug to reach the market was approved for the treatment of the neurological disorder narcolepsy in 2018. The antagonists for the most recently identified histamine receptor, H₄R, are currently under clinical evaluation for their potential therapeutic effects on inflammatory diseases such as atopic dermatitis and pruritus. In this chapter, we propose that H₄R antagonists are endowed with prominent anti-inflammatory and immune effects, including in the brain. To substantiate this proposition, we combine data from transcriptional analyses of postmortem human neurodegenerative disease brain samples, human genome-wide association studies (GWAS), and translational animal model studies. The results prompt us to suggest the potential involvement of the H₄R in various neurodegenerative diseases and how manipulating the H₄R may create new therapeutic opportunities in central nervous system diseases.

The Function of the Histamine H4 Receptor in Inflammatory and Inflammation-Associated Diseases of the Gut

Histamine is a pleiotropic mediator involved in a broad spectrum of (patho)-physiological processes, one of which is the regulation of inflammation. Compounds acting on three out of the four known histamine receptors are approved for clinical use. These approved compounds comprise histamine H1-receptor (H₁R) antagonists, which are used to control allergic inflammation, antagonists at H₂R, which therapeutically decrease gastric acid release, and an antagonist at H₃R, which is indicated to treat narcolepsy. Ligands at H₄R are still being tested pre-clinically and in clinical trials of inflammatory diseases, including rheumatoid arthritis, asthma, dermatitis, and psoriasis. These trials, however, documented only moderate beneficial effects of H₄R ligands so far. Nevertheless, pre-clinically, H₄R still is subject of ongoing research, analyzing various inflammatory, allergic, and autoimmune diseases. During inflammatory reactions in gut tissues, histamine concentrations rise in affected areas, indicating its possible biological effect. Indeed, in histamine-deficient mice experimentally induced inflammation of the gut is reduced in comparison to that in histamine-competent mice. However, antagonists at H₁R, H₂R, and H₃R do not provide an effect on inflammation, supporting the idea that H₄R is responsible for the histamine effects. In the present review, we discuss the involvement of histamine and H₄R in inflammatory and inflammation-associated diseases of the gut.

Impact of histaminergic H3 receptor antagonist on hypoglossal nucleus in chronic intermittent hypoxia conditions

RATIONALE

The hypoglossal nucleus (HN) controls the movement of the genioglossus (GG) muscle whose dysfunction leads to airway occlusion and occurrence of obstructive sleep apnea (OSA). Histamine produced by the tuberomammillary nucleus (TMN) has a potent excitatory action on GG muscle activity.

OBJECTIVES

The aim of the study was to investigate the role histaminergic neurons play in the regulation of the genioglossus.

METHODS

C57BL/6 mice were exposed to chronic intermittent hypoxia (CIH) for 3 weeks to resemble OSA. The histamine H3 receptor (H3R) antagonist ciproxifan was applied to increase histamine in the brain. Histamine levels and GG activity were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and electromyogram (EMG) separately. Neuronal activity and repair ability of the HN and TMN and key proteins of histamine were analyzed by immunohistochemistry and western blots.

RESULTS

Significant decline of histamine level and GG activity of the HN and TMN induced by CIH exposure could be ameliorated by ciproxifan. Application of ciproxifan could also partly reverse the decline of the histidine decarboxylase (HDC) by CIH.

CONCLUSIONS

This investigation studied the impacts of ciproxifan on the HN and TMN in CIH conditions and revealed that the negative effects on the HN and TMN caused by CIH could be partly ameliorated by ciproxifan, which might open new perspectives for the development of pharmacological treatment for OSA.

Evolutionary history of histamine receptors: Early vertebrate origin and expansion of the H3-H4 subtypes

Histamine receptors belonging to the superfamily of G protein-coupled receptors (GPCRs) mediate the diverse biological effects of biogenic histamine. They are classified into four phylogenetically distinct subtypes H₁-H₄, each with a different binding affinity for histamine and divergent downstream signaling pathways. Here we present the evolutionary history of the histamine receptors using a phylogenetic approach complemented with comparative genomics analyses of the sequences, gene structures, and synteny of gene neighborhoods. The data indicate the earliest emergence of histamine-mediated GPCR signaling by a H₂ in a prebilaterian ancestor. The analyses support a revised classification of the vertebrate H₃-H₄ receptor subtypes. We demonstrate the presence of the H₄ across vertebrates, contradicting the currently held notion that H₄ is restricted to mammals. These non-mammalian vertebrate H₄ orthologs have been mistaken for H₃. We also identify the presence of a new H₃ subtype (H₃B), distinct from the canonical H₃ (H₃A), and propose that the H₃A, H₃B, and H₄ likely emerged from a H₃ progenitor through the 1R/2R whole genome duplications in an ancestor of the vertebrates. It is apparent that the ability of the H₁, H₂, and H_3-4 to bind histamine was acquired convergently. We identified genomic signatures suggesting that the H₁ and H₃-H₄ shared a last common ancestor with the muscarinic receptor in a bilaterian predecessor whereas, the H₂ and the α-adrenoreceptor shared a progenitor in a prebilaterian ancestor. Furthermore, site-specific analysis of the vertebrate subtypes revealed potential residues that may account for the functional divergence between them.

Eosinophils adhesion assay as a tool for phenotypic drug screening - The pharmacology of 1,3,5 - Triazine and 1H-indole like derivatives against the human histamine H4 receptor

Histamine is a pleiotropic biogenic amine, having affinity towards four distinct histamine receptors. The existing pharmacological studies suggest the usefulness of histamine H₄ receptor ligands in the treatment of many inflammatory and immunomodulatory diseases, including allergic rhinitis, asthma, atopic dermatitis, colitis or pruritus. Up to date, several potent histamine H₄ receptor ligands were developed, none of which was registered as a drug yet. In this study, a series of potent indole-like and triazine derivatives were tested, in radioligand displacement and functional assays at histamine H₄ receptor, as well as in human eosinophils adhesion assay to endothelium. For selected compounds permeability, cytotoxicity, metabolic and in vivo studies were conducted. Adhesion assay differentiated the activity of different groups of compounds with a known affinity towards the histamine H₄ receptor. Most of the tested compounds downregulated the number of adherent cells. However, adhesion assay revealed additional properties of tested compounds that had not been detected in radioligand displacement and aequorin-based functional assays. Furthermore, for some tested compounds, these abnormal effects were confirmed during the in vivo studies. In conclusion, eosinophils adhesion assay uncovered pharmacological activity of histamine H₄ receptor ligands that has been later confirmed in vivo, underscoring the value of well-suited cell-based phenotypic screening approach in drug discovery.

Interacting Effects of Polystyrene Microplastics and the Antidepressant Amitriptyline on Early Life Stages of Brown Trout (Salmo trutta f. fario)

Whether microplastics themselves or their interactions with chemicals influence the health and development of aquatic organisms has become a matter of scientific discussion. In aquatic environments, several groups of chemicals are abundant in parallel to microplastics. The tricyclic antidepressant amitriptyline is frequently prescribed, and residues of it are regularly found in surface waters. In the present study, the influence of irregularly shaped polystyrene microplastics (<50 µm), amitriptyline, and their mixture on early life-stages of brown trout were investigated. In a first experiment, the impacts of 100, 104, and 105 particles/L were studied from the fertilization of eggs until one month after yolk-sac consumption. In a second experiment, eggs were exposed in eyed ova stages to 105, 106 particles/L, to amitriptyline (pulse-spiked, average 48 ± 33 µg/L) or to two mixtures for two months. Microplastics alone did neither influence the development of fish nor the oxidative stress level or the acetylcholinesterase activity. Solely, a slight effect on the resting behavior of fry exposed to 106 particles/L was observed. Amitriptyline exposure exerted a significant effect on development, caused elevated acetylcholinesterase activity and inhibition of two carboxylesterases. Most obvious was the severely altered swimming and resting behavior. However, effects of amitriptyline were not modulated by microplastics.

Conformational Restriction of Histamine with a Rigid Bicyclo[3.1.0]hexane Scaffold Provided Selective H3 Receptor Ligands

We designed and synthesized conformationally rigid histamine analogues with a bicyclo[3.1.0]hexane scaffold. All the compounds were selectively bound to the H₃ receptor subtype over the H₄ receptor subtype. Notably, compound 7 showed potent binding affinity and over 100-fold selectivity for the H₃ receptors (K_i = 5.6 nM for H₃ and 602 nM for H₄). These results suggest that the conformationally rigid bicyclo[3.1.0]hexane structure can be a useful scaffold for developing potent ligands selective for the target biomolecules.

Impact of histamine H4 receptor deficiency on the modulation of T cells in a murine breast cancer model

BACKGROUND

The histamine H4 receptor (H4R) is preferentially expressed in immune cells and is a potential therapeutic target for inflammatory and autoimmune diseases. This study aimed at further exploring the role of H4R in the immunobiology of breast cancer.

METHODS

We used wild type (WT) and H4R deficient mice (KO) to evaluate whether H4R genotypes show a different distribution of T cell subsets in spleens, tumours and tumour draining lymph nodes (TDLN) in a syngeneic ErbB2-positive breast cancer model developed orthotopically with LM3 cells and its impact on tumour growth.

RESULTS

The presence of tumours had a differential impact on the distribution of T cells in TDLN from KO mice compared to WT ones. At day 21 post-inoculation (p.i.) of cells, despite no significant changes in the tumour weight, TDLN from KO mice showed a significantly increased proportion of CD8⁺ T cells compared to WT mice. At day 38 p.i. of cells a reduced tumour weight was evident in KO mice. This was accompanied by a decreased proportion of CD4⁺CD25⁺FoxP3⁺ regulatory T cells in TDLN of KO compared to WT mice. Tumour-bearing KO mice showed a better survival compared to WT mice.

CONCLUSIONS

H4R-mediated mechanisms may modulate the immune tumour microenvironment, promoting an immunosuppressive milieu. Results suggest that H4R could be explored as an immunotherapeutic target with potential benefit in combination with immunotherapy. Further preclinical and clinical studies are necessary to confirm this hypothesis.

Genetic Deficiency of the Histamine H4-Receptor Reduces Experimental Colorectal Carcinogenesis in Mice

Colorectal cancer (CRC), a severe complication of inflammatory bowel diseases, is a common type of cancer and accounts for high mortality. CRC can be modeled in mice by application of the tumor promoter, azoxymethane (AOM), in combination with dextran sodium sulfate (DSS), which are able to induce colitis-like manifestations. Active colitis correlates with high mucosal concentrations of histamine, which, together with the histamine receptor subtype 4 (H₄R), provide a pro-inflammatory function in a mouse colitis model. Here, we analyzed whether H₄R is involved in the pathogenesis of AOM/DSS-induced CRC in mice. As compared to wild type (WT) mice, AOM/DSS-treated mice lacking H₄R expression (TM) demonstrate ameliorated signs of CRC, i.e., significantly reduced loss of body weight, stiffer stool consistency, and less severe perianal bleeding. Importantly, numbers and diameters of tumors and the degree of colonic inflammation are dramatically reduced in TM mice as compared to WT mice. This is concomitant with a reduced colonic inflammatory response involving expression of cyclooxygenase 2 and the production of C-X-C motif chemokine ligand 1 (CXCL1) and CXCL2. We conclude that H₄R is involved in the tumorigenesis of chemically-induced CRC in mice via cyclooxygenase 2 expression and, probably, CXCL1 and CXCL2 as effector molecules.

Differential Role of Serines and Threonines in Intracellular Loop 3 and C-Terminal Tail of the Histamine H4 Receptor in β-Arrestin and G Protein-Coupled Receptor Kinase Interaction, Internalization, and Signaling

The histamine H₄ receptor (H₄R) activates Gα_i-mediated signaling and recruits β-arrestin2 upon stimulation with histamine. β-Arrestins play a regulatory role in G protein-coupled receptor (GPCR) signaling by interacting with phosphorylated serine and threonine residues in the GPCR C-terminal tail and intracellular loop 3, resulting in receptor desensitization and internalization. Using bioluminescence resonance energy transfer (BRET)-based biosensors, we show that G protein-coupled receptor kinases (GRK) 2 and 3 are more quickly recruited to the H₄R than β-arrestin1 and 2 upon agonist stimulation, whereas receptor internalization dynamics toward early endosomes was slower. Alanine-substitution revealed that a serine cluster at the distal end of the H₄R C-terminal tail is essential for the recruitment of β-arrestin1/2, and consequently, receptor internalization and desensitization of G protein-driven extracellular-signal-regulated kinase (ERK)1/2 phosphorylation and label-free cellular impedance. In contrast, alanine substitution of serines and threonines in the intracellular loop 3 of the H₄R did not affect β-arrestin2 recruitment and receptor desensitization, but reduced β-arrestin1 recruitment and internalization. Hence, β-arrestin recruitment to H₄R requires the putative phosphorylated serine cluster in the H₄R C-terminal tail, whereas putative phosphosites in the intracellular loop 3 have different effects on β-arrestin1 versus β-arrestin2. Mutation of these putative phosphosites in either intracellular loop 3 or the C-terminal tail did not affect the histamine-induced recruitment of GRK2 and GRK3 but does change the interaction of H₄R with GRK5 and GRK6, respectively. Identification of H₄R interactions with these proteins is a first step in the understanding how this receptor might be dysregulated in pathophysiological conditions.

Investigation on beneficial role of l-carnosine in neuroprotective mechanism of ischemic postconditioning in mice: possible role of histidine histamine pathway

OBJECTIVE

The present study was undertaken to investigate the possible role of histidine-histamine pathway in the neuroprotective effects produced by L-carnosine hand in hand with ischemic postconditioning in the animal model of cerebral ischemia.

METHODS

Cerebral ischemia was induced in swiss albino mice by performing BCCAO surgery. Morris water-maze test was utilized to assess the learning ability and memory of the animals. The whole brain acetylcholinesterase (AChE) activity, TBARS, GSH levels and MPO activity were evaluated as the biochemical parameters. For histopathological evaluation of the cerebral infarct size, TTC staining was employed.

RESULTS

Administration of L-carnosine (500   mg/kg, i.p.) successfully attenuated the manifestations of cerebral ischemia. Higher levels of AChE, TBARS, and MPO were observed in BCCAO treated animals, which were successfully attenuated by treatment with L-carnosine and ischemic postconditioning. Whereas administration of L-carnosine and ischemic postconditioning significantly increased the level of GSH in BCCAO treated animals. Moreover, treatment with ranitidine, an H₂ blocker (30 NMol, i.c.v) antagonized the neuroprotective actions of L-carnosine evidenced by decrease in MWM performance, increase in the level of AChE and oxidative stress, while decrease in GSH level in brain. The cerebral infarct size was found to be more in BCCAO inflicted animals, which was improved by the administration of L-carnosine, while the cerebral infarct size worsened by treatment with ranitidine (3   nmol, i.c.v.).

CONCLUSION

It is concluded that L-carnosine exerts neuroprotective effect via involvement of histidine-histamine pathway since the beneficial effects of L-carnosine were abolished by the H₂-blocker.

Stimulation of histamine H4 receptor participates in cold-induced browning of subcutaneous white adipose tissue

Although many studies have shown that histamine and its signaling regulate energy homeostasis through the central nervous system, their roles in adipose tissues remain poorly understood. Here, we identified that the histamine H4 receptor (HrH4) was highly expressed in adipocytes at a level higher than that of the other three receptors (i.e., HrH1, HrH2, and HrH3). The HrH4 expression in adipocytes responded to cold through thermogenesis and lipolysis, supported by results from both mouse and cell models. When HrH4 expression was knocked down in the subcutaneous white adipose tissue (scWAT), browning and lipolysis effects triggered by cold were ablated, and the oxygen consumption was also lowered both at the normal and cold conditions. Moreover, mice exhibited browned scWAT, accelerated metabolic rates, and tolerance to hypothermia when 4-methylhistamine (4MH), a selective HrH4 agonist, was adjacently injected to the scWAT. Consistent with these findings, 4MH also triggered the browning and lipolytic effects in cultured C3H10T1/2 adipocytes. Mechanically, we demonstrated that p38/MAPK and ERK/MAPK pathways were involved in these processes. In conclusion, our findings have uncovered an effective role of HrH4 in adipose tissue browning.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.

Histamine pharmacology: from Sir Henry Dale to the 21st century

Histamine has been one of the most studied substances in medicine, playing a major role in diverse (patho)physiological processes. It elicits its multifaceted modulatory functions by activating four types of GPCRs, designated as H_1-4 . Despite the heterogeneity and the complexity of histamine receptor pharmacology, many discoveries over the past 100 years resulted in the development of H₁ antihistamines and H₂ -targeting 'blockbuster' therapeutics for the management of allergies and gastrointestinal disorders respectively. Recently, a first-in-class H₃ inverse agonist was approved for the treatment of narcolepsy, whereas H₄ antagonists are under clinical evaluation for their potential therapeutic exploitation in immune-related diseases. This review critically presents the past successes and drawbacks in histamine research, complemented by the modern conceptual innovations in molecular and receptor pharmacology. It targets both young and experienced researchers in an ongoing effort to stimulate novel insights for the dissection of the translational potential of histamine pharmacology. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

A Selective Histamine H4 Receptor Antagonist, JNJ7777120, Is Protective in a Rat Model of Transient Cerebral Ischemia

Cerebral ischemia is a multifactorial pathology characterized by different events evolving in time. The acute injury, characterized by excitoxicity, is followed by a secondary brain injury that develops from hours to days after ischemia. Extracellular levels of histamine increase in the ischemic area after focal cerebral ischemia induced by occlusion of the middle cerebral artery (MCAo). The histamine H4 receptor (H4R) is predominantly expressed in cell types of immune system where is involved in the regulation of immunological and inflammatory responses, and in numerous area of the Central Nervous System (CNS) including cortex and striatum. Our aim was to assess the putative neuroprotective effects of the potent and selective H4R antagonist, JNJ7777120 (JNJ), chronically administered (1 mg/kg, i.p., twice/day for 7 days) on damage parameters in a rat model of focal ischemia induced by transient MCAo (tMCAo). Chronic treatment with the H4R antagonist JNJ, significantly protected from the neurological deficit and from body weight loss after tMCAo. Seven days after the ischemic insult, JNJ reduced the volume of the ischemic cortical and striatal damage, the number of activated microglia and astrocytes in the ischemic cortex and striatum and decreased the plasma levels of IL-1β and TNF-α, while increased the levels of IL-10. Two days after ischemia, JNJ has reduced granulocyte infiltration in the ischemic area. Results demonstrate that the selective antagonist of H4R, JNJ, systemically and chronically administered after ischemia, reduces the ischemic brain damage, improves the neurological deficit and decreases blood pro-inflammatory cytokines, suggesting that H4R is a valuable pharmacological target after focal brain ischemia.

Histamine H4 receptor as a novel therapeutic target for the treatment of Leydig-cell tumours in prepubertal boys

Leydig-cell tumours (LCTs) are rare endocrine tumours of the testicular interstitium, with recent increased incidence. Symptoms include precocious puberty in children; and erectile dysfunction, infertility and/or gynaecomastia, in adults. So far, scientific evidence points to aromatase (CYP19) overexpression and excessive oestrogen and insulin-like growth factor (IGF) -1 production as responsible for Leydig-cell tumourigenesis. LCTs are usually benign; however, malignant LCTs respond poorly to chemo/radiotherapy, highlighting the need to identify novel targets for treatment. Herein, we investigated the potential role of the histamine receptor H4 (HRH4) as a therapeutic target for LCTs using R2C rat Leydig tumour cells, a well-documented in vitro model for Leydigioma. Also, we studied for the first time the expression of CYP19, IGF-1R, oestrogen receptor (ER) α, ERβ, androgen receptor (AR) and HRH4 in human prepubertal LCTs versus normal prepubertal testes (NPTs). HRH4 agonist treatment inhibited steroidogenesis and proliferation in R2C cells and also negatively affected their pro-angiogenic capacity in vitro and in vivo, as assessed by evaluating the proliferative activity of human umbilical vein endothelial cells and by means of the quail chorioallantoic membrane assay, respectively. Moreover, E2 and IGF-1 inhibited HRH4 mRNA and protein levels. In human prepubertal LCTs, CYP19, IGF-1R, ERα and ERβ were overexpressed compared with NPTs. In contrast, HRH4 staining was weak in LCTs, but moderate/strong and confined to the interstitium in NPTs. Importantly, HRH4 was absent or barely detectable in seminiferous tubules or germ cells. Overall, our results point to HRH4 as a novel therapeutic target in LCTs.

The Role of Histamine and Histamine Receptors in Mast Cell-Mediated Allergy and Inflammation: The Hunt for New Therapeutic Targets

Histamine and its receptors (H1R–H4R) play a crucial and significant role in the development of various allergic diseases. Mast cells are multifunctional bone marrow-derived tissue-dwelling cells that are the major producer of histamine in the body. H1R are expressed in many cells, including mast cells, and are involved in Type 1 hypersensitivity reactions. H2R are involved in Th1 lymphocyte cytokine production. H3R are mainly involved in blood–brain barrier function. H4R are highly expressed on mast cells where their stimulation exacerbates histamine and cytokine generation. Both H1R and H4R have important roles in the progression and modulation of histamine-mediated allergic diseases. Antihistamines that target H1R alone are not entirely effective in the treatment of acute pruritus, atopic dermatitis, allergic asthma, and other allergic diseases. However, antagonists that target H4R have shown promising effects in preclinical and clinical studies in the treatment of several allergic diseases. In the present review, we examine the accumulating evidence suggesting novel therapeutic approaches that explore both H1R and H4R as therapeutic targets for histamine-mediated allergic diseases.

Visualization of the activation of the histamine H3 receptor (H3R) using novel fluorescence resonance energy transfer biosensors and their potential application to the study of H3R pharmacology

Activation of the histamine-3 receptor (H3R) is involved in memory processes and cognitive action, while blocking H3R activation can slow the progression of neurological disorders, such as Alzheimer's disease, schizophrenia and narcolepsy. To date, however, no direct way to examine the activation of H3R has been utilized. Here, we describe a novel biosensor that can visualize the activation of H3R through an intramolecular fluorescence resonance energy transfer (FRET) signal. To achieve this, we constructed an intramolecular H3R FRET sensor with cyan fluorescent protein (CFP) attached at the C terminus and yellow fluorescent protein (YFP) inserted into the third intracellular loop. The sensor was found to internalize normally on agonist treatment. We measured FRET signals between the donor CFP and the acceptor YFP in living cells in real time, the results of which indicated that H3R agonist treatment (imetit or histamine) increases the FRET signal in a time- and concentration-dependent manner with Kon and Koff values consistent with published data and which maybe correlated with decreasing cAMP levels and the promotion of ERK1/2 phosphorylation. The FRET signal was inhibited by H3R antagonists, and the introduction of mutations at F419A, F423A, L426A and L427A, once again, the promotion of ERK1/2 phosphorylation, was diminished. Thus, we have built a H3R biosensor which can visualize the activation of receptor through real-time structure changes and which can obtain pharmacological kinetic data at the same time. The FRET signals may allow the sensor to become a useful tool for screening compounds and optimizing useful ligands.

Mechanisms of cholangiocyte responses to injury

Cholangiocytes, epithelial cells that line the biliary epithelium, are the primary target cells for cholangiopathies including primary sclerosing cholangitis and primary biliary cholangitis. Quiescent cholangiocytes respond to biliary damage and acquire an activated neuroendocrine phenotype to maintain the homeostasis of the liver. The typical response of cholangiocytes is proliferation leading to bile duct hyperplasia, which is a characteristic of cholestatic liver diseases. Current studies have identified various signaling pathways that are associated with cholangiocyte proliferation/loss and liver fibrosis in cholangiopathies using human samples and rodent models. Although recent studies have demonstrated that extracellular vesicles and microRNAs could be mediators that regulate these messenger/receptor axes, further studies are required to confirm their roles. This review summarizes current studies of biliary response and cholangiocyte proliferation during cholestatic liver injury with particular emphasis on the secretin/secretin receptor axis. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Role of the Histamine H4-Receptor in Bronchial Asthma

Histamine is a pro-inflammatory mediator with a prominent role in allergic diseases. Antagonists at the histamine receptor subtype 1 are central in anti-allergic therapies, with the exception of allergic asthma, where they are clinically without effect. The latest identified histamine receptor subtype 4, which is expressed mainly in hematopoietic cells, now provides a reasonable target for new therapeutic strategies inhibiting histamine function. The pathophysiology of allergy, esp. allergic asthma, and in its context the effects of antagonists at the histamine receptor subtype 4 in preclinical and clinical settings are discussed in this chapter.

Increased serum soluble vascular endothelial cadherin levels in patients with chronic spontaneous urticaria

BACKGROUND

Chronic spontaneous urticaria (CSU) is a common skin disease characterized by recurrent itchy wheals with or without angioedema that lasts longer than 6 weeks. Vascular endothelial (VE)-cadherin is an endothelial cell-specific adhesion molecule that plays critical roles in angiogenesis and endothelial permeability.

OBJECTIVE

To investigate serum levels of soluble VE (sVE)-cadherin in patients with CSU.

METHODS

Serum levels of sVE-cadherin in patients with CSU, patients with atopic dermatitis, and healthy controls were determined by enzyme-linked immunosorbent assay. In addition, changes in sVE-cadherin serum levels were compared in patients with CSU before and after H₁ antihistamine treatment. Furthermore, the effects of histamine on sVE-cadherin release by HMEC-1 cells were determined by enzyme-linked immunosorbent assay. The inhibition effects of H₁ antihistamine and H₂ antihistamine on sVE-cadherin release, VE-cadherin phosphorylation, and VE-cadherin disruption were evaluated in histamine-treated HMEC-1 cells by western blot and immunofluorescence.

RESULTS

Serum levels of sVE-cadherin in patients with CSU were significantly higher than those in patients with atopic dermatitis and healthy controls. Serum sVE-cadherin levels in patients with CSU were correlated with the severity of CSU according to Urticaria Activity Scores. Furthermore, serum sVE-cadherin levels in patients with CSU at pretreatment decreased after H₁ antihistamine treatment. In addition, histamine markedly induced sVE-cadherin release in HMEC-1 cells. Moreover, H₁ antihistamine, but not H₂ antihistamine, significantly inhibited sVE-cadherin release in histamine-treated HMEC-1 cells. Western blot data showed that histamine induced phosphorylation of VE-cadherin in HMEC-1 cells, which was blocked by H₁ antihistamine.

CONCLUSION

The present data showed serum levels of sVE-cadherin are increased in patients with CSU. Histamine-induced sVE-cadherin release from endothelial cells could play a role in the pathogenesis of CSU.

Suppression of osteogenic differentiation in mesenchymal stem cells from patients with ossification of the posterior longitudinal ligament by a histamine-2-receptor antagonist

Mesenchymal stem cells (MSCs) in ossification of the posterior longitudinal ligament (OPLL) patients have a high propensity toward osteogenesis. Histamine receptor H2 (H2R) antagonists (H2 blockers) like famotidine decrease ossification in patients, by an unclear mechanism. To confirm that MSCs express H2R and to clarify how H2 blockers suppress osteogenic differentiation, we used spinal-ligament MSCs from patients with OPLL or with cervical spondylotic myelopathy (CSM) (control). The MSCs were treated with 10, 30, or 100nM famotidine for 7 or 21 days. Flow cytometry revealed that cells from both groups expressed MSC surface markers CD44, CD90, and CD105 (> 97.5%) but not CD34 or CD45 (< 2.5%). Immunoblotting showed that the MSCs from both groups expressed H2R, but those from OPLL patients expressed it at higher levels. Real-time qPCR indicated the H2R expression was significantly suppressed by 30nM famotidine for 7 days or by 30 or 100nM for 21 days. However, histidine decarboxylase, a key enzyme in histamine production, did not change significantly after famotidine addition. Famotidine treatment at 100nM for 21 days significantly suppressed mRNA expression of the osteogenic markers osteocalcin (OCN), bone morphogenetic protein 2 (BMP2), and runt-related transcription factor 2 (RUNX2) only in OPLL-derived MSCs. Immunoblots showed that famotidine suppressed BMP2 and OCN in the OPLL group and H2R and RUNX2 in both groups. These results suggest famotidine inhibits osteogenic differentiation in OPLL-derived MSCs by acting as an H2R antagonist, but also by decreasing H2R expression, and support the clinical use of famotidine to treat OPLL.

Histamine and Histamine H4 Receptor Promotes Osteoclastogenesis in Rheumatoid Arthritis

Histamine H4 receptor (H4R) has immune-modulatory and chemotaxic effects in various immune cells. This study aimed to determine the osteoclastogenic role of H4R in rheumatoid arthritis (RA). The concentration of histamine in synovial fluid (SF) and sera in patients with RA was measured using ELISA. After RA SF and peripheral blood (PB) CD14+ monocytes were treated with histamine, IL-17, IL-21 and IL-22, and a H4R antagonist (JNJ7777120), the gene expression H4R and RANKL was determined by real-time PCR. Osteoclastogenesis was assessed by counting TRAP–positive multinucleated cells in PB CD14+ monocytes cultured with histamine, Th17 cytokines and JNJ7777120. SF and serum concentration of histamine was higher in RA, compared with osteoarthritis and healthy controls. The expression of H4R was increased in PB monocytes in RA patients. Histamine, IL-6, IL-17, IL-21 and IL-22 induced the expression of H4R in monocytes. Histamine, IL-17, and IL-22 stimulated RANKL expression in RA monocytes and JNJ7777120 reduced the RANKL expression. Histamine and Th17 cytokines induced the osteoclast differentiation from monocytes and JNJ7777120 decreased the osteoclastogenesis. H4R mediates RANKL expression and osteoclast differentiation induced by histamine and Th17 cytokines. The blockage of H4R could be a new therapeutic modality for prevention of bone destruction in RA.

Histamine therapeutic efficacy in metastatic melanoma: Role of histamine H4 receptor agonists and opportunity for combination with radiation

The aims of the work were to improve our knowledge of the role of H4R in melanoma proliferation and assess in vivo the therapeutic efficacy of histamine, clozapine and JNJ28610244, an H4R agonist, in a preclinical metastatic model of melanoma. Additionally, we aimed to investigate the combinatorial effect of histamine and gamma radiation on the radiobiological response of melanoma cells.Results indicate that 1205Lu metastatic melanoma cells express H4R and that histamine inhibits proliferation, in part through the stimulation of the H4R, and induces cell senescence and melanogenesis. Daily treatment with H4R agonists (1 mg/kg, sc) exhibited a significant in vivo antitumor effect and importantly, compounds reduced metastatic potential, particularly in the group treated with JNJ28610244, the H4R agonist with higher specificity. H4R is expressed in benign and malignant lesions of melanocytic lineage, highlighting the potential clinical use of histamine and H4R agonists. In addition, histamine increased radiosensitivity of melanoma cells in vitro and in vivo. We conclude that stimulation of H4R by specific ligands may represent a novel therapeutic strategy in those tumors that express this receptor. Furthermore, through increasing radiation-induced response, histamine could improve cancer radiotherapy for the treatment of melanoma.

Combined blockade of the histamine H1 and H4 receptor suppresses peanut-induced intestinal anaphylaxis by regulating dendritic cell function

BACKGROUND

Signaling through histamine receptors on dendritic cells (DCs) may be involved in the effector phase of peanut-induced intestinal anaphylaxis.

OBJECTIVES

The objective of this study was to determine the role of histamine H1 (H1R) and H4 receptors (H4R) in intestinal allergic responses in a model of peanut allergy.

METHODS

Balb/c mice were sensitized and challenged with peanut. During the challenge phase, mice were treated orally with the H1R antagonist, loratadine, and/or the H4R antagonist, JNJ7777120. Bone marrow-derived DCs (BMDCs) were adoptively transferred to nonsensitized WT mice. Symptoms, intestinal inflammation, and mesenteric lymph node and intestine mucosal DCs were assessed. Effects of the drugs on DC chemotaxis, calcium mobilization, and antigen-presenting cell function were measured.

RESULTS

Treatment with loratadine or JNJ7777120 individually partially suppressed the development of diarrhea and intestinal inflammation and decreased the numbers of DCs in the mesenteric lymph nodes and lamina propria. Combined treatment with both drugs prevented the development of diarrhea and intestinal inflammation. In vitro, the combination suppressed DC antigen-presenting cell function to T helper cells and DC calcium mobilization and chemotaxis to histamine.

CONCLUSION

Blockade of both H1R and H4R in the challenge phase had additive effects in preventing the intestinal consequences of peanut sensitization and challenge. These effects were mediated through the limitation of mesenteric lymph node and intestinal DC accumulation and function. Identification of this histamine H1R/H4R-DC-CD4⁺ T-cell axis provides new insights into the development of peanut-induced intestinal allergic responses and for prevention and treatment of peanut allergy.

Long Non-Coding RNAs (lncRNAs) of Sea Cucumber: Large-Scale Prediction, Expression Profiling, Non-Coding Network Construction, and lncRNA-microRNA-Gene Interaction Analysis of lncRNAs in Apostichopus japonicus and Holothuria glaberrima During LPS Challenge and Radial Organ Complex Regeneration

Long non-coding RNA (lncRNA) structurally resembles mRNA but cannot be translated into protein. Although the systematic identification and characterization of lncRNAs have been increasingly reported in model species, information concerning non-model species is still lacking. Here, we report the first systematic identification and characterization of lncRNAs in two sea cucumber species: (1) Apostichopus japonicus during lipopolysaccharide (LPS) challenge and in heathy tissues and (2) Holothuria glaberrima during radial organ complex regeneration, using RNA-seq datasets and bioinformatics analysis. We identified A. japonicus and H. glaberrima lncRNAs that were differentially expressed during LPS challenge and radial organ complex regeneration, respectively. Notably, the predicted lncRNA-microRNA-gene trinities revealed that, in addition to targeting protein-coding transcripts, miRNAs might also target lncRNAs, thereby participating in a potential novel layer of regulatory interactions among non-coding RNA classes in echinoderms. Furthermore, the constructed coding-non-coding network implied the potential involvement of lncRNA-gene interactions during the regulation of several important genes (e.g., Toll-like receptor 1 [TLR1] and transglutaminase-1 [TGM1]) in response to LPS challenge and radial organ complex regeneration in sea cucumbers. Overall, this pioneer systematic identification, annotation, and characterization of lncRNAs in echinoderm pave the way for similar studies and future genetic, genomic, and evolutionary research in non-model species.

Role of histamine H4 receptor ligands in bleomycin-induced pulmonary fibrosis

Fibrosis of lung tissue is a disease where a chronic inflammatory process determines a pathological remodelling of lung parenchyma. The animal model obtained by intra-tracheal administration of bleomycin in C57BL/6 mice is one of the most validated murine model. Bleomycin stimulates oxidative stress and the production of pro-inflammatory mediators. Histamine H4R have recently been implicated in inflammation and immune diseases. This study was focused to investigate the effects of H4R ligands in the modulation of inflammation and in the reduction of lung fibrosis in C57BL/6 mice treated with bleomycin. C57BL/6 mice were treated with vehicle, JNJ7777120 (JNJ, selective H4R antagonist) or ST-1006 (partial H4R agonist), ST-994 (H4R neutral antagonist) and ST-1012 (inverse H4R agonist) at equimolar doses, released by micro-osmotic pumps for 21days. Airway resistance to inflation was assayed and lung samples were processed to measure malondialdehyde (TBARS); 8-hydroxy-2'-deoxyguanosine (8OHdG); myeloperoxidase (MPO); COX-2 expression and activity as markers of oxidative stress and inflammation. Fibrosis and airway remodelling were evaluated throughout transforming growth factor-β (TGF-β), percentage of positive Goblet cells, smooth muscle layer thickness determination. Our results indicated that JNJ, ST-994 and ST-1012 decreased inflammation and oxidative stress markers, i.e. the number of infiltrating leukocytes evaluated as lung tissue MPO, COX-2 expression and activity, TBARS and 8OHdG production. They also reduced the level of TGF-β, a pro-fibrotic cytokine, collagen deposition, thickness of smooth muscle layer, Goblet cells hyperplasia; resulting in a decrease of airway functional impairment. The results here reported clearly demonstrated that H4R ligands have a beneficial effect in a model of lung fibrosis in the mouse, thus indicating that H4R antagonists or inverse agonists could be a novel therapeutic strategy for lung inflammatory diseases.

The histamine H4-receptor and the central and peripheral nervous system: A critical analysis of the literature

Expression and function of histamine H4R in central and peripheral nervous system have been a matter of controversy for more than a decade. The scientific discussion is often limited to a few publications postulating the presence of functional H4R on neurons of the central and peripheral nervous system, but the even larger number of reports showing negative data is often neglected. In this article, we critically review the existing literature on H4R in central and peripheral nervous system and discuss the weak points often overlooked by the community. We identified as most important problems (i) insufficient validation or quality of antibodies, (ii) missing knockout controls, (iii) uncritical interpretation of RT-PCR results instead of qPCR experiments, (iv) insufficient controls to confirm specificity of pharmacological tools, (v) uncritical reliance on results produced by a single method and (vi) uncritical reliance on results not reproduced by independent research groups. Additionally, there may be a publication as well as a citation bias favoring the awareness of positive results, but neglecting negative data. We conclude that H4R expression on neurons of the brain is not convincingly supported by the current literature, at least as long as the positive data are not reproduced by independent research groups. Expression and function of H4R on peripheral neurons or non-neuronal cells of the nervous system, specifically on microglia is an interesting alternative hypothesis that, however, requires further verification. This article is part of a Special Issue entitled 'Histamine Receptors'.

Conformational Restriction and Enantioseparation Increase Potency and Selectivity of Cyanoguanidine-Type Histamine H4 Receptor Agonists

2-Cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[2-(phenylsulfanyl)ethyl]guanidine (UR-PI376, 1) is a potent and selective agonist of the human histamine H4 receptor (hH4R). To gain information on the active conformation, we synthesized analogues of 1 with a cyclopentane-1,3-diyl linker. Affinities and functional activities were determined at recombinant hHxR (x: 1-4) subtypes on Sf9 cell membranes (radioligand binding, [(35)S]GTPγS, or GTPase assays) and in part in luciferase assays on human or mouse H4R (HEK-293 cells). The most potent H4R agonists among 14 racemates were separated by chiral HPLC, yielding eight enantiomerically pure compounds. Configurations were assigned based on X-ray structures of intermediates and a stereocontrolled synthetic pathway. (+)-2-Cyano-1-{[trans-(1S,3S)-3-(1H-imidazol-4-yl)cyclopentyl]methyl}-3-[2-(phenylsulfanyl)ethyl]guanidine ((1S,3S)-UR-RG98, 39a) was the most potent H4R agonist in this series (EC50 11 nM; H4R vs H3R, >100-fold selectivity; H1R, H2R, negligible activities), whereas the optical antipode proved to be an H4R antagonist ([(35)S]GTPγS assay). MD simulations confirmed differential stabilization of the active and inactive H4R state by the enantiomers.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Further evidence for the role of histamine H3, but not H1, H2 or H4, receptors in immepip-induced inhibition of the rat cardioaccelerator sympathetic outflow

Since histamine H3 and H4 receptors are coupled to heterotrimeric Gi/o proteins, a signal transduction pathway associated with inhibition of neurotransmitter release, the present study has investigated the inhibition of the rat cardioaccelerator sympathetic outflow induced by the H3/H4 receptor agonist immepip by using antagonists for histamine H1 (ketotifen), H2 (ranitidine), H3 (thioperamide) and H4 (JNJ7777120) receptors. For this purpose, 102 male Wistar rats were pithed, artificially ventilated and prepared for either preganglionic spinal (C7-T1) stimulation of the cardioaccelerator sympathetic outflow (n=90) or i.v. bolus injections of noradrenaline (n=12). This approach resulted in frequency-dependent and dose-dependent tachycardic responses, respectively. I.v. continuous infusions of immepip (3 and 10 μg/kg min), but not of saline (0.02 ml/min), dose-dependently inhibited the sympathetically-induced tachycardic responses. Moreover, the cardiac sympatho-inhibition induced by 10 μg/kg min immepip (which failed to affect the tachycardic responses to i.v. noradrenaline) was: (i) unaltered after i.v. treatment with 1 ml/kg vehicle, 100 μg/kg ketotifen, 3000 μg/kg ranitidine, 30 μg/kg thioperamide or 300 μg/kg JNJ7777120; and (ii) abolished after 100 μg/kg thioperamide (i.v.). These doses of antagonists, which did not affect per se the sympathetically-induced tachycardic responses, were high enough to block their respective receptors. In conclusion, the cardiac sympatho-inhibition induced by 10 μg/kg.min immepip involves histamine H3 receptors, with further pharmacological evidence excluding the involvement of H1, H2 and H4 receptors.