Histamine excites neurones in the human submucous plexus through activation of H1, H2, H3 and H4 receptors

Histamine in the neocortex: Towards integrating multiscale effectors

Histamine is a modulatory neurotransmitter, which has received relatively less attention in the central nervous system than other neurotransmitters. The functional role of histamine in the neocortex, the brain region that controls higher-order cognitive functions such as attention, learning and memory, remains largely unknown. This article focuses on the emerging roles and mechanisms of histamine release in the neocortex. We describe gaps in current knowledge and propose the application of interdisciplinary tools to dissect the detailed multiscale functional logic of histaminergic action in the neocortex ranging from sub-cellular, cellular, dendritic and synaptic levels to microcircuits and mesoscale effects.

Biopsy samples from patients with irritable bowel syndrome, but not from those with mastocytosis or unspecific gastrointestinal complaints reveal unique nerve activation in all gut regions independent of mast cell density, histamine content or specific gastrointestinal symptoms

Introduction

We previously showed enteric nerve activation after application of colonic mucosal biopsy supernatants from patients with irritable bowel syndrome (IBS). The question remains whether this is a region-specific or a generalized sensitization. We tested the nerve-activating properties of supernatants from large and small intestinal regions of IBS patients with diarrhea (IBS-D) in comparison to those from mastocytosis patients with diarrhea (MC-D) or non-IBS/non-MC patients with GI-complaints. MC-D patients were included to test samples from patients with an established, severe mast cell disorder, because mast cells are suggested to play a role in IBS.

Methods

Voltage-sensitive dye imaging was used to record the effects of mucosal biopsy supernatants from IBS-D, MC-D, and non-IBS/non-MC on guinea pig submucous neurons. Mast cell density and histamine concentrations were measured in all samples.

Results

The median neuroindex (spike frequency × % responding neurons in Hz × %) was significantly (all p < 0.001) increased for IBS-D (duodenum and colon, proximal and distal each, 49.3; 50.5; 63.7; 71.9, respectively) compared to non-IBS/non-MC (duodenum and colon, proximal and distal each, 8.7; 4.9; 6.9; 5.4, respectively) or MC-D supernatants (duodenum and colon, proximal and distal each, 9.4; 11.9; 0.0; 7.9, respectively). Nerve activation by MC-D and non-IBS/non-MC supernatants was comparable (p>0.05). Mast cell density or histamine concentrations were not different between IBS-D, MC-D, and non-IBS/non-MC samples.

Discussion

Nerve activation by biopsy supernatants is an IBS hallmark that occurs throughout the gut, unrelated to mast cell density or histamine concentration. At least as important is our finding that GI complaints per se were not associated with biopsy supernatant-induced nerve activation, which further stresses the relevance of altered nerve behavior in IBS.

Histamine H4 Receptor Antagonist Ameliorates the Progression of Experimental Autoimmune Encephalomyelitis via Regulation of T-Cell Imbalance

Multiple sclerosis (MS) is a degenerative condition characterized by immune-mediated attacks on the central nervous system (CNS), resulting in demyelination and recurring T-cell responses. The histamine H4 receptor (H4R) is mainly expressed in cellular populations and plays a vital role in inflammation and immunological responses. The role of H4R in neurons of the CNS has recently been revealed. However, the precise role of H4R in neuronal function remains inadequately understood. The objective of this work was to investigate the impact of JNJ 10191584 (JNJ), a highly effective and specific H4R antagonist, on the development of experimental autoimmune encephalomyelitis (EAE) and to gain insight into the underlying mechanism involved. In this study, we examined the potential impact of JNJ therapy on the course of EAE in SJL/J mice. EAE mice were administered an oral dose of JNJ at a concentration of 6 mg/kg once a day, starting from day 10 and continuing until day 42. Afterward, the mice's clinical scores were assessed. In this study, we conducted additional research to examine the impact of JNJ on several types of immune cells, specifically Th1 (IFN-γ and T-bet), Th9 (IL-9 and IRF4), Th17 (IL-17A and RORγt), and regulatory T (Tregs; Foxp3 and TGF-β1) cells in the spleen. In this study, we further investigated the impact of JNJ on the mRNA expression levels of IFN-γ, T-bet, IL-9, IRF4, IL-17A, RORγt, Foxp3, and TGF-β1 in the brain. Daily treatment of JNJ effectively reduced the development of EAE in mice. The percentages of CD4+IFN-γ+, CD4+T-bet+, CD4+IL-9+, CD4+IRF4+, CD4+IL-17A+, and CD4+RORγt+ cells were shown to decrease, whereas the percentages of CD4+TGF-β1+ and CD4+Foxp3+ cells were observed to increase in EAE mice treated with JNJ. Therefore, the HR4 antagonist positively affected the course of EAE by modulating the signaling of transcription factors. The identified results include possible ramifications in the context of MS treatment.

Inflammatory Bowel Disease: Crosstalk between Histamine, Immunity, and Disease

Inflammatory bowel disease (IBD) is increasingly recognized as a serious, worldwide public health concern. It is generally acknowledged that a variety of factors play a role in the pathogenesis of this group of chronic inflammatory diseases. The diversity of molecular actors involved in IBD does not allow us to fully assess the causal relationships existing in such interactions. Given the high immunomodulatory activity of histamine and the complex immune-mediated nature of inflammatory bowel disease, the role of histamine and its receptors in the gut may be significant. This paper has been prepared to provide a schematic of the most important and possible molecular signaling pathways related to histamine and its receptors and to assess their relevance for the development of therapeutic approaches.

P19-derived neuronal cells express H1, H2, and H3 histamine receptors: a biopharmaceutical approach to evaluate antihistamine agents

Histamine is a biogenic amine implicated in various biological and pathological processes. Convenient cellular models are needed to screen and develop new antihistamine agents. This report aimed to characterize the response of neurons differentiated from mouse P19 embryonal carcinoma cells to histamine treatment, and to investigate the modulation of this response by antihistamine drugs, vegetal diamine oxidase, and catalase. The exposure of P19 neurons to histamine reduced cell viability to 65% maximally. This effect involves specific histamine receptors, since it was prevented by treatment with desloratadine and cimetidine, respectively, H₁ and H₂ antagonists, but not by the H₃ antagonist ciproxifan. RT-PCR analysis showed that P19 neurons express H₁ and H₂ receptors, and the H₃ receptor, although it seemed not involved in the histamine effect on these cells. The H₄ receptor was not expressed. H₁ and H₂ antagonists as well as vegetal diamine oxidase diminished the intracellular Ca²⁺ mobilization triggered by histamine. The treatment with vegetal diamine oxidase or catalase protected against mortality and a significant reduction of H₂O₂ level, generated from the cells under the histamine action, was found upon treatments with desloratadine, cimetidine, vegetal diamine oxidase, or catalase. Overall, the results indicate the expression of functional histamine receptors and open the possibility of using P19 neurons as model system to study the roles of histamine and related drugs in neuronal pathogenesis. This model is less expensive to operate and can be easily implemented by current laboratories of analysis and by Contract Research Organizations.

Allergen-free extracts from birch, ragweed, and hazel pollen activate human and guinea-pig submucous and spinal sensory neurons

BACKGROUND

Non-allergenic, low molecular weight components of pollen grains are suspected to trigger changes in gut functions, sometimes leading to inflammatory conditions. Based on extensive neuroimmune communication in the gut wall, we investigated the effects of aqueous pollen extracts (APE) on enteric and spinal sensory neurons.

METHODS

Using Ca²⁺ and fast potentiometric imaging, we recorded the responses of guinea-pig and human submucous and guinea-pig dorsal root ganglion (DRG) neurons to microejection of low (<3 kDa) and high (≥3 kDa) molecular weight APEs of birch, ragweed, and hazel. Histamine was determined pharmacologically and by mass spectrometry (LC-MS/MS).

KEY RESULTS

Birch APE_<3kDa evoked strong [Ca⁺² ]_i signals in the vast majority of guinea-pig DRG neurons, and in guinea-pig and human enteric neurons. The effect of birch APE_≥3kDa was much weaker. Fast neuroimaging in human enteric neurons revealed an instantaneous spike discharge after microejection of birch, ragweed, and hazel APE_<3kDa [median (interquartile range) at 7.0 Hz (6.2/9.8), 5.7 Hz (4.4/7.1), and 8.4 Hz (4.3/12.5), respectively]. The percentage of responding neurons per ganglion were similar [birch 40.0% (33.3/100.0), ragweed 50.8% (34.4/85.6), and hazel 83.3% (57.1/100.0)]. A mixture of histamine receptor (H1-H3) blockers significantly reduced nerve activation evoked by birch and ragweed APEs_<3kDa , but was ineffective on hazel. Histamine concentrations in ragweed, birch and hazel APE's < 3 kDa were 0.764, 0.047, and 0.013 μM, respectively.

CONCLUSIONS

Allergen-free APEs from birch, ragweed, and hazel evoked strong nerve activation. Altered nerve-immune signaling as a result of severe pollen exposure could be a pathophysiological feature of allergic and non-allergic gut inflammation.

Enteric neuroanatomy and smooth muscle activity in the western diamondback rattlesnake (Crotalus atrox)

BACKGROUND

Gastrointestinal (GI) functions are controlled by the enteric nervous system (ENS) in vertebrates, but data on snakes are scarce, as most studies were done in mammals. However, the feeding of many snakes, including Crotalus atrox, is in strong contrast with mammals, as it consumes an immense, intact prey that is forwarded, stored, and processed by the GI tract. We performed immunohistochemistry in different regions of the GI tract to assess the neuronal density and to quantify cholinergic, nitrergic, and VIPergic enteric neurons. We recorded motility patterns and determined the role of different neurotransmitters in the control of motility. Neuroimaging experiments complemented motility findings.

RESULTS

A well-developed ganglionated myenteric plexus (MP) was found in the oesophagus, stomach, and small and large intestines. In the submucous plexus (SMP) most neurons were scattered individually without forming ganglia. The lowest number of neurons was present in the SMP of the proximal colon, while the highest was in the MP of the oesophagus. The total number of neurons in the ENS was estimated to be approx. 1.5 million. In all regions of the SMP except for the oesophagus more nitric oxide synthase+ than choline-acetyltransferase (ChAT)+ neurons were counted, while in the MP ChAT+ neurons dominated. In the SMP most nerve cells were VIP+, contrary to the MP, where numerous VIP+ nerve fibers but hardly any VIP+ neuronal cell bodies were seen. Regular contractions were observed in muscle strips from the distal stomach, but not from the proximal stomach or the colon. We identified acetylcholine as the main excitatory and nitric oxide as the main inhibitory neurotransmitter. Furthermore, 5-HT and dopamine stimulated, while VIP and the ß-receptor-agonist isoproterenol inhibited motility. ATP had only a minor inhibitory effect. Nerve-evoked contractile responses were sodium-dependent, insensitive to tetrodotoxin (TTX), but sensitive to lidocaine, supported by neuroimaging experiments.

CONCLUSIONS

The structure of the ENS, and patterns of gastric and colonic contractile activity of Crotalus atrox are strikingly different from mammalian models. However, the main excitatory and inhibitory pathways appear to be conserved. Future studies have to explore how the observed differences are an adaptation to the particular feeding strategy of the snake.

Metabotropic 5-HT receptor-mediated effects in the human submucous plexus

BACKGROUND

Serotonin (5-HT) is an important mediator in the gastrointestinal tract, acting on different neuronal 5-HT receptors. The ionotropic 5-HT₃ receptor mediates immediate but transient spike discharge in human enteric neurons. We studied the role of the metabotropic 5-HT_1P , 5-HT₄ , and 5-HT₇ receptors to activate human submucous neurons.

METHODS

Neuroimaging using the voltage sensitive dye Di-8-ANEPPS was performed in submucous plexus preparations from human surgical specimens of the small and large intestine. We synthesized a new, stable 5-HT_1P agonist, 5-benzyloxyhydrazonoindalpine (5-BOHIP).

KEY RESULTS

5-HT evoked a fast and late-onset spike discharge in enteric neurons. The fast component was blocked by the 5-HT₃ receptor antagonist cilansetron, while the remaining sustained response was significantly reduced by the 5-HT_1P receptor antagonist 5-hydroxytryptophanyl-5-hydroxytryptophan amide (5-HTP-DP). The newly synthesized 5-HT_1P agonist 5-BOHIP induced a slowly developing, long-lasting activation of submucous neurons, which was blocked by 5-HTP-DP. We could not demonstrate any 5-HT₇ receptor-induced spike discharge based on the lack of response to 5-carboxamidotryptamine. Similarly, the 5-HT₄ agonists 5-methoxytryptamine and prucalopride evoked no immediate or late-onset spike discharge.

CONCLUSIONS & INFERENCES

Our work demonstrated for the first time the presence of functional 5-HT_1P receptors on human submucous neurons. Furthermore, we found no evidence for a role of 5-HT₄ or 5-HT₇ receptors in the postsynaptic activation of human submucous neurons by 5-HT.

CNS-Sparing Histamine H3 Receptor Antagonist as a Candidate to Prevent the Diabetes-Associated Gastrointestinal Symptoms

Among the histamine receptors, growing evidence points to the histamine H₃ receptor as a pharmacological candidate to counteract the autonomic neuropathy associated with diabetes. The study aimed to evaluate the effect of PF00868087 (also known as ZPL-868), a CNS-sparing histamine H₃ receptor antagonist, on the autonomic neuropathy of the intestinal tract associated with diabetes. Diabetes was induced in male BALB/c mice by a single high dose of streptozotocin (150 mg/kg). Colorectal specimens from control and diabetic mice, randomized to vehicle or PF0086087 (10, 30, 100 mg/kg/day by oral gavage for 14 days), were processed for morphological and immunohistochemical analysis. A significant overproduction of mucus in the intestinal mucosa of diabetic mice compared to the controls was observed. PF0086087 at the highest dose prevented mucin overproduction. The immunohistochemistry analysis demonstrated that diabetes causes a decrease in the inhibitory component of enteric motility, measured as the percentage of neuronal nitric oxide synthase-positive neurons (p < 0.05) and a parallel increase in the excitatory component evaluated as substance P-positive fibres (p < 0.01). PF0086087 dose-dependently prevented these pathophysiological events. In conclusion, PF0086087 may be an essential tool in preventing nitrergic dysfunction in the myenteric plexus of the distal colon and diabetes-induced gastrointestinal complications.

Fast synaptic excitatory neurotransmission in the human submucosal plexus

BACKGROUND

Acetylcholine is the main excitatory neurotransmitter in the enteric nervous system (ENS) in all animal models examined so far. However, data for the human ENS is scarce.

METHODS

We used neuroimaging using voltage and calcium dyes, Ussing chamber, and immunohistochemistry to study fast synaptic neurotransmission in submucosal plexus neurons of the human gut.

KEY RESULTS

Electrical stimulation of intraganglionic fiber tracts led to fast excitatory postsynaptic potentials (fEPSPs) in 29 submucosal neurons which were all blocked by the nicotinic antagonist hexamethonium. The nicotinic agonist DMPP mimicked the effects of electrical stimulation and had excitatory effects on 56 of 73 neurons. The unselective NMDA antagonist MK-801 blocked fEPSPs in 14 out of 22 neurons as well as nicotine evoked spike discharge. In contrast, the application of NMDA showed only weak effects on excitability or calcium transients. This agreed with the finding that the specific NMDA antagonist D-APV reduced fEPSPs in only 1 out of 40 neurons. Application of AMPA or kainite had no effect in 41 neurons or evoked spike discharge in only one out of 41 neurons, respectively. Immunohistochemistry showed that 98.7 ± 2.4% of all submucosal neurons (n = 6 preparations, 1003 neurons) stained positive for the nicotinic receptor (α₁ , α₂ or α₃ -subunit). Hexamethonium (200 µM) reduced nerve-evoked chloride secretion by 34.3 ± 18.6% (n = 14 patients), whereas D-APV had no effect.

CONCLUSION & INFERENCE

Acetylcholine is the most important mediator of fast excitatory postsynaptic transmission in human submucous plexus neurons whereas glutamatergic fEPSPs were rarely encountered.

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.

Pruritogenic mechanisms and gut sensation: putting the "irritant" into irritable bowel syndrome

Chronic abdominal pain is a common clinical condition experienced by patients with irritable bowel syndrome (IBS). A general lack of suitable treatment options for the management of visceral pain is the major contributing factor to the debilitating nature of the disease. Understanding the underlying causes of chronic visceral pain is pivotal to identifying new effective therapies for IBS. This review provides the current evidence, demonstrating that mediators and receptors that induce itch in the skin also act as "gut irritants" in the gastrointestinal tract. Activation of these receptors triggers specific changes in the neuronal excitability of sensory pathways responsible for the transmission of nociceptive information from the periphery to the central nervous system leading to visceral hypersensitivity and visceral pain. Accumulating evidence points to significant roles of irritant mediators and their receptors in visceral hypersensitivity and thus constitutes potential targets for the development of more effective therapeutic options for IBS.

Biogenic amines in the colon

The gastrointestinal (GI) tract contains the highest concentration of biogenic amines in the human body. Neurons located in the GI tract, modulated by biogenic amines and various peptide and non-peptide transmitters, are called Enteric Nervous System (ENS). That explains why many medications used in neurology and psychiatry present side effects from the gut. Serotonin (5-hyroxytrypatamine, 5-HT), 95% of which is synthesized in the gut, is the most important amine (beside epinephrine and norepinephrine) colon functionality but another substances such as histamine, dopamine and melatonin are also potent in modulating intestine’s actions. Over 30 receptors for 5-HT were described in the human body, and 5-HT3, 5-HT4 and 5-HT7 are known to have the highest influence on motility and are a potent target for the drugs for treatment GI disorders, such as Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Diseases (IBD). Histamine is a key biogenic amine for pathogenesis of allergy also in the colon. Alteration in histaminergic system is found in patients with diarrhea and allergic enteropathy. Dopamine affects functions of the large intestine but its modulating actions are more presented in the upper part of GI tract. Melatonin is best known for regulating circadian circle, but may also be a potent anti-inflammatory agent within the gut. Despite many years of research, it seems that more studies are needed to fully understand human colon neurochemistry.

Vegetal diamine oxidase alleviates histamine-induced contraction of colonic muscles

Excess of histamine in gut lumen generates a pronounced gastrointestinal discomfort, which may include diarrhea and peristalsis dysfunctions. Deleterious effects of histamine can be alleviated with antihistamine drugs targeting histamine receptors. However, many antihistamine agents come with various undesirable side effects. Vegetal diamine oxidase (vDAO) might be a relevant alternative owing to its histaminase activity. Mammalian intestinal mucosa contains an endogenous DAO, yet possessing lower activity compared to that of vDAO preparation. Moreover, in several pathological conditions such as inflammatory bowel disease and irritable bowel syndrome, this endogenous DAO enzyme can be lost or inactivated. Here, we tested the therapeutic potential of vDAO by focusing on the well-known effect of histamine on gut motility. Using ex vivo and in vitro assays, we found that vDAO is more potent than commercial anti-histamine drugs at inhibiting histamine-induced contraction of murine distal colon muscles. We also identified pyridoxal 5′-phosphate (the biologically active form of vitamin B6) as an effective enhancer of vDAO antispasmodic activity. Furthermore, we discovered that rectally administered vDAO can be retained on gut mucosa and remain active. These observations make administration of vDAO in the gut lumen a valid alternative treatment for histamine-induced intestinal dysfunctions.

Mast Cell Regulation and Irritable Bowel Syndrome: Effects of Food Components with Potential Nutraceutical Use

Mast cells are key actors in inflammatory reactions. Upon activation, they release histamine, heparin and nerve growth factor, among many other mediators that modulate immune response and neuron sensitization. One important feature of mast cells is that their population is usually increased in animal models and biopsies from patients with irritable bowel syndrome (IBS). Therefore, mast cells and mast cell mediators are regarded as key components in IBS pathophysiology. IBS is a common functional gastrointestinal disorder affecting the quality of life of up to 20% of the population worldwide. It is characterized by abdominal pain and altered bowel habits, with heterogeneous phenotypes ranging from constipation to diarrhea, with a mixed subtype and even an unclassified form. Nutrient intake is one of the triggering factors of IBS. In this respect, certain components of the daily food, such as fatty acids, amino acids or plant-derived substances like flavonoids, have been described to modulate mast cells' activity. In this review, we will focus on the effect of these molecules, either stimulatory or inhibitory, on mast cell degranulation, looking for a nutraceutical capable of decreasing IBS symptoms.

Mouse Colonic Epithelial Cells Functionally Express the Histamine H4 Receptor

We hypothesized that, in mice, histamine via the histamine receptor subtype 4 (H₄R) on colon epithelial cells affects epithelial barrier integrity, perturbing physiologic function of the colonic mucosa and thus aggravating the severity of colitis. To test this hypothesis, bone marrow-chimeric mice were generated from H₄R knockout (H₄R^-/-) and wild-type (WT) BALB/cJ mice and subjected to the dextrane sodium sulfate (DSS)-induced acute colitis model. Clinical symptoms and pathohistological derangements were scored. Additionally, total RNA was extracted from either mouse whole-colon homogenates or primary cell preparations enriched for epithelial cells, and gene expression was analyzed by real-time quantitative polymerase chain reaction. The impact of the H₄R on epithelial barrier function was assessed by measurement of transepithelial electrical resistence of organoid-derived two-dimensional monolayers from H₄R^-/- and WT mice using chopstick electrodes. Bone marrow-chimeric mice with genetic depletion of the H₄R in nonhematopoietic cells exhibited less severe DSS-induced acute colitis symptoms compared with WT mice, indicating a functional proinflammatory expression of H₄R in nonimmune cells of the colon. Analysis of H₄R expression revealed the presence of H₄R mRNA in colon epithelial cells. This expression could be confirmed and complemented by functional analyses in organoid-derived epithelial cell monolayers. Thus, we conclude that the H₄R is functionally expressed in mouse colon epithelial cells, potentially modulating mucosal barrier integrity and intestinal inflammatory reactions, as was demonstrated in the DSS-induced colitis model, in which presence of the H₄R on nonhematopoietic cells aggravated the inflammatory phenotype. SIGNIFICANCE STATEMENT: The histamine H₄ receptor (H₄R) is functionally expressed on mouse colon epithelial cells, thereby aggravating dextrane sodium sulfate-induced colitis in BALB/cJ mice. Histamine via the H₄R reduces transepithelial electrical resistance of colon epithelial monolayers, indicating a function of H₄R in regulation of epithelial barrier integrity.

Differential expression of histamine receptors in the bladder wall tissues of patients with bladder pain syndrome/interstitial cystitis - significance in the responsiveness to antihistamine treatment and disease symptoms

Background

Activation of mast cells plays an important role in the pathogenesis of bladder pain syndrome/interstitial cystitis (BPS/IC). Histamine, a mast cell-derived mediators, induced inflammation and hypersensitivity of the bladder. The present study investigated the expressions of histamine receptors in the bladder wall tissues of patients with BPS/IC, and its association with the effectiveness of antihistamine therapy and disease symptoms.

Methods

Bladder tissues were collected from 69 BPS/IC patients and 10 control female patients. The expression of H3R in BPS/IC was further examined in an independent cohort of 10 female patients with BPS/IC and another 10 age-matched female patients. Immunohistochemistry, Western blotting, and quantitative RT-PCR were performed to quantify the expressions of histamine receptors. Statistical analyses of the correlation of histamine receptor expression with antihistamine therapy outcome and severity of disease symptoms were also performed.

Results

The expression of four histamine receptors was significantly elevated in BPS/IC (H1R, P < 0.001; H2R, P = 0.031; H3R, P = 0.008; H4R, P = 0.048). Western blotting revealed that H3R were significantly reduced in the patients, whereas the mRNA levels of H3R were significantly increased. The patients were further divided into antihistamine responders (n = 38) and nonresponders (n = 22). No significant correlation was found in the expression of histamine receptors between responder and nonresponder groups. However, significant correlations between OLS and H1R (P = 0.003) and H3R (P = 0.045) were found.

Conclusion

The present study showed that expression of all the 4 histamine receptors were elevated in BPS/IC. There were no statistical significant correlations between the expression levels of the four different histamine receptors and the treatment outcome of antihistamine therapy (amtitriptyline or cimetidine).

Influence of the Novel Histamine H3 Receptor Antagonist/Inverse Agonist M39 on Gastroprotection and PGE2 Production Induced by (R)-Alpha-Methylhistamine in C57BL/6 Mice

The role of histamine H3 receptors (H3Rs) in the regulation of gastroprotection and production of prostaglandin E2 (PGE2) as well as somatostatin remains contradictory. Therefore, the effects of the H3R antagonist/inverse agonist M39 on in vivo acidified ethanol-induced gastric ulcers and gastric acid secretion in the C57BL/6 mice were assessed. Results showed that acute systemic administration of H3R agonist (R)-α-methylhistamine (RAMH, 100 mg/kg, i.g.) significantly reduced the severity of ulcer index, increased gastric acid output, and increased mucosal PGE2 production without any alteration of somatostatin concentration in gastric juice. However, only acute systemic administration of the H2R agonist dimaprit (DIM, 10 mg/kg, p.o.) significantly decreased the level of somatostatin measured in gastric juice. Moreover, acute systemic administration of M39 (0.3 mg/kg, i.g.) abrogated the RAMH-induced increase of acid output as well as PGE2 production, but not the DIM (10 mg/kg, i.g.)-stimulated acid secretion, indicating that RAMH as well as M39 modulate the gastroprotective effects through interactions with histamine H3Rs. The present findings indicate that agonistic interaction with H3Rs is profoundly involved in the maintenance of gastric mucosal integrity by modulating PGE2 as well as gastric acid secretion, with no apparent role in the regulation of the inhibitory influence of somatostatin.

Histamine H3 Inverse Agonist BF 2649 or Antagonist with Partial H4 Agonist Activity Clobenpropit Reduces Amyloid Beta Peptide-Induced Brain Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is one of the leading causes for disability and death affecting millions of people worldwide. Thus, novel therapeutic strategies are needed to reduce brain pathology associated with AD. In view of increasing awareness regarding involvement of histaminergic pathways in AD, we explored the role of one H3 receptor inverse agonist BF 2649 and one selective H3 receptor antagonist with partial H4 agonist activity in amyloid beta peptide (AβP) infusion-induced brain pathology in a rat model. AD-like pathology was produced by administering AβP (1-40) intracerebroventricular (i.c.v.) in the left lateral ventricle (250 ng/10 μl, once daily) for 4 weeks. Control rats received saline. In separate group of rats, either BF 2649 (1 mg/kg, i.p.) or clobenpropit (1 mg/kg, i.p.) was administered once daily for 1 week after 3 weeks of AβP administration. After 30 days, blood-brain barrier (BBB) breakdown, edema formation, neuronal, glial injuries, and AβP deposits were examined in the brain. A significant reduction in AβP deposits along with marked reduction in neuronal or glial reactions was seen in the drug-treated group. The BBB breakdown to Evans blue albumin and radioiodine in the cortex, hippocampus, hypothalamus, and cerebellum was also significantly reduced in these drug-treated groups. Clobenpropit showed superior effects than the BF2649 in reducing brain pathology in AD. Taken together, our observations are the first to show that blockade of H3 and stimulation of H4 receptors are beneficial for the treatment of AD pathology, not reported earlier.

Imaging of mast cells

Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. The distribution, morphology and biochemical composition of mast cells has been studied in detail in vitro and on tissue sections both at the light microscopic and ultrastructural level. More recently, the development of fluorescent reporter strains and intravital imaging modalities has enabled first glimpses of the real-time behavior of mast cells in situ. In this review, we describe commonly used imaging approaches to study mast cells in cell culture as well as within normal and diseased tissues. We further describe the interrogation of mast cell function via imaging by providing a detailed description of mast cell-nerve plexus interactions in the intestinal tract. Together, visualizing mast cells has expanded our view of these cells in health and disease.

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.

Glioplasticity in irritable bowel syndrome

BACKGROUND

Growing evidence indicates a wide array of cellular remodeling in the mucosal microenvironment during irritable bowel syndrome (IBS), which possibly contributes to pathophysiology and symptom generation. Here, we investigated whether enteric glial cells (EGC) may be altered, and which factors/mechanisms lead to these changes.

METHODS

Colonic mucosal biopsies of IBS patients (13 IBS-Constipation [IBS-C]; 10 IBS-Diarrhea [IBS-D]; 11 IBS-Mixed [IBS-M]) and 24 healthy controls (HC) were analyzed. Expression of S100β and GFAP was measured. Cultured rat EGC were incubated with supernatants from mucosal biopsies, then proliferation and Ca²⁺ response to ATP were analyzed using flow cytometry and Ca²⁺ imaging. Histamine and histamine 1-receptor (H1R) involvement in the effects of supernatant upon EGC was analyzed.

KEY RESULTS

Compared to HC, the mucosal area immunoreactive for S100β was significantly reduced in biopsies of IBS patients, independently of the IBS subtype. IBS-C supernatants reduced EGC proliferation and IBS-D and IBS-M supernatants reduced Ca²⁺ response to ATP in EGC. EGC expressed H1R and the effects of supernatant upon Ca²⁺ response to ATP in EGC were blocked by pyrilamine and reproduced by histamine via H1R. IBS supernatants reduced mRNA expression of connexin-43. The S100β-stained area was negatively correlated with the frequency and intensity of pain and bloating.

CONCLUSION AND INFERENCES

Changes in EGC occur in IBS, involving mucosal soluble factors. Histamine, via activation of H1R-dependent pathways, partly mediates altered Ca²⁺ response to ATP in EGC. These changes may contribute to the pathophysiology and the perception of pain and bloating in patients with IBS.

Protease signaling through protease activated receptor 1 mediate nerve activation by mucosal supernatants from irritable bowel syndrome but not from ulcerative colitis patients

Background & aims

The causes of gastrointestinal complaints in irritable bowel syndrome (IBS) remain poorly understood. Altered nerve function has emerged as an important pathogenic factor as IBS mucosal biopsy supernatants consistently activate enteric and sensory neurons. We investigated the neurally active molecular components of such supernatants from patients with IBS and quiescent ulcerative colitis (UC).

Method

Effects of supernatants from 7 healthy controls (HC), 20 IBS and 12 UC patients on human and guinea pig submucous neurons were studied with neuroimaging techniques. We identify differentially expressed proteins with proteome analysis.

Results

Nerve activation by IBS supernatants was prevented by the protease activated receptor 1 (PAR1) antagonist SCHE79797. UC supernatants also activated enteric neurons through protease dependent mechanisms but without PAR1 involvement. Proteome analysis of the supernatants identified 204 proteins, among them 17 proteases as differentially expressed between IBS, UC and HC. Of those the four proteases elastase 3a, chymotrypsin C, proteasome subunit type beta-2 and an unspecified isoform of complement C3 were significantly more abundant in IBS compared to HC and UC supernatants. Of eight proteases, which were upregulated in IBS, the combination of elastase 3a, cathepsin L and proteasome alpha subunit-4 showed the highest prediction accuracy of 98% to discriminate between IBS and HC groups. Elastase synergistically potentiated the effects of histamine and serotonin–the two other main neuroactive substances in the IBS supernatants. A serine protease inhibitor isolated from the probiotic Bifidobacterium longum NCC2705 (SERPIN_BL), known to inhibit elastase-like proteases, prevented nerve activation by IBS supernatants.

Conclusion

Proteases in IBS and UC supernatants were responsible for nerve activation. Our data demonstrate that proteases, particularly those signalling through neuronal PAR1, are biomarker candidates for IBS, and protease profiling may be used to characterise IBS.

Calcium Imaging of Nerve-Mast Cell Signaling in the Human Intestine

Introduction: It is suggested that an altered microenvironment in the gut wall alters communication along a mast cell nerve axis. We aimed to record for the first time signaling between mast cells and neurons in intact human submucous preparations. Methods: We used the Ca2+ sensitive dye Fluo-4 AM to simultaneously image changes in intracellular calcium [Ca+2]i (%ΔF/F) in neurons and mast cells. Data are presented as median with interquartile ranges (25/75%). Results: We recorded nerve responses in 29 samples upon selective activation of 223 mast cells by IgE receptor cross linking with the antibody mAb22E7. Mast cells responded to mAb22E7 with a median [Ca+2]i increase of 20% (11/39) peaking 90 s (64/144) after the application. Only very few neurons responded and the median percentage of responding neuronal area was 0% (0/5.9). Mast cell activation remained in the presence of the fast sodium channel blocker tetrodotoxin. Specific neuronal activation by transmural electrical field stimulation (EFS) in 34 samples evoked instantaneously [Ca+2]i signals in submucous neurons. This was followed by a [Ca+2]i peak response of 8%ΔF/F (4/15) in 33% of 168 mast cells in the field of view. The mast cell response was abolished by the nerve blocker tetrododoxin, reduced by the Calcitonin Gene-Related Peptide receptor 1 antagonist BIBN-4096 and the Vasoactive Intestinal Peptide receptor antagonist PG97-269, but not by blockade of the neurokinin receptors 1-3. Conclusion: The findings revealed bidirectional signaling between mast cells and submucous neurons in human gut. In our macroscopically normal preparations a nerve to mast cell signaling was very prominent whereas a mast cell to nerve signaling was rather rare.

Hyperosmolarity evokes histamine release from ileum mucosa by stimulating a cholinergic pathway

Changes in extracellular osmolarity lead to alteration in cellular volume. In the study, we examined the effects of hyperosmolarity on short-circuit currents (Isc) in the rat ileum using the Ussing chamber technique. Mucosal exposure to 20 mM glucose evoked a decrease of I_SC in the rat ileum, which was antagonized by the stretch-activated channel blocker GdCl3, TTX and atropine, respectively. In contrast, it was not blocked by phlorizin, a Na⁺-glucose cotransporter SGLT1 inhibitor. Furthermore, the unabsorbed substances, such as sucrose, lactulose or urea, also induced a decrease of I_SC in rat ileum. ELISA results revealed that 20 mM glucose stimulated the release of histamine from rat ileum mucosa, which was attenuated by TTX. In addition, the glucose-induced I_SC was depressed by pyrilamine, a histamine H₁ receptor blocker (H₁ antagonist) whereas it was not affected by ranitidine (H₂ antagonist), clobenpropit (H₃ antagonists) or JNJ7777120 (H₄ antagonist), respectively. The ion substitution experiments suggest that the changes of Na⁺ and HCO₃^- ion flux underlie the glucose-induced I_SC. In conclusion, osmotic stimulus decreased the basal I_SC of rat ileum by evoking histamine release from ileum mucosa. The changes of Na⁺ and HCO₃^- ion transport are involved in the glucose-evoked decrease of basal I_SC.

Tryptase potentiates enteric nerve activation by histamine and serotonin: Relevance for the effects of mucosal biopsy supernatants from irritable bowel syndrome patients

BACKGROUND

We previously showed that mucosal biopsy supernatants from irritable bowel syndrome patients activated neurons despite low concentrations of tryptase, histamine, and serotonin which individually would not cause spike discharge. We studied the potentiating responses between these mediators on excitability of enteric neurons.

METHODS

Calcium-imaging was performed using the calcium-sensitive dye Fluo-4 AM in human submucous plexus preparations from 45 individuals. Histamine, serotonin, and tryptase were applied alone and in combinations to evaluate nerve activation which was assessed by analyzing increase in intracellular Ca²⁺ ([Ca²⁺ ]_i ), the proportion of responding neurons and the product of both defined as Ca-neuroindex (NI). Protease activated receptor (PAR) 2 activating peptide, PAR2 antagonist and the serine protease-inhibitor FUT-175 were used to particularly investigate the role of proteases.

KEY RESULTS

Histamine or serotonin (1 μmol/L each) evoked only few small responses (median NI [25%/75%]: 0 [0/148]; 85 [0/705] respectively). Their combined application evoked statistically similar responses (216 [21/651]). Addition of the PAR2 activator tryptase induced a significantly higher Ca-NI (1401 [867/4075]) compared to individual application of tryptase or to coapplied histamine and serotonin. This synergistic potentiation was neither mimicked by PAR2 activating peptide nor reversed by the PAR2 antagonist GB83, but abolished by FUT-175.

CONCLUSIONS & INFERENCES

We observed synergistic potentiation between histamine, serotonin, and tryptase in enteric neurons, which is mediated by proteolytic activity rather than PAR2 activation. This explained neuronal activation by a cocktail of these mediators despite their low concentrations and despite a relatively small PAR2-mediated response in human submucous neurons.

Activation of the ileal neuroendocrine tumor cell line P-STS by acetylcholine is amplified by histamine: role of H3R and H4R

Neuroendocrine tumors may present with pseudoallergic reactions like diarrhea and idiopathic anaphylaxis. Here we present the P-STS human ileal neuroendocrine cell line as a model cell line for these tumors. Neuroendocrine markers and changes in cytoplasmic calcium concentration ([Ca2+]i) in response to several possible activators of 5-hydroxytryptamine (5-HT) release were analyzed. P-STS cells still expressed chromogranin A and synaptophysin after 2 years of culture. Tryptophan hydroxylase 1 mRNA and a low amount of 5-HT were also detected. Acetylcholine (ACh) caused a rise in [Ca2+]i. Somatostatin inhibited, whereas histamine (HA) but not the HA receptor ligand betahistine enhanced activation by ACh. The [Ca2+]i response to ACh/HA was inhibited by the HA receptor H3 (H3R) agonist methimepip and by the antidepressant imipramine. Further [Ca2+]i response studies indicated the presence of H4Rs and of a functional calcium sensing receptor. High or low affinity IgE receptor protein or mRNA were not detected. Taken together, neuroendocrine markers and response to intestinal neurotransmitters approve the P-STS cell line as a valuable model for enterochromaffin cells. Enhancement of their ACh-induced pro-secretory response by HA, with a role for H3R and H4R, suggests an amplifying role of neuroendocrine cells in allergen-induced diarrhea or anaphylaxis.

Anti-Hu antibodies activate enteric and sensory neurons

IgG of type 1 anti-neuronal nuclear antibody (ANNA-1, anti-Hu) specificity is a serological marker of paraneoplastic neurological autoimmunity (including enteric/autonomic) usually related to small-cell lung carcinoma. We show here that IgG isolated from such sera and also affinity-purified anti-HuD label enteric neurons and cause an immediate spike discharge in enteric and visceral sensory neurons. Both labelling and activation of enteric neurons was prevented by preincubation with the HuD antigen. Activation of enteric neurons was inhibited by the nicotinic receptor antagonists hexamethonium and dihydro-β-erythroidine and reduced by the P2X antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid (PPADS) but not by the 5-HT₃ antagonist tropisetron or the N-type Ca-channel blocker ω-Conotoxin GVIA. Ca⁺⁺ imaging experiments confirmed activation of enteric neurons but not enteric glia. These findings demonstrate a direct excitatory action of ANNA-1, in particular anti-HuD, on visceral sensory and enteric neurons, which involves nicotinic and P2X receptors. The results provide evidence for a novel link between nerve activation and symptom generation in patients with antibody-mediated gut dysfunction.

Possible therapeutic role of IgE blockade in irritable bowel syndrome

Omalizumab is a humanized monoclonal antibody that binds to the high-affinity type-I IgE Fc receptors on mast cells (MCs) and basophils, inhibiting the IgE immune pathway. Irritable bowel syndrome (IBS) is the most common functional gastrointestinal disorder, and dysregulation of the immune system likely contributes to its etiology and/or symptomatology. Colonic biopsies from patients with IBS demonstrate considerable increase in the number of degranulating MCs releasing histamine in proximity to nerves, and this event may underlie the common IBS symptom of abdominal pain. Pharmacologic control of MC activation and mediator release is a current area of active interest in the field of IBS research. Recently, we and Pearson et al described 2 cases of patients with IBS with diarrhea (IBS-D) showing positive clinical response to omalizumab. In both cases, the female patients had severe, long-lasting IBS-D and achieved an almost complete resolution of IBS symptoms. Both patients were also able to discontinue all IBS medications after commencing the anti-IgE therapy. For both patients, the omalizumab treatment showed a relatively rapid onset of action, resembling the efficacy observed in and previously reported for patients with chronic spontaneous urticaria. In this Editorial, we discuss the possible biological mechanisms that may underlie the clinical efficacy of omalizumab in IBS. We suggest that there is a need for a well-designed prospective study to investigate the therapeutic effects of anti-IgE in IBS.

Identification of histamine receptors in the canine gastrointestinal tract

The important role of histamine in chronic gastrointestinal diseases has been increasingly recognized over the last two decades in human medicine. Histamine is released following mast cell activation and exerts its action through binding to four different histamine receptors (H1, H2, H3, and H4). Histamine receptors are dispersed throughout the body, and each different receptor mediates a unique response. Documentation of the presence and type of histamine receptors in the differing sections of the canine gastrointestinal tract will provide additional research opportunities to further explore the role of histamine and its receptors in chronic canine enteropathies, as well as potential therapeutic options. Full thickness gastric, duodenal, jejunal, ileal, and colonic biopsies were obtained from 6 clinically normal adult dogs immediately after humane euthanasia. Commercially available histamine receptor antibodies predicted to react with canine tissues were applied to paraffin-embedded tissue sections using standard immunohistochemistry techniques to identify different histamine receptors. Staining intensity was graded from negative to strong, and the specificity of each antibody was evaluated with western blot. The presence and distribution of histamine receptors varied by anatomic site and histologic level within sections of the canine gastrointestinal tract. All 4 histamine receptors were readily identified, although the distribution of H4 receptors was decreased in comparison to the other histamine receptors. The distribution of the various histamine receptors was similar to that seen in the normal human gastrointestinal tract. H1 receptors were located in the stomach, lymphoid tissue of the ileum and colon, and the smooth muscle and ganglia of all sections. H2 receptors were located in all sections of the gastrointestinal tract, with greatest staining intensity in the gastric mucosa. H3 receptors were located in the stomach and colonic mucosa, smooth muscle and ganglia of all sections, and ileal and colonic lymphoid tissue. H4 receptors were located in the ganglia and smooth muscle of the gastrointestinal tract, as well as the gastric and colonic mucosal and ileal lymphoid tissue. Western blot demonstrated both specific and non-specific staining with the H1 and H3 receptor antibody, but good specificity with the H4 receptor antibody. The H2 receptor antibody was not compatible with western blot techniques, despite excellent immunohistochemical specificity and consistency. Further studies to compare the density and distribution of the various histamine receptors in dogs with gastrointestinal disease are warranted.

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'.

Histamine induces the production of matrix metalloproteinase-9 in human astrocytic cultures via H1-receptor subtype

Accumulation of β-amyloid (Aβ) protein within the brain is a neuropathological hallmark of Alzheimer's disease (AD). One strategy to facilitate Aβ clearance from the brain is to promote Aβ catabolism. Matrix metalloproteinase-9 (MMP-9), a member of the family of Zn(+2)-containing endoproteases, known to be expressed and secreted by astrocytes, is capable of degrading Aβ. Histamine, a major aminergic brain neurotransmitter, stimulates the production of MMP-9 in keratinocytes through the histamine H1 receptor (H1R). In the present study, we show that histamine evokes a concentration- and calcium-dependent release of MMP-9 from human astrocytic U373 cells and primary cultures of human and rat astrocytes through the H1R subtype. Activation of H1R on astrocytes elevated intracellular levels of Ca(2+) that was accompanied by time-dependent increases in MAP kinase p44/p42 and PKC. In-cell western blots revealed dose-dependent increases in both enzymes, confirming involvement of these signal transduction pathways. We next investigated the extent of recombinant human MMP-9 (rhMMP-9) proteolytic activity on soluble oligomeric Aβ (soAβ). Mass spectrometry demonstrated time-dependent cleavage of soAβ (20 μM), but not another amyloidogenic protein amylin, upon incubation with rhMMP-9 (100 nM) at 1, 4 and 17 h. Furthermore, Western blots showed a shift in soAβ equilibrium toward lower order, less toxic monomeric species. In conclusion, both MAPK p44/p42 and PKC pathways appear to be involved in histamine-upregulated MMP-9 release via H1Rs in astrocytes. Furthermore, MMP-9 appears to cleave soAβ into less toxic monomeric species. Given the key role of histamine in MMP-9 release, this neurotransmitter may serve as a potential therapeutic target for AD.

Mast Cells and Irritable Bowel Syndrome: From the Bench to the Bedside

Irritable bowel syndrome (IBS) is traditionally defined as a functional disorder since it lacks demonstrable pathological abnormalities. However, in recent years, low grade inflammatory infiltration, often rich in mast cells, in both the small and large bowel, has been observed in some patients with IBS. The close association of mast cells with major intestinal functions, such as epithelial secretion and permeability, neuroimmune interactions, visceral sensation, and peristalsis, makes researchers and gastroenterologists to focus attention on the key roles of mast cells in the pathogenesis of IBS. Numerous studies have been carried out to identify the mechanisms in the development, infiltration, activation, and degranulation of intestinal mast cells, as well as the actions of mast cells in the processes of mucosal barrier disruption, mucosal immune dysregulation, visceral hypersensitivity, dysmotility, and local and central stress in IBS. Moreover, therapies targeting mast cells, such as mast cell stabilizers (cromoglycate and ketotifen) and antagonists of histamine and serotonin receptors, have been tried in IBS patients, and have partially exhibited considerable efficacy. This review focuses on recent advances in the role of mast cells in IBS, with particular emphasis on bridging experimental data with clinical therapeutics for IBS patients.

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.

Histamine Receptor Expression in the Gastrointestinal Tract of Dogs

Histamine is an important mediator of many physiological processes including gastrointestinal function that acts via four different histamine receptors (H1R to H4R). Elevated histamine levels and increased HR messenger ribonucleic acid (mRNA) have been shown in humans with gastrointestinal disorders such as irritable bowel syndrome or allergic intestinal diseases. As there is limited knowledge concerning the distribution of histamine receptors (HR) in dogs, one aim of this study was to investigate the expression of histamine 1 receptor (H1R), histamine 2 receptor (H2R) and histamine 4 receptor (H4R) in the canine gastrointestinal tract at protein level using immunohistochemistry. Histamine 1 receptor, H2R and H4R were widely expressed throughout the canine gastrointestinal tract including epithelial, mesenchymal, neuronal and immune cells. In addition, in situ hybridisation was established for detecting canine H4R mRNA. Results showed H4R mRNA to be present in enterocytes, lamina propria immune cells and submucosal plexus in the duodenum and colon of nearly all investigated animals. The results elucidate the importance of HR in the canine gut and represent the basis for investigating their possible impact on canine inflammatory gastrointestinal disorders.

Histamine 4 receptor promotes expression of costimulatory B7.1/B7.2 molecules, CD28 signaling and cytokine production in stress-induced immune responses

Recently, the expression of histamine 4 receptor (H4R) on neurons was reported, however its function in cells within the central nervous system (CNS) remains poorly understood. To this end, we used the H4R agonist, 4-methylhistamine (4-MeH), and the H4R antagonist, JNJ77777120 (JNJ), to investigate the function of H4R signaling in immune cells in a murine model of chronic stress. Treatment of stressed mice with 4-MeH resulted in an increase in the proportion of lymphocyte subsets (CD3(+), CD8(+), CD28(+), and CD4(+)CD28(+)) and cells expressing the co-stimulatory molecules CD80(+) (B7.1) and CD86(+) (B7.2) in heparinized blood as compared to normal control (NC) and stressed control (SC) groups. We also observed that as compared to NC and SC mice, 4-MeH-treated mice showed greater production of IL-2(+), IL-6(+), IL-9(+), IL-21(+), and IL-27(+) cytokines in the spleen and by splenic CD4(+) T cells. Furthermore, 4-MeH treatment of stressed mice led to an increase in the levels of serum Th1/Th17 cytokines and corticosterone, and a decrease in Th2 cytokines. Treatment of chronically-stressed mice with 4-MeH also augmented expression of IL-6, IL-21, NF-κB p65, and STAT3 mRNA. Moreover, Western blot analyses confirmed increased protein expression of NF-κB, iNOS, and STAT3 expression following 4-MeH treatment of chronically-stressed mice as compared to controls. These proteins provide a novel relevant targets for the manipulation of chronic stress induced immune regulation. In striking contrast, treatment of stressed mice with the H4R antagonist, JNJ, resulted in a substantial reduction in all of the aforementioned effects upon immune cell percentages and cytokine production.

Neuropharmacology of purinergic receptors in human submucous plexus: Involvement of P2X₁, P2X₂, P2X₃ channels, P2Y and A₃ metabotropic receptors in neurotransmission

RATIONALE

The role of purinergic signaling in human ENS is not well understood. We sought to further characterize the neuropharmacology of purinergic receptors in human ENS and test the hypothesis that endogenous purines are critical regulators of neurotransmission.

EXPERIMENTAL APPROACH

LSCM-Fluo-4/(Ca(2+))-imaging of postsynaptic Ca(2+) transients (PSCaTs) was used as a reporter of synaptic transmission evoked by fiber tract electrical stimulation in human SMP surgical preparations. Pharmacological analysis of purinergic signaling was done in 1,556 neurons (identified by HuC/D-immunoreactivity) in 235 ganglia from 107 patients; P2XR-immunoreactivity was evaluated in 19 patients. Real-time MSORT (Di-8-ANEPPS) imaging tested effects of adenosine on fast excitatory synaptic potentials (fEPSPs).

RESULTS

Synaptic transmission is sensitive to pharmacological manipulations that alter accumulation of extracellular purines: Apyrase blocks PSCaTs in a majority of neurons. An ecto-NTPDase-inhibitor 6-N,N-diethyl-D-β,γ-dibromomethyleneATP or adenosine deaminase augments PSCaTs. Blockade of reuptake/deamination of eADO inhibits PSCaTs. Adenosine inhibits fEPSPs and PSCaTs (IC50 = 25 µM), sensitive to MRS1220-antagonism (A3AR). A P2Y agonist ADPβS inhibits PSCaTs (IC50 = 111 nM) in neurons without stimulatory ADPbS responses (EC50 = 960 nM). ATP or a P2X1,2,2/3 (α,β-MeATP) agonist evokes fast, slow, biphasic Ca(2+) transients or Ca(2+) oscillations (ATP,EC50 = 400 mM). PSCaTs are sensitive to P2X1 antagonist NF279. Low (20 nM) or high (5 µM) concentrations of P2X antagonist TNP-ATP block PSCaTs in different neurons; proportions of neurons with P2XR-immunoreactivity follow the order P2X2 > P2X1 >> P2X3; P2X1 + P2X2 and P2X3 + P2X2 are co-localized. RT-PCR identified mRNA-transcripts for P2X1-7, P2Y1,2,12-14R.

CONCLUSIONS

Purines are critical regulators of neurotransmission in human ENS. Purinergic signaling involves P2X1, P2X2, P2X3 channels, P2X1 + P2X2 co-localization and inhibitory P2Y or A3 receptors. These are potential novel therapeutic targets for neurogastroenterology.

Histamine H₄ receptors in the gastrointestinal tract

Histamine is a well-established mediator involved in a variety of physiological and pathophysiological mechanisms and exerts its effect through activation of four histamine receptors (H1-H₄). The histamine H₄ receptor is the newest member of this histamine receptor family, and is expressed throughout the gastrointestinal tract as well as in the liver, pancreas and bile ducts. Functional studies using a combination of selective and non-selective H₄ receptor ligands have rapidly increased our knowledge of H₄ receptor involvement in gastrointestinal processes both under physiological conditions and in models of disease. Strong evidence points towards a role for H₄ receptors in the modulation of immune-mediated responses in gut inflammation such as in colitis, ischaemia/reperfusion injury, radiation-induced enteropathy and allergic gut reactions. In addition, data have emerged implicating H₄ receptors in gastrointestinal cancerogenesis, sensory signalling, and visceral pain as well as in gastric ulceration. These studies highlight the potential of H₄ receptor targeted therapy in the treatment of various gastrointestinal disorders such as inflammatory bowel disease, irritable bowel syndrome and cancer.

Reduced Responses of Submucous Neurons from Irritable Bowel Syndrome Patients to a Cocktail Containing Histamine, Serotonin, TNFα, and Tryptase (IBS-Cocktail)

BACKGROUND AND AIMS

Malfunctions of enteric neurons are believed to play an important role in the pathophysiology of irritable bowel syndrome (IBS). Our aim was to investigate whether neuronal activity in biopsies from IBS patients is altered in comparison to healthy controls (HC).

METHODS

Activity of human submucous neurons in response to electrical nerve stimulation and local application of nicotine or a mixture of histamine, serotonin, tryptase, and TNF-α (IBS-cocktail) was recorded in biopsies from 17 HC and 35 IBS patients with the calcium-sensitive-dye Fluo-4 AM. The concentrations of the mediators resembeled those found in biopsy supernatants or blood. Neuronal activity in guinea-pig submucous neurons was studied with the voltage-sensitive-dye di-8-ANEPPS.

RESULTS

Activity in submucous ganglia in response to nicotine or electrical nerve stimulation was not different between HC and IBS patients (P = 0.097 or P = 0.448). However, the neuronal response after application of the IBS-cocktail was significantly decreased (P = 0.039) independent of whether diarrhea (n = 12), constipation (n = 5) or bloating (n = 5) was the predominant symptom. In agreement with this we found that responses of submucous ganglia conditioned by overnight incubation with IBS mucosal biopsy supernatant to spritz application of this supernatant was significantly reduced (P = 0.019) when compared to incubation with HC supernatant.

CONCLUSION

We demonstrated for the first time reduced neuronal responses in mucosal IBS biopsies to an IBS mediator cocktail. While excitability to classical stimuli of enteric neurons was comparable to HC, the activation by the IBS-cocktail was decreased. This was very likely due to desensitization to mediators constantly released by mucosal and immune cells in the gut wall of IBS patients.

Antihistamines and itch

Histamine is one of the best-characterized pruritogens in humans. It is known to play a role in pruritus associated with urticaria as well as ocular and nasal allergic reactions. Histamine mediates its effect via four receptors. Antihistamines that block the activation of the histamine H₁receptor, H₁R, have been shown to be effective therapeutics for the treatment of pruritus associated with urticaria, allergic rhinitis, and allergic conjunctivitis. However, their efficacy in other pruritic diseases such as atopic dermatitis and psoriasis is limited. The other histamine receptors may also play a role in pruritus, with the exception of the histamine H₂receptor, H₂R. Preclinical evidence indicates that local antagonism of the histamine H₃receptor, H₃R, can induce scratching perhaps via blocking inhibitory neuronal signals. The histamine H₄receptor, H₄R, has received a significant amount of attention as to its role in mediating pruritic signals. Indeed, it has now been shown that a selective H₄R antagonist can inhibit histamine-induced itch in humans. This clinical result, in conjunction with efficacy in various preclinical pruritus models, points to the therapeutic potential of H₄R antagonists for the treatment of pruritus not controlled by antihistamines that target the H₁R.

Innervation of enteric mast cells by primary spinal afferents in guinea pig and human small intestine

Mast cells express the substance P (SP) neurokinin 1 receptor and the calcitonin gene-related peptide (CGRP) receptor in guinea pig and human small intestine. Enzyme-linked immunoassay showed that activation of intramural afferents by antidromic electrical stimulation or by capsaicin released SP and CGRP from human and guinea pig intestinal segments. Electrical stimulation of the afferents evoked slow excitatory postsynaptic potentials (EPSPs) in the enteric nervous system. The slow EPSPs were mediated by tachykinin neurokinin 1 and CGRP receptors. Capsaicin evoked slow EPSP-like responses that were suppressed by antagonists for protease-activated receptor 2. Afferent stimulation evoked slow EPSP-like excitation that was suppressed by mast cell-stabilizing drugs. Histamine and mast cell protease II were released by 1) exposure to SP or CGRP, 2) capsaicin, 3) compound 48/80, 4) elevation of mast cell Ca²⁺ by ionophore A23187, and 5) antidromic electrical stimulation of afferents. The mast cell stabilizers cromolyn and doxantrazole suppressed release of protease II and histamine when evoked by SP, CGRP, capsaicin, A23187, electrical stimulation of afferents, or compound 48/80. Neural blockade by tetrodotoxin prevented mast cell protease II release in response to antidromic electrical stimulation of mesenteric afferents. The results support a hypothesis that afferent innervation of enteric mast cells releases histamine and mast cell protease II, both of which are known to act in a diffuse paracrine manner to influence the behavior of enteric nervous system neurons and to elevate the sensitivity of spinal afferent terminals.