Presynaptic inhibition produced by histamine at nicotinic synapses in enteric ganglia

Sexually Dimorphic Effects of Histamine Degradation by Enteric Glial Histamine N-Methyltransferase (HNMT) on Visceral Hypersensitivity

Histamine is a neuromodulator that affects gut motility and visceral sensitivity through intrinsic and extrinsic neural pathways, yet the mechanisms regulating histamine availability in these pathways remain poorly understood. Here, we show that enteric glia contribute to histamine clearance in the enteric nervous system (ENS) through their expression of the enzyme histamine N-methyltransferase (HNMT). Glial HNMT expression was initially assessed using immunolabeling and gene expression, and functionally tested using CRISPR-Cas9 to create a Cre-dependent conditional Hnmt ablation model targeting glia. Immunolabeling, calcium imaging, and visceromotor reflex recordings were used to assess the effects on ENS structure and visceral hypersensitivity. Immunolabeling and gene expression data show that enteric neurons and glia express HNMT. Deleting Hnmt in Sox10+ enteric glia increased glial histamine levels and altered visceromotor responses to colorectal distension in male mice, with no effect in females. Interestingly, deleting glial Hnmt protected males from histamine-driven visceral hypersensitivity. These data uncover a significant role for glial HNMT in histamine degradation in the gut, which impacts histamine-driven visceral hypersensitivity in a sex-dependent manner. Changes in the capacity of glia to clear histamines could play a role in the susceptibility to developing visceral pain in disorders of the gut-brain interaction.

Further Understanding of Neuro-Immune Interactions in Allergy: Implications in Pathophysiology and Role in Disease Progression

The complicated interaction between the central and the autonomic (sympathetic, parasympathetic, and enteric) nervous systems on the one hand and the immune system and its components, on the other hand, seems to substantially contribute to allergy pathophysiology, uncovering an under-recognized association that could have diagnostic and therapeutic potentials. Neurons connect directly with and regulate the function of many immune cells, including mast cells, the cells that have a leading role in allergic disorders. Proinflammatory mediators such as cytokines, neurotrophins, chemokines, and neuropeptides are released by immune cells, which stimulate sensory neurons. The release of neurotransmitters and neuropeptides caused by the activation of these neurons directly impacts the functional activity of immune cells and vice versa, playing a decisive role in this communication. Successful application of Pavlovian conditioning in allergic disorders supports the existence of a psychoneuroimmunological interplay in classical allergic hypersensitivity reactions. Activation of neuronal homeostatic reflexes, like sneezing in allergic rhinitis, coughing in allergic asthma, and vomiting in food allergy, offers additional evidence of a neuroimmunological interaction that aims to maintain homeostasis. Dysregulation of this interaction may cause overstimulation of the immune system that will produce profound symptoms and exaggerated hemodynamic responses that will lead to severe allergic pathophysiological events, including anaphylaxis. In this article, we have systematically reviewed and discussed the evidence regarding the role of the neuro-immune interactions in common allergic clinical modalities like allergic rhinitis, chronic rhinosinusitis, allergic asthma, food allergy, atopic dermatitis, and urticaria. It is essential to understand unknown – to most of the immunology and allergy experts – neurological networks that not only physiologically cooperate with the immune system to regulate homeostasis but also pathogenetically interact with more or less known immunological pathways, contribute to what is known as neuroimmunological inflammation, and shift homeostasis to instability and disease clinical expression. This understanding will provide recognition of new allergic phenotypes/endotypes and directions to focus on specialized treatments, as the era of personalized patient-centered medicine, is hastening apace.

Histamine-dependent interactions between mast cells, glia, and neurons are altered following early-life adversity in mice and humans

Early-life adversity contributes to the development of functional bowel disorders later in life through unresolved mechanisms. Here, we tested the hypothesis that early-life adversity alters anatomical and functional interactions between mast cells and enteric glia. The effects of early-life stress were studied using the neonatal maternal separation (NMS) stress mouse model. Anatomical relationships between mast cells and enteric glia were assessed using immunohistochemistry and mast cell reporter mice (Mcpt5^Cre;GCaMP5g-tdT). Immunohistochemistry was used to assess the expression of histamine, histamine 1 (H1) receptors, and glial fibrillary acidic protein. Functional responses of glia to mast cell mediators were assessed in calcium imaging experiments using Sox10^CreERT2;GCaMP5g-tdT mice and cultured human enteric glial cells. NMS increases mast cell numbers at the level of the myenteric plexus and their proximity to myenteric ganglia. Myenteric glia respond to mediators released by activated mast cells that are blocked by H1 receptor antagonists in mice and humans and by blocking neuronal activity with tetrodotoxin in mouse tissue. Histamine replicates the effects of mast cell supernatants on enteric glia, and NMS increases histamine production by mast cells. NMS reduces glial responses to mast cell mediators in mouse tissue, while potentiating responses in cultured human enteric glia. NMS increases myenteric glial fibrillary acidic protein expression and reduces glial process length but does not cause neurodegeneration. Histamine receptor expression is not altered by NMS and is localized to neurons in mice, but glia in humans. Early-life stress increases the potential for interactions between enteric glia and mast cells, and histamine is a potential mediator of mast cell-glial interactions through H1 receptors. We propose that glial-mast cell signaling is a mechanism that contributes to enteric neuroplasticity driven by early-life adversity.NEW & NOTEWORTHY Early-life adversity places an individual at risk for developing functional gastrointestinal disorders later in life through unknown mechanisms. Here, we show that interactions between mast cells and glia are disrupted by early-life stress in mice and that histamine is a potential mediator of mast cell-glial interactions.

Histamine receptors mediate contraction of reticular groove smooth muscle of adult goats

The present study was conducted to evaluate the effect of histamine and to characterise its receptor subtypes in reticular groove (RG) smooth muscle of adult goats. The studies were done using floor and lip regions of RG. We used tension experiments on smooth muscle of RG isolated from adult goat for functional characterisation of H₁ and H₂ receptors. Western blotting and immunohistochemistry experiments were conducted for molecular characterisation of these receptors. Histamine evoked concentration-dependent contraction of isolated RG circular and longitudinal smooth muscle preparation. Pyrilamine antagonised the action of histamine. Histamine did not induce any relaxant effect on RG preparations. Additionally, cimetidine did not produce any significant effect on histamine-induced response. Non-selective histaminic receptor antagonist cyproheptadine attenuated the contraction response to histamine in the smooth muscle. Molecular characterisation and localisation of H₁ and H₂ receptor proteins confirmed the presence of these receptors in RG. It is most likely that histamine-induced contractile effect in RG smooth muscle of goats is mediated by H₁ histaminic receptors.

Involvement of gut microbiota in association between GLP-1/GLP-1 receptor expression and gastrointestinal motility

The microbiota in the gut is known to play a pivotal role in host physiology by interacting with the immune and neuroendocrine systems in gastrointestinal (GI) tissues. Glucagon-like peptide 1 (GLP-1), a gut hormone, is involved in metabolism as well as GI motility. We examined how gut microbiota affects the link between GLP-1/GLP-1 receptor (GLP-1R) expression and motility of the GI tract. Germ-free (GF) mice (6 wk old) were orally administered a fecal bacterial suspension prepared from specific pathogen-free (SPF) mice, and then after fecal transplantation (FT) GI tissues were obtained from the GF mice at various time points. The expression of GLP-1 and its receptor was examined by immunohistochemistry, and gastrointestinal transit time (GITT) was measured by administration of carmine red solution. GLP-1 was expressed in endocrine cells in the colonic mucosa, and GLP-1R was expressed in myenteric neural cells throughout the GI wall. GLP-1R-positive cells throughout the GI wall were significantly fewer in GF mice with FT than in GF mice without gut microbiota reconstitution. GITT was significantly shorter in GF mice with FT than in control GF mice without FT and correlated with the number of GLP-1R-positive cells throughout the GI wall. GITT was significantly longer in GF control mice than in SPF mice. When those mice were treated with GLP-1 agonist extendin4, GITT was significantly longer in the GF mice. The gut microbiota may accelerate or at least modify GI motility while suppressing GLP-1R expression in myenteric neural cells throughout the GI tract.NEW & NOTEWORTHY The gut microbiota has been intensively studied, because it plays a pivotal role in various aspects of host physiology. On the other hand, glucagon-like peptide 1 (GLP-1) plays important roles in metabolism as well as gastrointestinal motility. In the present study, we have suggested that the gut microbiota accelerates gastrointestinal motility while suppressing the expression of GLP-1 receptor in myenteric neural cells throughout the gastrointestinal tract. We believe that this article is very timely and suggestive work.

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.

Nonruminant Nutrition Symposium: Neurogastroenterology and food allergies

Neurogastroenterology is a subspecialty encompassing relations of the nervous system to the gastrointestinal tract. The central concept is emergence of whole organ behavior from coordinated activity of the musculature, mucosal epithelium, and blood vasculature. Behavior of each effector is determined by the enteric nervous system (ENS). The ENS is a minibrain positioned close to the effectors it controls. The ENS neurophysiology is in the framework of neurogastroenterology. The digestive tract is recognized as the largest lymphoid organ in the body with a unique complement of mast cells. In its position at the "dirtiest" of interfaces between the body and outside world, the mucosal immune system encounters food antigens, bacteria, parasites, viruses, and toxins. Epithelial barriers are insufficient to exclude fully the antigenic load, thereby allowing chronic challenges to the immune system. Observations in antigen-sensitized animals document direct communication between the mucosal immune system and ENS. Communication is functional and results in adaptive responses to circumstances within the lumen that are threatening to the functional integrity of the whole animal. Communication is paracrine and incorporates specialized sensing functions of mast cells for specific antigens together with the capacity of the ENS for intelligent interpretation of the signals. Immuno-neural integration progresses sequentially, beginning with immune detection, followed by signal transfer to the ENS, followed by neural interpretation and then selection of a neural program with coordinated mucosal secretion and a propulsive motor event that quickly clears the threat from the intestinal lumen. Operation of the defense program evokes symptoms of cramping abdominal pain, fecal urgency, and acute watery diarrhea. Investigative approaches to immuno-ENS interactions merge the disciplines of mucosal immunology and ENS neurophysiology into the realm of neurogastroenterology.

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

Histamine is a major mast cell mediator of immunoneural signalling in the gut and mast cells play a role in the pathophysiology of functional and inflammatory bowel diseases. Histamine receptors are therefore promising drug targets to treat gut disorders. We aimed to study the so far unknown effect of histamine on neural activity in the human enteric nervous system (ENS) and to identify the pharmacology of histamine response. We used fast imaging techniques in combination with the potentiometric dye di-8-ANEPPS to monitor directly membrane potential changes and thereby neuronal excitability in the human submucous plexus from surgical specimens of 110 patients (2137 neurones, 273 ganglia). Local microejection of histamine resulted in action potential discharge in 37% of neurones. This excitatory effect was mimicked by the H(1) agonist HTMT-dimaleat, H(2) agonist dimaprit, H(3) agonist (R)-(-)-alpha-methylhistamine and H(4) agonist 4-methylhistamine. The excitatory actions of the agonists were specifically and selectively blocked by the H(1), H(2), H(3) or H(4) receptor antagonists pyrilamine, ranitidine, clobenpropit or J1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methylpiperazine (JNJ 7777120), respectively. Clobenproprit reduced the excitatory response to histamine. Unlike in the guinea-pig ENS (R)-(-)-alpha-methylhistamine had no presynaptic actions in human submucous plexus. Application of agonists revealed receptor clustering which was as follows: 29% H(1)/H(3), 27% H(2), 20% H(1)/H(2)/H(3), 10% H(3), 7% H(1)/H(2) and 7% H(2)/H(3). Histamine excites human enteric neurones and this effect involves all four histamine receptors; most striking was the identification of an excitatory H(3) mediated component and the discovery of H(4) mediated neuronal excitation. These data may form the basis of identification of new targets to treat inflammatory and functional gut disorders.

Effects of bacteria on the enteric nervous system: implications for the irritable bowel syndrome

A unified scenario emerges when it is considered that a major impact of stress on the intestinal tract is reflected by symptoms reminiscent of the diarrhea-predominant form of irritable bowel syndrome. Cramping abdominal pain, fecal urgency, and explosive watery diarrhea are hallmarks not only of diarrhea-predominant irritable bowel syndrome, but also of infectious enteritis, radiation-induced enteritis, and food allergy. The scenario starts with stress-induced compromise of the intestinal mucosal barrier and continues with microorganisms or other sensitizing agents crossing the barrier and being intercepted by enteric mast cells. Mast cells signal the presence of the agent to the enteric nervous system (ie, the brain-in-the-gut), which uses one of the specialized programs from its library of programs to remove the "threat." This is accomplished by stimulating mucosal secretion, which flushes the threatening agent into the lumen and maintains it in suspension. The secretory response then becomes linked to powerful propulsive motility, which propels the secretions together with the offending agent rapidly in the anal direction. Cramping abdominal pain accompanies the strong propulsive contractions. Urgency is experienced when arrival of the large bolus of liquid distends the recto-sigmoid region and reflexly opens the internal anal sphincter, with continence protection now provided only by central reflexes that contract the puborectalis and external anal sphincter muscles. Sensory information arriving in the brain from receptors in the rapidly distending recto-sigmoid accounts for the conscious sensation of urgency and might exacerbate the individual's emotional stress. The symptom of explosive watery diarrhea becomes self-explanatory in this scenario.

Neuropathophysiology of functional gastrointestinal disorders

The investigative evidence and emerging concepts in neurogastroenterology implicate dysfunctions at the levels of the enteric and central nervous systems as underlying causes of the prominent symptoms of many of the functional gastrointestinal disorders. Neurogastroenterological research aims for improved understanding of the physiology and pathophysiology of the digestive subsystems from which the arrays of functional symptoms emerge. The key subsystems for defecation-related symptoms and visceral hyper-sensitivity are the intestinal secretory glands, the musculature and the nervous system that controls and integrates their activity. Abdominal pain and discomfort arising from these systems adds the dimension of sensory neurophysiology. This review details current concepts for the underlying pathophysiology in terms of the physiology of intestinal secretion, motility, nervous control, sensing function, immuno-neural communication and the brain-gut axis.

The bidirectional communication between neurons and mast cells within the gastrointestinal tract

Normal or disordered behaviour of the gastrointestinal tract is determined by a complex interplay between the epithelial barrier, immune cells, blood vessels, smooth muscle and intramurally located nerve elements. Mucosal mast cells (MMCs), which are able to detect noxious and antigenic threats and to generate or amplify signals to the other cells, are assigned a rather central position in this complex network. Signal input from MMCs to intrinsic enteric neurons is particularly crucial, because the enteric nervous system fulfils a pivotal role in the control of gastrointestinal functions. Activated enteric neurons are able to generate an alarm program involving alterations in motility and secretion. MMC signalling to extrinsic nerve fibres takes part in pathways generating visceral pain or extrinsic reflexes contributing to the disturbed motor and secretory function. Morphological and functional studies, especially studies concerning physiological stress, have provided evidence that, apart from the interaction between the enteric nervous system and MMCs, there is also a functional communication between the central nervous system and these mast cells. Psychological factors trigger neuronal pathways, which directly or indirectly affect MMCs. Further basic and clinical research will be needed to clarify in more detail whether basic patterns of this type of interactions are conserved between species including humans.

Fundamentals of neurogastroenterology: basic science

The focus of neurogastroenterology in Rome II was the enteric nervous system (ENS). To avoid duplication with Rome II, only advances in ENS neurobiology after Rome II are reviewed together with stronger emphasis on interactions of the brain, spinal cord, and the gut in terms of relevance for abdominal pain and disordered gastrointestinal function. A committee with expertise in selective aspects of neurogastroenterology was invited to evaluate the literature and provide a consensus overview of the Fundamentals of Neurogastroenterology textbook as they relate to functional gastrointestinal disorders (FGIDs). This review is an abbreviated version of a fuller account that appears in the forthcoming book, Rome III. This report reviews current basic science understanding of visceral sensation and its modulation by inflammation and stress and advances in the neurophysiology of the ENS. Many of the concepts are derived from animal studies in which the physiologic mechanisms underlying visceral sensitivity and neural control of motility, secretion, and blood flow are examined. Impact of inflammation and stress in experimental models relative to FGIDs is reviewed as is human brain imaging, which provides a means for translating basic science to understanding FGID symptoms. Investigative evidence and emerging concepts implicate dysfunction in the nervous system as a significant factor underlying patient symptoms in FGIDs. Continued focus on neurogastroenterologic factors that underlie the development of symptoms will lead to mechanistic understanding that is expected to directly benefit the large contingent of patients and care-givers who deal with FGIDs.

Functional gastrointestinal disorders and mast cells: implications for therapy

The pathophysiology of functional gastrointestinal disorders is poorly understood. Accepted common mechanisms include psychosocial factors, abnormal gastrointestinal motility and disturbed visceral sensory perception, but the underlying causes remain unclear. Mast cells (MCs) are immunocytes widely distributed throughout the gastrointestinal tract. Several stimuli (e.g. allergens, neuropeptides and stress) lead to MC activation with consequent mediator release (e.g. histamine, tryptase and prostanoids). The MC mediators interact with nerves supplying the gut leading to altered gut physiology and increased sensory perception. The intestinal mucosa of irritable bowel syndrome patients contains on average an increased number of MCs. These cells release an increased amount of mediators in close vicinity to mucosal innervation. The MC activation and their close proximity to nerve fibres is correlated with the severity of perceived abdominal painful sensations. These data provide a strong basis for considering MCs as important participants in visceral hypersensitivity and pain perception in irritable bowel syndrome. Inhibition of MC function may ameliorate irritable bowel symptoms. Novel drugs with an increased potential in the control of MC function (e.g., anti-IgE antibodies, the intracellular protein tyrosine kinase inhibitor Syk) and mediator release (e.g., second generation antihistamines, proteinase-activated receptor antagonists) may be useful pharmacological tools for these common disorders.

Human mast cell mediator cocktail excites neurons in human and guinea-pig enteric nervous system

Neuroimmune interactions are an integral part of gut physiology and involved in the pathogenesis of inflammatory and functional bowel disorders. Mast cells and their mediators are important conveyors in the communication from the innate enteric immune system to the enteric nervous system (ENS). However, it is not known whether a mediator cocktail released from activated human mast cells affects neural activity in the ENS. We used the Multi-Site Optical Recording Technique to image single cell activity in guinea-pig and human ENS after application of a mast cell mediator cocktail (MCMC) that was released from isolated human intestinal mucosa mast cells stimulated by IgE-receptor cross-linking. Local application of MCMC onto individual ganglia evoked an excitatory response consisting of action potential discharge. This excitatory response occurred in 31%, 38% or 11% neurons of guinea-pig submucous plexus, human submucous plexus, or guinea-pig myenteric plexus, respectively. Compound action potentials from nerve fibres or fast excitatory synaptic inputs were not affected by MCMC. This study demonstrates immunoneural signalling in the human gut and revealed for the first time that an MCMC released from stimulated human intestinal mast cells induces excitatory actions in the human and guinea-pig ENS.

Enteric neuroimmunophysiology and pathophysiology

Minute-to-minute behavior of the bowel, whether it is normal or disordered, is determined by integrative functions of the enteric nervous system (ENS). Information input processed by the ENS is derived from local sensory receptors, the central nervous system, and immune/inflammatory cells including mast cells. Enteric mast cells use the power of the immune system for detection of antigenic threats and for long-term memory of the identity of the specific antigens. Specific antibodies attach to the mast cells and enable the mast cell to detect sensitizing antigens when they reappear in the gut lumen. Should the sensitizing antigen reappear, mast cells detect it and signal its presence to the ENS. The ENS interprets the mast cell signal as a threat and calls up from its program library secretory and propulsive motor behavior that is organized to eliminate the threat rapidly and effectively. Operation of the alarm program protects the individual, but at the expense of symptoms that include cramping abdominal pain, fecal urgency, and diarrhea. Enteric mast cells use immunologic memory functions to detect foreign antigens as they appear and reappear throughout the life of the individual. Mast cells use paracrine signaling for the transfer of chemical information to the neural networks of the ENS. Integrative circuits in the ENS receive and interpret the chemical signals from the mast cells. Signals from the mast cells are interpreted by the ENS as a labeled code for the presence of a threat in the intestinal lumen.

Actions of galanin on neurotransmission in the submucous plexus of guinea pig small intestine

Electrophysiologic recording methods were used to study the actions of galanin on synaptic transmission in the submucous plexus of guinea pig ileum. Exposure to galanin resulted in concentration-dependent suppression of slow noradrenergic inhibitory postsynaptic potentials and fast nicotinic excitatory postsynaptic potentials in the majority of neurons. Failure of galanin to suppress nicotinic depolarizing responses to micropressure pulses of acetylcholine and failure to suppress hyperpolarizing responses to micropressure pulses of norepinephrine suggested that galanin acted at presynaptic inhibitory receptors to suppress release of acetylcholine and norepinephrine. Galanin suppressed slow excitatory postsynaptic potentials in eight of eight neurons with AH (after-hyperpolarization) type electrical behavior and in none of 26 neurons with S (synaptic) type electrical behavior. Suppression of excitatory neurotransmission in AH neurons was always associated with membrane hyperpolarization. Excitatory responses caused by experimentally applied substance P were also inhibited by galanin. Galanin-(1-16) and galanin-like peptide mimicked the inhibitory actions of galanin on neurotransmission. The selective galanin GAL2 receptor agonist [D-Trp(2)]galanin was inactive. The chimeric peptides, galanin-(1-13)-spantide I, galantide, galanin-(1-13)-neuropeptide Y(25-36) amide, galanin-(1-13)-bradykinin-(2-9)amide and galanin-(1-13)-Pro-Pro-Ala-Leu-Ala-Leu-Ala amide all produced varying degrees of suppression of the synaptic potentials. The evidence suggests that the galanin GAL1 receptor, but not the galanin GAL2 receptor, mediated the presynaptic and postsynaptic inhibitory actions of galanin.

Actions of cysteinyl leukotrienes in the enteric nervous system of guinea-pig stomach and small intestine

Conventional intracellular microelectrodes, neuronal tracer injection techniques and immunohistochemistry were used to study the actions of cysteinyl leukotrienes (CysLTs) on electrical and synaptic behavior of enteric neurons in guinea-pig stomach and small intestine. Bath application of leukotriene C(4), leukotriene D(4) or leukotriene E(4) evoked a slowly activating depolarizing response in most of the myenteric and submucous plexus neurons in the small intestine while no effect was observed in gastric neurons. The depolarization evoked by cysteinyl leukotrienes in intestinal neurons was associated with increased input resistance and enhanced excitability. Suppression of hyperpolarizing after-potentials occurred in AH type neurons. The depolarizing action of cysteinyl leukotrienes was resistant to tetrodotoxin and cyclooxygenase inhibitors. Neither the CysLT(1) receptor antagonists (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK 571), 1-[2-hydroxy-3-propyl-4-[4-(1H-tetrazol-5-yl)butoxy]phenyl]-ethanone (LY 171883) and alpha-pentyl-3-(2-quinolinylmethoxy)-benzenemethanol (REV 5901), nor the dual CysLT(1)/CysLT(2) receptor antagonist 6(R)-(4'-carboxyphenylthio)-5(S)-hydroxy-7(E),9(E),11(Z),14(Z)-eicosatetraenoic acid (BAY u9773) significantly altered the depolarizing action of the cysteinyl leukotrienes. Neurotransmission was unaffected by the cysteinyl leukotrienes. The results suggested involvement of cysteinyl leukotrienes in enteric immuno-neural communication through excitatory actions on enteric neurons. The receptor mediating these effects was distinct from currently recognized cysteinyl leukotriene receptor subtypes (CysLT(1) and CysLT(2) receptors) and may represent a new receptor subtype.

Disturbance of the prejunctional modulation of cholinergic neurotransmission during chronic granulomatous inflammation of the mouse ileum

The effect of chronic granulomatous inflammation of the intestine was studied on the prejunctional modulation of cholinergic nerve activity in the mouse ileum. Contractions to carbachol (0.01 - 0.3 microM) and to electrical field stimulation (EFS, 0.25 - 8 Hz) of enteric neurons were higher in inflamed ileum as compared to control ileum. However, when the neurally-mediated contractions to EFS were expressed as percentage of the direct smooth muscle contraction to carbachol, the responses to EFS were similar in control and inflamed ileum. Atropine (1 microM) abolished all contractions to EFS and carbachol in control and inflamed ileum. DMPP (3 - 30 microM), a nicotinic receptor agonist, induced concentration-dependent contractions that were more pronounced in inflamed ileum as compared to control ileum. Hexamethonium (100 microM), a nicotinic receptor blocker, significantly inhibited the contractions to EFS in inflamed ileum but not in control ileum. In control ileum, histamine (10 - 100 microM) and the histamine H(1) receptor agonist HTMT (3 - 10 microM) inhibited the contractions to EFS concentration-dependently without affecting the contractions to carbachol. The inhibitory effect of histamine and HTMT was prevented by the histamine H(1) antagonist mepyramine (5 - 10 microM) but not by the H(2)- and H(3)-receptor antagonists cimetidine and thioperamide (both 10 microM). In chronically inflamed ileum however, histamine (10 - 100 microM) and HTMT (3 - 10 microM) failed to inhibit the contractions to EFS. The histamine H(2) and H(3) receptor agonists dimaprit and R(-)-alpha-methylhistamine did not affect the contractions to EFS in control and inflamed ileum. The alpha(2)-receptor agonist UK 14.304 (0.01 - 0.1 microM) inhibited the contractions to EFS in control and inflamed ileum without affecting the contractions to carbachol. The effect of UK 14.304 was reversed by the alpha(2)-receptor antagonist yohimbine (1 microM). The inhibitory effect of UK 14.304 on contractions to EFS was of similar potency in control and inflamed ileum. Our results suggest that the prejunctional modulation of cholinergic nerve activity by nicotinic and histaminic H(1) receptors is disturbed during chronic intestinal inflammation whereas the modulation by alpha(2)-receptors is preserved. Such a disturbance of cholinergic nerve activity may contribute to the motility disturbances that are often observed during chronic intestinal diseases in humans.

Role of histamine H(3) receptors in the control of gastrointestinal motility. An overview

Over the last few years, the biochemical and functional characterization of H(3) receptors has been a matter for extensive investigation, culminating in the cloning of the human, guinea pig and rat receptor protein from brain tissues. This discovery contributed to determine the distribution of receptors in the body and to define the molecular mechanisms which follow activation. The major breakthrough in the histamine H(3) receptor field came with the synthesis of selective and potent agonists and antagonists, which unravelled the function of this receptor subtype in the different tissues. As expected from the ubiquitous location of histamine in the body, histamine H(3) receptors have also been identified in virtually every tissue, although they are quantitatively less abundant than H(1) and H(2) receptors. Concerning the gastrointestinal tract, this new receptor subtype seems to have multiple cellular locations, which include neurons, enteric ganglia, paracrine and immune cells and, in some tissues, also smooth muscle cells. Therefore it might be regarded as a general regulatory system of different digestive functions, including motility. The effects mediated by histamine H(3)-receptors mainly reflect the presynaptic inhibition of the release of either excitatory or inhibitory neurotransmitters from the myenteric plexus. The molecular mechanism of presynaptic inhibition seems to involve a restriction of calcium entry into the nerve endings, but other mechanisms (reduction of cAMP), possibly associated to different H(3) receptor subtypes, may be involved. Despite the widespread distribution and the well defined inhibitory effects evoked in the majority of in vitro models of intestinal motility, no clear cut evidence of its involvement in the control of peristalsis could be provided. In vivo models of gastrointestinal transit, indeed, did not reveal a defined effect of histamine H(3) receptor ligands, even though the possibility of a central inhibition was pointed out in several studies. Therefore, it is not clear at the present what is the physiological meaning of the histamine H(3) receptor in the control of gastrointestinal motility and whether it could represent a potential target for novel therapeutic interventions in deranged motility, taking into account that human gastrointestinal tissues are apparently devoid of this receptor.

Actions of reactive oxygen species on AH/type 2 myenteric neurons in guinea pig distal colon

With conventional intracellular recording methods, we investigated the mechanism of actions of reactive oxygen species (ROS) derived from hypoxanthine and xanthine oxidase (HX/XO) reactions on AH/type 2 myenteric neurons in the guinea pig distal colon. Of the 54 neurons to which HX/XO was applied, 32 neurons showed a transient membrane hyperpolarization(s) followed by a long-lasting membrane depolarization. Two additional groups of 10 myenteric neurons exhibited only a membrane hyperpolarization(s) or a late-onset membrane depolarization, respectively, and the remaining two neurons did not show any response to HX/XO. Analysis of changes of the input resistance induced by HX/XO indicated that suppression and augmentation of the conductance of Ca(2+)-dependent K(+) channels are the ionic mechanisms underlying the membrane hyperpolarization and depolarization, respectively. The effects of HX/XO on myenteric neurons were mimicked by application of caffeine or H(2)O(2). The results suggest that OH(.), but neither H(2)O(2) nor O(2)(.-), is responsible for HX/XO-induced responses. The intracellular Ca(2+) store may be the acting site of ROS in colonic AH/type 2 neurons.

Actions of histamine on muscle and ganglia of the guinea pig gallbladder

Histamine is an inflammatory mediator present in mast cells, which are abundant in the wall of the gallbladder. We examined the electrical properties of gallbladder smooth muscle and nerve associated with histamine-induced changes in gallbladder tone. Recordings were made from gallbladder smooth muscle and neurons, and responses to histamine and receptor subtype-specific compounds were tested. Histamine application to intact smooth muscle produced a concentration-dependent membrane depolarization and increased excitability. In the presence of the H(2) antagonist ranitidine, the response to histamine was potentiated. Activation of H(2) receptors caused membrane hyperpolarization and elimination of spontaneous action potentials. The H(2) response was attenuated by the ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide in intact and isolated smooth muscle. Histamine had no effect on the resting membrane potential or excitability of gallbladder neurons. Furthermore, neither histamine nor the H(3) agonist R-alpha-methylhistamine altered the amplitude of the fast excitatory postsynaptic potential in gallbladder ganglia. The mast cell degranulator compound 48/80 caused a smooth muscle depolarization that was inhibited by the H(1) antagonist mepyramine, indicating that histamine released from mast cells can activate gallbladder smooth muscle. In conclusion, histamine released from mast cells can act on gallbladder smooth muscle, but not in ganglia. The depolarization and associated contraction of gallbladder smooth muscle represent the net effect of activation of both H(1) (excitatory) and H(2) (inhibitory) receptors, with the H(2) receptor-mediated response involving the activation of K(ATP) channels.

Immunohistochemical localization of histamine N-methyltransferase in guinea pig tissues

Histamine plays important roles in gastric acid secretion, inflammation, and allergic response. Histamine N-methyltransferase (HMT; EC 2.1.1.8) is crucial to the inactivation of histamine in tissues. In this study we investigated the immunohistochemical localization of this enzyme in guinea pig tissues using a rabbit polyclonal antibody against bovine HMT. The specificity of the antibody for guinea pig HMT was confirmed by Western blotting and the lack of any staining using antiserum preabsorbed with purified HMT. There was strong HMT-like immunoreactivity (HMT-LI) in the epithelial cells in the gastrointestinal tract, especially in the gastric body, duodenum, and jejunum. The columnar epithelium in the gallbladder was also strongly positive. Almost all the myenteric plexus from the stomach to the colon was stained whereas the submucous plexus was not. Other strongly immunoreactive cells included the ciliated cells in the trachea and the transitional epithelium of the bladder. Intermediately immunoreactive cells included islets of Langerhans, epidermal cells of the skin, alveolar cells in the lung, urinary tubules in the kidney, and epithelium of semiferous tubules. HMT-LI was present in specific structures in the guinea pig tissues. The widespread distribution of HMT-LI suggests that histamine has several roles in different tissues.

Histamine H3 receptor-mediated suppression of inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine

Conventional intracellular microelectrodes and marker injection techniques were used to study the actions of histamine on inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine. Bath application of histamine (1-300 microM) reversibly suppressed both noradrenergic and non-adrenergic slow inhibitory postsynaptic potentials in a concentration-dependent manner. These effects of histamine were mimicked by the selective histamine H(3) receptor agonist R(-)-alpha-methylhistamine but not the selective histamine H(1) receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl) heptanecarboxamide (HTMT dimaleate), or the selective histamine H(2) receptor agonist, dimaprit. The histamine H(3) receptor antagonist, thioperamide, blocked the effects of histamine. Histamine H(1) and H(2) receptor antagonists did not change the action of histamine. Hyperpolarizing responses to focal application of norepinephrine or somatostatin by pressure ejection from micropipettes were unaffected by histamine and R(-)-alpha-methylhistamine. The results suggest that histamine acts at presynaptic histamine H(3) receptors on the terminals of sympathetic postganglionic fibers and intrinsic somatostatinergic nerves in the small intestine to suppress the release of the inhibitory neurotransmitters, norepinephrine and somatostatin.

Does the guinea-pig ileum obey the 'law of the intestine'?

1. We report the first simultaneous mechanical reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers of the guinea-pig ileum following mucosal stimulation and distension in vitro. 2. Dissection techniques were used to prevent mechanical interaction between the LM and CM layers both oral and anal to a stimulus site. 3. All graded stimuli produced graded contractions of both the LM and CM orally and anally to the stimulus. Contractions occurred synchronously in the LM and CM and under no circumstances were inhibitory responses recorded in either muscle layer, despite the presence of ongoing cholinergic tone in both the LM and CM. Contractions were abolished by tetrodotoxin (1.6 microM). 4. Local brush stroking of the mucosa evoked a peristaltic wave which readily conducted distally over 13 cm, without the presence of fluid in the lumen. No descending relaxation was observed. 5. Apamin (300 nM) disrupted evoked peristaltic waves and significantly increased the rate-of-rise of the LM and CM contractions anal to a stimulus, and the LM oral to a stimulus. 6. Nomega-nitro-L-arginine (100 microM), a nitric oxide synthesis inhibitor, had no overall significant effect on the characteristics of the LM and CM contractions, although on occasion an enhancement in their peak amplitude was noted. 7. It is suggested that the guinea-pig ileum does not conform to the 'law of the intestine' as postulated by Bayliss & Starling (1899). Rather, local physiological stimulation of the ileum elicits a contraction both orally and anally to a stimulus, which occurs synchronously in both the CM and LM layers. Apamin-sensitive inhibitory neurotransmission modulates the rate-of-rise of the anal contraction of the CM, possibly to generate distal propulsion.

The effects of excretions/secretions of Ostertagia circumcincta on ovine abomasal tissues in vitro

Products excreted/secreted by Ostertagia circumcincta stimulated the in vitro release of pepsinogen from intact abomasal mucosal sheets and caused the contraction of strips of abomasal smooth muscle, also in vitro. However, responses occurred only when tissues had been derived from animals that were assumed to have experienced prior exposure to the parasite. The overall median responses for pepsinogen secretion in response to ES, expressed as the ratio of stimulated secretion to basal secretion, were 1.8 for previously exposed animals and 0.9 for parasite-naive animals. For the smooth muscle from the previously exposed animals, the overall median response to ES, expressed as a percentage of the maximal response to carbachol in the same tissues, was 27.0. No responses were seen in muscle from any parasite-naive animal. These results suggest that the responses obtained were hypersensitivity reactions to antigens released by the worms during in vitro culture, and occurring in tissues from animals sensitised by exposure to O. circumcincta in the natural environment.

Stimulation of formation of adenosine 3',5'-phosphate by histamine in myenteric ganglia isolated from guinea-pig small intestine

Effects of histamine and related agonists and antagonists on formation of cAMP were determined for enzymatically dissociated ganglia from the myenteric plexus of the guinea-pig small intestine. Formation of cAMP was stimulated by histamine in both dose- and time-dependent manners. The stimulatory action of histamine was suppressed by the histamine H2 receptor antagonist, cimetidine. The histamine H1 receptor antagonists, tripelennamine or pyrilamine also suppressed the stimulatory action of histamine, but only at concentrations 3-4 orders higher than required for cimetidine. Formation of cAMP was stimulated dose-dependently by the histamine H2 receptor agonist, dimaprit. The histamine H1 receptor agonist, 2-methyl-histamine, also stimulated cAMP production, but required a threshold concentration 4-5 orders higher than dimaprit. We conclude that histamine acts at the histamine H2 receptor subtype to stimulate adenylate cyclase and the formation of cAMP in myenteric ganglia of the guinea-pig small bowel.

The source and action of histamine in the isolated guinea-pig gallbladder

We have investigated the effects of histamine on motility of the gallbladder and characterized the receptor types involved. Histamine and the histamine H1-receptor agonist, 2-thiazolylethylamine (2-TEA) contracted the isolated guinea-pig gallbladder strip in a dose dependent manner. The contractile response to histamine was shifted to the right by the H1-receptor antagonist, mepyramine. In pre-contracted gallbladder strips, the H2-receptor agonist dimaprit reduced the tension generated in a dose dependent fashion. The histamine H2-receptor antagonist, ranitidine shifted the histamine concentration effect curve to the left and attenuated the dose dependent relaxations elicited at high concentrations. The histamine H3-receptor agonist, (R)-alpha-methylhistamine (RMHA) elicited dose dependent contraction of the tissue which was significantly inhibited in the presence of mepyramine. The effects of electrical field stimulation (EFS) on the strips were not significantly altered by the presence of RMHA (10(-10) - 10(-7) M) indicating little pre-synaptic H3 activity in this tissue. Histamine immunoreactivity (IR) was detected in gallbladder whole mount preparations of the mucosa and the muscularis/serosa. The histamine IR appeared cell bound in cells of varying morphological characteristics but no IR was detected in nerve fibres or cell bodies (ganglia). Alcian blue staining was consistent with the distribution of histamine IR cells as mast cells. The results indicate that histamine is distributed in the guinea-pig gallbladder and it can regulate contractile activity via activation of H1 and H2 but not H3 receptors.

An update on histamine H3 receptors and gastrointestinal functions

The distribution and functions of histamine H3 receptors in the gastrointestinal tract is reviewed with particular reference to the effects on gastric acid secretion, mucosal protection, and intestinal motility. Histamine H3 receptor activation has negative effects on acid secretion induced by indirect secretagogues in cats, dogs, and rabbits; less clear effects were found in rats. An inhibitory effect on histamine release induced by different stimuli was observed in rats, rabbits, and dogs after H3 receptor agonists, thus supporting the idea that H3 receptors occur in ECL cells. (R)-alpha-methylhistamine has a marked protective effect against gastric lesions induced by ethanol in rats, being slightly less effective against aspirin and stress. H3 receptor activation decreases the intestinal motility induced by electrical stimulation in a variety of gut preparations, reducing both cholinergic and NANC neurotransmitter release. In this tissue the inhibitory effects mediated by histamine H3 receptors seem to be coupled, via a G protein, to a restriction of Ca2+ access into the nerve terminal; other mechanisms, however, have been suggested in the gastric mucosa. Histamine H3 receptors have already been subdivided into two receptor subtypes, H3A and H3B, the former being the subtype predominant in the gastrointestinal tissue. The increasing availability of selective agonists and antagonists of H3 receptors will unravel possible novel actions and physiological roles of histamine.

Regulation of ion transport by histamine in human colon

Histamine, added to the basolateral side of voltage clamped human colon in vitro, induced a rapid onset, transient inward short circuit current which was concentration dependent over the range 0.01-3 mM. This response was largely due to electrogenic chloride section since it was virtually abolished by bumetanide or by chloride replacement in the bathing solutions. Responses were unaffected by amiloride or acetazolamide. Neither the histamine H2 receptor agonist dimaprit (1 mM) nor the histamine H3 receptor agonist S-(+)-alpha-methyl histamine (1 mM) altered short circuit current. Responses to histamine were significantly reduced by the histamine H1 receptor antagonist mepyramine (1-10 microM) but not altered by the histamine H2 receptor antagonist cimetidine (100 microM) or by the histamine H3 receptor antagonist thioperamide (1 microM). Short circuit current responses to histamine were not altered by tetrodotoxin (1 microM). Piroxicam (10 microM) and nordihydroguaiaretic acid (100 microM) were without effect when used individually but significantly reduced responses to histamine when used simultaneously. These results indicate that histamine stimulates chloride secretion across human colonic epithelium by a mechanism which is mediated exclusively via histamine H1 receptors. This action does not involve intrinsic nerves but appears to be dependent upon eicosanoid synthesis.

Azines and diazines as potential histamine H3-receptor antagonists

In search of structure-activity relationships among histamine H3-receptor antagonists the imidazole ring of known H3-receptor antagonists was replaced by different heteroaromatic ring systems. Thus, azines and diazines with ether (6-13) and carbamate (15-24) moieties as functional groups were synthesized. The obtained compounds did not show significant H3-receptor antagonist activity in vitro (rat brain cortex) or in vivo (mice brain). The new compounds were also screened for H1-receptor antagonist activity on the isolated guinea-pig ileum and for H2-receptor antagonist activity on the isolated spontaneously beating guinea-pig right atrium. The substances showed only weak antagonistic activity at both histamine receptors, H1 and H2.

Histamine H1 and H3 vasodilator mechanisms in the guinea pig ileum

BACKGROUND/AIMS

Histamine dilates gastrointestinal blood vessels. Whether this is caused by direct activation of vascular histamine receptors or by activation of enteric neurons is not known. The aim of this study was to determine which of these pathways is activated by histamine and to examine the cellular mechanisms involved.

METHODS

The effects of histamine were studied in in vitro submucosal preparations from the guinea pig ileum using videomicroscopy to monitor changes in submucosal arteriolar diameter.

RESULTS

Histamine caused a tetrodotoxin-insensitive dose-dependent dilation (median effective concentration [EC50], 1 mumol/L), showing direct activation of vascular histamine receptors. The H1 antagonist pyrilamine, but not the H2 blocker ranitidine, competitively inhibited the histamine dilatation. The nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) inhibited histamine vasodilations by 66%. Indomethacin alone did not alter histamine vasodilations but, when combined with L-NMMA, caused a significantly greater inhibition of the histamine response compared with L-NMMA alone. L-Arginine prevented the actions of L-NMMA. In the presence of both H1 and H2 antagonists, periarteriolar stimulation of sympathetic nerves evoked a tetrodotoxin-sensitive vasoconstriction, which was inhibited by histamine (EC50, 0.8 mumol/L). This histamine action was blocked by the H3 antagonist thioperamide.

CONCLUSIONS

Histamine can produce vasodilation of submucosal arterioles by two distinct mechanisms: activation of vascular H1 receptors resulting in release of nitric oxide from endothelium and activation of H3 receptors on sympathetic nerve terminals resulting in presynaptic inhibition of vasoconstrictor tone.

H3 receptors: modulation of histamine-stimulated neural pathways influencing electrogenic ion transport in the guinea pig colon

The role of H3 receptors in neurally-evoked recurrent increases in chloride ion secretion evaluated from changes in short-circuit current (Isc) was examined during application of histamine or histamine analogs. Muscle-stripped or whole thickness segments of distal colon were set up in flux chambers. Histamine and dimaprit, an H2 receptor agonist, caused recurrent increases in Isc. Dimaprit-evoked recurrent responses were reduced by the H3 receptor agonists, N alpha-methylhistamine and R alpha-methylhistamine, and the inhibition was reversed by the H3 receptor antagonist, burimamide. Histamine-evoked recurrent increases in Isc were enhanced by the H3 receptor antagonists burimamide and thioperamide. The results indicate that H3 receptors play an inhibitory role in histamine-evoked, neurally-mediated recurrent increases in Isc in guinea pig colon.

Pharmacological characterization of the inhibitory effect of (R)-alpha-methylhistamine on sympathetic cardiopressor responses in the pithed guinea-pig

1. The effect of (R)-alpha-methylhistamine (R-alpha-mHA), a selective histamine H3-receptor agonist, on increases in blood pressure and heart rate mediated by activation of the sympathetic nervous system induced by electrical stimulation of the spinal cord, was characterized in the vagotomized, pithed guinea-pig. 2. The frequency-dependent nature of (R)-alpha-mHA's effect on sympathetic cardiopressor responses was studied at frequencies between 1 and 20 Hz. (R)-alpha-mHA (10-100 micrograms kg-1, i.v.) produced a dose-dependent inhibition of the stimulated increase in both blood pressure (BP) and heart rate (HR). The inhibition was inversely related to frequency and maximum inhibition (BP, 61% at 1 Hz; HR, 50% at 1 Hz) was seen with 100 micrograms kg-1 of (R)-alpha-mHA. Treatment with the H3 receptor inactive stereoisomer, (S)-alpha-methylhistamine (300 micrograms kg-1, i.v.) did not inhibit the neurogenic sympathetic cardiopressor responses. 3. Pretreatment with thioperamide (1 mg kg-1, i.v.), a histamine H3 receptor antagonist, blocked (R)-alpha-mHA's inhibitory effect on stimulation-induced sympathetic cardiopressor responses. 4. Combined pretreatment with the H2-receptor antagonist cimetidine (3 mg kg-1, i.v.) and the H1-receptor antagonist chlorpheniramine (0.3 mg kg-1, i.v.) did not attenuate (R)-alpha-mHA's inhibitory effects. 5. (R)-alpha-mHA (100 micrograms kg-1) had no effect on the hypertensive or tachycardia effects induced by adrenaline (1 and 3 micrograms kg-1, i.v.). 6. Treatment with a combination of prazosin (1 mg kg-1, i.v.) and yohimbine (1.5 mg kg-1, i.v.) to block alpha 1- and alpha 2-adrenoceptors, abolished the sympathetic hypertension without affecting the inhibition of sympathetic tachycardia induced by (R)-alpha-mHA. Conversely, pretreatment with the beta-adrenoceptor antagonist propranolol (1 mg kg-1, i.v.), which blocked the sympathetic tachycardia, did not block (R)-alpha-mHA's inhibition of sympathetic hypertensive responses. 7. In adrenalectomized guinea-pigs, (R)-alpha-mHA (100 micrograms kg-1, i.v.) also produced a frequency-dependent inhibition of sympathetic hypertensive cardiopressor responses that was not significantly different from intact animals. 8. These results demonstrate that (R)-alpha-mHA produces a frequency-dependent inhibition of the cardiopressor responses due to activation of the sympathetic innervation to the resistance vessels and the heart.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroimmune communication in the submucous plexus of guinea pig colon after infection with Trichinella spiralis

BACKGROUND/AIMS

Enteric neuroimmune communication in gastrointestinal hypersensitivity responses includes antigen detection by mast cells and release of chemical messages to the enteric nervous system. The aim of this study was to analyze the electrical and synaptic behavior of neurons in the colonic submucous plexus during exposure to Trichinella spiralis antigen in animals infected earlier with the parasite.

METHODS

Microelectrodes were used to record in submucous neurons of guinea pig distal colon during application of Trichinella antigen.

RESULTS

Neurons in sensitized animals were more excitable than in controls. Hyperexcitability was seen as a greater probability of spontaneous action potential discharge and repetitive firing to depolarizing current or exposure to acetylcholine. Application of histaminergic antagonists reversed the augmented excitability, suggesting endogenously released histamine as a responsible factor. Antigenic exposure increased neuronal excitability and suppressed nicotinic transmission at fast cholinergic synapses only in sensitized animals. Effects on excitability, but not presynaptic inhibitory effects, were blocked by cimetidine.

CONCLUSIONS

Signaling between mucosal mast cells and the enteric nervous system is involved in colonic anaphylactic responses to sensitizing antigens. Histamine is a paracrine signal in the communication pathway.

Histamine and serotonin released from the rat perfused heart by compound 48/80 or by allergen challenge influence noradrenaline or acetylcholine exocytotic release

Terminal nerve fibres of the autonomic nervous system closely approach mast cells in peripheral organs, and mutual influences between release of neurotransmitters or mast cell mediators may cause neuro-immunological interactions. We have studied the influence of mast cell degranulation on the release of endogenous noradrenaline and newly incorporated acetylcholine (such as 14C-choline/acetylcholine overflow) evoked by stimulation of extrinsic postganglionic sympathetic or preganglionic vagal nerves in the rat Langendorff heart perfused with Tyrode solution. Compound 48/80 perfused in normal hearts, or ovalbumin infused into hearts from rats sensitized to ovalbumin, enhanced the overflow of endogenous histamine and serotonin. Both stimuli increased the release of mediators to a similar extent and with fast kinetics. Maximum average concentrations in the perfusate of histamine were about 800 nmol/l, and of serotonin 40 nmol/l, in a sample collected within 4 min after mast cell degranulation. Stimulation of autonomic nerves did not affect basal histamine or serotonin overflow. Whereas basal overflows were unaffected, the stimulation-evoked releases of both noradrenaline and acetylcholine, were facilitated when compound 48/80 was perfused before and during nerve stimulation. The facilitation of noradrenaline overflow was more pronounced (by 60%) when compound 48/80-induced mediator overflow started 4 min before nerve stimulation as compared to 30 s (15%), and was reduced by cocaine (by 50%), and, in the presence of cocaine, abolished by cimetidine (but was unaffected by mepyramine and thioperamide) and NG-nitro-(L)-(-)-arginine. In the presence of cimetidine and cocaine, when the facilitatory components were abolished, the evoked noradrenaline overflow observed 30 s after the start of infusion of compound 48/80 was inhibited, and the inhibition was partly reduced by methiotepin and ketanserin. Ovalbumin infusion in hearts from sensitized animals caused an inhibition of evoked noradrenaline overflow sensitive to methiotepin and also partly to ketanserin, and no facilitation was observed. The facilitation (> 100%) of evoked overflow of acetylcholine observed at 4 min after the start of perfusion with compound 48/80 was partly reduced by thioperamide (but not mepyramine or cimetidine) and to a comparable extent either by tropisetron (3 mumol/l) alone or by tropisetron plus methiotepin. In conclusion, degranulation of immunological cells is followed by histamine and serotonin release in the rat heart and may affect the release of autonomic neurotransmitters in rather unusual ways, by i) an uptake1-dependent and ii) an H2-mediated facilitation which probably involves nitric oxide as a permissive mediator, and iii) a serotonergic inhibition, of noradrenaline release, and iv) an H3- and serotonergic facilitation of acetylcholine release.

Steroid-sensitivity of agonist binding to pituitary cell line histamine H3 receptors

Histamine H3 receptors have been identified in rat and guinea-pig pituitary glands and in the mouse pituitary tumor cell line, AtT-20. Histamine H3 receptor agonists are reported to stimulate adrenocorticotropic hormone (ACTH) release from AtT-20 cells, an effect blocked by histamine H3 but not H1 or H2 receptor antagonists. To determine whether negative feedback regulation of the histamine H3 receptor-mediated effect might occur, we tested the effects of steroid treatment upon binding of the agonist [3H]N alpha-methylhistamine to AtT-20 cell membranes. Consistent with feedback regulation, steroid treatment of the cells reduced [3H]N alpha-methylhistamine binding. The effect was dose-dependent and was greatest for glucocorticoids among the steroids tested. As the duration of steroid treatment increased, the amount of [3H]N alpha-methylhistamine binding decreased, to 15% of control at 36 h. However, the effect was not specific for histamine H3 receptors. Somatostatin inhibits ACTH release from these cells and its binding was similarly reduced by steroid treatment. Because steroids have been reported to modulate levels of guanine nucleotide-binding proteins, the lack of receptor specificity could reflect an indirect effect of steroids upon agonist binding and, in fact, we show that [3H]N alpha-methylhistamine binding to these cells, like somatostatin, is pertussis toxin-sensitive. However, steroid treatment does not alter the apparent levels of pertussis toxin substrate in these cells. Whether steroid treatment affects histamine H3 receptors of these cells directly or through some more subtle effect upon the guanine nucleotide-binding proteins to which they couple, the result is a negative feedback loop that attenuates [3H]N alpha-methylhistamine binding to these cells.

Histamine H3 receptor activation inhibits sympathetic-cholinergic responses in cats

Experiments were undertaken to determine the effect of the selective histamine H3 receptor agonist (R)-alpha-methylhistamine on the amplitude of neurally evoked electrodermal (sudomotor) responses in anesthetized cats. (R)-alpha-Methylhistamine produced comparable dose-related depressions of these evoked sympathetic-cholinergic electrodermal responses elicited by either pre- or postganglionic nerve stimulation. Responses evoked by i.a. methacholine were not depressed by pretreatment with (R)-alpha-methylhistamine. (R)-alpha-Methylhistamine inhibition of preganglionic evoked responses was antagonized by pretreatment with the histamine H3 receptor antagonist thioperamide, but not by pretreatment with selective blockers of histamine H1 or histamine H2 receptors (chlorpheniramine or cimetidine). Pretreatment with thioperamide did not antagonize presynaptic inhibition produced by i.v. (-)-epinephrine, nor did rauwolscine block the inhibition produced by (R)-alpha-methylhistamine. These results suggest that (R)-alpha-methylhistamine stimulates presynaptic histamine H3 receptors located on sudomotor postganglionic nerve endings to depress neurally evoked release of acetylcholine. (R)-alpha-Methylhistamine does not appear to act at an autonomic ganglionic site in this system.

Marked increase in [3H](R) alpha-methylhistamine binding in the superior colliculus of visually deprived rats after unilateral enucleation

The binding of [3H](R)alpha-methylhistamine to histamine H3-receptors in visual structures of unilaterally enucleated rats was examined by quantitative autoradiography to clarify the involvement of histamine neurons in the visual system. [3H](R)alpha-Methylhistamine binding in the visually deprived superior colliculus, contralateral to the enucleated eye, was significantly increased 5, 15, 30 and 45 days after unilateral enucleation. Slight time-dependent increases in ligand binding were observed in the visual cortex, but the change was significant only 45 days after unilateral enucleation. Unilateral enucleation had no significant effect in the dorsal lateral geniculate nucleus at any time after enucleation. Continuous injection of (S)-alpha-fluoromethylhistidine, a specific inhibitor of L-histidine decarboxylase, attenuated the effect of unilateral enucleation in the superior colliculus. These results suggest that retinal deafferentation induced an increase in histamine H3-receptor binding sites, probably by selective adjustment of histamine neurons in response to unilateral enucleation.

Modulation of neurotransmitter release via histamine H3 heteroreceptors

Presynaptic H3 receptors occur on histaminergic neurones of the CNS (autoreceptors) and on non-histaminergic neurones of the central and autonomic nervous system (heteroreceptors). H3 heteroreceptors, most probably located on the postganglionic sympathetic nerve fibres innervating the resistance vessels and the heart, have been identified in the model of the pithed rat. Furthermore, we could show in superfusion experiments that H3 heteroreceptors also occur on the sympathetic neurones supplying the human saphenous vein and the vasculature of the pig retina and on the serotoninergic, dopaminergic and noradrenergic neurones in the brain of various mammalian species, including man. The effects of three recently described H3 receptor ligands were studied in superfused mouse brain cortex slices. The potency of the novel H3 receptor agonist imetit exceeded that of R-(-)-alpha-methylhistamine (the reference H3 receptor agonist) by one log unit and that of histamine by almost two log units. Clobenpropit was shown to be a competitive H3 receptor antagonist, exhibiting a pA2 as high as 9.6 (exceeding the pA2 of the reference H3 receptor antagonist thioperamide by one log unit). The irreversible antagonism of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was also studied. Interactions of the H3 heteroreceptor with the dopamine autoreceptor in mouse striatal slices and the alpha 2-autoreceptor in mouse brain cortex slices could be demonstrated. Activation of alpha 2-autoreceptors decreases the H3 receptor-mediated effect. Blockade of alpha 2-autoreceptors increases the H3 receptor-mediated effect only if the alpha 2-autoreceptors are simultaneously activated by endogenous noradrenaline. The H3 receptor-mediated inhibition of noradrenaline release in mouse brain cortex slices was attenuated by the K+ channel blocker tetraethylammonium but this attenuation was abolished by reduction of the Ca2+ concentration in the medium (to compensate for the facilitatory effect of tetraethylammonium on noradrenaline release). Accordingly, we assume that the H3 receptors are not coupled to voltage-sensitive K+ channels. Pertussis toxin and N-ethylmaleimide attenuated the H3 receptor-mediated effect in the mouse brain cortex, suggesting that the H3 receptors are coupled to a G protein (eg Gi or Go). However, negative coupling to an adenylate cyclase does not appear to exist since an H3 receptor-mediated inhibition of cAMP accumulation was not obtained in mouse brain cortex membranes. H3 receptor ligands are currently undergoing clinical testing and might become new remedies for the treatment of disease of the gastrointestinal and bronchial system and the CNS.

Production by R-alpha-methylhistamine of a histamine H3 receptor-mediated decrease in basal vascular resistance in guinea-pigs

1. The effect of the selective histamine H3 receptor agonist, R-alpha-methylhistamine given intravenously (10-100 micrograms kg-1) was examined on baseline total peripheral resistance (TPR), and cardiovascular haemodynamics in bilaterally vagotomized, anaesthetized guinea-pigs. 2. R-alpha-methylhistamine produced a dose-dependent hypotension and fall in TPR at 30 and 100 micrograms kg-1. A decrease in heart rate (HR) was observed at a dose of 100 micrograms kg-1. R-alpha-methylhistamine (10-100 micrograms kg-1) also produced a dose-dependent fall in rate pressure product (RPP). There was no effect on cardiac output (CO) or stroke volume (SV) at these doses. 3. Histamine H1 and H2 blockade in animals pretreated with a combination of chlorpheniramine (0.3 mg kg-1) and cimetidine (3.0 mg kg-1) did not alter the haemodynamic actions of R-alpha-methyl-histamine (100 micrograms kg-1, i.v.). Pretreatment with the selective H3 antagonist, thioperamide (1 mg kg-1), completely blocked the action of R-alpha-methylhistamine on haemodynamic parameters. 4. To study the mechanism of action of R-alpha-methylhistamine, the vasodilator hydralazine (1 mg kg-1, i.v.) was used. Hydralazine lowered BP, TRP and RPP in guinea-pigs pretreated with ipratropium (50 micrograms kg-1, i.v.). Hydralazine had no effect on HR, SV or CO. 5. R-alpha-methylhistamine (100 micrograms kg-1) did not affect the vasopressor action and increases in TPR produced by adrenaline (1 and 3 micrograms kg-1). On the other hand, the vasodilator hydralazine (1 mg kg-1, i.v.) inhibited the effects of adrenaline (3 micrograms kg-1) on TPR and RPP. The effect of both doses of adrenaline on BP were attenuated by hydralazine. Therefore, the inhibitory effects of R-alpha-methylhistamine are not mediated through a direct action on vascular smooth muscle.6. In adrenalectomized guinea-pigs, R-alpha-methylhistamine (100 microg kg-1) produced a drop in BP and HR.There was no difference between the effects of R-alpha-methylhistamine on blood pressure and heart rate in adrenalectomized and non-adrenalectomized guinea-pigs.7. These results show that activation of peripheral H3 receptors lowers basal BP, HR and TPR, most likely by a peripheral prejunctional mechanism. The fall in BP and TPR is probably due to a decrease in noradrenaline release from sympathetic effector nerves innervating the resistance blood vessels.

Effects of (S)-alpha-fluoromethylhistidine and (R)-alpha-methylhistamine on locomotion of W/Wv mice

We studied the effects of inactivators of the central histaminergic neuron system, (R)-alpha-methylhistamine, a histamine H3 receptor agonist, and (S)-alpha-fluoromethylhistidine, a histamine synthesis inhibitor, on locomotor activity and brain histamine content of mast cell-deficient W/Wv mice using a recently developed high-performance liquid chromatography system coupled with a fluorometric detector. IP injection of (R)-alpha-methylhistamine (6-50 mg/kg) increased brain histamine content after 1 h but caused no significant change in locomotor activity. IP injection of (S)-alpha-fluoromethylhistidine decreased brain histamine content at doses of 6-50 mg/kg and locomotor activity at doses of 12.5-50 mg/kg. However, locomotor activity was decreased significantly (in Student's t-test) by sequential administrations of (S)-alpha-fluoromethylhistidine (6 mg/kg) and (R)-alpha-methylhistamine (12.5 or 25 mg/kg), but not by (S)-alpha-fluoromethylhistidine (6 mg/kg) and other doses of (R)-alpha-methylhistamine (6 or 50 mg/kg). These results support the hypothesis that the central histaminergic neuron system is involved in the control of spontaneous locomotion or alertness.

Prejunctional inhibition of sympathetically evoked pupillary dilation in cats by activation of histamine H3 receptors

Frequency-dependent pupillary dilations were evoked by electrical stimulation of the pre- or post-ganglionic cervical sympathetic nerve (sympatho-excitation) or the hypothalamus (parasympatho-inhibition) in sympathectomized anesthetized cats. Systemic administration of the selective histamine H3 receptor agonist (R)-alpha-methylhistamine (R alpha MeHA) produced a dose-dependent depression of mydriasis due to direct neural sympathetic activation but had no effect on responses elicited by parasympathetic withdrawal. The histamine H2 receptor agonist, dimaprit, was inactive. R alpha MeHA was much more effective in depressing sympathetic responses obtained at lower frequencies when compared to higher frequencies of stimulation. Responses evoked both pre- and postganglionically were inhibited by R alpha MeHA. This peripheral sympatho-inhibitory action of R alpha MeHA was antagonized by the histamine H3 receptor blocker thioperamide but not by intravenous pretreatment with the histamine H1 receptor antagonist chlorpheniramine. Histamine H2 receptor blockers cimetidine and ranitidine were also without effect. R alpha MeHA did not depress pupillary responses elicited by i.v. (-)-adrenaline. The results demonstrate that histamine H3 receptors modulate sympathetic activation of the iris at a site proximal to the iris dilator muscle. The predominant mechanism of action appears to the prejunctional inhibition of noradrenaline release from postganglionic sympathetic nerve endings. However, a concomitant ganglionic inhibitory action cannot be excluded.

Pharmacological profile of new thioperamide derivatives at histamine peripheral H1-, H2-, H3-receptors in guinea-pig

The recent availability of potent and selective ligands, namely R-(alpha)-methylhistamine and thioperamide, led to conclusive progresses as regards histamine H3-receptor knowledge. The aim of this work is to investigate by in vitro tests the pharmacological properties of new amino and methyl derivatives of the H3-antagonist thioperamide. Such original compounds, developed by the modulation of the thioperamide imidazolyl moiety, were assayed at guinea-pig ileal contractile H1-, atrial chronotropic H2- and enteric neuronal H3-receptors. None of the drugs exhibited interaction with H1 or H2 sites. On electrically stimulated ileum, two of the thioperamide methyl derivatives competitively antagonized the inhibitory effect of the H3-agonist R-(alpha)-methylhistamine. On the basis of the Schild analysis, the more active isomer (compound IV) displayed an affinity at H3-receptors only five times lower than thioperamide. These results could contribute to elucidate further the structural features required to develop potent and selective H3-antagonists. On the other hand, to prove the hypothesized apparent heterogeneity between peripheral and central H3-sites, as emerged by pharmacological and binding studies, autoradiographic investigations are in progress.

A detailed autoradiographic mapping of histamine H3 receptors in rat brain areas

[3H](R)alpha-methylhistamine, a selective histamine H3-receptor ligand, was used to perform binding studies with membranes and generate light microscopic autoradiograms in sections of the rat brain. High densities of H3 receptors were found in membranes from the anterior part of the cerebral cortex, the accumbens nucleus, the striatum, the olfactory tubercles and the substantia nigra. Autoradiography of sagittal and frontal sections evidenced specific labelling in a number of gray matter areas over a very low background, as determined using thioperamide, a selective H3-receptor antagonist, as competing drug. Labelled areas were identified by comparison with adjacent Nissl-stained sections and their labelling was rated visually. H3 receptors are heterogeneously distributed among areas known to receive histaminergic projections. In the cerebral cortex, H3 receptors are present in all areas and layers, with a rostrocaudal gradient and a higher density in deep layers (laminae IV-VI). In the hippocampal formation, H3 receptors are the most abundant in the dentate gyrus and the subiculum. In the amygdaloid complex, the highest densities are found in the central, lateral and basolateral groups of nuclei. In the basal forebrain, the accumbens nucleus, the striatum, the olfactory tubercles and the globus pallidus are highly labelled. In the thalamus in which histaminergic fibres are scarce, H3 receptors are present in a rather high density, particularly in the midline, median and intralaminar groups of nuclei. In the hypothalamus, where the densest network of histaminergic fibres is found, H3 receptors occur in moderate density, being slightly more abundant in the anterior and medial part. They are also present at the level of the tuberomammillary nuclei where they may reside on histaminergic perikarya. In mesencephalon and lower brainstem, H3 receptors are abundant in the reticular part of the substantia nigra and central gray. They are present in low density in areas of noradrenergic and serotoninergic perikarya and in the spinal cord, where a faint specific labelling is detected in the gray matter, particularly in the external layers of the dorsal horn. In the cerebellum and pituitary gland, H3 receptors are scarce. Kainic acid infusions into the striatum were followed by marked local decreases in H3 receptors evidenced in both membrane binding and autoradiographic studies. Unilateral interruption of the ascending histaminergic pathways via electrocoagulation of the lateral hypothalamic area was followed by ipsilateral increase in striatal [3H](R)alpha-methylhistamine binding, a process consistent with denervation up regulation of postsynaptic H3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)