Expression analysis of the histamine H(3) receptor in developing rat tissues

Long-Term Functional and Cytoarchitectonic Effects of the Systemic Administration of the Histamine H1 Receptor Antagonist/Inverse Agonist Chlorpheniramine During Gestation in the Rat Offspring Primary Motor Cortex

The transient histaminergic system is among the first neurotransmitter systems to appear during brain development in the rat mesencephalon/rhombencephalon. Histamine increases FOXP2-positive deep-layer neuron differentiation of cortical neural stem cells through H₁ receptor activation in vitro. The in utero or systemic administration of chlorpheniramine (H₁ receptor antagonist/inverse agonist) during deep-layer cortical neurogenesis decreases FOXP2 neurons in the developing cortex, and H₁R- or histidine decarboxylase-knockout mice show impairment in learning and memory, wakefulness and nociception, functions modulated by the cerebral cortex. Due to the role of H₁R in cortical neural stem cell neurogenesis, the purpose of this study was to evaluate the postnatal impact of the systemic administration of chlorpheniramine during deep-layer cortical neuron differentiation (E12–14) in the primary motor cortex (M1) of neonates (P0) and 21-day-old pups (P21). Chlorpheniramine or vehicle were systemically administered (5 mg/kg, i.p.) to pregnant Wistar rats at gestational days 12–14, and the expression and distribution of deep- (FOXP2 and TBR1) and superficial-layer (SATB2) neuronal cortical markers were analyzed in neonates from both groups. The qRT-PCR analysis revealed a reduction in the expression of Satb2 and FoxP2. However, Western blot and immunofluorescence showed increased protein levels in the chlorpheniramine-treated group. In P21 pups, the three markers showed impaired distribution and increased immunofluorescence in the experimental group. The Sholl analysis evidenced altered dendritic arborization of deep-layer neurons, with lower excitability in response to histamine, as evaluated by whole-cell patch-clamp recording, as well as diminished depolarization-evoked [³H]-glutamate release from striatal slices. Overall, these results suggest long-lasting effects of blocking H₁Rs during early neurogenesis that may impact the pathways involved in voluntary motor activity and cognition.

Histaminergic Control of Corticostriatal Synaptic Plasticity during Early Postnatal Development

A reduction in the synthesis of the neuromodulator histamine has been associated with Tourette's syndrome and obsessive-compulsive disorder. Symptoms of these disorders are thought to arise from a dysfunction or aberrant development ofcorticostriatal circuits. Here, we investigated how histamine affects developing corticostriatal circuits, both acutely and longer-term, during the first postnatal weeks, using patch-clamp and field recordings in mouse brain slices (C57Bl/6, male and female). Immunohistochemistry for histamine-containing axons reveals striatal histaminergic innervation by the second postnatal week, and qRT-PCR shows transcripts for H1, H2, and H3 histamine receptors in striatum from the first postnatal week onwards, with pronounced developmental increases in H3 receptor expression. Whole-cell patch-clamp recordings of striatal spiny projection neurons and histamine superfusion demonstrates expression of functional histamine receptors from the first postnatal week onwards, with histamine having diverse effects on their electrical properties, including depolarization of the membrane potential while simultaneously decreasing action potential output. Striatal field recordings and electrical stimulation of corticostriatal afferents revealed that histamine, acting at H3 receptors, negatively modulates corticostriatal synaptic transmission from the first postnatal week onwards. Last, we investigated effects of histamine on longer-term changes at developing corticostriatal synapses and show that histamine facilitates NMDA receptor-dependent LTP via H3 receptors during the second postnatal week, but inhibits synaptic plasticity at later developmental stages. Together, these results show that histamine acutely modulates developing striatal neurons and synapses and controls longer-term changes in developing corticostriatal circuits, thus providing insight into the possible etiology underlying neurodevelopmental disorders resulting from histamine dysregulation.SIGNIFICANCE STATEMENT Monogenic causes of neurologic disorders, although rare, can provide opportunities to both study and understand the brain. For example, a nonsense mutation in the coding gene for the histamine-synthesizing enzyme has been associated with Tourette's syndrome and obsessive-compulsive disorder, and dysfunction of corticostriatal circuits. Nevertheless, the etiology of these neurodevelopmental disorders and histamine's role in the development of corticostriatal circuits have remained understudied. Here we show that histamine is an active neuromodulator during the earliest periods of postnatal life and acts at developing striatal neurons and synapses. Crucially, we show that histamine permits NMDA receptor-dependent corticostriatal synaptic plasticity during an early critical period of postnatal development, which suggests that genetic or environmental perturbations of histamine levels can impact striatal development.

Neurotransmitters, neuropeptides and their receptors interact with immune response in healthy and psoriatic skin

Psoriasis is a chronic inflammatory disease with a multifactorial origin that affects the skin. It is characterized by keratinocyte hyperproliferation, which results in erythemato-squamous plaques. Just as the immune system plays a fundamental role in psoriasis physiopathology, the nervous system maintains the inflammatory process through the neuropeptides and neurotransmitters synthesis, as histamine, serotonin, calcitonin gene-related peptide, nerve growth factor, vasoactive intestinal peptide, substance P, adenosine, glucagon-like peptide, somatostatin and pituitary adenylate cyclase polypeptide. In patients with psoriasis, the systemic or in situ expression of these chemical mediators and their receptors are altered, which affects the clinical activity of patients due to its link to the immune system, provoking neurogenic inflammation. It is important to establish the role of the nervous system since it could represent a therapeutic alternative for psoriasis patients. The aim of this review is to offer a detailed review of the current literature about the neuropeptides and neurotransmitters involved in the physiopathology of psoriasis.

Histamine, histamine receptors, and neuropathic pain relief

Histamine, acting via distinct histamine H₁, H₂, H₃, and H₄ receptors, regulates various physiological and pathological processes, including pain. In the last two decades, there has been a particular increase in evidence to support the involvement of H₃ receptor and H₄ receptor in the modulation of neuropathic pain, which remains challenging in terms of management. However, recent data show contrasting effects on neuropathic pain due to multiple factors that determine the pharmacological responses of histamine receptors and their underlying signal transduction properties (e.g., localization on either the presynaptic or postsynaptic neuronal membranes). This review summarizes the most recent findings on the role of histamine and the effects mediated by the four histamine receptors in response to the various stimuli associated with and promoting neuropathic pain. We particularly focus on mechanisms underlying histamine‐mediated analgesia, as we aim to clarify the analgesic potential of histamine receptor ligands in neuropathic pain.

Linked Articles

This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc

The Systemic Administration of the Histamine H1 Receptor Antagonist/Inverse Agonist Chlorpheniramine to Pregnant Rats Impairs the Development of Nigro-Striatal Dopaminergic Neurons

The dopaminergic and histaminergic systems are the first to appear during the development of the nervous system. Through the activation of H₁ receptors (H₁Rs), histamine increases neurogenesis of the cortical deep layers, while reducing the dopaminergic phenotype (cells immunoreactive to tyrosine hydroxylase, TH⁺) in embryo ventral mesencephalon. Although the function of histamine in neuronal differentiation has been studied, the role of H₁Rs in neurogenesis has not been addressed. For this purpose, the H₁R antagonist/inverse agonist chlorpheniramine was systemically administered (5 mg/kg, i.p.) to pregnant Wistar rats (gestational days 12-14, E12-14), and control and experimental embryos (E14 and E16) and pups (21-day-old) were evaluated for changes in nigro-striatal development. Western blot and immunohistochemistry determinations showed a significant increase in the dopaminergic markers' TH and PITX3 in embryos from chlorpheniramine-treated rats at E16. Unexpectedly, 21-day-old pups from the chlorpheniramine-treated group, showed a significant reduction in TH immunoreactivity in the substantia nigra pars compacta and dorsal striatum. Furthermore, striatal dopamine content, evoked [³H]-dopamine release and methamphetamine-stimulated motor activity were significantly lower compared to the control group. These results indicate that H₁R blockade at E14-E16 favors the differentiation of dopaminergic neurons, but hampers their migration, leading to a decrease in dopaminergic innervation of the striatum in post-natal life.

Design, synthesis, and biological evaluation of novel oxadiazole- and thiazole-based histamine H3R ligands

Histamine H₃ receptor (H₃R) is largely expressed in the CNS and modulation of the H₃R function can affect histamine synthesis and liberation, and modulate the release of many other neurotransmitters. Targeting H₃R with antagonists/inverse agonists may have therapeutic applications in neurodegenerative disorders, gastrointestinal and inflammatory diseases. This prompted us to design and synthesize azole-based H₃R ligands, i.e. having oxadiazole- or thiazole-based core structures. While ligands of oxadiazole scaffold were almost inactive, thiazole-based ligands were very potent and several exhibited binding affinities in a nanomolar concentration range. Ligands combining 4-cyanophenyl moiety as arbitrary region, para-xylene or piperidine carbamoyl linkers, and/or pyrrolidine or piperidine basic heads were found to be the most active within this series of thiazole-based H₃R ligands. The most active ligands were in silico screened for ADMET properties and drug-likeness. They fulfilled Lipinski's and Veber's rules and exhibited potential activities for oral administration, blood-brain barrier penetration, low hepatotoxicity, combined with an overall good toxicity profile.

The Histamine H1 Receptor Participates in the Increased Dorsal Telencephalic Neurogenesis in Embryos from Diabetic Rats

Increased neuron telencephalic differentiation during deep cortical layer formation has been reported in embryos from diabetic mice. Transitory histaminergic neurons within the mesencephalon/rhombencephalon are responsible for fetal histamine synthesis during development, fibers from this system arrives to the frontal and parietal cortex at embryo day (E) 15. Histamine is a neurogenic factor for cortical neural stem cells in vitro through H₁ receptor (H₁R) which is highly expressed during corticogenesis in rats and mice. Furthermore, in utero administration of an H₁R antagonist, chlorpheniramine, decreases the neuron markers microtubuline associated protein 2 (MAP2) and forkhead box protein 2. Interestingly, in the diabetic mouse model of diabetes induced with streptozotocin, an increase in fetal neurogenesis in terms of MAP2 expression in the telencephalon is reported at E11.5. Because of the reported effects on cortical neuron differentiation of maternal diabetes in one hand and of histamine in the other, here the participation of histamine and H₁R on the increased dorsal telencephalic neurogenesis was explored. First, the increased neurogenesis in the dorsal telencephalon at E14 in diabetic rats was corroborated by immunohistochemistry and Western blot. Then, changes during corticogenesis in the level of histamine was analyzed by ELISA and in H₁R expression by qRT-PCR and Western blot and, finally, we tested H₁R participation in the increased dorsal telencephalic neurogenesis by the systemic administration of chlorpheniramine. Our results showed a significant increase of histamine at E14 and in the expression of the receptor at E12. The administration of chlorpheniramine to diabetic rats at E12 prevented the increased expression of βIII-tubulin and MAP2 mRNAs (neuron markers) and partially reverted the increased level of MAP2 protein at E14, concluding that H₁R have an important role in the increased neurogenesis within the dorsal telencephalon of embryos from diabetic rats. This study opens new perspective on the participation of HA and H₁R receptor in early corticogenesis in health and disease.

Multiple Targeting Approaches on Histamine H3 Receptor Antagonists

With the very recent market approval of pitolisant (Wakix®), the interest in clinical applications of novel multifunctional histamine H₃ receptor antagonists has clearly increased. Since histamine H₃ receptor antagonists in clinical development have been tested for a variety of different indications, the combination of pharmacological properties in one molecule for improved pharmacological effects and reduced unwanted side-effects is rationally based on the increasing knowledge on the complex neurotransmitter regulations. The polypharmacological approaches on histamine H₃ receptor antagonists on different G-protein coupled receptors, transporters, enzymes as well as on NO-signaling mechanism are described, supported with some lead structures.

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 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.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Histamine impairs midbrain dopaminergic development in vivo by activating histamine type 1 receptors

Background

Histamine (HA) regulates the sleep-wake cycle, synaptic plasticity and memory in adult mammals. Dopaminergic specification in the embryonic ventral midbrain (VM) coincides with increased HA brain levels. To study the effect of HA receptor stimulation on dopamine neuron generation, we administered HA to dopamine progenitors, both in vitro and in vivo.

Results

Cultured embryonic day 12 (E12) VM neural stem/progenitor cells expressed transcripts for HA receptors H₁R, H₂R and H₃R. These undifferentiated progenitors increased intracellular calcium upon HA addition. In HA-treated cultures, dopamine neurons significantly decreased after activation of H₁R. We performed intrauterine injections in the developing VM to investigate HA effects in vivo. HA administration to E12 rat embryos notably reduced VM Tyrosine Hydroxylase (TH) staining 2 days later, without affecting GABA neurons in the midbrain, or serotonin neurons in the mid-hindbrain boundary. qRT-PCR and Western blot analyses confirmed that several markers important for the generation and maintenance of dopaminergic lineage such as TH, Lmx1a and Lmx1b were significantly diminished. To identify the cell type susceptible to HA action, we injected embryos of different developmental stages, and found that neural progenitors (E10 and E12) were responsive, whereas differentiated dopaminergic neurons (E14 and E16) were not susceptible to HA actions. Proliferation was significantly diminished, whereas neuronal death was not increased in the VM after HA administration. We injected H₁R or H₂R antagonists to identify the receptor responsible for the detrimental effect of HA on dopaminergic lineage and found that activation of H₁R was required.

Conclusion

These results reveal a novel action of HA affecting dopaminergic lineage during VM development.

Developmental roles of brain histamine

Histamine appears early during brain development, has been shown to regulate fetal and adult brain-derived stem cells in a receptor type-dependent manner, and has widespread actions on systems involved in arousal and movement. Developmental studies in both rodents and zebrafish have elucidated the spatiotemporal patterning of the histaminergic system and, in zebrafish, have revealed the mechanisms whereby histamine regulates the number of hypocretin/orexin (hcrt) neurons, which in turn may regulate the number of histaminergic cells. Recent demonstrations of increased numbers of histaminergic neurons in patients with narcolepsy highlight the importance, for our understanding of both normal and pathological brain function, of understanding these interactions. Here, we review recent research into the developmental roles of histamine and suggest key areas for future research.

Comprehensive RNA-Seq expression analysis of sensory ganglia with a focus on ion channels and GPCRs in Trigeminal ganglia

The specific functions of sensory systems depend on the tissue-specific expression of genes that code for molecular sensor proteins that are necessary for stimulus detection and membrane signaling. Using the Next Generation Sequencing technique (RNA-Seq), we analyzed the complete transcriptome of the trigeminal ganglia (TG) and dorsal root ganglia (DRG) of adult mice. Focusing on genes with an expression level higher than 1 FPKM (fragments per kilobase of transcript per million mapped reads), we detected the expression of 12984 genes in the TG and 13195 in the DRG. To analyze the specific gene expression patterns of the peripheral neuronal tissues, we compared their gene expression profiles with that of the liver, brain, olfactory epithelium, and skeletal muscle. The transcriptome data of the TG and DRG were scanned for virtually all known G-protein-coupled receptors (GPCRs) as well as for ion channels. The expression profile was ranked with regard to the level and specificity for the TG. In total, we detected 106 non-olfactory GPCRs and 33 ion channels that had not been previously described as expressed in the TG. To validate the RNA-Seq data, in situ hybridization experiments were performed for several of the newly detected transcripts. To identify differences in expression profiles between the sensory ganglia, the RNA-Seq data of the TG and DRG were compared. Among the differentially expressed genes (> 1 FPKM), 65 and 117 were expressed at least 10-fold higher in the TG and DRG, respectively. Our transcriptome analysis allows a comprehensive overview of all ion channels and G protein-coupled receptors that are expressed in trigeminal ganglia and provides additional approaches for the investigation of trigeminal sensing as well as for the physiological and pathophysiological mechanisms of pain.

Histamine up-regulates fibroblast growth factor receptor 1 and increases FOXP2 neurons in cultured neural precursors by histamine type 1 receptor activation: conceivable role of histamine in neurogenesis during cortical development in vivo

Background

During rat development, histamine (HA) is one of the first neuroactive molecules to appear in the brain, reaching its maximal value at embryonic day 14, a period when neurogenesis of deep layers is occurring in the cerebral cortex, suggesting a role of this amine in neuronal specification. We previously reported, using high-density cerebrocortical neural precursor cultures, that micromolar HA enhanced the effect of fibroblast growth factor (FGF)-2 on proliferation, and that HA increased neuronal differentiation, due to HA type 1 receptor (H₁R) activation.

Results

Clonal experiments performed here showed that HA decreased colony size and caused a significant increase in the percentage of clones containing mature neurons through H₁R stimulation. In proliferating precursors, we studied whether HA activates G protein-coupled receptors linked to intracellular calcium increases. Neural cells presented an increase in cytoplasmic calcium even in the absence of extracellular calcium, a response mediated by H₁R. Since FGF receptors (FGFRs) are known to be key players in cell proliferation and differentiation, we determined whether HA modifies the expression of FGFRs1-4 by using RT-PCR. An important transcriptional increase in FGFR1 was elicited after H₁R activation. We also tested whether HA promotes differentiation specifically to neurons with molecular markers of different cortical layers by immunocytochemistry. HA caused significant increases in cells expressing the deep layer neuronal marker FOXP2; this induction of FOXP2-positive neurons elicited by HA was blocked by the H₁R antagonist chlorpheniramine in vitro. Finally, we found a notable decrease in FOXP2+ cortical neurons in vivo, when chlorpheniramine was infused in the cerebral ventricles through intrauterine injection.

Conclusion

These results show that HA, by activating H₁R, has a neurogenic effect in clonal conditions and suggest that intracellular calcium elevation and transcriptional up-regulation of FGFR1 participate in HA-induced neuronal differentiation to FOXP2 cells in vitro; furthermore, H₁R blockade in vivo resulted in decreased cortical FOXP2+ neurons.

Identification of histamine receptors and effects of histamine on murine and simian colonic excitability

BACKGROUND

Inflammatory responses can include recruitment of cells of hematopoietic origin to the tunica muscularis. These cells can secrete a variety of factors which can reset the gain of smooth muscle cells (SMC) and influence motor patterns. Histamine (H), a major mediator in inflammation, is released by mast cells and exerts diverse effects in SMC by binding to H receptors. The profiles of H receptor expression in animal models used to study inflammatory diseases are unknown.

METHODS

Histamine receptor expression and electro-mechanical responses to H were tested in simian and murine colonic smooth muscle using qualitative and quantitative PCR, isometric force measurements, microelectrode recordings and patch clamp techniques.

KEY RESULTS

H1, H2, and H4 receptor transcripts were expressed at similar levels in simian colonic tissue whereas only the H2 receptor transcript was detected in murine colonic tissue. Stimulation of simian colonic muscles with H caused depolarization and contraction in the presence of tetrodotoxin. Histamine activated non-selective cation channels in simian SMC. In contrast, H caused hyperpolarization and inhibited contractions of murine colon. The hyperpolarization was inhibited by the K(ATP) channel blocker, glibenclamide. Histamine-activated K(+) currents were inhibited by glibenclamide in murine colonic SMC.

CONCLUSIONS & INFERENCES

Histamine receptor expression in simian SMC was similar to that reported in humans. However, H receptor profile and responses to H were considerably different in mice. Thus, monkey colon may be a more suitable model to study how inflammatory mediators affect the gain of smooth muscle excitability.

H3 receptors and pain modulation: peripheral, spinal, and brain interactions

Histamine H(3) receptors (H(3)Rs), distributed within the brain, the spinal cord, and on specific types of primary sensory neurons, can modulate pain transmission by several mechanisms. In the skin, H(3)Rs are found on certain Aβ fibers, and on keratinocytes and Merkel cells, as well as on deep dermal, peptidergic Aδ fibers terminating on deep dermal blood vessels. Activation of H(3)Rs on the latter in the skin, heart, lung, and dura mater reduces calcitonin gene-related peptide and substance P release, leading to anti-inflammatory (but not antinociceptive) actions. However, activation of H(3)Rs on the spinal terminals of these sensory fibers reduces nociceptive responding to low-intensity mechanical stimuli and inflammatory stimuli such as formalin. These findings suggest that H(3)R agonists might be useful analgesics, but these drugs have not been tested in clinically relevant pain models. Paradoxically, H(3) antagonists/inverse agonists have also been reported to attenuate several types of pain responses, including phase II responses to formalin. In the periaqueductal gray (an important pain regulatory center), the H(3) inverse agonist thioperamide releases neuronal histamine and mimics histamine's biphasic modulatory effects in thermal nociceptive tests. Newer H(3) inverse agonists with potent, selective, and brain-penetrating properties show efficacy in several neuropathic and arthritis pain models, but the sites and mechanisms for these actions remain poorly understood.

Mechanisms underlying the radioprotective effect of histamine on small intestine

PURPOSE

To examine the protective effects of histamine on intestinal damage produced by gamma-radiation.

MATERIALS AND METHODS

56 mice were divided into 4 groups. Histamine and Histamine-10 Gy groups received a daily subcutaneous histamine injection (0.1 mg/kg) starting 20 hours before irradiation and continued until the end of the experimental period; the untreated group received saline. Histamine-10 Gy and untreated-10 Gy groups were irradiated with a single dose on whole-body using Cesium-137 source (7 Gy/min) and were sacrificed 3 days after irradiation. Small intestine was removed, fixed and stained with hematoxylin and eosin. The number of intestinal crypts per circumference, and other histological characteristics of intestinal cells were evaluated. We further determined by immunohistochemistry the expression of proliferating cell nuclear antigen (PCNA), Bax, Bcl-2 (pro- and anti-apoptotic protein, respectively), antioxidant enzymes (Superoxide dismutase (SOD), Catalase and Glutathione peroxidase), histamine content and apoptosis by terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate biotin nick end labeling (TUNEL) assay. Cells in the S phase of the cell cycle were identified by immunohistochemical detection of 5-bromo-2'-deoxyuridine (BrdU) incorporation.

RESULTS

Histamine treatment reduced mucosal atrophy, edema and preserved villi, crypts and nuclear and cytoplasmic characteristics of small intestine after radiation exposure. Additionally, histamine treatment increased PCNA expression and the BrdU-positive cell number, histamine content, decreased the number of apoptotic cells and significantly increased Catalase and copper-zinc-containing SOD of irradiated mice.

CONCLUSIONS

Histamine prevents radiation-induced toxicity by increasing proliferation of damaged intestinal mucosa and suppressing apoptosis that was associated with an increase in SOD and Catalase levels. This effect might be of clinical value in patients undergoing radiotherapy.

Autoregulation of McA-RH7777 hepatoma cell proliferation by histamine H3 receptors

Previous studies have suggested that histamine (HA) acts as an autocrine growth factor. We have explored the modulation of cell proliferation by HA using McA-RH7777 hepatoma cells. High L-histidine decarboxylase (HDC) expression and HA synthesis were found in McA-RH7777 cells. Whereas extracellular HA reached submicromolar concentrations, intracellular levels were very low, indicating that HA was secreted by the cells. McA-RH7777 cells also express H3-receptor (H3R) transcripts and proteins. Reverse transcriptase-polymerase chain reaction analysis detected only transcripts for the long isoform. Immunocytochemistry performed with a selective H3R antibody showed that most cells were immunoreactive. H3R binding sites (Bmax approximately 30 fmol/mg protein) were identified when [125I] iodoproxyfan binding was displaced by the agonist imetit. High-affinity binding also occurred at cytochrome P450 enzymes. This binding was not inhibited by HA, H3R agonists, or by a nonimidazole H3R antagonist but was displaced by imidazole H3R antagonists or by ketoconazole, a imidazole-containing cytochrome inhibitor. HA inhibited proliferation of McA-RH7777 hepatoma cells. The absence of uptake system, its much higher potency at H3Rs, and its low intracellular levels suggested that HA interacted with H3Rs rather than cytochromes. In agreement, both imidazole H3R antagonists, a nonimidazole H3R antagonist, and the HDC inhibitor alpha-monofluoromethyl histidine increased cell proliferation (up to approximately 60%), revealing a H3R-mediated inhibition by endogenous HA. Moreover, exogenous HA inhibited the increase induced by alpha-FMH or H3R antagonists with a nanomolar potency. In conclusion, our findings show that HA regulates proliferation of McA-RH7777 hepatoma cells by interacting with autoinhibitory H3Rs.

Stroke induces histamine accumulation and mast cell degranulation in the neonatal rat brain

Inflammatory processes are a major cause of hypoxic-ischemic brain damage. The present study focuses on both the cerebral histamine system and mast cells in a model of transient focal ischemia induced by permanent left middle cerebral artery, and homolateral transient common carotid artery occlusion (50 minutes) in the P7 newborn rat. Immunohistochemical analysis revealed that ischemia induces histamine (HA) accumulation in the core of the infarct 6-12 h post-ischemia, and in the penumbra at 24-48 h, although in situ hybridization failed to detect any histidine decarboxylase gene transcripts in these regions. Immunohistochemical co-localization of HA with the MAP2 marker revealed that HA accumulates in neuronal cells before they degenerate, and is accompanied by a very significant increase in the number of mast cells at 12 h and 48 h of reperfusion. In mast cells, histamine immunoreactivity is detected at 2, 6 and 12 h after ischemia, whereas it disappears at 24 h, when a concomitant degranulation of mast cells is observed. Taken together, these data suggest that the recruitment of cerebral mast cells releasing histamine may contribute to ischemia-induced neuronal death in the immature brain.

Activation of peripheral and spinal histamine H3 receptors inhibits formalin-induced inflammation and nociception, respectively

Pharmacological activation of histamine H3 receptors is known to reduce the release of inflammatory peptides, thereby reducing pain and inflammation, but the site(s) and mechanism(s) of these effects are currently unknown. The present study addressed these questions by examining the effects of the H3 agonist immepip and the H3 antagonist thioperamide on nociceptive behaviors and swelling produced during the rat formalin test. Systemic administration of immepip (5 and 30 mg/kg, s.c.) significantly attenuated formalin-induced flinching but not licking responses during both phases. This attenuation was reversed by either systemic (15 mg/kg, i.p.) or intrathecal (20 or 50 microg) administration of thioperamide. Furthermore, immepip (30 mg/kg, s.c.) significantly inhibited formalin-induced swelling, an action which was completely reversed by systemic (15 mg/kg, i.p.), but not intrathecal (50 microg) thioperamide. Also consistent with this pattern, intrathecal immepip (50 microg) reduced flinching responses, but had no effect on formalin-induced paw swelling. The present findings suggest that activation of H3 receptors located on peripheral and spinal terminals of deep dermal fibers attenuates formalin-induced swelling and flinching, respectively. Pharmacological stimulation of H3 receptors could be an important therapeutic approach for many disorders related to deep dermal or inflammatory pain.

Structurally novel histamine H3 receptor antagonists GSK207040 and GSK334429 improve scopolamine-induced memory impairment and capsaicin-induced secondary allodynia in rats

GSK207040 (5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide) and GSK334429 (1-(1-methylethyl)-4-({1-[6-(trifluoromethyl)-3-pyridinyl]-4-piperidinyl}carbonyl)hexahydro-1H-1,4-diazepine) are novel and selective non-imidazole histamine H(3) receptor antagonists from distinct chemical series with high affinity for human (pK(i)=9.67+/-0.06 and 9.49+/-0.09, respectively) and rat (pK(i)=9.08+/-0.16 and 9.12+/-0.14, respectively) H(3) receptors expressed in cerebral cortex. At the human recombinant H(3) receptor, GSK207040 and GSK334429 were potent functional antagonists (pA(2)=9.26+/-0.04 and 8.84+/-0.04, respectively versus H(3) agonist-induced changes in cAMP) and exhibited inverse agonist properties (pIC(50)=9.20+/-0.36 and 8.59+/-0.04 versus basal GTPgammaS binding). Following oral administration, GSK207040 and GSK334429 potently inhibited cortical ex vivo [(3)H]-R-alpha-methylhistamine binding (ED(50)=0.03 and 0.35 mg/kg, respectively). Functional antagonism of central H(3) receptors was demonstrated by blockade of R-alpha-methylhistamine-induced dipsogenia in rats (ID(50)=0.02 and 0.11 mg/kg p.o. for GSK207040 and GSK334429, respectively). In more pathophysiologically relevant pharmacodynamic models, GSK207040 (0.1, 0.3, 1 and 3mg/kg p.o.) and GSK334429 (0.3, 1 and 3mg/kg p.o.) significantly reversed amnesia induced by the cholinergic antagonist scopolamine in a passive avoidance paradigm. In addition, GSK207040 (0.1, 0.3 and 1mg/kg p.o.) and GSK334429 (3 and 10mg/kg p.o.) significantly reversed capsaicin-induced reductions in paw withdrawal threshold, suggesting for the first time that blockade of H(3) receptors may be able to reduce tactile allodynia. Novel H(3) receptor antagonists such as GSK207040 and GSK334429 may therefore have therapeutic potential not only in dementia but also in neuropathic pain.

Expression of histamine receptors and effect of histamine in the rat carotid body chemoafferent pathway

Chemosensory information from peripheral arterial oxygen sensors in the carotid body is relayed by petrosal ganglion neurons to the respiratory networks in the medulla oblongata. Biogenic amines, including histamine, released from glomus (type I) cells of the carotid body are considered to be primary transmitters in hypoxic chemosensitivity. Immunocytochemistry at light-and electron-microscopical levels, and RT-PCR, revealed the expression of histamine receptors 1 and 3 as well as histidine decarboxylase in the rat carotid body glomus cells and petrosal ganglion neurons. Histamine receptors 1 and 3, but not histidine decarboxylase, were also observed in the ventrolateral, intermediate and commissural subnuclei of the nucleus tractus solitarii in the medulla oblongata. In order to examine the possible role of histamine in the afferent branch of the respiratory system, we applied histamine receptor 1 and 3 agonists to the carotid body, which caused a mildly increased phrenic nerve activity in a working heart-brainstem preparation. Moreover, microinjection of antagonists of histamine receptors 1 and 3 into the nucleus tractus solitarii caused significant changes in the inspiratory timing and the chemoreceptor response. Our data show that histamine acting via histamine receptors 1 and 3 plays an important neuromodulatory role in the afferent control of chemosensitivity.

Immunohistochemical localization of histamine H3 receptors in rodent skin, dorsal root ganglia, superior cervical ganglia, and spinal cord: potential antinociceptive targets

Activation of histamine H3 receptors (H3Rs) reduces inflammation and nociception, but the existence of H3Rs on peripheral innervation has never been demonstrated. Here we use antibodies to locate H3Rs in whisker pads, hairy and glabrous hind paw skin, dorsal root ganglia (DRGs), and spinal cords of rats, wild type mice, and H3R knockout (H3KO) mice. Although H3Rs have been hypothesized to be on C and sympathetic fibers, H3R-like immunoreactivity (H3R-LI) was only detected on presumptive periarterial A delta fibers and on A beta fibers that terminated in Meissner's corpuscles and as lanceolate endings around hair follicles. The H3R-positive periarterial fibers were thin-caliber and coexpressed immunoreactivity for calcitonin gene-related peptide (CGRP), substance P, acid sensing ion channel 3, and 200 kDa neurofilament protein (NF). H3R-LI was also detected on epidermal keratinocytes and Merkel cells, but not on Merkel endings, C fibers, any other A delta fibers, or sympathetic fibers. In DRGs, H3R-LI was preponderantly on medium to large neurons coexpressing NF-LI and mostly CGRP-LI. In dorsal horn, CGRP-positive fibers with and without H3R-LI ramified extensively in lamina II; many of the former formed a plexus in lamina V. Low levels of H3R-LI were also present on A beta fibers penetrating superficial and into deeper laminae. The distribution of H3R-LI was similar in rats and wild type mice, but was eliminated or strongly reduced in A delta fibers and A beta fibers, respectively, in H3KO mice. Taken with recently published behavioral results, the present findings suggest that periarterial, peptidergic, H3R-containing A delta fibers may be sources of high threshold mechanical nociception.

Epithelial cell proliferation is promoted by the histamine H3 receptor agonist (R)-α-methylhistamine throughout the rat gastrointestinal tract

The temporal effect of (R)-alpha-methylhistamine on epithelial cell proliferation throughout the rat gastrointestinal tract was investigated. (R)-alpha-methylhistamine was administered at 100 mg/kg orally and the rats were sacrificed 1, 24, 48, 72 and 144 h later. All the animals received 5-bromo-2'-deoxyuridine, (BrdU), 200 mg/kg i.p., 2 h before sacrifice. Gastrointestinal tissue was processed for histology and immunohistochemistry. (R)-alpha-methylhistamine caused a progressive increase in mucosal thickness of gastric fundus, distal small intestine and distal colon. Statistically significant differences from control values were found between 48 and 72 h after (R)-alpha-methylhistamine. (R)-alpha-methylhistamine significantly increased the number of BrdU-positive cells in the gastric fundus and antrum, intermediate and distal small intestine and distal colon. Peak effects were observed between 1 and 24 h after (R)-alpha-methylhistamine administration. Proliferating cell number and mucosal thickness were comparable to those of control rats at 144 h. (R)-alpha-methylhistamine exerts a long lasting growth-promoting effect on the stomach, distal small intestine and distal colon. Present data support a role of histamine H(3) receptors in the normal regulation of cell cycle in epithelial tissue.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Inhibition of chemical and low-intensity mechanical nociception by activation of histamine H3 receptors

Histamine H 3 receptors have been suggested to inhibit the activity of a variety of central and peripheral neurons. Recent studies revealed that activation of spinal histamine H 3 receptors attenuates tail pinch, but not tail flick, nociception. To determine whether H 3 receptor-mediated antinociception is truly modality-specific, the effects of the selective H 3 agonist immepip were evaluated on nociceptive responses in rats induced by a range of thermal and mechanical intensities applied to the hind paw and the tail. In addition, the modulation of chemical nociceptive (ie, formalin) responses by immepip was evaluated. Immepip (5 to 30 mg/kg, subcutaneous) attenuated responses to low-intensity mechanical pinch, but not to high-intensity mechanical pressure applied to either the hind paw or the tail. The same doses of immepip had no effect on thermal nociceptive responses, regardless of the stimulus intensity. These results suggest that immepip-induced antinociception is modality- and intensity-specific. It is likely that immepip inhibits low-intensity mechanical nociception by activation of H 3 receptors located on the spinal terminals of Adelta and possibly C high-threshold mechanoreceptors. In addition, immepip (5 mg/kg, subcutaneous) significantly attenuated formalin-induced flinching, but not formalin-induced licking, during both phase 1 and phase 2, suggesting that H 3 agonists might be effective in treating some forms of clinically relevant pain. Certain classes of pain-transmitting fibers possess histamine H 3 receptors, but the localization and functional significance of these inhibitory receptors was not known. The present study shows that drugs that stimulate H 3 receptors can reduce behavioral responses produced by some, but not all, painful stimuli. Thus, H 3 agonists could be a new type of therapy for certain kinds of pain disorders.

Expression of the H3 receptor in the developing CNS and brown fat suggests novel roles for histamine

Histamine and histamine receptors have been implicated in signaling mechanisms in developmental processes in the brain and peripheral organs. Pharmacological studies have also implied that the histamine H(3) receptor, in addition to acting as a presynaptic auto- and heteroreceptor in the central nervous system, is active in peripheral tissues. We show that detectable histamine H(3)-receptor expression during development and in adult rat is restricted to specific areas of the brain and to adipocytes and the capillary network in brown adipose tissue. Histamine H(3)-receptor mRNA expression was not detected in other internal organs studied, or in spinal or sympathetic chain ganglia. These results support a histaminergic involvement in brain development through activation of the histamine H(3) receptor and indicate a possible novel involvement of the histamine H(3) receptor as a mediator of the effects of histamine in thermogenesis in brown adipose tissue.

Protean agonism at histamine H3 receptors in vitro and in vivo

G protein-coupled receptors (GPCRs) are allosteric proteins that adopt inactive (R) and active (R*) conformations in equilibrium. R* is promoted by agonists or occurs spontaneously, leading to constitutive activity of the receptor. Conversely, inverse agonists promote R and decrease constitutive activity. The existence of another pharmacological entity, referred to as "protean" agonists (after Proteus, the Greek god who could change shape), was assumed on theoretical grounds. It was predicted from the existence of constitutive activity that a same ligand of this class could act either as an agonist or an inverse agonist at the same GPCR. Here, we show that proxyfan, a high-affinity histamine H3-receptor ligand, acts as a protean agonist at recombinant H3 receptors expressed in the same Chinese hamster ovary cells. In support of the physiological relevance of the process, we show that proxyfan also behaves as a protean agonist at native H3 receptors known to display constitutive activity. On neurochemical and behavioral responses in rodents and cats, proxyfan displays a spectrum of activity ranging from full agonism to full inverse agonism. Thus, protean agonism demonstrates the existence of ligand-directed active states LR* different from, and competing with, constitutively active states R* of GPCRs, and defines a pharmacological entity with important therapeutic implications.

Activation of spinal histamine H3 receptors inhibits mechanical nociception

Previous studies have suggested a possible pain-modulatory role for histamine H(3) receptors, but the localization of these receptors and nature of this modulation is not clear. In order to explore the role of spinal histamine H(3) receptors in the inhibition of nociception, the effects of systemically (subcutaneous, s.c.) and intrathecally (i.t.) administered histamine H(3) receptor agonists were studied in rats and mice. Immepip (5 mg/kg, s.c.) produced robust antinociception in rats on a mechanical (tail pinch) test but did not alter nociceptive responses on a thermal (tail flick) test. In contrast, this treatment in mice (immepip, 5 and 30 mg/kg, s.c.) did not change either mechanically or thermally evoked nociceptive responses. When administered directly into the spinal subarachnoid space, immepip (15-50 microg, i.t.) and R-alpha-methylhistamine (50 microg, i.t.) had no effect in rats on the tail flick and hot plate tests, but produced a dose- and time-dependent inhibition (90-100%) of nociceptive responses on the tail pinch test. This attenuation was blocked by administration of thioperamide (10 mg/kg, s.c.), a histamine H(3) receptor antagonist. Intrathecally administered thioperamide also reversed antinociceptive responses induced by systemically administered immepip, which demonstrates a spinal site of action for the histamine H(3) receptor agonist. In addition, intrathecally administered immepip (25 microg) produced maximal antinociception on the tail pinch test in wild type, but not in histamine H(3) receptor knockout (H(3)KO) mice. These findings demonstrate an antinociceptive role for spinal histamine H(3) receptors. Further studies are needed to confirm the existence of modality-specific (i.e. mechanical vs. thermal) inhibition of nociception by these receptors, and to assess the efficacy of spinally delivered histamine H(3) receptor agonists for the treatment for pain.

A detailed mapping of the histamine H(3) receptor and its gene transcripts in rat brain

The detailed distribution of histamine H(3) receptor mRNAs in rat brain was analyzed by in situ hybridization using a 33P-labelled riboprobe and was combined for the first time with the detailed autoradiographic distribution of the receptor determined in the same animals with [(125)I]iodoproxyfan, a selective radioligand. The signals generated on adjacent brain sections by each probe were quantified and/or rated and were compared in order to identify neuronal populations expressing the receptor. In addition, the cellular localization of the transcripts within various brain structures was analyzed in sections dipped in a photographic emulsion. In the cerebral cortex, the strong mRNA expression in intermediate and deep layers indicates the presence of H(3) receptors on several types of neurons. The binding is dense except in layer V, suggesting that H(3) receptors are located on granule cells and apical dendrites of pyramidal cells. In addition to their localization on monoaminergic afferents, the dense binding in layer IV and strong mRNA expression in thalamic nuclei suggest the presence of heteroreceptors on thalamocortical projections. In the hippocampus, the strong mRNA expression but low binding in pyramidal layers of the CA1 and ventral CA3 fields suggest that H(3) receptors are abundant on efferent projections of pyramidal cells. In the dentate gyrus, some binding sites in the molecular layer may correspond to H(3) receptors synthesized in granule cells and coexpressed with H(1) and H(2) receptors in their dendrites. In the basal ganglia, H(3) receptors are highly expressed in the striatal complex and olfactory tubercles but not in islands of Calleja. Some of the striatal binding sites may correspond to presynaptic receptors present on afferents. The mRNAs in cortical layer V may encode for heteroreceptors on corticostriatal neurons. The presence of mRNAs in the substantia nigra pars compacta suggests that H(3) receptors are located upon nigrostriatal afferents. However, the absence of any signal in the ventral tegmental area indicates that some but not all dopaminergic neurons express H(3) receptors. In addition, the homogeneous mRNA expression within the caudate putamen and nucleus accumbens suggests that many striatal H(3) receptors are present on medium-sized, spiny projection neurons of both the direct and indirect movement pathways. In agreement, a dense binding, but low mRNA expression, is observed in external and internal pallidum and in substantia nigra pars reticulata. In the amygdala, the dense binding and mRNA expression indicate the presence of receptors on both afferents and projections. In the thalamus, the binding in some association nuclei may correspond to receptors present on neurons emanating from the deep cortical layers that strongly express the mRNAs, as well as receptors on the visual systems. However, the low binding and high mRNA expression in most nuclei indicate that many receptors are present upon thalamic projections. In the hypothalamus, the mRNA expression parallels the density of binding sites and is the highest in the tuberomammillary nucleus. Further investigation is needed to know if the dense binding and mRNA expression observed in other nuclei such as the paraventricular, ventromedial and medial tuberal nuclei correspond to pre- and/or postsynaptic receptors. mRNAs are also observed in several areas projecting to the tuberomammillary nucleus, such as the ventrolateral preoptic nucleus. In the lower brainstem, the high mRNA expression and very low binding in the locus coeruleus and raphe nuclei indicate that presynaptic rather than somatodendritic receptors regulate noradrenaline and serotonin release, respectively. A similar pattern in vestibular nuclei suggests that receptors located on projections account for the anti-vertigo properties of H(3) receptor antagonists. In the cerebellum, binding is hardly detectable but a strong mRNA expression is found in most, if not all, Purkinje cells as well as in several central cerebellar nuclei, suggesting the presence of H(3) receptors on efferent projections. The present study reports the first detailed quantification and/or rating of H(3) receptor mRNAs in the brain. The comparison, performed in the same animals, with the distribution of the H(3) receptor protein provides evidence for the presence of H(3) receptors on many neuronal perikarya, dendrites and projections. Although some localizations, mainly as auto- or heteroreceptors, are consistent with previous functional studies, the physiological role, if any, of most of these presynaptic or postsynaptic receptors remains to be established.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat

By using double in situ hybridization performed with proenkephalin and H3-receptor riboprobes on the same sections from rat brain, we show that histamine H3 receptors are expressed within striatopallidal neurons of the indirect movement pathway. The majority ( approximately 70%) of striatal enkephalin neurons express H3-receptor mRNAs. This important degree of coexpression of proenkephalin and H3-receptor mRNAs prompted us to explore the effect of H3-receptor ligands on the regulation of enkephalin mRNA expression in the striatum. Acute administration of ciproxifan, a H3-receptor antagonist/inverse agonist, did not modify the expression of the neuropeptide by itself but strongly increased the upregulation of its expression induced by haloperidol. This potentiation (1) was suppressed by the administration of (R)-alpha-methylhistamine, a H3-receptor agonist, (2) occurred both in the caudate-putamen and nucleus accumbens, and (3) was also observed with a similar pattern on c-fos and neurotensin mRNA expression. Similarly, whereas it was devoid of any motor effect when used alone, ciproxifan strongly potentiated haloperidol-induced locomotor hypoactivity and catalepsy, two behaviors in which striatal neurons are involved. The strong H3-receptor mRNA expression in enkephalin neurons suggests that the synergistic neurochemical and motor effects of ciproxifan and haloperidol result from direct H3/D2-receptor interactions, leading to an enhanced activation of striatopallidal neurons of the indirect movement pathway. The potentiation of the effects of haloperidol by ciproxifan strengthens the potential interest of H3-receptor antagonists/inverse agonists to improve the symptomatic treatment of schizophrenia.

Ligands for histamine H(3) receptors modulate cell proliferation and migration in rat oxyntic mucosa

1. (R)-alpha-methylhistamine, a selective agonist of histamine H(3) receptors, promotes mucus secretion and increases the number and volume of mucus-secreting cells. The hypothesis that the increased number of mucous cells could reside in an alteration of homeostasis in the gastric epithelium was investigated. 2. (R)-alpha-methylhistamine was administered to rats 1 h (10-100 mg kg(-1) by intragastric and by intraperitoneal route) and 24 h (100 mg kg(-1) by intragastric route) prior to killing. The (S)-isomer of alpha-methylhistamine (55.4 mg kg(-1)), 100 times less potent than the (R)-isomer at H(3) receptors, and the H(3)-receptor agonist FUB 407 (9.14-91.35 mg kg(-1)) were intragrastically administered 1 h prior to killing. The H(1)-receptor antagonist mepyramine (30 mg kg(-1)), the H(2)-receptor antagonist famotidine (3 mg kg(-1)), and the H(3)-receptor antagonists ciproxifan (3 mg kg(-1)) and clobenpropit (30 mg kg(-1)) were intragastrically administered 30 min before (R)-alpha-methylhistamine. Gastric tissue was processed for histology and immunohistochemistry. 3. Within 1 h, (R)-alpha-methylhistamine and FUB 407 dose-dependently increased the number of BrdU-positive cells and of apoptotic cells. (S)-alpha-methylhistamine failed to modify proliferation and apoptosis. The increase in proliferation by (R)-alpha-methylhistamine was reversed by ciproxifan and clobenpropit, but not by mepyramine and famotidine. 4. (R)-alpha-methylhistamine accelerated the differentiation towards pit cells and their outward migration 24 h after its administration. These effects were counteracted by ciproxifan. The apoptosis rate was unaffected at 24 h. 5. These findings reveal a primary role of histamine H(3)-receptor ligands in modulating cell proliferation and migration in rat fundic mucosa.

Histamine H3-receptor-mediated [35S]GTP gamma[S] binding: evidence for constitutive activity of the recombinant and native rat and human H3 receptors

Constitutive activity of the recombinant and native rat and human H(3) receptors (H(3)Rs) was studied using H(3)R-mediated [(35)S]GTPgamma[S] binding and [(3)H]-arachidonic acid release. Ciproxifan, an inverse agonist at the rat H(3)R (rH(3)R), decreased [(3)H]arachidonic acid release from CHO cells expressing moderate densities (approximately 200 - 300 fmol mg(-1) protein) of the human H(3)R (hH(3)R). This effect occurred with the same magnitude than at the rH(3)R. The expression of the hH(3)R was associated with an increase in [(35)S]GTPgamma[S] binding to membranes of CHO cells. Ciproxifan decreased [(35)S]GTPgamma[S] binding to membranes of CHO (hH(3)R) cells. Both effects were correlated to receptor density and revealed that constitutive activity of the hH(3)R, although lower than that of the rH(3)R in this assay, was again observed at physiological densities (<500 fmol mg(-1) protein). Ciproxifan was less potent at the human than the rat receptor, not only as an antagonist (K(i)=45 nM), but also as an inverse agonist (EC(50)=15 nM). Constitutive activity of the hH(3)R was also evidenced using inhibition of [(35)S]GTPgamma[S] binding by unlabelled GTPgammaS. The expression of the hH(3)R generated a high affinity binding for GTPgammaS which was increased by imetit, but partially decreased by ciproxifan, therefore acting as a partial inverse agonist. [(35)S]GTPgamma[S] binding to rat brain membranes was decreased in several regions by thioperamide, ciproxifan and FUB 465, three inverse agonists at the H(3)R, whose effects were blocked by proxyfan, a neutral antagonist. [(35)S]GTPgamma[S] binding was also decreased by an A(1)-adenosine receptor inverse agonist, but remained unchanged in the presence of inverse agonists at D(2)/D(3) dopamine, H(1) and H(2) histamine, alpha(2)-adrenergic and delta opioid receptors. In conclusion, the present study shows that the recombinant rat and human H(3) receptors expressed at physiological densities display constitutive activity and suggests that constitutive activity of native H(3)Rs is one of the highest among G-protein-coupled receptors present in rat brain.