Histamine H3 receptor activation inhibits glutamate release from rat striatal synaptosomes.

The release of glutamate from striatal synaptosomes induced by depolarisation with 4-aminopyridine (4-AP) was studied by a method based on the fluorescent properties of the NAPDH formed by the metabolism of the neurotransmitter by glutamate dehydrogenase.

Ca²⁺-dependent, depolarisation-induced glutamate release was inhibited in a concentration-dependent manner by the selective histamine H₃ agonist immepip. Best-fit estimates were: maximum inhibition 60±10% and IC₅₀ 68±10 nM. The effect of 300 nM immepip on depolarisation-evoked glutamate release was reversed by the selective H₃ antagonist thioperamide in a concentration-dependent manner (EC₅₀ 23 nM, K_i 4 nM).

In fura-2-loaded synaptosomes, the increase in the intracellular concentration of Ca²⁺ ([Ca²⁺]_i) evoked by 4-AP-induced depolarisation (resting level 167±14 nM; Δ[Ca²⁺]_i 88±15 nM) was modestly, but significantly reduced (29±5% inhibition) by 300 nM immepip. The action of the H₃ agonist on depolarisation-induced changes in [Ca²⁺]_i was reversed by 100 nM thioperamide.

Taken together, our results indicate that histamine modulates the release of glutamate from corticostriatal nerve terminals. Inhibition of depolarisation-induced Ca²⁺ entry through voltage-dependent Ca²⁺ channels appears to account for the effect of H₃ receptor activation on neurotransmitter release. Modulation of glutamatergic transmission in rat striatum may have important consequences for the function of basal ganglia and therefore for the control of motor behaviour.     

Histamine H3 receptor activation inhibits neurogenic sympathetic vasoconstriction in porcine nasal mucosa

Histamine release from mast cells is a primary mediator of rhinorrhea, nasal mucosal swelling, increased secretion, sneezing, pruritis and congestion that occur in allergic rhinitis. It is well known that histamine H(1) receptor antagonists inhibit the itch and rhinorhea, but do not block the allergic nasal congestion. A growing body of evidence shows that in addition to histamine H(1) receptors, activation of H(3) receptors may contribute to the procongestant nasal actions of histamine. Activation of the prejunctional histamine H(3) receptor modulates sympathetic control of nasal vascular tone and resistance. The present study was conducted to further characterize the role of histamine H(3) receptors on neurogenic sympathetic vascular contractile responses in isolated porcine nasal turbinate mucosa. We presently found that the histamine H(3) receptor agonist, (R)-alpha-methylhistamine (10-1000 nM), inhibited electrical field stimulation-induced sympathetic vasomotor contractions in a concentration-dependent fashion. Pretreatment with either of the selective histamine H(3) receptor antagonists, thioperamide and clobenpropit, blocked the sympathoinhibitory effect of (R)-alpha-methylhistamine in porcine turbinate mucosa. The effect of compound 48/80, an agent that elicits the release of endogenous histamine from mast cells on nasal sympathetic contractile responses, was also tested. The action of compound 48/80 to release mast cell-derived histamine in the nose mimics many of the nasal responses associated with allergic rhinitis, extravascular leakage and decreased nasal patency. We presently found that compound 48/80 also inhibited the electrical field stimulation-induced sympathetic response. Pretreatment with the H(3) receptor antagonist clobenpropit blocked the sympathoinhibitory action of compound 48/80 on sympathetic contractile responses in nasal mucosa. Taken together, these studies indicate that histamine H(3) receptors modulate vascular contractile responses by inhibition of noradrenaline release from sympathetic nerve terminals in nasal mucosa. It is further suggested that histamine H(3) receptors may play a role in the regulation of vascular tone and nasal patency in allergic nasal congestive disease.     

Pre-synaptic histamine H3 receptors regulate glutamate, but not GABA release in rat thalamus

We have investigated the presence of histamine H(3) receptors (H(3)Rs) on rat thalamic isolated nerve terminals (synaptosomes) and the effect of their activation on glutamate and GABA release. N-alpha-[methyl-(3)H]histamine ([(3)H]-NMHA) bound specifically to synaptosomal membranes with dissociation constant (K(d)) 0.78+/-0.20 nM and maximum binding (B(max)) 141+/-12fmol/mg protein. Inhibition of [(3)H]-NMHA binding by histamine and the H(3)R agonist immepip fit better to a two-site model, whereas for the H(3)R antagonist clobenpropit the best fit was to the one-site model. GTPgammaS (30 microM) decreased [(3)H]-NMHA binding by 55+/-4% and made the histamine inhibition fit better to the one-site model. Immepip (30 nM) induced a modest, but significant increase (113+/-2% of basal) in [(35)S]-GTPgammaS binding to synaptosomal membranes, an effect prevented by clobenpropit (1 microM) and by pre-treatment with pertussis toxin. In thalamus synaptosomes depolarisation-induced, Ca(2+)-dependent glutamate release was inhibited by histamine (1 microM, 25+/-4% inhibition) and immepip (30 nM, 38+/-5% reduction). These effects were reversed by clobenpropit (1microM). Conversely, immepip (up to 1 microM) had no effect on depolarisation-evoked [(3)H]-GABA release. Extracellular synaptic responses were recorded in the thalamus ventrobasal complex by stimulating corticothalamic afferents. H(3)R activation reduced by 38+/-7% the glutamate receptor-mediated field potentials (FPs), but increased the FP2/FP1 ratio (from 0.86+/-0.03 to 1.38+/-0.05) in a paired-pulse paradigm. Taken together, our results confirm the presence of H(3)Rs on thalamic nerve terminals and show that their activation modulates pre-synaptically glutamatergic, but not GABAergic neurotransmission.     

Histamine H4 receptor-RGS fusion proteins expressed in Sf9 insect cells: A sensitive and reliable approach for the functional characterization of histamine H4 receptor ligands

The human histamine H₄ receptor (hH₄R), co-expressed with Gα_i2 and Gβ₁γ₂ in Sf9 cells, is highly constitutively active. In the steady-state GTPase assay, the full agonist histamine (HA) induces only a relatively small signal (∼20-30%), resulting in a low signal-to background ratio. In order to improve this system for ligand screening purposes, the effects of the regulators of G-protein signaling (RGS) RGS4 and RGS19 (GAIP) were investigated. RGS4 and GAIP were fused to the C-terminus of hH₄R or co-expressed with non-fused hH₄R, always combined with Gα_i2 and Gβ₁γ₂. The non-fused RGS proteins did not significantly increase the relative effect of HA. With the hH₄R-RGS4 fusion protein the absolute GTPase activities, but not the relative HA-induced signal were increased. Fusion of hH₄R with GAIP caused a selective increase of the HA signal, resulting in an enhanced signal-to-noise ratio. A detailed characterization of the hH₄R-GAIP fusion protein (co-expressed with Gα_i2 and Gβ₁γ₂) and a comparison with the data obtained for the non-fused hH₄R (co-expressed with Gα_i2 and Gβ₁γ₂) led to the following results: (i) the relative agonist- and inverse agonist-induced signals at hH₄R-GAIP are markedly increased. (ii) Compared to the wild-type hH₄R, standard ligands show unaltered potencies and efficacies at hH₄R-GAIP. (iii) Like hH₄R, hH₄R-GAIP shows high and NaCl-resistant constitutive activity. (iv) hH₄R-GAIP shows the same G-protein selectivity profile as the non-fused hH₄R. Collectively, hH₄R-GAIP provides a sensitive test system for the characterization of hH₄R ligands and can replace the non-fused hH₄R in steady-state GTPase assays.     

JNJ-10181457, a selective non-imidazole histamine H3 receptor antagonist, normalizes acetylcholine neurotransmission and has efficacy in translational rat models of cognition

Histamine 3 (H₃) receptors are distributed throughout the brain and regulate histamine as well as the activity of other neurotransmitters including acetylcholine (ACh). Impaired ACh neurotransmission is associated with deficits of cognitive-related functioning in many species including humans. The goal of these studies was to evaluate the behavioral and neurochemical effects of JNJ-10181457, a selective non-imidazole histamine H₃ receptor antagonist, in rats. The pharmacokinetic profile and receptor occupancy of JNJ-10181457 were tested. The efficacy of JNJ-10181457 was evaluated, acutely, in the imetit-induced water licking model, delayed non-matching to position (DNMTP) task and microdialysis studies. In addition, the effects of repeated administration of JNJ-10181457 were evaluated in the reversal learning task. A single administration of JNJ-10181457 (10 mg/kg, i.p.) resulted in significant plasma and brain exposure and maximal H₃ receptor occupancy. In addition, JNJ-10181457 reversed imetit-induced water licking, similarly to thioperamide (10 mg/kg, i.p.). In the DNMTP task, scopolamine (0.06 mg/kg, i.p.) significantly decreased percentage correct responding. These effects were significantly reversed by JNJ-10181457 (10 mg/kg, i.p.) and also by donepezil (1 mg/kg, i.p.), an acetylcholinesterase inhibitor, and were associated with normalization of ACh neurotransmission in the cortex. Repeated administration of JNJ-10181457 (10 mg/kg, i.p.) significantly increased percentage correct responding in the reversal learning task. Treatment discontinuation was not associated with rebound effects on cognition. These results indicate that selective blockade of histamine H₃ receptors might have therapeutic utility for the treatment of working memory deficits and learning disorders, especially those in which ACh neurotransmission is compromised.     

Interactions between histamine H3 and dopamine D2 receptors and the implications for striatal function

The striatum contains a high density of histamine H(3) receptors, but their role in striatal function is poorly understood. Previous studies have demonstrated antagonistic interactions between striatal H(3) and dopamine D(1) receptors at the biochemical level, while contradictory results have been reported about interactions between striatal H(3) and dopamine D(2) receptors. In this study, by using reserpinized mice, we demonstrate the existence of behaviorally significant antagonistic postsynaptic interactions between H(3) and D(1) and also between H(3) and dopamine D(2) receptors. The selective H(3) receptor agonist imetit inhibited, while the H(3) receptor antagonist thioperamide potentiated locomotor activation induced by either the D(1) receptor agonist SKF 38393 or the D(2) receptor agonist quinpirole. High scores of locomotor activity were obtained with H(3) receptor blockade plus D(1) and D(2) receptor co-activation, i.e., when thioperamide was co-administered with both SKF 38393 and quinpirole. Radioligand binding experiments in striatal membrane preparations showed the existence of a strong and selective H(3)-D(2) receptor interaction at the membrane level. In agonist/antagonist competition experiments, stimulation of H(3) receptors with several H(3) receptor agonists significantly decreased the affinity of D(2) receptors for the agonist. This kind of intramembrane receptor-receptor interactions are a common biochemical property of receptor heteromers. In fact, by using Bioluminescence Resonance Energy Transfer techniques in co-transfected HEK-293 cells, H(3) (but not H(4)) receptors were found to form heteromers with D(2) receptors. This study demonstrates an important role of postsynaptic H(3) receptors in the modulation of dopaminergic transmission by means of a negative modulation of D(2) receptor function.     

Histamine H3 receptor modulates nociception in a rat model of cholestasis

Cholestasis is associated with changes including analgesia. The histaminergic system regulates pain perception. The involvement of histamine H₃ receptors in modulation of nociception in a model of elevated endogenous opioid tone, cholestasis, was investigated in this study using immepip and thioperamide as selective H₃ receptor agonist and antagonist respectively. Cholestasis was induced by ligation of main bile duct using two ligatures and transsection the duct between them. Cholestatic rats had increased tail-flick latencies (TFLs) compared to non-cholestatics. Administration of immepip (5 and 30 mg/kg) and thioperamide (10 and 20 mg/kg) to the cholestatic groups significantly increased and decreased TFLs compared to the saline treated cholestatic group. Immepip antinociception in cholestatic animals was attenuated by co-administration of naloxone. Immepip and thioperamide injections into non-cholestatic animals did not alter TFLs. At the doses used here, none of the drugs impaired motor coordination, as revealed by the rotarod test. The present data show that the histamine H₃ receptor system may be involved in the regulation of nociception during cholestasis in rats.     

Adenosine A2A receptor antagonism increases striatal glutamate outflow in dopamine-denervated rats

The objective of the work was to study, by in vivo microdialysis, the effect of the adenosine A(2A) receptor antagonist 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261) on glutamate outflow in the striata of unilateral 6-hydroxydopamine-infused rats. Two vertical microdialysis probes were implanted bilaterally in both the denervated striatum and in the intact striatum. Glutamate concentrations in the dialysate were determined by high-performance liquid chromatography (HPLC). Infusion of the adenosine A(2A) receptor antagonist SCH 58261 (50 nM), through the microdialysis fiber, significantly increased glutamate outflow from the denervated striatum while it decreased glutamate outflow from the intact striatum. The opposite effects of SCH 58261 on glutamate outflow in the intact and 6-hydroxydopamine-lesioned striatum might be attributed to blockade of striatal adenosine A(2A) receptors located on either striatal indirect output pathways or glutamatergic terminals. These results may be relevant to our understanding of the mechanism of action of adenosine A(2A) receptor antagonists in Parkinson's disease.     

Application of genomics to drug design: the example of the histamine H3 receptor

The histamine H₃ receptor was characterized in the 1980s as an autoreceptor regulating histamine release in brain. Since then, selective drugs have been designed, many of them displaying a high potency in vivo, and used in many studies to delineate the implications of cerebral histaminergic systems in physiological functions such as arousal or cognitive functions. The recent cloning of the H₃ receptor, more than 15 years later, has allowed to start molecular studies that led to important findings for optimization of drug design. In agreement some ligands display distinct affinities for the recombinant rat and human H₃ receptors, a difference that we assign to two amino acids in the third transmembrane domain. In addition, H₃ autoreceptors present in the brain display high constitutive activity including in vivo. As a consequence, inverse agonists enhance histamine neuron activity and constitute a novel potential therapeutic approach to schizophrenia and Alzheimer’s disease.     

组胺H2受体激动剂和拮抗剂及抗癌药对正常人造血祖细胞和HL-60白血病细胞生长的作用

采用体外微量克隆培养体系研究了组胺H2受体激动剂4-甲基组胺(4-MH)和拮抗剂雷尼替叮(ranitidine)及抗癌药阿糖胞苷分别对正常人外周血粒-巨噬系祖细胞(PBCFU-GM)和HL-60白血病细胞生长的作用。当4-MH的浓度为10^-9～10^-6mol·L^-1时,可促进PBCFU-GM的增殖,4-MH的浓度增加至10^-4mol·L^-1时则表现为抑制PBCFU-GM的增殖。Ranitidine的浓度为10^-9～10^-5mol·L^-1时,表现出对PBCFU-GM增殖的抑制作用,但在10-6mol·L^-1剂量时对PBCFU-GM的抑制率低于50%,而在该剂量时对HL-60白血病细胞的抑制率已达100%,具有一定的选择性。抗癌药阿糖胞苷(Ara-C)对HL-60白血病细胞的抑制作用比对PBCFU-GM的抑制作用较强,但两者的IC₅₀值处于同一个数量级。在强化化疗剂量10^-5mol·L^-1时,Ara-C对HL-60白血病细胞和PBCFU-GM正常造血祖细胞的抑制率均达100%。     

Differential thermodynamic driving force of first- and second-generation antihistamines to determine their binding affinity for human H1 receptors

Differential binding sites for first- and second-generation antihistamines were indicated on the basis of the crystal structure of human histamine H₁ receptors. In this study, we evaluated differences between the thermodynamic driving forces of first- and second-generation antihistamines for human H₁ receptors and their structural determinants. The binding enthalpy and entropy of 20 antihistamines were estimated with the van’t Hoff equation using their dissociation constants obtained from their displacement curves against the binding of [³H]mepyramine to membrane preparations of Chinese hamster ovary cells expressing human H₁ receptors at various temperatures from 4 °C to 37 °C. Structural determinants of antihistamines for their thermodynamic binding properties were assessed by quantitative structure–activity relationship (QSAR) analyses. We found that entropy-dependent binding was more evident in second- than first-generation antihistamines, resulting in enthalpy–entropy compensation between the binding forces of first- and second-generation antihistamines. QSAR analyses indicated that enthalpy–entropy compensation was determined by the sum of degrees, maximal electrostatic potentials, water-accessible surface area and hydrogen binding acceptor count of antihistamines to regulate their affinity for receptors. In conclusion, it was revealed that entropy-dependent hydrophobic interaction was more important in the binding of second-generation antihistamines, even though the hydrophilicity of second-generation antihistamines is generally increased. Furthermore, their structural determinants responsible for enthalpy–entropy compensation were explored by QSAR analyses. These findings may contribute to understanding the fundamental mechanisms of how the affinity of ligands for their receptors is regulated.     

H4 receptor antagonism exhibits anti-nociceptive effects in inflammatory and neuropathic pain models in rats

The histamine H₄ receptor (H₄R) is expressed primarily on cells involved in inflammation and immune responses. To determine the potential role of H₄R in pain transmission, the effects of JNJ7777120, a potent and selective H₄ antagonist, were characterized in preclinical pain models. Administration of JNJ7777120 fully blocked neutrophil influx observed in a mouse zymosan-induced peritonitis model (ED₅₀ = 17 mg/kg s.c., 95% CI = 8.5-26) in a mast cell-dependent manner. JNJ7777120 potently reversed thermal hyperalgesia observed following intraplantar carrageenan injection of acute inflammatory pain (ED₅₀ = 22 mg/kg i.p., 95% CI = 10-35) in rats and significantly decreased the myeloperoxide activity in the carrageenan-injected paw. In contrast, no effects were produced by either H₁R antagonist diphenhydramine, H₂R antagonists ranitidine, or H₃R antagonist ABT-239. JNJ7777120 also exhibited robust anti-nociceptive activity in persistent inflammatory (CFA) pain with an ED₅₀ of 29 mg/kg i.p. (95% CI = 19-40) and effectively reversed monoiodoacetate (MIA)-induced osteoarthritic joint pain. This compound also produced dose-dependent anti-allodynic effects in the spinal nerve ligation (ED₅₀ = 60 mg/kg) and sciatic nerve constriction injury (ED₅₀ = 88 mg/kg) models of chronic neuropathic pain, as well as in a skin-incision model of acute post-operative pain (ED₅₀ = 68 mg/kg). In addition, the analgesic effects of JNJ7777120 were maintained following repeated administration and were evident at the doses that did not cause neurologic deficits in rotarod test. Our results demonstrate that selective blockade of H₄ receptors in vivo produces significant anti-nociception in animal models of inflammatory and neuropathic pain.     

Analysis of the histamine H2-receptor in human monocytes

Histamine receptors are G-protein-coupled receptors (GPCRs). Canonically, the histamine H₂-receptor (H₂R) couples to G_s-proteins and activates adenylyl cyclases (ACs) with subsequent adenosine-3′,5′-cyclic monophosphate (cAMP) generation. Recently, the classic two-state model describing GPCR activation has been extended to a multiple-state model implying that any ligand stabilizes a ligand-specific receptor conformation, also referred to as functional selectivity. In our present study we pharmacologically characterized the H₂R in human monocytes. We found dissociations in the effects of histamine (HA) and H₂R agonists on cAMP accumulation and inhibition of formyl peptide-induced production of reactive oxygen species (ROS). In addition, we excluded participation of protein kinase A (PKA) in HA-induced ROS inhibition. Comparison of the potencies and efficacies of H₂R agonists in monocytes, neutrophils and eosinophils unmasked cell type-specific pharmacological profiles of individual ligands. Taken together, our data extend the concept of functional selectivity to the H₂R in human monocytes and demonstrate striking cell-type specificity in the pharmacological profile of the H₂R.     

Physiological roles of the melanocortin MC3 receptor

The melanocortin MC₃ receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC₃ receptor null humans have been identified and associations between variant alleles of the melanocortin MC₃ receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.     

Histamine H3 receptor antagonists in relation to epilepsy and neurodegeneration: a systemic consideration of recent progress and perspectives

The central histaminergic actions are mediated by H₁, H₂, H₃ and H₄ receptors. The histamine H₃ receptor regulates the release of histamine and a number of other neurotransmitters and thereby plays a role in cognitive and homeostatic processes. Elevated histamine levels suppress seizure activities and appear to confer neuroprotection. The H₃ receptors have a number of enigmatic features like constitutive activity, interspecies variation, distinct ligand binding affinities and differential distribution of prototypic splice variants in the CNS. Furthermore, this Gi/Go-protein-coupled receptor modulates several intracellular signalling pathways whose involvement in epilepsy and neurotoxicity are yet to be ascertained and hence represent an attractive target in the search for new anti-epileptogenic drugs. So far, H₃ receptor antagonists/inverse agonists have garnered a great deal of interest in view of their promising therapeutic properties in various CNS disorders including epilepsy and related neurotoxicity. However, a number of experiments have yielded opposing effects. This article reviews recent works that have provided evidence for diverse mechanisms of antiepileptic and neuroprotective effects that were observed in various experimental models both in vitro and in vivo. The likely reasons for the apparent disparities arising from the literature are also discussed with the aim of establishing a more reliable basis for the future use of H₃ receptor antagonists, thus improving their utility in epilepsy and associated neurotoxicity.     

M3受体在脑心综合征心律失常中的作用

观察M₃受体(M₃R)在脑心综合征(CCS)模型大鼠心室肌中的作用及其表达，以探讨其与CCS心律失常的关系。线栓法建立CCS模型，监测心电图，激光共聚焦扫描显微镜记录激动M₃R后心肌细胞内钙的变化，Western blotting检测心室肌中M₃R表达水平的改变。同对照组相比，模型组心电图QRS、QT间期显著延长(P<0.01)；Western blotting结果显示模型组心室肌中M₃R的表达水平较对照组显著降低(P<0.05)；胆碱激动M₃R可以抑制KCl除极诱导的模型组心肌细胞内钙的升高幅度(P<0.01)，其特异阻断剂4-DAMP可以阻断这一作用。M₃R表达降低可能是CCS心律失常的重要原因之一。激动M₃R可以降低心肌细胞内钙，M₃R可能是治疗CCS心律失常新的靶点。     

Catalepsy produced by striatal microinjections of the D1 dopamine receptor antagonist SCH 23390 in neonatal rats

Systemic injection of the D₁ dopamine receptor antagonist SCH 23390 produces catalepsy that is of lesser magnitude in neonatal than in adult rats. The present experiments were conducted in order to determine if SCH 23390 would produce catalepsy in neonatal rats following intrastriatal injection and if the ontogenetic pattern of catalepsy induced by intrastriatal SCH 23390 would be similar to the pattern observed with systemic injections. Rat pups (11 or 28 days of age) were microinjected unilaterally with SCH 23390 (0.2, 1, 5 or 10 μg) and tested for catalepsy using the forepow-on-horizontal-bar test. The results demonstrated that robust catalepsy occurred at both ages following intrastriatal injection and that catalepsy induced by 5 μg SCH 23390 was of lesser magnitude in 11-day-olds than in 28-day-olds. A separate study assessed the distribution of [³H]SCH 23390 (5 μg) following intrastriatal injection in 28-day-olds. Results of the distribution study indicated that [³H]SCH 23390 was localized primarily within the striatum. Taken together, these results suggest that the striatal mechanisms for catalepsy produced by D₁ receptor blockade are present, but not fully mature, in preweanling rat pups.     

Use of an inverse agonist radioligand [3H]A-317920 reveals distinct pharmacological profiles of the rat histamine H3 receptor

Selective radioligands for histamine H(3) receptors have been used to characterize H(3) receptor pharmacology by radioligand binding assays and to determine H(3) receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [(3)H]A-317920 (furan-2-carboxylic acid(2-[4-[3-([3,5-(3)H]4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H(3) receptor. The binding of [(3)H]A-317920 to rat cortical and cloned H(3) receptors revealed fast on- and slower off-rate kinetics with calculated K(d) values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [(3)H]A-317920 with the agonist [(3)H](N)-alpha-methylhistamine ([(3)H]NalphaMH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [(3)H]NalphaMH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [(3)H]A-317920 displacement by agonists was best fit by two-site binding models, and the potencies of the high affinity, GDP-sensitive sites correlated with the potencies defined in [(3)H]NalphaMH binding. Unlike [(125)I]iodoproxyfan, [(3)H]A-317920 exhibits potent and selective binding to rat H(3) receptors with low binding to non-H(3) sites, including cytochrome P450. These findings show that [(3)H]A-317920 is a potent rat H(3) receptor antagonist radioligand and has utility for studying H(3) receptor pharmacology.     

Activation of histamine H3 receptors in human nasal mucosa inhibits sympathetic vasoconstriction

The peripheral histamine H3 receptor is a presynaptic heterologous receptor located on postganglionic sympathetic nerve fibers innervating sympathetic effector systems such as blood vessels and the heart. An extensive body of evidence shows that activation of the histamine H3 receptor attenuates sympathetic tone by presynaptic inhibition of noradrenaline release. It is proposed that this sympathoinhibitory action, in vivo, leads to reduced vasoconstriction, thereby eliciting a vasodilatory effect. In humans, the peripheral histamine H3 receptor has also been shown to exert a sympathoinhibitory function on specific peripheral autonomic effector systems. For example, human saphenous vein and heart possess functional presynaptic histamine H3 receptors on the sympathetic nerve terminals that upon activation decrease the sympathetic tone to these respective organs. The present studies were conducted to define the role of histamine H3 receptors on neurogenic sympathetic vasoconstrictor responses in human nasal turbinate mucosa. Contractility studies were conducted to evaluate the effect of histamine H3 receptor activation on sympathetic vasoconstriction in surgically isolated human nasal turbinate mucosa. We found that the histamine H3 receptor agonist, (R)-alpha-methylhistamine (30 and 300 nM), inhibited electrical field stimulation-induced (neurogenic) sympathetic vasoconstriction in a concentration-dependent fashion. Pretreatment with the selective histamine H3 receptor antagonist, clobenpropit (100 nM), blocked the sympathoinhibitory effect of (R)-alpha-methylhistamine on the neurogenic sympathetic vasoconstriction. In addition, analysis of Taqman mRNA expression studies showed a specific, high level of distribution of the histamine H3 receptor localized in the human nasal mucosa.Taken together, these studies indicate that histamine H3 receptors modulate vascular contractile responses in human nasal mucosa most likely by inhibiting noradrenaline release from sympathetic nerve terminals in nasal mucosa. It is further suggested that histamine H3 receptors may play a role in the regulation of vascular tone and nasal patency in histamine-dependent allergic nasal congestive disease.     

Histamine H3 receptor-mediated inhibition of serotonin release in the rat brain cortex

Summary Rat brain cortex slices preincubated with ³H-serotonin were superfused with physiological salt solution (containing citalopram, an inhibitor of serotonin uptake) and the effect of histamine on the electrically (3 Hz) evoked ³H overflow was studied. Histamine decreased the evoked overflow in a concentration-dependent manner. The inhibitory effect of histamine was antagonized by impromidine and burimamide, but was not affected by pheniramine, ranitidine, metitepine and phentolamine. Given alone, impromidine facilitated the evoked overflow, whereas burimamide, pheniramine and ranitidine had no effect. The results suggest that histamine inhibits serotonin release in the rat brain cortex via histamine H3 receptors, which may be located presynaptically. Send offprint requests to E. Schlicker at the above address