Evidence for ligand-specific conformations of the histamine H2-receptor in human eosinophils and neutrophils

The histamine H₂-receptor (H₂R) couples to G_S-proteins and induces adenylyl cyclase-mediated cAMP accumulation. In human neutrophils and eosinophils, the H₂R reduces chemotactic peptide-stimulated superoxide anion (O₂^?) formation. However, pharmacological characterization of the H₂R in these cells is far from being complete. The aim of this study was to provide a comprehensive profiling of the H₂R in neutrophils and eosinophils. Histamine inhibited O₂^? formation in human neutrophils more effectively than in eosinophils. H₂R agonists mimicked the effects of histamine and H₂R antagonists blocked the effects of histamine. We noticed multiple discrepancies in the potencies and efficacies of H₂R agonists with respect to cAMP accumulation and inhibition of O₂^? formation in both cell types. There were also differences in the antagonist profiles between cAMP accumulation and inhibition of O₂^? formation in neutrophils. Moreover, the pharmacological profile of the recombinant H₂R did not match the H₂R profile in native cells. The H₂R sequence identified in human neutrophils corresponds to the published H₂R sequence, excluding the exclusive expression of a new H₂R isoform as explanation for the differences. Very likely, the differences between ligands are explained by the existence of ligand-specific receptor conformations with unique affinities, potencies and efficacies. Thus, our data provide evidence for the notion that the concept of ligand-specific receptor conformations can be extended from recombinant systems to native cells.     

A comparison of histamine effects on the sympathetic neurotransmission of testicular capsule and rat vas deferens

Histamine is an important modulatory agent of the sympathetic neurotransmission, but its exact action on the testicular capsule or rat vas deferens is not fully understood. The present study sought to further investigate the functional effects of histamine on the neuronal and exogenous noradrenaline-induced contraction of the testicular capsule and rat vas deferens as well as to evaluate the contractile properties of this drug. The testicular capsule or vas deferens from Wistar rats, 3–4 months old, weighing 300–400 g, was isolated and mounted in organ baths for functional experiments. The results indicated that the neuronally evoked contraction of the testicular capsule was affected by histamine (10^?10 to 10^?8 M) with participation of inhibitory (H₃ receptors) and excitatory (H₁ receptors) receptors. Histamine (10^?7 to 10^?4 M) modulated the field-stimulated vas deferens by excitatory (H₂ receptors) and inhibitory (H₁ receptors) receptors. Histamine was able to decrease the tonic response for noradrenaline-induced contractions with participation of H₁ receptors (testicular capsule) and H₃ receptors (vas deferens) followed by nitric oxide generation. At high concentration, histamine exerts contractile effects in both tissues. In the testicular capsule, the histamine-induced contractions were related to H₁ receptor activation followed by release of prostaglandins. In contrast, the contractile effects of histamine in the vas deferens were related to H₂ receptor activation followed by release of catecholamines from sympathetic nerve endings. Therefore, our results indicate that histamine induced several effects on the sympathetic neurotransmission of rat testicular capsule and vas deferens. These effects are dependent on the concentration used and with participation of multiple histamine receptors.     

Characterization of histamine H2-receptors in human neutrophils with a series of guanidine analogues of impromidine

Summary Human neutrophils possess an NADPH oxidase which catalyzes superoxide (O _inf2 ^sup– ) formation and is activated by chemotactic peptides. Histamine inhibits O _inf2 ^sup–1 formation via H₂-receptors (Burde et al. 1989). We characterized the neutrophil H₂-receptor with a series of new guanidine-type H₂-agonists structurally derived from impromidine. Histamine inhibited O _inf2 ^sup– formation with an IC₅₀ value of 6.7 ± 1.2 M. Five aryloxy- and arylthioalkylguanidines were less potent and effective than histamine. Several arpromidine-like phenyl(pyridylalkyl)guanidines were either full or partial H₂-agonists. Some guanidines possess a three-membered carbon chain connecting the aromatic rings and the guanidine group; they were similarly potent and effective as histamine. Shortening or elongation of the carbon chain substantially decreased the potency and intrinsic activity of the guanidines. Halogenation of the phenyl ring did not substantially affect the potency and intrinsic activity of the compounds in comparison to the non-substituted parent compound. The H₂-antagonist, famotidine, competitively antagonized inhibition of O _inf2 ^sup– formation caused by the guanidine, arpromidine, with a pA₂ value of 6.84. The H₂-antagonist, cimetidine, differentially counteracted inhibition caused by partial and full H₂-agonists. Partial H₂-agonists antagonized the effects of histamine. The inhibitor of phosphodiesterases, 3-isobutyl-lmethylxanthine, additively enhanced the inhibitory effects of histamine and guanidines. The properties of the neutrophil H₂-receptor were compared with literature data concerning properties of the H₂-receptor of the guinea pig atrium. In the latter system, guanidines are full H₂-agonists with potencies of up to 125-fold of that of histamine. Our data indicate that guanidines inhibit O _inf2 ^sup– formation in human neutrophils via H₂-receptors. The structure/activity relationship for the neutrophil H₂-receptor substantially differs from the one for the H₂-receptor in the guinea pig atrium, suggesting that the neutrophil H₂-receptor has cell type-specific properties. Other possibilities to explain the differences between H₂-receptors in these systems are discussed. Send offprint requests to R. Seifert at the above address     

Ciproxifan and chemically related compounds are highly potent and selective histamine H3-receptor antagonists

We determined the affinities of five newly synthesized histamine H₃-receptor antagonists in an H₃-receptor binding assay and their potencies in a functional H₃-receptor model. Furthermore, we determined their potencies in a histamine H₂- and H₁-receptor model. The compounds differ from histamine in that the ethylamine side chain is replaced by an aryl-substituted propyloxy chain and they differ from one another by varying substituents of the aryl rest. Iodoproxyfan, a highly potent and selective antagonist at H₃ receptors, is structurally related to these five compounds. The specific binding of [³H]-N ^α-methylhistamine to rat brain cortex membranes was monophasically displaced by each of the five compounds at pK _i values ranging from 8.24 to 9.27. Inhibition by histamine of the electrically evoked tritium overflow from mouse brain cortex slices preincubated with [³H]noradrenaline was antagonized by all compounds and the concentration-response curve was shifted to the right with apparent pA ₂ values ranging from 7.78 to 9.39. The five compounds under study possess negligible potencies at histamine H₂ and H₁ receptors studied in the guinea-pig right atrium and ileum, respectively (pD’₂ or pK _p values ≤5.2). The present paper shows that the five compounds under study possess high affinities and potencies at histamine H₃ receptors, four out of the five compounds in this respect being equipotent with iodoproxyfan. Like iodoproxyfan, the five compounds show an at least 1000-fold selectivity for H₃ as compared to H₂ and H₁ receptors. Received: 23 June 1998 / Accepted: 21 September 1998     

Separate receptors mediating the positive inotropic and chronotropic effect of histamine in guinea-pig atria

The direct positive inotropic effect of histamine was studied on paced left atrial preparations from guinea pigs. Histamine (10^?8 to 10^?4 M) increased the maximum tension developed in left atria incubated at 35°C and driven at 2 Hz. The maximum increase in tension was 60% of that observed with norepinephrine. Metiamide (3 × 10^?5 M; a specific H₂-receptor antagonist) did not alter the inotropic response of left atria to histamine. However, tripelennamine (a typical H₁-receptor antagonist) competitively shifted the histamine inotropic dose—response curve to the right at concentrations from 10^?8 to 10^?7 M. Higher concentrations (3 × 10^?7 and 10^?6 M) caused little further additional shift to the right. The positive chronotropic effect of histamine on spontaneously beating atria was competitively antagonized by metiamide (10^?6 and 3 × 10^?6 M). These results demonstrate that in guinea-pig atria histamine increases myocardial contractility by an interaction with receptors closely related to classical H₁-receptors while its chronotropic effects is mediated by interaction with H₂-receptors.     

Pharmacological profile of astemizole-derived compounds at the histamine H1 and H4 receptor—H1/H4 receptor selectivity

Astemizole, a H₁R antagonist shows high affinity to the histamine H₁ receptor but only a moderate affinity to the histamine H₄ receptor. This study aims to modify the astemizole to keep high affinity to the histamine H₁ receptor and to increase affinity to the histamine H₄ receptor. Therefore, 13 astemizole-derived compounds and astemizole-JNJ7777120-derived hybrid compounds were synthesized and pharmacologically characterized at the histamine H₁ and H₄ receptors. The new compounds show affinity to the histamine H₁ receptor in the pK _i range from 5.3 to 8.8, whereas the affinity of these compounds to the histamine H₄ receptor was surprisingly rather low (pK _i from 4.4 to 5.6). Three representative compounds were docked into the histamine H₁ receptor and molecular dynamic studies were performed to explain the binding mode and the experimental results on a molecular level. Furthermore, taking into account the binding mode of compounds with high affinity to the histamine H₄ receptor, a H₁/H₄-pharmacophore hypothesis was developed.     

Involvement of presynaptic H3 receptors in the inhibitory effect of histamine on serotonin release in the rat brain cortex

Summary Rat brain cortex slices or synaptosomes preincubated with ³H-serotonin were superfused with physiological salt solution (which, in the case of slices, contained citalopram, an inhibitor of serotonin uptake), and the effects of histamine and related drugs on the evoked tritium overflow were studied.The electrically (3 Hz) evoked tritium overflow from slices was inhibited by histamine and the H₃ receptor agonists R-(–)--methylhistamine and N-methylhistamine (pIC_12.5 values: 6.41, 7.28 and 6.12, respectively), but not affected by the H₁ receptor agonist 2-(2-thiazolyl)ethylamine and the H₂ receptor agonist dimaprit (each at 10 mol/l). The concentration-response curve for histamine was shifted to the right by the H₃ receptor antagonists impromidine, burimamide and thioperamide (apparent pA₂ values: 7.45, 5.97 and 7.88, respectively); the concentration-response curve of serotonin for its inhibitory effect on the electrically evoked overflow was not affected by the three drugs (apparent pA₂ values: < 5.5, < 5.5 and < 6.5). Given alone, impromidine, thioperamide and a low concentration of burimamide facilitated the electrically evoked overflow. In slices superfused with K⁺-rich, Ca²⁺-free solution containing tetrodotoxin throughout and in synaptosomes superfused with Ca²⁺-free solution, histamine inhibited the overflow evoked by introduction of Ca²⁺ (in synaptosomes, simultaneously with an increased amount of K⁺). In either tissue, the effect of histamine was counteracted by thioperamide.The results provide evidence that exogenous and probably also endogenous histamine inhibits serotonin release in the rat brain cortex via presynaptic histamine H₃ receptors.Send offprint requests to E. Schlicker at the above address     

H2 receptor-mediated facilitation and H3 receptor-mediated inhibition of noradrenaline release in the guinea-pig brain

The effect of histamine and related drugs on the tritium overflow evoked electrically (0.3 Hz) or by introduction of Ca²⁺ ions into Ca²⁺-free K⁺-rich (25 mmol/l) medium containing tetrodotoxin was studied in superfused guinea-pig brain cortex, cerebellum, hippocampus or hypothalamus slices and in mouse brain cortex slices preincubated with ³H-noradrenaline. The electrically evoked tritium overflow in guinea-pig cortex slices was inhibited by histamine; the H₃ receptor antagonist clobenpropit reversed the effect of histamine to a slight facilitation. The facilitatory effect of histamine (obtained in the presence of clobenpropit) was not affected by the H₁ receptor antagonist mepyramine but abolished by the H₂ receptor antagonist ranitidine. In the absence of clobenpropit, ranitidine augmented the inhibitory effect of histamine. In slices superfused in the presence of ranitidine, the evoked overflow was inhibited by histamine and, more potently, by the H₃ receptor agonist R-α-methylhistamine in a concentration-dependent manner (maximum inhibitory effect obtained for both agonists 30–35%). The concentration-response curve of histamine was shifted to the right by the H₃ receptor antagonist thioperamide. R-α-Methylhistamine inhibited the electrically evoked tritium overflow also in guinea-pig cerebellar, hippocampal and hypothalamic slices. In cortex slices superfused in the presence of clobenpropit, the H₂ receptor agonists impromidine and, less potently, R-sopromidine facilitated the evoked overflow in a concentration-dependent manner. S-Sopromidine only tended to increase the evoked overflow. The effect of impromidine was counteracted by the H₂ receptor antagonists ranitidine and cimetidine. The extent of the maximum facilitatory effect of impromidine (by 15–20%) was about the same when (i) the Ca²⁺ concentration in the medium was reduced from 1.3 to 0.98 mmol/l, (ii) the time of exposure to impromidine was reduced from 28 to 8 min or (iii) cerebellar, hippocampal or hypothalamic slices were used instead of cortical slices. The Ca²⁺-induced tritium overflow in guinea-pig cortex slices was inhibited by histamine (in the presence of ranitidine); this effect was abolished by clobenpropit. In slices superfused in the presence of clobenpropit, impromidine failed to facilitate the Ca²⁺-evoked tritium overflow. The electrically evoked tritium overflow in mouse brain cortex slices was inhibited by histamine by about 60% (both in the absence or presence of ranitidine). The inhibitory effect of histamine was abolished (but not reversed) by clobenpropit. In conclusion, noradrenaline release in the guinea-pig brain cortex is inhibited via presynaptic H₃ receptors and facilitated via H₂ receptors not located presynaptically. In the mouse brain cortex, only inhibitory H₃ receptors occur. The extent of the H₃ receptor-mediated effect is more marked in the mouse than in the guinea-pig brain cortex. Received: 25 September 1997 / Accepted: 17 November 1997     

Psychomotor stimulant effects of the stereoisomers of chlorpheniramine

The behavioral effects of the histamine H₁ antagonistsd- andl-chlorpheniramine and of the H₂ antagonist zolantidine were determined in squirrel monkeys responding under a fixed-interval (FI) 3-min schedule of stimulus-shock termination. Althoughd-chlorpheniramine is known to be much more potent thanl-chlorpheniramine for antagonizing H₁ receptor-mediated effects of histamine or displacing [³H]-mepyramine from histamine H₁ receptors, similar doses of racemic chlorpheniramine and thed- andl-isomers (3.0–10.0 mg/kg) produced comparable increases in rates of responding. Zolantidine (1.0–17.0 mg/kg) did not alter or, at the highest dose, markedly decreased responding. These findings suggest that the psychomotor stimulant effects of chlorpheniramine involve actions other than the blockade of histamine H₁ or H₂ receptors. Selected H₁ antagonists and cocaine are known to have comparable rateincreasing, reinforcing, and discriminative stimulus effects and, recently, the enantiomers of chlorpheniramine have been shown to displace [³H]-cocaine from binding sites in CNS with approximately equal potency. Possibly, such actions mediate behavioral effects common to H₁ antagonists and cocaine.     

Pharmacological characterization of GSK1004723, a novel, long-acting antagonist at histamine H(1) and H(3) receptors

BACKGROUND AND PURPOSE

Preclinical pharmacological characterization of GSK1004723, a novel, dual histamine H₁ and H₃ receptor antagonist.

EXPERIMENTAL APPROACH

GSK1004723 was characterized in vitro and in vivo using methods that included radioligand binding, intracellular calcium mobilization, cAMP production, GTPγS binding, superfused human bronchus and guinea pig whole body plethysmography.

KEY RESULTS

In cell membranes over-expressing human recombinant H₁ and H₃ receptors, GSK1004723 displayed high affinity, competitive binding (H₁ pKi = 10.2; H₃ pKi = 10.6). In addition, GSK1004723 demonstrated slow dissociation from both receptors with a t_1/2 of 1.2 and 1.5 h for H₁ and H₃ respectively. GSK1004723 specifically antagonized H₁ receptor mediated increases in intracellular calcium and H₃ receptor mediated increases in GTPγS binding. The antagonism exerted was retained after cell washing, consistent with slow dissociation from H₁ and H₃ receptors. Duration of action was further evaluated using superfused human bronchus preparations. GSK1004723 (100 nmol·L⁻¹) reversed an established contractile response to histamine. When GSK1004723 was removed from the perfusate, only 20% recovery of the histamine response was observed over 10 h. Moreover, 21 h post-exposure to GSK1004723 there remained almost complete antagonism of responses to histamine. In vivo pharmacology was studied in conscious guinea pigs in which nasal congestion induced by intranasal histamine was measured indirectly (plethysmography). GSK1004723 (0.1 and 1 mg·mL⁻¹ intranasal) antagonized the histamine-induced response with a duration of up to 72 h.

CONCLUSIONS AND IMPLICATIONS

GSK1004723 is a potent and selective histamine H₁ and H₃ receptor antagonist with a long duration of action and represents a potential novel therapy for allergic rhinitis.     

Possible Regulatory Role of Histamine in Human Platelet Function Examined by Thrombin-induced Serotonin Release

Abstract: The influence of histamine on human platelet function was studied by thrombin-induced serotonin release. The thrombin-induced ³H-serotonin release was confirmed to be a rapid process which does not require external calcium. Histamine was found to reduce the release of serotonin and the inhibition was abolished when H₁-plus H₂-antagonists were added together with histamine. H₁- and H₂-receptor stimulation was examined in two ways, by a combination of histamine with cimetidine or diphenhydramine and by the selective agonists 2-(2-pyridyl)-ethylamine and impromidine. In both instances H₁- and H₂-stimulation was found to reduce the platelet serotonin release. These results suggest a regulatory role of histamine in the platelet function by stimulation of platelet H₁- and H₂-receptors.     

Stimulation of histamine H2- (and H1)-receptors activates Ca2+ influx in all-atrans-retinoic acid-differentiated HL-60 cells independently of phospholipase C or adenylyl cyclase

In human neutrophils, histamine H₂-receptors mediate activation of adenylyl cyclase (AC) and inhibition of N-formyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP)-induced superoxide anion (0 _inf2 ^sup– ) formation, and in HL-60 promyelocytes, H₂-receptors mediate parallel activation of AC, phospholipase C (PLC) and non-selective cation (NSC) channels. As all-trans-retinoic acid (RA) is successfully used in the differentiation therapy of acute promyelocytic leukaemia, we studied signal transduction in RA-differentiated HL-60 cells. Histamine and the H₂-receptor agonist, impromidine, induced both rises in cAMP levels and cytosolic Ca²⁺ ([Ca²⁺]_i). Substances acting at post-receptor sites to increase cAMP did not increase [Ca²⁺]_i. H₂-but not H₁-receptor antagonists inhibited histamine-induced cAMP accumulation and rises in [Ca²⁺]_i were more effectively inhibited by H₂- than by H₁-receptor antagonists. Histamine-induced rises in [Ca²⁺]_i were completely dependent on the presence of extracellular Ca²⁺ and were abolished by the blocker of NSC channels, Gd³⁺, but were resistant to inhibition by pertussis toxin. Unlike FMLP, histamine did not activate PLC. The effects of FMLP on [Ca²⁺]_i were less sensitive to blockade by Gd³⁺ than those of histamine, and there was no cross-desensitization between the two stimuli. FMLP, but not histamine, inhibited transiently thapsigargin-induced rises in [Ca²⁺]₁. Taken together, our results show that histamine activates AC-mediated cAMP accumulation in RA-differentiated HL-60 cells via H₂-receptors and NSC channel-mediated Ca²⁺ influx via H₂- (and H₁)-receptors. Histamine-induced NSC channel activation is not the consequence of AC- or PLC stimulation and occurs, directly or indirectly, via pertussis toxin-insensitive guanine nucleotide-binding proteins. FMLP and histamine activate Ca²⁺ influx by different mechanisms. There are similarities in H₂-receptor-mediated signal transduction between RA-differentiated HL-60 cells and HL-60 promyelocytes and differences between the former cells and neutrophils, indicating that RA-differentiated HL-60 cells must be considered as partially immature.     

Mechanism of action of histamine in the estrogen-primed rat uterus

Histamine produced a dose-dependent relaxation of uterine strips obtained from the estrogen-primed rat uterus. The responses to histamine were blocked competitively by metiamide (10^?8 ?10^?8M), a specific H₂-receptor antagonist. Propranolol, a selective β-receptor blocker also produced competitive antagonism of the responses to histamine in the same dose range (10^?8?10^?6 M). The pA₂ value obtained for metiamide (8.9) was not significantly different from that obtained for propranolol (8.6). Nialamide (2.2 × 10^?6 M), the monoamine oxidase inhibitor, and cocaine (4.3 × 10^?6 M), the selective noradrenaline uptake blocker, potentiated the responses to histamine. However bretylium (2.4 × 10^?5 M), an adrenergic neuron blocker inhibited the responses to histamine. The combined effect of tyramine and histamine was found to be additive. Our data suggest that the histamine-induced relaxation of rat uterus may be produced through the stimulation of presynaptic H₂-receptors which causes the release of noradrenaline. The released noradrenaline acts on the postsynaptic β-receptors and produces relaxation of the rat uterus.     

Regulation of sterol synthesis by histamine in human mononuclear leukocytes: Roles of H1- and H2-receptors

Summary The effect of histamine on sterol synthesis has been investigated in freshly isolated human mononuclear leukocytes from healthy subjects.Incubation of cells for 6 h in a medium containing lipid depleted serum led to a threefold increase in the incorporation of (¹⁴C)-acetate or tritiated water into sterols. Histamine 0.3 µM added to the incubation medium at zero time inhibited this induction by 35% with a sigmoidal log dose-effect curve.The receptors mediating this action were characterised pharmacologically by using selective H₁- and H₂-agonists and -antagonists. The H₂-agonists impromidine and 4-methylhistamine mimicked the effect of histamine on sterol synthesis, the suppression being 42% and 31%, respectively, at a concentration of 1 µM. In contrast, the H₁-agonist 2-pyridylethylamine did not affect the pathway. The H₂-antagonist cimetidine (10 µM) but not the H₁-antagonist mepyramine (10 µM) totally reversed the inhibition of sterol synthesis by histamine.The results provide evidence that sterol synthesis in human mononuclear leukocytes is regulated by histamine, which appears to act predominantly via H₂-receptors.     

Characterization of H2-receptor mediated relaxation in rabbit aortic strips

Rabbit aortic strips contracted with either KC1 (60 mM), phenylephrine (10^?5 M) or angiotensin-II (10^?7 M) were exposed to cumulative concentrations of the H₂-receptor agonists impromidine, histamine, 4-methyl histamine and dimaprit in the presence of the H₁-receptor antagonist, pyrilamine (10^?6 M). All of the H₂-receptor agonists produced a similar degree of relaxation in strips contracted with phenylephrine. The order of potency was impromidine>histamine>4-methyl histamine?dimaprit. All of the H₂-receptor agonists except 4-methyl histamine were able to produce significant degrees of relaxation in aortic strips contracted with angiotensin-II. In contrast, none of the H₂-receptor agonists, in doses which relaxed phenylephrine and angiotensin-II contracted muscles, we able to relax aortic strips contracted by KCl.When the relaxant ability of the H₂-receptor agonist, impromidine, was compared to that of D-600, isoproterenol and adenosine, it was found to produce relaxation similar to that characterized by isoproterenol. Impromidine, like isoproterenol, more readily relaxed aortic strips contracted by either phenylephrine or angiotensin-II.     

Role of endothelium and nitric oxide in histamine-induced responses in human cranial arteries and detection of mRNA encoding H1- and H2-receptors by RT-PCR

1. Histamine induces relaxation of human cranial arteries. Studies have revealed that the relaxant histamine H₁-receptor predominates in human cerebral and the H₂-receptor in temporal arteries, while H₁- and H₂-receptors are of equal importance in the middle meningeal artery. The purpose of the present study was to examine the role of the endothelium and nitric oxide in histamine-induced responses and to show the presence of mRNA encoding H₁- and H₂-receptors in human cranial arteries.
2. Electrophoresis of polymerase chain reaction (PCR) products from human cerebral, middle meningeal and temporal arteries, demonstrated products corresponding to mRNA encoding both H₁- and H₂-receptors in arteries with and without endothelium. The amplified PCR products were sequenced and showed 100% homology with the published sequences of these histamine receptors.
3. A sensitive in vitro system was used to study vasomotor responses to histamine. In precontracted cerebral, middle meningeal and temporal arteries with and without endothelium, histamine caused a concentration-dependent relaxation with I_max values between 87% and 81% and pIC₅₀ values between 8.14 and 7.15. In arteries without endothelium the histamine-induced relaxation was significantly less potent (I_max values between 87% and 66% and pIC₅₀ values between 7.01 and 6.67) than in cranial arteries with an intact endothelium.
4. The addition of histamine to arteries without endothelium and pretreated with the histamine H₂-antagonist, cimetidine (10⁻⁵ M), caused a concentration-dependent contraction of the cranial arteries with E_max values between 86% and 29% and pEC₅₀ values between 7.53 and 6.77. This contraction was blocked by the histamine H₁-receptor antagonist, mepyramine (10⁻⁷ M), and even turned into a relaxation with I_max values between 84% and 14% and pIC₅₀ values between 7.42 and 5.86.
5. The nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (L-NAME, 3×10⁻⁵ M) significantly inhibited the relaxant response to histamine in cerebral and temporal arteries (pIC₅₀ values between 7.43 and 7.13). The combined treatment with L-NAME (3×10⁻⁵ M) and cimetidine (10⁻⁵ M) caused a further displacement of the concentration-response curve (pIC₅₀ values between 7.14 and 6.57) and decreased the maximum relaxant responses in all three cranial arteries (I_max values between 62% and 39%).
6. In conclusion, this is the first study which show mRNA encoding histamine H₁- and H₂-receptors in human cranial arteries. The results indicate that histamine-induced relaxation of human cranial arteries is partially mediated via an endothelial H₁-receptor coupled to the production of nitric oxide and partially via a H₂-receptor associated with the smooth muscle cells. In addition, there is evidence for a contractile H₁-receptor in the smooth muscle cells in these arteries.

     

A study of the actions of histamine on the isolated rat heart

1. The effects of histamine on cardiac force, heart rate and coronary perfusion pressure were studied in the isolated rat heart, using the Langendorff perfused heart preparation. 2. Single injections of histamine induced dose-dependent decreases in contractile amplitude, heart rate and coronary perfusion pressure. 3. Perfusions of metiamide (above 1 × 10^-4 M) had a depressant effect on contractile force and heart rate, whereas diphenhydramine (4 × 10^-6 M) reduced only the heart rate. Both agents caused a fall in coronary perfusion pressure. 4. The negative inotropic and chronotropic effects of histamine on the isolated rat heart were not significantly influenced by either metiamide or diphenhydramine, or a combination of these drugs. However, the fall in coronary perfusion pressure induced by injections of histamine was significantly antagonized by metiamide or diphenhydramine. 5. These results suggest that the effects of histamine on the isolated rat heart may not be due entirely to stimulation of H₁- or H₂-receptors on the cardiac muscle cells. Evidence is presented for the existence of histamine H₁- and H₂-receptors in the coronary vessels.     

Functional pharmacology of H1 histamine receptors expressed in mouse preoptic/anterior hypothalamic neurons

BACKGROUND AND PURPOSE

Histamine H₁ receptors are highly expressed in hypothalamic neurons and mediate histaminergic modulation of several brain-controlled physiological functions, such as sleep, feeding and thermoregulation. In spite of the fact that the mouse is used as an experimental model for studying histaminergic signalling, the pharmacological characteristics of mouse H₁ receptors have not been studied. In particular, selective and potent H₁ receptor agonists have not been identified.

EXPERIMENTAL APPROACH

Ca²⁺ imaging using fura-2 fluorescence signals and whole-cell patch-clamp recordings were carried out in mouse preoptic/anterior hypothalamic neurons in culture.

KEY RESULTS

The H₁ receptor antagonists mepyramine and trans-triprolidine potently antagonized the activation by histamine of these receptors with IC₅₀ values of 0.02 and 0.2 μM respectively. All H₁ receptor agonists studied had relatively low potency at the H₁ receptors expressed by these neurons. Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine had full-agonist activity with potencies similar to that of histamine. In contrast, 2-pyridylethylamine and betahistine showed only partial agonist activity and lower potency than histamine. The histamine receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl)heptanecarboxamide (HTMT) had no agonist activity at the H₁ receptors H1 receptors expressed by mouse preoptic/anterior hypothalamic neurons but displayed antagonist activity.

CONCLUSIONS AND IMPLICATIONS

Methylhistaprodifen and 2-(3-trifluoromethylphenyl)histamine were identified as full agonists of mouse H₁ receptors. These results also indicated that histamine H₁ receptors in mice exhibited a pharmacological profile in terms of agonism, significantly different from those of H₁ receptors expressed in other species.     

Receptor-specific functional efficacies of alkyl imidazoles as dual histamine H3/H4 receptor ligands

Histamine H₃ and H₄ receptors are highly related G protein-coupled receptors. Preclinical and clinical data strongly suggest the potential therapeutic application of selectively acting histamine H₄ receptor ligands to inflammatory conditions but also hint at a certain interference of the two receptors in diseases attended with itch and pain. The aim of this investigation was to identify dual acting ligands as pharmacological tools. Receptor binding profiles of ω-(1H-imidazol-4-yl)alkyl derivatives structurally defined as amides, carbamates, esters, ethers, ketones and ureas were evaluated with respect to their potencies at histamine H₃ and H₄ receptors. A two-step screening method based on in vitro radioligand binding studies and functional [³⁵S]GTPγS binding experiments was performed. The examined series of imidazole-containing compounds displayed both, selective histamine H₄ receptor and dual acting histamine H₃/H₄ receptor ligands. Slight structural modifications caused major differences in selectivity profiles and on functional properties at the human histamine H₄ receptor. N-(3-(1H-Imidazol-4-yl)propyl)-2-cyclohexylacetamide 11 was identified as most potent and selective human histamine H₄ receptor ligand in this series (K_i = 45 nM). Amide- and ether-containing structures consistently exhibited partial agonist efficacies, whereas ureas, ketones, esters and carbamates mainly acted as antagonists and inverse agonists. We identified novel dual acting histamine H₃/H₄ receptor ligands with varying efficacies at the histamine H₄ receptor subtype, whereas histamine H₃ receptor antagonism was kept constant, e.g. 3-(1H-imidazol-4-yl)propyl (cyclohexylmethyl)carbamate 19 or 4-(3-(3-phenylpropylthio)propyl)-1H-imidazole 30. These compounds state valuable pharmacological tools in studies of diseases, in which histamine H₃ and H₄ receptor signalling contributes to pathophysiological conditions.     

Brain histamine H3 receptors in rats with portacaval anastomosis: in vitro and in vivo studies

The long-term effects of portacaval anastomosis (PCA) on histamine H₃ receptors in rat brain were studied by in vitro and in vivo methods. The overflow of histamine from the anterior hypothalamus and from cortex after long-term PCA was determined by in vivo microdialysis. The binding properties of [³H]-R-α-methylhistamine in membranes from cortex, cerebellum, and rest of brain (ROB) were examined with saturation binding experiments. The regional distribution of [³H]-R-α-methylhistamine binding sites in the brain of sham- and PCA-operated rats was assessed also with autoradiography. The tissue levels of histamine were significantly elevated in cortex and ROB of PCA-operated rats. In addition, the spontaneous and K⁺-evoked overflow of histamine from anterior hypothalamus, and the thioperamide-induced overflow from both anterior hypothalamus and cortex were increased after chronic PCA. In spite of the significantly elevated tissue concentrations and the moderate increase in histamine release, the binding properties of [³H]-R-α-methylhistamine to cortical membranes were not significantly changed. However, the autoradiography study did reveal a decrease in [³H]-R-α-methylhistamine binding density, particularly in striatum and cortex, where H₃ receptors are located mainly at non-histaminergic neurons. In conclusion, we suggest that there is a region-selective increase in the histaminergic activity in chronic PCA, which leads to the down-regulation of somadendritic and pre-synaptic H₃ receptors located at non-histaminergic neurons. At the same time, the autoreceptor mediated control of histamine neuronal activity via pre-synaptic H₃ receptors located at histaminergic neurons is preserved after long-term PCA. Received: 11 May 1998 / Accepted: 10 August 1998