Binding of [3H]cimetidine to rat brain tissue

Binding of [³H]cimetidine to rat brain tissue was investigated, and a saturable binding with dissociation constant 0.22±0.05 M found. This binding is inhibited by a range of imidazole-derived histamine H₂-receptor antagonists, but not by a number of non-imidazole H₂-receptor antagonists. It is concluded that the [³H]cimetidine binding site in rat brain tissue that is labelled in these experiments is not the histamine H₂-receptor.     

Influence of histamine H3-antagonists on human leukocytes

To determine the role of the histamine H₃-receptor on basophils, different specific H₃-antagonists were investigated. Incubation of washed leukocytes with N-acylated histamine-derivatives (N-ahd) induced elevated histamine levels. This process turned out to be dependent on dose, time and temperature, but independent of Ca²⁺ and Mg²⁺ ions. IgE-mediated histamine release was not modulated. [³H]-l-histidine was not decarboxylated into [³H]-histamine in spite of the observed histamine increase. Highly purified basophils did not show any histamine elevation but purified neutrophils and eosinophils were found to have increased histamine levels even after disintegration and subsequent incubation with N-ahd. It seems that the increased histamine levels result from the cleavage of the applied histamine amides. Other potent H₃-antagonists (e.g. thioperamide) neither produced increased histamine levels nor influenced IgE-mediated release from basophil leukocytes. The existence of H₃-receptors on human basophils therefore seems unlikely.This work was supported by Grant No. KI 622/1-1 from the Deutsche Forschungsgemeinschaft, Bonn, FRG.     

Autoinhibition of histamine release by H3 receptors in rat brain cortex depends on stimulation frequency

Rat cortical slices preloaded with [³H]histidine released [³H]histamine upon electrical stimulation or after depolarization with elevated K⁺ levels. The release was dependent on the presence of Ca²⁺, suggesting a neurosecretory process. Histamine has been shown to inhibit its own release mediated by an autoreceptor belonging to the H₃-receptor subclass. In this study we have investigated the autoinhibition using different electrical field stimulation conditions (1, 10, 20 and 33.3 Hz). Applying electrical stimulation, the inhibition of [³H]histamine release by histamine is decreased when the stimulation frequency is elevated. When stimulated with 1 Hz histamine is able to block [³H]histamine release completely, with ap(EC₅₀) of 8.1±0.1. At higher frequencies histamine still blocks [³H]histamine release completely, but with a lowerp(EC₅₀).     

The degradation of histamine in some animal species

The aim of this work was 1. determine the major [³H]-metabolites of histaminein vivo by using the musculus gracilis of the dog, andin vitro in different tissues of the cat, 2. determine the endogenous levels of histamine and N^τ-methylhistamine in some tissues of the cat and the rat.

1. [³H] histamine injection into arterial supply of the muscle of the dog: after 60 minutes the content of [³H] histamine in the muscle was 1.9\+-0.6% and the content of [³H] N^\gt-methylhistamine and [³H] \lsAcid metabolites\rs was 47\+-6.7% and 51.0\+-6.5%. The cat tissuesin vitro showed the following t_1/2 for exogenous histamine: submandibular gland, 95.9 min, parotid gland, 36.1 min, stomach pylorus, 39.5 min, thyroid gland, 28.3 min, liver, 23.7 min.
2. Endogenous levels of histamine and N^\gt-methylhistamine were determined in the cat and the rat. In general, the contents of endogenous histamine and N^\gt-methylhistamine were similar to the data obtained with labelled [³H] histamine. In both species submandibular gland was among the richest tissues in N^\gt-methylhistamine content. The provenience of N^τ-methylhistamine in mast cells is discussed.

     

The effect of group selective reagentsN-ethylmaleimide and dithiothreitol on histamine H1-receptor binding sites in the vascular smooth muscle membranes

The chemical nature of the histamine H₁-receptors of beef aortic membranes has been elucidated by introducing two group selective reagents in the [³H]-mepyramine binding studies: dithiothreitol (DTT), a protein-disulphide group reducing reagent, andN-ethylmaleimide (NEM), a proteinthiol group alkylating agent.In the binding experiments, NEM independently inhibits [³H]-mepyramine binding. The inhibition is time and concentration dependent. DTT on the other hand potentiates the binding of the radioligand to its receptor and changes the affinity of histamine in competing for [³H]-mepyramine binding site. In the DTT-pretreated membranes (100 M), histamine shows a higher affinity for [³H]-mepyramine binding (K _i 0.35 M) than in the untreated membranes (K _i 3.7 M). Comparison of the pharmacological studies on the DTT-treated rabbit aortic strips and above binding studies, revealed a good correlation between the changes in the affinity of histamine for its receptor, when DTT was present. The results suggest an important role of the S-S and SH groups in the function of aortic histamine H₁-receptor.     

Evidence for H2-receptor-mediated inhibition of histamine release from isolated rat mast cells

Impromidine, an H₂-receptor agonist, inhibited the release of histamine from isolated purified rat mast cells evoked by compound 48/80 and acetylcholine. Pyridilethyl-amine (PEA), an H₁-receptor agonist, on the other hand, was only slightly effective at very high concentrations. The inhibitory effect of impromidine was blocked by preincubating the cells with cimetidine, but not by chlorpheniramine.The existence of an H₂-mediated inhibitory feed-back regulation of histamine release was also suggested by the demonstration of specific binding sites for [³H]-cimetidine in rat mast cell membranes.     

The distribution of histamine H1-receptors in the rat brain: An autoradiographic study

The localization of histamine H₁-receptors in the rat brain was studied at the light microscopic level by quantitative autoradiography. Receptors were labeled in vitro with [³H]mepyramine. The autoradiograms revealed a widespread distribution of these receptors throughout the brain. Areas with high receptor concentrations are: the bed nucleus of the stria terminalis, polymorphic layer of the hilus of the area dentata, ventromedial nuclei of the hypothalamus, pontine nuclei and other nuclei in the pons, the nucleus tractus solitarii and the dorsal motor nucleus of the vagus nerve. Possible relationships of the distributions of histamine H₁-receptors with (a) specific anatomical systems, (b) the known distribution of histaminergic terminals and (c) the central pharmacological effects of histamine and antihistaminics are discussed.     

Characterization of vascular histamine H1-receptors: Multiple affinity states of aortic histamine H1-receptor

We have shown that [³H]mepyramine labels histamine H₁-receptor-binding sites in bovine aortic membranes. Further characterization of H₁-receptors in this tissue was done by the interaction of an unlabelled histamine receptor agonist or antagonist, with the radioantagonist [³H]mepyramine-binding sites. The competition-binding assays have uncovered differences in the characteristics of the agonist/receptor interaction not shared by antagonists. Agonists interact in the heterogeneous manner with the radioantagonist-labelled sites, showing shallow competition curves with then _H 0.50–0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curvesn _H1). The results suggest the presence in this tissue of multiple affinity states of histamine H₁-receptor, differentiated by high and low affinity for agonists and the same affinity for antagonists.     

Specific binding of [3H]mepyramine to histamine H1-receptors in vascular smooth muscle membranes

The antagonist-sensitive binding of [³H]mepyramine to beef aortic membranes was as expected for binding to histamine H₁-receptors. [³H]mepyramine binds rapidly and in saturable fashion to the specific receptor sites, specific binding reaching equilibrium in 3 min at 37°CScatchard's analysis of the binding data gave a dissociation constant of 3.0 nM for the radioligand-receptor complex and maximal number of binding sites: 31 fmol/mg protein. In the competition studies histamine H₁-antagonists are more potent inhibitors of radioligand binding than H₂-antagonist. They inhibit [³H]mepyramine binding in the following order: mepyramine >triprolidine     

Effects of histamine and betahistine on rat medial vestibular nucleus neurones: possible mechanism of action of anti-histaminergic drugs in vertigo and motion sickness

The tonic discharge of 71 medial vestibular nucleus (MVN) neurones was recorded in slices of the dorsal brainstem of young adult rats. Bath application of histamine caused a dose-related excitation in 59 of the 71 cells (83%), the remaining 12 (17%) being unresponsive. Dimaprit, a selective H² agonist, also caused excitation in all 20 cells tested. The histamine-induced excitation and the response to dimaprit were antagonised by the selective H² antagonist ranitidine, confirming that the H² subtype of histamine receptor is involved in mediating the effects of histamine on these cells. Triprolidine, a selective H₁ antagonist, also antagonised the excitation caused by histamine, at a concentration (0.3 M) which left the H₂ receptor-mediated response to dimaprit unchanged. Thus the excitatory effects of histamine on MVN cells in the rat involve two components mediated through H₁ and H₂ receptor-linked mechanisms, respectively. Betahistine, a weak H₁ agonist and H₃ antagonist, had little excitatory action when applied on its own, but significantly reduced the excitation caused by histamine when the two drugs were applied together. The effects of betahistine were consistent with a partial-agonist action at H₁ receptors on MVN cells, reducing the excitatory responses to histamine presumably by occupying these receptor sites in competition with the exogenously applied neurotransmitter. This partial-agonist action of betahistine may be an important part of its mechanism of action in the symptomatic treatment of vertigo and motion sickness, since it is likely to occur not only in the MVN but also in many brain regions, including the thalamus and cortex, which express H₁ receptors and which are innervated by the hypothalamic histaminergic system. Thus the effectiveness of betahistine and other anti-H₁ drugs against motion sickness may be explained by their action in reducing the effects of the excess histamine release induced in such conditions in various brain areas, including the MVN.     

Mast cell receptors controlling histamine release: Influences on the mode of action of drugs used in the treatment of adverse drug reactions

Summary In drug-induced allergic diseases of the immediate type (anaphylactic and anaphylactoid reactions), the primary target cells are tissue mast cells, which discharge their granular content upon interaction with different secretagogues (immunological releasers; histamine liberators) on specific plasma membrane receptors.Experiments are reviewed here which report that IgE-mediated histamine release from mast cells, and the secretion of histamine induced by non-immunological secretagogues (dextran; compound 48/80; acetylcholine) are blocked by beta-adrenoceptor and H₂-receptor agonists, their inhibiting effect being surmountable by beta-adrenoceptor blocking drugs and by anti-H₂-antihistamines. Specific radioligands ([³H]-dihydroalprenolol; [³H]-cimetidine) binding to rat mast cell membranes points to the possibility that inhibition of histamine release is brought about by the activation of mast cell beta-adrenoceptors and H₂-receptors.Drugs used in therapy of anaphylactic or anaphylactoid reactions may act either on tissue receptors, competing with released mediators, or by inhibiting the release of allergic mediators from mast cells, on activation of specific receptors located in mast cell plasma membranes.     

A postsynaptic inhibitory histamine H2-receptor in the mouse isolated vas deferens

The effect of histamine on adrenergic neurotransmission in the mouse isolated vas deferens preparation was investigated. Concentrations of histamine ranging from 0.2 to 650 M depressed, in a dose-related manner, not only the contractile response elicited by field stimulation but also the response caused by the addition of exogenous noradrenaline and acetylcholine. However, the release of [³H]-NA evoked by field stimulation or by high K⁺ remained unchanged in the presence of these concentrations of histamine. The inhibitory effect of histamine on the contractile responses caused by various stimuli was reduced or completely antagonized by cimetidine, a histamine H₂-receptor antagonist but not by mepyramine, a conventional antihistamine. The inhibitory effect of histamine was found to be inversely proportional to both the Ca²⁺ concentration in the bathing medium and to the frequency of field stimulation. Further, the inhibitory effect of histamine was markedly reduced when Mg²⁺ was omitted from the bathing medium. It is concluded that the mouse vas deferens preparation contains a post-junctional inhibitory H₂-receptor. The stimulation of H₂-receptors by histamine inhibits the contractile response of the vas deferens, possibly by decreasing the availability of Ca²⁺ required for contraction by depressing the influx of Ca²⁺.     

Histaminergic H1-receptors in smooth muscle and endothelium of bovine thoracic aorta

The vascular endothelium modulates relaxation and contraction of blood vessels. Since endothelial cells respond to a variety of vasoactive substances, it was suggested that specific cell membrane receptors exist on the endothelial cells which are responsible for the modulatory role of the endothelium on the blood vessels. We therefore investigated the localization and binding characteristics of histaminergic H₁-receptors in the vascular model system of the bovine thoracic aorta. Our earlier binding experiments showed that histaminergic H₁-receptor binding sites labelled with [³H]mepyramine are present on the vascular smooth muscle membranes of this tissue. In addition a small number of specific H₁-receptor binding sites also exist on the endothelial cells of this tissue with the following binding characteristics: B_max=34.6 fmol [³H]mepyramine/mg protein, K_D=2.13 nM. [³H]mepyramine binding is more effectively inhibited by H₁- than H₂-receptor agonists and antagonists. These results provide evidence for the existence of endothelial histaminergic H₁-receptor binding sites in addition to vascular smooth muscle H₁-receptors in the bovine thoracic aorta.     

Influence of histamine release on postoperative vomiting (POV) following gynaecological laparoscopic surgery

Objective:The mechanisms leading to the high incidence of postoperative vomiting (POV) after gynaecological laparoscopic surgery are still unknown. The effectiveness of POV-prophylaxis using H₁ + H₂-receptor antagonists has been demonstrated, suggesting a role for histamine in the pathogenesis of POV. However, histamine levels were not measured in these studies. The aim of this study was to investigate the incidence of plasma histamine release and its association with POV after gynaecological laparoscopic surgery. Material or subjects:Twenty-two female patients, aged 20-56 y, classified ASA physical status I or II, undergoing elective gynaecological laparoscopic surgery were enrolled in the study. Blood samples for plasma histamine measurements were drawn at defined time points perioperatively. Emetic symptoms were recorded within the first 24 h after operation. A standardized balanced anaesthesia without any prophylactic antiemetic medication was applied. Formal causality analysis for histamine as a determinant for POV was performed. Results:The overall incidence of POV was 40.9% (9 out of 22 patients). Twelve out of 22 patients (54.5%) demonstrated a histamine release reaction during the whole observation period. Six out of 9 patients with POV (66.7%) had a histamine release. There was no difference in mean plasma histamine levels between POV-positive and POV-negative patients. The conditional probability for POV with histamine release was 6/12 = 0.5, in contrast to 3/10 = 0.3 for POV without histamine release. Conclusions:A high incidence of plasma histamine release was demonstrated in most but not all patients with POV. The probability of POV with histamine release (0.5) was higher than without histamine release (0.3), thus histamine release was shown to be one of the contributory determinants for POV in this clinical study. Thus, patients at risk for POV may benefit from a H₁ + H₂-receptor antagonists prophylaxis alone or in combination with other antiemetic strategies.     

Guanine nucleotides and pertussis toxin reduce the affinity of histamine H3 receptors on AtT-20 cells

Agonist occupancy of high affinity histamine H₃ receptors on AtT-20 cells induces increased ACTH release. However, the signal transduction process by which this occurs is presently unknown. As a first step in characterizing this pathway, we have examined the effects of a variety of nucleotides and nucleotide analogs on N^a-methylhistamine binding to these receptors. Nonhydrolyzable guanine nucleotide analogs inhibit up to 40% of the [³H]N^a-methylhistamine binding by increasing the dissociation rate of the ligand from the receptor and thereby, reducing receptor affinity. Pertussis toxin also decreases the affinity of the H₃ receptors and ADP ribosylates a 41 kDa protein. Neither GTPS nor pertussis toxin changeB _max. These data indicate that the H₃ receptors on these cells are coupled to a G protein of the G_i subclass.     

Labelling of non-H1-receptor binding sites by [3H]-mepyramine on the rat liver plasma membrane

In the present study we characterized [³H]-mepyramine binding to rat liver plasma membranes. Binding of [³H]-mepyramine proved to be of high affinity (K _d =7.7±0.4 nM) and saturable, resulting in a B_max-value of 70.4±9.5 pmol/mg protein. However, displacement studies revealed that this binding site was different from other H₁-receptor systems. The two stereoisomers of chlorpheniramine were rather ineffective in displacing [³H]-mepyramine and showed a stercospecificity in favour of thel-isomer. Also several H₁-receptor agonists were not potent in displacing [³H]-mepyramine from rat liver plasma membranes. Morcover, the histamine metabolite imidazole-4-acetic acid was about as potent as the H₁-agonists, whereas imidazole was even more potent. These data strongly suggest that [³H]-mepyramine labels a non-H₁-receptor binding site on the rat liver plasma membrane.     

Regulation of membrane histamine H1-Receptor binding sites by guanine nucleotides,mono- and divalent cations

Agonist interaction with histamine H₁-receptor in [³H] mepyramine bovine aortic membranes labeled with [³H] mepyramine is selectively regulated by cations and guanine nucleotides. GTP and his nonhydrolisable analog Gpp(NH)p' markedly decrease histamine affinity for [³H] mepyramine binding sites. The effect of GTP is reversed in the presence of divalent cation, magnesium. Calcium and sodium ions have little effect on histamine binding whereas magnesium ions decrease the affinity of histamine for the radioantagonist binding sites about tenfold.GTP has little effect on [³H] mepyramine binding and the interaction of H₁-antagonist triprolidine with histamine H₁-receptors. The above results indicate that the effect of guanine nucleotides, mono and divalent cations involves the effect on membrane signal transducing mechanism probably GTP-binding protein(s) cation regulatory site(s) rather than receptor binding site directly.     

Correlation between cholinergic histamine release and quinuclidinyl-benzilate ([3H]-QNB) binding in mast cell membranes

Isolated purified rat mast cells release histamine when exposed to acetylcholine according to a different pattern of sensitivity. No correlation was found between the release of histamine evoked by acetylcholine and the high affinity binding of [³H]-quinuclidinyl-benzilate (QNB), a specific cholinergic muscarinic ligand, to rat mast cell membranes, since QNB binding was the same in membrane isolated from cells which were sensitive or insensitive to acetylcholine.In murine neoplastic mast cells, a negative correlation was found between histamine release and [³H]-QNB binding, as no evidence of specific [³H]-QNB binding was present in murine neoplastic mast cell membranes which, accordingly, do not release histamine when exposed to acetylcholine.It is concluded that murine neoplastic mast cells are not provided with muscarinic cholinergic receptors. In rat mast cells, muscarinic cholinergic receptors are always present, but not always coupled with the effector mechanisms triggering the exocytosis.     

H3 receptor assay in electrically-stimulated superfused slices of rat brain cortex; effects of Nα-alkylated histamines and impromidine analogues

The release of the putative neurotransmitter histamine (HA) from rat brain cortex slices is under negative feedback control by an HA autoreceptor. This autoreceptor has been postulated to belong to a new class of HA receptors, H₃. To verify this hypothesis we have developed an assay using superfused rat brain cortex slices. The HA transmitter pool is labelled by incubation of the slices with the precursor³H-histidine;³H-HA is estimated after separation by column chromatography. Release of HA was found both after K⁺-induced depolarization and electrical field stimulation. The latter resulted in higher and more reproducible HA release. Electrically induced HA release could be fully inhibited in a concentration dependent way by exogenous HA in the superfusion buffer. N-alkylated histamines also showed agonistic activity. The action of exogenous HA was totally blocked by the potent H₂ agonist impromidine and some of its analogues.     

H3 Autoreceptors and muscarinic acetylcholine receptors modulate histamine release in the anterior hypothalamus of freely moving rats

To investigate the modulation of histamine release by autoreceptors and heteroreceptors, the rat anterior hypothalamus was superfused through a push-pull cannula with agonists or antagonists of histamine and acetylcholine muscarinic receptors. Superfusion with the H₃ receptor agonist (R)--methylhistamine inhibited, while superfusion with thioperamide (H₃ antagonist) enhanced histamine release. Superfusion with carbachol (a mixed M₁, M₂, M₃ agonist) inhibited the release of histamine. The release of endogenous histamine was enhanced on superfusion with atropine (a mixed M₁, M₂, M₃ antagonist). The M₃ muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine enhanced the release rate of histamine. It is concluded that in the anterior hypothalamus the release of endogenous histamine is modulated by H₃ autoreceptors. Moreover, acetylcholine released from cholinergic neurons also modulates the release of histamine via M₁ and/or M₃ heteroreceptors.