B. Malinowska

Modulation of neurotransmitter release via histamine H3 heteroreceptors

Summary— Presynaptic H3 receptors occur on histaminergic neurones of the CNS (autoreceptors) and on non‐histaminergic neurones of the central and autonomic nervous system (heteroreceptors). H3 heteroreceptors, most probably located on the postganglionic sympathetic nerve fibres innervating the resistance vessels and the heart, have been identified in the model of the pithed rat. Furthermore, we could show in superfusion experiments that H3 heteroreceptors also occur on the sympathetic neurones supplying the human saphenous vein and the vasculature of the pig retina and on the serotoninergic, dopaminergic and noradrenergic neurones in the brain of various mammalian species, including man. The effects of three recently described H3 receptor ligands were studied in superfused mouse brain cortex slices. The potency of the novel H3 receptor agonist imetit exceeded that of R‐(‐)‐α‐methylhistamine (the reference H3 receptor agonist) by one log unit and that of histamine by almost two log units. Clobenpropit was shown to be a competitive H3 receptor antagonist, exhibiting a pA2 as high as 9.6 (exceeding the pA2 of the reference H3 receptor antagonist thioperamide by one log unit). The irreversible antagonism of N‐ethoxycarbonyl‐2‐ethoxy‐1,2‐dihydroquinoline (EEDQ) was also studied. Interactions of the H3 heteroreceptor with the dopamine autoreceptor in mouse striatal slices and the α2‐autoreceptor in mouse brain cortex slices could be demonstrated. Activation of α2‐autoreceptors decreases the H3 receptor‐mediated effect. Blockade of α2‐autoreceptors increases the H3 receptor‐mediated effect only if the α2‐autoreceptors are simultaneously activated by endogenous noradrenaline. The H3 receptor‐mediated inhibition of noradrenaline release in mouse brain cortex slices was attenuated by the K+ channel blocker tetraethylammonium but this attenuation was abolished by reduction of the Ca2+ concentration in the medium (to compensate for the facilitatory effect of tetraethylammonium on noradrenaline release). Accordingly, we assume that the H3 receptors are not coupled to voltage‐sensitive K+ channels. Pertussis toxin and N‐ethylmaleimide attenuated the H3 receptor‐mediated effect in the mouse brain cortex, suggesting that the H3 receptors are coupled to a G protein (eg Gi or Go). However, negative coupling to an adenylate cyclase does not appear to exist since an H3 receptor‐mediated inhibition of cAMP accumulation was not obtained in mouse brain cortex membranes. H3 receptor ligands are currently undergoing clinical testing and might become new remedies for the treatment of diseases of the gastrointestinal and bronchial system and the CNS.

Mutual interaction of histamine H3-receptors and α2-adrenoceptors on noradrenergic terminals in mouse and rat rain cortex

SummaryBrain cortex slices were preincubated with 3H-noradrenaline and superfused with physiological salt solution containing desipramine. We studied the inhibition of the electrically evoked tritium overflow caused by histamine in the presence of α-adrenoceptor ligands (mouse and rat brain cortex), and the inhibition caused by talipexole (the former B-HT 920) in the presence of H3-receptor ligands (mouse brain cortex).In mouse brain cortex slices, the inhibitory effect of histamine on the tritium overflow evoked by 36 pulses, 0.3 Hz was not changed by the α1-adrenoceptor antagonist prazosin, but increased by the α2-adrenoceptor antagonist rauwolscine. When the current strength or the duration of electrical pulses was reduced to compensate for the increase in evoked tritium overflow produced by rauwolscine, the latter still. enhanced the effect of histamine. The histamine-induced inhibition of tritium overflow evoked by 360 pulses, 3 Hz was not affected by the α1-adrenoceptor agonist phenylephrine but attenuated by the α2-adrenoceptor- agonist talipexole. Finally, the inhibition by histamine of the tritium overflow evoked by 3 pulses, 100 Hz was attenuated by talipexole but not affected by rauwolscine. Conversely,. the inhibitory effect of talipexole on tritium overflow elicited by 360 pulses, 3 Hz was slightly attenuated by the H3-receptor agonist R-(−)-α-methylhistamine but not, affected by the H3- receptor antagonist thioperamide. In rat brain cortex slices, histamine only tended to inhibit tritium overflow evoked by 360 pulses, 3 Hz, both in the absence of α-adrenoceptor antagonists and in the presence of prazosin. However, histamine markedly inhibited the evoked overflow in the presence of rauwolscine. Again, enhancement of the histamine-induced inhibition also occurred when the current strength or the duration of pulses was reduced in order to compensate for the increase in evoked tritium overflow produced by rauwolscine.The results suggest that the α2-autoreceptors and the H3-heteroreceptors at the noradrenergic nerve endings in the brain of mouse and rat interact with each other. Activation of the α2-autoreceptors decreases, whereas blockade of the activated (but not of the non-activated) α2-autoreceptors increases, the inhibitory effect of histamine. Activation of the H3-heteroreceptors slightly decreases, whereas blockade of the H3-receptors fails to affect, the inhibitory effect of talipexole.

Identification of endothelial H1, vascular H2 and cardiac presynaptic H3 receptors in the pithed rat.

SummaryIn pithed and vagotomized rats the effects of the H3 receptor agonist R-(−)-α-methylhistamine, the H1 receptor agonist 2-(2-thiazolyl)ethylamine and the H2 receptor agonist dimaprit on basal diastolic blood pressure, basal heart rate and the electrically induced rise in heart rate were examined.Basal diastolic blood pressure was not altered by low, but increased by high doses of R-(−)-α-methylhistamine; the latter effect was not affected by selective H1, H2 or H3 receptor antagonists and by prazosin, but was attenuated by rauwolscine. Rauwolscine also unmasked a vasodepressor response to R-(−)-α-methylhistamine not affected by the H3 receptor antagonist thioperamide, but counteracted by the H1 receptor antagonist dimetindene or the H2 receptor antagonist ranitidine. The vasodepressor responses to 2-(2-thiazolyl)ethylamine and dimaprit were antagonized by dimetindene and ranitidine, respectively. The vasodepressor response to 2-(2-thiazolyl)ethylamine was not altered by indomethacin, but reduced by an inhibitor of endothelial nitric oxide synthase, Nω-nitro-L-arginine methyl ester (which, by itself, markedly increased blood pressure). Both drug tools did not alter the effect of dimaprit. Basal heart rate was not affected by 2-(2-thiazolyl)ethylamine (examined after administration of propranolol), dimaprit and R-(−)-α-methylhistamine. The electrically induced increase in heart rate (studied in animals which had received rauwolscine) was decreased by R-(−)-α-methylhistamine but not affected by 2-(2-thiazolyl)ethylamine and dimaprit. The effect of R-(−)-α-methylhistamine was abolished by thioperamide. R-(−)-α-methylhistamine did not influence the increase in heart rate produced by isoprenaline.In conclusion, the pithed rat offers the opportunity to study cardiac presynaptic H3 receptors, endothelial H1 receptors and vascular H2 receptors in the same experimental model. Cardiac presynaptic H1 and H2 receptors as well as postsynaptic H3 receptors in the heart and in the resistance vessels were not found. R-(−)-α-methylhistamine is a weak agonist at α2, H1 and H2 receptors.

Inhibition of noradrenaline release in the rat vena cava via prostanoid receptors of the EP3-subtype.

1 In segments of the rat vena cava preincubated with [3H]‐noradrenaline and superfused with physiological salt solution (containing desipramine and corticosterone), we studied the effects of prostaglandins of the D, E and F series, of a prostacyclin analogue and a thromboxane‐mimetic and of subtype‐selective prostaglandin E‐receptor (EP‐receptor) ligands on the electrically (0.66 Hz)‐evoked tritium overflow. 2 The electrically‐evoked tritium overflow was inhibited by prostaglandin E2 (maximum inhibition by about 80%; pIC40 7.49). The effect of prostaglandin E2 was not affected by rauwolscine, which, by itself, increased the evoked overflow; the α2‐adrenoceptor antagonist was added to the superfusion medium in all subsequent experiments. Indomethacin failed to affect either the evoked tritium overflow or its inhibition by prostaglandin E2. 3 The inhibitory effect of prostaglandin E2 on the electrically‐evoked tritium overflow was not altered by the EP1‐receptor antagonist, AH 6809 (6‐isopropoxy‐9‐oxoxanthene‐2‐carboxylic acid) at a concentration at least 30 fold higher than its pA2 value at EP1‐receptors. The following compounds mimicked the effect of prostaglandin E2 showing the following rank order of potencies: misoprostol (EP2‐/EP3‐receptor agonist) ≃ sulprostone (EP1/EP3‐receptor agonist) ≃ prostaglandin E1 = prostaglandin E2 ≫ iloprost (EP1‐/IP‐receptor agonist) = prostaglandin F2α. The evoked overflow was not affected by high concentrations of prostaglandin D2 or the thromboxane‐mimetic U46619 (9,11‐dideoxy‐11α, 9α‐epoxymethano‐prostaglandin F2α). 4 The present results suggest that the postganglionic sympathetic nerve fibres innervating the rat vena cava are endowed with presynaptic EP3‐receptors. They are not tonically activated by endogenously formed products of cyclo‐oxygenase and do not interact with the presynaptic α2‐adrenoceptors.

Noradrenaline release in the rat vena cava is inhibited by γ‐aminobutyric acid via GABAB receptors but not affected by histamine

1 Segments of the rat vena cava preincubated with [3H]‐noradrenaline were superfused with [3H]‐noradrenaline‐free solution containing desipramine and corticosterone and the effects of γ‐aminobutyric acid (GABA) receptor ligands and of histamine on tritium overflow evoked by transmural electrical stimulation were studied. 2 GABA inhibited, and histamine failed to affect, the electrically (0.66 Hz) evoked tritium overflow both in the absence and presence of rauwolscine (which was present in the superfusion medium in the subsequent experiments). The effect of GABA was less pronounced at a stimulation frequency of 2 Hz. 3 The inhibitory effect of GABA (pIC35 5.83) on the electrically (0.66 Hz) evoked overflow was mimicked by the GABAB receptor agonist, R‐(−)‐baclofen (6.07) and less potently by S‐(+)‐baclofen (3.30) and the GABAA receptor agonist, muscimol (3.70). The concentration‐response curve of GABA was shifted to the right by the GABAB receptor antagonist, CGP 35348 (P‐(3‐aminopropyl)‐P‐diethoxymethyl‐phosphinic acid; apparent pA2 4.76), but not affected by the GABAA receptor antagonist, (−)‐bicuculline methiodide 100 μmol1−1. Given alone, (−)‐bicuculline methiodide slightly increased, and CGP 35348 did not affect, the evoked overflow. 4 The effect of GABA in veins from rats treated for 14 days with RS‐baclofen (10 mg kg−1, i.p. once daily) did not differ from that in veins from rats which received the vehicle instead. The effect of GABA also did not differ in veins from rats treated once either with RS‐baclofen or with its vehicle. 5 The results suggest that the postganglionic sympathetic nerve fibres in the rat vena cava are endowed with presynaptic GABAB receptors, but not with receptors for histamine. (−)‐Bicuculline methiodide increases noradrenaline release by a mechanism not related to GABAA, GABAB, α2‐receptors or noradrenaline uptake.

Histaprodifen, methylhistaprodifen, and dimethylhistaprodifen are potent H1-receptor agonists in the pithed and in the anaesthetized rat

Selective H2- and H3-receptor agonists, exhibiting an at least tenfold higher potency than histamine itself at the respective receptors, have been known for several years. Selective H1-receptor agonists with a potency exceeding that of histamine have become available only recently; the most potent are methylhistaprodifen and dimethylhistaprodifen [Nα-methyl- and Nα,Nα-dimethyl-2-(3,3-diphenylpropyl)histamine, respectively] with 3.4- and 2.4-fold higher potencies than histamine in vitro (in the guinea-pig ileum). The aim of the present study was to examine whether these compounds and the parent compound histaprodifen are potent H1-receptor agonists in the pithed and in the anaesthetized rat. In pithed, vagotomized rats diastolic blood pressure was decreased by 2-(2-thiazolyl)ethanamine i.v. (which was used as a reference H1-receptor agonist) and by histaprodifen, methylhistaprodifen, and dimethylhistaprodifen; the maximum decrease was about 45 mmHg for each compound, and the potencies, expressed as pED50, the negative logarithm of the dose (in mole per kilogram body weight) eliciting a half-maximal response, were 7.23, 7.55, 8.43 and 8.12, respectively. The dose/response curves of the four compounds were shifted to the right to about the same extent by the H1-receptor antagonist dimetindene (1 µmol/kg i.v.). The vasodepressor response was not affected by combined i.v. administration of the H2- and H3-receptor antagonists ranitidine and thioperamide, by combined i.v. administration of the α1- and α2-adrenoceptor antagonists prazosin and rauwolscine, and by the β-adrenoceptor antagonist propranolol i.v. but was attenuated by the inhibitor of NO synthase, Nω-nitro-l-arginine methyl ester i.v. In anaesthetized rats 2-(2-thiazolyl)ethanamine, histaprodifen, methylhistaprodifen and dimethylhistaprodifen i.v. also decreased diastolic blood pressure in a manner sensitive to dimetindene i.v. Our data show that histaprodifen and, in particular, methyl- and dimethylhistaprodifen are highly potent H1-receptor agonists in vivo.

Effects of H1, H2 and H3 agonists on the neurogenic vasopressor response in the pithed rat

In pithed and vagotomized rats, the electrically induced vasopressor response was not affected by 2-(2-thiazolyl)ethylamine and dimaprit but was inhibited by Nα-methylhistamine (NαMH) in a thioperamidesensitive manner. NαMH did not affect the vasopressor response to exogenously added noradrenaline. NαMH produced a biphasic effect on basal diastolic blood pressure, i.e. a vasodepressor response (abolished by dimethindene plus ranitidine) and a subsequent vasopressor effect (counteracted by dimethindene or adrenalectomy). Basal heart rate was increased by NαMH (effect abolished by propranolol, dimethindene or adrenalectomy). In conclusion, presynaptic H3, but not H1 or H2, receptors are detectable in the resistance vessels of the rat. NαMH allows the characterization of another four histamine receptor-mediated effects, i.e vasodepressor effects mediated via H1 and H2 receptors and an increase in blood pressure and heart rate mediated via catecholamine-releasing H1, receptors in the adrenal medulla.

Effects of the novel H1 agonists 2-(3-trifluoromethylphenyl)- and 2-(3-bromophenyl)histamine and of 2-(2-thiazolyl)ethylamine on cardiovascular paramenters in the pithed rat

The effect of the newly synthetized H1 agonists 2-(3-trifluoromethylphenyl)histamine (2-TFMPH) and 2-(3-bromophenyl)histamine (2-BPH) and of the reference compound 2-(2-thiazolyl)ethylamine (2-TEA) on diastolic blood pressure and heart rate was studied in pithed and vagotomized rats. 2-TFMPH and 2-BPH were at least equipotent with 2-TEA in producinga dimethindene-sensitive, short-lasting vasodepressor response. At the highest dose, 2-TFMPH and 2-BPH produced an additional small vasopressor response, which was followed by a long-lasting vasodepressor effect not counteracted by dimethindene and ranitidine 2-TEA, at the highest dose, induced an additional vasopressor response abolished by prazosin plus rauwolscine. Basal heart rate was increased by 2-TEA (in a desipramine-sensitive manner) but not affected by 2-TFMPH or 2-BPH. In conclusion, 2-TFMPH and 2-BPH are potent H1 agonists, devoid of an indirect sympathomimetic effect; at high doses, they produce a vasodepressor response not mediated via histamine receptors.