Activation of histamine H3 receptors produces presynaptic inhibition of neurally evoked cat nictitating membrane responses in vivo

Summary This study was undertaken in order to determine the potential role of prejunctional histamine H₃ receptors in an in vivo adrenergic model system. Frequency-dependent nictitating membrane responses were elicited by sympathetic nerve stimulation in anesthetized cats. Systemic administration of the selective histamine H₃ receptor agonist, (R)--methylhistamine (RMeHA) produced a dose-related depression of amplitude of the evoked nictitating membrane responses with a threshold of about 10 g/kg and maximal effect (50% depression at the lowest frequency; 0.5 Hz) seen at 100–300 gg/kg. Responses obtained with low frequency stimulation were more sensitive to depression by RaMeHA than were responses evoked with higher frequencies of stimulation. Larger doses of RaMeHA given to the same animals, failed to produce additional inhibition.RMeHA depressed the amplitude of nictitating membrane responses evoked by either pre- or postganglionic nerve stimulation to an equivalent degree. This depressant action of RMeHA was antagonized by pretreatment with the specific histamine H₃ antagonist, thioperamide (3 mg/kg), but not by combined pretreatment with histamine H₁ and H₂ blockers chlorpheniramine (300 Ftg/kg) and cimetidine (5 mg/kg). Intravenous administration of adrenaline (1–30 wg/kg) also produced graded nictitating membrane responses that were not altered by subsequent administration of RMeHA.These results suggest that histamine H₃ receptors are involved in the modulation of neurally evoked noradrenaline release in the cat nictitating membrane by an inhibitory presynaptic action. The most likely site of drug action appears to be at the neuroeffector junction as no appreciable ganglionic effect of RMeHA was observed in this in vivo model system. Send offprint requests to M. C. Koss at the above address     

Prejunctional inhibition of sympathetically evoked pupillary dilation in cats by activation of histamine H3 receptors

Summary Frequency-dependent pupillary dilations were evoked by electrical stimulation of the pre- or post-ganglionic cervical sympathetic nerve (sympatho-excitation) or the hypothalamus (parasympatho-inhibition) in sympathectomized anesthetized cats. Systemic administration of the selective histamine H₃ receptor agonist (R)--methylhistamine (RMeHA) produced a dose-dependent depression of mydriasis due to direct neural sympathetic activation but had no effect on responses elicited by parasympathetic withdrawal. The histamine H₂ receptor agonist, dimaprit, was inactive. RMeHA was much more effective in depressing sympathetic responses obtained at lower frequencies when compared to higher frequencies of stimulation.Responses evoked both pre- and postganglionically were inhibited by RMeHA. This peripheral sympathoinhibitory action of RMeHA was antagonized by the histamine H₃ receptor blocker thioperamide but not by intravenous pretreatment with the histamine H₁ receptor antagonist chlorpheniramine. Histamine H₂ receptor blockers cimetidine and ranitidine were also without effect. RMeHA did not depress pupillary responses elicited by i.v. (-)-adrenaline.The results demonstrate that histamine H₃ receptors modulate sympathetic activation of the iris at a site proximal to the iris dilator muscle. The predominant mechanism of action appears to the prejunctional inhibition of noradrenaline release from postganglionic sympathetic nerve endings. However, a concomitant ganglionic inhibitory action cannot be excluded. Correspondence to M. C. Koss at the above address     

Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H₃ receptor selective agonists (R)--methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC₅₀ values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)--methyl-histamine to the right (pK_B value 8.38). The inhibitory effect of 1 mol/l (R)--methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC₅₀: 65 nmol/l) and dimaprit (IC₅₀: 8.6 mol/l); however, the effect of (R)--methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mol/l) and not significantly affected by other H₃ receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 mol/l). In conclusion, H₃ receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H₃ receptors may exist. The data suggest that histamine H₃ receptors modulating 5-HT release in pig small intestine do not belong to either H_3A or H_3B receptors as defined in rat tissue. Correspondence to: K. Racke at the above address     

Endogenous noradrenaline impairs the prostaglandin-induced inhibition of noradrenaline release

Summary The effects of prostaglandin E₂ (PGE₂) on electrically evoked noradrenaline release in rat brain cortex were studied under conditions under which autoinhibition of release was avoided. When stimulation was carried out with 36 pulses at 3 Hz, 1 mol/1 PGE2, produced about 50% inhibition of release. In the presence of the ₂-adrenoceptor antagonist yohimbine (1 gmol/1) the effect of PGE₂ was markedly increased. When release was elicited by 3 pulses/100 Hz the period of stimulation was too short to permit development of autoinhibition by released noradrenaline. Then the concentration-response-curve for PGE₂ was very similar to that obtained under the above conditions (36 pulses/3 Hz, in the presence of yohimbine). These data suggest that both the ₂-adrenoceptor and the PGE₂-receptor are linked to a common pathway. Since indometacin (10 mol/1) did not enhance evoked transmitter release, an influence of endogenous PG's on in vitro release of noradrenaline from rat brain cortex slices can be excluded.Abbreviation PG prostaglandin Send offprint requests to C. Allgaier at the above address     

Histamine H3A receptor-mediated inhibition of noradrenaline release in the mouse brain cortex

Summary Mouse brain cortex slices preincubated with ³H-noradrenaline were superfused with physiological salt solution containing desipramine plus a drug with ₂-adrenoceptor antagonist properties, and the effects of histamine receptor ligands on the electrically (0.3 Hz) evoked tritium overflow were studied. The evoked overflow (from slices superfused with phentolamine) was inhibited by histamine (pIC₃₅ 6.53), the H₃ receptor agonist R-(–)--methylhistamine (7.47) and its S-(+)-enantiomer (5.82) but not influenced by the H₁ receptor agonist 2-(2-thiazolyl)-ethylamine 3.2 mol/l and the H₂ receptor agonist dimaprit 10 mol/l. The inhibitory effect of histamine was not affected by the H₁ receptor antagonist dimetindene 1 mol/l and the H₂ receptor antagonist ranitidine 10 ol/l. The concentration-response curve of histamine (determined in the presence of rauwolscine) was shifted to the right by the H₃ receptor antagonists thioperamide (apparent pA₂ 8.67), impromidine (7.30) and burimamide (6.82) as well as by dimaprit (6.16). The pA₂ values of the four drugs were compared with their affinities for H_3A and H_3B binding sites in rat brain membranes (West et al. 1990 Mol Pharmacol 38:610); a significant correlation was obtained for the H_3A, but not for the H_3B sites. The results suggest that noradrenaline release in the mouse brain cortex is inhibited by histamine via H_3A receptors and that dimaprit is an H₃ receptor antagonist of moderate potency. Send offprint requests to E. Schlicker at the above address     

Sympathomimetic inhibition of noradrenaline release: Mediated by prostaglandins?

Summary In isolated perfused rabbit hearts, the effec of oxymetazoline and phentolamine on the stimulation-induced overflow of noradrenaline was tested after the synthesis of prostaglandins had been blocked by indomethacin. Indomethacin changed neither the decrease of overflow caused by oxymetazoline nor the increase caused by phentolamine. It is concluded that prostaglandins are not involved to any major degree in the modulation of noradrenaline release by drugs with affinity to -receptors, or in the feed-back inhibition of transmitter release mediated by the effect of liberated noradrenaline on -receptive sites.     

H3 receptor-mediated inhibition of noradrenaline release: an investigation into the involvement of Ca2+ and K+ ions,G protein and adenylate cyclase

The effects of ₂-adrenoceptor agonists (dexmedetomidine, oxymetazoline), alone or in combination with various -adrenoceptor subtype-selective antagonists (CH-38083, idazoxan, WB4101, BRL44408, ARC-239, prazosin), on noradrenaline- and isoprenaline-induced lipolysis were investigated in human isolated abdominal subcutaneous fat cells. The rank order of potency of antagonists in preventing dexmedetomidine- and oxymetazoline-evoked suppression of isoprenaline-induced lipolysis was (pA₂-values): CH-38083 (7.69 and 7.48) idazoxan (7.5 and 7.41) > BRL 44408 (7.23 and 7.19) WB 4101 (7.13 and 7.12) > prazosin (5.18 and 5.17) > ARC-239 (4.72, 4.9). While CH-38083 and idazoxan, non-subtype selective ₂-adrenoceptor antagonists and BRL44408, a selective a_2A-adrenoceptor antagonist as well as WB4101 potentiated the lipolytic effect of noradrenaline, ARC-239, the selective _2B-adrenoceptor antagonist failed to affect it. In addition since the _2A-adrenoceptor selective agonist, oxymetazoline concentration dependently inhibited the lipolytic effect of isoprenaline, and WB4101 and BRL44408 (a_2A-adrenoceptor antagonists) antagonised the effect of oxymetazoline in a competitive manner, it is concluded that the a_2A-adrenoceptor subtype is involved in antilipolysis. In addition, functional evidence was obtained that there is an interaction between _2A- and -adrenoceptors located on the cell surface of adipocytes, through which locally released noradrenaline and/or circulating circulating adrenaline influence lipolysis.On leave from Institute of Medical Pharmacology, University of Pisa, Via Roma 55, I-5626 Pisa, Italy Correspondence to: E. S. Vizi at the above address     

Chronic inhibition of monoamine oxidase type A increases noradrenaline release in rat frontal cortex

Summary Chronic but not acute treatment of rats with MAO inhibitory, as with other antidepressant drugs, has been shown to down-regulate the number of cerebro-cortical ß-adrenoceptors. In order to establish whether this effect is associated with an increase in cortical noradrenaline release, rats were treated for 1, 3 or 21 days with clorgyline (2 mg/kg i.p. single injection; 1 mg/kg i.p. repeated injections), and the frontal cortex was then perfused by microdialysis in the awake animal. Control animals were injected with saline.The concentration of noradrenaline in the microdialysate increased only slightly after 1 or 3 days of clorgyline treatment but increased fourfold over control levels after 21 days treatment. Yohimbine (20 mol/1) added to the perfusing solution caused a similar degree of enhancement in microdialysate noradrenaline concentration in all groups of rats. Tetrodotoxin (10 mol/1) reduced noradrenaline concentration to low levels in all groups of animals, but noradrenaline was still detectable in the microdialysate in rats treated with clorgyline for 21 days. Concentrations of the deaminated metabolites dihydroxyphenylacetic acid, dihydroxyphenylglycol and methoxy-hydroxyphenylglycol were lowest after the 21 day clorgyline treatment. Determination of enzyme activity ex vivo showed that MAO-A was inhibited more than 95% by all clorgyline treatments with less than 10% inhibition of MAO-B.The results indicate that cerebrocortical noradrenaline release increases gradually during chronic MAO inhibition. This may be the result of more complete inhibition of the enzyme with time, not detectable by the ex vivo assay, but shown by the progressive reduction in metabolite levels. The intraneuronal consequence of this effect could be increased vesicular packaging of noradrenaline, together with reduction in net uptake from the synaptic cleft because of increased axoplasmatic noradrenaline levels. Modification of neuronal firing rate may also play a role in the net change in cortical noradrenaline release. The increase in extracellular fluid noradrenaline concentration occurs over a time period similar to that required for ß-adrenoceptor down-regulation, and for the onset of clinical antidepressant action.Abbreviations DHPG dihydroxyphenylglycol - DOPAC dihydroxyphenylacetic acid - MHPG methoxy-hydroxyphenylglycol - TTX tetrodotoxin - MAO Monoamine oxidase     

Effects of imidazolines on noradrenaline release in rat isolated kidney

The aim of the present study was to investigate whether or not activation of imidazoline receptors modulates noradrenaline release in the rat isolated kidney. Kidneys were pre-exposed to ³H-noradrenaline and the renal nerves were stimulated with 6 pulses at 100 Hz. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. The imidazoline derivatives clonidine (1–1000 nmol/l) and moxonidine (10–1000 nmol/l) inhibited S-I outflow of radioactivity with an EC₅₀ of 6.8 nmol/l and 62.5 nmol/l and a maximum of 88% and 97%, respectively. The concentration response curves for clonidine and moxonidine were shifted to the right by the selective ₂-adrenoceptor antagonist rauwolscine (0.1 mol/l) in a parallel manner with identical pK_B's of 8.52 and 8.46, respectively. Furthermore, the -adrenoceptor agonist (–)--methylnoradrenaline (0.1–30nmol/l), which has no affinity for imidazoline binding sites, inhibited S-I outflow of radioactivity with an EC₅₀ of 2.4 nmol/l and a maximum of 94%. Rauwolscine (0.1 mol/l) again shifted the concentration response curve for this -adrenoceptor agonist to the right with a pK_B of 8.40. Moreover, the selective ₂-adrenoceptor antagonist 2-[2-(2-methoxy-1,4-benzodioxanyl)]imidazoline HCl(RX 821002,0.01 mol/l) shifted the concentration response curves for clonidine and moxonidine to the right with pK_B's of 9.46 and 9.18, respectively. The ₂-adrenoceptor antagonist 4-chloro-2-(2-imidazoline-2-ylamino)-isoindoline HCl (BDF 6143, 1–1000 nmol/l), which, in the presence of ₂-adrenoceptor blockade, has been shown to inhibit noradrenaline release through activation of imidazoline receptors, inhibited S-I outflow of radioactivity with an EC₅₀ of about 20.5 nmol/l (with 1000 nmol/l producing 60% inhibition). The inhibitory effects of BDF 6143 and clonidine were abolished after pretreatment of the kidneys with the irreversible ₂-adrenoceptor antagonist phenoxybenzamine (1 mol/l). However, RX 821002 did not alter and rauwolscine slightly antagonized the inhibitory effect of BDF 6143. It is concluded that the imidazoline derivatives clonidine and moxonidine as well as the phenylethylamine (–)--methylnoradrenaline inhibit noradrenaline release in rat isolated kidney exclusively through activation of prejunctional ₂-adrenoceptors. BDF 6143 inhibits noradrenaline release in rat isolated kidney through an ₂-adrenoceptor-independent, possibly an imidazoline receptor mechanism. Correspondence to: L. C. Rump at the above address     

Pharmacological characterization of the imidazoline receptor which mediates inhibition of noradrenaline release in the rabbit pulmonary artery

Summary Imidazoline receptors involved in modulation of noradrenaline release were characterized in the rabbit pulmonary artery preincubated with [³H]noradrenaline and superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Tritium overflow was evoked by transmural electrical stimulation.The ₂-adrenoceptor blocking imidazolines tolazoline, BDF 6100 [2-(2-imidazoline-2-ylamino)-isoindoline] and BDF 7572 (4,7-dichloro-derivative of BDF 6100) increased the electrically evoked ³H overflow; the concentration-response curves were bell-shaped. In contrast, two other imidazolines, i. e. moxonidine and clonidine, two guanidine derivatives structurally related to BDF 6100, i. e. aganodine and BDF 7579 [4-chloro(2-isoindolinel)-guanidine], as well as the catecholamine noradrenaline concentration-dependently inhibited the evoked ³H overflow. The concentration-response curves for moxonidine, clonidine, aganodine, BDF 7579 and noradrenaline were shifted to the right by rauwolscine. The apparent pA₂ value of rauwolscine against moxonidine was 8.22, whereas those against clonidine, aganodine, BDF 7579 and noradrenaline were in the range of 6.37–6.77 and, hence, considerably lower than reported for ₂-adrenoceptors. In the presence of rauwolscine an inhibitory effect was also observed with the ₂-adrenoceptor blocking imidazolines tolazoline, BDF 6100, BDF 7572, and the imidazolineST 587 [2-(2-chloro-5-trifluoromethylphenylimino)-imidazoline]; the rank order of potency of all guanidines and imidazolines investigated was: aganodine > BDF 7579 > BDF 7572 > BDF 6100 > clonidine > ST 587 > moxonidine > tolazoline. Amiloride, 1-benzylimidazole and histamine were ineffective. After irreversible blockade of -adrenoceptors by preexposure to phenoxybenzamine, evoked ³H overflow was still inhibited by aganodine, BDF 7579 and noradrenaline. Under this condition the maximal effects obtainable with the guanidines and in particular with noradrenaline were lower than in the presence of rauwolscine.These findings are compatible with our previous suggestion that imidazoline receptors mediating inhibition of noradrenaline release exist on the sympathetic nerve terminals of the rabbit pulmonary artery. Comparison of the present data with those obtained in other preparations containing imidazoline recognition sites revealed that those sites are different from the present ones. It is conceivable that the receptor characterized here represents an allosteric site of the ₂-adrenoceptor or a so far undescribed -adrenoceptor subtype since it can be activated not only by imidazolines and guanidines but also by noradrenaline and can be blocked by rauwolscine. Comparison of the properties of isoindolines substituted with either aminoimidazoline or guanidine reveals that the imidazolines (e.g. BDF 7572) possess both ₂-adrenoceptor antagonistic and imidazoline receptor agonistic properties, whereas the analogous guanidines (e. g. aganodine) are imidazoline receptor agonists as well as ₂-adrenoceptor agonists. Send offprint requests to M. Göthert at the above address     

Evidence that histamine H3 receptors are involved in the control of gastric acid secretion in the conscious cat

Summary In an attempt to assess the role of histamine H₃ receptors in the control of gastric acid secretion, the effects of the selective histamine H3 receptor agonist, (R)-methylhistamine and antagonist, thioperamide were evaluated in the conscious gastric fistula cat under basal conditions and against different stimuli. (R)-methylhistamine (0.05–0.2 mol/kg/h) was ineffective against spontaneous and dimaprit-induced acid secretion; it also did not reduce significantly pentagastr-ininduced acid output, but caused a dose-dependent (0.05–0.1 mol/kg/h) and significant inhibition of the acid response to 2-deoxy-d-glucose. Thioperamide (0.02–0.04 mol/kg/h) did not modify spontaneous acid secretion, whereas it evoked a significant enhancement of the acid response to submaximal doses (50 mg/kg i. v.) of 2-deoxy-d-glucose. Thioperamide completely reversed the inhibitory effect of (R)-methylhistamine against 2-deoxy-d-glucose-induced secretion, while leaving unaffected the inhibition induced by somatostatin. These data suggest that histamine H3 receptors may be involved in the control of acid secretion stimulated by indirectly acting secretagogues. Send offprint requests to G. Bertaccini at the above address     

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     

Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

Rationale The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.Objectives The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, ₂-adrenoceptor-mediated regulation) were characterized in the LC.Methods Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.Results NA in the LC decreased via local infusion of Ca²⁺-free medium (–42±5%) or the sodium channel blocker tetrodotoxine (TTX) (–47±8%) but increased (333±40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor -methyl-p-tyrosine (250 mg kg^–1, i.p.) and the vesicle depletory drug reserpine (5 mg kg^–1, i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the ₂-adrenoceptor agonist clonidine (0.1–100 M) decreased NA (E_max=–79±5%) in the LC, whereas the opposite effect (E_max=268±53%) was observed with the _2A-adrenoceptor antagonist BRL44408 (0.1–100 M). This suggests a tonic modulation of NA release through local _2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1–100 M) administered into the LC increased NA in the LC (E_max=223±40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.Conclusion Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.     

Dopaminergic modulation of hippocampal noradrenaline release

Summary ³H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with ³H-noradrenaline, continuously superfused in the presence of cocaine (30 mol/l) and subjected to electrical field stimulation. The electrically evoked tritium over-flow from the slices was reduced by 0.1 and 1 mol/l dopamine and apomorphine, but significantly enhanced by 10 mol/l apomorphine or by 0.1 and 1 mol/l bromocriptine. If the ₂-adrenoceptor antagonist yohimbine (0.1 mol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mol/l apomorphine and 1 mol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another ₂-antagonist, idazoxane (0.1 mol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (>1 mol/l) and bromocriptine (>0.01 mol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mol/l, each).At 1 mol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mol/l. Their marked facilitatory effects (50 to 60% increase at 1 mol/l) were reduced in the presence of idazoxane (0.1 mol/l) and almost abolished in the presence of 0.1 mol/l yohimbine, whereas the increase due to 1 mol/l (-)sulpiride persisted under these conditions.The evoked tritium efflux from rabbit hippocampal slices preincubated with ³H-serotonin was not affected by dopamine receptor agonists.From our results we conclude that hippocampal noradrenaline, but not serotonin release, is modulated via D2-dopamine receptors. In addition, our results provide evidence for more or less pronounced ₂-adrenoceptor agonistic properties of dopamine and ₂-adrenoceptor antagonistic properties of apomorphine, bromocriptine, SCH 23390 and bulbocapnine in this noradrenaline release model from CNS tissue.     

Inhibition of noradrenaline release via presynaptic 5-HT1Da receptors in human atrium

In segments of human right atrial appendages preincubated with [3^H]noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, we determined the effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on tritium overflow evoked by transmural electrical stimulation (2 Hz).Tritium overflow was inhibited by 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-McOT), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate (RU 24969) and sumatriptan. Yohimbine and oxymetazoline (in the presence of idazoxan) also inhibited tritium overflow. The inhibitory potency of the drugs was significantly correlated with their affinity for 5-HT_ID receptors in human brain and for cloned human 5-HT_1D and 5-HT_1D receptors, but not with their affinity for 5HT_1B, 5-HT_1E, 5-HT_1F, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT₃, 5-HT_5A, 5-HT_5B and 5-HT₇ receptors. The potency order 5-CT >5-HT >5-MeOT is opposite to the order of affinities reported for 5-HT₆ binding sites. The preferential 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (up to 0.3 M) and the selective 5-HT₄ receptor agonist cisapride (up to 1 M) failed to inhibit tritium overflow. L-694,247, a potent 5-HT_ID receptor agonist, did not inhibit tritium overflow, but counteracted the inhibitory effect of 5-HT. Ketanserin at a concentration which should block 5-HT_1Da but not 5-HT_1D receptors and methiothepin at a concentration which may be assumed to block both 5-HT_1D and 5-HT_1D receptors antagonized the inhibitory effect of 5-HT. Propranolol and ondansetron did not modify the 5-HT-induced inhibition of release. In conclusion, noradrenaline release in human right atrial appendages is inhibited via 5-HT receptors which are located on the noradrenergic axon terminals. These inhibitory presynaptic 5-HT receptors belong to the 5-HT_ID subfamily. The ability of ketanserin to antagonize the inhibitory effect induced by activation of these receptors suggests that they can be subclassified as 5-HT_1D.     

On the mechanism of alpha-receptor mediated modulation of 3H-noradrenaline release from slices of rat brain neocortex

Summary Brain cortical slices were superfused with Krebs-Ringer media and the effects of oxymetazoline (an -adrenoceptor agonist) and phentolamine (an -antagonist) on depolarization-induced ³H-NA release were examined. Depolarization was effected by various K⁺-concentrations or by electrical pulses.The effects of the -receptor agents on stimulated ³H-NA overflow appeared to be dependent on the strength of the depolarizing stimulus. Thus, at low K⁺-concentrations (13 or 26 mM) oxymetazoline decreased and phentolamine increased the stimulated overflow, while at 56 mM K⁺ little or no modulation was found. The agents acting on -receptors modulated ³H-NA release in a dose-dependent way (5 · 10^–8–10^–5 M).The lack of modulation by oxymetazoline of ³H-NA release induced by 56 mM K⁺ seems not to be due to a high concentration of NA released into the synaptic cleft, since reduction of the endogenous NA level by pretreatment with -methyl-para-tyrosine did not reveal such modulation.However, oxymetazoline was found to decrease 56 mM K⁺-induced ³H-NA release effectively, if the Ca²⁺-concentration in the medium was lowered from 1.2 to 0.2 mM. This suggests that -receptor mediated modulation of release may occur as a result of a change in Ca²⁺-availability to the depolarization-secretion process. In addition, hyperpolarization of nerve endings might be involved in the modulatory process, as concluded from calculations of the (theoretical) trans-membrane potential at various K⁺-concentrations.Although the -receptors modulating NA release seem to be localized presynaptically, their precise location remains uncertain. Experiments with tetrodotoxin suggested that the -receptor mediated modulation does not operate via a local interneuronal loop.     

Histamine H3 receptor-mediated inhibition of gastric acid secretion in conscious dogs

Summary The effect of (R)-methylhistamine (MH) and thioperamide (selective agonist and antagonist respectively of histamine H₃ receptors) was examined in conscious gastric fistula dogs to investigate the role of histamine H₃ receptors in the control of basal and stimulated gastric acid secretion. Intravenous infusion of MH at 0.3 and 0.6 mol/kg/h caused a significant reduction of the 2-deoxy-d-glucose (2-DG)-stimulated acid output, maximal inhibition being 60%. The inhibitory effect of MH was counteracted by thioperamide (0.1 mol/kg/h), which, by itself, did not modify the 2-DG-induced acid secretion. The increase in plasma gastrin levels induced by 2-DG was not significantly affected either by MH or by thioperamide. Under basal conditions MH (0.3 mol/kg/h) did not induce any significant change in acid secretion and in plasma gastrin levels; by contrast, thioperamide (0.1 mol/kg/h) produced a significant increase both in acid output and in plasma gastrin.These results suggest that activation of H₃ receptors can exert a negative control in stimulated acid secretion in conscious dogs, when cholinergic pathways to acid secretion are activated by 2-DG; moreover, the slight, but significant, stimulatory effect of thioperamide on basal acid output and basal plasma gastrin may be suggestive for a tonic inhibitory role of H₃ receptors in the regulation of basal acid secretion, however, a nonspecific effect of this drug cannot be excluded.Correspondence to G. Bertaccini at the above address     

In vivo release of neuronal histamine in the hypothalamus of rats measured by microdialysis

Summary Using an in vivo intracerebral microdialysis method coupled with an HPLC-fluorometric method, we investigated the extracellular level of endogenous histamine in the anterior hypothalamic area of urethaneanaesthetized rats. The basal rate of release of endogenous histamine in the anterior hypothalamic area measured by this method was 0.09 + 0.01 pmol/20 min. When the anterior hypothalamic area was depolarized by infusion of 100 mM K⁺ through the dialysis membrane or electrical stimulation at 200 A was applied through an electrode implanted into the ipsilateral tuberomammillary nucleus, histamine release increased to 175% and 188%, respectively, of the basal level. These increases were completely suppressed by removal of extracellular Ca²⁺. The basal release of histamine was also suppressed after infusion of 10^–6 M tetrodotoxin or i.p. administration of 100 mg/kg of -fluoromethylhistidine. On the other hand, 3-fold increase in the basal release was observed after i. p. administration of 5 mg/kg thioperamide. These results clearly indicate that both the basal and evoked release of histamine measured by our method are of neuronal origin. Send offprint requests to T. Mochizuki at the above address     

Effect of phorbol esters and polymyxin B on modulation of noradrenaline release in mouse atria

Summary Phorbol 12-myristate 13-acetate (PMA; 0.03, 0.1 and 1.0 mol/l), a protein kinase C activating phorbol ester, significantly enhanced the stimulation-induced (S-I) outflow of radioactivity at 5 Hz stimulation in mouse atria preincubated with [³H]-noradrenaline, whereas a phorbol ester which does not activate protein kinase C, phorbol 13-acetate (0.1 mol/l), had no effect. This suggests that protein kinase C may have a role in modulating sympathetic neurotransmission.Polymyxin B (7 and 21 mol/l), an inhibitor of protein kinase C, had no effect on the S-I outflow of radioactivity. However, it had a significant inhibitory effect in a concentration of 70 mol/l. Polymyxin B (21 mol/l) reduced the facilitation of the S-I outflow of radioactivity produced by PMA (0.03 mol/l), 8-bromo-cyclic AMP (90 mol/l), tetraethylammonium chloride (300 mol/l), and idazoxan (0.1 mol/l). Furthermore, when a higher frequency of stimulation was applied (10 Hz rather than 5 Hz), polymyxin B (21 pmol/1) by itself inhibited the S-I outflow of radioactivity.In the presence of a concentration of PMA (0.1 mol/l) that was maximally effective in enhancing the S-I outflow of radioactivity, both idazoxan (0.1 mol/l) and 8-bromocyclic AMP (90 mol/l) still enhanced the S-I outflow. This suggests that these agents are not operating through protein kinase C and further suggests that the inhibitory effect of polymyxin B on these agents cannot be due to inhibition of protein kinase C. The effects of clonidine on the S-I outflow were not affected by a maximally effective concentration of PMA (0.1 mol/l). These results suggest that protein kinase C is not involved in a ₂-adrenoceptor mediated modulation of noradrenaline release. Send offprint requests to I. F. Musgrave at the above address     

Alpha-receptor-mediated modulation of 3H-noradrenaline release from rat brain cortex synaptosomes

Summary The effects of oxymetazoline and noradrenaline (in the presence of desipramine) on the release of ³H-noradrenaline from rat brain cortex synaptosomes were studied using a superfusion technique. Both drugs (at 1M concentrations) were found to reduce the depolarization-induced (15 mM K⁺) release of ³H-noradrenaline. The release-modulating effect of noradrenaline was antagonized by phentolamine and yohimbine.The data provide direct evidence for the hypothesis that -receptors modulating the release of noradrenaline are localized on varicosities of central noradrenergic neurones.