Effects of imidazoline derivatives on cholinergic motility in guinea-pig ileum: involvement of presynaptic alpha2-adrenoceptors or imidazoline receptors?

Dexmedetomidine Has Differential Effects on the Contractility of Equine Jejunal Smooth Muscle Layers In Vitro

α2 agonists are frequently used in horses with colic, even though they have been shown to inhibit gastrointestinal motility. The aim of this study was to determine the effect of dexmedetomidine on small intestinal in vitro contractility during different phases of ischaemia. Experimental segmental jejunal ischaemia was induced in 12 horses under general anaesthesia, and intestinal samples were taken pre-ischaemia and following ischaemia and reperfusion. Spontaneous and electrically evoked contractile activity of the circular and longitudinal smooth muscles were determined in each sample with and without the addition of dexmedetomidine. During a second experiment, tetrodotoxin was added to determine if the effect was neurogenic. We found that the circular smooth muscle (CSM) contractility was not affected by ischaemia, whereas the longitudinal smooth muscle (LSM) showed an increase in both spontaneous and induced contractile activity. The addition of dexmedetomidine caused a decrease in the spontaneous contractile activity of CSM, but an increase in that of LSM, which was not mediated by the enteric nervous system. During ischaemia, dexmedetomidine also mildly increased the electrically induced contractile activity in LSM. These results may indicate a stimulatory effect of dexmedetomidine on small intestinal contractility. However, the influence of dexmedetomidine administration on intestinal motility in vivo needs to be further investigated.

Analysing the effect of I1 imidazoline receptor ligands on DSS-induced acute colitis in mice

Imidazoline receptors (IRs) have been recognized as promising targets in the treatment of numerous diseases; and moxonidine and rilmenidine, agonists of I₁-IRs, are widely used as antihypertensive agents. Some evidence suggests that IR ligands may induce anti-inflammatory effects acting on I₁-IRs or other molecular targets, which could be beneficial in patients with inflammatory bowel disease (IBD). On the other hand, several IR ligands may stimulate also alpha₂-adrenoceptors, which were earlier shown to inhibit, but in more recent studies to rather aggravate colitis. Hence, this study aimed to analyse for the first time the effect of various I₁-IR ligands on intestinal inflammation. Colitis was induced in C57BL/6 mice by adding dextran sulphate sodium (DSS) to the drinking water for 7 days. Mice were treated daily with different IR ligands: moxonidine and rilmenidine (I₁-IR agonists), AGN 192403 (highly selective I₁-IR ligand, putative antagonist), efaroxan (I₁-IR antagonist), as well as with the endogenous IR agonists agmatine and harmane. It was found that moxonidine and rilmenidine at clinically relevant doses, similarly to the other IR ligands, do not have a significant impact on the macroscopic and histological signs of DSS-evoked inflammation. Likewise, colonic myeloperoxidase and serum interleukin-6 levels remained unchanged in response to these agents. Thus, our study demonstrates that imidazoline ligands do not influence significantly the severity of DSS-colitis in mice and suggest that they probably neither affect the course of IBD in humans. However, the translational value of these findings needs to be verified with other experimental colitis models and human studies.

Effect of DA-9701 on the Normal Motility and Clonidine-induced Hypomotility of the Gastric Antrum in Rats

Background/Aims

DA-9701 is a novel prokinetic agent. In the present study, we investigated the effect of DA-9701 on the motility of the gastric antrum in the normal and clonidine-induced hypomotility in an in vivo animal model.

Methods

A strain gauge force transducer was sutured on the gastric antrum to measure the contractile activity in rats. A total of 28 rats were subclassified into the 4 groups: (1) the placebo group, (2) the DA-9701 group, (3) the placebo group in the clonidine-pretreated rats, and (4) the DA-9701 group in the clonidine-pretreated rats. After the basal recording, either placebo (3% [w/v] hydroxypropylmethyl cellulose) or DA-9701 was administered. Contractile signals were measured after the administration and after a meal. In the clonidine-pretreated rats, either placebo or DA-9701 was administered. Contractile signals were measured after the administration and after a meal.

Results

Oral administration of DA-9701 did not significantly alter the motility index of the gastric antrum in the preprandial and postprandial periods, compared with the placebo group. The administration of clonidine decreased the motility index of the gastric antrum in the preprandial and postprandial periods, compared with the administration of placebo. This reduction of the antral motility by the administration of clonidine was not observed in the clonidine-pretreated DA-9701 group. The percentage of the motility index in the postprandial period was significantly greater in the clonidine-pretreated DA-9701 group, compared with the clonidine-pretreated placebo group.

Conclusions

DA-9701 improves the hypomotility of the gastric antrum induced by clonidine, suggesting its gastroprokinetic effect in the pathologic condition.

Pretreatment with clonidine caused desensitization to WIN 55,212-2 in guinea pig ileum

Considering the existence of cross-tolerance between clonidine and morphine besides the same interaction between morphine and WIN 55,212-2 persuaded us to verify this fact between WIN 55,212-2 and clonidine in guinea pig ileum, which is a well-known model to examine the mode of action of cannabinoids and α2 -adenoceptor agonists The rectangular pulses were passed to the 0.5 g stretched ileum segments that were fixed in 20-ml organ bath. PowerLab system and Graphpad Prism were applied to record twitches and analyse the data. Electrically evoked contractions were dose-dependently inhibited by WIN 55,212-2 and clonidine (pD2 = 8.56 ± 0.41 and 7.65 ± 0.15, respectively). Tolerance to this effect could be induced by 4-h incubation with WIN 55,212-2 (3 × IC50 ) (pD2  = 6.36 ± 0.26, degree of tolerance: 159.32) (P < 0.01) but not with clonidine (2 × IC50 and 4 × IC50 ) for different time courses. Dose-response curve for inhibitory action of WIN 55,212-2 was shifted to the right after 4-h incubation with clonidine (3 × 10(-10) m) comparing to the untreated tissues (pD2  = 5.26 ± 0.69, degree of tolerance: 2000) (P < 0.001). This observation provides the evidence for the cannabinoid-noradrenergic systems interaction in the enteric nervous system as a simplified representative for central nervous system.

Imidazoline receptor antisera-selected/Nischarin regulates the effect of agmatine on the development of morphine dependence

Agmatine, an endogenous ligand for imidazoline receptor, has been shown to prevent opioid dependence, but not much is known about the mechanisms of the effect of agmatine. In the present study, we investigated the function of I1 imidazoline receptor and its candidate protein imidazoline receptor antisera-selected (IRAS)/Nischarin in morphine dependence as well as in the effect of agmatine inhibiting morphine dependence by pharmacological and molecular approaches. Results showed that inhibition of IRAS or Nischarin did not change the development of morphine dependence in vitro and in vivo under the basal condition. Agmatine could reduce the cyclic 3', 5' adenosine monophosphate (cAMP) overshoot at the concentration of 0.01-10 µM in the primary cultured rat hippocampal neurons and attenuated the withdrawal signals and the elevation of FosB and ΔFosB at the dose of 5 mg/kg in the morphine-dependent mice. The effect of agmatine was inhibited by efaroxan (I1 imidazoline receptor non-specific antagonist) and the RNA interference against IRAS or Nischarin. These findings indicate that I1 imidazoline receptor or IRAS/Nischarin mediates the effect of agmatine on morphine dependence and provide evidence that I1 imidazoline receptor may be a new target for treating morphine dependence.

Effects of imidazolines on neurogenic contraction in isolated urinary bladder detrusor strips from rabbit

Moxonidine and clonidine, which are imidazoline compounds, are sympathetic modulators used as centrally acting antihypertensive drugs. Moxonidine, clonidine, and agmatine produce extensive effects in mammalian tissues via imidazoline recognition sites (or receptors) or α(2)-adrenoceptors. To investigate the effects of imidazolines on the function of the urinary bladder, we tested the effects of moxonidine, clonidine, and agmatine on the neurogenic contraction induced by electric field stimulation, and on the post-synaptic receptors in isolated urinary bladder detrusor strips from rabbit. Both moxonidine at 1.0-10.0 µmol/L and clonidine at 0.1-10.0 µmol/L inhibited electric-field-stimulation-induced contraction in a concentration-dependent manner, but not agmatine (10.0-1000.0 µmol/L). Both moxonidine and clonidine failed to affect carbachol or adenosine-triphosphate-induced contractions; however, 1000.0 µmol/L agmatine significantly increased these contractions. Our study indicates that (i) moxonidine and clonidine produce a concentration-dependent inhibition of the neurogenic contractile responses to electric field stimulation in isolated detrusor strips from male New Zealand rabbits; (ii) post-synaptic muscarinic receptor and purinergic receptor stimulation are not involved in the responses of moxinidine and clonidine in this study; (iii) the inhibitory effects of these agents are probably not mediated by presynaptic imidazoline receptors.

Both α2B- and α2C-adrenoceptor subtypes are involved in the mediation of centrally induced gastroprotection in mice

α(2)-adrenoceptors are known to mediate gastroprotective effect in both acid-dependent and acid-independent ulcer models. The aim of the present study was to determine, which of the three α(2)-adrenoceptor subtypes (α(2A), α(2B) or α(2C)) is responsible for this protection. Various α(2)-adrenoceptor agonists and antagonists were administered intracerebroventricularly (i.c.v.) to C57BL/6 mice with deletion of genes encoding the different subtypes. The gastric mucosal damage was induced by orally injected acidified ethanol. Both the non-selective α(2)-adrenoceptor agonist clonidine (0.3-2.8 nmol) and the α(2B/C)-adrenoceptor subtype preferring agonist ST-91 (0.5-11.5 nmol) induced dose-dependent gastroprotective effect in wild type, α(2A)-, α(2B)- and α(2C)-KO mice. In contrast, the α(2A)-adrenoceptor subtype agonist oxymetazoline (0.07-84 nmol i.c.v.) reduced only slightly the development of ethanol-induced ulcers. The effect of clonidine was antagonized by the non-selective antagonist yohimbine (25 nmol) and the α(2B/C)-adrenoceptor antagonist ARC 239 (10.4 nmol), but not by the α(2A)-adrenoceptor antagonist BRL 44408 (7.5 nmol). ARC 239 also reversed the effect of clonidine in α(2A)-, α(2B)- and α(2C)-KO mice, while the selective α(2C)-adrenoceptor antagonist JP 1302 (52 nmol) antagonized that only in α(2B)-KO, but not in α(2A)- and α(2C)-KO mice. These results suggest that α(2B)- and α(2C)-adrenoceptor subtypes can equally contribute to the mediation of gastroprotective effect induced by α(2)-adrenoceptor agonists in mice.

α(2)-adrenoceptor agonist-induced inhibition of gastric motor activity is mediated by α(2A)-adrenoceptor subtype in the mouse

The role of α(2)-adrenoceptors in regulation of gastric motility has been well documented. However, only few data are available on the adrenoceptor subtype that mediates this effect. The purpose of the present work was to identify the α(2)-adrenoceptor subtype(s) responsible for the inhibition of gastric motor activity in isolated fundus strip of the mouse. It was shown that (i) the electrically evoked contraction of the gastric fundus strip of the mouse was inhibited by the non-selective α(2)-adrenoceptor stimulant clonidine (EC(50): 0.019±0.001μM), the α(2A)-adrenoceptor subtype selective agonist oxymetazoline (EC(50): 0.004±0.001μM) and the α(2B)-adrenoceptor subtype preferring ST-91 (EC(50): 0.029±0.004μM), (ii) the inhibitory effect of clonidine (1μM), oxymetazoline (0.1μM) and ST-91 (1μM) on the contractions of gastric fundus strip was reversed by the non-selective α(2)-adrenoceptor antagonist idazoxan and α(2A)-adrenoceptor antagonist BRL 44408, but not by the α(2B/2C)-adrenoceptor antagonist ARC-239. (iii) Clonidine and ST-91 inhibited the electrically induced gastric contractions in C57BL/6 wild type mice as well as in α(2B)- and α(2C)-adrenoceptor deficient mice in a concentration-dependent manner; however, neither of them was effective in α(2A)-deficient mice. As a conclusion, it was first demonstrated that the inhibitory effect of α(2)-adrenoceptor agonists on the gastric motor activity of isolated stomach strip of the mouse is mediated purely by α(2A)-adrenoceptors.

Non-adrenergic inhibition at prejunctional sites by agmatine of purinergic vasoconstriction in rabbit saphenous artery

We investigated the effects of agmatine, clonidine, xylazine and moxonidine on the purinergic vasoconstriction induced by electrical stimulation in the rabbit isolated saphenous artery without endothelium. Transmural electrical stimulations induced reproducible responses in the arterial preparations, which were abolished by tetrodotoxin at 0.1 microM or pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS, 30 microM), but were not affected by 1 microM prazosin. Clonidine, xylazine and moxonidine induced transient and concentration-independent vasoconstriction, with threshold concentrations of 1, 3 and 30 microM, respectively. Agmatine, in contrast, did not produce any vascular response even at 1 mM. Lower concentrations of clonidine, xylazine and moxonidine (0.01-0.3 microM) concentration-dependently decreased vasoconstrictor responses to electrical stimulation, whereas agmatine (0.1-1 mM) induced an inhibitory followed by a facilitatory effect on electrically evoked responses. Agmatine, clonidine and moxonidine but not xylazine significantly enhanced the vasoconstriction elicited by 1 mM ATP. The concentration-response curve for NA was shifted to the left slightly by 1 mM agmatine, but not affected by 0.3 microM of other three agonists. Phenoxybenzamine did not affect the vasoconstrictive responses to 1mM ATP and to electrical stimulations, but abolished those to NA. Agmatine at 1mM evoked only an inhibitory effect on electrical stimulation-induced vasoconstriction in the preparation pretreated with phenoxybenzamine, and the inhibitory action was enhanced to 38.6% from the control value (without treatment with phenoxybenzamine) of 22.5%. The non-imidazoline compound xylazine at 0.3 microM lost its inhibitory effect on the neurogenic vasoconstriction in the presence of phenoxybenzamine. In conclusion, agmatine produces a biphasic effect on the purinergic vasoconstriction induced by sympathetic nerve stimulation in the rabbit isolated saphenous artery. The monophasic inhibition of agmatine in the artery treated with phenoxybenzamine is due to an alpha-adrenoceptor-independent mechanism at prejunctional sites, and the potentiation effect of agmatine is mainly dependent on its enhancement of vasoconstriction at postjunctional sites.

Effects of clonidine and tricyclic antidepressants on gastric smooth muscle contractility

To determine if and how clonidine and tricyclic antidepressants affect gastric contractility. Guinea pig fundic and antral circular muscle strips were studied in vitro. The effects of clonidine or amitriptyline added in graded concentrations on contractions to electric field stimulation (EFS), acetylcholine (ACh), and SP in the presence of N(epsilon)-nitro-l-arginine methyl ester (l-NAME) were studied. EFS produced frequency dependent contractions of fundic and antral muscle that were abolished by atropine or tetrodotoxin (TTX). ACh contractions were abolished by atropine but not TTX. Clonidine reduced contractile response to EFS but had no effect on ACh contractions. The threshold concentration of clonidine to inhibit EFS contractions was lower in the fundus than in the antrum. Amitriptyline reduced contractions to both EFS and ACh but not to SP. The threshold concentration of amitriptyline to inhibit EFS contractions was lower in the antrum than in the fundus. Both clonidine and amitriptyline affect gastric contractility. At threshold concentrations, clonidine affects fundic contractility whereas amitriptyline affects antral contractility. Clonidine affects gastric contractility in response to EFS but not to ACh, suggesting alpha-2 receptors on cholinergic nerves that reduce ACh release. Amitriptyline inhibits gastric contractility to EFS and ACh suggesting an inhibitory muscle effect.

Alpha-2-adrenoceptor hyporesponsiveness in isolated tissues of cholestatic animals: involvement of opioid and nitric oxide systems

In the present study, the status of alpha(2)-adrenoceptors during cholestasis was investigated by the inhibitory effect of clonidine on the electrically stimulated contractions of mice vas deferens (MVD) and guinea pig ileum (GPI). Clonidine inhibited the contractions in both tissues in a dose-dependent manner. Compared to unoperated animals, there was a significant right-shift in the clonidine concentration-curves of both tissues obtained from 5-day bile-duct ligated (BDL) animals (p < 0.01), implying the hyporesponsiveness of alpha(2)-adrenoceptors during cholestasis. Chronic treatment with naltrexone (3 mg/kg/day) reversed the right-shift induced by cholestasis in both tissues. Administration of N(omega)-nitro-L-arginine methyl ester (20 mg/kg/day) also partially reversed cholestasis-induced effect on IC(50) of clonidine. These two treatments had no effect on IC(50) of tissues from controls. Chronic yohimbine treatment (5 mg/kg/day) recovered the effect of cholestasis on MVD, but sensitized the ileum of unoperated and BDL guinea pigs to clonidine to a similar extent, providing evidence for the role of the augmented adrenergic state of cholestasis in the hyporesponsiveness of norepinephrine-releasing neurons of MVD. We concluded that cholestasis is associated with the decreased responsiveness of alpha(2)-adrenoceptors and the cholestasis-associated augmented opioidergic tone and increased NO production contribute to this process.

Altered prejunctional modulation of intestinal cholinergic and noradrenergic pathways by alpha2-adrenoceptors in the presence of experimental colitis

1 This study investigates the influence of intestinal inflammation on: (1) the control of intestinal neurotransmission and motility by prejunctional alpha(2)-adrenoceptors and (2) the expression of intestinal alpha(2)-adrenoceptors. Experimental colitis was induced by intrarectal administration of 2,4-dinitrobenzenesulphonic acid (DNBS) to rats. 2 UK-14,304 inhibited atropine-sensitive electrically evoked contractions of ileal and colonic longitudinal muscle preparations. UK-14,304 acted with similar potency, but higher efficacy, on tissues from DNBS-treated animals; its effects were antagonized with greater potency by phentolamine than rauwolscine. 3 Electrically induced [(3)H]noradrenaline release from ileal preparations was reduced in the presence of colitis. Tritium outflow was decreased by UK-14,304 and stimulated by rauwolscine or phentolamine: these effects were enhanced in preparations from animals with colitis. 4 Reverse transcription-polymerase chain reaction and Western blot assay demonstrated the protein expression of alpha(2A)-adrenoceptors in mucosal and muscular tissues isolated from ileum and colon. The induction of colitis increased alpha(2A)-adrenoceptor expression in both ileal and colonic muscular layers, without concomitant changes in mucosal tissues. 5 Induction of colitis reduced gastrointestinal propulsion of a charcoal suspension in vivo. In this setting, the gastrointestinal transit was inhibited by intraperitoneal (i.p.) UK-14,304 and stimulated by i.p. rauwolscine. After pretreatment with guanethidine, the stimulant action of rauwolscine no longer occurred, and UK-14,304 exerted a more prominent inhibitory effect that was antagonized by rauwolscine. 6 The present results indicate that, in the presence of intestinal inflammation, prejunctional alpha(2)-adrenoceptors contribute to an enhanced inhibitory control of cholinergic and noradrenergic transmission both at inflamed and noninflamed distant sites. Evidence was obtained that such modulatory actions depend on an increased expression of alpha(2A)-adrenoceptors within the enteric nervous system.

Affinity profile at alpha(1)- and alpha(2)-adrenoceptor subtypes and in vitro cardiovascular actions of (+)-boldine

The present study examines the functional and binding affinities of the aporphine alkaloid, (+)-boldine, at different alpha(1)- and alpha(2)-adrenoceptor subtypes, namely, alpha(1A) (rat vas deferens and kidney) and its L-like state (rabbit spleen), alpha(1B) (guinea pig spleen, mouse spleen and rabbit aorta), alpha(1D) (rat aorta and pulmonary artery), at possible subtypes of prejunctional alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium, alpha(2D) in guinea pig ileum, cloned human alpha(1)-adrenoceptor subtypes A, B and D and alpha(2)-adrenoceptor subtypes A, B and C as well as rat alpha(2D)-adrenoceptors. Additionally, we investigated its Ca(2+) channel antagonism in vascular and cardiac preparations. (+)-Boldine had higher affinity at alpha(1)-adrenoceptor subtype A (pA(2)=7.46, pK(i)=7.21) compared with its L-like state (pA(2)=5.63) or subtype B (pA(2)=5.98- 6.12, pK(i)=5.79) and subtype D (pA(2)=6.18-6.37, pK(i)=6.09). Its affinities at alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium (pA(2)=6.02, 6.36, 6.06, respectively) were identical, but lower at guinea pig ileum alpha(2D)-adrenoceptors (pA(2)=4.38). (+)-Boldine displayed nearly undistinguishable affinity at cloned human alpha(2)-adrenoceptor subtypes A, B and C (pK(i)=6.26, 5.79 and 6.35, respectively), whereas its affinity at rat alpha(2D)-adrenoceptors was low (pK(i)=4.70). In perfused rat kidney, (+)-boldine inhibited K(+)-evoked vasoconstriction at doses 70-fold higher than diltiazem. In guinea pig Langendorff heart, (+)-boldine (10(-5) - 2 x 10(-4) M) was equieffective in increasing coronary flow and in depressing cardiac force, while lower concentrations already depressed heart rate. In papillary muscles from guinea pig, (+)-boldine (10(-6) - 10(-5) M) mainly prolonged the duration of action potential at levels >30% of repolarization. These data reveal that (+)-boldine, except for its moderate selectivity (15 to 25-fold) for alpha(1A)-adrenoceptors, does not discriminate between the alpha(1)-adrenoceptor subtypes B and D and alpha(2)-adrenoceptor subtypes A, B and C, at which the drug consistently displays micromolar affinity. In vascular and cardiac preparations, (+)-boldine, although being at least 50-fold weaker than diltiazem, shows Ca(2+) channel antagonistic properties but no specificity for coronary dilatation relative to cardiodepression.

Alpha 2-adrenoceptors in the enteric nervous system: a study in alpha 2A-adrenoceptor-deficient mice

Mammals possess three types of alpha(2)-adrenoceptor, alpha(2A), alpha(2B) and alpha(2C). Our aim was to determine the type of alpha(2)-adrenoceptor involved in the control of gastrointestinal motility. In transmitter overflow experiments, myenteric plexus longitudinal muscle (MPLM) preparations of the ileum were preincubated with [(3)H]-choline and then superfused. The alpha(2)-adrenoceptor agonist medetomidine reduced the electrically evoked overflow of tritium from preparations taken from wild type but not alpha(2A)-adrenoceptor-knockout mice. In a second series of overflow experiments, MPLM preparations were preincubated with [(3)H]-noradrenaline and then superfused. Again medetomidine reduced the electrically evoked overflow of tritium from wild type but not alpha(2A)-knockout preparations. In organ bath experiments, medetomidine reduced electrically evoked contractions of segments of the ileum from wild type but not alpha(2A)-knockout mice. In each of these three series, phentolamine antagonized the effect of medetomidine in wild-type preparations with greater potency than rauwolscine. In conscious mice, gastrointestinal transit was assessed by means of an intragastric charcoal bolus. In alpha(2A)-knockout mice, the speed of gastrointestinal transit was doubled compared to wild-type. Medetomidine, injected intraperitoneally, slowed gastrointestinal transit in wild type but not alpha(2A)-knockout mice. We conclude that the cholinergic motor neurons of the enteric nervous system of mice possess alpha(2)-heteroreceptors which mediate inhibition of acetylcholine release, of neurogenic contractions and of gastrointestinal transit. The noradrenergic axons innervating the intestine possess alpha(2)-autoreceptors. Both hetero- and autoreceptors are exclusively alpha(2A). It is the alpha(2A)-adrenoceptor which in vivo mediates the inhibition of intestinal motility by the sympathetic nervous system.

Characterization of a novel mechanism accounting for the adverse cholinergic effects of the anticancer drug irinotecan

1. This study investigates the mechanisms accounting for the adverse cholinergic effects of the antitumour drug irinotecan. The activity of irinotecan and its active metabolite, 7-ethyl-10-hydroxy-camptothecin (SN-38), was assayed in models suitable for pharmacological studies on cholinergic system. 2. Irinotecan moderately inhibited human or electric eel acetylcholinesterase activity, SN-38 had no effect, whereas physostigmine blocked both the enzymes with high potency and efficacy. 3. Irinotecan and SN-38 did not affect spontaneous or electrically-induced contractile activity of human colonic muscle. Acetylcholine and dimethylphenylpiperazinium (DMPP) caused phasic contractions or relaxations, respectively. Physostigmine enhanced the motor responses elicited by electrical stimulation. 4. Although irinotecan and SN-38 did not modify the basal contractile activity of guinea-pig ileum longitudinal muscle strips, irinotecan 100 microM moderately enhanced cholinergic twitch contractions. Acetylcholine or DMPP caused phasic contractions, whereas physostigmine enhanced the twitch responses. Electrically-induced [(3)H]-acetylcholine release was reduced by irinotecan (100 microM) or physostigmine (0.1 microM). 5. Intravenous irinotecan stimulated gastric acid secretion in rats, but no effects were obtained with SN-38, physostigmine or i.c.v. irinotecan. Hypersecretion induced by irinotecan was partly prevented by ondansetron, and unaffected by capsazepine. In the presence of atropine, vagotomy and systemic or vagal ablation of capsaicin-sensitive afferent fibres, irinotecan did not stimulate gastric secretion. 6. The present results indicate that irinotecan and SN-38 do not act as specific acetylcholinesterase blockers or acetylcholine receptor agonists. It is rather suggested that irinotecan promotes a parasympathetic discharge to peripheral organs, mediated by capsaicin-sensitive vagal afferent fibres, and that serotonin 5-HT(3) receptors are implicated in the genesis of vago-vagal reflex triggered by irinotecan.

Effect of alpha-2 adrenoceptor antagonists on colonic function in rats

We studied the effect of alpha-2 adrenoceptor antagonists on colonic function stimulated by water-avoidance stress, 5-hydroxytryptamine (5-HT), bethanechol and castor oil by comparison with the effects of atropine and a 5-hydroxytryptamine 3 (5-HT3) receptor antagonist, ondansetron. Yohimbine, idazoxan and atropine, but not ondansetron, significantly inhibited water-avoidance stress-stimulated faecal excretion. Yohimbine and idazoxan inhibited neither 5-HT- nor bethanechol-stimulated faecal excretion. In contrast, atropine inhibited both 5-HT- and bethanechol-stimulated faecal excretion and ondansetron inhibited 5-HT-stimulated faecal excretion. Yohimbine did not inhibit the incidence of diarrhoea induced by castor oil, but idazoxan significantly inhibited diarrhoea observed during a 1-h period after the administration of castor oil. Both atropine and ondansetron inhibited diarrhoea during a 2-h period after the administration of castor oil. These findings suggest that alpha-2 adrenoceptor antagonists specifically inhibit colonic motor function stimulated by stress in rats.

Histamine H(3) receptors mediate inhibition of noradrenaline release from intestinal sympathetic nerves

1. The present study investigates whether presynaptic histamine receptors regulate noradrenaline release from intestinal sympathetic nerves. The experiments were performed on longitudinal muscle-myenteric plexus preparations of guinea-pig ileum, preincubated with [(3)H]-noradrenaline. 2. In the presence of rauwolscine, electrically-induced [(3)H]-noradrenaline release was inhibited by histamine or R-alpha-methylhistamine, whereas it was unaffected by pyridylethylamine, impromidine, pyrilamine, cimetidine, thioperamide or clobenpropit. The inhibitory effects of histamine or R-alpha-methylhistamine were antagonized by thioperamide or clobenpropit, but not by pyrilamine or cimetidine. In the absence of rauwolscine, none of these drugs modified the release of [(3)H]-noradrenaline. 3. The modulatory action of histamine was attenuated by pertussis toxin and abolished by N-ethylmaleimide. Tetraethylammonium or 4-aminopyridine enhanced the evoked tritium outflow and counteracted the inhibitory effect of histamine. However, the blocking effects of tetraethylammonium and 4-aminopyridine were no longer evident when their enhancing actions were compensated by reduction of Ca(2+) concentration in the superfusion medium. 4. Histamine-induced inhibition of tritium output was enhanced by omega-conotoxin or low Ca(2+) concentration, whereas it was not modified by nifedipine, forskolin, rolipram, phorbol myristate acetate, H7 or lavendustin A. 5. The present results indicate that presynaptic H(3) receptors, located on sympathetic nerve endings, mediate an inhibitory control on intestinal noradrenergic neurotransmission. It is suggested that these receptors are coupled to G(i)/G(o) proteins which modulate the activity of N-type Ca(2+) channels through a direct link, thus reducing the availability of extracellular Ca(2+) at the level of noradrenergic nerve terminals.