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M Tonini

Publications and source records attributed to M Tonini.

At least 55 records · Page 3Linked to original sources
PubMed1998

Fluoroquinolone-induced motor changes in the guinea-pig isolated ileum.

The effects of norfloxacin and enoxacin were examined on spontaneous motor activity in the guinea-pig isolated ileum. Micromolar concentrations of both compounds caused a biphasic response consisting of relaxation followed by transient contraction. Relaxation to norfloxacin, which was unaffected by phentolamine, propranolol and hyoscine (each at 1 microM), was partially sensitive to tetrodotoxin (1 microM). This indicates that the response is partly mediated by non-adrenergic non-cholinergic (NANC) inhibitory nerves, and partly related to a direct action on the smooth muscle. Apamin (0.1 microM) and suramin (300 microM) inhibited norfloxacin-induced relaxations to an extent similar to that of tetrodotoxin. Conversely, NG-nitro-L-arginine (300 microM) was ineffective. In the presence of theophylline (100 microM) and 3-isobutyl-1-methylxanthine (10 microM), norfloxacin caused relaxation less effective than when added alone. Based on this observation, the NANC component of the relaxation apparently depends on ATP release, whereas the direct component might be due, at least in part, to phosphodiesterase inhibition. Norfloxacin-induced contractions were neurogenic and cholinergic in nature. They were reduced by indomethacin or S-ketoprofen (both at 0.01-1 microM) and suramin (300 microM), suggesting involvement of local prostaglandin production probably induced by ATP release. Previous findings revealed that norfloxacin acted as a non-competitive antagonist at enteric GABAA receptors. In this study the same property was shared by enoxacin against the contractile response to 3-aminopropane sulphonic acid (3-APS), a GABAA receptor agonist. In conclusion, fluoroquinolones exert inhibitory and excitatory effects in the guinea-pig ileum. These are mediated by ATP, prostaglandin and acetylcholine release that might underlie, at least in part, the alterations of gastrointestinal motility observed after fluoroquinolone administration. Furthermore, isolated intestinal preparations might be useful to predict the GABAA-antagonist potential of this class of compounds.

Animals↗
PubMed1998

Constipation: a common problem in patients with neurological abnormalities.

Constipation is a frequent complaint among patients with different neurological diseases. This review provides a brief account of the numerous conditions affecting the central, peripheral and intrinsic (enteric) nervous systems in which constipation can be the only clinical manifestation or a component of a complex syndrome. Recent neuropathophysiological acquisitions show that any structural or functional impairment of the intrinsic innervation of the gut, including both developmental (i.e., Hirschsprung's disease and intestinal neuronal dysplasia) and acquired (i.e., either degenerative or inflammatory neuropathies) disorders, can be associated with constipation. Constipation may also arise from derangements of the peripheral nervous system, including diabetes and primary chronic autonomic failure (pandysautonomias). Finally, in the central nervous system, a wide array of disorders (post-traumatic, degenerative, ischaemic or neoplastic) are recognized to determine bowel dysfunction, ultimately leading to constipation. Further understanding of the fine pathophysiological mechanisms through which the intrinsic and extrinsic nerve supplies to the digestive system are involved in idiopathic constipation or in diseases generating this symptom will hopefully lead to a better treatment of this frequent pathological condition.

Central Nervous System Diseases↗
PubMed1997

A re-appraisal of the nature of the atropine-resistant contraction to electrical field stimulation in the human isolated detrusor muscle.

We investigated whether in human isolated detrusor strips the atropine-resistant contractile response to electrical field stimulation was mediated by ATP (or a related purine), as previously shown in the urinary bladder of other mammalian species. Electrical stimulation (1-50 Hz for 5 s at 1 min intervals, 0.1 ms pulse width, 60 V) elicited reproducible, frequency-dependent twitch contractions, which were markedly reduced by atropine (10 microM). Tetrodotoxin (TTX: 1 microM) inhibited contractile responses to a similar degree. When applied together, atropine and TTX caused an inhibition which was superimposable to that caused by either drug alone. The TTX-resistant contractions were totally unaffected by omega-conotoxin GVIA (omega-CTX: 0.1 microM). The atropine-resistant contractions were unaffected by the P2-purinoceptor antagonists suramin (300 microM) and PPADS (30 microM), at concentrations which virtually suppressed the contractile response induced by applied ATP (10 microM(-1) mM). As previously described, antagonism of the ATP-induced contractions by suramin (30, 100, 300 microM) and PPADS (3, 10, 30 microM) was insurmountable, with apparent 'pA2' values (calculated at the lowest antagonist concentrations) of 4.9 and 5.2, respectively. It is concluded that, under our experimental conditions, the non-cholinergic (atropine-resistant) component of the excitatory transmission in the human detrusor is not mediated by neural release of ATP, in spite of the presence of excitatory P2-purinoceptors on the effector cells. The TTX- and omega-CTX-resistant, non-cholinergic component might be related to the release of unknown transmitter(s) through a mechanism independent of both Na+- and N-type Ca2+-channels. More likely, the atropine-resistant component may reflect direct smooth muscle excitation since the human detrusor has a very short chronaxie (Sibley 1984).

Adenosine Triphosphate↗
PubMed1997

Investigation into the contractile response of melatonin in the guinea-pig isolated proximal colon: the role of 5-HT4 and melatonin receptors.

1. The interaction of melatonin (N-acetyl-5-methoxytryptamine) with 5-hydroxytryptamine4 (5-HT4) receptors and/or with melatonin receptors (ML1, ML2 sites) has been assessed in isolated strips of the guinea-pig proximal colon. In the same preparation, the pharmacological profile of a series of melatonin agonists (2-iodomelatonin, 6-chloromelatonin, N-acetyl-5-hydroxytryptamine (N-acetyl-5-HT), 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT)) was investigated. 2. In the presence of 5-HT1/2/3 receptor blockade with methysergide (1 microM) and ondansetron (10 microM), melatonin (0.1 nM-10 microM), 5-HT (1 nM-1 microM) and the 5-HT4 receptor agonist, 5-methoxytryptamine (5-MeOT: 1 nM-1 microM) caused concentration-dependent contractile responses. 5-HT and 5-MeOT acted as full agonists with a potency (-log EC50) of 7.8 and 8.0, respectively. The potency value for melatonin was 8.7, but its maximum effect was only 58% of that elicited by 5-HT. 3. Melatonin responses were resistant to atropine (0.1 microM), tetrodotoxin (0.3 microM), and to blockade of 5-HT4 receptors by SDZ 205,557 (0.3 microM) and GR 125487 (3, 30 and 300 nM). The latter antagonist (3 nM) inhibited 5-HT-induced contractions with an apparent pA2 value of 9.6 GR 125487 antagonism was associated with 30% reduction of the 5-HT response maximum. Contractions elicited by 5-HT were not modified when melatonin (1 and 10 nM) was used as an antagonist. 4. Like melatonin, the four melatonin analogues concentration-dependently contracted colonic strips. The rank order of agonist potency was: 2-iodomelatonin (10.8) > 6-chloromelatonin (9.9) > or = N-acetyl-5-HT (9.8) > or = 5-MCA-NAT (9.6) > melatonin (8.7), an order typical for ML2 sites. In comparison with the other agonists, 5-MCA-NAT had the highest intrinsic activity. 5. The melatonin ML1B receptor antagonist luzindole (0.3, 1 and 3 microM) had no effect on the concentration-response curve to melatonin. Prazosin, an alpha-adrenoceptor antagonist possessing moderate/ high affinity for melatonin ML2 sites did not affect melatonin-induced contractions at 0.1 microM. Higher prazosin concentrations (0.3 and 1 microM) caused a non-concentration-dependent depression of the maximal response to melatonin without changing its potency. Prazosin (0.1 and 1 microM) showed a similar depressant behaviour towards the contractile responses to 5-MCA-NAT. 6. In the guinea-pig proximal colon, melatonin despite some structural similarity with the 5-HT4 receptor agonist 5-MeOT, does not interact with 5-HT4 receptors (or with 5-HT1/2/3 receptors). As indicated by the rank order of agonist potencies and by the inefficacy of luzindole, the most likely sites of action of melatonin are postjunctional ML2 receptors. However, this assumption could not be corroborated with the use of prazosin as this 'ML2 receptor antagonist' showed only a non-concentration-dependent depression of the maximal contractile response to both melatonin and 5-MCA-NAT. Further investigation with the use of truly selective antagonists at melatonin ML2 receptors is required to clarify this issue.

Animals↗
PubMed1997

Effects of mercuric chloride and methyl mercury on cholinergic neuromuscular transmission in the guinea-pig ileum.

The effects of mercuric chloride (HgCl2) and methyl mercury (MeHg) were examined on basal mechanical activity and electrically-induced neurogenic cholinergic contractions (twitch contractions) in longitudinal muscle-myenteric plexus strips from guinea-pig distal ileum. Both compounds at 0.33 microM slightly enhanced the amplitude of twitch contractions in approximately 50% preparations. This effect was probably due to facilitation of acetylcholine (ACh) release since 0.1 and 1 microM mercurials increased electrically-evoked tritium outflow from [3H]choline preloaded muscle layer with attached myenteric plexus. Conversely, higher mercury concentrations inhibited twitch contractions (HgCl2 IC50 = 21.3 +/- 6.4 microM; MeHg IC50 = 45.1 +/- 5.5 microM), as well as contractions to exogenous ACh (0.1 microM) in resting preparations, and concomitantly increased the basal tone. The former effects possibly reflected an antimuscarinic activity of mercury, while the latter was related to alterations of calcium homeostasis in the effector cells. Indeed, the effect of HgCl2 on basal tone was antagonized by the Ca2+ entry blocker nifedipine (3, 10, 30 nM), indicating Hg-induced facilitation of Ca2+ influx through voltage-dependent channels. On the whole, our results suggest that cholinergic neuromuscular transmission and Ca(2+)-dependent mechanisms underlying smooth muscle contractility are targets for mercury toxicity in the intestine.

Acetylcholine↗
PubMed1997

Cisapride enhances detrusor contractility and improves micturition in a woman with lazy bladder.

A 32-year-old woman with lazy bladder was treated with cisapride (10 mg q.i.d.) for three weeks, obtaining a progressive improvement of the symptomatology and urodynamic parameters, and the return to pretreatment conditions after drug withdrawal. This case agrees with the evidence that cisapride potentiates the release of acetylcholine from postganglionic cholinergic nerves in the human isolated detrusor by activating 5-HT4 receptors, i.e. through the same mechanism responsible for its gastrointestinal prokinetic action. It is concluded that selective 5-HT4 agonists could be potentially useful to improve bladder emptying in micturition disorders associated with detrusor hypocontractility.

Adult↗
PubMed1997

M4 muscarinic autoreceptor-mediated inhibition of -3H-acetylcholine release in the rat isolated urinary bladder.

A pharmacological analysis was carried out in the rat urinary bladder to assess the nature of muscarinic receptors subtypes functionally involved in the negative feedback mechanism regulating acetylcholine (ACh) secretion from postganglionic cholinergic nerve terminals and in smooth muscle contraction. Bladder strips were preincubated with 3H-choline, and the electrically evoked [3H]ACh release was detected simultaneously with contraction in the absence of acetylcholinesterase inhibitors. The effects were compared of seven muscarinic antagonists on [3H]ACh secretion (prejunctional effect) and muscle contraction (postjunctional effect). The rank order of postjunctional potencies (-log EC50) for the seven antagonists (atropine > 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > hexahydrosiladiphenidol hydrochloride (HHSiD) > tripitramine > pirenzepine > AF DX-116 > methoctramine) as well as their postjunctional affinity estimates (pA2) are in keeping with the notion that muscarinic receptors responsible for bladder contraction belong to the M3 subtype. The M3 subtype-preferring 4-DAMP and HHSiD did not discriminate between prejunctional and postjunctional effects. The M2/M4 subtype-preferring antagonists tripitramine, methoctramine and AF-DX 116 were more potent in facilitating the evoked [3H]ACh release than in inhibiting the contractile response. The rank order of prejunctional potencies was atropine > 4-DAMP > tripitramine > HHSiD > methoctramine > AF-DX 116 > pirenzepine, indicating the involvement of M4 receptors. Furthermore, when potency relationship was determined by correlating prejunctional-log EC50 values with published constants for cloned and natives muscarinic receptor subtypes, the correlations were significant for both M4 and M5 subtypes, but the best correlation found (P < .001) was for the M4 subtype. These findings suggest that the negative feedback mechanism inhibiting the release of ACh in the rat urinary bladder is mediated by prejunctional autoreceptors of the M4 subtype.

Acetylcholine↗
PubMed1996

Effects of ethanol administration on cerebral non-protein sulfhydryl content in rats exposed to styrene vapour.

Glutathione (GSH) and other non-protein sulfhydryls (NPS) are known to protect cells from oxidative stress and from potentially toxic electrophiles formed by biotransformation of xenobiotics. This study examined the effect of a simultaneous administration of styrene and ethanol on NPS content and lipid peroxidation in rat liver and brain. Hepatic cytochrome P450 and cytochrome b5 content, aniline hydroxylase and aminopyrine N-demethylase activities as well as the two major urinary metabolites of styrene, mandelic and phenylglyoxylic acids were also measured. Groups of rats given ethanol for 3 weeks in a liquid diet were exposed, starting from the second week, to 326 ppm of styrene (6 h daily, 5 days a week, for 2 weeks). In control pair-fed animals, styrene produced about 30% depletion of brain NPS and 50% depletion of hepatic NPS. Subchronic ethanol treatment did not affect hepatic NPS levels, but caused 23% depletion of brain NPS. Concomitant administration of ethanol and styrene caused a NPS depletion in brain tissue in the order of 60%. These results suggest that in the rat, simultaneous exposure to ethanol and styrene may lead to considerable depletion of brain NPS. This effect is seen when both compounds are given on a subchronic basis, a situation which better resembles possible human exposure.

Administration, Inhalation↗
PubMed1996

Dissociation of the ascending excitatory reflex from peristalsis in the guinea-pig small intestine.

Localized distension of the intestine evokes an ascending excitatory reflex and a descending inhibitory reflex in the circular muscle layer. The sequential activation of these two reflexes is believed to underlie the motor pattern of peristalsis, which is responsible for the co-ordinated propulsion of intestinal contents. In this study we have shown that the initiation of peristalsis involves mechanisms additional to those mediating the ascending excitatory reflex. A short length of guinea-pig small intestine was mounted in a partitioned organ bath so that the lumen was occluded by the partition, but neuronal continuity was maintained. The anal segment was distended by intraluminal fluid infusion to evoke a peristalsis; in the oral segment, an isotonic transducer was used to record circular muscle contractions due to ascending excitatory reflexes. Stepwise distension of the anal segment with 5 microliters increments at 10 s intervals, or with a large, single-step infusion, elicited both the ascending excitatory reflex and peristalsis, when carried out at 3 min intervals. The threshold volume for the ascending excitatory reflex was smaller than the threshold for peristalsis with either incremental or single-step distensions. The ascending excitatory reflex appeared with a shorter delay than peristalsis. Tetrodotoxin (0.6 microM) or hexamethonium (100 microM) added to the oral compartment abolished the ascending excitatory reflex but not peristalsis. These drugs abolished both the ascending excitatory reflex and peristalsis when added to the anal compartment. When stimuli were delivered at 1 min intervals, peristalsis failed completely after the first trial, but the ascending excitatory reflex persisted, at a slightly reduced amplitude. When the anal segment was distended to just-subthreshold volume, electrical field stimulation (0.25-0.5 ms, 1-5 Hz for 1 s), delivered at 3 min intervals, evoked ascending excitatory responses but not peristalsis. Higher frequency stimulation (10 Hz) consistently evoked both peristalsis and the ascending excitatory responses. When trains of electrical stimulation were repeated at 1 min intervals, peristalsis quickly failed, but the ascending excitatory response persisted, although reduced in amplitude. The initiation of peristalsis can be dissociated from the ascending excitatory reflex by its threshold volume, by the duration of distension or the intensity of electrical stimulation required, and by its susceptibility to fatigue with repeated mechanical or electrical stimuli. This suggests that the ascending excitatory reflex may be part of the mechanism underlying the initiation of peristalsis, but that additional mechanisms must also be involved. Peristalsis should not be regarded as a reflex response but rather as an all-or-nothing motor pattern, triggered by mechanical stimulation, similar to other co-ordinated motor patterns in vertebrates and invertebrates.

Animals↗
PubMed1996

Neural 5-HT4 receptors in the human isolated detrusor muscle: effects of indole, benzimidazolone and substituted benzamide agonists and antagonists.

1. In strips of human isolated detrusor muscle, the 5-hydroxytryptamine (5-HT) receptor (5-HT4) that mediates facilitation of neuromuscular cholinergic transmission was further characterized by using 5-HT and a series of ligands known for their 5-HT4 agonist (5-methoxytryptamine: 5-MeOT, cisapride, (R,S)-zacopride, BIMU 8) or antagonist (RS 23597, GR 125487, DAU 6285) properties. 2. In the presence of methysergide (1 microM) and ondansetron (3 microM) to isolate pharmacologically the 5-HT4 receptors, 5-HT (0.3 nM-1 microM), 5-MeOT (10 nM -30 microM), BIMU 8 (10 nM-3 microM), cisapride (0.1-10 microM) and (R,S)-zacopride (0.1-30 microM) potentiated cholinergic contractions to electrical field stimulation in a concentration-dependent manner. RS 23597 (10 nM-10 microM), a competitive 5-HT4 receptor antagonist in other systems, also showed agonist properties. The following rank order of potency as an agonist was obtained: 5-HT (pEC50 = 8.0) > RS 23597 (7.0) = BIMU 8 (6.9) > or = cisapride (6.6) > 5-MeOT (6.0) > or = (R,S)-zacopride (5.7). Relative to 5-HT (intrinsic activity = 1), 5-MeOT acted as a full agonist (1.03), while BIMU 8 (0.76), (R,S)-zacopride (0.61), RS 23597 (0.60) and cisapride (0.41) behaved as partial agonists. 3. The potentiation by 5-HT was competitively antagonized by the selective 5-HT4 receptor antagonist GR 125487 (0.3-3 nM) with a pA2 estimate of 9.75 (Schild slope of 1.09), and by DAU 6285 (1 microM; pK3 = 6.45). Additionally, GR 125487 (3 nM) antagonized the responses to 5-MeOT (pKB = 9.72) and reversed the potentiation induced by RS 23597. As an antagonist, RS 23597 (10, 30 and 100 nM) inhibited the response to 5-HT. In addition, 30 and 100 nM RS 23597 reduced the 5-HT response maximum by 30 and 50%, respectively. The pKB value calculated at 10 nM was 8.0. 4. Thus, in the human isolated detrusor muscle, the 5-HT4 receptors mediating facilitation of cholinergic neuromuscular transmission are activated by indoleamines (5-HT, 5-MeOT), substituted benzamide (cisapride, (R,S)-zacopride), benzoate (RS 23597) and benzimidazolone (BIMU 8) derivatives. The activities (in terms of both potency and efficacy) of most agonists, as well as the affinity estimates of the antagonists GR 125487 and DAU 6285, are comparable to those found in other peripheral tissues. Exceptions are RS 23597, which acted either as a partial agonist or as an antagonist of the response to 5-HT1 and 5-MeOT that showed an unusually low potency. The latter findings may be ascribed to differences in the efficiency of receptor coupling mechanisms and/or in the molecular structure (i.e. splice variants) of the 5-HT4 receptor.

Benzimidazoles↗
PubMed1996

Biochemical markers of neurotoxicity. A review of mechanistic studies and applications.

Neurotoxicology presents major challenges to the development of biological markers in accordance to conventional research strategies. Because of the inaccessibility of the nervous system, one of the proposed alternatives is the study of biochemical signals in peripheral tissues which can easily and ethically be obtained in humans, and which could represent surrogate indicators of equivalent parameters in the nervous tissue. Considerable scientific support to this approach is provided by the results of recent investigations in major areas of pharmacology and psychobiology. Studies examining parameters of neurotransmission and second messenger systems in peripheral blood cells, and variations in the peripheral body fluid content of endogenous substances reflecting nervous tissue dysfunction or damage are presented in this paper as examples of efforts toward rational development and validation of novel indicators of nervous system toxicity. Cholinergic muscarinic receptors and calcium signalling in peripheral blood lymphocytes, myelin basic protein in cerebrospinal fluid, and blood polyamines are discussed as potential surrogate indicators based on the results of in vitro or in vivo animal studies of neurotoxic metals (mercury, triethyltin), pesticides (disulfoton), drugs of abuse (d-fenfluramine) and model epileptogenic compounds (kainic acid). Data from investigations examining serum prolactin, type B monoamine oxidase (MAO-B) and dopamine beta-hydroxylase (DBH) in workers occupationally exposed to manganese, lead or styrene are also presented. Although research in this field is still at its very early stage, current evidence suggests that (i) certain neurochemical markers may be valuably used in animal studies as a complement to conventional laboratory tests to augment their sensitivity or predictivity; (ii) a mechanistic research approach is required to establish which markers offer the greatest promise for application in human biomonitoring.

Animals↗
PubMed1995

5-HT3 receptor involvement in descending reflex relaxation in the rabbit isolated distal colon.

In whole segments of rabbit distal colon with mucosa removed, descending reflex relaxations of the circular muscle (descending inhibition) elicited by inflating (0.1-1 ml) an intraluminal balloon, were partially antagonized by 100 microM hexamethonium and the 5-HT3 receptor antagonist, ondansetron (3 microM), and abolished by 1 microM tetrodotoxin. The inhibitory effects of hexamethonium and ondansetron were additive. Conversely, hexamethonium (100 microM) and ondansetron (3 microM) failed to reduce electrically induced non-adrenergic non-cholinergic (NANC) relaxations of colonic circular muscle. It is concluded that interneuronally released acetylcholine and 5-hydroxytryptamine (5-HT) activate descending inhibitory pathways supplying the circular coat, via nicotinic and 5-HT3 receptors, respectively. This evidence suggests a functional involvement of 5-hydroxytryptaminergic transmission in the descending inhibition of rabbit colon.

Animals↗
PubMed1995

Heterogeneity of cholinergic muscarinic receptors coupled to phosphoinositide metabolism in immature rat brain.

The effects of muscarinic agonists and antagonists on phosphoinositide (PtdIns) metabolism were examined in the cerebral cortex and brainstem of 7-day-old rats, in order to evaluate the role of muscarinic receptor subtypes in this process. Additionally, comparative experiments were performed in cortices from adult animals. Accumulation of [3H]inositol phosphates ([3H]InsPs) in [3H]inositol pre-labeled brain slices was taken as an index of PtdIns hydrolysis. In neonatal cortex, maximal stimulation induced by the full agonists acetylcholine, carbachol and methacholine was 8-10 fold over basal [3H]InsPs accumulation. The effect of the partial agonists bethanechol, pilocarpine and oxotremorine varied from 3 to 4 fold over basal. Smaller responses to cholinergic stimulation were found in the brainstem and in the adult cortex. In neonatal cortex, muscarinic antagonists inhibited the stimulatory responses with the following order of potency: 4-DAMP > pirenzepine > AF-DX 116 approximately p-F-HHSiD. Pirenzepine inhibition of full agonist-induced [3H]InsPs accumulation showed biphasic curves, with two thirds of the response being inhibited with high affinity. When partial agonists were used, the resulting pirenzepine curves were better described by interaction at one high affinity site. No differences were found between immature and adult rats in the effect of pirenzepine on [3H]InsPs accumulation induced by carbachol, methacholine, or bethanechol. Inhibition by pirenzepine of PtdIns hydrolysis induced by carbachol or methacholine showed biphasic curves also in the brainstem. In this area, only one third of the response was inhibited with high affinity, and p-F-HHSiD was more potent as an antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1995

Therapeutic potential of drugs with mixed 5-HT4 agonist/5-HT3 antagonist action in the control of emesis.

Drugs interacting with serotonin (5-hydroxytryptamine, 5-HT) receptors are of value in the treatment of several gastrointestinal disturbances. Selective 5-HT3 receptor antagonists (ondansetron, granisetron, tropisetron) are widely utilized to control emesis induced by chemotherapy and radiation, while agonists at 5-HT4 receptors (cisapride, renzapride, BIMU compounds) are endowed with gastrointestinal prokinetic action. Here we overview the therapeutic potential of drugs with potent mixed 5-HT4 agonist/5-HT3 antagonist properties (i.e. BIMU 1) in the management of anticancer therapy-induced emesis and of intestinal adynamic post-operative conditions associated with vomiting. In the former situation, the agonism at 5-HT4 receptors is expected to be of benefit via two possible mechanism: (i) inhibition of 5-HT release from enterochromaffin cells; (ii) restoration of anally driven peristaltic waves in the upper gastrointestinal tract. Moreover, 5-HT4 receptor-induced prokinetic activity may counteract colonic constipation, an unwanted effect which occurs in a number of patients treated with pure 5-HT3 receptor antagonists. Additionally, the above mentioned drugs might be of value in post-operative conditions associated with intestinal adynamia and emesis sensitive to 5-HT3 receptor blockade.

Animals↗
PubMed1995

The interaction of antidepressant drugs with central and peripheral (enteric) 5-HT3 and 5-HT4 receptors.

1. A combined study of receptor binding in central neuronal cell membranes and functional responses in isolated segments of guinea-pig small intestine allowed characterization of the interaction of four antidepressant drugs with central and peripheral 5-HT3 and 5-HT4 receptors. 2. Clomipramine, paroxetine and fluoxetine inhibited [3H]-DAU 6215 binding to 5-HT3 recognition sites in NG 108-15 cells with IC50 values in the range 1.3-4 microM. Litoxetine had an IC50 of 0.3 microM. The specific binding of [3H]-GR 113808 to 5-HT4 recognition sites in pig striatal membranes was inhibited by all four antidepressants with negligible potency (IC50 values > or = 20 microM). 3. In whole ileal segments, concentration-response curves to 5-HT were biphasic, with the high- and low-potency phases involving 5-HT4 and 5-HT3 receptors, respectively. Curves to 2-methyl-5-hydroxytryptamine (2-methyl-5-HT: a 5-HT3 receptor agonist) and 5-methoxytryptamine (5-MeOT: a 5-HT4 receptor agonist) were monophasic. All antidepressants were used at concentrations lacking anticholinoceptor properties, as demonstrated in both electrically stimulated longitudinal muscle-myenteric plexus preparations (LMMPs) and in unstimulated LMMPs following addition of acetylcholine (100 nM). 4. Fluoxetine (0.1-1 microM) and litoxetine (0.3-3 microM) antagonized both the high- and low-potency phases of the 5-HT curve. Schild analysis for the low-potency phase yielded pA2 estimates of 6.6 +/- 0.3 (Schild slope of 1.1) and of 6.6 +/- 0.1 (Schild slope of 1.1), respectively. At higher concentrations (3 microM), fluoxetine markedly inhibited the 5-HT response maximum. Clomipramine (10-300 nM) inhibited, by a mechanism independent of concentration, both phases of the 5-HT curve with a reduction of the maximum response. Paroxetine (1 microM) was ineffective on the high-potency phase, but caused a rightward shift of the low-potency phase (pKB: 6.1 +/- 0.01). 5. Responses to 2-methyl-5-HT were inhibited by 1 microM fluoxetine (pKB: 5.4 +/- 0.02). Like clomipramine(30 and 100 nM), litoxetine (1 and 3 microM) produced rightward displacements of 2-methyl-5-HT-induced contractions, which were virtually independent of antidepressant concentration (pKB values: 6.0 +/- 0.02 and 5.5 +/- 0.01, respectively). At higher concentrations, fluoxetine (3 microM) and clomipramine (300 nM)markedly reduced the 2-methyl-5-HT response maximum. Paroxetine (1 micro M) was ineffective.6. Responses to 5-MeOT were shifted to the right by fluoxetine (0.1-1 micro M) and litoxetine (1 and 3 microM)in a concentration-dependent manner. At higher concentrations, fluoxetine (3 microM) markedly reduced the 5-MeOT response maximum, an effect also observed with 100 and 300 nM clomipramine. Paroxetine(1 microM) was ineffective.7. In unstimulated LMMPs, the excitatory effects evoked by 5-HT, 2-methyl-5-HT and 5-MeOT and the antagonism produced by 300 nM clomipramine were comparable to those obtained in whole ileal segments. This suggests that 5-HT contained in the mucosa of whole preparations does not interfere with agonist-induced contractile responses and with the inhibitory effect of antidepressant drugs.8. In conclusion, our results show that clomipramine, fluoxetine, paroxetine and litoxetine possess low to moderate potency/affinity at both central and peripheral (enteric) 5-HT3 receptors. In contrast, all four antidepressants are virtually ineffective at central 5-HT4 receptors. Inhibition of 5-HT4 receptor mediated ileal contractions by fluoxetine, litoxetine and clomipramine may result from allostericant agonism or, more likely, from post-receptor blockade of second messenger generation. The interaction of antidepressants with central and peripheral 5-HT3 and 5-HT4 receptors may be relevant for both potential therapeutic action and adverse effects at gastrointestinal level.

Animals↗
PubMed1995

5-Hydroxytryptamine receptors that facilitate excitatory neuromuscular transmission in the guinea-pig isolated detrusor muscle.

1. In isolated detrusor strips from the guinea-pig urinary bladder, contractile responses to electrical field stimulation were mostly mediated by neurally released acetylcholine (ACh) and adenosine 5'-triphosphate (ATP). 2. 5-Hydroxytryptamine (5-HT) produced a concentration-dependent increase in the amplitude of stimulated detrusor strip contractions. The 5-HT concentration-response curve showed a biphasic profile: the high potency phase was obtained at sub-micromolar concentrations (10-300 nM), while the low potency phase in the range 1-30 microM. The maximum response of the first phase was 30% of the total 5-HT response. 3. Like 5-HT, the 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT: 0.3-100 microM), the 5-HT2 receptor agonist, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI: 30 nM-3 microM) and the 5-HT4 receptor agonist, 5-methoxytryptamine (5-MeOT: 0.1-30 microM) potentiated, though with lower potency, detrusor contractions. The resulting concentration-response curves were monophasic in nature. 2-Methyl-5-HT had a maximum effect comparable to that of 5-HT. By contrast, the maximal effects of DOI and 5-MeOT were only 20% and 30% of that elicited by 30 microM 5-HT, respectively. 4. The 5-HT3 receptor antagonist, granisetron (0.3 microM) had no effect on the high potency phase, but caused a rightward parallel shift of the low potency phase of the 5-HT curve (pKB = 7.3). Granisetron(0.3 microM) antagonized with comparable affinity (pKB = 7.1) 5-HT-induced responses after pharmacological isolation of 5-HT3 receptors with the 5-HT1/5-HT2 receptor antagonist, methiothepin (0.3 microM) and the 5-HT4 receptor antagonist, GR 125487 (30 nM). Granisetron (0.1, 0.3 and 1 microM) competitively antagonized the potentiating effect of 2-methyl-5-HT with an estimated pA2 of 7.3.5. Methiothepin (0.3 microM) and the 5-HT2A receptor antagonist, ketanserin (0.3 microM) produced a slight inhibition of the first phase of the 5-HT curve. In the presence of ketanserin, an equimolar concentration of methiothepin was ineffective in further reducing the effect of 5-HT. Similarly, the 5-HT4 receptor antagonist, GR 125487 (30 nM) slightly inhibited the first phase of the 5-HT curve. Conversely, this phase was suppressed when detrusor strips were coincubated with ketanserin (or methiothepin) and GR125487.6. In a separate set of experiments, the interactions of 5-HT with either the purinergic or cholinergic components of excitatory neuromuscular transmission were investigated. In the presence of hyoscine(1 microM), 5-HT was mostly effective at sub-micromolar concentrations, while in the presence of the P2-purinoceptor antagonist, suramin (300 microM), 5-HT-induced potentiation was mainly obtained with micromolar concentrations.7. Thus, in electrically stimulated detrusor strips from guinea-pig, 5-HT potentiated excitatory neuromuscular transmission by activating at least three separate neural 5-HT receptors. These include the 5-HT2A and 5-HT4 receptors, which mediate the 5-HT high potency phase mainly by activation of purinergic transmission. On the other hand, the potentiating effect caused by micromolar concentrations of 5-HT mostly involves cholinergic transmission and is mediated by the 5-HT3 receptors.

Acetylcholine↗
PubMed1994

The role of ascending excitatory and descending inhibitory pathways in peristalsis in the isolated guinea-pig small intestine.

1. The effects of experimental manipulations to alter the activation of ascending excitatory and descending inhibitory reflexes on peristalsis were studied in isolated segments of guinea-pig small intestine. 2. The normal site of initiation of the peristaltic contraction, the oral end, was always shifted to a point just anal to a crush which interrupted enteric neuronal pathways. 3. Shortening the functional length of the intestine by a series of crushes, thus effectively reducing the length of enteric neuronal reflex pathways, led to a progressive increase in the threshold distension for triggering the peristaltic contraction. 4. A sleeve placed around the intestine to prevent it from being distended by fluid led to a shift in the site of initiation of the peristaltic contraction to a point just anal to the sleeve. Furthermore, sleeves placed in the anal half of the intestine were able to stop propagation of the contraction. 5. The effect of these manipulations on peristalsis suggests that ascending excitatory and descending inhibitory enteric pathways, activated by distension, determine the pattern of peristaltic activity. The peristaltic contraction is initiated at the oral end as a result of the summation of ascending excitatory inputs and the relative absence of inputs from descending inhibitory pathways at this point. The magnitude of the distension for triggering this contraction is determined by a balance between ascending excitatory and descending inhibitory inputs to the circular muscle. Propagation of the circular muscle contraction requires the activation of ascending excitatory pathways at each point along the intestine and the sequential inactivation of the descending inhibitory reflex pathways anal to the contraction. The propagation of the circular muscle contraction stops when there is no longer a sufficient distension stimulus ahead.

Animals↗