A pharmacological analysis of receptors mediating the excitatory response to 5-hydroxytryptamine in the guinea-pig isolated trachea

5-HT2A, 5-HT1B/D, 5HT3 and 5-HT7 receptors as mediators of serotonin-induced direct contractile response of bovine airway smooth muscle

Background: Serotonin (5-hydroxytryptamine; 5-HT) performs a variety of functions in the body including the modulation of muscle tone in respiratory airways. Several studies indicate a possible role of 5-HT in the pathophysiology of bronchial hyperresponsiveness. However, the receptors and the molecular mechanisms by which 5-HT acts on airway smooth muscle (ASM) continue to be controversial. Most of the evidence suggests the participation of different subtypes of receptors in an indirect response. This study supports the proposal that 5-HT directly contracts ASM and characterizes pharmacologically the subtypes of serotonergic receptors involved. The characterization was carried out by using selective antagonists in an organ bath model allowing study of the smooth muscle of segments of bovine trachea. Results: The results obtained show that 5-HT_2A receptors are the main mediators of the direct contractile response of bovine ASM, with the cooperation of the 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors. Also, it was observed that the muscle response to serotonin is developed more slowly and to a lesser extent in comparison with the response to cholinergic stimulation. Conclusion: Overall, the receptors that mediate the direct serotonergic contraction of the smooth muscle of the bovine trachea are 5-HT_2A, 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors.

The extract from Nandina domestica THUNBERG inhibits histamine- and serotonin-induced contraction in isolated guinea pig trachea

Although the fruit of Nandina domestica THUNBERG (ND) has been used to treat respiratory disorders such as coughing and breathing difficulty in Japan for many years, very little is known about mechanisms underlying its action. In the present study, we investigated effects of the crude extract from ND (NDE) and one of its constituents, nantenine, on contractile responses in isolated guinea pig tracheal ring preparations. In normal experimental condition, guinea pig trachea remained tonically contracted during the resting state, and addition of NDE (1 mg/ml) caused a relaxation of tracheal smooth muscles, but had little effect on the responsiveness of trachea to acetylcholine. The basal, tonic contraction was abolished by the presence of atropine and indomethacin. In this condition, NDE at 0.1-1 mg/ml inhibited histamine-induced contraction in both competitive and non-competitive manners. NDE at 0.01-1 mg/ml inhibited serotonin-induced contraction in a competitive manner. Nantenine (2-20 microM) did not affect histamine-induced contraction, and slightly inhibited serotonin-induced contraction. These results suggest that NDE has inhibitory effects on tracheal smooth muscle contraction, and nantenine cannot account solely for this effect of NDE.

The effect of serotonin on airway transepithelial sodium ion pathways

The action of serotonin on ionic transport processes in the airways was revealed in a study of the isolated rabbit tracheal wall mounted in a Ussing apparatus. It was shown that the transepithelial electrical potential difference of the tissue was hyperpolarized transiently after mechanical stimulation by means of gentle rinsing of its mucosal side. This hyperpolarization was greatly influenced by the addition of serotonin and/or ambroxol to the stimulation (rinsing) fluid. By means of selective blocking of ion transport transepithelial pathways with amiloride and/or bumetanide, the effects of serotonin and/or ambroxol on the hyperpolarization after mechanical stimulation could be explained as changes of sodium ion currents. The importance of stimulated ionic currents for airway clearance and for the efficacy of drugs acting on airways is postulated.

Influence of fluoxetine and litoxetine on 5-HT4 receptor-mediated relaxation in the rat isolated oesophagus

The influence of two selective serotonin reuptake inhibitors (SSRIs), litoxetine and fluoxetine, has been studied on 5-HT4 receptor-mediated relaxation in the rat isolated oesophageal muscularis mucosae. In carbachol-precontracted oesophageal tissues, 5-hydroxytryptamine (5-HT) (0.1 nM-1 microM) induced concentration-dependent relaxations. Concentration-response curves were monophasic and reproducible. Litoxetine at concentrations without antimuscarinic properties (10 nM-1 microM) caused concentration-dependent relaxations, which reduced carbachol tone up to 37%. Higher litoxetine concentrations (3 microM-300 microM) were associated with marked relaxation up to the abolition of carbachol tone. The overall curve profile of litoxetine was biphasic in nature with a high (10 nM-1 microM) and a low (3 microM-300 microM) potency phase. Unlike 5-HT, the second curve of litoxetine was not reproducible, with a reduction involving mainly the low potency phase. Compared to litoxetine, fluoxetine caused minimal relaxation (less than 10% at 1 microM). Treatment of rats with parachlorophenylalanine (pCPA: 375 mg kg-1 per day, for two days), to deplete endogenous 5-HT stores, did not modify the relaxant effect of 5-HT, while it significantly reduced the high potency phase of litoxetine. In tissues from untreated rats, this phase was reduced by the 5-HT4 receptor antagonist GR 125487 (10 nM) to an extent similar (P = 0.20: ANOVA for continuous-by-class effects) to that induced by pCPA treatment. However, in tissues from pCPA treated animals GR 125487 (10 nM) exerted a slight but significant antagonism of litoxetine response (P = 0.037: ANOVA for continuous-by-class effects) mainly involving the high potency phase. In tissues from untreated rats, litoxetine (1 microM) increased the relaxant effects of 5-HT, while in tissues from pCPA treated animals it exerted a small but significant depression of the maximal response to 5-HT, without changing its potency value. Fluoxetine (1 microM) slightly, but significantly, antagonized the relaxant effect of 5-HT in an unsurmountable manner. In conclusion, litoxetine up to 1 microM relaxed the rat isolated oesophageal muscularis mucosae through a mechanism involving release of endogenous 5-HT, which in turn activates 5-HT4 receptors. However, based on results with GR 125487 in tissues from pCPA treated rats, a small component of litoxetine-induced relaxation may involve a direct activation of 5-HT4 receptors. It is unlikely that blockade of 5-HT reuptake can participate in the action of litoxetine, since fluoxetine, a 5-HT reuptake inhibitor equipotent to litoxetine, was ineffective in the same range of concentrations. The antimuscarinic activity of litoxetine, previously demonstrated in the isolated guinea-pig intestine, played a role at concentrations greater than 1 microM. The 5-HT-releasing action of litoxetine could account for the potentation by litoxetine of 5-HT-induced relaxation in tissues from untreated rats, which was reversed by pCPA treatment. Under these conditions, litoxetine depressed relaxations to high 5-HT concentrations only. In tissues from untreated rats, fluoxetine slightly but unsurmountably antagonized 5-HT-induced relaxations, thus confirming previous observations in the guinea-pig small intestine.

The serotonin uptake-enhancing drug tianeptine suppresses asthmatic symptoms in children: a double-blind, crossover, placebo-controlled study

Studies have shown that levels of free serotonin in plasma are increased in symptomatic patients with asthma. In addition, the concentration of free serotonin in symptomatic patients with asthma correlates positively with clinical status and negatively with pulmonary function. Thus, reducing the concentration of free serotonin in plasma might be useful in treating patients with asthma. We studied the effectiveness of tianeptine in treating patients with asthma. Tianeptine is the only drug known to be able to reduce levels of free serotonin in plasma and to enhance uptake by platelets. In this study, 69 children with asthma were assigned in randomized fashion to receive tianeptine and/or placebo in a double-blind crossover trial that lasted 52 weeks. Tianeptine provoked a dramatic and sudden decrease in both clinical rating and free serotonin plasma levels and an increase in pulmonary function.

Neuropharmacologic treatment of bronchial asthma with the antidepressant tianeptine: a double-blind, crossover placebo-controlled study

Studies have shown the levels of free serotonin in plasma are increased in symptomatic patients with asthma. In addition, the concentration of free serotonin in symptomatic children with asthma correlates positively with clinical status and negatively with pulmonary function (forced expiratory volume in 1 second [FEV1]). Thus, reducing the concentration of free serotonin in plasma may be useful in treating children with asthma. We studied the effectiveness of tianeptine in treating these patients. Tianeptine is the only drug known to be able to reduce the level of free serotonin in plasma and to enhance the uptake by platelets. Sixty-nine of the 82 children with asthma initially enrolled participated in this study. Children were randomized to receive tianeptine or placebo or both in a double-blind crossover trial. The trial lasted 52 weeks. Tianeptine provoked a dramatic and sudden decrease of both clinical rating and free serotonin plasma levels and an increase in pulmonary function.

In vitro pharmacological effects of S 12370 (2-[4-benzhydryloxypiperidinoethyl]isoxindole; an antibronchoconstrictor agent) in normal and sensitized tissue

The effects of S 12370 (2-[4-benzhydryloxypiperidinoethyl]isoxindole), were studied in vitro. In guinea pig isolated tracheal rings, S 12370 induced a similar competitive inhibition of the contractile responses produced by acetylcholine, histamine and serotonin. However, it did not affect the contractions induced by leukotriene D4 (LTD4), substance P and U 46619, a stable analogue of thromboxane A2. S 12370 induced a concentration dependent inhibition of the cholinergic component of the contraction induced by electrical field stimulation, whereas it did not influence the sustained nonadrenergic noncholinergic (NANC) excitatory response observed in guinea pig isolated bronchi. S 12370 did not influence the relaxations induced by prostaglandin E2, isoprenaline and salbutamol, and did not modify the nonadrenergic noncholinergic inhibitory response induced by electrical field stimulation. In isolated left atria, the negative inotropic effect of acetylcholine was competitively inhibited by S 12370. In binding experiments, S 12370 exhibited similar affinity for M1, M2, M3, M4 muscarinic receptors and also recognized 5-HT2 serotonin and H1 histamine receptor subtypes. In ovalbumin-sensitized animals, the contractile response of isolated tracheal rings produced by exposure to the allergen was not influenced by S 12370. Tracheal rings from sensitized animals preexposed in vitro to the allergen developed a hyporesponsiveness to beta-adrenoceptor stimulation. S 12370 prevented the inhibitory effect caused by ovalbumin immune sensitization in the relaxation to isoprenaline. In rat polymorphonuclear neutrophil (PMN) cells, S 12370 up to 10(-5) M did not inhibit the arachidonic acid metabolism. These results suggest that in guinea pig tracheal smooth muscle, S 12370 is a competitive inhibitor of muscarinic, serotonin and histamine receptors and can modulate the beta-adrenergic dysfunction induced by immune sensitization. S 12370 may present some therapeutic interest in inflammatory airway diseases.

Increased levels of free serotonin in plasma of symptomatic asthmatic patients

BACKGROUND

Previous research has shown that symptomatic asthmatic patients have increased levels of norepinephrine, epinephrine, dopamine, free serotonin, and cortisol in plasma when compared with asymptomatic patients.

OBJECTIVE

We investigated the relationship between plasma levels of catecholamines, free serotonin, and cortisol and clinical status and pulmonary function in symptomatic and asymptomatic patients with asthma.

METHODS

We compared clinical severity, spirometry, and neuroendocrine factors at weeks 0, 1, 2, 3, and 4 in 57 symptomatic (forced expiratory volume in one second [FEV1] < 70%) and 72 asymptomatic (FEV1 > 80%) asthmatic patients. We used multiple analyses of variance (repeated measures) to interpret the data. In addition, we used the Pearson Product Moment Test to investigate correlations among the different variables.

RESULTS

The clinical severity rating and levels of free serotonin, norepinephrine, epinephrine, dopamine, and cortisol were significantly higher in symptomatic asthmatic patients than those in asymptomatic patients (P < .001, in all cases). FEV1 was significantly lower in symptomatic patients than in asymptomatic patients. In symptomatic patients, the level of free serotonin correlated positively with the clinical severity rating (r = .564, P < .01) and negatively with FEV1 (r = -.959, P < .001). In addition, the clinical severity rating showed a negative correlation with FEV1 (r = -.359, P < .01). No significant correlations were found in asymptomatic patients.

CONCLUSION

Our finding that free serotonin was the only neuroendocrine factor closely associated with clinical severity and pulmonary function suggests that this factor plays an important role in the pathophysiology of acute asthma.

Interactions between acetylcholine and substance P effects on lung mechanics in the rabbit

The pharmacological mechanisms involved in the acetylcholine (ACh)- and substance P (SP)-induced changes in pulmonary mechanics were studied in isolated perfused rabbit lungs. Tracheal pressure (Ptr) and airflow were measured by a Fleisch pneumotachograph and pressure transducers. Air volume, lung resistance (RL) and dynamic compliance (Cdyn) were calculated. ACh induced a dose-dependent increase in Ptr and RL, and a decrease in Cdyn. These effects were strongly prevented by atropine, and partly by SR140333, a neurokinin NK1 receptor antagonist; SR48968, a neurokinin NK2 receptor antagonist; indomethacin and antihistaminics. Ketanserin had no significant protective effect against ACh. SP also induced concentration-dependent increases in RL and decreases in Cdyn. SR140333 and atropine strongly inhibited the effects of SP, while ketanserin, SR48968, antihistaminics and indomethacin did not protect the lungs against this drug. 5-hydroxytryptamine induced no significant change in lung mechanic parameters. Cumulative concentrations of histamine increased RL and decreased Cdyn. We conclude that ACh-induced changes in lung resistance and compliance are in part mediated by a direct effect on airway smooth muscle and in part by the stimulation of C fibers, by the release of histamine from mast cells and by the synthesis of arachidonic acid metabolites. In turn, the effects of SP on lung mechanics are partly due to cholinergic activation.

Effect of serotonin and calcium in normal and sensitized guinea pig isolated trachea

Tracheal strips from normal and actively sensitized guinea pigs were studied to determine the responses to serotonin (5-hydroxytryptamine, 5-HT; 1 nM-0.1 mM) and ouabain (0.1 microM-0.1 mM), and the effects of increasing the extracellular calcium (Cao) concentration on tonic contractions elicited by 5-HT. Sensitized trachea exhibited an increased responsiveness and sensitivity to 5-HT and ouabain. Increases in Cao to achieve final concentrations of 5, 10 and 20 mM caused concentration-related relaxations of normal and sensitized tissues contracted to a similar plateau level with 5-HT. Inhibition of the Na+/K(+)-ATPase by ouabain (10 microM) reversed the effects of Cao from relaxation to contraction in normal and sensitized tissues contracted with 5HT. Sensitized preparations showed reduced relaxations in response to Cao (10-20 mM), and sensitized, ouabain-treated, trachea showed augmented contractions to Cao (10-20 mM) when compared to normal tissues. These results demonstrate a decreased membrane-stabilizing effect of Cao in sensitized trachea and the implication of the Na+/K(+)-ATPase in the regulation of membrane stability by Cao, suggesting a possible relevance to those mechanisms underlying airway hyperreactivity.

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.