Inhibition of SK3 channels in the TE671 human medulloblastoma cell line by desipramine and imipramine

The TE671 human medulloblastoma cell line endogenously expresses SK3 channels. Using patch clamp, we tested the effects on this current of desipramine and imipramine. In both cases, we observed a complete, reversible and concentration-dependent block. The interaction of desipramine with the selective SK3 blocker, apamin, was studied in more detail. Co-application of desipramine and apamin at concentrations close to their IC(50) produced an additive effect that was significantly higher than that of each compound alone. This effect was also observed at IC(25) concentrations. Collectively, these data provide evidence against a common site of action for desipramine and apamin.     

Efflux of 86Rb from rat and mouse pancreatic islets: the role of membrane depolarization.

The efflux of 86Rb from rat or mouse perifused islets preloaded with the isotope has been used as an index of the potassium permeability of the islet beta-cell membrane. Cellular transmembrane potentials were altered by changing [K]O or by direct electrical stimulation and the effects on potassium permeability examined. Omission of KCl from the medium perifusing rat islets induced a biphasic change in 86Rb efflux, a brief decline being superseded by a pronounced increase in efflux. Re-introduction of KCl, 4.7 mM, caused a further increase in 86Rb efflux preceding a return to control values. Increasing [K]O from 4.7 mM to 10 mM, 20 mM or 47 mM caused a phasic increase in 86Rb efflux with the magnitude of both the peak and average rate of efflux being dependent upon the extent of the change in [K]O. The increase in 86Rb efflux produced by [K]O, 47 mM, was attenuated by Co2+, 2.56 mM (51% inhibition) or quinine, 10 microM (47% inhibition), but efflux remained significantly (P less than 0.001) above control values. Electrical stimulation of single microdissected mouse pancreatic islets by currents of 0.1 to 0.5 mA evoked a rapid, phasic increase in 86Rb efflux. The magnitude of the response was unaffected by EGTA, 2 mM, or nupercaine, 100 microM. These observations are discussed in relation to the mechanisms controlling the potassium permeability, membrane potential and insulin secretion of the pancreatic islet beta-cell.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of 5-hydroxytryptamine neuronal activity by the 5-HT agonist, DOI

Systemic, intra-raphe and microiontophoretic administration of the 5-hydroxytryptamine (5-HT)1C/5-HT2 agonist (1-(2,5-dimethoxy-4-iodophenyl)-2- aminopropane (DOI) inhibited the firing of 5-HT neurones in the dorsal raphe. DOI administered systemically and directly into the raphe also decreased the extracellular concentration of 5-hydroxytryptamine (5-HT) in the frontal cortex. In contrast, the administration of DOI directly into the frontal cortex did not significantly alter the concentration of frontal cortical extracellular 5-HT. The reduction of the firing rate of 5-HT neurons in the dorsal raphe and extracellular 5-HT concentration in the frontal cortex induced by systemic administration of DOI could not be blocked by the 5-HT2 antagonist ketanserin, ritanserin (5-HT2/5-HT1C antagonist) or the putative 5-HT1A antagonist, pindolol. These results suggest that the inhibition of 5-HT neuronal firing seen with administration of DOI is mediated via an action within the dorsal raphe and at least in close proximity to the 5-HT neurone cell bodies. The decrease in frontal cortical extracellular concentration of 5-HT release was not due to a direct action in the frontal cortex itself and may possibly be as a result of the decrease in the firing rate of the 5-HT neurones in the dorsal raphe. The mechanism of action of DOI to produce these effects is, however, unclear and warrants further investigation.     

Inhibition of human 5-HT(3A) and 5-HT(3AB) receptors by etomidate, propofol and pentobarbital

The actions of intravenous anaesthetics on 5-HT(3AB) receptors have not been studied. Using oocyte electrophysiology, the effects of etomidate, propofol, and pentobarbital on human 5-HT(3A) and 5-HT(3AB) receptors were studied and compared. Inhibition of peak currents by all three compounds in both receptor subtypes was anaesthetic concentration-dependant and non-competitive. Because the half-maximal inhibitory concentrations for etomidate, propofol and pentobarbital in 5-HT(3A) and 5-HT(3AB) receptors were all above their respective anaesthetic concentrations, the results of our study suggest that neither 5-HT(3) receptor subtype contributes to the anaesthetic actions of etomidate, propofol or pentobarbital.     

Characterization of the novel human serotonin receptor subunits 5-HT3C,5-HT3D, and 5-HT3E

Within the family of serotonin receptors, the 5-hydroxytryptamine-3 (5-HT(3)) receptor is the only ligand-gated ion channel. It is composed of five subunits, of which the 5-HT(3A) and 5-HT(3B) subunits are best characterized. Several studies, however, have reported on the functional diversity of native 5-HT(3) receptors, which cannot solely be explained on the basis of the 5-HT(3A) and 5-HT(3B) subunits. After our discovery of further putative 5-HT(3) serotonin receptor-encoding genes, HTR3C, HTR3D, and HTR3E, we investigated whether these novel candidates and the isoform 5-HT(3Ea) are able to form functional 5-HT(3) receptor complexes. Using immunofluorescence and immunoprecipitation studies of heterologously expressed proteins, we found that each of the respective candidates coassembles with 5-HT(3A). To investigate whether the novel subunits modulate 5-HT(3) receptor function, we performed radioligand-binding assays and calcium-influx studies in human embryonic kidney 293 cells. Our experiments revealed that the 5-HT(3C),5-HT(3D), 5-HT(3E), and 5-HT(3Ea) subunits alone cannot form functional receptors. Coexpression with 5-HT(3A), however, results in the formation of functional heteromeric complexes with different serotonin efficacies. Potencies of two agonists and antagonists were nearly identical with respect to homomeric 5-HT(3A) and heteromeric complexes. However, 5-HT showed increased efficacy with respect to 5-HT(3A/D) and 5-HT(3A/E) receptors, which is consistent with the increased surface expression compared with 5-HT(3A) receptors. In contrast, 5-HT(3A/C) and 5-HT(3A/Ea) receptors exhibited decreased 5-HT efficacy. These data show for the first time that the novel 5-HT(3) subunits are able to form heteromeric 5-HT(3) receptors, which exhibit quantitatively different functional properties compared with homomeric 5-HT(3A) receptors.     

Effects of ouabain on 86Rb-uptake, 3H-5-HT-uptake and aggregation by 5-HT and ADP in human platelets

In the search of sensitive models for actions of digitalis-like substances on intact cells or tissues, the effects of ouabain on human platelets were investigated. In a concentration-dependent manner ouabain 10(-8)-10(-4) M inhibited Na+-K+-ATPase activity measured as uptake of 86Rubidium (86Rb), with about 90% inhibition of the total uptake at ouabain greater than or equal to 10(-6) M. An almost identical concentration-effect curve was found for platelet uptake of 3H-serotonin (3H-5-HT). The platelet shape change reaction to exogenous 5-HT (1 X 10(-6) M) was suppressed by ouabain (10(-8)-10(-4) M) in a concentration-dependent manner, but with no clear maximum effect within the range tested. Aggregation induced by adenosine-di-phosphate (ADP 2 X 10(-6) M) was enhanced by ouabain 10(-8)-10(-6) M. At the highest concentration tested the rate of aggregation was increased by 31% and the change in light transmission by 54%. At low concentrations (less than 10(-9) M) of ouabain, there was a tendency towards increased aggregation as well as increased uptake of 86Rb, which may be a parallel to observations of positive inotropic effects of low concentration of glycosides, which do not inhibit Na+-K+-ATPase. The results show that human platelets can be used as a model tissue for studying effects of cardiac glycosides. This suggests that it may be useful for further investigations of the biological effects of agents with a similar effect profile, e.g. endogenous digitalis-like substances.     

Inhibition of 5-HT3 receptors by propofol: equilibrium and kinetic measurements

Patch-clamp/rapid solution exchange experiments as well as tracer ([14C]-guanidinium) influx measurements were applied to investigate effects of propofol on 5-HT3 receptor channels and compare the results with those obtained with pentobarbital. Currents induced by 30 microM 5-HT were recorded in outside-out patches from N1E-115 cells. Application of propofol 45 s before and during 5-HT application inhibited peak-currents and integrated current responses in a concentration-dependent manner (IC50 values=14.5 and 10.5 microM; Hill coefficients -1.5 and -1.3, respectively). The inhibitory effect of propofol in the current measurements was similar to the propofol-induced inhibition in tracer influx experiments in whole N1E-115 cells (Barann et al., 1993. Naunyn-Schmiedeberg's Archives of Pharmacology 347, 125-132). Pentobarbital-induced inhibition of 5-HT3 receptors in both patch-clamp (Barann et al., 1997. Neuropharmacology 36, 655-664) and tracer influx measurements indicated a lower potency and lower slope (IC50 values=130 and 55 microM; Hill coefficients -0.8 and -0.7, respectively) compared to propofol. Propofol, in contrast to pentobarbital, showed nearly the full potency when applied to the patches exclusively 45 s before 5-HT. Propofol was least effective when administered exclusively during 5-HT. The onset of inhibition of 5-HT-induced peak currents by propofol had a time constant of 220 ms, similar to the kinetics of 5-HT-induced desensitization.     

Modulation by 5-HT3 and 5-HT4 receptors of the release of 5-hydroxytryptamine from the guinea-pig small intestine

Summary The effects of agonists and antagonists of 5-hydroxytryptamine (5-HT) receptors on the release of endogenous 5-HT from enterochromaffin cells were studied in the vascularly perfused isolated guinea-pig small intestine. The experiments were done in the presence of tetrodotoxin in order to exclude a neuronally mediated influence on 5-HT release.The 5-HT₃ receptor agonist 2-methyl-5-HT increased 5-HT release, and this effect was antagonized by 1 nmol/l tropisetron. Nanomolar concentrations of tropisetron, MDL 72 222 and granisetron decreased 5-HT release. Ondansetron (0.1 and 1 mol/1) did not modify 5-HT release.5-Methoxytryptamine, BIMU8 and cisapride concentration-dependently inhibited 5-HT release. BIMU8 was more potent than 5-methoxytryptamine. Micromolar concentrations of tropisetron (1 and 10 mol/1) enhanced the release, whilst methiothepine (0.1 mol/l) did not affect the release of 5-HT.The results suggest that enterochromaffin cells of the guinea-pig ileum do not contain 5-HT₁ and 5-HT₂ receptors, but are endowed with 5-HT₃ and 5-HT₄ autoreceptors. Activation of the 5-HT₃ receptors triggers a positive feedback mechanism leading to an increase of 5-HT release. The 5-HT₃ receptors on the enterochromaffin cell differ from neuronal 5-HT3 receptors on guinea-pig myenteric plexus by their high affinity for tropisetron and MDL 72 222, and their very low affinity for ondansetron. Stimulation of 5-HT₄ receptors causes inhibition of release; the inhibitory 5-HT₄ receptor mechanism appears to predominate.Correspondence to H. Kilbinger at the above address     

Inhibition of 5-hydroxytryptamine-mediated behaviour by the putative 5-HT2 antagonist pirenperone

The effect of pirenperone, a putative 5-HT2 receptor antagonist, on various 5-HT-mediated behavioural responses has been examined. The head twitch response in mice, induced by administration of carbidopa (25 mg/kg) followed by 5-hydroxytryptophan (5-HTP) (200 mg/kg), was inhibited in a dose-dependent manner by pirenperone, with an ED50 of 76 micrograms/kg. The appearance of head weaving, forepaw treading and hind-limb abduction, which followed the administration of tranylcypromine (5 mg/kg) plus L-tryptophan (100 mg/kg) or quipazine (50 mg/kg) to rats, was also inhibited by pretreatment with pirenperone (100 micrograms/kg). Pirenperone did not alter the rate of 5-HT synthesis in the rat brain. Whilst pirenperone (100 micrograms/kg) did decrease methamphetamine-induced locomotor activity in rats, a dose of haloperidol producing a similar inhibition of this response did not alter the 5-HT-mediated behaviour. It is suggested, therefore, that the currently used 5-HT-induced behavioural models are 5-HT2 receptor-mediated.     

Ondansetron, a 5-HT3 receptor antagonist, partially attenuates the effects of amphetamine: a pilot study in healthy volunteers.

Preclinical studies suggest that 5-HT3 antagonists modulate dopamine-mediated responses in the limbic system and may therefore have a therapeutic role in psychiatry. We have examined the effect of ondansetron, a specific 5-HT3 antagonist, on the psychological and psychomotor changes induced by amphetamine in human volunteers. Nine healthy males took part in this double-blind placebo-controlled balanced-crossover study. Each subject received one of three treatments in a randomised manner: (a) placebo/placebo; (b) placebo/amphetamine (15 mg); (c) ondansetron (4 mg)/amphetamine (15 mg). Subjects were assessed for self-ratings of hunger, mood, energy, alertness, restlessness, irritability, and asked to rate the abnormality of their overall subjective state. In addition, systolic blood pressure, and performance on psychomotor tests were repeatedly assessed. Although amphetamine did not cause any significant changes in self-rating of mood, energy, alterness, restlessness or irritability, it induced a significant increase in self-ratings for overall subjective state, and a significant decrease in self-ratings of hunger. Amphetamine also caused an increase in systolic blood pressure and a decrease in the mean time taken to complete the psychomotor tests. Pretreatment with ondansetron attenuated the effects of amphetamine on hunger and subjective state, but not on blood pressure or psychomotor performance tests. These findings suggest that in humans 5-HT3 receptor antagonists may partially modify the subjective effects of amphetamine, and are in keeping with results from animal studies that 5-HT3 receptor antagonists might affect neurotransmission within mesolimbic brain regions. However, it was not possible to exclude a pharmacokinetic interaction to explain the effects of ondansetron.     

Increased Absorption of Digoxin from the Human Jejunum Due to Inhibition of Intestinal Transporter-Mediated Efflux

BACKGROUND AND OBJECTIVES: The contribution of transport in the small intestine by the apically located efflux pump P-glycoprotein to variable drug absorption in humans is still poorly understood. We therefore investigated whether inhibition of intestinal P-glycoprotein-mediated efflux by quinidine leads to increased absorption of the P-glycoprotein substrate digoxin. METHODS: Using a multilumen perfusion catheter, we investigated the impact of P-glycoprotein inhibition on absorption of two compounds: the P-glycoprotein substrate digoxin and the marker for passive transcellular absorption antipyrine. Two 20cm adjacent jejunal segments were isolated with the multilumen perfusion catheter in seven healthy subjects. Unlabelled and deuterated digoxin and antipyrine, respectively, were simultaneously infused into either of the intestinal segments. One of the segments was additionally perfused with the P-glycoprotein inhibitor quinidine. Intestinal perfusates were collected for 3 hours, and drug concentrations were determined in the intestinal perfusates, plasma and urine. RESULTS: Quinidine did not affect the disposition of antipyrine. In contrast, coadministration of quinidine into one jejunal segment caused a considerable increase in the amount of digoxin absorbed from this segment compared with the absorption from the other quinidine-free segment (22.3 +/- 8.9% vs 55.8 +/- 21.2% of the dose; p < 0.05). Accordingly, the area under the plasma concentration-time curve and the maximum plasma concentration of digoxin were considerably higher when luminal quinidine was coadministered (p < 0.05 and p < 0.001, respectively). Differences in digoxin absorption from the two intestinal segments were also reflected by pronounced differences in urinary digoxin elimination (5.5 +/- 3.3% vs 19.2 +/- 8.1% of the dose; p < 0.01). CONCLUSIONS: P-glycoprotein inhibition in enterocytes increases systemic exposure of orally administered drugs that are P-glycoprotein substrates. These data highlight the importance of the small intestine as an active barrier against xenobiotics.     

Inhibition by 5-HT3 receptor antagonists of release of cholecystokinin-like immunoreactivity from the frontal cortex of freely moving rats

Summary Effects of the 5-HT₃ receptor antagonists, ondansetron and tropisetron, on the release of cholecystokipin-like immunoreactivity (CCK-LI) in rat frontal cortex were investigated in conscious, unrestrained rats using intracerebral microdialysis. The release of CCK-LI was augmented by perfusion with 100 g/ml veratrine and was fully Ca²⁺-dependent and tetrodotoxin-sensitive. Ondansetron and tropisetron, each at 0.1–1 mol/l, decreased concentration-dependently the veratrine-evoked efflux of CCK-LI. The reduction of CCK-LI output was approximately 30%u when the antagonists were infused at 0.1 mol/l.The data suggest that 5-HT₃ receptor antagonists prevent the release of CCK evoked by endogenous 5-hydroxytryptamine. These drugs may thus represent a novel therapeutic approach in disease states, like anxiety, in which an inappropriately high release of brain CCK or 5-hydroxytryptamine seems to be involved.Correspondence to M. Raiteri at the above address     

黄芪多糖对人神经母细胞瘤SH-SY5Y细胞的增殖作用研究

目的:研究黄芪多糖(APS)对人神经母细胞瘤SH-SY5Y细胞增殖的抑制作用。方法:以0(空白对照)、25、50、100 mg/ml APS培养细胞6、12、24 h后,MTT法测定细胞活力并计算抑制率。0(空白对照)、25、50、100 mg/ml APS培养细胞24 h后,Hoechst33258荧光染色,荧光显微镜下观察细胞核形态;流式细胞仪检测细胞周期分布与凋亡情况;Western blot法检测细胞磷酸化丝裂原活化蛋白激酶(ERK)1/2蛋白表达;酶联免疫吸附(ELISA)法测定细胞白细胞介素2(IL-2)、IL-6、IL-12含量。结果:与空白对照比较,100 mg/ml APS培养细胞6 h,50、100 mg/ml APS培养细胞12 h与25、50、100 mg/ml APS培养细胞24 h后细胞抑制率升高。50、100 mg/ml APS培养细胞24 h后,G0/G1期比例升高,G2/M、S期比例降低;IL-2、IL-6含量增加。25、50、100 mg/ml APS培养细胞24 h后细胞凋亡率升高;细胞核出现碎块状致密浓染,并有凋亡小体出现;细胞ERK1/2蛋白磷酸化水平降低,IL-12含量增加。以上差异均具有统计学意义(P<0.01或P<0.05)。结论:APS能抑制SH-SY5Y细胞增殖,阻滞细胞周期进程,诱导细胞凋亡,其机制可能与降低ERK蛋白磷酸化水平和升高细胞因子水平有关。     

Inhibition of the 5-HT-induced cardiogenic hypertensive chemoreflex by the selective 5-HT3 receptor antagonist ICS 205-930

Summary We investigated the effect of ICS 205-930 [(3-tropanyl)-1H-indole-3-carboxylic acid ester], a selective antagonist at 5-HT₃ receptors, on the cardiogenic hypertensive chemoreflex in the anaesthetized dog. The reflex was elicited by injection of 5-HT (12.5–1600 g) into the left cardiac ventricle and consisted of a dose-dependent systemic hypertension associated with a decrease in heart rate. ICS 205-930 (10, 30, and 100 g/kg i.v.) caused a displacement to the right of both the dose-response curves of 5-HT-induced blood pressure increase and heart rate reduction. Its blocking effects upon the action of 5-HT could be surmounted by increasing the dose of the agonist. The selective 5-HT₂ receptor antagonist, ketanserin (0.1 mg/kg i.v.) and the combined 5-HT₁ and 5-HT₂ receptor antagonist, methiothepin (0.1 mg/kg i.v.) had no influence on the hypertensive reflex. When the reflex was elicited by the ganglionic stimulant, 1,1-dimethyl-4-phenyl-piperazinium (DMPP; 100–1600 g), ICS 205-930 had no blocking effect. The results suggest that the 5-HT-induced cardiogenic hypertensive chemoreflex is mediated by 5-HT₃ receptors. Send offprint requests to H. Berthold at the above address     

Ondansetron,an antagonist of 5-HT3 receptors,antagonizes the anti-exploratory effect of caerulein,an agonist of CCK receptors,in the elevated plus-maze

Systemic treatment with caerulein (0.25–5 µg/kg SC), non-selective agonist of cholecystokinin (CCK) receptors, dose-dependently suppressed the exploratory behaviour of rats in an elevated plus-maze without producing remarkable changes in the locomotor activity of animals in an open field test. Ondansetron, a selective antagonist of 5-HT₃ receptors, increased the number of open arm entries in the plus-maze test only at a dose 10 µg/kg. The other doses of ondansetron (0.1, 1 and 100 µg/kg IP) did not significantly change either the locomotor activity or the exploratory behaviour of rats. Pretreatment of rats with ondansetron (at 10 µg/kg, but not at 0.1, 1 or 100 µg/kg) completely reversed the anti-exploratory effect of caerulein (5 µg/kg). The concomitant treatment with caerulein and ondansetron did not cause any major change in the locomotor activity of animals in open field. Consequently, we propose that 5-HT-ergic mechanisms are involved not only in the regulation of CCK release in the cerebral cortex and nucleus accumbens, but also in the modulation of the anti-exploratory effect of caerulein, a CCK agonist, in the elevated plus-maze.     

Inhibition of cholera toxin-induced intestinal secretion by the 5-HT3 receptor antagonist ICS 205–930

Summary The secretory effect of cholera toxin on the gut has been ascribed to the activation of submucosal 5-hydroxytryptamine receptors by 5-hydroxytryptamine released from the enterochromaffin cells. The hypothesis that neuronally located 5-HT₃ receptors are involved in cholera toxin-induced diarrhoea has now been tested. Intestinal secretion was stimulated in mice by oral administration of pure cholera toxin and the effects of ICS 205-930, a potent and selective antagonist at 5-HT₃ receptors, and of ketanserin, a compound that blocks 5-HT₂ receptors, were investigated.Oral administration of cholera toxin resulted in a significant accumulation of fluid in the intestine. Pretreatment of the animals with ICS 205–930 partly prevented this effect and a maximal reduction of about 50% was achieved with doses of 0.3 mg/kg i. p. and higher. Ketanserin had only minimal effects up to a high dose of 1 mg/kg i. p. when a 20% reduction of fluid accumulation was seen. The data support the view that activation of 5-HT3 receptors plays a major role in the secretory effect of cholera toxin in the gut.     

Pharmacological characterization of the 5-HT1A, 5-HT2 and 5-HT3 receptors in the bovine ciliary muscle

We aimed to investigate the effects of serotonin (5-hydroxytryptamine, 5-HT) on the bovine ciliary muscle and subsequently to characterize and identify the subtypes of 5-HT receptors involved in the serotonin-evoked contractility muscle. The binding of [3H]ketanserin, [3H]granisetron and [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) was analyzed. All labelled compounds bound with high affinity to a single site in the membrane preparations studied. The affinity (K(d)) of the binding site was 7.5+/-1.2 nM for [3H]ketanserin, 6.9+/-0.8 nM for [3H]granisetron and 4.4+/-0.31 nM for [3H]8-OH-DPAT. The density of receptors (B(max)) was 1062+/-43.0 fmol/mg protein for [3H]ketanserin, 566+/-2.32 fmol/mg protein for [3H]granisetron and 205+/-4.63 fmol/mg protein for [3H]8-OH-DPAT. The serotonin-induced contraction appeared to be competitively antagonized by ketanserin (0.1, 1 and 10 microM) and ondansetron (0.1, 10 and 100 microM) which produced a pA(2) value of 8.5+/-0.12 and 8.0+/-0.19, respectively. 8-OH-DPAT and 5-carboxamidotryptamine (5-CT) proved to be completely ineffective. We conclude that serotonin induces bovine ciliary muscle contraction via 5-HT(2) and 5-HT(3) receptors while the 5-HT(1A) receptors, although present, do not mediate the contractile response.     

Inhibition by Y-25130 of the von Bezold-Jarisch effect evoked by 5-HT or 2-methyl-5-HT in anesthetized rats]

Effect of Y-25130 on 5-hydroxytryptamine3 (5-HT3) receptors was investigated using the von Bezold-Jarisch effect (BJE) in anesthetized rats. Intravenous or intraduodenal administration of Y-25130 antagonized the BJE evoked by 5-HT and its effect was over 100 times more potent than that of metoclopramide. Y-25130 also completely blocked the BJE induced by 2-methyl-5-HT, a selective 5-HT3 receptor agonist. The BJE induced by 5-HT was not antagonized by spiperone, ketanserin, phenoxybenzamine, yohimbine and haloperidol, but antagonized by atropine. Atropine inhibited the bradycardia caused by electrical stimulation of the vagus nerve, but Y-25130 had no inhibitory effect. These results indicate that Y-25130 possesses a potent and selective 5-HT3 receptor antagonistic property.     

The inhibition of cholera toxin-induced 5-HT release by the 5-HT(3) receptor antagonist, granisetron, in the rat

1. The secretagogue 5-hydroxytryptamine (5-HT) is implicated in the pathophysiology of cholera. 5-HT released from enterochromaffin cells after cholera toxin exposure is thought to activate non-neuronally (5-HT(2) dependent) and neuronally (5-HT(3) dependent) mediated water and electrolyte secretion. CT-secretion can be reduced by preventing the release of 5-HT. Enterochromaffin cells possess numerous receptors that, under basal conditions, modulate 5-HT release. 2. These include basolateral 5-HT(3) receptors, the activation of which is known to enhance 5-HT release. 3. Until now, 5-HT(3) receptor antagonists (e.g. granisetron) have been thought to inhibit cholera toxin-induced fluid secretion by blockading 5-HT(3) receptors on secretory enteric neurones. Instead we postulated that they act by inhibiting cholera toxin-induced enterochromaffin cell degranulation. 4. Isolated intestinal segments in anaesthetized male Wistar rats, pre-treated with granisetron 75 microg kg(-1), lidoocaine 6 mg kg(-1) or saline, were instilled with a supramaximal dose of cholera toxin or saline. Net fluid movement was determined by small intestinal perfusion or gravimetry and small intestinal and luminal fluid 5-HT levels were determined by HPLC with fluorimetric detection. 5. Intraluminal 5-HT release was proportional to the reduction in tissue 5-HT levels and to the onset of water and electrolyte secretion, suggesting that luminal 5-HT levels reflect enterochromaffin cell activity. 6. Both lidocaine and granisetron inhibited fluid secretion. However, granisetron alone, and proportionately, reduced 5-HT release. 7. The simultaneous inhibition of 5-HT release and fluid secretion by granisetron suggests that 5-HT release from enterochromaffin cells is potentiated by endogenous 5-HT(3) receptors. The accentuated 5-HT release promotes cholera toxin-induced fluid secretion.     

Inhibition of 5-HT cell firing in the DRN by non-selective 5-HT reuptake inhibitors: studies on the role of 5-HT1A autoreceptors and noradrenergic mechanisms

Improved clinical antidepressant efficacy may result if the acute inhibition of 5-HT cell firing induced by antidepressants is prevented. Here we examined whether inhibition of 5-HT cell firing by non-selective 5-HT uptake inhibiting antidepressant drugs is reversed by a selective 5-HT_1A receptor antagonist. In addition, we examined whether concomitant blockade of NA uptake offsets the inhibition of 5-HT cell firing resulting from 5-HT uptake blockade. Antidepressants which block 5-HT uptake (paroxetine, clomipramine, amitriptyline, venlafaxine), all caused dose-dependent and complete inhibition of 5-HT cell firing. Desipramine, a selective NA uptake blocker, caused a slight reduction in firing. The selective 5-HT_1A receptor antagonist, WAY 100635, reversed the inhibition of 5-HT cell firing induced by clomipramine, amitriptyline, venlafaxine, and paroxetine, but not that induced by the α₁ adrenoceptor antagonist, prazosin. Desipramine, at a dose which increased extracellular NA in the DRN, reversed the effect of prazosin but did not alter the ability of paroxetine to inhibit 5-HT cell firing. Our data indicate that antidepressant drugs with 5-HT uptake blocking properties inhibit 5-HT cell firing via activation of 5-HT_1A autoreceptors, and do so irrespective of their effects on NA uptake. These data are discussed in relation to the application of 5-HT_1A receptor antagonists to enhance the clinical efficacy of antidepressant drugs. Received: 15 July 1996 / Final version: 11 November 1996