Effect of ICS 205-930 (a specific 5-HT3 receptor antagonist) on gastric emptying of a solid meal in normal subjects.

The effects on gastric emptying of a solid meal of the specific 5-HT3-receptor antagonist ICS 205-930, 10 mg and 20 mg intravenously were assessed with a scintigraphic technique in 12 normals. The 50% emptying time was less, the lag phase was shorter and the post lag emptying rate was faster after 20 mg ICS 205-930 (p less than 0.02). After 10 mg ICS 205-930 the lag phase was significantly shorter compared with placebo (p less than 0.04). These results suggest that 5-HT3 receptors may be involved in the regulation of gastric emptying in man.     

Involvement of 5-HT1, 5-HT2, and 5-HT3 receptors in the mediation of the prolactin response to serotonin and 5-hydroxytryptophan.

Serotonin (5-HT) is involved in the neuroendocrine regulation of prolactin (PRL) secretion as a stimulator. Within the last decade several 5-HT receptor types have been identified, but their individual role in the mediation of the PRL response to 5-HT is only partly understood. We investigated in conscious male rats the effect of different 5-HT1, 5-HT2, and 5-HT3 receptor antagonists on the PRL response to 5-HT or to the 5-HT precursor 5-hydroxytrytophan (5-HTP) which was administered in combination with the 5-HT reuptake inhibitor fluoxetine. 5-HT (0.5-5.0 mg/kg BW i.v.) or 5-HTP (25-100 mg/kg i.p.) in combination with saline or fluoxetine (10 mg/kg i.p.) increased the plasma PRL concentration dose-dependently. Pretreatment with the 5-HT1+2 receptor antagonist methysergide (2.5 mg/kg i.p.) prevented the stimulatory effect of 5-HT or 5-HTP + fluoxetine. Pretreatment with the 5-HT2 receptor antagonists ketanserin or LY 53857 (2.5 mg/kg i.p.) inhibited the PRL response to 5-HT by approximately 80% and to 5-HTP + fluoxetine approximately 100%. A higher dose (10 mg/kg) of the 5-HT2 receptor antagonists possessed only 50% inhibitory effect. Pretreatment with the 5-HT3 receptor antagonists ICS 205-930 or GR 38032F (0.05-2.5 mg/kg i.p.) inhibited the PRL response induced by 5-HT or by 5-HTP + fluoxetine. The maximal inhibitory effect (approximately 80%) was obtained by a dose of 0.1 mg/kg of both compounds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Failure of tropisetron to inhibit jejunal water and electrolyte secretion induced by 5-hydroxytryptamine in healthy volunteers.

The effects of the 5-hydroxytryptamine (5-HT3) receptor antagonist, ICS 205-930 (tropisetron), on basal and 5-HT induced jejunal secretion of water and electrolytes were examined using a double blind, randomised crossover design. In seven healthy volunteers steady state perfusions of the proximal jejunum were performed twice with the Loc-I-Gut tube after 5+5 mg ICS 205-930 or placebo capsules were given. After equilibration for 60 minutes and completion of a 120 minute basal period 5-HT (10 micrograms/kg x min intravenously) was infused for 120 minutes. Net water absorption (mean (SEM)) in the basal period was 0.55 (0.84) ml/cm x h and 0.74 (0.72) ml/cm x h after placebo and ICS 205-930, respectively (p > 0.05). Infusion of 5-HT caused significant net secretion of water after placebo (2.05 (0.58) ml/cm x h; p < 0.02) as well as ICS 205-930 (2.60 (0.89) ml/cm x h; p < 0.05). As ICS 205-930 excerted no effects on either basal or 5-HT induced water and electrolyte transport in the intact human jejunum the compound is probably not efficacious as an anti-secretory drug in patients with 5-HT induced diarrhoea.     

Effect of dopamine and bromocriptine on rat ileal and colonic transport. Stimulation of absorption and reversal of cholera toxin-induced secretion

Water and electrolyte transport were determined in rat ileum and colon using the single-pass perfusion technique. Intraperitoneal dopamine caused prompt stimulation of both ileal and colonic water absorption. The dopamine effect was mediated by both specific dopamine and alpha 2-adrenergic receptors. Haloperidol, a specific dopamine antagonist, and yohimbine, an alpha 2-adrenergic antagonist, inhibited the effect of dopamine in ileal absorption; both antagonists alone had no effect on basal water transport. Bromocriptine (intravenous and intraluminal) stimulated ileal and colonic water absorption, which was inhibited by haloperidol and yohimbine, and reversed cholera toxin-induced ileal secretion. Magnitude and time-courses of the increased water absorption in ileal loops, inoculated with saline, were the same as in loops, inoculated with saline, suggesting that bromocriptine acted to reverse cholera toxin-induced secretion by stimulating absorption. Bromocriptine had no effect on the cyclic adenosine monophosphate increase caused by cholera toxin. We conclude (a) dopamine stimulates water absorption in vivo in rat ileum and colon; (b) this dopamine effect is via specific dopamine and alpha 2-receptors; (c) bromocriptine stimulates water absorption in ileum and colon and also acts by dopamine and alpha 2-receptors; and (d) bromocriptine reverses cholera toxin-induced secretion.     

Effect of serotonin 5-HT1, 5-HT2, and 5-HT3 receptor antagonists on the prolactin response to restraint and ether stress.

Serotonin (5-HT) appears to be involved in the central control of the prolactin (PRL) response to suckling and estrogen. Furthermore, 5-HT may participate in the mediation of stress-induced PRL release. In order further to elucidate the role of 5-HT and the type of 5-HT receptor(s) involved in the PRL response to stress, we investigated the effect of blockade of 5-HT1, 5-HT2 or 5-HT3 receptors on the restraint or ether stress-induced release of PRL in male rats. Pretreatment with the 5-HT1 + 2 receptor antagonist methysergide (0.5 or 2.5 mg/kg i.p.) inhibited or prevented the PRL response to restraint or ether stress. Pretreatment with the 5-HT2 receptor antagonists ketanserin or LY 53857 (0.5 or 2.5 mg/kg i.p.) inhibited the response to restraint or ether stress approximately 30 or 60%, respectively. Higher doses of both 5-HT2 receptor antagonists (10 mg/kg i.p.) had a minor inhibitory effect (5-30% for ketanserin and 50% for LY 53857). Prior intraperitoneal administration of the 5-HT3 receptor antagonists ICS 205-930 or GR 38032F (0.05-2.5 mg/kg i.p.) inhibited the restraint stress-induced PRL release dose-dependently. Both compounds inhibited the PRL response to ether stress, but only the effect of GR was dose-related. The maximal inhibitory effect (70% inhibition of the PRL response to restraint or ether stress) was obtained for both compounds at a dose of 0.1 mg/kg. We conclude that serotonergic neurons are involved in the mediation of the stress-induced PRL release by activation of 5-HT1, 5-HT2 as well as 5-HT3 receptors.     

Loperamide: studies on its mechanism of action.

The effects of loperamide on net solute and water absorption, and prostaglandin E2 (PGE2) and cholera toxin-induced secretion were studied in the rat jejunum using an in vivo steady-state perfusion technique. Loperamide stimulated absorption of fluid, electrolytes, and glucose and reversed PGE2 and cholera toxin-induced secretion to absorption; this opiate analogue had no effect on cholera toxin stimulation of adenylate cyclase activity or the rise of tissue cyclic AMP (cAMP) concentrations. The opiate antagonist, naloxone, reduced the antisecretory effects of loperamide without affecting tissue levels of cAMP. These results indicate that loperamide inhibits PGE2 and cholera toxin-induced secretion, and that this phenomenon is independent of any direct effect that cholera toxin has on the adenylate cyclase system. The action of naloxone suggests, but does not prove, that loperamide exerts its effect via opiate receptors.     

Activation of 5-HT(7) receptor in rat glomerulosa cells is associated with an increase in adenylyl cyclase activity and calcium influx through T-type calcium channels

Serotonin (5-HT) stimulates aldosterone secretion from the rat adrenal gland through 5-HT(7) receptors. The aim of the present study was to investigate the transduction mechanisms associated with activation of 5-HT(7) receptors in rat glomerulosa cells. The stimulatory effect of 5-HT on aldosterone secretion and cAMP formation was significantly reduced by the 5-HT(7) receptor antagonist LY 215840. Pretreatment of cells with the adenylyl cyclase inhibitor SQ 22536 or the PKA inhibitor H-89 markedly attenuated the effect of 5-HT on aldosterone secretion. Conversely, type 2 and 4 phosphodiesterase inhibitors potentiated the 5-HT-induced stimulation of aldosterone secretion. Administration of 5-HT in the vicinity of cultured glomerulosa cells induced a slowly developing and robust increase in cytosolic calcium concentration ([Ca(2+)](i)). The effect of 5-HT on [Ca(2+)](i) was suppressed by mibefradil, a T-type calcium channel blocker. Patch-clamp studies confirmed that 5-HT activated a T-type calcium current. Mibefradil also induced a dose-dependent inhibition of 5-HT-induced aldosterone secretion. The sequence of events associated with activation of 5-HT(7) receptors was investigated. The PKA inhibitor H-89 markedly attenuated both the [Ca(2+)](i) response and the activation of T-type calcium current induced by 5-HT. In contrast, reduction of the calcium concentration in the incubation medium did not affect 5-HT- induced cAMP formation. Preincubation of glomerulosa cells with cholera toxin abolished the stimulatory effect of 5-HT on aldosterone secretion, but pertussis toxin had no effect. Taken together, these data demonstrate that, in rat glomerulosa cells, activation of native 5-HT(7) receptors stimulates cAMP formation through a G(salpha) protein, which in turn provokes calcium influx through T-type calcium channels. Both the adenylyl cyclase/PKA pathway and the calcium influx are involved in 5-HT-induced aldosterone secretion.     

Role of 5-hydroxytryptamine type 3 receptors in rat intestinal fluid and electrolyte secretion induced by cholera and Escherichia coli enterotoxins.

Cholera toxin and Escherichia coli heat labile toxin (LT) induced intestinal secretion has in the past been attributed exclusively to an increase in intracellular cAMP whereas E coli heat stable toxin (ST) induced secretion is mediated through cGMP. Evidence is accumulating on the importance of 5-hydroxytryptamine (5-HT) in cholera toxin induced secretion, but its role in LT and ST is not well established. This study therefore investigated in vivo the effect of 5-HT3 receptor antagonist, granisetron, on intestinal fluid and electrolyte secretion induced by cholera toxin, LT, and ST. Granisetron (30, 75, 150, or 300 micrograms/kg) was given subcutaneously to adult male Wistar rats 90 minutes before instillation of 75 micrograms cholera toxin or 50 micrograms LT in isolated whole small intestine. In situ small intestinal perfusion was performed with an iso-osmotic plasma electrolyte solution (PES) to assess fluid movement. In a second group of animals, granisetron (300 micrograms/kg) was given subcutaneously and two hours later small intestinal perfusion with PES containing 200 micrograms/l ST was performed. Cholera toxin induced net fluid secretion (median -50.1 microliters/min/g (interquartile range -59.5 to -29.8)) was found to be dose dependently decreased or abolished by granisetron (plateau effect at 75 micrograms/kg: 18 (-7.8 to 28), p < 0.01). Granisetron in high dose (300 micrograms/kg), however, failed to prevent LT or ST induced secretion (-52 (-121 to -71) v -31 (-44 to -18), and (-39 (-49 to 17) v (-22 (-39 to -3)), respectively). Sodium and chloride movement paralleled that of fluid. In conclusion, these data show that 5-HT and 5-HT3 receptors play an important part in cholera toxin induced secretion but are not involved in E coli heat stable or heat labile toxin induced secretion.     

Peripheral neural serotonin receptors: identification and characterization with specific antagonists and agonists.

Serotonin (5-hydroxytryptamine, 5-HT) has been shown to be a neurotransmitter in the enteric nervous system (ENS). Although 5-HT is a mediator of slow excitatory postsynaptic potentials evoked by stimulation of interganglionic connectives, the precise role it plays in the physiology of the gut is unclear. Research has been hampered by an inadequate knowledge of the types of 5-HT receptor in the ENS and thus the lack of well-characterized antagonists. We now report the identification of two classes of enteric neural 5-HT receptor, the effects of activating these receptors on myenteric type II/AH neurons, and their characterization with specific agonists and antagonists. One class, which we propose to call 5-HT1P, is characterized by a high affinity for [3H]5-HT in radioligand binding assays. This class of receptor mediates a slow depolarization of myenteric type II/AH neurons associated with an increase in input resistance. Agonists at this receptor include, in addition to 5-HT (in order of potency), 5- and 6-hydroxyindalpine and 2-methyl-5-HT. 5-HT1P-mediated responses are specifically antagonized by 5-hydroxytryptophyl-5-hydroxytryptophan amide. The other class of 5-HT receptor, which we propose to call 5-HT2P, appears not to have a high affinity for [3H]5-HT. This receptor mediates a brief depolarization of myenteric II/AH neurons associated with a fall in input resistance. 2-Methyl-5-HT, at low concentrations, is a specific agonist at this receptor and ICS 205-930 is a specific antagonist. Binding of [3H]5-HT to enteric membranes is inhibited by 5-HT1P receptor agonists and antagonists but not by the 5-HT2P receptor antagonist ICS 205-930 or by MDL 72222, another compound reported to be an antagonist of 5-HT at peripheral receptors.     

5-HT1 and 5-HT2 receptor agonists blunt +/- -alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-stimulated GH secretion in prepubertal male rats

OBJECTIVE: Excitatory amino acids, gamma-amino butyric acid (GABA), serotonin and catecholamines are involved in the control of GH secretion. The actions of these neurotransmitters are interconnected, and recently we showed that the stimulatory effect of GABA was blocked by MK-801, an antagonist of N-methyl-D-aspartate receptors. The present experiments were carried out to analyze the interrelationships between +/- -alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors and serotoninergic and catecholaminergic pathways in the control of GH secretion in prepubertal (16-23-day-old) male rats. DESIGN AND METHODS: The GH response to AMPA was analyzed in animals pretreated with 5-hydroxytryptophan methyl ester (5-HTP) plus fluoxetine (a precursor of 5-hydroxytryptamine (5-HT) synthesis and a blocker of 5-HT re-uptake), R (+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT, an agonist of the 5-HT1 receptors), +/- -2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and alpha-methyl-5-hydroxytryptamine (agonists of 5-HT2 receptors), I-phenylbiguanide (an agonist of 5-HT3 receptors), or alpha-methyl-p-tyrosine (alpha-MPT) and diethyldithiocarbamate (DDC) (blockers of catecholamine synthesis). RESULTS: Basal GH secretion remained unchanged in prepubertal rats after activation of the serotoninergic system or blockade of catecholamine synthesis. The stimulatory effect of AMPA on GH secretion was blocked after activation of the serotoninergic system, through specific 5-HT1 and 5-HT2 receptor agonists. In contrast, activation of 5-HT3 receptors potentiated AMPA-stimulated GH secretion. CONCLUSIONS: Serotoninergic receptors modulate the stimulatory effect of AMPA on GH secretion in prepubertal male rats.     

Colonic secretion mediated by prostaglandin E2 and 5-hydroxytryptamine may contribute to diarrhea due to morphine withdrawal in the rat

Prostaglandins and cyclic adenosine monophosphate have been claimed to play a major role in the morphine withdrawal syndrome, but intestinal secretion has not been ruled out as being responsible, at least in part, for the accompanying diarrhea. Therefore, experiments were performed in which the effect of naloxone-induced morphine withdrawal on jejunal and on colonic fluid transport was assessed in tied-off loops of rat intestine in vivo simultaneously with mucosal cyclic adenosine monophosphate levels or colonic luminal release of prostaglandin E2 or 5-hydroxytryptamine. Naloxone-induced withdrawal reversed fluid absorption to secretion without changing cyclic adenosine monophosphate levels, but markedly enhanced local prostaglandin E2 and 5-hydroxytryptamine release (p less than 0.01). Indomethacin and the 5-hydroxytryptamine receptor antagonist ketanserin prevented withdrawal-induced fluid secretion and the increase in prostaglandin E2 release without influencing the release of 5-hydroxytryptamine. In addition, the alpha 2-adrenergic receptor agonist clonidine promoted absorption during withdrawal, whereas atropine failed to influence fluid transport. These data suggest that naloxone-precipitated intestinal fluid secretion may contribute to diarrhea due to morphine withdrawal and that 5-hydroxytryptamine may play an important role in mediating this secretion through stimulation of local prostaglandin formation.     

Serotonergic involvement in stress-induced vasopressin and oxytocin secretion

OBJECTIVE: To investigate the involvement of serotonin (5-hydroxytryptamine - 5-HT) receptors in mediation of stress-induced arginine vasopressin (AVP) and oxytocin (OT) secretion in male rats. DESIGN: Experiments on laboratory rats with control groups. METHODS: Different stress paradigms were applied after pretreatment with intracerebroventricular infusion of saline or different 5-HT antagonists. RESULTS: Restraint stress (5 min), hypotensive hemorrhage or dehydration for 24 h increased AVP secretion fivefold and OT secretion threefold. Swim stress for 3 min had no effect on AVP secretion, but increased OT secretion threefold. Ether vapor or hypoglycemia had no effect on AVP or OT secretion. The restraint stress-induced AVP response was inhibited by pretreatment with the 5-HT(2A+2C) antagonists ketanserin (KET) and LY-53857 (LY) and the 5-HT(3+4) antagonist ICS-205930 (ICS), whereas the 5-HT(1A) antagonist WAY-100635 (WAY) had no effect. The OT response to restraint stress was inhibited by WAY, KET and LY but not by ICS. KET and LY inhibited OT response to dehydration, and LY inhibited OT response to hemorrhage. Neither of the antagonists affected AVP responses to dehydration or hemorrhage, nor the swim stress-induced OT response. CONCLUSION: 5-HT(2A), 5-HT(2C) and possibly 5-HT(3) and 5-HT(4) receptors, but not 5-HT(1A) receptors, are involved in the restraint stress-induced AVP secretion. 5-HT does not seem to be involved in the dehydration- or hemorrhage-induced AVP response. The restraint stress-induced OT response seems to be mediated via 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptors. The dehydration and hemorrhage-induced OT responses are at least mediated by the 5-HT(2A) and 5-HT(2C) receptors. The 5-HT(3) and 5-HT(4) receptors are not involved in stress-induced OT secretion.     

Platelet derived serotonin drives the activation of rat cardiac fibroblasts by 5-HT2A receptors

Platelet activation occurs in different acute and chronic heart diseases including myocardial infarction, obstructive hypertrophic cardiomyopathy and valve stenosis. Recent studies suggested that some factors secreted by activated platelets may participate in cardiac remodeling. In the present study, we investigated whether platelets and platelet-released serotonin (5-HT) are directly involved in the functional regulation of cardiac fibroblasts. Treatment of neonatal rat cardiac fibroblasts with platelet lysate, 5-HT and the 5-HT2A receptor agonist DOI increased the expression of α-SMA protein, a marker of fibroblast differentiation into myofibroblasts. Platelet lysate, 5-HT and DOI also induced a time-dependent stimulation of cardiac fibroblast migration that was inhibited by the 5-HT2A receptor antagonist ketanserin. Finally, incubation of cardiac fibroblasts with platelet lysate or 5-HT enhanced secretion of TGF-β1 and expression of MMP-3 and MMP-13. As observed for fibroblast migration, these effects were prevented by ketanserin. These results demonstrated for the first time that factors released from platelet directly regulate cardiac fibroblasts by enhancing secretion of TGF-β1 and MMPs and promoting their migration and differentiation. 5-HT released by platelets appears to be a major contributor of platelet effects which are mediated through 5-HT2A receptors.     

Prevention of toxin-induced intestinal ion and fluid secretion by a small-molecule CFTR inhibitor

BACKGROUND & AIMS: The cystic fibrosis transmembrane conductance regulator (CFTR) provides an important apical route for Cl(-) secretion across intestinal epithelia. A thiazolidinone-type CFTR blocker (CFTR(inh)-172) reduced cholera toxin-induced fluid accumulation in mouse intestinal loops. Here, we characterize the efficacy and pharmacodynamics of CFTR(inh)-172 in blocking cAMP and cGMP induced Cl(-)/fluid secretion in rodent and human intestine. METHODS & RESULTS: CFTR(inh)-172 inhibited cAMP and cGMP agonist induced short-circuit current by >95% in T84 colonic epithelial cells (K(I) approximately 3 micromol/L) and in mouse and human intestinal sheets (K(I) approximately 9 micromol/L). A single intraperitoneal injection of CFTR(inh)-172 (200 microg) blocked intestinal fluid secretion in a rat closed-loop model by >90% for cholera toxin and >70% for STa Escherichia coli toxin. In mice, CFTR(inh)-172 (20 microg) inhibited cholera toxin-induced intestinal fluid secretion by 90% (persistence t(1/2) approximately 10 hours, K(I) approximately 5 microg) and STa toxin by 75% (K(I) approximately 10 microg). Tissue distribution and pharmacokinetic studies indicated intestinal CFTR(inh)-172 accumulation facilitated by enterohepatic circulation. An oral CFTR(inh)-172 preparation reduced fluid secretion by >90% in a mouse open-loop cholera model. CONCLUSIONS: A small molecule CFTR blocker markedly reduced intestinal ion and fluid secretion caused by cAMP/cGMP-dependent bacterial enterotoxins. CFTR inhibition may thus reduce fluid secretion in infectious secretory diarrheas.     

Serotonin released from intestinal enterochromaffin cells mediates luminal non-cholecystokinin-stimulated pancreatic secretion in rats

BACKGROUND & AIMS: Similar to cholecystokinin (CCK), non-CCK-dependent duodenal factors stimulate vagal mucosal afferent fibers to mediate pancreatic enzyme secretion via a common cholinergic pathway. We tested the hypothesis that 5-hydroxytryptamine (5-HT) released from enterochromaffin (EC) cells plays an important role in the transduction of luminal information to the central nervous system via vagal afferent fibers to mediate pancreatic secretion. METHODS: Pancreatic secretions were examined in conscious rats after intragastric administration of chopped rodent chow in the presence and absence of CCK or 5-HT(3) and 5-HT(2) antagonists. Pancreatic responses to intraduodenal administration of maltose, hyperosmolar NaCl, and light mucosal stroking were examined in rats pretreated with various pharmacological antagonists or after surgical or chemical ablation of vagal and 5-HT neural pathways. RESULTS: Administration of L364, 718 inhibited 54% of pancreatic protein secretion evoked by intragastric administration of rodent chow. L364,714 and ICS 205-930, a 5-HT(3) antagonist, combined produced a 94% inhibition. Vagal afferent rootlet section eliminated pancreatic secretions evoked by intraduodenal stimuli. p-Chlorophenylalanine, a 5-HT synthesis inhibitor, but not 5,7-hydroxytryptamine, a 5-HT neurotoxin, also eliminated the pancreatic response to these luminal stimuli. The 5-HT(3) antagonist markedly inhibited pancreatic secretion induced by maltose and hyperosmolar NaCl. 5-HT(2) and 5-HT(3) antagonists combined inhibited the pancreatic response to light stroking of the mucosa. CONCLUSIONS: Luminal factors such as osmolality, disaccharides, and mechanical stimulation stimulated pancreatic secretion via intestinal vagal mucosal afferent fibers. It is likely that 5-HT originating from intestinal EC cells activated 5-HT(3) and 5-HT(2) receptors on vagal afferent fibers to mediate luminal factor-stimulated pancreatic secretion.     

Effect of serotonin antagonists on prolactin and progesterone secretion in rats: evidence that the stimulatory and inhibitory actions of serotonin on prolactin release may be mediated through different receptors

The serotoninergic regulation of prolactin release was studied in female rats in different reproductive states using ketanserin, a specific S2 receptor blocker, ICS 205-930 ((3 alpha-tropanyl)1H-indol-3-carboxylic acid ester), a specific S3 receptor blocker and p-chlorophenylalanine (pCPA), a serotonin synthesis inhibitor. Administration of ketanserin to pro-oestrous rats inhibited the afternoon prolactin surge; this inhibition was prevented by progesterone. On day 3 of pregnancy, pCPA or ketanserin blocked the afternoon prolactin surge, and administration of oestrogen (on day 2) and progesterone (on day 3) in combination, but not alone, prevented this effect. On day 9 of pregnancy, treatment with oestrogen (on day 8) and progesterone (on day 9) induced an afternoon surge of prolactin which was prevented by administration of ketanserin or pCPA. On days 9 and 16, pCPA induced a slight increase in serum prolactin in rats not treated with steroids, but ketanserin had no effect. On day 13, ketanserin and pCPA had no effect on serum prolactin levels, but after increasing serotoninergic transmission by injecting fluoxetine and 5-hydroxytryptophan, serum prolactin levels were decreased. On day 19, ketanserin produced a transient increase in the serum concentration of prolactin, probably produced by the marked decrease in the serum concentration of progesterone induced by the S2 receptor blocker. Administration of ICS 205-930 to pro-oestrous rats or rats on day 19 of pregnancy had no effect on serum concentrations of prolactin and progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT1 and 5-HT2 receptor activation reduces N-methyl-D-aspartate (NMDA)-stimulated LH secretion in prepubertal male and female rats

OBJECTIVE: Excitatory amino acids and serotonin are involved in the control of gonadotropin secretion. The actions of these neurotransmitters are interconnected and recently we have reported that 5-HT(1) and 5-HT(2) receptor agonists blunted (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-stimulated GH secretion in prepubertal rats. The present experiments were carried out to analyze the effects of activation of different 5-hydroxytryptamine (5-HT) receptor subtypes on gonadotropin secretion and their role in the N-methyl-d-aspartate (NMDA)-stimulated LH release. DESIGN AND METHODS: We analyzed the gonadotropin secretion after manipulation of serotoninergic and aminoacidergic systems and their interactions in 5-, 16- and 23-day-old male and female rats. To this end, serum LH and FSH concentrations were measured in rats treated with 5-hydroxytryptophan methyl ester (5-HTP) (a precursor of 5-HT synthesis) plus Fluoxetine (Fx, a blocker of 5-HT reuptake), d,l-p-chlorophenyl-alanine methyl ester (PCPA, a blocker of 5-HT synthesis), R-(+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT, an agonist of 5-HT(1A) receptors), (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT, agonists of 5-HT(2) receptors), and 1-Phenylbiguanide (1-PHE an agonist of 5-HT(3) receptors). In addition, the effects of 8-OH-DPAT and DOI on NMDA-stimulated LH secretion were analyzed. RESULTS: Neither the activation nor blockade of the serotoninergic system modified LH secretion. Basal gonadotropin secretion remained unchanged in 23-day-old male and female rats after activation of 5-HT(1A), 5-HT(2) and 5-HT(3) receptors. The stimulatory effect of NMDA on LH secretion was blocked in both sexes after activation of the serotoninergic system, through specific 5-HT(1) and 5-HT(2) receptor agonists. CONCLUSIONS: Activation of serotoninergic receptors decreased the stimulatory effect of NMDA on LH secretion in prepubertal male and female rats.     

Influence of different serotonin receptor subtypes on growth hormone secretion

The role of serotonin (5-HT) in the regulation of growth hormone (GH) secretion remains unclear due to the existence of many different receptors that mediate the 5-HT actions, and the lack of suitable specific agonist and antagonist drugs. In the present work we have taken advantage of the recent development of new selective 5-HT drugs in order to clarify the role played by different 5-HT receptor types and subtypes on GH secretion. The experiments were carried out on beagle dogs. GH-releasing hormone (GHRH) increased basal canine GH (cGH) levels from 0.8 +/- 0.2 to 8.8 +/- 1.7 microg/l at 15 min. Administration of 5-HT(1D) receptor agonist sumatriptan (SUM) induced a cGH peak at 30 min of 12.9 +/- 2.7 microg/l. The combined administration of GHRH plus SUM strikingly potentiated cGH release with a peak of 36.9 +/- 6 microg/l at 30 min (p < 0.05). Pretreatment with the muscarinic receptor antagonist atropine completely abolished the cGH response to SUM, while the cholinergic agonist pyridostigmine (PYR) did not modify this response (15.3 +/- 5 microg/l PYR plus SUM vs. SUM alone 12.9 +/- 2. 7 microg/l). On the other hand, administration of drugs with activity at 5-HT(2A/C) receptors showed a stimulatory role for the 5-HT(2C) receptor subtype, since LY-53857 (antagonist 5-HT(2A/C)) and DOI agonist (5-HT(2A/C)) both modified the GH response stimulated by GHRH (AUC 88.5 +/- 30.4 and 400 +/- 64.6 vs. 267.3 +/- 52.6 respectively), while ketanserin (antagonist 5-HT(2A)) did not modify this response. The 5-HT(3) antagonist ICS-205-930 failed to modify either basal or GHRH induced GH responses. In conclusion, our data show that 5-HT(1D) receptors play a stimulatory role on GH secretion in the dog, possibly by acting through a decrease in hypothalamic somatostatin release. Similarly, the 5-HT(2C) receptor subtypes also appear to play a stimulatory role. However, 5-HT(2A) and 5-HT(3) receptors do not appear to be involved in the control of basal and GHRH-induced GH secretion.     

5-Hydroxytryptamine and cholera secretion: a histochemical and physiological study in cats.

The effect of cholera toxin on the content of 5-hydroxytryptamine (5-HT) in the enterochromaffin cells of the cat small intestine was estimated by cytofluorimetry of individual enterochromaffin cells at varying times after exposing the intestinal mucosa to the toxin. The observed changes in 5-HT levels in the enterochromaffin cells were correlated with the simultaneously measured rate of net fluid transport across the intestinal epithelium. Intestinal segments exposed to cholera toxin showed a statistically significant decrease in 5-HT levels of enterochromaffin cells compared with segments exposed to heat-inactivated cholera toxin. A good correlation (r = 0.73) was found between relative 5-HT fluorescence in enterochromaffin cells and net fluid transport across the intestinal epithelium. Thus, a diminished 5-HT content was associated with a decreased rate of fluid absorption or an increased rate of secretion. A hypothesis is presented for explaining the possible role of the enterochromaffin cells in the pathophysiology of cholera secretion.     

Serotonin increases protective duodenal bicarbonate secretion via enteric ganglia and a 5-HT4-dependent pathway

OBJECTIVE: Serotonin (5-HT) is present in much larger amounts in the gut than in the central nervous system and is predominantly synthesized and stored in mucosal enterochromaffin cells. Bicarbonate secretion by the duodenal mucosa is the major mechanism in maintaining mucosal integrity, neutralizing invading protons within the surface mucus gel. In this study the role of local 5-HT in the control of the protective secretion was investigated. MATERIAL AND METHODS: A segment of proximal duodenum was perfused in situ in anaesthetized rats and the alkaline secretion was continuously recorded by pH-stat. Intracellular calcium signalling was measured in clusters of human and rat duodenal enterocytes devoid of neural tissue. After loading with the fluorescent probe, fura-2, the clusters were attached to the bottom of a temperature-controlled perfusion chamber. RESULTS: Close intra-arterial infusion to the duodenal segment of 5-HT (20-200 nmol kg(-1) h(-1)) dose-dependently increased duodenal mucosal HCO3 secretion. A higher dose (2000 nmol kg(-1) h(-1)) did not further increase secretion. Responses were inhibited by the ganglionic blocker and nicotinic receptor antagonist hexamethonium, and were abolished by the 5-HT4 receptor antagonist SB 204070. The 5-HT3 antagonist tropisetron, in contrast, caused only slight inhibition. Viable human and rat duodenal enterocytes responded to 5-HT (100-500 nM) with an increase in intracellular calcium concentration. Pretreatment with SB 204070 or removal of external calcium abolished the response. CONCLUSIONS: Stimulation of the duodenal protective secretion by 5-HT thus involves receptors of the 5-HT4 subtype as well as nicotinic transmission, the myenteric plexus being a likely location. In addition, serotonin acts on enterocyte membrane receptors, inducing intracellular calcium signalling.