The effect of propranolol on the serotonin concentration in the portal plasma after vagal nerve stimulation in the cat

Serotonin infusions inhibit sensory input from the dural vasculature

Intravenous infusions of serotonin (5-hydroxtryptamine creatinine sulphate, 5HT, 50-300 microg/kg/min) in cats reversibly inhibited the responses of cervical spinal cord neurons to electrical stimulation of the superior sagittal sinus. Inhibition developed over 20-30 min and resolved over the same time course, suggesting a dependence on accumulation of 5HT in the central nervous system. Inhibition was suppressed by prior intravenous injection of the 5HT antagonists methysergide (1 mg/kg) and methiothepin (1 mg/kg). Infusions of 5HT (50 microg/kg/min) caused a rise in whole blood levels of 5HT by a factor of 1.5 of control values. 5HT levels in platelet-free plasma rose by a factor of 50. Levels of 5HT and 5 hydroxyindole acetic acid released into the cerebrospinal fluid rose significantly. The results suggest that earlier clinical observations that 5HT infusions can ameliorate the pain of migraine may not have been due to cranial vasoconstriction alone, but could have involved a central action of 5HT.

Regional 5-hydroxyindoleacetic acid production in humans

Veno-arterial plasma concentration differences and regional organ plasma flows were used to quantify the relative amounts of 5-hydroxyindoleacetic acid (5-HIAA) contributed by various sites into the peripheral circulation. Positive venoarterial concentration gradients were found in the hepatosplanchnic, forearm, cardiac and jugular vessels in the healthy subjects. The renal circulation was determined to be the principal site of 5-HIAA clearance, extracting 18 +/- 2 nmol/min. The gut was the greatest contributor to the total 5-HIAA plasma pool with the relative contributions of the various organs being as follows: hepatosplanchnic organs 58%, skeletal muscle 26%, brain 6% and the heart 3%. The source of 5-HIAA stemming from these regional beds remains unknown, it may derive from serotonin taken up by and deaminated in ubiquitous endothelial cells, enterochromaffin cells of the gut, peripheral serotonergic nerves, serotonin turnover in platelets or perhaps the metabolism of serotonin taken up by sympathetic nerves. To test the latter hypothesis we examined 23 patients with chronic congestive heart failure and 9 patients with pure autonomic failure to investigate the possible effects of sympathetic nervous system overactivity and underactivity on peripheral 5-HIAA production and plasma 5-HIAA concentration. The resting arterial plasma 5-HIAA concentration in the heart failure patients was increased three-fold. This elevated plasma 5-HIAA concentration was attributable to an increased rate of whole body 5-HIAA production. The arterial 5-HIAA plasma concentration in the autonomic failure patients was paradoxically elevated, being 70% greater than that of the healthy subjects. The increased 5-HIAA plasma concentration in these patients was accounted for by a reduction in 5-HIAA plasma clearance. In all subjects studied there was a weak relationship only between total body norepinephrine spillover to plasma and the arterial 5-HIAA plasma concentration. We found that in healthy subjects the overflow of 5-HIAA into the hepatic vein was significantly related to the underlying degree of sympathetic activity. It can be concluded that 5-HIAA is produced at a number of sites throughout the body with the arterial plasma concentration being dependent on both the level of production and plasma clearance. By far the majority of 5-HIAA in plasma is derived from the gut with only minimal contribution from the brain.

Carbamylcholine- and catecholamine-induced intracellular calcium dynamics of epithelial cells in mouse ileal crypts

BACKGROUND/AIMS

The intestinal epithelium is composed of various cells, and the enteric nervous system, which controls the epithelial functions, has different neurotransmitters and/or modulators. The aim of this study was to show whether the responses of intestinal epithelial cells to different neurotransmitters are elicited throughout the entire epithelium or are restricted to a certain cell.

METHODS

The spatiotemporal dynamics of cytosolic calcium ion ([Ca2+]c) were measured by digital imaging analysis in isolated crypts of mouse ileum loaded with [1-[2-(5'-carboxyoxazol-2'-yl)-6-amino-benzofuran-5- oxy]-2-(2'-amino-5'-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] pentakis (acetoxylmethyl) ester. Thereafter, the crypt cells were identified morphologically.

RESULTS

Carbamylcholine elicited [Ca2+]c dynamics in Paneth cells, showing a biphasic increase, but neither cholecystokinin octapeptide nor nicotine had any effect on the [Ca2+]c of the crypt cells including the Paneth cells. Adrenaline and noradrenaline, but not isoproterenol, induced a transient elevation of [Ca2+]c of some enterochromaffin cells. Increases in the [Ca2+]c of most crypt cells were elicited by thapsigargin. Propagation of a [Ca2+]c wave in the crypts was not evident.

CONCLUSIONS

Increases in [Ca2+]c can be induced by carbamylcholine in Paneth cells and catecholamines in some enterochromaffin cells. The digital imaging analysis showed the heterogeneity of the responses of intestinal crypt cells to different transmitters.

Enterochromaffin cells of the digestive system: cellular source of guanylin, a guanylate cyclase-activating peptide

Guanylin, a bioactive peptide, has recently been isolated from the intestine; this peptide activates intestinal guanylate cyclase (i.e., guanylate cyclase C) and thus is potentially involved in the regulation of water/electrolyte transport in the gastrointestinal mucosa. As yet, the cells involved in synthesis, storage, or secretion of guanylin have not been identified by immunocytochemistry. We raised antisera against guanylin and investigated the entire gastrointestinal tract of guinea pigs by light and electron microscopical immunocytochemistry. Extracts of various intestinal segments and plasma analyzed on a Western blot revealed a peptide band corresponding to the molecular mass of guanylin. Localization studies in the entire digestive tract showed that guanylin is exclusively confined to enterochromaffin (EC) cells. Remarkably, most EC cells contacted the gut lumen by cell processes that were highly immunoreactive for guanylin. In addition to the well known secretion in an endocrine fashion, EC cells by circumstantial evidence may release guanylin into the gut lumen to activate guanylate cyclase C that is immediately located on the brush border of adjacent enterocytes. The unique localization of guanylin in EC cells may indicate that these cells are involved in the regulation of fluid secretion in the gastrointestinal mucous membrane.

Potentiation of responses to sympathetic nerve stimulation and vasoconstrictor agents by SK&F 103829 in the feline mesenteric circulation

1. The amplification of vasoconstrictor effects of several agonists and sympathetic nerve stimulation, caused by 5-HT2 receptor activation, was studied in the autoperfused mesenteric circulation of anaesthetized cats. To produce long lasting and selective 5-HT2 receptor stimulation we used SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulphonyl]-1H3-benzazepin-7-ol methensulphonate). We assessed that SK&F 103829 was a strong contractile partial agonist in isolated preparations of rat tail artery and calf pulmonary artery. 2. The intrinsic activity of SK&F 103829 with respect to 5-hydroxytryptamine (5-HT) was 0.8 in rat tail artery and 0.6 in calf pulmonary artery. SK&F 103829-induced contractile responses were surmountably antagonized by ketanserin with a potency expected from its affinity for 5-HT2 receptors. SK&F 103829 surmountably antagonized the effects of 5-HT in rat tail artery with a pKp of 5.8. 3. Concentrations of SK&F 103829 causing greater than threshold constrictions enhanced vasoconstrictor responses of sympathetic nerve stimulation, noradrenaline, angiotensin II, methoxamine and alpha, beta-methylene ATP in the mesenteric arterial bed. Increases in mesenteric arterial pressure by noradrenaline, observed in the presence of prazosin, were also potentiated by SK&F 103829. 4. Ketanserin prevented both the constrictor effect of SK&F 103829 and the SK&F 103829-evoked potentiation of the responses to noradrenaline and angiotensin II in the mesenteric arterial bed. Ketanserin, however, failed to abolish (once established) the SK&F 103829-evoked potentiation of the constrictor effects caused by both noradrenaline and angiotensin II. 5. Short lasting constrictor effects of 5-HT were reversed to dilator effects by SK&F 103829 in both the mesenteric arterial and venous bed, thereby revealing the existence of vasodilator 5-HT receptors.6. The main finding is consistent with a sensitization of the mesenteric arterial bed to vasoconstrictor responses mediated through alpha 1- and alpha2-adrenoceptors, angiotensin II receptors and purinoceptors by SK&F 103829-evoked activation of 5-HT2 receptors. This property, together with the direct constrictor effect of the mesenteric arterial bed suggest that SK&F 103829 can reduce portal venous flow thereby being a useful therapeutic principle for the treatment of portal hypertension.

Regulation of serotonin release from the intestinal mucosa

In the mammalian intestine serotonin (5-hydroxytryptamine, 5-HT) is present in high concentrations in the enterochromaffin cells. The release of 5-HT from the intestinal mucosa is regulated by a complex pattern of neuronal and humoral inputs to the enterochromaffin cells. The enterochromaffin cells appear to be endowed with different inhibitory (alpha 2-adrenoceptors, GABAA- and GABAB-receptors, histamine H3-receptors, receptors for vasoactive intestinal polypeptide and somatostatin) as well as stimulatory receptors (beta-adrenoceptors, muscarine and nicotine receptors). The physiological significance of this complex system of receptors is suggested by experiments which demonstrate that the respective intrinsic neurotransmitters (catecholamines, acetylcholine, GABA and vasoactive intestinal polypeptide) released within the gut are involved in the regulation of the release of 5-HT from the enterochromaffin cells.

Effect of vasoactive intestinal polypeptide on the release of serotonin from the in vitro vascularly perfused small intestine of guinea pig

Isolated segments of the guinea pig small intestine were vascularly perfused and the release of endogenous serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) into the portal vein was measured. All test substances were intraarterially perfused. Vasoactive intestinal polypeptide (VIP, 1 pmol/l-100 nmol/l) inhibited the spontaneous release of 5-HT and 5-HIAA. The maximal inhibitory effect (about 60%) was seen at 100 pmol/l. The effect of VIP on the spontaneous release of 5-HT and 5-HIAA was not changed in the presence of 1 mumol/l tetrodotoxin (TTX). Raising intraluminal pressure by 500 Pa for 5 min increased the release of 5-HT and 5-HIAA by about 25%. Raising the intraluminal pressure in the presence of VIP reduced the release of 5-HT and 5-HIAA by about 75%. In the presence of TTX (1 mumol/l), raising intraluminal pressure also caused a decrease of the release of 5-HT and 5-HIAA which was unaffected by the additional presence of VIP. The fluid volume expelled during peristaltic activity was not affected by VIP, but reduced by about 90% in the presence of TTX. In conclusion the results demonstrate a direct inhibitory effect of VIP on the release of 5-HT from the enterochromaffin cells. In addition, VIP appears to interfere with the neuronally mediated stimulation of 5-HT release during peristaltic activity.

Increased plasma free serotonin but unchanged platelet serotonin in bipolar patients treated chronically with lithium

The effect of lithium salts administered chronically to bipolar patients on peripheral measures of the serotoninergic system has been studied. Plasma free serotonin (5HT), whole blood 5HT, plasma 5-hydroxyindoleacetic acid (5HIAA) and plasma total tryptophan (TP) have been analyzed in 22 patients treated with lithium carbonate (mean daily dose: 1280 mg, mean serum concentration: 0.73 mmol/l) and compared to 14 healthy controls and 11 patients treated chronically with antipsychotic drugs. Lithium salts induced significant increases in plasma free 5HT (+159% with respect to control values) and in plasma 5HIAA (+39%) without affecting 5HT contained in platelets. Plasma TP was also unchanged by chronic lithium treatment. The ratio between 5HT stored in platelets and 5HT free in plasma, a variable reduced after uptake inhibitors like clomipramine, was decreased in lithium-treated patients (-50%). These results are compatible with an enhanced synthesis of 5HT in the periphery (mainly enterochromaffin cells) as well as with an inhibition of platelet 5HT uptake (or increased 5HT efflux from intracellular stores) induced by lithium. The lack of effect of several antipsychotic drugs upon these variables is consistent with their predominant effect on the dopaminergic system and reinforces the specificity of the effect observed with lithium salts. Taken together, these results support the usefulness of using this "in vivo" 5HT peripheral model for the study of the actions elicited by drugs acting on the presynaptic components of the 5HT system.

Adrenergic modulation of the release of 5-hydroxytryptamine from the vascularly perfused ileum of the guinea-pig

1. Isolated segments of the guinea-pig ileum were vascularly perfused and the release of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) into the portal venous effluent was determined by h.p.l.c. with electrochemical detection. Test substances were applied via the arterial perfusion medium. 2. Isoprenaline (0.1 microM) increased the outflow of 5-HT and 5-HIAA maximally by about 75% and this was antagonized by propranolol (0.1 microM). Forskolin (1-10 microM) increased the outflow of 5-HT by approximately 105% and that of 5-HIAA by approximately 55%. The phosphodiesterase inhibitor AH 21-132 (0.1-1 microM) increased the outflow of 5-HT and 5-HIAA by about 70%. Isoprenaline (1 nM) and AH 21-132 (10 nM), which alone had no effect, increased the outflow of 5-HT and 5-HIAA by 75%, when applied in combination. 3. Clonidine (1 microM) reduced the outflow of 5-HT by 45%, an effect blocked by tolazoline (1 microM), but not by prazosin (0.1 microM). 4. The effects of isoprenaline, forskolin and clonidine were also observed in the presence of tetrodotoxin (1 microM) demonstrating a direct modulation of 5-HT release from the enterochromaffin cells. 5. In conclusion, the release of 5-HT from enterochromaffin cells is facilitated by activation of beta-adrenoceptors and inhibited via alpha 2-adrenoceptors. Enhancing intracellular cyclic AMP, by direct stimulation of adenylate cyclase with forskolin or by inhibition of phosphodiesterase, also facilitates the release of 5-HT. The beta-adrenoceptor-mediated effect on 5-HT release appears to involve an increase in cyclic AMP, as the effect of isoprenaline was potentiated after inhibition of phosphodiesterase.

Cholinergic modulation of the release of 5-hydroxytryptamine from the guinea pig ileum

Isolated segments of the guinea pig ileum were vascularly perfused and the release of 5-HT and its metabolite 5-HIAA into the portal venous effluent determined by HPLC with electrochemical detection. Test substances were applied via the arterial perfusion medium. Oxotremorine inhibited concentration-dependently the release of 5-HT and 5-HIAA (by 47% at 1 mumol/l). Scopolamine (0.1 mumol/l) did not affect the release of 5-HT and 5-HIAA, but antagonized the effect of oxotremorine. In the presence of TTX (1 mumol/l), oxotremorine (1 mumol/l) increased the release of 5-HT by 150% and that of 5-HIAA by 220%. This increase was completely blocked by scopolamine. Hexamethonium (100 mumol/l) and TTX (1 mumol/l) reduced the release of 5-HT by 32 and 40%, respectively. DMPP (10 mumol/l) increased the release of 5-HT by 57%, and this effect was prevented by hexamethonium. Neither DMPP nor hexamethonium significantly affected the release of 5-HIAA. The enhancing effect of DMPP on 5-HT release was increased and prolonged in the presence of TTX or scopolamine. Nicotine (1, 10 or 30 mumol/l) alone did not cause a consistent increase in the release of 5-HT. However, in the presence of scopolamine nicotine increased the release of 5-HT by 57%. In conclusion, the release of intestinal 5-HT is facilitated via muscarine and nicotine receptors located on the enterochromaffin cells. Indirect evidence suggests that the release of 5-HT is additionally modulated by an as yet unknown inhibitory neurotransmitter released by muscarine receptor activation.

Bipolarity of duodenal enterochromaffin cells in the rat

Enterochromaffin cells of the rat duodenum have been studied immunocytochemically by use of a specific antiserum to serotonin. At the light-microscopic level serotonin immunoreactivity was observed in enterochromaffin cells located in the epithelium of the duodenal mucosa. Most of the serotonin-immunoreactive material was localized to the basal portion of the enterochromaffin cells, but small amounts of immunoreactive material were regularly observed in the apical portion. At the electron-microscopic level serotonin immunoreactivity in enterochromaffin cells was found to be concentrated over the dense cores of the cytoplasmic granules. The majority of these granules was located in the basal cytoplasm of the enterochromaffin cells, but serotonin-immunoreactive granules were also observed in the apical cytoplasm immediately beneath the microvilli. These observations indicate that duodenal enterochromaffin cells are bipolar and that they secrete serotonin both basally, to the circulation, and apically, to the gut lumen. Rat duodenal enterochromaffin cells thus appear to have an exocrine as well as an endocrine function.

Cholinergic modulation of the release of serotonin in the gastric interstitial fluid. An in vivo study in rabbits

Dialysis fibers chronically implanted into the gastric submucosa of rabbits allowed us to simultaneously (a) collect an interstitial fluid dialysate in which 5-hydroxytryptamine concentrations were measured, and (b) locally perfuse drugs such as acetylcholine, neostigmine, and atropine, which stimulated the release of 5-hydroxytryptamine. The effect of acetylcholine was not blocked by atropine but was blocked by hexamethonium. Furthermore, 5-hydroxytryptamine concentrations in the interstitial fluid were lower when acetylcholine and hexamethonium were injected together than when hexamethonium was injected alone. We conclude that acetylcholine stimulates the release of 5-hydroxytryptamine into the gastric interstitial fluid by acting on nicotinic receptors, and has inhibitory effects by stimulating the muscarinic receptors.

Regional and selective changes in blood flow of the feline small intestine induced by endoluminal serotonin

A physiological role for endoluminal serotonin (5-HT) was explored in in vivo experiments, where feline jejunal segments were endoluminally perfused with saline or a low concentration of 5-HT. The upper jejunal segment was initially perfused with saline, followed by 5-HT in saline, and finally perfused with saline alone. The lower jejunal segment served as a control with constant perfusion with saline. The regional blood flow to the small intestine was determined by the microsphere technique. Endoluminal perfusion with 5-HT caused a selective muscular hyperaemia of the experimental gut segment, which was normalised upon subsequent saline perfusion. The blood flow to the saline perfused control segment was unchanged. This was also the case for control tissues; that is, adjacent small intestinal regions and kidneys. The 5-HT-induced muscular hyperaemia was confined to the experimental gut segment and seems to be mediated by a local cholinergic neural mechanism, activated from the mucosa, since the hyperaemic response was prevented by muscarinic blockade or local anaesthesia applied to the luminal surface of the experimental segment. The vascular response does not seem to involve 5-HT2 receptors, since selective blockade of such receptors did not prevent 5-HT-induced hyperaemia.

Local release of monoamines in the gastrointestinal tract: an in vivo study in rabbits

Dialysis fibers chronically implanted into the gastric submucosa of rabbits allowed us to collect an interstitial fluid (I.S.F.) dialysate in which biogenic amine concentrations were measured, and compared with those obtained from plasma and tissue samples. The results suggest that I.S.F. concentrations represent a good assessment of the local release of the amines by enteric nerves and/or paracrine cells, under basal conditions. The fact that acetylcholine and neostigmine, when perfused through the dialysis system, increased I.S.F. serotonin (5-HT) concentrations, supports a cholinergic modulation of the release of 5-HT within the gastrointestinal wall, and validates the dialysis method as a powerful tool to monitor, in vivo, dynamic changes in I.S.F. monoamine concentrations.

Dual presynaptic effects of 5-hydroxytryptamine on peripheral noradrenergic synapses

The aim of the present experiments was to study the effects of 5-hydroxytryptamine (5-HT) on the responses to postganglionic stimulation of two models of the peripheral sympathetic nervous system: the isolated nictitating membrane of the cat and the guinea-pig isolated atria. In the nictitating membrane of the cat, 5-HT (0.1 microM) shifted to the left the frequency-response curve to nerve stimulation. This potentiating effect was prevented by 5-HT receptor antagonists (0.1 microM methysergide, 0.1 microM pizotifen and 0.1 microM morphine) and also by the beta-adrenoreceptor blocker propranolol (0.1 microM). The alpha 2-adrenoreceptor antagonist yohimbine (0.1 microM) had no effect on the 5-HT-induced potentiation. In the guinea-pig isolated atria the responses to cardioaccelerans nerve stimulation were diminished by 5-HT (0.1 to 1.0 microM). The shift to the right in the frequency-response curve induced by 5-HT (1.0 microM) was additive to the antagonism caused in the atria by propranolol (0.1 microM). The inhibitory effect of 5-HT on the pacemaker responses to nerve stimulation was prevented by the 5-HT receptor antagonists methysergide (1.0 microM) and pizotifen (1.0 microM) and also by the alpha-adrenoreceptor antagonist phentolamine (0.1 microM). The selective alpha 2-adrenoreceptor agonist clonidine (0.01 microM) reduced to the same extent as 5-HT (1.0 microM) the responses to the guinea-pig atria to nerve stimulation. The inhibitory effect of clonidine was prevented by the alpha-adrenoreceptor blocker phentolamine (0.1 microM) but not by the 5-HT receptor blocker pizotifen (1.0 microM). With the exception of propranolol, which in the atria shifted to the right the concentration-response curve to exogenous noradrenaline (NA), neither 5-HT nor the different antagonists employed modified the sensitivity to NA in the tissues studied. The present observations show that 5-HT can produce a dual effect on the sympathetic neurotransmission. It is proposed that a modification in the overflow of NA in response to nerve stimulation is caused by 5-HT and results from the interaction of 5-HT with specific receptors located on the sympathetic fibres. These presynaptic 5-HT receptors behave as excitatory (cat nictitating membrane) or inhibitory (guinea-pig atria) depending on the tissue studied.

Light and propranolol suppress the nocturnal elevation of serotonin in the cerebrospinal fluid of rhesus monkeys

Markedly elevated nighttime concentrations of serotonin in rhesus monkey cerebrospinal fluid were reduced to daytime levels by exposing the monkeys to continuous light or to the beta-adrenergic antagonist propranolol. Nighttime elevations of melatonin in cerebrospinal fluid were also suppressed by propranolol and light. Serotonin released in large quantities at night appears to be regulated like melatonin, and may act as a cerebroventricular hormone to influence brain and pituitary function at night.

Cholinergic agonists induce vectorial release of serotonin from duodenal enterochromaffin cells

Serotonin-containing enterochromaffin cells in the rabbit duodenal mucosa span the tissue contacting both the luminal and serosal sides. When the serosal surface is stimulated with carbachol in vitro, serotonin is secreted on the serosal side but not the mucosal side. Carbachol added to the luminal side is ineffective. Atropine but not hexamethonium blocks the effect of carbachol. Acetylcholine on the serosal surface also stimulates serotonin release on the serosal side. These findings indicate that enterochromaffin cells possess on their serosal surfaces muscarinic receptors that mediate vectorial release of serotonin when activated by cholinergic agonists.