The enteric neural receptor for 5-hydroxytryptamine

Short- and Long-Term Effects of Cocaine on Enteric Neuronal Functions

Cocaine is one of the most consumed illegal drugs among (young) adults in the European Union and it exerts various acute and chronic negative effects on psychical and physical health. The central mechanism through which cocaine initially leads to improved performance, followed by addictive behavior, has already been intensively studied and includes effects on the homeostasis of the neurotransmitters dopamine, partly mediated via nicotinic acetylcholine receptors, and serotonin. However, effects on the peripheral nervous system, including the enteric nervous system, are much less understood, though a correlation between cocaine consumption and gastrointestinal symptoms has been reported. The aim of the present study was to gain more information on the effects of cocaine on enteric neuronal functions and the underlying mechanisms. For this purpose, functional experiments using an organ bath, Ussing chamber and neuroimaging techniques were conducted on gastrointestinal tissues from guinea pigs. Key results obtained are that cocaine (1) exhibits a stimulating, non-neuronal effect on gastric antrum motility, (2) acutely (but not chronically) diminishes responses of primary cultured enteric neurons to nicotinic and serotonergic stimulation and (3) reversibly attenuates neuronal-mediated intestinal mucosal secretion. It can be concluded that cocaine, among its central effects, also alters enteric neuronal functions, providing potential explanations for the coexistence of cocaine abuse and gastrointestinal complaints.

Uptake, recognition and responses to peptidoglycan in the mammalian host

Microbiota, and the plethora of signalling molecules that they generate, are a major driving force that underlies a striking range of inter-individual physioanatomic and behavioural consequences for the host organism. Among the bacterial effectors, one finds peptidoglycan, the major constituent of the bacterial cell surface. In the steady-state, fragments of peptidoglycan are constitutively liberated from bacterial members of the gut microbiota, cross the gut epithelial barrier and enter the host system. The fate of these peptidoglycan fragments, and the outcome for the host, depends on the molecular nature of the peptidoglycan, as well the cellular profile of the recipient tissue, mechanism of cell entry, the expression of specific processing and recognition mechanisms by the cell, and the local immune context. At the target level, physiological processes modulated by peptidoglycan are extremely diverse, ranging from immune activation to small molecule metabolism, autophagy and apoptosis. In this review, we bring together a fragmented body of literature on the kinetics and dynamics of peptidoglycan interactions with the mammalian host, explaining how peptidoglycan functions as a signalling molecule in the host under physiological conditions, how it disseminates within the host, and the cellular responses to peptidoglycan.

[Management of chemotherapy-induced emesis: what is the standard after 20 years of clinical research]

BACKGROUND

The knowledge of the importance, the physiopathological mechanisms, and the management of the chemotherapy-induced emesis has increased exponentially during the last 20 years. High-dosage metoclopramide (MCP) therapy has been introduced in the eighties and serotonine type-3 receptor antagonists (5-HT(3) antagonists) have been used since the late eighties and early nineties. Due to both classes of substances the results of the antiemetic therapies have improved drastically. After 20 years of intensive clinical research it seems to be appropriate to come to an intermediate conclusion.

METHOD

With the aid of an overview and a new analysis of the literature published on this topic so far, the current state of research is shown (including the fields in which further improvement will be necessary), and suggestions are made, wherever it seemed possible, to attain the "gold standard" in antiemetic therapy.

RESULTS AND CONCLUSIONS

In connection with all highly or very highly emetogenic chemotherapies, an antiemetic prophylaxis should be initiated on the day of therapy, especially when using platinum or most of the cyclophosphamide-based regimes for cancer treatment. The recommended prophylaxis consists of a combination of 5-HT(3) antagonists with a corticosteroid. To combat the so-called delayed emesis on the days following therapy, all patients should undergo an oral corticoid therapy, possibly in combination with MCP (especially platinum-therapy patients), less frequently with 5-HT(3) antagonists. With these means of prophylaxis emesis can be prevented/avoided completely in most patients, and nausea can at least be reduced. It is sufficient to administer a single dose of 5-HT(3) antagonists prior to chemotherapy. For ondansetron and granisetron, the best documented substances within this class of drugs, 8 mg (ondansteron) and 3 mg (granisetron) are considered standard dosages. Among the corticoids, most data have been accumulated for dexamethasone. A standard dose of 10 to 20 mg can be administered prior to chemotherapy. Right after and especially on the days following chemotherapy higher dosages seem to be indicated.

PROSPECT

Further therapy improvements, especially concerning emesis and nausea on the days following chemotherapy, are necessary and are currently object of clinical research.

Involvement of c-Src and protein kinase C delta in the inhibition of Cl(-)/OH- exchange activity in Caco-2 cells by serotonin

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter and intercellular messenger regulating various gastrointestinal functions, including electrolyte transport. To date, however, no information is available with respect to its effects on the human intestinal apical anion exchanger Cl(-)/OH- (HCO3-). The present studies were therefore undertaken to examine the direct effects of serotonin on OH- gradient-driven 4,4'-diisothiocyanato-stilbene-2, 2'-disulfonic acid-sensitive 36Cl- uptake utilizing the post-confluent transformed human intestinal epithelial cell line Caco-2. Our results demonstrate that serotonin inhibits Cl(-)/OH- exchange activity in Caco-2 cells via both tyrosine kinase and Ca(2+)-independent protein kinase C delta-mediated pathways involving either 5-HT3 or 5-HT4 receptor subtype. The data consistent with our inference are as follows. (i) The short term treatment of cells with 5-HT (0.1 microM) for 15-60 min significantly decreased Cl(-)/OH- exchange (50-70%, p < 0.05). (ii) The specific agonists for 5-HT3, m-chlorophenylbiguanide, and 5-HT4, 3-(4-allylpiperazin-1-yl)-2-quinoxaline chloronitrile, mimicked the effects of serotonin. (iii) Tropisetron dual inhibitor for both the 5-HT3/4 receptor subtypes significantly blocked the inhibition, whereas specific 5-HT3 (Y-25130) or 5-HT4 receptor (RS39604) antagonist failed to block the inhibitory effects of 5-HT. (iv) The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl ester) had no effect on the serotonin-induced inhibition. (v) The specific protein kinase C (PKC) inhibitors chelerythrine chloride or calphostin C completely blocked the inhibition by 5-HT. (vi) The specific inhibitor for PKC delta, rottlerin, significantly blocked the inhibition by 5-HT. (vii) The specific tyrosine kinase inhibitor, herbimycin, or Src family kinase inhibitor, PP1, abolished the 5-HT-mediated inhibition of Cl(-)/OH- exchange activity. (viii) 5-HT stimulated tyrosine phosphorylation of c-Src kinase and PKC delta.

Drug-nutrient interactions: inhibition of amino acid intestinal absorption by fluoxetine

Fluoxetine is one of the most widely used antidepressants and nowadays it is also being used to manage obesity problems. In our laboratory we demonstrated that the drug inhibited sugar absorption (Monteiro et al. 1993). The aim of the present work was to determine the effect of fluoxetine on intestinal leucine absorption. Using a procedure of successive absorptions in vivo the drug diminished amino acid absorption by 30% (P < 0.001). Experiments in vitro in isolated jejunum also revealed a reduction in leucine uptake of 37% (P < 0.001). In both cases fluoxetine only affected mediated transport without altering diffusion. In a preparation enriched in basolateral membrane, fluoxetine inhibited the Na+,K(+)-ATPase (EC 3.6.1.37) activity (55%; P < 0.001) in a non-competitive manner with an inhibition constant (Ki) value of 0.92 mM. Leucine uptake by brush-border membrane vesicles was diminished by the drug (a reduction of 48% was observed at 30s, P < 0.001); only the apical Na(+)-dependent transport system of the amino acid was modified and the inhibition was non-competitive. Leucine uptake in the presence of lysine indicated that transporter B was involved. These results suggest that fluoxetine reduces leucine absorption by its action on the basolateral and apical membrane of the enterocyte; the nutritional status of the patients under drug treatment may be affected as neutral amino acid absorption is decreased.

Mediation by protein kinases C and A of Go-linked slow responses of enteric neurons to 5-HT

5-HT activates the peristaltic reflex and is the neurotransmitter of a subset of myenteric interneurons. Hyperpolarizing afterpotential (AH)/type 2 neurons respond to 5-HT with a long-lived depolarization that is caused by the inhibition of a Ca(2+)-activated K+ conductance (gKCa). This effect is mediated by a G-protein-coupled receptor, 5-HT1P. 5-HT1P agonists specifically activate G alpha o, the immunoreactivity of which was found to be highly abundant and membrane-associated in almost all enteric neurons. Responses of hyperpolarizing AH/type 2 neurons to 5-HT were inhibited by intracellular injection of GDP beta S or anti-G alpha o Fab fragments but were potentiated and prolonged by intracellular GTP gamma S. Responses to 5-HT were antagonized by pertussis toxin, downregulation of protein kinase C (PKC) and inhibitors of phosphatidylcholine phospholipase C (PC-PLC), PKC (including pseudosubstrate peptides, chelerythrine, and the alpha/beta isoform-specific inhibitor Go 6976), protein kinase A (PKA), and adenylate cyclase. Responses to 5-HT were mimicked by activators of PKC, and 5-HT induced a concentration-dependent increase in the membrane-associated PKC activity in isolated myenteric ganglia. Immunocytochemical studies suggested that the most abundant isoforms of PKC in enteric neurons are alpha and delta. These data suggest that signal transduction of the 5-HT1P-mediated slow response to 5-HT involves activation of PC-PLC by G alpha o to liberate diacylglycerol, which stimulates PKC (most likely alpha). PKC probably activates adenylate cyclase, which through cAMP, activates PKA. Activation of both PKA and PKC lead to closure of gKCa.

Serotonin-induced changes in L-leucine transport across rabbit jejunum

The aim of the present study has been to determine the effect of serotonin (5-HT) on the absorption of L-leucine across the rabbit jejunum. The results show that serotonin significantly diminishes the uptake and steady-state tissue accumulation and the mucosal to serosal flux of L-leucine. This effect does not change with previous intestinal exposure of the mucosa to the 5-HT. Serotonin does not seem to modify amino acid simple diffusion across the intestinal epithelium. The effect on the amino acid uptake is due to an inhibition of the Na(+)-dependent system of transport, mainly through a reduction of the apparent Vmax. Moreover, this hormone, added to the incubation solution, does not affect the L-leucine uptake across brush border membrane vesicles. In presence of trifluoroperazine, TMB-8 and staurosporine, the serotonin effect disappears. These results suggest an effect mediated by intracellular processes related to protein kinase C which inhibit the intestinal absorption of L-leucine.

Serotonin-induced increase in cAMP in ganglia isolated from the myenteric plexus of the guinea pig small intestine: mediation by a novel 5-HT receptor

Serotonin (5-HT) is a mediator (through 5-HT1P receptors) of slow EPSPs in myenteric ganglia of the small intestine. The effect of 5-HT can be mimicked by elevating cAMP; therefore, we tested the hypothesis that the slow EPSP-like response to 5-HT is cAMP-mediated. Guinea pig gut was enzymatically dissociated; myenteric ganglia remained intact and were collected by filtration. Neurons in the isolated ganglia retained their ability to manifest the slow EPSP-like response to 5-HT. Exposure to 5-HT raised the ganglionic level of cAMP (ED50 0.3 microM). This effect was not antagonized by the 5-HT1P antagonist, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (100.0 microM), or mimicked by the 5-HT1P agonist, 5-hydroxyindalpine (10.0 microM). Increases in cAMP were also evoked by the 5-HT1 agonist, 5-carboxyamidotryptamine (10.0 microM), the 5-HT2 agonist, (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 1.0-10.0 microM), and by the 5-HT4 agonists, renzapride (1.0-10.0 microM) and 5-methoxytryptamine (1.0-10.0 microM); however, neither the 5-HT1/5-HT2 antagonists, spiperone, methysergide, and methiothepin, nor the 5-HT4 antagonist, tropisetron (ICS 205-930; 10.0 microM), were able to inhibit the rise in cAMP evoked by these compounds or by 5-HT (0.1-10.0 microM). The 5-HT-evoked elevation of cAMP was antagonized by ketanserin (10.0 microM), which also blocked the effects of 5-methoxytryptamine and DOI, but not those of renzapride. The effective concentration of DOI, however, was higher than that needed for activation of 5-HT2 receptors, and Northern analysis using a cDNA probe encoding the rat 5-HT2 receptor failed to reveal the presence of 5-HT2 mRNA in myenteric ganglia, although it hybridizes with mRNA of the right size in the guinea pig brain. Compounds that failed to change levels of cAMP or to antagonize the action of 5-HT included 8-hydroxy-di-n-propylamino tetralin, R58639, R88226, and sumatriptan. It is concluded that the receptor responsible for the 5-HT-induced rise in cAMP in ganglia isolated from the guinea pig myenteric plexus is not a known subtype of 5-HT receptor. Since the pharmacology of this novel receptor is different from that of the slow EPSP-like response to 5-HT, the receptor probably does not mediate the slow EPSP.

Pharmacokinetics of an immunomodulator peptidoglycan monomer in mice after intravenous administration

A 14C labeled low molecular weight immunomodulator, peptidoglycan monomer (14C-PGM), was injected intravenously (i.v.) into mice. At various time intervals thereafter (15 min-6 h), radioactivity in the urine, whole blood, plasma, kidneys, liver, spleen, lungs, intestines and the brain of the mice was determined. Shortly after injection, 14C-PGM was very rapidly excreted from the organism, so that 1 h following administration, 80% of the radioactivity was found in the urine (62% as unchanged PGM and the rest as the metabolites pentapeptide and disaccharide). At the same time, around 2% of the injected material was found in the blood. Six hours after injection, equal quantities were found in the intestines, liver and blood (0.5%), slightly less in the kidneys, lungs and spleen (0.2-0.3%) and the least quantity in the brain (0.04%). However, the dynamics of retention in the organs was evidently different. In the kidneys, lungs and spleen, radioactivity steadily decreased over the studied period. In the liver following an initial decrease, radioactivity remained the same 3 and 6 h after injection. On the other hand, in the intestines and brain PGM seemed to accumulate rather than disappear following i.v. administration. This fact should be considered when explaining different biological activities of low molecular weight bacterial peptidoglycans.

Serotonin: its role and receptors in enteric neurotransmission

Enteric neural 5-HT receptors were analyzed and related to possible physiological actions of 5-HT. Receptors were identified electrophysiologically with intracellular microelectrodes and by studies of the binding of radioligands. Radioligand binding was assessed by rapid filtration and by radioautography. Three subtypes of 5-HT receptor, 5-HT1P, 5-HT3, and 5-HT1A, were identified. 5-HT1P receptors were found to mediate slow depolarizations of myenteric neurons that were associated with a decrease in membrane conductance. These responses were inhibited by 5-HTP-DP and by BRL 24924 and mimicked by 5- and 6-hydroxyindalpine. 5-HT1P receptors were labeled with high affinity by 3H-5-HT and were located on both submucosal and myenteric neurons and on processes of intrinsic neurons in the lamina propria. Serotonergic EPSPs were found to be mediated by 5-HT1P receptors; it is postulated that 5-HT1P receptors may be involved in initiation of the peristaltic reflex and in the regulation of gastic emptying. 5-HT3 receptors have been shown to be responsible for fast depolarizations of myenteric and submucosal neurons associated with a rise in membrane conductance. These responses are antagonized by ICS 205-930 and mimicked by 2-methyl-5-HT. 5-HT1A receptors have been reported by others to mediate hyperpolarizing responses of myenteric neurons associated with a rise in membrane conductance. Hyperpolarizing responses are also elicited by the 5-HT1A agonist, 8-OH-DPAT. No physiological role has yet been identified for 5-HT3 or 5-HT1A receptors in the ENS.

Evaluation of the activity of chemically identified enteric neurons through the histochemical demonstration of cytochrome oxidase

The measurement of the density of the reaction product produced by the histochemical demonstration of cytochrome oxidase activity provides a method for the visual identification of physiologically active enteric neurons. The current study utilized the cytochrome oxidase technique in order to evaluate the metabolic history of neurons in different regions of the bowel and in chemically identified types of neuron. In addition, the effect of drugs or neurotoxins commonly used in the immunocytochemical identification of enteric neuronal phenotypes was also analyzed. Cytochrome oxidase activity was visualized with a blue-black reaction product resulting from the cobalt-intensified oxidation of 3,3'-diaminobenzidine. Peptides or 5-hydroxytryptamine (5-HT) were localized with biotinylated secondary antibodies and alkaline phosphatase-labeled avidin. Bound avidin or endogenous alkaline phosphatase was visualized with a red reaction product in the presence or absence, respectively, of levamisole. Use of measured without interference from a simultaneously demonstrated histo- or immunochemical marker. A multi-peptidergic class of cholinergic submucosal secretomotor neuron containing neuropeptide Y (NPY) and calcitonin gene related peptide (CGRP) immunoreactivities was found to be less metabolically active than the average of all submucosal neurons. In contrast, a non-cholinergic submucosal secretomotor neuron containing dynorphin (which is also known to contain vasoactive intestinal peptide) immunoreactivity was more metabolically active than submucosal neurons that do not contain this peptide. On average, submucosal neurons were more metabolically active than those of the myenteric plexus, and levels of metabolic activity in the myenteric plexus were found to be higher in the duodenum and the cecum than in the jejunum-ileum or colon. Myenteric neurons characterized by CGRP or NPY immunoreactivities or by endogenous alkaline phosphatase activity, were all less metabolically active than the average of all neurons in myenteric ganglia. Colchicine, which stimulates intestinal motility, was observed to increase cytochrome oxidase activity in enteric neurons, suggesting that an effect on the enteric nervous system contributes to its action on the bowel. The neurotoxins, 6-hydroxydopamine and 5,7-dihydroxytryptamine (5,7-DHT) were each found to stimulate neuronal metabolic activity. 5,7-DHT appeared to activate excitatory subtypes of 5-HT receptor since its effects were blocked or mimicked by compounds that act as antagonists or agonists, respectively, at these receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological and functional analysis of a novel serotonin receptor in the rat hippocampus

Administration of serotonin (5-hydroxytryptamine, 5-HT) to pyramidal cells of the CA1 region of the hippocampus results in a hyperpolarizing response which is followed by a rebound depolarization and a decrease in the calcium-activated afterhyperpolarization (AHP). While the hyperpolarizing response has been previously shown to be mediated by receptors of the 5-HT1A subtype, the identity of the receptor(s) involved in the depolarizing response and decrease of the AHP have not been identified. In the present study the effectiveness of a series of 5-HT receptor antagonists in blocking the membrane depolarization and reduction of the AHP was assessed. While a variety of 5-HT1 and 5-HT2 antagonists were found to be ineffective, the substituted benzamide BRL 24924 was found to be a potent and selective antagonist of the 5-HT-induced depolarization and decrease in the AHP in this region. This effect however appeared unrelated to the ability of this compound to block 5-HT3 receptors, as ICS 205-930 and MDL 72222 were markedly less efficacious in blocking these effects of 5-HT. Upon blockade of 5-HT1A receptors, 5-HT elicits a depolarization which is accompanied by a marked increase in excitability. These effects were also dose-dependently antagonized by BRL 24924. The present results thus suggest the presence in the CA1 region of the hippocampus of a novel 5-HT receptor at which BRL 24924 functions as a selective antagonist and which is capable of mediating slow excitatory responses in central neurons.

Correlation of anti-emetic efficacy and plasma levels of ondansetron

Intravenous ondansetron was administered at doses from 0.01 to 0.48 mg/kg every 4 h for three doses to patients receiving cisplatin 60-120 mg/m2 for the first time. Plasma samples were collected from 28 patients at baseline and at suitable times post-dose for pharmacokinetic analysis, and were assayed for ondansetron by high-pressure liquid chromatography. Plasma trough level was defined as the level before the third dose and 4 h area-under-the-curve (AUC4) was calculated with the linear trapezoidal method. Despite wide inter-patient variation, a correlation was seen between both trough level (r = 0.737, P less than 0.0001) and AUC4 (r = 0.903, P less than 0.001) related to dose. Trough level was also predictive of AUC4 (r = 0.824, P less than 0.0001). Frequency of complete protection (no emetic episodes) was equivalent throughout the AUC4 range, suggesting anti-emetic activity even at low AUC4. However, a trend toward better protection against failure (5 or more episodes) was seen when higher values of AUC4 were achieved, suggesting more consistent anti-emetic activity at moderate to high AUC4.

Advances in the management of nausea and vomiting induced by non-cisplatin containing chemotherapeutic regimens

Nausea and vomiting are the most feared toxicities of chemotherapy. Afferent impulses from the chemoreceptor trigger zone, peripheral sites, the cerebral cortex, or the vestibular center can initiate the emetic reflex. Antiemetic protection therefore requires interruption of appropriate emetogenic pathways. Since the chemoreceptor trigger zone contains numerous dopaminergic neurons, antidopaminergic agents including phenothiazines and metoclopramide have gained importance as antiemetics. However standard dose phenothiazines have limited efficacy while high dose metoclopramide may have excessive toxicity in the non-cisplatin setting. Recent advances have therefore centered on development of new classes of antiemetics. Corticosteroids have excellent activity alone or in combination with other antiemetic agents. Cannabinoids have recently been introduced into commercial use as antiemetics with particular activity against non-cisplatin chemotherapy. Benzodiazepines are active against anticipatory nausea and vomiting and are also used in combination antiemetic regimens. Although the vestibular center seems to have a lesser influence on chemotherapy-induced nausea and vomiting, vestibular blocking agents such as scopolamine may have a potential role as adjunctive antiemetics. Finally, appreciation of the role of serotonin (5-HT3) receptors in both peripheral and central emetic pathways may lead to a new class of antiserotonergic antiemetic agents.

Association of [3H]zacopride with 5-HT3 binding sites

An assay was developed for [3H]zacopride binding to 5-HT3 specific sites in membranes from rabbit ileum muscularis. The binding was rapid, saturable, reversible, salt-insensitive, unaffected by pH between 6.5 and 9.5, and of high affinity (apparent KD = 0.65 +/- 0.15 nM). ICS 205-930, a potent 5-HT3 antagonist that inhibited competitively, was utilized to define 5-HT3 specific binding. Other 5-HT3 antagonists and agonists, although exhibiting marked differences in potency, were also effective inhibitors; whereas, antagonists of other classes of serotonin receptors, guanyl nucleotides and numerous receptor-specific ligands, including peptide hormones, were inactive. Vagus nerve exhibited the greatest amount of 5-HT3 specific binding amongst rabbit tissues and virtually all of the [3H]zacopride was bound to 5-HT3 binding sites. In rabbit, rat and ferret a fairly uniform distribution of 5-HT3 binding sites was observed along the muscularis of the small bowel. [3H]Zacopride is a high-affinity ligand for detecting 5-HT3 binding sites and rabbit small bowel muscularis membranes are a sensitive system for evaluating the potency of 5-HT3 antagonists or agonists.

Time course of expression of neuropeptide Y, calcitonin gene-related peptide, and NADPH diaphorase activity in neurons of the developing murine bowel and the appearance of 5-hydroxytryptamine in mucosal enterochromaffin cells

Serotoninergic and cholinergic neurons are known to appear earlier in the ontogeny (day E12) of the murine gut than those containing substance P or vasoactive intestinal peptide (day E14). It has also been demonstrated that proliferating neural precursors coexist with mature neurons in developing enteric ganglia. These observations have led to the hypotheses that peptidergic neurons develop later than those that utilize small molecule neurotransmitters and that the activity of early developing neurons may affect the phenotypic expression of coexisting neuroblasts. As a partial test of these hypotheses we studied the phenotypic expression of neurons recognized by antisera to neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP), and of those visualized by the histochemical demonstration of reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity. NADPH diaphorase activity, which is coexpressed with NPY immunoreactivity in all submucosal and many myenteric neurons, was first found on day E11 in clusters of cells in the dorsal mesogastrium. These cells also expressed neurofilament reactivity and thus were developing along a neuronal lineage. Enteric neurons that expressed NADPH diaphorase activity were visualized in the stomach one day later, on day E12. At this time, NADPH diaphorase-containing cells could no longer be demonstrated in the dorsal mesogastrium. NPY immunoreactivity first appeared in the wall of the bowel on day E12, when it was seen in cells in the presumptive stomach. By day E13, the entire length of the bowel contained NPY-immunoreactive neurons. Cells that displayed NADPH diaphorase activity were found at this time at both ends of the alimentary tract, but did not appear in the ileum until day E18. In contrast, CGRP immunoreactivity could not be detected anywhere in the gut until day E17, but by day E18 all regions of the bowel contained CGRP-immunoreactive neurons. Endogenous 5-HT was first detected at day E16 in mucosal epithelial cells in all segments of the gut except the stomach, where it appeared at day E18. The NPY/NADPH diaphorase set of neurons thus develop before the acquisition of a detectable level of endogenous 5-HT or enteric neural 5-HT receptors (which arise in the foregut at day E14). These observations demonstrate that enteric neurons that express small molecule neurotransmitters do not necessarily develop earlier than peptidergic neurons as a class; however, various types of enteric neurons do appear in a sequential order.(ABSTRACT TRUNCATED AT 400 WORDS)

Rational pharmacotherapy of gastrointestinal motility disorders

Nervous control of gastrointestinal motility is extremely complex, is regulated by the enteric system, the "brain of the gut", and modulated by extrinsic nerves. This system with its multiplicity of transmitters and receptors does not always allow a clear interpretation of experimental data, especially with compounds lacking specificity. In this review the complex situation is described particularly in relation to receptor populations (cholinergic, adrenergic, dopamine, histamine, 5-hydroxytryptamine, opioid, gamma-aminobutyric acid (GABA), prostanoid and dihydropyridine receptors), therapeutic aspects of drugs and their usefulness in children. Newer principles with known drugs and promising new compounds with a more appropriate kinetic or fewer side-effects, deriving from distinct pharmacological groups, as candidates for the treatment of gastrointestinal disorders are considered e.g. anticholinergics (prifinium or actilonium bromide), adrenergic alpha 2-agonists (clonidine, lidamidine) for diarrhoea in diabetic neuropathy, adrenergic beta-blockers for shortening postoperative ileus (propranolol), dopamine receptor antagonists (metoclopramide, domperidone, alizapride) and another prokinetic substance (cisapride) which may be useful for a number of applications as gastro-oesophageal reflux, gastro-paresis, intestinal pseudo-obstruction, cystic fibrosis and constipation, morphine derivatives (e.g. loperamide) for intractable diarrhoea and calcium antagonists (e.g. nifedipine) for achalasia. Increasing experience in digestive tract pharmacology and reliable clinical studies will furthermore be the basis for a more specific and better tolerated therapy of gastrointestinal motility disorders in adults and children.

Involvement of 5-hydroxytryptamine, prostaglandin E2, and cyclic adenosine monophosphate in cholera toxin-induced fluid secretion in the small intestine of the rat in vivo

The diarrhea of cholera is considered to rely solely on a cyclic adenosine monophosphate-mediated secretory mechanism. However, both 5-hydroxytryptamine and prostaglandin E2 have been proposed to be involved in the pathogenesis of cholera. In vivo experiments were performed, therefore, in the rat jejunum to investigate the influence of purified cholera toxin on fluid secretion, luminal release of 5-hydroxytryptamine and prostaglandin E2, and formation of mucosal cyclic adenosine monophosphate. Also the effects of ketanserin, indomethacin, verapamil, and nifedipine on the named parameters were studied. Cholera toxin dose-dependently (0.1-0.5 microgram/ml) and time-dependently (1-5 h) increased mean net fluid secretion with a maximum response at 4 h. It also caused a significant (p less than 0.01) rise in release of 5-hydroxytryptamine and prostaglandin E2, in addition to formation of cyclic adenosine monophosphate. The dose-response curve for cholera toxin-induced fluid secretion was shifted to the right by indomethacin (10 mg/kg s.c.) and ketanserin (200 micrograms/kg s.c.), none of which caused a change in cholera toxin-induced release of 5-hydroxytryptamine. However, both agents significantly decreased the release of prostaglandin E2. Verapamil (0.2-9.5 micrograms/min i.a.) and nifedipine (0.05-0.5 microgram/min i.a.) dose-dependently reduced cholera toxin-induced fluid secretion. The estimated local concentrations at half-maximal inhibition were 5 x 10(-7) M verapamil and 5 x 10(-8) M nifedipine, respectively. The cholera toxin-induced increase in release of 5-hydroxytryptamine and prostaglandin E2 and formation of cyclic adenosine monophosphate was unaffected by verapamil. These results support the concept that cholera toxin-induced fluid secretion in vivo is caused, in part, by release of 5-hydroxytryptamine, which in turn stimulates formation of prostaglandin E2.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Serotonin neural receptors mediate motilin-induced motility in isolated, vascularly perfused canine jejunum

We previously reported that motilin induced both motility and endoluminal release of serotonin (5-HT) in canine jejunum. The motility response was blocked by nonspecific tachyphylaxis of 5-HT receptors. In this study, we investigated the effect of a selective neural serotonin receptor (5-HT3) antagonist, ICS 205-930, given into either the artery or the lumen, on motilin-induced motility. Segments of canine jejunum were arterially perfused with Kreb's HCO3 solution containing 15% RBCs in an organ preservation system. Following a 15-min basal period (Basal-1), 500 fM motilin was infused into the artery over 15 min (Motilin-1). After a 30-min rest, the cycle was repeated (Basal-2 and Motilin-2) in the presence and absence (control) of ICS 205-930, which was given as a continuous infusion either into the artery or into the lumen (0.1 to 1.0 microM). From continuous intraluminal pressure tracings, a motility index (MI) was calculated for 5- or 15-min periods from the product of the frequency and the mean amplitude of phasic waves for that period. In control segments, motilin reproducibly induced significant (P less than 0.05) increases in MI over basal levels during both Motilin-1 and Motilin-2. Arterial 0.1 microM ICS 205-930 abolished the motor response to rechallenge with motilin: 145 +/- 28 to 288 +/- 10 mm Hg (Motilin-1) versus 168 +/- 27 to 168 +/- 13 mm Hg (Motilin-2). An infusion of 1 microM ICS 205-930 into the lumen was required to inhibit the response. Thus, motilin-induced motility is mediated by neural 5-HT receptors. Since both arterial and luminal antagonist was effective, it is likely that 5-HT3 receptors on both myenteric plexus neurons and neurons in proximity to mucosal enterochromaffin cells mediate the motor response to motilin.

Prostaglandin E2 is a mediator of 5-hydroxytryptamine induced water and electrolyte secretion in the human jejunum

Studies in the rat jejunum in vivo have shown that 5-hydroxytryptamine (5-HT) causes secretion of fluid and luminal release of prostaglandin (PG) E2. These effects can be blocked by indomethacin and ketanserin, which suggests that PGE2 may be an important intermediate in the transduction mechanism leading to 5-HT induced fluid secretion. To test this hypothesis in man 'steady state' perfusions (9 ml/min) were done in eight healthy volunteers using the triple lumen technique. The proximal jejunum was perfused with Ringer's solution which contained 51Cr-EDTA as a non-absorbable marker. Before and after the administration of indomethacin (1.0 mg/kg iv) the effects of exogenous 5-HT (10 micrograms/kg/min iv) on jejunal net transport of fluid and electrolytes and jejunal flow rate (JFR) of PGE2 were measured in 15-min periods for 2 x 60 minutes after a 60 minute control period. 5-HT reversed fluid and electrolyte absorption into profuse secretion (p less than 0.01, Duncan's multiple range test) and significantly increased JFR of PGE2 (p less than 0.01). Indomethacin partly restored fluid and electrolyte absorption (p less than 0.01) and inhibited JFR of PGE2 (p less than 0.05). These results provide further evidence in favour of the theory that PGs are involved in 5-HT induced intestinal fluid secretion.

Effects of intravenous calcium on release of serotonin into jejunal lumen and portal circulation

The effect of intravenous calcium bolus (180 mg in 10 ml normal saline over 25-30 sec) on the release of serotonin into the jejunal lumen and the portal and peripheral venous circulation was studied. Proximal jejunal 25-cm cannulated Thiry-Vella loops were perfused with a neutral physiological buffer in an isoperistaltic direction at 2 ml/min. One minute after the calcium bolus, serum calcium levels increased from 8.7 +/- 0.3 to 14.2 +/- 0.8 mg/dl. Jejunal luminal concentrations of 5HT increased from 135 +/- 21 to 208 +/- 44 ng/ml at the same time; luminal levels peaked at 236 +/- 27 ng/ml at 7 min and slowly returned to baseline. In contrast, portal and systemic venous concentrations did not change after intravenous calcium bolus. The data support the contention that there are independent mechanisms for the release of serotonin into the bowel lumen and the blood stream.

Analgesia induced by N-acetylserotonin in the central nervous system

The relationship between N-acetylserotonin (NAS) in the central nervous system (CNS) and responses to pain was investigated. Using the rat tail-flick model, we initially replicated the work of others showing that intraventricular (IVC) injection of a dipeptide structurally similar to both NAS and serotonin was capable of inducing analgesia in the rat. We then showed that IVC-NAS, but not serotonin elicited analgesia in much the same manner as the dipeptide. This effect proved to be very specific as it required the presence of both an acetyl group on the terminal side chain amine as well as a hydroxyl group on the C-5 position of the indole ring. Substitution of the C-5 hydroxyl by a methoxyl group (melatonin) abolished the analgesic effect. Similarly, removing the N-acetyl substitution (serotonin) also eliminated the analgesia. IVC injection of highly specific antiserum to NAS induced hyperalgesia. Furthermore, an interaction was found between NAS and opiate systems. We demonstrated that while naloxone, the opiate antagonist, has no hyperalgesic properties of itself, it did counteract the analgesia induced by NAS. Similarly, NAS antiserum reversed the analgesia induced by the opiate morphine. This work provides evidence that NAS is an endogenously active substance within the CNS pain network.

Pharmacologically distinct actions of serotonin on single pyramidal neurones of the rat hippocampus recorded in vitro

1. The actions of serotonin (5-HT) on pyramidal cells of the CA1 region of the rat hippocampus were characterized using intracellular recording in in vitro brain slices. 2. 5-HT typically evokes a biphasic response consisting of a hyperpolarization which is followed by a longer-lasting depolarization. These effects on membrane potential are accompanied by a decrease in the calcium-activated after-hyperpolarization (a.h.p). 3. Detailed analysis using 5-HT antagonists and agonists indicates that the hyperpolarization is mediated by a 5-HT1A receptor. Spiperone is the most effective antagonist of the response and the selective 5-HT1A agonist, 8-OHDPAT, behaves as a partial agonist at this receptor. In agreement with the distribution of 5-HT1A binding sites, responses to 5-HT were most prominent in the stratum radiatum. 4. The hyperpolarizing response is associated with a decrease in input resistance, is blocked by extracellular barium and intracellular caesium, is unaffected by the chloride gradient, and its reversal potential shifts with the extracellular concentration of potassium as predicted for a response mediated by a selective increase in potassium permeability. 5. The depolarizing response and reduction in the a.h.p. could be studied in isolation by blocking the hyperpolarizing response with either pertussis toxin or spiperone. The pharmacology of these responses did not correspond to that of any of the 5-HT binding sites reported in C.N.S. tissue. Although the depolarization and blockade of the a.h.p. have the same time course it is unclear if they are mediated by the same or different receptors. 6. The depolarization most likely results from a decrease in resting potassium conductance. However, neither a blockade of the M current nor the a.h.p. current can account for the depolarization. 7. Blockade of phosphodiesterase activity by 3-isobutyl-1-methylxanthine (IBMX) did not enhance the depressant action of 5-HT on the a.h.p., making it unlikely that this action is mediated by cyclic AMP. 8. Blockade of the a.h.p. by 5-HT reduces spike frequency adaptation and counteracts the inhibitory action of 5-HT on 5-HT1A receptors. This excitatory action outlasts the hyperpolarizing action. 9. In summary 5-HT acts on at least two distinct receptors on hippocampal pyramidal cells, one coupled to the opening of potassium channels and a second coupled to a decrease in a resting potassium conductance and a decrease in the a.h.p.

Influence of a serotonin receptor antagonist, 5-HTP-DP-hex, on spinal and thalamic nociceptive neurons in rats

The antinociceptive properties of a new synthetic dipeptide (N-hexanoyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, or 5-HTP-DP-hex) were studied in rats by an electrophysiological method. After an i.p. injection of alpha-chloralose and urethane, the animals were prepared for stereotaxic approach to the nucleus ventralis posterolateralis of the thalamus. With tungsten microelectrodes, individual nociceptive neurons in the nucleus were identified by the sequence of spikes emitted in response to single-pulse stimulation of the sciatic nerve. In addition to the usual short-latency spikes, a nociceptive neuron fired late spikes at regular intervals within 500 ms following each stimulus. When the spikes were accumulated in poststimulus time histograms, the short-latency spikes compiled an intensity-related (I) peak. The late spikes formed modality-related (M) peaks with spacing characteristic of nociception. Intracarotid infusion of 5-HTP-DP-hex (1 mg/kg) elevated the delayed portion of the I peak and the first M peak. This effect was followed in 25 min by suppression of all M peaks. The control record could be reinstated at any time by 5-hydroxytryptophan (3.5 mg/kg), or by natural recovery in 2.5 h. Responses evoked from a thalamic nociceptive neuron by single-pulse stimulation of the spinothalamic tract were modified by 5-HTP-DP-hex in a similar manner, except that no elevation of the activity peaks was observed. As shown previously, elevation of the delayed I peak and M1 indicated an increased input of A-delta and C fibers, respectively. The increased input lowers the response threshold and may represent hyperalgesia. Suppression of the M peaks may result from altered function of the positive feedback loop in the nociceptive system at the thalamic level, and may represent analgesia. Naloxone, methysergide, as well as ketanserin had no significant effect on the response histograms. These findings suggested that 5-HTP-DP-hex, a known serotonin receptor antagonist, targeted its action on very specific receptors, and thus interfered with particular synaptic activity within the spinal cord and on the thalamic level.

Development of neural receptors for serotonin in the murine bowel

During the ontogeny of the enteric nervous system, the varicosities of mature neurons contact dividing neural precursors, which persist in the developing murine gut for several weeks postnatally. This phenomenon has led to the hypothesis that the release of transmitter from mature neurons may influence the subsequent development of uncommitted neuroblasts. In order to test this hypothesis, it is necessary to know the timing of the expression of postsynaptic receptors for enteric neurotransmitters. Since serotoninergic neurons are among the earliest of enteric neurons to develop (appearing on day E12 of development in the mouse), the ontogeny of enteric neural serotonin receptors (5-HTR) was studied. These receptors have previously been characterized in adult guinea pigs and rabbits. In the current experiments, 5-HTR were identified in the adult murine bowel; their properties were compared with the 5-HTR of guinea pig and rabbits; their ontogeny was followed throughout the length of the developing mouse gut; and the properties of 5-HTR in the developing murine bowel were compared with those of the mature gut. The 5-HTR were assayed by measuring the binding of 3H-serotonin (3H-5-HT) to isolated enteric membranes by rapid filtration, and to frozen sections of bowel by radioautography. A single saturable, high affinity 3H-5-HT binding site was found in membranes from the adult mouse gut (KD = 3.9 +/- 0.5 nM; Bmax = 1.6 +/- 0.3 pmoles/mg protein). Binding of 3H-5-HT at this site was not antagonized by compounds known to be antagonists at receptors for other neurotransmitters or at the 5-HT1 or 5-HT2 class of CNS 5-HTR. Hydroxylation of the indole ring of analogues of serotonin was required for affinity at the enteric 3H-5-HT binding site. Binding of 3H-5-HT was inhibited by N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan dipeptide, a compound that has been demonstrated to antagonize those responses of myenteric neurons to serotonin that resemble slow excitatory postsynaptic potentials, but not by ICS 205-930 (Sandoz), a serotonin antagonist that does not block these responses. All of these properties of adult murine 3H-5-HT binding sites are virtually identical of those of guinea pigs and rabbits, which have previously been shown to be 5-HTR; therefore, murine enteric 3H-5-HT binding sites are probably 5-HTR as well.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptors in the gastrointestinal tract

The receptor concept has been recently evolved and a new science was actually created, namely "receptorology". Receptors are now identified by means of different techniques (binding, agonist-antagonist interaction, autoradiography, etc.). The new techniques allowed the investigators to define new receptors and new subtypes of the "classical" ones. In the gastrointestinal (GI) tract a number of receptors have been identified and localized both on the effector organ and in the nerve terminal where they exert an important modulatory function on the neurotransmitter release. Recent biochemical studies have allowed a better understanding of the post-receptor event involving the second or third messenger regulation. Particular changes of receptors were recognized and they allow us to consider receptors not as static entities but as very dynamic components of the plasma membrane capable of different kinds of alterations, like interconversion, internalization, mobility, up- and downregulation, etc. Together with the "classical" receptors (cholinergic, adrenergic, opioid, etc.) also new receptors were identified: different subtypes of receptors for the tachykinins, for prostaglandin of the E type in the gastric parietal cell and the so-called dihydropyridine (DHP) receptor in the calcium channel of different areas of the gut. It is obvious that the precise knowledge of receptors and of their agonists and antagonists will represent the basis for a more specific and efficacious treatment of various gastrointestinal disorders.

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.

Analgesic properties of a systemically-administered synthetic dipeptide of 5-hydroxytryptophan

Synthetic peptides of 5-hydroxytryptophan (5-HTP), including N-acetyl-5-HTP-5-HTP amide (5-HTP-ACETYL-DP), specifically inhibit the binding of serotonin to serotonin binding protein. 5-HTP-ACETYL-DP also produces a long-lasting, opiate-sensitive analgesia following central, but not systemic administration. The present study evaluated an apolar derivative of 5-HTP dipeptide, N-hexanoyl-5-HTP-5-HTP amide (5-HTP-HEX-DP), for its analgesic properties in rats following systemic administration. 5-HTP-HEX-DP (5-50 mg/kg) significantly increased jump thresholds in a dose-dependent manner with peak analgesia occurring at 2.5 hr after injection, and lasting up to 5 hr. In the tail-flick assay, 5-HTP-HEX-DP (20 mg/kg) produced a significant antinociceptive effect at 1 hr post-injection using both high and low intensity levels of radiant heat. While 5-HTP-HEX-DP and morphine each elicited analgesia following acute administration, chronic (14 days) incremental dosing with 5-HTP-HEX-DP or morphine resulted in persistent analgesia in 5-HTP-HEX-DP-treated animals, and a loss of analgesia in morphine-treated rats. Thus, significant tolerance to morphine, but not 5-HTP-HEX-DP analgesia developed using this protocol. Hence, 5-HTP-HEX-DP is a systemically-active analgesic which fails to develop tolerance when administered daily over 14 days.

Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.