Detection of the 5-HT1A receptor and 5-HT1A receptor mRNA in the rat bowel and pancreas: comparison with 5-HT1P receptors

Gastrointestinal hormones and regulation of gastric emptying

PURPOSE OF REVIEW

In this review, we evaluate recent findings related to the association between gastrointestinal hormones and regulation of gastric emptying.

RECENT FINDINGS

Motilin and ghrelin, which act during fasting, promote gastric motility, whereas most of the hormones secreted after a meal inhibit gastric motility. Serotonin has different progastric or antigastric motility effects depending on the receptor subtype. Serotonin receptor agonists have been used clinically to treat dyspepsia symptoms but other hormone receptor agonists or antagonists are still under development. Glucagon-like peptide 1 agonists, which have gastric motility and appetite-suppressing effects are used as a treatment for obesity and diabetes.

SUMMARY

Gastrointestinal hormones play an important role in the regulation of gastric motility. Various drugs have been developed to treat delayed gastric emptying by targeting gastrointestinal hormones or their receptors but few have been commercialized.

The Role of Serotonin Neurotransmission in Gastrointestinal Tract and Pharmacotherapy

5-Hydroxytryptamine (5-HT, serotonin) is a neurotransmitter in both the central nervous system and peripheral structures, acting also as a hormone in platelets. Although its concentration in the gut covers >90% of all organism resources, serotonin is mainly known as a neurotransmitter that takes part in the pathology of mental diseases. Serotonin modulates not only CNS neurons, but also pain transmission and platelet aggregation. In the periphery, 5-HT influences muscle motility in the gut, bronchi, uterus, and vessels directly and through neurons. Serotonin synthesis starts from hydroxylation of orally delivered tryptophan, followed by decarboxylation. Serotonin acts via numerous types of receptors and clinically plays a role in several neural, mental, and other chronic disorders, such as migraine, carcinoid syndrome, and some dysfunctions of the alimentary system. 5-HT acts as a paracrine hormone and growth factor. 5-HT receptors in both the brain and gut are targets for drugs modifying serotonin neurotransmission. The aim of the present article is to review the 5-HT receptors in the gastrointestinal (GI) tract to determine the role of serotonin in GI physiology and pathology, including known GI diseases and the role of serotonin in GI pharmacotherapy.

Further investigation of the effects of 5-hydroxytryptamine, 8-OH-DPAT and DOI to mediate contraction and relaxation responses in the intestine and emesis in Suncus murinus

5-HT receptors are implicated in many gastrointestinal disorders. However, the precise role of 5-HT in mediating GI responses in Suncus murnius is still unclear. Therefore in this study, the effects of 5-HT and its agonists were investigated in Suncus. The involvement of 5-HT_2C receptors in mediating emesis was also investigated. The ability of 5-HT and its agonists/antagonists at 5-HT_1A and 5-HT₂ to modify GI motility was investigated in vitro and in vivo. WAY100635 (a 5-HT_1A antagonist) inhibited the contraction response to 5-HT in the proximal segments without affecting the maximum response; whilst enhancing the contraction to 5-HT (>30.0nM) in the distal intestine. The selective 5-HT_2A and 5-HT_2B receptor antagonists MDL-100907 and RS-127445 attenuated 5-HT-induced contractions (<10.0µM) in the distal segments. RS-127445 also attenuated 5-HT-induced contractions in the central segments. The selective 5-HT_2C receptor antagonist SB-242084, attenuated the responses to 5-HT (> 3.0nM) in the proximal and central but not the distal regions. 8-OH-DPAT-induced relaxation was resistant to the antagonism by 5-HT_1A/7 antagonists. DOI in the presence of 5-HT_1A/2A/2B/2C antagonists induced greater contraction responses (>1.0µM) in most tissues, whilst RS-127445, or SB-242084, reduced the responses to DOI (< 1.0µM) in some tissues. SB-242084 also suppressed emesis-induced by motion and intragastric CuSO₄. In conclusion, within different regions of intestine, 5-HT₂ receptors are differently involved in contraction and emetic responses and that 8-OH-DPAT induces relaxation via non-5-HT_1A/7 receptors. Suncus could provide a model to investigate these diverse actions of 5-HT.

The serotonin subtype 1A receptor regulates cortisol secretion in the Gulf toadfish, Opsanus beta

It is well established that serotonin (5-HT; 5-hydroxytryptamine) plays a role in mammalian regulation of the hypothalamic-pituitary-adrenal (HPA) axis via the 5-HT receptor subtype 1A (5-HT(1A)). To date, there has not been a comprehensive investigation of the molecular, pharmacological and physiological aspects of the 5-HT(1A) receptor and its role in the activation of the hypothalamic-pituitary-interrenal (HPI) axis in teleost fish. The 5-HT(1A) receptor of the Gulf toadfish (Opsanus beta) was cloned and sequenced, showing 67.5% amino acid similarity to the human homologue. The 5-HT(1A) receptor was distributed throughout the brain, with the whole brain containing significantly higher levels of 5-HT(1A) mRNA compared to all other tissues and the midbrain/diencephalon region containing significantly higher levels of transcript than any other brain region. Substantial levels of transcript were also found in the pituitary, while very low levels were in the kidney that contains the interrenal cells. Xenopus oocytes injected with toadfish 5-HT(1A) receptor cRNA displayed significantly higher binding of [(3)H]5-HT that was abolished by the mammalian 5-HT(1A) receptor agonist, 8-OH-DPAT, indicating a conserved binding site of the toadfish 5-HT(1A) receptor and a high specificity for the agonist. Supporting this, binding of [(3)H]5-HT was not affected by the mammalian 5-HT(1B) receptor agonist, 5-nonyloxytryptamine, the 5-HT(7) receptor antagonist, SB269970, or the 5-HT(2) receptor agonist, alpha-methylserotonin. Confirming these molecular and pharmacological findings, intravenous injection of 8-OH-DPAT stimulated the HPI axis to cause a 2-fold increase in circulating levels of cortisol. The present study of the 5-HT(1A) receptor in a single teleost species illustrates the high conservation of this 5-HT receptor amongst vertebrates.

Bone morphogenetic protein regulation of enteric neuronal phenotypic diversity: relationship to timing of cell cycle exit

The effects of bone morphogenetic protein (BMP) signaling on enteric neuron development were examined in transgenic mice overexpressing either the BMP inhibitor, noggin, or BMP4 under control of the neuron specific enolase (NSE) promoter. Noggin antagonism of BMP signaling increased total numbers of enteric neurons and those of subpopulations derived from precursors that exit the cell cycle early in neurogenesis (serotonin, calretinin, calbindin). In contrast, noggin overexpression decreased numbers of neurons derived from precursors that exit the cell cycle late (gamma-aminobutyric acid, tyrosine hydroxylase [TH], dopamine transporter, calcitonin gene-related peptide, TrkC). The numbers of TH- and TrkC-expressing neurons were increased by overexpression of BMP4. These observations are consistent with the idea that phenotypic expression in the enteric nervous system (ENS) is determined, in part, by the number of proliferative divisions neuronal precursors undergo before their terminal mitosis. BMP signaling may thus regulate enteric neuronal phenotypic diversity by promoting the exit of precursors from the cell cycle. BMP2 increased the numbers of TH- and TrkC-expressing neurons developing in vitro from immunoselected enteric crest-derived precursors; BMP signaling may thus also specify or promote the development of dopaminergic TrkC/NT-3-dependent neurons. The developmental defects in the ENS of noggin-overexpressing mice caused a relatively mild disturbance of motility (irregular rapid transit and increased stool frequency, weight, and water content). Although the function of the gut thus displays a remarkable tolerance for ENS defects, subtle functional abnormalities in motility or secretion may arise when ENS defects short of aganglionosis occur during development.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. The 5-HT receptor antagonists GR 127935 (5-HT(1B,D)), ketanserin (5-HT(2A)), SB 204741 (5-HT(2B)), RS 102221 (5-HT(2C)), granisetron (5-HT(3)), GR 113808 (5-HT(4)) and SB 269970 (5-HT(7)) had no influence on the 5-HT-induced response; the 5-HT(1A) receptor antagonists NAN 190 (pK(b)=8.13+/-0.06) and WAY 100635 (pK(b)=8.69+/-0.07), and the 5-HT(1,2,5,6,7) receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. The 5-HT(1,7) receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT(1A) receptors, had no contractile influence. These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.

Expression and function of 5-HT4 receptors in the mouse enteric nervous system

The aim of the current study was to identify enteric 5-HT(4) splice variants, locate enteric 5-HT(4) receptors, determine the relationship, if any, of the 5-HT(4) receptor to 5-HT(1P) activity, and to ascertain the function of 5-HT(4) receptors in enteric neurophysiology. 5-HT(4a), 5-HT(4b), 5-HT(4e), and 5-HT(4f) isoforms were found in mouse brain and gut. The ratio of 5-HT(4) expression to that of the neural marker, synaptophysin, was higher in gut than in brain but was similar in small and large intestines. Submucosal 5-HT(4) expression was higher than myenteric. Although transcripts encoding 5-HT(4a) and 5-HT(4b) isoforms were more abundant, those encoding 5-HT(4e) and 5-HT(4f) were myenteric plexus specific. In situ hybridization revealed the presence of transcripts encoding 5-HT(4) receptors in subsets of enteric neurons, interstitial cells of Cajal, and smooth muscle cells. IgY antibodies to mouse 5-HT(4) receptors were raised, affinity purified, and characterized. Nerve fibers in the circular muscle and the neuropil in ganglia of both plexuses were highly 5-HT(4) immunoreactive, although only a small subset of neurons contained 5-HT(4) immunoreactivity. No 5-HT(4)-immunoreactive nerves were detected in the mucosa. 5-HT and 5-HT(1P) agonists evoked a G protein-mediated long-lasting inward current that was neither mimicked by 5-HT(4) agonists nor blocked by 5-HT(4) antagonists. In contrast, the 5-HT(4) agonists renzapride and tegaserod increased the amplitudes of nicotinic evoked excitatory postsynaptic currents. Enteric neuronal 5-HT(4) receptors thus are presynaptic and probably exert their prokinetic effects by strengthening excitatory neurotransmission.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Catecholamines and 5-hydroxytryptamine in tissues of the rabbit exocrine pancreas

OBJECTIVES

Norepinephrine (NE), dopamine (DA), epinephrine (Epi), and 5-hydroxytryptamine (5-HT) all modulate pancreatic exocrine secretion, yet their concentrations in specific tissues of the exocrine pancreas are unknown.

METHODS

Concentrations of catecholamines and 5-HT in rabbit pancreatic ganglia, acini, ducts and ampullae, and arteries and veins were measured using HPLC.

RESULTS

Concentrations of NE in ganglia from the head/neck region were significantly higher than those from the body (1620 +/- 220 vs. 778 +/- 179 pmol/mg protein). Acini contained little NE, DA, or 5-HT (9 +/- 2, 0.9 +/- 0.2, 13 +/- 5 pmol/mg protein). Ducts and ampullae contained NE (314 +/- 74 and 156 +/- 24 pmol/mg protein), DA (43 +/- 14 and 13 +/- 4 pmol/mg protein), Epi (63 +/- 29 and 39 +/- 6 pmol/mg protein), and 5-HT (696 +/- 151 and 3563 +/- 288 pmol/mg protein). Arteries and veins contained the highest concentrations of NE (1962 +/- 463 and 736 +/- 80 pmol/mg protein, respectively).

CONCLUSIONS

Pancreatic ganglia and blood vessels, rather than acini, are the main sites of noradrenergic sympathetic innervation of the rabbit exocrine pancreas. These nerves preferentially target ganglionic transmission in the head/neck versus the body. Serotonergic nerves provide little or no innervation of rabbit pancreatic ganglia or acini.

Presynaptic modulation of cholinergic and non-cholinergic fast synaptic transmission in the myenteric plexus of guinea pig ileum

Abstract These studies investigated receptors modulating release of mediators of fast excitatory postsynaptic potentials (fEPSPs) in guinea pig ileum myenteric plexus using electrophysiological methods. Fast EPSPs inhibited by >95% by hexamethonium (100 micromol L(-1)) were cholinergic; mixed fEPSPs were inhibited <95% by hexamethonium. Non-cholinergic fEPSPs were studied in the presence of hexamethonium. The alpha2-adrenergic receptor agonist UK 14304 inhibited cholinergic (maximum inhibition = 76%, EC(50) = 18 nmol L(-1)), mixed (81%, 21 nmol L(-1)) and non-cholinergic (76%, 44 nmol L(-1)) fEPSPs equally. The 5-HT(1) receptor agonist 5-carboxamidotryptamine inhibited cholinergic, mixed and non-cholinergic fEPSPs equally. Renzapride, increased non-cholinergic (33%) less than mixed (97%, 13 micromol L(-1)) fEPSPs. Renzapride inhibited the purely cholinergic fEPSPs (-29%) but potentiated the cholinergic component of mixed fEPSPs (39%). Prucalopride potentiated all fEPSPs equally (30-33%). 5-HT (0.1 micromol L(-1)) induced potentiation of cholinergic (75%), mixed (97%) and non-cholinergic (84%) fEPSPs was not statistically different. The potentiating effects of renzapride and 5-HT on fEPSPs were inhibited by the 5-HT(4) receptor antagonist, SB 204070 (10 nmol L(-1)). Renzapride (0.3 micromol L(-1)) blocked 5-HT-induced increases in cholinergic fEPSPs. alpha2-Adrenergic and 5-HT(1) receptors mediate inhibition of transmitter release from cholinergic and mixed terminals. 5-HT and prucalopride, acting at 5-HT(4) receptors, facilitate all fEPSPs; renzapride facilitates the cholinergic and non-cholinergic components of mixed fEPSPs but not purely cholinergic fEPSPs. Cholinergic synapses may express few 5-HT(4) receptors or a renzapride-insensitive 5-HT(4) receptor isoform.

Neural hormonal regulation of exocrine pancreatic secretion

Exocrine pancreatic secretion is regulated by hormone-hormonal and neural-hormonal interactions involving several regulatory peptides and neurotransmitter from the gut, the pancreas and the vagus nerve. The roles of the gastrointestinal peptides including secretin, CCK, neurotensin, motilin, PYY and pancreatic islet hormones including insulin, pancreatic polypeptide and somatostatin have been established. Interactions among secretin, CCK and neurotensin produce synergistic stimulatory effect. Motilin modulates the cyclic pattern of pancreatic secretion while local insulin provides a permissive role for the action of secretin and CCK at physiological concentration. Somatostatin, PYY and pancreatic polypeptide are inhibitory regulators, acting either on the release of secretin and CCK or on the action of the two stimulatory hormones. The vagal afferent-efferent pathway mediates the actions of many of these regulatory peptides, particularly of secretin and CCK. Acetylcholine and nitric oxide are the neurotransmitters known to mediate the actions of secretin and CCK. Serotonin (5-HT) released from enterochromaffin cells in the intestinal mucosa and nerve terminals of the enteric nervous system and intrapancreatic nerves may be involved in both stimulatory and inhibitory mechanism through its various receptor subtypes. 5-HT also mediates the action of secretin and CCK. The regulatory roles of neuropeptides, PACP and GRP, are now established, whereas those of others are being uncovered. Pancreatic juice provides both positive and negative feedback regulation of pancreatic secretion through mediation of both secretin- and CCK-releasing peptides. Three CCK-releasing peptides have been purified: monitor peptide from pancreatic juice, diazepam-binding inhibitor from porcine intestine, and luminal CCK-releasing factor from rat intestinal secretion. All have been shown to stimulate CCK release and pancreatic enzyme secretion. Pancreatic phospholipase A2 from pancreatic juice and intestinal secretion appears to function as a secretin-releasing peptide. However, the detailed map of neurohormonal regulatory pathways of exocrine pancreatic secretion is yet to be constructed.

5-HT(2A) receptors: location and functional analysis in intestines of wild-type and 5-HT(2A) knockout mice

The distribution and function of the 5-hydroxytryptamine (5-HT(2A)) receptor were investigated in the intestines of wild-type (5-HT(2A) +/+) and knockout (5-HT(2A) -/-) mice. In 5-HT(2A) +/+ mice, rats, and guinea pigs, 5-HT(2A) receptor immunoreactivity was found on circular and longitudinal smooth muscle cells, neurons, enterocytes, and Paneth cells. Muscular 5-HT(2A) receptors were concentrated in caveolae; neuronal 5-HT(2A) receptors were found intracellularly and on the plasma membranes of nerve cell bodies and axons. Neuronal 5-HT(2A) immunoreactivity was detected as early as E14 in ganglia, intravillus nerves, and the deep muscle plexus. The 5-HT(2A) -/- colon did not express 5-HT(2A) receptors and did not contract in response to exogenous 5-HT. 5-HT(2A) -/- enterocytes were smaller, Paneth cells fewer, and muscle layers thinner (and showed degeneration) compared with those of 5-HT(2A) +/+ littermates. The 5-HT(2A) receptor may thus be required for the maintenance and/or development of enteric neuroeffectors and other enteric functions, although gastrointestinal and colonic transit times in 5-HT(2A) -/- and +/+ mice did not differ significantly.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors

G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.

Roles of 5-HT receptors in the release and action of secretin on pancreatic secretion in rats

5-Hydroxytryptamine (serotonin, 5-HT) is a hormone and neurotransmitter regulating gastrointestinal functions. 5-HT receptors are widely distributed in gastrointestinal mucosa and the enteric nervous system. Duodenal acidification stimulates not only the release of both 5-HT and secretin but also pancreatic exocrine secretion. We investigated the effect of 5-HT receptor antagonists on the release of secretin and pancreatic secretion of water and bicarbonate induced by duodenal acidification in anesthetized rats. Both the 5-HT(2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron at 1-100 microg/kg dose-dependently inhibited acid-induced increases in plasma secretin concentration and pancreatic exocrine secretion. Neither the 5-HT(1) receptor antagonists pindolol and 5-HTP-DP nor the 5-HT(4) receptor antagonist SDZ-205,557 affected acid-evoked release of secretin or pancreatic secretion. None of the 5-HT receptor antagonists affected basal pancreatic secretion or plasma secretin concentration. Ketanserin or ondansetron at 10 microg/kg or a combination of both suppressed the pancreatic secretion in response to intravenous secretin at 2.5 and 5 pmol x kg(-1) x h(-1) by 55-75%, but not at 10 pmol x kg(-1) x h(-1). Atropine (50 microg/kg) significantly attenuated the inhibitory effect of ketanserin on pancreatic secretion but not on the release of secretin. These observations suggest that 5-HT(2) and 5-HT(3) receptors mediate duodenal acidification-induced release of secretin and pancreatic secretion of fluid and bicarbonate. Also, regulation of pancreatic exocrine secretion through 5-HT(2) receptors may involve a cholinergic pathway in the rat.

Characterisation of 5-HT2 receptor subtypes in the Suncus murinus intestine

The involvement of 5-HT2 receptor subtypes in mediating a contraction response in the isolated intestine of Suncus murinus was investigated using DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane, a 5-HT2 receptor agonist) which produced a bell-shaped concentration response curve that was significantly (p < 0.05) reduced by methysergide (a 5-HT1/2 receptor antagonist, 1 microM) but not ketanserin (a 5-HT2A receptor antagonist, 1 microM), yohimbine (a 5-HT2B receptor antagonist, 1 microM) or a combination of ondansetron (a 5-HT3 receptor antagonist, 1 microM) plus SB204070 (8-amino-7-chloro(N-butyl-4-piperidyl) methylbenzo-1,4-dioxan-5-carboxylate hydrochloride, a 5-HT4 receptor antagonist, 1 nM). The contraction response to the lower concentrations of DOI (10 nM-0.3 microM) was reduced in the presence of SB206553 (5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2 ,3-f]indole, a 5-HT2B/2C receptor antagonist, 1 microM), whilst conversely, the reducing response to the higher concentrations of DOI (1-30 microM) was prevented. A repeated challenge with 3 microM DOI produced a smaller response (desensitisation) and also reduced the response to 5-HT (5-hydroxytryptamine, 0.3 microM) that was inhibited by SB206553 (1 microM). Data indicate that 5-HT2C receptors are likely candidates to mediate the contractile response to DOI and demonstrate desensitisation to repeated challenges.

Review article: roles played by 5-hydroxytryptamine in the physiology of the bowel

The application of 5-HT to the gut elicits a wide variety of effects because of the expression and wide distribution in the bowel of many subtypes of 5-HT. There is, however, no reason to believe that all of these receptors are stimulated by endogenous 5-HT. 5-HT has been found to be the neurotransmitter of a subset of myenteric interneurons, which evoke a slow excitatory postsynaptic response mediated by 5-HT1P receptors. The major enteric depot of 5-HT is found in mucosal enterochromaffin cells, which are sensory transducers that utilize 5-HT to activate both intrinsic (via 5-HT1P and 5-HT4 receptors) and extrinsic (via 5-HT3 receptors) primary afferent nerves. Mucosal 5-HT is inactivated by uptake into epithelial cells mediated by the same 5-HT transporter utilized by serotonergic neurons. Antagonism of 5-HT3 receptors by compounds such as alosetron should be useful in treating functional bowel disease because they can inhibit excitation of extrinsic sensory nerves by 5-HT without interfering with intrinsic enteric reflexes.

5-Hydroxytryptamine-1 receptor activation inhibits endocrine pancreatic secretion in humans

The selective 5-hydroxytryptamine-1 receptor agonist sumatriptan inhibits exocrine pancreatic function in humans. No data are available on the effect of sumatriptan on fasting and postprandial endocrine pancreatic function in humans. To elucidate the influence of 5-hydroxytryptamine-1 receptor activation by sumatriptan on endocrine pancreatic function and blood glucose homeostasis, we determined plasma levels of somatostatin, glucagon, pancreatic polypeptide, insulin, and C-peptide before and after subcutaneous administration of sumatriptan (6 mg) in seven healthy volunteers, and we measured blood glucose and insulin plasma levels during an oral glucose tolerance test after placebo and after subcutaneous administration of sumatriptan (6 mg) in seven healthy volunteers. Sumatriptan significantly decreased the mean plasma levels of somatostatin, glucagon, pancreatic polypeptide, insulin and C-peptide (P < 0.001) and also significantly decreased mean and peak plasma levels of insulin after an oral glucose challenge (P < 0.02 and P = 0.04, respectively) without affecting glucose homeostasis. From our study, we speculate that activation of the 5-hydroxytryptamine-1 receptor inhibits endocrine pancreatic secretion.

Guinea pig pancreatic neurons: morphology, neurochemistry, electrical properties, and response to 5-HT

The morphology, neurochemistry, and electrical properties of guinea pig pancreatic neurons were determined. The majority of neurons expressed choline acetyltransferase (ChAT) immunoreactivity; however, ChAT-negative neurons were also found. Both cholinergic and noncholinergic neurons expressed nitric oxide synthase (NOS) immunoreactivity. Three types of pancreatic neurons were distinguished. Phasic neurons fired action potentials (APs) at the onset of depolarizing current pulse, tonic neurons spiked throughout the duration of a suprathreshold depolarizing pulse, and APs could not be generated in nonspiking neurons, even though they did receive synaptic input. APs were tetrodotoxin sensitive, and all types of neurons received fast and slow excitatory postsynaptic potentials (EPSPs). Fast EPSPs had cholinergic and noncholinergic components. The majority of pancreatic neurons appeared to innervate the acini. NOS- and/or neuropeptide Y-immunoreactive phasic and tonic neurons were found. Microejection of 5-hydroxytryptamine (5-HT) caused a slow depolarization that was inhibited by the 5-HT1P antagonist N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide and mimicked by the 5-HT1P agonist 6-hydroxyindalpine. A pancreatic 5-HT transporter was located, and inhibition of 5-HT uptake by fluoxetine blocked slow EPSPs in 5-HT-responsive neurons by receptor desensitization.

Gastric antisecretory effect of serotonin: quantitation of release and site of action

Despite many reports that serotonin (5-HT) inhibits gastric acid output, the role and mechanism of action of endogenous 5-HT to modulate gastric secretion remain unclear. Vagal stimulation enhanced the basal rate of 5-HT release into both the gastric lumen (600%) and the portal circulation (265%) of the rat. The peak rate of 5-HT release into the portal circulation was 1,000-fold higher that luminal release (12 micrograms/min and 1.2 ng/min, respectively). To elucidate site(s) of action of 5-HT to inhibit acid secretion, several approaches were taken. Intraluminal perfusion of exogenous 5-HT to encompass enhanced levels seen after vagal stimulation did not reduce gastric acid output. In contrast, administration of systemic 5-HT, which raised portal venous 5-HT to similar levels as vagal stimulation, had a marked antisecretory effect. Chemical or surgical ablation of enteric or sympathetic nerves innervating the stomach did not attenuate the inhibitory effect of exogenous 5-HT on gastric acid output. The antisecretory effect of systemic 5-HT was insensitive to pretreatment with piroxicam, doxantrazole, close gastric intra-arterial sodium nitroprusside, somatostatin monoclonal antibody, or bilateral adrenalectomy. The results suggest that 5-HT is released from endogenous stores into the portal circulation in sufficient quantities after vagal stimulation to alter gastric physiology and that its action is independent of the autonomic nervous system, gastric mucosal prostaglandins or somatostatin, mucosal mast cell or adrenal constituents, or changes in gastric mucosal blood flow.

5-Hydroxytryptamine depolarizes neurons of cat pancreatic ganglia

Pancreatic ganglia contain 5-hydroxytryptamine (5-HT)-immunoreactive axons, some of which are extensions of myenteric neurons located in the pyloric antrum and proximal duodenum. The present study investigated the effect of 5-HT on the membrane potential of cat pancreatic ganglion neurons by means of intracellular recordings in vitro. Pressure application of 5-HT evoked a fast depolarization in 29 of 147 neurons and a slow depolarization in 89 of 147 neurons. A biphasic response was observed in 10 of 108 neurons. The 5-HT-induced slow depolarizing response was not altered in a low Ca2+ (0.1 mM), high Mg2+ (15 mM) solution nor by hexamethonium (10(-4) M) or atropine (10(-6) M). The fast depolarizing response was associated with a decrease of membrane input resistance (-17.2%). The slow depolarizing response was associated with either a decrease (-19.6%) in 24, an increase (+25.0%) in 20, or without a detectable change of membrane input resistance in 10 out of 54 neurons tested. Conditioning hyperpolarization increased the amplitude of both fast and slow depolarizing responses. A low Na+ (68.5 mM) solution and a high K+ (23.5 mM) solution significantly reduced the amplitude of the slow depolarizing response. A low Cl- (9.6 mM) solution had no significant effect on the slow depolarization. The 5-HT3 receptor antagonist MDL 72222 (Bemesetron) blocked the 5-HT-evoked fast depolarizing response. BRL 24924 (Renzapride) and 5 HT-DP, antagonists for the putative 5-HT1P receptor, blocked the slow depolarizing response. The 5-HT3 receptor agonist 2-methyl-5-HT evoked a fast depolarizing response and MCPP, an agonist for the putative 5-HT1P receptor, evoked a slow depolarizing response. Spiperone (a 5-HT1A receptor antagonist) and mianserin (a 5-HT2 receptor antagonist) had no effect on either depolarizing response to 5-HT. The results show that pancreatic ganglion neurons responded to 5-HT with fast and slow depolarizing responses. The data suggest that these responses were mediated by the 5-HT3 receptor and the putative 5-HT1P receptor, respectively.

Identification of cells that express 5-hydroxytryptamine1A receptors in the nervous systems of the bowel and pancreas

Although serotonin (5-HT)1A receptors are known to be present on neural elements in both the bowel and the pancreas, the precise location of these receptors has not previously been determined. Earlier investigations have suggested that 5-HT1A receptors are synthesized in enteric, but not pancreatic ganglia, and that they mediate pre-and postjunctional inhibition. Wholemount in situ hybridization was used to identify cells that contain mRNA encoding 5-HT1A receptors, and immunocytochemistry was employed to locate receptor protein. mRNA encoding 5-HT1A receptors was found in the majority of neurons in both submucosal and myenteric plexuses. 5-HT1A immunoreactivity, however, was abundant only on the surfaces of a limited subset of nerve cell bodies and processes. 5-HT-immunoreactive axons were found in close proximity to sites of 5-HT1A immunoreactivity. Myenteric, but not submucosal calbindin-immunoreactive neurons (with Dogiel type II morphology) were surrounded by rings of 5-HT1A immunoreactivity. The cytoplasm of the cell bodies and dendrites of a small subset of Dogiel type I neurons was also intensely 5-HT1A immunoreactive. Most of the Dogiel type I 5-HT1A-immunoreactive myenteric neurons, and some of the type II neurons that were ringed by 5-HT1A immunoreactivity became doubly labeled following injections of the retrograde tracer, FluoroGold (FG), into the submucosal plexus. 5-HT1A-immunoreactive neurons in distant submucosal ganglia also became labeled by retrograde transport of FG. None of the 5-HT1A-immunoreactive cells were labeled by the intraluminal administration of the beta-subunit of cholera toxin, a marker for vasoactive intestinal peptide-containing secretomotor neurons. These observations suggest that some of the myenteric 5-HT1A-immunoreactive neurons project to submucosal ganglia and that the submucosal 5-HT1A-immunoreactive cells are interneurons. In addition to neurons, a subset of 5-HT-containing enterochromaffin cells expressed 5-HT1A immunoreactivity, which was co-localized with 5-HT in secretory granules. In the pancreas, 5-HT1A immunoreactivity was observed in ganglia, acinar nerves, and glucagonimmunoreactive islet cells. Serotonergic enteropancreatic axons have been found to terminate in close proximity to each of these structures, which may thus be the targets of this innervation. The abundance of 5-HT1A receptor immunoreactivity on nerves of the gut and pancreas suggests that drugs designed to interact with these receptors may have unanticipated visceral actions.

8-OH-DPAT stimulates gastric acid secretion through a vagal-independent, adrenal-mediated mechanism

Serotonin (5-hydroxytryptamine, 5-HT) is a neuroendocrine component of the gastrointestinal tract. 5-HT1A receptors exist both in the brain and have been demonstrated autoradiographically in high density in the rat stomach. However, the physiologic role of 5-HT1A receptors in modulating gastric function is not known. The effect of the selective 5-HT1A receptor agonist, (+/-)-8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT), on gastric acid secretory function was compared to 5-HT in acute, urethane-anesthetized gastric-fistulated rats during pentagastrin infusion. 5-HT inhibited, but 8-OH-DPAT stimulated, gastric acid secretion in a dose-dependent manner. Bilateral cervical vagotomy or celiac ganglionectomy did not reverse the effect of 8-OH-DPAT on acid secretion. However, the enhancement of acid by 8-OH-DPAT was attenuated by acute adrenalectomy or close intra-arterial administration of spiperone, but not idazoxan. Thus, the data suggest that the selective 5-HT1A receptor agonist 8-OH-DPAT may augment gastric secretory function via an adrenal-dependent mechanism.