Electrophysiological characterization of functionally distinct 5-hydroxytryptamine receptors on guinea-pig submucous plexus

Expression Analysis of Serotonin Receptors, Serotonin Transporter and l-Amino Acid Decarboxylase in the Mouse Sphenopalatine Ganglion by RT-PCR, Northern Blot Analysis and In Situ Hybridization

The sphenopalatine ganglion (SPG) is a gathering of the cell bodies of parasympathetic fibers that dominate the nasal gland, lacrimal gland and cerebral blood vessels. The SPG controls nasal secretions, tears, and the dilation of cerebral blood vessels. However, it is unclear how serotonin regulates SPG functions. In this study, we investigated the expression of genes involved in the serotonergic system in the mouse SPG. We examined the mRNA expression levels of 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1F, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT_3A, 5-HT_3B, 5-HT₄, 5-HT_5A, 5-HT_5B, 5-HT₆ and 5-HT₇ receptors, as well as serotonin transporter, tryptophan hydroxylases 1 and 2, and L-amino acid decarboxylase (AADC) by RT-PCR. It revealed that the 5-HT_3A and 5-HT_3B ionotropic receptors and AADC were likely to be highly expressed in the SPG, as measured by RT-PCR. We next performed in situ hybridization on the SPG to examine the expression of these three genes at the cellular level after validating the specificity of each cRNA probe by northern blotting. The 5-HT_3A receptor, 5-HT_3B receptor, and AADC were expressed in 96.5% ± 1.0%, 29.7% ± 10.7%, and 57.4% ± 2.9% of neuronal cell bodies in the SPG, respectively, indicating that the 5-HT_3A receptor was virtually expressed in all SPG neurons. Our results on the expression of these critical serotonin system genes in the parasympathetic SPG provide insight into the pathogenetics of rhinitis, conjunctivitis and headache. Furthermore, our findings suggest that targeting the 5-HT_3A receptor might have therapeutic potential in the treatment of these ailments.

Ca(2+) transients in submucous neurons during the colonic migrating motor complex in the isolated murine large intestine

BACKGROUND

The colonic migrating motor complex (CMMC) is a spontaneous, rhythmic, and neurally mediated motor pattern generated by myenteric neurons, which can propel fecal pellets in mice. Our aim was to determine whether submucous neurons were also activated during the CMMC.

METHODS

:The isolated murine colon was opened and sections of mucosa were removed to expose the submucous ganglia, which were then loaded with Fluo-4.

KEY RESULTS

Colonic migrating motor complexes, which occurred spontaneously or by mechanically stimulating the mucosa, were identified by displacement of the tissue (duration = 23.3 s). Between CMMCs, spontaneous Ca(2+) transients (frequency = 0.9 Hz) were observed in 55% (n = 8) of submucous neurons. During the CMMC, 98% (seven ganglia, n = 7) of submucous neurons within the same ganglion exhibited rapid Ca(2+) transients (1.6 Hz) superimposed on a sustained rise in Ca(2+) (duration ∼23 s) that occurred 1.7 s following the mucosal stimulus; whereas other neurons exhibited a similar, but delayed response that occurred either at 7 or 13 s following the stimulus. The activity in submucous neurons was correlated with activity in adjacent nerve varicosities. Ondansetron (1 mm; 5-HT(3) antagonist) significantly reduced the frequency and duration of the Ca(2+) transient responses.

CONCLUSIONS & INFERENCES

Activity in the submucous neurons appears to be secondary to that in the myenteric plexus and appears to be generated largely by activity in myenteric descending (serotonergic) interneurons. During the CMMC, there is likely to be an increase in secretion to lubricate and facilitate fecal pellet propulsion.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

5-HT(1A), SST(1), and SST(2) receptors mediate inhibitory postsynaptic potentials in the submucous plexus of the guinea pig ileum

Vasoactive intestinal peptide (VIP) immunoreactive neurons are important secretomotor neurons in the submucous plexus. They are the only submucosal neurons to receive inhibitory inputs and exhibit both noradrenergic and nonadrenergic inhibitory synaptic potentials (IPSPs). The former are mediated by alpha(2)-adrenoceptors, but the receptors mediating the latter have not been identified. We used standard intracellular recording, RT-PCR, and confocal microscopy to test whether 5-HT(1A), SST(1), and/or SST(2) receptors mediate nonadrenergic IPSPs in VIP submucosal neurons in guinea pig ileum in vitro. The specific 5-HT(1A) receptor antagonist WAY 100135 (1 microM) reduced the amplitude of IPSPs, an effect that persisted in the presence of the alpha(2)-adrenoceptor antagonist idazoxan (2 microM), suggesting that 5-HT might mediate a component of the IPSPs. Confocal microscopy revealed that there were many 5-HT-immunoreactive varicosities in close contact with VIP neurons. The specific SSTR(2) antagonist CYN 154806 (100 nM) and a specific SSTR(1) antagonist SRA 880 (3 microM) each reduced the amplitude of nonadrenergic IPSPs and hyperpolarizations evoked by somatostatin. In contrast with the other antagonists, CYN 154806 also reduced the durations of nonadrenergic IPSPs. Effects of WAY 100135 and CYN 154806 were additive. RT-PCR revealed gene transcripts for 5-HT(1A), SST(1), and SST(2) receptors in stripped submucous plexus preparations consistent with the pharmacological data. Although the involvement of other neurotransmitters or receptors cannot be excluded, we conclude that 5-HT(1A), SST(1), and SST(2) receptors mediate nonadrenergic IPSPs in the noncholinergic (VIP) secretomotor neurons. This study thus provides the tools to identify functions of enteric neural pathways that inhibit secretomotor reflexes.

Selective stimulation of intrinsic excitatory and inhibitory motor pathways in porcine lower oesophageal sphincter

The mechanisms of stimulation of inhibitory and excitatory motor neurons (MNs) in the lower oesophageal sphincter (LOS) are not fully understood. The aim of this study was to assess the effect of selective stimulation of inhibitory and excitatory MNs in porcine LOS through nicotinic acetylcholine receptors (nAChRs), 5-HT(3) and P2X receptors. Circular LOS strips from adult pigs were studied in organ baths. We compared the effects of stimulation of MNs by electrical field stimulation (26 V, 0.3-20 Hz); nicotine (1-300 micromol L(-1)); 5-HT and 2-Me-5-HT (1 nmol(-1)-30 micromol L(-1)); and alpha,beta-methylene ATP (alpha,beta-meATP 1-100 micromol L(-1)); in standard Krebs solution; a non-adrenergic non-nitrergic non-purinergic (NANNNP) solution; and a non-adrenergic non-cholinergic (NANC) solution. Electrical stimulation of inhibitory MNs caused an intense LOS relaxation (-78.94 +/- 4.50% of LOS tone); and of excitatory MNs, a strong contraction (17.89 +/- 1.96 g). Nicotine 100 micromol L(-1) relaxed LOS (-84.67 +/- 3.98%) in standard Krebs solution, an effect reduced by Tetrodotoxin (TTX) 1 micromol L(-1). Nicotine induced a weak TTX-sensitive contraction (1.64 +/- 0.4 g) in NANNNP solution. 5-HT 10 micromol L(-1) and 2-Me-5-HT 30 micromol L(-1) contracted LOS in standard, NANC and NANNNP conditions, maximal responses (7.30 +/- 1.52 g, 3.50 +/- 0.18 g respectively) being reduced by TTX. alpha,beta-meATP 100 micromol L(-1) caused a LOS relaxation (-17.45 +/- 6.62%) unaffected by TTX in NANC solution, and a contraction (6.7 +/- 0.85 g) antagonized by TTX in NANNNP solution. Our results suggest selective mechanisms for stimulation of intrinsic excitatory and inhibitory motor pathways in porcine LOS. Inhibitory MNs are strongly stimulated by nAChRs and do not respond to stimulation of 5-HT(3) and P2X receptors. By contrast, excitatory MNs are stimulated through 5-HT(3) and P2X receptors, stimulation through nACRs being difficult and causing a weak response.

5-HT antagonists NAN-190 and SB 269970 block alpha2-adrenoceptors in the guinea pig

Serotonin (5-HT) plays a significant role in the regulation of intestinal secretion of water and electrolytes. The initial aim of this study was to use intracellular recording and specific antagonists to identify roles of 5-HT1A and 5-HT7 receptors of submucosal noncholinergic secretomotor neurons of guinea pig ileum, in vitro. However, it was found that the widely used 5-HT receptor antagonists NAN-190 (5-HT1A) and SB 269970 (5-HT7) both blocked alpha2-adrenoceptors, and hence depressed inhibitory synaptic potentials and hyperpolarizations evoked by noradrenaline, in these neurons. Both compounds enhanced neurally evoked contractions of the guinea pig vas deferens, an effect characteristic of blockade of alpha2-adrenoceptors. These results raise significant concerns about studies using NAN-190 and SB 269970 as specific antagonists of serotonin receptors.

mGluR(1) Receptors Contribute to Non-Purinergic Slow Excitatory Transmission to Submucosal VIP Neurons of Guinea-Pig Ileum

Vasoactive intestinal peptide (VIP) immunoreactive secretomotor neurons in the submucous plexus are involved in mediating bacterial toxin-induced hypersecretion leading to diarrhoea. VIP neurons become hyperexcitable after the mucosa is exposed to cholera toxin, which suggests that the manipulation of the excitability of these neurons may be therapeutic. This study used standard intracellular recording methods to systematically characterize slow excitatory postsynaptic potentials (EPSPs) evoked in submucosal VIP neurons by different stimulus regimes (1, 3 and 15 pulse 30 Hz stimulation), together with their associated input resistances and pharmacology. All slow EPSPs were associated with a significant increase in input resistance compared to baseline values. Slow EPSPs evoked by a single stimulus were confirmed to be purinergic, however, slow EPSPs evoked by 15 pulse trains were non-purinergic and those evoked by 3 pulse trains were mixed. NK₁ or NK₃ receptor antagonists did not affect slow EPSPs. The group I mGluR receptor antagonist, PHCCC reduced the amplitude of purinergic and non-purinergic slow EPSPs. Blocking mGluR1 receptors depressed the overall response to 3 and 15 pulse trains, but this effect was inconsistent, while blockade of mGluR₅ receptors had no effect on the non-purinergic slow EPSPs. Thus, although other receptors are almost certainly involved, our data indicate that there are at least two pharmacologically distinct types of slow EPSPs in the VIP secretomotor neurons: one mediated by P2Y receptors and the other in part by mGluR₁ receptors.

Neuropathophysiology of functional gastrointestinal disorders

The investigative evidence and emerging concepts in neurogastroenterology implicate dysfunctions at the levels of the enteric and central nervous systems as underlying causes of the prominent symptoms of many of the functional gastrointestinal disorders. Neurogastroenterological research aims for improved understanding of the physiology and pathophysiology of the digestive subsystems from which the arrays of functional symptoms emerge. The key subsystems for defecation-related symptoms and visceral hyper-sensitivity are the intestinal secretory glands, the musculature and the nervous system that controls and integrates their activity. Abdominal pain and discomfort arising from these systems adds the dimension of sensory neurophysiology. This review details current concepts for the underlying pathophysiology in terms of the physiology of intestinal secretion, motility, nervous control, sensing function, immuno-neural communication and the brain-gut axis.

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Sumatriptan is an agonist at 5-HT receptors on myenteric neurones in the guinea-pig gastric antrum

Sumatriptan, a 5-hydroxytryptamine(1D) (5-HT(1D))-receptor agonist used in the treatment in migraine, inhibits gastric motility via the enteric nervous system. As no studies have reported enteric neuronal 5-HT(1D) receptors, we used conventional intracellular recordings to characterize the actions of sumatriptan on 145 guinea-pig antral myenteric neurones. In 24 of 29 neurones with a 5-HT(1P) receptor-mediated depolarizing response to 5-HT, application of sumatriptan caused a dose-dependent depolarization, accompanied by increased membrane resistance and enhanced excitability. Depolarizing responses to sumatriptan occurred both in cholinergic and in nitrergic neurones. Sumatriptan did not mimic the 5-HT(3) receptor-mediated fast-depolarizing responses or 5-HT(1A) receptor-mediated inhibitory responses to 5-HT. Sumatriptan had no effect on neurones not responding to 5-HT. The depolarizing response to sumatriptan was inhibited by renzapride, but not by 5-HT(1-7) receptor antagonists. We conclude that sumatriptan behaves as an agonist at the 5-HT(1P) receptor on myenteric neurones in the guinea-pig gastric antrum. The actions of sumatriptan on gastric motility seem to be attributable to a direct action on enteric neurones.

Unbinding pathways of an agonist and an antagonist from the 5-HT3 receptor

The binding sites of 5-HT3 and other Cys-loop receptors have been extensively studied, but there are no data on the entry and exit routes of ligands for these sites. Here we have used molecular dynamics simulations to predict the pathway for agonists and antagonists exiting from the 5-HT3 receptor binding site. The data suggest that the unbinding pathway follows a tunnel at the interface of two subunits, which is approximately 8 A long and terminates approximately 20 A above the membrane. The exit routes for an agonist (5-HT) and an antagonist (granisetron) were similar, with trajectories toward the membrane and outward from the ligand binding site. 5-HT appears to form many hydrogen bonds with residues in the unbinding pathway, and experiments show that mutating these residues significantly affects function. The location of the pathway is also supported by docking studies of granisetron, which show a potential binding site for granisetron on the unbinding route. We propose that leaving the binding pocket along this tunnel places the ligands close to the membrane and prevents their immediate reentry into the binding pocket. We anticipate similar exit pathways for other members of the Cys-loop receptor family.

Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

Nicotinic transmission in the enteric nervous system (ENS) is extensive, but the role of individual nicotinic acetylcholine receptor (nAChR) subtypes in the functional connectivity of its plexuses has been elusive. Using monoclonal antibodies (mAbs) against neuronal alpha3-, alpha4-, alpha3/alpha5-, beta2-, beta4- and alpha7-subunits, combined with radioimmunoassays and immunocytochemistry, we demonstrate that guinea-pig enteric ganglia contain all of these nAChR-subunits with the exception of alpha4, and so, differ from mammalian brain. This information alone, however, is insufficient to establish the functional role of the identified nAChR-subtypes within the enteric networks and, ultimately, their specific contributions to gastrointestinal physiology. We have used voltage-sensitive dyes and a high-speed CCD camera, in conjunction with specific antagonists to various nAChRs, to elucidate some of the distinct contributions of the individual subtypes to the behaviour of enteric networks. In the guinea-pig, the submucous plexus has the extraordinary advantage that it is virtually two-dimensional, permitting optical recording, with single cell resolution, of the electrical activity of all of its neurones. In this plexus, the block of alpha3beta2-, alpha3beta4- and/or alpha7-nAChRs always results in a decrease in the magnitude of the synaptic response. However, the magnitude of the fast excitatory post-synaptic potentials (epsps) evoked by electrical stimulation of a neighbouring ganglion varies from cell to cell, reflecting the differential expression of subunits already observed using mAbs, as well as the strengths of the activated synaptic inputs. At the same time, we observe that submucous neurones have a substantial mecamylamine (Mec)-insensitive (non-nicotinic) component to their fast epsps, which may point to the presence of purinergic or serotonergic fast epsps in this system. In the myenteric plexus, on the other hand, the antagonist-induced changes in the evoked synaptic response vary depending upon the location of the stimulating electrode with respect to the ganglion under study. The range of activity patterns that follows sequential pharmacological elimination of individual subtypes suggests that nAChRs may be capable of regulating the activity of both excitatory and inhibitory pathways, in a manner similar to that described in the central nervous system.

Serotonin excites neurons in the human submucous plexus via 5-HT3 receptors

BACKGROUND & AIMS

Serotonin (5-hydroxytryptamine [5-HT]) is a key signaling molecule in the gut. Recently, the neural 5-HT3 receptor received a lot of attention as a possible target in functional bowel diseases. Yet, the 5-HT3 receptor-mediated changes in properties of human enteric neurons is unknown.

METHODS

We used a fast imaging technique in combination with the potentiometric dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine to monitor directly the membrane potential changes in neurons of human submucous plexus from surgical specimens of 21 patients. An Ussing chamber technique was used to study 5-HT3 receptor involvement in chloride secretion.

RESULTS

Local microejection of 5-HT directly onto ganglion cells resulted in a transient excitation of enteric neurons characterized by increased spike discharge. This response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, and blocked by the 5-HT3 receptor antagonist, tropisetron. The proportions of 5-HT-responsive nerve cells per ganglion ranged from 25.5% +/- 18.4% in the duodenum to 54.2% +/- 46.9% in the colon. Interestingly, 2-methyl-5-HT did not evoke chloride secretion in the human intestine but it did in the guinea-pig intestine. Specific 5-HT3A and 5-HT3B receptor subunit immunoreactivity as well as 5-HT3A and 5-HT3B receptor-specific messenger RNA were detected in the tissue samples. Based on co-labeling with the pan-neuronal marker HuC/D we conclude that submucous nerve cells potentially express heteromeric 5-HT3A/B receptors.

CONCLUSIONS

We show that 5-HT excited human enteric neurons via 5-HT3 receptors, which may comprise both 5-HT3A and 5-HT3B receptor subunits.

Enteric neuroimmunophysiology and pathophysiology

Minute-to-minute behavior of the bowel, whether it is normal or disordered, is determined by integrative functions of the enteric nervous system (ENS). Information input processed by the ENS is derived from local sensory receptors, the central nervous system, and immune/inflammatory cells including mast cells. Enteric mast cells use the power of the immune system for detection of antigenic threats and for long-term memory of the identity of the specific antigens. Specific antibodies attach to the mast cells and enable the mast cell to detect sensitizing antigens when they reappear in the gut lumen. Should the sensitizing antigen reappear, mast cells detect it and signal its presence to the ENS. The ENS interprets the mast cell signal as a threat and calls up from its program library secretory and propulsive motor behavior that is organized to eliminate the threat rapidly and effectively. Operation of the alarm program protects the individual, but at the expense of symptoms that include cramping abdominal pain, fecal urgency, and diarrhea. Enteric mast cells use immunologic memory functions to detect foreign antigens as they appear and reappear throughout the life of the individual. Mast cells use paracrine signaling for the transfer of chemical information to the neural networks of the ENS. Integrative circuits in the ENS receive and interpret the chemical signals from the mast cells. Signals from the mast cells are interpreted by the ENS as a labeled code for the presence of a threat in the intestinal lumen.

ATP participates in three excitatory postsynaptic potentials in the submucous plexus of the guinea pig ileum

Synaptic transmission between neurones intrinsic to the wall of the intestine involves multiple neurotransmitters. This study aimed to identify neurotransmitters responsible for non-cholinergic excitatory synaptic transmission in the submucous plexus of the guinea pig ileum. Intracellular recordings were made from secretomotor and vasodilator neurones. A single electrical stimulus to a fibre tract evoked excitatory postsynaptic potentials (EPSPs) with three different time courses - fast, slow and an EPSP with an intermediate time course (latency 96 ms, duration 1.2 s). In all neurones, blocking nicotinic receptors reduced fast EPSPs, but they were abolished in only 57 of 78 neurones. Fast EPSPs were also reduced by P2 purinoceptor blockade (5 of 27 neurones) or 5-HT(3) receptor blockade (3 of 20 neurones). The intermediate EPSP was abolished by P2 receptor blockade (13 of 13 neurones) or by the specific P2Y(1) receptor antagonist MRS 2179 (5 of 5 neurones) and was always preceded by a nicotinic or mixed nicotinic/purinergic fast EPSP. Intermediate EPSPs were observed in over half of all neurones including most non-cholinergic secretomotor neurones identified by immunoreactivity for vasoactive intestinal peptide. The slow EPSP evoked by a single pulse stimulus was also abolished by P2 receptor blockade (5 of 5 neurones) or by MRS 2179 (3 of 3 neurones). We conclude that fast EPSPs in submucous neurones are mediated by acetylcholine acting at nicotinic receptors, ATP acting at P2X receptors and 5-HT acting at 5-HT(3) receptors. Both the intermediate EPSP and the single stimulus slow EPSP are mediated by ATP acting at P2Y(1) receptors.

Neuronal location of 5-hydroxytryptamine3 receptor-like immunoreactivity in the rat colon

The presence of 5-hydroxytryptamine(3) receptors on enteric neurons is known from pharmacological data that date back more than 40 years. However, an adequate account of which neurons bear these receptors has not been made because suitable antisera have not been available. We have found that the majority of antisera that have been raised against sequences from the 5-hydroxytryptamine(3) receptor also recognize pre-prosomatostatin. We report that this source of false labeling can be eliminated by pre-incubating the antisera with a peptide designed for this purpose. We have used the pre-absorbed antiserum to localize 5-hydroxytryptamine(3) receptors in the rat colon. Immunoreactive nerve cell bodies occurred in the myenteric and submucosal ganglia. The majority had smooth cell bodies and long, smooth processes, that is, Dogiel type II morphology. The initial segments of the long processes of the Dogiel type II neurons were strongly immunoreactive. About 12% of immunoreactive myenteric nerve cells were of the same or smaller size, and had multiple short filamentous processes. Some of the immunoreactive Dogiel type II neurons were also immunoreactive for calretinin in both plexuses, and the majority were immunoreactive for calbindin in submucosal ganglia. Specific immunoreactivity occurred in non-varicose, but not in varicose, fibers in the myenteric and submucosal ganglia, and in fiber bundles that traversed the longitudinal and circular muscle layers. Immunoreactive varicose fibers were observed only in the mucosa. It is concluded that 5-hydroxytryptamine(3) receptors occur on intrinsic sensory neurons in the rat colon, and on extrinsic sensory nerve fibers that innervate the colon.

Ligand-gated ion channels in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). These receptors respond to synaptically released neurotransmitters, circulating hormones and locally released substances. Cell surface receptors are also targets for many therapeutically used drugs. This review will focus on ligand-gated ion channels, i.e. receptors in which the ligand binding site and the ion channel are parts of a single multimeric receptor. Ligand-gated ion channels expressed by enteric nerves are: nicotinic acetylcholine receptors (nAChRs), P2X receptors, 5-hydroxytryptamine3 (5-HT3) receptors, gamma-aminobutyric acid (GABAA) receptors, N-methyl-d-aspartate (NMDA) receptors,alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and glycine receptors. P2X, 5-HT3 and nAChRs participate in fast synaptic transmission in S-type neurones in the ENS. Fast synaptic transmission occurs in some AH-type neurones, and AH neurones express all the ligand-gated ion channels listed above. Ligand-gated ion channels may be localized at extra-synaptic sites in some AH neurones and these extra-synaptic receptors may be useful targets for drugs that can be used to treat disorders of gastrointestinal function.

The terminals of myenteric intrinsic primary afferent neurons of the guinea-pig ileum are excited by 5-hydroxytryptamine acting at 5-hydroxytryptamine-3 receptors

The aim of this study was to identify the receptor type(s) by which 5-hydroxytryptamine applied to the intestinal mucosa excites the terminals of myenteric AH neurons. The AH neurons have been identified as the intrinsic primary afferent (sensory) neurons in guinea-pig small intestine and 5-hydroxytryptamine has been identified as a possible intermediate in the sensory transduction process. Intracellular recordings were taken from AH neurons located within 1mm of intact mucosa to which 5-hydroxytryptamine was applied. Trains of action potentials and/or slow depolarizing responses were recorded in AH neurons in response to mucosal application of 5-hydroxytryptamine (10 or 20microM) or the 5-hydroxytryptamine-3 receptor agonist, 2-methyl-5-hydroxytryptamine (1 or 3mM), and to electrical stimulation of the mucosa. The 5-hydroxytryptamine-2 receptor agonist, alpha-methyl-5-hydroxytryptamine (100microM), and the 5-hydroxytryptamine-1,2,4 receptor agonist, 5-methoxytryptamine (10microM), did not elicit such responses. The 5-hydroxytryptamine-3 receptor-selective antagonist, granisetron (typically 1microM), and the 5-hydroxytryptamine-3,4 receptor antagonist, tropisetron (typically 1microM), each reduced or abolished the responses to 5-hydroxytryptamine, while the selective 5-hydroxytryptamine-4 receptor antagonist, SB 204070 (1microM), did not. It is concluded that application of 5-hydroxytryptamine to the mucosa activates a 5-hydroxytryptamine-3 receptor that triggers action potential generation in the mucosal nerve terminals of myenteric AH neurons.

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

BACKGROUND & AIMS

Similar to cholecystokinin (CCK), non-CCK-dependent duodenal factors stimulate vagal mucosal afferent fibers to mediate pancreatic enzyme secretion via a common cholinergic pathway. We tested the hypothesis that 5-hydroxytryptamine (5-HT) released from enterochromaffin (EC) cells plays an important role in the transduction of luminal information to the central nervous system via vagal afferent fibers to mediate pancreatic secretion.

METHODS

Pancreatic secretions were examined in conscious rats after intragastric administration of chopped rodent chow in the presence and absence of CCK or 5-HT(3) and 5-HT(2) antagonists. Pancreatic responses to intraduodenal administration of maltose, hyperosmolar NaCl, and light mucosal stroking were examined in rats pretreated with various pharmacological antagonists or after surgical or chemical ablation of vagal and 5-HT neural pathways.

RESULTS

Administration of L364, 718 inhibited 54% of pancreatic protein secretion evoked by intragastric administration of rodent chow. L364,714 and ICS 205-930, a 5-HT(3) antagonist, combined produced a 94% inhibition. Vagal afferent rootlet section eliminated pancreatic secretions evoked by intraduodenal stimuli. p-Chlorophenylalanine, a 5-HT synthesis inhibitor, but not 5,7-hydroxytryptamine, a 5-HT neurotoxin, also eliminated the pancreatic response to these luminal stimuli. The 5-HT(3) antagonist markedly inhibited pancreatic secretion induced by maltose and hyperosmolar NaCl. 5-HT(2) and 5-HT(3) antagonists combined inhibited the pancreatic response to light stroking of the mucosa.

CONCLUSIONS

Luminal factors such as osmolality, disaccharides, and mechanical stimulation stimulated pancreatic secretion via intestinal vagal mucosal afferent fibers. It is likely that 5-HT originating from intestinal EC cells activated 5-HT(3) and 5-HT(2) receptors on vagal afferent fibers to mediate luminal factor-stimulated pancreatic secretion.

Receptor-induced Ca(2+) signaling in cultured myenteric neurons

We studied the effect of excitatory neurotransmitters (10(-5) M) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of cultured myenteric neurons. ACh evoked a response in 48.6% of the neurons. This response consisted of a fast and a slow component, respectively mediated by nicotinic and muscarinic receptors, as revealed by specific agonists and antagonists. Substance P evoked a [Ca(2+)](i) rise in 68.2% of the neurons, which was highly dependent on Ca(2+) release from intracellular stores, since after thapsigargin (5 microM) pretreatment only 8% responded. The responses to serotonin, present in 90.7%, were completely blocked by ondansetron (10(-5) M), a 5-HT(3) receptor antagonist. Specific agonists of other serotonin receptors were not able to induce a [Ca(2+)](i) rise. Removing extracellular Ca(2+) abolished all serotonin and fast ACh responses, whereas substance P and slow ACh responses were more persistent. We conclude that ACh-induced signaling involves both nicotinic and muscarinic receptors responsible for a fast and a more delayed component, respectively. Substance P-induced signaling requires functional intracellular Ca(2+) stores, and the 5-HT(3) receptor mediates the serotonin-induced Ca(2+) signaling in cultured myenteric neurons.

A novel in vitro model of Brunner's gland secretion in the guinea pig duodenum

A novel in vitro model that combined functional and morphological techniques was employed to directly examine pathways regulating Brunner's gland secretion in isolation from epithelium. In vitro submucosal preparations were dissected from guinea pig duodenum. A videomicroscopy technique was used to measure changes in luminal diameter of glandular acini as an index of activation of secretion. Carbachol elicited concentration-dependent dilations of the lumen (EC(50) = 2 microM) by activating muscarinic receptors on acinar cells. Ultrastructural and histological analyses demonstrated that dilation was accompanied by single and compound exocytosis of mucin-containing granules and the accumulation of mucoid material within the lumen. Inflammatory mediators (histamine, PGE(1), PGE(2)) and intestinal hormones (CCK, gastrin, vasoactive intestinal polypeptide, secretin) also stimulated glandular secretion, whereas activation of submucosal secretomotor neurons by 5-hydroxytryptamine did not. This study directly demonstrates that multiple hormonal, inflammatory, and neurocrine agents activate Brunner's glands, whereas many have dissimilar effects on the epithelium. This suggests that Brunner's glands are regulated by pathways that act both in parallel to and in isolation from those controlling epithelial secretion.

Properties of synaptic inputs from myenteric neurons innervating submucosal S neurons in guinea pig ileum

This study examined synaptic inputs from myenteric neurons innervating submucosal neurons. Intracellular recordings were obtained from submucosal S neurons in guinea pig ileal preparations in vitro, and synaptic inputs were recorded in response to electrical stimulation of exposed myenteric plexus. Most S neurons received synaptic inputs [>80% fast (f) excitatory postsynaptic potentials (EPSP), >30% slow (s) EPSPs] from the myenteric plexus. Synaptic potentials were recorded significant distances aboral (fEPSPs, 25 mm; sEPSPs, 10 mm) but not oral to the stimulating site. When preparations were studied in a double-chamber bath that chemically isolated the stimulating "myenteric chamber" from the recording side "submucosal chamber," all fEPSPs were blocked by hexamethonium in the submucosal chamber, but not by a combination of nicotinic, purinergic, and 5-hydroxytryptamine-3 receptor antagonists in the myenteric chamber. In 15% of cells, a stimulus train elicited prolonged bursts of fEPSPs (>30 s duration) that were blocked by hexamethonium. These findings suggest that most submucosal S neurons receive synaptic inputs from predominantly anally projecting myenteric neurons. These inputs are poised to coordinate intestinal motility and secretion.

Morphological relationship between serotonergic neurons and nitrergic neurons for electrolytes secretion in the submucous plexus of the guinea pig distal colon

In the submucous plexus, double immunocytochemistry revealed that nitric oxide synthase (NOS)-immunoreactivity was found in both numerous nerve fibers and some nerve cell bodies, while 5-hydroxytryptamine (5-HT)-immunoreactivity was limited to many nerve fibers, but not any nerve cell bodies. About 30% of the total NOS positive neurons (978) had close or some contact with 5-HT positive nerve fiber, suggesting that NO may participate in the 5-HT-evoked chloride secretion.

5-HT is present in nerves of guinea pig sphincter of Oddi and depolarizes sphincter of Oddi neurons

This study involved immunohistochemistry and intracellular electrophysiology to investigate serotonergic neurotransmission in the sphincter of Oddi (SO). 5-Hydroxytryptamine (HT)-positive neurons (14 cells/preparation) and nerve fibers were observed in the ganglionated plexus. Serotonergic nerve fibers, which persisted under 2- to 6-day organ culture, were densely distributed, with varicose endings encircling some SO neurons. When 5-HT was applied to SO neurons, it elicited three different responses: 1) a fast depolarization to 5-HT in 31 of 62 cells was mimicked by 2-methyl-5-HT and blocked by LY-278584 (1 microM); 2) a prolonged depolarization to 5-HT in 21 of 62 cells evoked an increase in input resistance and was attenuated by the 5-HT1P antagonist renzapride (1 microM) but not by the 5-HT4 antagonist SDZ-205557 (0.1-10 microM); and 3) an indirect depolarization blocked by TTX or atropine was observed in 32 of 62 cells. 5-HT superfusion elicited a dose-dependent monophasic depolarization (EC50 = 2 microM, n=14). In conclusion, 5-HT is present in nerves of the SO and elicits both 5-HT3 and 5-HT1P receptor-mediated depolarizations, supporting the concept that 5-HT plays a role in SO regulation.

5-HT activates nitric oxide-generating neurons to stimulate chloride secretion in guinea pig distal colon

The participation of nitric oxide (NO) in serotonin (5-hydroxytryptamine; 5-HT)-evoked chloride secretion in guinea pig distal colon was examined. Submucosal/mucosal segments were mounted in Ussing flux chambers, and an increase in short-circuit current (Isc) was used as an index of secretion. Addition of 5-HT to the serosal side produced a concentration-dependent (10(-7)-10(-5) M) increase in Isc caused by chloride secretion. NG-nitro-L-arginine (L-NNA) significantly reduced the 5-HT-evoked early (P-1) and late (P-2) responses to 61.1 and 70.6% of control, respectively. Neurally evoked response was also inhibited by L-NNA. The NO donor sodium nitroprusside (SNP, 10(-4) M) increased basal Isc mainly because of chloride secretion. The SNP-evoked response was significantly reduced by tetrodotoxin but was unchanged by atropine or indomethacin. These results suggest that the 5-HT-evoked increase in Isc is associated with an NO-generating mechanism. Atropine significantly reduced the 5-HT (10(-5) M)-evoked P-1 and P-2 responses to 71.8 and 19.7% of control, respectively. Simultaneous application of atropine and L-NNA further decreased the 5-HT-evoked responses more than either drug alone; application of L-NNA and atropine decreased the 5-HT-evoked P-1 and P-2 responses to 68.5 and 39.2% of atropine-treated tissues, respectively. These results suggest that noncholinergic components of P-1 and P-2 responses are 71.8 and 19.7% of control, respectively, and that NO components of P-1 and P-2 responses are 32 and 61%, respectively, of the noncholinergic component of the 5-HT-evoked responses. The results provide evidence that NO may participate as a noncholinergic mediator of 5-HT-evoked chloride secretion in guinea pig distal colon.

Organization of intrinsic cholinergic neurons projecting within submucosal plexus of guinea pig ileum

Electrophysiological techniques were employed to examine the organization of the projections of submucosal neurons in the submucosal plexus of guinea pig ileum. These neurons were activated by focal pressure-pulse application of 5-hydroxytryptamine (5-HT) to single ganglia in submucosal preparations in vitro, and resulting fast excitatory postsynaptic potentials (EPSPs) were recorded intracellularly in S-type neurons. 5-HT-evoked fast EPSPs were blocked by TTX, hexamethonium, and ICS-205-930 (tropisetron). 5-HT was applied either directly to the ganglion containing the neuron recorded intracellularly or to adjacent ganglia positioned at increasing distances on either side of the impaled cell in circumferential or longitudinal orientations. All S-type neurons recorded in this study (n = 103) received nicotinic fast EPSPs from cholinergic neurons when 5-HT was applied directly to the ganglion containing the impaled neuron. Stimulation of adjacent ganglia also evoked nicotinic fast EPSPs, but the number of neurons that received this input decreased as the distance between the stimulus and the impaled cell increased. Maximal projections were 3 mm in the circumferential and orad-to-aborad orientations. There were no significant projections in the aborad-to-orad direction. These findings suggest that S-type neurons in the submucosal plexus are innervated by intrinsic cholinergic neurons that project over relatively short distances and have a distinct orad-to-aborad polarity.

Presynaptic calcium channels mediating synaptic transmission in submucosal neurones of the guinea-pig caecum

1. Intracellular recording techniques were used to examine the voltage-activated calcium channels mediating neurotransmitter release from nerve terminals of extrinsic, sympathetic origin and intrinsic (enteric) origin innervating submucosal neurones of the guinea-pig caecum. 2. The noradrenergic slow inhibitory postsynaptic potential (IPSP) was abolished by superfusion of omega-conotoxin (omega-CTX) GVIA (3-300 nM), with an apparent IC50 of 8.6 nM. Superfusion of omega-CTX MVIIC (500 nM) also suppressed the amplitude of slow IPSPs, but both omega-agatoxin IVA (100 nM) and nicardipine (1-10 microM) were ineffective. The hyperpolarization induced by exogenous noradrenaline was not affected by omega-CTX GVIA (100 nM). 3. In contrast to the slow IPSP, the amplitude of the cholinergic fast excitatory postsynaptic potential (EPSP) was partially inhibited, but not abolished, by omega-CTX GVIA (0.1-1 microM). Furthermore, omega-agatoxin IVA (0.1-1 microM) or omega-CTX MVIIC (0.1-1 microM) also affected the fast EPSP, but nicardipine (1-10 microM) was ineffective. In combination, omega-CTX GVIA (100 nM) and omega-agatoxin IVA (100 nM) inhibited the fast EPSP by 74 +/- 6 %; the residual fast EPSP was not affected by omega-CTX MVIIC (100 nM). The fast EPSP was completely abolished by low Ca2+, high Mg2+ Krebs solution or Krebs solution containing Co2+ (2 mM) and Cd2+ (400 microM). The depolarization induced by exogenous acetylcholine was not affected by either omega-CTX GVIA (100 nM), omega-agatoxin IVA (100 nM) or omega-CTX MVIIC (100 nM). 4. Taken together, these results suggest that, in the submucosal plexus of the guinea-pig caecum, release of noradrenaline from extrinsic nerve terminals is regulated by N-type calcium channels, whereas release of acetylcholine from intrinsic nerve terminals involves several types of calcium channel.

Direct and indirect actions of 5-hydroxytryptamine on the discharge of mesenteric afferent fibres innervating the rat jejunum

1. This study was performed to elucidate the actions of 5-hydroxytryptamine (5-HT) on mesenteric afferent discharge and to determine the receptor-mechanisms responsible for these effects. The activity of mesenteric afferents innervating the mid-jejunum of urethane-anaesthetized rats was recorded with extracellular microelectrodes. The discharge of single nerves within the whole nerve recording was monitored using waveform discriminator software. 2. The intravenous injection of 5-HT produced a complex pattern of afferent activation with two distinct components which could be distinguished both in terms of the response characteristics and the receptors involved. Initially, in 64% of nerve bundles, there was a brief (2.0 +/- 0.1 s) but intense activation of afferent discharge with peak afferent firing increasing with incremental doses of 5-HT. The discharge frequency in seventeen single units from these bundles during the initial response to 10 micrograms 5-HT was 13.0 +/- 1.8 impulses s-1 from a baseline discharge of 1.0 +/- 0.1 impulses s-1. 3. This initial response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, whereas 5-methoxytryptamine (5-MEOT, 10-100 micrograms) had no comparable effect. Similarly, the initial 4. 5-HT response was completely abolished by the 5-HT3 receptor antagonist, granisetron (0.5 mg kg-1). 5-HT also evoked, in approximately 35% of nerve bundles, a delayed response that single unit analysis showed to be mediated by an entirely different population of afferents from those activated during the initial response. This secondary response to 5-HT was characterized by a more prolonged (> 30 s) but less intense period of afferent activity which was coincident with an increase in intrajejunal pressure, and was mimicked by 5-MEOT (10-100 micrograms). The secondary response to 5-HT and the response to 5-MEOT were significantly attenuated by the 5-HT2A receptor antagonist, ketanserin (0.5 mg kg-1), which had no effect on the initial response. The initial response to 5-HT was unaffected by the L-type calcium channel inhibitor nifedipine (1 mg kg-1) or the N-type calcium channel inhibitor omega-conotoxin GVIA (25 micrograms kg-1). However, the secondary response to 5-HT was significantly reduced after treatment with nifedipine. 5. These results demonstrate that 5-HT activates different populations of afferent fibres innervating the rat jejunum. One population of afferents is activated directly via stimulation of 5-HT3 receptors, while another population responds to 5-HT with a time course consistent with secondary activation of mechanosensitive afferents following 5-HT2A-mediated contractile activity.

Signal transduction pathways for serotonin as an intestinal secretagogue

This review presents a signal transduction pathways for serotonin (5-hydroxytryptamine, 5-HT) as an intestinal secretagogue and some recently published related findings. 5-HT is a secretagogue in the small and large intestine of all studied species including pig and man. 5-HT mediates intestinal secretion through activation of at least the epithelial 5-HT2, and neuronal 5-HT3, and 5-HT4 receptors in the submucosal plexus, including a reflex arc. 5-HT activates both a cholinergic and a non-cholinergic pathway in its secretory response. Intracellular mediators include at least eicosanoids (prostaglandin E2), calcium, phosphoinositols (1,4,5-inositol trisphosphate) and maybe nitric oxide and cyclic nucleotides. Pig small intestine appears to be an appropriate model for the human small intestine with respect to the signal transduction pathways for 5-HT as an intestinal secretagogue. Species and segmental differences in the signal transduction pathways for 5-HT as an intestinal secretagogues are discussed together with related news on 5-HT receptors, 5-HT antagonists in clinical use, the enteric nervous system, and intracellular mediators.

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.

Electrophysiological studies of 5-hydroxytryptamine receptors on enteric neurons

Enteric nerves express multiple receptors for 5-hydroxytryptamine (5-HT). Three excitatory and 1 inhibitory receptor for 5-HT can be identified using electrophysiological methods. The excitatory receptors are the 5-HT1P, 5-HT3 and 5-HT4 subtypes. The 5-HT1P mediates slow depolarizations (> 10 s duration) of many enteric nerves and 5-HT1P receptors mediate some slow excitatory synaptic potentials. The 5-HT3 receptor is a ligand-gated cation channel that mediates fast depolarizations (< 2 s). The 5-HT4 receptor mediates presynaptic facilitation of fast excitatory neurotransmission. The inhibitory receptor is the 5-HT1A receptor. 5-HT1A receptors mediate hyperpolarizations in AH neurons and presynaptic inhibition of fast and slow excitatory neurotransmission.

Mucosal protection through active intestinal secretion: neural and paracrine modulation by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is widely distributed within enteroendocrine cells and neurons of the digestive tract. It stimulates active anion secretion in the intestinal epithelium, an effect which promotes the dilution and elimination of luminal pathogens. The intestinal secretory effects of 5-HT appear to be mediated in part by epithelial 5-HT2 like receptors that are linked to phosphatidylinositol turnover. In addition, 5-HT appears to interact with 5-HT3 or 5-HT4 receptors on submucosal neurons to elicit neuronal depolarization and an associated increase in intestinal secretion. Although the precise cellular mechanisms by which 5-HT modulates mucosal ion transport are incompletely understood, it is clear that 5-HT plays an important role as a intestinal secretagogue in certain diarrheal states, in laxation, and in intestinal hypersensitivity reactions.

Inhibitory effect of YM060 on 5-HT3 receptor-mediated depolarization in colonic myenteric neurons of the guinea pig

We used conventional intracellular recording methods to examine the effects of YM060 [(-)-(R)-5-[(1-methyl-1H-indol-3-yl)carbonyl]-4,5,6,7- tetrahydro-1H-benzimidazole monohydrochloride), a novel 5-HT3 receptor antagonist, on 5-hydroxytryptamine (5-HT, serotonin)-evoked fast membrane depolarization in myenteric neurons of the guinea pig distal colon, and compared its potency to that of other 5-HT3 receptor antagonists. Microapplication of 5-HT from fine-tipped pipettes evoked both fast and slowly activating depolarizing responses in 78% and 40% of colonic myenteric neurons, respectively. The selective 5-HT3 receptor agonist 2-methyl-5-HT applied with short pressure pulses (100-300 ms) mimicked the fast but not the slow response. The 5-HT3 receptor antagonists YM060, granisetron and ondansetron suppressed the 5-HT-evoked fast response in 98% of colonic myenteric neurons in a concentration-dependent manner with pIC50 values of 8.62, 7.77 and 6.90, respectively. Methysergide and GR113808 did not affect the fast responses at concentrations sufficient to block 5-HT1, 5-HT2 and 5-HT4 receptors, respectively. YM060 did not affect the slowly activating response to 5-HT or any other electrophysiological parameter of the neurons including resting membrane potential, input resistance and the amplitude of action potentials evoked by injection of depolarizing current. Stimulus-evoked fast excitatory postsynaptic potentials were unchanged by YM060 at concentrations up to 10(-8) M, excluding any possible local anesthetic or anticholinergic effects of YM060. The results confirm that the fast component of the two depolarizing responses to 5-HT in colonic myenteric neurons is mediated by 5-HT3 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural examination of the targets of serotonin-immunoreactive descending interneurons in the guinea pig small intestine

Serotonin neurons are descending interneurons in the myenteric plexus of the guinea pig small intestine. Preembedding single- and double-label immunocytochemistries at the ultrastructural level were used to identify the targets of these serotonin interneurons. Serial ultrathin sections were taken through a myenteric ganglion that had been processed for serotonin immunocytochemistry. The ganglion contained the cell bodies of 69 neurons, including 2 serotonin neurons and 6 neurons with the ultrastructural features of Dogiel type II cells. For each cell body in the ganglion, the number of serotonin inputs (synapses and close contacts) was determined. About 59% of the cell bodies did not receive any serotonin inputs. The most abundant serotonin terminals were related to two targets: other serotonin descending interneurons and a population of neurons with Dogiel type I morphology, but whose neurochemistry and function is unknown. The serotonin inputs to the serotonin cell bodies were located predominantly on the lamellar dendrites. Each of the Dogiel type II neurons received 3 or fewer serotonin inputs, and none of the serotonin inputs to Dogiel type II neurons formed a synapse. Overall, about 40% of the serotonin inputs formed synapses. The serotonin inputs to neurons that received many serotonin inputs were more likely to show synaptic specializations than serotonin inputs to neurons that received few serotonin inputs. Inhibitory motor neurons contain nitric oxide synthase (NOS). At the light microscope level, serotonin nerve fibers do not form dense pericellular baskets around NOS cell bodies. To determine whether there are serotonin inputs to NOS neurons, serial ultrathin sections were taken through a myenteric ganglion that had been processed for preembedding double-label immunocytochemistry, in which the NOS neurons were labeled with peroxidase-diaminobenzidine and the serotonin neurons with silver-intensified 1 nm gold. Only 1 out of 9 NOS cells examined in serial section received more than 5 serotonin inputs. The results suggest that, in the guinea pig small intestine, the serotonin descending interneurons are not an essential element of the descending inhibitory reflex.

Therapeutic potential of drugs with mixed 5-HT4 agonist/5-HT3 antagonist action in the control of emesis

Drugs interacting with serotonin (5-hydroxytryptamine, 5-HT) receptors are of value in the treatment of several gastrointestinal disturbances. Selective 5-HT3 receptor antagonists (ondansetron, granisetron, tropisetron) are widely utilized to control emesis induced by chemotherapy and radiation, while agonists at 5-HT4 receptors (cisapride, renzapride, BIMU compounds) are endowed with gastrointestinal prokinetic action. Here we overview the therapeutic potential of drugs with potent mixed 5-HT4 agonist/5-HT3 antagonist properties (i.e. BIMU 1) in the management of anticancer therapy-induced emesis and of intestinal adynamic post-operative conditions associated with vomiting. In the former situation, the agonism at 5-HT4 receptors is expected to be of benefit via two possible mechanism: (i) inhibition of 5-HT release from enterochromaffin cells; (ii) restoration of anally driven peristaltic waves in the upper gastrointestinal tract. Moreover, 5-HT4 receptor-induced prokinetic activity may counteract colonic constipation, an unwanted effect which occurs in a number of patients treated with pure 5-HT3 receptor antagonists. Additionally, the above mentioned drugs might be of value in post-operative conditions associated with intestinal adynamia and emesis sensitive to 5-HT3 receptor blockade.

Detection of 5-HT3R-A, a 5-HT3 receptor subunit, in submucosal and myenteric ganglia of rat small intestine using in situ hybridization

Physiological and radioligand binding studies have demonstrated the existence of 5-HT3 receptors in the enteric nervous system. In order to determine if the cloned 5-HT3 receptor subunit, 5-HT3R-A, was expressed in the enteric nervous system of rats, we have performed in situ hybridization with 33P-labeled cRNA antisense probes on sections of rat small intestine. Hybridization was detected in both submucosal and myenteric ganglia of the duodenum, jejunum, and ileum.

Evidence that the 5-HT3 receptors of the rat, mouse and guinea-pig superior cervical ganglion may be different

1. Using grease-gap recordings from the isolated superior cervical ganglion of mouse, rat and guinea-pig, we have compared the depolarization evoked by 5-hydroxytryptamine (5-HT) with that evoked by the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-Me-5-HT). 2. The maximum depolarization induced by 2-Me-5-HT was smaller than that induced by 5-HT in all three species, and particularly in the guinea-pig. 3. The 5-HT2 receptor antagonist ketanserin (1 microM) caused a clear rightward shift of the dose-response curve to 5-HT on the guinea-pig ganglion, but not on the mouse or rat ganglion. Spiperone (0.03 microM) had a quantitatively similar action to ketanserin (0.1 microM) on the 5-HT dose-response curve of the guinea-pig ganglion. Ketanserin had no significant effect on the dose-response curve to 2-Me-5-HT on any of these ganglia. 4. Using 2-Me-5-HT as the agonist, we determined the pA2 values for two 5-HT3 receptor antagonists. The potency of ICS 205-930 varied by approximately 100 fold between the species and that of (+)-tubocurarine varied by over 1000 fold. The differences in the pA2 values of these compounds varied independently among the species. 5. We conclude that 5-HT3 receptors are present on the superior cervical ganglion from the rat, mouse and guinea-pig, but these receptors may be pharmacologically distinct from each other. In addition, the depolarization of the guinea-pig superior cervical ganglion by low concentrations of 5-HT is largely mediated by ketanserin-sensitive receptors.

Projections of 5-hydroxytryptamine-immunoreactive neurons in guinea-pig distal colon

The presence of 5-hydroxytryptamine in enteric neurons of the guinea-pig distal colon was demonstrated by immunohistochemistry and the projections of the neurons were determined. 5-Hydroxytryptamine-containing nerve cells were observed in the myenteric plexus but no reactive nerve cells were found in submucous ganglia. Varicose reactive nerve fibres were numerous in the ganglia of both the myenteric and submucous plexuses, but were infrequent in the longitudinal muscle, circular muscle, muscularis mucosae and mucosa. Reactivity also occurred in enterochromaffin cells. Lesion studies showed that the axons of myenteric neurons projected anally to provide innervation to the circular muscle and submucosa and to other more anally located myenteric ganglia. The results suggest that a major population of 5-hydroxytryptamine neurons in the colon is descending interneurons, most of which extend for 10 to 15 mm in the myenteric plexus and innervate both 5-hydroxytryptamine and non-5-hydroxytryptamine neurons.

5-Hydroxytryptamine2 and 5-hydroxytryptamine3 receptors mediate serotonin-induced short-circuit current in pig jejunum

The purpose of this study was to determine the effect of methysergide, ketanserin, granisetron, cisapride, and renzapride on serotonin (5-hydroxytryptamine-evoked short-circuit current in muscle and myenteric plexus-stripped pig jejunum using the Ussing chamber technique. Ketanserin, granisetron, cisapride, and renzapride all reduced the 5-hydroxytryptamine-induced increase in short-circuit current by about 50%. Combination of ketanserin and granisetron only reduced the 5-hydroxytryptamine-induced peak increase in short-circuit current by 25%. Cisapride caused a small concentration-dependent increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Ketanserin, granisetron, methysergide, and renzapride did not alter the basal short-circuit current. These results suggest that 5-hydroxytryptamine elicits an increase in short-circuit current by activating epithelial and submucosal 5-hydroxytryptamine2 and 5-hydroxytryptamine3 receptor subtypes. Furthermore, the short-circuit current-increasing effect of cisapride, is due to activation of at least muscarinic and nicotinic receptors.

Investigation of the role of 5-HT3 and 5-HT4 receptors in ascending and descending reflexes to the circular muscle of guinea-pig small intestine

1. The present study was undertaken to ascertain whether 5-hydroxytryptamine (5-HT) acting at either 5-HT3 or 5-HT4 receptors plays a significant role in motility reflexes in the guinea-pig small intestine. 2. An isolated segment of small intestine was opened along its mesenteric border and pinned, mucosa uppermost, in a three chambered organ bath so that the oral, middle and anal regions of a single preparation could be separately superfused. 3. Conventional intracellular recording methods were used to monitor the responses of the circular muscle in the oral or the anal end chambers when distension was applied in either of the other two chambers or the mucosal villi were compressed in the middle chamber. Drugs were added to the middle chamber. 4. 5-HT3 receptor antagonists (tropisetron, 0.1-10 microM; granisetron, 1 microM and BRL 46470, 1 microM) depressed the ascending excitatory reflex evoked by these stimuli but had no effect on the descending inhibitory reflex. The depression of the excitatory reflex was observed whether the reflex was evoked from the chamber containing the drug or was simply conducted, via interneurones, through this chamber. 5. The 5-HT4 receptor antagonist, SDZ 205-557 (1 microM), had no significant effect on either the ascending or descending reflex pathways. However, 5-HT4 receptors were present as cisapride (0.1 microM) significantly enhanced the ascending excitation without affecting the descending inhibition. This effect of cisapride was converted to a significant depression of the ascending reflex by SDZ 205-557. 6. The results suggest that 5-HT3, but not 5-HT4, receptors play an important role in the ascending excitatory reflex and that these receptors may be on interneurones in the reflex pathway.

Pharmacological evidence for the involvement of calcium/calmodulin in serotonin 5-HT3 receptor-mediated cation permeability in NG 108-15 cells

In NG 108-15 clonal cells, extracellular application of micromolar concentrations of serotonin [5-hydroxytryptamine (5-HT)] and substance P induces the opening of a cation permeability monitored by the influx of [14C]-guanidinium. The serotoninergic component of this cation permeability is linked to 5-HT3 receptor activation, whereas the substance P component probably involves an "N-terminal-dependent substance P receptor." In this study, [14C]guanidinium influx triggered by 1 microM 5-HT plus 10 microM substance P was shown to be insensitive to tetrodotoxin, verapamil, diltiazem, nimodipine, and omega-conotoxin, as expected from a process independent of voltage-sensitive sodium and calcium channels. In contrast, [14C]guanidinium influx was inhibited by millimolar concentrations of extracellular calcium and by the chelation of intracellular calcium by bis-O-aminophenoxyethanetetraacetic acid. The inhibition by extracellular calcium apparently involved a competition between the divalent cation and [14C]guanidinium for the same channel. When NG 108-15 cells were exposed to X537A, an ionophore that specifically induces release of calcium from intracellular stores, [14C]guanidinium uptake was markedly increased even in the absence of 5-HT and/or substance P. Conversely, [14C]guanidinium influx due to the latter substances could be reversibly and dose-dependently blocked by various drugs that possess calmodulin-antagonizing properties. These results strongly suggest that the cation permeability opened by 5-HT and substance P in NG 108-15 cells involves a calcium/calmodulin-dependent process.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of 5-hydroxytryptamine receptors mediating mucosal secretion in guinea-pig ileum

1. The receptor subtypes through which 5-hydroxytryptamine (5-HT) increases electrolyte secretion across the mucosa of guinea-pig ileum were studied. 2. Flat sheep preparations of guinea-pig mucosa plus submucosa were placed in Ussing chambers and the short circuit current (ISC), an index of net electrogenic electrolyte transport across the mucosa, was measured under voltage clamp conditions. 3. Low concentrations of 5-HT (10-300 nM) evoked monophasic increases in ISC which were significantly reduced by hyoscine (100 nM), tetrodotoxin (TTX, 300 nM) and the 5-HT2 receptor antagonist, ketanserin (3-300 nM). 4. Higher concentrations of 5-HT (1-10 microM) produced biphasic responses which were reduced by hyoscine (100 nM), TTX (300 nM), ketanserin (3-300 nM) and also by the 5-HT3 receptor antagonists, granisetron (1 microM) and ICS 205-930 (100 nM). 5. 2-Methyl-5-HT (1-100 microM) and alpha-methyl-5-HT (30 nM-30 microM), agonists at 5-HT3 and 5-HT2 receptors respectively, also evoked ISC increases. These responses were reduced by hyoscine (100 nM) and abolished by TTX (300 nM) and the respective receptor antagonists, granisetron (1 microM) and ketanserin (30 nM). 6. The 5-HT4 receptor antagonist, SDZ 205-557 (300 nM) had no effect on the response to 5-HT. 7. The TTX-resistant response to 5-HT was not affected by 5-HT2,3 or 4 receptor antagonists. 8. These results indicate that 5-HT mediates secretion partly by an action on 5-HT3 receptors located on cholinergic and noncholinergic secretomotor neurones, partly by an action on higher affinity'5-HT2-like' receptors predominantly on noncholinergic neurones, and partly by a direct action on the epithelium.

5-HT1A and 5-HT4 receptor colocalization on hippocampal pyramidal cells

The mixed inhibitory and excitatory effects of 5-HT on hippocampal pyramidal cells were studied on hippocampal slices perfused with a low-Ca2+/high Mg2+ solution that blocked synaptic activity and induced spontaneous pyramidal cell discharge. Extracellular recordings of the spontaneous discharge revealed that, in 65% of the cells, 5-HT (0.5-10 microM) initially inhibited and then, upon washout, facilitated spontaneous discharge. Sometimes the off-stimulation persisted for the duration of the experiment. In 17% of the cells the response to 5-HT was only stimulatory, and in 15% the response was exclusively inhibitory. The 5-HT1 agonists, 8-hydroxy-dipropylamino-tetraline, and 5-carboxamidotryptamine produced inhibition with no excitatory responses upon washout. The inhibition was blocked by spiroxatrine indicating it was mediated by 5-HT1A receptors. The 5-HT3 agonist, 2-methyl 5-HT, had no effect, and the 5-HT2 antagonist, ketanserin, did not alter the excitatory responses to 5-HT. This indicates the excitatory response is not mediated by 5-HT2 or 3 receptors. Cisapride, a 5-HT4 agonist increased pyramidal cell discharge. The 5-HT3 & 4 antagonist, ICS 205-930 antagonized the excitatory responses to 5-HT, alpha-methyl 5-HT, and cisapride, indicating the excitatory response is mediated, in part, by 5-HT4 receptors. The phosphodiesterase inhibitor, isobutyl-methyl-xanthine, stimulated pyramidal cell discharge and potentiated the response to cisapride. This further suggests 5-HT4 receptor involvement since these receptors are positively coupled to adenylyl cyclase.

Gastrointestinal neurotransmitters

The enteric nervous system contains neurones that are intrinsic to the gastrointestinal tract and the axons of extrinsic neurones. More than 30 functional types of neurone are present and about 25 different possible neurotransmitters have been identified in enteric neurones. Most neurones utilize several transmitters; amongst the transmitters of an individual neurone, one is usually a primary transmitter and other substances are subsidiary transmitters or neuromodulators. The primary transmitter is the substance that has the major role in acutely changing the excitability of the innervated cell. Current evidence indicates that primary transmitters are strongly conserved; that is, the same substance will be the neurotransmitter in functionally equivalent neurones in different regions of the gastrointestinal tract and in different species. In contrast, subsidiary transmitters and neuromodulators of equivalent neurones in different regions are not necessarily the same. Only about seven of the approximately 25 enteric neurotransmitters are known to be primary transmitters. Acetylcholine is the primary transmitter of vagal and pelvic preganglionic neurones, of enteric interneurones, of one class of secretomotor neurone in the intestine and of motor neurones controlling gastric acid secretion. Acetylcholine and tachykinins are co-primary transmitters of muscle motor neurones, with acetylcholine appearing to have the greater role. Tachykinins are probably primary transmitters of enteric sensory neurones at neuroneuronal synapses. Serotonin may also be a transmitter to neurones in the enteric ganglia. Nitric oxide appears to be the usual primary transmitter of enteric inhibitory motor neurones to the muscle. ATP and vasoactive intestinal peptide are subsidiary transmitters of these neurones, although in some regions they may have a primary transmitter role. Vasoactive intestinal peptide is the primary transmitter of non-cholinergic secretomotor neurones. Gastrin releasing peptide is the primary transmitter of motor neurones to gastrin cells. Noradrenaline is the primary transmitter of sympathetic neurones that supply the intestine.

Role of 5-HT3 receptors in peristaltic reflex elicited by stroking the mucosa in the canine jejunum

1. The role played by the 5-HT3 receptor, a serotonin subtype receptor, in peristaltic reflexes was studied in dogs first given ketamine, then anaesthetized with urethane (1.0 g kg-1, I.V.) and alpha-chloralose (100 mg kg-1, I.V.). The jejunal loop was partitioned into two segments with respect to blood supply. Drugs were infused intra-arterially into each segment. 2. Stroking of the mucosa of the aboral and oral segments elicited an ascending contraction and a descending relaxation, respectively. 3. The ascending contraction was concentration-dependently inhibited by treatment of the aboral segment with the 5-HT3 receptor antagonists ICS 205-930 and ondansetron (1.4 pmol min-1 to 14 nmol min-1 for both). The maximal inhibition was 49.5 and 69.3%, respectively. The response was not affected by treatment of the oral segment with these drugs. The descending relaxation was inhibited by 51.4 and 60.8%, respectively, by treatment of the oral segment with ICS 205-930 and ondansetron (1.4 nmol min-1 for both). 4. The ascending contraction was markedly inhibited by treatment of either segment with hexamethonium (140 nmol min-1). The response was abolished by treating both segments with hexamethonium and by treating the oral segment with atropine (14 nmol min-1). 5. These results suggest firstly that, in the canine jejunum, enteric neurons with 5-HT3 receptors play a role as sensory neurons or interneurons in the ascending excitatory and the descending inhibitory pathways of the peristaltic reflex elicited by stroking the mucosa, and secondly, that the ascending limb is composed of cholinergic interneurons and motoneurons.

Characteristics of [14C]guanidinium accumulation in NG 108-15 cells exposed to serotonin 5-HT3 receptor ligands and substance P

In the presence of substance P (SP; 10 microM), serotonin (5-HT; 1 microM) triggered a cation permeability in cells of the hybridoma (mouse neuroblastoma x rat glioma) clone NG 108-15 that could be assessed by measuring the cell capacity to accumulate [14C]guanidinium for 10-15 min at 37 degrees C. In addition to 5-HT (EC50 0.33 microM), the potent 5-HT3 receptor agonists 2-methyl-serotonin, phenylbiguanide, and m-chlorophenylbiguanide, and quipazine, markedly increased [14C]guanidinium uptake in NG 108-15 cells exposed to 10 microM SP. In contrast, 5-HT3 receptor antagonists prevented the effect of 5-HT. The correlation (r = 0.97) between the potencies of 16 different ligands to mimic or prevent the effects of 5-HT on [14C]guanidinium uptake, on the one hand, and to displace [3H]zacopride specifically bound to 5-HT3 receptors on NG 108-15 cells, on the other hand, clearly demonstrated that [14C]guanidinium uptake was directly controlled by 5-HT3 receptors. Various compounds such as inorganic cations (La3+, Mn2+, Ba2+, Ni2+, and Zn2+), D-tubocurarine, and memantine inhibited [14C]guanidinium uptake in NG 108-15 cells exposed to 5-HT and SP, as expected from their noncompetitive antagonistic properties at 5-HT3 receptors. However, ethanol (100 nM), which has been reported to potentiate the electrophysiological response to 5-HT3 receptor stimulation, prevented the effects of 5-HT plus SP on [14C]guanidinium uptake. The cooperative effect of SP on this 5-HT3-evoked response resulted neither from an interaction of the peptide with the 5-HT3 receptor binding site nor from a possible direct activation of G proteins in NG 108-15 cells. Among SP derivatives, [D-Pro9]SP, a compound inactive at the various neurokinin receptor classes, was the most potent to mimic the stimulatory effect of SP on [14C]guanidinium uptake in NG 108-15 cells exposed to 5-HT. Although the cellular mechanisms involved deserve further investigations, the 5-HT-evoked [14C]guanidinium uptake appears to be a rapid and reliable response for assessing the functional state of 5-HT3 receptors in NG 108-15 cells.