Identification of sensory nerve cells in a peripheral organ (the intestine) of a mammal

Characterisation of calcitonin gene-related peptide-immunoreactive neurons in the myenteric plexus of rat colon

A mechanical or chemical stimulus applied to the intestinal mucosa induces motility reflexes in the rat colon. Enteric neurons containing calcitonin gene-related peptide (CGRP) have been suggested as intrinsic primary afferent neurons responsible for mediating such reflexes. In the present study, immunohistochemistry was performed on whole-mount stretch preparations to investigate chemical profiles, morphological characteristics and projections of CGRP-containing neurons in the myenteric plexus of the rat colon. CGRP-positive neuronal cell bodies were detected in preparations incubated with colchicine-containing medium, whereas CGRP-positive nerve fibres were found in colchicine-untreated preparations. These neurons had large oval or round cell bodies that were also immunoreactive for the calcium-binding protein calretinin and neurofilament 200. Myenteric neurons positive for both calretinin and neurofilament 200 had several long processes that emerged from the cell body, consistent with Dogiel type II morphology. Application of the neural tracer DiI to the intestinal mucosa revealed that DiI-labelled myenteric neurons each had an oval or round cell body immunoreactive for calretinin. Thus, CGRP-containing myenteric neurons are Dogiel type II neurons and are immunoreactive for calretinin and neurofilament 200 in the rat colon. These neurons probably project to the intestinal mucosa.

Lactobacillus reuteri enhances excitability of colonic AH neurons by inhibiting calcium-dependent potassium channel opening

Probiotics are live non-pathogenic commensal organisms that exert therapeutic effects in travellers' diarrhea, irritable bowel syndrome and inflammatory bowel disease. Little is known about mechanisms of action of commensal bacteria on intestinal motility and motility-induced pain. It has been proposed that probiotics affect intestinal nerve function, but direct evidence for this has thus far been lacking. We hypothesized that probiotic effects might be mediated by actions on colonic intrinsic sensory neurons. We first determined whether sensory neurons were present in rat colon by their responses to chemical mucosal stimulation and identified them in terms of physiological phenotype and soma morphotype. Enteric neuron excitability and ion channel activity were measured using patch clamp recordings. We fed 10(9)Lactobacillus reuteri (LR) or vehicle control to rats for 9 days. LR ingestion increased excitability (threshold for evoking action potentials) and number of action potentials per depolarizing pulse, decreased calcium-dependent potassium channel (IK(Ca)) opening and decreased the slow afterhyperpolarization (sAHP) in sensory AH neurons, similar to the IK(Ca) antagonists Tram-34 and clotrimazole. LR did not affect threshold for action potential generation in S neurons. Our results demonstrate that LR targets an ion channel in enteric sensory nerves through which LR may affect gut motility and pain perception.

Recent advances in enteric neurobiology: mechanosensitive interneurons

Until recently, it was generally assumed that the only intrinsic sensory neuron, or primary afferent neuron, in the gut was the after-hyperpolarizing AH/Type II neuron. AH neurons excited by local chemical and mechanical stimulation of the mucosa appear to be necessary for activating the peristaltic reflex (oral excitation and anal inhibition of the muscle layers) and anally propagating ring like contractions (peristaltic waves) that depend upon smooth muscle tone. However, our recent findings in the guinea-pig distal colon suggest that different neurochemical classes of interneuron in the colon are also mechanosensitive in that they respond directly to changes in muscle length, rather than muscle tone or tension. These interneurons have electrophysiological properties consistent with myenteric S-neurons. Ascending and descending interneurons respond directly to circumferential stretch by generating an ongoing polarized peristaltic reflex activity (oral excitatory and anal inhibitory junction potentials) in the muscle for as long as the stimulus is maintained. Some descending (nitric oxide synthase +ve) interneurons, on the other hand, appear to respond directly to longitudinal stretch and are involved in accommodation and slow transit of faecal pellets down the colon. This review will present recent evidence that suggests some myenteric S interneurons, in addition to AH neurons, behave as intrinsic sensory neurons.

Purinergic mechanisms in the control of gastrointestinal motility

For many years, ATP and adenosine have been implicated in movement regulation of the gastrointestinal tract. They act through three major receptor subtypes: adenosine or P1 receptors, P2X receptors and P2Y receptors. Each of these major receptor types can be subdivided into several different classes and is widely distributed amongst various neurons, muscle types, glia and interstitial cells that regulate intestinal functions. Several key roles for the different receptors and their endogenous ligands have been identified in physiological and pharmacological studies. For example, adenosine acting at A(1) receptors appears to inhibit intestinal motility in various pathological conditions. Similarly, ATP acting at P2Y receptors is an important component of inhibitory neuromuscular transmission, acting as a cotransmitter with nitric oxide. ATP acting at P2X and P2Y(1) receptors is important for synaptic transmission in simple descending excitatory and inhibitory reflex pathways. Some P2Y receptor subtypes prefer uridine nucleotides over purine nucleotides. Thus, roles for UTP and UDP as enteric transmitters in place of ATP cannot be excluded. ATP also appears to be important for sensory transduction, especially in chemosensitive pathways that initiate local inhibitory reflexes. Despite this evidence, data are lacking about the roles of either adenosine or ATP in more complex motility patterns such as segmentation or the interdigestive migrating motor complex. Clarification of roles for purinergic transmission in these common, but understudied, motility patterns will depend on the use of subtype-specific antagonists that in some cases have not yet been developed.

Local inhibitory reflexes excited by mucosal application of nutrient amino acids in guinea pig jejunum

The motility of the gut depends on the chemicals contained in the lumen, but the stimuli that modify motility and their relationship to enteric neural pathways are unclear. This study examined local inhibitory reflexes activated by various chemical stimulants applied to the mucosa to characterize effective physiological stimuli and the pathways they excite. Segments of the jejunum were dissected to allow access to the circular muscle on one-half of the preparation while leaving the mucosa intact on the circumferentially adjacent half. Chemicals were transiently applied to the mucosa, and responses were recorded intracellularly in nearby circular muscle cells. The amino acids l-phenylalanine, l-alanine, or l-tryptophan (all 1 mM) evoked inhibitory junction potentials (IJPs; latency 150-300 ms, amplitude 3-8 mV, each n > 6) that were blocked by TTX and partially blocked by antagonists of P2X receptors and/or a combination of antagonists at 5-HT(3) and 5-HT(4) receptors. The putative mediators 5-HT (10 microM), ATP (1 mM), and CCK-8 (1-10 microM) elicited IJPs mediated via 5-HT(3), P2X, and CCK-B receptors, respectively. Responses were only partially reduced by the effective antagonists. IJPs evoked by electrically stimulating the mucosa were unaffected by antagonists that reduced chemically evoked responses. Both chemically and electrically evoked IJPs were resistant to nicotinic, NK(1), NK(3), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-d-aspartate, or CGRP receptor blockade. We conclude that mucosal stimulation by amino acids activates local neural pathways whose pharmacology depends on the nature of the stimulus. Transmitters involved at some synapses in these pathways remain to be identified.

Postnatal development of the mucosal plexus in the porcine small and large intestine

Knowledge regarding the foetal and postnatal development of the enteric nervous system is crucial for the understanding of congenital disorders. While lot of information exists regarding the myenteric and submucosal plexuses, the development of the mucosal plexus has not been previously studied. The mucosal innervation seems to play an important role in the local reflex activity of the gut. In this study, we examined the development of enteric mucosal innervation in the pig at various ages of life. Small and large bowel paraffin-embedded specimens were stained with PGP 9.5 and neurofilament protein in three piglets from six age groups (60 and 90 days gestation, newborn, 4 and 12 weeks old, and adult pigs). Small and large bowel demonstrated identical innervation patterns. Myenteric and submucosal plexuses were stained with PGP 9.5 at 60 days gestation. However, the mucosal staining was first noted clearly at the newborn period. By 4 weeks, PGP 9.5 staining was noted in small amounts within the mucosa. Inner proprial and villous fibres were seen ahead in time to the subepithelial fibres. Both inner proprial and villous staining became quiet prominent by 12 weeks of age and remained unchanged into adulthood. However, the subepithelial fibres appear to increase in adulthood. This study demonstrates for the first time that enteric mucosal innervation first appears only at birth. The immaturity of the mucosa generated reflex activity, and secretory functions may have implication in the management of functional intestinal obstruction in the premature infant.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

Acidity induces c-Fos expression in a subpopulation of human colonic submucosal neurons

Enteric neurons responding to chemical challenge of the mucosa have been characterized in animal models mainly in the myenteric plexus. However, in humans, the existence of enteric neurons responding to chemical stimulation of the mucosa remains currently unknown. Therefore, the aim of our study was to identify and characterize human submucosal neurons activated by mucosal challenge with butyrate or hydrochloric acid. Segments of human colon were placed in a modified Ussing chamber and incubated on the mucosal side with butyric acid (20 mM, pH 6.5), sodium butyrate (20 mM, pH 7.5), hydrochloric acid (10 mM, pH 6.5) or culture medium (pH 7.5). After 90 min of culture, tissues were fixed and microdissected to obtain whole mount preparation of submucosa containing the Meissner's plexus. Neuron specific enolase (NSE), c-Fos, vasoactive intestinal peptide (VIP) and substance P (SP) were detected using immunohistochemical methods. Tetrodotoxin (TTX, 1 microM) was used to inhibit neuronal activity. After 90 min of culture, butyric acid induced a significant 5.6-fold increase in the proportion of c-Fos-immunoreactive neurons compared to control (19 +/- 4% versus 4 +/- 1%, respectively, p < 0.001). 41 +/- 5% of c-Fos-immunoreactive neurons were VIP-immunoreactive and 3 +/- 2% were SP-immunoreactive. Butyric acid did not modify the proportion of VIP-immunoreactive neurons. The increase in c-Fos-immunoreactive neurons induced by butyric acid was reproduced with hydrochloric acid at the same pH but not with sodium butyrate. Finally, preincubation of the tissue with TTX prevented the effect of butyric acid. In conclusion, our results demonstrate that acidic mucosal challenge induced the activation of a population of human submucosal neurons with a specific neurochemical coding.

Sensory elements within the circular muscle are essential for mechanotransduction of ongoing peristaltic reflex activity in guinea-pig distal colon

We have recently identified a population of mechanosensory myenteric S-interneurons in the distal colon of guinea-pigs. However, the role of the longitudinal (LM) and circular muscle (CM) in transducing these mechanosensory signals into enteric reflexes is unclear. In this study, we have investigated whether the LM or CM layer is necessary for activation of ascending excitatory and descending inhibitory neuronal pathways by static stretch of the paralysed isolated guinea-pig distal colon. Simultaneous intracellular recordings were made from pairs of CM cells at either end of isolated sheet preparations of distal colon that were devoid of mucosa and submucous plexus; and were maintained under circumferential stretch. In the presence of nifedipine (1 microm), an ongoing discharge of excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs) were recorded simultaneously at the oral and anal ends of the preparation. When the LM was sharp dissected off the myenteric plexus, the synchronized discharge of ascending EJPs and descending IJPs in the CM layer was unaffected. In contrast, when the majority of CM was sharp dissected off the myenteric plexus, ongoing neural activity was absent, or substantially decreased in both the LM and CM. In these preparations, immunohistochemical staining, together with transmural electrical stimuli confirmed that the myenteric plexus was always present and intact in these preparations. When full-thickness strips of CM were removed from progressively longer lengths of myenteric plexus, a graded reduction in the correlation of coordinated oral EJPs and anal IJPs occurred. However, removing approximately 40% of the thickness of CM layer from the entire preparation did not significantly disrupt, nor reduce the degree of correlation between oral EJPs and anal IJPs, suggesting that critical sensory elements did not lie adjacent to the submucosal plexus. It is concluded that mechanosensory transmission that underlies repetitive firing of ascending excitatory and descending inhibitory neuronal pathways is critically dependent upon sensory elements within the CM layer. These elements are likely to activate stretch-sensitive interneurons in the myenteric plexus. No evidence was found to suggest that the connectivity between the LM and the myenteric plexus was required for mechanotransduction.

Novel gut afferents: Intrinsic afferent neurons and intestinofugal neurons

Information about the conditions of all tissues in the body is conveyed to the central nervous system through afferent neurons. Uniquely amongst peripheral organs, the intestine has numerous additional afferent neurons, intrinsic primary afferent neurons that have their cell bodies and processes in the enteric plexuses and do not project to the central nervous system. They detect conditions within the gut and convey that information to intrinsic reflex pathways that are also entirely contained inside the gut wall. Intrinsic primary afferent neurons respond both to the presence of material in the gut lumen and to distension of the gut wall and initiate reflex changes in contractile activity, fluid transport across the mucosa and local blood flow. They also function as nociceptors that initiate tissue-protective propulsive and secretory reflexes to rid the gut of pathogens. The regulation of excitability of intrinsic primary afferent neurons is through multiple ion channels and ion channel regulators, and their excitability is critical to setting the strength of enteric reflexes. The intestine also provides afferent signals to sympathetic pre-vertebral ganglia. The signals are conveyed from the gut by intestinofugal neurons that have their cell bodies within enteric ganglia and form synapses in the sympathetic ganglia. Intestinofugal neurons form parts of the afferent limbs of entero-enteric inhibitory reflexes. Because the unusual afferent neurons of the small intestine and colon make their synaptic connections outside the central nervous system, the neurons and the reflex centres that they affect are potential targets for non-central penetrant therapeutic compounds.

P2X2 receptors are essential for [Ca2+]i increases in response to ATP in cultured rat myenteric neurons

We characterized ATP-induced changes in intracellular Ca2+ concentration ([Ca2+]i) and membrane current in cultured rat myenteric neurons using ratiometric Ca2+ imaging with fura-2 and the whole cell patch-clamp technique, respectively. Neuronal cells were functionally identified by [Ca2+]i responses to high K+ and nicotine, which occurred only in cells positive for neuron-specific protein gene product 9.5 immunoreactivity. ATP evoked a dose-dependent increase of [Ca2+]i that was greatly decreased by the removal of extracellular Ca2+ concentration ([Ca2+]o). The amplitude of the [Ca2+]i response to ATP was reduced by half in the presence of voltage-dependent Ca2+ channel blockers. In [Ca2+]o-free solution, ATP produced a small transient rise in [Ca2+]i similar to that induced by P2Y agonists. At -60 mV, ATP evoked a slowly inactivating inward current that was suppressed by the removal of extracellular Na+ concentration. The current-voltage relation for ATP showed an inward rectification with the reversal potential of about 0 mV. The apparent rank order of potency for the purinoceptor agonist-induced increases of [Ca2+]i was ATP > or = adenosine 5'-O-3-triphosphate > or = CTP > or = 2-methylthio-ATP > benzoylbenzoyl-ATP. A similar potency order was obtained with current responses to these agonists. P2 antagonists inhibited inward currents induced by ATP. Ca2+ and Mg2+ suppressed the ATP-induced current, and Zn2+, Cu2+, and protons potentiated it. RT-PCR and immunocytochemical studies showed the expression of P2X2 receptors in cultured rat myenteric neurons. These results suggest that ATP mainly activates ionotropic P2X2 receptors, resulting in a [Ca2+]i increase dependent on [Ca2+]o in rat myenteric neurons. A small part of the ATP-induced [Ca2+]i increase may be also mediated via a P2Y receptor-related mechanism.

The sphincter of Oddi: understanding its control and function

The most common functional disorders of the biliary tract and pancreas are associated with disordered motility of the sphincter of Oddi (SO). The SO is a neuromuscular structure located at the junction of the bile and pancreatic ducts with the duodenum. The primary functions of the SO are to regulate the delivery of bile and pancreatic juice into the duodenum, and to prevent the reflux of duodenal contents into the biliary and pancreatic systems. Disordered motility of the SO leads to the common and painful clinical conditions of SO dysfunction and acute pancreatitis. In order to understand normal SO motility, studies have been performed addressing SO function, control of spontaneous SO activity, responses to bioactive agents, SO innervation, and reflexes with other gastrointestinal organs. These studies have led to the current understanding of how the SO functions and may permit the development of targeted therapy for SO dysfunction and acute pancreatitis. This review summarizes the current knowledge regarding the control and regulation of SO motility, highlighting laboratory based and clinical research performed over the last 5 years.

Bursts of recurrent excitation in the activation of intrinsic sensory neurons of the intestine

Sensory neurons intrinsic to the wall of the intestine receive input from stimuli in the lumen. These stimuli interact with the mucosal epithelium causing release of sensory mediators that depolarize the sensory nerve terminals. The depolarization and the subsequent pattern of action potential (AP) discharge controls the type and magnitude of the reflex evoked. The characteristics of this AP discharge were investigated in 60 intrinsic sensory neurons from the myenteric plexus of the guinea-pig ileum. Intracellular electrophysiological recordings were made from neurons near intact mucosa during electrical stimulation of the mucosa and/or neuronal soma. Most neurons (87%) responded to mucosal stimulation with a burst of 3.8+/-0.3 APs (average instantaneous frequency, fINT 39+/-4 Hz). In 38%, a somatically evoked AP triggered a similar burst of 2.9+/-0.3 APs (fINT 52+/-6 Hz) while in 50% of neurons, there was ongoing spontaneous bursting (3.8+/-0.2 APs, fINT 48+/-6 Hz). APs in all of these bursts had an inflection on the rising phase and they persisted during somatic hyperpolarization indicating they were generated in a distal process rather than the soma. Collision experiments confirmed this and suggested that bursts originated near the mucosal sensory nerve terminals. A reduction in membrane excitability reduced the number of APs in a burst suggesting a brief depolarizing event, such as a voltage- or ligand-gated ion channel, was responsible. Bursting behavior in the intrinsic sensory neurons is common for mucosal stimuli and may involve a novel transmitter acting at the sensory nerve terminal. Further, some bursting involves positive feedback between the nerve terminals and other elements in the epithelium. This is a novel and potentially important component of intestinal sensory transduction.

MULTIPLE LEVELS OF SENSORY INTEGRATION IN THE INTRINSIC SENSORY NEURONS OF THE ENTERIC NERVOUS SYSTEM

1. The enteric nervous system (ENS) is present in the wall of the gastrointestinal tract and contains all the functional classes of neuron required for complete reflex arcs. One of the most important and intriguing classes of neuron is that responsive to sensory stimuli: sensory neurons with cell bodies intrinsic to the ENS. 2. These neurons have three outstanding and interrelated features: (i) reciprocal connections with each other; (ii) a slow excitatory post-synaptic potential (EPSP) resulting from high-speed firing in other sensory neurons; and (iii) a large after-hyperpolarizing potential (AHP) at the soma. Slow EPSP depolarize the cell body, generate action potentials (APs) and reduce the AHP. Conversely, the AHP limits the firing rate and, hence, reduces transmission of slow EPSP. 3. Processing of sensory information starts at the input terminals as different patterns of APs depending on the sensory modality and recent sensory history. At the soma, the ability to fire APs and, hence, drive outputs is also strongly determined by the recent firing history of the neuron (through the AHP) and network activity (through the slow EPSP). Positive feedback within the population of intrinsic sensory neurons means that the network is able to drive outputs well beyond the duration of the stimuli that triggered them. 4. Thus, sensory input and subsequent reflex generation are integrated over several hierarchical levels within the network on intrinsic sensory neurons.

Calbindin-immunopositive cells are cholinergic interneurons in the myenteric plexus of rabbit ileum

The 28-kDa calcium-binding protein (calbindin) is a widely studied neuronal marker in the enteric nervous system of numerous species. Calbindin has previously been detected in myenteric neurons of rabbit ileum in which 3% of all myenteric neurons are calbindin-immunopositive. We have studied the detailed morphology and chemical coding of calbindin-immunopositive neurons in this segment of the gut. We have found calbindin immunoreactivity in both strongly and weakly stained neurons. Of these, the strongly immunoreactive neurons belong to the Dogiel type I category. These neurons project only to other ganglia and primary strands of the plexus and their processes never run to the muscle or mucosal layers. The neurons within this group are 29.5+/-6.6 microm in length and 14.7+/-3.8 microm in width. The second smaller group of immunoreactive cells (27%) label faintly and have different morphological properties. They are characterized by their round medium-sized cell bodies (long axis: 24.4+/-5.2 microm; short axis: 15.5+/-2.9 microm) and do not exhibit immunoreactivity either in their dendrites or in their axonal processes. Double-label studies show that all calbindin-immunopositive neurons lack immunoreactivity for nitric oxide synthase, vasoactive intestinal peptide and substance P but all are immunoreactive for the synthesizing enzyme of acetylcholine, choline acetyltransferase. Thus, populations of neurons containing calbindin are cholinergic interneurons in the myenteric plexus of rabbit ileum.

R-type calcium channels in myenteric neurons of guinea pig small intestine

Currents carried by L-, N-, and P/Q-type calcium channels do not account for the total calcium current in myenteric neurons. This study identified all calcium channels expressed by guinea pig small intestinal myenteric neurons maintained in primary culture. Calcium currents were recorded using whole cell techniques. Depolarizations (holding potential = -70 mV) elicited inward currents that were blocked by CdCl(2) (100 microM). Combined application of nifedipine (blocks L-type channels), Omega-conotoxin GVIA (blocks N-type channels), and Omega-agatoxin IVA (blocks P/Q-type channels) inhibited calcium currents by 56%. Subsequent addition of the R-type calcium channel antagonists, NiCl(2) (50 microM) or SNX-482 (0.1 microM), abolished the residual calcium current. NiCl(2) or SNX-482 alone inhibited calcium currents by 46%. The activation threshold for R-type calcium currents was -30 mV, the half-activation voltage was -5.2 +/- 5 mV, and the voltage sensitivity was 17 +/- 3 mV. R-type currents activated fully in 10 ms at 10 mV. R-type calcium currents inactivated in 1 s at 10 mV, and they inactivated (voltage sensitivity of 16 +/- 1 mV) with a half-inactivation voltage of -76 +/- 5 mV. These studies have accounted for all of the calcium channels in myenteric neurons. The data indicate that R-type calcium channels make the largest contribution to the total calcium current in myenteric neurons. The relatively positive half-activation voltage and rapid activation kinetics suggest that R-type channels could contribute to calcium entry during somal action potentials or during action potential-induced neurotransmitter release.

Intrinsic primary afferent neurons and nerve circuits within the intestine

Intrinsic primary afferent neurons (IPANs) of the enteric nervous system are quite different from all other peripheral neurons. The IPANs are transducers of physiological stimuli, including movement of the villi or distortion of the mucosa, contraction of intestinal muscle and changes in the chemistry of the contents of the gut lumen. They are the first neurons in intrinsic reflexes that influence the patterns of motility, secretion of fluid across the mucosal epithelium and local blood flow in the small and large intestines. In the guinea pig small intestine, where they have been characterized in detail, IPANs have Dogiel type II morphology, that is they are large round or oval neurons with multiple processes, some of which end close to the luminal surface of the intestine, and some of which form synapses with enteric interneurons, motor neurons and with other IPANs. The IPANs have well-defined ionic currents through which their excitability, and their functions in enteric nerve circuits, is determined. These include voltage-gated Na(+) and Ca(2+) currents, a long lasting calcium-activated K(+) current, and a hyperpolarization-activated cationic current. The IPANs exhibit long-term changes in their states of excitation that can be induced by extended periods of low frequency activity in synaptic inputs and by inflammatory mediators, either applied directly or released during an inflammatory challenge. The IPANs may be involved in pathological changes in enteric function following inflammation.

Mechanosensory S-neurons rather than AH-neurons appear to generate a rhythmic motor pattern in guinea-pig distal colon

Simultaneous intracellular recordings were made from myenteric neurons and circular muscle (CM) cells in isolated, stretched segments of guinea-pig distal colon. We have shown previously that maintained stretch generates a repetitive and coordinated discharge of ascending excitatory and descending inhibitory neuronal reflex pathways in the distal colon. In the presence of nifedipine (1-2 microm) to paralyse the muscle, simultaneous recordings were made from 25 pairs of AH (after-hyperpolarization)-neurons and CM cells separated by 100-500 microm. In all 25 AH-neurons, proximal process potentials (PPPs) were never recorded, even though at the same time, all recordings from neighbouring CM cells showed an ongoing discharge of inhibitory junction potentials (IJPs) anally, or excitatory junction potentials (EJPs) orally. In fact, 24 of 25 AH-neurons were totally silent, while in one AH-cell, some spontaneous fast excitatory postsynaptic potentials (FEPSPs) were recorded. All 10 electrically silent AH-cells that were injected with neurobiotin were found to be multipolar Dogiel type II neurons. In contrast, when recordings were made from myenteric S-neurons, two distinct electrical patterns of electrical activity were recorded. Recordings from 25 of 48 S-neurons showed spontaneous FEPSPs, the majority of which (22 of 25) showed periods when discrete clusters of FEPSPs (mean duration 88 ms) could be temporally correlated with the onset of EJPs or anal IJPs in the CM. Nine S-neurons were electrically quiescent. The second distinct electrical pattern in 14 S-neurons consisted of bursts, or prolonged trains of action potentials, which could be reduced to proximal process potentials (PPPs) in six of these 14 neurons during membrane hyperpolarization. Unlike FEPSPs, PPPs were resistant to a low Ca(2+)-high Mg(2+) solution and did not change in amplitude during hyperpolarizing pulses. Mechanosensory S-neurons were found to be uniaxonal or pseudounipolar filamentous neurons, with morphologies consistent with interneurons. No slow EPSPs were ever recorded from AH- or S-type neurons when IJPs or EJPs occurred in the CM. In summary, we have identified a population of mechanosensory S-neurons in the myenteric plexus of the distal colon which appear to be largely stretch sensitive, rather than muscle-tension sensitive, since they generate ongoing trains of action potentials in the presence of nifedipine. No evidence was found to suggest that in paralysed preparations, the repetitive firing in ascending excitatory or descending inhibitory nerve pathways was initiated by myenteric AH-neurons, or slow synaptic transmission.

Computational model of the migrating motor complex of the small intestine

The migrating motor complex (MMC) is a cyclic motor pattern with several phases enacted over the entire length of the small intestine. This motor pattern is initiated and coordinated by the enteric nervous system and modulated by extrinsic factors. Because in vitro preparations of the MMC do not exist, it has not been possible to determine the intrinsic nerve circuits that manage this motor pattern. We have used computer simulation to explore the possibility that the controlling circuit is the network of AH/Dogiel type II (AH) neurons. The basis of the model is that recurrent connections between AH neurons cause local circuits to enter a high-firing-rate state that provides the maximal motor drive observed in phase III of the MMC. This also drives adjacent segments of the network causing slow migration. Delayed negative feedback within the circuit, provided by activity-dependent synaptic depression, forces the network to return to rest after passage of phase III. The anal direction of propagation is a result of slight anal bias observed in projections of AH neurons. The model relates properties of neurons to properties of the MMC cycle: phase III migration speed is governed by neuron excitability, MMC cycle length is governed by the rate of recovery of synaptic efficacy, and phase III duration is governed by duration of slow excitatory postsynaptic potentials in AH neurons. In addition, the model makes experimental predictions that can be tested using standard techniques.

Calcium imaging of gut activity

The major cell types regulating gut motility include enteric neurones, interstitial cells of Cajal (ICC) and their effector smooth muscle cells. These cells are arranged conveniently in nested layers through the gut wall. Our knowledge of how many of these cells in each layer are integrated to produce the various patterns of motility is largely unknown. So far, much of our knowledge of gut motility has usually been obtained by examining point sources of activity (e.g. intracellular recordings from enteric neurones, ICC and smooth muscle cells), rather than the spread of activity through these spatially distributed nerve and ICC networks, or smooth muscle syncitia. Our understanding of how these cells are integrated to produce gut movements would be greatly enhanced if we could image the activity in many of these cells in each layer, or many cells in several layers, simultaneously. Calcium (Ca2+) is a major signalling and regulatory molecule in most cells. In fact, electrical excitability in enteric neurones, ICC and smooth muscle is associated with robust rises in intracellular Ca2+ that long outlast the electrical events (e.g. action potentials in neurones and smooth muscle) that gave rise to them. These prolonged Ca2+ responses, together with the development of several high quality Ca2+ indicators, has provided a unique opportunity to image many cells in intact tissues simultaneously using ICCD video-rate cameras along with conventional microscopy. However, confocal microscopy has also been used, and has several advantages over the above systems. These include reduced photo-toxicity and bleaching and the elimination of out of focus light from different layers within the tissue. So far, despite some limitations with the calcium imaging techniques, the spread of activity through the two layers of smooth muscle, ICC networks and myenteric neurones in intact preparations, or cultured myenteric neuronal networks, is beginning to yield exciting new data about how these different cells interact and process information.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.

Inhibitory cotransmission or after-hyperpolarizing potentials can regulate firing in recurrent networks with excitatory metabotropic transmission

Recurrent networks of neurons communicating via excitatory connections are common in the nervous system. In the absence of mechanisms to control firing (collectively termed negative feedback), these networks are likely to be bistable and unable to meaningfully encode input signals. In most recurrent circuits, negative feedback is provided by a specialized subpopulation of interneurons, but such neurons are absent from some systems, which therefore require other forms of negative feedback. One such circuit is found within the enteric nervous system of the intestine, where AH/Dogiel type II neurons are interconnected via excitatory synapses acting through metabotropic receptors to produce slow excitatory postsynaptic potentials (slow EPSPs). Negative feedback in this recurrent network may come from either inhibitory postsynaptic potentials arising from the terminals that produce slow EPSPs or from the after hyperpolarizing potentials (AHPs) characteristic of these neurons. We have examined these possibilities using mathematical analysis, based on the Wilson-Cowan model, and computer simulations. Analysis of steady states showed that, under appropriate conditions, both types of negative feedback can provide robust regulation of firing allowing the networks to encode input signals. Numerical simulations were performed using large, anatomically realistic networks with realistic models for metabotropic transmission and suppression of the AHP. In the presence of constant exogenous input, parameters controlling aspects of synaptic events were varied, confirming the analytical results for static stimuli. The simulated networks also responded to time varying inputs in a manner consistent with known physiology. In addition, simulation revealed that neurons in networks with inhibitory contransmission fired in erratic bursts, a phenomenon observed in neurons in unparalysed tissue. Thus, either inhibitory contransmission or AHPs, or both, can allow recurrent networks of AH/Dogiel type II neurons to encode ongoing inputs in a biologically useful way. These neurons appear to be intrinsic primary afferent neurons (IPANs), which implies that the IPANs in a region act in a coordinated fashion.

Long vasodilator reflexes projecting through the myenteric plexus in guinea-pig ileum

This study examined enteric neural reflexes activating submucosal cholinergic vasodilator motoneurons, which innervate the final resistance vessels regulating mucosal blood flow. Videomicroscopy was employed to monitor dilatation of submucosal arterioles in in vitro preparations from guinea-pig ileum. Balloon distension of intact lumen evoked reflex vasodilatation and flat sheet preparations were employed to separate mucosal mechanical stimulation from intestinal distension. Mucosal stroking and balloon distension of the orad segment evoked vasodilatations > 1.5 cm from the stimulating site. Mucosal stimulation was blocked by combined 5-HT3/5HT4 antagonists but distension-evoked responses were unaffected. Distension-evoked responses were also unaffected by nifedipine (5 microM) or nifedipine (1 microM) and wortmannin (300 nM), suggesting stretch activation rather than stretch-activated contraction was involved. Mucosal and distension-evoked responses were completely blocked when the myenteric plexus was surgically lesioned and were significantly inhibited by hexamethonium. The muscarinic antagonist 4-DAMP, which inhibits vasodilatations evoked by submucosal cholinergic vasodilator neurons, blocked dilatations elicited by mucosal stimulation and balloon distension. Maximal dilatations evoked with either sensory modality could be further enhanced when stimulated with the second modality. Dilatations evoked by stimulation of the aborad segment were similar to those elicited in the orad segment. In conclusion, sensory mechanisms in the mucosa and muscularis propria activate vasodilator pathways in the myenteric plexus which project for significant distances in both ascending and descending directions before innervating submucosal arterioles. These reflexes could co-ordinate mucosal blood flow during multiple motor events such as peristalsis and intestinal mixing between propulsive events.

ATP and sensory transduction in the enteric nervous system

ATP is a neurotransmitter in the central and peripheral nervous systems and is also involved in peripheral inflammation and transmission of the sensation of pain. Recently, the regulated release of ATP from non-neuronal sources has been shown to play a role in the activation of sensory nerve terminals. Within the enteric nervous system, which is present in the wall of the gastrointestinal tract, ATP plays three major roles. ATP acts as an inhibitory transmitter from the enteric motor neurons to the smooth muscle via P2Y receptors. ATP is released as an excitatory neurotransmitter between enteric interneurons and from the interneurons to the motor neurons via P2Y and P2X receptors. Finally, ATP may act as a sensory mediator, from epithelial sources to the intrinsic sensory nerve terminals. Thus, ATP participates in the transduction of sensory stimuli from the gut lumen and in the subsequent initiation and propagation of enteric reflexes.

Latent and lytic infection of isolated guinea pig enteric ganglia by varicella zoster virus

Varicella zoster virus (VZV) has been demonstrated to infect guinea pig enteric neurons in vitro. Latent infection of isolated enteric neurons is established when the cultures predominantly consist of neurons and they are exposed to cell-free VZV. Neurons harboring latent infection survive for weeks in vitro and express mRNA encoding ORFs 4, 21, 29, 40, 62, and 63, but not 14(gC) or 68 (gE) (although DNA encoding the glycoproteins is present). The expressed proteins are the same as those that are also expressed in human sensory neurons harboring latent VZV. In addition to mRNA, the immunoreactivities of ORFs 4, 21, 29, 62, and 63 can be detected. ORF 62 and 29 proteins are cytoplasmic and not intranuclear. VZV does not preferentially infect and/or become latent in intrinsic enteric primary afferent neurons indicating that the virus is latent in these neurons. Lytic infection occurs when mixed cultures of neurons and non-neuronal cells of the bowel wall are exposed to cell-free VZV or when isolated enteric neurons are exposed to cell-associated VZV. When lytic infection occurs, enteric neurons die within 48 hr. Prior to their death, neurons express VZV glycoproteins, including gE and gB, and ORF 62 and 29 proteins are intranuclear. This new animal model should facilitate studies of VZV latency and the efficacy of therapies designed to prevent VZV infection, latency, and reactivation.

Electrical stimulation reveals complex neuronal input and activation patterns in single myenteric guinea pig ganglia

The myenteric plexus plays a key role in the control of gastrointestinal motility. We used confocal calcium imaging to study responses to electrical train stimulation (ETS) of interganglionic fiber tracts in entire myenteric ganglia of the guinea pig small intestine. ETS induced calcium transients in a subset of neurons: 52.2% responded to oral ETS, 65.4% to aboral ETS, and 71.7% to simultaneous oral and aboral ETS. A total of 41.3% of the neurons displayed convergence of oral and aboral ETS-induced responses. Responses could be reversibly blocked with TTX (10(-)6 M), demonstrating involvement of neuronal conduction, and by removal of extracellular calcium. omega-Conotoxin (5 x 10(-7) M) blocked the majority of responses and reduced the amplitude of residual responses by 45%, indicating the involvement of N-type calcium channels. Staining for calbindin and calretinin did not reveal different response patterns in these immunohistochemically identified neurons. We conclude that, at least for ETS close to a ganglion, confocal calcium imaging reveals complex oral and aboral input to individual myenteric neurons rather than a polarization in spread of activity.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Enhanced excitability of myenteric AH neurones in the inflamed guinea-pig distal colon

The electrical and synaptic properties of myenteric neurones in normal and inflamed guinea-pig distal colons were evaluated by intracellular microelectrode recording. Chronic inflammation was established 6 days following administration of trinitrobenzene sulfonic acid (TNBS). In S neurones, inflammation only altered synaptic inputs as the amplitude of fast excitatory postsynaptic potentials were significantly larger (31 +/- 2 mV compared to 20 +/- 1 mV) and they were more likely to receive slow excitatory synaptic input (85% compared to 55%). AH neurones displayed altered electrical properties in colitis compared to control tissues: they generated more action potentials during a maximal depolarising current pulse (7 +/- 1 compared to 1.6 +/- 0.2); they had a smaller after hyperpolarisation (9 +/- 2 mV s compared to 20 +/- 2 mV s); and they were more likely to receive fast excitatory synaptic input (74% compared to 17%), possess spontaneous activity (46% compared to 3%), and generate anodal break action potentials (58% compared to 19%). Although the resting membrane potential, input resistance and action potential characteristics were unaltered in AH neurones from inflamed tissues, they exhibited an enhanced Cs+-sensitive rectification of the current-voltage relationship. This suggests that the increase in excitability of AH neurones may involve a colitis-induced augmentation of the hyperpolarisation-activated cation current (Ih) in these cells. An increased excitability, selectively in AH neurones, suggests that the afferent limb of intrinsic motor reflexes is disrupted in the inflamed colon and this may contribute to dysmotility associated with inflammatory diseases.

Morphology and distribution of nitric oxide synthase-, neurokinin-1 receptor-, calretinin-, calbindin-, and neurofilament-M-immunoreactive neurons in the myenteric and submucosal plexuses of the rat small intestine

Characterization of the enteric neurons is vital for understanding their physiological role. We have used single and dual label fluorescence and peroxidase-based immunohistochemistry in myenteric and submucosal whole mounts from the rat small intestine to evaluate the morphology and distribution of enteric neurons immunoreactive for the following phenotypic antigens: neuronal nitric oxide synthase (NOS), neurokinin-1 receptor (NK-1R), calretinin (Calr), calbindin (Cal), and neurofilament-M (NF-M). NOS-immunoreactive neurons had Dogiel type I morphology, were abundant in the myenteric plexus compared to the submucosal plexus, and never coexpressed NK-1R immunoreactivity. NK-1R- and Calr-immunoreactive neurons had Dogiel type II morphology and were distributed comparably in both plexuses. NK-1R and Calr-immunoreactivity were coexpressed in many of the same neurons. Calbindin-immunoreactive neurons exhibited four distinct morphologies: small and large Dogiel type II neurons, Dogiel type I neurons, and small elongated neurons. These neurons were significantly fewer in number in the myenteric plexus compared to the submucosal plexus. Neurofilament-M-immunoreactive neurons had three morphologies, Dogiel type II neurons, small Dogiel type II neurons, and a less common subpopulation of small, elongated, multipolar neurons. These neurons were also fewer in number in the myenteric plexus compared to the submucosal plexus. The distribution of these phenotypic markers may assist future work that elucidates the functional activities of these enteric neurons such as control of intestinal motility and adaptation to the entry of gastric contents.

ATP as a putative sensory mediator: activation of intrinsic sensory neurons of the myenteric plexus via P2X receptors

The mucosal terminals of sensory neurons intrinsic to the wall of the intestine are sensitive to the chemical environment within the lumen. Lumenal stimuli probably release sensory mediators from the mucosal epithelium, which then activate the nerve terminals indirectly. Here, we tested the idea that ATP activates intrinsic sensory nerve terminals in a way consistent with its being a sensory mediator. We made intracellular recordings from intrinsic sensory neurons located in the myenteric plexus [identified as AH neurons, which are neurons with a long-lasting afterhyperpolarization following the action potential (AP)], located within 1 mm of intact mucosa. Focal electrical stimulation of the mucosa was used to locate and map regions innervated by each neuron. Application of ATP (1-2 mm in the pressure pipette) to these regions elicited trains of APs that originated at the sensory terminals. ATP-gamma-S produced a similar response, but alpha,beta-methylene ATP and 2-methylthio-ATP were only weakly active. The P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (PPADS) (60 microm in the bath) abolished the APs evoked by ATP and ATP-gamma-S but spared similar responses evoked by 5-hydroxytryptamine (5-HT). Another P2 receptor antagonist suramin (100 microm in the bath) did not significantly change the number of APs evoked by ATP. Either ATP or alpha,beta-methylene ATP desensitized the ATP-evoked APs; 50% recovery occurred after approximately 5 sec. The number of APs evoked by ATP was reduced, but not abolished, by the selective 5-HT3 receptor antagonist granisetron (1 microm in the bath). ATP was applied to the cell bodies of sensory neurons to investigate whether the cell bodies express the same P2X receptor as the terminals. ATP evoked a fast depolarization associated with a reduction in input resistance and a reversal potential of -11 mV. This depolarization was potentiated by suramin and blocked by PPADS. We conclude that activation of an atypical excitatory P2X receptor by ATP triggers AP generation in the mucosal processes of the sensory neurons; endogenous 5-HT release may also contribute to activation of the nerve terminals. A similar P2X receptor exists on the cell body of the sensory neuron. Together, these data are consistent with a role for ATP as a sensory mediator in gastrointestinal chemosensory transduction.

Sensitization of enteric reflexes in the rat colon in vitro

We have investigated sensitization of reflexes in the isolated rat colon in order to develop a model that might prove useful for investigating how the sensitivity of enteric reflexes can be altered by prior stimulation. Records were taken of circular muscle tension, 7-10 mm oral and anal to radial distension exerted by a hook passed through the wall of the colon. A test stimulus of 1.5 g produced consistent contractions both oral and anal to the distension. A conditioning protocol, consisting of repeated application of 3 g for 30 s with 30 s between the stimuli for 30 min, doubled the amplitudes of reflex contractions that were evoked by the test stimuli but did not change the sensitivity of the muscle to the direct action of carbachol. The enhanced responses persisted for at least 40 min. The enhancement of reflexes was not reduced by antagonists of tachykinin NK3 receptors or of 5-HT3 receptors, but the reflex oral to stimulation was reduced by NK1 and NK3 antagonists added together. Sensitization was abolished by the cyclo-oxygenase and thromboxane synthase inhibitor, indomethacin. We conclude that sensitization can be reliably induced in vitro and that the model described in the present work can be used to investigate drugs that interfere with the sensitization process.

Slow excitatory synaptic potentials evoked by distension in myenteric descending interneurones of guinea-pig ileum

The functional significance of the slow excitatory synaptic potentials (EPSPs) in myenteric neurones is unknown. We investigated this using intracellular recording from myenteric neurones in guinea-pig ileum, in vitro. In all, 121 neurones responded with fast EPSPs to distension of the intestine oral to the recording site. In 28 of these neurones, distension also evoked depolarizations similar to the slow EPSPs evoked by electrical stimulation in the same neurones. Intracellular injection of biocytin and immunohistochemistry revealed that neurones responding to distension with slow EPSPs were descending interneurones, which were immunoreactive for nitric oxide synthase (NOS). Other neurones, including inhibitory motor neurones and interneurones lacking NOS, did not respond to distension with slow EPSPs, but many had slow EPSPs evoked electrically. Slow EPSPs evoked electrically or by distension in NOS-immunoreactive descending interneurones were resistant to blockade of NK(1) or NK(3) tachykinin receptors (SR 140333, 100 nM; SR 142801, 100 nM, respectively) and group I metabotropic glutamate receptors (PHCCC, 10-30 microM), when the antagonists were applied in the recording chamber of a two-chambered organ bath. However, slow EPSPs evoked electrically in inhibitory motor neurones were substantially depressed by SR 140333 (100 nM). Blockade of synaptic transmission in the stimulation chamber of the organ bath abolished slow EPSPs evoked by distension, indicating that they arose from activity in interneurones, and not from anally directed, intrinsic sensory neurones. Thus, distension evokes slow EPSPs in a subset of myenteric neurones, which may be important for intestinal motility.

Maintenance of serotonin in the intestinal mucosa and ganglia of mice that lack the high-affinity serotonin transporter: Abnormal intestinal motility and the expression of cation transporters

The enteric serotonin reuptake transporter (SERT) has been proposed to play a critical role in serotonergic neurotransmission and in the initiation of peristaltic and secretory reflexes. We analyzed potential compensatory mechanisms and enteric function in the bowels of mice with a targeted deletion of SERT. The guts of these animals were found to lack mRNA encoding SERT; moreover, high-affinity uptake of 5-HT into epithelial cells, mast cells, and enteric neurons was present in the SERT +/+ bowel but absent in the SERT -/- bowel. However, both the SERT +/+ gut and the -/- gut expressed molecules capable of transporting 5-HT, but with affinities and selectivity much lower than those of SERT. These included the dopamine transporter (DAT) and polyspecific organic cation transporters OCT-1 and OCT-3. DAT and OCT immunoreactivities were present in both the submucosal and myenteric plexuses, and the OCTs were also located in the mucosal epithelium. 5-HT was found in all of its normal sites in the SERT -/- bowel, which contained mRNA encoding tryptophan hydroxylase, but no 5-HT was present in the blood of SERT -/- animals. Stool water and colon motility were increased in most SERT -/- animals; however, the increase in motility (diarrhea) occasionally alternated irregularly with decreased motility (constipation). The watery diarrhea is probably attributable to the potentiation of serotonergic signaling in SERT -/- mice, whereas the transient constipation may be caused by episodes of enhanced 5-HT release leading to 5-HT receptor desensitization.

Neurotrophin-3 is required for the survival-differentiation of subsets of developing enteric neurons

Neurotrophin-3 (NT-3) promotes enteric neuronal development in vitro; nevertheless, an enteric nervous system (ENS) is present in mice lacking NT-3 or TrkC. We thus analyzed the physiological significance of NT-3 in ENS development. Subsets of neurons developing in vitro in response to NT-3 became NT-3 dependent; NT-3 withdrawal led to apoptosis, selectively in TrkC-expressing neurons. Antibodies to NT-3, which blocked the developmental response of enteric crest-derived cells to exogenous NT-3, did not inhibit neuronal development in cultures of isolated crest-derived cells but did so in mixed cultures of crest- and non-neural crest-derived cells; therefore, the endogenous NT-3 that supports enteric neuronal development is probably obtained from noncrest-derived mesenchymal cells. In mature animals, retrograde transport of (125)I-NT-3, injected into the mucosa, labeled neurons in ganglia of the submucosal but not myenteric plexus; injections of (125)I-NT-3 into myenteric ganglia, the tertiary plexus, and muscle, labeled neurons in underlying submucosal and distant myenteric ganglia. The labeling pattern suggests that NT-3-dependent submucosal neurons may be intrinsic primary afferent and/or secretomotor, whereas NT-3-dependent myenteric neurons innervate other myenteric ganglia and/or the longitudinal muscle. Myenteric neurons were increased in number and size in transgenic mice that overexpress NT-3 directed to myenteric ganglia by the promoter for dopamine beta-hydroxylase. The numbers of neurons were regionally reduced in both plexuses in mice lacking NT-3 or TrkC. A neuropoietic cytokine (CNTF) interacted with NT-3 in vitro, and if applied sequentially, compensated for NT-3 withdrawal. These observations indicate that NT-3 is required for the normal development of the ENS.

Role of alpha(2)-adrenoceptors in the sympathetic inhibition of motility reflexes of guinea-pig ileum

1. Sympathetic regulation of the motility of guinea-pig ileum was investigated using mesenteric nerve (MN) stimulation to inhibit motility reflexes, in vitro. 2. Transmural electrical stimulation (5 Hz, 1 s) in intact intestinal segments, or inflation of a balloon against the mucosa in opened segments, evoked contractions of the circular and longitudinal muscles oral to the stimulus. 3. MN stimulation (10 Hz, 5 s) usually abolished contractions of the longitudinal and circular muscles evoked by either electrical or mechanical stimuli. 4. The inhibition was mimicked by UK14,304 (70-100 nM) and abolished by idazoxan (100 nM), revealing an enhancement of circular muscle contractions. There was no evidence for alpha(2)-receptors on the muscle, suggesting sympathetic inhibition was via the myenteric plexus. 5. Possible sites of action of noradrenaline released from sympathetic nerves were investigated using intracellular recordings from the circular muscle in a multichambered organ bath. 6. When in the stimulation chamber, UK14,304 depressed (by 50 %) excitatory junction potentials (EJPs) recorded oral to a distension stimulus, but did not affect inhibitory junction potentials (IJPs) recorded anal to the stimulus. When added to a chamber between the stimulus and recording chambers, UK14,304 depressed EJPs by 40 %, but did not alter IJPs. When in the recording chamber, UK14,304 depressed EJPs by 20 %, but had no effect on IJPs. IJPs were inhibited, however, when UK14,304 was applied to the whole bath. 7. It is concluded that sympathetic activity inhibits intestinal motility mainly via alpha(2)-adrenoceptors on ascending interneurons and intrinsic sensory neurons of the orally directed reflex pathway.

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.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

Distribution and chemical coding of orphanin FQ/nociceptin-immunoreactive neurons in the myenteric plexus of guinea pig intestines and sphincter of Oddi

Longitudinal muscle-myenteric plexus preparations of guinea pig intestines and sphincter of Oddi (SO) were immunostained for orphanin FQ/nociceptin. Orphanin FQ-immunoreactive (OFQ-IR) neurons and nerve fibers were relatively abundant in the SO, duodenum, ileum, cecum, and distal colon, with fewer neurons and nerve fibers observed in the proximal colon. Double staining with antibodies directed against the neuron-specific RNA binding protein Hu revealed that while the numbers of OFQ-IR neurons per ganglion decreased along the gut tube, similar proportions (7-9%) of neurons in these regions were OFQ-IR, whereas <1% of the neurons in the proximal colon were OFQ positive. In the ileum, where 8% of the myenteric neurons were OFQ-IR, all OFQ-IR neurons expressed choline acetyltransferase. In addition, multiple-label immunohistochemistry demonstrated that 58% of the OFQ-IR neurons were calretinin-IR, 52% were substance P-IR, and 28% were enkephalin-IR. Nitric oxide synthase immunoreactivity was observed in about 5% of OFQ-IR neurons, or 0.4% of the total population, and a similar proportion of the OFQ-IR neurons was positive for vasoactive intestinal peptide. No OFQ-IR neurons were immunoreactive for calbindin, somatostatin, or serotonin. These results, combined with previous studies of chemical coding and projection patterns in the guinea pig myenteric plexus, indicate that OFQ-IR is expressed preferentially in excitatory motor neurons projecting to the longitudinal and circular muscle layers, as well as a small subgroup of descending interneurons. Because OFQ is expressed by excitatory motor neurons, and because this peptide inhibits excitatory neurotransmission in the guinea pig ileum, it is likely that OFQ acts through a feedback autoinhibitory mechanism.

Classes of enteric nerve cells in the guinea-pig small intestine

The guinea-pig small intestine has been very widely used to study the physiology, pharmacology and morphology of the enteric nervous system. It also provides an ideal, simple mammalian preparation for studying how nerve cells are organised into functional circuits underlying simple behaviours. Many different types of nerve cells are present in the enteric nervous system and they show characteristic combinations of morphological features, projections, biophysical properties, neurochemicals, and receptors. To identify the different functional classes is an important prerequisite for systematic analysis of how the enteric nervous system controls normal gut behaviour. Based on combinations of multiple-labelling immunohistochemistry and retrograde tracing, it has been possible to account quantitatively for all of the neurones in the guinea-pig small intestine. This article summarises that account and updates it in the light of recent data. A total of 18 classes of neurones are currently distinguishable, including primary afferent neurones, motor neurones, interneurones, secretomotor and vasomotor neurones. It is now possible to take an individual nerve cell and use a few carefully chosen criteria to assign it to a functional class. This provides a firm anatomical foundation for the systematic analysis of how the enteric nervous system normally functions and how it goes wrong in various clinically important disorders.

Morphological and neurochemical identification of enteric neurones with mucosal projections in the human small intestine

Data on the axonal projections of enteric neurones in the human intestine are still scarce. The present study aimed to identify the morphology and neurochemical coding of enteric neurones in the human small intestine, which are involved in the innervation of the mucosa. The lipophilic neuronal tracer DiI was applied to one mucosal villus of small intestinal resection specimens. The tissue was kept in organotypic culture and subsequently processed for immunohistochemistry. Neurones labelled from the mucosa were located in all ganglionated nerve networks, including the myenteric plexus. In all plexuses, at least five neurochemical types of neurones could be observed, i.e. SOM-IR neurones, SP-IR neurones, SOM/SP-IR neurones, VIP-IR neurones and neurones lacking immunoreactivity for any of these markers. Most of the DiI-labelled neurones were multidendritic; a minority of neurones could be identified as Dogiel type II cells, suggesting the existence of a subgroup of primary afferent neurones in the DiI-filled cell population. The ratio of labelled multidendritic neurones (assumed to be secretomotor) to labelled Dogiel type II neurones (assumed to be primary afferent) in the myenteric plexus is higher in large mammals (pig and human) than in small mammals (guinea pig). This might point to the existence of a different topographical distribution of subsets of primary afferent neurones and/or topographically distinct intrinsic mucosal reflex circuits in large mammals, including humans.

Ryanodine-sensitive stores regulate the excitability of AH neurons in the myenteric plexus of guinea-pig ileum

Myenteric afterhyperpolarizing (AH) neurons are primary afferent neurons within the gastrointestinal tract. Stimulation of the intestinal mucosa evokes action potentials (AP) that are followed by a slow afterhyperpolarization (AHP(slow)) in the soma. The role of intracellular Ca(2+) ([Ca(2+)](i)) and ryanodine-sensitive Ca(2+) stores in modulating the electrical activity of myenteric AH neurons was investigated by recording membrane potential and bis-fura-2 fluorescence from 34 AH neurons. Mean resting [Ca(2+)](i) was approximately 200 nM. Depolarizing current pulses that elicited APs evoked AHP(slow) and an increase in [Ca(2+)](i), with similar time courses. The amplitudes and durations of AHP(slow) and the Ca(2+) transient were proportional to the number of evoked APs, with each AP increasing [Ca(2+)](i) by approximately 50 nM. Ryanodine (10 microM) significantly reduced both the amplitude and duration (by 60%) of the evoked Ca(2+) transient and AHP(slow) over the range of APs tested (1-15). Calcium-induced calcium release (CICR) was graded and proportional to the number of APs, with each AP triggering a rise in [Ca(2+)](i) of approximately 30 nM Ca(2+) via CICR. This indicates that CICR amplifies Ca(2+) influx. Similar changes in [Ca(2+)](i) and AHP(slow) were evoked by two APs in control and six APs in ryanodine. Thus, the magnitude of the change in bulk [Ca(2+)](i) and not the source of the Ca(2+) is the determinant of the magnitude of AHP(slow). Furthermore, lowering of free [Ca(2+)](i), either by reducing extracellular Ca(2+) or injecting high concentrations of Ca(2+) buffer, induced depolarization, increased excitability, and abolition of AHP(slow). In addition, activation of synaptic input to AH neurons elicited a slow excitatory postsynaptic potential (sEPSP) that was completely blocked in ryanodine. These results demonstrate the importance of [Ca(2+)](i) and CICR in sensory processing in AH neurons. Activity-dependent CICR may be a mechanism to grade the output of AH neurons according to the intensity of sensory input.

Growth of nerve fibres into murine peritoneal adhesions

Adhesions in the peritoneal cavity have been implicated in the cause of intestinal obstruction and infertility, but their role in the aetiology of chronic pelvic pain is unclear. Nerves have been demonstrated in human pelvic adhesions, but the presence of pain-conducting fibres has not been established. The purpose of this study was to use an animal model to examine the growth of nerves during adhesion formation at various times following injury and to characterize the types of fibres present. Adhesions were generated in mice by injuring the surface of the caecum and adjacent abdominal wall, with apposition. At 1-8 weeks post-surgery, adhesions were processed and nerve fibres characterized histologically, immunohistochemically, and ultrastructurally. Peritoneal adhesions had consistently formed by 1 week after surgery and from 2 weeks onwards, all adhesions contained some nerve fibres which were synaptophysin, calcitonin gene-related peptide, and substance P-immunoreactive, and were seen to originate from the caecum. By 4 weeks post-surgery, nerve fibres were found to originate from both the caecum and the abdominal wall, and as demonstrated by acetylcholinesterase histochemistry, many traversed the entire adhesion. Ultrastructural analysis showed both myelinated and non-myelinated nerve fibres within the adhesion. This study provides the first direct evidence for the growth of sensory nerve fibres within abdominal visceral adhesions in a murine model and suggests that there may be nerve fibres involved in the conduction of pain stimuli.

The soma and neurites of primary afferent neurons in the guinea-pig intestine respond differentially to deformation

1. Intrinsic primary afferent neurons in the small intestine are exposed to distortion of their processes and of their cell bodies. Recordings of mechanosensitivity have previously been made from these neurons using intracellular microelectrodes, but this form of recording has not permitted detection of generator potentials from the processes, or of responses to cell body distortion. 2. We have developed a technique to record from enteric neurons in situ using patch electrodes. The mechanical stability of the patch recordings has allowed recording in cell-attached and whole cell configuration during imposed movement of the neurons. 3. Pressing with a fine probe initiated generator potentials (14 +/- 9 mV) from circumscribed regions of the neuron processes within the same myenteric ganglion, at distances from 100 to 500 microm from the cell body that was patched. Generator potentials persisted when synaptic transmission was blocked with high Mg2+, low Ca2+ solution. 4. Soma distortion, by pressing down with the whole cell recording electrode, inhibited action potential firing. Consistent with this, moderate intra-electrode pressure (10 mbar; 1 kPa) increased the opening probability of large-conductance (BK) potassium channels, recorded in cell-attached mode, but suction was not effective. In outside-out patches, suction, but not pressure, increased channel opening probability. Mechanosensitive BK channels have not been identified on other neurons. 5. The BK channels had conductances of 195 +/- 25 pS. Open probability was increased by depolarization, with a half-maximum activation at a patch potential of 20 mV and a slope factor of 10 mV. Channel activity was blocked by charybdotoxin (20 nM). 6. Stretch that increased membrane area under the electrode by 15 % was sufficient to double open probability. Similar changes in membrane area occur when the intestine changes diameter and wall tension under physiological conditions. Thus, the intestinal intrinsic primary afferent neurons are detectors of neurite distortion and of compression of the soma, these stimuli having opposite effects on neuron excitability.

Types of neurons in the enteric nervous system

This paper, written for the symposium in honour of more than 40 years' contribution to autonomic research by Professor Geoffrey Burnstock, highlights the progress made in understanding the organisation of the enteric nervous system over this time. Forty years ago, the prevailing view was that the neurons within the gut wall were post-ganglionic neurons of parasympathetic pathways. This view was replaced as evidence accrued that the neurons are part of the enteric nervous system and are involved in reflex and integrative activities that can occur even in the absence of neuronal influence from extrinsic sources. Work in Burnstock's laboratory led to the discovery of intrinsic inhibitory neurons with then novel pharmacology of transmission, and precipitated investigation of neuron types in the enteric nervous system. All the types of neurons in the enteric nervous system of the small intestine of the guinea-pig have now been identified in terms of their morphologies, projections, primary neurotransmitters and physiological identification. In this region there are 14 functionally defined neuron types, each with a characteristic combination of morphological, neurochemical and biophysical properties. The nerve circuits underlying effects on motility, blood flow and secretion that are mediated through the enteric nervous system are constructed from these neurons. The circuits for simple motility reflexes are now known, and progress has been made in analysing those involved in local control of blood flow and transmucosal fluid movement in the small intestine.