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

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.

Distribution of neuronal nicotinic acetylcholine receptors containing different alpha-subunits in the submucosal plexus of the guinea-pig

The subunit composition and localisation of nicotinic acetylcholine receptors (nAChRs) in the submucosal plexus of the guinea-pig ileum were studied using both affinity-purified monoclonal and polyclonal antibodies against alpha3, alpha4, alpha5 and alpha7 nAChR subunits and specific alpha7-containing nAChRs blocker methyllycaconitine (MLA). By means of immunohistochemistry performed in non-dissociated preparations, it was found that only 4% of submucosal ganglia expressed nAChRs. Specific staining, associated with cell membranes, was found with alpha3-, alpha5- and alpha7-, but not alpha4-specific antibodies. Double staining using alpha5- and alpha7-specific antibodies demonstrated that about one-half of the nAChR-positive ganglia contained neurons immunoreactive to both antibodies, while the others possessed either alpha5- or alpha7-immunoreactivity. Nanomolar concentrations of MLA prevented alpha7-specific antibody binding and did not influence the alpha5-specific antibody binding even when applied in micromolar concentrations. In electrophysiological experiments performed using a patch-clamp 'whole-cell' recording method, the neurons were identified by their sensitivity to MLA. In conclusion, submucosal neurons of the guinea-pig ileum express nAChRs containing alpha3-, alpha5- and alpha7-subunits. The co localisation of alpha5- and alpha7-subunits found in immunohistochemical experiments as well as kinetic characteristics of MLA-blocked receptors found by electrophysiological experiments allow us to suggest the presence of heteromeric alpha7-containing nAChRs in the submucosal plexus of the guinea-pig ileum.

Distribution and function of the cannabinoid-1 receptor in the modulation of ion transport in the guinea pig ileum: relationship to capsaicin-sensitive nerves

We investigated the distribution and function of cannabinoid (CB)(1) receptors in the submucosal plexus of the guinea pig ileum. CB(1) receptors were found on both types of submucosal secretomotor neurons, colocalizing with VIP and neuropeptide Y (NPY), the noncholinergic and cholinergic secretomotor neurons, respectively. CB(1) receptors colocalized with transient receptor potential vanilloid-1 receptors on paravascular nerves and fibers in the submucosal plexus. In the submucosal ganglia, these nerves were preferentially localized at the periphery of the ganglia. In denervated ileal segments, CB(1) receptor immunoreactivity in submucosal neurons was not modified, but paravascular and intraganglionic fiber staining was absent. Short-circuit current (I(sc)) was measured as an indicator of net electrogenic ion transport in Ussing chambers. In the ion-transport studies, I(sc) responses to capsaicin, which activates extrinsic primary afferents, and to electrical field stimulation (EFS) were reduced by pretreatment with the muscarinic antagonist atropine, abolished by tetrodotoxin, but were unaffected by VIP receptor desensitization, hexamethonium, alpha-amino-3-hydroxy-5-methlisoxazole-4-proprionic acid, or N-methyl-d-aspartate glutamate receptor antagonists. The responses to capsaicin and EFS were reduced by 47 +/- 12 and 30 +/- 14%, respectively, by the CB(1) receptor agonist WIN 55,212-2. This inhibitory effect was blocked by the CB(1) receptor antagonist, SR 141716A. I(sc) responses to forskolin or carbachol, which act directly on the epithelium, were not affected by WIN 55,212-2. The inhibitory effect of WIN 55,212-2 on EFS-evoked secretion was not observed in extrinsically denervated segments of ileum. Taken together, these data show cannabinoids act at CB(1) receptors on extrinsic primary afferent nerves, inhibiting the release of transmitters that act on cholinergic secretomotor pathways.

Calbindin-immunoreactive neurones in the ovine rumen

In small laboratory animals, such as guinea pigs, immunoreactivity for the calcium-binding protein calbindin (CALB) can be used to distinguish functionally different classes of myenteric neurones. The rumen of sheep is a highly specialized gastrointestinal region, and the control of its functions requires specific intrinsic innervation patterns. The aim of this study was to neurochemically identify and characterize CALB-positive myenteric neurones of the ovine rumen. Therefore, we performed quadruple immunohistochemistry against CALB, substance P (SP), vasoactive intestinal peptide (VIP), and nitric oxide synthase (NOS) using whole-mount preparations of the ruminal myenteric plexus. On average, 3 +/- 2 and 1 +/- 0.4 myenteric neurones/ganglion were CALB-immunoreactive in suckling lambs and adult sheep, respectively. These neurones had Dogiel type-I morphology. Most of them (89.2% +/- 8.7% and 71.7% +/-44.8% in suckling lambs and adult sheep, respectively) did not colocalize any of the other antigenes. Since it has been shown in previous studies that ruminal myenteric neurones are immunoreactive for either choline acetyltransferase (ChAT) or NOS, we defined neurones which were CALB-positive and NOS-negative as CALB/ChAT. The other CALB-positive neurones were encoded CALB/NOS/+/-VIP (10.3% +/- 9.3% and 26.7% +/- 46.2% in suckling lambs and adult sheep, respectively) or CALB/ChAT/SP (0.5% +/- 1.0% and 1.7% +/- 1.9% in suckling lambs and adult sheep, respectively). We used cryostat sections of the ruminal wall to analyze the projections of the CALB-positive neurones. CALB-immunoreactive somata were exclusively located within the myenteric plexus. CALB-immunoreactive nerve fibers were found primarily in the lamina propria of the ruminal papillae. We conclude that CALB-positive myenteric neurones within the ovine rumen project to the epithelium; however, their functional role remains to be investigated.

Intrinsic primary afferent neurones of the digestive tract

Intrinsic primary afferent neurones of the intestine are specialized neurones that encode information about the state of the intestine by transducing mechanical and chemical stimuli that reflect tension in the gut wall and the chemical nature of its contents. They connect with interneurones and motor neurones in the gut to form the circuits of intrinsic muscle motor, secretomotor and vasomotor reflexes. A large range of ionic currents occur in these neurones. The neurones have voltage-activated inward sodium currents (both tetrodotoxin-sensitive and tetrodotoxin-insensitive) and inward calcium currents. Calcium entering during the action potential activates a slow after hyperpolarizing potassium current that has a profound influence on subsequent action potential firing. They also exhibit a prominent hyperpolarization-activated nonspecific cation current. The excitability of these neurones and sensory transduction are altered when the gut is inflamed. Changed excitability can persist after the inflammatory state has subsided. Intrinsic primary afferent neurones are thus important, both to the normal physiology and to pathophysiology of the small and large intestines.

Pharmacology and function of nicotinic acetylcholine and P2X receptors in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). Ligand-gated ion channels are an important class of receptors expressed by enteric neurones. This review will focus on nicotinic acetylcholine receptors (nAChRs) and P2X receptors for ATP, as these receptors contribute to fast synaptic transmission in identified pathways in the ENS. There are multiple subunit proteins that compose nAChRs and P2X receptors in the nervous system. Functional and pharmacological studies indicate that the predominant class of nAChR mediating fast synaptic transmission in enteric neurones is composed of alpha3 and beta4 subunits. P2X receptors mediating fast synaptic excitation are predominately P2X2 homomeric receptors.

Enteric motor and interneuronal circuits controlling motility

The enteric nervous system regulates intestinal motility. It contains intrinsic sensory neurones, several types of interneurones and excitatory and inhibitory motor neurones. This review summarizes our knowledge of motor neurones and interneurones in simple motility reflex pathways (ascending and descending excitation, descending inhibition) and it focuses on guinea-pig ileum. Excitatory circular muscle motor neurones contain choline acetyltransferase (ChAT) and tachykinins and project orally 0.5-10 mm. They transmit via muscarinic acetylcholine receptors and tachykinins acting at NK1 and NK2 receptors. Inhibitory circular muscle motor neurones contain nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating peptide (PACAP), project anally up to 25 mm and transmit via ATP, nitric oxide and/or VIP. Ascending interneurones project up to 10 mm orally and contain ChAT and tachykinins. They transmit to each other via ACh at nicotinic receptors (nAChR), but to excitatory motor neurones via both nAChR and NK3 receptors. There are at least three types of descending interneurones and one transmits to inhibitory motor neurones via ATP acting at P2X receptors. NOS-containing descending interneurones receive input via P2Y receptors from other interneurones. Transmission to and from the other descending interneurones (ChAT/5-HT, ChAT/somatostatin) is yet to be characterized.

Lessons from the porcine enteric nervous system

The porcine intestinal tract possesses functional and pathological similarities to the human digestive tract and the organization of the porcine enteric nervous system, like that of the human, appears to be more complex than that of commonly investigated guinea-pig intestine. Intrinsic primary afferent neurones appear to differ in the intestines of large and small animals in terms of their chemical coding, distribution over enteric neural networks, electrophysiological behaviour and synaptic properties. Opioid receptors on afferent and motor neurones in the porcine small intestine are predominately of the delta type, whereas those in guinea-pig ileum are mu. Moreover, delta-opioid receptors associated with the myenteric and submucosal plexuses of porcine ileum that, respectively, modulate neurogenic smooth muscle contractions and mucosal ion transport appear to differ in their pharmacological characteristics. These profound interspecies and interregional differences underscore the complexity of the enteric nervous system, and the development of new drugs designed to treat human neurogastrointestinal disorders should be based on the results of investigations in homologous animal models, such as the pig.

Enteric P2X receptors as potential targets for drug treatment of the irritable bowel syndrome

The irritable bowel syndrome (IBS) is a gastrointestinal motility disorder affecting millions of patients. IBS symptoms include diarrhea, constipation and pain. The etiology of IBS is due partly to changes in the function of nerves supplying the gastrointestinal tract, immune system activation and to psychological factors. P2X receptors are multimeric ATP-gated cation channels expressed by neuronal and non-neuronal cells. Sensory nerve endings in the gastrointestinal tract express P2X receptors. ATP released from gastrointestinal cells activates P2X receptors on sensory nerve endings to stimulate motor reflexes and to transmit nociceptive signals. Antagonists acting at P2X receptors on sensory nerves could attenuate abdominal pain in IBS patients. Primary afferent neurons intrinsic to the gut, and enteric motor- and interneurons express P2X receptors. These neurons participate in motor reflexes. Agonists acting at enteric P2X receptors may enhance gastrointestinal propulsion and secretion, and these drugs could be useful for treating constipation-predominant IBS. Antagonists acting at enteric P2X receptors would decrease propulsion and secretion and they might be useful for treating diarrhea-predominant IBS. Current knowledge of P2X receptor distribution and function in the gut of laboratory animals provides a rational basis for further exploration of the therapeutic potential for drugs acting at P2X receptors in IBS patients. However, more information about P2X receptor distribution and function in the human gastrointestinal tract is needed. Data on the distribution and function of P2X receptors on gastrointestinal immune cells would also provide insights into the therapeutic potential of P2X receptor agents in IBS.

Cyclooxygenase-2 contributes to dysmotility and enhanced excitability of myenteric AH neurones in the inflamed guinea pig distal colon

We have previously demonstrated that trinitrobenzene sulphonic acid (TNBS)-induced colitis in guinea pig is associated with hyperexcitability of myenteric AH neurones, enhanced synaptic activity in the myenteric plexus, increased serotonin (5-HT) availability in the mucosa, and decreased propulsive motor activity. The current study tested the hypothesis that the activation of cyclooxygenase (COX) contributes to these alterations in bowel functions. DFU inhibition of COX-2, but not SC-560 inhibition of COX-1, restored to normal levels the electrical properties of myenteric AH neurones, the proportion of S neurones exhibiting slow EPSPs, and the rate of propulsive motor activity. Neither inhibitor was effective in altering the level of inflammation, the increased availability of mucosal 5-HT, or the enhanced fast EPSPs in myenteric AH and S neurones. COX-2 expression is enhanced in the myenteric plexus and cells within the smooth muscle layers during colitis, possibly reflecting the site at which COX-2 inhibition acts to allow recovery of motor function. In support of this concept, COX-1, but not COX-2, inhibition was effective in restoring normal mucosal prostaglandin levels. These results indicate that the various changes that occur in the motor neural pathways of the distal colon in TNBS-induced colitis do not involve a single neuroimmune mechanism. COX-2 activation is a critical step in the enhanced excitability of AH neurones as well as diminished propulsive motility in TNBS colitis, whereas other yet to be resolved pathways, that do not involve COX-1 or COX-2 activation, lead to altered 5-HT content in the mucosa and an augmentation of fast EPSPs.

Mechanically evoked reflex electrogenic chloride secretion in rat distal colon is triggered by endogenous nucleotides acting at P2Y1, P2Y2, and P2Y4 receptors

Mechanical activation of the mucosal lining of the colon by brush stroking elicits an intestinal neural reflex and an increase in short circuit current (Isc) indicative of electrogenic chloride ion transport. We tested whether endogenous nucleotides are physiologic regulators of mucosal reflexes that control ion transport. The brush stroking-evoked Isc response in mucosa and submucosa preparations (M-SMP) of rat colon was reduced by the P2Y1 receptor (R) antagonist 2'deoxy-N6-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179) and further blocked by tetrodotoxin (TTX). M-SMP Isc responses to serosal application of the P2Y1 R agonist 2-methylthioadenosine-diphosphate (2MeSADP) or the P2Y2/P2Y4 R agonist 5'uridine-triphosphate (UTP) were reduced but not abolished by TTX. The potency profile of nucleotides for increasing Isc was 5'adenosine-triphosphate (ATP; effective concentration at half maximal response [EC50] 0.65 x 10(4) M) congruent with UTP (EC50 1.0 x 10(-4) M) congruent with 2MeSADP (EC50 = 1.60 x 10(-4) M). Mucosal touch and distention-induced Ca2+ transients in submucous neurons were reduced by apyrase and prevented by blocking the P2Y1 R with MRS 2179 and TTX; denervation of the mucosa. It did not occur by touching a ganglion directly. 2MeSADP Ca2+ responses occurred in subsets of neurons with or without substance P (SP) responses. The potency profile of nucleotides on the neural Ca2+ response was 2MeSADP (5 x 10(-7) M) > UTP (6 x 10(-6) M) > ATP (9 x 10(-5) M). The expression of P2Y R immunoreactivity (ir) in nerve cell bodies was in the order of P2Y1 R > P2Y4 R >> P2Y2 R. P2Y1R ir occurred in the cell somas of more than 90% of neuronal nitric oxide synthase, vasoactive intestinal peptide (VIP), calretinin, or neuropeptide Y (NPY)-ir neurons, 78% of somatostatin neurons, but not in calbindin or SP neurons. P2Y2 R ir was expressed in a minority of SP, VIP, NPY, vesicular acetylcholine transporter, and calcitonin gene-related peptide-ir varicose fibers (5-20%) and those surrounding calbindin (5-20%) neurons. P2Y4 ir occurred mainly in the cell somas of 93% of NPY neurons. Reverse transcriptase polymerase chain reaction of the submucosa demonstrated mRNA for P2Y1R, P2Y2, P2Y4, P2Y6, and P2Y12 Rs. Expression of P2Y1, P2Y2, and P2Y4 protein was confirmed by western blots. In conclusion, endogenous nucleotides acting at P2YRs transduce mechanically evoked reflex chloride ion transport in rat distal colon. Nucleotides evoke reflexes by acting primarily at postsynaptic P2Y1 Rs and P2Y4 R on VIP+/NPY+ secretomotor neurons, at P2Y2 Rs on no more than 2% of VIP+ secretomotor neurons, and 2Y2 Rs mainly of extrinsic varicose fibers surrounding putative intrinsic primary afferent and secretomotor neurons. During mucosal mechanical reflexes, it is postulated that P2Y1 R, P2Y2 R, and P2Y4 R are activated by endogenous ATP, UTP, and 5'uridine-diphosphate.

Blockade of Gi/o proteins modifies electrical activity of S-myenteric neurons from guinea-pig ileum

We have investigated the effect of blockade of G(i/o) proteins on the electrical activity of S-myenteric neurons from guinea-pig ileum longitudinal muscle-myenteric plexus preparations. Intracellular recordings were made from either control or pertussis toxin (PTX) treated tissues. PTX is known to disrupt the function of G(i/o) proteins. Incubation with PTX (300 ng/ml) induced three main changes in the electrophysiological characteristics of S-neurons: (1) resting membrane potential was more depolarised; (2) fast excitatory postsynaptic potentials (EPSPs) were faster and narrower; and (3) spontaneous fast EPSPs and action potentials were more frequent. Amplitude of slow EPSPs was not modified. This general increase in excitability after blockade of G(i/o) proteins shares characteristics with the slow depolarisation induced by long-lasting trains of low-frequency electrical stimulation and could be mediated by similar mechanisms. Electrophysiological activity of S-myenteric neurons could be modulated by an inhibitory tone, which PTX could modify by either pre- or post-synaptic mechanisms, such as inhibition of the tonic release of inhibitory neurotransmitters and/or intracellular signal transduction involving PTX-sensitive G(i/o) proteins.

Action of bradykinin in the submucosal plexus of guinea pig small intestine

Intracellular recording methods with "sharp" microelectrodes were used to study actions of bradykinin (BK) on electrical behavior of morphologically identified neurons and the identification and localization of BK receptors in the submucosal plexus of guinea pig small intestine. Exposure to BK depolarized the membrane potential and elevated excitability in submucosal neurons with AH-type electrophysiological behavior and Dogiel II multipolar morphology and in neurons with S-type electrophysiological behavior and uniaxonal morphology. BK-evoked depolarizing responses were associated with increased neuronal input resistance in AH-type neurons and decreased input resistance in S-type neurons. The selective B(2) BK receptor antagonists HOE-140 (icatabant acetate) and WIN64338 [(S)-4[2-bis(cyclohexylamino)methyleneamino]-3-(2-napthalenyl)-1-oxopropylamino]benzyl tributyl phosphonium chloride hydrochloride], but not the selective B(1) receptor antagonists des-arg(10)-HOE-140 and des-arg(9)-leu(8)-BK, suppressed the BK-evoked responses. The selective B(2) receptor agonist Kallidin, but not the selective B(1) receptor agonist des-arg(9)-BK mimicked the excitatory action of BK. Western blot analysis and reverse transcription-polymerase chain reaction confirmed the expression of B(2) receptor protein and mRNA. Binding studies with a fluorescently labeled BK(2) antagonist found expression of B(2) receptors on a majority of the ganglion cells. B(2) receptors occupied 82% of the neurons that expressed immunoreactivity for neuropeptide Y, 75% of the neurons that expressed vasoactive intestinal peptide, 84% of the neurons that expressed substance P, 71% of the neurons that expressed choline acetyltransferase, and all neurons that expressed calbindin immunoreactivity. The results suggest that the B(2) receptor mediates the excitatory action of BK on submucosal plexus neurons. Pathophysiological significance of the excitatory actions on secretomotor neurons might be stimulated mucosal secretion and the secretory diarrhea associated with intestinal inflammatory states.

Neurochemical coding of myenteric neurones in the human gastric fundus

The major functions of the stomach are under the control of the enteric nervous system (ENS), but the neuronal circuits involved in this control are largely unknown in humans. Enteric neurones can be characterized by their neuromediator or marker content, i.e. by neurochemical coding. The purpose of this study was to characterize the presence and co-localization of neurotransmitters in myenteric neurones of the human gastric fundus. Choline acetyltransferase (ChAT), neurone-specific enolase (NSE), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), substance P (SP) were detected by immunohistochemical methods in whole mounts of gastric fundus myenteric plexus (seven patients). Antibodies against ChAT and NOS labelled the majority of myenteric neurones identified by NSE (57.2 +/- 5.6% and 40.8 +/- 4.5%, respectively; mean +/- SD). The proportions of VIP- and SP-immunoreactive neurones were significantly smaller, constituting 19.6 +/- 6.9% and 16.0 +/- 3.7%, respectively. Co-localization studies revealed five major populations representing over 75% of the myenteric neurones: ChAT/-, 30.1 +/- 6.1%; NOS/-, 24.2 +/- 4.4%; ChAT/SP/-, 8.3 +/- 3.1%; NOS/VIP/-, 7.2 +/- 6.0%; ChAT/VIP/-, 4.9 +/- 2.6. Some similarities are apparent in the neurochemical coding of myenteric neurones in the stomach and intestine of humans, and between the stomach of humans and animals, but striking differences exist. The precise functional role of the neurochemically identified classes of neurones remains to be determined.

Reticular groove and reticulum are innervated by myenteric neurons with different neurochemical codes

The reticulum and the reticular groove are functional distinct compartments within the ovine forestomach. While the reticulum takes part in various motor functions, such as mixing, retaining, and rejecting the forestomach ingesta, the reticular groove serves mainly as a bypass between the esophagus and the abomasum. To accomplish these different tasks, the compartments develop specific motility patterns that are controlled by intrinsic neural circuits. In this study the intrinsic innervation by myenteric neurons was analyzed by quadruple immunohistochemistry against cholineacetyl transferase (ChAT), nitric oxide synthase (NOS), substance P (SP), and vasoactive intestinal peptide (VIP). Four neurochemically different subpopulations of myenteric neurons were found in the reticulum and the floor of the reticular groove: ChAT/-, ChAT/SP, NOS/-, and NOS/VIP. The neuronal proportions were calculated relative to all myenteric neurons. Neurons of the reticulum were mostly immunoreactive for ChAT (89% +/- 3%), whereas neurons adjacent to the reticular groove predominantly expressed a nitrergic phenotype (62% +/- 4%). ChAT-positive neurons were also immunoreactive for SP (ChAT/SP: 64% +/- 3% reticulum; 25% +/- 1% reticular groove) or were purely cholinergic (ChAT/-: 25% +/- 4% reticulum; 13% +/- 3% reticular groove). NOS-positive neurons colocalized VIP (NOS/VIP: 10% +/- 3% reticulum; 46% +/- 1% reticular groove) or none of the other neurotransmitters (NOS/-: 1% +/- 1% reticulum; 17% +/- 3% reticular groove). Analysis of the soma sizes revealed that in both compartments the nitrergic neurons were significantly larger than the cholinergic neurons. It is suggested that the specific neurochemical code in combination with a specific morphology leads to a precise regulation of the specialized tasks of the reticulum and reticular groove by subpopulations of myenteric neurons.

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.

Aging of the myenteric plexus: neuronal loss is specific to cholinergic neurons

Neuron loss occurs in the myenteric plexus of the aged rat. The myenteric plexus is composed of two mutually exclusive neuronal subpopulations expressing, respectively, nitrergic and cholinergic phenotypes. The goal of the present study, therefore, was to determine if neuron loss is specific to one phenotype, or occurs in both. Ad libitum fed virgin male Fischer 344 rats of 3 and 24 months of age were used in each of two neuronal staining protocols (n=10/age/neuron stain). The stomach, duodenum, jejunum, ileum, colon, and rectum were prepared as whole mounts and processed with either NADPHd or Cuprolinic Blue to stain, respectively, the nitrergic subpopulation or the entire population of myenteric neurons. Neuron numbers and sizes were determined for each preparation. Neuron counts from 24-month-old rats were corrected for changes in tissue area resulting from growth. There was no age-related loss of NADPHd-positive neurons for any of the regions sampled, whereas significant losses of Cuprolinic Blue-labeled neurons occurred in the small and large intestines of 24-month-old rats. At the two ages, the average neuron sizes were similar in the stomach and small intestine for both stains, but neurons in the large intestine were significantly larger at 24 months. In addition, numerous swollen NADPHd-positive axons were found in the large intestine at 24 months. These findings support the hypothesis that age-related cell loss in the small and large intestines occurs exclusively in the cholinergic subpopulation. It appears, however, from the somatic hypertrophy and the presence of swollen axons that the nitrergic neurons are not completely spared from the effects of age.

Expression of Notch1 and Jagged2 in the enteric nervous system

The Notch signaling pathway is a vitally important pathway in regulating brain development. To explore the involvement of the Notch pathway in neuronal cells of adult rat gut, we investigated the expression of Notch1 and Jagged2 by in situ hybridization (ISH) and immunohistochemistry (IHC). In the enteric nervous system, Notch1 and Jagged2 were expressed in ganglia of the submucosal and myenteric plexus. Notch1 was preferentially expressed in cholinergic neurons lacking calretinin or nitric oxide synthase (NOS), whereas Jagged2 was present in most neuron subtypes. We propose that Notch1 and Jagged2 have a continuing role in the maintenance and function of neuronal cells in the adult enteric nervous system.

Morphologies and projections of defined classes of neurons in the submucosa of the guinea-pig small intestine

Four types of neurons have previously been identified by neurochemical markers in the submucosal ganglia of the guinea-pig small intestine, and functional roles have been ascribed to each type. However, morphological differences among the classes have not been determined, and there is only partial information about their projections within the submucosa. In the present work, we used intracellular microelectrodes to fill neurons of each type with biocytin, which was then converted to a permanent dye, so that the shapes of the neurons could be determined and their projections within the submucosa could be followed. Cell bodies of noncholinergic secretomotor/ vasodilator neurons had Dogiel type I morphology. These neurons, which are vasoactive intestinal peptide immunoreactive, had single axons that ran through many ganglia without providing terminals around other neurons. Cholinergic secretomotor neurons with neuropeptide Y immunoreactivity had Stach type IV morphology, and cholinergic secretomotor/vasodilator neurons had stellate cell bodies. The axons of these two types ran short distances in the plexus and did not innervate other submucosal neurons. Neurons of the fourth type, intrinsic primary afferent neurons, had cell bodies with Dogiel type II morphology and their processes supplied networks of varicose processes around other nerve cells. It is concluded that each functionally defined type of submucosal neuron has a characteristic morphology and that intrinsic primary afferent neurons synapse with secretomotor neurons to form monosynaptic secretomotor reflex circuits.

Actions of bradykinin on electrical and synaptic behavior of neurones in the myenteric plexus of guinea-pig small intestine

1. Electrophysiologic methods were used to study actions of bradykinin (BK) in neurones of the myenteric plexus of guinea-pig small intestine in vitro. Exposure to BK depolarized the membrane potential and elevated excitability in AH- and S-type neurones. Neuronal input resistance associated with the depolarizing responses was either decreased or unchanged in S-type and increased in AH-type neurones. 2. The selective B(2) BK receptor antagonist HOE-140, but not the selective B(1) receptor antagonist des-arg(10)-HOE-140, suppressed the BK-evoked responses. RT-PCR confirmed the expression of B(2) receptor mRNA, but not B(1) receptor mRNA. 3. Binding of fluorescently- labeled HOE-140 (HOE741) was localized to ganglion cells in whole-mount preparations. BK B(2) receptors were coexpressed with immunoreactivity for calbindin or nitric oxide synthase. 4. Exposure to BK suppressed the amplitude of both fast and slow excitatory postsynaptic potentials. Depolarizing responses evoked by application of serotonin or substance P and nicotinic responses to acetylcholine were not reduced by BK. This suggested that BK action on neurotransmission was presynaptic suppression of neurotransmitter release. Presence of HOE-140 in the bathing solution suppressed or abolished the presynaptic inhibitory action of BK. 5. The cyclooxygenase inhibitor, piroxicam, suppressed both the direct excitatory action of BK and its presynaptic inhibitory action. Application of prostaglandin E(2), D(2), F(2alpha) or I(2) mimicked the BK-evoked responses. 6. The results suggest that BK acts at B(2) BK receptors on myenteric neurones to stimulate the formation of prostaglandins. Once formed and released, the prostaglandins act to elevate the excitability of ganglion cells in the myenteric plexus and to suppress the synaptic release of neurotransmitters.

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.

Correlation of electrophysiology, shape and synaptic properties of myenteric AH neurons of the guinea pig distal colon

Well-defined correlations between morphology, electrophysiological properties and the types of synaptic inputs received are established for myenteric neurons in the guinea pig ileum. However, in the distal colon, the correlations between AH electrophysiological properties, presence of fast excitatory post-synaptic potentials (EPSPs) and neuronal shape have been inadequately resolved and it is unknown whether any colon neurons receive synaptic inputs that generate sustained excitation. In this work, we have used intracellular recording, dye filling via the recording electrode, and immunohistochemistry to classify distal colon neurons. Neurons (24 of 168) had Dogiel type II morphology and 42% of these were dendritic type II neurons, compared to about 10% in the ileum. All Dogiel type II neurons had AH electrophysiological properties, including a prolonged post-spike after-hyperpolarization (AHP). None of these received fast excitatory post-synaptic potentials, 11 of 22 tested exhibited sustained slow post-synaptic excitation (SSPE) in response to 1 Hz pre-synaptic stimulation and 13 of 15 tested were immunoreactive for calbindin. Neurons (127) had Dogiel type I, filamentous or other uniaxonal cell shape and S type electrophysiology. Neurons of this group had fast excitatory post-synaptic responses to stimulation of synaptic inputs, but did not exhibit a prolonged post-spike after-hyperpolarization or sustained slow post-synaptic excitation. Another group of neurons (17) had both AH electrophysiological characteristics and fast excitatory post-synaptic potentials. These neurons had Dogiel type I, filamentous or other uniaxonal shapes, but none had Dogiel type II morphology and none showed sustained slow post-synaptic excitation. It is concluded that Dogiel type II neurons are all AH neurons and are probably intrinsic sensory neurons that could be involved in long-term changes in excitability in the colon. All other neurons are monoaxonal; these are motor neurons and interneurons, and most are S neurons, electrophysiologically. A small number of monoaxonal neurons display AH electrophysiology and also receive fast excitatory synaptic inputs. These include motor and interneurons, but not sensory neurons.

Correlation of electrophysiological and morphological characteristics of enteric neurons in the mouse colon

We report on the first correlative study of the electrophysiological properties, shapes, and projections of enteric neurons in the mouse. Neurons in the myenteric plexus of the mouse colon were impaled with microelectrodes containing biocytin, their passive and active electrophysiological properties determined, and their responses to activation of synaptic inputs investigated. Biocytin, injected into the neurons from which recordings were made, was converted to an optically dense product and used to determine the shapes of neurons. By electrophysiological properties, almost all neurons belonged to one of two classes, AH neurons or S neurons. AH neurons had a biphasic repolarization of the action potential, and slow afterhyperpolarizing potentials usually followed the action potentials. S neurons had monophasic repolarizations, no slow afterhyperpolarization, and fast excitatory postsynaptic potentials in response to fibre tract stimulation. By shape, neurons were divided into Dogiel type II (28/136 neurons) and uniaxonal neurons. Dogiel type II neurons had large, smooth-surfaced cell bodies and several long processes that supplied branches within myenteric ganglia. All Dogiel type II neurons had AH electrophysiology; conversely, most AH neurons had Dogiel type II morphology. The majority of uniaxonal neurons had lamellar dendrites, i.e., Dogiel type I morphology. They projected to the circular muscle (circular muscle motor neurons), to the longitudinal muscle (longitudinal muscle motor neurons), and to other myenteric ganglia (interneurons) and in some cases could not be traced to target cells. All S neurons were uniaxonal. A small proportion of uniaxonal neurons (3/70) had AH electrophysiology. Fast excitatory synaptic potentials were only recorded from uniaxonal neurons and were in most cases blocked by nicotinic receptor antagonists. A small component of fast excitatory transmission in some neurons was antagonized by the purine receptor antagonist PPADS. Slow excitatory postsynaptic potentials were observed in both AH and S neurons. Slow inhibitory postsynaptic potentials were recorded from S neurons. We conclude that the major classes of neurons are Dogiel type II neurons with AH electrophysiological properties and Dogiel type I neurons with S electrophysiological properties. The S/Dogiel type I neurons include circular muscle motor neurons, longitudinal muscle motor neurons, and interneurons.

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.

Requirement of signalling by receptor tyrosine kinase RET for the directed migration of enteric nervous system progenitor cells during mammalian embryogenesis

The majority of neurones and glia of the enteric nervous system (ENS) are derived from the vagal neural crest. Shortly after emigration from the neural tube, ENS progenitors invade the anterior foregut and, migrating in a rostrocaudal direction, colonise in an orderly fashion the rest of the foregut, the midgut and the hindgut. We provide evidence that activation of the receptor tyrosine kinase RET by glial cell line-derived neurotrophic factor (GDNF) is required for the directional migration of ENS progenitors towards and within the gut wall. We find that neural crest-derived cells present within foetal small intestine explants migrate towards an exogenous source of GDNF in a RET-dependent fashion. Consistent with an in vivo role of GDNF in the migration of ENS progenitors, we demonstrate that Gdnf is expressed at high levels in the gut of mouse embryos in a spatially and temporally regulated manner. Thus, during invasion of the foregut by vagal-derived neural crest cells, expression of Gdnf was restricted to the mesenchyme of the stomach, ahead of the invading NC cells. Twenty-four hours later and as the ENS progenitors were colonising the midgut,Gdnf expression was upregulated in a more posterior region —the caecum anlage. In further support of a role of endogenous GDNF in enteric neural crest cell migration, we find that in explant cultures GDNF produced by caecum is sufficient to attract NC cells residing in more anterior gut segments. In addition, two independently generated loss-of-function alleles of murine Ret, Ret.k— and miRet51, result in characteristic defects of neural crest cell migration within the developing gut. Finally, we identify phosphatidylinositol-3 kinase and the mitogen-activated protein kinase signalling pathways as playing crucial roles in the migratory response of enteric neural crest cells to GDNF.

Correlated morphological and chemical phenotyping in myenteric type V neurons of porcine ileum

The study was aimed at the immunohistochemical characterization of myenteric Stach type V neurons of the pig ileum that were not included in the widely used Dogiel classification. So far, this conspicuous population has been defined morphologically on the basis of silver-impregnated specimens only. By using neurofilament immunohistochemistry, type V neurons that occur singly or in aggregates could be identified unequivocally and could be distinguished from other smoothly contoured myenteric neurons, i.e., type II and type IV. Double-labeling immunohistochemistry revealed a number of potentially neuroactive substances or their synthesizing enzymes to be present in type V neurons. Choline acetyltransferase immunoreactivity (-ir) was found in all type V neurons, whereas neuronal nitric oxide synthase was detected in none. Leu-enkephalin-ir was found within 92.3%, somatostatin (SOM)-ir within 91.1%, calcitonin gene-related peptide (CGRP)-ir within 80.6% and met-enkephalin-ir within 74.7% of type V neurons. Triple-labeling immunohistochemistry was applied to address the question of a specific chemical coding for myenteric type V neurons. In contrast to other combinations of neuroactive substances/enzymes that were found in both type V and other, nontype V neurons, SOM/CGRP-ir was the only combination observed exclusively within type V neurons. Both substances were colocalized in 79.3% of type V neurons. This colocalization discriminates four-fifths of the type V neurons chemically from both type II neurons (CGRP positive, SOM negative) and type IV neurons (CGRP negative, SOM positive), which both share, at first glance, a similar morphology with type V neurons. These results further support the concept of a close correlation between morphologically defined neuronal type and chemical coding and, it is likely, also function in the enteric nervous system of larger mammals.

Serine proteases excite myenteric neurons through protease-activated receptors in guinea pig small intestine

BACKGROUND & AIMS

Serine proteases are postulated to influence gastrointestinal function by stimulating protease-activated receptors (PARs). This study identified the effects on myenteric neurons of activating PARs and investigated underlying mechanisms of action.

METHODS

Intracellular electrophysiologic methods were used to study the effects of proteases on electrical and synaptic behavior of morphologically identified neurons in the guinea pig enteric nervous system. Fluorescent immunohistochemistry was used to study the chemical coding of neurons that responded to PARs stimulation.

RESULTS

Application of thrombin, trypsin, or mast cell tryptase evoked slowly activating excitatory responses reminiscent of slow synaptic excitation in enteric neurons. Synthetic activating peptides for PAR-1, -2, and -4 receptors mimicked the actions of the proteases. The depolarizing responses evoked by PARs were insensitive to cyclooxygenase inhibitors and were suppressed by agents that inhibit phospholipase C (PLC) or block intraneuronal receptors for inositol triphosphate. A majority of PAR-sensitive uniaxonal neurons expressed immunoreactivity for nitric oxide synthase. Most of the PAR-sensitive AH Dogiel morphologic type II neurons were immunoreactive for calbindin.

CONCLUSIONS

Excitatory responses to the serine proteases are mediated by PAR-1, -2, and -4 receptors. The mechanism of signal transduction involves stimulation of PLC and intraneuronal calcium mobilization and is independent of prostanoid formation.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.

Regulation of K+ channels underlying the slow afterhyperpolarization in enteric afterhyperpolarization-generating myenteric neurons: role of calcium and phosphorylation

1. Myenteric afterhyperpolarization-generating myenteric (AH) neurons serve as intrinsic primary afferent neurons of the enteric nervous system and generate prolonged or slow afterhyperpolarizing potentials (slow AHP). The slow AHP is generated by an increase in a Ca2+-activated K+ conductance (gK-Ca) and is inhibited by enteric neurotransmitters leading to increased excitability. 2. Using cell-attached patch-clamp recordings from AH neurons, we have shown that K+ channels with an intermediate unitary conductance (IK channels) open following action potential firing. 3. In excised patches from AH neurons, we have identified an IK-like channel that can be activated by submicromolar levels of cytoplasmic Ca2+ and is not voltage dependent. 4. Application of the catalytic subunit of cAMP-dependent protein kinase to the cytoplasmic surface of inside-out patches inhibits the opening of IK-like channels previously activated by Ca2+. 5. The IK-like channels are resistant to external tetraethylammonium (5 mmol/L) and apamin (0.3-1 micro mol/L), but are inhibited by clotrimazole (10 micro mol/L). 6. Our present data support the idea that an increase in the open probability of IK-like channels in AH neurons following an increase in cytoplasmic [Ca2+] is responsible for the slow AHP and their opening is modulated by kinases.

Aging and neural control of the GI tract. I. Age-related changes in the enteric nervous system

As we enter the 21st century, the segment of the population that is the most rapidly expanding is that comprised of individuals 85 yr of age and older. Dysfunctions of the gastrointestinal (GI) system, including dysphagia, constipation, diarrhea, and irritable bowel syndrome are more common complaints of the elderly, yet our knowledge of the aging GI tract is incomplete. Compared with the rapid advances in the neurobiology of aging in the central nervous system, the understanding of age-related changes in the enteric nervous system (ENS) is poor. In this brief review, I recap experiments that reveal neurodegenerative changes and their functional correlates in the ENS of mice, rats, and guinea pigs. Clinical literature seems indicative of similar structural and functional age-related changes in the human ENS. Current studies that address the mechanisms underlying age-related changes in the ENS are introduced. The future directions for this field include physiological and pharmacological studies, especially at cellular and molecular levels. Research in the aging ENS is poised to make major advances, and this new knowledge will be useful for clinicians seeking to better understand and treat GI dysfunction in the elderly.

The projections of early enteric neurons are influenced by the direction of neural crest cell migration

The enteric nervous system arises from the neural crest. In embryonic mice, vagal neural crest cells enter the developing foregut at approximately embryonic day 9.5 (E9.5) and then migrate rostrocaudally to colonize the entire gastrointestinal tract by E14.5. This study showed that a subpopulation of vagal crest-derived cells, very close to the migratory wavefront, starts to differentiate into neurons early, as shown by the expression of neuron-specific proteins and the absence of Sox10. Many of the early differentiating neurons transiently exhibited tyrosine hydroxylase (TH) immunoreactivity. The TH cells were demonstrated to be the progenitors of nitric oxide synthase (NOS) neurons. Immunohistochemistry, lesions, and DiI tracing were used to examine the projections of developing enteric neurons. The axons of first neurons in the gut (the TH-NOS neurons) projected in the same direction (caudally), and traversed the same pathways through the mesenchyme, as the migrating, undifferentiated, vagal crest-derived cells. To examine if the direction of migration and direction of axon projection are linked, coculture experiments were set up in which vagal crest-derived cells migrated either rostrocaudally (as they do in vivo), or caudorostrally (which they do not normally do), to colonize explants of embryonic aneural hindgut. The direction in which neurons projected was correlated with the direction of cell migration, but migration direction appears to be not the only mechanism influencing axon projection. Peristaltic reflexes involve both orally (rostrally) projecting neurons and anally (caudally) projecting neurons. Because few rostrally projecting neurons could be detected before birth, the full circuitry for peristaltic reflexes appears to develop after birth.

Responses of myenteric S neurones to low frequency stimulation of their synaptic inputs

Previous experiments have shown that prolonged low frequency stimulation of presynaptic inputs causes excitation of AH neurones that considerably outlasts the period of stimulation in the guinea-pig small intestine. The present experiments compare the responses of S neurones (which are motor neurones and interneurones) with responses of AH neurones (intrinsic primary afferent neurones) to low frequency stimulation of synaptic inputs. Neurones in the myenteric plexus of isolated segments of guinea-pig small intestine were recorded from with intracellular microelectrodes. During their impalement, the neurones were filled with a marker dye and they were later processed to reveal their shapes and immunohistochemical properties. One group of neurones, inhibitory motor neurones to the circular muscle, was depolarised by stimulation of synaptic inputs at 1 Hz for 100 s to 4 min. With 4-min trains of stimuli, peak depolarisation was 21+/-2 mV (mean+/-S.E.M.), which was reached at about 110 s. Depolarisation was accompanied by increased excitability; before stimulation, a test intracellular pulse (500 ms) triggered 3 action potentials, at the peak of excitability this reached 16 action potentials. Depolarisation began to decline immediately at the end of stimulation. This contrasts with responses of AH neurones, in which depolarisation persisted after the end of the stimulus (peak depolarisation at 300 s). The excitation and depolarisation of inhibitory motor neurones was blocked by the neurokinin 1 tachykinin receptor antagonist, SR140333 (100 nM), but excitation of AH neurones was not affected. Small or no responses to 1 Hz stimulation were recorded from descending filamentous interneurones, longitudinal muscle motor neurones and excitatory circular muscle motor neurones. In conclusion, this study indicates that sustained slow postsynaptic excitation only occurs in AH neurones, and that one type of S neurones, inhibitory motor neurones to the circular muscle, responds substantially, but not beyond the period of stimulation, to activation of synaptic inputs at 1 Hz. This slow excitatory postsynaptic potential evoked by low frequency stimulation is mediated by tachykinins.

Enteric nervous system: development and developmental disturbances--part 1

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. In the first part, we describe the critical features of two principal abnormalities of ENS development: Hirschsprung's disease (HSCR) and intestinal neuronal dysplasia type B (INDB) in humans, and the similar abnormalities in animals. These represent the extremes of the diagnostic spectrum: HSCR has agreed and unequivocal diagnostic criteria, whereas the diagnosis and even existence of INDB as a clinical entity is highly controversial. The difficulties in diagnosis and treatment of both these conditions are discussed. We then review the genes now known which, when mutated or deleted, may cause defects of ENS development. Many of these genetic abnormalities in animal models give a phenotype similar or identical to HSCR, and were discovered by studies of humans and of mouse mutants with similar defects. The most important of these genes are those coding for molecules in the GDNF intercellular signaling system, and those coding for molecules in the ET-3 signaling system. However, a range of other genes for different signaling systems and for transcription factors also disturb ENS formation when they are deleted or mutated. In addition, a large proportion of HSCR cases have not been ascribed to the currently known genes, suggesting that additional genes for ENS development await discovery.

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.