Characterization of myenteric interneurons with somatostatin immunoreactivity in the guinea-pig small intestine

Identifying Types of Neurons in the Human Colonic Enteric Nervous System

Distinguishing and characterising the different classes of neurons that make up a neural circuit has been a long-term goal for many neuroscientists. The enteric nervous system is a large but moderately simple part of the nervous system. Enteric neurons in laboratory animals have been extensively characterised morphologically, electrophysiologically, by projections and immunohistochemically. However, studies of human enteric nervous system are less advanced despite the potential availability of tissue from elective surgery (with appropriate ethics permits). Recent studies using single cell sequencing have confirmed and extended the classification of enteric neurons in mice and human, but it is not clear whether an encompassing classification has been achieved. We present preliminary data on a means to distinguish classes of myenteric neurons in specimens of human colon combining immunohistochemical, morphological, projection and size data on single cells. A method to apply multiple layers of antisera to specimens was developed, allowing up to 12 markers to be characterised in individual neurons. Applied to multi-axonal Dogiel type II neurons, this approach demonstrated that they constitute fewer than 5% of myenteric neurons, are nearly all immunoreactive for choline acetyltransferase and tachykinins. Many express the calcium-binding proteins calbindin and calretinin and they are larger than average myenteric cells. This methodology provides a complementary approach to single-cell mRNA profiling to provide a comprehensive account of the types of myenteric neurons in the human colon.

The extraordinary partnership of Geoff Burnstock and Mollie Holman

Here, we recognise some of the extraordinary accomplishments of the partnership between Geoff Burnstock and Mollie Holman, and the everlasting impact they both made in autonomic neuroscience in Australia. Much of strength today in autonomic neuroscience can be traced back to a time when Geoff and Mollie commenced their seminal studies on autonomic neuroscience, initially at Oxford, then at The University of Melbourne in the mid 1960's. Mollie and Geoff published their first paper together, at Oxford, with their then mentor, and doyenne of smooth muscle, Professor Edith Bülbring. They did not always agree on the interpretation of their own scientific findings. Geoff was convinced early on that Adenosine triphosphate (ATP), or a related purine, was an excitatory neurotransmitter at peripheral sympathetic neuroeffector junctions. Mollie was reticent for decades. However, she began to take the notion seriously that ATP maybe a neurotransmitter, when receptors for purines were identified in the 1990's. What the partnership between Mollie and Geoff taught us in Australia was to not fear respectful criticism, but rather to be receptive to and embrace objective, collegial and constructive scientific peer-review. One of the many great legacies of Geoff and Mollie was the large number of researchers, who were fortunate disciples of their supervision, and who have now themselves gone on to make significant discoveries in autonomic and visceral neuroscience. This review summarizes some of their major legacies and represents a very personal historical perspective of the two authors, pupils respectively of Mollie and Geoff.

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

Activity in varicosities within the myenteric plexus between and during the colonic migrating motor complex in the isolated murine large intestine

BACKGROUND

  Neuronal communication within the myenteric plexus occurs when action potentials along nerve fibers produce Ca(2+) transients in varicosities leading to exocytosis of vesicles and neurotransmitters release. We used Ca(2+) transients in varicosities to monitor action potential activity in myenteric nerve pathways both between and during the colonic migrating motor complex (CMMC) in the isolated murine colon.

METHODS

  Strips of longitudinal muscle were removed to reveal the myenteric ganglia, which were then loaded with Fluo-4.

KEY RESULTS

  Many varicosities, including synaptotagmin 1 labeled varicosities, exhibited ongoing Ca(2+) transients (duration of unitary Ca(2+) transient 3.9 s). Between CMMCs, varicosities fired at a frequency of 0.6 Hz, which correlated with spontaneous inhibitory junction potentials in the circular muscle, suggesting they were mainly in inhibitory nerve pathways. During a CMMC other previously quiescent varicosities fired at 1.3 Hz (max. 2.0 Hz) for the duration (24 s) of the CMMC, suggesting they were on excitatory nerve pathways. Activity in varicosities was correlated with Ca(2+) transient responses in a number of neurons. Some varicosities appeared to release an inhibitory neurotransmitter that reduced activity in nNOS-positive neurons. Varicosities along the same nerve fiber exhibited identical patterns of activity that allowed nerve fibers to be traced throughout the myenteric plexus and internodal strands. Activity in varicosities was reduced by hexamethonium (100 μmol L(-1) ), and blocked by ω-conotoxin GVIA (200 nM) and tetrodotoxin (1 μmol L(-1) ; TTX).

CONCLUSIONS & INFERENCES

  Ca(2+) imaging of varicosities allows for a determination of activity in neural pathways within the enteric nervous system.

Synaptic transmission from the submucosal plexus to the myenteric plexus in Guinea-pig ileum

Stimulation of the myenteric plexus results in activation of submucosal neurons and dilation of arterioles, one way that motility and secretion can be coupled together. The present study aimed to examine the converse, whether myenteric neurons receive synaptic input from the submucosal plexus (SMP). Intracellular recordings were made from guinea-pig ileal myenteric neurons while the SMP was electrically stimulated. Of the 29 neurons studied (13 S and 16 AH neurons), stimulation of the SMP evoked a synaptic potential in only seven cells, or 24% of neurons. When the SMP was situated oral to the myenteric plexus, 4 of 13 (31%) myenteric neurons had synaptic input. When it was situated circumferential, 2 of 8 (25%) had input, and when the SMP was situated anal 1 of 8 (13%) had input. Overall, 5 of the 13 (38%) S neurons responded with fast excitatory post-synaptic potentials (EPSPs), one of which also showed a slow EPSP, while 2 of the 16 (13%) AH neurons responded with a slow EPSP. This study indicates that the synaptic input from the SMP to myenteric neurons is relatively sparse. Whether this input is less important than the myenteric to submucosal input or simply represents a more selective form of control is unknown.

Myenteric neurons of the ileum that express somatostatin are a target of prion neuroinvasion in an alimentary model of sheep scrapie

Neuroinvasion of the enteric nervous system by prions is an important step in dissemination to the brain, yet very little is known about the basic process of enteric neuroinvasion. Using an alimentary model of neonatal disease transmission, neuroinvasion by scrapie prions in the ileum of lambs was detected by immunohistochemical staining for the disease-associated form of the prion protein, PrPSc. Odds ratios (OR) were determined for the frequency of PrPSc staining within enteric somata categorized by plexus location (myenteric, submucosal) and neurochemical staining (PGP 9.5, neural nitric oxide synthase, somatostatin, substance P, and vasoactive intestinal polypeptide). PrPSc was observed in 4.48 +/- 4.26% of myenteric neurons and 2.57 +/- 1.82% of submucosal neurons in five lambs aged 208-226 days but not in a lamb aged 138 days. The relative frequency of PrPSc within enteric somata was interdependent on plexus location and neurochemical type. Interestingly, PrPSc was observed more frequently within myenteric neurons than in submucosal neurons (PGP 9.5; OR = 1.72, 95% confidence interval = 1.21-2.44), and was observed within the myenteric plexus approximately 4x (2.16-6.94) more frequently in somatostatin neurons than in the general neural population stained by PGP 9.5. Nerve fibers stained for somatostatin were present in the mucosa and near PrPSc staining within Peyer's patches. The results suggest that somatostatin-expressing enteric neurons, with fiber projections near Peyer's patches, but with somata present in greatest proportion within the myenteric plexus, are an early target for neuroinvasion by scrapie prions and could serve an important role in neural dissemination.

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.

Phenotypic changes of morphologically identified guinea-pig myenteric neurons following intestinal inflammation

We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca(2+)-dependent K(+) channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I(h)) antagonist Cs(+). It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.

Chemical coding of submucosal type V neurons in porcine ileum

In this study, we attempted to determine the proportion of type V neurons relative to the putative whole neuron population in the two submucosal plexuses of pigs identified by their neurofilament immunoreactivity. The total neuron number was estimated in cuprolinic blue (CB)/anti-Hu protein (HU) costained wholemounts as the sum of the number of CB+/HU+, CB+/HU- and CB-/HU+ neurons. In the external submucosal plexus (ESP), HU labelled 98.6% and CB 97.3% of neurons. In the internal submucosal plexus, HU labelled 98.3%, whereas CB only marked 92.5% of neurons. Furthermore, we investigated the chemical coding of submucosal type V neurons and searched for submucosal, non-type V neurons displaying the same chemical coding as the myenteric type V neurons described earlier, i.e. the colocalization of calcitonin gene-related peptide (CGRP) and somatostatin (SOM). In order to facilitate immunohistochemical detection of neuroactive peptides, ileal segments were pretreated with colchicine prior to fixation. Type V neurons in the ESP occurred either as single cells displaying one or few prominent dendrite(s) or within aggregates displaying a dendritic tangle. In this plexus, type V neurons amounted to between 0.9 and 1.6% of all CB-stained neurons. ESP type V neurons displayed immunoreactivities for choline acetyl transferase (95.8%) and leucine-enkephalin (73.9%). All type V neurons were negative for neuronal nitric oxide synthase. Fifty-eight percent of ESP CGRP/SOM co-immunoreactive neurons displayed type V morphology, whereas 42% were non-type V neurons. Thus, the chemical coding of ESP type V neurons is in principal similar to that of the myenteric type V neurons described earlier. In the internal submucosal plexus, we found no type V neurons. In this plexus, 0.2% of all neurons counterstained with HU displayed CGRP/SOM coreactivity. As had been observed earlier concerning the myenteric type V neurons, ESP type V neurons were also closely apposed by conspicuous accumulations of boutons reactive for the same markers as the neurons themselves. Although we cannot exclude that axons of CGRP/SOM-reactive enteric, non-type V or extrinsic neurons end synaptically on type V neurons, we suggest that the main synaptic input to type V neurons originates from other type V neurons. This presents an argument for an interneuronal role of type V neurons.

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

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

Expression of the sodium channel Nav1.2 in chemically identified myenteric neurons in the guinea pig

Our purpose was to identify Na(v)1.2-expressing myenteric neurons of the small and large intestine of the guinea pig by using antibodies directed against Na(v)1.2 and selected neurochemical markers. Na(v)1.2-like immunoreactivity (-li) co-localized with immunoreactivity for choline acetyltransferase in all regions, representing 45%-67% of Na(v)1.2-positive neurons. Na(v)1.2-li co-localized with immunoreactivity for the neural form of nitric oxide synthase more frequently in the colon (20% of neurons exhibiting Na(v)1.2-li) than in the ileum (8%). Co-localization of Na(v)1.2-li with immunoreactivity for a form of neurofilament (NF145) was infrequently observed in the ileum and colon. Enkephalin-immunoreactive cell bodies co-localized with Na(v)1.2-li in all regions. Few myenteric cell bodies immunoreactive for neuropeptide Y were observed in the ileum, but all co-localized with Na(v)1.2-li. This and our previous data suggest that Na(v)1.2 is widely expressed within the guinea pig enteric nervous system, including the three main classes of myenteric neurons (sensory, motor, and interneurons), and is involved in both excitatory and inhibitory pathways. Notable exceptions include the excitatory motor neurons to the longitudinal smooth muscle, the ascending interneurons of the ileum, and the myenteric neurons immunoreactive for NF145, few of which are immunoreactive for Na(v)1.2.

Trypanosoma cruzi: preliminary investigation of NADH-positive and somatostatin-immunoreactive neurons in the myenteric plexus of the mouse colon during the infection

In this paper, the distribution of NADH-positive and somatostatin (SOM) immunoreactive neurons in the myenteric plexus of the colon of mice infected with Trypanosoma cruzi was studied. Ten young, male, BALB/c mice were inoculated with the Y strain of T. cruzi, 60 days previously (chronic phase of the infection). Another 10 mice were uninfected controls. Distal and proximal colonic neurons from five chronically infected mice and their controls were stained using the NADH-diaphorase method. Quantitative results showed a significant decrease of 39% in the number of neurons in the proximal colon of infected mice and 58% in the distal colon (p<0.05). SOM was localized in five animals from each group by light microscopy, using an indirect immunofluorescence technique. It was observed that there were far fewer nerve cells and fibres and less intensely stained neuron bodies and varicose SOM-positive nerve fibres in both, control and chronic infected mice. These findings could be related to the disturbances in intestinal motility observed in patients in the chronic phase of Chagas' disease.

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.

Immunohistochemical characterization of putative primary afferent (sensory) myenteric neurons in human small intestine

Pseudouni- or multiaxonal Dogiel type II neurons are the intrinsic primary afferent (sensory) neurons (IPANs) in the guinea pig small intestine. Our aim was to decipher the chemical code of human myenteric type II neurons and to establish their putative vertical projections, i.e., from the myenteric plexus to the submucosa/mucosa. Additionally, we tried to distinguish them chemically from uniaxonal, dendritic type V neurons displaying, at first glance, similar shapes, i.e., smoothly contoured cell bodies with several long processes. Wholemount preparations of the myenteric plexus were immunohistochemically double or triple stained for neurofilaments (NF) and one or two of the following peptides: calbindin, calretinin (CR), calcitonin gene-related peptide (CGRP), somatostatin (SOM) and substance P (SP). In each triple stained wholemount three counts were conducted: (1) NF-positive pseudouni- or multiaxonal (type II) neurons including their reactivities for the above peptides, (2) uniaxonal or NF-negative neurons displaying coreactivities for the above peptides and (3) NF-reactive type V neurons taking into account their reactivities for the above markers. Additionally, type II neurons, which had an axon leading into (disrupted) interconnecting strands towards the submucosa were counted and somal areas of types II and V neurons were measured. The majority of myenteric type II neurons displayed coreactivities for SOM/CR (89.6%), SOM/SP (86.6%) and SP/CR (81.6%), respectively. A minority of type II neurons was positive for CGRP or calbindin. A small population with type III morphology (uniaxonal, long and slender dendrites) displayed the same coreactivities as type II neurons. In contrast, not one single type V neuron was coreactive for SOM/CR, SOM/SP or SP/CR. Out of 627 type II neurons counted in six wholemounts, 84 type II neurons displayed an axon which could be followed into disrupted interconnecting strands indicating a vertical projection pattern. Somal areas of type II neurons were twice as big as those of type V neurons (904+/-210 versus 449+/-110 microm(2)). In conclusion, most human myenteric type II neurons contain SOM, SP and CR. We suggest they are the human IPANs. Type V neurons are both morphologically and chemically distinctly different from type II neurons and may represent descending interneurons. Further studies have to decipher the type-specific chemical code of type II neurons distinguishing them also from type III neurons.

Molecular and functional analysis of hyperpolarisation-activated nucleotide-gated (HCN) channels in the enteric nervous system

Hyperpolarisation-activated non-specific cation currents (Ih currents) are important for the regulation of cell excitability. These currents are carried by channels of the hyperpolarisation-activated nucleotide-gated (HCN) family, of which there are four known subtypes. In the enteric nervous system (ENS), the Ih current is prominent in AH neurons. We investigated the expression and localization of HCN isoforms in the ENS of mice, rats and guinea-pigs. HCN1, HCN2 and HCN4 were expressed in enteric neurons. Immunoreactivity for HCN1 was observed on neuronal cell membranes of Dogiel type II neurons in rat and mouse. HCN2 channel immunoreactivity occurred in the majority of enteric neurons in the guinea-pig, rat and mouse. Immunoreactivity for HCN4 protein was revealed on the cell membranes of many neurons, including Dogiel type II neurons, in the guinea-pig. HCN4 was expressed by glial cells in guinea-pig. There was no evidence of HCN3 channel protein in any species with either immunohistochemistry or Western analysis. RT-PCR (polymerase chain reaction) using mouse HCN primers revealed mRNA for all four channels in the longitudinal muscle plus myenteric plexus of mouse distal colon. Sequencing confirmed the identity of the mRNA. Quantitative PCR demonstrated that HCN2 was the most highly expressed HCN channel subtype in the myenteric plexus of mouse distal colon. HCN1 and HCN4 were expressed at lower levels. HCN3 subtype mRNA was 0.2% of HCN2. We used intracellular recording to identify neurons having Ih currents and intracellular dye filling to locate the neurons for the immunohistochemical determination of channel expression. AH neurons with Ih currents were HCN2 and HCN4 channel positive. There was no correlation between the magnitude of the Ih and intensity of channel immunoreactivity. Our results indicate that HCN1, 2 and 4 genes and protein are expressed in the ENS. AH/Dogiel type II neurons, which have a prominent Ih, express HCN2 and 4 in guinea-pig and HCN1 and 2 in mouse and rat.

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.

Analysis of purinergic and cholinergic fast synaptic transmission to identified myenteric neurons

Types and projections of neurons that received cholinergic, purinergic and other fast excitatory synaptic inputs in myenteric ganglia of the guinea-pig distal colon were identified using combined electrophysiological recording, application of selective antagonists, marker dye filling via the recording microelectrode, and immunohistochemical characterisation. Fast synaptic inputs were recorded from all major subtypes of uniaxonal neurons including Dogiel type I neurons, filamentous interneurons, circular muscle motor neurons and longitudinal muscle motor neurons. Fast excitatory postsynaptic potentials were completely blocked by the nicotinic receptor antagonists hexamethonium or mecamylamine in 62% of neurons tested and were partially inhibited in the remaining neurons. The P2 purine receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, reduced the amplitudes of fast excitatory postsynaptic potentials in 20% of myenteric neurons. The 5-hydroxytryptamine(3) receptor antagonist granisetron reduced the amplitude of fast excitatory postsynaptic potentials in only one of 15 neurons tested. In five of five neurons tested, the combination of a nicotinic antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, granisetron and 6-cyano-7-nitroquinoxaline-2,3-dione did not completely block the fast excitatory postsynaptic potentials. Immunohistochemical studies of the neurons that had been identified electrophysiologically and morphologically imply that P2X(2) receptors may mediate fast transmission in some neurons, and that other P2X receptor subtypes may also be involved in fast synaptic transmission to myenteric neurons of the guinea-pig distal colon. Neurons with nicotinic and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid-sensitive fast excitatory postsynaptic potentials were present in both ascending and descending pathways in the distal colon. Thus, neither cholinergic nor mixed cholinergic/purinergic synaptic responses are confined to a particular class of neuron. The results indicate that acetylcholine and ATP are the major fast excitatory neurotransmitters in guinea-pig distal colon myenteric ganglia.

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.

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.

Ca2+ involvement in the action potential generation of myenteric neurones in the rat oesophagus

Intracellular recordings were used to study the physiological behaviour of rat oesophageal myenteric neurones, which are embedded in striated muscle. Injection of depolarizing pulses evoked action potentials with a clear 'shoulder' in all neurones. This shoulder disappeared under low Ca2+/high Mg2+ conditions. Tetrodotoxin (TTX; 1 micromol L-1) did not impede spike firing, whereas under combined TTX and low Ca2+/high Mg2+ conditions the action potentials were completely abolished, indicating that TTX- resistant action potentials are mediated by a Ca2+ current. Further experiments with omega-conotoxin GVIA (100 nmol L-1) revealed that these Ca2+ currents enter the cell via N-type voltage-activated Ca2+ channels (see also accompanying paper). Tetraethylammonium (10 mmol L-1) caused broadening of the action potentials, which probably resulted from prolonged Ca2+ influx due to blockade of the delayed rectifier K+ channel. Although Ca2+ appears to be involved in the spike generation of all rat oesophageal myenteric neurones, only a minority (14%) shows a slow afterhyperpolarization. Thus, no strict correlation exists between the presence of a shoulder and a slow afterhyperpolarization. Furthermore, morphological identification of 25 of the impaled neurones revealed that there was no strict correlation between morphology and electrophysiological behaviour. Consequently, rat oesophageal myenteric neurones appear to differ in several aspects from myenteric neurones in smooth muscle regions of the gastrointestinal tract.

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.

Analysis of whole-cell currents by patch clamp of guinea-pig myenteric neurones in intact ganglia

Whole-cell patch-clamp recordings taken from guinea-pig duodenal myenteric neurones within intact ganglia were used to determine the properties of S and AH neurones. Major currents that determine the states of AH neurones were identified and quantified. S neurones had resting potentials of -47 +/- 6 mV and input resistances (R(in)) of 713 +/- 49 MOmega at voltages ranging from -90 to -40 mV. At more negative levels, activation of a time-independent, caesium-sensitive, inward-rectifier current (I(Kir)) decreased R(in) to 103 +/- 10 MOmega. AH neurones had resting potentials of -57 +/- 4 mV and R(in) was 502 +/- 27 MOmega. R(in) fell to 194 +/- 16 MOmega upon hyperpolarization. This decrease was attributable mainly to the activation of a cationic h current, I(h), and to I(Kir). Resting potential and R(in) exhibited a low sensitivity to changes in [K(+)](o) in both AH and S neurones. This indicates that both cells have a low background K(+) permeability. The cationic current, I(h), contributed about 20 % to the resting conductance of AH neurones. It had a half-activation voltage of -72 +/- 2 mV, and a voltage sensitivity of 8.2 +/- 0.7 mV per e-fold change. I(h) has relatively fast, voltage-dependent kinetics, with on and off time constants in the range of 50-350 ms. AH neurones had a previously undescribed, low threshold, slowly inactivating, sodium-dependent current that was poorly sensitive to TTX. In AH neurones, the post-action-potential slow hyperpolarizing current, I(AHP), displayed large variation from cell to cell. I(AHP) appeared to be highly Ca(2+) sensitive, since its activation with either membrane depolarization or caffeine (1 mM) was not prevented by perfusing the cell with 10 mM BAPTA. We determined the identity of the Ca(2+) channels linked to I(AHP). Action potentials of AH neurones that were elongated by TEA (10 mM) were similarly shortened and I(AHP) was suppressed with each of the three omega-conotoxins GVIA, MVIIA and MVIIC (0.3-0.5 microM), but not with omega-agatoxin IVA (0.2 microM). There was no additivity between the effects of the three conotoxins, which indicates the presence of N- but not of P/Q-type Ca(2+) channels. A residual Ca(2+) current, resistant to all toxins, but blocked by 0.5 mM Cd(2+), could not generate I(AHP). This patch-clamp study, performed on intact ganglia, demonstrates that the AH neurones of the guinea-pig duodenum are under the control of four major currents, I(AHP), I(h), an N-type Ca(2+) current and a slowly inactivating Na(+) current.

Morphology, electrophysiology, and calbindin immunoreactivity of myenteric neurons in the guinea pig distal colon

The morphological and physiological characteristics of myenteric neurons in the guinea pig distal colon were determined using Lucifer yellow- or N-(2-aminoethyl) biotinamide-containing microelectrodes and intracellular recording and staining methods. The neurons in this study (n = 204) were classified on the basis of the shapes of their cell bodies and short processes or dendrites and the number of long processes or axons as Dogiel type I (n = 75 neurons; 36.8%), filamentous (n = 31 neurons; 15.2%), Dogiel type II (n = 38 neurons; 18.6%), and unclassified (n = 60 neurons; 29.4%). All Dogiel type II neurons had action potentials followed by an after-spike hyperpolarization (AH), and most of them (84%) had large, smooth somata and filamentous, short processes in addition to multiple, long processes or axons. Most of Dogiel type I, filamentous, and unclassified neurons (98%) had a single, long process, but four Dogiel type I neurons and one unclassified neuron had two long processes terminating as varicosities within other ganglia or on the surface of longitudinal muscle. The projections of monoaxonal neurons were distributed equally between oral and aboral directions, and most of them received fast excitatory postsynaptic potentials (EPSPs). All of the Dogiel type II neurons and seven Dogiel type I neurons were positive for calbindin immunoreactivity, but three filamentous neurons received fEPSPs, had spikes followed by AH, and were negative for calbindin. The presence of calbindin-immunoreactive(-IR) neurons was quite variable among the ganglia. These results confirm that neither the presence of calbindin immunoreactivity nor the absence of fEPSPs can be used as a predictor of cellular morphology or electrophysiological properties of myenteric neurons in the distal colon.

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.

Myenteric neurons activate submucosal vasodilator neurons in guinea pig ileum

This study examined whether myenteric neurons activate submucosal vasodilator pathways in in vitro combined submucosal-myenteric plexus preparations from guinea pig ileum. Exposed myenteric ganglia were electrically stimulated, and changes in the outside diameter of submucosal arterioles were monitored in adjoining tissue by videomicroscopy. Stimulation up to 18 mm from the recording site evoked large TTX-sensitive vasodilations in both orad and aborad directions. In double-chamber baths, which isolated the stimulating myenteric chamber from the recording submucosal chamber, hexamethonium or the muscarinic antagonist 4-diphenylacetoxy-N-(2-chloroethyl)-piperdine hydrochloride (4-DAMP) almost completely blocked dilations when superfused in the submucosal chamber. When hexamethonium was placed in the myenteric chamber approximately 50% of responses were hexamethonium sensitive in both orad and aboard orientations. The addition of 4-DAMP or substitution of Ca(2+)-free, 12 mM Mg(2+) solution did not cause further inhibition. These results demonstrate that polysynaptic pathways in the myenteric plexus projecting orad and aborad can activate submucosal vasodilator neurons. These pathways could coordinate intestinal blood flow and motility.

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.

Recording ionic events from cultured, DiI-labelled myenteric neurons in the guinea-pig proximal colon

To date investigations of enteric neurons by patch clamping/calcium imaging have been limited by studying unidentified heterogeneous populations of neurons. In DiI-labelled colonic myenteric neurons, the feasibility of recording ionic events was determined by applying DiI either to the mucosa or the circular muscle, dispersing neurons after 48 h organotypic culture, and patch-clamping/calcium imaging labeled neurons after 3-7 days in culture. Myenteric neurons with diffuse DiI fluorescence were typically smooth and agranular. Neurons labeled after DiI was applied to circular muscle, fired in either a phasic or a tonic manner, and exhibited fast afterhyperpolarizations (100-300 ms duration) at the end of a depolarizing pulse. They expressed a fast inward current and at least three different outward currents. Action potentials elicited in DiI-labeled sensory neurons were followed by a prolonged afterhyperpolarization (AH, 4-6 s). The offset of a suprathreshold depolarizing step elicited a prolonged outward tail current that approximated the timecourse of the prolonged AH. In addition, in response to membrane depolarization in DiI-labeled neurons loaded with fura-2, robust Ca(2+) transients were recorded using the perforated patch technique. These results demonstrate that DiI labeling of cultured myenteric neurons is feasible, and patch clamp/Ca(2+) fluorescence recordings can be made from specific populations of cultured DiI-labeled colonic myenteric neurons.

Action potential-dependent calcium transients in myenteric S neurons of the guinea-pig ileum

Simultaneous intracellular microelectrode recording and Fura-2 imaging was used to investigate the relationship between intracellular calcium ion concentration ([Ca2+]i) and excitability of tonic S neurons in intact myenteric plexus of the guinea-pig ileum. S neurons were impaled in myenteric ganglia, at locations near connections with internodal strands. The calcium indicator Fura-2 was loaded via the recording microelectrode. The estimated [Ca2+]i of these neurons was approximately 95 nM (n = 25). Intracellular current injection (200 ms pulses, 0.2 nA, delivered at 0.05 Hz) resulted in action potential firing throughout the stimulus pulse, accompanied by transient increases in [Ca2+]i (to approximately 240 nM, n = 12). Increasing the number of evoked action potentials by increasing stimulus duration (100-500 ms) or intensity (0.05-0.3 nA) produced correspondingly larger [Ca2+]i transients. Single action potentials rarely produced resolvable [Ca2+]i events, while short bursts of action potentials (three to five events) invariably produced resolvable [Ca2+]i increases. Some neurons demonstrated spontaneous action potential firing, which was accompanied by sustained [Ca2+]i increases. Action potential firing and [Ca2+]i increases were also observed by activation of slow synaptic input to these neurons, in cases where the slow depolarization initiated action potential firing. Action potentials (evoked or spontaneous) and associated [Ca2+]i transients were abolished by tetrodotoxin (1 microM). Omega-conotoxin GVIA (100 nM) reduced [Ca2+]i transients by approximately 67%, suggesting that calcium influx through N-type calcium channels contributes to evoked [Ca2+]i increases. The S neurons in this study showed prominent afterhyperpolarizations following bursts of action potential firing. The time-course of afterhyperpolarizations was correlated with the time-course of evoked [Ca2+]i transients. Afterhyperpolarizations were blocked by tetrodotoxin and reduced by omega-conotoxin GVIA, suggesting that calcium influx through N-type channels contributes to these events. The electrical properties of Fura-2-loaded neurons were not significantly different from properties of neurons recorded without Fura-2 injection, suggesting that Fura-2 injection alone does not significantly influence the electrical properties of these cells. These data indicate that myenteric S neurons in situ show prominent, activity-dependent increases in [Ca2+]i. These events can be generated spontaneously, or be evoked by intracellular current injection or synaptic activation. [Ca2+]i transients in these neurons appear to involve action potential-dependent opening of N-type calcium channels, and the elevation in [Ca2+]i increase may underlie afterhyperpolarizations and regulate excitability of these enteric neurons.

Correlation of morphology, electrophysiology and chemistry of neurons in the myenteric plexus of the guinea-pig distal colon

Intracellular recordings were made from myenteric neurons of the guinea-pig distal colon to determine their electrical behaviour in response to intracellular current injection and stimulation of synaptic inputs. The recording microelectrode contained the intracellular marker biocytin, which was injected into impaled neurons so that electrophysiology, shape and immunohistochemistry could be correlated. Myenteric neurons in the distal colon were divided into four morphological groups based on their shapes and projections. One group (29 of the 78 that were characterized electrophysiologically, morphologically and immunohistochemically) was the multiaxonal Dogiel type II neurons, the majority (25/29) of which were calbindin immunoreactive. Each of these neurons had an inflection on the falling phase of the action potential that, in 24/29 neurons, was followed by a late afterhyperpolarizing potential (AHP). Slow excitatory postsynaptic potentials were recorded in 20 of 29 Dogiel type II neurons in response to high frequency internodal strand stimulation and two neurons responded with slow inhibitory postsynaptic potentials. Low amplitude fast excitatory postsynaptic potentials occurred in 3 of 29 Dogiel type II neurons. Neurons of the other three groups were all uniaxonal: neurons with Dogiel type I morphology, filamentous ascending interneurons and small filamentous neurons with local projections to the longitudinal or circular muscle or to the tertiary plexus. Dogiel type I neurons were often immunoreactive for nitric oxide synthase or calretinin, as were some small filamentous neurons, while all filamentous ascending interneurons tested were calretinin immunoreactive. All uniaxonal neurons exhibited prominent fast excitatory postsynaptic potentials and did not have a late AHP following a single action potential, that is, all uniaxonal neurons displayed S type electrophysiological characteristics. However, in 6/19 Dogiel type I neurons and 2/8 filamentous ascending interneurons, a prolonged hyperpolarizing potential ensued when more than one action potential was evoked. Slow depolarizing postsynaptic potentials were observed in 20/29 Dogiel type I neurons, 6/8 filamentous ascending interneurons and 8/12 small filamentous neurons. Six of 29 Dogiel type I neurons displayed slow inhibitory postsynaptic potentials, as did 2/8 filamentous ascending interneurons and 4/12 small filamentous neurons. These results indicate that myenteric neurons in the distal colon of the guinea-pig are electrophysiologically similar to myenteric neurons in the ileum, duodenum and proximal colon. Also, the correlation of AH electrophysiological characteristics with Dogiel type II morphology and S electrophysiological characteristics with uniaxonal morphology is preserved in this region. However, filamentous ascending interneurons have not been encountered in other regions of the gastrointestinal tract and there are differences between the synaptic properties of neurons in this region compared to other regions studied, including the presence of slow depolarizing postsynaptic potentials that appear to involve conductance increases and frequent slow inhibitory postsynaptic potentials.

Projections of submucous neurons to the myenteric plexus in the guinea pig small intestine

The distribution of submucous neurons that project to the myenteric plexus of the guinea pig small intestine was established by retrograde transport of the carbocyanine dye 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) from myenteric ganglia in organ culture in combination with immunohistochemistry. Following the application of DiI to the serosal surface of a single myenteric ganglion, from 2 to 15 DiI-labelled nerve cell bodies were labelled in the submucous plexus up to 7.9 mm circumferentially, 4.5 mm orally, and 3.4 mm aborally to the DiI application site. No cells were labelled in preparations in which connections between myenteric and submucous plexuses had been severed prior to DiI application. Cells that were immunoreactive for vasoactive intestinal polypeptide (VIP) or for substance P (SP) accounted for about 75% and 11% of DiI-labelled cells, respectively. Neither neuropeptide Y- nor calretinin-immunoreactive submucous neurons were labelled by DiI, indicating that these classes of neurons do not project to the myenteric plexus. Retrograde tracing from the myenteric plexus with Neurobiotin revealed that labelled VIP-immunoreactive neurons had several short, filamentous processes and a single long axon that could be followed through the circular muscle to myenteric ganglia without branches to the mucosa. The previously described projection of submucous, SP-immunoreactive putative sensory neurons to the myenteric plexus was confirmed. However, this study has identified a considerably larger population of presumed interneurons that are immunoreactive for VIP that likely transmit information from the submucous plexus to the myenteric plexus and presumably coordinate activity between the two ganglionated plexuses.

Orally projecting interneurones in the guinea-pig small intestine

1. Orally projecting, cholinergic interneurones are important in mediating ascending excitatory reflexes in the small intestine. We have shown that there is just one major class of orally projecting interneurone, which we have characterized using retrograde labelling in organ culture, combined with immunohistochemistry, intracellular recording and dye filling. 2. Orally projecting interneurones, previously shown to be immunoreactive for choline acetyltransferase, tachykinins, enkephalin, calretinin and neurofilament protein triplet, have axons up to 14 mm long and are the only class of cells with orally directed axons more than 8.5 mm long. 3. They are all small Dogiel type I neurones with short dendrites, usually lamellar in form, and a single axon which sometimes bifurcates. Their axons give rise to short varicose collaterals in myenteric ganglia more than 3 mm oral to their cell bodies. 4. Orally projecting interneurones receive prominent fast excitatory post synaptic potentials (fast EPSPs). A major source of fast EPSPs is other ascending interneurones located further aborally. They also receive fast EPSPs from circumferential pathways. 5. In the stretched preparations used in this study, orally projecting interneurones were highly excitable, firing repeatedly to depolarizing current pulses and had negligible long after-hyperpolarizations following their action potentials. They did not receive measurable non-cholinergic slow excitatory synaptic inputs. 6. Ascending interneurones had a characteristic inflection in their membrane responses to depolarizing current pulses and their first action potential was typically delayed by approximately 30 ms. Under single electrode voltage clamp, ascending interneurones had a transient outward current when depolarized above -70 mV from more hyperpolarized holding potentials. Ascending interneurones also consistently showed marked inward rectification under both current clamp and voltage clamp conditions. 7. This class of cells has consistent morphological, neurochemical and electrophysiological characteristics and are important in mediating orally directed enteric reflexes.