Morphological properties and projections of electrophysiologically characterized neurons in the guinea-pig submucosal plexus

Neurochemical phenotypes of huntingtin-associated protein 1 in reference to secretomotor and vasodilator neurons in the submucosal plexuses of rodent small intestine

Huntingtin-associated protein 1(HAP1) is an immunohistochemical marker of the stigmoid body (STB). Brain and spinal cord regions with lack of STB/HAP1 immunoreactivity are always neurodegenerative targets, whereas STB/HAP1 abundant regions are usually spared from neurodegeneration. In addition to the brain and spinal cord, HAP1 is abundantly expressed in the excitatory and inhibitory motor neurons in myenteric plexuses of the enteric nervous system (ENS). However, the detailed expression of HAP1 and its neurochemical characterization in submucosal plexuses of ENS are still unknown. In this study, we aimed to clarify the expression and neurochemical characterization of HAP1 in the submucosal plexuses of the small intestine in adult mice and rats. HAP1 was highly expressed in the submucosal plexuses of both rodents. The percentage of HAP1-immunoreactive submucosal neurons was not significantly varied between the intestinal segments of these rodents. Double immunofluorescence results revealed that almost all the cholinergic secretomotor neurons containing ChAT/ CGRP/ somatostatin/ calretinin, non-cholinergic secretomotor neurons containing VIP/NOS/TH/calretinin, and vasodilator neurons containing VIP/calretinin expressed HAP1. Our current study is the first to clarify that STB/HAP1 is expressed in secretomotor and vasodilator neurons of submucosal plexuses, suggesting that STB/HAP1 might modulate or protect the secretomotor and vasodilator functions of submucosal neurons in ENS.

Calcimimetic R568 inhibits tetrodotoxin-sensitive colonic electrolyte secretion and reduces c-fos expression in myenteric neurons

AIMS

Calcium-sensing receptor (CaSR) is expressed on neurons of both submucosal and myenteric plexuses of the enteric nervous system (ENS) and the CaSR agonist R568 inhibited Cl^- secretion in intestine. The purpose of this study was to localize the primary site of action of R568 in the ENS and to explore how CaSR regulates secretion through the ENS.

MATERIALS AND METHODS

Two preparations of rat proximal and distal colon were used. The full-thickness preparation contained both the submucosal and myenteric plexuses, whereas for the "stripped" preparation the myenteric plexus with the muscle layers was removed. Both preparations were mounted onto Ussing chambers and Cl^- secretory responses were compared by measuring changes in short circuit current (I_sc). Two tissue-specific CaSR knockouts (i.e., neuron-specific vs. enterocyte-specific) were generated to compare the effect of R568 on expression of c-fos protein in myenteric neurons by immunocytochemistry.

KEY FINDINGS

In full-thickness colons, tetrodotoxin (TTX) inhibited I_sc, both in proximal and distal colons. A nearly identical inhibition was produced by R568. However, in stripped preparations, while the effect of TTX on I_sc largely remained, the effect of R568 was nearly completely eliminated. In keeping with this, R568 reduced c-fos protein expression only in myenteric neurons of wild type mice and mutant mice that contained CaSR in neurons (i.e., ^villinCre/Casr^flox/flox mice), but not in myenteric neurons of ^nestinCre/Casr^flox/flox mice in which neuronal cell CaSR was eliminated.

SIGNIFICANCE

These results indicate that R568 exerts its anti-secretory effects predominantly via CaSR-mediated inhibition of neuronal activity in the myenteric plexus.

Cholera Toxin Induces Sustained Hyperexcitability in Myenteric, but Not Submucosal, AH Neurons in Guinea Pig Jejunum

Background and Aims: Cholera toxin (CT)-induced hypersecretion requires activation of secretomotor pathways in the enteric nervous system (ENS). AH neurons, which have been identified as a population of intrinsic sensory neurons (ISNs), are a source of excitatory input to the secretomotor pathways. We therefore examined effects of CT in the intestinal lumen on myenteric and submucosal AH neurons.

Methods: Isolated segments of guinea pig jejunum were incubated for 90 min with saline plus CT (12.5 μg/ml) or CT + neurotransmitter antagonist, or CT + tetrodotoxin (TTX) in their lumen. After washing CT away, submucosal or myenteric plexus preparations were dissected keeping circumferentially adjacent mucosa intact. Submucosal AH neurons were impaled adjacent to intact mucosa and myenteric AH neurons were impaled adjacent to, more than 5 mm from, and in the absence of intact mucosa. Neuronal excitability was monitored by injecting 500 ms current pulses through the recording electrode.

Results: After CT pre-treatment, excitability of myenteric AH neurons adjacent to intact mucosa (n = 29) was greater than that of control neurons (n = 24), but submucosal AH neurons (n = 33, control n = 27) were unaffected. CT also induced excitability increases in myenteric AH neurons impaled distant from the mucosa (n = 6) or in its absence (n = 5). Coincubation with tetrodotoxin or SR142801 (NK3 receptor antagonist), but not SR140333 (NK1 antagonist) or granisetron (5-HT₃ receptor antagonist) prevented the increased excitability induced by CT. Increased excitability was associated with a reduction in the characteristic AHP and an increase in the ADP of these neurons, but not a change in the hyperpolarization-activated inward current, I_h.

Conclusions: CT increases excitability of myenteric, but not submucosal, AH neurons. This is neurally mediated and depends on NK3, but not 5-HT₃ receptors. Therefore, CT may act to amplify the secretomotor response to CT via an increase in the activity of the afferent limb of the enteric reflex circuitry.

VPAC1 receptors regulate intestinal secretion and muscle contractility by activating cholinergic neurons in guinea pig jejunum

In the gastrointestinal tract, vasoactive intestinal peptide (VIP) is found exclusively within neurons. VIP regulates intestinal motility via neurally mediated and direct actions on smooth muscle and secretion by a direct mucosal action, and via actions on submucosal neurons. VIP acts via VPAC1 and VPAC2 receptors; however, the subtype involved in its neural actions is unclear. The neural roles of VIP and VPAC1 receptors (VPAC1R) were investigated in intestinal motility and secretion in guinea pig jejunum. Expression of VIP receptors across the jejunal layers was examined using RT-PCR. Submucosal and myenteric neurons expressing VIP receptor subtype VPAC1 and/or various neurochemical markers were identified immunohistochemically. Isotonic muscle contraction was measured in longitudinal muscle-myenteric plexus preparations. Electrogenic secretion across mucosa-submucosa preparations was measured in Ussing chambers by monitoring short-circuit current. Calretinin(+) excitatory longitudinal muscle motor neurons expressed VPAC1R. Most cholinergic submucosal neurons, notably NPY(+) secretomotor neurons, expressed VPAC1R. VIP (100 nM) induced longitudinal muscle contraction that was inhibited by TTX (1 μM), PG97-269 (VPAC1 antagonist; 1 μM), and hyoscine (10 μM), but not by hexamethonium (200 μM). VIP (50 nM)-evoked secretion was depressed by hyoscine or PG97-269 and involved a small TTX-sensitive component. PG97-269 and TTX combined did not further depress the VIP response observed in the presence of PG97-269 alone. We conclude that VIP stimulates ACh-mediated longitudinal muscle contraction via VPAC1R on cholinergic motor neurons. VIP induces Cl(-) secretion directly via epithelial VPAC1R and indirectly via VPAC1R on cholinergic secretomotor neurons. No evidence was obtained for involvement of other neural VIP receptors.

Calretinin and somatostatin immunoreactivities label different human submucosal neuron populations

In human myenteric plexus, calretinin (CALR) and somatostatin (SOM) coexist in Dogiel Type II neurons, which were considered as intrinsic primary afferent neurons in the guinea pig. The aims of this study were to test if also human submucosal neurons costain immunohistochemically for CALR and SOM and whether these or other neurons display Type II morphology. Two sets of submucosal wholemounts of small and large intestine from 29 patients (median age 65 years) were triple stained for CALR, SOM, and human neuronal protein Hu C/D (HU, a pan-neuronal marker) as well as for CALR, SOM, and peripherin (PER), respectively. Only exceptionally, neurons coreactive for both CALR and SOM were found. The three major groups of neurons were CALR-/HU-coreactive (CALR-neurons), SOM-/HU-coreactive (SOM-neurons), and HU-alone-positive neurons. We observed significantly more CALR-neurons in the external submucosal plexus (ESP) of all regions and more SOM-neurons in the internal submucosal plexus (ISP), although with substantial interindividual variations. Comparisons of small vs. large intestine revealed more SOM-neurons (ESP: 29% vs. 4%, ISP: 40% vs. 13%) but fewer CALR-neurons (ESP: 37% vs. 77%, ISP: 21% vs. 67%) in small intestine. Morphologically, CALR-neurons had multiple processes; in some cases, we identified multidendritic/uniaxonal neurons. In contrast, SOM-neurons had mostly only one process. The functions of both populations as possible primary afferent neurons, interneurons, secretomotor neurons, or vasomotor neurons are discussed. Future morphochemical distinction of these groups may reveal different subgroups.

Nitric oxide enhances inhibitory synaptic transmission and neuronal excitability in Guinea-pig submucous plexus

Varicosities immunoreactive for nitric oxide synthase (NOS) make synaptic connections with submucosal neurons in the guinea-pig small intestine, but the effects of nitric oxide (NO) on these neurons are unknown. We used intracellular recording to characterize effects of sodium nitroprusside (SNP, NO donor) and nitro-l-arginine (NOLA, NOS inhibitor), on inhibitory synaptic potentials (IPSPs), slow excitatory synaptic potentials (EPSPs) and action potential firing in submucosal neurons of guinea-pig ileum in vitro. Recordings were made from neurons with the characteristic IPSPs of non-cholinergic secretomotor neurons. SNP (100 μM) markedly enhanced IPSPs evoked by single stimuli applied to intermodal strands and IPSPs evoked by trains of 2–10 pulses (30 Hz). Both noradrenergic (idazoxan-sensitive) and non-adrenergic (idazoxan-insensitive) IPSPs were affected. SNP enhanced hyperpolarizations evoked by locally applied noradrenaline or somatostatin. SNP did not affect slow EPSPs evoked by single stimuli, but depressed slow EPSPs evoked by stimulus trains. NOLA (100 μM) depressed IPSPs evoked by one to three stimulus pulses and enhanced slow EPSPs evoked by trains of two to three stimuli (30 Hz). SNP also increased the number of action potentials and the duration of firing evoked by prolonged (500 or 1000 ms) depolarizing current pulses, but NOLA had no consistent effect on action potential firing. We conclude that neurally released NO acts post-synaptically to enhance IPSPs and depress slow EPSPs, but may enhance the intrinsic excitability of these neurons. Thus, NOS neurons may locally regulate several secretomotor pathways ending on common neurons.

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

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

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

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

Pharmacological analysis of components of the change in transmural potential difference evoked by distension of rat proximal small intestine in vivo

The reflex response to distension of the small intestine in vivo is complex and not well understood. The aim of this study was to characterize the neural mechanisms contributing to the complex time course of the intestinal secretory response to distension. Transmucosal potential difference (PD) was used as a marker for mucosal chloride secretion, which reflects the activity of the secretomotor neurons. Graded distensions (5, 10, and 20 mmHg) of distal rat duodenum with saline for 5 min induced a biphasic PD response with an initial peak (rapid response) followed by a plateau (sustained response). The rapid response was significantly reduced by the neural blockers tetrodotoxin and lidocaine (given serosally) and by intravenous (iv) administration of the ganglionic blocker hexamethonium and the NK(1) receptor antagonist SR-140333. Serosal TTX and iv SR-140333 significantly reduced the sustained response, which was also reduced by the NK(3) receptor antagonist talnetant and by the vasoactive intestinal polypeptide (VPAC) receptor antagonist [4Cl-d-Phe(6), Leu(17)]-VIP. Serosal lidocaine and iv hexamethonium had no significant effect on this component. Inhibition of nitric oxide synthase had no effect on any of the components of the PD response to distension. The PD response to distension thus seems to consist of two components, a rapidly activating and adapting component operating via nicotinic transmission and NK(1) receptors, and a slow component operating via VIP-ergic transmission and involving both NK(1) and NK(3) receptors.

Mucosal stimulation activates secretomotor neurons via long myenteric pathways in guinea pig ileum

This study examined whether mucosal stimulation activates long secretomotor neural reflexes and, if so, how they are organized. The submucosa of in vitro full thickness guinea pig ileal preparations was exposed in the distal portion and intracellular recordings were obtained from electrophysiologically identified secretomotor neurons. Axons in the intact mucosa of the oral segment were stimulated by a large bipolar stimulating electrode. In control preparations, a single stimulus pulse evoked a fast excitatory postsynaptic potential (EPSP) in 86% of neurons located 0.7-1.0 cm anal to the stimulus site. A stimulus train evoked multiple fast EPSPs, but slow EPSPs were not observed. To examine whether mucosal stimulation specifically activated mucosal sensory nerve terminals, the mucosa/submucosa was severed from the underlying layers and repositioned. In these preparations, fast EPSPs could not be elicited in 89% of cells. Superfusion with phorbol dibutyrate enhanced excitability of sensory neurons and pressure-pulse application of serotonin to the mucosa increased the fast EPSPs evoked by mucosal stimulation, providing further evidence that sensory neurons were involved. To determine whether these reflexes projected through the myenteric plexus, this plexus was surgically lesioned between the stimulus site and the impaled neuron. No fast EPSPs were recorded in these preparations following mucosal stimulation whereas lesioning the submucosal plexus had no effect. These results demonstrate that mucosal stimulation triggers a long myenteric pathway that activates submucosal secretomotor neurons. This pathway projects in parallel with motor and vasodilator reflexes, and this common pathway may enable coordination of intestinal secretion, blood flow, and motility.

Restoration by VIP of the carbachol-stimulated Cl- secretion in TTX-treated guinea pig distal colon

To determine if vasoactive intestinal peptide (VIP) restores neural activity from tetrodotoxin (TTX) blockade, we studied the effects of VIP and related agents on carbachol (Cch)-induced Cl(-) secretion in control-isolated guinea pig distal colon and in that treated with TTX. The short circuit current (I(sc)) increased dose-dependently after serosal applications of Cch (10(-6) - 2 x 10(-5) M) and VIP (5 x 10(-9) - 10(-7) M). But no additive or synergistic increase in I(sc) was observed. Cch- and VIP-induced I(sc) was completely abolished by a serosal application of TTX (10(-6) M). However, a serosal application, not mucosal, of VIP (10(-7) M) and 8-bromo-cAMP (10(-3) M) restored the Cch-stimulated, TTX-inhibited I(sc) by 113% and 75.8%, respectively. Furthermore, mucosal and serosal applications of forskolin (aden late cyclase activator) restored the I(sc) by 43.9% and 65.3%, respectively. The restored I(sc) was completely abolished by atropine (muscarinic receptor antagonist). These results suggest that VIP may restore the cholinergic activity by increasing the level of intracellular cAMP, and that cholinergic neuron is very likely to be responsible for the regulation of Cl(-) secretion at neuroepithelial junctions. The exact mechanism of VIP's effect on the TTX-inhibited epithelial Cl(-) secretion, and its possible usefulness in the treatment of TTX-induced pathophysiological conditions, remain to be determined.

Recurrent networks of submucous neurons controlling intestinal secretion: a modeling study

Secretomotor neurons, immunoreactive for vasoactive intestinal peptide (VIP), are important in controlling chloride secretion in the small intestine. These neurons form functional synapses with other submucosal VIP neurons and transmit via slow excitatory postsynaptic potentials (EPSPs). Thus they form a recurrent network with positive feedback. Intrinsic sensory neurons within the submucosa are also likely to form recurrent networks with positive feedback, provide substantial output to VIP neurons, and receive input from VIP neurons. If positive feedback within recurrent networks is sufficiently large, then neurons in the network respond to even small stimuli by firing at their maximum possible rate, even after the stimulus is removed. However, it is not clear whether such a mechanism operates within the recurrent networks of submucous neurons. We investigated this question by performing computer simulations of realistic models of VIP and intrinsic sensory neuron networks. In the expected range of electrophysiological properties, we found that activity in the VIP neuron network decayed slowly after cessation of a stimulus, indicating that positive feedback is not strong enough to support the uncontrolled firing state. The addition of intrinsic sensory neurons produced a low stable firing rate consistent with the common finding that basal secretory activity is, in part, neurogenic. Changing electrophysiological properties enables these recurrent networks to support the uncontrolled firing state, which may have implications with hypersecretion in the presence of enterotoxins such as cholera-toxin.

Intrinsic primary afferent neurons and nerve circuits within the intestine

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

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.

Regulation of intestinal secretion involved in the interaction between neurotransmitters and prostaglandin E2

In this short review, it will be described that neurotransmitter-induced secretion in the intestine may be influenced by the tissue level of prostaglandin E2 (PGE2). In the normal condition, vasoactive intestinal polypeptide (VIP) and acetylcholine (ACh) are the predominant neurotransmitters of secretomotor neurones. VIP and ACh activate distinct second messenger systems in epithelial cells, i.e. adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium ion (Ca2+), respectively. An increase in intracellular cAMP induces a small amount of chloride (Cl-) secretion in epithelial cells, while simultaneous increases in intracellular Ca2+ and cAMP greatly enhances the cAMP-induced Cl- secretion. When the concentration of prostaglandins reaches a high level in the intestinal tissue substance P, which is a neurotransmitter of sensory neurones, can also induce a massive Cl- secretion by cross-potentiation of cAMP and Ca2+ in epithelial cells. In conclusion, it is considered that the concentration of tissue PGE2 may indicate tissue alert level, and when this level elevates, PGE2 enhances ACh and SP-induced Cl- secretion, thus mediating massive fluid secretion for host defence.

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

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

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.

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.

Endogenous neurokinins facilitate synaptic transmission in guinea pig airway parasympathetic ganglia

Neurokinin-containing nerve fibers were localized to guinea pig airway parasympathetic ganglia in control tissues but not in tissues pretreated with capsaicin. The purpose of the present study was to determine whether neurokinins, released during axonal reflexes or after antidromic afferent nerve stimulation, modulate ganglionic synaptic neurotransmission. The neurokinin type 3 (NK(3)) receptor antagonists SB-223412 and SR-142801 inhibited vagally mediated cholinergic contractions of bronchi in vitro at stimulation voltages threshold for preganglionic nerve activation but had no effect on vagally mediated contractions evoked at optimal voltage or field stimulation-induced contractions. Intracellular recordings from the ganglia neurons revealed that capsaicin-sensitive nerve stimulation potentiated subsequent preganglionic nerve-evoked fast excitatory postsynaptic potentials. This effect was mimicked by the NK(3) receptor agonist senktide analog and blocked by SB-223412. In situ, senktide analog markedly increased baseline tracheal cholinergic tone, an effect that was reversed by atropine and prevented by vagotomy or SB-223412. Comparable effects of intravenous senktide analog on pulmonary insufflation pressure were observed. These data highlight the important integrative role played by parasympathetic ganglia and indicate that activation of NK(3) receptors in airway ganglia by endogenous neurokinins facilitates synaptic neurotransmission.

Converging and diverging cholinergic inputs from submucosal neurons amplify activity of secretomotor neurons in guinea-pig ileal submucosa

The organization of synaptic connections between guinea-pig ileal submucosal neurons was examined using intracellular recordings from single or pairs of submucosal neurons. Synaptic inputs were elicited by stimulating cholinergic neurons using pressure-pulse application of 5-hydroxytryptamine (5-HT) in ganglia adjacent to those where intracellular recordings were obtained. In addition, when pairs of intracellular recordings were obtained, one neuron was activated by intracellular stimulation and synaptic responses were recorded in the other neuron. Neurobiotin-filled microelectrodes were employed to characterize cells electrophysiologically and immunohistochemically. Recordings were obtained from 176 (173 S-type and three AH-type) neurons; 81% of cells were classified as vasoactive intestinal peptide (VIP) neurons. No fast excitatory postsynaptic potentials and only rare slow excitatory postsynaptic potentials were recorded following intracellular stimulation of paired S-type neurons. However, when paired intracellular recordings were obtained from neurons within the same ganglion and 5-HT was applied to an adjacent ganglion, this stimulation evoked synchronized fast excitatory postsynaptic potentials in 94% of pairs. In contrast, when cell bodies of VIP-VIP pairs were located in different ganglia, fast synaptic activation evoked by 5-HT stimulation was not synchronized in 87% of pairs. When intracellular recordings were obtained from a single neuron and two separate ganglia were stimulated by 5-HT pressure-pulse activation, fast excitatory postsynaptic potentials originating from both sources were recorded in the same VIP neuron. Morphological study of 34 S-type and three AH-type horseradish peroxidase-labeled neurons was conducted. AH-type neurons had multiple axonal branches with dense arborization of collaterals containing numerous varicosities in three to nine ganglia, whereas axons of S-type neurons exhibited relatively rare collaterals and varicosities within adjacent ganglia. These results demonstrate that cholinergic neurons provide both diverging and converging inputs to VIP neurons, providing a mechanism to enhance activation of VIP secretomotor neurons. The axonal projections of AH-type neurons suggest they are likely candidates to provide diverging inputs to multiple VIP neurons.

Electrophysiological characteristics distinguish three classes of neuron in submucosal ganglia of the guinea-pig distal colon

Intracellular recordings were made from neurons in the submucosal ganglia of the guinea-pig distal colon. The recording electrode contained the intracellular marker biocytin, which was injected into neurons so that their electrophysiological characteristics could be correlated with their shape. Correlations of electrophysiology and shape have not been reported previously for neurons in this region. Three types of neuron were identified on electrophysiological grounds. Neurons of the first type (S neurons) had tetrodotoxin-sensitive soma action potentials, and received fast and slow excitatory synaptic inputs. They had uniaxonal morphologies and may function as secretomotor or possibly vasomotor neurons. The second type (AH neurons) received only slow synaptic input, while the soma action potential had tetrodotoxin-sensitive and -insensitive components with a shoulder on the falling phase and a long-lasting afterhyperpolarisation of the membrane potential following a single action potential. Neurons of this type had multipolar morphologies and provided dense innervation of adjacent submucosal ganglia. These neurons are similar to the submucosal intrinsic primary afferent neurons of the guinea-pig small intestine. The final type of neuron [the low-threshold (LT) neuron] had electrophysiological characteristics that set it apart from those described previously within enteric plexuses. They expressed tetrodotoxin-insensitive voltage-gated soma currents, did not have long-lasting afterhyperpolarisations and received only slow synaptic input. In addition, these neurons were very excitable: they had large input resistances and low thresholds for action potential discharge, and often fired action potentials in the absence of stimulation. Neurons with these characteristics were uniaxonal and thus are likely to be secretomotor or possibly vasomotor neurons. This study has shown that submucosal neurons of the distal colon fall into three distinct types, which can be distinguished by a combination of electrophysiological and morphological criteria.

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.

Activation of intrinsic afferent pathways in submucosal ganglia of the guinea pig small intestine

The enteric nervous system contains intrinsic primary afferent neurons that allow mucosal stimulation to initiate reflexes without CNS input. We tested the hypothesis that submucosal primary afferent neurons are activated by 5-hydroxytryptamine (5-HT) released from the stimulated mucosa. Fast and/or slow EPSPs were recorded in submucosal neurons after the delivery of exogenous 5-HT, WAY100325 (a 5-HT(1P) agonist), mechanical, or electrical stimuli to the mucosa of myenteric plexus-free preparations (+/- extrinsic denervation). These events were responses of second-order cells to transmitters released by excited primary afferent neurons. After all stimuli, fast and slow EPSPs were abolished by a 5-HT(1P) antagonist, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, and by 1.0 microM tropisetron, but not by 5-HT(4)-selective antagonists (SB204070 and GR113808A) or 5-HT(3)-selective antagonists (ondansetron and 0.3 microM tropisetron). Fast EPSPs in second-order neurons were blocked by hexamethonium, and most slow EPSPs were blocked by an antagonist of human calcitonin gene-related peptide (hCGRP(8-37)). hCGRP(8-37) also inhibited the spread of excitation in the submucosal plexus, assessed by measuring the uptake of FM2-10 and induction of c-fos. In summary, data are consistent with the hypothesis that 5-HT from enterochromaffin cells in response to mucosal stimuli initiates reflexes by stimulating 5-HT(1P) receptors on submucosal primary afferent neurons. Second-order neurons respond to these cholinergic/CGRP-containing cells with nicotinic fast EPSPs and/or CGRP-mediated slow EPSPs. Slow EPSPs are necessary for excitation to spread within the submucosal plexus. Because some second-order neurons contain also CGRP, primary afferent neurons may be multifunctional and also serve as interneurons.

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

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

Potassium channels of myenteric neurons in guinea-pig small intestine

Patch-clamp recording was used to study rectifying K+ currents in myenteric neurons in short-term culture. In conditions that suppressed Ca2+ -activated K+ current, three kinds of voltage-activated K+ currents were identified by their voltage range of activation, inactivation, kinetics and pharmacology. These were A-type current, delayed outwardly rectifying current (I(K),dr) and inwardly rectifying current (I(K),ir). I(K),ir consisted of an instantaneous component followed by a time-dependent current that rapidly increased at potentials negative to -80 mV. Time-constant of activation was voltage-dependent with an e-fold decrease for a 31-mV hyperpolarization amounting to a decrease from 800 to 145 ms between -80 and -100 mV. I(K),ir did not inactivate. I(K),ir was abolished in K+ -free solution. Increases in external K+ increased I(K),ir conductance in direct relation to the square root of external K+ concentration. Activation kinetics were accelerated and the activation range shifted to more positive K+ equilibrium potentials. I(K),ir was suppressed by external Cs+ and Ba2+ in a concentration-dependent manner. Ca2+ and Mg+ were less effective than Ba2+. I(K),ir was unaffected by tetraethylammonium ions. I(K),dr was activated at membrane potentials positive to - 30 mV with an e-fold decrease in time-constant of activation from 145 to 16 ms between -20 and 30 mV. It was half-activated at 5 mV and fully activated at 50 mV. Inactivation was indiscernible during 2.5 s test pulses. I(K),dr was suppressed in a concentration-, but not voltage-dependent manner by either tetraethylammonium or 4-aminopyridine and was insensitive to Cs+. The results suggest that I(K),ir may be important in maintaining the high resting membrane potentials found in afterhyperpolarization-type enteric neurons. They also suggest importance of I(K),ir channels in augmentation of the large hyperpolarizing after-potentials in afterhyperpolarization-type neurons and the hyperpolarization associated with inhibitory postsynaptic potentials. I(K),dr in afterhyperpolarization-type enteric neurons has overall kinetics and voltage behaviour like delayed rectifier currents in other excitable cells where the currents can also be distinguished from A-type and Ca2+ -activated K+ current.

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

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

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

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

Activation of 5-HT1P receptors on submucosal afferents subsequently triggers VIP neurons and chloride secretion in the guinea-pig colon

The role of vasoactive intestinal peptide (VIP) was investigated when mucosal stroking and 5-hydroxytryptamine (5-HT) were used to activate neural reflexes that stimulate chloride secretion in the guinea pig colon. Muscle-stripped segments of colon containing intact submucosal ganglia without myenteric ganglia were set up in modified flux chambers in order to record short-circuit current (Isc). Mucosal stroking with a brush for 1 s or a pulse of 5-HT (injection of 15 microliters of 100 microM 5-HT into 1.5 ml of mucosal solution) caused an increase in Isc that was reduced by the VIP antagonist, neurotensin6-11-VIP7-28, in a concentration-dependent manner. The Isc responses to mucosal stroking and a 5-HT pulse were reduced by 53% and 58%, respectively, by 2 microM neurotensin6-11-VIP7-28. The residual Isc response in the presence of neurotensin6-11-VIP7-28 was abolished by atropine. Blockade of 5-HT1P receptors on submucosal afferent neurons decreased Isc responses to stroking or a 5-HT pulse. The residual Isc response after 5-HT1P receptors were blocked was reduced by only 11-14% by neurotensin6-11-VIP7-28. In the presence of blockade of both 5-HT1P and VIP receptors, atropine abolished the Isc response to both stimuli. The observations suggest that the neural circuitry activated by stroking includes at least two independent pathways. One pathway contains VIP neurons which receive inputs directly or indirectly from 5-HT1P receptor-containing afferents. A second pathway involves muscarinic cholinergic transmission that is independent of 5-HT1P and VIP receptor activation.

Actions of Daiichi DQ-2511 on electrical and synaptic behavior of enteric neurons in the guinea-pig small intestine

Intracellular recording of electrical and synaptic behavior of neurons in the enteric nervous system of guinea-pig small intestine was used to evaluate actions of DQ-2511 (3-[[[2-(3, 4-dimethoxyphenyl)ethyl]carbamoyl]methyl]amino-N-methylbenzamide). DQ-2511 is a new drug with gastrointestinal prokinetic action. DQ-2511 was most effective in the nanomolar range. The drug depolarized some of the neurons and this was accompanied by increased input resistance and augmented excitability. DQ-2511 in nanomolar concentrations increased the amplitude of fast excitatory postsynaptic potentials at nicotinic synapses. Slow inhibitory postsynaptic potentials, produced by release of norepinephrine from sympathetic postganglionic fibers, were suppressed by DQ-2511. This appeared to reflect presynaptic suppression of release of norepinephrine because postsynaptic responses to exogenously applied norepinephrine were unaffected. The results suggest that the prokinetic action of DQ-2511 on gastrointestinal transit might emerge from actions that augment excitatory synaptic transmission in the microcircuits of the enteric nervous system while suppressing inhibitory sympathetic neurotransmission.

Neuropeptide Y in submucosal ganglia: regional differences in the innervation of guinea-pig large intestine

Since information about possible regional differences in the innervation of the guinea-pig large intestine is incomplete, a comparative study was made of the occurrence of neurones and nerve fibres of the submucosa showing immunoreactivity (IR) to neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). In addition, a quantitative analysis was made of submucosal neurones in regions of guinea-pig large intestine selected for probable differences in their function. There were two principal findings: First, the density of NPY-IR neurone somata was high in the ascending colon (mean +/- SEM 3148 +/- 464 neurones/cm2; n = 5 animals) and progressively declined in an anal direction, the descending colon having 348 +/- 125 neurones/cm2 (in the same 5 animals); immunoreactive cell bodies were rare in the rectum. The reduced density was also reflected in a fall in the number of NPY-IR neurones/ganglion from 3.0 +/- 0.3 in the ascending colon to 0.5 +/- 0.2 in the descending colon. Second, varicose NPY-IR intraganglionic fibres were a conspicuous feature of the duodenum, caecum, transverse colon, descending colon and rectum, but not of the ileum, ascending colon or distal spiral. Moreover, in the descending colon and rectum the fibres were arranged in a loose 'cobweb' structure around non-NPY-IR neurone somata; in the caecum, there was an apparent paucity of NPY-IR somata but the exceptionally dense intraganglionic varicose fibre network may have obscured NPY-IR somata. In all regions, fibre baskets were rare. In the ascending colon, only 25 +/- 5% of ganglia (compared to 92 +/- 2% of ganglia in the descending colon) showed any intraganglionic nerve fibres; furthermore, when they occurred, these were not of the 'cobweb' type but, rather, they gave the ganglia a speckled appearance. In very immature fetuses at a stage of development when no neuropeptide somata could be found in either the myenteric or submucosal plexuses, many NPY-IR nerve fibres were present in the submucosa with a distribution similar to that of adult guinea pigs. With respect to the density of VIP-IR neurones in the large intestine, there was only a 40% reduction in the number of neurones/cm2 from proximal to distal colon, in contrast to the corresponding 90% reduction in the density of NPY-IR neurones. The number of VIP-IR neurones/ganglion (6.4) and the proportion of ganglia with VIP-IR fibres (> 90%) were constant. It is concluded that the striking regional dissimilarities in (i) the occurrence of NPY-IR neurone somata and (ii) in the disposition of intraganglionic NPY-IR nerve fibres indicate potentially important regional differences in the functions of neuropeptide Y as an antisecretory peptide in the local regulation of chloride transport in the mucosa and as a modulator of ganglionic transmission, respectively.

Do vasoactive intestinal peptide (VIP)- and nitric oxide synthase-immunoreactive terminals synapse exclusively with VIP cell bodies in the submucous plexus of the guinea-pig ileum?

In the submucous plexus of the guinea-pig ileum, previous light-microscopic studies have revealed that vasoactive intestinal peptide (VIP)-immunoreactive and nitric oxide synthase (NOS)-immunoreactive terminals are found predominantly in association with VIP-immunoreactive nerve cell bodies. In this study, double-label immunohistochemistry at the light-microscopic level demonstrated co-localization of NOS-immunoreactivity and VIP-immunoreactivity in axon terminals in submucous ganglia. About 90% of nerve fibres with NOS-immunoreactivity or VIP-immunoreactivity were immunoreactive for both antigens; only about 10% of labelled varicosities contained only NOS-immunoreactivity or VIP-immunoreactivity. The VIP/NOS varicosities were more often seen in the central parts of the ganglia, close to the VIP-immunoreactive cell bodies. Ultrastructural immunocytochemistry with antibodies to VIP was used to determine if NOS/VIP terminals synapse exclusively with VIP-immunoreactive nerve cell bodies. We examined the targets of VIP-immunoreactive boutons in two submucous ganglia from different animals. Serial ultrathin sections were taken through the ganglia after they had been processed for VIP immunocytochemistry. For each cell body, the number of VIP inputs (synapses and close contacts) was determined. The number of VIP-immunoreactive synapses received by the cell bodies of submucous neurons varied from 0-4 and the number of VIP-immunoreactive close contacts varied from 3-10. There was no significant difference between VIP-immunoreactive nerve cell bodies and non-VIP nerve cell bodies in the number of VIP-immunoreactive synapses and close contacts they received. Thus, the implication from light microscopy that NOS/VIP terminals end predominantly on VIP nerve cells was not vindicated by electron microscopy.

Morphological study of the submucosal and mucosal plexuses of the sheep forestomach

Submucosal and mucosal nerve plexuses in the ovine forestomach were examined by immunohistochemical staining for protein gene-product 9.5 (PGP 9.5) and for S-100 protein (S-100), using whole-mount specimens that had been prepared by treatment with KOH. Nerve fibers of various sizes and glial cells (i.e., Schwann cells and satellite cells) were stained with antibodies against PGP 9.5 and S-100 respectively. The network of the submucosal plexus in the rumen is irregular and some nerve bundles in the plexus cross over other bundles. Some of the nerve bundles penetrate the ruminal papillae. The submucosal plexus in the reticulum consists of a network in the reticular wall and the reticular folds. The submucosal plexus in the omasum is also divided into two segments; namely, the sublaminar and the intralaminar plexuses. Most of the submucosal ganglion cells are unipolar and smooth-surfaced, being located singly or in small groups. A few perikarya were detected in the ruminal papillae. The number of perikarya per unit surface area is greater in the caudal portion of the omasal lamina (19.32 +/- 8.62 per cm2). In the mucosal plexuses, a well-developed network of beaded fibers with PGP 9.5-like immunoreactivity and a glial framework of S-100 like immunoreactivity was observed, in particular in the ruminal, reticular and omasal papillae. The intrapapillary nervous networks are interconnected by thin bundles of nerves in the interpapillary region. The present results suggest that some of the mucosal functions are intrinsically regulated by the submucosal and mucosal plexuses in the ovine forestomach.

Plurichemical transmission and chemical coding of neurons in the digestive tract

The enteric nervous system contains neurons with well-defined functions. However, when neurons of the same function are examined in different regions or species, they are found to show subtle differences in their pharmacologies of transmission and different chemical coding. Individual enteric neurons use more than one transmitter, i.e., transmission is plurichemical. For example, enteric inhibitory neurons have three or more primary transmitters, including nitric oxide, vasoactive intestinal peptide, and possibly adenosine triphosphate and pituitary adenylyl cyclase activating peptide. Primary transmitters are highly conserved, although their relative roles vary considerably between gut regions. Multiple substances, including transmitters and their synthesizing enzymes and nontransmitters (such as neurofilament proteins), provide neurons with a chemical coding through which their functions and projections can be identified. Although equivalent neurons in different regions have the same primary transmitters, other chemical markers differ substantially. Caution must be taken in extrapolating pharmacological and neurochemical observations between species or even between regions in the one species. On the other hand, careful interregion and interspecies comparisons lead to an understanding of the features of enteric neurons that are highly conserved and can be used in valid extrapolation.