Calbindin neurons of the guinea-pig small intestine: quantitative analysis of their numbers and projections

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

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

Neurochemically distinct classes of myenteric neurons express the mu-opioid receptor in the guinea pig ileum

The mu-opioid receptor (muOR), which mediates many of the opioid effects in the nervous system, is expressed by enteric neurons. The aims of this study were to determine whether 1) different classes of myenteric neurons in the guinea pig ileum contain muOR immunoreactivity by using double- and triple-labeling immunofluorescence and confocal microscopy, 2) muOR immunoreactivity is localized to enteric neurons immunoreactive for the endogenous opioid enkephalin, and 3) muOR immunoreactivity is localized to interstitial cells of Cajal visualized by c-kit. In the myenteric plexus, 50% of muOR-immunoreactive neurons contained choline acetyltransferase (ChAT) immunoreactivity, whereas about 43% of ChAT-immunoreactive neurons were muOR immunoreactive. Approximately 46% of muOR myenteric neurons were immunoreactive for vasoactive intestinal polypeptide (VIP), and about 31% were immunoreactive for nitric oxide synthase (NOS). MuOR immunoreactivity was found in about 68% of VIP-containing neurons and 60% of NOS-immunoreactive neurons. Triple labeling showed that about 32% of muOR neurons contained VIP and ChAT immunoreactivities. The endogenous opioid enkephalin (ENK) was observed in about 30% of muOR neurons; conversely, 48% of ENK neurons contained muOR immunoreactivity. MuOR was not detected in neurons containing calbindin, nor in interstitial cells of Cajal. MuOR-immunoreactive fibers formed a dense network around interstitial cells of Cajal in the deep muscular plexus. This study demonstrates that muOR is expressed by neurochemically distinct classes of myenteric neurons that are likely to differ functionally, is colocalized with the endogenous opioid ENK, and is not expressed by interstitial cells of Cajal.

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

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

Immunoreactivity of Hu proteins facilitates identification of myenteric neurones in guinea-pig small intestine

Hu proteins, together with neurone-specific enolase (NSE), protein gene product 9.5 (PGP-9.5), microtubule-associated protein-2 (MAP-2) and tubulin beta III isoform, were evaluated immunohistochemically as neuronal markers in whole-mount preparations and cultures obtained from the myenteric plexus of guinea-pig small intestine. Anti-Hu immunostaining marked the ganglion cell somas and nuclei without staining of the neuronal processes in the whole-mounts and cultures. The ganglion cell bodies were not obscured by staining of multiple neuronal fibres and this facilitated accurate counting of the neurones. MAP2 immunostaining also provided clear images of individual neurones in both whole mounts and cultures. Immunoreactivity for NSE, PGP-9.5 and tubulin beta III isoform provided sharp images of the ganglion cells in culture, but not in whole-mount preparations. Strong staining of the neuronal processes in the whole-mount preparations obscured the profiles of the ganglion cell bodies to such an extent that accurate counting of the total neuronal population was compromised. Anti-Hu immunostaining was judged to be an acceptable method for obtaining reliable estimates of total numbers of myenteric neurones in relation to other specific histochemical properties such as histamine binding.

Gene expression and localization of GABA(C) receptors in neurons of the rat gastrointestinal tract

The effects of GABA in the CNS are mediated by three different GABA receptors: GABA(A), GABA(B) and GABA(C) receptors. GABA(A) and GABA(B) receptors, but not yet GABA(C) receptors, have been demonstrated in the enteric nervous system, where GABA has been proposed to be a transmitter. The purpose of this study was to determine whether GABA(C) receptors are present and thus may play a role in mediating the effects of GABA in the myenteric plexus of the rat gastrointestinal tract. We examined the expression of the three known GABA(C) receptor subunits, rho1, rho2 and rho3, in the rat duodenum, ileum and colon using the reverse transcriptase-polymerase chain reaction. We determined the localization of GABA(C) receptors in the myenteric plexus of these regions using two different antisera directed against GABA(C) receptor subunits. The polymerase chain reaction revealed that all three subunits were expressed in the gastrointestinal tract. When the layers of the intestine were separated and the layer containing myenteric neurons was assayed, the rho3 subunit was found in the ileum and colon, whereas rho1 was expressed in the duodenum and weakly in the colon and rho2 was expressed in the ileum. Immunocytochemistry revealed numerous labeled neurons in the myenteric plexus of each region. Colocalization showed that a large proportion of calbindin plus calretinin immunoreactive neurons (intrinsic primary afferent neurons) were immunoreactive for the GABA(C) receptor, and that 56% of nitric oxide synthase immunoreactive neurons (inhibitory motor neurons) exhibited the receptor. These results indicate that GABA(C) receptors of differing subunit compositions are expressed by neurons in the rat gastrointestinal tract. The effects of GABA on intrinsic sensory and on inhibitory motor neurons are likely to be mediated in part through GABA(C) receptors.

Neurocalcin-alpha immunoreactivity in the enteric nervous system of young and aged rats

The distribution of the calcium binding protein neurocalcin a has been examined in the enteric nervous system of young adult (3 months) and aged (24+ months) male rats by immunofluorescence. Neurocalcin-immunoreactive (NC-ir) neurons were observed in the submucous and myenteric plexuses throughout the gastrointestinal tract from the oesophagus to the distal large intestine. NC-ir nerve terminals were also seen on NC-ir and NC-negative neurons. Semiquantitative estimates revealed fewer NC-ir neurons in the submucous plexus than in the myenteric plexus. The greatest occurrence of NC-ir neurons was in the small and large intestine. NC-ir axons were seen in the mucosa and also in between the ganglia of the myenteric plexus. In the aged rats, there were no discernible changes in the numbers of NC-ir neurons in th e oesophagus and stomach, with an increase in the pylorus and slight decreases in the small and large intestines. No decrease in NC-ir was observed in the distal large intestine. NC-ir neurons never contained lipofuscin age pigment and many enteric neuro ns devoid of NC-ir contained age pigment. Like other previously investigated calcium-binding proteins in enteric neurons, the distribution of NC shows much variability from one part of the intestine to another. The observed slight decreases in the number of NC-ir enteric neurons in aged rats may compromise the regulation of calcium in these neurons.

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.

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

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

Direct neuronal interactions between the duodenum and the sphincter of Oddi

The sphincter of Oddi (SO) is a complex structure that must function in coordination with the motor activities of the gallbladder and the duodenum. It is now clear that a neural circuit exists between the duodenum and the SO, and it is likely that this network is largely responsible for the regulation of SO motility. Recent studies have demonstrated that this circuit provides excitatory cholinergic input to SO ganglia that can be activated by electrical stimulation of the duodenal mucosa, distention of the duodenum, and increased motor activity of the duodenum.

Patch-clamp study of neurons and glial cells in isolated myenteric ganglia

Most of the physiological information on the enteric nervous system has been obtained from studies on preparations of the myenteric ganglia attached to the longitudinal muscle layer. This preparation has a number of disadvantages, e.g., the inability to make patch-clamp recordings and the occurrence of muscle movements. To overcome these limitations we used isolated myenteric ganglia from the guinea pig small intestine. In this preparation movement was eliminated because muscle was completely absent, gigaseals were obtained, and whole cell recordings were made from neurons and glial cells. The morphological identity of cells was verified by injecting a fluorescent dye by micropipette. Neurons displayed voltage-gated inactivating inward Na(+) and Ca(2+) currents as well as delayed-rectifier K(+) currents. Immunohistochemical staining confirmed that most neurons have Na(+) channels. Neurons responded to GABA, indicating that membrane receptors were retained. Glial cells displayed hyperpolarization-induced K(+) inward currents and depolarization-induced K(+) outward currents. Glia showed large "passive" currents that were suppressed by octanol, consistent with coupling by gap junctions among these cells. These results demonstrate the advantages of isolated ganglia for studying myenteric neurons and glial cells.

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.

Immunohistochemical evidence for the presence of calbindin containing neurones in the myenteric plexus of the guinea-pig stomach

Using immunohistochemistry we studied the presence of calbindin in myenteric neurones of the guinea-pig stomach. A rabbit anti recombinant rat calbindin-D28k (CALB) stained 12, 12 and 25% of all myenteric neurones in the fundus, corpus and antrum, respectively. A rabbit anti recombinant human CALB stained 4, 4 and 16%, respectively. A mouse monoclonal antibody against the chicken intestinal CALB showed no labelling. In all regions most calbindin neurones were additionally choline acetyltransferase (ChAT) positive while only a small proportion exhibited nicotinamide adenosine dinucleatide phosphate (NADPH)-diaphorase-activity. Numerous calbindin-positive varicose nerve fibres were present within myenteric ganglia, rarely detectable in the muscle layers and virtually absent in the mucosa. This study demonstrated that a supopulation of cholinergic myenteric neurones in the stomach contain calbindin and suggested that many of these neurones fulfil interneuronal tasks.

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.

Duodenal neurons provide nicotinic fast synaptic input to sphincter of Oddi neurons in guinea pig

We have investigated the existence of neural connections between the duodenum and the sphincter of Oddi (SO). Stimulation of duodenal myenteric fiber bundles elicited synaptic responses in SO neurons, which included nicotinic fast excitatory postsynaptic potentials (EPSPs), slow EPSPs, and alpha(2)-adrenoreceptor-mediated inhibitory postsynaptic potentials. After 48 h in organ culture, when extrinsic fibers had diminished, only the fast EPSPs persisted. Duodenal mucosal stimulation also elicited nicotinic fast EPSPs in SO neurons. There was no association between the SO neurons that received duodenal input and their chemical coding. A reciprocal projection also exists from the SO to the duodenum. In acute and cultured preparations, duodenal myenteric stimulation caused antidromic responses in 20% of SO neurons. Furthermore, 45.6 +/- 10.5 neurons in SO ganglia were retrogradely labeled from dye application sites in the duodenum. It is proposed that bidirectional neural communication occurs between the duodenum and the SO and that duodenal neurons provide excitatory fast synaptic input to SO neurons through a reflex that can be activated at the duodenal mucosa.

Topographical and electrophysiological characteristics of highly excitable S neurones in the myenteric plexus of the guinea-pig ileum

1. Most intracellular electrical recordings from myenteric neurones have been made from the centre of large ganglia. In this study, we examined the electrophysiological properties of neurones at the corners of large ganglia close to internodal strands and in microganglia. 2. Of 150 neurones in these locations: 111 were tonic S neurones; 9 were phasic S neurones and 30 were AH neurones. 3. Tonic S neurones were characterized by: (i) low resting membrane potentials (-50 +/- 1 mV, mean +/- s.e.m.); (ii) high input impedance (522 +/- 23 MOmega); (iii) low threshold for action potential (AP) generation (0.012 +/- 0.004 nA); (iv) firing of APs throughout a depolarizing pulse (duration <= 1 s) and one to four APs following a hyperpolarizing pulse and (v) spontaneous fast excitatory postsynaptic potentials (FEPSPs). A substantial proportion of tonic S neurones (43 %) also fired APs spontaneously (7.6 +/- 0.6 Hz; range, 0.3-19 Hz). All APs were blocked by tetrodotoxin (1 microM). 4. Tonic S neurones were subclassified, according to their post-stimulus responses, as SAH or SAD neurones. Following a burst of APs, SAH neurones exhibited a prominent after-hyperpolarization (duration, 711 +/- 10 ms) and SAD neurones an after-depolarization (duration, 170 +/- 10 ms). The after-hyperpolarization was reduced in four of ten neurones by apamin (0.3 microM). 5. FEPSPs were evoked in 20 of 38 S neurones by electrical stimulation applied both oral and anal to the recording site. Repetitive stimuli evoked slow excitatory postsynaptic potentials (SEPSPs) in some tonic S neurones. 6. Three functional classes of S neurones were identified after injection of neurobiotin through the recording microelectrode: (i) longitudinal muscle motor neurones, (ii) short circular muscle motor neurones, and (iii) ascending interneurones. 7. In conclusion, there appears to be topographical organization of highly excitable, tonic S neurones within the myenteric plexus, since, in contrast to other S neurones, they can be readily impaled in myenteric ganglia close to internodal strands and in microganglia.

Characteristics of mucosally projecting myenteric neurones in the guinea-pig proximal colon

1. Using retrograde tracing with 1,1'-didodecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI) in combination with electrophysiological and immunohistochemical techniques we determined the properties of the putative intrinsic primary afferent myenteric neurones with mucosal projections in the guinea-pig proximal colon. 2. Eighty-four out of eighty-five DiI-labelled myenteric neurones were AH neurones with a late after-hyperpolarization. Thirty-three per cent of them exhibited atropine- and tetrodotoxin-resistant spontaneously occurring hyperpolarizing potentials (SHPs) during which the membrane resistance and excitability decreased. 3. DiI-labelled AH neurones had multipolar Dogiel type II morphology, primarily of the dendritic type. Sixty-one per cent of the neurones were immunoreactive for choline acetyltransferase (ChAT) and calbindin (Calb) and 23 % were ChAT positive but Calb negative. 4. DiI-labelled neurones did not receive fast excitatory postsynaptic potentials but 94 % (34/36) received slow excitatory postsynaptic potentials (sEPSPs). The neurokinin-3 (NK-3) agonist (MePhe7)-NKB but not the NK-1 agonist [(SAR9,Met(O2)11]-SP mimicked this response. The NK-3 receptor antagonist SR 142801 (1 microM) significantly decreased the amplitude and duration of the sEPSPs; the NK-1 receptor antagonist CP-99,994 (1 microM) was ineffective. Atropine (0.5 microM) increased the duration but not the amplitude of the sEPSPs. 5. Microejection of 100 mM sodium butyrate onto the neurones induced in 90 % of the DiI-labelled neurones a transient depolarization associated with an increased excitability. In neurones with SHPs sodium butyrate evoked, additionally, a late onset hyperpolarization. Perfusion of 0.1-10 mM sodium butyrate induced a dose-dependent increase in neuronal excitability. Sodium butyrate was ineffective when applied directly onto the mucosa. 6. Mucosally projecting myenteric neurones of the colon are multipolar AH neurones with NK-3-mediated slow EPSPs and somal butyrate sensitivity.

The enteric nervous system and regulation of intestinal motility

The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intestine. The IPANs that have been physiologically characterized are in the intrinsic myenteric ganglia. They are numerous, about 650/mm length of small intestine in the guinea pig, and communicate with each other through slow excitatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the muscle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and with ascending and descending interneurons. The circular muscle contracts as an annulus, about 2-3 mm in minimum oral-to-anal extent in the guinea pig small intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how they are programmed to generate the motility patterns that are observed.

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.

Electrical mapping of the projections of intrinsic primary afferent neurones to the mucosa of the guinea-pig small intestine

The patterns of innervation of the mucosa by axons of individual primary afferent neurones with cell bodies in the myenteric plexus were studied by mapping sites from which electrical stimulation of the mucosa elicited action potentials (APs) in their cell bodies. Segments of guinea-pig ileum were dissected to reveal the myenteric plexus over half of the intestinal circumference, leaving the mucosa intact over the other half. Intracellular recordings were taken from myenteric neurones located within 1 mm of the intact mucosa. Focal electrical stimuli were applied to the mucosa at multiple locations separated by about 1 mm. Neurones that responded had round or oval cell bodies with several long processes (Dogiel type II) and APs that had an inflection on the falling phase (AH-neurones). Responses consisted of single APs or bursts of APs. Maps of the mucosal projections of 30 neurones were generated. The maximum distances from which individual neurones responded were 7 mm circumferential and 2 mm oral or anal to the cell body with a higher proportion of responses from the oral regions. The areas of intact mucosa calculated to be innervated ranged from 1 mm2 up to approximately 15 mm2 (mean 3.9 mm2; median 2.5 mm2). It is estimated that the areas innervated would be two to three times larger under conditions where part of the mucosa is not removed. Some neurones also responded to a chemical or a mechanical stimulus applied to the mucosa within the electrically mapped area. It is concluded that intrinsic primary afferent neurones have overlapping receptive fields with 230-350 neurones innervating the same region of mucosa.

Functional alterations in jejunal myenteric neurons during inflammation in nematode-infected guinea pigs

Intracellular recordings of jejunal myenteric neurons with an afterspike hyperpolarization (AH) from Trichinella spiralis-infected animals showed enhanced excitability on days 3, 6, and 10 postinfection (PI) compared with uninfected animals. Lower membrane potential, increased membrane input resistance, decreased threshold for action potential discharge, decreased AH amplitude and duration, and increased fast excitatory postsynaptic potential amplitude and duration were characteristic of neuronal recordings from infected animals. Concurrent with electrophysiological changes during T. spiralis infection, increased cytochrome oxidase activity, a marker of neuronal metabolic activity, and the expression of nuclear c-Fos immunoreactivity, an indicator of transcriptional-translational activity, were also observed in myenteric ganglion cells. Double-labeling for calbindin-immunoreactive myenteric neurons revealed that approximately 50% of these neurons also expressed increased c-Fos immunoreactivity during T. spiralis infection. Myeloperoxidase activity was significantly higher in the jejunum of T. spiralis-infected guinea pigs on days 3, 6, and 10 PI vs. uninfected counterparts. The expression of c-Fos in calbindin-immunoreactive neurons together with enhanced neuronal electrical and metabolic activity during nematode-induced intestinal inflammation suggests the onset of excitation-transcription coupled changes in enteric neural microcircuits.

Duodenal sensory neurons project to sphincter of Oddi ganglia in guinea pig

Retrograde labeling of duodenum-sphincter of Oddi (SO) preparations in vitro with the carbocyanine dye DiI revealed that duodenal neurons project to the SO. The duodenum-SO-projecting neurons were immunoreactive (IR) for choline acetyltransferase but not nitric oxide synthase or calretinin, indicating that this is a cholinergic projection and that this pathway is distinct from the circuitry involved in the ascending limb of the peristaltic reflex. Approximately 20% of the duodenum-SO projection neurons were IR for calbindin. Calbindin-IR nerves within SO ganglia degenerated when the SO was maintained in organ culture alone, but persisted when the SO was cultured with the duodenum intact. Therefore, SO ganglia are a target of the calbindin-positive duodenum-SO projection. Because calbindin is a marker of intrinsic sensory neurons that have processes that pass to the mucosa, these neurons are in position to detect the release of a compound from the mucosa and signal its release to SO ganglia. When applied to retrogradely labeled neurons, cholecystokinin (CCK) elicited a prolonged depolarization, indicating that duodenum-SO-projecting neurons could be capable of detecting CCK released from the mucosa. It is proposed that the role of the intrinsic sensory neurons that project to the SO may be to signal the postprandial release of CCK, thus providing an instruction to decrease SO resistance and facilitate the flow of bile into the duodenum.

Identification of neurons that express stem cell factor in the mouse small intestine

BACKGROUND & AIMS

Enteric neurons in the murine intestine express stem cell factor (SCF), which may provide an important signal in the development of the interstitial cells of Cajal (ICC). Our aim was to identify the subpopulation(s) of myenteric neurons that express SCF.

METHODS

Myenteric plexus preparations from postnatal SCF-lacZ mice were processed for beta-galactosidase histochemistry followed by immunohistochemistry.

RESULTS

Approximately 60% of the nitric oxide synthase-immunoreactive neurons, which projected to myenteric ganglia and to circular muscle, expressed SCF, and more than 80% of the calbindin-immunoreactive neurons, which projected exclusively to myenteric ganglia, expressed SCF. A small subpopulation of calretinin-immunoreactive neurons expressed SCF transiently. Many of the remainder of SCF-expressing neurons were choline acetyltransferase immunoreactive, but their projections are unknown.

CONCLUSIONS

SCF-expressing neurons that project within the myenteric plexus may be an important source of SCF for the development of Kit-expressing ICC at this level. The only possible neuronal source of SCF for the ICC of the deep muscular plexus is a subpopulation of nitric oxide synthase-immunoreactive neurons.

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.

Mediation by protein kinases C and A of Go-linked slow responses of enteric neurons to 5-HT

5-HT activates the peristaltic reflex and is the neurotransmitter of a subset of myenteric interneurons. Hyperpolarizing afterpotential (AH)/type 2 neurons respond to 5-HT with a long-lived depolarization that is caused by the inhibition of a Ca(2+)-activated K+ conductance (gKCa). This effect is mediated by a G-protein-coupled receptor, 5-HT1P. 5-HT1P agonists specifically activate G alpha o, the immunoreactivity of which was found to be highly abundant and membrane-associated in almost all enteric neurons. Responses of hyperpolarizing AH/type 2 neurons to 5-HT were inhibited by intracellular injection of GDP beta S or anti-G alpha o Fab fragments but were potentiated and prolonged by intracellular GTP gamma S. Responses to 5-HT were antagonized by pertussis toxin, downregulation of protein kinase C (PKC) and inhibitors of phosphatidylcholine phospholipase C (PC-PLC), PKC (including pseudosubstrate peptides, chelerythrine, and the alpha/beta isoform-specific inhibitor Go 6976), protein kinase A (PKA), and adenylate cyclase. Responses to 5-HT were mimicked by activators of PKC, and 5-HT induced a concentration-dependent increase in the membrane-associated PKC activity in isolated myenteric ganglia. Immunocytochemical studies suggested that the most abundant isoforms of PKC in enteric neurons are alpha and delta. These data suggest that signal transduction of the 5-HT1P-mediated slow response to 5-HT involves activation of PC-PLC by G alpha o to liberate diacylglycerol, which stimulates PKC (most likely alpha). PKC probably activates adenylate cyclase, which through cAMP, activates PKA. Activation of both PKA and PKC lead to closure of gKCa.

Projections of chemically identified myenteric neurons of the small and large intestine of the mouse

The projections of different subpopulations of myenteric neurons in the mouse small and large intestine were examined by combining immunohistological techniques with myotomy and myectomy operations. The myotomies were used to examine the polarity of neurons projecting within the myenteric plexus and showed that neurons containing immunoreactivity for nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), calbindin and 5-HT projected anally, while neurons with substance P (SP)-immunoreactivity projected orally, in both the small and large intestine. Neurons containing neuropeptide Y (NPY)- and calretinin-immunoreactivity projected locally. In the large intestine, GABA-immunoreactive neurons projected both orally and anally, with more axons tending to project anally. Myectomy operations revealed that circular muscle motor neurons containing NOS/VIP/ +/-NPY and calretinin neurons projected anally both in the small and large intestine, while SP-immunoreactive circular muscle motor neurons projected orally. In the large intestine, GABA-IR circular muscle motor neurons projected both orally and anally. This study showed that although some neurons, such as the NOS/VP inhibitory motor neurons and interneurons, SP excitatory motor neurons and 5-HT interneurons had similar projections to those in other species, the projections of other chemical classes of neurons in the mouse intestine differed from those reported in other species.

Influence of the mucosa on the excitability of myenteric neurons

Intracellular microelectrodes were used to examine the active and passive membrane properties of neurons in the myenteric plexus of the guinea-pig small intestine. Neurons of two types were examined: S neurons, which have prominent fast excitatory postsynaptic potentials and in which action potentials are not followed by long-lasting afterhyperpolarizations, and AH neurons, which have long-lasting afterhyperpolarizations following soma action potentials. In preparations in which the myenteric ganglia and longitudinal muscle, but no mucosa, were present, most S neurons (59/64) responded to intracellular depolarizing current with brief bursts of action potentials. Regardless of the strength of a depolarizing current of 500-ms duration, these neurons never fired action potentials beyond the first 250 ms. S neurons in this state were called rapidly accommodating. In contrast, within 600 microm circumferential to the intact mucosa, 26/58 S neurons fired action potentials for most or all of the period of a 500-ms insightful depolarizing pulse. S neurons in this state were called slowly accommodating. Depolarization of S neurons in the rapidly accommodating state caused a rapidly developing reduction in membrane resistance (outward rectification; onset about 7 ms). This rectification was absent from S neurons in the slowly accommodating state. Tetraethylammonium blocked the early rectification and the changed neuronal state from rapidly accommodating to slowly accommodating. Application of tetrodotoxin to neurons in the slowly accommodating state revealed the early rectification, indicating that its absence from these neurons before tetrodotoxin was applied had been due to ongoing activity in axons providing synaptic input to the neurons. After the mucosa was disconnected from the other layers and laid back in its original position, all S neurons close to the mucosa were in the rapidly accommodating state (17/17). Slow excitatory postsynaptic potentials, evoked by electrical stimulation of nerve tracts, converted 17 of 43 S neurons from rapidly accommodating to slowly accommodating and eliminated the early outward rectification in these neurons. These results indicate that the action potential firing properties of S neurons can be changed by external influences, including the activity of synaptic inputs that release a slowly acting transmitter. Spontaneous antidromic action potentials were recorded in 8/62 AH neurons within 600 microm circumferential to the intact mucosa. It is concluded that, when the mucosa is intact, a background firing of sensory neurons occurs which leads to a state change in many S neurons innervated by the active sensory neurons. We conclude that this state change is caused by the block of a voltage-sensitive outward rectification.

Neurochemical classification of myenteric neurons in the guinea-pig ileum

A strategy has been developed to identify and quantify the different neurochemical populations of myenteric neurons in the guinea-pig ileum using double-labelling fluorescence immunohistochemistry of whole-mount preparations. First, six histochemical markers were used to identify exclusive, non-overlapping populations of nerve cell bodies. They included immunoreactivity for the calcium binding proteins calbindin and calretinin, the neuropeptides vasoactive intestinal polypeptide, substance P and somatostatin, and the amine, 5-hydroxytryptamine. The sizes of these populations of neurons were established directly or indirectly in double-labelling experiments using a marker for all nerve cell bodies. Each of these exclusive populations was further subdivided into classes by other markers, including immunoreactivity for enkephalins and neurofilament protein triplet. The size of each class was then established directly or by calculation. These distinct, neurochemically-identified classes were related to other published work on the histochemistry, electrophysiology and retrograde labelling of enteric neurons and to the simple Dogiel morphological classification. A classification scheme, consistent with previous studies, is proposed. It includes 14 distinct classes of myenteric neurons and accounts for nearly all neurons in the myenteric plexus of the guinea-pig ileum.

Myenteric neurons with different projections have different dendritic tree patterns: a morphometric study in the pig ileum

Morphometric analysis was performed of two types of myenteric neurons with at first glance, very similar morphology, but different projections in the pig ileum. 50 type IV cells projecting vertically to the outer submucosal plexus, and 50 solitary type V neurons projecting aborally, within the myenteric plexus, were evaluated. Using a computer-aided morphometric device, the following parameters were recorded: somal area, longest somal diameter, number of primary dendrites, dendritic length, number of dendritic branching points and number of terminal segments. In addition to other significant differences, the most prominent discriminating parameter between the two populations of nerve cells estimated in this study was the length of the longest dendrite of each cell type. The longest dendrite of an individual type V neuron is a manifold longer than the corresponding somal diameter, in contrast to type IV neurons where it is at most twice the somal diameter. In addition, all type IV and type V single dendritic lengths were arranged in histogramms, where the type V dendrites showed two frequency peaks. Thus, we assume that solitary type V neurons can develop two populations of dendrites: a short and a long one. These results demonstrate that myenteric neurons with different projections (and hence different functions) display strikingly different dendritic morphology.

Experimental basis for realistic large-scale computer simulation of the enteric nervous system

1. The enteric nervous system is perhaps the most accessible part of the mammalian nervous system in which it is feasible to attempt large scale computer simulation that is based closely on experimentally determined data. Here we summarize the data obtained for simulation of motility reflexes in the guinea-pig small intestine. 2. The chemistry, morphology and connectivity of each type of neuron involved in intrinsic reflexes have been investigated and most classes of neurons are physiologically well characterized. This includes primary sensory neurons, ascending and descending interneurons and motor neurons to circular and longitudinal muscle. 3. The responses of primary sensory neurons and the physiology of synaptic transmission from sensory neurons to interneurons and motor neurons, from interneurons to interneurons and from interneurons to motor neurons have been recorded during reflexes and in some cases the pharmacology of transmission has also been investigated. 4. Computer simulation, in which the activities of up to 30,000 neurons are modelled, produces patterns of activity that closely mimic those recorded in physiological experiments.

In situ identification and visualization of neurons that mediate enteric and enteropancreatic reflexes

To identify neurons participating in enteric and enteropancreatic reflexes, we validated the use of the activity-dependent markers FM1-43 and FM2-10 as "on-line" probes for the visualization of activated guinea pig enteric and pancreatic neurons. FM1-43 or FM2-10 labeling of neuronal perikarya and processes was induced by KCl (70 mM), veratridine (1.0 microM), intracellular injection of depolarizing current pulses, stimulation of afferent inputs, evoking reflexes (by inflating an intraluminal balloon, blowing puffs of N2 at, or applying glucose to, the villous surface of the duodenum), or injury; labeling was prevented by tetrodotoxin (0.5 microM). Intracellular recording and injection of Neurobiotin confirmed that FM1-43 labeled neurons that spike, but not those that exhibit only fast excitatory postsynaptic potentials. Perikarya did not label if axonal transport was blocked by colchicine. When pulses of N2 or glucose were directed at duodenal villi in vitro, labeling by FM1-43 or FM2-10 was observed in myenteric and pancreatic neurons, as well as in subsets of cells in pancreatic islets and intestinal crypts. Hexamethonium blocked the spread of label via nicotinic synapses and thus enabled primary afferent neurons to be located. Balloon distension elicited hexamethonium-resistant labeling of epithelial cells, interstitial cells, and Dogiel type II neurons in each plexus; however, in preparations stimulated with pulses of N2 or glucose, hexamethonium-resistant labeling of neurons occurred only in the submucosal plexus and not in myenteric ganglia. These observations suggest that primary afferent neurons responsible for mucosal pressure- or glucose-induced enteric and enteropancreatic reflexes are submucosal, whereas myenteric afferent neurons become activated only when the wall of the bowel is distended. The data are compatible with the possibility that primary afferent neurons are activated by a signaling molecule released from intestinal epithelial cells.

Responsiveness to ATP with an increase in intracellular free Ca2+ is not a distinctive feature of calbindin-D28 immunoreactive neurons in myenteric ganglia

The aim of this study was to test the hypothesis that ATP elevates cytosolic free Ca2+ levels ([Ca2+]i) in myenteric neurons expressing the Ca2+ binding protein, calbindin-D28. A laser microbeam marked the location of cultured neurons on coverslips and provided unequivocal relocation of ATP-responsive neurons after immunocytochemistry. All myenteric multipolar neurons displayed ATP Ca2+ transients, and 42% also expressed calbindin-D28 reactivity. Statistical analysis of the kinetics and shape of ATP Ca2+ transients revealed no differences between calbindin and non-calbindin neurons. The identity of other responsive neurons is unknown. Less than 8% of ganglion cells with ATP Ca2+ transients were immunopositive for the glial protein S-100. We conclude that one of the actions of ATP in myenteric ganglia is to increase [Ca2+]i which may activate gKCa leading to membrane hyperpolarization in AH, Dogiel Type II neurons expressing calbindin-D28. An efficient buffering mechanism for handling large purinergic Ca2+ loads is a common feature of all types of myenteric ganglion cells.

Regional differences in the number of neurons in the myenteric plexus of the guinea pig small intestine and colon: an evaluation of markers used to count neurons

BACKGROUND

Subsets of myenteric neurons have been identified. To determine the proportional representation of neurons in each, it is necessary to relate the number of neurons in the subset to that of the complete set. Prior estimates of total numbers of neurons, obtained with many different markers, have varied widely.

METHODS

Markers were compared for counting myenteric neurons in dissected laminar preparations of guinea pig duodenum, jejunum-ileum, and colon; the effect of stretching preparations on these counts was also determined. Markers included the visualization of single-stranded nucleic acid with cuprolinic blue and the immunocytochemical demonstration of neuron specific enolase (NSE), PGP9.5, S-100, and the constitutive expression of a Fos related antigen (FRA).

RESULTS

Neurons could not be counted accurately by demonstrating NSE, PGP9.5, or S-100. The number of neurons detected by demonstrating FRA was consistently less than that determined with cuprolinic blue (approximately 65%). Cuprolinic blue-derived estimates of neuron numbers were higher than most reported in the literature, but comparable to those recently obtained with "a nerve cell body" antiserum. Ganglionic area was found to be stretch independent. The rank order of neurons/cm2 and ganglionic area/ unit resting length was colon > duodenum >> jejunum-ileum; more neurons were found in the myenteric plexus of the colon (7.3 x 10(6)) than in that of the entire small intestine (6.5 x 10(6)).

CONCLUSIONS

Prior studies that have obtained denominators for estimating the proportions of myenteric neuronal subsets with markers that do not reveal the entire population should be re-evaluated. The guinea pig colon contains a surprisingly large number of neurons, the physiological significance of which must be determined.

Identification of cells that express 5-hydroxytryptamine1A receptors in the nervous systems of the bowel and pancreas

Although serotonin (5-HT)1A receptors are known to be present on neural elements in both the bowel and the pancreas, the precise location of these receptors has not previously been determined. Earlier investigations have suggested that 5-HT1A receptors are synthesized in enteric, but not pancreatic ganglia, and that they mediate pre-and postjunctional inhibition. Wholemount in situ hybridization was used to identify cells that contain mRNA encoding 5-HT1A receptors, and immunocytochemistry was employed to locate receptor protein. mRNA encoding 5-HT1A receptors was found in the majority of neurons in both submucosal and myenteric plexuses. 5-HT1A immunoreactivity, however, was abundant only on the surfaces of a limited subset of nerve cell bodies and processes. 5-HT-immunoreactive axons were found in close proximity to sites of 5-HT1A immunoreactivity. Myenteric, but not submucosal calbindin-immunoreactive neurons (with Dogiel type II morphology) were surrounded by rings of 5-HT1A immunoreactivity. The cytoplasm of the cell bodies and dendrites of a small subset of Dogiel type I neurons was also intensely 5-HT1A immunoreactive. Most of the Dogiel type I 5-HT1A-immunoreactive myenteric neurons, and some of the type II neurons that were ringed by 5-HT1A immunoreactivity became doubly labeled following injections of the retrograde tracer, FluoroGold (FG), into the submucosal plexus. 5-HT1A-immunoreactive neurons in distant submucosal ganglia also became labeled by retrograde transport of FG. None of the 5-HT1A-immunoreactive cells were labeled by the intraluminal administration of the beta-subunit of cholera toxin, a marker for vasoactive intestinal peptide-containing secretomotor neurons. These observations suggest that some of the myenteric 5-HT1A-immunoreactive neurons project to submucosal ganglia and that the submucosal 5-HT1A-immunoreactive cells are interneurons. In addition to neurons, a subset of 5-HT-containing enterochromaffin cells expressed 5-HT1A immunoreactivity, which was co-localized with 5-HT in secretory granules. In the pancreas, 5-HT1A immunoreactivity was observed in ganglia, acinar nerves, and glucagonimmunoreactive islet cells. Serotonergic enteropancreatic axons have been found to terminate in close proximity to each of these structures, which may thus be the targets of this innervation. The abundance of 5-HT1A receptor immunoreactivity on nerves of the gut and pancreas suggests that drugs designed to interact with these receptors may have unanticipated visceral actions.

Sources of inputs to longitudinal muscle motor neurons and ascending interneurons in the guinea-pig small intestine

Light- and electron-microscopic studies were used to investigate connections between specific subgroups of neurons in the myenteric plexus of the guinea-pig small intestine. Inputs to two classes of calretinin-immunoreactive (IR) nerve cells, longitudinal muscle motor neurons and ascending interneurons, were examined. Inputs from calbindin-IR primary sensory neurons and from three classes of descending interneurons were studied. Electron-microscopic analysis showed that calbindin-IR axons formed two types of inputs, synapses and close contacts, on calretinin-IR neurons. About 40% of inputs to the longitudinal muscle motor neurons and 70% to ascending interneurons were calbindin-IR. Approximately 50% of longitudinal muscle motor neurons were surrounded by bombesin-IR dense pericellular baskets and 40% by closely apposed varicosities. At the electron-microscope level, the bombesin-IR varicosities were found to form synapses and close contacts with the motor neurons. Dense pericellular baskets with bombesin-IR surrounded 36% of all ascending interneurons, and a further 17% had closely apposed varicosities. Somatostatin- and 5-HT-IR descending interneurons provided no dense pericellular baskets to calretinin-IR nerve cells. Thus, calretinin-IR, longitudinal muscle motor neurons and ascending interneurons receive direct synaptic inputs from intrinsic primary sensory neurons and from non-cholinergic, bombesin-IR, descending interneurons.

Neurochemical coding of enteric neurons in the guinea pig stomach

The aim of this study was to investigate the neurochemical coding of myenteric neurons in the guinea pig gastric corpus by using immunohistochemical methods. Antibodies and antisera against calbindin (CALB), calretinin (CALRET), choline acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP), dopamine beta-hydroxylase (DBH), beta-endorphin (ENK), neuropeptide Y (NPY), neuron-specific enolase (NSE), nitric oxide synthase (NOS), protein gene product 9.5 (PGP), parvalbumin (PARV), serotonin (5-HT), somatostatin (SOM), substance P (SP), tyrosine hydroxylase (TH), and vasoactive intestinal peptide (VIP) were used. Double- and triple-labeling studies revealed colocalization of certain transmitters and enabled the identification of distinct subpopulations of gastric enteric neurons. NPY/VIP/NOS/ENK were present in 28% of all neurons, whereas 11% had NPY/VIP/DBH/ChAT; NOS-only neurons made up 2% of the population. The combination SP/ChAT/ENK occurred in 21% of the population, whereas SP/ChAT/ENK/CALRET and SP/CHAT/SOM/ +/- CALRET was identified in 5% and 6% of all cells, respectively. 5-HT-containing neurons comprised 2% of all cells and could be further classified by the presence of additional antigens as 5-HT/SP/(ChAT) or 5-HT/VIP/(ChAT). Approximately 21% of all neurons contained only ChAT with no additional antigen present and are referred to as ChAT/-. Gastric myenteric ganglion cells were not immunoreactive for CALB, PARV, CGRP, or TH. The results of this study indicate that gastric myenteric neurons can be characterized on the basis of different chemical coding. Neurochemical coding of corpus myenteric neurons revealed some similarities and significant differences in comparison with other regions of the gut. These differences might reflect adaptation of enteric nerves according to regional specialization and the distinct functions of the proximal stomach as a gastric reservoir.

Projections of neurochemically specified neurons in the porcine colon

The intramural projections of nerve cells containing serotonin (5-HT), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and nitric oxide synthase or reduced nicotinamide adenine dinucleotide phosphate diaphorase (NOS/NADPHd) were studied in the ascending colon of 5- to 6-week-old pigs by means of immunocytochemistry and histochemistry in combination with myectomy experiments. In control tissue of untreated animals, positive nerve cells and fibres were common in the myenteric and outer submucous plexus and, except for 5-HT-positive perikarya, immunoreactive cell bodies and fibres were also observed in the inner submucous plexus. VIP- and NOS/NADPHd-positive nerve fibres occurred in the ciruclar muscle layer while VIP was also abundant in nerve fibres of the mucosal layer. 5-HT- and CGRP-positive nerve fibres were virtually absent from the aganglionic nerve networks. In the submucosal layer, numerous paravascular CGRP-immunoreactive (IR) nerve fibres were encountered. Myectomy studies revealed that 5-HT-, CGRP-, VIP- and NOS/NADPHd-positive myenteric neurons all displayed anal projections within the myenteric plexus. In addition, some of the serotonergic myenteric neurons projected anally to the outer submucous plexus, whereas a great number of the VIP-ergic and nitrergic myenteric neurons send their axons towards the circular muscle layer. The possible function of these nerve cells in descending nerve pathways in the porcine colon is discussed in relation to the distribution pattern of their perikarya and processes and some of their morphological characteristics.

All calbindin-immunoreactive myenteric neurons project to the mucosa of the guinea-pig small intestine

The projections of Dogiel type II myenteric neurons to the mucosa of the guinea-pig ileum were quantified by combining retrograde transport of DiI, in vitro, with immunohistochemistry. After DiI application to the mucosa over an area of 1.5 x 10 mm2, virtually all (> 97%) calbindin-immunoreactive Dogiel type II neurons in the myenteric plexus underneath the mucosal DiI application site were labelled, indicating that essentially all of these neurons project to the mucosa. From cell counts, on average 5 calbindin-immunoreactive neurons project to each villus, and each calbindin-immunoreactive neuron supplies on average 10 villi. Since Dogiel type II neurons that were not immunoreactive for calbindin (19% of all labelled nerve cells) also projected to the mucosa, it is likely that all Dogiel type II neurons, which are putative sensory neurons of the gut, project to the mucosa.

Progressive reorganization of the myenteric plexus during one year following reanastomosis of the ileum of the guinea pig

The enteric nervous system appears to play a pivotal role in the functional recovery of the gastrointestinal tract after partial resection and reanastomosis, but the structural changes following surgery are not fully understood. The present study was designed to clarify the processes of myenteric plexus regeneration up to one year after transection and reanastomosis of the ileum of the guinea pig. The following techniques were used: nicotinamide adenine dinucleotide (NADH) diaphorase histochemistry, immunostaining of neuron-specific enolase (NSE) in whole-mount preparations, and transmission electron microscopy. Two months after transection and reanastomosis, myenteric ganglion cells with NADH diaphorase reactions were scarce in the center of the lesion, and were less numerous in adjacent areas (3 mm in width) than in the control ileum. In the areas adjacent to the lesion, a few large extraganglionic neurons that did not completely compensate for the loss of ganglion neurons were observed. The remaining ileum showed no changes in NADH diaphorase staining pattern at this stage. Two to 12 months after transection and reanastomosis, ectopic large neurons gradually increased in number not only in the areas adjacent to the lesion but also in part of the remaining ileum, up to 10 cm from the lesion. Concomitantly, large ganglion neurons decreased in number in these areas. In other ileal regions (more than 10 cm distant from the site of transection), no obvious changes in NADH diaphorase staining were noted throughout the observation period. The outgrowth of NSE-containing nerve fibers from the severed stumps was seen two weeks after transection. Six weeks later, numerous bundles of fine nerve fibers with NSE were shown to interconnect the oral and anal cut ends of the myenteric plexus, but they exhibited no subsequent alterations. Transmission electron microscopy revealed that regenerating nerve fiber bundles appeared initially among irregularly arranged smooth muscle cells eight weeks after the operation, as expected from light-microscopic observations. These findings suggest that myenteric ganglion cell bodies, unlike myenteric nerve fibers, require a longer term of reconstruction than previously believed after transection and reanastomosis of the ileum of the guinea pig.

The use of constitutive nuclear oncoproteins to count neurons in the enteric nervous system of the guinea pig

Immunohistochemical double labelling of the enteric nervous system of the guinea pig ileum was performed with a monoclonal antibody (anti-MYC 033) directed against a peptide sequence of the human c-Myc protein together with antibodies directed against either the neuron-specific antigens neuron-specific enolase or PGP 9.5 or the glia-specific marker S-100 to demonstrate that anti-MYC 033 labelled the nuclei of all enteric neurons but not glia. This strategy was also employed to demonstrate that another anti-c-Myc monoclonal antibody, anti-MYC 070, labelled the nuclei of all neurons and glia, as well as perhaps all other cells in these preparations. A polyclonal antiserum raised against a peptide sequence of the human c-Fos protein (anti-FOS 4) was shown to label the identical nuclei as anti-MYC 033. The ganglionic density of nuclei labelled by anti-FOS 4 was found to be similar to previous measures of the ganglionic density of neurons. Double labelling with anti-MYC 033 and an antiserum directed against vasoactive intestinal polypeptide was performed to reexamine the ganglionic density of neurons that express this neuropeptide. Our results suggest that the ganglionic density of these neurons might be less than previously determined.

Quantitative study of the NADPH-diaphorase-positive myenteric neurons of the rat ileum

The subpopulation of myenteric neurons able to synthesize nitric oxide was studied quantitatively in the adult rat, using the NADPH-diaphorase histochemical method on whole-mount preparations of distended distal ileum. The spatial density of NADPH-diaphorase-positive myenteric neurons was 2388 +/- 193/cm2 (S.D.; five rats), comprising about 27% of the nerve cell bodies per ganglion. Most neurons were intensely stained and displayed predominantly a Dogiel type I morphology; about 8% of the labelled nerve cells were ovoid neurons, exhibiting a pale cytoplasmic reaction product. The mean somatic size of all NADPH-diaphorase-positive myenteric neurons was 446 +/- 40 microns2, with a mean nuclear size of 96 +/- 18 microns2 (mean values +/- S.D.; five rats). Such values fell exactly within the range of neuronal sizes of the total myenteric population, marked by means of NADH-diaphorase histochemistry. Therefore, the morphometric analysis did not identify any peculiar cell size feature, characterizing this specific nerve cell subpopulation. Thus, the present study provides quantitative data on the size, density and proportion of those myenteric neurons that may synthesize nitric oxide in the distal ileum of the rat.

Local neural control of intestinal motility: nerve circuits deduced for the guinea-pig small intestine

1. Propulsion of digesta along the intestine appears to occur by the action of a series of local reflexes which cause contraction oral to the digesta and relaxation of circular muscle on the anal side. 2. There is now substantial evidence available about the identities of the enteric neurons that mediate these reflexes. 3. The motor neurons and interneurons of the reflex pathways lie within the myenteric plexus. These neurons can be classified electrophysiologically as S-neurons and have distinctive projections and neurochemistries. 4. The sensory neurons may lie in the myenteric plexus, but there is some evidence for sensory neurons in the submucous plexus. A contribution from extrinsic sensory neurons to local motility reflexes cannot be ruled out. Intrinsic sensory neurons are probably AH-neurons and are large multi-axonal cells.

Calretinin-immunoreactive neurons and their projections in the guinea-pig colon

The distribution of nerve cells and fibres with immunoreactivity for the calcium-binding protein, calretinin, was studied in the distal colon of the guinea-pig. The projections of the neurons were determined by examining the consequences of lesioning the myenteric plexus. Calretinin-immunoreactive neurons comprised 17% of myenteric nerve cells and 6% of submucous nerve cells. Numerous calretinin-immunoreactive nerve fibres were located in the longitudinal and circular muscle, and within the ganglia of the myenteric and submucous plexuses. Occasional fibres were found in the muscularis mucosae, but they were very rare in the lamina propria of the mucosa. Lesion studies revealed that myenteric neurons innervated the underlying circular muscle and provided both ascending and descending processes that gave rise to varicose branches in myenteric ganglia. Calretinin-immunoreactive fibres also projected to the tertiary component of the myenteric plexus, and are therefore likely to be motor neurons to the longitudinal muscle. Varicose fibres that supplied the submucous ganglia appear to arise from submucous nerve cells. Arterioles of the submucous plexus were sparsely innervated by calretinin-immunoreactive fibres. The submucous plexus was the principal source of immunoreactive nerve fibres in the muscularis mucosae. This work shows that calretinin-IR reveals different neuronal populations in the large intestine to those previously reported in the small intestine.

Electrophysiological and morphological classification of myenteric neurons in the proximal colon of the guinea-pig

Intracellular recordings were made from myenteric neurons in the proximal colon of the guinea-pig. The electrical behaviour of the neurons in response to intracellular depolarizing current pulses, and to internodal strand stimulation, was recorded. The intracellular electrode contained the intracellular marker biocytin which was injected into impaled neurons for subsequent histochemistry. Proximal colon myenteric neurons displayed electrophysiological properties similar to myenteric neurons in the small intestine, and were classified as either AH- or S-neurons. AH-neurons were characterized by the presence of a slow afterhyperpolarization following an action potential. Internodal strand stimulation evoked slow excitatory synaptic potentials in five out of six AH-neurons tested, but did not evoke fast excitatory synaptic potentials in 26 AH-neurons tested. S-neurons lacked a slow afterhyperpolarization, but internodal strand stimulation evoked fast excitatory synaptic potentials in all 113 neurons and slow excitatory synaptic potentials in seven out of 17 tested. A subpopulation of AH-neurons displayed a rhythmic oscillation in membrane potential which could be triggered by an action potential. S-neurons could be subdivided into those that fired tonically and those that fired phasically in response to long depolarizing current pulses. About 80% of the AH-neurons were immunoreactive for calbindin, as were 10% of S-neurons. A further 17% of S-neurons, but no AH neurons, were calretinin immunoreactive. Morphological analysis of filled neurons revealed eight distinct classes. Neurons electrophysiologically classified as AH typically had a large, oval soma and several long tapering processes. Processes of AH-neurons branched into many adjacent ganglia. Almost all S-neurons were uniaxonal and many axons ended in an expansion bulb in the myenteric plexus. S-neurons typically had broad, lamellar processes, or short, spiny processes. Roughly equal proportions of S-neurons had oral or anal projection. However, almost all S-neurons that were immunoreactive for calbindin or calretinin projected orally. The results indicate that myenteric neurons in the proximal colon of the guinea-pig are electrophysiologically similar to myenteric neurons in the small intestine, but there are a greater number of morphological and chemical categories.

Accommodation mediated by enteric inhibitory reflexes in the isolated guinea-pig small intestine

1. The aim of the present study was to investigate whether the guinea-pig small intestine shows accommodation to infused fluid, similarly to other regions of the gastrointestinal tract. Tetrodotoxin, papaverine and transmitter antagonists were used to establish the existence of reflex pathways and the nature of the neurotransmitters involved. 2. Compliance, measured as the change in volume of infused fluid divided by the intraluminal pressure change, was reduced by tetrodotoxin (0.6 microM), indicating that there is an overall neurally mediated relaxation of the circular muscle in response to low rates of distension. Papaverine (10 microM) did not have any significant effect on compliance at the low rates of distension, suggesting that the circular muscle is fully relaxed. 3. At each rate of distension, 400 microM N omega-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor) significantly decreased the compliance of the intestinal wall, indicating that the circular muscle was relaxed by a nitric oxide-mediated mechanism. Apamin (0.5 microM), which blocks a component of inhibitory transmission, did not have a significant effect. 4. In control preparations, the intestinal wall was less compliant when distended by fluid at a fast rate, compared with the lower rates of distension. This was not due to changes in passive components of the intestinal wall or a myogenic response to rapid stretch. 5. When the intestine was distended rapidly, 1 microM hyoscine and 100 microM hexamethonium increased intestinal compliance. However, they had no detectable effect on compliance with low rates of distension.(ABSTRACT TRUNCATED AT 250 WORDS)

Activity following colonic distension in enteric sensory fibres projecting to the inferior mesenteric ganglion in the guinea pig

In this investigation the characteristics of the response to colonic distension of afferent fibres projecting to the inferior mesenteric ganglion (IMG) of the guinea pig were studied in vitro. Intracellular membrane potential recordings were made from neurons in the IMG. Post-synaptic potentials arising from activity in afferent fibres were recorded in these cells following distensions of a small segment of colon placed in a separate and independently perfused organ bath. These afferent fibres showed both transient and sustained responses to distension. Application of a solution containing 0.25 mM Ca2+ and 10 mM2+ Mg2+ to the colon (but not to the IMG) reduced the overall response but did not abolish the activity in these fibres. It is concluded that some afferent fibres which respond to colonic distension project directly to the IMG.

Combined intracellular injection of Neurobiotin and pre-embedding immunocytochemistry using silver-intensified gold probes in myenteric neurons

We have developed methods to examine the neurochemistry of synaptic inputs to individual myenteric neurons labeled by dye injection through intracellular recording electrodes. Myenteric neurons of the guinea-pig ileum were filled with Neurobiotin, fixed, washed in 50% ethanol, exposed to sodium cyanoborohydride, incubated in avidin-biotin-horseradish peroxidase and incubated in antisera to calretinin or calbindin. The Neurobiotin-filled cells were revealed using the diaminobenzidine (DAB) reaction. The tissue was examined at the light microscope level to determine the morphology and projections of the Neurobiotin-filled neurons, and then incubated in 1 nm gold-labeled secondary antibodies. Following osmication, the gold probes were silver-intensified and the tissue embedded flat in resin. The tissue was re-examined at the light microscope level. Neurons containing a DAB reaction product could be distinguished from neurons containing a silver-intensified gold reaction product using oblique or epipolarized illumination. Ultrathin sections were taken through the injected neurons and examined. At the ultrastructural level, Neurobiotin-filled cell bodies and their processes (labeled with DAB) were easily distinguished from the structures labeled by silver-intensified gold. Gold-labeled terminals of enteric interneurons made synapses and close contacts with Neurobiotin-filled nerve cell bodies and their processes. This technique is valuable for the neurochemical identification of synaptic inputs to morphologically and/or functionally characterized myenteric neurons and could be easily applied to other preparations, such as brain slices.