Neurochemical coding of myenteric neurons in the guinea-pig antrum

Mechanosensitive enteric neurons in the guinea pig gastric fundus and antrum

BACKGROUND

Coping with the ingested food, the gastric regions of fundus, corpus, and antrum display different motility patterns. Intrinsic components of such patterns involving mechanosensitive enteric neurons (MEN) have been described in the guinea pig gastric corpus but are poorly understood in the fundus and antrum.

METHODS

To elucidate mechanosensitive properties of myenteric neurons in the gastric fundus and antrum, membrane potential imaging using Di-8-ANEPPS was applied. A small-volume injection led to neuronal compression. We analyzed the number of MEN and their firing frequency in addition to the involvement of selected mechanoreceptors. To characterize the neurochemical phenotype of MEN, we performed immunohistochemistry.

KEY RESULTS

In the gastric fundus, 16% of the neurons reproducibly responded to mechanical stimulation and thus were MEN. Of those, 83% were cholinergic and 19% nitrergic. In the antrum, 6% of the neurons responded to the compression stimulus, equally distributed among cholinergic and nitrergic MEN. Defunctionalizing the sensory extrinsic afferents led to a significant drop in the number of MEN in both regions.

CONCLUSION

We provided evidence for MEN in the gastric fundus and antrum and further investigated mechanoreceptors. However, the proportions of the chemical phenotypes of the MEN differed significantly between both regions. Further investigations of synaptic connections of MEN are crucial to understand the hardwired neuronal circuits in the stomach.

Topographical organization and morphology of substance P (SP)-immunoreactive axons in the whole stomach of mice

Nociceptive afferents innervate the stomach and send signals centrally to the brain and locally to stomach tissues. Nociceptive afferents can be detected with a variety of different markers. In particular, substance P (SP) is a neuropeptide and is one of the most commonly used markers for nociceptive nerves in the somatic and visceral organs. However, the topographical distribution and morphological structure of SP-immunoreactive (SP-IR) axons and terminals in the whole stomach have not yet been fully determined. In this study, we labeled SP-IR axons and terminals in flat mounts of the ventral and dorsal halves of the stomach of mice. Flat-mount stomachs, including the longitudinal and circular muscular layers and the myenteric ganglionic plexus, were processed with SP primary antibody followed by fluorescent secondary antibody and then scanned using confocal microscopy. We found that (1) SP-IR axons and terminals formed an extensive network of fibers in the muscular layers and within the ganglia of the myenteric plexus of the whole stomach. (2) Many axons that ran in parallel with the long axes of the longitudinal and circular muscles were also immunoreactive for the vesicular acetylcholine transporter (VAChT). (3) SP-IR axons formed very dense terminal varicosities encircling individual neurons in the myenteric plexus; many of these were VAChT immunoreactive. (4) The regional density of SP-IR axons and terminals in the muscle and myenteric plexus varied in the following order from high to low: antrum-pylorus, corpus, fundus, and cardia. (5) In only the longitudinal and circular muscles, the regional density of SP-IR axon innervation from high to low were: antrum-pylorus, corpus, cardia, and fundus. (6) The innervation patterns of SP-IR axons and terminals in the ventral and dorsal stomach were comparable. Collectively, our data provide for the first time a map of the distribution and morphology of SP-IR axons and terminals in the whole stomach with single-cell/axon/synapse resolution. This work will establish an anatomical foundation for functional mapping of the SP-IR axon innervation of the stomach and its pathological remodeling in gastrointestinal diseases.

Engaging biological oscillators through second messenger pathways permits emergence of a robust gastric slow-wave during peristalsis

Peristalsis, the coordinated contraction—relaxation of the muscles of the stomach is important for normal gastric motility and is impaired in motility disorders. Coordinated electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICC) and smooth muscle (SM) cells of the stomach wall as a slow-wave, underly peristalsis. Normally, the gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. Understanding of the integrative role of neurotransmission and intercellular coupling in the propagation of an entrained gastric slow-wave, important for understanding motility disorders, however, remains incomplete. Using a computational framework constituted of a novel gastric motility network (GMN) model we address the hypothesis that engaging biological oscillators (i.e., ICCs) by constitutive gap junction coupling mechanisms and enteric neural innervation activated signals can confer a robust entrained gastric slow-wave. We demonstrate that while a decreasing enteric neural innervation gradient that modulates the intracellular IP₃ concentration in the ICCs can guide the aboral slow-wave propagation essential for peristalsis, engaging ICCs by recruiting the exchange of second messengers (inositol trisphosphate (IP₃) and Ca²⁺) ensures a robust entrained longitudinal slow-wave, even in the presence of biological variability in electrical coupling strengths. Our GMN with the distinct intercellular coupling in conjunction with the intracellular feedback pathways and a rostrocaudal enteric neural innervation gradient allows gastric slow waves to oscillate with a moderate range of frequencies and to propagate with a broad range of velocities, thus preventing decoupling observed in motility disorders. Overall, the findings provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach, offer directions for future experiments and theoretical work, and can potentially aid in the design of new interventional pharmacological and neuromodulation device treatments for addressing gastric motility disorders.

The coordinated contraction and relaxation of the muscles of the stomach, known as peristalsis is important for normal gastric motility and primarily governed by electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICCs) and smooth muscle cells of the stomach wall as a slow-wave. Under normal conditions, a gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. However, the understanding of intrinsic and extrinsic mechanisms that ensure propagation of a robust entrained slow-wave remains incomplete. Here, using a computational framework, we show that in conjunction with an enteric neural innervation gradient along the rostrocaudal ICC chain, and intercellular electrical coupling, the intercellular exchange of inositol trisphosphate between ICCs prevents decoupling by extending the longitudinal entrainment range along the stomach wall, even when variability in intercellular coupling exists. The findings from our study indicate ways that ensure the rostrocaudal spread of a robust gastric slow-wave and provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach.

Immunohistochemical expression and neurochemical phenotypes of huntingtin-associated protein 1 in the myenteric plexus of mouse gastrointestinal tract

Huntingtin-associated protein 1 (HAP1) is a neural huntingtin interactor and being considered as a core molecule of stigmoid body (STB). Brain/spinal cord regions with abundant STB/HAP1 expression are usually spared from neurodegeneration in stress/disease conditions, whereas the regions with little STB/HAP1 expression are always neurodegenerative targets. The enteric nervous system (ENS) can act as a potential portal for pathogenesis of neurodegenerative disorders. However, ENS is also a neurodegenerative target in these disorders. To date, the expression of HAP1 and its neurochemical characterization have never been examined there. In the current study, we determined the expression of HAP1 in the ENS of adult mice and characterized the morphological relationships of HAP1-immunoreactive (ir) cells with the markers of motor neurons, sensory neurons, and interneurons in the myenteric plexus using Western blotting and light/fluorescence microscopy. HAP1-immunoreaction was present in both myenteric and submucosal plexuses of ENS. Most of the HAP1-ir neurons exhibited STB in their cytoplasm. In myenteric plexus, a large number of calretinin, calbindin, NOS, VIP, ChAT, SP, somatostatin, and TH-ir neurons showed HAP1-immunoreactivity. In contrast, most of the CGRP-ir neurons were devoid of HAP1-immunoreactivity. Our current study is the first to clarify that HAP1 is highly expressed in excitatory motor neurons, inhibitory motor neurons, and interneurons but almost absent in sensory neurons in myenteric plexus. These suggest that STB/HAP1-ir neurons are mostly Dogiel type I neurons. Due to lack of putative STB/HAP1 protectivity, the sensory neurons (Dogiel type II) might be more vulnerable to neurodegeneration than STB/HAP1-expressing motoneurons/interneurons (Dogiel type I) in myenteric plexus.

Characterization of putative interneurons in the myenteric plexus of human colon

BACKGROUND

The enteric nervous system contains multiple classes of neurons, distinguishable by morphology, immunohistochemical markers, and projections; however, specific combinations differ between species. Here, types of enteric neurons in human colon were characterized immunohistochemically, using retrograde tracing combined with multiple labeling immunohistochemistry, focussing on non-motor neurons.

METHODS

The fluorescent carbocyanine tracer, DiI, was applied to the myenteric plexus in ex vivo preparations, filling neurons projecting within the plexus. Limits of projection lengths of motor neurons were established, allowing them to be excluded from the analysis. Long ascending and descending interneurons were then distinguished by labeling for discriminating immunohistochemical markers: calbindin, calretinin, enkephalin, 5-hydroxytryptamine, nitric oxide synthase, and substance P. These results were combined with a previous published study in which nitric oxide synthase and choline acetyltransferase immunoreactivities were established.

KEY RESULTS

Long ascending neurons (with projections longer than 8 mm, which excludes more than 95% motor neurons) formed four types, in descending order of abundance, defined by immunoreactivity for: (a) ChAT+/ENK+, (b) ChAT+/ENK+/SP+, (c) ChAT+/Calb+, and (d) ChAT+/ENK+/Calb+. Long descending neurons, up to 70 mm long also formed at least four types, distinguished by immunoreactivity for (a) NOS + cells (without ChAT), (b) ChAT+/NOS+, (c) ChAT+/Calret+, and (d) ChAT+/5HT + cells (with or without NOS).

CONCLUSIONS AND INFERENCES

Long interneurons, which do not innervate muscularis externa, are likely to coordinate neural activity over distances of many centimeters along the colon. Characterizing their neurochemical coding provides a basis for understanding their roles, investigating their connectivity, and building a comprehensive account of human colonic enteric neurons.

Morphological and Immunohistochemical Characterization of Human Intrinsic Gastric Neurons

Our knowledge about human gastric enteric neuron types is even more limited than that of human intestinal types. Here, we immunohistochemically stained wholemounts and sections of gastric specimens obtained from 18 tumor-resected patients. Myenteric wholemounts were labeled for choline acetyl transferase (ChAT), neuronal nitric oxide synthase (NOS), and the human neuronal protein HuC/D (as pan-neuronal marker for quantitative analysis) or alternatively for neurofilament (for morphological evaluation). ChAT-positive neurons outnumbered NOS-positive neurons (56 vs. 27%), and neurons negative for both markers accounted for 17%. Two larger groups of neurons (each between 12 and 14%) costained for ChAT and vasoactive intestinal peptide (VIP) or for NOS and VIP, respectively. Clear morphochemical correlation was found for uniaxonal stubby type I neurons (ChAT+; putative excitatory inter- or motor neurons), for uniaxonal spiny type I neurons (NOS+/VIP+; putative inhibitory motor or interneurons), and for multiaxonal type II neurons (ChAT+; putative afferent neurons; immunostaining of additional wholemounts revealed their coreactivity for somatostatin). Whereas these latter neuron types were already known from the human intestine, the morphology of gastric myenteric neurons coreactive for ChAT and VIP was newly described: they had numerous short, extremely thin dendrites and resembled, together with their cell bodies, a "hairy" head. In our sections, nerve fibers coreactive for ChAT and VIP were commonly found only in the mucosa. We suggest these myenteric ChAT+/VIP+/hairy neurons to be mucosal effector neurons. In contrast to myenteric neurons, the much less common submucosal neurons were not embedded in a continuous plexus and did not display any clear morphochemical phenotypes.

Distribution and trafficking of the μ-opioid receptor in enteric neurons of the guinea pig

The μ-opioid receptor (MOR) is a major regulator of gastrointestinal motility and secretion and mediates opiate-induced bowel dysfunction. Although MOR is of physiological and therapeutic importance to gut function, the cellular and subcellular distribution and regulation of MOR within the enteric nervous system are largely undefined. Herein, we defined the neurochemical coding of MOR-expressing neurons in the guinea pig gut and examined the effects of opioids on MOR trafficking and regulation. MOR expression was restricted to subsets of enteric neurons. In the stomach MOR was mainly localized to nitrergic neurons (∼88%), with some overlap with neuropeptide Y (NPY) and no expression by cholinergic neurons. These neurons are likely to have inhibitory motor and secretomotor functions. MOR was restricted to noncholinergic secretomotor neurons (VIP-positive) of the ileum and distal colon submucosal plexus. MOR was mainly detected in nitrergic neurons of the colon (nitric oxide synthase positive, 87%), with some overlap with choline acetyltransferase (ChAT). No expression of MOR by intrinsic sensory neurons was detected. [d-Ala(2), MePhe(4), Gly(ol)(5)]enkephalin (DAMGO), morphiceptin, and loperamide induced MOR endocytosis in myenteric neurons. After stimulation with DAMGO and morphiceptin, MOR recycled, whereas MOR was retained within endosomes following loperamide treatment. Herkinorin or the δ-opioid receptor agonist [d-Ala(2), d-Leu(5)]enkephalin (DADLE) did not evoke MOR endocytosis. In summary, we have identified the neurochemical coding of MOR-positive enteric neurons and have demonstrated differential trafficking of MOR in these neurons in response to established and putative MOR agonists.

Excitatory and inhibitory enteric innervation of horse lower esophageal sphincter

The lower esophageal sphincter (LES) is a specialized, thickened muscle region with a high resting tone mediated by myogenic and neurogenic mechanisms. During swallowing or belching, the LES undergoes strong inhibitory innervation. In the horse, the LES seems to be organized as a "one-way" structure, enabling only the oral-anal progression of food. We characterized the esophageal and gastric pericardial inhibitory and excitatory intramural neurons immunoreactive (IR) for the enzymes neuronal nitric oxide synthase (nNOS) and choline acetyltransferase. Large percentages of myenteric plexus (MP) and submucosal (SMP) plexus nNOS-IR neurons were observed in the esophagus (72 ± 9 and 69 ± 8 %, respectively) and stomach (57 ± 17 and 45 ± 3 %, respectively). In the esophagus, cholinergic MP and SMP neurons were 29 ± 14 and 65 ± 24 vs. 36 ± 8 and 38 ± 20 % in the stomach, respectively. The high percentage of nitrergic inhibitory motor neurons observed in the caudal esophagus reinforces the role of the enteric nervous system in the horse LES relaxation. These findings might allow an evaluation of whether selective groups of enteric neurons are involved in horse neurological disorders such as megaesophagus, equine dysautonomia, and white lethal foal syndrome.

Gastric mucosal protection by aegle marmelos against gastric mucosal damage: role of enterochromaffin cell and serotonin

BACKGROUND/AIMS

Serotonin (5-hydroxytryptamine; 5-HT) released from enterochromaffin (EC) cells in gastric mucosa inhibits gastric acidity by increasing the gastric mucus secretion. In the present study, we evaluated the effect of aqueous extract of Aegle marmelos (AM) ripe fruit pulp (250 mg/kg body weight) on mean ulcer index (MUI), EC cells, 5-HT content, and adherent mucosal thickness of ulcerated gastric tissue in adult albino rats.

MATERIAL AND METHODS

Ulceration was induced by using aspirin (500 mg/kg, p.o.), cerebellar nodular lesion and applying cold-restraint stress.

RESULTS

In all cases increased MUI in gastric tissue along with decreased EC cell count was observed with concomitant decrease of 5-HT content and adherent mucosal thickness (P < 0.05). Pretreatment with AM for 14 days decreased MUI, increased EC cell count, and 5-HT content as well as adherent mucosal thickness in all ulcerated group (P < 0.05).

CONCLUSION

AM produces gastric mucosal protection mediated by increased EC cell count and 5-HT levels.

Quantitative changes of nitrergic neurons during postnatal development of chicken myenteric plexus

OBJECTIVE

Information regarding the development of the enteric nervous system (ENS) is important for understanding the functional abnormalities of the gut. Because fertilized chicken eggs provide easy access to embryos, chicken models have been widely used to study embryonic development of myenteric plexus; however, no study has been focused on the postnatal period. The aim of this study was to perform a qualitative and quantitative analysis of the nitrergic neurons in the myenteric plexus of developing chickens in the postnatal period.

METHODS

Whole-mount preparations of the myenteric plexus were made in 7-d, 15-d, and 40-d old (adult) chickens of either sex (n=15). The myenteric plexus was studied after nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry using light microscopy, digital photography, and Image-Pro Plus 6.0 software. The numbers of positively stained neurons and ganglia were counted in the duodenum, jejunum, ileum, caecum, and colon in the different age groups. Data were expressed as mean±standard deviation (SD), and statistical analysis was performed using a one-way analysis of variance (ANOVA) test.

RESULTS

The positively stained neurons showed various morphologies and staining intensities, and formed bead-shaped and U-shaped arrangements in the myenteric plexus. The densities of neurons and ganglia increased with age. However, the number of positive neurons per ganglion increased. The number of NADPH-d-positive neurons was highest in the colon, followed by the ileum, the jejunum, the duodenum, and the caeca in all age groups.

CONCLUSIONS

Developmental changes in the myenteric plexus of chickens continue in the postnatal period, indicating that the maturation process of the gastrointestinal function is gradual. In addition, no significant difference is happening among different intestinal segments during postnatal development, suggesting that the function of different intestinal segments had been determined after birth.

Serotonin-immunoreactive neurons and mast cells in the mouse esophagus suggest involvement of serotonin in both motility control and neuroimmune interactions

BACKGROUND

Serotonin is a major transmitter in the gastrointestinal tract, but little is known about the serotonergic system in the esophagus.

METHODS

The aim of this study was to use multilabel immunofluorescence to characterize serotonin-positive nerve cell bodies and fibers and their relationship with other neuronal and non-neuronal elements in the mouse esophagus. Antibodies against serotonin, vesicular acetylcholine transporter (VAChT), choline acetyltransferase (ChAT), protein gene product 9.5 (PGP 9.5), and α-bungarotoxin (α-BT), were used.

KEY RESULTS

Serotonin-containing perikarya represented ∼10% of all PGP 9.5-positive myenteric neurons. Serotonin-positive varicose nerve fibers were found in the lamina muscularis mucosae and present on ∼13% of α-BT-labeled motor endplates in addition to VAChT-immunoreactive motor terminals. As ChAT-positive neurons of the compact formation of the nucleus ambiguus were negative for serotonin, serotonin-positive varicosities on motor endplates are presumed to be of enteric origin. On the other hand, cholinergic ambiguus neurons were densely supplied with serotonin-positive varicosities. The tela submucosa and tunica adventitia contained large numbers of serotonin-positive mast cells, a few of which were in close association with serotonin-positive nerve fibers.

CONCLUSIONS & INFERENCES

The mouse esophagus is endowed with a rich serotonin-positive intrinsic innervation, including enteric co-innervation of striated muscles. Serotonin may modulate vagal motor innervation of esophageal-striated muscles not only at the central level via projections of the raphe nuclei to the nucleus ambiguus but also at the peripheral level via enteric co-innervation. In addition, mast cells represent a non-neuronal source of serotonin, being involved in neuroimmune processes.

Moringa oleifera induced potentiation of serotonin release by 5-HT(3) receptors in experimental ulcer model

ETHNOPHARMACOLOGICAL RELEVANCE

moringa oleifera (Moringaceae), a perennial plant is widely cultivated throughout the world. Extensive pharmacological studies revealed its promising role in modulation of various disorders like antispasmodic, diuretic, abortifacient, antimicrobial antibacterial, antitubercular, antiviral, antifertility, depressant, anti-inflammatory and anticancer property which promoted us to conduct the study to elucidate its role on experimental gastric ulceration.

AIM OF THE STUDY

the aim of the present study was to assess the efficacy of its aqueous leaf extract on protection of gastric ulceration and characterize the possible modulatory mechanism underlying the phenomenon.

MATERIALS AND METHODS

adult Holtzman strain albino rats (weight 150-200 g) of either sex were used for the study. Ulceration was induced using aspirin (500 mg/kg body weight) and using Moringa oleifera (MO), a herbal formulation, the modulatory mechanism has been studied and compared with a commonly used antagonist of 5-HT(3) receptors, ondansetron by assessing parameters like mean ulcer index, 5-HT content, EC cell count and mucosal thickness.

RESULTS

the results of our study suggest that MO protects ulcer formation by modulating 5-HT secretion through EC cell via 5-HT(3) receptors in gastrointestinal tract.

INTERPRETATION AND CONCLUSION

MO showed maximum protective activity at a dose of 300 mg/kg body weight against above-mentioned experimental rat ulcer model by modulating 5-HT secretion through EC cell via 5-HT(3) receptors in gastrointestinal tract which has given a glimpse of a therapeutic approach for gastric ulcer management, which may be beneficially used in contrast to the classical antacid, antihistamine or surgical treatment. Further investigations and proper screening regarding various phytochemicals, alkaloids present within MO leaf will help to formulate effective herbal preparation that will be used to combat gastrointestinal disorders in future.

Regional differences in neostigmine-induced contraction and relaxation of stomach from diabetic guinea pig

Delayed gastric emptying and autonomic neuropathy have been documented in patients with diabetes mellitus. Some medications used to treat delayed gastric emptying enhance release of acetylcholine from autonomic neurons to strengthen gastric contractions. Autonomic coordination among gastric regions may be altered in diabetes resulting in poor outcomes in response to prokinetic drugs. Fundus, antrum, and pylorus from STZ or control guinea pigs were treated with neostigmine to mimic release of acetylcholine from autonomic neurons by prokinetic agents. In diabetic animals, neostigmine-induced contractions were weaker in fundus and pylorus but similar in antrum. The muscarinic receptor antagonist 4-DAMP or the nicotinic receptor antagonist hexamethonium reduced neostigmine-induced contractions. Activation of presynaptic muscarinic receptors on nitrergic neurons was impaired in fundus and antrum from diabetic animals. Nerve-stimulated contractions and relaxations, number of nNOS myenteric neurons, and tissue choline content were reduced in fundus from diabetic animals. Despite reduced number of myenteric neurons, tissue choline content was increased in antrum from diabetic animals. Since cholinergic motility of each gastric region was affected differently by diabetes, prokinetic drugs that nondiscriminately enhance acetylcholine release from autonomic neurons may not effectively normalize delayed gastric emptying in patients with diabetes and more selective medications may be warranted.

Secretin-induced gastric relaxation is mediated by vasoactive intestinal polypeptide and prostaglandin pathways

Secretin has been shown to delay gastric emptying and inhibit gastric motility. We have demonstrated that secretin acts on the afferent vagal pathway to induce gastric relaxation in the rat. However, the efferent pathway that mediates the action of secretin on gastric motility remains unknown. We recorded the response of intragastric pressure to graded doses of secretin administered intravenously to anaesthetized rats using a balloon attached to a catheter and placed in the body of the stomach. Secretin evoked a dose-dependent decrease in intragastric pressure. The threshold dose of secretin was 1.4 pmol kg(-1) h(-1) and the effective dose, 50% was 5.6 pmol kg(-1) h(-1). Pretreatment with hexamethonium markedly reduced gastric relaxation induced by secretin (5.6 pmol kg(-1) h(-1)). Bilateral vagotomy also significantly reduced gastric motor responses to secretin. Administration of N(G)-nitro-L-arginine methyl ester (10 mg kg(-1)) did not affect gastric relaxation induced by secretin. In contrast, intravenous administration of a vasoactive intestinal polypeptide (VIP) antagonist (30 nmol kg(-1)) reduced the gastric relaxation response to secretin (5.6 pmol kg(-1) h(-1)) by 89 +/- 5%. Indomethacin (2 mg kg(-1)) reduced gastric relaxation induced by secretin (5.6 pmol kg(-1) h(-1)) by 87 +/- 5%. Administration of prostaglandin (48 mg kg(-1) h(-1)) prevented this inhibitory effect. Indomethacin also reduced gastric relaxation induced by VIP (300 pmol kg(-1)) by 90 +/- 7%. These observations indicate that secretin acts through stimulation of presynaptic cholinergic neurons in a vagally mediated pathway. Through nicotinic synapses, secretin stimulates VIP release from postganglionic neurons in the gastric myenteric plexus, which in turn induces gastric relaxation through a prostaglandin-dependent pathway.

Effects of muscarinic receptor stimulation and nitric oxide synthase inhibition on gastric tone and gastric myoelectrical activity in canines

AIMS

This study was designed to assess whether the muscarinic receptor stimulation and nitric oxide synthase inhibition were equally effective on gastric fundic tone or gastric myoelectrical activity (GMA) in canines, and the correlation between gastric fundic tone and GMA.

METHODS

Gastric fundic tone and GMA were recorded on seven dogs implanted with serosal electrodes and a gastric cannula.

RESULTS

Bethanechol and L-nitro-N-arginine (L-NNA) significantly increased gastric fundic tone; gastric volume was decreased with bethanechol or L-NNA (P < 0.05). Increased spike activities were observed after both bethanechol and L-NNA. The percentage of slow waves superimposed with spikes was increased with bethanechol (P < 0.001) and L-NNA (P < 0.05). There was a significant reduction in dominant frequency (DF) (P < 0.05), dominant power (DP) (P < 0.05) and percentage of normal slow waves (%N) (P < 0.05) with bethanechol, while no significant change was observed with L-NNA. The variation of gastric tone was not correlated with parameters of GMA.

CONCLUSIONS

Muscarinic receptor stimulation and nitric oxide synthase inhibition have similar effects on gastric tone and gastric spike activities, but different effects on gastric slow waves. Gastric fundic tone does not correlate with gastric slow waves.

Spatial organization of neurons in the dorsal motor nucleus of the vagus synapsing with intragastric cholinergic and nitric oxide/VIP neurons in the rat

The dorsal motor nucleus of the vagus (DMV) contains preganglionic neurons that control gastric motility and secretion. Stimulation of different parts of the DMV results in a decrease or an increase in gastric motor activities, suggesting a spatial organization of vagal preganglionic neurons in the DMV. Little is known about how these preganglionic neurons in the DMV synapse with different groups of intragastric motor neurons to mediate contraction or relaxation of the stomach. We used pharmacological and immunohistochemical methods to characterize intragastric neural pathways involved in mediating gastric contraction and relaxation in rats. Microinjections of L-glutamate (L-Glu) into the rostral or caudal DMV produced gastric contraction and relaxation, respectively, in a dose-related manner. Intravenous infusion of hexamethonium blocked these actions, suggesting mediation via preganglionic cholinergic pathways. Atropine inhibited gastric contraction by 85.5 +/- 4.5%. Gastric relaxation was reduced by intravenous administration of N(G)-nitro-L-arginine methyl ester (L-NAME; 52.5 +/- 11.9%) or VIP antagonist (56.3 +/- 14.9%). Combined administration of L-NAME and VIP antagonist inhibited gastric relaxation evoked by L-Glu (87.8 +/- 4.3%). Immunohistochemical studies demonstrated choline acetyltransferase immunoreactivity in response to L-Glu microinjection into the rostral DMV in 88% of c-Fos-positive intragastric myenteric neurons. Microinjection of L-Glu into the caudal DMV evoked expression of nitric oxide (NO) synthase and VIP immunoreactivity in 81 and 39%, respectively, of all c-Fos-positive intragastric myenteric neurons. These data indicate spatial organization of the DMV. Depending on the location, microinjection of L-Glu into the DMV may stimulate intragastric myenteric cholinergic neurons or NO/VIP neurons to mediate gastric contraction and relaxation.

Sumatriptan is an agonist at 5-HT receptors on myenteric neurones in the guinea-pig gastric antrum

Sumatriptan, a 5-hydroxytryptamine(1D) (5-HT(1D))-receptor agonist used in the treatment in migraine, inhibits gastric motility via the enteric nervous system. As no studies have reported enteric neuronal 5-HT(1D) receptors, we used conventional intracellular recordings to characterize the actions of sumatriptan on 145 guinea-pig antral myenteric neurones. In 24 of 29 neurones with a 5-HT(1P) receptor-mediated depolarizing response to 5-HT, application of sumatriptan caused a dose-dependent depolarization, accompanied by increased membrane resistance and enhanced excitability. Depolarizing responses to sumatriptan occurred both in cholinergic and in nitrergic neurones. Sumatriptan did not mimic the 5-HT(3) receptor-mediated fast-depolarizing responses or 5-HT(1A) receptor-mediated inhibitory responses to 5-HT. Sumatriptan had no effect on neurones not responding to 5-HT. The depolarizing response to sumatriptan was inhibited by renzapride, but not by 5-HT(1-7) receptor antagonists. We conclude that sumatriptan behaves as an agonist at the 5-HT(1P) receptor on myenteric neurones in the guinea-pig gastric antrum. The actions of sumatriptan on gastric motility seem to be attributable to a direct action on enteric neurones.

Imaging and modelling of digestion in the stomach and the duodenum

Gastroduodenal physiology is traditionally understood in terms of motor-secretory functions and their electrical, neural and hormonal controls. In contrast, the fluid-mechanical functions that retain and disperse particles, expose substrate to enzymes, or replenish the epithelial boundary with nutrients are little studied. Current ultrasound and magnetic resonance imaging allows to visualize processes critical to digestion like mixing, dilution, swelling, dispersion and elution. Methodological advances in fluid mechanics allow to numerically analyse the forces promoting digestion. Pressure and flow fields, the shear stresses dispersing particles or the effectiveness of bolus mixing can be computed using information on boundary movements and on the luminal contents. These technological advances promise many additional insights into the mechanical processes that promote digestion and absorption.

Gastric electrical stimulation inhibits postprandial antral tone partially via nitrergic pathway in conscious dogs

Gastric electrical stimulation (GES) has recently been explored as a therapeutic option for gastrointestinal motility disorders or obesity. The mechanism behind it is not fully elucidated. The aims of this study were to assess the effects of GES with different parameters on antral tone and to explore the involvement of the nitrergic pathway. Eight dogs equipped with a gastric cannula and one pair of serosal electrodes in the greater curvature 4 cm above the pylorus were studied on separate days. The study was composed of seven randomized sessions in the fed state [control, GES with different parameters, and GES plus neuronal nitric oxide synthase (nNOS) inhibitor]. Each session included three consecutive 30-min periods (baseline, GES, and recovery). GES was performed with long pulses or pulse trains. The antral volume was measured using an intragastric balloon connected with a barostat device. Behaviors of the dogs during each stimulation period were also noted. We found that 1) postprandial antral tone was reduced with GES with all tested parameter settings, reflected as a significant and substantial increase in antral volume ranging from 179 to 309%; 2) the inhibitory effect of GES on antral tone was partially blocked (decreased by 39.5%) with an nNOS inhibitor; and 3) mild symptoms were induced with GES and found to be correlated with the GES-induced increase in antral volume. We conclude that retrograde GES with long pulses or pulse trains inhibits antral tone, and this inhibitory effect is partially mediated via the nitrergic pathway. These results suggest that retrograde GES may have a therapeutic potential for obesity.

Neurochemical coding of myenteric neurones in the human gastric fundus

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

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

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

Chemical coding of the human gastrointestinal nervous system: cholinergic, VIPergic, and catecholaminergic phenotypes

The aim of this investigation was to identify the proportional neurochemical codes of enteric neurons and to determine the specific terminal fields of chemically defined nerve fibers in all parts of the human gastrointestinal (GI) tract. For this purpose, antibodies against the vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT), tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), serotonin (5-HT), vasoactive intestinal peptide (VIP), and protein gene product 9.5 (PGP 9.5) were used. For in situ hybridization (35)S-labeled VMAT1, VMAT2, and VAChT riboprobes were used. In all regions of the human GI tract, 50-70% of the neurons were cholinergic, as judged by staining for VAChT. The human gut unlike the rodent gut exhibits a cholinergic innervation, which is characterized by an extensive overlap with VIPergic innervation. Neurons containing VMAT2 constituted 14-20% of all intrinsic neurons in the upper GI tract, and there was an equal number of TH-positive neurons. In contrast, DBH was absent from intrinsic neurons. Cholinergic and monoaminergic phenotypes proved to be completely distinct phenotypes. In conclusion, the chemical coding of human enteric neurons reveals some similarities with that of other mammalian species, but also significant differences. VIP is a cholinergic cotransmitter in the intrinsic innervation of the human gut. The substantial overlap between VMAT2 and TH in enteric neurons indicates that the intrinsic catecholaminergic innervation is a stable component of the human GI tract throughout life. The absence of DBH from intrinsic catecholaminergic neurons indicates that these neurons have a dopaminergic phenotype.

Distribution of galanin receptor 1 immunoreactivity in the rat stomach and small intestine

Galanin affects gastrointestinal functions by activating different G protein-coupled receptors. Here, we identified the sites of expression of the galanin receptor 1 (GAL-R1) subtype in the rat stomach and small intestine by using immunohistochemistry with an antibody raised to the third intracellular loop of rat GAL-R1 (GAL-R1(Y225-238)) and confocal microscopy. Antibody specificity was confirmed by (1) the detection of a band at approximately 70 kDa in Western blot of membranes from GAL-R1 transfected cells, (2) the cell surface staining of GAL-R1 transfected cells, which was not detected in control cells, and (3) the abolition of Western signal and tissue immunostaining by preadsorbing the antibody with the peptide used for immunization. GAL-R1 immunoreactivity was localized to the cell surface of enterochromaffin-like cells, and of myenteric and submucous neurons, and to fibers distributed to the plexuses, interconnecting strands, muscle layers, vasculature, and mucosa. A dense network of GAL-R1 immunoreactivity was observed in the deep muscular plexus in very close association with interstitial cells of Cajal visualized by c-kit immunostaining. In the ileum, 81.6% of GAL-R1 myenteric neurons and 70.7% of GAL-R1 submucosal neurons were substance P immunoreactive. Vasoactive intestinal polypeptide immunoreactivity was found in 48.3% of GAL-R1 submucosal neurons, but not in GAL-R1 myenteric neurons. These findings support the hypothesis that GAL-R1 mediates galanin actions on gastrointestinal motility and secretion by modulating the release of other neurotransmitters and contributes to galanin-induced inhibition of gastric acid secretion by means of the suppression of endogenous histamine release.

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.

Neuronal nitric oxide synthase immunoreactivity in the guinea-pig liver: distribution and colocalization with neuropeptide Y and calcitonin gene-related peptide

AIMS/BACKGROUND

The innervation pattern of the guinea-pig liver is similar to that of the human liver. However, many aspects of the distribution of the neuronal isoform of the enzyme nitric oxide synthase (nNOS) in the guinea-pig liver and its colocalization with neuropeptides remain to be elucidated.

METHODS

The distribution of nNOS was studied in fixed guinea-pig liver by light microscopic immunohistochemistry. Confocal analysis was used to determine its colocalization with neuropeptide Y (NPY) or calcitonin gene-related peptide (CGRP).

RESULTS

nNOS-immunoreactive (nNOS-IR) nerves were observed in relation to hilar and interlobar vessels and in Glisson's capsule. A few nNOS-IR ganglia were observed in the extrahepatic bile duct and close to the interlobar portal triads. In addition, nNOS-IR fibers were located in the interlobular portal triads and pervading the parenchyma. Moreover, nNOS-IR nerves were demonstrated for the first time in the larger central veins and in the hepatic vein. nNOS-NPY and nNOS-CGRP colocalizations were detected in the fibromuscular layer of the bile duct and periductal plexus, respectively.

CONCLUSIONS

These results support the phylogenetic conservation of the nNOS-IR hepatic innervation and its possible contribution to the regulation of hepatic blood flow and certain hepatic functions.

Mucosal acid challenge activates nitrergic neurons in myenteric plexus of rat stomach

We tested the hypothesis that intrinsic neurons of the rat gastric myenteric plexus can be activated by an acid (HCl) challenge of the mucosa. Activated neurons were visualized by immunohistochemical detection of c-Fos, a marker for neuronal excitation. The neurochemical identity of the neurons activated by the HCl challenge was determined by colocalizing c-Fos with a marker for excitatory pathways, choline acetyltransferase (ChAT), and a marker for inhibitory pathways, nitric oxide synthase (NOS). Two hours after intragastric administration of HCl or saline, stomachs were removed and immunofluorescence triple labeling of myenteric neurons was carried out on whole mount preparations. Treatment with 0.35, 0.5, and 0.7 M HCl induced c-Fos in 8%, 56%, and 64%, respectively, of NOS-positive but not ChAT-positive neurons. c-Fos was also seen in glial cells of HCl-treated rats, whereas in saline-treated animals c-Fos was absent from the myenteric plexus. HCl treatment did not change the proportion of ChAT- and NOS-immunoreactive neurons in the myenteric ganglia. It is concluded that gastric acid challenge concentration-dependently stimulates a subpopulation of nitrergic, but not cholinergic, myenteric plexus neurons, which may play a role in muscle relaxation, vasodilatation, and/or secretion.

Intracisternal TRH analog induces Fos expression in gastric myenteric neurons and glia in conscious rats

Activation of gastric myenteric cells by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368, at a dose inducing near maximal vagal cholinergic stimulation of gastric functions, was investigated in conscious rats. Fos immunoreactivity was assessed in gastric longitudinal muscle-myenteric plexus whole mount preparations 90 min after intracisternal injection. Fos-immunoreactive cells were rare in controls (~1 cell/ganglion), whereas intracisternal RX-77368 (50 ng) increased the number to 24.8 +/- 1.8 and 26.8 +/- 2.2 cells/ganglion in the corpus and antrum, respectively. Hexamethonium (20 mg/kg sc) prevented Fos expression by 90%, whereas atropine (2 mg/kg sc) had no effect. The neuronal marker protein gene product 9.5 and the glial markers S-100 and glial fibrillary acidic proteins showed that RX-77368 induced Fos in both myenteric neurons and glia. Vesicular ACh transporter and calretinin were detected around the activated myenteric neurons. These results indicated that central vagal efferent stimulation by intracisternal RX-77368 activates gastric myenteric neurons as well as glial cells mainly through nicotinic ACh receptors in conscious rats.

Enteric pathways in the stomach

This report summarises the characteristics of target specific projection and neurochemical coding patterns of motor and interneuronal pathways in the gastric enteric nervous system (ENS) which are involved in the innervation of the mucosa, the circular and the longitudinal muscle. The pathways were identified by retrograde tracing and further characterised by optical and intracellular recordings of the synaptic activation of muscle motor neurones, and by recordings of pathway-specific muscle responses. All motor pathways had polarised projections consisting of ascending cholinergic and descending nitrergic populations. Thus, both muscle layers were innervated by excitatory and inhibitory motor neurones. Their projections indicated the presence of intrinsic circuits that mediate excitatory and inhibitory components of a peristaltic reflex and/or are involved in reflex mediated changes in gastric tone. Although polarised projections were also identified for interneuronal pathways, a substantial proportion of descending interneurones was cholinergic. Interneurones and longitudinal muscle motor pathways had longitudinal projection preferences whereas circular muscle motor pathways had circumferential projection preferences. Target-specific coding was primarily revealed for cholinergic populations; ChAT/ENK/+/-SP neurones projected to the muscle layers, ChAT/NPY/+/-VIP projected to the mucosa and ChAT/+/-SP/+/-5-HT/+/-Calret/+/-Calb were interneurones. Muscle strip recordings revealed the functional significance of ascending excitatory and descending inhibitory pathways to the circular muscle and the prominent influence of ascending and descending cholinergic interneurones which activated excitatory and inhibitory circular muscle motor neurones through nicotinic synapses. It is concluded that enteric pathways in the stomach have region specific features which reflect structural and functional adaptation of the gastric ENS.

Muscular innervation of the proximal duodenum of the guinea pig

We investigated the muscular structure and innervation of the gastroduodenal junction in the guinea pig. In the gastroduodenal junction, the innermost layer of the circular muscle contained numerous nerve fibers and terminals. Since this nerve network continued onto the deep muscular plexus (DMP) of the duodenum, we surmised that the numerous nerve fibers in the gastroduodenal junction were specialized DMP in the most proximal part of the duodenum. The innermost layer containing many nerve fibers was about 1,000 microm in length and 100 microm in thickness in the proximal duodenum. This layer contained numerous connective tissue fibers composed of collagen and elastic fibers. Five to 30 smooth muscle cells lay in contact with each other and were surrounded by fine connective tissue. The nerve fibers in the proximal duodenum contained nerve terminals immunoreactive for choline acetyltransferase, dynorphin, enkephalin, galanin, gastrin-releasing peptide, nitric oxide synthase, substance P, and vasoactive intestinal polypeptide. Adrenergic fibers which contained tyrosine hydroxylase immunoreactivity were rare in the proximal duodenum. In the innermost layer of the proximal duodenum, there were numerous c-Kit immunopositive cells that were in contact with nerve terminals. This study allowed us to clarify the specific architecture of the most proximal portion of the duodenum. The functional significance of the proximal duodenum in relation to the electrical connection and neural cooperation of the musculature between the antrum and the duodenum is also discussed.