Characterization of the peptidergic afferent innervation of the stomach in the rat, mouse and guinea-pig

Extrinsic and intrinsic sources of calcitonin gene-related peptide immunoreactivity in the lamb ileum: a morphometric and neurochemical investigation

To investigate extrinsic origins of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibres in the sheep ileum, the retrograde fluorescent tracer Fast Blue (FB) was injected into the ileum wall. Sections of thoraco-lumbar dorsal root ganglia (DRG) and distal (nodose) vagal ganglia showing FB-labelled neurons were processed for CGRP immunohistochemistry. The distribution of CGRP-IR in fibres and nerve cell bodies in the ileum was also studied. CGRP-IR enteric neurons were morphometrically analysed in myenteric (MP) and submucosal plexuses (SMP) of lambs (2-4 months). Sensory neurons retrogradely labelled with FB were scattered in T5-L4 DRG but most were located at the upper lumbar levels (L1-L3); only a minor component of the extrinsic afferent innervation of the ileum was derived from nodose ganglia. In the DRG, 57% of retrogradely labelled neurons were also CGRP-IR. In cryostat sections, a dense network of CGRP-IR fibres was observed in the lamina propria beneath the epithelium, around the lacteals and lymphatic follicles (Peyer's platches), and along and around enteric blood vessels. Rare CGRP-IR fibres were also present in both muscle layers. Dense pericellular baskets of CGRP-IR fibres were observed around CGRP-negative somata. The only CGRP-IR nerve cells were well-defined Dogiel type II neurons localised in the MP and in the external and internal components of the SMP. CGRP-IR neurons in the myenteric ganglia were significantly larger than those in the submucosal ganglia (mean profile areas: about 1,400 mum(2) for myenteric neurons, 750 mum(2) for submucosal neurons). About 6% of myenteric neurons and 25% of submucosal neurons were CGRP-IR Dogiel type II neurons. The percentages of CGRP-IR neurons that were also tachykinin-IR were about 9% (MP) and 42% (SMP), whereas no CGRP-IR neurons exhibited immunoreactivity for vasoactive intestinal peptide, nitric oxide synthase or tyrosine hydroxylase in either plexus. Thus, CGRP immunoreactivity occurs in the enteric nervous system of the sheep ileum (as in human small intestine and MP of pig ileum) in only one morphologically defined type of neuron, Dogiel type II cells. These are probably intrinsic primary afferent neurons.

Differential effects of intragastric acid and capsaicin on gastric emptying and afferent input to the rat spinal cord and brainstem

Background

Hydrochloric acid (HCl) is a potential threat to the integrity of the gastric mucosa and is known to contribute to upper abdominal pain. We have previously found that gastric mucosal challenge with excess HCl is signalled to the rat brainstem, but not spinal cord, as visualized by expression of c-fos messenger ribonucleic acid (mRNA), a surrogate marker of neuronal excitation. This study examined whether gastric mucosal exposure to capsaicin, a stimulant of nociceptive afferents that does not damage the gastric mucosa, is signalled to both brainstem and spinal cord and whether differences in the afferent signalling of gastric HCl and capsaicin challenge are related to different effects on gastric emptying.

Results

Rats were treated intragastrically with vehicle, HCl or capsaicin, activation of neurons in the brainstem and spinal cord was visualized by in situ hybridization autoradiography for c-fos mRNA, and gastric emptying deduced from the retention of intragastrically administered fluid. Relative to vehicle, HCl (0.5 M) and capsaicin (3.2 mM) increased c-fos transcription in the nucleus tractus solitarii by factors of 7.0 and 2.1, respectively. Capsaicin also caused a 5.2-fold rise of c-fos mRNA expression in lamina I of the caudal thoracic spinal cord, although the number of c-fos mRNA-positive cells in this lamina was very small. Thus, on average only 0.13 and 0.68 c-fos mRNA-positive cells were counted in 0.01 mm sections of the unilateral lamina I following intragastric administration of vehicle and capsaicin, respectively. In contrast, intragastric HCl failed to induce c-fos mRNA in the spinal cord. Measurement of gastric fluid retention revealed that HCl suppressed gastric emptying while capsaicin did not.

Conclusion

The findings of this study show that gastric mucosal exposure to HCl and capsaicin is differentially transmitted to the brainstem and spinal cord. Since only HCl blocks gastric emptying, it is hypothesized that the two stimuli are transduced by different afferent pathways. We infer that HCl is exclusively signalled by gastric vagal afferents whereas capsaicin is processed both by gastric vagal and intestinal spinal afferents.

The vagus regulates histamine mobilization from rat stomach ECL cells by controlling their sensitivity to gastrin

The ECL cells in the oxyntic mucosa secrete histamine in response to gastrin, stimulating parietal cells to produce acid. Do they also operate under nervous control? The present study examines histamine mobilization from rat stomach ECL cells in situ in response to acute vagal excitation and to food or gastrin following vagal or sympathetic denervation. Applying the technique of microdialysis, we monitored the release of histamine by radioimmunoassay. Microdialysis probes were placed in the submucosa on either side of the stomach, 3 days before experiments. The rats were awake during microdialysis except when subjected to electrical vagal stimulation. One-sided electrical vagal stimulation raised serum gastrin and mobilized gastric histamine. However, gastrin receptor blockade prevented the histamine mobilization, indicating that circulating gastrin accounts for the response. Vagal excitation by hypoglycaemia (insulin) or pylorus ligation did not mobilize either gastrin or histamine. The histamine response to food was almost abolished by gastrin receptor blockade, and it was halved on the denervated side after unilateral subdiaphragmatic vagotomy. While the histamine response to a near-maximally effective dose of gastrin was unaffected by vagotomy, the response to low gastrin doses was reduced significantly. Abdominal ganglionic sympathectomy failed to affect the histamine response to either food or gastrin. In conclusion, gastrin is responsible for most of the food-evoked mobilization of ECL-cell histamine. The histamine response to electrical vagal stimulation reflects the effect of circulating gastrin rather than a direct action of the vagus on the ECL cells. Vagal denervation was accompanied by an impaired histamine response to food intake, probably reflecting the right-ward shift of the serum gastrin concentration-histamine response curve. The results suggest that the vagus controls the sensitivity of the ECL cells to gastrin.

Nociceptive transmitter release in the dorsal spinal cord by capsaicin-sensitive fibers after noxious gastric stimulation

Little is known about transmitters that encode noxious gastric stimuli in the spinal cord. The release of glutamate, substance P, and CGRP from the spinal cord was therefore investigated in response to acid injury of the gastric mucosa. Dorsal halves of the caudal thoracic spinal cord (T7-T13) were removed 6 h after oral application of 0.5 M HCl or saline, transferred to a superfusion chamber, and the basal and capsaicin-stimulated (3.3 microM) transmitter release was determined. After acid injury, basal glutamate release increased 134% as compared to saline-treated animals. Capsaicin-stimulated release of CGRP and SP was 48% and 58% lower in acid- than in saline-treated animals, indicating that capsaicin-sensitive fibers in the dorsal spinal cord were already partially depleted by acid treatment. Capsaicin denervation reduced basal glutamate release by 33% after acid injury as compared to non-denervated acid-treated animals. Gastric origin and capsaicin sensitivity of glutamatergic, CGRP- and SP-containing primary afferents in thoracic dorsal root ganglia were then determined by retrograde tracing with True Blue and immunohistochemical labeling with the vanilloid receptor TRPV1. About 65% of True Blue-labeled cells were glutamatergic and more than 73% of this population expressed the TRPV1 receptor. Nearly all True Blue/CGRP (85%)- and True Blue/SP-positive cells (97%) coexpressed TRPV1. We conclude that noxious gastric stimulation with acid induces release of glutamate, SP, and CGRP from capsaicin-sensitive sensory afferents in the dorsal horn of the spinal cord where they may play an important role in gastric nociception and hyperalgesia.

Extracellular signal-regulated kinase-1 and -2 are activated by gastric luminal injury in dorsal root ganglion neurons via n-methyl-d-aspartate receptors

Mitogen activated protein kinases such as phosphorylated extracellular signal-regulated kinase-1 and -2 (pERK 1/2) have been recently demonstrated to play an important role in somatic nociception and hyperalgesia. In the present study we examined whether pERK 1/2 is involved in the response of sensory neurons to a noxious visceral stimulation, in particular, of the gastric mucosa. After induction of gastric injury by oral administration of 0.5M HCl pERK 1/2 expression was determined by Western blotting of caudal thoracic dorsal root ganglia and by immunohistochemistry in stomach-innervating dorsal root ganglion neurons which were retrogradely labeled with True Blue. The content of pERK 1/2 remained unchanged in dorsal root ganglia until 2 h post-HCl, however, was found elevated 4 (approximately 80%) and 6 h (approximately 100%) after HCl administration. True Blue-labeled pERK 1/2-immunoreactive neurons were likewise increased 6 h post-HCl (204%) and were mainly of small size (20-40 microm) and negative for neurofilament 200 (approximately 76%). The majority of these cells also expressed the nociceptive transient receptor potential vanilloid receptor 1 (approximately 70%). The gastric mucosa was simultaneously examined for lesion formation showing highest percentage of damage 6 h post-HCl. Application of a N-methyl-D-aspartate receptor antagonist (MK-801; 100 microg/kg s.c.) significantly reduced HCl-induced pERK 1/2 expression and mucosal lesions 6 h post-HCl. Activation of the extracellular signal-regulated kinase-1 and -2 signaling cascade indicates that visceral primary afferents may sensitize after gastric noxious stimulation involving N-methyl-D-aspartate receptors. The extracellular signal-regulated kinase-1 and -2 pathway therefore may not only be of importance for somatic but also for visceral nociception.

TRPV1 and the gut: from a tasty receptor for a painful vanilloid to a key player in hyperalgesia

Capsaicin, the pungent ingredient in red pepper, has been used since ancient times as a spice, despite the burning sensation associated with its intake. More than 50 years ago, Nikolaus Jancso discovered that capsaicin can selectively stimulate nociceptive primary afferent neurons. The ensuing research established that the neuropharmacological properties of capsaicin are due to its activation of the transient receptor potential ion channel of the vanilloid type 1 (TRPV1). Expressed by primary afferent neurons innervating the gut and other organs, TRPV1 is gated not only by vanilloids such as capsaicin, but also by noxious heat, acidosis and intracellular lipid mediators such as anandamide and lipoxygenase products. Importantly, TRPV1 can be sensitized by acidosis and activation of various pro-algesic pathways. Upregulation of TRPV1 in inflammatory bowel disease and the beneficial effect of TRPV1 downregulation in functional dyspepsia and irritable bladder make this polymodal nociceptor an attractive target of novel therapies for chronic abdominal pain.

Analysis of the cellular expression pattern of beta-CGRP in alpha-CGRP-deficient mice

In this study we compared the alpha-calcitonin gene-related peptide (alphaCGRP) and betaCGRP expression patterns in wild-type and knockout mice by using quantitative reverse transcriptase polymerase chain reaction and immunohistochemistry. In dorsal root ganglia and spinal cord of wild-type animals, alphaCGRP mRNA was about two times more abundant than betaCGRP mRNA. The betaCGRP mRNA was the only isoform expressed in the intestine. In alphaCGRP knockout mice, we found no change in betaCGRP mRNA levels in dorsal root ganglia and spinal cord compared with wild-type controls, but a twofold decrease in the intestine. CGRP immunoreactivity (IR) was detected in many small and some large neurons in the dorsal root ganglia, was found in sensory fibers and motor neurons in the spinal cord, and labeled neuromuscular junctions in wild-type mice. In the dorsal root ganglia of alphaCGRP knockout mice, punctate betaCGRP-IR again was predominantly found in small neurons. In the spinal cord, betaCGRP-IR fibers were localized to the outermost layer of the dorsal horn. IR was found in the cell bodies of motor neurons, but it was undetectable in neuromuscular junctions. In the intestine, CGRP-IR was localized to neurons of the myenteric plexus and to fibers in the mucosal folds, with similar staining intensity in both wild-type and knockout mice. Finally, CGRP-IR was undetectable in preganglionic fibers and postganglionic sympathetic neurons in mice from both genotypes. Our results indicate that alphaCGRP and betaCGRP are variably coexpressed in different functional aspects of the mouse nervous system. This pattern suggests distinct roles for betaCGRP in pain, neuromuscular, and gastrointestinal systems.

Gastrointestinal mechanisms of satiation for food

Satiation for food comprises the physiological processes that result in the termination of eating. Satiation is evoked by physical and chemical qualities of ingested food, which trigger afferent signals to the brain from multiple sites in the GI tract, including the stomach, the proximal small intestine, the distal small intestine and the colon. The physiological nature of each signal's contribution to satiation and overall control of food intake is likely to vary, depending on the level of the GI tract from which the signal arises. This article is a critical, though non-exhaustive, review of our current understanding of the mechanisms and adaptive value of satiation signals from the stomach and intestine.

Role of capsaicin-sensitive afferent nerves in different models of gastric inflammation in rats

Capsaicin-sensitive afferent nerves are described as being protective against gastric inflammation; their destruction leads to an exacerbation of inflammatory processes. However, these nerves have been shown to exert a pro-inflammatory action on stress-induced gastritis in rats. Our study aimed to investigate the role of capsaicin-sensitive afferent nerves in different experimental models of gastritis in rats. Functional ablation of sensory nerves was achieved by systemic capsaicin treatment (100 mg/kg). Gastritis was induced by mild (iodoacetamide, diquat, surgical duodeno-gastric reflux [DGR]) and strong (70% ethanol, indomethacin) inflammatory agents. Antagonists of the CGRP1 and NK1 receptors, hCGRP8-37 and SR140333, were administered in rats treated with iodoacetamide and ethanol. Macroscopic damage scores (MDS), myeloperoxidase (MPO) activity and malondialdehyde (MDA) concentration were evaluated after sacrifice. Macroscopic lesions appeared only in ethanol and indomethacin gastritis and were enhanced by capsaicin treatment. Gastric MPO activity was significantly increased by all agents compared to controls. Capsaicin treatment did not have any effect on MPO activity in indomethacin-treated rats or in rats submitted to surgery for duodeno-gastric reflux. However, it abolished the increase in MPO induced by iodoacetamide and diquat, and significantly enhanced that induced by ethanol. hCGRP8-37 and SR140333 abolished the increase in MPO activity and MDA concentration in iodoacetamide treated rats. In ethanol-treated rats, SR140333 diminished MPO activity. These results indicate that, depending upon the nature and duration of the experimental inflammation, capsaicin-sensitive afferent nerves may act differently to control gastric inflammatory processes, suggesting the involvement of a neurogenic component in some forms of gastric inflammation.

Expression of cannabinoid CB1 receptors by vagal afferent neurons is inhibited by cholecystokinin

Both inhibitory (satiety) and stimulatory (orexigenic) factors from the gastrointestinal tract regulate food intake. In the case of the satiety hormone cholecystokinin (CCK), these effects are mediated via vagal afferent neurons. We now report that vagal afferent neurons expressing the CCK-1 receptor also express cannabinoid CB1 receptors. Retrograde tracing established that these neurons project to the stomach and duodenum. The expression of CB1 receptors determined by RT-PCR, immunohistochemistry and in situ hybridization in rat nodose ganglia was increased by withdrawal of food for > or =12 hr. After refeeding of fasted rats there was a rapid loss of CB1 receptor expression identified by immunohistochemistry and in situ hybridization. These effects were blocked by administration of the CCK-1 receptor antagonist lorglumide and mimicked by administration of CCK to fasted rats. Because CCK is a satiety factor that acts via the vagus nerve and CB1 agonists stimulate food intake, the data suggest a new mechanism modulating the effect on food intake of satiety signals from the gastrointestinal tract.

Increased expression of TRPV1 receptor in dorsal root ganglia by acid insult of the rat gastric mucosa

It is still unknown which receptors of peripheral sensory pathways encode and integrate an acid-induced nociceptive event in the gastric mucosa. The transient receptor potential vanilloid receptor 1 (TRPV1) and the acid-sensing ion channel 3 (ASIC3) are two nociception-related receptors. Here we investigated (i) to what extent these receptors are distributed in stomach-innervating neurons of dorsal root and nodose ganglia, using immunohistochemistry and retrograde tracing, and (ii) whether their expression is altered in response to a noxious acid challenge of the stomach. We also explored the presence of TRPV1 in the gastric enteric nervous system because of its possible expression by intrinsic sensory neurons. Most stomach-innervating neurons in nodose ganglia were immunoreactive for TRPV1 (80%) and ASIC3 (75%), these results being similar in the dorsal root ganglia (71 and 82%). RT-PCR and Western blotting were performed up to 6 h after oral application of 0.5 m HCl to conscious rats. TRPV1 protein was increased in dorsal root but not in nodose ganglia whereas TRPV1 and ASIC3 mRNAs remained unchanged. TRPV1 mRNA was detected in longitudinal muscle-myenteric plexus preparations of control stomachs and was not altered by the acid challenge. Combined vagotomy and ganglionectomy abolished expression of TRPV1, indicating that it may derive from an extrinsic source. In summary, noxious acid challenge of the stomach increased TRPV1 protein in spinal but not vagal or intrinsic sensory afferents. The TRPV1 receptor may be a key molecule in the transduction of acid-induced nociception of the gastric mucosa and a mediator of visceral hypersensitivity.

Protective role of vanilloid receptor type 1 in HCl-induced gastric mucosal lesions in rats

BACKGROUND

Effects of vanilloid-receptor agonists and antagonists on HCl-induced gastric lesions in rats were investigated to elucidate the role of vanilloid receptor type 1 (VR1) in gastric mucosal defense mechanisms.

METHODS

Gastric lesions in rats were evaluated after intragastric administration of 0.6 N HCl. The localization of VR1 in the stomach was investigated immunohistochemically.

RESULTS

Intragastric administration of capsaicin inhibited the formation of gastric lesions in a dose-dependent manner (0.1-2.5 mg/kg). The functional VR1 antagonists ruthenium red and capsazepine markedly aggravated HCl-induced gastric lesions in rats. The gastroprotective effect of capsaicin was attenuated by ruthenium red or capsazepine. It is reported that resiniferatoxin, [6]-gingerol and lafutidine are compounds that activate VR1 and/or capsaicin-sensitive afferent neurons. These compounds significantly inhibited the formation of HCl-induced gastric lesions, and their gastroprotective effects were inhibited by treatment with ruthenium red. The immunohistochemical studies revealed that nerve fibers expressing VR1 exist along gastric glands in the mucosa, around blood vessels in the submucosa, in the myenteric plexus, and in the smooth muscle layers, especially the circular muscle layer.

CONCLUSION

The results of this study suggest that VR1 plays a protective role in the gastric defensive mechanism in rats.

Gastric enteric neurones that respond to luminal injury

Challenge of the rat gastric mucosa with HCl stimulates intrinsic neurones in the myenteric plexus of the stomach as demonstrated by immunohistochemical detection of c-Fos. In multiple labelling experiments of whole-mounts and sections of the gastric corpus we determined the chemical code of the stimulated neurones and investigated further whether neural pathways involving capsaicin-sensitive afferents, cholinergic neurones or the vagal system contribute to the stimulation of these neurones. Intragastric (IG) administration of 0.5 m HCl caused c-Fos expression in 12% of myenteric neurones, whereas IG saline failed to induce c-Fos. All stimulated neurones stained for nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY), but not for choline acetyltransferase (ChAT). Fibres coexpressing NOS/VIP/NPY were found predominantly in the external muscle layer and the muscularis mucosae of the stomach wall. Pretreatment with capsaicin or hexamethonium, combination of both pretreatments or vagotomy reduced HCl-induced c-Fos expression by 54%, 66%, 63% and 68%, respectively. The data indicate that mucosal acid challenge of the stomach stimulates inhibitory motor neurones in the myenteric plexus and that capsaicin-sensitive afferents as well as cholinergic neurones participate in the neuronal stimulation probably via a vago-vagal reflex.

Stomach-brain communication by vagal afferents in response to luminal acid backdiffusion, gastrin, and gastric acid secretion

Vagal afferents play a role in gut-brain signaling of physiological and pathological stimuli. Here, we investigated how backdiffusion of luminal HCl or NH(4)OH and pentagastrin-stimulated acid secretion interact in the communication between rat stomach and brain stem. Rats were pretreated intraperitoneally with vehicle or appropriate doses of cimetidine, omeprazole, pentagastrin, dexloxiglumide (CCK(1) receptor antagonist), and itriglumide (CCK(2) receptor antagonist) before intragastric administration of saline or backdiffusing concentrations of HCl or NH(4)OH. Two hours later, neuronal activation in the nucleus of the solitary tract (NTS) and area postrema was visualized by c-Fos immunohistochemistry. Exposure of the rat gastric mucosa to HCl (0.15-0.5 M) or NH(4)OH (0.1-0.3 M) led to a concentration-dependent expression of c-Fos in the NTS, which was not related to gender, gastric mucosal injury, or gastropyloric motor alterations. The c-Fos response to HCl was diminished by cimetidine and omeprazole, enhanced by pentagastrin, and left unchanged by dexloxiglumide and itriglumide. Pentagastrin alone caused an omeprazole-resistant expression of c-fos, which in the NTS was attenuated by itriglumide and prevented by dexloxiglumide but in the area postrema was reduced by dexloxiglumide and abolished by itriglumide. We conclude that vagal afferents transmit physiological stimuli (gastrin) and pathological events (backdiffusion of luminal HCl or NH(4)OH) from the stomach to the brain stem. These communication modalities interact because, firstly, acid secretion enhances afferent signaling of gastric acid backdiffusion and, secondly, gastrin activates NTS neurons through stimulation of CCK(1) receptors on vagal afferents and of CCK(2) receptors on area postrema neurons projecting to the NTS.

Role of nerve growth factor in the trinitrobenzene sulfonic acid-induced colonic hypersensitivity

The majority of patients with digestive disorders display visceral pain. In these troubles, visceral pain threshold is decreased, demonstrating visceral hypersensitivity. There is growing evidence that nerve growth factor (NGF) may function as a mediator of persistent pain states. This hypothesis was tested in a model of colonic hypersensitivity measured by isobaric distension in conscious rats. This study was designed to evaluate (1) the effect of exogenous NGF on colonic pain threshold, (2) the involvement of NGF in trinitrobenzene sulfonic acid (TNBS)-induced colonic hypersensitivity, by testing an anti-NGF antibody, and (3) finally the involvement of sensory nerves on NGF and TNBS effects using rats treated neonatally with capsaicin. Intra-peritoneal injection of NGF (0.1-100 ng/rat) decreased in a dose-related manner colonic pain threshold in naive rats. This effect was reversed by anti-NGF antibody (1/2000; 2 ml/kg). TNBS-induced colonic hypersensitivity was also reversed by anti-NGF antibody (1/2000; 2 ml/kg): 37.7 +/- 1.7 and 17.6 +/- 0.7 mmHg (p<0.01) for anti-NGF antibody- and vehicle-treated group, respectively. Neonatal capsaicin pre-treatment inhibited NGF- and TNBS-induced decrease in colonic pain threshold: 49.4 +/- 5.3 versus 22.3 +/- 1.6 mmHg (p<0.01) for capsaicin versus vehicle in NGF-treated rats and 39.6 +/- 3.3 versus 18.0 +/- 1.0 mm Hg (p<0.001) for capsaicin versus vehicle in TNBS-treated rats. These data suggest that the action of NGF on sensory neurons contributes to the development of visceral hypersensitivity and that anti-NGF strategy may be of some therapeutic benefits in digestive sensory disorders.

Neurochemical characterization of extrinsic innervation of the guinea pig rectum

The presence of markers for parasympathetic, sympathetic, and glutamatergic or peptidergic sensory innervation was investigated by using in vitro tracing with biotinamide, combined with immunohistochemistry, to characterise quantitatively extrinsic axons to myenteric ganglia of the guinea pig rectum. Of biotinamide-filled varicose axons, 3.6 +/- 1.3% were immunoreactive for tyrosine hydroxylase (TH) and 16.0 +/- 4.8% for vesicular acetylcholine transporter (VAChT). TH and vesicular monoamine transporter (VMAT1) showed high coexistence (83-100%), indicating that varicosities lacking TH immunoreactivity also lacked VMAT1. VAChT was detectable in 77% of choline acetyltransferase (ChAT)-immunoreactive varicosities. Calcitonin gene-related peptide (CGRP) was detected in 5.3 +/- 1.6% of biotinamide-labeled varicosities, the vesicular glutamate transporter (VGluT) 1 in 2.8 +/- 0.8%, and VGluT2 in 11.3 +/- 4.2% of varicosities of extrinsic origin. Varicosities from the same axon showed consistent immunoreactivity. A novel type of nerve ending was identified, with branching, flattened lamellar endings, similar to the intraganglionic laminar endings (IGLEs) of the proximal gut. Rectal IGLEs were frequently immunoreactive for VGluT1 and VGluT2. Thus most varicose axons of extrinsic origin, which innervate rectal myenteric ganglia, lack detectable levels of immunoreactivity for TH, VMAT1, VAChT, ChAT, VGluT1/2, or CGRP, under conditions in which these markers are readily detectable in other axons. Although some unlabeled varicosities may belong to afferent axons that lack detectable CGRP or VGluT1/2 in the periphery, this suggests that a large proportion of axons do not release any of the major autonomic or sensory transmitters. We speculate that this may vary under particular circumstances, for example, inflammation or obstruction of the gut.

Contribution of capsaicin-sensitive afferent nerves to rapid recovery from ethanol-induced gastric epithelial damage in rats

BACKGROUND AND AIM

It is well known that capsaicin-sensitive nerve signaling acts as a protective factor against various ulcerogens. However, the contribution of topical capsaicin-sensitive nerves within the stomach to rapid restitution has not been fully investigated. The present study was therefore conducted focusing on recovery from gastric mucosal damage induced by ethanol in vivo.

METHODS

Male Sprague-Dawley rats were fasted and anesthetized. 51Cr-EDTA was administered intravenously and gastric mucosal integrity was continuously monitored by measuring the blood to lumen 51Cr-EDTA clearance. Capsaicin or vehicle was irrigated before, together with or after the perfusion of 20% ethanol, followed by perfusion with saline. In another experiment, ruthenium red, a capsaicin-sensitive cation antagonist, was given before the ethanol-capsaicin perfusion. Furthermore, this study was verified using lafutidine, a histamine H2-receptor antagonist, which has a gastric mucosal protective action through the capsaicin-sensitive afferent nerves.

RESULTS

When capsaicin was administered before ethanol treatment, mucosal damage was significantly reduced and recovery was significantly rapid compared to the control. When capsaicin (160 micro M) and ethanol were administered simultaneously, the mucosal damage was exacerbated but recovery was nevertheless more rapid than the control group. With a lower dose of capsaicin (80 micro M), mucosal damage was not exacerbated and recovery was enhanced. When capsaicin or lafutidine was administered after the induction of ethanol injury no change was detected regarding the damage. However, recovery was significantly accelerated. Ruthenium red reversed the action of post-treatment with capsaicin on restitution.

CONCLUSIONS

These results indicate that luminal administration of capsaicin exerts protection against and accelerates restitution from gastric damage in the very early phase after ethanol injury. This action is probably due to activation of topical capsaicin-sensitive afferent nerves in the rat.

Immunohistochemical characterisation of pelvic autonomic ganglia in male mice

Pelvic ganglia are mixed sympathetic-parasympathetic ganglia and provide the majority of the autonomic innervation to the urogenital organs. Here we describe the structural and histochemical features of the major pelvic ganglion in the male mouse and compare two different mouse strains. The basic structural features of the ganglion are similar to those in the male rat. Almost all pelvic ganglion cells are monopolar and most are cholinergic. All contain either neuropeptide Y (NPY) or vasoactive intestinal peptide (VIP), or both peptides together. The peptide coexistence varies between strains, with C57BL/6 mice having similar proportions of neurons with NPY alone, VIP alone or both peptides. In contrast, virtually all pelvic neurons in the Quackenbush-Swiss (QS) strain express NPY, i.e. the level of VIP/NPY coexistence is much higher. Cholinergic axons provide the major nerve supply to epithelia of reproductive organs, bladder smooth muscle and, as described previously, penile erectile tissue. They also provide a minor component of the smooth muscle innervation of the prostate gland, seminal vesicles and vas deferens. Virtually all non-cholinergic pelvic ganglion cells are noradrenergic and contain NPY. Their major target is smooth muscle of reproductive organs. This study shows that the male mouse pelvic ganglion bears many similarities to that in the rat, but that VIP/NPY colocalisation is much more common in the mouse. We also show that there are differences in peptide expression in parasympathetic pelvic neurons between strains of mice. These studies provide the framework for future investigations on neural regulation of urogenital function, particularly in transgenic and knockout models.

Capsaicin-sensitive extrinsic afferents are involved in acid-induced activation of distinct myenteric neurons in the rat stomach

Challenge of the rat gastric mucosa with 0.5 mol L(-1) HCl activates nitrergic neurons in the myenteric plexus as visualized by c-Fos immunohistochemistry. In the present study, we characterized the activated neurons more extensively by their chemical coding and investigated whether a neural pathway that involves capsaicin-sensitive extrinsic afferents and/or cholinergic neurons transmitting via nicotinic receptors contributes to the activation of myenteric neurons. In multiple labelling experiments, c-Fos was examined for co-localization with nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), enkephalin (ENK), gastrin-releasing peptide (GRP), substance P (SP), calbindin D-28k (CALB) and neurofilament 145 (NF 145). All c-Fos-positive neurons were immunoreactive for NOS, VIP, NPY and NF 145, but not for SP, ENK, GRP and CALB. Nerve fibres co-expressing NOS, VIP and NPY were predominantly found in the external muscle layer and in the muscularis mucosae but rarely in the mucosa. Pre-treatment with capsaicin or hexamethonium or a combination of both pre-treatments reduced HCl-induced c-Fos expression by 54, 66 and 63%, respectively. Acid challenge of the stomach, therefore, leads to activation of presumably inhibitory motor neurons responsible for muscle relaxation. Activation of these neurons is partly mediated by capsaicin-sensitive afferents and involves ganglionic transmission via nicotinic receptors.

Non-electrical functions of neurons

Classical histological preparations of metasympathetic nervous system impregnated with silver salts were compared with the dynamics of the development of plexuses in cultures of isolated neurons and electron microscopic studies to investigate the non-electrical properties of neurons. Two modes of neurite contraction were demonstrated, along with a means of translocation of the neuron soma within neurites, and formation of tail processes and autotomy. It is hypothesized that nerve receptor endings have tissue trophic effects, with autoamputation of terminals with subsequent activation of their lysosomal proteases, which appear to be growth factors for surrounding tissues. The morphogenetic functions of neurocytes are analyzed and evidence is presented for the occurrence of constant reconstruction of previously formed plexuses in healthy adult animals.

Morphological examination of the termination pattern of substance P-immunoreactive nerve fibers in human antral mucosa

The termination pattern of substance P (SP)-containing axons in human antral mucosa was examined using immunohistochemical techniques at the light and electron microscopic level. SP-immunoreactive (IR) axons were found to extend towards the pit region of the glands, where intraepithelial axons were observed. Electron microscopy showed immunostained axon profiles in close contact with the basement membrane of surface mucous cells. Membrane-to-membrane contacts between labeled axons and myofibroblast-like cells were identified, and SP-IR axons that were apposed to the epithelium were also in contact with subjacent myofibroblast-like cells. The anatomical relationship between SP-IR axons and the cells of the muscularis mucosae was investigated by light microscopy. Immunoreactivity for alpha-smooth muscle actin (alpha-sma) was used to visualize the smooth muscle cells, and the alpha-sma-IR cells were found to create a network that surrounded the gastric glands. Immunostained varicose axons ran alongside and in close apposition to the labeled muscle strands. Ultrastructural examination showed close contacts between SP-IR axon profiles and smooth muscle-like cells. In conclusion, SP-containing neurons may be important for sensory and secretomotor functions in the human antral mucosa.

Localization of calcitonin gene-related peptide receptors in rat gastric mucosa

The location of calcitonin gene-related peptide (CGRP) receptors in the rat stomach has not been elucidated. It was recently reported that the CGRP receptor is formed when a calcitonin-receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP) 1 are co-expressed on the cell membrane. The aim of this study was to determine the location and the role of CGRP receptors in the rat gastric mucosa. Gene expressions of CRLR and RAMP1 were investigated by Northern blot analysis, reverse transcription-polymerase chain reaction (RT-PCR), and in situ hybridization. Immunohistochemical stainings for CGRP, somatostatin, gastrin, and chromogranin A were performed. Gastric endocrine cells were collected by counterflow-elutriation and their responses to CGRP were studied. CRLR and RAMP1 mRNA was expressed mainly in small gastric epithelial cells in the pyloric glands. The mRNA expression had a similar distribution to that of D cells. In cultured gastric endocrine cells, CGRP enhanced somatostatin production, while it inhibited the secretion of histamine and gastrin. Our results suggest that CGRP receptors are expressed in D cells in the rat gastric mucosa and control production and secretion of somatostatin.

Expression of the leptin receptor in rat and human nodose ganglion neurones

There is evidence for interactions between leptin and cholecystokinin in controlling food intake. Since cholecystokinin acts on vagal afferent neurones, we asked whether the leptin receptor was also expressed by these neurones. Primers for different forms of the leptin receptor were used in reverse transcriptase-polymerase chain reaction (RT-PCR) of rat and human nodose ganglia. RT-PCR yielded products corresponding to the long (functional) form as well as short forms of the rat leptin receptor. Moreover, RT-PCR revealed the long form of the leptin receptor in a human nodose ganglion. The identities of RT-PCR products were confirmed by sequencing. Primers corresponding to leptin itself did not give RT-PCR products in nodose ganglia. Immunocytochemical studies revealed leptin-receptor immunoreactivity in neuronal cell bodies. Many neurones co-expressed the leptin and cholecystokinin type A receptors, or leptin receptor and cocaine- and amphetamine-related transcript. We conclude that vagal afferent neurones that express the cholecystokinin type A receptor and cocaine- and amphetamine-related transcript, may also express the long form of the leptin receptor providing a neurochemical basis for observations of interactions between cholecystokinin and leptin.

Sensitizing effects of lafutidine on CGRP-containing afferent nerves in the rat stomach

1. Capsaicin sensitive afferent nerves play an important role in gastric mucosal defensive mechanisms. Capsaicin stimulates afferent nerves and enhances the release of calcitonin gene-related peptide (CGRP), which seems to be the predominant neurotransmitter of spinal afferents in the rat stomach, exerting many pharmacological effects by a direct mechanism or indirectly through second messengers such as nitric oxide (NO). 2. Lafutidine is a new type of anti-ulcer drug, possessing both an antisecretory effect, exerted via histamine H(2) receptor blockade, and gastroprotective activities. Studies with certain antagonists or chemical deafferentation techniques suggest the gastroprotective actions of lafutidine to be mediated by capsaicin sensitive afferent nerves, but this is an assumption based on indirect techniques. In order to explain the direct relation of lafutidine to afferent nerves, we conducted the following studies. 3. We determined CGRP and NO release from rat stomach and specific [(3)H]-resiniferatoxin (RTX) binding to gastric vanilloid receptor subtype 1 (VR1), which binds capsaicin, using EIA, a microdialysis system and a radioreceptor assay, respectively. 4. Lafutidine enhanced both CGRP and NO release from the rat stomach induced by a submaximal dose of capsaicin, but had no effect on specific [(3)H]-RTX and capsaicin binding to VR1. 5. In conclusion, our findings demonstrate that lafutidine modulates the activity of capsaicin sensitive afferent nerves in the rat stomach, which may be a key mechanism involved in its gastroprotective action.

Characterization of mechanosensitive splanchnic nerve afferent fibers innervating the rat stomach

Splanchnic nerve fibers innervating the stomach were studied in anesthetized rats; 997 fibers in the T(9) or T(10) dorsal roots were identified by electrical stimulation of the splanchnic nerve. Thirty-one fibers responded to gastric distension. Extrapolated response thresholds ranged between 0 and 53 mmHg; seven fibers had thresholds for response > or =30 mmHg. Thermo- and/or chemosensitivity was tested in 18 of the 31 fibers. Four of twelve fibers responded to intragastric perfusion of heated saline; none of eight fibers tested responded to perfusion of cold saline. Infusion of glucose, L-arginine, or potassium oleate produced no change in resting activity. Intragastric instillation of 12% glycerol or an inflammatory soup (bradykinin 10(-5) M, PGE(2) 10(-5) M, serotonin 10(-5) M, histamine 10(-5) M, and KCl 10(-3) M) and prior heat stimulation sensitized responses to distension. The results reveal the presence of low- and high-threshold mechanosensitive fibers in the splanchnic innervation of the stomach. These fibers have the ability to sensitize, and they likely contribute to pain and altered sensations that can arise from the stomach.

ECL-cell histamine mobilization in conscious rats: effects of locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators

1. The ECL cells control gastric acid secretion by mobilizing histamine in response to circulating gastrin. In addition, the ECL cells are thought to operate under nervous control and to be influenced by local inflammatory processes. 2. The purpose of the present study was to monitor histamine mobilization from ECL cells in conscious rats in response to locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators. 3. Microdialysis probes were implanted in the submucosa of the acid-producing part of the rat stomach. Three days later, the agents to be tested were administered via the microdialysis probe and their effects on basal (48 h fast) and stimulated (intravenous infusion of gastrin-17, 3 nmol kg(-1) h(-1)) mobilization of ECL-cell histamine was monitored by continuous measurement of histamine in the perfusate (radioimmunoassay). 4. Locally administered gastrin-17 and sulfated cholecystokinin-8 mobilized histamine as did pituitary adenylate cyclase-activating peptide-27, vasoactive intestinal peptide, peptide YY, met-enkephalin, endothelin and noradrenaline, adrenaline and isoprenaline. 5. While gastrin, sulfated-cholecystokinin-8, met-enkephalin and isoprenaline induced a sustained elevation of the submucosal histamine concentration, endothelin, peptide YY, pituitary adenylate cyclase activating peptide, vasoactive intestinal peptide, noradrenaline and adrenaline induced a transient elevation. 6. Calcitonin gene-related peptide, galanin, somatostatin and the prostanoid misoprostol inhibited gastrin-stimulated histamine mobilization. 7. The gut hormones neurotensin and secretin and the neuropeptides gastrin-releasing peptide, neuropeptide Y and substance P failed to affect ECL-cell histamine mobilization, while motilin and neuromedin U-25 had weak stimulatory effects. Also acetylcholine, carbachol, serotonin and the amino acid neurotransmitters aspartate, gamma-aminobutyric acid, glutamate and glycine were inactive or weakly active as was bradykinin. 8. In summary, a range of circulating hormones, local hormones, catecholamines, neuropeptides and inflammatory mediators participate in controlling the activity of rat stomach ECL cells in situ.

Gastrointestinal afferents as targets of novel drugs for the treatment of functional bowel disorders and visceral pain

An intricate surveillance network consisting of enteroendocrine cells, immune cells and sensory nerve fibres monitors the luminal and interstitial environment in the alimentary canal. Functional bowel disorders are characterized by persistent alterations in digestive regulation and gastrointestinal discomfort and pain. Visceral hyperalgesia may arise from an exaggerated sensitivity of peripheral afferent nerve fibres and/or a distorted processing and representation of gut signals in the brain. Novel strategies to treat these sensory bowel disorders are therefore targeted at primary afferent nerve fibres. These neurons express a number of molecular traits including transmitters, receptors and ion channels that are specific to them and whose number and/or behaviour may be altered in chronic visceral pain. The targets under consideration comprise vanilloid receptor ion channels, acid-sensing ion channels, sensory neuron-specific Na(+) channels, P2X(3) purinoceptors, 5-hydroxytryptamine (5-HT), 5-HT(3) and 5-HT(4) receptors, cholecystokinin CCK(1) receptors, bradykinin and prostaglandin receptors, glutamate receptors, tachykinin and calcitonin gene-related peptide receptors as well as peripheral opioid and cannabinoid receptors. The utility of sensory neuron-targeting drugs in functional bowel disorders will critically depend on the compounds' selectivity of action for afferent versus enteric or central neurons.

Effects of vagal and splanchnic section on food intake, weight, serum leptin and hypothalamic neuropeptide Y in rat

Truncal vagotomy can cause reduced food intake and weight loss in humans and laboratory animals. In order to investigate some of the factors that might contribute to this effect, we studied changes in ingestive behaviour, whole body and organ weights, serum leptin and hypothalamic neuropeptide Y in rats with bilateral vagal section, bilateral splanchnic nerve section and combined vagotomy plus splanchnectomy. Pyloromyotomy was combined with vagotomy to lessen effects of vagotomy on gastric emptying. Animals with vagotomy or vagotomy plus splanchnectomy lost weight and decreased their daily food intake relative to animals with splanchnectomy alone, rats with bilateral sham exposure of one or both nerve, or rats with pyloromyotomy alone. Serum leptin and white fat mass, 4 weeks after vagotomy, were about 20% of the values in the sham-operated animals at this time. No effect for splanchnic nerve section alone was observed. Pyloromyotomy caused no reduction in weight or fat mass, but reduced serum leptin. Following vagotomy with or without splanchnic nerve section, neuropeptide Y was elevated in the arcuate nucleus relative to values for the other four groups. Changes in neuropeptide Y were inversely correlated with levels of serum leptin. It is concluded that the effect of vagotomy could be due to the loss of a feeding signal carried by vagal afferent neurons, or to changed humoral signals, for example, increased production of a satiety hormone. However, it cannot be attributed to signals that reduce feeding (for example, gastric distension) reaching the central nervous system via the splanchnic nerves. The changes were sufficient to cause weight loss even though serum leptin was decreased, a change that would be expected to increase food intake.

Effect of calcitonin gene-related peptide (CGRP) on motility and on the release of substance P, neurokinin A, somatostatin and gastrin in the isolated perfused porcine antrum

We studied the effect of porcine CGRP (pCGRP) in concentrations from 10(-10) to 10(-8) mol L(-1) on the motility and on the release of substance P, neurokinin A, somatostatin and gastrin in the antrum using the isolated perfused porcine antrum as experimental model. In addition, we studied the localization of CGRP by immunohistochemistry in the porcine antrum. CGRP-immunoreactive nerve fibres were found mainly in the submucous layer and in the external muscle coat, where they were seen in all layers, and in the ganglia of the myenteric nervous plexus. The frequency of contraction was significantly and dose-dependently increased from a basal level of 11.8 +/- 0.5 contractions per 5 min to 24.4 +/- 3.6 contractions per 5 min at pCGRP 10(-8) mol L(-1). At this dose, the release of substance P and neurokinin A was significantly increased to 470 +/- 149% and 217 +/- 26%, respectively, compared to basal release. The effect of pCGRP was unaffected by the addition of the nonpeptide antagonists for the NK-1 (CP-99994) and NK-2 receptors (SR48968), both at 10(-6) mol L(-1), whereas atropine (10(-6) mol L(-1)) completely abolished the motor effect of pCGRP. The release of somatostatin was significantly increased by 154 +/- 15% in response to CGRP at 10(-8) mol L(-1). The release of gastrin was unaffected by pCGRP. In conclusion, pCGRP increases contractile activity in the porcine antrum, an effect that involves cholinergic mechanisms but is independent of the release of substance P and neurokinin A. in addition, pCGRP increases the release of somatostatin but has no effect on gastrin release in the isolated perfused porcine antrum.

Effects of octreotide on responses to colorectal distension in the rat

BACKGROUND AND AIMS

It has been suggested that the analgesic effect of the somatostatin analogue octreotide in visceral pain involves peripheral mechanisms. We evaluated the effect of octreotide on responses to noxious colorectal distension in rats.

METHODS

In a behavioural study, pressor and electromyographic responses to colorectal distension were evaluated before and after intravenous or intrathecal administration of octreotide. In pelvic nerve afferent fibre recordings, responses of mechanosensitive fibres innervating the colon to noxious colorectal distension (80 mm Hg, 30 seconds) were tested before and after octreotide.

RESULTS

Octreotide was ineffective in attenuating responses to colorectal distension in either normal or acetic acid inflamed colon when administered intravenously but attenuated responses when given intrathecally. Administration of octreotide over a broad dose range (0.5 microg/kg to 2.4 mg/kg) did not alter responses of afferent fibres to noxious colorectal distension in untreated, or acetic acid or zymosan treated colons.

CONCLUSIONS

In the rat, octreotide has no peripheral (pelvic nerve) modulatory action in visceral nociception. The antinociceptive effect of octreotide in this model of visceral nociception is mediated by an action at central sites.

Gastric mucosal integrity: gastric mucosal blood flow and microcirculation. An overview

The stomach is in a state of continuous exposure to potentially hazardous agents. Hydrochloric acid together with pepsin constitutes a major and serious threat to the gastric mucosa. Reflux of alkaline duodenal contents containing bile and pancreatic enzymes are additional important injurious factors of endogenous origin. Alcohol, cigarette smoking, drugs and particularly aspirin and aspirin-like drugs, and steroids are among exogenous mucosal irritants that can inflict mucosal injury. The ability of the stomach to defend itself against these noxious agents has been ascribed to a number of factors constituting the gastric mucosal defense. These include mucus and bicarbonate secreted by surface epithelial cells, prostaglandins, sulfhydryl compounds and gastric mucosal blood flow. The latter is considered by several researchers to be of paramount importance in maintaining gastric mucosal integrity. The aim of this paper is to review the experimental and clinical data dealing with the role of mucosal blood flow and in particular the microcirculation in both damage and protection of the gastric mucosa.

Capsaicin-sensitive afferents and their role in gastroprotection: an update

The pivotal role of capsaicin-sensitive peptidergic sensory fibers in the maintenance of gastric mucosal integrity against injurious interventions was suggested by the authors 20 years ago. Since then substantial evidence has accumulated for the local sensory-efferent function of the released CGRP, tachykinins and NO in this gastroprotective mechanism. This overview outlines some recent achievements which shed light on new aspects and further horizons in this field. (1) Cloning the capsaicin VR-1 receptor (an ion channel-coupled receptor) and raising the VR-1 knockout mice provided a definite molecular background for the existence of capsaicin-sensitive afferents with both sensory and mediator releasing functions in the stomach. This cation channel is also sensitive to hydrogen ions. (2) VR-1 agonists (capsaicin, resiniferatoxin, piperine) protect against gastric ulcer of the rat parallel with their sensory stimulating potencies. (3) Antidromic stimulation of capsaicin-sensitive vagal and somatic afferents results in the release of CGRP, tachykinins, NO and somatostatin. Somatostatin with gastroprotective effect is released from D cells and sensory nerve endings. (4) The recent theory for the existence of spinal afferents without sensory function [P. Holzer, C.A. Maggi, Dissociation of dorsal root ganglion neurons into afferent and efferent-like neurons, Neuroscience 86 (1998) 389-398] is discussed. Data proposed to support this theory are interpreted here on the basis of a dual sensory-efferent function of VR-1 positive afferents, characterized by a frequency optimum of discharges for release vasodilatory neuropeptides below the nociceptive threshold. (5) Recent data on the effect of capsaicin in healthy human stomach are summarized. These results indicate that the gastroprotective effect of capsaicin in the human stomach involves additional mechanisms to those already revealed in the rat.

Roles of central glutamate, acetylcholine and CGRP receptors in gastrointestinal afferent inputs to vagal preganglionic neurones

It is unknown which neurotransmitter receptors are involved in the transfer of sensory information from the digestive tract to the brainstem. We examined the potential involvement of central glutamate, acetylcholine, and CGRP receptors in vagal pathways arising from gut chemo- and mechanosensitive afferents. Unitary recordings were made from 21 vagal preganglionic efferent neurones. Neuronal discharge showed either excitation or inhibition in response to oesophageal balloon distension (1-2 ml), gastric distension (40-60 ml in whole stomach or 20 ml in proximal stomach), cholecystokinin-8s (100 pmol close intra-arterially) and bradykinin (18 nmol close intra-arterially). Effects of glutamatergic non-NMDA, muscarinic M1, and CGRP1 receptor antagonism on efferent responses were investigated using CNQX (75-155 nmol i.c.v.), pirenzepine (2.5-5.0 micromol/kg i.v.), and hCGRP8-37 (3.2-6.4 nmol i.c.v.), respectively. CNQX, pirenzepine, and hCGRP8-37, respectively, altered efferent responses in 65%, 23% and 41% of neurones. When both CNQX and hCGRP8-37 were administered, a further 58% of responses were reduced. CNQX and hCGRP8-37 reduced a proportion of efferent responses to all stimuli whereas pirenzepine selectively reduced only efferent responses to gastric distension. We conclude that central CGRP1 and non-NMDA receptors are involved in mediating a range of upper gastrointestinal mechano- and chemo-sensitive afferent inputs onto vagal efferents. M1 receptors, on the other hand, are selectively involved in neurotransmission from gastric mechanoreceptors.

Systemic anti-inflammatory effect of somatostatin released from capsaicin-sensitive vagal and sciatic sensory fibres of the rat and guinea-pig

The systemic anti-inflammatory effect induced by antidromic sensory nerve stimulation was investigated in rats and guinea-pigs. In atropine-pretreated rats, bilateral antidromic stimulation of vagal afferent fibres (8 Hz, 20 min, at C-fibre strength) inhibited plasma extravasation induced by 1% mustard oil on the acutely denervated hindlegs by 36.45+/-3.95%. Both the prevention of this inhibitory effect by cysteamine pretreatment and the stimulation-evoked rise of plasma somatostatin-like immunoreactivity in the two species suggest a mediator role of neural somatostatin. Since this response was blocked by systemic capsaicin pretreatment and slightly reduced after subdiaphragmal vagotomy, participation of thoracic capsaicin-sensitive afferents is indicated. In guinea-pigs pretreated with guanethidine and pipecuronium, antidromic sciatic nerve stimulation induced 45.46+/-5.08% inhibition on the contralateral leg and increased plasma somatostatin-like immunoreactivity. It is concluded that somatostatin released from the activated vagal capsaicin-sensitive sensory nerve terminals of the rat and somatic nerves of the guinea-pigs exerts a systemic humoral function.

Calcitonin gene-related peptide regulates amino acid absorption across rat jejunum

The calcitonin gene related peptide (CGRP) is widely distributed in the enteric nervous system and gut afferents. Its role in normal digestion and absorption is not characterised. This study is conducted to elucidate whether CGRP regulates amino acid absorption in the small intestine. In in vivo experiments using the single-pass perfusion technique, intravenous infusion of CGRP (250-750 pmol/kg-min) reduced alanine absorption by 35-40%. The effects were completely blocked by the antagonist hCGRP (8-37). Moreover, intravenous infusion of CGRP antagonist blocked the inhibitory effect of intraluminal capsaicin perfusion on alanine absorption. Similarly, intracerebral injection of CGRP decreased alanine absorption, an effect which was reduced by vagotomy. In vitro experiments using isolated jejunal strips showed that CGRP reduced alanine absorption in a dose-dependent manner. At 6 pM, CGRP decreased alanine absorption by 33%. Similarly, CGRP reduced the absorption of proline and taurine by 20 and 11.5%, respectively. Kinetic studies revealed that CGRP reduces alanine influx into intestinal epithelial cells by inhibiting the affinity of the carriers. It is demonstrated that CGRP is involved in the regulation of jejunal amino acid absorption through intrinsic (enteric) and extrinsic (central) neural mechanisms.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

The mechanism of action of amtolmetin guacyl, a new gastroprotective nonsteroidal anti-inflammatory drug

Amtolmetin guacyl (2-methoxyphenyl-1-methyl-5-p-methylbenzoyl-pyrrol-2-acetamido acetate) (MED15) is a new nonsteroidal anti-inflammatory drug (NSAID) with anti-inflammatory, analgesic and antipyretic properties similar to the traditional drugs, but with unexpected gastroprotective effects. In an in vivo rat model, amtolmetin guacyl administered orally demonstrates inhibition of gastric acid secretion following stimulation by various agonists, and up-regulation of gastric bicarbonate production. Pretreatment with MED15 also shows a significant reduction of indomethacin-induced gastric damage in the rat. The reason behind this behaviour appears to be bound to the presence in the MED15 molecule of a vanillic moiety known to stimulate capsaicin receptors. In fact, the antisecretive effect of MED15 is blocked by capsazepine (a specific capsaicin receptor antagonist). This effect is confirmed by the interference found with anti-histamine H(1) drugs. Owing to the connection between capsaicin and calcitonin gene-related peptide (CGRP), a possible effect of MED15 on CGRP receptors was hypothesized, considering the leading role played on gastric mucosa by the predominant sensory neuropeptide of the stomach wall, CGRP. In fact, the anti-secretive and gastroprotective effect of MED15 is abolished by CGRP-(8-37) (the specific CGRP receptor antagonist). The unmodified MED15 molecule is found throughout the gastroenteric tract for long periods of time following oral administration, as further confirmation of the mechanism of action being based on the presence of the vanillic moiety at receptor level.

Substance P may attenuate gastric hyperemia by a mast cell-dependent mechanism in the damaged gastric mucosa

Calcitonin gene-related peptide (CGRP) released from sensory neurons, which are closely apposed to mast cells and blood vessels, mediates gastric hyperemia in response to acid challenge of the damaged mucosa. Substance P (SP) is coreleased with CGRP from sensory neurons, but the role of this peptide in gastric blood flow regulation is largely unknown. Chambered rat stomachs were exposed to 1.5 M NaCl and acidic saline after treatment with SP, aprotinin (serine protease inhibitor), and the mast cell stabilizers ketotifen and sodium cromoglycate (SCG). Gastric hyperemia (measured with a laser Doppler flow velocimeter) after hypertonic injury and acid challenge was nearly abolished by SP. Aprotinin infused together with SP and pretreatment with ketotifen and SCG before SP restored the gastric hyperemia. Ketotifen and SCG inhibited mast cell degranulation in SP-treated rats. Preservation of gastric hyperemia was correlated with improved mucosal repair. These data suggest that impaired hyperemia by SP during acid challenge of the gastric mucosa may be mediated by a mast cell-dependent mechanism involving the release of proteases from mast cells.

Capsaicin inhibits the pentagastrin-stimulated gastric acid secretion in anaesthetized rats with acute gastric fistula

The effect of capsaicin on basal and pentagastrin-stimulated gastric acid secretion was investigated in the urethane anaesthetized acute gastric fistula rat. Gastric acid secretion was measured by flushing of the gastric lumen with saline every 15 min or by continuous gastric perfusion. Capsaicin given into the rat stomach at 120 ng x mL(-1) prior to pentagastrin (25 microg x kg(-1), iv) reduced gastric acid secretory response to pentagastrin by 24%. Intravenous (iv) capsaicin (0.5 microg x kg(-1)) did not reduce the pentagastrin-stimulated gastric acid secretion. After topical capsaicin desensitization (3 mg x mL(-1)), basal gastric acid secretion and that in response to pentagastrin (25 microg x kg(-1), intraperitonaeally) was unaltered compared with the control group. Data indicate that topical capsaicin inhibits gastric acid secretion stimulated with pentagastrin in anaesthetized rats.

Capsaicin-sensitive afferent sensory nerves in modulating gastric mucosal defense against noxious agents

In the rat stomach, evidence has been provided that capsaicin-sensitive sensory nerves (CSSN) are involved in a local defense mechanism against gastric ulcer. In the present study capsaicin or resiniferatoxin (RTX), a more potent capsaicin analogue, was used to elucidate the role of these sensory nerves in gastric mucosal protection, mucosal permeability, gastric acid secretion and gastrointestinal blood flow in the rat. In the rat stomach and jejunum, intravenous RTX or topical capsaicin or RTX effected a pronounced and long-lasting enhancement of the microcirculation at these sites, measured by laser Doppler flowmetry technique. Introduction of capsaicin into the rat stomach in very low concentrations of ng-microg x mL(-1) range protected the gastric mucosa against damage produced by topical acidified aspirin, indomethacin, ethanol or 0.6 N HCl. Resiniferatoxin exhibited acute gastroprotective effect similar to that of capsaicin and exerted marked protective action on the exogenous HCl, or the secretagogue-induced enhancement of the indomethacin injury. The ulcer preventive effect of both agents was not prevented by atropine or cimetidine treatment. Capsaicin given into the stomach in higher desensitizing concentrations of 6.5 mM markedly enhanced the susceptibility of the gastric mucosa and invariably aggravated gastric mucosal damage evoked by later noxious challenge. Such high desensitizing concentrations of capsaicin, however, did not reduce the cytoprotective effect of prostacyclin (PGI2) or beta-carotene. Capsaicin or RTX had an additive protective effect to that of atropine or cimetidine. In rats pretreated with cysteamine to deplete tissue somatostatin, capsaicin protected against the indomethacin-induced mucosal injury. Gastric acid secretion of the pylorus-ligated rats was inhibited with capsaicin or RTX given in low non-desensitizing concentrations, with the inhibition being most marked in the first hour following pylorus-ligation. Low intragastric concentrations of RTX reduced gastric hydrogen ion back-diffusion evoked by topical acidified salicylates. It is concluded that the gastropotective effect of capsaicin-type agents involves primarily an enhancement of the microcirculation effected through local release of mediator peptides from the sensory nerve terminals. A reduction in gastric acidity may contribute to some degree in the gastric protective action of capsaicin-type agents. The vasodilator and gastroprotective effects of capsaicin-type agents do not depend on vagal efferents or sympathetic neurons, involve prostanoids, histaminergic or cholinergic pathways.

Effect of lafutidine, a novel histamine H2-receptor antagonist, on monochloramine-induced gastric lesions in rats: role of capsaicin-sensitive sensory neurons

BACKGROUND

Lafutidine ((+/-)-2-(furfurylsulfinyl)-N-(4-(4-(piperidinomethyl)-2-pyr idyl)oxy-(Z)-2-butenyl)acetamide) is a novel histamine H2-receptor antagonist and has been shown to exhibit a potent gastroprotective activity in addition to its antisecretory action. In the present study, we examined the effects of lafutidine on the mucosal ulcerogenic and potential difference (PD) responses induced by monochloramine (NH2Cl) in rat stomachs.

METHODS

Oral administration of NH2Cl at 120 mmol/L produced haemorrhagic lesions in the stomach in unanaesthetized rats.

RESULTS

Lafutidine (3-30mg/kg), given p.o., showed a dose-dependent and significant inhibition against damage caused by NH2Cl: the effect was significant at 10 mg/kg or greater but disappeared almost totally in the sensory deafferented animals following capsaicin pretreatment. Likewise, capsaicin (10 mg/kg, p.o.), but not cimetidine (100 mg/kg, p.o.) exhibited a potent protection against NH2Cl-induced gastric lesions. Topical application of NH2Cl (10 mmol/L) reduced transmucosal PD in ex-vivo stomachs of anaesthetized rats, but this PD response was also prevented by pre-exposure to lafutidine, in a dose-dependent and sensory neuron-sensitive manner. Mucosal exposure to NH4OH (60 mmol/L) also caused PD reduction in ex-vivo stomachs made ischaemic by bleeding from the carotid artery (1 mL/100 g bodyweight), resulting in severe gastric lesions. These ulcerogenic and PD responses caused by NH4OH plus ischaemia were attenuated by prior application of lafutidine as well as taurine, a scavenger of NH2Cl. The former effect was, again, dependent on the sensory neurons. Intraluminal capsaicin but not cimetidine was also effective in preventing a PD response to NH2Cl.

CONCLUSIONS

These results suggest that lafutidine, but not cimetidine, protects the stomach against NH2Cl, whether occurring endogenously or administered exogenously and that this action may be mediated by capsaicin-sensitive sensory neurons.

Immunohistochemical localization of calcitonin gene-related peptide in the human gastric mucosa

BACKGROUND/AIM

There have been only a few studies on the distribution of calcitonin gene-related peptide (CGRP) in the human stomach, in which it was stated that CGRP fibers are rare in that organ. The aim of the present study was to investigate the immunohistochemical localization of CGRP in the human gastric mucosa obtained by endoscopic biopsy from patients with gastric ulcers.

METHODS

Immunohistochemistry was carried out according to the indirect immunoperoxidase method using an anti-human CGRP antibody. Biopsies were taken from the ulcer margin in 18 patients (age 37-78, average 57.4 years) and from two endoscopically normal portions (antrum and body) in 7 other patients (age 36-65, average 51.0 years). One biopsy specimen was obtained from each portion.

RESULTS

Twelve of the eighteen biopsy specimens from the ulcer margin, 6 of the 7 biopsy specimens from normal portions of the antrum and 3 of the 7 biopsy specimens from normal portions of the body showed CGRP-immunoreactive staining. Intense staining was more marked in the specimens from the ulcer margin compared to those of the normal portions.

CONCLUSIONS

CGRP immunoreactivity was observed in the human gastric mucosa in considerable abundance, and it is presumed that CGRP might participate in a restoration mechanism of the ulcer.

Central autonomic activation by intracisternal TRH analogue excites gastric splanchnic afferent neurons

Intracisternal (ic) injection of thyrotropin-releasing hormone (TRH) or its stable analogue RX 77368 influences gastric function via stimulation of vagal muscarinic pathways. In rats, the increase in gastric mucosal blood flow evoked by a low ic dose of RX 77368 occurs via release of calcitonin gene-related peptide from capsaicin-sensitive afferent neurons, most probably of spinal origin. In this study, the effect of low ic doses of RX 77368 on afferent impulse activity in splanchnic single fibers was investigated. The cisterna magna of overnight-fasted, urethan-anesthetized Sprague-Dawley rats was acutely cannulated, and fine splanchnic nerve twigs containing at least one fiber responsive to mechanical probing of the stomach were isolated at a site immediately distal to the left suprarenal ganglion. Unit mechanoreceptive fields were encountered in all portions of the stomach, both superficially and in deeper layers. Splanchnic afferent unit impulse activity was recorded continuously during basal conditions and in response to consecutive ic injections of saline and RX 77368 (15-30 min later; 1.5 or 3 ng). Basal discharge rates ranged from 0 to 154 impulses/min (median = 10.2 impulses/min). A majority of splanchnic single units with ongoing activity increased their mean discharge rate by >/=20% after ic injection of RX 77368 at either 1.5 ng (6/10 units; median increase 63%) or 3 ng (19/24 units; median increase 175%). Five units lacking impulse activity in the 5-min before ic RX 77368 (3 ng) were also excited, with the onset of discharge occurring within 1.0-5.0 min postinjection. In units excited by ic RX 77368, peak discharge occurred 15.6 +/- 1.3 min after injection and was followed by a decline to stable activity levels </=20-40 min thereafter. In a few cases (4/24), ic RX 77368 (3 ng) inhibited the impulse activity of initially active units, with a time course comparable to that seen in units excited by the same treatment. The pattern of discharge in most units was not suggestive of mechanical modulation of activity by rhythmic gastric contractions. The data demonstrate that low ic doses of TRH analogue induce sustained increases in afferent discharge in a substantial proportion of splanchnic neurons innervating the rat stomach. These findings support the notion that splanchnic afferent excitation occurs concomitantly with vasodilatory peptide release from gastric splanchnic afferent nerve terminals after ic TRH-induced autonomic activation.

The peptide content of colonic afferents decreases following colonic inflammation

Peripheral injury produces long term changes in peptide content in dorsal root ganglion (DRG) cells that contribute to the inflammatory process in the periphery and neuronal plasticity in the spinal cord. We report here the proportion of colonic afferents labeled for calcitonin gene-related peptide (CGRP), substance P (SP) or somatostatin (Som) in the T13-L2 and L6-S2 DRG and changes in the percentage of SP or CGRP labeled afferents 6, 24, and 72 h following induction of experimental colitis. Following injection of fluorogold (FG) into the descending colon, significantly more FG labeled DRG cells were observed in the T13-L2 than L6-S2 DRG. In noninflamed rats, in both spinal regions, 60-70% of the colonic afferents that were labeled with FG were double labeled for SP. Similar results were obtained when double labeling for CGRP. Only 20-30% of the FG labeled afferents were double labeled for Som. Following experimental colitis induced by intracolonic zymosan, there was a significant decrease in the percentage of cells double labeled for SP in the T13-L2 and L6-S2 DRG at 6, 24, and 72 h. The percentage of CGRP double labeled cells was decreased in the T13-L2 DRG at all time points, but only at 24 h in the L6-S2 DRG. The cell bodies of CGRP labeled colonic afferents were significantly larger than SP or Som in control rats. Inflammation did not affect the mean size of the double labeled cells. These results suggest that colonic inflammation increases SP and CGRP release in the spinal cord and the colon that is manifest as a decrease in peptide content in the cell bodies of the colonic afferents during the first 72 h following injury.

Central and peripheral vagal mechanisms involved in gastric protection against ethanol injury

Activation of medullary thyrotropin-releasing hormone (TRH), at a dose subthreshold to increase gastric acid secretion, protects the gastric mucosa against ethanol injury through vagal cholinergic pathways in urethane-anaesthetized rats. Peripheral mediators involve the efferent function of capsaicin-sensitive splanchnic afferents leading to calcitonin gene-related peptide (CGRP)- and nitric oxide (NO)-dependent gastric vasodilatory mechanisms. In addition, gastric prostaglandins participate in gastric protection through mechanisms independent of the stimulation of gastric mucosal blood flow and mucus secretion. Medullary TRH has physiological relevance in the vagal-dependent adaptive gastric protection induced by mild (acid or ethanol), followed by strong, irritants. Additional neuropeptides, namely peptide YY (PYY), somatostatin analogues, CGRP and adrenomedullin, also act in the brainstem to induce a vagal-dependent gastric protection against ethanol through interactions with their specific receptors in the medulla. Central PYY and adrenomedullin act through vagal cholinergic prostaglandins and NO pathways, while somatostatin analogue acts through vagal non-adrenergic, non-cholinergic vasoactive intestinal peptide and NO mechanisms. Although their biological relevance is still to be established, these peptides provide additional tools to investigate the multiple vagal-dependent mechanisms which increase the resistance of the gastric mucosa to injury.

Vagal mechanisms underlying gastric protection induced by chemical activation of raphe pallidus in rats

Peripheral mechanisms involved in kainic acid injected into the raphe pallidus (Rpa)-induced gastric protection were investigated in urethan-anesthetized rats. Gastric mucosal blood flow (GMBF), acid secretion, and gastric injury induced by intragastric ethanol (60%) were measured in response to kainic acid (25 pg) injected into the Rpa. Kainic acid reduced ethanol-induced gastric lesions by 57%. The protective effect was blocked by vagotomy, capsaicin deafferentation, and intravenous injection of the calcitonin gene-related peptide (CGRP) antagonist CGRP-(8-37) and NG-nitro-L-arginine methyl ester (L-NAME). L- but not D-arginine reversed the L-NAME action. Kainic acid injected into the Rpa, unlike outside sites, increased basal GMBF but not acid secretion. Indomethacin unmasked an acid secretory response to kainic acid. These results show that kainic acid injected into the Rpa at a dose that did not stimulate acid secretion, due to the inhibitory effect of prostaglandins, protects against ethanol-induced gastric injury through vagal-dependent activation of CGRP contained in capsaicin-sensitive afferents and nitric oxide-mediated gastric vasodilatory mechanisms.

Gastric acid-evoked c-fos messenger RNA expression in rat brainstem is signaled by capsaicin-resistant vagal afferents

BACKGROUND & AIMS

Gastric acid is known to contribute to ulcer pain, but the mechanisms of gastric chemonociception are poorly understood. This study set out to investigate the pathways and mechanisms by which gastric acid challenge is signaled to the brain.

METHODS

Neuronal excitation in the rat brainstem and spinal cord after intragastric administration of HCl (0.35-0.7 mol/L) was examined by in situ hybridization autoradiography for the immediate early gene c-fos.

RESULTS

Gastric acid challenge did not induce c-fos transcription in the spinal cord but caused many neurons in the nucleus tractus solitarii and area postrema to express c-fos messenger RNA (mRNA). The HCl concentration-dependent excitation of medullary neurons was in part associated with behavioral manifestations of pain but not directly related to the acid-induced injury and contraction of the stomach. Subdiaphragmatic vagotomy suppressed the c-fos mRNA response to intragastric acid, and morphine inhibited it in a naloxone-reversible manner, whereas pretreatment of rats with capsaicin was without effect.

CONCLUSIONS

Gastric acid challenge is signaled to the brainstem, but not the spinal cord, through vagal afferents that are sensitive to acid but resistant to capsaicin. It is hypothesized that the gastric acid-induced c-fos transcription in the brainstem is related to gastric chemonociception.