Sensory neurons signal for an increase in rat gastric mucosal blood flow in the face of pending acid injury

Gut homeostasis, injury, and healing: New therapeutic targets

The integrity of the gastrointestinal mucosa plays a crucial role in gut homeostasis, which depends upon the balance between mucosal injury by destructive factors and healing via protective factors. The persistence of noxious agents such as acid, pepsin, nonsteroidal anti-inflammatory drugs, or Helicobacter pylori breaks down the mucosal barrier and injury occurs. Depending upon the size and site of the wound, it is healed by complex and overlapping processes involving membrane resealing, cell spreading, purse-string contraction, restitution, differentiation, angiogenesis, and vasculogenesis, each modulated by extracellular regulators. Unfortunately, the gut does not always heal, leading to such pathology as peptic ulcers or inflammatory bowel disease. Currently available therapeutics such as proton pump inhibitors, histamine-2 receptor antagonists, sucralfate, 5-aminosalicylate, antibiotics, corticosteroids, and immunosuppressants all attempt to minimize or reduce injury to the gastrointestinal tract. More recent studies have focused on improving mucosal defense or directly promoting mucosal repair. Many investigations have sought to enhance mucosal defense by stimulating mucus secretion, mucosal blood flow, or tight junction function. Conversely, new attempts to directly promote mucosal repair target proteins that modulate cytoskeleton dynamics such as tubulin, talin, Ehm², filamin-a, gelsolin, and flightless I or that proteins regulate focal adhesions dynamics such as focal adhesion kinase. This article summarizes the pathobiology of gastrointestinal mucosal healing and reviews potential new therapeutic targets.

Stomach serosal arteries distinguish gastric regions of the rat

Because the stomach in situ has few distinctive surface features and changes shape dramatically with food intake, we have used micro-CT imaging combined with two distinct contrast agents to (1) characterize the pattern of arteries, potential landmarks, on the stomach wall and (2) evaluate how meal-related shape changes affect the size of the different regions. Images generated with a contrast agent injected directly into the heart during perfusion enabled a thorough look at the organizational features of the stomach angioarchitecture. The stomach receives its blood supply primarily from two pairs of vessels, the gastric and gastroepiploic arteries. Each of the three regions of the stomach is delineated by a distinctive combination of arterial fields: the corpus, consistent with its dynamic secretory activity and extensive mucosa, is supplied by extensive arterial trees formed by the left and right gastric arteries, travelling, respectively, on the ventral and dorsal stomach surfaces. These major arteries course circularly from the lesser towards the greater curvature, distally along both left (or ventral) and right (or dorsal) walls of the corpus, and branch rostrally to supply the region. The muscular antrum is characterized by smaller arterial branches arising primarily from the right gastroepiploic artery that follows the distal greater curvature and secondarily from small, distally directed arteries supplied by the large vessels of the left and right gastric arteries. The forestomach, essentially devoid of mucosal tissue and separated from the corpus by the limiting ridge, is vascularized predominantly by a network of small arteries issued from the left gastroepiploic artery coursing around the proximal greater curvature, as well as from higher order and smaller branches issued by the gastric and celiac arteries. These distinctive arterial fields appear to distinguish the major gastric regions, irrespective of the degree of fill of the stomach. Volume assessments of stomach compartments were made from images of iodine-stained stomachs. By varying the delay time between eating and perfusion, we were able to probe the emptying behavior of the stomach and demonstrate that the regions of the stomach empty at different rates, thus changing the relative dimensions of the organ regions. Notably, and despite these shape changes, the gastric arteries appear to form a regular, particularly recognizable, and lateralized pattern corresponding to the corpus that should be of use in guiding surgical and experimental interventions.

The mixture of Salvia miltiorrhiza-Carthamus tinctorius (Danhong injection) alleviates low-dose aspirin induced gastric mucosal damage in rats

BACKGROUND

Danhong injection (DHI) is quite often used in combination with low-dose aspirin (ASA, 75-325mg daily) in clinic, particularly for the treatment of cardiovascular diseases. Exploring their interaction profile is of great clinical importance.

PURPOSE

The current study aims to explore the interaction between DHI and low-dose ASA in rats.

METHODS

Sixty four rats were randomly divided into eight groups. Stomach and other four vital organs were collected for histological evaluation. Organs which exhibited histological changes were selected for a further study to evaluate the damage score and mode of action. We tested the protective effect of DHI on gastric mucosal damage in different regimes of administration. COX activity, gastric mucus secretion, pepsin activity, antioxidant activity and ROS level were assayed to reflect the protective effect of DHI on gastric mucosal damage induced by ASA.

RESULTS

Stomach was the target organ of interaction when DHI and ASA were used in combination. DHI alleviated gastric mucosal damage by 55.8% when DHI was injected before ASA (Group E) and by 53.5% when DHI was injected 2h after ASA administration (Group F). Additionally, if DHI treatment was appended to the long-term administration of ASA, DHI still decreased the gastric mucosal damage score in 52.0% from 2.50 to 1.20. DHI improved gastric mucus secretion, as well as decreased pepsin activity to maintain the integrity of gastric mucosal barrier (P<0.05). Furthermore, DHI recovered antioxidant activity which was impaired by ASA. In details, DHI decreased gastric mucosal ROS level, increased CAT, GSH-Px and SOD activity, and reduced MDA concentration (P<0.05). When ASA (71.9µM) was used in combination with DHI (23-fold dilution, presented in terms of concentrations of DSS, PA, SaD RA, SaB and SaA were 6.45-6.92, 1.10-1.14, 1.09-1.10, 0.86-0.90, 16.76-19.38 and 1.83-1.94µg/ml, respectively) in vitro, the inhibition rate of ASA increased from 38.6% (ASA alone) to 62.8% (ASA-DHI) on COX-1 and from 28.9% (ASA alone) to 38.8% (ASA-DHI) on COX-2 (P<0.05). DHI strengthened the inhibition activity of ASA on both COX-1 and COX-2, which showed that DHI alleviated ASA induced gastric mucosal damage but not antagonized anti-COX effect of ASA.

CONCLUSIONS

Gastric protective benefits were clearly produced when DHI and ASA were used in combination, which provided rational guidance for clinical combined application of DHI and ASA.

Peripheral corticotropin-releasing factor receptor type 2 activation increases colonic blood flow through nitric oxide pathway in rats

BACKGROUND

Corticotropin-releasing factor (CRF) peptides exert profound effects on the secretomotor function of the gastrointestinal tract. Nevertheless, despite the presence of CRF peptides and receptors in colonic tissue, their influence on colonic blood flow (CBF) is unknown.

AIM

To determine the effect and mechanism of members of the CRF peptide family on CBF in isoflurane-anesthetized rats.

METHODS

Proximal CBF was measured with laser-Doppler flowmetry simultaneously with mean arterial blood pressure (MABP) measurement. Rats were injected with intravenous human/rat CRF (CRF1 > CRF2 affinity), mouse urocortin 2 (mUcn2, selective CRF2 agonist), or sauvagine (SVG, CRF2 > CRF1 affinity) at 1-30 µg/kg. The nitric oxide (NO) synthase inhibitor, L-NAME (3 mg/kg, iv), the cyclooxygenase inhibitor, indomethacin (Indo, 5 mg/kg, ip), or selective CRF2 antagonist, astressin2-B (Ast2B, 50 µg/kg, iv) was given before SVG injection (10 µg/kg, iv).

RESULTS

SVG and mUcn2 dose-dependently increased CBF while decreasing MABP and colonic vascular resistance (CVR). CRF had no effect on CBF, but increased CVR. The hyperemic effect of SVG was inhibited by L-NAME but not by Indo, whereas hypotension was partially reduced by L-NAME. Sensory denervation had no effect on SVG-induced changes. Ast2B inhibited SVG-induced hyperemia and decreased CVR, and partially reduced the hypotension.

CONCLUSIONS

Peripheral CRF2 activation induces colonic hyperemia through NO synthesis, without involving prostaglandin synthesis or sensory nerve activation, suggesting a direct action on the endothelium and myenteric neurons. Members of the CRF peptide family may protect the colonic mucosa via the activation of the CRF2 receptor.

Functional properties of submucosal venules in the rat stomach

Venules in the stomach may have intrinsic properties for maintaining active microcirculation drainage even during gastric filling. Properties of spontaneous and nerve-mediated activity of submucosal venules in the rat stomach were investigated. Changes in vasodiameter and intracellular Ca(2+) in venular smooth muscle cells (SMCs) were monitored by video tracking and Fluo-8 Ca(2+) imaging, respectively. Venular SMCs developed synchronous spontaneous Ca(2+) transients and corresponding rhythmic constrictions of the venules. Nominally Ca(2+)-free solution or an L-type Ca(2+) channel blocker (1 μM nifedipine) disrupted the Ca(2+) transient synchrony and abolished spontaneous constrictions. Spontaneous constrictions were also prevented by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase (10 μM cyclopiazonic acid (CPA)), IP3 receptors (100 μM 2-APB) or Ca(2+)-activated Cl(-) channels (100 μM niflumic acid). Transmural nerve stimulation (TNS) induced a long-lasting venular constriction that was abolished by α-adrenoceptor antagonist (1 μM phentolamine), while TNS evoked a sympathetic transient constriction of arterioles that was abolished by a combination of phentolamine and a P2 purinoceptor antagonist (10 μM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS)). Consistently, P2X1 purinoceptor immunoreactivity was detected in arteriolar but not venular SMCs. Primary afferent nerve stimulation (300 nM capsaicin) caused a venular dilatation by releasing calcitonin gene-related peptide. Thus, Ca(2+) release from the sarcoplasmic reticulum may play a fundamental role in the generation of spontaneous Ca(2+) transients, while electrical coupling amongst venular SMCs via L-type Ca(2+) channel activation appears to be critical for Ca(2+) transient synchrony as well as spontaneous contractions. Sympathetic venular constrictions appear to be exclusively mediated by noradrenaline due to the lack of P2X1 receptor in venular SMCs.

Gastrointestinal HCO3- transport and epithelial protection in the gut: new techniques, transport pathways and regulatory pathways

The concept of a protective alkaline gastric and duodenal mucus layer is a century old, yet it is amazing how much new information on HCO3(-) transport pathways has emerged recently, made possible by the extensive utilization of gene-deleted and transgenic mice and novel techniques to study HCO3(-) transport. This review highlights recent findings regarding the importance of HCO3(-) for mucosal protection of duodenum and other gastrointestinal epithelia against luminal acid and other damaging factors. Recently, methods have been developed to visualize HCO3(-) transport in vivo by assessing the surface pH in the mucus layer, as well as the epithelial pH. New information about HCO3(-) transport pathways, and emerging concepts about the intricate regulatory network that governs duodenal HCO3(-) secretion are described, and new perspectives for drug therapy discussed.

Regulatory mechanism of duodenal bicarbonate secretion: Roles of endogenous prostaglandins and nitric oxide

The secretion of HCO(3)(-) in the duodenum is increased by exogenous prostaglandin (PG) E(2) and mucosal acidification, the latter being accompanied by a rise in mucosal PGE(2) content and nitric oxide (NO) release. The stimulatory effect of PGE(2) is mediated intracellularly by both Ca(2+) and 3',5'-adenosine cyclic adenosine monophosphate (cAMP), and this action is inhibited by EP3 and EP4 antagonists. The secretion is also increased by NOR3 (NO donor), and this response is mimicked by dibutyryl 3',5'-cyclic guanosine monophosphate (dbcGMP) and attenuated by indomethacin. Mucosal acidification stimulates HCO(3)(-) secretion with concomitant increases in mucosal PGE(2) production and NO release. The effects on HCO(3)(-) secretion and PGE(2) production are inhibited by indomethacin [nonselective cyclooxygenase (COX) inhibitor] and SC-560 (selective COX-1 inhibitor) but not rofecoxib (selective COX-2 inhibitor). N(G)-nitro-l-arginine methyl ester [l-NAME: nonselective NO synthase (NOS) inhibitor], but not aminoguanidine [selective inducible NOS inhibitor], attenuates the acid-induced HCO(3)(-) secretion and NO release in an l-arginine-sensitive manner. In addition, the response to PGE(2) is potentiated by vinpocetine [phosphodiesterase (PDE) 1 inhibitor] and cilostamide (PDE3 inhibitor), while the response to NOR3 is increased by vinpocetine. We conclude that endogenous PGs and NO are both involved in the local regulation of acid-induced duodenal HCO(3)(-) secretion; COX-1 and constitutive NOS are key enzymes responsible for the production of PGs and NO, respectively; NO stimulates HCO(3)(-) secretion by increasing PG production; PGE(2) stimulates HCO(3)(-) secretion via activation of EP3/EP4 receptors; and both PDE1 and PDE3 are involved in the regulation of duodenal HCO(3)(-) secretion.

Acid-induced CGRP release from the stomach does not depend on TRPV1 or ASIC3

BACKGROUND

Acid-sensing and regulating reactions are vitally important in the upper gastrointestinal tract and disturbances are common. Sensory neurons in the mucosa detect the intrusion of hydrogen ions and, by their release of vasoactive neuropeptides, seem to play a predominantly protective role in these tissues.

METHODS

The model to investigate sensory transduction of proton stimuli in the isolated everted mouse stomach was to measure the induced calcitonin gene-related peptide (CGRP) release as an index of neuronal activation.

KEY RESULTS

Proton concentrations in the range of pH 2.5-0.5 stimulated the release of CGRP and substance P and profoundly decreased the prostaglandin E2 formation in outbred CD mice. A similar linearly pH-dependent CGRP release was observed in inbred C57BL/6 mice, fully dependent on extracellular calcium at pH 2, partially at pH 1. Both transient receptor potential vanilloid type 1 (TRPV1) and acid-sensing ion channel type 3 (ASIC3) are expressed in the sensory neurons innervating the stomach walls and are responsible for the transduction of acidic stimuli in other visceral organs. However, the proton-induced gastric CGRP release in mice lacking the TRPV1 or the ASIC3 receptor-channels was the same as in corresponding wild-type mice. Nonetheless, the pharmacological blockers N-(4-tertiarybutylphenyl)-4-(3-chlorophyridin-2-yl)tetrahydropyrazine-1(2H)carboxamide and amiloride, respectively, inhibited the acid-stimulated CGRP release, although to the same extend in wild types as TRPV1 and ASIC3 knockout mice.

CONCLUSIONS & INFERENCES

Adequate proton concentrations inhibit prostaglandin and stimulate CGRP release from the stomach wall, however, the transduction mechanism in the gastric sensory neurons remains unclear.

Alosetron, cilansetron and tegaserod modify mesenteric but not colonic blood flow in rats

BACKGROUND AND PURPOSE

As the use of the 5-HT(3) receptor antagonist alosetron (GlaxoSmithKline) and the 5-HT(4) receptor agonist tegaserod (Novartis) in patients with irritable bowel syndrome has been associated with cases of ischaemic colitis, the effects of alosetron, cilansetron (Solvay) and tegaserod on the rat splanchnic circulation were evaluated.

EXPERIMENTAL APPROACH

Phenobarbital-anaesthetised rats were instrumented to record blood flow in the superior mesenteric artery and transverse colon and to calculate mesenteric and colonic vascular conductance.

KEY RESULTS

Intravenous alosetron (0.03-0.3 mg.kg(-1)) did not alter blood pressure or heart rate but reduced mesenteric blood flow and vascular conductance by 15-20%. This activity profile was also seen after intraduodenal alosetron and shared by the 5-HT(3) receptor antagonist cilansetron. In contrast, blood flow, vascular conductance and intraluminal pressure in the colon were not modified by alosetron and cilansetron. Intravenous or intraduodenal tegaserod (0.3-1.0 mg.kg(-1)) had no inhibitory effect on mesenteric and colonic blood flow. Peroral treatment of rats with alosetron or tegaserod for 7 days did not modify mesenteric haemodynamics at baseline and after blockade of nitric oxide synthesis. Mild inflammation induced by dextran sulphate sodium failed to provoke a vasoconstrictor effect of cilansetron in the colon.

CONCLUSIONS AND IMPLICATIONS

Alosetron and cilansetron, not tegaserod, caused a small and transient constriction of the rat mesenteric vascular bed, whereas blood flow in the colon remained unaltered. The relevance of these findings to the treatment-associated occurrence of ischaemic colitis in patients with irritable bowel syndrome remains open.

Afferent signalling from the acid-challenged rat stomach is inhibited and gastric acid elimination is enhanced by lafutidine

BACKGROUND

Lafutidine is a histamine H2 receptor antagonist, the gastroprotective effect of which is related to its antisecretory activity and its ability to activate a sensory neuron-dependent mechanism of defence. The present study investigated whether intragastric administration of lafutidine (10 and 30 mg/kg) modifies vagal afferent signalling, mucosal injury, intragastric acidity and gastric emptying after gastric acid challenge.

METHODS

Adult rats were treated with vehicle, lafutidine (10 - 30 mg/kg) or cimetidine (10 mg/kg), and 30 min later their stomachs were exposed to exogenous HCl (0.25 M). During the period of 2 h post-HCl, intragastric pH, gastric volume, gastric acidity and extent of macroscopic gastric mucosal injury were determined and the activation of neurons in the brainstem was visualized by c-Fos immunocytochemistry.

RESULTS

Gastric acid challenge enhanced the expression of c-Fos in the nucleus tractus solitarii but caused only minimal damage to the gastric mucosa. Lafutidine reduced the HCl-evoked expression of c-Fos in the NTS and elevated the intragastric pH following intragastric administration of excess HCl. Further analysis showed that the gastroprotective effect of lafutidine against excess acid was delayed and went in parallel with facilitation of gastric emptying, measured indirectly via gastric volume changes, and a reduction of gastric acidity. The H2 receptor antagonist cimetidine had similar but weaker effects.

CONCLUSION

These observations indicate that lafutidine inhibits the vagal afferent signalling of a gastric acid insult, which may reflect an inhibitory action on acid-induced gastric pain. The ability of lafutidine to decrease intragastric acidity following exposure to excess HCl cannot be explained by its antisecretory activity but appears to reflect dilution and/or emptying of the acid load into the duodenum. This profile of actions emphasizes the notion that H2 receptor antagonists can protect the gastric mucosa from acid injury independently of their ability to suppress gastric acid secretion.

Prostaglandins, NSAIDs, and gastric mucosal protection: why doesn't the stomach digest itself?

Except in rare cases, the stomach can withstand exposure to highly concentrated hydrochloric acid, refluxed bile salts, alcohol, and foodstuffs with a wide range of temperatures and osmolarity. This is attributed to a number of physiological responses by the mucosal lining to potentially harmful luminal agents, and to an ability to rapidly repair damage when it does occur. Since the discovery in 1971 that prostaglandin synthesis could be blocked by aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), there has been great interest in the contribution of prostaglandins to gastric mucosal defense. Prostaglandins modulate virtually every aspect of mucosal defense, and the importance of this contribution is evident by the increased susceptibility of the stomach to injury following ingestion of an NSAID. With chronic ingestion of these drugs, the development of ulcers in the stomach is a significant clinical concern. Research over the past two decades has helped to identify some of the key events triggered by NSAIDs that contribute to ulcer formation and/or impair ulcer healing. Recent research has also highlighted the fact that the protective functions of prostaglandins in the stomach can be carried out by other mediators, in particular the gaseous mediators nitric oxide and hydrogen sulfide. Better understanding of the mechanisms through which the stomach is able to resist injury in the presence of luminal irritants is helping to drive the development of safer anti-inflammatory drugs, and therapies to accelerate and improve the quality of ulcer healing.

Protection of the gastrointestinal tract epithelium against damage from low pH beverages

Extensive consumption of low pH beverages such as citrus juices (pHs 2.3 to 4.3), alcoholic beverages (pHs 2.7 to 4.5), and soft drinks (pHs 2.3 to 4.2) has raised the question of whether exposure of the gastrointestinal (GI) tract to acidic beverages will cause damage to the epithelial lining. To evaluate the potential effects of low pH beverages on the GI tract epithelium, a detailed examination of the literature was undertaken. In some animal models, there is evidence of damage to GI epithelial cells following exposure to low pH beverages; however, in these studies there is no definitive relationship between acidity and the amount or severity of damage. Results from several other studies, conducted in both animals and humans, indicate a lack of adverse effects on epithelial cells. Furthermore, there is no evidence that damage is irreversible. Permanent damage from routine exposure to acidic beverages in humans would not be expected because of repair mechanisms that are available to maintain a healthy epithelium. Additionally, numerous physical, chemical, and biological mechanisms are in place to prevent damage to the epithelial cells. Finally, the safe history of consumption of low pH beverages, including various fruit juices, supports the conclusion that low pH beverage ingestion does not cause damage to the GI epithelium.

Capsaicin and gastric ulcers

In recent years, infection of the stomach with the organism Helicobacter Pylori has been found to be the main cause of gastric ulcers, one of the common ailments afflicting humans. Excessive acid secretion in the stomach, reduction in gastric mucosal blood flow, constant intake of non-steroid anti-inflammatory drugs (NSAIDS), ethanol, smoking, stress etc. are also considered responsible for ulcer formation. The prevalent notion among sections of population in this country and perhaps in others is that "red pepper" popularly known as "Chilli," a common spice consumed in excessive amounts leads to "gastric ulcers" in view of its irritant and likely acid secreting nature. Persons with ulcers are advised either to limit or avoid its use. However, investigations carried out in recent years have revealed that chilli or its active principle "capsaicin" is not the cause for ulcer formation but a "benefactor." Capsaicin does not stimulate but inhibits acid secretion, stimulates alkali, mucus secretions and particularly gastric mucosal blood flow which help in prevention and healing of ulcers. Capsaicin acts by stimulating afferent neurons in the stomach and signals for protection against injury causing agents. Epidemiologic surveys in Singapore have shown that gastric ulcers are three times more common in the "Chinese" than among Malaysians and Indians who are in the habit of consuming more chillis. Ulcers are common among people who are in the habit of taking NSAIDS and are infected with the organism "Helicobacter Pylori," responsible for excessive acid secretion and erosion of the mucosal layer. Eradication of the bacteria by antibiotic treatment and avoiding the NSAIDS eliminates ulcers and restores normal acid secretion.

Distinct mechanisms of acid-induced HCO3- secretion in normal and slightly permeable stomachs

We investigated the regulatory mechanism of acid-induced HCO(3)(-) secretion in the slightly permeable rat stomach after an exposure to hyperosmolar NaCl. Under urethane anesthesia, a rat stomach was mounted on a chamber and perfused with saline, and the secretion of HCO(3)(-) was measured at pH 7.0 using a pH-stat method and by adding 2 mM HCl. Acidification of the normal stomach with 100 mM HCl increased HCO(3)(-) secretion, and this response was totally inhibited by pretreatment with indomethacin but not N(G)-nitro-l-arginine methyl ester (l-NAME) or chemical ablation of capsaicin-sensitive afferent neurons. Exposure of the stomach to 0.5 M NaCl deranged the unstirred mucus gel layer without damaging the surface epithelial cells. The stomach responded to 0.5 M NaCl by secreting slightly more HCO(3)(-), in an indomethacin-inhibitable manner, and responded to even 10 mM HCl with a marked rise in HCO(3)(-) secretion, although 10 mM HCl did not have an effect in the normal stomach. The acid-induced HCO(3)(-) response in the NaCl-treated stomach was significantly but partially attenuated by indomethacin, l-NAME, or sensory deafferentation and was totally abolished when these treatments were combined. These results suggest that gastric HCO(3)(-) secretion in response to acid is regulated by two independent mechanisms, one mediated by prostaglandins (PGs) and the other by sensory neurons and nitric oxide (NO). The acid-induced HCO(3)(-) secretion in the normal stomach is totally mediated by endogenous PGs, but, when the stomach is made slightly permeable to acid, the response is markedly facilitated by sensory neurons and NO.

Regulation of early and delayed radiation responses in rat small intestine by capsaicin-sensitive nerves

PURPOSE

Mast cells protect against the early manifestations of intestinal radiation toxicity, but promote chronic intestinal wall fibrosis. Intestinal sensory nerves are closely associated with mast cells, both anatomically and functionally, and serve an important role in the regulation of mucosal homeostasis. This study examined the effect of sensory nerve ablation on the intestinal radiation response in an established rat model.

METHODS AND MATERIALS

Rats underwent sensory nerve ablation with capsaicin or sham ablation. Two weeks later, a localized segment of ileum was X-irradiated or sham irradiated. Structural, cellular, and molecular changes were examined 2 weeks (early injury) and 26 weeks (chronic injury) after irradiation. The mast cell dependence of the effect of sensory nerve ablation on intestinal radiation injury was assessed using c-kit mutant (Ws/Ws) mast cell-deficient rats.

RESULTS

Capsaicin treatment caused a baseline reduction in mucosal mast cell density, crypt cell proliferation, and expression of substance P and calcitonin gene-related peptide, two neuropeptides released by sensory neurons. Sensory nerve ablation strikingly exacerbated early intestinal radiation toxicity (loss of mucosal surface area, inflammation, intestinal wall thickening), but attenuated the development of chronic intestinal radiation fibrosis (collagen I accumulation and transforming growth factor beta immunoreactivity). In mast cell-deficient rats, capsaicin treatment exacerbated postradiation epithelial injury (loss of mucosal surface area), but none of the other aspects of radiation injury were affected by capsaicin treatment.

CONCLUSIONS

Ablation of capsaicin-sensitive enteric neurons exacerbates early intestinal radiation toxicity, but attenuates development of chronic fibroproliferative changes. The effect of capsaicin treatment on the intestinal radiation response is partly mast cell dependent.

Gastroprotective action of glucocorticoid hormones in rats with desensitization of capsaicin-sensitive sensory neurons

The ability of glucocorticoid hormones to protect gastric mucosa during desensitization of capsaicin-sensitive afferent neurons has been investigated in rats. Functional ablation of the afferent neurons was performed by pre-treatment with neurotoxic doses of capsaicin (100 mg/kg s.c.). After 1 week of recovery, capsaicin-desensitized, as well as control rats were adrenalectomized or sham-operated. Seven days later, indomethacin at an ulcerogenic dose (35 mg/kg s.c.) was given to each group of rats. One half of adrenalectomized capsaicin-pre-treated rats were injected by corticosterone for replacement (4 mg/kg s.c., 15 min before indomethacin). Gastric lesions, plasma corticosterone and blood glucose levels were estimated 4 h after indomethacin administration. Indomethacin caused gastric erosions that were aggravated by adrenalectomy or desensitization of capsaicin-sensitive afferent neurons approximately with the same extension. Combination of adrenalectomy with the sensory desensitization profoundly potentiated the effect of sensory desensitization alone on indomethacin-induced gastric erosions: the mean gastric erosion area was increased approximately 10-fold. Corticosterone replacement completely prevented this profound effect of adrenalectomy. The results suggest a pivotal role of glucocorticoid hormones in the maintenance of gastric mucosal integrity in the case of impaired gastroprotective mechanisms provided by PGs and capsaicin-sensitive sensory neurons.

Efferent-like roles of afferent neurons in the gut: Blood flow regulation and tissue protection

The maintenance of gastrointestinal mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. To this end, the gastric mucosa is innervated by intrinsic sensory neurons and two populations of extrinsic sensory neurons: vagal and spinal afferents. Extrinsic afferent neurons constitute an emergency system that is called into operation when the gastrointestinal mucosa is endangered by noxious chemicals. The function of these chemoceptive afferents can selectively be manipulated and explored with the use of capsaicin which acts via a cation channel termed TRPV1. Many of the homeostatic actions of spinal afferents are brought about by transmitter release from their peripheral endings. When stimulated by noxious chemicals, these afferents enhance gastrointestinal blood flow and activate hyperaemia-dependent and hyperaemia-independent mechanisms of protection and repair. In the rodent foregut these local regulatory roles of sensory neurons are mediated by calcitonin gene-related peptide and nitric oxide. The pathophysiological potential of the neural emergency system is best portrayed by the gastric hyperaemic response to acid back-diffusion, which is governed by spinal afferent nerve fibres. This mechanism limits damage to the surface of the mucosa and creates favourable conditions for rapid restitution and healing of the wounded mucosa. Other extrinsic afferent neurons, particularly in the vagus nerve, subserve gastrointestinal homeostasis by signalling noxious events in the foregut to the central nervous system and eliciting autonomic, emotional-affective and neuroendocrine reactions. Under conditions of inflammation and injury, chemoceptive afferents are sensitized to peripheral stimuli and in this functional state contribute to the hyperalgesia associated with functional dyspepsia and irritable bowel syndrome. Thus, if GI pain is to be treated by sensory neuron-directed drugs it needs to be considered that these drugs do not inhibit nociception at the expense of GI mucosal vulnerability.

Endogenous neuropeptide Y depresses the afferent signaling of gastric acid challenge to the mouse brainstem via neuropeptide Y type Y2 and Y4 receptors

Vagal afferents signal gastric acid challenge to the nucleus tractus solitarii of the rat brainstem. This study investigated whether nucleus tractus solitarii neurons in the mouse also respond to gastric acid challenge and whether this chemonociceptive input is modified by neuropeptide Y acting via neuropeptide Y receptors of type Y2 or Y4. The gastric mucosa of female mice was exposed to different concentrations of HCl or saline, excitation of neurons in the nucleus tractus solitarii visualized by c-Fos immunohistochemistry, gastric emptying deduced from the gastric volume recovery, and gastric lesion formation evaluated by planimetry. Relative to saline, intragastric HCl (0.15-0.35 M) increased the number of c-Fos-expressing cells in the nucleus tractus solitarii in a concentration-dependent manner, inhibited gastric emptying but failed to cause significant hemorrhagic injury in the stomach. Mice in which the Y2 or Y4 receptor gene had been deleted responded to gastric acid challenge with a significantly higher expression of c-Fos in the nucleus tractus solitarii, the increases amounting to 39 and 31%, respectively. The HCl-induced inhibition of gastric emptying was not altered by deletion of the Y2 or Y4 receptor gene. BIIE0246 ((S)-N2-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6H)-oxodibenz[b,e] azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl] acetyl]-N-[2-[1,2-dihydro-3,5 (4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide; 0.03 mmol/kg s.c.), a Y2 receptor antagonist which does not cross the blood-brain barrier, did not modify the c-Fos response to gastric acid challenge. The Y2 receptor agonist peptide YY-(3-36) (0.1 mg/kg intraperitoneally) likewise failed to alter the gastric HCl-evoked expression of c-Fos in the nucleus tractus solitarii. BIIE0246, however, prevented the effect of peptide YY-(3-36) to inhibit gastric acid secretion as deduced from measurement of intragastric pH. The current data indicate that gastric challenge with acid concentrations that do not induce overt injury but inhibit gastric emptying is signaled to the mouse nucleus tractus solitarii. Endogenous neuropeptide Y acting via Y2 and Y4 receptors depresses the afferent input to the nucleus tractus solitarii by a presumably central site of action.

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.

Protective effect of lafutidine, a novel H2-receptor antagonist, on reflux esophagitis in rats through capsaicin-sensitive afferent neurons

We examined the effect of lafutidine, a novel histamine H(2)-receptor antagonist, on acid reflux esophagitis in rats in relation to capsaicin-sensitive afferent neurons. The esophagitis was induced in rats by ligating both the pylorus and forestomach for 4 h. Lafutidine (1 - 30 mg/kg) and cimetidine (100 mg/kg) were administered either intragastrically or intraduodenally, while capsaicin (1 - 30 mg/kg) was administered intragastrically after the dual ligation. Intragastrical administered lafutidine at >3 mg/kg significantly prevented the hemorrhagic esophageal damage induced by the dual ligation, and this effect was mimicked by neither capsaicin nor cimetidine given intragastrically, but totally abolished by sensory deafferentation. In contrast, lafutidine and cimetidine given intraduodenally were both protective against the esophageal damage in a sensory deafferentation-resistant manner. The acid secretion in pylorus-ligated stomachs was significantly inhibited by these agents given intraduodenally, but not intragastrically. Vanilloid receptor subtype 1 (VR1) was expressed abundantly in the stomach, but very weakly expressed in the esophagus as assessed by Western blotting. These results suggest that lafutidine is effective against the esophageal lesions induced by acid reflux through inhibition of acid secretion and capsaicin-sensitive afferent neurons. The latter mechanism, not shared by cimetidine, may be due to the interaction of lafutidine with unidentified sites on sensory neurons other than VR1.

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.

Role of bradykinin in gastric vasodilation caused by hypertonic saline and acid back diffusion

Protective vasodilation in response to tissue injury and acid back diffusion is associated with release of bradykinin in the rat stomach. We hypothesized that bradykinin might be involved in mechanisms behind such vasodilation via influence on mast cells and sensory neurons. Acid back diffusion after mucosal barrier disruption with hypertonic saline evoked degranulation of mast cells in the rat stomach wall. Acid back diffusion was also associated with increased luminal release of histamine and gastric blood flow in normal rats, but not in mast cell-deficient rats. Bradykinin (BK(2)) receptor blockade inhibited degranulation of submucosal mast cells in the stomach and attenuated gastric vasodilation both in response to acid back diffusion and after stimulation of sensory neurons with capsaicin. Gastric vasodilation caused by mucosal injury with hypertonic saline alone was associated with degranulation of mucosal mast cells. These events were unaffected by inhibition of prostaglandin synthesis, whereas bradykinin (BK(2)) receptor blockade was associated with abolished vasodilation and inhibition of mucosal mast cell degranulation. We conclude that bradykinin is involved in gastric vasodilation caused by hypertonic injury alone via influence on mast cells, and by acid back diffusion via influence on both sensory neurons and mast cells.

Cyclooxygenase isozymes involved in adaptive functional responses in rat stomach after barrier disruption

We investigated the preferential role of cyclooxygenase (COX) isozymes in various functional changes of the rat stomach after exposure to taurocholate (TC) as a mild irritant. Under urethane anesthesia, a rat stomach mounted in an ex vivo chamber was perfused with saline or acid (50 mM HCl), and transmucosal potential difference (PD), gastric mucosal blood flow (GMBF), and acid secretion were measured before and after exposure of the stomach to 20 mM TC for 30 min. Indomethacin, 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC-560) (a selective COX-1 inhibitor), or rofecoxib (a selective COX-2 inhibitor) was given intraduodenally 30 min before the TC treatment. Mucosal application of TC caused a marked reduction in PD, followed by a decrease of acid secretion and an increase of GMBF. Previous administration of indomethacin did not affect the reduction in PD but significantly mitigated the two other responses induced by TC, resulting in a delay in the recovery in PD. These effects were mimicked by SC-560 but not rofecoxib, although neither of these drugs had any effect on the reduction in PD. Perfusion of TC-treated stomachs with 50 mM HCl caused only minimal damage, yet this treatment produced gross lesions in the presence of indomethacin or SC-560. Mucosal exposure to TC increased prostaglandin E2 production, but the response was inhibited by both indomethacin and SC-560 but not rofecoxib. These results suggested that COX-1 but not COX-2 is a key enzyme for regulating the functional alterations of the stomach and for maintaining the mucosal integrity after barrier disruption.

Expression of vanilloid receptors in rat gastric epithelial cells: role in cellular protection

Vanilloid receptors subtype 1 (VR1), a nonselective cation channel responsive to capsaicin, protons, and noxious heat, has been recently identified in not only neural but also non-neural cells. In the present study, we demonstrated the peripheral expression of VR1 in gastric mucosal epithelial cells and investigated the role of the receptor in cellular protection. The rat gastric mucosal epithelial cell line was used. The expression of VR1 was examined by Western blotting and RT-PCR. Cell damage was induced by immersion in 10% ethanol or acid (pH 4.0) for 30 min, and cell viability was determined by MTT assay. Capsaicin or resiniferatoxin was added 30 min before the challenge with ethanol or acid, while capsazepine or ruthenium red (a VR1 antagonist) was added simultaneously with capsaicin. The distinct expression of VR1 protein and mRNA was detected in rat gastric mucosal epithelial cell line as well as in the rat stomach and spinal cord by Western blotting and RT-PCR, respectively. The cDNA sequence of the PCR product was found to be almost identical to that of the authentic VR1 (99.8%) when the product was subcloned and sequenced. On the other hand, the cell damage induced by ethanol or acid was dose-dependently prevented by pretreatment with capsaicin. The protective effect of capsaicin was mimicked by resiniferatoxin and almost totally abolished by co-addition of capsazepine or ruthenium red. These findings suggest that VR1 is expressed peripherally in gastric mucosal epithelial cells and plays a cellular protective role.

Transforming growth factor alpha-mediated gastroprotection against stress ulceration in the rat: involvement of capsaicin-sensitive sensory neurons

Exogenously administered TGF alpha has been shown to protect rodent gastric mucosa against injury caused by acid-dependent and acid-independent injury. The present study examined whether the gastroprotective effects of TGF alpha on stress-induced gastric ulceration in the rat involves activation of capsaicin-sensitive sensory neurons. Fasted male SD rats were subjected to water restraint stress (WRS) for four hours. Thereafter, rats were euthanized; the stomach opened and macroscopic areas of gastric ulceration quantitated (mm(2)). Gastric tissue contents of TGF alpha and the sensory neuropeptide, calcitonin gene-related peptide (CGRP) were determined by radioimmunoassay. Prior to stress rats received TGF alpha 50, 100 or 200 microg/kg by intraperitoneal injection. Sensory denervation was accomplished by high dose capsaicin treatment. WRS caused severe ulceration in the gastric corpus; 46.1 + 6.6 mm(2). Parenteral administration of TGF alpha caused dose-dependent reduction in gastric injury: 34.7 + 4.9 mm(2) with 50 microg/kg (p < 0.05); 25.4 + 3.6 mm(2) with 100 microg/kg (p < 0.001) and 9.4 + 0.8 mm(2) with 200 microg/kg (p < 0.001). The gastroprotective action of TGF alpha (200 microg/kg, i.p.) was abolished by capsaicin-induced sensory denervation. In addition, WRS ulceration was associated with significant reduction in gastric CGRP (-42%) and TGF alpha (-48%) content. Reduction in CGRP content was prevented by TGF alpha pretreatment. We conclude that: 1) TGF alpha caused dose-dependent gastroprotection against WRS ulceration, 2) TGF alpha-mediated gastric mucosal protection was prevented by capsaicin-induced sensory denervation and, 3) stress-induced injury was associated with significant reduction in gastric content of both TGF alpha and CGRP.

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.

Irsogladine Prevents Monochloramine-Induced Gastric Mucosal Lesions by Improving the Decrease in Mucosal Blood Flow Due to the Disturbance of Nitric Oxide Synthesis in Rats

The inhibitory effect of an anti-ulcer drug irsogladine [2,4-diamino-6-(2,5-dichlorophenyl)-s-triazine maleate] on monochloramine (NH(2)Cl)-induced gastric mucosal lesions and its mechanisms of action were clarified in rats. Irsogladine dose-dependently prevented the formation of gastric mucosal lesions induced by 60 mM NH(2)Cl. The mucosal protective effect of irsogladine was not influenced by capsaicin-sensitive sensory defunctionalization. On the other hand, its protective effect was diminished by the inhibitor of nitric oxide synthase N(G)-nitro-L-arginine methylester (L-NAME), but not by the inducible nitric oxide synthase selective inhibitor aminoguanidine. Irsogladine restored the NH(2)Cl-induced decrease in the gastric cGMP formation as an index of nitric oxide synthesis, while it alone had no influence on the cGMP formation in intact tissues. Pretreatment with L-NAME abolished the recovery of cGMP by irsogladine. Furthermore, irsogladine ameliorated the NH(2)Cl-induced decrease in gastric mucosal blood flow, which was also reversed by pretreatment with L-NAME. These findings suggest that the improvement of the decrease in mucosal blood flow subsequent to the disturbance of gastric nitric oxide synthesis is involved in the protective effect of irsogladine on gastric mucosal lesions caused by NH(2)Cl.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.

Histamine is involved in gastric vasodilation during acid back diffusion via activation of sensory neurons

Protective vasodilation during acid back diffusion into the rat gastric mucosa depends on activation of sensory neurons and mast cell degranulation with histamine release. We hypothesized that these two mediator systems interact and that histamine partly exerts its effect via sensory nerves. Gastric blood flow (GBF) and luminal histamine were measured in chambered stomachs, and mast cell numbers were assessed by morphometry. Ablation of sensory neurons and depletion of mast cells were produced by pretreatment with capsaicin or dexamethasone, respectively. Mucosal exposure to 1.5 M NaCl and then to pH 1.0 saline in ablated and control rats caused increased luminal histamine and reduced numbers of mast cells. Enterochromaffin-like cell marker pancreastatin remained unchanged. Only control rats responded with an increase in GBF. Capsaicin stimulation (640 microM) of the undamaged mucosa induced identical increase in GBF and unchanged mast cell mass in normal and dexamethasone-treated rats. Increase in GBF after topical exposure to histamine (30 mM) in rats pretreated with capsaicin or a calcitonin gene-related peptide (CGRP)(1) antagonist human CGRP(8-37) or exposed to the calcium pore blocker ruthenium red was less than one-half of that in control rats. These data suggest that mast cell-derived histamine is involved in gastric vasodilatation during acid back diffusion partly via sensory neurons.

Gastric ulcerogenic responses following barrier disruption in knockout mice lacking prostaglandin EP1 receptors

BACKGROUND/AIMS

Endogenous prostaglandins (PGs) are considered to play a pivotal role in maintaining the mucosal integrity of the stomach after injury. In the present study, we evaluated the mucosal ulcerogenic and mucosal blood flow (GMBF) responses in the stomach after damage by taurocholate (TC) in knockout mice lacking EP1 or EP3 receptors.

METHODS

Under urethane anaesthesia, a mouse stomach was mounted in an ex vivo chamber, exposed to 20 mmol/L TC for 20 min and treated with 20 mmol/L HCl before and after TC. GMBF was measured with a laser Doppler flowmeter.

RESULTS

Mucosal exposure to TC in wild-type mice caused a marked decrease in potential difference (PD), followed by an increase in H+ loss and GMBF. The decreased PD was gradually normalized after removal of TC from the chamber, with minimal damage in the mucosa 1 h after TC treatment. This hyperaemic response was inhibited by indomethacin, resulting in severe lesions in the mucosa without any change in PD or H+ loss. None of these responses induced by TC were altered in EP3-/- mice. However, in mice lacking EP1 receptors, TC treatment did not increase GMBF, despite causing PD reduction and acid loss, and resulted in severe damage in the mucosa. These responses were closely similar to those observed in animals pretreated with ONO-8711, a EP1 receptor antagonist. Mucosal PGE2 content was significantly increased after TC, similarly in all groups of mice.

CONCLUSION

These results confirm a mediator role for PGE2 in gastric hyperaemic response following mucosal exposure to TC and suggest that endogenous PGs may contribute to maintaining mucosal integrity after barrier disruption, mainly through activation of the EP1 receptor subtype.

A comparative study on the activity of lansoprazole, omeprazole and PD-136450 on acidified ethanol- and indomethacin-induced gastric lesions in the rat

1. The proton pump inhibitors lansoprazole (LP) and omeprazole (OP) and the cholecystokinin (CCK)-receptor antagonist PD-136450 (PD) provide a broad spectrum of activities in their ability to inhibit gastric acid secretion and protect the stomach against ulcerogens. In the present study, we investigated the protective effects of these compounds against gastric ulcers induced by acidified ethanol (AE) and indomethacin. 2. Both AE (60% ethanol in 150 mmol/L HCl, 1 mL/rat) and indomethacin (30 mg/kg) produced gastric haemorrhagic lesions in the rat 1 and 6 h after oral administration, respectively. 3. The gastric mucosal protective effects of LP (1-20 mg/kg), OP (0.5-10 mg/kg) and PD (1-20 mg/kg), administered either orally or subcutaneously (s.c.) 30 min before the administration of AE or indomethacin, were dose dependent against both models of ulcer induction. 4. To determine whether the cytoprotective effect of LP, OP and PD (each 10 mg/kg) was mediated by endogenous prostaglandins (PG), indomethacin (10 mg/kg, s.c.) was administered 15 min before AE to inhibit prostanoids biosynthesis. Indomethacin reduced the cytoprotective effects of OP, but not LP, administered either orally or s.c. Indomethacin reduced the cytoprotective effect of PD administered orally, although the effect was much less significant than when PD was administered s.c. The results exclude the role of PG in mediating the protective effects of LP, whereas the possibility exists for PG to have a role in mediating the protective effects of OP and PD. 5. To investigate the possible involvement of endogenous nitric oxide (NO) in the cytoprotective action of LP, OP and PD, we treated rats with a selective inhibitor of NO synthesis, namely NG-nitro-L-arginine methyl ester (L-NAME; 25 mg/kg, s.c.). Administration of L-NAME 15 min prior to LP, OP or PD (each 10 mg/kg) orally or s.c. and challenge with AE or indomethacin did not significantly increase the degree of the ulcer index and L-NAME was not able to antagonize the protective effects of LP, OP and PD, thus excluding the role of NO in mediating the protective effects of these drugs. However, the effects of PD in reducing the indomethacin-induced ulcer index were less significant in the presence than the absence of L-NAME (P < 0.05 vs P < 0.001, respectively), suggesting a role for NO. 6. In conclusion, the results of the present study suggest that LP and OP are equally effective against AE- as well as indomethacin-induced gastric ulcers and were more potent than PD in protecting the stomach against ulcer formation. Lansoprazole, OP and PD bring about their cytoprotective action through the reduction of acid secretion and some other unknown mechanisms. However, OP and PD may exert their cytoprotective action through PG and NO pathways.

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.

Pathogenesis of NSAID-induced gastroduodenal mucosal injury

The use of non-steroidal anti-inflammatory drugs (NSAIDs), even in the era of selective COX-2 inhibitors, remains limited by the ability of these agents to cause gastroduodenal ulceration and bleeding. This damage is caused mainly through the ability of these agents to inhibit prostaglandin synthesis, which has a negative impact on several components of mucosal defence. Many NSAIDs also have topical irritant effects on the epithelium which may be particularly important in the production of small intestinal injury. While the presence of acid in the lumen of the stomach may not be a primary factor in the pathogenesis of NSAID-induced gastroenteropathy it can make an important contribution to the chronicity of these lesions and to bleeding by impairing the restitution process, interfering with haemostasis and inactivating several growth factors that are important in mucosal defence and repair. Through better understanding of the pathogenesis of ulcers induced by NSAIDs, some new approaches to the development of more effective and safer anti-inflammatory drugs have been taken in recent years.

Mast cells are involved in the gastric hyperemic response to acid back diffusion via release of histamine

Acid back diffusion into the rat stomach mucosa leads to gastric vasodilation. We hypothesized that histamine, if released from the rat mucosa under such conditions, is mast cell derived and involved in the vasodilator response. Gastric blood flow (GBF) and luminal histamine were measured in an ex vivo chamber. Venous histamine was measured from totally isolated stomachs. Mucosal mast cells (MMC), submucosal connective tissue mast cells (CTMC), and chromogranin A-immunoreactive cells (CgA IR) were assessed morphometrically. After mucosal exposure to 1.5 M NaCl, the mucosa was subjected to saline at pH 5.5 (control) or pH 1.0 (H(+) back diffusion) for 60 min. H(+) back diffusion evoked a marked gastric hyperemia, increase of luminal and venous histamine, and decreased numbers of MMC and CTMC. CgA IR cells were not influenced. Depletion of mast cells with dexamethasone abolished (and stabilization of mast cells with ketotifen attenuated) both hyperemia and histamine release in response to H(+) back diffusion. GBF responses to H(+) back diffusion were attenuated by H(1) and abolished by H(3) but not H(2) receptor blockers. Our data conform to the idea that mast cells are involved in the gastric hyperemic response to acid back diffusion via release of histamine.

Protective effect of mannitol, glucose-fructose-sucrose-maltose mixture, and natural honey hyperosmolar solutions against ethanol-induced gastric mucosal damage in rats

BACKGROUND

We have previously shown that natural honey is able to protect the rat stomach against acute ethanol- and indomethacin-induced lesions. The present investigations were undertaken to examine the role of intraluminal osmolality in this protective effect.

METHODS

Mannitol, glucose-fructose-sucrose-maltose mixture (GFSM) and natural honey (300, 600, 1800 mOsmol/kg water) were given orally to rats 30 min before administration of 70% ethanol for a further 15-min period. Lesions area of the excised stomachs were evaluated. Pylorus-ligated stomachs were filled with mannitol, GFSM mixture and honey (1800 mOsmol/kg water) to test the effect of the hyperosmolar solutions on gastric fluid content and acid secretion. The rate of gastric emptying of the three test solutions (1800 mOsmol/kg) was measured by the phenol red method.

RESULTS

Intragastric administration of mannitol, GFSM mixture or honey prevented the formation of mucosal lesions in an osmolality-dependent manner. Using the pylorus-ligated stomach model, the test solutions led to a net increase of luminal fluid volume without affecting acid content. Hyperosmolar solutions presented a delayed gastric emptying if compared to a nonnutrient solution made of carboxymethyl cellulose.

CONCLUSIONS

The observed results suggest that hyperosmolar solutions can prevent the formation of hemorrhagic lesions by luminal dilution of the necrotising agent and acid, an effect which may be potentiated by a lowered gastric emptying rate.

Differential effects of clonidine, dopamine, dobutamine, and dopexamine on basal and acid-stimulated mucosal blood flow in the rat stomach

OBJECTIVE

To analyze the effects of clonidine, dopamine, dobutamine, and dopexamine on gastric mucosal blood flow (GMBF) at baseline and after stimulation by acid back diffusion through a disrupted gastric mucosal barrier.

DESIGN

Prospective, randomized, unblinded study.

SETTING

University research laboratory.

SUBJECTS

Adult Sprague-Dawley rats.

INTERVENTIONS

Mean arterial blood pressure (MAP) and heart rate (HR) were recorded from a carotid artery of the phenobarbital-anesthetized animals. A jugular vein was cannulated for continuous infusion of saline and intravenous drug administration. The stomach was prepared for luminal perfusion and for recording GMBF with the hydrogen gas clearance technique. Gastric mucosal vascular conductance (GMVC) was calculated as GMBF divided by MAP.

MEASUREMENTS AND MAIN RESULTS

Clonidine (37.5 and 112.5 nmol x kg(-1)) lowered MAP and HR and caused gastric vasodilation as shown by a rise of GMVC. The 2.5-fold increase in GMVC elicited by gastric perfusion with HCl (0.15 M) plus ethanol (25%) was depressed by clonidine. All cardiovascular effects of clonidine were prevented by the alpha2-adrenoceptor antagonist idazoxan (2 micromol x kg(-1)). Infusion of dopamine (15 and 45 micromol x kg(-1) x hr(-1)), dobutamine, or dopexamine (each at 5 and 15 micromol x kg(-1) x hr(-1)) caused tachycardia. GMVC at baseline was attenuated by the higher dose of dopamine and dopexamine, but not dobutamine. In contrast, the acid-induced vasodilation in the gastric mucosa was depressed by dobutamine and dopexamine, but not dopamine.

CONCLUSIONS

Clonidine, dobutamine, and dopexamine at high dosage suppress the gastric mucosal vasodilator response to acid back diffusion, which is an important defense mechanism. Although the dose equivalence between rats and humans is not known, the antivasodilator effects highlight an adverse action whereby large doses of dobutamine, dopexamine, and clonidine may compromise gastric mucosal homeostasis and facilitate stress ulcer formation. Dopamine lacks this detrimental activity.

Intraluminal acid and gastric mucosal integrity: the importance of blood-borne bicarbonate

The acid-secreting gastric mucosa resists intraluminal acid better than the nonsecreting. Here we investigated pH at the epithelial cell surface, mucosal permeability, and blood flow during intraluminal administration of acid (100 mM) in acid-stimulated and nonstimulated gastric corpus mucosae. Surface pH (H(+)-selective microelectrodes), permeability (clearance of (51)Cr-EDTA), and mucosal blood flow (laser-Doppler flowmetry) were studied in Inactin-anesthetized rats. Acid secretion was stimulated with pentagastrin (40 microg. kg(-1). h(-1)) or impromidine (500 microg. kg(-1). h(-1)), or HCO(3)(-) (5 mmol. kg(-1). h(-1)) given intravenously. Surface pH was only slightly reduced by intraluminal acid in acid secretion-stimulated or HCO(3)(-)-treated rats but was substantially lowered in nonstimulated rats. Clearance increased threefold and blood flow increased by approximately 75% in nonstimulated rats. During stimulated acid secretion or intravenous infusion of HCO(3)(-), clearance was unchanged and blood flow increased by only approximately 30% during intraluminal acid. Increased epithelial transport of HCO(3)(-) buffering the mucus gel is most probably the explanation for the acid-secreting mucosa being less vulnerable to intraluminal acid than the nonsecreting.

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.

Effects of pectin-induced passive linkage of gastric H+ on the gastric acid secretion on the development of ethanol- and salicylate-induced gastric mucosal lesions in rats

The aim of this study was to evaluate the effects of intragastrically given pectin-induced physicochemical properties and actions on active gastric acid secretion and on the development of ethanol- and aspirin-induced gastric mucosal lesions. The observations were carried out on CFY-strain rats, fasted for 24 h before the experiments with water ad libitum. The observations were carried out in two experimental series. A) The gastric mucosal lesions were produced by intragastrically given 96% ethanol or aspirin prepared with 0.2 M HCl. Different doses of pectin (100, 50 and 25 mg x kg(-1), respectively) were administered intragastrically 30 min before giving necrotizing agents. The number of gastric lesions was noted 1 h after the administration, while the severity of gastric mucosal lesions was scored by semi-quantitative scale. B) The effects of pectin were studied on the volume and H+ secretion of the stomach in 4-h pylorus-ligated rats. It has been found that: 1) the gastric mucosal lesions could be produced in 100% of rats by the application of both necrotizing agents. 2) Pectin in doses of 50-100 mg x kg(-1) increased the number of gastric mucosal lesions in both models, while no increase was produced by the application of 25-mg x kg(-1) dose. 3) The severity of mucosal lesions increased significantly after the administration of all doses of pectin. 4) The pectin-induced increase of gastric lesions (number) showed a dose-response effect. 5) The pectin produced a significant increase in the volume of gastric secretion and gastric H+ secretion. It has been concluded that: a) pectin-induced physicochemical changes are able to enhance the aggression to gastric mucosa produced by ethanol and aspirin; b) a positive correlation exists between the linkage of H+ to pectin and significant active metabolic response in the rat stomach; c) pectin alone stimulates the active metabolic process of the gastric H+ secretion.

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.

Visceral Afferent Neurons: Role in Gastric Mucosal Protection

Gastric mucosal homeostasis requires rapid alarm of protective mechanisms in the face of pending injury. This article summarizes the evidence that spinal afferent neurons monitor insults to the gastric mucosa and activate local mechanisms of defense and repair through release of transmitter peptides from their endings in the stomach.

Effect of monochloramine on recovery of gastric mucosal integrity and blood flow response in rat stomachs--relations to capsaicin-sensitive sensory neurons

Gastric mucosal blood flow (GMBF) response and the recovery of gastric mucosal integrity were investigated in anesthetized rat stomachs after damage by monochloramine (NH2Cl), in comparison with 20 mM taurocholate Na (TC). A rat stomach was mounted in an ex-vivo chamber, and the mucosa was exposed to 50 mM HCl during a test period. Mucosal application of 20 mM TC for 10 min caused a marked reduction of transmucosal potential difference (PD), but the PD recovered rapidly without development of gross lesions 90 min later. In contrast, the exposure of the mucosa to NH2Cl (5 to approximately 20 mM) produced a concentration-dependent decrease in gastric PD, and the values remained lowered even 90 min after removal of the agent, resulting in severe hemorrhagic damage in the stomach. TC caused a considerable H+ back-diffusion, followed by an increase in the GMBF. In the mucosa damaged by NH2Cl, such GMBF responses were not observed, except for the temporal increase during the exposure, although similar degrees of H+ back-diffusion were observed following NH2Cl treatment. In addition, the prior exposure of the mucosa to NH2Cl significantly attenuated gastric hyperemic response induced by capsaicin but not by misoprostol (a PGE1 derivative) or NOR-3 (a NO donor). Chemical ablation of capsaicin-sensitive sensory neurons had no effect on the PD reduction caused by TC but totally attenuated the GMBF response, resulting in hemorrhagic damage in the stomach. These results suggest that NH2Cl delayed the recovery of the mucosal integrity in the stomach after damage, and this effect may be attributable, at least partly, to the impairment of gastric hyperemic response associated with H+ back-diffusion, probably due to dysfunction of capsaicin-sensitive sensory neurons.

Effects of nociceptin and nocistatin on antidromic vasodilatation in hairless skin of the rat hindlimb in vivo

1. We tested whether nociceptin (NCE), the endogenous ligand of the opioid receptor-like 1 (ORL1) receptor, and nocistatin (NST), which reverses central NCE effects when applied intrathecally (i.t.), affect small-diameter afferent fibre-mediated vasodilatation in rat hairless skin. 2. Female Wistar rats were vagotomized. Ongoing sympathetic vasoconstrictor activity was abolished by bilateral section of the lumbar sympathetic trunk between ganglia L2 and L3. Sensory axons were selectively stimulated in the dorsal root L5 by 20 electrical impulses supramaximal for activating C-fibres at 1 Hz. Blood flow was measured on the plantar skin of the left hind paw in the L5 dermatome using laser Doppler flowmetry. 3. NCE injected intravenously (i.v.) as single boluses (1, 10 and 100 nmol kg(-1) 7 - 8 min before dorsal root stimulation (n=6) dose-dependently decreased blood pressure and local vascular resistance and suppressed antidromic vasodilatation maximally by 47% (P<0.01). When NCE was injected 2 min before stimulation (n=3), antidromic vasodilatation was reduced by 64% after NCE (1 nmol kg-1) and totally, or almost totally, abolished after the two higher doses. 4. NST (1 - 100 nmol kg(-1) i.v., n=6) was without significant effect on blood pressure and cutaneous vascular resistance. Applied 5 (n=6) or 2 min (n=3) before stimulation it also did not affect antidromic vasodilatation. NST (100 nmol kg(-1) i.v.) applied shortly before an equimolar dose of NCE did not antagonize NCE effects on vascular resistance, blood pressure and antidromic vasodilatation (n=4). 5. In conclusion, NCE inhibits antidromic vasodilatation, a component of neurogenic inflammation, in rat skin while NST is without effect. NST, at the small-diameter sensory ending, is not an effective antagonist of NCE.