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

Omeprazole treatment manifests anxiolytic effects in a cysteamine hydrochloride induced mouse model of gastrointestinal disorder

Omeprazole, a proton pump inhibitor (PPI), has widely been used to treat various gastrointestinal (GI) disorders. Notably, many clinical symptoms of GI disorders have been known to be associated with anxiety. In recent years, an exponentially increased number of subjects with abnormal ageing, neurological deficits, and psychiatric problems simultaneously exhibit GI dysfunctions as well as anxiety. Considering the fact, drugs that are used to treat GI disorders can be speculated to mitigate anxiety-related symptoms, and vice versa. Although, omeprazole treatment has been reported to result in development of anxiety and neurocognitive decline, ample reports suggest that omeprazole treatment is beneficial for the positive regulation of neuroplasticity. While underlying mechanisms of omeprazole-mediated neurological alterations remain obscure, the available scientific data on the omeprazole induced adverse effects in the brain appear to be inadequate, uncertain, and controversial. Hence, this study revisited the effect of omeprazole treatment on the degree of anxiety-like behaviours in a cysteamine hydrochloride (HCl) induced mouse model of GI disorder using open field test (OFT), light-dark box (LDB) test and elevated plus maze (EPM). Results revealed that omeprazole treatment mitigates anxiety-related behaviours in the cysteamine HCl induced animal model of GI disorder. Thus, this study assuredly supports and validates the anxiolytic properties of omeprazole. However, the adverse effects associated with inappropriate intake of omeprazole may not completely be excluded. Therefore, this study advocates the future direction in determining the long-term effects of omeprazole on the brain functions.

Role of the Duodenum in the Pathogenesis of Functional Dyspepsia: A Paradigm Shift

Functional dyspepsia (FD) is a common disorder characterized by chronic epigastric pain or burning, or bothersome postprandial fullness or early satiation, without a definitive organic cause. The pathogenesis of FD is likely heterogeneous. Classically, motor disorders, visceral hypersensitivity, and brain-gut interactions have been implicated in the pathophysiology of FD, but recently an important role for chronic low-grade inflammation and infection in FD has been reported and confirmed. Duodenal low-grade inflammation is frequently observed in FD in those with and without documented previous gastroenteritis. Duodenal eosinophils and in some cases mast cells may together or separately play a key role, and immune activation (eg, circulating homing small intestinal T cells) has been observed in FD. Low-grade intestinal inflammation in patients with FD may provoke impairment in motor-sensory abnormalities along the gastrointestinal neural axis. Among FD patients, the risk of developing dyspeptic symptoms after a bout of gastroenteritis is 2.54 (95% CI, 1.76–3.65) at more than 6 months after acute gastroenteritis. Gut host and microbial interactions are likely important, and emerging data demonstrate both quantitative and qualitative changes of duodenal mucosal and fecal microbiota in FD. Food antigens (eg, wheat proteins) may also play a role in inducing duodenal inflammation and dyspepsia. While causation is not established, the hypothesis that FD is a disorder of microscopic small intestinal inflammation in a major subset is gaining acceptance, opening the possibility of novel treatment approaches that may be able to alter the natural history of the disorder.

Meal-Sensing Signaling Pathways in Functional Dyspepsia

The upper gastrointestinal tract plays an important role in sensing the arrival, amount and chemical composition of a meal. Ingestion of a meal triggers a number of sensory signals in the gastrointestinal tract. These include the response to mechanical stimulation (e.g., gastric distension), from the presence of food in the gut, and the interaction of various dietary nutrients with specific "taste" receptors on specialized enteroendocrine cells in the small intestine culminating in the release of gut hormones. These signals are then transmitted to the brain where they contribute to food intake regulation by modulating appetite as well as feedback control of gastrointestinal functions (e.g., gut motility). There is evidence that the sensitivity to these food related stimuli is abnormally enhanced in functional dyspepsia leading to symptoms such nausea and bloating. In addition, these gut-brain signals can modulate the signaling pathways involved in visceral pain. This review will discuss the role of gut-brain signals in appetite regulation and the role dysregulation of this system play in functional dyspepsia.

Definition, Pathogenesis, and Management of That Cursed Dyspepsia

Dyspepsia is an umbrella term used to encompass a number of symptoms thought to originate from the upper gastrointestinal tract. These symptoms are relatively nonspecific; not surprisingly, therefore, a myriad of conditions may present with any one or a combination of these symptoms. Therein lays the clinician's first challenge: detecting the minority who may have a potentially life-threatening disorder, such as gastric cancer, from a population whose symptoms are, for the most part, considered functional in origin. The second challenge lies in the definition and management of those individuals with functional dyspepsia (FD); the major focus of this review. The Rome process has addressed the issue of FD definition and a look back at the evolution of Rome criteria for this disorder illustrates the complexities that have so frustrated us. There has been no shortage of hypotheses to explain symptom pathogenesis in FD; initially focused on gastric sensorimotor dysfunction, these have now strayed well into the duodenum and have come to entertain such factors as immune responses and the microbiome. FD has proven to be an equally challenging area for therapeutics; while the staple approaches of acid suppression and eradication of Helicobacter pylori have some limited efficacy in select populations, strategies to ameliorate symptoms in the majority of sufferers based on presumed pathophysiology have largely foundered. Lacking a validated biomarker(s) FD continues to be an elusive target and is likely to remain so until we can better define the various phenotypes that it must surely contain.

Involvement of Transient Receptor Potential Vanilloid Receptor 1, (TRPV1)-Expressing Vagal Nerve in the Inhibitory Effect of Gastric Acidification on Exogenous Motilin-Induced Gastric Phase III Contractions in Suncus murinus

BACKGROUND

Gastric acidification inhibits motilin-induced gastric phase III contractions. However, the underlying mechanism has not been thoroughly investigated. Here, we studied the inhibitory mechanism by gastric acidification on motilin-induced contraction in Suncus murinus (S. murinus).

METHODS

We measured interdigestive gastric phase III contractions in conscious, freely moving S. murinus, and examined the inhibitory effect of gastric acidification on motilin action and the involvement of the vagus nerve and transient receptor potential vanilloid receptor 1 (TRPV1) in the inhibitory mechanism.

RESULTS

A bolus injection of motilin evoked phase III-like contractions during intravenous infusion of saline. Intragastric acidification (pH 1.5-2.5) inhibited motilin-induced phase III contractions in a pH-dependent manner and significantly decreased the motility index at a pH below 2.0. In contrast, intraduodenal acidification (pH 2.0) failed to inhibit motilin-induced contractions. Vagotomy significantly alleviated the suppression of motilin-induced gastric contractions under acidic conditions (pH 2.0), suggesting vagus nerve involvement. Moreover, intragastric acidification (pH 2.0) significantly increased the number of c-Fos-positive cells in the nucleus tractus solitarii. In vagotomized S. murinus, the number of c-Fos-positive cells did not change, even under gastric acidification conditions. TRPV1 mRNA was highly expressed in the muscle and mucosal regions of the antrum and the nodose ganglion, whereas was not detected in the upper small intestine. Capsazepin, a TRPV1 antagonist, completely rescued the inhibitory effect of gastric acidification.

CONCLUSIONS

Gastric acidification in S. murinus inhibits motilin-induced contractions, a finding similar to results observed in humans, while TRPV1-expressing vagus nerves play a role in the inhibitory mechanism.

The effect of spinal cord injury on the neurochemical properties of vagal sensory neurons

The vagus nerve is composed primarily of nonmyelinated sensory neurons whose cell bodies are located in the nodose ganglion (NG). The vagus has widespread projections that supply most visceral organs, including the bladder. Because of its nonspinal route, the vagus nerve itself is not directly damaged from spinal cord injury (SCI). Because most viscera, including bladder, are dually innervated by spinal and vagal sensory neurons, an impact of SCI on the sensory component of vagal circuitry may contribute to post-SCI visceral pathologies. To determine whether SCI, in male Wistar rats, might impact neurochemical characteristics of NG neurons, immunohistochemical assessments were performed for P2X3 receptor expression, isolectin B4 (IB4) binding, and substance P expression, three known injury-responsive markers in sensory neuronal subpopulations. In addition to examining the overall population of NG neurons, those innervating the urinary bladder also were assessed separately. All three of the molecular markers were represented in the NG from noninjured animals, with the majority of the neurons binding IB4. In the chronically injured rats, there was a significant increase in the number of NG neurons expressing P2X3 and a significant decrease in the number binding IB4 compared with noninjured animals, a finding that held true also for the bladder-innervating population. Overall, these results indicate that vagal afferents, including those innervating the bladder, display neurochemical plasticity post-SCI that may have implications for visceral homeostatic mechanisms and nociceptive signaling.

Acid-sensing ion channels in gastrointestinal function

Gastric acid is of paramount importance for digestion and protection from pathogens but, at the same time, is a threat to the integrity of the mucosa in the upper gastrointestinal tract and may give rise to pain if inflammation or ulceration ensues. Luminal acidity in the colon is determined by lactate production and microbial transformation of carbohydrates to short chain fatty acids as well as formation of ammonia. The pH in the oesophagus, stomach and intestine is surveyed by a network of acid sensors among which acid-sensing ion channels (ASICs) and acid-sensitive members of transient receptor potential ion channels take a special place. In the gut, ASICs (ASIC1, ASIC2, ASIC3) are primarily expressed by the peripheral axons of vagal and spinal afferent neurons and are responsible for distinct proton-gated currents in these neurons. ASICs survey moderate decreases in extracellular pH and through these properties contribute to a protective blood flow increase in the face of mucosal acid challenge. Importantly, experimental studies provide increasing evidence that ASICs contribute to gastric acid hypersensitivity and pain under conditions of gastritis and peptic ulceration but also participate in colonic hypersensitivity to mechanical stimuli (distension) under conditions of irritation that are not necessarily associated with overt inflammation. These functional implications and their upregulation by inflammatory and non-inflammatory pathologies make ASICs potential targets to manage visceral hypersensitivity and pain associated with functional gastrointestinal disorders. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Identification of bladder and colon afferents in the nodose ganglia of male rats

The sensory neurons innervating the urinary bladder and distal colon project to similar regions of the central nervous system and often are affected simultaneously by various diseases and disorders, including spinal cord injury. Anatomical and physiological commonalities between the two organs involve the participation of shared spinally derived pathways, allowing mechanisms of communication between the bladder and colon. Prior electrophysiological data from our laboratory suggest that the bladder also may receive sensory innervation from a nonspinal source through the vagus nerve, which innervates the distal colon as well. The present study therefore aimed to determine whether anatomical evidence exists for vagal innervation of the male rat urinary bladder and to assess whether those vagal afferents also innervate the colon. Additionally, the relative contribution to bladder and colon sensory innervation of spinal and vagal sources was determined. By using lipophilic tracers, neurons that innervated the bladder and colon in both the nodose ganglia (NG) and L6/S1 and L1/L2 dorsal root ganglia (DRG) were quantified. Some single vagal and spinal neurons provided dual innervation to both organs. The proportions of NG afferents labeled from the bladder did not differ from spinal afferents labeled from the bladder when considering the collective population of total neurons from either group. Our results demonstrate evidence for vagal innervation of the bladder and colon and suggest that dichotomizing vagal afferents may provide a neural mechanism for cross-talk between the organs.

Acid sensing by visceral afferent neurones

Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics.

The importance of systemic response in the pathobiology of blast-induced neurotrauma

Due to complex injurious environment where multiple blast effects interact with the body parallel, blast-induced neurotrauma is a unique clinical entity induced by systemic, local, and cerebral responses. Activation of autonomous nervous system; sudden pressure increase in vital organs such as lungs and liver; and activation of neuroendocrine-immune system are among the most important mechanisms that contribute significantly to molecular changes and cascading injury mechanisms in the brain. It has been hypothesized that vagally mediated cerebral effects play a vital role in the early response to blast: this assumption has been supported by experiments where bilateral vagotomy mitigated bradycardia, hypotension, and apnea, and also prevented excessive metabolic alterations in the brain of animals exposed to blast. Clinical experience suggests specific blast-body-nervous system interactions such as (1) direct interaction with the head either through direct passage of the blast wave through the skull or by causing acceleration and/or rotation of the head; and (2) via hydraulic interaction, when the blast overpressure compresses the abdomen and chest, and transfers its kinetic energy to the body's fluid phase, initiating oscillating waves that traverse the body and reach the brain. Accumulating evidence suggests that inflammation plays important role in the pathogenesis of long-term neurological deficits due to blast. These include memory decline, motor function and balance impairments, and behavioral alterations, among others. Experiments using rigid body- or head protection in animals subjected to blast showed that head protection failed to prevent inflammation in the brain or reduce neurological deficits, whereas body protection was successful in alleviating the blast-induced functional and morphological impairments in the brain.

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.

Gastric hypersensitivity induced by oesophageal acid infusion in healthy volunteers

Distal oesophageal acid exposure has been shown to increase visceral sensitivity of the proximal oesophagus via central sensitization. Here we evaluated whether acidification of the distal oesophagus also affects the sensorimotor function of the proximal stomach. A gastric barostat study combined with a 30-min acid (HCl 0.15 mol L(-1)) or saline infusion in the distal oesophagus was performed in 18 healthy volunteers. Gastric and cutaneous sensitivity was assessed before and up to 2 h after the start of infusion. Directly after acid infusion, but not after saline, the threshold for discomfort decreased (-6.4 +/- 1.7 vs 0.4 +/- 0.4 mmHg; P = 0.028) and distension-induced symptoms increased significantly compared with the baseline (122 +/- 49% vs -3 +/- 9%). Cutaneous sensitivity remained unaffected by acid infusion. In contrast, when the infused liquid was aspirated 3 cm more distally, at the level of the lower oesophageal sphincter, the effect of acid infusion on gastric sensitivity was abolished and the increase in distension-induced symptoms was reduced (61 +/- 24%). Distal oesophageal acid infusion induces visceral hypersensitivity without affecting somatic sensitivity arguing against a similar mechanism of central sensitization as observed in non-cardiac chest pain. As reduction of the acid load to the stomach prevented this effect, our findings indicate that either gastric and/or duodenal acidification is involved. It should be emphasized though that aspiration from distal oesophagus may have attenuated the effect by reducing the acid-exposed area or by reducing the contact time.

Real-time evaluation of dyspeptic symptoms and gastric motility induced by duodenal acidification using noninvasive transnasal endoscopy

BACKGROUND

Although different pathophysiological mechanisms have been suggested to be involved in functional dyspepsia, a practical method to clarify them has not been established. The aim of this study was to evaluate dyspeptic symptoms and gastric motility induced by duodenal acidification using transnasal endoscopy.

METHODS

Fourteen healthy volunteers (mean age, 32 years) were enrolled. Transnasal endoscopy was performed on all fasting volunteers. Dyspeptic symptoms and antral contractions were evaluated before and after duodenal infusions of pure water (20 ml/min for 5 min) and acid (0.1 N HCl, 20 ml/min for 5 min). The severity of various symptoms was assessed by each subject using a 10-cm visual analog scale every 2 min. The maximum severity scale was calculated as the mean of the individual maximum values. The motility number was defined as the mean number of antral contractions in 1 min.

RESULTS

The maximum severity score for a heavy feeling in the stomach and other symptoms significantly increased after the acid infusion compared with after the pure water infusion. During pure water infusion, there were no changes in the motility number. On the other hand, the motility number significantly decreased after duodenal acidification (before vs. after, 2.93 +/- 0.12 times vs. 1.11 +/- 0.23 times, P < 0.0001).

CONCLUSIONS

Duodenal acid exposure induces dyspeptic symptoms and inhibits antral motility. Transnasal endoscopy enabled us to evaluate both dyspeptic symptoms and gastric motility simultaneously.

Effect of reflux-induced inflammation on transient receptor potential vanilloid one (TRPV1) expression in primary sensory neurons innervating the oesophagus of rats

A possible mechanism of oesophageal hypersensitivity is the acid-induced activation of transient receptor potential vanilloid receptor 1 (TRPV1) in the primary sensory neurons. We investigated TRPV1 expression and its colocalization with substance P (SP) and isolectin B4 (IB4)-positive cells in the thoracic dorsal root ganglia (DRGs) and nodose ganglia (NGs) of rats with reflux-induced oesophagitis (RO). RO was developed by fundus ligation and partial obstruction of the pylorus of Sprague-Dawley rats. Four groups of rats were used; fundus ligated acute (RO 48 h), chronic 7 days (RO 7D), RO 7D + omeprazole (7D + Omz, 40 mg kg(-1), i.p.) and sham-operated controls. Immunohistochemical analysis of TRPV1, SP and IB4 expression were carried out in spinal cord (SC), DRGs and NGs. RO rats exhibited significant inflammation and increase in TRPV1-ir and SP-ir expressions in the SC, DRGs and NGs. The maximum colocalization of TRPV1 and SP was observed in RO 7D rats, but Omz prevented inflammation and over expression of TRPV1 and SP. TRPV1-ir significantly increased in IB4-positive cells in DRGs and SC, but not in the NGs. Results document that acid-induced oesophagitis increases TRPV1 expression in both SP- and IB4-positive sensory neurons. The over expression of TRPV1 may contribute to oesophageal hypersensitivity observed in gastro-oesophageal reflux disease (GORD).

Significant increase in the aggressive behavior of transgenic mice overexpressing peripheral progastrin peptides: associated changes in CCK2 and serotonin receptors in the CNS

The gastrin precursor peptide, progastrin (PG), is secreted from enteroendocrine cells in the intestine and increased in patients with hypergastrinemia and colorectal cancers. In recent years, we and others have demonstrated an important role of PG peptides in colorectal carcinogenesis, and were surprised to note significant changes in the behaviors of transgenic mice overexpressing PGs. In the present studies, we examined emotional behaviors of transgenic mice overexpressing PG in the intestinal and peripheral circulation. Aggression, locomotor activity and anxiety-like behaviors of the homozygous transgenic (Tg/Tg) mice and the wild-type (WT) littermates were examined by intruder/resident test, open field and elevated plus maze, respectively. A significant increase in the aggression, locomotor activity, and anxiety-like behaviors was detected in the Tg/Tg vs WT mice. As CCK, CCK(2) receptors (CCK(2)R), and 5-HT(1A) receptors (5-HT(1A)R) in the CNS play an important role in these behaviors, possible changes in the expression of CCK and CCK(2)R and the density of CCK(2)R and 5-HT(1A)R were determined by either real-time RT-PCR or autoradiography of ligand binding assays. The results suggest that the expressions of CCK and CCK(2)R were increased in the hypothalamus, and the density of CCK(2)R were increased in the hypothalamus and amygdala of Tg/Tg vs WT mice. Similarly, the density of 5-HT(1A)R was increased in the hypothalamus. Our results suggest that an upregulation of the CCK response system and 5-HT(1A)R in the hypothalamus of Tg/Tg mice may mediate the alterations in the observed behaviors of these mice.

Increase in gastric acid-induced afferent input to the brainstem in mice with gastritis

Acid challenge of the gastric mucosa is signaled to the brainstem. This study examined whether mild gastritis due to dextrane sulfate sodium (DSS) or iodoacetamide (IAA) enhances gastric acid-evoked input to the brainstem and whether this effect is related to gastric myeloperoxidase activity, gastric histology, gastric volume retention or cyclooxygenase stimulation. The stomach of conscious mice was challenged with NaCl (0.15 M) or HCl (0.15 and 0.25 M) administered via gastric gavage. Two hours later, activation of neurons in the nucleus tractus solitarii (NTS) was visualized by c-Fos immunocytochemistry. Gastritis was induced by DSS (molecular weight 8000; 5%) or IAA (0.1%) added to the drinking water for 7 days. Relative to NaCl, intragastric HCl increased the number of c-Fos protein-expressing cells in the NTS. Pretreatment with DSS or IAA for 1 week did not alter the c-Fos response to NaCl but significantly enhanced the response to HCl by 54 and 74%, respectively. Either pretreatment elevated gastric myeloperoxidase activity and induced histological injury of the mucosal surface. In addition, DSS caused dilation of the gastric glands and damage to the parietal cells. HCl-induced gastric volume retention was not altered by IAA but attenuated by DSS pretreatment. Indomethacin (5 mg/kg) failed to significantly alter HCl-evoked expression of c-Fos in the NTS of control, DSS-pretreated and IAA-pretreated mice. We conclude that the gastritis-evoked increase in the gastric acid-evoked c-Fos expression in the NTS is related to disruption of the gastric mucosal barrier, mucosal inflammation, mucosal acid influx and enhanced activation of the afferent stomach-NTS axis.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Carbonic anhydrases and mucosal vanilloid receptors help mediate the hyperemic response to luminal CO2 in rat duodenum

BACKGROUND & AIMS

The duodenal mucosa is exposed to PCO(2) >200 mm Hg due to the luminal mixture of gastric acid with secreted bicarbonate, which augments mucosal protective mechanisms. We examined the hyperemic response to elevated luminal PCO(2) in the duodenum of anesthetized rats luminally exposed to high CO(2) saline to help elucidate luminal acid-sensing mechanisms.

METHODS

Blood flow was measured by laser Doppler, and intracellular pH of epithelial cells by measured by ratio microimaging. The permeant carbonic anhydrase (CA) inhibitor methazolamide, relatively impermeant CA inhibitor benzolamide, vanilloid receptor antagonist capsazepine, or sodium-hydrogen exchanger 1 (NHE-1) inhibitor dimethyl amiloride were perfused with or without the high CO(2) solution.

RESULTS

The high CO(2) solution increased duodenal blood flow, which was abolished by pretreatment with methazolamide or capsazepine or by dimethyl amiloride coperfusion. Sensory denervation with capsaicin also abolished the CO(2) effects. Benzolamide dose-dependently inhibited CO(2)-induced hyperemia and at 100 nmol/L inhibited CO(2)-induced intracellular acidification. The membrane-bound CA isoforms IV, IX, XII, and XIV and cytosolic CA II and the vanilloid receptor 1 (TRPV1) were expressed in duodenum and stomach. Dorsal root ganglion and nodose ganglion expressed all isoforms except for CA IX.

CONCLUSIONS

The duodenal hyperemic response to luminal CO(2) is dependent on cytosolic and membrane-bound CA isoforms, NHE-1, and TRPV1. CO(2)-induced intracellular acidification was inhibited by selective extracellular CA inhibition, suggesting that CO(2) diffusion across the epithelial apical membrane is mediated by extracellular CA. NHE-1 activation preceding TRPV1 stimulation suggests that luminal CO(2) is sensed as H(+) in the subepithelium.

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.

Basic and clinical pharmacology of new motility promoting agents

Recent research has provided new information about drugs that could be used to treat functional motility disorders. Promotility drugs accelerate gastric emptying or colonic transit and these properties may contribute to their efficacy in treating symptoms associated with gastroparesis, functional dyspepsia or constipation. 5-Hydroxytryptamine4 receptors are targets for drugs (tegaserod, renzapride) that treat symptoms in constipated irritable bowel syndrome patients and in gastroparesis. Drugs acting at motilin (erythromycin) and cholecystokinin-1 (dexloxiglumide) receptors accelerate gastric emptying. Dexloxiglumide might be useful in the treatment of functional dyspepsia particularly that associated with lipid intake. Alvimopan is a mu-opioid receptor antagonist that does not cross the blood brain barrier. Alvimopan is effective in treating postsurgical ileus and perhaps opiate-induced bowel dysfunction. Successes and failures of recent efforts to develop promotility agents revealed opportunities and challenges for developing new promotility drugs. The pharmacological properties of partial agonists might be exploited to develop effective promotility drugs. However, opposing actions of promotility agents on motility (increased contraction vs decreased accommodation) limit the clinical efficacy of drugs with these opposing actions. Selection of appropriate patient populations for evaluation of new drugs is also critical.

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.

Gastric secretion

PURPOSE OF REVIEW

The purpose of this chapter is to summarize and place into perspective the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

To prevent acid and pepsin from overwhelming mucosal defense mechanisms and causing injury, the secretion of gastric acid is precisely regulated by a variety of central (eg, neuropeptide Y, corticotropin-releasing factor, and neuromedin U) and peripheral (eg, gastrin, histamine, acetylcholine, somatostatin, cholecystokinin, calcitonin gene-related peptide, leptin, and parietal cell) pathways. These pathways regulate the acid-producing parietal cell directly and/or indirectly by regulating the secretion of histamine from enterochromaffin-like cells, gastrin from G cells, and somatostatin from D cells. Recently, genetically engineered mouse models have been used to reevaluate the neural, hormonal, and paracrine pathways that physiologically regulate acid secretion.

SUMMARY

An improved understanding of the pathways and mechanisms regulating gastric acid secretion should lead to the development of novel therapies to prevent and treat acid-peptic disorders as well as circumvent the adverse effects of currently used antisecretory medications such as the acid rebound observed after discontinuation of proton pump inhibitors.