Review: Chemosensing of nutrients and non-nutrients in the human and porcine gastrointestinal tract

The gastrointestinal tract (GIT) is an interface between the external and internal milieus that requires continuous monitoring for nutrients or pathogens and toxic chemicals. The study of the physiological/molecular mechanisms, mediating the responses to the monitoring of the GIT contents, has been referred to as chemosensory science. While most of the progress in this area of research has been obtained in laboratory rodents and humans, significant steps forward have also been reported in pigs. The objective of this review was to update the current knowledge on nutrient chemosensing in pigs in light of recent advances in humans and laboratory rodents. A second objective relates to informing the existence of nutrient sensors with their functionality, particularly linked to the gut peptides relevant to the onset/offset of appetite. Several cell types of the intestinal epithelium such as Paneth, goblet, tuft and enteroendocrine cells (EECs) contain subsets of chemosensory receptors also found on the tongue as part of the taste system. In particular, EECs show specific co-expression patterns between nutrient sensors and/or transceptors (transport proteins with sensing functions) and anorexigenic hormones such as cholecystokinin (CCK), peptide tyrosine tyrosine (PYY) or glucagon-like peptide-1 (GLP-1), amongst others. In addition, the administration of bitter compounds has an inhibitory effect on GIT motility and on appetite through GLP-1-, CCK-, ghrelin- and PYY-labelled EECs in the human small intestine and colon. Furthermore, the mammalian chemosensory system is the target of some bacterial metabolites. Recent studies on the human microbiome have discovered that commensal bacteria have developed strategies to stimulate chemosensory receptors and trigger host cellular functions. Finally, the study of gene polymorphisms related to nutrient sensors explains differences in food choices, food intake and appetite between individuals.

Chemoreceptors in the Gut

The gastrointestinal tract represents the largest interface between the human body and the external environment. It must continuously monitor and discriminate between nutrients that need to be assimilated and harmful substances that need to be expelled. The different cells of the gut epithelium are therefore equipped with a subtle chemosensory system that communicates the sensory information to several effector systems involved in the regulation of appetite, immune responses, and gastrointestinal motility. Disturbances or adaptations in the communication of this sensory information may contribute to the development or maintenance of disease. This is a new emerging research field in which perception of taste can be considered as a novel key player participating in the regulation of gut function. Specific diets or agonists that target these chemosensory signaling pathways may be considered as new therapeutic targets to tune adequate physiological processes in the gut in health and disease.

Manometric evaluation of the motilin receptor agonist camicinal (GSK962040) in humans

BACKGROUND

The gut hormone motilin stimulates gastrointestinal motility by inducing gastric phase III of the migrating motor complex (MMC) and enhancing the rate of gastric emptying. Camicinal (GSK962040), a small molecule motilin receptor agonist, has been shown to increase gastrointestinal motility.

METHODS

In this proof of concept study the effects of camicinal on MMC activity, esophageal and gastric pH was evaluated in eight healthy volunteers as a secondary endpoint. Doses of 50 and 150 mg were compared to placebo for a period of 24 hours in a double-blinded randomized crossover trial.

KEY RESULTS

The 50 mg dose (n=4) of camicinal had no significant impact on gastroduodenal manometry or pH parameters. A single dose of 150 mg (n=4) induced a gastric phase III after 0:34 h (0:25-0:58), which was significantly faster compared to placebo (18:15 h (4:32-22:16); P=.03). Moreover, the high dose significantly increased the occurrence of gastric phase III contractions compared to placebo (12% vs 39%; P=.0003). This increase in gastric phase III contractions during a period of 24 hour was due to an increased occurrence of gastric phases III during the daytime (5% vs 50%; P=.0001). The same dose however did not affect small bowel manometry parameters or esophageal and gastric pH.

CONCLUSIONS AND INFERENCES

Considering its stimulating effect on the MMC and gastric emptying, camicinal is an attractive candidate for the treatment of gastroparesis and gastroesophageal reflux disease. This trial was registered at clinicaltrials.gov as NCT00562848.

Intragastric infusion of the bitter tastant quinine suppresses hormone release and antral motility during the fasting state in healthy female volunteers

BACKGROUND

Intragastric administration of the bitter tastant denatonium benzoate inhibits the increase of motilin plasma levels and antral contractility. While these findings suggest that gastrointestinal bitter taste receptors could be new targets to modulate gastrointestinal motility and hormone release, they need confirmation with other bitter receptor agonists. The primary aim was to evaluate the effect of intragastric administration of the bitter tastant quinine-hydrochloride (QHCl) on motilin and ghrelin plasma levels. Secondly, we studied the effect on interdigestive motility.

METHODS

Ten healthy female volunteers were recruited (33±4 y; 22±0.5 kg/m²). Placebo or QHCl (10 μmol/kg) was administered intragastrically through a nasogastric feeding tube after an overnight fast in a single-blind randomized fashion. Administration started 20 min after the first phase III of the migrating motor complex. The measurement continued for another 2 h after the administration. Blood samples were collected every 10 min with the baseline sample taken 10 min prior to administration.

KEY RESULTS

The increase in plasma levels of motilin (administration; P=.04) and total ghrelin (administration; P=.02) was significantly lower after QHCl. The fluctuation of octanoylated ghrelin was reduced after QHCl (time by administration; P=.03). Duodenal motility did not differ. The fluctuation of antral activity differed over time between placebo and QHCl (time by administration; P=.03).

CONCLUSIONS AND INFERENCES

QHCl suppresses the increase of both motilin and ghrelin plasma levels. Moreover, QHCl reduced the fluctuation of antral motility. These findings confirm the potential of bitter taste receptors as targets for modifying interdigestive motility in man.

Abdominal vagus nerve stimulation as a new therapeutic approach to prevent postoperative ileus

BACKGROUND

Electrical stimulation of the cervical vagus nerve (VNS) prevents postoperative ileus (POI) in mice. As this approach requires an additional cervical procedure, we explored the possibility of peroperative abdominal VNS in mice and human.

METHODS

The effect of cervical and abdominal VNS was studied in a murine model of POI and lipopolysaccharide (LPS)-induced sepsis. Postoperative ileus was quantified by assessment of intestinal transit of fluorescent dextran expressed as geometric center (GC). Next, the effect of cervical and abdominal VNS on heart rate was determined in eight Landrace pigs to select the optimal electrode for VNS in human. Finally, the effect of sham or abdominal VNS on LPS-induced cytokine production of whole blood was studied in patients undergoing colorectal surgery.

KEY RESULTS

Similar to cervical VNS, abdominal VNS significantly decreased LPS-induced serum tumor necrosis factor-α (TNFα) levels (abdominal VNS: 366±33 pg/mL vs sham: 822±105 pg/mL; P<.01). In line, in a murine model of POI, abdominal VNS significantly improved intestinal transit (GC: sham 5.1±0.2 vs abdominal VNS: 7.8±0.6; P<.01) and reduced intestinal inflammation (abdominal VNS: 35±7 vs sham: 80±8 myeloperoxidase positive cells/field; P<.05). In pigs, heart rate was reduced by cervical VNS but not by abdominal VNS. In humans, abdominal VNS significantly reduced LPS-induced IL8 and IL6 production by whole blood.

CONCLUSIONS & INFERENCES

Abdominal VNS is feasible and safe in humans and has anti-inflammatory properties. As abdominal VNS improves POI similar to cervical VNS in mice, our data indicate that peroperative abdominal VNS may represent a novel approach to shorten POI in man.

Higher plasma motilin levels in obese patients decrease after Roux-en-Y gastric bypass surgery and regulate hunger

OBJECTIVE

Motilin-induced phase III contractions of the migrating motor complex (MMC) signal hunger in healthy volunteers. The current aim was to study the role of motilin as a hunger-inducing factor in obese patients and to evaluate the effect of Roux-en-Y gastric bypass (RYGB) surgery on plasma motilin levels and hunger scores.

DESIGN

Motilin and ghrelin plasma levels were determined during a complete MMC cycle in controls and obese patients selected for RYGB before, 6 months and 1 year after surgery. 20 min after the end of the second phase III, obese patients received an intravenous infusion of 40 mg erythromycin. Hunger was scored every 5 min. Hedonic hunger was assessed in obese patients with the Power of Food Scale questionnaire.

RESULTS

Obesity caused a switch in the origin of phase III from antrum to duodenum. Obese patients had significantly higher motilin levels compared with controls during the MMC but tended to lack the motilin peak prior to phase III necessary to trigger hunger. Hunger scores during phase III were significantly lower in obese patients, but could be restored to control levels through the administration of a low dose of the motilin agonist, erythromycin. After RYGB surgery motilin, but not ghrelin, levels decreased in parallel with hedonic hunger scores.

CONCLUSIONS

Motilin may be an important regulator involved in the pathogenesis of obesity.

Smooth muscle and neural dysfunction contribute to different phases of murine postoperative ileus

BACKGROUND

Postoperative ileus (POI) is characterized by a transient inhibition of gastrointestinal (GI) motility after abdominal surgery mediated by the inflammation of the muscularis externa (ME). The aim of this study was to identify alterations in the enteric nervous system that may contribute to the pathogenesis of POI.

METHODS

Gastrointestinal transit, contractility of isolated smooth muscle strips and inflammatory parameters were evaluated at different time points (1.5 h to 10 days) after intestinal manipulation (IM) in mice. Immune-labeling was used to visualize changes in myenteric neurons.

KEY RESULTS

Intestinal manipulation resulted in an immediate inhibition of GI transit recovering between 24 h and 5 days. In vitro contractility to K(+) (60 mM) or carbachol (10(-9) to 10(-4) M) was biphasically suppressed over 24 h after IM (with transient recovery at 6 h). The first phase of impaired myogenic contractility was associated with increased expression of TNF-α, IL-6 and IL-1α. After 24 h, we identified a significant reduction in electrical field stimulation-evoked contractions and relaxations, lasting up to 10 days after IM. This was associated with a reduced expression of chat and nos1 genes.

CONCLUSIONS & INFERENCES

Intestinal manipulation induces two waves of smooth muscle inhibition, most likely mediated by inflammatory cytokines, lasting up to 3 days after IM. Further, we here identify a late third phase (>24 h) characterized by impaired cholinergic and nitrergic neurotransmission persisting after recovery of muscle contractility. These findings illustrate that POI results from inflammation-mediated impaired smooth muscle contraction, but also involves a long-lasting impact of IM on the enteric nervous system.

The bitter truth about bitter taste receptors: beyond sensing bitter in the oral cavity

The bitter taste receptor (TAS2R)-family of G-protein-coupled receptors has been identified on the tongue as detectors of bitter taste over a decade ago. In the last few years, they have been discovered in an ever growing number of extra-oral tissues, including the airways, the gut, the brain and even the testis. In tissues that contact the exterior, protective functions for TAS2Rs have been proposed, in analogy to their function on the tongue as toxicity detector. However, TAS2Rs have also been found in internal organs, suggesting other roles for these receptors, perhaps involving as yet unidentified endogenous ligands. The current review gives an overview of the different proposed functions for TAS2Rs in tissues other than the oral cavity; from appetite regulation to the treatment of asthma, regulation of gastrointestinal motility and control of airway innate immunity.

Chronobesity: role of the circadian system in the obesity epidemic

Although obesity is considered to result from an imbalance between energy uptake and energy expenditure, the strategy of dietary changes and physical exercise has failed to tackle the global obesity epidemic. In search of alternative and more adequate treatment options, research has aimed at further unravelling the mechanisms underlying this excessive weight gain. While numerous studies are focusing on the neuroendocrine alterations that occur after bariatric Roux-en-Y gastric bypass surgery, an increasing amount of chronobiological studies have started to raise awareness concerning the pivotal role of the circadian system in the development and exacerbation of obesity. This internal timekeeping mechanism rhythmically regulates metabolic and physiological processes in order to meet the fluctuating demands in energy use and supply throughout the 24-h day. This review elaborates on the extensive bidirectional interaction between the circadian system and metabolism and explains how disruption of body clocks by means of shift work, frequent time zone travelling or non-stop consumption of calorie-dense foods can evoke detrimental metabolic alterations that contribute to obesity. Altering the body's circadian rhythms by means of time-related dietary approaches (chrononutrition) or pharmacological substances (chronobiotics) may therefore represent a novel and interesting way to prevent or treat obesity and associated comorbidities.

Motilin-induced gastric contractions signal hunger in man

RATIONALE

Hunger is controlled by the brain, which receives input from signals of the GI tract (GIT). During fasting, GIT displays a cyclical motor pattern, the migrating motor complex (MMC), regulated by motilin.

OBJECTIVES

To study the relationship between hunger and MMC phases (I-III), focusing on spontaneous and pharmacologically induced phase III and the correlation with plasma motilin and ghrelin levels. The role of phase III was also studied in the return of hunger after a meal in healthy individuals and in patients with loss of appetite.

FINDINGS

In fasting healthy volunteers, mean hunger ratings during a gastric (62.5±7.5) but not a duodenal (40.4±5.4) phase III were higher (p<0.0005) than during phase I (27.4±4.7) and phase II (37±4.5). The motilin agonist erythromycin, but not the cholinesterase inhibitor neostigmine, induced a premature gastric phase III, which coincided with an increase in hunger scores from 29.2±7 to 61.7±8. The somatostatin analogue octreotide induced a premature intestinal phase III without a rise in hunger scores. Hunger ratings significantly correlated (β=0.05; p=0.01) with motilin plasma levels, and this relationship was lost after erythromycin administration. Motilin, but not ghrelin administration, induced a premature gastric phase III and a rise in hunger scores. In contrast to octreotide, postprandial administration of erythromycin induced a premature gastric phase III accompanied by an early rise in hunger ratings. In patients with unexplained loss of appetite, gastric phase III was absent and hunger ratings were lower.

CONCLUSIONS

Motilin-induced gastric phase III is a hunger signal from GIT in man.

Role of the clock gene Bmal1 and the gastric ghrelin-secreting cell in the circadian regulation of the ghrelin-GOAT system

As adequate food intake is crucial to survival, organisms have evolved endogenous circadian clocks to generate optimal temporal patterns of food-related behavior and physiology. The gastric ghrelin-secreting cell is thought to be part of this network of peripheral food-entrainable oscillators (FEOs), regulating the circadian release of this orexigenic peptide. This study aimed to determine the role of the core clock gene Bmal1 and the gastric ghrelin-secreting cell as an FEO in the circadian rhythmicity of ghrelin expression and secretion in vivo and in vitro. Bmal1-deficient mice not only lacked circadian rhythmicity in plasma ghrelin levels and food intake, but also showed decreased gastric mRNA expression of ghrelin and ghrelin O-acyltransferase (GOAT), the ghrelin activating enzyme. Furthermore, in the absence of the hypothalamic master clock, food-related stimuli entrained the molecular clock of gastric ghrelinoma cells to regulate the rhythmic release of ghrelin. Divergent responses in octanoyl and total ghrelin release towards different food cues were observed, suggesting that the FEO also regulates the circadian rhythmicity of GOAT. Collectively, these findings indicate that circadian rhythmicity of ghrelin signaling requires Bmal1 and is driven by a food-responsive clock in the gastric ghrelin-secreting cell that not only regulates ghrelin, but also GOAT activity.

Ghrelin receptor modulates T helper cells during intestinal inflammation

BACKGROUND

The orexigenic peptide ghrelin has anti-inflammatory properties in colitis, however, the mechanism of action and the immune cells targeted remain still to be elucidated. Here, we assessed the possible effect of ghrelin on T helper (Th) cells in a T cell transfer model of chronic colitis.

METHODS

Disease was induced in the recombination activating gene 1 knockout mice (Rag1(-/-) ) by adoptive transfer of naïve Th cells from ghrelin receptor knockout mice (GRLN-R(-/-) ) or littermate wild-type (WT) mice. The course and severity of colitis was assessed by monitoring body weight, diarrhea score, histological analysis, gene expression, and flow cytometry analysis. The possible effects of ghrelin on Th cell proliferation, polarization, and apoptosis was examined in vitro.

KEY RESULTS

Our data showed that Rag1(-/-) mice injected with GRLN-R(-/-) Th cells displayed increased severity of colitis compared to mice injected with WT Th cells. In addition, Rag1(-/-) mice injected with GRLN-R(-/-) Th cells had significantly higher intestinal inflammation and increased accumulation of Th1 and Th17 cells in the colon. In vitro, ghrelin directly affected proliferation of Th cells and induced apoptosis whereas it did not influence Th cell polarization.

CONCLUSION & INFERENCES

Our observations suggest that ghrelin modulates Th effector cells in the gut controlling proliferation and inducing apoptosis. Our findings further support the use of ghrelin as a novel therapeutic option to treat intestinal inflammatory diseases.

Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

Endogenous motilin, but not ghrelin plasma levels fluctuate in accordance with gastric phase III activity of the migrating motor complex in man

BACKGROUND

Fluctuations in motilin plasma levels have been implicated in the control of the migrating motor complex (MMC). A plasma peak of motilin is present before a gastric phase III. Furthermore, not only exogenous administration of motilin but also ghrelin induces a gastric phase III in man. Aim of this study was to investigate the role of endogenous ghrelin in the regulation of the MMC.

METHODS

Plasma samples for motilin and ghrelin were taken in between two consecutive phases III of either origin measured using high-resolution manometry.

KEY RESULTS

The duration of 1 complete MMC cycle was on average 95 ± 12 min. Sixty percent of the first phases III and 40% of the second phases III had a gastric origin (p = 0.0574). Motilin (p < 0.05) plasma levels differed significantly between the phases of the MMC but total and octanoylated ghrelin did not. The percentage change in motilin during the MMC was dependent on the origin of phase III (p < 0.05). Motilin levels increased on average with 35 ± 10% right before a gastric phase III and with 3 ± 4% before a duodenal phase III (p < 0.05). The percentage change in total and octanoylated ghrelin plasma levels was not affected by the origin of phase III.

CONCLUSIONS & INFERENCES

These results confirm the role of motilin but not of ghrelin as an endogenous physiological regulator of the MMC with a gastric phase III.

Ghrelin signaling in the gut, its physiological properties, and therapeutic potential

BACKGROUND

Ghrelin, an orexigenic hormone secreted from the stomach, was soon after its discovery hypothesized to be a prokinetic agent, due to its homology to motilin. Studies in animals and humans, using ghrelin and ghrelin receptor agonists, confirmed this hypothesis, suggesting a therapeutic potential for the ghrelin receptor in the treatment of gastrointestinal motility disorders. Precilinical studies demonstrated that ghrelin can act directly on ghrelin receptors on the enteric nervous system, but the predominant route of action under physiological circumstances is signaling via the vagus nerve in the upper gastrointestinal tract and the pelvic nerves in the colon. Different pharmaceutical companies have designed stable ghrelin mimetics that revealed promising results in trials for the treatment of diabetic gastroparesis and post-operative ileus. Nevertheless, no drug was able to reach the market so far, facing problems proving superiority over placebo treatment in larger trials.

PURPOSE

This review aims to summarize the road that led to the current knowledge concerning the prokinetic properties of ghrelin with a focus on the therapeutic potential of ghrelin receptor agonists in the treatment of hypomotility disorders. In addition, we outline some of the problems that could be at the basis of the negative outcome of the trials with ghrelin agonists and question whether the right target groups were selected. It is clear that a new approach is needed to develop marketable drugs with this class of gastroprokinetic agents.

Ghrelin is involved in the paracrine communication between neurons and glial cells

BACKGROUND

Ghrelin is the only known peripherally active orexigenic hormone produced by the stomach that activates vagal afferents to stimulate food intake and to accelerate gastric emptying. Vagal sensory neurons within the nodose ganglia are surrounded by glial cells, which are able to receive and transmit chemical signals. We aimed to investigate whether ghrelin activates or influences the interaction between both types of cells. The effect of ghrelin was compared with that of leptin and cholecystokinin (CCK).

METHODS

Cultures of rat nodose ganglia were characterized by immunohistochemistry and the functional effects of peptides, neurotransmitters, and pharmacological blockers were measured by Ca(2+) imaging using Fluo-4-AM as an indicator.

KEY RESULTS

Neurons responded to KCl and were immunoreactive for PGP-9.5 whereas glial cells responded to lysophosphatidic acid and had the typical SOX-10-positive nuclear staining. Neurons were only responsive to CCK (31 ± 5%) whereas glial cells responded equally to the applied stimuli: ghrelin (27 ± 2%), leptin (21 ± 2%), and CCK (30 ± 2%). In contrast, neurons stained more intensively for the ghrelin receptor than glial cells. ATP induced [Ca(2+) ]i rises in 90% of the neurons whereas ACh and the NO donor, SIN-1, mainly induced [Ca(2+) ]i changes in glial cells (41 and 51%, respectively). The percentage of ghrelin-responsive glial cells was not affected by pretreatment with suramin, atropine, hexamethonium or 1400 W, but was reduced by l-NAME and by tetrodotoxin. Neurons were shown to be immunoreactive for neuronal NO-synthase (nNOS).

CONCLUSIONS & INFERENCES

Our data show that ghrelin induces Ca(2+) signaling in glial cells of the nodose ganglion via the release of NO originating from the neurons.

Systemic inflammation with enhanced brain activation contributes to more severe delay in postoperative ileus

BACKGROUND

The severity of postoperative ileus (POI) has been reported to result from decreased contractility of the muscularis inversely related to the number of infiltrating leukocytes. However, we previously observed that the severity of POI is independent of the number of infiltrating leukocytes, indicating that different mechanisms must be involved. Here, we hypothesize that the degree of tissue damage in response to intestinal handling determines the upregulation of local cytokine production and correlates with the severity of POI.

METHODS

Intestinal transit, the inflammatory response, I-FABP (marker for tissue damage) levels and brain activation were determined after different intensities of intestinal handling.

KEY RESULTS

Intense handling induced a more pronounced ileus compared with gentle intestinal manipulation (IM). No difference in leukocytic infiltrates in the handled and non-handled parts of the gut was observed between the two intensities of intestinal handling. However, intense handling resulted in significantly more tissue damage and was accompanied by a systemic inflammation with increased plasma levels of pro-inflammatory cytokines. In addition, intense but not gentle handling triggered enhanced c-Fos expression in the nucleus of the solitary tract (NTS) and area postrema (AP). In patients, plasma levels of I-FABP and inflammatory cytokines were significantly higher after open compared with laparoscopic surgery, and were associated with more severe POI.

CONCLUSIONS & INFERENCES

Not the influx of leukocytes, rather the manipulation-induced damage and subsequent inflammatory response determine the severity of POI. The release of tissue damage mediators and pro-inflammatory cytokines into the systemic circulation most likely contribute to the impaired motility of non-manipulated intestine.

Neurotransmitters involved in fast excitatory neurotransmission directly activate enteric glial cells

BACKGROUND

The intimate association between glial cells and neurons within the enteric nervous system has confounded careful examination of the direct responsiveness of enteric glia to different neuroligands. Therefore, we aimed to investigate whether neurotransmitters known to elicit fast excitatory potentials in enteric nerves also activate enteric glia directly.

METHODS

We studied the effect of acetylcholine (ACh), serotonin (5-HT), and adenosine triphosphate (ATP) on intracellular Ca(2+) signaling using aequorin-expressing and Fluo-4 AM-loaded CRL-2690 rat and human enteric glial cell cultures devoid of neurons. The influence of these neurotransmitters on the proliferation of glia was measured and their effect on the expression of c-Fos as well as glial fibrillary acidic protein (GFAP), Sox10, and S100 was examined by immunohistochemistry and quantitative RT-PCR.

KEY RESULTS

Apart from ATP, also ACh and 5-HT induced a dose-dependent increase in intracellular Ca(2+) concentration in CRL-2690 cells. Similarly, these neurotransmitters also evoked Ca(2+) transients in human primary enteric glial cells obtained from mucosal biopsies. In contrast with ATP, stimulation with ACh and 5-HT induced early gene expression in CRL-2690 cells. The proliferation of enteric glia and their expression of GFAP, Sox10, and S100 were not affected following stimulation with these neurotransmitters.

CONCLUSIONS & INFERENCES

We provide evidence that enteric glial cells respond to fast excitatory neurotransmitters by changes in intracellular Ca(2+). On the basis of our experimental in vitro setting, we show that enteric glia are not only directly responsive to purinergic but also to serotonergic and cholinergic signaling mechanisms.

Relationship between aquaporin-5 expression and saliva flow in streptozotocin-induced diabetic mice?

OBJECTIVE

To investigate the expression and distribution of AQP5 in submandibular acinar cells from sham- and streptozotocin (STZ)-treated mice in relation to the salivary flow.

METHODS

Mice were sham or STZ injected. Distribution of AQP5 subcellular expression in submandibular glands was determined by immunohistochemistry. AQP5 labelling indices (LI), reflecting AQP5 subcellular distribution, were determined in acinar cells. Western blotting was performed to determine the expression of AQP5 in submandibular glands. Blood glycaemia and osmolality and saliva flow rates were also determined.

RESULTS

AQP5 immunoreactivity was primarily located at the apical and apical-basolateral membranes of submandibular gland acinar cells from sham- and STZ-treated mice. No significant differences in AQP5 protein levels were observed between sham- and STZ-treated mice. Compared to sham-treated mice, STZ-treated mice had significant increased glycaemia, while no significant differences in blood osmolality were observed. Saliva flow rate was significantly decreased in STZ-treated mice as compared to sham-treated mice.

CONCLUSIONS

In STZ-treated mice, significant reduction in salivary flow rate was observed without any concomitant modification in AQP5 expression and localization.

GPR39, a receptor of the ghrelin receptor family, plays a role in the regulation of glucose homeostasis in a mouse model of early onset diet-induced obesity

GPR39, which may function as a Zn(2+) sensor, is a member of the G protein-coupled receptor family that also includes the receptor for the hunger hormone ghrelin. The down-regulation of GPR39 mRNA in adipose tissue of obese type 2 diabetic patients suggests that GPR39 may contribute to the pathogenesis of the disease. The present study aimed to investigate the role of GPR39 in the regulation of energy balance and glucose homeostasis in wild-type (GPR39(+/+) ) and GPR39 knockout mice (GPR39(-/-) ) with obesity-related type 2 diabetes. GPR39 mRNA levels in adipose tissue of fasted GPR39(+/+) mice fed a high-fat diet (HFD) for 30 weeks were reduced and correlated positively with blood glucose levels. Body weight, fat percentage and energy intake were increased in the HFD group but did not differ between both genotypes. Within the HFD group, blood glucose levels were lower in GPR39(-/-) than in GPR39(+/+) mice, despite significant reductions in prandial plasma insulin levels. The latter may not be a result of changes in β-cell hyperplasia because immunohistochemical staining of pancreata of mice on a HFD showed no differences between genotypes. The lower blood glucose levels may involve alterations in insulin sensitivity as revealed by glucose tolerance tests and respiratory quotient measurements that showed a preference of obese GPR39(-/-) mice for the use of carbohydrates as metabolic fuel. The increase in plasma ghrelin levels in GPR39(-/-) mice fed a HFD may contribute to the alterations in glucose homeostasis, whereas changes in gastric emptying or intestinal Zn(2+) absorption are not involved. The results obtained in the present study suggest that GPR39 plays a role in the pathogenesis of obesity-related type 2 diabetes by affecting the regulation of glucose homeostasis.

Review article: the role of gastric motility in the control of food intake

BACKGROUND

From a classical point of view, gastric motility acts to clear the stomach between meals, whereas postprandial motility acts to provide a reservoir for food, mixing and grinding the food and to assure a controlled flow of food to the intestines.

AIM

To summarise findings that support the role of gastric motility as a central mediator of hunger, satiation and satiety.

METHODS

A literature review using the search terms 'satiety', 'satiation' and 'food intake' was combined with specific terms corresponding to the sequence of events during and after food intake.

RESULTS

During food intake, when gastric emptying of especially solids is limited, gastric distension and gastric accommodation play an important function in the regulation of satiation. After food intake, when the stomach gradually empties, the role of gastric distension in the determination of appetite decreases and the focus will shift to gastric emptying and intestinal exposure of the nutrients. Finally, we have discussed the role of the empty stomach and the migrating motor complex in the regulation of hunger signals.

CONCLUSIONS

Our findings indicate that gastric motility is a key mediator of hunger, satiation and satiety. More specifically, gastric accommodation and gastric emptying play important roles in the regulation of gastric (dis)tension and intestinal exposure of nutrients and hence control satiation and satiety. Correlations between gastric accommodation, gastric emptying and body weight indicate that gastric motility can also play a role in the long-term regulation of body weight.

Endocannabinoid control of gastric sensorimotor function in man

BACKGROUND

Little is known about the physiological role of the endocannabinoid system in the regulation of the motility and the sensitivity of the stomach. Endocannabinoid system dysfunction has been hypothesized to contribute to the control of food intake and the pathogenesis of functional dyspepsia.

AIM

To study the influence of rimonabant, the endocannabinoid 1 (CB1) receptor antagonist, on gastric sensorimotor function in healthy controls.

METHODS

After 4 days of pre-treatment with rimonabant 20 mg/day or placebo, 12 healthy volunteers (mean age 34 +/- 12 years, six men) participated in a placebo-controlled, double-blind, randomized, crossover study with a gastric barostat assessment of gastric sensitivity to distension, gastric compliance, gastric accommodation and phasic motility on day 3 and a liquid nutrient challenge test on day 4.

RESULTS

Rimonabant did not influence gastric compliance and sensitivity to distension. The meal-induced gastric accommodation reflex was significantly inhibited by rimonabant (154.3 +/- 30.9 vs. 64.3 +/- 32.4 mL, P = 0.02). Rimonabant did not affect maximal nutrient tolerance or meal-related symptoms during the satiety drinking test.

CONCLUSION

Endocannabinoids acting on the CB1 receptor are involved in the control of gastric accommodation in man.

Influence of ghrelin on the gastric accommodation reflex and on meal-induced satiety in man

Ghrelin increases gastric tone in the fasting state and enhances gastric emptying in gastroparesis. The aims of the study were to evaluate the effect of ghrelin on postprandial gastric tone and on meal-induced satiety in health. Ten healthy volunteers underwent a barostat study on two occasions. After determination of intra-abdominal pressure (minimal distending pressure, MDP), isobaric volume measurement was performed for 90 min at MDP + 2 mmHg. After 20 min, ghrelin (40 microg) or saline was administered i.v. over 30 min in a double-blind-randomized cross-over design, followed 10 min later by a liquid meal (200 mL, 300 kcal). Stepwise isobaric distentions (+2 mmHg per 2 min) were performed 60 min after the meal. Data (mean +/- SEM) were compared using paired Student's t-test and ANOVA. Separately, a satiety drinking test (15 mL min(-1) until satiety score 5) was performed on 10 subjects twice, after treatment with placebo or ghrelin. Ghrelin infusion significantly inhibited gastric accommodation (mean volume increase adjusted means 108.0 +/- 50 vs 23.0 +/- 49 mL, P = 0.03, ANCOVA with the premeal postinfusion volume as covariate) and reduced postprandial gastric volumes (197.2 +/- 24.6 vs 353.5 +/- 50.0 mL, P = 0.01). Pressures inducing perception or discomfort during postprandial gastric distentions were not altered. During satiety testing, ghrelin did not alter nutrient volume ingested till maximal satiety (637.5 +/- 70.9 vs 637.5 +/- 56.2 mL, ns). Ghrelin administered during the meal significantly inhibits gastric accommodation in health, but this is not associated with early satiation.

Endogenous and exogenous ghrelin enhance the colonic and gastric manifestations of dextran sodium sulphate-induced colitis in mice

Ghrelin is an important orexigenic peptide that not only exerts gastroprokinetic but also immunoregulatory effects. This study aimed to assess the role of endogenous and exogenous ghrelin in the pathogenesis of colitis and in the disturbances of gastric emptying and colonic contractility during this process. Dextran sodium sulphate colitis was induced for 5 days in (i) ghrelin(+/+) and ghrelin(-/-) mice and clinical and histological parameters were monitored at days 5, 10 and 26 and (ii) in Naval Medical Research Institute non-inbred Swiss (NMRI) mice treated with ghrelin (100 nmol kg(-1)) twice daily for 5 or 10 days. Neural contractility changes were measured in colonic smooth muscle strips, whereas gastric emptying was measured with the (14)C octanoic acid breath test. Inflammation increased ghrelin plasma levels. Body weight loss, histological damage, myeloperoxidase activity and IL-1beta levels were attenuated in ghrelin(-/-) mice. Whereas absence of ghrelin did not affect changes in colonic contractility, gastric emptying in the acute phase was accelerated in ghrelin(+/+) but not in ghrelin(-/-) mice. In agreement with the studies in ghrelin knockout mice, 10 days treatment of NMRI mice with exogenous ghrelin enhanced the clinical disease activity and promoted infiltration of neutrophils and colonic IL-1beta levels. Unexpectedly, ghrelin treatment decreased excitatory and inhibitory neural responses in the colon of healthy but not of inflamed NMRI mice. Endogenous ghrelin enhances the course of the inflammatory process and is involved in the disturbances of gastric emptying associated with colitis. Treatment with exogenous ghrelin aggravates colitis, thereby limiting the potential therapeutic properties of ghrelin during intestinal inflammation.

Short exposure of oesophageal mucosa to bile acids, both in acidic and weakly acidic conditions, can impair mucosal integrity and provoke dilated intercellular spaces

BACKGROUND

Severe duodeno-gastro-oesophageal reflux (DGOR) is a risk factor for oesophagitis and Barrett's oesophagus. Patients with non-erosive reflux disease (NERD) have a slight increase in DGOR. Patients with gastro-oesophageal reflux disease (GORD), who are taking proton pump inhibitors (PPIs), still have reflux but of weakly acidic pH and persistence of bile. In these two groups of patients, heartburn might be due to increased oesophageal mucosal permeability and dilated intercellular spaces (DIS). We aimed to assess whether experimental short exposure of the oesophageal mucosa to bile acids, in low concentrations (at acidic, weakly acidic and neutral conditions) can increase mucosal permeability and provoke DIS.

METHODS

Rabbit oesophageal mucosa was studied in diffusion and Ussing chambers. We assessed the effects of different solutions containing bile acids, applied to the mucosal side, on transepithelial electrical resistance (R(T)) and permeability to fluorescein. The diameter of intercellular spaces was assessed by using transmission electron microscopy.

RESULTS

Incubation of oesophageal mucosa with acidic solutions (pH 2.0) containing a range of bile acids (0.5-5 mmol/l) markedly decreased R(T) and increased mucosal permeability. Weakly acidic solutions (pH 5.0), and to some extent neutral solutions (pH 7.4), containing some bile acids also decreased R(T) and increased permeability, although the effects were much less marked and in some combinations no effect was seen. Exposure to bile acids provoked DIS in acid and weakly acidic conditions but not in neutral (pH 7.4) solutions.

CONCLUSIONS

Experimental short exposure of the oesophageal mucosa to solutions with a bile acid concentration and acidity similar to that observed in the gastric contents of patients with NERD or ERD, and who are taking PPIs, may impair oesophageal mucosal integrity and even induce dilated intercellular spaces. Such a situation could, theoretically, underlie the occurrence and/or persistence of symptoms in these patients.

Characterization of myenteric neuropathy in the jejunum of spontaneously diabetic BB-rats

Decreased gastric expression and function of neuronal nitric oxide synthase (nNOS) has been proposed as a potential mechanism underlying diabetic gastroparesis. As gastric nNOS expression is vagally controlled, these changes might occur secondarily to vagal neuropathy. In addition, it is unclear whether other inhibitory neurotransmitters are also involved. We used the type 1 diabetic BioBreeding (BB)-rat model to study jejunal motor control and nNOS expression, which is independent of the vagus. Jejunal segments were used for in vitro contractility studies, and measurement of nNOS expression after 8 or 16 weeks of diabetes compared with age- and sex-matched controls. Unlike electrical field stimulation and acetylcholine (ACh)-induced contractions, non-adrenergic non-cholinergic (NANC) relaxations were significantly reduced in diabetic rats. In contrast to control rats, NANC relaxations in diabetic rats were N(omega)-nitro-L-arginine methyl ester (L-NAME) insensitive. Jejunal nNOS expression was significantly decreased in diabetic rats. Both in diabetic and in control animals, L-NAME resistant relaxations were sensitive to P(2)-receptor antagonists. In the jejunum of spontaneously diabetic rats, decreased nitric oxide responsiveness and decreased nNOS protein expression occur while purinergic transmission is unaffected. These findings indicate that nitrergic enteric neuropathy may be a primary dysfunction in diabetes, independent from vagal dysfunction.

Effect of peripheral obestatin on gastric emptying and intestinal contractility in rodents

Obestatin has recently been discovered in the rat stomach. It is encoded by the ghrelin gene and has been claimed to be a functional opponent of ghrelin and to be the natural ligand of the GPR39 receptor. The latter could not be confirmed by Holst et al. (Endocrinology, 2006). Yet, in GPR39 knockout mice, gastric emptying is accelerated. We verified the effects of obestatin on gastric emptying and intestinal contractility in rodents. Gastric emptying was measured with the (14)C octanoic breath test in mice. In vitro, the effect of obestatin was studied on electrically stimulated and non-stimulated strips from the fundus and small intestine of mice and rats. Obestatin (60, 125, 250 nmol kg(-1)) did not affect gastric emptying parameters (T(half) and T(lag)) and did not inhibit the prokinetic effects of ghrelin. Mouse and rat intestinal and fundic smooth muscle strips did not respond to obestatin either in the absence or in the presence of electrical field stimulation. Obestatin (125 nmol kg(-1)) did not inhibit fasting-induced hyperphagia. Our results suggest that peripheral obestatin is not a satiety signal that plays a role in the regulation of gastric emptying and do not support the concept that obestatin is a physiological opponent of ghrelin.

Role of endogenous ghrelin in the hyperphagia of mice with streptozotocin-induced diabetes

Ghrelin is an orexigenic peptide involved in the regulation of energy homeostasis. To investigate the role of ghrelin in the hyperphagia associated with uncontrolled streptozotocin-induced diabetes, food intake was followed in diabetic ghrelin knockout (ghrelin(-/-)) and control wild-type (ghrelin(+/+)) mice and diabetic Naval Medical Research Institute noninbred Swiss mice treated with either saline or the ghrelin receptor antagonist, D-Lys3-GH-releasing peptide-6 (D-Lys3-GHRP-6) for 5 d. In diabetic ghrelin(-/-) mice, hyperphagia was attenuated, and the maximal increase in food intake was 50% lower in mutant than in wild-type mice. The increased food intake observed during the light period (1000-1200 h) in ghrelin(+/+) mice was abolished in mutant mice. Diabetic ghrelin(-/-) mice lost 12.4% more body weight than ghrelin(+/+) mice. In diabetic ghrelin(+/+) mice, but not in ghrelin(-/-) mice, the number of neuropeptide Y (NPY)-immunoreactive neurons was significantly increased. Diabetic Naval Medical Research Institute noninbred Swiss mice were hyperphagic and had increased plasma ghrelin levels. Treatment with D-Lys3-GHRP-6 reduced daily food intake by 23% and reversed the increased food intake observed during the light period. The change in the number of NPY- (2.4-fold increase) and alpha-MSH (1.7-fold decrease)-immunoreactive hypothalamic neurons induced by diabetes was normalized by D-Lys3-GHRP-6 treatment. Our results suggest that enhanced NPY and reduced alpha-MSH expression are secondary to the release of ghrelin, which should be considered the underlying trigger of hyperphagia associated with uncontrolled diabetes.

Influence of ghrelin on interdigestive gastrointestinal motility in humans

BACKGROUND

Recent studies in animals have shown that ghrelin stimulates upper gastrointestinal motility through the vagus and enteric nervous system. The aim of the present study therefore was to simultaneously investigate the effect of administration of ghrelin on upper gastrointestinal motility and to elucidate its mode of action by measuring plasma levels of gastrointestinal hormones in humans.

MATERIALS AND METHODS

Nine healthy volunteers (four males; aged 22-35 years) underwent combined antroduodenal manometry and proximal stomach barostat study on two separate occasions at least one week apart. Twenty minutes after the occurrence of phase III of the migrating motor complex (MMC), saline or ghrelin 40 mug was administered intravenously over 30 minutes in a double blind, randomised, crossover fashion. Ghrelin, motilin, pancreatic polypeptide, glucagon, and somatostatin were measured by radioimmunoassay in blood samples obtained at 15-30 minute intervals. The influence of ghrelin or saline on MMC phases, hormone levels, and intraballoon volume was compared using paired t test, ANOVA, and chi(2) testing.

RESULTS

Spontaneous phase III occurred in all subjects, with a gastric origin in four. Administration of ghrelin induced a premature phase III (12 (3) minutes, p<0.001; gastric origin in nine, p<0.05), compared with saline (95 (13) minutes, gastric origin in two). Intraballoon volumes before infusion were similar (135 (13) v 119 (13) ml; NS) but ghrelin induced a longlasting decrease in intraballoon volume (184 (31) v 126 (21) ml in the first 60 minutes; p<0.05). Administration of ghrelin increased plasma levels of pancreatic polypeptide and ghrelin but motilin, somatostatin, and glucagon levels were not altered.

CONCLUSIONS

In humans, administration of ghrelin induces a premature gastric phase III of the MMC, which is not mediated through release of motilin. This is accompanied by prolonged increased tone of the proximal stomach.

Influence of ghrelin on gastric emptying and meal-related symptoms in idiopathic gastroparesis

BACKGROUND

Ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, is released from the stomach. Animal studies suggest that ghrelin stimulates gastrointestinal motor activity.

AIM

To investigate the influence of ghrelin on gastric emptying rate and meal-related symptoms in idiopathic gastroparesis.

METHODS

In six patients with idiopathic gastroparesis, a breath test was used to measure gastric emptying rates (t(1/2)) for solids and liquids after administration of saline or ghrelin 40 microg/30 min in a double-blind, randomized fashion. At each breath sampling, the patient was asked to grade the intensity of six different symptoms (epigastric pain, bloating, postprandial fullness, nausea, belching and epigastric burning) and these were added to obtain meal-related symptom severity score.

RESULTS

Ghrelin significantly enhanced liquid emptying (t(1/2): 86 +/- 7 vs. 53 +/- 6 min, P = 0.02) and tended to enhance solid emptying (144 +/- 45 vs. 98 +/- 15 min, P = 0.06). Ghrelin pre-treatment significantly decreased cumulative meal-related symptom score (196 +/- 30 vs. 136 +/- 23, P = 0.04) and individual scores for fullness (55 +/- 8 vs. 39 +/- 8, P = 0.02), and for pain (40 +/- 8 vs. 16 +/- 5, P < 0.05).

CONCLUSIONS

In idiopathic gastroparesis, administration of ghrelin enhances gastric emptying and improves meal-related symptoms. These observations suggest a potential for ghrelin receptor agonists in the treatment of gastroparesis.

Energy homeostasis and gastric emptying in ghrelin knockout mice

To elucidate the role of endogenous ghrelin in the regulation of energy homeostasis and gastric emptying, ghrelin knockout mice (ghrelin(-/-)) were generated. Body weight, food intake, respiratory quotient, and heat production (indirect calorimetry), and gastric emptying ((14)C breath test) were compared between ghrelin(+/+) and ghrelin(-/-) mice. In both strains, the effect of exogenous ghrelin on gastric emptying and food intake was determined. Ghrelin(-/-) mice showed some subtle phenotypic changes. Body weight gain and 24-h food intake were not affected, but interruption of the normal light/dark cycle triggered additional food intake in old ghrelin(+/+) but not in ghrelin(-/-) mice. Exogenous ghrelin increased food intake in both genotypes with a bell-shaped dose-response curve that was shifted to the left in ghrelin(-/-) mice. During the dark period, young ghrelin(-/-) mice had a lower respiratory quotient, whereas their heat production was higher than that of the wild-type littermates, inferring a leaner body composition of the ghrelin(-/-) mice. Absence of ghrelin did not affect gastric emptying, and the bell-shaped dose-response curves of the acceleration of gastric emptying by exogenous ghrelin were not shifted between both strains. In conclusion, ghrelin is not an essential regulator of food intake and gastric emptying, but its loss may be compensated by other redundant inputs. In old mice, meal initiation triggered by the light/dark cue may be related to ghrelin. In young animals, ghrelin seems to be involved in the selection of energy stores and in the partitioning of metabolizable energy between storage and dissipation as heat.

Gastric motor effects of peptide and non-peptide ghrelin agonists in mice in vivo and in vitro

BACKGROUND AND AIMS

The gastroprokinetic activities of ghrelin, the natural ligand of the growth hormone secretagogue receptor (GHS-R), prompted us to compare the effect of ghrelin with that of synthetic peptide (growth hormone releasing peptide 6 (GHRP-6)) and non-peptide (capromorelin) GHS-R agonists both in vivo and in vitro.

METHODS

In vivo, the dose dependent effects (1-150 nmol/kg) of ghrelin, GHRP-6, and capromorelin on gastric emptying were measured by the 14C octanoic breath test which was adapted for use in mice. The effect of atropine, N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME), or D-Lys3-GHRP-6 (GHS-R antagonist) on the gastroprokinetic effect of capromorelin was also investigated. In vitro, the effect of the GHS-R agonists (1 microM) on electrical field stimulation (EFS) induced responses was studied in fundic strips in the absence and presence of L-NAME.

RESULTS

Ghrelin, GHRP-6, and capromorelin accelerated gastric emptying in an equipotent manner, with bell-shaped dose-response relationships. In the presence of atropine or l-NAME, which delayed gastric emptying, capromorelin failed to accelerate gastric emptying. D-Lys3-GHRP-6 also delayed gastric emptying but did not effectively block the action of the GHS-R agonists, but this may be related to interactions with other receptors. EFS of fundic strips caused frequency dependent relaxations that were not modified by the GHS-R agonists. L-NAME turned EFS induced relaxations into cholinergic contractions that were enhanced by ghrelin, GHRP-6, and capromorelin.

CONCLUSION

The 14C octanoic breath test is a valuable technique to evaluate drug induced effects on gastric emptying in mice. Peptide and non-peptide GHS-R agonists accelerate gastric emptying of solids in an equipotent manner through activation of GHS receptors, possibly located on local cholinergic enteric nerves.

Effect of repeated cycles of acute esophagitis and healing on esophageal peristalsis, tone, and length

Severe esophagitis is associated with motor abnormalities in the esophageal body and lower esophageal sphincter. Reflux disease involves repeated episodes of mucosal inflammation and spontaneous or treatment-induced healing. The aims of this study were 1) to further assess changes induced by acute esophagitis on esophageal peristalsis, tone, and shortening and 2) to assess the effect of repeated sequences of acute esophagitis-healing on these motor parameters. Experiments were performed on adult cats. Esophageal manometry and barostat were performed before, 24 h after, and every 7 days after intraesophageal acid perfusion (0.1 N HCl, 80 min). Esophageal length was measured during manometry, and compliance of the esophageal body was assessed with barostat. The identical protocol was performed 8 and 16 wk after the first acid perfusion. The degree of esophageal mucosal damage was evaluated by endoscopy, histopathology, and myeloperoxidase activity. Acid perfusion induced severe esophagitis. At 24 h, distal peristaltic contractions disappeared, lower esophageal sphincter pressure was reduced by 60%, the esophagus length was 1-2 cm shorter, and esophageal compliance was reduced by 30%. Most parameters recovered in 4 wk. Subsequent repeated acute injuries induced similar endoscopic esophagitis but a different pattern of inflammatory infiltration and fibrosis in the mucosa and muscle layers, resulting in milder motor disturbances. Acute experimental esophagitis provokes severe but reversible hypomotility. Spaced repeated acute injuries provoke milder motor effects, suggesting an adaptive response.

Effect of ghrelin and growth hormone-releasing peptide 6 on septic ileus in mice

Ghrelin is an orexigenic peptide with prokinetic effects in the rat. We investigated the effect of ghrelin and growth hormone-releasing hormone 6 (GHRP-6) on gastric emptying and transit in control and septic mice. Mice were injected i.p. with lipopolysaccharides (LPS) or saline (control). After 16-17 h mice were pretreated with saline, ghrelin or GHRP-6 1 h before intragastric administration of Evans blue. Fifteen minutes later, after assessment of the behaviour scale, mice were killed and gastric emptying, transit and rectal temperature were measured. In control mice, ghrelin (100 microg kg(-1)) and GHRP-6 (20-100 microg kg(-1)) accelerated gastric emptying, whereas ghrelin and GHRP-6 failed to increase transit significantly. Septic mice developed a delay in gastric emptying and transit, hypothermia and a deterioration of the behaviour scale. In septic mice, ghrelin (20 microg kg(-1)) accelerated gastric emptying without effect on transit while GHRP-6 significantly accelerated gastric emptying dose-dependently and failed to increase transit significantly. Ghrelin and GHRP-6 had no effect on the endotoxin-induced hypothermia or deterioration of behaviour scale. Therefore, the beneficial prokinetic effect of ghrelin but mainly of GHRP-6 offers potential therapeutic options in the treatment of septic gastric ileus.

Effects of Schistosoma mansoni infection on somatostatin and somatostatin receptor 2A expression in mouse ileum

Intestinal schistosomiasis is accompanied by motility-related dysfunctions but the underlying mechanisms are not well-known. Therefore, the presence and effects on intestinal contractility of somatostatin (SOM) and its receptor, SSTR2A, were investigated in the ileum of normal and infected mice. The distribution of SOM and SSTR2A was visualized using immunocytochemistry. Radioimmunoassay combined with oogram studies was performed to determine SOM levels and contractility measurements were determined in organ bath experiments. Schistosomiasis resulted in a significant decrease in somatostatin-positive endocrine cells, whereas the number of somatostatin-immunoreactive (IR) neuronal cell bodies did not change. From 8 weeks postinfection onwards, an increase was noted in somatostatin-IR nerve fibres in both villi and granulomas. The staining intensity for SSTR2A, expressed in somatostatin-negative myenteric cholinergic neurones, increased during infection suggesting an upregulation of this receptor. SOM levels were negatively correlated with the number of eggs during the acute phase, and were elevated during the chronic phase. Pharmacological experiments revealed that schistosomiasis diminished the inhibitory effect of SOM on neurogenic contractions. We can conclude that schistosomiasis influences the distribution and expression levels of SOM and SSTR2A in the murine ileum, which might explain the changed motility pattern.

Motilin and motilin receptors: characterization and functional significance

In order to get more insight into the mechanism of action of the gastrointestinal peptide, motilin, and its role in human physiology, we aimed at characterizing motilin and motilin receptors. Motilin. Sequence analysis of the motilin precursor from several species indicated that the N- and C-terminal regions of the motilin precursor have evolved at different rates. Sequence analysis of the motilin precursor in brain tissue of rabbit and man and motilin radioimmunoassay on tissue extracts, proved that motilin is a brain-gut peptide. Plasma motilin levels are increased in patients with ulcerative colitis or Crohn's disease. A weak correlation between the motilin genotype and the susceptibility to inflammatory bowel disease was demonstrated. Motilin receptors. Motilin receptors are expressed early postnatally and can be regulated by changes in its plasma level. The pharmacophore of motilin consists of the aromatic rings from Phe1 and Tyr7 and the aliphatic side chains from Val2 and Ile4. In vivo and in vitro studies showed that in the rabbit and human antrum, smooth muscle and neuronal motilin receptors exist which have different characteristics. In the rabbit duodenum motilin's action depends upon the influx of extra- and intracellular Ca2+. Nevertheless, in primary smooth muscle cultures, Ca2+ influx through L-type Ca2+ channels is the major transduction mechanism. The existence of central motilin receptors was demonstrated by autoradiography. Receptor binding studies allowed the identification of two binding sites. In contrast to antral smooth muscle cells, the response to motilin in the human TE671 medulloblastoma cell line, expressing the motilin receptor, relies on intracellular IP3-sensitive Ca2+ stores. The antibiotic erythromycin-A (EM-A) binds to the motilin receptor and induces contractions with the same regional and species specificity as motilin. This interaction was supported by the discovery of motilin antagonists. Structure activity studies led to the development of more powerful erythromycin derivatives, which lack antibiotic properties and which are now in clinical trial for treatment of hypomotility disorders. Conclusion and perspectives. The physiological role of motilin and its receptors in the brain requires further investigation. Erythromycin and its derivatives act as motilin agonists with clinically useful prokinetic potential. The motilin receptor has recently been cloned and has substantial structural homology with the growth hormone secretagogue receptor. This may not only lead to the further characterization of motilin receptor subtypes and aid the development of safe and selective motilin receptor agonists and antagonists, useful for the treatment of GI disorders, but may also give a new dimension to the role of motilin in human physiology.

Demonstration of a functional motilin receptor in TE671 cells from human cerebellum

BACKGROUND

Our laboratory has described the presence of motilin receptors in the rabbit cerebellum. We discovered its presence in the human TE671 cell line, which is of cerebellar origin.

METHODS

Cytosolic Ca(2+) fluxes were monitored on a confocal microscope in cells loaded with Indo-1 and stimulated with motilin under various conditions. Binding studies were performed with 125I-[Nle(13)]porcine motilin. Using primers, PCR for the motilin receptor was performed.

RESULTS

Cells responded to motilin after 45+/-20 s. At different concentrations of motilin (10(-8), 10(-7), 10(-6.5), 10(-6) and 10(-5) M) the percentage of responding cells was 0+/-0, 0.6+/-1.5, 4.9+/-4.7, 21.7+/-15 and 35.7+/-12, respectively. The response was blocked by the motilin antagonists [Phe(3), Nle(13)]po-motilin (0.8+/-1.8%) and GM-109 (0.0+/-0.0%) and mimicked by the agonist ABT-229 (23.6+/-15%). After stimulation with motilin, ABT-229 or [Phe(3),Leu(13)]po-motilin, but not with the antagonist GM-109, cells were desensitized. The response to motilin persisted in Ca(2+)-free solution (22.8+/-14.7%), was not affected by nifedipine (44+/-11%) but was abolished by incubation with thapsigargin (0+/-0%). Neither ryanodine, nor a previous stimulation with caffeine (0+/-0%) in Ca(2+)-free Krebs, nor both could block the response to motilin (28, 32.0+/-5.7, 41.3+/-6.1%, respectively). Binding studies revealed two binding sites for motilin, with a pK(d) of 8.9+/-0.05 and 6.11+/-0.61 (n=4). There were 100 times more low than high affinity receptors per cell. The presence of receptor mRNA was confirmed by PCR.

CONCLUSION

Functional motilin receptors are present in TE671 cells. The response requires intracellular IP(3)-sensitive Ca(2+) stores. These cells may serve as a model of the central motilin receptor.

Effect of recombinant human interleukin-11 on motilin and substance P release in normal and inflamed rabbits

Recombinant human interleukin-11 (rhIL-11) normalizes depressed smooth muscle tension generation towards motilin and substance P (SP) in rabbits with colitis. The aim of this paper was to evaluate the effect of rhIL-11 treatment on motilin and SP release which could have an effect on the contractility changes. Rabbits received 4, 40, 72 or 720 microg/kg rhIL-11 s.c. or saline, 1 h later a continuous s.c. administration of rhIL-11 was started with or without the induction of colitis (135 mg/kg TNBS) for 5 days. Motilin and SP levels were measured by RIA, motilin mRNA expression by RT-PCR. TNBS-colitis did not affect plasma motilin levels but increased the motilin content of the duodenal mucosa 1.7-fold. rhIL-11 treatment dose-dependently increased plasma motilin levels (720 microg/kg day: 3.5-fold) and the motilin content of the duodenal mucosa (720 microg/kg day: 3.0-fold). The effects of rhIL-11 were similar in normal rabbits and were accompanied by an increased motilin mRNA expression. TNBS-colitis decreased plasma SP levels 2.7-fold and the SP content in the colonic muscle layer 7.1-fold. The decrease in the muscle layer, but not in the plasma, was normalized by rhIL-11 treatment. In normal rabbits, rhIL-11 caused a decrease in plasma SP levels, but had no effect on the tissue content of SP. In conclusion, treatment of inflamed or normal rabbits with rhIL-11 increases plasma and tissue levels of motilin in the duodenal mucosa via an increased expression of motilin in the endocrine cells and induces the release of SP from extrinsic neurons. These changes do not explain the beneficial effect of rhIL-11 on the lowered contractility in inflamed rabbits although a change in balance of neuropeptides may influence gastro-intestinal inflammation.

Identification and expression of the motilin precursor in the guinea pig

Motilin has never been isolated from rodents, the most frequently used laboratory animals, despite several attempts. We have isolated and sequenced the motilin precursor from duodenal mucosa of guinea pig (GenBank accession number AF323752) and studied its expression in several tissues. The percent homology with human motilin is the lowest yet observed due to several unique substitutions in the C-terminal end. As expected, the precursor was present in the gut mucosa with the exception of the gastric corpus. It was also present in medulla oblongata, nucleus of the solitary tract, hypophysis, spinal cord, hypothalamus, and cerebellum but not in the cerebral cortex. For the first time we demonstrated motilin expression in the thyroid.

Motilin receptor density in inflamed and noninflamed tissue in rabbit TNBS-induced colitis

Trinitrobenzenesulphonic acid (TNBS)-induced colitis decreases the contractile response of the rabbit colon to motilin, and inflammation may increase plasma motilin levels. We studied whether the decreased contractility could be due to a down-regulation of motilin receptors, caused by increased plasma motilin levels. As this would affect all tissues, uninflamed sites were studied as well. Colitis was induced by different doses (100-150 mg kg-1) of TNBS. In the colon, the TNBS dose-dependent decrease of the contractile response towards motilin was reflected in a decrease in motilin receptor density. In contrast, in the antrum, receptors were upregulated by 150 mg kg-1 TNBS, while central motilin receptors in the cerebellum were not affected. Plasma motilin levels were not influenced by inflammation, although the motilin content and mRNA expression in the duodenal and jejunal mucosa, but not in the colon, was significantly increased. The opposite was true for interleukin-1beta and interleukin receptor antagonist mRNA expression. We conclude that the decreased motilin contractility in rabbit colitis is due to a downregulation of motilin receptors in the colon, but this is not caused by chronic hormonal stimulation.

Contractile effects and intracellular Ca2+ signalling induced by motilin and erythromycin in the circular smooth muscle of human colon

Motilin has excitatory effects on the colon of the rabbit and the dog, but little is known of its effect on the human colon. The aim of this study was to investigate the effects induced by motilin and erythromycin A (EMA) on muscle strips and on single cells from primary cultures from human colon. Isotonic contraction was recorded in circular muscle strips from macroscopically normal resection specimens of patients operated on for colonic neoplasm. Agonist-induced intracellular Ca2+ ([Ca2+]i) signalling was studied in primary cultures of colonic smooth-muscle cells using the ratiometric Ca2+ indicator Indo 1, on a laser-scanning confocal epifluorescence microscope. In circular muscle strips, norleucine13-porcine motilin ([Nle13]-pm)and EMA induced tonic contractions with an EC50 of 92 +/- 21 nmol L(-1) and 31 +/- 16 micromol L(-1), respectively. The maximal contraction was 21 +/- 4% (motilin) and 33 +/- 12% (EMA) of the response to 10(-4) mol L(-1) acetylcholine (ACh). The motilin antagonist OHM-11526 (10(-5.5) mol L(-1)) abolished the effects of both [Nle13]-pm and EMA. Neither tetrodotoxin (10(-5.5) mol L(-1)), L-nitro-D-arginine methyl ester (L-NAME) (10(-3.5) mol L(-1)) nor guanethidine (10(-5) mol L(-1)) interfered with the effects of [Nle13]-pm or EMA. [Nle13]-pm (10(-11)-10(-6) mol L(-1)) induced rises of [Ca2+]i in cultured colonic myocytes. At 10(-6) mol L-1, 94% of the cells responded, and half of the cells responded at 1.4 nmol L(-1) [Nle13]-pm. 81% (35/43) and 95% (75/79) responded to EMA (10(-6) mol L(-1)) and acetylcholine (ACh, 10(-4) mol L(-1)), respectively. The motilin antagonist GM-109 inhibited motilin- and EMA-induced [Ca2+]i rises. In the absence of extracellular Ca2+, only 13% (7/52) of the cells responded to [Nle13]-pm (10(-6) mol L(-1)) vs. 90% (47/52) to ACh (10(-4) mol L(-1)). Motilin and EMA have direct excitatory effects on circular smooth muscle from the human colon and these effects are mediated via a smooth-muscle motilin receptor. These findings suggest that motilin may regulate colonic motility and that motilides may have therapeutic potential for the treatment of colonic hypomotility.

Dose-dependent effects of recombinant human interleukin-11 on contractile properties in rabbit 2,4,6-trinitrobenzene sulfonic acid colitis

We studied the effect of recombinant human interleukin-11 (rhIL-11), a cytokine with protective effects against injury to the intestinal mucosa, on inflammatory changes in the muscle layers of the gut, in rabbits with colitis. A single dose of rhIL-11 (4, 40, or 720 microg/kg) was given 1 h before colitis was induced with 135 mg/kg 2, 4,6-trinitrobenzene sulfonic acid (TNBS), followed by a continuous s. c. administration of 4, 40, or 720 microg/kg. day rhIL-11 or saline for 5 days. Colitis affected mucosal architecture, general mechanical properties (passive tension increased with 12.3 g/mm(2), optimal stretch decreased with 26%), and collagen content (decreased from 366 +/- 25 to 237 +/- 13 microg/mg of protein). Changes in passive tension and collagen content were normalized by the highest and lowest dose of rhIL-11, respectively, but neither dose could normalize the optimal stretch. Colitis also decreased maximal contractile tension in response to acetylcholine (ACh), motilin, substance P (SP), K(+), and prostaglandin E(2) but this was normalized with 40 microg/kg. day (motilin, SP) and 720 microg/kg. day (ACh, K(+)) rhIL-11 but not for prostaglandin E(2). For motilin and SP, receptor density was decreased in colitis and normalized in treated rabbits. Colitis also increased the contractile potency toward ACh, an effect already reversed by rhIL-11, 4 microg/kg. day. In conclusion, rhIL-11 partially normalizes disturbed tension generation in experimental colitis. The use of this cytokine in the treatment of irritable bowel disease may contribute to the restoration of motor dysfunction.

Sequence and characterization of cDNA encoding the motilin precursor from chicken, dog, cow and horse. Evidence of mosaic evolution in prepromotilin

Motilin is involved in the regulation of the fasting motility pattern in man and in dog, but may have a different role in other species. Immunoreactive motilin has been demonstrated in several species, but the sequence is mostly unknown. The aim of this study was to isolate and sequence the cDNA encoding the motilin precursor from several mammalian species and from chicken. Total RNA was isolated from the duodenal mucosa of the chicken, dog, cow and horse. In each case single stranded cDNA was synthesized. Motilin cDNA fragments were amplified by PCR, ligated into a plasmid and cloned. Clones which were positive after screening with an appropriate (32)P-labeled probe were sequenced. The 5'- and 3'-ends were determined by the rapid amplification of cDNA ends (RACE) method. Analysis of the cDNAs revealed an open reading frame coding for 115 (chicken and cow), or 117 (dog and horse) amino acids. It consists of a 25 amino acid signal peptide, motilin itself, and a 68 (chicken and cow) or 70 (dog and horse) amino acid motilin associated peptide (MAP). As in all motilin precursors already sequenced (man, monkey, pig and rabbit), an endoproteinase cleavage site is present at Lys(23)-Lys(24). Comparison of all known sequences shows considerable identity in amino acid and nucleotide sequence of the signal peptide and motilin. However, the MAPs differ not only in length but also, more strongly, in amino acid and nucleotide sequence. Our study demonstrates that the N- and C-terminal regions of the motilin precursor have evolved at different rates, which is evidence for 'mosaic evolution'.

Differential changes in ACh-, motilin-, substance P-, and K(+)-induced contractility in rabbit colitis

To test the hypothesis that the changes in intestinal contractility, which accompany inflammation of the gut, are agonist specific, we compared the response of inflamed strips to substance P (SP), motilin, ACh, and K(+) as a function of time. In parallel experiments, changes in the general mechanical properties (passive tension, optimal stretch) of the colitic tissue were evaluated. Colitis was induced by trinitrobenzenesulfonic acid, and rabbits were killed after 1, 2, 3, 5, or 8 days. Passive tension was increased starting from day 2 until day 8, and maximal active tension (T(max)) was generated at less stretch from day 5. A 50% decrease in T(max) was observed for ACh and K(+) between days 2 and 3 and for motilin and SP between days 3 and 5. For all compounds, T(max) returned to normal after 8 days. The pEC(50) value (negative logarithm of the concentration that induces 50% of the maximal contractile activity) for ACh was increased from day 3 until day 8 and for SP at day 3, whereas for motilin it was decreased at day 1. The changes in passive tension and optimal stretch indicate generalized structural alterations of smooth muscle tissue. However, the different time profiles of the changes in active tension and contractile potency for different contractile agents suggest that inflammation specifically affects receptor-mediated mechanisms.

Region-specific antiproliferative effect of VIP and PACAP-(1-38) on rabbit enteric smooth muscle

The ability of neuropeptides to modulate enteric smooth muscle proliferation was examined in primary explant cultures of rabbit gastric antrum and colon smooth muscle. Cell proliferation was determined by [3H]thymidine incorporation measurements and cell counting. Subcultured rabbit antrum and colon myocytes (passages 2-6) preserved a smooth muscle phenotype, as verified by immunohistochemistry for alpha-smooth muscle actin and electron microscopy. Both vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating peptide-(1-38) [PACAP-(1-38)] concentration dependently (10(-10) to 10(-6) M) inhibited the serum-induced [3H]thymidine incorporation [in colon, 48.2 +/- 5.8 and 55.6 +/- 9.3% of control with 10(-6) M VIP and 10(-7) M PACAP-(1-38)] and inhibited increase in cell numbers in cultures derived from the colon but not in those from the antrum. Effects of VIP and PACAP-(1-38) were mimicked by forskolin (10(-7) to 10(-6) M) but not by 8-bromo-cGMP, whereas theophylline enhanced the effects of VIP. Inhibition of nitric oxide synthase with NG-nitro-L-arginine methyl ester (10(-3.5) M) did not alter the effects of VIP. Substance P, motilin, calcitonin gene-related peptide, and somatostatin had no effect. A single class of 125I-labeled VIP binding sites was found in antrum and colon myocyte cultures with an equal affinity for VIP and PACAP-(1-38) [dissociation constant (Kd) in antrum = 3.4 +/- 0.8 nM for VIP and 2.0 +/- 1.0 nM for PACAP-(1-38); Kd in colon = 2.0 +/- 1.0 nM for VIP and 2.8 +/- 1.6 nM for PACAP-(1-38)]. Density of binding sites in the antrum was higher than in the colon. In disease states such as inflammatory bowel disease, inhibition of myocyte proliferation by VIP and PACAP may serve to control smooth muscle hyperplasia in the colon but not in the antrum.

Motilin in human milk: identification and stability during digestion

The presence of motilin in human milk and the influence of human milk on the degradation of [125I][Nle13] porcine motilin by gastric and duodenal fluids were investigated. Milk and plasma samples were collected from 14 mothers, and motilin was measured by radioimmunoassay. Plasma levels were 416 +/- 37 pg/mL. In 8 defatted samples the motilin level was 105 +/- 14 pg/mL, in the six others levels were above 1000 pg/mL but dilution curves were non-linear. After solid-phase extraction milk levels were 108 +/- 21 pg/mL in 13 samples, in 1 sample the dilution curve was still non-linear. The stability of motilin after ingestion was studied in vitro by incubating [121I][Nle13] porcine motilin with gastric and intestinal juices obtained from newborns (10 times diluted). Incubations were performed at 37 degrees C at pH 1.8, 3.2 and 5.8 for the gastric fluid and at pH 7.4 for the duodenal fluid. After different times of intervals (5, 10, 20 and 30 minutes) intact motilin was precipitated with trichloroacetic acid and the radioactivity of the supernatant was determined. Motilin was rapidly degraded by gastric juice. The breakdown was greatest at pH 3.2 (74% after 30 minutes) and lowest at pH 5.8 (29%), the pH after milk feeding in neonates. Degradation by intestinal juice at pH 7.4 was also very rapid (77% after 30 minutes). Human milk and BSA inhibited partially the gastric digestion at pH 3.2 (17 and 29%, respectively). Digestion by intestinal juice was not affected by human milk and BSA. These results suggest that digestion of motilin in the stomach may be sufficiently retarded by human milk in the newborn to exert a biological role.

Localization of motilin binding sites in subcellular fractions from rabbit antral and colonic smooth muscle tissue

BACKGROUND AND AIM

Motilin stimulates gastrointestinal motility, but in vitro studies reveal a direct smooth muscle effect, whereas in vivo studies reveal a neurally mediated effect. The aim of the present study was to determine if motilin binds to microsomes (smooth muscle) and/or synaptosomes (neurons).

METHODS

Subcellular fractions were prepared from tissue of rabbit gastric antrum and colon by differential centrifugation and density gradient centrifugation and characterized by determining motilin binding and the presence of membrane markers.

RESULTS

The purified microsomal fraction, enriched in the smooth muscle marker 5'-nucleotidase, was found to have the highest specific motilin binding in both antrum and colon. In the antrum, but not in the colon, the mitochondrial fraction also showed enrichment of [3H]-saxitoxin binding (marker for synaptosomes) and motilin binding, although the latter was much lower than in the microsomal fraction. Two receptor binding sites were characterized in both the antral mitochondrial/synaptosomal and colonic microsomal fraction (antrum: pKd,1 9.89+/-0.19, pKd,2 8.18+/-0.11, colon: pKd,1 9.72+/-0.31, pKd,2 8.39+/-0.58).

CONCLUSION

Motilin binding is predominantly associated with smooth muscle membranes in both antrum and colon of the rabbit. In both organs two motilin binding sites are present with comparable affinities, but the density in the colon is much higher for both sites. Whether they represent neural and smooth muscle receptors will require studies with isolated smooth muscle cells and neurons.

Isolation and sequence of cDNA encoding the motilin precursor from monkey intestine. Demonstration of the motilin precursor in the monkey brain

The motilin precursor cDNA has been isolated and sequenced from a cDNA library prepared from monkey small intestine. The sequence indicates a 345 bp open reading frame, a 63 bp 5' untranslated region and a 154 bp 3' untranslated region. The sequence encodes a 115 amino acid motilin precursor composed of a 25 amino acid signal peptide, the 22 amino acid motilin peptide and a 68 amino acid motilin associated peptide (MAP). Compared with the human motilin precursor cDNA, there are two amino acid substitutions in the signal peptide, one in motilin and four in the MAP. The presence of the motilin precursor in hypothalamus, hippocampus and cerebellum was demonstrated by RT-PCR.