Interplay between mast cells, enterochromaffin cells, and sensory signaling in the aging human bowel

BACKGROUND

Advanced age is associated with a reduction in clinical visceral pain perception. However, the underlying mechanisms remain largely unknown. Previous studies have suggested that an abnormal interplay between mast cells, enterochromaffin (EC) cells, and afferent nerves contribute to nociception in gastrointestinal disorders. The aim of this study was to investigate how aging affects afferent sensitivity and neuro-immune association in the human bowel.

METHODS

Mechanical and chemical sensitivity of human bowel afferents were examined by ex vivo afferent nerve recordings. Age-related changes in the density of mast cells, EC cells, sensory nerve terminals, and mast cell-nerve micro-anatomical association were investigated by histological and immune staining.

KEY RESULTS

Human afferents could be broadly classified into subpopulations displaying mechanical and chemical sensitivity, adaptation, chemo-sensitization, and recruitment. Interestingly human bowel afferent nerve sensitivity was attenuated with age. The density of substance P-immunoreactive (SP-IR) nerve varicosities was also reduced with age. In contrast, the density of ileal and colonic mucosal mast cells was increased with age, as was ileal EC cell number. An increased proportion of mast cells was found in close apposition to SP-IR nerves.

CONCLUSIONS & INFERENCES

Afferent sensitivity in human bowel was reduced with advancing age. Augmentation of mast cells and EC cell numbers and the mast cell-nerve association suggest a compensatory mechanism for sensory neurodegeneration.

Translational potential of a mouse in vitro bioassay in predicting gastrointestinal adverse drug reactions in Phase I clinical trials

BACKGROUND

Motility-related gastrointestinal (GI) adverse drug reactions (GADRs) such as diarrhea and constipation are a common and deleterious feature associated with drug development. Novel biomarkers of GI function are therefore required to aid decision making on the GI liability of compounds in development.

METHODS

Fifteen compounds associated with or without clinical GADRs were used to assess the ability of an in vitro colonic motility bioassay to predict motility-related GADRs. Compounds were examined in a blinded fashion for their effects on mouse colonic peristaltic motor complexes in vitro. For each compound concentration-response relationships were determined and the results compared to clinical data. Compounds were also assessed using GI transit measurements obtained using an in vivo rat charcoal meal model.

KEY RESULTS

Within a clinically relevant dosing range, the in vitro assay identified five true and three false positives, four true and three false negatives, which gave a predictive capacity of 60%. The in vivo assay detected four true and four false positives, four false and three true negatives, giving rise to a predictive capacity for this model of 47%.

CONCLUSIONS & INFERENCES

Overall these results imply that both assays are poor predictors of GADRs. Further analysis would benefit from a larger compound set, but the data show a clear need for improved models for use in safety pharmacology assessment of GI motility.

Curcumin acts via transient receptor potential vanilloid-1 receptors to inhibit gut nociception and reverses visceral hyperalgesia

BACKGROUND

An antinociceptive effect has been reported for curcumin in animal models and in humans, but the molecular mechanisms of curcumin's effect remain undefined. In this study, we explored the possibility that curcumin inhibit visceral nociception via antagonizing the transient receptor potential vanilloid-1 (TRPV1) receptor.

METHODS

The effects of curcumin were explored using two experimental models: viscero-motor response (VMR) to colorectal distension (CRD) in rats and jejunal afferent firing in the ex vivo mouse jejunum preparations [TRPV1 knockout (KO) and wild-type mice, naive and trinitrobenzene sulfonic acid (TNBS)-treated Kunming mice]. In addition, capsaicin-induced calcium transients and whole-cell currents were examined in acutely dissociated dorsal root ganglia (DRG) neurons.

KEY RESULTS

In the anesthetized rat, curcumin (4 mg kg(-1)  min(-1) for 3 min) caused a marked and rapidly reversible inhibition of CRD-induced VMRs. In the mouse jejunum, the mesenteric afferent nerve response to ramp distension was attenuated by curcumin (3, 10 μmol L(-1) ), an effect that was significantly reduced in TRPV1 KO mice compared with wild-type (WT) controls. Moreover, in WT mice, curcumin (1-30 μmol L(-1) ) was found to inhibit the afferent responses to capsaicin in a concentration-dependent manner. Trinitrobenzene sulfonic acid-induced hypersensitivity of jejunal afferents was also attenuated by curcumin. Curcumin potently inhibited capsaicin-induced rise in intracellular calcium and inward currents in mouse or rat DRG neurons.

CONCLUSIONS & INFERENCES

Our results provide strong evidence that curcumin inhibit visceral nociception via antagonizing TRPV1 and may be a promising lead for the treatment of functional gastrointestinal diseases.

Endocannabinoid modulation of jejunal afferent responses to LPS

BACKGROUND

Endocannabinoids influence immune function and nociceptive signaling. This study examines cannabinoid modulation of sensory signaling from the GI tract following an acute inflammatory response triggered by systemic administration of bacterial lipopolysaccharide (LPS).

METHODS

A segment of proximal jejunum was intubated, to measure intraluminal pressure, in anesthetized rats. Afferent impulse traffic was recorded from a single isolated paravascular nerve bundle supplying the jejunal loop. Drugs and LPS were administered intravenously and changes in afferent firing were determined.

KEY RESULTS

The non-selective cannabinoid agonist, WIN55,212-2 (1 mg kg(-1) i.v.) and the anandamide transport inhibitor, VDM11 (1 mg kg(-1) i.v.) but not the fatty acid amide hydrolase (FAAH) inhibitor, URB597 (0.3 mg kg(-1)) caused a significant increase in afferent activity. The WIN55,212-2-induced afferent response was mediated by activation of CB(1) receptors whereas the VDM11 response was mediated by both CB(1) and CB(2) receptor mechanisms. LPS (10 mg kg(-1)) evoked an increase in afferent activity which was significantly reduced in the presence of WIN55,212-2 and VDM11 but not URB597. The inhibitory effect of WIN55,212-2 was prevented by CB(1) but not CB(2) receptor antagonism. In contrast, the inhibitory effect of VDM11 remained unaltered after CB(1) or CB(2) receptor blockade.

CONCLUSIONS & INFERENCES

Endocannabinoids play a role in modulating afferent signaling and may represent a target for the treatment of visceral hypersensitivity. In contrast to the effects of blocking endocannabinoid uptake (VDM11), inhibiting breakdown of endocannabinoids (URB597) had no effect on baseline or LPS induced afferent firing. Therefore, uptake of cannabinoids rather than breakdown via FAAH terminates their action in the GI tract.

Neurogenic adaptation contributes to the afferent response to mechanical stimulation

This study aimed to characterize the effect of mechanical stimuli on mesenteric afferent nerve signaling in the isolated rat jejunum in vitro. This was done to determine the effect of mechanical stresses and strains relative to nonmechanical parameters (neurogenic adaptation). Mechanical stimulations were applied to a segment of jejunum from 15 rats using ramp distension with water at three rates of distension, a relaxation test (volume maintained constant from initial pressure of 20 or 40 mmHg), and a creep test (pressure maintained constant). Circumferential stress and strain and the spike rate increase ratio were calculated for evaluation of afferent nerve activity during the mechanical stimulations. Ramp distension evoked two distinct phases of afferent nerve signaling as a function of circumferential stress or strain. Changing the volume distension rate did not change the stress-strain relationship, but faster distension rate increased the afferent firing rate (P < 0.05). In the stress relaxation test, the spike rate declined faster and to a greater extent than the stress. In the creep test, the spike rate declined, despite a small increase in the strain. Three classes of mechanosensitive single-afferent units (low, wide dynamic range, and high threshold units) showed different response profiles against stress and strain. Low-threshold units exhibited a near linear relationship against the strain (R(2) = 0.8095), whereas high-threshold units exhibited a linear profile against the stress (R(2) = 0.9642). The afferent response is sensitive to the distension speed and to the stress and strain level during distension. However, the afferent nerve response is not a simple function of either stress or strain. Nonmechanical time-dependent adaptive responses other than those related to viscoelasticity also play a role.

The differential effect of CB1 receptors on the discharge of afferent and efferent fibres supplying the rat jejunum

BACKGROUND

The cannabinoid receptor (CB(1) ) is expressed on GI sensory neurons and is suggested to play a role in food intake, inflammation and nociception. Expression of CB(1) in the nodose is influenced by nutritional status. Our aim was to determine the functional response of afferent and efferent fibres supplying the proximal jejunum to the CB(1) agonist docosatetraenylethanolamide (DEA) in fed and fasted animals.

METHODS

Experiments were performed on anesthetized rats, either fed ad libitum or fasted for 24 h. Blood pressure was recorded via the carotid artery and the proximal jejunum intubated to measure intraluminal pressure. A single paravascular nerve bundle was isolated and attached to an electrode for recording either afferent or efferent impulse traffic.

KEY RESULTS

Docosatetraenylethanolamide (1 mg kg(-1) , i.v.) had a depressor effect on blood pressure but surprisingly had little effect on afferent nerve activity in fed animals. In fasted animals the afferent response to DEA was augmented, however, the blood pressure effect was attenuated. In contrast, DEA caused a significant and prolonged increase in efferent firing, which was diminished in fasted animals. Bilateral cervical vagotomy had no effect on the DEA-mediated efferent response, however, hexamethonium (10 mg kg(-1) ) abolished excitation and unmasked an inhibitory effect of DEA.

CONCLUSIONS & INFERENCES

Docosatetraenylethanolamide has only a modest effect on intestinal afferent firing but a profound effect on efferent function, which is modulated by changes in nutritional status. The persistent response after vagotomy and block by hexamethonium suggests DEA is acting centrally, although there may be an inhibitory effect at the level of the postganglionic sympathetic neuron.

Signaling mechanisms involved in the intestinal pro-secretory actions of hydrogen sulfide

BACKGROUND

H(2) S actions in the gut involve neural activation. This study aimed to reveal the signaling mechanisms responsible for the pro-secretory effect of H(2) S by using TRPV1 and unselective TRP blockers and inhibitors of other signaling cascades hitherto described to be targeted by H(2) S elsewhere.

METHODS

Ussing chamber voltage clamp technique was used to study actions of the H(2) S donor NaHS on secretion in guinea-pig and human colon. NaHS effects on guinea-pig primary afferents were also evaluated.

KEY RESULTS

NaHS evoked secretion was significantly reduced in guinea-pig and human tissue by the selective TRPV1 blockers capsazepine, AMG9801, SB705498, BCTC; LY294002 (Phosphatidylinositol-3 kinase (PI3K) inhibitor), SKF96365 (store operated calcium channel blocker), 2-APB (inositol triphosphate blocker), and atropine but not by HC030031 (TRPA1 blocker) or L- and T-type calcium channel antagonists. Actions of TRPV1 antagonists suggested non-competitive inhibition at multiple sites. In guinea-pig colon, Gd(3+) and La(3+) (unselective TRP blockers) had no effects while ruthenium red reduced NaHS effects; in human colon Gd(3+) attenuated NaHS response. NaHS response was inhibited by neurokinin-1 and -3 receptor blockers in guinea-pig and neurokinin-1 and -2 receptor blockade in human tissue. There was cross-desensitization between NaHS and capsaicin responses. NaHS induced capsazepine and LY294002 sensitive afferent discharge.

CONCLUSIONS & INFERENCES

H(2) S evokes mucosal secretion by targeting TRPV1 expressing afferent nerves which activate cholinergic secretomotor neurons via release of substance P acting in a species dependent manner on neurokinin-1, -2 or -3 receptors. Besides TRPV1 signaling H(2) S may target intracellular calcium dependent pathways and PI3K.

Differential afferent sensitivity to mucosal lipopolysaccharide from Salmonella typhimurium and Escherichia coli in the rat jejunum

Postinfectious irritable bowel syndrome may develop subsequent to acute bacterial enteritis. We therefore hypothesized that intestinal afferents may develop hypersensitivity upon exposure to luminal lipopolysaccharide (LPS) from pathogens but not from commensal bacteria and that this may be prostaglandin mediated. Extracellular recordings of jejunal afferents were obtained in vivo from male Wistar rats (n = 5 per group; 300-400 g). Lipopolysaccharide from Escherichia coli (E-LPS), Salmonella typhimurium (S-LPS) or vehicle were infused into the intestinal lumen at 5 mg mL(-1). The selective 5-HT(3)-receptor agonist 2-methyl-5-HT (2m5-HT, 15 microgkg(-1), i.v.) was administered at 15-min intervals before and up to 2 h after S-LPS administration. Intraluminal E-LPS had no effect on mesenteric afferent nerve discharge at baseline. By contrast, afferent discharge increased from 21.7 +/- 0.3 impsec(-1) to 28.8 +/- 3.4 impsec(-1) 40 min after S-LPS administration (mean +/- SEM; P < 0.05) and reached 38.8 +/- 4.1 impsec(-1) after 2 h (P < 0.05). The afferent response to 2m5-HT was enhanced 30 min following S-LPS by 30.9 +/- 3.9% (P < 0.05) and remained elevated thereafter. The increase in baseline discharge and sensitivity to 2m5-HT following S-LPS was prevented by pretreatment with naproxen (COX inhibitor, 10 mgkg(-1) i.v.) or AH-6809 (EP1/EP2 receptor antagonist, 1 mg kg(-1)). Intestinal afferents do not alter their discharge rate to LPS from E. coli but to LPS from the pathogenic bacterium S. typhimurium. The latter response entails afferent sensitisation to 2m5-HT that depends on prostanoid release. This acute sensitisation may prime the intestinal afferent innervation for a later development of persistent hypersensitivity.

Purinergic contribution to small intestinal afferent hypersensitivity in a murine model of postinfectious bowel disease

Increased sensitivity of the afferent innervation of the gastrointestinal tract reportedly underlies symptoms of discomfort and pain in functional bowel disorders. The present investigation aimed to examine whether the purinergic P2X(2) and P2X(3) receptor subunits contribute to the mechanosensitivity of small intestinal afferents in normal mice and in a murine model of postinfectious gut dysfunction. Mesenteric afferent nerve activity was recorded in a mouse jejunum preparation maintained in vitro. As has been shown previously, ramp distension of the jejunal segment evoked biphasic afferent discharge, reflecting activation of low and high threshold fibres. The average pressure-afferent response curve in mice deficient in both P2X(2) and P2X(3) subunits (n = 14) was not significantly different from that of the wild-type control preparations (n = 13). Application of pyridoxal 5-phosphate 6-azophenyl-2 ,4-disulphonic acid (PPADS) (30 micromol L(-1)), a P2X and P2Y antagonist, or 2,4,6-trinitrophenol-adenosine 5'-triphosphate (10 micromol L(-1)), an antagonist selective for homomeric P2X(3) and heteromeric P2X(2/3) receptors, had no effect on the averaged pressure-afferent response curve in wild-type animals. In Trichinella spiralis-infected mice, the magnitude of mesenteric afferent responses to jejunal distension was greater at day 21 and day 56 postinfection compared with the sham control preparations demonstrating the development of afferent hypersensitivity. PPADS had no significant effect upon mechanically evoked afferent discharge rates in sham treated preparations (n = 5), but significantly inhibited afferent sensitivity to jejunal distension in preparations from mice at day 21 (n = 6) and day 56 (n = 7) postinfection. These results suggest that purinergic mechanisms play no role in mechanosensory transduction in the normal small intestine but contribute significantly to postinfectious mechano-hypersensitivity.

Impairment of rectal afferent mechanosensitivity in experimental diabetes in the rat

Diabetes mellitus results in neuropathy of both somatic and visceral nerves. In diabetic patients with faecal incontinence, impaired rectal sensory function, manifested by a decreased sensitivity to balloon distention is common. This may contribute to unawareness of rectal filling and incontinence. There has been little study to date of visceral mechanosensation in experimental diabetes however. We hypothesized that experimental diabetes would impair mechanosensitivity in rectal afferent nerves. Diabetes was induced in rats by i.p. injection of streptozotocin. Controls were injected with citrate. In vitro recordings were performed from rectal afferent nerves innervating isolated segments of rectum. In control animals, three distinct populations of mechanosensitive fibres were identified. Low threshold fibres responded at low intensity stretch and reached a maximal firing rate at less than 10 g of stretch (11/24 units). Wide dynamic sensitivity units responded at low intensity stretch (<2 g) but encoded stimulus intensity in a linear fashion up to 20 g (12/24 units). High threshold units responded at greater than 5 g. In diabetic animals there was a near complete loss of LT units (1/19) and most (16/29) had properties similar to WD units. However, their response threshold was significantly increased. Firing rates in response to maximal distention did not change in diabetic animals. We conclude that experimental diabetes selectively affects the detection of low threshold 'physiologic' rectal distention, such as that which might occur during rectal filling, prior to defaecation.

Neuronal activation in the nucleus of the solitary tract following jejunal lipopolysaccharide in the rat

INTRODUCTION

Inflammation during systemic lipopolysaccharide (LPS) seems to be modulated by the CNS via afferent and efferent vagal pathways. We hypothesized that similar to systemic inflammation, local LPS in the gut lumen may also activate central neurons and aimed to identify potential molecular mechanisms.

METHODS

Male Wistar rats were equipped with an exteriorized canula in the proximal jejunum. LPS or vehicle were administered into the jejunum (10 mg ml(-1)). For further study of molecular mechanisms, LPS or vehicle were administered systemically (1 mg kg(-1)). Brain stem activation was quantified by Fos-immunohistochemistry in the vagal nucleus of the solitary tract (NTS) and the Area postrema which is exposed to systemic circulation. Serum LPS concentrations were also determined.

RESULTS

Jejunal LPS exposure entailed 91+/-12 (n=7) Fos-positive neurons in the NTS compared to 39+/-9 in controls (n=6; p<0.01), while serum LPS concentrations and Fos-positive neurons in the Area postrema were not different. Systemic LPS triggered 150+/-25 (n=6) and vehicle 52+/-6 Fos-positive neurons (n=7; p<0.01). The Fos count after systemic LPS was reduced to 99+/-30 following pretreatment with the cyclooxygenase inhibitor Naproxen (10 mg kg(-1); p>0.05 versus vehicle controls) and increased to 242+/-66 following the iNOS-inhibitor Aminoguanidine (15 mg kg(-1); p<0.01). In the Area postrema, 97+/-17 (n=6) neurons were counted in animals pretreated with systemic LPS compared to 14+/-4 in controls (n=7, p<0.001).

CONCLUSIONS

Central neuronal activation following inflammation after systemic LPS is modulated by cyclooxygenase and NO pathways. Local exposure to bacterial LPS in the gut lumen activates the NTS which may set the stage for efferent vagal modulation of intestinal inflammation.

An educational and audit tool to reduce prescribing error in intensive care

AIM

To reduce prescribing errors in an intensive care unit by providing prescriber education in tutorials, ward-based teaching and feedback in 3-monthly cycles with each new group of trainee medical staff.

METHODS

Prescribing audits were conducted three times in each 3-month cycle, once pretraining, once post-training and a final audit after 6 weeks. The audit information was fed back to prescribers with their correct prescribing rates, rates for individual error types and total error rates together with anonymised information about other prescribers' error rates.

RESULTS

The percentage of prescriptions with errors decreased over each 3-month cycle (pretraining 25%, 19%, (one missing data point), post-training 23%, 6%, 11%, final audit 7%, 3%, 5% (p<0.0005)). The total number of prescriptions and error rates varied widely between trainees (data collection one; cycle two: range of prescriptions written: 1-61, median 18; error rate: 0-100%; median: 15%).

CONCLUSION

Prescriber education and feedback reduce manual prescribing errors in intensive care.

The gaseous mediator, hydrogen sulphide, inhibits in vitro motor patterns in the human, rat and mouse colon and jejunum

Hydrogen sulphide (H2S) has been recently proposed as a transmitter in the brain and peripheral tissues. Its role in the gastrointestinal tract is still unknown despite some data which suggest an involvement mediating smooth muscle relaxation. The aim of this study was to investigate the effect of this gas on intestinal segments from mouse jejunum and colon, and muscular strips from the human and rat colon. In isolated segments of mouse colon and jejunum, bath applied sodium hydrogen sulphide (NaHS) (a H2S donor) caused a concentration-dependent inhibition of spontaneous motor complexes (MCs) (IC(50) 121 micromol L(-1) in the colon and 150 micromol L(-1) in the jejunum). This inhibitory effect of NaHS on MCs was (i) unaffected by tetrodotoxin (TTX), capsaicin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and N-nitro-L-arginine suggesting a non-neural effect and (ii) significantly reduced by apamin 3 micromol L(-1). NaHS concentration-dependently inhibited the spontaneous motility in strips from human colon (IC(50) 261 micromol L(-1)) and rat colon (IC(50) 31 micromol L(-1)). The inhibitory effect of NaHS on colonic strips was (i) unaffected by the neural blocker TTX (1 micromol L(-1)) with IC(50) 183 micromol L(-1) for the human colon and of 26 micromol L(-1) for the rat colon and (ii) significantly reduced by glybenclamide (10 micromol L(-1)), apamin (3 micromol L(-1)) and TEA (10 mmol L(-1)) with IC(50) values of 2464, 1307 and 2421 micromol L(-1) for human strips, and 80, 167 and 674 micromol L(-1) for rat strips respectively. We conclude that H2S strongly inhibits in vitro intestinal and colonic motor patterns. This effect appears to be critically dependent on K channels particularly apamin-sensitive SK channels and glybenclamide-sensitive K (ATP) channels.

The gaseous mediator, hydrogen sulphide, inhibits in vitro motor patterns in the human, rat and mouse colon and jejunum

Hydrogen sulphide (H2S) has been recently proposed as a transmitter in the brain and peripheral tissues. Its role in the gastrointestinal tract is still unknown despite some data which suggest an involvement mediating smooth muscle relaxation. The aim of this study was to investigate the effect of this gas on intestinal segments from mouse jejunum and colon, and muscular strips from the human and rat colon. In isolated segments of mouse colon and jejunum, bath applied sodium hydrogen sulphide (NaHS) (a H2S donor) caused a concentration-dependent inhibition of spontaneous motor complexes (MCs) (IC(50) 121 micromol L(-1) in the colon and 150 micromol L(-1) in the jejunum). This inhibitory effect of NaHS on MCs was (i) unaffected by tetrodotoxin (TTX), capsaicin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and N-nitro-L-arginine suggesting a non-neural effect and (ii) significantly reduced by apamin 3 micromol L(-1). NaHS concentration-dependently inhibited the spontaneous motility in strips from human colon (IC(50) 261 micromol L(-1)) and rat colon (IC(50) 31 micromol L(-1)). The inhibitory effect of NaHS on colonic strips was (i) unaffected by the neural blocker TTX (1 micromol L(-1)) with IC(50) 183 micromol L(-1) for the human colon and of 26 micromol L(-1) for the rat colon and (ii) significantly reduced by glybenclamide (10 micromol L(-1)), apamin (3 micromol L(-1)) and TEA (10 mmol L(-1)) with IC(50) values of 2464, 1307 and 2421 micromol L(-1) for human strips, and 80, 167 and 674 micromol L(-1) for rat strips respectively. We conclude that H2S strongly inhibits in vitro intestinal and colonic motor patterns. This effect appears to be critically dependent on K channels particularly apamin-sensitive SK channels and glybenclamide-sensitive K (ATP) channels.

5-HT system in the gut: roles in the regulation of visceral sensitivity and motor functions

5-Hydroxytryptamine (5-HT) is a major transmitter molecule within the gastrointestinal tract. It is contained in enterochromaffin (EC) cells, which form part of the epithelial lining of the gut and in enteric neurones in the submucosal and myenteric plexuses. 5-HT is present in murine mucosal mast cells in the lamina propria and some studies have suggested that human mast cells may also contain 5-HT especially in conditions associated with mastocytosis. The strategic positioning of the enteric and extrinsic sensory innervation in close proximity to these sources of 5-HT, in conjunction with their demonstrated sensitivity to this mediator, suggests the involvement of 5-HT in the transduction of visceral stimuli and reflex responses affecting motor and secretory function. Under physiological conditions, the release of 5-HT from these storage sites may result in the orchestration of reflexes responsible for transit of material along the bowel at a rate that is appropriate for digestion and absorption of nutrients. However, in the pathophysiological state, 5-HT acting together with other inflammatory mediators may cause inappropriate intestinal secretomotor activity and/or initiate sensations such as nausea or discomfort/pain. Current evidence suggests that the bioavailability of 5-HT within the gut wall is altered in a number of post-inflammatory models of gut dysfunction with increased numbers of EC cells and mast cells with increased 5-HT content in proximity to sensory nerve endings, and decreased serotonin reuptake mechanisms. Changes may also occur in the sensory innervation or pathways within the central nervous system. These processes may contribute to pain mechanisms in the irritable bowel syndrome, in which visceral hypersensitivity is a predominant feature and may also contribute to motor dysfunction leading to altered bowel habit.

Influence of the pattern of jejunal distension on mesenteric afferent sensitivity in the anaesthetized rat

Vagal, spinal and intestino-fugal fibres all potentially transmit mechanosensory afferent information from the gastrointestinal tract. We aimed to characterize the relative mechanosensitivity of these three different afferent populations supplying the rat jejunum. Afferent nerve discharge was recorded from pentobarbitone-anaesthetized rats during different distension protocols. Saline ramp distension (1 mL min(-1)) and barostat ramp distension (2 mmHg 4 s(-1)) each evoked biphasic responses but with the latter significantly attenuated especially at low distending pressures. Barostat controlled phasic distensions (10-50 mmHg, 25 s) evoked an afferent response with a peak at the onset of distension adapting to a plateau level that was maintained and comparable to the barostat ramp responses at the corresponding pressures. Chronic subdiaphragmatic vagotomy significantly attenuated the low pressure component of the response to balloon ramp distension and both peak and plateau responses to phasic distension. Single unit analysis showed an absence of low threshold afferent activity after vagotomy while the response to fibres with wide-dynamic range and high threshold sensitivity were preserved hexamethonium had no effect on the responses to either ramp or phasic distension. These findings suggest that the nature of the distension stimulus is critical in determining the pattern of response observed from the various subpopulations of afferents supplying the bowel wall.

Intestinal secretory and absorptive function in Trichinella spiralis mouse model of postinfective gut dysfunction: role of bile acids

OBJECTIVE

Observations showing that bile acid malabsorption is frequent in irritable bowel syndrome (IBS) suggest that alterations in bile acid-induced secretion and absorption could contribute to IBS-associated diarrhoea. The secretory response to bile acids, fluid transport and bile absorption was examined in intestinal tissues from a Trichinella spiralis mouse model of postinfectious gut dysfunction in vitro. Changes in the protein expression of apical sodium-dependent bile acid transporter (ASBT) were also measured.

DESIGN

T. spiralis-infected mice were killed at 18 and 25 days postinfection. Jejunal, ileal, proximal and distal colon segments were exposed to taurodeoxycholic acid (TDCA) or cholic acid. Short circuit current (SCC) increases were determined. Tritiated taurocholic acid (3H-TCA) absorption was determined in everted jejunal and ileal sacs. ASBT protein expression was determined by Western blot analysis and immunohistochemistry.

RESULTS

Basal SCC increased in ileum and distal colon at 18 and 25 days postinfection, respectively. Ileal SCC responses to TDCA and cholic acid were enhanced at 18 days postinfection. Distal colon SCC response to TDCA was raised at 18 days postinfection but was significantly reduced by 25 days. Ileal 3H-TCA uptake was significantly reduced at 18 and 25 days postinfection. Surprisingly, increased ASBT expression was observed in infected animals.

CONCLUSIONS

In a T. spiralis model of postinfectious gut dysfunction, decreased bile absorption and enhanced secretion in response to bile acids was observed. Decreased absorption was not, however, caused by decreased ASBT as increased expression was observed. If similar events occur postinfection, the combined effects of these disturbances may contribute to some symptoms observed in postinfectious IBS patients.

Activation of the cannabinoid 2 (CB2) receptor inhibits murine mesenteric afferent nerve activity

Abstract Cannabinoid 2 (CB2) receptors have both antinociceptive and antihypersensitivity effects, although the precise mechanisms of action are still unclear. In this study, the modulatory role of CB2 receptors on the mesenteric afferent response to the endogenous immunogenic agent bradykinin (BK) was investigated. Mesenteric afferent recordings were obtained from anaesthetized wild-type and CB2(-/-) mice using conventional extracellular recording techniques. Control responses to BK were obtained in all experiments prior to administration of either CB2 receptor agonist AM1241, or AM1241 plus the CB2 receptor antagonist AM630. Bradykinin consistently evoked activation of mesenteric afferents (n = 32). AM1241 inhibited the BK response in a dose dependent manner. In the presence of AM630 (10 mg kg(-1)), however, AM1241 (10 mg kg(-)1) had no significant effect on the BK response. Moreover, AM1241 had also no significant effect on the BK response in CB2(-/-) mice. Activation of the CB2 receptor inhibits the BK response in mesenteric afferents, demonstrating that the CB2 receptor is an important regulator of neuroimmune function. This may be a mechanism of action for the antinociceptive and antihypersensitive effects of CB2 receptor agonists.

Mechanisms underlying mechanosensitivity of mesenteric afferent fibers to vascular flow

Spinal afferent neurons, with endings in the intestinal mesenteries, have been shown to respond to changes in vascular perfusion rates. The mechanisms underlying this sensitivity were investigated in an in vitro preparation of the mesenteric fan devoid of connections with the gut wall. Afferent discharge increased when vascular perfusion was stopped ("flow off"), a response localized to the terminal vessels just prior to where they entered the gut wall. The flow-off response was compared following pharmacological manipulations designed to determine direct mechanical activation from indirect mechanisms via the vascular endothelium or muscle. Under Ca(2+)-free conditions, responses to flow off were significantly augmented. In contrast, the myosin light chain kinase inhibitor wortmannin (1 microM, 20 min) did not affect the flow-off response despite blocking the vasoconstriction evoked by 10 microM l-phenylephrine. This ruled out active tension, generated by vascular smooth muscle, in the response to flow off. Passive changes caused by vessel collapse during flow off were speculated to affect sensory nerve terminals directly. The flow-off response was not affected by the N-, P-, and Q-type Ca(2+) channel blocker omega-conotoxin MVIIC (1 muM intra-arterially) or the P2X receptor/ion channel blocker PPADS (50 microM). However, ruthenium red (50 microM), a blocker of nonselective cation channels, greatly reduced the flow-off response and also abolished the vasodilator response to capsaicin. Our data support the concept that mesenteric afferents sense changes in vascular flow during flow off through direct mechanisms, possibly involving nonselective cation channels. Passive distortion in the fan, caused by changes in blood flow, may represent a natural stimulus for these afferents in vivo.

Bladder afferent sensitivity in wild-type and TRPV1 knockout mice

Understanding bladder afferent pathways may reveal novel targets for therapy of lower urinary tract disorders such as overactive bladder syndrome and cystitis. Several potential candidate molecules have been postulated as playing a significant role in bladder function. One such candidate is the transient receptor potential vanilloid 1 (TRPV1) ion channel. Mice lacking the TRPV1 channel have altered micturition thresholds suggesting that TRPV1 channels may play a role in the detection of bladder filling. The aim of this study was therefore to investigate the role of TRPV1 receptors in controlling bladder afferent sensitivity in the mouse using pharmacological receptor blockade and genetic deletion of the channel. Multiunit afferent activity was recorded in vitro from bladder afferents taken from wild-type (TRPV+/+) mice and knockout (TRPV1-/-) mice. In wild-type preparations, ramp distension of the bladder to a maximal pressure of 40 mmHg produced a graded increase in afferent activity. Bath application of the TRPV1 antagonist capsazepine (10 mum) caused a significant attenuation of afferent discharge in TRPV1+/+ mice. Afferent responses to distension were significantly attenuated in TRPV1-/- mice in which sensitivity to intravesical hydrochloric acid (50 mm) and capsaicin (10 microm) were also blunted. Altered mechanosensitivity occurred in the absence of any changes in the pressure-volume relationship during filling indicating that this was not secondary to a change in bladder compliance. Single-unit analysis was used to classify individual afferents into low-threshold and high-threshold fibres. Low threshold afferent responses were attenuated in TRPV1-/- mice compared to the TRPV1+/+ littermates while surprisingly high threshold afferent sensitivity was unchanged. While TRPV1 channels are not considered to be mechanically gated, the present study demonstrates a clear role for TRPV1 in the excitability of particularly low threshold bladder afferents. This suggests that TRPV1 may play an important role in normal bladder function.

Impact of 5-HT3 receptor blockade on colonic haemodynamic responses to ischaemia and reperfusion in the rat

5-HT(3) receptor antagonists are clinically available for treating patients with irritable bowel syndrome (IBS) but their use is restricted because of a link with some episodes of ischaemic colitis. However, the role of 5-HT3 receptors in regulating colonic blood flow has not been systematically investigated. Thus, we examined acute and chronic treatment with alosetron, a potent and selective antagonist of the 5-HT3 receptor, on baseline colonic blood flow and haemodynamic responses during occlusion and reactive hyperaemia in the pentobarbitone-anaesthetized rat. Colonic haemodynamics were assessed using ultrasonic recordings of superior mesenteric blood flow (MBF) and laser Doppler recordings of colonic vascular perfusion (VP). Blood pressure (BP) was also monitored and in some experiments tissue oxygen was detected polarographically. Alosetron (10, 30, 100 microg kg(-1), i.v.) had no effect on baseline haemodynamics nor responses to nitric oxide synthase inhibition with N(omega)-nitro-l-arginine methyl ester (l-NAME) (16 mg kg(-1)). Arterial occlusion (5 min) reduced MBF (-98.6 +/- 0.6%) and VP (-70.7 +/- 5.4%) followed by a post-occlusion reactive hyperaemia (MBF = +94.5 +/- 19.1%; VP = +60.0 +/- 22.3%) the magnitude of which was unchanged following acute (30 microg kg(-1)) or chronic alosetron administration (0.5 mg kg(-1) twice daily, 5 days). Alosetron did not significantly alter baseline colonic blood flow in the anaesthetized rat; nor did it interfere with vascular control mechanisms activated during occlusion and reactive hyperaemia.

Vagal modulation of intestinal afferent sensitivity to systemic LPS in the rat

The central nervous system modulates inflammation in the gastrointestinal tract via efferent vagal pathways. We hypothesized that these vagal efferents receive synaptic input from vagal afferents, representing an autonomic feedback mechanism. The consequence of this vagovagal reflex for afferent signal generation in response to LPS was examined in the present study. Different modifications of the vagal innervation or sham procedures were performed in anesthetized rats. Extracellular mesenteric afferent nerve discharge and systemic blood pressure were recorded in vivo before and after systemic administration of LPS (6 mg/kg iv). Mesenteric afferent nerve discharge increased dramatically following LPS, which was unchanged when vagal efferent traffic was eliminated by acute vagotomy. In chronically vagotomized animals, to eliminate both vagal afferent and efferent traffic, the increase in afferent firing 3.5 min after LPS was reduced to 3.2 +/- 2.5 impulses/s above baseline compared with 42.2 +/- 2.0 impulses/s in controls (P < 0.001). A similar effect was observed following perivagal capsaicin, which was used to eliminate vagal afferent traffic only. LPS also caused a transient hypotension (<10 min), a partial recovery, and then persistent hypertension that was exacerbated by all three procedures. Mechanosensitivity was increased 15 min following LPS but had recovered at 30 min in all subgroups except for the chronic vagotomy group. In conclusion, discharge in capsaicin-sensitive mesenteric vagal afferents is augmented following systemic LPS. This activity, through a vagovagal pathway, helps to attenuate the effects of septic shock. The persistent hypersensitivity to mechanical stimulation after chronic vagal denervation suggests that the vagus exerts a regulatory influence on spinal afferent sensitization following LPS.

Sensory transmission in the gastrointestinal tract

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.

STW 5 (Iberogast) reduces afferent sensitivity in the rat small intestine

INTRODUCTION

A limited number of drugs are available for the treatment of functional dyspepsia and irritable bowel syndrome. The efficacy of STW 5 (Iberogast) was previously shown in clinical trials. Since visceral hypersensitivity seems to be the prime pathomechanism of functional gastro-intestinal disorders, the aim of this study was to explore whether STW 5 reduces intestinal afferent sensitivity in the upper gastrointestinal tract.

METHODS

Two groups of male Wistar rats were pretreated with either the herbal preparation STW 5 or its vehicle (30.8% ethanol). Then, after 2h, general anesthesia was induced by pentobarbitone (60 mg kg(-1)i.p.) and extracellular multi-unit afferent recordings were obtained from mesenteric afferents innervating the proximal jejunum. The intestinal afferent nerve response to increasing doses of 5-HT and bradykinin were quantified as well as afferent discharge following a ramp distension of the adjacent intestinal loop from 0 to 60 cm H(2)O.

RESULTS

Afferent discharge to 5-HT and bradykinin increased dose-dependently. Following the different doses of 5-HT, the peak in afferent nerve discharge was always reduced after pretreatment with STW 5 compared to controls with a response of 110+/-5 imp s(-1) after STW 5 and 128+/-3 in vehicle controls at the maximum dose (40 microg kg(-1); p<0.05; mean+/-SEM). For bradykinin, afferent responses were reduced following STW 5 at the 20 and 40 microg kg(-1) dose but not at 10 microg kg(-1) (40 microg kg(-1)176+/-7 imp s(-1) following STW 5 versus 200+/-6 imp s(-1) in controls; p<0.05). The ramp distension of the intestinal loop stimulated a rise in intestinal afferent nerve discharge that was always lower in the STW 5 pretreated group compared to vehicle controls with the exception of the discharge rate at the pressure level of 0 and 20 cm H(2)O (all other pressures up to 60 cm H(2)O p<0.05).

CONCLUSIONS

Sensitivity of intestinal afferents to mechanical and chemical stimuli is reduced following treatment with the herbal preparation STW 5. This mechanism may help to explain why STW 5 relieves dyspeptic and bowel symptoms in patients.

Lipopolysaccharide-induced changes in mesenteric afferent sensitivity of rat jejunum in vitro: role of prostaglandins

Bacterial translocation across the intestinal mucosal barrier leads to a macrophage-mediated inflammatory response, visceral hyperalgesia, and ileus. Our aim was to examine how mediators released into mesenteric lymph following LPS treatment influence intestinal afferent sensitivity and the role played by prostanoids in any sensitization. Intestinal lymph was collected from awake rats following treatment with either saline or LPS (5 mg/kg ip). Extracellular multiunit afferent recordings were made from paravascular mesenteric nerve bundles supplying the rat jejunum in vitro following arterial administration of control lymph, LPS lymph, and LPS. Mesenteric afferent discharge increased significantly after LPS lymph compared with control lymph. Peak discharge occurred within 2 min and remained elevated for 5 to 8 min. This response was attenuated by pretreatment with naproxen (10 microM), and restored upon addition of prostaglandin E(2) (5 microM) in the presence of naproxen, but AH6809 (5 microM), an EP(1)/EP(2) receptor(s) antagonist, failed to decrease the magnitude of LPS lymph-induced response. LPS itself also stimulated mesenteric afferent discharge but was unaffected by naproxen. TNF-alpha was significantly increased in LPS lymph compared with control lymph (1,583 +/- 197 vs. 169 +/- 38 pg/ml, P < 0.01) but exogenous TNF-alpha failed to evoke any afferent nerve discharge. We concluded that inflammatory mediators released from the gut into mesenteric lymph during endotoxemia have a profound effect on afferent discharge. These mediators influence afferent firing via the release of local prostaglandins.

Sensitization of mesenteric afferents to chemical and mechanical stimuli following systemic bacterial lipopolysaccharide

BACKGROUNDS AND AIMS

The mechanisms underlying endotoxin-induced hyperalgesia remain unknown. We aimed to study the mechanisms underlying the sensitizing action of lipopolysaccharide (LPS) on intestinal afferent responses to mechanical and chemical stimuli.

METHODS

Extracellular recordings of jejunal afferent nerve discharge were obtained from pentobarbitone-anaesthetized rats.

RESULTS

Lipopolysaccharide (6 mg kg(-1), i.v.) stimulated a short-term, transient (<30 min) increase in chemosensitivity to systemic 5-HT (6 microg kg(-1)) and responses to mechanical distension and a delayed but maintained (>30 min) increase in spontaneous afferent discharge. Naproxen (10 mg kg(-1)) and the prostaglandin receptor antagonist AH6809 (1 mg kg(-1)) significantly attenuated both the short-term sensitization to mechanical distension and 5-HT and the long-term increase in baseline afferent firing following LPS. In contrast, the iNOS inhibitor aminoguanidine (15 mg kg(-1)) and the L-type calcium channel antagonist nifedipine (1 mg kg(-1)) both prolonged the period of afferent sensitization to distension and 5-HT without influencing the augmented baseline-firing rate. omega-Conotoxin GVIA attenuated the increase in afferent discharge to LPS, without any change in mechano- and chemosensitivity.

CONCLUSIONS

The long-term (>30 min) increase in afferent firing following systemic LPS involves neurogenic release of prostanoids. The short-term (<30 min) sensitization also appears to depend on prostanoid release, while nitric oxide production may serve to down-regulate LPS-induced afferent hypersensitivity.

Vagal selective effects of ruthenium red on the jejunal afferent fibre response to ischaemia in the rat

A variety of inflammatory mediators and local metabolites, have been implicated in the sensitivity of intestinal afferent fibres to brief periods of ischaemia and reperfusion. As yet, the contribution of the vanilloid transient receptor potential (TRPV)1 receptor to the response to intestinal ischaemia remains undetermined. In the present study, the effect of pretreatment with the competitive TRPV1 antagonist capsazepine and the non-selective TRPV channel antagonist ruthenium red, on the mesenteric afferent fibre response to ischaemia was examined. In control animals there was a reproducible biphasic increase in whole nerve afferent fibre activity during two brief periods of ischaemia. Treatment with ruthenium red significantly attenuated the early phase increase in afferent fibre activity during ischaemia. However, capsazepine treatment did not significantly alter the afferent fibre response to either ischaemia or reperfusion. Further experiments in chronically vagotomized animals indicated that the early phase response to ischaemia was mediated via vagal afferent fibres. The mechanism via which ruthenium red selectively inhibited vagal afferent fibres during ischaemia is unknown, but it does not appear to involve blockade of the TRPV1 receptor.

The herbal preparation STW 5 (Iberogast) desensitizes intestinal afferents in the rat small intestine

INTRODUCTION

Visceral hypersensitivity in the upper gastrointestinal tract is a potential pathomechanism of functional dyspepsia. The herbal preparation STW 5 (Iberogast) provides symptomatic relief for this condition. We aimed to investigate whether STW 5 modulates intestinal afferent sensitivity.

METHODS

The herbal preparation STW 5 or vehicle (30.8% ethanol) were administered orally in male Wister rats. After 2 h animals were anaesthetized and extracellular multi-unit intestinal afferent nerve recordings were secured from the neurovascular bundle of the mesentery in the proximal jejunum. Afferent discharge to ramp distension of the intestinal loop (0-60 cm H2O) and dose-response curves for i.v. bradykinin (10, 20 and 40 microg kg(-1)) and 5-HT (5, 10, 20 and 40 microg kg(-1)) were recorded.

RESULTS

Baseline discharge was not different between the vehicle and treatment group. Ramp distension was followed by a pressure dependent increase in afferent nerve discharge that was decreased following STW 5 pretreatment for all distending pressures reaching 147 +/- 8 impulses s(-1) (imp s(-1)) following STW 5 vs 171 +/- 5 imp s(-1) following vehicle at 60 cm H2O (mean +/- SEM; P < 0.05). A dose-dependent increase in afferent discharge was observed for 5-HT and bradykinin. Following STW 5 pretreatment, afferent discharge was reduced at all doses of 5-HT to 110 +/- 5 at the maximum dose after STW 5 and 128 +/- 3 imp s(-1) in controls (all P < 0.05). Afferent discharge to bradykinin was similarly reduced at 20 and 40 microg kg(-1) but not at 10 microg kg(-1) of bradykinin with a discharge rate of 176 +/- 7 imp s(-1) following STW 5 and 200 +/- 6 imp s(-1) in controls at 40 microg kg(-1) (P < 0.05).

CONCLUSIONS

The preparation STW 5 reduces intestinal afferent nerve discharge following chemical and mechanical stimuli, while baseline discharge is not affected. This effect of STW 5 on afferent sensitivity may contribute to its therapeutic relief of dyspeptic symptoms.

Peripheral opiate action on afferent fibres supplying the rat intestine

The aim of the present study was to examine the sensitivity of mesenteric afferents supplying the rat small intestine to mu-opioid receptor ligands. Mesenteric afferent discharge was recorded electrophysiologically in response to [D-ALA2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; 100 mug kg(-1) i.v.), before and after treatment with the mu-receptor antagonist alvimopan (1 mg kg(-1) i.v.). DAMGO markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05), an effect completely blocked by alvimopan. The response of mesenteric afferents to 2-methyl-5-hydroxytryptamine (30 microg kg(-1) i.v.), bradykinin (0.1-1 microg kg(-1) i.a.) and both low- and high-threshold distension (0-60 mmHg) was unaffected by alvimopan. In chronically vagotomized animals, the low-threshold response to distension was attenuated while the remaining high-threshold response was unaffected by alvimopan. In conclusion, mesenteric afferent fibres are markedly stimulated by mu-opioid receptor agonists, an effect blocked by alvimopan, which may contribute to the gastrointestinal reflex and behavioural responses to opiate treatment or abuse. However, alvimopan did not influence the normal sensitivity of intestinal afferents to chemical and mechanical stimuli that activate different subpopulations of vagal and spinal afferents. Thus, alvimopan may be useful for the treatment of gastrointestinal sequelae following opiate treatment for postoperative or chronic pain.

Neuroanatomy of extrinsic afferents supplying the gastrointestinal tract

Here we discuss the neuroanatomy of extrinsic gastrointestinal (GI) afferent neurones, the relationship between structure and function and the role of afferents in disease. Three pathways connect the gut to the central nervous system: vagal afferents signal mainly from upper GI regions, pelvic afferents mainly from the colorectal region and splanchnic afferents from throughout. Vagal afferents mediate reflex regulation of gut function and behaviour, operating mainly at physiological levels. There are two major functional classes - tension receptors, responding to muscular contraction and distension, and mucosal receptors. The function of vagal endings correlates well with their anatomy: tracing studies show intramuscular arrays (IMAs) and intraganglionic laminar endings (IGLEs); IGLEs are now known to respond to tension. Functional mucosal receptors correlate with endings traced to the lamina propria. Pelvic afferents serve similar functions to vagal afferents, and additionally mediate both innocuous and noxious sensations. Splanchnic afferents comprise mucosal and stretch-sensitive afferents with low thresholds in addition to high-threshold serosal/mesenteric afferents suggesting diverse roles. IGLEs, probably of pelvic origin, have been identified recently in the rectum and respond similarly to gastric vagal IGLEs. Gastrointestinal afferents may be sensitized or inhibited by chemical mediators released from several cell types. Whether functional changes have anatomical correlates is not known, but it is likely that they underlie diseases involving visceral hypersensitivity.

What activates visceral afferents?

Vagal and spinal afferents represent the information superhighways that convey sensory information from the gut to the central nervous system. These afferents are sensitive to both mechanical and chemical stimuli. Vagal afferents terminate in the muscle layers and in the mucosa. Muscle afferents are activated at physiological levels of distension and during peristalsis. In contrast, spinal afferents encode supraphysiological levels of intestinal pressure. Vagal and spinal afferents also express a wide range of membrane receptors to a variety of chemical mediators generated from both within and outside the gut wall. Some of these receptors are part of a modality specific transduction pathway involved in sensory signalling from the gut lumen to vagal afferent endings in the mucosa. Others, which are activated by substances derived from multiple cellular sources during ischaemia, injury, or inflammation act in a synergistic way to cause acute or chronic sensitisation of the afferent nerves to mechanical and chemical stimuli. Understanding the mechanisms that underlie hypersensitivity may have implications for the pharmaceutical approach to the treatment of functional bowel disorders like irritable bowel syndrome.

Stress-induced attenuation of brain stem activation following intestinal anaphylaxis in the rat

Intestinal anaphylaxis triggers neuronal activation in the nucleus tractus solitarius (nTS) of the rat brain stem. Stress may modulate reflex circuitry in the brain stem and facilitate intestinal inflammatory responses. We hypothesized that stress would modulate central neuronal activation during intestinal anaphylaxis. NTS neurons were activated following intestinal antigen challenge in sensitized Hooded Lister rats but not in negative controls (P < 0.05). The number of Fos-positive neurons following intestinal anaphylaxis decreased in animals exposed to water-avoidance stress (P < 0.05), although serum levels of rat mast cell protease II were not different in stressed and unstressed animals, indicating a similar degree of mast cell degranulation. Stress seems to inhibit neuronal activation in the rat brain stem during intestinal inflammation without modulation of the inflammatory response itself. This may have implications for a potential efferent neuronal modulation of inflammatory responses in the gut.

Mesenteric afferent nerves are sensitive to vascular perfusion in a novel preparation of rat ileum in vitro

Using novel in vitro preparations of vascularly perfused rat ileum, we investigated mesenteric afferent sensitivity to vascular perfusion. Gut (GPP) and vascular (VPP) perfusion pressures were recorded simultaneously with afferent discharge (AD). After preconstriction (L-phenylephrine), capsaicin (100 microM, gut lumen) caused a transient increase in AD and a sustained fall in VPP, supporting afferent modulation of vascular tone. In turn, AD was affected by vascular perfusion rate (VPR). Increasing VPR step-wise (0.6 to 1.0, 1.4 and 1.8 ml/min) caused concomitant falls in AD, returning at 0.6 ml/min. Terminating flow (5 min) increased AD. Afferent responses were independent of changes in GPP, vascular O2, or the gut "tube" ("gut-off"). In gut-off studies, where capsaicin (100 nM ia) still reduced VPP, flow-associated falls in AD were abolished by the enzyme neuraminidase (0.2 U/ml ia or extravascularly over 20 min). In contrast, increased AD after stopped flow was unaffected. We propose that mesenteric afferents "sense" changes in vascular perfusion. The precise stimuli (pressure and/or flow) and the physiological relevance to control of local circulation remain to be determined.

Cosensitivity of vagal mucosal afferents to histamine and 5-HT in the rat jejunum

A complex sensitivity of afferent nerves in the mesentery of the rat jejunum to systemic administration of histamine has recently been demonstrated. In the present study, we aimed to characterize subpopulations of mesenteric afferents that mediate this afferent nerve response. Multiunit afferent discharge was recorded from mesenteric nerves supplying the proximal jejunum in anesthetized rats. The majority of mesenteric bundles (84%) exhibited biphasic responses to histamine (8 micromol/kg), and these bundles also responded to 2-methyl-5-HT (2m5HT). In contrast, monophasic responses lacked a short-latency component, and these bundles failed to respond to 2m5HT. Single-unit analysis revealed a population of afferents that possessed cosensitivity for 2m5HT and histamine. This population of afferents was absent in chronically vagotomized animals, whereas mucosal anesthesia with luminal lidocaine reversibly converted the biphasic profile to a monophasic one. Ondansetron (500 microg/kg) blocked the response to 2m5HT with no effect on the profile of the histamine response, whereas pyrilamine (5 mg/kg) blocked the histamine response without affecting the response to 2m5HT. We conclude that histamine-sensitive afferents exist in the rat proximal jejunum that also respond to 5-HT via the 5-HT3 receptor. These fibers appear to be vagal afferents originating in the intestinal mucosa and may be involved in the organization of mast cell-mediated responses.

Neuroanatomy of visceral nociception: vagal and splanchnic afferent

Afferent fibres convey sensory information from the upper gastrointestinal tract to the central nervous system but the nature of this information is different for vagal and spinal pathways. Vagal afferents convey predominantly physiological information while spinal afferents are able to encode noxious events. Because of the different response profiles following activation of these pathways, it is likely that vagal and splanchnic afferents play different roles in mediating sensation.

Effects of duodenal fat, protein or mixed-nutrient infusions on epigastric sensations during sustained gastric distension in healthy humans

Duodenal fat modulates sensory and motor responses to gastric distension and raises plasma cholecystokinin compared with glucose. The effects of protein (also releasing cholecystokinin), or mixed nutrients (with a balanced macronutrient composition), on gastrointestinal sensations in relation to gastric relaxation and plasma cholecystokinin concentrations are not known. The aim of this study was therefore to compare the effects of duodenal infusion of fat, protein or mixed nutrients during sustained gastric distension (mimicking the intragastric presence of food) on these parameters. In 10 healthy subjects, gastric distension to fullness was maintained for 90 min, while gastric volume, sensations and plasma cholecystokinin were monitored during duodenal infusion of isotonic saline or nutrients (2 kcal min-1). During saline infusion, all parameters remained unchanged for 90 min. Initially, only lipid increased plasma cholecystokinin, gastric volume and scores for sensations. Cholecystokinin and gastric volume responses to protein and mixed nutrients were delayed and not associated with significant changes in sensations. In conclusion, the intensity of gastrointestinal sensations is related to, but not entirely explained by, the magnitude in intragastric volume and plasma cholecystokinin changes. Our results offer new insights into the role of dietary nutrient composition in gastrointestinal sensations, and may have implications for the dietary management of digestive symptoms.

Vagal afferent responses to fatty acids of different chain length in the rat

The role of cholecystokinin (CCK) in the effect of dietary lipid on proximal gastrointestinal function and satiety is controversial. Recent work suggests that fatty acid chain length may be a determining factor. We investigated the mechanism by which long- and short-chain fatty acids activate jejunal afferent nerves in rats. Whole mesenteric afferent nerve discharge was recorded in anaesthetized male Wistar rats during luminal perfusion of saline, sodium oleate, and sodium butyrate (both 10 mM). Both fatty acids evoked characteristic afferent nerve responses, distinct from the mechanical response to saline, that were abolished in rats following chronic subdiaphragmatic vagotomy. The effect of oleate was abolished by the CCK-A receptor antagonist Devazepide (0.5 mg/kg), whereas the effect of butyrate persisted despite pretreatment with either Devazepide or a combination of the calcium channel inhibitors nifedipine (1 mg/kg) and the omega-conotoxins GVIA and SVIB (each 25 microg/kg). In summary, long- and short-chain fatty acids activate intestinal vagal afferents by different mechanisms; oleate acts via a CCK-mediated mechanism and butyrate appears to have a direct effect on afferent terminals.

Somatostatin sst(2) receptor-mediated inhibition of mesenteric afferent nerves of the jejunum in the anesthetized rat

BACKGROUND & AIMS

Octreotide inhibits visceral sensations in clinical studies, but the site of action and the receptor type(s) involved are unknown. Our aim was to investigate the effects of octreotide, the selective sst(2) receptor agonist (BIM 23027), and the sst(2) antagonist (Cyanamid154806) on the activity of mesenteric afferent fibers innervating the rat jejunum. Their effects were investigated on baseline discharge, mechanosensitivity, and responses to algesic chemicals.

METHODS

Extracellular multiunit recordings of jejunal afferent nerve firing were made in pentobarbitone-anesthetized (60 mg/kg intraperitoneally) male Wistar rats.

RESULTS

Octreotide and BIM23027 (0.001-100 microg/kg intravenously) each evoked a long-lasting inhibition of baseline discharge, which was blocked by cyanamid 154806 (3 mg/kg) and absent in chronically vagotomized animals. Afferent responses to bradykinin were also inhibited by an sst(2) receptor-mediated mechanism but were unaffected by vagotomy. Ramp distentions of the jejunum evoked a biphasic activation of afferent nerve discharge, the low threshold component of which was attenuated in vagotomized animals. Sst(2) receptor agonists significantly inhibited the mechanosensitivity of spinal, but not vagal, afferents.

CONCLUSIONS

These data suggest that activation of somatostatin sst(2) receptors inhibit populations of mesenteric afferents likely to be involved in nociceptive transmission.

Modulation of gastric motor activity by a centrally acting stimulus, circular vection, in humans

The aims of this study were to investigate gastric motor correlates of vection, a centrally acting stimulus, and relate these responses to the induction of motion sickness symptoms. Antral contractile activity and gastric volume retained after a liquid nutrient meal (600 ml) were assessed by magnetic resonance imaging in healthy subjects during two different protocols. Vection was induced by an optokinetic drum, and subjects repeatedly rated the intensity of vection and nausea on 0-10 analog scales. Vection delayed gastric emptying [99% (89-102%) [median (interquartile ranges)] of volume retained at 28 min; control situation: 79% (69-81%), P < 0.05]. Antral contractile activity followed a distinct time course of rapid decrease [-64% (-72 to -59%) change from baseline activity] immediately after onset of drum rotation followed by gradual recovery upon withdrawal of the stimulus. No relationship was found between the severity of nausea and inhibition of gastric emptying or antral contractile activity. The inhibition of antral contractile activity appears to be a good measure of the peripheral response to vection but is probably independent of subjective symptom induction.

Receptors and transmission in the brain-gut axis: potential for novel therapies. I. Receptors on visceral afferents

Visceral afferents are the information superhighway from the gut to the central nervous system. These sensory nerves express a wide range of membrane receptors that can modulate their sensitivity. In this themes article, we concentrate on those receptors that enhance the excitability of visceral afferent neurons. Some receptors are part of a modality-specific transduction pathway involved in sensory signaling. Others, which are activated by substances derived from multiple cellular sources during ischemia, injury, or inflammation, act in a synergistic fashion to cause acute or chronic sensitization of the afferent nerves to mechanical and chemical stimuli. Such hypersensitivity is the hallmark of conditions such as irritable bowel syndrome. Accordingly, these receptors represent a rational target for drug treatments aimed at attenuating both the inappropriate visceral sensation and the aberrant reflex activity that are the foundation for alterations in bowel function.

Modulation of gastric distension-induced sensations by small intestinal receptors

Duodenal lipid exacerbates gastrointestinal sensations during gastric distension. Using luminal application of the local anesthetic benzocaine, we investigated the role of intestinal receptors in the induction of these sensations. Nine healthy subjects were studied on five occasions, during which isotonic saline or 20% lipid (2 kcal/min), combined with (duodenal or jejunal) 0.75% benzocaine or vehicle at 2.5 ml/min, was infused intraduodenally before and during gastric distension. Intragastric pressures and volumes, gastrointestinal sensations, and plasma CCK levels were determined. Duodenal lipid combined with vehicle increased gastric volume (in ml: saline, -10 +/- 18; lipid/vehicle, 237 +/- 30) and plasma CCK [mean levels (pmol/l): saline, 2.0 +/- 0. 2; lipid/vehicle, 8.0 +/- 1.6] and, during distensions, induced nausea (scores: saline, 3 +/- 2: lipid/vehicle, 58 +/- 19) and decreased pressures at which fullness and discomfort occurred. Duodenal but not jejunal benzocaine attenuated the effect of lipid on gastric volume, plasma CCK, and nausea during distension (135 +/- 38 and 216 +/- 40 ml, 4.6 +/- 0.6 pmol/l and not assessed, and 37 +/- 12 and 64 +/- 21 for lipid + duodenal benzocaine and lipid + jejunal benzocaine, respectively) and on pressures for sensations. In conclusion, intestinal receptors modulate gastrointestinal sensations associated with duodenal lipid and gastric distension. There is also the potential for local neural mechanisms to regulate CCK release and thereby reduce afferent activation indirectly.