Modulation of murine gastric vagal afferent mechanosensitivity by neuropeptide W

AIM

Neuropeptide W (NPW) is an endogenous ligand for the receptors GPR7 and GPR8 and is involved in central regulation of energy homeostasis. NPW in the periphery is found in gastric gastrin (G) cells. In the stomach, energy intake is influenced by vagal afferent signals, so we aimed to determine the effect of NPW on mechanosensitive gastric vagal afferents under different feeding conditions.

METHODS

Female C57BL/6 mice (N > 10 per group) were fed a standard laboratory diet (SLD), high-fat diet (HFD) or were food restricted. The relationship between NPW immunopositive cells and gastric vagal afferent endings was determined by anterograde tracing and NPW immunohistochemistry. An in vitro gastro-oesophageal preparation was used to determine the functional effects of NPW on gastric vagal afferents. Expression of NPW in the gastric mucosa and GPR7 in whole nodose ganglia was determined by quantitative RT-PCR (QRT-PCR). The expression of GPR7 in gastric vagal afferent neurones was determined by retrograde tracing and QRT-PCR.

RESULTS

Neuropeptide W immunoreactive cells were found in close proximity to traced vagal afferents. NPW selectively inhibited responses of gastric vagal tension receptors to stretch in SLD but not HFD or fasted mice. In the nodose ganglia, GPR7 mRNA was specifically expressed in gastric vagal afferent neurones. In fasted mice gastric mucosal NPW and nodose GPR7, mRNA was reduced compared with SLD. A HFD had no effect on gastric NPW mRNA, but down-regulated nodose GPR7 expression.

CONCLUSION

Neuropeptide W modulates gastric vagal afferent activity, but the effect is dynamic and related to feeding status.

Visceral pain readouts in experimental medicine

Visceral pain is studied at the level of the primary afferent fiber, spinal cord, subcortical, and cortical levels electrophysiologically and using brain imaging, which provides an objective measure of excitation at each level. However, correlation of these with actual perception of pain in conscious animal models has been problematic, and we rely on indirect measures in most preclinical research. The main method is electromyographic recording of abdominal muscle contractions in response to colorectal distension (CRD), which may reflect reflexes set up at several levels of the above pathway. Several experimental treatments for visceral pain have failed in clinical trials, possibly because of failure to translate from preclinical observations on CRD responses in animals to perception of spontaneous events in patients. Therefore, we need more objective outcomes. In this NGM issue, Hultin et al. show feasibility of routine recordings of cortical evoked electrical potentials (CEP) using implanted cranial electrodes in response to graded CRD in rats. CEP comprised three temporal components with latencies of approximately 20-50 ms, 90-180 ms, and 300 ms, which were reproducible and graded in intensity and latency with distension pressure. From this basic study it is clear that colorectal evoked potentials can be recorded reliably in awake rats and may serve as an objective marker for centrally projecting visceral sensory signals in rodents. It remains to be seen how these responses are affected by drugs under development for clinical management of visceral pain, and if there is improved translation.

Different in vitro and in vivo profiles of substituted 3-aminopropylphosphinate and 3-aminopropyl(methyl)phosphinate GABA(B) receptor agonists as inhibitors of transient lower oesophageal sphincter relaxation

BACKGROUND AND PURPOSE

Gastro-oesophageal reflux is predominantly caused by transient lower oesophageal sphincter relaxation (TLOSR) and GABA(B) receptor stimulation inhibits TLOSR. Lesogaberan produces fewer CNS side effects than baclofen, which has been attributed to its affinity for the GABA transporter (GAT), the action of which limits stimulation of central GABA(B) receptors. To understand the structure-activity relationship for analogues of lesogaberan (3-aminopropylphosphinic acids), and corresponding 3-aminopropyl(methyl)phosphinic acids, we have compared representatives of these classes in different in vitro and in vivo models.

EXPERIMENTAL APPROACH

The compounds were characterized in terms of GABA(B) agonism in vitro. Binding to GATs and cellular uptake was done using rat brain membranes and slices respectively. TLOSR was measured in dogs, and CNS side effects were evaluated as hypothermia in mice and rats.

KEY RESULTS

3-Aminopropylphosphinic acids inhibited TLOSR with a superior therapeutic index compared to 3-aminopropyl(methyl)phosphinic acids. This difference was most likely due to differential GAT-mediated uptake into brain cells of the former but not latter. In agreement, 3-aminopropyl(methyl)phosphinic acids were much more potent in producing hypothermia in rats even when administered i.c.v.

CONCLUSIONS AND IMPLICATIONS

An enhanced therapeutic window for 3-aminopropylphosphinic acids compared with 3-aminopropyl(methyl)phosphinic acids with respect to inhibition of TLOSR was observed and is probably mechanistically linked to neural cell uptake of the former but not latter group of compounds. These findings offer a platform for discovery of new GABA(B) receptor agonists for the treatment of reflux disease and other conditions where selective peripheral GABA(B) receptor agonism may afford therapeutic effects.

Post-inflammatory colonic afferent sensitisation: different subtypes, different pathways and different time courses

OBJECTIVE

Intestinal infection evokes hypersensitivity in a subgroup of patients with irritable bowel syndrome (IBS) long after healing of the initial injury. Trinitrobenzene sulfonic acid (TNBS)-induced colitis in rodents likewise results in delayed maintained hypersensitivity, regarded as a model of some aspects of IBS. The colon and rectum have a complex sensory innervation, comprising five classes of mechanosensitive afferents in the splanchnic and pelvic nerves. Their plasticity may hold the key to underlying mechanisms in IBS. Our aim was therefore to determine the contribution of each afferent class in each pathway towards post-inflammatory visceral hypersensitivity.

DESIGN

TNBS was administered rectally and mice were studied after 7 (acute) or 28 (recovery) days. In vitro preparations of mouse colorectum with attached pelvic or splanchnic nerves were used to examine the mechanosensitivity of individual colonic afferents.

RESULTS

Mild inflammation of the colon was evident acutely which was absent at the recovery stage. TNBS treatment did not alter proportions of the five afferent classes between treatment groups. In pelvic afferents little or no difference in response to mechanical stimuli was apparent in any class between control and acute mice. However, major increases in mechanosensitivity were recorded from serosal afferents in mice after recovery, while responses from other subtypes were unchanged. Both serosal and mesenteric splanchnic afferents were hypersensitive at both acute and recovery stages.

CONCLUSIONS

Colonic afferents with high mechanosensory thresholds contribute to inflammatory hypersensitivity, but not those with low thresholds. Pelvic afferents become involved mainly following recovery from inflammation, whereas splanchnic afferents are implicated during both inflammation and recovery.

Post-inflammatory modification of colonic afferent mechanosensitivity

1. The present review discusses interactions between the immune and nervous systems in post-infectious irritable bowel syndrome (PI-IBS). 2. Visceral pain is the single symptom that most affects the quality of life of patients with irritable bowel syndrome (IBS), yet it is the least successfully managed. An underlying hypersensitivity of colonic afferents to mechanical stimuli has long been implicated in visceral pain in IBS, but little more is known of the physiological aetiology. 3. The PI-IBS patients are a cohort of IBS patients who attribute their symptoms to a preceding gastrointestinal infection by pathogens such as Campylobacter or Salmonella. Current evidence suggests that the immune system remains activated in these patients and contributes to their visceral hypersensitivity. This is characterized by a shift in the phenotype of circulating immune cells towards a Type 1 (Th1 predominating) state. Products from these immune cells sensitize colonic afferents to mechanical stimuli. 4. Rectal instillation of trinitrobenzene sulphonic acid induces a Th1-mediated inflammatory response, consistent with clinical observations in PI-IBS. The visceral hypersensitivity observed in this model is biphasic, with an initial onset characterized by visceral hypersensitivity correlating with histological damage followed by a delayed phase that occurs after histological recovery. Interestingly, this chronic visceral hypersensitivity is mediated by afferents in closest apposition to blood vessels, but furthest from the initial site of damage. 5. Both clinical and experimental evidence indicates that chronic dysregulation of the immune system induces visceral afferent hypersensitivity and, therefore, may be the central mechanism underlying PI-IBS.

Expression of taste molecules in the upper gastrointestinal tract in humans with and without type 2 diabetes

OBJECTIVE

Nutrient feedback from the small intestine modulates upper gastrointestinal function and energy intake; however, the molecular mechanism of nutrient detection is unknown. In the tongue, sugars are detected via taste T1R2 and T1R3 receptors and signalled via the taste G-protein alpha-gustducin (G alpha(gust)) and the transient receptor potential ion channel, TRPM5. These taste molecules are also present in the rodent small intestine, and may regulate gastrointestinal function.

SUBJECTS AND METHODS

Absolute transcript levels for T1R2, T1R3, G alpha(gust) and TRPM5 were quantified in gastrointestinal mucosal biopsies from subjects with and without type 2 diabetes; immunohistochemistry was used to locate G alpha(gust). Effects of luminal glucose on jejunal expression of taste molecules were also quantified in mice.

RESULTS

T1R2, T1R3, G alpha(gust) and TRPM5 were preferentially expressed in the proximal small intestine in humans, with immunolabelling for G alpha(gust) localised to solitary cells dispersed throughout the duodenal villous epithelium. Expression of T1R2, T1R3, TRPM5 (all p<0.05) and G alpha(gust) (p<0.001) inversely correlated with blood glucose concentration in type 2 diabetes subjects but, as a group, did not differ from control subjects. Transcript levels of T1R2 were reduced by 84% following jejunal glucose perfusion in mice (p<0.05).

CONCLUSIONS

Taste molecules are expressed in nutrient detection regions of the proximal small intestine in humans, consistent with a role in "tasting". This taste molecule expression is decreased in diabetic subjects with elevated blood glucose concentration, and decreased by luminal glucose in mice, indicating that intestinal "taste" signalling is under dynamic metabolic and luminal control.

A novel preparation to study rat pancreatic spinal and vagal mechanosensitive afferents in vitro

The management of pancreatic pain is a significant clinical problem so understanding of how sensory signals are generated in pancreatic tissue is fundamental. We aimed to characterize mechanosensitive and chemosensitive properties of pancreatic spinal and vagal afferents in vitro. Spinal and vagal afferent preparations from Sprague-Dawley rats were established incorporating the left splanchnic nerve or vagus nerves respectively. The common bile duct was cannulated for distension of the pancreatic duct with fluid. Nerve discharge evoked by blunt probing, duct distension or electrical stimulation was obtained from teased nerve bundles using standard extra-cellular recording. Discharge from 197 spinal afferent bundles was recorded, of which 57% displayed spontaneous activity. Blunt probing revealed 61 mechanosensitive receptive fields which were associated primarily with arteries/blood vessels (33/61) and the parenchyma (22/61). All mechanosensitive responses were slowly adapting, with 33% continuing to discharge after termination of the stimulus and 60% displaying a response threshold <10 g. Application of chemical mediators (bradykinin, histamine, 5-hydroxytryptamine, cholecystokinin octapeptide) evoked a response from 31/57 units, with 33% excitatory and 23% inhibitory. Spontaneous discharge was recorded from 72% of 135 vagal bundles. Mechanosensitive receptive fields were not identified in the pancreas but were evident in adjacent organs. No spinal or vagal afferent response to duct distension was obtained. In conclusion, pancreatic mechanosensitive spinal afferents are common, in contrast to pancreatic mechanosensitive vagal afferents indicating that pancreatic sensory innervation is predominantly spinal. Chemosensitive spinal afferent nerve endings are present in the pancreas and respond to a variety of inflammatory and physiological mediators.

Chemical coding and central projections of gastric vagal afferent neurons

Vagal afferents that innervate gastric muscle or mucosa transmit distinct sensory information from their endings to the nucleus of the tractus solitarius (NTS). While these afferent subtypes are functionally distinct, no neurochemical correlate has been described and it is unknown whether they terminate in different central locations. This study aimed to identify gastric vagal afferent subtypes in the nodose ganglion (NG) of ferrets, their terminal areas in NTS and neurochemistry for isolectin-B4 (IB4) and calcitonin gene-related peptide (CGRP). Vagal afferents were traced from gastric muscle or mucosa and IB4 and CGRP labelling assessed in NG and NTS. 7 +/- 1% and 6 +/- 1% of NG neurons were traced from gastric muscle or mucosa respectively; these were more likely to label for CGRP or for both CGRP and IB4 than other NG neurons (P < 0.01). Muscular afferents were also less likely than others to label with IB4 (P < 0.001). Less than 1% of NG neurons were traced from both muscle and mucosa. Central terminals of both afferent subtypes occurred in the subnucleus gelatinosus of the NTS, but did not overlap completely. This region also labelled for CGRP and IB4. We conclude that while vagal afferents from gastric muscle and mucosa differ little in their chemical coding for CGRP and IB4, they can be traced selectively from their peripheral endings to NG and to overlapping and distinct regions of NTS. Thus, there is an anatomical substrate for convergent NTS integration for both types of afferent input.

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.

Pancreatobiliary afferent recordings in the anaesthetised Australian possum

The sensory innervation to the pancreatobiliary system is poorly characterized. Afferent signals from the gastrointestinal tract and biliary tree are transmitted to the central nervous system via the vagus and spinal nerves. We aimed to record afferent discharge in order to characterize the vagal and splanchnic afferent signals from the possum upper gastrointestinal tract, biliary tree and pancreas. In 21 anaesthetised possums nerve fibres were teased from the vagus or splanchnic nerve for multi-unit recording. Mechanical stimuli consisted of balloon distension of the gallbladder and duodenum (2-7 ml) and fluid distension (0-20 mm Hg) of the bile or pancreatic ducts. Approximately 60% of fibres from all nerves displayed spontaneous discharge. Spinal afferent responses to mechanical stimuli were infrequent (n=13). Increased discharge occurred in response to duodenal (12/99 fibres) or gallbladder (7/96 fibres) distension, but not to bile duct (0/73 fibres) or pancreatic duct (0/51 fibres) distension. Vagal afferent responses to distension of the duodenum or stomach (5-30 ml) were more common (n=8). Increased discharge was recorded in response to duodenal (49/134 fibres), or gastric (22/70 fibres) distension. Responses to gallbladder distension were less frequent (6/99 fibres) and as with the spinal afferent no response to bile duct (0/66) or pancreatic duct (0/70) distension were recorded. We conclude that mechanosensitive afferents in the pancreatobiliary system are relatively rare, particularly within the ducts, and/or that they are adapted to monitor stimuli other than luminal distension.

Inhibition of mechanosensitivity in visceral primary afferents by GABAB receptors involves calcium and potassium channels

GABA(B) receptors inhibit mechanosensitivity of visceral afferents. This is associated with reduced triggering of events that lead to gastro-esophageal reflux, with important therapeutic consequences. In other neuronal systems, GABA(B) receptor activation may be linked via G-proteins to reduced N-type Ca(2+) channel opening, increased inward rectifier K(+) channel opening, plus effects on a number of intracellular messengers. Here we aimed to determine the role of Ca(2+) and K(+) channels in the inhibition of vagal afferent mechanoreceptor function by the GABA(B) receptor agonist baclofen. The responses of three types of ferret gastro-esophageal vagal afferents (mucosal, tension and tension mucosal receptors) to graded mechanical stimuli were investigated in vitro. The effects of baclofen (200 microM) alone on these responses were quantified, and the effects of baclofen in the presence of the G-protein-coupled inward rectifier potassium channel blocker Rb(+) (4.7 mM) and/or the N-type calcium channel blocker omega-conotoxin GVIA (0.1 microM). Baclofen inhibition of mucosal receptor mechanosensitivity was abolished by both blockers. Its inhibitory effect on tension mucosal receptors was partly reduced by both. The inhibitory effect of baclofen on tension receptors was unaffected. The data indicate that the inhibitory action of GABA(B) receptors is mediated via different pathways in mucosal, tension and tension mucosal receptors via mechanisms involving both N-type Ca(2+) channels and inwardly rectifying K(+) channels and others.

Activation of splanchnic and pelvic colonic afferents by bradykinin in mice

BACKGROUND

Lumbar splanchnic (LSN) and sacral pelvic (PN) nerves convey different mechanosensory information from the colon to the spinal cord. Here, we determined whether these pathways differ also in their chemosensitivity to bradykinin.

METHODS

Using a novel in vitro mouse colon preparation, serosal afferents were recorded from the LSN and PN and distinguished based on their mechanosensitivity to von Frey filaments (70-4000 mg) and insensitivity to colonic stretch (1-5 g) or fine mucosal stroking (10 mg). Bradykinin was applied into a ring around mechanoreceptive fields.

RESULTS

The LSN and PN afferents had different dynamic responses to mechanical stimuli: PN afferents required lower intensity stimuli, evoked larger responses, and displayed more maintained responses than LSN afferents. Bradykinin (1 micromol L-1) excited 66% (27 of 41) of LSN afferents. Responses to probing were potentiated after bradykinin. The concentration-dependent (EC50: 0.16 micromol L-1) response was reversed by the B2-receptor antagonist HOE-140 (10 nmol L-)). Twelve bradykinin responsive afferents were mechanically insensitive. More LSN serosal afferents responded to bradykinin than PN afferents (11%, P<0.001) , with larger responses (P<0.05). No mechanically insensitive PN afferents were recruited by bradykinin.

CONCLUSIONS

Bradykinin potently stimulates most splanchnic serosal afferents via B2-receptors, but few pelvic afferents. Mechanically insensitive afferents recruited by bradykinin are exclusive to the LSN.

Different contributions of ASIC channels 1a, 2, and 3 in gastrointestinal mechanosensory function

AIMS

Members of the acid sensing ion channel (ASIC) family are strong candidates as mechanical transducers in sensory function. The authors have shown that ASIC1a has no role in skin but a clear influence in gastrointestinal mechanotransduction. Here they investigate further ASIC1a in gut mechanoreceptors, and compare its influence with ASIC2 and ASIC3.

METHODS AND RESULTS

Expression of ASIC1a, 2, and 3 mRNA was found in vagal (nodose) and dorsal root ganglia (DRG), and was lost in mice lacking the respective genes. Recordings of different classes of splanchnic colonic afferents and vagal gastro-oesophageal afferents revealed that disruption of ASIC1a increased the mechanical sensitivity of all afferents in both locations. Disruption of ASIC2 had varied effects: increased mechanosensitivity in gastro-oesophageal mucosal endings, decreases in gastro-oesophageal tension receptors, increases in colonic serosal endings, and no change in colonic mesenteric endings. In ASIC3-/- mice, all afferent classes had markedly reduced mechanosensitivity except gastro-oesophageal mucosal receptors. Observations of gastric emptying and faecal output confirmed that increases in mechanosensitivity translate to changes in digestive function in conscious animals.

CONCLUSIONS

These data show that ASIC3 makes a critical positive contribution to mechanosensitivity in three out of four classes of visceral afferents. The presence of ASIC1a appears to provide an inhibitory contribution to the ion channel complex, whereas the role of ASIC2 differs widely across subclasses of afferents. These findings contrast sharply with the effects of ASIC1, 2, and 3 in skin, suggesting that targeting these subunits with pharmacological agents may have different and more pronounced effects on mechanosensitivity in the viscera.

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.

The pharmacology of gastrointestinal nociceptive pathways

The principal conscious sensations that arise from the gastrointestinal tract are discomfort and pain. Chronic visceral discomfort and pain are generally managed poorly with currently available pharmacological agents. Receptors and ion channels present on extrinsic visceral primary afferent (sensory) neurons are targets for the development of new pharmacological strategies for control of visceral pain.

Vagal mechanoreceptors and chemoreceptors in mouse stomach and esophagus

We used a novel in vitro mouse vagus-gastro-esophageal preparation to study the properties of peripheral vagal afferent endings. We found two types of mechanoreceptive fiber, mucosal receptors and tension receptors. These were distinguished by their sensitivity to mucosal stroking with von Frey hairs and circular tension applied via a claw-cantilever system. A comparison was made with gastro-esophageal afferents found in a similar preparation of ferret tissue. Responses of mouse tension receptors to circular tension were significantly greater than ferret tension and tension/mucosal receptors. Similarly the responses of mouse mucosal receptors to mucosal stroking were significantly greater than ferret mucosal and tension/mucosal receptors. Forty-seven percent of mouse mucosal receptors and 50% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. These included alpha,beta-methylene ATP (10(-6) to 10(-3) M), 5-hydroxytryptamine (10(-6) to 10(-3) M), and hydrochloric acid (10(-2) to 10(-1) M). Drug responses were concentration dependent. One hundred percent of mucosal receptors and 61% of tension receptors tested responded to bile (1:8 to 1:1 dilution). A third type of fiber was recruited by bile. These fibers were mechanically insensitive and silent prior to bile exposure. In conclusion, we have shown three types of gastro-esophageal vagal afferent fibers in the mouse: mucosal mechanoreceptors, tension receptors, and specific chemoreceptors activated by bile.

GABA(B)R expressed on vagal afferent neurones inhibit gastric mechanosensitivity in ferret proximal stomach

GABA(B)-receptor (GABA(B)R) agonists reduce transient lower esophageal sphincter relaxation (TLESR) and reflux episodes through an action on vagal pathways. In this study, we determined whether GABA(B)R are expressed on vagal afferent neurones and whether they modulate distension-evoked discharge of vagal afferents in the isolated stomach. Vagal mehanoreceptor activity was recorded following distensions of the isolated ferret proximal stomach before and after perfusion with the GABA(B)R-selective agonists baclofen and 3-aminopropylphosphinic acid (3-APPiA). Retrograde labeling and immunohistochemistry were used to identify GABA(B)R located on vagal afferent neurones in the nodose ganglia. Vagal afferent fibers responded to isovolumetric gastric distension with an increase in discharge. The GABA(B)-receptor agonists baclofen (5 x 10(-5) M) and 3-APPiA (10(-6) to 10(-5) M) but not muscimol (GABA(A)-selective agonist: 1.3 x 10(-5) M) significantly decreased afferent distension-response curves. The effect of baclofen (5 x 10(-5) M) was reversed by the GABA(B)-receptor antagonist CGP 62349 (10(-5) M). Over 93% of retrogradely labeled gastric vagal afferents in the nodose ganglia expressed immunoreactivity for the GABA(B)R. GABA(B)R expressed on vagal afferent fibers directly inhibit gastric mechanosensory activity. This is likely a contributing mechanism to the efficacy of GABA(B)-receptor agonists in reducing TLESR and reflux episodes in vivo.

Receptors and transmission in the brain-gut axis: potential for novel therapies. IV. GABA(B) receptors in the brain-gastroesophageal axis

GABA(B) receptors are inhibitory G protein-coupled receptors that are commonly associated with presynaptic inhibition of transmitter release in the central nervous system. In the brain-gastroesophageal axis, a role has recently been demonstrated for GABA(B) receptors on extrinsic afferent endings within the stomach and esophagus, where they reduce mechanosensitivity. This action is compounded by inhibition of communication centrally from these afferents in the brain stem and within central circuits. There is a final peripheral action on the motor pathway where GABA(B) receptors reduce output of acetylcholine from vagal preganglionic motoneurons. These potent, multiple actions of GABA(B) receptors may have therapeutic benefit by reducing the triggering of transient lower esophageal relaxations, which are the major cause of gastroesophageal reflux. An important clinical application is therefore emerging for this recent discovery.

Colonic mechanoreceptor inputs to rat lumbo-sacral dorsal horn neurones: distribution, thresholds and chemosensory modulation

In order to better understand the central processing of visceral sensory information, we studied the responses of lumbo-sacral dorsal horn (L4-S1) neurones to colonic stimuli in anaesthetized rats. Twenty-four neurones responded to distal colonic distension with a 2.5-cm balloon; six of these were tested with proximal colonic distension, to which none responded. All neurones tested responded to somatic non-noxious inputs (tail movement). Responses to colonic distension were excitatory (n=22) or inhibitory (n=2). Sixteen neurones responded at a threshold of 20 mmHg or less, five at 20-40 mmHg, and three at 40-80 mmHg. Three of 10 neurones tested showed increased responses to colonic distension after intraluminal perfusion with bile. Bile itself did not evoke a response. We conclude that lumbo-sacral spinal neurones selectively receive mechanosensory inputs from the distal colon. Neurones respond at thresholds within and above the physiological range. Dorsal horn neurones receiving colonic mechanosensory inputs are not directly modulated by chemosensory inputs, but their responsiveness to distension may be augmented.

Vagal and spinal mechanosensors in the rat stomach and colon have multiple receptive fields

Mechano- and chemosensitive extrinsic primary afferents innervating the gastrointestinal tract convey important information regarding the state of ingested nutrients and specific motor patterns to the central nervous system via splanchnic and vagal nerves. Little is known about the organization of peripheral receptive sites of afferents and their correspondence to morphologically identified terminal structures. Mechano- and chemosensory characteristics and receptive fields of single vagal fibers innervating the stomach as well as lumbar splanchnic nerves innervating the distal colon were identified using an in vitro perifusion system. Twenty-three (17%) of one-hundred thirty-six vagal units identified were found to have multiple, punctate receptive fields, up to 35 mm apart, and were distributed throughout the stomach. Evidence was based on similarity of generated spike forms, occlusion, and latency determinations. Most responded with brief bursts of activity to mucosal stroking with von Frey hairs (10-200 mg) but not to stretch, and 32% responded to capsaicin (10(-5) M). They were classified as rapidly adapting mucosal receptors. Four (8%) of fifty-three single units recorded from the lumbar splanchnic nerve had more than one, punctate receptive field in the distal colon, up to 40 mm apart. They responded to blunt probing, particularly from the serosal side, and variously to chemical stimulation with 5-hydroxytryptamine and capsaicin. We conclude that a proportion of gastrointestinal mechanosensors has multiple receptive fields and suggest that they integrate mechanical and chemical information from an entire organ, constituting the generalists in visceral sensation.

GABA(B) receptors on vagal afferent pathways: peripheral and central inhibition

To investigate GABA(B) receptors along vagal afferent pathways, we recorded from vagal afferents, medullary neurons, and vagal efferents in ferrets. Baclofen (7-14 micromol/kg i.v.) reduced gastric tension receptor and nucleus tractus solitarii neuronal responses to gastric distension but not gastroduodenal mucosal receptor responses to cholecystokinin (CCK). GABA(B) antagonists CGP-35348 or CGP-62349 reversed effects of baclofen. Vagal efferents showed excitatory and inhibitory responses to distension and CCK. Baclofen (3 nmol i.c.v. or 7-14 micromol/kg i.v.) reduced both distension response types but reduced only inhibitory responses to CCK. CGP-35348 (100 nmol i.c.v. or 100 micromol/kg i.v.) reversed baclofen's effect on distension responses, but inhibitory responses to CCK remained attenuated. They were, however, reversed by CGP-62349 (0.4 nmol i.c.v.). In conclusion, GABA(B) receptors inhibit mechanosensitivity, not chemosensitivity, of vagal afferents peripherally. Mechanosensory input to brain stem neurons is also reduced centrally by GABA(B) receptors, but excitatory chemosensory input is unaffected. Inhibitory mechano- and chemosensory inputs to brain stem neurons (via inhibitory interneurons) are both reduced, but the pathway taken by chemosensory input involves GABA(B) receptors that are insensitive to CGP-35348.

Vagal ganglionic and nonadrenergic noncholinergic neurotransmission to the ferret lower oesophageal sphincter

In the present study we aimed to discretely characterise ganglionic and neuroeffector transmission to the ferret lower oesophageal sphincter (LOS) using a novel preparation of LOS muscle with intact vagal innervation in conjunction with isolated LOS muscle strips. In this way we could compare vagally mediated LOS relaxation with that of enteric inhibitory motorneurones which were directly stimulated. Preparations of LOS muscle, with or without attached vagus nerves, were dissected from adult ferrets and maintained under preload in organ baths, where LOS muscle developed spontaneous tone. LOS relaxations in response to vagal stimulation (0.5-5 Hz, 30 V) were recorded, alone and following pretreatment with tetrodotoxin (TTX), hexamethonium (Hex), Hex and atropine and NG-nitro-L-arginine (L-NNA). Direct activation of enteric inhibitory motorneurones was performed via electrical field stimulation (EFS). Vagal stimulation elicited frequency-dependent relaxations of the LOS that were abolished by tetrodotoxin (1 microM) and markedly reduced following L-NNA pretreatment (100 microM), but unaltered following pretreatment with the selective VIP or PACAP receptor antagonists VIP (10-28) or PACAP (6-38), respectively (each at 5 microM). The potent NOS inhibitor S-methyl-L-thiocitrulline (100 microM) inhibited LOS relaxation to the same degree at 5 Hz. Hex alone (500 microM) reduced maximal relaxation by 50%; in combination with atropine (2 microM), relaxation was almost abolished. In isolated LOS muscle strips, neither VIP (10-28) nor PACAP (6-38) altered EFS-induced relaxation. Taken together, these results suggest ganglionic neurotransmission to the ferret LOS occurs mainly through a combination of nicotinic and muscarinic receptors and utilises nitroxidergic enteric inhibitory motorneurones to relax the LOS. Moreover, LOS relaxation due to direct activation of inhibitory motorneurones also utilises primarily nitric oxide and other as yet undefined neurotransmitters. Neither VIP nor PACAP are involved in vagally mediated or direct enteric neuronally stimulated LOS relaxation in the ferret.

Vagal neurotransmission to the ferret lower oesophageal sphincter: inhibition via GABA(B) receptors

GABA(B) receptors modulate the function of the lower oesophageal sphincter (LOS) in vivo by inhibiting neurotransmitter release in the vagal pathway controlling LOS relaxation. We aimed to determine whether this effect was mediated peripherally on vagal motor outflow to the ferret LOS in vitro. The LOS, with intact vagal innervation, was prepared from adult ferrets and LOS tension measured. Vagal stimulation (0.5 - 10 Hz, 30 V) evoked a tetrodotoxin-sensitive, frequency-dependent relaxation. Both GABA (3x10(-4) M) and (+/-)baclofen (2x10(-4) M) inhibited vagally-stimulated LOS relaxation. The potent GABA(B) receptor-selective agonist 3-APPA dose-dependently inhibited vagally-stimulated LOS relaxation, with an EC(50) value of 0.7 microM Decreased responses following vagal stimulation in the presence of (+/-)baclofen or 3-APPA were reversed with the potent GABA(B) receptor antagonist CGP 62349. Neither CGP 62349 nor muscimol (GABA(A) receptor agonist) alone affected LOS responses following vagal stimulation. Agonists of other G protein-coupled receptors (clonidine (alpha(2)-adrenoceptor) (5x10(-6) M), U50488 (kappa opioid) (10(-5) M), neuropeptide Y (10(-6) M)) did not affect vagally-mediated LOS relaxation. The present study supports a discrete presynaptic inhibitory role for GABA(B) receptors on vagal preganglionic fibres serving inhibitory motorneurones in the ferret LOS.

Divergence of mucosal and motor effects of insulin-like growth factor (IGF)-I and LR3IGF-I on rat isolated ileum following abdominal irradiation

BACKGROUND AND AIMS

In addition to its beneficial effects on small intestinal mucosal development and repair, insulin-like growth factor (IGF)-I has also been reported to improve neural function in toxic neuropathies. It has recently been recognized that enteric neural abnormalities contribute to the small intestinal dysmotility observed during and after abdominal radiotherapy for gynecological and pelvic malignancy. The aim of the present study was to evaluate the effects of IGF-I (5 mg/kg per day) and the more potent analog LR3IGF-I (5 mg/kg per day) on neurally mediated ileal dysmotility following irradiation.

METHODS

Intestinal motor activity was recorded from 6-8 cm segments of explanted rat ileum using a miniaturized manometric technique during arterial perfusion with oxygenated fluorocarbon solution. Studies were performed 4 days after treatment with 10 Gy abdominal irradiation. At the time of irradiation, all rats underwent implantation of an osmotic mini-pump that contained 100 mmol/L acetic acid vehicle (n = 8), IGF-I (n = 8) or LR3IGF-I (n = 7). For each experiment, the total number of pressure waves, high-amplitude long-duration (defined as > 20 mmHg, > 6 s; HALD) pressure waves and long bursts (> 20) of pressure waves were determined. Ileal segments from 12 non-irradiated rats were used as controls for manometric studies. In radiotherapy treated animals, the degree of mucosal damage was determined using a standardized histologic scoring system.

RESULTS

The HALD pressure waves were infrequent in non-irradiated rats but occurred in all irradiated animals. Insulin-like growth factor-I and LR3IGF-I had no effect on the frequency, amplitude or migration characteristics of HALD pressure waves compared with vehicle. Histologic damage was reduced in animals that received IGF-I and LR3IGF-I compared with vehicle-treated animals.

CONCLUSIONS

In radiation enteritis, IGF-I has no effect on neurally mediated small intestinal dysmotility while improving mucosal histology. The disparity between these results suggests that parallel but separate pathologic processes underlie mucosal and motor abnormalities in radiation enteritis.

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

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

Triggering of transient LES relaxations in ferrets: role of sympathetic pathways and effects of baclofen

Activation of gastric vagal mechanoreceptors by distention is thought to be the trigger for transient lower esophageal sphincter relaxations (TLESR), which lead to gastroesophageal reflux. The contribution of higher-threshold gastric splanchnic mechanoreceptors is uninvestigated. GABA(B) receptor agonists, including baclofen, potently reduce triggering of TLESR by low-level gastric distention. We aimed to determine first whether this effect of baclofen is maintained at high-level distention and second the role of splanchnic pathways in triggering TLESR. Micromanometric/pH studies in conscious ferrets showed that intragastric glucose infusion (25 ml) increased triggering of TLESR and reflux. Both were significantly reduced by baclofen (7 micromol/kg ip) (P < 0.05). When 40 ml of air was added to the glucose infusion, more TLESR occurred than with glucose alone (P < 0.01). These were also reduced by baclofen (P < 0.001). TLESR after glucose/air infusion were assessed before and after splanchnectomy (2-4, 9-11, and 23-25 days), which revealed no change. Baclofen inhibits TLESR after both low- and high-level gastric distention. Splanchnic pathways do not contribute to increased triggering of TLESR by high-level gastric distention.

Neuromuscular function of the human lower oesophageal sphincter in reflux disease and Barrett's oesophagus

BACKGROUND

Columnar lined (Barrett's) oesophagus is often considered a sequel to chronic severe reflux disease. Aberrant lower oesophageal sphincter (LOS) motility associated with Barrett's oesophagus includes reduced basal LOS pressures. The aim of this study was to characterise neuromuscular function of the LOS in normal (squamous cell carcinoma (SCC) with uninvolved LOS) and reflux affected (Barrett's) oesophagus in vitro.

METHODS

Strips of LOS muscle were prepared at biopsy following oesophagectomy from 16 patients with SCC and seven patients with oesophageal adenocarcinoma and Barrett's oesophagus associated with a history of reflux disease. LOS smooth muscle responses were recorded in response to electrical field stimulation (EFS), potassium chloride (KCl), DMPP, isoprenaline, capsaicin, bethanechol, and tachykinins.

RESULTS

Basal LOS tone and LOS relaxations in response to isoprenaline, EFS, and DMPP were not significantly altered in the Barrett's group. After tetrodotoxin pretreatment, responses to KCl and DMPP were significantly reduced in the SCC but not in Barrett's LOS. Maximal contraction in response to bethanechol was significantly decreased in Barrett's LOS while substance P and NK-2 receptor mediated contraction was unaltered. Capsaicin, NK-1, and NK-3 receptor agonists exerted negligible effects on LOS tone.

CONCLUSIONS

LOS muscle strips from patients with reflux associated Barrett's oesophagus exhibit a reduction in cholinergic muscle contraction while retaining similar features of basal tone, responses to tachykinins, and inhibitory muscle and neural function. Enteric inhibitory neurones in LOS muscle strips from patients with reflux associated Barrett's oesophagus display resistance to axonal sodium channel blockade. No evidence for functional NK-1 or NK-3 receptors or capsaicin sensitive axon collateral reflexes was observed in the human LOS.

GABA(B) receptor-mediated effects on vagal pathways to the lower oesophageal sphincter and heart

GABA(B) receptors influencing vagal pathways to the lower oesophageal sphincter and heart were investigated. In urethane-anaesthetized ferrets, the GABA(B) agonist baclofen (7 micromol kg(-1) i.v.) increased basal lower oesophageal sphincter (LOS) pressure. This was reversed by antagonism with CGP35348 (100 micromol kg(-1) i.v.). Baclofen's effect was abolished by vagotomy, suggesting a central action, yet it was ineffective when given centrally (3 - 6 nmol i.c.v.). Peripheral vagal stimulation (10 Hz, 5 s duration) caused LOS inhibition, followed by excitation, then prolonged inhibition. Bradycardia was also evoked during stimulation. Bradycardia and LOS responses were abolished after chronic supranodose vagotomy, indicating that they were due to stimulation of vagal pre-ganglionic neurones, not antidromic stimulation of afferents. Baclofen (1 - 10 micromol kg(-1)) reduced bradycardia and enhanced LOS excitation, which was also seen in animals pretreated with atropine (400 microgram kg(-1) i.v.) and guanethidine (5 mg kg(-1) i.v.), but not in those pretreated with L-NAME (100 mg kg(-1) i.v.). Effects of baclofen (7 micromol kg(-1) i.v.) on vagal stimulation-induced LOS and cardiac responses were unchanged by the GABA(B) antagonists CGP35348 or CGP36742 (up to 112 micromol kg(-1) i.v.), but were reversed by CGP62349 (ED(50) 37 nmol kg(-1) i.v.) or CGP54626 (ED(50) 100 nmol kg(-1) i.v.). Responses of isolated LOS strips to electrical stimulation, capsaicin, NK-1, NK-2 and nicotinic receptor agonists were all unaffected by baclofen (</=200 microM). We conclude that baclofen reduces vagal output at two peripheral sites: one presynaptically on pre-ganglionic neurones (CGP35348-insensitive), and another (CGP35348-sensitive) that could not be identified. This demonstrates heterogeneity of GABA(B) receptors through differential sensitivity to antagonists.

Acute effects of capsaicin on gastrointestinal vagal afferents

Capsaicin is an important tool for investigation of thin afferent fibres, but its acute effects on subtypes of vagal afferent endings are unknown. In the gastrointestinal tract, these subtypes are: muscle endings (thought to be purely tension sensitive), mucosal endings (sensitive to stroking and chemical stimuli) and endings in the oesophagus with both properties. Acute capsaicin sensitivity was investigated in ferrets using in vivo and in vitro methods. Single-fibre activity was recorded from 63 vagal afferents: 12 Adelta-fibres, 15 C-fibres and 36 unclassified fibres with endings in the oesophagus (n=42), stomach (n=19) and duodenum (n=2). Responses to capsaicin occurred independently of motility changes and were therefore due to direct activation of the receptor ending. In the oesophagus in vivo, two of 10 tension receptors and one of one mucosal receptor responded to intraluminal application of 3.25 mM capsaicin. In the stomach and duodenum, five of 14 tension receptors and two of four mucosal receptors responded to close-systemic (32-164 nmol) capsaicin. In an in vitro gastro-oesophageal preparation, three of five tension, four of 21 mucosal and two of eight tension/mucosal receptors responded to topical application of 1mM capsaicin. Occurrence of responses was therefore unrelated to location of endings and isolation of tissue. Responsiveness was also unrelated to conduction velocity. Capsaicin caused desensitization of responses to further capsaicin application in 37% of afferents. It additionally caused cross-desensitization to mechanical stimuli, which was also seen in afferents that did not respond directly to capsaicin. In conclusion, capsaicin acutely activates all subtypes of gut vagal afferents in vivo and in vitro, although responsiveness is restricted to 30% of fibres and follows no specific pattern. Acute desensitization may be induced with or without a response.

P2X purinoceptor-induced sensitization of ferret vagal mechanoreceptors in oesophageal inflammation

1. Using an in vitro single unit recording technique we studied the changes in mechanical and chemical sensitivity of vagal afferent fibres in acute oesophagitis, with particular attention to inflammatory products such as purines. 2. Histologically verified oesophagitis was induced by oesophageal perfusion of 1 mg ml-1 pepsin in 150 mM HCl in anaesthetized ferrets for 30 min on two consecutive days. Controls were infused with 154 mM NaCl. 3. The number of action potentials evoked in oesophageal mucosal afferents by mucosal stroking with calibrated von Frey hairs (10-1000 mg) was stimulus dependent. In oesophagitis responsiveness was reduced across the range of stimuli compared with controls. 4. Topical application of the P2X purinoceptor agonist alphabeta-methylene ATP had no direct excitatory effect on afferents. In oesophagitis, but not in controls, there was a significant increase in responses to stroking with von Frey hairs during superfusion with alphabeta-methylene ATP (1 microM). 5. Mucosal afferents responded directly to one or more chemical stimuli: 26 % (5/19 afferents) responded in controls, and 47 % (7/15 afferents) in oesophagitis. There were no differences in responsiveness to bradykinin (1 microM), prostaglandin E2 (100 microM), 5-hydroxytryptamine (100 microM), capsaicin (1 mM) or hydrochloric acid (150 mM) between control and oesophagitis groups. 6. We conclude that a sensitizing effect of a P2X purinoceptor agonist on mechanosensory function is induced in oesophagitis. This effect is offset by a decrease in basal mechanosensitivity.

GABA(B) receptors inhibit mechanosensitivity of primary afferent endings

The modulatory effects of baclofen on the sensitivity of peripheral afferent endings to mechanical stimulation were investigated using an in vitro ferret gastroesophageal vagal afferent preparation. Changes in sensitivity of three types of gastroesophageal vagal afferent endings previously categorized as mucosal, tension, and tension-mucosal (TM) receptors according to their mechanoreceptive field characteristics were investigated. Baclofen (30-200 microM) dose dependently reduced responses of mucosal afferents to mucosal stroking with calibrated von Frey hairs (10-1000 mg). This was reversed by the GABA(B) receptor antagonist SCH50911 (1 microM). TM afferent responses to mucosal stroking (10-1000 mg) were unaffected by baclofen (30-200 microM). However, baclofen (30-200 microM) significantly inhibited the response of 11 of 18 TM afferents to circumferential tension. This was reversed by SCH50911 (1 microM). Baclofen (100 and 200 microM) significantly inhibited the response of all tension receptor afferents to circumferential tension in the lower range (1-3 gm) but not in the higher range (4-7 gm). This inhibition was reversed by SCH50911 (1 microM; n = 3). This study provides the first direct evidence for the inhibitory modulation of peripheral mechanosensory endings by the G-protein-coupled GABA(B) receptor. Inhibition was dose-dependent, pharmacologically reversible, and selective to certain aspects of mechanosensitivity. These findings have important relevance to strategies for selective reduction of sensory input to the CNS at a peripheral site.

Inhibition of transient LES relaxations and reflux in ferrets by GABA receptor agonists

Transient lower esophageal sphincter (LES) relaxation is the major mechanism of gastroesophageal reflux. This study uses an established ferret model to evaluate GABA(B) receptor agonists' ability to reduce triggering of transient LES relaxations. One hundred sixty manometric/pH studies were performed on 18 conscious ferrets. In untreated animals, intragastric infusion of 25 ml glucose (pH 3.5) led to 2.0 +/- 0.6 reflux episodes over the first 30 min. Twenty-nine of forty-seven reflux episodes occurred during transient LES relaxation, and 18 occurred after downward drifts (<1 mmHg/s) in basal LES pressure. The GABA(B) receptor agonists baclofen (7 micromol/kg ip), CGP-44532, and SKF-97541 (both ED(50) <0.3 micromol/kg) reduced reflux episodes and transient LES relaxations. The putative peripherally selective GABA(B) receptor agonist 3-aminopropylphosphinic acid (80-240 micromol/kg) was ineffective, as was the GABA(A) receptor agonist muscimol (5 micromol/kg). Baclofen's inhibition of transient LES relaxations and reflux was unaffected by low-affinity GABA(B) receptor antagonists CGP-35348 and CGP-36742 at 100 micromol/kg but was reversed by higher-affinity CGP-54626 and CGP-62349 (0.7 micromol/kg) or by CGP-36742 at 200 micromol/kg. Therefore, GABA(B) receptor inhibition of reflux shows complex pharmacology. Our and other data indicate the therapeutic potential for these drugs.

In vitro recordings of afferent fibres with receptive fields in the serosa, muscle and mucosa of rat colon

1. Colonic afferent fibres were recorded using a novel in vitro preparation. Fibres with endings in the colonic mucosa are described, along with those in muscle and serosa, and their responses to a range of mechanical and chemical luminal stimuli. 2. Mechanical stimuli were applied to the tissue, which included stretch, blunt probing of the mucosa and stroking of the mucosa with von Frey hairs (10-1000 mg). Chemical stimuli were applied into a ring that was placed over the mechanoreceptive field of the fibre; these were distilled water, 154 and 308 mM NaCl, 100 microM capsaicin, 50 mM HCl, and undiluted and 50% ferret bile. 3. Recordings were made from 52 fibres, 12 of which showed characteristics of having endings in the mucosa. Mucosal afferents were sensitive to a 10 mg von Frey hair and were generally chemosensitive to >= 1 chemical stimulus. 4. Ten fibres showed characteristics of having receptive fields in the muscular layer. These fibres responded readily to circumferential stretch, as well as to blunt probing. 5. Twenty-seven fibres showed characteristics of having endings in the serosal layer. They adapted rapidly to circumferential stretch and responded to blunt probing of the serosa. Fifteen of 19 serosal fibres tested also responded to luminal chemicals. 6. Three fibres were unresponsive to all mechanical stimuli but were recruited by chemical stimuli. 7. This is the first characterization of colonic afferent fibres using an in vitro method and the first documentation of afferent fibres with their endings in the mucosa of the colon. These fibres are likely to be important in aspects of colonic sensation and reflex control.

An in vitro study of the properties of vagal afferent fibres innervating the ferret oesophagus and stomach

1. A novel preparation of the oesophagus with attached vagus nerve from the ferret maintained in vitro was used to study the properties of single vagal afferent nerve fibres with identified receptive fields. 2. Recordings were made from three types of gastro-oesophageal vagal afferent fibres that were classified on the basis of their sensitivity to mechanical stimulation. There were those responding to mucosal stroking (mucosal receptors), to circular tension (tension receptors) and those responding to mucosal stroking and circular tension, which we have termed tension/mucosal (TM) receptors. 3. The conduction velocities for mucosal, TM and tension receptor fibres were 6.38 +/- 1.22 m s-1 (n = 22), 6.20 +/- 1.49 m s-1 (n = 13) and 5.33 +/- 0.86 m s-1 (n = 22), respectively. 4. Receptive fields of afferents showed random topographical distribution by fibre type and conduction velocity. They were found mainly distal but also occasionally proximal to the point of vagal dissection. 5. Twenty-eight per cent of mucosal, 63% of TM and 43% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. 6. In conclusion, this experimental preparation provides evidence for the existence of three types of oesophageal vagal afferent fibre, namely mucosal, tension and the newly identified tension/mucosal receptors.

Oesophagitis-induced changes in capsaicin-sensitive tachykininergic pathways in the ferret lower oesophageal sphincter

Prolonged oesophageal acidification may impair lower oesophageal sphincter (LOS) function in reflux disease. The aim of this study was to investigate aspects of altered LOS innervation in a model of oesophagitis. Oesophagitis was induced by acid (HCl, 0.15 M) and pepsin (0.1% w/v) infusions in anaesthetized ferrets. LOS muscle strip responses to the following stimuli were measured in vitro from control and acid/pepsin-treated ferrets: electrical field stimulation (EFS; 1-50 Hz), potassium chloride KCl; 20 mM), substance P, [beta-Ala8]-neurokinin A 4-10, [Sar9, Met (O2)11]-substance P (all 10(-10) to 10(-6) M) and capsaicin (10(-8) to 10(-6) M). LOS relaxation occurred in response to all stimuli except [beta-Ala8]-neurokinin A 4-10, which evoked contraction. In muscle strips from acid/pepsin-treated animals there were no differences in amplitude or sensitivity of relaxation following EFS, KCl or substance P vs controls. However, the inhibitory response to capsaicin was increased four-fold (10(-8) M; P < 0.05) and an increased sensitivity of the inhibitory response to [Sar9, Met (O2)11]-substance P occurred (pD2 = 8.64 +/- 0.12 acid/pepsin-treated vs 7.94 +/- 0.24 control, P < 0.05). We conclude that in acute oesophagitis, increased sensitivity of capsaicin-activated inhibitory pathways occurs in which activation of NK-1 receptors plays an integral role in the ferret LOS.

Small intestinal dysmotility following abdominal irradiation in the rat small intestine

Abdominal symptoms such as diarrhoea, abdominal cramps and vomiting are common during and after abdominal radiotherapy for gynaecological and pelvic malignancy. It has recently been recognized that small intestinal dysmotility may contribute to these symptoms but the underlying mechanisms are unclear in part because of the technical difficulties inherent in performing studies in irradiated small intestine. The aim of the current study was to evaluate small intestinal motor activity using perfused micromanometric techniques in 6-8-cm segments of ileum during arterial perfusion with isotonic oxygenated fluorocarbon solution. Intestinal segments from six rats were studied 4 days after treatment with 10 Gy abdominal irradiation. Ileal segments from nine nonirradiated animals acted as controls. For each experiment the total number of pressure waves, high-amplitude (> 20 mmHg, long-duration > 6 sec) pressure waves, and long (> 20 associated) bursts of pressure waves were determined. Irradiation had no effect on the overall number of pressure waves, but increased high-amplitude long-duration (HALD) pressure waves (248 vs 7, P < 0.01). In control animals HALD waves were localized to a single recording site but after radiotherapy 74% of HALD waves were temporally associated with similar pressure waves in other manometric channels. Forty-seven per cent of associated HALD waves migrated aborally. Retrograde migration of HALD waves was seen in five segments following irradiation. Irradiation abolished bursts of > 20 pressure waves.

Activation of non-adrenergic non-cholinergic inhibitory pathways by endogenous and exogenous tachykinins in the ferret lower oesophageal sphincter

Repeated oesophageal acidification causes lower oesophageal sphincter (LOS) relaxation in the anaesthetized ferret which is mediated by a peripheral neurokinin (NK-1) receptor mechanism. Our aim in this study was to characterize neural pathways in the LOS activated by capsaicin and tachykinin receptor agonists in vitro. Circular muscle strips of LOS (two per animal) from a total of 24 ferrets were maintained in organ baths. Electrical field stimulation (EFS, 50 V, 5-50 Hz) caused frequency-dependent LOS relaxation which was abolished by tetrodotoxin (TTX; 10(-6) M: P < 0.001) and reduced by N(G)-nitro-L-arginine (L-NNA; 10(-4) M: P < 0.01). Substance P and [Sar9, Met (O2)11]-substance P (selective NK-1 agonist) caused dose-dependent relaxation, while the NK-2 receptor agonist [beta-Ala8]-NKA 4-10 evoked excitation. Capsaicin (10(-6) M) caused relaxation and desensitization that was overcome by long recovery periods and substance P dosing (10(-8) M). After pretreatment with the NK-1 receptor antagonist CP 99994 (10(-7) M), substance P (10(-8) M; P < 0.001) and capsaicin (10(-6) M: P < 0.01)-induced relaxations were reduced. In the presence of TTX (10(-6) M), excitation resulted in response to substance P (10(-8) M; P < 0.05) and [Sar9, Met (O2)11]-substance P (10(-8) M; P < 0.001), while the response to [beta-Ala8]-NKA 4-10 (10(-7) M) was unaffected. In the presence of L-NNA (10(-4) M), substance P and [Sar9, Met (O2)11]-substance P-induced relaxations were reduced (10(-8) M; P < 0.01), while the response to [beta-Ala8]-NKA 4-10 (10(-7) M) was unaffected. These results show that functional coupling between capsaicin-sensitive sensory neurones and NANC inhibitory neural pathways occurs via NK-1 receptors in the ferret LOS. NK-2 (and some NK-1) receptors activate non-neural excitatory mechanisms. Substance P and NK-1 receptors coupling sensory and NANC inhibitory neurones may be important in the reflex control of LOS motility.

Mechanisms of gastro-oesophageal reflux in the ferret

Transient lower oesophageal sphincter (LOS) relaxation is the major mechanism of gastro-oesophageal reflux in humans--an event unassociated with swallowing. Mechanisms involved in triggering transient LOS relaxation are poorly understood, and their further study requires a small animal model. In this study we aimed to establish methods for prolonged ambulant oesophageal manometry in ferrets, and to determine motor events associated with reflux episodes and their triggering by different gastric nutrient loads. Forty-two studies were performed on nine ferrets with chronic cervical oesophagostomies, through which a manometric assembly was introduced and secured to a collar, which incorporated a microphone for detection of swallows. The assembly included a gastric feeding channel, one gastric and four oesophageal manometric sideholes, a 2.5-cm-long LOS sleeve sensor, and an oesophageal pH electrode. Intragastric infusions were given over 2 min, the first after a 30-min control recording period, and in 29/42 studies, a second infusion was given 60 min later. Infusions were either 25 mL 10% dextrose solution, pH 3.5 (22 studies), 25 mL triglyceride emulsion (Intralipid) pH 3.5 (11 studies), or 25 mL air (nine studies). Episodes of oesophageal acidification were absent before gastric infusions. After infusion, 2.1 +/- 0.2 episodes occurred over the first 30 min. After glucose infusion, 15/18 acidification episodes (83%) occurred during transient LOS relaxation, and 3/18 (17%) occurred after gradual (< 1 mmHg sec-1) downward drifts in basal LOSP to < 2 mmHg. After lipid infusion two acidification episodes occurred, both during transient LOS relaxation. Mean duration of transient LOS relaxation was 8.0 +/- 0.4 sec. All infusions increased occurrence of transient LOS relaxation to a similar extent, each of which ended with primary peristalsis. We conclude that gastric infusion of glucose, lipid and gas are all effective in provoking gastro-oesophageal reflux in ferrets. Reflux occurs through similar mechanisms to those seen in humans, i.e. increased triggering of transient LOS relaxation. The conscious ferret is therefore an appropriate model for future studies of manipulation of mechanisms giving rise to gastro-oesophageal reflux.

Vagal and sympathetic influences on the ferret lower oesophageal sphincter

This study has investigated the relative involvement of cholinergic, adrenergic, nitric oxide and tachykininergic transmission in extrinsic neural influences on the lower oesophageal sphincter (LOS) in urethane anaesthetized ferrets. A micromanometric assembly (OD 1.75 mm) incorporating a sleeve sensor was used for high-fidelity oesophageal, LOS and gastric pressure measurement at low perfusion rates (< 0.1 ml/min). The LOS response to vagal and splanchnic nerve stimulation (0.5 ms pulse width, 10 s duration) was frequency- and voltage-dependent. LOS responses to stimulation at 20 V, 10 Hz were investigated in separate groups of animals with either L-NAME (100 mg/kg), hexamethonium (15 mg/kg), guanethidine (5 mg/kg), CP96,345 (NK-1 antagonist, 4 mg/kg), atropine (0.4 mg/kg) or propranolol (1 mg/kg). Propranolol treatment was followed by yohimbine (1 mg/kg) and prazosin (0.25 mg/kg). Vagal stimulation caused an immediate decrease in LOS pressure, followed by increase on cessation of stimulation, followed by a prolonged decrease (77 +/- 2%) for up to 5 min. L-NAME did not affect inhibition, but increased excitation 4-fold (p < 0.001). Guanethidine and CP96,345 had no major effect. Hexamethonium decreased the inhibitory (p < 0.05) and excitatory (p < 0.01) responses. Atropine reduced the excitatory response (p < 0.05). Some inhibition still remained if all treatments were combined. Splanchnic stimulation reduced LOS pressure by 70 +/- 6% for 101 +/- 17 s. L-NAME, guanethidine, hexamethonium and CP96,345 all independently significantly reduced inhibition. The combination of guanethidine and CP96,345 usually abolished splanchnic-induced inhibition. Atropine was without effect. Propranolol (1 mg/kg) changed the splanchnic-induced response from mainly inhibition to excitation (100 +/- 44% increase). LOS responses to noradrenaline (1-10 micrograms close IA) showed similar features to responses to splanchnic stimulation. We conclude that vagal stimulation evokes LOS relaxation via activation of established cholinergic and NANC mechanisms and other, unidentified mechanisms. Splanchnic stimulation activates adrenergic neurones probably via nicotinic and non-nicotinic ganglionic mechanisms, which in turn elicit beta adrenergic inhibitory effects on the LOS. Splanchnic stimulation also antidromically activates spinal afferent fibres. These may release substance P from peripheral myenteric plexus and prevertebral ganglionic endings causing activation of myenteric NANC inhibitory neurones and sympathetic neurones, respectively.

Vagal efferent fibre responses to gastric and oesophageal mechanical and chemical stimuli in the ferret

Gastric and oesophageal afferent inputs to vagal efferent fibres were investigated in Urethane anaesthetized ferrets. Mechanical, chemical, and pharmacological stimuli were tested and efferent activity recorded from single cervical vagal fibres. Fibres showed either no basal discharge or low frequency, irregular patterns of resting discharge; only those which showed > 50% excitation or inhibition of basal activity with both gastric distension and oesophageal balloon distension were studied further. These responses were rapid and maintained only for the duration of the stimuli. 18/32 efferent fibres tested also showed changes in discharge in response to acid infused slowly into the distal oesophagus. These responses were larger after repeated acid infusions. Subsequent intra-oesophageal capsaicin elicited a similar response in 7/8 fibres. These responses were reproducible with repeated capsaicin infusions in 2/4 fibres and desensitized in 2/4 fibres. 2 capsaicin-responsive fibres were unresponsive to oesophageal acidification. 4/12 fibres tested responded to close intraarterial injections of capsaicin and 9/12 to close intraarterial bradykinin. These responses were brief and of short latency. Vagal efferent responses to mechanical and chemical stimuli above were unchanged after the NK-1 receptor antagonist CP96,345 (4 mg/kg i.v.). Subsequently, bilateral vagotomy caudal to the recording site abolished the basal activity in 4/7 fibres. In the 3 fibres where spontaneous activity remained, none of these responded to oesophageal distension or intra-oesophageal acid (2/2 fibres tested) after vagotomy, whereas 2/2 fibres tested still responded to gastric distension. The response of 1 fibre to intraarterial bradykinin and capsaicin was unchanged by vagotomy. We conclude that vagal efferent neurones respond to gastro-oesophageal mechanical inputs and also receive convergent input from oesophageal acid-sensitive and gastrointestinal bradykinin- and capsaicin-sensitive afferents. These afferent inputs are not mediated via NK-1 receptors. There also exists a nonvagal afferent input onto vagal efferent neurones which is probably spinal and likewise non NK-1 receptor mediated.

Lower oesophageal sphincter responses to noxious oesophageal chemical stimuli in the ferret: involvement of tachykinin receptors

Repeated oesophageal acidification is a definitive feature of gastro-oesophageal reflux disease, which in turn is caused by relaxation of the lower oesophageal sphincter (LOS). This study in anaesthetised ferrets investigates the reflex pathways involved in effects of oesophageal acidification on motor function of the LOS, with particular focus on the role of tachykinins. LOS pressure was monitored with a perfused micromanometric sleeve assembly. Oesophageal acidification reduced LOS pressure by 48 +/- 5% until washout with saline. This reduction became larger with repeated tests, and was unaffected in amplitude by acute bilateral vagotomy, although the response became slower in onset. Intra-oesophageal capsaicin (0.5% solution) caused a 68 +/- 17% decrease in LOS pressure which remained unchanged with repeated tests. The NK-1 receptor antagonist CP96,345 (1-5 mg/kg intravenous (i.v.) blocked the post-vagotomy LOS responses to both intra-luminal acid and capsaicin. Close intra-arterial (i.a.) injections of capsaicin (1-100 micrograms) gut induced LOS relaxation which was neither vagally nor NK-1 receptor-mediated. Substance P or the selective NK-1 receptor agonist [Sar9, Met(O2)11] substance P (25-500 ng close i.a.) caused a biphasic LOS response, consisting of initial brief contraction followed by prolonged, dose-dependent relaxation. Tetrodotoxin (10 micrograms/kg close i.a.) changed the biphasic response to substance P to excitation only. The neurokinin-1 (NK-1) receptor antagonist CP96,345 (0.3-10 mg/kg i.v.) dose-dependently reduced the inhibitory response to substance P. The excitatory phase of the response to substance P was larger and prolonged after guanethidine (5 mg/kg, i.v.), or propranolol (1 mg/kg, i.v.). L-NAME (100 mg/kg i.v.) reduced the inhibitory phase. The selective NK-2 receptor agonist [beta-Ala8] neurokinin A(4-10) caused LOS excitation only. These data indicate that intra-oesophageal acid causes substance P release from extrinsic afferent nerve endings which activates local inhibitory pathways to the LOS via NK-1 receptors.

Pyloric motor response to central and peripheral nitric oxide in the ferret

This study has investigated the relative importance of central nervous and peripheral nitroxidergic mechanisms in the control of pyloric motility. In 10 urethane-anaesthetized ferrets, drugs were administered directly to the CNS via a 0.5-mm-diameter cannula inserted into the 4th ventricle, approximately at the obex. Drugs were also given directly to the upper GI tract by close intra-arterial (i.a.) injection at the coeliac axis. Antropyloroduodenal pressures were recorded with a five-channel sleeve/sidehole micromanometric assembly (1.35 x 1.75 mm o.d.), which was introduced via the duodenum. Pyloric motility was stimulated throughout the main part of each study with a continuous i.v. infusion of CCK-8 (30 pmol min-1). This infusion produced an immediate and sustained increase in tonic and phasic pyloric activity, and sustained abolition of antral pressure waves. CCK-8 also induced a duodenal motor response, but this was short-lived (11.4 +/- 7.9 min). Coeliac axis injection of the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) decreased phasic pyloric activity (from 330 +/- 35 to 148 +/- 21 mmHg min-1 after SNAP 5 micrograms, P < 0.01). By comparison central SNAP administration over the same dose range had no effect on CCK-stimulated pyloric motlity. Inhibition of endogenous NO synthase with L-Nitro Arginine Methyl Ester (L-NAME, 100 mg kg-1 close i.a.) caused a marked increase of phase pyloric motor activity from 349 +/- 59 to 1044 +/- 140 mmHg min-1 (P < 0.01). In addition, SNAP caused marked stimulation of pyloric tone from 2.6 +/- 0.5 to 13.1 +/- 2.8 mmHg (P < 0.01). Central nervous administration of L-NAME caused modest enhancement of phasic pyloric activity (248 +/- 31 to 283 +/- 32 mmHg min-1 P < 0.05) and pyloric tone (2.6 +/- 0.5 to 3.7 +/- 0.7 mmHg, P < 0.05). Our data indicate that motor activity of the ferret pylorus is potently modulated by NO released within the upper gut. Additionally, there is potential for modulation of pyloric motility by central nervous system production of NO.

Responses of the rat lower oesophageal sphincter (LOS) to vagal efferent activation

Nitric oxide (NO) is a major candidate in vagal-induced LOS relaxation. Vagal adrenergic fibres also innervate the gastrointestinal tract including the LOS. This study investigates the role of these two and other mechanisms in LOS responses to vagal activation in the rat, and provides functional and anatomical evidence for a smooth muscle LOS in this species. LOS, gastric and oesophageal pressures were measured in urethane anaesthetized rats during vagal stimulation. The LOS pressure (LOSP) response to Vagal stimulation (5 mA, 10 Hz, 0.5 msec pulses, 5 sec) comprised three consecutive stages: (1) brief reduction of LOSP, (2) transient increase of LOSP and (3) prolonged reduction of LOSP. The influences of additive treatment with several antagonist drugs on the LOS response to vagal stimulation were investigated. L-NAME (100 mg kg-1) reduced stage 1 and increased stage 2. Subsequent treatment with either phentolamine (1 mg kg-1) or prazosin (200 micrograms kg-1) abolished stage 1. After phentolamine, atropine treatment (400 micrograms kg-1) abolished stage 2. Stage 3 was evident throughout experiments. In five additional studies, treatment with hexamethonium (30 mg kg-1) abolished stages 2 and 3 leaving stage 1, which was later abolished by phentolamine or atropine. In the LOS response to vagal stimulation, the following major mechanisms are therefore evident: nicotinic transmission in both excitation and inhibition, alpha-adrenergic and NO-mediated inhibition, muscarinic excitation, and non-adrenergic, non-NO inhibition (not characterized further). Characteristics of these different neurotransmitter influences may be important in LOS relaxation associated with swallowing and gastro-oesophageal reflux.

Responses of ferret lower esophageal sphincter to 5-hydroxytryptamine: pathways and receptor subtypes

In urethananesthetized ferrets, basal lower esophageal sphincter pressure (LESP) was unaffected by the 5-hydroxytryptamine3 (5-HT3) receptor antagonist granisetron (0.5 mg/kg) or by greater splanchnic nerve section (GSX), but increased after bilateral vagotomy. Peripheral vagal nerve stimulation caused LES relaxation, often followed by a brief contraction and a prolonged inhibition of LESP. Close intra-arterial injection of 5-HT (5-100 micrograms) had a biphasic effect on LESP, with a brief drop followed by a prolonged increase. Granisetron (0.5 mg/kg i.v.) abolished the initial relaxation and revealed an earlier peak of excitation. This was not influenced by subsequent vagotomy and GSX. In a series of eight additional experiments (series 2), granisetron was given after vagotomy and GSX. In series 2, 5-HT-induced relaxation was unaffected by vagotomy but was significantly reduced after GSX and was further reduced after granisetron, indicating that 5-HT3 receptor mechanisms may lie on a sympathetic neural pathway. Vagotomy had no effect on the excitatory component. GSX had no effect on the amplitude of excitation, but reduced its latency. Granisetron had no further effect on excitation in series 2. In a separate series of 13 experiments (series 3), the excitatory component of the LES response to 5-HT was abolished by ketanserin (2.5 mg/kg i.v.) , after which only relaxation occurred. Both 5-HT2 and 5-HT3 antagonists in combination abolished all effects of 5-HT on LESP. Atropine (400 micrograms/kg i.v., n = 7) had no effect on 5-HT-induced LES responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of 5-hydroxytryptamine in gastrointestinal chemosensitivity

The present electrophysiological investigation examines the effect of 5-hydroxytryptamine (5-HT) on gastrointestinal afferent fiber discharge. 5-HT markedly and specifically stimulated vagal mucosal chemosensitive afferents. The response was mediated by 5-HT3 receptors as demonstrated by the action of 2-methyl-5-HT and antagonism by granisetron. At doses of granisetron that completely block the response to 5-HT, the afferent fibers still responded to both mechanical and chemical stimulation of the mucosa. This sensitivity of extrinsic afferents is in marked contrast to that reported for intrinsic afferents, suggesting fundamental differences in the organization of enteric and vagal reflexes.

Gastro-oesophageal afferent and serotonergic inputs to vagal efferent neurones

Peripheral 5-HT3 receptor mechanisms are involved in activation of gastrointestinal (GI) mucosal vagal afferent fibres. 5-HT3 receptor mechanisms in the central nervous system (CNS) may be involved in behavioural and reflex motility responses. This study investigates the processing of different sensory inputs in the CNS and the involvement of 5-HT3 receptors at these different levels. In Urethane (1.5 g/kg, i.p.) anaesthetized, splanchnectomized ferrets, the jugular vein was cannulated for intravenous (i.v.) drug injection, and the coeliac axis for intraarterial (i.a.) injection close to the upper GI tract. The carotid artery was intubated with a T-cannula for CNS-directed intracarotid (i.c.) injections. An intragastric cannula was used for fluid distension (40-50 ml), and an oesophageal catheter for balloon distension (2 ml). Efferent fibres were dissected from the right cervical vagus for single-unit recording. Nineteen single vagal efferent fibres were selected, with low frequency resting discharge (2.5 +/- 0.3 impulses/s), but no respiratory or cardiovascular phasic input. All responded rapidly (< 2.5 s) to gastric distension (532 +/- 230% change in firing rate) and oesophageal distension (300 +/- 170%). Gastric distension caused excitation in 14 fibres, inhibition in 4 fibres, and a biphasic response in 1. Oesophageal distension excited 16 and inhibited 3. Discharge was also influenced by i.a. injection of 5-HT or the 5-HT3 receptor agonist 2-methyl 5-HT (10-100 micrograms) in all fibres tested. These responses consisted of rapid (< 2.5 s) and powerful changes in firing rate, with excitation, inhibition or biphasic responses. 65% of responses to i.c. or i.v. injection were opposite in direction to those after close i.a. injection, indicating the activation of a different population of receptors. No differences were seen between effects of i.c. and i.v. injections. The 5-HT3 receptor antagonist granisetron (100 micrograms/kg, i.v.) blocked or reduced efferent responses to 5-HT receptor agonists, whereas responses to gastric and oesophageal distension were unchanged. Thus there is extensive convergence of inputs from gastric and oesophageal mechanoreceptors onto vagal motorneurones. These central effects of mechanical stimuli do not involve 5-HT3 receptor mechanisms. Other 5-HT3 receptor inputs are evident, probably peripherally from GI mucosal afferent fibres and from within the CNS.