Gastric relaxation induced by hyperglycemia is mediated by vagal afferent pathways in the rat

A multivariate physiological model of vagus nerve signalling during metabolic challenges in anaesthetised rats for diabetes treatment

Objective.This study aims to develop a comprehensive decoding framework to create a multivariate physiological model of vagus nerve transmission that reveals the complex interactions between the nervous and metabolic systems.Approach.Vagus nerve activity was recorded in female Sprague-Dawley rats using gold hook microwires implanted around the left cervical vagus nerve. The rats were divided into three experimental cohorts (intact nerve, ligation nerve for recording afferent activation, and ligation for recording efferent activation) and metabolic challenges were administered to change glucose levels while recording the nerve activity. The decoding methodology involved various techniques, including continuous wavelet transformation, extraction of breathing rate (BR), and correlation of neural metrics with physiological signals.Main results.Decrease in glucose level was consistently negatively correlated with an increase in the firing activity of the intact vagus nerve that was found to be conveyed by both afferent and efferent pathways, with the afferent response being more similar to the one on the intact nerve. A larger variability was observed in the sensory and motor responses to hyperglycaemia. A novel strategy to extract the BR over time based on inter-burst-interval is also presented. The vagus afferent was found to encode breathing information through amplitude and firing rate modulation. Modulations of the signal amplitude were also observed due to changes in heart rate in the intact and efferent recordings, highlighting the parasympathetic control of the heart.Significance.The analytical framework presented in this study provides an integrative understanding that considers the relationship between metabolic, cardiac, and breathing signals and contributes to the development of a multivariable physiological model for the transmission of vagus nerve signals. This work progresses toward the development of closed-loop neuro-metabolic therapeutic systems for diabetes.

The effect of timing and dosing of 5% and 10% Dextrose versus Saline on postoperative nausea and vomiting in high-risk women following ambulatory surgeries: a randomized double-blind controlled trial

BACKGROUND

A common complication after outpatient surgeries is postoperative nausea and vomiting (PONV) which is concomitant with high levels of patient distress and dissatisfaction. Perioperative dextrose-containing fluid administration has been used as a non-pharmacologic preventive measure against postoperative nausea and vomiting (PONV). However, its efficacy remains unclear. This study aimed to compare different concentrations of dextrose solution on PONV for 24 hours after surgery.

METHODS

A randomized double-blind controlled study of 120 women were divided into three equal groups. Group C, was infused with IV 0.9% saline 100 ml/h for 2 hours (200 ml) starting 1 hour before anesthesia and ending during surgery. Group D5%, was infused with dextrose 5%. Group D10%, was infused with D10%. PONV within 24 hours was compared between groups by PONV score. Other outcomes were the antiemetic drugs needed, VAS score, total fentanyl consumption, hemodynamic data, unplanned admission, and complications RESULTS: The incidence of nausea and vomiting was significantly reduced in dextrose 10% and 5% patients in comparison to the control group (5 patients (12.5%), 18 (42.5%), and 39 (87.5%) respectively). The request for antiemetic drugs decreased significantly in groups D10% and 5% than in group C (36 patients did not receive any antiemetics (90%), 29 (72.5%) and 19 (47.5%) respectively, P-value=0.002). Postoperative pain prevalence in patients with PONV was significantly different only at 24 h, P-value=0.03.

CONCLUSIONS

There was a potential role for preoperative IV infusion of dextrose 10% solution that significantly reduces the incidence and severity of PONV and pain in patients at high risk for PONV.

Impact of Vagotomy on Postoperative Weight Loss, Alimentary Intake, and Enterohormone Secretion After Bariatric Surgery in Experimental Translational Models

Obesity may be treated by bariatric procedures and is related to enterohormone release modulation. Nevertheless, a majority of commonly used surgical procedures have a significant impact on vagus nerve function by breaking the connections with its gastric branches. In the case of an intragastric balloon (BAL), this interaction is unclear. However, BAL-induced weight reduction is not long-lasting. Interestingly, this method has not been used in combination with vagotomy (VAG). Thus, we evaluated, for the first time, the short- and long-term effects of combined BAL and VAG using the animal-based translational model and compared these effects with sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB). Wistar rats were fed a high-calorie diet for 8 weeks to induce obesity before SG, RYGB, BAL + / - VAG. Animals' weight and eating behaviors were monitored weekly. After 90 days, serum samples were collected to evaluate postprandial and fasting GLP-1, GIP, PYY, ghrelin, glucagon, insulin, leptin, and pancreatic polypeptide concentrations by fluorescent assay. VAG, SG, RYGB, and BAL + VAG significantly reduced body weight 30 and 90 days after surgery. BAL alone induced temporal weight reduction observed after 30 days, reversed after 90 days. Calories intake was reduced at the first half of the observation period in all groups. Fluid intake was reduced in all groups except SG and BAL. Enterohormone profile for BAL + VAG was comparable to SG and RYGB but not BAL. VAG and BAL + VAG but not BAL alone maintain weight reduction, alimentary intake changes, and enterohormone release after long-term observation. VAG may improve the effectiveness of bariatric procedures for obesity treatment in clinical practice.

Effects of intravenous dextrose on preventing postoperative nausea and vomiting: A systematic review and meta-analysis with trial sequential analysis

BACKGROUND

It is reported that postoperative nausea and vomiting, common general anesthesia complications, may be prevented by perioperative intravenous dextrose administration, but with controversial clinical effectiveness.

OBJECTIVE

To evaluate perioperative intravenous dextrose for preventing postoperative nausea and vomiting through a systematic review and meta-analysis of randomized controlled trials with trial sequential analysis.

DATA SOURCES

MEDLINE, the Cochrane Central Register of Controlled Trials, Embase, Web of Science, clinicaltrials.gov, and the University Hospital Medical Information Network Clinical Trials Registry were searched from inception until 22 June 2019.

ELIGIBILITY CRITERIA

Trials investigating intravenous dextrose effects vs. placebos on postoperative nausea and vomiting in patients who underwent general anesthesia.

RESULTS

Eleven trials (1,250 patients) were included. All participants were ASA1-2. The nine trials included laparoscopic surgeries, and 92.2% of the participants were women. The timing of dextrose infusion was before, during, and after surgery in three, five, and three trials, respectively. Our results revealed intravenous dextrose administration significantly reduced postoperative nausea, but not vomiting, during early and late postoperative periods (risk ratio [95% confidence interval], early nausea: 0.76 [0.59-0.99], late nausea: 0.65 [0.48-0.89]; early vomiting: 1.00 [0.81-1.25], late vomiting: 0.96 [0.43-2.16]). Evidence quality was downgraded to low because the trial sequential analysis indicated more trials are needed for firm conclusions.

CONCLUSIONS

Compared with placebos, perioperative intravenous dextrose administration may decrease postoperative nausea but not vomiting.

TRIAL REGISTRATION

University Hospital Medical Information Network Clinical Trials Registry (registration number: UMIN000030901).

Perioperative Dextrose Infusion and Postoperative Nausea and Vomiting: A Meta-analysis of Randomized Trials

BACKGROUND

Perioperative IV dextrose infusions have been investigated for their potential to reduce the risk of postoperative nausea and vomiting. In this meta-analysis, we investigated the use of an intraoperative or postoperative infusion of dextrose for the prevention of postoperative nausea and vomiting.

METHODS

Our group searched PubMed, Embase, Cochrane library, and Google Scholar for relevant randomized controlled trials examining the use of perioperative IV dextrose for prevention of postoperative nausea and vomiting. The primary outcome was the incidence of postoperative nausea and vomiting (both in the postanesthesia care unit and within the first 24 h of surgery). Secondary outcomes included postoperative antiemetic administration and serum glucose level.

RESULTS

Our search yielded a total of 10 randomized controlled trials (n = 987 patients) comparing the use of a perioperative dextrose infusion (n = 465) to control (n = 522). Perioperative dextrose infusion was not associated with a significant reduction in postoperative nausea and vomiting in the postanesthesia care unit (risk ratio = 0.91, 95% CI, 0.73-1.15; P = .44) or within the first 24 h (risk ratio = 0.76, 95% CI, 0.55-1.04; P = .09) of surgery. Although the use of dextrose was associated with a significant reduction in antiemetic administration within the first 24 h (risk ratio = 0.55, 95% CI, 0.45-0.69; P < .001), it also increased postoperative plasma glucose levels compared to controls.

CONCLUSIONS

The use of perioperative dextrose did not result in a statistically significant association with postoperative nausea and vomiting. When utilized, plasma glucose monitoring is recommended to assess for postoperative hyperglycemia. Further prospective trials are necessary to examine the potential impact of timing of administration of a dextrose infusion on incidence of postoperative nausea and vomiting and rescue antiemetic requirements.

Evidence that hindbrain astrocytes in the rat detect low glucose with a glucose transporter 2-phospholipase C-calcium release mechanism

Astrocytes generate robust cytoplasmic calcium signals in response to reductions in extracellular glucose. This calcium signal, in turn, drives purinergic gliotransmission, which controls the activity of catecholaminergic (CA) neurons in the hindbrain. These CA neurons are critical to triggering glucose counter-regulatory responses (CRRs) that, ultimately, restore glucose homeostasis via endocrine and behavioral means. Although the astrocyte low-glucose sensor involvement in CRR has been accepted, it is not clear how astrocytes produce an increase in intracellular calcium in response to a decrease in glucose. Our ex vivo calcium imaging studies of hindbrain astrocytes show that the glucose type 2 transporter (GLUT2) is an essential feature of the astrocyte glucosensor mechanism. Coimmunoprecipitation assays reveal that the recombinant GLUT2 binds directly with the recombinant Gq protein subunit that activates phospholipase C (PLC). Additional calcium imaging studies suggest that GLUT2 may be connected to a PLC-endoplasmic reticular-calcium release mechanism, which is amplified by calcium-induced calcium release (CICR). Collectively, these data help outline a potential mechanism used by astrocytes to convert information regarding low-glucose levels into intracellular changes that ultimately regulate the CRR.

MicroRNA 375 modulates hyperglycemia-induced enteric glial cell apoptosis and Diabetes-induced gastrointestinal dysfunction by targeting Pdk1 and repressing PI3K/Akt pathway

Diabetic neuropathy can damage systemic nervous system, including alteration of enteric nervous system and subsequent gastrointestinal dysfunction. The effect of diabetes on enteric glia cell (EGC) is not clear. We investigated the effect of diabetes and hyperglycemia on EGC, and the role of microRNA375 in modulating EGC survival in vivo and in vitro. Streptozotocin-induced diabetic mice were intraperitoneally injected with microRNA375 inhibitor or its negative control. EGC was transfected with microRNA375 inhibitor or its mimic. Diabetes mice with gastrointestinal dysfunction showed increased apoptosis of EGC (no difference in cell numbers) and gene expression of micorRNA375 in the myenteric plexus. Hyperglycemia triggered apoptosis of EGC in vitro with decreased expression of Pdk1 and p-Akt, but increased expression of micorRNA375. MicorRNA375 mimic induced apoptosis of EGC in vitro with repressed Pdk1and p-Akt. MicorRNA375 inhibitor could both prevent hyperglycemia-induced apoptosis of EGC in vitro and diabetes-induced gastrointestinal dysfunction in vivo. Our results suggest that diabetes-induced gastrointestinal dysfunction is related to increased apoptosis of EGC in the myenteric plexus. Hyperglycemia can increase the expression of microRNA375 and damage EGC survival through PI3K/Akt pathway. MicroRNA375 specific inhibition can prevent hyperglycemia induced EGC damage and diabetes-induced gastrointestinal dysfunction.

Response of catecholaminergic neurons in the mouse hindbrain to glucoprivic stimuli is astrocyte dependent

Hindbrain catecholaminergic (CA) neurons are required for critical autonomic, endocrine, and behavioral counterregulatory responses (CRRs) to hypoglycemia. Recent studies suggest that CRR initiation depends on hindbrain astrocyte glucose sensors (McDougal DH, Hermann GE, Rogers RC. Front Neurosci 7: 249, 2013; Rogers RC, Ritter S, Hermann GE. Am J Physiol Regul Integr Comp Physiol 310: R1102-R1108, 2016). To test the proposition that hindbrain CA responses to glucoprivation are astrocyte dependent, we utilized transgenic mice in which the calcium reporter construct (GCaMP5) was expressed selectively in tyrosine hydroxylase neurons (TH-GCaMP5). We conducted live cell calcium-imaging studies on tissue slices containing the nucleus of the solitary tract (NST) or the ventrolateral medulla, critical CRR initiation sites. Results show that TH-GCaMP5 neurons are robustly activated by a glucoprivic challenge and that this response is dependent on functional astrocytes. Pretreatment of hindbrain slices with fluorocitrate (an astrocytic metabolic suppressor) abolished TH-GCaMP5 neuronal responses to glucoprivation, but not to glutamate. Pharmacologic results suggest that the astrocytic connection with hindbrain CA neurons is purinergic via P2 receptors. Parallel imaging studies on hindbrain slices of NST from wild-type C57BL/6J mice, in which astrocytes and neurons were prelabeled with a calcium reporter dye and an astrocytic vital dye, show that both cell types are activated by glucoprivation but astrocytes responded significantly sooner than neurons. Pretreatment of these hindbrain slices with P2 antagonists abolished neuronal responses to glucoprivation without interruption of astrocyte responses; pretreatment with fluorocitrate eliminated both astrocytic and neuronal responses. These results support earlier work suggesting that the primary detection of glucoprivic signals by the hindbrain is mediated by astrocytes.

Effect of Hyperglycemia on Purinergic and Nitrergic Inhibitory Neuromuscular Transmission in the Antrum of the Stomach: Implications for Fast Gastric Emptying

Background

Hyperglycemia has been reported to enhance vagovagal reflex that causes the release of inhibitory neurotransmitter, nitric oxide (NO), at the neuromuscular junction in the antrum to relax the antrum and slow gastric emptying by stimulating glucose-sensitive afferent neurons. However, hyperglycemia has also been reported to cause fast gastric emptying that may be due to suppression of the inhibitory motor neurons.

Aims

The purpose of the present study was to investigate changes in inhibitory neuromuscular transmission in the gastric antrum due to hyperglycemia.

Methods

Inhibitory electrical junction potentials were recorded from gastric antral muscle strips, using intracellular electrodes under non-adrenergic, non-cholinergic conditions. Studies were performed in non-hyperglycemic NOD (NH-NOD), NOD mice as they develop hyperglycemia (H-NOD) and their age-matched controls. The purinergic inhibitory junction potential (pIJP) and nitrergic IJP (nIJP) were isolated pharmacologically.

Results

The control pIJP was large, around −18 mV and nIJP was small, around −9 mV. In NH-NOD the IJPs were not affected, but in H-NOD pIJP was nearly abolished and nIJP was significantly reduced. In H-NOD mice, membrane hyperpolarization caused by exogenous α,β-MeATP or diethylenetriamine NO adduct was similar to that in wild-type controls (P > 0.05). H-NOD smooth muscles were significantly depolarized as compared to NH-NOD smooth muscles.

Conclusion

These observations show that hyperglycemia causes suppression of purinergic and nitrergic transmission by acting on the motor neurons that form the last neuron in the vagovagal circuit. Moreover, the loss the neurotransmission is due to a defect in neurotransmitter release rather than a defect in signal transduction. Hyperglycemia also causes depolarization of smooth muscles that may increase their excitability.

Synchronized dual pulse gastric electrical stimulation improves gastric emptying and activates enteric glial cells via upregulation of GFAP and S100B with different courses of subdiaphragmatic vagotomy in rats

Previous research and clinical practice have indicated that damage to the vagal nerve may seriously affect gastrointestinal physiological movement behavior. The aim of the current study was to observe the change of gastric motility, as well as enteric glial cells (EGCs) in the stomach with different courses of vagal nerve transection in rats prior to and following synchronized dual pulse gastric electrical stimulation. The gastric emptying rates were measured to assess the gastric motility. The glial markers, containing calcium binding protein (S100B) and glial fibrillary acidic protein (GFAP), were detected by reverse transcription‑quantitative polymerase chain reaction and double‑labeling immunofluorescence analysis. Ultrastructural changes of EGCs were observed using transmission electron microscopy. Gastric emptying was delayed in the terminal vagotomy group, compared with the terminal control group. The effect of long‑term synchronized dual pulse gastric electrical stimulation (SGES) was superior to short‑term SGES in terminal groups. The expression levels of S100B/GFAP were markedly decreased in the terminal vagotomy group compared with the terminal control group. Following short‑term or long‑term SGES, S100B/GFAP gene and protein expression increased in terminal groups. However, long‑term SGES was more effective than short‑term SGES and the difference was statistically significant. Vagal nerve damage leads to gastric motility disorder and weakens the function of EGCs. Therefore, SGES may improve stomach movement behavior and restore the impaired EGCs. The underlying mechanism of the effect remains elusive, but maybe associated with activation of EGCs.

High fat diet attenuates glucose-dependent facilitation of 5-HT3 -mediated responses in rat gastric vagal afferents

KEY POINTS

Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Diet-induced obesity compromises the excitability and responsiveness of vagal afferents. In this study, we assessed whether exposure to a high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. We show that HFD does not alter the response of gastric vagal afferent nerves and neurones to 5-HT but attenuates the ability of glucose to amplify 5-HT3-induced responses. These results suggest that glucose-dependent vagal afferent signalling is compromised by relatively short periods of exposure to HFD well in advance of the development of obesity or glycaemic dysregulation. Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Since diet-induced obesity attenuates the responsiveness of gastric vagal afferents to several neurohormones, the aim of the present study was to determine whether high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. Rats were fed control or HFD (14% or 60% kilocalories from fat, respectively) for up to 8 weeks. Neurophysiological recordings assessed the ability of 5-HT to increase anterior gastric vagal afferent nerve (VAN) activity in vivo before and after acute hyperglycaemia, while electrophysiological recordings from gastric-projecting nodose neurones assessed the ability of glucose to modulate the 5-HT response in vitro. Immunocytochemical studies determined alterations in the neuronal distribution of 5-HT3 receptors. 5-HT and cholecystokinin (CCK) induced dose-dependent increases in VAN activity in all rats; HFD attenuated the response to CCK, but not 5-HT. The 5-HT-induced response was amplified by acute hyperglycaemia in control, but not HFD, rats. Similarly, although 5-HT induced an inward current in both control and HFD gastric nodose neurones in vitro, the 5-HT response and receptor distribution was amplified by acute hyperglycaemia only in control rats. These data suggest that, while HFD does not affect the response of gastric-projecting vagal afferents to 5-HT, it attenuates the ability of glucose to amplify 5-HT effects. This suggests that glucose-dependent vagal afferent signalling is compromised by short periods of exposure to HFD well in advance of obesity or glycaemic dysregulation.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Decreased L-type calcium current in antral smooth muscle cells of STZ-induced diabetic rats

BACKGROUND

Diabetic gastroparesis (delayed gastric emptying) is associated with antral hypomotility. L-type Ca(2+) channels play an important role in generation of action potentials and activation of contractions. This study was designed to investigate if the function of the L-type Ca(2+) channels of antral circular smooth muscle cells (SMCs) is impaired in streptozotocin (STZ)-induced diabetic rats.

METHODS

Eight weeks after the injection of STZ or vehicle, whole-cell patch clamp was used to record Ca(2+) currents, and isometric tension recording was used to measure Ca(2+) influx-induced contractions in circular muscle strips. Solid gastric emptying was measured in diabetic and control rats. Protein expression of Ca(2+) αlC-subunit in antral smooth muscles was compared between diabetic and control rats.

KEY RESULTS

(1) Solid gastric emptying, independent of age or bodyweight, was slower in the diabetic rats, even after acute correction of hyperglycemia. (2) Verapamil, a potent calcium channel blocker, dose dependently reduced solid gastric emptying in normal rats. (3) Current density of L-type Ca(2+) channel at 10 mV in antral circular SMCs was significantly decreased in the diabetic rats (-9.8 ± 0.7 pA/pF vs -15.9 ± 1.0 pA/pF in control, p < 0.001). However, protein expression of the Ca(2+) channel in antral muscles did not differ between diabetic and control rats. (4) Contractile responses to 1 and 3 mM [Ca(2+) ] were significantly reduced in the diabetic antral circular muscle strips, indicative of reduced Ca(2+) influx.

CONCLUSIONS & INFERENCES

These data suggested that the decreased L-type Ca(2+) current in antral SMCs may contribute to antral hypomotility in STZ-induced diabetic rats.

Vagal anandamide signaling via cannabinoid receptor 1 contributes to luminal 5-HT modulation of visceral nociception in rats

Serotonin (5-HT) plays pivotal roles in the pathogenesis of postinfectious irritable bowel syndrome (PI-IBS), and luminal 5-HT time-dependently modulates visceral nociception. We found that duodenal biopsies from PI-IBS patients exhibited increased 5-HT and decreased anandamide levels and that decreased anandamide was associated with abdominal pain severity, indicating a link between 5-HT and endocannabinoid signaling pathways in PI-IBS. To understand this, we investigated the role of endocannabinoids in 5-HT modulation of visceral nociception in a rat model. Acute intraduodenally applied 5-HT attenuated the visceromotor response (VMR) to colorectal distention, and this was reversed by the cannabinoid receptor 1 (CB1) antagonist AM251. Duodenal anandamide (but not 2-arachidonoylglycerol) content was greatly increased after luminal 5-HT treatment. This effect was abrogated by the 5-HT 3 receptor (5-HT3R) antagonist granisetron, which was luminally delivered to preferentially target vagal terminals. Chemical denervation of vagal afferents blocked 5-HT-evoked antinociception and anandamide release. Chronic luminal 5-HT exposure for 5 days increased baseline VMR and VMR post-5-HT (days 4 and 5). Duodenal levels of anandamide and N-acyl-phosphatidylethanolamine-specific phospholipase D (NAPE-PLD, the anandamide-synthesizing enzyme) protein gradually declined from day 1 to 5. The time-dependent effects of 5-HT were abolished by daily granisetron pretreatment. Daily pretreatment with CB1 agonists or anandamide from day 3 attenuated 5-HT-induced hyperalgesia. These data suggest that vagal 5-HT3R-mediated duodenal anandamide release contributes to acute luminal 5-HT-induced antinociception via CB1 signaling, whereas decreased anandamide is associated with hyperalgesia upon chronic 5-HT treatment. Further understanding of peripheral vagal anandamide signaling may provide insights into the mechanisms underlying 5-HT-related IBS.

Mapping glucose-mediated gut-to-brain signalling pathways in humans

Objectives

Previous fMRI studies have demonstrated that glucose decreases the hypothalamic BOLD response in humans. However, the mechanisms underlying the CNS response to glucose have not been defined. We recently demonstrated that the slowing of gastric emptying by glucose is dependent on activation of the gut peptide cholecystokinin (CCK₁) receptor. Using physiological functional magnetic resonance imaging this study aimed to determine the whole brain response to glucose, and whether CCK plays a central role.

Experimental design

Changes in blood oxygenation level-dependent (BOLD) signal were monitored using fMRI in 12 healthy subjects following intragastric infusion (250 ml) of: 1 M glucose + predosing with dexloxiglumide (CCK₁ receptor antagonist), 1 M glucose + placebo, or 0.9% saline (control) + placebo, in a single-blind, randomised fashion. Gallbladder volume, blood glucose, insulin, and GLP-1 and CCK concentrations were determined. Hunger, fullness and nausea scores were also recorded.

Principal observations

Intragastric glucose elevated plasma glucose, insulin, and GLP-1, and reduced gall bladder volume (an in vivo assay for CCK secretion). Glucose decreased BOLD signal, relative to saline, in the brainstem and hypothalamus as well as the cerebellum, right occipital cortex, putamen and thalamus. The timing of the BOLD signal decrease was negatively correlated with the rise in blood glucose and insulin levels. The glucose + dex arm highlighted a CCK₁-receptor dependent increase in BOLD signal only in the motor cortex.

Conclusions

Glucose induces site-specific differences in BOLD response in the human brain; the brainstem and hypothalamus show a CCK₁ receptor-independent reduction which is likely to be mediated by a circulatory effect of glucose and insulin, whereas the motor cortex shows an early dexloxiglumide-reversible increase in signal, suggesting a CCK₁ receptor-dependent neural pathway.

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We have identified two distinct CNS responses to glucose in man.

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A CCK1 receptor (CCK1R)-dependent BOLD signal increase in the motor cortex.

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A CCK1R-independent BOLD signal decrease in the brainstem and hypothalamus.

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The BOLD signal decrease was mediated by changes in blood glucose and insulin,

MicroRNA 375 mediates palmitate-induced enteric neuronal damage and high-fat diet-induced delayed intestinal transit in mice

BACKGROUND & AIMS

A high-fat diet (HFD) can cause serious health problems, including alteration of gastrointestinal transit, the exact mechanism of which is not clear. Several microRNAs (miRNAs) are involved in energy homeostasis, lipid metabolism, and HFD-induced weight gain. We investigated the role of miRNAs in HFD-induced damage to the enteric nervous system.

METHODS

Male mice were fed a HFD (60% calories from fat) or regular diets (18% calories from fat) for 11 weeks. Mice on regular diets and HFDs were given intraperitoneal injections of Mir375 inhibitor or a negative control. Body weights, food intake, stool indices, and gastrointestinal transit (following Evans blue gavage) were measured. An enteric neuronal cell line (immorto-fetal enteric neuronal) and primary enteric neurons were used for in vitro studies.

RESULTS

HFD delayed intestinal transit, which was associated with increased apoptosis and loss of colonic myenteric neurons. Mice fed a low-palmitate HFD did not develop a similar phenotype. Palmitate caused apoptosis of enteric neuronal cells associated with mitochondrial dysfunction and endoplasmic reticulum stress. Palmitate significantly increased the expression of Mir375 in vitro; transfection of cells with a Mir375 inhibitor prevented the palmitate-induced enteric neuronal cell apoptosis. Mir375 expression was increased in myenteric ganglia of mice fed HFD and associated with decreased levels of Mir375 target messenger RNAs, including Pdk1. Systemic injection of a Mir375 inhibitor for 5 weeks prevented HFD-induced delay in intestinal transit and morphologic changes.

CONCLUSIONS

HFDs delay colonic transit, partly by inducing apoptosis in enteric neuronal cells. This effect is mediated by Mir375 and is associated with reduced levels of Pdk1. Mir375 might be targeted to increase survival of enteric neurons and gastrointestinal motility.

Modulation of gastrointestinal vagal neurocircuits by hyperglycemia

Glucose sensing within autonomic neurocircuits is critical for the effective integration and regulation of a variety of physiological homeostatic functions including the co-ordination of vagally-mediated reflexes regulating gastrointestinal (GI) functions. Glucose regulates GI functions via actions at multiple sites of action, from modulating the activity of enteric neurons, endocrine cells, and glucose transporters within the intestine, to regulating the activity and responsiveness of the peripheral terminals, cell bodies and central terminals of vagal sensory neurons, to modifying both the activity and synaptic responsiveness of central brainstem neurons. Unsurprisingly, significant impairment in GI functions occurs in pathophysiological states where glucose levels are dysregulated, such as diabetes. A substantial obstacle to the development of new therapies to modify the disease, rather than treat the symptoms, are the gaps in our understanding of the mechanisms by which glucose modulates GI functions, particularly vagally-mediated responses and a more complete understanding of disease-related plasticity within these neurocircuits may open new avenues and targets for research.

Melanocortin signaling in the brainstem influences vagal outflow to the stomach

Activation of melanocortin 4 receptors (MC4-Rs) in brain nuclei associated with food intake profoundly influences consummatory behavior. Of these nuclei, the dorsal motor vagal nucleus (DMV), which has a dense concentration of MC4-Rs, is an important regulator of gastric tone and motility. Hence, the present study sought to examine the role of MC4-Rs in this nucleus on these activities. Using an in vivo approach, MC4-R agonists, melanotan-II (MT-II) or α-melanocyte stimulating hormone (α-MSH), were unilaterally microinjected into the DMV of rats, and their effects were noted on gastric activity. MT-II decreased phasic contractions, whereas α-MSH increased their amplitude. Both effects were blocked by the MC4-R antagonist SHU9119 or by ipsilateral vagotomy. Microinjection of the agonists (MT-II and α-MSH) into the overlying nucleus of the solitary tract (NTS), an important component of "vago-vagal" gastric circuitry, decreased phasic contractions. In addition, α-MSH reduced gastric tone and mean arterial blood pressure. To study the underlying mechanisms of the effect of MC4-R stimulation on gastric activity, electrophysiological recordings were made from labeled DMV antrum neurons in rat pups and MC4-R(-/-) mice. Bath application of MT-II or α-MSH significantly reduced spontaneous action potentials (but not in MC4-R(-/-) mice). However, in low-calcium ACSF, MT-II decreased neuronal firing, whereas α-MSH increased it. These effects mirror those of our in vivo DMV studies. Altogether, our novel findings show that activation of MC4-Rs in the brainstem, particularly in the medial NTS by the endogenous peptide α-MSH, modulates gastric activity, which may have physiological relevance for food intake and gastric function.

The Neural Mechanism by Which the Dorsal Vagal Complex Mediates the Regulation of the Gastric Motility by Weishu (RN12) and Zhongwan (BL21) Stimulation

A large number of studies have been conducted to explore the mechanism of Back-Shu and Front-Mu points. While several lines of evidence addressed the acupuncture information of Shu acupoints and Mu acupoints gathering in the spinal cord, whether the convergence is extended to the high centre still remains unclear. The study selected gastric Mu points (RN12) and gastric Shu points (BL21) regulating gastric motility and its central neural mechanisms as the breakthrough point, using the technique of immunochemistry, nuclei lesion, electrophysiology, and nerve transection. Here, we report that gastric motility regulation of gastric Shu and Mu acupoints and their synergistic effect and the signals induced by electroacupuncture (EA) stimulation of acupoints RN12 and RN12 gather in the dorsal vagal complex (DVC), increasing the levels of gastrointestinal hormones in the DVC to regulate gastric motility through the vagus. In sum, our data demonstrate an important role of DVC and vagus in the regulation of gastric motility by EA at gastric Shu and Mu points.

Essential elements for glucosensing by gastric vagal afferents: immunocytochemistry and electrophysiology studies in the rat

Glucosensing nodose ganglia neurons mediate the effects of hyperglycemia on gastrointestinal motility. We hypothesized that the glucose-sensing mechanisms in the nodose ganglia are similar to those of hypothalamic glucose excited neurons, which sense glucose through glycolysis. Glucose metabolism leads to ATP-sensitive potassium channel (K(ATP)) channel closure and membrane depolarization. We identified glucosensing elements in the form of glucose transporters (GLUTs), glucokinase (GK), and K(ATP) channels in rat nodose ganglia and evaluated their physiological significance. In vitro stomach-vagus nerve preparations demonstrated the gastric vagal afferent response to elevated glucose. Western blots and RT-PCR revealed the presence of GLUT1, GLUT3, GLUT4, GK, and Kir6.2 in nodose ganglia neurons and gastric branches of the vagus nerve. Immunocytochemistry confirmed the expression of GLUT3, GK, and Kir6.2 in nodose ganglia neurons (46.3 ± 3%). Patch-clamp studies detected glucose excitation in 30% (25 of 83) of gastric-projecting nodose ganglia neurons, which was abolished by GLUT3 or GK short hairpin RNA transfections. Silencing GLUT1 or GLUT4 in nodose ganglia neurons did not prevent the excitatory response to glucose. Elevated glucose elicited a response from 43% of in vitro nerve preparations. A dose-dependent response was observed, reaching maximum at a glucose level of 250 mg/dl. The gastric vagal afferent responses to glucose were inhibited by diazoxide, a K(ATP) channel opener. In conclusion, a subset of neurons in the nodose ganglia and gastric vagal afferents are glucoresponsive. Glucosensing requires a GLUT, GK, and K(ATP) channels. These elements are transported axonally to the gastric vagal afferents, which can be activated by elevated glucose through modulation of K(ATP) channels.

Glucose-dependent trafficking of 5-HT3 receptors in rat gastrointestinal vagal afferent neurons

BACKGROUND

Intestinal glucose induces gastric relaxation via vagally mediated sensory-motor reflexes. Glucose can alter the activity of gastrointestinal (GI) vagal afferent (sensory) neurons directly, via closure of ATP-sensitive potassium channels, and indirectly, via the release of 5-hydroxytryptamine (5-HT) from mucosal enteroendocrine cells. We hypothesized that glucose may also be able to modulate the ability of GI vagal afferent neurons to respond to the released 5-HT, via regulation of neuronal 5-HT(3) receptors.

METHODS

Whole-cell patch clamp recordings were made from acutely dissociated GI-projecting vagal afferent neurons exposed to equiosmolar Krebs' solution containing different concentrations of d-glucose (1.25-20 mmol L(-1)) and the response to picospritz application of 5-HT assessed. The distribution of 5-HT(3) receptors in neurons exposed to different glucose concentrations was also assessed immunohistochemically.

KEY RESULTS

Increasing or decreasing extracellular d-glucose concentration increased or decreased, respectively, the 5-HT-induced inward current and the proportion of 5-HT(3) receptors associated with the neuronal membrane. These responses were blocked by the Golgi-disrupting agent Brefeldin-A (5 μmol L(-1)) suggesting involvement of a protein-trafficking pathway. Furthermore, l-glucose did not mimic the response of d-glucose implying that metabolic events downstream of neuronal glucose uptake are required to observe the modulation of 5-HT(3) receptor mediated responses.

CONCLUSIONS & INFERENCES

These results suggest that, in addition to inducing the release of 5-HT from enterochromaffin cells, glucose may also increase the ability of GI vagal sensory neurons to respond to the released 5-HT, providing a means by which the vagal afferent signal can be amplified or prolonged.

Severe intestinal ischemia can trigger cardiovascular collapse and sudden death via a parasympathetic mechanism

Hemorrhagic shock and splanchnic arterial occlusion (SAO) followed by reperfusion are associated with high mortality. However, rapid cardiovascular failure and death may also occur before reperfusion in hemorrhagic shock and SAO. We show in a rat SAO model that, upon gut ischemia, mean arterial blood pressure transiently elevates and then drops fatally in one of two time courses: (i) gradually over ∼1 to 3 h or (ii) rapidly (often by >80 mmHg) over a period of 1 to 6 min. We hypothesize that fast fatal pressure drops (FFPDs) are due to failure of autonomic nervous system control. To test this, we treated rats with Glucose (10%) in the small intestinal lumen and intramuscularly administered xylazine to activate the parasympathetic nervous system or with a muscarinic anticholinergic (glycopyrrolate) or by total subdiaphragmatic vagotomy to attenuate parasympathetic nervous system activity. We also tested nafamostat mesilate (ANGD [6-amidino-2-naphthyl p-guanidinobenzoate dimethanesulfonate]), a protease inhibitor efficacious in preventing blood pressure loss in SAO with reperfusion, in the intestinal lumen. Fifty percent of animals receiving xylazine and Glucose died by FFPD (vs. 33% with neither, not statistically significant). Total subdiaphragmatic vagotomy or glycopyrrolate treatment significantly reduced the incidence to 0% (P < 0.008), although slow fatal pressure drops still occurred. ANGD did not prevent FFPDs, but delayed onset of slow fatal pressure drops (P < 0.013). These results suggest that gut ischemia can cause sudden death via an autonomic nervous system mechanism and that SAO with Glucose and xylazine may serve as a useful model for the study of neurogenic shock or autonomic dysregulation associated with sudden death.

Tetrahydroberberine, an isoquinoline alkaloid isolated from corydalis tuber, enhances gastrointestinal motor function

Because delayed gastric emptying and impaired gastric accommodation are regarded as pathophysiological mechanisms underlying functional dyspepsia (FD), prokinetics and fundic relaxants have been suggested as a new treatment for FD. We isolated tetrahydroberberine (THB), an isoquinoline alkaloid (5,8,13,13a-tetrahydro-9,10-dimethoxy-6H-benzo[g]-1,3-benzodioxolo[5,6-a]quinolizine) from Corydalis tuber, and found that it has micromolar affinity for dopamine D(2) (pK(i) = 6.08) and 5-HT(1A) (pK(i) = 5.38) receptors but moderate to no affinity for other relevant serotonin receptors (i.e., 5-HT(1B), 5-HT(1D), 5-HT(3), and 5-HT(4); pK(i) < 5.00). Oral administration of THB not only resulted in significantly accelerated gastric emptying of normal rats in a bell-shaped relationship, with a maximal efficacy at a dose of 30 μg/kg, but also restored the delayed gastric emptying caused by apomorphine, which might be mediated by an antidopaminergic effect. Data from electromyography indicated enhanced motor function of the upper gastrointestinal tract by THB, which occurred through strengthening contractility and shortening the contraction interval. Furthermore, in rats stressed by repeated restraint, a significantly higher shift in the pressure-volume curve by THB (10 μg/kg, p < 0.05), which was inhibited by [O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY-100635), a 5-HT(1A) antagonist, and N(ω)-nitro-l-arginine methyl ester, a nitric-oxide synthase inhibitor but not a vasoactive intestinal peptide antagonist, was observed. Oral administration of THB resulted in a drastic increase of gastric accommodation in Beagle dogs. Area under the volume versus time curve was increased significantly by THB (30 μg/kg, p < 0.01) and comparable with that of sumatriptan (3 mg/kg), a potent fundic relaxant. Taken together, our data suggested that THB, with D(2) receptor antagonist and 5-HT(1A) receptor agonist properties, has significant potential as a therapeutic for treatment of FD.

Plasticity of vagal brainstem circuits in the control of gastrointestinal function

The afferent vagus transmits sensory information from the gastrointestinal (GI) tract and other viscera to the brainstem via a glutamatergic synapse at the level of the nucleus of the solitary tract (NTS). Second order NTS neurons integrate this sensory information with inputs from other CNS regions that regulate autonomic functions and homeostasis. Glutamatergic and GABAergic neurons are responsible for conveying the integrated response to other nuclei, including the adjacent dorsal motor nucleus of the vagus (DMV). The preganglionic neurons in the DMV are the source of the parasympathetic motor response back to the GI tract. The glutamatergic synapse between the NTS and DMV is unlikely to be tonically active in regulating gastric motility and tone although almost all neurotransmitters tested so far modulate transmission at this synapse. In contrast, the tonic inhibitory GABAergic input from the NTS to the DMV appears to be critical in setting the tone of gastric motility and, under basal conditions, is unaffected by many neurotransmitters or neurohormones. This review is based, in part, on a presentation by Dr Browning at the 2009 ISAN meeting in Sydney, Australia and discusses how neurohormones and macronutrients modulate glutamatergic transmission to NTS neurons and GABAergic transmission to DMV neurons in relation to sensory information that is received from the GI tract. These neurohormones and macronutrients appear to exert efficient "on-demand" control of the motor output from the DMV in response to ever-changing demands required to maintain homeostasis.

Inhibition of gastric motility by hyperglycemia is mediated by nodose ganglia KATP channels

The inhibitory action of hyperglycemia is mediated by vagal afferent fibers innervating the stomach and duodenum. Our in vitro studies showed that a subset of nodose ganglia neurons is excited by rising ambient glucose, involving inactivation of ATP-sensitive K(+) (K(ATP)) channels and leading to membrane depolarization and neuronal firing. To investigate whether nodose ganglia K(ATP) channels mediate gastric relaxation induced by hyperglycemia, we performed in vivo gastric motility studies to examine the effects of K(ATP) channel activators and inactivators. Intravenous infusion of 20% dextrose induced gastric relaxation in a dose-dependent manner. This inhibitory effect of hyperglycemia was blocked by diazoxide, a K(ATP) channel activator. Conversely, tolbutamide, a K(ATP) channel inactivator, induced dose-dependent gastric relaxation, an effect similar to hyperglycemia. Vagotomy, perivagal capsaicin treatment, and hexamethonium each prevented the inhibitory action of tolbutamide. Similarly, N(G)-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthase, also blocked tolbutamide's inhibitory effect. To show that K(ATP) channel inactivation at the level of the nodose ganglia induces gastric relaxation, we performed electroporation of the nodose ganglia with small interfering RNA of Kir6.2 (a subunit of K(ATP)) and plasmid pEGFP-N1 carrying the green fluorescent protein gene. The gastric responses to hyperglycemia and tolbutamide were not observed in rats with Kir6.2 small interfering RNA-treated nodose ganglia. However, these rats responded to secretin, which acts via the vagal afferent pathway, independently of K(ATP) channels. These studies provide in vivo evidence that hyperglycemia induces gastric relaxation via the vagal afferent pathway. This action is mediated through inactivation of nodose ganglia K(ATP) channels.

Luminal serotonin time-dependently modulates vagal afferent driven antinociception in response to colorectal distention in rats

BACKGROUND

Compelling evidence shows that vagal afferents mediate antinociception in response to visceral insults. Our recent findings implied that luminal serotonin (5-hydroxytryptamine, 5-HT) might mediate chronic food allergen sensitized visceral hyperalgesia, in which vagal afferents might be implicated. Here, to test this hypothesis, we investigated the effects of luminal infused 5-HT on visceral nociception and the involvement of vagal antinociceptive pathway.

METHODS

The vagus-intact or vagotomized rats were given acute intraluminally or intraperitoneally administered 5-HT, or chronic luminal infusion of 5-HT. The visceromotor response (VMR) to colorectal distension (CRD) was electrophysiologically recorded.

KEY RESULTS

Acute intraluminal infusion of 5-HT (10 or 100 nmol) significantly attenuated VMR to CRD, while systemic administered 5-HT at similar doses resulted in markedly augmented nociception. Pretreatment with luminal application of granisetron or lidocaine, or pharmacological depletion of endogenous 5-HT with injection of p-chlorophenylalanine, a 5-HT synthesis inhibitor, and subdiaphragmatic vagotomy or functional deafferentation with capsaicin abolished the effect of luminal (but not systemic) 5-HT. Chronic infusion of 5-HT (10 nmol d(-1) for 5 days) produced gradual augmentation of baseline VMR. And, the VMR to CRD after 5-HT infusion decreased on day 1 and 2, then gradually increased from day 3. Surgical vagotomy or daily preperfusion with granisetron canceled these time-dependent patterns.

CONCLUSIONS & INFERENCES

Luminal 5-HT time-dependently modulates vagal afferent driven antinociception. Acute infusion of 5-HT attenuates visceral nociception via activation of vagal afferent 5-HT type 3 receptors (5-HT(3)Rs)within intestinal mucosa; while chronic luminal 5-HT caused gradually developed visceral hyperalgesia, which may also involve vagal 5-HT(3)Rs.

Targeting type 2 diabetes

The evolving concept of how nutrient excess and inflammation modulate metabolism provides new opportunities for strategies to correct the detrimental health consequences of obesity. In this review, we focus on the complex interplay among lipid overload, immune response, proinflammatory pathways and organelle dysfunction through which excess adiposity might lead to type 2 diabetes. We then consider evidence linking dysregulated CNS circuits to insulin resistance and results on nutrient-sensing pathways emerging from studies with calorie restriction. Subsequently, recent recommendations for the management of type 2 diabetes are discussed with emphasis on prevailing current therapeutic classes of biguanides, thiazolidinediones and incretin-based approaches.

Gut sensing mechanisms

The mechanisms by which the gut senses and responds to nutrients involve the interplay of multiple complex pathways. In addition to regulating digestion and absorption, the pathways stimulated by molecules in the gut lumen mediate gastric motility, food intake, and satiety. Furthermore, protective mechanisms are activated as necessary to prevent injury, promote healing, and limit intake and absorption of potentially toxic substances. This review provides an update on the current knowledge and recent findings related to gastric sensing of nutrients, highlighting recent research and future endeavors in the field.

Gastric emptying of enterally administered liquid meal in conscious rats and during sustained anaesthesia

BACKGROUND

Gastric motility studies are frequently conducted with anaesthetized animal models. Some studies on the same animal species have reported differences in vagal control of the stomach that could not be explained solely by slightly different experimental conditions. A possible limitation in the comparison between similar studies relates to the use of different anaesthetic agents. Furthermore, anaesthetic effects may also limit generalizations between mechanistic studies of gastric function and the gastric function of conscious animals. In the present study, we used the [(13)C]-breath test following a liquid mixed-nutrient test meal (Ensure), 1 ml) with the aim to investigate the rate of gastric emptying in animals that were either conscious or anaesthetized with either Inactin or urethane.

METHODS

One week after determining the maximum (13)CO(2) concentration, time to peak [(13)C] recovery and gastric half emptying time in control, conscious rats, we repeated the experiment in the same rats anaesthetized with Inactin or urethane.

KEY RESULTS

Our data show that Inactin anaesthesia prolonged the time to peak [(13)C] recovery but did not significantly reduce the maximum (13)CO(2) concentration nor delay gastric half emptying time. Conversely, urethane anaesthesia resulted in a significant slowing of all parameters of gastric emptying as measured by the maximum (13)CO(2) concentration, time to peak [(13)C] recovery and half emptying time.

CONCLUSIONS & INFERENCES

Our data indicate that Inactin(R) anaesthesia does not significantly affect gastric emptying while urethane anaesthesia profoundly impairs gastric emptying. We suggest that Inactin(R), not urethane, is the more suitable anaesthetic for gastrointestinal research.

Electrophysiological identification of glucose-sensing neurons in rat nodose ganglia

The vagal afferent system is strategically positioned to mediate rapid changes in motility and satiety in response to systemic glucose levels. In the present study we aimed to identify glucose-excited and glucose-inhibited neurons in nodose ganglia and characterize their glucose-sensing properties. Whole-cell patch-clamp recordings in vagal afferent neurons isolated from rat nodose ganglia demonstrated that 31/118 (26%) neurons were depolarized after increasing extracellular glucose from 5 to 15 mm; 19/118 (16%) were hyperpolarized, and 68/118 were non-responsive. A higher incidence of excitatory response to glucose occurred in gastric- than in portal vein-projecting neurons, the latter having a higher incidence of inhibitory response. In glucose-excited neurons, elevated glucose evoked membrane depolarization (11 mV) and an increase in membrane input resistance (361 to 437 M). Current reversed at 99 mV. In glucose-inhibited neurons, membrane hyperpolarization (13 mV) was associated with decreased membrane input resistance (383 to 293 M). Current reversed at 97 mV. Superfusion of tolbutamide, a K(ATP) channel sulfonylurea receptor blocker, elicited identical glucose-excitatory but not glucose-inhibitory responses. Kir6.2 shRNA transfection abolished glucose-excited but not glucose-inhibited responses. Phosphatidylinositol bisphosphate (PIP(2)) depletion using wortmannin increased the fraction of glucose-excited neurons from 26% to 80%. These results show that rat nodose ganglia have glucose-excited and glucose-inhibited neurons, differentially distributed among gastric- and portal vein-projecting nodose neurons. In glucose-excited neurons, glucose metabolism leads to K(ATP) channel closure, triggering membrane depolarization, whereas in glucose-inhibited neurons, the inhibitory effect of elevated glucose is mediated by an ATP-independent K(+) channel. The results also show that PIP(2) can determine the excitability of glucose-excited neurons.

Secretin-induced gastric relaxation is mediated by vasoactive intestinal polypeptide and prostaglandin pathways

Secretin has been shown to delay gastric emptying and inhibit gastric motility. We have demonstrated that secretin acts on the afferent vagal pathway to induce gastric relaxation in the rat. However, the efferent pathway that mediates the action of secretin on gastric motility remains unknown. We recorded the response of intragastric pressure to graded doses of secretin administered intravenously to anaesthetized rats using a balloon attached to a catheter and placed in the body of the stomach. Secretin evoked a dose-dependent decrease in intragastric pressure. The threshold dose of secretin was 1.4 pmol kg(-1) h(-1) and the effective dose, 50% was 5.6 pmol kg(-1) h(-1). Pretreatment with hexamethonium markedly reduced gastric relaxation induced by secretin (5.6 pmol kg(-1) h(-1)). Bilateral vagotomy also significantly reduced gastric motor responses to secretin. Administration of N(G)-nitro-L-arginine methyl ester (10 mg kg(-1)) did not affect gastric relaxation induced by secretin. In contrast, intravenous administration of a vasoactive intestinal polypeptide (VIP) antagonist (30 nmol kg(-1)) reduced the gastric relaxation response to secretin (5.6 pmol kg(-1) h(-1)) by 89 +/- 5%. Indomethacin (2 mg kg(-1)) reduced gastric relaxation induced by secretin (5.6 pmol kg(-1) h(-1)) by 87 +/- 5%. Administration of prostaglandin (48 mg kg(-1) h(-1)) prevented this inhibitory effect. Indomethacin also reduced gastric relaxation induced by VIP (300 pmol kg(-1)) by 90 +/- 7%. These observations indicate that secretin acts through stimulation of presynaptic cholinergic neurons in a vagally mediated pathway. Through nicotinic synapses, secretin stimulates VIP release from postganglionic neurons in the gastric myenteric plexus, which in turn induces gastric relaxation through a prostaglandin-dependent pathway.

Effects of gastric vagotomy on visceral cell proliferation induced by ventromedial hypothalamic lesions: role of vagal hyperactivity

In rats, ventromedial hypothalamic (VMH) lesions induce cell proliferation in the visceral organs (stomach, small intestine, liver, and pancreas) due to hyperactivity of the vagus nerve. To investigate the effects of selective gastric vagotomy on VMH lesion-induced cell proliferation and secretion of gastric acid, we assessed the mitotic index (the number of proliferating cell nuclear antigen (PCNA)-immunopositive cells per 1,000 cells in the gastric mucosal cell layer) and measured the volume of secreted basal gastric acid. Furthermore, to explore whether or not ethanol-induced acute gastric mucosal lesions (AGML) lead to ulcer formation in VMH-lesioned rats, we assessed the ulcer index of both sham-operated and VMH-lesioned rats after administration of ethanol. VMH lesions resulted in an increased mitotic index and thickness of the gastric mucosal cell layer and gave rise to the hypersecretion of gastric acid. Selective gastric vagotomy restored these parameters to normal without affecting cell proliferation in other visceral organs. Ethanol-induced AGML caused ulcers in sham VMH-lesioned rats, whereas VMH-lesioned rats were less likely to exhibit such ulcers. These results suggest that VMH lesion-induced vagally mediated cell proliferation in the visceral organs is associated with hyperfunction in these organs, and VMH lesion-induced resistance to ethanol may be due to thickening of the gastric mucosal cell layer resulting from cell proliferation in the gastric mucosa-this in turn is due to hyperactivity of the vagus nerve.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

Glucose increases synaptic transmission from vagal afferent central nerve terminals via modulation of 5-HT3 receptors

Acute hyperglycemia has profound effects on vagally mediated gastrointestinal functions. We have reported recently that the release of glutamate from the central terminals of vagal afferent neurons is correlated directly with the extracellular glucose concentration. The present study was designed to test the hypothesis that 5-HT(3) receptors present on vagal afferent nerve terminals are involved in this glucose-dependent modulation of glutamatergic synaptic transmission. Whole-cell patch-clamp recordings were made from neurons of the nucleus tractus solitarius (NTS) in thin rat brainstem slices. Spontaneous and evoked glutamate release was decreased in a concentration-dependent manner by the 5-HT(3) receptor selective antagonist, ondansetron. Alterations in the extracellular glucose concentration induced parallel shifts in the ondansetron-mediated inhibition of glutamate release. The changes in excitatory synaptic transmission induced by extracellular glucose concentration were mimicked by the serotonin uptake inhibitor, fenfluramine. These data suggest that glucose alters excitatory synaptic transmission within the rat brainstem via actions on tonically active 5-HT(3) receptors, and the number of 5-HT(3) receptors on vagal afferent nerve terminals is positively correlated with the extracellular glucose concentration. These data indicate that the 5-HT(3) receptors present on synaptic connections between vagal afferent nerve terminals and NTS neurons are a strong candidate for consideration as one of the sites where glucose acts to modulate vagovagal reflexes.