Changes in serotonin levels and 5-HT receptor activity in duodenum of streptozotocin-diabetic rats

Irritable Bowel Syndrome Is Associated With Brain Health by Neuroimaging, Behavioral, Biochemical, and Genetic Analyses

BACKGROUND

Irritable bowel syndrome (IBS) interacts with psychopathology in a complex way; however, little is known about the underlying brain, biochemical, and genetic mechanisms.

METHODS

To clarify the phenotypic and genetic associations between IBS and brain health, we performed a comprehensive retrospective cohort study on a large population. Our study included 171,104 participants from the UK Biobank who underwent a thorough assessment of IBS, with the majority also providing neuroimaging, behavioral, biochemical, and genetic information. Multistage linked analyses were conducted, including phenome-wide association analysis, polygenic risk score calculation, and 2-sample Mendelian randomization analysis.

RESULTS

The phenome-wide association analysis showed that IBS was linked to brain health problems, including anxiety and depression, and poor cognitive performance. Significantly lower brain volumes associated with more severe IBS were found in key areas related to emotional regulation and higher-order cognition, including the medial orbitofrontal cortex/ventromedial prefrontal cortex, anterior insula, anterior and mid-cingulate cortices, dorsolateral prefrontal cortex, and hippocampus. Higher triglycerides, lower high-intensity lipoprotein, and lower platelets were also related (p < 1 × 10-10) to more severe IBS. Finally, Mendelian randomization analyses demonstrated potential causal relationships between IBS and brain health and indicated possible mediating effects of dyslipidemia and inflammation.

CONCLUSIONS

For the first time, this study provides a comprehensive understanding of the relationship between IBS and brain health phenotypes, integrating perspectives from neuroimaging, behavioral performance, biochemical factors, and genetics, which is of great significance for clinical applications to potentially address brain health impairments in patients with IBS.

Type 1 diabetes human enteroid studies reveal major changes in the intestinal epithelial compartment

Lack of understanding of the pathophysiology of gastrointestinal (GI) complications in type 1 diabetes (T1D), including altered intestinal transcriptomes and protein expression represents a major gap in the management of these patients. Human enteroids have emerged as a physiologically relevant model of the intestinal epithelium but establishing enteroids from individuals with long-standing T1D has proven difficult. We successfully established duodenal enteroids using endoscopic biopsies from pediatric T1D patients and compared them with aged-matched enteroids from healthy subjects (HS) using bulk RNA sequencing (RNA-seq), and functional analyses of ion transport processes. RNA-seq analysis showed significant differences in genes and pathways associated with cell differentiation and proliferation, cell fate commitment, and brush border membrane. Further validation of these results showed higher expression of enteroendocrine cells, and the proliferating cell marker Ki-67, significantly lower expression of NHE3, lower epithelial barrier integrity, and higher fluid secretion in response to cAMP and elevated calcium in T1D enteroids. Enteroids established from pediatric T1D duodenum identify characteristics of an abnormal intestinal epithelium and are distinct from HS. Our data supports the use of pediatric enteroids as an ex-vivo model to advance studies of GI complications and drug discovery in T1D patients.

Duodenal Mucosa: A New Target for the Treatment of Type 2 Diabetes

OBJECTIVE

After a high-fat and high-sugar diet, the duodenal mucosa of rodents proliferate and trigger the signal of insulin resistance, which may be the cause of type 2 diabetes (T2D). In response to this phenomenon, researchers have designed the duodenal mucosal resurfacing (DMR) procedure, mainly through the hydrothermal ablation procedure, to restore the normal mucosal surface, thereby correcting this abnormal metabolic signal. This article aims to understand the changes in duodenum before and after the onset or treatment of T2D, and the potential mechanisms of DMR procedure.

METHODS

A literature search of PubMed and Web of Science was conducted using appropriate keywords.

RESULTS

Both animal and clinical studies have shown that the villus thickness, intestinal cells, glucose transporters, enteric nerves, and gut microbiota and their metabolites in the duodenum undergo corresponding changes before and after the onset or treatment of T2D. These changes may be related to the pathogenesis of T2D. DMR procedure may produce beneficial glycemic and hepatic metabolic effects by regulating these changes.

CONCLUSION

The duodenum is an important metabolic signaling center, and limiting nutrient exposure to this critical region will have powerful metabolic benefits. The DMR procedure may regulate glycemic and hepatic parameters through various mechanisms, which needs to be further confirmed by a large number of animal and clinical studies.

Administration of Different Doses of Acrylamide Changed the Chemical Coding of Enteric Neurons in the Jejunum in Gilts

Excessive consumption of highly processed foods, such as chips, crisps, biscuits and coffee, exposes the human to different doses of acrylamide. This chemical compound has a multidirectional, adverse effect on human and animal health, including the central and peripheral nervous systems. In this study, we examined the effect of different doses of acrylamide on the enteric nervous system (ENS) of the porcine jejunum. Namely, we took into account the quantitative changes of neurons located in the jejunum wall expressing substance P (SP), galanin (GAL), a neuronal form of nitric oxide synthase (nNOS), the vesicular acetylcholine transporter (VAChT) and cocaine- and amphetamine-regulated transcript (CART). The obtained results indicate that acrylamide causes a statistically significant increase in the number of neurons immunoreactive to SP, GAL, VAChT and CART in all types of examined enteric plexuses and a significant drop in the population of nNOS-positive enteric neurons. Changes were significantly greater in the case of a high dose of acrylamide intoxication. Our results indicate that acrylamide is not indifferent to ENS neurons. A 28-day intoxication with this substance caused marked changes in the chemical coding of ENS neurons in the porcine jejunum.

Streptozotocin-Induced Diabetes Causes Changes in Serotonin-Positive Neurons in the Small Intestine in Pig Model

Serotonin (5-hydroxytryptamine or 5-HT) is an important neurotransmitter of the central and peripheral nervous systems, predominantly secreted in the gastrointestinal tract, especially in the gut. 5-HT is a crucial enteric signaling molecule and is well known for playing a key role in sensory-motor and secretory functions in the gut. Gastroenteropathy is one of the most clinical problems in diabetic patients with frequent episodes of hyperglycemia. Changes in 5-HT expression may mediate gastrointestinal tract disturbances seen in diabetes, such as nausea and diarrhea. Based on the double immunohistochemical staining, this study determined the variability in the population of 5-HT-positive neurons in the porcine small intestinal enteric neurons in the course of streptozotocin-induced diabetes. The results show changes in the number of 5-HT-positive neurons in the examined intestinal sections. The greatest changes were observed in the jejunum, particularly within the myenteric plexus. In the ileum, both de novo 5-HT synthesis in the inner submucosal plexus neurons and an increase in the number of neurons in the outer submucosal plexus were noted. The changes observed in the duodenum were also increasing in nature. The results of the current study confirm the previous observations concerning the involvement of 5-HT in inflammatory processes, and an increase in the number of 5-HT -positive neurons may also be a result of increased concentration of the 5-HT in the gastrointestinal tract wall and affects the motor and secretory processes, which are particularly intense in the small intestines.

Role of serotonin in regulation of pancreatic and mesenteric arterial function in diabetic mice

The aim of this study was to investigate the reaction of pancreatic and mesenteric artery to 5-hydroxytryptamine (5-HT, serotonin) and the mechanism of nitric oxide in diabetes. Diabetic mice were induced by streptozotocin through intraperitoneal injection. The vascular tension of the pancreatic, mesenteric and brain basilar arteries in diabetic and control mice were measured by myograph in the applications of angiotensin II, 5-HT, 5-HT_2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI), 5-HT_1B/1D receptor agonist sumatriptan, 5-HT_2B receptor agonist BW723C86, 5-HT_1D receptor antagonist Palonosetron and 5-HT₂ receptor antagonist Sarpogrelate. The effect of 5-HT on arteries pretreated with L-NAME and sodium nitroprusside (SNP) on arteries pretreated with norepinephrine were measured. The mRNA expressions of eNOS, 5-HT_1B, 5-HT_1D, 5-HT_2A and 5-HT_2B in pancreatic and mesenteric arteries were measured by Real-time PCR. The concentration of 5-HT in plasma and eNOS in pancreatic and mesenteric arteries were tested. Our results showed that the tension of pancreatic and mesenteric arteries in diabetic mice impaired to 5-HT, but not Ang II, and to DOI and sumatriptan, but normalized by incubation with L-NAME. Pancreatic and mesenteric arteries showed no differences to SNP after pretreated with NE between diabetic and control mice. The mRNA of eNOS and 5-HT receptors in pancreatic and mesenteric artery showed no difference between control and diabetic mice. We conclude that the effect of 5-HT on the tension of pancreatic and mesenteric arteries decrease in diabetic mice. It may due to the decreased activity of 5-HT receptors and the activation of eNOS, which causes nitric oxide to release more and makes the tension of vessels decreased.

Interdependency between mechanical parameters and afferent nerve discharge in remodeled diabetic Goto-Kakizaki rat intestine

Background

Gastrointestinal disorders are very common in diabetic patients, but the pathogenesis is still not well understood. Peripheral afferent nerves may be involved due to the complex regulation of gastrointestinal function by the enteric nervous system.

Objective

We aimed to characterize the stimulus–response function of afferent fibers innervating the jejunum in the Goto-Kakizaki (GK) type 2 diabetic rat model. A key question is whether changes in afferent firing arise from remodeled tissue or from adaptive afferent processes.

Design

Seven 32-week-old male GK rats and seven age-matched normal Wistar rats were studied. Firing from mesenteric afferent nerves was recorded in excised jejunal segments of seven GK rats and seven normal Wistar rats during ramp test, stress relaxation test, and creep test. The circumferential stress–strain, spike rate increase ratio (SRIR), and single unit firing rates were calculated for evaluation of interdependency of the mechanical stimulations and the afferent nerve discharge.

Results

Elevated sensitivity to mechanical stimuli was found for diabetic nerve bundles and single unit activity (P<0.05). The stress relaxed less in the diabetic intestinal segment (P<0.05). Linear association between SRIR and the thickness of circumferential muscle layer was found at high stress levels as well as for SRIR and the glucose level.

Conclusion

Altered viscoelastic properties and elevated mechanosensitivity were found in the GK rat intestine. The altered nerve signaling is related to muscle layer remodeling and glucose levels and may contribute to gastrointestinal symptoms experienced by diabetic patients.

Vitamin C Improves Gastroparesis in Diabetic Rats: Effects on Gastric Contractile Responses and Oxidative Stress

BACKGROUND

Diabetic gastroparesis is a common complication of diabetes mellitus, which mainly affects women. Previous studies have demonstrated that oxidative stress is involved in its onset and development.

AIMS

This study evaluated the role of vitamin C on diabetes-associated gastric dysmotility.

METHODS

Female rats with streptozotocin-induced diabetes were treated with vehicle (water, 1 mL/kg, p.o.), vitamin C (300 mg/kg/day, p.o.), or insulin (6 IU/day, s.c.). Gastric emptying, in vitro gastric contractility, and biochemistry parameters were analyzed at the end of the treatment (i.e. 8 weeks after the diabetes induction).

RESULTS

Vitamin C reversed the delayed gastric emptying of diabetic rats to normal levels, and avoided the changes in the contractile responses to acetylcholine (0.1 nM-1 µM), but not to 5-hydroxytryptamine (0.1 nM-1 µM), in the pylorus and fundus from diabetic rats. Moreover, the contraction evoked by KCl (40 mM) in the fundus, but not in the pylorus, was intensely increased in diabetic rats treated with vitamin C. Notably, the vitamin C reestablished the reduced glutathione levels by 77% and decreased the reactive oxygen species content by 60% in the gastric tissue from diabetic rats. Despite the effects on gastric motility, vitamin C treatment did not change the fasting glycaemia or the glycated hemoglobin of diabetic rats. Unsurprisingly, insulin treatment normalized all parameters evaluated.

CONCLUSIONS

Vitamin C exhibited a remarkable beneficial effect on gastric emptying dysfunction in diabetic rats, which was mediated by attenuation of oxidative stress and maintenance of the cholinergic contractile responses in fundus and pylorus.

Desired and side effects of the supplementation with l-glutamine and l-glutathione in enteric glia of diabetic rats

BACKGROUND/AIMS

Enteric neuropathy associated with Diabetes Mellitus causes dysfunction in the digestive system, such as: nausea, diarrhea, constipation, vomiting, among others. The aim of this study was to compare the effects of supplementation with 2% l-glutamine and 1% l-glutathione on neurons and enteric glial cells of ileum of diabetic rats.

METHODS

Thirty male Wistar rats have been used according to these group distributions: Normoglycemic (N), Normoglycemic supplemented with l-glutamine (NG), Normoglycemic supplemented with l-glutathione (NGO), Diabetic (D), Diabetic supplemented with l-glutamine (DG) and Diabetic supplemented with l-glutathione (DGO). After 120days, the ileum was processed for immunohistochemistry of HuC/D and S100β. Quantitative and morphometric analysis have been performed.

RESULTS

Diabetic rats presented a decrease in the number of neurons when compared to normoglycemic animals. However, diabetes was not associated with a change in glial density. l-Glutathione prevented the neuronal death in diabetic rats. l-Glutathione increased a glial proliferation in diabetic rats. The neuronal area in diabetic rats increased in relation to the normoglycemics. The diabetic rats supplemented with l-glutamine and l-glutathione showed a smaller neuronal area in comparison to diabetic group. The glial cell area was a decreased in the diabetics. The diabetic rats supplemented with l-glutamine and l-glutathione did not have significant difference in the glial cell body area when compared to diabetic rats.

CONCLUSION

It is concluded that the usage of l-glutamine and l-glutathione as supplements presents both desired and side effects that are different for the same substance in considering normoglycemic or diabetic animals.

Supplementation with L-Glutamine and L-Alanyl-L-Glutamine Changes Biochemical Parameters and Jejunum Morphophysiology in Type 1 Diabetic Wistar Rats

We evaluated the effects of the supplementation with L-glutamine and glutamine dipeptide (GDP) on biochemical and morphophysiological parameters in streptozotocin-diabetic rats. For this purpose, thirty animals were distributed into six groups treated orally (gavage) during thirty days: non diabetic rats (Control) + saline, diabetic + saline; Control + L-glutamine (248 mg/kg), Diabetic + L-glutamine (248 mg/kg), Control + GDP (400 mg/kg), Diabetic + GDP (400 mg/kg). Diabetes was induced by an intravenous injection of streptozotocin (60 mg/kg) and confirmed by fasting glucose ≥ 200 mg/dL. Physiological parameters, i.e., body mass, food intake, blood glucose, water intake, urine and faeces were evaluated during supplementation. After the period of supplementation, the animals were euthanized. The blood was collected for biochemical assays (fructosamine, transaminases, lipid profile, total protein, urea, ammonia). Moreover, the jejunum was excised and stored for morphophysiological assays (intestinal enzyme activity, intestinal wall morphology, crypt proliferative index, number of serotoninergic cells from the mucosa, and vipergic neurons from the submucosal tunica). The physiological parameters, protein metabolism and intestinal enzyme activity did not change with the supplementation with L-glutamine or GDP. In diabetic animals, transaminases and fructosamine improved with L-glutamine and GDP supplementations, while the lipid profile improved with L-glutamine. Furthermore, both forms of supplementation promoted changes in jejunal tunicas and wall morphometry of control and diabetic groups, but only L-glutamine promoted maintenance of serotoninergic cells and vipergic neurons populations. On the other hand, control animals showed changes that may indicate negative effects of L-glutamine. Thus, the supplementation with L-glutamine was more efficient for maintaining intestinal morphophysiology and the supplementation with GDP was more efficient to the organism as a whole. Thus, we can conclude that local differences in absorption and metabolism could explain the differences between the supplementation with L-glutamine or GDP.

Autonomic neuropathy in experimental models of diabetes mellitus

Autonomic neuropathy complicates diabetes by increasing patient morbidity and mortality. Surprisingly, considering its importance, development and exploitation of animal models has lagged behind the wealth of information collected for somatic symmetrical sensory neuropathy. Nonetheless, animal studies have resulted in a variety of insights into the pathogenesis, neuropathology, and pathophysiology of diabetic autonomic neuropathy (DAN) with significant and, in some cases, remarkable correspondence between rodent models and human disease. Particularly in the study of alimentary dysfunction, findings in intrinsic intramural ganglia, interstitial cells of Cajal and the extrinsic parasympathetic and sympathetic ganglia serving the bowel vie for recognition as the chief mechanism. A body of work focused on neuropathologic findings in experimental animals and human subjects has demonstrated that axonal and dendritic pathology in sympathetic ganglia with relative neuron preservation represents one of the neuropathologic hallmarks of DAN but it is unlikely to represent the entire story. There is a surprising selectivity of the diabetic process for subpopulations of neurons and nerve terminals within intramural, parasympathetic, and sympathetic ganglia and innervation of end organs, afflicting some while sparing others, and differing between vascular and other targets within individual end organs. Rather than resulting from a simple deficit in one limb of an effector pathway, autonomic dysfunction may proceed from the inability to integrate portions of several complex pathways. The selectivity of the diabetic process appears to confound a simple global explanation (e.g., ischemia) of DAN. Although the search for a single unifying pathogenetic hypothesis continues, it is possible that autonomic neuropathy will have multiple pathogenetic mechanisms whose interplay may require therapies consisting of a cocktail of drugs. The role of multiple neurotrophic substances, antioxidants (general or pathway specific), inhibitors of formation of advanced glycosylation end products and drugs affecting the polyol pathway may be complex and therapeutic elements may have both salutary and untoward effects. This review has attempted to present the background and current findings and hypotheses, focusing on autonomic elements including and beyond the typical parasympathetic and sympathetic nervous systems to include visceral sensory and enteric nervous systems.

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.

Glucose sensing by gut endocrine cells and activation of the vagal afferent pathway is impaired in a rodent model of type 2 diabetes mellitus

Glucose in the gut lumen activates gut endocrine cells to release 5-HT, glucagon-like peptide 1/2 (GLP-1/2), and glucose-dependent insulinotropic polypeptide (GIP), which act to change gastrointestinal function and regulate postprandial plasma glucose. There is evidence that both release and action of incretin hormones is reduced in type 2 diabetes (T2D). We measured cellular activation of enteroendocrine and enterochromaffin cells, enteric neurons, and vagal afferent neurons in response to intestinal glucose in a model of type 2 diabetes mellitus, the UCD-T2DM rat. Prediabetic (PD), recent-diabetic (RD, 2 wk postonset), and 3-mo diabetic (3MD) fasted UCD-T2DM rats were given an orogastric gavage of vehicle (water, 0.5 ml /100 g body wt) or glucose (330 μmol/100 g body wt); after 6 min tissue was removed and cellular activation was determined by immunohistochemistry for phosphorylated calcium calmodulin-dependent kinase II (pCaMKII). In PD rats, pCaMKII immunoreactivity was increased in duodenal 5-HT (P < 0.001), K (P < 0.01) and L (P < 0.01) cells in response to glucose; glucose-induced activation of all three cell types was significantly reduced in RD and 3MD compared with PD rats. Immunoreactivity for GLP-1, but not GIP, was significantly reduced in RD and 3MD compared with PD rats (P < 0.01). Administration of glucose significantly increased pCaMKII in enteric and vagal afferent neurons in PD rats; glucose-induced pCaMKII immunoreactivity was attenuated in enteric and vagal afferent neurons (P < 0.01, P < 0.001, respectively) in RD and 3MD. These data suggest that glucose sensing in enteroendocrine and enterochromaffin cells and activation of neural pathways is markedly impaired in UCD-T2DM rats.

New method of manometric measurement of gastroduodenal motility in conscious mice: effects of ghrelin and Y2 depletion

Since no previous studies have reported dual measurements of stomach and duodenal motility in conscious mice, we developed a manometric method to measure the gastroduodenal motility in the physiological fed and fasted states of conscious mice. By this method we measured, for the first time, the gastroduodenal motility in Y2 knockout mice and analyzed the effects of ghrelin on the gastroduodenal motility in conscious mice. To evaluate this new method, we provide the comparison on the effects of CCK-8 examined by present and previous methods. In the fasted state of mice, phase III-like contractions with frequencies of 7.8 +/- 0.5 contractions/h in the antrum and 6.6 +/- 0.7 contractions/h in the duodenum were observed. This fasted pattern was disrupted and replaced by the fed pattern after feeding, with an increase of the motor index (MI) immediately after feeding. Intravenous injection of ghrelin induced the fasted pattern in the duodenum when injected in the fed state and increased %MI (114.3 +/- 9.8%) compared with saline-injected controls (64.4 +/- 9.6%) in the antrum. Intravenous injection of CCK-8 disrupted phase III-like contractions in both antrum and duodenum, which were replaced by fed-like motor patterns accompanied with the elevation of baseline pressure. In Y2 knockout mice, the frequency of phase III-like contractions was decreased in the antrum compared with wild-type mice and the immediate increase of MI after feeding seen in wild-type mice was disrupted in Y2 knockout mice. Our model provides a new method for studies of gastrointestinal motility in various mouse models, including transgenic and knockout ones.

Increased frequency of prediabetes in patients with irritable bowel syndrome

The aim of this study was to compare the occurrence of prediabetes [impaired fasting glucose and/or impaired glucose tolerance are considered to be precursors to type 2 diabetes mellitus (DM)] in irritable bowel syndrome (IBS) cases and matched controls. Ninety-two patients with IBS and 104 healthy matched controls were included in this study. Type 2 DM was considered an exclusion criterion in both groups. Fasting blood glucose, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and triglyceride levels were examined; after 1 night of fasting, an oral glucose tolerance test with 75 g glucose was administered, and the blood glucose levels after 2 hours were examined. Although there were no significant differences in the triglyceride levels, significant differences were found for total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol levels (P < 0.001, 0.001, and <0.001, respectively). These measures were found to be elevated in the IBS group compared with the control group. The frequency of prediabetes, which is regarded as the first stage of type 2 DM, was also found to be significantly higher in the IBS group (P < 0.001). After adjusting for potential confounders, such as age, lipid levels, and anthropometric measures in the analysis of covariance models, prediabetes was significantly more frequent in the IBS group than in the control group (P < 0.001). Thus, given the higher prediabetes occurrence in IBS, IBS may indirectly indicate a higher risk of DM. Further investigations will be necessary to fully elucidate the mechanisms behind these observations.

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.

Therapy Insight: gastrointestinal complications of diabetes--pathophysiology and management

Patients with diabetes often have gastrointestinal symptoms, but the extent and severity of this problem and the specificity of the symptoms are not nearly as well defined as frequently assumed. Any part of the gastrointestinal tract can be affected, and the presenting symptoms depend on the composite of dysfunctional elements. Gastroesophageal reflux, Candida esophagitis, gastroparesis, diarrhea and constipation are among the many common gastrointestinal complications of diabetes. No specific risk factor for the development of these complications has been identified and their etiology is most likely to be multifactorial, involving both reversible and irreversible processes. Treatment should be directed at tighter glycemic and symptom control, which can bring about clinical improvement for many patients. For other patients, however, effective clinical management is problematic because no therapies are available to prevent or correct the underlying disease mechanisms. Studies now suggest that reduced levels of key trophic factors cause transdifferentiation of pacemaker interstitial cells of Cajal into a smooth-muscle-like phenotype. If this really is the case, therapies directed at restoring the normal milieu of trophic signals could correct the dysfunction of the interstitial cells of Cajal and resolve many gastrointestinal complications. Advances in stem cell technology also hold promise to provide a cure for diabetes and to correct abnormalities in gastrointestinal pathology.

Diabetes and the enteric nervous system

Diabetes is associated with several changes in gastrointestinal (GI) motility and associated symptoms such as nausea, bloating, abdominal pain, diarrhoea and constipation. The pathogenesis of altered GI functions in diabetes is multifactorial and the role of the enteric nervous system (ENS) in this respect has gained significant importance. In this review, we summarize the research carried out on diabetes-related changes in the ENS. Changes in the inhibitory and excitatory enteric neurons are described highlighting the role of loss of inhibitory neurons in early diabetic enteric neuropathy. The functional consequences of these neuronal changes result in altered gastric emptying, diarrhoea or constipation. Diabetes can also affect GI motility through changes in intestinal smooth muscle or alterations in extrinsic neuronal control. Hyperglycaemia and oxidative stress play an important role in the pathophysiology of these ENS changes. Antioxidants to prevent or treat diabetic GI motility problems have therapeutic potential. Recent research on the nerve-immune interactions demonstrates inflammation-associated neurodegeneration which can lead to motility related problems in diabetes.

Diabetes-induced changes in the 5-hydroxytryptamine inhibitory receptors involved in the pressor effect elicited by sympathetic stimulation in the pithed rat

1. We investigated the effect of alloxan-induced diabetes on the inhibitory mechanisms of 5-hydroxytryptamine (5-HT) in the pressor responses induced by stimulation of sympathetic vasopressor outflow in pithed rats, and analysed the type and/or subtype of 5-HT receptors involved. 2. Diabetes was induced in male Wistar rats by a single s.c. injection of alloxan, then 4 weeks later, they were anaesthetized, pretreated with atropine and pithed. Electrical stimulation of the sympathetic outflow from the spinal cord (0.1, 0.5, 1 and 5 Hz) resulted in frequency-dependent increases in blood pressure. 3. Intravenous infusions of 5-HT (1-80 microg kg(-1) min(-1)) reduced the pressor effects obtained by electrical stimulation. The 5-HT(1) receptor agonist 5-carboxamidotryptamine, 5-CT (5 microg kg(-1) min(-1)), caused an inhibition of the pressor response, whereas the selective 5-HT(2) receptor agonist, alpha-methyl-5-HT (5 microg kg(-1) min(-1)) and the selective 5-HT(3) receptor agonist, 1-phenylbiguanide (40 microg kg(-1) min(-1)), did not modify the sympathetic pressor responses. 5-HT had no effect on exogenous noradrenaline (NA)-induced pressor responses. 4. The inhibition of electrically induced pressor responses by 5-HT (10 microg kg(-1) min(-1)) was unable to be elicited after i.v. treatment with methiothepin (100 microg kg(-1)) because of the marked inhibition produced by methiothepin alone. The 5-HT-induced inhibition was blocked after i.v. administration of WAY-100,635 (100 microg kg(-1)) and not affected by ritanserin (1 mg kg(-1)), MDL 72222 (2 mg kg(-1)). 5. The selective 5-HT(1A) receptor agonist, 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT) (5-20 microg kg(-1) min(-1)) but neither the rodent 5-HT(1B) receptor agonist, CGS-12066B (5 microg kg(-1) min(-1)), nor the selective nonrodent 5-HT(1B) and 5-HT(1D) receptor agonist, L-694,247 (5 and 40 microg kg(-1) min(-1)), inhibited the electrically induced pressor response. The selective 5-HT(1A) receptor antagonist, WAY-100,635 (100 microg kg(-1)), blocked the inhibition induced by 8-OH-DPAT (10 microg kg(-1) min(-1)). 8-OH-DPAT had no effect on exogenous NA-induced pressor responses. 6. Experimental diabetes produces changes in the inhibitory effect induced by 5-HT on electrically induced sympathetic pressor responses, such that the inhibitory action induced by 5-HT in diabetic pithed rats is mediated by prejunctional 5-HT(1A) receptors.

Persistent and selective effects of inflammation on smooth muscle cell contractility in rat colitis

Intestinal inflammation affects smooth muscle contractility contributing to altered motility, but changes to the individual smooth muscle cells are not well described. We used video microscopy to study the contractility of circular smooth muscle cells (CSMC) isolated from the rat mid-descending colon throughout the course of TNBS-induced colitis, measuring their shortening response to carbachol (CCh), 5-HT, histamine or high K(+). In control CSMC, CCh caused a maximal shortening response of 28 (2%), similar to that for 5-HT of 27 (1%), but by day 4 of colitis, these responses were decreased by 35% and 37%, respectively. By day 36, all aspects of cholinergic contraction returned to control levels, while 5-HT-induced contraction remained significantly attenuated. In contrast, the contractile responses to histamine remained similar at all time points. K(+)-induced contraction was impaired only on day 4, and the maximal response remained substantially greater than CCh or 5-HT. Colitis caused a 121% increase in CSMC length by day 2 that persisted through day 36, independent evidence for phenotypic change. We conclude that impaired CSMC contractility at both the receptor and non-receptor levels contribute to altered smooth muscle function during colitis. Persistent changes in contractile response remained detectable after resolution of inflammation, and similar events may occur in post-enteritis syndromes seen in humans.

Serotonin stimulates endotoxin translocation via 5-HT3 receptors in the rat ileum

Because few previous studies have investigated the mechanisms of endotoxin translocation induced by intestinal obstruction, we aimed to clarify whether or not serotonin [5-hydroxytryptamine (5-HT)], which is released from enterochromaffin (EC) cells, is responsible for alterations of the mucosal permeability to endotoxin and to identify the 5-HT receptor subtypes that mediate this action. FITC-labeled LPS (FITC-LPS) was injected into the ileum of rats, and the FITC-LPS level in the superior mesenteric vein was subsequently measured by using a fluorescence spectrophotometer. To measure the 5-HT release induced by high intraluminal pressure, ex vivo preparation of vascularly and luminally perfused rat ileum was used. Results demonstrated that elevated intraluminal pressure stimulates the translocation of FITC-LPS and the release of 5-HT from the EC cells into the intestinal lumen but not into the portal circulation. This FITC-LPS translocation, which was stimulated by exogenously applied 5-HT in the lumen and the jugular vein, was inhibited by 5-HT(3) receptor antagonist administration both intaluminally and intravenously. The stimulatory effect of elevated intraluminal pressure on the translocation of FITC-LPS was inhibited by the intraluminal and intravenous administration of 5-HT(3) receptor antagonist. These results suggest that 5-HT released from EC cells may be involved in the translocation of FITC-LPS induced by elevated intraluminal pressure and that this effect is mediated by 5-HT(3) receptors that may be located in the intestinal epithelium.