Effects of chronic experimental streptozotocin-induced diabetes on the noradrenergic and peptidergic innervation of the rat alimentary tract

The Influence of a Hyperglycemic Condition on the Population of Somatostatin Enteric Neurons in the Porcine Gastrointestinal Tract

Somatostatin (SOM) is the most common agent in the gastrointestinal (GI) tract that is involved in the regulation of several gastric functions, as well as in gastric disorders. Hyperglycemia, which develops as a consequence of improperly treated diabetes, can cause numerous disturbances in the appropriate functioning of the gastrointestinal tract. High glucose level is toxic to neurons. One of the lines of defense of neurons against this glucotoxicity are changes in their chemical coding. To better understood the role of SOM secreted by enteric neurons in neuronal response on elevated glucose level, pancreatic β cells were destroyed using streptozotocin. Due to the close similarity of the pig to humans, especially the GI tract, the current study used pigs as an animal model. The results revealed that the number of enteric neurons immunoreactive to SOM (SOM-IR) in a physiological state clearly depend on the part of the GI tract studied. In turn, experimentally induced diabetes caused changes in the number of SOM-IR neurons. The least visible changes were observed in the stomach, where an increase in SOM-IR neurons was observed, only in the submucosal plexus in the corpus. However, diabetes led to an increase in the population of myenteric and submucosal neurons immunoreactive to SOM in all segments of the small intestine. The opposite situation occurred in the descending colon, where a decrease in the number of SOM-IR neurons was visible. This study underlines the significant role of SOM expressed in enteric nervous system neurons during diabetes.

Somatostatin as an Active Substance in the Mammalian Enteric Nervous System

Somatostatin (SOM) is an active substance which most commonly occurs in endocrine cells, as well as in the central and peripheral nervous system. One of the parts of the nervous system where the presence of SOM has been confirmed is the enteric nervous system (ENS), located in the wall of the gastrointestinal (GI) tract. It regulates most of the functions of the stomach and intestine and it is characterized by complex organization and a high degree of independence from the central nervous system. SOM has been described in the ENS of numerous mammal species and its main functions in the GI tract are connected with the inhibition of the intestinal motility and secretory activity. Moreover, SOM participates in sensory and pain stimuli conduction, modulation of the release of other neuronal factors, and regulation of blood flow in the intestinal vessels. This peptide is also involved in the pathological processes in the GI tract and is known as an anti-inflammatory agent. This paper, which focuses primarily on the distribution of SOM in the ENS and extrinsic intestinal innervation in various mammalian species, is a review of studies concerning this issue published from 1973 to the present.

Changes in Immunoreactivity of Sensory Substances within the Enteric Nervous System of the Porcine Stomach during Experimentally Induced Diabetes

One of the most frequently reported disorders associated with diabetes is gastrointestinal (GI) disturbance. Although pathogenesis of these complications is multifactorial, the complicity of the enteric nervous system (ENS) in this respect has significant importance. Therefore, this paper analysed changes in substance P- (SP-), calcitonin gene-related peptide- (CGRP-), and leu5-enkephalin- (L-ENK-) like immunoreactivity (LI) in enteric stomach neurons caused by chemically induced diabetes in a porcine model. Using double immunofluorescent labelling, it was found that acute hyperglycaemia led to significant changes in the chemical coding of stomach enteric neurons. Generally, the response to artificially inducted diabetes depended on the "kind" of enteric plexus as well as the stomach region studied. A clear increase in the percentage of neurons immunoreactive to SP and CGRP was visible in the myenteric plexus (MP) in the antrum, corpus, and pylorus as well as in the submucosal plexus (SmP) in the corpus. For L-ENK, an increase in the number of L-ENK-LI neurons was observed in the MP of the antrum and SmP in the corpus, while in the MP of the corpus and pylorus, a decrease in the percentage of L-ENK-LI neurons was noted.

Increased peripherin in sympathetic axons innervating plantar metatarsal arteries in STZ-induced type I diabetic rats

A common characteristic of axonopathy is the abnormal accumulation of cytoskeletal proteins. We recently reported that streptozotocin (STZ)-induced type 1 diabetes produced a change in the morphology of sympathetic nerve fibers supplying rat plantar metatarsal arteries (PMAs). Here we investigated whether these morphological changes are associated with axonal accumulation of the type III intermediate filament peripherin and the microtubule protein β-tubulin III, as both are implicated in axonal remodeling. PMAs from hyperglycemic STZ-treated rats receiving a low dose of insulin (STZ-LI) were compared with those from normoglycemic STZ-treated rats receiving a high dose of insulin (STZ-HI) and vehicle-treated controls. Western blotting revealed an increase in protein expression level for peripherin in PMAs from STZ-LI rats but no change in that for β-tubulin III. In addition, there was an increase in the number of peripherin immunoreactive nerve fibers in the perivascular nerve plexus of PMAs from STZ-LI rats. Co-labeling for peripherin and neuropeptide Y (a marker for sympathetic axons) revealed that peripherin immunoreactivity increased in sympathetic axons. None of these changes were detected in PMAs from STZ-HI rats, indicating that increased peripherin in sympathetic axons of STZ-LI rats is likely due to hyperglycemia and provides a marker of diabetes-induced nerve damage.

High-fat diet ingestion correlates with neuropathy in the duodenum myenteric plexus of obese mice with symptoms of type 2 diabetes

Obesity and type 2 diabetes are increasing in prevalence at an alarming rate in developed and developing nations and over 50% of patients with prolonged stages of disease experience forms of autonomic neuropathy. These patients have symptoms indicating disrupted enteric nervous system function including gastric discomfort, gastroparesis and intestinal dysmotility. Previous assessments have examined enteric neuronal injury within either type 1 diabetic or transgenic type 2 diabetic context. This study aims to assess damage to myenteric neurons within the duodenum of high-fat diet ingesting mice experiencing symptoms of type 2 diabetes, as this disease context is most parallel to the human condition and disrupted duodenal motility underlies negative gastrointestinal symptoms. Mice fed a high-fat diet developed symptoms of obesity and diabetes by 4 weeks. After 8 weeks, the total number of duodenal myenteric neurons and the synaptophysin density index were reduced and transmission electron microscopy showed axonal swelling and loss of neurofilaments and microtubules, suggesting compromised neuronal health. High-fat diet ingestion correlated with a loss of neurons expressing VIP and nNOS but did not affect the expression of ChAT, substance P, calbindin and CGRP. These results correlate high-fat diet ingestion, obesity and type 2 diabetes symptoms with a loss of duodenal neurons, biasing towards those with inhibitory nature. This pathology may underlie dysmotility and other negative GI symptoms experienced by human type 2 diabetic and obese patients.

The participation of the sympathetic innervation of the gastrointestinal tract in disease states

Knowledge of neural circuits, neurotransmitters and receptors involved in the sympathetic regulation of gastrointestinal (GI) function is well established. However, it is only recently that the interaction of sympathetic neurons, and of sympathetic transmitters, with the GI immune system and with gut flora has begun to be explored. Changes in the behaviour of sympathetic nerves when gut function is compromised, for example in ileus and in inflammation, have been observed, but the roles of the sympathetic innervation in these and other pathologies are not adequately understood. In this article, we first review the principal roles of the sympathetic innervation of the GI tract in controlling motility, fluid exchange and gut blood flow in healthy individuals. We then discuss the evidence that there are important interactions of sympathetic transmitters with the gut immune system and enteric glia, and evidence that inflammation has substantial effects on sympathetic neurons. These reciprocal interactions contribute to pathological changes in ways that are not yet clarified. Finally, we focus on inflammation, diabetes and postoperative ileus as conditions in which there is sympathetic involvement in compromised gut function.

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 mellitus and the peripheral nervous system: manifestations and mechanisms

Diabetes targets the peripheral nervous system with several different patterns of damage and several mechanisms of disease. Diabetic polyneuropathy (DPN) is a common disorder involving a large proportion of diabetic patients, yet its pathophysiology is controversial. Mechanisms considered have included polyol flux, microangiopathy, oxidative stress, abnormal signaling from advanced glycation endproducts and growth factor deficiency. Although some clinical trials have demonstrated modest benefits in disease stabilization or pain therapy in DPN, robust therapy capable of reversing the disease is unavailable. In this review, general aspects of DPN and other diabetic neuropathies are examined, including a summary of recent therapeutic trials. A particular emphasis is placed on the evidence that the neurobiology of DPN reflects a unique yet common and disabling neurodegenerative disorder.

A potent sorbitol dehydrogenase inhibitor exacerbates sympathetic autonomic neuropathy in rats with streptozotocin-induced diabetes

We have developed an animal model of diabetic sympathetic autonomic neuropathy which is characterized by neuroaxonal dystrophy (NAD), an ultrastructurally distinctive axonopathy, in chronic streptozotocin (STZ)-diabetic rats. Diabetes-induced alterations in the sorbitol pathway occur in sympathetic ganglia and therapeutic agents which inhibit aldose reductase or sorbitol dehydrogenase improve or exacerbate, respectively, diabetes-induced NAD. The sorbitol dehydrogenase inhibitor SDI-711 (CP-470711, Pfizer) is approximately 50-fold more potent than the structurally related compound SDI-158 (CP 166,572) used in our earlier studies. Treatment with SDI-711 (5 mg/kg/day) for 3 months increased ganglionic sorbitol (26-40 fold) and decreased fructose content (20-75%) in control and diabetic rats compared to untreated animals. SDI-711 treatment of diabetic rats produced a 2.5- and 4-5-fold increase in NAD in the SMG and ileal mesenteric nerves, respectively, in comparison to untreated diabetics. Although SDI-711 treatment of non-diabetic control rat ganglia increased ganglionic sorbitol 40-fold (a value 8-fold higher than untreated diabetics), the frequency of NAD remained at control levels. Levels of ganglionic sorbitol pathway intermediates in STZ-treated rats (a model of type 1 diabetes) and Zucker Diabetic Fatty rats (ZDF, a genetic model of type 2 diabetes) were comparable, although STZ-diabetic rats develop NAD and ZDF-diabetic rats do not. SDI failed to increase diabetes-related ganglionic NGF above levels seen in untreated diabetics. Initiation of Sorbinil treatment for the last 4 months of a 9 month course of diabetes, substantially reversed the frequency of established NAD in the diabetic rat SMG without affecting the metabolic severity of diabetes. These findings indicate that sorbitol pathway-linked metabolic alterations play an important role in the development of NAD, but sorbitol pathway activity, not absolute levels of sorbitol or fructose per se, may be most critical to its pathogenesis.

Depressed ventilatory reflexes after capsaicin challenge in streptozotocin-treated rats

Diabetic sensory neuropathy is an affliction that decreases sensory perception in a number of organ systems. Although little is known of its pulmonary effects certain diabetic patients have reduced airway reactivity to cold air and elevated cough threshold to irritant inhalation, reflexes reported to be mediated by pulmonary C-fibers. Therefore we studied the effects the selective C-fiber activator capsaicin (0.01% aerosol, 30 s) on variables of ventilation using a whole-body plethysmograph in age-matched rats treated with streptozotocin (STZ) or its vehicle at 6 and 12 weeks after treatment. Body weight increased and plasma glucose and glycosylated hemoglobin were stable in vehicle-treated rats. In STZ-treated rats body weight decreased and plasma glucose and glycosylated hemoglobin increased. Capsaicin challenge decreased tidal volume, respiratory rate and therefore minute ventilation in non-treated and vehicle-treated rats. However capsaicin challenge increased tidal volume thereby altering minute ventilation in STZ-treated rats. Specific airway resistance increased in both groups after capsaicin challenge. Changes in ventilation in response to capsaicin challenge in STZ-treated rats may involve C-fiber sensory neuropathy.

Inhibition of sorbitol dehydrogenase exacerbates autonomic neuropathy in rats with streptozotocin-induced diabetes

We have developed an animal model of diabetic autonomic neuropathy that is characterized by neuroaxonal dystrophy (NAD) involving ileal mesenteric nerves and prevertebral sympathetic superior mesenteric ganglia (SMG) in chronic streptozotocin (STZ)-diabetic rats. Studies with the sorbitol dehydrogenase inhibitor SDI-158, which interrupts the conversion of sorbitol to fructose (and reactions dependent on the second step of the sorbitol pathway), have shown a dramatically increased frequency of NAD in ileal mesenteric nerves and SMG of SDI-treated versus untreated diabetics. Although lesions developed prematurely and in greater numbers in SDI-treated diabetics, their distinctive ultrastructural appearance was identical to that previously reported in long-term untreated diabetics. An SDI effect was first demonstrated in the SMG of rats that were diabetic for as little as 5 wk and was maintained for at least 7.5 months. As in untreated diabetic rats, rats treated with SDI i) showed involvement of lengthy ileal, but not shorter, jejunal mesenteric nerves; ii) demonstrated NAD in paravascular mesenteric nerves distributed to myenteric ganglia while sparing adjacent perivascular axons ramifying within the vascular adventitia; and, iii) failed to develop NAD in the superior cervical ganglia (SCG). After only 2 months of SDI-treatment, tyrosine hydroxylase immunolocalization demonstrated marked dilatation of postganglionic noradrenergic axons in paravascular ileal mesenteric nerves and within the gut wall versus those innervating extramural mesenteric vasculature. The effect of SDI on diabetic NAD in SMG was completely prevented by concomitant administration of the aldose reductase inhibitor Sorbinil. Treatment of diabetic rats with Sorbinil also prevented NAD in diabetic rats not treated with SDI. These findings indicate that sorbitol pathway-linked metabolic imbalances play a critical role in the development of NAD in this model of diabetic sympathetic autonomic neuropathy.

Loss of interstitial cells of cajal and inhibitory innervation in insulin-dependent diabetes

BACKGROUND & AIMS

Gastrointestinal complications of long-standing diabetes include nausea, vomiting, abdominal pain, diarrhea, and constipation. The pathophysiology underlying these symptoms is poorly understood. Recent evidence suggests an important role for interstitial cells of Cajal in controlling gastrointestinal motility. The aim of this study was to determine changes in interstitial cells of Cajal and enteric innervation in a patient with insulin-dependent diabetes.

METHODS

A full thickness jejunal biopsy was obtained from a 38-year-old insulin-dependent diabetic with evidence for diabetic gastroenteropathy. Immunohistochemistry, confocal microscopy, and 3-dimensional reconstruction techniques were used to quantify changes in the volume of interstitial cells of Cajal and enteric innervation.

RESULTS

Interstitial cells of Cajal were markedly decreased throughout the entire thickness of the jejunum. A decrease in neuronal nitric oxide synthase, vasoactive intestinal peptide, PACAP, and tyrosine hydroxylase immunopositive nerve fibers was observed in circular muscle layer while substance P immunoreactivity was increased.

CONCLUSIONS

The data suggest that long-standing diabetes is associated with a decrease in interstitial cells of Cajal volume and a decrease in inhibitory innervation, associated with an increase in excitatory innervation. The changes in interstitial cells of Cajal volume and enteric nerves may underlie the pathophysiology of gastrointestinal complications associated with diabetes and suggest future therapeutic targets.

Changes in peptidergic nerves in the atrioventricular valves of streptozotocin-induced diabetic rats: a confocal microscopy study

Several previous studies have described the distribution of neuropeptide Y (NPY)-like and calcitonin gene related peptide (CGRP)-like immunoreactive nerve fibres in the atrioventricular valves of humans and various animals. It has been suggested that peptide-containing nerve fibres might have motor or sensory roles in valvular function. Although there is evidence that diabetic changes occur in the sympathetic (preganglionic and postganglionic), parasympathetic (vagal) and peptidergic nerves of rats, the changes of peptide-containing nerve fibres in the atrioventricular valves of the diabetic rat have not been studied. The distribution, relative density and staining intensity of NPY-like and CGRP-like immunoreactive nerve fibres in the mitral and tricuspid valves were studied in whole mount preparations using confocal microscopy with a computer-assisted image analysis system. Streptozotocin-induced diabetic and control rats were sacrificed at 12 and 24 months. The nerve staining intensity within the tricuspid valve was greater than the mitral valve in both control (P < 0.01) and diabetic (P < 0.001) rats. Nerve density in the anterior leaflet was greater than the posterior leaflet of the mitral valve. However, the anterior leaflet of the mitral and tricuspid valves showed a decreased number of nerve fibres, followed by drastic reduction in the staining intensities for both the peptides studied (P < 0.001) in the long-term diabetic rat. The decrease in the number of nerve fibres that follow the mechanical interruption of nerves raises the possibility that cycles of degeneration may occur. It is suggested that these peptide-containing nerve fibres in the atrioventricular valves may be involved in valvular dysfunction in the diabetic state.

Distinct adhesive properties of ciliary and choroid neurons from the avian ciliary ganglion

The avian ciliary ganglion (CG) contains two populations of neurons: ciliary neurons, which innervate striated muscle, and choroid neurons, which innervate vascular smooth muscle. We used cell size (ciliary cells are larger) and somatostatin immunoreactivity (which is restricted to choroid cells) as markers to compare the adhesive properties of these two neuronal types. Similar numbers of freshly dissociated embryonic chick ciliary and choroid neurons adhered to laminin (laminin 1) and polylysine, consistent with the fact that each population comprises about half of the ganglionic neurons. In contrast, severalfold more ciliary neurons than choroid neurons adhered to a recombinant fragment of a synapsespecific basal lamina protein, s-laminin/laminin beta 2. Moreover, severalfold more ciliary neurons than choroid neurons adhered to a plastic surface when assayed by the method of Needels et al. in serum-free medium. Adhesion to s-laminin and plastic appears to be mediated by different cell surface components, as adhesion to recombinant s-laminin is inhibited by the tripeptide, LRE, and by Ca2+ ions, but not by heparin, whereas adhesion to plastic is LRE and Ca2+ insensitive but heparin sensitive. Both adhesive differences are apparent at embryonic day 8, soon after the ciliary and choroid neurons have begun to form synapses. Thus, two sets of neurons in the CG that send axons through different nerves and innervate different targets also show distinct adhesive behaviors.

Effect of streptozotocin-induced diabetes mellitus on release of vasoactive intestinal polypeptide from rodent small intestine

Representative longitudinal muscle strips (6 x 10 mm) from proximal and distal small intestine were excised from control and streptozotocin-treated rats after one month of untreated and insulin-treated diabetes. Untreated diabetes significantly reduced tissue concentrations of vasoactive intestinal polypeptide (VIP) at both intestinal loci. Insulin treatment of the diabetic animals restored tissue VIP concentrations to control group levels, although the beneficial effect of insulin treatment was only significant in the duodenum. Spontaneous release of VIP was significantly attenuated by untreated diabetes at both intestinal sites. In the duodenum, insulin treatment of the diabetic animals restored VIP release to levels indistinguishable from control group values. In the ileum, insulin treatment produced levels of VIP release that were not significantly different from those of the control and untreated diabetic groups. Tetrodotoxin (5 x 10(-6) M) significantly--but incompletely--inhibited VIP release from control group animals at both intestinal sites. These observations indicate that diabetes mellitus significantly diminishes VIP tissue concentrations and release from intestinal myenteric nerves. These abnormalities improve with insulin treatment. However, the mechanisms of VIP release from proximal and distal intestine appear to differ not only in their response to the diabetic state, but also in their response to insulin treatment.

Immunohistochemical study of enteric nervous system after small bowel transplantation in humans

The neurohormonal structures of two human intestines removed due to rejection 22 months and eight months after intestinal transplantation were studied by an indirect immunohistochemical method and compared with normal ileum. The distribution and density of neurons immunoreactive for tyrosine hydroxylase, substance P, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal peptide, galanin, gastrin-releasing peptide, L-enkephalin, and somatostatin were examined. Mucosal endocrine cells immunoreactive for somatostatin, peptide YY, and glucagon were also examined. Extrinsic adrenergic fibers and perivascular fibers were absent in all intestinal layers of the failed grafts. The distribution of intrinsic neurons was unchanged; however, the density was decreased by one rank. Distribution of endocrine cells of the first graft was similar to the normal. Extrinsic fibers were not detected by immunohistochemistry in human small intestinal grafts following long-term survival and eventual rejection, while the immunohistochemical expression of intrinsic neural and endocrine transmitters were well preserved.

Adrenergic innervation of the choroid and iris in diabetic rats

We compared the distribution and density of ocular adrenergic nerves in rats after 3 to 13 months of streptozotocin diabetes and in age-matched control animals to learn whether diabetic sympathetic neuropathy is evident in the eye as it is in other organs. An aqueous aldehyde method for histochemical demonstration of catecholamines provided a clear and complete view of the adrenergic innervation in whole flat preparations of choroid and iris. In the choroid, a dense plexus of varicose nerve fibers invested all of the branching arterial blood vessels. A less dense network of nerves was present in the choroidal stroma between the arteries, but there was no obvious association of nerves with the venules draining choroidal capillaries. Using a stereological method to measure the density of the adrenergic plexus of choroidal arteries, we found the mean innervation density to be normal in diabetic animals sampled at 3, 9, and 13 months after onset of hyperglycemia. Microscopic examination also failed to reveal diabetes-associated changes in the diffuse stromal nerves of the choroid or in the rich adrenergic innervation of the iris. Diabetes of relatively long duration, therefore, does not obviously affect the density or distribution pattern of catecholamine-containing nerves supplying the rat eye.

Changes of substance P and somatostatin contents in the gastrointestinal tract of streptozotocin diabetic rats

Substance P and somatostatin contents were measured in the gastrointestinal tract of streptozotocin diabetic rats, 1 month after streptozotocin administration (60 mg/kg), and of age-matched controls with radioimmunoassay. Substance P and somatostatin contents were statistically increased in the extrafundus of the diabetic stomach, but not in the diabetic fundus. Substance P was significantly decreased in the diabetic ileum and caecum. Similarly, somatostatin was decreased in the diabetic caecum. Contrarily, slight increase of somatostatin contents in the diabetic duodenum, jejunum and proximal colon was not statistically significant.

Gracile nucleus of streptozotocin-induced diabetic rats

This study reports ultrastructural changes in the gracile nucleus of male Wistar rats after streptozotocin-induced diabetes. During the acute phase (3-7 days) degenerating electron-dense dendrites and axon terminals were dispersed in the neuropil. Degenerating dendrites were characterized by an electron-dense cytoplasm, swollen mitochondria, dilated endoplasmic reticulum and scattered ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals may form part of a synaptic glomerulus with a central electron-dense dendrite, or they may form the central element of a synaptic glomerulus. These degenerating profiles were absent in the gracile nucleus of the 3 and 7 days insulin-treated post-streptozotocin rats. Macrophages were present in the neuropil and were in the process of engulfing neuronal elements. During the medium phase (1-6 months), most of the degenerating dendrites and axon terminals had been engulfed or removed by macrophages. During the late phase (9-12 months) a second wave of degeneration occurred in the gracile nucleus, similar to the acute phase. During the medium and late phases, dystrophic axonal profiles were also significantly increased in the rats after streptozotocin treatment. It is concluded that the ultrastructural changes observed in the gracile nucleus in the present study were the result of streptozotocin-induced diabetes rather than a toxic effect of streptozotocin, even in the acute phase.