Denervation and hyperinnervation in the nervous system of diabetic animals. I. The autonomic neuronal dystrophy of the gut

The neuropeptide substance P/neurokinin-1 receptor system and diabetes: From mechanism to therapy

Diabetes is a significant public health issue known as the world's fastest-growing disease condition. It is characterized by persistent hyperglycemia and subsequent chronic complications leading to organ dysfunction and, ultimately, the failure of target organs. Substance P (SP) is an undecapeptide that belongs to the family of tachykinin (TK) peptides. The SP-mediated activation of the neurokinin 1 receptor (NK1R) regulates many pathophysiological processes in the body. There is also a relation between the SP/NK1R system and diabetic processes. Importantly, deregulated expression of SP has been reported in diabetes and diabetes-associated chronic complications. SP can induce both diabetogenic and antidiabetogenic effects and thus affect the pathology of diabetes destructively or protectively. Here, we review the current knowledge of the functional relevance of the SP/NK1R system in diabetes pathogenesis and its exploitation for diabetes therapy. A comprehensive understanding of the role of the SP/NK1R system in diabetes is expected to shed further light on developing new therapeutic possibilities for diabetes and its associated chronic conditions.

Traumatic brain injury, diabetic neuropathy and altered-psychiatric health: The fateful triangle

Traumatic brain injury is a detrimental medical condition particularly when accompanied by diabetes. There are several comorbidities going along with diabetes including, but not limited to, kidney failure, obesity, coronary artery disease, peripheral vascular disease, hypertension, stroke, neuropathies and amputations. Unlike diabetes type 1, diabetes type 2 is more common in adults who simultaneously suffer from other comorbid conditions making them susceptible to repetitive fall incidents and sustaining head trauma. The resulting brain insult exacerbates current psychiatric disorders such as depression and anxiety, which, in turn, increases the risk of sustaining further brain traumas. The relationship between diabetes, traumatic brain injury and psychiatric health constitutes a triad forming a non-reversible vicious cycle. At the proteomic and psychiatric levels, cellular, molecular and behavioral alterations have been reported with the induction of non-traumatic brain injury in diabetic models such as stroke. However, research into traumatic brain injury has not been systematically investigated. Thus, in cases of diabetic neuropathy complicated with traumatic brain injury, utilizing fine structural and analytical techniques allows the identification of key biological markers that can then be used as innovative diagnostics as well as novel therapeutic targets in an attempt to treat diabetes and its sequelae especially those arising from repetitive mild brain trauma.

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.

Sympathetic axonopathies and hyperinnervation in the small intestine smooth muscle of aged Fischer 344 rats

It is well documented that the intrinsic enteric nervous system of the gastrointestinal (GI) tract sustains neuronal losses and reorganizes as it ages. In contrast, age-related remodeling of the extrinsic sympathetic projections to the wall of the gut is poorly characterized. The present experiment, therefore, surveyed the sympathetic projections to the aged small intestine for axonopathies. Furthermore, the experiment evaluated the specific prediction that catecholaminergic inputs undergo hyperplastic changes. Jejunal tissue was collected from 3-, 8-, 16-, and 24-month-old male Fischer 344 rats, prepared as whole mounts consisting of the muscularis, and processed immunohistochemically for tyrosine hydroxylase, the enzymatic marker for norepinephrine, and either the protein CD163 or the protein MHCII, both phenotypical markers for macrophages. Four distinctive sympathetic axonopathy profiles occurred in the small intestine of the aged rat: (1) swollen and dystrophic terminals, (2) tangled axons, (3) discrete hyperinnervated loci in the smooth muscle wall, including at the bases of Peyer's patches, and (4) ectopic hyperplastic or hyperinnervating axons in the serosa/subserosal layers. In many cases, the axonopathies occurred at localized and limited foci, involving only a few axon terminals, in a pattern consistent with incidences of focal ischemic, vascular, or traumatic insult. The present observations underscore the complexity of the processes of aging on the neural circuitry of the gut, with age-related GI functional impairments likely reflecting a constellation of adjustments that range from selective neuronal losses, through accumulation of cellular debris, to hyperplasias and hyperinnervation of sympathetic inputs.

Concurrent activation of the somatosensory forebrain and deactivation of periaqueductal gray associated with diabetes-induced neuropathic pain

We combined behavioral testing with brain imaging using (99m)Tc-HMPAO (Amersham Health) to identify CNS structures reflecting alterations in pain perception in the streptozotocin (STZ) model of type I diabetes. We induced diabetic hyperglycemia (blood glucose >300 mg/dl) by injecting male Sprague-Dawley rats with STZ (45 mg/kg i.p.). Four weeks after STZ-diabetic rats exhibited behaviors indicative of neuropathic pain (hypersensitivity thermal stimuli) and this hypersensitivity persisted for up to 6 weeks. Imaging data in STZ-diabetic rats revealed significant increases in the activation of brain regions involved in pain processing after 6 weeks duration of diabetes. These regions included secondary somatosensory cortex, ventrobasal thalamic nuclei and the basolateral amygdala. In contrast, the activation in habenular nuclei and the midbrain periaqueductal gray were markedly decreased in STZ rats. These data suggest that pain in diabetic neuropathy may be due in part to hyperactivity in somatosensory structures coupled with a concurrent deactivation of structures mediating antinociception.

Effect of aminoguanidine treatment on diabetes-induced changes in the myenteric plexus of rat ileum

The aim of this study was to investigate the ability of aminoguanidine (AG) to prevent diabetes-induced changes in nitric oxide synthase- (nNOS), vasoactive intestinal polypeptide- (VIP) and noradrenaline- (NA) containing nerves of the rat ileum using immunohistochemical and biochemical techniques. Diabetes was induced in adult male Wistar rats by a single intraperitoneal injection of streptozotocin (65 mg/kg). AG was administered in the drinking water to control (1.8 g/l) and diabetic (0.9 g/l) rats over a period of 8 weeks. Diabetes caused a significant increase in the thickness of nNOS-containing nerve fibres (p<0.001) in the circular muscle, in nNOS activity (p<0.05) and in the size distribution of nNOS-containing myenteric neurons (p<0.001). The thickness of VIP-containing nerve fibres was significantly greater (p<0.01) and there was a significant increase in varicosity size (p<0.01) and proportion of VIP-positive myenteric neurons (p<0.01) in diabetes. NA levels were significantly reduced (p<0.01) and the size of varicosities containing tyrosine hydroxylase (TH) was significantly increased (p<0.001) in diabetes. AG treatment completely or partially prevented the diabetes-induced increase in nNOS activity, in VIP-containing varicosity size, and in fibre width of both VIP- and nNOS-containing fibres in the circular muscle but had no effect on the diabetes-induced increase in nNOS-containing neuronal size or proportion of VIP-containing myenteric neurons. In contrast to VIP, AG treatment had no effect on the increase in TH-containing varicosity size in diabetes and also failed to prevent the decrease in NA levels induced by diabetes. These results indicate that AG treatment for neuropathy is not equally effective for all autonomic nerves supplying the ileum and that diabetes-induced changes in NA-containing nerves are particularly difficult to treat.

Choline acetyltransferase and inducible nitric oxide synthase are increased in myenteric plexus of diabetic guinea pig

Alterations in enzymes in myenteric neurons from ileum were investigated in guinea pigs treated with either the pancreatic beta cell toxin streptozotocin or vehicle. After 5-6 weeks, expressions of choline acetyltransferase, neuronal nitric oxide synthase and inducible nitric oxide synthase were determined in longitudinal and myenteric plexus preparations using indirect immunohistochemistry. In ileum from streptozotocin-treated animals, the density of choline acetyltransferase-immunoreactive nerve fibers within the tertiary plexus and the percent total myenteric neurons expressing inducible nitric oxide synthase were increased, but the percent total myenteric neurons expressing neuronal nitric oxide synthase was not changed. Diabetes resulted in selective alterations in myenteric neurons including an increased density of cholinergic tertiary fibers and percentage of neurons expressing the inducible isoform of nitric oxide synthase. These adaptive changes by myenteric neurons to diabetes may contribute to gastrointestinal dysfunctions associated with diabetes.

Effects of age and streptozotocin-induced diabetes on biogenic amines in rat tail artery

The changes in amine concentrations in different segments of the rat tail artery have been investigated at different ages and after different durations of streptozotocin-induced diabetes. There was a significant positive slope to the relationship between amine concentrations and age in proximal portion of the normal tail artery, and a significant additional increase in amine concentrations following induction of diabetes. The peak of the latter response occurred between 10 and 20 weeks following the induction of diabetes. There was also a significant dependence on the length of the post-ganglionic neurones, which may be related to the failure of axonal transport of some of the essential enzymes or transporters for these biogenic amines. This model of altered catecholamine metabolism and handling requires further investigation so that alterations in the mechanisms involved in processing of these amines in diabetic autonomic neuropathy may be elucidated.

Neuropathology and pathogenesis of diabetic autonomic neuropathy

Autonomic neuropathy is a significant complication of diabetes resulting in increased patient morbidity and mortality. A number of studies, which have shown correspondence between neuropathologic findings in experimental animals and human subjects, have demonstrated that axonal and dendritic pathology in sympathetic ganglia in the absence of significant neuron loss represents a neuropathologic hallmark of diabetic autonomic neuropathy. A recurring theme in sympathetic ganglia, as well as in the pot-ganglionic autonomic innervation of various end organs, is the involvement of distal portions of axons and nerve terminals by degenerative or dystrophic changes. In both animals and humans, there is a surprising selectivity of the diabetic process for subpopulations of autonomic ganglia, nerve terminals within sympathetic ganglia and end organs, from end organ to end organ, and between vascular and other targets within individual end organs. Although the involvement or autonomic axons in somatic nerves may reflect an ischemic pathogenesis, the selectivity of the diabetic process confounds simple global explanations of diabetic autonomic neuropathy as the result of diminished blood flow with resultant tissue hypoxia. A single unifying pathogenetic hypothesis has not yet emerged from clinical and experimental animal studies, and it is likely that diabetic autonomic neuropathy will be shown to have multiple causative mechanisms, which will interact to result in the variety of presentations of autonomic injury in diabetes. Some of these mechanisms will be shared with aging changes in the autonomic nervous system. The role of various neurotrophic substances and the polyol pathway in the pathogenesis and treatment of diabetic neuropathy likely represents a two-edged sword with both salutary and exacerbating effects. The basic neurobiologic process underlying the diabetes-induced development of neuroaxonal dystrophy, synaptic dysplasia, defective axonal regeneration, and alterations in neurotrophic substance may be mechanistically related.

Gastrointestinal transit in nonobese diabetic mouse: an animal model of human diabetes type 1

Gastrointestinal transit (GI) in the nonobese diabetic (NOD) mouse, an animal model of human diabetes type 1, was examined in animals with short- (duration 1-5 days) and long-term (duration 28-35 days) diabetes. Blood glucose level, serum insulin concentration, and gut neuroendocrine peptide content were also measured. GI was significantly rapid in NOD mice with long-term diabetes (LTD), but was not correlated with blood glucose level, serum insulin concentration, or pancreatic insulin content. GI was correlated with duodenal secretin content, but not with the content of other neuroendocrine peptides in the different segments investigated. Whereas antral vasoactive intestinal peptide (VIP) content in NOD mice with LTD was significantly higher, colonic VIP was lower in NOD mice with short-term diabetes (STD). In the duodenum, whereas the concentration of secretin in NOD mice with both STD and LTD was lower, the gastrin content was higher. Duodenal somatostatin content in NOD mice with LTD was lower. In colon, the content of galanin in NOD mice with LTD was higher than in controls. The decreased content of secretin may be among the factors that cause rapid GI in NOD mice with LTD. Changes in the antral content of VIP, duodenal somatostatin, and colonic galanin in NOD mice with LTD may cause low intestinal secretion and, together with rapid GI, give rise to diarrhoea, which is a common symptom in diabetes.

Effects of diabetes on non-adrenergic, non-cholinergic relaxation induced by GABA and electrical stimulation in the rat isolated duodenum

1. The effects of streptozotocin (STZ)-induced experimental diabetes on nitrergic-mediated responses to GABA and electrical field stimulation (EFS) have been evaluated in rat isolated duodenum. 2. In the presence of noradrenergic and cholinergic blockade, EFS (60 V, 1 ms, 0.1-32 Hz) induced frequency dependent relaxations of the preparation. GABA also caused submaximal relaxation of the rat duodenum. The relaxations induced by GABA and EFS were reduced in duodenal tissues from diabetic rats compared with control rats. 3. Neither ATP- nor sodium nitroprusside-induced relaxations were altered in diabetic duodenal tissues. GABA- and EFS-induced relaxations were inhibited by NG-nitro-L-arginine methyl ester (L-NAME; 300 mmol/L) in both diabetic and control rats. Although the inhibition caused by L-NAME of GABA- and EFS-induced relaxation was partially reversed by L-arginine (1 mmol/L), L-arginine alone had no effect on GABA- and EFS-induced relaxation in diabetic rats. 4. These results suggest that STZ-induced diabetes impairs non-adrenergic, non-cholinergic relaxation induced by EFS and GABA. Impairment of nitrergic innervation of the rat duodenum may contribute to the abnormalities of intestinal motility abnormalities associated with diabetes.

Neuroendocrine peptides in stomach and colon of an animal model for human diabetes type I

Twelve prediabetic and 12 diabetic non-obese diabetic (NOD) mice, all females aged 22-24 weeks, were investigated. As controls, 12 BLAB/cJ Bom mice of the same age and gender as the NOD mice were used. The concentration of several neuroendocrine peptides was determined by radioimmunoassay of tissue extracts of transmural specimens of antrum and distal colon. The neuroendocrine peptides investigated were peptide YY (PYY), somatostatin, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), neurotensin, and galanin. In the antrum, VIP, NPY, and galanin concentrations were all significantly lower in prediabetic and diabetic NOD mice than in controls. There was no statistical difference between NOD mice and controls regarding neurotensin content. In the colon, the concentrations of PYY, somatostatin, VIP, NPY, and galanin were lower in prediabetic and diabetic NOD mice than in controls. The concentration of neurotensin in prediabetic, but not in diabetic NOD mice was lower than that of controls. The present observations support the previously reported studies on animal models for human type I diabetes that the neuroendocrine system of the gut is disturbed. It also shows that the neuroendocrine system of the stomach and large intestine is affected. The present findings may have some implications for the gastrointestinal dysfunction observed in patients with human type I diabetes.

Diabetes and the gastrointestinal tract

Gastrointestinal motility disorders are common in patients with diabetes. The entire gastrointestinal tract may be involved from the esophagus to the anal sphincter. Before instituting therapy, people with diabetes first require a careful diagnostic evaluation. Treatment includes tight glucose control and the use of antiemetics and prokinetic agents.

Large intestinal endocrine cells in non-obese diabetic mice

Colonic endocrine cells from prediabetic and diabetic non-obese diabetic mice as well as of the sister strain, BALB/cJ, were investigated by immunocytochemistry and computer image analysis. In prediabetic mice, enteroglucagon-and serotonin-immunoreactive cells were significantly increased in number, whereas the cell secretory index of these two cell types was significantly reduced. No significant differences were found in numbers or cell secretory index of peptide YY (PYY)-immunoreactive cells. In diabetic mice, PYY-immunoreactive cells were significantly fewer, but there were no significant differences in the numbers of enteroglucagon-and serotonin-immunoreactive cells. Whereas the cell secretory index was reduced in serotonin-producing cells, no significant differences were found between diabetic and control mice regarding the cell secretory index of PYY- and enteroglucagon-immunoreactive cells. Nor was any statistically significant difference found between controls, prediabetic and diabetic non-obese diabetic mice, regarding the thickness of submucosa, of circular and longitudinal-muscle layers, or of the mucosal area/microm baseline. The present study showed that abnormalities in colonic endocrine cells do occur, in both prediabetic and diabetic mice, but they are different in nature and can be divided into primary and secondary to the diabetes onset. The present findings of abnormal colonic endocrine cells in non-obese diabetic mice, an animal model for human insulin-dependent diabetes mellitus, might help explain the gastrointestinal disorders observed in patients with diabetes. The study also showed that the change in the colonic endocrine cells is dynamic and started before the onset of the diabetic condition.

Abnormalities of small intestinal endocrine cells in non-obese diabetic mice

The endocrine cells in the duodenum of pre-diabetic and diabetic female non-obese diabetic (NOD) mice aged 22-24 weeks were studied by means of immunohistochemistry and computed image analysis as well as by radioimmunoassays of tissue extracts. As controls, 12 female BALB/cJ mice of the same age as NOD mice were used. The number of secretin-immunoreactive cells increased in diabetic but not in pre-diabetic NOD mice. The level of extractable secretin was higher in both pre-diabetic and diabetic NOD mice. The number of GIP-, CCK/gastrin-, and serotonin-immunoreactive cells was significantly reduced in both pre-diabetic and diabetic NOD mice. There was no statistical difference in the number of somatostatin-immunoreactive cells between the NOD mice and controls. The level of GIP was higher and gastrin was lower in NOD mice compared to controls. There was no statistical difference in the somatostatin level between the NOD mice and controls. The cell secretory index was elevated in all the endocrine cell types except CCK/gastrin cells. It has been suggested that some of the changes in the duodenal endocrine cells could be attributed to the diabetes state, but most of the changes seem to take place before the onset of diabetes. The abnormalities in the duodenal endocrine cells observed here in an animal model for diabetes type I might have relevance for the gastrointestinal dysfunction displayed in human diabetes.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Alterations of protein mono-ADP-ribosylation and diabetic neuropathy: a novel pharmacological approach

This study monitored the extranuclear endogenous mono ADP-ribosylation of proteins. At least 10 proteins were ADP-ribosylated in a crude extract from control superior cervical ganglia, and 7 were labeled in control dorsal root ganglia; whereas in the diabetic rat the extent of labeling was reduced. These data suggest that proteins of peripheral ganglia are excessively ADP-ribosylated in vivo. Treatment of diabetic animals with silybin, a flavonoid with ADP-ribosyltransferase inhibitory activity, did not affect hyperglycemia, but prevented the alterations in the extent of mono-ADP-ribosylation of proteins. This effect was associated with the prevention of substance P-like immunoreactivity loss in the sciatic nerve. In the membrane fraction of sciatic nerve Schwann cells, at least 9 proteins were ADP-ribosylated, in diabetic rats the extent of labeling was increased. A comparable increase involving the same proteins was triggered by chronic nerve injury and by corticosteroid treatment. Silybin treatment of diabetic rats prevented such an increase. We propose that the inhibition of excessive protein mono-ADP-ribosylation by silybin prevented the onset of diabetic neuropathy, while the silybin effect on mono-ADP-ribosylation of Schwann cells is likely indirect and secondary to the improvement of diabetic axonopathy.

Enteric neuropeptides in streptozotocin-diabetic rats; effects of insulin and aldose reductase inhibition

The aim of the present study was to determine whether diabetes-induced changes in the distribution of enteric neuropeptides, could be prevented in 12-week streptozotocin-diabetic rats, by rigorous control of glycaemia, using daily adminstration of insulin, or an aldose reductase inhibitor (ponalrestat). The pattern of distribution of nerve fibres and cell bodies, containing immunoreactive vasoactive intestinal polypeptide (VIP), galanin (GAL), calcitonin gene-related peptide (CGRP) and substance P was examined in the myenteric plexus of ileum from control, untreated diabetic, insulin-treated diabetic and aldose reductase inhibitor-treated diabetic rats. The increase in VIP- and GAL-like immunoreactivity, seen in the myenteric plexus of untreated diabetic rat ileum, was not present in the myenteric plexus of ileum from insulin- and aldose reductase inhibitor-treated diabetic rats. With CGRP-like immunoreactive fibres, there was a clear decrease in the ileum of untreated diabetic rats. This was prevented by insulin treatment, but aldose reductase inhibitor treatment had no effect. No alterations in substance P-like immunoreactivity were seen in the myenteric plexus of ileum from any of the groups investigated. Generally, the similarity of effect of ponalrestat and insulin on VIP and galanin expression in this study supports a primary effect of insulin via glycaemic control. The dissimilarity of the effect of the two treatments on CGRP expression may imply a neurotrophic effect of insulin, although there are certainly consequences of hyperglycaemia other than exaggerated flux through the polyol pathway.

Effect of substance P and capsaicin on stomach fundus and ileum of streptozotocin-diabetic rats

The in vitro responses of longitudinal preparations of rat stomach fundus and ileum to capsaicin at 1, 8, 4, 16 and 26 weeks and to substance P at 1 and 8 weeks from diabetes induction were studied. The results were compared with those obtained in age-matched control rats. The contractile responses to exogenous substance P and capsaicin were not affected in the stomach fundus from diabetic rats. Atropine (1 microM) did not antagonize the substance P-induced response whereas it inhibited about 90% of the capsaicin-induced response in controls and about 60% of the response in diabetic rats. At the resting tone, capsaicin induced a relaxation followed by a contraction in stomach fundus of control rats. Only a contraction was evoked in diabetic rats. In carbachol (0.05-0.1 microM) pre-stimulated strips, a complete restoration of the biphasic response was obtained in the diabetic state. The contractile response elicited by exogenous substance P was not significantly increased in the ileum preparations from diabetic rats; nevertheless the EC50 value for substance P was reduced 8 weeks after the onset of diabetes. The response elicited by capsaicin in the ileum of control rats was also biphasic. The capsaicin-induced contraction was greater in tissue from diabetic rats as compared with controls and relaxation was not evident. An age-related decrease of the contraction was also evident in both groups. Atropine (1 microM) partially antagonized the responses to substance P and capsaicin. The inhibition of the responses with atropine was more evident in control than in diabetic rats. These results suggest that the myogenic actions of several agonists in these two tissues are differently modified in experimental diabetes.

Diabetic neuropathy in the rat: 1. Alcar augments the reduced levels and axoplasmic transport of substance P

This study examined the sciatic nerve axonal transport of substance P-like immunoreactivity (SPLI) and its basal content in stomach, sciatic nerve and lumbar spinal cord of 8- and 12-week alloxan-diabetic rats, respectively. One group of diabetic rats received acetyl-l-carnitine (ALCAR) throughout the experimental period. Alloxan treatment caused hyperglycemia and reduced boy growth. Axonal transport of SPLI was studied by measurement of 24-hour accumulation at a ligature on the sciatic nerve. There was a marked reduction (from 50% to 100% according to the nerve segment examined) of anterograde and retrograde accumulation of SPLI in the constricted nerve of 8-week diabetic rats. In the sciatic nerve of ALCAR-treated diabetic rats, the accumulation of SPLI was comparable to control values. In the sciatic nerve, lumbar spinal cord and stomach of 12-week diabetic rats, there is a significant reduction of SPLI content. ALCAR treatment prevented SPLI loss in these tissues. Sciatic nerves showed the typical sorbitol increase and myo-inositol loss that were significantly counteracted by ALCAR. This study suggests that ALCAR treatment prevents diabetes-induced sensory neuropathy by improving altered metabolic pathways such as polyol activity and myo-inositol synthesis, and by preventing the reduction of synthesis and axonal transport of substance P.

The effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastrointestinal tract in rats with non-insulin-dependent diabetes

1. Decreased beta-adrenergic responses have been reported in gastro-intestinal tract of rats with diabetes mellitus. Effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastro-intestinal tract due to non-insulin-dependent diabetes were investigated using duodenum, jejunum and ileum from rats which were injected with alloxan in their neonatal periods. 2. Insulin treatment of non-insulin-dependent diabetic rats for 10 days corrected the decreased beta-adrenergic responses of the isolated duodenum, jejunum and ileum confirming the previous results obtained from insulin-dependent diabetic rats. 3. Glyburide treatment alone for 3 weeks also reversed the changes in the gastro-intestinal beta-adrenergic responses of non-insulin-dependent diabetic rats. Combination of glyburide with insulin, however, did not cause an additive or supra-additive interaction in terms of beta-adrenergic sensitivities of the diabetic tissues. 4. The results obtained in the present study strongly suggested that non-insulin-dependent diabetes may cause a decrease in the number of gastro-intestinal beta-adrenoceptors, while glyburide and insulin treatments correct the changes related to beta-adrenoceptors. The effect of insulin on the beta-adrenergic sensitivity of diabetic rat duodenum, jejunum and ileum may occur via a direct mechanism, whereas glyburide seems to be effective on the beta-adrenergic responses through the increases in the insulin secretion and/or in the number of gastro-intestinal insulin receptors.

The effects of streptozotocin-induced diabetes on norepinephrine and cholinergic enzyme activities in rat parotid and submandibular glands

Autonomic neuropathy is a common complication of diabetes mellitus, and both morphological and physiological data suggest that salivary gland function in diabetic rats is affected by neuropathies involving sympathetic and parasympathetic nerves. Therefore, glandular levels of the adrenergic neurotransmitter, norepinephrine (NE) and two cholinergic enzymes, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), were investigated in 6-month streptozotocin-diabetic rats. Significant, but variable, increases in total parotid NE (ng/gland) were observed in diabetic rats, whereas total submandibular NE was lower in diabetic animals than in controls. However, on a ng/mg tissue basis, NE levels in both the parotid gland, and less dramatically, in the submandibular gland were increased. Somewhat different results were observed for AChE and ChAT. AChE was marginally greater in the parotid glands of diabetic rats, whereas AChE and ChAT levels were significantly lower in diabetic than control submandibular glands. Expressed as enzymatic activity per mg tissue, submandibular gland ChAT, but not AChE, was increased. Short-term (3-day) insulin treatment of diabetic animals had no significant effects on total NE, AChE or ChAT in the parotid gland, but led to a further reduction in submandibular ChAT. With regard to function, changes in AChE appeared to be correlated with previously reported morphological assessments of parotid gland innervation in diabetic animals. Thus, the decreased response of the parotid gland in diabetic rats to parasympathetic stimulation may be related in part to the increase in AChE.

Perinatal morphine treatment inhibits pruning effect and regeneration of serotoninergic pathways following neonatal 5,7-HT lesions

Lesion of the serotoninergic system in neonate rats is an ideal model for assessing the activity of chemical substances capable of affecting neuronal plasticity and regeneration (Jonsson et al., Dev Brain Res 16: 171-180, 1984). Treatment of newborn rats within 6 hr from birth with the selective neurotoxin 5,7-dihydroxytryptamine causes degeneration of the most distal serotoninergic axons. In our experimental conditions we have observed that after such neurotoxic treatment there is spinal cord denervation, which is particularly remarkable in the lumbar segment. This degenerative event is followed by gradual regeneration of the lesioned axons, with good reinnervation of the entire cord within 8 weeks. The degeneration-regeneration process is correlated with a transient hyperinnervation of the pons-medulla and hypothalamus by the short collaterals (pruning effect), as evidenced by increased serotonin content. Perinatal morphine exposure markedly impairs serotonin regeneration in the spinal cord. In addition, opiate treated rats are more susceptible to lesions, as shown by the neurotoxin induced denervation of the cortex, pons-medulla, and hypothalamus, which does not occur in lesioned controls. Therefore, our observations suggest that perinatal exposure to morphine affects the plasticity and regeneration of the developing serotoninergic system by increasing its susceptibility to neurotoxic lesions and reducing its regenerative capacity.

β-Adrenergic Responsiveness of the Gastrointestinal Tract in Diabetic Rats

Recently, decreased gastrointestinal beta-adrenergic responses in experimental diabetes have been demonstrated. Gastrointestinal responses to beta-adrenoceptor agonists are impaired in both insulin-dependent and non-insulin-dependent diabetic rat. Insulin treatment improves the impaired gastrointestinal beta-adrenergic responsiveness of diabetic rats. The improvement seen with insulin treatment on beta-adrenergic responsiveness is closely related to protein biosynthesis. The decreased beta-adrenergic responses in diabetic rat gastrointestinal tract seem to result from a decrease in the number of beta-adrenoceptors. It is most likely that the decreased gastrointestinal beta-adrenergic responsiveness is related to an impairment in the turnover of beta-adrenoceptors as a consequence of diabetes and that insulin has a beneficial effect on the impaired receptor turnover.

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.

Effects of diabetes on cholinergic transmission in two rat gut preparations

The gut may be a site of early diabetic neuropathy in humans and rats. The latter may provide appropriate models of these conditions. Therefore, cholinergic function was examined in two gut smooth muscle preparations from control, 30-day, and 6-month streptozotocin-diabetic and similarly diabetic rats that had received continuous treatment with an aldose reductase inhibitor, ponalrestat. Responses of terminal ileum longitudinal muscle to transmural nerve stimulation were depressed in preparations from untreated 30-day diabetic animals. Responses to exogenous acetylcholine were also depressed, by at least the same extent, in preparations from both 30-day and 6-month diabetic groups. Ponalrestat treatment prevented both changes in the 30-day study but did not prevent a depression of responses to acetylcholine in the 6-month study. Neither diabetes nor ponalrestat affected responses of esophageal muscularis mucosa to electrical stimulation or to exogenous acetylcholine. These observations suggest a change in the smooth muscle and/or noncholinergic innervation rather than in the cholinergic nerves of the ileal preparation. Cholinergic function in the ileum did not, therefore, seem to be an appropriate model of diabetic neuropathy.

Endogenous levels of nerve growth factor (NGF) are altered in experimental diabetes mellitus: a possible role for NGF in the pathogenesis of diabetic neuropathy

Sympathetic and neural-crest derived sensory neurons consisting of unmyelinated and small myelinated fibers are known to be affected at an early stage in diabetes mellitus (DM). Since these peripheral neurons need nerve growth factor (NGF) for their development and maintenance of function in adulthood, changes in endogenous NGF levels could be of relevance for the pathogenesis of diabetic neuropathy (DNP). Using an improved two-site enzyme immunoassay for NGF, we have investigated whether endogenous NGF levels are altered in Sprague-Dawley rats with DM induced by a single injection of streptozotocin (STZ). STZ-treated rats are known to develop in many respects equivalents to neuropathic complications observed in human DM. We found in some sympathetically innervated target organs decreased NGF contents by maximally 56%: transiently in the iris 2 weeks and in the ventricle 12-24 weeks after DM induction and permanently in the submandibular gland already 3 days after DM induction. Several weeks after onset of DM, NGF content was increased by maximally 145-300% in most peripheral targets investigated, such as in iris, cardiac atrium and ventricle, spleen, prostate gland, and vas deferens. This is suggestive for an impaired NGF removal by NGF-sensitive neurons in diabetic rats. Moreover, NGF levels were decreased to minimally 42.6 +/- 4% of control in the NGF-transporting sciatic nerve. NGF levels began to decrease not before 3 weeks after DM induction and remained decreased with 54.0 +/- 5% of control even after 6 months duration of DM. About the same time (i.e., 2 weeks after induction of DM) NGF levels began to decrease in the superior cervical ganglion (where the sympathetic perikarya are located) to minimally 53.2 +/- 4% of control 12 weeks after DM induction. No altered NGF levels were observed during a 3-month duration of DM in the terminal ileum and sensory trigeminal ganglion. Since NGF exerts its neurotrophic action in the perikarya after its retrograde transport from the NGF-producing periphery, our results are consistent with the hypothesis that an alteration in NGF levels may play a role in the pathogenesis of DNP as far as sympathetic neurons are concerned. Thus, our results suggest that DM influences the production and/or transport of endogenous NGF and consequently, that a deprivation of this neurotrophic factor may account for some of the functional deficits known to occur in DNP, such as impaired catecholaminergic transmitter synthesis. This hypothesis possibly opens the way for new concepts in the therapy of DNP.

Denervation and hyperinnervation in the nervous system of diabetic animals: III. Functional alterations of G proteins in diabetic encephalopathy

G protein-mediated effects on cAMP production were evaluated in the corpus striatum of diabetic rats 5 and 14 weeks after alloxan injection by measuring both D1-receptor-induced stimulation and D2-receptor-mediated inhibition of adenylate-cyclase activity. At 5 weeks of diabetes, no obvious alterations of G protein functions were detected. Both dopamine-stimulated adenylate cyclase and bromocriptine-induced inhibition of enzyme activity were indeed similar in control and diabetic animals. Fourteen weeks after alloxan injection, profound alterations were observed. Dopamine-stimulated cAMP production was markedly increased in diabetic rats, whereas bromocriptine ability to reduce cAMP formation was almost abolished at this late stage of diabetes. Hypoactivity of Gi/Go proteins was also confirmed by the reduced ability of the GTP non-hydrolyzable analog GTP-gamma-S to inhibit forskolin-stimulation of adenylate cyclase. These results show an apparent functional imbalance between Gs and Gi/Go-mediated transduction mechanisms, with an increased efficacy of Gs activity likely due to the loss of Gi/Go inhibitory functions. Concomitantly with such transductional alteration detected in chronic diabetes, we observed a marked increase of the striatal content of met-enkephalin, which is known to utilize Gi/Go proteins for inhibition of adenylate cyclase. The measurement of other transmitters (vaso-active intestinal peptide, substance P, serotonin, noradrenaline, and dopamine) did not reveal any difference with respect to controls. The observed transductional defect in diabetic animals and the increased content and/or hyperinnervation by the metenkephalinergic system could be correlated as mutual compensatory mechanisms.