Myo-inositol normalizes decreased sodium permeability of the blood-brain barrier in streptozotocin diabetes

ASK1 modulates the expression of microRNA Let7A in microglia under high glucose in vitro condition

Hyperglycemia results in oxidative stress and leads to neuronal apoptosis in the brain. Diabetes studies show that microglia participate in the progression of neuropathogenesis through their involvement in inflammation in vivo and in vitro. In high-glucose-induced inflammation, apoptosis signal regulating kinase 1 (ASK1) triggers the release of apoptosis cytokines and apoptotic gene expression. MicroRNA-Let7A (miR-Let7A) is reported to be a regulator of inflammation. In the present study, we investigated whether miR-Let7A regulates the function of microglia by controlling ASK1 in response to high-glucose-induced oxidative stress. We performed reverse transcription (RT) polymerase chain reaction, Taqman assay, real-time polymerase chain reaction, and immunocytochemistry to confirm the alteration of microglia function. Our results show that miR-Let7A is associated with the activation of ASK1 and the expression of anti-inflammatory cytokine (interleukin (IL)-10) and Mycs (c-Myc and N-Myc). Thus, the relationship between Let-7A and ASK1 could be a novel target for enhancing the beneficial function of microglia in central nervous system (CNS) disorders.

High glucose stimulates TNFα and MCP-1 expression in rat microglia via ROS and NF-κB pathways

AIM

To investigate whether high glucose stimulates the expression of inflammatory cytokines and the possible mechanisms involved.

METHODS

ELISA and real-time PCR were used to determine the expression of the inflammatory factors, and a chemiluminescence assay was used to measure the production of reactive oxygen species (ROS).

RESULTS

Compared to low glucose (10 mmol/L), treatment with high glucose (35 mmol/L) increased the secretion of tumor necrosis factor (TNF)α and monocyte chemotactic protein-1 (MCP-1), but not interleukin (IL)-1β and IL-6, in a time-dependent manner in primary cultured rat microglia. The mRNA expression of TNFα and MCP-1 also increased in response to high glucose. This upregulation was specific to high glucose because it was not observed in the osmotic control. High-glucose treatment stimulated the formation of ROS. Furthermore, treatment with the ROS scavenger NAC significantly reduced the high glucose-induced TNFα and MCP-1 secretion. In addition, the nuclear factor kappa B (NF-κB) inhibitors MG132 and PDTC completely blocked the high glucose-induced TNFα and MCP-1 secretion.

CONCLUSION

We found that high glucose induces TNFα and MCP-1 secretion as well as mRNA expression in rat microglia in vitro, and this effect is mediated by the ROS and NF-κB pathways.

Hyperglycemia not hypoglycemia alters neuronal dendrites and impairs spatial memory

BACKGROUND/OBJECTIVE

We previously reported that chronic hyperglycemia, but not hypoglycemia, was associated with the reduction of neuronal size in the rat brain. We hypothesized that hyperglycemia-induced changes in neuronal structure would have negative consequences, such as impaired learning and memory. We therefore assessed the effects of hyperglycemia and hypoglycemia on neuronal dendritic structure and cognitive functioning in young rats.

DESIGN/METHODS

Experimental manipulations were conducted on male Wistar rats for 8 wk, beginning at 4 wk of age. At the completion of the treatments, all rats were trained in the radial-arm water maze, a spatial (hippocampus-dependent) learning and memory task. Three groups of rats were tested: an untreated control group, a streptozotocin-induced diabetic (STZ-D) group, and an intermittent hypoglycemic group. Following behavioral training, the brains of all animals were examined with histologic and biochemical measurements.

RESULTS

Peripheral hyperglycemia was associated with significant increases in brain sorbitol (7.5 +/- 1.6 vs. 5.84 +/- 1.0 microM/mg) and inositol (9.6 +/- 1.4 vs. 7.1 +/- 1.1 microM/mg) and reduced taurine (0.65 +/- 0.1 vs. 1.3 +/- 0.1 mg/mg). Histologic evaluation revealed neurons with reduced dendritic branching and spine density in STZ-D rats but not in control or hypoglycemic animals. In addition, the STZ-D group exhibited impaired performance on the water maze memory test.

CONCLUSIONS

Hyperglycemia, but not hypoglycemia, was associated with adverse effects on the brain polyol pathway activity, neuronal structural changes, and impaired long-term spatial memory. This finding suggests that the hyperglycemic component of diabetes mellitus has a greater adverse effect on brain functioning than does intermittent hypoglycemia.

High glucose stimulates GRO secretion from rat microglia via ROS, PKC, and NF-kappaB pathways

Hyperglycemia causes direct neuronal damage in diabetic encephalopathy. Microglia have been found to be activated in diabetic encephalopathy, presumably mediating and amplifying neuron degeneration. Chemokine IL-8 plays an important role in the pathogenesis of encephalopathy. Therefore, we investigated whether high glucose could activate microglia and stimulate IL-8 secretion and if so, the possible mechanisms that were involved. ELISA results showed that treatment with high glucose (35 mM) compared with treatment with low glucose (10 mM) time-dependently elevated secretion of GRO (the rat ortholog of human IL-8) in primary cultured rat microglia. Real-time PCR results showed GRO mRNA expression also increased in response to high glucose in a time-dependent manner. These effects were specific to high glucose because the osmolality control had no such effect. High-glucose treatment stimulated the formation of ROS, as seen in the DCF fluorescence assay, increased phosphorylation of PKC, as seen in the Western blot analysis, and activated NF-kappaB, as seen in the luciferase reporter assay. In addition, treatment with the ROS scavenger NAC (2 mM) significantly reduced the high glucose-induced phosphorylation of PKC and GRO secretion. Treatment with the PKC activator PMA (10-50 nM) stimulated GRO secretion, and the PKC inhibitors calphostin C (300 nM) or chelerythrine (1 microM) attenuated the high glucose-induced GRO secretion. Furthermore, the NF-kappaB inhibitors MG132 (10 microM) or PDTC (5 microM) completely blocked the high glucose-induced GRO secretion. In conclusion, high glucose induces GRO secretion and mRNA expression in activated rat microglia, which is mediated by the ROS, PKC, and NF-kappaB pathways. High glucose-induced IL-8 production by microglia may contribute to diabetic encephalopathy.

Cerebral dysfunction in type 1 diabetes: effects of insulin, vascular risk factors and blood-glucose levels

Type 1 diabetes can lead to several well-described complications such as retinopathy, nephropathy and peripheral neuropathy. Evidence is accumulating that it is also associated with gradually developing end-organ damage in the central nervous system. This relatively unknown complication can be referred to as "diabetic encephalopathy" and is characterised by electrophysiological and neuroradiological changes, such as delayed latencies of evoked potentials, modest cerebral atrophy and (periventricular) white matter lesions. Furthermore, individuals with type 1 diabetes may show performance deficits in a wide range of cognitive domains. The exact mechanisms underlying this diabetic encephalopathy are only partially known. Chronic metabolic and vascular changes appear to play an important role. Interestingly, the differences in the "cognitive profile" between type 1 and type 2 diabetes also suggest a critical role for disturbances of insulin action in the central nervous system.

Cognition and synaptic plasticity in diabetes mellitus

Diabetes mellitus is associated with cognitive deficits and an increased risk of dementia, particularly in the elderly. These deficits are paralleled by neurophysiological and structural changes in the brain. In animal models of diabetes, impairments of spatial learning occur in association with distinct changes in hippocampal synaptic plasticity. At the molecular level these impairments might involve changes in glutamate-receptor subtypes, in second-messenger systems and in protein kinases. The multifactorial pathogenesis of diabetic encephalopathy is not yet completely understood, but clearly shares features with brain ageing and the pathogenesis of diabetic neuropathy. It involves both metabolic and vascular changes, related to chronic hyperglycaemia, but probably also defects in insulin action in the brain. Treatment with insulin might therefore not only correct hyperglycaemia, but could also directly affect the brain.

Neurophysiological changes in the central and peripheral nervous system of streptozotocin-diabetic rats. Course of development and effects of insulin treatment

Diabetes mellitus can affect both the peripheral and the central nervous system. However, central deficits are documented less well than peripheral deficits. We therefore compared the course of development of neurophysiological changes in the central and peripheral nervous systems in streptozotocin-diabetic rats. Sciatic nerve conduction velocities and auditory and visual evoked potentials were measured prior to diabetes induction, and then monthly after diabetes induction for 6 months. In addition, the effect of insulin treatment was examined. Treatment was initiated after a diabetes duration of 6 months and was continued for 3 months. During treatment, evoked potentials and nerve conduction were measured monthly. In a third experiment, conduction velocities in ascending and descending pathways of the spinal cord were examined after 3 and 6 months of diabetes. Impairments of sciatic nerve conduction velocities developed fully during the first 2-3 months of diabetes. In contrast, increased latencies of auditory and visual evoked potentials developed only after 3-4 months of diabetes, and progressed gradually thereafter. Insulin treatment, initiated 6 months after induction of diabetes, improved both nerve conduction velocities and evoked potential latencies. Conduction velocities in the spinal cord tended to be reduced after 3 months of diabetes and were significantly reduced after 6 months of diabetes. The present study demonstrates that in streptozotocin-diabetic rats the course of development of peripheral and central neurophysiological changes differs. Peripheral impairments develop within weeks after diabetes induction, whereas central impairments take months to develop. Insulin can reverse both peripheral and central neurophysiological alterations.

Treatment with dimethylthiourea prevents impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that the synthesis/release of hydroxyl radical accounts for impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin, 50-60 mg/kg ip) in response to nitric oxide synthase-dependent agonists (acetylcholine and substance P) and a nitric oxide synthase-independent agonist (nitroglycerin). Reactivity of the basilar artery was measured in untreated nondiabetic and diabetic rats and in nondiabetic and diabetic rats treated with a daily intraperitoneal injection of dimethylthiourea (DMTU; 50 mg/kg). Injection of DMTU was started 48 h after injection of streptozotocin and was continued throughout the diabetic period (3-4 wk). Topical application of acetylcholine (0.1, 1.0, and 10 microM) and substance P (0.1 and 1.0 microM) produced similar dilatation of the basilar artery in untreated and DMTU-treated nondiabetic rats. In untreated diabetic rats, the magnitude of vasodilation produced by acetylcholine and substance P was significantly less than in untreated nondiabetic rats. However, in DMTU-treated diabetic rats, dilatation of the basilar artery in response to acetylcholine and substance P was similar to that observed in nondiabetic rats. Dilatation of the basilar artery in response to nitroglycerin was similar in untreated and DMTU-treated nondiabetic and diabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related to the synthesis/release of hydroxyl radical.

Superoxide dismutase partially restores impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of superoxide dismutase restores nitric oxide synthase-mediated dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin; 50-60 mg/kg i.p.) in response to nitric oxide synthase-dependent agonists (acetylcholine and bradykinin) and a nitric oxide synthase-independent agonist (nitroglycerin) before and during application of superoxide dismutase. Topical application of acetylcholine (1.0 and 10 microM) and bradykinin (1.0 and 10 microM) produced dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. However, the magnitude of vasodilation produced by acetylcholine and bradykinin was significantly less in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. Treatment with superoxide dismutase (150 U/ml) did not alter baseline diameter of the basilar artery in nondiabetic and diabetic rats. However, topical application of superoxide dismutase partially restored nitric oxide synthase-dependent dilatation of the basilar artery in diabetic rats towards that observed in nondiabetic rats. Superoxide dismutase did not alter dilatation of the basilar artery in nondiabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related, in part, to enhanced release of oxygen-derived free radicals.

Central nervous system complications of diabetes mellitus--a perspective from the blood-brain barrier

A host of diabetes-related changes in the central nervous system (CNS) has been recognized. The underlying causes of these changes are multiple. An important contributor to the changes in the CNS is the blood-brain barrier (BBB). Diabetes is associated with changes in both the barrier and transport functions of the cerebral microvessels. Structural changes in cerebral microvessels may account for some of the observed changes. Additional mechanisms include alterations in hemodynamic variables such as arteriovenous shunting, changes in biophysical properties and biochemical compositions of the endothelial cells including changes in lipid fluidity and composition, and alterations of neurotransmitter activity in the cerebral microvessels, notably altered beta adrenergic neurotransmission. These observations indicate that the CNS is not immune against the microangiopathic complications commonly found in various tissues of diabetic animals.

Regulation of phosphoinositide cycle by intracellular sodium in the blood-brain barrier

In the present study of cerebral microvessels, we report that monensin, a Na+ ionophore, elicits a decrease in 32P radioactivity incorporation into phosphoinositides in cerebral microvessels. In addition, monensin evokes enhanced production of inositol-1-monophosphate (IP) and inositol-1,4-bisphosphate (IP2), together with an increase in the diacylglycerol (DAG) mass. These results indicate that monensin evokes a phosphoinositide hydrolysis by phospholipase C (PLC). The absence of inositol-1,4,5-trisphosphate (IP3) production leads us to think that although phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis occurs in this process, there is a very rapid disappearance of IP3. The net decrease in 32P radioactivity incorporated into phosphoinositides suggests that a partial inhibition of their re-synthesis is also evoked. Experimental evidence with pharmacological tools suggests that: (1) these effects are secondary to an increase in Ca2+ through the Na+/Ca2+ exchanger; and (2) the intracellular Ca2+ release is not involved in these effects of monensin. Since some neuropeptide receptors in cerebral microvessels have been reported to be coupled to either the Na+/H+ exchanger or to PLC, we discuss the possibility that cross-talk exists between these intracellular signalling pathways (phosphoinositide metabolism and Na+ transport) in the blood-brain barrier (BBB).

L-Arginine does not restore dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of L-arginine, a substrate for the synthesis of nitric oxide, restores endothelium-dependent dilatation of the basilar artery during diabetes mellitus. We measured the diameter, of the basilar artery in vivo in nondiabetic and diabetic (streptozotocin; 50-60 mg/kg i.p.) rats in response to endothelium-dependent agonists (acetylcholine and bradykinin) and an endothelium-independent agonist (nitroglycerin) before and during application of L-arginine. Topical application of acetylcholine (1.0 and 10 muM) and bradykinin (1.0 and 10 microM) produced dilatation in nondiabetic rats of the basilar artery which was impaired in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dilatation of the basilar artery in nondiabetic and diabetic rats. Topical application of L-arginine (0.1 and 3 mM) did not enhance dilatation of the basilar artery in response to acetylcholine and bradykinin in diabetic rats. Thus, impairment of dilatation of the basilar artery in diabetic rats in response to acetylcholine and bradykinin appears to be related to a mechanism unrelated to the availability of L-arginine for nitric oxide synthase.

Tolrestat treatment prevents modification of the formalin test model of prolonged pain in hyperglycemic rats

This study examined the effects of hyperglycemia and treatment with the aldose reductase inhibitor, Tolrestat, on the pain behavior evoked by injection of formalin into the dorsum of a single hind paw. In control rats, injection of formalin (50 microliters of a 5% solution) evoked two phases of flinching of the injected paw (phases 1 and 2), separated by a quiescent period. Four weeks of streptozotocin-induced diabetes or galactose intoxication did not alter the frequency of flinching during either of the active phases but significantly (P < 0.001 and P < 0.05, respectively) enhanced flinch frequency during the quiescent period. Concurrent treatment with Tolrestat (50 mg/kg/day by gavage) during hyperglycemia prevented the accumulation of the polyol pathway metabolites sorbitol and fructose in the nerve and spinal cord of streptozotocin-diabetic rats and also significantly (P < 0.05) reduced the enhanced flinching of diabetic rats during the quiescent period. These data demonstrate that hyperglycemia induces a period of Tolrestate-preventable hyperalgesia in a paradigm that is used to model persistent pain and suggest that exaggerated flux through aldose reductase may initiate changes in nociceptive pathways that could contribute to some of the pain states experienced by patients with diabetic neuropathy.

Cerebral function in diabetes mellitus

Diabetes mellitus is a common metabolic disorder associated with chronic complications such as nephropathy, angiopathy, retinopathy and peripheral neuropathy. Diabetes is not often considered to have deleterious effects on the brain. However, long-term diabetes results in a variety of subtle cerebral disorders, which occur more frequently than is commonly believed. Diabetic cerebral disorders have been demonstrated at a neurochemical, electrophysiological, structural and cognitive level; however, the pathogenesis is still not clear. Probably alterations in cerebral blood supply and metabolic derangements play a role, as they do in the pathogenesis of diabetic neuropathy. Furthermore, the brain is also affected by recurrent episodes of hypoglycaemia and poor metabolic control. We describe herein the cerebral manifestations of diabetes and discuss the putative pathogenetic mechanisms.

Cerebral circulation during diabetes mellitus

Diabetes mellitus is characterized by hyperglycemia, a decrease in circulating insulin and the development of macro- and microvascular pathology. Hyperglycemia appears to be a primary determinant for the structural, biochemical and functional changes that occur in large and small blood vessels during diabetes mellitus. While much research has focused on the effects of diabetes mellitus on the peripheral circulation, it is clear that diabetes mellitus also has profound effects on the cerebral circulation. Thus, the focus of this review is to discuss morphological and functional alterations in the cerebral circulation during diabetes mellitus.

Evidence that a proconvulsant action of lithium is mediated by inhibition of myo-inositol phosphatase in mouse brain

Lithium inhibits myo-inositol mono- and polyphosphatase activity in brain at concentrations similar to those optimal for the treatment of manic depressive psychosis. A consequence of this inhibition is the possibility that the availability of myo-inositol for the regeneration of polyphosphoinositides involved in cellular signalling mechanisms may be reduced. While there are no good models of manic depressive disorders in rodents, lithium is known to alter their behavioural responsiveness to a number of neurotransmitter receptor agonists, but the role of the phosphatidylinositol second messenger system in these effects is unknown. Consistent with the myo-inositol depletion hypothesis, when injected directly into the CNS, myo-inositol, but not its biologically inactive epimer, scyllo-inositol or D-mannitol, has been found to reverse a proconvulsant action of lithium in mice given the muscarinic receptor agonist, pilocarpine.