Impaired modulation of AMPA receptors by calcium-dependent processes in streptozotocin-induced diabetic rats

Targeting microbiota-immune-synaptic plasticity to explore the effect of tea polyphenols on improving memory in the aged type 2 diabetic rat model

OBJECTIVES

The study aimed to explore whether TP could improve memory in the aged type 2 diabetic rat model by regulating microbiota-immune-synaptic plasticity axis.

METHODS

The experiment was divided into two parts. Firstly, to investigate the effects of TP on the physiopathology of the aged T2DM model rats, rats were randomly divided into the Normal control group, the aged group, the Aged T2DM model group, the TP 75, 150, 300 mg/kg groups, the 150 mg/kg Piracetam group and the 3 mg/kg Rosiglitazone group. Then, to further verify whether TP improved memory in aged T2DM rat model by regulating intestinal flora, the fecal microbiota transplantation (FMT) from the rats in the 300 mg/kg TP group into the rats in the aged T2DM model group was carried out. Effects on gut microbiota, colonic integrity (epithelial tight junction proteins), and endotoxemia (serum LPS) were examined, along with synaptic structure, synaptic plasticity-related structural proteins and inflammation signaling of the hippocampus in our study.

RESULTS

Our results demonstrated that TP alleviated memory impairments in the aged T2DM rat model. The specific outcomes were as follows: TP 300 mg/kg corrected the gut dysbacteriosis, alleviated intestinal permeability reduction and peripheral/central inflammation, inhibited the TLR4/NF-κB signaling pathway. Meanwhile, TP improved the synaptic plasticity in the hippocampus of the aged T2DM model rats, whose expressions of SYN, PSD 95, NMDAR1 and GluR1 in hippocampus were significantly up-regulated. Surprisingly, rats of the FMT group displayed the same changes.

DISCUSSION

TP improves the memory in aged T2DM rat model. The mechanism may be related to the alteration of gut flora, which can inhibit hippocampal TLR4/NF-κB signaling to attenuate neuroinflammation, then improve synaptic plasticity. The study proposes that TP interventions aimed at manipulating the gut microbiota may hold great potential as an effective approach for preventing and treating this disease.

Hippocampal acetylcholine receptor activation-dependent long-term depression in streptozotocin-induced diabetic rats

Cholinergic innervation of the hippocampus correlates with memory formation. In a well-established animal model of type 1 diabetes mellitus, obtained by injecting young adult rats with streptozotocin (STZ), reductions have been reported in the expression of acetylcholine receptors and choline acetyltransferase. In this study, we showed that long-term synaptic depression (LTD) induced by carbachol (CCh), a nonselective cholinergic receptor agonist, at Schaffer collateral-CA1 synapses in hippocampal slices was significantly weaker in streptozotocin-induced diabetic rats (STZ rats) than in age-matched control rats. No significant change was observed in the paired-pulse ratio between before and 80 min after the application of CCh in control and STZ rats. Moreover, CCh-induced LTD in control and STZ rats was not affected by an NMDA receptor antagonist. Although the application of CCh down-regulated the surface expression of GluA2 in the hippocampus of control rats, but not STZ rats. Therefore, the present results suggest that acetylcholine receptor-mediated LTD in STZ rats requires the internalization of AMPA receptors on the postsynaptic surface and their intracellular effects in the hippocampus.

Alterations of the glutamatergic system in diabetes mellitus

Diabetes mellitus (DM) is a chronic disease characterized by elevated blood glucose levels caused by a lack of insulin production (type 1 diabetes) or insulin resistance (type 2 diabetes). It is well known that DM is associated with cognitive deficits and metabolic and neurophysiological changes in the brain. Glutamate is the main excitatory neurotransmitter in the central nervous system that plays a key role in synaptic plasticity, learning, and memory processes. An increasing number of studies have suggested that abnormal activity of the glutamatergic system is implicated in the pathophysiology of DM. Dysfunction of glutamatergic neurotransmission in the central nervous system can provide an important neurobiological substrate for many disorders. Magnetic resonance spectroscopy (MRS) is a non-invasive technique that allows a better understanding of the central nervous system factors by measuring in vivo the concentrations of brain metabolites within the area of interest. Here, we briefly review the MRS studies that have examined glutamate levels in the brain of patients with DM. The present article also summarizes the available data on abnormalities in glutamatergic neurotransmission observed in different animal models of DM. In addition, the role of gut microbiota in the development of glutamatergic alterations in DM is addressed. We speculate that therapeutic strategies targeting the glutamatergic system may be beneficial in the treatment of central nervous system-related changes in diabetic patients.

Impaired Insulin Signaling Alters Mediators of Hippocampal Synaptic Dynamics/Plasticity: A Possible Mechanism of Hyperglycemia-Induced Cognitive Impairment

Alzheimer's disease (AD) is a neurological condition that affects the elderly and is characterized by progressive and irreversible neurodegeneration in the cerebral cortex [...].

Investigating the effects of Citrullus colocynthis on cognitive performance and anxiety-like behaviors in STZ-induced diabetic rats

Background: Diabetes can impair cognitive performance and lead to dementia. Patients with type 1 diabetes mellitus (T1DM) are reported with different levels of cognitive dysfunctions in various cognitive domains ranging from general intellectual testing to specific deficits with visuospatial abilities, motor speed, writing, attention, reading, and psychomotor efficiency. The present study aimed to investigate the effect of Citrullus colocynthis on cognitive functions.Methods: A total of 42 male Wistar rats (3-4  months old and weighing 200-250  g) were tested in the current study. Rats were randomly allocated into 3 groups of control, Diabetes, and Diabetes + Drug. The diabetic rats received Citrullus colocynthis extraction orally. The behavioral tests included the open field, elevated plus maze (EPM), novel object recognition (NOR), passive avoidance tests, and Morris Water Maze (MWM) tests. Data were analyzed using student and paired t-tests via SPSS software version 16.Results: Our results showed the protective effects of Citrullus colocynthis administration against cognitive impairments. This is followed by STZ-induced diabetes in the MWM, novel object recognition, and passive avoidance tasks. Also, it was found that Citrullus colocynthis improved anxiety in diabetic rats.Conclusion According to the findings of this study, the administration of 200  mg/kg C. colocynthis once per day for 40  days can lead to ameliorated cognitive impairments and antidiabetic effects such as increasing body weight and decreasing FBS.

Pathology and prevention of brain microvascular and neuronal dysfunction induced by a high-fructose diet in rats

A high-fructose diet causes metabolic abnormalities in rats, and the cluster of complications points to microvascular and neuronal disorders of the brain. The aim of this study was to evaluate i) the involvement of microvascular disorders and neuronal plasticity in the deleterious effects of a high-fructose diet on the rat brain and ii) a comparative assessment of the effectiveness of Phytocollection therapy (with antidiabetic, antioxidant, and acetylcholinesterase inhibitory activities) compared to Galantamine as first-line therapy for dementia and Diabeton as first-line therapy for hyperglycemia. The calcium adenosine triphosphate non-injection histoangiological method was used to assess capillary network diameter and density. A high-fructose diet resulted in a significant decrease in the diameter and density of the capillary bed, and pharmacological manipulations had a modulatory effect on microcirculatory adaptive mechanisms. In vivo single-unit extracellular recording was used to investigate short-term plasticity in the medial prefrontal cortex. Differences in the parameters of spike background activity and expression of excitatory and inhibitory responses of cortical neurons have been discovered, allowing for flexibility and neuronal function stabilization in pathology and pharmacological prevention. Integration of the coupling mechanism between microvascular function and neuronal spike activity could delay the progressive decline in cognitive function in rats fed a high fructose diet.

The Role of Glucagon-Like Peptide-1 Receptor Agonists (GLP-1 RA) in Diabetes-Related Neurodegenerative Diseases

Recent clinical guidelines have emphasized the importance of screening for cognitive impairment in older adults with diabetes, however, there is still a lack of understanding about the drug therapy. Glucagon-like peptide 1 receptor agonists (GLP-1 RAs) are widely used in the treatment of type 2 diabetes and potential applications may include the treatment of obesity as well as the adjunctive treatment of type 1 diabetes mellitus in combination with insulin. Growing evidence suggests that GLP-1 RA has the potential to treat neurodegenerative diseases, particularly in diabetes-related Alzheimer’s disease (AD) and Parkinson’s disease (PD). Here, we review the molecular mechanisms of the neuroprotective effects of GLP-1 RA in diabetes-related degenerative diseases, including AD and PD, and their potential effects.

RAGE signaling is required for AMPA receptor dysfunction in the hippocampus of hyperglycemic mice

Diabetes in humans has been associated for a long time with cognitive dysfunction. In rodent animal models, cognitive dysfunction can manifest as impaired hippocampal synaptic plasticity. Particular attention has been concentrated on the receptor for advanced glycation end products (RAGE), which is implicated in multiple diabetic complications involving the development of vascular and peripheral nerve abnormalities. In this study, we hypothesize that RAGE signaling alters glutamate receptor function and expression, impairing synaptic transmission in the hippocampus. Using preparations of hippocampal slices from male mice, we show a RAGE-dependent decrease in long-term potentiation (LTP) and an increase in paired-pulse facilitation (PPF) following streptozotocin (STZ)-induced diabetes. Consistently, in hippocampal cultures from male and female neonatal mice, high glucose caused a RAGE-dependent reduction of AMPA- but not NMDA-evoked currents, and an increase in cytosolic reactive oxygen species (ROS). Consistently, when cultures were co-treated with high glucose and the RAGE antagonist FPS-ZM1, AMPA-evoked currents were unchanged. Hippocampi from STZ-induced hyperglycemic wild type (WT) mice showed increased RAGE expression concomitant with a decrease of both expression and phosphorylation (Ser 831 and 845) of the AMPA GluA1 subunit. We found these changes correlated to activation of the MAPK pathway, consistent with decreased pJNK/JNK ratio and the JNK kinase, pMEK7. As no changes in expression or phosphorylation of regulatory proteins were observed in hippocampi from STZ-induced hyperglycemic RAGE-KO mice, we report a RAGE-dependent impairment in the hippocampi of hyperglycemic WT mice, with reduced AMPA receptor expression/function and LTP deficits.

Changes in Synaptic Plasticity and Glutamate Receptors in Type 2 Diabetic KK-Ay Mice

In the present study, the progressive alteration of cognition and the mechanisms of reduction in long-term potentiation (LTP) in spontaneous obese KK-Ay type 2 diabetic mice were investigated. In the study, 3-, 5-, and 7-month-old KK-Ay mice were used. The results indicated that KK-Ay mice showed cognitive deficits in the Morris water maze test beginning at the age of 3 months. LTP was significantly impaired in KK-Ay mice during whole study period (3 to 7 months). The above deficits were reversible at an early stage (3 to 5 months old) by diet intervention. Moreover, we found the underlying mechanisms of LTP impairment in KK-Ay mice might be attributed to abnormal phosphorylation or expression of postsynaptic glutamate receptor subunits instead of alteration of basal synaptic transmission. The expression levels of NR1, NR2A, and NR2B subunits of N-methyl-d-aspartate receptors (NMDARs) were unchanged while the Tyr-dependent phosphorylation of both NR2A and NR2B subunits were significantly reduced in KK-Ay mice. The level of p-Src expression mediating this process was decreased, and the level of αCaMKII autophosphorylation was also reduced. Meanwhile, the GluR1 of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) was decreased, and GluR2 was significantly increased. These data suggest that deficits in synaptic plasticity in KK-Ay mice may arise from the abnormal phosphorylation of the NR2 subunits and the alteration of subunit composition of AMPARs. Diet intervention at an early stage of diabetes might alleviate the cognitive deficits and LTP reduction in KK-Ay mice.

Higher body mass index is associated with episodic memory deficits in young adults

Obesity has become an international health crisis. There is accumulating evidence that excess bodyweight is associated with changes to the structure and function of the brain and with a number of cognitive deficits. In particular, research suggests that obesity is associated with hippocampal and frontal lobe dysfunction, which would be predicted to impact memory. However, evidence for such memory impairment is currently limited. We hypothesised that higher body mass index (BMI) would be associated with reduced performance on a test of episodic memory that assesses not only content, but also context and feature integration. A total of 50 participants aged 18–35 years, with BMIs ranging from 18 to 51, were tested on a novel what–where–when style episodic memory test: the “Treasure-Hunt Task”. This test requires recollection of object, location, and temporal order information within the same paradigm, as well as testing the ability to integrate these features into a single event recollection. Higher BMI was associated with significantly lower performance on the what–where–when (WWW) memory task and all individual elements: object identification, location memory, and temporal order memory. After controlling for age, sex, and years in education, the effect of BMI on the individual what, where, and when tasks remained, while the WWW dropped below significance. This finding of episodic memory deficits in obesity is of concern given the emerging evidence for a role for episodic cognition in appetite regulation.

Inside the Diabetic Brain: Role of Different Players Involved in Cognitive Decline

Diabetes mellitus is the most common metabolic disease, and its prevalence is increasing. A growing body of evidence, both in animal models and epidemiological studies, has demonstrated that metabolic diseases like obesity, insulin resistance, and diabetes are associated with alterations in the central nervous system (CNS), being linked with development of cognitive and memory impairments and presenting a higher risk for dementia and Alzheimer's disease. The rising prevalence of diabetes together with its increasing earlier onset suggests that diabetes-related cognitive dysfunction will increase in the near future, causing substantial socioeconomic impact. Decreased insulin secretion or action, dysregulation of glucose homeostasis, impairment in the hypothalamic-pituitary-adrenal axis, obesity, hyperleptinemia, and inflammation may act independently or synergistically to disrupt neuronal homeostasis and cause diabetes-associated cognitive decline. However, the crosstalk between those factors and the mechanisms underlying the diabetes-related CNS complications is still elusive. During the past few years, different strategies (neuroprotective and antioxidant drugs) have emerged as promising therapies for this complication, which still remains to be preventable or treatable. This Review summarizes fundamental past and ongoing research on diabetes-associated cognitive decline, highlighting potential contributors, mechanistic mediators, and new pharmacological approaches to prevent and/or delay this complication.

Insulin treatment restores glutamate (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor function in the hippocampus of diabetic rats

Type 1 diabetes is associated with cognitive dysfunction. Cognitive processing, particularly memory acquisition, depends on the regulated enhancement of expression and function of glutamate receptor subtypes in the hippocampus. Impairment of memory was been detected in rodent models of type 1 diabetes induced by streptozotocin (STZ). This study examines the functional properties of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the expression of synaptic molecules that regulate glutamatergic synaptic transmission in the hippocampus of STZ-diabetic rats. The AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) and single-channel properties of synaptosomal AMPA receptors were examined after 4 weeks of diabetes induction. Results show that amplitude and frequency of mEPSCs recorded from CA1 pyramidal neurons were decreased in diabetic rats. In addition, the single-channel properties of synaptic AMPA receptors from diabetic rat hippocampi were different from those of controls. These impairments in synaptic currents gated by AMPA receptors were accompanied by decreased protein levels of AMPA receptor subunit GluR1, the presynaptic protein synaptophysin, and the postsynaptic anchor protein postsynaptic density protein 95 in the hippocampus of diabetic rats. Neural cell adhesion molecule (NCAM), an extracellular matrix molecule abundantly expressed in the brain, and the polysialic acid (PSA) attached to NCAM were also downregulated in the hippocampus of diabetic rats. Insulin treatment, when initiated at the onset of diabetes induction, reduced these effects. These findings suggest that STZ-induced diabetes may result in functional deteriorations in glutamatergic synapses in the hippocampus of rats and that these effects may be reduced by insulin treatment.

Short-lived diabetes in the young-adult ZDF rat does not exacerbate neuronal Ca(2+) biomarkers of aging

Results from clinical studies provide evidence that cognitive changes relatively late in life may be traced to antecedent conditions including diabetes, obesity, a sedentary lifestyle, and an atherogenic diet. As such, several traits of Type 2 diabetes (T2DM) could be considered pathogenic factors of aging, contributing to age-dependent cognitive decline and our susceptibility to Alzheimer's disease. It appears that both the duration of metabolic condition and the age of the individual, together can contribute to the potential impact on peripheral as well as brain health. Because of robust evidence that in animal models of aging, Ca(2+) dysregulation alters neuronal health, synaptic plasticity, and learning and memory processes, we tested the hypothesis that peripheral metabolic dysregulation could exacerbate Ca(2+) dysfunction in hippocampal CA1 neurons. Using intracellular/ extracellular electrophysiological and Ca(2+) imaging techniques, we show that Ca(2+)levels at rest or during synaptic stimulation, the Ca(2+)-dependent afterhyperpolarization, baseline field potentials, and short-term synaptic plasticity were not significantly altered in young-adult male Zucker diabetic fatty rats compare to their lean counterparts. Our observations suggest that early phases of T2DM characterized by high levels of glucose and insulin may be too transient to alter hippocampal CA1 physiology in this animal model of diabetes. These results are supported by clinical data showing that longer T2DM duration can have greater negative impact on cognitive functions. This article is part of a Special Issue entitled SI: Brain and Memory.

Sweet memories: 20 years of progress in research on cognitive functioning in diabetes

This paper appears in a special issue of the European Journal of Pharmacology that commemorates the retirement of Professor Willem Hendrik Gispen as distinguished professor of Utrecht University and as editor of the European Journal of Pharmacology. The paper provides an overview of a research line on the impact of diabetes on cognition that we started together 20 years ago, and that continues to this day. I will report how we more or less stumbled upon this topic, that was understudied, but proved to be of definite clinical relevance. I will discuss how we tried to establish animal models, how developments from clinical and experimental studies from around the world led us to reconsider our concepts, and how findings from research on diabetic neuropathy, insulin signaling in the brain, Alzheimer's disease and dementia, and vascular disease and stroke converged and helped to create new ideas and refute others. This voyage has not ended yet, because the ultimate goal is to offer patients with diabetes treatment that can protect them against accelerated cognitive decline. Although this could take another 20 years, the research from Willem Hendrik and his group brought us an important step in the right direction.

NMDA and AMPA receptor mediated excitotoxicity in cerebral cortex of streptozotocin induced diabetic rat: ameliorating effects of curcumin

Functional activity of neurotransmitter receptor and their sensitivity to regulation are altered in DM. We evaluated the neuroprotective effect of curcumin in glutamate mediated excitotoxicity in cerebral cortex of streptozotocin induced diabetic rats. Gene expression studies in diabetic rats showed a down regulation of glutamate decarboxylase mRNA leading to accumulation of glutamate. Radioreceptor binding assays showed a significant increase in α-amino-3-hydroxy-5-methyl-4-isoxazole propionate and N-methyl-D-aspartate receptors density which was confirmed by immunohistochemical studies. Decreased glutathione peroxidases gene expression indicates enhanced oxidative stress in diabetic rats. This leads to decreased expression of glutamate aspartate transporter, which in turn reduces glutamate transport. All these events lead to excitotoxic neuronal death in the cerebral cortex, which was confirmed by the increased expression of caspase 3, caspase 8 and BCL2-associated X protein. Curcumin and insulin treatment reversed these altered parameters to near control. We establish, a novel therapeutic role of curcumin by reducing the glutamate mediated excitotoxicity in cerebral cortex of diabetes through modulating the altered neurochemical parameters.

Subclinical vascular disease and cerebral glutamate elevation in metabolic syndrome

Metabolic syndrome (MetS), the co-occurrence of obesity, hypertension, hyperglycemia and dyslipidema, is an important risk factor for diabetes, cardiovascular disease and end-organ damage in the brain. Our goal was to determine if metabolic syndrome (MetS) differentially affects cerebral metabolism in middle-aged adults with varying degrees of subclinical vascular disease. Sixty-five neurologically healthy adults aged 40 to 60 years (19 with MetS and 46 controls) underwent ultrasound examination of carotid artery intima-media thickness (IMT), a measure of peripheral vascular disease, a full neuropsychological evaluation, and a proton magnetic resonance spectroscopy ((1)H MRS) scan of occipitoparietal grey matter. The Johnson-Neyman technique and pick-a-point approach were used to test if MetS-related neurochemical changes were moderated by IMT. The MetS and control groups were comparable in age, education, gender distribution, average IMT, and cognitive performance. MetS individuals with low IMT values (1 SD below sample mean) demonstrated comparable neurochemical concentrations to the healthy controls (t = -0.21, p = 0.84, 95 % CI -0.106 to 0.086), while MetS individuals with high IMT values (1 SD above sample mean) exhibited significantly elevated glutamate concentrations (t = 2.84, p = 0.006, 95 % CI 0.038 to 0.220). We found that the level of peripheral atherosclerosis moderated the level of elevation of cerebral glutamate concentrations in patients with MetS. These results suggest that peripheral metabolic dysfunction in midlife likely acts in conjunction with subclinical vascular disease to foster pro-neurotoxic conditions in the central nervous system creating early brain vulnerability.

Elevated glucose concentration changes the content and cellular localization of AMPA receptors in the retina but not in the hippocampus

Diabetic retinopathy and diabetic encephalopathy are two common complications of diabetes mellitus. The impairment of glutamatergic neurotransmission in the retina and hippocampus has been suggested to be involved in the pathogenesis of these diabetic complications. In this study, we investigated the effect of elevated glucose concentration and diabetes on the protein content and surface expression of AMPA receptor subunits in the rat retina and hippocampus. We have used two models, cultured retinal and hippocampal cells exposed to elevated glucose concentration and an animal model of streptozotocin-induced type 1 diabetes. The immunoreactivity of GluA1, GluA2 and GluA4 was evaluated by Western blot and immunocytochemistry. The levels of these subunits at the plasma membrane were evaluated by biotinylation and purification of plasma membrane-associated proteins. Elevated glucose concentration increased the total levels of GluA2 subunit of AMPA receptors in retinal neural cells, but not of the subunits GluA1 or GluA4. However, at the plasma membrane, elevated glucose concentration induced an increase of all AMPA receptor subunits. In cultured hippocampal neurons, elevated glucose concentration did not induce significant alterations in the levels of AMPA receptor subunits. In the retinas of diabetic rats there were no persistent changes in the levels of AMPA receptor subunits comparing to aged-matched control retinas. Also, no consistent changes were detected in the levels of GluA1, GluA2 or GluA4 in the hippocampus of diabetic rats. We demonstrate that elevated glucose concentration induces early changes in AMPA receptor subunits, mainly in GluA2 subunit, in retinal neural cells. Conversely, hippocampal neurons seem to remain unaffected by elevated glucose concentration, concerning the expression of AMPA receptors, suggesting that AMPA receptors are more susceptible to the stress caused by elevated glucose concentration in retinal cells than in hippocampal neurons.

Diabetes as a chronic metabolic stressor: causes, consequences and clinical complications

Diabetes mellitus is an endocrine disorder resulting from inadequate insulin release and/or reduced insulin sensitivity. The complications of diabetes are well characterized in peripheral tissues, but there is a growing appreciation that the complications of diabetes extend to the central nervous system (CNS). One of the potential neurological complications of diabetes is cognitive deficits. Interestingly, the structural, electrophysiological, neurochemical and anatomical underpinnings responsible for cognitive deficits in diabetes are strikingly similar to those observed in animals subjected to chronic stress, as well as in patients with stress-related psychiatric illnesses such as major depressive disorder. Since diabetes is a chronic metabolic stressor, this has led to the suggestion that common mechanistic mediators are responsible for neuroplasticity deficits in both diabetes and depression. Moreover, these common mechanistic mediators may be responsible for the increase in the risk of depressive illness in diabetes patients. In view of these observations, the aims of this review are (1) to describe the neuroplasticity deficits observed in diabetic rodents and patients; (2) to summarize the similarities in the clinical and preclinical studies of depression and diabetes; and (3) to highlight the diabetes-induced neuroplasticity deficits in those brain regions that have been implicated as important pathological centers in depressive illness, namely, the hippocampus, the amygdala and the prefrontal cortex.

Diabetes induces early transient changes in the content of vesicular transporters and no major effects in neurotransmitter release in hippocampus and retina

Diabetes induces changes in neurotransmitter release in central nervous system, which depend on the type of neurotransmitter and region studied. In this study, we evaluated the effect of diabetes (two and eight weeks duration) on basal and evoked release of [(14)C]glutamate and [(3)H]GABA in hippocampal and retinal synaptosomes. We also analyzed the effect of diabetes on the protein content of vesicular glutamate and GABA transporters, VGluT-1, VGluT-2 and VGAT, and on the α(1A) subunit of P/Q type calcium channels, which are abundant in nerve terminals. The protein content of vesicular glutamate and GABA transporters, and of the α(1A) subunit, was differently affected by diabetes in hippocampal and retinal synaptosomes. The changes were more pronounced in the retina than in hippocampus. VGluT-1 and VGluT-2 content was not affected in hippocampus. Moreover, changes occurred early, at two weeks of diabetes, but after eight weeks almost no changes were detected, with the exception of VGAT in the retina. Regarding neurotransmitter release, no major changes were detected. After two weeks of diabetes, neurotransmitter release was similar to controls. After eight weeks of diabetes, the basal release of glutamate slightly increased in hippocampus and the evoked GABA release decreased in retina. In conclusion, diabetes induces early transient changes in the content of glutamate and/or GABA vesicular transporters, and on calcium channels subunit, in retinal or hippocampal synaptosomes, but only minor changes in the release of glutamate or GABA. These results point to the importance of diabetes-induced changes in neural tissues at the presynaptic level, which may underlie alterations in synaptic transmission, particularly if they become permanent during the later stages of the disease.

Glutamate (mGluR-5) gene expression in brain regions of streptozotocin induced diabetic rats as a function of age: role in regulation of calcium release from the pancreatic islets in vitro

Metabotrophic glutamate receptors (mGluRs) modulate cellular activities involved in the processes of differentiation and degeneration. In this study, we have analysed the expression pattern of group-I metabotropic glutamate receptor (mGlu-5) in cerebral cortex, corpus striatum, brainstem and hippocampus of streptozotocin induced and insulin treated diabetic rats (D+I) as a function of age. Also, the functional role of glutamate receptors in intra cellular calcium release from the pancreatic islets was studied in vitro. The gene expression studies showed that mGlu-5 mRNA in the cerebral cortex increased siginficantly in 7 weeks old diabetic rats whereas decreased expression was observed in brainstem, corpus striatum and hippocampus when compared to control. 90 weeks old diabetic rats showed decreased expression in cerebral cortex, corpus striatum and hippocampus whereas in brainstem the expression increased significantly compared to their respective controls. In 7 weeks old D+I group, mGlu-5 mRNA expression was significantly decreased in cerebral cortex and corpus striatum whereas the expression increased significantly in brainstem and hippocampus. 90 weeks old D+I group showed an increased expression in cerebral cortex, while it was decreased significantly in corpus striatum, brainstem and hippocampus compared to their respective controls. In vitro studies showed that glutamate at lower concentration (10(-7) M) stimulated calcium release from the pancreatic islets. Our results suggest that mGlu-5 receptors have differential expression in brain regions of diabetes and D+I groups as a function of age. This will have clinical significance in management of degeneration in brain function and memory enhancement through glutamate receptors. Also, the regulatory role of glutamate receptors in calcium release has immense therapeutic application in insulin secretion and function.

Diazoxide reduces status epilepticus neuron damage in diabetes

Diabetic hyperglycemia is associated with seizure severity and may aggravate brain damage after status epilepticus. Our earlier studies suggest the involvement of ATP-sensitive potassium channels (K(ATP)) in glucose-related neuroexcitability. We aimed to determine whether K(ATP) agonist protects against status epilepticus-induced brain damage. Adult male Sprague-Dawley rats were divided into two groups: the streptozotocin (STZ)-induced diabetes (STZ) group and the normal saline (NS) group. Both groups were treated with either diazoxide (15 mg/kg, i.v.) (STZ + DZX, NS + DZX) or vehicle (STZ + V, NS + V) before lithium-pilocarpine-induced status epilepticus. We evaluated seizure susceptibility, severity, and mortality. The rats underwent Morris water-maze tests and hippocampal histopathology analyses 24 h post-status epilepticus. A multi-electrode recording system was used to study field excitatory postsynaptic synaptic potentials (fEPSP). RNA interference (RNAi) to knockdown Kir 6.2 in a hippocampal cell line was used to evaluate the effect of diazoxide in the presence of high concentration of ATP. Seizures were less severe (P < 0.01), post-status epilepticus learning and memory were better (P < 0.05), and neuron loss in the hippocampal CA3 area was lower (P < 0.05) in the STZ + DZX than the STZ + V group. In contrast, seizure severity, post-status epilepticus learning and memory, and hippocampal CA3 neuron loss were comparable in the NS + DZX and NS + V groups. fEPSP was lower in the STZ + DZX but not in the NS + DZX group. The RNAi study confirmed that diazoxide, with its K(ATP)-opening effects, could counteract the K(ATP)-closing effect by high dose ATP. We conclude that, by opening K(ATP), diazoxide protects against status epilepticus-induced neuron damage during diabetic hyperglycemia.

Treadmill running improves long-term potentiation (LTP) defects in streptozotocin-induced diabetes at dentate gyrus in rats

OBJECTIVES

It has been demonstrated that exercise has neuroprotective effects in the central nervous system (CNS), especially in hippocampus. Previous studies have indicated that diabetes mellitus affects synaptic plasticity in the hippocampus leading to impairments in learning and memory. The aim of this study was to evaluate the effects of treadmill running on synaptic plasticity at dentate gyrus (DG) of streptozotocin-induced diabetic rats.

STUDY DESIGN

Experimental groups were the control, the diabetes and the diabetes-exercise groups. Long-term potentiation (LTP) in perforant path-DG synapses was assessed (by 400Hz tetanization) in order to investigate the effect of exercise on synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slope and population spike (PS) amplitude were measured.

RESULTS

With respect to the control group, fEPSP were significantly decreased in the diabetes group. However, there were no differences between responses of the diabetes-exercise group and the control.

CONCLUSION

The present results suggest that LTP induction in the dentate gyrus is affected under diabetic conditions and that treadmill running prevents these effects. The data suggest that treadmill running protect against diabetes-induced decrease of learning ability and memory function of the hippocampus.

Metabolism-induced oxidative stress is a mediator of glucose toxicity in HT22 neuronal cells

Oxidative stress has been widely considered as a key player in the adverse effects of hyperglycaemia to various tissues, including neuronal cells. This study examined the participation of oxidative stress in injurious effects of high glucose on HT22 cells along with the activity of proteasome, a proteolytic system responsible for degradation of oxidized proteins. Although 10-fold glucose concentration caused non-significant viability changes, a significant reduction of cell proliferation was found. Moreover, the cell morphology was also altered. These changes were followed by an enhancement of intracellular ROS generation, however without any significant boost of the carbonyl group concentration in proteins. Correspondingly, only a slight decline in the 20S proteasome activity was found in high-glucose-treated cells. On the other hand, substances affecting glucose metabolism or antioxidants partially preserved the oxidative stress in high glucose treated cells. In summary, these results highlight the role of metabolic oxidative stress in hyperglycaemia affecting neurons.

EFFECTS OF DIETARY LONG CHAIN PUFAs ON HIPPOCAMPAL LIPID PEROXIDATION AND NMDA RECEPTOR SUBUNITS A AND B CONCENTRATION IN STREPTOZOTOCIN-DIABETIC RATS

This study examined the effects of streptozotocin (STZ)-diabetes and dietary long chain polyunsaturated fatty acids (LC-PUFAs) on hippocampal N-methyl-D-aspartate (NMDA) receptor subunit expression and lipid peroxidation. MDA level was significantly increased after 8 weeks of STZ-diabetes. LC-PUFAs administration significantly reduced MDA levels in diabetic rats. NR2A and NR2B protein concentrations were significantly decreased by about 30% in diabetic rats. Dietary LC-PUFAs partially restored NR2A and NR2B in diabetic rats whereas the most significant increase was seen in nondiabetic rats. Consequently, dietary LC-PUFAs can partially restore hippocampal NMDA receptors and decrease lipid peroxidation in diabetes. LC-PUFAs are thus a possible prophylactic means for preventing the cognitive deficiencies of diabetes.

Muscarinic M(1), M(3) receptors function in the brainstem of streptozotocin induced diabetic rats: their role in insulin secretion from the pancreatic islets as a function of age

In the present study, we have investigated acetylcholine esterase (AChE) activity and muscarinic M(1), M(3) receptors kinetics in the brainstem of both young and old streptozotocin induced and insulin treated diabetic rats (D + I). Also, the functional role of acetylcholine and muscarinic receptors in insulin secretion from the pancreatic islets was studied in vitro. 90 week old control rats showed decreased V(max) (P < 0.001) for AChE compared to 7 week old control rats. V(max) was decreased (P < 0.001) in 7 week diabetic groups whereas 90 week old diabetic groups showed increased (P < 0.001) V(max) when compared to their respective controls. Binding studies using [(3)H]QNB and [(3)H]DAMP of 90 week old control showed significant increase in the B(max) (P < 0.001) and K(d) (P < 0.01) of muscarinic M(1) receptors whereas M(3) receptor number was decreased significantly (P < 0.001) with no change in affinity when compared to 7 week old control respectively. M(1) receptor number was decreased significantly (P < 0.001) whereas M(3) receptor number was increased significantly (P < 0.001) in both 7 week and 90 week old diabetic rat groups compared to their respective controls. The competition curve for [(3)H]QNB fitted for two sited model in 7 week old groups whereas fitted for one sited model in 90 week old groups. [(3)H]DAMP was fitted for two sited model in both 7 week and 90 week old groups. Insulin treatment significantly reversed (P < 0.001) the binding parameters to near control level. In vitro studies showed that acetylcholine through muscarinic M(1) and M(3) receptors stimulated insulin secretion from the pancreatic islets. Thus our studies suggest that both brainstem and pancreatic muscarinic M(1), M(3) receptors differentially regulate the cholinergic activity and insulin secretion which will have clinical significance in the management of diabetes and insulin treatment as a function of age.

Determination of the extracellular basal levels of glutamate and GABA at dentate gyrus of streptozotocin-induced diabetic rats

Previous studies have indicated an association between diabetes mellitus and impairments in synaptic plasticity in the hippocampus. However, it is not clear if the impairments of synapses are pre- or post-synaptic or both. The aim of this study was to evaluate the extracellular basal levels of glutamate and GABA at dentate gyrus of anesthetized streptozotocin-induced diabetic rats, after 12 weeks of diabetes induction. Extracellular levels of glutamate and GABA were investigated by using the microdialysis technique coupled to high performance liquid chromatography (HPLC) with fluorescent detection. Experimental groups were the control group and the diabetes group. The results showed that glutamate levels were significantly decreased in diabetes group compared to the control group, while GABA levels showed no changes. The findings support the possibility that alterations in transmission may account, in part, for synaptic plasticity deficits induced in diabetes.

Effects of treadmill running on spatial learning and memory in streptozotocin-induced diabetic rats

Previous studies have shown an association between diabetes mellitus and impairments in learning and memory. These deficits were partially reversed by the use of insulin. Due to the fact that exercise has positive effects on many physiological systems, including the central nervous system, the present study, evaluated the effects of treadmill running on spatial learning and memory in streptozotocin (STZ)-induced diabetic rats. The exercise program was treadmill running at 17 meters per minute (m/min) at 0 degrees inclination for 40 minutes per day (min/day), 7 days/week, for 12 weeks. Experimental groups were: the control-rest, the control-exercise, the diabetes-rest and the diabetes-exercise. Spatial learning and memory was investigated by Morris water maze test in the rats after 12 weeks of diabetes induction and the exercise period. Our data showed that spatial learning and memory was significantly impaired in the diabetes-rest group with respect to the control-rest group. However, there were no differences between the other groups. The present results suggest that spatial learning and memory is affected under diabetic conditions and that treadmill running prevents these effects. The data correspond to the possibility that treadmill running is helpful in the prevention and alleviation of the cognitive decline in diabetes mellitus.

Diabetic hyperglycemia aggravates seizures and status epilepticus-induced hippocampal damage

Epileptic seizures in diabetic hyperglycemia (DH) are not uncommon. This study aimed to determine the acute behavioral, pathological, and electrophysiological effects of status epilepticus (SE) on diabetic animals. Adult male Sprague-Dawley rats were first divided into groups with and without streptozotocin (STZ)-induced diabetes, and then into treatment groups given a normal saline (NS) (STZ-only and NS-only) or a lithium-pilocarpine injection to induce status epilepticus (STZ + SE and NS + SE). Seizure susceptibility, severity, and mortality were evaluated. Serial Morris water maze test and hippocampal histopathology results were examined before and 24 h after SE. Tetanic stimulation-induced long-term potentiation (LTP) in a hippocampal slice was recorded in a multi-electrode dish system. We also used a simulation model to evaluate intracellular adenosine triphosphate (ATP) and neuroexcitability. The STZ + SE group had a significantly higher percentage of severe seizures and SE-related death and worse learning and memory performances than the other three groups 24 h after SE. The STZ + SE group, and then the NS + SE group, showed the most severe neuronal loss and mossy fiber sprouting in the hippocampal CA3 area. In addition, LTP was markedly attenuated in the STZ + SE group, and then the NS + SE group. In the simulation, increased intracellular ATP concentration promoted action potential firing. This finding that rats with DH had more brain damage after SE than rats without diabetes suggests the importance of intensively treating hyperglycemia and seizures in diabetic patients with epilepsy.

The influences of juvenile diabetes on memory and hippocampal plasticity in rats: improving effects of glucagon-like peptide-1

Previous studies in children with diabetes found that hyperglycemia induces memory dysfunction. In this study, we investigated memory and synaptic plasticity in streptozotocine (STZ)-induced diabetic rats during the juvenile period. We further investigated the effects of glucagon-like peptide-1 (GLP-1) on the diabetes-induced profiles. STZ (85 mg/kg, i.p.) was administered to 17-day-old Wistar rats to induce type-1 juvenile diabetes mellitus (JDM). In the Y-maze test, JDM rats showed significant impairment of learning and memory, which were improved by GLP-1 (7-36) amide (1 microg/5 microl/rat, i.c.v.). Extracellular recording at Schaffer collateral synapses in the CA1 region of hippocampal slices showed that long-term potentiation and paired-pulse facilitation in JDM rats were similar to age-matched control rats. However, the input-output relation was strengthened, and long-term depression (LTD) and responses of N-methyl d-aspartic acid through NR2B subunits were weakened in the JDM rats. GLP-1 (7-36) amide (100 nM) increased the magnitude of LTD and the responses through NR2B in the JDM rats. These results indicate that the lack of LTD and NR2B responses may contribute to impairment of memory associated with JDM, suggesting the potential usefulness of GLP-1 in the treatment of memory dysfunction in JDM.

Neuronal apoptosis in neurodegeneration

Apoptosis mediates the precise and programmed natural death of neurons and is a physiologically important process in neurogenesis during maturation of the central nervous system. However, premature apoptosis and/or an aberration in apoptosis regulation is implicated in the pathogenesis of neurodegeneration, a multifaceted process that leads to various chronic disease states, such as Alzheimer's (AD), Parkinson's (PD), Huntington's (HD) diseases, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and diabetic encephalopathy. The current review focuses on two major areas (a) the fundamentals of apoptosis, which includes elements of the apoptotic machinery, apoptosis inducers, and emerging concepts in apoptosis research, and (b) apoptotic involvement in neurodegenerative disorders, neuroprotective treatment strategies/modalities, and the mechanisms of, and signaling in, neuronal apoptosis. Current and new experimental models for apoptosis research in neurodegenerative diseases are also discussed.

Alterations in mRNA expression of myelin proteins in the sciatic nerves and brains of streptozotocin-induced diabetic rats

Diabetic neuropathy is the most common complication of diabetes. We examined the levels and the mRNA expression of myelin proteins in the sciatic nerves and the brains of streptozotocin-induced diabetic rats. The diabetic rats exhibited a decrease in body weight, elevation of the blood glucose level and a decrease in motor nerve conduction velocity at 2 weeks after streptozotocin injection. In the sciatic nerves of diabetic rats, the level of P0 protein and its mRNA expression were markedly reduced at 20 weeks after the injection. In the brains, the levels of proteolipid protein and myelin-associated glycoprotein and their mRNA expression were selectively decreased at 20 weeks after the injection. This affected expression of myelin proteins was found even when no histological abnormalities were detectable. Considering the functional significance of myelin proteins, this impairment of protein expression is possibly involved in the pathogenesis of diabetic neuropathy, including that in brain disorders.

Increase of cannabinoid CB1 receptor density in the hippocampus of streptozotocin-induced diabetic rats

In the hippocampus, impaired neurophysiology, compromised neurogenesis, and eventually apoptosis accompany cognitive deficits in insulinopenic (type-1) diabetes (T1D). The underlying pathological mechanisms remain to be defined. The hippocampus has a high density of the cannabinoid type 1 receptor (CB(1)R), which has been shown to control several brain functions affected by diabetes, such as synaptic plasticity, glucose utilisation, memory consolidation and cognition, and maturation and survival of hippocampal neurons. However, the role of this receptor has not been investigated yet in diabetic encephalopathy. We report now that in the streptozotocin animal model of T1D, the hippocampal densities of CB(1)R protein and of specific CB(1)R binding sites are significantly increased both in the nerve terminals and in total membranes (changes between 13% and 38%), whereas CB(1)R mRNA expression is decreased by 25%, suggesting that CB(1)Rs might play a role in diabetic encephalopathy.

Current status of clinical and experimental researches on cognitive impairment in diabetes

This article reviews the clinical and experimental researches on cognitive impairment related to diabetes in the recent decade. Most clinical studies indicate that the cognitive impairment in patients with type 1 diabetes mellitus is related to recurrent hypoglycemia closely. There is little research about whether or not hyperglycemia is related to cognitive impairment in patients with type 1 diabetes mellitus. Most studies indicate that the cognitive impairment in type 2 diabetes involves multiple factors through multiple mechanisms, including blood glucose, blood lipid, blood pressure, level of insulin, medication, chronic complication, etc. But, there has been no large-scale, multi-center, randomized controlled clinical trial in China recently. And what is more, some problems exist in this field of research, such as the lack of golden criterion of cognitive function measurement, different population of studied objects, and incomprehensive handling of confounding factors. Experimental studies found that hippocampal long-term potentiation (LTP) was impaired, which were manifested by impairment of spatial memory and decreased expression of LTP, but it's relation to hyperglycemia, the duration of diabetes, learning and memory has always been differently reported by different researches. Thus, there are a lot of unknown things to be explored and studied in order to clarify its mechanism. TCM has abundant clinical experience in treating cerebral disease with medicine that enforces the kidney and promotes wit. However, there has been no research on treating diabetic cognitive impairment, which requires work to be done actively and TCM to be put into full play, in order to improve the treatment of diabetes and enhance living quality of patients.

Hippocampal neuropathology of diabetes mellitus is relieved by estrogen treatment

1. A recently recognized complication of uncontrolled diabetes mellitus is the encephalopathy involving, among other regions, the hippocampus. Since estrogens bring neuroprotection in cases of brain injury and degenerative diseases, we have studied if estradiol (E2) administration counteracts some hippocampal abnormalities of streptozotocin (STZ)-diabetic adult mice. 2. We first report the ability of E2 to modulate neurogenesis in the dentate gyrus (DG) and subventricular zone (SVZ) of diabetic mice. Using bromodeoxyuridine (BrdU) to label newly generated cells, a strong reduction in cell proliferation was obtained in DG and SVZ of mice sacrificed 20 days after STZ administration. The reduction was completely relieved by 10 days of E2 pellet implantation, which increased 30-fold the circulating E2 levels. 3. Diabetic mice also showed abnormal expression of astrocyte markers in hippocampus. Thus, increased number of GFAP(+) cells, indicative of astrogliosis, and increased number of apolipoprotein-E (Apo-E)(+) astrocytes, a marker of ongoing neuronal dysfunction, was found in stratum radiatum below the CA1 hippocampal subfield of diabetic mice. Both parameters were reverted to normal by the E2 regime that upregulated cell proliferation. 4. The studies demonstrated that hippocampal neuropathology of uncontrolled diabetes is a reversible condition and sensitive to estrogen treatment. Studies in animal models may open up new venues for understanding the beneficial role of steroid hormones in diabetic encephalopathy.

Reduced hippocampal neurogenesis and number of hilar neurones in streptozotocin-induced diabetic mice: reversion by antidepressant treatment

Cerebral dysfunctions, including a high incidence of depression, are common findings in human type 1 diabetes mellitus. An association between depression and defective hippocampal neurogenesis has been proposed and, in rodents, antidepressant therapy restores neuronal proliferation in the dentate gyrus. Hippocampal neurogenesis is also deficient in diabetic mice, which led us to study whether the selective serotonin reuptake inhibitor fluoxetine influences cell proliferation in streptozotocin-diabetic animals. Diabetic and control C57BL/6 mice received fluoxetine (10 mg/kg/day, i.p., 10 days) and dentate gyrus cell proliferation was measured after a single injection of 5-bromo-2'-deoxyuridine (BrdU). Diabetic mice showed reduced cell proliferation. Fluoxetine treatment, although having no effect in controls, corrected this parameter in diabetic mice. The phenotype of newly generated cells was analysed by confocal microscopy after seven daily BrdU injections, using Tuj-1/beta-III tubulin as a marker for immature neurones and glial fibrillary acidic protein for astrocytes. In controls, the proportion of Tuj-1-BrdU-positive cells over total BrdU cells was approximately 70%. In vehicle-treated diabetic mice, immature neurones decreased to 56% and fluoxetine brought this proportion back to control values without affecting astrocytes. Therefore, fluoxetine preferentially increased the proliferation of cells with a neuronal phenotype. In addition, neurones were counted in the hilus of the dentate gyrus; a 30% decrease was found in diabetic mice compared with controls, whereas this neuronal loss was prevented by fluoxetine. In conclusion, fluoxetine treatment restored neuroplasticity-related hippocampal alterations of diabetic mice. These findings may be potentially important to counteract diabetes-associated depression in humans.

The impact of diabetes on cognition: What can be learned from rodent models?

Diabetes mellitus is associated with modest impairments in cognition, particularly in the elderly. In addition, the risk of dementia is increased. We review herein studies in rodent models that may help to identify the mechanisms that underlie these adverse effects of diabetes on the brain. Abnormalities in learning and memory, synaptic plasticity, and glutamatergic neurotransmission have now been identified in a number of these models. In general, observations in models characterized by chronic hyperglycaemia and hypoinsulinaemia (referred to as models of type 1 diabetes) are quite consistent, and these models are being increasingly used to study the pathogenesis and to develop new treatments. However, results from models characterized by insulin resistance, hyperinsulinaemia, and modest hyperglycaemia (referred to as models of type 2 diabetes) are much more variable. Moreover, the possible interaction between diabetes and aging has not been examined in sufficient detail. Because clinically relevant cognitive deficits mainly occur in elderly patients with type 2 diabetes, the challenge for researchers in this field will be to further develop adequate models.

Diabetes mellitus concomitantly facilitates the induction of long-term depression and inhibits that of long-term potentiation in hippocampus

Memory impairments, which occur regularly across species as a result of ageing, disease (such as diabetes mellitus) and psychological insults, constitute a useful area for investigating the neurobiological basis of learning and memory. Previous studies in rats found that induction of diabetes (with streptozotocin, STZ) impairs long-term potentiation (LTP) but enhances long-term depression (LTD) induced by high- (HFS) and low-frequency stimulations (LFS), respectively. Using a pairing protocol under whole-cell recording conditions to induce synaptic plasticity at Schaffer collateral synapses in hippocampal CA1 slices, we show that LTD and LTP have similar magnitudes in diabetic and age-matched control rats. But, in diabetic animals, LTD is induced at more polarized and LTP more depolarized membrane potentials (V(ms)) compared with controls: diabetes produces a 10 mV leftward shift in the threshold for LTD induction and 10 mV rightward shift in the LTD-LTP crossover point of the voltage-response curve for synaptic plasticity. Prior repeated short-term potentiations or LTP are known to similarly, though reversibly, lower the threshold for LTD induction and raise that for LTP induction. Thus, diabetes- and activity-dependent modulation of synaptic plasticity (referred to as metaplasticity) display similar phenomenologies. In addition, compared with naïve synapses, prior induction of LTP produces a 10 mV leftward shift in Vms for inducing subsequent LTD in control but not in diabetic rats. This could indicate that diabetes acts on synaptic plasticity through mechanisms involved in metaplasticity. Persistent facilitation of LTD and inhibition of LTP may contribute to learning and memory impairments associated with diabetes mellitus.