NMDA receptor subunits 2A and 2B decrease and lipid peroxidation increase in the hippocampus of streptozotocin-diabetic rats: effects of insulin and gliclazide treatments

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.

Melatonin attenuates the high-fat diet and streptozotocin-induced reduction in rat hippocampal neurogenesis

A deviant level of melatonin in blood circulation has been associated with the development of diabetes and with learning and memory deficiencies. Melatonin might have an important function in diabetes control; however, the mechanism of melatonin in diabetes remains unknown. The present study aimed to investigate the hyperglycemic condition induced by high-fat diet (HFD) feeding and streptozotocin (STZ) injection and to examine the effect of melatonin on adult hippocampal functions. HFD-fed and STZ-treated rats significantly increased blood glucose level. The present study showed that HFD-fed and STZ-treated rats significantly impaired memory in the Morris Water Maze task, reduced neurogenesis in the hippocampus shown by a reduction in nestin, doublecortin (DCX) and β-III tubulin immunoreactivities, reduced axon terminal markers, synaptophysin, reduced dendritic marker including postsynaptic density 95 (PSD-95) and the glutamate receptor subunit NR2A. Moreover, a significant downregulation of melatonin receptor, insulin receptor-β (IR-β) and both p-IR-β and phosphorylated extracellular signal-regulated kinase (p-ERK) occurred in HFD-fed and STZ-treated rats, while the level of glial fibrillary acidic protein (GFAP) increased. Treatment of melatonin, rats had shorter escape latencies and remained in the target quadrant longer compared to the HFD-fed and STZ-treated rats. Melatonin attenuated the reduction of neurogenesis, synaptogenesis and the induction of astrogliosis. Moreover, melatonin countered the reduction of melatonin receptor, insulin receptor and downstream signaling pathway for insulin. Our data suggested that the dysfunction of insulin signaling pathway occurred in the diabetes may provide a convergent mechanism of hippocampal impaired neurogenesis and synaptogenesis lead to impair memory while melatonin reverses these effects, suggesting that melatonin may reduce the pathogenesis of diabetes.

Type 1 diabetes mellitus in mice increases hippocampal D-serine in the acute phase after streptozotocin injection

Diabetes mellitus (DM) is known to be a risk factor in the development of deficits in cognition, learning, and memory. In DM animal models, including the streptozotocin (STZ)-induced diabetic rodent model, abnormalities in the regulation of several neurotransmitters have been reported. However, the role in DM of d-serine, an endogenous co-agonist of glutamatergic N-methyl-d-aspartate receptors, remains unknown. Here, we measured the amounts of d-/l-serine and l-glutamate in the hippocampi of STZ-treated mice using a 2D-HPLC system from acute to chronic phases after the induction of DM. STZ treatment significantly increased the d-serine level by 23.7% in the hippocampus compared with vehicle treatment at 1 week after the injection, whereas it did not affect the levels of l-serine. In contrast, l-glutamate levels in the hippocampus were elevated at 3 days after STZ injection and rather decreased at 1 week after that. Such alterations in the amino acids were not evident in the chronic phases. We further tested whether the STZ-induced d-serine increase was caused by DM pathophysiology. In vivo, subcutaneous insulin implants into STZ-treated mice restored the elevated d-serine levels in the hippocampus. An in vitro study using primary cultured hippocampal neurons revealed that treatments of STZ did not directly affect the level of d-serine secreted in the cultured media. These results indicate that DM pathology caused by insulin deficiency triggers transient d-serine increase and l-glutamate alteration in the hippocampus. Such aberrant regulations of excitatory neurotransmitters may be relevant to the formation of DM-related dysfunction of the central nervous system (CNS).

Protective effects of beta glucan and gliclazide on brain tissue and sciatic nerve of diabetic rats induced by streptozosin

There have not been yet enough studies about effects of beta glucan and gliclazide on oxidative stress created by streptozotocin in the brain and sciatic nerve of diabetic rats. The aim of this paper was to investigate the antioxidant effects of gliclazide and beta glucan on oxidative stress and lipid peroxidation created by streptozotosin in brain and sciatic nerve. Total of 42 rats were divided into 6 groups including control, diabetic untreated (DM) (only STZ, diabetic), STZ (DM) + beta glucan, STZ (DM) + gliclazide, only beta glucan treated (no diabetic), and only gliclazide treated (no diabetic). The brain and sciatic nerve tissue samples were analyzed for malondialdehyde (MDA), total oxidant status (TOS), total antioxidant status (TAS), oxidative stress index (OSI), and paraoxonase (PON-1) levels. We found a significant increase in MDA, TOS, and OSI along with a reduction in TAS level, catalase, and PON-1 activities in brain and sciatic nerve of streptozotocin-induced diabetic rats. Also, this study shows that in terms of these parameters both gliclazide and beta glucan have a neuroprotective effect on the brain and sciatic nerve of the streptozotocin-induced diabetic rat. Our conclusion was that gliclazide and beta glucan have antioxidant effects on the brain and sciatic nerve of the streptozotocin-induced diabetic rat.

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.

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.

Dietary n-3 polyunsaturated fatty acid depletion activates caspases and decreases NMDA receptors in the brain of a transgenic mouse model of Alzheimer's disease

Epidemiological data indicate that low n-3 polyunsaturated fatty acids (PFA) intake is a readily manipulated dietary risk factor for Alzheimer's disease (AD). Studies in animals confirm the deleterious effect of n-3 PFA depletion on cognition and on dendritic scaffold proteins. Here, we show that in transgenic mice overexpressing the human AD gene APPswe (Tg2576), safflower oil-induced n-3 PFA deficiency caused a decrease in N-methyl-D-aspartate (NMDA) receptor subunits, NR2A and NR2B, in the cortex and hippocampus with no loss of the presynaptic markers, synaptophysin and synaptosomal-associated protein 25 (SNAP-25). n-3 PFA depletion also decreased the NR1 subunit in the hippocampus and Ca2+/calmodulin-dependent protein kinase (CaMKII) in the cortex of Tg2576 mice. These effects of dietary n-3 PFA deficiency were greatly amplified in Tg2576 mice compared to nontransgenic mice. Loss of the NR2B receptor subunit was not explained by changes in mRNA expression, but correlated with p85alpha phosphatidylinositol 3-kinase levels. Most interestingly, n-3 PFA deficiency dramatically increased levels of protein fragments, corresponding to caspase/calpain-cleaved fodrin and gelsolin in Tg2576 mice. This effect was minimal in nontransgenic mice suggesting that n-3 PFA depletion potentiated caspase activation in the Tg2576 mouse model of AD. Dietary supplementation with docosahexaenoic acid (DHA; 22 : 6n-3) partly protected from NMDA receptor subunit loss and accumulation of fodrin and gelsolin fragments but fully prevented CaMKII decrease. The marked effect of dietary n-3 PFA on NMDA receptors and caspase/calpain activation in the cortex of an animal model of AD provide new insights into how dietary essential fatty acids may influence cognition and AD risk.