Diabetic type II Goto-Kakizaki rats show progressively decreasing exploratory activity and learning impairments in fixed and progressive ratios of a lever-press task

A systematic review of preclinical studies exploring the role of insulin signalling in executive function and memory

Beside its involvement in somatic dysfunctions, altered insulin signalling constitutes a risk factor for the development of mental disorders like Alzheimer's disease and obsessive-compulsive disorder. While insulin-related somatic and mental disorders are often comorbid, the fundamental mechanisms underlying this association are still elusive. Studies conducted in rodent models appear well suited to help decipher these mechanisms. Specifically, these models are apt to prospective studies in which causative mechanisms can be manipulated via multiple tools (e.g., genetically engineered models and environmental interventions), and experimentally dissociated to control for potential confounding factors. Here, we provide a narrative synthesis of preclinical studies investigating the association between hyperglycaemia - as a proxy of insulin-related metabolic dysfunctions - and impairments in working and spatial memory, and attention. Ultimately, this review will advance our knowledge on the role of glucose metabolism in the comorbidity between somatic and mental illnesses.

Diabetic rats with high levels of endogenous dopamine do not show retinal vascular pathology

Purpose

Limited research exists on the time course of long-term retinal and cerebral deficits in diabetic rodents. Previously, we examined short term (4-8 weeks) deficits in the Goto-Kakizaki (GK) rat model of Type II diabetes. Here, we investigated the long-term (1-8 months) temporal appearance of functional deficits (retinal, cognitive, and motor), retinal vascular pathology, and retinal dopamine levels in the GK rat.

Methods

In GK rats and Wistar controls, retinal neuronal function (electroretinogram), cognitive function (Y-maze), and motor function (rotarod) were measured at 1, 2, 4, 6, and 8 months of age. In addition, we evaluated retinal vascular function (functional hyperemia) and glucose and insulin tolerance. Retinas from rats euthanized at ≥8 months were assessed for vascular pathology. Dopamine and DOPAC levels were measured via HPLC in retinas from rats euthanized at 1, 2, 8, and 12 months.

Results

Goto-Kakizaki rats exhibited significant glucose intolerance beginning at 4 weeks and worsening over time (p < 0.001). GK rats also showed significant delays in flicker and oscillatory potential implicit times (p < 0.05 to p < 0.001) beginning at 1 month. Cognitive deficits were observed beginning at 6 months (p < 0.05), but no motor deficits. GK rats showed no deficits in functional hyperemia and no increase in acellular retinal capillaries. Dopamine levels were twice as high in GK vs. Wistar retinas at 1, 2, 8, and 12 months (p < 0.001).

Conclusion

As shown previously, retinal deficits were detectable prior to cognitive deficits in GK rats. While retinal neuronal function was compromised, retinal vascular pathology was not observed, even at 12+ months. High endogenous levels of dopamine in the GK rat may be acting as an anti-angiogenic and providing protection against vascular pathology.

Peripheral versus central insulin and leptin resistance: Role in metabolic disorders, cognition, and neuropsychiatric diseases

Insulin and leptin are classically regarded as peptide hormones that play key roles in metabolism. In actuality, they serve several functions in both the periphery and central nervous system (CNS). Likewise, insulin and leptin resistance can occur both peripherally and centrally. Metabolic disorders such as diabetes and obesity share several key features including insulin and leptin resistance. While the peripheral effects of these disorders are well-known (i.e. cardiovascular disease, hypertension, stroke, dyslipidemia, etc.), the CNS complications of leptin and insulin resistance have come into sharper focus. Both preclinical and clinical findings have indicated that insulin and leptin resistance are associated with cognitive deficits and neuropsychiatric diseases such as depression. Importantly, these studies also suggest that these deficits in neuroplasticity can be reversed by restoration of insulin and leptin sensitivity. In view of these observations, this review will describe, in detail, the peripheral and central functions of insulin and leptin and explain the role of insulin and leptin resistance in various metabolic disorders, cognition, and neuropsychiatric diseases.

Examination of nicotine and saccharin reward in the Goto-Kakizaki diabetic rat model

Morbidity and mortality attributed to type 2 diabetes have exponentially increased in the US. At exceptionally high risk is a subpopulation of persons with type 2 diabetes who smoke, which are shown to have decreased success rates of smoking cessation than euglycemic smokers. Preclinical research in our laboratory has shown that the rewarding effects of nicotine are enhanced in the streptozotocin and high-fat diet rodent model of diabetes. It is presently unclear whether this enhancement of nicotine reward can be demonstrated in other insulin resistant rat models. This study aimed to determine if a similar increase in nicotine reward is found in Goto-Kakizaki (GK) rats, a model of the spontaneous formation of insulin resistance in an inbred sub-strain of Wistar rat. Nicotine conditioned place preference (CPP) was examined in Sprague-Dawley (SD), Wistar, and GK rats. A robust nicotine CPP was found in SD and Wistar rats, but nicotine CPP was not detected in GK rats. Locomotor activity was also evaluated in all three strains, and GK rats demonstrated significantly less activity as compared to SD and Wistar rats. To further assess reward behavior in GK rats, consumption of saccharin solution was measured over a 48 -h period. GK rats showed a significant increase in saccharin intake compared to SD rats. These findings suggest that GK rats experience an enhanced hedonic processing as compared to SD rats. The lack of nicotine CPP in GK rats may be due to deficits in learning and memory, thus hindering their ability to acquire or express a place preference.

Sugar in mind: Untangling a sweet and sour relationship beyond type 2 diabetes

It is widely recognised that type 2 diabetes (T2D) represents a major disease burden but it is only recently that its role in neurodegeneration has attracted more attention. This research has shown that T2D is associated with impaired cerebral health, cognitive decline and dementia. However, the impact on the brain of progressive metabolic changes associated with the pre-clinical development of the disease is less clear. The aim of this review is to comprehensively summarise how the emergence of risk factors and co-morbid conditions linked to the development of T2D impact cerebral health. Particular attention is directed at characterising how normal but elevated blood glucose levels in individuals without T2D contribute to neurodegenerative processes, and how the main risk factors for T2D including obesity, physical activity and diet modulate these effects. Where available, evidence from the animal and human literature is contrasted, and sex differences in risk and outcomes are highlighted.

Retinal Deficits Precede Cognitive and Motor Deficits in a Rat Model of Type II Diabetes

Purpose

To investigate the temporal appearance of retinal, cognitive, and motor deficits in Goto-Kakizaki (GK) rats, a spontaneously occurring, polygenic model of type II diabetes. GK rats develop impaired insulin secretion at 2 weeks and fasting hyperglycemia at 4 weeks.

Methods

In male and female GK rats and Wistar controls, glucose tolerance test (hyperglycemia) and electroretinogram (ERG, retinal function) were performed at 4 and 8 weeks of age. Spectral domain-optical coherence tomography (retinal structure) was assessed at 6 weeks. Spatial alternation (cognitive function) and number of entries (exploratory behavior) were assessed via Y-maze at 4, 5, 6, 7, and 8 weeks. Rotarod (motor function) was performed at 4, 6, and 8 weeks.

Results

By 4 weeks, the GK rats exhibited significant glucose intolerance (P < 0.001) and retinal deficits, including delays in ERG implicit times (flicker, P < 0.01; oscillatory potentials, P < 0.001). In addition, the GK rats showed greater ERG amplitudes (P < 0.001) and thinner retinas (P < 0.001). At 7 weeks, the GK rats showed deficits in cognitive function (P < 0.001) and exploratory behavior (P < 0.01). However, no motor function deficits were observed by 8 weeks. Interestingly, the male GK rats showed greater hyperglycemia (P < 0.05), but the female rats showed greater ERG delays (P < 0.001).

Conclusions

In GK rats, retinal function deficits developed prior to cognitive or motor deficits. Future studies will investigate common mechanistic links, long-term functional and vascular changes, and whether early retinal deficits can predict cognitive dysfunction or late-stage retinal disease.

Type 2 diabetes impairs odour detection, olfactory memory and olfactory neuroplasticity; effects partly reversed by the DPP-4 inhibitor Linagliptin

Recent data suggest that olfactory deficits could represent an early marker and a pathogenic mechanism at the basis of cognitive decline in type 2 diabetes (T2D). However, research is needed to further characterize olfactory deficits in diabetes, their relation to cognitive decline and underlying mechanisms.

The aim of this study was to determine whether T2D impairs odour detection, olfactory memory as well as neuroplasticity in two major brain areas responsible for olfaction and odour coding: the main olfactory bulb (MOB) and the piriform cortex (PC), respectively. Dipeptidyl peptidase-4 inhibitors (DPP-4i) are clinically used T2D drugs exerting also beneficial effects in the brain. Therefore, we aimed to determine whether DPP-4i could reverse the potentially detrimental effects of T2D on the olfactory system.

Non-diabetic Wistar and T2D Goto-Kakizaki rats, untreated or treated for 16 weeks with the DPP-4i linagliptin, were employed. Odour detection and olfactory memory were assessed by using the block, the habituation-dishabituation and the buried pellet tests. We assessed neuroplasticity in the MOB by quantifying adult neurogenesis and GABAergic inhibitory interneurons positive for calbindin, parvalbumin and carletinin. In the PC, neuroplasticity was assessed by quantifying the same populations of interneurons and a newly identified form of olfactory neuroplasticity mediated by post-mitotic doublecortin (DCX) + immature neurons.

We show that T2D dramatically reduced odour detection and olfactory memory. Moreover, T2D decreased neurogenesis in the MOB, impaired the differentiation of DCX+ immature neurons in the PC and altered GABAergic interneurons protein expression in both olfactory areas. DPP-4i did not improve odour detection and olfactory memory. However, it normalized T2D-induced effects on neuroplasticity.

The results provide new knowledge on the detrimental effects of T2D on the olfactory system. This knowledge could constitute essentials for understanding the interplay between T2D and cognitive decline and for designing effective preventive therapies.

Sevoflurane Induces Exaggerated and Persistent Cognitive Decline in a Type II Diabetic Rat Model by Aggregating Hippocampal Inflammation

Recent studies show that a moderate duration of sevoflurane, one of the most commonly used volatile anesthetics in clinical practice, does not induce cognitive impairment in animals under physiological conditions. However, the influence of sevoflurane on cognitive function under diabetic conditions remains unclear. The aim of this study was to determine whether sevoflurane causes cognitive decline in a rat model of type 2 diabetes mellitus (DM) and if so, to explore a possible underlying mechanism. Diabetic Goto–Kakizaki (GK) rats and non-diabetic Wistar rats underwent 2.6% sevoflurane for 4 h or sham (control) exposure. Cognitive function and hippocampal inflammation were assessed 1 week and 5 months after sevoflurane or sham exposure. Compared with Wistar control rats, GK control rats exhibited shorter freezing times in Trace fear conditioning task 1 week after exposure, took longer to locate the submerged platform and had shorter dwell-time in the target quadrant in Morris Water Maze task 5 months after exposure. GK rats that received sevoflurane not only exhibited less freezing times 1 week after exposure, but also spent more time to locate the submerged platform and had less dwell-time in the target quadrant, compared with GK control rats. Molecular studies revealed that the levels of pro-inflammatory cytokines and activated microglia in the hippocampus were higher in GK control rats than those in Wistar control rats at both time points and were further increased in GK rats receiving sevoflurane. Wistar rats that received sevoflurane and Wistar control rats did not differ in any cognitive performance and molecular assessment. The results suggest that diabetic GK rats exhibit cognitive dysfunction probably due to increased hippocampal inflammation, and that sevoflurane induces exaggerated and persistent cognitive decline in GK rat by aggregating hippocampal inflammation.

Type 2 diabetes-induced neuronal pathology in the piriform cortex of the rat is reversed by the GLP-1 receptor agonist exendin-4

Type 2 diabetes (T2D) patients often present olfactory dysfunction. However, the histopathological basis behind this has not been previously shown. Since the piriform cortex plays a crucial role in olfaction, we hypothesize that pathological changes in this brain area can occur in T2D patients along aging. Thus, we determined potential neuropathology in the piriform cortex of T2D rats, along aging. Furthermore, we determined the potential therapeutic role of the glucagon-like peptide-1 receptor (GLP1-R) agonist exendin-4 to counteract the identified T2D-induced neuropathology.

Young-adult and middle-aged T2D Goto-Kakizaki rats were compared to age-matched Wistars. Additional Goto-Kakizaki rats were treated for six weeks with exendin-4/vehicle before sacrifice. Potential T2D-induced neuropathology was assessed by quantifying NeuN-positive neurons and Calbindin-D28k-positive interneurons by immunohistochemistry and stereology methods. We also quantitatively measured Calbindin-D28k neuronal morphology and JNK phosphorylation-mediated cellular stress. PI3K/AKT signalling was assessed by immunohistochemistry, and potential apoptosis by TUNEL.

We show T2D-induced neuronal pathology in the piriform cortex along aging, characterized by atypical nuclear NeuN staining and increased JNK phosphorylation, without apoptosis. We also demonstrate the specific vulnerability of Calbindin-D28k interneurons. Finally, chronic treatment with exendin-4 substantially reversed the identified neuronal pathology in correlation with decreased JNK and increased AKT phosphorylation.

Our results reveal the histopathological basis to explain T2D olfactory dysfunction. We also show that the identified T2D-neuropathology can be counteracted by GLP-1R activation supporting recent research promoting the use of GLP-1R agonists against brain diseases. Whether the identified neuropathology could represent an early hallmark of cognitive decline in T2D remains to be determined.

Middle-Aged Diabetic Females and Males Present Distinct Susceptibility to Alzheimer Disease-like Pathology

Type 2 diabetes (T2D) is a highly concerning public health problem of the twenty-first century. Currently, it is estimated that T2D affects 422 million people worldwide with a rapidly increasing prevalence. During the past two decades, T2D has been widely shown to have a major impact in the brain. This, together with the cognitive decline and increased risk for dementia upon T2D, may arise from the complex interaction between normal brain aging and central insulin signaling dysfunction. Among the several features shared between T2D and some neurodegenerative disorders (e.g., Alzheimer disease (AD)), the impairment of insulin signaling may be a key link. However, these may also involve changes in sex hormones' function and metabolism, ultimately contributing to the different susceptibilities between females and males to some pathologies. For example, female sex has been pointed as a risk factor for AD, particularly after menopause. However, less is known on the underlying molecular mechanisms or even if these changes start during middle-age (perimenopause). From the above, we hypothesized that sex differentially affects hormone-mediated intracellular signaling pathways in T2D brain, ultimately modulating the risk for neurodegenerative conditions. We aimed to evaluate sex-associated alterations in estrogen/insulin-like growth factor-1 (IGF-1)/insulin-related signaling, oxidative stress markers, and AD-like hallmarks in middle-aged control and T2D rat brain cortices. We used brain cortices homogenates obtained from middle-aged (8-month-old) control Wistar and non-obese, spontaneously T2D Goto-Kakizaki (GK) male and female rats. Peripheral characterization of the animal models was done by standard biochemical analyses of blood, plasma, or serum. Steroid sex hormones, oxidative stress markers, and AD-like hallmarks were given by specific ELISA kits and colorimetric techniques, whereas the levels of intracellular signaling proteins were determined by Western blotting. Albeit the high levels of plasma estradiol and progesterone observed in middle-aged control females suggested that they were still under their reproductive phase, some gonadal dysfunction might be already occurring in T2D ones, hence, anticipating their menopause. Moreover, the higher blood and lower brain cholesterol levels in female rats suggested that its dysfunctional uptake into the brain cortex may also hamper peripheral estrogen uptake and/or its local brain steroidogenic metabolism. Despite the massive drop in IGF-1 levels in females' brains, particularly upon T2D, they might have developed some compensatory mechanisms towards the maintenance of estrogen, IGF-1, and insulin receptors function and of the subsequent Akt- and ERK1/2-mediated signaling. These may ultimately delay the deleterious AD-like brain changes (including oxidative damage to lipids and DNA, amyloidogenic processing of amyloid precursor protein and increased tau protein phosphorylation) associated with T2D and/or age (reproductive senescence) in female rats. By demonstrating that differential sex steroid hormone profiles/action may play a pivotal role in brain over T2D progression, the present study reinforces the need to establish sex-specific preventive and/or therapeutic approaches and an appropriate time window for the efficient treatment against T2D and AD.

Impairment of synaptic development in the hippocampus of diabetic Goto-Kakizaki rats

Insulin receptor signaling has been shown to regulate essential aspects of CNS function such as synaptic plasticity and neuronal survival. To elucidate its roles during CNS development in vivo, we examined the synaptic and cognitive development of the spontaneously diabetic Goto-Kakizaki (GK) rats in the present study. GK rats are non-obese models of type 2 diabetes established by selective inbreeding of Wistar rats based on impaired glucose tolerance. Though they start exhibiting only moderate hyperglycemia without changes in plasma insulin levels from 3 weeks postnatally, behavioral alterations in the open-field as well as significant impairments in memory retention compared with Wistar rats were observed at 10 weeks and were worsened at 20 weeks. Alterations in insulin receptor signaling and signs of insulin resistance were detected in the GK rat hippocampus at 3 weeks, as early as in other insulin-responsive peripheral tissues. Significant reduction of an excitatory postsynaptic scaffold protein, PSD95, was found at 5w and later in the hippocampus of GK rats due to the absence of a two-fold developmental increase of this protein observed in Wistar control rats between 3 and 20w. In the GK rat hippocampus, NR2A which is a NMDA receptor subunit selectively anchored to PSD95 was also reduced. In contrast, both NR2B and its anchoring protein, SAP102, showed similar developmental profiles in Wistar and GK rats with expression peaks at 2 and 3w. The results suggest that early alterations in insulin receptor signaling in the GK rat hippocampus may affect cognitive performance by suppressing synaptic maturation.

Early onset of cognitive impairment is associated with altered synaptic plasticity and enhanced hippocampal GluA1 expression in a mouse model of depression

Memory deficit is a common manifestation of age-related cognitive impairment, of which depression is a frequently occurring comorbidity. Previously, we developed a submissive (Sub) mouse line, validated as a model of depressive-like behavior. Using learning paradigms testing hippocampus-dependent spatial and nonspatial memory, we demonstrate here that Sub mice developed cognitive impairments at earlier age (3 months), compared with wild-type mice. Furthermore, acute hippocampal slices from Sub animals failed to display paired-pulse facilitation, whereas primed burst stimulation elicited significantly enhanced long-term potentiation in region CA1, relative to control mice. Changes in synaptic plasticity were accompanied by markedly reduced hippocampal messenger RNA expression of insulin-like growth factor and brain-derived neurotrophic factor. Finally, we identified markedly elevated protein levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 in the hippocampi of Sub mice, which was exacerbated with age. Taken together, the results point to a linkage between depressive-like behavior and the susceptibility to develop age-related cognitive impairment, potentially by hippocampal α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated glutamatergic signaling.

Cerebral antioxidant enzyme increase associated with learning deficit in type 2 diabetes rats

In this study, we examined alterations in the enzymatic antioxidant defenses associated with learning deficits induced by type 2 diabetes, and studied the effects of the peroxisome proliferator-activated receptor γ agonist pioglitazone on these learning deficits. Learning ability was assessed by visual discrimination tasks in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, as a model of spontaneous type 2 diabetes. Levels of the antioxidant enzymes glutathione peroxidase (GPx), Cu(2+)-Zn(2+) superoxide dismutase (CuZn-SOD) and manganese SOD were measured in the cortex, hippocampus and striatum. Half the rats received oral pioglitazone (20mg/kg/day) from the early stage of diabetes (22 weeks old) to 27 weeks old. OLETF rats showed learning deficits compared with control, Long-Evans Tokushima Otsuka (LETO) rats. GPx levels in the cortex and hippocampus were increased in OLETF rats compared with LETO rats, with an inverse correlation between GPx in the hippocampus and learning score. CuZn-SOD levels were also increased in the hippocampus in OLETF rats. Pioglitazone reduced blood glucose and increased serum adiponectin levels, but had no effect on learning tasks or antioxidant enzymes, except for CuZn-SOD. These results suggest that an oxidative imbalance reflected by increased brain antioxidant enzymes plays an important role in the development of learning deficits in type 2 diabetes. Early pioglitazone administration partly ameliorated diabetic symptoms, but was unable to completely recover cerebral oxidative imbalance and functions. These results suggest that diabetes-induced brain impairment, which results in learning deficits, may have occurred before the appearance of the symptoms of overt diabetes.

The GK rat: a prototype for the study of non-overweight type 2 diabetes

Type 2 diabetes mellitus (T2D) arises when the endocrine pancreas fails to secrete sufficient insulin to cope with the metabolic demand because of β-cell secretory dysfunction and/or decreased β-cell mass. Defining the nature of the pancreatic islet defects present in T2D has been difficult, in part because human islets are inaccessible for direct study. This review is aimed to illustrate to what extent the Goto Kakizaki rat, one of the best characterized animal models of spontaneous T2D, has proved to be a valuable tool offering sufficient commonalities to study this aspect. A comprehensive compendium of the multiple functional GK abnormalities so far identified is proposed in this perspective, together with their time-course and interactions. A special focus is given toward the pathogenesis of defective β-cell number and function in the GK model. It is proposed that the development of T2D in the GK model results from the complex interaction of multiple events: (1) several susceptibility loci containing genes responsible for some diabetic traits; (2) gestational metabolic impairment inducing an epigenetic programming of the offspring pancreas and the major insulin target tissues; and (3) environmentally induced loss of β-cell differentiation due to chronic exposure to hyperglycemia/hyperlipidemia, inflammation, and oxidative stress.

The effects of chromium complex and level on glucose metabolism and memory acquisition in rats fed high-fat diet

Conditions in which glucose metabolism is impaired due to insulin resistance are associated with memory impairment. It was hypothesized that supplemental chromium (Cr) may alleviate insulin resistance in type 2 diabetes and consequently improve memory acquisition, depending upon its source and level. In a complete randomized design experiment, male Wistar rats (n=60; weighing 200-220 g) were fed either normal (8%, normal diet (ND)) or high-fat (40%, high-fat diet (HFD)) diet and supplemented with Cr as either chromium-glycinate (CrGly) or chromium-acetate (CrAc) at doses of 0, 40, or 80 μg/kg body weight (BW) via drinking water from 8 to 20 weeks of age. Feeding HFD induced type 2 diabetes, as reflected by greater glucose/insulin ratio (2.98 vs. 2.74) comparing to feeding ND. Moreover, HFD rats had greater BW (314 vs. 279 g) and less serum (53 vs. 68 μg/L) and brain (14 vs. 24 ng/g) Cr concentrations than ND rats. High-fat diet caused a 32% reduction in expressions of glucose transporters 1 and 3 (GLUTs) in brain tissue and a 27% reduction in mean percentage time spent in the target quadrant and a 38% increase in spatial memory acquisition phase (SMAP) compared with ND. Compared with supplemental Cr as CrAc, CrGly was more effective to ameliorate response variables (i.e., restoration of tissue Cr concentration, enhancement of cerebral GLUTs expressions, and reduction of the glucose/insulin ratio and SMAP) in a dose-response manner, especially in rats fed HFD. Supplemental Cr as CrGly may have therapeutic potential to enhance insulin action and alleviate memory acquisition in a dose-dependent manner, through restoring tissue Cr reserve and enhancing cerebral GLUTs expressions.

Upregulation of the heme oxygenase system ameliorates postprandial and fasting hyperglycemia in type 2 diabetes

In type 2 diabetes (T2D), postprandial and fasting hyperglycemia are important predictors of cardiovascular diseases; however, few drugs are currently available to simultaneously suppress these conditions. Here, we report an enduring antidiabetic effect of the heme oxygenase (HO) inducer hemin on Goto-Kakizaki rats (GK), a nonobese insulin-resistant T2D model. HO breaks down the heme-moiety-generating antioxidants (biliverdin/bilirubin and ferritin) and carbon monoxide, which stimulate insulin secretion. Hemin induces HO-1 to potentiate HO activity and the HO-derived products. Chronically applied hemin (30 mg/kg ip) for a month reduced and maintained fasting glucose at physiological levels for 3 mo. Before therapy, glucose levels were 9.3 +/- 0.3 mmol/l (n = 14). At 1, 2, and 3 mo posttherapy, we recorded 6.7 +/- 0.13, 5.9 +/- 0.2, and 7.2 +/- 0.2 mmol/l, respectively. Hemin was also effective against postprandial hyperglycemia (14.6 +/- 1.1 vs. 7.5 +/- 0.4 mmol/l; n = 14; P < 0.01), and the effect remained sustained for 3 mo after therapy. The reduction of hyperglycemia was accompanied by enhanced HO-1, HO activity, and cGMP of the soleus muscle, alongside increased plasma bilirubin, ferritin, SOD, total antioxidant capacity, and insulin levels, whereas markers/mediators of oxidative stress like urinary-8-isoprostane and soleus muscle nitrotyrosine, NF-kappaB, and activator protein-1 and -2 were abated. Furthermore, inhibitors of insulin signaling including soleus muscle glycogen synthase kinase-3 and JNK were reduced, while the insulin-sensitizing adipokine, adiponectin, alongside AMPK were increased. Correspondingly, hemin improved glucose tolerance, suppressed insulin intolerance, reduced insulin resistance, and overturned the inability of insulin to enhance glucose transporter 4, a protein required for glucose uptake. Hemin also upregulated HO-1/HO activity and cGMP and lowered glucose in euglycemic Sprague-Dawley control rats albeit less intensely, suggesting greater selectivity of the HO system in diabetic conditions. In conclusion, reduced oxidative stress alongside the concomitant and paradoxical enhancement of insulin secretion and insulin-sensitizing pathways may account for the 3-mo-enduring antidiabetic effect. The synergistic interaction among HO, adiponectin, and GLUT4 may be explored against insulin-resistant diabetes.

Insulin resistance and amyloidogenesis as common molecular foundation for type 2 diabetes and Alzheimer's disease

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Abeta), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Abeta aggregation may not only be the consequence of excitotoxicity, aberrant Ca(2+) signals, and proinflammatory cytokines such as TNF-alpha, but may also promote these pathological effectors. At the molecular level, insulin resistance and Abeta disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

Rosiglitazone attenuates the cognitive deficits induced by high fat diet feeding in rats

The present study was designed to test the hypothesis that insulin resistance plays a role in high fat diet feeding induced cognitive deficits. Rats consuming the high fat diet exhibited characteristic features of insulin resistance viz. mild hyperglycemia, hypertriglyceridemia, hypercholesterolemia, and hyperinsulinemia. Further, these rats showed a severe deficit in learning and memory. In contrast, rosiglitazone at the dose of 5 mg/kg, p.o. for 7 days prior to biochemical and behavioral testing significantly lowered the plasma glucose, triglycerides, cholesterol, and insulin levels. These animals also performed better on Morris water maze task, suggesting improved spatial memory. Our data demonstrate that the insulin sensitizers can overcome the cognitive deficits arising from high fat diet feeding, which may be in part mediated through the development of peripheral insulin resistance.

Mitochondrial nutrients improve immune dysfunction in the type 2 diabetic Goto-Kakizaki rats

The development of type 2 diabetes is accompanied by decreased immune function and the mechanisms are unclear. We hypothesize that oxidative damage and mitochondrial dysfunction may play an important role in the immune dysfunction in diabetes. In the present study, we investigated this hypothesis in diabetic Goto-Kakizaki rats by treatment with a combination of four mitochondrial-targeting nutrients, namely, R-α-lipoic acid, acetyl-L-carnitine, nicotinamide and biotin. We first studied the effects of the combination of these four nutrients on immune function by examining cell proliferation in immune organs (spleen and thymus) and immunomodulating factors in the plasma. We then examined, in the plasma and thymus, oxidative damage biomarkers, including lipid peroxidation, protein oxidation, reactive oxygen species, calcium and antioxidant defence systems, mitochondrial potential and apoptosis-inducing factors (caspase 3, p53 and p21). We found that immune dysfunction in these animals is associated with increased oxidative damage and mitochondrial dysfunction and that the nutrient treatment effectively elevated immune function, decreased oxidative damage, enhanced mitochondrial function and inhibited the elevation of apoptosis factors. These effects are comparable to, or greater than, those of the anti-diabetic drug pioglitazone. These data suggest that a rational combination of mitochondrial-targeting nutrients may be effective in improving immune function in type 2 diabetes through enhancement of mitochondrial function, decreased oxidative damage, and delayed cell death in the immune organs and blood.