Intracerebroventricular insulin enhances memory in a passive-avoidance task

Targeting amyloid-beta by glucagon-like peptide -1 (GLP-1) in Alzheimer's disease and diabetes

INTRODUCTION

Epidemiological evidence suggests an association between type 2 diabetes (T2DM) and Alzheimer's disease (AD), in that one disease increases the risk of the other. T2DM and AD share several molecular processes which underlie the tissue degeneration in either disease. Disturbances in insulin signaling may be the link between the two conditions. Drugs originally developed for T2DM are currently being considered as possible novel agents in the treatment of AD.

AREAS COVERED

This review discusses the potential role of glucagon-like peptide -1 (GLP-1) treatment in AD. GLP-1 receptors are expressed in areas of the brain important to memory and learning, and GLP-1 has growth-factor-like properties similar to insulin. A key neuropathological feature of AD is the accumulation of amyloid-beta (Aβ). In preclinical studies, GLP-1 and longer lasting analogues have been shown to have both neuroprotective and neurotrophic effects, and to protect synaptic activity in the brain from Aβ toxicity.

EXPERT OPINION

A convincing amount of evidence has shown a beneficial effect of GLP-1 agonist treatment on cognitive function, memory and learning in experimental models of AD. GLP-1 analogues may therefore be the new therapeutic agent of choice for intervention in AD.

Homocysteine levels are associated with hippocampus volume in type 2 diabetic patients

BACKGROUND

Elevated total plasma homocysteine (tHcy) levels are associated with cognitive dysfunction, in which changes in the hippocampus plausibly play a pivotal role. We tested the hypothesis that elevated tHcy levels are correlated with hippocampus volume and insulin resistance in nondementia patients with type 2 diabetes.

MATERIALS AND METHODS

The study included 43 nondementia patients with type 2 diabetes, who were divided into two groups: a high tHcy group (age: 65 ± 8 years, mean ± standard deviation, n = 16) and a normal tHcy group (64 ± 9 years, n = 27). Hippocampus volume was quantified with a computer-assisted analysis using a magnetic resonance imaging (MRI) voxel-based specific regional analysis system developed for the study of Alzheimer's disease (VSRAD), which yields a Z-score as the end point for the assessment of hippocampal volume. Results  The Z-score was higher in the high tHcy group compared to the normal tHcy group (P < 0·0001). The fasting plasma glucose (P < 0·01) and insulin (P < 0·0001) concentrations and the homoeostasis model assessment (HOMA) index (P < 0·0001) were higher in the high tHcy group than in the normal tHcy group. Multiple regression analysis showed that the main factors that influenced tHcy levels may be the Z-score and the HOMA index.

CONCLUSIONS

Our results indicate that the elevated levels of tHcy in Japanese nondementia patients with type 2 diabetes are characterised by hippocampal atrophy and insulin resistance and that the Z-score and HOMA index may be the primary factors that influence tHcy levels.

Amyloid-β as a modulator of synaptic plasticity

Alzheimer's disease is associated with synapse loss, memory dysfunction, and pathological accumulation of amyloid-β (Aβ) in plaques. However, an exclusively pathological role for Aβ is being challenged by new evidence for an essential function of Aβ at the synapse. Aβ protein exists in different assembly states in the central nervous system and plays distinct roles ranging from synapse and memory formation to memory loss and neuronal cell death. Aβ is present in the brain of symptom-free people where it likely performs important physiological roles. New evidence indicates that synaptic activity directly evokes the release of Aβ at the synapse. At physiological levels, Aβ is a normal, soluble product of neuronal metabolism that regulates synaptic function beginning early in life. Monomeric Aβ40 and Aβ42 are the predominant forms required for synaptic plasticity and neuronal survival. With age, some assemblies of Aβ are associated with synaptic failure and Alzheimer's disease pathology, possibly targeting the N-methyl-D-aspartic acid receptor through the nicotinic acetylcholine receptor, mitochondrial Aβ alcohol dehydrogenase, and cyclophilin D. But emerging data suggests a distinction between age effects on the target response in contrast to the assembly state or the accumulation of the peptide. Both aging and Aβ independently decrease neuronal plasticity. Our laboratory has reported that Aβ, glutamate, and lactic acid are each increasingly toxic with neuron age. The basis of the age-related toxicity partly resides in age-related mitochondrial dysfunction and an oxidative shift in mitochondrial and cytoplasmic redox potential. In turn, signaling through phosphorylated extracellular signal-regulated protein kinases is affected along with an age-independent increase in phosphorylated cAMP response element-binding protein. This review examines the long-awaited functional impact of Aβ on synaptic plasticity.

Mechanisms of brain aging regulation by insulin: implications for neurodegeneration in late-onset Alzheimer's disease

Insulin and IGF seem to be important players in modulating brain aging. Neurons share more similarities with islet cells than any other human cell type. Insulin and insulin receptors are diffusely found in the brain, especially so in the hippocampus. Caloric restriction decreases insulin resistance, and it is the only proven mechanism to expand lifespan. Conversely, insulin resistance increases with age, obesity, and sedentarism, all of which have been shown to be risk factors for late-onset Alzheimer's disease (AD). Hyperphagia and obesity potentiate the production of oxidative reactive species (ROS), and chronic hyperglycemia accelerates the formation of advanced glucose end products (AGEs) in (pre)diabetes—both mechanisms favoring a neurodegenerative milieu. Prolonged high cerebral insulin concentrations cause microvascular endothelium proliferation, chronic hypoperfusion, and energy deficit, triggering β-amyloid oligomerization and tau hyperphosphorylation. Insulin-degrading enzyme (IDE) seems to be the main mechanism in clearing β-amyloid from the brain. Hyperinsulinemic states may deviate IDE utilization towards insulin processing, decreasing β-amyloid degradation.

Insulin receptor substrate 2 is a negative regulator of memory formation

Insulin has been shown to impact on learning and memory in both humans and animals, but the downstream signaling mechanisms involved are poorly characterized. Insulin receptor substrate-2 (Irs2) is an adaptor protein that couples activation of insulin- and insulin-like growth factor-1 receptors to downstream signaling pathways. Here, we have deleted Irs2, either in the whole brain or selectively in the forebrain, using the nestin Cre- or D6 Cre-deleter mouse lines, respectively. We show that brain- and forebrain-specific Irs2 knockout mice have enhanced hippocampal spatial reference memory. Furthermore, NesCreIrs2KO mice have enhanced spatial working memory and contextual- and cued-fear memory. Deletion of Irs2 in the brain also increases PSD-95 expression and the density of dendritic spines in hippocampal area CA1, possibly reflecting an increase in the number of excitatory synapses per neuron in the hippocampus that can become activated during memory formation. This increase in activated excitatory synapses might underlie the improved hippocampal memory formation observed in NesCreIrs2KO mice. Overall, these results suggest that Irs2 acts as a negative regulator on memory formation by restricting dendritic spine generation.

Western diet consumption and cognitive impairment: links to hippocampal dysfunction and obesity

Intake of saturated fats and simple carbohydrates, two of the primary components of a modern Western diet, is linked with the development of obesity and Alzheimer's Disease. The present paper summarizes research showing that Western diet intake is associated with cognitive impairment, with a specific emphasis on learning and memory functions that are dependent on the integrity of the hippocampus. The paper then considers evidence that saturated fat and simple carbohydrate intake is correlated with neurobiological changes in the hippocampus that may be related to the ability of these dietary components to impair cognitive function. Finally, a model is described proposing that Western diet consumption contributes to the development of excessive food intake and obesity, in part, by interfering with a type of hippocampal-dependent memory inhibition that is critical in the ability of animals to refrain from responding to environmental cues associated with food, and ultimately from consuming energy intake in excess of that driven solely by caloric need.

Carbohydrates for improving the cognitive performance of independent-living older adults with normal cognition or mild cognitive impairment

BACKGROUND

Mild cognitive impairment (MCI) is an intermediate state between normal cognition and dementia in which daily function is largely intact. This condition may present an opportunity for research into the prevention of dementia. Carbohydrate is an essential and easily accessible macronutrient which influences cognitive performance. A better understanding of carbohydrate-driven cognitive changes in normal cognition and mild cognitive impairment may suggest ways to prevent or reduce cognitive decline.

OBJECTIVES

To assess the effectiveness of carbohydrates in improving cognitive function in older adults.

SEARCH STRATEGY

We searched ALOIS, the Cochrane Dementia and Cognitive Improvement Group Specialized Register on 22 June 2010 using the terms: carbohydrates OR carbohydrate OR monosaccharides OR disaccharides OR oligosaccharides OR polysaccharides OR CARBS. ALOIS contains records from all major healthcare databases (The Cochrane Library, MEDLINE, EMBASE, PsycINFO, CINAHL, LILACS) as well as from many trial databases and grey literature sources.

SELECTION CRITERIA

All randomised controlled trials (RCT) that have examined the efficacy of any form of carbohydrates in normal cognition and MCI.

DATA COLLECTION AND ANALYSIS

One review author selected and retrieved relevant articles for further assessment. The remaining authors independently assessed whether any of the retrieved trials should be included. Disagreements were resolved by discussion. 

MAIN RESULTS

There is no suitable RCT of any form of carbohydrates involving independent-living older adults with normal cognition or mild cognitive impairment.

AUTHORS' CONCLUSIONS

There are no suitable RCTs on which to base any recommendations about the use of any form of carbohydrate for enhancing cognitive performance in older adults with normal cognition or mild cognitive impairment. More studies of many different carbohydrates are needed to tease out complex nutritional issues and further evaluate memory improvement.

High-sensitivity C-reactive protein is associated with hippocampus volume in nondementia patients with type 2 diabetes mellitus

The elevated level of high-sensitivity C-reactive protein (HSCRP) is associated with cognitive dysfunction, for which changes in the hippocampus plausibly play a pivotal role. We tested the hypothesis that an elevated level of HSCRP correlates with hippocampus volume and insulin resistance in nondementia patients with type 2 diabetes mellitus. Subjects included 45 nondementia patients with type 2 diabetes mellitus, who were divided into 2 groups: high-HSCRP group (age, 65 ± 6 years [mean ± SD]; n = 17) and normal-HSCRP group (65 ± 7 years, n = 28). Hippocampus volume has been quantitated with computer-assisted analysis using a magnetic resonance imaging voxel-based specific regional analysis system developed for the study of Alzheimer disease (VSRAD), which yields a z score as the end point for assessment of hippocampal volume. The z score was higher in the high-HSCRP group than in the normal-HSCRP group (P < .0001). The fasting plasma glucose (P < .05) and insulin concentrations (P < .0001) and the homeostasis model assessment (HOMA) index (P < .0001) were higher in the high-HSCRP group than in the normal-HSCRP group. Multiple regression analysis showed that HSCRP levels were independently predicted by z score and HOMA index. Our results indicate that the elevated level of HSCRP in Japanese nondementia patients with type 2 diabetes mellitus is characterized by increased hippocampus volume and insulin resistance, and that the z score and HOMA index are independent predictors of HSCRP.

No effect of single-dose intranasal insulin treatment on verbal memory and sustained attention in patients with schizophrenia

BACKGROUND

Impairments in verbal memory and attention are among the most severe and disabling cognitive deficits in patients with schizophrenia. Whereas efficacy for cognition has not yet been established for any pharmacologic strategy in schizophrenia, an accumulating body of evidence suggests a possible beneficial role of insulin.

METHODS

We conducted a double-blind, placebo-controlled trial to examine the effect of single-dose intranasal insulin treatment on cognition in nondiabetic patients with schizophrenia. After fasting for 12 hours, subjects received either 40 IU regular human insulin or placebo administered by intranasal pump. The Hopkins Verbal Learning Test and the Continuous Performance Test-Identical Pairs were administered before and 30 minutes after intranasal treatment.

RESULTS

Thirty patients were enrolled and completed the study. The 2 treatment groups (insulin vs placebo, n = 15 in each group) did not differ on any demographic or general clinical variable (P > 0.40). There was no significant difference between the 2 treatment groups in change on Hopkins Verbal Learning Test immediate recall total score and delayed recall score, or on CPT d', hits rate, reaction time of hits, or false-alarm rate (P > 0.1).

CONCLUSIONS

Results of the present study suggest that single-dose intranasal insulin treatment does not have a large-enough effect on verbal memory or sustained attention to be detected by a sample of this size in patients with schizophrenia but was safe and well tolerated. Longitudinal studies to explore cognitive benefits of repeated dosing of intranasal insulin treatment are needed.

The role of the brain in the regulation of metabolism and energy expenditure: the central role of insulin, the insulin resistance of the brain

Regulatory role of the brain in energy expenditure, appetite, glucose metabolism, and central effects of insulin has been prominently studied. Certain neurons in the hypothalamus increase or decrease appetite via orexigenes and anorexigenes, regulating energy balance and food intake. Hypothalamus is the site of afferent and efferent stimuli between special nuclei and beta- and alpha cells, and it regulates induction/inhibition of glucose output from the liver. Incretines, produced in intestine and in certain brain cells (brain-gut hormones), link to special receptors in the hypothalamus. Central role of insulin has been proved both in animals and in humans. Insulin gets across the blood-brain barrier, links to special hypothalamic receptors, regulating peripheral glucose metabolism. Central glucose sensing, via “glucose-excited” and “glucose-inhibited” cells have outstanding role. Former are active in hyperglycaemia, latter in hypoglycaemia, via influencing beta– and alpha cells, independently of traditional metabolic pathways. Evidence of brain insulin resistance needs centrally acting drugs, paradigm changes in therapy and prevention of metabolic syndrome, diabetes, cardiovascular and oncological diseases. Orv. Hetil., 2011, 152, 83–91.

Insulin deficiency exacerbates cerebral amyloidosis and behavioral deficits in an Alzheimer transgenic mouse model

Background

Although increasing evidence has indicated that brain insulin dysfunction is a risk factor for Alzheimer disease (AD), the underlying mechanisms by which insulin deficiency may impact the development of AD are still obscure. Using a streptozotocin (STZ)-induced insulin deficient diabetic AD transgenic mouse model, we evaluated the effect of insulin deficiency on AD-like behavior and neuropathology.

Results

Our data showed that administration of STZ increased the level of blood glucose and reduced the level of serum insulin, and further decreased the phosphorylation levels of insulin receptors, and increased the activities of glycogen synthase kinase-3α/β and c-Jun N-terminal kinase in the APP/PS1 mouse brain. We further showed that STZ treatment promoted the processing of amyloid-β (Aβ) precursor protein resulting in increased Aβ generation, neuritic plaque formation, and spatial memory deficits in transgenic mice.

Conclusions

Our present data indicate that there is a close link between insulin deficient diabetes and cerebral amyloidosis in the pathogenesis of AD.

Intranasal insulin as a therapeutic option in the treatment of cognitive impairments

The brain is a major target of circulating insulin. Enhancing central nervous insulin action has been shown to improve memory functions in animals as well as in humans, benefitting in particular hippocampus-dependent (declarative) memory. As Alzheimer's disease (AD) is associated with reduced central nervous insulin signaling and attenuated permeation of blood-borne insulin across the blood-brain-barrier, the cognitive decline in AD patients may at least in part be derived from impaired brain insulin signaling. Thus, therapeutic strategies to overcome central nervous system insulin deficiency and resistance might be an attractive option in the treatment of cognitive impairments like AD. Insulin can be effectively delivered directly to the brain via the intranasal route that enables the hormone to bypass the blood-brain barrier and modulate central nervous functions. This review summarizes a series of studies demonstrating beneficial effects of intranasal insulin on memory functions both in healthy humans and in patients with cognitive impairments such as AD. These experiments in humans consistently indicate that enhancing brain insulin signaling by intranasal administration of the hormone improves hippocampus-dependent memory in the absence of adverse side effects. Considering that insulin also acts as a neuroprotective signal, up-regulating brain insulin levels by intranasal insulin administration appears to be a promising approach in the treatment and prevention of central nervous system insulin deficiency and resistance as found in AD.

The inositol phosphatase SHIP2 negatively regulates insulin/IGF-I actions implicated in neuroprotection and memory function in mouse brain

Impairment of insulin and IGF-I signaling in the brain is one of the causes of dementia associated with diabetes mellitus and Alzheimer's disease. However, the precise pathological processes are largely unknown. In the present study, we found that SH2-containing inositol 5'-phosphatase 2 (SHIP2), a negative regulator of phosphatidylinositol 3,4,5-trisphosphate-mediated signals, is widely expressed in adult mouse brain. When a dominant-negative mutant of SHIP2 was expressed in cultured neurons, insulin signaling was augmented, indicating physiological significance of endogenous SHIP2 in neurons. Interestingly, SHIP2 mRNA and protein expression levels were significantly increased in the brain of type 2 diabetic db/db mice. To investigate the impact of increased expression of SHIP2 in the brain, we further employed transgenic mice overexpressing SHIP2 and found that increased amounts of SHIP2 induced the disruption of insulin/IGF-I signaling through Akt. Neuroprotective effects of insulin and IGF-I were significantly attenuated in cultured cerebellar granule neurons from SHIP2 transgenic mice. Consistently, terminal deoxynucleotide transferase-mediated dUTP nick end labeling assay demonstrated that the number of apoptosis-positive cells was increased in cerebral cortex of the transgenic mice at an elderly age. Furthermore, SHIP2 transgenic mice exhibited impaired memory performance in the Morris water maze, step-through passive avoidance, and novel-object-recognition tests. Importantly, inhibition of SHIP2 ameliorated the impairment of hippocampal synaptic plasticity and memory formation in db/db mice. These results suggest that SHIP2 is a potent negative regulator of insulin/IGF-I actions in the brain, and excess amounts of SHIP2 may be related, at least in part, to brain dysfunction in insulin resistance with type 2 diabetes.

Rosiglitazone monotherapy in mild-to-moderate Alzheimer's disease: results from a randomized, double-blind, placebo-controlled phase III study

BACKGROUND/AIMS

A phase II study of the peroxisome proliferator-activated receptor-γ agonist rosiglitazone extended release (RSG XR) in mild-to-moderate Alzheimer's disease (AD) detected a treatment benefit to cognition in apolipoprotein E(APOE)-ε4-negative subjects. The current phase III study with prospective stratification by APOE genotype was conducted to confirm the efficacy and safety of RSG XR in mild-to-moderate AD. An open-label extension study assessed the long-term safety and tolerability of 8 mg RSG XR.

METHODS

This double-blind, randomized, placebo-controlled study enrolled 693 subjects. Within 2 APOE allelic strata (ε4-positive, ε4-negative), subjects were randomized (2:2:2:1) to once-daily placebo, 2 mg RSG XR, 8 mg RSG XR or 10 mg donepezil (control). Coprimary endpoints were change from baseline to week 24 in the Alzheimer's Disease Assessment Scale-cognitive subscale (ADAS-Cog) score, and week 24 Clinician's Interview-Based Impression of Change plus caregiver input (CIBIC+).

RESULTS

At week 24, no significant differences from placebo in change from baseline in coprimary endpoints were detected with either the RSG XR dose in APOE-ε4-negative subjects or overall. For donepezil, no significant treatment difference was detected in ADAS-Cog; however, a significant difference was detected (p = 0.009) on the CIBIC+. Peripheral edema was the most common adverse event for 8 mg RSG XR (15%) and placebo (5%), and nasopharyngitis for 2 mg RSG XR (7%).

CONCLUSION

No evidence of efficacy of 2 mg or 8 mg RSG XR monotherapy in cognition or global function was detected in the APOE-ε4-negative or other analysis populations. The safety and tolerability of RSG XR was consistent with its known pharmacology.

Insulin signaling plays a dual role in Caenorhabditis elegans memory acquisition and memory retrieval

Insulin signaling plays a prominent role in regulation of dauer formation and longevity in Caenorhabditis elegans. Here, we show that insulin signaling also is required in benzaldehyde-starvation associative plasticity, in which worms pre-exposed to the odor attractant benzaldehyde in the absence of food subsequently demonstrate a conditioned aversion response toward the odorant. Animals with mutations in insulin-related 1 (ins-1), abnormal dauer formation 2 (daf-2), and aging alteration 1 (age-1), which encode the homolog of human insulin, insulin/IGF-1 receptor, and PIP3 kinase, respectively, demonstrated significant deficits in benzaldehyde-starvation associative plasticity. Using a conditional allele, we show that the behavioral roles of DAF-2 signaling in associative plasticity can be dissociated, with DAF-2 signaling playing a more significant role in the memory retrieval than in memory acquisition. We propose DAF-2 signaling acts as a learning-specific starvation signal in the memory acquisition phase of benzaldehyde-starvation associative plasticity but functions to switch benzaldehyde-sensing amphid wing C neurons into an avoidance signaling mode during memory retrieval.

Rosiglitazone reversal of Tg2576 cognitive deficits is independent of peripheral gluco-regulatory status

Converging lines of evidence associate gluco-regulatory abnormalities and peroxisome-proliferator-activated receptor (PPAR) gamma function with increased risk for Alzheimer's disease (AD). In this study, we used the Tg2576 AD mouse model to test the hypothesis that cognitive improvement following 1 month of PPAR gamma agonism with rosiglitazone (RTZ) correlates with peripheral gluco-regulatory status. We assessed cognition and peripheral gluco-regulatory status of Tg2576 mice following 1 month treatment with RTZ initiated prior to, coincident with, or after, the onset of peripheral gluco-regulatory abnormalities (4, 8, and 12 months of age, respectively). Whereas 5 months old (MO) and 13 MO Tg2576 did not gain cognitive improvement after 1 month treatment with RTZ, 9 MO Tg2576 mice exhibited reversal of associative learning and memory deficits. Peripheral gluco-regulatory abnormalities were improved in 9 and 13 MO Tg2576 with RTZ treatment; RTZ treatment had no effect on the normal glucose status of 5 MO Tg2576 mice. These findings suggest that RTZ-mediated cognitive improvement does not correlate with peripheral gluco-regulatory abnormalities per se, but reflects the age-dependent mechanistic differences that underlie cognitive decline in this mouse model.

Meta-analysis of Alzheimer's disease risk with obesity, diabetes, and related disorders

BACKGROUND

Late-onset Alzheimer's disease (AD) is a multifactorial and heterogeneous disorder with major risk factors including advanced age, presence of an apolipoprotein E epsilon4 (APOE4) allele, and family history of AD. Other risk factors may be obesity and diabetes and related disorders, which are highly prevalent.

METHODS

We reviewed longitudinal epidemiological studies of body mass, diabetes, metabolic syndrome, and glucose and insulin levels on risk for AD. We conducted meta-analyses of the results from these studies.

RESULTS

For obesity assessed by body mass index, the pooled effect size for AD was 1.59 (95% confidence interval [CI] 1.02-2.5; z = 2.0; p = .042), and for diabetes, the pooled effect size for AD was 1.54 (95% CI 1.33-1.79; z = 5.7; p < .001). Egger's test did not find significant evidence for publication bias in the meta-analysis for obesity (t = -1.4, p = .21) or for diabetes (t = -.86, p = .42). Since these disorders are highly comorbid, we conducted a meta-analysis combining all studies of obesity, diabetes, and abnormal glucose or insulin levels, which yielded a highly significant pooled effect size for AD of 1.63 (95% CI 1.39-1.92; z = 5.9; p < .001).

CONCLUSIONS

Obesity and diabetes significantly and independently increase risk for AD. Though the level of risk is less than that with the APOE4 allele, the high prevalence of these disorders may result in substantial increases in future incidence of AD. Physiological changes common to obesity and diabetes plausibly promote AD.

Hippocampal memory processes are modulated by insulin and high-fat-induced insulin resistance

Insulin regulates glucose uptake and storage in peripheral tissues, and has been shown to act within the hypothalamus to acutely regulate food intake and metabolism. The machinery for transduction of insulin signaling is also present in other brain areas, particularly in the hippocampus, but a physiological role for brain insulin outside the hypothalamus has not been established. Recent studies suggest that insulin may be able to modulate cognitive functions including memory. Here we report that local delivery of insulin to the rat hippocampus enhances spatial memory, in a PI-3-kinase dependent manner, and that intrahippocampal insulin also increases local glycolytic metabolism. Selective blockade of endogenous intrahippocampal insulin signaling impairs memory performance. Further, a rodent model of type 2 diabetes mellitus produced by a high-fat diet impairs basal cognitive function and attenuates both cognitive and metabolic responses to hippocampal insulin administration. Our data demonstrate that insulin is required for optimal hippocampal memory processing. Insulin resistance within the telencephalon may underlie the cognitive deficits commonly reported to accompany type 2 diabetes.

Mini-review: impact of recurrent hypoglycemia on cognitive and brain function

Recurrent hypoglycemia (RH), the most common side-effect of intensive insulin therapy for diabetes, is well established to diminish counter-regulatory responses to further hypoglycemia. However, despite significant patient concern, the impact of RH on cognitive and neural function remains controversial. Here we review the data from both human studies and recent animal studies regarding the impact of RH on cognitive, metabolic, and neural processes. Overall, RH appears to cause brain adaptations which may enhance cognitive performance and fuel supply when euglycemic but which pose significant threats during future hypoglycemic episodes.

Insulin and Insulin-Sensitizing Drugs in Neurodegeneration: Mitochondria as Therapeutic Targets

Insulin, besides its glucose lowering effects, is involved in the modulation of lifespan, aging and memory and learning processes. As the population ages, neurodegenerative disorders become epidemic and a connection between insulin signaling dysregulation, cognitive decline and dementia has been established. Mitochondria are intracellular organelles that despite playing a critical role in cellular metabolism are also one of the major sources of reactive oxygen species. Mitochondrial dysfunction, oxidative stress and neuroinflammation, hallmarks of neurodegeneration, can result from impaired insulin signaling. Insulin-sensitizing drugs such as the thiazolidinediones are a new class of synthetic compounds that potentiate insulin action in the target tissues and act as specific agonists of the peroxisome proliferator-activated receptor gamma (PPAR-γ). Recently, several PPAR agonists have been proposed as novel and possible therapeutic agents for neurodegenerative disorders. Indeed, the literature shows that these agents are able to protect against mitochondrial dysfunction, oxidative damage, inflammation and apoptosis. This review discusses the role of mitochondria and insulin signaling in normal brain function and in neurodegeneration. Furthermore, the potential protective role of insulin and insulin sensitizers in Alzheimer´s, Parkinson´s and Huntington´s diseases and amyotrophic lateral sclerosis will be also discussed.

Central nervous insulin resistance: a promising target in the treatment of metabolic and cognitive disorders?

Research on functions and signalling pathways of insulin has traditionally focused on peripheral tissues such as muscle, fat and liver, while the brain was commonly believed to be insensitive to the effects of this hormone secreted by pancreatic beta cells. However, since the discovery some 30 years ago that insulin receptors are ubiquitously found in the central nervous system, an ever-growing research effort has conclusively shown that circulating insulin accesses the brain, which itself does not synthesise insulin, and exerts pivotal functions in central nervous networks. As an adiposity signal reflecting the amount of body fat, insulin provides direct negative feedback to hypothalamic nuclei that control whole-body energy and glucose homeostasis. Moreover, insulin affects distinct cognitive processes, e.g. by triggering the formation of psychological memory contents. Accordingly, metabolic and cognitive disorders such as obesity, type 2 diabetes mellitus and Alzheimer's disease are associated with resistance of central nervous structures to the effects of insulin, which may derive from genetic polymorphisms as well as from long-term exposure to excess amounts of circulating insulin due to peripheral insulin resistance. Thus, overcoming central nervous insulin resistance, e.g. by pharmacological interventions, appears to be an attractive strategy in the treatment and prevention of these disorders. Enhancement of central nervous insulin signalling by administration of intranasal insulin, insulin analogues and insulin sensitisers in basic research approaches has yielded encouraging results that bode well for the successful translation of these effects into future clinical practice.

Insulin-stimulated translocation of GLUT4 to the plasma membrane in rat hippocampus is PI3-kinase dependent

In the central nervous system (CNS) insulin mediates a variety of effects including feeding, metabolism and cognition. The cognitive enhancing effects of insulin are proposed to be mediated through activation of insulin receptors in the hippocampus, an important integration center for learning and memory in the mammalian brain. Since less is known regarding insulin signaling events in the hippocampus, the aim of the current study was to determine whether insulin stimulates similar signaling cascades and GLUT4 translocation in the rat hippocampus as has been described in peripheral tissues. Intracerebroventricular administration of insulin increases hippocampal insulin levels and also stimulates the phosphorylation of Akt in a time-dependent manner. Insulin also stimulates the translocation of GLUT4 to hippocampal plasma membranes in a time course that mirrors the increases in glucose uptake observed during the performance of hippocampal-dependent tasks. Insulin stimulated phosphorylation of Akt and translocation of GLUT4 were blocked by pretreatment with the PI3-kinase inhibitor LY294002. Confocal immunofluorescence determined that insulin stimulated phosphorylation of Akt was localized to neurons and colocalized with the insulin receptor and GLUT4 in the rat hippocampus, thereby identifying the functional anatomical substrates of insulin signaling in the hippocampus. These results demonstrate that insulin-stimulated translocation of GLUT4 to the plasma membrane in the rat hippocampus occurs via similar mechanisms as described in peripheral tissues and suggests that insulin-mediated translocation of GLUT4 may provide a mechanism through which hippocampal neurons rapidly increase glucose utilization during increases in neuronal activity associated with hippocampal-dependent learning.

Awake intranasal insulin delivery modifies protein complexes and alters memory, anxiety, and olfactory behaviors

The role of insulin pathways in olfaction is of significant interest with the widespread pathology of diabetes mellitus and its associated metabolic and neuronal comorbidities. The insulin receptor (IR) kinase is expressed at high levels in the olfactory bulb, in which it suppresses a dominant Shaker ion channel (Kv1.3) via tyrosine phosphorylation of critical N- and C-terminal residues. We optimized a 7 d intranasal insulin delivery (IND) in awake mice to ascertain the biochemical and behavioral effects of insulin to this brain region, given that nasal sprays for insulin have been marketed notwithstanding our knowledge of the role of Kv1.3 in olfaction, metabolism, and axon targeting. IND evoked robust phosphorylation of Kv1.3, as well as increased channel protein-protein interactions with IR and postsynaptic density 95. IND-treated mice had an increased short- and long-term object memory recognition, increased anxiolytic behavior, and an increased odor discrimination using an odor habituation protocol but only moderate change in odor threshold using a two-choice paradigm. Unlike Kv1.3 gene-targeted deletion that alters metabolism, adiposity, and axonal targeting to defined olfactory glomeruli, suppression of Kv1.3 via IND had no effect on body weight nor the size and number of M72 glomeruli or the route of its sensory axon projections. There was no evidence of altered expression of sensory neurons in the epithelium. In mice made prediabetic via diet-induced obesity, IND was no longer effective in increasing long-term object memory recognition nor increasing anxiolytic behavior, suggesting state dependency or a degree of insulin resistance related to these behaviors.

Insulin and the brain

Circulating insulin crosses the blood-brain barrier (BBB) into the central nervous system (CNS). There are many insulin receptors in various areas of the brain; they are expressed by both astrocytes and neurons. The two main insulin actions in the brain are (a) control of food intake and (b) effect on cognitive functions. In obesity there is a relative insulin deficiency in the CNS despite increased circulating levels. Insulin plays an important role in cognitive functions as demonstrated by the intranasal administration of insulin bypassing the liver. Brain insulin decreases with aging and may be related to the decrease in cognitive functions, as has also been reported in Alzheimer's disease. Certain brain tumours over-express insulin receptors. Whether the larger insulin analogues pass the BBB is as yet not known.

The insulin/Akt signaling pathway is targeted by intracellular beta-amyloid

Intraneuronal beta-amyloid (Abeta(i)) accumulates early in Alzheimer's disease (AD) and inclusion body myositis. Several organelles, receptor molecules, homeostatic processes, and signal transduction components have been identified as sensitive to Abeta. Although prior studies implicate the insulin-PI3K-Akt signaling cascade, a specific step within this or any essential metabolic or survival pathway has not emerged as a molecular target. We tested the effect of Abeta42 on each component of this cascade. In AD brain, the association between PDK and Akt, phospho-Akt levels and its activity were all decreased relative to control. In cell culture, Abeta(i) expression inhibited both insulin-induced Akt phosphorylation and activity. In vitro experiments identified that beta-amyloid (Abeta), especially oligomer preparations, specifically interrupted the PDK-dependent activation of Akt. Abeta(i) also blocked the association between PDK and Akt in cell-based and in vitro experiments. Importantly, Abeta did not interrupt Akt or PI3K activities (once stimulated) nor did it affect more proximal signal events. These results offer a novel therapeutic strategy to neutralize Abeta-induced energy failure and neuronal death.

Extracts of Liriopsis tuber protect AMPA induced brain damage and improve memory with the activation of insulin receptor and ERK I/II

The brain insulin receptor and ERK I/II are known to play an important role in memory formation and neuroprotection. A series of experiments was designed to explore if Liriopsis tuber (LT) extracts could exhibit neuroprotection and memory enhancing actions. LT was extracted with 70% methanol and subsequently fractionated into chloroform (fraction C), chloroform/methanol-(3:1) (fraction CM), methanol-soluble (fraction M) and methanol-insoluble, water-soluble fractions (fraction A). The LT fractions (T, C, M, A) significantly inhibited the cortical depolarization induced by AMPA in cortical slices of rats. In addition, these fractions were also effective in promoting memory in the passive avoidance test in mice. To gain insight into the mechanism of memory enhancing effects by Liriopsis tuber extracts, the activities of hippocampal insulin receptors and ERK I/II were tested in rats. Extract of LT (T) dramatically stimulated tyrosine phosphorylation of the insulin receptor, while fraction C of LT also significantly stimulated the same. In addition, ERK I/II were stimulated and cholinesterase activities were inhibited by fractions T, C, M and A in the rat hippocampus. These results suggest that Liriopsis tuber extracts may exert neuroprotection and memory enhancing effects via activation of the insulin receptor and ERK I/II as well as inhibiting cholinesterase.

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.

Curcumin ameliorates impaired insulin/IGF signalling and memory deficit in a streptozotocin-treated rat model

Increased serum insulin levels and reduced peripheral insulin activities seen in insulin resistance syndrome are associated with age-dependent cognitive impairment and Sporadic Alzheimer's Disease (SAD), suggesting a disturbance in the insulin signalling system in the brain and possibly being one of the causes of dementia. Therefore, the streptozotocin (STZ)-induced animal may be an appropriate model for the investigation of SAD and related dementia. This study was designed to investigate the beneficial effect of Curcumin (CUR), a neuroprotective agent, on intracerebroventricular (ICV) STZ-induced cognitive impairment in rats. For this purpose, adult male Wistar rats were bilaterally ICV injected with STZ (3 mg/kg). An artificial cerebrospinal fluid (aCSF) was given to the control group (SHAM) instead of STZ on days 1 and 3. Learning and memory performance were assessed using the "passive avoidance task" and the "Morris water maze test". After confirmation of acquisition impairment with these tests, the STZ group was divided into two subgroups: STZ + vehicle (Vh) and STZ + CUR. The rats in the SHAM and STZ + Vh groups were administered intraperitoneally with 0.5 ml Vh and the rats in the STZ + CUR group were treated intraperitoneally with CUR (300 mg kg(-1) day(-1) in Vh) for 10 days starting from the 25th day after STZ injection. The Morris water maze test was reapplied on the 35th day after STZ injection and all of the rats were sacrificed on day 36 for quantitation of IGF-1 and for histopathological evaluation. Rats in the STZ + CUR group were found to have a higher performance in cognitive tests than rats in the STZ + Vh group (P < 0.01). In parallel with the cognitive tests, IGF-1 levels were decreased in all of the STZ-injected groups (1.78 +/- 0.34) compared to the SHAM group (3.46 +/- 0.41). In contrast, CUR treatment significantly increased IGF-1 levels (P < 0.001). The degree of neuronal loss decreased after CUR treatment compared to the SHAM group (P < 0.02). These results clearly indicate that CUR treatment is effective in reducing the cognitive impairment caused by STZ in rats, and may be a potential therapeutic agent for altering neurodegeneration in SAD.

Déficit cognitivo: mais uma complicação do diabetes melito?

Com o envelhecimento da população, as doenças crônicas serão mais prevalentes, como o diabetes melito (DM) e aquelas caracterizadas por disfunções cognitivas, como as demências. Alguns estudos mostraram associação do DM e outros fatores de risco cardiovascular associados a distúrbios cognitivos. Além das complicações vasculares, estudos sugerem ação da hiperglicemia e dos produtos avançados finais de glicação (PAFG) em estresse oxidativo e acúmulo de substância β-amilóide intracerebral. Outros fatores também vêm sendo investigados, como o papel da insulinemia, da genética e do IGF-1 (insulin-like growth factor-1). Estudos mostraram que o bom controle glicêmico e a ingestão de dieta rica em gordura poliinsaturada, ômega-3 ou alimentos antioxidantes podem ter papel protetor contra os déficits cognitivos. Esclarecimentos sobre a associação entre DM e cognição e sua fisiopatologia podem ser essenciais para a prevenção e o tratamento de déficits cognitivos, levando a impacto positivo sobre a qualidade de vida dos pacientes idosos com DM.

Branched-chain amino acids may improve recovery from a vegetative or minimally conscious state in patients with traumatic brain injury: a pilot study

OBJECTIVE

To investigate whether supplementation with branched-chain amino acids (BCAAs) may improve recovery of patients with a posttraumatic vegetative or minimally conscious state.

DESIGN

Patients were randomly assigned to 15 days of intravenous BCAA supplementation (n=22; 19.6g/d) or an isonitrogenous placebo (n=19).

SETTING

Tertiary care rehabilitation setting.

PARTICIPANTS

Patients (N=41; 29 men, 12 women; mean age, 49.5+/-21 y) with a posttraumatic vegetative or minimally conscious state, 47+/-24 days after the index traumatic event.

INTERVENTION

Supplementation with BCAAs.

MAIN OUTCOME MEASURE

Disability Rating Scale (DRS) as log(10)DRS.

RESULTS

Fifteen days after admission to the rehabilitation department, the log(10)DRS score improved significantly only in patients who had received BCAAs (log(10)DRS score, 1.365+/-0.08 to 1.294+/-0.05; P<.001), while the log(10)DRS score in the placebo recipients remained virtually unchanged (log(10)DRS score, 1.373+/-0.03 to 1.37+/-0.03; P not significant). The difference in improvement of log(10)DRS score between the 2 groups was highly significant (P<.000). Moreover, 68.2% (n=15) of treated patients achieved a log(10)DRS point score of .477 or higher (3 as geometric mean) that allowed them to exit the vegetative or minimally conscious state.

CONCLUSIONS

Supplemented BCAAs may improve the recovery from a vegetative or minimally conscious state in patients with posttraumatic vegetative or minimally conscious state.

Insulin increases excitability via a dose-dependent dual inhibition of voltage-activated K+ currents in differentiated N1E-115 neuroblastoma cells

A role in the control of excitability has been attributed to insulin via modulation of potassium (K(+)) currents. To investigate insulin modulatory effects on voltage-activated potassium currents in a neuronal cell line with origin in the sympathetic system, we performed whole-cell voltage-clamp recordings in differentiated N1E-115 neuroblastoma cells. Two main voltage-activated K(+) currents were identified: (a) a relatively fast inactivating current (I(fast) - time constant 50-300 ms); (b) a slow delayed rectifying K(+) current (I(slow) - time constant 1-4 s). The kinetics of inactivation of I(fast), rather than I(slow), showed clear voltage dependence. I(fast) and I(slow) exhibited different activation and inactivation dependence for voltage, and have different but nevertheless high sensitivities to tetraethylammonium, 4-aminopyridine and quinidine. In differentiated cells - rather than in non-differentiated cells - application of up to 300 nm insulin reduced I(slow) only (IC(50) = 6.7 nm), whereas at higher concentrations I(fast) was also affected (IC(50) = 7.7 microm). The insulin inhibitory effect is not due to a change in the activation or inactivation current-voltage profiles, and the time-dependent inactivation is also not altered; this is not likely to be a result of activation of the insulin-growth-factor-1 (IGF1) receptors, as application of IGF1 did not result in significant current alteration. Results suggest that the current sensitive to low concentrations of insulin is mediated by erg-like channels. Similar observations concerning the insulin inhibitory effect on slow voltage-activated K(+) currents were also made in isolated rat hippocampal pyramidal neurons, suggesting a widespread neuromodulator role of insulin on K(+) channels.

The As and Ds of stress: metabolic, morphological and behavioral consequences

Unlike responses to acute stressful events that are protective and adaptive in nature, chronic stress elicits neurochemical, neuroanatomical and cellular changes that may have deleterious consequences upon higher brain functioning. For example, while exposure to acute stress facilitates memory formation and consolidation, chronic stress or chronic exposure to stress levels of glucocorticoids impairs cognitive performance. Chronic stress or glucocorticoid exposure, as well as impairments in hypothalamic-pituitary-adrenal (HPA) axis function are proposed to participate in the etiology and progression of neurological disorders such as depressive illness, anxiety disorders and post-traumatic stress disorder (PTSD). HPA axis dysfunction, impaired stress responses and elevated basal levels of glucocorticoids are also hallmark features of experimental models of type 1 and type 2 diabetes, as well as diabetic subjects in poor glycemic control. Such results suggest that stress and glucocorticoids contribute to the neurological complications observed in diabetes patients. Interestingly, many of the hyperglycemia mediated changes in the brain are similar to those observed in depressive illness patients and in experimental models of chronic stress. Such results suggest that common mechanisms may be involved in the development of the neurological complications associated with Anxiety, Depressive illness and Diabetes: the As and Ds of stress. The aim of the current review will be to discuss the mechanisms through which limbic structures such as the hippocampus and amygdala respond and adapt to the deleterious consequences of chronic stress and hyperglycemia.

Insulin signaling effects on memory and mood

The escalating obesity/diabetes epidemic is an important health-care issue that has crucial socio-economic ramifications. The complications of diabetes/obesity phenotypes extend to the central nervous system (CNS), including the hippocampus, a brain region that is particularly vulnerable to hyperglycemia and insulin resistance. Deficits in hippocampal synaptic plasticity observed in diabetes ultimately have deleterious consequences upon cognitive function. For example, recent studies using brain imaging technologies have identified cerebral atrophy in diabetic patients, suggesting that the neuroanatomical changes observed in experimental models of diabetes may accurately reflect what is occurring in the clinical setting. Deficits in insulin receptor (IR) signaling and impairments in hypothalamic-pituitary-adrenal (HPA) axis function also contribute to the neurological complications of diabetes phenotypes. The pathophysiological similarities between diabetes and stress-related mood disorders suggest that common mechanistic mediators may be involved in the etiology and progression of the neurological complications of these disorders. When combined with the accumulating evidence from pre-clinical models, these data support the hypothesis that a long-term consequence of diabetes/obesity phenotypes is accelerated brain aging that results in neuropsychological deficits and increased vulnerability to co-morbidities such as depressive illness.

Insulin and ghrelin: peripheral hormones modulating memory and hippocampal function

Peptide hormones, initially identified in the periphery and best known for regulation of food intake and appetite, have increasingly been shown to regulate brain functions not only within the hypothalamus but elsewhere. The hippocampus, in particular, expresses receptors for many hormones. Both insulin and ghrelin are now known to enhance hippocampal memory processes; in addition, insulin acts to increase local hippocampal metabolism and regulate synaptic plasticity, while administration of ghrelin has been shown to promote dendritic spine synaptic formation and to increase anxiety. While insulin's effects appear to be specifically within the hippocampus, ghrelin may act at a range of sites within the limbic system.

Obese men respond to cognitive but not to catabolic brain insulin signaling

CONTEXT AND OBJECTIVE

Insulin acts in the brain to reduce food intake and body weight and is considered a major adiposity signal in energy homeostasis. In normal-weight men, intranasal insulin administration reduces body fat and improves declarative memory. The present experiments aimed to generalize these findings to obese patients, with a view to evaluate the therapeutic potential of the compound.

DESIGN, SUBJECTS AND MEASUREMENTS

Insulin and placebo, respectively, were intranasally administered four times a day (amounting to 160 IU day(-1)) over 8 weeks to two groups of 15 obese men each.

RESULTS

Contrasting with the catabolic effects in normal-weight men, insulin treatment did not induce any significant reduction of body weight (P>0.50) and body fat (P>0.44) in the obese subjects. However, in accordance with the effects in normal-weight men, declarative memory and mood were improved (P<0.05) and hypothalamic-pituitary-adrenal axis activity as assessed by circulating ACTH (P<0.01) and cortisol levels (P<0.04) was reduced.

CONCLUSIONS

Our results indicate that in obese men, intranasal insulin is functionally active in the central nervous system but fails to affect the neuronal networks critically involved in body weight regulation. We conclude that obesity in men is associated with central nervous resistance to the adiposity signal insulin. This defect likely contributes to the persistence of obesity in spite of elevated levels of circulating insulin in obese patients.

Taurine Induces Anti-Anxiety by Activating Strychnine-Sensitive Glycine Receptor in vivo

Taurine has a variety of actions in the body such as cardiotonic, host-defensive, radioprotective and glucose-regulatory effects. However, its action in the central nervous system remains to be characterized. In the present study, we tested to see whether taurine exerts anti-anxiety effects and to explore its mechanism of anti-anxiety activity in vivo. The staircase test and elevated plus maze test were performed to test the anti-anxiety action of taurine. Convulsions induced by strychnine, picrotoxin, yohimbine and isoniazid were tested to explore the mechanism of anti-anxiety activity of taurine. The Rotarod test was performed to test muscle relaxant activity and the passive avoidance test was carried out to test memory activity in response to taurine. Taurine (200 mg/kg, p.o.) significantly reduced rearing numbers in the staircase test while it increased the time spent in the open arms as well as the number of entries to the open arms in the elevated plus maze test, suggesting that it has a significant anti-anxiety activity. Taurine's action could be due to its binding to and activating of strychnine-sensitive glycine receptor in vivo as it inhibited convulsion caused by strychnine; however, it has little effect on picrotoxin-induced convulsion, suggesting its anti-anxiety activity may not be linked to GABA receptor. It did not alter memory function and muscle activity. Taken together, these results suggest that taurine could be beneficial for the control of anxiety in the clinical situations.